Oxidative-reforming of model biogas over NiO/Al2O3 catalysts: The influence of the variation of support synthesis conditions
Graphical abstract
Introduction
Biogas is produced by anaerobic fermentation of biomass and is composed primarily of methane and carbon dioxide (molar ratio CH4:CO2 = 1.5), and other gaseous by-products such as H2S, NH3, H2. Its composition can vary with the origin of the biomass feedstock and the operational conditions during fermentation [1], [2]. Most of the hydrogen and syngas for industrial use is produced through steam reforming of methane. The biogas can be used as a substitute for natural gas in reforming processes, which is advantageous since biogas is considered a renewable source.
Thus, due to its principal components, biogas can be transformed into synthesis gas (syngas = H2/CO) through the dry reforming of methane (DRM, reaction (1)) in a CH4:CO2 molar ratio of 1, and the excess methane can be partially oxidized by the POM reaction (reaction (2)). These coupled processes result in the oxidative-reforming of biogas [3], [4]:DRM: 1.5CH4 + 1CO2 → 2CO + 2H2 + 0.5 CH4 ΔH° = 260.5 kJ mol−1POM: 0.5CH4 + 0.25O2 → 0.5CO + 1H2 ΔH° = −22.6 kJ mol−1
It is known that the POM reaction over nickel catalysts follows the combustion-reforming pathway [5], [6], where the total combustion of methane (TCM), dry reforming of methane (DRM) and steam reforming of methane (SRM) occur in parallel:TCM: CH4 + 2O2 → CO2 + 2H2O ΔH° = −890 kJ mol−1DRM: CH4 + CO2 → 2CO + 2H2 ΔH° = 260.5 kJ mol−1SRM: CH4 + H2O → CO + 3H2 ΔH° = 225.4 kJ mol−1
All these reactions (1), (2), (3), (4), (5), occur in parallel during the oxidative-reforming of biogas. The water–gas shift reaction (WGSR) is a reversible and exothermic reaction, and its reverse direction (RWGSR, reaction (6)) is favored at high temperatures where the reforming of methane takes place; so the occurrence of the RWGSR is very likely to occur during the reforming of biogas:RWGSR: CO2 + H2 ↔ CO + H2O ΔH° = 34.3 kJ mol−1
The syngas (H2/CO) produced by the reforming of the biogas can be used in the production of dimethyl ether (DME), a promising substituent for commercial diesel, through the syngas to DME process (known as STD process). Syngas can also be used as a feedstock in the Fischer-Tropsch process. The oxidative-reforming of biogas (reactions (1), (2), (3), (4), (5)) produces syngas with a H2/CO ratio of 1.2, which allows its direct use in the STD process [7], [8].
Several metal catalysts have been proposed for methane decomposition: (i) the non-noble group VIII: Ni, Co, Fe; (ii) the noble group VIII: Ru, Rh, Pd, Pt, Ir; and (iii) transition metal carbide catalysts. Within non-noble group VIII, nickel based catalysts are the most promising as they are inexpensive, of high availability and show catalytic activity comparable to that of noble metals [9]. Nevertheless, nickel based catalysts are affected by carbon incrustation in the metallic nickel particles which leads to deactivation, additionally it was found that the carbon is more soluble in nickel than in noble metals (e.g. Rh, Ru, Ir, Pt) [10].
The methane reforming industry uses the commercial catalyst, nickel supported on alumina (Ni/Al2O3); however, it presents problems due to carbon deposits that lead to deactivation. This problem is related to coking (deposition of coke) caused by nickel incrustation, and can also be promoted by the acidity of the alumina that also favors the carbon deposition. Some pathways have been proposed to explain the carbon deposition during the reforming of methane, among them, are the cracking reaction of methane (reaction (7)) and the Boudouard reaction (8):CH4 → C + 2H2 ΔH° = 74.8 kJ mol−12CO → C + CO2 ΔH° = −172.8 kJ mol−1
Coke formation is inevitable under the reforming process operating conditions (<500 °C), hence various alternatives have been proposed to minimize the carbon deposits including: the use of solid solutions as catalytic supports [3], [6], [11], [12], the reduction of the metallic nickel crystallite size [13], [14] and the addition of promoters [4], [15], [16], among others. Another proposal to improve the catalytic performance, thereby reducing the coke deposits, is to modify the acidic–basic properties of the catalyst by the addition of alkaline-earth cations (e.g. Ca2+, Mg2+, Ba2+) or rare earth metal oxides (e.g. La2O3, Y2O3) [17].
The γ-alumina is a solid-acid which is extensively used as catalytic support due to its high surface area, acidic properties (due to Bronsted acid centers: Al–OH), thermal stability, low toxicity and low cost [18], [19], [20]. The properties of alumina depend on the synthetic method which can affect the catalytic properties of the resulting catalyst for the methane reforming reaction, and can lead to different conversion rates and different types of carbon deposits [21]. Currently, there are not many studies focused on the variation of the synthesis conditions of γ-alumina obtained by the sol–gel method, and its influence on the catalysis of the reforming of biogas.
Aluminum is an abundant raw material on earth and is mainly extracted from bauxite, much of this aluminum is recycled. In our previous work [18], various high-surface-area aluminas (the highest values recorded 371 m2 g−1) were obtained from aluminum scrap (aluminum cans of >99% purity) by the sol–gel method under moderate synthesis conditions using inexpensive starting materials. In that report, the influence of the variation of the synthesis conditions (precipitation pH and ageing temperature) in the physicochemical properties of the resulting γ-aluminas was studied. The present paper is a continuation of that previous report [18]; hence, the present study focuses on the influence of this variation, on the catalytic properties of the resulting nickel catalysts. The physicochemical properties of the resulting Ni/γ-Al2O3 catalysts were correlated with their catalytic behavior in the oxidative-reforming of model biogas and with their coke deposition rates; these results were compared with a nickel catalyst supported on a commercial γ-alumina.
Section snippets
Preparation of catalysts
The aluminas were prepared by the sol–gel method from aluminum scrap (>99.9% Al). The methodology was detailed in our previous report [18]. In this methodology, the sodium aluminate was obtained from the stoichiometric reaction between a beverage can (>99% Al purity) and NaOH solution (2N). The resulting sodium aluminate solution was filtered and then separated in different aliquots to be precipitated with an H2SO4 solution (2N). The precipitation pH was varied between 6 and 7, once obtaining
Characterization
The scanning electron micrograph (SEM) images of the fresh catalysts are shown in Fig. 1a–e. The SEM images of samples NiAl6-25 (Fig. 1c), NiAl7-25 (Fig. 1d) and NiAl6-80 (Fig. 1e) are similar to each other, in these images it is possible to observe the formation of agglomerates in the form of round distorted platelets, the platelets in NiAl6-25 and NiAl7-25 are larger and have similar sizes. The SEM image of NiAl_commercial (Fig. 1a) shows an amorphous morphology. NiAl7-80 (Fig. 1b) formed
Conclusions
The variation of the parameters of synthesis of sol–gel alumina (precipitation pH and ageing temperature) influenced the catalytic performance of the resultant catalysts. The sol–gel aluminas precipitated at pH 7 and aged at 25 or 80 °C were optimal catalytic supports for nickel based catalysts applied in the reforming of the model biogas.
The catalysts supported on alumina precipitated at pH7 (NiAl7-80 and NiAl7-25), favored high conversion rates. The highest conversion values was recorded by
Acknowledgements
The authors thank the Brazilian National Council for Scientific Development (CNPq) for the fellowship, the FAPESP agency for the financial support, the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas, Brazil, for the XANES analysis, and the Department of Chemical Engineering of the Universidade Federal de São Carlos for the N2 adsorption–desorption analysis.
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