Solvent-effect, hydrogen donor

The hydrogen donor solvent-effect should still operate with an aqueous vehicle, since product oil would dissolve most of the early reaction products these being organic in nature, would tend to dissolve readily in oil already formed. Hydrogenation of intermediate products would thus still be expected to occur, even though water is a poor donor solvent. 

The Exxon Donor Solvent (EDS) Process, developed by the Exxon Research and Engineering Co., differed from the typical process in that, before being recycled, the solvent was hydrogenated in a fixed-bed reactor using a hydrotreating catalyst, such as cobalt or nickel molybdate. Exxon found that use of this hydrogen donor solvent with carefully controlled properties improved process performance. Exxon developed a solvent index, based on solvent properties, which correlated with solvent effectiveness. 

The second effect of the reduction in the rate of radical propagation reactions is to slow down the rate of radical recombination or polymerization. This effect has been demonstrated in hydrogenation reactions of preasphaltene in the presence of the non-hydrogen donor solvent, decalin. The results are summarized in Table 3. 

Solvent effectiveness in the benzophenone-photoinitiated polymerization of methyl methacrylate was in the order tetrahydrofuran > isopropanol > toluene > benzene (46). In the case of TCMB, isopropanol gaw slightly higher polymerization rates than tetrahydrofuran, but both wore again considerably more effective than toluene and benzene. As might be expected, monomers which thonselves contain ether grouj , e.g. diethylene glycol diacrylate (7), do not require addition of a separate hydrogen-donor solvent for efficient photoinitiation by benzophenone. 

Russell and coworkers reported thermal and photochemical cyclizations of cyclic pyrimidine enediynol and enediynone (Scheme 30.10) [24]. Again, the cyclic structure favors the cycUzation, and irradiation of the alcohol provides 83% of the cyclized product. On the other hand, irradiation of the corresponding ketone leads to a complex mixture with only 10% of cyclized products. No solvent or hydrogen donor concentration effects were studied. 

Data for the kinetics of coal liquefaction have been published in the literature (1-11). A review of the reported studies has recently been given by Oblad (12). The reported data were mostly obtained in bench-scale reactors. Guin et al. (7) studied the mechanism of coal particle dissolution, whereas Neavel (7), Kang et al. (8), and Gleim (10) examined the role of solvent on coal liquefaction. Tarrer et al. (9) examined the effects of coal minerals on reaction rates during coal liquefaction, whereas Whitehurst and Mitchell (11) studied the short contact time coal liquefaction process. It is believed that hydrogen donor solvent plays an important role in the coal liquefaction process. The reaction paths in a donor solvent coal liquefaction process have been reviewed by Squires (6). The reported studies examined both thermal and catalytic liquefaction processes. So far, however, very little effort has been made to present a detailed kinetic model for the intrinsic kinetics of coal liquefaction. 

Using aryl ketones in combination with hydrogen donor solvents or conventional free radical photoinitiators, it is, therefore, possible to effect a new kind of photosensitization of the photolysis of onium salts which results in an expansion in the spectral sensitivity of these cationic photoinitiators. This photosensitization occurs not as a result of a direct interaction between the onium salt and the excited photosensitizer but by a secondary dark nonphotochemical reaction of the onium salt with the radical photoproducts of the photosensitizer. 

The solvent effect on the diastereofacial selectivity in the reactions between cyclopentadiene and (lR,2S,5R)-mentyl acrylate is dominated by the hydrogen bond donor characteristics of the solvent... 

Different types of other coal liquefaction processes have been also developed to convert coals to liqnid hydrocarbon fnels. These include high-temperature solvent extraction processes in which no catalyst is added. The solvent is usually a hydroaromatic hydrogen donor, whereas molecnlar hydrogen is added as a secondary source of hydrogen. Similar but catalytic liquefaction processes use zinc chloride and other catalysts, usually under forceful conditions (375-425°C, 100-200 atm). In our own research, superacidic HF-BFo-induced hydroliquefaction of coals, which involves depolymerization-ionic hydrogenation, was found to be highly effective at relatively modest temperatnres (150-170°C). 

Hydrogen donors are, however, not the only important components of solvents in short contact time reactions. We have shown (4,7,16) that condensed aromatic hydrocarbons also promote coal conversion. Figure 18 shows the results of a series of conversions of West Kentucky 9,14 coal in a variety of process-derived solvents, all of which contained only small amounts of hydroaromatic hydrocarbons. The concentration of di- and polyaromatic ring structures were obtained by a liquid chromatographic technique (4c). It is interesting to note that a number of these process-derived solvents were as effective or were more effective than a synthetic solvent which contained 40% tetralin. The balance between the concentration of H-donors and condensed aromatic hydrocarbons may be an important criterion in adjusting solvent effectiveness at short times. 

Kleinpeter and Burke have recently reported (24) that solvents can also be over hydrogenated and thus become less effective in short time processes. Figure 19 shows some of their work in which a process-derived SRC recycle solvent was hydrogenated to various severities and used for the conversion of an Indian V bituminous coal. The results clearly show a maximum at intermediate hydrogenation severities. Our assessment of this observation is that the loss in conversion was due primarily to the loss in condensed aromatic nucleii rather than conversion of hydrogen donors to saturates. 

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