Hydrolysis of Cellulose and Glucose Using Recyclable α

Oct 9, 2017 - A principal challenge in using biomass as a replacement for petroleum has been to develop cost-competitive and sustainable processing ...
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Cite This: Ind. Eng. Chem. Res. 2017, 56, 12529-12537

Hydrolysis of Cellulose and Glucose Using Recyclable α‑Hydroxysulfonic Acids W. Brett Barclay,†,‡ M. Clayton Wheeler,† Anna Moh,†,§ and G. Peter van Walsum*,† †

Department of Chemical and Biological Engineering, University of Maine, Orono, Maine 04469, United States ABSTRACT: A principal challenge in using biomass as a replacement for petroleum has been to develop cost-competitive and sustainable processing technology. Cellulose in biomass can be converted into levulinic and formic acids through acidcatalyzed hydrolysis and dehydration, conventionally done using sulfuric acid. These products can be used as chemical feedstocks or converted to advanced biofuels. Product separation and sulfuric acid recovery are significant hurdles hindering the commercialization of the acid-catalyzed process. Hydrolysis of cellulose using α-hydroxysulfonic acids (αHSAs) presents a novel approach to simplifying acid recovery from biomass hydrolysates. We present the results of comparing six different αHSAs, including two derived from molecules produced by the hydrolysis itself, for conversion of glucose or cellulose to levulinic acid. After evaluating the six compounds for qualities of catalytic activity, thermal stability, and recyclability, we found that acetone-derived αHSA compares favorably in terms of these metrics. Highest yields of levulinic acid derived from cellulose were 40% of theoretical, achieved at conditions of 150 °C, 24 h reaction time, 3% cellulose feed, and 1.875 M acetone-derived αHSA. 1.2. Hydrolysis of Biomass. The carbohydrate components of biomass can be converted into various organic chemicals, often by starting with hydrolysis to produce monomer sugars. Acid-catalyzed hydrolysis processes will typically reach an optimum combination of conversion and sugar yield at a given reaction severity, with further increases in severity resulting in degradation of hydrolysis products and reduction of final yield. Once cellulose is hydrolyzed to glucose, the glucose can dehydrate to form 5-hydroxymethyl furfural (5HMF) which eventually breaks down further to levulinic and formic acids. Meanwhile, under the acidic high temperature conditions, pentoses, primarily xylose, will dehydrate to form furfural. For chemical applications, levulinic acid and furfural may be desired end products. Figure 1 shows the mechanism for the desired reactions producing levulinic acid, formic acids, and furfural. Figure 2 represents the pathways resulting in humin degradation products. 1.2.1. Optimization of Levulinic Acid Yield. In a review of the conversion of lignocellulosics to levulinic acid, Rackemann and Doherty recite yields of levulinic acid from different feedstocks under different operating conditions varying from 45.6% to 82.7% on a molar basis with cellulose or hexose sugars.11 On a mass basis, full conversion of six carbon starting materials to levulinic and formic acids together account for all of the mass of initial cellulose, or 90% of the initial glucose mass. Because it undergoes substantial dehydration, at full

1. INTRODUCTION Of all the major renewable resources, biomass is the only one that can provide a source of fixed carbon for production of hydrocarbon transportation fuels and organic materials.1 Replanting previously harvested biomass allows CO2 to be recycled back through the growth of new foliage.2 Conversion of biomass to liquid fuels and chemicals can be completed through the use of biological or thermochemical conversion methods. The specific conversion method and conditions used will depend on the type of biomass and desired final products.2,3 1.1. Lignocellulosic Biomass. Substantial research efforts have been dedicated to sustainable methods for the conversion of lignocellulosic biomass to liquid transportation fuels.4−9 This is due in part to its abundance and relatively low cost; the sugars in the cell walls of plants contain more than 50% of Earth’s organic carbon.10 Lignocellulosic biomass is mainly composed of three components: cellulose, hemicelluloses, and lignin. Cellulose is highly crystalline and polymeric, with unbranched chains of β(1,4)-D-glucose linkages, and is the most abundant of the three main components, ranging from about 40−50% of the total mass. Hemicellulose is also a polymeric carbohydrate, but it is amorphous, branched, and composed of several different pentose and hexose sugar monomers. Hemicellulose is more easily hydrolyzed than cellulose, and it has a lower degree of polymerization. Lignin fills in spaces between cellulose and hemicellulose in the cell wall and binds the cellulose fibers together. Lignin is a complex polymer primarily made up of three phenylpropenes with a phenyl group in the para position and zero, one, or two methoxy groups in the meta positions.2 © 2017 American Chemical Society

Received: Revised: Accepted: Published: 12529

August 13, October 5, October 9, October 9,

2017 2017 2017 2017 DOI: 10.1021/acs.iecr.7b03355 Ind. Eng. Chem. Res. 2017, 56, 12529−12537

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Industrial & Engineering Chemistry Research

Figure 1. Chemical pathway for production of levulinic acid, formic acid, and furfural from glucose and xylose starting materials in an acid environment.

conversion, furfural will achieve only a 64% mass yield on initial xylose. As illustrated in Figure 2, the balance of converted starting material (either cellulose of glucose) is assumed to degrade to humic materials through pathways labeled Cd, Gd, and Hd in Figure 2. Most of these studies were done in a singlestage reactor system. One of the cited works reported results from a patented, continuous 2-stage method for the production of levulinic acid, formic acid, and furfural from carbohydrate containing feedstocks. Based on the hexose content of the feed, a yield of >70% of theoretical is claimed.12,13 Most of the systems reviewed by Rackemann and Doherty, including the 2stage system, made use of H2SO4 as their acid catalyst, though some used HCl.11 1.3. α-Hydroxsulfonic Acids (αHSAs). Although hydrolysis using high acid concentrations can give high sugar yields, it is necessary to recover the acids for both economic and environmental reasons. The recovery and recycling of the strong acid has proven to be a significant hurdle affecting the scale up of biomass hydrolysis processes. In 2012, Blackbourn and Weider patented the use of α-hydroxysulfonic acids (αHSAs) for hydrolysis of hemicellulose in the context of

Figure 2. Schematic for cellulose decomposition pathways.

Figure 3. Acid structure, formation, and dissociation: (a) sulfuric acid, (b) generic αHSA, (c) equilibrium reaction of a carbonyl, sulfur dioxide, and water forming a generic αHSA, and (d) dissociation of generic αHSA. 12530

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Industrial & Engineering Chemistry Research biomass pretreatment.10 They reported that biomass hemicellulose was successfully hydrolyzed at temperatures between 80 and 120 °C with acid concentrations between 1 and 41 wt %. The focus of the technology was on releasing pentoses for fermentation, for which they reported optimal conditions to be 100 °C for 4 h in CSTR with 10% wt αHSA. They identified two αHSAs as leading candidates for hydrolysis of hemicellulose, α-hydroxyethanesulfonic acid and α-hydroxymethanesulfonic acid, derived from the carbonyls acetaldehyde and formaldehyde, respectively. For this current study, we hypothesize that under sufficiently severe conditions αHSAs may also be useful for hydrolyzing glucose or cellulose to levulinic acid. αHSAs are structured similarly to sulfuric acid with one of the two hydroxyl groups substituted as shown in Figure 3a and b. R-group branches can either be hydrogen or hydrocarbon chains. The αHSAs can be synthesized by combining a carbonyl with SO2 and water (Figure 3c) in a 1:1:1 mol ratio, which will exist in equilibrium with the αHSA. The specific αHSA created corresponds directly to the carbonyl used. The hydrogen that dissociates is the one bound to the oxygen on the central sulfur atom (Figure 3d). The formation of αHSA is exothermic; thus, higher temperature will shift the equilibrium toward the individual starting components. Due to the volatility of sulfur dioxide, the reaction can be reversed by increasing the temperature and/or lowering the pressure to release the SO2. As the SO2 is removed, the pH of the reaction mixture rises. The outcome of carbonyl recovery depends on the nature of the carbonyl, most importantly its volatility and its solubility in the aqueous phase. It has been previously reported that αHSAs will react with NaCl, subsequently freeing HCl to create an alkali metal sulfite.14 This is indicative of the acid strength achievable with αHSAs. Blackbourn and Weider10 state that αHSAs were effective at xylan hydrolysis and could be removed from the reaction mixture directly following treatment, simplifying downstream processing. Because the formation of these acids is reversible, high concentrations of acid can be employed and recovered. The position of the equilibrium presented in Figure 3d is sensitive to the nature of the carbonyl used to create the αHSA. Blackbourn and Weider claim that greater steric bulk around the carbonyl will lower the thermal stability of the acid. Research presented in this study suggests that there may be other important aspects playing into the role of equilibrium as well, including electron-withdrawing effects which can negate the role of steric bulk. The equilibrium of the αHSA system is dependent on the aqueous solubility of the carbonyl and the SO2, which is in turn dependent on its partial pressure and the temperature. Other contributing influences on the equilibrium include the presence of competing carbonyl compounds, including in this system furfural and levulinic acid, which are products of the reaction, and other potential byproducts as detailed in Rydholm’s pulping processes.15 1.4. Levulinic Acid as a Platform Chemical. In 2004, the United States Department of Energy issued a report that included levulinic acid as one of the 12 leading candidates for value-added biomass-derived chemicals. The final list of 12 leading candidates was created after studying the prospective markets for chemical building blocks and their derivatives alongside the complexity of the synthesis processes.16 The report lists levulinic acid derivatives including gasoline and biodiesel additives, herbicides, acrylate polymers, copolymeriza-

tion enhancers for other polymers, and lactones for use in the solvent market, suggesting that levulinic acid could be a significant building block in biorefining. Recently, levulinic acid has been identified as a useful molecule for production of near drop in fuels via the thermal deoxygenation (TDO) process.17−19

2. MATERIALS AND METHODS 2.1. Materials. The carbonyls acetaldehyde, furfural, methyl ethyl ketone, 4-methyl-2-pentanone, acetone, and levulinic acid were all reagent grade chemicals from either Fisher Scientific or Acros Organics. Cellulose was in the microcrystalline form from Aldrich Chemical Co., Inc. (No. 43, 523-6). Glucose used was D-(+)-glucose, SigmaUltra, from Sigma (CAS 50-99-7). 2.2. α-Hydroxysulfonic Acid Synthesis. Two methods were employed to generate αHSAs. In one method, the acids were synthesized under SO2 pressure in a Parker Autoclave Engineers 300 mL reactor, operated with a 100 mL working volume. Soluble carbonyls were charged to a final concentration of 1.875 mol/L, equal to the molar solubility of SO2 in water at 20 °C. Less soluble carbonyls were added up to their solubility limit. After dissolving the carbonyl, SO2 was added to the headspace at the pressure of the source (∼35 psi) for at least 6 h to allow for full solvation and reaction with the carbonyl (determined when there was no pressure decrease after closing the valve to the source of SO2). During preparation, the vessel was agitated at 200 rpm, and pressure and temperature were continuously monitored using a Sentinel series controller. When formation of the αHSA acid was complete, the reactor was vented down to atmospheric pressure, capturing excess SO2 with a NaOH scrubbing solution.20 The solution was then transferred to a sealed glass container that had been flushed with nitrogen. The second method for generating αHSAs was done at ambient pressure. These αHSAs were prepared in a vertical 1 L cylinder fitted with an inlet tube terminating with a diffuser at the bottom. For soluble carbonyls, the cylinder was filled with a solution of 1.875 mol carbonyl/L. Less soluble carbonyls were added up to their solubility limit. SO2 was then sparged into the cylinder at low pressure, with any excess gas discharged through the top directly to a NaOH scrubber solution. The sparging apparatus was mounted on a balance, allowing real time determination of the amount of SO2 dissolved into solution. The temperature at which the synthesis occurred was noted and was usually ambient room temperature (∼20 °C), but in some experiments, the sparging was carried out immersed in an ice bath to enhance SO2 solubility. 2.3. Carbonyl Screening and Selection. Criteria applied to identify candidate carbonyls included: good solubility in water, thermal stability under the acid conditions, and high volatility. These criteria were deemed necessary to achieve sufficient equilibrium concentration of acid, thermal stability of the catalyst, and ease of separation and recycle, respectively. Since generation of the αHSAs was done under continual SO2 replenishment, acid concentrations were assumed to be carbonyl limited. Using a carbonyl that would volatilize out of aqueous solution under reduced pressure at high temperature would enable the envisioned separation and recycle of the acid species. An additional criterion for selecting carbonyls was to consider using carbonyls that are themselves products of the hydrolysis and dehydration reactions, in particular, levulinic acid and furfural. Being products of the reaction, these 12531

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Industrial & Engineering Chemistry Research Table 1. Carbonyls with Their Water Solubility and Corresponding α-Hydroxysulfonic Acid Carbonyl

Solubility (g/100 mL water)

Boiling point (°C)

Acid name

Product of the reaction?

acetaldehyde acetone methyl ethyl ketone 4-methyl-2-pentanone furfural levulinic acid

miscible miscible 27.5 1.9 8.3 miscible

20.2 57 79.6 118 162 246

α-hydroxyethanesulfonic acid acetone α-hydroxysulfonic acid methyl ethyl ketone α-hydroxysulfonic acid 4-methyl-2-pentanone α-hydroxysulfonic acid α-hydroxyfurfuralsulfonic acid levulinic α-hydroxysulfonic acid

no no no no yes yes

2.5. Small-Scale Batch Hydrolysis Conditions. The same tube reactor system used for thermal stability tests was also used to conduct hydrolysis of cellulose and glucose using different acids. A volume of 5 mL of αHSA was used with carbohydrates added in initial concentrations of 1.5 or 3 wt %. The temperatures ranged from 150 to 200 °C, with reactions enduring from 15 min to 24 h. 2.6. Stirred Tank Reactor (STR) System. For scale up and chemical recovery purposes, reactions were performed in a 300 mL Parr reactor using a Parr 4875 series power controller. The reactor was equipped with stirring and internal cooling capabilities and inlet and outlet ports for feeding or removing gas from the system. The reactor was heated using an external heating jacket. Experiments were heated from 20 to 175 °C at a rate of 7.75 °C/min for a total heating up time of 20 min. The reactor was able to maintain temperatures of ±5 °C at a set temperature of 175 °C. Reaction times ranged from 30 min to 2 h. For all experiments, a liquid volume of 100 mL of αHSA was used. The concentration of carbohydrates used in STR hydrolysis experiments was 3 wt %. At the conclusion of the experiment, the controller was set to 20 °C (initiating the cooling loop), the heating jacket was removed, and external cooling was implemented through the use of an ice bath. Cooling to 20 °C from 175 °C took approximately 8 min. Before opening the reactor, nitrogen was passed through the system at a rate of 100 sccm to sweep the headspace of free SO2. Purge gases were vented to a saturated NaOH solution to capture the SO2. For SO2 removal/αHSA reversion purposes, the Parr reactor was reheated to 80 °C while under nitrogen flow at a rate of 100 sccm. Reversion experiments involving levulinic acidderived αHSA were held at 80 °C for 10 min, while acetoneand methyl ethyl ketone-derived αHSA experiments were immediately cooled to 20 °C after reaching 80 °C. 2.7. Analytical Methods. Hydrolyzates were prepped for analysis by dilution with water by either 1:50 or 1:100, then filtered using a Fisherbrand PTFE 0.45 μm syringe filter. They were then analyzed via a Shimadzu LC-10AT HPLC with a Shimadzu RID-10A refractive index detector. The column was a Bio-Rad Aminex HPX-87H ion exclusion column measuring 300 mm × 7.8 mm. The mobile phase was 5 mM H2SO4. The flow rate and oven temperature combination was either 0.5 mL/min and 45 °C or 0.6 mL/min and 60 °C. It was noticed that the levulinic acid-derived αHSA had the same retention time as formic acid. Accordingly, the HPLC protocol was modified (oven temp changed from 60 to 40 °C and flow rate decreased from 0.6 to 0.5 mL/min) to separate the levulinic acid αHSA peak from the formic acid peak during HPLC analysis. Running appropriate standards with this modified method confirmed that the coelution had been resolved. Calibrations for levulinic acid, acetone, glucose, and formic acid were performed at each temperature.

compounds would not need to be completely recycled and would therefore represent options for a less volatile carbonyl. A total of six different αHSAs were synthesized. The carbonyls that they are derived from are listed in Table 1 along with the carbonyl water solubility, boiling points, and their assigned αHSA name. Figure 4 shows structures of these αHSAs studied in this work.

Figure 4. Structure and assigned names of αHSAs investigated in this study.

2.4. Thermal Stability Testing. Thermal stability tests of specific αHSAs were carried out in a batch reactor system. These reactors were constructed of 1/2 in. 316 stainless steel tubing (wall thickness of 0.065 in.) sealed with Swagelock caps. The tubes had a total volume of 11 mL and were filled with 5 mL of αHSA reagent. Temperature of the reactors was controlled by immersion into a Thermo Haake W19 oil bath coupled with a Thermo Haake DC30 circulator and SIL180 silicone oil heat transfer liquid. The circulator was able to maintain temperatures in the bath to ±0.3 C with respect to the set value. Each acid was prepared and cycled through thermal cycles from room temperature to at least 150 °C and as high as 200 °C. To terminate the experiments, the reaction tubes were removed from the bath and immediately placed into an ice bath to cool before analysis. Results were evaluated though HPLC analysis and visible changes in physical appearance. Acidity of the αHSA was also taken into account. 12532

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Industrial & Engineering Chemistry Research Table 2. pH and Thermal Stability of Various α-Hydroxysulfonic Acid by Carbonyl Carbonyl

Initial pH

Final pH

Visible changes

Hydrolysis attempted

acetaldehyde acetone methyl ethyl ketone 4-methyl-2-pentanone furfural levulinic acid

0 0 0 2 0 0

0 0 variable 2 0 0

black precipitate, dark yellow liquid clear to yellow/green color, little occasional precipitate turns increasingly dark green with severity appears to decompose, although no visible changes yellow color turns darker above 150 °C, oily appearance clear to yellow-green color, occasional precipitate

no yes yes no yes yes

3. RESULTS AND DISCUSSION 3.1. Thermal Stability Experiments. The tested conditions and results of thermal stability tests are described below and summarized in Table 2. Four tested αHSAs had pH levels below zero through the thermal cycling. Acetaldehyde-derived αHSA showed poor thermal stability. At temperatures above 150 °C, a significant amount of black, sweet-smelling solid precipitated, and the remaining liquid turned dark yellow to black, although the solution pH did remain at ≤0. Due to this thermal instability, no hydrolysis was attempted with acetaldehyde derived αHSA. Acetone-derived αHSA was thermo-cycled to 150, 175, and 200 °C with the resulting solutions being clear to light yellow/ green with only occasional and minimal black sediment at high severity. Methyl ethyl ketone- derived αHSA showed no major precipitates on heating, although the original clear solution developed a greenish tinge which darkened with increasing severity. The αHSA was heated to 150, 175, and 200 °C for up to 4 h. It was observed that the pH increased with increasing cycle severity. After thermal cycles to 175 °C or higher, no remaining αHSA was detected in the solution. From these results, it appeared that high temperatures had the effect of permanently shifting the equilibrium in favor of the starting components, which may have been caused by a loss of the SO2 at high temperatures. 4-Methyl-2-pentanone-derived αHSA had a pH of 2, the highest initial pH out of all acids synthesized. This weaker acidity appears to be a function of the acid itself and not concentration since the equilibrium appeared to favor formation of the acid over the starting components (no 4methyl-2-pentanone was detected by HPLC in the equilibrium solution). After thermo-cycling, the αHSA detected by HPLC decreased with increasing severity and eventually disappeared entirely after only 2 h at 150 °C. Although there were minimal apparent decomposition reactions observed, the complete disappearance of the αHSA after heating to 150 °C for 2 h and the low acidity dropped this αHSA from consideration for hydrolysis testing. Furfural is originally yellow in color, and the resulting αHSA is also yellowish. When exposed to air, the surface of the solution began turning very dark after 24 h, and this darkness propagated through the remaining acid with increasing time. After heating to 150 °C, the acid became very dark, almost black, and developed an oily appearance. However, there were no solids present, and the pH remained at ≤0. Furfural is known to undergo acid-catalyzed decomposition.21,22 This study shows that decomposition increases with increasing furfural concentration and temperature. In addition to acidcatalyzed decomposition, furfural may be able to undergo an aldol condensation in the presence of a strong acid due to possible enol formation from its carbonyl group. Although it appeared to have poor thermal stability, because furfural is a

product of the conversion process, it was still tested for hydrolysis capability. While being the desired product of cellulose acid hydrolysis, levulinic acid also contains a carbonyl group and therefore is able to combine with SO2 and water to form an αHSA. Levulinic acid alone has a pH of 3. The αHSA synthesized using levulinic acid has a pH of ≤0. Levulinic acid-derived αHSA was thermo-cycled to 150 175, and 200 °C with the resulting solutions being clear to yellow/green with occasional and minimal black sediment which increasing with severity. 3.2. Hydrolysis of Cellulose and Glucose Using αHydroxysulfonic Acids. 3.2.1. Equilibrium and Competitive Presence of Different α-Hydroxysulfonic Acids. As detailed in Section 3.2.5, levulinic acid has been shown to form an αHSA. As cellulose and glucose decompose to levulinic acid, some of this levulinic acid will combine with unbound SO2 and water to form levulinic acid-derived αHSA. This is significant because accurate quantization of levulinic acid thus requires the removal of SO2 to release levulinic acid bound in its αHSA. An experiment was performed to determine a rough estimate for the selectivity of carbonyl for αHSA formation. Levulinic acid was added to acetone-derived αHSA in the amount that would be produced if 3% glucose (0.167 M) were to be completely converted to levulinic acid. Immediate HPLC analysis showed that about 6% of the levulinic acid was rendered undetectable. It is assumed that this lost levulinic acid had complexed with SO2 and H2O to create levulinic acid-derived αHSA. Time before HPLC analysis and prior temperature variations could affect the observed equilibrium. Nevertheless, yields reported from batch reactions, which were not vented to remove SO2, are subject to this variablility in measurement of levulinic acid. Accordingly, actual yields of levulinic acid in some cases may then have been higher than measured. Because of uniform sample handling procedures pre- and post-hydrolysis, comparable trends of levulinic acid production and yield in batch experiments should at least represent relative trends. Reactions performed in the stirred tank reactor (STR) could opt to have the SO2 vented from the system after reaction to improve the quantification of this reaction. 3.2.2. Hydrolysis with Acetone-Derived α-Hydroxysulfonic Acid. Hydrolysis of cellulose and glucose using acetone-derived αHSA was studied using both the batch tubes and the STR. Temperatures tested were 150, 175, and 200 °C with the acid catalyst concentration kept constant. Concentration of carbohydrate was either 1.5 or 3 wt %. 3.2.2.1. Glucose Conversion Using Acetone αHSA in Tube Batch Reactors. A total of 24 experiments were performed using acetone αHSA to hydrolyze glucose. These were performed at 150, 175, and 200 °C with 3 wt % of glucose. A typical concentration profile observed for decomposition of glucose to levulinic acid is shown in Figure 5a. The rate and yield of glucose conversion to levulinic acid is a function of 12533

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αHSA was performed in an analogous manner to hydrolysis of glucose. Hydrolysis at temperatures of 150, 175, and 200 °C were used. Two initial cellulose concentrations were tested: 1.5 and 3 wt %. Yields of levulinic acid are expressed on a per mole basis using the number of moles of glucose derived from a given mass of cellulose. A typical concentration versus time profile is shown in Figure 6a. When reaction rates were slow

Figure 5. (a) Typical concentration profile for glucose conversion to levulinic acid using acetone-derived αHSA (T = 150 °C, 3 wt % glucose, tube reactor). (b) Yield of levulinic acid from glucose versus time at different temperatures (3 wt % glucose, acetone-derived αHSA, tube reactor).

temperature and time as evident in Figure 5b, where lower temperature and longer reaction times optimize yields. The time required for full conversion at 150 °C was 8 h, while being less than 2 h at 175 °C and less than 15 min at 200 °C. The highest ultimate yield observed was about 40 mol % at 150 °C. The trends associated with these results follow the trends observed and predicted by Girisuta et al.23 The higher final yields with decreasing temperatures are consistent with the higher activation energies associated with conversion of glucose and 5-HMF to humin, relative to the activation energies for the conversion of glucose to 5-HMF and then to levulinic acid. A notable difference between hydrolysis using acetonederived αHSA versus sulfuric acid is the delay of levulinic acid accumulation, while still showing immediate glucose decomposition. These trends are not seen in studies by Girisuta et al.23 In their study, it is mentioned that the reaction of glucose to 5-HMF is slower than that of 5-HMF to levulinic acid; hence, 5-HMF is not expected to accumulate. However, in the sulfonic acid system, it could be that some of the 5-HMF may form an αHSA, given that it has a carbonyl group, which could result in effective accumulation of bound 5-HMF, that may in turn delay the appearance of levulinic acid. Likewise, the measurement delay in levulinic acid could also be attributed to the immediate formation of levulinic acid-derived αHSA. The complex and changing availability of carbonyl compounds in the system is liable to result in variable acid concentration over the course of the reaction. 3.2.2.2. Cellulose Conversion Using Acetone αHSA in Tube Batch Reactors. Hydrolysis of cellulose using acetone-derived

Figure 6. (a) Typical product concentration profile for cellulose conversion to glucose and levulinic acid using acetone-derived αHSA (T = 150 °C, 3 wt % cellulose, tube reactor). (b) Levulinic acid yield over time at temperatures of 150, 175, and 200 °C, using acetonederived αHSA in tube reactor. (c) Effect of initial cellulose concentration on levulinic acid yield (T = 150 °C, acetone-derived αHSA, tube reactor).

enough to capture it (reactions at 150 °C), the concentration of glucose exhibited a maximum. This maximum correlates to about a 15 mol % glucose yield. Levulinic acid yield is again influenced by temperature with higher temperatures leading to lower yields. This is demonstrated in Figure 6b, which shows levulinic acid yields with respect to time at the three different temperatures. The highest yield observed for hydrolysis of 3 wt % cellulose was about 40% on a per molar basis of glucose contained in the given mass of cellulose. As with glucose, the conversions were more rapid at higher temperatures, but final yield was higher at the lowest temperature. Temperature seemed to have a slightly larger influence on hydrolysis of cellulose than hydrolysis of glucose, with the maximum yields at 175 and 200 °C reaching lower levels than in the glucose decomposition cases. This may be due to cellulose decomposing to unwanted decomposition products. Girisuta et al. claim that in the sulfuric acid-catalyzed 12534

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improved mixing is likely one contributing factor to this improvement. In addition to this, the SO2 removal from the STR system post-hydrolysis allowed additional levulinic acid to be detected. The additional levulinic acid measured correlated to 4−10% of the total detected after SO2 removal. Unlike in batch tube reactions, no decrease in levulinic acid was observed beyond the maximum reaction times. 3.2.3. Methyl Ethyl Ketone Derived α-Hydroxysulfonic Acid. Using methyl ethyl ketone-derived αHSA as the catalyst for glucose hydrolysis was attempted at 150 °C in the presence of 30 g/L glucose. After 2 h, there was evidence of glucose conversion. After 4 h, there was evidence of levulinic acid production. A separate experiment starting with 22.3 g/L glucose showed complete consumption of glucose in the first 2 h but relatively little levulinic acid production. The levulinic acid reached it maximum at the end of the experiment (4 h), attaining only 2.4 g/L after 4 h at 150 °C. This represented a yield of about 11%, well below other reported conversions. 3.2.4. Furfural-Derived α-Hydroxysulfonic Acid. Despite its low thermal stability, hydrolysis of carbohydrates with the furfural-derived αHSA was investigated because furfural is a product of the reaction and would be present even if not added to the reaction system. Cellulose and furfural-derived αHSA were combined in the tube reactors and heated to 95, 125, and 150 °C for up to 3 h. Evidence of degradation to glucose was seen at 150 °C after 1 h, and conversion to levulinic acid was seen after 3 h. Although this demonstrated its catalytic capability, no further tests were performed due to the lack of thermally stability of furfural-derived αHSA. 3.2.5. Levulinic Acid-Derived α-Hydroxysulfonic Acid. Like furfural, levulinic acid is a product of acid-catalyzed biomass decomposition and might allow hydrolysis to become autocatalytic in the presence of SO2. In addition, utilizing levulinic acid as the carbonyl would avoid the need for a dedicated αHSA recycle system and allow for a lower required separation efficiency. Levulinic acid-derived αHSA successfully hydrolyzed both glucose and cellulose and was able to produce newly formed levulinic acid at 175 °C. Glucose consumption was clearly observed in the glucose-fed system, with complete conversion of glucose in tube batch reactors observed after 60 min. The hydrolysis of cellulose was evident through the detection of an initial increase in the concentration of glucose in the cellulose-fed system. Subsequently, this glucose was decomposed. Rates of levulinic acid degradation were assessed by running control samples that contained no carbohydrates. An increase in levulinic acid levels relative to this control demonstrated that conversion of cellulose and glucose to levulinic acid was occurring. Experiments were run to revert the αHSA to its starting components. Based on sugar conversion results, reactions done in the STR at 100 mL scale were performed for 45 min and 1 h. Glucose loading was varied between 5 and 10 wt % and initial αHSA strength of 14 and 28 wt %. It was noticed that in several cases the net concentration of levulinic acid relative to the control (no carbohydrate in feed) had decreased. This suggested that the levulinic αHSA was not reverting entirely back to its original components. Table 3 lists the net levulinic acid change for each of the eight combinations of time, acid strength, and initial glucose concentration. Only two of the eight conditions registered a net increase in levulinic acid. Girisuta et al. did not witness a decrease in levulinic acid in either of their levulinic acid production studies, although they only tested one scenario at very low levulinic acid

hydrolysis system the highest activation energies out of all the reactions, either desired or undesired, are associated with the degradation of cellulose to decomposition products.24 Thus, if cellulose degradation reactions are most affected at the highest temperatures, it makes sense that levulinic acid yields from cellulose are most jeopardized at the highest temperatures. The effect of initial cellulose loading was also examined with initial concentrations being 1.5 or 3 wt %. At 150 °C, there appeared to be no difference in levulinic acid yield with changing carbohydrate concentration as shown in Figure 6c. This is consistent through all temperatures. In most cases where data were collected sufficiently late toward the end of the reaction, one phenomenon witnessed was the slight decrease in levulinic acid once it reached its maximum. This was especially evident on experiments performed at 200 °C. This could be caused by decomposition of the levulinic acid as severity increases. Such decomposition may be happening over the course of the entire reaction but only becomes evident once all available glucose is converted. The second explanation could be that the αHSA system equilibria shift in the evolving mixture and less levulinic acid remains detectable by HPLC. 3.2.2.3. Cellulose Decomposition Using Acetone αHSA in STR. Scaling up reactions to 100 mL carried out in a 300 mL Parr reactor allowed for more complete mixing (100 rpm) of the reactor contents and also for selective removal of SO2 and volatiles at the end of the experiment. By removing SO2 from the system, the αHSA equilibrium shifts to the left (nonacid side), and the acids revert to their nonacidic starting compounds. In particular, generated levulinic acid and furfural, which may have reacted to form their respective αHSA, could be reverted to the carbonyl form and more accurately quantified. Figure 7 shows levulinic acid yields at 175 °C with respect to time for cellulose decomposition with and without SO2 removal

Figure 7. Levulinic acid yield from cellulose in a stirred tank reactor, with and without αHSA reversion (T = 175 °C, 3 wt % cellulose, acetone-derived αHSA, 100 rpm mixing).

at the end of the STR experiments. Experiments were performed at 175 °C. They were run for 1, 2, and 3 h. Glucose was not detected in experiments longer than 1 h. Comparing maximum levulinic acid yields in stirred tank conditions versus tube batch reactors, where in both cases no SO2 removal was possible, reactions in STR produced higher yields at this temperature (36 versus 33 mol %). This increase from batch to STR reactor configuration is consistent with trends observed using sulfuric acid as the acid catalyst.24 Thus, 12535

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the system by reducing it to thiosulfate.28 SO2 first reacts with water to form sulfurous acid. Sulfurous acid reacts with formic acid to form thiosulfate, water, and CO2. Identifying an increasing thiosulfate concentration throughout hydrolysis was not attempted but could be done through ion chromatography to support this claim. Since formic acid has been detected in sulfuric acid systems, it seems likely that this second hypothesis may be more applicable to this αHSA system. 3.3. Validation of α-Hydroxysulfonic Acid Reversion and SO2 Removal. Experiments separate from the hydrolysis experiments were performed to confirm the recyclability of αHSAs. An initial qualitative approach was used to test the reversion of acetone and methyl ethyl ketone αHSAs to their starting components. These acids were heated while venting to a NaOH solution. The pH of the system increased and precipitation of Na2SO3 confirmed SO2 removal from the solution. Quantitative assessment of the same procedure was done using acetone- and levulinic acid-derived αHSAs. Analysis by HPLC confirmed αHSA reversion with the diminishing of the αHSA peak and increase in the carbonyl peak after venting. Trace amounts of αHSA were still detected after the SO2 removal procedure. It was not desirable to extend the duration of the removal process because vaporization of water and possibly the carbonyl would increase. For acetone, which volatizes fairly easily under the SO2 removal conditions, the volume change of the system was taken into account by measuring the liquid volume lost and by measuring the volume of condensed vapors in the vapor stream. The condensable components were analyzed by HPLC and were primarily made up of acetone with trace amounts of αHSA. It is presumed that the αHSA did not vaporize in that form but instead was produced in the condenser from the SO2, acetone, and water vapor that escaped. Typical HPLC chromatograms for an αHSA before and after SO2 removal are shown in Figure 8.

Table 3. Net Levulinic Acid Change for Various Reaction Conditions Converting Glucose to Levulinic Acid Using Levulinic Acid-Derived αHSA Experiment ID

Time (min)

αHSA concentration (wt %)

Glucose (g)

Net levulinic acid change (%)

1 2 3 4 5 6 7 8

45 45 60 60 45 45 60 60

28 28 28 28 14 14 14 14

5 10 5 10 5 10 5 10

−1.5 −5.3 −4.6 −5.3 −4.0 + 3.2 −3.0 + 7.0

concentrations.23,24 An important difference between this study and Girisuta et al. is the presence of SO2 and αHSA. By analysis of the levulinic acid remaining from posthydrolysis, it can be determined that the degradation (or disappearance) of levulinic acid increases with increasing αHSA concentration and time. The two cases that netted positive levulinic acid yield were the 45 and 60 min cases for the low level (14 wt %) of acid catalyst with the higher (10 wt %) glucose concentration (ID 6 and ID 8, respectively). Net levulinic acid change was higher for ID 8 than ID 6. This can be attributed to having enough time to approach 100% conversion. For the most part, the 14 wt % acid cases all had more positive net levulinic acid changes than the 28 wt % acid cases. The low net yields and kinetics surrounding possible levulinic acid decomposition do not recommend it being used as an acid catalyst for hydrolysis. 3.2.6. Fate of Formic Acid. Formic acid is formed in an equimolar ratio to levulinic acid from glucose decomposition. Therefore, assuming no further decomposition of levulinic or formic acid, hydrolysis of cellulose and glucose should be quantifiable by measuring either one of these two products. However, in reaction hydrolysates, formic acid was not able to be detected after hydrolysis, even with definitive proof of levulinic acid production. While reported to be present in the studies performed by Girisuta et al., there was no evidence of formation of formic acid in the studies presented here.23,24 Samples spiked with formic acid before hydrolysis showed little evidence of it remaining afterward: depending on the severity of the reaction, in most cases these spike experiments resulted in zero formic acid detection, with the few cases showing a formic acid peak had less than 2% of the initial amount remaining. For comparison, samples that were spiked with formic acid posthydrolysis still did have it present when analyzed by HPLC. It was suspected that there may be effects on formic acid when heated in the presence of carbohydrate and/or αHSA. While it may be possible for formic acid to form a geminol diolbased αHSA, it its rather unlikely due to the instability that would come with having such an electron-positive carbon. Two hypotheses were formulated to explain this disappearance of formic acid through the hydrolysis. First, formic acid is reported to decompose both in the gas phase and in the aqueous phase by decarboxylation to CO2 and H2 or by dehydration to CO and H2O.25,26 The selectivity between decomposition methods can be influenced by the presence of water and how it can act as both a catalyst for the decarboxylation pathway and a stabilizer for the final product.27 The second hypothesis is that formic acid in an αHSA system can also react with the SO2 present in

Figure 8. (A) Typical HPLC chromatogram of levulinic acid αHSA immediately after synthesis. (b) HPLC of the same system after thermo-cycle to 175 °C followed by SO2 removal. 12536

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Industrial & Engineering Chemistry Research Figure8A shows an αHSA analyzed after synthesis. In this specific case, about 60% of the levulinic acid used in the synthesis is present in the αHSA form. Figure 8B show the result after the same acid is thermally cycled to 175 °C and then subjected to the SO2 removal process. Only 88% of the levulinic acid was recovered in this case, and it can be seen that there are still trace amounts of the αHSA.

Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. The path forward for biofuels and biomaterials. Science 2006, 311 (5760), 484−489. (9) Lin, Y.-C.; Huber, G. W. The critical role of heterogeneous catalysis in lignocellulosic biomass conversion. Energy Environ. Sci. 2009, 2 (1), 68−80. (10) Blackbourn, R. L.; Weider, P. R. Treating Biomass to Produce Materials Useful for Biofuels. World Patent Patent WO 2012061596A1, 2012. (11) Rackemann, D. W.; Doherty, W. O. S. The conversion of lignoicellulosics to levulinic acid. Biofuels, Bioprod. Biorefin. 2011, 5, 198−214. (12) Fitzpatrick, S. W. Production of Levulinic Acid from Carbohydrate Containing Materials. U.S. Patent 5608105A, 1997. (13) Bozell, J. J.; Moens, L.; Elliott, D. C.; Wang, Y.; Neuenscwander, G. G.; Fitzpatrick, S. W.; Bilski, R. J.; Jarnefeld, J. L. Production of levulinic acid and use as a platform chemical for derived products. Resour. Conserv. Recycl. 2000, 28 (3−4), 227−239. (14) Wilson, W. J.; Das Bejoy, S. Production of Alkali Metal Sulfites or Bisulfites. U.S. Patent 3549319A, 1970. (15) Rydholm, S. A. Chemical Pulping; Robert E. Krieger Publishing Co., Inc.: Malabar, FL, 1965. (16) Werpy, T.; Peterson, G.; Aden, A.; Bozell, J.; Holladay, J.; White, J.; Manheim, A. Top Value Added Chemicals from Biomass Volume I: Results of Screening for Potential Candidates from Sugars and Synthesis Gas; PNNL-14808; U.S. Department of Energy, 2004; DOI: 10.2172/926125. (17) Schwartz, T. J.; van Heiningen, A. R. P.; Wheeler, M. C. Energy densification of levulinic acid by thermal deoxygenation. Green Chem. 2010, 12 (8), 1353−1356. (18) Case, P. A.; van Heiningen, A. R. P.; Wheeler, M. C. Liquid hydrocarbon fuels from cellulosic feedstocks via thermal deoxygenation of levulinic acid and formic acid salt mixtures. Green Chem. 2012, 14 (1), 85−89. (19) Eaton, S. J.; Beis, S. H.; Bunting, B. G.; Fitzpatrick, S. W.; van Walsum, G. P.; Pendse, H. P.; Wheeler, M. C. Characterization and Combustion of Crude Thermal Deoxygenation Oils Derived From Hydrolyzed Woody Biomass. Energy Fuels 2013, 27 (9), 5246−5252. (20) Kohl, A.; Nielson, R. Sulfur Dioxide Removal; Gulf Publishing Company: Houston, TX, 1997. (21) Williams, D. L.; Dunlop, A. P. Kinetics of Furfural Destruction in Acidic aqueous Media. Ind. Eng. Chem. 1948, 40 (2), 239−241. (22) Rose, I.; Epstein, N.; Watkinson, A. Acid-catalyzed 2furaldehyde (furfural) decomposition kinetics. Ind. Eng. Chem. Res. 2000, 39 (3), 843−845. (23) Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J. A kinetic study on the conversion of glucose to levulinic acid. Chem. Eng. Res. Des. 2006, 84 (A5), 339−349. (24) Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J. Kinetic study on the acid-catalyzed hydrolysis of cellulose to levulinic acid. Ind. Eng. Chem. Res. 2007, 46 (6), 1696−1708. (25) Blake, P. G.; Hinshelwood, C. 1960. The Homogeneous Decomposition Reactions of Gaseous Formic Acid. Proc. R. Soc. London, Ser. A 1960, 255 (1283), 444−455. (26) Ruelle, P.; Kesselring, U. W.; Nam-Tran, H. Ab initio quantumchemical study of the unimolecular pyrolysis mechanisms of formic acid. J. Am. Chem. Soc. 1986, 108 (3), 371−375. (27) Akiya, N.; Savage, P. E. Role of water in formic acid decomposition. AIChE J. 1998, 44 (2), 405−415. (28) Gibson, H. W. Chemistry of formic acid and its simple derivatives. Chem. Rev. 1969, 69 (5), 673−692.

4. CONCLUSIONS Acetone-, methyl ethyl ketone-, furfural-, and levulinic acidderived αHSAs catalyzed reactions of glucose and cellulose at various combinations of temperature, carbohydrate concentration, and reactor configuration to produce levulinic acid. Levulinic acid and furfural are products of the conversion process, but neither performed well enough to serve as promising αHSA autocatalysts. Acetone was the best of the carbonyls tested for production of levulinic acid, giving a yield of 40 mol % in batch operation. The equilibrium between αHSAs and their constituent reagents was shifted toward the reagents by heating and depressurizing the reaction vessel, confirming that separating and recycling αHSAs can be accomplished through simple physical process manipulations.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

M. Clayton Wheeler: 0000-0003-1113-1324 G. Peter van Walsum: 0000-0003-0921-5369 Present Addresses ‡

W. Brett Barclay: Enersys, Reading, Pennsylvania, 19605, United States. § Anna Moh: Virginia Commonwealth University, Richmond, Virginia 23298, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by a grant from the NSF Sustainable Energy Pathways program, project Award No. 1230908 and NSF REU Award No. 1461116. We thank Dr. Adriaan van Heiningen for his insights into this process, in particular, the potential for production of thiosulfates as a cause for disappearance of formic acid.

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REFERENCES

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