Dethiophenation of Benzene Using Formaldehyde
Abstract Generated abstract
This study examines the removal of thiophene sulfur from benzene by condensation with formaldehyde, with emphasis on catalyst choice and reagent ratios. Experiments with artificial benzene, thiophene mixtures and commercial KMK benzene grades compared several potential catalysts and then varied formalin and sulfuric acid consumption at room temperature. The tested substitutes for sulfuric acid were ineffective, while sulfuric acid with formalin gave high or complete dethiophenation at relatively low thiophene concentrations and in industrial benzene samples, with evidence of an optimum formalin amount for a given acid consumption. The work concludes that the formaldehyde method is promising but requires more detailed investigation, particularly for treatment of unwashed BT fractions.
Full Text
Dethiophenation of Benzene Using Formaldehyde
N. M. Smolyaninova
(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)
One of the promising methods for purifying benzene from thiophene sulfur is the formaldehyde method. This purification method is based on the known ability of thiophene to enter into a condensation reaction with formaldehyde in the presence of a catalyst, with the formation of high-molecular-weight products (resins).
Taking into account the exceptional simplicity of the process, the availability of the reagent, and the encouraging results obtained in the formaldehyde purification of benzene [1, 2], we carried out experiments to determine the possibilities of this method.
The first series of experiments was set up with the aim of replacing scarce sulfuric acid, which is an excellent catalyst in the polycondensation of thiophene with formaldehyde, by other catalysts.
The experiments were carried out at room temperature in a round-bottom flask equipped with a stirrer and a dropping funnel for introducing the reagents. Artificial mixtures of benzene for cryoscopy and thiophene, 36% formalin, and catalysts usually used in processes of condensation of formaldehyde with various reagents were employed.
After the reaction was completed, the benzene was distilled off, washed, if necessary, free of unreacted formaldehyde, dried over ignited calcium chloride, and analyzed for its thiophene content according to GOST 8448-61.
The results of the experiments, given in Table 1, showed that none of the substances tested is an effective catalyst in the indicated process.
The next series of experiments, presented in Table 2, was carried out with 92% sulfuric acid as the catalyst, in order to find the optimal ratios between the reagents that would ensure practically complete dethiophenation of benzene.
It follows from Table 2 that, at a high content of thiophene in benzene (about 0.1%), formaldehyde purification is ineffective: only with a very large consumption of sulfuric acid (10% by volume) is it possible to bring the degree of purification to 97–98% (experiments of the 4th series). The presence of formalin here apparently has no substantial effect on the removal of thiophene, since at such a high acid consumption dethiophenation also occurred without the addition of formaldehyde.
At a lower concentration of thiophene in benzene (on the order of 0.40–0.46%), the situation is entirely different. Here, at a volume ratio of formalin to sulfuric acid of 1 : 2, a high degree of purification is achieved—on the order of 99% (experiment 7), whereas without formalin, but with the same consumption of sulfuric acid (2% by volume), the degree of dethiophenation was only 58.33% (experiment 17).
Table 1
Effect of catalyst type on the dethiophenation of benzene with formaldehyde
| Experiment No. | Benzene content in mixture, cm³ | CH₂O content in mixture, cm³ | Catalyst | Amount of catalyst, cm³ | Stirring time, h | Thiophene content before experiment, % | Thiophene content after experiment, % | Degree of purification, % |
|---|---|---|---|---|---|---|---|---|
| 1 | 99 | 1 | 40% caustic soda | 1.6 | 2 | 0.1074 | 1024 | 4.65 |
| 2 | 99 | 1 | Glacial acetic acid | 1.6 | 2 | 0.1074 | 0.1000 | 6.89 |
| 3 | 99 | 1 | 25% aqueous ammonia solution | 1.6 | 2 | " | 0.1010 | 5.95 |
| 4 | 99 | 1 | Hydrochloric acid | 1.6 | 2 | " | 0.0994 | 7.44 |
| 5 | 49 | 0.5 | Nitric acid (conc.) | 0.8 | 2 | " | 0.0880 | 18.63 |
| 6 | 49 | 0.5 | " | 1.6 | 1 | 0.1040 | 0.0910 | 12.50 |
| 7 | 49 | 1 | " | 1.6 | 1 | 0.1040 | 0.0920 | 11.50 |
| 8 | 49 | 0.5 | Nitric acid (conc.) | 0.8 | 1 | 0.1040 | 0.0840 | 19.20 |
| 9 | 50 | 1 | Aluminum chloride | 2 g | 1 | 0.0462 | 0.0397 | 14.07 |
| 10 | 50 | 1 | Potassium chlorate | 2 g | 1 | 0.0910 | 0.0910 | 0.00 |
| 11 | 50 | 1 | Phosphoric acid | 2 g | 1 | 0.0429 | 0.0352 | 16.19 |
| 12 | 50 | 1 | " | 2 g | 1 | 0.0462 | 0.0357 | 22.70 |
| 13 | 50 | 1 | Ammonium chlorate | 2 g | 1 | 0.0408 | 0.0398 | 2.45 |
In experiment 14, at a volume ratio of formalin to sulfuric acid of 1 : 4 and an initial thiophene content of 0.0408%, practically complete desulfurization of benzene was achieved.
The following series of experiments was carried out with plant benzene of the grades “pure” and “for nitration,” from KMK coke-chemical production. In all cases, 50 ml of benzene was taken for dethiophenation; the stirring time was 1 h, and the temperature was room temperature.
The experimental data are given in Table 3.
It follows from Table 3 that formaldehyde purification from thiophene is effective for both investigated grades of benzene at a volume ratio of formalin to sulfuric acid of 1 : 2 and at their consumption of 1 and 2% (by volume), respectively. At a ratio of 0.8 : 2, 100% dethiophenation of benzene for nitration was also obtained.
The experiments also showed that, at the given consumption of sulfuric acid, there is an optimum consumption of formalin at which complete dethiophenation is ensured. The same was observed in experiments with artificial mixtures of benzene with thiophene.
Table 2
Effect of the ratio of reagents on the degree of dethiophenation of benzene with formaldehyde
| No. of experiments | Mixture composition, ml: benzene | Mixture composition, ml: formalin | Mixture composition, ml: sulfuric acid | Reaction duration, h | Thiophene content in benzene, %: before experiment | Thiophene content in benzene, %: after experiment | Degree of purification, % |
|---|---|---|---|---|---|---|---|
| 1 | 50 | 0.5 | 1.0 | 1 | 0.0964 | 0.0720 | 25.30 |
| 2 | 50 | 0.5 | 2.0 | 1 | 0.0964 | 0.0530 | 45.2 |
| 3 | 50 | 0.5 | 3.0 | 1 | 0.0924 | 0.0320 | 65.3 |
| 4 | " | " | " | " | 0.0900 | 0.0019 | 97.4 |
| 5 | " | 1.0 | 3.0 | " | 0.0910 | 0.0214 | 76.5 |
| 6 | 50 | 0.25 | 0.5 | 1 | 0.0924 | 0.0380 | 58.8 |
| 7 | 50 | 0.5 | 1.0 | 1 | 0.0420 | 0.0003 | 98.3 |
| 8 | 50 | 1.0 | 1.0 | 1 | 0.0462 | 0.0152 | 67.1 |
| 9 | 50 | 1.5 | " | " | 0.0420 | 0.0255 | 39.3 |
| 10 | " | " | 2.0 | " | 0.0420 | 0.0003 | 98.3 |
| 11 | " | " | " | 0.5 | 0.0462 | 0.0020 | 95.4 |
| 12 | " | 0.5 | 5.0 | 1.0 | 0.0462 | 0.0000 | 100.0 |
| 13 | 50 | — | 2 | 0.5 | 0.0462 | 0.0012 | 97.4 |
| 14 | 50 | 0.25 | 1 | 1.0 | 0.0408 | 0.0000 | 100.0 |
| 15 | 50 | 0.25 | 0.5 | 1.0 | 0.0408 | 0.0085 | 79.1 |
| 16 | " | 0.10 | 0.5 | " | " | 0.0078 | 73.5 |
| 17 | " | — | 1.0 | " | 0.0420 | 0.0175 | 58.3 |
| 18 | " | — | 5.0 | " | " | 0.0013 | 97.1 |
Table 3
Effect of the amount of sulfuric acid and formalin on the degree of dethiophenation of commercial KMK benzenes
| No. of experiments | Experiments with “pure” benzene containing 0.062% thiophene: amount of reagents, cm³: formalin | Experiments with “pure” benzene containing 0.062% thiophene: amount of reagents, cm³: sulfuric acid | Experiments with “pure” benzene containing 0.062% thiophene: degree of discoloration, % | No. of experiments | Experiments with benzene “for nitration,” containing 0.046% thiophene: amount of reagents, cm³: formalin | Experiments with benzene “for nitration,” containing 0.046% thiophene: amount of reagents, cm³: sulfuric acid | Experiments with benzene “for nitration,” containing 0.046% thiophene: degree of discoloration, % |
|---|---|---|---|---|---|---|---|
| 1a | 0.5 | 1.0 | 100.0 | 1a | 0.5 | 1.0 | 101.0 |
| 2a | 1.5 | 2 | 100.0 | 2a | 0.4 | 1.0 | 100.0 |
| 3a | 0.25 | 1.0 | 93.9 | 3a | 0.25 | 1.0 | 93.05 |
| 4б | 0.25 | 0.5 | 86.3 | 4a | 1.0 | 1.0 | 63.04 |
| 5a | 0.1 | 0.5 | 31.6 | 5б | 0.25 | 0.5 | 100.00 |
| 6a | — | 0.5 | 12.6 | 6a | 0.1 | 0.5 | 79.30 |
| 7б | 0.5 | 0.5 | 50.00 | ||||
| 8a | 0.25 | 2.0 | 95.70 | ||||
| 9 | — | 2.0 | 91.90 |
It should be noted that formaldehyde purification of industrial benzenes proceeds more effectively than the purification of artificial mixtures.
To verify the formaldehyde method of dethiophenation, a more detailed investigation must be carried out, since this method deserves serious attention.
Of particular interest is the study of the possibility of removing thiophene sulfur from the unwashed fraction of BT.
REFERENCES
- J. Walker. Formaldehyde. Moscow: Goskhimizdat, 1957.
- L. M. Cherkasova and L. K. Gorin. Coke and Chemistry, No. 1, 44–45, 1963.
- N. K. Moshchinskaya, V. N. Nosach, B. S. Bouden. Proceedings of the Dnepropetrovsk Chemical-Technological Institute named after F. E. Dzerzhinsky, 57–61, issue 7, 1960.