Wang Zhipeng, Zhang Yixiang, Liu Jingjing, Song Lianjun, Wang Xueyu, Yang Xiuying, Xu Chao, Li Jun, Ding Songdong
College of Chemistry, Sichuan University Chengdu 610064 China
Department of Chemistry, Key Laboratory of Organic Optoelectronics, Molecular Engineering of the Ministry of Education, Tsinghua University Beijing 100084 China.
RSC Adv. 2020 Nov 4;10(66):40035-40042. doi: 10.1039/d0ra08841b. eCollection 2020 Nov 2.
Dithiophosphinic acids (DPAHs, expressed as RRPSSH) are a type of sulfur-donor ligand that have been vastly applied in hydrometallurgy. In particular, DPAHs have shown great potential in highly efficient trivalent actinide/lanthanide separation, which is one of the most challenging tasks in separation science and is of great importance for the development of an advanced fuel cycle in nuclear industry. However, DPAHs have been found liable to undergo oxidative degradation in the air, leading to significant reduction in the selectivity of actinide/lanthanide separation. In this work, the atmospheric degradation of five representative DPAH ligands was investigated for the first time over a sufficiently long period (180 days). The oxidative degradation process of DPAHs elucidated by ESI-MS, P NMR, and FT-IR analyses is RRPSSH → RRPSOH → RRPOOH → RRPOO-OOPRR, RRPSSH → RRPSS-SSPRR, and RRPSSH → RRPSOH → RRPOS-SOPRR. Meanwhile, the determination of p values through pH titration and oxidation product by PXRD further confirms the S → O transformation in the process of DPAH deterioration. DFT calculations suggest that the hydroxyl radical plays the dominant role in the oxidation process of DPAHs and the order in which the oxidation products formed is closely related to the reaction energy barrier. Moreover, nickel salts of DPAHs have shown much higher chemical stability than DPAHs, which was also elaborated through molecular orbital (MO) and adaptive natural density portioning (AdNDP) analyses. This work unambiguously reveals the atmospheric degradation mechanism of DPAHs through both experimental and theoretical approaches. At the application level, the results not only provide an effective way to preserve DPAHs but could also guide the design of more stable sulfur-donor ligands in the future.
二硫代次膦酸（DPAHs，以RRPSSH表示）是一类供硫配体，已在湿法冶金中得到广泛应用。特别是，DPAHs在高效三价锕系元素/镧系元素分离方面显示出巨大潜力，这是分离科学中最具挑战性的任务之一，对核工业先进燃料循环的发展至关重要。然而，已发现DPAHs在空气中易于发生氧化降解，导致锕系元素/镧系元素分离的选择性显著降低。在这项工作中，首次在足够长的时间（180天）内研究了五种代表性DPAH配体的大气降解情况。通过电喷雾电离质谱（ESI-MS）、磷核磁共振（P NMR）和傅里叶变换红外光谱（FT-IR）分析阐明的DPAHs氧化降解过程为RRPSSH→RRPSOH→RRPOOH→RRPOO - OOPRR、RRPSSH→RRPSS - SSPRR和RRPSSH→RRPSOH→RRPOS - SOPRR。同时，通过pH滴定法测定p值以及利用粉末X射线衍射（PXRD）确定氧化产物，进一步证实了DPAH降解过程中的硫到氧的转化。密度泛函理论（DFT）计算表明，羟基自由基在DPAHs的氧化过程中起主导作用，形成氧化产物的顺序与反应能垒密切相关。此外，DPAHs的镍盐表现出比DPAHs高得多的化学稳定性，这也通过分子轨道（MO）和自适应自然密度分配（AdNDP）分析进行了阐述。这项工作通过实验和理论方法明确揭示了DPAHs的大气降解机制。在应用层面，研究结果不仅提供了一种保存DPAHs的有效方法，还可为未来设计更稳定的供硫配体提供指导。
