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

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.