Catalytic Applications Of Porphyrins And Phthalocyanines

Introduction

Porphyrins are a class of macromolecular heterocyclic compounds formed by the interconnection of the α-carbon atoms of four pyrrole subunits through a methine bridge (=CH-). The molecular structure is shown in the figure. Phthalocyanine is a compound with a large conjugated system of 18 electrons, and its structure is very similar to porphyrins, which are widely found in nature. Porphyrins and phthalocyanines are structurally related macrocyclic compounds with highly conjugated π-electron systems that exhibit strong absorption and extraordinary thermal and chemical stability in the visible region. Meanwhile, their photophysical properties and electronegativity can be tuned by changing the metallic central or peripheral substituents. A series of metalloporphyrins and metallophthalocyanines showed increased catalytic activity in different chemical/photochemical processes.

Metalloporphyrin Catalyst

The metalloporphyrin homogeneous catalytic system catalyzes the oxidation of hydrocarbons by molecular oxygen with high selectivity to the corresponding aldehydes/ketones and alcohols. Metalloporphyrin catalysts have good catalytic activation ability for different oxygen donors such as PhIO, peroxide, NaClO, etc., especially for the selective oxidation of saturated hydrocarbons, which reflects the incomparable superiority of other catalysts.

Metal Phthalocyanine Catalyst

Metal phthalocyanine catalysts are mainly used in aerobic reactions, reactions involving hydrogen and decomposition reactions. Among the various reactions catalyzed by metal phthalocyanines, the research on oxidation reaction is the main one. Since Calvin et al. first used phthalocyanine and copper phthalocyanine as catalysts to catalyze the activation of hydrogen molecules and hydrogen exchange reactions in 1936, many types of metal phthalocyanines have been synthesized so far. They were prepared as homogeneous, heterogeneous and enzyme-mimicking catalysts for different types of organocatalytic reactions.

Typical Applications

Air Oxidation of Cyclohexane

Cyclohexane oxidation can give a mixture of cyclohexanol and cyclohexanone. Cyclohexane oxidation technology has greatly promoted the rapid development of polyamide fiber production. Studies have shown that Fe, Mn, Co and other monometalloporphyrins have a high conversion rate for the oxidation of cyclohexane.

Catalytic Oxidation of Cumene

The α-position hydrogen atom in the cumene molecule has high chemical reactivity, and can be oxidized by air (oxygen) to generate cumene hydrogen peroxide (CHP) under certain conditions. Cumene hydroperoxide has been widely used in other fields such as organic synthesis, production of polymer materials and fine chemicals, in addition to the synthesis of phenol. Studies have shown that copper phthalocyanine, cobalt phthalocyanine and iron phthalocyanine have good catalytic effects on the oxidation of cumene.

Catalytic Oxidative Desulfurization

The highly planarity of metallophthalocyanines enables catalytic reactions to take place in the axial position of this plane, especially the highly selective axial coordination of metallophthalocyanines to organosulfides. Therefore, metal phthalocyanines can be used in fuel oil desulfurization and catalytic oxidation of thiophene sulfides.

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