Characterization of Acid Sites in Ion-exchanged and Solid State-exchanged Zeolites 2001-01-3571

Brønsted acidity of solution ion-exchanged and solid-state exchanged zeolites was compared for NaY, BaY, CaY, NaX, and CaX zeolites. These materials were chosen because they all exhibit catalytic activity in SCR of NO_x in combination with a non-thermal plasma. Brønsted acidity was characterized qualitatively with retinol as an indicator dye. Our results show that the solid-state exchange using a chloride salt creates zeolites with lower acidity than zeolites obtained by conventional solution ion-exchange. NO₂ adsorption was also found to create a significant quantity of acid sites at room temperature and a slight increase in acidity at 200°C. We speculate that the acid sites created by NO₂ adsorption, because of their vicinity to metal cation sites in the zeolite, may lead to preferential reactions that lead to NO_x reduction. BaY made by solution ion-exchange and BaY made by solid-state exchange using a chloride salt were tested for NO_x reduction in a plasma-catalyst reactor system. Both catalysts showed similar catalytic activity. BaY prepared by solid-state exchange shows no coking due to absence of acid sites. In contrast, the BaY material prepared by conventional solution ion-exchange shows significant coke formation. Thus, the solid state exchanged BaY is a potentially more stable catalyst.