Enhanced CO₂ separation performance for tertiary amine‐silica membranes via thermally induced local liberation of CH₃Cl

Abstract

in Wiley Online Library (wileyonlinelibrary.com) A facile method for the fabrication of amine-silica membranes with enhanced CO 2 separation performance was proposed via the thermally induced liberation of small molecules from quaternary ammonium salt. Quaternary ammoniumsilica (QA-SiO 1.5) xerogel powders/films were fabricated via sol-gel processing and their thermal stability was systematically studied using thermogravimetric mass spectrometer, Fourier transform infrared, energy dispersive spectroscopy, and positron annihilation lifetime spectroscopy analysis. CO 2 sorption performances of QA-SiO 1.5 derived xerogel powders were quantitatively compared after assigning their relevant parameters to a dual-mode sorption model. The gas permeation performances of membranes derived from QA-SiO 1.5 were evaluated in terms of kinetic diameter and temperature dependence of gas permeance, and activation energy (E p) required for gas permeation. The results indicate that liberation of the CH 3 Cl molecules from these membranes significantly improved both CO 2 permeation and CO 2 /N 2 separation capabilities. Therefore, the present study provides insight that should be useful in the development of highperformance CO 2 separation membranes via the effect of the thermally induced liberation of small molecules.

Role of Amine Type in CO2 Separation Performance within Amine Functionalized Silica/Organosilica Membranes: A Review

“…It should be apparent that the xerogel powders with unhindered amines demonstrated irreversible Langmuir-dominant adsorption/desorption isotherms with hysteresis loops probably due to the formation of stable carbamates after CO2 adsorption, whereas tertiary and sterically hindered amines showed almost reversible Henry-dominant adsorption/desorption isotherms. Generally, CO2 adsorption behaviors on amine-functionalized solid materials can be described by a dual-mode sorption model as a combination of Langmuir-type and Henry-type sorption as discussed in our earlier studies [23]. Note that SA-Si displayed a much higher adsorption capacity than the other three samples, which were comparable, presumably due to a somewhat stronger basicity that could improve the reaction capacity [10,37,38].…”

“…Figure 7 shows a typical example of PALS measurement on quaternary ammonium-silica films fired at different temperatures. Although N2 adsorption measurement reveals no differences between those two films, PALS measurement can obviously detect the differences between them due to the differences in calcination temperatures that result in a local liberation of small molecules (CH3Cl) and in the subsequent formation of ultra-micropores [23]. The pore sizes of these materials in the ultramicro level (0.2-0.7 nm) have been discussed in detail elsewhere [23].…”

“…Note that amorphous silica/organosilica materials/membranes fabricated via sol-gel routes generally demonstrate ultramicropores (<0.7 nm), whereas N 2 adsorption/desorption methods fail to characterize these very narrow pores. Alternatively, positron annihilation lifetime spectroscopy (PALS) measurement sometimes can be adopted to assess these dual flexible-rigid microstructures in terms of pore radius (rigid pores or free-volume pores) and fractional free volume (FFV) [23]. Figure 7 shows a typical example of PALS measurement on quaternary ammonium-silica films fired at different temperatures.…”

“…In addition to amine type, microporosity is another important factor impacting permeation performance of CO 2 as well as that of other gases. In one of our previous studies [23], we used a similar method to develop a microporosity-enhanced, amine-functionalized, organosilica membrane via the thermally induced local liberation effect of QA-Si. In that study, we observed a dealkylation reaction of quaternary ammonium groups at~230 • C in polycondensed QA-Si derived membranes with the liberation of CH 3 Cl molecules.…”

“…Unfortunately, only a very limited number of systematic studies providing simultaneous CO 2 adsorption and diffusion/desorption kinetic properties have been devoted to amine-functionalized, silica-based membranes. Given this, in our earlier studies, we developed a series of amine-containing organosilica membranes including unhindered (PA-Si, SA-Si) and sterically hindered (TA-Si, BTPP) amines, as well as a quaternary ammonium salt (QA-Si), and investigated the effect that amine type exerted on the CO 2 adsorption/desorption properties of solid powders and the related membrane performance (see Figure 3) [21][22][23][24]. It seems really important to more systematically compare the CO 2 separation performance of amine-functionalized silica-based membranes, despite many review papers for CO 2 separation membranes in the literature, which mostly review hydrocarbon-based membranes [4,25].…”