Iron porphyrins immobilised on silica surface and encapsulated in silica matrix: a comparison of their catalytic activity in hydrocarbon oxidation

Bringing Homogeneous Iron Catalysts on the Heterogeneous Side: Solutions for Immobilization

Noble metal catalysts currently dominate the landscape of chemical synthesis, but cheaper and less toxic derivatives are recently emerging as more sustainable solutions. Iron is among the possible alternative metals due to its biocompatibility and exceptional versatility. Nowadays, iron catalysts work essentially in homogeneous conditions, while heterogeneous catalysts would be better performing and more desirable systems for a broad industrial application. In this review, approaches for heterogenization of iron catalysts reported in the literature within the last two decades are summarized, and utility and critical points are discussed. The immobilization on silica of bis(arylimine)pyridyl iron complexes, good catalysts in the polymerization of olefins, is the first useful heterogeneous strategy described. Microporous molecular sieves also proved to be good iron catalyst carriers, able to provide confined geometries where olefin polymerization can occur. Same immobilizing supports (e.g., MCM-41 and MCM-48) are suitable for anchoring iron-based catalysts for styrene, cyclohexene and cyclohexane oxidation. Another excellent example is the anchoring to a Merrifield resin of an FeII-anthranilic acid complex, active in the catalytic reaction of urea with alcohols and amines for the synthesis of carbamates and N-substituted ureas, respectively. A SILP (Supported Ionic Liquid Phase) catalytic system has been successfully employed for the heterogenization of a chemoselective iron catalyst active in aldehyde hydrogenation. Finally, FeIII ions supported on polyvinylpyridine grafted chitosan made a useful heterogeneous catalytic system for C-H bond activation.

Catalytic oxidation of primary and secondary alcohols over a novel TCPP/Zn–Fe2O4@ZnO catalyst

The catalytic activity of TCPP/Zn–Fe₂O₄@ZnO as a recyclable and high efficiency heterogeneous catalyst for oxidation of primary and secondary alcohols using H₂O₂as a green oxidant under reflux conditions in CH₃CN was investigated.

Anchorage of a ruthenium complex into modified MOF: synergistic effects for selective oxidation of aromatic and heteroaromatic compounds

The structure of IRMOF-3 was modified with pyridine-2-aldehyde and then the Schiff base moieties were used to anchor ruthenium complex to this metal–organic framework.

Graphene-supported hemin as a highly active biomimetic oxidation catalyst

Well supported: stable hemin-graphene conjugates formed by immobilization of monomeric hemin on graphene, showed excellent catalytic activity, more than 10 times better than that of the recently developed hemin-hydrogel system and 100 times better than that of unsupported hemin. The catalysts also showed excellent binding affinities and catalytic efficiencies approaching that of natural enzymes.

Catalysts for heterogeneous oxidation reaction based on metalloporphyrins immobilized on kaolinite modified with triethanolamine

Raw kaolinite was modified with triethanolamine (TEA), in an attempt to create a new support for the immobilization of metalloporphyrins. Anionic metalloporphyrins containing Fe(3+) or Mn(3+) as metallic centers were immobilized on the prepared support, and the obtained solids were characterized by Fourier-transform infrared (FTIR) spectroscopy, X-ray powder diffraction (XRPD), thermal analysis (thermogravimetric and differential thermal analyses--TGA/DTA), and scanning electron microscopy (SEM). The solids were used in heterogeneous oxidation catalysis of cyclooctene and cyclohexane. The yields from the oxidation of cyclooctene depended on the amount of TEA and/or water present in the solids. Good reaction yields were obtained for the oxidation of cyclohexane, with selectivity for the alcohol. In one specific case, a possible co-catalytic activity was verified for TEA during the oxidation of cyclohexane.

Manganese(III) Porphyrin Covalently Bound to sol-gel Derived Silica (Mn(III) Porphyrinosilica): A Reusable and Green Heterogeneous Photocatalyst for Oxidative Decarboxylation of α-Arylacetic Acids with H2O2

Manganese(III) (5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin)chloride, MnIII(TDCPP)Cl, was chemically (covalently) bound to a silica matrix by a sol-gel method and used as a new reusable heterogeneous photocatalyt for the selective and efficient oxidative decarboxylation of α-arylacetic acids with H2O2 as a mild and clean oxidant at room temperature. The activity of this heterogeneous photocatalytic system is higher than that of a corresponding homogeneous systems and the catalyst can be reused several times without loss of its activity and selectivity.

Realizing metalloporphyrin functionalization of 4-vinylpyridine copolymer via axial coordination reaction

Cobalt tetra(para-chlorophenyl)porphyrin ( CoTCPP ) and zinc tetraphenyl porphyrin ( ZnTPP ) were linked on the side chains of the copolymer of 4-vinylpyridine (4VP) and styrene (St), P(4VP-co-St), via axial coordination reactions, respectively, and the metalloporphyrin-functionalized macromolecules, CoTCPP -P(4VP-co-St) and ZnTPP -P(4VP-co-St), were prepared. Their chemical structures were characterized by FTIR and 1H NMR. The spectral properties of the two macromolecular axial coordination complexes were mainly studied, and their photophysical behavior were discussed in depth. The experimental results show that the metalloporphyrin-functionalized macromolecules, CoTCPP -P(4VP-co-St) and ZnTPP -P(4VP-co-St), can be prepared favorably through axial coordination reaction with the side pyridine groups of the copolymer P(4VP-co-St) as ligands. The two complexes have characteristic spectra similar to that of the small molecular metalloporphyrins, CoTCPP and ZnTPP , respectively. At the same time, they also display the characteristic spectroscopic property of axial coordination complexes: the electronic adsorption spectra of CoTCPP -P(4VP-co-St) and ZnTPP -P(4VP-co-St) red-shifted obviously as compared to that of CoTCPP and ZnTPP , and the fluorescence emission of ZnTPP -P(4VP-co-St) blue-shifted apparently with respect to that of ZnTPP . For CoTCPP -P(4VP-co-St) and ZnTPP -P(4VP-co-St), some polymer effects were found: (1) the bonding degree of the small molecular metalloporphyrin, CoTCPP or ZnTPP , on the side chains of the copolymer P(4VP-co-St) has a limit value because of the steric hindrance and there is a bonding degree difference between the actual value and the theoretical value; (2) for ZnTPP -P(4VP-co-St), slight energy transfer between adjacent ZnTPP units on an identical macromolecule occurs, leading to slight static quench of the fluorescence emission as the bonding density of ZnTPP units on the side chains of the copolymer P(4VP-co-St) reaches a certain value.

Phase-transfer of porphyrins by polypeptide-containing hyperbranched polymers and a novel iron(iii) porphyrin biomimetic catalyst

Water-soluble porphyrins can be phase transferred by the hyperbranched multiarm copolymer PEI-PZLys; good catalytic activities and recyclabilities were observed for oxidation catalyzed by the encapsulated porphyrins.

Spectroscopic investigation of the encapsulation and the reactivity towards NO of a Co(II)-porphyrin inside a cross-linked polymeric matrix

A highly dispersed cobalt(II) porphyrin (PhCo(II)) encapsulated inside a highly cross-linked poly(4-ethylstyrene-co-divinylbenzene) matrix (St-DVB) is investigated by means of FTIR, Raman, UV-Vis and EPR spectroscopies. The adoption of the highly porous St-DVB support is the key factor in reaching an optimum porphyrin dispersion in undiluted conditions ( approximately 10 wt% of porphyrin). It is demonstrated that the encapsulated PhCo(II) are characterized by the absence of any axial interactions involving the polymeric matrix, that can be considered a "non-coordinating" solvent. Finally, the permanent porosity of the encapsulating matrix allows gaseous molecules to reach the Co(II) cations also in the absence of swellable solvents. In particular, the reactivity of the isolated Co(II) porphyrin species towards nitric oxide (NO) is investigated, with possible implications in the understanding of the crucial role played by NO in biological systems.

Comparison of activity of immobilised and soluble metalloporphyrins by NO release of oximes by chemiluminescence

The ongoing interest in very efficient systems for the imitation of cytochrome P-450-dependent monooxygenase reactions, consisting of metalloporphyrin and oxygen donor, prompted us to develop a method to compare the catalytic activity of soluble metalloporphyrins with those which have been immobilised on different silica surfaces. In this article, we describe a method which allows direct comparison between the catalytic activities of soluble and immobilised metalloporphyrins by measuring the NO release of cyclohexanone oxime. We observed that all metalloporphyrins are able to enhance the NO release of cyclohexanone oxime in the presence of iodosobenzene as oxygen donor. In case of cationic metalloporphyrins, no difference in the catalytic activity was determined. In contrast, for perfluorinated metalloporphyrins we ascertained a major loss of activity after immobilisation. These results led us to the assumption that it is not easy to compare quantitative results of NO releases if soluble and immobilised metalloporphyrins were used.

Electrochemical olefin epoxidation employing an iron(IV)porphyrin/trisbipyridineruthenium(III) polymer catalyst

A porphyrin molecule containing four meso-appended 2,2'-bipyridyl ligands has been prepared. Each bipyridine is attached to the porphyrin core at the 4-position (pseudo-para to one of the pyridine nitrogens). Subsequently, each of the four bipyridines was complexed with a RuL2(2+) moiety and iron coordinated in the porphyrin core. When L = 4-vinyl-4'-methyl-2,2'-bipyridine, reduction of the ruthenium centers resulted in the formation of robust electroactive polymer films which deposited on the electrode surface. In the presence of aqueous acid, these films electrocatalytically epoxidize cyclohexene at positive potentials from the formal iron(IV) oxidation state. Although product analysis has only been conducted for cyclohexene, the catalytic activity extends to a large variety of olefins including ethylene and propylene.