(674g) Quantification of External and Pore Mouth Acid Sites in Unit-Cell Thick Pillared MFI and Pillared MWW Zeolites
The quantification of acid sites on the external surface and in pore mouth of pillared MFI (PMFI) and pillared MWW (PMWW or MCM-36) zeolites was studied. A combined techniques of organic base titration, atomic layer deposition (ALD) of silicon (ALD-Si) and alkylation of benzyl alcohol in mesitylene were carried out to identify the acid site locations, their quantities and catalytic behaviors. The organic base titrations with triphenylphosphine (TPP, ? nm, only access to external acid sites) and 2,6-di-tert-butylpyridine (DTBP, ? nm, access to both external and pore mouth acid sites), respectively, in the course of methanol dehydration reactions indicated that PMFI contains 10% external and 26% pore mouth active sites, while PMWW consists of 22% and 45%, respectively. The ALD-Si treatment of PMFI and PMWW zeolites led to loss of external surface acid sites, which is consistent with that determined from TPP titrations. The catalysis tests in the absence and presence of TPP and DTBP, respectively, comparatively indicated the catalytic behaviors of external and pore mouth acid sites in PMFI and PMWW zeolites. The alkylation and etherification reaction rates as a function of cumulative DTBP addition suggested Brønsted acid sites with different strengths on the external surface or in pore mouth environments of the pillared zeolites, which influenced the external etherification reaction, but not as significantly as the alkylation reaction. The PMWW and PMFI zeolites are 2-dimensional (2D) catalytically active materials with single- or near single-unit-cell thick 2D microporous layers which expose higher number of external surface and pore mouth acid sites than their microporous counterparts. The present study exemplified for the first time to differentiate and quantify surface acid sites (external and pore mouth) in the 2D unit-cell thick zeolites.