Consideration of Acid Strength of a Single OH Group on Zeolites by Isotope Exchange Reaction with Ethane at High Temperatures

Infrared probe (IR) methods to estimate the acid strength of OH groups on H-form zeolites were compared on mordenites (HM90, Si/Al = 45 and HM20, Si/Al = 10), ZSM-5 (Z5-90H, Si/Al = 45) and Y (HY5.6, Si/Al = 2.8). Heat of adsorption as well as the internal frequency of adsorbed CO was not sensitive enough to evaluate the acid strength of different zeolites. Thus, H/D isotope exchange reaction of acidic OD groups with a small hydrocarbon, C₂H₆, was employed. For the estimation of intrinsic activation energy, heat of adsorption of C₂H₆ was first measured. However, C₂H₆ molecules could not enter the 8 membered ring (MR) of HM20, and the secondary interaction of C₂H₆ with pore walls of Z5-90H increased the heat of adsorption, both due to the observation at low temperatures. This prohibited the comparison of heat of adsorption on a single Brønsted acid site in different spaces of zeolite with different topologies. Considering that the structural restrictions of such small molecules as C₂H₆ become negligible at high temperatures as a result of the vibrational excitation of pore mouth-breathing modes of zeolites, and that the topology difference of a single Brønsted acid site in heat of adsorption is relatively small, we proposed that the apparent activation energy for H/D isotope exchange reaction would reflect the acid strength of single sites. The present results are compared and discussed with conclusions led by other methods.