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Carbon dioxide mediated transesterification of mixed triacylglyceride substrates Lindsay Soh, Mary Kate Mitchell Lane, Junwei Xiang, Thomas Alan Kwan, and Julie B. Zimmerman Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02207 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018
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Carbon dioxide mediated transesterification of mixed triacylglyceride substrates Lindsay Soha,*, Mary Kate M. Laneb, Junwei Xianga, Thomas A. Kwanb, Julie B. Zimmermanb,c,* a
Chemical and Biomolecular Engineering, Lafayette College, 740 High St, Easton PA, 18042
b
Chemical and Environmental Engineering, Yale University, 17 Hillhouse Ave, New Haven,
CT 06511 c
School of Forestry and Environmental Studies, Yale University, 195 Prospect Street, New
Haven, CT, 06511
ABSTRACT Conversion of oil feedstocks to fatty acid alkyl esters is a necessary step to produce biodiesel and other biobased products. These conversion processes must be robust for a number of different feedstocks containing varied oil profiles and contaminants. Furthermore, the final products may have specific compositional requirements to provide desirable properties for a particular application. In this work, conversion processes are used to modify fatty acid methyl ester (FAME) profiles from triacylglyceride (TG) feedstocks. A system using an acid heterogeneous catalyst with carbon dioxide as a reaction co-solvent is used to convert mixed TG feedstocks to FAME while illustrating resilience to reaction contaminants, such as water and free fatty acids (FFA). The presence of water does not show negative impacts on the reaction at loadings of up to 15 wt% (mass water/mass TG). Likewise, the presence of FFA does not negatively impact the reaction and in some cases improves TG conversion in the mixed system. Single FFA and TG reaction kinetics are evaluated across a range of alkyl chain lengths and degrees of unsaturation.
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FFA show faster kinetics compared to TG while both feedstocks’ conversions vary based on alkyl chain degree of unsaturation. Conversion of commercial vegetable oil feedstocks is evaluated illustrating that certain FAME formation is favored based on chain length and relative abundance. Overall a FAME yield of 86% from palm oil was obtained in 4 h of reaction with methyl palmitate and methyl oleate yields dominating the produced FAME (87% and 95% yields, respectively). This selective conversion may be used to improve FAME product properties for select applications and should be considered in process design. INTRODUCTION Among renewable energy sources, biomass is unique in the fact that it directly stores energy in chemical bonds. These bonds serve as a storage mechanism for energy and have been exploited for the formation of biofuels and chemicals.1 One set of molecules that are of recent interest are triacylglycerides (TG) whose fatty acids can be converted to fatty acid methyl esters (FAME) and used as biodiesel or other products such as solvents, lubricants, and surfactant feedstocks.1,2 TG fatty acids come in a mixture of different chain lengths and degrees of unsaturation, which may vary in type and concentration based on the oil feedstock. The identity of these chains can greatly alter the properties of the resulting FAME mixture. For instance, biodiesel must provide an energy dense fuel with high cetane number and oxidative stability, low viscosity and melting point.3 Customization of specific FAME can help to make a high functioning fuel but the properties must be balanced due to contrasting requirements – for example, high cetane numbers can be attained with longer chain FAME with increased saturation while low cloud points require shorter chain lengths or unsaturated FAME.3,4 Guidelines have been made that recommend certain chain lengths be used for biodiesel production; for example, Moser and Vaughn recommend a mix with saturated 62%, trienoic 90%) could be attained at 180°C using AlCl3 as a lewis acid catalyst.30 These studies determined that CO2 plays an essential role in allowing for high conversions at reduced temperatures and pressures in supercritical methanol conversions. In the current study, a multi-phase CO2-mediated methanol transesterification is evaluated at moderate pressures and temperatures. The work presented here systematically surveys the reaction yields in a CO2-expanded-methanol and heterogeneous acid catalyst (Nafion®) system with TG feedstocks of varying chain length and degree of unsaturation and, in some cases, with contaminants, such as water and fatty acids. Structural differences significantly affect the substrate properties and in turn impact the system reactivity. Therefore, mixed substrate systems are also tested to explore the possibility of
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enrichment for FAME.
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Reaction kinetics are explored as a means to gain a fundamental
understanding of the system selectivity based on fatty acid chains as applied to the conversion of oil feedstocks. EXPERIMENTAL SECTION Materials All TG and FFA feedstocks as well as fatty acid methyl ester standards (with the exceptions noted below) were purchased from Nu-Check Prep, Inc (Elysian, MN) and were of >99% purity. Triolein was purchased from Sigma-Aldrich® (>99% purity). Methyl laurate and palm oil, coconut oil, and canola oil analytical standards were purchased from Sigma Aldrich. The FAME mixtures (Sigma-Aldrich) came with corresponding certificates of analysis that included the fatty acid composition by weight percent in each oil. Methanol, pyridine, and N-Methyl-N(trimethylsilyl)trifluoroacetamide (MSTFA) were purchased from Sigma Aldrich. Isopropanol was purchased from Acros Organics (> 99.5% purity). Bone-dry carbon dioxide with a siphon tube was obtained from Airgas, Inc. The commercially available catalyst, Nafion NR50,® was purchased in bead-form from Ion Power, Inc. and stored in a desiccator. Reactor and Reaction Conditions As the work was performed as a collaboration between different institutions, multiple different reactors of different scales (25, 50, and 100 mL) were used. In order to ensure comparable results, calibration was done on each reactor to achieve equivalent or improved reaction conditions as compared to our previous work (Supporting Information (SI), Figures S1 and S2).24,25 Using multiple reactors illustrates the reaction can be repeated between different setups
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and scales, indicating the results are reproducible and transferrable. All reactions were conducted at 9.65 MPa and 95°C (unless otherwise noted) with appropriate catalyst loading and mixing as determined by the reactor size and in proportion to previous results.24 This temperature and pressure reflects the conditions of highest triolein conversion from our previous work and also represents moderate reaction temperatures compared to other transesterifications. At these conditions, carbon dioxide is above its critical point but the mixture (with methanol and feedstock) exists as a 3-phase system with each liquid phase expanded by CO2.24,25 As noted in previous work, the excess methanol used in the reaction is used to create favorable phase conditions in the reactor as well as provide a relatively constant methanol concentration. Single TG experiments were performed using the reactor and setup as described previously (50 mL).24 TG contamination experiments (with water and FFA) were performed in a 100 mL, stainless steel HPR-Series Reactor (Supercritical Fluid Technologies, Inc) stirred at 500 RPM. Multiple TG and oil feedstock experiments were performed in a 25 mL stainless steel reactor vessel (Parr Instrument Co.) equipped with a stainless steel stirrer (300 RPM) and in situ temperature probe. Feedstock, catalyst, and methanol were added directly to the vessel which was then sealed, heated to the reaction temperature, and pressurized with carbon dioxide via a two-way valve. At the end of the reaction time, the vessel was depressurized by venting carbon dioxide through a two-way valve and sparging through a collection solvent. The remaining products in the vessel were added to the solvent for sample analysis. Chemical Analysis Fatty acid methyl esters: FAME were analyzed using liquid or gas chromatography by methods detailed previously.24,32
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Free fatty acids: A Waters Acquity Ultra Performance C2 system) equipped with a mass spectrometer (Waters Xevo TQD) was used to quantify reaction FFA content. Supercritical carbon dioxide with a modifier of 90:10 v/v acetonitrile: methanol solution was used as the mobile phase at a column temperature of 45 °C. The back pressure was kept at 1500 psi. The modifier was increased from 15% to 40% over 3.5 min and held isocratically for 1 min before returning to initial conditions. The mass spectrometer was run with a desolvation temperature of 600°C and gas flow rate of 1000 L/h (N2). Compounds were analyzed using APCI in positive mode with cone voltages varying from 28-50 V and collision energies varying from 18-42 V. Oils and intermediates: FAME from palm, coconut, and canola oil was analyzed using a gas chromatography-flame ionization detector (GC-FID) HP 6890 equipped with an Agilent Select Biodiesel column (15 m, 0.32 mm, 0.10 µm). Products from the tripalmitin, tristearin, and triarachidin mixture were analyzed using an SP-2560 Capillary Column (100m, 0.25mm, 0.20 µm) as previously reported.18 For oils products, the GC inlet temperature was set to 250 °C with an 80:1 split, helium carrier gas at 4 mL/min, the initial oven temperature was set to 40 °C and held for 5 min, ramped up to 140°C at 3°C per min, then to 147°C at 0.5°C per min, then to 230°C at 10°C per min, then to 395°C at 30°C per min and held for 10 min. Silylation using NMethyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) was performed before analyzing in the GC-FID to enhance elution properties of remaining unreacted triacylglycerides, diacylglycerides, and monoacylglycerides. Reaction yield: The yield of FAME was calculated based on the mass of the FAME produced divided by the theoretical mass of FAME that could be produced by a given feedstock. For TG and FFA experiments, the theoretical mass was calculated based on the mass of the starting material and reaction stoichiometry. For the oil feedstocks, the theoretical mass was determined
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based on the expected FAME masses from the certificate of analysis provided with each oil sample.
RESULTS AND DISCUSSION TG conversion in this CO2-mediated system was systematically evaluated with a variety of model and commercial oil feedstocks to determine the impact of composition and contamination on yield. Single TG feedstocks were first used to ascertain the yield of FAME and kinetic differences of varied chain length and degree of unsaturation. Next, the impact of oil contaminants, water and FFA, on yield were evaluated. FFA and TG mixtures with varied alkyl chain lengths and degrees of unsaturation were also used to evaluate their role in the reaction. Finally, vegetable oil feedstocks were evaluated to determine the yield and kinetics of actual mixed TG systems. Triacylglyceride conversion of varying chain lengths and degrees of unsaturation Conversion: Triacylglyceride feedstocks comprise a variety of alkyl groups with varying chain lengths and degrees of unsaturation, which lead to differing thermophysical properties such as polarity. As TG conversion in the mixed CO2-methanol and catalyst system depends on substrate interaction with the methanol and catalyst, these property differences could significantly impact yields within the multi-phase system.24,25 As the CO2 allows for better interaction between each of the these phases, a large increase in yield compared to a control experiment with no CO2 is seen (SI, Figure S3),25 a phenomenon which is dependent on the CO2 solubility of individual TG. It was found that TG chain length significantly impacts the conversion to FAME for each individual chain TG feedstock tested (Figure 1). Namely, shorter
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chain lengths displayed higher yield over the 1 h time period tested. The same trend generally holds at a lower temperature of 80 ºC, however yields at 95 ºC were much higher than the lower temperature. FAME yields at 110ºC (not shown) resulted in a decreased final yield compared to the 95 ºC trials.
While chain length had a significant impact on the reaction yields, the effect of degree of unsaturation is less clear (SI, Figure S4). For alkyl chains with 20 carbons (C20), increasing the degree of unsaturation did increase the yield while the position of the double bond for C20:3 did not seem to affect the yield. For the C18 family, however, C18:2 had lower yield than C18:1 and C18:3, which had comparable yields. This trend was also observed for the same experimental conditions after 4 h. As conversion in this system is a result of complex interactions – phase behavior, expanded liquid phase solubility, and substrate-catalyst interactions – the relative overall FAME yield and kinetics for different alkyl chains can vary depending on competing phenomena. As chain length decreases and degree of unsaturation increases, the polarity of the substrate increases which allows for greater interaction with the methanol for reaction leading to faster reaction kinetics. Reaction kinetics: The difference in the conversion of individual TG at a fixed time point (e.g., 1 h in Figure 1) could be attributed to either a difference in reaction equilibrium or reaction kinetics for each TG substrate. In order to determine if the reactions are kinetically limited, the reaction kinetics were explored for a range of TG chain lengths and degrees of unsaturation (Figure 2). While the shorter chain lengths had faster initial kinetics, the overall yields after 4 h did not vary greatly indicating that the yield differences in Figure 1 may be due to a difference in reaction kinetics.
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Previous literature has studied the effect of chain length on fatty acid conversion (carboxylic acid) via esterification33 showing that for a heterogeneous acid catalyzed reaction, chain length significantly affected reaction rate; namely shorter chain length TG had faster reaction kinetics. Therefore, it is reasonable to expect that for a fixed reaction time of 1 h in this system, TG with shorter chain lengths would react faster and thus have a greater yield. There are several possible causes for this chain length effect – Liu et al. postulated that these yield differences could be due to an inductive effect or steric hindrance for bulkier molecules,33 but this difference could also be due to the different structural properties of the TG attributing to differential interactions of the TG with the CO2-expanded-methanol or catalyst surface. The role of the catalyst in this complex system has been previously characterized using triolein as a feedstock.25 The acidic catalyst is essential to the TG reaction progress with negligible FAME production observed in its absence. Due to the interaction of CO2 and the fluoropolymer, Nafion® undergoes significant expansion in the presence of CO2. This expansion leads to an increase in the catalyst bead’s surface area and acid site availability, which changes the effective system catalyst concentration. The greater site availability leads to a rate change as observed in Figure 2 at the 1 h timepoint for all TG, corresponding with previous results.25 Triacylglyceride properties: In order to better understand the properties that may affect the reaction yield, structural properties of the TG feedstocks were compared (Table 1). Molecular Modeling Pro34 was used to estimate van Krevelen35 solubility parameters for the range of TG feedstocks tested (Table 1). The specific parameters of the pure substances represent the overall solubility parameter (δ) as well as the contributions from dispersion forces (δd), dipole interactions (δp), and hydrogen bonding (δh). The van Krevelen estimate for molar volume (V) is also provided. Statistical analysis using linear regression of reaction yield based on structural
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parameters indicates that the δp, δh, and molecular volume (V) have a statistically significant impact on the reaction yield (p < 0.05) with R2 values provided in Table 1. These property differences indicate that high substrate polarity improves reaction yields and could be due to greater substrate-methanol interaction. The lower molecular volumes also have a significant correlation and could indicate that smaller substrates have less stearic hindrance for reaction with the Nafion® catalyst. These physical parameters also have an impact on thermophysical properties of the molecules; for instance, longer chain length and saturated TG have higher melting points and viscosity.36 These properties could have an impact on the interactions of the TG within the multiphase system as well as with other TG or contaminants in the system. Effect of potential feedstock contaminants: Water and Free Fatty Acids Water: The impact of water contamination on triolein conversion to methyl oleate was tested after 4 h of reaction to evaluate the impact on overall yield. A range of moisture contents were added to the triolein feedstock at concentrations ranging from 1 wt% to 20 wt%. The presence of water does not exhibit a detrimental effect on the yield of methyl oleate in the system up to 15 wt% (g water/g triolein) (Figure 3). Typical base catalyst transesterification may be hindered with water contents less than 1 wt%.37 In acidic systems water may also lead to partial TG hydrolysis to FFA with subsequent conversion to FAME.38 Here, the slight decrease in yield may be due to impacts of the water on the catalyst such as decreases in swelling, lowered acid site activity, and/or deactivation due to hydrolysis.39 Previous studies have indicated that water may poison the heterogenous catalyst by blocking access to acid sites27 and is particularly salient for porous catalysts40 unlike Nafion®.
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Pure free fatty acid conversion: Free fatty acid content may range from
