# Ionic Liquids Hybridization for Carbon Dioxide Capture: A Review
## 4 Application of ILs Hybrid Material in Carbon Dioxide
### ILs/Mesoporous Silica  
Due to its exceptional features, namely, highly ordered pore structure, large surface area and uniform pore size, mesoporous silica had become a center of attention in research field since late 1970s. Generally, mesoporous silica such as SBA-16, MCM-48 and MCM-41 are greatly favored for applications such as gas separation due to high BET surface area and pore sizes. Furthermore, the presence of silanol groups that act as adsorption main sites [94] combined with their capability to functionalize with various organic molecules [95] has assisted in elevating the performance of mesoporous silica in CO2 capture. The existence of a hydroxyl group also allows amine functionalization, thus enhancing the CO2 affinity in silica. Several studies related to the functionalization of various mesoporous silica with amine have been conducted with the aim to improve the material ability in CO2 adsorption. [PERSON] and co-workers introduced amine functional groups onto pores of SBA-15 [96]. The modified SBA-15 adsorbed 1.6164 mmol/g of CO2 at 1 bar, which is twice as much as the amount adsorbed by pristine SBA-15 due to high affinity effect of CO2 towards amine functional groups. Despite the promising result, there are still significant limitations which could contribute to the reduction in CO2 capture capacity by amine modified silica. For example, an ambient steam exposure led to degradation of the modified mesoporous silica structure due to amine evaporation, thus reducing the CO2 adsorption performance [95]. Due to the low vapor pressure of ILs, their confinement within the mesoporous silica structure is expected to improve the stability and regenerability of solid adsorbent. [PERSON] and co-workers studied the effect of ILs immobilization with different anions on commercial SBA-15 9S (SBA) and silica obtained from rice husk (SIL) towards CO\({}_{2}\) separation process [97]. Apart from becoming a good alternative to solve ILs viscosity issue, the loss of grafted or immobilized solvent can be avoided due to the low vapor property of ILs. In this work, each silica was immobilized using [BMIM][CI] and [BMIM][OAc] through the wet impregnation method. The immobilized samples were prepared by loading 10 wt% of ILs into dichloromethane. The mixture was added dropwise into dry solid silica. The immobilized silica was dried in oven at 120 \({}^{\circ}\)C for 1 h. SAP analysis indicated the pore radius distribution (R\({}_{\text{p}}\)) of SBA and SIL are 7.20 and 3.88 nm, respectively. While [BMIM][CI] and [BMIM][OAc] impregnation showed insignificant effect towards the R\({}_{\text{p}}\) of SIL, the SBA R\({}_{\text{p}}\) showed noticeable reduction. Analysis on XRD showed a similar pattern between pristine and modified samples, which indicates that the silica structure was still preserved after ILs immobilization. The result in the sorption study showed reduction in CO\({}_{2}\) uptake for all modified silica compared to its pristine material due to reduction in specific surface area and pore volume after immobilization. Among immobilized samples, SBA-OAc and SIL-OAc recorded the highest CO\({}_{2}\) adsorption performance due to excellent CO\({}_{2}\) permeation ability provided by [OAc]\({}^{-}\) anion. The CO\({}_{2}\)/CH\({}_{4}\) selectivity of the immobilized materials, SBA-Cl and SIL-Cl, improved by 1.51 and 1.37 times, respectively, compared to raw silica. In the CO\({}_{2}\)/CH\({}_{4}\) selectivity study, the performance of immobilized materials with Cl-based IL were better than acetate-based IL. This is due to the strong interaction existing between the chloride (Cl)\({}^{-}\) and imidazolium ring which, in turn, provides binding site for CO\({}_{2}\). To summarize, though modified SBA showed better CO\({}_{2}\) selectivity, the high cost of commercial SBA should be taken into consideration. Furthermore, the utilization of SIL coupled with [BMIM][CI] will be the best alternative for development of low-cost adsorbent for CO\({}_{2}\) capture.