The effect of ceramic supports on partial oxidation of hydrocarbons over noble metal coated monoliths

Support effects on the production of synthesis gas and olefins by the partial oxidation of light hydrocarbons has been examined on noble metal catalysts at contact times of similar to 5 ms. We consider the effect of the following parameters on selectivities and conversions: adding a washcoat, varying pore size, ceramic support material, and loading of noble metal. In oxidation of methane on rhodium-coated monoliths, maximum hydrogen selectivity improves from 89 to 95% on the addition of a washcoat. It is also a strong function of the catalyst pore size, changing from 83% on a catalyst with 20 ppi (pores per inch) to 93% on a catalyst with 80 ppi. It varies from 86 to 91% on different ceramic supports with ZrO2 giving the best results. Using n-butane as the fuel, the hydrogen selectivity improves from 70 to 95% on adding a washcoat and changes from 70 to 95% on changing the pore size. In the oxidation of ethane on platinum-coated monoliths, the addition of a washcoat reduces ethylene selectivity from 63 to 35%, while changing the pore size results in minor variations. On different ceramic supports, the ethylene selectivity varies from 60 to 64% with mullite giving the best results. We find that washcoat addition, decreasing pore size, and replacement of zirconia for alumina as the support material increase syngas selectivity and reduce olefin selectivity irrespective of the fuel, catalyst, or amount of diluent used. Most of these results can be explained on the basis of differences in mass transfer rates to the catalytic site between catalysts of different support geometries. It is argued that homogeneous reactions play a minor role in these short contact time processes. (C) 1998 Academic Press