Zeolites metal-loaded

Infrared spectroscopy can be used to obtain a great deal of information about zeolitic materials. As mentioned earlier, analysis of the resulting absorbance bands can be used to get information about the structure of the zeolite and other functional groups present due to the synthesis and subsequent treatments. In addition, infrared spectroscopy can be combined with adsorption of weak acid and base probe molecules to obtain information about the acidity and basicity of the material. Other probe molecules such as carbon monoxide and nitric oxide can be used to get information about the oxidation state, dispersion and location of metals on metal-loaded zeolites. 

Instead of the seed zeolite crystals, a mixture of one part y-alumina ( 0.2 pm ) and three parts a-alumina ( ca. 1 im ) was used as seed material. Fortunately, a similar effect on the crystallization was observed, although about 2 times the quantity of the seed materials was necessary (5)). The metal pre-loaded y and a-alumina mixture was then tried as the seed material and confirmed the same effect on crystallization. In the case of 0.4 wt% Ru ( ) and 0.7 wt% Rh (12) in the catalyst product a longer catalyst life was obtained, i.e., 1.32 times and 1.65 times, respectively, without any significant change in the activity and selectivity compared with the non metal-loaded zeolite catalyst. 

There are numerous examples of reactant shape selectivity in the hydrogenation of olefins over noble-metal loaded zeolites (49). This can be important to remove impurities from olefin feedstocks, or as a criterion to assess the location of the noble metal, at the outer or inner surface of the zeolite. However, shape selectivity is also increasingly used in reductive conversion of (poly)functional molecules. 

The adsorption property was measured by a static method at 30 °C with a conventional volumetric apparatus as well as by the temperature programmed desorption (TPD) method. The details of the pretreatment and adsorption procedures were shown in Results and Discussion section. Metal-loaded zeolite samples were characterized by XRD, diffuse reflectance UV-Vis spectroscopy (DRS) and electron spin resonance (ESR). 

Metal-loaded zeolites are expressed in the form M1/M2Z, where Mi = reduced metal, Mz = charge-compensating metal ion, and Z = zeolite type. For example, Pt/KL stands for platinum in the channels of an L-zeolite, with K as the charge-compensating cation. Details of zeolite types with their conventional abbreviations can be found in Zeolite Molecular Sieves (D. W. Breck Robert E. Krieger Publishing Company, Malabar, Florida, 1974). 

Fig. 6.—Reaction of noble-metal-loaded zeolites with hydrogen. The initial temperature on exposure to hydrogen was 300°C in all cases.

The metal functions can be elegantly combined with the acidic functions of the zeolitic support to obtain a very effective bifunctional catalyst. For example the selective isomerisation followed by dehydrogenation of limonene to give p-cymene (Scheme 24) can be carried out in one step over a multifunctionalised zeolite [213]. With an acidic boron zeolite (Si/B= 21) 21% selectivity to p-cymene was obtained at 100% conversion. Addition of 3 wt% Pd increased the selectivity to 70% at the same conversion. Further addition of Ce (1.5 wt% Pd, 3.5 wt% Ce) to the metal loaded zeolite led to 87% selectivity. 

Polyfunctional and composite catalysts offer many new shape selective uses for zeolites and other microporous materials. In metal-loaded zeolites, monoatomic dispersion is usually preferred. Pt- and Pd-zeolites are usually prepared by ion-exchange or impregnation. 

Generally, preparation of metal-loaded zeolite catalysts involves initial introduction of the metal component by impregnation, cation exchange, or—occasionally—physical adsorption of a volatile inorganic (such as Ni(CO)4), followed by an in situ thermal decomposition or reduction step. Thus, a Pt-containing zeolite catalyst was prepared by Rabo et al. 

Direct Decomposition of Methane to Hydrogen on Metal-Loaded Zeolite Catalyst... 

Sample Preparation. Cobalt catalysts were prepared by subliming Co2(C0)g into the pores of dehydrated NaX zeolite in a vacuum line at pressures of 1 x 10- f torr. Argon was flowed over the metal loaded zeolite sample at a pressure of 0.3 torr. A microwave plasma was induced with a static gun and the decomposition of the metal carbonyl precursor occurred for two hours. After total decomposition of the metal carbonyl which can be determined by the color of the plasma, the argon flow was stopped and the sample was sealed off by closing the Teflon stopcocks at both ends of the reactor. The sample was then brought into a drybox and loaded into catalytic reactors or holders for spectroscopic experiments. Further details of this procedure can be found elsewhere (11, 25). Iron samples were prepared in a similar fashion except ferrocene was used as a metal precursor. 

Interesting applications of metal-loaded zeolites as intracrystalline electrodes are proposed. The use of feeder electrodes and dispersions of metalated zeolites results in electrode functions without a direct electrical contact (Fig. 5). It was demonstrated that dispersion electrolysis can be achieved on platinum-loaded NaY zeolites [69]. 

Table 1. Adsorbed amounts of C2H4 on various metal-loaded zeolite adsorbents (30 p(piF0.IS) ...

The hydroconversion of -alkanes over noble metal-loaded zeolites follows a bifunctional mechanism, in which the reactant is dehydrogenated on the metal particles and the formed alkene is isomerized and/or cracked over the acid sites of the zeolite, followed by a subsequent hydrogenation of all unsaturated products on the metallic sites (see reaction Scheme 7). 

These mechanistic features were elucidated in detail in the 1960s. Based on the pioneering work of Mills et al. and Weisz ", a carbenium ion mechanism was proposed, similar to catalytic cracking plus additional hydrogenation and skeletal isomerization. More recent studies of paraffin hydrocracking over noble metal-loaded, zeolite based catalysts have concluded that the reaction mechanism is similar to that proposed earlier for amorphous, bifunctional hydrocracking catalysts. ... 

L6pez-Fonseca R, Gutidrrez-Ortiz JI, Gonzalez-Velasco JR. Noble Metal Loaded Zeolites for the Catalytic Oxidation of Chlorinated Hydrocarbons. React Kinet Catal Lett 2005 86 127-133. 

In the first instance UV-VIS and Raman spectroscopy are the traditional complementary methods yielding additional support to IR spectroscopic studies. Besides its application in exploring the coordination sphere of transition metal-loaded zeolites [3] and the interaction of zeolites with SO2 [136], UV-VIS spectroscopy has been successfully applied to the detection of carbenium ions formed during the reaction of hydrocarbons on the zeolite surface (see e.g. [135,137,138] and the references cited therein). 

With metal-loaded zeolites, this technique can be used to determine the quantitative distribution of molecules chemisorbed on the metal particles, the nximber of particles in the sample, and, consequently, the average number of atoms per metal particle. It can also be used to determine quantitatively the distribution of several gases simultaneously chemisorbed on these encaged particles and to determine the percentage of metal located inside or outside the zeolite crystallites. 

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