Article
Selective Conversion of Biomass Hemicellulose to Furfural Using Maleic Acid with Microwave Heating Eurick S. Kim,† Shuo Liu,† Mahdi M. Abu-Omar,*,† and Nathan S. Mosier*,‡ †
Department of Chemistry and the Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio), Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907 ‡ School of Agricultural and Biological Engineering, the Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio), and Laboratory of Renewable Resources Engineering, Purdue University, 500 Central Drive, West Lafayette, Indiana 47907 S Supporting Information *
ABSTRACT: Maleic acid has interesting properties versus other organic and mineral acids in furfural formation. The use of maleic acid as a catalyst in biomass conversion has the benefit in comparison to the use of other acids (mineral/organic) due to the efficient conversion of solid xylans to xylose at high yields in aqueous solution at the reasonably mild temperature of 160 °C. This manuscript reports the kinetics of xylose dehydration to furfural using maleic acid over the temperature range 180−210 °C. These kinetics were used to determine the optimal temperatures and times for a two-step process to first hydrolyze plant hemicellulose to furfural and then to subsequently dehydrate the resulting xylose to furfural. High selectivity in furfural formation was observed when maleic acid was in equimolar or higher concentrations compared to the initial xylose concentration. High selectivity for furfural, 67%, was observed for xylose derived from corn stover, switch grass, and poplar, in comparison to modest selectivity, 39%, for pine wood. In all instances, xylose from biomass was found to be more reactive toward furfural formation than pure xylose. Maximum furfural concentrations were observed in shorter times than required for pure xylose. The rate increase is attributed to chloride salts extracted concurrently into condensed phase with hemicellulose-derived xylose.
1. INTRODUCTION Biomass is a plentiful and renewable carbon source.1−4 With oil supplies diminishing, there is a pressing need to obtain liquid fuels and chemicals from suitable biomass, including lignocellulosic plant biomass. The three main components of lignocellulosic biomass are hemicellulose (8.8−22.4%), cellulose (14.3−49.9%), and lignin (8.4−29.4%).5 One of the major challenges in lignocellulose utilization is its natural recalcitrance to depolymerization and low selectivity in conversion of biomass components, together resulting in low product yield.6,7 Furfural is a useful chemical and a potential building block for hydrocarbon fuels.7−9 It can be obtained by dehydrating C5 sugars derived from hemicellulose (primarily xylose), using strong acid catalysts (e.g., sulfuric acid10−12 and hydrochloric acid13−15). The dehydration of xylose to furfural is primarily catalyzed by Brønsted acids (donors of H+), which catalyze the formation of a 1,2-enediol, which dehydrates to furfural.25 While strong acids depolymerize hemicellulose and cellulose, they exhibit low selectivity of xylose conversion to furfural as a result of further decomposition of furfural to humin and other products7,15 Because of these factors, typical industrial methods produce furfural in only 45−55% yield.7 Improved processes must address both lignocellulose depolymerization and catalytic conversion of xylose to furfural at high yields, rates, and concentrations and at low cost.3,4,7 For the purposes of this work, we define yield (in %) as the product recovered compared to the stoichiometric amount of product that could be produced from the initially available substrate (xylose). We define selectivity (in %) as the product measured compared to the stoichiometric amount of product that could be produced © 2012 American Chemical Society
from the substrate that was reacted. When all of the substrate has reacted, yield and selectivity are equivalent. While the proposed mechanisms for furfural formation require protons to protonate xylose, it has been reported that the presence of other ions in the reaction system also affects the rate of xylose decomposition.27 Prior work has focused primarily on the effect of halides (especially Cl−) on xylose decomposition. In this case, the presence of chloride ions increases the observed rate of xylose decomposition to furfural while exhibiting less influence on the rates of decomposition of furfural to undesired products. This is proposed to occur because the presence of Cl− favors the formation of the 1,2enediol while not catalyzing undesired side reactions. By contrast, mineral acids such as sulfuric or hydrochloric acid increase the rate of enolization, xylose dehydration, and furfural decomposition to a similar degree. These data suggest that rates and selectivity toward xylose decomposition products may be affected by the presence and type of other ions in the acid catalyzed reaction system. Prior papers report that maleic acid is effective for hydrolyzing hemicellulose without resulting in significant furfural formation. This suggests that maleate may impart an effect similar to chloride in this reaction system.18,19,21 We report herein the use of maleic acid as a Brønsted acid for both depolymerizing plant hemicellulose to xylose and for dehydrating the resulting xylose to furfural in a two-step reaction using microwave heating. Microwave heating offers the Received: May 20, 2011 Revised: January 10, 2012 Published: January 10, 2012 1298
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before use. The approximate compositions of the lignocellulosic biomass are given in Table 1.
advantages of rapid heat-up times and efficient energy absorption versus conventional heating methods (fluidized sand-baths). Preliminary comparisons between conventional heating and microwave heating methods resulted no significant differences in yields of xylose from the hydrolysis of the hemicellulose (see the Supporting Information). In addition, the microwave heating method was very similar to the method previous published for similar kinetic studies of furfural formation.15−17 In a two-step procedure, maleic acid with microwave heating (Figure 1) is used to both depolymerize hemicellulose18−21 and
Table 1. Composition of Lignocellulose Used for Furfural Generation biomass corn stover18,19 switchgrassa lodgepole pinea poplar22 a
% xylose
% arabinose
% glucose
% lignin
% other (ash, protein, etc.)
23
4.2
35
11
26
21 20
1.8 1.6
35 26
20 28
22 24
18
0.6
45
21
15
Analyses performed for this work.
2.2. Methods. Plant composition was determined by the LAP002 sulfuric acid digestion analysis method.23 All reactions were done in a CEM DISCOVER SP microwave system reactor set to reaction temperatures (180−210 °C) at 100 W, control function, and cooled via nitrogen flow to 60 °C to end the reaction. For kinetic runs, 6 time points were generally used (0, 1, 2.5, 5, 7.5, and 10 min). The 0 min time points denote reactions that are brought up to reaction temperature and immediately cooled. Heat up times (to 180−210 °C) averaged between 60 and 90 s with a 2 min cool down to 60 °C (see the Supporting Information). Reaction temperatures were recorded in real time by an IR fiber optic probe in the reactor. 2.3. Instrumentation and Chromatographic Conditions. All reaction samples were filtered through a 0.22 μm cutoff syringe filter (25 mm diameter) prior to chemical analysis. High pressure liquid chromatography (HPLC) analyses were performed on a Waters e2696 separations module equipped with an Aminex HPX-87H column (300 × 7.8 mm) set at 65 °C. A 5% (w/w) acetonitrile in 0.005 M H2SO4 solution was employed as the mobile phase through isocratic elution. The acetonitrile was used to help separate sugars (e.g., xylose and glucose) from the maleic acid.19 The flow rate of the mobile phase was set to 0.6 mL/min. The elution of compounds was detected via refractive index on a Waters 2412 refractive index (RI) detector calibrated with external standards, as described previously.18,19,21 Standard curves (maleic acid, xylose, and furfural) were made from the pure substances. Fractions were taken from elution peaks and submitted for MS/MS to confirm the RI results (see the Supporting Information).
Figure 1. Selective hydrolysis and conversion of biomass xylose to furfural.
dehydrate the resulting xylose to furfural. The key features of this process are (a) highly selective fractionation of xylose from biomass into the aqueous stream and (b) high selectivity and yield for furfural from biomass. Further, we demonstrate the utility of this approach for the conversion of C-5 sugars from a variety of lignocellulosic biomasses that represent the major categories of biofuel/bioproduct feedstocks (e.g., agricultural residue, perennial grass, hardwood, and softwood). Furthermore, this process does not require corrosive salts,27 ionic liquids12,13 or biphasic solvent reaction systems14,15 to achieve comparable yields and selectivity.10−15
2. MATERIALS AND METHODS 2.1. Materials. Xylose, maleic acid, malic acid, furfural, and dichloromethane were purchased from Sigma-Aldrich (≥99% purity). Maleic acid and dichloromethane were used as received without further purification. The xylose was dried (100 °C) under vacuum overnight and kept in a desiccator. The furfural was distilled, kept in a Schlenk flask under nitrogen in a −10 °C freezer, and wrapped with aluminum foil. Corn stover was obtained from Mr. James Beaty at the Purdue University Agronomy Center for Research and Education, West Lafayette, Indiana. The samples were dried to Ea2). However, we observed that the selectivity remained consistently around 70% for the temperatures tested. The high activation energy for decomposition of xylose to undesirable products (Ea3) also results in a faster rate of xylose as temperature is increased; thus, for the temperature range tested (180 to 210 °C), the selectivity of furfural production remains consistent. The ΔS‡ of this system deserves a comment. There is a small increase in entropy during the xylose degradation. This implies that there is a small decrease in order in the transition state of the xylose to furfural reaction. The furfural degradation reaction has a small decrease in entropy. This implies that the intermediate or transition state of furfural degradation reaction (humin formation) is more highly ordered in comparison to furfural itself in water. An alternate reaction scheme to Figure 2 has been proposed where xylose and furfural react together to form insoluble degradation products. This coupling reaction has been reported to be an important degradation pathway in furfural formation in an aqueous phase system, which contributes significantly to the degradation of the product furfural.7,15 To avoid this additional furfural degradation, flash steam separation7,11 and biphasic reaction systems where furfural is extracted into the organic
Figure 2. Kinetic scheme for xylose conversion to furfural and furfural degradation catalyzed by maleic acid. 1300
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Figure 3. (A) Rate constants (k1,app) for xylose conversion at different temperatures; (B) rate constants (k2) for furfural degradation at different temperatures; (C and D) Eyring plots of xylose and furfural decomposition for calculations of ΔH‡ andΔS‡ (using k1 and k2).
(k1,app) in our coupling reactions are slightly smaller (but within experimental error) than those for xylose alone, which may be explained by the reason proposed by Antal et al.25 In conclusion, little to no coupling of the product furfural to the reactant xylose was observed when maleic acid was used as a catalyst. After the rate constants were determined, eqs 1 and 2 were used to predict the concentrations of xylose and furfural over the time course of pure xylose (10 g/L) dehydration to furfural at varying temperatures in the presence of 0.25 M maleic acid (Figure 4). The curves from the fitted parameters (Figure 4)
Table 3. Kinetic Parameters for Xylose Dehydration in Aqueous Solution rate constant (L mol−1 s−1)
log10A
Ea kJ/mol
ΔH‡ kJ/mol
ΔS‡ J/mol K
k1 k2 k3
14 ± 2 6±2 14 ± 5
121 ± 10 68 ± 18 148 ± 45
117 ± 10 63 ± 18 144 ± 45
45 ± 36 −172 ± 39 17 ± 96 (∼0)
liquid phase have been proposed.14,15 To test this alternative reaction scheme, 10 g/L xylose and 10 g/L furfural were reacted in 0.25 M maleic acid solutions under the same temperatures conditions described above. The observed degradation rate of the xylose in the xylose/furfural reaction was very similar to that of the reactions with only xylose (Table 4). Table 4. Xylose (10 g/L Initial) Degradation Rate Constants T (°C) 180 190 200 210 a
xylose alone k1,app (L mol−1 s−1)a 0.58 1.24 2.32 4.28
× × × ×
10−3 ± 0.02 10−3 ± 0.20 10−3 ± 0.20 10−3 ± 0.20
× × × ×
with 10 g/L furfural k1,app (L mol−1 s−1)a 10−3 10−3 10−3 10−3
0.53 1.17 2.26 3.67
× × × ×
10−3 ± 0.02 10−3 ± 0.10 10−3 ± 0.20 10−3 ± 0.20
× × × ×
10−3 10−3 10−3 10−3
95% confidence interval.
These data suggest that the coupling of xylose and furfural does not occur to a significant degree in the aqueous maleic acid reaction. Antal et al.25 have proposed that excess furfural plays the role of a Brønsted base, which reacts with H3O+ and thus decreases the total acid concentration of the system, slowing the degradation of xylose. Xylose degradation rates
Figure 4. Fittings of furfural formation at 10 g/L xylose and 0.25 M maleic acid at different temperatures (180−210 °C). Lines represent model prediction for furfural production, and points represent experimental results. 1301
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conversion, although slightly lower than when maleic acid is equimolar or in excess to xylose. This observation is true even at relatively low xylose conversion (70%). Furthermore, this high selectivity is relatively constant over a wide range of conversion of xylose (30−75% of initial concentration). The higher xylose concentration solution (100 g/L) still displays high selectivity with respect to 1302
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Several factors may explain the differences between biomassderived xylose and pure xylose in the presence of maleic acid. First, biomass contains salts (listed as ash).5 The presence of halides, especially chloride ions, has been shown to increase the rate of xylose degradation and furfural formation in aqueous acid systems.11,15,27 The chloride ion salts released from the biomass during fractionation could explain the higher rate of xylose conversion and the lower selectivity, as compared to the pure xylose results. Second, some salts found in biomass are alkaline and other compounds released during aqueous pretreatment of biomass are alkaline or weak acids (such as acetic acid), which may neutralize maleic acid.5 The final pH of the pine-derived solutions after xylose degradation were slightly higher than the other biomass-derived samples, which may account for the differences observed in xylose degradation and furfural yield. More in-depth experiments to identify all of the contributing factors are beyond the scope of this paper. 3.5. Maleic Acid Recycling. For maleic acid to be used as a catalyst in commercial applications, it must be recyclable for repeated use. The recycling of the maleic acid was investigated using switchgrass under the optimal conditions (Table 6). The xylose was converted to furfural, and the resulting furfural was extracted with dichloromethane. The aqueous phase, which contained maleic acid, was then used a second time to both hydrolyze the hemicellulose in a second batch of switchgrass and convert the resulting xylose to furfural. The results of three consecutive cycles (hydrolysis, furfural formation, and extraction) are given in Table 7. The amount of xylose that was
Table 8. Recovery of Maleic Acid in the Product Solution of Pure Xylose Dehydration after 10 min Reaction at Various Temperatures
a
xylose recovery (%)
selectivity (%)
xylose conversion (%)
furfural yield (%) HPLC
1st run 2nd run 3rd run
>90 >85 >85
67 65 60
85 80 80
57 51 48
maleic acida
malic acida
180 190 200 210
94 89 85 81
4 5 7 10
% of initial maleic acid.
recycling experiments (Table 7). These results indicate that maleic acid is at least partially recyclable.
4. CONCLUSIONS Maleic acid possesses interesting properties for converting xylose to furfural. Its ability to selectively form high yields of xylose in water from various biomass sources is unique. Hydrolysis of the xylan in biomass is heavily favored in comparison to xylose degradation. Thus, a single step can produce an easily separable lignocellulosic solid and a xyloserich solution.18,19,21 This selective, controllable separation of hemicellulose (xylose) from the lignocellulosic components allows for separate processing streams for xylose from cellulose and lignin in biomass conversion processes. Maleic acid also has the ability to convert the xylose for furfural in an aqueous system without an additional catalyst. In addition, the xylose/ furfural coupling reaction is minimal in the presence of maleic acid/maleate. Together, this results in increased selectivity toward furfural formation from whole biomass. The kinetic analysis can accurately predict furfural formation from pure sugar. However, the observed rates were much higher for xylose derived from biomass, possibly due to the effects of other ions (metal salts, etc.) also extracted from the biomass. Maleic acid also shows some promise in regards to reusability, although maleic acid is slowly hydrated to malic acid under the conditions used for the xylose conversion to furfural.
Table 7. Recycling of Maleic Acid Solution after Organic Extraction switchgrass xylose
temp (°C)
■
reacted remained constant, but the selectivity, and thus the yield, of furfural decreased slightly with additional uses of the maleic acid. This could be attributed to reactions to maleic acid to other products. The concentration of maleic acid was observed to decrease over time during the reaction (up to 20% at the highest temperatures and longest reaction times) for all reaction substrates (pure xylose or plant biomass). A new peak observed in the HPLC analysis of the reaction product was suspected to be the degradation product of maleic acid. Mass spectrometry of the HPLC fraction collected at the elution of this peak was identified as malic acid (hydroxybutanedioic acid), which we hypothesize is formed by the hydration of the CC double bond of maleic acid. The fraction of maleic acid hydrated to malic acid after 10 min of reaction at the tested temperatures for pure xylose are shown in Table 8. The molar sum of maleic acid and malic acid in the product accounts for 91−98% of the initial maleic acid, which suggests that malic acid is the only significant degradation product. Similar conversion of maleic acid to malic acid was observed in experiments where xylose derived from plant biomass was dehydrated to fufural (data not shown). Malic acid (pKa1 = 3.40, pKa2 = 5.20) is a weaker acid than maleic acid (pKa1 = 1.9, pka2 = 6.07), which may also contribute to the lower selectivity found in the catalyst
ASSOCIATED CONTENT
S Supporting Information *
General procedure, heating curves, and HPLC data. This material is available free of charge via the Internet at http:// pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected].
■
ACKNOWLEDGMENTS
This research was supported as part of the Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number DE-SC0000997. We thank Purdue University Department of Agriculture for providing corn stover. We thank Dr. Keith Johnson (Purdue University) for supplying the switchgrass, Mr. Jerry Warner (Defence LifeSciences, LLC) for supplying the lodgepole pine, and the USDA Forest Product Lab (CAFI Consortium) for supplying the poplar. We thank Dr. Hilkka Kenttämaa and Dr. Nelson Vinueza for the mass spectroscopy analysis. 1303
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