Ni/Mg/Al anionic clay derived catalysts for the catalytic partial oxidation of methane - Residence time dependence of the reactivity features

The catalytic partial oxidation (CPO) of methane was investigated with four Ni-based catalysts prepared through reduction of hydrotalcite-type precursors, The calcination of the precursors, generated materials in which the Ni species were differently distributed between NiO, (Ni, Mg)Al2O4 phases and NiO-MgO periclase structures. The relative amount of the different phases depended on Ni content and affected the reactivity of the solids towards reduction and towards CPO step, Catalysts with high Ni-content required mild reduction conditions, but deactivated rapidly with time-on-stream due to carbon formation, Instead, catalysts with low Ni-content were activated only after a severe reduction treatment but showed high stability during the reaction. The effect of residence time was investigated with a particularly stabilized catalyst, in order to understand if selectivities and conversions could be kinetically controlled. Results were grouped considering three reactivity regions (tau less than or equal to 70 ms, 70< tau <150 ms, and 300< tau < 720 ms), In the shorter residence time region, large chemical composition variations and large temperature gradients were observed along the catalytic bed and relevant differences between surface and gas temperatures were determined, These differences, not observed in the other two residence time regions, are discussed and related to heat transfer limitation, occurrence of direct oxidation routes and hot spot phenomenon effects, (C) 1998 Academic Press.