Comparison of Structural Features of Humins Formed Catalytically

Article pubs.acs.org/EF

Comparison of Structural Features of Humins Formed Catalytically from Glucose, Fructose, and 5‑Hydroxymethylfurfuraldehyde Sushil K. R. Patil, Jacob Heltzel, and Carl R. F. Lund* Department of Chemical and Biological Engineering, University at Buffalo, SUNY, Buffalo, New York 14260, United States ABSTRACT: The IR spectra of humins formed during the acid-catalyzed conversion of glucose, fructose, and 5hydroxymethylfurfuraldehyde were compared. The spectra are quite similar except for three groups of features that can be attributed to furan rings and carbonyl groups conjugated with carbon−carbon double bonds. IR spectroscopy further revealed that benzyl groups could be added to the humins as they formed or in a separate aldol addition/condensation reaction after they had been recovered. The IR spectra are consistent with a model where each of the three reactants must first be converted to 2,5dioxo-6-hydroxyhexanal (DHH) before humins can form via subsequent aldol addition and condensation. The differences in the IR spectral features can then be explained by variations in the concentrations of other aldehydes and ketones that can react with DHH.

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INTRODUCTION Cellulosic biomass is a renewable resource that can be converted into fuels and chemicals. During acid-catalyzed hydrolysis, cellulose depolymerizes forming glucose; the glucose is further converted to 5-hydroxymethylfurfuraldehyde (HMF) and the HMF is converted to equimolar amounts of levulinic acid (LA) and formic acid (FA).1,2 Fructose also may be an intermediate during the conversion of glucose to HMF.3−8 Levulinic acid is stable at reaction conditions,9 so it can be recovered and subsequently used as a platform chemical for the production of a variety of chemicals and fuels.9−11 A major shortcoming of acid-catalyzed hydrolysis, especially when compared to enzymatic processes, is that an undesired side reaction also occurs. Dark-colored, tarry solids known as humins are coproduced2,12,13 as indicated in Scheme 1. If humin formation could be eliminated, acid-catalyzed hydrolysis would be a significantly more attractive option for the conversion of cellulosic biomass to fuels and chemicals.14 Horvat proposed a more detailed pathway, Scheme 2, for the later steps of Scheme 1 where HMF is converted to LA and to humins.15,16 Horvat’s original designations are used in Scheme 2; the intermediates designated with numbers were isolated and detected using NMR, and those designated with letters could not be detected but were suggested or inferred on the basis of similar reactions. Horvat’s pathway stops short of showing how humins form and grow, but it does indicate 2,5-dioxo-6hydroxyhexanal (DHH) as the monomer for humin growth. Horvat could not detect DHH, suggesting that it is highly reactive. We recently suggested that aldol addition/condensation of DHH, via one of its four enols, is responsible for the growth of humins.17 Since DHH is highly reactive, it is expected to react with the first aldehyde/ketone it encounters. As such, the aldehydes and ketones present in the highest concentration are most likely to be added to DHH. In doing so, the carbonyl group of the added aldehydes/ketones will be consumed. If HMF is the starting reactant, it will be the carbonyl-containing reagent most likely to add to DHH, Scheme 3. Presumably, this “side chain” addition generates a more stable species, and since © 2012 American Chemical Society

it retains all three of the carbonyl groups originally present in DHH, it is capable of polymerizing to form humins by further aldol addition/condensation. Infrared spectra of the humins resulting from acid-catalyzed conversion of HMF were consistent with this postulate, showing features that could be attributed to the furan ring of HMF but not its aldehyde group. Infrared spectroscopy additionally demonstrated that if another aldehyde, such as benzaldehyde, was present in high concentration, it, too, was incorporated into the humins, again consuming its carbonyl group. Scanning electron microscopy showed that the morphology of the humins changed when different aldehydes were added in this manner. The present investigation examines the formation of humins when starting with either fructose or glucose as compared to HMF. It considers whether humins form directly from these hexoses, as indicated by the dashed arrows in Scheme 1, or whether the hexoses must first be converted to HMF and then DHH in order for humins to form. Kinetic models including direct conversion of the hexoses to humins are commonly used and found to provide accurate representations of the overall kinetics and selectivity.1,2,13,18−21 Mechanisms for the conversion of the hexoses to HMF have been suggested,5,7,22−25 but to our knowledge no mechanism for direct catalytic conversion of these hexoses to humins has been proposed. Additionally, if fructose is converted under nonaqueous conditions where HMF is stable, no humins form,26−28 suggesting the direct conversion of fructose to humins is insignificant. At the same time, for glucose and fructose conversion in the absence of a catalyst, both direct pathways and indirect pathways have been advocated.29−33

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EXPERIMENTAL SECTION

The experimental methods used in this study were previously described.17 Glucose (99.9%, Sigma-Aldrich), fructose (98%, Fisher Scientific), HMF (98+%, Aldrich), benzaldehyde (99%, Aldrich), Received: May 2, 2012 Revised: July 2, 2012 Published: July 2, 2012 5281

dx.doi.org/10.1021/ef3007454 | Energy Fuels 2012, 26, 5281−5293

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Scheme 1. Possible Pathways for Cellulose Conversion to Humins

cellobiose (98%, Sigma-Aldrich), and sulfuric acid (96%, Acros) were used without further purification. Reactions were studied in 10 mL, thick-walled glass vials with conical magnetic stir bars, sealed with Teflon-lined septa and screw-on plastic caps. In each experiment, five vials were used simultaneously. Heating was effected using aluminum blocks with holes sized to fit the vials, with the blocks immersed in a controlled-temperature oil bath. One such system was used to rapidly heat the samples to a temperature close to the desired reaction temperature whereupon the vials were simultaneously transferred to a second system operating at the desired reaction temperature. To terminate the reaction, vials were removed one at a time and quenched in an ice bath. A small aliquot of liquid was removed from the vial using a syringe with a 0.2 μm filter and injected into an Agilent 1200 Series HPLC. A Biorad Aminex HPX-87H column was used in the HPLC to separate the species present in the reactor solution, and a refractive index detector was used to detect their presence in the 5 mM sulfuric acid solution that was used as the mobile phase. The detector response was calibrated for the major reaction components using reference solutions of the pure components at known concentrations. The remaining contents of the reaction vials were repeatedly centrifuged, decanted, and washed with distilled water until no other components could be detected in the wash water. The resulting solids were then dried overnight at 90 °C, after which they were stored in sealed bottles for further characterization. The conversions versus time were measured separately at 118, 125, and 135 °C for glucose, fructose, and HMF reactants. In all experiments the initial concentration of the reactant and that of sulfuric acid were 0.1 M. This corresponds to a pH of 1.0 for the initial mixture at room temperature. A few of the runs were replicated, and in

some runs the HPLC analysis of each reactor vial, removed at a different reaction durations, was repeated four times. From these repeat analyses we estimate the uncertainty in the measured concentrations to be approximately 10% of the measured value. Solid samples, in air, were placed on the ZnSe crystal of a PIKE MIRacle ATR accessory of a Bruker Vertex 70 infrared spectrometer. The IR spectrum was then recorded using an instrument resolution of 4 cm−1. The spectrum of air was recorded as the background; both measurements used a 100 s scan time. To prepare humin samples for scanning electron microscopy (SEM), they were coated with a thin carbon film and then observed in field free mode using a Hitachi SU70 SEM with the beam energy set at 20 keV. The compositional HPLC analysis yielded the concentrations of glucose, fructose, HMF, LA, and formic acid in the reaction vials at the time they were removed and quenched. The initial concentration of reactant, either glucose, fructose, or HMF, was also known. The reactant conversion could thus be computed directly. Similarly, the yields of all detected solution phase species could be computed directly. The humin concentration could not be measured directly, so the yield of humins, expressed as moles of reactant converted to humins per total moles of reactant converted, had to be calculated by difference. Horvat’s mechanism indicates that LA and formic acid are produced in a 1:1 molar ratio in which case either one could be used to calculate the yield of humins. Surprisingly, the yields of LA and formic acid were not always found to be equal, so in this work, the measured yield of formic acid was used in the calculation of the humins yield. Ab-initio free energies were calculated using the GAMESS computational chemistry program.34 A complete conformational analysis of each molecule studied was not feasible. The molecular 5282


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Scheme 2. HMF Conversion Pathway Proposed by Horvat

mechanics feature of the open-source computer program, Avogadro, performs a random search of rotational conformations, computing their energies using the MMFF94 force field. This type of search was performed on each molecule studied, sampling 100 or more rotational conformations, where possible. The conformation with the lowest energy was then used as the initial geometry guess for the ab-initio calculations. While this does not guarantee that the conformation used for each molecule is the absolute lowest energy conformation, it is expected to yield reaction free energies that are representative of the chemistry taking place. The free energies of the molecules were computed at the G3(MP2, CCSD(T)) level of theory.35−37 When used on the G2/97 test set of 299 molecules, this computational method gave a 1.34 kcal/mol (5.61 kJ/mol) average deviation from experiment. This was deemed to be sufficiently accurate, given that there was no guarantee that the lowest energy conformer had been found for each of the molecules studied. The G3(MP2, CCSD(T)) free energy was calculated at 298 K. In addition, the free energy of solvation in water at 298 K was calculated for each species using the SMD polarized continuum model.38 The solvation energy was calculated using the gas phase geometry;

calculations on a subset of the molecules studied showed that the additional change in free energy resulting from geometry optimization in solution was negligible. The remainder of this paper will refer to these as G3 free energy calculations, but in every case the actual calculations used the G3(MP2,CCST(T)) methodology.

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RESULTS AND DISCUSSION The concentration vs time data during fructose conversion to LA and FA, Figure 1a, are typical of a series reaction network. The HMF concentration initially increases from zero, passes through a maximum and then decreases; this is typical of an intermediate product. In contrast, Figure 1b shows that the HMF concentration never reaches an appreciable level during glucose conversion, and fructose is not detected at all during glucose conversion. This is because the rate of glucose conversion is much smaller than the rate of fructose conversion as reflected in the different time scales plotted in Figure 1; intermediate species react faster and therefore do not accumulate. In both cases the liquid phase HPLC analysis 5283


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Scheme 3. Four Possible Sites for Aldol Condensation of DHH and HMFa

a

In the case of the 1-2 enol, only addition can occur because no adjacent hydrogen atom can be eliminated with an OH group.

compares fructose humins at 37% and 88% fructose conversion. There are fewer and fewer spherical humin particles present as the reaction proceeds while the number of irregularly shaped particles increases. At present it is not clear whether this change in morphology is a function of reactant conversion or more simply just a function of time under reaction conditions. In either case, the relative invariance of the IR spectra suggests that the morphological change is not accompanied by a substantial change in the chemical structure of the humins. The observation, Figure 5, of few spherical glucose humin particles at 18%, glucose conversion which required 9 h vs. 37%, fructose conversion which required only 50 min, suggests that the changing morphology is predominantly a consequence of time at reaction conditions and not a function of conversion. Figure 6 compares the IR spectra of humins formed from glucose, fructose, and HMF, and it also shows the spectrum of HMF, itself. The figure shows that humins from all three reactants show similar broad absorbance peaks spanning the wavenumbers from ca. 1100 to 1400 cm−1. All three humins show IR peaks at the same wavenumbers, but there are two groups of spectral features where the relative intensities differ appreciably. The first group, indicated in the figure by dashed gray lines, includes two peaks in the 750 to 850 cm−1 range, a peak at 1030 cm−1, and a peak at 1525 cm−1. A comparison of the humins spectra to the spectrum of HMF in Figure 6 shows that these peaks arise from the furan ring of HMF.12,39 This is clear for the 1525 cm−1 feature, but in the 750 to 850 cm−1 range the bands in the humins are shifted relative to HMF. Nonetheless, we believe these peaks do arise from furan rings; DFT frequency calculations for products of the condensation of

reveals a number of unidentified species, but they are all present at very low concentrations. The only species present in solution in appreciable quantity during fructose conversion were fructose, HMF, LA, and FA; and during glucose conversion the only species detected in appreciable quantity were glucose, LA, and FA. The rate of conversion of fructose is slightly faster than that of HMF, and, as previously noted, both of those are much, much greater than the rate of conversion of glucose. Table 1 presents representative conversion and yield values for the conversion of 0.1 M reactant using 0.1 M H2SO4 at a temperature of 398 K. It can be seen that the yield of humins is greatest for glucose and smallest for HMF. In all three cases, the humins yield did not vary within experimental uncertainty as a function of reactant conversion. Figure 2 compares the IR spectra of humins recovered at different levels of glucose conversion, and Figure 3 presents the same information for humins formed during fructose conversion. It is immediately apparent that, in both cases, the spectra do not change appreciably as a function of reactant conversion. A minor trend in the glucose humins spectra is an increase in intensity of the broad peak at 1200 cm−1 relative to the two peaks at 1625 and 1710 cm−1 as the conversion increases. For fructose humins, as the conversion increases the features at ∼800, 1525, 1625, and 1710 cm−1 all increase relative to the broad feature centered around 1200 cm−1. The spectra shown in these figures correspond to reaction of 0.1 M glucose or fructose in 0.1 M H2SO4 at 408 K. While the IR spectra change little with reactant conversion, the humin particle morphology does change. For example, Figure 4 5284


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Figure 3. IR spectra of humins formed at varying conversion of 0.1 M fructose using 0.1 M H2SO4 at 408 K.

Figure 1. Concentration versus time during (a) fructose conversion and (b) glucose conversion at 135 °C in 0.1 M sulfuric acid.

Table 1. Rate and Selectivity Data for 0.1 M Carbohydrate Conversion Using 0.1 M H2SO4 at 125 °C reactant

reaction time (h)

conversion (%)

humins yield (%)

glucose fructose HMF

5.0 2.0 2.0

12 85 65

29 24 18

Figure 4. Scanning electron microscope images of humins formed from fructose after 10 min (top) of reaction (37% conversion) and after 50 min (bottom) of reaction (88% conversion); the marks on the scale are 1.0 μm apart. Figure 2. IR spectra of humins formed at varying conversion of 0.1 M glucose using 0.1 M H2SO4 at 408 K.

corresponding modes in HMF. These features are present in all three of the humins, but they are much stronger in the fructose and HMF humins than in the glucose humins. In contrast to the spectral features attributed to the furan ring, the second set of spectral features, two strong peaks at ca. 1625 and 1710 cm−1

DHH with HMF indicate vibrational modes of the furan ring in this range that do not appear at the same frequencies as the 5285


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Scheme 4. Ring and Straight-Chain Forms of Glucose (Top) and Fructose (Bottom)

humins directly. It is not likely that glycosidic bonds could be responsible because at the reaction conditions used, the glycosidic bonds of cellulose are hydrolyzed to produce glucose. While molecular dynamics simulations suggest protonation of the hydroxyl groups of glucose to be the rate determining step in its degradation,44 condensation involving the hydroxyl groups of glucose to form ether linkages does not seem likely. HMF could not polymerize in this way; it could only dimerize. Therefore if etherification was important in the formation of glucose humins, one would expect the IR spectrum of humins from HMF to differ from that of humins from glucose in the 1050 to 1300 cm−1 range where ether linkages are typically observed. In contrast, Figure 6 shows the HMF and glucose humins to be quite similar at these wavenumbers. Similarly, it does not seem likely that the hydroxyl groups of glucose undergo dehydration to produce a carbonyl group. Sorbitol is quite similar to glucose; it has six hydroxyl groups but no aldehyde/hemiacetal group. Sorbitol is stable at reaction conditions and does not form humins.45−47 This again indicates that neither alcohol condensation nor carbonyl formation via alcohol dehydration is involved in humin growth but instead that the aldheyde/hemiacetal group is responsible. If glucose does directly form humins via its aldehyde/ hemiacetal group, aldol addition/condensation would be the most likely mechanism. Scheme 5 shows that polymerization of glucose via aldol addition/condensation would have two important consequences. First, while the resulting polymer could contain conjugated CC bonds, it could only contain a single carbonyl group at the growing end of the polymer chain. While the relative molar absorptivities of the carbonyl and

Figure 5. Scanning electron microscope images of humins formed from glucose after 9 h of reaction (18% conversion); the marks on the scale are 1.0 μm apart.

Figure 6. IR spectra of (a) HMF and of humins formed using 0.1 M H2SO4 at 408 K from (b) 0.1 M HMF at 90% conversion, (c) 0.1 M fructose at 88% conversion, and (d) 0.1 M glucose at 75% conversion. The dashed gray lines indicate humin features assigned to furan rings, and the solid gray lines indicate features assigned to carbonyl groups conjugated with carbon−carbon double bonds.

indicated by solid gray lines in the figure, are prominent in the glucose and fructose humins, while they cannot be distinguished in the HMF humins. The location of these two peaks is characteristic of a carbonyl group conjugated to an alkene group.39 They might also be attributed to a conjugated diene structure such as that proposed to exist in polymers of furfuryl alcohol,40,41 but in that case we would expect them to be most evident in the humins formed from HMF. The intensity of these two peaks helps to answer the question of whether humins are formed by the direct polymerization of glucose and fructose, or whether glucose and fructose must first be converted to DHH via HMF before humins can form. In aqueous media, glucose exists as pyranose rings and as a linear aldohexose, Scheme 4. At equilibrium in aqueous media, 99% of the glucose is in the pyranose form; the ring forms of fructose are similarly more abundant than the acyclic form.22,42,43 These structures suggests several possibilities for mechanisms by which glucose might polymerize to form

Scheme 5. Structure Resulting from Aldol Condensation of Glucose

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expectation, humins were produced from fructose, separated from any unreacted fructose, and recovered. In a subsequent experiment, the recovered humins were then reacted with benzaldehyde. Figure 8 shows that this resulted in the

carbon−carbon double bonds are not known, and having previously ruled out the possibility that a carbonyl group is formed by dehydration of the glucose hydroxyl groups, the strong IR band at 1710 cm−1, characteristic of carbonyl groups, in the glucose humins appears to be inconsistent with the aldol addition/condensation pathway shown in Scheme 5. Additionally, it would not be possible for humins formed via aldol addition/condensation of glucose to incorporate any “side chains” such as a furan group. Noting that neither a growing glucose chain like that shown in Scheme 5 nor HMF can form an enol, they could not undergo aldol addition with each other. At most, a single furan group from HMF or benzyl group from benzaldehyde could be present at the starting end of the humin polymer. The IR spectrum of glucose humins in Figure 6 does not show strong furan features, but that is not conclusive since the concentration of HMF is quite low during glucose conversion, Figure 1. Therefore, an experiment was performed where glucose was converted in the presence of added benzyaldehyde. The IR spectrum of the resulting humins is shown in Figure 7. It clearly shows a prominent peak at 700

Figure 8. IR spectra of humins formed from fructose (bottom spectrum) and humins formed from fructose, separated and subsequently reacted with benzaldehyde.

appearance of a strong IR peak at 700 cm−1, indicating that benzyl side groups were added to the humins after they had formed. That would not be possible if the fructose humins had been formed by direct aldol self-addition/condensation. As just discussed, the experimental results are not consistent with direct formation of humins from glucose or from fructose. The alternative possibility is that glucose and fructose must first be converted to HMF, and that humins from all three sources, glucose, fructose, and HMF, are produced via DHH. This might lead to the expectation that all three types of humins should have the same, or nearly the same, IR spectrum. Figure 6 clearly shows that this is not true; we believe the differences in the spectra can be attributed to differences in the identity of aldehydes and ketones that are available to react with DHH. That is, the humin polymers have the same DHH-derived backbone, but they have different “side groups.” As discussed in the introduction, when HMF is the starting reactant, it will be present in much, much greater concentration than any other aldehyde or ketone. Consequently aldol addition/condensation between HMF and the highly reactive DHH, as represented in Scheme 3, is expected to occur almost exclusively. The resulting addition/condensation product can then polymerize as represented in Scheme 6 where the “R” side group would be the one containing a furan ring. The resulting humin polymer structure shown in Scheme 6 immediately raises the question: why are not the two strong peaks at 1625 and 1710 cm−1 present in the HMF humins? Specifically, those IR peaks are characteristic of carbon−carbon double bonds conjugated to carbon−oxygen double bonds, and the polymer structure shown on the right side in Scheme 6 does contain those structures. Upon closer examination, however, it is seen that in every case an additional functional group such as a furan ring or a hydroxyl group is also coordinated to the carbon−carbon double bond. The conjugated carbon−carbon vibrations in the furan ring and the vibrations of the hydroxyl group are expected to couple with those of the carbon−carbon double bonds conjugated to carbon−oxygen double bonds. In addition, the polymer structure in Scheme 6 is highly idealized; it suggests that the

Figure 7. IR spectra of humins formed from glucose (bottom spectrum) and humins formed from a mixture of glucose and benzaldehyde.

cm−1 indicative of the incorporation of benzyl groups in the humins via aldol addition/condensation. Benzaldehyde alone does not form humins at these reaction conditions. Thus, the direct formation of humins from glucose is inconsistent with the strong carbonyl feature in the IR of the humins and with the ability to incorporate benzyl groups in the humins. The direct conversion of fructose to humins is similarly inconsistent with experimental results, but there are some differences. The linear form of fructose is a ketone whereas the linear form of glucose is an aldehyde. Being a ketone, fructose could add a “side group,” such as a furan group, without losing its ability to form an enol and thereby undergo a second aldol addition. Nonetheless, once a species such as HMF had been added, the addition of the next fructose would consume the carbonyl so that, like glucose, the resulting polymer could only contain a single carbonyl group at the growing end of the polymer chain. Again, the strong 1710 cm−1 IR feature in the fructose humins is characteristic of carbonyl groups and inconsistent with aldol addition/condensation polymerization of fructose. Furthermore, if only one carbonyl is present at the growing end of the humins polymer then, once they have formed, it would not be possible to add “side groups”; they could only be added during humin formation. To test this 5287


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are expected to be present in sufficient quantity to react with DHH. Considering recent results for the conversion of furfuryl alcohol to LA,48 it seems likely that additional intermediates may also be involved in the mechanism, but those being considered here were present in sufficient quantity to be detected by Horvat. In acid media, the aldol addition of any carbonyl-containing species to DHH begins with the conversion of DHH into an enol. G3 calculations were performed to determine the free energy change associated with conversion of DHH to each of the four enols shown in Scheme 3. The top section of Table 2 shows that forming the enol with a double bond between the second and third carbon atoms requires the smallest increase in free energy at 298 K, using either gas phase species free energies or free energies corrected for solvation in water. Consequently, the DHH 2−3 enol was used in subsequent calculation of the free energy change for aldol addition to DHH of the aldehydes and ketones identified in the preceding paragraph. Specifically G3 calculations were performed to calculate the free energy change at 298 K for adding the linear forms of glucose and fructose; intermediate 17; intermediate 16 at carbon 1, at carbon 2, and at carbon 5; intermediate 15; HMF and LA, all at carbon 3 of the 2−3 enol of DHH. The results are shown in the second section of Table 2. Additional G3 calculations were performed to determine the free energy change for the condensation of the DHH−HMF adduct and the DHH−intermediate 16 adduct with the lowest free energy. These results are shown in the third section of Table 2. According to Table 2, the addition of HMF to DHH is not the most favorable reaction of those considered. Nonetheless, if HMF is the starting reactant, its concentration will be so much greater than any of the other species that the formation of the DHH−HMF adduct is expected to predominate. The table shows that the subsequent condensation of the DHH−HMF adduct is favorable. Considering the conversion of glucose and the early stages of fructose conversion when HMF is not present in high concentration, Table 2 shows that the addition of intermediate 16 at its aldehyde carbon, C1 is by far the most favorable reaction. Table 2 further shows that the subsequent condensation of the resulting DHH−intermediate 16 adduct is also favorable. The overall free energy changes, addition plus condensation, are comparable when DHH reacts with either HMF or intermediate 16, but the former has a significantly higher intermediate free energy. On the basis of Table 2, the most favored “R” group when humins form under low HMF conditions is the one shown in Scheme 6 that is derived from intermediate 16. The resulting humin structure can be seen to contain a large concentration of carbonyl groups conjugated to a carbon−carbon double bond because each time intermediate 16 adds to DHH, two sets of carbonyl groups coordinated to carbon−carbon double bonds are created in the “side group.” In contrast to the situation when HMF adds to DHH, these conjugated CC−CO pairs are not simultaneously bonded to additional functionalities such as furan rings or hydroxyl groups that would broaden their IR bands. As such they are expected to cause sharper, more intense peaks in the IR. This explains why the peaks at 1625 and 1710 cm−1 are so predominant in the IR spectra of the glucose and fructose humins, compared to the HMF humins. Thus, the IR spectral differences at 1625 and 1710 cm−1 between HMF humins and glucose/fructose humins can be explained in terms of the postulate that all three types of

Scheme 6. Idealized Humin Structure

polymerization of DHH always occurs via the two end carbons. In fact, DHH could polymerize through its first, third, fifth, and sixth carbon atoms. This would result in additional structural variations adjacent to the carbonyl and carbon−carbon double bonds of the polymer. These factors are expected to result in a distribution of less intense peaks instead of two very strong peaks and may explain why the humins formed from HMF do not have the two strong IR peaks at 1625 and 1710 cm−1. A second, less likely explanation is that the HMF side groups add to DHH producing a carbon−carbon single bond, but the subsequent condensation step to form a carbon−carbon double bond does not occur. In any case, if the postulate that humins from all three sources all form via DHH, then the strong peaks at 1625 and 1710 cm−1 must be attributed to differences in the identity of the “R” group in Scheme 6 and not to the DHH-derived backbone. As just mentioned, the furan-containing “R” groups of Scheme 6 are expected when HMF is the starting reactant. Throughout the glucose conversion process and during the early stages of fructose conversion the HMF concentration will be small, Figure 1. Under these conditions, when a reactive DHH forms, it will not necessarily add an HMF; there may be several other aldehydes or ketones present in concentrations comparable to the concentration of HMF. As already noted, approximately 1% of the total glucose present is expected to be present in the linear form, and this could add to DHH via its aldehyde group. A similarly small fraction of the fructose is expected to be in the linear form and capable of adding to DHH. In addition, examination of Horvat’s mechanism, Scheme 2, reveals several additional aldehydes and ketones that could add to DHH. Horvat was not able to isolate or detect those labeled B-5, C-5, D-5, and E-5, making it unlikely that DHH reacts with them. The others, LA and those labeled 15, 16, and 17, were isolated and detected by Horvat, so they 5288


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Table 2. Calculated G3(MP2,CCSD(T)) Free Energies in kJ/mol

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Table 2. continued

several locations. Having the ability to form an enol, the humins could add additional benzyl groups during or after the humin formation process. This is consistent with the experimentally

humins originate from DHH polymerization. Furthermore the humin structure resulting from DHH aldol condensation polymerization, Scheme 6, retains the ability to form enols at 5290


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glucose and HMF in both number of furan rings in the humins and yield of humins. Generally speaking, the formation of humins is undesirable because they have a low value. Ideally one would prefer to eliminate their formation entirely. If that is not possible, however, overall process economics would be better if the humins could be converted to a higher valued material. One way to do this is to add chemical functionality to the humins so that they can be used for other purposes. For example, if basic chemical groups could be added to the humins, then they might become useful as adsorbents for acidic gases such as H2S or CO2. Indeed a wide spectrum of uses has been suggested for functionalized carbon nanospheres.32,49 The incorporation of benzyl groups from benzaldehyde into HMF humins in previous work provided a demonstration that adding chemical functionality to humins is possible. The present results extend those findings in two important ways. First, the present results show that fructose and glucose humins also can be functionalized in the same way. If this had to be done simultaneously with the conversion of the starting reactant, it would not be particularly convenient to do so. The present results, Figure 8, have demonstrated that the functionalization does not have to occur at the same time the starting reactant is being processed. Instead it is possible to separate the humins after the starting reactant has all been converted and then functionalize the humins in a subsequent process. Of course the addition of benzyl groups, as was done here, does not increase the humins value, but it strongly suggests that aldol addition/condensation chemistry could be used to add other functional groups just as easily. The work reported here focused upon determining whether fructose and glucose could form humins directly or whether they first had to be converted to HMF and subsequently DHH before humins would form. The ultimate goal, however, is the conversion of cellulosic biomass to LA as shown in Scheme 1. This leaves open the possibility that cellulose itself could be converted directly to humins. To this end, the conversion of cellobiose, effectively a cellulose polymer containing just two glucose units, was studied at the same conditions as were used for glucose and fructose. Figure 9 shows that the IR spectrum of the humins formed from glucose and that from cellobiose are nearly indistiguishable. This very strongly suggests that cellobiose, and by extension cellulose, is first hydrolyzed to glucose, that glucose is then converted to HMF and eventually DHH, and that only then can humins start to form.

demonstrated ability to add benzyl groups to the glucose and fructose humins, Figures 7 and 8, by adding benzaldehyde either during or after humin formation. The postulate that all three types of humins form via aldol addition/condensation polymerization of DHH is also consistent with the other spectral difference. The dashed lines in Figure 6 suggest that the number of “R” groups containing furan rings is very low for glucose humins relative to fructose and HMF humins. This can be explained in a straightforward way by considering the concentration of HMF during the humin formation reaction. Figure 1 shows that during glucose conversion, the HMF concentration never becomes appreciable, so the very weak IR intensity for the group of peaks attributed to the furan ring is expected. As already discussed, the very high HMF concentration during HMF conversion results in DHH adding HMF almost exclusively, explaining the greater intensity of these peaks in the IR spectrum of HMF humins. During fructose conversion, the concentration of HMF is initially zero, but as the reaction proceeds, the concentration of HMF increases. At 398 K, the HMF concentration becomes greater than the fructose concentration at approximately 60% fructose conversion and remains so for the duration of the reaction. Hence during the early stages of fructose humin formation, the side groups are expected to be predominantly derived from intermediate 16, giving rise to the strong IR spectral features at 1625 and 1710 cm−1, but as the reaction proceeds and the HMF concentration builds, “R” groups containing a furan ring are expected to predominate. Hence the strong intensity of both groups of spectral features in fructose humins can be explained by the postulate that they are produced via aldol addition/condensation polymerization of DHH. The IR spectra for humins from all three sources can be explained in terms of the postulate that they all form by DHH polymerization. It remains to explain why the yield of humins is not the same in all three cases; it increases as one moves from HMF to fructose to glucose as the starting reactant. Specifically, Scheme 2 indicates that the addition of water to HMF is the branching point in the reaction pathway. If water adds at carbons 2 and 3, LA is formed and if water adds at carbons 4 and 5, humins form. In both cases the reactants and the catalyst are the same. It would initially appear that, upon taking the ratio of the corresponding rate expressions, the concentration terms would cancel out, giving a constant selectivity irrespective of the composition of the mixture. The postulated humin formation mechanism also affords a qualitative explanation for the increasing yield of humins, as the reactant changes from HMF to fructose to glucose. In all cases, the high reactivity of DHH is expected to result in the rapid addition of a “side group” before polymerization, Scheme 6. In the case of glucose, that side group is postulated here to be intermediate 16. According to Horvat’s mechanism shown in Scheme 2, intermediate 16 would otherwise have been converted into LA. In effect, the possibility of DHH reacting with intermediate 16 opens up a new pathway that diverts products from becoming LA to become humins instead. In the case of HMF conversion, DHH is much, much more likely to add HMF as a side group and not intermediate 16. This effectively shuts down the pathway by which intermediate 16 can be converted to humins, so instead it is converted to LA and the humin yield is smaller. This is observed experimentally: the side groups in HMF humins are almost all furan rings, and the humin yield is smaller. Fructose is intermediate between

Figure 9. IR spectra of humins formed (a) from glucose and (b) from cellobiose. 5291


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AUTHOR INFORMATION

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Humins that are formed during the acid-catalyzed conversion of either glucose or fructose show two strong IR absorption peaks at 1625 and 1710 cm−1. These peaks are attributed to a carbonyl group appearing at 1710 cm−1 conjugated with a carbon−carbon double bond appearing at 1625 cm−1. Experiments also showed that if benzaldehyde was added during glucose conversion, benzyl groups were incorporated in the humins. Similarly, if humins from fructose were recovered and subsequently reacted with benzaldehyde, benzyl groups were again incorporated in the humins. Obvious ways that either of these hexoses might directly form humins such as glycosidic bonding, hydroxyl condensation to form ether linkages, and aldol addition/condensation were considered, but none could explain the presence of these two intense peaks in the IR spectrum or the ability to incorporate benzyl groups from benzaldehyde. An alternative postulate is that glucose and fructose are first converted into HMF and then DHH. DHH is highly reactive, and as a consequence it is proposed that the DHH rapidly undergoes aldol addition/condensation with any aldehyde or ketone that is available. G3(MP2,CCSD(T)) free energy calculations showed that addition and condensation of Horvat’s intermediate 16 with DHH was the most favorable of the possible DHH reactions. If this is taken to be the predominant reaction of DHH during glucose conversion or during the early stages of fructose conversion, then the experimental results for glucose and fructose can be explained. While this proposal is consistent with the experimental observations, there could be other mechanistic pathways that are also consistent. The IR spectrum of HMF humins differs from those of glucose and fructose in that the strong IR peaks at peaks at 1625 and 1710 cm−1 are not observed. This discrepancy can also be explained by noting that during HMF conversion and during the later stages of fructose conversion the HMF concentration is very much larger than any other aldehyde or ketone. Under these conditions, the addition of HMF to DHH is expected to predominate instead of the addition of Horvat’s intermediate 16. This permits an explanation of all of the experimental results for humins from all sources, and it qualitatively explains the differences in humins yield between the three starting reactants. Hence, upon consideration of all the experimental results, the direct formation of humins from fructose, glucose, and cellobiose does not appear to be significant. Instead, it appears that all of these species must be first be converted to HMF and then DHH in order to form humins. Acid-catalyzed conversion of cellobiose yields humins that are nearly identical to those from glucose. It is concluded that the most likely pathway for humin formation from cellulose entails sequential hydrolysis to glucose, dehydration of the glucose to HMF, formation of DHH from HMF, and aldol addition/condensation polymerization of the DHH. The direct conversion of cellobiose, glucose, or fructose to humins does not appear to be significant. Finally, the humins that are recovered can be functionalized by aldol addition/condensation with added aldehydes or ketones, as demonstrated by the incorporation of benzyl groups upon reaction of the humins with benzaldehyde. This might represent a route by which humins can be converted into higher-valued materials.

*E-mail: lund@buffalo.edu. Voice: (716) 645-1180. Fax: (716) 645-3822. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The use of the computational facilities of the Center for Computational Research at the University at Buffalo are gratefully acknowledged.

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REFERENCES

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Comparison of Structural Features of Humins Formed Catalytically

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