Development of oxorhenium complexes and bimetallic Re-Pd/C catalysts for perchlorate reduction in water

Abstract

Perchlorate (ClO4-) is a highly inert endocrine disrupting contaminant that is ubiquitous in drinking water sources. Efficient and sustainable technologies are needed to treat ClO4- in drinking water and ion exchange (IX) waste brine. This thesis study focuses on the development, mechanistic study and application of novel bimetallic heterogeneous catalysts combining oxophilic rhenium (Re) species with metallic palladium (Pd) nanoparticles on activated carbon support materials (Re-Pd/C). An oxorhenium(V) complex, Re(O)(hoz)2Cl (hoz = 2-(2’-hydroxyphenyl)-2-oxazoline), is synthesized as the precursor for a highly active metal center for oxygen atom transfer (OAT) reaction with ClO4-. Aqueous immobilization of Re(O)(hoz)2Cl into Pd/C yields the hybrid catalyst Re(hoz)2−Pd/C, which shows the highest activity reported to date for reducing aqueous ClO4- to Cl- at room temperature using 1 atm H2. Re speciation and reaction mechanisms at the heterogeneous water-catalyst interface, including redox transformation, reactivity with ClO4- and decomposition of the immobilized Re complex, are systematically studied. Meanwhile, the structure-activity relationship of Re(O)(hoz)2Cl isomers is investigated. Based on the studied isomer formation mechanism, a facile synthetic strategy for converting the less reactive N,N-cis Re(O)(hoz)2Cl to the more reactive N,N-trans isomer is established. The kinetics and stability of Re(hoz)2−Pd/C catalyst in practical water matrices are also studied. With the aim of recycling perchlorate-contaminated waste IX regenerant brines, a salt-resistant ReOx−Pd/C catalyst prepared from perrhenate (ReO4-) precursor is used to treat ClO4- in waste brine collected from a water treatment plant from California. Deactivation of ReOx−Pd/C in the waste brine is prevented by implementing a sequential treatment strategy where nitrate (NO3-) is first reduced first by an indium (In)-based Pd catalyst. Results from these studies demonstrate the promise of developing bimetallic catalysts combining a hydrogenation metal and a secondary promoter metal to facilitate reduction of recalcitrant oxyanion contaminants.