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Mar 18, 2017 - In our study, heterogeneous catalysts based on different active metal nanoparticles (Pt, Pd, and Ru) and supports (carbon, silica, and ...
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Highly Selective Hydrogenation of R‑(+)-Limonene to (+)-p1‑Menthene in Batch and Continuous Flow Reactors Giuliana Rubulotta,†,‡ Kylie L. Luska,‡ César A. Urbina-Blanco,‡ Tobias Eifert,‡ Regina Palkovits,‡ Elsje Alessandra Quadrelli,† Chloé Thieuleux,*,† and Walter Leitner*,‡,§ †

Université de Lyon, Institut de Chimie de Lyon, UMR 5265 CNRS-Université Claude Bernard Lyon 1-ESCPE Lyon, Laboratory of Chemistry, Catalysis, Polymers and Processes (C2P2, UMR 5265), ESCPE Lyon 43 Bd du 11 Novembre 1918, 69616 Villeurbanne, France ‡ Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 2, 52074 Aachen, Germany § Max-Planck-Institut für Kohlenforschung, 45470 Mülheim an der Ruhr, Germany S Supporting Information *

ABSTRACT: In our study, heterogeneous catalysts based on different active metal nanoparticles (Pt, Pd, and Ru) and supports (carbon, silica, and alumina) were systematically tested in hydrogenation of limonene under mild reaction conditions (room temperature, 3 bar H2). The heterogeneous catalyst Pt/C was found highly active and selective for the reduction of limonene into the partial hydrogenation product (+)-p-1-menthene. Pt/C and Pt/Al2O3 are the most active systems among the series of commercially available catalysts that have been investigated. The catalytic activity and stability of Pt/C remained high throughout the recycling tests under batch conditions and thus allowed for the implementation of this catalytic system into continuous flow operations. The selective hydrogenation of the terminal over the internal CC bond of limonene was rationalized by detailed kinetic studies which revealed an 8-fold difference in reaction rate between the two reactions. KEYWORDS: Limonene, Selective hydrogenation, Continuous flow, Heterogeneous catalysis, Biomass conversion



INTRODUCTION R-(+)-limonene is a monocyclic terpene, which is a member of the family of organic compounds biosynthetically derived from the dimerization of isoprene. Limonene, the principal component of essential oils from the rinds of various citrus fruits,1 is widely used in cosmetic and perfume industries due to its pleasant smell, relative low toxicity, and high abundance (>70000 t/a in 2013).2 Recent efforts have been made to transform R-(+)-limonene into useful platform chemicals or value-added products for use in the chemical industry.3 The production of the partially hydrogenated product, (+)-p-1menthene, has been of particular interest since it is an attractive intermediate due to the retention of the chiral center and the presence of a double bond which can be further functionalized. Indeed, (+)-p-1-menthene has been widely used as a starting material for the synthesis of bulk (e.g., additives for adhesives, coatings, food) and fine chemicals (e.g., steroids, antimalaria agent, menthol).4−9 Furthermore, the fully hydrogenated products from (+)-p-1-menthene, p-menthanes, have been shown to be promising biobased fuel additives.10,11 Therefore, the study of selective hydrogenation of monoterpenes, which has industrial relevant applications as shown above, has attracted several studies. The partial hydrogenation © 2017 American Chemical Society

of limonene has been subjected to different studies and was performed in media as different as MeOH, EtOH, heptane, benzene, and recently supercritical CO2.6,12−14 Beside molecular hydrogen, reducing agents such as hydrazine and alcohols have also been reported.15−17 In general, two possible reaction pathways have been identified for the hydrogenation of limonene (Scheme 1). The preferred reaction pathway involves reduction of the terminal CC bond to form (+)-p-1menthene, independently from the nature of the catalyst and the hydrogen source. Alongside this main reaction pathway, isomerization and/or disproportion products as byproducts are also reported.18 The alternative hydrogenation of the cyclic CC bond to yield p-8-menthene has only been observed when the terminal CC bond was first protected to avoid the thermodynamically favorable hydrogenation of the external double bond over the internal one.19 Subsequent hydrogenation of p-1-menthene or p-8-menthene leads to the fully saturated products, p-menthanes, as a mixture of cis- and transisomers (Scheme 1). Received: October 3, 2016 Revised: February 17, 2017 Published: March 18, 2017 3762

DOI: 10.1021/acssuschemeng.6b02381 ACS Sustainable Chem. Eng. 2017, 5, 3762−3767

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pressure reactor equipped with a digital manometer. The hydrogenation of limonene was monitored at 15 and 30 bar H2, and the reaction mixture was analyzed by GC-FID. Equipment description and the calculation used to determine the hydrogenation rate are annexed in the SI. Continuous Flow Reactor. Continuous flow experiments were conducted using a ThalesNano H-Cube Pro equipped with a H-Cube Gas Module (for external H2(g) usage) and employing 70 mm CatCart cartridges with an internal volume of 10 mL. The cartridge was filled with ca. 260 mg of 5 wt % Pt/C catalyst. Once the powder was transferred into the cartridge, the cartridge was placed into the HCube and flushed with solvent (1 mL/min) for 30 min. Then, the substrate solution (3.0−0.3M) was pumped through the catalyst with a flow of hydrogen (20 bar) at 0.3 mL/min, and the reaction parameters (substrate flow = 0.3−2.5 mL/min, H2 pressure = 20−80 bar) were varied. The system was allowed to equilibrate for 30 min before collecting ca. 9 mL of reaction solution, which was analyzed via GC using n-dodecane as the external standard. GC Analysis. Gas chromatography (GC) analyses for limonene hydrogenation were performed on a trace GC Thermo Scientific GC apparatus equipped with a flame ionization detector (FID) and an OV1701 50m column (0.25 μm film thickness, 0.25 mm ID). The separation was conducted with the following temperature program: 80−250 °C with a 20 min isotherm 8 °C/min and a 10 min holding. NMR. 1H NMR spectra were recorded in CDCl3 solutions at 25 °C on a Bruker Avance-300 or 400 MHz spectrometer. High Resolution TEM and STEM. Micrographs were performed at the “centre technologique des microstructures (CTμ)”, Villeurbanne, France, on a Jeol 2010 transmission electron microscope. The acceleration voltage was 200 kV. H2 and O2 Chemisorption. Measurements were performed on a BELSORB-max from BEL JAPAN.

Scheme 1. Reaction Pathway of the Hydrogenation of Limonene

In the literature, limonene hydrogenation has been catalyzed with different metallic nanoparticles (Pt, Pd, Ni, Cu, Rh, Ru)20,21 using a variety of supports.22−26 In general, it was observed that when high pressure and/or high temperature are involved in the reaction the favorite product of reaction is the p-menthane.20,26 In certain cases, promising selectivities toward (+)-p-1-menthene were reached under less harsh conditions.27,28 However, a comparative and systematic study of different catalysts under mild reaction conditions for the hydrogenation of limonene has not been reported in literature. Herein, we report the activities of several commercially available metal nanoparticle-based catalysts for the hydrogenation of limonene. The hydrogenation of limonene was performed in neat limonene and under mild conditions, e.g., low temperature (30 °C) and low dihydrogen pressure (3 bar), aiming at an effective synthetic procedure for (+)-p-1menthene.





RESULTS AND DISCUSSION All the catalytic reactions in batch were performed in neat limonene using molecular H2. Since, to the best of our knowledge, the solubility of dihydrogen in limonene has not been previously reported in the literature, it was determined by 1 H NMR, and the Henry coefficient (Hcp) was found to be 0.0095 mmol mL−1bar−1, which at constant temperature depends linearly on the pressure (see SI for full experimental details). From our study, the solubility of dihydrogen in limonene was found comparable to its solubility in common solvents such as cyclohexane.29 The hydrogenation of limonene was investigated using commercially available catalysts where the active metals (Pt, Pd, and Ru) and supports (carbon, silica, alumina) were varied (Table 2). All the catalysts in use were characterized via HRTEM (STEM), N2 adsorption, and gas chemisorption (H2 was used for Pt and Ru; O2 was used for Pd), and the results (NPs size, metal dispersion, and surface areas) are summarized in Table 1. The TEM micrographs and size histograms based on the analysis of ca. 200 particles along with the chemisorption isotherms are provided in the SI. Overall, the characterization data show that all the catalysts, with the exception of Pt/SiO2, exhibit very small metal nanoparticles with a mean size ranging from 1.1 to 2.8 according to TEM images. The NPs size as given by TEM was confirmed by gas chemisorption which gave similar values from 1.1 to 2.9 nm. Pt/SiO2 was found to contain big Pt particles of about ca. 20 nm according to TEM and H2 chemisorption. This difference in metal dispersion is further taken into account when evaluating the catalytic performances, reporting TOF, which is calculated with respect to total metal loading, and TOFMs, which is calculated per surface metal atom for all catalysts (vide infra).

EXPERIMENTAL SECTION

Safety Warning. High-pressure experiments with compressed H2(g) must be carried out only with appropriate equipment and under rigorous safety precautions! General. All catalysts were purchased from commercial sources and were activated under hydrogen flow before use (see SI for activation procedure): 5 wt % Pt/Al2O3 (Aldrich), 5 wt % Pt/C and 5 wt % Pd/ C (Fluka), 5 wt % Pt/SiO2 (Alfa Aesar), 5 wt % Ru/Al2O3 (Johnson Mathey), 5 wt % Ru/C (ABCR) and Ru/SiO2 (Alfa Aesar). R(+)-Limonene was purchased from Fluka (purity 98%), dried with molecular sieves, and degassed prior to use. Dihydrogen used for the catalytic test was purchased as ultrahigh pure from Air Products, Premier Plus. Pentane (technical grade) prior to use was distilled and dried over activated alumina using a purification system. Heptane and ethanol (technical grades) was distilled, degassed, and stored under argon (heptane also over 4 Å molecular sieves). High Pressure NMR. Due to the fact that molecular dihydrogen was used as a hydrogen source, the solubility of dihydrogen in limonene was determined. Parameters, equipment description, and formulas used for calculations of the solubility of dihydrogen in neat limonene are annexed in the SI. Hydrogenation of Limonene under Batch Conditions. Experiments were carried out using an in-house engineered 20 mL stainless steel high pressure reactor, equipped with a glass insert and a 2 cm stirring bar. Catalyst (1.5 μmol of metal, based on the intrinsic metal loading), substrate (6.0 mmol, ca. 4000 equiv), and internal standard (dodecane, 1.06 mmol) were weighed into the glass insert. The reaction mixture was pressurized with 3 bar H2 and stirred at 1200 rpm; limonene hydrogenation was performed in a preheated water bath at 30 °C. The reaction mixture was analyzed by GC-FID and NMR during the reaction (sampling at t = 0, 2, 3, 5, 10, 19, 24 h). Hydrogen Uptake Measurement for Hydrogenation of Limonene. Hydrogenation uptake experiments were conducted as outlined above using an in-house engineered 20 mL stainless steel high 3763

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selectivity at 100% conversion) albeit with a low reaction rate (full limonene conversion at 24 h of reaction, Chart 1b) and a TOF and TOFMs of ca. 280 and 667 h−1, respectively. Knowing that the three Ru-based catalysts all exhibit a very high metal dispersion corresponding to Ru NPs from 1.3 to 1.5 nm for C and Al2O3, respectively, to 2.5 nm (for SiO2), this result highlights the strong influence of the support, leading to inactive Ru catalysts when carbon or alumina were used as supports and active catalysts when silica was chosen. Given the comparable specific surface areas of the alumina and the silicasupported catalysts, the positive influence of the silica support discussed above is chemical in nature rather than textural. The platinum-based catalysts are all active, with the silicasupported one being slowest and least selective than the carbon- or the alumina-supported ones: the Pt/SiO2 catalyst required 19 h to reach full conversion rather than 5 h, as for the two other catalysts. The very different metal dispersions (6% dispersion for Pt/ SiO2, i.e., NPs size of ca. 20 nm; 37% dispersion for Pt/Al2O3, i.e., NPs size of ca. 2.9 nm; 63% of dispersion for Pt/C, i.e., a NPs size of ca. 1.5 nm) confirm that the silica-supported material is substantially different from the other two catalysts. The slow rate of the silica-supported Pt catalyst is to be directly correlated to its low metal dispersion given the large difference between TOF and TOFMs (TOF = 300 h−1 vs TOFMs of 5000 h−1). At the same time, the very large activity of the silicasupported catalyst when the dispersion is taken into account (substantially larger than the corresponding value for the alumina and the carbon-supported catalysts) is in line with our previous findings on the ruthenium catalysts (vide supra): silica appears to be a support with a beneficial chemical effect on the activity of the catalyst. This said, the very low dispersion observed on the silica-supported catalyst leading to overall poor catalytic performances led us to dismiss this commercial catalyst for further studies. Moreover, the selectivity of the silicasupported catalyst toward (+)-p-1-menthene was only 83% after 10 h and fell to 43% at full conversion, which is in strong contrast with what was observed for the two other supports. Conversely, Pt/C and Pt/Al2O3 exhibit similar performances in terms of rate (TOFMs = 1238 h−1 for Pt/C and 2270 h−1 for Pt/ Al2O3) and in terms of selectivity with a (+)-p-1-menthene selectivity of ca. 95% at 5 h of reaction (Chart 1).

Table 1. Catalysts Features (Nanoparticles Size, Metal Dispersion, and Surface Area) As Given by TEM, H2 or O2 chemisorption, and N2 Adsorption/Desorption TEM

gas chemisorption

N2 adsorption

catalyst

NPs size (nm)

metal dispersion (%)

NPs size (nm)

surface area (m2/g)

Ru/C Pd/C Pt/C Ru/Al2O3 Ru/SiO2 Pt/Al2O3 Pt/SiO2

1,5 2,5 2 1,3 2,5 2,8 20

71 38 63 77 47 37 6

1,3 2,8 1,5 1,1 2,6 2,9 22

914 752 753 44 70 100 66

At first, a comparison of the catalysts was performed considering a molar ratio between substrate and metal equal to 4000, taking into account the total amount of the metal present in the catalyst. Where relevant, comparison taking into account the dispersion was also discussed. Preliminary experiments examined the hydrogenation of limonene under mild reaction conditions (neat limonene (ca. 4000 equiv), 3 bar H2, 30 °C) employing carbon-supported catalysts. Pd/C was found active for the reduction of limonene to p-1menthene and p-menthane (a mixture of cis- and transisomers) with an initial TOF = 1100 h−1 (and TOFMS = 2895 h−1 when the dispersion of metal NPs is taken into account). Pd/C also catalyzed the isomerization of p-1-menthene to p-2menthene and p-3-menthene, as well as to the formation of bicyclic compounds, 5-isopropyl-2-methylbicyclo[3.1.0]hex-2ene and 2,6,6-trimethylbicyclo[3.1.1]hept-2-ene, reducing the selectivity toward p-menthene (Chart S1 and Table 2). Of the carbon-based catalysts, the most active was Pt/C as full conversion of limonene was reached after 5 h of reaction (Chart 1a). Most significantly, this catalyst showed high product selectivity toward (+)-p-1-menthene with a 95% selectivity after 2 h and 83% selectivity at full conversion, the only side product resulting from deep hydrogenation of (+)-p1-menthene to p-menthane (no isomerization was detected). Ru/C and Ru/Al2O3 were completely inactive for the hydrogenation of limonene under these reaction conditions. In sharp contrast, Ru/SiO2 was found active, and it very selectively transformed limonene into (+)-p-1-menthene (95%

Table 2. Selective Production of (+)-p-1-Menthene from Hydrogenation of Limonene in Batch Reactor under Mild Conditions Selectivity (+)-p-1-menthene, S (%)a

conversion, X (%)a catalyst

t1/2 (h)

Xt1/2 (%)

Tmax (h)

Xtmax (%)

St1/2

Stmax

TON (TON Ms)b

TOF (h−1)c

TOFMs (h−1)

Pd/C Ru/C Pt/C Ru/Al2O3 Ru/SiO2 Pt/Al2O3 Pt/SiO2

2 − 2 − 10 2 5

55 ± 2 − 39 ± 1 − 54 ± 1 42 ± 6 57 ± 10

19 − 5 − 24 5 19

89 ± 7 − 100 ± 1 − 100 ± 1 100 ± 1 100 ± 1

53 − 95 − 97 97 91

45 − 83 − 95 78 43

3500 (9210) − 4000 (6450) − 4000 (8510) 4000 (10,810) 4000 (66,670)

1100 − 780 − 280 840 300

2895 − 1238 − 667 2270 5000

Values of limonene conversion (X) and selectivity (S) in (+)-p-1-menthene have been calculated at a value of half-conversion (Xt1/2 = 50 ± 10%) and at maximum conversion (Xtmax); for each, the time when this conversion was reached is reported (t1/2 and tmax, respectively). bTON = maximum moles of limonene converted/mols of metal, and TON Ms is the TON taking into account the surface metal only as given by the metal dispersion from gas chemisorption. cInitial TOF was calculated based on conversion of limonene after 2h of reaction. TOFMs per surface metal was calculated taking into account metal dispersion determined by gas chemisorption. Experimental conditions: batch reactor (V = 20 mL), 30 °C, 3 bar of H2, catalyst 3−6 mg (metal ca. 1.5 μmol), 1 mL neat limonene (6.0 mmol), molar ratio limonene/metal ca. 4000, GC standard = n-dodecane, 1200 rpm. a

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Chart 1. Reaction Profile for the Hydrogenation of Limonene over Pt/C (a), Ru/SiO2 (b), Pt/SiO2 (c), and Pt/Al2O3 (d)a

a Curves in the graphs are spine curves as guide for the eyes. Experimental conditions: batch reactor (V = 20 mL), 30°C, 3 bar of H2, catalyst 3−6 mg (metal = 1.5 μmol), 1 mL neat limonene (6.0 mmol), molar ratio limonene/metal ca. 4000, GC standard = n-dodecane, 1200 rpm.

mixture by centrifugation, dried under argon for 30 min, and then under vacuum at 70 °C for 1 h before the next run. As shown in Chart 2, the profile of the reaction for three different cycles was maintained, and the selectivity toward (+)-p-1-menthene was stable over each cycle, indicating no activity or selectivity losses. During the fourth cycle, a split test was performed, and no active metal leaching was observed (see SI for complete details). To further assess the very low Pt

On the basis of these observations, we selected Pt/Al2O3 and Pt/C as the best catalysts with high reaction rates and a selectivity toward (+)-p-1-menthene above 80% even during extended exposure to hydrogen after full conversion of limonene. Moreover, the optical rotation of the p-menthene obtained over these two catalysts was found to be [α] ≈ 100° (0.03 mol/L of p-menthene in EtOH, at 23 °C at a λ of 589 nm), meaning that over these catalysts no inversion of the chiral center was occurring. Of the two catalysts, we selected Pt/C for further recycling tests, kinetic studies, and catalytic tests under continuous flow conditions. The kinetics of the limonene hydrogenation over Pt/C were studied by monitoring the hydrogen uptake under batch conditions after having ensured that the reaction was under chemical regime with no mass transfer limitation (see SI for full kinetic details). The hydrogenation of the external double bond in limonene and the subsequent reduction of the internal double bond in (+)-p-1-menthene followed a pseudo-first-order kinetics with reaction rates of 0.376 and 0.044 mol L−1 h−1, respectively, under 15 bar of dihydrogen. Thus, the rate of hydrogenation of the terminal CC bond was over 8 times higher than that of the internal CC bond. These data provide kinetic insight into the capacity of Pt/C to discriminate between the terminal and internal CC bonds in limonene leading to high selectivity toward (+)-p-1-menthene under these mild reaction conditions. Once kinetics of limonene and p-menthene hydrogenation were determined, further tests were performed to define the catalyst stability. In particular, recycling tests were carried out using Pt/C under standard batch reaction conditions (30 °C, 3 bar of H2, neat limonene (6.0 mmol), ca. 4000 equiv). After 24 h of reaction, the catalyst was separated from the reaction

Chart 2. Reaction Profile for the Hydrogenation of Limonene over Pt/C (a), Ru/SiO2 (b), Pt/SiO2 (c), and Pt/ Al2O3 (d)a

a

Curves in the graphs are spine curves as guide for the eyes. Experimental conditions: batch reactor (V = 20 mL), 30°C, 3 bar of H2, catalyst 3−6 mg (metal = 1.5 μmol), 1 mL neat limonene (6.0 mmol), molar ratio limonene/metal ca. 4000, GC standard = ndodecane, 1200 rpm. 3765

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Al2O3 showed excellent selectivities toward p-menthene even at high substrate conversion (>90% of (+)-p-1-menthene selectivity at quantitative conversion after 5 h). Kinetic studies showed that Pt/C can differentiate between the terminal and internal CC bonds in limonene under mild reaction conditions due to a greater than 8 times faster reaction rate for the reduction of the terminal double bond. The stability of Pt/C under these mild reaction conditions allowed for the hydrogenation of limonene under continuous flow conditions where almost quantitative conversion (96%) and (+)-p-1menthene selectivity (>87%) were maintained for at least 5 h on-stream. Pt/C was thus identified as a highly productive catalyst for the synthesis of the value-added product (+)-p-1menthene under mild reaction conditions.

leaching, Pt in the supernatant was titrated by ICP-MS and only 0.0016% of Pt was detected. These investigations allowed us to identify a stable and suitable catalytic system for the production of (+)-p-1-menthene under mild conditions. In light of the stability of Pt/C over the recycling tests and the absence of catalytically active metal leaching, the activity of this catalyst was further tested under continuous flow conditions using an H-Cube Pro reactor from ThalesNano. A parameter screening was first conducted to determine suitable flow conditions for the hydrogenation of limonene (see SI for complete continuous flow details). Under optimized conditions, a 0.3 M solution of limonene in EtOH (substrate flow = 2.5 mL/min) was flowed through a cartridge packed with Pt/ C (260 mg) under 20 bar of dihydrogen (hydrogen flow = 2 mmol/min), leading to full conversion of the substrate initially. As shown in Chart 3, a selectivity of (+)-p-1-menthene between



ASSOCIATED CONTENT

S Supporting Information *

Chart 3. Hydrogenation of Limonene under Continuous Flow Conditionsa

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02381. General information, catalyst preparation and characterization, batch reactor and continuous flow reactors experiments. (PDF)



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C. Thieuleux). *E-mail: [email protected] (W. Leitner). ORCID

Kylie L. Luska: 0000-0003-3477-4648 César A. Urbina-Blanco: 0000-0001-7973-192X Chloé Thieuleux: 0000-0002-5436-2467 Notes

a Experimental conditions: substrate flow = 2.5 mL/min, hydrogen flow = 2 mmol/min, dihydrogen pressure 20 bar, residence time = 3 s.

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was cofunded through a SINCHEM Grant. SINCHEM is a Joint Doctorate programme selected under the Erasmus Mundus Action 1 Programme (FPA 2013-0037). Additional support was given by the Cluster of Excellence “Tailor-Made Fuels from Biomass”, which is funded by the Excellence Initiative of the German federal and state governments to promote science and research at German universities. Preliminary experiments for the selection of possible catalysts for hydrogenation of terpenes were carried out as part of the Ph.D. thesis by Dr. Dominique Peters. We also want to thank the co-workers at the laboratories, workshops, and analytical departments of ITMC in Aachen, CPE, CNRS, and UCBL in Lyon.

85% and 92% was maintained after a short induction period over more than 5 h on stream. Conversion remained also high, reaching still 96% at the end of the experiment. The performance under continuous-flow conditions confirmed the potential of commercially available Pt/C for synthetically useful limonene hydrogenation with high selectivity toward the partially hydrogenated product (+)-p-1-menthene at high substrate conversions.



CONCLUSIONS In this study, a series of commercially available catalysts were studied for the hydrogenation of limonene under mild reaction conditions in order to systematically evaluate the influence of the active metals and support materials on catalytic activities and selectivities. The supports have a significant influence on the catalytic properties of the catalysts, with silica exhibiting the highest activity for both Ru- and Pt-based systems when the metal dispersion is taken into account. In the latter case, the detrimental very poor dispersion induced a decrease in the (+)-p-1-menthene selectivity since Pt/SiO2 also catalyzed the hydrogenation of the internal CC bond to p-menthane. Moreover, from the comparison of these catalysts, Pt- and Rubased catalysts showed highest selectivities toward the (+)-p-1menthene as compared to Pd-based catalysts which isomerized drastically the CC double bond. In particular, Pt/C and Pt/



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DOI: 10.1021/acssuschemeng.6b02381 ACS Sustainable Chem. Eng. 2017, 5, 3762−3767