Adsorption equilibrium of xylene isomers and ethylbenzene on MIL-125(Ti)_NH2: the temperature influence on the para-selectivity

Abstract

The separation of xylene isomers feedstock containing ethylbenzene is still a relevant industrial challenge. The increase in p-xylene consumption worldwide promotes the search for more efficient separation processes. One possible way to achieve such a goal is to upgrade the existing SMB units with selective adsorbents operating at lower temperatures. Therefore, new adsorbents such as the functionalized metal–organic framework MIL-125(Ti)_NH2 appear as strong candidates. The MIL-125(Ti)_NH2 was tested in this work for the selective adsorption and separation of xylene isomers, including ethylbenzene, in liquid phase and using n-heptane as eluent. In this way, a detailed experimental study of binary and multicomponent adsorption equilibrium of xylene isomers in MIL-125(Ti)_NH2, at three temperatures (299 K, 313 K and 343 K) in liquid phase, was performed and is reported in this manuscript. Ternary breakthrough experiments indicate competitive adsorption between p-xylene over the two other isomers. Indeed the selectivity values range from 1.4 to 1.9 and 1.5 to 2.3 for the p-xylene/m-xylene and p-xylene/o-xylene pairs, respectively, increasing with the temperature. Similar trends are observed for the quaternary mixtures, where p-xylene and ethylbenzene are adsorbed preferentially. However, the MIL-125(Ti)_NH2 does not discriminate between the two more linear molecules at lower temperatures. Yet, when temperature is increased to 343 K, the adsorption of p-xylene becomes more favorable than ethylbenzene, presenting a selectivity of 1.3. Therefore, within the studied temperature range a temperature, 343 K is the most favorable temperature to operate the SMB process, for the p-xylene production from mixtures containing ethylbenzene.