Geani Maria Ucoski

Chemical Reactions Catalyzed by Metalloporphyrin-Based Metal-Organic Frameworks

The synthetic versatility and the potential application of metalloporphyrins (MP) in different fields have aroused researchers’ interest in studying these complexes, in an attempt to mimic biological systems such as cytochrome P-450. Over the last 40 years, synthetic MPs have been mainly used as catalysts for homogeneous or heterogeneous chemical reactions. To employ them in heterogeneous catalysis, chemists have prepared new MP-based solids by immobilizing MP onto rigid inorganic supports, a strategy that affords hybrid inorganic-organic materials. More recently, materials obtained by supramolecular assembly processes and containing MPs as building blocks have been applied in a variety of areas, like gas storage, photonic devices, separation, molecular sensing, magnets, and heterogeneous catalysis, among others. These coordination polymers, known as metal-organic frameworks (MOFs), contain organic ligands or complexes connected by metal ions or clusters, which give rise to a 1-, 2- or 3-D network. These kinds of materials presents large surface areas, Brønsted or redox sites, and high porosity, all of which are desirable features in catalysts with potential use in heterogeneous phases. Building MOFs based on MP is a good way to obtain solid catalysts that offer the advantages of bioinspired systems and zeolitic materials. In this mini review, we will adopt a historical approach to present the most relevant MP-based MOFs applicable to catalytic reactions such as oxidation, reduction, insertion of functional groups, and exchange of organic functions.

Preparation of Catalysts based on Iron(III) Porphyrins Heterogenized on Silica obtained by the Sol-Gel Process for Hydroxylation and Epoxidation Reactions

Solid catalysts have been prepared by chemical interaction of iron(III) porphyrins with the surface of the pores of a silica matrix obtained by the sol-gel method. The presence of the complexes in the silica matrix and the morphology of the obtained particles were studied by UV-Vis spectroscopy, powder X-ray diffractometry, infrared spectroscopy, transmission electron microscopy, electron paramagnetic resonance and thermogravimetric analysis. The catalytic activity of the immobilized iron(III) porphyrins in the oxidation of (Z)-cyclooctene, cyclohexene and cyclohexane was evaluated in dichloromethane/acetonitrile 1:1 solvent mixture (v/v) using iodosylbenzene as oxidant. Results were compared with those achieved with the homogeneous counterparts.

New highly brominated Mn-porphyrin: a good catalyst for activation of inert C–H bonds

This work describes the synthesis and characterization of a novel third-generation catalyst 5,10,15,20-tetrakis-(4′-bromine-3′,5′-dimethoxyphenyl)-2,3,7,8,12,13,17,18-octabromoporphyrinatomanganese chloride [MnIIIBr12T3,5DMPP]Cl (Cat.2). The catalytic activity of Cat.2 in cyclohexane, adamantane, and n-hexane oxidation by iodosylbenzene (PhIO) or iodobenzene diacetate (PhI(OAc)2) was compared with the catalytic activity of [MnIIIT3,5DMPP]Cl (Cat.1), a second generation catalyst. The Cat.2/PhI(OAc)2 system led to higher yields of cyclohexane oxidation products (65%) with high selectivity for cyclohexanol (86%) as compared with Cat.1 (19% and 74%, respectively) and addition of water essentially did not alter total product yield. Addition of a small amount of imidazole to the Cat.1/PhIO system gave superior yields of cyclohexane oxidation products (64%) as compared with Cat.2 (52%). In all systems Cat.2 afforded significantly higher yields of 2-adamantanol, a product with great commercial value compared with 1-adamantanol. n-Hexane oxidation gave low total product yield; Cat.2 was more selective for alcohol products (2-hexanol and 3-hexanol).

Selective oxidation catalysts obtained by immobilization of iron(III) porphyrins on thiosalicylic acid-modified Mg-Al layered double hydroxides.

Nitrate-intercalated Mg-Al layered double hydroxides (LDHs) were synthesized and exfoliated in formamide. Reaction of the single layer suspension with thiosalicylic acid under different conditions afforded two types of solids: LDHA1, in which the outer surface was modified with the anion thiosalicylate, and LDHA2, which contained the anion thiosalicylate intercalated between the LDH layers. LDHA1 and LDHA2 were used as supports to immobilize neutral (FeP1 and FeP2) and anionic (FeP3) iron(III) porphyrins. For comparison purposes, the iron(III) porphyrins (FePs) were also immobilized on LDH intercalated with nitrate anions obtained by the co-precipitation method. Chemical modification of LDH facilitated immobilization of the FePs through interaction of the functionalizing groups in LDH with the peripheral substituents on the porphyrin ring. The resulting FePx-LDHAy solids were characterized by X-ray diffraction (powder) and UV-Vis and EPR spectroscopies and were investigated as catalysts in the oxidation of cyclooctene and cyclohexane. The immobilized neutral FePs and their homogeneous counterparts gave similar product yields in the oxidation of cyclooctene, suggesting that immobilization of the FePs on the thiosalicylate-modified LDHs only supported the catalyst species without interfering in the catalytic outcome. On the other hand, in the oxidation of cyclohexane, the thiosalicylate anions on the outer surface of LDHA1 or intercalated between the LDHA2 layers influenced the catalytic activity of FePx-LDHAy, leading to different efficiency and selectivity results. FeP1-LDHA2 performed the best (29.6% alcohol yield) due to changes in the polarity of the surface of the support and the presence of FeP1. Interestingly, FeP1 also performed better in solution as compared to the other FePs. Finally, it was possible to recycle FeP1-LDHA2 at least three times.

Anionic Iron(III) Porphyrin Immobilized on/into Exfoliated Macroporous Layered Double Hydroxides as Catalyst for Oxidation Reactions

The oxidation of organic substrates via catalytic routes is an important class of reactions to produce industrial input materials such as epoxides, alcohols, and ketones. Metalloporphyrins display recognized catalytic activity that mimics oxidation processes in living organisms. Their immobilization in different inert supports allows their recovery and reuse in heterogeneous catalytic processes. Layered double hydroxides (LDHs) are inorganic materials consisting of di- and trivalent metal hydroxides that afford bidimensional positively charged layers. This work reports on the preparation of the solid based on macroporous LDHs (LDHMs) by the co-precipitation method, which involved the use of polystyrene as template, oxides reconstruction, and exfoliation, to furnish LDHME. We also describe the immobilization of an iron(III) porphyrin (FeP) in LDHME and in LDH intercalated with nitrate anions, obtained by the co-precipitation method. Application of the immobilized catalysts in (Z)-cyclooctene and cyclohexane oxidation will help to assess their catalytic activity.

Biomimetic oxidation of cyclic and linear alkanes: high alcohol selectivity promoted by a novel manganese porphyrin catalyst

A novel third-generation metalloporphyrin, chloro-5,10,15,20-tetrakis-(3′-bromine-4′-methoxyphenyl)-2,3,7,8,12,13,17,18-octabromoporphyrinatomanganese(III), [MnIIIBr12T4(-OMe)PPCl], designated as MnBr12Por, was synthesized and characterized. The catalytic activity of the new manganese porphyrin was investigated in the oxidation of cyclohexane, adamantane and n-hexane by iodosylbenzene (PhIO) or iodobenzene diacetate (PhI(OAc)2) in comparison to the catalytic activity of the second-generation catalyst [MnIIIT4(-OMe)PPCl], designated as MnPor. All yields were based on oxidants. The MnBr12Por/PhIO system led to higher yields of cyclohexane oxidation products (45%) with high selectivity for cyclohexanol (80%) as compared to the MnPor/PhIO system (23% and 77%, respectively). Addition of imidazole to MnPor/PhIO increased the total product yield from 23 to 43%; addition of imidazole to MnBr12Por/PhIO did not alter the total product yield at all. For the MnPor/PhI(OAc)2 and MnBr12Por/PhI(OAc)2 systems, addition of imidazole increased the product yields from 19 to 45% and from 35 to 66%, respectively. Addition of water increased the total product yields during cyclohexane and adamantane oxidation for all the studied systems. In all cases, MnBr12Por performed better than MnPor.

Magnetically recyclable nanocatalysts based on magnetite: an environmentally friendly and recyclable catalyst for esterification reactions

Solid magnetic nanoparticles (magnetite = Mag) composed of Fe3O4 and magnetite coated with silica (Fe3O4/SiO2 = Mag/Si) were prepared from inexpensive starting materials. The catalytic activity of the solids was investigated for palmitic acid esterification with methanol under solvothermal conditions. Both pure Fe3O4 (Mag) and silica-coated (Mag/Si) nanoparticles exhibited high catalytic activities and were easy to recover from the reaction environment using an external magnet. Furthermore, the magnetic nanoparticle catalysts were reused without significant loss of catalytic activity and showed high durability in typical acid-catalyzed reactions. XRD and SEM analyses were conducted before and after esterification, showing almost identical particle distribution in both fresh and reused catalysts.