Triphase catalysis over titanium-silicate molecular sieves under solvent-free conditions: I. Direct hydroxylation of benzene

Under solvent-free triphase conditions (solid catalyst TS-1, organic substrate and aqueous H₂O₂, S-L-L) a significant enhancement in the reaction rate (ca by 15-25 times) during the oxidation of benzene, compared to that obtained under conventional biphase system using an organic solvent (solid TS-1 catalyst in the presence of a cosolvent to homogenize the organic and aqueous layers, S-L), was observed. Time-dependent studies in the hydroxylation of benzene over TS-1/H₂O₂ system indicated that while in the presence of a cosolvent (like acetone, acetonitrile, or methanol) a long induction period was observed; in solvent-free conditions the induction period was almost absent. The effect of various reaction parameters such as benzene/H₂O₂ molar ratio, mode of addition of H₂O₂, reaction temperature, catalyst concentration, stirring speed, and the dilution level on benzene conversion and phenol selectivity was also studied. Around 85 ± 5 mol% H₂O₂ utilization for oxygenated benzene products (reaction time = 2 h) as well as phenol selectivity could be obtained under the present solvent-free, triphase conditions (at 333 K and benzene to H₂O₂ mole ratio = 1-3). The corresponding values under similar reaction conditions, except the presence of organic solvent (reaction time = 8 h), in conventionally used biphase system were 17 ± 3 and 85 ± 5, respectively. Further, the present solvent-free method offers distinct advantages of easier workup and being environmentally safer, because of the absence of organic solvents. Competitive adsorption experiments suggest that under triphase condition benzene competes favorably with water for diffusion inside the TS-1 channels. However, under biphase conditions benzene faces strong competition with organic solvents for diffusion inside relatively hydrophobic TS-1 channels resulting in low conversion.