(483d) Adsorptive Separation of Xylene Isomers Using Zn2(BDC)2(DABCO) [MOF1] Metal Organic Framework

Adsorptive Separation of
Xylene Isomers using Zn₂ (BDC)₂ (DABCO)  [MOF1]  Metal Organic Framework

Arun G and Sudeep N Punnathanam

            Isomeric
forms of dimethylbenzene(xylene) especially the ortho and para
isomers, find wide applications in the polymer, pharmaceutical and dye
industries[1-3]. However, the separation of these isomers, especially the ortho
isomer, remains a challenging task till date, due to the closeness of its
melting and boiling points with the meta isomer[3]. The metal-organic
framework MOF1, also known as Zn₂ (BDC)₂(DABCO), has been
shown to selectively adsorb ortho-xylene from a mixture of xylenes[5]. In this
work, Grand Canonical Monte Carlo(GCMC)[4] simulations  are used to study the adsorption of
xylenes on MOF1 in order to understand the reasons for selectivty towards
o-Xylene. MOF1 shows guest dependent framework flexibility[6], i.e., the
structure of the adsorbent changes with the chemical nature and loading of the
adsorbate. In the case of MOF1, two distinct structures, namely a large pore
and a narrow pore, are observed. Comparison of GCMC simulations with
experiments reveal that at high loading of xylenes, the MOF1 undergoes a
structural transition from the large pore to the narrow pore structure. An
examination of density distributions inside the pore reveal two adsorption
sites for ortho-xylene but  only
one for meta and para isomers. The presence of an additional adsorption site is
the source of selectivity of MOF1 towards the ortho-isomer. The existence of
this additional site is due to favorable alignment of the CH₃ groups
in the ortho isomer towards the oxygen atoms of the benzene dicarboxylate
linkers which make up the pore walls of the adsorbent. Such favorable alignment
is not possible with the meta and the para isomers.

References:

1. Reynold, L.A,
Quinizarin, Org. Synth. Coll., 1941. 1, p. 476.

2. Neurath, A. R..,
Design of Microbicide for Prevention of Sexually Transmitted Diseases Using A
Pharmaceutical Excipient. AIDS Patient Care ST, 1999, 27(1), p. 11-21

3. Fabri, J., Graeser,
Ulrich, and Simo, Thomas A, Xylenes, Ullmann's Encyclopedia of Industrial
Chemistry. 2002: Wiley-VCH, Weinheim.

4. Frenkel, D., Smit, B.,
Understanding Molecular Simulation 2e, Academic Press, 2002.

5.Nicolau, M.,
Marco.P.M., et al., Single- and Multicomponent Vapor-Phase Adsorption

of Xylene Isomers and
Ethylbenzene in a Microporous Metal – Organic Framework., J. Phys. Chem.
C 2009, 113, p.13173–13179.

6. Dybtsev, D.N., H.
Chun, and K. Kim, Rigid and Flexible: A Highly Porous Metal-Organic Framework
with Unusual Guest-Dependent Dynamic Behavior. Angew. Chem., 2004. 43: p.
5033-6.