Silicate structural design

In 1992 scientists of the Mobil Oil Corporation published the surfactant controlled synthesis of mesoporous silicate material, designated MCM-41 [1]. Previous studies concerning the preparation and characterization [2-4] and the mechanism of MCM-41 formation [5] have been published. Based on the synthesis mechanism the idea was further developed to design mesostructured transition metal oxides analogous to the MCM-41 structure. The mechanism is based on ionic interactions of the cationic surfactant headgroups with the anionic silicate species. In the synthesis of MCM-41 related materials the charged silicate species are substituted by metal oxides which are able to form polyanions. The metal oxide polyanions perform an analogous function to the silicate species in the synthesis of MCM-41. 

The silica carrier of a sulphuric acid catalyst, which has a relatively low surface area, serves as an inert support for the melt. It must be chemically resistant to the very corrosive pyrosulphate melt and the pore structure of the carrier should be designed for optimum melt distribution and minimum pore diffusion restriction. Diatomaceous earth or synthetic silica may be used as the silica raw material for carrier production. The diatomaceous earth, which is also referred to as diatomite or kieselguhr, is a siliceous, sedimentary rock consisting principally of the fossilised skeletal remains of the diatom, which is a unicellular aquatic plant related to the algae. The supports made from diatomaceous earth, which may be pretreated by calcination or flux-calcination, exhibit bimodal pore size distributions due to the microstructure of the skeletons, cf. Fig. 5. 

The stability of MCM-41 is of great interest because, from the practical point of view, it is important to evaluate its potential application as a catalyst or adsorbent. It is known that purely-siliceous MCM-41 (designated here as PSM) has a high thermal stability in air and in oxygen containing low concentration (2.3 kPa) of water vapor at 700 °C for 2 h [1], However, the uniform mesoporous structure of PSM was found to be collapsed in hot water and aqueous solution due to silicate hydrolysis [2], limiting its applications associated with aqueous solutions. After MCM-41 samples were steamed in 100% water vapor at 750°C for 5 h. their surface areas were found to be lower than amorphous silica-alumina and no mesoporous structure could be identified by XRD measurement [3]. In addition, PSM showed poor stability in basic solution [4]. 

---