Extraction of Pd(ii) and Pt(ii) from aqueous hydrochloric acid with 1,3-diaminocalix[4]arene: switching of the extraction selectivity by using different extraction modes

Abstract

The extraction ability of 1,3-diaminocalix[4]arene (2: H₂L) toward platinum group metals (PGMs) has been investigated, which revealed that 2 is able to extract Pd(II) and Pt(II) from hydrochloric acid via different extraction modes. The extraction species for Pd(II) and Pt(II) are [PdL] and [Pt₂Cl₆(H₃L)₂], respectively, as evidenced by equilibrium analysis and X-ray crystallography. In [PdL], the two phenoxide oxygens and two amino nitrogens of L²⁻ coordinate to the Pd ion. On the other hand, in [Pt₂Cl₆(H₃L)₂], two anionic trichloro complexes PtCl₃⁻ are sandwiched between two H₃L⁺, in which one amino nitrogen directly coordinates to a PtCl₃⁻ species and another protonated amino group forms an ion pair with another PtCl₃⁻. Utilizing the different extraction modes, switching of the extraction selectivity has been achieved in the competitive extraction between Pd(II) and Pt(II) by varying the concentrations of H⁺ and Cl⁻ in the aqueous phase. Finally, from the extracted organic phase, back-extraction of Pd(II) and Pt(II) was easily performed, respectively.