CHEMISTRY

Academician A. V. TOPCHIEV, G. M. MAMEDALIEV, A. N. KISLINSKII
and G. N. ANIKINA

PREPARATION OF XYLENES BY ALKYLATION AND DEALKYLATION OF AROMATIC HYDROCARBONS IN THE PRESENCE OF SYNTHETIC ALUMINOSILICATES

The reaction of dealkylation and coupled alkylation of aromatic hydrocarbons in the presence of various catalysts has been studied by many authors. Most of these investigations were devoted to developing a method for obtaining toluene. In the earliest studies \((^{1-4})\) it was shown that aluminum chloride promotes the course of the reaction of dealkylation and coupled alkylation of aromatic hydrocarbons. Later, results were published from a number of experimental works on the catalytic conversion of various aromatic hydrocarbons in the presence of aluminum chloride \((^{5-8})\). Recently, in connection with the wide use of aluminosilicate catalysts in petroleum processing, they have also been successfully used for the alkylation and dealkylation of aromatic hydrocarbons \((^{9-11})\).

The results of a study on the synthesis of toluene by demethylation of various methylbenzenes and coupled methylation of benzene in the presence of aluminosilicates were reported in our previous works \((^{12,13})\).

Over the last decade, the production of various types of synthetic fiber has undergone broad industrial development. The success in the use of polyester fibers, in particular the growth of terephthalene production abroad, has made necessary the comprehensive development of industrial production of para-xylene. One of the promising directions for the production of \(p\)-xylene, as was shown by us \((^{14})\), is the process of isomeric conversion of \(m\)- and \(o\)-xylenes into the \(p\)-isomer, readily carried out in the presence of synthetic aluminosilicates. Xylenes can be synthesized on the basis of the reaction of dealkylation and coupled alkylation of aromatic hydrocarbons.

The transfer of the methyl group and the reaction of coupled alkylation of toluene in the process of dealkylation of various polyalkylbenzenes have not been studied. The study of these reactions is at present of important practical significance and is of scientific interest.

In the present work are presented the results of a study of the reaction of dealkylation of industrial samples of the polymethylbenzene fraction and coupled alkylation of toluene in the presence of aluminosilicate catalysts.

Experimental Part

In the process of producing toluene on the basis of the condensation reaction of benzene with methyl alcohol, along with toluene a considerable amount of polymethylbenzenes is formed. These products are waste from production and do not find valuable practical application.

As starting material we used an industrial sample of the aforementioned polymethylbenzene fraction, characterized by the following main parameters: initial boiling point \(152^\circ\), specific gravity 0.9256, coef-

refractive coefficient 1.5339, molecular weight 140, iodine number 18.3, sulfur content 100%. The content of the trimethylbenzene fraction boiling within the range 165–175° was about 31%. The sum of the fractions boiling up to 195° was 38.5%. The main mass of the starting product (about 60%) boiled above 200° and consisted of a mixture of tetra-, penta-, and hexamethylbenzenes (predominantly solid compounds with high melting points). The somewhat elevated iodine number of the product is due to the presence in it of a small amount of derivatives of styrene and other unsaturated compounds.

As the second component of the feedstock, toluene from coke-oven gas production was used. The latter contained traces of benzene with admixtures of a small amount of light benzene fractions (less than 1%). Its specific gravity was 0.8669, refractive coefficient 1.4970, bromine number 0.2, sulfur content 100%. The experiments were carried out in a laboratory installation of a flow reactor (Fig. 1).

The mixture of toluene with the polymethylbenzene fraction was subjected to catalytic processing. Their weight ratio in the mixture was 2 : 1. Synthetic aluminosilicates from the Baku and Grozny catalyst plants were used as catalysts.

Experiments were carried out to study the influence of temperature, pressure, and space velocity, and the optimum conditions of the xylene process were determined.

Fig. 1. Laboratory installation of a flow reactor operating under pressure:
1 — feed burette, 2 — pump, 3 — reactor, 4 — manometer, 5 — electric furnace, 6 — reducing valve, 7 — refrigerator-condenser, 8 — receiver, 9 — flowmeter, 10 — gasometer, 11 — thermoregulator, 12 — relay, 13 — galvanometer.

At temperatures below 420°, the conversion of high-boiling polymethylbenzenes, as well as the alkylation of toluene with the formation of low-molecular aromatic hydrocarbons, was of limited significance. Raising the temperature favored the course of the reaction, and the maximum effect of xylene formation was achieved in the range 470–480°.

At atmospheric pressure and a temperature of 480°, catalytic processing of the product was not accompanied by the formation of large amounts of xylenes and benzene. Increasing the pressure (3–10 atm.) noticeably intensified the course of the reaction of dealkylation and coupled alkylation of the initial aromatic hydrocarbons and led to the formation of up to 26–29% xylenes and 10–12% benzene.

Under the optimum process conditions, a series of experiments was carried out. The analytical data for the starting mixture and a representative sample of catalyst are given in Table 1.

Table 1

At a temperature of 480°, a pressure of 10 atm, a feed rate of 0.5 : 1, and a cycle duration of 1 hour, the yields of catalyzate, coke, and gas are, respectively, 92; 3.3; and 2 wt. % based on the feedstock. A deep conversion of the polymethylbenzene fraction takes place. Its content decreases from 33% in the feedstock to 11.4% in the catalyzate. A significant part of the toluene is alkylated.

with the formation of xylenes. Its amount decreases from 66 to 48%. The specific gravity of the product decreases from 0.8834 to 0.8692; the end point of boiling of the catalyzate is 180°; iodine number 2.9; sulfonatability 100%.

The yield of xylenes and benzene, as is evident from the material-balance data (Table 2), is 35.6 g, which amounts to 81.5% of the total amount of toluene and polymethylbenzene fraction consumed in one pass. The consumption of toluene for the formation of the xylene fraction and of the sum of the xylene plus benzene fractions is, respectively, 86 and 59%.

Table 2

Spectral investigation of the xylene fraction showed the presence in it of 20–25% p-xylene, 45–50% m-xylene, ~20–25% o-xylene, and an insignificant amount of ethylbenzene (2–3%). The low ethylbenzene content favorably distinguishes the product obtained from products of other industrial processes for the production of xylenes.

The resources of polyalkylaromatic hydrocarbons from pyrolysis, coke-oven-gas production, and industrial processes of platforming, hydroforming, etc., create a real basis for practical use of the reaction of dealkylation and coupled alkylation of aromatic hydrocarbons. The process studied has a number of advantages over existing industrial methods for obtaining xylenes, and its practical implementation will make it possible to increase substantially the resources of paraxylene.

Institute of Petroleum
Academy of Sciences of the USSR

Received
6 II 1957

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