Bio-Based Methacrylic Acid via Selective Catalytic ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Biobased Methacrylic Acid via Selective Catalytic Decarboxylation of Itaconic Acid James C. Lansing,†,‡ Rex E. Murray,† and Bryan R. Moser*,† †

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Bio-Oils Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, 1815 North University Street, Peoria, Illinois 61604, United States ‡ Oak Ridge Institute for Science and Education, United States Department of Energy, 1299 Bethel Valley Road, Oak Ridge, Tennessee 37830, United States S Supporting Information *

ABSTRACT: We report a biobased route to methacrylic acid via selective decarboxylation of itaconic acid utilizing catalytic ruthenium carbonyl propionate in an aqueous solvent system. High selectivity (>90%) was achieved at low catalyst loading (0.1 mol %) with high substrate concentration (5.5 M) at low temperature (200−225 °C) and pressure (≤425 psig) relative to previous contributions in this area. Direct decarboxylation of itaconic acid was achieved as opposed to the conjugate base reported previously, thereby avoiding basification and acidification steps. Also investigated was catalytic manganese(II) oxalate (5 mol %), but low yield (4.8%) and evolution of carbon monoxide via oxalate decomposition was problematic. Attempts at stabilization of the catalyst with triphenylphosphine were unsuccessful, but it exhibited greater catalytic efficacy (14.0% yield) than the manganese catalyst (4.8% yield) at 5 mol %. Neither carbon monoxide nor propylene (excessive decarboxylation) were detected during ruthenium-catalyzed decarboxylation. In addition, cosolvents such as tetraglyme lowered vapor pressures within the reaction vessel by >100 psig while minimizing decomposition of starting acids. In combination, these findings represent improvements over existing methodologies that may facilitate sustainable production of methacrylic acid, an important petrochemically based monomer for the plastics industry. KEYWORDS: Decarboxylation, Itaconic acid, Methacrylic acid, Ruthenium, Tetraglyme

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numerous industrially significant plastics.8 The principal application of MMA is homopolymerization to provide the light-weight, shatter-resistant, thermoplastic poly(methyl methacrylate) (Plexiglas) (PMMA) as a versatile alternative to glass. MMA is also an essential component of copolymers found in surface coatings, paints, adhesives, and emulsion polymers.8−10 The most significant petrochemical route to MAA/MMA is the acetone cyanohydrin process (ACH) depicted in Figure 1. Acetone is initially reacted with hydrogen cyanide to form an acetone-cyanohydrin intermediate, which is then converted to methacrylamide sulfate upon treatment with stoichiometric sulfuric acid at 140 °C. Production of MAA or MMA is achieved by reaction of the sulfate with water or anhydrous

INTRODUCTION

Renewable materials are increasingly preferred over crude petroleum oil and its refinery products as feedstocks for industrial chemicals.1 Various environmental, social, economic, legislative, and geopolitical incentives motivate the commercial transition to biobased chemicals. Renewable alternatives are either direct drop-in replacements or structurally different but with similar properties and performance. Direct drop-in replacements are preferred, as industry is already familiar with and has accepted their structure, properties, and performance. Examples include biobased ethylene and butadiene from ethanol; ethanol and succinic acid from glucose; propylene, propylene glycol, syngas, and epichlorohydrin from glycerol; diesel and jet-fuel-range hydrocarbons from plant oils; and aromatics from depolymerization of lignin.2−7 Methacrylic acid (2-methylpropenoic acid, MAA) and methyl methacrylate (MMA) are important commodity monomers for © 2017 American Chemical Society

Received: December 1, 2016 Revised: February 15, 2017 Published: February 23, 2017 3132

DOI: 10.1021/acssuschemeng.6b02926 ACS Sustainable Chem. Eng. 2017, 5, 3132−3140

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ACS Sustainable Chemistry & Engineering

Figure 1. Production of methacrylic acid from acetone by the conventional petrochemical acetone-cyanohydrin route.

Figure 2. Biobased route to methacrylic acid from itaconic acid. Note that mesaconic and citramalic acids are intermediates along the way to methacrylic acid. Also note that 2-hydroxyisobutyric acid is the hydrated form of methacrylic acid.

methanol, respectively.11 Along with production of MAA/ MMA, an excess of ammonium bisulfate is obtained in a molar ratio of 1.5:1.8,9 Disadvantages of the ACH process include utilization of nonrenewable materials and stoichiometric amounts of harmful, toxic, and corrosive reagents; generation of toxic intermediates; and production of low-value ammonium bisulfate, which must be further reacted with ammonia to yield ammonium sulfate for use as a fertilizer. Additional petrochemical routes to MAA/MMA are reviewed elsewhere.8,9 Relatively few biobased routes to MAA are reported. One such process is dehydration and decarboxylation of citramalic (2-hydroxy-2-methylbutanedioc) acid at elevated temperatures (250−400 °C) and pressures (450−4000 psi; 31−276 bar) in the presence of catalytic sodium hydroxide (NaOH).12 Application of this methodology to maleic (2-Z-butenedioc) acid yields acrylic (2-propenoic) acid (AA).12 The Alpha Process relies on carbon monoxide, methanol, and ethylene to yield MMA following a two-stage sequence.13,14 Another methodology entails dehydration and decarboxylation of citric

acid to itaconic (2-methylidenebutanedioc) acid (IA), followed by a second decarboxylation to afford MAA in the presence of stoichiometric bases under near-critical and supercritical water conditions.15 However, accumulation of byproducts such as 2hydroxybutyric and crotonic [(E)-2-butenoic] acids (CA) at the expense of MAA was problematic. In addition to low selectivity, high temperatures (>350 °C) were required. In another approach, selectivity was improved to over 90%, but high temperatures (245−270 °C) and pressures (450−3000 psi; 31−207 bar) were needed; stoichiometric bases were used, and byproducts such as CA, 2-hydroxybutyric acid, and propylene were observed.16 Propylene arises via decarboxylation of MAA and/or CA, thereby reducing the yield of the intended product. A possible bio-based route to MAA from IA is shown in Figure 2. More recently, either IA or citric acid was selectively decarboxylated using catalytic palladium, platinum, and ruthenium.17 Higher selectivity was achieved (up to 84%), and lower reaction temperatures (200−250 °C) and pressures 3133


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ACS Sustainable Chemistry & Engineering Table 1. Comparison of Existing Biobased Routes to MAA with the Current Study feedstock reactor catalysts [catalyst] (mol %) [starting acid] temp (°C) pressure (psi) time solvent reaction mode yield of MAA selectivitya propylene detected?b

ref 17

ref 16

ref 15

current study

monosodium salt of IA high water diluent low [IA] heterogeneous Pt, Pd, and Ru 2.5 0.15 M 200−250 551 1h supercritical water batch (unvented) up to 51% (isolated) up to 84% yes

monosodium salt of IA high water diluent low [IA] group I and II bases 30−100 0.3−3.0 255−265 450−3000 150 s supercritical water continuous (vented) up to 63% (HPLC) up to 99% not determined

citric acid or IA high water diluent low [IA] NaOH 0−50 0.01−0.50 230−400 4000−5000 1−250 s supercritical water continuous (vented) up to 75% (HPLC) up to 100% yes

IA low water diluent high [IA] Mn(II) oxalate [Ru(CO)2(CH3CH2COO)]n 0.1−10 1.0−10.0 190−250 1−425 1−3 h subcritical water batch (unvented) up to 40% (HPLC) up to 95% no

Selectivity is defined as [MAA/(MAA + crotonic acid) × 100]. bPropylene is indicative of over-reaction (double decarboxylation of IA or triple for citric) and is therefore undesirable. a

Figure 3. Production of methacrylic acid through decarboxylation of monosodium itaconate, the conjugate base of itaconic acid. In this example, the sodium salt is depicted, but others such as potassium, ammonium, and others may be utilized.

(550 psi; 38 bar) were utilized relative to previous contributions (Table 1). However, similar to previous reports, stoichiometric bases such as NaOH were utilized during the reaction. In such embodiments, NaOH reacts with the starting acid to form a conjugate base, which is then decarboxylated to sodium methacrylate. The rationale behind formation of monosodium salts is that the rate of decarboxylation at 280− 330 °C and 4000 psi (276 bar) is enhanced relative to the free acid and the disodium salt.18 However, subsequent acidification of the sodium salt is required to generate MAA, thereby adding basification and acidification steps to the process (Figure 3). In essence, these approaches convert the conjugate base of IA to sodium methacrylate, rather than IA directly to MAA. Consequently, a sustainable technology is needed that avoids basification and acidification steps through direct decarboxylation of the starting acid to MAA. The objective of our study was to develop a sustainable route to MAA that is free from the technical difficulties discussed above. Presented in Table 1 is a comparison of existing biobased routes to the present work. IA was chosen as the starting material since it is derived from dehydration and decarboxylation of citric acid or from fermentation of simple sugars such as glucose (Figure 4).19−21 Specific objectives included identifying catalysts that are efficient at direct decarboxylation of IA to yield MAA, increasing throughput by increasing the concentration of IA, and lowering reaction parameters such as temperature and pressure. In combination, these changes would represent techno-economic improvements over existing methodologies, thus yielding a sustainable route to MAA that is more amenable to commercialization.

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Figure 4. Biobased methacrylic acid from simple sugars and citric acid via an itaconic acid intermediate. dihydrate (MnC2O4·2H2O; 99%) were purchased from Alfa Aesar (Ward Hill, MA). CCA (98%), MAA (99%), MSA (98%), triphenylphosphine (PPh3; 99%), 4-methoxyphenol (MeHQ; 99%), and trifluoroacetic acid (CF3CO2H, TFA, 99%) were obtained from Sigma-Aldrich Corp. (St. Louis, MO). Ruthenium(I) dicarbonyl propionate was prepared according to literature precedent.22 All other materials were obtained from Sigma-Aldrich Corp. and used as received. Catalysts and IA were stored prior to use in a nitrogen filled Innovative Technologies (Amesbury, MA) model IL-2GB inert atmosphere glovebox with oxygen and water concentrations kept below 1 ppm. Catalytic Decarboxylation. Decarboxylations were performed using a Parr Instrument Company (Moline, IL) 50 mL stainless steel (T316) reactor with a maximum allowable working pressure of 3000 psi (207 bar) at 350 °C. The reactor was lined with a glass liner and equipped with an internal thermocouple, cooling loop, variable speed overhead magnetic stirrer with a standard shaft and impeller, heater assembly, and a Span (Waukesha, WI) overhead pressure gauge rated up to 3000 psi (207 bar). Internal reaction temperature and stirring rate (rpm) were controlled by a Parr model 4848 reactor controller unit, which also provided internal vessel pressure information. In a typical experiment, IA (10.0 g; 76.9 mmol; 5.5 M in water), manganese(II) oxalate (5 mol %), or ruthenium dicarbonyl propionate

EXPERIMENTAL SECTION

Materials. Water for decarboxylation reactions and as a component of the mobile phase for HPLC was ultrapure (18 MΩ), obtained from a Barnstead (Lake Balboa, CA) Easy Pure II RF/UV ultrapure water system. Prior to decarboxylation, ultrapure water was degassed by a nitrogen sparge. CA (98%), IA (99%), and manganese(II) oxalate 3134


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Figure 5. Comparison of pressure (psig) as a function of temperature (Celsius) at constant volume among water, a 1:1 (mole ratio) mixture of water and tetraglyme, and tetraglyme, demonstrating that vapor pressure can be decreased with prudent choice of an appropriate low-vapor-pressure organic cosolvent. [(Ru(CO)2(CH3CH2COO)]n (0.1 mol %) and MEHQ (5000 ppm) were weighed in the inert atmosphere glovebox and placed in a glass liner. A rubber septum was affixed to the glass liner, which was then removed from the glovebox. Ultrapure and degassed water (14.0 mL) was added to the closed system using a syringe, and the septum was removed before the liner was introduced into the reaction vessel. After the reactor was closed, a headspace pressure of 400 ± 5 psig N2 was applied and the mixture stirred (500 rpm) and heated to the desired reaction temperature. Typically, heating to the reaction temperature was achieved in 20 min. The reaction time started when the desired temperature was reached. Parameters such as time and temperature were varied to determine the optimum conditions. In some instances, triphenylphosphine (PPh3) (0.5−5 mol %) was added to the reaction mixture. After the allocated reaction time, the reactor was equilibrated to room temperature (1−1.5 h), and the gas pressure was released. MAA was isolated via azeotropic distillation, and yield was reported as the percentage recovered of theoretical (6.63 g). High-Performance Liquid Chromatography. Samples (50 μL) collected in pairs were diluted to a volume of 1.5 mL of 1:1 v/v methanol/water and analyzed in duplicate by HPLC. Levulinic acid (1.0 M) was added to all samples as an internal standard. Analyses were performed using an Agilent (Santa Clara, CA) 1100 series degasser, pump, and autosampler connected to a spectrophotometric ultraviolet diode array detector and a Sedere (Alfortville, France) Sedex 85 LT-ELSD in series. Samples (5.0 μL) were injected onto a GL Sciences (Tokyo, Japan) Inertsil ODS-3 column (5.0 μm; 4.6 mm × 250 mm). The mobile phase consisted of a mixture of methanol/ water with a flow rate of 1.0 mL/min. A gradient method was employed with the following conditions: 85% (v/v) water initially, changing to 82% by 10 min, 78% at 11 min, 75% at 12 min, 80% at 49 min, 83% at 51 min, 85% at 53 min, and hold at 85% until 55 min. The water phase was spiked with 0.25% v/v TFA. Compounds were identified by retention time comparison to reference standards using both ultraviolet detection at a wavelength of 254 nm and the signal from the ELSD. Individual compounds were quantified by external calibration with pure standards. Approximate retention times follow: levulinic acid (6.6 min), citraconic acid (9.4 min), itaconic acid (10.0 min), mesaconic acid (12.0 min), methacrylic acid (19.6 min), and trans-crotonic acid (16.0 min). Micro-Gas-Chromatography. Gas analysis was performed in triplicate using a model 490 Micro GC by Agilent operating at a sample line temperature of 30 °C with helium as the carrier gas. Gas samples were collected from the Parr reactor using a 3 oz pressure reaction bottle from Lab Crest-Andrews Glass (Vineland, NJ) and infused at 10−20 psi (0.7−1.4 bar) into the instrument through a

model 170 Lab Series membrane separator by Genie Filters (A+ Corporation, Gonzalez, LA). Analyses were isothermal with a duration of 3 min. Channel 1 (Cox 1 m) was used for characterization, and its temperature and operating pressure were 80 °C and 29 psi (2 bar). Compounds were identified by retention time comparison to reference standards, and approximate retention times were 0.37 min (nitrogen), 0.47 min (carbon monoxide), 2.2 min (carbon dioxide), and 1.0 min for hydrocarbons such as methane, ethane, propane, and butane. Spectroscopy. 1H nuclear magnetic resonance (NMR) spectra were collected at 26.9 °C using a Bruker Avance-500 spectrometer (Billerica, MA) operating at 500 MHz using a 5 mm BBO probe. Chemical shifts (δ) are reported as parts per million (ppm) from tetramethylsilane in CDCl3 (Cambridge Isotope Laboratories, Andover, MA). Vapor Pressure of Solvent Systems. In a typical experiment, the solvent (14 mL) of interest (water; 1:1 mol ratio of tetraglyme to water; and tetraglyme) was introduced into the reactor with a syringe. No other reactants or catalysts were added. The Parr reactor was pressurized with nitrogen to ensure no leaks were present. The extra pressure was then relieved such that the initial pressure of the reactor was 0 psig. The mixture was then stirred (500 rpm) and heated (20 min) to the desired temperature. The recorded pressure (psig) was measured after equilibration (40 min) at the desired temperature. Pressure was measured at 100, 150, 175, 200, 212, 225, and 250 °C (Figure 5).

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RESULTS AND DISCUSSION

Catalyst Screening. Numerous metal catalysts were screened for production of MAA, with manganese(II) oxalate and ruthenium(I) dicarbonyl propionate identified as the most efficient at selective decarboxylation of a 1.0 M solution of IA at 250 °C for 3 h. Other catalysts screened included simple metal salts of chromium, cobalt, copper, iron, manganese, molybdenum, nickel, and zinc, but they provided lower yields of MAA (data provided in Table S1) and were thus not explored further. Optimization Using Manganese(II) Oxalate. Once the most effective catalyst was identified, we modified reaction conditions in an effort to maximize yield. Lower reaction temperatures, pressures, and catalyst loadings were of interest, along with short reaction times and enhanced selectivity. Another objective was to increase the concentration of IA for process economic and engineering reasons. A significant consequence of low starting material concentration is that the 3135


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Table 2. Results of Catalytic Decarboxylation of 5.5 M IA with Mn(II) Oxalate and Triphenylphosphine (PPh3) with 400 ± 5 psig N2 Headspace Pressure entry time (h) temp (°C) 1 2 3 4 5 6 7 8

1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

225 200 212 225 250 212 225 225

ΔP (psig)

Mn(II) oxalate (mol %)

PPh3 (mol %)

CA (area %)

MAAa (isolated)

IA + CCA + MCAb (area %)

Selc

pH2Od (psig)

69 67 135 250 342 196 326 282

0 5.0 5.0 5.0 5.0 5.0 5.0 0

0 0 0 0 0 5.0 5.0 5.0

0.8 0.8 0.8 0.7 0.9 2.1 2.4 2.9

2.6 1.5 2.4 4.8 5.7 5.3 13.0 14.0

69.5 56.4 58.5 41.4 6.2 64.6 33.4 19.4

76.5 65.2 75.0 87.2 86.4 71.6 84.4 82.8

312 191 244 312 499 243 311 311

a

Yield of MAA is reported as percent of theoretical after distillation. bIA = itaconic acid; CCA = citraconic acid; MSA = mesaconic acid. Values were determined by HPLC. cSelectivity is [MAA/(MAA + CA) × 100]. dPartial vapor pressure of water (pH2O, psig) was calculated from mole fraction of water and vapor pressure data from ref 26.

Figure 6. Micro-GC chromatogram of samples of headspace gas after decarboxylation of IA using 5.0 mol % manganese(II) oxalate (a) with and (b) without 5.0 mol % PPh3 showing evolution of both carbon monoxide (0.47 min) and carbon dioxide (2.2 min). Chromatogram c depicts the results of an experiment conducted in the presence of excess oxalic acid. The signal at 0.38 min corresponds to nitrogen.

that MAA in the vapor phase might condense in the upper reaches of the reactor and undergo polymerization. As seen in Table 2, optimization reactions conducted at lower temperatures (200−225 °C) and with shorter reaction times (1.5−2.0 h) than during catalyst screening failed to give MAA in excess of 7.2%. In addition, carbon monoxide was detected among the headspace gases (Figure 6a), along with nitrogen and carbon dioxide. Carbon dioxide is the expected gaseous coproduct from decarboxylation of IA. We suspected that carbon monoxide arose from decarbonylation of oxalic acid

majority of the reaction vessel is occupied by water, thereby resulting in high internal pressure due to the high vapor pressure of water (see Figure S1). Table 2 depicts initial attempts at optimization using manganese(II) oxalate at 5.0 mol %, which was half of the amount used during screening studies. In addition, the concentration of IA was increased to 5.5 M. A headspace pressure of 400 psig nitrogen (at room temperature) was added in an effort to keep MAA in the liquid phase. We hypothesized 3136


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Figure 7. Representative micro-GC chromatogram of a sample of headspace gas collected after decarboxylation of IA at 200 °C for 2.0 h utilizing 0.1 mol % [Ru(CO)2(CH3CH2COO)]n. Signals corresponding to nitrogen (N2) and carbon dioxide (CO2) are at 0.38 and 2.2 min, respectively. If present, propylene would appear at 1.0 min.

7) at 225 °C than without added PPh3 but with otherwise similar reaction conditions (entry 4). Lastly, PPh3 alone (entry 8) demonstrated greater efficacy at decarboxylation of IA than did manganese(II) oxalate (entry 4) at similar reaction conditions (225 °C and 1.5 h), as indicated by higher ΔP (282 versus 221 psig) and yield of MAA (14.0 versus 7.0%). The control experiment conducted in the absence of catalyst at 225 °C for 1.5 h (entry 1) gave MAA in 2.6% yield, which was lower than catalyzed reactions conducted utilizing otherwise similar conditions (entries 4, 5, 7, and 8). However, suppression of carbon monoxide was unsuccessful with PPh3 (Figure 6b). Lower partial vapor pressures (pH2O; 191−499 psig) were achieved under the conditions described in Table 2 than during catalyst screening experiments due to lower temperatures investigated as well as higher concentrations of IA in water. For comparison, a pH2O of 540 psig was calculated according to Raoult’s law with a reaction temperature of 250 °C and a concentration of IA of 1.0 M (Table S1). Increasing the concentration of IA to 5.0 M with otherwise similar conditions resulted in a pH2O of 499 psig (entry 5, Table 2). Further reductions in reaction temperature afforded progressively lower pH2O at constant IA concentration (Table 2). Optimization with Ruthenium Catalysis. Due to the relatively low yield of MAA given by manganese(II) oxalate along with evolution of carbon monoxide, ruthenium catalysis was explored. Ruthenium carbonyl carboxylates were of interest because of our previous work on tandem isomerization− decarboxylation of oleic acid.23−25 When using catalytic ruthenium(I) dicarbonyl propionate in the present system, IA was first isomerized to mesaconic acid, which was in turn decarboxylated to MAA (Figure 2). Isomerization was confirmed by a control experiment subjecting dibutyl itaconate

via dissociation of manganese(II) oxalate under our conditions, thereby leading to decarbonylation of oxalic acid to carbon monoxide and other species. This was confirmed by conducting an experiment in the presence of 10 equiv of oxalic acid for every equivalent of manganese(II) oxalate with otherwise similar conditions. The headspace gases from this experiment displayed a spike in carbon monoxide (Figure 6c), thus indicating that carbon monoxide was evolving from decomposition of oxalic acid. Reaction temperature affected ΔP and yield of MAA. As seen in Table 2, ΔP and yield of MAA increased with increasing temperature at constant reaction time (1.5 h) and with longer reaction times at constant temperature (200 °C). For example, entries 2−5 represent decarboxylations conducted at progressively higher temperatures (200−250 °C) and otherwise similar reaction conditions. Both ΔP and yield of MAA increased as the temperature was increased from 200 to 250 °C. In an effort to stabilize manganese(II) oxalate, an equimolar amount of PPh3 (5.0 mol %) was added to the reaction mixture (entries 6−8, Table 2). The rationale behind addition of phosphines originated with Crooks et al., who noted that ruthenium carbonyl carboxylate polymers form more stable ruthenium dimers capped by phosphines upon addition of tributylphospshine and pyridine.22 He proposed that these dimers were the active catalyst species in the decarboxylation of organic acids. We hypothesized that phosphines would form stable dimeric species with manganese upon addition to our system. Addition of PPh3 enhanced the yield of MAA. For instance, comparison of decarboxylation at 212 °C for 1.5 h with (entry 6) and without (entry 3) PPh3 (5.0 mol %) in the presence of manganese(II) oxalate (5.0 mol %) revealed higher ΔP (196 versus 135 psig) and yield of MAA (5.3 versus 2.4%) with PPh3. Furthermore, higher ΔP (326 versus 221 psig) and yield of MAA (13.0 versus 7.0%) were noted with PPh3 (entry 3137


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ACS Sustainable Chemistry & Engineering Table 3. Results of Catalytic Decarboxylation of 5.5 M IA with 0.1 mol % [Ru(CO)2(CH3CH2COO)]na entry

time (h)

temp (°C)

ΔP (psig)

mol % PPh3

CA (area %)

MAAb (isolated)

IA + CCA + MCAc (area %)

seld

pH2Oe (psig)

1 2 3 4 5 6 7 8 9 10

0.5 1.0 2.0 1.5 1.5 1.5 1.5 1.5 1.0 1.5

212 212 212 200 212 225 200 212 225 225

75 128 201 120 184 277 116 206 251 289

0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.5 0.5 0.5

0 0 0 0 0 0 0 0 0 0

8.2 13.6 13.7 8.6 17.0 14.9 14.9 21.6 25.4 33.8

92.5 57.8 27.7 74.9 34.4 3.8 46.0 25.2 12.3 6.2

100 100 100 100 100 100 100 100 100 100

245 245 245 191 245 313 190 245 313 313

All headspace pressures are 400 ± 5 psi. bYield of MAA is reported as percent of theoretical after distillation. cIA = itaconic acid; CCA = citraconic acid; MSA = mesaconic acid. Values were determined by HPLC. dSelectivity was calculated according to the same method as in Table 2. ePartial vapor pressure of water was calculated according to the same method in Table 2. a

increase, reaching a maximum of 33.8% at 225 °C and 1.5 h (entry 10). This compares favorably to decarboxylations performed in the absence of PPh3, where a yield of only 14.9% was observed at otherwise similar conditions (entry 6). The addition of PPh3 had a positive effect on the isolated yield in these reactions, and it is possible that other additives could further increase the isolated yield. Last, as seen in Figure 7, carbon monoxide was not observed with ruthenium catalysis, thereby indicating that carbon monoxide was from oxalate decomposition. Comparison of manganese-catalyzed (Table 2) to ruthenium-catalyzed decarboxylations revealed several important differences. Ruthenium-catalyzed reactions provided MAA in higher yield (33.8% versus 14.0%) with lower catalyst concentration (0.1 versus 5.0 mol %), as seen by comparison of entry 10 in Table 3 to entry 10 in Table 2. Both rutheniumand manganese-catalyzed reactions were facilitated by addition of PPh3. However, the percentage of PPh3 (relative to IA) was an order of magnitude lower for ruthenium-catalyzed reactions. A further demonstration of the efficacy of ruthenium relative to that of manganese is a comparison of experiments conducted at similar reaction temperature and time. For instance, manganese-catalyzed decarboxylations conducted at 200 °C for 1.5 h (entry 3, Table 2) compared to the corresponding ruthenium experiment (entry 4, Table 3) revealed a 6-fold difference in yield of MAA (8.6 versus 1.5%) despite the lower catalyst loading for ruthenium. Lowering Vapor Pressure with Cosolvents. Lower pressures are desirable from engineering and economic standpoints due to safety and cost considerations. Lower pressure is achieved primarily by lower reaction temperatures yielding lower pressures according to Gay−Lussac’s law and/or by replacement of a fraction of water with an appropriate cosolvent possessing lower vapor pressure according to Raoult’s law. Replacement of a portion of the solvent (water) with tetraglyme to form a mixture (1:1 mol ratio) of water and tetraglyme facilitated considerably lower vapor pressures relative to water at constant volume, as seen in Figure 5. Tetraglyme was chosen because it is chemically inert to our process and has a lower vapor pressure than water. Note that the data collected for Figure 5 was conducted in the absence of IA and catalyst. Application of a tetraglyme/water solvent system (1:1 mol ratio) to decarboxylation of IA in the presence of catalytic ruthenium carbonyl propionate yielded a considerably lower ΔP (133 versus 277 psig) while reducing the yield of MAA.

to the reaction conditions, thereby inhibiting decarboxylation via protection of the carboxylic acid moieties as butyl esters. When the reaction was performed at 225 °C for only 15 min, dibutyl mesaconate was present in 44% with dibutyl itaconate constituting most of the remaining product (see Figures S2 and S3). As expected, a negligible amount of citraconic dibutyl ester was detected, as it is the cis isomer of mesaconate and is therefore less thermodynamically favored. Isomerization was anticipated because extensive isomerization of double bond location and geometry was observed in our previous studies using oleic acid as the substrate.23−25 Under the same conditions with manganese(II) oxalate, less than 5% isomerization to mesaconic dibutyl ester was observed. Catalytic ruthenium on carbon (Ru/C) was previously investigated for decarboxylation of IA, but the authors concluded that the catalyst was inferior to Pd/C and Pt/C due to higher propylene production via unwanted decarboxylation of MAA.17 However, we did not observe propylene during the course of our study (Figure 7). Depicted in Table 3 is catalytic decarboxylation of a 5.5 M solution of IA in water using ruthenium dicarbonyl propionate (0.1 mol % Ru) at 200−225 °C and 0.5−2.0 h with a headspace pressure of 400 psig nitrogen. Yields of MAA ranged from 8.2% to 33.8%, which were higher than those observed in Table 2 despite the lower catalyst loading, generally shorter reaction times, and lower reaction temperatures explored in Table 3. Similar to manganese-catalyzed decarboxylations, the yield of MAA increased when reaction time was increased from 0.5 to 1.0 h at constant temperature (212 °C), as seen by comparison of entries 1 and 2. However, a further increase in reaction time to 2.0 h (entry 3) at otherwise similar conditions did not provide higher yield relative to a reaction time of 1.0 h. In addition, ΔP increased with increasing reaction time at constant temperature (entries 1−3, Table 3), despite yield plateauing at 1.5 h. These results indicate that headspace pressure does not necessarily correlate with yield of MAA. Entries 7−10 (Table 3) were conducted in the presence of 0.5 mol % PPh3 in an effort to stabilize the ruthenium catalyst. A higher yield of MAA was achieved at 200 °C and 1.5 h with 0.5 mol % PPh3 added to the system (14.9%; entry 7) versus without (8.6%; entry 4) and otherwise similar conditions. In addition, a higher ΔP was observed without PPh3 addition (120 versus 116 psig) despite the lower yield, thus providing further evidence that headspace pressure does not correlate to yield of MAA. When investigated at progressively higher temperatures in the presence of PPh3 (entries 7−10), the isolated yield of MAA continued to 3138


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Table 4. Results of Catalytic Decarboxylation of 5.5 M IA Using 0.1 mol % [Ru(CO)2(CH3CH2COO)]n with Varied Solvent Compositions entry

mole ratio TG:water

mole ratio IA:water

ΔP (psi)

CA (area %)

MAAa (isolated)

IA+CCA+MCAb (area %)

selc

pH2Od (psig)

1 2 3 4 5

0:1 1:0 1:1 1:3 1:4

1:10 N/A 0.72:1 1:2 1:2.5

277 108 133 139 153

0.9 3.7 3.0 3.8 3.3

14.9 3.9 12.6 7.6 9.2

16.8 67.6 75.1 82.6 49.9

94.3 51.3 80.8 66.7 73.6

313 0 101 188 209

a

Yield of MAA is reported as percent of theoretical after distillation. bIA = itaconic acid; CCA = citraconic acid; MSA = mesaconic acid. Values were determined by HPLC. cSelectivity was calculated according to the same method as in Table 2. dPartial vapor pressure of water was calculated according to the same method in Table 2.

This is seen by comparison of the first and third entries of Table 4. While yields were typically lower in water/tetraglyme mixtures than in water alone, the water/tetraglyme mixtures preserved considerably more of the starting acids, which would facilitate recycling to produce additional MAA. Replacement of all of the water with tetraglyme (entry 2, Table 4) resulted in further yield reduction (3.9%). We speculate that this was due to anhydride formation, as anhydrides are anticipated to be less reactive toward decarboxylation than carboxylic acids. Furthermore, anhydride formation in the absence of water is probably thermodynamically favored under our conditions. For instance, the equilibrium constants determined in a previous study for diacid ↔ anhydride + water in aqueous solution at 20 °C ranged from 3.2 to 25 for dimethylmaleic acid and other maleic acid derivatives.27 As a consequence, water is a necessary component in our reaction medium, as it mitigates formation of anhydrides in situ. Finally, as anticipated from the data presented in Figure 5, replacement of water with tetraglyme afforded lower pH2O, as seen by comparison of the first entry in Table 4 (100% water; pH2O of 313 psig) with the other entries (pH2O ≤ 209 psig).

Additionally, ruthenium-catalyzed yields increased further with the addition of PPh3, and could potentially be further enhanced with the appropriate additive. Furthermore, we achieved direct decarboxylation of IA as opposed to its conjugate base, thereby avoiding basification and acidification steps. We also report that utilization of cosolvents such as tetraglyme lowers the vapor pressure of water within the reaction vessel by >100 psig while minimizing decomposition of starting acids. We continue to investigate additives, reaction conditions, and solvent conditions that will lead to increased yield, conversion, and selectivity, as well as stabilization of products within the reactor system once they have formed.

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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/acssuschemeng.6b02926. Table S1 depicting screening results of catalytic decarboxylation using various catalysts; vapor pressure of steam as a function of temperature, shown in Figure S1; and Figures S2 and S3 displaying GC−MS and NMR data of isomerized dibutyl itaconate (PDF)

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CONCLUSIONS Ruthenium(I) dicarbonyl propionate was identified as the most efficient catalyst at selective decarboxylation of IA to afford sustainable MAA with high selectivity. Also investigated was manganese(II) oxalate (5 mol %), which was identified as providing the highest yield of MAA after screening over 20 simple transition metal catalysts. However, accumulation of carbon monoxide during the course of manganese-catalyzed decarboxylation suggested that decomposition of oxalate under our conditions was problematic. This was confirmed with a control experiment that demonstrated elevated carbon monoxide levels when oxalic acid was added to the system. Attempts at stabilization of the catalyst with triphenylphosphine were unsuccessful, but it exhibited greater catalytic efficacy (14.0% yield) than the manganese catalyst (4.8% yield) at 5 mol % with otherwise similar reaction conditions (1.5 h and 225 °C). Evolution of carbon monoxide was not observed during ruthenium-catalyzed decarboxylation. Additionally, lower catalyst loadings (90%), and avoidance of propylene production (excessive decarboxylation) represent advances over previous contributions in this area.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 309-681-6534. Phone: 309-681-6511. E-mail: Bryan. [email protected]. ORCID

Bryan R. Moser: 0000-0002-4019-3738 Author Contributions

All authors contributed to conceptual design of experiments as well as manuscript preparation. All authors have given approval to the final version of the manuscript. Funding

This work was part of the in-house research of the Agricultural Research Service of the United States Department of Agriculture. Notes

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. The authors declare no competing financial interest. 3139


Research Article


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carboxylate complexes of ruthenium and osmium carbonyls. J. Chem. Soc. A 1969, 2761−2766. (23) Murray, R. E.; Doll, K. M.; Liu, Z. Process for isomerization and decarboxylation of unsaturated organic compounds with a metal catalyst or catalyst precursor. U.S. Patent Application 2014/0275592, September 18, 2014. (24) Murray, R. E.; Walter, E. L.; Doll, K. M. Tandem isomerizationdecarboxylation for converting alkenoic fatty acids into alkenes. ACS Catal. 2014, 4, 3517−3520. (25) Moser, B. R.; Knothe, G.; Walter, E. L.; Murray, R. E.; Dunn, R. O.; Doll, K. M. Analysis and properties of the decarboxylation products of oleic acid by catalytic triruthenium dodecacarbonyl. Energy Fuels 2016, 30, 7443−7451. (26) Perry, R. H.; Green, D. W. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill, 1997; pp 2-48−2-75. (27) Eberson, L.; Welinder, H. Studies on cyclic anhydrides. III. Equilibrium constants for the acid-anhydride equilibrium in aqueous solutions of certain vicinal diacids. J. Am. Chem. Soc. 1971, 93, 5821− 5826.

ACKNOWLEDGMENTS Benetria Banks, Daniel Knetzer, and Erin Walter are acknowledged for excellent technical assistance.

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