ARTICLE pubs.acs.org/EF
Formation and Growth of Humins via Aldol Addition and Condensation during Acid-Catalyzed Conversion of 5-Hydroxymethylfurfural Sushil K. R. Patil and Carl R. F. Lund* Department of Chemical and Biological Engineering, University at Buffalo, SUNY, Buffalo, New York 14260, United States ABSTRACT: The acid-catalyzed conversion of 5-hydroxymethylfurfural (HMF) produces levulinic and formic acids in equal amounts. Dark-colored solids, known as humins, are also formed in a parallel reaction. Aldol addition and condensation are proposed as important reactions in the acid-catalyzed growth of humins, adding HMF to 2,5-dioxo-6-hydroxy-hexanal. Consistent with this proposal, infrared (IR) spectra of humins formed from HMF indicate that the furan ring and hydroxymethyl group of HMF are present in the humins, but the carbonyl group is not. Similarly, if a mixture of HMF and benzaldehyde are processed, IR spectra of the humins indicate the additional presence of the aromatic ring from benzaldehyde but not its carbonyl group. The incorporation of the aromatic ring from benzaldehyde in the humins demonstrates the possibility of functionalizing humins to increase their value.
C
ellulosic biomass represents an attractive feedstock for the sustainable production of fuels and chemicals.14 It can be converted enzymatically,2,5 catalytically,2,6 or thermally,710 and each approach offers advantages and disadvantages. When mineral acid catalysts are used to convert cellulose, a variety of compounds are generated, but the important reaction pathway proceeds sequentially through glucose, perhaps fructose and 5-hydroxymethylfurfural (HMF), ending with levulinic acid (LA), which is stable at processing conditions.1118 The selectivity for LA is decreased by the parallel formation of darkcolored solids that are commonly referred to as humins. It is not clear whether humins form directly from each of the primary intermediates or whether they form from a single intermediate. It is well established, however, that acid-catalyzed processing of HMF involves two pathways, one leading to LA and the other to humin formation.1921 Consequently, during processing of cellulose, some, if not all, of the generated humins are derived from HMF. The present study seeks to understand how HMF is converted into humins and the resulting chemical and physical structure of those humins. The focus here is on acid-catalyzed conversion of HMF, but humins will also form hydrothermally (without a catalyst) if HMF is processed at higher temperatures and pressures. It appears that humins formed hydrothermally have been more thoroughly characterized than those resulting from acid-catalyzed conversion. The reason may be that, in studies where the goal is to produce functionalized carbon micro- or nanoparticles,2224 hydrothermal processing is preferred because it avoids the use of mineral acids and makes the process more environmentally friendly. The resulting hydrothermal carbons have been characterized in a variety of ways, but the findings are contradictory in some respects. For example, Yao et al.24 observed the individual carbon spheres to display different morphologies when produced from glucose (bumpy surfaces) as opposed to fructose (smooth), while Titrici et al.23 noted that carbons derived from hexoses (which include both glucose and fructose) were similar but distinct from carbons derived r 2011 American Chemical Society
from pentoses. At very similar processing conditions, glucosederived carbons are reported in one case to have very uniform particles sizes,22 in a second case to have a significantly broader distribution of particles sizes,23 and in a third case to not form at all.25 Chemical characterization of hydrothermally formed carbons leads to similar inconsistencies. In one study, carbons produced from fructose and HMF show infrared (IR), NMR, and Raman evidence of an aromatic component being present,24,26 and in a different study, similar results were reported for carbons derived from glucose.22 These results contradict a separate investigation wherein evidence for an aromatic component was found for pentose-derived carbons, but hydroxylated methylenes predominated in carbons from hexoses (including glucose and fructose).23 While their structure and the mechanism by which they form remain uncertain, carbons prepared via hydrothermal hydrolysis of carbohydrates display promising surface functionality. They can be prepared with hydrophobic cores and hydrophilic shells,24 and it is possible to add specific functionalities to their surfaces.27,28 They retain functional groups that can be used to decorate their surfaces with smaller metal particles, as demonstrated with silver and palladium, and they have also been formed with metallic nanoparticles at their center.22 One might reasonably expect that carbons formed during acid-catalyzed hydrolysis could be similarly modified, though this remains to be demonstrated. In the case of acid-catalyzed hydrolysis, the humins that form have not been characterized nearly as thoroughly, probably because the focus has been on improving selectivity to other products such as LA. Often, all that is reported is that humins did form and in what yield. When it is reported, the elemental composition of humins is typically found to be on the order of 5565% carbon, 45% hydrogen, and 3040% oxygen.6,11,19,29 Received: July 11, 2011 Revised: August 24, 2011 Published: August 25, 2011 4745
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Energy & Fuels Scheme 1. HMF Conversion Pathway Suggested by Horvat
Microscopy (scanning electron microscope (SEM)) generally reveals agglomerated spherical particles with a broad distribution of diameters.6,19 Sievers et al.30,31 found strong NMR similarities between the starting cellulose and the humins derived from it via hydrolysis in an ionic liquid solution. They concluded that saccharides are a primary component of humins but that other species must be involved in their formation and final structure. The kinetics of HMF acid-catalyzed conversion can be modeled accurately using only two parallel reactions, one forming LA and the other forming humins. The actual pathway, however, is considerably more complicated.1921 At higher temperatures, characteristic of hydrothermal processing, benzenetriol has been suggested to be the intermediate that introduces aromatic character into the humins.29 In the case of acid-catalyzed conversion of HMF, Horvat20,21 proposed that HMF is converted to LA and humins according to Scheme 1, which stops short of identifying the steps by which humins grow. Instead, it simply indicates that 2,5-dioxo-6hydroxy-hexanal somehow polymerizes, leading to humin formation. This paper will present results suggesting that aldol addition and condensation involving 2,5-dioxo-6-hydroxy-hexanal are important steps in the acid-catalyzed growth of humins and their resulting chemical structure.
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’ EXPERIMENTAL SECTION HMF 98+% (Aldrich), sulfuric acid 96% (Acros), levulinic acid 98% (Alfa Aesar), formic acid 98% (Alfa Aesar), benzaldehyde >99% (Aldrich), hydroxybenzaldehyde 98% (Aldrich), formaldehyde 37% (Acros), and acetaldehyde 99.5% (Acros) were used without further purification. Solutions of the desired composition were prepared in larger quantity, and then, 5 mL was added to each of the 10 mL, thickwalled glass vials that were to be used as reactors. A conical, magnetic stir bar was added to each, and the vials were topped with Teflon-lined silicone septa and sealed with screw-on caps. Five such vials were processed in each experimental run along with a sixth vial that was fitted with a thermocouple. The six vials were simultaneously transferred to an aluminum block with holes sized to accommodate the vials. This preheating block was maintained at a temperature of 180 to 190 °C. When the solution in the vials approached the desired reaction temperature (as measured using the extra vial fitted with the thermocouple), the six vials were transferred to a second aluminum block that was immersed in an oil bath maintained at the desired temperature. In this way, the solutions were heated as rapidly as possible to the desired temperature. When the temperature of the solution had stabilized, one vial was removed and quenched in an ice bath. This time marked the start of the isothermal reaction; the composition of the quenched solution was measured via HPLC and taken to be the starting composition for the reaction. (This was necessary; as otherwise, the measured apparent rate coefficients were observed to depend upon the heating rate.) The remaining four vials were then removed and quenched, one at a time, after selected periods of time had elapsed. After quenching, part of the liquid solution was withdrawn using a syringe fitted with a 0.2 μm filter. The withdrawn sample was analyzed using an Agilent 1200 Series HPLC fitted with a degasser, quaternary pump, Biorad Aminex HPX-87H column, and refractive index detector (RID). A 5 mM solution of sulfuric acid was used as the mobile phase at a flow rate of 0.6 mL/min. The column and RID were operated at 65 and 40 °C, respectively. Reference solutions containing known concentrations of the pure components were used to prepare calibration curves that were then used to determine the composition of the quenched reaction solutions. The remaining contents of the quenched vials were centrifuged and washed multiple times with distilled water until no other reagents could be detected in the wash water. The resulting solids were further dried overnight in air at 90 °C and, then, were stored in sealed bottles. The kinetic analysis of the data assumed the reaction vials to be perfectly mixed and isothermal during the entire period of reaction. It further assumed that the system could be modeled as two parallel, pseudofirst-order reactions, eqs 1 and 2, wherein the fluid volume was constant. As such, the rate expressions are empirical and convey no mechanistic information. With these assumptions, differential mole balances on HMF and LA can be written and integrated, leading to equations for the concentrations of these two species as a function of time, eqs 3 and 4. In those equations, square brackets denote concentrations (subscript 0 denotes at time zero), and k1 and k2 are the apparent first-order rate coefficients for reactions 1 and 2. A plot of the negative of logarithm of [HMF]/[HMF]0 vs t will then have a slope equal to the sum of k1 plus k2 according to eq 3, and a plot of [LA] vs [HMF] will have a slope equal to k1/(k1 + k2). The individual values of k1 and k2 for a given kinetic run can thereby be determined from these two slopes.
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HMF þ 2H2 O f LA þ HCOOH
ð1Þ
HMF f humins
ð2Þ
½HMF ¼ ½HMF0 expfðk1 þ k2 Þtg
ð3Þ
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Figure 1. Representative kinetic data analysis for the determination of apparent first-order rate coefficients k1 and k2 using (a) eq 3 and (b) eq 4. (Data shown are for conversion at 118 °C starting with 0.1 M HMF and 0.1 M H2SO4.) ½LA ¼ ½LA0 þ ¼ ½LA0 þ
k1 ½HMF0 ð1 expfðk1 þ k2 ÞtgÞ k1 þ k2
k1 ð½HMF0 ½HMFÞ k1 þ k2
ð4Þ
Infrared (IR) spectra were measured using a Bruker Vertex 70 Infrared spectrometer fitted with a ZnSe crystal PIKE MIRacle ATR accessory. The resolution of the instrument was set to 4 cm1. The IR spectrum of air was recorded as background, and then, samples were placed on a ZnSe crystal for measuring their absorbance. Scan times of 100 s were used for both the background and the samples. Imaging of humins was performed using a Hitachi SU-70 Scanning Electron microscope (SEM). Samples were coated with a thin carbon film and then were observed under field free mode at an electron beam energy of 20 keV. DFT calculations were performed using GAMESS.32 All calculations reported here used B3LYP exchange-correlation functionals, 6-311++G(2d,p) basis sets and a polarized continuum model for the solvent (water). Prior to DFT geometry optimization, each structure was optimized using molecular mechanics. Vibrational frequencies were computed following each successful geometry optimization, and the results were only accepted if there were no imaginary frequencies. Vibrational frequencies were not scaled, and zero point corrections were not applied. The numerical force calculations employed double perturbations for each coordinate direction. The energies reported here are the free energies in the solvent.33,34
’ RESULTS AND DISCUSSION The results of a typical kinetics run are plotted in Figure 1. A total of 14 kinetics runs were performed, yielding 28 plots of this
Figure 2. Selectivity of acid-catalyzed HMF conversion at three different temperatures: (a) ratio of levulinic acid to formic acid and (b) moles of levulinic acid formed per mole of HMF converted.
type. Experiments at the three temperatures studied were replicated at least three times. The correlation coefficient, r2, was greater than 0.98 in all but 2 of these plots, where it equaled 0.94 and 0.92. The values of the apparent rate coefficients k1 and k2 were determined for each experimental run. The resulting values were averaged for each of the three experimental temperatures (118, 125, and 135 °C), and from the averages, the apparent activation energies for reactions 1 and 2 were found to equal 89 ( 5.5 and 94 ( 8.8 kJ mol1, respectively (the r2 value for each of the corresponding Arrhenius plots was greater than 0.99). These values fall nearly midway between those reported by Chang et al.12,18 and by Girisuta et al.19 Figure 2a shows that, at all three temperatures studied, the ratio of LA to formic acid was essentially equal to one over the full course of the reaction. No other solution phase products were detected in appreciable amounts. This indicates that humins do not form from one or the other of these products. Girisuta et al.19 previously have shown that a mixture of LA and formic acid does not form humins in the presence of 1 M H2SO4 at 141 °C. Figure 2b indicates that the selectivity for LA relative to humins was not a particularly strong function of temperature. Initially, it was on the order of 0.85, and it decreased slightly as the reaction progressed. While the kinetics were modeled as first-order reactions, one would expect the acid concentration to affect the kinetics, too.17 Figure 3 shows that this is true and, further, indicates that the effect of acid concentration on k1 is much greater than its effect on k2. Higher acid concentration leads to a greater selectivity for LA relative to humins. The first-order kinetic network used here is quite accurate in modeling the reaction rates, and it could be useful for process 4747
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Figure 3. Effect of sulfuric acid concentration on the apparent firstorder rate coefficients at 135 °C. The circles represent k1 and the squares, k2.
design and modeling. However, the very fact that a first-order kinetic network is accurate is an indication that kinetics studies will not be particularly revealing or helpful for understanding the mechanism of the reactions. At some point, the growth of humins must entail bimolecular reaction between two carbohydrate molecules. Kinetic modeling will not be useful in identifying such reactions. That is, apparent first-order kinetics can result if any one of the steps shown in Scheme 1 is rate-determining, and even so, Scheme 1 does not go so far as to suggest what bimolecular carbohydrate reaction(s) is(are) responsible for humin growth. The SEM images presented in Figure 4 show that the humins take the form of spherical particles. The average diameter of the spheres increases continually as the HMF conversion reaction progresses. Growth appears to involve both single particles increasing in size as well as coalescence of two or more particles. There is a subtle change in the morphology of the humins particles after complete conversion of HMF has occurred. In addition to the spherical particles, one also observes more flattened shapes. We believe that the latter result from coalescence at the walls of the reaction vials, but a detailed study has not been performed. As mentioned in the introduction, hydrothermal conversion of carbohydrates (that is, without acid catalyst) has been suggested as a green, sustainable, and economical means of producing functionalized carbon microspheres and nanoparticles for use across a broad spectrum of applications including drug delivery, biodiagnostics, combinatorial synthesis, electronics, catalysis, adsorbents, and optics.22 Hydrothermal preparation also yields spherical particles, but the temperature is typically 160 to 180 °C as compared to the 118 to 135 °C temperatures used here for acid-catalyzed processing. A different growth mechanism has also been proposed for hydrothermal humin formation.29 Figure 5 presents SEM images of humins that were formed hydrothermally from HMF at the same conditions as the humins of Figure 4, except that no acid was present. The hydrothermal humins are not spherical but instead display a very different morphology. This may reflect a difference in the chemical composition and structure arising from different growth processes and not just a slower version of the acidcatalyzed mechanism. It should also be noted that the hydrothermal process required much longer reaction times. When acid was used at 135 °C, the HMF was completely consumed in
Figure 4. SEM images of humins recovered after 0.1 M sulfuric acidcatalyzed conversion of 0.1 M HMF at 135 °C for (a) 22 min (43% HMF conversion), (b) 2 h (90% HMF conversion), and (c) 18 h (100% HMF conversion).
less than 3 h, but without acid, 15% of the HMF remained even after 120 h. Figure 6 compares the IR spectra of humins formed hydrothermally and via acid catalysis at two different HMF conversion levels. For both the hydrothermal humins and the acid-catalyzed humins, the IR spectrum does not change appreciably over the time required to reach 85 to 90% conversion of the HMF. In addition, all four spectra are similar; the same functional groups appear to be present in both types of humins. The biggest 4748
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Energy & Fuels
Figure 5. SEM images of humins recovered after hydrothermal conversion of 0.1 M HMF at 135 °C for (a) 49 h (45% HMF conversion) and (b) 120 h (85% HMF conversion).
difference between the hydrothermal humins and the acidcatalyzed humins is in the relative amounts of different functional groups. This is most evident by comparison of the intensity of the envelope of peaks in the range of ca. 1550 to 1750 cm1 to the envelope of peaks in the 1150 to 1350 cm1 range. Hydrothermal humins show more intensity in former envelope than do acid-catalyzed humins. This is consistent with the differences in morphology revealed by SEM and suggestive of differences in the growth chemistry. The focus for the remainder of this paper will be on the humins formed during acid-catalyzed conversion of HMF. While kinetics measurements did not provide any mechanistic insight, the detailed work of Horvat does suggest the conversion pathway leading from HMF to LA and the first few steps of the pathway leading to humin formation, Scheme 1. DFT was used to calculate the free energy changes associated with the mechanistic steps of Scheme 1. The level of theory used in doing so represented a trade-off between computational speed and high accuracy. As such, the results are expected to show relative trends, but for high accuracy, the calculations should be repeated using a higher level of theory and a full exploration of the conformations of all of the species involved.35,36 The results are presented in Figure 7. In performing these calculations, only one conformation was used for each of the species, except for HMF, where six different low energy conformations were used. The six different conformations of HMF differed by less than 10 kJ/mol, so
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conformational effects are not expected to be significant. The calculations suggest that the two pathways involve comparable free energy changes, at least for as far as the humins pathway extends. As such, the selectivity of the overall process appears to be controlled by reaction kinetics and not thermodynamic equilibrium. It should be noted that Horvat did not detect all the intermediates shown in Scheme 1; some were detected by NMR while the presence of others was inferred. The final intermediate shown on the humin formation pathway of Scheme 1, that is 2,5-dioxo-6-hydroxy-hexanal, was not detected experimentally. It was proposed by analogy to the conversion of 2-methylfuran, in which case the corresponding intermediate could be detected. The fact that 2,5-dioxo-6-hydroxy-hexanal was not present in sufficient quantity to be detected means that if it truly is an intermediate in the humin formation mechanism, then it must be highly reactive. Furthermore, if it is present in such low concentration, it is unlikely that humins are the result of its self-polymerization. When a molecule of 2,5dioxo-6-hydroxy-hexanal forms, it is rapidly converted, but given the undetectably low concentration of 2,5-dioxo-6-hydroxyhexanal, that conversion is likely to involve a more abundant species. Thus, if Horvat’s mechanism is correct, then the first bimolecular reaction leading to actual humin growth is most likely to involve reaction of 2,5-dioxo-6-hydroxy-hexanal with some other, more abundant species. In the present investigation, the most abundant species at the start of the reaction is HMF. As reaction proceeds, LA and formic acid will accumulate, but as already noted, these species are not converted to humins. This leads us to postulate that humin growth involves reaction between 2,5-dioxo-6-hydroxy-hexanal and HMF. We additionally suggest that the pertinent reactions are the acid-catalyzed aldol addition and condensation of 2,5-dioxo-6-hydroxy-hexanal with HMF. Aldol addition is a reaction between two molecules containing carbonyl groups, one of which must possess an alpha hydrogen so that it can tautomerize to an enol. If the carbonyl compound contains two alpha hydrogens, the adduct resulting from aldol addition can subsequently undergo condensation by eliminating water and forming a carboncarbon double bond. HMF does not possess an alpha hydrogen, so in the present case, 2,5-dioxo-6-hydroxy-hexanal must be the reactant that forms the enol. In fact, 2,5-dioxo-6-hydroxy-hexanal has three carbonyl groups, but only carbons 2 and 5 possess alpha hydrogens. As such, 2,5-dioxo-6-hydroxy-hexanal can undergo aldol condensation with HMF at carbons 3, 4, and 6, and it can undergo aldol addition with HMF at carbon 1. If 2,5-dioxo-6-hydroxy-hexanal adds HMF at carbon 1, carbon 1 will then also possess an alpha hydrogen, permitting addition/condensation with an additional HMF. Reaction 5 shows the product that would be formed if 2,5dioxo-6-hydroxy-hexanal underwent all possible aldol addition/ condensation reactions with HMF. The final product shown in reaction 5 is like HMF in the sense that it can no longer form an enol, but it does possess three carbonyl groups. As such, when another 2,5-dioxo-6-hydroxy-hexanal is formed, it could add either HMF or the product shown in reaction 5. This represents one extreme possibility for humin growth from HMF via aldol addition/condensation. The distinguishing feature of this first extreme is that the growing humins do not possess carbonyl groups with alpha hydrogens. As such, once all the HMF has been consumed, there would be no carbonyl compounds remaining that were capable of forming an enol. Thus, humin growth, at 4749
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Figure 6. IR spectra of humins recovered after hydrothermal (HT) and acid-catalyzed (AC) HMF conversion at the indicated percentage conversion levels.
Figure 7. Calculated free energy changes for the HMF conversion pathways proposed by Horvat.
addition of HMF. For example, 2,5-dioxo-6-hydroxy-hexanal could add HMF according to reaction 6 and then undergo condensation according to reaction 7. Having added an HMF molecule, the product of reaction 6 or 7 might be much less reactive toward addition/condensation of another HMF. Lower reactivity could be a result of changes in the electronic structure as well as steric limitations introduced by adding the relatively bulky HMF species. If the product of reaction 6 or 7 was indeed much less reactive than 2,5-dioxo-6-hydroxy-hexanal, then it might survive long enough to react with a second, newly formed 2,5-dioxo-6-hydroxy-hexanal. This represents the other extreme possibility for humin growth from HMF via aldol addition/ condensation. The distinguishing feature in this extreme possibility is that the humins would still have the capability to form enols, and consequently, they could continue to react via aldol addition/condensation even after all of the HMF had been consumed.
least via aldol addition/condensation, would cease when the HMF conversion was complete.
The scenario just described assumes that the reactivity for aldol addition/condensation remains very high as long as the product possesses even one carbonyl with an alpha hydrogen. Alternatively, it is possible that the reactivity decreases with each
The actual humin growth process could fall anywhere between the two extremes presented here. However, there are some structural consequences that will result irrespective of that detail. Because HMF can only participate as the added reactant, its incorporation into humins via aldol addition/condensation will result in the disappearance of its carbonyl group. Therefore, IR and other characterizations are expected to show the presence of the furan ring and the hydroxymethyl group but not the carbonyl. 4750
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Table 2. Calculated Free Energy Changes for Enol Formation, HMF Addition, and Condensation Involving 2,5-Dioxo-6-hydroxy hexanala carbon atoms involved in
free energy of formation (kJ/mol) condensation productb
enol
HMF addition
enol
adduct
C1, C2
C1
84.2
10.5
C2, C3
C3
29.1
13.9
7.0
C4, C5
C4
53.4
8.3
8.5
C5, C6
C6
54.5
5.1
44.5
C5, C6
C6
54.5
5.1
31.5
a
C1 through C6 designate the carbon atoms of 2,5-dioxo-6-hydroxy hexanal. b When HMF adds at C6, two different condensation products can form, by elimination of either the hydroxyl group at C6 or the one created from the carbonyl of HMF.
Figure 8. Comparison of the (a) low wavenumber and (b) high wavenumber IR spectra of humins recovered after acid-catalyzed conversion of HMF (percentage HMF conversion given) to the IR spectrum of HMF.
Table 1. Calculated Carbonyl Stretching Frequencies of 2,5-Dioxo-6-hydroxy Hexanal and the Product of Its Aldol Condensation/Addition with HMFa added species
reaction and site
C1 carbonyl
C2 carbonyl
C5 carbonyl
none
n.a.
92
67
53
HMF
addition C1
71
63
49
HMF
addition C3
95
62
43
HMF
condensation C3
72
58, 13
59
HMF
addition C4
94
66
59
HMF HMF
condensation C4 addition C6
92 94
71 72
12 54
HMF
condensation C6
92
67
46, 2
HMF
alt. condensation C6
90
62
69, 37
Frequencies in cm1 relative to the calculated carbonyl stretch in HMF.
Figure 9. Comparison of the (a) low wavenumber and (b) high wavenumber IR spectrum of benzaldehyde (BA) to those of humins recovered after acid-catalyzed conversion of HMF and an equimolar mixture of HMF and benzaldehyde.
a
However, most of the carbonyl groups from 2,5-dioxo-6-hydroxy-hexanal are expected to remain in the humin structure (reaction 5 or 7). Figure 8 compares the IR spectra of humins formed during the acid-catalyzed conversion of HMF to the IR spectrum of HMF, itself. The spectra of the humins contain many overlapping peaks,
and it is not immediately clear which spectral features might correspond to the carbonyl groups from 2,5-dioxo-6-hydroxyhexanal. To assist in assigning the peaks, equilibrium geometries and vibrational frequencies were calculated using DFT. This was done for HMF, 2,5-dioxo-6-hydroxy-hexanal, the four possible products of aldol addition of a single HMF to 2,5-dioxo-6hydroxy-hexanal and the four possible aldol condensation products of the latter. The calculated IR spectra were not scaled, and 4751
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Figure 10. SEM images of humins recovered after the acid-catalyzed conversion of an equimolar mixture of HMF with (a) benzaldehyde, (b) hydroxybenzaldehyde, (c) formaldehyde, and (d) acetaldehyde.
for each molecule, only one conformation was considered. For these reasons, the calculations are not expected to yield frequencies that are absolutely correct. However, the DFT frequencies can be used, for example, to compute the frequency of the carbonyl stretch of each of the three carbonyls of 2,5-dioxo-6hydroxy-hexanal relative to the carbonyl stretch of HMF. These relative frequencies are expected to be reasonably accurate, and they are very useful for assigning experimental IR peaks in the spectra of the humins. The peaks at 1522, 1275, 1198, 1020, 962, 826, and 777 cm1 in the IR spectrum of HMF can all be assigned to vibrations associated with either the furan ring or the hydroxymethyl group. Compared to the IR spectra of the humins formed during the acid-catalyzed conversion of HMF in Figure 8, there is evidence of corresponding features in the humins. This is consistent with the postulate that HMF is incorporated into the humins via aldol addition/condensation, because aldol addition/condensation would not change the furan ring or the hydroxymethyl group. The other very strong feature in the spectrum of HMF is a peak at 1665 cm1 that arises from the carbonoxygen stretch of the carbonyl group. The humins spectra do not show a strong feature at this wavenumber, but instead, they show a group of weak features. The aldol addition/condensation postulate is consistent with the absence of a feature at 1665 cm1 because the carbonyl of HMF would be destroyed by the reaction. As Table 1 shows, the DFT calculations suggest that the carbonyl groups of 2,5dioxo-6-hydroxy-hexanal would appear as three distinct features
at wavenumbers 50 to 90 cm1 higher than the carbonyl stretch of HMF. The humins spectra do indeed display a shoulder in this vicinity. Additional DFT calculations involving single, double, quadruple, and quintuple addition of HMF to 2,5-dioxo-6hydroxy-hexanal indicate that the carbonyl stretches can be observed at wavenumbers 45 to 65 cm1 smaller than that of the carbonyl in HMF. These smaller wavenumbers result when there is coupling between the carbonyl stretch and that of a carboncarbon double bond resulting from aldol condensation. The experimental spectra can be seen to be consistent in this regard, as well. The DFT calculations also permit an assessment of the free energy changes that accompany the possible aldol addition/ condensation reactions between 2,5-dioxo-6-hydroxy-hexanal and HMF. These are presented in Table 2 for each possible addition of a single HMF to 2,5-dioxo-6-hydroxy-hexanal. The table also includes calculated values of the free energy change associated with the required step of forming the enol. The results indicate that the energetics of aldol addition/condensation are comparable to the energetics of the other steps in the mechanism proposed by Horvat, Figure 7. Both the IR results and the DFT calculations are consistent with humin growth via aldol addition/condensation involving 2,5-dioxo-6-hydroxy-hexanal, but they do not prove it to be true. If aldol addition/condensation is a primary growth reaction, then the addition of a second reagent that could only participate as the added reactant (i.e., by adding a different 4752
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Table 3. Calculated Free Energy Changes for the Formation of Enols from the Addition and Condensation Products of HMF and 2,5-Dioxo-6-hydroxy Hexanal product from
of HMF at
forming enol at a
free energy (kJ/mol) 48.6
addition
C1
1
addition
C1
C2, C3
19.7
addition
C1
C4, C5
51.9
addition
C1
C5, C6
50.1
addition addition
C3 C3
C1, C2 C2, C3
70.6 48.4
addition
C3
C4, C5
53.7
addition
C3
C5, C6
51.8
addition
C4
C1, C2
83.1
addition
C4
C2, C3
40.0
addition
C4
C4, C5
75.6
addition
C4
C5, C6
52.6
addition addition
C6 C6
C1, C2 C2, C3
83.7 26.2
addition
C6
C4, C5
51.2
addition
C6
C5, C6
60.4
condensation
C3
C1, C2
69.9
condensation
C3
C4, C5
50.1
condensation
C3
C5, C6
47.9
condensation
C4
C1, C2
78.8
condensation condensation
C4 C4
C2, C3 C5, C6
1.9 62.5
condensation
C6
C1, C2
83.3
condensation
C6
C2, C3
28.3
condensation
C6
C4, C5
82.3
condensation
C6b
C1, C2
86.1
condensation
C6b
C2, C3
17.5
condensation
C6b
C4, C5
66.7
condensation
C6b
C5, C6
133.0
a
The second carbon involved in the formation of the enol is the carbonyl carbon of HMF. b The condensation product formed by elimination of the hydroxyl group at C6.
ketone or aldehyde that does not have an alpha hydrogen) would be expected to lead to the incorporation of that second reactant in the humins that form. To test this, benzaldehyde alone was first subjected to acid-catalyzed conversion conditions. Even after processing for 68 h at a nominal temperature of 135 °C, no humins formed; the solution remained clear. A second experiment was then performed where a mixture of HMF and benzaldehyde (0.1 M in each) reacted at 135 °C in the presence of 0.1 M sulfuric acid. Figure 9 compares the IR spectrum of the resulting humins to the spectrum of humins that formed from HMF only and to the spectrum of benzaldehyde. The figure reveals that the IR spectra of the two humins samples are very similar. The most significant difference is at wavenumbers below ca. 775 cm1. Humins from HMF only display a small peak at ca. 777 cm1 and no discernible peaks below that while the humins from a mixture of HMF and benzaldehyde show a stronger peak at 760 cm1 and a new peak at 700 cm1. Frequencies calculated using DFT for benzaldehyde added to 2,5-dioxo-6-hydroxy-hexanal via aldol condensation include two stronger peaks in these positions, and these peaks can be assigned to vibrations in the benzyl group. Figure 9
shows that benzaldehyde also has peaks in this range that are associated with its aromatic ring. Thus, the IR results are consistent with expectations based upon aldol addition/condensation being an important reaction for humin growth. Specifically, the IR shows that when benzaldehyde is added to HMF during processing, the resulting humins show vibrations associated with a benzyl group, but they do not include benzaldehyde’s carbonyl stretch at 1697 cm1. That carbonyl would be destroyed if the benzaldehyde added to 2,5-dioxo-6-hydroxy-hexanal via aldol condensation in the same way as described above in relation to HMF’s carbonyl stretch at 1665 cm1. Further evidence that the benzaldehyde has been incorporated into the humins is found in the SEM images presented in Figure 10. Humins formed from a mixture of HMF and benzaldehyde, Figure 10a, show a very different morphology than those formed from HMF only, Figure 4, the latter being predominantly spherical particles. Figure 10 also shows humins formed from mixtures of HMF and other aldehydes. All the results presented here are consistent with the postulate that, during acid-catalyzed conversion of HMF, humins grow via aldol addition/condensation involving 2,5-dioxo-6-hydroxy-hexanal and other aldehydes/ketones present in the system. The ability to incorporate other species in the humins (as demonstrated here using benzaldehyde) could provide a means of imparting specific functionality to the humins. This, in turn, could impact the economics of processes like the acid-catalyzed hydrolysis of cellulose. Humins could then become a value added product instead of a low-value waste material. Indeed, a variety of uses have been suggested for functionalized humins that are produced hydrothermally from glucose, fructose, or HMF.22 While it remains to conclusively prove that aldol addition/ condensation is a primary route for humin growth, the postulate that it is raises several other interesting questions. Further kinetic studies for a system including both HMF and BA might be revealing. Another important need is to determine which of the two extremes discussed earlier is closer to the actual process. Recall that the possibilities are either that 2,5-dioxo-6-hydroxyhexanal rapidly adds 5 HMF molecules, at which point it is no longer capable of forming an enol, or that the reactivity decreases with each successive HMF molecule that is added to 2,5-dioxo-6-hydroxy-hexanal. In the latter case, the final humins would retain sites where an enol could be formed, and this could provide a means of imparting functionalization without having to add a second reactant during the initial acidcatalyzed conversion. Instead, the humins could be recovered and functionalized in a subsequent process. Table 3 shows the free energy change associated with forming an enol after one HMF molecule has been added. The values in that table can be compared to the free energies for forming an enol from 2,5dioxo-6-hydroxy-hexanal, shown in the third column of Table 2. No clear conclusion is apparent, but on average, the free energy change for forming the enol appear to be comparable whether an HMF molecule has been added or not. Of course, even while the overall energies may be comparable, the rates may change appreciably after an HMF molecule has been added, both due to changes in electronic structure and to steric effects. Additionally, the observation that the morphology of the humins continued to change after all the HMF had been consumed might be interpreted to indicate that they retain some ability to form enols and participate in aldol addition/condensation for an extended period of time. 4753
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Energy & Fuels Another interesting question, assuming aldol chemistry to be an important reaction in humin growth, is why species like levulinic acid do not become incorporated in the humins. Levulinic acid has a ketone group that can participate in aldol chemistry either as the enol-forming reactant or as the added reactant, but the selectivity data indicate that this does not occur. It is clear that much additional experimentation and computation will be needed in order to develop a detailed understanding of how humins form and grow during acid-catalyzed conversion of carbohydrates. However, as a better understanding of chemistry of humin formation develops, it could lead to a means of eliminating formation of humins. Even if the formation of humins cannot be completely suppressed, a better understanding still could lead to procedures for converting the humins that do form into value-added products.
’ CONCLUSIONS As reported in previous studies, the kinetics of the acidcatalyzed conversion of HMF can be accurately modeled using two parallel reactions that are first order in the concentration of HMF. The humins display a spherical shape with the average sphere diameter increasing as the reaction proceeds. The observed selectivity strongly suggests that humins are derived from HMF and not from LA or formic acid. This is consistent with a mechanism proposed by Horvat that indicates that humins are formed from 2,5-dioxo-6-hydroxy-hexanal, which, in turn, is formed from HMF. IR spectra indicate that the humins retain the furan ring and hydroxymethyl group of HMF but that the carbonyl group of HMF is not present. This is consistent with humin growth via aldol addition/condensation of HMF with 2,5-dioxo-6-hydroxyhexanal. Reacting alone, benzaldehyde does not form humins at these conditions, but if benzaldehyde is additionally present during acid-catalyzed HMF conversion, IR spectra indicate that its aromatic ring is also incorporated in the humins, while its carbonyl group is not present. Again, this is consistent with aldol addition/condensation as a primary humin growth reaction. It further shows that it is possible to introduce new functionality (in this case an aromatic ring) into the humins, and this could eventually lead to a means for increasing their value. ’ AUTHOR INFORMATION Corresponding Author
*Phone: 716-645-1180. Fax: 716-645-3822. E-mail: lund@buffalo. edu.
’ ACKNOWLEDGMENT The experimental assistance of Ryan Barton, Michelle D’Lima, Jacob Heltzel, and Erick Dustin is gratefully acknowledged. ’ REFERENCES (1) Farrell, A. E. Ethanol Can Contribute to Energy and Environmental Goals. Science 2006, 311 (5760), 506–508. (2) Galbe, M.; Zacchi, G. A review of the production of ethanol from softwood. Appl. Microbiol. Biotechnol. 2002, 59 (6), 618–628. (3) Hayes, D. J. An examination of biorefining processes, catalysts and challenges. Catal. Today 2009, 145 (12), 138–151. (4) Mascal, M.; Nikitin, E. B. Direct, high-yield conversion of cellulose into biofuel. Angew. Chem., Int. Ed. 2008, 47 (41), 7924–7926.
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