New insights in evaluation of acid sites in micro-mesoporous zeolite-like materials using potentiometric titration method

Potentiometric titration was used to characterize the acid-base properties of micro-mesoporous zeolite-like materials obtained via dual template synthesis using sol-precursors of Beta zeolite and micellar templates. The obtained results were compared with other methods of acidity determination (ammonia TPD, pyridine ad(de)sorption). The acid sites with the strength comparable to Beta zeolite (lg K values 4.3–4.7) were found in low-crystalline and X-ray amorphous micro-mesoporous materials. The strongest acid sites were observed in conventional Beta zeolite and a low-crystalline (degree of crystallinity ca. 0.1) micro-mesoporous material prepared on its basis (lg K 4.3) in concentration of 450 and 100 μmol/g, respectively. A change from acidic to alkaline reaction media in the second stage of micro-mesoporous materials preparation led to an increase of the acid sites concentration from 190 to 600 μmol/g concomitant with formation of a high fraction of comparatively strong acid sites (lg K 4.7, 200 μmol/g). A high content of acid sites in low-crystalline and X-ray amorphous samples was assigned to acidity of zeolite precursors rather than zeolite crystals.