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Ind. Eng. Chem. Res. 2005, 44, 4726-4736
Synthesis and Evaluation of Metal-Ligand Complexes for Selective Olefin Solubilization in Reactive Solvents Annebart E. Wentink, Norbert J. M. Kuipers,* Andre B. de Haan, Japie Scholtz,† and Harko Mulder† Faculty of Science and Technology, University of Twente, 7500 Enschede, The Netherlands
The separation of 1-hexene from Fischer-Tropsch streams is an expensive and difficult separation because of the low relative volatilities encountered. Applying metal-ligand complexes, which π-complex olefins, can increase the selectivity and capacity of these separations. In this paper, potential metal-ligand combinations are screened for this purpose using ethylene as the olefin. Ethylene solubility measurements with the metal-ligand complexes show a qualitative relationship between the stability and ability of the metal-ligand complex. The ligands LIX 26 (a hydroxyquinoline) and LIX 54 (a diketone) do not easily extract silver, and the resulting metalligand complex is unstable as silver precipitates. In contrast, Cyanex 301 and 302, thiophosphoric acids, easily extract silver and are very stable, but here the silver ion no longer forms π complexes. The ligands, bis(2-ethylhexyl)phosphoric acid and dinonylnaphthalenesulfonic acid, result in a stable metal-ligand complex that can also selectively complex ethylene relative to ethane. 1. Introduction R-Olefins such as 1-butene, 1-pentene, 1-hexene, or 1-octene have found widespread application in the production of oxoalcohols, in the synthesis of bromoalkenes, and as comonomers in the production of polymers such as linear low-density polyethylene.1 Currently, the major production route for higher R-olefins (>6) is based on the oligomerization of ethylene.1 Alternative sources of R-olefins are Fischer-Tropsch product streams. The recovery, and especially the purification, of R-olefins (e.g., 1-hexene or 1-octene) from these streams is a difficult and expensive operation because of the large component spectrum in which some of the encountered relative volatilities are close to unity. For example, 1-hexene needs to be recovered from a component spectrum of at least 29 different components, which comprise oxygenates, branched, internal, and cyclic olefins (paraffins and aromatics), in which the key split between 1-hexene and 2-methyl-1-pentene is characterized by a relative volatility of only 1.06. These low relative volatilities render normal distillation impractical and uneconomical. Also, extractive distillation will not significantly improve this separation because all isomers respond in a way similar to that of the solvent, leaving the relative volatility almost unchanged.2 In earlier work,3 the use of π complexation in combination with (reactive) extractive distillation (RED) was investigated. It was shown that enriching a polar solvent with an inorganic transition-metal salt like AgNO3 can dramatically increase the selectivity. For example, the selectivity between 1-hexene and 2-methyl1-pentene was increased from 1.06 to 1.8. This increase is the result of the specific complexation between the metal ion and the olefin isomer. The disadvantage of the developed solvent was the low capacity and potentially the instability of the metal ion. * To whom correspondence should be addressed. Tel.: + 31 489 4289. Fax: +31 489 4821. E-mail: N.J.M.kuipers@ tnw.utwente.nl. † Present address: Sasol Technology, Sasolburg, South Africa.
To overcome the low capacity, new solvents are developed that have a lower polarity and thus a higher olefin capacity. However, now it becomes increasingly difficult or even impossible to dissolve an inorganic metal salt. To overcome this problem, it is opted to use an organic metal salt. Here, the metal ion is bonded to an organic group, the ligand that can complex with a metal ion (silver). This organic metal salt is easily dissolvable in apolar organic solvents. In this paper, potential suitable ligands are identified, validated, and compared for their effectiveness in π complexation referring to the stability and ability of the metal-ligand complex. Stability means that the interaction between the ligand and metal ion is sufficiently strong to keep the metal ion in the solution. This is investigated with different silver(I)-ligand complexes because these are more stable than copper(I). A ligand that is not able to keep silver(I) in the solution is also unlikely to keep copper(I) in the solution. Various metal-ligand complexes are synthesized by liquidliquid extraction of silver(I) from aqueous silver nitrate solutions. The ability of the metal-ligand complex refers to whether the metal is still able to complex an olefin once bonded with the ligand. This is tested by gravimetric measurement of the ethylene solubility in the synthesized (reactive) metal-ligand-containing solvents. Ethylene is used because it can be considered as the simplest R-olefin available, which shows the strongest interaction of all R-olefins (so if ethylene does not complex, a higher (R-)olefin will certainly not4). To show that this reaction can also increase the selectivity between an olefin and paraffin, also the ethane solubility is determined. Finally, the effect of the polarity of the diluent on the ethylene solubility and complexation strength is investigated. The ligands that are selected in this paper will be used in experiments to verify their applicability in higher olefin isomer separations. To minimize the amount of liquid required in these measurements, a rather new technique to measure gas solubilities is introduced: an intelligent gravimetric analyzer (IGA).
10.1021/ie0487890 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/18/2005
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Figure 1. Schematic picture of a transition metal-ligand complex and its π complex with an olefin.
2. Metal-Ligand Complexes Selected from Hydrometallurgy A schematic representation of a transition metalligand complex bonded with an olefin is given in Figure 1. Two interactions of the metal ion are important: (1) π complexation between the metal ion and olefin and (2) the interaction between the metal ion and stabilizing ligand. The ligand can be divided into two parts: an organic tail and a functional group. The functional group of the ligand is involved in the bonding of the metal ion. It directly interacts with the metal ion and strongly determines the stability and ability of the metal ion. The organic tail ensures that the metal-ligand complex can be dissolved in apolar diluents. Although not directly involved in the metal-ligand interaction, it can also be of influence on the stability and ability if it influences the properties of the functional group. The size and structure of the organic tail is therefore important: an organic tail that is too small might yield a metal-ligand complex that solidifies, but once too long, it limits the maximum attainable metal concentration. The structure of the organic tail can be optimized to improve the physical properties such as viscosity, eliminating the need for a diluent. In case a diluent is required, it should be nonreactive toward the metal-ligand complex. Aliphatic or aromatic diluents are therefore suitable, but amines or nitriles are excluded because these strongly compete with the olefin for metal complexation.5 The ligands can be tailor-designed, but a wealth of information on potentially suitable ligands is already available from hydrometallurgical applications. In addition, these ligands are commercially used and widely applied for the reactive extraction of metal ions present in aqueous streams into an organic phase. Utilization of these hydrometallurgical extractants might therefore yield new and better ligands that are useful in olefin separations. In the area of olefin isomers or olefinparaffin purifications, the use of metal-ligand complexes is already known. Different silver(I) or copper(I) ligands are mentioned, for the separation of ethyleneethane, propylene-propane, or olefin isomers in the C4 and higher range.4,5 2.1. Ligand and Diluent Requirements. Both the metal-ligand complex and the diluent (if used) must fulfill a number of requirements for application in olefin separations. These demands are set by the processes where the olefins as well as safety constraints, costs, etc., will be used. The latter means that the ligand should be simple (thus cheap), safe, and noncorrosive and should have a low viscosity. In the production of polymers or in oxosynthesis, catalysts are used that are
easily deactivated by impurities asking for a high purity of the feed streams.1 These impurities comprise components such as halides, sulfurous compounds, or oxygenates (water). So, both the ligand and diluent should (preferably) not comprise halogenated hydrocarbons, but if sulfurous compounds or oxygenates are used, these should at least be stable and have a low volatility. In hydrometallurgy, the aqueous solubility of the extractant is kept low, by using large organic tails to make the extractant apolar. This also ensures a low volatility, such that no ligands are lost to the vapor phase. This is vital for applying RED for the separation of medium-boiling R-olefins such as 1-hexene or 1-octene. Furthermore, the ligand and metal-ligand complex should be thermally stable; i.e., it should not decompose at higher temperatures because the decomposition products can foul the product or result in the precipitation of the metal, rendering the process infeasible. The ligand should not negatively influence the π-complexation abilities of the metal because this also makes RED infeasible. 2.2. Selected Ligands. The potential ligands to be discussed are listed in Table 1 and are selected because they interact with silver or extract the noble metals [Au(I,III)]. In the following section, each extractant-ligand is discussed with its reason of selection and whether it has been used experimentally to extract silver. Phosphoric Acids. Phosphoric acids form dimers in the organic phase because of hydrogen bonding.6 Bis(2-ethylhexyl)phosphoric acid (D2EHPA) is the most common. It can extract silver from aqueous solutions at a pH of around 5.6 with a 1:2 ratio (metal-ligand).7 The use of phosphoric acids with copper(I) or silver(I) is also mentioned in patents for olefin separation.8 Sulfonic Acids. Sulfonic acids are strong acids that can extract transition metals at very low pH. In hydrometallurgy, dinonylnaphthalenesulfonic acid (DNNSA) is the most common, but also didodecylnaphthalenesulfonic acid is used.9 Sulfonic ligands (e.g., dodecylbenzenesulfonic acid) with copper(I) or silver(I) are suggested for olefin separations.10-12 (Thio)phosphinic Acids. Cyanex 272, 302, and 301 are structurally similar but with the oxygen atoms gradually replaced by sulfur. The increasing degree of substitution with sulfur decreases the pKa from 6.37 (Cyanex 272), to 5.63 (Cyanex 302), to 2.61 (Cyanex 301) and therefore increases the degree of extraction of metal ions at low pH.13 For example, both Cyanex 302 and 301 extract silver from nitrate solutions at a pH of 0. Sulfur strongly interacts with silver so the difference in the pKa diminishes, while Cyanex 272 only achieves this around a pH of 8.13 However, the bonding between Cyanex 301 or Cyanex 302 with silver is so strong that nondestructive stripping of silver will be extremely difficult. For a complete extraction of copper(II), Cyanex 272 requires a pH of around 6, whereas Cyanex 302 and 301 do this already at a very low pH (0). Copper is very difficult to strip from Cyanex 301 and 302 because copper(II) is reduced to copper(I), whereas two ligands are oxidized to a disulfide [R2P(X)-S-S-(X)PR2, X ) S or O]. Phosphine Oxides-Sulfides. Different phosphine oxides, e.g., Cyanex 921 (trioctylphosphine oxide, TOPO), are described for the extraction of Au(I,III) from chloride or cyanide solutions.14 In Cyanex 471X, the oxygen atom is replaced by sulfur, whereas a different tail length is
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Table 1. Overview of Selected Ligandsa
a
EM ) extraction mechanism, A ) acidic, IP ) ion pair, S ) solvating, and n.r. ) not reported.
used. Sulfur increases the selectivity for silver(I) because it strongly interacts with silver.15 Hydroxyquinolines. 8-Hydroxyquinoline (8HQ) extracts silver from aqueous solutions at high pH (89.5).16 8HQ is not used in hydrometallurgy because it has a relatively high solubility in water. This can be reduced by introducing a large organic group as found in, e.g., Kelex 100 (4-ethyl-1-methyloctylquinoline) and LIX 26 (large but unknown R group). β-Diketones. β-Diketones exhibit tautomerism such that one of the central hydrogen atoms is acidic and exchangeable for a metal ion.16 The ketonic oxygen can complete the ring formation, resulting in the formation of a stable metal-ligand complex. The interaction with silver is weak and probably the reason that commercial extraction of silver is not described. However, benzoylacetone [(C6H5)C(O)CH2C(O)CH3] is known to complex silver.16 Other metals such as Cu(II) or Zn(II) can be extracted by LIX 54 [(C6H5)C(O)CH2C(O)(C7H15)].16 The diketone hexafluoroacetylacetonate complexes with copper(I), as described for olefin (isomer) separation.18 Miscellaneous Extractants. A guanidine-based extractant, LIX 79, is used to extract gold(I).19 Recently, also the use of crown ethers, e.g., 15-crown-5 or 18crown-6, is described for the extraction of silver.20,21 Other silver-interacting ligands are dithizone, tropole, and dithiocarbamates. However, these are light-sensitive and are not commercially applied in hydrometallurgy.16
3. Modeling of the Gas Solubility in Reactive Solvents The aim of this section is to present a simple equilibrium model that can adequately describe the ethylene solubility in a solvent containing reactive metal-ligand complexes and can also be used for the calculation of the selectivity between ethylene and ethane. The development of a complete and detailed thermodynamic framework for the gas solubility of the studied systems would be too time-consuming, requires too many additional data, and is therefore beyond the scope of this paper. In Figure 2, a schematic presentation is given for the distribution of ethylene between a gas and a reactive liquid phase. The liquid phase contains the metalligand complexes (MLn) that are capable of forming π complexes with ethylene. Predominantly, a complex with a 1:1 stoichiometry is formed.22 This complex is referred to as the primary complex, and its reversible formation in phase II proceeds according to (see also Figure 2) d
Cd 2
2 KC r,1
+ MLn y\z (Cd 2 )MLn
primary complexation
Under certain conditions (high silver loading, high ethylene partial pressure, and low temperature), complexes with different stoichiometries can exist: the d secondary complex with complexation constant KCr,22
Ind. Eng. Chem. Res., Vol. 44, No. 13, 2005 4729 d
d
[MLn] ) [-(1 + KCr,12 HC2 PC2 ) + d
x(1 + KCr,1HC PC )2 + 8KCr,1 KCr,3HC PC [MLn]T]/ d 2
d 2
d 2
d 2
d 2
d 2
d 2
d
d
d
d
4KCr,12 KCr,32 HC2 PC2 (5) Combining eqs 4 and 5 gives the total ethylene solubility as a function of the pressure and concentration of the metal-ligand complex. In case only primary complexes are formed, eqs 4 and 5 can be simplified by substituting Kr,3 ) 0. The total ethylene solubility then becomes
Figure 2. Schematic representation of ethylene solubility in a (reactive) solvent.
(1:2 silver-olefin) and the tertiary complex with comd plexation KCr,32 (2:1).22 In the experimental results, no indications were found that significant amounts of this secondary complex are formed, in contrast to tertiary complexes. The tertiary complex is formed from a consecutive reaction of the primary complex with another metal-ligand complex: d
2 KC r,1
(Cd \z (Cd 2 )MLn + MLn y 2 )(MLn)2 tertiary complexation In our model, the reaction complexation constants are expressed in concentrations instead of activities. Furthermore, it is assumed that the metal-ligand complex has no influence on the physical solubility of ethylene, which therefore can be determined in the absence of metal-ligand complexes. The physical solubility of ethylene and ethane as a function of its pressure is described by Henry’s law (eq 1). Because ethane does not react with the metal-ligand complexes, eq 1 is also valid in systems containing these metal-ligand complexes: C2 C2 [Cd 2] ) H P d
d
(1)
The equilibrium constants for the primary and tertiary complexations are defined as d KCr,12
[(Cd [(Cd 2 )MLn] 2 )(MLn)2] Cd 2 , Kr,3 ) ) d d [C2 ][MLn] [(C2 )MLn][MLn]
(2a,b)
The total ethylene solubility is the sum of physically dissolved ethylene ([Cd 2 ]) and that bonded in the primary ([Cd ][ML ]) and tertiary ([(Cd n 2 2 )(MLn)2)]) complexes: T d d d [Cd 2 ] ) [C2 ] + [(C2 )MLn] + [(C2 )(MLn)2]
(3)
Combining eq 3 with eqs 1 and 2a,b gives T C2 C2 2 C2 C2 C2 [Cd 2 ] ) H P (1 + Kr,1 [MLn] + Kr,1 Kr,3 [MLn] ) (4) d
d
d
d
d
The free metal-ligand concentration ([MLn]) is equal to the total concentration of the metal-ligand complex ([MLn]T) minus the metal-ligand concentrations in the d primary ([(Cd 2 )MLn]) and tertiary (2[(C2 )(MLn)2]) complexes. Combining with eqs 1 and 2a,b yields after rewriting
T [Cd 2]
Cd Cd 2 2
)H P
(
1+
d KCr,12
[MLn]T
C2 1 + HKCr,12 [Cd 2 ]P d
d
)
(6)
The equilibrium constants are determined by minimizing the weighted square error between the model and experimental data. The selectivity of the metal-ligand complex for ethylene over ethane is calculated by7
S)
d I T [Cd 2 ] /[C2 ] T d I [Cd 2 ] /[C2 ]
(7)
4. Experimental Methods 4.1. Synthesis of Metal-Ligand Complexes. The silver-ligand complexes are synthesized by liquidliquid extraction. A stirrer magnet is put in a glass vessel of sufficient volume filled with an aqueous silver nitrate solution. The stirrer is started, and then the organic solution, containing the extractant in an appropriate diluent, is added slowly. An equilibration time of 30 min is used for all extractants and diluents. Samples are taken after settling and filtered with a 10mL glass syringe, equipped with a Spartan 30/0.45 RC filter unit (0.45-µm mesh). The density change is used as an indication for the amount of metal extracted and to estimate whether equilibrium is reached. The density is measured with an Anton-Paar DMA 5000 density meter (accuracy ) (1 × 10-5 kg/m3; ∆T ) (0.01 °C). Organic-phase samples are stored in airtight brown bottles under nitrogen to prevent degradation of the samples. All extractions are done at room temperature (291 K). The metal content in the organic phase is measured directly with atomic absorption spectroscopy (AAS). Initially, this was done by oxidizing the organic matrix with HNO3-H2O2, dissolving the remaining metal salts in water, and measuring the metal concentration via AAS. However, this procedure is cumbersome such that later measurements were executed without chemical oxidation on a Varian SpectrAA 110/SIPS with samples that are diluted with 2-propanol. Solutions of Ag-D2EHPA in 1-methylnaphthalene (1MN) and N-methylpyrrolidone (NMP) were not made directly. Instead, D2EHPA is dissolved in n-hexane, and then silver is extracted as described, after which nhexane is evaporated by flushing nitrogen, followed by dilution of the Ag-D2EHPA complex in an appropriate solvent. The metal extraction of acidic extractants is enhanced by adding a 1 M NaOH solution. The volumes of the aqueous solutions were selected such that the volume ratio between aqueous and organic phases at the end of the extraction is approximately unity. The procedure for making copper(I)-TOPO is the same as the described liquid-liquid procedure, but here
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Figure 4. Typical IGA sample mass and pressure profile during stabilization and ethylene absorption for aqueous AgNO3 ((6 M) at 298 K: (s) pressure; (0) sample mass.
Figure 3. (Left) Schematic picture of the IGA from Hiden Analytical. Dotted line: electrical connections. Solid line: fluid connections. (Right) Schematic of the IGA balance head (adapted from ref 23).
an aqueous CuICl-HCl solution is used because CuICl is unstable in and does not dissolve in pure water. The acidity of LIX 26 and 54 is not that high, so a lot of NaOH was added to extract sufficient amounts of silver. However, at the same time, silver hydroxide precipitates, does not redissolve, and is thus lost. Silver complexes with these extractants were made by reversing the procedure, so starting with NaOH and slowly adding the silver nitrate solution. Some silver hydroxide precipitated, but nevertheless also small amounts of silver could be extracted. A copper(I)-Cyanex 301 complex can be synthesized by extraction of copper(II) because upon extraction, copper(II) is reduced to copper(I). At the same time, Cyanex 301 is oxidized into a disulfide,19 reducing the maximum copper concentration that can be achieved by a factor of 2. Therefore, the copper(I)-Cyanex 301 complex is synthesized by reacting the ligand dissolved in dodecane with an excess of copper(I) oxide under continuously flowing nitrogen. After reaction, the excess of oxide settles, liquid samples are taken and filtered as in the liquid-liquid experiments, and its metal content was determined via AAS. In this way, higher copper(I) concentrations can be achieved. 4.2. Measuring Gas Absorption in the IGA. General Description. A schematic representation of the IGA (model 003) is given in Figure 3. The temperature, pressure, gas flow, and (if required) gas-phase composition for multicomponent systems are measured as a function of time.23 Temperature control is accomplished with an external water bath (Julabo F25 MW). A vacuum pump (Baltzers TCP 121) is used to remove air and noncondensable gases from the IGA measurement chamber. The pressure is measured by a Transinstruments BHL-4240-01-
01MO pressure sensor (Pmax ) 25 ( 0.008 bar) and is of maximum 20 bar. Figure 3 shows the detailed scheme of the balance head of the IGA. Two glass bulbs in two separate interconnected chambers, sample and reference, are suspended via golden chains from the balance head. The head is balanced by electrically compensating for the mass change on the sample side, with a stable resolution of 1 µg. Absorption isotherms can be measured fully automatically within a specified pressure range (0-20 bar) with varying pressure steps (0.05% span). In gravimetric methods, buoyancy effects influence the measured weight change. To compensate for these effects, the IGA has software where all items present in the chambers (golden chain, glass bulb, etc.) are specified (mass and density), and the software compensates for the buoyancy effects. This is an accepted method of measurement, and more details can be found in ref 23. 4.3. Experimental Procedure. A Pyrex glass bulb (supplied by Hiden Analytical) is used to contain the liquid sample and weighted on an analytical balance (Mettler-Toledo AT200). The glass bulb is suspended on the golden chain, whereafter the absorption chamber is closed. All measurements are done with approximately the same sample mass ((0.4 g). A similar glass bulb with Teflon pieces is used to balance the head and serves as the reference. The water bath is installed around the chamber, and its temperature is set such that the chamber temperature is approximately 298 ( 0.2 K. Measurements are performed at a single temperature, and the effect of the temperature on π-complexing systems is discussed in detail in ref 3. Air is removed from both chambers using a vacuum pump to degas the liquid, lasting typically between 7 and 10 min, after which the vacuum pump is switched off. The sample mass now decreases because of degassing of the liquid and evaporation of the solvent. The absorption experiment can be initiated only once the sample mass no longer decreases. A typical stabilization process is shown in Figure 4 for a 6 M aqueous silver nitrate solution. After approximately 1000 min, the sample mass has been stabilized. This process is slow because it is diffusion-controlled because the sample cannot be agitated. The equilibrium time depends on the used liquid and could last up to 1 week. After stabilization, ethylene is supplemented to the chamber in pressure steps of 0.6 or 0.4 bar up to a maximum pressure of about 6 bar. This causes the sample mass to increase because ethylene dissolves. The next pressure step commences only when the sample mass no longer increases because the liquid is saturated.
Ind. Eng. Chem. Res., Vol. 44, No. 13, 2005 4731 Table 2. Extraction Results of the Tested Ligands (Reverse Extraction: Contact of the Organic Solution with the NaOH Solution First, Followed by the Addition of a Silver Nitrate Solution) ligand concn [L] (mol/L)
solvent during extraction
metal salt used
Cyanex 302 Cyanex 272
1 1 2 1 2 2 2 1 1 1 1 2
n-hexane dodecane 1MN 1MN dodecane n-hexane n-hexane dodecane dodecane dodecane dodecane dodecane
AgNO3 AgNO3 AgNO3 CuCl AgNO3 AgNO3 AgNO3 AgNO3 AgNO3 Cu2O AgNO3 AgNO3
Cyanex 471X
1
n-hexane
AgNO3
TBPO LIX 79
1 1
1MN n-hexane
AgNO3 AgNO3
15C5
1
n-hexane
AgNO3
ligand (L) LIX 26 LIX 54 TOPO D2EHPA DNNSA Cyanex 301
a
solvent absorption
[MLn]T (M)
density at 298 K (kg/m3)
during extraction NaOH/KCl added
n-hexane dodecane 1MN 1MN dodecane 1MN NMP dodecane dodecane dodecane dodecane organic phase polymerizes upon contact with 1 NaOH organic phase polymerizes upon contact with aqueous phase no extraction detected extraction capacity insufficient for practical application (99%), and o-xylene (99%)], Fluka [1MN (97%) and TOPO (99%)], Acros [NMP (99%)], and Praxair [ethylene (99.95%), ethane (99.9%), and helium (99.99%)]. DNNSA (50 wt % nheptane) was a gift from King Industries; n-heptane is removed before extraction by evaporation with flowing nitrogen (checked by gas chromatography). Cyanex 272 (87.3%), 301 (80.3%), 302 (83.2%), and 471X were gifts from Cytec Industries and LIX 26, 54, and 79 from Cognis. All chemicals were used as received. Deionized water was used in all experiments (Milli-Q 185 PLUS). 5. Results and Discussion 5.1. Synthesis Complexes. Table 2 shows the extraction results for the tested ligands. The results for the different extractants are discussed in detail in section 5.1. The ethylene solubility measurements are discussed in section 5.2. LIX 26 and 54. Silver complexes of LIX 26 and 54 were made by reverse extraction. The obtained silver concentrations were not high ([MLn]T e 0.2 M; see Table 2), and the silver-ligand complexes were also unstable. Although stored under nitrogen, silver precipitated from a silver-LIX 26 solution within 24 h. The silver complex with LIX 54 was more stable. However, an ethylene absorption experiment indicated that the silver-LIX 54 complex was nonreactive, whereafter visual inspection
showed that silver had precipitated. The inherent instability of the silver complexes with these extractants implied that these ligands were discarded for further investigation. Cyanex 921 (TOPO). Both silver(I) and copper(I) were extracted from the aqueous phase by Cyanex 921, although the achieved concentrations were not very high (see Table 2). However, these concentrations were sufficient to measure any possible enhancement of the ethylene solubility by reactive absorption and will therefore be investigated. Cyanex 921 extracted metals by a solvating reaction, indicating that an overall neutral metal complex, i.e., AgNO3 or CuICl, was extracted.24 Silver was extracted from a nitrate solution, but this was not enhanced by adding HNO3. In contrast, the copper(I) extraction was enhanced by adding HCl. The copper(I)-containing solution is light yellow/green; with silver, it is dark. DNNSA and D2EHPA. D2EHPA and DNNSA are not as strong extractants as Cyanex 302 or 301. Therefore, NaOH was added to enhance the extraction of silver. A 2 M solution of D2EHPA in dodecane was used because phosphoric acids extract silver ions in a 1:2 ratio (metal-ligand) (see ref 7 and Figure 9). The complexation is different for DNNSA because it extracts silver as a monomer. With both extractants, the addition of NaOH resulted in the appearance of a black precipitate (silver hydroxide) that dissolved in time. The color of the Ag-D2EHPA solution is red/yellow, while the AgDNNSA solution is black. Cyanex 301 and 302. Extraction of silver with Cyanex 301 and 302 diluted in dodecane is relatively easy such that high silver loadings are achieved without the addition of NaOH. Both ligands extract silver in a 1:1 stoichiometry.13 The organic solutions of both ligands with or without silver were black. In the case of Cyanex 301, the organic solution needed to be added stepwise. Adding it all at once resulted in a strong temperature increase, pointing at a strongly exothermic complexation. The nonreversibility of the complexation by Cyanex 301 is also mentioned in the literature.13 The reaction between copper(I) oxide and Cyanex 301 resulted in a copper concentration of 0.88 M. The solution was also black.
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Figure 5. Absolute and relative mass losses during stabilization as a function of time at 298 K. (Left) Absolute mass loss: (b) pure dodecane; (2) dodecane (0.6 bar of He); ([) TOPO/o-xylene; (9) (6 M AgNO3 in water. (Right) Relative mass loss: (O) pure dodecane; (4) dodecane (0.6 bar of He); (]) TOPO/o-xylene; (0) (6 M AgNO3 in water.
Figure 6. (Left) Molar ethylene solubility in pure dodecane as a function of the ethylene pressure at 298 K: (9) run 1; (2) run 2; (b) run 3; (s) literature data.25 (Right) Molar ethylene solubility in AgNO3 in water ((6 M) as a function of the ethylene pressure at 298 K: (0) Cho et al.;26 (O) Keller et al.;27 (4) Hughes et al.;28 (b, 9, [) IGA results; (s) trend line.
Table 3. Molar Mass, Saturation Vapor Pressure, and Density at 298 K for 6 M AgNO3 in Water, o-Xylene, and Dodecane and the Saturation Vapor Density Relative to That of 6 M AgNO3 in Water
and 4000 min, respectively. However, even after 8000 min, the mass of the dodecane sample did not stabilize sufficiently, although the measured mass loss was small (3.2 wt % after 4000 min). Extrapolation of this mass loss profile suggests that it could take up to 200 000 min for stabilization. To speed up the measurements with dodecane, an inert gas can be used. Figure 5 shows that the presence of helium (0.6 bar) decreases the amount of dodecane that evaporates. In addition, stabilization is much faster. Although the sample mass has stabilized sufficiently at (1100 min to start the absorption of ethylene, it still decreases by approximately 0.8 µg/min. A power law was used to extrapolate the solvent mass loss by evaporation during the absorption of ethylene. In the calculation of the ethylene solubility, this correction is taken into account. 5.3. Ethylene Solubility of Model Systems. The d 2 , mol/L) at pressure Pi molar ethylene solubility (SCP,1 (see Figure 6) is calculated using eq 8. The mass of
6 M AgNO3 in water o-xylene dodecane
molar mass (g/mol)
vapor pressure (mbar)
vapor density (g/m3)
relative vapor density
18 106 170
28.3 8.55 0.2
20.6 36.6 1.4
1.00 1.78 0.07
Cyanex 272 and 471X, TBPO, LIX 79, and 15C5. The remaining five extractants were discarded for further investigation for different reasons. In two cases, the organic phase thickened because of “polymerization”: for Cyanex 471X, this immediately occurred after contact with the aqueous silver nitrate solution, and for Cyanex 272, it occurred after the addition of aqueous NaOH because of the formation of multinuclear complexes.13 This is not described elsewhere for Cyanex 471X, but in the literature, much lower concentrations (6 × 10-3 mol/L) are used.14 In the case of TBPO, no detectible extraction of silver was measured. The guanidine extractant LIX 79 did extract small amounts of silver nitrate (
