A DFT study on the structure–reactivity relationship of aliphatic oxime derivatives as copper chelating agents and malachite flotation collectors

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Abstract

Aliphatic oxime derivatives C7H15CXdouble bondNOH (X = H, CH3, NH2 or OH) have been commercially used as copper(II) extractants or flotation collectors, but the true nature of their reactivity toward Cu2+ or mineral surfaces still remains elusive. Using density functional theory (DFT) method, the structure–reactivity relationship of these aliphatic oxime derivatives was evaluated at B3LYP/6-311 + G(d, p) level. The results indicated that the O or N atoms in the head group of octanaldoxime (OTAO), methyl n-heptyl ketoxime (MHKO), N-hydroxyoctanimidamide(HOIM) and n-octanohydroxamic acid (OTHA) are the chemical reaction center. The reactivity of the aliphatic oxime ionic species increases successively with the replacement of hydrogen atom by methyl, amino and hydroxyl, suggesting that the affinity of them to copper species is as follows: OTHA > HOIM > OTAO > MHKO, which coincides with the order of their binding energy toward Cu2+. The flotation performance of aliphatic oxime derivatives to malachite was in the order of OTHA > OTAO > HOIM > MHKO, which was in line with the combination effect of their reactivity and hydrophobicity. The established structure–reactivity relationship provides an atomic level understanding of the structural requirements for aliphatic oximes to recover cupric ions or copper oxide minerals.

Introduction

Oximes (RR′Cdouble bondNsingle bondOH), a unique family of organic compounds with a wide spectrum of practical applications, can act as chelating agents with various metal ions, including Cu, Fe, Ni, Pd, Pt, Co, Rh, Zn and Mn [1], [2], [3], [4], [5], [6], [7]. Owing to their unique chemical reactivity, oxime derivatives, such as aldoxime, ketoxime, amidoxime and hydroxamic acid, have found their ways into a wide range of industrial practices and engineering applications, including corrosion inhibitors for metal or metal alloys [8], [9], [10], [11], extraction agents for noble and transition metals [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], and flotation collectors in mineral processing [22], [23], [24], [25], [26], [27], [28], [29]. For example, Soldenhoff [13] studied the extraction of cupric ions from sodium chloride solutions using octanal oxime (OCOX) and thought it existed in CuCl2 (OCOX)3 and CuCl2(OCOX)4 complexes. Verraest et al. [19] have investigated the coordination of Cu2+ with inulin modified with amidoxime groups, suggesting that one Cu(II) ion was coordinated by two bidentate amidoxime ligands to form 2:1 complexes. In each Cu-amidoxime chelates, Cu atom bonded with the oxygen atom in the oximido and the nitrogen atom in the amino. The complexes of monohydroxamic acids with Cu(II), usually involving the chelation via both oxygen atoms of the donor ligand, have been extensively studied [20], [21], [30], [31], [32], [33], [34], [35]. Peterson et al. [35] identified that the configuration of copper hydroxamate complex might be (RCOHNO)2 Cu in which coordination bonds were present with the O atoms of the two bidentate hydroxamate ligands, which was in good agreement with other experimental observations [36].

In the last decades, computational chemistry has been widely recognized as a valuable tool to understand the reactive chemical system and predict their physiochemical properties. Among the myriads of powerful computational chemistry techniques, the density functional theory (DFT), with acceptable reliability and affordable computing power, has been commonly used to study the electronic structure and property of various chemical species [37], [38], [39], [40]. Furthermore, the chemical reactivity [41], [42], [43], [44], [45] and structure–reactivity relationship [46], [47], [48], [49], [50] as well as interface chemistry reaction [51], [52], [53], [54], [55], [56] have been extensively investigated by employing DFT methods to throw light on the possible interaction mechanism underling the experimental phenomena.

The chemical reactivity of a extractant or collector toward metal ions or its mineral surfaces depends on certain quantum-chemical parameters, e.g., the composition and energy of the frontier molecular orbitals (FMO) [57] and atomic charges of some reactive atoms. Particularly, the highest occupied molecular orbital (HOMO), second highest occupied molecular orbital (SOMO) and lowest unoccupied molecular orbital (LUMO) are of great importance for determining the chemical reactivity of a functional molecule, largely due to the fact that the chemical bonds between organic molecules and metal species are usually the product of valence electrons. On the one hand, the HOMO orbital plays an important role when extractants or collectors, acting as electron-donating molecules, react with metal atoms. On the other hand, the energy and composition of LUMO orbital are closely related to the electron-accepting capacity of extractants or collectors. In addition, the atomic charges of active atoms contribute to strengthen the electron-donating ability of extractants or collectors and the electrostatic interaction with metal species.

Since aliphatic oxime derivatives uniformly exhibited particular affinity toward Cu2+ ion, it stands to reason that these compounds could make their way into hydrometallurgical industry, especially for flotation recovery of copper oxide minerals, as evidenced by scores of experimental investigations [22], [23], [24], [25], [26]. Folkers et al. [58] found that the long-chain alkanehydroxamic acids formed stable monolayers on copper oxide minerals. Hope et al. [59] investigated the interaction of n-octanohydroxamic acid with copper oxide minerals, confirming that the surfaces of malachite and cuprite contained bulk copper n-octanohydroxamate in which the Cu–N and Cu–O coordination were both involved.

In spite of the large number of experimental studies conducted for aliphatic oxime derivatives, however, the comparative theoretical study of their reactivity has rarely been reported. It has been well established that the structure–reactivity relationship study is of great significance to develop a theoretical framework to guide the design of flotation collectors of high performance [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], [74], [75]. In order to understand the interaction and bond formation between the chelating aliphatic oximes with cupric ions or copper oxide minerals, in the present study, we attempted to develop a structure–reactivity relationship of substituted aliphatic oximes, namely octanaldoxime (OTAO), methyl n-heptyl ketoxime (MHKO), N-hydroxyoctanimidamide (HOIM) and n-octanohydroxamic acid (OTHA), which would help in molecular design of flotation collectors for copper oxide minerals.

Section snippets

Computational method

All the computational analysis in this work was carried out using Gaussian 09 software package [76] and ChemBioOffice 2014. The initial molecular structure of all the considered aliphatic oximes was preliminarily optimized by MM2 (a modified version of Allinger’s MM2 force field) and PM3 (Parameterized Model Revision 3) methods. The molecular geometry obtained was further analyzed and optimized employing DFT method at the B3LYP/6-311 + G (d, p) level [77], [78]. The atomic charge values were

Results and discussion

OTHA may exist as ketone and oxime tautomers, and both can be further ionized in aqueous solutions as shown in Fig. 1. The predicted pKa values for OTAO, MHKO, HOIM and OTHA, which was calculated using Advanced Chemistry Development (ACD/Labs) Software [89], were about 11.29, 12.44, 7.58 and 9.56, respectively. HOIM may exist in the “amino oxime” form or the “imino hydroxylamine” structure. From the view point of structure chemistry, identifying the exact form of a molecule is of great

Interaction energies

In order to further investigate the effect of substitution on the reactivity of aliphatic oxime derivatives, the interactions of Cu(II) metal ion with aliphatic oxime derivatives in solution were studied using density functional theory (DFT). Employing B3LYP functional, a split-valence basis set with polarization and diffuse functions viz. 6-311 + G (d, p), was applied for all elements except Cu for which LanL2DZ was used. The interaction energy of Cu2+ with OTAO was calculated according to Eq.

Micro-flotation experimental results

The flotation recovery of malachite as a function of pH with the collector concentration 1 × 10−4 mol L−1 is illustrated in Fig. 7(a). The preferable pH values of these flotation collectors for recovery of malachite were 7.5–11 (OTHA), 7.5–9.5 (OTAO), 5.5–7.5 (HOIM) and around 7.5 (MHKO), respectively. The effect of collector concentration on malachite recovery under the preferable pH value (8.7 for OTHA, 8.7 for OTAO, 7.5 for HOIM and 7.5 for MHKO) is presented in Fig. 7(b). The results

Conclusion

In this work, the molecular structure and geometrical parameters of aliphatic oxime derivatives were optimized using DFT at the B3LYP/6-311 + G (d, p) level in both vacuum and aqueous phases. The interaction energies of aliphatic oxime derivatives with copper ions have been calculated. The flotation response of aliphatic oxime derivatives to malachite mineral has been investigated. Based on the detailed analysis of theoretical and experimental results, the following conclusions can be drawn:

  • (1)

    The O

Acknowledgements

The authors would like to thank the National Natural Science Foundation of China (51474253 and 51074183), the National Basic Research Program of China (973 program) (2014CB643403) and Central South University (CSU) for the financial supports of this research. The work was kindly supported by the High Performance Computing Center of CSU. Special thanks must be extended to Dr. Nemykin, V.N. (University of Minnesota Duluth) for providing the VMOdes7.1 program package.

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