Oxidative Coupling of Methane Using Catalysts Synthesized by Solution Combustion Method

Ranjita Ghose, Purdue University

Abstract

Ethylene is a precursor to many industrially important chemicals (e.g. polyethylene, polystyrene, PVC, etc.) and is primarily manufactured via high-temperature steam cracking of naphtha. Methane is the main constituent of natural gas (typically >95%), for which the reserves are vast and estimated to exceed those of crude oil. Thus, there is a strong interest in developing processes which enable methane conversion to higher valued products. In this context, oxidative coupling of methane (OCM), is an attractive alternative for the production of C2+ hydrocarbons, as compared to current processes based on crude oil. Solution combustion synthesis (SCS) is a one-step technique for the preparation of nanostructured complex metal oxides and is expected to be especially suitable for preparation of OCM catalysts, which are typically multimetallic and/or complex metal oxides. Details of this method and certain applications from literature are described in Chapter 2. In this work, SCS was used for the synthesis of several OCM catalyst series with varying metal ratios: (a) Sr-Al complex oxides, (b) La2O 2, (c) La-Sr-Al complex oxides, and (d) Na2WO4-Mn/SiO 2. The C2 yield and ethylene/ethane ratio were measured for each catalyst over a range of temperatures under a standard set of flow conditions. All the catalysts tested in this study showed good C2 yields and ethylene/ethane ratio, indicating that SCS is a promising method to prepare OCM catalysts, the details of which are discussed in Chapter 3. The measurement of catalyst activity at varying Sr to Al ratios suggested that perhaps the double perovskite phase in the Sr-Al oxides is active for OCM. The Na2WO4-Mn/SiO2 catalyst was the most promising among the tested catalysts, and was further optimized with respect to (a) addition of different metals, (b) Si precursor used and (c) process conditions including temperature and CH4/O2 feed ratio. The use of 5% La and tetraethoxysilane as Si precursor led to the best OCM performance, and the optimized catalyst (5%-La-10%Na2WO 4-5%Mn/SiO2) exhibited C2 yield 27% at ethylene/ethane ratio 3.6 under the optimum operating conditions. These values are among the highest reported in the literature. These optimization results are summarized in Chapter 4. The kinetics for OCM were also investigated systematically on the optimized catalyst, and the results are presented in Chapter 5. The kinetic studies provide an insight into the possible oxygen species responsible for methane activation. Based on power rate law expressions for methane conversion, it appears likely that methane activation step is rate-limiting, and oxygen adsorption on the catalyst is dissociative. The formation rates of primary products (C 2H6, CO and CO2) could be well described by the reaction scheme proposed in prior work. Finally, the possibility of increasing OCM performance by the use of two reactors in series with oxygen distribution is explored. Two catalysts, namely La2O3 and 5%La- Na2WO4-Mn/SiO2, were tested under high and low gas hourly space velocities. The results of the study indicate that a single reactor performs better than two reactors in series for the conditions and catalysts tested. An attempt is made to rationalize these results to provide recommendations for future work in Chapter 6.

Degree

Ph.D.

Advisors

Varma, Purdue University.

Subject Area

Chemistry|Chemical engineering

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