Mechanistic Explanation for Differences Between Catalytic Activities of Dissolved and Aerogel Immobilized Cu(II) Cyclen

Forgács, Attila and Balogh, Zoltán and Andrási, Melinda and Len, Adél and Dudás, Zoltán and May, Nóra V. and Herman, Petra and Juhász, Laura and Fábián, István and Lihi, Norbert and Kalmár, József (2022) Mechanistic Explanation for Differences Between Catalytic Activities of Dissolved and Aerogel Immobilized Cu(II) Cyclen. APPLIED SURFACE SCIENCE, 579. pp. 1-11. ISSN 0169-4332

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Abstract

The copper(II) complex of 1,4,7,10-tetraazacyclododecane [Cu(II)-cyclen] was covalently immobilized in mesoporous silica aerogel. This immobilization significantly alters the catalytic activity of Cu(II)-cyclen when referenced to the dissolved complex in the oxidation of phenol by H2O2 in aqueous solution. In order to understand this phenomenon, the functionalized aerogel was characterized by scanning electron microscopy (SEM), N2 porosimetry, small angle neutron scattering (SANS), infrared spectroscopy (IR) and electron paramagnetic resonance spectroscopy (EPR). Aerogel morphology is typical of mesoporous silica aerogels, and the coordination mode of Cu(II) in the immobilized complex is well-related but not identical to solution phase Cu(II)-cyclen. The mechanisms of the catalytic reactions involving dissolved and immobilized Cu(II)-cyclen were explored by fine kinetic experiments using capillary electrophoresis (CE) and on-line UV–vis spectrophotometry. Hydroquinone, pyrocatechol and the related benzoquinones were identified as the main intermediates in both reaction systems. A detailed kinetic model is postulated based on global data fitting, which clearly highlights the mechanistic differences in the two systems. Interestingly, the activation of the catalyst by H2O2 is more effective in the case of the aerogel, but the total conversion of phenol is slower due to hindered mass transport compared to using dissolved Cu(II)-cyclen.

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