Encapsulation, Stabilization, and Catalytic Properties of Flexible Metal Porphyrin Complexes in MCM-41 with Minimal Electronic Perturbation by the Environment

Abstract

meso-Tetrakis(5-trimethylammoniopentyl)porphyrin (TMAP) was incorporated in MCM-41 directly during a hydrothermal synthesis or by a surfactant ↔ porphyrin ion-exchange reaction with as-synthesized MCM-41. Both methods permitted encapsulation of the porphyrin within the mesoporous channel system. UV−vis absorption spectra indicated that porphyrin molecules dimerized or formed larger agglomerates in the ion-exchanged samples, while TMAP molecules remained isolated in hydrothermally prepared samples. Spectra of the latter samples closely resembled those of TMAP in solution with no significant broadening of the Soret and Q-band absorptions. Acid extraction of the surfactant converted the encapsulated free-base porphyrin to the dication TMAP-H22+, which could be further metalated with Cu2+, Ni2+, or Fe2+ without leaching porphyrin from the mesoporous sieve. The catalytic activity of the copper porphyrins was evaluated in the oxidative bleaching reaction of the azo dye β-naphthol violet. During this reaction, TMAP-Cu2+ also degraded when it was free in solution or incorporated in MCM-41 by ion exchange. However, in samples where the porphyrin was directly incorporated during the hydrothermal synthesis, TMAP-Cu2+ was stabilized and exhibited greater catalytic activity for longer time periods. It is suggested that isolation of the porphyrin molecules within the MCM-41 channels prevented their mutual oxidation.