Influence of Surface Groups on the Diffusion of Gases in MCM-41: A Molecular Dynamics Study

Abstract

The diffusion of binary mixtures of CO2 and N2 in a series of functionalized MCM-41 materials was studied using molecular dynamics (MD) simulations. Transport diffusion coefficients were obtained in order to assess the effect of surface functionalization with organic groups. Unmodified MCM-41 was found to be transport selective for N2. The spatial distribution of the two coadsorbed species showed that N2 molecules permeated faster and occupied the central pore region to a greater extent than CO2 molecules. CO2 was found to have a much lower permeability than N2 due to stronger interactions with the surface of MCM-41 and a tendency to become trapped in nooks in the amorphous pore wall. This effect was not reproduced when using a more simplistic model pore with a smooth surface demonstrating the importance of using a realistic pore model. Larger aminophenyl surface groups were found to significantly reduce the N2 selectivity due to a decrease in the flux of N2 and an increase in the flux of CO2. We explain this effect in terms of the marked difference in the transport mechanisms of CO2 and N2 in functionalized MCM-41 materials. Lastly, we remark on the importance of calculating the cross-term diffusion coefficients for this series of materials.

Dexamethasone-loaded poly(ɛ-caprolactone)/silica nanoparticles composites prepared by supercritical CO2 foaming/mixing and deposition

“…This CO 2 self-diffusion coefficient (inside the pores of MCM-41) is much lower than the gaseous CO 2 self-diffusion coefficient at 25 • C and atmospheric pressure (∼13.8 × 10 −6 m 2 s −1 ) (Massman, 1998). This fact was attributed to the interactions that may be established between CO 2 and the hydroxyl groups at the MCM-41 surface as well as to the tendency of CO 2 to become trapped in nooks in the amorphous pore walls (Williams et al, 2011). It is expected that similar phenomena may also occur at the higher pressure conditions employed in this work and for both tested SNPs .…”

“…To the best of our knowledge, there is no literature data regarding the diffusivity of scCO 2 inside MCM-41 SNPs at the experimental conditions employed in this work. Diffusivity was calculated to be ∼1.45 × 10 −8 m 2 s −1 (25 • C, atmospheric pressure) by molecular simulation studies (Williams et al, 2011). This CO 2 self-diffusion coefficient (inside the pores of MCM-41) is much lower than the gaseous CO 2 self-diffusion coefficient at 25 • C and atmospheric pressure (∼13.8 × 10 −6 m 2 s −1 ) (Massman, 1998).…”

Simulation of H2/CH4 mixture permeation through MOF membranes using non-equilibrium molecular dynamics

“…NEMD simulations have been recently used to examine water transport across various nanoporous materials such as zeolites,11 carbon nanotubes,12 graphene13 and graphene oxide nanosheets,14 and aquaporin 15. In contrast to water simulations, a small variety of materials, such as polymers of intrinsic microporosity,5 zeolites,10 inorganic mesoporous silica,16 and MOFs,17–20 were investigated using different NEMD simulation approaches such as an external field-NEMD (EF-NEMD), dual control volume grand canonical molecular dynamics (DCV-GCMD), and impermeable moving wall approach in gas simulation studies. Snurr's group10 estimated the transport properties of binary mixtures of short n -alkanes in faujasite type zeolite by using both the EMD and NEMD simulations.…”

Alkylsilane-Functionalized Microporous and Mesoporous Materials: Molecular Simulation and Experimental Analysis of Gas Adsorption

“…The most used generalization consists of not taking into account the formation of siloxane bridges between neighboring silane chains. [21][22][23]43 This assumption was also employed in this work. The hybrid simulated material was generated starting from the O atom covalently bonded to the silica surface, whereas the second atom was the Si bonded to the organic chain, which was added segment by segment.…”