Metal doping of porous materials via a post-synthetic mechano-chemical approach: a general route to design low-loaded versatile catalytic systems

Mesoporous silica materials (Al-SBA-15 and Al-MCM-41) were successfully doped with two different transition metals (Zr and Fe) by following a mechano-chemical approach. Different metal precursors (metal salts versus MOF particles) were studied in order to explore their impact on the ultimate catalytic performance of the doped materials. Importantly, doping levels as low as <1 wt% led to significant catalytic improvements for selective mild oxidation reactions under microwave irradiation in a very short reaction time (3 min). Fe-doped materials (0.175 mol% catalyst) resulted in the highest yields, achieving a maximum conversion of 46% and 100% selectivity in the oxidation of benzyl alcohol to benzaldehyde, and 61% conversion and 44% selectivity in the production of vanillin from isoeugenol. Results clearly indicated that the incorporation of Zr or Fe into the structure increased the total acidity of the support, which is then translated into an improved catalytic performance toward the studied oxidation reactions. The recyclability (up to 5 reuses without any treatment; although more reuses are possible after regeneration) of the doped catalysts pointed out their true potential applicability in a sustainable manner. Overall, this work demonstrates that the proposed post-synthetic mechano-chemical approach is an effective good alternative for the preparation of metal-doped porous materials, and is remarkably applicable to different doping metals, allowing thus the straightforward preparation of a library of catalysts with specific catalytic properties depending on the metal incorporated.