Zeolites Cu-exchanged

Cu-exchanged zeolites have been examined in the nucleophilic substitution of halobenzenes towards aminated and oxygenated systems. Selectivities are dependent on the zeolite s pore sizes. 

Schneider, W.F., Hass, K.C., Ramprasad, R. et al. (1998) Density functional theory study of transformations of nitrogen oxides catalyzed by Cu-exchanged zeolites, J. Phys. Chem. B, 102, 3692. 

Tajima, N., Hashimoto, M., Toyama, F. et al. (1999) A theoretical study on the catalysis of Cu-exchanged zeolite for the decomposition of nitric oxide, Phys. Chem. Chem. Phys., 1, 3823. 

For these reasons, recent investigations have focused on the development of more practical reaction conditions. Bases such as DBU or DBN increase the reaction rate, and the reaction can be performed at room temperature within a few hours in the presence of these additives [12]. The combination of Cu(CH3CN)4BF4 as catalyst and benzotrifluoride as solvent enables the reaction to be conducted without excess olefin in good yields and short reaction times [13]. Recyclable catalysts have also been developed. Fluorous [14] or water-soluble catalysts [15] and Cu-exchange zeolites [16] can be reused and enable several catalytic cycles without loss of activity. 

Kinetic and mechanism. Table 16.1 presents the reaction order with respect to NO, NH3 and O2 for the SCR on various Cu-zeolites. On Cu-exchanged zeolites it is... 

Table 4 Water tolerance of Cu-exchanged zeolite catalysts for the selective reduction of NO by HCs... 

Copper ions in zeolites are mainly responsible for NO reduction activity therefore, deactivation would occur if alteration of their state or the zeolite structure were accompanied. The origin of the deactivation of Cu-exchanged zeolites due to their hydrothermal excursion has been published in several literatures. Solid state Al MAS NMR spectra of a Cu-MFI catalyst appeared to have a loss of 23% of framework aluminum after hydrothermal aging at 410°C in flowing 10% H20/90% air for 113 h. Kharas et reported the formation of... 

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-... 

Cu-exchanged zeolite catalyst containing strong water tolerance and hydrothermal stability, although a method for preparation by using a solid ion exchange chemical transport technique has been suggested. 

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. 

Zeolites are also able to decompose N2O. Li and Armor (11] prepared 25 zeohte samples by exchanging the cation Na+. These catalysts were tested for the catalytic decomposition of N2O. The Co- and Cu-exchanged zeolites are active at temperatures ranging from 623 K to 673 K. Rhodium and ruthenium supported on ZSM-5 are active catalysts between 523 and 573 K. 

We conclude that Cu strongly prefers type I and type II six-rings above the hexagonal prisms, conclusion which is experimentally confirmed by the XRD studies on dehydrated Cu-exchanged zeolites (8,9). 

Cyclodimerization of 1,3-butadiene over Cu-exchanged zeolites FAU and EMT was followed in situ by DRIFT in a study by Voskoboinikov et al. [906]. The reaction was Brqnsted acid-catalyzed. Even though the deactivation was rather Uttle, it could be also monitored in situ through the IR spectra showing the formation of polycyclic naphthenes, which then in part consecutively transformed into aromatics. 

The preferred method for controlling NOx, once formed, is to catalytically decompose it to nitrogen and oxygen. Although thermodynamically favored, steady-state direct NO decomposition in the presence of excess O2 has only been demonstrated recently. Iwamoto and co-workers have demonstrated that Cu-exchanged zeolites, and in particular Cu-ZSM5, are the most active catalysts for this... 

Goodman BR, Schneider WF, Hass KC, Adams JB (1998) Theoretical analysis of oxygen-bridged Cu pairs in Cu-exchanged zeolites. Catal Lett 56 183... 

The Cu-exchanged zeolite ZSM-5 revealed a particularly high and steady activity for the direct catalytic conversion of NO into N2 and O2 ( DeNOx activity) [8612]. The same system is also active for the selective catalytic reduction of NO by hydrocarbons in the presence of excess oxygen [95 S2, 96T1 ]. 

---