Sensitivity of the selective oxidation of methane over Fe/ZSM-5 zeolites in a micro fixed-bed reactor for the catalyst preparation method

Abstract

Selective oxidation of methane in a micro fixed-bed reactor by using aqueous H2O2 solution as oxidant was studied. Sub-micrometer sized crystals of HZSM-5 and silicalite-1 with Fe contents below 10 ppm were synthesized. Different methods were used to introduce iron to produce Fe/ZSM-5 zeolites with similar but defined Fe contents. Commercial ZSM-5 with trace amount (175 ppm) of Fe impurity showed considerable methane conversion while over the own nearly Fe-free HZSM-5 no conversion could be measured. Fe/ZSM-5 catalysts prepared by different methods starting from the own nearly Fe-free HZSM-5 showed similar methane conversions and catalysts based on Fe(II) as the precursor exhibited a better performance with regard to retarding over-oxidation. The productivities of oxygenates were enhanced more than two orders of magnitude while at the same time obtaining high turnover frequencies (TOFs) of 303 h−1. A methane conversion as high as 23.5% together with oxygenates selectivity of 90.1% was reached. In contrast to Fe/ZSM-5, Fe/silicalite-1, prepared from Fe-free silicalite-1 by activation via the same post-synthetic preparation methods that were also used to prepare Fe/ZSM-5 and having similar Fe contents, never led to an active catalyst.