Effects of Pore Connectivity on the Sorption of Fluids in Nanoporous Material: Ethane and CO2 Sorption in Silicalite

Abstract

Adsorption of fluids in nanoporous materials is important for several applications including gas storage and catalysis. The pore network in natural, as well as engineered, materials can exhibit different degrees of connectivity between pores. While this might have important implications for the sorption of fluids, the effects of pore connectivity are seldom addressed in the studies of fluid sorption. We have carried out Monte Carlo simulations of the sorption of ethane and CO2 in silicalite, a nanoporous material characterized by sub-nanometer pores of different geometries (straight and zigzag channel like pores), with varied degrees of pore connectivity. The variation in pore connectivity is achieved by selectively blocking some pores by loading them with methane molecules that are treated as a part of the rigid nanoporous matrix in the simulations. Normalized to the pore space available for adsorption, the magnitude of sorption increases with a decrease in pore connectivity. The increased adsorption in the systems where pore connections are removed by blocking them is because of additional, albeit weaker, adsorption sites provided by the blocker molecules. By selectively blocking all straight or zigzag channels, we find differences in the absorption behavior of guest molecules in these channels.

Effects of pore connectivity and tortuosity on the dynamics of fluids confined in sub-nanometer pores

“…This study employed models of silicalite with different number of pore connections generated in a previous study. 19 In brief these models were made by selectively blocking some channels of a supercell of silicalite 21 made up of 2 × 2 × 3 unit cells obtained with VESTA. 22 Methane molecules were used as blockers which were treated as an immobile part of the silicalite substrate in the MD simulations described here.…”

“…For example, we have recently reported a systematic study on the effects of pore connectivity on the sorption of fluids using grand canonical Monte Carlo (GCMC) simulations. 19 While pore connectivity varied between 48 and 0 pore connections was found to affect the sorption amounts, the effect of pore connectivity is expected to have a stronger effect on the dynamical properties. 19 In an earlier study, we used molecular dynamics (MD) simulations to address the effects of inter-connectivity of pores on the structure and dynamics of confined fluids by comparing the behavior of CO 2 and ethane in ZSM-22, a zeolite characterized by unidimensional pores of 0.5 nm diameter, with that in silicalite (all silica analogue of ZSM-5), a zeolite with pores of the similar size but characterized by unidimensional pores connected to each other via zig-zag channel like pores running in a perpendicular plane.…”

“…19 While pore connectivity varied between 48 and 0 pore connections was found to affect the sorption amounts, the effect of pore connectivity is expected to have a stronger effect on the dynamical properties. 19 In an earlier study, we used molecular dynamics (MD) simulations to address the effects of inter-connectivity of pores on the structure and dynamics of confined fluids by comparing the behavior of CO 2 and ethane in ZSM-22, a zeolite characterized by unidimensional pores of 0.5 nm diameter, with that in silicalite (all silica analogue of ZSM-5), a zeolite with pores of the similar size but characterized by unidimensional pores connected to each other via zig-zag channel like pores running in a perpendicular plane. 14 Connecting the one-dimensional pores with quasi one-dimensional channels was found to suppress the dynamics of both fluids while the effect of connecting the pores by artificially inserting two-dimensional inter-crystalline space was different for the two fluids.…”

“…It is also worth noting that because the pore size in silicalite is similar regardless of pore type (straight versus zigzag), the pore size distribution is unimodal and sharply peaked. In the previous study addressing sorption properties, 19 we created 12 models of silicalite with different degrees of pore connectivity by selectively blocking some channels via loading immobile methane molecules. These 12 models had pores connected via different combinations of connections ranging between 0 and 48.…”

“…Silicalite is an all-silica analogue of ZSM-5 zeolite and has a network of one-dimensional channel-like pores inter-connected at regular intervals with quasi one-dimensional zigzag channels in a perpendicular plane. 19 This intricate pore network structure provides an opportunity to systematically vary the connections between pores by selectively blocking some of them, thus resulting in different scenarios of pore connectivity in the same substrate. Further, with different channel geometries – straight ellipsoidal channels oriented along the crystallographic b -axis and sinusoidal (zigzag) channels running in a perpendicular plane, selectively blocking all channels of one type can also help constrain the effects of pore geometry and tortuosity.…”