Characterization of Ni and Pd supported on H-mordenite catalysts: Influence of the metal loading method

Characterization of metal/HM catalysts has been carried out by the BET surface, TPDA, FTIR, TPR, TEM and CO pulse chemisorption techniques. The isomerization of n-butane was used as a test reaction for the activity of the prepared catalysts. The results show no difference between loading by ion exchange or impregnation when Ni is used as metal function. At low Ni contents (metal function control) an increase in conversion and selectivity to isobutane is obtained when the Ni content is raised. However, at high Ni content, the hydrogenolytic activity of the catalyst is enhanced, which leads to a decrease in its selectivity to isobutane. TEM pictures reveal no difference between Ni loaded by ion exchange or impregnation. This indicates that Ni atoms have certain mobility even if the catalysts were prepared by ion exchange. A very different behavior is found with Pd/HM catalysts. Pd is present as small particles in the ion-exchange catalysts, but forms clusters in the impregnated catalysts. Higher conversion and isobutane selectivity are obtained for the impregnated catalysts. At high metal contents, Pd-exchanged catalysts behave ina way similar to impregnated catalysts. It is proposed that bimolecular processes, such as isomerization via C-8 formation, cannot take place inside the one-dimensional pore system of mordenite. (C) 1998 Elsevier Science B.V.