Preparation of silica-based porous materials from starch-derived low molecular weight organic gelator template

Abstract

Starch, as readily available biomass source, low in cost and renewable biodegradable material, has not been paid enough attention by researchers as a promising candidate for developing sustainable materials. Starch is also referred to as a polysaccharide, mainly composed of two homopolymers of D-glucose, amylose and amylopectin building unit. The present study reports on the chemical modification of starches derived from locallygrown rice and sago to low molecular weight organic gelators (LMWOGs) for application in template-assisted synthesis of porous materials. Both rice and sago starches were modified chemically via acid hydrolysis in aqueous solutions (pH < 2) in order to break down the long branched chain of polysaccharides into much smaller monosaccharide chains of ß-D-glucose. The presence of ß-D-glucose was confirmed by Benedict’s test, FTIR and NMR spectroscopy. The starch derived either from rice or sago showed similar chemical characteristics but exhibited significant differences in their granular arrangements. The rice starch granules were polygonal in shape while those of sago starch were oval shaped as revealed by FESEM micrographs. Synthesis of mesoporous silica-based materials with high surface areas (756 m2 g-1) were performed by employing ß-D-glucose, as organic gelator template and tetraethyl orthosilicate (TEOS) as silica precursor, in a typical HCl-catalyzed sol-gel process. The nature of interaction between silica and LMWOGs was investigated. The results show that LMWOGs act as template for the structuration of silica and the electrostatic interactions at the template-silica interface contribute to the porosity of the materials. The template removal by water as an extraction solvent and followed by calcination at 400°C were evaluated as the best template removal method. Based on the nitrogen adsorptiondesorption isotherms, the pore parameters of the mesoporous silica depend primarily on the amount ratio of modified starch to silica precursor (in % v/v). At low amount of template used (< 40 % v/v) materials with micropores dominant were formed. As the template concentration is increased in medium amount ranging from 45 to 65 %v/v, the relative contribution from mesopores becomes dominant while the presence of excess amount of template resulted in low pressure hysteresis, suggesting the presence of ultramicropores. The mesoporous silica material was inserted with different titanium loadings (1, 3 and 5 wt.%) to generate titanium-silicate catalysts for the oxidation of 1- naphtol to 1,4-naphtoquinone. The catalyst containing 1 wt.% titanium which possessed the highest amount of tetrahedral titanium species as active sites exhibited the highest conversion (44 %) of 1-naphtol towards 1,4-naphtoquinone.