Optimization of Zeolite Catalysts for Removing NOx & CH4

Abstract

Owing to the narrow crystallization field, the synthesis of the large-pore zeolite UZM-35 with MSE topology starting from amorphous reagents in the presence of dimethyldipropylammonium ions as an organic structure-directing agent is often non-reproducible. Here we report the synthesis of UZM-35 via an interzeolite conversion route using USY zeolites, as the sole source of Al and Si, and a small amount of previously synthesized UZM-35 as seed crystals. We also present the one-pot synthesis of Fe-UZM-35 with the best low-temperature NH3-SCR activity among the iron-exchanged zeolites reported so far. These synthesis routes showed high reproducibility, together with considerably faster crystallization kinetics (≤1.5 vs 7 days) compared to conventional UZM-35 synthesis. When the Fe loading level was properly adjusted, the NH3-SCR activity of one-pot-synthesized Fe-UZM-35 was found to be essentially the same as that of post-synthetically exchanged Fe- UZM-35 due to the similarity in the nature of their iron active sites. NH3-SCR over copper- and iron-exchanged zeolites is a state-of-the-art technology for removal of nitrogen oxides (NOx, NO, and NO2) from exhaust emissions, but suffers from poor low-temperature (i.e., 150 °C) activity. Here we show that hydrothermal aging of Fe-beta, Fe-ZSM-5, and Fe-ferrierite at 650 °C or higher leads to a remarkable increase in NOx conversion from ~30 to ~80% under fast NH3-SCR conditions at 150 °C. The practical relevance of this finding becomes more evident as an aged Fe-beta/fresh Cu-SSZ-13 composite catalyst exhibits ~90% conversion. We propose that a neutral heteronuclear bis-μ-oxo ironaluminium dimer might be created within iron zeolites during hydrothermal aging and catalyze ammonium nitrate reduction by NO at 150 °C. Density functional theory calculations reveal that the activation free energy (125 versus 147 kJ mol-1) for the reaction of NO with adsorbed NO3 - species, the rate-determining step of ammonium nitrate reduction, is considerably lower on the bis-μ-oxo ironaluminium site than on the well-known mononuclear iron-oxo cation site, thus greatly enhancing the overall SCR activity. Natural gas engines are the most viable alternative to diesel and gasoline ones. However, the current state-of-art catalyst, palladium (Pd) on alumina, for eliminating unburnt methane from engine exhaust suffers from high light-off temperature (> 400 °C) and poor water tolerance. Here we report that a 3.0 wt% Pd catalyst supported on Na+-post-exchanged IWV zeolite with a Si/Al ratio of 45 has a light-off temperature as low as 290 °C for methane combustion in the presence of 10% water vapor while maintaining over 85% methane conversion at 330 °C for 100 h. We simultaneously achieved these sustainable development goals in wet methane combustion by combining the effects of the zeolite structure and its Si/Al ratio with those of air calcination at 500 °C after Na+ ion exchange. This study provides a new direction for bringing zeolite-supported Pd catalysts close to real-world applications. CONTENS