Catalytic steam reforming of acetic acid as a model compound of bio-oil
Graphical abstract
Introduction
The depletion and environmental pollution from fossil fuels necessitate new energy resource development [1]. Hydrogen is recognized as the most promising alternative to fossil fuels for the future because of its cleanness, recyclability and high energy density [2], [3]. Hydrogen can be produced from a variety of sources such as fossil fuels, nuclear energy, water and biomass [4]. Currently, hydrogen is produced mainly from fossil fuels; roughly 96% of hydrogen is produced by steam reforming natural gas and oil fractions [5], [6]. The hydrogen production from fossil fuels presents some disadvantages such as additional consumption of fossil fuel as energy source and release of large amounts of CO2 into the atmosphere [7]. In addition, the production of hydrogen from water using non-fossil primary energy sources (atomic, solar, geothermal and others) is still disadvantageous economically [8]. This is expected to result in increased usage of alternative biomass energy. In general terms, production of H2 from biomass wastes can reduce waste disposal problems and substrate costs, thus becoming attractive. Moreover, it does not contribute to global warming since carbon dioxide from the atmosphere is recycled [9], [10].
Hydrogen can be produced from biomass via thermochemical processes, such as flash pyrolysis, which produces mainly liquids (bio-oils), and then the bio-oils can be converted to hydrogen by catalytic steam reforming [11], [12]. Steam reforming of bio-oil produced from fast pyrolysis of biomass has been described by many authors as one of the most promising and economical methods for hydrogen production [13], [14], [15]. Bio-oil is a complex mixture of organic compounds like acids, ketones, esters, alcohols, phenols and guaiacols, and its steam reforming is characterized with many difficulties; the most important is coke accumulation over the catalytic surface [16], [17]. Therefore, designing efficient catalyst requires the use of model oxygenated components for preliminary tests. Acetic acid (HAc), which is soluble in water, is chosen as a model compound because it is one of the most representative constituents of bio-oil [18]. Thermodynamic calculations have proven and accounted for the feasibility of acetic acid steam reforming [19]. Stoichiometrically, steam reforming of HAc can be represented as follows (Eq. (1)):CH3COOH + 2H2O → 2CO2 + 4H2 ΔH°298 K = 32.21 kcal/mol
Nickel catalysts are widely used as low-cost non-noble metal catalysts in industry for a number of chemical reaction processes. This metal is commonly employed as the active phase to conduct steam reforming of organic compounds such as methane, ethanol and glycerol, due to the high C–C bond breaking activity [20], [21], [22], [23]. Moreover, the nature of the support strongly influences the catalytic performance of supported Ni catalysts destined for steam reforming, as it affects dispersion and stability of the metal as well as may participate in the reaction [24]. Among oxide supports, alumina-based supports are commonly used for steam reforming due to their chemical and physical properties, since they have good mechanical strength and thermal stability, moreover it is possible to control their textural properties [25], [26]. However, the main problem for most of the Ni catalysts supported on alumina is the high deactivation rate related to the formation of coke deposits. The deactivation can occur by covering the active phase due to encapsulating carbon and also by filamentous carbon formation [27], [28]. Thus, basic additives or promoters produce highly dispersed Ni0 species, and consequently, can drastically enhance the resistance to carbon deposition over Ni catalysts. Addition of alkaline earth oxides (MgO, CaO) is widely used in reforming formulations to neutralize acidity of Al2O3 [29]. These basic additives, besides acting as a poison for the acid sites on alumina, also favor H2O adsorption and the OH mobility on the surface, accelerating the carbon oxidation and reducing the coke deposition [23].
In order to improve the stability of the nickel catalysts in steam reforming of acetic acid, we prepared and characterized nickel catalysts supported on γ-Al2O3 modified by MgO used as promoter; therefore, the total amount of Mg in the nickel catalyst range from 1 to 10 wt.%. The Ni loading was fixed at 15 wt.% in all cases. γ-Al2O3 was used as support to obtain Ni catalysts with high specific surface area. The Ni-MgO/γ-Al2O3 catalysts were prepared by sequential impregnation method and tested in steam reforming of acetic acid at 500 and 600 °C.
Section snippets
Catalyst preparation
Ni catalysts supported on (MgO)-modified γ-Al2O3 were prepared by the sequential impregnation method. The γ-Al2O3 powder (200 m2 g−1; Vpore = 0.60 cm3 g−1; Alfa Aesar) was initially calcined under air at 500 °C for 2 h in a muffle furnace in order to remove organic impurities. γ-Al2O3 was first impregnated with a solution of Mg(NO3)2·6H2O (Alfa Aesar) with loadings of 1, 5 and 10 wt.% of Mg, under stirring at 70 °C followed by drying overnight in air at 110 °C. The final samples (1%Mg/γ-Al2O3, 5%Mg/γ-Al2O
XRF analysis
The results of X-ray fluorescence showed that the contents were very close to the nominal composition (Table 1), indicating that there was no loss of Ni and Mg by the sequential impregnation method.
XRD analysis
The XRD diffraction patterns for calcined and reduced catalysts are shown in Fig. 1, Fig. 2 and their textural properties in Table 1. The typical diffraction peaks of γ-Al2O3 support at 2θ = 36.9°, 46.8° and 66.7° (JCPDS 86-1410) virtually coincided with those of the NiAl2O4 and MgAl2O4 spinel-like
Conclusion
Through analysis of X-ray diffractometry it was observed that with increased Mg load in the catalysts, the main diffraction peaks of γ-Al2O3 shift to a small angle, due to the formation of the MgAl2O4 phase. This effect is more evident in the 15Ni5Mg/Al and 15Ni10Mg/Al catalysts. The EXAFS analyses corroborate the XRD findings. The local chemical environment of Ni after calcination may be described as Ni aluminate phase. It was observed that the addition of Mg in the calcined catalysts does not
Acknowledgements
The authors thank the Brazilian National Council for Scientific Development (CNPq) and the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas, Brazil, for the XAS analysis.
References (86)
Int. J. Hydrogen Energy
(2010)Curr. Appl. Phys.
(2010)- et al.
Int. J. Hydrogen Energy
(2005) Appl. Catal. B
(2005)Technol. Soc.
(2003)- et al.
Renew. Sust. Energ. Rev.
(2012) Biomass Bioenerg.
(2012)- et al.
Appl. Catal. A
(2000) - et al.
Bioresour. Technol.
(2011) - et al.
Energy
(2011)
Catal. Today
Catal. Today
J. Anal. Appl. Pyrolysis
Appl. Catal. B
Catal. Today
Int. J. Hydrogen Energy
Int. J. Hydrogen Energy
J. Catal.
J. Catal.
Fuel
Chem. Eng. Sci.
Int. J. Hydrogen Energy
Appl. Catal. A
J. Catal.
Appl. Catal. A
Stud. Surf. Sci. Catal.
Fuel
J. Catal.
J. Catal.
Appl. Catal. B
Catal. Today
Fuel Process. Technol.
Int. J. Hydrogen Energy
Appl. Catal. A
Appl. Catal. A
Appl. Catal. A Gen.
J. Catal.
Solid State Ionics
Appl. Catal. A
Appl. Catal. A
Catal. Today
Catal. Today
Appl. Catal. A
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