Abstract
A novel Mn-derived catalyst was prepared starting from the biomass of Mn-hyperaccumulating plants growing on metal-rich soils. Recovery of this biomass as value-added ``ecocatalyst'' provides incentives for the development of phytoextraction programs on soils degraded by mining activities. Characterization of the resulting plant-based catalyst Eco-Mn by inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), and X-ray photoelectron spectroscopy (XPS) demonstrated the presence of unusual polymetallic complexes of Mn(II) in the catalyst, along with Fe(III). Incorporation of these species into montmorillonite K10 as solid support provided a supported Eco-Mn catalyst, whose properties were investigated for alkene epoxidation with H2O2 (30 wt %)/NaHCO3 (0.2 M) as a green terminal-oxidizing reagent. The supported Eco-Mn catalyst demonstrated a high efficiency for styrene epoxidation, with only 0.31 mol % Mn, a much lower content of Mn than in previously described Mn-derived heterogeneous catalysts. Although Fe was also present in the supported Eco-Mn catalyst, comparison experiments showed that Fe had only a limited role in the catalysis. The water content in the reaction medium had a beneficial effect, increasing the reaction efficiency. The supported Eco-Mn catalyst was recycled four times without any loss of activity. Comparison of its properties to those of heterogeneous catalysts made by incorporation of commercial MnCl2 center dot 4H(2)O and FeCl3 center dot 6H(2)O highlighted the superior catalytic activity of polymetallic species present in the biosourced catalyst. The substrate scope of the method was extended to various alkenes, including bulky natural products which were epoxidized with high yields (up to 99%), sometimes much higher than those obtained with already described Mn-derived heterogeneous catalysts. Finally, by simple adjustments of reaction conditions, the method allowed controlled access to aldehydes by oxidative cleavage of various styrene-derived substrates (up to 93% yield). The method thus constitutes a valuable alternative not only to classical epoxidation reagents but also to oxidative cleavage of styrene-derived molecules, which usually involves toxic and hazardous reagents.