Metal loading, zeolites with

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

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. 

The process of loading zeolites with organometallie complexes always brings to the forefront the question of internal versus external confinement of the metal guest. In this paper we present some experiments based on size exclusion, metal loading and intrazeolite chemistry which in conjunction with FT-FAR-IR, EPR and UV-visible reflectance spectroscopy, critically probe the question of internal versus external location for the case of five representative organometallics,... 

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. 

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). 

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. 

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. 

Transition metal-incorporated zeolites have been shown to be effident catalysts for direct conversion of methane to benzene and toluene under nonoxidative conditions [45,46]. Bao and co-workers revealed that Mo/ H-MCM-22 catalysts are desirable bifiinctional catalysts for methane dehydroaromatization reaction [47]. In terms of catalytic performances of Mo/H-MCM-22 with varied metal loading, catalyst with a Mo loading of ca. 6 wt% was found to exhibit the optimal benzene selectivity, suppressed naphthalene yield, and prolonged catalyst hfe under a moderate methane conversion. Although both Bronsted and Lewis acid sites are capable of catalysing methane conversion reaction, active sites with higher acidic strengths are anticipated to play the dominant role. 

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. 

Molybdenum/zeolite catalysts prepared by impregnating zeolites with ammonium hepiamolybdate solution generally give rise to poor dispersion of molybdenum [2]. In contrast, ion exchange would be an ideal method for loading active metal species onto supports. Few cationic forms are available as simple salts of molybdenum of high oxidation... 

It is well known that Rh(I) complexes can catalyze the carbonylation of methanol. A heterogenized catalyst was prepared by ion exchange of zeolite X or Y with Rh cations.126 The same catalytic cycle takes place in zeolites and in solution because the activation energy is nearly the same. The specific activity in zeolites, however, is less by an order of magnitude, suggesting that the Rh sites in the zeolite are not uniformly accessible. The oxidation of camphene was performed over zeolites exchanged with different metals (Mn, Co, Cu, Ni, and Zn).127 Cu-loaded zeolites have attracted considerable attention because of their unique properties applied in catalytic redox reactions.128-130 Four different Cu sites with defined coordinations have been found.131 It was found that the zeolitic media affects strongly the catalytic activity of the Cd2+ ion sites in Cd zeolites used to catalyze the hydration of acetylene.132... 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

Isomerization of olefins or paraffins is an acid-catalyzed reaction that can be carried out with any number of strong acids, including mineral acids, sulfated metal oxides, zeolites and precious metal-modified catalysts [10]. Often the catalyst contains both an acid function and a metal function. The two most prevalent catalysts are Pt/chlorided AI2O3 and Pt-loaded zeolites. The power of zeoHtes in this reaction type is due to their shape selectivity [11] and decreased sensitivity to water or other oxygenates versus AICI3. It is possible to control the selectivity of the reaction to the desired product by using a zeoHte with the proper characteristics [12]. These reactions are covered in more detail in Chapter 14. 

They have been used to obtain well dispersed metal catalysts. Early experiments dealt with platinum-loaded zeolites (1,2). 

The scheme implies that in the presence of a metal which establishes the olefin-paraffin equilibrium, the carbonium ion concentration on the surface depends on the hydrogen partial pressure. The stabilizing effect of a given metal load will depend on its dispersion and distribution and on the prevailing hydrogen pressure. Similar experiments show that for zeolite Y based catalysts the reaction mechanism is identical with that discussed above for mordenite. 

This paper describes some new zeolite organometallic impregnation experiments, in which a combination of metal loading, size exclusion, intrazeolite chemistry and diffusion considerations in conjunction with epr, far-IR and UV-visible reflectance spectroscopic probes, serve to distinguish metal guests located in the intracrystalline voids of the zeolite from those located on the external surface. 

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