Cubic Pm3n supermicroporous silicas and aluminosilicates templated by short-chain alkyltrimethylammonium surfactants

Abstract

Highly ordered supermicroporous silicas and aluminosilicates with cubic Pm3n symmetry were prepared under weakly alkaline condition using alkyltrimethylammonium bromide with a short hydrophobic chain and relatively small hydrophilic head as a template. As the length of the carbon chain was increased from C₁₀ to C₁₆, the surfactant packing parameter increased and the favored symmetry of the silicas shifted from cubic Pm3n to hexagonal P6mm. Different mineral acids used to pre-hydrolyze TEOS could tune the silica mesostructures when dodecyltrimethylammonium bromide (C₁₂TMAB) was used as the template: sulfuric acid (H₂SO₄) led to cubic Pm3n symmetry and nitric acid (HNO₃) resulted in hexagonal P6mm symmetry. Under similar conditions, octyltrimethylammonium bromide (C₈TMAB) templating led to a disordered structure. The Pm3n symmetry could be well preserved after Al was incorporated into the framework and the molar ratio of SiO₂/Al₂O₃ in the aluminosilicates reached as low as 25. The pore size of the cubic Pm3n silicas and aluminosilicates was in the supermicroporous range. All the silica and aluminosilicate samples exhibited high specific area and large pore volume. Compared to zeolite USY-1 and AlSBA-1 catalysts synthesized in concentrated acid solution, aluminosilicates prepared herein exhibited more pronounced resistance to deactivation, higher reactivity for the acetalization of cyclohexanone with pentaerythritol, and a higher yield of 4-t-butyl-catechol in the tert-butylation of catechol.