Porphyrins catalysts are a class of macromolecular heterocyclic compounds formed by the interconnection of the alpha-carbon atoms of four pyrrole subunits through a methine bridge (=CH-). The parent compound is porphin (C20H14N4), and the porphin having a substituent is called porphyrin. The porphyrin ring has 26 π electrons and is a highly conjugated system. Many porphyrins exist in nature in the form of complexes with metal ions, such as chlorophyll containing chlorine and magnesium coordination structures and heme coordinating with iron. The porphyrin or the modified porphyrin can be coordinated with a metal such as iron, cobalt or aluminum to catalyze the copolymerization of carbon dioxide and the epoxy compound by the cocatalyst. When the porphyrin accumulation in the human body is excessive, it will cause porphyria, also known as porphyria.
Figure 1. Porphyrin
Applications
- Oxidation catalysts：The catalytic system of metalloporphyrin supramolecular compounds and iodide benzene (PhIO) shows high catalytic activity and stability in the epoxidation of styrene. Using Mn(TDCPP)CI as a catalyst and sodium hypochlorite as an oxidant, cyclohexane can be effectively oxidized to form cyclohexanol and cyclohexanone.
- Hydroxylation catalysts: Under the catalysis of stable metalloporphyrin-sodium hypochlorite, cyclohexane is hydroxylazed to cyclohexanol and trace cyclohexanone at normal temperature and pressure.
- Photocatalysts: Porphyrins with the conjugated double bond ring can transfer electrons under appropriate conditions or stimulate electrons by light. It has been reported that macromolecular metalloporphyrins have high photosensitivity, and good photocatalytic degradation efficiency under sunlight irradiation. It can completely degrade mixed dyes, and can also be used for catalytic degradation of various wastewaters, such as dye wastewater, chemical wastewater, and domestic sewage.
- Electrocatalysts: The metalloporphyrin compound has a high conjugated structure and chemical stability, and has good electrocatalytic reduction activity for molecular oxygen under both acidic and basic conditions. Moreover, it avoids the positive electrode potential loss caused by the methanol permeated through the negative electrode in the direct methanol fuel cell (DMFC), which makes it the most promising catalyst material for the air electrode. It is used to replace precious metals as oxygen reduction catalysts for fuel cells and metal-air batteries.
- Supramolecular catalysts: The special structure of cyclodextrin makes it hydrophobic in the inner cavity and hydrophilic in the outer cavity. Certain properties of the porphyrin, such as water solubility, can be improved by the effective attachment of the cyclodextrin to the metalloporphyrin. The polymerization of metalloporphyrin molecules is prevented, and the porphyrin ring and its aromatic substituents are protected from other reactive groups during catalytic oxidation. After the cyclodextrin encapsulates the metalloporphyrin, the metalloporphyrin is provided with a hydrophobic environment, which hinders the formation of micelles by the metalloporphyrin, further improving the stability and catalytic activity of the supramolecular system and bringing the metalloporphyrin to an appropriate environment facilitates the catalytic reaction.
Classification
Generally, porphyrin catalysts are classified into two types, the fat-soluble porphyrin catalysts, and water-soluble porphyrin catalyst. The porphyrin catalyst usually has a melting point of more than 300 degrees, and is a purple-red solid powder or a crystalline solid having a certain photosensitive property, and can effectively release singlet oxygen under the action of ultraviolet or visible light.
- Fat-soluble porphyrin catalysts: It is usually soluble in organic solvents, such as chloroform, dichloromethane, ethyl acetate, benzene, and other fat-soluble solvents. Solubility in petroleum ether, n-hexane is very small. For example, the phenyl substituted metalloporphyrin catalyst is a fat- soluble porphyrins catalyst.
- Water-soluble porphyrin catalyst: It is usually soluble in water, methanol, ethanol, acetone, acetonitrile, and other hydrophilic organic solvents. For example, sulfonate porphyrin catalyst is a fat- soluble porphyrins catalyst.
Figure 2. Rosman method for synthesizing porphyrin
References
- P. Rothemund. (1936). “A New Porphyrin Synthesis. The Synthesis of Porphin.” J. Am. Chem. Soc. 58 (4): 625-627.
- P. Rothemund. (1935). “Formation of Porphyrins from Pyrrole and Aldehydes.” J. Am. Chem. Soc. 57 (10): 2010-2011
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