Abstract
MWW type zeolites are characterized by the presence of zeolitic layers of 2.5 nm thickness, containing 10-member ring sinusoidal channels inside and supercavities with 12-member ring openings located on their surfaces. Expansion and pillaring of layered zeolites increase the access to active sites and can enable or facilitate catalytic activity towards larger reactant molecules. This goal is explored in this work reporting the pillaring of layered zeolite MCM-56 with MWW topology by tetraethylorthosilicate (TEOS) treatment with the assistance of isopropanol, aimed at obtaining hierarchical micro-mesoporous systems. MCM-56 (Si/Al = 12) was synthesized with hexamethyleneimine as a structure-directing and aniline as a structure-promoting agent. Hierarchical porous systems were obtained using two different pillaring methods: (1) with TEOS only and (2) with TEOS mixed with isopropanol. The MWW framework was preserved during swelling/pillaring in both methods. Pillared zeolites obtained via alcohol-assisted pillaring possessed unique intermediate micro-mesopores with the size of about 2 nm. IR study revealed a decrease in the concentration of accessible acid centers upon pillaring. However, the fraction of acid sites on the external surface, accessible for adsorption of large molecules, increased by up to 90%. Catalytic activity was evaluated in the Friedel-Crafts alkylation of mesitylene with benzyl alcohol. Pillaring resulted in reduction of the acid site concentrations, but the materials retained high catalytic activity. Pillaring in the presence of alcohol produced increased turnover frequency values based on the concentrations of the external acid sites.
Highlights
The layered material MCM-56 is a unique representative of two-dimensional (2D)zeolites as a delaminated form obtained by direct synthesis with high activity and ease of modification [1,2,3,4,5]
Xing et al [7] reported that addition of aniline to the MCM-56 synthesis delays the conversion to MCM-49 so it can be viewed as the final product
The analysis of the X-ray powder diffraction (XRD) patterns has shown that the basic MCM-56 (MWW) structure of all studied materials has been preserved during applied modifications
Summary
Zeolites as a delaminated form obtained by direct synthesis with high activity and ease of modification [1,2,3,4,5] It is the first layered zeolite exfoliated directly by soft-chemical treatment. Jiang et al [8] proposed an easy double templating method to control the crystallization of MCM56, involving addition of tetraethyl ammonium hydroxide (TEAOH) to the HMI-containing synthesis mixture. This synthetic route yielded pure, delaminated MCM-56 material, with an extended phase stabilization period. To the best of our knowledge, there seems to be no reports concerning modification of the MCM-56 obtained with the additives mentioned above [9]
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