Modeling cyclohexane dehydrogenation

The catalytic activity of membrane catalyst, obtained by IR-pyrolysis of PAN and ammonium perrhenate, was studied in the flow membrane reactor in a model reaction of cyclohexane dehydrogenation at the temperatures from 500 to 700 K. 

Using a developed plug-flow membrane reactor model with the catalyst packed on the tube side, Mohan and Govind [1986] studied cyclohexane dehydrogenation. They concluded that, for a fixed length of the membrane reactor, the maximum conversion occurs at an optimum ratio of the permeation rate to the reaction rate. This effect will be discussed in more detail in Chapter 11. They also found that, as expected, a membrane with a highly permselective membrane for the product(s) over the reactant(s) results in a high conversion. 

Mohan and Govind [1988c] applied their isothermal packed-bed porous membrane reactor model to the same equilibrium-limited reaction and found that the reactor conversion easily exceeds the equilibrium value. The HI conversion ratio (reactor conversion to equilibrium conversion) exhibits a maximum as a function of the ratio of the permeation rate to the reaction rate. This trend, which also occurs with other reactions such as cyclohexane dehydrogenation and propylene disproportionation, is the result of significant loss of reactant due to increased permeation rate. This loss of reactant eventually negates the equilibrium displacement and consequently the conversion enhancement effects. 

Third, the doublet and, especially, sextet models require very precise superimposing of the molecule on the catalyst lattice. We have found that the cyclohexane derivatives, in accordance with the sextet model, smoothly dehydrogenate only on the following metals nickel, cobalt, iridium, palladium, platinum, ruthenium, osmium, and rhenium, all of which crystallize in Al, A3 lattices with certain interatomic distances. These results extend to the alloys of these metals. The catalytic activity of rhenium for this reaction was predicted by the multiplet theory as this metal maintains the square of activity this prediction was realized experimentally in the laboratory of the author. Similar correlations take place in the exchange of cyclanes with deuterium. 

During the early years of development of the multiplet theory, attention was paid chiefly to the correspondence of the structure of reacting molecules and catalyst, especially in relation to the sextet model of dehydrogenation of six-membered cycles on metal catalysts. This work permitted the determination of the group of metals that can act as catalysts for the dehydrogenation of cyclohexane (the so-called Blandin s square of activity ) and the prediction of catalytic activity, e.g., for Re which was unknown as a catalyst for this reaction. 

Different model reactions were used in order to study the interaction between the modifier and the parent metal. It was observed that an inert additive introduced by a redox reaction generally poisons, more or less, the activity of the parent metal or strongly modifies the selectivity of the reaction, which indicates a deposition of the additive on the parent metal. For example, a decrease in activity for structure insensitive reactions, such as toluene hydrogenation [41] or cyclohexane dehydrogenation [43, 78] proves the existence of bimetallic nanoparticles. Likewise, in the case of the 2,2-dimethylpropane reaction, the modification of both the selectivity and the apparent activation energy, demonstrates an interaction between Pd and Au introduced by direct redox reaction. Conversely, no modification was observed on the catalysts prepared by incipient wetness co-impregnation [75]. 

The CFD model clearly shows that large temperature and concentration distributions are formed both in the radial and axial directions. Simulation results with cyclohexane dehydrogenation were in good agreement with the experimental data for the both membrane reactors. Furthermore, it is demonstrated that the multi-tube model developed is applicable for changing the reactor design, for instance the membrane dimension, the length of catalyst-packed layer, and the operation conditions such as temperature, feed rate and pressure. 

The following reaction kinetics model is used for the catalytic reaction of cyclohexane dehydrogenation (Itoh and Wu, 1997). 

In order to verify the reactor model, a numerical simulation was carried out for the cyclohexane dehydrogenation experiment. Table 13.3 shows the conditions of the seven cases (Cases 30-36) studied. 

T. Kokugan, A. Trianto, and H. Takeda, Dehydrogenation of pure cyclohexane in the membrane reactor and prediction of conversion by pseudo equilibrium model, J. Chem. Eng. Jpn. 31,596-603 (1998). 

In bimetallic catalysts, Cu-Ru is an important system. Combinations of the Group Ib metal (Cu) and Group VIII metal (Ru)-based catalysts are, for example, used for the dehydrogenation of cyclohexane to aromatic compounds and in ethane hydrogenolysis involving the rupture of C-C bonds and the formation of C-H bonds (Sinfelt 1985). Here we elucidate the structural characteristics of supported model Cu-Ru systems by EM methods, including in situ ETEM. 

A model compound here is benzene. Both the benzene hydrogenation (de-aro-matization of oil) as well as the dehydrogenation of cyclohexane into benzene (e.g. upon naphtha reforming) are of practical interest. Hydrogenation of benzene looks similar to the hydrogenation of olefins. For example, in a certain region of reaction conditions, the reaction is near to first order in hydrogen pressure and zero order in benzene pressure [78]. (N.B. A more exact analysis of the kinetics is available too, see ref. 81). However, there also seem to be some important differences. 

Jeong BH, Sotowa KI, and Kusakabe K. Modeling of an FAU-type zeolite membrane reactor for the catalytic dehydrogenation of cyclohexane. Chem Eng J 2004 103 69-75. 

Facile dehydrogenation is consistent with kinetic models derived from catalytic conversion studies of cyclohexane to benzene. These models predict an ensemble size for the active site of only one atom. On the other hand, surface science studies propose a model where several metal atoms, on the order of seven, are required, and suggest that specific orientation with respect to subsurface metal atoms is needed. Theoretical studies suggest that the key is to bring cyclohexane sufficiently close to the metal such that strong orbital overlap will occur. Small clusters may be even more effective than surfaces. Further experiments are needed to identify the chemical state of the products of the cluster reactions in order to connect the results with the surface science and catalysis results. 

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