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A novel method for synthesis of styryl phosphonate monoester and its application in La(III) extraction
Journal Pre-proof A novel method for the synthesis of styryl phosphonate monoester and its application in La (III) extraction Kaihua Huang, Yun Jia, Shuai Wang, Jia Yang, Hong Zhong PII:
S1002-0721(19)30247-9
DOI:
https://doi.org/10.1016/j.jre.2019.09.003
Reference:
JRE 592
To appear in:
Journal of Rare Earths
Received Date: 27 March 2019 Revised Date:
12 August 2019
Accepted Date: 10 September 2019
Please cite this article as: Huang K, Jia Y, Wang S, Yang J, Zhong H, A novel method for the synthesis of styryl phosphonate monoester and its application in La (III) extraction, Journal of Rare Earths, https:// doi.org/10.1016/j.jre.2019.09.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
A novel method for the synthesis of styryl phosphonate monoester and its application in La (III) extraction Kaihua Huang a, Yun Jia a, Shuai Wang a, Jia Yang a, *, Hong Zhong a, * a College of Chemistry and Chemical Engineering, and Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha 410083, Hunan, China Abstract: Herein, styryl phosphonate monoester (SPE) was synthesized and first introduced as rare earth extractant. The solvent extraction of lanthanum (III) from nitrate solution using styryl phosphonate mono-iso-octyl ester (SPE108), di-2-ethylhexyl phosphoric acid (D2EHPA) and 2-ethylhexyl phosphonic acid-mono-2-ethylhexyl ester (EHEHPA) as extractants was investigated. The effects of experimental parameters including equilibrium time, extractant concentration, aqueous pH, phase ratio and salt concentration on the extraction process have been studied. The results indicate that the extraction ability and capacity of the extractants follow the order: SPE108 > D2EHPA > EHEHPA. What’s more, the extraction process is less affected by ammonium sulfate in the aqueous phase with SPE108. The results of the separation between lanthanum and adjacent lanthanides (Ce, Pr, Nd, Sm) show that SPE108 can separate lanthanides efficiently at low pH. The extraction mechanism of SPE108 is proved to be similar to D2EHPA, and the density functional theory (DFT) calculation results infer that SPE108 exhibits superior extraction ability due to its strong electron-accepting ability. Key words: styryl phosphonate monoester; rare earth; extraction; extractant 1. Introduction Rare earth elements (REEs) are of great importance in the high-tech field owing to its excellent optical, electrical and magnetic properties. Global demand for rare earth metals has been increasing every year 1. However, there are many problems need to be solved in rare earth metallurgy, including resource waste, environmental pollution and low value-added product etc. Solvent extraction has been proved to be an efficient technique to separate and extract rare earth metals 2-4. However, some shortcomings including the use of high amount of solvent and extractant, low extraction efficiency and generation of pollutants make the process expensive and environmentally unfriendly 5. Researchers have been devoted to address these problems by a variety of means by developing more efficient process 6,7 and extractants. Extractant is critical in the extraction process which contains both hydrophobic group and complexing group. The complexing group can form complexes with metal elements and the hydrophobic group increases the oil solubility of the compound to ensure the separation of the extracted complexes from the aqueous phase. The extraction capacity of Foundation item: Project supported by National High Technology Research and Development Program of China (863 Program, No.2013AA064102), Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007). * Corresponding authors: E-mail addresses: [email protected] (J. Yang); [email protected](H. Zhong) Tel./fax: +86 73188836263. ☆
extractant is a critical factor in its industrial use since it will affect the required amount of extractant and extraction times. Organic chemists have synthesized various types of extractants of different properties to enrich the field of solvent extraction 8,9. However, some problems still exist in the industrial practice such as low recovery, high extractant dosage and consequent pollution, which are important economic factors for their commercial use 10. Among different kinds of REE extractants, acidic organophosphorous extractants play an important role in industrial production. D2EHPA and EHEHPA are two representatives of this class of extractant 11 . Extensive researches have been done to investigate their extraction properties on REEs 12,13. Peppard et al. explored the possibility of using D2EHPA as extractant for REEs separation and described the mechanism of extraction of REEs 14,15. They found that D2EHPA form dimers in the organic phase and chelate with rare earth ions through cation exchange. The mechanism was further confirmed by other researchers 16. Geist et al. investigated the kinetics of rare earth extraction with D2EHPA and found that the extraction rate was limited by diffusion and the aqueous-phase composition has no effect on extraction kinetics 17. The effect of diluents on the solvent extraction by D2EHPA was also investigated by Batchu N K et al. The results showed that aliphatic diluents provided higher extraction efficiencies while the aromatic diluents could suppress the formation of emulsions or gels 18. EHEHPA attracts many researchers’ attention as it can be stripped at lower acidity, which will reduce the industrial cost and equipment corrosion 19. Sato T et al. confirmed that the extraction efficiency for trivalent REEs by D2EHPA and EHEHPA both rises with increasing atomic number of elements 20. Lee et al. calculated the equilibrium constant for the solvent extraction of REEs with EHEHPA and saponified EHEHPA and illustrated that saponified EHEHPA greatly enhanced the extraction of REEs 21,22. Other researchers utilized complexing agents as a replacement of the saponified extractants to overcome the environment pollution issues 23. Styryl phosphonate monoester is a novel acidic organophosphorous extractant for REEs. Zheng et al. first introduced styryl phosphonate monoester as extractant and studied its separation coefficients for different metal ions including Fe, Co, Ni, Cu, Zn etc. Styryl phosphonate monoester with ester alkyl containing 11-13 carbon atoms was proved to be a good extractant for the separation of Cu, Co and Ni ions 24. Qiu et al. investigated the extraction of cobalt with D2EHPA, EHEHPA and styryl phosphonate monoesters. The results showed that styryl phosphonate monoesters had smaller extraction enthalpy and exhibited stronger extraction ability than D2EHPA and EHEHPA 25. However, to the best of our knowledge, the application of styryl phosphonate monoester in REEs extraction has not been reported in the previous literatures, which means it has a great potential in this area due to its superior extraction performance and low-cost advantage. Herein, we report a new method for synthesizing styryl phosphonate compound and its extraction property for REEs was investigated. Lanthanum was selected as a representative rare earth element as it’s one of the most abundant REEs in the earth’s crust. The extraction property of styryl phosphonate mono-iso-octyl ester (SPE108) for lanthanum was studied and compared with D2EHPA and EHEHPA. The effects of equilibrium time, extractant concentration, aqueous pH and phase volume ratio on the extraction process were investigated. Stripping of lanthanum (III) with hydrochloride acid was also studied. In order to evaluate the practicability of the extractants in industrial production, the effect of aqueous ammonium sulfate concentration on the extraction process was further studied. Finally, the extraction mechanism was discussed and analyzed by DFT calculations.
2. Materials and Methods 2.1 Material Styryl phosphonate monoester was synthesized and characterized by NMR (AVANCE III HD 400 MHz, Switzerland) and FTIR spectra (IR-960, Tianjin Ruian Technology Co. Ltd, China). Styryl phosphonate monoester with ester alkyl of 1-4 carbon chain length has poor solubility in kerosene, so SPE108 was used in the extraction study for this research. D2EHPA and EHEHPA were purchased from commercial companies with a purity of higher than 95% and used as obtained without further purification, and the phosphorus-containing impurities in the reagents were determined by quantitative 31 P NMR spectroscopy to be less than 2% (Fig. S1-S2). Other chemicals were purchased from commercial suppliers with analytical-grade purity. All experiments were carried out with distilled water. 2.2 Solvent extraction Solutions of rare earth metals used in the experiments were prepared by dissolving metal nitrates in distilled water. The organic phase was prepared by diluting the extractants in kerosene. The extraction of rare earth metals was performed by shaking mixtures of the aqueous phase and the organic phase in a conical flask at 25 °C using a thermostatic oscillator (SHA-C, Changzhou Aohua Instrument Co. Ltd, China). The phase ratio (O/A) was 1(10 mL: 10 mL) if not specified. Complete equilibrium was achieved in only 5 min. The metal concentrations in the aqueous phase before and after extraction were measured by spectrophotometric method with a UV/VIS spectrophotometer (UV-1750, Shimadzu Corporation). In case of solutions of mixed lanthanides in the separation study, the metal concentrations were measured by an ICP-AES (Optima5300DV, Perkin EImer, US). The pH of the aqueous solution was measured by using a digital pH meter (PHS-3C, INESA Scientific Instrument Co. Ltd, China) and adjusted to desired value with ammonium hydroxide or hydrochloric acid solution. The percentage of extraction (E) was calculated by Eq. (1). (1) = ( − )⁄ Where and stand for the metal concentrations in the aqueous phase before and after extraction, respectively. From E values, the distribution coefficient (D) was calculated by Eq. (2).
= ⁄(1 − ) (2) The stripping experiments were carried out by shaking equal volume of loaded organic phase and hydrochloride acid in the separatory funnel. The stripping rate (S) was calculated by Eq. (3). = ⁄( − ) (3) Where represents the metal concentration in the hydrochloride acid solution after stripping. 2.3 DFT calculations The calculations were performed using the Gaussian 09 program package with GaussView 5.0. Firstly, the molecule structure was optimized successively by MM2 force field and PM3 methods on chem3D software. Then further optimization and calculation were carried out using the basis set and the method of B3LYP/6-311+G(d) on GaussView 5.0 26. The integral equation formalism for the polarizable continuum model (IEF-PCM) was used to optimize the structure of the molecule in aqueous solutions, and the dielectric constant for water was set as 78.56. The energy of HOMO and LUMO orbitals were calculated to describe the molecule reactivity.
3. Results and discussion 3.1 Synthesis and characterization of styryl phosphonate monoester 3.1.1 Synthesis of styryl phosphonate mono ester Styryl phosphonate monoesters were synthesized with phosphorus pentachloride, styrene and alcohol as raw materials. The synthesis route is shown in Fig.1. First, 0.1 mol phosphorus pentachloride was added to 80 mL chloroform in a three-necked glass flask and dissolved by magnetic stirring at 50 °C in a thermostat water bath. 0.1 mol styrene was then added into the flask dropwise. After 4 hours, 0.12 mol alcohol was added to the mixture and reacted for 2 h. Then 50 mL distilled water was added into the flask and reacted for 1 h until the reaction was finished. Styryl phosphonate monoester was extracted from the crude with the help of NaOH as the sodium salt of the ideal product can be dissolved in water and separated from the organic solution. The yield of the reaction was 40%~50%. 3.1.2 Characterization of products Styryl phosphonate mono-methyl ester (SPE101): 1H NMR (400 MHz, DMSO d6) δ 7.66–7.65 (m, 2H), 7.42–7.37 (m, 3H), 7.29 (dd, J1 = 17.6 Hz, J2 = 22.0 Hz, 1H), 6.51 (t, J = 17.2 Hz, 1H), 3.56 (d, J = 11.2 Hz, 3H). 13C NMR (100.6 MHz, DMSO d6) δ 146.15 (d, J = 6.1 Hz), 135.48 (d, J = 22.7 Hz), 130.32, 129.32, 128.13, 117.56 (d, J = 183.8 Hz), 51.86 (d, J = 5.2 Hz). 31P NMR (162 MHz, DMSO d6) δ 17.27. FTIR analysis 27: 3442 cm-1 (intermolecular hydrogen bonding), 2344, 984 cm-1 (P-O-H stretching vibration), 1613 cm-1 (C=C stretching vibration), 1210 cm-1 (P=O stretching vibration), 1042 cm-1 (P-O-C stretching vibration), 742 cm-1 (P-C stretching vibration). The pure product of SPE101 is white solid and the characterization results are shown in Fig. S3~S6. Styryl phosphonate mono-ethyl ester (SPE102): 1H NMR (400 MHz, DMSO d6) δ 7.66–7.64 (m, 2H), 7.41–7.37 (m, 3H), 7.28 (dd, J1 = 17.6 Hz, J2 = 22.0 Hz, 1H), 6.52 (t, J = 17.2 Hz, 1H), 3.93 (m, 2H), 1.23 (t, J = 7.2 Hz, 3H). 13C NMR (100.6 MHz, DMSO d6) δ 145.74 (d, J = 6.2Hz), 135.52 (d, J = 22.6 Hz), 130.27, 129.31, 128.11, 118.29 (d, J = 183.5 Hz), 60.82 (d, J = 5.1 Hz), 16.83 (d, J = 6.2 Hz). 31 P NMR (162 MHz, DMSO d6) δ 15.90. FTIR analysis: 3445 cm-1 (intermolecular hydrogen bonding), 2260, 984 cm-1 (P-O-H stretching vibration), 1617 cm-1 (C=C stretching vibration), 1237 cm-1 (P=O stretching vibration), 1049 cm-1 (P-O-C stretching vibration), 741 cm-1 (P-C stretching vibration). The pure product of SPE102 is white solid and the characterization results are shown in Fig. S7~S10. Styryl phosphonate mono-iso-octyl ester (SPE108): 1H NMR (400 MHz, CDCl3) δ 11.93 (s, 1H), 7.42–7.32 (m, 3H), 7.25–7.24 (m, 3H), 6.23 (t, J = 17.6 Hz, 1H), 3.90–3.82 (m, 2H), 1.49–1.47 (m, 1H), 1.36–1.17 (m, 8H), 0.80–0.77 (m, 6H). 13C NMR (100.6 MHz, CDCl3) δ 146.55 (d, J = 6.0 Hz), 133.96 (d, J = 23.4 Hz), 129.02, 127.74, 126.64, 113.59 (d, J = 197.0 Hz), 66.72 (d, J = 6.0 Hz), 39.07 (d, J = 6.8 Hz), 28.86, 27.85, 22.22, 21.93, 12.99, 9.85. 31P NMR (162 MHz, CDCl3) δ 20.96. FTIR analysis: 3418 cm-1 (intermolecular hydrogen bonding), 2293, 984 cm-1 (P-O-H stretching vibration), 1620 cm-1 (C=C stretching vibration), 1198 cm-1 (P=O stretching vibration), 1030 cm-1 (P-O-C stretching vibration), 741 cm-1 (P-C stretching vibration). The pure product of SPE108 is yellowish transparent liquid and the characterization results are shown in Fig. S11~S13. 3.2 Extraction tests 3.2.1 Effect of equilibrium time The effect of equilibrium time on the extraction process was investigated. The extraction of La(III) with 50 g/L SPE108, D2EHPA and EHEHPA were carried out for different shaking time (2~30 min) at
an initial pH of 5.38. The results shown in Fig. 2 indicate that the extraction of 1 g/L La (III) with both three extractants can reach extraction equilibrium in less than 5 min. The results indicate that all the three extractants have very fast extraction kinetics in the extraction of La (III). All other extraction experiments in this study were carried out for 5 min to reach extraction equilibrium. 3.2.2 Effect of aqueous pH The effect of aqueous pH on the extraction rate of 1 g/L La(III) with 50 g/L SPE108, D2EHPA and EHEHPA was studied. As shown in Fig.3, the extraction rate of La(III) rises with the increase of aqueous pH for both three extractants. SPE108 exhibits stronger extraction ability than D2EHPA and EHEHPA under same experimental conditions. The results indicate that SPE108 could extract La (III) at lower aqueous pH. It has been proved that acidic organophosphorous extractants belong to cation exchanger extractants and form dimers in diluents 28,29. The extraction equilibrium of trivalent REEs by such extractants was generally expressed as Eq. (4): (4) Where M3+ denotes the trivalent REE, A denotes the organic anion, the overbar denotes species in the organic phase. The extraction of La(III) with the three extractants both accompanies with a decrease of aqueous pH, which proves the release of hydrogen ions during the extraction process. When the aqueous pH gets higher, the equilibrium moves to the right and the extraction rate increases. The pH50 value of the extractants has been determined to be 0.53, 1.27 and 5.39 for SPE108, D2EHPA and EHEHPA, respectively. Smaller pH50 value infers that SPE108 could reach extraction rate of 50% (or other desired extraction rate) at a lower aqueous pH (i.e. higher hydrogen ion concentration), which is due to the fact that SPE108 form more stable complex with lanthanum ion than D2EHPA and EHEHPA. The strong electron inductive effect of styryl group 30 may be the explanation for why SPE108 has stronger ability to release hydrogen ions and form stable complex with lanthanum ions. 3.2.3 Effect of extractant concentration Fig. 4 shows the effect of extractant concentration on the extraction rate of La(III) at an initial pH of 5.38. For both three acidic organophosphorous extractants, the extraction rates increase with increasing extractant concentration and reach peak value at certain points. SPE108 and D2EHPA exhibit strong extraction ability for La(III) under neutral condition while EHEHPA is a relatively weaker extractant by comparison. It takes 50 g/L SPE108 or 100 g/L D2EHPA to reach extraction rate of higher than 99%, while only 96.1% lanthanum is extracted with 300 g/L EHEHPA. The results further prove that SPE108 possesses higher extraction equilibrium constant than D2EHPA and EHEHPA. SPE108 is the strongest extractant among the three extractants, which means it requires smaller extractant dosage when it’s used for the extraction of lanthanum. 3.2.4 Effect of phase volume ration As shown in Fig. 5, the effect of phase ratio (O/A) on the extraction rates and loaded La (III) in the organic phase was investigated. The experiments were conducted by mixing 10 mL 25g/L extractants with 1g/L La (III) solution of different volumes (10-50 mL) at an initial pH of 5.38. The results show that the extraction rate decreases as the phase ratio (O/A) decreases. The loaded La (III) in the organic phase is calculated and shown in Fig. 5(b). It’s observed that the extracted La (III) in the organic phase increases with the increasing aqueous volume and almost reached stable when the aqueous volume
reached 50 mL for the three extractants 31. The loading capacity of SPE108, D2EHPA and EHEHPA at a phase ratio of 1/5 was about 1.80, 1.29 and 0.75 g/L, respectively, indicating SPE108 possesses higher extraction capacity compared with D2EHPA and EHEHPA under same experimental conditions. 3.2.5 Stripping studies and recycling capacity Fig. 6(a) shows the effect of HCl concentration on the stripping rate. The loaded organic phase with high extraction rates of 1 g/L La(III) (99.3% with 50 g/L SPE108, 95% with 50 g/L D2EHPA, 96% with 300 g/L EHEHPA) was chosen for the stripping study. The stripping rate reaches 95.5%, 97.5% and 98.1% with 1 mol/L HCl for SPE108, D2EHPA and EHEHPA, respectively. The effect from the concentration of La (III) in the loaded organic phase was also studied with 1 mol/L HCl as stripping acid as shown in Fig. 6(b). Higher stripping rate is found at higher HCl concentration and lower rare earth concentration, which can be well explained by the extraction equilibrium since the stripping reaction goes in the opposite direction of the extraction equation. The results also show that it was relatively harder to strip lanthanum from loaded SPE108 than D2EHPA and EHEHPA, which is not difficult to understand because the stripping reaction and extraction goes in the opposite direction and SPE108 has higher extraction rate in the extraction reaction. The recycling capacity of the extractants was evaluated by carrying out ten successive extractions and stripping cycles for La (III) 32. The organic phase was washed with distilled water until the washings were neutral after stripping in each cycle. The extraction rate in each cycle was calculated and shown in Fig. 7. An insignificant change in the extraction rate is observed from the experimental results. Good recycling capacity may be a result of low water solubility of the extractants. By calculation with ChemBioOffice software, the LogP value of SPE108, D2EHPA and EHEHPA is 4.9, 6.12 and 5.61, respectively. The calculation results indicate that all the three extractants have high oil-water partition coefficient, which is a proof for their good oil solubility and poor water solubility. 3.2.6 Effect of salts Ion-adsorption type rare earth ore is a kind of rare earth ore abundant in southern China. Different from normal rare earth ores, ion-adsorption type rare earth exist in form of hydrated or hydroxyl hydrated cation which is adsorbed on the surface of clay minerals. Instead of acid leaching, it’s usually processed by in-situ leaching with 10~40 g/L ammonium sulfate currently 33. The effect of ammonium sulfate (0~50 g/L) on the extraction of 1 g/L La(III) by 50 g/L SPE108, 50g/L D2EHPA and 300 g/L EHEHPA was investigated. As we can see in Fig. 8, when SPE108 is used as extractant, the extraction rate is hardly affected by ammonium sulfate. For D2EHPA and EHEHPA, the extraction rate decreases when 10 g/L (NH4)2SO4 was added, but when the (NH4)2SO4 concentration increases the extraction rate slightly increases instead. The explanation of this effect may be, when (NH4)2SO4 id added, the extraction rate decreases due to the competitive complexation of La(III) by the inorganic anion in the aqueous phase. However, with increasing concentration of (NH4)2SO4, the aqueous phase is saturated to produce the salting-out effect which will counterbalance the effect of competitive complexation and increases the extraction percentage 34. These effects are negligible when SPE108 is used as extractant because the extraction constant for SPE108 is much higher. The fact that SPE108 is not very sensitive to the concentration of (NH4)2SO4 is a benefit for its industrial use. 3.2.7 Separation studies It’s quite difficult to separate lanthanides from each other since they have very similar chemical
properties. As indicated in the introduction, the extraction rate of trivalent REEs by D2EHPA and EHEHPA both rises with increasing atomic number of elements 20. The extraction rate of 1g/L La, Ce, Pr, Nd, Sm by 50g/L SPE108 from aqueous phase of different HCl concentration was investigated in this study and the results show the same trends with the order: La < Ce < Pr < Nd < Sm as described in Figure 9. The separation study was performed by mixing 1g/L La (III) and 1g/L Ce, Pr, Nd and Sm in the aqueous phase. The separation experiments were carried out with 50g/L SPE108, D2EHPA and EHEHPA under the conditions where the aqueous pH was 0.4, 1 and 2, respectively. The distribution factors and separation factors were calculated and shown in Table 1. The results show that the separation factor of SPE108 is close to that of D2EHPA under the experimental conditions, while EHEHPA exhibits relatively higher separation factor in the separation study. For both three extractants, the highest separation factor is observed in the separation of La-Sm pair and the lowest with the La-Ce pair. It can be inferred that SPE108 can separate lanthanides efficiently at low pH. 3.3 Extraction mechanism The equilibrium constant for the extraction of La(III) can be represented as Eq. (5):
=
() ()
= × ()
(5)
By taking the logarithm of both sides of the Eq. (4), the distribution coefficient D can be written in a linear form as Eq. (6): (6) lg = lg − 3 lgH ! + nlg(HA)% + Fig. 10 shows the plot of lgD versus lg[H ] corresponding to the experimental results in section 3.2.2. The linear fitting results are in good accordance with Eq. (5) for both three extractants. The lg-lg plots of D versus extractant concentration are shown in Fig. 11. The data for D2EHPA and EHEHPA corresponds to the experimental results in section 3.2.3. Since the extraction rate of La (III) by SPE108 is very high under the experimental conditions in section 3.2.3 which results in a big error in calculation, the data for SPE108 were obtained from experiments conducted at an initial aqueous pH of 0.7. The results indicate the equilibrium equations for the studied extractions as follows: La(III) extraction with SPE108 and D2EHPA in kerosine as Eq. (7): (7) La(III) extraction with EHEHPA in kerosine as Eq. (8): (8) The extraction mechanism of SPE108 is similar to D2EHPA, while EHEHPA exhibits otherwise. Acharya S etc. studied the extraction and separation of lanthanum with D2EHPA and EHEHPA with petrofin as diluents. Their experimental results indicated the same equilibrium expression as Eq. (7) and (8) 35. Eq. (7) indicates the extractant saturation for SPE108 and D2EHPA at a 1:6 mole ratio of M3+ to HA, while Eq. (8) indicates a 1:3 mole ratio for the extractant saturation of EHEHPA. Peppard et al. proposed a six-membered ring structure of D2EHPA dimer and its corresponding complex with metal ions (Peppard et al., 1958). The reaction between the extractant dimer and lanthanum ion involves the release of one hydrogen ion from the dimer and formation of O-M-O bond.
It’s likely that SPE108 chelate with lanthanum ion in a same way since it has similar structure and extraction mechanism to D2EHPA. The possible bonding model of SPE108 with La complex is shown in Fig. 12. The difference of the three extractants in their extraction ability may be explained by the electron inductive effect of their substituent groups. The molecular structure of the extractants can be written as R-PO(C8H17)(OH), where R represents different substituent groups (styryl for SPE108, iso-octoxyl for D2EHPA, iso-octyl for EHEHPA). It’s well known the inductive effect of alkoxyl group is stronger than its corresponding alkyl group, while the styryl group is even stronger due to its high degree of unsaturation. Thus, the electron-accepting ability of substituent groups follows the order: styryl > iso-octoxyl > iso-octyl, strong electron inductive makes it easier for the organophosphorus acid to release hydrogen ions and form complex with with lanthanum ions. The frontier orbital theory can be used to describe the reactivity of molecules, where the HOMO (higherst occupied molecule orbital) reflects the electron-donating ability and the LUMO (lowest unoccupied molecule orbital) reflects the electron- accepting ability. The density functional theory (DFT) based quantum chemical method was a credible method for understanding the electronic structure of molecules 36. Therefore it was carried out to calculate the HOMO and LUMO values for the three compounds. The calculation results are shown in Table 2. The extractant molecule accepts electrons in the complexation reaction with metal ions, thus the LUMO is a key factor of its complexation ability. Lower LUMO value corresponds to stronger electron-accepting ability. The LUMO values for SPE108, D2EHPA and EHEHPA are -0.07107, -0.01059 and -0.00826, respectively, indicating the electron-accepting ability follows the order: SPE108 > D2EHPA > EHEHPA. The calculation results are in accordance with the experimental results and verify the explanation of electron inductive effect. 4
Conclusions Styryl phosphonate ester is synthesized with a new method and characterized. The extraction performance and mechanism of SPE108 in the extraction of La(III) ions were studied by extraction tests and compared with D2EHPA and EHEHPA. SPE108 exhibits stronger extraction ability and higher extraction capacity than D2EHPA and EHEHPA. Meanwhile, the extraction of lanthanum with SPE108 is less affected by (NH4)2SO4 in the aqueous phase than D2EHPA and EHEHPA, which is an advantage for its industrial use. Stripping and recycling studies show that SPE108 has good recycling capacity. The separation study show that SPE108 can separate lanthanides efficiently at low pH. The extraction mechanism of SPE108 is proved similar to D2EHPA with the formation of LaA3(HA)3 complex, and the bonding model of the complex is proposed. In consideration of the cheap raw material and simple method for its synthesis, it can be concluded that SPE108 is a promising extractant for REE extraction, especially for the condition of leaching solution with high acidity or high salt concentration. Acknowledgements The authors express their appreciation for the support of the High Performance Computing Center of Central South University, China.
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TOC:
Styryl phosphonate mono-iso-octyl ester (SPE108) exhibits stronger extraction ability and higher extraction capacity than D2EHPA and EHEHPA in the extraction of lanthanum (III). The extraction mechanism of SPE108 is similar to that of D2EHPA with the formation of LaA3(HA)3 complex.
Fig.1. Synthesis route of styryl phosphonate monoester.
100
E /%
80 SPE108 D2EHPA EHEHPA
60
0
5
10
15
20
25
30
Equilibrium time / min
Fig. 2. Effect of equilibrium time on the extraction of La(III) with SPE108, D2EHPA and EHEHPA.
100 80
SPE108 D2EHPA EHEHPA
E /%
60 40 20 0 0
1
2
3
4
5
6
pHaq
Fig. 3. Effect of pH on the extraction of La(III) with SPE108, D2EHPA and EHEHPA.
100
E/%
80
60
40 SPE108 D2EHPA EHEHPA
20 0
5
10
15
20
25
30
Extractant concentration (g/L)
Fig. 4. Effect of extractant concentration on the extraction of La(III) with SPE108, D2EHPA and EHEHPA.
100
2.0
(a)
(b)
SPE108 D2EHPA EHEHPA
1.6
[La3+]organic / (g/L)
80
E/%
60
40
SPE108 D2EHPA EHEHPA
1.2
0.8
20 0.4
0 1/1
1/2
1/3 1/4 Phase ratio (O/A)
1/5
1/1
1/2
1/4 1/3 Phase ratio (O/A)
1/5
Fig. 5. Effects of phase ratio on the (a) extraction rates and (b) loaded La (III) in the organic phase.
100
100
(a)
(b) 98 Stripping rate / %
Stripping rate / %
80
60
40
20
SPE108 D2EHPA EHEHPA 0.0
0.5 1.0 1.5 2.0 2.5 Hydrochloride concentration / (mol/L)
3.0
96
94 SPE108 D2EHPA EHEHPA
92
90
0.2
0.4
0.6 0.8 3+ [La ] organic / (g/L)
1.0
Fig. 6. Effect of HCl concentration on the stripping of La(III) from loaded SPE108, D2EHPA and EHEHPA in kerosene.
100
E/%
95
SPE108 D2EHPA EHEHPA
90
0
1
2
3
4
5 6 Cycles
7
8
9
10
11
Fig. 7. Recycling capacity of SPE108, D2EHPA and EHEHPA.
100 98 96
E/%
94 92 90 SPE108 D2EHPA EHEHPA
88 86 0
10
20
30
40
50
(NH4)2SO4 concentration / (g/L)
Fig. 8. Effect of ammonium sulfate on the extraction of La(III).
100
La(III) Ce(III) Pr(III) Nd(III) Sm(III)
80
E/%
60 40 20 0 1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
HCl concentration / (mol/L)
Fig.9. Extraction rate of La, Ce, Pr, Nd, Sm with SPE108 at different acidity.
2
7 .4
lgD
-1
-2.0
-1.5
3
06 .5 -5 8x 99 2.
-2
-3
4
y=
-1
1x
.45
03
-3
0
- 3.
6x
y=
99
- 2.
y=
1
SPE108 D2EHPA EHEHPA
-1.0
-0.5
0.0
+
lg[H ]eq
Fig. 10. Variation of lgD with lg[H+]eq for the extraction of La(III).
2
SPE108 D2EHPA EHEHPA
lgD
2.9 y= 0
y= -2
-1.6
7 2.9
4x
90
.5 +3
4.7 x+
71
-1.2
39
.652 7x+1 1 5 . y=1
-0.8
lg[(HA)2]
Fig. 11. lgD versus lg[(HA)2] for the extraction of La(III).
Fig. 12. Proposed bonding model of SPE108 dimer with La (III).
Table 1 Separation of lanthanides with 50g/L SPE108, D2EHPA and EHEHPA at an initial pH of 0.4, 1 and 2, respectively Extractant La(g/L) DLa Ce(g/L) DCe β=（DCe/DLa） SPE108 1 0.38 1 1.67 4.37 D2EHPA 1 0.18 1 0.65 3.54 EHEHPA 1 0.26 1 1.32 5.04 Pr(g/L) DPr β=(DPr/DLa） SPE108 1 0.32 1 2.39 7.54 D2EHPA 1 0.3 1 1.61 5.32 EHEHPA 1 0.23 1 1.92 8.26 Nd(g/L) DNd β=（DNd/DLa） SPE108 1 0.36 1 2.86 7.95 D2EHPA 1 0.14 1 1.27 8.98 EHEHPA 1 0.19 1 2.48 12.86 Sm(g/L) DSm β=（DSm/DLa） SPE108 1 0.30 1 17.09 57.90 D2EHPA 1 0.13 1 7.78 61.89 EHEHPA 1 0.15 1 12.91 85.20
Table 2 HOMO and LUMO values for SPE108, D2EHPA and EHEHPA Molecule HOMO SPE108 -0.24934 D2EHPA -0.30409 EHEHPA -0.29449
LUMO -0.07107 -0.01059 -0.00826
A novel method for the synthesis of styryl phosphonate monoester was proposed.
Styryl phosphonate mono-iso-octyl ester (SPE108) exhibited stronger extraction ability and higher extraction capacity than D2EHPA and EHEHPA in the extraction of lanthanum (III).
The extraction mechanism of SPE108 is similar to that of D2EHPA.
DFT calculations indicated that the electron inductive effect might be an explanation for the superior extraction performance of SPE.
S1002-0721(19)30247-9
DOI:
https://doi.org/10.1016/j.jre.2019.09.003
Reference:
JRE 592
To appear in:
Journal of Rare Earths
Received Date: 27 March 2019 Revised Date:
12 August 2019
Accepted Date: 10 September 2019
Please cite this article as: Huang K, Jia Y, Wang S, Yang J, Zhong H, A novel method for the synthesis of styryl phosphonate monoester and its application in La (III) extraction, Journal of Rare Earths, https:// doi.org/10.1016/j.jre.2019.09.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
A novel method for the synthesis of styryl phosphonate monoester and its application in La (III) extraction Kaihua Huang a, Yun Jia a, Shuai Wang a, Jia Yang a, *, Hong Zhong a, * a College of Chemistry and Chemical Engineering, and Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha 410083, Hunan, China Abstract: Herein, styryl phosphonate monoester (SPE) was synthesized and first introduced as rare earth extractant. The solvent extraction of lanthanum (III) from nitrate solution using styryl phosphonate mono-iso-octyl ester (SPE108), di-2-ethylhexyl phosphoric acid (D2EHPA) and 2-ethylhexyl phosphonic acid-mono-2-ethylhexyl ester (EHEHPA) as extractants was investigated. The effects of experimental parameters including equilibrium time, extractant concentration, aqueous pH, phase ratio and salt concentration on the extraction process have been studied. The results indicate that the extraction ability and capacity of the extractants follow the order: SPE108 > D2EHPA > EHEHPA. What’s more, the extraction process is less affected by ammonium sulfate in the aqueous phase with SPE108. The results of the separation between lanthanum and adjacent lanthanides (Ce, Pr, Nd, Sm) show that SPE108 can separate lanthanides efficiently at low pH. The extraction mechanism of SPE108 is proved to be similar to D2EHPA, and the density functional theory (DFT) calculation results infer that SPE108 exhibits superior extraction ability due to its strong electron-accepting ability. Key words: styryl phosphonate monoester; rare earth; extraction; extractant 1. Introduction Rare earth elements (REEs) are of great importance in the high-tech field owing to its excellent optical, electrical and magnetic properties. Global demand for rare earth metals has been increasing every year 1. However, there are many problems need to be solved in rare earth metallurgy, including resource waste, environmental pollution and low value-added product etc. Solvent extraction has been proved to be an efficient technique to separate and extract rare earth metals 2-4. However, some shortcomings including the use of high amount of solvent and extractant, low extraction efficiency and generation of pollutants make the process expensive and environmentally unfriendly 5. Researchers have been devoted to address these problems by a variety of means by developing more efficient process 6,7 and extractants. Extractant is critical in the extraction process which contains both hydrophobic group and complexing group. The complexing group can form complexes with metal elements and the hydrophobic group increases the oil solubility of the compound to ensure the separation of the extracted complexes from the aqueous phase. The extraction capacity of Foundation item: Project supported by National High Technology Research and Development Program of China (863 Program, No.2013AA064102), Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007). * Corresponding authors: E-mail addresses: [email protected] (J. Yang); [email protected](H. Zhong) Tel./fax: +86 73188836263. ☆
extractant is a critical factor in its industrial use since it will affect the required amount of extractant and extraction times. Organic chemists have synthesized various types of extractants of different properties to enrich the field of solvent extraction 8,9. However, some problems still exist in the industrial practice such as low recovery, high extractant dosage and consequent pollution, which are important economic factors for their commercial use 10. Among different kinds of REE extractants, acidic organophosphorous extractants play an important role in industrial production. D2EHPA and EHEHPA are two representatives of this class of extractant 11 . Extensive researches have been done to investigate their extraction properties on REEs 12,13. Peppard et al. explored the possibility of using D2EHPA as extractant for REEs separation and described the mechanism of extraction of REEs 14,15. They found that D2EHPA form dimers in the organic phase and chelate with rare earth ions through cation exchange. The mechanism was further confirmed by other researchers 16. Geist et al. investigated the kinetics of rare earth extraction with D2EHPA and found that the extraction rate was limited by diffusion and the aqueous-phase composition has no effect on extraction kinetics 17. The effect of diluents on the solvent extraction by D2EHPA was also investigated by Batchu N K et al. The results showed that aliphatic diluents provided higher extraction efficiencies while the aromatic diluents could suppress the formation of emulsions or gels 18. EHEHPA attracts many researchers’ attention as it can be stripped at lower acidity, which will reduce the industrial cost and equipment corrosion 19. Sato T et al. confirmed that the extraction efficiency for trivalent REEs by D2EHPA and EHEHPA both rises with increasing atomic number of elements 20. Lee et al. calculated the equilibrium constant for the solvent extraction of REEs with EHEHPA and saponified EHEHPA and illustrated that saponified EHEHPA greatly enhanced the extraction of REEs 21,22. Other researchers utilized complexing agents as a replacement of the saponified extractants to overcome the environment pollution issues 23. Styryl phosphonate monoester is a novel acidic organophosphorous extractant for REEs. Zheng et al. first introduced styryl phosphonate monoester as extractant and studied its separation coefficients for different metal ions including Fe, Co, Ni, Cu, Zn etc. Styryl phosphonate monoester with ester alkyl containing 11-13 carbon atoms was proved to be a good extractant for the separation of Cu, Co and Ni ions 24. Qiu et al. investigated the extraction of cobalt with D2EHPA, EHEHPA and styryl phosphonate monoesters. The results showed that styryl phosphonate monoesters had smaller extraction enthalpy and exhibited stronger extraction ability than D2EHPA and EHEHPA 25. However, to the best of our knowledge, the application of styryl phosphonate monoester in REEs extraction has not been reported in the previous literatures, which means it has a great potential in this area due to its superior extraction performance and low-cost advantage. Herein, we report a new method for synthesizing styryl phosphonate compound and its extraction property for REEs was investigated. Lanthanum was selected as a representative rare earth element as it’s one of the most abundant REEs in the earth’s crust. The extraction property of styryl phosphonate mono-iso-octyl ester (SPE108) for lanthanum was studied and compared with D2EHPA and EHEHPA. The effects of equilibrium time, extractant concentration, aqueous pH and phase volume ratio on the extraction process were investigated. Stripping of lanthanum (III) with hydrochloride acid was also studied. In order to evaluate the practicability of the extractants in industrial production, the effect of aqueous ammonium sulfate concentration on the extraction process was further studied. Finally, the extraction mechanism was discussed and analyzed by DFT calculations.
2. Materials and Methods 2.1 Material Styryl phosphonate monoester was synthesized and characterized by NMR (AVANCE III HD 400 MHz, Switzerland) and FTIR spectra (IR-960, Tianjin Ruian Technology Co. Ltd, China). Styryl phosphonate monoester with ester alkyl of 1-4 carbon chain length has poor solubility in kerosene, so SPE108 was used in the extraction study for this research. D2EHPA and EHEHPA were purchased from commercial companies with a purity of higher than 95% and used as obtained without further purification, and the phosphorus-containing impurities in the reagents were determined by quantitative 31 P NMR spectroscopy to be less than 2% (Fig. S1-S2). Other chemicals were purchased from commercial suppliers with analytical-grade purity. All experiments were carried out with distilled water. 2.2 Solvent extraction Solutions of rare earth metals used in the experiments were prepared by dissolving metal nitrates in distilled water. The organic phase was prepared by diluting the extractants in kerosene. The extraction of rare earth metals was performed by shaking mixtures of the aqueous phase and the organic phase in a conical flask at 25 °C using a thermostatic oscillator (SHA-C, Changzhou Aohua Instrument Co. Ltd, China). The phase ratio (O/A) was 1(10 mL: 10 mL) if not specified. Complete equilibrium was achieved in only 5 min. The metal concentrations in the aqueous phase before and after extraction were measured by spectrophotometric method with a UV/VIS spectrophotometer (UV-1750, Shimadzu Corporation). In case of solutions of mixed lanthanides in the separation study, the metal concentrations were measured by an ICP-AES (Optima5300DV, Perkin EImer, US). The pH of the aqueous solution was measured by using a digital pH meter (PHS-3C, INESA Scientific Instrument Co. Ltd, China) and adjusted to desired value with ammonium hydroxide or hydrochloric acid solution. The percentage of extraction (E) was calculated by Eq. (1). (1) = ( − )⁄ Where and stand for the metal concentrations in the aqueous phase before and after extraction, respectively. From E values, the distribution coefficient (D) was calculated by Eq. (2).
= ⁄(1 − ) (2) The stripping experiments were carried out by shaking equal volume of loaded organic phase and hydrochloride acid in the separatory funnel. The stripping rate (S) was calculated by Eq. (3). = ⁄( − ) (3) Where represents the metal concentration in the hydrochloride acid solution after stripping. 2.3 DFT calculations The calculations were performed using the Gaussian 09 program package with GaussView 5.0. Firstly, the molecule structure was optimized successively by MM2 force field and PM3 methods on chem3D software. Then further optimization and calculation were carried out using the basis set and the method of B3LYP/6-311+G(d) on GaussView 5.0 26. The integral equation formalism for the polarizable continuum model (IEF-PCM) was used to optimize the structure of the molecule in aqueous solutions, and the dielectric constant for water was set as 78.56. The energy of HOMO and LUMO orbitals were calculated to describe the molecule reactivity.
3. Results and discussion 3.1 Synthesis and characterization of styryl phosphonate monoester 3.1.1 Synthesis of styryl phosphonate mono ester Styryl phosphonate monoesters were synthesized with phosphorus pentachloride, styrene and alcohol as raw materials. The synthesis route is shown in Fig.1. First, 0.1 mol phosphorus pentachloride was added to 80 mL chloroform in a three-necked glass flask and dissolved by magnetic stirring at 50 °C in a thermostat water bath. 0.1 mol styrene was then added into the flask dropwise. After 4 hours, 0.12 mol alcohol was added to the mixture and reacted for 2 h. Then 50 mL distilled water was added into the flask and reacted for 1 h until the reaction was finished. Styryl phosphonate monoester was extracted from the crude with the help of NaOH as the sodium salt of the ideal product can be dissolved in water and separated from the organic solution. The yield of the reaction was 40%~50%. 3.1.2 Characterization of products Styryl phosphonate mono-methyl ester (SPE101): 1H NMR (400 MHz, DMSO d6) δ 7.66–7.65 (m, 2H), 7.42–7.37 (m, 3H), 7.29 (dd, J1 = 17.6 Hz, J2 = 22.0 Hz, 1H), 6.51 (t, J = 17.2 Hz, 1H), 3.56 (d, J = 11.2 Hz, 3H). 13C NMR (100.6 MHz, DMSO d6) δ 146.15 (d, J = 6.1 Hz), 135.48 (d, J = 22.7 Hz), 130.32, 129.32, 128.13, 117.56 (d, J = 183.8 Hz), 51.86 (d, J = 5.2 Hz). 31P NMR (162 MHz, DMSO d6) δ 17.27. FTIR analysis 27: 3442 cm-1 (intermolecular hydrogen bonding), 2344, 984 cm-1 (P-O-H stretching vibration), 1613 cm-1 (C=C stretching vibration), 1210 cm-1 (P=O stretching vibration), 1042 cm-1 (P-O-C stretching vibration), 742 cm-1 (P-C stretching vibration). The pure product of SPE101 is white solid and the characterization results are shown in Fig. S3~S6. Styryl phosphonate mono-ethyl ester (SPE102): 1H NMR (400 MHz, DMSO d6) δ 7.66–7.64 (m, 2H), 7.41–7.37 (m, 3H), 7.28 (dd, J1 = 17.6 Hz, J2 = 22.0 Hz, 1H), 6.52 (t, J = 17.2 Hz, 1H), 3.93 (m, 2H), 1.23 (t, J = 7.2 Hz, 3H). 13C NMR (100.6 MHz, DMSO d6) δ 145.74 (d, J = 6.2Hz), 135.52 (d, J = 22.6 Hz), 130.27, 129.31, 128.11, 118.29 (d, J = 183.5 Hz), 60.82 (d, J = 5.1 Hz), 16.83 (d, J = 6.2 Hz). 31 P NMR (162 MHz, DMSO d6) δ 15.90. FTIR analysis: 3445 cm-1 (intermolecular hydrogen bonding), 2260, 984 cm-1 (P-O-H stretching vibration), 1617 cm-1 (C=C stretching vibration), 1237 cm-1 (P=O stretching vibration), 1049 cm-1 (P-O-C stretching vibration), 741 cm-1 (P-C stretching vibration). The pure product of SPE102 is white solid and the characterization results are shown in Fig. S7~S10. Styryl phosphonate mono-iso-octyl ester (SPE108): 1H NMR (400 MHz, CDCl3) δ 11.93 (s, 1H), 7.42–7.32 (m, 3H), 7.25–7.24 (m, 3H), 6.23 (t, J = 17.6 Hz, 1H), 3.90–3.82 (m, 2H), 1.49–1.47 (m, 1H), 1.36–1.17 (m, 8H), 0.80–0.77 (m, 6H). 13C NMR (100.6 MHz, CDCl3) δ 146.55 (d, J = 6.0 Hz), 133.96 (d, J = 23.4 Hz), 129.02, 127.74, 126.64, 113.59 (d, J = 197.0 Hz), 66.72 (d, J = 6.0 Hz), 39.07 (d, J = 6.8 Hz), 28.86, 27.85, 22.22, 21.93, 12.99, 9.85. 31P NMR (162 MHz, CDCl3) δ 20.96. FTIR analysis: 3418 cm-1 (intermolecular hydrogen bonding), 2293, 984 cm-1 (P-O-H stretching vibration), 1620 cm-1 (C=C stretching vibration), 1198 cm-1 (P=O stretching vibration), 1030 cm-1 (P-O-C stretching vibration), 741 cm-1 (P-C stretching vibration). The pure product of SPE108 is yellowish transparent liquid and the characterization results are shown in Fig. S11~S13. 3.2 Extraction tests 3.2.1 Effect of equilibrium time The effect of equilibrium time on the extraction process was investigated. The extraction of La(III) with 50 g/L SPE108, D2EHPA and EHEHPA were carried out for different shaking time (2~30 min) at
an initial pH of 5.38. The results shown in Fig. 2 indicate that the extraction of 1 g/L La (III) with both three extractants can reach extraction equilibrium in less than 5 min. The results indicate that all the three extractants have very fast extraction kinetics in the extraction of La (III). All other extraction experiments in this study were carried out for 5 min to reach extraction equilibrium. 3.2.2 Effect of aqueous pH The effect of aqueous pH on the extraction rate of 1 g/L La(III) with 50 g/L SPE108, D2EHPA and EHEHPA was studied. As shown in Fig.3, the extraction rate of La(III) rises with the increase of aqueous pH for both three extractants. SPE108 exhibits stronger extraction ability than D2EHPA and EHEHPA under same experimental conditions. The results indicate that SPE108 could extract La (III) at lower aqueous pH. It has been proved that acidic organophosphorous extractants belong to cation exchanger extractants and form dimers in diluents 28,29. The extraction equilibrium of trivalent REEs by such extractants was generally expressed as Eq. (4): (4) Where M3+ denotes the trivalent REE, A denotes the organic anion, the overbar denotes species in the organic phase. The extraction of La(III) with the three extractants both accompanies with a decrease of aqueous pH, which proves the release of hydrogen ions during the extraction process. When the aqueous pH gets higher, the equilibrium moves to the right and the extraction rate increases. The pH50 value of the extractants has been determined to be 0.53, 1.27 and 5.39 for SPE108, D2EHPA and EHEHPA, respectively. Smaller pH50 value infers that SPE108 could reach extraction rate of 50% (or other desired extraction rate) at a lower aqueous pH (i.e. higher hydrogen ion concentration), which is due to the fact that SPE108 form more stable complex with lanthanum ion than D2EHPA and EHEHPA. The strong electron inductive effect of styryl group 30 may be the explanation for why SPE108 has stronger ability to release hydrogen ions and form stable complex with lanthanum ions. 3.2.3 Effect of extractant concentration Fig. 4 shows the effect of extractant concentration on the extraction rate of La(III) at an initial pH of 5.38. For both three acidic organophosphorous extractants, the extraction rates increase with increasing extractant concentration and reach peak value at certain points. SPE108 and D2EHPA exhibit strong extraction ability for La(III) under neutral condition while EHEHPA is a relatively weaker extractant by comparison. It takes 50 g/L SPE108 or 100 g/L D2EHPA to reach extraction rate of higher than 99%, while only 96.1% lanthanum is extracted with 300 g/L EHEHPA. The results further prove that SPE108 possesses higher extraction equilibrium constant than D2EHPA and EHEHPA. SPE108 is the strongest extractant among the three extractants, which means it requires smaller extractant dosage when it’s used for the extraction of lanthanum. 3.2.4 Effect of phase volume ration As shown in Fig. 5, the effect of phase ratio (O/A) on the extraction rates and loaded La (III) in the organic phase was investigated. The experiments were conducted by mixing 10 mL 25g/L extractants with 1g/L La (III) solution of different volumes (10-50 mL) at an initial pH of 5.38. The results show that the extraction rate decreases as the phase ratio (O/A) decreases. The loaded La (III) in the organic phase is calculated and shown in Fig. 5(b). It’s observed that the extracted La (III) in the organic phase increases with the increasing aqueous volume and almost reached stable when the aqueous volume
reached 50 mL for the three extractants 31. The loading capacity of SPE108, D2EHPA and EHEHPA at a phase ratio of 1/5 was about 1.80, 1.29 and 0.75 g/L, respectively, indicating SPE108 possesses higher extraction capacity compared with D2EHPA and EHEHPA under same experimental conditions. 3.2.5 Stripping studies and recycling capacity Fig. 6(a) shows the effect of HCl concentration on the stripping rate. The loaded organic phase with high extraction rates of 1 g/L La(III) (99.3% with 50 g/L SPE108, 95% with 50 g/L D2EHPA, 96% with 300 g/L EHEHPA) was chosen for the stripping study. The stripping rate reaches 95.5%, 97.5% and 98.1% with 1 mol/L HCl for SPE108, D2EHPA and EHEHPA, respectively. The effect from the concentration of La (III) in the loaded organic phase was also studied with 1 mol/L HCl as stripping acid as shown in Fig. 6(b). Higher stripping rate is found at higher HCl concentration and lower rare earth concentration, which can be well explained by the extraction equilibrium since the stripping reaction goes in the opposite direction of the extraction equation. The results also show that it was relatively harder to strip lanthanum from loaded SPE108 than D2EHPA and EHEHPA, which is not difficult to understand because the stripping reaction and extraction goes in the opposite direction and SPE108 has higher extraction rate in the extraction reaction. The recycling capacity of the extractants was evaluated by carrying out ten successive extractions and stripping cycles for La (III) 32. The organic phase was washed with distilled water until the washings were neutral after stripping in each cycle. The extraction rate in each cycle was calculated and shown in Fig. 7. An insignificant change in the extraction rate is observed from the experimental results. Good recycling capacity may be a result of low water solubility of the extractants. By calculation with ChemBioOffice software, the LogP value of SPE108, D2EHPA and EHEHPA is 4.9, 6.12 and 5.61, respectively. The calculation results indicate that all the three extractants have high oil-water partition coefficient, which is a proof for their good oil solubility and poor water solubility. 3.2.6 Effect of salts Ion-adsorption type rare earth ore is a kind of rare earth ore abundant in southern China. Different from normal rare earth ores, ion-adsorption type rare earth exist in form of hydrated or hydroxyl hydrated cation which is adsorbed on the surface of clay minerals. Instead of acid leaching, it’s usually processed by in-situ leaching with 10~40 g/L ammonium sulfate currently 33. The effect of ammonium sulfate (0~50 g/L) on the extraction of 1 g/L La(III) by 50 g/L SPE108, 50g/L D2EHPA and 300 g/L EHEHPA was investigated. As we can see in Fig. 8, when SPE108 is used as extractant, the extraction rate is hardly affected by ammonium sulfate. For D2EHPA and EHEHPA, the extraction rate decreases when 10 g/L (NH4)2SO4 was added, but when the (NH4)2SO4 concentration increases the extraction rate slightly increases instead. The explanation of this effect may be, when (NH4)2SO4 id added, the extraction rate decreases due to the competitive complexation of La(III) by the inorganic anion in the aqueous phase. However, with increasing concentration of (NH4)2SO4, the aqueous phase is saturated to produce the salting-out effect which will counterbalance the effect of competitive complexation and increases the extraction percentage 34. These effects are negligible when SPE108 is used as extractant because the extraction constant for SPE108 is much higher. The fact that SPE108 is not very sensitive to the concentration of (NH4)2SO4 is a benefit for its industrial use. 3.2.7 Separation studies It’s quite difficult to separate lanthanides from each other since they have very similar chemical
properties. As indicated in the introduction, the extraction rate of trivalent REEs by D2EHPA and EHEHPA both rises with increasing atomic number of elements 20. The extraction rate of 1g/L La, Ce, Pr, Nd, Sm by 50g/L SPE108 from aqueous phase of different HCl concentration was investigated in this study and the results show the same trends with the order: La < Ce < Pr < Nd < Sm as described in Figure 9. The separation study was performed by mixing 1g/L La (III) and 1g/L Ce, Pr, Nd and Sm in the aqueous phase. The separation experiments were carried out with 50g/L SPE108, D2EHPA and EHEHPA under the conditions where the aqueous pH was 0.4, 1 and 2, respectively. The distribution factors and separation factors were calculated and shown in Table 1. The results show that the separation factor of SPE108 is close to that of D2EHPA under the experimental conditions, while EHEHPA exhibits relatively higher separation factor in the separation study. For both three extractants, the highest separation factor is observed in the separation of La-Sm pair and the lowest with the La-Ce pair. It can be inferred that SPE108 can separate lanthanides efficiently at low pH. 3.3 Extraction mechanism The equilibrium constant for the extraction of La(III) can be represented as Eq. (5):
=
() ()
= × ()
(5)
By taking the logarithm of both sides of the Eq. (4), the distribution coefficient D can be written in a linear form as Eq. (6): (6) lg = lg − 3 lgH ! + nlg(HA)% + Fig. 10 shows the plot of lgD versus lg[H ] corresponding to the experimental results in section 3.2.2. The linear fitting results are in good accordance with Eq. (5) for both three extractants. The lg-lg plots of D versus extractant concentration are shown in Fig. 11. The data for D2EHPA and EHEHPA corresponds to the experimental results in section 3.2.3. Since the extraction rate of La (III) by SPE108 is very high under the experimental conditions in section 3.2.3 which results in a big error in calculation, the data for SPE108 were obtained from experiments conducted at an initial aqueous pH of 0.7. The results indicate the equilibrium equations for the studied extractions as follows: La(III) extraction with SPE108 and D2EHPA in kerosine as Eq. (7): (7) La(III) extraction with EHEHPA in kerosine as Eq. (8): (8) The extraction mechanism of SPE108 is similar to D2EHPA, while EHEHPA exhibits otherwise. Acharya S etc. studied the extraction and separation of lanthanum with D2EHPA and EHEHPA with petrofin as diluents. Their experimental results indicated the same equilibrium expression as Eq. (7) and (8) 35. Eq. (7) indicates the extractant saturation for SPE108 and D2EHPA at a 1:6 mole ratio of M3+ to HA, while Eq. (8) indicates a 1:3 mole ratio for the extractant saturation of EHEHPA. Peppard et al. proposed a six-membered ring structure of D2EHPA dimer and its corresponding complex with metal ions (Peppard et al., 1958). The reaction between the extractant dimer and lanthanum ion involves the release of one hydrogen ion from the dimer and formation of O-M-O bond.
It’s likely that SPE108 chelate with lanthanum ion in a same way since it has similar structure and extraction mechanism to D2EHPA. The possible bonding model of SPE108 with La complex is shown in Fig. 12. The difference of the three extractants in their extraction ability may be explained by the electron inductive effect of their substituent groups. The molecular structure of the extractants can be written as R-PO(C8H17)(OH), where R represents different substituent groups (styryl for SPE108, iso-octoxyl for D2EHPA, iso-octyl for EHEHPA). It’s well known the inductive effect of alkoxyl group is stronger than its corresponding alkyl group, while the styryl group is even stronger due to its high degree of unsaturation. Thus, the electron-accepting ability of substituent groups follows the order: styryl > iso-octoxyl > iso-octyl, strong electron inductive makes it easier for the organophosphorus acid to release hydrogen ions and form complex with with lanthanum ions. The frontier orbital theory can be used to describe the reactivity of molecules, where the HOMO (higherst occupied molecule orbital) reflects the electron-donating ability and the LUMO (lowest unoccupied molecule orbital) reflects the electron- accepting ability. The density functional theory (DFT) based quantum chemical method was a credible method for understanding the electronic structure of molecules 36. Therefore it was carried out to calculate the HOMO and LUMO values for the three compounds. The calculation results are shown in Table 2. The extractant molecule accepts electrons in the complexation reaction with metal ions, thus the LUMO is a key factor of its complexation ability. Lower LUMO value corresponds to stronger electron-accepting ability. The LUMO values for SPE108, D2EHPA and EHEHPA are -0.07107, -0.01059 and -0.00826, respectively, indicating the electron-accepting ability follows the order: SPE108 > D2EHPA > EHEHPA. The calculation results are in accordance with the experimental results and verify the explanation of electron inductive effect. 4
Conclusions Styryl phosphonate ester is synthesized with a new method and characterized. The extraction performance and mechanism of SPE108 in the extraction of La(III) ions were studied by extraction tests and compared with D2EHPA and EHEHPA. SPE108 exhibits stronger extraction ability and higher extraction capacity than D2EHPA and EHEHPA. Meanwhile, the extraction of lanthanum with SPE108 is less affected by (NH4)2SO4 in the aqueous phase than D2EHPA and EHEHPA, which is an advantage for its industrial use. Stripping and recycling studies show that SPE108 has good recycling capacity. The separation study show that SPE108 can separate lanthanides efficiently at low pH. The extraction mechanism of SPE108 is proved similar to D2EHPA with the formation of LaA3(HA)3 complex, and the bonding model of the complex is proposed. In consideration of the cheap raw material and simple method for its synthesis, it can be concluded that SPE108 is a promising extractant for REE extraction, especially for the condition of leaching solution with high acidity or high salt concentration. Acknowledgements The authors express their appreciation for the support of the High Performance Computing Center of Central South University, China.
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TOC:
Styryl phosphonate mono-iso-octyl ester (SPE108) exhibits stronger extraction ability and higher extraction capacity than D2EHPA and EHEHPA in the extraction of lanthanum (III). The extraction mechanism of SPE108 is similar to that of D2EHPA with the formation of LaA3(HA)3 complex.
Fig.1. Synthesis route of styryl phosphonate monoester.
100
E /%
80 SPE108 D2EHPA EHEHPA
60
0
5
10
15
20
25
30
Equilibrium time / min
Fig. 2. Effect of equilibrium time on the extraction of La(III) with SPE108, D2EHPA and EHEHPA.
100 80
SPE108 D2EHPA EHEHPA
E /%
60 40 20 0 0
1
2
3
4
5
6
pHaq
Fig. 3. Effect of pH on the extraction of La(III) with SPE108, D2EHPA and EHEHPA.
100
E/%
80
60
40 SPE108 D2EHPA EHEHPA
20 0
5
10
15
20
25
30
Extractant concentration (g/L)
Fig. 4. Effect of extractant concentration on the extraction of La(III) with SPE108, D2EHPA and EHEHPA.
100
2.0
(a)
(b)
SPE108 D2EHPA EHEHPA
1.6
[La3+]organic / (g/L)
80
E/%
60
40
SPE108 D2EHPA EHEHPA
1.2
0.8
20 0.4
0 1/1
1/2
1/3 1/4 Phase ratio (O/A)
1/5
1/1
1/2
1/4 1/3 Phase ratio (O/A)
1/5
Fig. 5. Effects of phase ratio on the (a) extraction rates and (b) loaded La (III) in the organic phase.
100
100
(a)
(b) 98 Stripping rate / %
Stripping rate / %
80
60
40
20
SPE108 D2EHPA EHEHPA 0.0
0.5 1.0 1.5 2.0 2.5 Hydrochloride concentration / (mol/L)
3.0
96
94 SPE108 D2EHPA EHEHPA
92
90
0.2
0.4
0.6 0.8 3+ [La ] organic / (g/L)
1.0
Fig. 6. Effect of HCl concentration on the stripping of La(III) from loaded SPE108, D2EHPA and EHEHPA in kerosene.
100
E/%
95
SPE108 D2EHPA EHEHPA
90
0
1
2
3
4
5 6 Cycles
7
8
9
10
11
Fig. 7. Recycling capacity of SPE108, D2EHPA and EHEHPA.
100 98 96
E/%
94 92 90 SPE108 D2EHPA EHEHPA
88 86 0
10
20
30
40
50
(NH4)2SO4 concentration / (g/L)
Fig. 8. Effect of ammonium sulfate on the extraction of La(III).
100
La(III) Ce(III) Pr(III) Nd(III) Sm(III)
80
E/%
60 40 20 0 1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
HCl concentration / (mol/L)
Fig.9. Extraction rate of La, Ce, Pr, Nd, Sm with SPE108 at different acidity.
2
7 .4
lgD
-1
-2.0
-1.5
3
06 .5 -5 8x 99 2.
-2
-3
4
y=
-1
1x
.45
03
-3
0
- 3.
6x
y=
99
- 2.
y=
1
SPE108 D2EHPA EHEHPA
-1.0
-0.5
0.0
+
lg[H ]eq
Fig. 10. Variation of lgD with lg[H+]eq for the extraction of La(III).
2
SPE108 D2EHPA EHEHPA
lgD
2.9 y= 0
y= -2
-1.6
7 2.9
4x
90
.5 +3
4.7 x+
71
-1.2
39
.652 7x+1 1 5 . y=1
-0.8
lg[(HA)2]
Fig. 11. lgD versus lg[(HA)2] for the extraction of La(III).
Fig. 12. Proposed bonding model of SPE108 dimer with La (III).
Table 1 Separation of lanthanides with 50g/L SPE108, D2EHPA and EHEHPA at an initial pH of 0.4, 1 and 2, respectively Extractant La(g/L) DLa Ce(g/L) DCe β=（DCe/DLa） SPE108 1 0.38 1 1.67 4.37 D2EHPA 1 0.18 1 0.65 3.54 EHEHPA 1 0.26 1 1.32 5.04 Pr(g/L) DPr β=(DPr/DLa） SPE108 1 0.32 1 2.39 7.54 D2EHPA 1 0.3 1 1.61 5.32 EHEHPA 1 0.23 1 1.92 8.26 Nd(g/L) DNd β=（DNd/DLa） SPE108 1 0.36 1 2.86 7.95 D2EHPA 1 0.14 1 1.27 8.98 EHEHPA 1 0.19 1 2.48 12.86 Sm(g/L) DSm β=（DSm/DLa） SPE108 1 0.30 1 17.09 57.90 D2EHPA 1 0.13 1 7.78 61.89 EHEHPA 1 0.15 1 12.91 85.20
Table 2 HOMO and LUMO values for SPE108, D2EHPA and EHEHPA Molecule HOMO SPE108 -0.24934 D2EHPA -0.30409 EHEHPA -0.29449
LUMO -0.07107 -0.01059 -0.00826
A novel method for the synthesis of styryl phosphonate monoester was proposed.
Styryl phosphonate mono-iso-octyl ester (SPE108) exhibited stronger extraction ability and higher extraction capacity than D2EHPA and EHEHPA in the extraction of lanthanum (III).
The extraction mechanism of SPE108 is similar to that of D2EHPA.
DFT calculations indicated that the electron inductive effect might be an explanation for the superior extraction performance of SPE.