Comparison of Acids and Sulfates for Producing Levoglucosan and

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Comparison of Acids and Sulfates for Producing Levoglucosan and Levoglucosenone by Selective Catalytic Fast Pyrolysis of Cellulose Using Py-GC/MS Xin Meng, Huiyan Zhang,* Chao Liu, and Rui Xiao* Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China ABSTRACT: Levoglucosan (LG) and levoglucosenone (LGO) are two high-valued chemicals, which can be produced by catalytic fast pyrolysis of cellulose with proper catalysts. This work investigated the catalytic characteristics of different acids and metal salt catalysts to produce LG and LGO using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetric analysis (TGA). The results showed that weak acids (such as formic acid and acetic acid) enhanced LG yield, whereas sulfuric acid and phosphoric acid promoted the yield and selectivity of LGO with inhibitory effects on LG production. The maximum LGO selectivity of 61.8% was obtained with 10% phosphoric acid, while the maximum LG selectivity of 87.6% was obtained with 10% acetic acid. In comparison with two impregnation methods, the filtration method can increase LGO yield, while the evaporation method enhanced the selectivity of LGO (up to 82.6%) with lower concentration of phosphoric acid. The effects of metal salts on pyrolysis of cellulose were investigated by impregnating with different sulfates and chlorates. The results indicated that sulfates can increase the LGO yield significantly, which can be attributed to the effect of sulfate anions. etc.13−16 Cellulose, which is composed of 1−4-linked β-D-glucan units, is one of the most important components of biomass. The structure of cellulose favors the production of LG and LGO by dehydration reaction with selective catalytic pyrolysis method directly.4,10,17 During direct fast pyrolysis of cellulose or biomass, LG is one of the main products, whereas there is almost no LGO in the products. However, the products contain more than 400 compounds, and LG is hard to separate. Therefore, selective catalytic pyrolysis should be used to improve LG and LGO yields, and selectivities, which saves a lot of energy for their separation. Catalyst is the key factor for LGO production. Until now, some studies have been done to find out suitable catalysts for producing LGO.18−21 Dobele et al. carried out catalytic pyrolysis of impregnated microcrystalline cellulose with phosphoric acid using Py-GC/MS, and the maximum LGO selectivity was 30%.22,23 Dobele et al. also estimated the effect of Fe3+ ions on the LG and LGO from wood, and high contents of LG (27.3%) and LGO (25.7%) were obtained from wood.24 Zandersons et al. carried out catalytic pyrolysis of lignocellulose with orthophosphoric acid, and the highest content of LGO in volatile products from pyrolysis was obtained from lignocellulose (29.8%).25 Branca et al. investigated acid-catalyzed pyrolysis of corncobs to produce chemicals in a packed-bed, and the production of levoglucosenone was significantly enhanced from trace amounts to 4.5%.26 The literature indicates that acid pretreatment can increase the contents of 1,6-anhydrosaccharides by pyrolysis. Meanwhile, studies about solid superacids, such as TiO2, ZrO2, SO42−/ZrO2, SO42−/TiO2, and SO42−/TiO2−Fe3O4, have been also proposed to produce LGO by Py-GC/MS.27−30 In addition,

1. INTRODUCTION Fast pyrolysis is one of the most promising technologies for biomass utilization. Solid biomass can be converted into charcoal, liquid bio-oil, and noncondensable gases.1 Because of the advantages of universality, renewability, easy transport, and high energy density, bio-oil attracts a lot of attention.2 Besides, there are hundreds of organic compounds that exist in bio-oil, including various valuable chemicals, so it is an excellent chance to obtain the specific value-added chemicals.3,4 However, because of the low concentration of all components in bio-oils, the obtainment of value-added chemicals from bio-oils is uneconomical at present. The commercialization of getting valuable chemicals from bio-oils needs to meet the requirement of producing specific bio-oils with high contents of target products. To solve the problem, selective pyrolysis has been put forward to produce some specific chemicals via controlling the process properly, such as choosing suitable raw materials, adding specific catalysts, pretreating feedstocks, and selecting appropriate pyrolysis conditions.5−7 The production of levoglucosan (LG, 1,6-anhydro-β-Dglucopyranose) and levoglucosenone (LGO, 1,6-anhydro-3,4dideoxy-β-D-pyranosen-2-one) has been proposed from biomass or cellulose.8−11 LG, whose price is $110 per gram sold in SigmaAldrich Co. LLC, can be used in the synthesis of valuable chemicals and new materials, including esters, ethers, film, adhesive, and UV-polymer, etc.12 LGO, of which the price is 10 times higher than LG, about $1100 per gram supplied by SigmaAldrich Co. LLC, is easy to have modification and polymerization reactions with oxygen-containing compounds due to the unique character with its carbonyl and olefinic bonds. Besides, because of the distinct structure of the double-ring, LGO has been considered as a promising ingredient with high versatility in modern organic synthesis, for preparing different bioactive compounds, disaccharides, chiral inductors, and tetrodotoxin, © XXXX American Chemical Society

Received: June 13, 2016 Revised: September 14, 2016

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DOI: 10.1021/acs.energyfuels.6b01436 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels the effect of ionic liquid31 was also reported. The above studies show that proper catalysts can increase LGO yield significantly. However, the effects of different acids and metal salts have not been studied systematically, such as the strength of acids, feedstock’s pretreatment methods, and cations and anions performances in the reaction. In this work, catalytic pyrolysis of cellulose with a variety of acid catalysts was carried out using analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetric analysis (TGA). The effects of various factors including different catalysts, a series of concentration, and different impregnation methods on the pyrolytic products’ yields and selectivities were investigated. The effects of acid strength and feedstock’s pretreatment methods on LG and LGO selectivity were disclosed. In addition, several metal salts were also considered to explore the influence of cations and anions. The formation mechanism of LG and LGO was also provided by result analysis.

Figure 1. Experimental sample installation pattern for in situ catalytic fast pyrolysis. In this work, the peak area of every product can be represented by the ratio of the specific peak area and mass of feedstocks, and the selectivities were defined as the ratio of specific peak area and the total peak area: peak area =

specific peak area mass of feedstocks (mg)

selectivity =

specific peak area × 100% total peak area

2. EXPERIMENTAL SECTION 2.1. Materials. Raw material used in this study was microcrystalline cellulose, which was purchased from Sigma-Aldrich Co., LLC. Cellulose particles were sieved to obtain the size less than 75 μm. Chemicals, including formic acid (AR, 98%), acetic acid (AR, 99.5%), hydrochloric acid (AR, 36.0−38.0%), sulfuric acid (AR, 98%), phosphoric acid (AR, 85%), Fe2(SO4)3 (AR, 75%−82%), CuSO4·5H2O (AR, 99%), and FeCl3·6H2O (AR, 99%), were supplied by Sinopharm Chemical Reagent Co., Ltd. (China) and used without further purification. 2.2. Impregnation Methods. Cellulose was impregnated with catalysts in two different ways. The first way was filtration method. Acid solutions (50 mL) of 1, 3, 5, and 10 wt % concentrations were prepared, and the screened cellulose (1 g) was added into these solutions, respectively. The suspensions were stirred at room temperature for 2 h, then filtered, and dried in a vacuum oven (−0.1 MPa) at 40 °C for 24 h. The other method was evaporation method. The suspension liquid was prepared as the former filtration method, then stirred in a magnetic stirrer at 75 °C for 6 h, until most moisture of the suspension evaporated. Subsequently, it was dried in an oven at 40 °C for 24 h in a vacuum condition (−0.1 MPa). Samples were also prepared with Fe2(SO4)3, CuSO4, and FeCl3 solutions of 1, 3, 5, and 10 g/L using filtration method. In this Article, all percentages meant weight percentages, and “%” was used on behalf of “wt %” later. 2.3. Thermogravimetric Analysis (TGA). TGA of cellulose was performed on a thermogravimetric analyzer (ZRT-1, Beijing jingyi hightech Co., Ltd., China). High-purity nitrogen (99.999%) was used as the carrier gas with a flow rate of 50 mL/min. The operation temperature ranged from room temperature to 800 °C with a heating rate of 20 °C/ min. 2.4. Analytical Py-GC/MS Experiments. Analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) experiments were conducted to investigate the pyrolysis characteristics of cellulose using the fast pyrolyzer (CDS 5200) connected with the gas chromatography/ mass spectrometry (GC/MS) instrument (Agilent 7890A/5975C). The experimental sample installation pattern is shown in Figure 1. Cellulose has been loaded as catalysts by the impregnation method. Each experimental sample quality was strict to be 0.5 mg. The whole volatiles were directly analyzed by the coupled GC/MS, of which the injector temperature was held at 280 °C. Helium was the carrier gas with the flow rate of 1 mL/min and a split ratio of 1:60. The capillary column was HP-5MS (30 m × 0.25 mm × 0.25 μm) where the chromatographic separation was performed. The temperature program started from 50 °C and remained for 2 min, and then heated to 290 °C with the heating rate of 8 °C/min, holding for 1 min. The mass spectra were operated in EI mode at 70 eV, which was obtained from 15 to 550 (m/z). The chromatographic peaks were identified according to the NIST MS library v2.0.

According to the former studies,27,28,32 the analytical pyrolysis experiments were carried out at 350 °C with the heating rate of 20 000 °C/s and heat time of 5 s. It is an overall consideration of suitable degradation temperature of cellulose and energy saving. For each experimental condition with different catalysts and concentration, the experiments were conducted repeatedly to confirm the reliability of the reported work. Furthermore, pure cellulose samples without catalysts were also prepared and used for comparison.

3. RESULTS AND DISCUSSION 3.1. Effects of Impregnated Concentration on LG and LGO Production. Sulfuric acid and phosphoric acid were used to study the impact of different concentrations with filtration method for the better production and selectivity of LG and LGO. Because of the flash heating mode (heating rate from 10 to 20 000 °C/s) and tiny amounts of samples (normally less than 1 mg), pyrolysis is relatively complete and secondary reaction or repolymerization of products can be avoided in Py-GC/MS, so qualitative analysis is reliable. Peak areas and selectivities were the evaluation criteria in this work. 3.1.1. Effects of Sulfuric Acid and Phosphoric Acid. Analytical Py-GC/MS experiments of cellulose were carried out under the temperature of 350 °C. LG, LGO, 1,4:3,6dianhydro-α-D-glucopyranose (DGP), and 1,6-anhydro-β-Dglucofuranose (AGF) were analyzed as the main products. According to the ion chromatograms, peak areas of main pyrolytic products that can be identified are listed in Table 1. It is shown that the major identified compounds were saccharides and furan compounds from Table 1, and saccharides occupied more than half proportion. Figure 2a shows the ion chromatograms of sulfuric acid catalytic pyrolysis under the temperature of 350 °C, marked with the selectivity of main saccharides. Impregnated by sulfuric acid, it is visible that the peak areas significantly improved, especially for LG and LGO. The peak area of LG showed a trend of decline with the increasing concentration from 1% to 10%. Although the selectivity of LGO always rose, its peak area also kept slightly decreasing from 1% to 5% and was roughly stable from 5% to 10%. Total peak areas and LG peak area fell sharply, while that of LGO remained stable. After 1% sulfuric acid pretreatment, excellent catalytic effects of considerable LG and LGO production have been shown, B

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Table 1. Peak Area of Compounds from Microcrystalline Cellulose Pyrolysis Catalyzed by Sulfuric Acid and Phosphoric Acid via Py-GC/MS (×107 mg−1) H2SO4 no.

retention time/min

compounds

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

4.83 5.39 6.88 7.50 9.67 9.98 10.13 10.39 10.60 10.91 11.75 12.33 12.68 12.94 13.16 13.30 13.40 13.55 14.22 14.65 18.63 19.71

furfural 2(3H)-furanone, 5-methyl2(5H)-furanone, 5-methyl2-furancarboxaldehyde, 5-methylpentanoic acid, 4-oxo3-furancarboxylic acid, methyl ester furyl hydroxymethyl ketone 3-methyl-2-furoic acid 2-furanmethanol levoglucosenone unknown compound 1 unknown compound 2 2(5H)-furanone, 3-methyl1,4:3,6-dianhydro-anonglucopyranose 2-furancarboxaldehyde, 5-(hydroxymethyl)2(3H)-furanone, 3-acetyldihydrounknown compound 3 2H-pyran-2-one, tetrahydro-6-methyl3-methyl-3-hexene acetaldehyde, O-ethyloximelevoglucosan 1,6-anhydro-β-D-glucofuranose

H3PO4

1%

3%

5%

10%

3.30

2.67

1.01

4.18 1.24 1.13

1.51

1.32

2.19

3.16

2.62 0.50

2.19 0.44

0.81 0.83

1.31 1.89

131.52

117.76

31.60

60.72

4.11

4.36

1.67

2.15

32.82 2.18 0.76

27.08 1.56 0.68

7.12

10.29

2.28 1.74 5.99 223.45 19.27

1.69 1.59 3.93 86.56 11.58

although the selectivity of LGO was not very high. Total proportion of LGO and LG was over 80%. With gradually strengthened dehydration reactions, the selectivity of LG was always decreasing, whereas that of LGO was the opposite, which was shown in Figure 3a. Catalyzed by sulfuric acids, the selectivity of LGO significantly increased up to 61.5% at 10%. Along with the increase of sulfuric acid concentration, the selectivity of DGP increased slightly, and downtrend of AGF was observed. It is seen that GC/MS spectra of phosphoric acid catalytic pyrolysis are in Figure 2b. Main pyrolytic products were also displayed in Table 1. Under phosphoric acid catalysis, it was similar to sulfuric acid that the yield of LGO improved significantly, yet gradually to be the main product of volatile contents. In contrast with sulfuric acid, the catalytic effects were not visible at the low concentration of 1% with the low content of LG and LGO. With the increment of concentration, total production and LG first rose and then declined while LGO was always rising. The trend of selectivities of LG and LGO with increasing concentration was shown in Figure 3b. It is visible that the selectivity of LG decreased very fast, LGO rose both in yield and in selectivity, and all achieved the best value at 10% concentration with the highest selectivity of 61.8%. Phosphoric acid can enhance dehydration reaction and substantially inhibit the other products, so it is the ideal catalysts to obtain oriented high-yield LGO. The selectivity of DGP remained over 5%, while the selectivity of AGF slightly dropped. The impact of phosphoric acid was milder than that of sulfuric acid, and the dehydration reaction occurred prevalently. After phosphoric acid treatment, the hole of cellulose was enlarged, and the glucosidic bonds became easier to fracture to form small molecule products. For both phosphoric acid and sulfuric acid, the highest selectivities were all above 60% under the best condition of 10% concentration. However, the production of LGO from cellulose pyrolysis with phosphoric acid was more

0.89

1.44

4.10 0.55

4.35

1%

3%

5%

10%

7.92

9.74

10.29

1.17 1.85 0.61 4.71

6.29 0.50 19.93 1.2

0.75 1.07 0.70 5.00 0.70 4.47 136.74 1.19 2.43 0.61 19.08 2.96

0.44 0.86 64.2 5.26

0.31 20.90

15.83 1.97

0.57 5.32 0.61 3.63 155.56 1.26 3.48 0.77 22.56 3.57

1.42 1.10

0.67 0.53

1.21 1.22

6.27 122.93 9.68

3.93 63.64 5.22

6.40 30.11 1.35

5.85 134.62 0.51 2.19

than 3 times that of samples impregnated by 10% sulfuric acid solution. 3.1.2. Effects of Enhanced Impregnation Methods. According to the result of the previous section, phosphoric acid can enhance dehydration substantially to improve the production of LGO effectively. Therefore, this study used the evaporation method to further strengthen the phosphoric acid catalytic process. The procedure and result of the first method, which was in the condition of room temperature for a short time, have been discussed in the last section. The second method of evaporation was to impregnate the raw materials for longer time under the heating condition of 75 °C, until most moisture evaporated. The samples then were directly dried in the vacuum oven. Figure 4 shows the comparison of the two methods. After treating samples by filtration method, LGO yield was constantly rising with the increase of concentration. With 5% concentration pretreatment, the selectivity of LGO was the highest and production was also considerable. As for samples treated by the evaporation method, it is noted that the production of LG declined to a very low extent, hardly to be detected. On the contrary, the yield of LGO increased apparently as compared to the noncatalytic. Yet the degree of production improvement was lower than the filtration method, about one-seventh. From the aspect of LGO selectivity, evaporation method was superior to filtration method. The best selectivity of LGO using evaporation method was 82.6% at 5% concentration. As we can see, the production of LGO using evaporation method was lower than that using filtration method, but as compared to the blank, the highest yield still increased by more than 50 times. However, in the aspect of selectivity, evaporation method can inhibit other products visibly to enhance the selectivity of LGO up to 82.6%. It was also the highest selectivity among all operating conditions, so phosphoric acid is the ideal catalyst to produce LGO. C

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Figure 3. Selectivity of main products from cellulose catalytic pyrolysis with sulfuric acid (a) and phosphoric acid (b) at different concentrations.

etry (TG) and differential thermogravimetry (DTG) curves of cellulose loaded with acid catalysts at the heating rate of 20 °C/ min were presented in Figure 5. Three degradation stages can be recognized in the thermogravimetric process of microcrystalline cellulose.34 The first step was from room temperature to 310 °C, when a small amount of weight loss occurred. It was characterized that significant moisture was lost and cellulose depolymerized slightly to the state of glass transition slowly. The second step, ranging from 310 to 390 °C, was the main pyrolysis process, which can be connected with the cellulose devolatilization, generating chemicals such as furfural, 1,6-anhydrosaccharides, and furan. In addition, it can be noted that the last step was correlated with the slow decomposition of residues to leave carbon and ash contents, occurring in the temperature range of 390−800 °C. However, after pretreating with acid catalysts, the main decomposition stage of cellulose shifted to the low temperature area. Catalyzed with three kinds of weak acids respectively, pyrolysis processes only changed a little. The temperature of maximum weight loss rate was advanced to 334, 340, and 340 °C by pretreatment of hydrochloric acid, formic acid, and acetic acid. During pretreatment with phosphoric acid, the main degradation step was observed at temperatures ranging from 220 to 300 °C in advance with maximum weight loss rate temperature of 280 °C. It can be noticed that a stable weight loss rate was still kept over 300 °C. Catalytic pyrolysis of cellulose with sulfuric acid was similar to that of phosphoric acid, and the main degradation period was ahead. However, DTG data shown in Figure 3b indicated that there were two separate rapid weight loss steps of cellulose devolatilization. The first rapid weight loss stage

Figure 2. Ion chromatograms of cellulose catalytic pyrolysis with sulfuric acid (a) and phosphoric acid (b) at different concentrations.

The main effect of the phosphoric acid catalyst is the character of swelling, which leads to an easy diffusion of catalysts and an increased accessibility of reaction centers. Appropriate acid can break the 1,4-glycosidic bond of cellulose molecules to severely damage and decrease polymerization. The amorphous region of cellulose molecules is first destroyed, so reorientation occurs to form a more orderly state in the process. Some of the hydrolysis products are the high-molecular-weight powder cellulose and hydrocellulose, etc., and these hydrolysis products tend to be firmly adhesive on the surface of cellulose.23 If heated slightly, hydrolysis is enhanced to generate hexaose, tetrose, and trisaccharide, and cellobiose and glucose come out at last. Obviously, this kind of intense pretreatment not only leads to the secondary changes of raw materials, but also can affect the results of subsequent pyrolysis.22,33 3.2. Pyrolysis Characteristics of Different Acids Impregnated in Cellulose Using TGA. Cellulose impregnated with sulfuric acid, phosphoric acid, hydrochloric acid, formic acid, and acetic acid in filtration method with the same concentration of 10% was used in this part. The thermogravimD

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occurred at temperatures from 140 to 290 °C. Furthermore, after a short flat period, a second devolatilization step of rapid weight loss was also observed at temperature ranging from 320 to 500 °C, with a slower rate than the former one. Cellulose pretreated with acids, which can advance the starting time of heat decomposition, extend the cellulose degradation process, particularly with sulfuric acid and phosphoric acid. The difficulty of depolymerization and decomposition reaction was reduced dramatically. It is mainly because the hydrolysis process has caused cellulose chain segments damaged to be broken easily, and the surface of cellulose has many low molecular weight chains with a lot of breaking points.35,36 It is favorable to generate small molecular volatile products. 3.3. Effects of Different Acid Catalysts on LG and LGO Production. Followed by the exploration of the slow catalytic pyrolysis characteristic in TGA, fast pyrolysis characteristic was studied via Py-GC/MS. Cellulose was impregnated using the same method as TGA. Peak areas of several main products after impregnation with acids of 10% concentration were shown in Table 2. LG was the main product of cellulose pyrolysis without catalysts. The selectivity of LG was 80.2%, whereas the production was low. Catalyzed by acids, the total yields were all promoted to a certain degree, especially phosphoric acid. The proportion of four main saccharides was nearly beyond 70% among all identified volatile matters except hydrochloric acid. It is obviously indicated that depolymerization was promoted in the catalytic pyrolysis process. From the aspect of LG, all catalysts advanced LG production except sulfuric acid. Acetic acid had particular positive effects on LG; the peak area has increased 6 times in comparison with the noncatalytic, and the selectivity increased to 85.7%. The improvements with formic acid or hydrochloric acid were nonsignificant. Despite a strong acid, hydrochloric acid had weak effects on cellulose pyrolysis because of its volatility during the period of long time impregnation. Pretreated by sulfuric acid and phosphoric acid, the selectivities of LG fell sharply, and the production catalyzed by sulfuric acid was even lower than that of the noncatalytic. However, LGO was totally opposite from LG. LGO peak areas of hydrochloric acid, formic acid, and acetic acid were always staying as low as noncatalytic, whereas sulfuric acid and phosphoric acid can significantly promote LGO production by hundreds of times with the selectivity of more than 60%. Depolymerization and dehydration reactions were carried out during the fast catalytic pyrolysis. On the one hand, previous studies showed that β-1,4glycosidic bonds of cellulose are sensitive to acid, and it is easy to fracture through hydrolysis, so that the cellulose polymerization degree drops rapidly.37 More LG was generated after fast pyrolysis, such as under 10% acetic acid, and the selectivity of LG reached 85.72%. On the other hand, dehydration reactions were catalyzed only to a low extent if acidity was low. Further pyrolysis of cellulose cannot be carried out easily, so LGO yield and selectivity remained low. On the contrary, strong acids like sulfuric acid and phosphoric acid can considerably advance LGO. Because acids worked as catalysts during pyrolysis, catalytic dehydration was enhanced strongly. 22 In addition, the production of LG and LGO was in the opposite direction. High selectivity of LG or LGO meant the low selectivity of the other one. It is concluded that LG is a first-order product of cellulose due to the high yield without catalysts. LGO is a smaller molecule product. LGO can be obtained from cellulose catalytic pyrolysis directly; meanwhile, it also may be from secondary

Figure 4. Peak area and selectivity of phosphoric acid catalytic pyrolysis of cellulose with two impregnation methods: (a) LG; and (b) LGO.

Figure 5. TG (a) and DTG (b) curves of different acids impregnated cellulose catalytic pyrolysis at the heating rate of 20 °C/min.

E

DOI: 10.1021/acs.energyfuels.6b01436 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 2. Peak Area and Selectivity of Main Products from Acids-Catalyzed Pyrolysis of Cellulose at 350 °C catalysts

noncatalytic

sulfuric acid

phosphoric acid

hydrochloric acid

formic acid

concentration (%):

0

10

10

10

10

10

1.26 0.04 0.06 0.10 1.46 80.29 2.29 3.71 6.48 92.78

0.44 6.08 1.02

3.02 15.56 2.26 0.14 20.96 11.94 61.67 8.94 0.53 83.09

3.92 0.10 0.12 0.20 4.34 46.58 1.17 1.45 2.46 51.65

2.52 0.14 0.16 0.18 3.02 73.06 4.15 4.66 5.50 87.37

7.36 0.12 0.14 0.42 8.06 85.72 1.41 1.72 4.96 93.81

peak area/×108 mg−1

selectivity/%

LG LGO DGP AGF total LG LGO DGP AGF total

7.54 4.40 61.47 10.42 76.29

acetic acid

Figure 6. Peak area (left column) and selectivity (right column) of LG and LGO from Fe2(SO4)3, FeCl3, and CuSO4 catalyzed pyrolysis of cellulose: (a,b) Fe2(SO4)3; (c,d) FeCl3; and (e,f) CuSO4.

acid can improve the production of LGO. Therefore, phosphate and sulfate are also studied to produce high-yield LGO. It is shown that Fe2(SO4)3 has advanced effects on LGO production, according to former studies.24,38 Yet it is hard to explain that the

reaction of LG under the appropriate catalytic condition. There exists the transformation path from LG to LGO by catalyst.20 3.4. Effects of Metal Salts on LG and LGO Production. In the previous section, it is seen that phosphoric acid and sulfuric F

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main effects are from metal cations or anions. Considering FePO4 and Cu3(PO4)2 are insolubles, we choose several soluble sulfates and chlorates to be the catalysts loaded on the cellulose surface. Therefore, samples were pretreated with Fe2(SO4)3, FeCl3, and CuSO4 in filtration method. As shown in Figure 6a,b, impregnated with Fe2(SO4)3, dehydration reaction was enhanced during cellulose pyrolysis process and total production significantly increased. As the increment of concentration, the yields of LG and LGO increased. However, when the concentration grew to over 10 g/L, the yield of LG remained stable while the LGO yield kept rising. Therefore, the selectivity of LGO increased from 2.3% to 22.5% at the concentration of 50 g/L, whereas LG was opposite from 83.8% to 35.8%. Meanwhile DGP significantly increased to 16%, and the total sorts of pyrolytic products went down. It is a competitive advantage of selective pyrolysis with the catalysts. However, impregnated with FeCl3, the total and LGO peak area has little improvement, which was shown in Figure 6c,d. In terms of selectivity, LG dropped to 46.2%, and LGO only rose to 9.7%. The LGO promotion of FeCl3 was far less effective than that of Fe2(SO4)3, both in the peak area and in selectivity. The effect of CuSO4 was similar to that of Fe2(SO4)3; the yields of LG and LGO rose first then fell. It is worth mentioning that LGO yield was first higher than LG at 30 g/L, where the highest LGO selectivity (30%) and the lowest LG selectivity (27%) were obtained. Sulfates can advance the yields of LGO and DGP, whereas they inhibit LG.24 However, pyrolysis of cellulose impregnated with FeCl3 cannot produce substantial LGO; meanwhile, the improvement of selectivity was too little. Considering the different effects of hydrochloric acid and sulfuric acid, the promotion of LGO with FeCl3 derives from the Fe3+. For appreciable catalytic effects of Fe2(SO4)3 or CuSO4, it is mainly because sulfates were hydrolyzed and ionized into sulfate anions and metal ions in the solution, so sulfuric acid and weak base worked. FeCl3 solution was also converted to hydrochloric acid and ferric hydroxide. Considering the different effects of hydrochloric acid and sulfuric acid in section 3.3, the weak promotion of LGO with FeCl3 mostly derives from the Fe3+. Yet sulfates can be ionized to generate sulfuric acid, which can promote LGO significantly.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Natural Science Foundation of China (Grant Nos. 51561145010, 51525601 and 51676045), and the Excellent Young Teachers Program of Southeast University.

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REFERENCES

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4. CONCLUSION Selective catalytic pyrolysis of cellulose with proper catalysts can produce high-valued chemicals, such as LGO and LG with high selectivity. Acid catalysts with low acidity can increase LG yield by enhancing the depolymerization of cellulose, and the maximum LG selectivity of 87.6% was obtained with 10% acetic acid. LGO is the product that needs further pyrolysis with dehydration. Sulfuric acid and phosphoric acid had positive effects on the production of LGO, with the highest selectivity 61.8% pretreated by 10% phosphoric acid. Feedstock’s pretreatment methods were also one of the key factors for the reaction. Evaporation method with heating pretreatment resulted in the common increase of LGO yield, but enhanced its selectivity to 82.6% with 5% phosphoric acid. Furthermore, sulfates had positive effects on LGO production, while chlorates did not have the same advantages. Thus, sulfate anions played the main role in catalysis rather than metal ions. It is promising to obtain the optimum catalytic conditions with high content of LGO for highvalue utilization of biomass with high cellulose content. G

DOI: 10.1021/acs.energyfuels.6b01436 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b01436 Energy Fuels XXXX, XXX, XXX−XXX