Nano-TiO2 on Dodecyl-Sulfated Silica: As an Efficient Heterogeneous Lewis Acid-Surfactant-Combined Catalyst (HLASC) for Reaction in Aqueous Media

A reusable Lewis acid-surfactant-combined catalyst (LASC), using immobilization of nano-TiO2 on dodecyl-sulfated silica support (NTDSS), has been developed. To obtain this catalyst system, sodium dodecyl sulfate (SDS) was reacted with silica chloride (SC) to generate a dodecyl-sulfated silica (DSS) substrate. Subsequently, the addition of TiCl4 to a water solution of DSS resulted in the immobilization of nano-TiO2 on the surface of a DSS substrate. The NTDSS catalyst was characterized using different techniques such as TEM, SEM, XRD, ICP, and FT-IR. The catalyst was successfully used for the synthesis of coumarin derivatives via Pechmann condensation in water as a green solvent. High catalytic activity of the catalyst in water media can be a result of colloidal particle formation on the surface of the NTDSS catalyst due to the grafted DS moieties on the surface of silica, which act as a surfactant. It seems that the generation of emulsion droplets containing nano-TiO2 on the surface of the catalyst would function as an effective reaction reactor to accelerate the rate of the reaction in water media.