Hydrogen:hydrocarbon ratio effect

It can be seen that yields up to 86-98% could be obtained with slow space velocities and high hydrogen to hydrocarbon ratio. The use of a coprecipitated chromia-alumina catalyst did not improve the hydrogenation, but the presence of carrier (alumina) had a purely diluting effect on active chromia. Experiments at superatmospheric pressure (33 atm.) in a flow system showed that hydrogenation of octene could be achieved to the extent of 82% at much lower temperatures (200°) and higher space velocities (4.0-5.0 hourly liquid space velocities). 

The results of chain transfer studies with different polymer radicals are compared in Table XIV. Chain transfer constants with hydrocarbon solvents are consistently a little greater for methyl methacrylate radicals than for styrene radicals. The methyl methacrylate chain radical is far less effective in the removal of chlorine from chlorinated solvents, however. Vinyl acetate chains are much more susceptible to chain transfer than are either of the other two polymer radicals. As will appear later, the propagation constants kp for styrene, methyl methacrylate, and vinyl acetate are in the approximate ratio 1 2 20. It follows from the transfer constants with toluene, that the rate constants ktr,s for the removal of benzylic hydrogen by the respective chain radicals are in the ratio 1 3.5 6000. Chain transfer studies offer a convenient means for comparing radical reactivities, provided the absolute propagation constants also are known. 

The isotope effect in peroxyl radical reactions with C—H/C—D bonds of attacked hydrocarbon shows the direct hydrogen atom abstraction as the limiting step of this reaction [15], For example, the cumylperoxyl radical reacts with the C—D bond of a-deuterated cumene (PhMe2CD) ninefold slower than with the C—H bond (cumene, 303 K [118]). The second isotope effect (ratio fcp(PhMe2CH)//tp(Ph(CD2)2CH) is close to unity, i.e., 1.06 per C—D bond [118],... 

Phenols decrease the intensity of CL 7chi in oxidized hydrocarbons as a result of chain termination by the reaction with peroxyl radicals. Since Icu [R02 ]2 (see Chapter 2), the ratio (/0//)12 was found to be proportional to [ArOH] [7]. The kinetic isotope effect (k0K/k0n 1) proves that the peroxyl radical abstracts a hydrogen atom from the O—H bond of phenol [2,8]. 

Fischer-Tropsch synthesis requires a stochiometric H2 CO ratio of 2.1 1. If coal or biomass are used as feedstock, the raw syngas contains much less hydrogen than needed. Hence, CO is reacted with water to form C02 and hydrogen in the shift reactor. As the C02 cannot be used in the Fischer-Tropsch synthesis, part of the carbon for fuel production is lost in this process. If external hydrogen is added to increase the H2 CO ratio, the carbon of the coal or biomass is more effectively used and the hydrocarbon product yield is improved. 

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

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