Introduction of Ionic liquid–silica Xerogels for CO2 capture

“Incorporating ILs in solid matrices emerged as a new and challenging field, allowing the use of heterogeneous systems for CO2 capture [22,23,24,25]. The immobilization process is usually carried out by cationic or anionic incorporation, or by cationic anchorage, or through in-situ polymerization via a sol–gel process [26,27]. In the case of cationic or anionic incorporation, the IL is synthesized and then fixed into the inorganic material through an IL cation or anion anchorage. In the case of the in situ polymerization via sol–gel process, the IL is used as a templating agent for the synthesis of silica xerogels due to its surfactant character and the low surface tension which enables control of the silica particle size, gelation time, structures, and morphology [28,29].

In the classical sol–gel process, an alkoxide precursor is hydrolyzed leading to the formation of Si-OH bonds (silanol groups) that condensate forming an inorganic three-dimensional network. The targeted material is then obtained after drying—xerogel or aerogel [30].
The IL acts like a solvent, catalyst or template in in-situ polymerization, bringing with it the advantage of being easier to handle and having wider applicability in various fields. ILs improve the efficiency and selectivity and allow easy separation of the products and catalyst recycle, especially in catalysis [31,32].”

“With the immobilization of ILs in gels leading to ionogels, these materials are applied in electrochemistry devices to encapsulate functional molecules (catalysts, sensing molecules, fluorescent metal complexes) offering a new method to functionalize nanostructured inorganic materials [33,34]. The xerogel silica final structure is highly influenced by the nature of the alkyl-alkoxysilanes precursor such as the chain length, functionality, and its hydrophobic character. These properties can be modified adding a wide range of compounds to the alkyl-alkoxysilanes precursor, such as surfactants like ionic liquids [35].

The combination of the well-known affinity between pure ILs and CO2 [9,11,16,17,18,19] and solid adsorbents leads to a synergic effect reported by Aquino et al. [25]. Therefore, this study aims to synthesize hybrid silica–IL xerogels via a sol–gel process using different alkyl-alkoxysilanes compounds: the tetramethoxysilane (TMOS), the methyltrimetoxysilane (MTMS), and a based 1-methyl-3-(3-trimethoxysylilpropyl) imidazolium IL associated to the anion Cl or Tf2N incorporated in the range 1–40% IL/ alkyl-alkoxysilanes v/v. During the synthesis, TMOS and MTMS are hydrolyzed leading to the formation of silanols groups. Part of these silanol groups condensate producing a three-dimensional network and the remaining silanol groups will react with the IL ensuring its participation in the structure building.”

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