**2. Supported ionic liquid phase (SILP)**

In view of recycling and reusing the transition metal catalytic materials, the technology of immobilization or heterogenization was exploited, and the notable methodologies include polymer supports, macrocellular silica-based foams, magnetic Fe3O4, carbon nanotubes (CNT), multiwalled carbon nanotubes (MWCNT), mesoporous solids, functionalized magnetic Fe3O4, metal organic framework (MOF), monolithic ionogels, and SILP. Among them, the SILP technology is the most recent one; there are debates that the use of ILs is not cost-effective; however, the whole SILP system is reusable and recyclable and hence the cost can be abruptly reduced; the technology ranks to be green in nature since there is no escape of volatile organic compounds (VOCs). Metal in the form of oxides, ions, metal complexes coordinated to the cation or anion, or as nanoparticles was supported in an IL matrix through a thin film. In the SILP system, IL is a part of the support material; accordingly, the bulk properties of IL such as solvation strength, conductivity, viscosity and density are altered. ILs with hydrophobic in nature can offer excellent surface support. ILs can also act as nanoparticles stabilizers. Even though the stabilization mechanism has not been proved, ILs seem to create an electrostatic and steric barrier between nanoparticles; a stabilization type Derjaguin-Landau-Verwey-Overbeek (DLVO) has been proposed due to their polymeric structure.

### **2.1 SILP in macrocellular silica**

The preparation of macrocellular silica is quite simple; the transition metal compounds of interest is suspended in a solution employing an ionic liquid in an volatile organic solvent. Upon stripping the volatile organic compound, the free-flowing immobilized catalytic material can be obtained where the transition metals are trapped within the SILP system. The materials thus trapped within the SILPs were found to be useful in promoting the organic functional group transformations catalyzed by transition and/or noble metals. Mainly the aryl-X activation (Mizoroki-Heck reaction) was carried out using palladium containing SILPs (in imidazolium ILs). With low palladium loading, the Heck-type coupling reaction was performed, and the reaction of iodobenzene with cylohexyl acrylate yielded the corresponding cinnamates with high turnover numbers (TON) and turnover frequency (TOF) [8]. Palladium acetate Pd(OAc)2 supported on amorphous silica immobilized in [bmim] PF6 (1-butyl-3-methylimidazolium hexafluorophosphate) was reported to be highly efficient towards the promotion of the Heck-type coupling reaction without any additional ligands, gave the trans-cinnamates in preparative yields [9], and the TON and TOF reached 68,400 and 8000 h<sup>−</sup><sup>1</sup> , respectively.

## **2.2 Heterogeneous ionic liquid (HIL)-encapsulated IL on silica**

A variation for the synthesis of heterogeneous IL-based palladium-containing catalytic system was developed by Shi et al. [10], through a one pot synthesis of heterogeneous ionic liquid encapsulated on silica involving sol-gel method. These catalysts provided excellent activity for the carbonylation of aniline and nitrobenzene to synthesize diphenyl urea in (Pd or Rh)-EMImBF4/silica gel (1-ethyl-3-methylimidazolium tetrafluoroborate) or (Pd or Rh)-DMImBF4/silica gel (0.1 g, 50–60 mesh), (1-decyl-3-methylimidazolium tetrafluoroborate), or DMImBF4 ionic liquid (2.5 g) containing Rh complex (1 mg) or 0.11 wt%Rh 35 wt% DMImBF4/SiO2 (0.1 g, for the purpose of comparison). The selectivity, yield, and turnover number (TON) were excellent. In the same publication, the identical catalytic system was

**129**

*On the Technology of Heterogenization of Transition Metal Catalysts towards the Synthetic…*

also oxime transfer between cyclohexanone oxime and acetone.

**2.3 Palladacycles on macroporous aluminosilicate**

reuse showed increase in homocoupling [14].

*moieties (CCPIL)*

was air and moisture stable [15].

proved to be excellent for the carbonylation of aniline to synthesize carbamates and

Yokoyama et al. [11] reported that the Mizoroki-Heck reaction of iodobenzene and 4-methyl-iodobenzene with olefins using silica-supported palladium complex catalysts was fruitful in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), exhibiting higher activities than in DMF (N,Ndimethylformamide) in addition to easy product isolation and catalyst recycling. Silica-supported Pd catalysts were synthesized in 1-butylimidazolium hexafluorophosphate with low palladium loadings (0.35 and 0.08%); fascinatingly, both samples thus synthesized proved to be effective catalytic system for Heck-type coupling reactions between bromo- and chloroarenes with ethylacrylate. They also reported that the presence of electron withdrawing groups attached to the aromatic moiety increases the conversion and selectivity towards trans-selectivity. They could recycle the catalyst but pointed out that there is some loss of activity of the catalytic material upon recycling; the hot filtration measurements proved the leaching of Palladium [12]. A multiple-layered SILP system was developed for a hydroformylation process and found to be effective in the hydroformylation

In a variation, oxime Pd cycle was derivatized to increase its ionophilicity by a tethered imidazolium group; the resulting IL forms an imidazolium complex which is supported on high surface area Al/MCM-41, aluminosilicate (1-butyl-3-methylimidazolium bromide is tethered onto oxime Pd cycle which is then heterogenized on Al/MCM-41). This is a solid active catalyst for the Mizoroki-Heck and Suzuki coupling. Results are moderate to good yields. Ionophilicity of the oxime Pd cycle can be significantly increased, limited by the need for basic conditions. Catalytic

*2.3.1 Palladium on clay composite carrying phosphinite-functionalized ionic liquid* 

A clay composite comprising of phosphinite-functionalized ionic liquid moieties was synthesized, in which palladium nanoparticles were uniformly ligated, and this system is called as clay carrying phosphinite-functionalized ionic liquid (CCPIL) moieties or systems. The preparation involves the synthesis of an epoxy functionalized clay, which is reacted with 1-methyl imidazole and chlorodiphenyl phosphine yielding the required CCPIL. The CCPIL material thus obtained was separated by simple filtration to which later palladium was complexed. This novel catalytic material combines the advantages of homogeneous IL phase, phosphorylated ligands, and a new heterogeneous clay support for Pd catalyzed C-C bond formation in green media. The catalytic material is pictorially represented as **Figure 1**.

The heterogeneous catalyst was successfully exploited for the Suzuki coupling of aryl iodides in aqueous media to give biaryls in high yields. Bromide and chloride substrates too gave the biaryls but required longer reaction times. Recycling was repeated for five runs with little leaching observed where each run required the same reaction times. These catalysts were also used for solvent-free MH reaction. Further, copper-free Sonogashira-Hagihara reaction of aryl iodides, bromides, and chlorides with phenylacetylene was also catalyzed by these Pd on CCPIL. Recycling was also possible with little depletion, and the added advantage is that the catalyst

*DOI: http://dx.doi.org/10.5772/intechopen.86846*

reactions [13].

## *On the Technology of Heterogenization of Transition Metal Catalysts towards the Synthetic… DOI: http://dx.doi.org/10.5772/intechopen.86846*

proved to be excellent for the carbonylation of aniline to synthesize carbamates and also oxime transfer between cyclohexanone oxime and acetone.

Yokoyama et al. [11] reported that the Mizoroki-Heck reaction of iodobenzene and 4-methyl-iodobenzene with olefins using silica-supported palladium complex catalysts was fruitful in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), exhibiting higher activities than in DMF (N,Ndimethylformamide) in addition to easy product isolation and catalyst recycling. Silica-supported Pd catalysts were synthesized in 1-butylimidazolium hexafluorophosphate with low palladium loadings (0.35 and 0.08%); fascinatingly, both samples thus synthesized proved to be effective catalytic system for Heck-type coupling reactions between bromo- and chloroarenes with ethylacrylate. They also reported that the presence of electron withdrawing groups attached to the aromatic moiety increases the conversion and selectivity towards trans-selectivity. They could recycle the catalyst but pointed out that there is some loss of activity of the catalytic material upon recycling; the hot filtration measurements proved the leaching of Palladium [12]. A multiple-layered SILP system was developed for a hydroformylation process and found to be effective in the hydroformylation reactions [13].
