Frameworks, cations, clusters

14-0-01 - Imaging the mesopores in zeolite Y using three-dimensional transmission electron microscopy 

A 3D-TEM study on a series of Y zeolites reveals the mesopores (generated by steaming and/or acid leaching) with great clarity. Both the diameters and shapes of the visualised pores correspond very well with nitrogen physisorption measurements of the entire sample. Also cimorphous alumina in the mesopores and on the external surface can be visualised, which is in agreement with results obtained by XPS on these samples. From these results a more detailed model for the formation of mesopores in zeolite Y is proposed. 

Universitdt Leipzig, Abteilmg Grenzfldchenphysik, freude mi-leipzig.de 

14-0-03 - Theoretical interpretation of UV-VIS spectra of Cu- and Ag-species in zeolites structure vs. transition energies 

Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic and Center for Complex Molecular Systems and Biomolecules, DolejSkova 3, 182 23 Prague 8, Czech Republic 

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The existence of the carbon monoxide dimer is currently accepted as a weakly bound molecular complex [CO CO] [124], Besides, it has been found that weakly bound molecular clusters of carbon monoxide dimers and cyclic trimers, confined in zeolite cavities and channels, are stabilized under the zeolite framework cationic field [125], These structures are similar in geometric configuration to those reported in Figure 9.14 however these are not covalently bonded, but are only weakly bound. 

A notable example is the wheel-shaped cationic cluster [Er36([X3-OH)3o([i3-0)6(BDC)6] " " (Figure 6.41) present in the lanthanide-transition metal sandwich framework comprising (Cus) cluster pillars and layered networks of (Erse) wheels [25, 77]. The Er + ions are linked by hydroxo and oxo bridges to give two types of smaller cluster cores cubic [Er4([i3-0)([i3-0H)3] + (Ere) and dimeric [Er2([i3-OH)2]" + (Er2) cores (Figure 6.41a). Different from the familiar cubane [Ln4([X3-OH)4] + which contains four [X3-OH groups, the present (Er4) cluster... 

Figure 6.43 (a) A ball-and-stick view of the cationic cluster of [Eu54Ni54(IDA)4g (0H)i44(C03)6(H20)25] and (b) a four-shell presentation showing only its metal frameworks [25]. (Reproduced from Z.P. Zheng, Cluster compounds of the f-elements, in K.A. Gschneidner, Jr., J.C.G. Bilnzli, and V.K. Pecharsky (eds.). Handbook on the Physics and Chemistry of Rare Earths, volume 40, 2010, with permission from Elsevier.)... 

Generally, a metal cluster can be regarded as a tiny metal particle. However, close inspection of the internal structure and electron configuration of a metal cluster will reveal that metal clusters are distinct from metal particles. Correspondingly, their physicochemical properties are also very different from those of metal particles. A metal cluster may be naked, but, in most cases, they are enclosed by ligands, because otherwise they would not be stable. Because in a zeolite there are framework O and extraframework cations, and these O or extra-framework cations can protect the metal clusters in the channels of zeolites, the metal clusters may be stabilized.131... 

FIGURE 97 A ball-and-stick view of the cationic cluster [Eu54Ni54(IDA)48(OH)i44 (C03)6(H20)2s] (left) and a four-shell presentation showing only its metal frameworks (right) (redrawn after Kong et al., 2008a). 

Thanks to the synergistic coordination of BDC and IN, lanthanide-transition metal sandwich framework comprising CU3 cluster pillars and layered networks of Erag wheels has been reported. X-ray diffraction studies revealed a stunningly beautiful 3D network structure composed of a 2D network of wheel-shaped cationic clusters [Er36(/i3-OH)3o(/is-... 

The elements of group 16 in the Periodic Table have a comparatively large number of valence electrons to be distributed over the cluster framework in which they are incorporated. Their homoatomic clusters are therefore often electron-rich in the sense that they have an electron count higher than 5n. The more electron-poor character of the elements of group 15 results in a borderline situation where cationic clusters are found to be electron-poor (Sec. 1.29.4.4), whereas the anionic ones are electron-rich. The structurally characterized, electron-rich naked clusters of the post-transition elements are listed in Table 4. 

Xu YH, La YQ, Zhao YH, Du D-Y, Xu G-J, Shao K-Z, et al. An unprecedented [Cu.yIJ cationic cluster-based metal-organic framework (MOF) including ID nanochannels. Inorg Chem Commun 2009 12 169-72. 

Metal clusters (M ,) can be roughly described as small pieces of metal,but closer studies show that their atomic and electronic structures deviate from those of bulk metals either because of intrinsic size effects (deviation from normal electronic structure at very small nuclearities) or because of their interaction with framework anions and extra-framework species including cations. Clusters can be naked or bonded to various extra-framework ligands. They may be charged as a result of incomplete reduction or of interactions (inductive and/or field effects) with framework or extra-framework species. Clusters can involve two metals homogeneously alloyed or segregated. 

Advances in crystal characterization have been made in X-ray crystallography and in electron microscopy by the use of reiterative refinements using independently determined parameters. NMR-MAS has grown to be the most effective tool to describe the environment, and possibly the ordering, of framework cations. Far infrared spectroscopy has the potential to provide detailed information characterizing the chemical environment of extra-framework cations and small occluded metal clusters. 

Ms " clusters have 12 framework bonding electrons as has [BsHs]- (p. 161) the anions are also isoelectronic with the well-known cation [Bis]. Similarly, the alloy NaSn. 2.23 reacts with cryptand in ethylenediamine to give dark-red crystals of [Na(ciypt)]4 [Sng] the anion is the first example of a C41, unicapped Archi-median antiprism (Fig. 10. lOc) and differs from the >3/, structure of the isoelectronic cation [Bis] + which, in the salt Bi+[Bi9] +[HfCl6]5 (p. 591), features a tricapped trigonal prism, as in [BgHg] " (p. 153). The emerald green species [Pb9] , which is stable in liquid NH3 solution, has not so far proved amenable to isolation via ciyptand-complexed cations. 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

The electrostatically favored cation (Li) and anion (RE) arrangement implies the presence of two different E-, Si- and Li sorts, which has been established by solution and solid-state NMR spectroscopy. The electronic structures of the mixed-valent pnictides 10 and 11 have been simply described as electron-deficient clusters with delocalized framework electrons. Formally the latter consist of two low-valent anediyl moieties RE and eight andiides (RE)2- (E = P, As). The relatively large E-E distances of >4 A exclude the occurrence of localized E-E bonds. However, delocalization of the cluster valence electrons is achieved without Li-Li bonds via Li-mediated multiple bonding. Evidence for this has been seen in the NMR spectra (31P, 7Li, 29Si), which are in accordance with the electron delocalization model (see later discussion). 

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