CO 2 activation and synthesis of cyclic carbonates and alkyl/aryl carbamates over adenine-modified Ti-SBA-15 solid catalysts

Abstract

Cyclic carbonates were synthesized by the cycloaddition of CO 2 to epoxides (epichlorohydrin, propene oxide, and styrene oxide), and carbamates were synthesized by reaction of alkyl or aryl amines, CO 2, and n-butyl bromide. Solvents and cocatalysts/promoters like N,N-dimethylaminopyridine (DMAP) and quaternary ammonium salts, which are usually essential with conventional catalyst systems, could be avoided with the use of adenine-modified Ti-SBA-15 catalysts. The catalysts were reused in several recycle experiments. The structural and textural properties of the catalysts were determined by X-ray diffraction (XRD), transmission electron microscopy (TEM), N 2 adsorption, and Fourier transform infrared (FTIR) and diffuse reflectance ultraviolet–visible (UV–vis DRS) techniques. Acid–base properties of the solid catalysts were investigated by temperature-programmed desorption (TPD) of NH 3 and CO 2 and DRIFTIR spectroscopy of adsorbed pyridine and CO 2 techniques. The studies reveal that the Ti 4+ ions increase catalytic activity by enhancing the adsorption of the epoxide and alkyl or aryl amine substrates. CO 2 molecules are activated at the basic nitrogen groups of adenine. Increasing the surface concentrations of either CO 2 (by anchoring basic molecules like adenine or increasing the partial pressure of CO 2) or epoxides and alkyl or aryl amines (by increasing the concentration of Lewis acidic Ti 4+ ions) enhances the catalytic activity. CO 2 molecules activated at the covalently anchored adenine sites react with epoxide/amines adsorbed on the silica surface to form carbonates/carbamates.