Changing the molecular sieve

It is equally important to change the molecular sieve regularly. If this is done each time a tank of gas is changed, there will be little doubt as to the effectiveness of the drying tube. The indicators that change color in presence of moisture are not satis-... 

Furthermore, the implantation of various boron-nitrogen compounds inside the zeolite framework can reduce, in a controlled way, the effective pore size of zeolites. When NH3 is added to the boranated zeolite (before hydrolysis reaction), at room temperature, the formation of amine-boranes can be detected, which changes the molecular sieving properties of the zeolite. 

The free dimensions of the windows in zeolite A are 4.3A and those in mordenite are 6.7 X 7.0A. Any of the sorbates of Table VIII could migrate therefore into the crystals with minimal values of E, were it not for an influence of the ions (diameter 2.66A). These ions must obstruct the diffusion paths partially in each case. Indeed, ion exchange is one of the most effective ways of changing the molecular sieve characteristics of zeolites. Decationation, as in H-mordenite or H-clinoptilolite, produces maximum clearance of diffusion paths from metallic ions, so... 

The heating of the gas stream is commonly carried out by using hot oil. The molecular sieves used for drying the nitrogen stream have to be changed... 

The sensible heat required to heat the adsorbed water on the molecular sieve again over the same temperature range. The properhes of the adsorbed phase may safely be assumed to be those of liquid water and the calculahons of the enthalpy change can be made from available data. 

The amount of n-paraffins not desorbed from the molecular sieves could not be determined from the weight change of the molecular sieves due to the ammonia adsorbed. 

The question of which lattice components to include in the interpretive analysis reduces to the experimental problem of comparing systems of similar geometry but which contain different species in the lattice—e.g., Ge or Th in substitution for Si (1) S or F in substitution for O or OH, and say 2Na+ for lCa+2. The effect of change of cation is well known. In limiting cases it modifies the Molecular Sieve effect in zeolites. For host substituents, the availability of suitable systems is limited. The present work offers a comparison between a normal aluminosilicate and the same species after subjection to a substitution of halogen for oxygen and hydroxyl. 

The latter authors found that a reversible water isothenn was obtained after the low-temperature (i.e. 40°C) evacuation of a carbon cloth, which had been activated by oxidative HNO, treatment. The molecular sieve character of this material was reduced by evacuation at 400°C and this also led to the appearance of hysteresis in the water vapour isothenn. Barton and Koresh (1983) conclude that such hysteresis is mainly due to the concentration of surface oxides which dictate the adsorption value at which the change from cluster adsorption to a continuous adsorbed phase takes place . The relationship between the adsorption of water and the surface concentration of chemisorbed oxygen was first established by Walker and Janov (1968). Bansal et al. (1978a,b) also investigated the influence of the surface oxygen on the adsorption of water they concluded that at p/p° < 0.5 the level of water uptake is determined by the concentration of surface oxygen-containing structures. 

Electron microscopy, with its high spatial resolution, plays an important role in the physical characterization of these catalysts. Scanning electron microscopy (SEM) is used to characterize the molecular sieve particle sizes and morphologies as a function of preparation conditions. Transmission electron microscopy (TEM) is used to follow the changes in the microstructure of the iron silicates caused by different growth conditions and subsequent thermal and hydrothermal treatments. 

This technique (Fig. 3.8), which is examined in greater detail in connecdon with the upgrading of aromatic Cs cuts, is based on the use of a pseudo-countercurrent between the liquid feedstock and the adsorbent bed. The displacement of the solid is in fact simulated by means of a rotary valve with multiple inlets and outlets, which causes a gradual change ih the injection and collection points of the liquid streams in the molecular sieve placed in a rould-stage column. 

Zeolitic materials have been widely used in the last decades in the chemical and petrochemical industries. This increasing interest on these materials is based in their unique properties a uniform intra-crystalline microporosity that provides aceess to a large and well-defined surface, the molecular sieve effect, and the electrostatic field centered at zeolite cations. Furthermore, some properties of zeolites can be tailored by changing the nature of the compensating cation located in the inner part of the cavities by means of their ion-exchange capability. In this way, the pore accessibility of some zeolites used in gas separation processes, as well as the adsorbent-adsorbate interactions, can be tailored by the introduction of cations with different size and chemical nature. Similarly, different cations can be used to introduce new chemical properties (acid-base, redox, etc.), which are needed for a given application in catalytic processes. 

It has been shown (Fig. 9) that for most of the molecular sieve catalysts the selectivity for isobutylene improves with TOS. Furthermore, it has been demonstrated with ferrierite that the change with TOS from the nonselective form (the noncoked state) to the selective form (the coked state) is accompanied by a change in the reaction mechanism from bimolecular to monomolecular. Here, the physicochemical properties of the molecular sieves and their variation with TOS are described and analyzed in terms of the correlations between the state of the molecular sieves and their activities and selectivities for n-butenc isomerization. 

The difficulty of incorporating metal ions into the molecular sieve lattice results from the fact that actually two requirements have to be fulfilled, i.e., (i) the metal cation must have approximately the size of the atom it replaces (Si, A1 or P) and (ii) it must be able to coordinate in a tetrahedral position in the firamework. Fiuthermore, to function as a successful redox catalyst, a change in the valency and/or the coordination of the oxidant must be realized via reversible change of the coordination of the metal cation. Only a limited number of cations have been reported to be incorporated in the fiamework of zeolite and metal-aluminophosphate molecular sieves. These cations include Co, V, Mn, Cr. Ti [158,159] and a short compilation of the structures available (isomorphously substituted molecular sieves) is compiled in Table 1. Generally, it seems that aluminophosphate lattices are more easily adaptable for isomorphous substitution, but that the resulting materials have a lower stability than the corresponding zeolite frameworks [160]. 

A change in entropy will thus have a significant effect on the selectivity when molecular sieving is considered. This is thoroughly discussed by Singh and Koros [9]. The flux may be described as in Equation 4.16 where Eq.ms is the activation energy for diffusion in the molecular sieving media. 

With VO(HPS)-Y [E] as the catalyst, reaction in the presence of the zeolite gave a much better selectivity towards cis-epoxide, than without zeolite present. This suggests that carveol is able to complex with the vanadium present within the molecular sieve. Moreover, the selectivity on TBHP consumed decreased from 95% to 45%, suggesting that the ligand environment has changed. 

Based on their regular pore structure, it should be possible to impart the molecular sieving capabilities of zeolites on the surface of electrodes in electrochemical reactions. Several research groups have addressed this issue by (i) developing ways to modify electrodes with thin zeolite films and (ii) by studying the resulting changes in electrode behaviour in electrochemical reactions. The subject has been reviewed.[91]... 

Finally, we mention a novel transduction concept based on the heal evolved from a reaction such as combustion. Microcalorimetric devices can now be made using lithographic techniques. One of the two sensitive areas of such a device (were evolved heat can be measured) was coated with a thin film of CoA1P04-5, the other was kept open as a reference 135] The additional benefit of a zeolite with catalytic activity for such a device is the molecular sieving effect that can be combined in the response of the sensor (a molecule too big to enter the catalytically active interior of the zeolite should only show a weak response). The change in temperature was measured with a meandering Pt-wire resistor. This device was examined in the detection of CO and cyclohexane, and sensitivity and selectivity in the low ppm-range was observed. 

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