Fundamental study of adsorption of associating fluids in carbonaceous materials – a simulation study

Nguyen, Van Thuy (2013). Fundamental study of adsorption of associating fluids in carbonaceous materials – a simulation study PhD Thesis, School of Chemical Engineering, The University of Queensland.

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Author Nguyen, Van Thuy
Thesis Title Fundamental study of adsorption of associating fluids in carbonaceous materials – a simulation study
School, Centre or Institute School of Chemical Engineering
Institution The University of Queensland
Publication date 2013
Thesis type PhD Thesis
Supervisor Duong D. Do
David Nicholson
Total pages 125
Total colour pages 33
Total black and white pages 92
Language eng
Subjects 0904 Chemical Engineering
0104 Statistics
Formatted abstract
Associating fluids belong to an important class of adsorbates well-known for their non-ideal fluid-phase behaviour due to the formation of aggregates even at low-densities. Water, ammonia, alcohols and other molecules which are capable of hydrogen bonding are typical examples of associating fluids. The formation of hydrogen bonds usually leads to strong molecular association and greatly affects the macroscopic properties of bulk fluids. Beside the fundamental aspects of hydrogen bonding, these associating fluids have been the subject of considerable experimental and computational interest in recent years because of their important roles in chemistry, biology and atmospheric sciences. Until recently, the prediction of the behaviour of adsorption systems involving associating fluids and carbonaceous adsorbents remains a challenge to adsorption scientists and engineers. The aim of this thesis is to gain a fundamental understanding of the adsorption mechanism of associating fluids in carbonaceous solids and to reconcile the differences between experimental data and simulation results.

Molecular simulations, particularly Monte Carlo simulations, have shown great potential for adsorption studies. This is because a Monte Carlo simulation with grand canonical ensemble can mimic experiments carried out in a volumetric apparatus. In this thesis grand canonical Monte Carlo simulations are used to obtain adsorption isotherms. At zero loading, all adsorption isotherms are linear and one of the most important pieces of information that can be derived from an isotherm is its slope at zero loading (Henry constant), which provides information about the relative strength between various interactions: fluid-basal plane, fluid-fluid and fluid-functional group interactions, with regard to the initial adsorption.

Alcohols are associating fluids that show an intermediate adsorption behaviour between non-polar adsorbates, such as noble gases and nitrogen, and strongly polar adsorbates, such as water. In the family of alcohols, methanol is the smallest molecule whose adsorption mechanism has been studied on non-porous carbon adsorbents in the sub-monolayer region. To gain a deeper understanding on methanol adsorption beyond the monolayer coverage, a detailed computer simulation and experimental measurement of methanol are carried out on a highly graphitized thermal carbon black, Carbopack F, over a wide range of temperature. The solid is modelled as a homogeneous flat surface to investigate the interactions between methanol molecules and the basal-plane of graphite surface. The adsorption is shown to be strongly affected by hydrogen bonding which results in strong fluid-fluid interaction between the adsorbed molecules. The adsorption isotherms are initially sigmoidal; however, in the multilayer region the isotherms follow typical type II behaviour. The isosteric heat of adsorption is distinctly different from that of simple gases because of the dominance of the strong interactions between adsorbed molecules over the solid-fluid interactions. The configurations of adsorbed molecules on the carbon black are also investigated from simulations.

Next part of this thesis reports a comprehensive simulation study of methanol adsorption in the confined space of graphitic slit-like micropores having widths ranging from 0.65nm to 2nm at temperatures between 273K and 422K. Simulation results show that the amount adsorbed increases gradually with pressure in 0.65nm and 0.8nm pores (where only one molecular layer can be accommodated), while for pores having widths greater than 1nm the adsorption isotherms exhibit a sharp jump at low temperatures and they become gradual as the temperature is increased above the critical pore temperature, which is found to increase with pore width. For a given pore size, the pressure, at which a large uptake of adsorption occurs, increases and the excess amount adsorbed decreases with temperature. The interaction between adsorbate molecules and a pore is studied via the solvation pressure, which exhibits oscillations with pore size. The peaks of this oscillation correspond to pores that have an integer number of layers of methanol molecules. The configurations of methanol molecules in pores are also studied and compared with those on an open surface to evaluate the effects of the confinement.

Among associating fluids, water is the most interesting due to its unique behaviour which has been an intriguing challenge to experimentalists and theoreticians studying this ubiquitous liquid for many decades. The unusual behaviour of water is, to a large extent, a consequence of the very strong electrostatic interaction between the positively charged hydrogen atom of one molecule and the lone pair on the oxygen atom of another molecule, giving rise to strongly directional hydrogen bond. Adsorption of water on carbon surfaces has been the subject of many experimental studies. The topic is of practical interest because of its implications for the use of carbon adsorbents in separation processes where the presence of small amounts of water can be of crucial importance. In its pure graphitic form, carbon should have only a very weak interaction with water, since the attractive part of the interaction is primarily through dispersion forces. The observation that water is adsorbed strongly at low relative pressures by carbon adsorbents has prompted a number of simulation studies. One possible explanation is the chemical heterogeneities, specifically functional groups attached to the surface. This thesis proposes a new molecular model for graphitized thermal carbon black (GTCB), with phenol groups fixed at edge sites of the stacked graphene layers that make up the adsorbent, in order to study water adsorption at various temperatures. Clearly it is important, and could be critical in any application of carbons for the adsorption of mixtures involving water to determine the concentration of functional groups. The last part of this thesis proposes a new and simple method, using water as a molecular probe, for the estimation of the concentration of surface oxygen groups on the carbonaceous adsorbents. The idea is to determine the interaction between a water molecule and a functional group using a statistical mechanical calculation of the Henry constant from the volume integration of the Boltzmann factor over the accessible space around the functional group. The results obtained are in good agreement with those measured by Boehm titration. The method can be applied to adsorbents containing small concentrations of functional groups where the Boehm titration method fails.
Keyword Adsorption
Molecular simulation
Associating fluids

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