Alkylation of isoparaffins

The Heterogeneity of Catalyst Surfaces for Chemisorption Hugh S. Taylor Alkylation of Isoparaffins V. N. Ipatieff and Louis Schmerling Surface Area Measurements. A New Tool for Studying Contact Catalysts P. H. Emmett... 

The alkylation of isoparaffins with olefins is the reaction involved in the large scale processes in the petroleum industry. Any isoparaffin and almost any olefin can be used and although the product in largest percentage is usually with the number of carbon atoms equal to the sum of the carbon atoms in the paraffin and olefin, this is not always the case. A great mixture of branch chain products is obtained (Phillips Petroleum Company, 13). 

Just as in the case of aromatic compounds isoparaffins can be alkylated with sources of alkyl groups other than olefins. Alkyl halides, alcohols, ethers, mercaptans, sulfides, etc., can be used. When olefins are used some alkyl fluorides from a combination of olefin and hydrogen fluoride are always formed. The quantity of this in the product can be greatly reduced by providing conditions under which the alkyl fluoride is used in alkylation. The apparent paradox is provided, in that the fluoride content of the product is lessened by further treatment with hydrogen fluoride. A more thorough treatment of the details of the alkylation of isoparaffins with olefins is found elsewhere in this volume. 

In certain reactions, such as the isomerization of butane and the alkylation of isoparaffins, problems of handling hydrogen chloride and acidic sludge are encountered. The corrosive action of the aluminum chloride-hydrocarbon complex, particularly at 70 to 100°C, has long been recognized and various reactor liners have been found satisfactory. 

Excellent reviews of the alkylation of aromatic hydrocarbons have been published. The production of paraffins by the alkylation of isoparaffins by olefins is important industrially " but is not common on a laboratory scale for the preparation of pure hydrocarbons. 

Besides the addition of such reagents as halogens, hydrogen halides, or the hypohalous acids, there are other reactions which appear to proceed through addition mechanisms, but the exact course of the process is sometimes considerably obscured. The Friedel-Crafts addition of alkyl halides to olefins (p. 145), the self-condensation of olefins, and the alkylation of isoparaffins are examples in which attack at the double bond seems to be led by a carbonium ion. 

The Alkylation of Isoparaffins. It has also been found that branched-Ichain paraffins will combine with olefins in the presence of an acid catalyst. Isobutylene and isobutanc, for example, combine to form a mixture of hydrocarbons from which both normal products (those having a multiple of four carbon atoms) and abnormal products (those having a number of carbon atoms not a multiple of four) have been isolated. A flow sheet is outlined in Chart 1 (see p. 144) which will account for some of the products commonly obtained. 

Alkylation of isoparaffin Synthetic rubber Reforming of naphtha... 

It is the object of this review to present a critical discussion of the catalytic alkylation of isoparaffins and of the reaction mechanism. A comprehensive and detailed summary of the papers which had been published on this subject up to the middle of 1945, appeared in Chemical Reviews (Egloff and Hulla, 2). 

Acid-t T)e catalysts. Sulfuric acid of 96-98% concentration catalyzes the alkylation of isoparaffins with butenes and higher molecular weight olefins at 0-20°. Alkylation with propene requires acid of 98-100% concentration and a temperature of 30 . Alkylation with ethylene has not been very successful, presumably because stable ethyl hydrogen sulfate or diethyl sulfate forms too readily. 

The probable function of hydrogen halides as promoters for the metal halides and boron fluoride in the alkylation of isoparaffins seems to be to initiate and maintain the formation of f-alkyl halide by the reaction of step 1. Also, it may convert the metal halide to the more active form, such as, for example, hydrogen aluminum tetrachloride or an ester thereof. 

Hydrogen transfer resulting in the conversion of more or less of the olefin to paraffin always accompanies the catalytic alkylation of isoparaffins with olefins. At the same time, some of the isoparaffin is converted to paraffin containing twice the number of carbon atoms thus, for example, trimethylpentanes are formed in all isobutane alkylations regardless of the olefin used. 

Boron fluoride, promoted by a minor amount of hydrogen fluoride or water, seems to catalyze the alkylation of isoparaffins with ethylene at lower temperatures than does aluminum chloride (Grosse and Ipatieff, 25). The reaction of isobutaiie with ethylene to —30 to —40° and 6 atmospheres pressure yielded a liquid produet, 20% of which was hexanes, to the extent of 180% by weight of the ethylene charged. At 0-5° and 10 atmospheres pressure, a 224% yield of alkylate, 45% of which was hexanes, was obtained. Like the aluminum chloride product formed at 25-35°, the hexanes consisted of 70-90% of 2,3-dimethylbutane, l( -25% of 2-methyl-pentane, and 3% of 2,2-dimethylbutane. 

The action of zirconium cliloride in the alkylation of isoparaffins with olefins and in other hydrocarbon reactions is very similar to that of aluminum chloride, the principal difference being that it requires higher temperatures. Comparatively little information concerning its use has, however, been published. 

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