Investigation of coking behaviors of model compounds in bio-oil during steam reforming

Fuel 265 (2020) 116961

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Investigation of coking behaviors of model compounds in bio-oil during steam reforming

T

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Xianglin Lia, Zhanming Zhanga, Lijun Zhanga, Huailin Fana, Xueli Lib, , Qing Liuc, Shuang Wangd, ⁎ Xun Hua, a

School of Materials Science and Engineering, University of Jinan, Jinan 250022, China Shaanxi Key Laboratory of Chemical Reaction Engineering, Department of Chemistry and Chemical Engineering, Yan′an University, China c College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China d School of Energy and Power Engineering, Jiangsu University, Jiangsu 212013, China b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Bio-oil Steam reforming Model compounds Molecular structures Coke properties

Sugars and the derivatives of sugars are important fractions in bio-oil, understanding coking behaviors of which are important for development of the robust coking–resistant catalyst for steam reforming of bio-oil. In this study, steam reforming of glucose, xylose, acetic acid and furfuryl alcohol (FA) was carried out, aiming to correlate coking behaviors with molecular structure of these organics. Acetic acid, as a small aliphatic molecule, could be effectively reformed and produced the lowest amount of coke deposit. The carbonyl functionality, the multiple hydroxyl groups in the sugars and the furan ring in FA made polymerisation/cracking to form coke as the dominant reaction route in their steam reforming, diminished hydrogen production while led to rapid catalyst deactivation. The coke formed from acetic acid and FA was more aromatic, containing more C]C species, while that from glucose and xylose was more aliphatic, containing more carbonyl functionalities, which projected structural characteristics of the feedstock. In addition, morphologies of the coke formed from acetic acid was mainly carbon nanotube. In comparison, the coke from the sugars and FA was mainly the amorphous coke

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Corresponding authors. E-mail address: [email protected] (X. Hu).

https://doi.org/10.1016/j.fuel.2019.116961 Received 9 September 2019; Received in revised form 17 November 2019; Accepted 25 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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with cobalt particles wrapped inside, which was more thermally stable, especially for that from FA, relating to the aromatic ring in FA.

1. Introduction

2. Experimental

Bio-oil is a liquid product from the pyrolysis of the renewable and sustainable biomass [1,2], which is a complex mixture of organics including carboxylic acids, sugars, furans, phenolic monomer or oligomers [3,4]. The organics in bio-oil can be converted into biofuels via removing the excess oxygen in the organic mixtures [5]. The hydrotreatment of bio-oil requires the use of hydrogen as a reactant [6,7]. How to obtain hydrogen via cost–effective ways determines the efficiency from bio-oil to biofuel. As mentioned above, bio-oil contains abundant organics and also abundant water, especially in the aqueous phase of bio-oil [8]. Steam reforming of the organics in bio-oil with the water inside bio-oil is a viable way to produce the hydrogen for hydrotreatment of bio-oil in a biorefinery. During the steam reforming of the organics in bio-oil, coking is a major challenge in the process, which accumulated on surface of catalysts, leading to deactivation of catalyst and/or blockage of the reactors [9,10]. Coking results from the incomplete gasification of the carbonaceous species formed on surface of the catalyst [11,12]. Nevertheless, it has also reported that the properties of the coke formed in steam reforming were significantly affected by the molecular structure/properties of the reaction substrate [13,14]. In bio-oil, sugars and derivatives of sugars are the major components [15,16]. This is because that the pyrolysis of cellulose and hemicellulose, the major components of biomass, mainly produced sugars and derivatives of sugars [17,18]. Understanding the reaction behaviors, especially the coking behaviors of sugars and the derivatives of sugars during steam reforming is of importance for optimization of the process for steam reforming of biooil. In this study, glucose, xylose, acetic acid and furfuryl alcohol (FA) were selected as the model compounds for the investigation of the coking behaviors of the sugars and their derivatives in bio-oil over Co/ SBA–15, a catalyst with the support of ordered structure [19,20] and with cobalt as the active species for steam reforming of varied organics [21–24]. Glucose is a typical C6 sugar from the degradation of cellulose [25,26], while xylose is a typical C5 sugar from the degradation of hemicellulose [27,28]. Acetic acid and FA are the typical derivatives from the further degradation of sugars [29,30]. In existing literatures, there have been some studies about steam reforming of glucose, xylose, acetic acid, furfuryl alcohol (FA) [31–35]. Nevertheless, the focus of the studies was mainly on the development of catalysts for improving the catalytic activities and especially stabilities. The correlation of the structure of the reaction substrates with the properties of the resulting coke has not been fully understood yet. The study of the properties of coke is of significance for achieving at least the following three aims: (a) understanding the reaction network for coke formation; (b) understanding the mechanism for deactivation of the catalyst; (c) optimizing the catalyst formulation for obtaining the coke–resistant catalysts. Glucose, xylose, acetic acid and FA have distinct molecular structures. The properties of the coke formed from steam reforming of these individual representative compounds might also be different, which were investigated in this study over the same catalyst and under the similar reaction conditions. The results showed that the morphologies and other physiochemical properties of the coke formed were closely related to the molecular structure of the reaction substrates.

2.1. Preparation of the catalysts SBA–15 molecular sieve was used as the carrier of the cobalt catalyst, which was synthesized according to the procedures specified in the reference [20]. The typical procedures for the synthesis of SBA–15 were shown below. 57 mL of hydrochloric acid with a concentration of 36.5% was mixed with 10 g of P123 and 307 g of distilled water to form a solution. The mixture was stirred in a water bath at 35 °C for 2 h and then 23 mL TEOS was added, which was stirred continuously at 35 °C for 24 h. The solution was then aged at 80 °C for another 24 h. The filtrated after washing was calcined in a muffle furnace at 550 °C for 5 h. The Co/SBA–15 catalyst was prepared using an equivalent wetness impregnation method [36]. The given amount of Co(NO3)2·6H2O, which was calculated to achieve a Co loading (in metallic form) of 22% to SBA–15, was dissolved in distilled water to prepare a solution. The prepared solution was then mixed with the pre–dried SBA–15 for the impregnation. The impregnated samples were dried at 105 °C for 12 h to remove the moisture absorbed. After that, the sample was calcined at 600 °C for 2 h in a muffle furnace in air atmosphere to obtain the Co/ SBA–15 catalyst in calcined form. Finally, the prepared sample was pressed to tablets and crashed to the particles with the size in the range of 0.4 to 1.25 mm. 2.2. Catalytic tests Steam reforming of the organics were carried out in a fixed bed continuous–flow reactor (inner diameter: 20 mm; outer diameter: 25 mm) heated in a furnace (OTF-1200X-S-VT; length of heating zone: 200 mm; thermocouple: K type; max temperature: 1000 °C; power rating: 1.2 kW; in-tube air pressure: < 0.02 MPa). 0.5 g of Co/SBA–15 catalyst with the bed height of ca. 0.4 cm was placed in the constant zone of the furnace. The calcined catalyst was reduced with a mixture of H2 and N2 at a flow rate of 120 mL/min (flow rate of each: 60 mL/ min) at 600 °C for 0.5 h before the reforming experiment. For steam reforming, the specific reaction conditions were as follows: carrier gas: N2 with the flow rate of 60 mL/min; steam to carbon ratio (S/C) = 5; liquid hourly space velocity (LHSV) = 5.7 h−1; reforming temperature: 300–600 °C, holding time at each investigated temperature: 30 min. The liquid products were trapped in two gas–liquid separators connected in tandem. It needs to note that the liquid products were collected at each investigated temperature and analyzed, respectively. The non–condensable gaseous products were collected and quantified with gas chromatography. At the end of the test, the catalyst was cooled to room temperature in a 60 mL/min N2 stream to prevent the oxidization of the catalyst. The spent catalyst was then recovered for further analysis.

LHSV =

S/C =

Volumetric flow rate of feed liquid (cm3h−1) Catalyst bed volume (cm3)

Moles of steam in the fed Moles of carbon in the fed

The reaction formula for steam reforming of the varied reaction substrates to produce hydrogen is:

m Cn Hm Ok + (2n−k)H2 O→ nCO2 + ⎛2 n+ −k⎞ H2 2 ⎝ ⎠ The conversion of the reactant was calculated via the following 2

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accurately weighed) was reduced at the temperature from 50 to 900 °C with a ramping rate of 10 °C/min by using a flow rate of 5 vol% H2/Ar of 30 mL/min. The signal for hydrogen consumption was monitored by a thermal conductivity detector (TCD). The reaction intermediates formed in steam reforming of the different reactant substrates were determined by a Nicolet IS 50 spectrometer with an in situ Harrick Cell. To prevent overheating during the heating process, the reactor is cooled by circulating water. The specific conditions for characterization of in–situ Diffused Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) of steam reforming are as follows: S/C = 5; nitrogen carrier gas: 75 mL/min; reaction temperature: 100–600 °C; heating rate: 10 °C/min. X–ray diffraction (XRD) characterization of the calcined or reduced cobalt–based catalyst was analyzed by using Rigaku Ultima IV diffractometer with the Cu Kα target and λ of 1.5406. The scan range was 10.0° to 80.0° and the scan rate was 20°/min. The operating voltage was 40 kV and the current was 40 mA. The particle size of nickel was calculated from the results of the XRD characterization with the Scherrer formula. The liquid products after the catalytic tests or steam reforming of acetic acid versus the prolonged reaction time were analyzed with GC–MS fitted with a capillary column (DB–Wax) (length: 30 m; internal diameter: 0.25 mm; film thickness: 0.25 mm). Helium was used as the carrier gas at a flow rate of 4.0 mL/min. The sample was diluted to a certain concentration with ethanol, and then 0.5 μL of the sample was injected into the inlet with a split ratio of 50:1. The column temperature was maintained at 35 °C for 3 min, then raised to 250 °C at a ramp rate of 10 °C/min and held for 3 min. Compounds in the MS spectrum were identified by comparison to the standard spectra of the National Institute of Standards and Technology (NIST 2014). The conjugated ring structures of polymers in the liquid products were measured with a UV–fluorescence spectrometer (Shimadzu,

equation:

The conversion of the reactant Mole of the reactant in product ⎞ = ⎛1 − − × 100% Mole of reactant in feedstock ⎠ ⎝ Yield of CO2 /CO/CH 4 The mole of CO2 /CO/CH 4 in the product ⎞ =⎛ × 100% The mole of carbon in feedstock ⎝ ⎠ H2 yield of acetic acid =

H2 yield of glucose =

H2 yield of xylose =

H2 yield of FA =

The mole of H2 in the product The mole of acetic acid consumed ×4

The mole of H2 in the product The mole of glucose consumed ×12

The mole of H2 in the product The mole of xylose consumed×10

The mole of H2 in the product The mole of FA consumed×11

2.3. Characterization of the catalysts The specific surface area of the support and the calcined catalyst was determined by N2 adsorption–desorption measurement at 77 K by using the Brunauer–Emmet–Teller (BET) method with an SSA–6000 apparatus. Prior to N2 adsorption, the sample was degassed at 200 °C to remove the moisture adsorbed on and in the porous structure, followed by adsorption and desorption of nitrogen. Hydrogen temperature programmed reduction characterization (H2–TPR) was carried out in a U–tube quartz reactor in a PCA–1200 chemisorption analyzer. The sample of Co/SBA–15 (about 30 mg but

Fig. 1. Characterization of the catalyst: (a) XRD; (b) TPR; (c) the isothermal curves, the symbol of triangle means adsorption isotherm curves and the round means desorption isotherm; (d) Volume distribution diagram of BJH desorption hole. 3

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sample was then dried in a vacuum oven at 80 °C for 1 h to remove physically absorbed water. The dried sample was pressed into a thin disc sheet having good light transmittance for the FT–IR measurement. Transmission electron microscopy (TEM) analysis was performed using a JEOL 2010 instrument to observe the microscopic morphology of the used catalyst. About 6 mg of the catalyst fine powder was dispersed in an ethanol solution and sonicated into a suspension. The dispersed mixture was then dropped on a copper mesh for drying and subsequent characterization with electron microscope transmission.

Table 1 Characterizations of the Co/SBA–15 and SBA–15 catalysts.a Properties 3

Pore volume (cm /g) Mean pore radius (Å) Surface area (m2/g)

Co/SBA–15

SBA–15

1.0 41.0 486.0

0.79 25.0 630.0

a Pore volume, mean pore radius and surface area were calculated with BET method.

RF–6000). The testing conditions were as follows: excitation wavelength range: 230–480 nm; wavelength range: 210–500 nm; scanning speed: 600 nm/min. Before testing, ethanol was used as solvent to dilute the sample to 400 ppm. The amount of coke deposit on the catalyst after the reforming test was investigated by using TG/DTA (TG8121 Rigaku Corp.). A certain amount of sample (ca. 10 mg) was heated from room temperature to 900 °C in air or in nitrogen (50 mL/min) at a heating rate of 20 °C/min and the weight change was recorded. Temperature programmed oxidation (TPO) characterization of the spent catalysts was performed in a U quartz tube to evaluate the reactivity of coke towards oxidation by using PCA–1200 instrument. Approximately 30 mg of catalyst was placed in a U–shaped quartz tube and heated to 800 °C in a 5% O2/He stream (30 mL/min) at a temperature ramping rate of 20 °C/min. The coke formed on the catalyst was analyzed by using EuroEA3000–Single instrument. The content of elements such as C, H, and N is measured by the combustion at the high temperature and the quantification of the gaseous products with TCD detectors. The spent catalysts after the steam reforming reaction was characterized by Fourier transform infrared spectroscopy (FT–IR) on a Nicolet iS50 instrument to analyze its functionalities. The sample and KBr were mixed at a ratio of 1:200 and uniformly ground. The ground

3. Results and discussion 3.1. Physicochemical properties of the catalysts 3.1.1. XRD analysis Fig. 1a shows the XRD patterns of the calcined and reduced Co/ SBA–15 catalyst. In the calcined catalyst, the diffraction peaks at 2θ = 19.0°, 31.3°, 36.8°, 38.5°, 44.8°, 55.5°, 59.4°, 65.3° and 77.8° were the characteristic diffraction of Co3O4 phase (PDF# 43–1003); In the reduced catalyst, the diffraction angles of 2θ = 44.3°, 51.4°, and 75.8° were the characteristic diffraction peaks of metallic Co phase (PDF#15–0806). In addition, the diffraction angle of 2θ = 41.8° for CoO phase (PDF#09–0402) was also observed. The broad diffraction peak of SiO2 was observed at the diffraction angle from 15° to 30° (PDF#18–1170). The particle size of cobalt species was calculated and shown in Table S1. After the reduction to remove the oxygen in Co3O4, the particle size of metallic Co should be decreased, which, however, did not decrease remarkably, indicating the agglomeration of metallic cobalt species during the reduction. The interaction between cobalt oxides and the support was probably not high, which was subsequently investigated by the TPR characterization to probe the reduction behavior of cobalt oxides on the surface of SBA–15.

Fig. 2. Steam reforming of the varied organics. Reaction conditions: LHSV = 5.7 h−1; S/C = 5; P = atmospheric pressure. 4

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products increased significantly. Among the four reaction substrates, only acetic acid could be effectively reformed. The yield of H2 reached ca. 80% at 400 °C. The yields of hydrogen from glucose, xylose and FA were much lower. Nevertheless, the conversion of the sugars and FA reached 100% even at 300 °C, which was converted, but not via steam reforming. The organic by–products formed from glucose and xylose were shown in Fig. S1. The results showed that the sugars were mainly cracked or degraded, forming ketones, aldehydes, etc. as the by–products. These ketones and aldehydes were prone to polymerisation or cracking to form coke. In addition to this, it has also reported that the sugars showed the very tendency towards coking [39,40]. The formation of coke, especially at the low temperatures where the organics could not be activated enough to be reformed, would rapidly deactivate the catalyst. This should also apply to the steam reforming of FA, in which few volatile products could be identified via the GC–MS characterization. FA was prone to polymerization [41], especially at the high reaction temperature employed.

3.1.2. TPR analysis Fig. 1b shows the TPR spectrum of the Co/SBA–15 catalyst. The catalyst had three main peaks, locating at 361 °C, 384 °C and 683 °C, respectively. The reduction of Co3O4 to metallic Co species proceeded step wisely with CoO oxide as the intermediate phases [37]. This led to the appearance of the several reduction peaks. In addition to this, the cobalt oxides that strongly interacted with SBA–15 also shifted the reduction temperatures to a higher range, as evidenced by the reduction temperature at 683 °C. The main reduction peaks of cobalt oxides were below 400 °C, indicating that majority of the cobalt oxide had weak interaction with the SBA–15 support. 3.1.3. BET analysis Fig. 1c and 1d shows the hysteresis loop and the distribution the pores for SBA–15 and Co/SBA–15 catalyst. The type of hysteresis loop for SBA–15 and Co/SBA–15 catalyst belonged to the H3 type, representing the mesoporous structure. The area of the hysteresis loop of Co/SBA–15 catalyst was lower than that of SBA–15, which was mainly caused by the blockage of the carrier by loading cobalt species. The result in Fig. 1d also showed that the pores in the range of 20 to 30 Å in semi–diameter also decreased in abundance, confirming the filling of some of the pores with cobalt species. Nevertheless, some additional pores were formed at ca. 18 Å, suggesting the formation of some piled pores. The formation of the piled pores also increased the total pore volume and the mean pore radius, as shown in Table 1. Nevertheless, in overall, the specific area decreased with the loading of cobalt species.

3.2.1. In–situ DRIFTS test for steam reforming The in situ DRIFTS characterization in Fig. 3 can be used to explore the reaction intermediates formed on surface of the catalysts during steam reforming. The types of the reaction intermediates in steam reforming of acetic acid and FA were basically the same. In the case of acetic acid, the hydroxyl group (3740 cm−1) was derived from the dissociative absorption of water. CHx group appeared in steam reforming of acetic acid, which was mainly caused by the break of the CeC bond in acetic acid or the radical reactions. The appearances of the absorption peak of C]O and C]C functionalities were probably due to the dehydration of acetic acid ketene (CH2]C]O), which occurred at the temperature as low as 100 °C [42,43]. The carbonyl functionalities could also be due to the formation of other reaction intermediates with abundance in the whole range of temperature investigated, which could be the precursors of coke species. In addition, the absorption of

3.2. Catalytic performances The conversion of the different reactants and the yield of the main products in steam reforming of the varied reaction substrates were shown in Fig. 2. Temperature had a significant effect on the catalytic efficiency [38]. As the temperature raised, the yields of the gaseous

Fig. 3. In situ DRIFTS characterization of steam reforming of the different reactants. (a) and (b): acetic acid; (c) and (d): FA. 5

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induction time for the catalysts for reaching the maximum catalytic efficiency. With the prolonged reaction time, the catalysts deactivated progressively. For steam reforming of FA, the deactivation of the catalyst proceeded much quicker. The coking mechanism and the impacts of the formed coke on stabilities of the catalysts in steam reforming of FA was clearly different from that in steam reforming of the sugars. The heavy liquid products with the fused ring structures were the precursors for coke formation, which were subsequently analyzed with UV fluorescence.

C]C]C species were also observed, agglomeration of which would form the coke species. In addition, the carbonate species were also formed from steam reforming of acetic acid. The cracking of the CeC bonds in acetic acid formed *CH3 and *COOH species. The further dissociation of the *COOH species formed COO*, which further integrated with *OH to form bicarbonate species and then the further degradation to give carbonate species. The higher reaction temperature promoted the formation of carbonate species. In addition, the CeOeC aliphatic structure was also important reaction intermediates, formation of which was more significant with the increase of temperature. Comparing the DRIFTS spectra in steam reforming of acetic acid, the main feature for the absorption patterns of the DRIFTS spectra of steam reforming of FA was the broad absorption peaks for the carbonyl functionalities. FA contains only a hydroxyl group and a oxygen in the furan ring with no carbonyl functionality. The eminent absorption of the carbonyls clearly indicated the degradation or polymerisation of FA on surface of the catalyst, as FA could not be effectively gasified. The coking behaviors FA together with the sugars and acetic acid during the steam reforming were further investigated with the prolonged reaction time.

3.3.1. UV–fluorescence analysis Fig. 5 shows UV–fluorescence characterization of the soluble polymer after steam reforming of acetic acid and glucose versus the prolonged reaction time. In the experiment, the collected liquid products were no longer transparent, but showed brown to black color. From the results of fluorescence spectra, it could be found that polymerization occurred and formed the condensed products with the π–conjugated structures. In steam reforming of acetic acid, the fluorescence peaks spanned from 250 to ca. 360 nm, indicating the formation of the π–conjugated structures with the equitant of 1 to 3 benzene ring structures. With the prolonging reaction time from 1 to 5 h, the abundance of the conjugated structure of the soluble polymer increased, indicating progress of the polymerization of acetic acid in the steam reforming reaction. The peaks of soluble polymers formed in steam reforming of glucose (Fig. 5e) and xylose (Fig. S2a) were concentrated at the lower wavelength range at ca. 295 nm, indicating that the π–conjugated structures in the soluble polymer from the sugars were smaller than that from acetic acid, even though the molecular structure of glucose or xylose is much bigger than that of acetic acid. In addition, for the steam reforming of xylose and FA, the abundance of

3.3. Steam reforming of the varied organics versus the prolonged reaction time The results for steam reforming of the varied reaction substrates were shown in Fig. 4. For steam reforming of acetic acid, the conversion of acetic acid reached and maintained at 100%, and the catalyst did not show the sign for significant deactivation. The main by–products were CO and CH4 with negligible amount of acetone, the organic by–product, formed. For steam reforming of glucose or xylose, there was an

Fig. 4. Steam reforming of the organics versus the prolonged reaction time. Reaction conditions: LHSV = 5.7 h−1; S/C = 5; catalyst loading = 0.5 g; Reaction temperature = 600 °C. 6

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Fig. 5. UV fluorescence analysis of the polymeric products in steam reforming of acetic acid and glucose versus the prolonged reaction time. (a), (b), (c) and (d): acetic acid; (e), (f), (g), and (h): glucose.

7

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Fig. 6. Characterization of the Co/SBA–15 catalysts after the steam reforming of the organics versus the prolonged reaction time: (a): TG–Air test; (b): DTG test; (c): TG–N2; (d): TPO; (e): XRD test of the spent catalysts after the prolonged reaction time; (f) XRD test of the spent catalysts after the catalytic tests (the conditions were specified in Fig. 2; (g) and (h): FT–IR test and the enlarged view.

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polymers formed. The amounts of solid coke deposit formed on the catalysts were further investigated with TG analysis.

Table 2 The C and H content and C/H ratio of the coke in the used catalysts.a Reactant

Acetic acid

Glucose

Xylose

FA

C/wt% H/wt% C/H molar ratio

14.9 0.72 1.7

60.1 3.8 1.3

64.6 3.7 1.5

28.2 0.78 3.0

3.3.2. TG analysis Fig. 6a and b are TG and DTG profiles of the spent catalyst after the steam reforming of the organics. Coking was most significant for steam reforming of the two sugars, followed by FA and was the minimum in steam reforming of acetic acid. The DTG curves showed that the maximum for the weight decrease for the spent catalyst in steam reforming of acetic acid located at 480.9 °C, which was much lower than the coke from steam reforming of the sugars and FA. The subsequent characterization of the coke species with TG in nitrogen indicated the distinct thermal stability of the coke. The coke formed in steam reforming

a Catalysts after steam reforming of the varied organics versus the prolonged reaction time.

the soluble polymers did not change significantly and the size of the π–conjugated structures were relatively small (Fig. S2). Obviously, the sugars and FA might direct formed solid coke deposit with little soluble

Fig. 7. The TEM characterization of the spent catalysts after steam reforming of different reactants versus the prolonged reaction time. (a), (b), (c) and (d): acetic acid; (e): glucose; (f): xylose. 9

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temperature–time history, the changes of the particle size of cobalt was very different. The increment of the particle size of cobalt in steam reforming of xylose, followed by that in reforming of glucose, was the most significant, while was the lowest in steam reforming of acetic acid. The steam reforming of the different reaction substrates also affected the sintering of cobalt species, mechanism of which needed further investigation. In addition, the catalysts after the steam reforming of the varied organics under the conditions specified in the caption of Fig. 2 were also characterized with XRD and showed in Fig. 6f. The results showed the diffraction patterns were similar to that after the prolonged reaction time. In the spent catalyst, we also found the presence of CoO. During the steam reforming of various organics, these unreduced cobalt oxides could continue to be reduced with the hydrogen generated during the steam reforming reaction or with the organic species such as the reaction intermediates generated. In steam reforming of the various organics, the yields of hydrogen and the reaction intermediates generated were different, which all affected their capabilities for the reduction of the cobalt oxides during the steam reforming reaction. The presence of the crystal phase of carbon species could also be observed.

of acetic acid showed the lowest thermal stability, with majority of the coke species gasified at the elevated temperature. The coke formed from steam reforming of the sugars and FA was much stable, especially for the coke formed from that of FA. Only at 599.8 °C, the coke started to decompose. In comparison, the coke formed from steam reforming of glucose or xylose started to lose weight earlier and the coke from xylose was more stable than that from glucose (weight loss ratio: 41.7% for that of glucose versus 33.1% for that of xylose), originating from their structural difference. In addition to the thermal stability, the difference in terms of reactivity of the coke species towards oxidation was further investigated. 3.3.3. TPO analysis Fig. 6d shows the TPO characterization of coke species formed from steam reforming of the different reaction substrates. The oxidation peaks of the coke deposits located at 517 °C in steam reforming of acetic acid. The maximum oxidation temperature for the coke in steam reforming of glucose or xylose was similar, locating at 585 °C. Nevertheless, the profiles still showed that the oxidation of the coke in steam reforming of glucose proceeded at relatively lower temperatures. The coke formed in steam reforming of FA was the most difficult to be oxidized, which was probably due to the retaining of the aromatic ring (the furan ring) in the coke. The results were also in line with the result in Fig. 6b where the coke showed the highest thermal stability.

3.3.5. FT–IR analysis The results for FT–IR characterization of the functionalities of the coke species in the spent catalysts were shown Fig. 6g, with the enlarged view shown in Fig. 6h. There were eOH, C]O, C]C, aliphatic CeOeC structure and aryl–H functionalities present on surface of the catalyst. The coke in steam reforming of acetic acid and FA contained mainly the C]C species, while the coke in steam reforming of glucose and xylose contained abundant C]O functionalities. Glucose and xylose contain multiple hydroxyl group and the dehydration of them produced the carbonyl functionalities. In addition, the characterization

3.3.4. XRD analysis The XRD patterns of the spent catalysts were shown in Fig. 6e. The diffraction patterns of C species (PDF #03–0401) were more significant. The particle sizes of Co in the spent catalysts were calculated and shown in Table S1. Although the catalysts experienced the same

Fig. 8. The TEM characterization of the spent catalysts after steam reforming of FA versus the prolonged reaction time. 10

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and rapidly decomposed through pyrolysis, leading to the formation of coke particles blocking the bed. The solid residue (char) obtained from the thermal decomposition of sugars is composed of polymerised degradation products such as humic acids and large organic compounds with the carbon chain ranging from C8 to C15 [32]. The coke deposition from glucose is globular type and usually with a porous structure, which was referred to as “soft coke” in some literature articles [48]. Thus, this does not restrict reactants from entering the active metal site. However, the pyrolysis of glucose will generate a large amount of coke, which will still wrap the active site, leading to the gradual inactivation of the catalyst [49]. In this study, we found that the C]O species were the main functionalities in the coke formed in steam reforming of glucose and xylose and the coke formed was more aliphatic, originating from the long aliphatic chains in the sugars. In addition, the carbon deposition after sugar reforming is relatively stable and has a strong resistance towards oxidation. Steam reforming of acetic acid have also been widely reformed. For example, An et al. studied the effects of catalysts with different Ni loadings on the carbon deposition in steam reforming of acetic acid [50]. They concluded that catalytic cracking was the main pathway of coke formation. With the change of Ni loading, morphology of the coke changed from carbidic-like to graphitic-like. In addition, the rate of carbon deposition also related to the particle size of the active component in the catalyst [51]. In general, large particles can accelerate the rate of carbon deposition on the catalyst, while carbon adsorbed on small particles is more difficult to diffuse, thus reducing the rate of carbon deposition [51]. In addition, it has also been reported that coke from steam reforming of acetic acid was usually produced by secondary reactions between the intermediates [52]. For example, acetone produced by condensation/dehydration of acetic acid is considered to be a precursor of coke, which can be converted into oligomeric coke species [52]. In this study, the intermediate products during steam reforming of acetic acid were also studied. The results showed that the soluble polymer with the π–conjugated structures were formed, which were the precursors of coke. In addition to carbohydrates and acetic acid, the steam reforming of the organics with the ring structures were also performed. Xu et al. studied the characteristics in steam reforming various compounds such as furfural and m-cresol, which contain aromatic ring [35]. Their results showed that the gas yield of these compounds was generally low due to the difficulty of destroying the stable ring structure. In this study, the steam reforming of FA, the furan ring in FA was also difficult to be reformed, leading to very low yield of hydrogen yield and rapid deactivation of the catalyst. Owing to the influence of furan ring structure, the coke from steam reforming of FA reaction was more aromatic and more stable. The layer of carbon deposited on the surface of the catalyst leads to rapid deactivation of the catalyst. The above analysis clearly indicated that properties of coke were closely related to the structures/ properties of the reaction substrates.

of the liquid products with GC–MS showed that a number of carbonyl–containing organics were produced from steam reforming of the sugars. The carbonyl functionalities were retained in the coke species. The EA test showed that the C/H in the spent catalyst in steam reforming of FA was much higher than others, indicated the coke was less aliphatic but more aromatic (Table 2). The lower C/H in the coke from steam reforming of glucose or xylose indicated that the coke was more aliphatic. The C/H ratio in the coke from steam reforming of FA was 3.0, which was 3.6 times higher than that in FA feedstock (0.83). The C/H ratio in the coke from steam reforming of acetic acid was 3.4 times higher than that in acetic acid feedstock (0.83). In comparison, C/H ratio in the coke from steam reforming of glucose and xylose to that in the sugar feedstock was 2.6 and 3.0, indicating that the hydrogen in the sugars was prone to retain in the polymer. 3.3.6. TEM analysis Morphologies of the spent catalysts after the steam reforming versus the prolonged reaction times was characterized by TEM and the results were shown in Figs. 7 and 8. The results showed that carbon deposit in steam reforming of acetic acid was mainly in the form of carbon nanotubes (Fig. 7a, b and d). Some amorphous coke deposit was also observed (Fig. 7c). The nanotubes grown on the tip of metallic cobalt, which have the distinct diameters. This was possible related to the dispersion of metallic cobalt with the different particle size on surface of the catalyst. It has reported that carbon nanotubes have the high thermal stability [44,45]. Nevertheless, the results in Fig. 6d showed that the coke in the spent catalyst in steam reforming of acetic acid did not have a high thermal stability, and the detailed reason of which had not been cleared yet. For the spent catalysts in steam reforming of glucose or xylose, the coke in the carbon nanotube form could not be identified. In addition, the cobalt particles, which could be clearly identified in the spent catalyst in steam reforming of acetic acid, could almost not be identified in steam reforming of either glucose or xylose. The cracking, or most likely the polymerization, of the sugar formed the polymeric coke, which covered Co species and inhibited the formation of the carbon nanotubes. This also led to the deactivation of the catalysts. For the spent catalyst in steam reforming of FA, the coverage of cobalt particles with the carbon layer was observed (Fig. 8a to c), which inhibited the formation of the coke with the nanotube form and resulted in the rapid deactivation of the catalyst. Glucose and xylose are thermally unstable. During heating, they could crack or polymerize, which could even occur before reaching the catalyst bed and the formed coke deposited on surface of the catalyst. The deactivation of the catalyst would further render the coking reactions the dominant route for the conversion of the sugars in the steam reforming. FA also has a high tendency towards polymerisation. The coke deposits wrapped the metallic cobalt size and led to the rapid deactivation of the catalyst. Although the coke formed from the steam reforming of glucose, xylose and FA was not in the form of carbon nanotube, they had a high thermal stability or high resistivity towards oxidation, which were the characteristic features of the coke formed from these type of organics. Production of hydrogen from carbohydrates has received some research interest. The deactivation of catalysts induced by coking is a major challenge in steam reforming of carbohydrates for hydrogen production. Remon et al. conducted aqueous phase reforming of xylitol and sorbitol and compared the influence of the structure of the reaction substrates [46]. They found the length of carbon chain has an important effect on the yield of the target product. In addition, a reaction network was proposed to explain the similarity in the aqueous phase reforming of xylitol and sorbitol. In terms of specific reaction substrates, Marquevich et al. reported the influence of temperature and S/C ratio on the catalytic steam reforming of xylose, glucose and sucrose [47]. Their results showed that the increase of temperature was beneficial to the formation of target products and the reduction of carbon deposition, while S/C affects the gasification of carbon-containing substances. However, Sugar molecules are unstable at higher reaction temperatures

4. Conclusions In summary, the results showed that the coking behaviors the sugars (glucose, xylose) and the sugar derivatives (acetic acid, FA) as well as the properties of the resulting coke were closely related to the molecular structure of the feedstock. Acetic acid is a small aliphatic molecule, which could be effectively reformed and showed the least tendency towards coking. Glucose and xylose contain the multiple hydroxyl groups, making the polymerisation but not the steam reforming the dominant reaction route. The furan ring in FA was also difficult to be reformed, leading to very low yield of hydrogen yield and rapid deactivation of the catalyst. The tendency towards coking was the highest for the sugars, as they polymerized even before the temperature was high enough to activate the catalyst for reform these heavy sugars. During the polymerisation/cracking process to form coke, the soluble polymer with the π–conjugated structures were formed, and the size of 11

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the π–conjugated structures was bigger from steam reforming of acetic acid than that from sugars, even though the sugars is much bigger in size. In addition, the in situ DRIFTS characterization showed that dissociative absorption of acetic acid and FA to form C]C species, the precursor of coke, could occur at the temperature as low as 100 °C. The C]C species were the main functionalities in the coke formed in steam reforming of acetic acid and FA, while C]O species were the main ones in that from steam reforming of glucose and xylose, resulting from the dehydration reactions. The coke formed in steam reforming of acetic acid and FA was more aromatic, while that from steam reforming of glucose and xylose was more aliphatic, originating from the long aliphatic chains in the sugars. The coke formed in steam reforming of acetic acid showed the lowest thermal stability, while that from the sugars and FA was more stable, especially from FA. In addition, the coke from xylose was more stable than that from glucose, resulting from their structural difference. The morphologies of the coke from steam reforming of acetic acid was mainly carbon nanotubes, while that from glucose, xylose and FA was mainly in amorphous form. The amorphous coke wrapped cobalt particles inside with a carbon layer, leading to rapid catalyst deactivation. In addition, the coke produced from the sugars and especially from FA was also more resistant towards oxidation. The distinct morphologies and the reactivity of the coke produced from the varied organics towards oxidation needs to be paid particular attention during the regeneration of the deactivated catalysts.

[4] Hu X, Gunawan R, Mourant D, Hasan MM, Wu L, Song Y, et al. Upgrading of bio-oil via acid–catalyzed reactions in alcohols—a mini review. Fuel Process Technol 2017;155:2–19. [5] Luo Y, Guda VK, Hassan EB, Steele PH, Mitchell B, Yu F. Hydrodeoxygenation of oxidized distilled bio-oil for the production of gasoline fuel type. Energy Convers Manage 2016;112:319–27. [6] Zacher AH, Olarte MV, Santosa DM, Elliott DC, Jones SB. A review and perspective of recent bio-oil hydrotreating research. Green Chem 2014;16:491–515. [7] Zhang X, Wang T, Ma L, Zhang Q, Jiang T. Hydrotreatment of bio-oil over Ni–based catalyst. Bioresource Technol 2013;127:306–11. [8] Zhang Z-J, Wang Q-W, Yang X-L, Chatterjee S, Pittman CU. Sulfonic acid resin–catalyzed addition of phenols, carboxylic acids, and water to olefins: model reactions for catalytic upgrading of bio-oil. Bioresource Technol 2009;101:3685–95. [9] Choong CKS, Zhong Z, Huang L, Wang Z, Ang TP, Borgna A, et al. Effect of calcium addition on catalytic ethanol steam reforming of Ni/Al2O3: I. catalytic stability, electronic properties and coking mechanism. Appl Catal A: Gen 2011;407:145–54. [10] Wu C, Liu R. Carbon deposition behavior in steam reforming of bio-oil model compound for hydrogen production. Int J Hydrogen Energy 2010;35:7386–98. [11] Liu M, Bai J, Kong L, Bai Z, He C, Li W. The correlation between coal char structure and reactivity at rapid heating condition in TGA and heating stage microscope. Fuel 2020;;260:116318. [12] Alenazey F, Cooper CG, Dave CB, Elnashaie SSEH, Susu AA, Adesina AA. Coke removal from deactivated Co-Ni steam reforming catalyst using different gasifying agents: an analysis of the gas-solid reaction kinetics. Catal Commun 2009;10:406–11. [13] Li J, Mei X, Zhang L, Yu Z, Liu Q, Wei T, Wu W, Dong D, Xu L, Hu X. A comparative study of catalytic behaviors of Mn, Fe, Co, Ni, Cu and Zn–Based catalysts in steam reforming of methanol, acetic acid and acetone. Int. J. Hydrogen Energy. DOI: 10. 1016/j.ijhydene.2019.03.269. [14] Li Y, Zhang L, Zhang Z, Liu Q, Zhang S, Liu Q, Hu G, Wang Y, Hu X. Steam reforming of the alcohols with varied structures: impacts of acidic sites of Ni catalysts on coking. Appl. Catal. A: Gen. DOI: 10.1016/j.apcata.2019.117162. [15] Hu X, Wang Y, Mourant D, Gunawan R, Lievens C, Chaiwat W, et al. Polymerization on heating up of bio-oil: a model compound study. AIChE J 2013;59:888–900. [16] Patwardhan PR, Satrio JA, Brown RC, Shanks BH. Product distribution from fast pyrolysis of glucose–based carbohydrates. J Anal Appl Pyrol 2009;86:323–30. [17] Stefanidis SD, Kalogiannis KG, Iliopoulou EF, Michailof CM, Pilavachi PA, Lappas AA. A study of lignocellulosic biomass pyrolysis via the pyrolysis of cellulose, hemicellulose and lignin. J Anal Appl Pyrol 2014;105:143–50. [18] Shen DK, Gu S, Bridgwater AV. The thermal performance of the polysaccharides extracted from hardwood: cellulose and hemicellulose. Carbohyd Polym 2010;82:39–45. [19] Kruk M, Jaroniec M, Ko CH, Ryoo R. Characterization of the porous structure of SBA–15. Chem Mater 2000;12:1961–8. [20] Hiyoshi N, Yogo K, Yashima T. Adsorption characteristics of carbon dioxide on organically functionalized SBA–15. Microporous Mesoporous Mater 2005;84:357–65. [21] Bayram B, Soykal II, von Deak D, Miller JT, Ozkan US. Ethanol steam reforming over Co–based catalysts: investigation of cobalt coordination environment under reaction conditions. J Catal 2011;284:77–89. [22] Sun J, Karim AM, Mei D, Engelhard M, Bao X, Wang Y. New insights into reaction mechanisms of ethanol steam reforming on Co–ZrO2. Appl Catal B: Environ 2015;162:141–8. [23] Hu X, Dong D, Shao X, Zhang L, Lu G. Steam reforming of acetic acid over cobalt catalysts: effects of Zr, Mg and K addition. Int J Hydrogen Energy 2017;42:4793–803. [24] Hu X, Lu G. Steam reforming of acetic acid to hydrogen over Fe–Co catalyst. Chem Lett 2006;35:452–3. [25] Onda A, Ochi T, Yanagisawa K. Selective hydrolysis of cellulose into glucose over solid acid catalysts. Green Chem 2008;10:1033–7. [26] Pang J, Wang A, Zheng M, Zhang T. Hydrolysis of cellulose into glucose over carbons sulfonated at elevated temperatures. Chem Commun 2010;46:6935–7. [27] Sahu R, Dhepe PL. A one–pot method for the selective conversion of hemicellulose from crop waste into C5 sugars and furfural by using solid acid catalysts. ChemSusChem 2012;5:751–61. [28] Dhepe PL, Sahu R. A solid–acid–based process for the conversion of hemicellulose. Green Chem 2010;12:2153–6. [29] Jin F, Enomoto H. Rapid and highly selective conversion of biomass into value–added products in hydrothermal conditions: chemistry of acid/base–catalysed and oxidation reactions. Energy Environ Sci 2011;4:382–97. [30] Kobayashi H, Fukuoka A. Synthesis and utilisation of sugar compounds derived from lignocellulosic biomass. Green Chem 2013;15:1740–63. [31] Azadi P, Otomo J, Hatano H, Oshima Y, Farnood R. Hydrogen production by catalytic near–critical water gasification and steam reforming of glucose. Int J Hydrogen Energy 2010;35:3406–14. [32] Remón J, García L, Arauzo J. Cheese whey management by catalytic steam reforming and aqueous phase reforming. Fuel Process Technol 2016;154:66–81. [33] Hu X, Lu G. Comparative study of alumina–supported transition metal catalysts for hydrogen generation by steam reforming of acetic acid. Appl Catal B: Environ 2010;99:289–97. [34] Zhang Z, Hu X, Li J, Gao G, Dong D, Westerhof R, et al. Steam reforming of acetic acid over Ni/Al2O3 catalysts: correlation of nickel loading with properties and catalytic behaviors of the catalysts. Fuel 2018;217:389–403. [35] Xu Q, Xie D, Wang F, Yan Y. Mechanism of hydrogen production by the catalytic steam reforming of bio-oil. Energy Source Part A 2013;35:1028–38. [36] Yu Z, Hu X, Jia P, Zhang Z, Dong D, Hu G, et al. Steam reforming of acetic acid over

CRediT authorship contribution statement Xianglin Li: Investigation, Data curation, Writing - original draft, Validation, Visualization. Zhanming Zhang: Investigation, Validation. Lijun Zhang: Investigation, Visualization, Writing - review & editing. Huailin Fan: Investigation, Visualization, Writing - review & editing. Xueli Li: Investigation, Supervision. Qing Liu: Investigation, Supervision. Shuang Wang: Resources, Supervision. Xun Hu: Resources, Supervision, Writing - review & editing, Project administration, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51876080), the Strategic International Scientific and Technological Innovation Cooperation Special Funds of National Key Research and Development Program of China (No. 2016YFE0204000), the Program for Taishan Scholars of Shandong Province Government, the Recruitment Program of Global Experts (Thousand Youth Talents Plan), the Natural Science Foundation of Shandong Province (ZR2017BB002) and the Key Research and Development Program of Shandong Province (2018GSF116014). Collaborative Innovation Center of Technology and Equipment for Biological Diagnosis and Therapy in Universities of Shandong. References [1] Hu X, Lu G. Investigation of the effects of molecular structure on oxygenated hydrocarbon steam re-forming. Energy Fuel 2009;23:926–33. [2] Feng D, Zhao Y, Zhang Y, Xu H, Zhang L, Sun S. Catalytic mechanism of ion-exchanging alkali and alkaline earth metallic species on biochar reactivity during CO2/H2O gasification. Fuel 2018;212:523–32. [3] Wang S, Li X, Zhang F, Cai Q, Wang Y, Luo Z. Bio-oil catalytic reforming without steam addition: application to hydrogen production and studies on its mechanism. Int J Hydrogen Energy 2013;38:16038–47.

12

Fuel 265 (2020) 116961

X. Li, et al.

[37] [38]

[39]

[40]

[41]

[42]

[43]

[44]

[45] Kundu S, Wang Y, Xia W, Muhler M. Thermal stability and reducibility of oxygen–containing functional groups on multiwalled carbon nanotube surfaces: a quantitative high–resolution XPS and TPD/TPR study. J Phys Chem C 2008;112:16869–78. [46] Kirilin AV, Tokarev AV, Kustov LM, Salmi T, Mikkola JP, Murzin DY. Aqueous phase reforming of xylitol and sorbitol: comparison and influence of substrate structure. Appl Catal A-Gen 2012;435:172–80. [47] Marquevich M, Czernik S, Chornet E, Montané D. Hydrogen from biomass: steam reforming of model compounds of fast-pyrolysis oil. Energy Fuel 1999;13:1160–6. [48] Jovanović MR, Putanov PS. Nature and distribution of coke formed on mono-metallic platinum and bimetallic platinum-rhenium catalysts. Appl Catal A-Gen 1997;159:1–7. [49] Tanksale A, Wong Y, Beltramini JN, Lu GQ. Hydrogen generation from liquid phase catalytic reforming of sugar solutions using metal-supported catalysts. Int J Hydrogen Energy 2007;32:717–24. [50] An L, Dong C, Yang Y, Zhang J, He L. The influence of Ni loading on coke formation in steam reforming of acetic acid. Renew Energy 2011;36:930–5. [51] Nogueira FGE, Assaf PG, Carvalho HW, Assaf EM. Catalytic steam reforming of acetic acid as a model compound of bio-oil. Appl Catal B: Environ 2014;160:188–99. [52] Chen G, Tao J, Liu C, Yan B, Li W, Li X. Hydrogen production via acetic acid steam reforming: a critical review on catalysts. Renew Sustain Energy Rev 2017;79:1091–8.

nickel-based catalysts: the intrinsic effects of nickel precursors on behaviors of nickel catalysts. Appl Catal B: Environ 2018;237:538–53. Potoczna-Petru D, Kępiński L. Reduction study of Co3O4 model catalyst by electron microscopy. Catal Lett 2001;73:41–6. Du ZY, Zhang ZH, Xu C, Wang XB, Li WY. Low-temperature steam reforming of toluene and biomass tar over biochar-supported Ni nanoparticles. ACS Sustain Chem Eng 2018;7:3111–9. Tanksale A, Wong Y, Beltramini JN, Lu GQ. Hydrogen generation from liquid phase catalytic reforming of sugar solutions using metal–supported catalysts. Int J Hydrogen Energy 2006;32:717–24. He L, Chen D. Hydrogen production from glucose and sorbitol by sorption–enhanced steam reforming: challenges and promises. ChemSusChem 2012;5:587–95. Hu X, Jiang S, Kadarwati S, Dong D, Li CZ. Effects of water and alcohols on the polymerization of furan during its acid–catalyzed conversion into benzofuran. RSC Adv 2016;6:40489–501. Duan X, Page M. Theoretical investigation of competing mechanisms in the thermal unimolecular decomposition of acetic acid and the hydration reaction of ketene. J Am Chem Soc 1995;117:5114–9. Zhang Z, Wei T, Chen G, Li C, Dong D, Wu W, et al. Understanding correlation of the interaction between nickel and alumina with the catalytic behaviors in steam reforming and methanation. Fuel 2019;250:176–93. Srivastava A, Srivastava ON, Talapatra S, Vajtai R, Ajayan PM. Carbon nanotube filters. Nat Mater 2004;3:610–4.

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