Catalytic Oxidation of Nitric Oxide Using Hyper-Cross-Linked Porous Polymers: Impact of Physiochemical Properties on Conversion Efficiency
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Emissions of nitrogen oxides (NOx) to the atmosphere remain a threat to human health and the environment despite stringent regulations and decades of control technology development. A majority of emitted NOx is generated from fuel combustion in stationary and mobile sources, where selective catalytic or non-catalytic reduction (SCR or SNCR) are established for its control. These technologies, however, cannot individually prevent more than ~90% of NOx emissions without risking ammonia slip. High capital and operation costs associated with SCR and SNCR further necessitate the development of an efficient and inexpensive alternative. Liquid absorption of NOx, similar to how SO2 is controlled, may be viable, but upstream oxidation of NO to NO2 is required to increase NOx solubility; NO2 is 250 times more soluble in water than NO. While the NO oxidation challenge has been extensively considered in the literature, current low temperature catalysts cannot be applied under representative conditions because relative humidity in flue gas streams decreases performance. For example, activated carbon (AC) fails to oxidize any NO at 50% RH, restricting its use for industrial NOx control.In this work, microporous, hydrophobic polymers are prepared through the polymerization of 4,4′-Bis(chloromethyl)-1,1′-biphenyl (BCMBP) using the Friedel-Crafts reaction. These polymers are microporous (i.e., up to 0.38 cm3/g of micropore volume) with extensive specific surface area (i.e., up to 1430 m2/g). Combined, these properties allow the polymers to accommodate the N2O4 transition state, while their hydrophobicity prevents interference from moisture. At 25 °C, NO conversion in dry conditions is higher than NO conversion efficiency at wet conditions (22% reduced efficiency in humid condition). However, relative to other catalysts reported in the literature, such as AC, this is an improvement for NO conversion in moist conditions. There is potential to use these polymers in industrial and realistic situations. To improve performance further, polymers were modified through treatment with dimethyl amine (DMA) or benzene to improve catalytic activity without sacrificing hydrophobicity. Although DMA functionalization increased conversion in dry conditions compared to the BCMBP control (8.3% increase at 25 °C), it was less favorable in 1.6 vol% humidity (22% reduction at 25 °C). On the other hand, functionalization with benzene increased conversion for both dry and wet conditions at 25 °C, by 6.1% and 17.6%, respectively. Overall, this research not only proposes that polymers are inherently valuable as catalysts for NO oxidation, but also develops methods to manipulate their physical and chemical properties to improve performance further.