THE RELATION BETWEEN DEACTIVATION OF CUZSM-5 IN THE SELECTIVE REDUCTION OF NO AND DEALUMINATION OF THE ZEOLITE

The durability of CuZSM-5 in the selective reduction of NO has receive d scant attention notwithstanding the prospective use at h igh tempera tures in streams containing high levels of steam, i.e., conditions tha t cause zeolite dealumination. Dealumination is synonymous with the lo ss of Bronsted acid sites and with the destruction of the geometric in tegrity of the exchanged Cu-ion sites. If either of these sites is imp ortant in NO reduction, dealumination will result in deactivation. Ano ther issue is the effect of copper exchange on dealumination. We have followed the decrease in the tetrahedrally coordinated(27) Al MAS n.m. r. peak at 54 ppm to assess the extent of the dealumination. The treat ment of 43%-exchanged CuZSM-5 (SiO2/Al2O3 = 30) in a static system at 700 degrees C established that Cu inhibits the dealumination. For a di fferent CuZSM-5 with a high SiO2/Al2O3 ratio of 80 and ''overexchanged '' at 147%, it was observed that the activity decreases rapidly at 410 degrees C if the catalyst is exposed to steam-containing air, whereas it is not affected by dry air. The decrease in activity is accompanie d by dealumination. The activity decreased to about half its initial v alue and remained constant thereafter. The extent of dealumination was somewhat lower, proportionately, than the loss of activity. Some infe rences are drawn from the experimental results.