Metal-exchanged zeolites

ASPECTS OF CATALYST DEVELOPMENT FOR MOBILE UREA-SCR SYSTEMS - FROM VANADIA-TITANIA CATALYSTS TO METAL-EXCHANGED ZEOLITES... 

Comparison of metal-exchanged zeolites and vanadia catalysts under standard-SCR conditions... 

Metal macrocycles encapsulated in zeolites can be synthesized in different ways.50-53 The flexible ligand51 or intrazeolite complexation53 method involves the diffusion of a ligand into the pores of a metal-exchanged zeolite, where upon complexation with the metal ion, it becomes too large to exit. This... 

Ever since the first study of metal-exchanged zeolites as new catalysts for selective catalytic reduction (SCR) of NOx with methane in the presence of oxygen was undertaken (Li and Armor, 1993), the simultaneous catalytic removal of NOx and CH4 at the exhaust of lean-bum natural gas engines has remained a challenge. 

We have recently shown that metal-exchanged zeolites give rise to carbocationic reactions, through the interactions with alkylhalides (metal cation acts as Lewis acid sites, coordinating with the alkylhalide to form a metal-halide species and an alkyl-aluminumsilyl oxonium ion bonded to the zeolite structure, which acts as an adsorbed carbocation (scheme 2). We were able to show that they can catalyze Friedel-Crafts reactions (9) and isobutane/2-butene alkylation (70), with a superior performance than a protic zeolite catalyst. 

Scheme 2 Reaction of an alkylchloride with a metal-exchanged zeolite.

The next homologues are 1- and 2-butyne, where similar isomerizations have been observed [20] a recent report describes the reaction on a basic, alkali metal-exchanged zeolite [21]. As an unexpected product, an allene was obtained in reactions with hydrogen and a samarium catalyst [16, 22]. 

Alkoxyl species form very readily from the reaction of alkyl halides on alkali, alkaline earth, transition metal, and lanthanide exchanged zeolites (128, 129). The more basic the zeolite, the more readily the reaction proceeds. Alkyl halides have been used to generate methoxyl, ethoxyl, isopro-poxyl, and ferf-butoxyl species on metal-exchanged zeolites. The mechanistic significance of alkoxyl species in zeolite acid catalysis is not in general clear in some reactions they may be true intermediates, and in others mere spectators. 

Only a few attempts have been made to relate the catalytic activity to the properties of cations on the transition metal-exchanged zeolites. Cross, Kemball, and Leach (5) studied the isomerization of 1-butenes over a series of the ion-exchanged X zeolites. Their results with CeX zeolite and the majority of other zeolites indicated a carbonium ion mechanism however a radical mechanism was operative with NiX and in some cases with ZnX. 

The catalytic activity for the aniline formation from chlorobenzene and ammonia of the Y zeolites with various cations was studied at 395° C (Table I). It is clear that the transition metal-exchanged zeolites have the catalytic activity for the reaction, while alkali metal and alkaline earth metal zeolites do not. The fact that alkaline earth metal-exchanged zeolites usually have high activity for carbonium ion-type reactions denies the possibility that Bronsted acid sites are responsible for the reaction. Thus, catalytic activity of zeolites for this reaction may be caused by the... 

Propene Alkali-metal exchanged zeolites X and Y T chemical shifts, NOE enhancements, effects of paramagnetics (301)... 

Scheme 10.3 Formation of MePc complexes in the supercages of FAU-type zeolites via tetramerization of 1,2-dicyanobenzene around transition metal exchanged zeolite.

The selective catalyst reduction of both NOx and N2O can take place when a second metal-exchanged, zeolite catalyst is combined with the catalyst that removes N20. Propane is used as the reductant. Destruction efficiency of both NOx and N20 is higher than 80% at a tail gas temperature above 280°C, a pressure of 4 bara and a space velocity of 15,000 h 1. At atmospheric pressure, conversions of NOx and N2O are somewhat lower. A space velocity of 15,000 h 1 and a temperature of 350°C results in 80% destruction of both NOx and N20221. 

Despite the enormous importance of zeolites (molecular sieves) as catalysts in the petrochemical industry, few studies have been made of the use of zeolites exchanged with transition metal ions in oxidation reactions.6338- 634a-f van Sickle and Prest635 observed large increases in the rates of oxidation of butenes and cyclopentene in the liquid phase at 70°C catalyzed by cobalt-exchanged zeolites. However, the reactions were rather nonselective and led to substantial amounts of nonvolatile and sieve-bound products. Nevertheless, the use of transition metal-exchanged zeolites in oxidation reactions warrants further investigation. 

Pyrrole has also been applied as a probe molecule in FTIR specroscopic studies. Upon interaction with a base site, the N—H stretching vibration is found to shift to lower wavenumber and in alkali metal-exchanged zeolites this behavior has been found to correlate with both N Is XPS data and the negative charge calculated from Sanderson electronegativities [4, 26]. 

Other Metal-Exchanged Zeolites. Nitrogen oxides have been selectively reduced by HCs over zeolite catalyst exchanged with other metal ions including Pd, Pt, Rh, In, Mn, Ga and Ce, besides Fe, Cu and Co. Pd-zeolites ° ° were most frequently investigated in this catalyst group. 

Pt-zeolite catalysts were also employed for the selective reduction of NO by HCs and were very active at lower reaction temperatures compared to Fe-, Cu- and Co-zeolites, but little, if any, literature was found for the Pt-exchanged zeolite catalysts deahng with the water tolerance. Iwamoto and coworkers conducted a comparative investigation of Pt- and Cu-MFI and Fe-MOR for their performance in NO reduction by C2H4. Pt-MFI-97 was found to be more active than the other two metal-exchanged zeolite catalysts at low temperatures and its activity for NO conversion at 200"C is hardly alfected by the addition of 8.6% H2O into the feed gas stream (Table 6), whereas the conversion of N2O formed during the course of the reaction slightly decreases by less than 10 /o. In... 

Table 6 Water tolerance of other metal-exchanged zeolite catalysts for the selective reduction of NOx by HCs...

Beneficial Modification of HC-SCR DeNO Catalysts to Improve Hydro-thermal Stability. - Impairment of the hydrothermal durability of HC-SCR catalysts, particularly metal-exchanged zeolites which are of interest in high-temperature deNOx applications, can be either prevented or at least reduced by selecting appropriate parent zeolite structure, employing suitable preparation technique, modifying physicochemical properties, and adding subsequent cation. The former two approaches have been extensively described in the previous section and the other ones will be mainly discussed. 

---