Are mesoporous silicas and aluminosilicas assembled from zeolite seeds inherently hydrothermally stable? Comparative evaluation of MCM-48 materials assembled from zeolite seeds

Abstract

Well ordered cubic (MCM-48) mesoporous silica and aluminosilica materials were assembled from zeolite seeds (ZSM-5, Beta or silicalite-1) via a two-step crystallization process and their hydrothermal stability in steam and boiling water was investigated using a variety of techniques including powder XRD, nitrogen sorption studies, elemental analysis and monitoring of acid content. The presence of zeolite seeds within the framework of the resulting mesoporous materials was inferred from infrared spectroscopy and thermogravimetric analysis (TGA) studies. The proton exchanged forms of MCM-48 materials assembled from zeolite seeds (ZSM-5 or Beta) were found to exhibit hydrothermal stability which is much higher than that of conventional (direct mixed-gel synthesized) Al-MCM-48 but comparable or lower than that of equivalent Al-grafted MCM-48 materials. A purely siliceous MCM-48 material assembled from silicalite-1 zeolite seeds was found to exhibit much lower hydrothermal stability. The hydrothermal stability of Al-containing materials was in all cases (regardless of whether they contained zeolite seeds) higher than that of pure silica MCM-48 assembled from silicalite-1 zeolite seeds. This suggests that mesoporous materials assembled from zeolite seeds are not inherently hydrothermally stable. However, Al-MCM-48 materials assembled from zeolite seeds exhibited the highest acidity.