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Gas Adsorption at the nanoscale: Effect of molecular morphology and pore geometry

Harry Cardenas Mansilla

Department of Chemical Engineering

Monday 25th March 2019
Time: 12.00pm
Venue: Room G05, Royal school of Mines, Imperial College London
Contact: Ms Hafiza Bibi
Tel: 020 7594 7252

Abstract:

Nanoporous materials have become a common staple in modern membrane separation, catalysis, and adsorption engineering. At this level of confinement, it is the energetical and morphological molecular-level interactions that dominate the observed macroscopic behaviour, as opposed to the more classical macroscopic fluid descriptors, such as viscosity and pressure, whose sheer definition breaks down in this highly anisotropic nanodomain.

It is well known that the structure and energy of the pore walls control the shape of adsorption isotherms. An important feature of the pore structure is the geometry as it directly influences the interactions between the particles and the walls. The pore geometry has a significant influence on the ultimate shape of the adsorption isotherm [1], and it can affect other aspects of interfacial phenomena such as nucleation kinetics due to orientational order effects [2]. Moreover, angles and wedges within pores influence the hysteresis behaviour in the adsorption isotherms when comparing adsorption and desorption [1].

When considering adsorption, besides the structure of the pore, the morphology of the fluid molecules (i.e., their overall shape) is equally relevant as the unique peculiarities of the molecular configuration may lead to different adsorption behaviour. Depending on the pore structure, there are some molecular morphologies that are preferred over others leading to unexpected selectivities [3] and/or enhanced permeability [4].

The main objective of this work is to perform molecular simulations to study both the effect of molecular shape and pore geometry on the adsorption isotherms. For the former, Molecular Dynamics (MD) simulations are used to analyse how the molecular morphology affects the adsorption of model trimers into cylindrical nanopores. The model fluids are composed of three tangentially bonded Lennard-Jones interactions sites with three distinct morphologies; a flexible chain, a rigid “stiff” linear configuration, and a rigid triangular ring. The adsorption and diffusion of these three different morphologies in cylindrical nanopores are studied here for different pore sizes and different values of fluid-solid energy interaction.

The effect of pore geometry is studied by conducting Grand Canonical Monte Carlo (GCMC) simulations of model fluids in well-defined pores with different cross section shapes but identical pore cross-sectional area. The proposed pores have triangle, square, pentagon, hexagon, octagon, decagon and circular cross-sections. The study quantifies the effect that the energetical and morphological heterogeneities have on adsorbed fluid behaviour. In order to do this, three different cross-sectional areas and three different fluid-solid interaction values are used. It is observed that the key property in the porous structure is the presence of the energetic heterogeneities at the corners of the pores, and that otherwise the influence of the pore shape is rather unnoticeable. The results have relevance as guidelines to the design of nanoporous adsorbents and membranes.

[1] L. Sarkisov, P. A. Monson, Langmuir, 17, 7600-7604 (2001).

[2] Y. Diao, T. Harada, A. S. Myerson, T. A. Hatton, B. L. Trout, Nature Materials, 10, 867-871 (2011).

[3] B. Smit, T. L. M. Maesen, Chemical Reviews, 108, 4125-4184 (2008).

[4] H. Cardenas, E. A. Müller, Molecules, 24 (3), 1-13 (2019

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