Impact of Trifluoroacetic Acid on Tetraethoxysilane and Amine-Functionalized Tetraethoxysilane Silica Membranes for CO2 Separation

Abstract

Amorphous silica derived from tetraethoxysilane (TEOS) is known for its remarkable properties, including high chemical and thermal stabilities. However, its inherent structure presents challenges for effective CO₂/N₂ separation, owing to the difficulty in controlling the silica pore size, considering the similar sizes of CO₂ (0.33 nm) and N₂ (0.36 nm) molecules. In this study, we investigated the impact of trifluoroacetic acid (TFA) and amine (APTES: 3-aminopropyltriethoxysilyl) concentrations, aiming to leverage tailored silica structures with enhanced CO₂ affinity. Specifically, a two-stage investigation was conducted by first examining the influence of TFA on the pore structure of the TEOS networks, followed by an analysis of the CO₂ separation performance using composite TEOS–APTES membranes in the presence of TFA. While the TEOS (TFA) membrane exhibited a CO₂ permeance of 10^–6 mol m^–2 s^–1 Pa^–1, its CO₂/N₂ permselectivity remained low. However, introducing TFA into the TEOS–APTES structure resulted in a notable transformation of the primary amine (NH₂) groups into amide (−NHCOCF₃) functionalities, along with improved microporous properties. This was confirmed by FT-IR spectroscopy, reversible CO₂ adsorption/desorption, and the high uptake of adsorbed N₂. The resulting composite TEOS–APTES (TFA) membranes with APTES concentrations of 2 and 5 mol % demonstrated enhanced CO₂ permeation properties, achieving a CO₂/N₂ selectivity of 15 and 35, respectively. This improvement is attributed to the increased pore volume and the introduction of amide functionalities (−NHCOCF₃), which exhibit mild affinity for CO₂. These findings suggest that the developed composite (TEOS–APTES) membranes are promising for industrial applications that require efficient CO₂ separation.