Applying Alkyl-Chain Surface Functionalizations in Mesoporous Inorganic Structures: Their Impact on Gas Flow and Selectivity Depending on Temperature

Abstract

Porous inorganic capillary membranes are prepared to serve as model structures for the experimental investigation of the gas transport in functionalized mesopores. The porous structures possess a mean pore diameter of 23 nm which is slightly reduced to 20 nm after immobilizing C-alkyl chains on the surface. Gas permeation measurements are performed at temperatures ranging from 0 to 80 °C using Ar, N, and CO. Nonfunctionalized structures feature a gas transport according to Knudsen diffusion with regard to gas flow and selectivity. After C-functionalization, the gas flow is reduced by a factor of 10, and the ideal selectivities deviate from the Knudsen theory. CO adsorption measurements show a decrease in total amount of adsorbed gas and isosteric heat of adsorption. It is hypothesized that the immobilized C-chains sterically influence the gas transport behavior without a contribution from adsorption effects. The reduced gas flow derives from an additional surface resistance caused by the C-chains spacially limiting the adsorption and desorption directions for gas molecules propagating through the structure, resulting in longer diffusion paths. In agreement, the gas flow is found to correlate with the molecular diameter of the gas species (CO < Ar < N) increasing the resistance for larger molecules. This affects the ideal selectivities with the relation [Formula: see text]. The influence on selectivity increases with increasing temperature which leads to the conclusion that the temperature induced movement of the C-chains is responsible for the stronger interaction between gas molecules and surface functional groups.

Surface Functionalization of Mesoporous Membranes: Impact on Pore Structure and Gas Flow Mechanisms

“…A decrease in the flow rate upon membrane functionalization was observed many times for a large variety of functional group types, and the decrease in the pore size and volume has been considered to be the reason. − However, in many cases, the flow rate decreases by multiple orders of magnitude upon surface functionalization. This extent cannot be explained by the decrease in the pore size and porosity, and the decrease in flow is related to the functional group density, as exemplarily shown for C 16 -alkyl functionalizations in our previous work. , In conclusion, it is hypothesized that the gas molecules are forced to travel on longer trajectories due to limited adsorption/desorption angles caused by the functional groups. Recently, experiments of Sadilov and co-authors lead to similar results and another possible explanation is presented: increased residence times of the gas molecules on the functional surface cause a decrease in flow.…”

“…Single-gas permeation measurements are performed similarly to our previous work . The gases used, namely, He (5.0), Ne (5.0), Ar (5.3), N 2 (5.0), CO 2 (4.5), CO (4.7), CH 4 (4.5), C 2 H 4 (3.5), C 2 H 6 (3.5), and C 3 H 8 (3.5) were purchased from Linde AG, Germany.…”

A Role of Activators for Efficient CO2 Affinity on Polyacrylonitrile-Based Porous Carbon Materials

“…CO 2 capture, such as amine-modified mesoporous adsorbents (Chen et al, 2013;Sim et al, 2015), silica (Belmabkhout et al, 2009), microporous carbon-based materials (Wickramaratne and Jaroniec, 2013;Kamran and Park, 2020), zeolites (Bae et al, 2013), inorganic-capillary membranes (Besser et al, 2016), porous hexagonal boron nitride (h-BN) sheets (Kamran et al, 2019a), and nitrogen-doped carbon adsorbents (Heo and Park, 2015), are being investigated as promising alternatives because of their cost-effective preparation, broad availability, high specific surface area, controllable surface properties, physiochemical sustainability, and minimal energy utilization. Among these adsorbents, porous carbon sorbents, commonly referred to as activated carbons, have demonstrated several advantages and are considered efficient adsorbents for gas uptake, metal recovery, and catalysis (Lee et al, 2006;Liang et al, 2014;Kamran et al, 2019b), because of their high adsorptive capacity, economical processing, easy regeneration, high thermal sustainability, rapid adsorption kinetics, high specific surface area, adjustable porosity, and functionality and low sensitivity to moisture (Zhou et al, 2013).…”