From biodiversity to catalytic diversity: how to control the reaction mechanism by the nature of metallophytes

Abstract

Phytoextraction is widely used for the reclamation of degraded sites, particularly to remove trace metals from contaminated soils. Whereas this technique demonstrates several advantages, the biomass resulting from phytoextraction processes is highly enriched in metallic elements and constitutes therefore a problematic waste. We show here that this biomass can be used for the preparation of novel polymetallic extracts, with high potential as catalysts or reagents in organic synthesis. This new concept of ecocatalysis constitutes an innovative recycling of metallic elements whose current known reserves could be exhausted in the coming decades. The ecocatalysts Eco-Zn and Eco-Ni prepared respectively from Zn and Ni hyperaccumulating plants display two distinct chemical reactivities, starting from the same substrates. Eco-Zn led to the formation of esters of commercial interest for the fragrance industry, following a hydro-acyloxy-addition reaction pathway. In contrast, Eco-Ni afforded chlorinated products thank to the hydrochlorination of alkenes. Both ecocatalysts allowed the synthesis of valuable products in high yields through methodologies in line with the spirit of sustainable chemistry.

From Conventional Lewis Acids to Heterogeneous Montmorillonite K10: Eco‐Friendly Plant‐Based Catalysts Used as Green Lewis Acids

“…The yellow solution is concentrated under vacuum then filtered through a pad of Celite if a purification step is necessary. The resulting solid residue, stored in a desiccator, may be used as an ecocatalyst or may be supported on silica or MK10 …”

“…However, a number of scientists used hazardous reagents, nonenvironmentally friendly substances, or non‐reusable catalysts. Two eco‐friendly plant‐based catalysts have recently been developed and used in hydroacyloxy addition . These ecocatalysts are derived from two metal hyperaccumulating plants (e.g., N. caerulescens , A. vulneraria , and Geissois pruinos ), one of them is a Zn metallophyte and the other is a Ni‐metallophyte.…”

“…The authors revealed several advantages in using the Eco‐Zn and Eco‐Ni catalysts (Table ), including short reaction times, reaction without solvent or in the solid state, low catalyst loading, and the possibility of catalyst recovery and reuse to limit the cost of the reaction . However, the best yields were obtained with traditional Lewis acids (e.g., ZnCl 2 and NiCl 2 ), rather than Eco‐Zn and Eco‐Ni prepared from metal hyperaccumulating plants.…”

“…Moreover, under the most appropriate conditions, the authors showed that Eco‐Ni afforded chlorinated products through hydrochlorination of the alkenes (60–88 %), instead of the hydroacyloxy addition of carboxylic acids to alkenes. Eco‐Ni and Eco‐Zn catalysts were composed of the same transition metals (Al, Ca, Cd, Fe, Mg, and Pb), but at different concentrations . The authors showed that the Lewis acidity of Eco‐Ni was lower than that of Eco‐Zn.…”

“…The authors showed that the Lewis acidity of Eco‐Ni was lower than that of Eco‐Zn. The X‐ray diffractogram of Eco‐Ni highlighted the presence of tachhydrite (CaMg 2 Cl 6 (H 2 O) 12 ), Ca 2.5 Fe 15.5 O 25 , KNiCl 3 , NiSO 3 (H 2 O) 6 , Na 2 SO 4 , chlorocalcite (KCaCl 3 ), and KClO 3 , whereas the crystalline species in Eco‐Zn were CaMg 2 Cl 6 (H 2 O) 12 , anhydrite (CaSO 4 ), K 2 ZnCl 4 , sylvite (KCl), and MgP 4 O 11 …”