From Colloidal Dispersions of Zeolite Monolayers to Effective Solid Catalysts in Transformations of Bulky Organic Molecules: Role of Freeze-Drying and Dialysis.

Katarzyna Kałahurska,Barbara Gil,Wieslaw J Roth,Pawel P Ziemiański

doi:10.3390/molecules26072076

Abstract

We investigated the properties and catalytic activity of zeolites with MWW topology obtained by unprecedented liquid exfoliation of the MCM-56 zeolite into solutions of monolayers and isolation/reassembly of the dispersed layers by various methods, with optional purification by dialysis or ammonium exchange. The layers were recovered by flocculation with alcohol or ammonium nitrate and freeze-drying. Flocculation alone, even with ammonium nitrate, did not ensure removal of residual sodium cations resulting in catalysts with low activity. Dialysis of the solutions with dispersed monolayers proved to be efficient in removing sodium cations and preserving microporosity. The monolayers were also isolated as solids by freeze-drying. The highest BET area and pore volume obtained with the freeze-dried sample confirmed lyophilization efficiency in preserving layer structure. The applied test reaction, Friedel–Crafts alkylation of mesitylene, showed high benzyl alcohol conversion due to increased concentration of accessible acid centers caused by the presence of secondary mesoporosity. The applied treatments did not change the acid strength of the external acid sites, which are the most important ones for converting bulky organic molecules. Zeolite acidity was not degraded in the course of exfoliation into monolayers, showing the potential of such colloid dispersions for the formation of active catalysts.

Highlights

Catalytic reactions take place at the active centers located at the available surfaces of a solid material
Colloidal dispersions of MCM-56 zeolite monolayers were produced by the treatment with 10–11.5% solutions of tetrabutylammonium hydroxide (TBAOH), which resulted in expansion, i.e., opening of interlayer galleries across original multilayered particles and producing total delamination/exfoliation of monolayers in solution
The region 8–10◦ 2θ with the broad band characterizing MCM-56 is preserved in some samples, mostly without the appearance of a dip shown by the solid MCM-56 material (MCM-56 exch, Figure 1a)

Summary

Introduction

Catalytic reactions take place at the active centers located at the available surfaces of a solid material Their accessibility depends on the size of the reactant molecule on one hand, and on the number, nature and distribution of the active centers in a given catalyst, on the other. The intrinsic acid centers of such tens-tohundreds-of-nanometers-wide- and only a few-nanometer-thin-crystals are all located near the external surface and are well accessible to bulky reactants. Even more important, such natural “flattening” of crystals does not seem to penalize either the acid sites strength or the microporous structure; the intrinsic high activity and shape-selectivity may be retained [12,13]

Methods

Results

Conclusion