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

• Published in: Journal of catalysis Vol. 337; pp. 177 - 187
• Main Authors: Wu, Yiqing, Lu, Zheng, Emdadi, Laleh, Oh, Su Cheun, Wang, Jing, Lei, Yu, Chen, Huiyong, Tran, Dat T., Lee, Ivan C., Liu, Dongxia
• Format: Journal Article
• Language: English
• Published: San Diego Elsevier Inc 01.05.2016; Elsevier BV
• Subjects: Acidity; Acids; Atomic layer deposition (ALD); Brønsted acid sites; Catalysts; Hierarchical zeolite; Methane aromatization; Unit-cell thick zeolite; Brønsted acid sites; Hierarchical zeolite; Acidity; Unit-cell thick zeolite; Methane aromatization; Atomic layer deposition (ALD)
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2016.01.031

[Display omitted] •Atomic layer deposition (ALD) was used to modify surface properties of pillared zeolites.•ALD-Al deposited Al-species on exterior and mesopores of 2D PMFI and PMWW zeolites.•ALD-Si deposited Si-species preferentially on exterior of 2D PMFI and PMWW zeolites.•Composition and acidity of 2D zeolites were tuned spatially by ALD of Si- or Al-species.•Catalytic performance of ALD-modified 2D pillared zeolites was tested. The pillared MWW (PMWW or MCM-36) and pillared MFI (PMFI) zeolites are 2-dimensional (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.