Silver ion-exchanged zeolites as highly effective adsorbents for removal of NO x by pressure swing adsorption

Abstract

The increasing global concern in recent years over air pollution problems caused by nitrogen oxides (NOx) has brought extensive research on catalytic NOx decomposition and reduction [1, 2]. Adsorption is one of the most suitable techniques for removal of dilute adsorbates [3]. In particular, pressure swing adsorption (PSA) is expected to be an effective method to remove or concentrate NOx exhausted into air. In a previous paper, we reported that the copper ion-exchanged ZSM-5 zeolite is the most effective adsorbent for PSA at 273 K among the various cation-exchanged zeolites [4]. However, the adsorptive properties were tested only at 273 K. The optimum temperature of adsorption for PSA depends on adsorbent and adsorbate; for example, the optima for CO2 adsorption were 230 K and 323 K for activated carbon and zeolite, respectively [5]. In this study, the adsorption properties of metal ion-exchanged zeolites were examined as a function of adsorption temperature, and it was found that silver ion-exchanged ZSM-5 and mordenite zeolites with exchange level of around 100% have greater capacity at 195-273 K for reversible adsorption of NO than copper ion-exchanged zeolites. The concentration of NO was set at 1000 ppm in this work to enable quick evaluation of the adsorption properties of adsorbents; the concentration is much lower under practical conditions. Parent zeolites, ZSM-5 (SIO2/A1203=23.3), mordenite (15.0), ferrierite (12.3), offretite/erionite (7.7), Y-(5.6), L-(6.0), X-(2.6), and A-type (2.0) were supplied by Tosoh Corporation and denoted as MFI, MOR, FER, OFF/ERI, FAU, LTL, FAU, and LTA, respectively. Copper and silver ionexchanged zeolites were prepared as described previously [6]. The exchange levels of metal ions were determined by atomic absorption spectroscopy. Hereafter the samples are abbreviated as Ag-MFI(23.3)-100 (cation-zeolite structure (SiO2/ A120 3 ratio)-degree of exchange). The adsorption and desorption measurements were carried out in a fixed bed adsorption apparatus [4]. The adsorbent (0.5 g) was placed in a stainless steel column and heated at 773 K for 5 h under a helium stream (50 cm 3 min -1) just before the adsorption run. In the adsorption experiment, 1000 ppm of NO in He was introduced at a rate of 100 cm 3 min -1 into the column. After the adsorption run, He