Catalytic activity of dealuminated Y and HZSM-5 zeolites measured by the temperature-programmed desorption of small amounts of preadsorbed methanol and by the low-pressure flow reaction of methanol

Abstract

The activity of HZSM-5, NH₄Y and Y zeolites, dealuminated using SiCl₄, in methanol transformation has been compared and correlated with the acid properties and the Si:Al ratio of the respective zeolites. The temperature-programmed desorption of small amounts of methanol preadsorbed at ambient temperature together with measurements in a flow reactor under low pressure allowed the primary reaction steps to be observed. HZSM-5 and Y zeolites which had been moderately dealuminated showed the highest activity; however, the product distribution was different above 630 K:HZSM-5 gave predominantly methane and formaldehyde (aromatics appearing at higher temperatures), while C_3–5 olefins and aromatics were formed above the Y zeolite. HY and highly dealuminated Y zeolite with Si/Al 20 were less active than moderately dealuminated Y and HZSM-5 zeolites. The high activity of the latter zeolites is caused by the action of strong proton-donor sites and electron-acceptor centres (extralattice aluminium in dealuminated zeolites). The difference between HZSM-5 and dealuminated Y zeolites is caused by their different structures, which lead to different reaction mechanisms. For comparison, the temperature-programmed desorption of preadsorbed ethanol was also studied. This resulted predominantly in ethene formation via intramolecular dehydration of ethanol.