Research Article pubs.acs.org/journal/ascecg
Role of CO2 in Mass Transfer, Reaction Kinetics, and Interphase Partitioning for the Transesterification of Triolein in an Expanded Methanol System with Heterogeneous Acid Catalyst Lindsay Soh,*,†,○ Chun-Chi Chen,‡,○ Thomas A. Kwan,§ and Julie B. Zimmerman§,∥ †
Department of Chemical and Biomolecular Engineering, Lafayette College, 740 High Street, Easton, Pennsylvania 18042, United States ‡ Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, 195 Chung Hsing Rd, Chutung, Hsinchu County 310, Taiwan § Department of Chemical and Environmental Engineering, Yale University, 9 Hillhouse Avenue, New Haven, Connecticut 06520, United States ∥ School of Forestry and Environmental Studies, Yale University, 195 Prospect St, New Haven, Connecticut 06520, United States S Supporting Information *
ABSTRACT: Fatty acid methyl ester (FAME) production via transesterification of triglycerides (TGs) over a heterogeneous acid catalyst is mediated by carbon dioxide in an expanded methanol system. A representative TG, triolein, is used to determine the mechanisms and interactions responsible for the improved yields over Nafion-NR50 in these system conditions. Namely, the system mass transfer limitations, reaction kinetics, and interphase partitioning behavior are explored by varying mixing conditions and catalyst characteristics over a time series of 4 h. It is found that CO2 enhances mass transfer leading to improved reaction yields and product profiles due to increased substrate transport to and from the catalyst surface. CO2 also contributes to catalyst expansion, leading to greater exposure of active sites and faster reaction kinetics. Initial reaction rate constants using methanol-soaked Nafion reflect pseudo-first-order kinetics. Finally, the substrate-reagent mediating properties of CO2 are discussed in reference to the varying reaction rates of TG and intermediate products. All three of these mechanisms contribute to CO2’s multifaceted role in facilitating heterogeneously catalyzed reactions. KEYWORDS: Biodiesel, CO2-exanded methanol, Heterogeneous catalysis, Methanolysis, Green chemistry and engineering, Gas-expanded liquids
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byproducts.2 Additionally, the presence of water with basic catalysts will lead to the formation of FFA via hydrolysis with subsequent saponification.1 Instead of alkaline catalysts, acidic catalysts can be used as they are less sensitive to water contamination and even catalyze interesterification of FFA to FAME, thus leading to improvements in biodiesel yields.2 Unfortunately the kinetics of acid catalyzed transesterification are significantly slower than with alkaline catalysts; for example beef tallow transesterification catalyzed by sulfuric acid required 24 h to realize greater than 98% yield.2 One strategy to mitigate FFA contamination and maintain heightened reaction kinetics is the use of serial reactions starting with an acid-catalyzed followed by an alkaline-catalyzed system. This technique requires expensive and numerous operations including those for catalyst neutralization and separation.1
INTRODUCTION
Biodiesel production entails conversion of triglycerides (TGs) to form fatty acid alkyl esters via transesterification with an alcohol. Commonly, this alcohol is methanol and the TGs are converted stepwise into diglycerides (DGs), monoglycerides (MGs), and glycerol, producing a fatty acid methyl ester (FAME) at each step. This reaction suffers from technical hindrances with respect to kinetics, selectivity, and yield.1 The typical transesterification process involves reaction of TG-rich feedstocks with methanol and sodium or potassium hydroxide as the dissolved catalyst.1 At a temperature of 60 °C, FAME yields range between 93−98% for various extracted oils after a 1 h reaction time.2 While effective for pure oils, significant improvements, such as utilization of acidic and/or heterogeneous catalysts, must be made to successfully convert feedstocks of varying composition and to reduce requirements for downstream separations.1,3 Although the use of alkaline catalysts offers favorable reaction kinetics, feedstock contamination with free fatty acids (FFA) will decrease biodiesel yields through the creation of saponified © XXXX American Chemical Society
Received: May 28, 2015 Revised: September 16, 2015
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DOI: 10.1021/acssuschemeng.5b00472 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
postulated that the two liquid phasesmethanol-rich and TGrichhave enhanced interphase mass transfer due to the presence of CO2 given that CO2 is known to directly affect interphase interactions in heterogeneously catalyzed systems,11 subsequently influencing transesterification kinetics, yield, and selectivity. However, the role of CO2 on heterogeneous acid catalyzed transesterification at near-critical conditions is not well studied and would be necessary to develop a mechanistic understanding to develop, model, and optimize parameters for TG feedstocks of commercial relevance. The results presented in this work provide additional insight into optimizing transesterification with Nafion in CO2-methanol mixtures at relatively mild temperatures and pressures to improve reaction yields, kinetics, and selectivity.
The use of heterogeneous instead of homogeneous catalysts minimizes downstream separations required postreaction.4 Heterogenous catalysts also mitigate exposure to caustic liquid phase catalysts. These solid catalysts include basic and acidic varieties as well as immobilized enzymes.3 While biological catalysts are quite effective, these enzymes are expensive, intolerant of high reaction temperatures, and costly. 5 Heterogeneous acid catalysts thus show great potential for a transesterification reaction that would mitigate saponification using FFA or water contaminated feedstocks and ease downstream separations. Acid exchange resins, such as Nafion,6−8 have been shown to be effective heterogeneous catalysts for the conversion of both FFA9 and TG6 to FAME. Nafion, a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, is available in various forms including NR50 powder (smaller diameter) and beads (larger diameter) as well as immobilized in an amorphous silica matrix (SAC-13).10 López et al.8 found that Nafion (both NR50 beads and SAC-13) catalyzed the conversion of the short chain TG, triacetin (C9H14O6), to methyl acetate with turnover frequencies comparable to sulfuric acid. However, overall yields were low (∼10% yield of methyl acetate after 2 h at 60 °C) due to the slow reaction kinetics of the acidic reaction. Furthermore, the heterogeneous catalyst is hampered by both external and internal mass transfer resistances.1 External resistances can be mitigated by appropriate mixing and/or fluid manipulation to allow for increased system turbulence.11 For nonporous materials, such as Nafion, internal mass transfer can be facilitated by swelling the catalyst prior to use allowing for access of internal active sites.8 For example, soaking Nafion in methanol prior to reaction expands the polymeric resin and leads to a greater availability of active sites (37−1104 μmol/g in the unexpanded and maximally expanded, respectively).8 Compressed carbon dioxide (CO2) has been used as a reaction solvent/cosolvent that reduces both external and internal mass transfer resistances in heterogeneous catalyst applications.6,7,12 CO2 has been used as a cosolvent for extractions and reactions in methanol13−15 where the high CO2 solubility in methanol changes the mixture properties including reductions in critical point, liquid viscosity, and solvent polarity. Specific to transesterification, CO2 has been used in both supercritical and expanded liquid applications over heterogeneous catalysts.6,7,12,16 These studies report FAME yields from 62 to 98% using pressures between 11.0 and 25 MPa and temperatures between 50 and 200 °C in both continuous and batch processes with reaction times ranging from 0.03 to 8 h.7,12,16 Unfortunately these studies required high pressures/temperatures, and/or long reaction times to achieve high yields. These results show the potential feasibility of CO2-mediated transesterification with a variety of different TG feedstocks; yet the applicability of these systems is limited as they are performed at constrained system conditions and do not provide fundamental mechanistic understanding for further optimization. High yields of methyl oleate transesterified from triolein over Nafion-NR50 using a mixed CO2-methanol system with a 1 h reaction have been reported.6 The results showed that the yields of FAME were dependent on system phase behavior, which can be controlled through the operating conditions. The highest yields were found at moderate temperature and pressure (95 °C, 9.65 MPa) in a ternary phase system consisting of CO2-rich, methanol-rich, and TG-rich phases (not including the solid catalyst phase). Based on these findings, it is
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EXPERIMENTAL MATERIALS AND METHODS
Chemicals. All oleate species standards were purchased from Sigma-Aldrich (purity: ≥ 99%) except diolein (1,2 and 1,3 DG isomers, 2:1) from MP Biomedicals LLC (purity: ≥ 99%). ACS grade methanol was obtained from J.T. Baker. HPLC grade heptane and ultrapure isopropanol were obtained from Alfa Aesar and SigmaAldrich, Inc. Bone-dry CO2 with a siphon tube and nitrogen gas were supplied by Airgas, Inc. Both Nafion NR50 beads and powder catalyst were purchased from Sigma-Aldrich and stored in a desiccator. Catalyst Characteristics. Both beads and powder consist of Nafion NR50 1100 EW polymer mesh sizes of 7−9 mesh (3−4 mm) and 35−60 mesh (0.25−0.50 mm), respectively (Table 1). To assess
Table 1. Catalyst Characterization site availability (μmol/g) Nafion type SAC-13 NR50, bead NR50, powder
particle size (mm)
theoretical maximuma
surface
measured
3−4 0.25−0.50
131−144 939−1104 939−1104
37a 212b
758 976
a Data from Lopez et al.8 Lower values reflect results from S elemental analysis. High values represent titration results. bExtrapolated from bead data.
CO2’s effect on particle swelling, all Nafion NR50 was presoaked in methanol for at least 72 h before reaction. NafionSAC-13 was used as received from BASF and consists of 13 wt % dispersed Nafion in a nonswelling, silica support matrix.8,17 Reactor and Reaction Conditions. All reactions were investigated as detailed in previous works.6 Note that a large methanol stoichiometric molar excess of 366 was used in order to provide a constant methanol concentration throughout the reaction. For each reaction, the catalyst and substrates were added directly into the reactor. Control experiments were performed to ensure that no significant reaction occurred during the heating or cooling steps. An agitation test established that mixing at 300 rpm provided sufficient turbulence to avoid inter- and intraphase concentration gradients18 during kinetic trials.6 Typically all reactions were performed in triplicate with an initial substrate loading of 100 mg except for reaction rate experiments. Analysis. All oleate species were measured using liquid chromatography−mass spectrometry as detailed previously.6
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RESULTS AND DISCUSSION When considering a heterogeneously catalyzed system, the overall reaction may be controlled by three mechanismsmass transfer, kinetics, and interphase partitioningall of which are differentially impacted by the interaction of CO2 with each reactant, TG, 1,2-DG, 1,3-DG, and MG.4 The following results represent a methodical study using oleates as representative B
DOI: 10.1021/acssuschemeng.5b00472 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering glycerides to elucidate the role of CO2 on each reactant by first characterizing the impact of CO2 on the catalyst followed by quantifying mass transfer limitations, reaction kinetics, and the substrate-catalyst/substrate−substrate interphase partitioning. Catalyst Characterization. Active site density characterization by Lopez et al.8 for Nafion SAC-13 and NR50 beads as well as measured site availabilities are reported in Table 1. The theoretical maximum number of active sites for Nafion is an intrinsic property of the resin whereas the available surface sites is extrinsically determined. For SAC-13, the active sites measure between 131−144 μmol/g.8 Based on the assumption that catalyst dispersion allows for all active sites to be exposed, this value should not change between the bulk material and the surface. Conversely, for the nondispersed Nafion-NR50 material, catalyst active site availability will vary drastically based on available surface area. Comparing NR50 beads and powder, the calculated surface area is 0.0014 for the beads and 0.013 m2/g for the powder (assuming spherical particles with an average of the reported diameters). Because surface site availability is expected to be proportional to surface area, the powder surface site density is calculated to be 212 μmol/g (Table 1). While the number of surface sites is highly variable, the effect of swelling can drastically change the available active sites. The measured total sites in the catalyst polymer are approximately 25.4−29.8 times higher than that found on the surface of virgin NR50 beads and approximately 4.4−5.2 times that of the powder. Swelling for NR50 particles in methanol and CO2 has not previously been characterized and is measured here by correlation. Nafion SAC-13 with known acid site density was used to provide a calibration measuring effective acid site availability for the bead and powder catalyst morphologies under swelling from both methanol and CO2 (SI, Figure S1). Within the range of SAC-13 loadings tested, the reaction yield increases linearly with active site availability indicating that the acid site concentration has a direct impact on yield. While the supported catalyst is porous and thus subject to internal mass transfer resistance, Lopez et al.8 noted that triacetin transesterification was not mass transfer limited, even without the aid of CO2 as a mass transfer reducing agent.8 Thus, it is assumed that all active sites are available for reaction in the 4 h time frame. Compared to SAC-13, Nafion NR50 unsupported resins will swell in the presence of both methanol8 and CO27,19 leading to a net increase in active site availability. To isolate the effect of swelling from CO2 beyond that of methanol, catalyst particles were presoaked in methanol prior to reaction. While CO2 is known to swell both methanol20 and the Nafion resin,21 the diffusivity of CO2 into methanol will be much higher.22,23 Figure S2 (SI) illustrates the difference in particle swelling due to different bead treatment. It is assumed that within the 4 h reaction time frame, that catalyst CO2 swelling is complete and the yield can be used to estimate the catalysts’ site availabilities. Comparing the yields from 50 mg of catalyst beads and powder (expected site availabilities 1.85−55.2 μmol) to SAC-13 provides yield-based site availabilities estimated at 758 and 976 μmol/g, respectively, or 20.5 and 4.6 times the expected surface sites. Using this correlation with nitrogen atmosphere controls (SI, Figure S3) indicates that the methanol soaked beads not expanded by CO2 have a site density of 173 μmol/g. These results demonstrate that swelling plays an important role in the effectiveness of the catalyst resin, increasing the site availability by a factor of 4.7 for methanol-soaked beads and an additional 4.4 for those beads additionally treated with CO2. In
addition, CO2 can impact resin properties such as crystallinity and proton conductivity as seen in Nafion 1100 EW membranes, but these affects are estimated to be small compared to that of swelling.24 Mixing and Controls. In order to examine the impact of CO2 on mass transfer in the system, the reaction conditions were replicated using nitrogen (N2) gas instead of CO2 for both mixed and unmixed systems. Compared to N2 controls, CO2 significantly increased reaction yields in both the mixed and unmixed cases (Figure 1) regardless of TG loading. Com-
Figure 1. Product yield distribution for nitrogen and CO2 mediated reactions at varying mixing rates. Reaction conditions: 9.65 MPa, 95 °C, for 2 h with 200 mg Nafion beads and 5 mL methanol.
pressed CO2 increases the diffusivity and decreases the liquid viscosity of both liquid phases20 subsequently allowing for lower resistances to external and internal mass transfer (i.e., greater contact between the substrate and catalyst)20 enhancing reaction rate beyond those observed at elevated pressure alone. Differences in contact between substrate and catalyst were tested by varying the substrate loading (10 and 100 mg of TG) at excess catalyst concentrations. The N2 control results at the lower TG loading indicate that without CO2, the FAME yield is almost negligible (Figure 1). Operating with higher TG loadings lowers the catalyst to substrate ratio creating a competition for active sites that lowers FAME yields and increases the residual intermediate (DG and MG) content. In the CO2 system, the reverse phenomenon occursat low TG loadings the FAME yield is >90% with insignificant fractions of reaction intermediates. Experiments at higher TG loadings reveal a slight decrease in FAME yield and increased DG and MG content, but this affect was mitigated in the presence of CO2 compared to the N2 system. Thus, the improved mass transport resulting from CO2 improves the interaction between catalyst and substrates (TG and reaction intermediates) by facilitating substrate and catalyst contact. In the low TG loading case, the higher catalyst to substrate ratio coupled with this improved mass transfer allows for the more complete reaction with undetectable intermediate products. Note that the low substrate to catalyst ratios are initially mass transport limited (see Kinetic Rate Constants section below) and are also unrealistic on a larger scale, thus a TG loading of 100 mg was used for further testing. Kinetics of control reactions using N2 over 4 h (SI, Figure S3) demonstrate that the initial reaction rate is greater for the mixed versus unmixed case indicating that contact at the catalyst surface can impact the reaction yield. The reaction yields stagnate after 1 h with final FAME reaction yields
