Zeolites metal particle formation

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. 

Mixed clusters much more active. Formation of bimetallic clusters depends on surface migration of Re oxide to hydrogen covered Pt particle and coalescence of migrating metal particles (especially on zeolites).41 ... 

Tailoring of the product distribution is possible by a limitation of chain growth by pore size. This has been demonstrated by Ballivel Tkatchenko and Tkatchenko using zeolite catalysts. Ruthenium, iron or cobalt metal particles in Y-zcolilc supcrcagcs were prepared by thermal decomposition of the carbonyls. These metal-zeolite catalysts give selective formation of )- hydrocarbons [471. 

Another strategy for the rejuvenation of metal/zeolite catalysts with very large metal particles achieves mobilization by the formation of volatile... 

If this stereochemical concept is valid, one would predict that MCP ring opening leads to the preferred formation of 3-methylpentane whenever the oriented molecule hits a metal particle in a zeolite pore. The opposite effect, preferential formation of 2-methylpentane, would be predicted for a situation in which a metal site is anchored in a niche in the wall of a pore through which the MCP molecules are diffusing. This has indeed recently been observed for Pt/mordenite—Pt atoms have been located in the side pockets of the main channel 321). Figure 28 shows that for very low metal... 

Metals are modified by a second element due to the change of either electronic or geometric environment. Formation of stable bimetallic particles or alloys is prerequisite for improved catalytic properties. It seems even more important when metal particles are entrapped inside zeolite cages, like faujasite-type zeolite which is remarkably different from oxide supports. 

Studies of catalysts deactivation by coke are abundant in the literature most of them are usually conducted at high temperatures (around 500°C) using metal catalysts supported on oxides with low surface area such as silica, aluminas or silica-alumina [2 and references therein]. The deactivation by coke of zeolite catalysts has also been studied and such studies have mostly been done for high temperature reactions such as the conversion of n-hexane or the isomerization of xylenes [2,4]. However, low temperature coke formation (20-25°C) combining the effect of high acidity and size specificity for a high coking component such as nickel, has not yet been considered from the point of view of the presence of compounded effects of crystalline structure and location of metal particles. 

B. Xu and L. Kevan, Formation of Silver Ionic Clusters and Silver Metal Particles in Zeolite Rho Studied by Electron Spin Resonance and Far-infrared Spectroscopies. J. Phys. Chem., 1991, 95, 1147-1151. 

Transition metal clusters in zeolite cages form another important class of supported species. Zeolites are very suitable supports/hosts for small metal particles because the dimensions of their cavities affect the formation of encaged moieties of nanometer and sub-nanometer scale, thus stabilizing clusters of desired sizes and shapes. Experimental investigations in this direction resulted in preparation procedures that yield nearly uniform small encapsulated metal moieties in zeolites [18-20]. 

Bimetallic bifunctional catalysts containing different proportions of Ni and Pt supported in HUSY zeolite were prepared and characterized by TEM, punctual EDX analysis and -hexane isomerization. The EDX analysis of the Ni and Pt bimetallic catalysts shows that the metal particles contain both metals and from HRTEM it was observed that the bimetallic particles have crystallographic parameters of metallic nickel. The presence of small platinum amounts in the nickel catalysts produces more active catalysts for the -hexane isomerization, and presents also higher selectivity for the formation of dibranched hexane than the ones containing only platinum. 

Models for metal-support interactions occurring at moderate temperatures include (i) M + cations adjacent to the metal particle (Figure 2.9A), (ii) unreduced M + cations by proton acquisition in acidic zeolites (Figure 2.9B) and (iii) formation of M"+ cations by proton acquisition in acidic zeolites (Figure 2.9C) these all give the metal an electron-deficient... 

Alumina supports for metal particles were synthesized in the form of fibers and thin plates suitable both for catalyst studies and electron microscopic examination.(106) Light scattering, proton resonance, viscosity measurements, were used to study the formation of silica gels and monodisperse sols.(114) The same techniques were used to study synthesis of zeolites. 

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. 

Incorporation of metals or metal oxides into zeolite cavities leads to the formation of nanosized clusters exhibiting different catalytic properties from the bulk materials. These metal particles are usually introduced into zeolite channels through ion-exchange followed by reduction or oxidation/reduction to get their final dispersions. Metal clusters can also be formed via zeolite impregnation by corresponding azides from methanolic solutions followed by thermal decomposition. " Catalytic activities of the bifunctional or basic catalysts prepared using these methods can be successfully combined with shape-selective properties of parent zeolites. 

Romanovsky et al (19-22,32) have used this method extensively. The decomposition of Fe(C0)5 is critical according to the work of Bein et al. (33). There exist specific conditions in which it is possible to decompose adsorbed ironpentacarbonyl quantitatively into occluded iron metal particles. The major disadvantage of this method is the easy formation of extra-zeolitic iron clusters. Moreover, it is impossible to isolate a single Fe atom per supercage and consequently to transform Fe in a quantitative way into isolated and zeolite occluded FePc complexes. 

The reduced Pt/K-LTL zeolite has been studied by EXAFS at RT, before and after CO adrttission [%M2]. After reduction, very small platinttm metal particles were present consisting of five to six atoms. The CO admission, at RT, leads to complete decortqrosition of the platinum metal particles arrd the formation of a platinrtm carboxyl clrrster most probably stabilized by the zeolite walls. The platinrrm carbortyl cluster just fits irtside the pores of the zeolite LTL. 

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