Catalytic Upgrading of Extractives to Chemicals: Monoterpenes to

Review pubs.acs.org/CR

Catalytic Upgrading of Extractives to Chemicals: Monoterpenes to “EXICALS” Mikhail Golets,*,† Samikannu Ajaikumar,‡ and Jyri-Pekka Mikkola*,†,‡ †

Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, FI-20500 Åbo-Turku, Finland ‡ Technical Chemistry, Department of Chemistry, Chemical-Biological Centre, Umeå University, SE-90187 Umeå, Sweden renewable raw materials are key goals of any biorefinery-related research.1−3 The chemical and mechanical pulping of wood generates large amounts of side streams, of which crude turpentine (CT) is one of the most interesting.1,4 In fact, the estimated worldwide production of turpentine is about 350000 t/year.5,6 Currently, in chemical pulping, the crude sulfate turpentine (CST) is condensed from digester vapors. Although the sulfate method (the Kraft process) is the current predominant technology, some sulfite mills also still exist. Sulfite turpentine emerges when crude tall oil is skimmed from pulping liquor, neutralized with NaOH or lime, and subsequently distilled.7,8 In addition, minor amounts of high-purity wood turpentines are steamed directly from chopped tree trunks.9 Still, in mechanical CONTENTS pulping processes, terpenes are recovered from the mill by 1. Introduction A steam distillation.10 The estimated amount of turpentine 2. Turpentine Composition and Upgrading B produced depends highly on the tree species processed and 3. α-Pinene Upgrading Techniques B reaches 0.3−1.0 and 2−16 kg/ton of pulp for the sulfite and 3.1. Isomerization B sulfate processes, respectively. In most cases, roughly 75% of 3.2. Isomerization of “Oxygenated Terpenoids” G the native turpentine composition remains unchanged during 3.2.1. α-Pinene Epoxide G the extraction.7,8,11 Still, thermomechanical pulping (TMP) 3.2.2. Limonene Epoxide I processes typically allow for the recovery of 0.3 kg of turpentine 3.3. Dehydroisomerization J per ton of pulp.10,11 Very often, these fractions are utilized on3.4. Oxidation and Epoxidation M site; they are commonly burned in recovery boilers to generate 3.4.1. Oxidation and Epoxidation of α-Pinene M steam used by the mills, and also to produce electricity.8 3.4.2. Epoxidation of Limonene and CamIn addition, for a long time, turpentine has been utilized by phene O the perfumery industry and as a solvent for dyes and 3.4.3. Oxidation of ρ-Cymene Q varnishes.10,12 Furthermore, purified CST is potentially 3.5. Hydrogenation S applicable as a 5−20% fuel blend because of its low volatility 3.6. Acetoxylation V and advantageous volumetric net heat of combustion.10,13 3.7. Dimerization W Herein, we review a series of important scientific advances 4. Conclusions Y made within the time frame of ca. 2000−2014 in the upgrading Author Information Y of α-pinene, the predominant compound of crude turpentine, Corresponding Authors Y into value-added products through heterogeneous catalysis Notes Y involving various different active species (such as metal Biographies Y functions or acid sites) on the catalyst surface. Moreover, Acknowledgments Z various transformations of limonene, camphene, β-pinene, and References Z ρ-cymene are also considered. Furthermore, several examples of successful applications utilizing crude turpentine are described. Evidently, several approaches taking advantage of heteroge1. INTRODUCTION neous catalysis have been developed and improved during recent decades involving materials that are highly selective Evidently, the global demand for oil products is bound to toward the desired products, namely, catalysts with good increase and is predicted to reach 5700 × 106 t/year by 2030.1 reusability, low toxicity, simple handling, and easy disposal. In In addition to expected oil price increases, further increases in general, investigations concerning catalytic aspects, reaction greenhouse-gas (GHG) emissions are foreseen as a result of the extraction of oil, particularly from unconventional sources such as oil sands and oil shale. Decreasing our current oil Received: July 30, 2014 dependence and reducing GHG emissions by shifting toward © XXXX American Chemical Society

A

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Table 1. Approximate Compositions (wt %) of Various Turpentines1,7,8,10−12,14 turpentine

a

country

α-pinene

United States/Canada Russia Sweden Finland

40−70 55−70 50−70 55−70

Sweden

2

United States/Canada

75−85

Sweden

40−60

β-pinene

camphene

Sulfate (Kraft) 1−2 1−8 1−10 ≤1 Sulfite ≤1 2 Steam-Distilled (Wood) Turpentine 0−3 4−15 Thermomechanical Turpentine 25 2 15−35 1−5 4−20 2−6

3-carene

limonene

ρ-cymene

2−10 10−25 10−40 7−30

5−10 3−8 1−9 ≤4

npa npa ≤1 npa

≤1

≤1

90

npa

5−15

npa

3−5

5−17

≤1

np, not provided.

3. α-PINENE UPGRADING TECHNIQUES α-Pinene is a bicyclic terpene exhibiting a high reactivity that has been broadly studied in various reactions such as isomerization, dehydroisomerization, oxidation, hydrogenation, hydration, dimerization, and acetoxylation reactions.12,14 Optical activity is an important aspect of monoterpene chemistry, and in some cases, it helps in elucidating the reaction mechanisms, providing explanations in terms of the interactions between the substrate and the catalyst and even in terms of tuning the catalyst stereoselectivity. Unfortunately, many authors do not specify the optical nature of the terpene reactant employed or the terpenoid obtained. The geometric (cis/trans) and stereoisomeric structures of monoterpenes, described in the current review, are depicted in Figure 1. The optical activities of α- and β-pinenes are represented by the chiral centers at C1 and C5 and defined by the spatial configuration of the sixth carbon atom. Other terpenoids with such bicyclic structures demonstrate similar stereoactivities (Figure 1).21,22 In nature, two optical isomers exist for limonene: R-(+) and S-(−), with orange and lemon odors, respectively.23,24 In this molecule, the chiral center is situated at the fourth carbon atom of the limonene ring skeleton. Also, monocyclic terpene derivatives with related isomers demonstrate similar optical activities.25 However, an additional chiral center could also exist at the first carbon atom of the monoterpenoid ring if it were hydrogenated or functionalized at that position (Figure 1). Of the two above-mentioned isomers, the R-(+) isomer is the more abundant and accessible. Currently, related studies as described in this review are restricted to R-(+)-limonene.23,24,26−29 By default, most studies have been performed with R-(+)-limonene as the more reactive substrate, although studies comparing the relative activities of R-(+)- and S(−)-limonene substrates have also appeared. Furthermore, the formation of various optical isomers is schematically presented for all described reactions.

parameters, and mechanistic pathways are also discussed and compared.

2. TURPENTINE COMPOSITION AND UPGRADING Turpentine is a collective term used for a mixture comprising numerous C10H16 monoterpene isomers. In general, bicyclic compounds such as 3-carene, camphene, and α- and β-pinenes, together with monocyclic limonene, are the principal compounds of this raw material.1,4,7,9 The chemical composition of crude turpentine (CT) varies strongly with the wood species, biomass growth region, pulping process or mill, and even harvesting season (Table 1). For example, Kraft turpentine from the United States can contain more β-pinene than α-pinene.7 However, in turpentine originating from sulfite pulping, ρ-cymene is typically the predominant compound.7,8,11,14 Because of the use of sulfur-containing cooking chemicals upon pulping, the sulfur content in CT can reach 3 wt %, whereupon the three main species present are methanethiol, dimethyl sulfide (DMS), and dimethyldisulfide (DMDS). The organoleptic properties of the aforementioned malodorous organics complicate the further use and upgrading of CT, as well as the isolation and utilization of specific terpenes.1,4,7,15 Furthermore, sulfur is a well-known catalyst poison that efficiently compromises catalytic activity by means of poisoning and deterioration of the active sites, particularly in noble-metal catalysts.4 For further utilization of bicyclic monoterpenes, sulfur removal is required in such a way that the initial CT composition is maintained and formation of degradation products is minimized.1 Through the combination of various absorption, fractionation, hypochlorite, and metal treatment techniques, the sulfur content can be decreased.16,17 In addition, Knuuttila successfully used the hydrodesulfurization (HDS) of CST over mixed NiMo/γ-Al2O3 and NiW-NiO catalysts.1 Also, up to 80 wt % sulfur was eliminated upon HDS treatment with low substrate degradation in a study by Casbas et al., who used sodium doped Co and Mo catalysts.18,19 In contrast to that obtained from the sulfate and sulfite processes, TMP turpentine is sulfur-free and therefore of higher value.10,11 Direct catalytic upgrading of turpentine is, in general, complicated because of the high variety of its constituents.14 Evidently, in most pulping processes, pinene is the predominant turpentine compound, and thus, the appropriate catalyst selection and the desired process optimization should reflect this fact.20

3.1. Isomerization

Isomerization is the fastest reaction or primary reaction in essentially every α- and β-pinene upgrading process.32−34 It is possible to classify α- and β-pinene isomerization derivatives as monocyclic (limonene, α- and γ-terpinene, terpinolene, etc.), bicyclic (camphene), and tricyclic (tricyclene) compounds.35 Upon selective isomerization of α-pinene, it is desirable to maximize the camphene yield. Roughly 10 t/year of camphene is utilized directly as a fragrance, and another 12 t/year is used B

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Figure 1. Absolute configurations of monocyclic and bicyclic monoterpenes and their derivatives described in the current review: (A) pinene and related, (B) limonene and related, (C) camphene and related, (D) campholenic aldehyde (five-membered ring). Adapted from refs 21−25, 30, and 31.

as a solvent for varnishes by, for example, the automobile industry.14 However, camphor is perhaps the most attractive product and is currently obtained from camphene. Camphor is a compound used in insect repellents, films, plastics, and explosives.9,14,36,37 On the other hand, camphene is conventionally synthesized in stirred-tank reactors over TiO2/H2SO4 catalytic systems and is associated with common problems related to the use of homogeneous catalysis.9,14,36,38 Both camphor and camphene exist in two enantiomeric forms (Figure 1). The (+)-camphor enantiomeric form is the naturally occurring one and could be obtained, for example, by direct steam distillation of Cinnamomum camphora wood. In addition, (+)- and (−)-camphors can also be synthesized in the laboratory (Figure 1).39

The second well-known isomerization product, limonene, is widely available from orange and lemon juice production and pulping in approximate amounts of 50000 t/year.20,40,41 Limonene is of less value, but is still utilized by pharmaceutical industry, resin synthesis and perfumery industry; for example in menthol synthesis.14,38,42,43 Menthol is an important pharmaceutical, alimentary, and perfume chemical existing as eight isomers: (+)-menthol, (−)-menthol, (+)-neo-menthol, (−)-neo-menthol, (+)-isomenthol, (−)-isomenthol, (+)-neo-isomenthol, and (−)-neoisomenthol. However, among these isomers, (−)-menthol (Figure 1) is the only one with high value because of its characteristic odor and physiological cooling effect.44 During the past decade, conventional extraction from mint could not C

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Scheme 1. Reaction Network for the Isomerization of α- and β-Pinenes14,20,21,30,32,33,42,49−53

Table 2. Current Achievements in the Isomerization of α-Pinene ref

yield (wt %)

53

66

2.2 wt % Au/γ-Al2O3

20 42

61 50

Fe3+-exchanged clinoptilolite 15% H3PW12O40/TiO2-500

54 43 9 55

45 41 32 38

20% H3PW12O40/SiO2 AlCl3/γ-Al2O3 ammonium forms of ferrierite H4SiW12O40/SiO2

56

35

montmorillonite K10

36

25

Amberlyst 35

43 57

53 45

AlCl3/γ-Al2O3 H3PW12O40/SiO2

52 35

42 25

Amberlyst 70 clinoptilolite zeolite

catalyst

reactor type

process conditions

Substrate, α-Pinene; Product, Camphene flow-through 0.4 vol % substrate in octane, 0.2 g of catalyst, H2, 200 °C, 7−8 h reactor glass reactor 2 mL of substrate, 0.3 g of catalyst, 10 mL/min N2, 155 °C, 8 h flow-through continuous flow of substrate (liquid), 0.04 mL/min, 0.1 g of catalyst, 20 mL/min N2, reactor 200 °C, 10 h glass reactor 2 mL of substrate, 0.2 mL of dodecane, 0.6 wt % catalyst, 100 °C, 1 h glass reactor 2 mL of substrate, 0.1 g of catalyst, 40 °C, 2 h glass reactor 5 mL of substrate, 0.25 g of catalyst, 40−80 °C, 3 h glass reactor 23.3 mL of substrate, 0.2 g of catalyst, 100/0.25 substrate/catalyst ratio, 100 °C, 12 h Substrate, β-Pinene; Product, Camphene glass reactor 40.7 mL of substrate slowly added to slurry (0.5 g of catalyst/10 mL heptane) at 0 °C, further heating to 30 °C glass reactor 100 mL of substrate, 1−4 g of catalyst, 100−140 °C, 4 h Substrate, α-Pinene; Product, Limonene glass reactor 2 mL of substrate, 0.1 g of catalyst, 40 °C, 2 h Radleys carousel 10 mL of substrate, 0.1 g of catalyst, 30 °C, 24 h reactor pressurized vessel 5.4 mL of substrate, 120 mL of acetic acid, 0.25 g of catalyst, 20 bar O2, 100 °C, 12 h glass reactor 50 mL of substrate, 1 g of catalyst, 155 °C, 5 h

D

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Table 3. Product Distributions for the Isomerization of α-Pinene over Iron-Loaded Clinoptilolites as a Function of Iron Contenta selectivityb (%) Fe content (%)

conversion (%)

1

2

3

4

5

6

7

8

9

2.7 2.4 1.1 0.7 0.3 −c

100 98.6 100 93.7 100 96.0

38.2 16.6 1.0 65.6 5.8 32.5

1.9 8.3 18.0 4.4 1.4 35.4

14.0 15.8 4.4 7.7 15.4 3.6

7.1 7.7 10.5 6.5 − −

10.5 11.8 3.2 6.8 32.4 −

14.2 15.5 18.3 8.9 45.0 28.5

14.1 15.4 12.2 − − −

− 4.2 − − − −

− 4.7 2.4 − − −

Adapted from ref 20. bIsomers: 1, camphene; 2, limonene; 3, α-terpinene; 4, ρ-cymene; 5, γ-terpinene; 6, α-terpinolene; 7, cis-ocymene; 8, αfenchene; 9, β-fenchene. cRaw CL. a

phenomenon occurs when water molecules coordinate to the Fe3+ or Cr3+ ions. The polarizing effect of trivalent metal ions created the possibility for water dissociation, thus increasing the Brønsted acidity and favoring isomerization. In addition, higher Brønsted acidity was demonstrated for the FeCl catalyst in comparison to the chromium one.20 A nonlinear dependence of the selectivity on the Fe content was also demonstrated. It was found that an Fe loading of 0.7 wt % was optimal, thus providing 65.6 wt % selectivity toward camphene. Meanwhile, the nonloaded catalyst was more selective toward camphene than catalysts containing 1.1 or 2.4 wt % iron. Presumably, the Brønsted acidity of the catalyst varied at various loadings, resulting in fluctuations in the selectivity toward camphene. The selectivity toward limonene was always low for this catalyst, and the maximum value of 18.0 wt % was reached at a loading of 1.1 wt % Fe. The authors suggested that secondary isomerization of limonene occurred on FeCl catalysts with high Brønsted acidities. They reported that the concentrations of α-terpinolene and γ-terpinene were maximized at low iron content. However, with increased iron loadings, substantial amounts of cis-ocymene formed. Furthermore, the enhanced metal loading did not influence the conversion significantly. Consequently, the product distributions obtained over FeCl catalysts with various Fe content levels are described in Table 3. On the contrary, the catalyst loaded with 0.4% chromium resulted in more uniform selectivities of 43.6 and 45.4 wt % toward limonene and camphene, respectively. Hence, different behaviors of the loaded cations were demonstrated and lower acidity of the CrCl catalyst resulted in an increased selectivity toward limonene by suppressing its secondary isomerization. On the other hand, the selectivities toward both camphene and limonene decreased significantly with higher loadings of chromium and promoted the formation of ρ-cymene, cisocymene, and γ-terpinene at the expense of α-terpinene and αterpinolene. The authors suggested that an optimum loading was needed for the best performance in catalytic tests.20 Several earlier studies reported the application of supported phosphotungstic heteropolyacids (H3PW12O40/SiO2) in the isomerization of α-pinene. Typically, these catalysts were prepared by wet impregnation. Recently, Szucs-Balazs et al. tested a 10 wt % H4SiW12O40 catalyst supported on SiO2, TiO2, and H-ZSM-5.55 In the case of reaction for 12 h at 100 °C over the silica support, the concentrations of limonene and camphene increased to 21.8 and 38.4 wt %, respectively. A similar product distribution was observed after 2 h at 160 °C. In addition, the selectivity of camphene decreased with high catalyst loadings. Disappointingly, the catalyst supported on titania was less active. Furthermore, the H4SiW12O40/H-ZSM-5

meet the annual demand for (−)-menthol, even after an increase in availability from 6300 to 20000 t (2010). Therefore, several successful synthetic methods were developed, including an elegant route starting from myrcene (Takasago International Corporation, Tokyo, Japan). The method is based on the catalytic asymmetric isomerization of geranyldiethylamine over the chiral catalyst (S)-BINAP-Rh.45 As an alternative, the Symerise and BASF processes allow the production of menthol from m-cresol and citral, respectively.46 On the other hand, 3-carene can be isomerized to 2-carene and further hydrated to menthol.14 Selective isomerization of 3carene was achieved over alkali X and Y zeolites and basic oxides, such as MgO or CaO.14 Other isomerization products, namely, terpinenes and terpiniolenes, are usually considered to be byproducts and could, in principle, be used to synthesize menthol or ρ-cymene, for example.12 The isomerization of pinenes is initiated by the protonation of the double bond with the subsequent formation of a pinyl cation, which subsequently rearranges to monocyclic limonenelike and bicyclic camphene-like terpenes by bond migration to the positively charged carbon.14,20 Commonly, the isomerization of pinenes occurs irreversibly through parallel steps and isomerization products; for example, limonene and camphene cannot readily be converted back to the α- and β-pinene substrates.13,20 Furthermore, secondary isomerization of limonene to terpinolene is a nonselective process that, nevertheless, can occur over either acidic or basic catalysts.14 In addition, a fraction of α-pinene can be isomerized to βpinene or fenchenic compounds (Scheme 1).6,47,48 The challenge of efficient synthesis of camphene and limonene implementing heterogeneous catalysts is exemplified by the trials listed in Table 2. Efficient isomerization of α-pinene to camphene was recently demonstrated by Akgul et al.20 Their Fe3+-loaded clinoptilolites resulted in a 61 wt % yield of camphene. Upon studying the Xray diffraction (XRD) profiles of their catalyst before and after the reaction, they found that the catalyst did not suffer from deterioration of its structural properties. However, the authors reported that the pore dimensions of clinoptilolite were not large enough to accommodate the α-pinene molecule. Hence, the internal acid sites of clinoptilolite remained inaccessible, and the reaction occurred only at the acid sites of the mesopore walls. As demonstrated by the catalyst characterization, the sample surface area, pore diameter, and crystallinity of the catalyst remained unchanged upon the exchange of natural zeolites with Fe3+ and Cr3+ ions. As evidenced by ammonia adsorption methods, an increase in the amount of Brønsted sites was reported after Fe3+ and Cr3+ ions were introduced in to the framework. The authors hypothesized that this E

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Table 4. Characteristics of Two Hydrogen Forms of Ferrieritea acid site strengthe

concentration of Brønsted acid sites (mmol/g) b

sample

Si/Al

H-FER (Z) H-FER (T)

10.0 8.9

c

2

SSA (m /g) 287 275

1

d

e

H MAS NMR

FTIR

0.904 1.389

0.84 1.172

average

ΔνCO (cm−1)

PAf(kJ/mol)

0.872 1.281

295 286

1162 1168

a

Adapted from ref 9. bDetermined by chemical wet analysis. cSSA, specific surface area. dDetermined by 1H MAS NMR spectroscopy eDetermined by ammonia adsorption using FTIR spectroscopy fPA, proton affinity

catalyst, operated at 160 °C, provided a camphene selectivity comparable to that of the silica-supported catalyst. According to the XRD studies, SiW was completely dispersed on the tested supports.55 Also, a similar catalyst was investigated by da Silva Rocha et al. in 2009.54 The reaction performed over a 20 wt % H3PW12O40/SiO2 catalyst at 100 °C resulted in 90 wt % conversion, giving limonene and camphene selectivities of 28 and 50 wt %, respectively. The catalyst was stable to leaching and was recoverable under the studied reaction conditions (Table 2).54 Paying attention to the catalyst preparation, Newman et al. investigated the function of the acid loading.57 They reported that the catalytic activity scaled directly with the number of accessible tungstate sites perturbed on the SiO2 surface. To confirm the ordinary formation of the catalyst structure, the authors applied surface-sensitive X-ray photoelectron spectroscopy (XPS) coupled with Raman and infrared (IR) spectroscopy and X-ray diffraction (XRD) methods. Elemental analysis confirmed the successful impregnation of silica by the HPW precursor. In fact, the heteropolyacid was distributed across the silica surface at loadings up to 44 wt %. Higher loadings resulted in surface area deterioration and pore blockage. Hence, the change in growth mode and bulk HPW agglomeration was indicated. The product distribution was analyzed, and predominantly limonene was obtained, with a maximum α-pinene conversion of 45 wt %. Also, rapid conversion was observed with increasing HPW loading within the monolayer regime, although loadings exceeding 44 wt % resulted in a dramatic activity decline. This phenomenon was explained by the surface coverage of exposed WO centers coordinated to the silica support: As these centers became encapsulated and inaccessible to the reactants, multilayer HPW was formed. Hence, the α-pinene isomerization occurred at the interfacial tungstate sites.57 Also, Alsalme et al. studied this process under continuous-flow conditions.42 Up to 50 wt % camphene was the best result obtained over H3PW12O40 supported on TiO2. A strong Brønsted acidity profile was provided by the bulk and SiO2-supported H3PW12O40 catalysts, as well as the Cs2.5H0.5PW12O40 catalyst. Interestingly, 15 wt % H3PW12O40, when supported on Nb2O5, ZrO2, and TiO2, gave rise to only moderate Lewis and Brønsted acidities. Evidently, α-pinene conversion was stimulated by the highly acidic catalysts, although, at the same time, rapid deactivation followed. Catalysts with moderate acidities demonstrated improved stability and camphene selectivity.42 In an another study, Rachwalik et al. obtained 32 wt % camphene over commercial H-ferrierite type zeolite.9 As confirmed by nuclear magnetic resonance (NMR) spectroscopy, Brønsted acid sites dominated this catalytic system. In one of their catalyst samples, the acidity was also enhanced by the presence of excess extraframework aluminum detected by solidstate NMR and Fourier transform infrared (FTIR) spectroscopies. High selectivity (51 wt %) toward camphene at 90 °C was observed over H-FER (T) ferrierite material, which was obtained from Tosoh Corporation (Tokyo, Japan). The

authors explained this result with the high stability of camphene and the lower strength of the Brønsted acid sites present in this catalyst. On the contrary, H-FER (Z), obtained from Zeolyst International (Conshohocken, PA), provided higher acid strength, hence promoting the selectivity toward limonene (45 wt %) at identical reaction temperatures. Interestingly, the number of Brønsted acid sites was found to be less important for these catalysts (Table 4).9 Simakova et al. obtained a beneficial camphene yield of 66 wt % under continuous-flow conditions.53 Their Au/γ-Al2O3 catalyst was prepared by impregnation. During the catalyst preparation process, the acidic sites on the support surface of the reduced sample were neutralized with NaOH, and good camphene selectivities were demonstrated when H2 or N2 carrier gas was used. Poor results obtained with neat alumina support indicated the necessity for the participation of gold particles in the α-pinene isomerization rearrangements. In the case of low substrate concentrations, the catalyst provided stable conversion over a run time of up to 7−8 h. However, significant coke deposition and rapid catalyst deactivation occurred at increased α-pinene concentrations. The authors also studied the successful catalyst recovery at 650 °C under an oxygen flow.53 Yihui et al. demonstrated the highly selective isomerization of α-pinene over AlCl3/γ-Al2O3.43 In essence, 41 and 53 wt % camphene and limonene, respectively, were obtained following the one-pot synthesis strategy because of the strong acidity of this catalyst. Conversely, AlCl3, when supported on silica, gave a lower activity.43 Previously, our group has demonstrated good isomerization performance for the reaction of α-pinene to form limonene (43 wt %) in anhydrous acetic acid solutions over highly acidic commercial polymer catalysts such as Amberlyst 70. Also, the limonene selectivity was enhanced when 5 wt % water was added to the reaction mixture. This phenomenon is explained by the stronger nucleophilic properties of water in comparison to glacial acetic acid.52 In terms of the general principles of the isomerization reactions, the influence of both reaction temperature and catalyst acidity should be taken into account. For instance, camphene is conventionally produced in homogeneous acidic environments within the temperature range from 150 to 170 °C. The observed reaction gives yields of camphene ranging from 35% to 50%.36 Interestingly, the camphene byproduct was formed even in the temperature range of 250−300 °C. This occurred under continuous operation over catalysts with various acidities (Pd/γ-Al2O3, Pd−Zn/Al-SBA-15, ZnO/ CrO).32,33,58 However, the activation energies in the case of higher-boiling byproducts (e.g., ρ-cymene) are more sensitive to the temperature.20,33,36 Upon further transformations of camphene to, for example, ρ- and m-cymenes, highly acidic catalysts are required to open the internal ring of the bicyclic terpene.32 Tricyclene and bornylene are other well-known isomerization products of camphene,32,38,59 and their emergence as reaction products depends strongly on the catalyst F

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acidity.42 Furthermore, in suitable catalytic systems, the desired formation of camphene was reported at temperatures below 150 °C (which is commonly reported). Consequently, aiming for terpene dimers, Harvey et al. obtained ca. 35 wt % camphene in 4 h over a montmorillonite K10 (MMT-K10) catalyst.56 In fact, this reaction step was performed with (−)-βpinene at 30 °C. The authors reported that the acidity of MMT-K10 can vary by several orders of magnitude and that such variations arise from the amount of water present in the sample. Furthermore, this catalyst contains both Lewis and Brønsted acid sites.56 Also, Chimal-Valencia et al. tested a commercial Amberlyst 35 catalyst at 100−140 °C and reported yields equivalent to those obtained in the conventional process.36 They reported that the reaction temperature was proportional to the α-pinene conversion and inversely proportional to the camphene selectivity.36 Furthermore, the reaction temperature strongly influenced the camphene/limonene ratio, which also depends on the catalyst acidity (Figure 2). Consequently, Akgul et al. suggested

the framework Brønsted sites favor the formation of monocyclic terpenes.38,61 However, overly Brønsted acidic catalysts cause secondary isomerization of limonene; the formation of oligomeric byproducts results in faster catalyst deactivation and deteriorating selectivity.20,33,58,60,62 Camphene formation is promoted by weak acid sites, whereas strong acid sites promote the formation of limonene-like monocyclics.9,14,42 Optimal acidity is required in cases where the isomerization is only a preliminary step in a consecutive series of reactions such as dehydrogenation.33,62 3.2. Isomerization of “Oxygenated Terpenoids”

3.2.1. α-Pinene Epoxide. The oxidation product of αpinene (section 3.4.1), namely, α-pinene epoxide, which is itself a key intermediate in the synthesis of flavors, fragrances, and therapeutic agents, can also be isomerized to more valuable fragrances: campholenic aldehyde and trans-carveol.37,63−65 The current reaction procedure is commercially interesting because of the high reactivity of α-pinene oxide and its high susceptibility to various heterogeneous catalysts. The conventional process involves homogeneous catalysis (benzene with ZnCl2 or ZnBr2), and being a “dirty” process, it should be circumvented.66,67 In addition, these products are commercially available as a mixture of isomers, and obtaining pure fractions would be beneficial.63 Campholenal is applicable as a musk additive, fabric softener, laundry detergent ingredient, fragrance, and fragrance synthesis precursor (naturanol, santalol, Polysantol, ebanol, etc.).68−71 Obviously, the epoxide group of α-pinene oxide is activated by the Lewis acid; the epoxy ring is thus opened, and a carbenium ion is formed (Scheme 2). Further movement of the electron pair from the carbon−carbon σ-bond to either C-1 or C-7 causes the rearrangement of the aforementioned carbenium ion to carbocation A or B, respectively. Campholenic aldehyde is formed through C-2C-3 bond cleavage of cation A. As claimed, (−)-α-pinene epoxide forms (+)-campholenal. Meanwhile, (−)-campholenal is formed by the isomerization of

Figure 2. Isomerization reaction: product distribution as a function of temperature over 0.7 FeCl catalyst (2 mL of substrate, 0.3 g of catalyst, 10 mL/min N2, 155 °C, 8 h). (1) Camphene, (2) limonene, (3) α-terpinolene. Adapted from ref 20.

Scheme 2. Overall Mechanism for the Isomerization of (+)-trans-α-Pinene Epoxide22,30,63,65,66,71,74

that limonene formation is strongly dependent on the nature of the metal sites present in the catalyst.20 As the reaction temperature increased above 70 °C, better and better camphene yields were observed, and finally, the maximum yield (61.5 wt %) was reached at 155 °C. In addition, a high yield of limonene (30 wt % at 70 °C) was initially demonstrated. With increasing temperature, the limonene concentration decreased to 5 wt % (because of rearrangements into α- and γ-terpinenes and α-terpinolene).20 Under the actual reaction conditions, secondary isomerization appears to occur in the case of limonene, thus causing oligomerization and decreasing selectivities toward the desired products.20,36,37,60 In comparison to camphene, limonene is more reactive and is easily transformed into different monocyclic terpenes (Scheme 1).14 In fact, formation of camphene, limonene, or other isomerization products is a strong function of the acid strength of the catalyst chosen.9 For example, in the case of zeolites or other alumina-containing structured silica materials, the number of aluminum atoms in a framework is the key factor.14,33,38 The accessibility of active sites is also important.6 More precisely, the formation of bicyclic compounds is favored by both framework and nonframework Lewis sites. Meanwhile, G

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Table 5. Current Achievements in the Isomerization of α-Pinene Oxide ref

yield (wt %)

catalyst

reactor type

Substrate, α-Pinene glass reactor Cu(BF4)2 (homogeneous) Cu3(benzene-1,3,5 tricarboxylate)2 glass reactor H3PW12O40(PW)/SiO2 glass reactor H-US-Y zeolites glass reactor Zn(OTf)2/HMS silica glass reactor Fe−H-MCM-41 glass reactor

75 76 77 73 78 74

88 86 70 70 69 66

66 71

58 56

67

48

B2O3/SiO2 tungsten-containing indenyl complex Zn(OTF)2/K60

65 79

72 41

Ce/SiO2 32 wt % Ce−Si−MCM-41

63

22

molecularly imprinted polystyrenes

glass reactor microreactor

process conditions Oxide; Product, Campholenic Aldehyde 0.16 mL of substrate, 10 mL of CH2Cl2, 0.059 g of catalyst, 25 °C, 20 min 0.1 mL of substrate, 5 mL of acetonitrile, 0.1 g of catalyst, 25 °C 0.15−0.30 M substrate, 0.1 M dodecane, 0.6 wt % catalyst, 25 °C, 5 min 7.8 mL of substrate, 34.5 mL of toluene, 7.5 g of catalyst, 0 °C, 24 h 1.16 mL of substrate, 0.68 mL of decane, 100 mL of 1,2-dichlorethane, 25 °C, 1 h 2 mL of substrate, 150 mL of toluene, 0.075 g of catalyst (previously activated: 250 °C, Ar, 0.5 h), 70 °C, 3 h 1.75 mL of substrate, 7 mL of toluene, 0.15 g of catalyst, 25 °C, 4 h 0.0268 mL of substrate, 0.5 mL of 1,2-dichloroethane, 35 °C, 3 h

spinning-disk continuous flow of mixture (6 mL/s, 1.16 mL of substrate, 100 mL of 1,2-dichlorethane), reactor 1 g of catalyst, 85 °C, 1500 rpm Substrate, α-Pinene Oxide; Product, trans-Carveol glass reactor 0.38 mL of substrate, 5 mL of dimethylacetamide, 0.1 g of catalyst, 140 °C, 8 h glass reactor 0.315 mL of substrate, 100 mL of N-methylpyrrolidone, 0.075 g of catalyst; (1) 30 min of activation at 250 °C, (2) 30 min of reaction under reflux at 140 °C glass reactor 0.026 mL of substrate, 10 mL of toluene, 0.9 g of catalyst, 25 °C, 1 h

(+)-α-pinene epoxide.22,30 The proton shift from the C-9 methyl group of the cation B gives trans-carveol. Meanwhile, trans-pinocarveol is formed without skeletal isomerization when a proton from the C-10 methyl group is attached to the oxygen atom.63,65,71,72 The ion-solvating ability of polar solvents is able to stabilize the carbenium ion, leading to trans-carveol.65 In fact, further dehydration/isomerization of trans-carveol to ρ-cymene occurs in small amounts.66,73,74 Table 5 includes information on the α-pinene oxide isomerization studies implementing heterogeneous catalysis. Recently, Stekrova et al. described how to elegantly isomerize α-pinene oxide over Fe-modified MCM-41, SiO2, TiO2, and zeolites beta-75 and ZSM-5 catalysts.74 Utilizing Fe−H-MCM41 with moderate Brønsted and Lewis acidities, a 66 wt % yield of campholenal was obtained upon complete conversion of αpinene oxide (Table 5). Nitrogen physisorption measurements revealed a decline in the textural properties of Fe-MCM-41 because of pore blockage by coke and decay of the Brønsted acidic centers; still, the catalyst activity and reaction rate could be nearly completely restored by catalyst rejuvenation.74 The influence of the tungsten- and molybdenum-containing indenyl complexes was studied by Bruno et al.71 The tungsten sites demonstrated significantly higher activity than the molybdenum ones. The main product of their reaction was campholenal (56 wt %). As observed, the solvent type had a more dramatic impact on the reaction rate than on the campholenal selectivity.71 Robinson et al. obtained an 88 wt % yield of campholenic aldehyde utilizing the homogeneous reagent Cu(BF4)2.75 However, they failed to synthesize a functional heterogeneous catalyst, and the distribution of the mentioned Cu(II) salts on high-surface-area silica resulted in poor activity.75 Alaerts et al. demonstrated excellent results over a copper/benzene-1,3,5-tricarboxylate (Cu-BTC) metal−organic network, yielding 86 wt % yield of campholenal.76 Infrared (IR) spectroscopy was instrumental in demonstrating the high Lewis acidity and the absence of Brønsted sites in the catalyst. The synthetic precursor organic ligand H3BTC demonstrated low isomerization activity. Meanwhile, a Cu(NO3)2 precursor, as tested, revealed a higher activity of Cu(II) and a lower selectivity when compared to the final catalyst Cu3(BTC)2. The catalyst performance recovered after the reaction was

performed in a 1,2-dichloroethane/ethanol/water system.76 Shortly before this, da Silva Rocha et al. reported a highly selective isomerization on silica-supported heteropolyacids.77 In 5 min on stream, they obtained 28 and 70 wt % yields of transcarveol and campholenal, respectively. The catalyst was reused six times without any notable deactivation.77 Boron oxide loaded on silica was tested by Ravindra et al.66 In this case, a 58 wt % yield of campholenal was observed, and the catalyst investigated contained a mixture of Lewis and Brønsted acid sites. Boron oxide loadings of up to 15 wt % influenced the initial reaction rate, but not the campholenic aldehyde selectivity. The authors postulated that active Lewis acidic sites were located on the Si atoms and that the electrondeficient boron increased the Lewis acidity of Si through an inductive effect. At catalyst loadings reaching 20 wt %, all of the silica sites were covered with boron oxide, and Brønsted acidity dominated.66 Earlier, Wilson et al. obtained a 69 wt % yield of campholenal over silica-supported zinc triflates.78 As pyridine adsorption tests showed, their catalysts demonstrated substantial Lewis acidity combined with low Brønsted acidity. Some catalysts were fully recoverable, but longer run times were needed to reach total conversion upon recycle. The authors reported the deposition of heavy organic residues on the catalytic surface as the main reason for deactivation.78 Hölderich et al. obtained a good yield of campholenal (70 wt %) over a stable H-US-Y zeolite.73 High dispersion of Lewis sites was demonstrated and resulted in good campholenal selectivity. Still, the authors reported that a small amount of Brønsted acidity is required for efficient performance.73 On the other hand, Costa et al. maximized the yield of transcarveol (72 wt %) in high-temperature reactions using heterogeneous sol−gel Ce/SiO2.65 However, the catalyst recovery was challenging, and leaching was observed. Evidently, their catalysts were unstable in basic dimethylacetamide solutions. However, a comparable yield of trans-sobrerol was obtained in weakly basic acetone, whereby the catalyst recovery and recycling were successful.65 Stekrova et al.79 also previously targeted trans-carveol, utilizing Ce-modified mesoporous MCM-41 and SBA-15. They found that the activity was proportional to the amount of Brønsted acid sites. Meanwhile, the selectivity toward trans-carveol was determined by the H

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grance intermediates (Scheme 3).82−85 In analogy with the αpinene oxide isomerization, the isomerization of limonene

Lewis acidity of the catalyst. In light of the above, the least basic catalyst, namely, 32 wt % Ce−Si−MCM-41, was the most active one among those studied and resulted in 41 wt % selectivity toward trans-carveol upon complete conversion of αpinene epoxide. The authors pointed out the key role of basic polar solvents in the preparation of trans-carveol. More precisely, the solvent N-methylpyrrolidone resulted in the highest selectivity toward this product.79 The solvent polarity is crucial in determining whether campholenal or trans-carveol is formed. The overall reaction rate is increased by more polar solvents as a a result of polarization effects. Also, the solvent participates in hydrogen bonding and influences the reactivity of the intermediates.71 Evidently, relatively basic solvents could decrease the Lewis acidity of the catalyst and thus diminish the reaction rate. ρMenthadienic structures (trans-carveol) are formed in basic polar solvents such as dimethylacetamide. In contrast, campholenal formation occurs in nonpolar solvents, such as cyclohexane.65,71,79 Toluene was recommended by Stekrova et al.74 They also observed faster deactivation of their catalyst when acetonitrile was applied.74 Evidently, the reaction occurs even at low temperatures because of the high reactivity of αpinene oxide (Table 5).65,66,73 In fact, low temperatures favor campholenal formation, whereas trans-carveol formation is favored by higher temperatures.73,79 The formations of campholenal and trans-carveol occur simultaneously over Lewis and Brønsted acid sites, respectively.63,66,71,73,80 As emphasized by Stekrova et al., the number of Brønsted sites has a dramatic effect on the initial reaction steps: the stronger the Brønsted acidity, the lower the yield of campholenal.74 Thus, a relatively low concentration of Brønsted sites is beneficial. The oxygen atom of the epoxide ring reacts with Lewis acidic sites, causing splitting of the contiguous C C bond in the α-pinene oxide carbon ring; as a result, campholenic aldehyde is obtained.74 As expected, a high surface area was found to promote campholenal formation.73,74,76,78 α-Pinene oxide is a relatively large molecule, and consequently, the pore sizes of the catalyst should be large enough.74 For example, as in the aforementioned study, Stekrova et al. compared the performance of Femodified layered aluminosilicates and zeolites in the isomerization of α-pinene oxide. The authors reported that the catalysts supported on MCM-41 and zeolite beta-75 allowed for the rapid and complete conversion of substrate because of its well-ordered mesoporous and microporous structure and enhanced acidity. In contrast, the lowest conversion was demonstrated for ZSM-5-supported catalysts, although Lewis and Brønsted acid sites were present in high amounts. Despite the fact that all tested supports exhibited high surface areas, the ZSM-5 material is characterized by its small pore diameter (0.5 nm). Thus, the preparation of specialty chemicals with larger kinetic diameters was problematic for this material. The authors suggested that the isomerization of α-pinene oxide proceeded only on the external surface of this catalyst. Furthermore, in her recently published doctoral thesis, Stekrova supported this statement with computational calculations.81 The total critical diameter calculated for the α-pinene oxide molecule was determined to be 7.1 Å (0.71 nm). Hence, the authors were able to define the minimum pore size required for the effective isomerization of α-pinene oxide.74,81 3.2.2. Limonene Epoxide. Upon isomerization of limonene epoxide, the aim is to obtain carveol, cyclopentylcarboxaldehyde, and dihydrocarvone, all valuable fra-

Scheme 3. Overall Mechanism for the Isomerization of Limonene Epoxide25,30,63,85

epoxide starts with the opening of the epoxy ring, which is induced by the protonation of the oxygen atom. The resulting carbenium ion undergoes several competing transformations. Hence, a C2C1 hydride shift followed by the proton abstraction forms ion B, from which dihydrocarvone is produced. Alternatively, the deprotonation of ion A at C6 or C7 produces carveols. Furthermore, ion A rearranges into carbenium ion C through electron-pair transfer from the C2 C3 σ-bond to C3. Cyclopentanecarboxyaldehyde is formed when ion C loses a proton (Scheme 3).85 In a new study by Costa et al., isomerization of limonene epoxide was targeted using silica-supported tungstophosphoric acid.85 The reaction in 1,4-dioxane solvent gave 82 wt % yield of the valuable fragrance component, dihydrocarvone, with an equal enantiomeric composition (Figure 1). Meanwhile, the blank reaction resulted in negligible conversions. Moreover, the product selectivity was improved upon use of more polar solvents (i.e., dichloroethane). The catalyst acid strength, and thus also the reaction rate, was hampered by highly basic solvents, namely, dimethylacetamide and 1,4-dioxane. However, the increased solvent polarity had a positive influence on the dihydrocarvone selectivity. The authors assumed that the reaction was more kinetically controlled when “slow” polar solvents were applied and the catalyst was stable to leaching.85 Kolomeyer and Ferone recently patented the isomerization of a mixture of cis- and trans-limonene epoxides in the presence of a chromium octoate catalyst.86 trans-Isocarveol was the main product observed, with a 51 wt % yield. Interestingly, cislimonene epoxide reacted readily, giving 95 wt % conversion. In contrast, only 13 wt % of trans-limonene epoxide was converted.86 The analogous reaction performed on amorphous silica−alumina resulted in a 77 wt % yield of cyclopentylcarboxaldehyde.87 Also, in a patent by Fujiwara et al., a 90 wt % yield of carvenone was achieved from limonene epoxide on CaA zeolite at 50−110 °C.88 I

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3.3. Dehydroisomerization

limonene or terpinenes, no initial ring-opening isomerization reactions are required, thus leading to increased yields of ρcymene.14,33,40,41 Recent advances in ρ-cymene synthesis are detailed in Table 6. In our previous study, successful dehydroisomerization of αpinene was demonstrated as a result of the bifunctionality of the catalyst, promoting both hydrogenation and dehydrogenation. A Pd−Zn/Al-SBA-15 catalyst yielded 80 wt % ρ-cymene and retained its textural properties and was thus found to be recyclable. The optimal Pd/Zn ratio was found to be 1:1 at a 5 wt % loading. An individual test of a monometallic Zn/Al-SBA15 catalyst demonstrated strong dehydrogenation activity, whereas Pd/Al-SBA-15 provided only hydrogenation activity (Figures 3 and 4). Ammonia temperature-programmed desorption (NH3 TPD) revealed an increase in catalyst acidity after the reaction, presumably because of the accumulation of residual organic compounds on the catalyst surface.33 Comparable results were also obtained under flow-through conditions by Al-Wadaani et al. with highly acidic mixed zinc and chromium oxides. Their catalyst was stable up to 30 h on stream (Figure 5).58 Recently, we also tested TiO2/SBA-15 supported catalysts.92 TiO2 was introduced onto the amorphous surface of Si-SBA-15 by chemical grafting methods. Furthermore, various metal nanoparticles, namely, Au and AuM (M = Co, Ni, Cu, and Zn), were distributed on the aforementioned support by deposition−precipitation (DP) methods, and a good dispersion of metal nanoparticles over the well-ordered hexagonal mesopores of Si-SBA-15 was obtained. The catalysts were stable upon prolonged time on stream (TOS), especially when Ni sites were present. AuNi−TiO2/SBA-15 prepared by the deposition−precipitation method was the most successful in dehydroisomerization of α-pinene, producing 63 wt % ρcymene. Additionally, AuCo, AuCu, and monometallic Au, when supported on TiO2, demonstrated high tendencies toward the isomerization of camphene.92 Although aiming for cis-pinane, Bazhenov et al. obtained 80 wt % ρ-cymene over dealuminated zeolite Y, an outstanding result in a liquid-phase operation.90 A slightly lower yield (70 wt %) was also obtained by Jaramillo et al. with a H3PW12O40· xH2O/SiO2 catalyst (liquid phase).91 Buhl et al.40 and Kamitsou et al.41 reported 90 and 99 wt % yields, respectively, in the conversion of R-(+)-limonene to ρ-cymene. Understandably, Buhl et al. emphasized that porous supports give better results than nonporous ones. The Pd/SiO2 catalyst appeared to be stable for 532 h on stream.40 Slightly later, the same group obtained up to 68 wt % ρ-cymene from α-pinene over a Pd/γ-Al2O3 catalyst.32 Martin-Luengo et al. converted R-(+)-limonene to 88 wt % ρ-cymene using microwave irradiation. The reaction was initiated by the microwave-adsorbing paramagnetic centers. The authors used natural clay (sepiolite) modified with nickel, iron, or manganese (Sep-Me). Compared with conventional heating, increased reaction rates and selectivities were obtained with solids combining both acidic and paramagnetic sites. The highest activities were demonstrated by SepFe and SepNi catalysts because of their high acidities and good dispersions of metal oxide particles (Table 6).93 Evidently, the temperature range of 300−350 °C is optimal for continuous operations. However, in batch operations, 150− 160 °C is sufficient (Table 6). Some of the aforementioned isomerization studies (Table 2) were performed at temperatures comparable to those used for liquid-phase dehydrogen-

High-purity ρ-cymene could be converted to valuable ρ-cresol and terephthalic acid by oxidation and serves as a component in non-nitrate musk synthesis.14,32,37,89 Furthermore, ρ-cresol is a precursor in the synthesis of antioxidants, such as 2,6-di-tertbutyl-ρ-cresol.58 Alternatively, ρ-cymene is applicable as a solvent, heat-transfer media, and fragrance, among other uses.14,29,37,89 Friedel−Crafts alkylation of benzene or toluene with propene is the traditional way to obtain ρ-cymene (ρisopropyltoluene). The reaction is catalyzed by homogeneous acids such as AlCl3 or H2SO4.29 Initially, a mixture of o, m, and ρ isomers is obtained and further separated. Roughly 4000 t of ρ-cymene is produced annually by this process.29 In addition, as mentioned previously, ρ-cymene is found in high concentrations in sulfite turpentine, albeit heavily contaminated. Alternatively, ρ-cymene could be obtained with high selectivity from terpene feedstocks through heterogeneously catalyzed dehydrogenation; in this case, coproduction of H2 is an additional benefit.33,41 Furthermore, the application of heterogeneous catalysts could minimize the reactor corrosion and facilitate facile catalyst reuse.12 Consequently, isomerization is promoted by acid sites of the catalysts, whereas the following rate-limiting dehydrogenation step is catalyzed by metal sites (Scheme 4).14,32,33,58 A Scheme 4. Overall Mechanism for the Dehydroisomerization of α-Pinenea

a

Adapted from ref 33.

successful catalyst should combine both minor isomerization and predominant dehydrogenation activities.14,33,56 As also stated by Al-Wadaani et al., Lewis acidity should dominate over Brønsted acidity to suppress the tendency toward isomerization.58 In addition, enhanced acidity of the catalyst accelerates the isomerization of the limonene double bonds, which further promotes the dehydrogenation.41 Still, m-cymene is formed by the same mechanism, but the isomerization proceeds by camphene−fenchene pathways (Scheme 4).12,33 In the case of the dehydrogenation of pinenes, the term dehydroisomerization describes more accurately the whole process, as isomerization occurs prior to the hydrogen removal−initially monocyclics are formed which are further dehydrogenated.12,14,33 However, if the starting compounds are J

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Table 6. Current Achievements in the Heterogeneously Catalyzed Dehydroisomerization of Terpenes ref

a

yield (wt %)

catalyst

33

80

Pd−Zn/Al-SBA-15

90 58

80 78

50% dealuminated modernite Y ZnO/CrO

91 32

70 68

H3PW12O40·xH2O (33%)/SiO2 Pd/γ-Al2O3 (D-10−10)

92

63

AuNi−TiO2/SBA-15

40

99

Pd/SiO2

41

90

TiO2 anatase

93

88

Fe-modified sepiolite (SepFe)

reactor type

reaction conditions

Substrate, α-Pinene; Product, ρ-Cymene flow-through reactor continuous flow of substrate (liquid), 0.09 mL/min, 0.25 g of catalyst, 10 mL/min H2, 300 °C, 6 h pressurized vessel 50 mL of substrate, 0.4−4 g of catalyst, 3 bar N2, 150 °C, 2 h flow-through reactor continuous flow of substrate (gas), 0.15 mL/min, 0.3 g of catalyst, 10 mL/min N2, 350 °C, 2 h batch reactor (npa) 160 °C (npa) flow-through reactor continuous flow of substrate (liquid), 0.03 mL/min, 1 g of catalyst, 1 bar H2, 300 °C, 3.5 h flow-through reactor continuous flow of substrate (liquid), 0.03 mL/min, 0.25 g of catalyst, 10 mL/min H2, 300 °C, 6 h Substrate, Limonene; Product, ρ-Cymene flow-through reactor continuous flow of substrate (liquid), 0.12 mL/min, 2 g of catalyst, 12.5 mL/min H2 and 12.5 mL/min N2, 300 °C, 1 h flow-through reactor continuous flow of substrate (gas), 0.12 g of catalyst, 26 mL/min He, 300 °C, 24 h monomode programmable 5 mL of substrate, 0.5 g of catalyst; (1) reaction at 165 °C for 20 min, (2) focalized microwave oven extraction with 2 mL of ethanol

np, not provided.

Figure 3. Dehydroisomerization reaction: evolution of reaction products, 1:1 Pd−Zn/Al-SBA-15 (left, 5 wt %; right, 3 wt %) (0.09 mL/min substrate, 0.25 g of catalyst, 10 mL/min H2, 300 °C, 5 h). Symbols: (●) total cymene, (○) ρ-cymene, (■) limonene, (□) mcymene, (Δ) ρ-menthene. Reprinted with permission from ref 33.

Figure 5. Dehydroisomerization reaction: α-pinene conversion and product selectivity vs time on stream (TOS) (350 °C, 0.3 g of ZnCr (1:1), 2 vol % α-pinene in N2, 7.5 mL/min flow rate, 1.7-s contact time, 2-h reaction time). From top to bottom: conversion of α-pinene, selectivity of ρ-cymene, selectivity of other products. Adapted from ref 58.

terpenes with menthane structures: limonene, 1,4-menthadiene, α-phellandrene, α-terpineol, trans-piperitol, and carveol. These substrates were treated with iodine at 170 °C, allowing for 65− 90 wt % yields of ρ-cymene.95 In 1993, Newman prepared 67 wt % 8-hydroxycymene through the dehydrogenation of αterpineol over a Pd/Al2O3 catalyst. The catalyst was stable upon prolonged times on stream in a six-tube parallel reactor.96 Renewable cineole is the main chemical component of eucalyptus oil (ca. 90 wt %) and an alternative substrate for this reaction. Leita et al. published two works on the conversion of this raw material, comparing the performances of Pd-, Mo-, Cr-, Fe-, and Co-doped γ-Al2O3 catalysts.97,98 Among the tested materials, the Pd-doped catalyst demonstrated the best results. The solventless pyrolysis was performed in a custom-built down-flow fixed-bed pyrolysis rig and resulted in an 88 wt % yield of ρ-cymene. The authors reported that the catalyst demonstrated dual-functional catalytic activity: the initial dehydration was followed by a dehydrogenation step.97 Furthermore, the authors optimized the catalyst performance in a trickle-bed reactor and provided extensive characterization studies of their material. As a result, the yield of ρ-cymene

Figure 4. Dehydroisomerization reaction: evolution of reaction products (left, 5 wt % Zn/Al-SBA-15; right, 5 wt % Pd/Al-SBA-15) (0.09 mL/min substrate, 0.25 g of catalyst, 10 mL/min H2, 300 °C, 5 h). Symbols: (●) total cymene, (○) ρ-cymene, (■) limonene, (□) mcymene, (Δ) ρ-menthene, (+) trans-pinane. Reprinted with permission from ref 33.

ations. Presumably, isomeric compounds are initially formed with high selectivity, followed by dehydroisomerization. In contrast, higher temperatures are needed in trickle-bed operations to overcome possible mass-transfer limitations. Oxygenated terpenoids can also be converted to ρ-cymene. In the late 1940s, Sondhi et al. studied the vapor-phase pyrolysis of crude terpineol, obtaining up to 31 wt % ρcymene.94 Later, in 1987, Ho studied the aromatization of K

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Scheme 5. Reaction Network for the Oxidation and Epoxidation of R-(+)-α-Pinene30,68,80

Table 7. Current Achievements in the Heterogeneously Catalyzed Oxidation and Epoxidation of α-Pinene ref

yield (wt %)

107

84

64

46

108

26

109

20

110

27

64

16

111

97

112

95

113

93

103

91

114

67

catalyst

reactor type

process conditions

substrate, α-Pinene; Product, Verbenone Fe-pillared montmorillonite glass reactor 0.23 mL of substrate, 1.8 mL of TBHP, 2 mL of dichloromethane, 0.005 g of catalyst, Ar, 40 °C, 50 h AuCu/TiO2 glass reactor 0.8 mL of substrate, 0.48 mL of TBHP, 15 mL of acetonitrile, 0.01 g of catalyst, 70 °C, 24 h Ru(III)Saloph-Y pressurized 1.7 mL of substrate, 23.4 mL of acetonitrile, 0.05 g of catalyst, vessel 30 bar O2, 100 °C, 3 h Si−Ti gel glass reactor 3 g of substrate, 0.2 g of catalyst, 20 mL of chlorbenzene, 4.2 g of H2O2, 80 °C, 8 h Substrate, α-Pinene; Product, Verbenol O2 followed by Pd/C pressurized (1) Oxidation: 50 mL of substrate, 4 bar O2, 100 °C. (2) Reduction: vessel 50 mL of oxidized substrate, Na2SO3/H2O (8 g/24 mL), 0.2 g of catalyst, 4 bar H2, 50 °C. 5 h in total AuRu/TiO2 glass reactor same as mentioned above (product verbenone) Substrate, α-Pinene; Product, α-Pinene oxide [R]3[PW12O40]3− modified with SiO2 gel glass reactor 3 mmol of substrate, 0.015 mmol of catalyst (0.5 mol %), 6 mmol of 15% H2O2, 70 °C, 2 h immobilized Co(salen-5) complex pressurized 0.3 mL of substrate, 0.5 mL of pivalaldehyde, 10 mL of fluorobenzene, vessel 0.25 g of catalyst, 25 °C, 3 h chiral SO3(−) salen Mn(III) complex/Zn(II)Al(III) glass reactor 0.585 mL of substrate, 1.1 mL of pivalaldehyde, layered double hydroxide 0.15 mL of 1-methylimidazole, 18.5 mL of acetone, 0.1 g of catalyst, atm P (1 bar O2 bubbling), 25 °C, 6 h Ti-SBA-15 glass reactor 1.13 mL of substrate, 1.05 mL of TBHP (5 M in decane), 10 mL of acetonitrile, 0.1 g of catalyst, 80 °C, 24 h titanium-containing mesoporous aluminophophates glass reactor 2.5 mL of substrate, 2.21 mL of H2O2 (30% aqueous), (TAPs) 0.93 mL of acetonitrile, 50 mL of methanol, 0.2 g of catalyst, 60 °C, 20 h

reached 95 wt % with good stability.98 The recent study by Meylemans et al. focused on the low-temperature deoxygenation reactions of 1,4- and 1,8-cineoles over Amberlyst-15, Nafion SAC-13, and montmorillonite K10 catalysts.99 In fact, terpinolene and α-terpinene were the main products of the dehydration reactions of 1,4-cineole. For example, approx-

imately 30 wt % of each of these terpenoids was produced over the Amberlyst 15 catalyst, the most active among those studied. Small amounts of ρ-cymene were also formed (ca. 5 wt %). In contrast, significant amounts of ρ-cymene were obtained over the montmorillonite K10 catalyst. Furthermore, when the reaction time was prolonged or higher temperatures were L

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catalytic structures verified the formation of a layer-pillared structure with Fe2O3 pillars and developed micropores. However, less-ordered structures of mechanically activated Fe-montmorillonite were found to be significantly more active toward oxidation products, especially at prolonged times on stream. The authors explained that free diffusion of the substrate can occur in a more porous catalytic material.107 Ancel et al., on the other hand, studied the two-step oxidation/ reduction of α-pinene over Pd/C.110 The oxidation process was selective at low α-pinene conversions, giving α-pinene oxide and verbenyl hydroperoxide. The minor product was further selectively hydrogenated to verbenol (27 wt %). Kinetic studies showed that the ratio of oxidation products is influenced by the temperature, oxygen pressure, and α-pinene conversion.110 Over the past decade, several reports have emerged with αpinene oxide as the target molecule. Specifically, Bhattacharjee et al. was able to produce 93 wt % (−)-α-pinene oxide implementing a fully recoverable chiral sulfonatosalenmanganese(III) complex intercalated into a Zn(II) Al(III) layered double hydroxide.113 By oxidizing R-(+)-limonene, they also produced 87 wt % (+)-cis-limonene epoxide over the same catalyst. The catalyst was successful and worked regardless the choice of solvent or gas atmosphere (Table 9). Advantageously, either air or oxygen at atmospheric pressure could be used.113

applied, the authors were able to convert these products further into dimer biofuels.99 3.4. Oxidation and Epoxidation

3.4.1. Oxidation and Epoxidation of α-Pinene. The oxidation of α-pinene provides an attractive route to vitamins, fragrances, and fine chemicals.100,101 Conventionally, environmentally harmful and costly metallic oxidants with compromised profitability are used.102 Thus, the application of heterogeneous catalysts should be beneficial. Verbenone, verbenol, and α-pinene oxide are well-known fragrance products obtained from α-pinene oxidation (Figure 1). Verbenone has also been used for the synthesis of a taxol intermediate.68 Epoxidation involves the catalytic oxidation of CC bonds by peroxides. In the case of terpene upgrading, H2O2 and tertbutyl hydroperoxide (TBHP) are commonly applied as initiators.14,103 As in the case of α-pinene epoxidation, the most common product is α-pinene oxide.14 Obviously, molecular oxygen reacts with α-pinene by a freeradical chain mechanism rather than a direct oxidation mechanism (Scheme 5).68,80,104 The oxidation is initiated by the catalyst metal sites involving the solvent in the electron transfer. As a result, the formed alkoxy radical (RO) abstracts hydrogen from the carbon atoms of α-pinene. This is followed by isomerization, resulting in allylic radicals of α-pinene, which are capable of reacting with oxygen.2,68,105 The subsequent formation of oxidation products is a competing process and strongly depends on both the nature of the specific metal sites and the stability of the radicals formed.2,106 Throughout an epoxidation process, peroxy radicals (RO2) are decomposed and isomerized to produce alkoxy radicals (RO). As the next step, alkoxy radicals can be treated with oxygen in a conventional manner.2,68,80 The catalytic oxidation of α-pinene was previously studied by several groups and is summarized in Table 7. The oxidation of α-pinene on bimetallic catalysts AuM/TiO2 (where M = Cu, Co, or Au) was studied by our group. We were able to produce 46 wt % verbenone over AuCu/TiO2. Verbenol and α-pinene oxide were also identified among the major reaction products (Table 8).64 An excellent yield of verbenone, 84 wt %, was obtained by Romanenko et al. from R-(+)-α-pinene through tert-butyl hydroperoxide- (TBHP-) catalyzed epoxidation.107 Fe-pillared montmorillonite and its mechanically activated analogue were studied. Extensive characterization analysis of the formed

Table 9. Influence of Solvent and Choice of Oxidant on the Epoxidation of Limonene and α-Pinenea,b

α-pinene oxide

AuCu/TiO2 (1:1) AuCu/TiO2 (5:1) AuCo/TiO2 AuRu/TiO2 Au/TiO2 Cu/TiO2 no catalyst

97 87 88 73 78 80 62

6.5 6.1 6.3 5.7 4.6 8.5 19.2

conversion (%)

selectivity (%)

96.6 97.0 98.3 100.0 100.0

88.5d 87.0d 88.5d 90.0f 91.7f

Analogous epoxidation was studied by Chiker et al. utilizing a Ti-SBA-15 catalyst.115 They obtained 91 and 97 wt % yields of the corresponding oxides from α-pinene and R-(+)-limonene substrates, respectively. The catalyst was prepared by gas-phase reactions of Ti precursors with an SBA-15 support. XPS and transmission electron microscopy (TEM) images identified a properly ordered hexagonal structure and good dispersion of Ti species. The catalyst was successfully reused three times, and no leaching was observed.115 The highest yield of α-pinene oxide (97 wt %) was obtained from α-pinene by Sakamoto and Pac.111 They proposed a polyoxometalate catalyst supported on chemically modified hydrophobic mesoporous silica gel and discussed the hydrophobic nature of the catalyst surface as the reason for the selective epoxidation.111 Meanwhile, Schuster and Hölderich obtained a 96 wt % yield of (−)-α-pinene oxide from S-(−)-α-pinene over immobilized manganese salen complexes.112 At first, zeolites X, Y, and DAY were dealuminated. The resulting structures consisted of mesopores that were completely surrounded by micropores. Bulky transitionmetal salen complexes were further occluded on this support, and the catalysts were reusable.112

verbenol verbenone 12.1 14.7 13.2 22.1 17.5 8.8 12.8

oxidant dioxygenc aire aire dioxygenc airc

Reaction conditions: 3.7 mmol of substrate, 9.2 mmol of pivalaldehyde, 18.5 mL of solvent, 0.100 g of catalyst, and temperature of 298 K. Catalyst: LDH-[Mn(Cl)(L)] (LDH = ZnO-Al2O3 layered double hydroxide). bAdapted from ref 113. cReaction time of 6 h. d Product: (+)-cis-1,2-limonene epoxide. eReaction time of 8 h. f Product: (−)-α-pinene epoxide.

selectivity (%) conversion (%)

solvent toluene toluene acetone toluene acetone

a

Table 8. TiO2-Supported Transition-Metal Catalysts Studied for the Oxidation of α-Pinenea,b

catalyst

substrate limonene limonene limonene α-pinene α-pinene

47.9 38.0 28.2 32.1 24.6 22.8 21.6

a Reaction conditions: temperature, 70 °C; α-pinene, 5 mmol; TBHP, 5 mmol; acetonitrile, 15 mL; catalyst loading, 100 mg. bAdapted from ref 64.

M

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Table 10. Current Achievements in the Heterogeneously Catalyzed Epoxidation of Limonene ref

a

yield (wt %)

25

99

26

98

103

97

124

90

113

87

24

85

125

83

27

82

126

81

127

70

28

52

128

50

catalyst

reactor type

Substrate, Limonene; Product, Mn porphyrins glass reactor methyloxorhenium/polystyrene glass reactor Ti-SBA-15 glass reactor Mo/polybenzimidazole glass reactor glass chiral SO3(−) salen Mn(III) complex/Zn(II)Al(III) layered double hydroxide reactor salen manganese(III) complex glass reactor glass Al2O3 (weakly acidic) reactor indenyl Mo(II)-tricarbonyl complex glass reactor glass MoOx/SiO2 reactor molybdenum metallosilicates glass reactor zeolite-immobilized Fe(III) complex glass reactor aluminophosphate molecular sieves glass reactor

process conditions 1,2-Limonene Epoxidea 3.2 mL of substrate, 0.1 M sodium-N-dodecanoyl-L-prolinate, H2O2, 3 × 10−3 M catalyst, 9 × 10−2 M imidazole, 25 °C, 10 min CH2Cl2/MeCN (1:1), H2O2 (35 wt % aqueous), 1 wt % catalyst, −10 °C 1.13 mL of substrate, 1.05 mL of TBHP (5 M in decane), 10 mL of acetonitrile, 0.3 g of catalyst, 80 °C, 24 h 250 mL of substrate/TBHP (5:1), 70 °C, 2 h same as mentioned above (α-pinene substrate) 0.32 mL of substrate, 4 mmol of KHSO5, 1.2 mmol of NaHCO3, 10 mL of acetone, 4 mL of H2O2, 0.05 mmol of catalyst, pH 8.5, 20 °C, 25 min 1.62 mL of substrate, 0.17 mL of dibutyl ether, 10 mL of ethyl acetate, 0.5 mL of H2O2 (60 wt %); reflux with Dean−Stark trap under N2 0.27 mL of substrate, 0.25 mL of TBHP (TBHP/DCE = 0.9), 17 μmol of catalyst, 10 min at 55 °C or 35 min at 35 °C 1.62 mL of substrate, 7 mL of pentane, 1.5 mL of anhydrous TBHP (10% in decane), 0.1 mmol of catalyst, 20 °C, 24 h 1.62 mL of substrate, 1.45 mL of TBHP, 2.5 mmol of n-butylether, 0.05 g of catalyst, 60 °C, 24 h 0.16 mL of substrate, 0.25 mL of acetonitrile, 0.13 mmol of ammonium acetate, 0.13 mmol of H2O2, 0.02 g of catalyst, 60 °C, 48 h 1.26 mL of substrate, 10 mL of acetone, 1 mL of H2O2, 0.1 g of catalyst, 60 °C, 12 h

Cis/trans diastereomer ratios specified in the text (if available in the original articles).

Scheme 6. Possible Products of the Epoxidation of R-(+)-Limonene24,25,30,121,123

Bhattacharjee et al. observed that an oxygen atmosphere gave better results in comparison with atmospheric air.113 Schuster and Hö lderich speculated that elevated pressures could suppress the selectivity; thus, determination of an optimal pressure is required.112 Obviously, the yields of verbenol and verbenone are highly dependent on temperature, even more so than that of α-pinene oxide.64,100

Acetonitrile has been widely discussed as an optimal solvent for oxidation reactions.64,100,108,114 This is rationalized by the strong interactions between the active sites of the catalyst and CH3CN molecules. Also, a facile desorption of the reaction products from the active sites occurs. In addition to acetonitrile, solvents with small dielectric constants, such as ethyl acetate and tert-butyl alcohol, have also been recommended.64 N

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In fact, the epoxidation yields with H2O2 were lower in the absence of molecular oxygen.68,100,103 However, the catalytic process appears to dominate, given that 97 wt % α-pinene oxide was obtained by Sakamoto and Pac with a H2O2 promoter.111 However, Chiker et al. observed catalyst leaching in cases where H2O2 was used.103 H2O2 rapidly decomposes to H2O and O2, thus limiting the yield of oxidation products, and additional injections of H2O2 are needed to stimulate the substrate conversion.68,100,108 In fact, TBHP is a more stable and efficient oxidant.103,109,115 Large-pore-volume catalysts are required for the efficient epoxidation of α-pinene. At this point, the smaller molecule limonene, which also contains reactive electron-rich endocyclic bonds, gives higher epoxide yields.103,115 The chirality transition in the epoxidation of S-(−)-α-pinene to α-pinene epoxide was recently studied by Qiu et al.116 Following the epoxidation method proposed by Bhattacharjee et al., a mixture of cis and trans isomers of α-pinene oxide was obtained (Figure 1).113 However, according to a Raman optical activity (ROA) analysis, in the case of S-(−)-α-pinene, the trans isomer was predominantly favored. The authors reported that, throughout the epoxidation, the chiral configurations of the first and fifth carbon atoms remained unchanged. Meanwhile, the achiral CC bond was transformed into a COC epoxide ring, and two new chiral centers were created.116 The epoxidation of α-pinene is a diastereofacially selective reaction. Commonly, the reaction affords excellent diastereofacial selectivity in the case of both S-(−)- and R-(+)-αpinene substrates. Normally, the interaction of the electrophillic reagent with the less sterically hindered face demonstrates equal yields of α-pinene oxides from both S-(−)- and R-(+)-αpinenes under homogeneous catalytic conditions.22 3.4.2. Epoxidation of Limonene and Camphene. The oxidation products of camphene and limonene are valuable precursors for fragrances, food additives, and agrochemicals. Consequently, the homogeneous oxidations of camphene and limonene have been studied by a number groups.117−120 The heterogeneous oxidations of these terpenes are summarized in Table 10. In fact, the valuable fragrance and surfactant compound limonene-1,2-epoxide can be synthesized by limonene epoxidation over Ti-substituted zeolites, porphyrins, cobalt and ruthenium complexes, and polyoxometalates.14,113 Limonene epoxide is formed by the electron transfer of oxygen addition to the CC bond. Meanwhile, carveol and carvone are formed by allylic H-abstraction (Scheme 6).24,121,122 Pena and co-workers recently reported the oxidation of limonene with molecular oxygen under solvent-free conditions.121 The reaction was catalyzed by NiAl hydrotalcites and resulted in a maximum conversion of 53 wt % at 80 °C over 6 h. Despite the fact that the combined selectivity to epoxides was only 39 wt % (the endocyclic epoxide was 75 wt % of the total amount), the performance of the catalyst was impressive for solventless operations. The nickel loading determined the catalytic activity upon reuse. The authors also reported that limonene peroxide initially formed in the substrate upon storage. Indeed, the peroxide was formed by a spontaneous loss of the allylic H atom and further reaction of the formed radical with oxygen. Presumably, this peroxide caused the chain oxidation of the substrate in the absence of the catalyst. Hence, in the presence of the catalyst, the overall oxidation rate was also promoted by the aforementioned peroxide formation.121

Madadi and Rahimi examined zeolite-immobilized Mn(III), Fe(III), and Co(III) complexes for the epoxidation of R(+)-limonene using a mixture of H2O2 and ammonium acetate.28 For the catalyst preparation, the authors applied a unique “ship-in-a-bottle” method requiring four pyrrole and four 4-methoxybenzaldehyde molecules to diffuse into the pores of metal-exchanged zeolite-Y. They reported that the Fe(III) complex demonstrated the highest activity and recoverability among the studied materials. A formidable number of characterization techniques were applied to confirm the immobilization of the complexes and the stability of the obtained catalysts. Among the reaction products, 1,2- and 8,9limonene epoxides were predominant. An R-(+)-limonene conversion of 81 wt % and a 1,2-limonene epoxide selectivity of 64 wt % were achieved over an iron-immobilized catalyst (Table 10). This material was successfully reused, although, after two reaction cycles, the authors observed a significant decline in catalytic activity, which presumably occurred because of the blockage of the zeolite channels. The R-(+)-limonene conversion was inversely proportional to the R-(+)-limonene/ H2O2 ratio.28 Egusquiza et al. studied the epoxidation of limonene with H2O2.123 The reaction was catalyzed by copper(II) heteropolytungstates (PWs) supported on γ-alumina. Interestingly, pure aqueous PWCu was more selective but less active than the supported catalyst prepared by impregnation. As explained by the authors, the biphasic homogeneous system prevented the hydrolysis of the formed 1,2-limonene epoxide.123 The epoxidation of R-(+)-limonene on indenyl molybdenum(II) tricarbonyl as a precatalyst was proposed by Abrantes et al.27 To initiate the epoxidation, the authors used various TBHP blends. In fact, aqueous TBHP premixed with 1,2-dichloroethane (DCE) resulted in the highest yield of 1,2limonene epoxide (82 wt %) (Table 10). Meanwhile, TBHP mixed with decane produced 96 wt % mixed limonene epoxides: 73 and 23 wt % 1,2-monoepoxide and 1,2:8,9diepoxide, respectively. In addition, minor amounts of the formed epoxides were hydrolyzed to diols. Oxidation in the absence of the catalyst was tested for 24 h and gave no products. The molar ratios of mono- and diepoxides indicated a high probability for the epoxidation of the endocyclic double bond of R-(+)-limonene. A fair stability of the catalyst was demonstrated. The authors also observed partial allylic oxidation of decane. Thus, the application of TBHP/decane mixtures is limited. The authors also assumed that the reaction rate was inversely proportional to the amount of water in the reaction mixture, because water hydrolyzed the formed epoxides to diols. Application of a predrying treatment for the TBHP/DCE mixture significantly improved the reaction performance.27 Another group studied salen manganese(III) complexes in the epoxidation of R-(+)-limonene with dimethyldioxirane as the oxidizing agent.24 The noncatalytic reaction gave rise to moderate conversions, and limonene diepoxides were the main products observed. Interestingly, during a catalytic run under optimal reaction conditions, total conversion of the substrate was obtained, and at the same time, the selectivities toward mono- and diepoxides were dependent on the acetone-tolimonene ratio. In particular, 86 wt % yields of diepoxides were obtained with a solvent-to-substrate ratio of 2 mL/mmol. Meanwhile, the corresponding ratio of 10 mL/mmol resulted in 85 wt % conversion of R-(+)-limonene to its endocyclic monoepoxide (58 wt % cis-epoxide) (Table 10). However, the O

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reaction converted only 1 wt % of the substrate. Upon 7 h of catalytic reaction, a 52 wt % conversion was reached with a 60 wt % selectivity toward epoxylimonene, giving a mixture of 1,2and 8,9-limonene epoxides. The catalyst was successfully reused, and no leaching was observed. Nevertheless, leaching was observed at limonene-to-H2O2 ratios below unity. Hence, an excess of H2O2 was undesirable. Interestingly, in the absence of the catalyst, the conversion remained unaltered even after 15 min of run time. Because no Ti leaching was observed, this phenomenon can be explained by the radical mechanism. This was also confirmed by simultaneous increases in the carveol and carvone selectivities up to 40 wt %. In fact, the presence of the catalyst favored catalytic oxygen transfer. Simultaneously, the catalyst assisted in suppressing the undesirable radical mechanism.131 Vanadium-containing hexagonal mesoporous aluminophosphate molecular sieves were synthesized by Venkatathri and Srivastava.128 This material demonstrated high selectivities (ca. 97 wt %) toward both 1,2-limonene and α-pinene oxides. However, the conversions of the corresponding substrates were only around 50 wt %. Numerous characterization techniques revealed the high textural purity of the catalyst with uniform incorporation of vanadium in the form of tetrahedral and square-pyramidal clusters.128 Saladino et al. obtained good results using methyltrioxorhenium catalysts prepared by microencapsulation techniques.26 Throughout the epoxidation process, 98 wt % α-pinene oxide, trans-3,4-epoxycarene, and 1,2-limonene epoxide were produced from the corresponding terpenes [α-pinene, carene, and R-(+)-limonene, respectively] (Table 10). It was reported that the properties of the Lewis base adducts of methyloxorhenium were successfully retained after their encapsulation in polystyrene. The authors also reported that the predominant catalytic activity did not require a large excess of the ligand. Among the catalysts synthesized, the monoencapsulated complexes with bidentate ligands were more active than those with monodentate ligands. Aliphatic ligands were also found to boost the activity more than aromatic ligands. Furthermore, the catalyst was found to be reusable.26 In an unorthodox study, Bussi et al. used palladium sites in the liquid-phase oxidation of limonene in the form of Pd/Cu MnAl hydrotalcite catalysts.132 The coprecipitation and impregnation synthesis techniques were compared. Tests were carried out with both molecular oxygen and pure limonene substrates. The experimental results were explained by a chain-reaction mechanism. Consequently, palladium was assumed to activate the carbon−hydrogen bonds and, at the same time, to cleave the carbon−carbon double bonds of limonene. Copper has also been utilized in the decomposition of hydroperoxides, leading to radical intermediates. Product mixtures including 1,2- and 8,9-limonene epoxides (ratio of ca. 1:1) with conversions up to 70 wt % were obtained, albeit with low selectivity.132 Meanwhile, another group investigated the H2O2-initiated epoxidation of limonene and α-pinene over various types of alumina: basic, neutral, weakly acidic, and acidic.125 Throughout the study, the amount of water was found to critically influence the reaction rate. Moreover, anhydrous hydrogen peroxide proved to exhibit a much higher activity. As demonstrated, an experiment performed under “Dean−Stark” conditions allowed for a lower average water content and led to slightly higher yields in comparison with anhydrous H2O2. Hence, under these conditions, 83 wt % limonene epoxide

excess acetone complicated the separation and further recovery of the catalyst. The catalytic material was stable to oxidative degradation.24 Ambroziak et al. reported the batch epoxidation of α-pinene and limonene over a polymer-supported Mo(VI) catalyst.124 The reaction was initiated by TBHP. Throughout limonene epoxidation, the presence of water reduced the TBHP conversion. Furthermore, the water content influenced the ratio between cis- and trans-limonene epoxides: 50:50 for anhydrous TBHP versus 25:75 for aqueous TBHP. The mentioned observation was explained by the faster hydrolysis of the cis isomer over trans-1,2-limonene epoxide. Hence, throughout the epoxidation reaction, the composition drifted in favor of the less reactive trans isomer (Figure 1). Analogous differences in the reactivities of cis- and trans-1,2-limonene epoxides were also reported by Salles et al.129 The authors reported only minimal participation of molecular oxygen in the studied reaction. Also, the polymeric support was characterized as a thermally and oxidatively stable material, and it was also found to exhibit good leaching resistance. A linear dependence of the limonene conversion on the Mo loading for values up to 0.6 wt % was observed. Upon oxidation of α-pinene, the mentioned loadings gave rise to negligible variations in the reaction rate, and the selectivity toward limonene epoxide did not depend on the limonene/TBHP ratio. For the two substrates, 90 wt % 1,2-limonene and α-pinene epoxides, respectively, were produced (Table 10).124 Another group experimented with three different salen-based complexes supported on MCM-41 using TBHP as the oxidant.122 In this case, XRD, TEM, and nitrogen physisorption were used to confirm the preservation of the obtained MCM-41 structures. The catalyst allowed for a limonene conversion of 80 wt % and exhibited stable behavior during four consecutive experiments. However, only poor selectivity toward limonene epoxide was recorded, and polymerization products were in the majority. The authors mentioned that the polymerization commonly occurred by free-radical-pathway reactions.122 Santa et al. carried out limonene epoxidation to its endocyclic epoxide with hydrogen peroxide using zincophosphate and zincochromate molecular sieves.130 Although the authors aimed to use environmentally friendly conditions, both the conversion and the selectivity were low.130 Highly dispersed silica-supported molybdenum oxide catalysts were studied by Bakala et al.126 In fact, the impregnation with oxoperoxo molybdenum complex precursors resulted in a strong interaction with hydroxyl groups on the surface of the support. In contrast, polyoxo precursors resulted in materials with inferior interactions. Raman spectroscopy indicated the absence of undesired Mo clusters when oxoperoxo precursors were used; only two-dimensional MoOx monomers were detected. As determined by energy-dispersive X-ray (EDX) analysis, oxoperoxo precursors led to effective Mo dispersions. Despite some Mo leaching during the first run, during the second run, the catalyst demonstrated high leaching stability and reusability. A high yield of 1,2-limonene epoxide (81 wt %, 3:4 cis/trans ratio) was obtained from R-(+)-limonene (Table 10). The authors reported that the cis isomer was favored with the molybdenum systems.126 A Ti/MCM-41 catalyst was synthesized by Cagnoli et al. using the sol−gel method.131 This material was employed for the epoxidation of limonene with H2O2. As evidenced by nitrogen physisorption analysis, a porous catalyst with cylindrical, equal, and straight pores was obtained. A blank P

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(mono- and diepoxides) and 69 wt % α-pinene oxide were produced. The limonene oxidation occurred with a clear preference for the 1,2-monoepoxide. The catalyst maintained its initial activity over three runs, and no leaching was observed. Members of the same group also studied sol−gel-prepared molybdenum silicate.133 This catalyst demonstrated a high stability and activity. After the fifth run, significant leaching of molybdenum was observed, presumably because of the formation of α-dihydroxy species in the case of the ring opening of the epoxide. Using this catalyst, the authors reported the highest 1,2-limonene epoxide selectivity of 89 wt % (80 °C, 8 h). Skrobot et al. proposed an α-pinene and R-(+)-limonene oxidation process over a γ-zeolite-supported manganese(III) tetraporphyrin complex.134 Despite the successful inclusion of the complex into the zeolite pores, the catalyst was not recoverable because of the leaching of the porphyrin complex in the presence of H2O2. Use of environmentally friendly conditions allowed selectivities of 1,2-limonene epoxide and α-pinene oxide approaching 58 and 71 wt %, respectively. However, low conversions of the corresponding substrates were demonstrated.134 An interesting study was previously performed by Borocci and Marotti.25 They utilized both R-(+)- and S-(−)-limonenes to evaluate the stereoselectivity of their manganese porphyrin catalysts, which they aimed to use further in the synthesis of pharmaceuticals. The room-temperature epoxidation was performed in a microreactor and was initiated by NaClO and H2O2. As a result, 99 wt % 1,2-limonene epoxide was obtained from R-(+)-limonene. In contrast, the S-(−)-limonene substrate provided only 30 wt % of the corresponding epoxide. Interestingly, the stereoselectivity remained stable for both substrates, and 1:1 ratios were observed between the cis and trans isomers. In fact, the epoxidation favored the same face of the six-membered ring in both isomers. The authors reported that the small diastereomeric difference was caused by the chiral center of the surfactant and not by the chiral center of limonene. In fact, the difference was observed only in the presence of the chiral surfactant.25 In another study, analogous stereoselectivities of R-(+)- and S-(−)-limonenes were demonstrated, yielding R-(+)- and S-(−)-α-terpineols, respectively.135 Furthermore, the S-(−)-limonene substrate was also more inert.135 Eight optical isomers of limonene epoxide exist for either stereoisomeric form of limonene (Figure 1). Obviously, various geometric isomers could demonstrate different behaviors if mixtures thereof are utilized.24,25 Aramendia et al. tested Mg/Al double-layered hydroxides intercalated with dodecylsulfate (DS) and dodecylbenzenesulfonate (DBS) anions.136 Hydrogen peroxide was used as the initiator. The authors also studied the influence of nitriles on the oxidation reaction. XRD analysis of the final catalyst revealed the effective intercalation of both DS and DBS. In addition, a broad characterization study was conducted concerning the surface and interlayer species of the catalysts using Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies. 1,2-Limonene epoxide was obtained with a yield of 52 wt %. Throughout the noncatalytic trials, limonene conversion was significantly improved by the presence of benzonitrile. In fact, peroxicarboximidic acid is formed upon contact of benzonitrile with peroxide. Consequently, oxygen atoms are transferred from the acid to the olefin. The authors suggested that, in aqueous medium, undesired glycols could form by hydrolysis of previously

formed limonene epoxides. In addition, glycol could form by direct hydroxylation of the endocyclic double bond of limonene. At this point, the recommended conclusion was to aim at increasing the epoxidation rate to avoid prolonged contact of limonene with the aqueous medium in the presence of the catalyst.136 Arnold et al. studied the epoxidation of α-pinene and limonene over microporous metallosilicates containing molybdenum.127 Catalytic materials were prepared by the acidcatalyzed sol−gel process with molybdenum(V) isopropoxide as the metal precursor. This precursor resulted in a selective catalyst that was also stable to leaching. In the case of limonene oxidation, a constant selectivity of around 87 wt % was demonstrated, with a 1,2-limonene epoxide yield of 70 wt % (Table 10). At the same time, only 14 wt % α-pinene oxide was produced from α-pinene, presumably because of the steric restrictions and secondary isomerization of α-pinene oxide.127 The epoxidation of camphene opens the way toward fragrances and synthetic intermediates such as camphene epoxide and camphyl aldehyde. In a previous study, Adam et al. focused on the epoxidation of camphene with methyloxorhenium(VII) heterogenized on a NaY zeolite.137 The authors used H2O2 and obtained ca. 95 wt % selectivity toward camphene epoxide at 89 wt % conversion.137 An analogous catalyst was previously studied by another group and provided up to 85 wt % camphene epoxide.138 Meanwhile, another group obtained up to 97 wt % of the same product using sulfonated manganese(III) tetraphenylporphyrin supported on Amberlite IRA-400.139 Also, aluminumfree zeolite-titanium beta was applied by van der Waal et al. in 1998.140 The catalyst gave high selectivity toward camphyl aldehyde (92 wt %); however, poor conversion of 4 wt % was demonstrated.140 Furthermore, homogeneous epoxidation of camphene was recently reported by Carari and da Silva.141 Unfortunately, moderate conversion (45 wt %) was reported, and only isomerization and etherification products were formed. In contrast, the use of α- and β-pinene substrates gave rise to better reactivity.141 3.4.3. Oxidation of ρ-Cymene. Terephthalic acid (TPA) is a valuable precursor in the synthesis of poly(ethylene terephthalate) (PET), a thermoplastic polymer.5,12 PET is commonly used as a material for lemonade bottles, tire cords, and textile fibers. In 2011, PepsiCo announced that, beginning in 2012, its PET production line would be based on renewable resources only.5 Conventional synthetic methods to obtain terephthalic acid involve the homogeneous oxidation of ρxylene by dioxygen in the presence of a Co/Mn/Br− catalyst system in an acetic acid medium (the AMOCO process). In terms of the sustainability of this process, the bromine promoter in particular should be replaced to avoid potential corrosion and environmental problems. Furthermore, significant consumption of acetic acid occurs on-site through the decomposition to CO and CO2. Moreover, in this process, the final yield of TPA is decreased by its decarboxylation to benzoic acid.142,143 As an alternative process solution, Okkerse and van Bekkum previously suggested two potential green routes to terephthalic acid: a two-step route through terpenes and a fourstep route through fructose.144 Unfortunately, the second route involves a difficult Diels−Alder addition of ethylene to 2,5furandicarboxylic acid, and no experimental evidence was presented by the authors.144 Q

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Scheme 7. Overall Mechanism for the Oxidation of ρ-Cymene151,152,155,157

Table 11. Current Achievements in the Oxidation of ρ-Cymene and α-Pinene Aiming for Terephthalic Acid ref

yield (wt %)

catalyst

reactor type

152

38

Mn/MCM-41

flow-through reactor

151

41

glass reactor

158

26

Ru/carbon nanofibers (CNFs) (VO)2P2O7

156

51

Mn(III) porphyrin complex

159

20

V2O5

process conditions Substrate, ρ-Cymene; Product, ρ-Methylacetophenone continuous flow of substrate (liquid), 0.015 mL/min, 0.3 g of catalyst, 50 mL/min air, 350 °C, 1 h Substrate, ρ-Cymene; Product, ρ-Cymene Hydroperoxides 17.8 mL of substrate, 0.4 mL of 70% tert-butyl hydroperoxide, 0.1 g of catalyst, 30 mL/min O2, 90 °C, 5 h

glass reactor

40.7 mL of substrate, 2.3 mL of oxidized cymene oil (58 wt % TCHP), 0.0175 g of catalyst, 40 mL/min O2, 100 °C, 3 h Substrate, ρ-Cymene; Product, 4-Isopropylbenzoic Acid glass reactor 50.6 mL of substrate, 2 mL of acetonitrile, 0.005 g of amonium acetate, 5.18 mmol of catalyst, 0.4 mL of H2O2/acetonitrile mixture (1:10) injected every 40 min, room temperature, 5 h Substrate, α-Pinene; Product, Terephthalic Acid flow-through (1) continuous flow of substrate (liquid), 9.3 mL/h, 0.4-m catalytic bed, 600 L/h air, 36 mL/h H2O (vapor), reactor 370 °C, 10 h; (2) extraction with hot benzene, 8 h

given that the σ-bond enthalpy of the methyl group is higher than that of the isopropyl group.152,155 However, an unexpected formation of 4-isopropylbenzoic acid was reported by Martins et al.156 Various metals were found to differ in terms of their abilities to activate the methyl or acetyl groups of PMA. For example, toluic acid was formed at Co sites, whereas Mn sites provided the oxidation of the methyl group of PMA.152,155 Recent advances in ρ-cymene oxidation are presented in Table 11. Makgwane and Ray reported the efficient oxidation of ρcymene to its primary and secondary hydroperoxides, which could subsequently be further utilized in the synthesis of PMA, TPA, or ρ-cresol.151 Ruthenium particles (3 wt %) were anchored on carbon nanofibers to obtain 41 wt % ρ-cymene hydroperoxides, at 55% substrate conversion (Figure 6). The size of the Ru particles, the effect of the metal loading, the nature of the metal species, the structural properties of the resulting materials, and the reaction conditions all significantly influenced the catalyst activity. The catalyst was involved in ρcymene CH bond activation through TCHP initiator decomposition rather than through direct catalytic H-atom

On the contrary, in the past several decades, many good homogeneous oxidation strategies of monoterpenes allowing for terephthalic acid yields of up to 85 wt % were proposed.145−150 Still, oxidizing ρ-cymene in the absence of a catalyst significantly lowers the conversion (15−20 wt %), distorts the selectivity, and requires batch times up to (and sometimes above) 14 h.151 Alternatively, ρ-cymene oxidation also gives ρ-methylacetophenone (PMA), a valuable perfumery and pharmaceutical product.152,153 In fact, ρ-celecoxib, a nonsteroidal antiinflammatory drug, was synthesized by the condensation of ρ-methylacetophenone and ethyl trifluoroacetate.154 Syam et al. also tested PMA in the synthesis of chalcones, which are antiinflammatory and antibacterial pharmaceutical compounds.153 The overall mechanism of ρ-cymene oxidation is described in Scheme 7. The reaction starts with metal-catalyzed hydrogen transfer from the methyl or isopropyl group of ρ-cymene. The corresponding group is further oxidized, and the formed hydroperoxide is thermally decomposed to ρ-methylacetophenone (PMA) or cumic aldehyde. The concentration of PMA is predicted to be 8 times higher than that of cumic aldehyde, R

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causing a gradual decline in catalytic activity.152 However, Makgwane and co-workers managed to demonstrate the conversion of ρ-cymene at ca. 100 °C, with ρ-cymene hydroperoxides as the main reaction products.151,158 This observation supports the intermediate role of ρ-cymene hydroperoxides in the described reaction mechanism (Scheme 7). Similarly to the case of continuous flow-through operations, intensive coking was also reported at prolonged times on stream. The ρ-cymene hydroperoxide decomposition step could be the major source of coke.152 As is generally observed, ρ-cymene is stable in heterogeneously catalyzed oxidations (Figure 7). However, the low Figure 6. Comparison of the catalytic activities of Ru-supported catalysts in ρ-cymene oxidation. Reaction conditions: ρ-cymene, 114 mmol; TBHP initiator, 6 mmol; catalyst, 0.1 g; O2 flow rate, 30 mL/ min; temperature, 90 °C, time, 5 h. (1) 3 wt % Ru/CNF, (2) 5 wt % Ru/CNF, (3) 7 wt % Ru/CNF, (4) 1 wt % Ru/CNF, (5) pure CNF, (6) no catalyst. Adapted from ref 151.

abstraction. Thus, the Ru activity was significantly improved when small volumes of an initiator were added, and the recoverability of the catalyst was demonstrated.151 Makgwane et al. also studied the application of vanadium phosphate oxide catalysts, obtaining 26 wt % tert-ρ-cymene hydroperoxide (TCHP) in a 4-h on-stream experiment.158 The improved surface area of (VO)2P2O7 resulted in 85 wt % selectivity toward TCHP. The high selectivity was also explained by the slow catalytic decomposition of TCHP, especially at substrate conversions of 35 wt % or less. In contrast, the catalyst contributed significantly to the TCHP decomposition under an oxygen atmosphere.158 Vetrivel and Pandurangan studied the oxidation of ρ-cymene over mesoporous Si- and Al-MCM-41 molecular sieves impregnated with manganese. The reaction yielded 38 wt % PMA. The catalysts were synthesized by a hydrothermal method, and XRD measurements showed the formation of well-ordered mesoporous structures. Mn-MCM-41 had the finest dispersion of nonframework manganese oxide particles, hence giving rise to the highest activity. 4-Isopropylbenzaldehyde, 1,2-epoxyisopropylbenzaldehyde, and 4-methylstyrene were the main byproducts identified.152 Previously, Martins et al. obtained 51 wt % 4-isopropylbenzoic acid catalyzed by Mn(III) porphyrin complexes. Their catalyst was an exceptionally active one, whic, h, even at room temperature was capable of converting up to 74 wt % ρcymene. ρ-Cymenol, ρ-isopropylbenzyl alcohol, thymoquinone, and cumic aldehyde were other reaction products observed.156 Much earlier, the method of direct oxidation of α-pinene to TPA involving V2O5 in a trickle-bed column was proposed at an elevated temperature of 370 °C and a large air flow of 600 L/h. Moreover, significant amounts of the catalyst were used: Only 9 mL of α-pinene passed through 43 cm of the catalytic bed per hour. Furthermore, extraction with benzene was required to purify the reaction products. The mentioned setup gave only a 20 wt % yield of TPA (Table 11). Nevertheless, this experiment was apparently the only reported successful synthesis of terephthalic acid over a heterogeneous catalyst.159 For the highest ρ-cymene conversion, Vetrivel and Pandurangan recommended a temperature range of 350−400 °C because of the intensive decomposition of the intermediate ρ-cymene hydroperoxides.152 They reported that significant coke formation was initiated at temperatures above 400 °C,

Figure 7. Comparison of catalyzed and noncatalyzed ρ-cymene oxidation rates. (1) (VO)2P2O7, (2) VO(PO3)2, (3) no catalyst. Other reaction conditions: ρ-cymene, 35 g; initiator, 2 g; O2 flow rate, 40 mL/min; catalyst, 0.0175 g; temperature, 100 °C; agitation rate, 2.000 rpm. Adapted from ref 158.

conversion of ρ-cymene is compensated by the substantial selectivity of the process and is further combined with low amounts and numbers of byproducts, in comparison to corresponding reactions of α-pinene or limonene substrates.151,152,158 A prolonged reaction time should decrease the selectivity while increasing the conversion of stable ρcymene. The initiation of the autoxidation reactions by activation of the CH bonds is hampered in the noncatalyzed ρ-cymene oxidation; hence, longer induction periods are required in the absence of a catalyst (Figure 7).158 For heterogeneously catalyzed ρ-cymene oxidation, both batch and continuous operating conditions could be recommended. The effectiveness of Co, Mn, Br, and V with bromic or acetic acids as oxidation promoters has been highlighted.145−150,155,159 Nair et al. noted a significant bromine influence in the catalytic oxidation of aromatic hydrocarbons.143 Nevertheless, the synthesis of terephthalic acid over heterogeneous catalysts remains a challenge. 3.5. Hydrogenation

The hydrogenation of terpene substrates gives rise to pharmaceutical, perfume, and agrochemical compounds.160 From α- and β-pinenes, trans- and cis-pinanes are obtained with chair and boat conformations, respectively.160,161 Furthermore, (+) and (−) optical isomers emerge in accordance with the orientation of the C(CH3)2 group (Scheme 8).161 Hydroperoxides of cis- and trans-pinanes are applicable as initiators of low-temperature copolymerization of butadiene with styrene.90,162 In line with the fragrance synthesis, pinanes can be further upgraded to linalool through subsequent S

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Scheme 8. Overall Mechanism for the Hydrogenation of αPinene30,104,161

group of camphor from below.164 To generate enantiomeric products, the less hindered endo face of the camphor skeleton is preferably attacked, and further synthetic steps are commonly determined by the stereoselectivity of the resulting derivatives. As a result of their steric bulk or polarity effects, the attack at one face or another could be either blocked or promoted.39 Bazhenov et al. synthesized 95 wt % cis-pinane from α-pinene using a Ni/kieselguhr catalyst at elevated temperatures.90 Two decades earlier, Pavlin patented a process involving batch hydrogenation over a row of Ru/Al2O3 and Ru/C catalysts.165 Depending on the reaction parameters and catalyst type, up to 99 wt % cis-pinane was obtained at room temperature. Recently, utilizing β-pinene as the substrate, Agarwal and Ganguli produced 80 wt % (−)-cis-pinane.160 In the case of limonene as the substrate, the same group obtained 99.8 wt % ρ-menthene. As the catalyst, rhodium nanoparticles stabilized on montmorillonite K10 clay were employed. Characterization methods confirmed the good dispersion of Rh(0) nanoparticles in the 1−3-nm diameter range. Also, the product selectivity could be tuned by controlling the hydrogen pressure.160 As presented by a few authors, highly selective hydrogenation can be performed even at room temperature;160 however, the hydrogenation reaction is accelerated by higher temperatures (Ea = 37.5 kJ/mol).104,161 The crucial influence of the hydrogen pressure on the cis-pinane selectivity was reported, whereas higher temperatures favored trans-pinane formation.33,104,160,161,168 These authors recommended Pt, Pd, Rh, Ni, and Ru sites supported on SiO2, TiO2, or carbon nanotubes, with hydrogenation promoters such as organic acids, esters, amines, and amides.31,113,160,165 Under elevated pressures, the metal-π-complex elimination is suppressed.104,161 In this case, the selectivity of cis-pinane formation is strongly dependent on the substrate adsorption and is stimulated by low temperatures. A decrease in pressure brings the ratio between cis- and trans-pinanes to the thermodynamic equilibrium composition when the role of temperature is determinative. Higher temperatures boost the number of free adsorption sites while having no influence on the concentration of adsorbed α-pinene or on the concentration of the metal hydride species. Thus, utilization of elevated temperatures results in lower cis-pinane formation independent of the α-pinene concentration or hydrogen pressure.104,161

oxidation, hydrogenation, and isomerization stages.104 Linalool is utilized in the synthesis of vitamins12 and can also be efficiently converted to methylcyclopentadiene dimers and high-density missile fuels.163 Some α-pinene oxidation products (such as verbenone) are also recognized as substrates for hydrogenation reactions producing other fragrances (such as verbanone).164 Successful examples of terpene feedstock hydrogenations are listed in Table 12. Silica-supported platinum catalysts were successfully implemented by Casella et al., yielding 86 wt % cis-pinane from αpinene, at moderate temperatures.164 As reported by the authors, monometallic Pt catalysts performed better than bimetallic ones; thus, the hydrogenation sites should not be blocked. In fact, modification of Pt/SiO2 with Sn resulted in a decreased catalytic activity through a combination of electronic and geometrical effects. This group also converted 95 wt % of verbenone to cis-verbenol with 80 wt % selectivity through chemical reduction with NaBH4. The optical activity of camphor is crucial to the synthesis of chemicals applicable as medicines and vitamins. In a similar study, Casella et al. also achieved 20 wt % conversion of camphor in 6 h over a Pd/SiO2 catalyst. Throughout the hydrogenation process the authors noticed high stereoselectivity toward camphor. Still, both exoand endo-borneols were formed (Figure 1). However, in the case of the monometallic catalyst, the formation of endoborneol prevailed, although the reaction was slower. Presumably, exo-borneol was formed when the camphor molecule coordinated to the catalyst surface at its interior endo face. Hence, the adsorbed hydrogen was allowed to attack the CO

Table 12. Current Achievements in the Heterogeneously Catalyzed Hydrogenation of Terpenes ref

yield (wt %)

catalyst

165 90 164

96 96 86

Ru/Al2O3 Ni/kieselguhr Pt/SiO2

160 166

99 99

Rh/montmorillonite K10 Ru/Al2O3 coated with ionic liquid

31

99

Pd/C (SCN type)

167

72

1 wt % Pd/C

160

80

Rh/montmorillonite K10

reactor type

process conditions

Substrate, α-Pinene; Product, cis-Pinane pressurized vessel 100:7 substrate/catalyst ratio, 13.8 bar H2, 25 °C, 3 h bubbling flow batch reactor 50 mL of substrate, 100 mL of pelletized catalyst, 6 bar H2, 150 °C, 2 h pressurized vessel 0.26 mL of substrate, n-heptane solvent, 0.25 g of catalyst, 10 bar H2, 80 °C, 1.5 h Substrate, Limonene; Product, ρ-Menthene glass reactor 0.16 mL of substrate, 15 mL of methanol, 0.03 g of catalyst, 2.5 bar H2, 20 °C, 0.7 h flow-through semibatch continuous circulation of 2 mL of substrate (3.3 mL/min), 50 °C, 40 bar H2, reactor 125 bar CO2, 2 mL of ionic liquid, 1.5 g of catalyst Substrate, Limonene; Product, ρ-Menthane flow-through reactor with 1 mL of substrate (3.3 mL/min circulation), 0.2 g of catalyst, 38 bar H2, 87 bar N2, close circulation 50 °C pressurized vessel (1) activation of the catalyst in H2; (2) 400 mL of substrate, 20 g of catalyst, 11 bar H2, 200 °C, 60 h Substrate, β-Pinene; Product, cis-Pinane glass reactor 0.16 mL of substrate, 15 mL of methanol, 0.05 g of catalyst, 3 bar H2, 20 °C, 0.4 h T

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blend of these products with diesel fuel was tested according to standard method ASTM D975, and the properties were found to be acceptable for a basic diesel fuel. Positive effects of the additives on the viscosity and cloud point were also observed. It was stated that the unsaturated bonds of terpenes are highly reactive and are thus undesirable in fuel blends. Hence, complete hydrogenation is required. Trials were carried out over 1 wt % loaded Pd/C and Pt/Al2O3. The yield of ρmenthane from limonene was 73 wt %, and m-cymene was the second main product.167 Bogel-Lukasik et al. provided a consecutive series of studies of R-(+)-limonene hydrogenation under high pressures of CO2 using a flow-through reactor.31,169 They observed that the H2/ CO2 ratio determined the reaction kinetics in terms of the R(+)-limonene concentration. In fact, increasing CO2 pressure caused the volumetric expansion of the liquid phase. Palladium supported on activated carbon was applied as the catalyst. The reaction proceeded through intermediates ρ-menth-1-ene and ρ-menth-3-ene (Scheme 9). Saturated ρ-menthane (cis/trans ratio of 1:2) was the only product obtained at total conversion (Figure 1, Table 12). Interestingly, low flow rates provided side-isomerization of R-(+)-limonene, decreasing the yield of ρmenthane.31,169 In a recent study by the same group, a reaction setup was used that contained ionic-liquid-coated Ru/Al2O3.166 The reaction allowed for the complete and selective hydrogenation of R-(+)-limonene, whereupon ρ-menthene was the main reaction product (Table 12). The addition of the ionic liquid significantly promoted the ρ-menthene selectivity.166 In a new study by Mokhov et al., the hydrogenation of camphene in a colloid solution of Ni was carried out at 50 °C under atmospheric pressure.170 Ultimately, a 55 wt % conversion of camphene to 2,2,3-trimethylnorbornane was

The stereoselectivity of this reaction was explained by several authors. The sp2-hybridized CC double bond of α-pinene generates a plane around which all other functional groups are oriented. In fact, in this plane, CH2 and C(CH3)2 groups are located on opposite sides. With respect to the double bond, the adsorption of the α-pinene molecule to the metal surface can take place in two opposite positions. As a result, two different π-complexes can be produced.104,161,164 The consecutive hydrogenolysis of metal−carbon bonds in metal−δ-pinanyl complexes results in the formation of the pinane molecule.161 Therefore, the preferable geometric arrangement is strongly dependent on the steric hindrance of the specific metal site.161,164 Beyond the synthesis of fragrances and pharmaceuticals, hydrogenation of limonene also provides a route to diesel fuel additives. Tracy et al. hydrogenated branched myrcene and monocyclic limonene, obtaining 2,6-dimethyloctane and ρmenthane, respectively (Scheme 9).167 Furthermore, a 10% Scheme 9. Hydrogenation of Limonene and Myrcene30,31,167,169

Scheme 10. Reaction Network for the Acetoxylation and Hydration of α-Pinene30,51,60,176−180

U

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Table 13. Current Achievements in the Heterogeneously Catalyzed Acetoxylation of Terpenes ref

yield (wt %)

catalyst

49 50

50 47

H-beta zeolite H3PW12O40 (homogeneous)

52

35

no catalyst

173 175

30 29

H-beta zeolite PdCl2CuCl2

49

85

H-beta zeolite

51

41

[HSO3(CH2)3NEt3]HSO4

52

40

Amberlyst 70

174

50

beta zeolite

reactor type

process conditions

Substrate, α-Pinene; Product, α-Terpinyl Acetate glass reactor 8.4 mL of substrate, 15.8 mL of methanol, 2−5 g of catalyst, 40 °C, 5 h glass reactor 2.32 mL of substrate/acetic acid mixture (1:1.2) (acetic acid/H2O = 95:2.5), 0.05 g of catalyst, 15 °C, 1 h pressurized 5.4 mL of substrate, 120 mL of acetic acid, 0.25 g of catalyst, 20 bar O2, 100 °C, 12 h vessel glass reactor 1.15 mL of substrate, 10 mL of acetic acid, 0.5 g of catalyst, 20 °C, 24 h glass reactor 1.0 mol/L substrate, [PdCl2] = 0.01 mol/L, [CuCl2·2H2O] = 0.1 mol/L, [LiCl] = 0.7 mol/L, 10 mL of acetic acid, 1 bar O2, 80 °C, 0.4 h Substrate, Limonene; Product, α-Terpinyl Acetate glass reactor 11.2 mL of substrate, 25 mL of methanol, 2−5 g of catalyst, 40 °C, 5 h Substrate, α-Pinene; Product, Bornyl Acetate glass reactor 7.9 mL of substrate, 3.1 mL of chloracetic acid, 8.6 mL of acetic acid, 0.001 mol of catalyst, 30 °C, 10 h pressurized 5.4 mL of substrate, 120 mL of acetic acid, 0.25 g of catalyst, 20 bar O2, 100 °C, 12 h vessel Substrate, Camphene; Product, Isobornyl Acetate glass reactor 0.28 mL of substrate, 30 mL of acetic acid, 0.13 g of catalyst, 95 °C, 6 h

Liu et al. obtained 41 wt % bornyl acetate from α-pinene using acidic ionic liquids.51 In addition, α-terpinyl and fenchyl acetates were produced in minor amounts. The catalytic activity was found to be influenced by the cation of the ionic liquid, whereas the properties of the corresponding anion predominantly affected the selectivity obtained. Successful product separation and catalyst reusability were also reported.51 Previously, α-terpinyl acetate yields from S-(−)-α- and S(−)-β-pinenes were maximized by Robles-Dutenhefner et al.50 Up to 50 wt % of this product was obtained in a system containing a heteropoly acid, namely, H3PW12O40(PW), in acetic acid and acetic acid/water solutions. The same group utilized limonene substrates, obtaining up to 45 wt % αterpineol. Homogeneous reactions were performed at room temperature, giving two products with 85% selectivity at 90% conversion. In addition, the heteropoly acid catalyst was recoverable by precipitation. However, the authors failed to transfer the same activity into the heterogeneous system. The use of a silica-supported heteropoly acid resulted in a low yield of 20% from monoterpene substrates.50 Hensen et al. proposed an excellent method implementing H-beta zeolites in methanol solutions.49 Both batch and continuous operations were successfully demonstrated, and the authors managed to synthesize up to 50 and 85 wt % αterpinyl acetate from α-pinene and limonene substrates, respectively. Furthermore, 50% and 95% selectivities were observed in the case of α-pinene and limonene substrates, respectively. Interestingly, linear alcohols were the only successful solvents because other branched alcohols dehydrated to the corresponding alkenes and water. An inverse dependence of the selectivity on the reaction temperature was demonstrated. Meanwhile, the substrate conversion increased with increasing temperature (Figure 8). The zeolites used were stable and demonstrated reasonable recoverability because of the moderate reaction temperature. The authors assumed that the high catalytic activity was a result of appropriately sized and shaped acid sites present in the pores of the catalyst. Large amounts of acid sites and high surface areas were reported, supported by TPD and nitrogen physisorption (Brunauer− Emmett−Teller, BET) measurements, respectively.49 In 2014, Czapiewski and Meier studied the direct catalytic regioselective acetoxylation of limonene by palladium-catalyzed

reached. The authors assumed high reversibility of camphene hydrogenation and thus indicated that high pressures of hydrogen should be used.170 Previously, camphene hydrogenation was also tested over rhodium(I) complexes and chirally modified ruthenium clusters, resulting in high diastereoselectivity, with the selectivity favoring the cis hydrogenation products.171,172 3.6. Acetoxylation

The acetoxylation of α-pinene provides access to valuable perfumery precursors, such as α-terpinyl and bornyl acetates. Use of complicated mixtures of terpene acetates as a fragrance is also permitted in the fragrance industry.52,60 α-Terpinyl acetate has a grapefruit-like smell and is widely used in soaps and shampoos,49,51,60,173 whereas bornyl acetate is a less valuable fragrance but can be used by the pharmaceutical industry.51,60,174 The acetoxylation of limonene, because of its monocyclic structure, gives improved yields of α-terpinyl acetate.49,50 In the case of camphene acetoxylation, bornyl acetate and borneol are produced predominantly.174,175 The homogeneous catalytic method is traditionally executed in two stages: initial treatment of α-pinene with sulfuric or phosphoric acid to obtain α-terpineol, followed by esterification in acetic acid solution.60,173 The consecutive formations of pinyl, terpinyl, fenchyl, and bornyl ions follow from parallel acetoxylation steps (Scheme 10).51,60,176,177 The presence of acidic groups in the potential catalyst is beneficial and thus promotes the addition of acetic acid to α-pinene.176 Recent achievements in heterogeneous catalysis of acetoxylation are presented in Table 13. We also recently studied the high-temperature acetoxylation of α-pinene over the commercial catalyst Amberlyst 70.52 Interestingly, 35 wt % of the targeted α-terpinyl acetate was obtained in a predominantly nonheterogeneous catalytic reaction, presumably because of the optimized reaction conditions, promoted by acetic acid. The high acidity of Amberlyst 70 increased the yield of the desired product, with up to 40 wt % bornyl acetate being obtained. Still, fenchyl and verbenyl acetates were also formed. In total, up to 70 wt % terpene acetates were obtained. However, the textural properties of the catalyst seriously deteriorated, and catalyst recovery was not practical.52 V

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oxide occurred. This oxidation product could be an intermediate reacting further with acetic acid to produce more terpene acetates (e.g., verbenyl acetate), thus leading to an increased yield of acetoxylated products.52,60,181 Lajunen et al. studied the homogeneous oxidation of (−)-α-pinene by molecular oxygen, using Co(II) salts as catalysts and TBHP or acetic acid as an initiator.181 They reported that increasing concentrations of acetic acid led to the rearrangement of αpinene oxide to verbenyl acetate. Verbenone was also formed from α-pinene oxide through a competitive route.181 Optimal catalyst acidity is required to stabilize the desired carbonium ion for the consecutive acetoxylation. Obviously, the role of acidity is more important than that of temperature in terms of α-terpinyl acetate formation.49−51 α-Terpineol (Scheme 10) is the most valuable monocyclic terpene alcohol; its mild lilac odor makes it popular for use in bath, pharmaceutical, and perfumery products. α-Terpineol forms through hydration and acetoxylation reactions.12,14,50,179,182 α-Terpineol is conventionally synthesized through partial dehydration of cis-terpin hydrate, which is formed upon the hydration of α-pinene, limonene, 3-carene, or turpentine.12,14,182 Hydrolysis of terpene acetate is also a synthetic route to α-terpineol.14 It should be noted that hydration of limonene generally gives higher α-terpineol yields.14,50 Several examples of studied hydration reactions are given in Table 14.

Figure 8. Acetoxylation reaction: influence of temperature on αpinene conversion in an integral continuous-flow fixed-bed reactor. Conditions: weight hourly space velocity (WHSV), 2.0 h−1; pressure, 10 bar; methanol/α-pinene mass ratio, 2:1; beta zeolite, 5 g. Adapted from ref 49.

CH activation utilizing ρ-benzoquinone as a reoxidation agent in acetic acid solution.177 The exocyclic double bond of limonene was selectively acetoxylated when dimethyl sulfoxide was added to the reaction mixture and used in combination with palladium(II) acetate and ρ-benzoquinone. In this case, 78 wt % of the corresponding acetate was obtained. Interestingly, under the optimized reaction conditions, both (S)-(−)- and R(+)-limonenes reacted with similar conversions and selectivities.177 Elevated temperatures resulted in decreased selectivity toward α-terpinyl acetate, whereas the substrate conversion and selectivity toward bornyl acetate increased.60,173,176 At elevated temperatures (100−125 °C) and prolonged times on stream, partial decomposition of α-terpinyl acetate to limonene was detected.60 The above factors, in combination with the high acidity of the catalyst, also stimulated the formation of isomerization products and subsequent ρ-cymene formation from γ-terpinene.49,60,173 A strong Arrhenius-type temperature dependence was observed in all cases: bornyl acetate, limonene, and α-terpinyl acetate.52 Because of the nucleophilic properties of water, adding small amounts of water to glacial acetic acid is beneficial in terms of the selectivity of acetoxylation products. While increasing the proton abstraction rate, water promotes the formation of terpene acetates and increases the conversion rate.50,52,60,173 Acetic acid itself is both a strong catalyst and an acetylating agent in the studied reaction, resulting in high yields of αterpinyl acetate even under homogeneous conditions.52 Furthermore, the positive influence of an oxygen atmosphere was detected, in contrast to hydrogen and nitrogen gases. Under an oxygen atmosphere, partial formation of α-pinene

3.7. Dimerization

The dimerization of terpenes is of importance to the synthesis of lubrication oil and aerospace fuels.6,56 Strained molecules commonly have a higher heat of combustion than unstrained or linear ones.56 In dimerization reactions, the density of the substrate is significantly increased, thus improving the volumetric heat of combustion.56 Furthermore, the obtained dimers should be hydrogenated over catalysts such as platinum. This procedure improves the net heat of combustion, stability, and density of the obtained dimers to values nearly identical to those of the widely used tactical fuels.56,183 The dimerization starts with the common isomerization step as described next . Initially, isomerized products, such as camphene, limonene, terpinenes, and menthenes (Scheme 1), dominated but were later consumed upon formation of dimers (Figure 9).

Figure 9. Proposed structures of hydrogenated dimers. Adapted from ref 56.

Table 14. Current Achievements in the Hydration of Terpenes ref

yield (wt %)

catalyst

179

43

USY zeolite

47

32

H-beta zeolite

50

44

182

33

H3PW12O40 (homogeneous) H-beta zeolite

reactor type

process conditions

Substrate, α-Pinene; Product, α-Terpineol membrane 37 mL/min circulation, 0.6 mL of substrate, 28.5 mL of acetone (50 wt % H2O), 0.25 g of catalyst, 50 °C reactor glass reactor 0.25 mL of substrate, 30 mL of acetone, 0.017 mL of H2SO4, 0.5 g of catalyst, 56 °C, 2.5 h Substrate, Limonene; Product, α-Terpineol glass reactor 2.32 mL of substrate/acetic acid mixture (1:1.2) (10 wt % H2O), 0.05 g of catalyst, 60 °C, 0.5 h glass reactor

2 mL of substrate, 10 mL of acetic acid, 0.1 g of catalyst, 50 °C, 24 h W

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Table 15. Current Achievements in the Heterogeneous Dimerization of Terpenes ref

yield (wt %)

catalyst

13

79

Nafion SAC-13

6

71

H3PW12O40/MCM-41

56

90

Nafion SAC-13

6

71

H3PW12O40/MCM-41

13

81

Nafion SAC-13

6

71

H3PW12O40/MCM-41

99

59

Amberlyst 15

reactor type glass reactor glass reactor glass reactor glass reactor glass reactor glass reactor glass reactor

process conditions Substrate, α-Pinene; Product, Dimers (1) 0.5 g of catalyst and 30 mL of heptane heated to 100 °C, (2) 100 mL of substrate added dropwise, (3) 18 h of treatment 23 mL of substrate, 0.4 g of catalyst, purging with N2 for several minutes, 160 °C, 3 h Substrate, β-Pinene; Product, Dimers prior to heating, 40.7 mL of substrate added dropwise to a slurry of 10 mL of heptane and 0.1 g of catalyst, 100 °C, 6 h same as mentioned above (α-pinene substrate) Substrate, Crude Turpentine; Product, Dimers (1) 5.5 g of catalyst and 150 mL of heptane heated to 100 °C, (2) 250 mL of substrate added dropwise, (3) 12 h of refluxing under N2 same as mentioned above (α-pinene substrate) Substrate, 1,4-Cineole; Product, Dimers 5.4 mL of substrate, 0.25 g of catalyst, N2, 85 °C, 24 h

Furthermore, stable ρ-cymene and ρ-menthene were continuously accumulated.6,13,56 In fact, less-viscous dimers were obtained from limonene substrates because of its monocyclic structure and its possibility for linear dimerization.13 Conventional homogeneous processes involve the use of phosphoric, sulfuric, and hydrofluoric acids; thus, a shift toward heterogeneous catalysis is desirable.56,184 Table 15 summarizes significant advances in the heterogeneously catalyzed dimerization chemistry of terpenes. Utilizing β-pinene as the reactant, Harvey et al. synthesized a fuel with a density of 0.94 g/cm3 and a net volumetric heat value of 39.5 MJ/L,56 values that are comparable to those of some widely used tactical fuels (Table 15). Despite the simplicity of the structures proposed in their study as target dimer molecules (Figure 9), the original product mixture was more complicated. A complex mixture of peaks was observed by means of gas chromatography (GC)/mass spectrometry (MS), and molecular weights of ∼272 were mainly demonstrated. Meanwhile, some peaks with molecular weights of ∼274 were also observed. The results can be partly explained either by hydrogenation after dimerization of two monomers or by coupling of terpinenes and menthenes.56 Members of the same group also investigated α-pinene, R-(+)-limonene, camphene, and crude turpentine substrates for dimerization over Nafion SAC-13.13 Depending on the feedstock, yields of up to 90 wt % were obtained. The catalyst could be recovered by simple filtration and reused eight times without loss in activity (Table 15).13 Further, Nie et al. performed the isomerization and dimerization of turpentine and α- and βpinenes over H3PW12O40/MCM-41.6 They obtained 40 wt % camphene in the isomerizations of both α- and β-pinene. In the case of turpentine as a substrate, 22 wt % dimer was obtained. Interestingly, the three substrates demonstrated comparable behaviors. In fact, turpentine was able to form more dimers than α- or β-pinenes. This phenomenon can be explained by the presence of isomeric compounds existing in crude turpentine undergoing direct dimerization. The same group synthesized up to 70 wt % dimers of turpentine and α- or βpinenes. Obviously, the dimerization activity was strongly dependent on the heteropolyacid loading and was optimal for an 80% H3PW12O40/MCM-41 catalyst. This catalyst exhibited

fairly good stability. The product distribution of the reaction with α-pinene is presented in Figure 10.6

Figure 10. Product distribution from the dimerization of α-pinene catalyzed by H3PW12O40/MCM-41. Reaction conditions: 23 mL of substrate, 0.4 g of catalyst, purging with N2 for several minutes, 160 °C. Reprinted with permission from ref 6.

In a recent study by Meylemans et al., up to 60 wt % dimers were obtained from 1,4-cineole.99 The composition of the mixture of intermediate terpinenes and terpinolenes depended strongly on the acid strength, acid type, and hydrophilicity of the catalysts. Consequently, the authors suggested that knowledge of the reaction intermediate distribution might help in predicting and characterizing the final complex mixture of dimers.99 Interestingly, as suggested by Meylemans et al. in 2012, the dimerization of terpenes has probably occurred in many research trials, although the dimers were not detected because of their high boiling points. A specific gas chromatography (GC) procedure is required for the precise determination of terpene dimers.13 Dimers could also be confused with X

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AUTHOR INFORMATION

oligomers or other waste products and, thus, not be reported. In a typical GC chromatogram, the dimers are represented as a group of inseparable peaks at the end of the run.56 As described below, the process depends mainly on the temperature, gas atmosphere, and acidic nature of the catalyst. Isomerization and dehydrogenation rates surpass that of dimerization under elevated reaction temperatures of 150 °C and above.6,56 Previous studies recommended applying atmospheres of inert gas (nitrogen, argon, carbon dioxide) to avoid the oxidation or hydrogenation steps.13,184 In dimerization processes, the concentration of moderate and strong acid sites is crucial. For high concentrations of acid sites, the yield of dimers becomes independent of the terpene concentration.6 As reported by Harvey et al., the presence of Lewis acidic sites is desirable in the initial stage of dimerization reaction.56 In contrast, Brønsted acid sites are active only at high temperatures, providing isomerization/dehydrogenation activity.13

Corresponding Authors

*E-mail: Mikhail.Golets@abo.fi. Tel.: +46 0 76 104 3381. *E-mail: [email protected]. Tel. +46 0 70 620 0371. Notes

The authors declare no competing financial interest. Biographies

4. CONCLUSIONS This review provides an overview of current research trends concerning the upgrading of α-pinene and related terpenes involving heterogeneous catalysis. The following conclusions were drawn: (1) Clearly, the isomerization step is present in the dehydroisomerization, hydration, and acetoxylation reactions of α-pinene. Moreover, the main transformations proceed through similar intermediates: pinyl, bornyl, fenchyl, and terpinyl cations. In fact, this simplifies the potential kinetic modeling. Hence, each investigated catalyst should display either minor or major isomerization activity to allow for the final transformation steps in the aforementioned reactions. In terms of the isomerized monocyclic product limonene, it can be proposed as the most appropriate reactant in the acetoxylation, hydration, and dehydrogenation reactions, provided that monocyclic products are ultimately desired. (2) In general, high yields have been demonstrated in all studied reactions; however, the dehydrogenation and acetoxylation performances could be improved in cases when αpinene is applied. Furthermore, the possibility for straightforward utilization of crude turpentine should be considered, in particular in hydrogenation, dehydroisomerization, and acetoxylation reactions. (3) Finding green routes to TPA, ρ-cresol, ρ-methylacetophenone, and other valuable oxidation products of ρ-cymene is still a challenging task, especially considering heterogeneous catalysis. Obviously, the strategy of aiming at low conversion of the stable ρ-cymene molecule, in most cases, has paid off in terms of high selectivity toward the targeted products. (4) Catalytic activity can be significantly promoted by correct surface properties and acidity of the applied catalytic materials. (5) Over recent decades, many trials have contributed to the understanding of the reaction pathways. In most cases, the effects of reaction parameters have been elucidated. (6) The optical activity of terpenes is not always considered by researchers, but could affect the reaction performance. Furthermore, the optical purity of some products (e.g., menthol) could be the critical factor determining their final properties and commercial value.

Dr. Mikhail Golets was born in 1987 in Saint Petersburg, Russia. He received his M.Sc. degrees in Chemical Engineering from Lappeenranta University of Technology and Saint Petersburg State Forestry Academy in 2010 through a double-degree program. In 2010, he joined the Chemical Reaction Engineering Laboratory at Umeå University, Sweden, as a Ph.D. student. He graduated in December 2014. Currently, he is employed as a postdoctoral researcher at the Åbo Akademi University of Finland. His research interests are focused on heterogeneous catalysis, including the development of supported metal catalysts for organic synthesis and the catalytic upgrading of wood extractives and other biorefinery effluents.

Dr. Ajaikumar Samikannu received his Ph.D. (2004−2008) in Heterogeneous Catalysis from Anna University, Chennai, India. The focus of his Ph.D. thesis was the synthesis and characterization of supported mesoporous metal oxide catalysts and their catalytic evaluation in organic transformations. Since 2009, he has worked as a postdoctoral researcher with Prof. Jyri-Pekka Mikkola at Umeå University, Sweden, and his research has focused on the development of gold-containing bimetallic catalysts for fine chemical synthesis from renewable biomass feedstocks. He currently holds a position as a Senior Research Engineer and studies the catalytic conversion of biomass-derived feedstocks into various hydrocarbons and value-added fine chemicals. Y

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Prof. Jyri-Pekka Mikkola was born in 1966, in Nousiainen, Finland. In 1992, he graduated with an M.Sc. degree in chemical engineering from Åbo Academi University, Åbo-Turku, Finland. He then spent several years in industry, later returning to academia. In 1999, he completed his Ph.D. degree in chemical engineering from Åbo Academi University. Since 2008, he has been a full professor in technical chemistry at both Umeå University, Sweden, and Åbo Akademi University, Finland. The principal areas of his research interest are ionic liquids and novel materials development, green chemistry, chemical kinetics, and catalysis. Throughout his career he has coauthored ca. 250 articles and holds a number of patents. Currently, Prof. Mikkola is a member of the editorial boards of the following journals: Frontiers in Chemistry and Progress in Industrial Ecology, An International Journal. Furthermore, he is a member of the scientific advisory board of the “Biorefinery of the Future”, Finnish Catalysis Society, and Finnish Society for Industrial Ecology; steering group member of the “Bio4Energy” research programme; member of the scientific advisory board of Nordic ChemQuest AB and SLU Biofuel Technology Center; and management committee member of COST action CM0903 “UBIOCHEM” and COST action CM1206 (EXIL) 2013. In 2004, he was appointed as Academy Research Fellow and received the Incentive Award of the Academy of Finland in 2006.

ACKNOWLEDGMENTS Processum Biorefinery Initiative AB, Domsjö Aditya Birla AB, Metsä Board AB, Holmen AB, Kempe Foundations, and the Bio4Energy program are gratefully acknowledged for financial support. This work is also part of the activities of the Wallenberg Wood Science Center (WWSC) under the auspices of the Knut and Alice Wallenberg Foundation, as well as the activities of the Johan Gadolin Process Chemistry Centre, a center of excellence funded by the Åbo Akademi University. REFERENCES (1) Knuuttila, P. Fuel 2013, 104, 101. (2) Capouet, M.; Peeters, J.; Noziere, B.; Muller, J.-F. Atmos. Chem. Phys. 2004, 4, 2285. (3) Librando, V.; Tringali, G. J. Environ. Manage. 2005, 75, 275. (4) Yermakova, A.; Chibiryaev, A. M.; Kozhevnikov, I. V.; Anikeev, V. I. J. Supercrit. Fluids 2009, 48, 139. (5) Colonna, M.; Berti, C.; Fiorini, M.; Binassi, E.; Mazzacurati, M.; Vannini, M.; Karanam, S. Green Chem. 2011, 13, 2543. (6) Nie, G.; Zou, J.; Feng, R.; Zhang, X.; Wang, L. Catal. Today 2014, 234, 271. (7) Haneke, K. E. Toxicological Summary for Turpentine; Report 800664-2; National Institute of Environmental Health Sciences: Research Triangle Park, NC, 2002; available at http://ntp.niehs.nih.gov/ntp/ htdocs/Chem_Background/ExSumPdf/turpentine_508.pdf (accessed August 28, 2013). Z

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AC

DOI: 10.1021/cr500407m Chem. Rev. XXXX, XXX, XXX−XXX