Hosts zeolites

We have observed large variations in the sorption capacities of zeolite samples characterized by (ID) channel systems, as for instance AFI (AIPO4-5 zeolite) and MTW (ZSM-12 zeolite) architectural framework types. Indeed, for such unconnected micropore networks, point defects or chemisorbed impurities can annihilate a huge number of sorption sites. Detailed analysis, by neutron diffraction of the structural properties of the sorbed phase / host zeolite system, has pointed out clear evidence of closed porosity existence. Percentage of such an enclosed porosity has been determined. 

Centi, G. Perathoner, S. Vazzana, F., Catalysis using guest single and mixed oxides in host zeolite matrices, in Catalysis by Unique Metal Ion Structures in Solid Matrices, Centi, G. Bell, A.T. Wichterlova, B. (eds.), Springer-Verlag. Dordrecht (The Netherlands), 2001,165-186. 

Mo, W) decreases with the increase in basic strength of the framework oxygen of the host zeolite [32]. The M(CO)3(Oz)3 (Oz = zeolite framework oxygen) species generated by the thermal treatment are active in the stereoselective hydrogenation of butadiene to cis-2-butene [33, 34]. 

However, zeolite-like materials with ultralarge pores such as Cloverite (20 T, 0.6 x 1.32 nm), VPI-5 (18 T, 1.27 nm), AIPO4-8 (14 T, 0.79 x 0.87 nm) were recently synthesized. A comparison between the pore openings of zeolites and the kinetic diameter of guest molecules shows clearly that zeolites can be used for molecular sieving. It should however be stressed that these values are temperature-dependant temperature increases both the flexibility of the guest molecules and the breathing motions of the host zeolite pore mouth and framework. 

The adsorption properties of silver- and cupper-loaded zeolites for C2 and C3 hydrocarbons were investigated to explore excellent materials for cold-start hydrocarbon trap. The adsorption property and the stability of the adsorbents depended significantly on the metal species and host zeolites. It has turned out that silver-loaded ferrierite zeolite is the promising material with excellent olefin selectivity, high adsorption capacity, desirable storage ability and hydrothermal stability. 

The polymer/zeolite host-guest precursor for preparation of porous carbon can also be obtained through direct contact of monomers in a carrier gas with zeolite molecular sieves followed by polymerization. For example, propylene can enter into zeolite Y under the carriage of N2 and polymerize to form polypropylene. After pyrolysis, the polypropylene undergoes carbonization, and the host zeolite framework of the carbonization product can be removed by dissolution in acids, leaving carbon material with characteristic pores.[93] However, the pore-size distribution of the porous carbons obtained through this approach is not uniform, and hence they are hardly used as molecular sieves for sieving small molecules. 

Consider now the effect of presorbed polar molecules on the diffusivity of nonpolar molecules in host zeolites. Blockage of polyhedral cavities by strongly sorbed polar molecules is found experimentally to lead to a... 

Figure 4.37. The effect of exterior boundary conditions on the sorption of guest molecules in a host zeolite system. Plotted is the percentage...

Intrazeolite molybdenum sulfide catalysts (MoSx/NaY) were prepared by a CVD technique using Mo(CO)6. On the basis of the Mo-Mo atomic distance, it has been shown that a MoSs-like molybdenum sulfide dimer clusters are formed when the Si/Al ratio is between 3.5 and 2.4. When the Si/Al ratio was very high, finely dispersed M0S2 particles were formed. The specific activity of molybdenum sulfide clusters for the HDS of thiophene decreased as the Mo-Mo atomic distance increased. It has been substantiated that the morphology, structure, and catalytic activity of intrazeolite molybdenum sulfides are controlled by the host zeolite composition. 

In the present study, we have shown that the morphology (dimer clusters or finely dispersed M0S2 particles), structure (Mo-Mo atomic distance), and catalytic activity (the HDS of thiophene) of molybdenum sulfide clusters encaged in NaY zeolite can be controlled by the factors such as the host zeolite composition (Si/Al ratio) through Mo-Oz interactions. 

As shown by studies on various zeolites (ZSM-5, mordenite, H-Y) [44,55], it is difficult to compare adsorptions of ammonia and pyridine, because of their difference in basicity and the specific interactions of each of these molecules with the host zeolite. The adsorption heats should be correlated, not to the basicity of the probe molecules in liquid phase or in aqueous solutions (where NH3 is more basic dian pyridine by about 20 kJ mol", because the pKa values are 9.3 and 5.2, respectively), but to their basicity in the gas phase, which can be expressed in terms of the proton affinities (PA). In the gas phase, isopropylamine is a stronger base than pyridine and tiian ammonia the PA value of isopropylamine (pKa = 10.7) is about 924 U mol, and tiiat of pyridine is about 922.2 kJ mol", while that of ammonia is only about 857.7 kJ mol". This feature is illustrated in Figure 3, which represents the differential heats of ammonia, pyridine and isopropylamine vs uptake on a H-mordenite zeolite [54]. Other parameters that have to be taken into account are size (the kinetic diameters of pyridine and ammonia are 0.533 and 0.375 nm, respectively) and the secondary interactions between these adsorbates and the zeolite structure (hydrogen bonds may lead to the formation of a monodentate bond with pyridine and a bidentate bond with ammonia), as shown by Parillo et al. [55-57], These differences are reflected in the heats of adsorption, which are usually 20-30 kJ mof higher in the case of pyridine, than in the case of ammonia. A similar difference is observed for the chemisorption limit [44,54]. 

Wu, C.G. Bein, T. Conducting polymer wires in mesopore hosts. Zeolites and Related Mieroporous Materials State of the Art 1994 1994, 84. 2269. 

Precursor cations can be introduced into the host zeolite framework and subsequently converted into the semiconductor phase by chemical or thermal treatment. The ion exchange process can lead to changes in the cation locations, as mentioned above. This process must be carefully controlled in order to give materials that can be consistently reproduced. 

Particular attention was paid to Fe-zeolite using ammonia as a reducing agent A comparative study carried out by Guzman-Vargas et al. [46] revealed that this transformation is feasible when isolated Fe species are stabilized. The host zeolite structure and the nuclearity of iron oxocation were found to influence the catalytic properties. By way of illustration, MeUan-Cabrera et al. [47] show a remarkable stability of exchange iron species stabilized as Fe-hydr(oxo) complexes in ferrierite framework, much better than Fe/ZSM5, under realistic conditions. 

Na clusters were formed when Na-Y was exposed to either sodium or potassium vapour, and blue compounds containing K + clusters when K-Y was exposed to either vapour. Rb was not observed with Rb-Y under comparable conditions. Thus, the formation of Na + and K + was found to be independent of the nature of the incoming alkali atom, but dependent on the nature of the host zeolite, implying that the following reaction schemes are appropriate ... 

The interaction of tungsten oxide species with the host zeolite and especially the anchoring site can be studied by Na MAS NMR spectroscopy in combination with Na+ far-infrared and proton mid-inffared spectroscopy [247]. For 16 (WO)3/Hj6Na4o-Y it was observed that the OH stretching vibrations of supercage... 

Inclusion Complexes in the Solid State Cyclodextrin Hosts Zeolites 73-10... 

---