Tuning external surface of unit-cell thick pillared MFI and MWW zeolites by atomic layer deposition and its consequences on acid-catalyzed reactions

Abstract

The pillared MWW (PMWW or MCM-36) and pillared MFI (PMFI) zeolites are 2dimensional (2D) catalytically active materials made by pillaring of layered MCM-22(P) and multilamellar MFI precursors, respectively. The single-or near single-unit-cell thick 2D microporous layers in PMWW and PMFI expose comparable number of external surface acid sites (i.e., acid sites enclosed in mesopores between zeolitic layers) to those in micropores, which become important or dominant contributor to the catalytic properties. Although the acidity and catalytic activities of PMWW and PMFI have been studied, modification of their external surfaces and its implications on catalytic reactions are not available. In the present study, we report the tailoring of external surfaces of PMWW and PMFI zeolites by atomic layer deposition (ALD) of silicon (ALD-Si) and aluminum (ALD-Al), respectively. The textural, acidic and catalytic properties of the ALD modified pillared zeolites were investigated using a variety of characterization methods. ALD-Al and ALD-Si modifications kept micropore almost intact, but resulted in significant reduction in mesopore volume and considerable changes in external surface composition and acidity. The catalytic tests showed that intrinsic catalytic behavior of Brønsted acid sites in ALD modified pillared zeolites was similar to their parent counterparts. In diffusion constrained parallel reactions, ALD of Al-or Si-species altered catalyst selectivity. In addition, ALD of Si-species on PMFI zeolite deactivated surface active sites, which resulted in improved catalytic activity in direct methane aromatization reactions under optimal ALD cycles. The study exemplified for the first time that ALD is an effective tool for tuning the surface properties of 2D unit-cell thick zeolites.

Mechanistic Understanding of Catalytic Conversion of Ethanol to 1-Butene over 2D-Pillared MFI Zeolite

“…The most preferable BAS ratio of internal to external pore was calculated to be 2:1, as determined from the DFT energetic preference for different sets of internal and external BAS (the details of the searching procedure are presented in Figure S2 of SI). Such a BAS ratio of internal to external pore on the 2D-PMFI model matches closely with the reported experimental findings, where the experimentally measured fraction of external sites on the PMFI catalyst is determined as ∼32% by dimethyl ether titration and methanol dehydration. , …”

Analysis and control of acid sites in zeolites

“…The set of experiments showed that the intrinsic catalytic behavior of Brønsted acid sites in PMWW and PMFI was not altered significantly by the ALD treatment. However, a certain selectivity of external acid sites in the alkylation reaction of benzyl alcohol with mesitylene was established [287].…”

Two-Dimensional Zeolite Materials: Structural and Acidity Properties

“…For example, the FTIR spectra of adsorbed pyridine, collidine, and DTBP molecules have shown that the 2D unilamellar and multilamellar MFI zeolites have a higher fraction of Lewis acidity than that of 3D MFI, while the accessibility of acid sites is improved due to the open structure [247][248][249][250]. The acid site accessibility of pillared MFI and SPP zeolites has been measured by direct DTBP uptake measurements, which maintain the characteristics of the high acid site accessibility in 2D MFI zeolites [173,245,251,252]. In particular, the concentrations of external surface and pore mouth Brønsted acid sites in the pillared MFI zeolite was quantified by a combined dimethyl ether (DME) titration and methanol dehydration in the presence of DTBP or triphenylphosphine (TPP) titrants, respectively [232].…”

“…DTBP titration, in combination with DME titration or the ethanol dehydration reaction, was used to evaluate the accessibility of acid sites in the MCM-36 prepared under mild conditions [173,245,251,252]. The ethanol dehydration rates as a function of cumulative DTBP or pyridine titrant addition on the MCM-22 and MCM-36 zeolites, respectively, were measured.…”