Quantification of external surface and pore mouth acid sites in unit-cell thick pillared MFI and pillared MWW zeolites

Abstract

The pillared MWW (PMWW or MCM-36) and pillared MFI (PMFI) zeolites consist of a single- or near single-unit-cell thick two-dimensional (2D) zeolitic layer structure. The concentrations of external surface and pore mouth Brønsted acid sites in these zeolites were quantified by a combined dimethyl ether (DME) titration and methanol dehydration in the presence of 2,6-di-tert-butylpyridine (DTBP) or triphenylphosphine (TPP), respectively. DME can access to all Brønsted acid sites in PMFI and PMWW zeolites and thus determine the total number of acid sites. DTBP cannot access to acid sites in micropores, but on the external surface and at the pore mouth regions. In contrast, TPP can only access to acid sites on external surface. The degree of decrease in methanol dehydration rate in the presence of DTBP or TPP indicates the fraction of the sum of external surface and pore mouth acid sites and the sole fraction of external surface acid sites, respectively. The study shows that PMFI contains ∼32% external surface and ∼6% pore mouth acid sites, while PMWW consists of ∼33% external surface and ∼31% pore mouth acid sites. The individual external surface or pore mouth acid site performed distinctively in alkylation of mesitylene by benzyl alcohol reactions.