Hydrophobic titanium oxide and gold loaded magnesium oxide aerogel as catalysts in the oxidation of styrene

Abstract

Magnesium oxide aerogel (MgO-A) was synthesized via alkoxylation, sol-gel and supercritical extraction processes. The surface properties of MgO-A system were further modified by several methods in order to enhance its catalytic activity. Modification methods of the surface of MgO-A were impregnation of titanium oxide from titanium(IV)isopropoxide (TiO2/MgO-A) and deposition-precipitation of gold from gold chloride (Au/MgO-A). The metal impregnated TiO2/MgO-A and Au/MgO-A systems were further functionalised by alkylsilylation of n-octadecyltrichlorosilane (OTS) and chlorotrimethylsilane (CTMS) and the products were labelled as OTS-TiO2/MgO-A, CTMS-TiO2/MgO-A, OTS-Au/MgO-A and CTMS-Au/MgO-A respectively. Characterization by X-ray diffraction analysis, Diffuse Reflectance-Ultraviolet-Visible spectroscopy (DR-UV-Vis), Fourier Transformation Infrared spectroscopy (FTIR), nitrogen adsorption analysis and Field Emission Scanning Electron Microscopy (FESEM) showed the existence of titanium in octahedral coordination in TiO2/MgO-A and metallic gold in Au/MgO-A. Catalytic tests were conducted by oxidation of styrene using tert-butyl hydroperoxide (TBHP) as the oxidant. Products of the catalyzed reaction were analysed as benzaldehyde, styrene oxide and phenylacetaldehyde; where the products amount in decreasing order are: benzaldehyde, styrene oxide and phenylacetaldehyde. Despite similar selectivity, there was considerable increase in the conversion of styrene over all catalysts tested. The degree of catalytic activity of MgO-A catalysts in decreasing order are: CTMS-Au/MgO-A, OTS-Au/MgO-A, CTMS-TiO2/MgO-A, OTS-TiO2/MgO-A, Au/MgO-A and TiO2/MgO-A. The increase in the oxidation activity of both alkylsilylated TiO2/MgO-A and Au/MgO-A was attributed to the increasing hydrophobicity of the catalyst, whereby the active sites were being protected by the alkylsilyl groups from being deactivated by adsorption of hydrophilic molecules. Alkylsilylation also increased the adsorptivity of hydrophobic substrate on the surface of the catalysts. The result indicates that introduction of gold on the surface of MgO-A further increase the catalytic activity and acidity plays a role in the product distribution of styrene oxidation.