Evaluation of Reactivities of Various Compounds in Steam Reforming

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Evaluation of Reactivities of Various Compounds in Steam Reforming over RuNi/BaOAl2O3 Catalyst Jianglong Pu, Takashi Toyoda, and Eika W. Qian Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03244 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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Evaluation of Reactivities of Various Compounds in Steam Reforming over RuNi/BaOAl2O3 Catalyst Jianglong Pu, Takashi Toyoda, Eika W. Qian* Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16, Nakacho, Koganei, Tokyo 184-8588, Japan *Corresponding author. Tel.: +81 42 388 7410; fax: +81 42 388 7410. E-mail address: [email protected] (E.W. Qian)

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Abstract: Hydrogen production from steam reforming of bio-oil is a potential method to reduce the dependence on the conventional fossil fuels. To investigate the reactivity of bio-oil and its difference with gasification tar and conventional fossil fuels, the steam reforming of various compounds (benzene, toluene, m-xylene, m-cresol, n-hexane, cyclohexane, 1-propanol and acetic acid) were conducted in a fixed-bed flow reactor at various temperatures over a high-performance RuNi/BaOAl2O3 catalyst. As a whole, the reactivities of these compounds in steam reforming decrease in the following trend: n-hexane > cyclohexane > benzene > toluene > m-xylene > 1-propanol > m-cresol > acetic acid. For the C6 hydrocarbons, benzene showed lower reactivity than n-hexane and cyclohexane, due to the stable benzene ring. The reactivities of the aromatic hydrocarbons decrease with addition of methyl groups to benzene ring due to electronic and steric effects. m-cresol showed lower reactivity than benzene, toluene and m-xylene, suggesting that the incorporation of hydroxyl to the benzene ring hindered the steam reforming reaction. Besides the steam reforming reactions, the side reactions such as hydrogenolysis, demethylation, decomposition, and methanation of CO and CO2 also occurred. Benzene ring can be formed by the dehydroaromatization of n-hexane or cyclohexane, while the reverse reaction cannot occur due to the limit of thermodynamic. The largely containing acetic acid in bio-oil needs higher reforming temperature than other compounds, and is easy to be thermally decomposed into coke at low temperatures, which increases the difficulty of bio-oil steam reforming. Keywords: Hydrogen production; Steam reforming; Bio-oil; Kinetics; Model compound


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1. Introduction The demand for energy is increasing with the rapid development of economy in recent years,1, 2 and the excessive use of fossil fuels leads to many environmental problems such as energy crisis, environmental pollution and global warming.3, 4 It is urgent to replace the conventional fossil fuels with some renewable and clean energies.5 The utilization of biomass has attracted many researchers because the abundant biomass can be renewed quickly,6 and the carbon dioxide emitted during the utilization of biomass can be recycled by the plant photosynthesis, which greatly alleviates the global warming.3 The fast pyrolysis of biomass in the absence of oxygen with a high temperature rising rate can easily convert biomass into bio-oil.1, 7 However, the obtained bio-oil contains large amount of water, and has a poor stability due to the strong acidity.1, 8, 9 The composition of bio-oil is rather complex, including aldehyde, alcohols, acids, ketones, phenols, etc.,1, 3, 6, 10, 11 which are mainly derived from the depolymerization of cellulose, hemicellulose and lignin in biomass.10, 12 The large amount of oxygenates greatly decrease the energy density of bio-oil.1 Moreover, the complex composition of bio-oil also greatly depends on the source of biomass and the pyrolysis conditions.8, 11, 13, 14 Therefore, the direct use of bio-oil as fuels is very difficult and it is necessary to be further upgraded.8 As the large demand for hydrogen in the field of fuel cell,15 hydrogenation reactions and ammonia synthesis,6, 16 one of the upgrading methods is to convert the aqueous phase of bio-oil into hydrogen by steam reforming.4, 15, 17 Generally, the steam reforming of bio-oil can be described as Eq. (1) and water gas shift reaction (Eq. (2)) also occurs simultaneously to further convert the formed CO into CO2.5

C H O + n − k H O → nCO + n − k + H CO + H O ⇌ CO + H

(1)

(2)

Therefore, the overall reaction equation can be expressed by Eq. (3).

C H O + 2n − k H O → nCO + 2n − k + H (3)


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Similar to typical hydrocarbons, the main difficulties in the process of bio-oil steam reforming are high operating temperatures, coke deposition, high steam to carbon ratio, large energy consumption for the endothermic reactions, etc.15 Noble metal supported catalyst and Ni-based catalyst are usually used in steam reforming reactions due to their superior C-C or C-H scission ability.16, 18 However, thermal decomposition of various oxygenated compounds in bio-oil greatly accelerated catalyst deactivation from coke formation.14, 15 High temperatures and severe hydrothermal conditions make the catalyst lose its activity due to metal sintering.19 Additionally, because of the complex composition of bio-oil, reactions during steam reforming are rather complicated and the conversion of different components requires different catalysts,15 therefore, most studies of steam reforming of bio-oil were conducted using model compounds.19-23 Although using some model compounds helps understand the steam reforming of bio-oil,13 the understanding for the effect of composition on the reactivity of bio-oil is limited. The composition of aromatic hydrocarbons and alkanes in pyrolysis bio-oil is negligible, while the contents of aromatic hydrocarbons such as benzene, toluene and xylene markedly increase in the tar from biomass or coal gasification.24 On the other hand, in the conventional steam reforming of fossil fuels such as naphtha, the feedstocks usually contain large amount of hydrocarbons, such as benzene, xylene, toluene and hexane.25 The reactivities of tar and fossil fuels in steam reforming are much different from bio-oil; however, to date, no systematic comparisons were reported. Therefore, in the present study, with an aim to better understand the behavior of various bio-oil components in the steam reforming process and its difference with steam reforming of gasification tar and fossil fuels, the reactivities of compounds including aromatic hydrocarbons, cycloalkanes, alkanes, phenols, alcohols and organic acids, were systematically investigated and compared. Benzene, toluene, m-xylene, m-cresol, n-hexane, cyclohexane, 1-propanol and acetic acid were selected as representatives of above components. In


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previous study, we found that the addition of Ru to the Ni-based catalyst could remarkably improve the catalytic activity and stability of catalyst in the steam reforming of m-cresol, and the optimal loading amount of Ni and Ru was 15 wt. % and 2% wt. %, respectively.21 It was reported that addition of barium to Ni/LaAlO3 markedly improved the activity and coke resistance of catalyst in the steam reforming of toluene, owing to its promoted adsorption and activation of water.26 We also investigated the effect of BaO addition on the performance of RuNi/BaOAl2O3 in the steam reforming of m-cresol. It is shown that the addition of barium improves the stability and enhances the coke-resistant ability of catalyst. The catalyst with 10 wt. % BaO showed the best performance with a stable catalytic activity in a 24-h stability test as well as the smallest coke deposition. Therefore, in this study, the steam reforming of these various compounds were conducted over the best-performance RuNi/BaOAl2O3 in a fixed-bed flow reactor at various reaction temperatures. The effect of methyl or hydroxyl addition on the reactivity of benzene ring in steam reforming reactions was investigated by comparing the reactivities of benzene, toluene, m-xylene and m-cresol. Furthermore, the reactivities of C6 hydrocarbons were also compared by using benzene, cyclohexane and n-hexane as model compounds.

2. Experimental 2.1 Chemical materials γ-alumina (Nippon Ketjen, >99%), barium nitrate (Ba(NO3)2, >99.9%, Kishida Reagents Chemicals), ruthenium chloride n-hydrate (RuCl3·nH2O, 40% Ru, Kanto Chemical), nickel nitrate hexahydrate (Ni(NO3)2·6H2O, >99.9%, Wako), benzene (>99.0%, Wako), toluene (>99.0%, Wako), m-xylene (>98.0%, Wako), m-cresol (>98.0%, Wako), n-hexane (>96.0%, Wako), cyclohexane (>98.0%, Wako), 1-propanol (>99.5%, TCI) and acetic acid (>99.7%, Wako) were used as received without further purification. 2.2 Catalyst preparation


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The alumina support modified with 10 wt. % of BaO was prepared by impregnation method. Firstly, γ-alumina with sizes of 425~850 µm was calcined at 500 °C for 6 h to remove the impurities. Subsequently, barium nitrate was dissolved in the diluted water and mixed with the alumina support, followed by 5 min of ultrasonication. Then, the obtained mixture was loaded on a heated sand bath to evaporate the water. Finally, the support was dried at 120 °C for 3 h and calcined at 750 °C for 6 h. The catalyst containing 2 wt. % of Ru and 15 wt. % of Ni were prepared by impregnation method. Firstly, Ru and Ni were incorporated to the support in sequence using RuCl3·nH2O and Ni(NO3)2·6H2O as metal precursors, respectively. Then, the catalyst precursor was dried at 120 °C for 3 h. The physical properties of prepared RuNi/BaOAl2O3 catalyst are listed in Table 1. 2.3 Catalytic activity tests The steam reforming of various model compounds were conducted in a stainless steel (SS316) fix-bed flow reactor (8 mm internal diameter; 350 mm length). A schematic diagram of apparatus for steam reforming reactions is shown in Fig. 1. 0.50 g of a catalyst diluted by quartz sand was loaded into the constant-temperature zone of the reactor. The model compounds and water with a molar ratio of steam to carbon (S/C) of 3.35, were continuously fed by two pumps. Prior to the steam reforming reactions, the catalyst was reduced using a 100 ml/min of hydrogen (99.99%) at 600 °C for 3 h. The steam reforming reactions were conducted at various temperatures under 1.0 MPa, and the weight hourly space velocity (WHSV) of each feedstock was kept at 20.8 h-1. A splash condenser in the outlet of the reactor was used to separate the residual liquid and gaseous products. The gas composition was analyzed by two gas chromatographs with TCD detector, of which the GL Science GC-323 using nitrogen as the carrier gas was used to quantify the hydrogen, and the one (SHIMADZU GC-8A) with helium as the carrier gas was used to quantify the carbonaceous gas (CO, CH4 and CO2). The oil phase and aqueous phase in the residual liquid were separated using a separatory


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funnel, and the composition was analyzed by a gas chromatograph with FID detector (GC-14B SHIMADZU). Prior to the data collection, all the reactions were kept under the reaction conditions for 2.5 h to ensure the experimental data were collected during the stable activity period of catalyst. The gaseous products were analyzed for three times and no significant disparity was found, indicating the activity of catalyst was stable. The residual liquid was collected every 1 h and analyzed for three times. The conversion of each model compound was calculated by the equation below: % =

feed flow rate mol⁄ℎ − unreacted feed flow ratemol⁄ℎ × 100% feed flow rate mol⁄ℎ

The yield of H2 was defined by the stoichiometric H2 formation: ()* % =

moles of hydrogen produced × 100% m 2n − k + 2 × moles of reactant in the feedstock

The yields of carbonaceous products were estimated on a carbon basis: (1 % =

moles of carbon of each carbonaceous product × 100% moles of carbon in the feedstock

3. Results and discussion 3.1 Thermodynamic analysis Without consideration of the side reactions, steam reforming of bio-oil occurs following Eq. (1) and Eq. (2). The feedstocks of this reaction system are steam and each model compound with S/C=3.35, and the reactants are H2, CO and CO2. The equilibrium compositions of reactant, water, H2, CO and CO2 were calculated by minimizing the Gibbs free energy in the reaction system. The equilibrium yield of H2 is defined by the following equation: (34)* % =

moles of hydrogen under equilibrium × 100% m 2n − k + 2 × moles of reactant in feedstock

Although the steam reforming of these reactants at high temperatures are irreversible, the exothermic


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water-gas-shift reaction (Eq. (2)) is reversible, by which the hydrogen yield should have a maximum value. In other words, the hydrogen yield is limited by the water-gas-shift reaction. The equilibrium yields of hydrogen in the steam reforming of various compounds at different temperatures are shown in Fig. 2. These compounds showed various equilibrium hydrogen yields below 450 °C, suggesting that Eq. (1) of these reactants has different thermodynamic equilibriums at low temperatures. At the same temperature, hydrogen equilibrium yields for the oxygenated compounds increased in the following trend: acetic acid > 1-propanol > m-cresol. The hydrogen equilibrium yields for hydrocarbons (benzene, toluene, m-xylene, cyclohexane and n-hexane) are similar but lower than these for oxygenated compounds (m-cresol, 1-propranol and acetic acid). Since Eq. (1) of all the reactants are endothermic reactions, the hydrogen equilibrium yields increase with reaction temperature, while at high temperatures (above 500 °C), they suffer decreases due to the reverse shift of exothermic water-gas-shift reaction (Eq. (2)). However, under the present reaction conditions (550~700 °C), these various compounds showed similar hydrogen equilibrium yields, indicating that the hydrogen equilibrium yields were not limited by Eq. (1) and were only limited by the water-gas-shift reaction (Eq. (2)). This suggests that the following hydrogen yields in steam reforming reactions are comparable without consideration of the various thermodynamic equilibriums. 3.2 Steam reforming of various compounds 3.2.1 Aromatic hydrocarbons The concentration of aromatic hydrocarbons in bio-oil is negligible but largely exist in gasification tar.24 Regarded as a comparison with the steam reforming of bio-oil and to investigate the effect of methyl addition on the reactivity of benzene ring, the steam reforming of benzene, toluene and m-xylene were conducted at 550~650 °C. The benzene conversion and yields of products in the steam reforming of benzene are presented in Fig. 3a. Since a high temperature is favorable for steam reforming reactions, the benzene conversion and


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hydrogen yield increased with the reaction temperature. Benzene was almost converted from 600 °C. The main composition of the unreacted oil phase is benzene. Trace amount of toluene (less than 1%) was also detected, indicating that the methylation of benzene occurred during the steam reforming of benzene. However, cyclohexane and n-hexane were not detected in the unreacted oil phase. According to the principle of minimum Gibbs free energy, the benzene equilibrium conversions to form cyclohexane, n-hexane and methane were calculated and the results are shown in Fig. 4a. It is shown that although the hydrogen in the feedstock is excess (H2/benzene (mol/mol) = 10), the benzene equilibrium conversions into cyclohexane and n-hexane are almost zero under the present reaction conditions. On the other hand, the equilibrium conversion of benzene to give methane (Eq. (4)) was almost 100%. This well agreed with the activity test results that large amount of methane was formed, while no cyclohexane or n-hexane was detected. The yield of methane increased with reaction temperature and was up to 28% at 600 °C, which was from the hydrogenolysis of benzene (Eq. (4)), as well as the methanation of CO (Eq. (5)) and CO2 (Eq. (6)) due to the efficient conversion ability of the Ni-based catalysts.10 + 9H2

6CH4

CO + 3H ⇌ CH7 + H O CO + 4H ⇌ CH7 + 2H O

(4)

(5) (6)

Catalysts with high activity and good stability are extensively developed for the steam reforming of toluene,26-32 since toluene is a typical model tar derived from the pyrolysis of biomass.27 As shown in Fig. 3b, the toluene conversion and yields of hydrogen, CO and CO2 in the steam reforming of toluene increased with the reaction temperature. Only toluene and benzene were detected in the unreacted oil, indicating that the demethylation of toluene (Eq. (7)) occurred simultaneously with the steam reforming of toluene.32 It should be noted that methyl group on the benzene ring could also be detached by a steam dealkylation reaction (Eq.


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(8)).33 Similar to benzene, toluene was almost completely converted above 600 °C. The yield of methane increased with the reaction temperature and was up to 24.2% at 600 °C mainly due to the hydrogenolysis and demethylation of toluene, as well as the methanation of CO and CO2. + H2

+ CH4

+ H2 O

+ CO + 2H2

(7) (8)

The m-xylene conversion and yields of products in the steam reforming of m-xylene are presented in Fig. 3c. The conversion of m-xylene and hydrogen yield showed similar trend with those of benzene and toluene. m-xylene was almost converted at 625 °C, which was higher than benzene and toluene (about 600 °C), indicating that the reactivity of m-xylene was lower than those of benzene and toluene. Besides the m-xylene feed, toluene and benzene were also produced, suggesting that the demethylation reactions of m-xylene (Eq. (9) and Eq. (10)) occurred during the reaction.33 The yields of toluene and benzene increased with the reaction temperature at low temperatures but decreased above 600 °C, because the steam reforming of toluene and benzene became significant at high temperatures. The yield of benzene is higher than that of toluene, suggesting that both the two methyl groups prefer to be detached during the reaction rather than only one. Thermodynamic analysis of the demethylation of m-xylene also showed the similar results. As shown in Fig. 4b, the equilibrium yields of products in the demethylation of m-xylene were calculated. With temperature increasing, the benzene equilibrium yield increased, while the toluene equilibrium yield decreased. The equilibrium yield of benzene is much higher than that of toluene under the present reaction conditions. It is noteworthy that even with a low hydrogen in feed (H2/m-xylene (mol/mol) = 2), the methane equilibrium yield is very high, suggesting that hydrogenolysis of benzene ring is more prone to occur. Methane in the products was mainly formed by the hydrogenolysis of m-xylene, as well as the methanation of CO (Eq. (5)) and CO2 (Eq. (6)) and demethylation reactions (Eq. (9~10)). As shown in Fig. 3c, the yield of methane decreased with


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the reaction temperature above 625 °C, implying the methane steam reforming became significant. + H2

+ CH4

+ 2H2

+ 2CH4

(9) (10)

Generally, for aromatic hydrocarbons, it was found that the hydrogen yields decreased in the following trend: benzene > toluene > m-xylene. Particularly, at a low temperature (550 °C), benzene, toluene and m-xylene showed hydrogen yields of 17.0%, 11.7% and 3.3%, respectively. This indicates that reactivity of aromatic hydrocarbons in steam reforming decreases with the addition of methyl groups to the benzene ring. In the previous study, it was also found that benzene showed a more reactivity in steam reforming reactions than other aromatic hydrocarbons.34 Although studies about the effect of methyl groups addition to benzene ring on the reactivity in steam reforming reactions were not reported in the previous research, similar trend was found in the hydrogenation of benzene, toluene and xylene, i.e., the reactivity decreased in the order: benzene > toluene > xylene.35 The authors attributed this trend to an electronic effect on the aromatic ring raised by the addition of methyl groups although the steric hindrance also appeared to be responsible for the low reactivity of xylene. The addition of methyl groups to benzene ring can introduce both steric and electronic effects. Based on the experimental data, it is difficult to distinguish which contributes to the reactivity decrease. The addition of methyl groups decreases the ionization potential (I.P.) in the following order: benzene (9.56 eV) > toluene (9.18 eV) > m-xylene (9.05 eV) due to the electronic effect.36 On the other hand, the chemisorption studies of benzene and toluene on the Pt (111) sites showed that no differentiable adsorption sites were found for these two molecules and the adsorption mode is π-bonded molecules lying flat on the surface.37 This suggests that the low reactivity of toluene is more attributed to the electronic effect rather than the steric hindrance by the methyl group. However, a relatively small decrease of ionization potential from toluene to m-xylene (9.18→9.05 eV) was shown in comparison with that from benzene to


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toluene (9.56→9.18 eV), but a much significant decrease of reactivity was shown from toluene to m-xylene. This suggests that the decrease of reactivity of m-xylene appears to be attributed to both electronic and steric effects. Furthermore, it is noteworthy that the methane yields of aromatic hydrocarbons are relatively large and decrease with the addition of methyl group to the benzene ring, i.e., benzene > toluene > m-xylene. Below 575 °C, the methane yield of m-xylene is almost negligible. This suggests that the addition of methyl group also suppresses the hydrogenolysis of benzene ring into methane under the steam reforming conditions. 3.2.2 Phenols The steam reforming of phenols using m-cresol as a model compound was conducted at 550~650 °C. The m-cresol conversion and yields of products are shown in Fig. 3d. The conversion of m-cresol was almost 100% at 650 °C. However, at 600 °C, the conversion of m-cresol (52.6%) is much lower than those of benzene (97.3%), toluene (97.1%) and m-xylene (70.6%), indicating the low reactivity of m-cresol in comparison with the aromatic hydrocarbons. The main compositions in the unreacted oil are m-cresol, phenol, toluene and benzene. This suggests that during the steam reforming reaction, the methyl and hydroxyl groups are also removed from the benzene rings.4 The yields of phenol, toluene and benzene suffered a decrease above 600 °C, because the steam reforming of these compounds became significant at high temperatures. Similarly, the yield of methane decreased from 625 °C due to the steam reforming of methane. It was also found that m-cresol showed much lower hydrogen yields than toluene (Fig. 3b), indicating that the addition of hydroxyl group to the benzene ring decreased the reactivity of feedstock. Moreover, m-cresol also showed much lower methane yields below 600 °C than toluene, indicating that the hydrogenolysis of benzene ring was significantly inhibited by adding a hydroxyl group to it. 3.2.3 C6 hydrocarbons To investigate the reactivity of C6 hydrocarbons, the steam reforming of n-hexane and cyclohexane were


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conducted at 525~625 °C. The conversion of n-hexane and yields of products in the steam reforming of n-hexane are shown in Fig. 5a. The complete conversion temperature of n-hexane is about 575 °C, which is lower than those of benzene, toluene, m-xylene, and m-cresol, indicating the high reactivity of n-hexane in comparison with aromatic hydrocarbons. Besides n-hexane, trace of benzene was also detected in the unreacted oil at 550 °C, indicating that the dehydroaromatization of n-hexane occurred forming the benzene rings during the reaction.38, 39 In addition, the methane yield for n-hexane is much larger than those for aromatic compounds, which is mainly attributed to the hydrogenolysis of n-hexane (Eq. (11)) as well as the methanation of CO and CO2. C9 H:7 + 5H → 6CH7

(11)

The conversion of cyclohexane and yields of products in the steam reforming of cyclohexane are shown in Fig. 5b. Similar to the results of n-hexane, cyclohexane also showed a high reactivity and was almost completely converted from 575 °C. The main compositions of the unreacted oil are cyclohexane, benzene and toluene. This suggests that dehydroaromatization of cyclohexane forming benzene (Eq. (12))40 as well as the methylation of benzene forming trace of toluene, occurs during the reaction. It is interesting that no n-hexane is detected in the products, which is probably because that once the ring-opening reaction of cyclohexane occurs, the formed n-hexane is immediately converted into H2, CO, CO2 and CH4. Similarly, the large amount of methane formed in the products is mainly from the hydrogenolysis of cyclohexane (Eq. (13)) as well as the methanation of CO and CO2. Generally, for C6 hydrocarbons, i.e., benzene, n-hexane and cyclohexane, it was found that the conversion and hydrogen yield in the case of cyclohexane (Fig. 5b) were higher than those in the case of benzene (Fig. 3a), but lower than those in the case of n-hexane (Fig. 5a). The reactivity of C6 hydrocarbons decreases in the following sequence: n-hexane > cyclohexane > benzene. This is because that the aromatic hydrocarbons


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contain the stable skeleton of benzene ring, which needs higher temperature to be reformed than alkanes and cycloalkanes.10 + 2H2

+ 7H2

6CH4

(12) (13)

3.2.4 Alcohols and organic acids To investigate the reactivity of alcohols and organic acids, the steam reforming of 1-propanol and acetic acid were conducted at 525~650 °C and 650~750 °C, respectively. The conversion of 1-propanol and yields of products in the steam reforming of 1-propanol are shown in Fig. 6a. Only 1-propanol was detected in the unreacted liquid, and the conversion of 1-propanol was almost 100% at 650 °C. Besides the steam reforming reaction, the formation of methane by decomposition of 1-propanol (Eq. (14)) and the methanation of CO and CO2 also occurred.41 The yield of methane suffered a decrease from 625 °C due to the reforming of methane at high temperatures. C= H> OH → 2CH7 + CO

(14)

The conversion of acetic acid and yields of products in the steam reforming of acetic acid are shown in Fig. 6b. In comparison with other compounds, higher reaction temperatures (650~750 °C) were needed for the steam reforming of acetic acid, indicating its low reactivity. The yield of methane decreased with the reaction temperature due to the steam reforming of methane at high temperatures. The formation of methane is mainly from the decomposition of acetic acid (Eq. (15)).42 The steam reforming of acetic acid below 650 °C was stopped by the blocking of reactor because a large amount of coke was formed on the catalyst. Bimbela et al. investigated the influence of reaction temperature on the stability of Ni-based catalyst in the steam reforming of acetic acid, and found that no decrease of gas yield was observed at 750 °C, while a significant decrease was observed at 650 °C and 550 °C, indicating that the low reaction temperature accelerated coke


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deposition.43 In the steam reforming of acetic acid, coke can be formed in several ways, but the most possible of them are:44 1) via the Boudouard reaction (Eq. (16)); 2) via the acetic acid decomposition reaction (Eq. (17)); 3) via decomposition of ethylene and 4) via oligomers derived from acetic acid or acetone. Wang et al. found that acetic acid underwent rapid decomposition forming coke on the catalyst surface in steam reforming, and proposed a coke formation mechanism.45 Due to the instability of acetic acid, it is easy to be decomposed into alkene and further converted into coke.18, 42 The coke covered on the active sites leads to the deactivation of catalysts. It is shown that acetic acid is notorious for the coke formation in the steam reforming reactions, and the content of organic acids is relatively large in bio-oil. Therefore, the study of steam reforming of bio-oil using acetic acid as model compound has arouse great attention in recent years.18, 19, 42, 46 CH= COOH → CH7 + CO 2CO ⇌ C + CO

(15)

(16)

CH= COOH → 2H + CO + C?@A

(17)

3.3 Comparison of the reactivities of various compounds As discussed above, the reactivities of these compounds were evaluated based on both the reactant conversion and hydrogen yield. Besides the steam reforming reactions, some parallel reactions, such as demethylation, also affect the conversion of reactants. However, the yields of toluene and benzene derived from the demethylation of reactants is small, and consequently conversion can be used for the reactivity comparison. To better compare the reactivities of various compounds in steam reforming, the pseudo-first-order reaction rate constants k were calculated according the following equation assuming that all were first-order reactions: B=

C 1 × EF D 1−

where F (g/h) is the flow rate of feedstock, W (g) is the weight of catalyst and X is the conversion of


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

feedstock. As shown in Fig. 7, the obtained reaction rate constants of various model compounds in steam reforming were correlated with reaction temperature using Arrhenius equation. Parameters of the model are summarized in Table 2. The apparent activation energy reflects the sensitivity of reaction rate against temperature. It should be noted that the apparent activation energy is also affected by the parallel reactions since the reaction rates are calculated from the conversion. Generally, as shown in Fig. 7, the reactivities of these model compounds decreased in the following trend: n-hexane > cyclohexane > benzene > toluene > m-xylene > 1-propanol > m-cresol > acetic acid. This indicates that the cleavage of C-C bond in alkanes or cycloalkanes is easier than that of the pi bond in the benzene ring in the steam reforming reaction. The little higher reactivity of n-hexane is probably due to the one less C-C bond than that in the cyclohexane structure. The addition of methyl group to the benzene ring decreases the reactivity of aromatic hydrocarbons in steam reforming. m-cresol showed lower reactivity than toluene, suggesting that the addition of hydroxyl hindered the steam reforming reaction. The oxygenated compounds (acetic acid, m-cresol and 1-propanol) showed lower reactivities than these hydrocarbons, indicating that oxygen-containing groups in the feedstocks are unfavorable for the steam reforming reactions. The great difference of bio-oil, gasification tar and naphtha is that the composition of former contains a large amount of oxygenated compounds, which makes the steam reforming reaction much difficult. Moreover, the oxygenated compounds also accelerated the deactivation of catalyst because of their easy coke formation. Among these compounds, acetic acid showed the lowest reactivity towards steam reforming, needed a higher reaction temperature (650~750 °C) than other compounds (550~650 °C), and leaded to a severe coke formation at low temperatures. Therefore, it is significant to use acetic acid as model compound to develop the coke-resistant catalyst with high activity for steam reforming of bio-oil.


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Energy & Fuels

4. Conclusions The steam reforming of a series of model compounds over the RuNi/BaOAl2O3 catalyst was conducted in a fixed-bed flow reactor at various temperatures. As a whole, the reactivities of these compounds in steam reforming decreased in the following trend: n-hexane > cyclohexane > benzene > toluene > m-xylene > 1-propanol > m-cresol > acetic acid. For aromatic hydrocarbons, the reactivity decreased with the addition of methyl groups to the benzene ring. The reactivity of toluene is higher than that of m-cresol, indicating that the incorporation of hydroxyl to the benzene ring increases the difficulty of steam reforming. Besides the steam reforming reactions, the side reactions such as demethylation, hydrogenolysis, dehydroxylation of aromatic compounds also occurred. Methane was mainly formed from the hydrogenolysis or decomposition of feedstocks and the methanation of CO and CO2. For C6 hydrocarbons, benzene showed lower reactivity than n-hexane and cyclohexane, indicating the cleavage of C-C bond in the stable benzene ring is more difficult. Cyclohexane and n-hexane can be converted into benzene by dehydroaromatization, while the reverse reactions cannot occur due to the thermodynamic limit. Acetic acid is easy to be thermally decomposed into coke at low temperatures and high reaction temperature is needed, which increases the difficulty of steam reforming bio-oil.


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

References: (1) Zhang, Q.; Chang, J.; Wang, T.; Xu, Y., Energy Convers. Manage. 2007, 48, (1), 87-92. (2) Pu, J.; Weng, H., China Pet. Process Pe. 2013, 15, (3), 86-90. (3) Trane, R.; Dahl, S.; Skjøth-Rasmussen, M. S.; Jensen, A. D., Int. J. Hydrogen Energy 2012, 37, (8), 6447-6472. (4) Wu, C.; Liu, R., Energy & Fuels 2010, 24, (9), 5139-5147. (5) Rioche, C.; Kulkarni, S.; Meunier, F. C.; Breen, J. P.; Burch, R., Appl Catal B: Environ. 2005, 61, (1), 130-139. (6) Wang, Z.; Pan, Y.; Dong, T.; Zhu, X.; Kan, T.; Yuan, L.; Torimoto, Y.; Sadakata, M.; Li, Q., Appl Catal A: Gen. 2007, 320, (Supplement C), 24-34. (7) Czernik, S.; Evans, R.; French, R., Catal. Today 2007, 129, (3), 265-268. (8) Xiu, S.; Shahbazi, A., Renew. Sustainable Energy Rev. 2012, 16, (7), 4406-4414. (9) Vagia, E. C.; Lemonidou, A. A., Int. J. Hydrogen Energy 2007, 32, (2), 212-223. (10) Hu, X.; Lu, G., Appl Catal B: Environ. 2009, 88, (3), 376-385. (11) Lyu, G.; Wu, S.; Zhang, H., Frontiers in Energy Research 2015, 3, (28). (12) Mukarakate, C.; Evans, R. J.; Deutch, S.; Evans, T.; Starace, A. K.; ten Dam, J.; Watson, M. J.; Magrini, K., Energy & Fuels 2017, 31, (2), 1600-1607. (13) Chen, J.; Sun, J.; Wang, Y., Ind. Eng. Chem. Res. 2017, 56, (16), 4627-4637. (14) Zhang, L.; Liu, R.; Yin, R.; Mei, Y., Renew. Sustainable Energy Rev. 2013, 24, (Supplement C), 66-72. (15) Kechagiopoulos, P. N.; Voutetakis, S. S.; Lemonidou, A. A.; Vasalos, I. A., Energy & Fuels 2006, 20, (5), 2155-2163. (16) Hou, T.; Zhang, S.; Chen, Y.; Wang, D.; Cai, W., Renew. Sustainable Energy Rev. 2015, 44, (Supplement C), 132-148. (17) Xu, Q.; Lan, P.; Zhang, B.; Ren, Z.; Yan, Y., Energy & Fuels 2010, 24, (12), 6456-6462. (18) Chen, G.; Tao, J.; Liu, C.; Yan, B.; Li, W.; Li, X., Renew. Sustainable Energy Rev. 2017, 79, (Supplement C), 1091-1098. (19) Pu, J.; Ikegami, F.; Nishikado, K.; Qian, E. W., Int. J. Hydrogen Energy 2017, 42, (31), 19733-19743. (20) Galdámez, J. R.; García, L.; Bilbao, R., Energy & Fuels 2005, 19, (3), 1133-1142. (21) Ishihara, A.; Qian, E. W.; Finahari, I. N.; Sutrisna, I. P.; Kabe, T., Fuel 2005, 84, (12), 1462-1468. (22) Bimbela, F.; Oliva, M.; Ruiz, J.; García, L.; Arauzo, J., J. Anal. Appl. Pyrolysis 2009, 85, (1), 204-213. (23) Xie, H.; Yu, Q.; Yao, X.; Duan, W.; Zuo, Z.; Qin, Q., Journal of Energy Chemistry 2015, 24, (3), 299-308. (24) Nurul Islam, M.; Zailani, R.; Nasir Ani, F., Renewable Energy 1999, 17, (1), 73-84. (25) Melpolder, F. W.; Brown, R. A.; Young, W. S.; Headington, C. E., Industrial & Engineering Chemistry 1952, 44, (5), 1142-1146. (26) Higo, T.; Saito, H.; Ogo, S.; Sugiura, Y.; Sekine, Y., Appl Catal A: Gen. 2017, 530, (Supplement C), 125-131. (27) Koike, M.; Li, D.; Nakagawa, Y.; Tomishige, K., ChemSusChem 2012, 5, (12), 2312-2314. (28) Takise, K.; Manabe, S.; Muraguchi, K.; Higo, T.; Ogo, S.; Sekine, Y., Appl Catal A: Gen. 2017, 538, (Supplement C), 181-189. (29) Takise, K.; Imori, M.; Mukai, D.; Ogo, S.; Sugiura, Y.; Sekine, Y., Appl Catal A: Gen. 2015, 489, (Supplement C), 155-161. (30) Świerczyński, D.; Libs, S.; Courson, C.; Kiennemann, A., Appl Catal B: Environ. 2007, 74, (3), 211-222. (31) Sekine, Y.; Mukai, D.; Murai, Y.; Tochiya, S.; Izutsu, Y.; Sekiguchi, K.; Hosomura, N.; Arai, H.; Kikuchi,


Page 18 of 29

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Energy & Fuels

E.; Sugiura, Y., Appl Catal A: Gen. 2013, 451, (Supplement C), 160-167. (32) Bona, S.; Guillén, P.; Alcalde, J. G.; García, L.; Bilbao, R., Chem. Eng. J. 2008, 137, (3), 587-597. (33) Sarıoğlan, A., Int. J. Hydrogen Energy 2012, 37, (10), 8133-8142. (34) Coll, R.; Salvadó, J.; Farriol, X.; Montané, D., Fuel Process. Technol. 2001, 74, (1), 19-31. (35) Vasiur Bahaman, M.; Albert Vannice, M., J. Catal. 1991, 127, (1), 251-266. (36) Crable, G. F.; Kearns, G. L., The Journal of Physical Chemistry 1962, 66, (3), 436-439. (37) Abon, M.; Bertolini, J. C.; Billy, J.; Massardier, J.; Tardy, B., Surf. Sci. 1985, 162, (1), 395-401. (38) Ahuja, R.; Punji, B.; Findlater, M.; Supplee, C.; Schinski, W.; Brookhart, M.; Goldman, A. S., Nat Chem 2011, 3, (2), 167-171. (39) Thawani, A.; Rajeev, R.; Sunoj, R. B., Chemistry – A European Journal 2013, 19, (12), 4069-4077. (40) Tsai, M. C.; Friend, C. M.; Muetterties, E. L., JACS 1982, 104, (9), 2539-2543. (41) Mizuno, T.; Nakajima, T., J. Chem. Eng. Jpn. 2002, 35, (5), 485-488. (42) Pu, J.; Nishikado, K.; Wang, N.; Nguyen, T. T.; Maki, T.; Qian, E. W., Appl Catal B: Environ. 2018, 224, (Supplement C), 69-79. (43) Bimbela, F.; Oliva, M.; Ruiz, J.; García, L.; Arauzo, J., J. Anal. Appl. Pyrolysis 2007, 79, (1), 112-120. (44) Basagiannis, A. C.; Verykios, X. E., Int. J. Hydrogen Energy 2007, 32, (15), 3343-3355. (45) Wang, D.; Montané, D.; Chornet, E., Appl Catal A: Gen. 1996, 143, (2), 245-270. (46) Yang, X.; Wang, Y.; Li, M.; Sun, B.; Li, Y.; Wang, Y., Energy & Fuels 2016, 30, (3), 2198-2203.


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

Figure captions:

Table 1 Physicochemical properties of the prepared RuNi/BaOAl2O3 catalyst. Table 2 Apparent activation energies and frequency factors for steam reforming of various model compounds.

Fig. 1 Schematic diagram of apparatus for steam reforming reactions. Fig. 2 Equilibrium yields of hydrogen as function of temperature in the steam reforming of various compounds. Conditions: 1.0 MPa, S/C=3.35. Fig. 3 Conversions of feedstocks and yields of products in the steam reforming of (a) benzene, (b) toluene, (c) m-xylene and (d) m-cresol at various temperatures. Reaction conditions: WHSV=20.8 h-1, 1.0 MPa, and S/C=3.35. Fig. 4 (a) Equilibrium conversions of benzene into cyclohexane, n-hexane and methane, and (b) equilibrium yields of products in demethylation of m-xylene as function of temperature. Conditions: 1.0 MPa. Fig. 5 Conversions of feedstocks and yields of products in the steam reforming of (a) n-hexane and (b) cyclohexane at various temperatures. Reaction conditions: WHSV=20.8 h-1, 1.0 MPa, and S/C=3.35. Fig. 6 Conversions of feedstocks and yields of products in the steam reforming of (a) 1-propanol and (b) acetic acid at various temperatures. Reaction conditions: WHSV=20.8 h-1, 1.0 MPa, and S/C=3.35. Fig. 7 Arrhenius plots of the reaction rate constants of the feedstock conversions in the steam reforming of various compounds.


Page 20 of 29

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Energy & Fuels

Table 1 Physicochemical properties of the prepared RuNi/BaOAl2O3 catalyst. Property Ni loadinga (wt. %) Ru loadinga (wt. %) Surface areab (m2/g) Pore volumeb (cm3/g) a b

Value 14.71 1.89 160 0.541

Data were obtained by XRF. Data were obtained by BET method.


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

Table 2 Apparent activation energies and frequency factors for steam reforming of various model compounds. Model compound Ea (kJ/mol) R2 lnA Benzene 292 44.54 0.98 Toluene 204 32.16 0.99 m-xylene 221 33.63 0.98 n-hexane 261 41.45 0.96 Cyclohexane 230 36.86 0.98 m-cresol 190 28.96 0.99 1-propanol 91.0 16.27 0.98 Acetic acid 116 18.63 0.98


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Energy & Fuels

1 2

Fig. 1 Schematic diagram of apparatus for steam reforming reactions.

3

23


Energy & Fuels

100

Equilibrium yield of hydrogen (%)

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

1

80

acetic acid 1-propanol

60 m-cresol 40

benzene

20

toluene m-xylene cyclohexane n-hexane

0

0

100

200

300

400

Reaction temperature 500

600

700

Temperature (°C)

2

Fig. 2 Equilibrium yields of hydrogen as function of temperature in the steam reforming of various

3

compounds. Conditions: 1.0 MPa, S/C=3.35.

4 5

24


Page 24 of 29

Page 25 of 29

H2

CO

CH4

CO2

toluene

(a)

100

CO

CO2

benzene

toluene 100

60

30

40

20 20 10 0 540

560

580

600

620

640

60

40 30

40

20 20 10

0 660

0 540

560

Temperature (°C)

1 H2

CO

CH4

CO2

benzene

(b)

H2 70

100

620

640

0 660

CO

CH4

CO2

benzene

toluene

(d)

phenol 100

60

30

40

20 20 10

560

580

600

620

640

50

Yield (%)

60

40

80

Toluene conversion (%)

80 50

60

40 30

40

20 20 10

0 660

0 540

560

Temperature (°C)

2

600

Temperature (°C)

60

0 540

580

Xylene conversion (%)

60

40

80 50

Yield (%)

50

Benzene conversion (%)

80

70

CH4

(c)

60

Yield (%)

H2

70

580

600

620

640

Cresol conversion (%)

70

Yield (%)

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

Energy & Fuels

0 660

Temperature (°C)

3

Fig. 3 Conversions of feedstocks and yields of products in the steam reforming of (a) benzene, (b) toluene, (c)

4

m-xylene and (d) m-cresol at various temperatures. Reaction conditions: WHSV=20.8 h-1, 1.0 MPa, and

5

S/C=3.35.

6

25


Energy & Fuels

25

80

(b)

methane

(a)

CH4

20 cyclohexane

Equilibrium yield (%)

100

Benzene conversion (%)

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

Page 26 of 29

60 n-hexane 40 H2:benzene=10:1 20

Reaction temperature

15

H2:xylene=2:1

10

benzene

5

Reaction temperature

toluene m-xylene

0 100

1

200

300

400

500

600

0 100

700

200

Temperature (°C)

300

400

500

600

700

Temperature (°C)

2

Fig. 4 (a) Equilibrium conversions of benzene into cyclohexane, n-hexane and methane, and (b) equilibrium

3

yields of products in demethylation of m-xylene as function of temperature. Conditions: 1.0 MPa.

4

26


CO

CH4

CO2

benzene

50

100

60

80

50

40 60 30 40 20 20

10 0 540

1

560

580

600

620

H2

CO

CH4

CO2

benzene

(b)

toluene 100

80

40 60 30 40 20 20

10

0 640

0 540

560

Temperature (°C)

580

600

620

Cyclohexane conversion (%)

H2 (a)

Yield (%)

60

Yield (%)

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

Energy & Fuels

Hexane conversion (%)

Page 27 of 29

0 640

Temperature (°C)

2

Fig. 5 Conversions of feedstocks and yields of products in the steam reforming of (a) n-hexane and (b)

3

cyclohexane at various temperatures. Reaction conditions: WHSV=20.8 h-1, 1.0 MPa, and S/C=3.35.

4

27


Energy & Fuels

H2

CO

CH4

CO2

propanol

(a)

H2

80

100

CO

CH4

CO2

60

40 30

40

20 20 10 0 520

1

540

560

580

600

620

640

80 60

Yield (%)

80

Propanol conversion (%)

60 50

100

(b)

60 40 40 20 20

0 640

0 660

660

680

700

720

740

Acetic acid conversion (%)

70

Yield (%)

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

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0 760

Temperature (°C)

Temperature (°C)

2

Fig. 6 Conversions of feedstocks and yields of products in the steam reforming of (a) 1-propanol and (b)

3

acetic acid at various temperatures. Reaction conditions: WHSV=20.8 h-1, 1.0 MPa, and S/C=3.35.

4

28


Page 29 of 29

7

benzene hexane

6

toluene acetic acid

5

ln[k(h-1)]

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

Energy & Fuels

cyclohexane

xylene propanol

4 3

cresol

2 1 0.95

1.00

1.05

1.10

1.15

1.20

1.25

-1

1000/T (K )

1 2

Fig. 7 Arrhenius plots of the reaction rate constants of the feedstock conversions in the steam reforming of

3

various compounds.

4

29


Evaluation of Reactivities of Various Compounds in Steam Reforming

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