Comparison of selective catalytic reduction of NO with C3H6 and C3H8 over Cu(II)-ZSM-5 and Co(II)-ZSM-5

Abstract

Selective catalytic reduction (SCR) of NO with propene (C₃H₆-SCR) and propane (C₃H₈-SCR) over different levels of Cu(II) or Co(II) ion exchanged into ZSM-5 zeolite has been evaluated under flowing conditions with 1500 ppm of NO + 4500 ppm of C₃H₆ or C₃H₈ + 3 vol.% of oxygen in helium at a gas hourly space velocity of 20000 h⁻¹s. increasing temperature. Cu-ZSM-5 (Si/Al = 15, Cu/Al = 0.46) shows its highest NO conversion at 440–500°C for C₃H₆-SCR and at 340–600°C for C₃H₈-SCR. However, Co-ZSM-5 (Si/Al = 14, Co/Al = 0.43) shows its highest NO conversion at 460°C for C₃H₆-SCR and at 550°C for C₃H₈-SCR. The conversion efficiency at the optimum reaction temperatures was 98% for C₃H₆-SCR over Cu-ZSM-5 (Si/Al = 15, Cu/Al = 0.46), 96% for C₃H₈-SCR over Cu-ZSM-5 (Si/Al = 15, Cu/Al = 0.46), 75% for C₃H₈-SCR over Co-ZSM-5 (Si/Al = 14, Co/Al = 0.43) and 31% for C₃H₆-SCR over Co-ZSM-5 (Si/Al = 14, Co/Al = 0.43). All NO_x species adsorbed on the copper ion in ZSM-5 are desorbed below 430°C and are not observed in the temperature range for the highest NO conversion for C₃H₆-SCR. However, in Co-ZSM-5, Co-(NO)₂ species are observed up to 450°C by IR absorption. A reaction pathway at 450°C for C₃H₆-SCR over Cu-ZSM-5 is proposed based on Cu-allyl, Cu-allyl oxime, Cu-ethenyl isocyanate, Cu-primary amine and Cu-ethenyl reaction intermediates identified by IR absorption. A different pathway is proposed for C₃H₈-SCR over Cu-ZSM-5 at 250–340°C and for C₃H₈-SCR over Co-ZSM-5 at 230–550°C based on CH₃NO₂, HNCO, CO₂, Al-NCO, Si-NCO and Co-NCO reaction intermediates identified by IR absorption.