Parameters Governing Ruthenium Sawhorse-Based Decarboxylation

Article pubs.acs.org/IECR

Parameters Governing Ruthenium Sawhorse-Based Decarboxylation of Oleic Acid Kenneth M. Doll,*,† Grigor B. Bantchev,† Erin L. Walter,† Rex E. Murray,† Michael Appell,‡ James C. Lansing,†,§ and Bryan R. Moser† †

ARS, National Center for Agricultural Utilization Research, Bio-Oils and ‡Mycotoxin Prevention and Applied Microbiology Research Units, U.S. Department of Agriculture, 1815 N. University St., Peoria, Illinois 61604, United States § Oak Ridge Institute for Science and Education, U.S. Department of Energy, 1299 Bethel Valley Road, Oak Ridge, Tennessee 37830, United States S Supporting Information *

ABSTRACT: Ruthenium-catalyzed decarboxylation of 9-cisoctadecenoic is a path to produce biobased olefins. Here, a mechanistic study of this reaction was undertaken utilizing a closed reaction system and a pressure reactor. The proposed mechanism of an isomerization followed by a decarboxylation reaction was consistent with a mathematical kinetic model. That same model was able to accurately predict CO2 evolution. Additionally, computational chemistry was used to determine that the barrier of the oleic acid decarboxylation reaction is 249 kJ mol−1. Using the new information, the efficacy of the decarboxylation reaction was improved to an overall catalytic efficiency of 850 total turnovers.

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INTRODUCTION The development of industrially useful polymers from nonpetroleum sources is a topic that continues to attract interest.1−3 The conversion of biobased components into materials that are identical to those already used by industry, a drop-in replacements strategy, may lead to viable biorefinery platforms.4 One target is deoxygenation of lipids to form unsaturated hydrocarbons, where many different methodologies are reported,5−23 including the deoxygenation of oleic acid with platinum, palladium, or activated carbon in tandem with active porous supports.24−26 Results vary among the systems, where one of the best ones reports 250 catalytic turnovers per mole of RhCl3.21 However, some require enzymes or co-reagents, such as peroxydisulfate22,23 or carboxylic anhydride,7,8 to perform the conversion. One older reference even requires lead.27 Perhaps the most promising technology28−30 utilizes a ruthenium compound to catalytically decarboxylate a substrate without the need for co-reagents. Triruthenium dodecacarbonyl [Ru(CO)12] was shown to react with carboxylic acids at elevated temperatures to form a variety of species. In the presence of excess carboxylic acid, a more stable dinuclear ruthenium species referred to as a sawhorse is formed.31,32 Initial decarboxylation studies28,29 reported decarboxylation of 10-undecenoic, 9-cis-octadecenoic (oleic), and cinnamic acids using catalytic amounts of these ruthenium sawhorse complexes. Specifically of industrial interest for the manufacture of fuels, polymers, and lubricants are unsaturated linear fatty acids such as oleic acid. The effectiveness was reported to be a consequence of a tandem mode reaction (Scheme 1), where decarboxylation of oleic acid was preceded by isomerization to more reactive positional isomers, which © 2017 American Chemical Society

Scheme 1. Tandem Isomerization Decarboxylation of 9-cisOctadecenoic Acid

after decarboxylation, underwent further isomerization. The overall process yielded an apparent equilibrium mixture of heptadecene isomers that were previously characterized and evaluated as a potential biofuel.28 What is needed is a systematic approach to monitor these reactions and to elucidate parameters to increase yield and total turnovers. The key to improving decarboxylation of oleic acid Received: Revised: Accepted: Published: 864

November 23, 2016 January 5, 2017 January 11, 2017 January 11, 2017 DOI: 10.1021/acs.iecr.6b04555 Ind. Eng. Chem. Res. 2017, 56, 864−871

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Industrial & Engineering Chemistry Research Scheme 2. Synthesis of Ruthenium Sawhorse Compoundsa

a

Carboxylic acids employed here include propanoic acid and 2-ethylhexanoic acid.

the following equation: [2.00/(2.00 + x)] × 100. Terminal olefin yield was determined by difference (100 − internal olefins = terminal olefins). The ratio of internal to terminal olefins was calculated by dividing the percentage of internal olefins by the percentage of terminal olefins. Purification of product mixtures was via silica gel chromatography (70−230 mesh; 1:1 ethyl acetate/hexane) and removal of solvent by rotary evaporation and drying under high vacuum. Decarboxylations. Decarboxylation reactions were performed in a 25 mL high pressure steel reactor (Parr, Moline, IL, 5500 mini and 4836 controller). Inside an inert atmosphere glovebox, ∼9 g of oleic acid was combined with the catalyst, (1−7 mg catalyst (g oleic acid)−1. The mixture was stirred and heated to ∼90 °C while still in the glovebox to produce a uniform yellow solution. The solution was transferred to the argon purged reactor under positive argon flow. The reactor was sealed, then purged three additional times by pressurizing to ∼345 kPa (50 lbs in.−2). The initial reaction pressure was set to 350 kPa of argon pressure and the stirring and heating started. Pressure was monitored by a Dell Optiplex 745 computer running the Parr communication program, version 1.0.0.9. The reaction was run the allotted time, cooled, and vented, and the reaction progress was measured by analysis of the reaction solution. Sawhorse Synthesis. The synthesis of sawhorse compounds dates to the 1960s (Scheme 2).33 As well as in the work described in the Introduction,29,30 combinations of Ru3(CO)12, the sawhorse compounds, and appropriate ligands have demonstrated efficacy for isomerization34−39 and transvinylation40 reactions. Starting from Ru3(CO)12, the synthesis of ruthenium(I) dicarbonyl propionate [Ru(CO)2(CH3CH2CH2COO)]n was performed according to literature methods,33 and the synthesis of the ruthenium(I) dicarbonyl 2-ethylhexanoate precatalyst, [Ru(CO)2(CH3(CH2)3C(CH2CH3)HCOO)]n, was performed analogously with toluene as the solvent. The IR spectra, Nicolet (Madison, WI) Nexus 470 spectrophotometer, of the products were as expected, with appropriate peaks for the carbonyl and carboxylic groups, and confirm complete reaction of Ru3(CO)12. Gas Chromatography. In the chromatographic analysis of the reactions, two different Agilent 7890 (Santa Clara, CA) gas chromatographs were employed. For product identification, a 5975 mass spectrometry detector was used, whereas flame

reaction was the utilization of a pressurized reactor. Because carbon dioxide produced during the course of reaction is captured in that approach, pressure generation can be used to estimate reaction progress, and the data can be fit to a putative mechanism. Additionally, the temperature can be varied without artifacts that convolute the results such as the vaporization of the product. Finally, a gaseous reagent, carbon monoxide, can be added at the start of the reaction to presumably stabilize the ruthenium catalyst. Using these new techniques, this more rational approach led to a nearly 3-fold increase in total turnover number calculated on a per ruthenium basis relative to the 288 turnovers Ru1− demonstrated in prior work.

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MATERIALS AND METHODS Reagents. Triruthenium dodecacarbonyl [Ru3(CO)12; 99%, Acros, Pittsburgh, PA, or 99%, Strem Chemical, Newburyport, MA], triosmium dodecacarbonyl [Os3(CO)12; 99%, Strem Chemical, Newburyport, MA], acetatodicarbonyl ruthenium polymer ([Ru(CO)2(CH3COO)]n; Strem Chemical, Newburyport, MA), 2-ethylhexanoic acid (99%, Sigma-Aldrich, St. Louis, MO), propanoic acid (99%, Sigma-Aldrich, St. Louis, MO), acetone (99%, Fisher, Fairlawn, NJ), toluene (99.8%, Fisher, Fairlawn, NJ), diethyl ether (99%, Sigma-Aldrich, St. Louis, MO), pentadecane (99%, Sigma-Aldrich, St. Louis, MO), pentadecene (97%, Alfa Aesar, Fairlawn, NJ), argon (prepurified industrial grade, Ilmo, Jacksonville, IL), and methyl 10undecenoate (M10U, 96%, Sigma-Aldrich, St. Louis, MO) were used as received. Methyl 9-decenoate (M9D) was prepared synthetically via esterification of 9-decenoic acid (>95%; SigmaAldrich, St. Louis, MO) with methanol using catalytic sulfuric acid at reflux for 4 h. Isomerization of Methyl 9-Decenoate and Methyl 10Undecenoate. There was significant overlap in the GC-MS chromatograms of the internal isomers on M9D and M10U. Therefore, the percentages of internal and terminal olefins were determined via 1H NMR spectroscopy (500 MHz) by measuring the ratio of internal vinylic protons (2H, ∼5.4−5.5 ppm) to the internal vinyl proton of the terminal olefin (1H, ∼5.8−5.9 ppm). Integration values were normalized to yield a value of 2.00 for the peak corresponding to the internal vinylic protons. The integration value for the internal olefin peak at 5.8−5.9 ppm was then multiplied by 2, hereafter referred to as x. The percentage of internal olefins was then determined by 865

DOI: 10.1021/acs.iecr.6b04555 Ind. Eng. Chem. Res. 2017, 56, 864−871

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Industrial & Engineering Chemistry Research Scheme 3. Outline of the Mechanism Used for Fitting the Data of CO2 Release vs Reaction Timea.

a Number of the reaction in the list is used as a number of the reaction rate constant in the fitting algorithm and all six curves of CO2 pressure vs time were fitted simultaneously. Each curve was obtained at similar experimental conditions, with a varying initial amount of catalyst. Ruthenium-catalyzed tandem isomerization−decarboxylation of oleic acid; in all the equations below, the Ru ligands are omitted for simplicity.

Table 1. Isomerization of M9D and M10U by Ru3(CO)12 or [Ru(CO)2(CH3COO)]n (0.4 mol % Ru) in 24 h substrate

catalyst

temperature (°C)

internal (%)

terminal (%)

internal:terminal

M9D M9D M9D M9D M9D M9D M9D M9D M10U M10U M10U M10U

Ru3(CO)12 Ru3(CO)12 Ru3(CO)12 Ru3(CO)12 [Ru(CO)2(CH3COO)]n [Ru(CO)2(CH3COO)]n [Ru(CO)2(CH3COO)]n [Ru(CO)2(CH3COO)]n Ru3(CO)12 Ru3(CO)12 [Ru(CO)2(CH3COO)]n [Ru(CO)2(CH3COO)]n

100 150 200 250 100 150 200 250 200 250 200 250

97.2 97.0 98.5 98.5 98.3 99.3 97.1 97.7 98.4 98.2 98.2 98.2

2.8 3.0 1.5 1.5 1.7 0.7 2.9 2.3 1.6 1.8 1.8 1.8

34.7:1 32.3:1 65.7:1 65.7:1 57.8:1 141.9:1 33.5:1 42.5:1 61.5:1 54.6:1 54.6:1 54.6:1

320 μm, 0.25 μm film thickness) samples of ∼10 μL diluted in 1 mL of acetone and 1 μL were injected by autosampler using a 50:1 split ratio; the temperature program started at 40 °C for 3 min, then was raised 10 °C min−1 to 190 °C and held for 5 min, then raised 25 °C min−1 to 340 °C.

ionization detection (FID) was used for quantitation. The GCFID was calibrated to account for the differences in alkene vs fatty acid response factors. Identical columns, sample preparation, and temperature/injection programs were used for each instrument: Agilent/J&W DB35-MS column (30 m × 866


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Industrial & Engineering Chemistry Research Computational Chemistry. The B3LYP calculations on the reaction coordinate of the decarboxylation of the unsaturated fatty acid derivatives were performed using the 631+G** basis set as implemented in the Spartan’16 software package v.1.0.0 (Irvine, CA). Calculated free energy values were obtained in fully geometry optimized structures based on vibrational frequency calculations at 298.15 K using default settings and scales. The initial structures were built using Hyperchem v8.0.10 (Gainesville, FL) and data was translated using OpenBabel v 2.3.0. Modeling Using Ordinary Differential Equations. The chemical kinetic of the system can be described by a simple stepwise mechanism (Scheme 3), which can be fitted by a series of ordinary differential equations. A detailed discussion on the methods used for the kinetic modeling is discussed in the Supporting Information.41−45

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RESULTS AND DISCUSSION Isomerization. Isomerization reactions were performed on M9D and M10U, both containing terminal alkenes, in order to evaluate the first step of the mechanistic proposal (isomerization). Both Ru3(CO)12 and the acetate version of the sawhorse complex, [Ru(CO)2(CH3COO)]n, were tested. The ratio of terminal to internal alkenes was determined by NMR spectroscopy (Table 1), and the amount of internal olefin was 97% or greater under all of the reaction conditions employed, despite the initial substrate consisting of entirely terminal olefin. The theoretical yield of each isomer assuming an equal distribution of olefins is 12.5% and 11.1% for M9D (eight olefin isomers) and M10U (nine isomers), respectively. However, terminal olefins should be less energetically favorable than internal olefins due to lower alkyl substitution as was confirmed in the current study. This supports a mechanism with initial isomerization as a potential first step. These results indicate that stabilization of the catalyst to form sawhorse complexes was not necessary for isomerization to proceed to an equilibrium mixture, as seen by comparison of Ru3(CO)12 to ruthenium(I) dicarbonyl acetate (Table 1). For example, Ru3(CO)12catalyzed isomerization of M10U at 250 °C yielded a ratio of internal to terminal olefins of 54.6:1, which was identical to ruthenium(I) dicarbonyl acetate-catalyzed isomerization at similar conditions. Decarboxylation. Decarboxylations were run in a pressurized reactor at oxygen-free, inert conditions. GC-MS analysis was used to identify the products, and GC-FID analysis was used for quantification due to its more consistent response factor. The light yellow product solution was shown by GC-MS analysis to be the expected alkene products. Reactions were performed at reactions times varying from 4 to 140 h, all of which showed the expected alkene reaction products. The product distribution was similar to that observed in a paper where the product was used in fuel applications.28 It is a mixture of heptadecene isomers where all 15 positional and conformational isomers were found. The reactions reported here, except where noted, were ran for 22 h, which was a convenient time to assess decarboxylation progress since at that point they were substantially complete as evidenced by pressure monitoring. Specific effects of temperature, catalyst type, and catalyst concentration are discussed below. Temperature Effect. Reaction temperatures between 200 and 300 °C were employed. As anticipated, conversions performed using the sawhorse catalyst were higher at higher temperatures (Figure 1). However, chromatography indicated

Figure 1. Decarboxylation of 9-cis-octadecenoic acid, Ru3(CO)12, ∼3 mg (g substrate)−1, in 22 h at various temperatures. The GC-FID was calibrated to account for the differences in alkene vs fatty acid response factors. Experiments were performed up to three times, and the standard deviation of the experiments were calculated. The reported error bars are observed standard deviation of the experiment with the most experimental repetitions.

the presence of significant amounts of oligomeric side products when the reaction was conducted at 300 °C. It appears that 250 °C represented the best trade-off in reaction time versus obtaining product that had not undergone polymerization. Catalyst Type, Amount, and Recyclability. Different precatalyst loadings, from 0.5 to 5 mg precatalyst per g substrate, were placed in the reactor, and the results were as anticipated (Figure 2). At low levels, increases in catalyst

Figure 2. Decarboxylation of 9-cis-octadecenoic acid, 250 °C in 22 h, with various concentrations of Ru3(CO)12 precatalyst. The GC-FID was calibrated to account for the differences in alkene vs fatty acid response factors. Experiments were performed up to three times, and the standard deviation of the experiments were calculated. The reported error bars are observed standard deviation of the experiment with the most experimental repetitions. 867


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Industrial & Engineering Chemistry Research amounts gave more reaction, although the effect became less pronounced at the higher levels. Further increases did not result in further increases in reactivity. Observations of the reaction in similar experiments done in glassware indicate that the solution is not homogeneous when higher amounts of precatalyst are added. In a previous report,29 a catalytic system where the sawhorse was synthesized deliberately instead of in situ, [Ru(CO)2(CH3CH2COO)]n, was also shown to isomerize M10U, methyl oleate and 7-trans-tetradecene. It was interesting to see if this sawhorse, or other precatalysts of similar structure, could also be used for decarboxylation of oleic acid. The acetate version of the material, [Ru(CO)2(CH3COO)]n, is commercially available and yields results that are within experimental error (Figure 2) to those obtained using the propanoic analogue, [Ru(CO)2(CH3CH2COO)]n. However, when a larger carboxylic acid, 2-ethylhexenoic acid, is used, the resulting precatalyst [Ru(CO) 2 (CH 3 CH 2 CH 2 CH 2 CH(CH2CH3)COO)]n provided even higher conversion. This may be due to increased solubility of the active catalyst species, but that hypothesis has not been confirmed. Comparison of these results to those obtained utilizing catalytic Ru3(CO)12 revealed that higher conversions are achieved with ruthenium(I) dicarbonyl carboxylates, thereby indicating that catalyst stabilization via sawhorse formation improves efficacy of decarboxylation. The data also suggest the ruthenium(I) dicarbonyl carboxylate catalysts share a common catalyst motif. In unreported results, molybdenum carbonyl and manganese carbonyl were tested under similar conditions and showed no reaction. Triosmium dodecacarbonyl and the chloro (1,5cyclooctadiene) iridium(I) dimer were also tested. They did show a reaction but at a rate of ∼1/2 to 1/3 of that of the sawhorse complex. Attempts to reuse spent catalyst solution were also undertaken. Used catalyst solutions were replaced in the reactor, and it was charged with additional substrate. The reactions were performed under the same reaction conditions, but no significant reaction occurred demonstrating complete catalyst deactivation. Visual observation of the catalyst solution shows it to be significantly darker than productive reaction solutions indicating the probability of metal agglomeration. Kinetic Modeling. The kinetic model (Scheme 3) suggests that there will be an induction period in the reaction, which was demonstrated as an S-shape in the plot. This is a consequence of the time for equilibration of the double bond position (Figure 3) and was supported by the experimental data on

isomerization (Table 1). The model was designed to include the positional isomerization using two equal rate constants, k+1 and k−1. This modeling resulted in an S-shaped concentration curve that was approximated by the step function (Figure 4) and by setting the isomerization equal to 6.5/k+1.

Figure 4. Formation of 2-octadecenoic acid-modeled (solid line) and approximated (dashed line) relative concentrations. The two curves have the same area beneath them. The jump of the approximation line occurs at approximately 6.5 units.

The number of steps and the time to fit the data with 49 sets of rate constants (Table S1), and the concentration profiles as a function of reaction times, revealed that the concentrations of all the compounds changed initially from fast to slower toward the end of the simulation. We see that the RK5 algorithm, on average, failed in this respect, due to the stiffness of the problems shown by an increase in the average number of steps with the reaction time. The semi-implicit methods successfully decreased the numbers of steps with the increase in the reaction time. The number of the aborted fits due to insufficiently large decreases in the time step was high for RK5. The SI-Euler showed better overall performance even though it was slower through each step. It was more successful in adjusting the numbers of the calculation steps and therefore used less overall computations. However, it was not as precise, and its solution set did not match exactly the solution by other methods. The semi-implicit RKR methods showed the best performance they calculated all the test cases, gave very close answers, and used a small number of steps for the calculation. Among the RKR methods, the differences were small. In the current test, the RKR3-M was the fastest by a small margin. The 13 successfully solved cases from utilization of RK5 were separated into “stiff” and “non-stiff” catagories (Table S2). If the number of steps for the last interval was less than the one for the first interval, the case was classified as “stiff” otherwise as “non-stiff”. We see that the algorithm was efficient for nonstiff casesit required a smaller number of steps, which combined with the small computational time per step resulted in much faster overall calculations. On the other hand, only a small number of stiff cases were successfully solved, and they required a large number of steps and computational time. We observed that the optimization procedure stopped at several very different combinations of reaction rates, and the calculated CO2 profiles were identical (Figure 5 and Table 2). Reaction Coordinate. The reaction coordinate for the spontaneous decarboxylation of the unsaturated fatty acids was modeled using the B3LYP density functional theory methods at the 6-31+G** level of theory. Medium and long chain fatty acid calculations possess shallow wells associated with numerous unrestricted torsions along the single bond chain; therefore,

Figure 3. Illustration of the time distribution of the positional isomers of oleic acid with time. 868


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Figure 5. Experimental data for decarboxylation with different amounts of Ru catalyst, CO2 released (dashed lines), and best fit curves (solid lines) with different sets of rate constants (Table 2). The six sets of best fit curves practically overlap, despite the large difference in the reaction rates used to describe them.

Figure 6. Calculated reaction coordinate of an α−β unsaturated carboxylic acid which uncatalyzed proceeds through a transition state with a barrier of 249 kJ.

experiments were carried out on α−β unsaturated butanoic acid in order to appropriately calculate the fatty acid energy levels. The noncatalyzed decarboxylation proceeds through a transition state that is 249.0 kJ mol−1 higher in free energy (Figure 6). The products are energetically favored by 51.8 kJ mol−1. Theoretical investigation of the ruthenium sawhorse-type transition state complexes revealed multiple complexes that require further investigation that are beyond the scope of this experimental manuscript. In all, the spontaneous decarboxylation of the unsaturated fatty acids must go through a significant reaction barrier, and the ruthenium catalyst plays a noteworthy role to enable this chemistry accessible for a commercial process. Carbon Monoxide Addition. The conversion of the precatalyst Ru3(CO)12 to the active catalyst species involves dissociation of two carbon monoxides per ruthenium, so it was of interest to determine if added carbon monoxide would reduce catalyst efficacy. At the start of the reaction, 345 kPa (∼50 lbs in.−2) of pressurized carbon monoxide was added to the reactor. Although this procedure convoluted the pressure monitoring data, an increase in pressure could still be observed. It was approximately the same amount as seen in the reaction without added carbon monoxide. Different reaction times were employed in these trials, and the results showed no substantial effect on these reactions. The gas composition of the reactions at the end of the reaction were analyzed on an Agilent 490 micro GC and were as expected; i.e., a low level of carbon monoxide was evident in reactions where none was added, much higher levels when it was. The amount of carbon dioxide was dominant in the unadulterated reactions, whereas those with added carbon monoxide showed a more even mixture of

the two gases. Therefore, addition of carbon monoxide to the reaction medium did not appear to retard catalyst efficacy. Other Reactions and Highest Total Turnover Amount. Addition of high boiling solvents was tested for their effect on this reaction. The added solvent has the advantages of keeping the media consistent, possibly improving catalyst stability and allowing more consistent temperature control during the reaction. In other words, when the substrate is used as the solvent, it is transformed from a carboxylic acid to an alkene and undergoes changes in boiling point and solvency characteristics. The fully saturated pentadecane, which does not have functional groups to interact with the catalytic species, and the unsaturated analog, 1-pentadecene, were added to the reaction medium in separate experiments. The results appear to indicate that alkanes gave a positive effect on the reaction and alkenes had the opposite effect, reducing the conversion. This could be due to competition between 1-pentadecene and oleic acid for catalyst sites. In other words, the catalyst may be performing nonproductive equilibrium isomerization of the solvent instead of productive reaction turnover. This was confirmed by the observation of many isomers of pentadecene in the reaction solution. It also points to the potential advantage of an industrial reaction process where the biobased olefin product is removed in a reactive distillation to mitigate competition. For demonstration purposes, an experiment designed to give a record number of total turnovers was set up. A low catalyst loading was used, 0.5 mg (g substrate)−1, such that a high number of turnovers was possible. The reaction was allowed to proceed for ∼137 h, significantly past the point of further

Table 2. Example of 6 Out of the 40 Best Fits, Illustrating the Degeneracy of the Problem: Sets with Very Different Values for the Rate Constants Yield Close Fitting Curves fit

A

B

C

D

E

F

sum of errors k2, L mol−1 s−1 k3, s−1 k4, s−1 k5, s−1 k6, L mol−1 s−1 k7, s−1

47.85581 0.051 311 17 21 47 1.18 × 10−5

47.8746 0.0032 0.39 1.6 1.8 3.9 1.22 × 10−5

48.50757 0.65 907 3.1 5.7 8.9 1.92 × 10−5

48.31415 0.0028 0.00034 2.7 5.1 14.5 5.85 × 10−6

47.90779 0.0028 70 1397 19 47 9.09 × 10−6

48.0369 0.0029 5.7 × 10−6 2.05 × 106 0.10 0.28 8.82 × 10−6

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New Syntheses with Oils and Fats as Renewable Raw Materials for the Chemical Industry. Angew. Chem., Int. Ed. 2000, 39, 2206−2224. (3) Bozell, J. J.; Moens, L.; Elliott, D. C.; Wang, Y.; Neuenscwander, G. G.; Fitzpatrick, S. W.; Bilski, R. J.; Jarnefeld, J. L. Production of levulinic acid and use as a platform chemical for derived products. Resour. Conserv. Recycl. 2000, 28, 227−239. (4) Yao, K.; Tang, C. Controlled Polymerization of Next-Generation Renewable Monomers and Beyond. Macromolecules 2013, 46, 1689− 1712. (5) Popov, S.; Kumar, S. Rapid hydrothermal deoxygenation of oleic acid over activated carbon in a continuous flow process. Energy Fuels 2015, 29, 3377−3384. (6) Dawes, G. J. S.; Scott, E. L.; Le Nôtre, J.; Sanders, J. P. M.; Bitter, J. H. Deoxygenation of biobased molecules by decarboxylation and decarbonylation - A review on the role of heterogeneous, homogeneous and bio-catalysis. Green Chem. 2015, 17, 3231−3250. (7) Miller, J. A.; Nelson, J. A.; Byrne, M. Process of Making Olefins. U.S. Patent 5077447, December 31, 1991. (8) Miller, J. A.; Nelson, J. A.; Byrne, M. P. A highly catalytic and selective conversion of carboxylic acids to 1-alkenes of one less carbon atom. J. Org. Chem. 1993, 58, 18−20. (9) Kraus, G. A. Method for Producing Olefins. U.S. Patent 8629312, April 25, 2013. (10) Kraus, G. A.; Riley, S. A Large-Scale Synthesis of alpha-Olefins and alpha-omega-Dienes. Synthesis 2012, 44, 3003−3005. (11) Stern, R.; Hillion, G. Process for Manufacturing a Linear Olefin from a Saturated Fatty Acid or Fatty Acid Ester. U.S. Patent 4554397, November 19, 1985. (12) Goossen, L. J.; Linder, C.; Rodriguez, N.; Lange, P. P.; Fromm, A. Silver-catalysed protodecarboxylation of carboxylic acids. Chem. Commun. 2009, 7173−7175. (13) Goossen, L. J.; Rodriguez, N. A mild and efficient protocol for the conversion of carboxylic acids to olefins by a catalytic decarbonylative elimination reaction. Chem. Commun. 2004, 724−725. (14) Tanaka, S.; Shimizu, K.; Yamamoto, I. Syntheis of 1-nonene form decanoic acid by polymer-bound palladium complexes. Chem. Lett. 1997, 26, 1277−1278. (15) Murzin, D. Y.; Kubickova, I.; Snare, M.; Maki-Arvela, P.; Myllyoja, J. Method for the Manufacture of Hydrocarbons. U.S. Patent 7491858, February 17, 2009. (16) Snare, M.; Kubickova, I.; Maki-Arvela, P.; Chichova, D.; Eranen, K.; Murzin, D. Y. Catalytic deoxygenation of unsaturated renewable feedstocks for production of diesel fuel hydrocarbons. Fuel 2008, 87, 933−945. (17) Snåre, M.; Kubičková, I.; Mäki-Arvela, P.; Eränen, K.; Murzin, D. Y. Heterogeneous Catalytic Deoxygenation of Stearic Acid for Production of Biodiesel. Ind. Eng. Chem. Res. 2006, 45, 5708−5715. (18) Maki-Arvela, P.; Kubickova, I.; Snare, M.; Eranen, K.; Murzin, D. Y. Catalytic Deoxygenation of Fatty Acids and Their Derivatives. Energy Fuels 2007, 21, 30−41. (19) Liu, Y.; Kim, K. E.; Herbert, M. B.; Fedorov, A.; Grubbs, R. H.; Stoltz, B. M. Palladium-Catalyzed Decarbonylative Dehydration of Fatty Acids for the Production of Linear Alpha Olefins. Adv. Synth. Catal. 2014, 356, 130−136. (20) Miranda, M. O.; Pietrangelo, A.; Hillmyer, M. A.; Tolman, W. B. Catalytic decarbonylation of biomass-derived carboxylic acids as efficient route to commodity monomers. Green Chem. 2012, 14, 490−494. (21) Foglia, T. A.; Barr, P. A. Decarbonylation dehydration of fatty acids to alkenes in the presence of transition metal complexes. J. Am. Oil Chem. Soc. 1976, 53, 737−741. (22) van der Klis, F.; Le Notre, J.; Blaauw, R.; van Haveren, J.; van Es, D. S. Renewable linear alpha olefins by selective ethenolysis of decarboxylated unsaturated fatty acids. Eur. J. Lipid Sci. Technol. 2012, 114, 911−918. (23) van der Klis, F.; van den Hoorn, M. H.; Blaauw, R.; van Haveren, J.; van Es, D. S. Oxidative decarboxylation of unsaturated fatty acids. Eur. J. Lipid Sci. Technol. 2011, 113, 562−571.

detectable carbon dioxide evolution. The results were 480 total turnovers, which was followed by a reaction under these conditions with a high initial carbon monoxide pressure of 1870 kPa (270 lbs in.−2). With added carbon monoxide, catalytic decarboxylation of oleic acid achieved 850 total turnovers. This result confirmed the earlier experiment which demonstrated that carbon monoxide addition is not an inhibitor of the reaction. In fact, it may stabilize the active species, thus giving longer catalyst lifetime. A potential method to reactivate spent catalyst using carbon monoxide pressure is also suggested by the high turnover result.

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CONCLUSIONS The decarboxylation of oleic acid, a reaction which computational chemistry determines to have a barrier of 249 kJ mol−1, was found to run efficiently at 250 °C with a catalyst concentration of 5 mg (g substrate)−1 using several different ruthenium sawhorse catalysts. This reaction was studied, and a mechanism was postulated. Isomerization is the first step in the proposed model, which was demonstrated on a substrate analog which cannot undergo decarboxylation. Using this data, a mathematical model was proposed and shown to accurately model the carbon dioxide evolution of the reaction performed in a closed system. The decarboxylation reaction total turnover number was increased from 288 Ru1−, in previous reports, to a new value of 850.

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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.6b04555. Discussion of the method used in the kinetic modeling. Table S1: Performance of each of the methods on 49 test combinations. Table S2: Performance of the RK5 method. (PDF) Output using kinetic modeling equations. (XLSX)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 309-681-6103. Fax: 309-681-6524. E-mail: Kenneth. [email protected]. ORCID

Kenneth M. Doll: 0000-0002-5328-7848 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Benetria N. Banks for technical assistance. This work was a part of the in-house research of the Agricultural Research Service of the United States Department of Agriculture. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.

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Parameters Governing Ruthenium Sawhorse-Based Decarboxylation

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