Enzymatic Reactive Distillation: Kinetic Resolution of rac-2-Pentanol

Sep 7, 2015 - Harmsen , G. J. Reactive distillation: the front-runner of industrial process intensification: a full review of commercial applications,...
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Enzymatic Reactive Distillation: Kinetic Resolution of rac-2-Pentanol with Biocatalytic Coatings on Structured Packings Rene Heils,*,† Jan-Hendrik Jensen,† Steffen Wichert,† Natalie Behrens,† Mario Fabuel-Ortega,† Andreas Liese,‡ and Irina Smirnova† †

Institute of Thermal Separation Processes, Hamburg University of Technology, Eissendorfer Strasse 38, D-21073 Hamburg, Germany ‡ Institute of Technical Biocatalysis, Hamburg University of Technology, Denickestrasse 15, D-21073 Hamburg, Germany S Supporting Information *

ABSTRACT: An enantioselective biocatalytic reaction was carried out for the first time in a fully integrated batch reactive distillation setup. The investigated reaction was the lipase catalyzed kinetic resolution of a racemic mixture of (R/S)-2-pentanol with ethyl butyrate. The reaction is strongly limited by the reaction equilibrium so that the reactive distillation helped to shift the equilibrium toward the product side. The enantioselectivity of the applied lipase was high with ee values >99% for 2-pentanol at conversions of 69 ± 3%. As established in our previous work, the biocatalyst was again immobilized within a hydrophobic silica coating for structured packings. The production of the biocatalytic coating was further developed as a spray-coating method to allow reproducible coatings which also can be applied on larger surfaces. The influence of the coating on separation efficiency and pressure drop was studied as well as the stability of the coating under the required process conditions. Overall, this work demonstrates the first kinetic resolution in a reactive distillation setup with structured packings and presents the catalytic coating as an alternative structure for (bio)catalytic columns internals. work, an enzymatic reaction was carried out for the first time in a reactive distillation column with structured packings. The enzyme Candida antarctica lipase B (CALB) was immobilized in a hydrophobic silica gel, which was applied as coating on the column internals.4 The hydrophobicity of the silica matrix helps to increase the activity of the lipase in an organic environment as studied previously by Reetz et al.5,6 The US company Akermin followed a similar approach in a reactive CO2 absorption process with a silica based coating to immobilize a carbon anhydrase on metal sheet packings.7 More recently, the feasibility of an enzymatic reaction in a continuous reactive distillation column was shown using a simulation-based approach.8 Up to now, no enantioselective enzyme reaction was investigated in a fully integrated reactive distillation setup which is the goal of the present work. Recently, Au-Yeung proposed a horizontal reactive distillation setup for the enantioselective acylation of two chiral substrates catalyzed by a lipase in order to produce four enantiopure compounds. The horizontal setup enhanced the liquid phase residence time that was proposed to be necessary for the enzyme to obtain adequate conversion. The immobilized lipase was not fully integrated into the column, but it was placed in a fixed bed reactor that was incorporated in an external loop for every stage of the column.9 In this work it was proposed, that the design of catalytic column internals for an enzymatic reaction requires special attention. For rather slow enzyme kinetics, higher

1. INTRODUCTION Reactive distillation (RD) is an integrated separation process that is well-established in industry with more than 150 commercial operations.1 By integrating the reaction and separation process into one unit operation, the equipment and operating costs can be reduced, and in case of equilibriumlimited reactions, the yield can be enhanced by removal of the product. Chemical catalysts (e.g., ion exchange resins) are still the first choice for the majority of reactive distillation processes. The provision of biological catalysts in reactive distillation columns is restricted mainly because of the thermal instability as well as the increased costs for the biocatalyst. On the other hand, diverse examples demonstrate that enzymes can work in industrial scale processes with excellent selectivity.2 Many thermostable enzymes fall into the class of the hydrolases, with process temperatures ranging up to 115 °C (α-amylase, EC 3.2.1.1 in the high fructose corn syrup production). The particular advantage in the application of enzymes is their selectivity in the synthesis of chiral molecules. The selectivity of the biocatalysts is based on different steric hindrance of the chiral substrates at the active center which results in different reaction rates and thereby an excess of one enantiomeric product. Therefore, the integration of biocatalysts in a reactive distillation processes would allow a different set of highly selective biocatalytic reactions to be considered for this integrated separation process. The proof of concept for the enzymatic reactive distillation was provided by Paiva et al. with a lipase immobilized on an acryl resin.3 The setup consisted of a laboratory scale fractionating column (hcolumn ≈ 25 cm) with 13 inverted pear bulbs that were filled with lipase granulate. In our previous © 2015 American Chemical Society

Received: Revised: Accepted: Published: 9458

July 30, 2015 September 2, 2015 September 6, 2015 September 7, 2015 DOI: 10.1021/acs.iecr.5b02802 Ind. Eng. Chem. Res. 2015, 54, 9458−9467

Article

Industrial & Engineering Chemistry Research

Germany); ethanol (>99.5%), methanol (>99.9%), acetonitrile (99.9%), isoamylalcohol (>98.5%), and sodium hydroxide from Carl Roth (Karlsruhe, Germany); tetramethyl orthosilicate (>98.0%) and trimethoxymethyl silane (>98.0%) from Fluka (Buchs, Switzerland); sodium fluoride and acetone from Prolabo (East Grinstead, United Kingdom); n-heptane (>99.5%) and 2-(R/S)-pentanol (>99.0%) were purchased from ABCR (Karlsruhe, Germany). All chemicals were used as received. 2.2. Enzyme. Lipase B from Candida antarctica (CALB) was kindly provided by Novozymes (Bagsvaerd, Denmark) as liquid solution under the trade name lipozyme CALB L. The activity of the free enzyme solution was 91 ± 18 U/mL in a pnitrophenol acetate assay (pNP-acetate = 50 mM, 30 °C, pH 7, adsorption was measured at 400 nm). The protein concentration in the liquid enzyme solution was determined to 6 wt % with the Bradford assay. According to the manufacturer, the enzyme is produced by submerged fermentation of a genetically modified microorganism. The enzyme protein, which itself is not genetically modified, is separated and purified from the production organism. 2.3. Production of Coated Column Internals. The catalytic coating is based on a porous silica gel which subsequently dries out into a xerogel at ambient condition. The gel is formed during the sol−gel reaction, and by addition of enzyme into the reacting sol, the enzyme becomes entrapped. The sol was produced according to the method described in a previous work.4 Two solutions A and B were prepared according to the composition in Table 1. The sol−gel reaction is initiated by mixing solutions A and B.

residence times are necessary which are difficult to be met by a standard packing column. On the other hand, tray columns show higher residence time for the liquid phase, but then pressure drop increases so that the higher heat duty in the bottom could possibly deactivate the thermosensitive enzymes. In spite of numerous designs for catalytic packings,10 there is no catalytic packing which can satisfy an increased reaction efficiency as well as a high separation capacity. In industrial processes, most catalytic packings consist of wire gauze baskets filled with catalyst granulate, like e.g. KATAPAK from Sulzer or KATAMAX from Koch Glitsch. The design is similar to a standard distillation packing, but the implementation of catalyst baskets can lower the separation efficiency of the packing. Furthermore, the manufacturing costs for these catalytic structures are rather high. The coating of structured packings with a catalytic active layer is another promising approach for the integration of catalysts into reactive distillation processes. The original structure of the packing is only slightly altered by the coating so that low pressure drops and high separation efficiency can be achieved. In addition, the catalytic layers offer a high surface area for increased reaction efficiencies. The application of catalytically active coatings on column internals has been investigated for different materials, like zeolite on ceramic monoliths and metal wire gauze structured packings,11 magnesium oxide on ceramic Norton saddles12 as well as a silica-based coating containing ion exchange resins on aluminum packings.13 In our previous work, a silica based coating was developed to immobilize enzymes (lipase) on structured wire-gauze packings which were successfully tested in a batch reactive distillation setup.4 The biocatalytic coatings were produced by means of a dip-coating method, i.e. the packing elements were immersed in the reacting sol solution containing the enzymes and the gel was dried out on the surface of the packing. During the sol−gel reaction, the viscosity of the sol steadily changed which made it difficult to control the coating process. An immersion right before the gel point e.g. generated partially thicker coatings which were more unstable than thinner layers. In addition, the liquid sol accumulated in the depressions of the corrugated sheets forming thicker unstable layers. The size of the packing segment was limited because of inefficient cooling of larger vessels to remove the heat of the reaction and likely enzyme denaturation. Hence, another goal of this work is the optimization of the coating procedure. In this work, a spray-coating method is presented that enables the production of more reproducible biocatalytic coatings. The effect of the spray coating procedure on the structure of the coating is addressed in the first part of the work. In the second part of the work, the packings with the biocatalytic coating are applied to demonstrate the feasibility of an enzymatic kinetic resolution in a batch reactive distillation column. Time- and space resolved concentration profiles of the reactive distillation column are presented to show the effect of distillate product removal on the conversion of the kinetic resolution. In addition, the influence of the coating on the separation efficiency and the pressure drop of the packing were investigated along with the stability of coating and enzyme under given process conditions.

Table 1. Composition of Biocatalytic Coatinga solution A

solution B

component

wt % (final concn)

methanol trimethoxymethylsilane (MTMS) tetramethyl orthosilicate (TMOS) water enzyme CALB L NaF, 1 M polyethylene glycol

33.9 28.7 8.0 12.6 10.5 4.9 1.4

a

The concentration is given as weight fraction in the combined solution.

The sodium fluoride was added as catalyst for the hydrolysis as well as for the polycondensation reaction of TMOS and MTMS. Methanol was used as solvent for the water insoluble silane precursors. Due to the heat liberation in the beginning of the reaction, the sol had to be cooled in an ice bath for the first 3 min (for the usual preparations in a beaker of up to 140 g sol). The coating of the structured wire gauze packings could be applied in two different ways, the dip-coating and the spraycoating method. In the dip-coating method, the sol was prepared in a vessel, and the packing segments were immersed into the reacting sol for approximately 5 s and dried at room temperature for approximately 20 s. These steps were repeated until the sol in the vessel solidified to a gel monolith. The coatings were left for drying until the weight became constant. Alternatively, a spray coating setup was developed to gain control over the production process of the coating (e.g., coating thickness control, extension to larger packing pieces, and results’ reproducibility). The setup consists of a commercially

2. MATERIALS AND METHODS 2.1. Chemicals. Ethyl butyrate (>98.0% purity), cyclohexane (>99.5%), and polyethylene glycol (average Mn 400, water 0.54 mol/mol.18 Further experimental VLE data is

Figure 6. Reaction equation of kinetic resolution of racemic (R/S)-2-pentanol with ethyl butyrate. 9463

DOI: 10.1021/acs.iecr.5b02802 Ind. Eng. Chem. Res. 2015, 54, 9458−9467

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Figure 7. Time dependent concentration progress curves of all reaction components in the bottom and top of the column for the kinetic resolution of (R/S)-2-pentanol with ethyl butyrate. p = 60−115 mbar, Tbottom = 63 ± 1 °C, xi,EtBu = 0.66, xi,2‑PeOH = 0.33, mi,substrate = 800 and 20.6 g immobilized enzyme (coating). Reflux ratio rr = 20 was set within the framed time period.

Figure 8. Concentration profiles of all components over the height of the column at two different time points. Conditions: p = 115 mbar (t = 1 h) and p = 100 mbar (t = 6 h), Tbottom = 63 ± 1 °C, xi,EtBu = 0.66, xi,2‑PeOH = 0.33, mi,substrate = 800 and 20.6 g of immobilized enzyme (coating). Lines serve as visual aid.

The investigated reaction is strongly limited by the reaction equilibrium which was observed in batch experiments in stirred tank reactors (STR). The immediate removal of products in the reactive distillation (RD) setup helped to shift the conversion beyond the equilibrium concentration and fully convert the (R)-2-pentanol to (R)-2-pentyl butyrate. Figure 9 shows the time dependent conversion of (R)-2-pentanol in the reactive distillation setup compared to the conversion in a stirred tank

corresponding product (S)-2-pentyl butyrate was only slowly increasing and remained below 0.1 mol/mol after 14 h. In the distillate, the concentration of ethanol was higher than 0.84 mol/mol during the first 8 h of the experiment (see Figure 7, right). After 8 h the reaction slows down and less ethanol is formed due to depletion of the fast reacting enantiomer (R)-2pentanol. Instead the medium boiler ethyl butyrate accumulates in the distillate stream to 0.39 ± 0.04 mol/mol at around 14 h. At this time point (t = 14 h), the main constituents in the bottom mixture were 0.39 ± 0.01 mol/mol (R)-2-pentyl butyrate, 0.35 ± 0.04 mol/mol ethyl butyrate, and 0.19 ± 0.01 mol/mol (S)-2-pentanol. Due to differences in boiling points, the resolved product (S)-2-pentanol could be easily separated from the other components in the mixture by fractional distillation in the same setup. For this experiment, the reactive section with the coated packings was located in the lower half of the column with a length of 0.36 m. Figure 8 shows the concentration profiles at two different time points, 1 h and 6 h. During this time, the medium boiling substrates ethyl butyrate and 2-pentanol were accumulating more in the upper half of the column due to removal of distillate and decreasing operating pressures. However, both substrates ethyl butyrate and (R/S)-2-pentanol were always present in the reactive section. Furthermore, the products (R)-2-pentyl butyrate and ethanol are enriched outside of this section which reveals that the position of the catalyst was properly chosen.

Figure 9. Conversion of (R)-2-pentanol in a batch stirred tank reactor in comparison with batch reactive distillation experiment with product removal. Conditions reactive distillation: see caption of Figure 8. Conditions batch STR: p = 1013 mbar, T = 60 ± 1 °C, xi,EtBu = 0.66, xi,2‑PeOH = 0.33, mi,substrate = 30 g and 84 mg of immobilized enzyme (coating). 9464

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internals was investigated by separation of the binary system cyclohexane/n-heptane. The gauze packings had an average loading of 100 ± 20 mg coating per g of packing. The loss of silica coating due to washing off effects with the cyclohexane/nheptane mixture can be neglected (max. 1.5 wt % weight loss after 80 h at 60 °C). The separation efficiency was determined for the coated gauze packing and compared with the efficiencies obtained for catalytic KATAPAK-like as well as noncatalytic MELLAPAK-like column internals in the same setup (see Figure 11).

reactor experiment. For the calculation of the (R)-2-pentanol conversion, the liquid holdup within the column was estimated to be constant at 5% of the column volume. Furthermore, the volume reduction within the column by sampling and distillate removal was included. The high selectivity of lipase CALB for (R)-2-pentanol reported previously by Orrenius et al.20 was also evident during this experiment. The enantiomeric ratio, i.e. the ratio of the reaction constants of the fast reacting enantiomer to the slow reacting, was 390 according to the method of Rakels.21 The efficiency of the kinetic resolution can be realized from the plot of the enantiomeric excess ee as a function of rac-2-pentanol conversion (see Figure 10). The enantiomeric excess ee is

Figure 11. Separation efficiency structured gauze packings with and without coating in comparison with KATAPAK-like and MELLAPAK-like structures. Figure 10. Selectivity of the kinetic resolution of rac-2-pentanol in the batch reactive distillation setup as enantiomeric excess plotted over conversion of 2-pentanol.

For F-factors between 0.39−1.02 ± 0.08 Pa0.5, the HETP value for the coated packings ranges between 0.19−0.21 ± 0.02 m in comparison to 0.12−0.15 ± 0.02 m for the noncoated packings in a similar F-factor range. Apparently, the coating decreases the separation efficiency by an average of 21% for comparable F-factors. This reduction was expected because the silica coating blocks the open structures of the wire gauze sheets and lowers to some extent the specific surface area for the heat- and mass transfer. A further effect on the separation performance might be caused by the irregular morphology of the coating which leads to maldistribution of the liquid/vapor streams. On the other hand, the efficiencies determined for standard metal sheet packings (MELLAPAK-like structures) were still lower compared to the efficiency of the coated gauze packings. This might indicate that the coated gauze structures still offer a higher surface area. In addition, the hydrophobic character of the silica coating (high MTMS/TMOS ratio) improves the wettability of the packing surface for the organic reactants and therefore enhances the mass transfer at the vapor/liquid interphase. KATAPAK-like structures, similar to those currently used in industrial processes, show a lower separation efficiency compared to the coated gauze packings. The HETP value for the KATAPAK-like structures ranged between 0.40−0.47 ± 0.02 m for the investigated F-factors. The dense catalyst baskets lowers the surface area and thereby the separation performance of the KATAPAK-like structures. This effect is much less pronounced for our coated internals. At the same time in comparison to the original gauze packing, the coating caused an increase in pressure drop by an average of 0.9 mbar/m, probably due to the blockage of the wire gauze structure with silica-gel coating and thus a smaller void fraction within the column. However, the absolute value of the pressure drop observed for the coated internals was

defined as the difference in the amount of both enantiomers over the sum of both enantiomers (see eq 5): ee% =

m(R) − m(S) m(R) + m(S)

·100% (5)

m(R) = mass of (R )‐enantiomer m(S) = mass of (S)‐enantiomer

For the kinetic resolution of rac-2-pentanol, the ee of the product 2-pentyl butyrate remained >89% up to conversions of 50% which demonstrates the excellent selectivity of the lipase CALB in this reaction. At a conversion of 69%, the racemic 2pentanol was completely resolved (ee2‑PeOH > 0.99). Overall, we can conclude that the integrated setup of a reactive distillation column was successfully used to carry out the enzymatic kinetic resolution of racemic (R/S)-2-pentanol with conversions beyond the equilibrium concentration. In addition, the high enantioselectivity of lipase CALB for the kinetic resolution of rac-2-pentanol was also evident in the RD experiments. 3.3. Separation Efficiency of Gauze Packings with Biocatalytic Coatings. The intention of applying a catalytic coating on structured packings was to achieve higher separation efficiencies compared to standard catalytic packings with catalyst pockets. Since in a reactive distillation process, the effects of reaction and separation are overlapping with each other, the impact of the coating on the mass transfer efficiency (height equivalent to theoretical plate - HETP) of the column 9465

DOI: 10.1021/acs.iecr.5b02802 Ind. Eng. Chem. Res. 2015, 54, 9458−9467

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3.5. Reproducibility. All errors shown in this work were calculated with a confidence interval of 95% (student’s tdistribution). For dependent variables (conversion X2‑PeOH, xEtBu) the maximum error was estimated. The errors presented for the reactive distillation experiments (section 3.2) account for the error of the gas chromatography (n = 3), the error of the slope of the calibration line (n = 5), and the estimated error of the bottom mass. The mass balance for all reactive distillation experiments presented in this section was closed by a deviation less than 1%. The errors of the HETP values were calculated based on triplicate runs for different F-factors on one set of packings. The errors of the gas loadings were estimated including the error of the temperature difference of the cooling water inlet and outlet as well as the error of the cooling water mass flow. A sensitivity study showed that in particular the temperature difference of cooling water contributes to the overall error of the gas loadings. Because of the coarse scaling of the analog thermometer, the error for the temperature difference was estimated at SDΔT = 0.72 °C. For the stability experiments, the relative coating mass was averaged for 12 coated packing segments that were used together within one RD experiment. The weight loss of all packings was monitored for 6 batch experiments in the reactive distillation column.

maximum 5 mbar/m for the maximum F-factors studied (1.3 Pa0.5) which is not significant for the presented setup. The results from the pressure drop measurements can be found in the Supporting Information (see Figure S5). 3.4. Stability of the Biocatalytic Coatings. If the catalyst preparation in a reactive distillation process is applied as coating on column internals, the stability under process conditions is of substantial interest. This becomes even more crucial for processes with enzymatic catalysts because of their thermal sensitivity. The stability of the catalytic coating on the structured packings was studied by determining the mass of the coatings after repeated batch reactive distillation experiments (Figure 12). A washing step was required before the first

4. CONCLUSIONS This work presents the first enantioselective enzymatic reaction that was carried out in a fully integrated batch reactive distillation setup. The enzyme was fixed on the internals of the column by a sol−gel reaction using either a dip-coating or a spray-coating procedure. The spray-coating method is more flexible for application on larger surfaces and the coating thickness variation. The coated packings had a reasonable stability under process conditions (T = 30−60 °C, p = 60−115 mbar) with an average weight loss of 2 wt % per run and showed a higher separation performance compared to standard catalytic structures with catalyst pockets. However, the catalyst density for the coated structures is restricted to the surface of the packing and the loading of enzyme in the gel. Overall, the reactive distillation could become a promising tool for selected enzyme reactions, where equilibrium limitation or an enzyme-specific product inhibtion needs to be overcome and at least one of the products can be separated from the reacting mixture by distillation. The catalytic coating of structured packings is a potential new way to introduce (enzymatic) catalysts in reactive distillation without significant loss of the separation performance.

Figure 12. Relative coating mass after each batch experiment in the reactive distillation (RD) column. The duration of each experiment was between 8−14 h. The temperature in the reactive section was between 40−60 °C.

application of the coated packings to remove loosely attached gel particles and/or unreacted sol constituents (washing was done in ethyl butyrate, 24 h, 60 °C, and 60 min−1). Assuming a linear correlation, the loss of biocatalytic coating was approximately 2 wt % per batch for experiments with duration between 8−14 h each. The decrease was not significant for the last RD experiments (runs 4−6). In addition, the long-term activity of lipase CALB was determined by immersion of the immobilized enzyme as granulate in an equimolar solution of the substrates. The temperature was adjusted to 60 °C which corresponds to the highest expected process temperature. In the RD experiments, the temperature in the reactive section ranged between 40−60 °C. The corresponding temperature profiles along the column can be found in the Supporting Information (see Figure S3). The long-term test demonstrated that the activity of lipase CALB could be preserved for at least 27 days without any loss of activity. The results for long-term activity test can be found in the Supporting Information (see Figure S4). Although the stability of the biocatalytic coatings is low compared to traditional (chemical) catalysts applied in industrial reactive distillation processes, the application of enzymes might enable the production of specific chiral products which might compensate for the higher costs of the catalyst. In addition, the batch operation mode presented within this work offers a certain flexibility for small scale production of fine chemicals. To further facilitate the application of thermosensitive enzymes in RD columns, the focus of our current research is the development of an in situ production method to avoid long shut down times and decrease the investment costs for new catalytic packings.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02802. Figures S1−S5 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 9466

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Industrial & Engineering Chemistry Research Notes

(7) Rambo, B. M.; Bucholz, T. L.; Powell, D. C.; Weber, L. E.; Linder, A. J.; Duesing, C. M.; Zaks, A. Polysilicate-Polysilicone enzyme immobilization materials. 13/840,696, Mar 15, 2013. (8) Heils, R.; Niesbach, A.; Wierschem, M.; Claus, D.; Soboll, S.; Lutze, P.; Smirnova, I. Integration of Enzymatic Catalysts in a Continuous Reactive Distillation Column: Reaction Kinetics and Process Simulation. Ind. Eng. Chem. Res. 2014, 50, 19612−19619. (9) Au-Yeung, P. H.; Resnick, S. M.; Witt, P. M.; Frank, T. C.; Donate, F. A.; Robbins, L. A. Horizontal reactive distillation for multicomponent chiral resolution. AIChE J. 2013, 59 (7), 2603−2620. (10) Keller, T. Reactive Distillation. In Distillation: Equipment and Processes; Gorak, A., Olujic, Z., Eds., 2014; pp 261−294. (11) Beers, A. E.; Nijhuis, T.; Aalders, N.; Kapteijn, F.; Moulijn, J. BEA coating of structured supportsperformance in acylation. Appl. Catal., A 2003, 243 (2), 237−250. (12) Dechaine, G. P.; Ng, F. T. T. A New Coated Catalyst for the Production of Diacetone Alcohol via Catalytic Distillation. Ind. Eng. Chem. Res. 2008, 47 (23), 9304−9313. (13) Mehrabani, A.; Akbarnejad, M. M.; Hosseini, H. Structural Catalytic Packing for Reaction−Distillation Columns. Ind. Eng. Chem. Res. 2002, 41 (23), 5842−5847. (14) Olujić, Ž . Standardization of structured packing efficiency measurements. Delft University of Technology. http://www.tkk.fi/ Units/ChemEng/efce/2008/presentations/Olujic-document.pdf 2008. (15) Perry, R. H.; Green, D. W.; Maloney, J. O. Perry’s chemical engineers’ handbook, 7th ed.; McGraw-Hill: New York, 1997. (16) Aferka, S.; Viva, A.; Brunazzi, E.; Marchot, P.; Crine, M.; Toye, D. Liquid load point determination in a reactive distillation packing by X-ray tomography. Can. J. Chem. Eng. 2010, 88, 611−617. (17) Anderson, E. M.; Larsson, K. M.; Kirk, O. One Biocatalyst− Many Applications: The Use of Candida Antarctica B-Lipase in Organic Synthesis. Biocatal. Biotransform. 1998, 16 (3), 181−204. (18) Lecat, M. Tables Azeotropiques. Tome Premier: Azeotropes binaires orthobares, 2nd ed.; L’auteur: Uccle, Bruxelles, 1949, retrieved from DDB on 05/22/2014. (19) Ortega, J.; Ocon, J.; Pena, J. A.; de Alfonso, C.; Paz-Andrade, M. I.; Fernandez, J. Vapor-liquid equilibrium of the binary mixtures CnH2n + 1(OH) (n = 2,3,4) + Propyl Ethanoate and + Ethyl Propanoate. Can. J. Chem. Eng. 1987, 65 (6), 982−990. (20) Orrenius, C.; Hbffner, F.; Rotticci, D.; öhrner, N.; Norin, T.; Hult, K. Chiral Recognition Of Alcohol Enantiomers In Acyl Transfer Reactions Catalysed By Candida Antarctica Lipase B. Biocatal. Biotransform. 1998, 16 (1), 1−15. (21) Rakels, J.; Straathof, A.; Heijnen, J. J. A simple method to determine the enantiomeric ratio in enantioselective biocatalysis. Enzyme Microb. Technol. 1993, 15 (12), 1051−1056.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the German Research Association (DFG SM 82/9-2). We highly appreciate the supply of packings from Sulzer Chemtech and Montz as well as enzyme solution from Novozymes. Furthermore, I would also like to thank Kathleen McAllister for performing several repetition experiments.

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FORMULA SYMBOLS α = relative volatility (−) ρG = density of vapor (kg·m−3) ΔHV = heat of vaporization (J·kmol−1) dc = column diameter (m) FG = superficial vapor load, F-factor (Pa−0.5) Nmin = minimum number of theoretical stages (−) mG = vapor mass flow (kg·s−1) mi = initial mass (g) Tr = reduced temperature (K) uG = superficial vapor velocity (m·s−1) xB = liquid molar fraction in the bottom (mol·mol−1) xD = liquid molar fraction in the distillate (mol·mol−1) xi = initial molar fraction (mol·mol−1)

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ABBREVIATIONS CALB = Candida antarctica lipase B DIPPR = Design Institute for Physical Properties ee = enantiomeric excess EtBu = ethyl butyrate EtOH = ethanol HETP = height equivalent to a theoretical plate MTMS = trimethoxymethylsilane PeBu = pentyl butyrate PEG = polyethylene glycol rac-2-PEOH = racemic 2-pentanol RD = reactive distillation rpm = revolutions per minute rr = reflux ratio STR = stirred tank reactor TMOS = tetramethoxysilane VLE = vapor−liquid equilibria

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REFERENCES

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DOI: 10.1021/acs.iecr.5b02802 Ind. Eng. Chem. Res. 2015, 54, 9458−9467