Heterogenized “homogeneous”catalysts

Two basic mechanisms of fixation of a homogeneous catalyst onto a polymeric support have been cited. The first involves different means of catalyst component precipitation, impregnation, inclusion in gel, microencapsulation, etc. The second concerns fixation of catalytic sites by valence, coordination and ionic bonds, immobilization, ion exchange, etc. 

The influence of the structure of polymeric supports on the selectivity and activity of catalysts of platinum group metals supported as high-dispersed powders on polyamide, PAN, PVA, PETP and other polymers was studied during the hydrogenation of unsaturated organic compounds [20] and during hydrogen peroxide decomposition [21, 22]. In some cases an optimum metal/polymer ratio was found [23-26]. 

The effect of the chemical properties of a support on the activity of dispersed zero-valent metals has been shown [27]. Chloromethylated polystyrene, cross-linked with 4-7% DVB with high porosity and surface area (0.1 -0.03 pm particles), was functionalized by the following groups rr-donor and 7r-acceptor groups capable of forming molecular associates with the substrate hydrophobic and hydrophilic groups 

The polymeric supports shown above have the following functional groups n-donor and 7t-acceptor groups (polymers V, VI, and VII) and hydrophobic or hydrophilic groups (polymers I-IV). Polymers with similar functional groups but different matrices constitute a separate group. 

All the experiments were conducted with the same amount of active metal (0.54 mg Pd) at 40 °C and at a H2-partial pressure of SOOmmHg. The molar ratio of Pd to the substrate was 1 2070. It was shown that catalysts, the functional groups of which decreased the retention time of the substrate in the polymer matrix or enhanced the substrate solubility in the polymer matrix, catalyzed the hydrogenation of styrene more effectively. Such catalyst types included Jt-acceptor or hydrophobic supports. During the hydrogenation of allyl acrylate of the polar substrate model, the catalytic activity depended on both the -acceptor and polar properties of the polymeric supports. Thus, a definite relationship was determined between properties of functional groups and the respective polymers. 

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The reaction of iodobenzene with acrylate is a good synthetic method for the cinnamate 17[7]. In the competitive reaction of acrylate with a mixture of 0-and /i-iodoanisoles (18 and 19), the o-methoxycinnamate 20 was obtained selectively owing to the molecular recognition by interlamellar montmorillonite ethylsilyldiphenylphosphine (L) as a heterogenized homogeneous catalyst used as a ligand[28]. 

Increasing efforts to heterogenize homogeneous catalysts for LPO are apparent (2,206—209). Significant advantages in product recovery, catalyst use, and catalyst recovery are recognized. In some instances, however, the active catalyst is reported to be material dissolved from the sotid catalyst (210). 

The situation has now changed and currently an area of considerable research interest is in heterogenizing homogeneous catalysts. One such instance is to be found in the ethylene based manufacture of vinyl acetate (11). A homogeneous catalytic process based on palladium and copper salts was first devised, but corrosion problems were made much less serious in a heterogeneous system based on the same chemical principles. 

Keywords Catalysis, polymer-supported oxidovanadium (IV) complex, Solid supported catalysts, heterogenized homogeneous catalysts... 

DENs can be immobilized on electroactive substrates to prepare heterogenized homogeneous catalysts. Electrochemical grafting of hydroxyl-terminated Pt DENs... 

The use of supported (i.e., heterogenized) homogeneous catalysts offers another possibility for easy catalyst separation. New examples include polymer-anchored Schiff-base complexes of Pd(TT),446 PdCl2(PhCN)2 supported on heterocyclic polyamides,447 various Pd complexes supported on crosslinked polymers 448 sol-gel-encapsulated Rh-quatemary ammonium ion-pair catalysts,449 and zwitterionic Rh(T) catalysts immobilized on silica with hydrogen bonding.450... 

The valuable advantages of heterogenizing homogeneous catalysts within the zeolite porous structure was then envisioned. 

The polysiloxane - SiOz hybrids were tested for their capability for heterogenizing homogeneous catalysts. The platinum(II) complex was obtained by replacement of ligands in the parent metal complex PtCl2(PhCN)2 by Vi groups. 

Classification of Methods for Immobilizing (or Heterogenizing) Homogeneous Catalysts... 

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