Propane, surface intermediates

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

In the platinum-catalysed reaction, it has been observed [155] that the effect of increasing the hydrogen/propadiene reactant ratio was to increase the yield of propane without affecting the yield of propene. This has been interpreted as showing that propane may be formed directly from adsorbed propadiene by a mechanism which does not involve adsorbed propene as an intermediate. However, no conclusions were drawn regarding the nature of the surface intermediates involved in such an interconversion. 

The oxidative dehydrogenation (ODH) of lower alkanes is an attractive process for the formation of alkenes. The ODH of propane to produce propene has been particularly studied, given its high demand for the production of polypropene, acrylonitrile and propene oxide. There is a combined influence of the redox and acid-base properties of the surface of the oxides used for propane ODH. Intermediate reducibility, weak Lewis acid centers and oxygen mobility represent the essential requirements for selective ODH, as they are consistent with the trends in ODH rates observed in VO, MoO and WO based catalysts. 

It was also interesting that the specific radioactivity of propane was higher than that of ethane, under the same experimental conditions. This was taken as strong evidence for the participation, in the exchange reaction, of the terminal C atoms of propylene via a 7T-allyl surface intermediate. 

The surface intermediates formed in the photocatalytic oxidation of propane has been investigated over anatase, rutile, and mixed-phase anatase-rutile Ti02 nanoparticles [26]. Detailed spectroscopic analysis of surface intermediates was conducted via simultaneous in situ FTIR with online mass spectrometry (MS), which was used to follow the decline of propane and formation of carbon dioxide under irradiation by a 400 W Xe lamp. 

V-Sb-oxide based catalysts show interesting catal)dic properties in the direct synthesis of acrylonitrile from propane [1,2], a new alternative option to the commercial process starting from propylene. However, further improvement of the selectivity to acrylonitrile would strengthen interest in the process. Optimization of the behavior of Sb-V-oxide catalysts requires a thorough analysis of the relationship between structural/surface characteristics and catalytic properties. Various studies have been reported on the analysis of this relationship [3-8] and on the reaction kinetics [9,10], but little attention has been given to the study of the surface reactivity of V-Sb-oxide in the transformation of possible intermediates and on the identification of the sxirface mechanism of reaction. 

Ammonia also reacts with the acrolein intermediate, via the formation of an imine or possibly oxime intermediate which transforms faster to the acrylonitrile than to the acrylamide intermediate. This pathway of reaction occurs at lower temperatures in comparison to that involving an acrylate intermediate, but its relative importance depends on the competitive reaction of the acrolein intermediate with the ammonia species and with catalyst lattice oxygens. NH3 coordinated on Lewis sites also inhibits the activation of propane differently from that absorbed on Brsurface reaction network in propane ammoxidation. 

Catalytic hydrogenolysis of light alkanes (propane, butanes, pentanes) with the exception of ethane has been accomplished under very mild conditions over silica-supported hydride complexes.502 The hydrogenolysis proceeds over (=SiO)3 ZrH,503 (=SiO)3HfH,504 and (=SiO)3TiH505 by stepwise cleavage of carbon-carbon bonds by P-alkyl elimination from surface metal-alkyl intermediates. 

There was yet another possibility that the enhancement could be due to the fact that some stable products formed on the catalyst wafer, upon desorption into the void volume, underwent further sequential reaction with propane. If so, the enhancement would not require the immediate adjacency of the catalyst wafer and the void volume and should be observable when the catalyst and the void volume were physically separated. Such separation, however, would quench any desorbed reactive intermediates. This was tested. The wafer was separated from the void volume, and the two were separately heated to the appropriate temperatures. The result was that only a small enhancement was observed in the separated mode. This confirmed that the enhancement was due not to sequential reaction of stable products but to desorption of reactive intermediates from the catalyst surface. The small enhancement could be attributed to the higher temperature throughout the separately heated void volume in the separated mode than in the other mode. 

The surface gas is very rich in intermediates and often is processed to remove liquid propane, butanes, pentanes, and heavier hydrocarbons. These liquids often are called plant liquids. The gas-oil ratios in the rules of thumb discussed above do not include any of these plant liquids. 

C-C scission in the unsaturated acyl formed from acrolein would release vinyl (CH2=CH-) ligands to the surface. Isomerization of these would lead to stable ethylidynes, hydrogenation, to volatile ethylene and ethane. The observation that acrolein decarbonylates at lower temperature than does propanal suggests that these hydrogenation steps must follow C-C scission propanal cannot be an intermediate in the acrolein decarbonylation sequence. 

In the case of normal heptane it is immediately observed that there is something wrong stoichiometrically - whereas the moles of propane and butane produced are nearly equivalent, there is a substantial discrepancy between the moles of methane and hexane as well as between the moles of ethane and pentane. It is inevitable that the first step is to check the analysis - any departure from stoichiometry is initially inconceivable. But the analysis is correct and this is where the interdisciplinary forces must come to the rescue - ability to understand what is happening on the surface and what is the effect of the composition of the surface upon the series of reactions that must take place. What is happening has been very aptly named by Professor Burwell as organometallie Zoo . This is shown in figure 1. It is the intermediate formation of butyl and propyl carbonium ions on the surface which can then react with adjacent heptyl carbonium ions to produce C and carbonium ions. These can then split to... 

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