Alkanes selectivity

Figure 3.11 Propane metathesis catalyzed by (=SiO)2TaH (3) in a continuous flow reactor. Contact time effect (520mg of 5.3% Ta/SiOj 1 atm, 150 C, 1 to lOOmLmin or VHSV = 38 to 3800 h ) (a) alkane selectivity (b) alkene selectivity (c) hydrogen selectivity (d) alkane alkene ratio.

As another variation, the production of alkanes can be accomplished by modifying the support with a mineral acid (such as HCl) that is co-fed with the aqueous sorbitol reactant. In general, the selectivities to heavier alkanes increase as more acid sites are added to a non-acidic Pt/alumina catalyst by making physical mixtures of Pt/alumina and silica-alumina. The alkane selectivities are similar for an acidic Pt/silica-alumina catalyst and a physical mixture of Pt/alumina and silica-alumina components, both having the same ratio of Pt to acid sites, indicating that the acid and metal sites need not be mixed at the atomic level. The alkane distribution also shifts to heavier alkanes for the non-addic Pt/alumina catalyst when the pH of the aqueous sorbitol feed is lowered by addition of HCl. The advantages of using a solid acid are... 

Figure 6.11 Hydrogen and alkane selectivity for reforming of 10wt% aqueous EG at 225 °C and 22 bar (adapted from Ref [289]).

Alkanes, selective oxidation, cobalt catalysis, 44 291 Alkene... 

The presence of a large fraction of adsorbed species on the catalyst surface thtis not only decreases the number of available active sites for alkane selective oxidation, but also decreases their s >ecific turnover number, thus increasing the surface lifeiime of adsorbed intermediates on the PVO deactivated surface. This increases the probability of parallel unselective reactions with a decrease in the selectivity to anhydrides. 

Adsorption and separation processes involve also the active sites existing on the external surface of (CH3)4N - montmorillonite, their role being more important in the case of adsorption of isoparaffins and cyclohexane molecules. This is indicated by a significantly smaller differences between the specific retention volumes of iso- and cycloparaffins on natural and tetramethylammonium samples than the difference in Vm values characteristic for n-paraffines on the same adsorbents. Thus, the tetramethylammonium montmorillonite adsorbs n-alkanes selectively from the mixtures containing iso- and cycloparaffines, which is confirmed by the values of relative retention volumes for such hydrocarbon pairs as, for instance, n-heptane / 2,4- dimethylpentane. These can be easily calculated from the data presented in Table 5. 

Noteworthy also is the selectivity for higher alkanes. For example the metathesis of propane gives mainly butanes rather than pentanes, and butane rather than isobutane. This is in agreement with stereoelectronic factors that favor the transfer of one carbon on to, preferentially, primary alkyl surface species, leaving tertiary alkyl species unreactive (Scheme 10). The alkane selectivity also depends on the structure of the starting alkane (Figure 1). Overall, this reaction shows the potential of surface organometallic chemistry, and new, unprecedented reactions will be probably discovered in the near future. 

Albert [53] developed a GC method for determining the types of hydrocarbons (aromatics, unsaturated, n-alkanes and isoalkanes) in mixtures of Cs-Cn hydrocarbons. The method is based on the use of a selective liquid stationary phase, N,N-bis(2-cyanethyl) formamide, from which aromatic hydrocarbons are eluted after the other compoimds, molecular sieves, which retain n-alkanes selectively, and an absorber containing mercury perchlorate, in which unsaturated compounds are absorbed. 

The aqueous-phase components also affect the and alkane selectivities at 225 C [56]. Figure 9.8 shows the selectivity and alkane selectivity over a Pt(3 wt %)/A1 0 catalyst The selectivity is highest when methanol and ethylene glycol are us as the model compounds, and they give low formation of alkanes. However, with glucose and sorbitol as substrates, the formation of alkanes is also possible. In fact, at 265°C, the alkane selectivities for glucose and sorbitol are over two times higher than at 225 C (>30%). 

Further reaction of CO and/or CO2 with H2 would lead to methanation or Fischer-Tropsch reactions [125]. The selectivity towards hydrogen or alkanes can be adjusted by the choice of the active metal and the supports of the catalysts. In this way, higher selectivities towards hydrogen could be achieved if Pt, Ni, and Ni-Sn are used on alumina and titania as support material, whereas Ru, Rh, and Ni and also Si02—AI2O3 as support revealed higher alkane selectivities [124,126]. 

For branched-chain alkanes, select the longest chain of carbon atoms as the parent chain its name becomes the root name. 

Although highly active pincer-PCP-li catalysts have been designed for the dehydrogenation of alkanes, selective systems remain elusive [139]. Pincer-type NHC-based iridium complexes have been also tested in such a challenging reaction. Iridium complexes supported by pincer-NHC ligands described by... 

Baudry, Ephritikhine and Felkin found the first soluble organometallic system capable of functionalizing alkanes selectively and catalytically under mild conditions. The bis(phosphine)rhenium heptahydrides L2ReH7 14 react with cycloalkanes C H2 n = 6, 7, 8) in the presence of dimethylbutene to give the corresponding cycloolefins (reaction 16) [32, 33] these are not further dehydrogenated to dienes or benzene. The... 

Cellulose acetate/polyvinylchloride (PVC), alkane selective (Farajzadeh and Hatami, 2003)... 

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