Dry Reforming and Oxidative Conversion of Methane on Composite Materials Prepared by Self-Propagating Combustion Synthesis and Impregnation Method
Svetlana A. Tungatarova1,2*, Galina Xanthopoulou3, Tolkyn S. Baizhumanova1,2, Zauresh T. Zheksenbaeva1,2, Manapkhan Zhumabek1, Gulnar N. Kaumenova1,2, Rabiga O. Sarsenova1, Gulzeinep U. Begimova1,2
1 – D.V. Sokolsky Institute of Fuel, Catalysis and Electrochemistry, 142 Kunaev str., Almaty, 050010, Kazakhstan
2 - al-Farabi Kazakh National University, 71 al-Farabi ave., Almaty, 050040, Kazakhstan
3 – Institute of Nanoscience and Nanotechnology, NCSR Demokritos, Athens, 15310, Greece
* corresponding tungatarova58@mail.ru
Introduction
The utilization of the greenhouse gases, carbon dioxide and methane, has received increasing attention in the recent years. Particularly, methane catalytic reforming with carbon dioxide can transform two greenhouse gases simultaneously and thereby contribute to reduce global warming. The reaction product, syngas, contains a lower H2/CO ratio than those available from steam reforming and partial oxidation of methane, which is more preferable as a feedstock for the synthesis of valuable oxygenated chemicals and sulfur free synthetic liquid fuels.
Self propagating high temperature synthesis (SHS) of catalysts offers several benefits with respect to obtaining a homogeneous product, minimizing operating time, simplifying procedure and equipment, much lower energy costs, ease of manufacture and capability for producing materials with unique properties and characteristics. Method is attractive for industrial production: much lower energy consumption than traditional production methods, much lower energy costs, possibility for “just-in-time” manufacturing, high productivity, cheap catalysts, relatively simple process - easily adaptable to industrial scale, controlled physico-chemical properties of the products, large range of new materials which can be used in catalysis, it has wide diapason of structural forms of products - from granules of different size to blocks of honeycomb structure and different geometric forms. In addition, the environmental impact of SHS is very much lower than that of the traditional method, a fact which decreases even further the indirect cost of production. Therefore, the purposes of this work are to produce catalysts by using SHS to characterize the catalyst and experimentally examine their catalytic activity.
Materials and Methods
The SHS catalysts on the base of NiO – Al - α-Al2O3 were prepared from powder mixtures consisting of nitrates, metals, and oxides. Cylindrical specimens 10 mm in diameter and about 20 mm long were produced by uniaxial compaction under a pressure of about 10 MPa. Prior to initiation of SHS reaction, the specimens were preheated in an electric furnace at 700 – 900°C for several minutes. The resulting materials were characterized by X-ray diffraction (XRD) using CuKa1 radiation and Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDX). Further analyses were carried out using TEM and the particle size distributions were estimated by XRD peak broadening analysis. Surface area of the samples was determined by BET method. The catalytic activity of synthesized catalysts for dry reforming of methane was measured using coarsely crushed materials with an average pellet size of 3 mm, as dictated by industrial needs. The tests were carried out in a fixed bed free flow quartz reactor without any pre-reduction. All of the tests were conducted at atmospheric pressure in a flow of CO2-CH4-N2 mixture (1:1:1). The total flow rate of reactants was set at 860 h-1 and 3300 h-1 and the catalytic reaction was carried out at 700 - 900°C.
Results and Discussion
During the SHS experiments the combustion velocity was measured. The specific surface area of the catalyst system is low and ranges from 0.3 to 2.1 m2/g. This is due to the high temperatures of combustion during SHS, reaching 1500oC. SHS catalysts on the base of initial batch Al-NiO-Al2O3 have a similar qualitative composition, but there are differences in the phase ratio. The increase of NiO concentration and decrease of the Al concentration in the initial charge increases combustion velocity. It is connected with approach the stoichiometric composition, and therefore a greater heat generation affects the increase in the reaction rate.
The study of SHS catalysts structure was carried out using a scanning electron microscope (catalysts with NiΟ: 24.1 and 34.4 wt% concentration in the initial batch). It was found that the phase analysis by method of chemical analysis is corresponding to the data of XRD analysis: Al, α-Al2O3, Al-Ni, Ni, NiO, NiAl2O4.
Obtained SHS catalysts were tested for catalytic activity in the carbon dioxide dry reforming of methane in the temperature range 750 - 900°C. Hydrogen and CO yields for the studied SHS catalysts on the base of system NiO – Al - α-Al2O3 were investigated. Nickel, aluminum and oxygen ratio varies in different areas of catalyst. A high content of nickel, aluminum and oxygen corresponds to the spinel phase. The virtual absence of oxygen at a high content of nickel and aluminum corresponds to NiAl.
The best results for SHS catalysts based on systems ΝiΟ – Al - Αl2O3 are: 93% CH4 conversion, 100% conversion of CO2, product yield reaches 92% H2 and 99 % CO. Ratio hydrogen to carbon monoxide in the reaction product varies in the range of 0.7 – 1.35. Increasing the reaction temperature, in most cases, increases the ratio of H2/CO due to the amplification of dehydrogenation reaction.
Effect of catalyst composition on the conversion of CH4, CO2 and the ratio of H2/CO also seen in relation to nickel oxide, because from nickel oxide concentration depends content of nickel spinel - active catalyst component. The optimum concentration (maximum conversion of CO2) of nickel oxide in the starting material is 24 - 30%, and for the conversion of methane - optimum is clearly revealed 29% of nickel oxide. The catalyst with 29% nickel oxide and 51% aluminum in the starting material, after the SHS reaction contains a maximal concentration of NiAl, NiAl2O4 active phases into carbon dioxide reforming of methane. The optimal lattice parameter for maximum conversion of carbon dioxide and methane are 3.48 - 3.485 Å for aluminum oxide, which plays the role of a catalyst carrier and 1.42 Å - for NiAl2O4 playing the role of catalyst.
Effect of structural deformation and the composition of catalysts on the conversion of CH4, CO2 and H2/CO (at 900°C) were investigated too. The data indicate a clear structural dependence of the catalyst activity. The optimal lattice parameter for maximum conversion of carbon dioxide and methane are 3.48 - 3.485 Å for aluminum oxide, which plays the role of a catalyst carrier and 1.42 Å - for NiAl2O4 playing the role of catalyst.
The study of catalysts of similar composition but prepared by the traditional method of wetness impregnation and further their comparison with SHS catalysts was carried out. Investigation of catalysts was carried out under the following conditions: CH4 : CO2 : Ar = 1 : 1 : 1, GHSV - 860 h-1. Analysis of the data shows that the conversion of feedstock has similar values both on SHS catalysts, and on traditional supported samples. However, target products yield is significantly higher for SHS catalysts: hydrogen yield is about 48 - 51% on the supported catalysts, and ~ 80% on SHS catalysts; CO yield is about 40 - 42% on supported catalysts, while about 90% on SHS catalysts.
The catalysts of NiO-Al-α-Al2O3 series prepared by SHS method and by incipient wetness supporting were tested in the partial oxidation of methane. SHS catalysts show higher activity in CH4 partial oxidation, H2 yields (52.0 - 67.0%) are higher compared with supported samples (53.9 - 57.2), and for CO - (21.0 - 27.1) instead of (21.9 - 24.2). Ratio of Í2/ÑÎ = 2.0 – 2.9. The ideal ratio of H2/CO = 2 was obtained for the SHS sample (29.2% NiO + 50.8% Al + 20% Al2O3).
Received data indicate a significant advantage of the new composite materials produced by combustion synthesis process. The obtained target products on above-mentioned catalysts are cleaner, which do not require additional treatment.
This publication has been made within the project Grant No AP05132348, which is funded by the Ministry of Education and Science of the Republic of Kazakhstan.
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