Zeolite Cu-ZSM

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

The use of a copper exchanged H-ZSM-5 zeolite (Cu-ZSM-5) has been reported for the production of acetic acid and methyl acetate from methanol (87). Copper incorporated into H-ZSM-5 performs better at 70 atm pressure and 285-375°C compared to 10 atm (76), but the space time yield or productivity of acetyls (acetic acid -l-methyl acetate) is low, at about 0.03 g/(gcat. h) and the maximum selectivity is 17% at 33% methanol conversion. Higher productivities have been reported for cobalt/zeolite catalysts but pressures of approximately 700 atm are needed (88). 

Evaluating the results a clear kinetic picture of the catalysts has been obtained. In the steady state the active sites in Fe- and Cu-ZSM-5 are nearly fully oxidized, while for Co only -50% of the sites are oxidized. The former catalysts oporate in an oxidation reduction cycle, Fe /Fe and CuVCu. Coi in zeolites is hardly oxidized or reduced, but ESR studies on diluted solid solutions of Co in MgO indicate that Co -0 formation is possible, rapidly followed by a migration of the deposited oxygen to lattice oxygen and reduction back to Co [36]. For Fe-ZSM-5 such a migration has been observed, so a similar model can be proposed for the zeolitic systems. Furthermore, it is obvious that application of these catalysts strongly depends on the composition of the gas that has to be treated. 

Based on previous studies [15, 22-25], the band at 1941 cm-i is assigned to Co2+(NO), and the pair of bands at 1894 and 1815 cm-i, to Co2+(NO)2- The shoulders at 1874 and 1799 cm may be due to a second dinitrosyl species. While little is known about the location and coordination of the Co 2+ in ZSM-5, it is likely that cobalt ions are associated with both [Si-0-Al]- and [Al-0-Si-0-AI]2- structures in the zeolite. In the former case, the cobalt cations are assumed to be present as Co2+(OH-) cations and in the latter case as Co2+ cations. The presence of cobalt cations in different environments could account for the appearance of two sets of dinitrosyl bands. The band at 2132 cm-> is present not only on Co-ZSM-5 but also on H-ZSM-5 and Na-ZSM-5, and has been observed by several authors on Cu-ZSM-5 [26-28]. 

Figure 2.15. X-band EPR spectra recorded after NO adsorption (1 -5 torr) onto ConZSM-5, FenZSM-5 (after [64]), and Cu ZSM-5 (after [41]) zeolites.

Sojka, Z., Che, M. and Giamello, E. (1997) EPR investigation of the electronic structure of mononuclear copper(I) nitric oxide adduct formed upon low-pressure adsorption of NO onto Cu/ZSM-5 zeolite, J. Phys. Chem. B, 101, 4831. 

Another way to work in transient conditions is to stop suddenly (or conversely to instantaneously introduce) one of the reactants, in order to destabilize the system and to enhance the concentration of labile species. With this method, for example, Poignant et al. studied the DeNO. reaction mechanism on a H—Cu-ZSM-5 catalyst, using propane or propene as reducing agents. The introduction of 2000 ppm of hydrocarbon in a flow of NO (2000 ppm) + 5% 02 allowed to evidence the formation of acrylonitrile, which behaved as an intermediate. Its reactivity with NO+ species constituted a fundamental point to describe a detailed SCR mechanism for NO removal on zeolitic compounds [137],... 

Steam is invariably present in a real exhaust gas of motor vehieles in relatively high concentration due to the fuel combustion. The influence of water vapor on catalytic performances should not be ignored when dealing with the aim to develop a practical TWCs. Cu/ZSM-5 catalysts once were regarded as suitable substitutes to precious metal catalysts for NO elimination[78], nevertheless, they are susceptible to hydrothermal dealumination leading to a permanent loss of activity[79], Perovskites have a higher hydrothermal stability than zeolites[35]. Although perovskites were expected to be potential autocatalysts in the presence of water[80], few reports related to the influence of water on the reactants adsorption, the perovskite physicochemical properties, and the catalytic performance in NO-SCR were previously documented. The H2O deactivation mechanism is also far from well established. 

The catalytic properties of Cu-ZSM-5 zeolites, first found by Iwamoto and ooworkers, will be outlined in the next section. Here I discuss how it occurred to us that Cu-zeolites are suitable as... 

Since 1981, three-way catalytic systems have been standard in new cars sold in North America.6,280 These systems consist of platinum, palladium, and rhodium catalysts dispersed on an activated alumina layer ( wash-coat ) on a ceramic honeycomb monolith the Pt and Pd serve primarily to catalyze oxidation of the CO and hydrocarbons, and the Rh to catalyze reduction of the NO. These converters operate with a near-stoichiometric air-fuel mix at 400-600 °C higher temperatures may cause the Rh to react with the washcoat. In some designs, the catalyst bed is electrically heated at start-up to avoid the problem of temporarily excessive CO emissions from a cold catalyst. Zeolite-type catalysts containing bound metal atoms or ions (e.g., Cu/ZSM-5) have been proposed as alternatives to systems based on precious metals. 

In addition, DOCs are also capable of reducing NOx, under certain conditions. Early work on these lean NOx catalysts concentrated on Cu/ZSM-5 catalysts (Amiridis et al., 1996 Walker, 1995), but platinum (Amiridis et al., 1996 Burch and Millington, 1995) and silver (Breen and Burch, 2006) -based catalysts, with better hydrothermal resistance than the zeolite systems, are also available. Unfortunately, NOx reduction under lean conditions only occurs over a narrow temperature range and therefore modelling can aid in optimisation of the catalyst and emissions system. 

The abovementioned rate acceleration and selectivity enhancement brought about by catalysts are particularly marked when unactivated dienes and dienophiles are involved. Two molecules of 1,3-butadiene can react in a Diels-Alder reaction, one acting as diene and the other as a dienophile to produce 4-vinylcyclohexene (in 0.1% yield at 250°C in the absence of a catalyst). Cs+, Cu,+ and trivalent transition-metal exchanged montmorillonites534 as well as large-pore sodium zeolites (Na ZSM-20, NaY) and carbon molecular sieves,535 result in 20-35% yields with 95% selectivity. Large rate enhancement was observed when 1,3-cyclohexadiene underwent a similar cycloaddition536 in the presence of K10 montmorillonite doped with Fe3+ ... 

Fig. 4. IR spectra of CO dosed, at liquid-nitrogen temperature, onto Cu -ZSM-5 (bottom dotted lines), Cu -mordenite (bottom full lines), Cu -P (top dotted lines), and Cu -Y (top full lines) zeolites. Parts (a), (b), and (c) report low, medium, and high Pco spectra, respectively, approximately corresponding to mono-, di- and tri-carbonyl complexes. (Adapted with permission from Bordiga et al. (P5).)...

If the Cu -zeolite/NO systems are allowed to progressively reach room temperature, then kT first starts to be comparable to and then greater than A/sino Oj+Nj, and the activity of the catalyst is switched on. The evolution of the Cu ZSM-5/NO... 

Copper ions exchanged microporous molecular sieves, in particular Cu-ZSM-5, are active catalysts for the selective catalytic reduction of NO and N2O with hydrocarbons in the presence of O2 (HC-SCR). It has been reported that the catalytic activity may be controlled by intra-crystalline diffiisivity and by geometry-limited diffusion depending on the hydrocarbon molecular size and the zeolite pore size [1]. Therefore, it is of interest to prepare Cu-Al-MCM-41 mesoporous molecular sieves and to compare their activity with that of Cu-ZSM-5. 

The H-ZSM-5 (Si/Al=25, PQ Corp.) and Silicalite (S-1) zeolites were used to prepare the microporous copper-containing catalysts, Cu-ZSM-5 and Cu-S-1. ( We recall that S-1 has the same framework topology of ZSM-5, but without AP+ ions in the framework, and therefore S-1 is an all silica materials as MCM-41.) S-1 was prepared using TEOS and a 20% aqueous solution of tetrapropylammonium hydroxide (TPA-OH) (Fluka-purum). TEOS was poured in the TPA-OH solution. The resulting mixture was kept at 333 K for 3 h and then heated under autogenous pressure in a 350 mL stainless autoclave in an oven at 448 K for 24 h, without stirring. The solid was washed with water, dried 2 h at 383 K and finally treated in air at 823 K for 5 h. Further details were reported in Ref 4. 

Cu-Exchanged Zeolites. Copper ions and/or complexes exchanged into such commercially manufactured zeolites as MFI, MOR, FAU, FER, BEA and so forth have been shown to be active for deNOx catalysis with HCs. Catalytic deNOx activity for this reaction can be maximized when combining copper with the MFI structure zeolites, representatively ZSM-5, depending mainly on the nature of reductant and physicochemical properties of the zeolite employed. These Cu-based zeolites reveal the peak NOx reduction activity at higher tern-... 

With increasing temperatures the NO conversion over most catalysts of this group passes throu a maximum (33). Cu/ZSM-5 has the highest maximum at the lowest temperature, namely, 500°C. The activity of Cu in other zeolites, e.g., mordenite, ferrierite, faujasite, /3 and L, is distinctly lower (33,35). A rough correlation seems to exist between activity and Si/Al ratio of these zeolites (33) Li and Hall state, however, that the Si/Al cannot be the sole controlling factor, because Cu/Y and dealuminated Cu/Y have very similar activities (i5). 

The formation of Cu —O—Cu pairs in ZSM-5 was also reported on the basis of fluorescence measurements (346)-, this assignment was confirmed in later work (35). In the absence of a reducing agent NO is decomposed to N2 + O2 over Cu/ZSM-5 at high temperatures (347). However, downstream of the catalyst bed O2 reacts with undecomposed NO to NO2. N2 yields close to 100% have been reported for Cu/ZSM-5, with the Cu load exceeding 100% ion exchange capacity of the zeolite (348). 

MO -zeolites prepared by incorporation of metal complexes followed by mild oxidation offer unique control of activity. Comparison of Cu-ZSM-5 catalysts... 

S Sato, Y Yu-u H Yahiro, N Mizuno, and M Iwamoto, Cu ZSM 5 zeolite as highly active catalyst for removal of nitrogen monoxide from emission of diesel engines, AppL Catal 70U (1991)... 

Basaldella El, Kikot A, Quincoces CE, and Gonzalez MG. Preparation of supported Cu/ZSM-5 zeolite films for DeNO(x) reaction. Mater Lett 2001 51(4) 289-294. 

Another important zeolite-catalyzed chemical reaction is the decomposition of NO. Cu-exchanged zeolites, especially Cu-ZSM-5, have been shown to catalyze the decomposition of NO in the presence of hydrocarbons and excess oxygen. The increasing awareness of the detrimental effects of automobile exhaust has prompted several theoretical studies on the active site and reaction mechanism. ° Cu-ZSM-5 was described using an empirical force field and energy minimization to locate the copper ions in ZSM-5. Isolated copper atoms and copper clusters were found in the micropores, mostly associated with framework aluminium species. A cluster of two copper ions bridged via an OH species not part of the zeolite framework ( extra-framework ) was proposed as the active site. Quantum mechanical cluster calculations were carried out to study the elementary steps in the NO decomposition. A single T-site model was used to represent the zeolite framework. 

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