Phase-boundary catalysis of alkene epoxidation with aqueous hydrogen peroxide using amphiphilic zeolite particles loaded with titanium oxide

A new heterogeneous catalytic system, phase-boundary catalysis, for epoxidation of alkene with aqueous H2O2 is proposed. Amphiphilic titanium-loaded zeolite particles, a part of the external surface of which was covered with hydrophobic alkyl groups and the rest being left hydrophilic, were prepared by deposition of titanium species from titanium(IV) tetra-2-propoxide and attachment of octadecylsilyl groups from n-octadecyltrichlorosilane (ODS) onto an NaY zeolite powder. Due to their amphiphilicity, the catalyst particles lay at the liquid-liquid phase boundary between upper alkene and lower aqueous phases, and they showed catalytic activity for epoxidation of 1-alkenes (e.g., 1-octene) with aqueous hydrogen peroxide. The phase-boundary catalytic system required neither stirring to make an emulsion nor addition of a cosolvent to make a homogeneous solution to drive the epoxidation. The yield of 1,2-epoxyoctane, a sole oxidation product from 1-octene, strongly depended on the apparent interphase area of the aqueous-organic phase boundary. The amphiphilic catalyst exhibited much higher catalytic activity than that of hydrophilic titanium-loaded NaY, without modification by ODS, or of a hydrophobic catalyst with almost full coverage by the alkyl groups. A similar trend in the activities of these catalysts was also observed when the reaction was carried out with vigorous stirring, in the presence of a cosolvent, or in a water-carbontetrachloride (CCl4) Mixture, where the aqueous hydrogen peroxide phase lies above the CCl4 phase. The phase-boundary catalytic system could also be applied to epoxidation of other normal alkenes. Compared with nonporous silica particles, the use of microporous NaY with a relatively large surface area had a beneficial effect, probably due to an increase both in the surface contact between the aqueous and organic layer and in the number of effective active sites of titanium species on its external surface. On the basis of these experimental results, a reaction model is proposed. (C) 2001 Elsevier Science.