Grafting of Imidazolium Based Ionic Liquid on the Pore Surface of Nanoporous Materials—Study of Physicochemical and Thermodynamic Properties

Abstract

Supported ionic liquid phase (SILP) systems were prepared by immobilizing a methylimidazolium cation based ionic liquid onto the pore surface of two types of support, MCM-41 and Vycor. The "grafting to" method was applied, involving (3-chloropropyl)-trialkoxysilane anchoring on the supports' silanol groups, followed by treatment with 1-methylimidazole and ion exchange with PF(6)(-). Optimum surface pretreatment procedures and reaction conditions for enhanced ionic liquid (IL) loading were properly defined and applied for all modifications. A study on the effect of different pore sizes on the physical state of the grafted 1-(silylpropyl)-3-methylimidazolium-hexafluorophosphate ([spmim][PF(6)(-)]) was also conducted. The [spmim][PF(6)(-)] crystallinity under extreme confinement in the pores was investigated by modulated differential scanning calorimetry (DSC) and X-ray diffraction (XRD) and was further related to the capacity of the developed SILP to preferentially adsorb CO(2) over CO. For this purpose, CO(2) and CO absorption measurements of the bulk ionic liquid [bmim][PF(6)(-)] and the synthesized alkoxysilyl-IL were initially performed at several temperatures. The results showed an enhancement of the bulk IL performance to preferentially adsorb CO(2) at 273 K. The DSC analysis of the SILPs revealed transition of the melting point of the grafted alkoxysilyl-IL to higher temperatures when the support pore size was below 4 nm. The 2.3 nm MCM-41 SILP system exhibited infinite CO(2)/CO separation capacity at temperatures below and above the melting point of the bulk IL phase, adsorbing in parallel significant amounts of CO(2) in a reversible manner. These properties make the developed material an excellent candidate for CO(2)/CO separation with pressure swing adsorption (PSA) techniques.

Characterization of Supported Ionic Liquid Phase (SILP) materials prepared from different supports

“…There are a number of advantages of ILs in separation and reaction systems, but in many cases there is a need to immobilize them on porous materials (Wasserscheid and Welton 2008). The supported ILs can be spread on the inner surface of the porous structure, maintaining large specific surface area and mechanical properties of the support, thus circumventing the problem of mass transport limitation and the need for high amounts of ILs (Zhang et al 2009a;Vangeli et al 2010). The resulting material is usually referred as supported ionic liquid phase (SILP).…”

“…They can be grouped into three classes based on their pore diameter: microporous, d < 2.0 nm; mesoporous, 2.0 < d < 50 nm; and macroporous, d > 50 nm (Zhang et al 2009b). A diversity of support materials were selected of different chemical nature among those commonly used in SILP preparation, like TiO 2 (Chen et al 2007), SiO 2 (Zhang et al 2009a;Vangeli et al 2010;Fang et al 2010;Mehnert 2005) or AC (Virtanen et al 2009a;Kohler et al 2010) among others (Riisager et al 2006b). In an earlier work, Palomar et al (2009) showed the possibility of following the amount of imidazolium-based IL on AC supports by measuring the percentage of elemental nitrogen in SILP from EA and it has been herein extended to several supports and SILP systems in order to generalize the use of this technique for direct quantification of IL loaded on SILP materials.…”

“…Structural properties of highly porous supports and SILP materials were analyzed, using adsorption-desorption isotherms of N 2 at 77 K and mercury porosimetry. Both techniques give information on the available volume of micro, meso and macropores available and have been used in previous works showing the difference between a fresh support and a SILP material (Vangeli et al 2010;Zhang et al 2009b;Chen et al 2007;Shi and Deng 2005). Thus, a systematic study on the variation of the porous structure has been carried out with the aim of better understanding how the IL is distributed onto the supports.…”

“…In addition, results on the thermal stability of SILP materials at high temperature by means of TGA and DSC analysis are collected herein together for the first time. To the best of our knowledge, only thermal studies of ILs have been so far reported (Fernandez et al 2007;Holbrey et al 2002;Karna et al 2009), but without analyzing the relationship between IL and support, except two recent works where a thermal study of SILP materials has been reported (Vangeli et al 2010;Kohler et al 2010). Some of the works published in the literature (Zhang et al 2009a;Vangeli et al 2010;Kohler et al 2010;Muldoon 2010;Kim et al 2002) showed SILP applications at high temperatures based on different supports.…”

Phosphotungstate-Based Ionic Silica Nanoparticles Network for Alkenes Epoxidation

“…Inorganic-organic hybrid [1][2][3][4][5][6][7][8] materials have attracted considerable interest as the combination of the features of different parts can generate high performance. As the development of nanoscale materials, SiO 2 has indeed stimulated remarkable scientific interest because of its excellent performance and promising applications in scientific and technological fields.…”