Sulfolane

Sulfolane (also tetramethylene sulfone, systematic name: 2,3,4,5-tetrahydrothiophene-1,1-dioxide) is a clear, colorless liquid commonly used in the chemical industry as an extractive distillation solvent or reaction solvent. Sulfolane was originally developed by the Shell Oil Company in the 1960s as a solvent to purify butadiene. Sulfolane is an aprotic organosulfur compound, and it is readily soluble in water.

Chemical Properties
Sulfolane is classified as a sulfone, a group of organosulfur compounds containing a sulfonyl functional group. The sulfonyl group is a sulfur atom doubly bonded to two oxygen atoms. The sulfur-oxygen double bond is highly polar, allowing for its high solubility in water, while the four carbon ring provides non-polar stability. It is these properties that allow it to be so miscible in both water and hydrocarbons, resulting in its widespread use as a solvent for purifying hydrocarbon mixtures.

Synthesis
The original method developed by the Shell Oil Company was to first allow butadiene to react with sulfur dioxide. This yields sulfolene, which was then hydrogenated using Raney nickel as a catalyst to give sulfolane.



Shortly thereafter, it was discovered that both the product yield and the lifetime of the catalyst could be improved by adding hydrogen peroxide and then neutralizing to a pH of roughly 5-8 before hydrogenation.

Developments have continued over the years, including in the catalysts used. Recently, it was found that Ni-B/MgO showed superior catalytic activity to that of Raney nickel and other common catalysts that have been used in the hydrogenation of sulfolene.

Other syntheses have also been developed, such as oxidizing tetrahydrothiophene with hydrogen peroxide. This first produces tetramethylene sulfoxide, which can then be further oxidized to tetramethylene sulfone. Because the first oxidation takes place at low temperature and the second at relatively higher temperature, the reaction can be controlled at each stage. This gives greater freedom for the manipulation of the reaction, which can potentially lead to higher yields and purity.

Uses
Sulfolane is widely used as an industrial solvent, especially in the extraction of aromatic hydrocarbons from hydrocarbon mixtures and to purify natural gas.

The first large scale commercial use of sulfolane, the sulfinol process, was first implemented by Shell Oil Company in March of 1964 at the Person gas plant near Karnes City, Texas. The sulfinol process purifies natural gas by removing H2S, CO2, COS and mercaptans from natural gas with a mixture of alkanolamine and sulfolane.

Shortly after the sulfinol process was implemented, sulfolane was found to be highly effective in separating high purity aromatic compounds from hydrocarbon mixtures using liquid-liquid extraction. This process is still widely used today in refineries and the petrochemical industry. Because sulfolane is the most efficient industrial solvent for purifying aromatics, they operate at the lowest solvent-to-feed ratio, making sulfolane units highly cost effective. In addition, it is selective in a range that complements distilliation; where sulfolane can’t separate two compounds, distillation easily can and vice versa, keeping sulfolane units useful for a wide range of compounds with minimal additional cost.

While sulfolane is highly stable and can therefore be reused many times, it does eventually break down into acidic byproducts. A number of measures have been developed to remove these byproducts, allowing the sulfolane to be reused and increase the lifetime of a given supply. Some methods that have been developed to regenerate spent sulfolane include vacuum and steam distillation, back extraction, adsorption, and anion-cation exchange resin columns.