Adsorption and Separation of C₈ Aromatic Hydrocarbons in Zeolites

Abstract

The separation of C8 aromatic hydrocarbons (e.g. xylenes) is one of the most important processes in the petrochemical industry. Current research efforts are focused on materials that can decrease the energy consumption and increase the efficiency of the separation process. Industrial processing of C8 aromatics typically considers adsorption in a zeolite from a vapor or liquid stream of mixed aromatics. Adsorption in porous materials can be used to separate the isomers or to promote catalytic reactions to transform aromatics into high value products. However, little is known about the chemical equilibrium of the adsorbed phase at reaction conditions. Most studies of adsorption of aromatics in zeolites, either experimental or computational, have focused on adsorption of pure components from the vapor phase. Experimentally, it is very difficult to determine adsorption equilibrium at saturation conditions. In molecular simulations, very difficult insertions and deletions of molecules make simulations very inefficient. Nowadays, advanced simulations techniques can be used to overcome this issue. Computer simulations of adsorption of aromatics in zeolites are typically performed using rigid zeolite frameworks. However, it is known that adsorption isotherms for aromatics are very sensitive to small differences in the atomic positions of the zeolite. In this thesis, the following types of questions are addressed: (1) how does framework flexibility influence adsorption and diffusion of C8 aromatics in zeolites?; (2) what is the role of the pore topology? For the separation and catalytic conversion of xylenes; (3) how does the type of framework influence the product distribution of xylene isomers?; (4) are there any possible zeolite structures that may have been overlooked for the processing of aromatics? For this, the different aspects that affect the interactions between aromatic molecules and the aromatics/zeolite systems in the simulations are discussed. The intermolecular interactions between aromatic molecules are studied by computing the vapor-liquid equilibria of pure xylenes and binary mixtures using four different force fields. The densities of pure p-xylene and m xylene can be well estimated using the TraPPE-UA and AUA force fields. The largest differences of computed VLEs with experiments are observed for o-xylene. Binary mixtures of p xylene and o-xylene are simulated, leading to an excellent agreement for the predictions of the composition of the liquid phase compared to experiments. For the vapor phase, the accuracy of the predictions of the composition are linked to the quality of the density predictions of the pure components of the mixture. The phase composition of the binary system of xylenes is very sensitive to small differences in vapor phase density of each xylene isomer, and how well the differences are captured by the force fields. Most of the models commonly used for framework flexibility in zeolites include a combination of Lennard-Jones and electrostatic intra framework interactions. The effect of these models for framework flexibility on the predictions of adsorption of aromatics in zeolites is studied. It is observed that the intra framework interactions in flexible framework models induce small but important changes in the atom positions of the zeolite, and hence in the adsorption isotherms. Framework flexibility is differently ’rigid’: flexible force fields produce a zeolite structure that vibrates around a new equilibrium configuration with limited capacity to accommodate to bulky guest molecules. The simulations show that models for framework flexibility should not be blindly applied to zeolites and a general reconsideration of the parametrisation schemes for such models is needed. The effect of framework flexibility on the adsorption and diffusion of aromatics in MFI-type zeolite is systematically studied. It is found that framework flexibility has a significant effect on the adsorption of aromatics in zeolites, specially at high pressures. For very flexible zeolite frameworks, loadings up to two times larger than in a rigid zeolite framework are obtained at a given pressure. Framework flexibility increases the rate of diffusion of aromatics in the straight channel of MFI-type zeolites by many orders of magnitude compared to a rigid zeolite framework. The simulations show that framework flexibility should not be neglected and that it significantly affects the diffusion and adsorption properties of aromatics in a MFI-type zeolite. The interactions of aromatic molecules inside different zeolite types are studied by computing adsorption isotherms of pure xylenes and a mixture of xylenes at chemical equilibrium. It is observed that for zeolites with one dimensional channels, the selectivity for a xylene isomer is determined by a competition of entropic and enthalpic effects. Each of these effects is related to the diameter of the zeolite channel. For zeolites with two intersecting channels, the selectivity is determined by the orientation of the methyl groups of xylenes. m-Xylene is preferentially adsorbed if xylenes fit tightly in the intersection of the channels. If the intersection is much larger than the adsorbed molecules, p-xylene is preferentially adsorbed. This thesis provides insight on how the zeolite framework can influence the competitive adsorption and selectivity of xylenes at reaction conditions. Different selectivities are observed when molecules are adsorbed from a vapor phase compared to the adsorption from a liquid phase. This suggests that screening studies that consider adsorption only from a vapor phase may have overlooked well-performing candidates for C8 aromatics processing. This insight has a direct impact on the design criteria for future applications of zeolites in industry. It is observed that MRE-type and AFI-type zeolites exclusively adsorb p-xylene and o-xylene from a mixture of xylenes in the liquid phase, respectively. These zeolite types show potential to be used as high-performing molecular sieves for xylene separation and catalysis.