Selective Separation of HNO3 and HCl by Extraction: The Investigation on the Noncovalent Interaction between Extractants and Acids by Density Functional Theory.

There is a huge demand for the highly selective separation of HNO3 and HCl in many industries, and solvent extraction is considered a feasible method. In this article, DFT calculations were performed to investigate the interactions between acids and extractants including alcohols, ketones, phosphorus, and amines. One of the significant findings to emerge from this study is that amines bind to acids through ion association. Nevertheless, the interaction between acids and alcohols, ketones, and phosphorus with a (RO)3P═O structure is mainly dominated by hydrogen bonds. The change of Gibbs free energy in the extraction process shows that the phosphorus ((RO)3P═O) is superior to other types of extractants in the selective separation of HNO3 and HCl. Furthermore, after the alkoxyl group (RO-) in phosphorus ((RO)3P═O) is replaced by RN- or R- with less electronegativity, the interaction between HCl and the substituted extractants transitions from a hydrogen bond to ion association, but there are still strong hydrogen bonds between them and HNO3. That will lead to a decrease in the selectivity of phosphorus due to the change in interaction types. This new understanding should help the design and screening of efficient extractants for the separation of mineral acids.