Ketonization of Model Pyrolysis Bio-oil Solutions in ... - ACS Publications

Jun 20, 2013 - Michael A. Jackson*. Renewable Product Technology Unit, National Center for Agricultural Utilization Research (NCAUR), Agricultural ...
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Ketonization of Model Pyrolysis Bio-oil Solutions in a Plug-Flow Reactor over a Mixed Oxide of Fe, Ce, and Al Michael A. Jackson* Renewable Product Technology Unit, National Center for Agricultural Utilization Research (NCAUR), Agricultural Research Service (ARS), United States Department of Agriculture (USDA), 1815 North University Street, Peoria, Illinois 61604, United States ABSTRACT: The stabilization and upgrading of pyrolysis oil requires the neutralization of the acidic components of the oil. The conversion of small organic acids, particularly acetic acid, to ketones is one approach to addressing the instability of the oils caused by low pH. In the ketonization reaction, acetic acid is converted to acetone, water, and CO2. Here, a 16 vol % acetic acid solution is converted to acetone in a flow reactor over the mixed oxide Fe0.2Ce0.2Al0.6Ox in the presence of several other components meant to represent pyrolysis oil. These components include hydroxyacetone, furfural, phenol, cresol, guaiacol, and eugenol. Hydroxyacetone also undergoes ketonization, forming acetone, 2-butanone, 3-pentanone, and 2- and 3-hexanone. A mechanism for the ketonization of hydroxyacetone through propanal is proposed and supported by incorporation of isotopically labeled water. Inhibition of the reaction occurs to a significant degree by the addition of furfural, guaiacol, and eugenol. stream, known as red mud.9 The selective reduction of the double bonds of bio-oil components, e.g., acids, ketones, and aldehydes, while maintaining aromaticity has been accomplished using a Shvo-type homogeneous ruthenium catalyst.10 Moderate-pressure membrane filtration has been demonstrated to be a promising technology for the removal of acetic acid from model bio-oil solutions containing glucose and phenolics.11 The ketonization of small acids and aldehydes is another approach to stabilizing and upgrading pyrolysis oil.12 There have been many recent studies on acid ketonization, with most focused on the CeIII−CeIV transition and the related high oxygen exchange capacity of cerium being at the heart of the reaction.13−18 Pham et al. have demonstrated high ketonization activity in a Ru/TiO2/C catalyst and suggested the availability of a “Ti−acetate species” forming because of oxygen mobility enabling the reaction.19 Surface basicity of the catalysts has also been suggested as important as Gaertner et al. co-fed CO2 with hexanoic acid over CeZrO2, noted the decrease in ketone formation, and attributed this to competition at basic active sites on the catalyst.13 Debate also remains on the mechanism of the reaction. In the ketonization of acetic acid on heterogeneous catalysts, it is proposed that surface reactions occur to deprotonate or dehydrate the acid to the acetate ion or ketene,20 respectively. Dependent upon the lattice energy of the catalyst, two routes can then be followed. Low-energy salts, such as BaO, form metal acetates, which decompose to form acetone. Upon high lattice energy solids, the surface-bound acetate reacts with an adjacent intermediate and an adsorbed proton to give acetone. The formation of the proposed intermediate lying planar on the catalyst surface requires an abstractable proton adjacent to the acid group.21 However, Mekhemer et al. have shown with spectroscopic results that the ketonization of acetic acid on MgO proceeds

1. INTRODUCTION The search for renewable fuels beyond ethanol and biodiesel has led to fast pyrolysis as arguably the most efficient route for converting biomass to liquid fuel. Fast pyrolysis can capture up to 65% of the energy inputs of the feedstock in the resulting bio-oil, with another 15% recoverable in the biochar and noncondensable gases that form as co-products.1 However, biooil suffers from some serious drawbacks, notably instability of the liquid, high oxygen content, and high acidity. Upon standing, bio-oil tends to repolymerize to a degree that it becomes a viscous gel that cannot be moved through a pipeline. The high oxygen content, in the form of small ketones, aldehydes, and acids, keeps the energy density to a low 16−19 MJ/kg, which is less than half of that of diesel or gasoline.2 The high acidity, attributable to high acetic and formic acid levels, makes the bio-oil corrosive and, as such, hard on equipment used in its handling. The acidity likely contributes to the repolymerization as well, acting to catalyze esterification reactions. Therefore, for bio-oil to become a fungible replacement for petroleum, substantial upgrading is required. Upgrading has been approached on several paths. Hydrotreating to remove oxygen using conventional hydrodesulfurization (HDS) catalysts has produced gasoline- and diesel-like fractions from bio-oil prepared from pine sawdust.3,4 Bio-oil has been subjected to a high-pressure thermal treatment that removes oxygen as water and CO2 and produces an oil phase with a heating value of 29 MJ/kg but also produces a phase that is higher in molecular weight than the starting bio-oil.5 Bio-oil has been subjected to steam reforming over commercial Ni catalysts6 to produce hydrogen, primarily from the watersoluble fraction. Reduced pressure distillation of bio-oil has been shown to give a less corrosive product with a higher heating value.7 The instability of bio-oil and the complex nature of the components have compelled several groups to work with proxy solutions. A model bio-oil solution has been upgraded over Pt/Al2O3 to produce small alkanes and aromatics,8 and acetic and formic acids have been thermally decomposed over a sacrificial iron catalyst from the aluminum-processing waste This article not subject to U.S. Copyright. Published 2013 by the American Chemical Society

Received: January 9, 2013 Revised: June 12, 2013 Published: June 20, 2013 3936

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through two routes, one catalytic and one pyrolytic.22 In these studies, surface-adsorbed acetic acid underwent catalytic conversion, whereas the absorbed acid formed magnesium acetate in the bulk oxide, which decomposed to acetone, CO2, and water. Snell and Shanks have examined the ketonization of acetic acid over cerium oxide in the condensed phase. Using catalysts that had been calcined at either 450 or 900 °C, they determined that, in either the gas phase or condensed phase, acetic acid forms acetates with the bulk ceria calcined at the lower temperature.23 This paper describes using ketonization in the upgrading of a model solution composed of 16 vol % acetic acid progressively charged with pyrolysis oil oxygenates, such as hydroxyacetone, furfural, and phenolics, to study the inhibition or poisoning caused by these compounds. The chemistry of hydroxyacetone, 4-hydroxy-2-butanone, and propanal over a ketonization catalyst is also described. The catalyst used is a mixed oxide of Fe, Ce, and Al, similar to a catalyst that was previously shown to effectively catalyze the cross-ketonization of acetic acid and triacylglycerol from the seeds of Cuphea.24 The reactions were carried out in a fixed-bed plug-flow reactor operating at 400− 430 °C. This project resulted in three angles of study. First, the applied aspect of the effectiveness of the catalyst in converting acetic acid in the presence of potential inhibitors and poisons was examined. The stability of the Fe0.2Ce0.2Al0.6Ox catalyst over the time on stream was studied with regard to effectiveness and surface features of the oxide before and after use. Finally, the mechanisms of hydroxyacetone and propanal ketonization were explored.

room temperature. The ammonia was removed under vacuum for 30 min and under helium flow for 30 min at 40 °C. The desorption measurement was performed using He as the carrier gas at a heating rate of 10 °C/min. Water was removed using a trap at −15 °C. NH3 was monitored at m/e 17 using a Pfeiffer Prisma Plus detector. Ammonia was quantified by integrating under the line generated by its desorption and a comparison to a standard curve made using the decomposition of (NH3)6Mo7O24·4H2O as a standard.25 CO2-TPD measurements were performed likewise. The CO2 (ILMO gas, Charleston, IL) was adsorbed by passing the gas over the sample at room temperature for 2 h at 30 mL/min, followed by evacuation and purging with He for 30 min each. CO2 was quantified using a standard curve generated by the decomposition of calcium oxalate. Temperature-programmed reductions (TPRs) were performed using 10% H2 in Ar (ILMO gas, Charleston, IL). The analysis regimen was as follows. Approximately 45 mg samples were outgassed at 150 °C for 30 min; the sample cell was purged with the H2/Ar mixture for 15 min at 40 mL/min; and then the analysis was run from 150 to 900 °C at 10 °C/min. Water was removed by a dry ice/ethanol trap. X-ray diffraction (XRD) data were collected on a STOE θ/θ diffractometer using Cu Kα radiation and a scintillation counter. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) images were collected using a Hitachi S-2460N variable pressure scanning electron microscope with an Oxford Instruments Isis X-ray analyzer. The mixed metal oxides were examined at 20 kV with a beam current of 0.5 nA, producing a count rate of approximately 3000 counts per second (cps). 2.4. Model Pyrolysis Oil. The ultimate model solution contained (as volume percents) 16% acetic acid, 10% hydroxyacetone, 5% guaiacol, 6% furfural, and 1% each of phenol, 2-cresol, and eugenol. The balance was deionized water. Reactions carried out on solutions with fewer components were at these same concentrations, with deionized water used as a diluent. 2.5. Product Analyses. Products were analyzed using a Shimadzu QP2010 SE mass spectrometer using a Supelco Petrocol DH 50.2 (50 m × 0.2 mm × 0.5 μm) column for separations. The oven program was as follows: initial temperature of 50 °C for 3 min, ramp at 10 °C/ min to 170 °C, and then ramp at 20 °C/min to 300 °C. The product mixture was collected as 5 min cuts, giving about 1 g of solution. A total of 10 μL of methyl octanoate was added as an internal standard, and then the solution was diluted with 2.0 mL of methanol. The methanol solubilized water-insoluble products. A total of 200 μL of this solution was then diluted with diethyl ether to about 1 mL for analysis. Molar response factors for components and available products were calculated against methyl octanoate. 2.6. Catalytic Reaction. The reactions were performed in a fixedbed plug-flow reactor (Parr Instruments, Moline, IL) operating in a downward flow mode. The reactor tube was 50 cm long and 12 mm in diameter. A total of 4 g of catalyst was loaded in the center zone between wads of quartz wool, such that the controlling/recording thermocouple rested at the top of the catalyst bed. The zone below the bed was filled with glass wool and glass beads and maintained at 150 °C. The top zone was empty and maintained at 25 °C warmer than the catalyst bed. The substrate was delivered to the catalyst by a highperformance liquid chromatography (HPLC) pump at 200 μL/min. Using the mass of the aqueous solution, this is a weight hourly space velocity (WHSV) of 3. Ar flow through the reactor was 125 mL/min. The substrate entered the Ar stream at a T junction outside of the heated zone and was swept into the reactor. The reactor effluent was cooled in a condenser operating at 5 °C and collected in an iced receiver vented of gaseous products, which were carried to a Pfeiffer Prismaplus mass spectrometer.

2. EXPERIMENTAL SECTION 2.1. Materials. Fe(NO3)3, Al(NO3)3, phenol, cresol, guaiacol, furfural, hydroxyacetone, and eugenol were purchased from SigmaAldrich (Milwaukee, WI). Propionaldehyde and Ce(NO3)3 were from Alfa Aesar (Ward Hill, MA). Acetic acid and ammonium hydroxide were from Fisher Scientific. 10% H218O was from Icon Isotopes (Summit, NJ). 2.2. Catalyst Preparation. Catalysts were prepared by coprecipitation of the appropriate ratios of the nitrate salts with precipitation initiated by the addition of concentrated ammonium hydroxide. Typically, 10.5 g of Fe(NO3)3, 29.3 g of Al(NO3)3, and 11.3 g of Ce(NO3)3 were dissolved in 400 mL of deionized water. This orange solution was stirred using an overhead stirrer with a broad paddle at 250 rpm. The pH of this solution was 1.72. The addition of 55 mL of 28% ammonium hydroxide in a steady stream caused precipitation of a brown solid. The final pH was 9.40. This stirred for 5 min and was then aged, static, at room temperature overnight. The solid was collected by filtration and washed with deionized water. The heavy, sticky mass was allowed to air-dry overnight prior to being dried in vacuo at 100 °C. During this drying period, larger chunks of the solid were broken up, first with a spatula and then with a pestle. The solid was then calcined under flowing air from 25 to 500 °C in 5 h and then held at this temperature for 30 min. The yield was 11.8 g of Fe0.2Ce0.2Al0.6Ox. 2.3. Catalyst Characterization. Surface textures were determined using a Quantachrome ASiQ (Quantachrome Instruments, Boynton Beach, FL). Samples were outgassed at 200 °C for 10 h prior to analysis. Analyses were performed at −196 °C using N2 as the adsorptive. Surface areas were determined using the Brunauer− Emmett−Teller (BET) equation within 0.05 < P/Po < 0.30. Pore sizes were determined using the Barrett-Joyner−Halenda (BJH) method on the adsorption branch of the isotherms. Temperature-programmed desorptions (TPDs) were performed as follows: A 300 mg sample of catalyst was cleaned under nitrogen flow for 30 min at 150 °C and then evacuated for 30 min. Ammonia (electronic grade, Scott Gas) was then added to the sample cell and allowed to adsorb for 30 min at

3. RESULTS AND DISCUSSION 3.1. Characterization of the Fresh and Used Catalyst. Fe0.2Ce0.2Al0.6Ox was characterized both fresh and after approximately 150 h on stream, the effect of which is seen in surface features but not in activity. Figure 1 shows the N2 isotherms of the fresh and used catalyst. From the isotherms, it 3937

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Figure 3. Powder XRD patterns of the (A) fresh and (B) used catalyst. The asterisks mark peaks assigned to CeO2.

Figure 1. Isotherms of the catalyst (A) fresh and (B) after being on stream for about 150 h.

can be seen that the catalyst is affected by the reaction conditions. The fresh catalyst exhibits a H2 hysteresis loop common to metal oxides and indicating an irregular pore structure. After time on stream, this structure changes toward a type H1 hysteresis loop. This change also causes the surface area to fall from 181 to 70 m2/g as the mean pore size increases from 6.6 to 17.5 nm. The SEM−EDX map of the fresh and used catalyst is shown in Figure 2, where no differences can be seen. The chemical composition of the mixed oxide taken from the EDX analysis shows the atomic ratio of Fe/Ce/Al to be 19.4:21.3:59.2 for the fresh catalyst and 19.4:22.1:58.5 for the used catalyst, essentially the as-synthesized ratios. Figure 3 shows the powder XRD patterns of the fresh and used catalyst. The fresh catalyst shows weak, broad signals from CeO2, whereas these are much better defined in the used sample as the catalyst became more crystalline. Using Scherrer’s equation on the (111) plane at 28° 2θ gives a CeO2 crystallite size of 7 nm. No iron oxide or alumina phases were discernible in either sample. Figure 4 shows the CO2-TPD and NH3-TPD traces of the used catalyst. The used catalyst is weakly amphoteric with 0.15 mmol of CO2 and 0.22 mmol of NH3 adsorbing per gram of catalyst. Spread evenly over the catalyst, these equate to 1.3 basic sites and 1.9 Brønsted acid sites per square nanometer of catalyst surface. Figure 5 shows the H 2 -TPR of Fe0.2Ce0.2Al0.6Ox, alongside α-Fe2O3 and CeO2. This profile is very similar to that reported for ferrite-based water−gas shift catalysts.26 A notable difference between the ferrite reduction and that of the Fe0.2Ce0.2Al0.6Ox mixed oxide is the higher temperature in the latter for the first reduction peak, 430 versus

Figure 4. NH 3 -TPD and CO 2 -TPD profiles from used Fe0.2Ce0.2Al0.6Ox.

380 °C. This could be the result of the Ce4+ ↔ Ce3+ couple, which has a peak at 500 °C, being buried under this peak.27 For the ketonization studies, the catalyst was used as prepared and was not reduced prior to the addition of reagents to the catalyst bed, but it can be seen in the gas evolution profile in Figure 6 that the initial surge in gas production is likely from the oxidation of the substrate by the catalyst, resulting in a reduced catalyst. 3.2. Catalytic Activity. Figure 7 shows the results of the ketonization of acetic acid alone and in the presence of

Figure 2. EDX map of the (left) fresh and (right) used catalyst Fe0.2Ce0.2Al0.6Ox: (A) surface image at 1000× magnification, (B) Al, (C) Ce, and (D) Fe. 3938

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Figure 5. H2-TPR profiles of Fe0.2Ce0.2Al0.6Ox, α-Fe2O3, and CeO2.

Figure 7. Acetone and 2-butanone yields from the ketonization of acetic acid and hydroxyacetone in model pyrolysis oil solutions over a fixed bed of Fe0.2Ce0.2Al0.6Ox operating at 400 and 430 °C. Key: A, 16 vol % acetic acid; B, A + 10 vol % hydroxyacetone; C, B + 2 vol % furfural; D, C + 1 wt % phenol; E, D + 1 vol % guaiacol; F, D + 1 vol % m-cresol; G, F + 1% eugenol; and H, B + 6 vol % furfural + 1 vol % mcresol + 5 vol % guaiacol.

upgraded product containing 1.4 mmol/mL acetone. This is reported as 1.1 mmol/mL on the chart, with the remainder lost, as noted above, as coke, tar, and in the gas stream. Although it was expected that hydroxyacetone could be a potential inhibitor of the reaction, solution B in the bar graph, containing 16% acetic acid and 10 vol % hydroxyacetone, did not show reduced acetone production. In fact, not only was the acetic acid reaction not inhibited at 400 °C, other ketones, particularly 2butanone, were produced. This suggests that hydroxyacetone is converted to acids, and these proceed to the higher ketones. Specifically, the production of 2-butanone indicates that hydroxyacetone is converted to propionic acid, which then reacts with acetic acid to give 2-butanone. This is in agreement with the results by Mansur et al., who upgraded pyroligneous acids from a torrefaction unit.28 Low levels of 3-pentanone were also found, resulting from the self-ketonization of the propionic acid. The reactions of hydroxyacetone on this catalyst are discussed in detail below. At 400 °C, residual acetic acid and propionic acid are found in the product mixture, along with 2methylpropionic acid and 2-methylbutyric acid. Because of the poor peak shape and separation, the acids were not quantified. The associated ketones resulting from the reaction of these acids with acetic acid, which is the most abundant free acid, are also found. At 430 °C, ketonization of the acids is complete and only ketones are detected. These are listed in Table 1. These results make solution B, 16 vol % acetic acid and 10 vol % hydroxyacetone, the solution that the following bio-oil model solutions should be compared to in the bar graph. As the solutions were incrementally loaded with bio-oil components, the impact of these compounds is noted in the acetone and 2-butanone yields. Solution C containing 2 vol % furfural had little impact on acetone and butanone yields. A total of 77% of the furfural was recovered in the receiver, with about 3% of the furfural having been converted to the aldol condensation product 4-(2-furanyl)-3-butene-2-one and the partially hydrogenated analogue, 4-(2-furanyl)-2-butanone, along with a small amount of 1-(2-furanyl)-ethanone. The addition of 1 vol % each of phenol, guaiacol, and m-cresol in solutions D−F also had a minimal effect on conversion. Adding 1 vol % eugenol to the solution, however, caused about 40%

Figure 6. Gas evolution profile resulting from the ketonization of solutions B and H. The CO and H2 lines from solution B were omitted for clarity. Solution B contained 16% acetic acid and 10% acetol, whereas solution H contains 16% acetic acid, 10% acetol, 6% furfural, 1% cresol, and 5% guaiacol. The reaction temperature was 400 °C. The dips in the CO2 line from solution B and the small peaks in the CO line from solution H are the result of sampling.

components of bio-oil. These analyses were performed on samples collected after the reaction had been running for at least 2 h. The reaction and system were taken to be stable once a steady level of CO2 in the effluent gas was observed. This can be seen in Figure 6. Acetone production was used as the measure of ketonization, and its lowered production was used as a sign of catalyst inhibition or poisoning. It should be noted that, despite having a condenser cooled to 5 °C and the receiver in ice, not all of the acetone made from acetic acid was collected and carbon balances were about 75%. Upon passing a solution of aqueous acetone through the loaded reactor, it was found that, dependent upon catalyst loading and temperature, 70− 85% of the acetone was collected. Some acetone was detected in the gas stream (as m/e 58 and 43) along with CO, and coke formation was also noted by CO2 produced upon burnoff in flowing air of the catalyst bed. Tar was also found on the glass beads downstream of the catalyst bed. The ketonization of 16 vol % aqueous acetic acid over Fe0.2Ce0.2Al0.6Ox at a WHSV of 3 was complete at 400 °C, giving acetone without any unreacted acid remaining. This is solution A in the bar graph and has the potential to give an 3939

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Table 1. Distribution of Ketones, Expressed as Percentages of Total Ketones, from the Model Pyrolysis Oil Solutions at 400 and 430 °Ca solution B

solution C

solution D

solution E

solution F

solution G

solution H

ketone

400 °C

430 °C

400 °C

430 °C

400 °C

430 °C

400 °C

430 °C

400 °C

430 °C

400 °C

430 °C

400 °C

430 °C

acetone 2-butanone 3-methyl-2-butanone 2-pentanone 3-pentanone 3-methyl-2-pentanone 3-hexanone 2-hexanone 3-methyl-2-cyclopentene-1-one

72.8 17.9 2.6 1.5 1.1 0.0 0.4 0.0 3.3

73.2 17.2 3.6 1.4 1.2 0.4 0.0 0.3 2.7

77.8 16.2 1.3 1.1 0.7 0.0 0.0 0.0 2.8

74.4 15.9 3.2 1.7 1.4 0.0 0.0 0.0 3.4

85.3 11.8 0.0 0.7 0.0 0.0 0.0 0.0 2.2

83.2 13.1 0.0 0.9 0.0 0.0 0.0 0.0 2.7

88.0 9.4 0.0 0.0 0.0 0.0 0.0 0.0 2.6

83.2 13.1 0.0 0.9 0.0 0.0 0.0 0.0 2.7

78.8 15.9 1.1 1.2 0.0 0.0 0.0 0.0 3.0

74.9 15.9 3.0 1.6 1.3 0.5 0.0 0.0 2.8

89.4 6.2 0.0 0.0 0.0 0.0 0.0 0.0 4.4

82.4 10.3 3.9 0.0 0.0 0.0 0.0 0.0 3.4

91.5 5.1 0.0 0.0 0.0 0.0 0.0 0.0 3.4

89.9 7.1 0.0 0.0 0.0 0.0 0.0 0.0 3.0

a

A, 16 vol % acetic acid; B, A + 10 vol % hydroxyacetone; C, B + 2 vol % furfural; D, C + 1 wt % phenol; E, D + 1 vol % guaiacol; F, D + 1 vol % mcresol; G, F + 1% eugenol; and H, B + 6 vol % furfural + 1 vol % m-cresol + 5 vol % guaiacol.

inhibition of the ketonization at 400 °C but less with the reactor at 430 °C. The eugenol was recovered at about 25% of the starting stock solution, with the balance apparently lost as coke because no new compounds were found. Likewise for the solutions containing guaiacol, no new compounds associated with guaiacol were found and guaiacol was recovered at 85%. The final mixture, solution H, containing 6% furfural, 1% each of phenol and m-cresol, and 5% guaiacol had the lowest level of conversion. There are two messages to be taken from the bar graph. First, there is little inhibition by furfural or phenolics at low concentrations, but inhibition becomes significant at furfural and guaiacol levels that are more representative of pyrolysis oil.29 Second, much of the inhibition can be overcome by running at 430 °C rather than at 400 °C. The higher temperature affects ketone distribution as well. Table 1 shows that, at each of the two temperatures, acetone yields were the highest because of the high acetic acid concentration but the higher molecular weight ketones increased in yield as the temperature was increased to 430 °C. Although the high temperatures used here may seem severe, they are in the range of bio-oil vapors exiting as a pyrolyzer. It is envisioned that a ketonization catalyst bed would be placed in the hot gas stream of a pyrolysis unit. Other components of pyrolysis oils that are potentially problematic for a fixed-bed catalyst that were not examined here include the minerals from ash, anhydrosugars, and pyrolytic lignin that can form in the vapor phase of pyrolysis.30 Figure 6 shows the gas evolution profiles of solutions B and H for a comparison of CO2 production from the solutions that yielded the highest and lowest levels, respectively, of conversion. It can be seen that CO2 production from solution B is quite steady at 10.8 mol % over the course of the reaction. Solution H is significantly inhibited initially and continues to yield less CO2 over time. The decline is about 10% between 100 and 300 min. This suggests that competition at active sites keeps ketone production low at the start and catalyst poisoning, perhaps simply by coking, causes ketone production to fall further. 3.3. Reactions of Hydroxyacetone. The reaction of a 10 vol % hydroxyacetone solution over Fe0.2Ce0.2Al0.6Ox produces a mixture of acids and ketones at 400 °C and only ketones at 430 °C. At either temperature, the ketones are primarily acetone and 2-butanone but other longer chain ketones and branched ketones are also found. These are shown in Table 2. The gases formed by this reaction are CO2, CO, and H2 in approximately a 2:1:3 molar ratio, although this ratio has to be

Table 2. Distribution of Ketones Formed from Hydroxyacetone Ketonization at 400 °C over Fe0.2Ce0.2Al0.6Ox ketone

percentage of ketones formed (%)

acetone butanone 3-methyl-2-butanone 2-pentanone 3-pentanone 3-methyl-2-pentanone 3-hexanone 2-hexanone 3-methyl-2-cyclopentene-1-one 2-methylcyclopentanone

44.4 34.3 3.3 4.3 5.2 1.0 0.9 0.9 4.6 1.0

regarded with caution because each can be formed as a result of the reactant or product coking. The formation of acetone requires that C−C bond breaking occurs, while the formation of 2-pentanone from acetic and butyric acids as well as the branched ketones is indicative of C−C bond formation and chain rearrangement. The formation of acetic acid from hydroxyacetone would require a bound hydroxyacetone splitting between C1 and C2, giving a catalyst-bound acetyl group and formyl group. Attack of the acetyl group by water would give acetic acid. Likewise, the formyl group could form formic acid, which can undergo ketonization with acetic acid, forming acetaldehyde, which can go on to acetone. The proposed mechanism of water attacking a surface-bound η2-aldehyde was examined by following isotope labels through the ketonization of propanal. These results are discussed below. The branching and chain-lengthening reactions are harder to explain. It is possible that the decarbonylation reaction leaves behind a reactive methyl group, such as that proposed for the Pd- and Pt-catalyzed decomposition of ethanol.31 This surfacebound methyl group could then become added to a carbon chain. 3.4. Ketonization of Propanal. To further examine the mechanism by which hydroxyacetone is converted to 2butanone, the ketonization of propanal was studied because this is a proposed intermediate at the catalyst surface. Figure 8 shows the gas evolution resulting from the ketonization of 10 vol % propanal in H2O and in deuterium oxide. This experiment was performed by starting with propanal in H2O, allowing that reaction to run until stable gas yields were obtained, and then promptly switching the feed to a solution of 3940

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parent ion and molecular ion, respectively. These form largely as a result of the natural abundance of 18O in 3-pentanone. Table 3 shows the mass spectral results, and it can be seen that 18 O enrichment does not meet the expected levels from a reaction of acids enriched 24.3-fold in 18O. This adds support to the dual mechanism route to 3-pentanone. The enrichment is lower than expected because some of the 3-pentanone forms directly from propanal through decarbonylation and reaction with surface-bound acid. It should also be noted that no C18O was detected in the gas stream, indicating that only the aldehyde and not bound acids undergo decarbonylation. As noted above, the oxygen atoms of surface-bound acids are equivalent; therefore, decarbonylation of this species would yield C18O. Both of the isotope-labeling experiments support the two routes to 3-pentanone. However, the ratio of H2/HD is reflective of the presence of each mechanism: about 87% of the hydrogen gas is HD, indicating that 87% of the propanal goes through propionic acid, while the remaining 13% undergoes the ketonization reaction directly. The mixed oxide catalyst is weakly basic, leaving open the possibility that the ketonization of propanal is the result of propionic acid formation via the Cannizzaro reaction, which is base-catalyzed.32 To examine this possibility, the ketonization of 1-propanol, propanal, and propionic acids were performed under conditions of low turnover. The reactor was loaded with 1 g of catalyst, and the reaction was run at both 400 and 430 °C. Substrates were 1.6 M aqueous solutions and were delivered at a rate of 0.32 mmol of substrate/min. At 400 °C, the propanal reaction showed propanal distributed as 36% unreacted, 8.5% found as 3-pentanone, and 4.5% found as propionic acid. No 1-propanol was found. Carbon recovery was only 49%, but the gases formed from the reaction suggest that ketonization was greater than reflected by the 3-pentanone isolated; 2.8 times as much CO2 was formed as ketone. Also, the large amounts of CO and H2 that formed (0.12 and 0.13 mmol/min, respectively) suggest the ketone may have been lost as coke or tar. At 430 °C, 43% of the propanal ends up in 3pentanone, 2.3% as propionic acid, and 5.5% unreacted. Of course, the 1-propanol that may form by the Cannizzaro reaction could be lost as coke, but the reaction of 1-propanol at 400 °C suggests otherwise. Only 2.9% of the 1-propanol forms 3-pentanone, but 87% is recovered unreacted. At 430 °C, these numbers change to 15 and 74%, respectively. The reaction of propionic acid at 400 °C led to 54% of this substrate to be found in the 3-pentanone and 4% left unreacted, and at 430 °C, 55% ended as ketone, with none left unreacted. In light of the relative reactivities of 1-propanol and propionic acid, it seems unlikely that the Cannizzaro reaction is in play over Fe0.2Ce0.2Al0.6Ox. It would be expected that, if it were, 1propanol rather than propionic acid would be found in the product mix from the reaction of propanal.

Figure 8. Gas evolution from the ketonization of propanal in H2O and, to the right of the dashed line, D2O.

propanal in D2O. In the figure, to the left of the dashed vertical line, the H2O solution was in the reactor and, to the right of the line, the deuterium oxide solution was reacting. The gases produced are CO2, CO, H2, HD, and a small amount of D2. The plot clearly indicates that the D2O reacts with the aldehyde proton of adsorbed propanal to produce HD. This suggests that the OH(D) of water reacts with the bound propanal to make propionic acid. Two propionic acids then react to give 3pentanone. CO2, as always, is the co-product of this ketonization. This mechanism is shown schematically in Figure 9. The concomitant formation of CO through decarbonylation suggests two mechanisms are at work on this catalyst. The ratio of CO2/CO is 8.2, indicating that 89% of the propanal goes through the decarboxylation pathway. Decarbonylation of propanal would give a surface-bound C2 fragment and proton. The C2 would have to react directly with another propanal to give the ketone and H2. The surface-bound proton must be exchangeable with the D2O. Such a mechanism explains the small amount of D2 produced by the reaction in deuterium oxide. To further examine these mechanisms, the ketonization of propanal was performed in water enriched to 10% H218O. This is a 48.8-fold increase over the natural abundance of 18O, which is 0.205%. Gas evolution from the reaction performed using this solution showed the formation of CO18O from the reaction well above natural abundance (Figure 10). Because the surfacebound acid would have equivalent oxygen atoms, ketonization between these species would give CO18O and 18O-enriched 3pentanone. With the 18O being split between three products, the CO2 should be 48.8-fold enriched, meaning 20% of the CO2 formed from the reaction should be CO18O. This was found to be 13. The ketone, forming from an acid that is 10% enriched in 18O but splits these oxygens with the water that forms as a co-product, should be 24.3-fold enriched in 18O. This enrichment is manifested in the mass spectral data at m/e 59 and 88. These masses are two mass units greater than the

Figure 9. Proposed mechanism of the formation of bound propionic acid from propanal on the surface of Fe0.2Ce0.2Al0.6Ox. The incorporation of hydrogen and oxygen isotopes is included. For simplicity, these are left out of the product water. 3941

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Table 3. Mass Spectral Data of 3-Pentanone Fragments Containing Natural Abundance Oxygen and the 3Pentanone Made in 18O-Enriched Watera relative intensity m/e

natural abundance

57 59 86 88

100 0.28 23.1 0.08

18

O enriched 100 4.6 22.3 1.1

expected intensity 100 6.8 23 1.9

a

Expected intensities are those expected from 3-pentanone fully 25fold enriched.

4. CONCLUSION This work has demonstrated that the mixed metal oxide Fe0.2Ce0.2Al0.6Ox is effective in converting the acetic acid and hydroxyacetone in the vapors of a pyrolysis oil model solution to ketones. Inhibition of the catalytic activity was seen by other components of pyrolysis oil, such as eugenol, guaiacol, and to a lesser extent, furfural, but this could be overcome by operating the catalyst bed at 430 °C rather than at 400 °C. The mechanism of hydroxyacetone ketonization was shown to include a surface-bound aldehyde that reacts with water to form the corresponding acid. This then goes on to form the ketones.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 309-681-6255. Fax: 309-681-6524. E-mail: [email protected]. Notes

Disclaimer: 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 USDA. The USDA is an equal opportunity provider and employer. The author declares no competing financial interest.

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ACKNOWLEDGMENTS The author thanks Judith A. Blackburn, Dr. Rex Murray, and Dr. Mike Appell of NCAUR for technical assistance and helpful discussions and Dr. Brent H. Shanks and Dr. Ryan W. Snell of Iowa State University for the SEM images and XRD data.

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REFERENCES

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