Novelties of selective triphasic synthesis of bis-(p-chlorobenzyl) sulfide using hydrogen sulfide and reusable phase transfer catalyst

• Methyldiethanolamine was used for selective absorption of toxic H2S gas.
• The product bis-(para-chlorobenzyl) sulfide was synthesized with 100% selectivity.
• The green PTC catalyst was regenerated and reused up to 2 cycles.
• The catalyst used was found to perform better than other catalysts tested.
• A generalized empirical kinetic model was developed and validated.

The present investigation is based on the advancement in hydrogen sulfide (H2S) capture and utilization (HSCU). Commercially H2S is absorbed efficiently using alkanolamines such as methyldiethanolamine (MDEA). Aqueous H2S-rich MDEA is proposed to act as a sulfiding agent for aromatic halides such as p-chlorobenzyl chloride (p-CBC) to synthesize value-added thioethers. The objective of the present investigation is to synthesize bis-(p-chlorobenzyl) sulfide (BPCBS), a value-added thioether, selectively using p-CBC and H2S-laden aqueous MDEA. For the immiscible bi-phasic system, reusable solid phase transfer catalyst, polymer-bound tributylmethylammonium chloride (PBTBMAC) was employed under liquid-liquid-solid (L-L-S) mode in the presence of solvent toluene to enhance the reaction rate and product selectivity. Full conversion of p-CBC was obtained with 100% selectivity towards the desired product BPCBS at optimized specific level of process parameters. The catalyst has shown substantial activity even after three times of reuse, which leads to waste minimization and economic benefits. A generalized empirical kinetic model was developed and successfully validated against the experimental results. The triphasic reaction thus leads to process intensification, waste minimization and selectivity enhancement. The process can be utilized as an alternative to many other HSCU processes to utilize the sour gas in synthesizing value-added chemicals.

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