Polyethylene glycol: an efficient solid–liquid transfer agent for K₂CO₃-catalyzed oxidative coupling reaction of alkyl mercaptan with sulfur as oxidant

The solid–liquid phase transfer catalyzed (S-L PTC) properties of polyethylene glycol (PEG) were investigated in the preparation of dialkyl disulfide (R₂S₂) by oxidative coupling of alkyl mercaptan and sulfur with K₂CO₃ as catalyst under atmospheric pressure and solvent-free conditions. The results have showed that the S-L PTC properties of PEG are not significantly different from those of crown ether. The S-L PTC properties of PEG are reduced due to methylation of the terminal hydroxyl groups, indicating that the terminal hydroxyl group in PEG plays an important role in stabilizing the [PEG-K₂⁺ CO₃²⁻] complex. The results of infrared spectral analysis confirm that CO₃²⁻ in K₂CO₃ has a strong interaction with the terminal hydroxyl group in PEG. At the fixed molar ratio of –CH₂CH₂O-/K₂CO₃, the increase of PEG chain length has a positive correlation with the S-L PTC performance. The reaction rate and yield of R₂S₂ can be improved by increasing the amount of K₂CO₃ and the molar ratio of -CH₂CH₂O-/K₂CO₃ appropriately. The PEG400/K₂CO₃ system exhibits very high catalytic activity for the oxidative coupling reaction of alkyl mercaptan with sulfur as oxidant. The yield of the reaction product (n-C₈H₁₇)₂S₂ can still reach more than 97% by using 0.1 mol% K₂CO₃ and 0.006 mol% PEG400 (based on sulfur).