Fundamental study of adsorption and desorption process in porous materials with functional groups

Abstract

Adsorption is used in industries for gas separation and purification because of its less energy intensive than other traditional separation processes, such as distillation and gas absorption. However, its effective application depends on the theoretical understanding of the underlying phenomena of adsorption of molecules in porous solid adsorbents. With the advances in molecular simulation techniques, investigation into the microscopic mechanisms of adsorption phenomena can be realized and this will lead to a development of an unambiguous approach for the characterization of porous solids. This is the aim of this project to understand adsorption and desorption mechanisms in porous materials, especially porous carbons with functional groups because they are not fully studied in the literature. One of the significant points of this thesis is the development of a novel molecular model for porous carbon. Graphitized thermal carbon black (GTCB) was used as model adsorbent modelled as a composite of basal plane of graphene layers with crevices (ultrafine micropores) and oxygen functional groups attached at the edges of the graphene layers. This model was used in adsorption of various gases, and was validated with high resolution experimental data and theoretically analysed with simulation results obtained with a grand canonical Monte Carlo simulation. Excellent agreement between the experimental data and the simulation results has led us to derive the structural properties of GTCB and the nature of the functional group. Furthermore, the experimental Henry constant and the isosteric heat at zero loading (in the region of very low loadings) are described correctly with the Monte Carlo integration of the Boltzmann factor of the pairwise interaction between an adsorbate molecule and the porous carbon. It was found that adsorbate dominantly adsorbs in the fine crevices at very low loadings because of the enhancement of the solid-fluid potential energy, followed by adsorption on the basal plane of graphene layers. This is the case for non-polar fluids, such as argon and nitrogen. On the other hand, polar fluids, such as ammonia and water, the dominance of the functional group in adsorption is manifested, especially water. This novel model for carbon can be extended to describe practical porous carbons containing both micropores (for adsorptive capacity) and mesopores (for transport). Adsorption in mesopores is associated with capillary condensation and evaporation, and these are commonly used in the literature to derive the mesopore size distribution. For this determination to be realized, the fundamental understanding of condensation and evaporation must be understood, and this is the second objective of this thesis. We chose graphitic slit pores to model the mesopore, and investigated the effects of various parameters on the capillary condensation and evaporation. Grand canonical Monte Carlo technique is used to obtain the isotherm and the isosteric heat, and we particularly investigate the mechanisms of adsorption and desorption and derived conditions under which hysteresis occurs. The microscopic understanding of hysteresis was particularly studied for pores of different topology: pores with both ends opened to the surrounding, pores with one end closed, ink-bottle pores composing of a cavity connected to the bulk surrounding by a neck smaller in size, wedge type pore. Analysing the adsorption isotherms of these pores led us to capture features of how molecules adsorb and are structured in pores which result from the interplay between a number of fundamental processes: (1) molecular layering, (2) clustering, (3) capillary condensation and evaporation and (4) molecular ordering. The results derived from this comprehensive study not only guide engineers and scientists to substantially improve characterisation methods using gas adsorption but also to better design adsorptive processes in separation and purification.