Na2ZrO3 as an Effective Bifunctional CatalystâSorbent during

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NaZrO as an effective bifunctional catalyst-sorbent during cellulose pyrolysis Muhammad Zaki Hassan Memon, Guozhao Ji, Jinhui Li, and Ming Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00309 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 7, 2017

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Industrial & Engineering Chemistry Research

Na2ZrO3 as an effective bifunctional catalyst-sorbent during cellulose pyrolysis Muhammad Zaki Memon †,#, Guozhao Ji†,#, Jinhui Li†, Ming Zhao*,†,‡ †

School of Environment, Tsinghua University, Beijing 100084, China

‡

Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education,

Beijing, 100084, China KEYWORDS. Na2ZrO3, bifunctional material, catalyst-sorbent, sorption enhanced reforming, cellulose.

ABSTRACT. Na2ZrO3 was tested as bifunctional catalyst sorbent using cellulose as model biomass under pyrolytic conditions. Thermogravimetric analyzer connected to a mass spectrometer (TG-MS) was used to study the influence of Na2ZrO3 on the gas evolution from cellulose pyrolysis. The weight loss data and gas evolution was analyzed over a temperature range of 200-800 °C. Na2ZrO3 showed a clear catalytic influence during cellulose pyrolysis and it was actively catalyzing tar cracking and reforming reactions at elevated temperatures. A comparison with CaO was conducted under identical conditions and results showed that Na2ZrO3 mixed samples were able to produce higher yield of hydrogen from cellulose, mainly due to participating in tar-cracking and reforming reactions at lower temperatures than CaO (500 °C for Na2ZrO3, compared to 600 °C for CaO). The study showed that Na2ZrO3 can act as catalyst for pyrolysis reactions of cracking and reforming, and subsequently remove CO2 produced in-situ.

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The results suggest that Na2ZrO3 has potential to participate in gasification of biomass as an effective bi-functional catalyst-sorbent which may enhance hydrogen yield.

1. INTRODUCTION Hydrogen is a vital chemical for multiple industrial applications and it has the potential to be a clean burning energy carrier for the future. Hydrogen offers multiple pathways for its production from different sources but the primary feedstock of hydrogen production are still fossil fuels, accounting for approximately 96% of hydrogen production worldwide.

1, 2

By harnessing

biomass feedstock, which are currently the largest non-conventional energy source of the world, hydrogen can be produced from a renewable and carbon neutral energy resource, thus addressing major concerns with the fossil fuel hydrogen production. 3, 4 Sorption enhanced reforming (SER) is a thermochemical conversion pathway for hydrogen production that relies upon in-situ removal of carbon dioxide (CO2) via sorbent to shift the thermodynamic equilibrium in favor of enhanced hydrogen production.

5

One segment of research focused on SER has been upon

improving the performance of catalyst and sorbent. Integrated catalysis-sorption sites on a single particle have theoretically shown to improve chemisorption kinetics and lead to process intensification in the reactor.

6-9

Catalyst and sorbent

functionalities sharing a single particle instead of conventional separate particles for each functionality brings catalyst site, where product, in this case CO2, is formed closer to sorbent site, where CO2 can be absorbed. With reduced distance the CO2 partial pressure at the intraparticle level will not decrease, which in turn will enhance chemisorption kinetics. Additionally using a single material for both functions will lower the cost of operations. This approach has been adopted for CaO based sorbents, which are known for their infirmity under high-


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temperature multicycle conditions.

10, 11

CaO sorbents need inert support to endure operating

conditions of SER and researchers have dispersed catalyst (Nickel) on the same support practically making metal support based CaO as a bifunctional catalyst-sorbent. Bifunctional CaO has been used for sorption enhanced steam reforming (SESR) of methane and the material was able to produce syngas with hydrogen purity of > 90 %.

12-14

But even with addition of support

CaO based bi-functional material loses it chemisorption potency as number of cycles increase. 15, 16

Hydrotalciltes (HTlc) have also been synthesized as bifunctional material for sorption

enhanced reforming of ethanol, which is a good fit for HTlc based material because ethanol reforming is relatively low temperature process, < 500 °C, and high steam to carbon molar ratio in reactor rehydrates HTlcs.

17, 18

Syngas generated by ethanol reforming in presence of

bifunctional HTlcs has hydrogen concentration of 90% and above. 17, 19 The drawbacks of HTlcs based material lie in their limited range of operation temperature as other hydrocarbon feedstock such as methane and biomass require a temperature for reforming (> 500 °C) which is adversarial for HTlcs stability. Na2ZrO3 possesses highest chemisorption kinetics among alkali metal sorbents (Eq. 1) and sustains its chemisorption capability under multicycle high-temperature process.

20-22

Na2ZrO3

can actively capture CO2 with as low partial pressure as 0.1 bar, reaching saturation within 10 minutes.

23

The carbonates formed as result of chemisorption are also reported as effective

catalyst for tar cracking, methane reforming and water-gas shift reaction (WGS). 24 Na2ZrO3(s) + CO2(g) ⇌Na2CO3(s) + ZrO2(s),

∆H298= − 149 kJ mol-1

Na2ZrO3 has been studied as sorbent for steam methane reforming.

22, 25, 26

(1)

The experimental

conditions used sorbent and catalyst separately and the results showed H2 yield of above 90%.


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Further examples of catalytic ability of Na2ZrO3 include the ability to reform methane for hydrogen production at temperatures of > 800 °C

27

and ability to catalytically convert CO into

CO2 via oxidation reaction.28 Cellulose is a major constituent of biomass and it has been used as a model biomass for performance prediction of various catalysts for gasification. Florin and Harris29 studied mechanistic pathways that are involved in sorption enhanced reforming of biomass in gasifiers using cellulose as model biomass. Zhao et al.30-32 examined the effects of Ni, Co and bimetallic Ni-Co on hydrogen production taking cellulose as biomass sample. Widyawati et al.33 analyzed hydrogen synthesis from biomass samples which included cellulose, xylan, lignin and pine with and without addition of CaO as CO2 sorbent. In this study the performance of Na2ZrO3 as a bifunctional catalyst and sorbent was tested while using cellulose as biomass sample. The results of different loadings and their influence on the cellulose decomposition was recorded.

2. EXPERIMENTAL 2.1. Synthesis and Characterization of material. Na2ZrO3 was prepared by soft-chemistry routine taking sodium oxalate Na2C2O4, (abbreviated to NaOx) and zirconium nitrate Zr(NO3)4 as sodium and zirconia precursors, respectively. The molar ratio of precursors was set at 2:1 for Na to Zr. The appropriate amounts of NaOx and Zr(NO3)4 were dissolved in 100 ml deionized (DI) water. The addition of precursors into DI water led to greyish colored solution indicating that precursors were not homogenously dissolved in the solution. To remedy this, oxalic acid was added dropwise until clear solution was obtained.34 The solution was placed in 60 °C hot water bath to initiate evaporation drying. The dried precursor powder was clacined in the furnace at


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850 °C for 8 hours to form Na2ZrO3. Methyl cellulose (Vetec) was mixed with prepared material with weight ratio of 1:0.5, 1:1 and 1:2 (cellulose:Na2ZrO3), subsequently the sample were labeled according to Na2ZrO3 presence i.e. NZ-0.5, NZ-1 and NZ-2. CaO (Damao Chemical Company, China) was used for comparison. The phase detection of material was undertaken by the use of X-ray diffraction (XRD, D/Max 2500V+/PC). 2.2. Determination of catalyst-sorption activity. Thermogravimetric analyzer (SDT Q600) connected to mass spectrometer (HPR20) (TG-MS) was used to study the pyrolysis and the subsequent gas evolution of samples. Experiment regime was carried out at a constant Argon (Ar) flowrate of 500 ml min-1, which purged the evolved gas species from the reaction zone of TGA where pyrolysis occurred. A heated capillary collected a sample of purged gases and delivered it to MS to determine the composition of the gas species. In this manner, evolved gas species from pyrolysis can be rapidly analyzed in real time corresponding to the ongoing pyrolysis. The heating rate of pyrolysis was set at 40 °C min-1 for all experiments and pyrolysis was carried up to 900 °C. To ensure that no air was in reaction zone, an Ar purge (500 ml min-1) was conducted before initiating the pyrolysis until the all signals were stable. At the end of pyrolysis phase, air was introduced to burn off the char left in the crucible. MS ionizes the gas molecules collected by heated capillary and differentiates the resulting positive ions on the basis of their mass to charge ratio (m/z). The ions set to be scanned by MS and their corresponding evolved gas species are given in Table 1. The scanning species were limited to products of major reactions that occur during pyrolysis. Ar (m/z = 40) was observed as a measure to ensure consistency of evolved gases delivery through heated capillary. Table 1. Key ion fragments and representative evolved gas species.


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m/z Ion fragments

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Representative species

2

H2+

Hydrogen

15

CH3+

Methane

18

H2O+

Water

28

CO+

Carbon monoxide

40

Ar+

Argon

44

CO2+

Carbon dioxide

The calculation to estimate generation rate and production of gas species was done by recorded signals by the method described by Zhao et al.

31

Experiments were repeated with

differing ratios of cellulose to Na2ZrO3.

3. RESULTS AND DISCUSSIONS 3.1. Catalyst Characteristics. XRD analysis revealed that synthesis method was able to produce pure Na2ZrO3 nanocrystals and no evidence of impurity i.e. ZrO2, Na2CO3 or NaHCO335 was found (Figure 1). Na2ZrO3 obtained consisted of two phases: monoclinic and hexagonal Na2ZrO3. Studies on the effects of phases on the chemisorption performance of Na2ZrO3 show that monoclinic phase is more active towards CO2 chemisorption, hence it is a more desirable phase for Na2ZrO3 sorbent.

23

In order to investigate which phase was dominant in the material

the relative intensity of the peaks present in the material were compared with peaks of model monoclinic and hexagonal compounds available in JCPDS database.

23, 36

Figure 1 labels the

most intense peaks achieved from data analyses and Table 2 compares the labeled peaks with model peaks of monoclinic and hexagonal compounds. With the exception of peak Ia/Imax of Na2ZrO3 which was closer to hexagonal peak at Ia/Imax, the rest of peaks of Na2ZrO3 i.e. Ib-e/Imax


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were closer to monoclinic peaks intensities at Ib-e/Imax indicating that although the sample has a hexagonal Na2ZrO3 phase but the dominant phase of sample was monoclinic Na2ZrO3.

Figure 1. XRD diffraction patterns of synthesized Na2ZrO3. Table 2. Intensity ratios of diffraction peaks for Na2ZrO3 samples. Label

2θ

Hexagonal

Monoclinic

Na2ZrO3

Ia/Imax

16.16

1

0.37

0.78

Ib/Imax

32.2

0.06

0.23

0.285

Ic/Imax

33.6

0.1

0.4

0.48

Id/Imax

38.73

0.2

1

1

Ie/Imax

56.6

0.06

0.27

0.277


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3.2. Influence of Na2ZrO3 on cellulose pyrolysis. Cellulose is a well-researched glucose polymer which breaks down into lighter species upon heating. The main reactions governing the decomposition of cellulose are presented in Table 3. Under pyrolysis, cellulose registered a major weight loss at the temperatures of 350-400 °C (Figure 2) and the polymer chains of cellulose degraded into tar, chars and multiple gaseous species i.e. H2, CH4, CO, CO2, H2O, aldehydes, ketones, and organic acids. The volatile products further interacted with each other via secondary gas-phase and gas-solid reforming reactions. Reforming reactions occurred in concurrence with thermal cracking of tars and the resulting products also took part in reforming reactions. Table 3. Reactions of pyrolysis of cellulose. Reaction

Eqs.

Equation

Pyrolysis

(2)

Biomass →H2 + CO + CO2 + CH4 + H2O + tar + char

Catalytic Tar Cracking

(3)

Tars → H2O + H2 + CO + CO2 + CH4 + CnHm + CH3OH + CH2O + CH3CHO

∆H298 (kJ mol-1) >0 >0

(4)

CH3OH → CH2O +H2

85

(5)

CH2O → CO + H2

5.4

(6)

CH3CHO → CH4 + CO

-19.6

Dry Reforming

(7)

CnHm + nCO2 ⇌ 2nCO + (m/2)H2

247.3

Hydrocracking

(8

CnHm + H2 ⇌ Cn-xHm-y ...+ H2 + CH4 ...+ C

CH4 Steam Reforming

(9)

CH4 + H2O ⇌ CO + 3H2

Tar Steam Reforming

(10)

CnHm + nH2O ⇌ nCO + (n + m/2)H2

>0

Thermal Cracking

(11)

CnHm ⇌ Cn-xHm-y ...+ H2 + CH4 ...+ C

>0

(12)

C + H2O ⇌ CO + H2

131.3

(13)

C + 2H2O ⇌ CO2 + H2

90.1

Boudouard

(14)

C + CO2 ⇌ 2CO

172.5

Water-Gas Shift

(15)

CO + H2O ⇌ CO2 + H2

-41.2

Methanation

(16)

C + H2 ⇌ CH4

-74.9

Catalytic Cracking

Water Gas

*

>0 206.2

represents one or many lighter compounds than CnHm.


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Figure 2. DTG of cellulose and NZ-1 under pyrolysis. Figure 2 showed mass loss rates by plotting the DTG curve to detect the influence of addition of Na2ZrO3 on the cellulose pyrolysis. As it can be seen Na2ZrO3 caused an early degradation at approximately 295 °C, and reached peak weight loss at 337 °C, a difference of 58 °C from pure cellulose sample. This indicated catalytic participation of Na2ZrO3 in decomposition of cellulose, specifically lowering the activation energy of cellulose decomposition (Eq. 2). Alkali metal species as catalyst are generally used in the form of alkali metal carbonates and are reported to promote unzipping of cellulose during biomass gasification. 37, 38 This is not the only instance of alkali metal zirconates acting as catalyst for biomass conversion, Li2ZrO339 and Na2ZrO340 have been used as catalyst for esterification/transesterification reactions to a good effect.


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Figure 3. Hydrogen yield rate of different samples. The presence of Na2ZrO3 made a distinct impression on the profile of thermal degradation of cellulose. Figure 3 showed the hydrogen yield rates for cellulose only (sample marked as Cellulose) and Na2ZrO3 mixed with cellulose samples (labeled as NZ-0.5, NZ-1 and NZ-2 according to their weight ratio). Na2ZrO3 was acting as catalyst to initiate tar cracking at a lower temperature. NZ-2 appeared first to set off pyrolysis reaction in cellulose followed by NZ-1 and NZ-0.5. The lag in degradation can be accounted by the fact that the sample with earliest degradation had the highest mass ratio of Na2ZrO3 and all the cellulose was in contact with catalyst sites on Na2ZrO3, which resulted in a near instantaneous breakdown of cellulose chains. The hydrogen release from cellulose-only sample was noted by MS as a small peak but the samples with the presence of Na2ZrO3 produced a sharper peak of hydrogen caused by primary pyrolysis and subsequent cracking reactions (Eqs. 3-6) which involves further degradation of methanol, aldehyde and formaldehyde into H2, H2O and CO, signifying C-C and C-H bond


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breaking ability from Na2ZrO3. Increased release of CO2 during pyrolysis was linked to promotion of C=O double bond formation by Na2ZrO3, an activity which has been reported for potassium catalyst. 41 Hydrocarbon reforming reactions (Eqs. 7-10), might also be involved in H2 production at this stage. There was a cessation in hydrogen production before a second hydrogen peak appeared at temperature above 450 °C. Na2ZrO3 has exhibited CO2 chemisorption affinity at the temperatures as low as 500 °C. 21 Na2ZrO3 also possesses catalytic capacity to oxidize CO into CO2 and, subsequently, absorb generated CO2 in-situ.27 It was observed in all Na2ZrO3 samples that a continued H2 production between the temperatures of 500 ~ 700 °C. It should be noted that TGA recorded a tiny but steady weight loss during the period of H2 production between 500 ~ 700 °C. This suggested that H2 production was derived from secondary reactions like tar cracking and reforming Eqs. 4-6, 9-11. TGA registered a visible weight loss at temperature around 700 °C, but this it did not correspond to any H2 production. This weight loss was related to tar cracking and CO2 release. The effects are discussed in coming sections of the paper.


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Figure 4. XRD patterns (from bottom to top) of freshly calcined Na2ZrO3, sorbent after capturing CO2, cellulose residue after pyrolysis up to 600 °C and Na2ZrO3 mixed cellulose after pyrolysis up to 600 °C. 3.3. In-situ CO2 chemisorption. Evidence of CO2 chemisorption by Na2ZrO3 can be observed in Figure 4, where XRD patterns showed the presence of Na2CO3 phase. Four sample were analyzed for diffraction patterns, those samples were freshly calcined Na2ZrO3, Na2ZrO3 after capturing CO2 to saturation, cellulose residual after pyrolysis up to 600 °C, and Na2ZrO3 mixed cellulose after pyrolysis up to 600 °C. To obtain fully chemisorbed Na2ZrO3, fresh Na2ZrO3 was placed in TGA and then the temperature was raised to 600 °C. At 600 °C, CO2 was


12

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introduced at 1 bar and the conditions were held for 15 minutes until weight gain by Na2ZrO3 due to CO2 chemisorption stopped. XRD patterns for fresh Na2ZrO3 and Na2ZrO3 after absorbing CO2 were completely different because after CO2 chemisorption the Na+ together with O2- in the sorbent will convert into Na2CO3 leaving ZrO2 behind.42 Hence, the diffraction profile of this Na2CO3+ZrO2 phase will be different from Na2ZrO3. Na2ZrO3 mixed cellulose after heating exhibited similar peak patterns to the fresh Na2ZrO3. However, in addition to the peaks which were also presented in fresh Na2ZrO3, several evident extra peaks (2θ = 30º, 35º and 61º) were observable in Na2ZrO3 mixed cellulose. There were two possible contributors to these extra peaks. One may come from the change of Na2ZrO3 to Na2CO3+ZrO2 phase; the other possibility is from the pyrolyzed cellulose. The peak pattern of pyrolyzed cellulose residual didn't show peaks at these angles, which eliminated the second possibility. As such the extra peaks could only be the formation of Na2CO3+ZrO2 phase. Moreover, these extra peaks matched the standard major peaks of Na2CO3 and ZrO2, further confirming the CO2 chemisorption by Na2ZrO3 Eq. 1. Hence the evidence from XRD patterns suggested that Na2ZrO3 was able to chemisorb the CO2 formed from catalytic reactions in-situ, demonstrating the application of Na2ZrO3 as bifunctional catalyst-sorbent in practice.


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Figure 5. Gas yield rate of CO2 (a) and CO (b) from different samples. Desorption of CO2 was not observed by MS, however a visible CO peak was witnessed at the temperature of 700 °C (Figure. 5). A likely possibility was Boudouard’s reaction Eq. 14 due to release of CO2 and presence of C as char. In Boudouard’s reaction the equilibrium increasingly shifts towards the formation of CO after the temperature exceeds 700 °C.43 Alkali carbonates are active catalysts for gasification reactions.44

Figure 6. Mass loss rate of cellulose, NZ-0.5, NZ-1 and NZ-2. 3.4. Different ratios of cellulose to Na2ZrO3. The different ratios of cellulose to Na2ZrO3 had no significant effect in the final gas yield (Table 4). Apart from a weight loss from cellulose


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degradation, a second weight loss was registered at in the vicinity of 690 ~ 700 °C for all mixed samples (Figure 6). The second weight loss was due to release of CO2. It should be noted that even though the weight loss curves remains stable at the temperatures between 500 ~ 700 °C, there was a slight steady decline in curve denoting the mass loss of sample. This weight loss indicated that continuous reforming and cracking reactions, Eqs. 9-11, were taking place. Figure 6 displayed the weight loss curves of cellulose and mixed samples. NZ-0.5 displayed two consecutive peaks of weight loss during initial stage which may be due to its low Na2ZrO3 to cellulose ratio. As Na2ZrO3 in NZ-0.5 was not in contact with all the available cellulose, so the first peak only corresponded to the catalyzing of the cellulose which was in contact with the Na2ZrO3. The second peaks corresponded to catalysis of the remaining cellulose as it comes in contact with Na2ZrO3. These peaks appeared to act as batch reactions instead of continuous reactions which suggested that there were kinetics limitation governing the catalysis of pyrolysis reaction by Na2ZrO3. Another effect of different ratio was recorded for the production CO (Figure 5), where NZ-2 exhibited an enhanced CO peak during CO2 desorption stage. The reason could be that NZ-2 was able to absorb larger quantity of CO2 because it contained more Na2ZrO3 than other samples. This captured CO2, when released, might have acted as a reactant for Boudouard’s reaction Eq. 14 leading to an increased CO synthesis. Moreover, more Na2ZrO3 were likely to increase the activity of tar cracking, and produce more CO. Table 4. Accumulative gas production from cellulose pyrolysis up to 800 °C. CH4

Sample

H2

CO

CO2

ml mg-1

Cellulose

0.02

0.03

0.08

0.10

NZ-0.5

0.04

0.18

0.11

0.14


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NZ-1

0.03

0.19

0.12

0.13

NZ-2

0.03

0.19

0.14

0.14

CaO + Cellulose

0.04

0.14

0.15

0.15

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Figure 7. Gas yield rate by NZ-1 and CaO. (a) H2, (b) CO, and (c) CO2 3.5. Comparison with CaO. To get a better understanding of Na2ZrO3’s performance, CaO, which has been known as the most widely used CO2 sorbent, was mixed with cellulose (1:1 mass ratio) and pyrolyzed under same conditions. CaO has been reported for enhancing hydrogen production from cellulose sample.

33

Figure 7 displayed and contrasted the gas evolution from

pyrolysis of cellulose in the presence of Na2ZrO3 and CaO. Both materials were able to derive hydrogen from pyrolysis of cellulose (Figure 7 (a)), however Na2ZrO3 lowered the thermal degradation temperature significantly, which implied lower activation energy of pyrolysis


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reaction was required. Moreover, the intensity of the first H2 generation peak was higher for Na2ZrO3 mixed cellulose, which demonstrated higher catalytic activity of Na2ZrO3 in volatile matter decomposition. Both Na2ZrO3 and CaO gave secondary peaks of H2 generation. The peak intensity and width also showed the advantage of Na2ZrO3 over CaO in producing H2. In the temperature range of secondary H2 peak, the major reactions are tar cracking. The comparison of Na2ZrO3 and CaO over the two H2 generation peaks suggested that Na2ZrO3 was more active in catalyzing volatile decomposition and tar cracking. Along with the first H2 peak, CO and CO2 also showed rapid generation in the same temperature range (Figure 7 (b) and (c)). Na2ZrO3 mixed cellulose produced more CO2 (200 ~ 500 ºC). This was likely attributed to the lower CO2 chemisorption capacity in Na2ZrO3 than in CaO. A secondary CO peak was observed for Na2ZrO3 at 700 ºC (Figure 7 (b)). Decarbonation test was undertaken for a fully sorbed Na2ZrO3 (mainly contained Na2CO3 and ZrO2) in pure N2 with 1 ºC min-1 ramping rate, the CO2 releasing temperature was from 700 to 930 with peak rate at 909 ºC (Figure S1). This temperature range favors the formation of CO from Boudouard reaction, which could partially explain the CO generation at round 700 ºC. To further inspect the cause for the secondary CO, pure cellulose was pyrolyzed until 600 ºC, and then the residual tar was collected for a subsequent pyrolysis test in the presence of Na2ZrO3 and CaO. The gas generation during the pyrolysis from residual tar was provided in Figure S2. An evident CO peak appeared also at 700 ºC for Na2ZrO3 mixed tar, suggesting the secondary CO generation was also from tar cracking. A secondary CO2 peak started from about 620 ºC for CaO mixed cellulose (Figure 7 (c)). Figure S1 implied that the CO2 came from decarbonation of CaCO3 (617 ºC). This temperature was relatively low to favor Boudouard reaction, as such the secondary CO peak was evident for CaO.


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Figure 8 summarized the accumulative yield of H2, CO, CO2 and CH4 from cellulose, cellulose mixed Na2ZrO3 and cellulose mixed with CaO. The presence of functional materials significantly enhanced the total gas yield, owing to the catalytic activity of the functional material. Moreover, the most remarkable enhancement was for H2 gas. On top of the catalytic function, both materials could capture CO2, which in turn intensified the steam reforming reactions as well as water gas shift, thus H2 yield was enriched in the product gas. Compared to CaO, Na2ZrO3 produced H2 by two times more, mainly due to its higher catalytic activity in volatile decomposition and tar cracking, as well as a wider temperature range for CO2 chemisorption.

Figure 8. Accumulative gas production for CH4, H2, CO and CO2.

4. CONCLUSIONS


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Soft chemistry synthesized Na2ZrO3 was tested as bifunctional catalyst–sorbent for cellulose pyrolysis. The results showed catalytic behavior and in-situ CO2 removal in the presence of Na2ZrO3. Na2ZrO3 had a prominent effect on the degradation temperature of cellulose, lowering the activation energy required for pyrolysis reaction and initiating cellulose breakdown 55 °C lower than pure cellulose pyrolysis. Na2ZrO3 enhanced the yields of H2 during pyrolysis via cleavage of C-H and C-C bonds. At elevated temperatures (>500 °C), Na2ZrO3 was able to participate in tar cracking and reforming reactions and delivered H2 from side reactions. Furthermore, Na2ZrO3 as a CO2 sorbent was able to absorb CO2 produced by different reactions in-situ. XRD suggests that Na2ZrO3 was able to chemisorb CO2 generated in-situ, causing an increase in the hydrogen yield due to sorption enhanced reforming. The comparison between Na2ZrO3 and CaO showed that more H2 could be produced for Na2ZrO3 mixed cellulose, ascribed to the higher catalytic activity of Na2ZrO3 in volatile decomposition and tar cracking, together with a wider range of CO2 capture temperature.

AUTHOR INFORMATION Corresponding Author * Phone: +86 10 6278 4701; Emails: [email protected] (MZ) Author Contributions #

These authors contributed equally.

Funding Sources


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This work was supported by the National Natural Science Foundation of China (grant number: 51506112) and the Tsinghua University Initiative Scientific Research Program (grant number: 20161080094). ACKNOWLEDGEMENTS This contribution was identified by symposium chair as the Best Presentation in the session “Chemistry of Biomass Waste Conversion to Energy & Chemicals” in the Division of Environmental Chemistry of the 2016 ACS Fall National Meeting in Philadelphia, PA.

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