An Explanation for the Enhanced Activity for Light Alkane Conversion in Mildly Steamed Dealuminated Mordenite : The Dominant Role of Adsorption

Abstract

This paper presents a catalytic, spectroscopic, calorimetric study of mildly steam-dealuminated mordenite (H-MOR). With increasing steam partial pressures at 673 K there is increasing dealumination, the loss of Bronsted acid sites and the presence of extra-framework Al (Al{E}{F}) likely in the zeolite pores. Additionally, the number of Lewis sites increases with increasing Al{E}{F}. As observed in previous studies, the catalytic activity for hydroisomerization of n-hexane and monomolecular cracking of n-butane and n-hexane initially 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 change the strength of the Bronsted acid sites. In the absence of Al{E}{F}, the enthalpy of alkane adsorption is constant at all surface coverages. However, in the presence of Al{E}{F}, the initial heat of alkane sorption increases by about 5-10 kJ/mol. After covering these adsorption sites, the heat of adsorption is identical to that on the unsteamed H-MOR until saturation coverage has been reached. The increased enthalpies of adsorption are suggested to occur on the Al{E}{F}, Lewis acid sites. Since the surface coverage of paraffins under typical reaction conditions is low, it is proposed that the enhanced rate is due to the increased initial heat of adsorption leading to a higher concentration of reactants in the steamed H-MOR zeolites.