Adsorptive purification of NO_x by HZSM-5 zeolites: Effects of Si/Al ratio, temperature, humidity, and gas composition

Adsorption is a promising technology for capturing nitrogen oxides (NO_x) from humid flue gases, for which as the preferred NO_x adsorbent, zeolite with an appropriate silica to alumina (Si/Al) ratio provides the key step for success in practical applications. In this work, the effects of Si/Al ratio, temperature, relative humidity (RH), and feed gas (NO_x-H₂O(g)–CO₂–O₂-N₂) compositions on NO_x sorption behavior on HZSM-5 zeolites were systematically investigated, based on a series of characterizations, fixed-bed adsorption and temperature programmed desorption (TPD) tests. The low-silica HZSM-5(25) showed the best performance (326.6 μmol/g and 39.2 μmol/g at dry and 90%RH) as compared to others with higher Si/Al ratios at each RH, in which the roll-up effect was positively correlated with the Si/Al ratios and humidity. O₂ enhanced NO_x adsorption and reached an optimal adsorption capacity at the concentration of 14%, while CO₂ had little effect on NO_x adsorption. The decrease in temperature increased NO_x adsorption capacity on HZSM-5(25), benefitting from NO₂ physisorption as well as satisfactory NO oxidation within the active adsorbent structure. The facile thermal desorption of NO_x and significant NO–NO₂ conversion on HZSM-5(25) was exhibited by bimodal TPD peaks at 343–353K and 533–543K respective for NO and NO₂, in contrast to those on Silicalite-1. A potential “water-resistant” strategy via the enhancement of NO_x adsorption competitiveness was revealed, providing a theoretical guideline for NO_x adsorbent screening in industrial applications.