Hydrocarbons dimers, conversion

Steroids are heavily modified triterpenoids that are biosynthesized in living organisms from farnesyl diphosphate (Cl5) by a reductive dimerization to the acyclic hydrocarbon squalene (C30), which is converted into lanosterol (Figure 27.12). Further rearrangements and degradations then take place to yield various steroids. The conversion of squalene to lanosterol is among the most... 

Keller and Bhasin were first to report in 1982 [1] on the catalytic one-step oxidative dimerization or coupling of methane (OCM) to C2 hydrocarbons, ethane and ethylene. Numerous investigations have followed this seminal work and a large number of catalysts have been found which give total selectivity to C2 hydrocarbons higher than 90% at low (<2%) methane conversion [2-6]. 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

Propene also undergoes conjunct polymerization in the presence of dilute phosphoric acid at high temperatures and pressures (Monroe and Gilliland, 58). When propene was treated with 10-30% phosphoric acid at 260-305° and at 170-410 atmospheres pressure, the only operating variable which appreciably affected the composition of the polymer was the extent to which the feed was polymerized. At constant percentage reaction of the feed under these conditions, the temperature, pressure, and acid catalyst concentration had no effect on the product composition. At low conversions, the polymer consisted of nearly pure dimer at 50% polymerization, two-thirds of the total was dimer and even when the feed was almost completely polymerized, the dimer fraction amounted to 35-40 % of the total polymer. The dimer and trimer fractions obtained at temperatures of 305° or lower using a acid concentrations below 30% contained about 25% paraffins and little or no naphthenes or aromatic hydrocarbons. 

Table III. Maximum Conversion of. Hydrocarbons to Aldehydes, Alcohols, and Dimers...

From their scheme of dimerization of methylene radicals, Eidus and co-workers assigned exceptional importance to ethylene in the mechanism of hydrocarbon synthesis. Because both of its carbons can add new methylene radicals, its conversion is expected to occur at a high rate,... 

Obolentsev (264) has published researches on the conversions of individual hydrocarbons over silica-alumina. Depolymerization of triisobutylene as the temperature was raised from 200 to 365° resulted in a decrease of the liquid products from 45 to 60 % progressively to 24 to 26 % with a corresponding increase in the yield of the gaseous products the dimer was also depolymerized, although other workers reported it to be stable. A mechanism was postulated which involved the intermediate formation of an alkylated cyclobutane (264). Isopropylbenzene was dealkylated with simultaneous disproportionation to diisopropylbenzene at 350-450° (266). Disproportionation may become the principal reaction at the higher temperature (267). 

I to 5. 0 mol per mol of hydroperoxide. The presence of sodium naphtheoate, by prevenling side reaction, helps to reduce the excess propylene required (from lO/l to 2/1 in moles). In the Shell technology, epoxidation is catalyzed by metallic oxides (molybdenum, vanadium, titanium, etc.) supported on sih cau The liighiy exothe c reaction takes place around 100 to 130 at 3.5.10 Pa absolute. Hydroperoxide conver> sion is very hi (> 97 per cent). Propylene oxide molar selectivity exceeds 70 per cent and that of the styrene precursors 93 per cent As for propylene, its once-through conversion is about 15 per cent, for a oxide molar selectivity greater than 90 per cent, and the main by-products are dimers and heavier hydrocarbons. 

Methane can be catalytically oxidized in the fuel cell mode to simultaneously generate electricity and C2 hydrocarbons by dimerization of methane using a yttria-stabilized zirconia membrane. A catalyst, used as the anode, is deposited on the side of the membrane that is exposed to methane and the cathode is coated on the other side of the membrane. When the catalyst Ag>Bi2C>3 is used as the anode for the reaction at 750> 900X and atmospheric total pressure, the selectivity to ethane and ethylene exceeds 90%. But this high selectivity is at the expense of low power output and low overall methane conversion (less than about 2%). 

Examples illustrating the application of DPSCA include the cleavage of the carbon-halogen bond in radical anions derived from aromatic compounds [215], the protonation of radical anions derived from aromatic hydrocarbons [90,213,216], the dimerization of radical anions [112,113,217], radical cations [218], and neutral radicals [219], and the conversion of the B form to the A form for 10,10 -dimethyl-9,9 -biacridylidene [220]. 

Crabtree, by combining ease of free radical generation with the site selectivity imparted by functional groups that stabilize adjacent free radicals, has developed a very practical approach to hydrocarbon carbafunctionalization, as illustrated by the conversion of cyclohexane to (13 equation 5). The real elegance of this method is that it does not require highly reactive reagents. This photochemically initiated dimerization of hydrocarbons proceeds efficiently in the presence of even a very low concentration of mercury vapor. 

A trimer of norbornadiene, m.p. 201°, was obtained from the reaction of the olefin with Ni[P(CaH5)3]4 as the catalyst 58). Except that it is unsaturated, no further structural information is presently available on this hydrocarbon. Rhodium on carbon also catalyzes norbornadiene dimerization and trimerization (45). Three dimers, (XXIII), (XXIX), and (XXX) or (XXXI) are formed in the ratio of 4 84 12, in 70-80% yield at 90% conversion (45). In addition, 20-30% of a crystalline trimer, m.p. 67-68°, was isolated. On the basis of NMR and IR measurements it could have either structure (LIV) or (LV). 

Direct evidence for homologation is not readily available because reactions between olefin molecules are so much more facile than homologation. Recently Behrsing et al. (ref. 19 see also ref. 20) obtained C7 hydrocarbons singly labelled with 3C by reaction of 3C-labelled methanol with hexenes under conditions where hexene oligomerisation is incomplete and methanol conversion is low. Table 1 shows a typical result from one of these experiments. Single C-labelling is even more pronounced in the Cjg products (from hexene dimer and 3C-methanol) than in the C7 products. 

The oxidative dimerization of methane in the presence of oxygen or air often gives substantial amounts of carbon oxides as byproducts [1]. Conversion of methane to methylchloride and further condensation to higher hydrocarbons and hydrogen chloride avoids this problem. The reoxidation of hydrogen chloride to chlorine, as well as the formation of methylchloride from methane is known technology. 

The difference in Cs-hydrocarbon concentrations observed for Pt/Ga-silicate and Ga-silicate can be related to the Pt reactivity in hydrogen spillover. Since Cs-hydrocarbons are likely to be Cs-dimer cracking products produced over acidic sites, they can also be involved in subsequent oligomerization reactions over acidic sites, too. Cs-hydrocarbon reactions are more probable than Cs-hydrocarbon reactions, because C3-hydrocarbons are more volatile. However, Cs-unsaturated intermediates are rapidly hydrogenated over Pt/Ga-silicate by hydrogen which was activated on platinum and spills over the acidic sites. Consequently, they did not participate in further conversion and their concentration remained constant. The platinum hydrogenation activity was confirmed by the data on the hydrogen effect on the activity for Pt/Ga-silicate and Ga-silicate. 

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