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 ha s been studied as a function of pH. temperature and type of gemini surfacta nt template. SBA-2 is formed from reaction mixtures with a pH between 10 an d 12, with the best ordered samples being obtained in the pH range 11-12. I n the most highly ordered samples domains may extend to microns in dimensio n. 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 decreas e to between 253(8) and 136(5) nm(3), and the average pore size decreases b y 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 a dsorption characteristics of these aluminosilicates are very similar to pur e silica SBA-2 prepared with the same surfactant. Upon calcination the alum inium adopts tetrahedral, 5-fold and octahedral environments, and imparts s olid acidity of sufficient strength to catalyse the isomerisation of butene with high selectivities to butenes and conversions to isobutene of up to 2 6%. The catalysts deactivate only slowly on stream as the pore volume is fi lled with carbonaceous residue. (C) 2001 Elsevier Science B.V. All rights r eserved.