A process for the preparation of tertiary dodecanethiol

Abstract

The application provides a preparation method of tertiary dodecanethiol, comprising the following steps: dissolving anhydrous aluminum chloride in a linear dialkyl sulfide to obtain a catalyst liquid phase; introducing hydrogen sulfide gas into the catalyst liquid phase to make hydrogen sulfide reach a dissolution equilibrium in the catalyst liquid phase; under the condition of continuously introducing hydrogen sulfide, slowly adding C12 branched olefin into the pre-saturated catalyst liquid phase to react; and after the reaction is completed, a tertiary dodecanethiol product is separated. The linear dialkyl sulfide is used as a complexing solvent of aluminum chloride, which can not only dissolve anhydrous aluminum chloride to form a homogeneous catalytic system, but also plays a role of a hydrogen sulfide solubilizing medium. The lone pair electrons on the sulfur atom in the linear dialkyl sulfide molecule can form a weak hydrogen bond or a dipole interaction with a hydrogen sulfide molecule, and the chemical solubilization effect cooperates with the physical solubilization effect brought by the large molar volume of the long-chain alkyl group.

Description

Technical Field

[0001] This application relates to the field of organic synthesis, specifically to a method for preparing a tert-dodecyl mercaptan. Background Technology

[0002] Tertiary dodecyl mercaptan (TDM) is an important class of organosulfur compounds, primarily used as a molecular weight regulator in the manufacture of butadiene and styrene-based polymers, and widely applied in the production of synthetic rubber, synthetic resins, and synthetic fibers. Furthermore, TDM is also an important intermediate in the manufacture of bactericides, insecticides, rust inhibitors, lubricant additives, and pharmaceuticals. Industrially, TDM is typically produced through an addition reaction of hydrogen sulfide with propylene tetramer or isobutylene trimer in the presence of a catalyst.

[0003] In acid-catalyzed addition reactions of olefins with hydrogen sulfide, the selectivity of thiols directly determines the economics of the process. Existing aluminum trichloride catalytic processes typically employ a batch, single-feed operation, adding the olefin, hydrogen sulfide, and catalyst to the reactor all at once. However, the limited solubility of hydrogen sulfide in conventional organic solvents results in insufficient effective concentrations of hydrogen sulfide in the liquid phase. To increase the hydrogen sulfide concentration, existing technologies often require higher operating pressures, which undoubtedly increases the pressure resistance requirements of the equipment and the safety risks of operation. Furthermore, the single-feed mode leads to a much higher olefin concentration than hydrogen sulfide concentration in the initial stage of the reaction. Carbocation intermediates are easily captured by the already formed thiols, resulting in the formation of thioether byproducts and reducing the selectivity of the target product.

[0004] Therefore, how to increase the effective concentration of hydrogen sulfide in the liquid phase under lower pressure conditions and maintain a high hydrogen sulfide to olefin molar ratio throughout the reaction, thereby improving the selectivity of tertiary alkyl thiols and reducing the formation of thioether byproducts, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the problems of low hydrogen sulfide solubility leading to numerous thioether byproducts and low selectivity in existing technologies, this application provides a method for preparing tert-dodecyl mercaptan. This method, by employing a specific solvent system combined with a semi-batch operation process, significantly increases the concentration of hydrogen sulfide in the liquid phase and effectively suppresses thioether side reactions.

[0006] The first aspect of this application provides a method for preparing tert-dodecyl mercaptan, comprising the following steps: dissolving anhydrous aluminum trichloride in a straight-chain dialkyl sulfide to obtain a catalyst liquid phase; introducing hydrogen sulfide gas into the catalyst liquid phase to achieve a dissolution equilibrium of hydrogen sulfide in the catalyst liquid phase; under the condition of maintaining a continuous introduction of hydrogen sulfide, slowly adding a C12 branched olefin to the pre-saturated catalyst liquid phase for reaction; and after the reaction is completed, separating to obtain the tert-dodecyl mercaptan product.

[0007] This application uses straight-chain dialkyl sulfides as the complexing solvent for aluminum trichloride. These sulfides not only effectively dissolve anhydrous aluminum trichloride to form a homogeneous catalytic system, but more importantly, they act as a solubilizing medium for hydrogen sulfide. The lone pair electrons on the sulfur atoms in the straight-chain dialkyl sulfide molecule can form weak hydrogen bonds or dipole interactions with hydrogen sulfide molecules. This chemical solubilizing effect, combined with the physical solubilizing effect brought about by the larger molar volume provided by the long-chain alkyl group, results in a significantly higher solubility of hydrogen sulfide in the catalyst liquid phase compared to its solubility in inert solvents such as dichloromethane. This means that under the same pressure conditions, the effective concentration of hydrogen sulfide in the liquid phase system of this application is higher.

[0008] Meanwhile, this application employs a semi-batch operation mode, i.e., pre-saturation with hydrogen sulfide followed by slow addition of the olefin. This operation mode ensures that the reaction always proceeds under conditions of excess hydrogen sulfide, maintaining an extremely high instantaneous molar ratio of hydrogen sulfide to olefin. Kinetically, this suppresses the capture of the carbocation intermediate by the thiol product, thereby significantly reducing the formation of thioether byproducts. Furthermore, anhydrous aluminum trichloride, as a strong Lewis acid, can form a complex with the generated tertiary alkyl thiol product, reducing the nucleophilic activity of the thiol product and further inhibiting the secondary addition reaction.

[0009] Further, the straight-chain dialkyl sulfide is selected from one or more of di-n-octyl sulfide, di-n-decyl sulfide, and di-n-decyl straight-chain dialkyl sulfide. Di-n-dodecyl sulfide is preferred, as its longer alkyl chain provides a larger free volume, which is beneficial for the physical solubility of hydrogen sulfide, and its higher boiling point makes it easier to separate from the product. Specifically, the molar ratio of anhydrous aluminum trichloride to the straight-chain dialkyl sulfide is 0.25-0.60:1, preferably 0.3-0.5:1. When this ratio is lower than 0.25:1, there are insufficient catalytic active centers, which may lead to a low reaction rate and a decrease in conversion rate; when this ratio is higher than 0.60:1, excess aluminum trichloride may consume too much of the sulfide coordination capacity, reducing the solubilizing effect of the system on hydrogen sulfide. Within the preferred range, sufficient catalytic active centers are ensured while maintaining the good solubility of the solvent system for hydrogen sulfide.

[0010] Furthermore, the partial pressure of the hydrogen sulfide gas is 1.5-5 bar, and the reaction temperature is 15-40°C. Within this temperature and pressure range, hydrogen sulfide has high solubility in straight-chain dialkyl sulfides and can effectively avoid carbocation rearrangement side reactions.

[0011] Furthermore, the addition rate of the C12 branched olefin is 1.0-3.0 g / min. Strictly controlling the addition rate of the olefin is key to maintaining a high molar ratio of hydrogen sulfide to olefin, thereby ensuring high selectivity. The C12 branched olefin is preferably a propylene tetramer or an isobutylene trimer.

[0012] Furthermore, the method of this application also includes the step of introducing hydrogen chloride gas into the catalyst liquid phase, wherein the amount of hydrogen chloride gas introduced is 1-5% of the molar amount of anhydrous aluminum trichloride. The introduction of trace amounts of hydrogen chloride can rapidly establish catalytic activity through the synergistic effect of protic acid and Lewis acid, eliminate the hysteresis phenomenon in the initial stage of reaction initiation, and maintain stable catalytic efficiency during the reaction process.

[0013] Furthermore, the method of this application also includes the step of adding a polycyclic aromatic hydrocarbon (PAH) synergist to the liquid phase of the catalyst. The PAH is selected from one or more of anthracene, phenanthrene, and pyrene, and its addition amount is 1-3% of the molar amount of anhydrous aluminum trichloride. The presence of the PAH helps to regulate the acidic environment of the catalyst system, further improving the stability of the reaction.

[0014] Furthermore, after the olefins have been added, hydrogen sulfide is continuously introduced and the mixture is stirred for 0.5–2 hours. This post-reaction step helps to completely convert any remaining trace amounts of olefins, further improving the conversion rate.

[0015] Further, the separation step includes: washing the reaction solution with alkali to remove catalyst residue, washing and neutralizing with water, drying, and removing light components by vacuum distillation at 50-80℃ / 10mmHg, and then collecting the fraction at 100-140℃ / 10mmHg, which is the tert-dodecyl mercaptan product, and recovering and recycling the straight-chain dialkyl sulfide solvent in the residue.

[0016] The present invention has the following beneficial effects: This application uses linear dialkyl sulfides as bifunctional solvents, which both dissolve the catalyst and significantly solubilize hydrogen sulfide, enabling high concentrations of hydrogen sulfide in the liquid phase to be achieved at relatively low pressures (1.5-5 bar), thus avoiding the safety hazards and increased equipment costs associated with high-pressure operation.

[0017] This application employs a semi-batch process with hydrogen sulfide presaturation combined with slow addition of olefins, ensuring that the reaction is in a state of hydrogen sulfide excess throughout, effectively suppressing the formation of thioether byproducts, significantly improving the selectivity of tert-dodecyl mercaptan, and reducing the thioether content to below 10 wt%, and up to 5 wt% under optimal conditions.

[0018] The in-situ complexation effect of anhydrous aluminum trichloride with the product thiol reduces the nucleophilic activity of the thiol, mechanistically blocking the secondary addition reaction pathway and further ensuring high selectivity.

[0019] The introduction of hydrogen chloride co-catalyst eliminates the reaction start-up lag period, improves reaction efficiency, and provides mild process conditions with strong operational controllability. Detailed Implementation

[0020] To facilitate understanding of this application, a more complete description will be provided below. This application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0021] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0022] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention. Unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0023] In some preferred embodiments, the linear dialkyl sulfide can be selected from any one of di-n-octyl sulfide, di-n-decyl sulfide, or di-n-dodecyl sulfide; specifically, di-n-dodecyl sulfide has a particularly significant solubilizing effect on hydrogen sulfide due to its high free volume resulting from its long-chain structure.

[0024] In some preferred embodiments, the molar ratio of anhydrous aluminum trichloride to linear dialkyl sulfide can be any value among 0.25:1, 0.30:1, 0.35:1, 0.40:1, 0.45:1 or 0.50:1; within this range, the catalyst liquid phase can maintain both high catalytic activity and good hydrogen sulfide solubility.

[0025] In some preferred embodiments, the partial pressure of hydrogen sulfide gas can be arbitrarily selected between 1.5 bar, 2.0 bar, 2.5 bar, 3.0 bar, 3.5 bar, or 4.0 bar; a high liquid phase concentration can be achieved at a lower pressure, reducing the pressure resistance requirements of the equipment.

[0026] In some preferred embodiments, the reaction temperature can be arbitrarily selected between 18°C, 20°C, 25°C, 30°C, 35°C or 38°C; low temperature operation is beneficial to suppress carbocation rearrangement and improve the purity of product isomers. Example

[0027] This embodiment provides a method for preparing tert-dodecyl mercaptan.

[0028] Raw materials for preparation: Di-n-dodecyl sulfide: 40.0 g (0.108 mol), as a solvent.

[0029] Anhydrous aluminum trichloride 5.8 g (0.043 mol) was used as a catalyst.

[0030] Anthracene: 0.10 g (0.56 mmol), as an additive.

[0031] Hydrogen chloride (HCl) gas: approximately 0.05 g (approximately 1.4 mmol), as a co-catalyst.

[0032] Propylene tetramer ( Olefins: 100.0 g (0.595 mol) were used as reactants.

[0033] hydrogen sulfide ( Gas: continuously introduced, with partial pressure controlled at 2.5 bar.

[0034] Preparation method: Catalyst liquid-phase preparation: In a 500 mL three-necked round-bottom flask, under nitrogen protection, 40.0 g of di-n-dodecyl sulfide was added, followed by the addition of 5.8 g of anhydrous aluminum trichloride in portions with stirring. (Sulphide molar ratio = 0.40:1), stir at 25°C for 30 min until... Completely dissolves, forming a homogeneous, deep amber-colored catalyst liquid phase. Add 0.10 g of anthracene (1.3 mol% relative to the catalyst liquid phase). Stir for 15 minutes until the anthracene is dissolved.

[0035] Introduction of HCl co-catalyst: Slowly introduce approximately 0.05 g (3.2 mol% relative to the catalyst liquid phase) of HCl gas into the catalyst liquid phase. The infiltration time was about 5 minutes, and the liquid phase color darkened slightly.

[0036] Pre-saturation: Transfer the reaction flask to a 2L pressure vessel and seal it. Continuously bubble gas into the liquid phase through the gas feed pipe. Gas, regulation Reduce the partial pressure to 2.5 bar, maintain the temperature at 25°C, and continue bubbling for 15 minutes until the liquid phase reaches its maximum viscosity. Dissolution equilibrium.

[0037] Olefin slow addition reaction: 100.0 g of propylene tetramer is loaded into a metering feed tank. While maintaining... Under continuous flow (2.5 bar) and stirring (300 rpm), propylene tetramer was slowly added dropwise to the reaction solution at a constant rate of 2.0 g / min using a metering pump. The total olefin addition time was approximately 50 min. The reaction temperature was controlled at 25 ± 2 °C using a water bath.

[0038] Post-reaction heat preservation: After the olefin is completely added, continue to maintain the temperature. Introduce (2.5 bar) and stir, then keep at 25°C for 1.0 h.

[0039] Post-treatment: Release the remaining H2S gas (collect and recover), and transfer the reaction solution to a separatory funnel. Wash twice with 5% NaOH aqueous solution (100 mL each time), and separate the aqueous phase (containing... (Hydrolysis products). The organic phase was washed with deionized water until neutral, dried with anhydrous sodium sulfate, and filtered. Light components were removed by vacuum distillation at 50-80℃ / 10mmHg. The fraction collected at 100-140℃ / 10mmHg was used as the tert-dodecyl mercaptan product. The residue in the reactor was a di-n-decalinear dialkyl sulfide solvent, which could be recycled after recovery. Example

[0040] This embodiment provides a method for preparing tert-dodecyl mercaptan.

[0041] Raw materials for preparation: Di-n-dodecyl sulfide: 40.0 g (0.108 mol).

[0042] waterless 3.6g (0.027mol) / Sulphide molar ratio = 0.25:1.

[0043] Propylene tetramer: 100.0g.

[0044] : 4.0 bar partial pressure.

[0045] Preparation method: The operation steps are the same as in Example 1, except that: the amount of AlCl3 is adjusted to 3.6g, the partial pressure of H2S is adjusted to 4.0bar, anthracene is not added, HCl is not introduced, the olefin addition rate is adjusted to 1.5g / min (total time is about 67min), and the reaction holding time is extended to 1.5h. Example

[0046] This embodiment provides a method for preparing tert-dodecyl mercaptan.

[0047] Raw materials for preparation: Di-n-octyl sulfide: 30.0 g (0.116 mol).

[0048] waterless 5.0g (0.037mol) The sulfide molar ratio is 0.32:1.

[0049] Anthracene: 0.08g.

[0050] Propylene tetramer: 100.0g.

[0051] : 2.5 bar partial pressure.

[0052] Preparation method: The operating steps are the same as in Example 1, except that the linear dialkyl sulfide is replaced with di-n-octyl sulfide. The dosage is 5.0g, the anthracene dosage is 0.08g, and the other conditions are the same as in Example 1. Example

[0053] This embodiment provides a method for preparing tert-dodecyl mercaptan.

[0054] Raw materials for preparation: Isobutylene trimer: 100.0g.

[0055] : 3.0 bar partial pressure.

[0056] Preparation method: The operating steps are the same as in Example 1, except that the olefin is replaced with isobutylene trimer. The partial pressure was adjusted to 3.0 bar, the reaction temperature was adjusted to 30°C, and the other conditions were the same as in Example 1. Example

[0057] This embodiment provides a method for preparing tert-dodecyl mercaptan.

[0058] Raw materials for preparation: waterless : 8.0g (0.060mol), / Sulphide molar ratio = 0.56:1.

[0059] HCl: Approximately 0.08 g (approximately 2.2 mmol).

[0060] : 1.5 bar partial pressure.

[0061] Preparation method: The operation steps are the same as in Example 1, except that: The dosage was adjusted to 8.0g, and the reaction temperature was adjusted to 18℃. The partial pressure was adjusted to 1.5 bar, the HCl flow rate was increased to approximately 0.08 g, and the olefin addition rate was reduced to 1.2 g / min (total time approximately 83 min).

[0062] Comparative Example 1 This comparative example provides a method for preparing tert-dodecyl mercaptan.

[0063] The raw materials were prepared in the same manner as in Example 1.

[0064] Preparation method: The liquid-phase preparation of the catalyst was the same as in Example 1. The key difference was that no liquid-phase preparation was performed. Pre-saturation and slow olefin addition operation. 100.0 g of propylene tetramer was added to the catalyst liquid phase in one step, and then the catalyst was introduced at 25 °C and an H2S partial pressure of 2.5 bar. The mixture was stirred and reacted for 3.0 h. The post-treatment method was the same as in Example 1.

[0065] Comparative Example 2 This comparative example provides a method for preparing tert-dodecyl mercaptan.

[0066] Raw materials for preparation: The solvent was replaced with dichloromethane. ).

[0067] Preparation method: 5.8g of anhydrous Dissolve in 80 mL of dichloromethane. Add 0.10 g of anthracene and bubble with HCl. Then proceed with the semi-batch process as described in Example 1. Pre-saturation (2.5 bar, 25 °C) and slow addition of olefins were performed. The post-treatment vacuum distillation conditions were adjusted according to the characteristics of the dichloromethane solvent: the dichloromethane solvent (boiling point 40 °C) was first removed under atmospheric pressure, and then the tert-dodecyl mercaptan product fraction was collected according to the method in Example 1.

[0068] Comparative Example 3 This comparative example provides a method for preparing tert-dodecyl mercaptan.

[0069] The preparation method is the same as in Example 1, the only difference being that HCl gas is not introduced.

[0070] Comparative Example 4 This comparative example provides a method for preparing tert-dodecyl mercaptan.

[0071] The preparation method is the same as in Example 1, the only difference is that The pressure is introduced at normal atmospheric pressure (1 atm), without pressurization.

[0072] Olefin conversion: determined by gas chromatography-FID (GC-FID). An Agilent 7890B gas chromatograph with an HP-5ms capillary column (30m × 0.25mm × 0.25μm) was used. The conversion rate was calculated by comparing the change in peak area of ​​olefins in the reaction solution before and after the reaction, and straight-chain dialkyl sulfides were used as internal standards for correction.

[0073] Thiol selectivity and thioether content: determined by gas chromatography-mass spectrometry (GC-MS) and area normalization method. The chromatographic peaks of the tert-dodecylthiol isomer (characteristic ion m / z 202) and thioether byproduct (characteristic ion m / z 370) were qualitatively confirmed by mass spectrometry, and the selectivity and content were calculated based on the peak area percentage.

[0074] Markovnikov selectivity: employing 1 1H NMR spectroscopy (1H NMR spectroscopy). Determined in CDCl3 solvent, the content of non-tertiary thiol isomers is calculated by detecting the characteristic signals of hydrogen atoms on the α-carbon (the α-CH2 signal for primary thiols -CH2-SH appears at δ 2.4-2.6 ppm, and the α-CH signal for secondary thiols -CH(SH)- appears at δ 3.0-3.5 ppm). The proportion of non-tertiary thiol isomers is then determined by the ratio of their integrated area to the total alkyl hydrogen signal.

[0075] Thiol purity (SH value): determined by potentiometric titration (refer to ASTM D3227). Using silver nitrate-isopropanol solution as the titrant, the thiol sulfur content in the sample was measured, and the purity was calculated.

[0076] Table 1 Test results of the examples and comparative examples

[0077] As shown in Table 1, Examples 1-5 all exhibited excellent catalytic performance, with olefin conversion rates generally exceeding 94%, thiol selectivity maintained between 90% and 95%, and thioether byproduct content controlled below 10 wt%. This indicates that the preparation method provided in this application can be carried out under mild conditions (1.5-5 bar). Highly efficient and selective synthesis of tert-dodecyl mercaptan can be achieved under pressure (15-40℃). Example 1, as the optimal solution, exhibits the best overall performance, with a thioether content of only 5wt%, significantly superior to other solutions. In Comparative Example 1, the single-feed reaction resulted in excessively high local olefin concentrations in the initial stages of the reaction. Compared to other methods, carbocation intermediates are more prone to secondary reactions with the already formed thiols to form sulfides. Although Comparative Example 2 employed a semi-batch process, replacing straight-chain dialkyl sulfides with dichloromethane resulted in a decrease in thiol selectivity to 85% and an increase in sulfide content to 15 wt%. This verifies that straight-chain dialkyl sulfides, as solvents, not only serve as reaction media but also play a crucial role in the reaction. "Solubilizing" effect. Dichloromethane cannot improve its solubility through specific intermolecular interactions. The concentration in the liquid phase leads to the effective concentration within the reaction system. The low concentration increased the likelihood of side reactions. In Comparative Example 3, compared to Example 1, the conversion and selectivity decreased slightly without the addition of HCl co-catalyst. Comparative Example 4 demonstrates that although the thioether solvent has solubilizing ability, a certain pressure-driven force is still required to ensure a sufficiently high concentration in the liquid phase. concentration.

[0078] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0079] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for preparing tert-dodecyl mercaptan, characterized in that, Includes the following steps: Anhydrous aluminum trichloride was dissolved in a straight-chain dialkyl sulfide to obtain the catalyst liquid phase; Hydrogen sulfide gas is introduced into the catalyst liquid phase to allow the hydrogen sulfide to reach dissolution equilibrium in the catalyst liquid phase; Under the condition of continuous hydrogen sulfide flow, C12 branched olefins are slowly added to the pre-saturated catalyst liquid phase for reaction. After the reaction was completed, the tert-dodecyl mercaptan product was obtained by separation.

2. The preparation method according to claim 1, characterized in that, The linear dialkyl sulfide is selected from one or more of di-n-octyl sulfide, di-n-decyl sulfide, and di-n-dodecyl sulfide.

3. The preparation method according to claim 1, characterized in that, The molar ratio of the anhydrous aluminum trichloride to the linear dialkyl sulfide is 0.25-0.60:

1.

4. The preparation method according to claim 1, characterized in that, The partial pressure of the hydrogen sulfide gas is 1.5-5 bar, and the reaction temperature is 15-40℃.

5. The preparation method according to claim 1, characterized in that, The addition rate of the C12 branched olefin is 1.0-3.0 g / min.

6. The preparation method according to claim 1, characterized in that, The C12 branched olefin is selected from propylene tetramer or isobutylene trimer.

7. The preparation method according to claim 1, characterized in that, Hydrogen chloride gas is introduced into the liquid phase of the catalyst, and the amount of hydrogen chloride gas introduced is 1-5% of the molar amount of anhydrous aluminum trichloride.

8. The preparation method according to claim 1, characterized in that, A polycyclic aromatic hydrocarbon (PAH) synergist is added to the liquid phase of the catalyst. The PAH is selected from one or more of anthracene, phenanthrene, and pyrene, and the amount added is 1-3% of the molar amount of anhydrous aluminum trichloride.

9. The preparation method according to claim 1, characterized in that, After the olefins have been added, continue to introduce hydrogen sulfide and stir for 0.5-2 hours.

10. The preparation method according to claim 1, characterized in that, The separation process includes: washing the reaction solution with alkali to remove catalyst residue, washing and neutralizing with water, drying, and removing light components by vacuum distillation at 50-80℃ / 10mmHg. The fraction at 100-140℃ / 10mmHg, i.e., tert-dodecyl mercaptan product, is then collected, and the straight-chain dialkyl sulfide solvent in the residue is recovered and recycled.

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