Creation of acidic sites by hydrogen spillover in model hydrocracking systems

Abstract

The results concern mechanical mixtures of sulfided CoMo (14% MoO 3 , 3% CoO) deposited on a non-acidic support, i.e. SiO 2 , with silica-aluminas (6.5, 12 or 60 wt.% Al 2 O 3 ). A strong synergy between the two components is observed: (i) in the extremely selective cracking of diphenyl methane (DPM) to benzene and toluene under hydrogen pressure (5 MPa; 673 K; feed (wt.%): 29.5 DPM, 70 n-dodecane and 0.5 benzothiophene added for keeping the catalyst sulfided); (ii) FTIR band at 1540 cm −1 of pyridine relatively strongly adsorbed on Brönsted acid sites; and (iii) extend of D-H exchange as detected by FTIR OD bands. No hydrogenation of the benzenic rings is observed. Spillover hydrogen increases the acidity, the rate of cracking of DPM and the rate of D-H exchange. A new mechanism for explaining the cracking of DPM in the presence of hydrogen is proposed. In addition to creating Brönsted sites, spillover hydrogen would act as a dual hydrogenizing agent, with spillover H + creating first a carbonium ion, which would decompose to carbenium, this carbenium reacting with a spillover hydride ion H − to form toluene.