Structure insensitive hydrocarbon reactions

The deactivation of metal sites and acid sites on heterogeneous catalysts during structure insensitive reactions of hydrocarbons is illustrated. 

Recently, FT synthesis reactions were shown to be independent of metal dispersion on Si02-supported catalysts with 6-22% cobalt dispersion (103). Turnover rates remained nearly constant (1.8-2.7 x 10 s ) over the entire dispersion range. Dispersion effects on reaction kinetics and product distributions were not reported. These tests were performed at very low reactant pressures (3 kPa CO, 9 kPa H2), conditions that prevent the formation of higher hydrocarbons and lead to methane with high selectivity and to CO hydrogenation turnover rates 10 times smaller than those obtained at normal FT synthesis conditions and reported here. These low reactant pressures also lead to kinetics that become positive order in CO pressure. Thus, the reported structure insensitivity (103) may agree only coincidentally with the similar conclusions that we reach here on the basis of our results for the synthesis of higher hydrocarbons on Co. 

Acidic forms of zeolites are well suited as supports for metal functions which are employed for hydrogenation, since they can also withstand the presence of traces of sulfur compounds frequently found in feedstocks of petrochemical industry. It should be noted, however, that hydrogenation is a structure insensitive reaction so it will primarily depend upon the concentration of the accessible metal particles and the adsorption constant of the unsaturated hydrocarbon. This may offer an explanation as to why Pt catalysts, for example, are still active for hydrogenation, when their activity for dehydrocyclization or hydrogenolysis (i.e., for structure sensitive reactions) is completely lost (e.g., by poisoning). 

Metal catalysed reactions are differentiated introducing the concept of facile and demanding reactions. In principle a single atom should be adequate for a facile (structure insensitive) reaction, while an ensemble of surface atoms is required to form a catalytic site adequate for demanding (structure sensitive) reactions. Consequently, there are reactions, which requires more than one species to form multiplets " or ensembles. In other words, some reactions depend on the surface geometry e.g. hydrogenolysis of hydrocarbons), while other may not e.g. hydrogenation of olefinic double bond). 

One reaction model that explains these results has been proposed [165]. The hydrogen atom is transferred to the ethylene molecule that is weakly adsorbed on top of the ethylidyne and in the second layer perhaps by forming an ethyl idene intermediate. This model of hydrogen transfer from hydrocarbons to ethylene was first proposed by Thomson and Webb [175]. This mechanism is of the Eley-Rideal type and is characterized by low activation energy and structure insensitivity. 

Accepting the existence of active sites implies also that there are also inactive centres or perhaps overactive centres that are quickly inactivated by the destructive chemisorption that occurs so easily with hydrocarbons. The accidental or deliberate removal of such sites by autogenic toxins (i.e. carbonaceous deposits , ethylidyne etc.), or by other poisonous species such as sulfur compounds, allows the remaining active centres to exhibit reactions of a structure-insensitive type that could not take place while the overactive centres were in existence. This situation may arise not only through surface heterogeneity, but also on a plane surface by operation of the Principle of Maximum Occupancy (Section 4.2) by which the preferred first reaction is that which utilises the largest size of active... 

Kinetic results were consistent with a bimolecular termination reaction whereas reaction products and mechanisms were something of a mystery. At that time it was known that the termination rate constant for autoxidation of cumene ( ) is about three orders of magnitude smaller than the termination rate constant for autoxidation of tetralin (7.). It was, however, generally accepted that the tennination rate constants for tertiary ( ) and secondary (9 ) alkylperoxy radicals are insensitive to the structure of the hydrocarbon residue in the radical. 

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