Ion-exchanged ZSM-5 Zeolites

Nitric oxide can be reduced by low molecular weight hydrocarbons in the presence of oxygen. This avoids the use of ammonia and, potentially, makes the metal exchanged ZSM-5 suitable for NOX removal from some exhaust emissions. Palladium exchanged ZSM-5 was found to be more active than the copper exchanged catalysts, at significantly lower temperature.  

So far, the practical problems of commercial operation and the effects of poisons limit the efficiency of metal exchanged ZSM-5 catalysts. No large-scale operations have been reported for the treatment of effluents from either stationary or mobile sotrrces. 

---

Iwamoto, M Yahiro, H Tanda, K Mizuno, N Mine, Y Kagawa, S. Removal of nitrogen monoxide through a novel catalytic process. 1. Decomposition on excessively copper-ion-exchanged ZSM-5 zeolites. J., Phys. Chem., 1991, Volume 95, 3727-3730. 

The catalytic properties of a copper ion-exchanged ZSM-5 zeolite (Cu-ZSM-5) can be compared with others. 

Transition metal ion-exchanged ZSM-5 zeolites were prepared by using conventional and solid-state ion-exchange procedures. The samples proved to be crystalline and no structural destruction was found as a consequence of the modifications. 

Zhang, Y., and M. Flytzani-Stephanopoulos, 1994, Catalytic decomposition of nitric oxide over promoted copper-ion-exchanged ZSM-5 zeolites, in Environmental Catalysis, ed. J.M. Armor, Vol. 552 of ACS Symposium Series (American Chemical Society, Washington, DC) pp. 7-21. 

The reaction over alkali metal ion exchanged X, Y, and ZSM-5 zeolites was also studied by Wierzchowski and Zatorski using trioxane as the source of HCHO. It is concluded that the alkali-metal-ion-exchanged ZSM-5 zeolite catalysts are not suitable as catalyst for this reaction. 

Alkaline earth and rare earth metal cocation effects are reported in this paper for copper ion-exchanged ZSM-5 zeolites used for the catalytic decomposition of nitric oxide in 02- free, 02- rich, and wet streams. Severe steaming (20% H2O) of Na-ZSM-5 at temperatures above 6(X)°C leads to partial vitreous glass formation and dealumination. Unpromoted Cu-ZSM-5 catalysts suffer drastic loss of NO decomposition activity in wet gas streams at 500°C. Activity is partially recovered in dry gas. Copper migration out of the zeolite channels leading to CuO formation has been identified by STEM DX. In Ce/Cu-ZSM-5 catalysts the wet gas activity is greatly improved. CuO particle formation is less extensive and the dry gas activity is largely recovered upon removal of the water vapor. 

Iwamoto M, Yahiro H, Tanda K, Mizuno N, Mine Y, Kagawa S (1991) Removal of Nitrogen Monoxide through a Novel Catalytic Process. 1. Decomposition on Excessively Copper Ion Exchanged ZSM-5 Zeolites. J Phys Chem 95 (9) 3727-3730... 

Fig. 4.15 Differential heat of adsorption versus NH3 uptake on HZSM-5 and ion exchanged ZSM-5 zeolites [37]...

The ferrocene-exchanged ZSM-5 zeolite shows a Mossbauer spectrum that when analyzed gives a 40% ferrocinium ion signal with an isomer shift of 0.53 mm/second. It is likely that water in the zeolite is being reduced as the ferrocene is oxidized. Reduction of this sample in hydrogen causes a complete loss of both the ferrocinium ion and the ferrocene Mossbauer features and a new appearance of alpha iron(0) metal and a new quadrupole split doublet of 0.4 mm/second and a quadrupole splitting of... 

The polymerization of pyrrole over Cu(II)-exchanged ZSM-5 zeolites was studied with resonance Raman spectroscopy. The authors found that a critical concentration of cupric ions must be exceeded to observe polymerization. Hosts with low Si/Al ratios gave partially oxidized pol5rpyrrole (having quinoidal and aromatic structures) and pyrrole monomer. The quinoidal structure was associated with the charge carriers. Residual oxygen degraded the polymer. 

Cu - and Fe -ion containing ZSM-5 zeolites woe prepared by conventional and solid-state ion-exchange methods described in ref [15]. The Si/Al ratio of parent Na-ZSM-5 was 40. The samples were characterized by X-ray diffiaction, IR spectroscopy, thermal analytical method and BET measuronent. Related data are shown in Table 1. 

Metals other than A1 have been successfully introduced in numerous zeolite frameworks. Aluminum substitution by other metals, such as Fe, Ga, Zn, Co or Cu in the zeolite framework results in modified acidity, and subsequently modified catalytic activity, for certain reactions such as selective catalytic reduction of NOx by hydrocarbons. For example, a calorimetric and IR spectroscopic study of the adsorption of N2O and CO at 303 K on Cu(II)-exchanged ZSM-5 zeolites with different copper loadings has been performed by Rakic et al. [92]. The active sites for both N2O and CO are Cu (I) ions, which are present on the surface as a result of the pre-treatment in vacuum at 673 K. The amounts of chemisorbed species adsorbed by the investigated systems and the values of the differential heats of adsorption of both nitrous oxide (between 80 and 30 kJ mof ) and carbon monoxide (between 140 and 40 kJ mol ) demonstrate the dependence of the adsorption properties on the copper content. 

The Norrish type II reaction of various phenyl alkanones in zeohtes has been studied by Ramamurthy and his collaborators. " The size and shape of zeohte reaction cavities control the behavior of triplet ketone as well as the 1,4-biradical intermediate that is consequently monitored as a difference in photoproduct distribution and triplet lifetimes. Norrish type I and type II reactions of 2-pentanone included within cavities of various types of zeohtes and the alkah metal cation-exchanged ZSM-5 zeolite have been investigated by experimental and theoretical approaches. " Ion-exchanged cations had significant effects not only on the adsorption state but also on the photochemical reactions of the ketone. The... 

From the comparison of the results, it can be inferred that copper ions exchanged in the ZSM-5 zeolites assumes a bidentate (sites 12 and II) or tridentate coordination (sites M5, Z6, and M7). These two groups differ also in the molecular properties (Table 2.2). The I-centers are characterized by lower values of the valence index and greater partial charges, QCu, in comparison to the M and Z centers, which is associated with the deeper laying HOMO and LUMO levels. In the M5, Z6, and M7 sites Cu1 ions exhibit more covalent character, and the frontier orbitals have less negative energies. As a result, the chemical hardness of the I-centers, located at the channel intersections, is smaller than those located on the walls of the ZSM-5 zeolite. 

The catalyst samples were prepared in our laboratory. The synthesized Na-ZSM-5 zeolite was modified by conventional or solid state ion-exchange [11] to form H-, Fe-, Cu-, Ni- and Ti-ZSM5 samples, while the mesoporous catalysts (Fe- and Ti-MCM-41) were synthesized by isomorphous substitution [12], as well as the hydrotalcites containing Fe-, Cu-, Cr- or Ca-oxide in the Mg,Al-LDH structure [13]. 

The ZSM-5 zeolite had a SAR (silica-to-alumina ratio) value of 38 and were supplied by CENPES/PETROBRAS. These samples, as received, were submitted to two ion exchange processes with ammonium chloride solution at 323K for sodium content reduction, followed by calcinations at 773K under dry air flow for transformation to its acid form. 

---