Metallic in zeolites

The distribution of iron and other metals in zeolites has been studied by many authors. In general, Fe can occupy three positions in ZSM-5 matrix [26] (1) as isolated ions in the tetrahedral lattice positions (2) as isolated ions or small complexes outside the lattice but inside... 

The cluster size of the transition metal in zeolites was determined for a number of different preparations. In the mesoporous MCM-41 materials [48, 49] isolated clusters were observed, whereas for some solid-state exchanged and chemical vapor deposition samples dimeric species similar to methane monooxygenase were suggested [50, 51]. To date the discussion centers on clustered versus isolated species present in the various zeoHtes. 

Removal of coordinating ligands by careful calcination prior to reduction, is therefore extremely important for metal/zeolite catalysts, because it controls the cation locations and thus the metal particle growth mechanism during subsequent reduction. It has been demonstrated that the ultimate metal dispersion depends on the temperature of the calcination (50,69,71,79,107). An optimum calcination temperature can be defined for obtaining maximum dispersion of metals in zeolites. 

Structure-rate relationships W. M. H. Sachtler and Z. Zhang present a view of many aspects of catalysis and catalysts utilizing transition metals in zeolites E. Iglesia et al. discuss catalysts, mechanisms, and performance in the Fischer-Tropsch reaction and Y. Morikawa makes us aware of a class of intracrystalline catalysts other than zeolites. 

Incorporation of metals in zeolites occurs usually via ion exchange with the respective metal salt and subsequent reduction. A large body of information exists on the preparation and characterization of mono and bimetallic particles in molecular sieves and the reader is referred to that literature for details [197,198,199]. 

Although it has long been recognized that very highly dispersed metals in zeolites can be prepared, the literature of these materials indicates the difficulty of the characterizations and the recent advances resulting from the use of modem physical methods, especially EXAFS spectroscopy. These points are illustrated by the research with zeolite-supported platinum reported over a period of years by Boudart and coworkers. 

Almost all the examples involve acid catalysis, and researchers have been enticed by the possibilities of shape selective catalysis by metals in zeolites. [104-106]... 

Cations of noble metals in zeolites are easily reduced by H2, and this is the method most commonly applied. Proper activation and reduction treatments are essential to obtain the highest metal dispersions. Empirically determined treatment conditions are usually employed to obtain well dispersed metals, and generalizations about them are difficult. For example, prior to the final reduction in H2, it is usually necessary to eliminate any NH3 produced during the thermal decomposition of an amine complex or a NH4 ion in the zeolite. The reduction of metal ions in the presence of evolving NH3 can easily lead to the formation of agglomerated metal. Furthermore, Dalla Betta and Boudart [129] pointed out... 

Zeolite supported metal dusters have been characterized by several kinds of NMR spectroscopy. Xe NMR spectroscopy provides a sensitive probe of the contents of the cages and NMR of spin active metals provide evidence for the various metals in zeolites. Spectra of the sorbed xenon give information about the chemical shifts assodated with the collisions of the xenon atoms with the cage walls and with the encaged spedes. Hence, the method can be used to estimate the average number of metal atoms per duster. [148] The line shape and the chemical shift are sensitive to the metal spedes present within the cages or channels. For example, zeolite supported metal dusters show much larger chemical shifts than the zeolite alone under the same experimental conditions. 

The results based on chemical shifts should be interpreted with caution because the difference between the chemical shifts of the supported sample and those of the zeolite support depends on the xenon pressure, [149] on the type of cations exchanged into zeolite (e.g. divalent vs. monovalent, [150-153]) and on the temperature and size of the zeolite crystals. [154, 155] A recent publication by Ryoo et al. [149] illustrates the application of the technique in characterizing dusters of several different transition metals in zeolite Y. 

TPR experiments have been applied for investigations into the reduction behavior of bulk and supported reducible species, solid solutions, promoted metal catalysts, metals in zeolites, and of supported sulfides and nitrides (66). The reduction profile of a Pd0/Si02 sample is shown in Figure 10. The negative peak at 300 K corresponds to the consumption of H2 when Pd + is reduced to metallic Pd°. Simultaneously, a small amount of hydrogen is dissolved in the growing Pd° particles, which is then released at 345 K, as indicated by the positive peak. 

Although to date there are no clear examples of mixed alkali metal clusters in zeolites, the knowledge that the featureless singlet ESR lines often observed from alkali metals in zeolites derive from interacting clusters rather than metal particles [67] provides evidence that such clusters may exist Blatter et al. [68], for instance, reported ESR signals with a range of g-values characteristic of electrons interacting with both sodium and caesium nuclei. 

---