Temperature swing adsorption of NOx over Keggin type heteropolyacids

Abstract

Temperature swing NO_x adsorption–desorption cycles with gas mixtures consisting of 1000 ppm NO_x, 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 NO_x 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 NO_x adsorption. Supported W-based Keggin heteropolyacid showed lower NO_x adsorption capacity compared to the unsupported heteropolyacid.