Influence of Mg/CTAB ratio on the structural, physicochemical properties and catalytic activity of amorphous mesoporous magnesium silicate catalysts

Abstract

This study investigated the physicochemical and catalytic properties of mesoporous magnesium silicate catalysts prepared at various Mg/CTAB ratios (0.25, 0.50, 0.75 and 1.00). The XPS analysis detected a mixture of enstatite and magnesium carbonate species when the Mg/CTAB ratio was 0.25, and 0.50. A mixture of forsterite and magnesium carbonate species were detected when the Mg/CTAB ratio was 0.75 whereas for the Mg/CTAB ratio of 1.00, enstatite and magnesium metasilicate species were detected. A catalyst with the Mg/CTAB ratio of 1.00 demonstrated the highest catalytic activity in the oxidation of styrene. The styrene conversion rate was 59.0%, with 69.2% styrene oxide (StO) selectivity. The H₂O₂ molecules were activated regio-specifically by the magnesium species to prevent rapid self-decomposition while promoting selective interaction with styrene. All the parameters that influence the styrene conversion and product selectivity were evaluated using analysis of variance (ANOVA) with Tukey's test. The ANOVA analysis showed that the reaction time (h), Mg/CTAB ratio, styrene/H₂O₂ ratio, catalyst loading (mg) and temperature (°C) affect styrene conversion and product selectivity (StO) significantly (p < 0.05). The oxidation of styrene was well fitted to the pseudo-first-order model. The activation energy, E_a of the catalysed styrene epoxidation reaction was calculated to be 27.7 kJmol⁻¹. The catalyst can be reused several times without any significant loss in its activity and selectivity. The results from this study will be useful in designing and developing low cost, high activity catalysts from alkaline earth metals.