Synthesis and characterisation of the mesoporous silicate SBA-2 and its performance as an acid catalyst

The synthesis of the three-dimensional hexagonal mesoporous silica SBA-2 has been studied as a function of pH. temperature and type of gemini surfactant template. SBA-2 is formed from reaction mixtures with a pH between 10 and 12, with the best ordered samples being obtained in the pH range 11-12. In the most highly ordered samples domains may extend to microns in dimension. Increasing pH within this range gives SBA-2. with unit cells decreasing from 380(10) to 220(7) nm(3) Upon calcination the unit cell volumes decrease to between 253(8) and 136(5) nm(3), and the average pore size decreases by approximate to 20%. Incorporation of aluminium in tetrahedral sites at Si/Al ratios of > 11 also gives SBA-2, but reduces the degree of order. The adsorption characteristics of these aluminosilicates are very similar to pure silica SBA-2 prepared with the same surfactant. Upon calcination the aluminium adopts tetrahedral, 5-fold and octahedral environments, and imparts solid acidity of sufficient strength to catalyse the isomerisation of butene with high selectivities to butenes and conversions to isobutene of up to 26%. The catalysts deactivate only slowly on stream as the pore volume is filled with carbonaceous residue. (C) 2001 Elsevier Science B.V. All rights reserved.