Solubility and Modifications of Metal Chelates in Supercritical Carbon

Chapter 5

Solubility and Modifications of Metal Chelates in Supercritical Carbon Dioxide Bernd W. Wenclawiak, H. Beer, A. Ammann, and A. Wolf

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch005

Analytical Chemistry, Department of Chemistry, University of Siegen, Adolf Reichwein Strasse 9, D-57078, Siegen, Germany

The solubilities of chelates with palladium, rhodium, lead and copper have been measured with static and dynamic spectroscopic methods at different temperatures and different pressures. The influence of different ligands, ligand modifications and of the metal ions coordination sphere on the solubility were studied and compared to the influence of pressure and temperature of the supercritical carbon dioxide (scCO ). In a series of C , C , C , C copper dithiocarbamates a maximum solubility was measured with the butyl substituents. 2

2

4

6

8

In recent years the use of supercritical fluids (ScF) for analytical and process scale extraction has increased dramatically in effort to reduce the amount of organic solvents used. For a wide variety of low-polarity solutes pure s c C 0 can quantitatively extract organics from a wide variety of matrices, (i-3) Because of practical considerations of low toxicity, high purity, and good ability to solvate a range of organics, s c C 0 has received the most use for analytical scale extractions. Many methods to dissolve and extract metals as chelates into s c C 0 have been described and different ways to determine chelate solubility data in s c C 0 have been reported (4-27). 2

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2

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© 2003 American Chemical Society

Gopalan et al.; Supercritical Carbon Dioxide ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

51 Modem catalytic exhaust converters used in cars contain the precious metals palladium and rhodium in the gram level. (25) Common techniques of recycling heavy metals use the transformation of the metals into chloro complexes which are extracted by means of organic solvents. In order to evaluate the replacement of organic solvents by s c C 0 by a different extraction process the solubility of different chelates of palladium(II), rhodium(III), lead(H) and copper(II) was investigated. 2


There are principally two ways of modifying the solubility of metal chelates such as dithiocarbamates or beta-diketonates: 1. Change of residual groups - e.g. replacement of hydrogen by fluorine or increase the carbon chain length. (6,29) 2. Adding of a synergism or additional modifier to the chelates - e.g. T B P or other. (J) We have systematically studied this behaviour for a group of palladium, rhodium, lead and copper chelates. Linear and branched dialkyldithiocarbamates (dtc) as S,S-type, 2,2,6,6-tetramethyl-3,5-heptanedionate (thd) as Ο,Ο-type and methylglycolates (mtg) as S,0-type were selected. The diisopropyldithiocarbamate (DPDTC) ligand was used as one dtc representative to investigate the influence of fluid density on the solubility of the dtc chelates. The influence of alkylchain length (R=C -C ) of dialkyldithiocarbamates on the solubility of palladium(II), rhodium(III) and copper(II) chelates was measured at constant conditions with two different methods. We observed that branching and length increase of dtc-ligand alkylchain increases the solubility of the respective palladium, rhodium and copper dtc-chelate but only to a certain content. After a solubility maximum the solubility decreases again. The optimum solubility showed dtc with linear or branched C or C alkyl chains. The ligand thd, belonging to the beta-diketon group, was used to measure the influence of the central ion coordination number and therefore the chelates spherical structure on the solubility of the palladium(II)- and rhodium(III) chelates in scC0 . As another type of ligand we tested methyiglycolate and measured the solubility of palladium(II), rhodium(III) and lead(II) methyiglycolate at different conditions. We can compare independent results measured with different apparatuses: Most working groups use a static-spectroscopic or dynamic-spectroscopic method with offline quantification to determine chelate solubilities. We present a new dynamic-spectroscopic method with online quantification compared to a static-spectroscopic method. Both methods depend on photometrical absorption measurements of chelates dissolved in scC0 . 2

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52 To increase the understanding of factors determining the solubility of metal chelates in ScF we will report our concept and recent results. We discuss the influence of ligand structure modifications and the influence of the s c C 0 density on the solubility of the chosen chelates. 2

Methods


Apparatus for Dynamic Spectroscopic Online Measurements The center of the dynamic-spectroscopic apparatus consiste of a modified Suprex MPS 225 (Suprex Corp. Pittsburgh, PA.) system as shown in Figure 1.

Figure 1. Apparatus for online dynamic-spectroscopic measurement of chelate solubility

A 250 mL syringe pump is used to pressurize C 0 . A l l heated components, except the pneumatic driven valve and the capillary connected to the detector are placed in an oven. The pressure of the scC0 and the .temperature of the oven, detection cell and of the restriction system can be programmed via a control panel. The C 0 we used was heated in a 1.0 m preheating coil of 1.6 mm χ 762 μπι i.d. (1/16 in. χ 0,03 in.) stainless steel tubing and then passes through the saturation cell (V=l,027 mL, 14,5 mm χ 9,5 mm i.d. SFE cell, Keystone 2

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53 Scientific, Bellfonte, P A ) containing approx. 100 mg of the chelate. The saturation cell with the chelate was equilibrated to the desired temperature before C 0 was filled in. The preheating coil was necessary to ensure that the C 0 is at operating temperature prior to entering the saturation cell. After pressurizing the cell the pneumatic valve is switched to the "load" position where the ScF moves directly from the pump to the detector. In this position, the flow rate is adjusted to a value of 0.3 mL/min while the chelate dissolves in the fluid (see Figure 2). 2

2

ScF C 0 or HPLC pump


2

ScF C 0 or HPLC pump 2

saturation cell

saturation cell load

detector

flood

detector

Figure 2: Positions of the pneumatic valve. The dissolution of the chelates is accelerated by shaking the cell with a mixer motor (Cenco Intrumenten, Netherlands). After equilibration the valve is switched for 500 ms to a position where the saturation cell is flooded and an aliquot of the saturated solution is transferred to a "Spectra Focus" (Spectra Physics) forward scanning absorbance detector where the absorbance of the chelate aliquot is measured time-resolved (Figure 3). The flow rate, measured by an Aalborg mass flow meter is recorded in parallel with the signal of the U V detector. The spectra of five extractions were recorded and afterwards the chelate is eluted from the steel capillary and restrictor by pumping ethanol through it. The ethanol of the extract is removed and the chelate is treated with hydrochloric acid (1:5 v:v). This solution is analyzed with an ICP-OES (Leeman Labs, Inc.). To calibrate the system a correlation between known amounts of dissolved chelates and the area of the time resolved chelate absorption was derived. With the analyzed quantity of five extractions, the known switching time of valve and the measured flow rate, the concentration of the extracted solution can be calculated. This calculated concentration is set in relationship to the area under the spectra measured. Therefore a correlation between the area of the on line absorption area of the chelate aliquots and the solubility was found.



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Figure 3. Representative time-resolved absorption measurement of a Cu(thd)i aliquot passing the scanning Spectrafocus UV detector

Apparatus for Static Solubility measurements The second apparatus to measure chelate solubilities in s c C 0 used here already is described in the work of Carrott and Wai. Details to this method can be found there. (16) With this static method the U V absorption of a saturated chelate solution is measured directly in a capillary tubing connected to the fiber optic (Polymicro Technologies, Phoenix, Arizona) of a Cary I E U V - V I S Spektrometer (Varian Instruments, Sugarland, Texas). 2

Classification and Characterization of Chelates Preparation of metal complexes A l l metal beta-diketonates and methylglycolates were prepared according to methods described in the literature. (30-31) A detailed way to get long alkyl chained dtc of palladium and rhodium is described in the thesis of Ammann. (32) In Figure 4 the structure of the ligands is shown. In Table I the systematic names of dtc ligands we have chosen is given.


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thd

dtc

mtg

Figure 4. Structure of2,2,6,6-tetramethyl-3,5-heptanedion (thd), dialkyldithiocarbarnate (dtc) and methyiglycolate (mtg) ligands with R representing different alkyl chains


Table I. Dtc ligands used in this study

chemical name

formula [S CN(C H ) ]2

2

5

2

[S CN(C H ) y 2

3

7

2

[SjCNiQHeW[S CN(C H ) r 2

6

13

2

[S CN(C H ) ]2

8

17

2

Diethyldithiocarbamate (DEDTC) Diisopropyldithiocarbamate (DPDTC) Dibutyldithiocarbamate (DBDTC) Dihexyldithiocarbamate (DHDTC) Dioctyldithiocarbamate (DODTC)

A l l chelates have been characterized by means of elementary analysis and melting points (see Table II). Dtc chelates also have been characterized by F A B mass spectrometry and H - N M R . (32) The structures of the Pd(DPDTC) and of Pd(thd) were solved by x-ray diffraction measurements. Further information to crystal structures and spectroscopic data can be achieved from the authors. (5734). l

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Results of Dynamic Solubility Measurements

Solubility of Pd(thd) and Rh(thd) 2

3

The solubilities of chelates presented in this chapter have been measured with the dynamic spectroscopic method. In Figure 5 the solubility data of Pd(thd) and Rh(thd) measured with this system are presented. In this diagram the molar solubilities are plotted against the reduced density of the s c C 0 at a temperature of 70 °C. The solubility of Pd(thd) is approximatelya factor of 50 higher than the solubility of Rh(thd) . This is quite surprising because in general the solubility of chleates with coordination number Z=3 (for example Cr(thd) ) 2

3

2

2

3

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Table IL Melting points of dithiocarbamates with different metals *not determined

chelate Pd(DEDTC) Rh(DEDTC) Cu(DETDC)

2


3

Pd(DPDTC) Rh(DPDTC)

2

chelate

225-226 >235 189-190

Pd(DHDTC) Rh(DHDTC) Cu(DHTDC)

2

Pd(DBDTC) Rh(DBDTC) Cu(DBTDC)

65-66 176-177 49-56

2

3

>300

Pd(DODTC)

>235

Rh(DODTC)

not determined

Cu(DOTT)C)

2

3

Cu(DPTDC)

Melting point °C

Melting point "C

2

Solubility and Modifications of Metal Chelates in Supercritical Carbon

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