Molecular Dynamics Investigation of the Potentiality of Zeolite NaY for Trimethylbenzene Isomer Separation: An Insight in Terms of Structure, Energetics, and Dynamics.

Isomers of trimethylbenzene (TMB) are the important constituent chemicals used in a variety of industrial applications, including the production of synthetic resins, insecticides, dyes, pigments, and pharmaceuticals. However, these applications require them in their purest form, mandating them to separate from their mixtures. Due to the similar physicochemical properties of the TMB isomers, traditional methods such as cryogenic distillation are very energy-expensive. Thus, adsorption-based separation methods using nanoporous adsorbents such as zeolite are a cost-effective alternate. Such adsorption-based separation methods, however, require the isomers to exhibit significant differences in their adsorption and diffusion properties. In the present report, we carried out a thorough investigation of the structural, energetic, and dynamical properties of the three isomers of TMB in a faujasite-type zeolite NaY containing cage-like pores. Our results indicate that 1,2,4-TMB exhibits the most facile translational and out-of-plane rotational motions in contrast to the other two isomers. However, the in-plane rotational motions are seen to be more facile in 1,2,3-TMB compared with the other two isomers. It is evident from our analysis that the dynamics are dominantly driven by entropy. These results highlight the importance of the porous material in the case of the separation of a given hydrocarbon mixture.

Thermodynamics of translational and rotational dynamics of C9 hydrocarbons in the pores of zeolite-beta.

There has been a growing interest in the separation of aromatic hydrocarbon molecules from the petroleum stream using zeolite-based technologies. This led to numerous experimental and molecular simulation studies of the structural and dynamical properties of aromatic hydrocarbons under the confinement of microporous materials like zeolites. The understanding of the behavior of the isomers of the trimethylbenzene under confinement is crucial for their separation and purification from industrial streams. Here, we investigate the translational and rotational dynamics and associated thermodynamics of three isomers of trimethyl benzene, namely, 1,2,3-trimethyl benzene (1,2,3-TMB), 1,2,4-trimethyl benzene (1,2,4-TMB), and 1,3,5-trimethylbenzene (1,3,5-TMB) under the confinement of zeolite-beta (BEA) using molecular dynamics (MD) simulations. The trends in the diffusion coefficients of the TMB isomers calculated from our MD simulation data are in good agreement with the data already available in the literature. Analysis of dynamics and associated thermodynamic properties indicate that 1,2,4-TMB is translationally more facile than the other two isomers. The rotational motion of TMB isomers is largely anisotropic and it is relatively more significant for both 1,2,4-TMB and 1,3,5-TMB. The thermodynamic properties reveal that the distinguishability in the dynamic properties among these three isomers is essentially caused by entropy. These results are not only critical to engineer the separation process of TMB isomers across the zeolite beds but also to understand the different catalytic processes such as trans-alkylation, conversion, cracking etc.