Zeolite metal containing

Zeolite-like Transition Metal Containing Porous Compounds... 

Together with the fast oxidation (at low temperatures) of NO to N02, the plasma causes the partial HC oxidation (using propylene, the formation of CO, C02, acetaldehyde and formaldehyde was observed). Both the effects cause a large promotion in activity of the downstream catalyst [86]. For example, a "/-alumina catalyst which is essentially inactive in the SCR of NO with propene at temperatures 200°C allows the conversion of NO of about 80% (in the presence of NTP). Formation of aldehydes follows the trend of NO concentration suggesting their role in the reaction mechanism. Metal oxides such as alumina, zirconia or metal-containing zeolites (Ba/Y, for example) have been used [84-87], but a systematic screening of the catalysts to be used together with NTP was not carried out. Therefore, considerable improvements may still be expected. 

Work with the objective of comparing oxo-ions with oxide particles in order to test the validity of this reasoning has been reported by Chen et al. who used a catalyst that initially contains Fe oxo-ions, [HO-Fe-0-Fe-OH] +. These sites were first converted to Fc203 particies by a simpie chemical treatment. This was followed by another treatment, which redispersed these Fc203 particies back to oxo-ions. The change in particle size was monitored by a spectroscopic method based on the observation that in zeolites metal ions and oxo-ions, that are attached to the wall of a cage, give rise to a typical IR band caused by the perturbation of the vibrations of the zeolite lattice. 

Full catalyst formulations consist of zeolite, metal and a binder, which provides a matrix to contain the metal and zeolite, as well as allowing the composite to be shaped and have strength for handling. The catalyst particle shape, size and porosity can impact the diffusion properties. These can be important in facile reactions such as xylene isomerization, where diffusion of reactants and products may become rate-limiting. The binder properties and chemistry are also key features, as the binder may supply sites for metal clusters and affect coke formation during the process. The binders often used for these catalysts include alumina, silica and mixtures of other refractory oxides. 

The paper deals with some new data concerning the state of the metal after reduction and the catalytic functions of zeolite catalysts containing nickel and platinum. By using the molecular sieve selectivity in the hydrogenation of mesitylene it has been proved that metal (platinum) is contained in the volume of the zeolite crystal. The temperature dependence of the formation of nickel crystals was investigated. The aluminosilicate structure and the zeolite composition influence mainly the formation of the metal surface which determines the catalytic activity. In the hydrocracking of cumene and disproportionation of toluene a bifunctional action of catalysts has been established. Hydrogen retarded the reaction. 

Feolite catalysts modified by transition metals are interesting and difficult subjects to study. In one of the first studies of zeolites as catalysts, Rabo and co-workers (1) used a zeolite catalyst containing 0.5% platinum for isomerization of n-paraffins. In this reaction the metal-zeolite system acted as a typical representative of the bifunctional catalysts. Studies of zeolites modified by transition metals ( 2, 3, Jf) showed that their polyfunctional properties are determined by the structural and chemical properties of the zeolite and by the state of the metal in it. In this paper we discuss new data on the metal state after reduction as well as the catalytic functions of zeolite catalysts containing nickel and platinum. 

Significant improvements have also been introduced with the use of heterogeneous catalysts that are less water-sensitive than homogeneous Lewis acids and more convenient because of easier reaction mixture work-up. An important class of MPVO solid catalysts consists of zeolite beta and its metal-containing derivatives, especially Sn-, Zr- and Ti-beta. Several examples are known and the reduction or oxidation can be performed either in the gas phase [11, 12] or in solution [13, 14]. A very recent paper also reports the use of a bifunctional Zr-beta-sup-ported Rh catalyst able to promote both arene and carbonyl reduction [15],... 

In this paper, we review primary and secondary shape selective acid catalysis with zeolites. Next, we discuss shape selectivity with metal containing zeolites.We conclude with a section that deals with future trends in shape selective catalysis. 

In zeolite NH4Y, about 73% of the ammonium ions are situated in large cavities and can be replaced by Pt(NH3)4+ ions [38-40]. The saturation of ail of these exchangeable sites corresponds to the fixing of more than 25wt% of platinum in relation to the anhydrous zeolite. Yet, for obvious reasons of price, industrial catalysts must contain small amounts of noble metal, less than 1 wt% in relation to the zeolite, and the catalyst manufacturer will try to fix on the zeolite all the metal contained in the solution. This amount of metal contained thus represents 3-4% at the most of the exchangeable sites of the zeolite. 

In conclusion, the rapid crystallization method is very effective not only for rapid synthesis, but also for synthesis of metal-containing uniform zeolitic materials which show higher catalytic activity and selectivity. 

Recently, a nickel zeolite hydrogenation catalyst has been prepared by a novel route (94) involving the adsorption and decomposition of nickel carbonyl onto NaX, which would not be expected to result in the formation of acid sites. In general, the platinum metal-containing zeolites are more active than those containing other transition metals. For example, in zeolite Y the following activity series has been found,... 

Metal containing zeolites are of great interest as practical catalysts for petrochemical reactions. Such catalysts are usually considered to be bifunctional, with the zeolite framework acting as a support for the metal centers. In order to optimize reaction conditions and parameters in the processes for which metal-zeolite catalysts are used, it i s necessary to understand the chemistry of the support and the metal and their interaction. 

Whilst the metal containing xerogels show reasonable catalytic activity with hydrogen peroxide, they are still inferior to the redox zeolites mentioned earlier. 

Dimerization presumably takes place on the transition metal-containing sites, and alkylation on the acidic sites of zeolltic surface. The sodium form of zeolite exchanged with transition metal cations Is capable of dimerization (and further polymerization), but does not practically exhibit alkylating capacity. This explains the composition of the product obtained from ethylene and Isobutane over this catalyst (Table V, column 3). 

Zeolite catalysts containing platinum and other precious metals are prepared by ion exchange of species such as Pt(NH3)4 +, for example, the catalyst Pt/zeolite KL that catalyzes hexane aromatization to selectively form benzene. 

Transition-metal containing zeolites such as CoY and NiY (but not the Cu, Mn and Zn forms) polymerize acetylene to give trans-polyacetylene with relatively short conjugation length, as indicated by resonance Raman spectroscopy.70 The pol3nmerization products appeared to be restricted to the zeolite crystal surfaces. The authors also point to die importance of Lewis acidic centers for the polymerization. 

---