The application of cobalt containing acidic zeolites as catalysts for hydrodesulfurization reactions

Cobalt containing HY and HZSM5 zeolites were prepared by impregnation and solid state ion exchange methods. The type and concentration of acid sites of non sulfided and presulfided faujasites, HZSM5 and one CoHZSM5 zeolite prepared by impregnation were studied by adsorption and temperature programmed desorption (TPD) of pyridine followed by infrared spectroscopy. The concentration of Brönsted acid sites of unsulfided cobalt containing zeolites was low compared to the starting materials, indicating reaction between the OH groups of the support and the cobalt species. Three types of Lewis acid sites were observed on the CoHY samples. They were assigned to weakly acidic Lewis sites (1440 cm^-1), extralattice Al (1448 cm^-1) and Co (1448 cm^-1) species. After sulfidation the Brönsted acidity of cobalt containing zeolites highly increased and the number of Lewis acidic cobalt species strongly decreased, which means that cobalt is sulfided at high extent. The faujasite samples are more active in the thiophene hydrodesulfurization (HDS) reaction compared to CoHZSM5 because of their higher Co dispersion. The overall catalytic activity can be divided into C-S bond hydrogenolysis activities without C-C bond breaking, cracking and to the reaction to coke formation. Coking and cracking are connected with the acidity, cracking is a consecutive reaction of the hydrogenolysis products. The C-S bond hydrogenolysis activity can be attributed to sulfided cobalt species.