Improvement of surface acidity and structural regularity of Zr-modified mesoporous MCM-41

Abstract

This work reports the synthesis and surface characterization of a Zr-modified mesoporous MCM-41 solid with an ordered hexagonal arrangement, prepared through a templated synthesis route, using cetyltrimethylammonium chloride as the template. The surface features, crystalline structure, textural properties and surface acidity of the materials were characterized by in situ Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), N 2 physisorption isotherms, 29Si MAS-NMR and in situ FT-IR of pyridine adsorption. It is evident that the surfactant cations inserted into the network of the solids during the preparation could be removed by calcination of the sample above 500 °C. The resultant material showed a large surface area of 680.6 m 2 g −1 with a uniform pore diameter distribution in a very narrow range centered at approximately 2.5 nm. Zirconium incorporation into the Si-MCM-41 framework, confirmed by 29Si MAS-NMR analysis, increased not only the wall thickness of the mesopores but also the long-range order of the periodically hexagonal structure. Both, Lewis and Brönsted acid sites, were formed on the surface of the Zr-modified MCM-41 solid. Compared to Si-MCM-41 on which only very weak Lewis acid sites were formed, the densities of both Lewis and Brönsted acid sites and the strength of the acidity on the Zr-modified sample were significantly increased, indicating that the incorporation of zirconium greatly enhances the acidity of the material.