A New Bioinspired Perchlorate Reduction Catalyst with Significantly Enhanced Stability via Rational Tuning of Rhenium Coordination Chemistry and Heterogeneous Reaction Pathway

Rapid reduction of aqueous ClO4- to Cl- by H-2 has been realized by a heterogeneous Re(hoz)(2)-Pd/C catalyst integrating Re(O)(hoz)(2)Cl complex (hoz = oxazolinyl-phenolato bidentate ligand) and Pd nanoparticles on carbon support, but ClOx- intermediates formed during reactions with concentrated ClO4- promote irreversible Re complex decomposition and catalyst deactivation. The original catalyst design mimics the microbial ClO4- reductase, which integrates Mo(MGD)(2) complex (MGD = molybdopterin guanine dinucleotide) for oxygen atom transfer (OAT). Perchlorate reducing microorganisms employ a separate enzyme, chlorite dismutase, to prevent accumulation of the destructive ClO2- intermediate. The structural intricacy of MGD ligand and the two-enzyme mechanism for microbial ClO4- reduction inspired us to improve catalyst stability by rationally tuning Re ligand structure and adding a ClOx- scavenger. Two new Re complexes, Re(O)(htz)(2)Cl and Re(O)(hoz)(htz)Cl (htz = thiazolinylphenolato bidentate ligand), significantly mitigate Re complex decomposition by slightly lowering the OAT activity when immobilized in Pd/C. Further stability enhancement is then obtained by switching the nanoparticles from Pd to Rh, which exhibits high reactivity with ClOx- intermediates and thus prevents their deactivating reaction with the Re complex. Compared to Re(hoz)(2)-Pd/C, the new Re(hoz)(htz)-Rh/C catalyst exhibits similar ClO4- reduction activity but superior stability, evidenced by a decrease of Re leaching from 37% to 0.25% and stability of surface Re speciation following the treatment of a concentrated challenge solution containing 1000 ppm of ClO4-. This work, demonstrates the pivotal roles of coordination chemistry control and tuning of individual catalyst components for achieving both high activity and stability in environmental catalyst applications.