Manganese-sulfonato porphyrin adsorbed on amino kaolinite as heterogeneous catalyst for oxidation and polymerization reactions

Due to its medical applications, poly-caprolactone (PCL) is one of the most studied polymers. Herein, adsorption was used as a route to synthesize a new kaolinite/metalloporphyrin-based material for application in caprolactone (ε-CL) polymerization. Kaolinite (Kaol) was modified with tris(hydroxymethyl)aminomethane (TRIS), and the resulting Kaol-TRIS was used as adsorbent for 5,10,15,20-tetrakis(4-sulfonatophenyl)manganese(III) porphyrinate (MnTSPP). Kaol-MnTSPP was also prepared for comparison purposes. Kaolinite grafting with TRIS molecules increased the Kaol basal value from 7.2 to 12.7 Å. MnTSPP immobilization on Kaol-TRIS decreased the intensity and broadened the Kaol-TRIS reflection peaks, evidencing Kaol-TRIS exfoliation. Mathematical equation models of Sips and Langmuir were used to fit the adsorption data, indicating a monolayer adsorption model (maximal adsorption = 8.78 mg·g ⁻¹). Kaol-TRIS-MnTSPP stability and efficiency as a catalyst of cis -cyclooctene (C8) and cyclohexane oxidations were tested. Kaol-TRIS-MnTSPP obtained from 600 mg·L ⁻¹ MnTSPP aqueous solution provided the highest C8 conversion to cyclooctenoxide (81%). MnTSPP did not leach from Kaol-TRIS-MnTSPP, evidencing that TRIS acted as a binder for MnTSPP and enhanced its attachment to the matrix. In contrast, purified Kaol interacted with MnTSPP weakly: MnTSPP completely leached from Kaol-MnTSPP after the first catalytic cycle. Finally, Kaol-TRIS-MnTSPP was used to initiate ε-CL polymerization. Monitoring of the reaction by infrared absorption spectroscopy showed that the bands around 3438 cm ⁻¹, attributed to free hydroxyls in the ε-CL monomer aliphatic structure, became less intense. This indicated that the ε-CL cyclic monomer structure was lost, with subsequent polymerization. This work has proven that the chemical modification of Kaol with subsequent adsorption of a metalloporphyrin is an important route for synthesizing initiator catalysts of polymerization reactions in the synthesis of new materials.