English abstract
Aromatic hydrocarbons such as the isomers of trimethylbenzene (TMB) are extracted
from crude oil along with other aromatic hydrocarbon groups. The isomers of
trimethylbenzene are mostly abundant as a mixture in the crude oil stream. These
isomers are used as precursors in industries such as pharmaceutical, agriculture,
plastics, dyes, textiles, food and so on. However, these are needed in their purest form
in all the applications mentioned above. Efficient separation of these isomers from
their mixtures is very critical to their application in the industry, however, the
conventional separation techniques such as chromatography, crystallization, fractional
distillation etc. face challenges due to close proximity of these isomers with respect to
their physical properties such as mass, density, melting point, boiling point etc. and
cost intensive. Separation of these isomers using membrane-based techniques is not
only efficient but also cost-effective. In these techniques, the membranes are usually
made up of porous materials such as zeolites.
Zeolites are the microporous crystalline silicate materials, consisting of void
networks in the shape of cages/channels of various topologies. These networks allow
only molecules of certain shape and size, blocking the rest, and thus acting as
molecular sieves. Known for their stability even at higher temperatures, the zeolites
are particularly used as molecular sieves at the industry scale for the hydrocarbon
separation. The choice of the zeolite is dependent on the hydrocarbons that are to be
separated, usually the effective pore diameter of the zeolite should be close to the size
of the hydrocarbon molecule. The design of the separation method is entirely based on
the structure, energetics, and the dynamic properties of the hydrocarbon molecule
under the confinement of the chosen zeolite framework.
The present thesis investigates the structural, energetic, and dynamic properties
of three isomers of trimethylbenzenes (TMB), namely, 1,2,3-TMB, 1,2,4-TMB and
1,3,5-TMB, under the confinement of two structurally contrasting zeolites, namely,
zeolite-ꞵ (BEA) and zeolite-NaY (FAU) through molecular dynamics simulations at
different temperatures. zeolite-ꞵ (BEA) is a channel type porous framework consisting
IV
of straight channels along x and y-axes, tortuous channels along z-axis, whereas
zeolite-NaY (FAU) is a cage type framework, consisting of large super cages separated
by 12MR ring (12 Oxygens) windows.
The results from our molecular dynamics simulations indicate that the 1,2,4-
TMB diffuses faster than the other two in both the zeolites, whereas 1,3,5-TMB is
strongly confined particularly in zeolite-ꞵ at all temperatures. The activation energies
calculated from Arrhenius relation indicate that the 1, 3, 5-TMB encounters a relatively
large energy barrier for translation as compared to the other two, in both the zeolites,
however, the absolute entropies calculated from two-phase thermodynamics (2PT)
method indicates that the associated translational barrier is purely entropic in nature.
Rotational dynamics of these isomers has been studied through the time decay of
Legendre polynomials corresponding to the autocorrelation of two chosen unit vectors,
namely, a vector on the plane of the phenyl ring and a vector perpendicular to the plane
of the phenyl ring representing the in-plane and out-of-plane rotations, respectively.
In-plane rotations of 1,2,3-TMB are more facile than those of 1,2,4-TMB in both the
zeolites while the out-of-plane rotations are largely hindered in 1,2,3-TMB than in
1,2,4-TMB. The in-plane and out-of-plane rotations are severely hindered for 1,3,5-
TMB in both the zeolites.
Therefore, our results demonstrate that the three isomers of the trimethylbenzene
are distinguishable in their structure, energetic and dynamic properties under the
confinement of zeolite-ꞵ and zeolite-NaY. We are confident that these results will be
helpful in designing a zeolite-based separation technique.
