PURIFICATION OF BENZENE FROM THIOPHENE BY SELECTIVE OXIDATION

N. M. Smolyaninova, V. A. Pykhtin

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

The production of especially pure low-sulfur grades of benzene at coke-chemical enterprises is of great interest in connection with the ever-increasing demand for it as a raw material for organic synthesis.

We have carried out a study to develop a method for purifying benzene from thiophene by selective oxidation.

Hypochlorous acid (chlorine water) was chosen as the oxidizing agent, since the reagents indicated in the literature [1, 2, 3, 4]—potassium permanganate and dichromate, potassium chromate, the potassium salt of persulfuric acid, and bleaching powder—did not give positive results in our experiments.

In order to determine the optimal conditions for purifying benzene from thiophene, the influence of the following factors was studied: a) the ratio of the reagents—benzene: chlorine water; b) the concentration of active chlorine in the chlorine water; c) the rate and duration of stirring of the reagents; d) the process temperature.

Commercial “benzene for nitration” from the Kuznetsk Metallurgical Combine, with characteristics according to GOST 8448—61, was subjected to purification, as were, in some cases, artificial mixtures of benzene and thiophene containing from 0.023 to 0.033% of the latter.

Oxidation was carried out with freshly prepared hypochlorous acid, obtained by complete saturation of water with gaseous chlorine at room temperature (20°C). The oxidizing agent was mixed with the product being purified in a round-bottom flask of 250 cm³ capacity with vigorous stirring for a specified time. The benzene was then separated from the chlorine water in a separatory funnel, neutralized with alkali, washed with water, dried over calcined calcium chloride for 24 hours, and analyzed for thiophene content according to GOST 8448—61.

The experimental results are given in Tables 1–5 and in Fig. 1. From the data in Table 1 it is clear that complete removal of thiophene is achieved within 1 minute at a benzene—chlorine-water ratio of 1:2, whereas at a ratio of 5:1 complete de-thiophenization does not occur even at the maximum contact time (60 min.).

The results presented in Table 2 show that the concentration of active chlorine in the reagent has a substantial influence on

process. At a constant ratio of the components, the degree of purification decreases with decreasing concentration of hypochlorous acid. However, increasing the amount of chlorine water to a ratio of 1:5 (experiment 4) even at fivefold dilution ensures complete removal of thiophene owing to better contact of the reagents.

Table 1

Effect of the reagent ratio on the degree of benzene desulfurization
(temperature 20°C; thiophene content in benzene—0.033%)

Experiments carried out at stirrer speeds of 420 and 600 rpm showed that increasing the stirring rate makes it possible to shorten the contact time and to achieve a better degree of desulfurization at a minimum reagent ratio.

Table 2

Effect of oxidant concentration on the degree of desulfurization

Data on the effect of the duration of reagent mixing (Fig. 1) show that the optimum contact time is 5 min at a benzene:oxidant ratio of 1:2, 15 min at a ratio of 1:1, and 50 min at a ratio of 2:1. At a chlorine-water consumption of 1 volume per 5 volumes of benzene, desulfurization does not occur at all because of a deficiency of oxidant.

To determine the effect of temperature on the benzene purification process, experiments were carried out in the temperature range 10–40°C at various ratios of benzene and oxidant and with different contact times.

The experiments presented in Table 3 show that increasing the temperature has a positive effect on the thiophene oxidation process; however, the ratio of benzene to oxidant and the phase contact time have a much greater effect. The optimum temperature, at a benzene-to-oxidant ratio of 1:1 and a contact time of 5 min, is 20–40°C. Such temperatures can easily be created and maintained under production conditions. However, from the standpoint of saturating the water with chlorine, it is more expedient to operate at the lower limit.

Table 3

Effect of process temperature on the degree of benzene desulfurization

To determine the optimum alkali consumption, the distribution of chlorine in the reaction products was investigated (the experiments were carried out at a temperature of 20°C). The results given in Table 4 show that

Table 4

Distribution of chlorine in the reaction products

most of the chlorine passes from the initial chlorine water, containing 3.75 g/L chlorine, into the benzene (to a chlorine content of 2.75 g/L). During mixing, the benzene becomes colored a greenish color characteristic of chlorine. After washing with alkali, the benzene is decolorized. An insignificant portion (0.1%) of the chlorine remains in the water in bound form.

Table 5

Effect of alkali concentration on the neutralization of benzene

The effectiveness of the process of neutralizing chlorine with alkali and its optimum consumption are substantially affected by the alkali concentration (Table 5). The data show that it is most expedient to use 10% sodium hydroxide for neutralizing chlorine in benzene. Lower alkali concentrations lead to an increase in the volume of solution and in the number of washings.

Thus, on the basis of the experiments carried out, the following optimum conditions can be recommended for the process of purifying benzene from thiophene by oxidation with hypochlorous acid.

1. Volume ratios benzene : chlorine water — 1 : 1.
2. Process temperature — 20°C.
3. Contact time of the reagents — 15 min.
4. Stirrer rotation speed — 600 rpm.

It should be noted that the chemistry of the process of oxidizing thiophene has not yet been clarified.

Apparently, the direction of the reaction and the nature of the products of thiophene oxidation depend on the type of oxidizing agent and the conditions under which the process is carried out.

Our experimental data indicate that the reaction proceeds partially with the formation of carbon disulfide.

In order to determine the yield of thiophene-free benzene, purification losses, consumption indices for benzene, chlorine water, sodium hydroxide solution, and the duration of each operation, balance experiments were carried out with large quantities of benzene (400–500 ml).

Fig. 1. Dependence of benzene de-thiophenization on reagent contact time. Temperature: 10°C. Component ratios: curve 1—1:2, curve 2—1:1, curve 3—2:1, curve 4—5:1.

On the basis of these experiments, the following principal consumption indices of the purification process were determined, calculated per ton of benzene:

oxidizing agent (chlorine water)—1200 kg/t,
alkali (10%)—78.5 kg/t,
alkali recalculated as 92%—8.5 kg/t,
gaseous chlorine—4.0 kg/t.

It should be noted that the proposed purification method is distinguished by low benzene losses, amounting to no more than 0.5% (as against 6–9% in the sulfuric-acid method), with total losses not exceeding 1.58%, calculated on the initial benzene.

REFERENCES

1. Heterocyclic Compounds. Vol. I. Edited by R. Elderfield. IL, 1953.

2. K. Ellis. Chemistry of Petroleum Hydrocarbons and Their Derivatives. Vol. I, ONTI, 1936.

3. A. S. Broun and A. P. Sivers. Chemistry of Sulfur Compounds of Liquid Fuel. GONTI, 1936.

4. S. S. Nametkin and A. I. Sosnina. ZhPKh, No. 7, 124–126, issue 1–2, 1934.