Paraffins, hydrogenolysis

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

Fig. 14. Qualitative sketch of reactivities for olefin reactions (on acidic catalyst) and paraffin hydrogenolysis (on platinum metal) vs. carbon number of hydrocarbon.

Secondly, this mechanism (1,3-carbon-carbon bond activation) applies to both acyclic and cychc paraffins such as hexane and cyclohexane (Scheme 40 and Table 8). Kinetic studies on the hydrogenolysis of these alkanes are note-... 

Schultz and Linden Ind. Eng. Chem. Process Design and Development, 1 (111), 1962] have studied the hydrogenolysis of low molecular weight paraffins in a tubular flow reactor. The kinetics of the propane reaction may be assumed to be first-order in propane in the regime of interest. From the data below determine the reaction rate constants at the indicated temperatures and the activation energy of the reaction. 

S. Kempling, J. C., Hydrogenolysis of Small Paraffins Over Ruthenium, Ph.D. Thesis, McMaster University, Hamilton, Ontario, 1971. 

Paracrystallinity, Cu/ZnO/AIjOj, 31 295 2"-Paracyclophanes, 32 453 Paracyclophanes, macrocycles, 29 206-208 Paraffin, see also Alkanes alkylation, 10 165, 27 98 carbon selectivity, bed residence time effects, 39 249-250 cracking, 39 283 cyclization, 28 295 rates, 28 301, 302 double cyclization, 28 312-314 in exhaust gases, 24 66, 67 hydrogenolysis, 30 43-44 hydroisomerization, 39 183 oxidation, 32 118-121 solubility enhanced hydrogeolysis, 39 285 Parahydrogen conversion rate correlations, 27 48-50... 

Various catalytic reactions are known to be structure sensitive as proposed by Boudart and studied by many authors. Examples are the selective hydrogenation of polyunsaturated hydrocarbons, hydrogenolysis of paraffins, and ammonia or Fischer-Tropsch synthesis. Controlled surface reactions such as oxidation-reduction reactions ° or surface organometallic chemistry (SOMC) " are two suitable methods for the synthesis of mono- or bimetallic particles. However, for these techniques. 

Cracking reactions can take place on either the acid site or the platinum site. Acid cracking is characterized by the formation of C3 and C4 paraffins due to the carbonium ion mechanism. Metal cracking (hydrogenolysis), as shown by Sinfelt (19), is random and forms more C, and C2 gases relative to acid cracking. 

Another reaction which has been studied over a bifunctional catalyst is the hydrogenolysis of benzene and toluene over platinum-alumina, which has been reported by Rohrer and Sinfelt (Rl). The reaction was studied over a temperature range of 471 to 527°C. and hydrogen pressure range of 9 to 26 atm. It was found that the rate of formation of the hy-drogenolysis products (Ce and lighter paraffins) was essentially independent... 

Transition metals catalyze a very wide variety of hydrocarbon reactions ranging from hydrogenation of olefins and exchange of paraffins with deuterium at lower temperatures to skeletal rearrangement, cyclization, hydrogenolysis, cracking, and carbiding under more severe conditions. Because of this flexibility in... 

A side reaction of paraffin isomerization is that of hydrogenative cracking to lower molecular weight paraflSns. Besides hydrogenolysis of hydrocarbons, which we visualize as occurring on metal catalyst sites alone, polystep hydrocracking according to the scheme exemplified by... 

When the required intimacy has been provided, the polystep conversion to six-carbon ring products is seen to be successfully accomplished even at this low temperature by the coaction of the separate catalyst components. In addition, we observe here again the phenomenon of reducing the rate of production of certain products by providing more intimate contact with a second catalyst component the hydrogenolysis reaction of C—C bonds to Ci to Ce paraffins is inhibited i.e., diverted to a new reaction path. 

---