Investigation of the Effects of Molecular Structure on Oxygenated

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Energy & Fuels 2009, 23, 926–933

Investigation of the Effects of Molecular Structure on Oxygenated Hydrocarbon Steam Re-forming Xun Hu†,‡ and Gongxuan Lu*,† State Key Laboratory for Oxo Synthesis and SelectiVe Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China, and Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China ReceiVed October 8, 2008. ReVised Manuscript ReceiVed NoVember 27, 2008

This paper presents the effects of molecular structure on the catalytic behavior of bio-oil components in the steam-re-forming reactions. Methanol, ethanol, 1-propanol, butanol, 2-propanol, 1,2-propanediol, glycerol, propionaldehyde, acetone, and propionic acid were used as model compounds. Steam re-forming of the alcohols with a long carbon chain was relatively difficult and yielded large amounts of CH4 and coke deposits. An increased number of hydroxyl groups in the alcohols suppressed the generation of CH4 while promoting the production of CO and coke deposits. Furthermore, the location of the hydroxyl group also impacted both the product distribution and carbon deposition in the re-forming process. The type of functional group significantly affected steam re-forming as well. Alcohol steam re-forming tended to produce a significant amount of CH4, while aldehyde (propionaldehyde) steam re-forming produced only small amounts of CH4 and other organic byproducts. Severe coke deposition was encountered in the steam re-forming of ketone compounds such as acetone, since acetone showed a high tendency for polymerization to coke.

Introduction Presently, hydrogen production has attracted great interest in the energy area because of its potential application in transportation and production of electricity with fuel cell systems.1,2 The traditional processes for hydrogen production are catalytic steam re-forming of methane, light hydrocarbons, and naphtha, or gasification of coal to yield syngas followed by water-gas shift conversion.3-5 These hydrogen production routes lead to carbon dioxide emissions and a strong greenhouse effect. Furthermore, the shortage of fossil fuel in the near future will cause serious energy problems. Hydrogen from biomass may be an alternative way to meet environmental needs because of the renewable, available, and carbon-neutral features of biomass.6-9 Two typical routes of producing hydrogen from biomass have been reported: gasification plus water-gas shift and fast pyrolysis coupled with steam re-forming of bio-oil. Bio-oil is a complex mixture of various types of organic compounds, and it typically consists of 4-6% of acids, 6-16% of aldehydes, 8-16% of phenolics, and * To whom correspondence should be addressed. Tel: +86 931 4968178. Fax: +86 931 8277088. E-mail: [email protected]. † Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences. (1) Chen, Y. Z.; Shao, Z. P.; Xu, N. P. Energy Fuels 2008, 22, 1873– 1879. (2) Liu, S.; Zhang, K.; Fang, L. N.; Li, Y. D. Energy Fuels 2008, 22, 1365–1370. (3) Tailleur, R. G.; Davila, Y. Energy Fuels 2008, 22, 2892–2901. (4) Pena, M. A.; Gomez, J. P.; Fierro, J. L. G. Appl. Catal. A 1996, 144, 7–57. (5) Zhou, H. C.; Cao, Y.; Zhao, H. Y.; Liu, H. Y.; Pan, W. P. Energy Fuels 2008, 22, 2341–2345. (6) Adhikari, S.; Fernando, S. D.; Filip To, S. D.; Bricka, R. M.; Steele, P. H.; Haryanto, A. Energy Fuels 2008, 22, 1220–1226. (7) Jefferson, M. Renew Energy 2006, 31, 571–582. (8) Wang, Z. X.; Dong, T.; Yuan, L. X.; Kan, T.; Zhu, X. F.; Torimoto, Y.; Sadakata, M.; Li, Q. X. Energy Fuels 2007, 21, 2421–2432. (9) Lee, D. H.; Lee, D. J. Energy Fuels 2008, 22, 177–181.

unidentified oxygenates.10,11 Steam re-forming of bio-oil as a whole meets many difficulties because of its complexity.12 Some work has been done on production of hydrogen from the single compound, such as methanol,13-16 ethanol,17-20 ethylene glycol,21-24 glycerol,25-27 and acetic acid.28-34 The development of active catalyst needs further understanding of the structure(10) Oasmaa, A.; Meier, D. Fast Pyrolysis of Biomass: A Handbook; CPL Press: Newbury, UK, 2002; Vol. 2, pp 41-58. (11) Radlein, D.; Piskorz, J.; Scott, D. S. J. Anal. Appl. Pyrolysis 1991, 19, 41–63. (12) Garcia, L.; French, R.; Czernik, S.; Chornet, E. Appl. Catal. A 2000, 201, 225–239. (13) Conant, T.; Karim, A. M.; Lebarbier, V.; Wang, Y.; Girgsdies, F.; Schlogl, R.; Datye, A. J. Catal. 2008, 257, 64–70. (14) Fukuhara, C.; Ohkura, H. Appl. Catal. A 2008, 344, 158–164. (15) Makarshin, L. L.; Andreev, D. V.; Gribovskiy, A. G.; Parmon, V. N Int. J. Hydrogen Energy 2007, 32, 3864–3869. (16) Papavasiliou, J.; Avgouropoulos, G.; Ioannides, T. Appl. Catal. B 2007, 69, 226–234. (17) Wang, W.; Wang, Z.; Ding, Y.; Xi, J.; Lu, G. Catal. Lett. 2002, 81, 63–68. (18) Byrd, A. J.; Pant, K. K.; Gupta, R. B. Energy Fuels 2007, 21, 3541– 3547. (19) Morgenstern, D. A.; Fornango, J. P. Energy Fuels 2005, 19, 1708– 1716. (20) Campos-Skrobot, F. C.; Rizzo-Domingues, R. C. P.; FernandesMachado, N. R. C.; Cantao, M. P. J. Power Sources 2008, 183, 713–716. (21) Shabaker, J. W.; Simonetti, D. A.; Cortright, R. D.; Dumesic, J. A. J. Catal. 2005, 231, 67–76. (22) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal. B 2005, 56, 171–186. (23) Dauenhauer, P. J.; Salge, J. R.; Schmidt, L. D. J. Catal. 2006, 244, 238–247. (24) Kechagiopoulos, P. N.; Voutetakis, S. S.; Lemonidou, A. A.; Vasalos, I. A. Catal. Today 2007, 127, 246–255. (25) Douette, A. M. D.; Turn, S. Q.; Wang, W.; Keffer, V. I. Energy Fuels 2007, 21, 3499–3504. (26) Slinn, M.; Kendall, K.; Mallon, C.; Andrews, J. Bioresour. Technol. 2008, 99, 5851–5858. (27) Adhikari, S.; Fernando, S.; Haryanto, A. Energy Fuels 2007, 21, 2306–2310. (28) Guell, B. M.; Babich, I.; Seshan, K.; Lefferts, L. J. Catal. 2008, 257, 229–231.

10.1021/ef8008647 CCC: $40.75  2009 American Chemical Society Published on Web 01/12/2009

Oxygenated Hydrocarbon Steam Re-forming

Energy & Fuels, Vol. 23, 2009 927

Table 1. Physicochemical Properties of Ni/Al2O3 Catalyst Ni specific Vporea Dporea Dredb Ddispc Ni sized loading surface sample wt.% areaa (m2/g-1) (cm3/g-1) (nm) (%) (%) (nm) Ni/Al2O3

30

126.2

0.3

9.8

10.9

7.8

23.6

a

Data were obtained by the BET method. b Reduction degree of Ni: the ratio of reduced Ni to total Ni × 100. c Dispersion of metal particles: ((2 × H2 adsorption)/reduced Ni) × 100. d Ni particle size calculated by the Scherrer formula from the X-ray diffraction spectra of the reduced Ni/Al2O3 catalyst.

reactivity of bio-oil components on catalyst. On the other hand, rapid deactivation of catalyst induced by coke deposition requires thorough study of the catalytic behavior of different components in bio-oil, for example, the coke formation tendency in the steam re-forming process. Nevertheless, less attention has been devoted to the relationship between molecular structure and catalytic behavior of a compound in steam re-forming reactions so far. The aim of this paper was to study the steam re-forming reaction of selected model compounds systematically and to compare their behavior in the steam re-forming reaction in detail. Methanol, ethanol, 1-propanol, butanol, 2-propanol, 1,2-propanediol, glycerol, propionaldehyde, acetone, and propionic acid were selected as model compounds. Ni/Al2O3 catalyst, a typical re-forming catalyst, was used. The effects of the carbon chain and functional groups on steam re-forming were presented in terms of conversion of the feedstock and product distribution. In addition, the coke formation rates of the feedstock in both the presence and the absence of steam were measured to assess the feasibility of the feedstock as a hydrogen resource. Experimental Section Catalyst Preparation. Ni/Al2O3 catalyst was prepared by impregnation method using Ni(NO3)2 · 6H2O as a precursor. The nickel loading was 30 wt.% to Al2O3. Before impregnation, the support γ-Al2O3 (129 m2/g, 30-45 mesh) was stabilized in air at 600 °C for 6 h. After impregnation, the catalyst precursors were dried at room temperature for 24 h and at 110 °C for another 24 h. Finally, the precursor was calcined at 500 °C for 4 h. Ni/Al2O3 catalyst was characterized by the BET method, hydrogen temperature-programmed reduction (H2-TPR), hydrogen temperatureprogrammed desorption (H2-TPD), temperature-programmed oxidation (TPO), and X-ray diffraction spectra (XRD) techniques. H2TPD measurements were performed in the range of 50 to 800 °C using Ar as a carrier gas (40 mL/min). Before measurements, catalyst samples (100 mg) were reduced with a 50 vol% H2/N2 mixture (flow rate: 60 mL/min) at 600 °C for 3 h. The sample was then cooled in flowing H2 to room temperature and then purged by the carrier stream until baseline stabilized. The H2 desorbed was monitored and quantified by a thermal conductivity detector (TCD). An adsorption stoichiometry of H/Ni ) 1 was assumed to calculate the dispersion of Ni. The reduction degree of Ni was defined as the fraction of the reduced Ni species with respect to total Ni species on the catalyst surface. The number of reduced Ni species was calculated by the TPO technique over the freshly reduced Ni/Al2O3 catalyst. The physicochemical properties of Ni/Al2O3 catalyst was summarized in Table 1. (29) Hu, X.; Lu, G. Chem. Lett. 2006, 35, 452–453. (30) Basagiannis, A. C.; Verykios, X. E. Int. J. Hydrogen Energy 2007, 32, 3343–3355. (31) Hu, X.; Lu, G. Chem. Lett. 2008, 37, 614–615. (32) Takanabe, K.; Aika, K.; Inazu, K.; Baba, T.; Seshan, K.; Lefferts, L. J. Catal. 2006, 243, 263–269. (33) Hu, X.; Lu, G. J. Mol. Catal. A: Chem. 2007, 261, 43–48. (34) Iulianelli, A.; Longo, T.; Basile, A. Int. J. Hydrogen Energy 2008, 33, 4091-4096.

Figure 1. Effects of the carbon chain on conversion of the alcohols: S/C ) 6; LHSV ) 12.2 h-1; P ) 1 atm.

Catalytic Activity Measurements. A catalytic test was carried out at atmospheric pressure in a fixed-bed continuous flow quartz reactor (i.d. 8 mm), consisting of a flow controller unit, a reactor unit, and an analysis unit. The reaction temperature increased from 200 to 600 °C at a 50 °C increment, and a sample was taken for analysis after stabilizing for 1 h at each investigated temperature. Typically, 0.5 g of catalyst diluted with an equal amount of quartz was used in each run. Prior to the experiment, the calcined catalyst was reduced in situ at 600 °C (heating rate 10 °C/min) for 3 h with a 50 vol% H2/N2 mixture (flow rate: 60 mL/min). The reaction mixture was then fed into the reactor by a syringe pump with a liquid hourly space velocity (LHSV) of 12.2 h-1 and a given steam to carbon ratio (S/C). N2 (20 mL/min) was used as the carrier gas. Separation and quantification of the products were attained on two online chromatographs equipped with thermal-conductivity detectors (TCD) and flame ionization detectors (FID). Conversion of the feedstock was calculated by dividing the total carbon in gaseous and liquid products with the carbon in feed. H2 selectivity was defined as the fraction of H2 produced with respect to the theoretical “full” conversion of the feedstock to H2 according to the equations presented in Table 2. Calculated method of the carbon-containing product’s selectivities was defined by the formula: Scarbon-containing product (%) ) 100 × (moles of carbon-containing compounds)/(moles of carbon reacted). S/C was defined as (moles of steam in feed)/ (moles of carbon in feed), while LHSV was defined as (volumetric flow rate of feed solution (cm3 h-1))/(catalyst bed volume (cm3)). Catalyst Characterization. The amount of carbon deposition on catalyst surface was analyzed by thermogravimetric analysis in a Perkin-Elmer TG/DTA apparatus. The catalyst was heated from 50 to 800 at 10 °C/min under synthetic air flow and the mass loss was measured.

Results and Discussion Effects of the Carbon Chain in Alcohols on Steam Re-forming. To investigate the effects of the carbon chain on steam re-forming, we selected methanol, ethanol, 1-propanol, and butanol as model compounds because of the linear increase of carbon atoms in these molecules. The re-forming reactions were carried out in a wide temperature range of 200-600 °C with an S/C of 6 and a LHSV of 12.2 h-1. Conversion of the feedstock versus reaction temperature was plotted in Figure 1. As could be seen, reactivity of the alcohols with steam decreased in the following order: methanol > ethanol > 1-propanol > butanol. Furthermore, the temperature required for complete conversion of the alcohols seemed to increase at a ca. 50 °C increment with the increase of one C-C bond in the alcohols. Clearly, the increase of carbon chain length in alcohols resulted in the difficulty for steam re-forming. Steam re-forming of the alcohols, with the exception of methanol, involved the cracking

928 Energy & Fuels, Vol. 23, 2009

Hu and Lu Table 2. “Full” Steam Re-forming of the Feedstock

feedstock

“full” steam re-forming reactions

∆H298 (kJ/mol)

Methanol ethanol 1-propanol butanol 1,2-propanediol glycerol 2-propanol propionaldehyde acetone propionic acid

CH3OH + H2O f 3 H2 + CO2 CH3CH2OH + 3 H2O f 6 H2 + 2 CO2 CH3CH2CH2OH + 5 H2O f 9 H2 + 3 CO2 CH3CH2CH2CH2OH + 7 H2O f 12 H2 + 4 CO2 CH3CHOHCH2OH + 4 H2O f 8 H2 + 3 CO2 CH2OHCHOHCH2OH + 3 H2O f 7 H2 + 3 CO2 CH3CHOHCH3 + 5 H2O f 9 H2 + 3 CO2 CH3CH2CHO + 5 H2O f 8 H2 + 3 CO2 CH3COCH3 + 5 H2O f 8 H2 + 3 CO2 CH3CH2COOH + 4 H2O f 7 H2 + 3 CO2

49.6 173.4 285.2 393.7 211.0 130.3 301.2 220.8 246.3 241.9

Table 3. Effects of the Carbon Chain in Alcohols on Product Selectivitiesa product selectivity (%)

a

feedstock

H2

CO2

CH4

CO

C2H4

C3H6

C4H8

acetaldehyde

others

methanol ethanol 1-propanol butanol

76.5 72.6 67.3 61.2

80.1 71.4 69.1 58.1

18.9 22.1 25.2 27.7

1.0 1.4 1.1 5.6

s 0.82 s 2.12

s s 0.57 s

s s s 1.13

s 1.2 0.22 0.19

s 0.16 1.26 2.1

Reaction conditions: T ) 400 °C; S/C ) 6; LHSV ) 12.2 h-1; P ) 1 atm.

of C-C bonds to form C1 products and hydrogen. Thus, higher reaction temperature was needed to re-form the alcohols with longer carbon chain. Carbon chain also had a pronounced impact on the product distribution, as seen in Table 3. The re-forming products became complex with the increase of carbon chain length. Only two simple byproducts, CH4 and CO, were detected in methanol re-forming process. In converse, complex organic byproducts, which led to the low yield of H2, were detected in steam re-forming of the higher alcohols. There are many pathways for the conversion of the higher alcohols in the steam re-forming process, such as dehydration, decomposition, or degradation. Ethanol, 1-propanol, and butanol all dehydrated in the re-forming reactions, giving ethylene, propylene, and butene, respectively. In addition, steam reforming of the long-chain alcohols seemed to be more favorable for the production of CH4, as evidenced by the remarkably high CH4 selectivity in butanol re-forming process. Methanation of carbon oxides and decomposition of feedstock were reported as two main routes for the formation of CH4 in the steam reforming reaction.30,33 Both the catalyst employed and the concentrations of hydrogen and carbon oxides in the effluent gas determined the occurrence and extent of methanation reaction, while the molecular structure played an essential role in determining the decomposition of a feedstock to CH4. Compared to that of methanol, complete rupture of the C-C bonds in the butanol molecule will result in the formation of more CHx (x e 3) species that could be hydrogenated, forming the byproduct CH4.35 Therefore, it is reasonable to propose that the decomposition of the long-chain alcohols is responsible for the high CH4 selectivity in their steam re-forming processes. Although the catalytic steam re-forming of hydrocarbon is often carried out with an excessive steam-to-carbon ratio (S/C > 3), online catalyst deactivation due to coking invariably affects catalytic performance.36 We therefore carried out the endurance tests under conditions that deliberately favored coking to assess the carbon formation tendency of the alcohols during long-term experiments. Steam re-forming of the alcohols was performed at stoichiometric S/C ratios and 600 °C for 12 h. The stoichiometric S/C ratios for methanol, ethanol, 1-propanol, and butanol re-forming, which were calculated according to the (35) Ren, J.; Huo, C.; Wang, J.; Cao, Z.; Li, Y.; Jiao, H. Surf. Sci. 2006, 600, 2329–2337. (36) Hardiman, K. M.; Ying, T. T.; Adesina, A. A.; Kennedy, E. M.; Dlugogorski, B. Z. Chem. Eng. J. 2004, 102, 119–130.

“full” steam re-forming reactions (Table 2), were 1, 1.5, 1.67, and 1.75, respectively. The conversion of the alcohols and the distribution of the products as a function of reaction time were illustrated in Figure 2. Ni/Al2O3 catalyst was deactivated to a different extent versus the prolonged reaction time in steam reforming reactions, which depended on the feedstock re-formed or rather the byproduct produced. Although the endurance test was performed at the stoichiometric S/C ratio, methanol steam re-forming gave only two simple byproducts, CH4 and CO. In comparison, dehydration of the higher alcohols occurred to a significant extent at the low S/C ratio, resulting in the presence of large amounts of ethylene, propylene, and butene in the products. In addition, a small amount of ethylene, which came from the degradation of butanol, was observed in the butanol re-forming process. The coke formation rates of the alcohols in the endurance tests are presented in Figure 3, where the coke formation tendency of the alcohols decreased in the following order: butanol > 1-propanol > ethanol . methanol. The byproducts CO, CH4, and olefin could be carbon precursors because of the potential occurrence of disproportionation of CO,37 decomposition of CH4,38 and polymerization of olefin39 in the steam reforming process. Nevertheless, although steam re-forming of methanol gave large amounts of CO and CH4, the carbon deposition was slight, implying that the disproportionation of CO or decomposition of CH4 did not occur to a significant extent under the reaction conditions employed. The experimental results also indicated that both CO and CH4 were not the main carbon precursors in the re-forming processes of the higher alcohols. Hence, another byproduct olefin was speculated as one of the main carbon precursors in the steam re-forming of the higher alcohols because of its high concentration in the products and its high tendency of polymerization to coke.40 In addition, direct decomposition of alcohols to carbonaceous deposits might be another route for coke formation, since alcohols were unstable at high temperature.41 Furthermore, the low S/C ratio employed (37) Trimm, D. L. Catal. Today 1999, 49, 3–10. (38) Guo, J.; Lou, H.; Zheng, X. Carbon 2007, 45, 1314–1321. (39) Michiue, K.; Jordan, R. F. J. Mol. Catal. A: Chem. 2008, 282, 107– 116. (40) Ni, M.; Leung, D. Y. C.; Leung, M. K. H. Int. J. Hydrogen Energy 2007, 32, 3238–3247. (41) Soma, Y.; Onishi, T.; Tamaru, K. Trans. Faraday Soc. 1969, 65, 2215–2223. (42) Paul, J. F.; Sautet, P. J. Phys. Chem. B 1998, 102, 1578–1585.



Figure 2. The endurance tests of methanol, ethanol, 1-propanol, and butanol re-forming: T ) 600 °C; stoichiometric S/C; LHSV ) 12.2 h-1; P ) 1 atm. (9) Conversion; (2) H2; (O) CO2; (b) CO; (1) CH4; (×) ethylene; (4) propylene; (0) butene; ()) acetaldehyde.

Figure 3. Coke formation rates of the alcohols in the endurance and decomposition tests.

Figure 4. Effects of the number of hydroxyl groups on the conversion of the alcohols: S/C ) 6; LHSV ) 12.2 h-1; P ) 1 atm.

here would result in incomplete steam re-forming, which favored the decomposition of alcohols. Therefore, we conducted the thermal decomposition experiments of the alcohols using pure alcohol solution with a LHSV of 12.2 h-1 at 600 °C for 3 h to measure the coke formation tendency of the alcohols in the absence of steam. The results were also summarized in Figure 3. The coke formation rate in methanol decomposition was far less than that of the higher alcohols. The methanol molecule contains high oxygen content, and therefore the decomposition of methanol gave high concentrations of carbon oxides rather than coke deposits. Conversely, decomposition of the higher alcohols gave a large amount of coke and that of butanol was the most serious. The butanol molecule contains a longer carbon chain, the complete rupture of which will generate more CHx species that is the precursor of coke.42 Effects of the Number of Hydroxyl Groups in Alcohols on Steam Re-forming. Functional groups play an essential

role in determining various physicochemical properties of a compound. Therefore, the catalytic behavior of a compound in steam re-forming is expected to be significantly affected by its functional group. Effects of the number of functional groups on the steam re-forming reaction were investigated using 1-propanol, 1,2-propanediol, and glycerol as the model compounds. Conversion of the alcohols as a function of reaction temperature is presented in Figure 4. As can be seen, the temperature required for complete conversion of the alcohols shifted to a higher temperature region with the increase in number of hydroxyl groups in the alcohols. Obviously, the existence of multiple hydroxyl groups in alcohol did not “activate” the molecule for re-forming and conversely led to difficulty in steam re-forming. An increasing number of hydroxyl groups in the molecule remarkably increased the gasification enthalpy of the alcohols (the gasification enthalpy for 1-propanol, 1,2-propanediol, and


Hu and Lu

Table 4. Effects of the the Number of Hydroxyl Groups in Alcohols on Product Selectivitiesa product selectivity (%) feedstock

H2

CO2

CH4

CO

C2H4

C3H6

acetaldehyde

acetone

1-propanol 1,2-propanediol glycerol

67.3 72.8 75.1

69.1 70.7 74.5

25.2 20.2 16.8

1.1 1.26 1.69

s 0.68 0.28

0.57 s s

0.22 s 1.21

s 0.28 s

a

others 1.26 1.92 3.1


glycerol at 20 °C is 50.5, 70.5, and 88.2 kJ/mol,43 respectively), which might further negatively affect activation of the molecule on metal sites, especially at low reaction temperatures. The number of hydroxyl groups in the alcohols also affected the product distribution, as shown in Table 4. Compared to that of 1-propanol and 1,2-propanediol, steam re-forming of glycerol yielded more unidentified organic byproduct. Moreover, the production of CH4 decreased greatly with the increase of hydroxyl group, while the production of CO presented the reverse trend in the steam re-forming reactions. The 1-propanol molecule contains low oxygen content, and therefore its cracking is more favorable for the formation of CHx species that can be the precursor of CH4. In contrast, cracking of glycerol favors the formation of COx intermediate that is the precursor of CO. In consequence, a higher CH4 selectivity was obtained in 1-propanol re-forming process while a higher CO selectivity was obtained in the glycerol re-forming process. The endurance tests of 1,2-propanediol and glycerol reforming were carried out at the stoichiometric S/C ratio and 600 °C for 12 h. Catalytic results and coke formation rates in the tests were presented in Figures 5 and 6, respectively. Similar to that of 1-propanol, conversions of both 1,2-propanediol and glycerol decreased versus the prolonged reaction time, but the decrease became more significant, especially for that of glycerol. CO and CH4 were the main gaseous byproducts and their production was still affected by the number of hydroxyl groups in alcohols even at the low S/C ratio. The coke formation rates of the alcohols in the endurance tests decreased in the following order: glycerol > 1,2-propanediol > 1-propanol. Steam reforming of glycerol gave only a small amount of ethylene and no propylene whose amount was significant in 1-propanol reforming reaction. However, glycerol steam re-forming gave the highest coke formation rate. The carbohydrate that has a C/O ratio of 1 such as glucose was unstable at the elevated temperature,44 which decomposed directly to a carbonaceous deposit to a significant extent before reaching the catalyst bed. Glycerol is the compound with a C/O ratio of 1. Therefore, direct decomposition of glycerol to coke was speculated to be the main route for carbon deposition in the endurance test because of the high reaction temperature employed and the high concentration of glycerol in the feed. Decomposition of 1,2propanediol and glycerol were carried out using the conditions similar to that of 1-propanol. The coke formation rates in the decomposition experiments were also summarized in Figure 6. In the absence of steam, the coke formation rates of 1-propanol and 1,2-propanediol were comparable but much slower than that of glycerol, thus supporting the speculation that decomposition of glycerol to coke was the main route for carbon formation in glycerol steam re-forming. Effects of the Location of the Hydroxyl Group in Alcohols on Steam Re-forming. We selected 1-propanol and 2-propanol as the model compounds to investigate the effects of location of hydroxyl group on steam re-forming. Steam re(43) Liu, G. Q; Zhang, L. J, Thermochemical data of organics: A Handbook, Chemical industry Press: Beijing, China, 2003; pp 609-610.

Figure 5. The endurance tests of 1,2-propanediol and glycerol re-forming: T ) 600 °C; stoichiometric S/C; LHSV ) 12.2 h-1; P ) 1 atm. (9) Conversion; (2) H2; (O) CO2; (b) CO; (1) CH4; (4) acetone; (×) ethylene; ()) acetaldehyde.

Figure 6. Coke formation rates of 1,2-propanediol and glycerol in the endurance and the decomposition tests.

forming of 2-propanol was carried out using the conditions similar to that used for 1-propanol. The conversion of 2-propanol versus reaction temperature was close to that of 1-propanol; hence, the conversion-temperature curve of 2-propanol was not presented. The product distributions of 1-propanol and 2-propanol re-forming were summarized in Table 5. Although the location of hydroxyl group showed a negligible effect on the



Table 5. Effects of Location of the Hydroxyl Group in Alcohols on Product Selectivitiesa product selectivity (%)

a

feedstock

H2

CO2

CH4

CO

C3H6

C2H4

acetaldehyde

others

1-propanol 2-propanol

67.3 64.5

69.1 68.6

25.2 23.6

1.1 0.95

0.57 1.12

s 0.33

0.22 0.49

1.26 1.1


Figure 7. The endurance test of 2-propanol re-forming: T ) 600 °C; stoichiometric S/C; LHSV ) 12.2 h-1; P ) 1 atm. (9) Conversion; (2) H2; (O) CO2; (b) CO; (1) CH4; (×) ethylene; (∆) propylene.

conversion of the alcohols, it showed an impact on product selectivities. The byproduct in 2-propanol re-forming became more complex. A small amount of ethylene, which probably formed through cracking of the C-C bond of 2-propanol associated with the subsequent dehydration, was detected in the 2-propanol re-forming process. Moreover, the amount of propylene was higher in the 2-propanol re-forming than in the 1-propanol re-forming. Evidently, 2-propanol showed a higher tendency for dehydration, which might be related to the bonding of the hydroxyl group with the middle carbon atom of 2-propanol molecule. The endurance test of 2-propanol reforming was also performed. Conversion of 2-propanol and the product selectivities in the endurance tests are depicted in Figure 7. The coke formation rate in the 2-propanol steam re-forming (around 67 mgC/gCat/h) was a little higher than that for 1-propanol, leading to a severe loss of activity of the Ni/Al2O3 catalyst. The severe carbon deposition in the 2-propanol steam re-forming was induced by the generation of ethylene and the high amount of propylene in the products because the coke formation rate for 2-propanol in the absence of steam was comparable with that of 1-propanol (results are not shown). Effects of the Type of Functional Group on Steam Re-forming. Effects of the type of functional group on steam re-forming were investigated using 1-propanol, propionaldehyde, acetone, and propionic acid as model compounds. The selected compounds contain the same carbon number and are representative of alcohol, aldehyde, ketone, and organic acid in bio-oil, respectively. Conversion-temperature curves for the feedstock in the steam re-forming reactions were plotted in Figure 8. The different reactivity of the feedstock with steam was evident at 350 °C, where conversion of the feedstock decreased in the following order: 1-propanol > propionaldehyde > propionic acid > acetone. Functional groups showed different capacities to activate the molecules for re-forming, and consequently distinct conversion patterns in the steam re-forming were observed. Steam re-forming of organic compounds involves the cracking of the C-C, C-H, C-O, or O-H bonds, forming hydrogen, carbon oxides, or other organic byproducts. The different functional groups imposed distinct effects on the

Figure 8. Effects of type of functional group on conversion of the feedstock: S/C ) 6; LHSV ) 12.2 h-1; P ) 1 atm.

compounds, such as acidity of the molecules, the bond energy, bond polarity, and the spatial configurations of the atoms, which may further affect the catalytic activity of the molecules in the steam re-forming process. Steam re-forming of the different types of compounds also gave distinct product distributions. The product selectivities in the feedstock re-forming at 400 °C were summarized in Table 6. 1-Propanol steam re-forming gave the lowest hydrogen yield because of the production of significant amounts of CH4 and other complex organic byproducts. Conversely, steam re-forming of propionaldehyde gave the highest hydrogen yield, since only CH4, CO, and ethane were produced as the byproducts and their amounts were mild. Although the steam re-forming of acetone and propionic acid also yielded complex organic byproducts, the hydrogen yields were remarkably higher than that for 1-propanol. These results could be ascribed to the much lower amount of CH4 in both the acetone and propionic acid re-forming processes. Taking the results presented in Tables 2, 3, and 4 into consideration, we found that steam re-forming of not only 1-propanol but also the short-chain alcohols (methanol), long-chain alcohols (butanol), and polyhydric alcohol (glycerol) produced a large amount of CH4. Evidently, the alcohol steam re-forming showed a high tendency to form CH4, which greatly diminished hydrogen yield. As previously stated, methane may come from the decomposition of organic molecules and methanation of carbon oxides in the steam re-forming reactions.30,33 Decomposition of the organic molecules dominates when the steam re-forming was of low efficiency, while the methanation reaction is favorable at mild temperatures, for example, 400 °C. The Ni/Al2O3 catalyst employed was probably more favorable for the occurrence of alcohol decomposition and the methanation reaction in the steam re-forming of the alcohols, leading to high methane selectivity. In addition to the product distribution, the functional group may affect coke formation tendency of the feedstock in the steam re-forming reaction. The endurance tests of propionaldehyde, acetone, and propionic acid steam re-forming were performed using conditions similar to that for 1-propanol. The catalytic results and the coke formation rates in the tests are summarized in Figures 9and 10, respectively. As can be seen,


Hu and Lu

Table 6. Effects of Type of Functional Group in Molecules on Product Selectivitiesa product selectivity (%) feedstock

H2

CO2

CH4

CO

C2H4

C3H6

C2H6

ketene

acetaldehyde

others

1-propanol propionaldehyde acetone propionic acid

67.3 89.1 84.9 87.7

69.1 90.5 82.6 86.2

25.2 8.2 8.8 4.8

1.1 1.2 1.0 0.94

s s s 0.56

0.57 s s s

s 0.12 s 1.36

s s 2.1 s

0.22 s 0.16 s

1.26 s 1.9 2.1

a


Figure 10. Coke formation rates of 1-propanol, propionaldehyde, propionic acid, and acetone in the endurance and decomposition tests.

re-forming, and ketene in the acetone re-forming.45 Direct decomposition or polymerization of the feedstock to coke was another possible reaction pathway for coke formation. The coke formation rates of the feedstock in the absence of steam are also depicted in Figure 10. Very interestingly, the coke formation rate for decomposition of propionic acid was comparable with that of 1-propanol, whereas propionic acid steam re-forming gave a higher coke formation rate in the presence of steam. The discrepancy was ascribed to the generation of a significant amount of ethylene in the propionic acid re-forming process. Carbon deposition in the decomposition of acetone was the most significant among the feedstocks investigated. Acetone, as a ketone compound, was easily polymerized, forming carbonaceous deposits on the catalyst surface.46 Furthermore, ketene, which was produced from the degradation of acetone, also had a high tendency to polymerize. Therefore, acetone acted not as only the reactant but also as the carbon precursor in the steam re-forming process, leading to severe coke deposition. Conclusions

Figure 9. Endurance tests of propionaldehyde, propionic acid, and acetone re-forming: T ) 600 °C; stoichiometric S/C; LHSV ) 12.2 h-1; P ) 1 atm. (9) Conversion; (2) H2; (O) CO2; (b) CO; (1) CH4; (×) ethylene; (]) ethane; (+) ketene.

the extent of deactivation of Ni/Al2O3 catalyst was approximately in accordance with the coke formation rates in the endurance tests. The carbon deposition in propionaldehyde reforming was the slightest, and conversion of propionaldehyde remained almost stable during the entire time on stream. Conversely, significant coke formation in the acetone re-forming led to a rapid decrease in the conversion of acetone and in hydrogen yield. Apart from CH4 and CO, significant amounts of byproduct were produced in steam re-forming of the feedstock and some were identified as carbon precursors, such as propylene in the 1-propanol re-forming, ethylene in the propionic acid

Results presented above clearly indicated the direct relationships between molecular structure and catalytic behavior of the compounds in the steam re-forming reaction. An increase of carbon number in alcohols resulted in an increase of re-forming temperature as well as the amounts of CH4 and other organic byproducts. Furthermore, severe carbon deposition was observed in the steam re-forming of long-chain alcohols at low S/C, resulting from the decomposition of the alcohols to coke and the production of large amounts of byproduct olefin. An increase of the number of hydroxyl groups in alcohols negatively affected activation of the alcohols for re-forming and (44) Marquevich, M.; Czernik, S.; Chornet, E.; Montane, D. Energy Fuels 1999, 13, 1160–1166. (45) Egret, H.; Couvercelle, J.; Belleney, J.; Bunel, C. Eur. Polym. J. 2002, 38, 1953–1961. (46) Zhang, B.; Tang, X.; Li, Y.; Cai, W.; Xu, Y.; Shen, W. Catal. Commun. 2006, 7, 367–372.


the production of CH4, while promoting the production of CO and coke formation. Direct decomposition to coke was the main route for coke formation in the glycerol steam re-forming. Location of the hydroxyl group mainly affected product distribution and coke formation. The type of functional group significantly affected steam re-forming. Compared to that of other feedstocks, the steam re-forming of 1-propanol was relatively easy. Nevertheless, the steam re-forming of 1-propanol and other alcohols presented a very high tendency for the formation of CH4. The coke formation rates in the steam reforming of the feedstock decreased in the following order:


acetone > propionic acid > 1-propanol > propionaldehyde. The ketone compounds such as acetone presented a high tendency to polymerize via carbonaceous deposits and therefore gave the highest coke formation rate, while both decomposition of the feedstock and polymerization of the byproduct contributed to carbon deposition in propionic acid, 1-propanol, and propionaldehyde re-forming reactions. Acknowledgment. We acknowledge the financial support of the 973 Project of China (No. 2007CB210204 and 2007CB613305). EF8008647

Investigation of the Effects of Molecular Structure on Oxygenated

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