Porosity and accessibility of acid sites in desilicated ZSM-5 zeolites studied using adsorption of probe molecules

Porosity of zeolites ZSM-5 (Si/Al = 32 or 164) desilicated with NaOH or NaOH/TBAOH (tetrabutylammonium hydroxide) solutions was studied by means of quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) of selected hydrocarbons. QE-TPDA of n-nonane confirmed substantial formation of mesopores upon desilication, observed also in N2 adsorption/desorption isotherms. Very narrow mesopore size distributions with maxima at about 4 nm, typical for partially constricted mesopore networks, were obtained for all desilicated zeolites. It was also observed that desilication with NaOH/TBAOH results in formation of additional slightly larger mesopores (ca. 4–6 nm) while even larger mesopores (6–10 nm) were found after leaching with NaOH only. Thermodesorption of n-hexane revealed that the micropores in the desilicated zeolites remained unchanged. Acidity of the zeolites was characterized by quantitative IR measurements of pyridine and collidine (2,4,6-trimethylpyridine) adsorbed on acid sites. Increased numbers of Brønsted acid sites accessible for large collidine molecules found for the modified zeolites indicate that during desilication redistribution of the framework Al atoms to the surface of mesopores occurred. The influence of TBAOH presence in desilicating mixture on porosity and accessibility of the acid sites was also discussed.