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 CO 2 /N 2 separation, owing to the difficulty in controlling the silica pore size, considering the similar sizes of CO 2 (0.33 nm) and N 2 (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 CO 2 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 CO 2 separation performance using composite TEOS–APTES membranes in the presence of TFA. While the TEOS (TFA) membrane exhibited a CO 2 permeance of 10 –6 mol m –2 s –1 Pa –1, its CO 2 /N 2 permselectivity remained low. However, introducing TFA into the TEOS–APTES structure resulted in a notable transformation of the primary amine (NH 2 ) groups into amide (−NHCOCF 3 ) functionalities, along with improved microporous properties. This was confirmed by FT-IR spectroscopy, reversible CO 2 adsorption/desorption, and the high uptake of adsorbed N 2 . The resulting composite TEOS–APTES (TFA) membranes with APTES concentrations of 2 and 5 mol % demonstrated enhanced CO 2 permeation properties, achieving a CO 2 /N 2 selectivity of 15 and 35, respectively. This improvement is attributed to the increased pore volume and the introduction of amide functionalities (−NHCOCF 3 ), which exhibit mild affinity for CO 2 . These findings suggest that the developed composite (TEOS–APTES) membranes are promising for industrial applications that require efficient CO 2 separation.

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