Synthesis, characterisation and catalytic properties of selected mesoporous solids

Abstract

The syntheses of mesoporous zeolitic materials of the type MCM-48 and AlPO with incorporation of heteroatoms in the framework have been carried out using CTAB as structure-directing agent under hydrothermal conditions. All the materials synthesized in the present study were found to have mesoporous texture with an average pore size of 28 Å. The acidity of Al or Fe incorporated system was found to be less compared to the conventional zeolites. However, these systems can be suitably employed for reactions where moderate acidity is the driving force for the formation of products. For example, alkylation of naphthalene with alcohols over mesoporous MCM-48 results in monoalkylated naphthalenes. In a similar manner, transition metal incorporated MCM-48s have been examined for phenol oxidation and the results indicate that Ti-incorporated system shows higher activity and selectivity for diphenol formation. A relatively easier approach to synthesize mesoporous aluminophosphates has been achieved and transition elements like V, Cr and Fe have been incorporated into the framework. Various physico-chemical studies confirmed the formation of mesophase as well as the presence of heteroatom within the framework of mesoporous aluminophosphates. Even though the thermal stability of aluminophosphates is less when compared to that of aluminosilicate analogous materials, aluminophosphates exhibit higher activity for the selective side chain oxidation of aromatics like toluene, ethylbenzene and benzyl alcohol. It has been established that less expensive and readily available oxidants like molecular oxygen/air can also be employed as an oxidizing agent for the oxidation of toluene and cyclohexane. The observed activity of these systems can be attributed to the site isolation of the desecrate redox centers. Mesoporous solids have been successfully supported on unconventional supports like stainless steel, which probably eliminate the mass transfer limitations experienced in actual industrial processes. The successful synthesis of thermally stable titania has also been carried out using CTAB as template.