Hydrogen donors hydrocarbons

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). 

A schematic diagram of the liquid solvent extraction process is illustrated in Figure 1. Where the production of liquid hydrocarbons is the main objective an hydrogenated donor process solvent is used, whereas in the production of needle coke this is not necessary and a coal derived high boiling aromatic solvent may be used (e.g. anthracene oil). An essential economic requirement of the process is that a high extraction yield of the coal is obtained and this will depend upon the coal used and the digestion conditions. 

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. 

Next, a consideration of the kinetics for Step 2 raises some questions. The transfer of hydrogen in similar reactions has been well studied, and Table III presents data for the relative rates of transfer of hydrogen from a number of hydrocarbons to the free radical C130 at 350°C. The donor hydrocarbons are listed in order of increasing ease of H-transfer to the free radical. Tetralin is near the middle of the list. The most reactive donor in the table, 1,4-dihydronaphthalene, is about four times as active as Tetralin. 

MHDV [Mobil Hydrogen Donor Visbreaking] A modified visbreaking process in which a hydrogen donor stream from the oil refinery is added to the heavy hydrocarbon stream before thermal cracking. Developed by Mobil Corporation... 

The hydrogen donors vary widely from heteroatom-containing compounds such as alcohols, amines, acids and cyclic ethers to hydrocarbons such as alkanes (Table 20.1). The choice of donor is largely dependent on several issues ... 

The hydrogen donor (RHl may be an alkane, but often the use of a saturated hydrocarbon leads to inefficient reaction the balance of... 

At the end of reaction the initial hydrocarbon concentration is still important, but there are also many other hydrogen donors. Thus, in the absence of oxygen, reactions of alkoxy radicals will lead to aldehydes by pyrolysis and to alcohols by abstraction. Let us examine these two possible reactions for each alkoxy radical. 

Stepwise decarboxylation also occurs, particularly in reactions in which the carboxylate radical (RC02 ) is formed. This radical usually decomposes to a hydrocarbon radical (R-) and C02- The overall decarboxylation product is determined by what R- reacts with If a good hydrogen donor is present, RH is formed if a halogen donor such as Br2 is present, RBr is formed ... 

The most effective hydrogen donors are reported [1,2,5] to be alcohols and ethers, hydrocarbons behaving similarly although to a lesser extent. 

Commercial tetralin contains naphthalene as the principal impurity and this interferes with the preparation of tetralin-1-hydroperoxide or with use of the hydrocarbon as hydrogen donor in hydrogen-transfer reactions. An early purification procedure is uninviting fractionation extraction in turn with mercury (to remove sulfur impurities), with mercuric acetate solution (to remove olefins), and with sulfuric acid fractionation. More recently Bass sulfonated the crude hydrocarbon with coned, sulfuric acid and added ammonium chloride to precipitate ammonium tetralin-6-sulfonate. The salt was crystallized until pure and hydrolyzed by steam distillation from sulfuric acid solution. Distillation from sodium gave material showing no ultraviolet bands characteristic of naphthalene. 

A new decarboxylative route to free radicals, which has proved particularly successful in preparative work, embodies the thermal (or photochemical) decomposition reaction of 1-hy-droxypyridine-2(l/f)-thione esters 23 with tributyltin hydride, /er/-butanethiol, or a similar hydrogen donor.These esters can be easily prepared from acyl halides and the sodium salt of l-hydroxypyridine-2(l//)-thione, or from the carboxylic acid, dicyclohexylcarbodiimide and l-hydroxypyridine-2(l/f)-thione. The intermediate radicals were readily reduced to the corresponding hydrocarbons 24 in efficient chain reactions with organotin hydrides or thiols as reaction partners, and the proportion of rearranged to unrearranged products could be controlled by the choice of hydrogen donor, its concentration and the temperature. This system was sufficiently quantitative and well behaved for use in kinetic studies, and the rate constants of the (S-scission reactions of the listed cyclopropylmethyl species were determined. 

The hydrogen donors for which dimers of the corresponding radical have been reported are hydrocarbons. Thus biacetyl 20 79> and camphorquinone 15> reactions with cyclohexene afforded, in part, bicyclohexenyl. Dimers have also been reported in reactions of alkylbenzenes 15,20,101, I29,i33)( diphenylmethane 148>, and thioxanthene 148> (64). 

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