The “silica garden’' as a Brønsted acid catalyst

Abstract

The “silica garden’', an amorphous aluminosilicate produced from a solution of sodium silicate seeded with crystalline aluminium nitrate, has been examined by FTIR, TGA, solid-state NMR and catalytic testing. The silica garden is a Brønsted acid catalyst and we characterize its porosity, acidity and catalytic activity. The as-synthesized (AS) and H⁺-exchanged (H-AS) samples have BET surface areas of 148 and 161 g^-1, respectively, with corresponding pore volumes of 0.36 and 0.31 cm³ g^-1 and a broad distribution of pore sizes (30–1000 Å in diameter). The average pore size for AS and H-AS is 116 and 88 Å, respectively. The presence of Brønsted sites in the H⁺-exchanged samples was confirmed by FTIR of adsorbed pyridine. The population of Brønsted acid sites in the H⁺-exchanged sample with Si/Al=5.3 is lower than in a commercial amorphous aluminosilicate (ASA) with Si/Al=12 because, as NMR reveals, H-AS has proportionally more non-acid generating 6-coordinate aluminium. Catalytic tests reveal that, despite this, the acid sites in H-AS are stronger than in ASA.