Effect of Functional Groups on Autothermal Partial Oxidation of Bio-oil

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Effect of Functional Groups on Autothermal Partial Oxidation of Bio-oil. Part 2: Role of Homogeneous and Support-Mediated Reactions Jacob S. Kruger, David C. Rennard, Tyler R. Josephson, and Lanny D. Schmidt* University of Minnesota, 421 Washington Avenue Southeast, Minneapolis, Minnesota 55455, United States ABSTRACT: This work aims to clarify preferred chemical routes in an autothermal system by investigating individually two-carbon molecules containing the functional groups found in bio-oil. In part 2, conversion and selectivity to major compounds are compared in the presence and absence of R-Al2O3 support and oxygen. Oxygen significantly increases conversion, even at feed rates insufficient to sustain autothermal operation. In general, below 450 °C, homogeneous reactions in the absence of oxygen are insignificant for all molecules; in the presence of oxygen, the reaction onset temperature is somewhat lower. The role of the R-Al2O3 support appears to be a combination of improved heat transfer and the radical quenching ability of the foam structure. Acid catalysis appears minor compared to these other functions. With no O2 co-feed, acids, ethers, and aldehydes appear to be more stable than alcohols and esters. With O2 co-feed, acids and ethers may be less reactive in the gas phase than the other functionalities studied. Implications for autothermal systems are discussed.

’ INTRODUCTION Pyrolysis is a promising technique to convert renewable feedstocks, such as biomass, into a relatively easily transported, energy-dense liquid. The liquid or bio-oil is chemically complex but consists largely of organic molecules that can be categorized by functional group. Bio-oil requires further processing to be useful with current infrastructure, and numerous attempts at applying fossil fuel technology to bio-oil upgrading have met with varying degrees of success.1 One potential technology for processing bio-oil is autothermal partial oxidation to synthesis gas. We previously reported on the autothermal partial oxidation of two-carbon fuels representing functional groups found in biomass pyrolysis oil2 but for brevity included only whole-system (autothermal, supported noble metal catalyst) and catalyst without O2 experiments, for the time, neglecting the activity of the support and homogeneous reactions. Here, we complete our analysis of C2 bio-oil model compounds by investigating the role of reactions that occur on the alumina support and in the gas phase, as well as the oxygen dependence of those reactions. ’ EXPERIMENTAL SECTION The experimental apparatus was similar to that used in the O2-free experiments in part 1 (10.1021/ef200455q) of this work2 and is shown in Figure 1. The O2 flow used in the autothermal experiments was substituted with N2, such that the residence time for each fuel in these experiments was approximately equal to the experiments in part 1 (10.1021/ef200455q). The catalyst bed from the autothermal experiments was replaced with uncoated 65 pores per linear inch (ppi) R-Al2O3 foam monoliths, with a fourth uncoated monolith as a back heat shield and thermocouple support. The use of an external heat source in these experiments rather than the in situ heat generation of the autothermal experiments likely created different temperature profiles in these experiments than in the autothermal experiments. As a result, we cannot quantitatively determine contributions of different reaction pathways or the exact amount of homogeneous chemistry in the autothermal system. However, the trends in selectivity and the variation r 2011 American Chemical Society

between fuels warrant some interesting qualitative conclusions, and the product spectra provide some insight into the reactions that may be taking place at a given temperature in the autothermal reactor. Four experiments were performed with each fuel. Uncoated foam experiments with and without O2 co-feed, heated to the autothermal operating temperatures with a clamshell furnace, provided insight into the role of the support. Empty tube experiments with and without O2 cofeed elucidated purely homogeneous reaction pathways. The O2 co-feed in uncoated foam and empty tube experiments was substantially less than in the autothermal experiments because of safety concerns. The C/O molar ratio (defined as the ratio of carbon in the feed molecule to atomic oxygen in the O2 co-feed) was approximately 7, which is outside the flammability limits of every fuel, except acetaldehyde. However, as will be seen in the following sections, even a small amount of oxygen addition clearly demonstrates the role of O2 in reaction initiation.

’ RESULTS Conversion of each feed molecule in each of the four regimes is shown in Figure 2, along with results for the autothermal experiments over Pt and Rh for comparison.2 In general, conversion was significantly lower in the current experiments than in the autothermal situation, although in the presence of 0.1 standard liter per minute (SLPM) O2, conversion of ethylene glycol and acetaldehyde and ethanol over uncoated foams was comparable to conversion over Pt in the autothermal experiments. In the case of acetaldehyde in the empty tube, the addition of 0.1 SLPM O2 yielded higher conversion than the autothermal Pt experiments. Also, the product distribution in these experiments more closely approximated Pt than the Rh product distribution in the autothermal experiments, consistent with previous work by this group.3
5 This observation may be in part due to the similarity in apparent reaction pathways for some of Received: March 24, 2011 Revised: May 31, 2011 Published: June 01, 2011 3172

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Figure 1. Reactor configuration for the (left) uncoated foam and (right) empty tube experiments.

these molecules homogeneously and over Pt and the lower water
gas shift and reforming activity of Pt relative to Rh. In some cases, temperature control was precluded by ignition of the fuel; that is, despite varying the reactor configuration and increasing the diluent (N2) flow rate, the steady-state backface temperature was higher than that observed in the autothermal experiments, regardless of the presence of the external heat source. In these cases (methyl formate over the uncoated foams at all temperatures below 624 °C and ethylene glycol uncoated foam at target temperatures below 500 °C), conversions and selectivities are not shown. The reason ignition occurs for some fuels but not others and at low but not high temperatures is not well-understood at this point, but it does not appear to be due to the longer residence time or lower dilution of O2, because increasing the N2 flow rate by a factor of 2 does not affect the phenomenon. In general, O2 appears to have a larger effect than the monolith, although acetic acid, which is difficult to oxidize, is an exception to this trend. Another feature of these experiments is the shift in the role of the monolith between fuels. For some fuels, it appears that the monolith acts primarily to increase heat transfer from the walls of the tube, while in other cases, the monolith seems have more effect as a radical quencher. We speculate that the primary role depends upon both the physical properties (e.g., boiling point, heat capacity, and heat of vaporization) and the decomposition mechanism of the feed molecule, which leads to results that may appear contradictory (i.e., in some cases, higher conversion is observed in the empty tube, while in others, the uncoated foam yields higher conversion). We note that the two lowest boiling fuels (dimethyl ether and acetaldehyde, which also have the lowest ΔHvap) both exhibit higher conversion in the empty tube than over the blank foam (when there is a significant difference between the tube and the foam), possibly suggesting that the heat-transfer ability of the monolith is less important for easily vaporized fuels. For other fuels, a hot monolith may assist in initiating reactions, possibly explaining the higher conversion over the monolith versus the empty tube for some fuels, while selectivity is unchanged. We attempt to clarify these effects in the

following sections, but more studies are necessary to fully understand the role of the monolith. At conversions less than 0.5%, selectivities are not quantitative and have been omitted from the figures. A cutoff conversion of 3.5% was used for methyl formate because of the presence of 3% methanol in the feed and 1% for dimethyl ether because of 1% isobutene and propane in the feed. Acetaldehyde. Conversion (Figure 2) was negligible in the absence of O2 below 525 °C but became comparable to the autothermal experiments over Pt with only 0.1 SLPM O2 in the feed.2 Conversion in the empty tube with O2 exceeded conversion over the Pt catalyst at temperatures below 525 °C. The major carbonaceous product from acetaldehyde was CO, although CH4, CO2, C2H6, and CH3OH were also significant. The latter three were only detected in significant quantities when O2 was present, as shown in Figure 3. In that case, selectivities to CO2, C2H6, and CH3OH were similar to each other in the presence of O2 and decreased slightly as the temperature increased, while selectivity to CH4 was slightly higher and increased slightly as the temperature increased. Selectivity to H2 was below 10% in these experiments, although selectivity to water reached 30% in the empty tube and was somewhat suppressed by the presence of the foam, as shown in panels a and b of Figure 4. Selectivity is generally not influenced by the presence of the monolith. In the absence of O2, CO and CH4 were the only products that were detected in significant quantities, likely because of the low conversion. Selectivities are generally consistent with the acetaldehyde pyrolysis mechanism first proposed by Rice and Herzfeld6 and later slightly modified by several experiments and calculations.7,8 In such a mechanism, the reaction is initiated by C
C bond scission, generating CH3 and CHO radicals. Methane and CO are then produced primarily by chain-propagation steps involving hydrogen abstraction from acetaldehyde (primarily from the R-C) and dissociation of the resulting CH3CO radical, respectively.7 Chain-terminating combination of CH3 radicals would form the minor product C2H6. For the case of O2 addition, the Rice
Herzfeld mechanism can likely be modified to include 3173

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Figure 2. Conversion of C2 feeds. For autothermal experiments, the point furthest to the right corresponds to the feed carbon/atomic oxygen co-feed (C/O) ratio = 1.2, while the point furthest to the left corresponds to C/O = 2.0, except for ethanol over Pt, in which case the point farthest to the left represents C/O = 1.8. (2) Uncoated foam and no O2 (F), (9) empty tube and no O2 (T), (4) uncoated foam and 0.1 SLPM O2 (FO), (0) empty tube and 0.1 SLPM O2 (TO), () Pt autothermal, and (þ) Rh autothermal.2

propagation steps involving oxygenated radical species, including O and OH, which can lead to the formation of CO2 and CH3OH in chain-propagating or chain-terminating steps. In the presence of O2, conversion was much higher, likely because of the initial presence of the diradical O2. The higher conversion in the empty tube experiments than in the uncoated foam or autothermal Pt experiments may be due to the radical

quenching ability of the foam monolith. The pore size in a 65 ppi monolith is such that the distance between walls is likely small enough to extinguish homogeneous flames because of radicalquenching reactions at the pore walls.9,10 Liu and Laidler7 found that increasing the surface area of their reaction vessel slightly increases the rate of acetaldehyde pyrolysis, which they attributed to the surface participating in initiation reactions. Here, however, 3174

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Figure 3. Carbon selectivity to major products from acetaldehyde. For autothermal experiments, the point furthest to the right corresponds to C/O = 1.2, while the point furthest to the left corresponds to C/O = 2.0. (2) Uncoated foam and no O2 (F), (9) empty tube and no O2 (T), (4) uncoated foam and 0.1 SLPM O2 (FO), (0) empty tube and 0.1 SLPM O2 (TO), () Pt autothermal, and (þ) Rh autothermal.2

the opposite is observed, which we tentatively attribute to the radical-quenching ability of the foam structure. The foam has a different geometry than the packed vessel used by Liu and Laidler,7 and the surface area/volume ratio in these experiments is roughly 3 orders of magnitude larger than in Liu and Laidler’s experiment. This suggests that, if the monolith surface were more active for reaction initiation from acetaldehyde than reaction

quenching, a significant increase in conversion would be observed. In comparison to the autothermal reactor, we surmise that there may be significant homogeneous chemistry for acetaldehyde in the portion of the reactor with O2 present (generally the first few millimeters of the catalyst bed5). However, after O2 is consumed, surface reactions may be insignificant over Pt, while 3175

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Figure 4. Hydrogen selectivity from ethanol and acetaldehyde. For autothermal experiments, the point furthest to the right corresponds to C/O = 1.2, while the point furthest to the left corresponds to C/O = 2.0, except for ethanol over Pt, in which case the point farthest to the left represents C/O = 1.8. (2) Uncoated foam and no O2 (F), (9) empty tube and no O2 (T), (4) uncoated foam and 0.1 SLPM O2 (FO), (0) empty tube and 0.1 SLPM O2 (TO), () Pt autothermal, and (þ) Rh autothermal.2

Rh may continue to convert acetaldehyde and intermediate C2H6 and CH3OH to syngas. Ethanol. Carbon selectivities to major products from ethanol are shown in Figure 5; hydrogen selectivities are shown in panels c and d of Figure 4. In the absence of O2 and below 600 °C, the major product is acetaldehyde, which may be formed by H abstractions by oxygenated radicals or by non-O2-mediated pyrolysis of ethanol. In the former, H2O is likely the other main product; in the later, H2 would be formed as well. In the presence of O2, the selectivity to CO and CH4 is higher than without O2, while selectivity to acetaldehyde is correspondingly lower. This can likely be explained by the decomposition of acetaldehyde, which occurred readily in the presence of O2 as discussed in the previous section. However, a direct decomposition of ethanol by C
C bond scission may also be possible.11 The foam gives nearly equal selectivities to CO and CH4 in the presence of O2, while the empty tube gives higher selectivity to CO and lower selectivity to CH4. This phenomenon suggests that, over the uncoated foam, O2 may be consumed almost entirely by initiation reactions, producing primarily H2O and acetaldehyde. In the empty tube, the absence of a solid surface to transfer heat from the reactor walls to the reactor center may allow for some O2 to be consumed by reactions with CHx species from acetaldehyde

decomposition, resulting in increased selectivity to CO. It is also worth noting that conversion is also higher over the foam than in the empty tube; therefore, if O2 is consumed primarily in the production of H2O, increased conversion of ethanol by non-O2mediated reactions would yield a lower selectivity to H2O, which is observed over the foam. Ethylene is also significant at 15
25% selectivity. Selectivity to ethylene was significantly higher in all four of these experiments than in the catalytic experiments,2 where ethylene selectivity was always less than 5%. Most notably, the selectivity to ethylene was relatively insensitive to the presence of the alumina support, which could potentially act as a solid acid catalyst for the dehydration reaction. It is worth noting at this point that the foam monoliths in this study consist of 99% polycrystalline R-Al2O3, which is generally considered a poor catalyst.12,13 The formation of ethylene in an autothermal system can thus be attributed almost exclusively to homogeneous chemistry that does not require oxygen. Indeed, dehydration is a well-known pyrolysis reaction of ethanol14 and is especially predominant in the absence of radical chemistry.15 Here, we observe only a minor effect with the addition of O2 and the addition of a monolith, suggesting that, for ethanol, any changes in radical chemistry introduced by O2 or the monolith may be offset by changes in thermal chemistry for this system. 3176

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Figure 5. Carbon selectivity to major products from ethanol. For autothermal experiments, the point furthest to the right corresponds to C/O = 1.2, while the point furthest to the left corresponds to C/O = 2.0, except for Pt autothermal, in which case the point farthest to the left represents C/O = 1.8. (2) Uncoated foam and no O2 (F), (9) empty tube and no O2 (T), (4) uncoated foam and 0.1 SLPM O2 (FO), (0) empty tube and 0.1 SLPM O2 (TO), () Pt autothermal, and (þ) Rh autothermal.2

The condensation of ethanol to form diethyl ether has not been observed in ethanol pyrolysis or combustion experiments. In the homogeneous and uncoated foam experiments here, as in the autothermal experiments,2 the selectivity to diethyl ether was negligible. However, in the catalyst and no O2 experiments,2 selectivity to diethyl ether approached 20% over Pt as the

temperature decreased (feed conversion was a relatively low 7% at that point), suggesting that the Pt
support interface may be active for condensation reactions. It is also possible that diethyl ether is formed over the metal surface, although we have not found evidence for this reaction in the surface science literature. 3177

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Energy & Fuels As with acetaldehyde, significant homogeneous chemistry may be present in the autothermal reactor, especially in the oxidation zone and at low C/O ratios. Also, as with acetaldehyde, the conversion of ethanol here was was similar to that over Pt in the autothermal experiments, both of which were much lower than over Rh. The primary route here, as with the Pt catalyst, appears to be through an acetaldehyde intermediate, which is relatively stable in the absence of O2. A different route was supposed to be active over Rh,2 which may account in part for the different product spectra. Ethylene Glycol. Conversion of ethylene glycol displayed some interesting trends (Figure 2), namely, that conversion over the uncoated foam was significantly higher than the autothermal Pt catalyst at the highest temperatures and lower at the lowest temperatures, regardless of the presence of O2. Over the uncoated foam with O2, homogeneous ignition prevented collection of data below 500 °C. With O2 in the empty tube, conversion was comparable to the autothermal Pt catalyst at the highest temperatures but dropped off rapidly below 450 °C. In the empty tube without O2, conversion increased significantly above 500 °C, consistent with the literature.16 The initial temperatures of significant decomposition are higher than those observed by Rudenko et al.,17 which may be due to longer residence times or the presence of potentially active carbon steel in those experiments. The seemingly counterintuitive result that conversion would be higher over an uncoated foam and with less O2 than the autothermal experiments over Pt above 600 °C may result from the likely differences in the temperature profile between these experiments and the autothermal experiments. Such effects would be expected more with ethylene glycol than the other fuels investigated, because it has a higher boiling point, heat capacity, and ΔHvap than the other fuels.18 Despite these differences, the general trends in the selectivity data, shown in Figure 6, can still provide important insight into the reactions of ethylene glycol over the uncoated foam and in the empty tube. In the absence of O2, the major product is acetaldehyde. This product likely results from the molecular dehydration of ethylene glycol, because the most stable conformation of ethylene glycol is the gauche configuration, in which a hydrogen bond exists between the two hydroxyl groups.19 Another possibility is a water-mediated concerted dehydration, which is more energetically favorable than an elimination mechanism.20 Acetaldehyde selectivity displays a maximum at around 525 °C, which may be due to multiple effects. At lower temperatures, a minor pathway in the absence of O2 may involve dehydrogenation and C
C bond scission of a C2HxO2 species (0 e x e 6), resulting in species that ultimately decompose to CO and H2. As the temperature increases, dehydration kinetics may become more favorable to the point that dehydration is essentially the only reaction. As the temperature further increases, decomposition of acetaldehyde would yield CO and CH4; selectivities to H2, CO, and CH4 are consistent with this hypothesis. Alternatively, a dehydration could yield ethylene oxide, although this potential product was not observed and would be expected to rapidly isomerize to the more stable acetaldehyde.21 Additionally, ethylene oxide greatly enhances the decomposition of acetaldehyde;21 the high selectivity to acetaldehyde observed here is further evidence that ethylene oxide is not produced in significant quantities. In the presence of O2, radical chemistry is likely much more prominent. As with ethanol, initiation reactions may be H-atom abstractions from ethylene glycol. The overall mechanism may be similar to the one proposed by Hafner et al.22 for ethylene glycol combustion and by Kim and Hoffmann23 for ethylene glycol photodegradation. Both mechanisms involve C
C bond

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scission to yield a formaldehyde intermediate, which further decomposes. In the current experiments, a decrease in acetaldehyde selectivity is observed with the addition of O2, similar to the ethanol results and possibly indicating the similarity of the homogeneous oxidation mechanism for those two fuels. Additionally, the decomposition of acetaldehyde, which would be expected to be enhanced by O2, may contribute to its lower selectivity. However, the minor selectivity to CH4 suggests that a parallel ethylene glycol decomposition channel may be in operation. The primary role of the foam in the O2 addition experiments may be to improve heat transfer to the center of the reactor, with the hot surface leading to more uniform consumption of O2. The result may be higher selectivity to the C
C bond scission products CO, CO2, and H2 and lower selectivity to the dehydration product acetaldehyde. Comparison to the autothermal experiments is difficult for ethylene glycol because of the conversion trends discussed earlier, although significant homogeneous chemistry may be present in the autothermal system. However, it appears that acetaldehyde formed in those experiments may be formed after O2 consumption is complete and it may be primarily homogeneous. Along the same line, it appears that O2 promotes a C
C bond scission pathway relative to the dehydration path; the larger amount of O2 in the autothermal experiments likely enhances the effect. Acetic Acid. Consistent with its low conversion in the autothermal experiments2 and the observations of other researchers,24 acetic acid was the least reactive in these experiments, displaying conversion less than 20% at all temperatures investigated (Figure 2). Conversion was slightly higher over the uncoated foam than in the empty tube, and acetic acid is different from the other five fuels, in that the presence of the foam appears to have a larger effect than the presence of O2. Acetic acid is well-known to resist oxidation,24 which may allow for the foam to play a larger role than O2. Major product selectivities are shown in Figure 7. In the absence of O2 over the uncoated foam, the major reaction, as observed in the catalyst and no O2 experiments,2 was a bimolecular reaction yielding acetone, CO2, and H2O, likely catalyzed by the oxide surface.25 The selectivity to acetone, CO2, and H2O is nearly at the stoichiometry predicted if this were the only reaction. In the empty tube without O2, this reaction is minor, as evidenced by the significantly lower selectivity to acetone. Instead, decarboxylation, yielding CO2 and CH4, becomes more apparent; other products with significant selectivity included acetaldehyde, ethylene, and benzene (not shown). Additionally, the uncoated foam exhibits significant selectivity to H2 in the absence of O2, which falls off rapidly as the temperature increases. There appears to be two main channels for acetic acid decomposition in this system that do not require a metal catalyst or O2. The bimolecular, oxide-catalyzed reaction yielding acetone, CO2, and H2O dominates in the presence of the foam. The much lower selectivity to acetone in the absence of the foam suggests that, in this case, the foam may promote the ketonization reaction. The extent of this reaction was small, however, because conversion was only around 1%. In the autothermal experiments, this reaction was likely negligible (as evidenced by the negligible selectivity to acetone), which is consistent with the relative inertness of the alumina foam. In the empty tube, some evidence of the ketonization reaction is also present, possibly catalyzed by the reactor walls. Additionally, decarboxylation likely leads to the observed CO2 and CH4. 3178

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Figure 6. Selectivity to major products from ethylene glycol. For autothermal experiments, the point furthest to the right corresponds to C/O = 1.2, while the point furthest to the left corresponds to C/O = 2.0. (2) Uncoated foam and no O2 (F), (9) empty tube and no O2 (T), (4) uncoated foam and 0.1 SLPM O2 (FO), (0) empty tube and 0.1 SLPM O2 (TO), () Pt autothermal, and (þ) Rh autothermal.2

CO2 was the major carbonaceous product in the presence of O2 but displayed slightly different trends depending upon the presence of the foam. Over the foam, CO2 selectivity proceeded through a minimum at 500 °C, at which point selectivity to CO is nearly equal. Methane selectivity was less than 10% throughout but increased slightly as the temperature increased. In the empty tube, CO2

selectivity is initially high but falls off as the temperature increases above 450 °C, accompanied by an increase in CO and CH4. In the presence of O2, three primary routes appear to be active. Oxidation pathways appear to dominate below 500 °C, yielding CO, CO2, and H2O almost exclusively. Selectivity to CH4 remains low as selectivity to CO increases, suggesting that a 3179

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Figure 7. Carbon selectivity to major products from acetic acid. For autothermal experiments, the point furthest to the right corresponds to C/O = 1.2, while the point furthest to the left corresponds to C/O = 2.0. (2) Uncoated foam and no O2 (F), (9) empty tube and no O2 (T), (4) uncoated foam and 0.1 SLPM O2 (FO), (0) empty tube and 0.1 SLPM O2 (TO), () Pt autothermal, and (þ) Rh autothermal.2

significant portion of CO is generated from methyl carbon. However, it cannot be ruled out that dehydration, followed by decomposition of the intermediate ketene, contributes significantly to the CO selectivity. Over the foam, selectivity to CO is higher, possibly because of improved heat transfer through the monolith, yielding a hot surface that may increase conversion as

in the ethylene glycol experiments. As the temperature increases, thermal decomposition may begin to compete with the oxidation reactions. Acetic acid pyrolyzes by competetive decarboxylation and dehydration routes, leading to increased CH4 from decarboxylation and increased CO from dehydration (via a ketene intermediate).26
29 The decarboxylation reaction appears to be 3180

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Energy & Fuels less prominent over the foam than in the tube, because selectivity to CH4 is lower over the foam. The reason for this result is not immediately clear, because neither pyrolysis reaction requires radical chemistry.25,26 It may be that oxidation reactions benefit more than the pyrolysis reactions from the hot surface of the foam and the complex kinetics of the oxidation reactions determine the relative ratios of CO and CO2. It is possible that the foam may be promoting the dehydration reaction, which ultimately yields the same products as the oxidation reactions, but this hypothesis would not be consistent with the other experiments, because the foam did not appear to be active for dehydration of ethanol or ethylene glycol. In the empty tube, the relative contributions of dehydration and decarboxylation are expected to be competitive and the dehydration may proceed directly or indirectly by a 1,3-H shift.26
29 The indirect dehydration has been found to be slightly more energetically favorable, at least at higher temperatures,27,28 and the rate of dehydration changes from second to first order as the temperature increases just above the range explored here.26 Thus, the phenomena observed in these experiments may be explained by the complex kinetics of these reactions. Oxygen breakthrough was observed at all temperatures in the empty tube and all but the highest temperature over the uncoated foam, suggesting that the combustion reactions are kinetically limited and proceed at a similar rate to pyrolysis, at least above 500 °C. Further investigation on the mechanism of acetic acid decomposition is clearly necessary, but at this point, it does not appear that one pathway is dominant and rather that multiple reaction channels are competitive. Still, conversion was much lower in these experiments than in the autothermal experiments,2 suggesting that the contribution of homogeneous and support chemistry in an autothermal system may be minor for acetic acid. Methyl Formate. Conversion of methyl formate (Figure 2) was a strong function of the temperature and O2 feed, and consistent with the autothermal experiments,2 it was the most reactive feed investigated. Over the uncoated foam in the presence of O2, the temperature control could not be maintained at an acceptable accuracy below 624 °C. At that temperature, selectivities were different than either the empty tube plus O2 or the uncoated foam and no O2 experiment, but as discussed below, the differences may be due to the heat-transfer effects of the monolith; chemistry catalyzed by the support surface is likely minor compared to gasphase oxidation and decomposition. The major products are shown in Figure 8. In the absence of O2, the dominant pathway appears to be decarbonylation to yield methanol and CO, both over the foam and in the empty tube. A minor pathway comprises decarboxylation to yield CO2 and CH4; the relative contribution of each of these pathways is generally consistent with the literature.30,31 The dominant pathway, leading to CO and CH3OH, likely passes through a threemember cyclic transition state between the carbonyl carbon, the carbonyl hydrogen, and the bridging oxygen.30,31 In the presence of O2, the selectivity to methanol is significantly decreased, while selectivity to CO2 and H2 are significantly increased. Dooley et al.31 found that the decomposition reaction to CO and methanol was still the primary source of methanol in the oxidation of methyl formate but radical reactions are responsible for CO, CO2, H2, and H2O formation. More specifically, they found that hydrogen abstraction reactions account for the majority of fuel consumption and that abstraction of carbonyl hydrogen is slightly less favorable than abstraction of methyl hydrogen, which is consistent with the lower methanol selectivity.

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Furthermore, they found that hydrogen abstractions by H and OH radicals are the most common, which is consistent with the increased selectivity to H2 and H2O. At the one comparable data point between the uncoated foam and empty tube with O2 experiments, the difference in selectivity to CO and CO2 may be due to improved heat transfer in the uncoated foam experiments, resulting in more uniform consumption of O2. Higher selectivity to CO over the foam relative to in the empty tube was also observed for acetic acid and ethylene glycol, possibly for the same reason. Furthermore, selectivities, especially to CO2 and H2O, were considerably less reproducible than to the other species in the empty tube with O2 experiments, also suggesting complex flame phenomena that may be quenched by the monolith. In summary, however, the oxidation mechanism by Dooley et al.31 is not inconsistent with the results observed here. For these reactions, the radical-quenching ability of the monolith may play a secondary role to the reactivity of methyl formate, which is significantly less thermally stable than the other fuels and is highly flammable (i.e., reactive with O2). With respect to the autothermal reactor, it is conceivable that much of the observed methyl formate chemistry is homogeneous, with the noble metals primarily catalyzing secondary reactions, such as decomposition of the intermediate methanol and water
gas shift. This hypothesis is supported by the very similar trends in selectivity and conversion observed over the Pt and Rh catalysts for methyl formate,2 while trends were somewhat different for the other molecules. Dimethyl Ether. Conversion of dimethyl ether (Figure 2) was less than 5% in the absence of O2 and less than 1% over the foam in the absence of O2. With 0.1 SLPM O2 added, conversion was 10% in both the empty tube and uncoated foam experiments, although it increased to 25% in the empty tube at the highest temperature investigated. The presence of O2 has significantly more of an effect than the presence of the monolith, but the monolith may have some radical-quenching role, as evidenced by higher conversion in the empty tube experiments at the upper end of the temperature range. In the absence of O2, the major products were CH4, CO, and H2 (Figure 9). The apparently stoichiometry-defying selectivity to CH4 in the empty tube experiments may be due to the production of formaldehyde, which cannot be detected in the current configuration. Incomplete dissociation of formaldehyde in the reactor system would yield an artificially high selectivity to CH4. It is important to note that conversion is lower for dimethyl ether than other fuels, where a formaldehyde intermediate is also expected (e.g., ethylene glycol); the result is that the error in apparent CH4 selectivity is higher. A corollary is that the calculated selectivity to H2O is artificially high, because undetected formaldehyde contains an oxygen atom. The important observation is that the major products are CH4, CO, H2, and a miniscule amount of C2H6, which is consistent with the dimethyl ether pyrolysis literature.32,33 The mechanism suggested in those references involves homolytic C
O bond scission to generate CH3 and CH3O radicals, with the latter decomposing to formaldehyde and a H radical. The formaldehyde then decomposes to CO and H2. The CH3 and H radicals can react further to form CH4 by a termination step or by abstracting a H from another molecule (dimethyl ether or formaldehyde) to form CH4 or H2, respectively. The main termination step, as with acetaldehyde, is the combination of CH3 radicals to form C2H6. With the monoliths, conversion was below 1% over the entire temperature range investigated; therefore, selectivity data are not shown. The lower conversion in the presence of the foam also 3181

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Figure 8. Carbon selectivity to major products from methyl formate. For autothermal experiments, the point furthest to the right corresponds to C/O = 1.2, while the point furthest to the left corresponds to C/O = 2.0. (2) Uncoated foam and no O2 (F), (9) empty tube and no O2 (T), (4) uncoated foam and 0.1 SLPM O2 (FO), (0) empty tube and 0.1 SLPM O2 (TO), () Pt autothermal, and (þ) Rh autothermal.2

suggests that the foam may be acting as a radical quencher, because dimethyl ether pyrolyzes via a radical mechanism.32,33 The reaction scheme in the presence of O2 is likely similar to the pyrolysis scheme, except that oxygenated radicals (especially OH and HO2) may be present in the radical pool, generating CO2 and H2O in addition to the products observed in the O2-free

experiments.33 The selectivity to CH4 is significantly reduced, which is consistent with reactions of oxygenated radicals with both CH3 radicals and unconverted fuel. Fischer et al.33 and Curran et al.34 developed high- and low-temperature models for the oxidation of dimethyl ether in a variety of conditions and were able to correlate much of the experimental data to that 3182

dx.doi.org/10.1021/ef200456m |Energy Fuels 2011, 25, 3172–3185

Energy & Fuels

ARTICLE

Figure 9. Carbon selectivity to major products from dimethyl ether. For autothermal experiments, the point furthest to the right corresponds to C/O = 1.2, while the point furthest to the left corresponds to C/O = 2.0. (2) Uncoated foam and no O2 (F), (9) empty tube and no O2 (T), (4) uncoated foam and 0.1 SLPM O2 (FO), (0) empty tube and 0.1 SLPM O2 (TO), () Pt autothermal, and (þ) Rh autothermal.2

point; their mechanisms also appear to be valid in this system. Conversion at the highest temperature explored was higher in the empty tube, in both the presence and absence of O2, again suggesting the possible radical-quenching ability of the monolith. With respect to the autothermal experiments, there is likely significant homogeneous chemistry in the autothermal reactor,

but it may be limited primarily to the oxidation zone of the catalyst; conversion over the foams with no O2 was