Ex-framework FeZSM-5 for control of N2O in tail-gases

In this paper, the performance of a novel ex-framework FeZSM-5 catalyst for direct N2O decomposition in simulated tail-gas from nitric acid plants (containing O-2, NOx, and H2O) and combustion processes (containing O-2, CO2, H2O, and SO2) is presented. This catalyst was prepared by isomorphous substitution of Fe in the MFI framework followed by calcination and steam activation. The specific activity per mole of Fe of the ex-framework catalyst in N2O/He was 4-10 times higher than observed for FeZSM-5 catalysts prepared by conventional solid and liquid-ion exchange and sublimation methods. The presence of NO, and SO2 has a positive effect on the N2O conversion over ex-FeZSM-5, decreasing the inhibition effect of H2O, while 02 and CO2 do not influence catalytic performance. The operation temperature is decreased similar to100 K by addition of propene to the feed mixture. The stability of ex-framework FeZSM-5 for direct N2O decomposition, in the absence of any reductant, was remarkable, showing no significant deactivation (at N2O conversion levels ranging from 20 to 65%) after 600 It time-on-stream. Sublimed and especially ion-exchanged FeZSM-5 catalysts showed a strong irreversible deactivation in feed mixtures containing H2O and SO2. The performance of the ex-framework catalyst has also been compared with that of different Rh-based catalysts. Co-Rh, Al mixed oxide, derived from a hydrotalcite precursor containing these metals, shows a remarkably high N2O decomposition activity in N2O/He. The initial activity of this catalyst in a simulated tail-gas mixture is higher compared to ex-FeZSM-5, but is severely inhibited by NO and deactivated in the presence of H2O and SO2. This also applies to other Rh-based catalysts. Application of ex-framework FeZSM-5 appears to allow a flexible and cost-effective end-of-pipe catalytic technology in chemical production plants and combustion processes. Different abatement options have been considered, depending on the source and the operating conditions of the process. Aspects of (monolithic) reactor design for an optimal catalyst implementation are also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.