Direct Hydrogenolysis of Cellulose into Methane under Mild

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Biofuels and Biomass

Direct Hydrogenolysis of Cellulose into Methane under Mild Conditions Haiyong Wang, Caihong Zhang, Qiying Liu, Changhui Zhu, Lungang Chen, Chenguang Wang, and Longlong Ma Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02235 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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Direct Hydrogenolysis of Cellulose into Methane under Mild Conditions Haiyong Wang*

a,b,c,d

a,b,c,d

, Caihong Zhang*

, Qiying Liu†,a,b,c , Changhui Zhua,b,c,d, Lungang Chena,b,c,

Chenguang Wanga,b,c, Longlong Maa,b,c a Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, P. R. China b Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, P. R. China c Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, P. R. China d University of Chinese Academy of Sciences, Beijing 100049, P. R. China * These authors contributed equally to this work E-mail: [email protected]; Fax: +86-20-8705-7673；Tel: +86-20-3702-9721

ABSTRACT: CH4 is a clean fuel due to the highest H/C atom ratio among the other hydrocarbon fuels, showing less carbon emission during combustion. The traditional CH4 production by fermentation presents the obvious disadvantages of low CH4 yield (50-75%) and high CO2 emission (25-50%). Herein, an efficient direct conversion of cellulose to CH4 was investigated by using several hydrogenation catalysts. The Ru/C catalyst showed the excellent catalytic performance among all the studied catalysts. The optimized reaction parameters including temperature, time, pressure and catalyst dosage were further investigated by using Ru/C catalyst, and the maximum CH4 yield of 88.1% was obtained at mild reaction conditions (220 °C and 1 MPa initial H2). This value obtained is the highest yield for the production of CH4 from cellulose to date. The reaction network suggests that the hydrolysis, hydrogenation, decarbonylation, retro-aldol reaction as well as aqueous-phase-reforming (APR) are probably taken place, in which cellulose hydrolysis was proved to be the rate determined step. This work provided an efficient approach to produce biomass derived CH4 with extreme 1

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CO2 emission (below 5%) and a better understanding of the cellulose hydrogenolysis process. Key words: cellulose, Ru/C catalyst, hydrogenolysis, methane

1. INTRODUCTION With the growing depletion of resources and the increasing concern about the environmental problems, fossil fuels are facing challenges as its non-renewability and production of CO2. Thus an abundant renewable resource, which can be used as the alternative for producing fuels and chemicals, must be proposed. One of the promising renewable resources is known as the lignocellulosic biomass. As the most abundant renewable resource in nature, lignocellulosic biomass is an energy carrier which can be converted into value added organic compounds1-4. Production of biomass derived methane has also been commonly developed during recent years5. As a kind of clean energy resource with the highest H/C ratio, CH4 can be used as both energy and chemical sources6-8. Generally, it can be obtained from the natural gas or formed as a by-product in the petroleum industry. On the other hand, production of methane from biomass is another widely studied subject and the typical production method is known as the anaerobic digestion9,10. Other methods involve the pyrolytic gasification of biomass11 and the catalytic hydrothermal process12,13. Anaerobic digestion (AD), a microbial mediated process accomplished by an assortment of microbes, is considered to be a very energy-efficient and environmentally beneficial technology among all these methods for producing 50-75% CH4 yield under mild reaction conditions14. According to the literature15, despite the digestion process is 2


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usually carried out under 35-60 °C, the existence of the microorganisms makes the AD process sensitive to the temperature change. Moreover, a large amount of CO2 emission (25-50% yield) is a serious unsolved problem in this process. Other limitations like low production efficiency, long reaction time and requirement for biomass pretreatment are also restricted its further application. Considering these limitations, researchers did many works to solve the problems. Despite some progress have been made in recent years, the above limitations for fermentation technology still remain. For this reason, some other researchers try to focus their attention on the preparation of biogas by chemical method16-18. The pyrolytic gasification, for example, is a typical method for biogas production, but this method is not considered a feasible option as it requires high temperatures (˃1100 °C) and produces soot. Tars and char are also formed during the gasification process, which reduces the gasification efficiency19. The hydrothermal gasification, which is carried out near or above the critical point of water (374 °C, 22 MPa), may be a better choice for CH4 production since the high temperature (˃500°C) in biomass gasification lead to the hydrogen-rich gas production. More importantly, Modell has proved that biomass could be gasified in supercritical water without the formation of tars and char20. A relatively moderate temperature (near critical temperatures to 500°C,) was found to be beneficial for the methane-rich gas production21. A maximum CH4 yield of 0.33 (g CH4)/(g wood) with 49 vol.% CH4 in product gas was gained by using a laboratory batch reactor operated at 400 °C and 30 MPa for 98 min and Raney nickel as the catalyst (the rest gas components mainly contained 43 vol.%

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of CO2)17. In another work, porous Ni was employed as the catalyst for methane production. In the presence of Zn as reductant, the maximum CH4 yield of 73.8% was achieved at 325 °C for 2 h, and the conversion of cellulose to CH4 still remained good activity with gaseous hydrogen after three runs of transformation22. There is no doubt that reducing reaction temperature and energy consumption is an important direction for biomass methanation by chemical methods. Though compared to the pyrolytic gasification, the catalytic hydrothermal method can achieve higher throughputs and energy efficiency at lower temperature, but it still suffers from the drawbacks of harsh reaction conditions (high temperature and pressure). At the same time, the catalysts deactivation and carbonization issues are usually occurred. Similar to AD method, the problem of high CO2 emission still remains. Thus, the more mild reaction conditions, for example, lower environmental burden, high efficiency and single-step processing are required. The present study is aimed at exploring an efficient process for the methane production from cellulose with less CO2 emission under mild conditions. Cellulose methanation needs to break both C-C and C-O bonds under hydrogenation. Herein, several common hydrogenation catalysts were firstly used to investigate their abilities to break C-C and C-O bonds. The reaction conditions including reaction temperature, reaction time, initial H2 pressure and catalyst dosage were investigated to maximize the CH4 yield. The stability test of Ru/C catalyst was conducted. In addition, the degradation pathways for cellulose methanation were also explored by analyzing the detected main intermediates and used them as the reactants to produce the target

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product. In contrast to the previous work that using high temperature and high pressured water, this work was designed to produce CH4 under mild conditions in the presence of low pressure of H2. The proposed reaction pathways were also conducted for further understanding this cellulose methanation process.

2. MATERIALS AND METHOD 5wt% Ru/C was obtained from Sigma-Aldrich. 5wt% Pd/C, 5wt% Pt/C, 5wt% Rh/C and Raney nickel, and cellulose, cellobiose, D-glucose, D-sorbitol, xylitol, glycerol, ethylene glycol, 1,2-propanediol, ethylene glycol, ethanol and 1-butanol were purchased from Aladin Co. Ltd. KBr for FT-IR was purchased Sigma Aldrich Co. Ltd. (FT-IR grade). All these reagents were used without further purification. The conversion of cellulose was carried out in a stirred batch reactor with an inner volume of 100 mL. The typical experiment was carried out by loading 0.25 g cellulose, 0.05 g catalyst and 30 mL deionized water into the reactor. After that, a certain amount of nitrogen was charged into the sealed reactor to exclude the air residue, and then 1 MPa hydrogen gas was loaded at room temperature. The reaction was carried out at 220 °C for 12 h with mechanical stirring at 800 rpm. At this temperature, the final pressure was about 3.5-4 MPa. After reaction, the autoclave was cooled rapidly to room temperature with cooling water, and the monitored pressure was about 1 MPa. The gas products were then collected by a sampling bag for further analysis, while the mixture of liquid and solid was recovered by centrifugation. The separated solid material was dried at -48 °C for 24 h in vacuum using a freeze dryer. For reusability, the solid residue involving Ru/C and unreacted cellulose was

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recovered by centrifugation from the reaction mixture, thoroughly rinsed by deionized water and dried by a freeze dryer as the above mentioned procedure. For the next cycle, the introduced cellulose amount is determined by the unreacted cellulose to kept 0.25 g of feedstock, assuming the Ru/C is constant. The liquid was obtained by centrifugation and the solid residue was rinsing by deionized water, dried with a freeze dryer for further analysis. The obtained gas products were measured by using gas chromatography (GC, 7890A, Agilent, USA; CP-Wax 58 (FFAP) capillary column, 0.25 mm × 25 m) with FID and TCD detector. For C1-C6 alkane, the FID condition as follows, the injection port and the detector were held at 200 oC and 300 oC, respectively. The column flow rate was 28 mL/min with a He carrier gas. The GC oven temperature was initially held at 60 °C for 2.5 min, ramped to 250 °C at 10 °C/min, and kept at 250 °C for 5 min. For two TCD detectors, the detectors’ temperatures were 200 oC, and the carrier gas was He with a flow rate at 5 mL/min. The water-soluble products in the liquid fraction were analyzed by high-performance liquid chromatography (HPLC) with a differential reflective index detector (Waters e2695 series, USA) and a Shodex SUAGER SH1011 column. Separation was achieved at 50 °C with 5 mM H2SO4 as the mobile phase, flowing at a rate of 0.5 mL/min. GC-MS (7890A-5975C, Agilent, USA) was used to confirm those unknown compounds. The injection port and the detector were held at 240 oC and 260 oC, respectively. The column flow rate was 40 mL/min with a N2 carrier gas. The GC oven temperature was initially held at 60 °C for 2 min, ramped to 240 °C at 10 °C/min, and kept at 240 °C for 10 min. The mass

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spectrometer was operated at 70 eV over the scan range of 33-400 amu. For the mass spectrometer (MS), temperatures of the transfer line and ion source were 260 °C and 240 °C, respectively. All these products were quantified by the external standard method. Total amount of liquid products in the resultant solution was measured using a TOC-VCSH analyzer (Elementar, Vario EL Cube). The conversion of cellulose was determined by weight difference before and after reaction: Conversion(%)=

M cellulose ,o ( g ) − M cellulose ( g ) M cellulose ,o ( g )

(1)

Where Mcellulose,0 is the mass of cellulose loaded, M cellulose is the mass of residual cellulose after reaction: Mcellulose = Mresidue – Mcatalyst

(2)

Where Mresidue is the weight of remained solid after reaction; Mcatalysis is the weight of catalyst. The yield of products was calculated on the molar carbon basis and defined as follows: Yield(%)=

carbon moles of product × 100% carbon moles of cellulose put into reactore

(3)

For further characterizing the solid samples such as the catalyst, cellulose and the residues, other characterization methods were also used. The particle distribution and mean particle size of the Ru nanoparticles were determined from TEM and HRTEM analyses employing the JEM‐2100F transmission electron microscope operated at 200 kV with a LaB6 source. The X-ray powder diffraction (XRD) diffractogram was measured by a X-ray diffractometer (X’Pert Pro MPD, Philip) with Cu Kα radiation

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(λ=0.154 nm) operated at 40 kV and 40 mA. The structure and group information were obtained by using a Fourier transform infrared (FT-IR) spectrometer (NICOLET iS50, Thermo Fisher, Waltham, MA, USA) using a KBr pelleting method, 2.0 mg samples were diluted with 200 mg KBr and mixed in the agate mortar.

3. RESULTS AND DISCUSSION 3.1 Evaluation of catalysts Preliminary studies were conducted in order to investigate the potential catalysts which are efficient in cellulose conversion and methane production. For this purpose, several common hydrogenation catalysts were used to conduct the reaction, and the results obtained from these experiments are presented in Table 1 and Table 2. The using of porous Ni catalyst obtained 5.4% of CH4 yiled, indicating that the Raney nickel catalyst was effective in cellulose methanation17,22. Over Ru/C catalyst, the highest 60.7% of CH4 was gained at the cellulose conversion of 83.6%, showing the best performance among these catalysts tested. This is due to that Ru/C presented the superior C-C/C-O bonds cracking properties for biomass gasification. However, the other supported noble metal catalysts, which are known for its good hydrogenation performance like Pd/C, Rh/C and Pt/C were proved to be inefficient in methane formation since the obtained target product yields were less than 1%. A larger amount of CO and CO2 as well as a higher aqueous phase yield were observed by using these catalysts, indicating that the Rh, Pt and Pd based catalysts were preferable for the formation of liquid than the gaseous products23. Similar experimental results can also be obtained from Table 2. When using the Raney Ni,

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Pd/C, Pt/C and the Rh/C catalyst, the liquid products detected were mainly sugar alcohols and small molecular polyols, such as sorbitol, glycerol, ethanol via hydrogenation and hydrogenolsis. Some other undetectable chemicals were also observed by TOC analysis, which is likely responsible for the soluble polysaccharides with no signal in the HPLC measurements24,25. Table 1. Hydrogenolysis of cellulose to CH4 over different metal catalysts

Products yield/% Catalysts

CO2

CO

Aqueous phase yield/%

Conversion/%

CH4

C2-C5 hydrocarbon

Ru/C

83.6±0.50

60.7±1.35

12.2±1.5

3.8±0.9

0

5.8±1.15

Raney Ni

77.1±0.75

5.4±0.94

0.5±0.05

1.4±0.05

0.2±0.05

50.0±3.34

Pd/C

86.0±0.60

0.6±0.15

0.4±0.04

1.4±0.04

6.9±1.20

66.3±2.90

Pt/C

94.8±1.15

0.3±0.10

1.3±0.05

7.9±0.85

0

54.9±2.50

Rh/C

93.2±2.10

0.9±0.10

1.1±0.07

10.4±2.40

2.9±0.75

76.4±3.60

Reaction condition: 0.25 g cellulose; 0.05 g catalyst, 30 mL H2O; 220 °C; 1 MPa initial pressure with hydrogen; 12 h. Table 2. Aqueous products of cellulose methanation over different metal catalysts Products yield/% Catalysts sorbitol

xylitol

glycerol

EG

1,2-PG

ethanol

1(2)-propanol

othersa

Ru/C

0

0

0

0

0

0

0

5.8±0.85

Raney Ni

2.3±0.10

2.0±0.65

3.2±1.12

1.3±0.20

4.2±0.20

15.5±0.75

4.4±0.40

17.1±2.50

Pd/C

1.4±0.05

0.3±0.04

1.6±0.52

2.0±0.30

7.8±0.32

25.2±1.55

19.0±0.75

9.0±1.45

Pt/C

3.5±0.20

1.2±0.60

5.0±0.46

0.9±0.25

5.0±0.75

12.2±0.95

5.2±0.60

21.9±1.40

Rh/C

0.8±0.05

5.0±1.55

0.8±0.11

1.7±0.30

9.1±0.45

3.0±0.70

7.4±1.25

48.6±2.10

Reaction condition: 0.25 g cellulose; 0.05 g catalyst, 30 mL H2O; 220 °C; 1 MPa initial pressure with hydrogen; 12 h. EG: ethylene glycol; 1,2-PG: 1, 2- propanediol. a

Others include soluble sugar compounds and unidentified ones.

3.2 Effect of processing parameters Several parameters that might affect cellulose hydrogenolysis were tested to optimize the reaction condition. It is proved that the enhancement of the amount of catalyst, reaction time, initial pressure and reaction temperature can all result in the 9


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increased CH4 yields. The gaseous products were shown in Figure 1. The C6 alkane is not stated here as its yield was usually below 0.1%, and the liquid products analyzed by HPLC were not discussed in this part since these liquid compounds were below the detection limit except for using trace amount of catalyst (below 0.03 g Ru/C). More details are discussed as below. 80

100

80

70

100

70

Yield (%)

60

40 40

30 20

Conversion (%) Yield (%)

50

80

60

20

10

50

60

40 40

30 20

Conversion (%)

80

60

20

10

0 180

200

0

0 300

220 240 260 280 Reaction temperature (oC)

2

4

6

8 10 12 14 Reaction time (h)

16

0 18

100

100

80

80

80

80

60

60

40

40

20

20

0

0

1

2 3 Pressure (MPa)

4

60

60

40

40

20

20

0 0.00

0

0.02

0.04

0.06 0.08 0.10 Ru/C dosage (g)

0.12

Conversion (%)

100

Conversion (%) Yield (%)

100

Yield (%)

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0 0.14

CH4; C2-C4; C5-C6; CO2 Figure 1. Effects of reaction parameters on the cellulose conversion and main products yield over Ru/C catalyst. All these reactions were conducted by using 0.25 g cellulose and 30 mL H2O.

The influence of reaction temperature (200-260 °C) on cellulose methanation is shown in Figure 1. A. 44.8% of cellulose conversion and 22.0% of CH4 yield were gained when the reaction was conducted at 200 °C. Except for the CH4, other components detected in the gas were C2-C5 alkanes. With the reaction temperatures gradually increased, the cellulose conversion and CH4 increased and obtained the

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highest 99.2% and 67.5% at the temperature of 260 °C, respectively. Meanwhile, a risen CO2 yield can also be observed at increased temperature, indicates that higher temperature is favorable for promoting aqueous-phase reforming (APR) process besides cellulose methanation26,27. The promotion effect of high temperature on cellulose conversion and methane production were probably attributed to the autogenetic H3O+ produced from hot compressed water dissociation and byproduct organic acids from cellulose degradation28,29. These acidic species promote cellulose hydrolysis and thereby accelerates the methanation process. However, even though a higher temperature is proved to be beneficial to the CH4 formation, the increased rates for CH4 slowed down when the temperature was higher than 220 °C. Hence, considering both the CH4 yield and the energy consumption, the catalytic temperature of 220 °C was used in the following experiments. The reaction time is one of the most important factors that influences the cellulose conversion and CH4 formation. The gaseous products from cellulose methanation proceeded at different reaction time (4 -16 h) are presented in Figure 1. B. The promoting effects of reaction time on cellulose conversion and product yield were similar to that of the reaction temperature. A gradually increased trend for cellulose conversion was observed with the reaction time elongated to 12 h. However, a lower increased rate was observed as the reaction time further increasing to 16 h. At the same time, the corresponding CH4 formation rate was steadily decreased with the reaction time prolonged. Other gaseous products like the C2-C5 alkanes and CO2 also followed this similar trend.

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The reactions under different initial H2 pressures were conducted and the results are shown in Figure 1. C. It was found that the higher hydrogen pressure could obviously lead to the higher CH4 yields as well as the cellulose conversions. As the reaction was conducted at the relative low initial H2 pressure as 0.7 MPa, the CH4 yield of 71% can obtained. In this case, the amount of hydrogen added was comparable to the theoretical consumption value according to 0.25 g cellulose (only CH4 was considered during cellulose methanation), indicating the high H2 utilization efficiency. Moreover, from the hydrogen consumptions at different pressures (Table S1), there was the significant portions of external hydrogen remained unconsumed and the actual hydrogen consumption was less than the theoretical values based on the produced CH4, which suggests that hydrogen production reactions occurred via APR process and the real H2 consumption was the sum of the amount of external H2 consumed and the consumption of hydrogen generated by APR process. And with the initial hydrogen pressure increased, the real H2 consumption as well as the external hydrogen consumption were both increased. Another interesting note from Figure 1. C is that the byproducts were gradually increased with the initial H2 pressure, obtaining the yields of C2-C5 hydrocarbons increased from 6.7% to 25% as the initial pressure increased from 0.7 MPa to 4 MPa. Considering the external hydrogen consumption and the CH4 yield, the higher hydrogen pressure is not actually benefit for the cellulose methanation process. The suitable pressure of 1 MPa H2 was used for further investigation. The CH4 formation usually needs a hydrogenation catalyst30. In this research, a

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commercial Ru/C catalyst was used and the gaseous components produced during the cellulose conversion are shown in Figure 1. D. When 0.03 g Ru/C was used, the CH4 yield were only 48.6%, whereas with the catalyst dosage increased to 0.12 g, a maximum CH4 yield of 88.1% was achieved. It is obvious that the Ru/C catalyst played an important role in CH4 production, but too much catalyst will significant increase the production cost. Thus, there is no need for further study on more catalyst loading. The achieved high CH4 yield could be attributed to the outstanding hydrogenolysis activity of Ru/C. It is reported that the Ru/C catalyst has a good performance for broking the C–O bonds and the C–C bonds31,32. Researchers33 have also proved that Ru catalyst was quite efficient in hydrodeoxygenation of cellulose and xylan to lower alkanes because of its high activity in C–C bonds cleavage and transformation of hydrogen. Another reason that might contribute to the high CH4 yield is the well distributed small Ru particles on carbon support. XRD pattern (Figure 2. A) of the Ru/C catalyst showed that no obvious diffraction peak of metal Ru was detected, indicating that metal Ru is highly dispersed on the carbon support. The TEM images give more intuitive evidence that the Ru particles are homogeneously disperse on the surface of carbon with the mean diameter of about 1.9 nm (Figure. S1). A few bigger particles detected are attributed to the aggregation or overlapping of smaller Ru particles. It’s noted that the 88% of CH4 yield in this study is significantly higher than that obtained by the previous report33 and no CO2 was produced. This is preferable for its direct utilization because no additional CO2 separation is needed, which is superior to the fermentation and hydrothermal

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gasification processes where the separation of CO2 is necessary to improve the combustion efficiency of CH4. Moreover, the 220 ℃ of mild reaction temperature is remarkably reduced by 100 ℃ as comparing to the reported Ni based catalyst33.

3.3 Characterization of raw materials and solid residues In order to figure out whether the unreacted reactant in the residues kept the cellulose structure, the XRD and FT-IR characterizations were used to analyze the physical-chemical properties of the raw cellulose and the residual solid. Since the residue are the possible mixture of Ru/C catalyst and the cellulose or its derivatives, the fresh Ru/C catalyst and raw cellulose were also characterized for comparison. From Figure 2. A, it can be seen that the residual sample has both the remarkable characteristic peaks of cellulose and Ru/C catalyst, but the characteristic peak strength as well as the crystallinity of cellulose in solid residue is totally decreased since the diffractions at 15 °, 22 ° and 34.5 ° become lower and less sharp than those in the raw cellulose. This indicates the conversion of cellulose. Besides, no new diffraction peaks are discovered from the XRD pattern of the solid residue, suggesting that no solid cellulose derivatives are produced in this cellulose methanation process. After reaction, no obvious diffractions corresponding to metal Ru species demonstrated that the Ru particles are quite stable in the current hydrothermal conditions (no Ru particle sintering takes place).

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A)

B)

(c)

(b)

Absorbance (a.u.)

1106

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(c)×5 898

(b) 1057 1168

(a) 10

20

30

40 50 60 2 Theta(degree)

70

80

1031

(a) 2000

1600

1200

800

cm-1

Figure 2. XRD patterns A) and FT-IR spectra B) of (a) fresh Ru/C, (b) raw cellulose, and (c) residual solid.

The FT-IR measurements were also employed to analyze the chemical bonding and functional groups information of the raw cellulose, the fresh Ru/C catalyst and the solid residue. The absorption peaks at 1168 cm-1, 1106 cm-1, 1057 cm-1, 1031 cm-1 and 898 cm−1 were generally considered to be the vibrations of cellulose34,35. From the FT-IR spectra in Figure 2.B Figure S2, it can be noticed that, with the conversion of cellulose, the characteristic peaks of cellulose at 1168 cm-1, 1106 cm-1, 1057 cm-1 and 1031 cm-1 were retained but the intensities decreased compared to the raw cellulose. In spectrum a (Figure B), the peaks were mainly attributed to atmospheric water and CO2. The peak at 898 cm-1 which represents the crystalline structure of cellulose, disappeared in the detected residual sample, further supporting the fact that the crystalline cellulose is partially converted into the amorphous nature36. This result is well consistent with the XRD analysis (Figure 2. A). According to these characterizations, it can be concluded that in the atmosphere of hydrogen, the Ru/C catalyst was efficient in cellulose methanation and the remained reaction material can still be regarded as the unreacted cellulose. This supported that the calculation for cellulose conversion was reasonable by weight

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subtraction.

3.4 Stability of Ru/C Figure 3 shows the stability of Ru/C during cellulose hydrogenolysis to CH4. The cellulose conversion and the target product yield slightly decreased from the initial 99% and 87% to the 90% and 81%, respectively, after the third cycle, indicating slightly deactivation was observed on Ru/C catalyst. After the third runs, however, the cellulose conversion and CH4 yield could be stabilized at 90% and 81%, respectively, showing the good reusability of this catalyst. The TEM analysis showed that the average Ru particle of the used Ru/C was slightly increased to about 3 nm after the fifth cycle, which was still highly dispersed on the carbon surface (Figure 4). Therefore, the good anti-sintering property of Ru/C is relative to the good stability in this cellulose hydrogenolysis process. Cellulose conversion CH4 yield

100

Conversion & yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40 20 0

1

2

3 Run number

4

5

Figure 3. Stability test of Ru/C in cellulose hydrogenolysis to CH4. Reaction conditions: 0.25 g cellulose, 0.12 g Ru/C, 30 mL water, 220 °C and 12 h.

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Page 17 of 26

40

D)

C)

35

25 20 15

Intensity(a.u.)

30

Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(a)

10 5

(b)

0 1.0 1.6 2.2 2.8 3.4 4.0 4.6 5.2 5.8 6.4 7.0 7.6 10 Particle size (nm)

20

30

40 50 60 2 Theta (degree)

70

80

Figure 4. TEM images of used Ru/C after the fifth cycle (A-B) and the statistical Ru particles distribution (C), (D) XRD patterns of fresh Ru/C (a) and used Ru/C after the fifth cycle (b).

3.5 Reaction pathway By-products and intermediates play the important roles in understanding the reaction pathways during this cellulose methanation. From the above research we can see that the by-products such as C2-C5 alkanes and CO2 in the gaseous phase were detected, liquid intermediates, however, were only observed when the reaction was carried out with less catalyst (less than 0.03 g). From Figure 5 it can be seen that the dominant intermediates involve the polyols and alcohols at different degradation stages, which means the hydrogenation and hydrogenolysis reactions were probably happened under the current hydrogen environment.

17


Energy & Fuels

sorbitol

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

xylitol

glycerol 1,2-PG ethanol EG

glucose

10

1-propanool

20

30 40 Time (min)

1-butanol

50

60

Figure 5. HPLC profile of liquid intermediates produced during cellulose hydrogenolysis.

For further elucidating the process of cellulose methanation, comparison experiments were conducted by using cellulose and different intermediates as the feedstocks. The results are shown in Table 3. The blank reaction was conducted without catalyst or H2, the obtained gaseous products include only 4% of CO2, 0.4% of CH4 and a small amount of H2 (5.7 vol.%). However, about 17% of CH4 along with 26% CO2 were obtained when the reaction was carried out with 0.12 g Ru/C catalyst and 1 MPa N2. Apparently, the addition of catalysts greatly promoted the effective conversion of cellulose and the formation of CH4 and CO2. The reason may be the catalytic decomposition of the hydrolyzed sugar to form CO and H2, followed by the conversion of CO and H2O to produce CO2 and H2 via water gas shift31,37. The produced CO2, however, was difficult to be further fully hydrogenated to CH4 since the hydrogen source was insufficient. Obviously, the catalyst as well as the hydrogen environment played the important roles in CH4 formation from cellulose hydrogenolysis.

18


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Energy & Fuels

As is shown in Table 3, when cellulose was reacted only for 1 h in the presence of Ru/C catalyst, about 19% of CH4 was gained. By comparison, about 2 and 4 times higher CH4 yields were obtained when cellobiose and glucose were used as the feedstock, respectively. This may suggest that the CH4 formation rate is probably relative to the polymerization degree (glucosidic bonds) of reactants, and the one with less glucosidic bonds would favor the methanation reaction. Thus, comparing to cellulose and cellobiose, a higher CH4 yield was obtained by using glucose as the feedstock. Another interesting result was observed between glucose and sorbitol. The methane yield obtained from glucose is 10% higher than that of sorbitol. This is probably originated from the easy decarbonylation of glucose comparing to sorbitol. From the verification tests, it is now clear that the products detected from cellulose methanation reaction such as glucose, sorbitol and glycerol can all be converted into CH4 under the studied conditions, which confirms that these chemicals are intermediates of cellulose methanation process. This could also be verified by the comparison experiments by using different Ru/C dosages. With increasing the catalyst dosage, the variety and content of the liquid products were all decreased (Figure. S3). Another phenomenon which is worth to note in Table 3 is that the CO2 were observed in almost all reactions, but no CO was detected. The reason is possibly due to the easy hydrogenation (CO + 3H2 CH4 +H2O; CO2 + 4H2 CH4 + 2H2O) and/or the water–gas shift reaction (CO + H2O CO2 + H2) over Ru catalyst18,38. Based on the results above, the possible reaction pathways were proposed in Scheme 1. Four distinct pathways are possibly included during this process: (1) path A

19


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

went through the hydrolysis, decarbonylation and hydrogenolysis; (2) path B experienced cellulose hydrolysis, hydrogenation and hydrogenolysis; and in (3) path C, hydrolysis, retro-aldol reaction as well as hydrogenolysis are included; (4) path D presents the APR reactions. Because the reactant of APR process may be cellulose or a variety of alcohols after cellulose hydrolysis, pathway D is not specified in Scheme 1. Table 3. Gaseous products yields obtained from different feedstocks

Yield /% Entry

Substrate a

CH4

C2H6

C3H8

n-C4H10

n-C5H12

CO2

CO

Conversion/ %

1

cellulose

0.4±0.05

0.1±0.05

0±0.005

0.2±0.03

0

4.0±0.40

0.3±0.05

100

2

celluloseb

17.1±0.45

3.4±0.25

0.9±0.15

0.2±0.05

0

25.6±0.70

0

85.2±2.5

3

cellulose

18.9±0.85

2.8±0.25

0.9±0.10

0.4±0.10

0

0.2±0.02

0

20.0±2.0

4

cellobiose

43.1±0.55

4.0±0.30

1.9±0.20

0.7±0.08

0.5±0.04

3.3±0.20

0

100

5

glucose

77.1±1.00

7.5±0.35

2.4±0.30

2.2±0.10

0.4±0.10

0.6±0.10

0

100

6

sorbitol

64.1±0.90

7.0±0.10

2.4±0.15

1.6±0.10

0.6±0.04

0.3±0.05

0

100

7

xylitol

63.7±1.20

5.2±0.20

1.6±0.20

0.3±0.05

0

2.2±0.20

0

100

8

Glycerol

53.4±0.80

2.8±0.05

0.5±0.05

0.01

0

2.0±0.45

0

95.8±1.3

9

1,2-PG

70.5±0.90

3.7±0.45

0.8±0.05

0

0

4.1±0.55

0

100

10

EG

70.9±1.00

2.2±0.45

0.2±0.05

0

0

2.7±0.35

0

100

11

ethanol

68.4±0.80

2.4±0.25

0.2±0.06

0

0

0.6±0.3

0

100

1-butanol

24.9±1.40

2.7±0.45

56.3±2.0

1.1±0.10

0

0.12±0.1

0

100

12

Reaction conditions: 0.25 g feedstock, 0.12 g Ru/C, 30 mL H2O, 1 MPa initial pressure with hydrogen gas, 220 °C, 1 h. a

Reacted at 220 °C for 12 h without catalyst or H2.

b

Reacted at 220 °C for 12 h with 0.12 g Ru/C and 1 MPa N2.

In all these possible pathways, cellulose was first hydrolyzed to cellobiose and glucose, and the CH4 was finally released via various hydrogenation products with Ru/C as the catalyst. In this process, the cellulose methanation can be regarded as a process of C─C and C─O cleavage reactions. The C─C cleavages can take place at

20


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Energy & Fuels

the end of the carbon chain involved C=O groups (decarbonylation)

39-42

or in the

middle of the carbon chain (retro-aldol reaction)43 along with hydrogenation, dehydrogenation, and water–gas shift (WGS) reactions. While the C─O cleavages mainly consist in dehydration and hydrogenation reactions in hydroxyl groups32, 44-47. These reactions occur simultaneously or successively, leading to the complex of reaction network. It seems that decarbonylation is the main pathway for CH4 formation at the final stages since CH4 was presented as the major product after hydrogenolysis of the C2-C3 oxygenated intermediates (1,2-PG, EG and ethanol, Table 3). When using 1-butanol as the feedstock, CH4 and propane were detected as the major products via decarbonylation and hydrogenation, indicating that CH4 is probably produced by first splitting the C-C bonds of the oxygenated intermediates followed by cracking the C-O bonds during the carbon and oxygen number reduction process. In the current cases, the CH4 produced from kinds of liquid chemicals by hydrogenation, decarbonylation may be the ‘‘primary methane’’, and the ‘‘secondary methane’’ may come from the hydrogenation of CO and/or CO25. In addition, the by-products such as C2-C5 alkanes (can be co-combusted with CH4) are possibly produced via hydrodeoxygenation of corresponding intermediates during cellulose methanation.

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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

OH

OH OH O

Page 22 of 26

O

O OH

O

OH n cellulose

OH Hydrolysis OH

Path A Decarbonylation

OH

OH

OH

OH

Retro-Aldol/Hydrogenation

OH Hydrogenation

Path B OH

Path C

O

OH OH

OH

OH

OH

OH +

CO +

OH OH

OH

OH

OH

OH

OH

Decarbonylation

Retro-aldol/ Hydrogenation OH

OH OH

O

O

OH OH

Retro-aldol/ Hydrogenation

OH

O

+ OH

OH Retro-aldol/ Hydrogenation

OH CO +

OH OH

OH

OH

Decarbonylation

OH

OH

+ OH

OH

OH OH


OH

OH

CO +

Decarbonylation

OH

OH

OH

OH OH

+ OH OH

CO +

OH

Decarbonylation

OH OH

OH

OH

OH


Decarbonylation

CH3OH

CO +

+ OH

OH

OH

OH

CH3OH

CH4 C2-C6 intermediates + H 2O CO +

H2O

CO 2 +

H2

(APR)

CO + 3H 2

CH4 + H2O

CO2 +

H2

(WGS)

CO2 + 4H2

CH4 + 2H2O

Scheme 1. Proposed reaction pathways for the catalytic conversion of cellulose to CH4.

However, in spite of many reactions happened in this cellulose methanation process, a conclusion can be drawn that cellulose hydrolysis is a speed control step, since only a small amount of glucose (can not be quantified due to detection limit) was observed with the reaction time prolonged from 0.5 h to 12 h, but in the meantime, a complex liquid sample containing a variety of sugar alcohols and small molecular alcohols was gained after a 12 hours hydrothermal reaction with 0.01 g or 0.03 g Ru/C catalyst (Figure. S2 and Figure. S3). It is clear that in the case of less catalyst dosage, the hydrogenation was restrained, thus kinds of incomplete hydrogenation products/intermediates can be detected. On the contrary, the sufficient amount of catalyst results in the intermediate glucose and other intermediates rapidly 22


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Energy & Fuels

converted into CH4.

4. CONCLUSIONS A highly efficient approach for cellulose hydrogenolysis to CH4 was achieved by using Ru/C catalyst, and the maximum CH4 yield of 88.1% was gained under the optimized hydrothermal conditions. The Ru/C showed good stability for obtaining 90% of initial activity and CH4 yield after the fifth cycle due to the good anti-sintering property in the current hydrothermal conditions. Further study showed that four possible pathways of hydrogenation, decarbonylation, retro-aldol reactions and APR process are involved using comparison experiments of possibly intermediates and cellulose hydrolysis is proposed as the speed controlled step. Comparing to the previously reports by fermentation and hydrothermal gasification, this study presents the obvious advantage for highly efficient CH4 production (88.1% yield) at rather mild reaction conditions while keeping the extremely low CO2 emissions (below 5%).

ACKNOWLEDGEMENTS This research was financially supported by the National Natural Science Foundation of China (51576199, 51536009), the Natural Science Foundation of Guangdong Province (2017A030308010) and the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01N092).

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Direct Hydrogenolysis of Cellulose into Methane under Mild

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