Low-Temperature Torrefaction of Phyllostachys heterocycla cv

Research Article pubs.acs.org/journal/ascecg

Low-Temperature Torrefaction of Phyllostachys heterocycla cv. pubescens: Effect of Two Torrefaction Procedures on the Composition of Bio-Oil Obtained Xiaoyan Lv, Zhicheng Jiang, Jindong Li, Yue Wang, Dongmei Tong, and Changwei Hu* Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu, Sichuan 610064, P. R. China S Supporting Information *

ABSTRACT: Low-temperature torrefaction of pubescens was studied by one-step (RT−200 °C) and two-step (RT−120 °C first, and then 120−200 °C) ways. For one-step torrefaction, the cracking of intermolecular and intramolecular hydrogen bonds in cellulose promoted the formation of oligosaccharide, anhydrosugar, and levoglucosan intermediates with monosaccharide, carboxylic acid, furans, etc. The oligomers with 4O-5, β-O-4 linkages were mainly derived from the cleavage of C−O−C bonds between lignin and cellulose, and phydoxyphenyl (H), ferulate (F) intermediates were derived from the cracking of β-O-4 and Cα−Cβ in lignin. In two-step torrefaction, the first step (RT−120 °C) tended to generate the intermediates of oligosaccharide, levoglucosan and pyranose, which mainly derived from the cleavage of intermolecular hydrogen bonds or ether bonds in amorphous cellulose. The lignin-based oligomers with 4-O-5 and β-O-4 linkages were mainly derived from the cracking of β-O-4 and dehydration. For the second step (120−200 °C), the cracking of intramolecular hydrogen bonds in amorphous cellulose promoted the formation of oligosaccharide and anhydrosugar intermediates. While the oligomers with β-5, 4-O-5, and β-O-4 linkages were derived from the cracking of Cα−Cβ bonds and dehydration process, the cleavage of C−O−C bonds promoted the formation of ferulate (F) intermediates. KEYWORDS: Pubescens, Torrefaction, Degradation, Intermediates, Oligomers

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INTRODUCTION With increasing exhaustion of fossil fuels, lignocellulosic biomass is widely regarded as green and renewable resources to produce fuels and chemicals.1 Torrefaction is a low temperature pyrolytic technology, leading to the cleavage of weakly bonded parts in biomass to yield gaseous, liquid, and solid products in the absence of oxygen.2,3 However, the commonly obtained bio-oil from pyrolysis is a complex mixture, which is not possible to be used directly as fuel or commercial chemical, whereas the upgrading of these products is proved to be very difficult. At the same time, the separation of these chemicals from pyrolytic oil is also very difficult, because there are usually more than 300 kinds of products contained in the thus obtained bio-oil.4,5 The upgrading and separation processes would need intensive cost. Therefore, low temperature torrefaction of biomass provides a possible route for simplify the diversity of the products in thermal degradation. Two strategies have been used to enhance the selectivity of target products, that is, fractional condensation and fractional pyrolysis.6−11 In fractional condensation, because of their unique chemical and physical properties, the bio-oil products could be separated by cooling at different temperatures.6,7 © 2017 American Chemical Society

Using a series of condensers maintained at different and gradually decreasing temperature, bio-oil recovered was widely divided into three fractions: light, middle, and heavy bio-oil.8 Because of the accumulation of water in one of the condenser, the bio-oil attained via fractional condenser systems, contained lower water content and possessed higher heat value than that of traditional one-condenser based bio-oil recovered. In a word, fractional condensation method provided an effective way for the separation of the multicomponent liquid products according to their boiling points. However, because enormous kinds of products formed in pyrolytic process and the differences in condensing temperature of the components were relatively small, it was difficult to fully separate different compounds, and the yield of specific products at each temperature was relatively small. Taking advantage of different decomposition characteristics of different components (cellulose, hemicellulose, and lignin, etc.) in lignocellulosic biomass, selective conversion of the Received: January 26, 2017 Revised: March 5, 2017 Published: April 24, 2017 4869

DOI: 10.1021/acssuschemeng.7b00283 ACS Sustainable Chem. Eng. 2017, 5, 4869−4878

Research Article

ACS Sustainable Chemistry & Engineering

control and monitor the temperature variation with time in the torrefaction process of pubescens. A thermocouple was placed next to the outer tube, which was used to monitor and control the temperature variation in the whole process. Another thermocouple was placed next to the inner tube linked with temperature detector, which was used to monitor the actual temperature variation in the torrefaction process. The detailed variation of temperature simultaneously showed that the difference between the setting temperature and actual temperature was about 8−10 °C in the temperature rising process, while the difference between the setting temperature and actual temperature was about 2−3 °C in the temperature holding process. It was usually thought that the temperature holding process was the main torrefaction stage. Carrier gas of nitrogen controlled by rotameters was introduced into the reaction system from the bottom of the outer tube and inner tube, respectively. Volatile compounds formed in the reactions were immediately swept away from the heating area by N2 flow. The liquid and gaseous products were collected separately at each temperature. Condensable products were collected in the outer quartz tube and condensers as liquid products, and all the noncondensable products were collected with a vacuum gas bag at each torrefaction temperature. About 4.0 g of sample was put in the inner quartz vessel, and the system was swept with N2 at a flow rate of 60 mL min−1 in the outer tube and 40 mL min−1 in the inner tube to get a completely inert atmosphere. Two procedures, that is, one-step and two-step torrefaction, were adopted. According to TG analysis (see Figure S2), in two-step procedure, pubescens was first torrefied from room temperature to 120 °C at a heating rate of 2.5 °C min−1, and then the reactor was kept at 120 °C for 2 h, and this step was donated as RT− 120 °C. The residues remained was consecutively torrefied from 120 to 200 °C at the same heating rate and holding for 2 h at 200 °C, and this step was donated as 120−200 °C. One-step control experiment was also performed directly from room temperature to 200 °C and hold for 2 h or 4 h, and this process was donated as RT−200 °C. After each trial, the reacted sample was cooled down to room temperature under nitrogen flow and then weighed to calculate the yield of solid residues. The liquid products consisted of bio-oil and water, and its yield was calculated by the weight difference of both the condensers and outer tube before and after torrefaction. The content of water was detected by automatic trace moisture meter. The gas quantity was calculated by overall mass balance. All the data provided for the product yield were based on the initial weight of pubescens sample. The final result of conversion and yield were the average value of at least three parallel experiments, and the error was less than ±0.2%. Characterization of the Solid Residue. The content of the three major components (i.e., cellulose, hemicellulose, and lignin) in pubescens and residues after torrefaction were determined using classical chemical titration methods. The experimental details could be obtained from our previous work.17,18 To characterize the surface morphology of pubescens and residues after torrefaction, SEM (FEIINSPECT F) analysis was performed. The instrument was run at an acceleration voltage of 20 kV. The crystalline forms of pubescens and residues after torrefaction under one-step and two-step reaction conditions were examined by XRD measurements on a Dandong Fangyuan DX-1000 instrument. The diffracted intensity of Cu Kα radiation (k = 0.1540 nm; 40 kV and 25 mA) was measured over the 2θ range of 5° to 40°. The crystallinity index (CrI) of cellulose in the samples was calculated according to the literature.19 The FT-IR spectra were recorded on a Nicolet Nexus 670 Fourier transform-infrared spectrometer in the range of 4000−400 cm−1 with a resolution of 2 cm−1. In total, 1 mg of dried sample was blended with 100 mg of KBr and pressed into thin pellets. The 13C cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) of pubescens and solid residues were conducted with a Bruker Avance III 500 MHz instrument. A total of 800 scans were accumulated for each sample. The spinning rate was 37878 Hz, and the relaxation delay was 5 s. Spinal 128 decoupling was used during acquisition. Characterization of the Small Molecular Weight Products. First, the liquid product was dissolved in acetone and identified

components in lignocellulosic biomass was realized by fractional pyrolysis. Furans and phenols were separately obtained through a two-step hydrothermal conversion of pubescens.9 Two-step conversion of raw biomass was first performed at a moderate temperature in a shorter time to obtain furans and then carried out at a higher temperature for a comparatively longer time to produce phenols, which also improved the efficiency of lignocellulosic resource utilization. The fractional catalytic pyrolysis of hybrid poplar wood was introduced to generate stable and low-viscosity biomass pyrolysis bio-oil.10,11 Fractional catalytic conversion was used to convert carbohydrate and lignin components to gaseous products and phenols, respectively. The catalytic and pyrolytic reactions were performed simultaneously, resulting in the conversion of biomass to desirable liquids. Fractional pyrolysis was proved to be an effective process for selective conversion of the three major components in biomass.12 With the multiple step thermal chemical reaction instead of pretreatment, the feedstock of each step was simplified and it becomes possible to obtain high-grade biooil. The fractional pyrolysis and one-step direct pyrolysis of natural algae cyanobacteria from Taihu Lake was studied comparatively from 200 to 500 °C, and catalyst was added to improve the selectivity of liquid products, where high quality bio-oil was obtained.13,14 Fractional pyrolysis of natural algae could produce bio-oil with concentrated components, while in one-step pyrolysis the composition of the bio-oil was very complex. Therefore, fractional pyrolysis is considered as an appropriate way for selective conversion of biomass to attain high quality bio-oil without necessarily going through secondary extraction and upgrading processes. Thus, the design of pyrolytic processes is important to obtain specific product with high selectivity. In order to help the design of pyrolytic process, low temperature torrefaction of phyllostachys heterocycla cv. pubescens (shortened to pubescens herein) was performed. Pubescens is a kind of typical fast growing lignocellulosic biomass, mainly consisting of hemicellulose, cellulose, and lignin. Pubescens forests are distributed extensively in South and Southeast Asia, while China has a pubescens forest area of approximately 6.01 million hectares. Because of its fast grow and usefulness, pubescens will continue to play an important role in the world. The torrefaction was carried out via one-step way from room temperature to 200 °C, and two-step way from room temperature to 120 °C and then from 120 to 200 °C in the present work. We also aimed to probe the formation mechanism of various products and the variation of different components in the biomass samples with temperature. The breaking of different kinds of linkages among and within cellulose, hemicellulose, and lignin were also discussed.

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EXPERIMENTAL SECTION

Materials. Pubescens samples purchased from Anji county of Zhejiang Province in China were used in this study. The sample used for experiment was ground to 80 meshes and then washed by distilled water three times. Finally, the samples were dried in an oven at 110 °C overnight before use in the torrefaction experiments.3,15 Experimental Apparatus and Procedure. A fixed bed reactor was modified on the basis of our previous apparatus as experimental apparatus.16 The schematic diagram of torrefaction process was shown in Figure S1. The reactor was composed of an outer quartz tube and an inner quartz tube with a vessel in the top, and the inner tube was inserted into the outer tube. The reaction device was placed in an electric furnace, and a temperature controller (SKW-100) was used to 4870


Research Article

ACS Sustainable Chemistry & Engineering Table 1. Variation of Three Major Components in pubescens and Residues hemicellulose

a

cellulose

lignin

T/°C

remaining ratiob (wt %)

Cov. (wt %)


Cov. (wt %)


Cov. (wt %)

Pubescens RT−120, 2 h 120−200, 2 h ΣRT−200, 2 ha RT−200, 2 h

18.8 18.8 18.8 18.8 18.8

0 0 0 0 0

45.1 42.2 41.3 41.3 40.8

0 6.4 2.0 8.4 9.5

24.2 21.9 20.0 20.0 21.7

0 9.5 7.9 17.4 10.3

The accumulated conversion (Cov.) and yield (wt %). bBased on the weight of pubescens feedstock (wt %).

Table 2. Product Distribution in Two-Step and One-Step Torrefaction of pubescens T/°C residues liquid gas conversion

RT-120, 2 ha 91.1 7.0 1.9 8.9

120−200, 2 hb 98.0 (89.3) 0.6 (0.6) 1.4 (1.2) 2.0 (1.8)

a

ΣRT-200, 2 hc

RT-200, 2 ha

89.3 7.6 3.1 10.7

89.2 8.0 2.8 10.8

a

The conversion and yields (wt %) were all based on the initial weight of pubescens. bThe conversion and yields (wt %) were based on the residues derived from the first torrefaction step. cThe accumulated conversion and yields (wt %) via two-step torrefaction based on the initial weight of pubescens.

structural variation of biomass at about 120 °C might include the cleavage of hydrogen bonds, C−C, and C−OH(R) bonds, etc.21 The information about these variations is scientifically important, which will help us to design more reasonable biomass conversion processes. So the first step torrefaction temperature was selected to be 120 °C. As shown in Table 1, the dried pubescens was composed of 18.8 wt % hemicellulose, 45.1 wt % cellulose, and 24.2 wt % lignin. When one-step torrefaction of pubescens was conducted at 200 °C, the conversion of cellulose and lignin were 9.5 and 10.3 wt %, respectively. The yields of the products in different phase were given in Table 2. The conversion of pubescens was 10.8 wt %, and the yield of liquid products was 8.0 wt %, among which water accounted for around 82 wt % in all liquid products and 2.8 wt % of gaseous products were obtained. We also performed the one-step torrefaction (RT−200 °C) with residence time of 4 h, and the conversion of pubescens and the yields of products were almost the same as those obtained within 2 h. In the two-step way, the conversion of cellulose and lignin in the first step were noticeably different. The easy degradation components in pubescens were initially converted at RT−120 °C, where the conversion of cellulose was 6.4 wt % and that of lignin was 9.5 wt %. The conversion of pubescens was 8.9 wt %. A relatively low gas yield of 1.9 wt % was obtained, and the yield of liquid products was 7.0 wt %, where water also accounted for the majority of liquid products (about 80 wt %) at this step. Then the residues derived from the first torrefaction step were subjected to the second torrefaction step from 120 to 200 °C, and the conversion of cellulose and lignin was 2.0 and 7.9 wt %, respectively. The yield of liquid decreased to 0.6 wt % and that of gas to 1.4 wt %, while the conversion of the residues obtained at 120 °C was about 2.0 wt %. As demonstrated in Table 2, the accumulated conversion of pubescens (10.7 wt %) and the yield of liquid products (7.6 wt %) obtained from twostep torrefaction were nearly the same as those via one-step torrefaction at 200 °C, respectively. That is, the effect of twostep torrefaction on the total conversion of pubescens at low temperature range was not significant.

qualitatively by GC-MS, and then the retention time was determined using standard substances. The GC-FID (Fuli 9750) was equipped with an HP-innowax column (30 m × 0.25 mm × 0.25 μm) and the temperature of both the detector and injector were 280 °C. The temperature program was set as rising from 50 to 250 °C at the heating rate of 5 °C min−1, holding for 10 min at 250 °C. Benzyl alcohol was used as an internal standard to quantify the content of the products attained from the degradation of lignin. Products obtained from the conversion of cellulose were analyzed by Dionex U-3000 high-performance liquid chromatography (HPLC) equipped with Dionex PG-3000 pump, an Aminex HPX-87 column (Bio-Rad), and Shodex 101 refractive index detector (RID). The temperature of column oven and detector were 50 and 35 °C, and the mobile phase was 0.005 M H2SO4 solution at a flow rate of 0.6 mL min−1. The content of products such as monosaccharides, carboxylic acids, and furans were quantified by the external standard method. Characterization of the Oligomer-Based Liquid Products. The molecular weight distribution of liquid products was determined by gel permeation chromatography (GPC) conducted by a HLC-8320 analyzer with two columns of TSK gel super HZM-M and TSK gel super HZ3000 (6.0 mm × 150 mm). The liquid products were also characterized by ESI-MS (Shimadzu). The nebulizer gas (N2) flow rate was 1.5 L min−1, and the detector voltage was 1.58 kV. Methanol solvent was used as the eluent with a flow rate of 0.6 L min−1, and an injection volume of 15 μL was used. The 2D HSQC NMR spectra of the liquid products were qualitatively determined on a Bruker Advance 400 MHz spectrometer. About 50 mg of viscous samples after removing the solvent were fully dissolved in 0.5 mL of deuterated dimethyl sulfoxide (DMSO-d6). The parameters of the instrument were recorded according to the literature.20 Characterization of Gaseous Products. The composition of gaseous products was identified by GC 9710 with a thermal conductivity detector (TCD) using a TDX-1 carbon molecular sieve packed column (2 m × 3 mm i.d.). The temperatures of the column and detector were 120 and 160 °C, and the flow rate of carrier gas nitrogen was 20 mL min−1. The online mass spectrometry detector (HPR-20QIC) was also used to monitor and identify in real time the gas species with molecular weights between 2 and 300 Da, evolving from the process of torrefaction.

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RESULTS AND DISCUSSION Variation of the Three Major Components in Torrefaction. As shown in Figure S2, TG analysis of pubescens indicated that the weight loss of the sample was stepwise, and we observed the first step weight loss at about 120 °C. The 4871


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Figure 1. FT-IR spectra of pubescens and the solid residues. The left panel is the whole spectrum ranging from 400−4000 cm−1. The right panel is the partial spectrum ranging from 800−1800 cm−1.

below 2000 cm−1. The characteristic absorption peaks of the lignin in pubescens occurred at 1632, 1603, 1511, 1462, 1426, 1034, and 833 cm−1.25 The small band at 1632 cm−1 was attributed to the conjugated CO stretching to the aromatic ring.26 The bands at 1603 and 1511 cm−1 were assigned to C− C and CC stretching of the aromatic ring of lignin, while the band at 1034 cm−1 was due to the C−O stretch for the O−CH3 and C−OH.27 The peaks at 1374, 1331, 1165, 1114, 1054, and 896 cm−1 were assigned to the absorbance of cellulose, where those at 1331, 1114, and 1054 cm−1 were assigned to C−H, CO, and C−O stretching vibrations in cellulose, respectively.28 In one-step torrefaction at RT−200 °C, the relative intensity of band at 1632 cm−1 slightly decreased after torrefaction. It could be indicated that the one-step torrefaction promoted the rearrangement of carbonyl bond, and the cleavage of C−O−C between lignin and cellulose further affected the CC stretching within the benzene ring. The shift of the band at 1511 cm−1 to 1514 cm−1 suggested the variation of side chains, which demonstrated that the cleavage of C−O−C bonds in glucosidic linkage or Cα−Cβ in the side-chains of lignin in turn influenced the conjugation of the benzene ring structure. The bands at 1331 and 1054 cm−1 shift to higher wavenumbers of 1333 and 1056 cm−1, which indicated the variation of the hydrogen bonds and C−O−C linkage between major components in pubescens. In the first torrefaction step at RT−120 °C, the FT-IR variations of residue were not significant compared to the initial sample. For 120−200 °C, the band at 1635 cm−1 shifted to 1630 cm−1, reflecting the dehydration of water from the hydroxyl groups in the cellulose or lignin enhanced the conjugation between benzene ring and aliphatic side chains. The band at 1601 cm−1 shifted to high wavenumber of 1605 cm−1, which indicated the cleavage of C−O−C bonds between lignin and cellulose affecting the C−C stretching in the aromatic skeleton.29 The band at 1462 cm−1 assigned to the C−H vibration of − CH3, −CH2− and − OCH3 groups, shifted to 1464 cm−1, indicating the cleavage of aliphatic side chains or hydrogen bonds in cellulose. 13 C CPMAS Solid-State NMR. To further investigate the changes of cellulose and lignin in pubescens residues after torrefaction, the 13C CPMAS solid-state NMR experiment was performed. Figure 2 exhibited the 13C CPMAS NMR spectra of pubescens and solid residues after two-step and one-step torrefaction. For cellulose, the signals of C-1, C-4 ordered, C-4 disordered, C-2, C-3, C-5, C-6 ordered, and C-6 disordered

Over the whole temperature range, both processes result in no significant degradation of hemicellulose. The conversion of cellulose in pubescens was 9.5 wt % with one-step torrefaction at 200 °C for 2 h, which was slightly higher than the total conversion of cellulose via two-step torrefaction process. While the total conversion of lignin in pubescens increased from 10.3 wt % at one-step torrefaction to 17.4 wt % in two-step torrefaction processes. That is to say, more amount of lignin was converted via two-step torrefaction, while slightly more amount of cellulose was converted via one-step torrefaction. It further demonstrated that the interactions among the three major components could alter the torrefaction performance.22−24 It could be observed that two-step torrefaction over this temperature range provided an effective way to priorly degrade the lignin in pubescens. SEM Analysis of pubescens and Solid Residues. The pubescens sample and solid residues were characterized by scanning electron microscopy (SEM) (see Figure S3) and X-ray diffraction XRD (see Figure S4). After one-step torrefaction, the surface of reaction residues became rougher, which suggested that a fraction of lignin attached to the surface was peeled off from the raw material. The bundle structure and the surface of cellulose were arranged disorderly and split away from the bundle structure at RT−200 °C. It could be inferred that the amorphous cellulose and part of crystalline cellulose component were obviously varied in the process of thermal treatment, resulting from the disruption of hydrogen bonding in cellulose. SEM analysis indicated that the surface of reaction residues after RT−120 °C became rougher compared to the pubescens feedstock, which supported that the component located in the outer cell wall, that is, lignin in pubescens was partially degraded at low torrefaction temperature. However, the bundle structure of cellulose located within a lignin shell arranged orderly, indicating that there was little degradation of cellulose component occurred. For the second torrefaction step at 120−200 °C, the bundle structure of crystalline cellulose was still kept orderly. While the component of lignin attached to the surface was peeled off from the surface and cracked into fragments. The degradation of lignin in the outer cell wall further affected the linkage structure of cellulose, promoting the irregularly arrangement of cellulose units. FT-IR Analysis of pubescens and Solid Residues. The FT-IR spectra of pubescens feedstock, the residues obtained at two-step torrefaction and one-step torrefaction are displayed in Figure 1. The main difference in the FT-IR spectra of the biomass components and residues appeared at wavenumbers 4872


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Distribution of Gaseous Products. Online mass spectrometry analysis is in favor of the detection and identification in real time of gas species with molecular weight between 2 and 300 Da, evolving from the process of torrefaction. As demonstrated in Figure S5, gaseous products of CO2 and CH4 were the only observed species in all the torrefaction processes. The relative contents of gaseous products were listed in Table 3. Table 3. Relative Content of the Gaseous Product in All Torrefaction Processes T/°C CH4 CO2

RT−120, 2 h

120−200, 2 h

ΣRT−200, 2 ha

RT−200, 2 h

100.0

16.1 83.9

15.2 84.8

10.9 89.1

a

The gaseous products attained via two-step torrefaction were put together for GC analysis, and then the accumulated yield was determined (wt %).

13

Figure 2. C CPMAS solid-state NMR spectra of pubescens and solid residues.

For the one-step direct torrefaction at RT−200 °C, small amount of CH4 were found in gas products in addition to abundant CO2, revealing that dehydrogenation, demethanation, and decarboxylation reactions occurred. Over the temperature range from RT to 120 °C, only CO2 was detected in the gaseous products. The emission of CO2 mainly resulted from the cleavage of thermolabile carboxyl, carbonyl, and ether groups in lignin phenylpropane side chains in the lowtemperature torrefaction.32,33 With the temperature increased from 120 to 200 °C, remarkable change was found, where a small quantity of CH4 was attained, which was primarily attributed to the demethylation of the lignin methoxyl groups. Moreover, the cracking of methylene groups and saturated alkyl side chains with the γ-hydroxyl group could also contribute to the formation of CH4.34 As presented in Table 3, the accumulated content of CO2 (84.8 wt %) from two-step torrefaction was lower than that via one-step torrefaction (89.1 wt %) at 200 °C. While the yield of CH4 attained to 15.2 wt % via two-step torrefaction. It turned out that two-step torrefaction would be more beneficial to the cleavage of the methoxyl bond and alkyl side chains with the γhydroxyl group in lignin. That is, the first torrefaction step altered the linkage between major components in pubescens, which promoted the partial structure units in lignin more easily to be degraded compared to one-step torrefaction. Analysis of the Small Molecular Weight Products. Liquid products were dissolved by acetone and then quantitatively analyzed by GC-FID and HPLC, respectively. The yield of detected small molecular weight products were calculated based on the amount of the starting materials, which were presented in parentheses in Table 4. For comparison goal, we calculated the yields based on the converted cellulose and lignin components, respectively. The composition of the liquid products obtained at one-step and two-step torrefaction were comparatively presented in Table 4. Given the extremely low yield of products based on the amount of pubescens, the following discussion on the variation of small molecular weight products were based on the converted cellulose and lignin components. For one-step torrefaction, glucose (0.67 wt %), carboxylic acids (3.93 wt %), furfural (0.11 wt %), and 5-HMF (0.21 wt %) were the main small molecular weight products, which might be derived from the degradation of cellulose. Although the small amount of acetic acid might also be derived from the

were presented at 106, 89, 84, 74, 69, and 64 ppm, respectively. The C-4 disordered and C-4 ordered signals are related to the amorphous and crystal structures of cellulose. The signals of guaiacol C-6 and aryl −OCH3 in lignin were at 124 and 58 ppm.2,30 For one-step torrefaction at RT−200 °C, the signals assigned to C-4 ordered and C-4 disordered were significantly decreased. The result indicated that the C4−O linkage to saccharide H intermolecular hydrogen bonds within crystalline cellulose and amorphous cellulose were all disrupted. The signals assigned to C-2, C-3, C-5 was also decreased, which indicated that the amorphous (or disordered) cellulose and part of crystalline cellulose were degraded in thermal treatment over this temperature range. The other signals of guaiacyl C-6, which might link the components of cellulose via ether bond and aryl methoxyl carbon (−OCH3) in the lignin component weakened as well. It demonstrated that C−O−C linkage between guaiacyl and the structure unit in cellulose was the mainly cracked bonds. After the first torrefaction step at RT−120 °C, the signals assigned to C-4 disordered in cellulose distinctly decreased accompanied by the signals of C-4 ordered increased; that is, the cracking of amorphous cellulose made the crystalline cellulose arrange more orderly. This feature indicated that the degradation of amorphous cellulose was prior to crystalline cellulose. The signals assigned to C-2, C-3, C-5 in cellulose and guaiacyl C-6 in the lignin component were simultaneously weakened. It further demonstrated that the intermolecular hydrogen bonds or ether bonds were the initial cracked linkage at low-temperature torrefaction. For the second torrefaction step at 120−200 °C, the C-6 disordered signals greatly decreased accompanied by C-2, C-3, C-5 signals in cellulose slightly decreased. That is, C6-OH···O2 intramolecular hydrogen bonds were significantly cracked and the structure of amorphous cellulose was also influenced resulting from the degradation of lignin. The aryl methoxyl carbon (−OCH3) greatly decreased, indicating the structure unit with −OCH3 groups in lignin were degraded at this step. The above structural variation in components might also take place in the torrefaction pretreatment of biomass under different conditions, which altered the formation mechanism and distribution of the final products obtained.3,31 4873


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ACS Sustainable Chemistry & Engineering Table 4. Yield of Small Molecular Weight Products via One-Step and Two-Step Torrefaction entry

compound

RT-120 °C

120-200 °C

ΣRT-200 °Ca

RT-200 °C

Σmonomers glucose fructose lactic acid formic acid acetic acid levulinic acid furfural 5-HMF 2-furanmethanol 5-methylfurfural Σmonophenolsc 2,3-dihydrobenzofuran phenol 4-ethylphenol 4-vinylphenol guaiacol 4-methylguaiacol 4-ethylguaiacol 4-vinylguaiacol 2,6-dimethoxyphenol 4-allyl-2,6-dimethoxyphenol vanillin syringaldehyde

0.83 0.18(0.008) 0.18(0.008)

1.60 0.14(0.005) 0.56(0.02) 0.41(0.01) 0.44(0.02) 0.34(0.01) 0.38(0.01) 0.05(0.002) 0.03(0.001) 0.02(0.001) 0.04(0.001) 2.52 0.05(0.001)

2.43 0.32(0.01) 0.74(0.03) 0.41(0.01) 0.44(0.02) 0.64(0.02) 0.38(0.01) 0.12(0.004) 0.25(0.01) 0.09(0.004) 0.05(0.002) 2.55 0.056(0.001) 0.005(0.002) 0.02(0.001) 2.14(0.05) 0.005(0.002) 0.006(−) 0.004(−) 0.15(0.003) 0.04(0.001) 0.002(−) 0.12(0.002) 0.006(−)

2.67 0.67(0.04) 0.12(0.007) 0.20(0.01) 0.67(0.04) 3.03(0.18) 0.28(0.02) 0.11(0.007) 0.21(0.01) 0.03(0.002) 0.03(0.002) 1.11

b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

0.30(0.01) 0.004(0.002) 0.22(0.01) 0.07(0.003) 0.01(0.001) 0.03 0.006(−) 0.005(0.002)

0.02(0.001) 2.14(0.05) 0.005(0.002) 0.002(−) 0.004(−)

0.004(−) 0.15(0.003) 0.04(0.001)

0.002(−) 0.12(0.002) 0.006(−)

0.55(0.04) 0.44(0.03) 0.04(0.002)

0.08(0.006)

a

The accumulated yield of small molecular weight products via two-step torrefaction (wt %). bThe accumulated yield of small molecular weight derived from cellulose according to the carbon balance (wt %). cThe accumulated yield of monophenols derived from lignin (wt %).

Figure 3. Effect of one-step and two-step torrefaction on the molecular weight distribution of liquid products.

via ring-opening, dehydration, and elimination reactions. The evolution of water mainly derived from the dehydration and elimination reactions rather than simple dewatering step, which might result from the scission of the glucosidic bond between polysaccharide structure units in cellulose and the cleavage of hydroxyl groups in the aliphatic side chains in lignin. Meanwhile, water could be further generated in the process of ring-opening and rearrangement reactions.37 The monophenols derived from lignin were nearly negligible accompanied by partial conversion of lignin, resulting in the products mainly existing in the form of oligomers derived from the degradation of lignin. For the second torrefaction step at 120−200 °C, fructose (0.56 wt %), lactic acid (0.41 wt %), and levulinic acid (0.38 wt %) were the cellulose-based products detected in bio-oil. The content of 4-vinylphenol increased to 2.14 wt %, 4-vinylguaiacol to 0.15 wt %, and vanillin to 0.12 wt %. The bond fragmentation in the phenylpropane side chains and/or the conversion of guaiacol-type to phenol-type released CO2, which could accelerate the production of 4-vinylphenol.38 The

cleavage of the acetyl group in lignin in the formation process of 4-methylguaiacol;35 in the present work, the products of acetic acid were attributed to the conversion of cellulose as a result of low conversion of major components over the low temperature range. In addition, 4-vinylphenol (0.55 wt %) and 4-vinylguaiacol (0.04 wt %) could be derived from the Cα−OH units of lignin via dehydration into CαCβ and the cleavage of coniferyl aldehyde and alcohol side chains. With the cleavage of saturated alkyl side-chain structure, 4-methylguaiacol (0.44 wt %) possibly formed via the cracking of Cα−Cβ bonds. Vanillin (0.08 wt %) likely derived from the considerable precursor with α-carbonyl group through the cleavage of Cα−Cβ in guaiacyl units. For the first torrefaction step at RT−120 °C, the main small molecular weight products in bio-oil were glucose (0.18 wt %), fructose (0.18 wt %), acetic acid (0.30 wt %), and 5-HMF (0.22 wt %). The monosaccharides might be originated from the cleavage of β-1,4-glycosidic linkage in cellulose36 and were further converted to small molecular weight compounds (acetic acid, furfural, 5-HMF, 2-furanmethanol, 5-methylfurfural, etc.) 4874


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ACS Sustainable Chemistry & Engineering cracking of Cα−Cβ, Cβ−Cγ bonds on saturated and α-carbonyl alkyl side-chains might promote the formation of 4methylguaiacol, 4-vinylguaiacol, and vanillin. From what have been discussed above, it could be inferred that stepwise torrefaction greatly influenced the product distribution. For one-step torrefaction, slightly more amount of cellulose was converted, and it was favorable to obtain more amount of glucose (0.67 wt %), formic acid (0.67 wt %), acetic acid (3.03 wt %), etc. A greater amount of lignin was converted via two-step torrefaction and the amount of 4-vinylphenol (2.14 wt %), 4-vinylguaiacol (0.15 wt %), and vanillin (0.12 wt %) attained were relatively higher than those from one-step torrefaction. Analysis of Oligomer-Based Liquid Products. The liquid products were analyzed by gel permeation chromatography (GPC) (Figure 3) and electrospray ionization-mass spectrometry (ESI-MS) (see Figures S6−S8). GPC analysis provided the detailed data on weight-average and numberaverage molecular weights (see Table S1). In one-step torrefaction, the products with a molecular weight ranged from 200 to 400 Da were 57%, while the intermediates with the molecular weight ranged from 400 to 800 Da were 25%. The bio-oil with the molecular weight above 800 Da accounted for 14%. In addition, the liquid products of low molecular weight (below 200 Da) were only 4%, which was approximately equal to the total yields (see Table 4) of the small molecular weight products. The detailed data on weightaverage and number-average molecular weights were listed in Table S1. Polydispersity of the liquid products was 1.24−1.02 at one-step torrefaction. For the first torrefaction step at RT−120 °C, the products with low molecular weight below 200 Da were only 1%, which was consistent with the total yields of the small molecular weight products. The products with molecular weight ranged from 200 to 400 Da accounted for most of the products (60%), while the intermediates with the molecular weight ranging from 400 to 800 Da was 33%. The polydispersity of the liquid products was slightly larger (1.67). When the torrefaction was performed from 120 to 200 °C, the products with molecular weight below 200 Da increased to 3%.While the oligomers with molecular weight from 200 to 400 Da accounted for 61%, and the intermediates with the molecular weight ranged from 400 to 800 Da reduced from 33% to 24%. The products with the molecular weight above 800 Da also existed, and the polydispersity of the bio-oil was 1.22−1.02. Via two-step torrefaction the polydispersity of products slightly decreased, suggesting that the molecular weight distribution became more concentrated with the increase of temperature. That is to say, most of the degraded cellulose and lignin mainly existed in the form of oligomers.39,40 The results of GPC analysis indicated that low temperature stepwise torrefaction was favorable for the formation of oligomers with molecular weight ranged from 200 to 800 Da. Furthermore, 2D HSQC NMR was employed to analyze the lignin fraction in bio-oil (see Table S2).41,42 The 2D NMR spectra exhibited two regions, which were aromatic C−H correlation (δC/δH 95−150/5.5−8.0 ppm) regions and the aliphatic side-chain C−H correlation (δC/δH 50−95/2.5−6.0 ppm) regions. As shown in Figure 4, signals from phydoxyphenyl (H), guaiacyl (G), and syringyl (S) lignin units were observed, especially intermediates derived from the phydoxyphenyl (H) unit were more concentrated in the liquid fraction via one-step torrefaction at RT−200 °C. In the

Figure 4. 2D HSQC NMR spectra (aliphatic and side-chain region) of liquid fraction obtained from one-step torrefaction at RT−200 °C.

aliphatic side-chain C−H correlation regions, methoxyl correlation was the most prominent signal. For the first torrefaction step at RT−120 °C, the signals were significantly affected by the water contained in bio-oil. As shown in Figure 5, signals from cellulose degraded products,

Figure 5. 2D HSQC NMR spectra (aliphatic and side-chain region) of liquid fraction obtained from the first torrefaction step at RT−120 °C.

that is, the signals of pyranose OH, were detected. On the other hand, the lignin-based products with signals of ferulate (F) and β-O-4 (β-aryl ether) also appeared. Ferulate (F) was the only unit concentrated in the liquid fraction, and the guaiacyl (G) and syringyl (S) units were barely. This confirmed the fact that nearly no monophenol was detected by GC in the first torrefaction step. In the second torrefaction step at 120−200 °C, signals of phydoxyphenyl (H), guaiacyl (G), and syringyl (S) units in the lignin substructure appeared. Figure 6 also exhibited other signals of ferulate (F) and 4-O-5 linkages in lignin, which suggested that two-step torrefaction greatly affected the torrefaction performance of lignin. In the aliphatic side-chain C−H correlation regions, methoxyl correlation, β-O-4 (β-aryl ether) and β-5 (resinol) signals strengthened gradually in the 4875


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linkage, while the cracking of β-O-4 promoted the formation of oligomers with 4-O-5 linkage generated (m/z 415). The further cleavages of β-O-4 and C−O−C intermolecular linkages were in favor of the formation of various intermediates with pcoumarate (H) and ferulate (F) units. The intermediates with p-coumarate (H) and ferulate (F) were also derived from the cracking of Cγ−O−CAr and dehydration process. The β-O-4 (m/z 255, 279), p-coumarate (m/z 164, 177), and ferulate (m/ z 182, 195, 209) were the main intermediates derived from lignin via the one-step torrefaction process. Figure S5 demonstrated the ESI-MS results from RT to 120 °C, and the peaks of m/z 98, 116, and 163 (H+) corresponded to furfural, pyranose, and levoglucosan, which originated from the conversion of amorphous cellulose. The peak of m/z 339 could be also assigned to the cellulose-based oligomers. The lignin-based oligomers were almost the same as those from one-step torrefaction, where the intermediates of m/z 255 (H+) and 256 might be the intermediates of β-O-4 linkages. There also existed oligomers with β-O-4 and 4-O-5 linkages, which presented in ESI-MS with the peaks of m/z 255(H+), 359 (H+), and 415 (H+). The proposed formation processes of various products were presented in Scheme 2, where the formation of

Figure 6. 2D HSQC NMR spectra (aliphatic and side-chain region) of liquid fraction obtained from the second torrefaction step at 120−200 °C.

second torrefaction step. The cleavage of β-O-4 linkage in lignin was responsible for the formation of various oligomers. Proposed Formation Route of Lignin-Based Oligomers. High-resolution mass spectrometry (ESI-MS) was used to detect the compounds in bio-oil.43,44 The biooils analyzed by ESI-MS spectra were divided into two major groups: the small molecular weight compounds are located in a first group at m/z between 80 and 200 Da, while the oligomers could be detected in a second group between 200 and 800 Da. Obviously, the oligomers with m/z among 200 and 800 Da were the main products obtained at each temperature range, which was in accordance with GPC analysis. Further detailed description of oligomers was shown in Figure S4. Combined with the results of 2D HSQC and ESIMS, a mechanistic scheme was tentatively proposed. In onestep torrefaction, the peaks at m/z 96 and 162 (163 H+) were the furfural and levoglucosan derived from the conversion of cellulose, while m/z 339 might be the cellulose-based oligomers. The formation mechanism of anhydro-sugar oligomers was demonstrated in the literature.45,46 The other peaks (m/z 80−200 Da) were the small molecular weight products confirmed by GC at one-step torrefaction. As shown in Scheme 1, the cracking of C−O−C intermolecular ether bonds promoted the formation of oligomers with a β-O-4

Scheme 2. Formation of Lignin-Based Oligomers Derived from the First Torrefaction Step at RT−120 °C

β-O-4 linked oligomers might derive from the cracking of the C−O−C bond. The oligomers with a 4-O-5 linkage possibly originated from the cracking of the β-O-4 bond. Accompanied with the cleavage of Cβ−Cγ and the dehydration reaction, various oligomers with β-O-4 and 4-O-5 were generated. For the second torrefaction step at 120−200 °C, furfural (m/ z 96), anhydrosugar (m/z 146), and oligosaccharide (m/z 339) were presented in Figure S6. The small molecular weight products presented in ESI-MS were nearly consistent with the GC results. The intermediates mainly consisted of 4-O-5, β-O4, and β-5 linkages in lignin. The proposed cleavage route of lignin oligomers via the second torrefaction step were shown in Scheme 3, where the cracking of Cβ−Cγ and the dehydration reaction might promote the oligomers linked with β-5 and C− O−C ether bonds (m/z 510). The cracking of ether bonds promoted the β-5 linkage and methyl ferulate (m/z 208) intermediates obtained. The cracking of the 4-O-5 linkage and C−O−C promoted the oligomers with β-O-4 linkage (m/z 434), and the further cracking of β-O-4, Cα−Cβ, and the dehydration reaction facilitated the formation of intermediates of the β-O-4 linkage (m/z 255, 281) and p-hydoxyphenyl. The intermediates with a 4-O-5 linkage derived from the cracking of β-O-4 bonds. The various oligomers with 4-O-5 linkages (m/z 359, 415) were detected mainly via the cracking of β-O-4 and the Cβ−Cγ bond.

Scheme 1. Formation of Lignin-Based Oligomers Derived from One-Step Torrefaction at RT−200 °C

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ACS Sustainable Chemistry & Engineering Scheme 3. Formation of Lignin-Based Oligomers Derived from the Second Torrefaction Step at 120−200 °C

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CONCLUSIONS Oligomers with molecular weight between 200 and 800 Da were the main products formed in all the torrefaction processes. For one-step torrefaction, the cracking of intermolecular and intramolecular hydrogen bonds in the cellulose structure unit promoted the formation of oligosaccharide, anhydrosugar, and levoglucosan intermediates. The oligosaccharide were further degraded to small molecular weight products (monosaccharide, carboxylic acid, furans, etc.) via the cleavage of hydrogen and β1,4-glycosidic bonds, ring-opening, elimination reactions, etc. The oligomers with 4-O-5, β-O-4 linkages were mainly derived from the cleavage of C−O−C bonds between lignin and cellulose, and the intermediates of p-hydoxyphenyl (H), ferulate (F) were derived from the cracking of β-O-4 and Cα−Cβ in lignin. The cracking of Cα−Cβ bonds and dehydration in 4-O-5, β-O-4 linkages promoted the formation of 4-vinylphenol, 4-vinylguaiacol, 4-methylguaiacol, and vanillin, etc. The first torrefaction step tended to generate the intermediates of oligosaccharide, levoglucosan, and pyranose with monosaccharide, acetic acid, and furans, etc., which mainly derived from the degradation of amorphous cellulose. The intermolecular hydrogen bonds and ether bonds were the initial cracked linkages at RT−120 °C. The lignin-based oligomers with 4-O-5 and β-O-4 linkages were mainly derived from the cracking of β-O-4 and the dehydration process, and there were a few monophenols formed in this step. Amorphous cellulose and the guaiacyl unit in lignin were the initially degraded structure at RT−120 °C. For the second torrefaction step at 120−200 °C, the cracking of intramolecular hydrogen bonds in amorphous cellulose promoted the formation of oligosaccharide and anhydrosugar intermediates with monosaccharide, carboxylic acid, and furans, etc. While the oligomers with β-5, 4-O-5, and β-O-4 linkages were derived from the cracking of the Cα−Cβ bonds and the dehydration process. The cleavage of C−O−C bonds promoted the formation of ferulate (F) intermediates. The further cracking of Cα−Cβ and Cβ−Cγ bonds facilitated the formation of various monophenols, that is, 4-ethylphenol, 4vinylphenol, 4-methylguaiacol, 4-vinylguaiacol, 2,6-dimethoxyphenol, and vanillin, etc.

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Experimental schematic diagram, Thermogravimetric (TG) curve of pubescens, SEM micrographs of pubescens and solid residues, XRD graphs of pubescens and solid residues, online MS spectrometry of the off-gas during torrefaction processes, GPC analysis of weight-average molecular weight (Mw) and number-average (Mn) molecular weight, and ESI-MS spectra of liquid products derived from torrefaction of pubescens (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/fax: (+86) 02885411105. ORCID

Changwei Hu: 0000-0002-4094-6605 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (973 Program, Grant No. 2013CB228103). The characterization from the Analytical and Testing Center of Sichuan University was greatly appreciated.

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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.7b00283. 4877


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Low-Temperature Torrefaction of Phyllostachys heterocycla cv

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