Valorization of agriculture waste: Preparation of alkoxysilanes from mixed rice straw and rice husk ash

Abstract

Synthesis of alkoxysilanes from renewable silica resources could promote their sustainability but remain grand challenges. Herein, we present a simple, effective strategy to synthesize tetraethyl orthosilicate (TEOS) directly from bio-derived silica through supercritical ethanol processing, where no catalyst nor additives are required. Crucially, calcination temperature is demonstrated to play a critical role in altering the structure of silica in rice husk ash (RHA), which directly influences its conversion activity to alkoxysilanes. Specifically, under optimized conditions at 280 °C and 1 h, RHA obtained at 400–625 °C calcination allows for an amorphous silica structure and as high as 85 mol% yield to alkoxysilanes. Moreover, the ash obtained from mixed rice straw and rice husk yields over 94 mol% alkoxysilanes, demonstrating the self-catalytic potential of the natural complexity of the ash. The effects of typical minerals contained in ash on the reaction are probed. It is revealed that potassium oxide is the main catalytic species in ash, which provide a suitable alkaline strength. The concentration of potassium ion exhibits a pronounced positive effect on catalyzing the synthesis of alkoxysilanes, and the co-existed phosphorus species exhibits promotional effect to TEOS selectivity. Furthermore, 29Si MAS NMR and Laplacian bond order analysis reveal that the cleavage of Si-O-Si bond predominates in the supercritical ethanol environment, providing a viable path for the production of TEOS.