Roles of zeolite support and noble metal in Sulfur-tolerant catalyst for low temperature hydrogenation of aromatics

This dissertation focuses on fundamental understanding of the roles of supports and noble metals in the sulfur-tolerant catalysts for low-temperature hydrogenation of aromatics in the presence of sulfur. Emphasis was placed on investigating the effect of supports and supported metals for high sulfur tolerance and verifying a catalyst design concept for the sulfur-tolerant noble metal catalyst based on shape selective exclusion of sulfur and hydrogen spillover for lowtemperature hydrogenation of aromatics [C.S. Song, Chemtech, 29 (1999) 26-30].;The effect of supports was explored for sulfur tolerance of the supported Pd catalyst on tetralin hydrogenation in the presence of benzothiophene by varying the Pd catalysts on different supports (mordenite, zeolite Y, zeolite A, amorphous silica-alumina). The pore structures and pore sizes were both found to be the important factors in determining the catalytic activity of sulfur-tolerant metal catalyst. Among the catalysts tested, the acidic zeolite Y-supported Pd catalyst showed excellent sulfur tolerance while maintaining high catalytic activity, which may be mainly due to its high surface area, acidity and three-dimensional zeolite framework.;The hybrid zeolite-supported Pd catalyst was prepared to improve sulfur tolerance, based on the proposed catalyst design concept. The hybrid catalyst consists of Pd supported on Y and A type zeolites. For further investigation on small pore system in hybrid catalyst, surface metal passivation by silica coating and pore size control by potassium ion exchange were employed to zeolite A-supported Pd catalyst. Although Pd on Zeolite A showed no catalytic activity for hydrogenation of tetralin, adding the small-pore catalyst to Pd/Y significantly enhanced sulfur tolerance of the catalyst for both naphthalene and tetralin hydrogenation in the presence of sulfur in the form of benzothiophene. Sulfur tolerance of the hybrid catalyst is mainly attributed to the small pore system, inducing size-selective exclusion of bulky sulfur compounds as well as hydrogen spillover from metal inside small pore component. Hydrogen spillover plays two roles in maintaining high sulfur tolerance of the hybrid catalyst: first, regeneration of sulfur-poisoned metal active sites in the large pores of Pd/Y as well as hydrodesulfurization of aromatic sulfur compounds over the zeolite Y support.;The effect of metal species for the zeolite Y-supported metal catalyst was investigated using various group 10 metals (Ni, Pd, Pt) as well as Pd-Pt bimetal. The metal type also has major impact on sulfur tolerance of the zeolite supported catalyst for tetralin hydrogenation. Even Pd shows better sulfur-tolerance than other monometallic catalysts, Pd-Pt bimetallic catalyst exhibited exceptionally high sulfur-tolerance, maintaining 100% conversion for tetralin hydrogenation in the presence of sulfur, which can be ascribed to high metal dispersion and electron-deficiency of Pd-Pt. Addition of small pore catalyst (silica-coated Pd/KA catalyst) to Pd- Pt/Y also enhances sulfur tolerance of the Pd-Pt catalyst on tetralin hydrogenation even in the presence of 300ppmw sulfur as benzothiophene.;On the basis of the above results and discussion focusing on the importance of the supports and metal types for improving sulfur tolerance, the hybrid catalyst system based on the new design concept of sulfur tolerant catalyst is applicable in the development of sulfur tolerant catalysts for low-temperature hydrogenation of aromatics.