Role of silanol groups and hydrothermal rehydroxylation on the functionalization of SBA-15 with APTES for CO2 capture

Abstract

To identify the most suitable initial characteristics of an APTES-functionalized SBA-15 material, a series of SBA-15 samples was synthesized. Firstly, to remove the template, two methods were employed: i) water and ethanol washing method, and ii) calcination method where three calcination temperatures were explored (350, 450 and 550 °C), all carried out in a constant air flow. The purpose of avoiding calcination of the material is to prevent the loss of silanol groups, which play a key role in the chemical modification with amino groups. And secondly, a portion of these materials were subjected to a hydrothermal process to improve the amount of surface silanol groups, groups on which APTES molecules are anchored. As a result, eight materials were obtained and subsequently functionalized with APTES. Based on the nitrogen adsorption isotherms, an increase in pore size was observed when the material is rehydroxylated, which demonstrates the formation of silanol groups. Upon functionalization, the average pore size decreased by approximately 1.0 nm, suggesting the formation of new bonds between the surface silanol groups with the ethoxy groups of the APTES molecule. The CO₂ uptake capacity showed promising results for most of the materials, with the uncalcined materials presenting the highest values, 2.52 and 2.32 mmol g⁻¹ at 1 bar and 25 °C for S15WC-F and S15WC-FR, respectively.