Temperature swing adsorption of NOx over Keggin type heteropolyacids

Abstract

Temperature swing NOx adsorption–desorption cycles with gas mixtures consisting of 1000 ppm NOx, 5% O2, 3% H2O and N2 were carried out on heteropolyacid adsorbents in the temperature range 80–170 °C. Keggin type heteropolyacids having tungsten, molybdenum or their combination in octahedral coordination in combination with P, Si, Ge or B atoms in tetrahedral coordination were investigated. The heteropolyacids were used as pure compounds or supported on zeolite Y or the metal–organic framework Cu3(BTC)2 (BTC represents 1,3,5-benzenetricarboxylic acid). The property of reversible adsorption of equimolar mixtures of NO and NO2 was observed with Keggin type heteropolyacids based on W and combinations of W and Mo. Regeneration of the saturated adsorbent was achieved by cooling in a water vapor containing gas stream. The investigated heteropolyacids showed different water adsorption behavior. The capability of NOx adsorption was found to be related with crystal hydrate formation. Crystal water was much stronger retained in W-based compared to Mo-based Keggin compounds. Co-adsorption of NO and NO2 molecules occurred only on Keggin compounds containing crystal water. The formation of H2NO2+ compound out of NO, NO2 and H2O in the interstitial spaces between Keggin units is proposed. The H-form and Co, Cu, Fe, Ni and Mn salts of Wells–Dawson type heteropolyacids were found to be readily dehydrated upon heating and did not show reversible NOx adsorption. Supported W-based Keggin heteropolyacid showed lower NOx adsorption capacity compared to the unsupported heteropolyacid.