Lower alkanes

Several of the lower alkanes, from C2 to C7, have been used from time to time for surface area determination. They possess the virtue of chemical inertness towards the majority of adsorbents, and their saturation pressures... 

A factor militating against the use of other adsorptives for pore size determination at the present time is the lack of reliable r-curves. The number of published isotherms of vapours such as benzene, carbon tetrachloride or the lower alkanes, or even such simple inorganic substances as carbon dioxide, on a reasonable number of well-defined non-porous adsorbents, is very small. 

Lower alkanes such as methane and ethane have been polycondensed ia superacid solutions at 50°C, yielding higher Hquid alkanes (73). The proposed mechanism for the oligocondensation of methane requires the involvement of protonated alkanes (pentacoordinated carbonium ions) and oxidative removal of hydrogen by the superacid system. 

Sulfonic acids may be hydrolytically cleaved, using high temperatures and pressures, to drive the reaction to completion. As would be expected, each sulfonic acid has its own unique hydrolytic desulfonation temperature. Lower alkane sulfonic acids possess excellent hydrolytic stability, as compared to aromatic sulfonic acids which ate readily hydrolyzed. Flydrolytic desulfonation finds use in the separation of isomers of xylene sulfonic acids and other substituted mono-, di-, and polysulfonic acids. 

Explicit mechanisms attempt to include all nonmethane hydrocarbons believed present in the system with an explicit representation of their known chemical reactions. Atmospheric simulation experiments with controlled NMHC concentrations can be used to develop explicit mechanisms. Examples of these are Leone and Seinfeld (164), Hough (165) and Atkinson et al (169). Rate constants for homogeneous (gas-phase) reactions and photolytic processes are fairly well established for many NMHC. Most of the lower alkanes and alkenes have been extensively studied, and the reactions of the higher family members, although little studied, should be comparable to the lower members of the family. Terpenes and aromatic hydrocarbons, on the other hand, are still inadequately understood, in spite of considerable experimental effort. Parameterization of NMHC chemistry results when NMHC s known to be present in the atmosphere are not explicitly incorporated into the mechanism, but rather are assigned to augment the concentration of NMHC s of similar chemical nature which the... 

Finally, the reaction has also been applied to a wide range of paraffinic materials including polyolefins, which can transformed into lower alkanes... 

In some catalytic processes, it is necessary to avoid carbon-carbon bond cleavage. For example, isobutane is mainly transformed into its lower alkane homologues (hydrogenolysis products) on metal surfaces, while it can be converted more and more selectively into isobutene when the Pt catalysts contain an increasing amount of Sn (selective dehydrogenation process) [131]. 

This catalyst can catalyze a new reaction, called alkane metathesis. By this reaction, alkanes are transformed into higher and lower alkanes.265 Silica-supported zirconium catalysts were also used for the mild oxidation of alkenes by H202 266... 

Earlier transition metals, as zirconium and hafnium, are still more active in hydrogenolysis, which allows zirconium hydrides to be used in depolymerization reactions (hydrogenolysis of polyethylene and polypropylene) [89], In this case, the zirconium hydride was supported on silica-alumina. Aluminum hydrides close to [(=SiO)3ZrH] sites would increase their electrophilicity and, thus, their catalytic activity. A catalyst prepared in this way was able to convert low-density polyethylene (MW 125000) into saturated oligomers (after 5h) or lower alkanes at 150°C (100% conversion). It was also able to cleave commercial isotactic polypropylene (MW 250000) under hydrogen at about 190 °C (40% of the starting polypropylene was converted into lower alkanes after 15 h of reaction). 

D.5.4. Carbon Dioxide and Lower Alkanes. Carbon dioxide has a remarkably high solubility in ionic liquids (22). For example, about 0.5 mol fraction of CO2 was present in [BMIMJPFg at 40°C and 50 bar, although the total volume increased by only 10% as a result of the CO2 addition. 

As mentioned in Section 9.3, Jackson (141) has obtained estimates of the chain-transfer coefficient of the growing radical with polymer in the free-radical polymerization of ethylene, C,p, by choosing the value so as to fit the MWD. As the polymerization conditions for the polymers mentioned in Table 10.1 are not disclosed, it is necessary to choose typical conditions 220° C and 2000 atm will be selected. Under these conditions Ctp, the ratio of the rate constant for attack on polymer (per monomer unit) to that for propagation, in a homogeneous phase, was found to be about 4.0 x 10 3. This is in good agreement with the known transfer coefficients for the lower alkanes (160), when allowance is made for the differences in pressure and temperature (100). The relation between Ctp and k is ... 

However, at least the high vinylidene PIB type may be produced using a concentrated feedstock with an isobutylene content of 90%. Non-reactive hydrocarbons, such as isobutane, n-butane or other lower alkanes commonly present in petroleum fractions, may also be included in the feedstock as diluents. Further, the feedstock... 

Oxidative Dehydrogenation. Specific reviews deal with oxidative dehydro-genation of lower alkanes, particularly over vanadium oxide-based cata-... 

To improve the understanding of these alkane alkylation reactions, Olah and his group carried out experiments involving the alkylation of lower alkanes by stable car-benium ions under controlled superacidic stable ion conditions128 145 146 [Eq. (5.60)]. 

Oxidation of Lower Alkanes Over Oxide Catalysts... 

To summarize, N20 studies have contributed significantly to a better understanding of the activation mechanism of lower alkanes. This facilitates progress in the oxidation by 02, which is being made step by step [49-51]. 

The second factor is the unique feature of N20 as an oxygen donor - first demonstrated with the gas-phase oxidation of lower alkanes over metal oxides. Later, the high catalytic efficiency of the FeZSM-5 zeolites was discovered, suggesting an opportunity for developing new processes of gas-phase oxidation with N20, especially the hydroxylation of benzene and other aromatics to the corresponding phenols. 

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