Hydro-dehydrogenation

Three catalyst samples were prepared to identify and qnantify the coke related with the hydro-dehydrogenation capacity of contaminant metals. The first sample corresponds to the fresh catalyst deactivated 20 honrs, at 788°C, with 80% steam, the second and third samples were deactivated at the same conditions bnt impregnated with 4100 ppm V, and with 4100 ppm V and 4000 ppm Ni, respectively. The MAT test was performed using gas oil. 

The Figures 10.1 and 10.3 present the TPO spectra of the samples with and without metals. For the sample impregnated with 4100 ppm vanadium, it was observed the appearance of a shoulder around 680°C that translates in a 10% increase in peak C area, compared to the metal-free catalyst as illustrated in Figure 10.3. Then, the signal C located around 61TC apparently corresponds to the contaminant coke produced by the hydro-dehydrogenation properties of vanadium. 

In A, a Raney copper catalyst would be able to hydro-dehydrogenate alcoholic functions (-H, +H) on metallic copper sites. About 10 to 15% of the copper would be hydroxylated copper able to catalyze the degradation reactions DOH, RC, RM. These sites would be more reactive than Cu towards Mn + in the oxido-reduction modification of the initial Raney copper, so that, beeing first exchanged, the rates of DOH, RC, RM decrease. 

Hydrogenolytic reactions can also be suppressed by carbonaceous layer deposition or by modification of the active surfaces by — for example — sulphur. Suppression of hydrogenolysis gives more chance to a reaction the rate of which decreases only proportionally to the active metal surface concentration, like hydro/dehydrogenation, or some forms of isomerisation (5C cyclic mechanism see the chapter on elementary steps). 

The heterogeneous catalysis process requires the formulation of a multifunctional catalyst which at a first approximation presents (i) acidic properties (amine adsorption, dehydration,...) and (ii) a hydro-dehydrogenating function (methanol dehydrogenation, hydrogenation of imine and enamine intermediates). 

From this study the following reaction scheme describing the transformation of ethylamine to the main product DMEA and by-products was established. From a kinetic point of view, steps 2 and 3 are the rate determining reactions. It follows that the DMEA selectivity is increased by modifying the acido-basicity of copper chromite used as a catalyst. In fact, the change of the catalyst basicity can decrease the MEA condensation to form DEA without modification of the hydro-dehydrogenating properties of the catalyst which are necessary for the methylation of ethylamine with methanol (steps 1 and 3). 

These reactions proceed essentially through a bifimctional mechanism involving two types of catalytic sites hydro-dehydrogenating sites and acidic sites. A saturated hydrocarbon (paraffin... 

Hydroisomerization reactions are generally intimately associated with hydrocracking reactions. The overall scheme is rather complex. It involves the independent action of both types of catalytic sites and the existence of a transport mechanism for olefins between these sites. Therefore, the catalyst must be designed according to this bifunctional mechanism. The relative strength of the hydro-dehydrogenation and acidic components must be adjusted for the desired operation. 

There are a number of hydrocracking catalysts available they are generally tailored to the process, the feed material and the products desired. Most of these catalysts consist of a mixture of silica-alumina with a small, uniformly-distributed amount of metads. The silica-alumina portion, amorphous or crystalline, provides the cracking activity while the metals promote hydrogenation. The main types of hydro-dehydrogenation components are mixed metal sulfides and noble metals. The main noble metal used is palladium (less than 1 weight %) it is used preferably with zeolite as the acidic component. 

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