Synthesis of iron chlorophyllins and their catalytic performance for aerobic oxidation of cyclohexene

Abstract

A series of iron(III) chlorophyllins was prepared from silkworm excrement crude chlorophyll extract as a raw material. Aerobic oxidation of cyclohexene by using the prepared iron(III) chlorophyllins as biomimetic catalysts was studied under atmospheric pressure in the absence of reducing agent and solvent. The results indicate that chlorophyll iron porphyrins have better catalytic performance than the industrial-applied iron tetraphenylporphyrin and cobalt tetraphenylporphyrin, and possess a higher selectivity for 2-cyclohexen-1-one. The smaller the polarity of iron(III) chlorophyllin’s ligand is, the easier the catalytic oxidation of cyclohexene will be. Esterification products of iron(III) chlorophyllins can catalyze the oxidation of cyclohexene better than non-esterified iron(III) chlorophyllins, and therefore show a higher conversion of cyclohexene and a higher selectivity for 2-cyclohexen-1-one than the non-esterified ones. Among the six synthesized iron(III) chlorophyllin catalysts, iron(III) methyl-pyropheophorbide-a is the best biomimetic catalyst for the highest conversion of cyclohexene. The influences of catalyst’s substituent, polarity and ring structure on the catalytic performance were discussed. The catalytic performance of iron(III) chlorophyllins improves with decreasing polarity, increasing conjugated degree of porphyrin’s ring or enhancing chlorophyllins’ stability. Possible mechanism of cyclohexene aerobic oxidation catalyzed by iron(III) chlorophyllins was also discussed.