Study of Stationary Phases for Chromatographic Separation of Lanthanides

Transcription

1 Study of Stationary Phases for Chromatographic Separation of Lanthanides Jonas Bigelius Department of Chemical Engineering, Lund University, Sweden Abstract The lanthanide series is composed of fifteen metallic elements ranging from lanthanum to lutetium, their atomic number reaches from 57 to 71. The elements have very desirable properties and are used in a variety of applications. The demand is increasing and new approaches of how to retrieve lanthanides are constantly investigated. Lanthanides have very similar chemical properties and are therefore known to be difficult to separate individually. Usage of complexing agents, also referred to as ligands, is the most utilized strategy for separation of lanthanides. Liquid-liquid extraction is used in largest extension, however, chromatography raise interesting possibilities and aspects. The aim of this study was to evaluate the ability of bis(2-ethylhexyl) phosphate (HDEHP), tris(2-ethylhexyl) phosphate (TOP), decanoic acid (DA) and butyronitrile (CN) as lanthanide complexing agents in a chromatographic system. The CN column was bought and the other columns had to be prepared manually. This was done by means of reversed phase chromatography during which the complexing agents got immobilized in the column. The columns were validated by injecting a sample containing all lanthanides after which the columns were processed with a nitric acid gradient. In dissociated form, HDEHP and DA possess high electron density and were expected to be the most selective. The CN group showed no interactions at all and was found non-usable. TOP had the ability to retain all the lanthanides but indicated no selectivity and can mainly be used for bulk recovery. The stability of the lanthanide complexes formed with HDEHP increased exponentially with atomic number, from lanthanum to lutetium. HDEHP is therefore most usable for separation of lanthanides from samarium to lutetium. DA proved good separation abilities for all elements, especially from lanthanum to samarium. However, stabilities of the formed complexes varied a lot and DA is therefore only usable for treating subgroups. Keywords: Extraction chromatography, Complexing agents, Lanthanides, Electron density, Separation, Hard and Soft Acids and Bases Introduction Lanthanides are most often found together, which is a consequence of their striking similarity regarding chemical properties. Lanthanides have strong affinity for oxygen and are mainly found as oxides, carbonates, phosphates and silicides. Based on the difference in properties the lanthanides are divided into subgroups. From lanthanum to neodymium is referred to as light, from samarium to gadolinium, as SEG and from terbium to lutetium, heavy lanthanides. Because of their chemical similarity it is

2 commonly known that lanthanides are difficult to separate individually. (1) Liquid-liquid extraction, LLE, is the most used separation method and chromatography is mainly used in small scale for production of valuable heavy rare earths (2). When complexing agents, used in LLE, are immobilized inside a column it is called extraction chromatography. This combination is described as a technique that combines the selectivity of solvent extraction with the ease of operation of the chromatographic method. (3) Complexing agents used in the study were partially based on ligands used in other studies and what was found in the litterateur. Another aspect that was considered was the theory of hard and soft acids and bases, HSAB, which characterizes compounds in order to predict their reactivity. Theory Lanthanides Decreasing ionic radii as the atomic number is increased is denoted as lanthanide contraction, lutetium has the smallest radius and lanthanum has the greatest. This phenomenon is caused by imperfect shielding between electrons located in the subshell. Along the lanthanide series nuclear charge and number of f electrons increase. Because of the poor shielding, of one electron by another, the nuclear charge will affect the electrons more as the atomic number increases. This results in decreased ionic radii along the series. The basicity of the lanthanides is an effect of the contraction. This property is very important and forms the basis to many of their chemical features, among them their extractive properties. Cations having great attraction to anions are considered as least basic, the basicity of lanthanides ions decreases along the series. (1) La 3+ > Ce 3+ > Pr 3+ > Nd 3+ > Pm 3+ > Sm 3+ > Eu 3+ > Gd 3+ > Tb 3+ > Dy 3+ > Ho 3+ > Y 3+ > Er 3+ > Tm 3+ > Yb 3+ > Lu 3+ Separation processes Precipitation and crystallization were used as separation processes in beginning of the lanthanide era but nowadays only in rare cases. Chromatography is mainly used in small scale for production of valuable heavy lanthanides. Liquid-liquid extraction, LLE, is the most used separation method and holds a great advantage as it can handle very high concentrations of metals throughout the process. (2) Organophophorous acids are typical cation exchange extractants and HDEHP is the most well used compound in this category. The separation efficiency for all phosphorous acids increases along the lanthanide series, from La to Lu. Tributyl phosphate, TBP, is a natural extractant, i.e. it is uncharged, and not recommended to use for individual separation but useful regarding bulk recovery. Carboxylic acids also extract by cation exchange. The most common carboxylic acid is Versatic 911 which can extract all lanthanides. (1) A pyridine based resin has the ability to retain all lanthanides from a diluted nitric acid solution. Elution with concentrated acid results in elution order from Lu to La. (4) HDEHP can also be used as complexing agent in chromatography. It can be used to separate all the lanthanides and by varying the concentration of ligands in the column it is possible to target the different groups of lanthanides. (3) A bis(carboxylmethyl)amino, CMA, based resin can also retain all lanthanides and provides high selectivity (5). Hard and Soft Acids and Bases Hard and soft acids and bases, referred to as HSAB, is a theory used to classify chemical substances and how they interact with each other. The definition of acids and bases is according to the Lewis theory; a base is an electron donor while an acid is an electron acceptor. The gist of the theory is that hard acids interact strong with hard bases whereas soft acids bind strong to soft bases. Hard acids are recognized by low electronegativity, small

3 size and high charge. Not surprisingly, soft acids have intermediate to high electronegativity, large size and therefore often polarized. Hard bases are characterized by small size and very high electronegativity. Soft bases have intermediate electronegativity and larger size, which often results in polarization. (6) According to HSAB all lanthanides are hard acids. Since the basicity of lanthanides decreases along the series the hardness of them will increase. Hard metal ions form stable complexes with a ligand atom if it belongs to the first elements in group 15, 16 and 17, i.e. N, O and F. High charge and low polarization often results in an electron configuration where the electrons will be located close to the nucleus and therefore harder to share. Since the hardness is governed by such properties hardhard interactions are mainly represented by ionic bonds while soft-soft interactions are most often covalent (7). The density functional theory, DFT, is a theory based on the electron density distribution of a compound. DFT is closely related to HSAB and signifies that the hardness of a compound correlates to the charge density (8). The electron density is therefore a suitable property to utilize when determine the hardness of a base. Complexing agents Phosphate based minerals are rich in rare earth elements and it is therefore not surprising that HDEHP in particular has good ability to interact with all the rare earths. The structure of HDEHP is illustrated in FIGURE 1. In dissociated form, the hydrogen in the hydroxyl group will be absent and the oxygen will be negatively charged and able to interact by ion exchange. Figure 1. Bis(2-etylhexyl) phosphate, HDEHP. Just as HDEHP, DA will also have the ability to interact with two partially negatively charged oxygen when deprotonated. The structure of DA is shown in FIGURE 2. Figure 2. Decanoic acid, DA. Tris(2-ethylhexyl) phosphate, TOP, has the same functional group as TBP but has longer alkyl groups in comparison. Since the active part is identical, their separation behavior is supposed to be similar. The structure of TOP is shown in FIGURE 3. Figure 3. Tris (2-etylhexyl) phosphate, TOP. Butyronitrile, CN, is a nitrile and illustrated in FIGURE 6. The structure differs compared to pyridine which is a tertiary amine and has an aromatic structure. However, in both cases the nitrogen has its lone electron pair exposed to the environment, making it possible to serve as an electron donor. Figure 4. Butyronitrile, CN. Electron densities for all the ligand were calculated by means of the software SPARTAN and are shown in TABLE 1. Table 1. Electron densities. Ligand Electron density (kj/mole) HDEHP -748 DA -674 TOP -292 CN -232 Experimental design The column containing CN was bought and columns containing HDEHP, TOP and DA

4 were prepared manually by immobilizing the ligands into the column. Material The columns used for impregnation were of the kind Kromasil C18 and the CN column was a Kromasil CN. Impregnation Impregnation of ligands was performed on an ÄKTA Purifier HPLC system. Hydrophobic interaction chromatography formed the bases for the impregnation procedure during which the hydrophobic part of the ligands interacted with the C-18 groups covering the particles. Application buffer, containing the ligand, was added until a breakthrough curve occurred. At this point no more ligands could be adsorbed. Starting from a breakthrough curve, the amount of ligand adsorbed on the column was calculated, given that the feed concentration was known. The area between when the application began and left of the breakthrough curve corresponded to the adsorbed amount. Retention experiments The retention experiments were performed using an Agilent HPLC lab scale system connected to an ICP-MS. Initially 10 µl lanthanide sample, containing g/l of each element having a ph of 5, was injected into the column. The column was then processed by running a linear gradient of nitric acid. Gradient length and acid concentration were individually adjusted to fit the different stationary phases. By adapting the acid gradient it was possible to determine during which circumstances the complexing agent interacted with the lanthanides and at which it did not and thereby if the complexing agent could be used for individual separation. All experiments were performed as triplicates to assure reproducible experiments. All experiments were performed at room temperature and at a flow rate of 2 ml/min. Acid gradient conditions used to retrieve represented results are stated in TABLE 2. Table 2. Experimental conditions. Gradient length is given in column volumes, cv. Ligand Gradient length (cv) Initial ph Ending ph CN TOP HDEHP DA Specifications of the columns used in the retention experiments are shown in TABLE 3. The ligand density of a column is defined as the amount of complexing agents per milliliter column, µeq/ml. Table 3. Column specifications. Ligand Volume (ml) Ligand density ( µeq/ml) CN TOP HDEHP DA Result Impregnation TOP could not be detected by conductivity, refractive index detector or a spectrophotometer and the ligand density could therefore not be determined. Repeated experiments with HDEHP and DA showed a very small deviation, the achieved ligand density never varied more than 5 %. Retention experiments The CN column showed no sign of interaction as the lanthanides eluted at the dead volume of the system independent of mobile phase conditions. All lanthanides got retained when running a ph 3 through the TOP column and at ph 2 all of them eluted simultaneously. Clearly formation of complex occurs but indicates no signs of selectivity.

5 FIGURE 5 illustrates the result achieved with the HDEHP column. The trend clearly shows how the stability of lanthanide complexes with HDEHP increases significantly with atomic number. The light and SEG elements are almost co-eluting and the heavy lanthanides elutes individually. Light SEG Heavy Figure 5. Retention times for lanthanides retrieved using the HDEHP column. DA showed increased stability for complexes up to samarium after which it dropped down to erbium before increasing again, see FIGURE 6. Treated as groups, light and SEG lanthanides can be individual separated while the heavy lanthanides co-elute. Light SEG Heavy Figure 6. Retention times of the lanthanides when using DA as ligand. Discussion Since the pyridine based resin is stated to serve as a good ligand the result from the CN column was surprising. The most probable reason for the absent interaction is the low electron density, it would therefore be interesting to compare the electron density of pyridine and see if they differ from CN. The difference in structure could also be an explanation of why the amines interact differently. Pyridine is aromatic and might have properties which make it significantly better as complexing agent. The result from the TOP column was expected. It cannot be used for individual separation of lanthanides but can be used for bulk recovery, just as in LLE. However, since the ligand density was not defined, and might have been to low, it could be more usable than the result indicates. If a column has a high ligand density the metals will yield more complexes before it elutes. This is directly related to extent of separation possible to achieve with the column. Since the heavy lanthanides formed very stable complex with HDEHP, a steep acid concentration was needed in order to elute all of them. However, by changing the acid gradient it is possible to target different groups of the lanthanides. If a more diluted acid gradient was used the light fraction would probably have been separated in the same way as the heavy ones. Due to the strong complexes formed with HDEHP a lot of nitric acid is needed to achieve elution. This is a drawback since it will be a large cost in large scale. An advantage with HDEHP is that the load does not have to be done in form of groups. Retention times of the lanthanides retrieved from the DA column was surprising and an explanation to this behavior could not found in literature. Based on the achieved result DA is a suitable ligand for separation of light and SEG elements. However, in order to achieve individual separation the load must be done in form of groups which is not preferable.

6 A low need of nitric acid is positive since it reduces the chemical costs. HSAB appears to be a good method to predicting suitable complexing agents in extraction chromatography as the selectivity of the ligands was in accordance with the charge density. According to HSAB, DA should be a better complexing agent than HDEHP since it has the highest electron density, especially for the heavy lanthanides which are least basic. This could possibly be explained by that the pka of the ligands differ and therefore affected by the acid environment to different extent. Since TOP had the ability to interact and the fact that it is an uncharged complexing agent suggest that ion exchange is not the only interaction mechanism occurring. This could possibly also be the reason for why the stabilities of DA and HDEHP formed complexes vary. Conclusions Compared to LLE, extraction chromatography has as a drawback of lower loading capacity but also holds a great advantage which is the low need of complexing agents and easy regeneration of them. HDEHP can be used to separate heavy lanthanides and maybe SEG as well. Decanoic acid serves well as complexing agent for light and SEG lanthanides. However, due to variation in complex stabilities formed with DA, the lanthanides can only be separated if loaded as subgroups. TOP works as a complexing agent but indicates no selectivity and can only be used for bulk recovery. Cyano groups do not interact at all and are not suitable as complexing agents. The HSAB theory is good to utilize when determining if a compound is suitable as a complexing agent. If all lanthanides should be individually separated several columns must probably be used, each targeting a certain group of lanthanides. References 1. C.K. Gupta, N. Krishnamurthy. Extractive metallurgy of rare earths. u.o. : CRR Press, Rare Earth Elements. McGill, Ian. 2000, Ulmman s Encyclopedia, Vol. 31, ss Impregnation and characterization of high performance extraction columns for separation of metal ions. Kifle, Dejene, o.a., o.a. Accepted for publication, Journal of Solvent Extraction and Ion Exchange. 4. Separation of rare earth elements by tertiary puridine resin. Suzuki, Tatsuya, o.a., o.a. 2006, Journal of alloys and compounds, Vol , ss Chromatographic separation of rare earth pairs by a chelating resin having bis(carboxymethyl)amino groups. Kanesato, Mastoshi, o.a., o.a. 1989, The chemical society of Japan, ss Hard and Soft Acids and Bases. Pearson, Ralph G. 1963, Journal of the american chemical society, Vol. 85, ss Absolute Hardness: Companion Parameter to Absolute Electronegativity. Parr, Robert G. och Pearson, Ralph G. 1983, Journal of the american chemical society, Vol. 105, ss Density Functional Theory of Electronic Structure. Kohn, W., Becke, A. D. och Parr, R. G. 1996, the Journal of Physical Chemistry, Vol. 100(31), ss