Effect of surfactant type, substitution by aluminium and additives on direct liquid crystal templated monolithic silica

Abstract

A range of monolithic silicas and aluminosilicates with disordered crystalline-like pore structures and high surface areas have been synthesised based on a direct crystal templating approach to surfactant-assisted sol–gel synthesis of porous materials. Commercially available oxyethylated non-ionic surfactants, including sorbitan-based molecules and those possessing a straight alkyl or alkylphenol tail, were used as structure-directing agents in high-concentration surfactant solutions (about 50 wt%). Incorporation of aluminium into monolithic silica increases pore wall thickness. Thermoprogrammed desorption of ammonia in the temperature range from 373 K to 923 K was used for the evaluation of the number and the strength distribution of surface acid sites. The mean strength of acid sites reaches a maximum value for the material prepared with a Si∶Al ratio of 20, as evidenced by flow-microcalorimetry measurements of ammonia chemisorption at 373 K. The calcined silica and aluminosilicate materials were characterised by nitrogen gas adsorption at 77 K. The mean pore diameter ranges from 2 nm to 4 nm. The addition of lithium nitrate and 1,3,5-trimethylbenzene to the reaction mixture was investigated to prepare materials with mesopores up to 6 nm. Mean pore diameters increase with increasing length of the hydrophobic tail of the template, decreasing number of oxyethylene units, increasing amount of lithium nitrate or hydrophobic solubilisate added to the reaction mixture, and decreasing synthesis temperature. The pore size appears to be sensitive to the orientation of the polyoxyethylene chain within the aqueous domains of the lyotropic mesophases as a result of its interaction with the growing inorganic precursor units.