Ternary Phase-Separation Investigation of Sol-Gel Derived Silica from Ethyl Silicate 40.

Shengnan Wang,João C Diniz Da Costa,Simon Smart,David K Wang

doi:10.1038/srep14560

Abstract

A ternary phase-separation investigation of the ethyl silicate 40 (ES40) sol-gel process was conducted using ethanol and water as the solvent and hydrolysing agent, respectively. This oligomeric silica precursor underwent various degrees of phase separation behaviour in solution during the sol-gel reactions as a function of temperature and H2O/Si ratios. The solution composition within the immiscible region of the ES40 phase-separated system shows that the hydrolysis and condensation reactions decreased with decreasing reaction temperature. A mesoporous structure was obtained at low temperature due to weak drying forces from slow solvent evaporation on one hand and formation of unreacted ES40 cages in the other, which reduced network shrinkage and produced larger pores. This was attributed to the concentration of the reactive sites around the phase-separated interface, which enhanced the condensation and crosslinking. Contrary to dense silica structures obtained from sol-gel reactions in the miscible region, higher microporosity was produced via a phase-separated sol-gel system by using high H2O/Si ratios. This tailoring process facilitated further condensation reactions and crosslinking of silica chains, which coupled with stiffening of the network, made it more resistant to compression and densification.

Highlights

Silica porous materials have attracted growing scientific interest due to their unique properties in terms of large surface area, thermal stability and chemical inertness and have found diverse applications in absorption[1], catalysis[2], energy[2] and separation applications[3]
Such behaviour is manifested by its longer molecular chains of the precursor, as well as the ability to form larger silica particles during the hydrolysis and condensation sol-gel process[17,19]
This time indicates that hydrolysis is being overtaken by condensation reactions, and more siloxane species are being generated at the expense of the silanol species regardless of their molar absorptivities. This further indicates that the shift in the reaction equilibrium is favoured towards the condensation reactions. These results demonstrate that the sol-gel process in phase-separated system is suppressed by low temperature due to a slower hydrolysis reaction and subsequently inhibits the condensation reactions

Summary

Introduction

Silica porous materials have attracted growing scientific interest due to their unique properties in terms of large surface area, thermal stability and chemical inertness and have found diverse applications in absorption[1], catalysis[2], energy[2] and separation applications[3]. Neville et al followed the sol-gel process of methyltrimethoxysilane by measuring the peak intensity variation of silanol (Si-OH) groups generated from hydrolysis and siloxane bridges (Si-O-Si) from condensation and in so doing, introduced the silica particle growth mechanism[15]. These studies strongly suggest that FTIR is a strong characterisation tool for assessing the silica sol-gel process. ES40 has been found to improve the hydrothermal stability of silica when prepared at high water and low ethanol contents[18] Considering these desirable aspects, it is important to study the ES40 sol-gel process in order to better tailor materials. The evolution of the phase-separated ES40 sol-gel method is studied as a function of reaction temperatures and molar ratios of water to ES40 (H2O/ES40)

Methods

Results

Conclusion