Selective low-temperature pyrolysis of microcrystalline cellulose to produce levoglucosan and levoglucosenone in a fixed bed reactor

Fuel Processing Technology 167 (2017) 484–490

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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Research article

Selective low-temperature pyrolysis of microcrystalline cellulose to produce levoglucosan and levoglucosenone in a fixed bed reactor

MARK

Huiyan Zhang, Xin Meng, Chao Liu, Yao Wang, Rui Xiao⁎ Ministry of Education of Key Laboratory of Energy Thermal Conversion and Control, School of Energy and Environment, Southeast University, Nanjing 210096, PR China.

A R T I C L E I N F O

A B S T R A C T

Keywords: Biomass Microcrystalline cellulose Bio-char Selectively catalytic pyrolysis Levoglucosenone Fixed bed

Selective low-temperature pyrolysis of microcrystalline cellulose was carried out to produce levoglucosan (LG) and levoglucosenone (LGO) using a fixed bed reactor. The effects of temperature, self-produced bio-char, different catalysts and catalytic pattern on product's yields and selectivities were studied. The results showed the self-catalysis of bio-char increased LGO yield from 0.14 wt% to 1.35 wt% by increasing almost 10 times with reaction feedstock mass increasing 3 times. The maximum LGO selectivity of 77.67% was obtained with H3PO4 catalysts at 270 °C. Through comparing different catalysts including H3PO4, H2SO4, Fe2(SO4)3 and FePO4, the highest LGO yield of 2.65 wt% was obtained with Fe2(SO4)3. Acid catalysts can obviously increase LGO selectivity and in-situ pattern performed better than ex-situ pattern during the pyrolysis for LGO. Finally, the mechanism with bio-char effect for producing LGO and LG from selective pyrolysis of microcrystalline cellulose was proposed.

1. Introduction Fast pyrolysis of biomass, which can convert solid biomass into biooil in several seconds, is one of most promising technologies for biomass utilization [1,2]. The bio-oil includes a lot of high-value oxygenated chemicals, which offers a useful way to produce chemicals from biomass. However, because bio-oil contains hundreds of organic compounds and most of the components are in very low selectivities, it is uneconomical to obtain value-added chemicals especially pure chemicals from bio-oils directly by separation. How to get a specific bio-oil with high selectivities of one or several high-value chemicals is one of most important issues for the process development. Selective pyrolysis technology has been proposed to improve the selectivities and yields of some chemicals via directional control of the pyrolysis process [3–5]. Suitable raw materials, specific catalysts, proper feedstock pretreatment, coke adjustment and appropriate pyrolysis conditions should be taken into consideration in the process [6–10]. Herein two important oxygen-content chemicals, that is levoglucosan (LG, 1,6-anhydro-β-D-glucopyranose) and levoglucosenone (LGO, 1,6-anhydro-3,4-dideoxy-β-D-pyranosen-2-one), were chosen as the targeted products during biomass selective pyrolysis process. Because of complex structures and their high oxygen-content, these two chemicals usually producing from petrochemicals via cyclization followed by several catalytic oxidation reactions [11,12]. Catalytic oxidation reaction is regarded as one of the most difficult reactions in controlling

⁎

Corresponding author. E-mail address: [email protected] (R. Xiao).

http://dx.doi.org/10.1016/j.fuproc.2017.08.007 Received 17 January 2017; Received in revised form 2 August 2017; Accepted 3 August 2017 0378-3820/ © 2017 Published by Elsevier B.V.

concern, which results in the high price of LG and LGO. LG having the price about 100 dollars per gram is used as raw material in the synthesis of esters, ethers, film, adhesive, UV-polymer, etc. [13]. The price of LGO is ten times more than that of LG. LGO mainly has three reaction centers, including carbon‑carbon double bond, carbonyl and glycosidic bond, which make it widely used in chiral synthesis as different bioactive compounds, disaccharides, chiral inductors, tetrodotoxin, etc. [14,15]. On the other hand, cellulose includes 1,4-linked β-D-glucan units, which can be easily converted to LG and LGO by depolymerization and dehydration reactions using selectively catalytic pyrolysis technology [16–19]. It is proved that LG is one of the main products during fast pyrolysis of cellulose, while there is very little LGO existed in the bio-oil [20–22]. Many studies have been conducted to increase LGO yield by adding proper catalysts. Dobele et al. investigated catalytic pyrolysis of cellulose by impregnating phosphoric acid using Py-GC/MS and 34% LGO was obtained at 2% phosphoric acid solution [23]. Zandersons et al. investigated LGO production from birch lignocellulose pyrolysis with orthophosphoric acid by Py-GC/MS, and the selectivity of LGO in volatile products from lignocellulose was 29.8% at 375 °C [24]. The literatures show that suitable pretreatment of raw materials can visibly increase the production of LGO. Meanwhile, solid super acids were reported to have the preferable effects on LGO. Lu et al. tested SO42 −/ TiO2–Fe3O4, and Wei et al. tested TiO2, ZrO2, SO42 −/ZrO2 and SO42 −/ TiO2 for producing LGO using Py-GC/MS, and the highest LGO

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selectivity of 71.1% with SO42 −/TiO2-Fe3O4 was obtained [25]. Zhang et al. studied selective pyrolysis of biomass with solid phosphoric acid catalysts, and the results showed the maximum LGO yield of 8.2% from poplar wood was obtained at the pyrolysis temperature of 300 °C and the catalyst-to-biomass ratio of 1 [26]. In addition, the effects of ionic liquid for LGO production were also reported [27,28]. It is shown that LGO yield and selectivity can be significantly improved if the reaction conditions were adjusted properly. However, a lot of studies were performed using Py-GC/MS, it is a very small pyrolysis tube directly connected with GC/MS analysis. Only less than 0.5 mg biomass can be converted [29]. The selective pyrolysis of cellulose to produce LGO should be tested in a fixed bed or fluidized bed to commercialize the process. LGO with relatively complicated structure can be regards as one of intermediates of biomass pyrolysis. So its yield is strongly influenced by the amount of feedstocks (more vapors concentration, more secondary cracking). That is, the collision of produced LGO with other compounds in vapors would reduce LGO yield significantly since LGO is very active in high temperature. We can reduce the collision by enhancing carrier gas flow rate in a fixed bed. However, we need high vapor concentration in pyrolysis flue gas to make sure enough partial pressure for condensation. Thus, there is a conflict between the less collision of produced LGO with other compounds and higher vapor concentration for condensation during performing in a fixed bed or fluidized bed. Besides, There are more factors needed to be considered in the experiments of fixed bed, such as retention time, fast condensation method, etc. Besides, the amount of continuously generated bio-char has significantly effect on LGO yield and selectivity. Therefore, the influence of continuously generated bio-char and its functional mechanisms should be studied systematically. In this work, a lab-scale fixed-bed reactor was setup to investigate the effect of continuously generated bio-char during fast pyrolysis on the yields and selectivities of LGO and LG. The effects of various factors including the amount of bio-char, a series of temperatures and different catalysts on the pyrolytic product yields and selectivities were investigated. The proper amount of bio-char, catalyst, temperature for producing LGO and LG from cellulose were obtained. Finally, a proper reaction mechanism including bio-char effect of cellulose catalytic pyrolysis to produce LG and LGO was proposed according to the experimental results.

solution of 20 g/L were prepared. Then the same amount of screened cellulose was added into the solutions, respectively. The suspensions were stirred at room temperature for 6 h, then filtered and dried in a vacuum oven (− 0.1 MPa) at 40 °C for 24 h. All the impregnated samples were grounded and seized out to 0.1–0.3 mm in particle size for experiments. 2.3. Catalytic conversion experiments Fig. 1 shows the schematic diagram of a fixed bed for catalytic pyrolysis of microcrystalline cellulose to produce LG and LGO. It mainly consisted of a gas-supplying unit, a fixed bed reactor, an electrical furnace, a temperature controlling unit, three condensers, two filters and a gas-collecting unit. The fixed bed reactor was made of quartz glass, with the inside diameter and height of 36 mm and 30 mm for the reaction zone, respectively. In each experiment, 4 g feedstock was added into the reactor through the feed hopper. Pure nitrogen (99.999%) was used as carrier gas with the flow rate of 160 mL/min. A mass flow controller was used to control the flow rate of the carrier gas. Before the experiment started, the reactor was first purged with nitrogen for a certain time. A cylindrical furnace was employed to supply the heat needed in the pyrolysis reactions. The product vapors were passed through three condensers to collect liquid products. Following the condensers, a cotton filter and silica gel filter were used to ensure all the condensable vapors were captured. The non-condensable gas fraction was collected by gas-sampling bags for analysis. The feeding rate and total pyrolysis time were 0.4 g/min and 15 min, respectively. The condensed liquids were trapped to be analyzed by GC/MS (Agilent, 7890A-5975C) to obtain accurate compounds. An HP-5MS capillary column (30 m × 0.25 mm × 0.25 μm) was employed to separate the components of products. High-purity helium (99.999%) was used as the carried gas with a constant flow rate of 1.0 mL/min. The temperature of the injector was held at 280 °C. The injection volume was 1 μL with split ratio of 1:60. The GC oven was programmed with the following temperature regime: hold at 50 °C for 2 min, ramp to 290 °C with 8 °C/min and hold at 290 °C for 1 min. Some typical operating conditions were ionization energy of 70 eV and scan per second over the electron range of (m/z) 15–550 amu. The chromatographic peaks were identified according to the NIST MS library v2.0. The liquid product was also analyzed by gas chromatography (Shimadu 2014 GC) to obtain the real yield of specific compounds by external standard method. A 30 m × 0.25 mm × 0.25 m fused-silica capillary column (SE54, China) was employing. The carrier gas was helium (99.999%) with the flow rate of 6.3 mL/min. The temperature of the injector was 280 °C. The injection volume was 1 μL in split mode with split ratio of 1:2. The detailed operating conditions were as follows: the oven temperature was maintained at 40 °C for 3 min and then heated to 180 °C at a rate of 5 °C/min, holding for 2 min, finally heated to 280 °C at a rate of 10 °C/min and maintained for 2 min at this temperature. For each spectrogram, the absolute peak area and relative peak area (%) of each kind of products was recorded. In this work, selectivities and yields which were used to evaluate the production of chemicals are defined as follows:

2. Materials and methods 2.1. Materials Microcrystalline cellulose chosen as feedstock was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). The particle of cellulose was screened ranging of 0.2–0.3 mm. The proximate and ultimate analysis was shown in Table.1. Chemicals, including sulfuric acid (AR, 98%), phosphoric acid (AR, 85%), Fe2(SO4)3 (AR, Fe 21.0–23.0%) and FePO4 were supplied by Sinopharm Chemical Reagent Co., Ltd. (China). 2.2. Impregnation methods Cellulose was impregnated with catalysts using filtration method. According to the former studies which were conducted in Py-GC/MS [30], phosphoric acid of 10 wt%, sulfuric acid of 1 wt% and Fe2(SO4)3

Selectivity =

Yield = Table 1 Proximate and ultimate analysis of cellulose. Raw materials

Cellulose

Proximate analysis (war%)

Peak area of one compound × 100% Peak area of all compounds

Mass of one compound × 100% Mass of feedstocks

Ultimate analysis (war%)

3. Results and discussion

moisture

ash

volatiles

fixed carbon

C

H

O

3.1. Comparison of non-catalytic and catalytic pyrolysis

4.00

0.16

90.44

5.40

41.57

6.27

52.16

Direct pyrolysis of cellulose can produce chemicals such as furfural, 1,6-anhydrosaccharides, furan, etc. Non-catalytic and catalytic 485

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Fig. 1. The schematic diagram of a fixed bed for selective pyrolysis of microcrystalline cellulose to produce levoglucosan and levoglucosenone. 1. N2; 2. Mass flow controlling box; 3. Mass flow controller; 4. Feed hopper; 5. Temperature controller; 6, 7. K type thermocouple; 8. Electrical furnace; 9. Fixed bed reactor; 10. Condensers; 11. Cotton wool filter; 12. Silica gel filter; 13. Gas-sampling bag.

pyrolysis of cellulose were compared in this work. In this part, GC/MS was used to analyze the change of products' distribution. 4 g feedstock was put into the fixed bed under the temperature of 350 °C. The main compounds in bio-oil from pyrolysis reaction identified by GC/MS were displayed in Table. 2. It is indicated that LGO was a minor product while the main product was LG from pyrolysis of cellulose. It is consistent with results of experiments in former Py-GC/MS research [29]. Meanwhile, the distribution of products was complicated. As shown in Table. 2, The total selectivity of LG, LGO, 1,4:3,6-Dianhydro-à-D-glucopyranose and 1,6-Anhydro-à-d-galactofuranose was occupied more than 80%. While catalyzed by H3PO4, the production of LGO significantly increased to be the main product. It is shown that the selectivity of LGO was 41.27%, and was about 10 times higher than that in the non-catalytic run. LG almost disappeared under the catalytic effect, and some furan and furanone compounds were generated. Therefore, catalytic effects were highly effective to produce LGO.

Table 2 The main compounds in bio-oil identified by GC/MS from non-catalytic and H3PO4 catalytic pyrolysis at 350 °C. Compounds

Ethanone, 1-(2-furanyl)Furfural 2(5H)-Furanone, 5-methyl2(3H)-Furanone, 5-methyl2-Furancarboxaldehyde, 5methylEthoxyacetaldehyde diethylacetal 2(5H)-Furanone, 5-methyl2-Furancarboxaldehyde, 5methyl1,2-Cyclopentanedione, 3methylPentanoic acid, 4-oxo-, ethyl ester Pentanoic acid, 4-oxo2-Furaldehyde diethyl acetal 3-Furancarboxylic acid, methyl ester 1,3-Cyclohexanedione, 2-methylLevoglucosenone 2-Pentanone, 5,5-diethoxy1-Butanol, 2-methyl-, propanoate 1,4:3,6-Dianhydro-à-Dglucopyranose 2,4:3,5-Dimethylene-l-iditol cis‑1,2-Cyclohexanediol 2-Butene-1,4-diol, (Z)Undecanol-5 2-Butene, 1,4-diethoxy3-Cyclopentene-1,2-diol, cis‑ á-D-Glucopyranose, 1,6-anhydroà-D-Glucopyranose, 4-O-á-Dgalactopyranosyl1,6-Anhydro-à-d-galactofuranose cyclohexane, 1,1′-[1,2ethenediyl]bisCyclohexanol,2-methyl-5-(1methylethenyl)6-Undecyl-5,6-dihydro-2Hpyran-2-one Sucrose, 8TMS derivative

Non-catalytic

Catalytic

Peak area (× 107)

Peak area (%)

1.49

1.37

0.53

0.49

Peak area (× 107)

Peak area (%)

36.70 125.44 63.34 9.49 82.11

1.40 4.78 2.41 0.36 3.13

19.75 13.59

0.75 0.52

15.93

0.61

11.90

0.45

0.49

0.45

38.58 488.17 12.30

1.47 18.59 0.47

5.26

4.86

16.62 1083.75 28.47

0.63 41.27 1.08

0.33 11.29

0.31 10.42

103.17

3.93

0.41

0.38 20.74

0.79

3.08 2.38 1.93 0.48 70.45 0.60

2.84 2.19 1.78 0.44 64.99 0.56

261.87

9.97

6.81 1.09

6.29 1.00

166.30

6.33

0.44

0.40 9.97

0.38

18.09

0.69

1.34

1.24

3.2. Effects of pyrolysis biochar on the yields and selectivities of LG and LGO During the process of continuous feeding, bio-char was generated constantly. Char has special catalytic effects on the distribution of pyrolytic products. The pyrolysis experiments in this part were conducted at the temperature of 350 °C. The impacts of bio-char were investigated in this work by controlling different feed quantity. The yields and selectivities were based on the results of GC analysis. Fig. 2 showed the selectivities and yields of LGO and LG with different masses of feedstock ranging from 1 g to 5 g. It is obviously seen that LG was always the main product. The selectivity of LG is 56.15% while that of LGO was only 2.49% at the condition of 1 g. With the increasing feed quantity from 1 g to 4 g, the selectivity of LGO was monotonically rising. The highest LGO selectivity was 17.49% at 5 g. In contrast, LG selectivity generally decreased from 1 g to 5 g with the lowest selectivity of 29.41%. The yields of LGO and LG both increased first and then decreased. The yield of LGO increased from 0.14% at 1 g to 1.35% at 4 g by increasing almost 10 times. The yields of LG also had an obvious improvement, and the highest yield of 9.03% was obtained at 4 g. However, if the bio-char was further increased, the production of LG and LGO decreased fast. It can be attributed to that too more bio-char resulted in LG and LGO further cracking. This result indicated that catalytic effects of bio-char on pyrolysis of cellulose were significant, which is promising for the production of LGO and LG. The bio-char obtained in pyrolysis process mainly contains volatile matter because cellulose nearly has no ash and little fixed carbon. During the reaction process, the chemical bonds of biomass 486

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(a)

60

LGO LG

70

Selectivity (%)

Selecticity (%)

50

(a)

80

40 30 20 10

LGO LG

60 50 40 30 20 10 0

0 1

2

3

4

250

5

300

Mass of feedstocks (g)

400

450

(b)

LGO LG

1.5

Yield (%)

8 6

LGO LG

4

500

(b)

2.0

10

Yield (%)

350

Temperature (°C)

1.0

0.5

2

0.0 250

0 1

2

3

4

5

300

350

400

Temperature (°C)

450

500

Fig. 3. The effects of temperature on the yields and selectivities of LG and LGO with H3PO4 as the catalyst.

Mass of feedstocks (g) Fig. 2. The effects of pyrolysis bio-char on the yields and selectivities of LG and LGO.

H3PO4 was less than that produced without catalyst. Temperature is a key factor to influence the distribution of products in the pyrolysis process. Fig. 3 shows the selectivities and yields of LG and LGO from the phosphoric acid catalytic pyrolysis of cellulose with the temperature ranging of 250–500 °C. It can be seen that the selectivity of LGO are much higher than that of LG when the reactions catalyzed by phosphoric acid. According to Fig. 3, the selectivity of LGO first increased and then decreased with increasing pyrolysis temperature. The LGO selectivity was 72.69% at 250 °C, while it increased to the maximum value of 77.67% at 270 °C. Then its selectivity decreased monotonically from 270 °C to 500 °C, and it decreased fast after 400 °C. LGO yield first increased and then decreased with the maximum value of 1.98 wt% at 270 °C. The results indicated that proper lower temperature favors LGO production. LG almost disappeared after pyrolysis under the catalysis of phosphoric acid. The selectivity of LG was no more than 0.5%.

restructured and formed many functional groups in the internal and external surface of bio-char. Carboxyl, ester bond and hydroxyl are the main functional groups of pyrolysis bio-chars, which makes bio-chars have strongly catalytic effects [31,32]. For example, bio-char with carboxyl can be used as solid acid catalysts. The acid effects of bio-char can enhance dehydration reaction and produce LGO. However, overmuch bio-char could result in diffusional limitations originating from mass transport resistance in the catalysis, which led to the prolongation of reaction time, causing a secondary thermal cracking of volatile. Therefore, when the feeding mass was over 4 g in our reactor, the LG and LGO yield decreased fast. 3.3. Effects of temperature on the yields and selectivities of LG and LGO with phosphoric acid as the catalyst The solid and liquid yields at different pyrolysis temperatures were shown in Table.3. It is obvious that solid and liquid yields were in the opposite. The yield of bio-oil increased with increasing temperature, while that of solid bio-char decreased. Liquid yield produced with

3.4. Comparison of different catalysts for catalytic pyrolysis of microcrystalline cellulose to produce LG and LGO in the fixed bed reactor This part of experiments was conducted to investigate the effects of different catalysts and catalytic reaction patterns (in-situ and ex-situ) in the fixed bed at the temperature of 300 °C. Cellulose impregnated with the catalysts is called as “in-situ” pattern. The “ex-situ” pattern was that the catalysts were used as solid catalysts, and the catalysts were put in the reactor before the experiments, and the pyrolysis vapors passed the catalyst layer during the experiments. The combining methods of catalysts and patterns are listed below: H3PO4, H2SO4, Fe2(SO4)3 of in-situ pattern, Fe2(SO4)3, FePO4 of ex-situ pattern and bio-char of self-

Table 3 Solid and liquid yields of microcrystalline cellulose selective pyrolysis with H3PO4 as the catalyst at different temperature. Temperature (°C)

250

270

300

350

400

450

500

Liquid yield (%) Solid yield (%)

38.53 54.32

42.38 49.85

45.79 43.41

48.88 38.85

49.89 36.1

50.01 36.41

53.66 32.7

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80 70

(I ) --in situ (E) --ex stiu

60

Selectivity (%)

3.5. The reaction mechanism for selective pyrolysis of microcrystalline cellulose to produce LG and LGO

(a)

Fig. 5 shows the possible pathway for selective pyrolysis of microcrystalline cellulose to produce LG and LGO. Cellulose is composed of βD-glucopyranose, which is the monomer of cellulose, and they are connected by β-1,4-glycosidic links [33]. There are three main pathways for the conversion of cellulose in catalytic pyrolysis. At first, after the direct fast pyrolysis of cellulose without catalysts, most of the volatiles are converted into LG (P1). During this process, LG is generated by the cleavage of the glycosidic links. Thermal pyrolysis can realize the procedure, and it also can be enhanced by adding proper catalysts, for example weak acid catalysts. The second path is the way to form LGO. The important and essential first step is the dehydration reaction, which hardly occurs without catalysts. In the procedure, IM1 with the carbon‑carbon double bonds is produced through 3-OH obtaining hydrogen [17,25]. Then keto-enol tautomerism reaction occurs and carbon‑oxygen double bonds are formed (IM2). After these conversions, the concerted cleavage of the glycosidic links on both sides takes place and the monomers IM3 are formed. The monomer unit loses a molecule of H2O and then LGO (P2) is formed. The third and the last pathway is that cellulose can be decomposed to form other small molecule products, such as 1,4:3,6-Dianhydro-α-D-glucopyranose, furfural, and other furan compounds, etc. According to the first path, LG is the main products without catalysts. LGO almost does not appear due to the difficulty of dehydration reaction. Other small molecular products are also generated. As indicated in the previous Section 3.1, bio-char has certain catalytic effects and acts as weak solid acid catalysts due to having the linked carboxyl. Bio-char has positive catalytic effects on path one and path two. Because of the weak acidity, the dehydration is promoted to a relatively low extent. The pathway one is still the main path and the cleavage of the glycosidic links is also enhanced through the catalysis. The good catalytic performance of bio-char is worthy of attracting more attention. However, if the pyrolysis is catalyzed by strong acids or metal salts, the pathway two becomes the main reaction path and the catalysis can inhibit another two pathways [20]. Dehydration reaction has higher priority which is necessary for the formation of LGO. The cleavage of the glycosidic links can occur spontaneously under the thermal effects and be enhanced by catalysts.

50 40 30 20 10 0 Bla

nk

4( PO H3

I)

) I) (I) (E) (E 4( 4)3 O4 4)3 SO O P 2 O e S H F 2( 2( S Fe Fe

Catalysts 3.0

(I ) --in situ (E) --ex stiu

2.5

(b)

Yield (%)

2.0 1.5 1.0 0.5 0.0 n Bla

k P H3

(I) O4

I) (I) (E) (E) 4( O4 4 )3 4 )3 SO P 2 O O e H F 2 (S 2 (S Fe Fe

Catalysts Fig. 4. The effect of different catalysts on the yields and selectivities of LG and LGO.

catalysis. The selectivities and yields of LGO were shown in Fig.4. The selectivities of LGO in all runs increased due to the inhibition of other products. Impregnated by acid catalysts, LGO selectivity had significant enhanced over 55%. Ex-situ catalytic pyrolysis with Fe2(SO4)3 increased LGO selectivity to 50.41%. The highest selectivity of 61.4% was obtained with H3PO4 as catalyst. H3PO4 can effectively catalyzed cellulose pyrolysis to produce LGO, whose yield was 2.62 wt%. Although the LGO selectivity with H2SO4 was high, the yield was only 0.93%. In-situ catalytic pyrolysis with Fe2(SO4)3 produced the highest LGO yield of 2.65 wt% with selectivity of 32.7%. It is worth to state that bio-char's self-catalysis obtained 1.35% yield of LGO, which is better than some other catalysts. Considering the different catalytic pattern, the yield of LGO from insitu catalytic pyrolysis is generally better than those of ex-situ. During the in situ catalytic pyrolysis process, the catalysts were physically or chemically absorbed on the surface of feedstocks, and pyrolysis with catalytic reaction occurred compulsively and simultaneously. However, as to ex-situ catalysis, the pyrolysis of cellulose was conducted firstly and then the vapor was in touch with the solid catalysts. The time and space between the two steps could convert some of the vapors into other small compounds. According to the results, it is able to infer that depolymerization and dehydration reactions occur simultaneously during the formation of LGO period.

4. Conclusion Selective pyrolysis was proved to be a promising way to obtain LGO and LG from cellulose using Py-GC/MS. This work is meaningful to conduct the reactions in a fixed bed. The maximum LGO selectivity of 77.67% was obtained with H3PO4 at 270 °C. Both acid catalysts and metal salts have positive effects on LGO production with the highest yield of 2.65 wt% using Fe2(SO4)3. The proposed reaction pathway shows thermal effect produced LG, self-produced bio-char with week acid characteristics enhanced both LG and LGO yields, while strongacid catalysts improved LGO yield and restrained LG yield by accelerating the dehydration reaction.

Acknowledgements The authors acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51676045), Sino-Thai Cooperation Project from the National Science Foundation of China for international academic exchanges (Grant No. 51561145010), the Jiangsu Natural Science Foundation (Grant No. BK20170081) and the Excellent Young Teachers Program of Southeast University (Grant No. 2242016R30005).

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O

O

O HO

O

O

O

O

O

OH

HO

O

O

O HO

Craking

Craking

HO

1 O

2

3

OH

H

4 HO

6 OH 5 O 2

3 HO

n

1 O

H OH

6 OH 5 O 2 1 O

4 HO

1 O OH

H

3

H

n

IM1

6

6 4

O OH 1

Cleavage

5

HO

O 2

O

H n

TS3

Bi och ar

5 3

2

3

n

O H

HO

HO

H

6 OH 5 O

Enol Tautomerism

6 4

4

TS2

Thermal Effect

R

HO

Dehydration

Craking

HO

Acid Catalyst

6 OH 5 O

4

Others

O

HO

O

O

2 O

3

O

HO

O 2 O

3

1

IM3

1

4

H

6 OH 5 O 3

2 O

n

TS4

1 O

H n

IM2

Cleavage

TS1

5

HO

H n

Acid Catalyst

O H

4

5

OH OH 3

O 2OH

P1

6

6

6 4

O 1

4

5

HO H

3

Dehydration

O 2 O

O 1

4

5 3

TS5

O 2 O

O 1

P2

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