Synthesis of Methacrylic Acid by Catalytic ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Synthesis of Methacrylic Acid by Catalytic Decarboxylation and Dehydration of Carboxylic Acids Using a Solid Base and Subcritical Water Maryam Pirmoradi and James R. Kastner* Biochemical Engineering, College of Engineering Driftmier Engineering Center, The University of Georgia, 597 D.W. Brooks Drive, Athens, Georgia 30602, United States

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S Supporting Information *

ABSTRACT: Methacrylic acid was synthesized from the biobased substrates citric acid, itaconic acid, and 2-hydroxyisobutyric acid (2-HIBA). Hydrotalcite, a solid base catalyst, was employed to form methacrylic acid (MAA) through decarboxylation of itaconic acid and citric acid. The effect of temperature, catalyst mass, residence time, substrate concentration, and fermentation media, on carboxylic acid conversion and methacrylic acid yield was determined. Optimum MAA yields occurred at a substrate to catalyst mass ratio of 9.6 g-substrate/g-catalyst and 21% for citric acid and 6.4 g/g and 23% for itaconic acid (250 °C, 15 min). Catalyst reusability experiments resulted in higher methacrylic acid yields for both citric and itaconic acid. Methacrylic acid was also formed from 2-hydroxyisobutyric acid in a single-step dehydration reaction. Among these three substrates, the highest yield of methacrylic acid (71.5%) was achieved at 275 °C (1 min) using 2-HIBA and subcritical water. Finally, we tested conversion of these three acids in a simulated residual fermentation broth (0.1 M NaOH, 0.04 M Na2SO4, 0.04 M Na2HPO4, 0.06 M glucose, 0.12 wt % albumin) and MAA yields from itaconic acid and citric acid using hydrotalcite increased in the presence of these fermentation “impurities” and decreased slightly from 2-HIBA. KEYWORDS: Methacrylic acid, Citric acid, Itaconic acid, 2-Hydroxyisobutyric acid, Solid base, Catalyst, Hydrotalcite, Subcritical water

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INTRODUCTION

biomass, such as methacrylic acid (MAA) and methyl methacrylic acid (MMA). MAA esters form the basis of flexible but hard polymers with excellent clarity, weatherability, and UV stability.8 Methacrylic acid (MAA) is used in production of plastics, moldings, fibers, resins and other organic compounds. The major product of MAA and methyl methacrylate (MMA) is poly(methyl methacrylate), with an annual consumption of 2.1 million tons.8,9 Currently, the hazardous acetone−cyanohydrin process is the main route for producing MAA.8 Toxic starting materials, high process cost, and the large amount of bisulfate waste are problems with this process. Another industrial method to

There is an increasing need to synthesize chemicals and materials for societal needs (e.g., chemicals, solvents, pharmaceuticals, and engineered polymers) from renewable carbon sources.1 Compared to the research and commercialization efforts devoted to biofuels, comparatively less focus has been directed to the biobased synthesis of chemicals, especially industrial polymers from biomass.1 Only recently has research focused on a chemical and materials based biorefinery.2−4 This research was initiated by the identification of platform chemicals and intermediates that could be produced from biomass.5 Subsequently, work focused on metabolically engineering microbes to produce these biochemicals2 and catalytic conversion of these chemicals to drop-in ready products.6,7 One area in which there has been little research is on the synthesis of industrial important vinyl polymers from © 2016 American Chemical Society

Received: September 12, 2016 Revised: November 1, 2016 Published: December 14, 2016 1517

DOI: 10.1021/acssuschemeng.6b02201 ACS Sustainable Chem. Eng. 2017, 5, 1517−1527

Research Article

ACS Sustainable Chemistry & Engineering produce MAA involves two oxidation steps. The first step is oxidation of isobutylene to methacrolein using mixed metal oxides of Mo and Fe, with promoters such as Co, Ni, and an alkali metal.10 The second step involves oxidation of methacrolein to MAA that takes place over a phosphomolybidic catalyst containing Cu and V and alkali metal promoters. Other routes to MAA based on carbonylation of C2 compounds have also been studied.11 These processes involve propionate as an intermediate, which is condensed with formaldehyde to produce either MAA or a mixture of MAA and MMA. Synthesis of MAA by vapor-phase aldol condensation of propionic acid with formaldehyde over silica-supported metal phosphate catalysts12 and acid−base bifunctional catalysts such as MgO, SiO2, Al2O3, and ZrO2, with and without cesium10 has also been reported. Current routes to MAA and MMA use petroleum-derived feedstocks, although biobased routes are being explored. Synthesis of MAA from 2-methyl-1,3-propanediol (2M1,3PD), which is reportedly produced by glycerol fermentation or a byproduct of butanediol production, via a biocatalytic hybrid approach has been reported.9 In this process, 2M1,3PD is selectively oxidized to 3-hydroxy-2methylpropionic acid (3H2MPA) using nongrowing cells (i.e., a biotransformation) and 3H2MPA is catalytically dehydrated to MAA using TiO2. Other more direct biocatalytic routes may be possible. Considering the C3 route, a possible biobased entry point at isobutyric acid (IBA) or C4 carboxylic acids in general are indicated. IBA can be oxidatively dehydrogenated to MAA using O2 and a catalyst generating 80% yields (95% conversion, Fe−P catalysts), but requires high temperatures (300−400 °C). However, IBA and MAA (155 °C vs 162 °C) have similar boiling points which makes separation difficult. Continuing this line of inquiry, Carlsson et al. (1994) studied sequential conversion of fermentation derived citric acid to itaconic acid and then to MAA in near critical (220−370 °C) and supercritical water (375−400 °C).13 In plug flow reactors using sodium hydroxide as a homogeneous base catalyst, they reported rapid decarboxylation of itaconic acid to MAA above 350 °C, although significant byproducts formed resulting in lower yields (20−59%). Additionally, decarboxylation of citric acid to itaconic and citraconic acid with high selectivity at 250 °C was achieved (∼90% combined selectivity, 6% methacrylic acid or MAA yield at 320 °C with no reported formation of MAA below 280 °C).13 At all tested temperatures from either itaconic acid or citric acid, almost identical byproducts were obtained (acetone, acetic and pyruvic acid). Results from the effect of itaconic acid residence time on MAA yield indicated that MAA yield rapidly rises to a steady value (∼70%, 25−60 s, 10 mM NaOH, 360 °C). In another biocatalytic approach, synthesis of MAA by decarboxylation of itaconic acid or citric acid (both of which can be derived from fermentations) using solid-transition metal catalysts has recently been reported.14 The transition metal catalyst Pd/C, Ru/C, Pt/C and Pt/Al2O3 were tested for decarboxylation of itaconic acid and citric acid (in water) to MAA in batch reactors at 200−250 °C and 38 bar argon. The highest reported MAA yield (65%) was achieved with Pd/C and Pt/C at 250 °C and a 1 h residence time. Le Nôtre et al. (2014) also reported that water was added across the MAA double bond, forming 2-hydroxyisobutyric acid, which then degraded to pyruvic acid, acetone and other volatile compounds.14

Patent searches indicate much recent industrial interest in biobased MAA production. Lucite has filed patent applications focused on the homogeneous (NaOH) catalytic conversion of itaconic (IA), citraconic (CA), mesaconic (MA), and citramalic (CMA) acids to MAA. MAA yields ranged from 48%, 40%, 52%, to 81% for IA, CA, MA, and CMA respectively (0.33−15 min, 2000−5000 psig, 190−300 °C).15,16 A separate patent application by Lucite focuses on the microbiological production of MA, CA, and CMA, yet there has been no work reported on coupling the microbial processes with catalysis.17 The CMA production application indicates overexpression of citramalate synthase in Escherichia coli to enhance CMA formation from acetyl-CoA and pyruvate via the reductive TCA branch. Limited information on pH, DO, temperature, time, and yields are provided. In an approach focused on hydroxyacids instead of carboxylic acids, a Genomatica patent theorizes the microbial formation of 2-hydroxyisobutyric (2-HIBA) acid.18 The key concept is overexpression of 3-hydroxybutyryl-CoA mutase in an organism which converts 3-hydroxybutyryl-CoA, generated from acetate metabolism, to 2-hydroxybutyryl-CoA, which is enzymatically converted internally to either methacrylic acid directly or 2-HIBA. The patent claims microbial production of MAA or chemical conversion of 2-HIBA to MAA, yet results supporting these claims are not presented.18 In a similar approach, a microbial fermentation using recombinant Cupriavedus necator has been studied. This PHB-producing bacterium uses a cobalamin-dependent mutase in the synthesis of 2-hydroxyisobutyric acid. In the process of producing PHB from sugar, two acetyl-CoA molecules are transformed to acetoacetyl-CoA using 3-ketothiolase, then acetoacetyl-CoA is converted to (R)-3-hydroxybutyryl-CoA by acetoacetyl-CoA reductase in the presence of NADPH. In the last step toward PHB, (R)-3-hydroxybutyryl-CoA is polymerized by PHB synthase. To produce 2-hydroxyisobutyric acid instead, this last enzymatic step is blocked, and the (R)-3-hydroxybutyrylCoA converted to 2-HIBA using a cobalamin-dependent mutase.19,20 2-HIBA levels of 6 g/L were produced in a fedbatch fermentation, yet this fermentation processes was not coupled with the chemical or catalytic conversion of the formed 2-HIBA to MAA.20 Because a liquid base cannot be recovered and would have to be neutralized, and transition metals such as Pt or Ru are very expensive, we wanted to explore the possibility of using a solid base catalyst, called hydrotalcite, to catalyze these reactions replacing NaOH and precious metals. Hydrotalcite (HT) is a layered (brucite-like layers) anionic clay (Mg4Al2CO3(OH)12· 4(H2O)) with base sites originating from HCO3− on the surface. In addition to active base sites, the presence of acid sites on calcined hydrotalcite have been reported.21 Active acid−base sites are obtained through formation of Mg−O−Al bonds after calcination (heating typically at 300−500 °C) of hydrotalcite.21,22 Given, that at some point in the future production costs for citric (a TCA cycle intermediate) and itaconic acid (a TCA cycle derivative) may be reduced due to increased demand and improved fermentation technology (e.g., metabolic engineering of E. coli for overproduction of itaconic acid),23 and anticipating that microbial overproduction of 2HIBA will succeed (i.e., higher titers will be generated), we wanted to (1) better understand heterogeneous selective decarboxylation and dehydration of TCA cycle intermediates and their possible derivatives, and (2) contrast the potential single step dehydration of 2-HIBA to MAA with the multiple 1518


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ACS Sustainable Chemistry & Engineering

port was held at 100 °C and the following method was used: an initial oven temperature of 35 °C was held for 5 min, followed by a ramp of 20 °C/min for 8.25 min and a final holding time of 27 min at 200 °C (detector at 140 °C). The injection volume was 50 μL and helium was the mobile phase at 30 mL/min. The concentration of each compound was determined using standard curves for both HPLC and GC/TCD analysis. CO2 was the only observed product in gas sample. The standard curve for CO2 was performed using nitrogen as the balance gas and 5, 10, 25, 50, and 75% by volume CO2 (run in triplicate) was used to make the standard curve on GC/TCD. Methacrylic acid (Sigma-Aldrich), mesaconic acid (Sigma-Aldrich), citraconic acid (Acros), acetone (Fisher Scientific), acetic acid (Sigma-Aldrich), pyruvic acid (Sigma-Aldrich), citric acid, itaconic acid, and 2-hydroxyisobutyric acid 5-point standard curves were prepared using DI water. Catalytic Reactions. Production of methacrylic acid was performed in a 75 mL autoclave batch reactor (PARR Series 5000 Multiple Reactor System). A working volume of 40 mL (substrate in DI water) and catalyst was mixed using a magnetic stir bar (750 rpm). The reactor headspace was purged for 1 min with helium, pressurized at 34.5 bar (500 psi, He), then heated to the set-point at 9 °C/min. Typically, heat up times were 15 min and reaction pressure rose from 34.5 bar (500 psi) to 59 bar (855 psi) at a temperature of 250 °C. Residence times are reported from the time the temperature reached the set-point. Once the reaction was completed, the vessel was cooled down using a water bath at room temperature. After taking a gas sample using a 1 L Tedlar bag, the headspace pressure was released, and the catalyst recovered via filtration (Whatman, 11 μm pore size). The effect of key reaction variables on methacrylic acid yield (mole of MAA formed/mol of carboxylic acid charged) was determined and included catalyst mass, reaction temperature, and substrate concentration. Initial reactions were performed with 20 g L−1 of substrate (in DI water) at 250 °C and 1 g catalyst (calcined HT or Pd/C, Pd/HT). To determine the effect of subcritical water only (in absence of catalyst), a set of blank reactions was also performed in varying substrate concentrations (5, 10, 20, 30 g L−1) and temperatures (200, 225, and 250 °C for itaconic acid and citric acid; 200, 225, 250, and 275 °C for 2-hydroxyisobutyric acid). After determining optimum conditions for methacrylic acid formation from each substrate, the effect of temperature (200−250 °C for itaconic acid and citric acid, 200−275 °C for 2-hydroxyisobutyric acid) and catalyst mass (0.125.0.25,0.5, 1 g) was determined. Catalyst reuse reactions and reactions in the presence of palladium on hydrotalcite catalyst were performed at 20 g L−1 and 250 °C. Finally, the effect of residual fermentation media components on methacrylic acid yield was determined for each substrate. The selection of the model media was based on previous work by Zhang et al. (2008).24 To determine the effect of fermentation media residuals on methacrylic acid yield, selected components including, NaOH (0.1 M), Na2SO4 (0.04 M), Na2HPO4 (0.04 M), glucose (0.06 M), and albumin (0.12 wt %) were added to the reaction medium and tested. Statistical Analysis. Reactions in the presence of fermentation residuals, reused catalysts, NaOH only, water only without catalyst, 0.125 g cHT, and 2-HIBA were replicated at all residence times. Error bars indicate deviation from the mean based on the range. Error bars for reactions in the presence of 0.25, 0.5, 1, and 2 g cHT were calculated from the error of reactions in the presence of 0.125 g cHT under the same conditions.

step dehydration/decarboxylation of citric acid and the single step decarboxylation of itaconic acid to MAA. Finally, anticipating the challenge of interfacing fermentation with catalysis, we studied the effect of select fermentation residual components on these transformations.

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EXPERIMENTAL SECTION

Materials and Catalysts. Itaconic acid and citric acid monohydrate were purchased from Sigma-Aldrich and 2-hydroxyisobutyric acid (2-HIBA) was purchased from TCI Co. Desired concentrations of each substrate was prepared using deionized water. Sodium hydroxide (Fisher Scientific), albumin (bovine, Sigma), D-(+)-glucose (Sigma), sodium phosphate dibasic heptahydrate (Sigma), and sodium sulfate (Sigma) were purchased in order to study the effect of fermentation medium on catalyst activity and methacrylic acid yield. The raw hydrotalcite (HT) powder, Mg6Al2(CO3) (OH)16·4H2O, with a MgO/Al2O3 ratio of 4.0−5.0 was purchased from SigmaAldrich. The HT was calcined at 400 °C overnight and then allowed to cool. The powder was then stirred in DI water to make a paste, and the paste was placed in a 105 °C drying oven overnight. Then the dried hydrotalcite was crushed and sieved to a particle size between 0.5 and 1 mm. A Pd on activated carbon (5 wt % Alfa Aesar, fine powder) catalyst was also tested in this work and was reduced for 8 h in flowing 100% hydrogen (100 mL min−1) at 450 °C before testing. In addition, a solid base catalyst impregnated with a transition metal was synthesized in this work. Palladium on HT (5 wt % Pd) was prepared using incipient wetness impregnation. A 5 wt % palladium(II) nitrate dihydrate (40% Pd basis, Sigma-Aldrich) solution in deionized water was prepared and added to a solution of the hydrotalcite powder that had been precalcined at 400 °C. This solution was then dried at 120 °C for 2 h, calcined at 400 °C for 2 h, and then crushed and sieved to the desired particle size. Prior to reaction the sample was reduced in a tubular reactor system at 450 °C for 8 h in flowing 100% hydrogen (Airgas). Catalyst Characterization. Surface areas of the catalysts were measured using a 7-point BET analysis equation (Quantachrome AUTOSORB-1C; Boynton Beach, FL). Pore size distribution, average pore radius, and total pore volume were estimated from N2 desorption curves using BJH analysis. Hydrotalcite (HT) was analyzed by CO2 TPD/TCD analysis. Samples (∼0.2 g) were degassed in helium at 450 °C for 1 h before CO2 TPD analysis. Samples were then cooled to 50 °C and saturated with CO2 (100%) for 15 min, flushed with helium at 40 °C for 15 min, then desorbed with helium from 50 to 650 °C at 10 °C/min (flow at 80 mL/min). Base site density (μmol CO2/g catalyst) was estimated using a CO2 TCD standard curve and calculating the peak areas for CO2 desorption via numerical integration. A four-point standard curve was generated via triplicate pulse injection of known volumes of pure CO2. Recovered catalyst was washed with DI water, dried at 105 °C for 1 h, cooled to room temperature, and weighed to determine the mass of tar (residual substrates or soluble materials bound to the HT) removed. Catalyst coke formation was determined by heating the washed catalyst in a thermogravimetric analyzer (TGA) at 10 °C/min from 25 to 500 °C under air flow (50 mL/min). The change in the mass of catalyst was assumed to be due to the complete combustion of coke. Analytical. Liquid product concentrations were determined using high performance liquid chromatography (HPLC). HPLC (Shimadzu LC-20 AT) was performed using a Coregel 64-H Transgenomic column (7.8 × 300 mm) and a 7 mN H2SO4 mobile phase (0.6 mL min−1) at 60 °C (5 μL injection volume, 30 min run time). The methacrylic acid peak was verified with UV detection at 210 nm on the HPLC and GC/MS analysis (HP-6890, HP-5973 MS, HP-5 MS column). The byproducts acetaldehyde (not quantified), acetone, and acetic acid were also verified by GC/MS analysis (Figure S6 and details are provided in the Supporting Information). Permanent gases (H2, O2, CO, CO2) and low molecular weight hydrocarbons (C1−C2 hydrocarbons, formaldehyde and methanol) were analyzed by GC/TCD analysis (HP 5890 Series II) using a Carboxen 1000 column (2.1 mm internal diameter). The injection

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RESULTS AND DISCUSSION Catalyst Characterization. The calcined HT (granulated, crushed, and sieved) had a relatively low surface area compared to the Pd/C and Pd/HT and broad pore size distribution (Table 1, Figure 1). After one batch reaction with itaconic acid we observed a decrease in surface area for the recovered catalyst. If the catalyst was recovered and reused, a significant increase in surface area, pores of diameter 40 Å, and hysteresis in the N2 adsorption isotherm was observed (Figure 1). This 1519


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trend was most apparent with itaconic acid (Figure 1 and S1). These results suggest that upon reuse the HT was rehydrated in a manner leading to a significant increase in micro and macro pores. Such responses have previously been reported for HT.25 As detailed later, these changes in physical property correlated with an increase MAA yields. The surface area for the Pd/HT was larger than cHT and in the range previously reported for 1−5 wt % Pd/HT (60−160 m2/g).26,27 CO2-TPD analysis of the calcined and granulated HT indicated two peaks, one at 130 °C and at 495 °C (Figure 1). The CO2-TPD profile for Pd/HT did not change (data not shown). The CO2 peak at 130 °C (100−180 °C)26,27 has been reported as a weak base site and the peak at 495 °C a strong base site.27 TPR analysis of the Pd/ HT indicated peak H2 consumption at 300 °C which has been assigned to reduction of Pd2+.27 TPR analysis of the Pd/C catalyst indicated lower H2 consumption relative to Pd/HT and a shift to higher temperatures (Figure S2). The response of the Pd/C in H2-TPR may have resulted from previous reduction by the catalyst supplier. Catalytic Conversion of Citric and Itaconic Acids. One of the first steps in our concept was to determine if cHT (calcined at 400 °C) could replace a liquid base, NaOH, in transformation of the carboxylic acids to MAA. Initially a range of temperatures was screened (200, 225, and 250 °C) for MAA formation from the carboxylic acids using cHT. MAA formation was only measured at 250 °C and thus all subsequent reactions, excluding work with 2-HIBA, was conducted at this temperature. The effect of initial substrate concentration (5−30 g L−1) on MAA yields was also studied. MAA yields increased with initial substrate concentration (250 °C, 15 min) reaching a maximum between 20 to 30 g L−1 (Figure S3). Because our goal was to explore the use of cHT as a solid base catalyst and not perform a detailed kinetic analysis at this point, a majority of our work was conducted using 20 g L−1 substrate, a temperature of 250 °C, and a 15 min residence time (after heatup). Next the effect of catalyst loading was determined. At the lowest catalyst loading tested, cHT generated significantly higher MAA yields for transformation of both citric (CA) and itaconic acid (IA), compared to a thermal reaction (Figure 2). After these positive results, further testing was initiated. As the catalyst loading was decreased or substrate to catalyst ratio increased, MAA yields increased for both citric and itaconic acid (Figure 3). At the lowest cHT loading, the MAA yield from citric acid was similar to that for NaOH, but lower for itaconic acid (Figure 3). Itaconic acid (from citric acid), mesaconic acid, citraconic acid, acetone (data not shown), acetic acid, and pyruvic acid (data not shown) were the byproducts measured, with acetic acid the largest (Figure 4). As the cHT loading was decreased, the acetic acid yield approached a maximum (0.25 g HT, citric; 0.5 g HT, itaconic) and declined for both citric and itaconic acid (Figure 4). A carbon balance in both the liquid and gas phase was performed on reactions with and without homogeneous and heterogeneous catalysts. Results indicate that blank reactions and reactions with 0.15 M NaOH had the highest recovered carbon ranging from 66% to 80%, and reactions with cHT generated lower carbon recoveries (50−75%), with citric acid having lower recoveries compared to itaconic acid (Figure S4). The lower carbon recover using cHT might be attributed to coke formation, since TGA analysis of the recovered catalyst indicated a coke level of 13.6% and 7.2% for itaconic acid and citric acid, respectively. Finally, we studied the reusability of

Table 1. Effect of Reaction Conditions on Hydrotalcite Physical Properties and Pd Catalyst Properties Properties State/ Substrate

Surface Area (m2/g)

Pore Volumea (cm3/g)

Average Pore Sizea (diameter, Å)

Calcined HTC Reactedb,c/ Itaconic Reusedb,d/ Itaconic Reusedb,d/Citric Pd/HTCe Pd/Activated Carbonf

30.71 17.58

0.0481 0.02738

62.7 62.3

98

0.1316

53.64

0.0870 0.1645 0.4824

46.13 41.14 28.1

75.46 159 686

evaluated at P/Po = 0.83−0.88. b250 °C, 5 min. eNot reduced with H2. fAnalyzed as received. cReacted: Indicates one reaction, recovered, and analyzed. dReused: Indicates one reaction, recovered, and reused in another reaction. a

Figure 1. Effect of reaction conditions on pore size distribution (top), adsorption isotherm of hydrotalcite (middle), and CO2-TPD of the calcined HT (bottom). IA is itaconic acid reacted with HT for 5 min (250 °C). Adsorption isotherms are dashed lines and solid lines are for desorption (middle). Reused indicates one reaction, recovered, and reused in another reaction. Reacted indicates one reaction, recovered, and analysis.

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Figure 2. Conversion of itaconic (A) and citric (B) acid to MAA with and without HT (water only) at 250 °C, 20 g L−1, and 0.125 g HT. Reported data includes a 15 min unsteady state, heat-up time.

forming byproducts.13,28 High pH (>1 base equivalent) generates the dianion, which has a significantly lower decarboxylation rate than the monoanion.28 We suggest that base sites in the calcinced hydrotalcite (cHT) dissociate itaconic acid (IA) forming the anion form that has a higher rate of decarboxylation. Because we observed mesaconic, citraconic, and itaconic acids, yet not citramalic, in the formation of MAA from citric acid (CA), we suggest citric acid is dehydrated forming aconitic acid, which undergoes decarboxylation to MAA, through IA, and is promoted by the base sites on cHT. Integrating the strong base peak from CO2TPD (Figure 1), we estimated the base density at 610 μmol/g, which is consistent with previously reported values for calcined HT (550−2100 μmol/g).26,27 Using the initial substrate concentration, estimated base density and catalyst loading, we calculated that the moles of base sites required to form the anion is significantly lower than 1 base equivalent (e.g., 0.0012−0.0198 mol base/mol IA vs 0.975 for NaOH alone). The optimum molar base site to substrate ratio using HT in our work was 0.0012 for IA and 0.0018 for CA, which is significantly lower than the 0.10 to 0.20 ratio reported for conversion of IA to MAA (∼70% yield, 360 °C, ∼25−60 s) using NaOH.13 These results suggest that in addition to base sites, other active sites in the catalysts, which we did not characterize, may have contributed to the activity responsible for MAA formation and parasitic side reactions leading to byproducts (acetic acid, acetone, and pyruvic acid).13,14 Calcined hydrotalcites form three types of base sites, OH− (weak), Mg2+−O2− pairs (medium), O2− (strong), and two

the cHT for citric and itaconic acid conversion to MAA. Similar itaconic acid conversion was achieved with both fresh and reused hydrotalcite; in fact, the MAA yield increased by a factor of 1.65 (Figure 5). Similar results were obtained using citric acid: a 2.55 factor increase in MAA yield was observed upon reusing the hydrotalcite (Figure 5). Given the previously reported high MAA yields from itaconic and citric acid using Pd/C and Pt/C catalysts, we were curious to determine if a bifunctional, metal/base catalyst could be developed from HT that would increase MAA yields over HT alone and eliminate the need for a homogeneous base (e.g., NaOH). Unfortunately, our results using Pd/C and Pd/HT were disappointing. The Pd/C material (in the presence of 0.15 M NaOH) generated MAA yields lower than NaOH alone (0.15 M), and Pd/HT generated MAA yields similar to cHT alone, but still significantly lower than NaOH (itaconic acid and citric acid − Figure S5). At the moment, we do not have a clear understanding as to why the MAA yields were not significantly improved using these catalytic materials. Relative to the reactions conducted in water only, it is clear that the calcined HT (cHT) significantly increased the rate of MAA formation and yield from citric and itaconic acid (Figures 1 and 2). These results are consistent with previous work indicating that a homogeneous base (NaOH) increased the rate of MAA formation and subsequent yield from itaconic acid in near to supercritical water (300−375 °C)13,28 It is theorized that base addition promotes carboxylic dissociation forming a monoanion (one base equivalent), which more readily decarboxylates relative to the neutral form that decomposes 1521


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Figure 3. Effect of catalyst (HT) loading on conversion and methacrylic acid yield (250 °C, 15 min, 20 g L−1, reported data includes a 15 min unsteady state, heat-up time). (Substrate: itaconic (A) and citric (B) acid.)

allowed reincorporation of CO 3 2− in place of OH − . Subsequently during the reactions under subcritical conditions, CO32− may have been evolved allowing restructuring of the cHT. Thermal treatment of hydrotalcite followed by rehydration in carbonate free water generates a material with irregularly stacked platelets, enhanced mesoporosity, more accessible base sites, and significantly higher reaction rates in base catalyzed condensation.25 Catalytic Conversion of 2-Hydroxyisobutyric Acid. As previously mentioned, methacrylic acid formation is possible through a one-step dehydration of 2-hydroxyisobutyric (2HIBA), of which there are possible fermentation routes for production. Because dehydration can be catalyzed by acids, our interest was in determining the possibility of using subcritical water for dehydration of 2-HIBA to MAA. Thus, we first screened the effect of reaction temperature and residence time

Lewis acid sites, Mg2+ and Al3+.29 The Lewis acid sites may contribute to the dehydration step in citric acid conversion to itaconic acid (IA) and decarboxylation of IA to MAA. Lewis acid sites in γ-Al2O3 have been shown to decarboxylate unsaturated carboxylic acids.30 The increase in MAA yield upon cHT reuse for conversion of both citric and itaconic acid suggests a structural change in cHT that promotes IA decarboxylation to MAA. The reused cHT for both CA and IA showed a significant increase in surface area and pore volume, but only the cHT reused for IA showed significant hysteresis in the adsorption isotherm. In total these results suggest a restructuring of cHT upon rehydration and reaction, increasing active sites that promote the dehydration and decarboxylation steps leading to MAA. In our preparation of cHT, water added to the calcined hydrotalcite had not been decarbonated, which may have 1522


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Figure 4. Effect of HT loading on intermediate and byproducts. MS is mesaconic acid, CC is citraconic acid, IT is itaconic acid, and AA is acetic acid. (Substrate: citric [A] and itaconic [B] acid.) Reported data includes a 15 min unsteady state, heat-up time.

A carbon balance on these reactions indicated that the amount of recovered carbon was above 90% (Figure 6). Less than 5% of the recovered carbon was associated with byproducts and carbon dioxide in each reaction. Acetone, acetic acid, and pyruvic acid were the byproducts of 2hydroxyisobutyric reactions and carbon dioxide was the only gas phase product. At 200 and 225 °C the highest recovered carbon is in the unconverted 2-hydroxyisobutyric acid, but at higher temperatures, 250 and 275 °C, methacrylic acid becomes the main product of reactions and the highest amount of carbon is recovered by methacrylic acid (Figure 6). Critical analysis of our results for 2-HIBA relative to the literature is difficult, because we could not find published material on the dehydration of 2-HIBA or similar compounds using subcritical water. The significant increase in MAA yield with increasing temperature under subcritical conditions does suggest an increase in acidity resulting in increased rates of 2HIBA decarboxylation. The ion product (Kw = [H+] [OH−]) increases significantly up to 300 °C and then decreases,

on conversion and MAA yield. MAA yield increased with both increasing temperature and residence time. It is clear that the 2HIBA reaction rate was significantly slower at 200 °C and rapidly increased with temperature (Figure 6). The largest MAA yield, 71.5% (83% conversion), occurred at a 1 min residence time and 275 °C. At 275 °C, as residence time increased, methacrylic acid yield decreased and additional byproducts were formed. These results imply that in the higher acidity of the subcritical reaction medium, methacrylic acid is not stable at higher temperatures and starts decomposing to byproducts, as time increases. In general, at lower temperatures (200 and 225 °C), methacrylic acid yield increased with increasing residence time (Figure 6). However, at 250 and 275 °C after 15 min, methacrylic acid yield decreased. Thus, the higher acidity of the reaction medium (at higher temperatures) results in a faster dehydration reaction rate of 2-HIBA, yet since methacrylic acid decomposes in acidic medium, the MAA yield decreases with time. 1523


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Figure 5. Catalyst reuse results for citric (A, 1 g fresh HT, 0.7 g recovered HT) and itaconic acid (B, 1 g fresh HT, 0.92 g recovered HT) conversion to MAA Effect of adding reused hydrotalcite on MAA yield at 250 °C, 20 g L−1 substrate, and 15 min. Note, 1 g HT was used and not the optimum catalyst loading (see Figures 2 and 3) due to the need to recover the catalyst for reuse and analysis. Reported data includes a 15 min unsteady state, heat-up time.

indicating subcritical water can act as an acid or base catalyst.31,32 Primary and secondary alcohols, and glycerol have been shown to undergo acid catalyzed dehydration in subcritical and supercritical water.32,33 Ethanol primarily forms ethene via an E2 mechanism (ionic reaction mechanism) and glycerol primarily forms acrolein in a similar mechanism.32,34 Supercritical conditions promote a radical reaction based pathway for conversion of glycerol, significantly lowering acrolein yields and forming a range of byproducts (aldehydes and ketones).34 We suggest that 2-HIBA undergoes an ionic based dehydration mechanism, under subcritical conditions, leading to MAA formation. Effect of Fermentation Components. Given that citric and itaconic acid are commercially produced via fermentation, and anticipating that 2-HIBA will be generated in a similar manner, we determined the effect of potential residual fermentation media components on formation of MAA. Complete conversion of citric acid and itaconic acid was achieved in these reactions. Mesaconic acid, citraconic acid, acetic acid, pyruvic acid and acetone, and a very small trace of itaconic was observed after the reaction with citric acid. HPLC chromatograms indicated the presence of three unknown byproducts not produced when using pure citric acid. A yield of 28.5% was achieved for MAA in the presence of fermentation residuals, which was higher than the MAA yield without fermentation impurities (22%, Figure 7). The presence of fermentation impurities lowered the acetic acid yield and resulted in a higher pyruvic acid yield. In the case for itaconic

acid, the results were similar to those for citric acid; the same byproducts were produced, two unknowns were produced not formed when using pure itaconic acid, and the MAA yield increased from 22 to 31% (Figure 7). Although MAA yields did not increase for conversion of 2-HIBA to MAA in the presence of the fermentation media component, the results were still encouraging. MAA yield decreased from 66 to 60%, along with a decrease in 2-HIBA conversion (Figure 7). The lower MAA yield was due to a reduction in 2-HIBA conversion and it should be noted that MAA selectively (S = mol of MAA/mol of 2-HIBA converted) did not change (S = 0.85 in both cases). These results indicate that the presence of fermentation impurities may have inhibited or reduced the rate of 2-HIBA dehydration to MAA. We postulate the unknown peaks may have been due to Maillard reactions. We did observe samples that received fermentation residuals (i.e., glucose and albumin) turned a light brown color after the reaction. Thus, we suggest that a network of Maillard reactions occurred between glucose and amino acids formed from the hydrolysis of albumin. In Maillard reactions, the amino acids can react with glucose forming a range of nitrogenated compounds.35 We can only speculate on the mechanism by which the fermentation components increased MAA yields from CA and IA using cHT, but decreased MAA yields from 2-HIBA. It is possible that Na+ ions (Na2SO4, Na2HPO4) exchanged for CO32+ in cHT increasing base sites or altered its structure to increase accessibility to active sites. Na, Li, and K doped 1524


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Figure 7. Effect of fermentation media components on methacrylic acid yield and byproduct formation from citric (A), itaconic acid (B), and 2-hydroxyisobutyric acid (C) at 250 °C, 30 g L−1 substrate, 15 min, and 0.125 g HT). Reported data includes a 15 min unsteady state, heat-up time.

and MAA yields. The similar MAA yields and intermediate product formation (mesaconic [MA], citraconic [CC], and itaconic acids [IA]) using cHT compared to the homogeneous base catalyst (NaOH) indicate it acts in similar manner to the base and can be used to replace these basic liquids. The increase in cHT activity and MAA yield from CA and IA upon recovery and reuse suggest potential for prolonged use of the solid base material. The rapid conversion of CA and subsequent formation of MAA, without observed formation of citramalate and hydroxyisobutyric acid, indicates CA is first dehydrated and then decarboxylated generating the intermediate pool (MA, CC, IA), of which IA is decarboxylated to MAA. Comparison of MAA yields and byproduct formation between CA, IA, and 2HIBA reactions clearly indicate that the dehydration step is favored over decarboxylation, when using subcritical water as a catalyst. The significantly higher MAA yield and lower byproduct levels from 2-HIBA (single step dehydration) compared to CA (dehydration followed by two decarboxylation steps) and IA (single step decarboxylation) support this conclusion. Given the high MAA yields (∼71%) in short residence times (1 min), 2-HIBA seems a superior biobased substrate for production of MAA, via coupling with continuous dehydration, if a metabolically engineered pathway for its synthesis and overproduction can be developed. The minimal impact of fermentation media components, in addition to the high yields at residence times in the range for continuous processing, support this conclusion.

Figure 6. Effect of temperature and residence time on catalytic dehydration of 2-HIBA to MAA using subcritical water. Reported data includes an unsteady state, heat-up time.

calcined hydrotalcites (cHT) have been shown to have significantly higher aldol condensation activity relative to cHT without alkaline metals.36 The alkali doped cHT had a significantly higher number of base sites, and an increase in strong base sites (as measured by CO2-TPD), which was subsequently attributed to an increase the catalytic activity.36 Concerning 2-HIBA conversion to MAA, we can only speculate that components in the fermentation media (e.g., Na2SO4, Na2HPO4, NaOH) may have altered the ionic character of the subcritical water (e.g., lowered acidity) resulting in a lower 2HIBA conversion and subsequent MAA yield.

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CONCLUSIONS Relative to subcritical water alone, the calcined hydrotalcite (cHT) increased the rate of MAA formation from CA and IA, 1525


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(10) Tai, J.; Davis, R. J. Synthesis of methacrylic acid by aldol condensation of propionic acid with formaldehyde over acid−base bifunctional catalysts. Catal. Today 2007, 123 (1), 42−49. (11) Spivey, J. J.; Gogate, M. R.; Zoeller, J. R.; Colberg, R. D. Novel catalysts for the environmentally friendly synthesis of methyl methacrylate. Ind. Eng. Chem. Res. 1997, 36 (11), 4600−4608. (12) Ai, M.; Fujihashi, H.; Hosoi, S.; Yoshida, A. Production of methacrylic acid by vapor-phase aldol condensation of propionic acid with formaldehyde over silica-supported metal phosphate catalysts. Appl. Catal., A 2003, 252 (1), 185−191. (13) Carlsson, M.; Habenicht, C.; Kam, L. C.; Antal, M. J. J.; Bian, N.; Cunningham, R. J.; Jones, M. J. Study of the sequential conversion of citric to itaconic to methacrylic acid in near-critical and supercritical water. Ind. Eng. Chem. Res. 1994, 33 (8), 1989−1996. (14) Le Nôtre, J.; Witte-van Dijk, S.; van Haveren, J.; Scott, E. L.; Sanders, J. P. Synthesis of Bio-Based Methacrylic Acid by Decarboxylation of Itaconic Acid and Citric Acid Catalyzed by Solid Transition-Metal Catalysts. ChemSusChem 2014, 7 (9), 2712−2720. (15) Johnson, D. W.; Eastham, G. R.; Poliakoff, M. Method of Producing Acrylic Acid and Methacrylic Acid. WO 2011/077140 A2, June 30, 2011. (16) Eastham, G. R.; Johnson, D. W.; Waugh, M. A. Process for the Production of Methacrylic Acid and its Derivatives and Polymers. WO 2013/160703 A1, October 31, 2013. (17) Eastham, G. R.; Johnson, D. W.; Archer, I.; Carr, R. A. Process for the Production of Methacrylic Acid and its Derivatives Thereof. WO 2015/022496 A2, February 19, 2015. (18) Burgard, A. P.; Burk, M. J.; Osterhout, R. E.; Pharkya, P. Microorganisms for the Production of 2-Hydroxyisobutyric acid. U.S. Patent US 8900837 B2, December 2, 2014. (19) Rohwerder, T.; Müller, R. H. Biosynthesis of 2-hydroxyisobutyric acid (2-HIBA) from renewable carbon. Microb. Cell Fact. 2010, 9 (1), 13. (20) Hoefel, T.; Wittmann, E.; Reinecke, L.; Weuster-Botz, D. Reaction engineering studies for the production of 2-hydroxyisobutyric acid with recombinant Cupriavidus necator H 16. Appl. Microbiol. Biotechnol. 2010, 88 (2), 477−484. (21) Yamaguchi, K.; Ebitani, K.; Yoshida, T.; Yoshida, H.; Kaneda, K. Mg-Al mixed oxides as highly active acid-base catalysts for cycloaddition of carbon dioxide to epoxides. J. Am. Chem. Soc. 1999, 121 (18), 4526−4527. (22) Sels, B. F.; De Vos, D. E.; Jacobs, P. A. Hydrotalcite-like anionic clays in catalytic organic reactions. Catal. Rev.: Sci. Eng. 2001, 43 (4), 443−488. (23) Harder, B.-J.; Bettenbrock, K.; Klamt, S. Model-based metabolic engineering enables high yield itaconic acid production by Escherichia coli. Metab. Eng. 2016, 38, 29−37. (24) Zhang, Z.; Jackson, J. E.; Miller, D. J. Effect of biogenic fermentation impurities on lactic acid hydrogenation to propylene glycol. Bioresour. Technol. 2008, 99 (13), 5873−5880. (25) Winter, F.; Xia, X.; Hereijgers, B. P. C.; Bitter, J. H.; Jos van Dillen, A.; Muhler, M.; de Jong, K. P. On the Nature and Accessibility of the Brønsted-Base Sites in Activated Hydrotalcite Catalysts. J. Phys. Chem. B 2006, 110, 9211−9218. (26) Nikolopoulos, A. A.; Jang, B.W.-L.; Spivey, J. J. Acetone condensation and selective hydrogenation to MIBK on Pd and Pt hydrotalcite-derived Mg−Al mixed oxide catalysts. Appl. Catal., A 2005, 296, 128−136. (27) Naresh, D.; Kumar, V. P.; Harisekhar, M.; Putrakumar, N. N. B.; Chary, K. V. R. Characterization and functionalities of Pd/hydrotalcite catalysts. Appl. Surf. Sci. 2014, 314, 199−207. (28) Li, J.; Brill, T. B. Spectroscopy of Hydrothermal Reactions 18: pH-Dependent Kinetics of Itaconic Acid Reaction in Real Time. J. Phys. Chem. A 2001, 105 (105), 6171−6175. (29) Di Cosimo, J. I.; Dı ́ez, V. K.; Xu, M.; Iglesia, E.; Apesteguıa, C. R. Structure and Surface and Catalytic Properties of Mg-Al Basic Oxides. J. Catal. 1998, 178, 499−510.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02201. (1) GC/MS analysis method and sample chromatograms, (2) additional pore size distribution (BJH) data for reacted cHT, Pd/HT, and Pd/C, (3) temperatureprogrammed reduction (TPD-H2) for Pd/HT and Pd/C materials, (4) results for the conversion of itaconic acid to MAA using Pd/HT and Pd/C, (5) effect of substrate (IA, CA) concentration using cHT, and carbon balances for the reactions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*J. R. Kastner. Phone: 706-583-0155; fax: 706-542-8806; email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the College of Engineering at UGA for providing a Strategic Research Initiative Grant that supported Maryam Pirmoradi’s research assistantship and her effort in obtaining a MS in Biochemical Engineering. Funding for the high pressure, temperature reactor used in this work was provided by the U.S. Dept. of Energy (Funding Opportunity Announcement DEFOA-0000686: Bio-Oil Stabilization and Commoditization and Award No. DE-EE0006201).

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REFERENCES

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