Application and integration of ruthenium catalysts for water treatment and resource recovery

Abstract

Water contaminants in oxidized form can be preferably removed or transformed to less harmful species by chemical or biological reduction. Hydrogenation metal-catalyzed reduction has emerged as a promising treatment technology for oxidized pollutants (e.g., oxyanions, halo- and nitro-organics). To date, Pd-based catalysts have received significant attention and demonstrate good activity and stability in reducing a number of contaminants relevant to drinking water or groundwater, but the deployment of catalytic reduction systems remains limited, in large part, by the high cost and volatile market price of this metal. The narrow focus on Pd-based materials also hinders the advancement of catalytic reduction technology because other hydrogenation metals are being overlooked which may have exhibited higher activity for specific contaminants. In addition, demonstrating catalytic activity with multiple metals can reduce uncertainty in the cost of the technology by allowing for metal substitution during market price spikes. Thus, it is necessary to expand catalyst “toolbox” for the water treatment applications and to integrate catalysts with other technologies (e.g., separations processes) to advance the development of practical water catalysis technologies. To develop alternative hydrogenation metal catalysts for water purification, several supported platinum group metals catalysts were assessed with a suite of representative oxyanion pollutants. Rh, Ru, Pt and Ir were found to exhibit higher activity, wider substrate selectivity or variable pH dependence in comparison to Pd. A detailed investigation, coupling experiments with computational work, was then conducted to identify mechanisms controlling nitrate and nitrite reduction by supported Ru catalysts. Pseudo-first-order rate constants and turnover frequencies were determined for carbon- and alumina-supported Ru, and this work demonstrated Ru’s high activity for hydrogenation of nitrate at ambient temperature and H2 pressure. Pretreatment of the catalysts was found to enhance nitrate reduction activity by removing catalyst surface contaminants and exposing highly reducible surface Ru oxides. Ru reduces nitrate selectively to ammonia and nitrite to a mixture of ammonia and N2, with the product distribution determined by the initial aqueous nitrite concentrations. Experimental observation and Density Functional Theory calculations together support a reaction mechanism wherein sequential hydrogenation of nitrate to nitrite and NO is followed by parallel pathways involving the adsorbed NO that lead to ammonia and N2. The activity of supported Ru catalysts was further evaluated for reducing N-nitrosamines, including the toxic disinfection byproduct N-nitrosodimethylamine (NDMA) and other organic water contaminants. Using NDMA as a representative contaminant, commercial Ru/Al2O3 catalyst showed high activity with an initial turnover frequency (TOF0) of 58.0 ± 7.0 h-1. A second Ru/Al2O3 catalyst was synthesized using an incipient wetness impregnation technique, and this catalyst exhibited higher initial pseudo-first-order rate constant than the commercial catalyst due to higher dispersion of Ru nanoparticles on the catalyst support. NDMA was reduced to dimethylamine (DMA) and ammonia end-products, and a small amount of 1,1-dimethylhydrazine (UDMH) was detected as a transient intermediate. Experiments with a mixture of five N-nitrosamines spiked into tap water (1 g L-1 each) demonstrated that Ru catalysts are very effective in reducing a range of N-nitrosamine structures at environmentally relevant concentrations. These results encourage the further development of Ru catalysts as part of the water purification and remediation toolbox. Supported Ru catalyst was then integrated into a hybrid catalytic hydrogenation/membrane distillation process to improve nitrate-contaminated ion exchange waste brine management and recover valuable nitrogen resources. The ability of a commercial Ru/C catalyst to reduce concentrated nitrate was demonstrated in a semi-batch reactor under typical waste brine conditions. Nitrate hydrogenation exhibited zero-order kinetics, attributed to saturation of available surface reaction sites, and the apparent rate constant was influenced by both solution chemistry and reaction temperature. The resulting ammonia product was efficiently recovered using membrane distillation. At low temperatures (<35 °C), solution pH showed significant impact on ammonia mass transfer coefficient by controlling the free ammonia species fraction. Ammonia recovery efficiency was not affected by salt levels in the brine, indicating the feasibility of membrane distillation for recovering ammonia from waste ion exchange brine. The hybrid catalytic hydrogenation/membrane distillation process was also applied to a real ion exchange waste brine and demonstrated high nitrate hydrogenation and ammonia recovery efficiency. These findings provide alternative catalyst for catalytic treatment of ion exchange waste brine and design option of efficient, low footprint system for nitrogen resource recovery from waste ion exchange brines. In addition, the efforts of catalyst and process development were extended to the field of bio-renewable energy. Leveraging fuel property predictive models, a non-cyclic branched C14 hydrocarbon (5-ethyl-4-propylnonane) was identified to be a potential target molecule for renewable diesel applications. This target molecule is accessible from butyric acid through sequential catalytic reactions of acid ketonization, ketone condensation, and hydrodeoxygenation. Catalytic activity, product selectivity, and catalyst stability for individual conversion step were first evaluated, followed by demonstration of hydrocarbon blendstock production from butyric acid through integrated conversion process scheme. Experimental fuel property testing of the conversion product validated its suitability for use as diesel blendstock.