Tuning the electronic structure of phosphonic acid-based deep eutectic solvents for synergistic catalytic oxidative desulfurization†

Abstract

Deep eutectic solvents (DESs) hold immense potential in extraction-coupled oxidative desulfurization; however, their efficient utilization, in terms of catalytic activity and cycle-regeneration stability, remains a significant challenge. Herein, we propose a strategy for constructing bifunctional phosphonic acid-based DESs (PDESs) using zinc chloride (ZnCl₂) combined with organic phosphonic acids to achieve ultradeep desulfurization by inducing strong electronic interaction via coordination regulation. Through experimental and theoretical screening, the PDES ZnCl₂/phenylphosphinic acid (ZnCl₂/PIA = 1 : 2), demonstrating strong electron transfer capability and high adsorption energy for oxidants, exhibits remarkable catalytic performance towards the removal of heterocyclic thiophenes. Notably, PDESs can simultaneously function as extractants and catalysts, maintaining a desulfurization efficiency of up to 98.4% even after 12 consecutive cycles under mild conditions, which is much higher than that of previously reported DESs-based desulfurization systems. Furthermore, a possible reaction mechanism is proposed, wherein heterocyclic thiophenes are extracted by the ZnCl₂/2PIA PDES via strong interactions (e.g. hydrogen bonding, C–H⋯π and π⋯π) and are then rapidly oxidized by reactive oxygen radicals and peroxy acid in the presence of an oxidant. This study provides a feasible strategy for achieving strong electronic transfer via coordination regulation, aimed at developing high-performance DESs for deep desulfurization and other related application.