N-HEPTANE HYDROCRACKING OVER A SULFIDED NIMO Y ZEOLITE CATALYST - REVERSIBLE DEACTIVATION OF THE ZEOLITE BY SULFUR-CONTAINING MOLECULES/

Hydrocracking of n-heptane was carried out over a sulfided NiMo/Y zeol ite catalyst in the presence of ammonia and hydrogen sulfide. An incre ase in the hydrogen sulfide pressure decreased the activity of the cat alyst and increased the isomerization/cracking ratio, which indicated a poisoning of the acid sites. This poisoning was accompanied by an in crease in the formation of coke on the catalyst when the hydrogen sulf ide pressure increased. The coke was composed mainly of sulfur-contain ing molecules, namely dimethyl polysulfides (tri-, tetra- and pentasul fides), and also pure sulfur: S8. The deactivation of the catalyst was due to a blockage of the porosity of the zeolite by the sulfur polyme rs because of the low volatility of these compounds. The higher the hy drogen sulfide pressure the greater the amount of sulfur polymers, thi s phenomenon being totally reversible.