Toward Fast Pyrolysis-Based Biorefinery: Selective ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Toward Fast Pyrolysis-Based Biorefinery: Selective Production of Platform Chemicals from Biomass by Organosolv Fractionation Coupled with Fast Pyrolysis Anqing Zheng,†,‡,§ Tianju Chen,∥ Jiangwei Sun,⊥ Liqun Jiang,†,‡,§ Jinhu Wu,∥ Zengli Zhao,*,†,‡,§ Zhen Huang,†,‡,§ Kun Zhao,†,‡,§ Guoqiang Wei,†,‡,§ Fang He,†,‡,§ and Haibin Li†,‡,§ †

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China CAS Key Laboratory of Renewable Energy, Guangzhou 510640, People’s Republic of China § Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, People’s Republic of China ∥ Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, People’s Republic of China ⊥ Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei 230026, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: The heterogeneous structure of biomass causes the complex compositions of bio-oil, thereby posing huge challenges for the extraction of value-added chemicals from bio-oil and the catalytic upgrading of bio-oil in existing petroleum-refining infrastructures. In order to overcome these challenges, a new advanced biorefinery based on organosolv fractionation coupled with fast pyrolysis is first proposed. The experimental results showed that biomass can be effectively divided into cellulose-rich fractions, organosolv lignins, and xylose by organosolv fractionation, thus improving the relative yields of platform chemicals (levoglucosan (LG) and phenols) in subsequent fast pyrolysis. The relative LG yields from eucalyptus, pine, and bagasse increased from 4.8, 3.5, and 2.1 wt % to 42.1, 22.7, and 59.8 wt %, respectively. These findings provide a simple and efficient integrated process to selective production of platform chemicals, which is different from the existing processes, e.g. catalytic fast pyrolysis and postpyrolysis separation. In addition, the fast pyrolysis of acid-passivated cellulose-rich fractions with varying cellulose contents revealed that the LG yields were linearly related to the cellulose contents of feedstocks, and the gap between actual and theoretical yields of LG decreased with increasing cellulose contents of feedstocks, suggesting that the interactions between cellulose and other components (lignin and hemicellulose) were the main controlling factor of LG yields from pyrolysis of acid-passivated cellulose-rich fractions. KEYWORDS: Biorefinery, Biomass, Platform chemicals, Organosolv fractionation, Fast pyrolysis

1. INTRODUCTION The limited fossil resources and the global efforts to alleviate climate change demand advanced biorefinery methods to partly replace petroleum refinery methods for the sustainable production of fuels, chemicals, and materials.1−3 The advanced biorefinery methods must convert lignocellulosic biomass into a variety of products through modern and proven green technologies such as fermentation, gasification, pyrolysis, catalytic conversion, and their hybrid processes.4−6 In the last decades, the fast pyrolysis-based biorefinery method has received considerable attention from both academia and industry, since fast pyrolysis can realize simultaneous thermal degradation of the three major components contained in biomass (hemicellulose, cellulose, and lignin) within several seconds for high yield of bio-oil production with low cost.7−13 However, bio-oil is a highly complex mixture including © 2017 American Chemical Society

hundreds of oxygenated compounds, such as alcohols, carboxylic acids, furans, aldehydes, ketones, esters, ethers, phenols, and sugars.14−16 In addition, their respective concentrations are very low.17 There are three main reasons responsible for the complex compositions of bio-oil: (1) the very different chemical structure and corresponding pyrolytic product distribution from cellulose, hemicellulose, and lignin;18−23 (2) the catalytic effects of alkali and alkaline-earth metals (AAEM);24−26 and Received: February 28, 2017 Revised: June 1, 2017 Published: June 15, 2017 6507

DOI: 10.1021/acssuschemeng.7b00622 ACS Sustainable Chem. Eng. 2017, 5, 6507−6516

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ACS Sustainable Chemistry & Engineering

Figure 1. Proposed biorefinery scheme based on organosolv fractionation coupled with fast pyrolysis.

glycerol, γ-valerolactone, and so on.53,54 However, little information is provided on the organosolv fractionation coupled with fast pyrolysis. Our previous study reported that microwave-assisted glycerolysis of biomass at ∼240 °C is an efficient pretreatment method prior to fast pyrolysis for anhydrosugar production, because of the high boiling point and dielectric loss factor of glycerol.55 The feasibility of selective production from organosolv fractionation in low boiling solvents and conventional heating coupled with fast pyrolysis are still not understood. Moreover, there is a lack of quantitative information on the yield of the desired product from this proposed system. In brief, the main objective of the present work is to verify the feasibility of obtaining selective platform chemicals from organosolv fractionation coupled with fast pyrolysis. In addition, the formation of levoglucosan influenced by the interactions between cellulose and other components (lignin and hemicellulose) during fast pyrolysis is quantitatively elucidated.

(3) the biomass recalcitrant caused by the interactions of cellulose, hemicellulose, and lignin during fast pyrolysis.23,27−30 The complex compositions of bio-oil pose huge challenges for the extraction of value-added chemicals and catalytic upgrading in existing petroleum-refining infrastructure.31,32 Pretreatment, catalytic fast pyrolysis, and post-pyrolysis separation are the most common methods to overcome these challenges. The passivation of AAEM in biomass by mineral acid can effectively improve the yield of sugars in subsequent fast pyrolysis.33−35 Catalytic fast pyrolysis of biomass over zeolites, metals, or metal compounds can switch the product selectivity toward aromatics, furans, and so on.36−43 Postpyrolysis separation can achieve the stepwise extraction of phenols-rich and sugar-rich fractions from bio-oil.44 However, there are several drawbacks in the latter two methods, e.g., serious coking can cause the rapid deactivation of catalysts in catalytic fast pyrolysis, and post-pyrolysis separation usually requires several cost-intensive and time-consuming steps. Hence, the first route, pretreatment coupled with fast pyrolysis, could be the most effective and simple route for the selective production of platform chemicals from biomass. Here, a new advanced biorefinery based on organosolv fractionation coupled with fast pyrolysis is proposed. As shown in Figure 1, biomass is first divided into cellulose-rich fractions, organosolv lignins, and xylose by mineral acid (H2SO4, HCl, or H3PO4)-assisted organosolv fractionation. The organic solvents can be easily recovered for reuse.45 The cellulose-rich fractions and organosolv lignins then undergo fast pyrolysis to the highly selective production of levoglucosan and phenols, respectively. The obtained platform chemicals (xylose, levoglucosan, and phenols) can be easily extracted by a series of methods, including molecular distillation, solvent extraction, membrane filtration, and supercritical CO2 separation.44,46−49 The platform chemicals can be further converted to fuels, chemicals, and materials using a wide range of processes. The critical technical barriers to this proposed biorefinery method are to obtain selective platform chemicals from organosolv fractionation coupled with fast pyrolysis. Recently, organosolv fractionation coupled with enzymatic saccharification for glucose has been well-studied by many researchers.45,50−52 The organic solvents used in organosolv fractionation include acetic acid, formic acid, ethanol, methanol, acetone,

2. EXPERIMENTAL SECTION 2.1. Organosolv Fractionation of Biomass. The organosolv fractionation of biomass (eucalyptus, pine, and bagasse) was conducted in a 250 mL round-bottom flask in an oil bath at 108 °C under reflux for 3 h. Typically, 10 g of biomass was mixed with 100 g of solvent (consisting of 59.5 g of acetic acid, 25.5 g of formic acid, 15 g of deionized water, and 1 or 2 wt % H2SO4, according to our preliminary experiments and literature results).56 After organosolv fractionation, the mixtures were filtered off onto Xinxing filter paper, and the solid residuesnamely, cellulose-rich fractionswere collected and washed thoroughly with deionized water. The filtrate was precipitated by the addition of 700 mL of deionized water for recovery organosolv lignins. The organosolv lignins were filtered off and transferred to the vacuum freeze-dryer for dewatering at approximately −40 °C for 24 h. The xylose in the filtrate was quantitatively analyzed by high-performance liquid chromatography (Waters 2695/Waters 2489, Waters Corporation, USA) with an Aminex HPX-87P column after neutralization. 2.2. Characterization of Cellulose-Rich Fractions and Organosolv Lignins. The elemental analysis (C, H, N, and S) of raw biomass, cellulose-rich fractions and organosolv lignins were conducted on a Vario EL (Elementar Analysensysteme, Germany). The composition analysis of raw biomass is shown in Table S1 in the Supporting Information, according to our previous study.57 The cellulose contents of cellulose-rich fractions were calculated by that original compositions of raw biomass subtract the amounts of cellulose-derived sugars (mainly glucose) in the liquids from organsolv 6508


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ACS Sustainable Chemistry & Engineering fractionation. The structural characterization of cellulose-rich fractions was conducted on a Fourier transform infrared (FTIR) spectroscopy system (Model TENSOR27, Bruker, Germany) with a resolution of 4 cm−1 and 32 scans. The resulting spectra were normalized to the highest peak in the fingerprint region (2000−600 cm−1). The thermogravimetric analysis of cellulose-rich fractions and organosolv lignins was carried out in a thermogravimetric analyzer (TGA) (Model STA409PC, Netzsch, Germany). In each run, the samples with same mass was heated from 40 °C to 900 °C and held there for 20 min with a heating rate of 20 °C/min, and high-purity nitrogen (99.999%) with a flow rate of 40 mL/min was used as the purge gas. The evolved gases were online monitored using a mass spectrometer (MS) (OmniStarTM, Pfeiffer Vacuum, Germany). For two-dimensional heteronuclear single-quantum coherence nuclear magnetic resonance (2D HSQC NMR), ∼140 mg of organosolv lignins were dissolved in 0.6 mL DMSO-d6. 2D HSQC NMR experiments used the “hsqcetgpsisp2” pulse program with spectral widths of 400 MHz (from 11 ppm to 0 ppm) and 100 MHz (from 200 ppm to 0 ppm) for the 1H and 13C dimensions. The NMR spectra were recorded on a liquid-state NMR spectrometer (Model AscendTM 400, Bruker BioSpin GmbH, Germany) at room temperature. 2.3. Fast Pyrolysis of Cellulose-Rich Fractions and Organosolv Lignins. The fast pyrolysis experiments were performed in a commercial micropyrolysis reactor (Pyroprobe 5200, CDS Analytical, USA). The mass of reactants (∼0.1 mg for cellulose-rich fractions and 0.5−1 mg for raw feedstocks and organosolv lignins) were weighed on a microbalance with an accuracy of 0.001 mg (Model XP6152, Mettler Toledo, Germany). The reactants were rapidly heated to 550 °C and held for 20 s by a heating rate of 10 °C ms−1. The pyrolysis products were analyzed online using a gas chromatograph (Model 7890A, Agilent Technologies, USA) with an HP-INNOwax capillary column coupled to a mass spectrometer (Model 5975C, Agilent Technologies, USA). The online analysis of pyrolysis vapors could avoid the mass loss during the condensation and collection of bio-oil. The identified compounds were quantified by external standard method. All experiments were carried out at least two times and averaged to compensate experimental reproducibility. The reproducibility of identified compounds from fast pyrolysis of cellulose-rich fractions is shown in the Supporting Information. The relative yield of specific compound was calculated based on the mass of sample used in PyGC/MS. The theoretical levoglucosan yield was calculated from fast pyrolysis of commercial pure cellulose (Microcrystalline Cellulose, Sigma−Aldrich).

Figure 3. Van Krevelen diagram of cellulose-rich fractions and organosolv lignins from organosolv fractionation of biomass, CF and OL are the abbreviations for cellulose-rich fractions and organosolv lignins, respectively. And P, E and B are the abbreviations for pine, eucalyptus and bagasse, respectively. All the organosolv lignins and cellulose-rich fractions were obtained from organosolv fractionation of raw biomass with 1% H2SO4, except CF-E-2 and OL-E-2, which was produced from organosolv fractionation of eucalyptus with 2% H2SO4.

organosolv lignins, and xylose from organosolv fractionation of pine, eucalyptus, and bagasse are shown in Figure 2. It is evident that biomass can be effectively separated into celluloserich fractions, organosolv lignins, and xylose by organosolv fractionation. The possible reactions during acid-catalyzed organosolv fractionation of biomass included the following:58 (1) the cleavage of hydrogen bonds and covalent bonds between lignin and carbohydrates, e.g., benzylether-, benzylester-, phenylglycoside-, and acetal-type bonds; (2) the scission of glycosidic bonds in hemicellulose to form oligosaccharide/monosaccharide; (3) the breakage of β-O-4 and α-O-4 bonds in lignin to generate lignin fragments; and (4) acid-catalyzed degradation of saccharides and lignin to furfural, phenols, acetic acid, levulinic acid, and formic acid. The yields of cellulose-rich fractions were dependent on the feedstock species. The yields of cellulose-rich fractions from pine, eucalyptus, and bagasse were 69.5, 56.8, and 46.5 wt %, respectively, while the yields of organosolv lignins were 6.5%, 16.4%, and 14.9%. The low yields of organosolv lignins could be due to severe decomposition of lignin into small molecular compounds during organosolv fractionation that cannot be trapped by filters. The rank order of yields of cellulose-rich fractions was bagasse < eucalyptus < pine, whereas the rank order of xylose yields was pine < eucalyptus < bagasse, indicating that woody plants (pine and eucalyptus) exhibited more resistance to organosolv fractionation, compared to herbaceous plants (bagasse), and softwood was more recalcitrant than hardwood. The results could be explained by the fact that the structure and chemical compositions of hemicellulose and lignin in softwood (pine), hardwood (eucalyptus), and herbaceous plants (bagasse) are very different. The hemicellulose in softwood mainly consists of mannan and xylan, and the lignin in softwood usually has a greater degree of cross-linking structure. The xylose yields from organosolv fractionation of eucalyptus increased from 5.1 wt %

3. RESULTS AND DISCUSSION 3.1. Mass Yields of Xylose, Organosolv Lignins, and Cellulose-Rich Fractions from Organosolv Fractionation of Biomass. The mass yields of cellulose-rich fractions,

Figure 2. Mass yields of cellulose-rich fractions, organosolv lignins, and xylose from organosolv fractionation of biomass. 6509


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Figure 4. FTIR analysis of cellulose-rich fractions from organosolv fractionation of biomass.

Figure 5. TG/DTG analysis of cellulose-rich fractions from organosolv fractionation of biomass (dry basis).

Van Krevelen diagram of cellulose-rich fractions and organosolv lignins resulting from elemental analysis is depicted in Figure 3. The O/C ratios of cellulose-rich fractions from eucalyptus and bagasse obviously increased, compared with that of raw eucalyptus and bagasse. At the same time, their H/C ratios were increased slightly. In addition, their O/C and H/C ratios were close to those of commercial cellulose. The H/C ratios of cellulose-rich fractions from pine were lower than that of raw pine, whereas their O/C ratios were higher than that of raw pine. The result indicated that organosolv fractionation cannot effectively remove the lignin fractions of pine. The H/C and O/

to 8.5 wt % when the concentration of H2SO4 increased from 1 wt % to 2 wt %; at the same time, the yield of organosolv lignins improved slightly. Very small amounts of glucose were detected in the filtrate, implying that cellulose was hardly decomposed during organosolv fractionation. The unquantified compounds in the filtrate included oligosaccharide, mannose, arabinose, sorbose, and the degraded products of lignin and sugars.52 3.2. Characterization of Cellulose-Rich Fractions and Organosolv Lignins by Elemental Analysis, FTIR, TG/ DTG, and 2D HSQC NMR. The Van Krevelen diagram is widely used as an indication to the quality of solid fuel. The 6510


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Figure 6. Online-evolved gas analysis from pyrolysis of cellulose-rich fractions by mass spectroscopy (MS).

contrast, the intensities of these signals changed slightly when pine was subjected to organosolv fractionation, suggesting that lignin cannot be effectively removed from raw pine by organosolv fractionation. These results were consistent with the mass yields of organosolv lignins from organosolv fractionation of biomass. The weight loss and weight loss rate (TG/DTG) curves of cellulose-rich fractions are illustrated in Figure 5. It should be noted that the char yields from all the cellulose-rich fractions were lower than those from raw samples. It could be explained by the removal of the ash and lignin fractions from raw samples by organosolv fractionation. It is well-known that lignin generally produces more char than cellulose and hemicellulose during pyrolysis. The shoulder peaks centered at around 290− 300 °C in DTG curves were mainly attributed to the devolatilization of hemicellulose and lignin during pyrolysis. Our previous study revealed that the thermal stability of biomass components was hemicellulose < lignin < cellulose when biomass was pyrolyzed at 200−300 °C.60 The shoulder peaks in cellulose-rich fractions were disappeared in the DTG curves, suggesting that the lignin and hemicellulose fractions were removed from raw feedstocks to form cellulose-rich

C ratios of organosolv lignins from pine, eucalyptus, and bagasse were very low, and they were similar to those of commercial alkali lignin. Moreover, no sulfur was found in organosolv lignins, implying that high-purity lignins without sulfur content were produced from this organosolv fractionation. The FTIR spectra of cellulose-rich fractions are drawn in Figure 4. The signals in the fingerprint (600−1800 cm−1) assigned to the specific functional groups are listed as follows:59 (1) 1594 and 1504 cm−1 for aromatic skeletal vibrations in lignin, (2) 1465 and 1427 cm−1 for C−H deformation vibrations in lignin and carbohydrates, (3) 1331 cm−1 for C−H vibration in C−O vibration in syringyl ring and cellulose, and (4) 1244 for syringyl ring and C−O stretch in lignin and xylan. It is evident that the intensities of these aromatic carbons in cellulose-rich fractions from eucalyptus and bagasse were much weaker than that in raw eucalyptus and bagasse. It could be attributed to the fact that lignin was efficiently separated from raw eucalyptus and bagasse by organosolv fractionation. In 6511


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Figure 7. Two D HSQC NMR spectra of organosolv lignins: (A) organosolv lignins from eucalyptus, (B) organosolv lignins from pine, and (C) organosolv lignins from bagasse.

Table 1. Comparison of Relative Mass Yields of Anhydrosugars from Fast Pyrolysis of Raw Feedstocks and Their Derived Cellulose-Rich Fractions Eucalyptus

Pine

Bagasse

feedstock

raw

CF-E

CF-E-2

raw

CF-P

raw

CF-B

levoglucosan 2,3-anhydro-d-mannosan 1,4:3,6-dianhydro-α-D-glucopyranose 5-hydroxymethylfurfural hydroxy-acetaldehyde 1-hydroxy-2-propanone 2-hydroxy-2-cyclopenten-1-one

4.8 1.0 0.2 0.2 2.4 0.6 0.9

42.1 1.2 0.3 0.9 5.0 1.1 0.7

44.4 1.2 0.3 0.8 5.2 1.1 0.6

3.5

22.7

2.1

0.3 2.1 0.6 0.7

0.8 4.8 1.9 1.9

0.3 1.4 0.6 1.1

59.8 1.8 0.5 1.4 4.6 1.8 1.7

fractions by organosolv fractionation. The releases of H2O, CO2, CO and CH4 during pyrolysis of cellulose-rich fractions are plotted in Figure 6. The peak areas of H2O, CO2, CO and CH4 from cellulose-rich fractions, corresponding to their respective gaseous amounts, were decreased compared with that from raw biomass, indicating that cellulose-rich fractions yielded lower water and permanent gases than raw biomass. Brown reported that alkali and alkaline metals (AAEM) in ash can strongly catalyze the fragmentation of biomass to generate permanent gases and light oxygenates.25 Organosolv fractionation with H2SO4 can effectively passivate the AAEM in cellulose-rich fractions, since the AAEM can react with H2SO4 to form thermally stable sulfates thereby reducing their catalytic activity. The shoulder peaks (centered at about 290−300 °C) of released gases from cellulose-rich fractions were also obviously reduced, since they were linked to the emission of H2O, CO2, CO and CH4 from the pyrolysis of hemicellulose and lignin fractions. The organosolv fractionation can effectively remove the hemicellulose and lignin fractions from raw feedstocks to form cellulose-rich fractions. The results were in accordance with the DTG curves of cellulose-rich fractions. The 2D HSQC NMR spectra of organosolv lignins from eucalyptus, pine, and bagasse are graphed in Figure 7. It is clear that the organosolv lignins from different sources exhibited very different chemical structures. The absence of carbohydrate

signals between 90 and 102 ppm in the three NMR spectra implies very low carbohydrate content in these organosolv lignins, suggesting that high-purity lignins were obtained by the organosolv fractionation of biomass. Lignin is a complex network of three phenylpropane units with varying degrees of methoxylation, which are sinapyl, coniferyl, and p-coumaryl alcohol. These basic units were linked through various C−C and C−O−C bonds, such as β-O-4, α-O-4, β-5, β−β, β-1, and so on.61 As shown in Figure 7, all the organosolv lignins were rich in methoxyl group and β-O-4 structure. The organosolv lignins from eucalyptus predominantly contained syringyl and guaiacyl units. In contrast, the orgnosolv lignins from pine mainly comprised of guaiacyl units. More-complex structures were observed in the organosolv lignins from bagasse. They primarily included guaiacyl, syringyl, and p-hydroxyphenyl units. p-Coumarate and ferulate, mainly esterified to the phenylpropanoid side chains of syringyl units, were also found in the orgnosolv lignins from bagasse. 3.3. Selective Production of Levoglucosan and Phenols from Fast Pyrolysis of Cellulose-Rich Fractions and Organosolv Lignins, Respectively. The yields of selected compounds from fast pyrolysis of raw feedstocks and their derived cellulose-rich fractions are listed in Table 1. The compounds mainly included anhydrosugars, aldehydes, ketones, furans, phenols, and so on. The identified anhydrosugars involved levoglucosan (LG), 2,3-anhydro-d-mannosan (AM), 6512


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Figure 8. Mechanistic insight into organosolv fractionation coupled with fast pyrolysis for levoglucosan production.

was dominant during all of the pyrolysis products. It is widely accepted that cellulose pyrolysis follows two competing pathways: the heterolytic fission of glycosidic bonds to anhydrosugar, and pyranose ring-breaking to low-molecularweight aldehydes and ketones through a retro-aldol type mechanism. As shown in Table 1, the raw feedstocks produced relatively few anhydrosugars. The LG yields from pyrolysis of raw eucalyptus, pine, and bagasse were 4.8, 3.5 and 2.1 wt %, respectively. The anhydrosugar yields were dramatically promoted by organosolv fractionation, especially for LG. The LG yields from pyrolysis of cellulose-rich fractions from eucalyptus, pine, and bagasse reached 42.1, 22.7, and 59.8 wt %, respectively, when raw feedstocks were applied to organosolv fractionation with 1% H2SO4. The yields of low molecular compounds, such as hydroxy-acetaldehyde and 1hydroxy-2-propanone, were also improved by organosolv fractionation. As shown in Figure 8, the yield gaps of LG between raw feedstocks and their derived cellulose-rich fractions were predominantly ascribed to two critical factors: (1) the dissolution and passivation of AAEM during organosolv fractionation; AAEM could catalyze the C− C cleavage through retro-aldol type reactions during fast

Figure 9. Yields of levoglucosan from fast pyrolysis of cellulose-rich fractions, as a function of cellulose contents.

1,4:3,6-dianhydro-α-D-glucopyranose (AGP). Moreover, levoglucosan, which was mainly derived from cellulose pyrolysis, 6513


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Table 2. Comparison of Relative Mass Yields of Selected Phenols from Fast Pyrolysis of Raw Feedstocks and Their Derived Organosolv Lignins Eucalyptus feedstock

origin

phenol 4-vinylphenol guaiacol syringol o-creosol 4-ethylguaiacol p/m-creosol 3-allylguaiacol 4-vinylguaiacol isoeugenol methoxyeugenol vanillin 4-hydroxy-2-methoxycinnamaldehyde vanillic acid syringaldehyde

H H G S G G G G G G S G G G S

raw

OL-E

Pine OL-E-2

Bagasse

raw

OL-P

raw

OL-B

0.09

0.15

0.28 1.95 0.08 0.53 0.12

0.76 5.09 0.86 1.42 0.63 0.40 0.66

0.65

0.83

0.42

0.80

0.07 0.84 0.17 0.02

0.62 1.42 0.57 0.17

0.77 1.57 0.61 0.12

0.81

1.83

0.56 0.10

0.26

0.06 0.27

0.04 0.25

0.20 0.65 0.10

1.99 0.73 0.18 0.21 0.70 0.83

0.84 0.12

0.92 0.30

0.80 0.40

0.13 1.47

0.40 3.22

0.59 0.22

1.56 0.99

1.41 0.70

yields of 4-vinylphenol from raw bagasse and its derived organosolv lignins were 1.95 and 5.09 wt %, respectively. However, the absolute yield of 4-vinylphenol from organosolv lignin (equal to the relative yield of 4-vinylphenol times the yield of its original organosolv lignin from Figure 2) was less than that from raw bagasse. The 4-vinylphenol was the pyrolysis product of H units in lignin. In addition, it could be resulted from the degradation of p-coumarate and ferulate. Organosolv fractionation could cleave the ester bonds between ferulic acid (or p-coumaric acid) and the phenylpropanoid side chain of lignin (or the arabinose side chain of arabinoxylans), which could cause the release of ferulic acid and p-coumaric acid during organosolv fractionation of bagasse, thus reducing the formation of 4-vinylphenol in subsequent fast pyrolysis of organosolv lignins from bagasse. Note that, although the relative yields of all phenols based on organosolv lignins increased, their absolute yields based on raw feedstocks decreased, except guaiacol and 4-ethylguaiacol. Their absolute yields from eucalyptus lignins and pine lignins increased remarkably when comparing with those from raw feedstocks. The results could be attributed to that a number of the methoxyl groups in S units can be removed to form G units during organosolv fractionation.

pyrolysis, where they inhibit the LG formation and favor the production of light oxygenates; and (2) the cleavage of covalent bonds between lignin and carbohydrates; the covalent linkage between cellulose and lignin/hemicellulose could suppress the heterolytic fission of glycosidic bond to form LG. It is speculated that all of the residual AAEM in cellulose-rich fractions were passivated by the H2SO4 used in organosolv fractionation. Hence, the yield gaps of LG were mainly determined by the biomass recalcitrant in this study. The LG yields from raw feedstocks and their derived cellulose-rich fractions, as a function of cellulose contents, are represented in Figure 9. As shown in Figure 9, The cellulose contents of CF-P, CF-E, CF-E-2, and CF-B were 67.10, 77.30, 78.70, and 97.70 wt %, respectively. The LG yields were linearly related to the cellulose contents of feedstocks. In addition, the differences between actual and theoretical yields of LG decreased as the cellulose contents of feedstocks increased, indicating that LG yields were mainly dependent on the interactions between cellulose and other components during the pyrolysis of passivated cellulose-rich fractions. The results strongly supported the above-mentioned speculation. The yields of selected phenols from fast pyrolysis of raw feedstocks and their derived organosolv lignins are tabulated in Table 2. Apparently, the product distributions were determined by the nature of the different lignin origins. No carbohydratederived compounds were found in the pyrolysis products of organosolv lignins, indicating that the selectivity of phenols were strongly enhanced. The phenols could be classified into three types, according to their derived chemical groups: guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H).62 Organosolv lignins from eucalyptus, pine, and bagasse mainly yielded S-, G-, and H-type phenols, respectively. Very few Stype phenols were observed from the fast pyrolysis of organosolv lignins from pine. The results were consistent with the 2D HSQC NMR spectra of organosolv lignins. The major phenols (yield >1%) included 4-vinylphenol, guaiacol, syringol, creosol, vanillic acid, and 4-hydroxy-2-methoxycinnamaldehyde. The selectivities of all phenols were remarkably promoted by organosolv fractionation. The low selectivities of phenols from raw feedstocks were mainly due to the dilution effect of carbohydrate-derived pyrolysis products. The relative

4. CONCLUSION Biomass can be effectively separated into cellulose-rich fractions, organosolv lignins, and xylose by organosolv fractionation, thus improving the yields or selectivities of platform chemicals (LG and phenols) in subsequent fast pyrolysis. The relative LG yields from eucalyptus, pine, and bagasse increased from 4.8, 3.5, and 2.1 wt % to 42.1, 22.7, and 59.8 wt %, respectively. The proposed advanced biorefinery method based on organosolv fractionation, coupled with fast pyrolysis, is a new integrated process to the selective production of LG and phenols. From a technical point of view, these findings provide a simple and efficient method to sugar production in several seconds, rather than hours or days in hydrolysis. However, evaluating the economic feasibility of the proposed process is another important issue that should be considered in the near future. In addition, the impacts of different solvents used in organosolv fractionation on subsequent fast pyrolysis must be further clarified. 6514


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00622. The chemical composition analysis and proximate analysis of biomass, as well as the reproducibility of the total ion chromatograms resulting from fast pyrolysis of cellulose-rich fractions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+86 02087057721. Fax: +86 02087057737. E-mail: [email protected]. ORCID

Zengli Zhao: 0000-0002-8069-8757 Guoqiang Wei: 0000-0002-2501-2185 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grant Nos. 21406227 and 51376186), the Natural Science Foundation of Guangdong Province, China (Grant No. 2014A030313672) and the Science and Technology Planning Project of Guangdong Province, China (Grant Nos. 2014B020216004 and 2015A020215024) for financial support of this work. The authors also gratefully acknowledge the Foundation of Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences (No. CASKLB201508).

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REFERENCES

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