Mg–Al hydrotalcite-like compounds containing iron-phthalocyanine complex: effect of aluminum substitution on the complex adsorption features and catalytic activity

Mg–Al hydrotalcite-like materials (Mg_(1−x)Al_x(OH)₂(CO₃)_x/2·nH₂O with x=0.33, 0.25 and 0.20) were used as supports for the immobilization of Fe(III) tetrasulfonated phthalocyanine (FePcTs). Electron paramagnetic resonance (EPR) and X-ray absorption (XANES) spectroscopies were used to characterize the solids after FePcTs adsorption on the hydrotalcite-like materials (HTs). An adsorption study was carried out by monitoring in situ the FePcTs UV/visible electronic spectra during its addition to suspensions of HTs. The results showed that the HT composition controls the nature of adsorbed species: for HTs with higher Al³⁺ contents (x=0.33 and 0.25), the FePcTs was adsorbed mainly in the μ-oxo complex form whereas for HT with x=0.20, non-oxo-bridged FePcTs dimeric species prevailed. The heterogeneous catalytic studies of the HT-FePcTs materials in the oxidation of catechol, using hydrogen peroxide as oxidant, showed an enhanced catalytic activity and longevity, compared to the homogeneous counterpart. The catechol conversion was 63%, 73% and 88% for the materials containing HTs with x=0.20, 0.25 and 0.33, respectively. Therefore, the reactivity of HT-FePcTs materials was improved when the Al³⁺ content in the HTs increases. These catalytic tests associated to the adsorption studies showed that the μ-oxo complex of the FePcTs, mainly adsorbed on the HTs with x=0.25 and 0.33, seems to be the active species in catechol oxidation. These findings suggest that a cooperative effect take place in the HT-FePcTs materials, showing that HTs do not act as an inert support.