Julia HoetsSynthesis of Mesoporous MaterialsMesoporous materials are those with pores in the range 20-500 in diameter.1 www. mesoporous. com, International Mesostructured Material Association.(From Yahoo search engine.) They have huge surface areas, providing a vast number of sites where sorption processes can occur. These materials have numerous applications in catalysis, separation and many other fields.2 Davies, S., Goltner, C., Mesoporous Silica from Supramolecular Templating, University of Bristol, Inorganic Practical level II, experiment 8.(From relevant practical handout.) The synthesis of these materials is therefore of considerable interest and is constantly being developed to introduce different properties.Originally porosity was introduced to layered systems by pillaring. This process involves the intercalation of a guest species to create a porous network.3 Schllhorn, R., Intercalation Systems as Nanostructured Functional Materials, Chem.Mater.,1996, 8, 1747-1757.(From list of references in reference number 13.) However, the interlayer voids would collapse on removal of the guest species, so pillars were introduced to the framework to maintain the lateral spacing. Pore size could thus be controlled by the size of the pillar. The spacing in materials synthesised in this fashion was however irregular and the size distribution broad.In 1992 researchers at Mobil demonstrated the synthesis of hexagonally packed mesoporous silicate and aluminosilicate materials with uniform pore sizes, evenly distributed throughout the material.4 Kresge, C.T. et al. Ordered Mesoporous Molecular Sieves Synthesised by a Liquid Crystal Template Mechanism, Nature, 1992, 359, 710-712. (Starting reference given.) 5 Beck, J.S. et al. A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates, J.Am.Chem.Soc., 1992, 114, 10834-10843.(Starting reference given.)These materials were termed MCM-41. Their synthesis was achieved by the use of surfactant micelles as structure directing agents in a sol-gel process. Amphiphillic surfactants self assemble into cylindrical micelles, which are encapsulated by an inorganic material (silicate or aluminosilicate in this case), which balances the charge on the micellular surfaces. Calcination, a thermal processing technique is then used to remove the organic surfactant, leaving a hexagonal arrangement of mesopores.This process employs a liquid crystal templating mechanism, and is used to synthesise pores up to 100 in diameter with surface areas of up to 700m/g.Sol-gel technology, first used in 1846 by J.Ebelman6 Lev, O., Sol-gel Materials in Electrochemistry, Chem.Mater., 1997, 9, 2354-3375.(From list of references in reference 15.) , is now commonly used in the synthesis of ceramics. It involves the formation of a solid phase, through the gelation of a colloidal suspension (sol). There are two possible routes for this process7 Pringle, P.G., University of Bristol Level II lecture notes. (From relevant lecture material.):i)Inorganic route ; this hydrothermal technique involves the formation of a sol from a very fine powder of the silicate starting material. This is then heated and dehydrolysed to form the ceramic via a gel state.ii)Organic route ; here the starting material has an organic component, for example TEOS (tetraethylorthosilicate) which is hydrolysed to form a gel, thus losing ethanol, and then heated to form the ceramic. This is a more expensive route but forms pure products of optical and electronic quality. The liquid crystal templating mechanism used by Mobil in 1992 involved the dissolution of a surfactant species in the pre-hydrolysed inorganic pre-cursor8 Ogawa, M., Simple Sol-gel Route for Preparation of Silica-Surfactant Mesoporous Materials, Chem.Commun., 1996, 1149-1150.(From computer citation search using Mesoporous Materials as a subject.). The mechanism is strongly influences by electrostaic and steric interactions between the solvent molecules, the inorganic species and the self assembled organic surfactants9 Vaudry, F., et al. Synthesis of Pure Alumina Mesoporous Materials, Chem.Mater., 1996, 8, 1451-1464. (From list of references in reference 15.) .A wide variety of surfactants, with different properties; anionic, lipid, zwitterionic even two-tailed species, are used to introduce the mesopores10 Huo, Q., Organisation of Organic Molecules with Inorganic Molecular Species into Nanocomposite Biphase Arrays, Chem.Mater., 1994, 6, 1176 1184. (From list of references in reference 14.).Ionic surfactants self assemble into micelles, governed by the matching of charge densities at surfactant inorganic interfaces11 Monnier, A. et al. Cooperative Formation of Inorganic-Organic Interfaces in the Synthesis of Silicate Mesostructures. Science, 1993, 261, 1299 - 1303.(From list of references in ref