Alkenic hydrocarbon conversion

Alkanes and Strong Solid Acids. Since the early reports by Nenizetscu and Dragan67 on alkane isomerization on wet aluminum chloride in 1933, all mechanistic studies have led to a general agreement on the carbenium-ion-type nature of the reaction intermediates involved in acid-catalyzed hydrocarbon conversions. In contrast with this statement, the nature of the initial step is still under discussion and a variety of suggestions can be found in the literature among which direct protolysis of C—H and C—C bonds, protonation of alkenes present as traces, and oxidative activation are the most often quoted.54,55... 

Here hydrocarbon conversion reactions occur wholly or at least partly on the carbonaceous overlayer on the metal and oxide surfaces, as reported by others (13,15-20). Poly-condensed EDA complexes may behave as giant alkenes in which by reversible catalytic hydrogenation/dehydrogenation occurs. This mechanism is similar to the intermolecular hydrogen transfer mechanism proposed (IS) for hydrogenation of unsaturated hydrocarbons. 

Technologies have been developed to reduce the hydrocarbon emissions during cold start. One of the technologies, the hydrocarbon adsorber [1-3] has been most attractive to automotive makers because it has the advantage in view of cost and performance. Hydrocarbons of the cold start depend on the condition of a vehicle, fuel and driver. About 100 hydrocarbon species are present in the exhaust of the cold start. They consist of about 10% methane, about 30% alkenes such as ethylene or propene, about 30% alkanes such as pentane or hexane, about 20% aromatics such as toluene or xylene, and about 10% other species. Therefore, a hydrocarbon adsorber has to show a good selectivity for the hydrocarbons. In order to remove the adsorbed hydrocarbons effectively, the hydrocarbon adsorber needs to have an additional function, hydrocarbon conversion. 

It was shown in Fig. 53 that the hydrocarbon conversion which is reached over the catalyst at fixed reaction conditions depends on the type of the hydrocarbon. Alkenic and aromatic hydrocarbons are more reactive than alkanic hydrocarbons. The reactivity of alkanic hydrocarbons increases with the number of carbon atoms in the molecule. 

The nature of the hydrocarbon also effects the conversion of CO and NO (Fig. 56). With a lean exhaust gas composition, the level of 50% conversion of an alkanic hydrocarbon is reached at a higher temperature than the corresponding temperature for CO and NO. Alkenic hydrocarbons reach the 50% conversion level at the same temperature as CO. These phenomena strongly depend on the detailed kinetic mechanism of the eonversion reactions and are therefore influenced by the composition and the design of the catalyst. 

At 375°C with the ZSM-5, the main products formed are n-alkanes. Other products are observed ramified alkanes and alkenes, 1-alkenes, aromatics and cyclic saturated hydrocarbons. The majority of hydrocarbons formed have a carbon number between 3 to 6. In the case of the zeolite Y, the n-alkanes and similar secondary products are formed but their repartition is different i.e. the normal and ramified alkanes are the main products and no cyclic compound can be observed. All these products are in higher quantity with the ZSM-5 than with the zeolite Y. This is in agreement with the calculated n-dodecane conversions. With the increase of the temperature, the same products are formed but their quantities increase. The analysis of the gaseous phase shows the presence of hydrogen, light normal and ramified alkanes and 1-alkenes. 

The hydroformylation of alkenes was accidentally discovered by Roelen while he was studying the Fischer-Tropsch reaction (syn-gas conversion to liquid fuels) with a heterogeneous cobalt catalyst in the late thirties. In a mechanistic experiment Roelen studied whether alkenes were intermediates in the "Aufbau" process of syn-gas (from coal, Germany 1938) to fuel. He found that alkenes were converted to aldehydes or alcohols containing one more carbon atom. It took more than a decade before the reaction was taken further, but now it was the conversion of petrochemical hydrocarbons into oxygenates that was desired. It was discovered that the reaction was not catalysed by the supported cobalt but in fact by HCo(CO)4 which was formed in the liquid state. 

Svelle, S., Joensen, F., Nervlov, J., Olsbye, U., lillerud, K.-P., Kolboe, S., and Bjorgen, M. (2005) Conversion of methanol into hydrocarbons over the zeolite H-ZSM-5 ethene formation is mechanistically separated from the formation of higher alkenes. /. Am. Chem. Soc., 128,14770-14771. 

K. It too gives the characteristic adducts obtained from the singlet, but the addition does not occur directly between the bicyclic hydrocarbon (28) and the alkene. Instead, the reaction occurs in two steps, first the reversible unimolecular ring opening of 28 to singlet biradical 14b, followed by a bimolecular capture of the latter (Scheme 5.5). Another hydrocarbon isomer 29 can be prepared as a transient intermediate. Its thermal conversion to the biradical 14b apparently occurs at even lower temperature. 

The 02 ion appears to play an important role in a number of photooxidation reactions (see Section VI,C) for example, the photo-oxidation of alkenes over TiOz. However, it seems likely that OJ is not, in many cases, active in the oxidation step but further conversion occurs to give a mononuclear species, not detected directly, which then oxidizes the adsorbed hydrocarbons. Photo-oxidation of lattice oxygen in the M=0 systems (e.g., V2Os supported on PVG) gives rise to an excited charge transfer state such as V4 + -0 . This excited state can react as O- either by addition to a reactant molecule or by an abstraction reaction (see Section V of Ref. /). In the presence of oxygen, 03 is formed which then reacts further with organic molecules. 

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