Surface Assembly of Ionic Liquid Functionalities on Mesoporous Silica to Enhance Paraffin Affinity and Induce Reverse Selectivity for Ethane/Ethylene Separation

Low molecular weight olefins, such as ethylene, constitute an important feedstock not only in polymer production but also as reactants for the synthesis of several compounds, with a global production rate of over 200 million tons per year. Ethylene is mainly produced by steam cracking of naphtha, and, depending on its final application, it can be obtained in different grades. For example, a high-purity grade (&gt;99.5%) is required for polymer production. To obtain polymer-grade olefins, it is imperative to separate light olefins from paraffins. The state-of-the-art technology for light olefin/paraffin separation is cryogenic distillation, yet because of the similar sizes and close volatilities of the C₂H₄/C₂H₆ molecules, the process is energy-intensive, such that even subtle improvements in the quality of separation can significantly affect the operating cost.<br/>Adsorptive separations based on porous adsorbents exhibit high potential for light-olefin/paraffin separation. As the industrial feed is rich in olefins, developing a paraffin-selective adsorbent is energetically favorable. In this study, we developed surface-functionalized silica-based adsorbents for C₂H₄/C₂H₆ separation which selectively take up ethane because of their enhanced van der Waal’s interactions with ethane molecules. Fluorine-containing anions for instance, have been reported to have stronger interactions with C₂H₆ molecules, attributed to the C–H—F van der Waal’s forces between the six hydrogen atoms of the C₂H₆ molecule and the fluorine atoms. Therefore, in our work, a fluorine-based ionic liquid (IL) with longer alkyl chains, 1-propyl-3-methyl-imidazolium bis(tri-fluoro-methyl-sulfonyl) imide was selected as the primary IL for grafting on MCM-41 surface <i>via</i> silanization to develop a reverse-selective adsorbent.<br/>To study the effect of the type of anion and cation on adsorbent performance, anions such as dicyanamide and stearate were also investigated by switching the anions of the primary IL. In addition, the effect of the cation on the selectivity was investigated by swapping the imidazolium-based cation with a phosphonium-based cation. These novel functionalized adsorbents were observed to switch selectivity after IL modification and exhibited reverse selectivity. The developed adsorbents exhibited also low enthalpies of adsorption, i.e., 12 kJ/mol and 9 kJ/mol for C₂H₆ and C₂H₄, respectively, which implies facile regeneration with reduced energy consumption.