Isomerization of α-Pinene to Monocyclic Monoterpenes in Hot

May 9, 2017 - The isomerization of α-pinene in hot compressed water (HCW) at 250 °C and 7 ... A plausible explanation for this is that Lewis acid si...
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Isomerization of α‑Pinene to Monocyclic Monoterpenes in Hot Compressed Water Using TiO2 and WOx/TiO2 Catalysts Makoto Akizuki* and Yoshito Oshima Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan S Supporting Information *

ABSTRACT: The isomerization of α-pinene in hot compressed water (HCW) at 250 °C and 7 MPa using TiO2 and WOx/TiO2 catalysts was investigated. In HCW, α-pinene mainly converts into monocyclic monoterpenes rather than into the polycyclic monoterpenes that form in He gas or water vapor. A plausible explanation for this is that Lewis acid sites on the surfaces of the catalysts convert to Brönsted acid sites in HCW owing to the stabilization of H+. In HCW, α-pinene is converted into acyclic monoterpenes less with the catalysts than without. Furthermore, the selectivities of the TiO2 and the WOx/TiO2 catalysts for monocyclic monoterpenes were quite different, which was attributed to the rapid formation of α-terpinene and γ-terpinene from α-pinene and isoterpinolene from terpinolene over the strongly acidic WOx/TiO2 catalyst. No significant coking of the TiO2 and the WOx/TiO2 catalysts occurred in HCW, and both retained their activity for 6 h.

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INTRODUCTION α-Pinene, a major component of turpentine oil, is an important raw material for the synthesis of monoterpenes and monoterpenoids, and numerous studies concerning its thermal or acid-catalyzed isomerization reactions have been performed.1,2 The thermal isomerization of α-pinene produces acyclic and monocyclic monoterpenes such as alloocimene, ocimene, limonene, α-pyronene, and β-pyronene.3−5 Stolle et al. investigated the mechanism of this thermal isomerization and reported that α-pinene is first converted to ocimene and limonene through a biradical transition state and that the ocimene produced is readily converted to alloocimene.5 They also reported that the ratio of ocimene and limonene formed in the first step is almost unity and that it remains constant between 350 and 500 °C. In addition to the thermal isomerization of α-pinene in the liquid phase or in inert gas, the reaction in supercritical alcohols has also been investigated.6,7 It has been reported that the reaction rate in supercritical alcohols is significantly larger than that in the liquid or gas phase. The acid-catalyzed isomerization of α-pinene has also been widely researched, and the reaction catalyzed by solid acids has attracted particular attention. The acid-catalyzed isomerization of α-pinene has been reported to produce monocyclic, bicyclic, and tricyclic monoterpenes such as camphene, β-pinene, bornylene, α-fenchene, tricyclene, limonene, terpinolene, αterpinene, γ-terpinene, and isoterpinolene.8−20 One of the most important of these products is camphene, and a number of © XXXX American Chemical Society

researchers have investigated its selective production. For example, Simakova et al. reported that the selectivity for camphene reaches 60−80% at a 99.9% conversion of α-pinene when using a Au/γ-Al2O3 catalyst.18 With regards to product selectivity, it has been reported that bicyclic monoterpenes (such as camphene) are preferentially formed on Lewis acid sites while monocyclic monoterpenes (such as limonene) are preferentially formed on Brönsted acid sites.14,15,17 In the presence of both solid-acid catalysts and water, the hydration of monoterpenes occurs, and alcohols such as α-terpineol, 4terpineol, and 1,8-terpene are produced in addition to monoterpenes.21−23 Mochida et al.23 examined the hydration of α-pinene over zeolite catalysts with different SiO2/Al2O3 ratios and reported that more hydrophobic zeolites show higher activity because only a small number of water molecules competitively adsorb on the acid sites. Recently, the conversion of α-pinene in hot compressed water (HCW) has also been attracting attention. When the temperature of water is increased to 250 °C, its dielectric constant (ε) is decreased to about 27,24 which is almost equal to that of ethanol (25) at ambient temperature. Therefore, the miscibility of organics with HCW is significantly increased, making the use of water as a solvent for this process possible. In addition, the ion product of water (KW) for HCW (pKW = 11 at Received: Revised: Accepted: Published: A

March 21, 2017 May 7, 2017 May 9, 2017 May 9, 2017 DOI: 10.1021/acs.iecr.7b01172 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 250 °C)25 is larger than that for ambient water (pKW = 14); thus, HCW itself can act as acid and/or base catalysts. Chamblee et al.26 were the first to investigate reactions of βpinene, a structural isomer of α-pinene, in HCW and reported that 90% conversion of β-pinene was achieved in 20 min at 200 °C. Although their aim was to convert β-pinene into αterpineol, they reported that hydrocarbons were the main products. Furthermore, Szuppa et al.27 investigated the isomerization of α-pinene in HCW generated by microwave heating using NaCl as a microwave absorber. They reported that quantitative conversion of α-pinene was achieved in 1 h at 300 °C and 8 MPa mainly because of acid-catalyzed reactions rather than thermal reactions and that the main products were γ-terpinene (24%), terpinolene (20%), limonene (14%), αterpinene (12%), and alloocimene (9%). Moreover, Kawahara et al.28 investigated the isomerization of α-pinene and reported that a ∼70% yield of limonene, which is comparable to the largest yield of limonene from α-pinene reported in the literature, was obtained in 20 min at 300 °C or in 1 min at 400 °C. They also reported that the main byproduct was alloocimene, and dimeric and polymeric materials were not formed significantly, which is a unique characteristic of reactions in HCW. In this research, we investigated the isomerization of αpinene in HCW using solid-acid catalysts. In HCW, the acidity of solid acids is strongly affected by water properties such as density and KW,29,30 which leads to atypical catalyst activity and the possibility of controlling reactions by tuning water properties. Thus, the product selectivity of the solid-acidcatalyzed isomerization of α-pinene in HCW is expected to be different from that in the absence of water or in the presence of water vapor because the acidity of solid acids affects product selectivity. In this study, we focused on the conversion of αpinene to monocyclic monoterpenes such as limonene, terpinolene, α-terpinene, and γ-terpinene. α-Terpinene and γterpinene are attracting particular attention owing to their antioxidant effects31 and trypanocidal activity against Trypanosoma evansi.32 Two different metal oxides, i.e., TiO2 and 5 wt % WOx/TiO2, which have been reported to act as acid catalysts in HCW and to have different acidities,29 were used as catalysts. The purpose of this study was to use kinetic analysis to elucidate the effects of HCW and solid-acid catalysts on the selectivity of products in the isomerization of α-pinene.

Experimental Procedure for Reactions in HCW. The reactions in HCW were conducted using a fixed-bed flow reactor made of SUS316 at 250 °C and 7 MPa [water density (ρwater) = 8.0 × 102 kg/m3 and pKW = 11].25,33 A schematic diagram of the reactor is shown in Figure 1a. Distilled water

Figure 1. Schematic diagrams of fixed bed flow reactors: (a) for the reactions in HCW or water vapor and (b) for the reaction in He gas.

and α-pinene were pumped separately using two pumps (PU2080; JASCO Corp., Japan, and 260D; Teledyne ISCO, Inc., United States), and distilled water was preheated to the reaction temperature in a preheat line. The two streams were mixed and fed into the fixed-bed reactor. The preheat line and the reactor were set in a heating oven. The α-pinene concentration at the reactor entrance was 0.10 mol/dm3. The stream emitted from the reactor was immediately cooled with a water-cooled heat exchanger and then depressurized using a back-pressure regulator (SCF-Bpg; JASCO Corp., Japan). The organics dispersed in the liquid sample were extracted using dichloromethane (Wako Pure Chemical Industries, Ltd., Japan.). Space time, defined as the volume of the reactor divided by the volumetric flow rate in the reactor, was used to indicate the reaction time in the HCW so that the reaction rates of the reactions with and without catalysts could be compared. Space time was controlled by adjusting both the reactor volume (0.22−5.33 cm3) and the volumetric flow rate in the reactor (1.83 × 10−2 to 7.31 × 10−2 cm3/s). The amounts of catalyst loaded were 0.59−3.97 g for the TiO2 catalyst and 0.18−2.05 g for the WOx/TiO2 catalyst. To investigate the reaction without a catalyst, the reactor was filled with SUS304 balls of 1.0 mm diameter (Azuma Jikuuke Corp., Japan) instead of the catalysts. Experimental Procedure for Reactions in He Gas or Water Vapor. To investigate the effects of the presence of water and HCW on the reaction, reactions in He gas and in water vapor were performed. The experiments in He gas were conducted at 250 °C and atmospheric pressure using a fixed-bed flow reactor made of quartz glass. A schematic diagram of the reactor is shown in

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EXPERIMENTAL METHODS Reagents. α-Pinene was purchased from Tokyo Chemical Industries Co., Ltd., Japan and used as received. Distilled water was prepared using distillation equipment (RFD240HA; Advantec Tokyo Kaisha, Ltd., Japan) and degassed by bubbling with N2 gas prior to use. TiO2 powder (anatase-type containing 12 wt % rutile-type) and (NH4) 10 W12 O41 ·5H 2 O were purchased from Wako Pure Chemical Industries, Ltd., Japan. Catalyst Preparation. The WOx/TiO2 powder used in this study was prepared by the following impregnation method: The TiO2 powder was added to an aqueous solution of (NH4)10W12O41·5H2O and stirred for 4 h at 80 °C. After the water was evaporated, the powder obtained was dried in a vacuum oven for 2 h and then calcined at 600 °C for 6 h. The W content of the catalyst was 5 wt % as WO3. For the reaction in HCW, the TiO2 and WOx/TiO2 powders were pressed into pellets and then crushed and sieved into granular forms of 0.30−0.50 mm diameters. For reactions in He gas, the TiO2 and the WOx/TiO2 powders were sieved to less than 200 mesh. B

DOI: 10.1021/acs.iecr.7b01172 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Figure 1b. Prior to flowing α-pinene, the reactor was preheated by flowing He gas only. After the temperature in the reactor reached 250 °C, He gas saturated with α-pinene vapor at 50 °C was fed into the reactor by bubbling He gas (Nihon Helium Corp., Japan) through liquid α-pinene set in a water bath. The initial concentration of α-pinene in the reactor was 1.6 × 10−4 mol/dm3. Gas lines installed before and after the reactor were made of SUS316, and they were heated to 100−130 °C to avoid condensing α-pinene and reaction products. The gas stream emitted from the reactor was fed into n-hexane (Wako Pure Chemical Industries, Ltd., Japan.) to collect unreacted αpinene and reaction products. The volumetric flow rate of He gas was 1.17 cm3/s, and the amounts of catalysts loaded were 5.96 × 10−2 g for the TiO2 catalyst and 1.00 × 10−2 g for the WOx/TiO2 catalyst. The reactions in water vapor were performed using the same reactor described in Figure 1a at 250 °C and 1 MPa (ρwater is 4.3 kg/m3 and pKW is 36).25,33 The molar ratio of α-pinene and water was identical to that used for the reactions in HCW. The volumetric flow rate of water vapor was 2.44 cm3/s, and the amounts of catalysts loaded were 1.21 g for the TiO2 catalyst and 0.53 g for the WOx/TiO2 catalyst. Analysis. Organics in the solvents were qualified using gas chromatography−mass spectroscopy (GC-MS) and quantified using gas chromatography−flame ionization detection (GCFID). Both GC apparatuses were equipped with a TC-1701 capillary column (GL Sciences, Inc., Japan). Typical chromatograms are shown in Figures S1−S3. In the GC-FID analyses, some compounds (tricyclene, isoterpinolene, α-phellandrene, β-phellandrene, and 3,8-p-mentadiene) were quantified based on effective carbon number rules owing to the difficulty in obtaining standard materials.34 The crystal structures of the catalysts were analyzed using X-ray diffraction (XRD, SmartLab; Rigaku Corp., Japan). The Brunauer−Emmett− Teller (BET) surface areas of the catalysts were analyzed using an N2 adsorption method (NOVA e; Quantachrome Corp., United States). The acidity of the catalysts was analyzed using a Fourier transform infrared spectrometry (FTIR, FT/IR4200; JASCO Corp., Japan) instrument equipped with a diffuse reflectance unit (DR-600 Bi; JASCO Corp., Japan). The IR peaks of pyridine adsorbed were measured under vacuum at 200 °C. The amount of coke deposited on the catalysts was measured by thermogravimetric analysis (TGA, Thermo plus EVO2; Rigaku Corp., Japan). The weight loss between ambient temperature and 600 °C indicated by TGA in air was regarded as the mass of the deposited coke.

Figure 2. α-Pinene conversion in HCW at 250 °C and 7 MPa: (□) without catalyst, (●) TiO2, and (◊) WOx/TiO2.

Figure 3. Yields of the major products in HCW at 250 °C and 7 MPa: (a) without catalyst, (b) TiO2, and (c) WOx/TiO2; (□) limonene, (●) terpinolene, (◊) α-terpineol, (▲) alloocimene, (○) α-terpinene, (■) γ-terpinene, and (▼) isoterpinolene.

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and alloocimene are also formed in relatively high yields. In the initial stage of the reaction, the product selectivity for αterpineol is high, but decreases with increasing space time. From the results of the GC analysis, α-terpinene, γ-terpinene, and camphene are also produced during the reaction. Figure 3c shows the yields of the WOx/TiO2-catalyzed reaction. For short space times, limonene and terpinolene are produced with relatively high selectivity, and their yields decrease with increasing space time. As the reaction proceeds, the yields of α-terpinene, γ-terpinene, and isoterpinolene increase. The yields of α-terpinene and γ-terpinene are comparable or even larger than those in previous reports of acid-catalyzed or HCWinduced isomerization of α-pinene.9,16,20,27 In addition to these products, tricyclene, camphene, α-phellandrene, β-phellandrene, p-cymene, and 3,8-p-mentadiene are also detected as minor products. The differences in the reaction rate and reaction products for these three reaction systems will be discussed based on kinetic analysis later.

RESULTS AND DISCUSSION α-Pinene Conversion and Yields of Products in HCW. Figure 2 shows the α-pinene conversion, defined as moles of consumed α-pinene divided by moles of initial α-pinene, of the reactions at 250 °C and 7 MPa with and without catalysts. The reaction proceeds even in the absence of catalyst. Both the TiO2 and the WOx/TiO2 catalysts promote the reaction, and the WOx/TiO2 catalyst shows the higher activity. Figure 3a shows the yields of the major products, defined as moles of the product divided by moles of initial α-pinene, for the reaction without catalyst. The major products are limonene and alloocimene, which is consistent with a previous report on the reaction of α-pinene in HCW without catalyst.28 In addition, α-terpineol and terpinolene are also produced. The yields of the TiO2-catalyzed reaction are shown in Figure 3b. The major products are limonene and terpinolene. α-Terpineol C

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Industrial & Engineering Chemistry Research Table 1. α-Pinene Conversion and the Yields of the Major Products in Gas Phase and in Water Vapor at 250 °C yield [−] catalyst TiO2

WOx/ TiO2

TiO2

WOx/ TiO2

a

operating time [h]

α-pinene conversion [−]

0.50 0.83 1.2 0.50

0.654 0.572 0.488 0.945

0.83 1.2

0.927 0.927

0.50 1.5 2.0 0.50 1.5 2.0

γ-terpinene

isoterpinolene

carbon balance [−]

In He Gas at 250 °C and Atmospheric Pressure 0.229 0.044 0.083 0.010 0.010 0.203 0.038 0.061 0.005 0.006 0.195 0.035 0.058 0.005 0.006 0.290 0.100 0.011 0.003 0.074

0.005 0.003 0.002 0.012

n.d.a n.d. n.d. 0.009

0.730 0.745 0.815 0.578

0.327 0.315

camphene

tricyclene

limonene terpinolene

α-terpinene

0.012 0.004 0.013 0.006 at 250 °C and 1 MPa 0.168 0.050 0.158 0.046 0.160 0.046 0.004 0.008

0.068 0.065

0.011 0.012

0.008 0.012

0.639 0.623

0.429 0.393 0.408 0.973

0.112 0.105 In Water Vapor 0.159 0.012 0.148 0.011 0.152 0.011 0.195 0.092

0.009 0.008 0.008 0.217

0.008 0.007 0.007 0.069

n.d. n.d. n.d. 0.095

0.982 0.990 0.981 0.739

0.976 0.987

0.211 0.217

0.004 0.004

0.231 0.237

0.074 0.076

0.105 0.108

0.788 0.797

0.094 0.089

0.009 0.009

n.d.: not detected.

or adsorbed terpene molecules,10 which is consistent with our experimental results for the TiO2-catalyzed reaction. The reaction in water vapor was also examined at a constant flow rate. Operating time = 0 was defined as the time at which the temperature in the reactor oven reached 250 °C. The αpinene conversion and the yields of the major products at each operating time are shown in Table 1. The α-pinene conversion and the product distribution are independent of the operating time. The major products of the TiO2-catalyzed reaction are camphene and limonene, and those of the WOx/TiO2-catalyzed reaction are camphene, tricyclene, α-terpinene, γ-terpinene, and isoterpinolene. Similar to the reactions in He gas, the yields of camphene and tricyclene in water vapor are significantly larger than those in HCW. The carbon balance values for both catalysts in water vapor are larger than those in He gas, suggesting that the production of polymerized products is suppressed in the presence of water. For the WOx/TiO2catalyzed reaction, even though the carbon balance values are large in water vapor, they are still smaller than those in HCW. Effects of HCW and Solid-Acid Catalysts on Reaction Kinetics. Based on the experimental data and previous reports,5,15,17,18,28 a lumped reaction scheme for kinetic analysis was proposed (Figure 4). In acid-catalyzed reactions, the carbon double bond in α-pinene is first protonated and a pinanyl cation is produced. Some of the pinanyl cations are skeletally isomerized, and camphene and tricyclene are produced (R5). Another portion of the pinanyl cations convert into terpinyl cations upon the opening of the four-membered ring. From this terpinyl cation, limonene and terpinolene are formed, and α-terpineol is also formed in the presence of water (R1, R2). If carbocation rearrangement of the terpinyl cation occurs, α-terpinene and γ-terpinene are formed. Because αterpinene and γ-terpinene are produced both directly from αpinene (R3) and indirectly via limonene, terpinolene, and αterpineol (R7), both reactions are included in the reaction scheme. In addition, because terpinolene is also formed from the rearranged terpinyl cation, conversion of limonene and αterpineol into terpinolene (R6) is considered. From terpinolene, isoterpinolene is formed by protonating the other carbon double bond in the six-membered ring (R8). In addition to the

The carbon balances for each reaction system, calculated by summing the yields of the unreacted α-pinene and the reaction products, are 1.00−1.06 (no catalyst), 0.94−1.06 (TiO2), and 0.88−1.03 (WOx/TiO2). The values are close to unity, which indicates that α-pinene is mainly converted into the aforementioned monocyclic monoterpenes, acyclic monoterpenes, and polycyclic monoterpenes in HCW. With increasing space time, the carbon balance for the TiO2 and the WOx/ TiO2-catalyzed reactions slightly decreases, indicating that these identified products convert to other products under longer reaction times. α-Pinene Conversion and Yields of Products in He Gas and in Water Vapor. In order to compare the product distributions of the reactions in HCW, the reactions in He gas and in water vapor were investigated. The reaction in He gas was conducted at a constant flow rate (i.e., constant space time). Operating time = 0 was defined as the time at which the feeding of α-pinene began. The α-pinene conversion and the yields of the major products at each operating time are shown in Table 1. Regardless of the catalyst species, camphene and tricyclene are the major products, and this product distribution is quite different from that for the reactions in HCW. Besides these polycyclic products, limonene is produced in the TiO2-catalyzed reaction and α-terpinene is produced in the WOx/TiO2-catalyzed reaction, and the production of these monocyclic monoterpenes is the same as in the reactions in HCW. TGA results for the catalysts after reaction indicate that coke is deposited on catalyst surfaces at 4.7 wt %-cat (TiO2) and 2.9 wt %-cat (WOx/TiO2) (Figure S4). The formation of cokes on catalyst surfaces during the short operation times suggests that polymerized products are formed during the reaction and that some of them are deposited as cokes. Thus, a considerable reason for the small carbon balance is that polymerized products are formed, though they could not be identified under our experimental conditions. In the reaction with the TiO2 catalyst, the deposition of coke on the catalyst surface is believed to cause the decrease in αpinene conversion. Alsalme et al. also reported a considerable decrease of α-pinene conversion with increasing operating time at 200 °C in N2 gas owing to the blocking of acid sites by coke D

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terpineol is a bimolecular reaction of α-pinene and water, this reaction is assumed to be a pseudo-first-order reaction with respect to the reactant because the water concentration is high and does not change significantly during the reaction. Using the equation for ri, the α-pinene conversion (Xα‑pinene) and products yield (Yproducts) can be described as a function of space time and ki. ki values were evaluated by fitting equations for Xα‑pinene and Yproducts to the experimental data. The data fitting was performed using Microsoft Excel with the solver add-in. In the reaction without catalyst, Xα‑pinene and Yproducts can be explained without including R3, R6, R8, and R9 because the contributions of these reactions are small under these reaction conditions. In addition, R4 is not an acid-catalyzed reaction; thus, its reaction rate is assumed to be the same for all reaction systems. Therefore, the value of k4 determined from the reaction without catalyst was used for the reactions with the TiO2 and WOx/TiO2 catalysts. Figure 5 shows the fitting results for each reaction system, illustrating that the experimental data are well-explained using the proposed reaction scheme. In Table 2, the kinetic rate constants obtained at 250 °C and 7 MPa are summarized. To discuss the effects of HCW and solid-acid catalysts on the selectivity of the α-pinene conversion quantitatively, the selectivity for acyclic monoterpenes (Sacyclic), monocyclic monoterpenes (Smonocyclic), and di- or tricyclic monoterpenes (Spolycyclic) are defined in the following equations:

Figure 4. Lumped reaction scheme for kinetic analysis.

acid-catalyzed mechanism, α-pinene converts into alloocimene (R4) and limonene by a radical mechanism. Because the yield of alloocimene decreases with increasing space time, the degradation reaction of alloocimene (R9) is also included. Kinetic analysis was conducted based on the reaction scheme shown in Figure 4. Assuming each reaction obeys first-order kinetics, the reaction rate of Ri (ri) can be represented as follows: ri = kiCreactant

(i = 1−9)

Sacyclic =

(1)

where ki is the kinetic rate constant and Creactant is the concentration of each reactant. Although water concentration may affect R1 because the conversion of α-pinene into α-

k4 k1 + k 2 + k 3 + k4 + k5

Smonocyclic =

k1 + k 2 + k 3 k1 + k 2 + k 3 + k4 + k5

(2)

(3)

Figure 5. Fitting results: (a) without catalyst, (b) TiO2, and (c) WOx/TiO2; (■) Xα‑pinene, (●) Ylimonene+α‑terpineol, (◊) Yterpinolene, (▲) Yα‑terpinene+γ‑terpinene, (○) Yisoterpinolene, (□) Yalloocimene, (▼) Ycamphene+tricyclene, and (- - -) fitting results. E

DOI: 10.1021/acs.iecr.7b01172 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 2. Kinetic Rate Constants in HCW at 250 °C and 7 MPa kinetic rate constants [s−1] without catalyst TiO2 WOx/ TiO2 a

k1

k2

2.25 × 10−3

6.63 × 10−4

−a

1.07 × 10−3

3.40 × 10−5

−a

3.62 × 10−4

−a

−a

4.75 × 10−3 1.79 × 10−2

1.87 × 10−3 2.55 × 10−2

2.91 × 10−4 4.49× 10−2

same value as without catalyst

2.13 × 10−4 4.60 × 10−3

1.75 × 10−3 1.22 × 10−2

9.11 × 10−4 6.48 × 10−3

3.03 × 10−4 3.28 × 10−2

3.68 × 10−3 2.81 × 10−3

k3

k4

k5

k6

k7

k8

k9

Conversion and products yield could be explained without including these reactions.

Table 3. Selectivity of the α-Pinene Conversion in HCW without catalyst TiO2 WOx/TiO2

Spolycyclic =

in He gas

in water vapor

Sacyclic

Smonocyclic

Spolycycric

Scamphene+tricyclene [−]

Scamphene+tricyclene [−]

0.266 0.130 0.011

0.726 0.844 0.940

0.008 0.026 0.049

− 0.42−0.47 0.41−0.47

− 0.40 0.29−0.31

k5 k1 + k 2 + k 3 + k4 + k5

assigned as the peaks of the pyridine adsorbed on Lewis acid site,35 and the peak of the pyridine adsorbed on Brönsted acid site, which typically appeared at 1540 cm−1, is not detected. In a previous study, Fourier transform infrared measurements were used to ascertain that anatase TiO2 possesses only Lewis acid sites in the absence of water and that these acid sites act as Lewis acid sites even in the presence of water vapor.36 This is consistent with our finding that Scamphene+tricyclene for the TiO2catalyzed reaction both in He gas and in water vapor is not significantly different. Conversely, Scamphene+tricyclene for the WOx/TiO2-catalyzed reaction in water vapor is smaller than that in He gas, suggesting that the contribution of Brönsted acid sites is larger in water vapor. The pyridine-IR spectrum of the WOx/TiO2 catalyst shown in Figure 6 indicates that the peak of the pyridine adsorbed on Brönsted acid site is not detected in the absence of water. Thus, a considerable reason for the smaller Scamphene+tricyclene value in water vapor is that Brönsted acid sites are formed by the dissociation of water molecules on Lewis acid sites. Another explanation is that weak Brönsted acid sites, which cannot be detected by pyridine-IR, exist in the absence of water, and these acid sites are poisoned in water vapor. In a previous study on the dehydration of 1,2alkanediols over WOx/SiO2, the catalyst exhibited both Brönsted and Lewis acid sites inherently, and the contribution of Brönsted acid sites on the reaction increased in the presence of water vapor because water molecules poisoned the Lewis acid sites.37 Therefore, in the present study, the smaller value for Scamphene+tricyclene in water vapor over the WOx/TiO2 catalyst can be explained by the dissociation and/or the poisoning of Lewis acid sites by water molecules. However, the small value of Spolycyclic for both the TiO2 and the WOx/TiO2 catalysts in HCW cannot be explained by the poisoning effect of water molecules on Lewis acid sites alone because TiO2 has no Brönsted acid sites inherently. It has been reported that the contributions of Lewis and Brönsted acid sites on reactions in HCW near and above the critical point (374 °C and 22.1 MPa) vary with the ion product of water (KW) because the dissociation of water molecules on the acid sites is promoted with increasing KW, and this causes a change in acid type from Lewis to Brönsted.29,30 Because KW in the liquid phase is particularly large around 250 °C (pKW = 11 at 250 °C and 7 MPa),25 a plausible explanation for the considerably smaller value of Spolycyclic in HCW than in He gas and in water

(4)

The values are summarized in Table 3. In HCW, Spolycyclic for both the TiO2 and the WOx/TiO2 catalysts are small. When the same catalysts are used in He gas or water vapor, the selectivities for camphene and tricyclene (Scamphene+tricyclene), calculated by summing the yields of both products divided by the α-pinene conversion, are 0.29−0.47, as shown in Table 3, and these values are notably larger than Spolycyclic in HCW. Though the selectivities for polycyclic monoterpenes in He gas or water vapor might be underestimated by using the Scamphene+tricyclene values because we have no experimental evidence that camphene and tricyclene are stable in He gas or water vapor, the difference of the selectivities in HCW and in He gas or water vapor becomes larger even in that case. These results suggest that the acid properties of catalysts in the HCW are different from those in He gas and in water vapor. It has been reported that bicyclic monoterpenes are preferentially formed from α-pinene on Lewis acid sites while monocyclic monoterpenes are preferentially formed on Brönsted acid sites.14,15,17 Therefore, a large Spolycyclic value indicates that the contribution of Lewis acid sites to α-pinene conversion is large. When the value of Scamphene+tricyclene for the TiO2-catalyzed reaction in He gas is compared to that in water vapor, the value for water vapor is slightly smaller but not significantly different. The IR spectrum of pyridine adsorbed on the TiO2 catalyst is shown in Figure 6. The four peaks are

Figure 6. IR spectra of pyridine adsorbed on the catalysts under vacuum at 200 °C. F

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Industrial & Engineering Chemistry Research

The BET surface areas of the TiO2 and the WOx/TiO2 catalysts before and after use in HCW are given in Table 4,

vapor is that Lewis acid sites present on the catalyst surface are changed to Brönsted acid sites in HCW. In contrast to the low selectivity for polycyclic monoterpenes, α-pinene conversion to acyclic monoterpenes (Sacyclic), which does not proceed as a solid-acid-catalyzed reaction in He gas or water vapor, occurs to some extent in HCW. The values of Sacyclic for both catalysts are smaller than that without catalyst because the conversion of α-pinene to acyclic monoterpenes is a homogeneous reaction and is not promoted by solid-acid catalysts. Because solid-acid-catalyzed reactions with the WOx/TiO2 catalyst proceeded faster than those with the TiO2 catalyst (k1−k3, k5), α-pinene is mainly consumed in solid-acid-catalyzed reactions and Sacyclic for the WOx/TiO2-catalyzed reaction is significantly smaller. Because of the small value of Spolycyclic in HCW and the small value of Sacyclic when using the catalysts, the selectivity of the αpinene conversion to monocyclic monoterpenes (Smonocyclic) is high for the reactions in HCW with the solid-acid catalysts. Furthermore, when the kinetic rate constants for the TiO2catalyzed reaction and the WOx/TiO2-catalyzed reaction are compared, almost all the rate constants for the WOx/TiO2catalyzed reaction are greater than those for the TiO2-catalyzed reaction. Particularly, k3 and k8 for the WOx/TiO2-catalyzed reaction are considerably greater than those for the TiO2catalyzed reaction compared to the other kinetic rate constants. The pyridine-IR spectra for the catalysts (Figure 6) indicate that the peaks for the WOx/TiO2 catalyst appeared in 1447 and 1610 cm−1 shift to larger wavenumber compared to those for the TiO2 catalyst, suggesting that the WOx/TiO2 catalyst possesses stronger acid sites,35 and R3 and R8 may be promoted by these stronger acid sites. Because the contributions of R3 and R8 are large in the reaction with the WOx/TiO2 catalyst, α-terpinene, γ-terpinene, and isoterpinolene are produced as the major products, as shown in Figure 3c. Operating-Time Dependence of α-Pinene Conversion and Product Yields. To investigate the stability of the catalysts, the reactions were conducted at a constant flow rate (i.e., constant space time) for 6 h, and the dependence of conversion on operating time was examined. Figure 7 shows the α-pinene conversion and the yields of the major products in the TiO2-catalyzed reaction (a) and the WOx/TiO2-catalyzed reaction (b) at 250 °C and 7 MPa. The α-pinene conversion and the yields for both the TiO2- and the WOx/TiO2-catalyzed reactions do not change during the 6 h runs.

Table 4. BET Surface Area of Catalysts before and after the Use in Hot Compressed Water

TiO2 WOx/TiO2

before use

after 1 h use

after 19 h (TiO2), 15 h (WOx/TiO2) use

51.8 43.7

49.9 48.4

45.5 47.7

showing that the BET surface areas of both catalysts are virtually unchanged after use for 19 h (TiO2) and 15 h (WOx/ TiO2). In the XRD analyses of the catalysts, only peaks for anatase- and rutile-TiO2 structures are observed (Figure S5). The weight fractions of anatase-TiO2 (fA) were estimated from the peak intensities using eq 5,38 as follows: IA fA = IA + 1.265IR (5) where IA is the intensity of anatase (101) and IR is the intensity of rutile (110). The values of fA for the TiO2 catalyst are 0.843 (before use), 0.831 (used for 1 h), and 0.794 (used for 19 h), while those for the WOx/TiO2 catalyst are 0.818 (before use), 0.821 (used for 1 h), and 0.825 (used for 15 h). These data indicate that the crystalline structures of the catalysts are unchanged during use in HCW. The TGA results for the catalysts after reaction indicate that 1.4 wt %-cat of coke is deposited on the surfaces of both the TiO2 and the WOx/TiO2 catalysts (Figure S4). Despite the fact that the catalysts were used for ten times longer in HCW (TiO2, 19 h; WOx/TiO2, 15 h) than that in He gas (1.3 h), less organic matter is deposited during the reactions in HCW than that in He gas (4.7 wt %-cat for the TiO2 catalyst and 2.9 wt %-cat for the WOx/TiO2 catalyst, as described in the previous section). Considering the fact that there is a decrease in the αpinene conversion with increasing operating time in the TiO2catalyzed reaction in He gas due to coking, the low level of coking for HCW is thought to be responsible for the stable activity of the catalysts, as shown in Figure 7. Our experimental finding that the carbon balance is close to unity in HCW suggests that the polymerization of monoterpenes is not significant. Therefore, an explanation for the low level of coking is that the polymerization reaction is suppressed because reactant molecules are isolated by the solvent cage effect of HCW, 39 thus reducing surface coking caused by the polymerized products.

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CONCLUSIONS We investigated the isomerization of α-pinene in HCW at 250 °C and 7 MPa over TiO2 and the WOx/TiO2 catalysts and found that α-pinene mainly converts into monocyclic monoterpenes. Limonene and terpinolene are the major products in the TiO2-catalyzed reaction, and α-terpinene, γterpinene, and isoterpinolene are the major products in the WOx/TiO2-catalyzed reaction. Kinetic analyses were conducted to elucidate the effect of HCW and solid-acid catalysts on the product selectivity quantitatively. In solid-acid-catalyzed reactions in HCW, the conversion of α-pinene into polycyclic monoterpenes (camphene and tricyclene), which is the major reaction in He gas and water vapor, is not significant. It is thought that the large ion product of HCW at 250 °C and 7 MPa leads to the

Figure 7. Dependence of the α-pinene conversion and the yields of the major products on operating time at 250 °C and 7 MPa: (a) TiO2, space time = 100 s; (b) WOx/TiO2, space time = 50 s; (⧫) Xα‑pinene, (□) Ylimonene, (●) Yterpinolene, (◊) Yα‑terpineol, (▲) Yalloocimene, (○) Yα‑terpinene, (■) Yγ‑terpinene, and (▼) Yisoterpinolene. G

DOI: 10.1021/acs.iecr.7b01172 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

(4) Fuguitt, R. E.; Hawkins, J. E. The Liquid Phase Thermal Isomerization of α-Pinene. J. Am. Chem. Soc. 1945, 67, 242. (5) Stolle, A.; Ondruschka, B.; Findeisen, M. Mechanistic and Kinetic Insights into the Thermally Induced Rearrangement of α-Pinene. J. Org. Chem. 2008, 73, 8228. (6) Yermakova, A.; Chibiryaev, A. M.; Kozhevnikov, I. V.; Mikenin, P. E.; Anikeev, V. I. Thermal isomerization of α-pinene in supercritical ethanol. Chem. Eng. Sci. 2007, 62, 2414. (7) Chibiryaev, A. M.; Anikeev, V. I.; Yermakova, A.; Mikenin, P. E.; Kozhevnikov, I. V.; Sal’nikova, O. I. Thermolysis of α-pinene in supercritical lower alcohols. Russ. Chem. Bull. 2006, 55, 987. (8) Comelli, N. A.; Ponzi, E. N.; Ponzi, M. I. α-Pinene isomerization to camphene: Effect of thermal treatment on sulfated zirconia. Chem. Eng. J. 2006, 117, 93. (9) Sidhpuria, K. B.; Tyagi, B.; Jasra, R. V. ZrO2−SiO2 Mixed Oxides Xerogel and Aerogel as Solid Acid Catalysts for Solvent Free Isomerization of α-Pinene and Dehydration of 4-Methyl-2-Pentanol. Catal. Lett. 2011, 141, 1164. (10) Alsalme, A.; Kozhevnikova, E. F.; Kozhevnikov, I. V. α-Pinene isomerisation over heteropoly acid catalysts in the gas-phase. Appl. Catal., A 2010, 390, 219. (11) Tzompantzi, F.; Valverde, M.; Pérez, A.; Rico, J. L.; Mantilla, A.; Gómez, R. Synthesis of Camphene by α-Pinene Isomerization Using W2O3−Al2O3 Catalysts. Top. Catal. 2010, 53, 1176. (12) Kitano, T.; Hayashi, T.; Uesaka, T.; Shishido, T.; Teramura, K.; Tanaka, T. Effect of High-Temperature Calcination on the Generation of Brønsted Acid Sites on WO3/Al2O3. ChemCatChem 2014, 6, 2011. (13) Wu, Y.; Tian, F.; Liu, J.; Song, D.; Jia, C.; Chen, Y. Enhanced catalytic isomerization of α-pinene over mesoporous zeolite beta of low Si/Al ratio by NaOH treatment. Microporous Mesoporous Mater. 2012, 162, 168. (14) Rachwalik, R.; Olejniczak, Z.; Jiao, J.; Huang, J.; Hunger, M.; Sulikowski, B. Isomerization of α-pinene over dealuminated ferrieritetype zeolites. J. Catal. 2007, 252, 161. (15) Luque, R.; Campelo, J. M.; Conesa, T. D.; Luna, D.; Marinas, J. M.; Romero, A. A. Ga-MCM-41 synthesis and catalytic activity in the liquid-phase isomerisation of α-pinene. Microporous Mesoporous Mater. 2007, 103, 333. (16) Chimal-Valencia, O.; Robau-Sánchez, A.; Collins-Martínez, V.; Aguilar-Elguézabal, A. Ion exchange resins as catalyst for the isomerization of α-pinene to camphene. Bioresour. Technol. 2004, 93, 119. (17) Ö zkan, F.; Gündüz, G.; Akpolat, O.; Beşün, N.; Murzin, D. Y. Isomerization of α-pinene over ion-exchanged natural zeolites. Chem. Eng. J. 2003, 91, 257. (18) Simakova, I. L.; Solkina, Y. S.; Moroz, B. L.; Simakova, O. A.; Reshetnikov, S. I.; Prosvirin, I. P.; Bukhtiyarov, V. I.; Parmon, V. N.; Murzin, D. Y. Selective vapour-phase α-pinene isomerization to camphene over gold-on-alumina catalyst. Appl. Catal., A 2010, 385, 136. (19) Frattini, L.; Isaacs, M. A.; Parlett, C. M. A.; Wilson, K.; Kyriakou, G.; Lee, A. F. Support enhanced α-pinene isomerization over HPW/SBA-15. Appl. Catal., B 2017, 200, 10. (20) Costa, M. C. C.; Johnstone, R. A. W.; Whittaker, D. Catalysis of gas and liquid phase ionic and radical rearrangements of α- and βpinene by metal(IV) phosphate polymers. J. Mol. Catal. A: Chem. 1996, 104, 251. (21) Vital, J.; Ramos, A. M.; Silva, I. F.; Castanheiro, J. E. The effect of α-terpineol on the hydration of α-pinene over zeolites dispersed in polymeric membranes. Catal. Today 2001, 67, 217. (22) Comelli, N.; Avila, M.; Volzone, C.; Ponzi, M. Hydration of αpinene catalyzed by acid clays. Cent. Eur. J. Chem. 2013, 11, 689. (23) Mochida, T.; Ohnishi, R.; Horita, N.; Kamiya, Y.; Okuhara, T. Hydration of α-pinene over hydrophobic zeolites in 1,4-dioxane-water and in water. Microporous Mesoporous Mater. 2007, 101, 176. (24) Fernández, D. P.; Goodwin, A. R. H.; Lemmon, E. W.; Levelt Sengers, J. M. H.; Williams, R. C. A Formulation for the Static Permittivity of Water and Steam at Temperatures from 238 to 873 K

dissociation of water on Lewis acid sites; thus, Brönsted acid sites are dominant on the catalyst surfaces. In addition, by using solid-acid catalysts in HCW, the conversion of α-pinene into acyclic monoterpenes (e.g., alloocimene), which occurs significantly in HCW without a catalyst, does not significantly proceed because it is not promoted by solid-acid catalysts. As a result, α-pinene selectively converts into monocyclic monoterpenes in HCW using solid-acid catalysts. Furthermore, the reaction rate constant for the conversion of α-pinene to αterpinene and γ-terpinene (k3) and that of terpinolene to isoterpinolene (k8) are particularly large in the WOx/TiO2catalyzed reaction, probably because of the strong acidity of the WOx/TiO2 catalyst. Therefore, α-terpinene, γ-terpinene, and isoterpinolene are mainly formed in HCW using the WOx/ TiO2 catalyst. The use of HCW also affects the stability of the catalysts. The TGA results indicated that less catalyst coking occurs during reaction in HCW than in He gas. A plausible explanation for this is that the production of polymerized products is suppressed owing to the solvent cage effect of HCW. Both the TiO2 and the WOx/TiO2 catalysts retain their activity over 6 h of reaction in HCW.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b01172. Typical GC chromatograms (Figures S1−S3), TGA results of the catalysts after the reaction (Figure S4), and XRD patterns of the catalysts (Figure S5) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +81 4 7136 4694. ORCID

Makoto Akizuki: 0000-0001-9350-0036 Yoshito Oshima: 0000-0002-6222-3774 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (Grant 15K18262) and Kurita Water and Environment Foundation Grant (Grant 16D018). GC-MS analyses were performed using facilities at Kashiwa Branch, Environmental Science Center, The University of Tokyo; XRD analyses were performed using facilities at Institute of Solid State Physics, The University of Tokyo; and IR analyses were conducted in Shimada Laboratory, Faculty of Textile Science and Technology, Shinshu University. We very much appreciate all of this support.

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REFERENCES

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DOI: 10.1021/acs.iecr.7b01172 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.7b01172 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX