Immobilization of a biomimetic catalyst on clay minerals. Study of the activity of the supported catalysts on the oxidative coupling of humic molecules and precursors.
Nuzzo, Assunta (2011) Immobilization of a biomimetic catalyst on clay minerals. Study of the activity of the supported catalysts on the oxidative coupling of humic molecules and precursors. [Tesi di dottorato] (Inedito)
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Two novel heterogeneous biomimetic catalysts were synthesized by immobilizing a meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrinate of manganese (III) chloride [Mn(TDCPPS)Cl] on both kaolinite (K) and montmorillonite (M) clay minerals, previously functionalized with a molecular spacer ending with an imidazole group, whose nitrogen atom firmly coordinated the metal in the porphyrin ring. The clay functionalization by a 3-(1-imidazolyl)propylcarbamoyl-3′-aminopropyl-triethoxysilane spacer was proved by DRIFT-IR, 13C- and 29Si-CPMAS-NMR spectroscopies, and the percent of Mn(TDCPPS)Cl immobilized by the spacer coordination was calculated. The activity of the novel catalysts was evaluated in the oxidative coupling reaction of catechol, a humic phenolic precursor, in the presence of H2O2 as oxygen donor. The rate of catechol oxidation catalyzed by both K and M heterogeneous catalysts was about four times as rapid as that catalyzed by the free manganese-porphyrin in a homogeneous catalysis, and depended on the percent of catalyst immobilized on the clay minerals. Moreover, the activity of the heterogeneous catalysis remained effective for at least two sequential reaction cycles, although a rate decrease in catechol transformation was observed. Afterwards, Mn(TDCPPS)Cl immobilized on spacer –functionalized kaolinite was employed to catalyze the oxidative polymerization of a lignite humic acid under H2O2 or exposure to UV light. The humic polymerization was followed by high-performance size exclusion chromatography (HPSEC), with both spectrophotometric and refractive index (RI) detectors. A significant increase of apparent weight-average molecular weight (Mw) of the humic acid following the oxidative polymerization under heterogeneous biomimetic catalysis, after 72 hours of reaction time. The enhancement in the apparent molecular mass of humic matter subjected to catalyzed oxidative polymerization was also confirmed by HPSEC chromatograms recorded after acetic acid addition to lower the solution pH to 3.5. These results showed that the immobilization of a biomimetic catalyst on clay minerals increased the catalytic efficiency and allowed the catalyst recycling and reuse for additional reactions. Moreover, the heterogeneous biomimetic catalysis increased the molecular mass of a humic acid by formation of intermolecular covalent bonds, thus suggesting its potential use for the soil carbon stabilization and the consequent reduction of CO2 emissions from arable soils.
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