Catalytic Behaviour with Periodic Properties

The behaviour of the oxides of Group IV which contains thoria is quite complex. Titania can exist in two forms, anatase and rutile, the former usually giving selective dehydration with Saytzev alkene orientation from 2-alcohols. Hafnia also gives selective dehydration irrespective of any 

The above model is not too disimilar to that proposed by Knozinger et al except that instead of the latter s OH bonded species (see later), a co-ordinative interaction is suggested. In the case of methanol, not only could bimolecular interaction lead to ether formation, but also saturated and unsaturated hydrocarbons were produced via the formation of titanium alkyls, similar to a Ziegler-Natta process.  

In the same group, the oxide of tungsten was observed to be a selective dehydration catalyst, irrespective of hydrogen or air pretreatment.Nevertheless, the octene distribution from octan-2-ol was affected by the pretreatment and as O ions were removed, fra s-oct-2-ene became favoured and geometric factors are probably of importance. It was suggested that high cisitrans ratios may occur either when basic oxygen sites crowd closely around the catalytic site or when the metal ion has a lower co-ordination number. 

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During the last period considerable attention has been given to Ti-silicalite, a zeolite derived from silicalite by partial substitution of framework Si with Ti (refs. 1-3). This zeolite exhibits very valuable catalytic properties for a variety of reactions of industrial interest, in particular for cyclohexanone ammoximation, phenol hydroxilation and olefins epoxidation (refs. 4, 5). Despite that, the stability of Ti-silicalite, as well as fundamental aspects of its characteristics and catalytic behaviour, has not been studied. In this work sorption measurements of small (N ) and large (p- and m-xylene) probe molecules were performed to characterize the porous structure of Ti-silicalite, before and after various thermal treatments. The results, compared to those obtained by spectroscopic and diffractometric techniques, provided information on stability of both zeolite lattice and titanium inserted in the framework. 

Electronic properties such as electrical conductance, magnetic behaviour and band structure typically show dramatic changes with alloy composition, especially where the electronic structures of the pure components differ greatly, as happens for example when the d-shell is filled. Alloys of this type (Ni-Cu, Pd-Ag, Pd-Au) were the subject of intensive research in the period 1945-1970, as it was believed that the presence of an incompletely-filled d-shell was an important feature in determining catalytic activity, and that filling would occur at some composition that could be deduced from electronic properties. The experimental results and the theoretical models that form our present state of understanding of the behaviour of electrons in alloys will be considered in the following section. 

When investigating a model of a chemical plant or process, one of the most important tasks is to determine the influence of model parameters like operation conditions or geometric dimensions on performance and dynamics. Because in most cases a large number of parameters has to be examined, an efficient tool for the determination of parameter dependencies is required. Continuation methods in conjunction with the concepts of bifurcation theory have proved to be useful for the analysis of nonlinear systems and are increasingly used in chemical engineering science. They offer the possibility to compute steady states or periodic solutions directly as a function of one or several parameters and to detect changes in the qualitative behaviour of a system like the appearance or disappearance of multiple steady states. In this paper, numerical methods for the continuation of steady states and periodic solutions for large sparse systems with arbitrary structural properties are presented. The application of this methods to models of chemical processes and the problems which arise in this context are discussed for the example of a special type of catalytic fixed bed reactor, the so-called circulation loop reactor. 

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