Bridging the Gap between Pyrolysis and Fermentation: Improving

Research Article pubs.acs.org/journal/ascecg

Bridging the Gap between Pyrolysis and Fermentation: Improving Anhydrosugar Production from Fast Pyrolysis of Agriculture and Forest Residues by Microwave-Assisted Organosolv Pretreatment Anqing Zheng, Kun Zhao, Liqun Jiang, Zengli Zhao,* Jiangwei Sun, Zhen Huang, Guoqiang Wei, Fang He, and Haibin Li Key Laboratory of Renewable Energy, Chinese Academy of Sciences, 510640, Guangzhou, People’s Republic of China Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, People’s Republic of China Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China ABSTRACT: Fast pyrolysis is a potential alternative route to obtain fermentable anhydrosugar (levoglucosan) from biomass. However, the low yield of anhydrosugar from biomass fast pyrolysis hampers its rapid development. Microwaveassisted organosolvolysis could be an effective pretreatment method prior to fast pyrolysis of biomass for addressing this challenge. Here, in order to examine the feedstock flexibility of microwave-assisted organosolv pretreatment on anhydrosugar production from different agricultural and forest residues, three kinds of representative agricultural and forest residues, pine, eucalyptus, and straw, were selected as feedstocks in this study. Pretreatment of them using microwave-assisted glycerolysis was performed in an atmospheric microwave reactor. The pretreated agricultural and forest residues was subsequently fast pyrolyzed in a commercial micropyrolysis reactor. The results demonstrated that microwave-assisted glycerolysis is a versatile and feedstock flexible pretreatment method prior to biomass fast pyrolysis for enhancing anhydrosugar production. The highest yield of levoglucosan (59.4%) was reached by fast pyrolysis of eucalyptus pretreated at 150 W for 20 min. The yield was boosted by 13.5 times compared to that obtained from fast pyrolysis of raw eucalyptus. KEYWORDS: Fast pyrolysis, Feedstock flexibility, Organosolv pretreatment, Anhydrosugar, Microwave

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INTRODUCTION Diversifying the world energy matrix by incorporating renewable energy will help prolong the existence of fossil fuel reserves, address the threats of global warming, and enable better security of the energy supply on a global scale.1−5 Lignocellulosic biomass resources such as agricultural and forest residues are the only inexpensive renewable source of organic carbon that can be used for producing the high volumes of liquid fuels that our transportation sector has historically favored, e.g., ethanol.6 A key step in the cellulosic ethanol process is depolymerization of recalcitrant biomass, especially crystalline cellulose, into fermentable sugars.7,8 Currently, the most common method for deconstruction of cellulose into glucose is by enzymatic hydrolysis. However, the low hydrolysis rate and the high cost of enzymes hamper its commercial development.9,10 Another possible route to obtain fermentable sugar is fast pyrolysis.11−13 Brown and So demonstrated that the hybrid thermochemical/biochemical process, containing fast pyrolysis and subsequent fermentation, can provide a costcompetitive alternative to existing biochemical processes for cellulosic ethanol production.14 Biomass fast pyrolysis is a © XXXX American Chemical Society

thermal decomposition process that occurs in an inert atmosphere using high heating rates (103 to 104K/s) and short residence times ( eucalyptus > straw. It could be attributed to the different chemical composition and structure of softwood, hardwood, and herbaceous plants, especially the composition and structure of hemicellulose and lignin. Softwood usually contains higher lignin content and a greater degree of cross-linking structure. In addition, the hemicellulose and lignin in softwood is mainly comprised of xylan, arabinogalactan, and guaiacyl type lignin, respectively.29,30 The hemicellulose and lignin in hardwood and herbaceous plants is primarily composed of mannan, xylan, syringyl, and guaiacyl type lignin, respectively.29,30 The elemental analysis of raw and pretreated pine, eucalyptus, and straw is also shown in Table 1. The carbon content of pine, eucalyptus, and straw dropped gradually, whereas the oxygen content of them increased. Hence, the O/C

EXPERIMENTAL SECTION

Pretreatment of Agricultural and Forest Residues by Microwave-Assisted Glycerolysis. The microwave-assisted glycerolysis of pine, eucalyptus, and straw was conducted in a homemade atmospheric microwave reactor. The mixtures of biomass and glycerol (the solid/liquid ratio was 8) in a 40 mL beaker was placed in the reaction zone. The microwave power was fixed to 150 W, and residence times were varied at 6 or 20 min, since high power can lead to the rapid heating up and subsequent evaporation of glycerol. The reaction temperature was measured by a K-type thermocouple. The microwave temperature increased linearly from room temperature and reached about 250 °C at the sixth minute, then kept constant until the end of the reaction. After microwave pretreatment, the pretreated agricultural and forest residues were filtered off, washed thoroughly with deionized water/ethanol, and then dried at 105 °C for 12 h. The filtrate can be treated for recycling use. Characterization of Raw and Pretreated Agricultural and Forest Residues. The elemental analysis (C, H, N, and S) of raw and pretreated agricultural and forest residues was conducted on a Vario EL (Elementar Analysensysteme, Germany). The analyses of alkali and alkaline earth metals (AAEMs) in raw and pretreated agricultural and forest residues were performed on an Optima 8000 inductively coupled plasma optical emission spectroscope (ICP-OES; PerkinElmB

DOI: 10.1021/acssuschemeng.6b01416 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Table 2. Analysis of AAEMs in Raw and Pretreated Agricultural and Forest Residues feedstocks pretreatment conditions K (mg/kg) Na (mg/kg) Ca (mg/kg) Mg (mg/kg)

raw 538 25 817 165

pine 150W, 6 min 25 8 562 90

150W, 20 min 13 17 299 58

eucalyptus 150W, 6 min 150W, 20 min 29 5 23 1 483 299 96 24

raw 1311 233 565 135

raw 27303 1344 2822 3173

straw 150W, 6 min 345 60 2264 1338

150W, 20 min 228 39 1591 1452

specific component divided by the total weight loss), are listed in Table 3 to provide an insight into the variation of the biomass constituents affected by microwave-assisted organosolv pretreatment. As shown in Table 3, the weight loss fractions of raw and pretreated agricultural and forest residues were significantly influenced by applying microwave-assisted organosolv pretreatment. The notable differences in their weight loss fractions and residues were primarily due to their different chemical compositions and AAEMs contents. When applying microwave-assisted organosolv pretreatment, the weight loss fractions for lignin dropped obviously, and it further decreased with increasing residence time of pretreatment. However, the weight loss fractions for cellulose raised gradually. Compared to raw agricultural and forest residues, the weight loss fractions of agricultural and forest residues pretreated at 150 W and 6 min for extractives and hemicellulose were slightly lower. As the residence time further increased from 6 to 20 min, the weight loss fractions for extractives and hemcellulose decreased obviously. It is concluded that the weight loss of agricultural and forest residues during pretreatment was mostly ascribed to the removal of lignin and hemicellulose. The results were in line with the literature.28,35 And the rank order of the changes in weight loss fractions was eucalyptus > straw > pine. It is worthy to note that the weight loss fraction of eucalyptus pretreated at 150 W and 20 min for cellulose can be achieved as high as 93.84%, indicating that lignin and hemicellulose fractions of eucalyptus can be almost totally removed by microwaveassisted organosolv pretreatment. Structural Characterization of Raw and Pretreated Agricultural and Forest Residues by Solid-State 13C CP/ MAS NMR. The solid-state 13C CP/MAS NMR spectra of raw and pretreated agricultural and forest residues are shown in Figure 2. The signals assigned to the different types of carbons in agricultural and forest residues are tabulated in Table 4.36,37 And the representation of carbons in hemicellulose, lignin, and cellulose by Greek letters and numbers are those from the literature.37 The signals H1 at 172 ppm and H5 at 22 ppm are mainly associated with the carbonyl and methyl carbons of hemicellulose, respectively. When the agricultural and forest residues were subjected to microwave-assisted glycerolysis, their intensities declined and further decreased with the increasing residence time of pretreatment, indicating that the carbonyl and methyl in hemicellulose can be efficiently reduced by pretreatment. The signals H2 at 103 ppm, H3 at 70−80 ppm, and H4 at 63−66 ppm were assigned to C1, C-2/3/5, and C6 in hemicellulose, respectively. Their intensities decreased when applying microwave-assisted glycerolysis, implying that hemicellulose can be partly removed by pretreatment. The different intensities of signals at 153−110 ppm assigned to aromatic structure in raw agricultural and forest residues indicated that they exhibited different lignin structures. The signal L1 at 153 ppm is related to C-3/5 of etherified syringyl units. And the signal L2 at 148 ppm is associated with C3/4 etherified and nonetherified guaiacyl units and C3/5 nonethe-

of them increased gradually. It was primarily due to the removal of the lignin fraction during microwave-assisted organosolv pretreatment. The contaminants present in biomass such as sulfur and nitrogen must be removed before it can be converted into transportation fuels. As shown in Table 1, the nitrogen and sulfur content of agricultural and forest residues could be effectively reduced by microwave-assisted glycerolysis, especially for straw. The nitrogen content in straw decreased from 1.31 to 0.29%, while the sulfur content dropped from 0.13% to ND on applying microwave-assisted glycerolysis. It is demonstrated that microwave-assisted organosolv pretreatment is an efficient method to remove the contaminants (sulfur and nitrogen) present in biomass. The contents of AAEMs of raw and pretreated agricultural and forest residues are tabulated in Table 2. When applying microwave-assisted glycerolysis, the potassium content of pine, eucalyptus, and straw decreased from 538, 1311, and 27303 mg/kg to 13, 5, and 228 mg/kg, respectively. The sodium content of pine, eucalyptus, and straw dropped from 25, 233, and 1344 mg/kg to 17, 1, and 39 mg/kg, respectively. The alkali metals (K and Na) in eucalyptus and pine can be almost completely removed by pretreatment. A certain amount of alkali metals remained in the pretreated straw because of its very high initial content in raw straw. Compared to alkali metal, the alkaline earth metals exhibited relatively lower removal efficiency. Brown et al. demonstrated that the AAEMs have significant impacts on the product distribution from cellulose pyrolysis. The AAEMs can strongly catalyze the fragmentation of biomass to permanent gases and low-molecular-weight oxygenates compared to the thermally induced cleavage of glycosidic bonds in cellulose that yield anhydrosugars.31 And the rank order of the effect of AAEMs on the reduction in levoglucosan yield from cellulose pyrolysis is K > Na > Ca > Mg.32 The effective removal of AAEMs could play positive roles in the improving anhydrosugar production from fast pyrolysis of agricultural and forest residues. The Variation in Compositions of Raw and Pretreated Agricultural and Forest Residues Characterized by TG/ DTG Analysis. The weight loss (thermogravimetry, TG) and weight loss rate (differential thermogravimetry, DTG) curves of raw and pretreated agricultural and forest residues are illustrated in Figure 1. It is assumed that there is no interaction among the pyrolyses of extractives, hemicellulose, cellulose, and lignin. In order to separate the contributions of different components, each DTG profile was deconvolved into four peaks corresponding to the minimum number to obtain a good superposition of the experimental and fitting profile using Gaussian fitting.33,34 As shown in Figure 1, the first two signals centered at 286−216 and 298−355 °C were mainly assigned to the mass loss rates of extractives and hemicellulose. And the last two signals centered at 334−384 and 360−389 °C were primarily related to the mass loss rates of cellulose and lignin, respectively. The normalized integration values of the four signals, indicating the weight loss fractions (the weight loss of a C


Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. TG/DTG curves of raw and pretreated agricultural and forest residues. (The raw pine, pine pretreated at 150 W for 6 min, and pine pretreated at 150 W for 20 min are respectively denoted P0, P6, and P20. The raw eucalyptus, eucalyptus pretreated at 150 W for 6 min, and eucalyptus pretreated at 150 W for 20 min are respectively denoted E0, E6, and E20. The raw straw, straw pretreated at 150 W for 6 min, and straw pretreated at 150 W for 20 min are respectively denoted S0, S6, and S20.) D


Research Article

ACS Sustainable Chemistry & Engineering Table 3. Weight Loss Fractions of Raw and Pretreated Agricultural and Forest Residues feedstocks pretreatment conditions weight loss fractions (%)

residue (%)

extractives and hemicellulose cellulose lignin

51.39

pine 150W, 6 min 40.53

150W, 20 min 28.22

26.13 22.46 16.8

39.30 20.16 13.2

60.60 11.20 12.2

raw

46.83

eucalyptus 150W, 6 min 42.53

150W, 20 min 4.35

32.66 20.50 15.6

46.62 10.85 10.8

93.84 1.81 7.2

raw

42.77

straw 150W, 6 min 41.61

150W, 20 min 22.96

41.17 16.06 28.0

48.52 9.87 20.1

72.86 4.16 19.7

raw

Figure 2. 13C CP/MAS NMR spectra of raw and pretreated agricultural and forest residues.

identified and quantified. Their mass yields are given in Table 5. The levoglucosan yield from fast pyrolysis of raw pine, eucalyptus, and straw were 4.0, 4.1, and 1.6%, respectively. The levoglucosan yield increased obviously when microwaveassisted organosolv pretreatment was performed. And it continued to improve with the increasing residence time of pretreatment. The yield of levoglucosan from pine, eucalyptus, and straw pretreated at 150 W for 20 min reached as high as 33.2, 59.4, and 36.0%, respectively, implying that microwaveassisted organosolv pretreatment is a versatile and feedstock flexible pretreatment method prior to biomass fast pyrolysis for enhancing anhydrosugar production. The rank order of the effect of organosolv pretreatment on the improvement in anhydrosugar production from different agricultural and forest residues is eucalyptus > straw > pine. It is worthy of note that the mass yields of pretreated eucalyptus and straw were close, and their levoglucosan yield displayed a significant difference. The improvements in levoglucosan yields were mainly attributed to the effective removal of AAEMs and lignin during pretreatment. Brown et al. demonstrated that AAEMs can

rified syringyl units. The high intensities of L1, L3, and L4 in raw eucalyptus and straw demonstrated that they are mainly composed of syringyl and guaiacyl units, while the high intensities of L2 in raw pine showed that it is primarily comprised of guaiacyl units. The tiny signals of the aromatic region in eucalyptus and straw pretreated at 150 W for 20 min implied that the lignin fraction in eucalyptus and straw can be effectively removed by pretreatment. However, the obvious signals at 148 ppm in pine pretreated at 150 W for 20 min suggested that considerable guaicayl-type lignin is still presented in pine after pretreatment. Anhydrosugar Production from Fast Pyrolysis of Raw and Pretreated Forest and Agricultural Residues. The total ion chromatograms resulting from fast pyrolysis of raw and pretreated forest and agricultural residues are shown in Figure 3. The peaks of CO/CO2 and levoglucosan were tagged in Figure 3. As shown in Figure 3, the intensities of most of the peaks were evidently decreased when applying microwaveassisted organosolv pretreatment. Some of the relative highconcentration compounds in each chromatogram were E


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also plays positive roles in enhancing the levoglucosan yield.23 The highest levoglucosan yield obtained by fast pyrolysis of eucalyptus pretreated at 150 W for 20 min can be explained by the almost total removal of alkali metals and lignin fractions during preatreatment of eucalyptus. The results were in accordance with the previous AAEM analysis and TG/DTG analysis. The yields of microbial inhibitors such as carboxylic acid, furans, and phenols are also shown in Table 5. The acetic acid was predominantly derived from the decarbonylation of hemicellulose. When applying microwave-assisted organosolv pretreatment, the yield of acetic acid from fast pyrolysis of pine, eucalyptus, and straw decreased sharply from 1.2, 2.9, and 1.1% to 0.2, 0.2, and 0.3%, respectively, indicating that the hemicellulose fraction of agricultural and forest residues can be efficiently removed by microwave-assisted organosolv pretreatment. The furans (furfural and 5-hydroxymethylfurfural) are generally considered the products of acid-catalyzed dehydration of hemicellulose and cellulose. Their yield improved with increasing pretreatment severity. It could be ascribed to the increasing cellulose content in pretreated forest and agricultural residues. The yields of phenols, including 2methoxy-phenol, 2-methoxy-4-vinylphenol, 2-methoxy-4-(1propenyl)-phenol, vanillin, and 2,6-dimethoxy-4-(2-propenyl)phenol, were also tabulated in Table 5. It is evident that the yields of phenols declined after microwave-assisted organosolv pretreatment, and they further dropped with increasing residence time of microwave-assisted organosolv pretreatment. The phenols were primarily the result of the pyrolysis of lignin. It was mainly due to the effective removal of lignin during pretreatment. By applying microwave-assisted organosolv pretreatment, the total yields of selected microbial inhibitors from fast pyrolysis of pine, eucalyptus, and straw decreased from 2.2, 4.4, and 1.6% to 1.1, 1.0, and 1.2%, respectively. The rank order of reducing rate of selected microbial inhibitors was eucalyptus > pine > straw. Eucalyptus exhibited the highest reducing rate of selected microbial inhibitors since its alkali metal, hemicellulose, and lignin fractions can be almost totally removed by microwave-assisted glycerolysis.

Table 4. Assignments of the Signals in 13C CP/MAS NMR Spectra of Raw and Pretreated Agricultural and Forest Residues36,37 signal number H1 L1 L2 L3 L4 L5 L6 C1 H2 C2 C3, L7 C4, L8 C5 H3 C6 H4 C7 L9 L10 H5

chemical shift (ppm) 173 153 148 138 133 121 110 106 103 89 84 75 72 70−80 66 63−66 63 62 56 22

assignment hemicellulose: −COO−R,CH3−COO−R lignin: S-3(e), S-5(e) lignin: S-3(ne), S-5(ne), G-3(ne,e), G4(ne,e) lignin: S-1(e), S-4(e), G-1(e) lignin: S-1(ne), S-4(ne), G-1(ne) lignin: G-6 lignin: G-5, G-6, S-2, S-6 cellulose: C-1 hemicellulose: C-1 cellulose: C-4(crystalline) cellulose: C-4(amorphous), lignin: Cβ cellulose: C-2/3/5, lignin: Cα cellulose: C-2/3/5 hemicellulose: C-2/3/5 cellulose: C-6(crystalline) hemicellulose: C-6 cellulose: C-6(amorphous), lignin: Cγ lignin: −OCH3 hemicellulose: −CH3

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CONCLUSION

A comparative study of anhydrosugar production from different agricultural and forest residues pretreated by microwaveassisted glycerolysis was conducted in this study. It is demonstrated that microwave-assisted glycerolysis can serve as an effective pretreatment method for improving the yield of levoglucosan from fast pyrolysis of agricultural and forest residues. The yield of levoglucosan from pine, eucalyptus, and straw pretreated at 150 W for 20 min reached as high as 33.2%, 59.4%, and 36.0%. In addition, the yield of microbial inhibitors (carboxylic acids and phenols) can simultaneously be effectively reduced by microwave-assisted glycerolysis. These results could be attributed to the efficient removal of AAEMs and the lignin fraction during microwave-assisted organosolv pretreatment of agricultural and forest residues. These findings suggested that microwave-assisted glycerolysis is a versatile and feedstock flexible pretreatment method to bridge the gap between biomass pyrolysis and fermentation toward cellulosic ethanol production.

Figure 3. Total ion chromatograms resulting from fast pyrolysis of raw and pretreated forestry and agricultural residues.

catalyze the fragmentation of biomass to form permanent gases and low-molecular-weight oxygenates.31 The absorption of AAEM ions (K+, Na+, Ca2+, and Mg2+) on cellulose could enhance the homolytic cleavage of pyranose ring resulting in low-molecular-weight oxygenates compared to the heterocyclic cleavage of glycosidic linkages that yield levoglucosan.32 In addtion, AAEMs could strongly catalyze the secondary reactions of levoglucosan. Levoglucosan can bind to AAEM ions (e.g., Na+) to form a complex of levolgucosan-Na+. The complex then undergoes retro-aldol reaction to yield lowmolecular-weight oxygenates (aldehydes and ketones).38,39 Brown et al. also reported that the inherent covalent linkages between cellulose and lignin can inhibit the formation of levoglucosan during fast pyrolysis of herbaceous biomass.40 Zhao et al. found that the selective removal of lignin and hemicellulose fractions for overcoming biomass recalcitrance F


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Table 5. Comparison of Mass Yields of Sugars from Fast Pyrolysis of Raw and Pretreated Forestry and Agricultural Residues (Based on Pretreated Forestry and Agricultural Residues) feedstocks pretreatment conditions sugars levoglucosan 1,4:3,6-dianhydro-alpha-Dglucopyranose total microbial inhibitors acetic acid furfural (FF) 5-hydroxymethylfurfural (HMF) phenol, 2-methoxy2-methoxy-4-vinylphenol phenol, 2-methoxy-4-(1-propenyl)vanillin phenol, 2,6-dimethoxy-4-(2propenyl)total others acetol glycolaldehyde 2(5H)-furanone 1,2-cyclopentanedione total a

origin

raw

Ca C

4.0 1.4

Hb H/C C Lc L L L L

H/C H/C H/C H/C

pine 150W, 6 min

150W, 20 min

raw

16.2 1.8

33.2 1.7

4.1 1.2

5.4

18.0

34.9

1.2 0.1 0.2 0.1 0.3 0.2 0.1 0.0

0.7 0.1 0.4 0.1 0.1 0.2 0.1 0.0

2.2 1.0 2.0 0.2 0.2 3.4

eucalyptus 150W, 6 min

straw 150W, 6 min

150W, 20 min

raw

150W, 20 min

17.6 2.1

59.4 2.1

1.6 1.3

12.2 1.8

36.0 1.9

5.3

19.7

61.5

2.9

14.0

37.9

0.2 0.2 0.5 0.1 0.1 0.0 0.0 0.0

2.9 0.2 0.2 0.1 0.2 0.3 0.1 0.4

1.9 0.2 0.4 0.1 0.1 0.0 0.0 0.3

0.2 0.2 0.5 0.0 0.0 0.0 0.0 0.1

1.1 0.2 0.0 0.1 0.2 0.0 0.0 0.0

0.8 0.2 0.3 0.1 0.2 0.0 0.0 0.0

0.3 0.3 0.5 0.1 0.0 0.0 0.0 0.0

1.7

1.1

4.4

3.0

1.0

1.6

1.6

1.2

0.6 1.8 0.1 0.2 2.7

0.7 2.2 0.1 0.2 3.2

1.1 1.9 0.2 0.3 3.5

0.6 1.9 0.1 0.2 2.8

0.2 1.9 0.0 0.1 2.2

1.1 0.9 0.1 0.2 2.3

0.8 1.7 0.1 0.2 2.8

0.7 2.0 0.1 0.1 2.9

C for cellulose. bH for hemicellulose. cL for lignin.

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+86 2087057721. Fax: +86 2087057737. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grants 51376186 and 21406227), the Natural Science Foundation of Guangdong Province, China (Grant 2014A030313672), and the Science and Technology Planning Project of Guangdong Province, China (Grants 2014B020216004 and 2015A020215024) for financial support of this work.

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