Extraction and Recovery of Thiols from Petroleum Distillates RALPH L. HOPKINS and H. M. SMITH Petroleum Experiment Station, Bureau o f Mines, Bartlerville, O k l a .
The study of sulfur compounds in crude oils requires methods for separating the various types of sulfur compounds quantitatively from petroleum distillates. This report describes the qualitative testing and quantitative application of a method for thiols (mercaptan) using sodium aminoethoxide in ethylenediamine solution. This method has been successfully used in the separation of thiols from a distillate boiling between 111"and 150' C.
T
H E research program of American Petroleum Institute Research Project 48A being conducted a t the Petroleum Experiment Station of the Bureau of Mines is concerned in part with the separation and identification of the sulfur compounds in petroleum. A general procedure for separations has been devised which includes isolation of the sulfur compounds from a petroleum distillate by adsorption, fractional distillation in a semimicro column, and identification of the components by infrared abmrption spectra and through chemical derivatives This scheme is satisfactory for a distillate boiling below 100" C., but above this temperature the number of compounds present becomes 80 large that it is necessary t o devise methods of further separating the mixture into types, each of which may then be distilled and studied separately. Such a method for separating and recovering thiols has been developed. Several methods are described in the literature for the chemical separation of thiols (1, %), but because of solubility limitations, or possible decomposition or rearrangements, they did not meet the requirements of this project, Thus, aqueous sodium and potassium hydroxides are effective only on the lowest boiling members of the series and on aromatic thiols having the thiol group attached to the ring or in an alpha position with respect to the ring. Heavy metal mercaptides have been used as a means of isolation, but these are gelatinous and difficult to handle and purify, as they adsorb large amounts of other substances. Some metal mercaptides, particularly those of silver, are sensitive to light.
Table I.
Efficiency of Thiol Extraction
Thiol Cyclohexanethiol 1-Octanethiol 2-Octanethiol tert-Octyl mercaptan0 1-Decanethiol 1-Dodecanethiol tert-Dodecvl mercaDtana tert-Tetradecyl mercaptana 1-Hexadecanethiol tert-Hexadecyl mercaptan" tert-Hexadecyl mercaptana Structure not known.
Recovery, 3' % 96 9: 98 YO 90 85 96 85 80 70
4.5
Residue. % 5
2 2 3 2
3 Solid 20 3
EXPERIMENTAL
The thiol-containing distillate, preferably concentrated by adsorption, is diluted with isopentane (2-methylbutane) and brought in contact with sodium aminoethoxide (H2NC2H40Na) dissolved in anhydrous ethylenediamine (H2NC2H&H2). From this treatment an extract of sodium mercaptides dissolved in the ethylenediamine is obtained. The thiols are regenerated from this solution by acid hydrolysis and recovered by steam distillation The method of Moss, Elliott, and Hall ( 3 ) for titration of phenols and other weak acids in ethylenediamine suggested the use of this reagent for extraction of thiols. Solutions of sodium hydroxide (up to 50% aqueous) in ethylenediamine and of sodium aminoethoxide in anhydrous ethylenediamine were tested for this purpose. Thiols up to Clo dissolve easily in the aqueous solutions, where the anhydrous reagent will dissolve readily every thiol tested (including the difficultly reacting tert-hexadecyl mercaptan), except 1-octadecanethiol and high concentrations of I-hexadecanethiol. Both of these readily react but yield salts only slightly soluble a t room temperature which dissolve on warming.
Efficiency of Thiol Extraction. A test procedure was designed for evaluating the efficiency of extraction with various thiols using 1.1N sodium aminoethoxide in anhydrous ethylenediamine. One milliliter of thiol dissolved in 25 ml. of isopentane is extracted with 50-ml. and 10-ml. portions of the reagent. Dissolved and entrained isopentane is removed from the extract by reducing the pressure with a water pump.
206
Figure 1.
Distillation Trap
An approximately equal weight of crushed ice is then added
to the extract to prevent volatilization of thiols from the heat of
the hydration, followed by about 2 volumes of water containing enough hydrochloric acid to neutralize the sodium salts. The addition of acid is necessary only to prevent severe corrosion of the flask. The flask is attached to a trap (Figure l), and the thiols are separated by steam distillation. The trap was designed to give better recovery and more exact observation of the end of the distillation than is possible with conventional traps. A series of thiols ranging from Csto Cle, including primary, secondary, and tertiary members, was tested by this procedure, with the results shown in Table I. Commercial materials were used in most cases without purification. I n several instances where yields were lowered appreciably by impurities, the isopentane was washed and evaporated and the residue measured. Tests for thiols in the residue were negative in every case.
V O L U M E 2 6 , NO. 1, J A N U A R Y 1 9 5 4
207
Table 11. Treatment of 111 O to 150" C. Boiling-Range Wasson Distillate to O b t a i n T h i o l C o n c e n t r a t e Distillate (8973 grams)
I
Alumina adsorption I
I
I I
I
Hvdrocarbons
Sulfur concentrate
- 1
Alcohol eluent
Dilute with salt water and extract with isopentane
Discard
I *Tal
O
Sulfur compounds and isopentane
I
Extract three times with sodium aminoethoxide
I
Raffinate II
I
I
Discard
Extract I
Neutral sulfur compounds and hydrocarbons
I
Sodium mercaptides
I
Extract with isopentane I Raffinate II Treat with ice and dilute HCI. Steam distill
I
Extract I
'
Aromatics neutral sulfur compounds
I
7
Thiols
1
Water
II
Water wash I
I
Extract with isopentane and dietill
For further study
I
I
Thiols
Thiols I
I
wax extracted directly without any previous treatment. The yields by both mpthods were essentially equal, but treatment of the whole distillate not only required handling much larger batches of material but also resulted in the accumulation of some aromatic hydrocarbons in the extract, and several backextractions with isopentane were required to remove them. The concentration process requires the time-consuming and expensive adsorption operation, but this is still necessary for subsequent steps in the separation scheme. About 8 ml. of material resulting from these extractions was distilled in a semimicro column. Cyclohexanethiol was identified by infrared spectroscopy, and subsequent spectral data have shown that a plateau on the distillation curve was caused by substantial amounts of 2hexanethiol. Large Scale. The above procedure (Table 11) was used on a more comprehensive study of the sulfur compounds in Wasson crude oil boiling between 111 and 150' C. The distillate, which weighed 8973 grams (11.7 I.), was first passed through activated alumina to concentrate the sulfur compounds. This yielded about 500 grams of material. However, some of the sulfur compounds were dissolved in the alcohol used for desorbing the alumina columns. This material was recovered by diluting the alcohol with salt water and extracting the mixture with about 2.5 liters of isopentane. The main sulfur-containing fraction was then added to the isopentane extract from the alcohol, and this mixture was extracted twice with 125 ml. and once with 100 ml. of 1.9N sodium aminoethoxide in anhydrous ethylenediamine. The three extracts were combined and extracted four times with 50 ml. of isopentane to remove aromatic hydrocarbons and neutral sulfur compounds dissolved or entrained in the extract. Distillation of the isopentane after washing free of amines yielded 3.5 ml., which was predominantly aromatic hydrocarbons. This was returned to the raffinate from the sodium aminoethoxide extraction. The extract was diluted with about 500 grams of crushed ice, followed by 78 ml. of concentrated hydrochloric acid in 500 ml. of water in a 2-liter boiling flask. A trap was connected to the flask and the mixture refluxed until no more oil separated from the distillate. The regenerated thiols were drawn from the trap, and an additional 200 ml. of water was distilled over to complete volatilization of the thiols. The thiol distillate was washed to remove small amounts of ethylenediamine. The washings were combined with the distillate water and extracted with isopentane, which was then distilled off. The thiols recovered from the washings were combined with the main portion, and the total recovered amounted to 22.2 grams of thiols.
I
Sodent
I
22.5 grams
Thiols containing 16 carbon atoms seem to be the upper limit of usefulness for the distillation method of recovery. Several hours of refluxing were required in this boiling range, and in the cme of 1-hexadecanethiol some oxidation to disulfide probably occurred during this time as indicated by a solid residue in the flask after completion of the distillation and cooling. For the lower members of the series, the steam distillation required only a few minutes of boiling. Sulfide Interference. Similar tests were made on a mixture of a thiol and an alkyl sulfide to determine whether appreciable amounts of sulfides are extracted along with the thiols. When a mixture of 1 ml. of 1-octanethiol and 1 ml. of 5-thianonane in 25 ml. of isopentane was extracted in the manner described, the extract yielded 1.21 ml. of distillate. The extraction was repeated and the extract washed with 50-ml. and 25-ml. portions of isopentane before distillation. The extract yielded 0.95 ml., and 1.00 ml. of residue was obtained by distillation of the isopentane. The purity of the extract as determined by silver nitrate titration was 98%. A similar test with 1-dodecanethiol and 7-thiatridecane yielded 0.87 ml. (87%) of thiol similarly determined to be 99% pure. PROCEDURE
Preliminary. In applying the method to extraction of distillates two variations in procedure were investigated on two parts of a distillate. One part was passed through activated alumina to concentrate the sulfur compounds, while the other
CONCLUSION
After the raffinate was washed and the isopentane distilled, it was discovered that the raffinate still contained traces of thiols. The sample was again extracted with sodium aminoethoxide and yielded an additional 0.3 gram of thiols (total yield 22.5 grams). Analysis of the extract for aromatic hydrocarbons by ultraviolet absorption showed the presence of not more than 0.5%. I t has been determined that 2iV sodium aminoethoxide in anhydrous ethylenediamine is an effective reagent for removing thiols from petroleum distillates. The data indicate that the procedure, for practical purposes, is quantitative. Aromatics and other materials, which are somewhat soluble in the reagent, can be removed by washing the extract with isopentane. The thiols are quantitatively recovered by acidifying the sodium salts and steam-distilling the mixture. Ethylenediamine present in the recovered thiols may be washed out with water. Although "whole" distillates can be treated in this manner, it is preferable to concentrate the thiols by adsorption on alumina before P X traction to avoid handling eweaqivc quantities of material. LITERATURE CITED
(1) Birch, 8. F.,a n d Norris, W. S., J . Chem. SOC.,127, 898-907: 193444 (1925). (2) Emmott, R., p r i v a t e communication. (3) Moss, M. L., Elliott, J. W., and Hall. It. T., ANAL.CHEM.,2 0 , 784-8 (1948). RECEIVEDfor review September 24, 1953. Accepted October 27, 1953. Presented before t h e Division of Petroleum Chemistry at the 123rd Meeting of the AMERICAN C H E M I C ASOCIETY, L Los Angeles, Calif. Investigation performed as a part of the work of American Petroleum Institute Research Project 48.4 on the production, isolation, a n d purification of sulfur compounds a n d measurement of their properties which the Bureau of Mines conducts a t Bartlesville, Okla., and Laramie, Wyo.
