Alkaline Thermal Treatment of Cellulosic Biomass for H2 Production

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Alkaline Thermal Treatment of Cellulosic Biomass for H2 Production Using Ca-based Bi-functional Materials Ming Zhao, Xiaomin Cui, Guozhao Ji, Hui Zhou, Arun K Vuppaladadiyam, and Xiao Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04855 • Publication Date (Web): 27 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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ACS Sustainable Chemistry & Engineering

Alkaline Thermal Treatment of Cellulosic Biomass for H2 Production Using Cabased Bi-functional Materials Ming Zhaoa,b, Xiaomin Cuia,b, Guozhao Jia,c, Hui Zhoud, Arun K Vuppaladadiyama,b, Xiao Zhaoa,e* a

School of Environment, Tsinghua University, Beijing, 100084, China.

b

Key Laboratory for Solid Waste Management and Environmental Safety, Ministry of

Education Beijing 100084, China. c

School of Environmental Science and Technology, Dalian University of Technology,

Linggong Road 2, Dalian 116024, China. d

Department of Mechanical and Process Engineering, ETH Zürich, Zürich CH-8092,

Switzerland. e

College of Water Resources & Civil Engineering, China Agricultural University,

Beijing 100083, China. Email: [email protected]

ABSTRACT. Hydrogen production from cellulosic biomass not only provides a sustainable approach to cope with the growing demand for energy, but also facilitates the relief of environmental burden. In this study, we developed a series of Ca-based bifunctional materials (Ca(OH)2 & Ni composites) for alkaline thermal treatment (ATT) of cellulose to produce high purity hydrogen at moderate temperatures (350-450 ºC). Ca(OH)2 served predominantly as a CO2 carrier and a H2O donor, and enlarged the surface area of the materials to improve H2 production. However, excess Ca(OH)2 1

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tended to cover Ni particles and block pore structures resulting in a suppressed H2 production. Ni promoted tar cracking and enhanced H2 production, but the surface area of catalyst decreased with an increment in Ni, which suppressed H2 generation. The yield of hydrogen was improved at elevated temperature. The maximum hydrogen yield, 34.5 mmol·g-1 with 77% volume fraction, was obtained by adopting a molar ratio of cellulose: Ca(OH)2: Ni as 1:6:2 at 450 ºC for 10 min. GC-MS analysis results of tar products revealed that Ca(OH)2 promoted primary pyrolysis of cellulose, and Ni promoted the decomposition of furan ring derivatives. The reaction temperature affected the distribution rather than the composition of the tar products.

KEYWORDS. Cellulosic biomass, alkaline thermal treatment, hydrogen production, bi-functional materials INTRODUCTION Hydrogen (H2) plays an important role in chemical and energy industry with several uses for upgrading oil,1 refining metals,2 synthesizing ammonia or other nitrogenated fertilizer3 and producing liquid fuel.4 Most of all, H2 is admitted as a future energy source considering long-term energy and climate change issues.5 Hitherto, steam reforming of methane (SRM, CH4(g) + H2O(g)↔CO(g) + 3H2(g)) is the most efficient and commercially available process for H2 production, accounting 48% of the global H2 production.6 Nevertheless, this hydrogen producing route is not sustainable with a high CO2 footprint.7 H2 production form thermochemical conversion of cellulosic biomass is economically viable and environmental-friendly as cellulosic biomass is inexpensive, abundant, renewable and carbon neutral. However, compared with fossil 2


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fuels, cellulosic biomass has lower energy density and higher moisture content, which makes it a flawed substitute feedstock.8 To improve H2 production from cellulosic biomass, several enhanced thermochemical conversion processes have been developed, such as, catalytic steam gasification,9,

10

sorption enhanced gasification11,

12,

supercritical hydrothermal treatment,13 etc.. Despite showing positive results regarding H2 yield and purity, these approaches require strict operating conditions (high temperature and/or high pressure), which results in high energy penalty and relatively poor economic performance.14 Thus, efficient techniques capable of converting cellulosic biomass to H2 under moderate conditions are highly in need. Alkaline thermal treatment (ATT), as a novel pathway for thermal decomposition of biomass, has gained increasing interest recently. High purity H2 (>80%) can be produced through this process at atmospheric pressure and relatively low temperature (usually 300-450 °C). Saxena et al.15 pioneered the concept of producing high purity hydrogen through the reaction of carbon with NaOH in a steam atmosphere. The proposed reaction is: C(s) + 2NaOH(s) + H2O(g)→1.5H2(g) + Na2CO3(s)

(1)

Since then, Ishida et al.16 developed a similar process using cellulose as the model biomass: C6H10O5(s) +12NaOH(s) + H2O(g)→12H2(g) +6Na2CO3(s)

(2)

It is reported that ca. 100% yield of H2 was obtained by adding nickel-based catalyst to suppress the formation of methane. So far, Group I hydroxides (i.e., NaOH, KOH and LiOH)16-18 and Group II hydroxides (i.e., Ca(OH)2, and Mg(OH)2)19, 20 have been investigated in ATT of biomass. Stonor et al.19 found comparable H2 production performance could be achieved when using Ca(OH)2 with the assistance of nickel-based 3



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catalysts instead of NaOH to treat cellulose. A new reaction was proposed (Eqn.3). C6H10O5(s) +6Ca(OH)2(s) + H2O(g)→12H2(g) + 6CaCO3(s)

(3)

Several researches have been conducted on applying bi-functional catalyst-sorbent materials of Ni (or NiO)-CaO to biomass pyrolysis.12, 21-23 Bi-functional materials show better performance on H2 production as they contain disparate reactive sites that promote complementary reaction steps.24 To the best of our knowledge, there is no study on application of Ca-based bi-functional materials in the ATT reaction. Certain studies provide the basic information about the mechanism of cellulose pyrolysis with alkali or Ni,20,

25-28

but the influence of Ca-based bi-functional materials on the

mechanism of ATT of cellulose is not yet clear and needs an in-depth study. In the present study an attempt has been made to synthesize bi-functional materials Ca(OH)2Ni (where Ni is made available in nano size). Furthermore, the impact of Ca(OH)2-Ni in ATT of cellulose was investigated by optimizing the molar ration of cellulose to Ca(OH)2 and Ni, and the effects of Ca(OH)2-Ni on the mechanism of ATT of cellulose at different temperatures were elucidated. EXPERIMENTAL

Synthesis of bi-functional materials The precursors for Ca(OH)2-Ni, namely calcium acetate (Xilong Chemical Industry, China) and nickel (II) nitrate hexahydrate (Xilong Chemical Industry, China) at a designed amount, were dissolved together. The molar ratios of Ca to Ni were controlled as 1:0, 1.5:1, 3:1, 6:1 and 12:1. After being chilled for ~ 6 h at -80 ºC, the solution was freeze-dried for 24 h at -60 ºC and 8.0 Pa. Before each test, the freeze-dried precursors 4


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sequentially went through calcination, reduction and hydration. The details of these three steps are given in supporting information (SI) Text S1.

Bi-functional materials characterization The crystalline structure of the synthesized bi-functional materials were characterized by X-ray diffraction (XRD, Cu Kα radiation, D8/Aduance, Bruker, Germany) with a continuous scanning rate of 5 degree·min-1 in the 2θ range of 10-90°. The surface morphology of materials and the distribution of Ni on Ca(OH)2 were characterized via scanning electron microscopy (SEM, ZEISS Merlin, Germany) coupled to an energy dispersive spectroscopy (EDS). The textual properties, including Brunauer-EmmettTeller (BET) surface area, Barrett-Joyner-Halenda (BJH) pore volume and pore size distribution, were measured based on N2 adsorption-desorption isotherms from a Quantachrome autosorb instrument (Autosorb-1, Quantachrome, USA).

Catalytic-absorptive activity test Microcrystalline Cellulose (Sinopharm, China) is adopted as a representative compound of cellulosic biomass. The fresh bi-functional materials were blended mechanically with cellulose powder according to molar ratios of C6H10O5 to Ni 1:1 and C6H10O5 to Ca(OH)2 1:6, respectively. Cellulose without the bi-functional material was considered as control for comparison. All the samples were named by the molar ratio of these three components, i.e. sample C6H10O5: Ca(OH)2: Ni with the molar ratio 1:6:1 was named “1Cel-6Ca-1Ni” or control group as “1Cel-0Ca-0Ni”. The effects of the bi-functional material on gases production were investigated 5



using a thermogravimetric analyzer (Q600 SDT, TA, USA) coupled with a mass spectrometer (HPR20, Hiden, UK) (TGA-MS). A crucible loaded with ca. 5 mg was placed in the TGA furnace. Prior to pyrolysis, the furnace was purged with Ar (99.99%, v/v) at 500 mL·min-1 for 30 min to create an inert atmosphere and then heated from 30 ºC to 600 ºC at a ramping rate of 40 ºC·min-1 while the signals of key ion fragments were recorded by MS. The main targeted ion fragments and representative gas species are listed in SI Table S1. The production rate and yield of gases were estimated by adopting a semi-quantitative method used in previous studies of Zhao et al.28, 29 The total volume of gas was converted to the number of moles of gas by adapting a room temperature.

Analysis of condensed gases and liquid products (tar) The condensed gases and liquid products (tar) available after pyrolysis were qualitatively analyzed on the gas chromatograph (Agilent 7890, USA) coupled with a mass spectrometer (Agilent 5975C, USA) (GC–MS). The details of GC-MS analysis are given in SI Text S2. RESULTS AND DISCUSSION

Characterization of the bi-functional materials The X-ray diffraction patterns of fresh materials are plotted in Fig. 1. The profiles reveal the existence of Ca(OH)2 (JCPDS No. 44-1481) and elemental Ni (JCPDS No. 04-0850) in fresh samples. As expected, the intensity of signals corresponding to Ca(OH)2 became higher with the increment of Ca(OH)2 molar fraction, while the peaks 6


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corresponding to Ni decreased. Moreover, there is no Ca-based impurities detected suggesting the fully transform of Ca(OH)2. After pyrolysis of cellulose with bifunctional material, the presence of CaCO3 and CaO and absence of Ca(OH)2 (SI Fig. S1) indicating that reactions of CO2 capture by Ca(OH)2 and decomposition of Ca(OH)2 occurred. Whereas no oxidation of Ni implying Ni play the role as a catalyst in this process.

Fig. 1 XRD patterns of fresh bi-functional materials.

According to SEM investigation, all types of the materials present lax structures with rough surface (Fig. 2), which might be resulted from the hydration of Ca(OH)2. It was reported that Ca(OH)2 could contribute to enhanced pore structure and high surface area of the materials.30-32 The particles dispersed on the surface of Ca(OH)2 (as shown in Fig. 2b, 2d, 2f, and 2h) are in size of 50-100 nm. These nanoparticles are further conformed to be with high loads of Ni by EDS.

7



Fig. 2 SEM images of fresh bi-functional materials and mass fraction of elements based on EDS in specified areas. (a and b) molar ratio of Ca(OH)2 to Ni as 1.5:1, (c and d) molar ratio of Ca(OH)2 to Ni as 3:1, (e and f) molar ratio of Ca(OH)2 to Ni as 6:1, (g and h) molar ratio of Ca(OH)2 to Ni as 12:1.

The N2 physisorption isotherms were plotted in SI Fig. S2 for the bi-functional materials with varied molar ratios of Ca(OH)2 and Ni. The hysteresis implied the presence of meso-pores in these materials. Fig. 3 presents the results of BET surface area and pore volume for the materials with varied molar ratios of Ca(OH)2 to Ni. Again, specific surface area and pore volume of materials increase with the increment of Ca(OH)2 fraction. With an increase in molar ratio of Ca(OH)2 : Ni from 1.5:1 to 3:1, the specific surface area and pore volume of materials increased from 3.6 to 11.7 m2·g-1 8


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and 0.05 to 0.11 cm3·g-1, respectively. However, further increase in Ca(OH)2 molar fraction slightly improved specific surface area and pore volume. The specific surface area of the material even decreased when the molar ratio of Ca(OH)2 : Ni was increased from 6:1 to 12:1, suggesting materials with much higher Ca(OH)2 may start to agglomerate.

Fig. 3 BET surface area and pore volume of fresh bi-functional materials with varied molar ratios of Ca(OH)2 to Ni

Effects of molar ratio of cellulose to Ca(OH)2 on H2 production The effects of different molar ratios of cellulose to Ca(OH)2 on H2 production were studied in TGA-MS by fixing the molar ratio of cellulose to Ni, and the results are displayed in Fig. 4. Fig. 4a shows that negligible H2 was detected for the case 1Cel-0Ca0Ni, suggesting that cellulose solely cannot generate H2 within the temperature range of 300-500 ºC. For the cases of cellulose with bi-functional materials, two distinct H2 production rate peaks appeared over this temperature range. The first peak occurred at ca. 400 ºC, and the second appeared at ca. 450 ºC. In general, H2 formation from the pyrolysis of cellulose involves primary decomposition, secondary cracking, steam 9



reforming and water-gas shift (WGS) reaction.33,

34

Most of these reactions are

endothermic, and H2 production via pyrolysis of cellulose can only be triggered at temperature > 500 ºC.33 Therefore, this bi-functional material can promote H2 production at lower temperature by lowering the activation energy of these reactions. As the molar ratio of cellulose: Ca(OH)2 was increased from 1:1.5 to 1:6, the first peak of H2 production increased greatly. This is because Ca(OH)2 captured CO2 (i.e., Ca (OH)2(s) + CO2(g)↔CaCO3(s) + H2O(g)), and shifted the equilibrium of WGS reaction (i.e., CO(g) + H2O(g)↔CO2(g) + H2(g)) in favor of H2 production.20, 35 Moreover, more Ca(OH)2 can provide more dissociative OH- which can reduce the C–H bond energies in cellulose and promote H2 production.25, 36 However, an adverse trend was observed for the case of 1Cel-12Ca-1Ni, suggesting that agglomeration of Ca(OH)2 might not only degrade the performance in promoting the WGS reaction, but also reduce the reactive sites of Ni resulting in the inhibition of secondary cracking. These results suggested that the reactions of secondary cracking and WGS determined the H2 production at lower temperature. The second peak of H2 production rate tended to shift to higher temperature with the higher moles of Ca(OH)2 suggesting that steam reforming for the derived compounds from the cellulose decomposition predominantly contributed to H2 production above 450 ºC, as Ca(OH)2 underwent decomposition at ca. 400 ºC (SI Fig. S3a). On the other hand, steam-reforming reactions intend to occur at elevated temperature. According to Fig. 4d, 1Cel-6Ca-1Ni gave the highest yield of H2 (29.70 mmol·g-1). It is noteworthy that despite of its inferior performance in H2 production at low temperature (ca. 400 ºC), 1Cel-12Ca-1Ni achieved the second highest H2 yield 10


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(24.35 mmol·g-1). For both CO and CO2, the production peaked at around 400 ºC. The CO production rate as shown in Fig. 4b exhibited a similar tendency with H2 and its generation is highly affected by SRM. According to Fig. 4c, some of the generated CO2 might not be captured (the “CO2 slip”) during the ATT of cellulose. The yield of CO2 increased with the moles of Ca(OH)2 (Fig. 4d). The generation of CO2 is supposed to be associated with SRM and WGS reaction, and those two reactions can be intergraded to one (i.e., C H4(g) +2H2O(g)↔CO2(g) +4H2(g)). This intergraded reaction will move forward when H2O is added or CO2 is removed. The production rate of H2O during the ATT of cellulose process (SI Fig. S3a and Fig. S4a) shows that the production of H2O was increased with the decomposition of Ca(OH)2 from 400 ºC to 450 ºC. In addition, the XRD patterns of bi-functional materials after ATT of cellulose (SI Fig. S1) indirectly proved the decomposition and carbonation of Ca(OH)2. Therefore, Ca(OH)2 can serve as a CO2 carrier or a H2O donor to affect the production of CO2 and H2. The “CO2 slip” is the most significant in case 1Cel-12Ca-1Ni, which suggests that Ca(OH)2 played a role of a H2O donor rather than a CO2 carrier in this case. In all cases, the extremely low production rate of CH4 (SI Fig. S3b) might be attributed to intensive consumption of CH4 due to SRM. Based on the results, the process of ATT for cellulose with Ca(OH)2-Ni without steam was supposed to be developed by successive steps. CH4 was considered to be an intermediate and was converted to H2. The involved reactions might be described by the following equations: Ni

C6H10O5(s) +5Ca(OH)2(s) 5CaCO3(s) +8H2(g) + CH4(g) 11


(4)


Ca(OH)2(s)↔ CaO(s) + H2O(g) CH4(g) + H2O(g)

Ni

CO(g) + 3H2(g)

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(5) (6)

Ni

Total equation: C6H10O5(s) + 6Ca(OH)2(s) 5CaCO3(s) + CaO(s) + 11H2(g) + CO(g) (7) In summary, Ca(OH)2 in the bi-functional materials played a key role on H2 production. The overall effects greatly depend on the fraction of Ca(OH)2 which directly impacted the physical structure and CO2 chemisorption capability of the bifunctional materials. Besides, it could also be an in situ steam source for enhancing steam reforming reactions during the dry pyrolysis progress.

Fig. 4 Effect of molar ratio of cellulose to Ca(OH)2 on the production rates of (a) H2, (b) CO and (C) CO2, and (d) accumulated yield (300-600 ºC) of major gases.

12


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Effects of molar ratio of cellulose to Ni on H2 production Considering integrality of the bi-functional material, we investigated the effects of molar ratio of cellulose to Ni by changing the molar ratio of cellulose to Ca-Ni. In the case of 1Cel-6Ca-2Ni, the molar ratio of cellulose to material 3Ca-1Ni is 1:2. Fig. 5 shows the performance of the materials with different fractions of Ni on gases production. As indicated in Fig. 5a, 5b and 5c, once Ni was introduced into the ATT of cellulose, production rates of H2 and CO were significantly increased, whilst CO2 production rate remained unchanged compared with the non-catalytic pyrolysis of cellulose (case 1Cel-0Ca-0Ni) or the one with only the presence of Ca(OH)2 (case 1Cel6Ca-0Ni). With respect to the first peak of H2 production rate, it was improved with increasing the molar ratio of Ni: cellulose from 1:1 to 2:1. However, further increasing the molar ratio of Ni: cellulose (case 1Cel-6Ca-4Ni) suppressed H2 production rate. Studies have revealed that Ni promoted both solid phase reactions (i.e., primary pyrolysis) and gas phase reactions (i.e., tar cracking and steam reforming) associated with H2 formation.20, 35, 37 1Cel-6Ca-1Ni gave the lowest H2 production rate at the first peak and one plausible reason might be the lowest quantity of exposed Ni, since the H2 formation during the first peak was supposed to be predominated by solid-phase catalysis. For the second peak, gas phase reforming became more significant and this step was mainly controlled by mass transfer limitation through the material pores.20, 35 Therefore, the lowest second peak was obtained in case 1Cel-6Ca-4Ni, since it has the lowest specific surface area (3.6 m2·g-1 for 1.5Ca-1Ni). Based on Fig. 5d, the total H2 yield followed the order of 1Cel-6Ca-4Ni < 1Cel-6Ca-1Ni< 1Cel-6Ca-2Ni. Hence, both 13



the exposed catalytic metal and the surface area of the bi-functional materials are important for the hydrogen production in ATT. Again, methane formation was negligible (SI Fig. S4b), and a certain amount of CO and CO2 produced. Therefore, to improve the synergistic effect of the materials for suppressing the COx formation needs further study.

Fig. 5 Effect of molar ratio of cellulose to Ni on the production rates of (a) H2, (b) CO and (C) CO2, and (d) accumulated yield (300-600 ºC) of major gases.

Effect of reaction temperature on H2 production In order to investigate the effect of reaction temperature on H2 production, three temperature schemes, with terminal temperatures 350, 400, and 450 ºC, were selected with a set ramping rate of 40 ºC min-1. After reaching the terminal temperature, an 14


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isothermal condition was maintained for 10 minutes to ensure the completion of reactions. Fig. 6a, 6b and 6c shows the production rate of gas as a function of time for three cases. Apparently, higher temperature resulted in higher gas production rate. According to Fig. 6a, the H2 production rate was greatly enhanced from 350 to 450 ºC, due to higher temperatures promotes thermal cracking. An interesting phenomenon was found that the H2 formation rate went through two decline stages after reaching the maximum in the case of 450 ºC, while it decreased gradually in the other two cases, maybe due to enhanced steam reforming at high temperature. The accumulated H2 yield for three cases are shown in Fig. 6d, and the amount of H2 is notably improved during the heating step at elevated temperature. Previous studies indicated cellulose was supposed to undergo decomposition at the temperature ranges of 300-400 ºC, and could be completely decomposed at around 500 ºC.34, 38 Thus, cellulose may go through a partly decomposed at 350 ºC in this study. One the other hand, secondary cracking rate was so low that little H2 was formed at 350 ºC. However, the least H2 yield was obtained during isothermal step at 450 ºC. These results indicated that there was a gas formation tradeoff between the heating and isothermal steps. Over all, the highest H2 yield obtained was 34.5 mmol·g-1 with volume fraction of 77% (dry basis).

15



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Fig. 6 Effect of reaction temperature on the production rate of gases and H2 yield in the ATT of cellulose (cellulose: 3Ca-1Ni=1: 2, in moles). (a) H2, (b) CO and (C) CO2,and (d) the accumulated yield of H2

Analysis of tar produced from ATT of cellulose Minerals content and pyrolysis temperature were considered to be the key factors controlling the composition of tar, and hence they could influence the gas production indirectly. Therefore, we compared the samples from varied materials and at different temperatures. The key identified compounds for different cases are listed in Table 1. We

found

that

intermediates

with

low

molecular

weight,

such

as

hydroxymethyldihydrofuran-2-one and 3-hydroxy-butanal, were detected in cases 1Cel-6Ca-0Ni and 1Cel-6Ca-2Ni, but were not detected in case 1Cel-0Ca-0Ni. It 16


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implies that Ca-based additives had positive effects on cellulose decomposition. Levoglucosan, was the most prevalent product formed through primary pyrolysis of cellulose,39 however, conflicting results have been reported regarding the effect of calcium on its formation.25, 26, 40 In this study, levoglucosan was available only in the products of pure cellulose pyrolysis (case 1Cel-0Ca-0Ni), while the introduction of Ca(OH)2 reduced the yield of levoglucosan. Furan ring derivatives were found in all the samples, and their formation was considered to compete with the formation of levoglucosan, indicating that Ca(OH)2 could promote the primary pyrolysis of cellulose. However, previous studies revealed that either the formation of levoglucosan or furan ring derivatives was undesirable for gas production, since they lead to tar formation. 41, 42

Based on the qualitative analysis, it is hard to confirm that Ca(OH)2 is beneficial for

tar cracking. A reduction in furan ring derivatives and more short-chain organics are found in the tar composition of in case of 1Cel-6Ca-2Ni and case of 1Cel-6Ca-4Ni (SI Table S2) indicating that Ni promoted gas production by involving in tar cracking. In addition, the presence of butanoic acid, 3-oxo-1, 2-propenyl ester (case 1Cel-6Ca-2Ni), vinyl Ether, propane, acetaldehyde etc. (case 1Cel-6Ca-4Ni) and the absence of hydroquinone and 1,4:3,6-Dianhydro-α-d-glucopyranose in might be attributed to the breakage of C-C bonds by Ni. Concerning temperature effects, we found similar category of main compounds in all cases with different terminal temperatures for 1Cel6Ca-2Ni, which implies that temperature affected the distribution rather than the composition of the tar products.

17



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Table 1. Compounds of tar from ATT of cellulose identified by GC-MS analysis under different conditions RTa (min) 15.06 17.11 21.35 22.43 22.44 23.44 23.76

24.76

25.17

25.71 25.98 26.69 27.24 28.51 28.52

1Cel-6Ca-2Ni

Identified compounds 2(5H)-Furanone 5-methylfuran-2carboxaldehyde 2,5Furandicarboxaldehyde 2-Furanmethanol Levoglucosenone Butanoic acid, 3-oxo-, 2propenyl ester 4-Hydroxydihydrofuran2(3H)-one 5Hydroxymethyldihydrofur an-2-one 5-(1,2Dihydroxyethyl)dihydrofur an-2-one 1,4:3,6-Dianhydro-.alpha.d-glucopyranose 5-(Hydroxymethyl)furan2-carboxaldehyde 2,3-Anhydro-d-mannosan Hydroquinone Formic acid, 1-methylethyl ester 3-hydroxy-butanal

1Cel-6Ca-0Ni

1Cel-0Ca-0Ni

350 ºC

400 ºC

450 ºC

400 ºC

400 ºC

+

+

+

+

+

-

-

-

+

-

-

-

-

+

-

+ -

+ -

-

-

+

+

+

+

-

-

+

+

+

+

+

+

+

+

+

-

-

-

+

-

-

+

-

-

+

+

-

-

-

+

-

-

-

-

+ +

+ -

-

-

+

-

-

+

+

+

+

-

“+”: detected; “-”: not detected a RT: retention time

CONCLUSION In this work, we found that both components of the bi-functional materials, Ca(OH)2 and Ni, played important roles in H2 production. Ca(OH)2 increased H2 production by promoting WGS reaction, enhancing steam reforming reactions, and enlarging the surface area of bi-functional materials. Increasing reaction temperature was favor to the 18


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decomposition of cellulose, but there was a H2 production trade-off between the heating stage and isothermal stage. The optimized molar ratio of cellulose to Ca(OH)2 and Ni was 1: 6: 2, which gave the highest yield of H2 of 34.5 mmol•g-1 with volume fraction of 77% (dry basis) at 450 ºC. According to the tar analysis, we found that in ATT of cellulose, Ca(OH)2 could promote the primary pyrolysis of cellulose，and Ni mainly promoted tar cracking to increase H2 production. Similar components for different temperature cases indicated that temperature affected the distribution rather than the composition of the tar products. These results show that the bi-functional material Ca(OH)2-Ni has a promising potential on producing high purity H2 under moderate conditions. ASSOCIATED CONTENT Supporting Information Details of experiments on synthesis of bi-functional materials and GC-MS analysis please refer to Text S1 and S1, respectively. Targeted ion fragments and representative gas species for TGA-MS test are listed in Table S1. The results of the production rates of H2O and CH4 for the different molar ratio of cellulose to Ca(OH)2, and for the different molar ratio of cellulose to Ni can be found in Fig.S1 and Fig.S2, respectively. AUTHOR INFORMATION Corresponding Author * Phone: +86 10 6278 4701; Email: [email protected] (XZ) ACKNOWLEDGEMENTS

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This work was supported by the Tsinghua University Initiative Scientific Research Program (grant number: 20161080094) & The National Natural Science Foundation of China (grant number: 51506112). REFERENCES (1)

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Table of Content:

Synopsis: Alkaline Thermal Treatment of biomass provides a sustainable approach for clean enengry production under moderate conditions.

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Alkaline Thermal Treatment of Cellulosic Biomass for H2 Production

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