Clusters in zeolites

Rhenium is one of the oxophilic atoms effective for oxidation reactions. ReOx species are likely to have chemical interaction with various oxide supports and exhibit unique catalytic properties that cannot be observed on monomeric rhenium oxides. A new active six-membered octahedral Re cluster in zeolite pores (H-ZSM-5 [HZ]) is produced from inactive [Re04] monomers in situ under selective propene oxidation to acrolein (C3H6+02 - CH2=CHCH0+H20) in the presence of ammonia that is not involved in the reaction equation [16], The cluster is transformed back to the original inactive monomer in the absence ammonia. Note that coexistence of spectator NH3 is indispensable for the selective oxidation. 

FIGURE 9.3. FTIR spectrum of Pd carbonyl cluster in zeolite cages. 

Ship-in-a-Bottle Synthesis of Bimetallic Clusters in Zeolites... 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

Bonn M, Bakker H J, Kleyn A W, Santen R A Van. 1996. Dynamics of infrared photodissociation of methanol clusters in zeolites and in solution. JPhys Chem 100 15301-15304. 

There does seem to be some quite good evidence for the existence of small electron-deficient metal clusters in zeolites (101-105), which may be related to their increased resistance to sulfur poisoning. Further studies are, however, necessary in order to provide a more detailed understanding in this area. 

Several major areas of interest can be recognized, namely (1) the state and location of metal cations and clusters in zeolites, (2) the nature of carbocations and unsaturated hydrocarbon species (exclusively in zeolites), and (3) the nature of oxide-supported highly dispersed transition metal oxide species. The first and the third of these areas show that UV-vis spectroscopy is a useful laboratory method for analyzing the local environment of metal cations in cases where long-range order is absent. 

Metal clusters in zeolites can be regarded as intermediate forms between the naked clusters and clusters which are stabilized chemically by a ligand shell. Fe and Co clusters with diameters of 4-11 A have been prepared from bis(toluol)metal complexes and NaY zeolite93 ). 

What is the structure and the dynamics of hydrated/solvated electron in hot/supercritical water In dispersed clusters of polar liquids in nonpolar liquids In microheterogeneous media (e.g. water clusters in zeolite cavities) In mixed and complex solvents of practical importance (e.g. Ref. 107) on surfaces ... 

The interaction of metal clusters in zeolites with protons has been studied by isotope exchange with D2 280-283). The presence of metals or metal impurities is essential for this exchange to occur at moderate temperatures absolutely no exchange between D2 and zeolite protons in metal-free HY is detected at room temperature. In the presence of Pt or Pd, however, exchange is fast and includes all protons in the zeolite, which are detectable by their O—H vibration bands in FTIR. 

A recent, well-defined example of such clusters is the existence of CdS superclusters in zeolites (86,87). The authors prepared very small CdS clusters in zeolites. In zeolite Y, for example, there are sodalite cages (5... 

A. Ship-in-Bottle Synthesis of Metal Clusters in Zeolites and Clay Minerals Structures and Reactivity of Metal Clusters in Zeolites... 

Ichikawa M, Pan W, Imada Y, Yamaguchi M, Isobe K, Shido T (1996) Surface-grafted metal oxide clusters and metal carbonyl clusters in zeolite micropores XAES/ETIR/TPD characterization and catalytic behavior. J Mol Catal A Chem 107 23... 

The activity of platinum clusters in zeolites for the hydrogenolysis of neopentane, ethane, and butane exceeds that of conventional platinum catalysts, and in two cases the results were interpreted in terms of an intrinsic change of electronic properties with particle size. In butane hydrogenolysis the sticking probability of alkane on the electrophilic platinum cluster was high compared with that on conventional platinum crystallites. ... 

These and other aspects of catalysis by metal clusters in zeolites have been reviewed recently by Gallezot, and by Uytterhoeven. ... 

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