Synthesis of enamel-protected catalysts for microchannel reactors: Application to methane oxidative coupling

Abstract

Lanthanum based oxides are good catalysts for the oxidative coupling of methane (OCM). However, when applied in stainless steel microstructured reactors, these catalysts quickly lose selectivity to ethane and ethylene. This is due to the incorporation of chromium, originating from the steel, into the catalyst surface. This study explores the possibility of protecting the catalyst layer from chromium poisoning by applying a dense inert protective layer inside the microchannels on which the catalyst is then deposited. Comparison of three different protective layers has revealed the incompatibility of alumina primer coatings with lanthanum catalysts and the insufficient blocking efficiency of spinels layers at 900°C. The coating of enamel layers on micro-reactor platelets has shown good efficiency against the migration of chromium into OCM catalysts even at 900°C. The enamel protective ability appears to be correlated to its density. The firing procedure responsible of the enamel densification requires heat treatments above 1000°C. Above these temperatures the chromium oxide solid-state diffusion through the enamel becomes an important pathway during the enamel densification step. Densification of the enamel is optimal at 1050°C with short dwell times. Influence of the enamel composition has also been studied, revealing that zirconium and barium oxides are undesirable.La/Sr catalysts deposited on enamel layers show good ethylene and ethane selectivity that only slightly decreases with time on stream. Exposure of the enamel/catalyst to unprotected steel at 800°C, however, leads to rapid deactivation by chromium contamination. This is in line with the diffusion of gaseous CrO3 as the main pathway for chromium contamination.