Oxidative coupling of methane on Sr/La2O3 catalysts: Improving the catalytic performance using cordierite monoliths and ceramic foams as structured substrates

Abstract

This work shows how the use of structured catalysts results in an important increase of the C2 yield in the oxidative coupling of methane over Sr/La2O3 catalysts. Solids with 2 and 5wt.% of Sr (Sr2 and Sr5) were prepared and washcoated on cordierite monoliths and alumina foams. Homogeneous and mechanically stable catalytic films were obtained in both cases with the addition of silica as binder. Other types of foams were also prepared (aluminum silicate and magnesia stabilized zirconia) but in such cases the adhesion of coatings was rather poor. The catalytic surface XPS characterization of both powders and structured catalysts indicates that lanthanum is mainly present as hydroxide and carbonate and that strontium is mainly present as carbonate. However, in the case of washcoated cordierite monoliths, a modification in La 3d spectra measured by XPS suggests a strong interaction of La with its chemical environment, most probably with silica and/or magnesia. This interaction could be originated by migration of the cordierite components towards the catalytic film, as seen by EDX. The decomposition of La(OH)3 and La2O2CO3 into La2O3 under reaction conditions was confirmed by in situ LRS. While hydroxide prevails at low temperature, the presence of oxycarbonates starts to be relevant when the reaction takes place at temperatures above 400°C and the oxycarbonates decompose into oxide over 700°C. The monolithic catalysts (M)Sr2 and (M)Sr5 resulted twofold more active towards ethane and ethylene than the corresponding Sr2 and Sr5 powders. Meanwhile, the washcoated alumina foam (FAl)Sr5 showed methane conversion values similar to those of the powder but higher C2 selectivity; thus, the C2 yield was also higher. In the case of the monolithic catalysts, the increase in methane conversion and C2 yield could be related to the interaction of Sr and La with silica and/or magnesia present in the cordierite structure. In the case of the alumina foam catalyst, the higher selectivity could be originated by the lower gas phase/catalytic surface ratio (as compared with the monolith), that inhibits gas-phase combustion of C2 products. In order to assess the stability of (M)Sr5, which was the best catalyst among the ones studied in this work, time-on stream runs at 600 and 800°C were performed. Surprisingly, at 800°C, the C2 yield steeply increased from 18% to 22.5% during the first 70h and then remained constant until 100h. This phenomenon could be due to a gradual increase in chemical interactions between the cordierite components and the catalytic coat. At lower temperature (600°C),C2 yield was constant during the 100h of time-on-stream, indicating that this temperature is not enough to produce the said effect.