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 (R 2 S 2 ) by oxidative coupling of alkyl mercaptan and sulfur with K 2 CO 3 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-K 2 + CO 3 2− ] complex. The results of infrared spectral analysis confirm that CO 3 2− in K 2 CO 3 has a strong interaction with the terminal hydroxyl group in PEG. At the fixed molar ratio of –CH 2 CH 2 O-/K 2 CO 3 , the increase of PEG chain length has a positive correlation with the S-L PTC performance. The reaction rate and yield of R 2 S 2 can be improved by increasing the amount of K 2 CO 3 and the molar ratio of -CH 2 CH 2 O-/K 2 CO 3 appropriately. The PEG400/K 2 CO 3 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 -C 8 H 17 ) 2 S 2 can still reach more than 97% by using 0.1 mol% K 2 CO 3 and 0.006 mol% PEG400 (based on sulfur). Graphical Abstract