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.

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