A NOVEL DESIGN OF HETEROGENEOUS CATALYST FOR LIQUID-GAS REACTION SYSTEM WITH MARS-VAN KREVELEN TYPE MECHANISM Hadi Nur Ibnu Sina Institute for Fundamental Science Studies Universiti Teknologi Malaysia, 80130 UTM Skudai, Johor Malaysia. E-mail: hadi@kimia.fs.utm.my ABSTRACT The oxidation of cyclohexene with molecular oxygen, in the presence of hydrophobic niobium oxide/silica as catalysts, has been investigated. The catalysts have been prepared by impregnation of niobium penta-ethoxide on the silica surface followed by attachment of alkylsilyl groups from octadecyltrichlorosilane (ODS) on the surface with partial coverage. Cyclohexene was oxidized to 2- cyclohexen-1-ol, 2-cyclohexen-1-one and 7-oxabicyclo[4.1.0]hexane by molecular oxygen at atmospheric pressure in the presence modified Nb 2 O 5 /silica particles without any solvent at 70°C. These catalysts are efficient for the oxidation of cyclohexene with molecular oxygen which probably occurs with Mars-van Krevelen type mechanism. Based on these results, a catalyst design for liquid- gas reaction system is proposed. 1. INTRODUCTION Solid-gas catalyzed-liquid reactions are often encountered in the chemical process industry, most frequently in hydroprocessing operations and in the oxidation of liquid phase organic [1-4]. In respect of the latter reaction type, the system usually has been applied for mineralization of toxic organics in wastewaters. However, synthesis of useful organic materials with this system is also one of particular field of application. Along this line, we have attempted to make an effective heterogeneous catalytic system for this application. Recent results about the evidence for a Mars-van Krevelen type mechanism for oxidation with molecular oxygen in the liquid phase have been reported in several journals [5-7], making this system a good starting point for further extensive research on this kind of mechanism in liquid system. Those are the only reports on the liquid-phase Mars–van Krevelen mechanism-type oxidation in liquid phase system. This is interesting because by controlling the lattice oxygen diffusion, an effective catalyst for liquid-gas system could be obtained with a proper selection of oxide (as catalyst/active site). Recently, we proposed a novel concept of "phase-boundary catalysis (PBC)” in order to utilize the immiscible liquid-liquid reaction system with solid catalysts [8-12]. In the PBC system, it was demonstrated that the bimodal amphiphilic zeolite particles could be placed at the liquid-liquid phase boundary between aqueous hydrogen peroxide and water-immiscible organic phases and act as efficient catalyst for epoxidation reaction. In this paper, it is demonstrated that the phase-boundary catalysts not only show catalytic activity in liquid-liquid boundary, but also in liquid-gas system. For this purpose, the oxidation of cyclohexene with molecular oxygen using hydrophobic niobium oxide/silica particles is chosen as a reaction model. Creating active sites which is suitable for Mars- van Krevelen mechanism is proposed in this paper. Despite the numerous reports on selective oxidation of hydrocarbons using niobium oxide, the oxidation of alkenes by molecular oxygen using Nb 2 O 5 /silica system has never been reported. The use of molecular oxygen as the oxidant makes the reaction interesting from both economic and environmental points of view. 2. EXPERIMENTAL Catalyst preparation Silica sol with a primary particle size of 500 nm (Nissan Chemical Industries), was used after evaporation of water in order to get dry powder. Partially modified silica, whose surface was partly covered with alkylsilane, was prepared in two steps. First, niobium penta-ethoxide Nb(OC 2 H 5 ) 5 ; Wako Pure Chemical) in ethanol solution was impregnated into silica powder and heated at 383 K