Functionalized Silicas for Metal‐Free and Metal‐Based Catalytic Applications: A Review in Perspective of Green Chemistry

Abstract

Heterogeneous catalysis plays a key role in promoting green chemistry through many routes. The functionalizable reactive silanols highlight silica as a beguiling support for the preparation of heterogeneous catalysts. Metal active sites anchored on functionalized silica (FS) usually demonstrate the better dispersion and stability due to their firm chemical interaction with FSs. Having certain functional groups in structure, FSs can act as the useful catalysts for few organic reactions even without the need of metal active sites which are termed as the covetous reusable organocatalysts. Magnetic FSs have laid the platform where the effortless recovery of catalysts is realized just using an external magnet, resulting in the simplified reaction procedure. Using FSs of multiple functional groups, we can envisage the shortened reaction pathway and, reduced chemical uses and chemical wastes. Unstable bio‐molecules like enzymes have been stabilized when they get chemically anchored on FSs. The resultant solid bio‐catalysts exhibited very good reusability in many catalytic reactions. Getting provoked from the green chemistry aspects and benefits of FS‐based catalysts, we confer the recent literature and progress focusing on the significance of FSs in heterogeneous catalysis. This review covers the preparative methods, types and catalytic applications of FSs. A special emphasis is given to the metal‐free FS catalysts, multiple FS‐based catalysts and magnetic FSs. Through this review, we presume that the contribution of FSs to green chemistry can be well understood. The future perspective of FSs and the improvements still required for implementing FS‐based catalysts in practical applications have been narrated at the end of this review.

Heterogeneous Catalysis with the Participation of Ionic Liquids

“…However, there are considerably more ionic liquid-containing heterogeneous systems that were not presented here, because of the article volume limitations. For instance, we left out nanocatalysis (in a broad sense) with the participation of dispersed metal nanoparticles stabilized by ionic liquids [133,134], ionosilicas [135,136], and poly(ionic liquids) [107,137], which can be modifying agents for polymers, copolymers, as well as play the role of catalyst in polymerization processes. Moreover, they can take part in metal-organic frameworks (MOFs) that were only very superficially mentioned when presenting porous ionic liquids [127,138], although they certainly deserve more attention.…”

Recent Advances in Heterogeneous Catalytic Systems Containing Metal Ions for Phosphate Ester Hydrolysis

“…Although there are few reports of the use of catalysts supported on silica for the hydrolysis of phosphate esters (see below), the immobilisation of organic compounds on silicon matrices is used frequently, since this approach affords good chemical stability and easy surface functionalisation. Examples include the immobilisation of dyes, [51–55] biomolecules, [56–59] and catalysts [60–63] . Other uses of organo‐functionalised silicon matrices include the coating of different nanomaterials, such as magnetic nanoparticles, [64–67] metallic nanoparticles, [68–70] and quantum dots [71] …”

“…The immobilisation of coordination compounds on silicon‐based materials through covalent bonds, rather than electrostatic interactions, requires the use of silylating agents. In a routine protocol for the synthesis of inorganic–organic hybrid compounds based on condensed silicates, it is necessary to consider the insertion of functional groups, such as (3‐glycidyloxypropyl)trimethoxysilane (GPTMS), (3‐aminopropyl)trimethoxysilane (APTES), and N 1 ‐(3‐trimethoxysilylpropyl)diethylenetriamine (TMSP), that can bind to organoalkoxysilane reagents in such a manner as not to disrupt the metal coordination sphere of the potential catalytic moiety [62] . The solid catalysts can be obtained by self‐condensation of the silylating agents bound to the ligands or by co‐condensation with tetraethyl orthosilicate (silica gel monomer) [62] …”

“…Examples include the immobilisation of dyes, [51][52][53][54][55] biomolecules, [56][57][58][59] and catalysts. [60][61][62][63] Other uses of organo-functionalised silicon matrices include the coating of differentn anomaterials, such as magnetic nanoparticles, [64][65][66][67] metallic nanoparticles, [68][69][70] andq uantum dots. [71] Silica is av ery versatile materialt hat can be prepared with good controlo fp articles ize and morphology.F or example, silica nanospheresc an be synthesised by means of microemulsions [31,51,52] or by modified synthesis of Stçber sol-gels.…”

“…In a routine protocol for the synthesis of inorganic-organic hybrid compounds based on condensed silicates,i ti sn ecessary to consider the insertion of functional groups, such as (3-glycidyloxypropyl)trimethoxysilane (GPTMS), (3-aminopropyl)trimethoxysilane (APTES), and N 1 -(3-trimethoxysilylpropyl)diethy-lenetriamine (TMSP), that can bind to organoalkoxysilane reagentsi ns uch am anner as not to disrupt the metal coordination sphere of the potential catalytic moiety. [62] The solid catalysts can be obtained by self-condensation of the silylating agents bound to the ligands or by co-condensation with tetraethyl orthosilicate (silica gel monomer). [62] Piovezan et al [60,73] and Muxel et al [74] synthesised compounds 8-10 ( Figure 3) by using the same strategy,i nvolving, firstly,t he synthesis of coordination compounds and then the reactionw ith the silylatinga gents.…”