Formation of hydrocarbons

As the cation becomes progressively more reluctant to be reduced than [53 ], covalent bond formation is observed instead of electron transfer. Further stabilization of the cation causes formation of an ionic bond, i.e. salt formation. Thus, the course of the reaction is controlled by the electron affinity of the carbocation. However, the change from single-electron transfer to salt formation is not straightforward. As has been discussed in previous sections, steric effects are another important factor in controlling the formation of hydrocarbon salts. The significant difference in the reduction potential at which a covalent bond is switched to an ionic one -around -0.8 V for tropylium ion series and —1.6 V in the case of l-aryl-2,3-dicyclopropylcyclopropenylium ion series - may be attributed to steric factors. 

As an example we quote the formation of hydrocarbons from synthesis gas with Group 8 metal catalysts. 

Propionaldehyde is incorporated to an extent similar to that of 1-propanol. For formation of hydrocarbons the hydroxoalkyl group of incorporated alcohol... 

These observations, coupled with the findings of Muetterties et al. that none of a large number of mononuclear coordination catalysts examined showed any activity for the H2/CO reaction (53), lend further support to the idea that more than one metal center is necessary for the catalytic formation of hydrocarbon products from synthesis gas. 

Organometallic compounds or carbanions undergo a number of reactions in which the carbanion or carbanion-like moiety of the organometallic compound acts as a nucleophilic displacing agent. Examples are the formation of hydrocarbons from alkyl halides, alkyl halides from halogens, and ketones from acid chlorides or esters. The latter two reactions are closely related to the base-catalyzed condensations and are perhaps additions as well as displacement reactions. Related addition reactions are the carbonation of organometallic compounds and the addition to ketones or aldehydes. 

No evidence of ruthenium metal formation was found in catalytic reactions until temperatures above about 265°C (at 340 atm) were reached. The presence of Ru metal in such runs could be easily characterized by its visual appearance on glass liners and by the formation of hydrocarbon products (J/1J) The actual catalyst involved in methyl and glycol acetate formation is therefore almost certainly a soluble ruthenium species. In addition, the observation of predominantly a mononuclear complex under reaction conditions in combination with a first-order reaction rate dependence on ruthenium concentration (e.g., see reactions 1 and 3 in Table I) strongly suggests that the catalytically active species is mononuclear. 

Formation of Hydrocarbons by Hydridic Reduction of Carbon Monoxide on Cp2Fe2(CO)4... 

Mechanistic observations on formation of hydrocarbons in Cp2Fe2(C0K with LAH. There is little doubt that the initial step in the reaction of Cp2Fe2(C0K with LAH involves formation of a formyl complex by addition of a hydride to coordinated CO. 

We have presented a new system for formation of hydrocarbons from coordinated carbon monoxide. By preparing and reducing possible intermediates we have shown that an insertion step is important in the chain formation and suggest a scheme involving... 

Unless otherwise said, our preferred sources for enthalpies of formation of hydrocarbons are Reference 8 by Roth and his coworkers, and J. B. Pedley, R. D. Naylor and S. P. Kirby, Thermochemical Data of Organic Compounds (2nd ed.), Chapman Hall, New York, 1986. In this chapter these two sources will be referred to as Roth and Pedley , respectively, with due apologies to their coworkers. We will likewise also occasionally take enthalpies of fusion from either E. S. Domalski, W. H. Evans and E. D. Hearing, Heat Capacities and Entropies of Organic Compounds in the Condensed Phase , J. Phys. Chem Ref. Data, 13, 1984, Supplement 1, or E. S. Domalski and E. D. Hearing, J. Phys. Chem Ref. Data, 19, 881 (1990), and refer to either work as Domalski . 

This reaction has been studied, and it was assumed that the hydride attacks electrophili-cally on nitrogen in a polar reaction forming a tin-tin bond. This means that the amine behaves as a catalyst. No formation of hydrocarbons or ammonia was observed in this reaction. 

Fu Q, Sherwood Dollar B, horita J, Lacrampe-Couloume G, Seyfried WE (2007) Abiotic formation of hydrocarbons under hydrothermal conditions constraints from chemical and isotope data. Geochim Cosmochim Acta 71 1982-1998... 

TABLE 1. Enthalpies of formation of hydrocarbon-substituted hydroperoxides and peroxides (kJ mol )... 

The highest conversions of the substrates are obtained, analogous to methanol homologation, with HI as promoter. This assures a high concentration of HRu(CO)3l3. The best selectivities to ethyl acetate + acetic acid and the lowest formation of hydrocarbons are obtained with Nal, which generates NaRu(CO)3l3 and supplies the Lewis acid Na ... 

Thus a decrease in the formation of hydrocarbons and etherification and further homologation products is observed with selectivities to... 

Carbohydrate resources. Carbohydrate resourees, sueh as hydrolyzed stareh and suerose as well as xylose and glueose, ean be proeessed into hydroearbons in a proeess similar to the one performed with bio-oils as described above (section 3.2.2), i.e. by using a HZSM-5 catalyst operated at around 510 °C and ambient pressure." This process is perhaps a little surprising since carbohydrates do not resemble the desired hydrocarbon product as much as the bio-oils do. However, formation of hydrocarbon compounds was found to occur as a result of oxygen removal from the carbohydrate by decarbonylation and decarboxylation reactions." This process is probably one of the first attempts to conduct catalytic cracking of biomass. 

Solutions of ruthenium carbonyl complexes in acetic acid solvent under 340 atm of 1 1 H2/CO are stable at temperatures up to about 265°C (166). Reactions at higher temperatures can lead to the precipitation of ruthenium metal and the formation of hydrocarbon products. Bradley has found that soluble ruthenium carbonyl complexes are unstable toward metallization at 271°C under 272 atm of 3 2 H2/CO [109 atm CO partial pressure (165)]. Solutions under these conditions form both methanol and alkanes, products of homogeneous and heterogeneous catalysis, respectively. Reactions followed with time exhibited an increasing rate of alkane formation corresponding to the decreasing concentration of soluble ruthenium and methanol formation rate. Nevertheless, solutions at temperatures as high as 290°C appear to be stable under 1300 atm of 3 2 H2/CO. 

Siloxane polymerization differs mechanistically from the formation of hydrocarbon polymers in that it is essentially an acid-base process, as might be expected from the strong alternation of electronegativites along the het-eroatomic chain, and the radical initiators that catalyze the homocatenation of alkenes do not work for siloxanes. Long, unbranched polysiloxane chains are favored by higher condensation reaction temperatures and basic catalysts such as alkali metal hydroxides. Acidic condensation catalysts tend to produce polymers of lower molar mass, or cyclic oligomers. 

Kolbe electrolysis is generally useful for the formation of hydrocarbons from monocarboxylic acids and for the preparation of many difunctional compounds as well. A specific illustration is the synthesis of esters of long-chain dicarboxylic adds from monoesters of appropriate dicarboxylic acids (see p. 33). A number of these syntheses are discussed by Fichter.4 In the present preparation, a two-compartment cell is employed to avoid, or at least greatly reduce, undesired reduction of the nitro group at the cathode. It seems likely that the procedure could be adapted to the preparation of other difunctional compounds containing groups that are easily reduced. 

Water, carbon dioxide, olefin hydrocarbons, and alcohols are shown as products. It is obvious that other equations could be written showing the formation of hydrocarbons of other types—that is CH4, C2H6—and of the other oxygenates produced in this synthesis. Although Equations 8, 9, and 10 do not represent the reaction mechanism but simply express the stoichiometry of the system, they do indicate certain fundamental actions that... 

B.2. Simultaneous Characterization of the Formation of Hydrocarbon Pool Compounds on Acidic Zeolites by MAS NMR- UV/Vis Spectroscopy... 

Recently, a novel CF MAS NMR-UV/Vis technique (Fig. 17, Section III.B) was applied to characterize the formation of hydrocarbons by the conversion of methanol on a weakly dealuminated zeolite HZSM-5 6S). The C MAS NMR spectrum recorded at 413 K during the continuous conversion of C-enriched methanol (Fig. 37a, left) consists of signals at 51 and 61 ppm attributed to methanol and DME, respectively. The very weak signal at ca. 23 ppm is probably an indication of alkanes or alkylated cyclic compounds. The appearance of the signals at 23 and 61 ppm indicates that the conversion of methanol on weakly dealuminated zeolites HZSM-5 starts even at 413 K. The simultaneously recorded UV/Vis spectrum (Fig. 37a, right) consists of bands at 275, 315, and 375 nm. The band at 275 nm indicates the formation of neutral aromatic compounds 301,302), and those at 315 and 375 nm may be assigned to mono- and dienylic carbenium ions (301,302), respectively. Because the UV/Vis spectrum of the non-dealuminated zeolite HZSM-5, that... 

The simultaneous investigation of the methanol conversion on weakly dealuminated zeolite HZSM-5 by C CF MAS NMR and UV/Vis spectroscopy has shown that the first cyclic compounds and carbenium ions are formed even at 413 K. This result is in agreement with UV/Vis investigations of the methanol conversion on dealuminated zeolite HZSM-5 performed by Karge et al (303). It is probably that extra-framework aluminum species acting as Lewis acid sites are responsible for the formation of hydrocarbons and carbenium ions at low reaction temperatures. NMR spectroscopy allows the identification of alkyl signals in more detail, and UV/Vis spectroscopy gives hints to the formation of low amounts of cyclic compounds and carbenium ions. 

TABLE IIA. Heats of Formation of Hydrocarbon Ions at 25°C. in Real. Mole... 

Comparing the effects of alkali cations of various sizes applied in reduction of C02 in HCOJ solution with a Cu cathode, Na+, K+, and Cs+ were shown to favor the formation of hydrocarbons.138 The selectivity of ethylene formation surpasses that of methane with increasing cation size. Deactivation of the Cu cathode... 

Metal molybdates421 and cobalt-thoria-kieselguhr422 also catalyze the formation of hydrocarbons. It is believed, however, that methanol is simply a source of synthesis gas via dissociation and the actual reaction leading to hydrocarbon formation is a Fischer-Tropsch reaction. Alumina is a selective dehydration catalyst, yielding dimethyl ether at 300-350°C, but small quantities of methane and C2 hydrocarbons423 424 are formed above 350°C. Heteropoly acids and salts exhibit high activity in the conversion of methanol and dimethyl ether.425-428 Acidity was found to determine activity,427 130 while hydrocarbon product distribution was affected by several experimental variables.428-432... 

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