Dimethyl sulfoxide (DMSO) is an organosulfur compound represented by the chemical formula C2H6OS, or more commonly (CH3)2SO. This colorless liquid is prized in industrial and laboratory settings as a highly polar aprotic solvent. Its ability to dissolve a wide range of both polar and nonpolar compounds makes it versatile. The chemical structure of DMSO, which includes a sulfur atom bonded to two methyl groups and one oxygen atom, gives it distinct characteristics, including its miscibility with water and many organic solvents.
Primary Raw Material Source
The manufacturing of Dimethyl sulfoxide begins with its chemical precursor, Dimethyl Sulfide (DMS), which is obtained from two main industrial sources. Historically, the most significant source was the wood pulp industry, specifically the Kraft process used to convert wood into wood pulp. This process generates “black liquor,” a byproduct containing various sulfur compounds, including DMS, which are captured and separated for use. Modern production methods often involve a purely synthetic route, where methanol and hydrogen sulfide are reacted over a catalyst to directly produce high-purity Dimethyl Sulfide.
The Core Manufacturing Process
The transformation of Dimethyl Sulfide (DMS) into Dimethyl sulfoxide (DMSO) is achieved through a precise chemical reaction called controlled oxidation. This process involves adding a single oxygen atom to the sulfur atom of the DMS molecule. The predominant industrial method for this oxidation is the Nitrogen Dioxide Method, which utilizes nitrogen dioxide (NO2) or nitric acid as the primary oxidizing agent.
The reaction is typically carried out in a specialized oxidation tower at controlled temperatures, usually ranging between 60°C and 80°C. The presence of oxygen gas is often required in the reaction chamber, with the nitrogen dioxide effectively acting as a catalyst to facilitate the transfer of the oxygen atom to the DMS.
Control over the reaction conditions is mandatory to ensure DMSO is the primary outcome. If the oxidation process proceeds further, a second oxygen atom can be added to the DMSO molecule. This over-oxidation results in the formation of Dimethyl Sulfone (DMSO2), a related compound that must be minimized. The reaction is stopped once the majority of the DMS has been converted to DMSO, yielding a crude product that contains various impurities, unreacted precursors, and the DMSO2 byproduct.
Ensuring Purity and Grade
Following the oxidation reaction, the crude Dimethyl sulfoxide product requires extensive purification to remove water, unreacted DMS, and the DMSO2 byproduct. The primary purification technique is fractional distillation, which separates the compounds based on their different boiling points. Since DMSO has a relatively high boiling point of 189°C, distillation is often performed under reduced pressure to prevent thermal decomposition. For extremely high-purity requirements, manufacturers may employ melt crystallization, a process that relies on the precise freezing point of DMSO to separate it from impurities.
This crystallization step is effective for removing trace amounts of inorganic salts and organic byproducts that may remain after distillation. The final level of purification dictates the grade of the finished product, which ranges from technical or industrial grade to pharmaceutical grade. Pharmaceutical-grade DMSO, intended for medical use, demands the most stringent purity standards, often requiring a final product purity exceeding 99.95%. The purified solvent is then carefully dried, often using desiccants like molecular sieves, because DMSO is highly hygroscopic and readily absorbs moisture from the air.