Pressure Chemistry

PRESSURE CHEMISTRY.) A carbonyl group in an organic compound is reduced similarly; thus acetone (q.v.) very easily into isopropyl alcohol, in the presence of nickel, copper or a catalyst of the platinum group; or with nickel at a higher temperature, pro pane is obtained. Catalytic reduction is also applicable to a car bonyl group in a ring : thus, menthone is hydrogenated to menthol, or by further treatment to menthane. In place of the elimination of water, a hydrogen halide may be eliminated. Benzaldehyde is formed by hydrogenating benzoyl chloride in the presence of palladium black: Oxygen attached to nitrogen is easily rep'aced by hydrogen catalytically. The oxides of nitrogen themselves yield ammonia when passed with hydrogen over copper at 300-400° C. Aniline is obtained by shaking nitrobenzene with platinum in hydrogen at room temperature, whereas in alkaline solution azo- and hydrazo benzene are produced successively, with aniline as final product.

The catalytic reduction of sulphur compounds is of interest on account of the catalytically poisonous nature of many sulphur derivatives. Carbon disulphide is reduced at low temperatures to methylene dithiol, and at about 45o° C hydrogen sulphide is liberated. The latter reaction has been utilized by Car penter and Evans as the basis of a process for the removal of sulphur from coal gas, the gas being passed over a nickel catalyst maintained at the temperature stated. The hydrogen sulphide is easily removed from the resulting gas by iron-oxide purifying boxes, and the deposit of carbon on the nickel is burnt off occa sionally by treatment with air.

Dehydrogenation.

In certain cases hydrogenation is rever sible, loss of hydrogen taking place on leading a hydrogenated body over a catalyst, usually at a relatively high temperature. Two important types of this reaction are the dehydrogenation of reduced benzene rings and the conversion of alcohols, by loss of hydrogen, to aldehydes or ketones. For the dehydrogenation of the reduced benzene nucleus, nickel may be used as a catalyst ; but it is liable to induce side reactions. More satisfactorily, the hexa hydrobenzene is distilled over palladium at 3oo-35o° C. For alcohols, copper is a very suitable catalyst ; for instance, dehy drogenation of ethyl alcohol to acetaldehyde, without the forma tion of appreciable quantities of subsidiary products, takes place on distillation of this compound over copper at 250-350° C; moreover, isopropyl alcohol yields acetone.

Preparation of Catalysts and of Hydrogen.

The cata lysts employed may be divided into classes containing, re spectively, the platinum metals (which for many reactions are active at room temperature) and the remaining elements of the transition group, nickel, cobalt and iron, together with copper, all of which are usually used at I o0-200° or at higher tempera tures in the case of iron. The catalytic activity of the first class is, moreover, considerably higher than that of the second, o• i % or less being a common proportion of a platinum catalyst, whereas the amount of nickel employed in practice is frequently about 1% of the weight of the substance to be hydrogenated. Platinum and palladium are the commonest catalysts of their group; and nickel is usually the most suitable base-metal catalyst, save for reactions in which it is too active. In such cases, copper is substituted for nickel.

If a platinum catalyst is required in its most active condition, it is prepared in a colloidal state, preferably in the presence of a protective colloid to prevent coagulation during use. Paal em ploys, as the protective colloid, sodium lysalbate, a product ob tained by the action of alkalis on egg-albumen. For reactions in acid media, e.g., in acetic acid solution, this protective colloid is replaced by that obtained by the partial hydrolysis of gluten by means of acetic acid (Kelber and Schwarz). In many cases a colloidal catalyst is not necessary. A simple method of preparing a non-colloidal platinum catalyst consists in suspending finely ground platinic oxide in the liquid to be hydrogenated, or in a suitable solvent, reduction to metal being effected either by bub bling hydrogen through the heated mixture or with hydrazine. Reduction by formaldehyde also gives a catalyst of very good activity.

In preparing a nickel catalyst, basic nickel carbonate is con veniently taken as the starting point ; and a porous support, such as kieselguhr, is frequently used to increase The activity and stability of the nickel, the carbonate being precipitated on the support, and reduced with hydrogen at 300-320°. Iron and co balt catalysts, although prepared similarly, are reduced at 45o° C, and copper catalysts at 200° C. For the treatment of gases or vapours, asbestos or fragments of pumice soaked in nickel nitrate solution are heated just sufficiently to form black nickel sesquioxide.

In preparing catalysts, and indeed in carrying out the process of hydrogenation itself, great care must be taken to avoid con tamination by catalyst poisons. Minute quantities of arsenic, sulphur, mercury, phosphorus and certain of their compounds are sufficient to inhibit completely the activity of the catalyst ; and many of the early failures in hydrogenation were due to neglect of this precaution. Similarly, the hydrogen used should be free from hydrogen sulphide or carbon monoxide. A catalyst may also lose its activity by sintering caused by exposure to an ex cessive temperature, either in reduction or during use. Such sup ports as kieselguhr, etc., considerably reduce the liability to sin tering; indeed, a supported nickel catalyst may be heated to 500° C without great loss in activity, whereas with unsupported nickel, temperatures higher than 35o° C are to be avoided. In general, a relatively low temperature of reduction leads to a catalyst of relatively high activity.

The hydrogen required for the hardening of oils is prepared either electrolytically or by the alternate action of water gas and steam on iron, and subsequently purified from carbon monoxide and sulphur compounds. It is also usual to dry the hydrogen be fore use, since water vapour tends to cause the splitting of the oil into free fatty acid, which forms nickel soap with the catalyst and inhibits its action.

Technical Applications of Hydrogenation.

The oldest ap plication of hydrogenation in industry is for the so-called harden ing of oils (see above). The natural oils which can be hardened in this way consist of the glyceryl esters of various unsaturated fatty acids, the most important of which contain IS carbon atoms and are transformed into stearic acid. Thus, with oleic acid : Linoleic acid, which differs from oleic acid in containing two double bonds, also linolenic acid, which is present in linseed oil and contains three ethylenic bonds, and clupanodonic acid, the typical constituent of whale and fish oils, all pass similarly into stearic acid by absorption, respectively, of 2, 3 and 4 molecules of hydrogen.

For use in soaps or for the manufacture of candles, whale oil, various sorts of fish oil, linseed oil and cotton oil are of great importance, while ground-nut (arachis) oil and superior qualities of cotton oil are used in large quantities for the manufacture of artificial lard compounds and similar edible fats. On an indus trial scale, the oil is usually first refined, to remove free fatty acids and albuminoid impurities, both of which tend to inhibit the activity of the nickel catalyst. The two refining processes commonly employed consist, first in gently agitating the oil with water containing the calcu lated amount of soda to neu tralize the free fatty acid pres ent, and in treatment of the oil with a small percentage of fuller's earth, to remove the albuminoid impurities.

A simple form of hydrogena tion vessel of 5-ton capacity is show diagrammatically in fig. 3. It consists of a tall cylinder, containing the oil, which is main tained at the required tempera ture by means of the steam coil. The charge is subjected to a vacuum for several minutes to re move air and any traces of mois ture that may be present in the oil ; hydrogen is subsequently ad mitted under a pressure of several atmospheres and circulated through the oil by means of the pump shown, fresh hydrogen being supplied continuously to replace that absorbed by the oil.

In order to increase the degree of contact between hydrogen and oil, baffles may be introduced into the reaction vessel, or the liquid mixture of oil and catalyst may be sprayed into the gas phase. Alternatively the oil is allowed to flow slowly over a sta tionary catalyst in a hydrogen atmosphere.

The period of reaction varies from i to 4 hours. The process of hydrogenation is appreciably exothermic, i.e., the temperature of the charge usually rises during the course of the conversion, even if the passage of steam through the heating coil is discon tinued. When the required volume of hydrogen has been ab sorbed, the mixture of oil and catalyst is run out into a closed tank and filtered in a steam-heated filter press. The catalyst thus recovered is used for a fresh charge of unhardened oil, while the hydrogenated oily filtrate is allowed to solidify. Hardened oil is a white, neutral, solid fat, having a degree of hardness and a melt ing point dependent on the volume of hydrogen introduced. For complete saturation, cotton oil requires about 3,5oocu.ft. of hy drogen per ton, but considerably less is usually employed.

Nickel catalyst is prepared on a large scale in a manner similar to that already described, from solutions of nickel sulphate and of sodium carbonate in the presence of kieselguhr (reduction tern perature 300-350° C). The easily oxidized, and often pyrophoric, catalyst is allowed to cool in hydrogen, and a current of carbon dioxide is then passed through the reduction chamber; or the hydrogen may be displaced by carbon dioxide while the catalyst is still hot. Nickel catalyst which has been protected by carbon dioxide in this way is sufficiently stable, when cold, to be handled for a short time in air without losing its activity. It is usually mixed with enough oil to form a paste, in which form it can easily be stored and added, as required, to charges of oil which are to be hardened.

For the technical hydrogenation of aromatic compounds a some what higher gas pressure, for instance 15-2o atmospheres, is usually employed, the reaction temperature being about 200° C. It is stated that the capacity of the plant of the Tetralin Co. at Rodleben, Germany, exceeds ioo tons of tetrahydro- and deca hydro-naphthalene a day. The products are liquids which are used as solvents and as turpentine substitutes. The principal diffi culty, in the case of naphthalene, lies in the presence of a small trace of a cyclic sulphur compound, difficult to separate from com mercial naphthalene, which rapidly poisons the nickel catalyst employed. In order to eliminate this, the crude naphthalene is refluxed over metallic sodium or heated in contact with a finely divided metal such as iron, when the sulphur compound is de composed.

Acetone is hydrogenated on a large scale under similar condi tions, save that the temperature should not be allowed to exceed 12o° C—a lower temperature still is preferable—in view of the reversibility of the hydrogenation. Nickel is a suitable catalyst, and the acetone is maintained by means of an increased pressure of hydrogen in the liquid state at the temperature employed.

(E. B. M.)