MOLECULAR-SIEVING SILICA OVERLAYER ON GAMMA-ALUMINA - THE STRUCTURE AND ACIDITY CONTROLLED BY THE TEMPLATE MOLECULE

A silica overlayer was prepared on gamma-alumina by a method of chemic al vapor deposition (CVD) of silicon alkoxide using preadsorbed benzal dehyde and alpha-naphthaldehyde as a template. Infrared spectroscopy c onfirmed that the preadsorbed benzoate anion species remained after th e deposition of silica, indicating that silica was deposited on the ex posed surface of alumina. On the basis of the adsorption property of m odified alumina, it is considered that the silica overlayer had a vaca ncy whose size was as large as the template molecule. Si-29 NMR spectr oscopy showed that the Si species deposited using the template was Si( OAl)(1)(OSi)(1)(OH)(2) or Si(OAl)(2)(OSi)(1)(OH)(1), while the deposit ion without the template formed such a species as Si(OAl)(1)(OSi)(3) a nd Si(OAl)(1)(OSi)(2)(OH)(1) bonded via a two-dimensional siloxane net work. Lack of activity for double-bond isomerization of 1-butene to 2- butene on the silica overlayer prepared using the template was consist ent with the previous results that the two-dimensional network of silo xane on the alumina surface induced Bronsted acidity on the Al-O-Si-OH species. Both the adsorption and acidic properties on silica-deposite d alumina using alpha-naphthaldehyde were different from those on the sample prepared using benzaldehyde, showing that the structure of the silica overlayer was controlled by the template molecules.