Sulfur tolerance of zeolite beta-supported Pd-Pt catalysts for the isomerization of n-hexane

Bifunctional bimetallic M-Pt/H-Beta (M=Cu, Ga, Ni, and Pd) catalysts were prepared and applied to the isomerization of n-hexane. The sulfur-containing feed was prepared by addition of thiophene in pure n-hexane to have 500 ppmw sulfur. Sulfur in the feed brought about a substantial decrease in the catalyst performance and the sulfur deactivation of bifunctional Pt/H-Beta turned out to be a two step irreversible process caused by metal poisoning followed by coking. To test their effect on the sulfur tolerance, various second metals (Cu, Ga, Ni, and Pd) were added to monometallic Pt/H-Beta catalysts. Unfortunately, all of these, except for Pd, greatly decreased the sulfur tolerance of the original Pt/H-Beta catalyst. Regardless of the preparation method or the Pd/Pt atomic ratio of the bimetallic Pd-Pt series, all the bimetallic catalysts showed high sulfur tolerance, in comparison to the monometallic Pt/H-Beta and Pd/H-Beta. The metal dispersion and the hydrogenation activity decreased in the Pd-Pt series compared to Pt/H-Beta. However, the amounts of sulfur adsorbed and coke deposited on the sulfur-deactivated Pd-Pt/H-Beta were much lower than those on Pt/H-Beta, Pd/H-Beta, and the other M-Pt/H-Beta catalysts. It seems obvious that the Pd-Pt bimetallic interaction in Pd-Pt/H-Beta increased the amount of electron deficient metal sites. Therefore, it may be concluded that Pd-Pt bimetallic interaction inhibits irreversible sulfur adsorption and thereby reduces sulfur-induced coke formation. This is why the Pd-Pt series maintained high activity under sulfur deactivation conditions. (C) 1998 Academic Press.