Byproduct Formation Mechanisms and Effect of Mass Transfer in Plant

Article pubs.acs.org/IECR

Byproduct Formation Mechanisms and Effect of Mass Transfer in Plant Sterol Hydrogenation Ville Nieminen,*,† Esa Toukoniitty,‡ Thomas Holmbom,§ and Victor A. Sifontes Herrera∥ †

Raisio GroupBenecol Unit, Raisionkaari 55, P.O. Box 101, 21201 Raisio, Finland Metropolia University of Applied Sciences, P.O. Box 4071, 00079 Metropolia, Finland § Separation Research Ltd., Porthaninkatu 3, 20500 Turku, Finland ∥ Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Åbo Akademi University, FI-20500 Turku/Åbo, Finland ‡

S Supporting Information *

ABSTRACT: Plant sterols, a mixture of several des-4-methyl sterols, were hydrogenated over a Pd/C catalyst by varying catalyst amount and stirring rate. Reactions were carried out under kinetic regime and under the influence of external mass transfer limitations. All reaction were done in the absence of internal diffusion limitations. Under external mass transfer limitations of hydrogen more byproducts due to hydrogenolysis and double bond migration were formed. Under mass transfer limited reactions higher catalyst amounts led to a more extensive byproduct formation. Because of double bond migration two very similar byproducts from sitosterol were formed having trans- and cis-fused rings. Interestingly hydrogenolysis also resulted in two similar stereoisomers of sitostane, in the same way as sitostanone, indicating that the hydrogenolysis occurred during the double bond migration when the double bond is in the Δ3 position. An intermediate product of stigmasterol was observed, in which the ring structure is hydrogenated but the alkyl chain double bond is intact, proving that the ring double bond is hydrogenated prior to alkyl chain double bond hydrogenation. The results show that under mass transfer limitation an optimal amount of catalyst should be used to minimize unwanted byproduct formation.

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INTRODUCTION

Plant stanols are produced industrially by hydrogenating plant sterols, which have a CC double bond located at the Δ5-position in the ring. High selectivity is hindered by side reactions generating mainly stanes (plant stanols without a hydroxyl group) and stanones (plant stanols with a carbonyl group instead of hydroxyl group), see Scheme 1. Sterol hydrogenation to stanols has been studied in several papers.3−5 Especially catalyst deactivation, an economically important issue in sterol hydrogenation, has been studied in detail by the group of Professor Salmi.6−8 In this paper, we will discuss the influence of mass transfer on the byproduct formation in sterol hydrogenation. Because catalyst deactivation typically affects

Phytosterols consisting of plant sterols and stanols, are a group of naturally occurring compounds similar to cholesterol having four trans-fused rings; three cyclohexane rings and one cyclopentane ring, see Figure 1 for the most relevant phytosterols discussed in this paper. They typically differ from each other by the side chain length and saturation. Phytosterols are present, for example, in cereals, vegetables, vegetable oils, fruits, and berries. Plant stanols lower serum cholesterol level and thereby confer health benefits when consumed in sufficient amounts.1 However, plant stanols are not present in high enough concentrations in food to induce a clinically meaningful reduction in serum cholesterol levels and therefore so-called functional foods are enriched with plant stanols in fat soluble esterified form to give the cholesterollowering effect. Plant stanols have been reported to be stable during storage in functional foods.2 © XXXX American Chemical Society

Special Issue: Tapio Salmi Festschrift Received: Revised: Accepted: Published: A

April 4, 2017 July 17, 2017 July 17, 2017 July 17, 2017 DOI: 10.1021/acs.iecr.7b01341 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Chemical structures for the reacting compounds (1, 3, 5, 6), main end-products (2, 4, 7−10) and atom numbering. Stereochemistry is not considered.

is to give an insight into the reaction mechanisms of sterol hydrogenation byproduct formation. The impact of stirring rate on the product distribution during sterol hydrogenation is also discussed.

the product distribution, special care has been taken to minimize the amount of catalyst poisons by a proper selection of plant sterol raw material. Concerning the reaction mechanism of the byproducts, stanes are hydrogenolysis products of the corresponding sterol/ stanol (e.g., sitostane for sitosterol/sitostanol) and stanones originate from the double bond migration resulting in a carbonyl group at position 3 (see numbering in Figure 1). Product distribution is affected not only by the catalyst type, but also by stirring rate, hydrogen pressure, and temperature. In the laboratory scale the reactions are typically carried out under a kinetic regime with very good mixing. However, in the industrial scale reactions the mixing is rarely as efficient as in the laboratory scale and reduced mass transfer affects the product distribution and reaction rate. Our focus in this paper

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MATERIALS AND METHODS Plant sterols are natural compounds extracted from for example, tall oil pitch, soy oil, and rapeseed oil (canola oil). Basically plant sterols are a mixture of several different sterols, mainly sitosterol, campesterol, stigmasterol, brassicasterol, and some stanols, mainly sitostanol and campestanol. In this work we used sterol having a very low amount of catalyst poisons in order to minimize effects caused by the catalyst deactivation. The sterol assay of the used plant sterol is as follows (based on B

DOI: 10.1021/acs.iecr.7b01341 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Scheme 1. Three Main Reactions of Plant Sterols, Hydrogenation, Hydrogenolysis, and Double Bond Migration. The End Products Are Presented Only for Sitosterol

the gas chromatogram peak area %): cholesterol 0.3%, brassicasterol 0.6%, campesterol 25.2%, campestanol 0.6%, stigmasterol 27.0%, sitosterol 41.4%, and sitostanol 1.8%, other 4-desmethyl sterols 3.0%. The commercial plant sterol was extracted from soy oil distillate and purified by solvent crystallization. Liquid-phase sterol hydrogenations (nsterol = 0.134 mol, 20 mass-% in a solvent) were carried out in a 0.5 L Büchi batch reactor over a 5% Pd/C catalyst (Degussa E101 NO/W purchased from Sigma-Aldrich) using 1-propanol as a solvent at 80 °C and 4 bar of hydrogen partial pressure. A hollow shaft-type impeller was used without any baffles. The mass of the catalyst (70, 100, 130, and 190 mg dry basis) and the stirring rate (400−1500 rpm) were varied in order to do hydrogenations in the absence and presence of external mass transfer limitations. Initial hydrogenation rates were determined from sterol conversion vs time plots using linear regression at t = 0. Small liquid samples were taken during the hydrogenation and analyzed by gas chromatography (GC). The GC analysis was performed according to the NMKL 198 procedure.9 Catalyst Characterization. The catalyst was characterized with X-ray photon spectroscopy (XPS), H2-temperature programmed reduction (H2-TPR) and H2-temperature programmed desorption (H2-TPD), transmission electron microscopy (TEM), N2-physisorption, and CO-pulse chemisorption. In N2-physisorption the catalyst was first outgassed at 150 °C for 3 h. The nitrogen physisorption was carried out at atmospheric liquid nitrogen temperatures (−196 °C). Data from the adsorption/desorption isotherms were analyzed by a three-parameters BET equation. The pore volume and area distributions were obtained by using the Dollimore and Heal method departing from the adsorption isotherm.

Details of the TPD, TPR, TPO, and chemisorption analysis method have been given in Table S1. The as-received catalyst was flushed at room temperature with a 10 mL/min flow of pure hydrogen for 60 min prior to the H2-TPD analysis. Then a TPR step (reduction under 5% H2/Ar gas mixture and heating to 300 °C) was introduced to monitor the complete reduction of the catalyst. After the TPR step, a second H2-TPD analysis was done to study the metallic sites on the catalyst. In the H2-TPR analysis the catalyst was first reduced by means of TPR (Step I, Table S1) to (i) study the reducible species present in the sample and (ii) obtain metallic Pd for the subsequent chemisorption analysis (Step II). Due to the ability of Pd to absorb H2 forming palladium hydrides, COchemisorption was used instead of hydrogen. TPO (Step IV) was performed to oxidize the sample to obtain a reference TPR pattern for palladium oxide (Step V, Table S1). An energy-filtered transmission electron microscope (EFTEM LEO 912 OMEGA, 120 kV) was employed to study the particle size distribution of the active metal. A PerkinElmer PHI 5400 ESCA (XPS) spectrometer with an Mg Kα X-ray source was used with a 17.9 eV pass energy to analyze the different samples. The samples were not neutralized during the measurements. Small binding energy shifts in the C 1s and Si 2p can be seen; these shifts are taken into account in the fitting of the Pd 3d peaks. The GC−MS assay was carried out with a Shimadzu QP2010Plus instrument. The detailed method parameters are given in the Supporting Information.

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RESULTS Catalyst Characterization. The surface area according to 3-parameter BET equation was 976 m2/g. The pore volume C


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Industrial & Engineering Chemistry Research and area distributions are given in Table S2. In the TPD spectrum of the unreduced sample, two desorption peaks (71 and 273 °C) were observed. In the case of the reduced sample several peaks were observed. The positions of one peak at 66 °C and another at around 700 °C were directly identified in the spectrum, while additional peak positions at 137, 449, and 602 °C, were detected by numerical deconvolution. Peaks at and above 650−800 °C correspond to spillover hydrogen,10 which usually desorbs at much higher temperatures than that adsorbed on metallic clusters. The peaks at the lowest temperatures are assumed to correspond to Pd hydride (βPdHx), which decomposes between 66 and 111 °C.10 It has been suggested that such hydrides serve as a reservoir of activated hydrogen that promotes the activity of the catalyst in hydrogenation reactions.11 TPD peaks are much smaller for the reduced samples, which may be explained by the fact that high temperature reductions promote a strong metal−support interaction (SMSI) that hinders hydrogen uptake, which is the case when the samples are first submitted to a TPD at 700 °C followed by TPR at 300 °C. In the TPR spectra hydrogen uptake peaks at 52 and 57 °C for the fresh and the oxidized catalyst were detected, respectively. Pd oxide species were determined by XPS: PdO (88% of Pd content) and PdO2 (12% of Pd content). No metallic Pd was detected by XPS. The observed hydrogen uptake peaks in the TPR spectra are attributable to the reduction of Pd oxide species. On the basis of the CO-chemisorption, the mean particle size was 2.6 nm and metal dispersion was 36%. Additionally, TEM images (see Figure S1) were digitally analyzed in order to obtain the particle size distribution; the mean particle size by TEM was 2.5 nm, 40% of particles were 1.13−2 nm and 53% were between 2 and 4 nm. These particle sizes are in line with what is expected from Pd/C catalysts.11−13 The results are summarized in Table S3. Hydrogenation. Hydrogenation results are given in Table 1 and Figures 2−6. In the results the sum of all sterol

Figure 2. Sterol concentration as a function of time with the 700 rpm stirring rate and varying catalyst masses. Symbols: (●) 0.19 g; (×) 0.13 g; (■) 0.10 g; (○) 0.07 g.

and verified experimentally. Hydrogenation of CC double bond is the desired reaction yielding saturated stanols. Sterol hydrogenation showed first order kinetics (see Figure 2) and stanols were the main products. External Mass Transfer of Hydrogen. In three-phase hydrogenations it is of interest to identify if obtained reaction rates are influenced by mass transfer limitations or are under a kinetic regime. The latter is desired in mechanistic studies, whereas the former is often the case under the operating conditions in industrial production. Both internal and external mass transfer limitation can be present. In practice due to low hydrogen solubility the mass transfer limitations of hydrogen are of concern. Less frequently, the reaction is limited by the diffusion of the organic reactant. An often-used experimental approach to verify a kinetic regime (absence of external mass transfer limitation) is to increase the stirring rate up to a point after which the reaction rate becomes independent of the stirring rate. As long as the reaction rate is independent of the stirring rate external mass transfer limitations of hydrogen no longer influence the reaction rate. The stirring rate was varied using 100 mg and 190 mg of catalyst, both experiment series (Figure 3) exhibited a similar behavior, that is, first the initial rate increases with increasing rpm and then reaches a broad maximum. Using 100 mg of catalyst the maximum rate and kinetic regime was

Table 1. Stanones and Stanes at the End of the Reactions with Varying Catalyst Amount and Stirring Rate catalyst mass (g)

stirring rate (rpm)

stanes (%)

stanones (%)

0.19 0.19 0.19 0.10 0.10 0.10 0.13 0.07

400 500 700 400 500 700 700 700

11.3 9.1 5.9 4.4 3.7 3.8 4.4 2.9

4.5 2.9 1.9 4.0 3.1 2.5 2.3 3

compounds (i.e., sitosterol, campesterol, stigmasterol and other 4-desmethyl sterols) are reported and denoted as sterols. The sum of desired products(i.e., campestanol and sitostanol) are denoted as stanols. The main byproducts are stanones (having a CO group at position 3, see sitostanone in Scheme 1) and stanes (no −OH or CO group at position 3, see sitostane in Scheme 1). We group all stanones together (sitostanone and campestanone) as stanones and use similar grouping for stenes. Small amounts of other compounds (mostly very small unidentified peaks in the chromatogram) than sterols, stanols, stanones, and stanes were present. In the sterol hydrogenation three parallel competing reactions were identified (Scheme 1)

Figure 3. Initial rate vs stirring rate. Reaction conditions: T = 80 °C, P(H2) = 4 bar. Symbols: (●) 0.19 g of catalyst; (○) 0.10 g catalyst. The line is just to guide the eye. D


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by GC (confirmed by GC−MS) instead of one. Analogous observation was made for stenes and stanes: four stene compounds (two sitostene and two campestene) and four stane compounds (two sitostane and two campestane) were obtained. However, these stenes are intermediates, which react further and disappear at long reaction times, but the amount of stanes increases during the reaction (see Figure 5). One

obtained around 500−700 rpm, whereas with 190 mg of catalyst the maximum rate was around 700 rpm. The gas− liquid mass transfer was also studied by varying the catalyst mass at 700 rpm stirring rate (Figure 4) which provided the

Figure 4. Initial hydrogenation rate as a function of catalyst mass at 700 rpm. Reaction conditions: T = 80 °C, P(H2) = 4 bar, 700 rpm.

Figure 5. Relative concentrations of (●) stenes and (○) stanes as a function of time with the catalyst amount of 0.10 g and stirring rate of 400 rpm.

optimum agitation efficiency and higher initial reaction rates. Initial rate increased linearly with increasing catalyst mass indicating absence of gas−liquid mass transfer limitations. It can be concluded that reactions carried out at 700 rpm were done under kinetic regime in the absence of gas−liquid and liquid−solid mass transfer limitation of hydrogen. Furthermore, experiments carried out using the lowest stirring rate (400 rpm) were influenced by external mass transfer limitations of hydrogen (Figure 3). The effects of external mass transfer limitation vs kinetic control can be evaluated qualitatively by comparing results at 400 and 700 rpm, respectively (Figure 3 and Table 1). Internal Diffusion. Internal diffusion can be studied varying the catalyst particle size or by calculating catalyst effectiveness factor (ηeff). Diffusion coefficient of hydrogen in 1-propanol was calculated from the Wilke-Chang equation.14 The catalyst effectiveness factor was calculated according to a published procedure15 considering an irreversible reaction of first order on the spherical catalyst particles. When ηeff approaches 1, the rate is not influenced by pore diffusion limitations. The commercial 5% Pd/C catalyst used in the experiments was in the form of fine powder (particle size < 50 μm) and therefore, the internal diffusion limitations of hydrogen can be expected to be negligible. The obtained results, ηeff = 0.999, indicate negligible internal mass transfer limitations of hydrogen. The parameters used for the calculations can be found in Table S4. Under kinetic control (i.e., at 700 rpm, see below) the selectivity and maximum amount of the desired products (stanols) was independent of the reaction rate. External mass transfer limitations of hydrogen result in a decreasing main product yield and selectivity. This can also be detected as an increase in the amounts of stanes and stanones, see Table 1. Byproduct Formation. There is a clear trend that at a lower stirring rate, that is, under mass transfer limitations, the amount of byproducts, stanes, and stanones, are higher than at the optimal 700 rpm stirring rate, with no mass transfer limitation (Table 1). With increasing catalyst amounts byproduct formation is increased. The product distribution needs also another important observation. Two sitostanone compounds and two campestanone compounds were observed

additional intermediate product is also observed with the mass number of 486 m/z. It appears in the very beginning and is only detected with slow reactions. It increases prior to vanishing (see Figure 6) and no new products are formed; this is an

Figure 6. Relative concentrations of (●) sterol, (○) stanol, and (■) intermediate compound (“stigmastanol”) as a function of time with the catalyst amount of 0.10 g and stirring rate of 400 rpm.

intermediate product which will react further to form one of the major final products. When the reaction reaches its end, that is, no more sterols are converted, no other reactions are observed. There are always approximately 1.6% of sterols remaining at the end of the reaction and the sterol content does not decrease to a lower level at extended reaction times. Moreover, at this point, no change in the product distribution is observed even if the reaction solution with the catalyst is kept under hydrogen pressure for a longer period of time. A similar observation was reported by Augustine.16

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DISCUSSION Despite several articles related to sterol hydrogenation, byproduct formation is not extensively studied. In fact, in several reports with a Pd/C catalyst4,5,7,16 no byproduct formation was reported. Formation of stanes and stanones was reported by Wärnå et al.3 They successfully estimated E


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Formation of Intermediate Products and Stanes. Hydrogenolysis leads to a formation of water and a sterol/ stanol analogue without a hydroxyl group. During the reaction up to eight compounds without hydroxyl group were detected: two sitostenes, two sitostanes, two campestenes, and two campestanes. Stanes content increased during the whole reaction, but stenes content first increased before they almost disappeared. Stenes content always remained low during the whole reaction (Figure 5). Sitostane and campestane were the two final products, and they were formed from sitostene and campestene. Stenes were formed as a result of a double-bond migration to the position Δ3 followed by hydrogenolysis. Stanes are hydrogenated forms of Δ3 stenes. Double-bond migration plausably explains the two observed compound for each stane and stene: They are 5α and 5β forms analogously to stanones (cis and trans-fused rings, see Scheme 2 and Scheme S1). The hydrogenolysis must take place during the doublebond migration when the double bond is in the position Δ3 between carbons C3 and C4. There is no hydrogenolysis taking place for the stanols. Apparently a double bond in the position Δ3 is necessary for the formation of stenes (and stanes). Previously an additional intermediate product formation and disappearance was described (see Figure 6). By a GC−MS assay it was concluded that the structure is something between sitostanol and stigmasterol with a mass of 486 m/z. A reasonable explanation is that this intermediate product was stigmasterol, of which the ring CC at the position Δ5 is hydrogenated, and the alkyl chain double bond is intact. We would call this intermediate “stigmastanol”. Please note that sometimes in the scientific literature sitostanol has also been called stigmastanol, to our mind erroneously, and we would like to call this intermediate product stigmastanol. In the next step, the alkyl chain double bond is hydrogenated forming one of the main products, sitostanol. This reaction mechanism has another important conclusion. More strongly adsorbed bulky sterol rings are hydrogenated first while the less strongly adsorbed alkyl chain double bond hydrogenate later indicating a typical behavior of competitive adsorption. Furthermore, there is only one-sided adsorption (α-side) of sterol molecules, since only 5α-stereoisomers were observed as stanol products, opposite to stanones and stanes. Notably, in the case of stanones (and stanes) the reason for two stereoisomers is double-bond migration, not adsorption on α- and β-phases. Apparently the methyl groups at positions C18 and C19 hinder adsorption and lead to adsorption only on one side. A similar type of adsorption has been observed and discussed by Nishikori et al. for 5α- and 5β-cholestan-3-ones over unsupported Pd.18 Role of Mass Transfer Limitation of Hydrogen on Product Distribution. Hydrogenation rate is clearly dependent on the hydrogen availability: at lower stirring rates the hydrogenation reaction is clearly slower (see Figure 3). The drop in hydrogenation rate as stirring rate decreases is much more pronounced at higher catalyst amounts. At the same time, byproduct formation is strongly increased; for example, the stanes amount with 0.19 g of catalyst and a 400 rpm stirring rate is 11.3%, but it is just 2.9% with 0.07 g of catalyst and a 700 rpm stirring rate (Table 1). With a higher 0.19 g of catalyst amount, maximum stanol yields were 90% and 81% with 700 and 400 rpm stirring rates, respectively. Stanol yields increased with a lower 0.10 g catalyst amount to 92% and 90% with 700 and 400 rpm stirring rates, respectively. The Pd/C catalyst catalyzed all three reactions: hydrogenation reaction, double bond migration, and hydrogenolysis. Under hydrogen-poor

kinetic parameters under the kinetic regime for the reactants (sterols) and products (stanols), but the fit of the model was not as accurate for the byproducts, stanes, and stanones. Several kinetic models were developed in sterol hydrogenation over a Pd supported on polymer fiber catalyst under strong catalyst deactivation.17 Formation of three byproducts were reported, sitostane, sitostene, and Δ7-sitosterol, but without clear conclusions how they are formed. Very little information is available in sterol hydrogenation on byproduct distribution with different mixing and how the byproducts are formed. Furthermore, the stereochemistry of different byproducts has received very little attention. Sterol molecules are rather large; the dimensions of the sterol ring structure is approximately 1 nm × 0.5 nm and the alkyl chain length is approximately 0.7 nm. The catalyst characterization results indicated that basically no micropores are present and the catalyst has rather large pores: 70% of the surface is in the range of 3−6.7 nm. The pores are accessible for sterol molecules. On the other hand, the average Pd particle size was characterized as 2.5 and 2.6 nm by TEM and COchemisorption, respectively. The dimensions of one sterol molecule are very close to that of the average Pd particle, which serve as active sites for hydrogenation. It is important to note that the complete hydrogenation of both brassicasterol and campesterol leads to campestanol. Analogously, the complete hydrogenation of sitosterol and stigmasterol will give sitostanol. In the case of stigmasterol and brassicasterol, the double bond in the alkyl chain is also hydrogenated in addition to the ring double bond at the Δ5 position. Therefore, there are only two main products, sitostanol and campestanol. Stanone Formation Mechanism. Stanones are products of double bond migration. In Scheme 2 a double bond Scheme 2. Double Bond Migration from Δ5 to Δ3 Position

migration reaction is presented. Interestingly, no double-bond migration products other than the two sitostanones and two campestanones were detected. When the double bond migrates through carbons C5C4 (Δ4 position) to C4C3 (Δ3 position), the hybridization of carbon C5 is changed from sp2 to sp3. As a result, the stereochemistry of carbon C5 has two possibilities: 5α or 5β. In other words, the fusion of the two cyclohexane rings can become either cis or trans. In sitostanol (or actually 3-β-sitostanol) the cyclohexane ring is trans-fused (5α) and the OH-group is in the equatorial position (βposition). Since there are two stereoisomers possible, two different sitostanone (and campestanone) products are formed. The identification of two sitostanone and campestanone compounds was confirmed by GC−MS. Formation of two campestanones and sitostanones can be plausibly explained by two different stereoisomers of these compounds. F


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conditions the latter two reactions become faster while the rate of hydrogenation is suppressed. Double-bond migration is favored at hydrogen-poor conditions.19 Hydrogenolysis is catalyzed by a surface acidity of carbon support. It has been reported that the initial rates of hydrogenolysis are higher with a higher acidity for the Pd/C catalyst.20 Under the mass transfer limitations the hydrogenation rate was decreased and the relative importance of the acid (or base) catalyzed double bond migration reaction was increased followed by the hydrogenolysis reaction. In the industrial scale, the mass transfer limitations result in increased side product formation, and undesired products must be separated by, for example, crystallization. Higher byproduct formation leads to increased production expenses. These results emphasize the importance of good mixing in the hydrogenation of plant sterols. In the case of mass transfer limitations, which often occurs at the industrial scale production, it is also important to use an optimal amount of catalyst to avoid unnecessary byproduct formation.

Ville Nieminen: 0000-0002-8389-3419 Thomas Holmbom: 0000-0001-9898-1190 Victor A. Sifontes Herrera: 0000-0002-9890-9575 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L. Koponen, T. Suominen and S. Kaatrasalo are acknowledged for the precise operation of the hydrogenation reactor and GC assays. A-R. Rautio, Microelectronics and Materials Physics Laboratories, Oulu University, is acknowledged for collecting the TEM image. A. Aho at the Laboratory of Industrial Chemistry, Åbo Akademi University, is acknowledged for acquiring and analyzing the XPS spectra.

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CONCLUSIONS The reaction mechanisms of plant sterols hydrogenation were studied under kinetic regime and mass transfer limitations by varying the stirring rate and mass of catalyst. The products and intermediates were characterized in detail. The main products were stanols, and the main byproducts were stanes and stanones. Under the kinetic regime the hydrogenation rate was fast and byproducts due to double bond migration and hydrogenolysis were formed to a lesser extent. However, under mass transfer limitations the initial hydrogenation rates were decreased and more byproducts were formed. Increasing catalyst mass under mass transfer limitation increases hydrogenolysis and double-bond migration more than hydrogenation rate is increased. Under mass transfer limitations an increase in catalyst amount did not result in an optimal result due to extensive byproduct formation. In double-bond migration there were two compounds formed having either trans- or cis-fused rings at the position 5. Therefore, two sitostanone and two campestanone compounds were observed. Interestingly, two similar hydrogenolysis products were also observed, namely two sitostanes and two campestanes. This can plausably be explained if the hydrogenolysis took place as a competitive reaction for stanone formation when the double bond is migrated to the Δ3 position. No hydrogenolysis was observed for the main products, sitostanol and campestanol, indicating that a double bond is necessary for hydrogenolysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications Web site. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b01341. Tables for catalyst characterization results (Tables S1− S4); GC−MS assay method parameters; TEM image of the catalyst (Figure S1); reaction scheme (Scheme S1) (PDF)

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REFERENCES

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*E-mail: [email protected] or [email protected]. G


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Byproduct Formation Mechanisms and Effect of Mass Transfer in Plant

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