An explanation for the enhanced activity for light alkane conversion in mildly steam dealuminated mordenite: The dominant role of adsorption

This paper presents a catalytic, spectroscopic, calorimetric study of mildl y steam-dealuminated mordenite (H-MOR). With increasing steam partial press ures at 673 K there is increasing dealumination, the loss of Bronsted acid sites and the presence of extraframework Al (Al-EF) likely in the zeolite p ores. Additionally, the number of Lewis sites increases with increasing Al- EF. As observed in previous studies, the catalytic activity for hydroisomer ization of n-hexane and monomolecular cracking of n-butane and n-hexane ini tially increases and goes through a maximum with increasing steam severity. While the maximum increase in the rate of hexane cracking is about 5 times , the activation energies indicate that steaming does not significantly cha nge the strength of the Bronsted acid sites. In the absence of Al-EF, the e nthalpy of alkane adsorption is constant at all surface coverages. However, in the presence Of Al-EF, the initial heat of alkane sorption increases by about 5-10 kJ/mol. After covering these adsorption sites, the heat of adso rption is identical to that on the unsteamed H-MOR until saturation coverag e has been reached. The increased enthalpies of adsorption are suggested to occur on the Al-EF, Lewis acid sites. Since the surface coverage of paraff ins under typical reaction conditions is low, it is proposed that the enhan ced rate is due to the increased initial heat of adsorption leading to a hi gher concentration of reactants in the steamed H-MOR zeolites. (C) 2001 Aca demic Press.