Search for surface brönsted acidity over sulphided Ni-W/Al2O3 hydrotreating catalysts

Abstract A series of catalysts containing tungsten and nickel on a γ-alumina carrier were examined with respect to surface acidity. The studies focused on the behaviour of sulphided samples to which various techniques were applied. In agreement with prior studies on Co-Mo catalysts, infrared spectroscopy of adsorbed pyridine showed that both Bronsted and Lewis acidic sites are present on the calcined Ni-W samples, but Bronsted acidity can no longer be detected after sulphiding. Other probes (ammonia or piperidine) substantiated these conclusions. Moreover, gravimetric adsorption-desorption studies on ammonia confirmed that Bronsted acidity is not a major characteristic of the sulphide catalysts. The lack of strong Bronsted acidity was also demonstrated by measuring the rates of cyclohexanol dehydration or cyclohexylamine coupling carried out on the series of sulphides Ni-W catalysts. Therefore, it was proved that acidic mechanisms do not play any major role in catalysis by sulphides. In contrast, the sulphide catalysts show a better aptitude than typical cracking catalysts for isomerizing and cracking 3-methylpentane under hydrodesulphurization conditions but the product selectivity is different. The occurrence of a metallic-type catalysis is suggested, which could also operate in hydrodenitrogenation.