Solubility measurements of noble metal acetylacetonates in supercritical carbon dioxide by high performance liquid chromatography (HPLC)

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J. of Supercritical Fluids 44 (2008) 139–147

Solubility measurements of noble metal acetylacetonates in supercritical carbon dioxide by high performance liquid chromatography (HPLC) Satoshi Yoda a,∗ , Yoko Mizuno b , Takeshi Furuya a , Yoshihiro Takebayashi a , Katsuto Otake c , Tomoya Tsuji b , Toshihiko Hiaki b a

Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b College of Industrial Technology, Nihon University, 1-2-1 Izumicho, Narashino, Chiba 275-8575, Japan c Faculty of Engineering, Tokyo University of Science, 12-1 Ichigaya Funakawaramachi, Shinjuku-ku, Tokyo 162-0826, Japan Received 8 June 2007; received in revised form 5 November 2007; accepted 6 November 2007

Abstract A method has been developed for measuring the solubility of noble metal acetylacetonates by the direct injection of the supercritical carbon dioxide (scCO2 ) solution into a high pressure liquid chromatograph. We found the use of (1) reverse phase high pressure liquid chromatography (RP-HPLC) to separate the sample peak from the noise peak on chromatogram and (2) a scCO2 -philic eluent to eliminate the noise peak, to be effective. Using this improved method, we measured the solubilities of noble metal complexes of Pt, Pd, Ru, Rh and Ag acetylacetonates in scCO2 from 10 to 30 MPa at 313 K. Measurements were conducted with smaller amounts of samples (less than 0.1 g) and with shorter experimental times than by the standard dynamic flow method. The molar fraction y2 was of the order 10−5 to 10−4 for Ru and Rh acetylacetonate, 10−5 for Pd and Pt acetylacetonate, and 10−7 for Ag acetylacetonate. The solubility data for Pd, Pt, Ru, and Rh acetylacetonates were successfully correlated using the Chrastil model. © 2007 Elsevier B.V. All rights reserved. Keywords: Solubility; Noble metal acetylacetonates; Supercritical carbon dioxide; High performance liquid chromatography

1. Introduction The development of supercritical carbon dioxide (scCO2 ) technology has reached a practical stage. A number of authors have investigated the use of scCO2 as an alternative solvent for various applications, and have employed a variety of new substances in scCO2 systems. The availability of an appropriate solubility measurement technique is a key to scCO2 process design. For such measurements, ease of use, speed, and efficiency are the most important criteria. Conventional solubility measurements are not always sufficient for new substances and improvements of measurement techniques are required. Noble metal complexes that are soluble in scCO2 have been used for the fabrication of catalysts and nano-functional materials [1,2]. In these efforts, an important issue is how to control the

∗

Corresponding author. Tel.: +81 29 861 4656; fax: +81 29 861 4567. E-mail address: [email protected] (S. Yoda).

0896-8446/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2007.11.002

size and dispersion of the noble metal nanoparticles. These characteristics depend on events occurring during nucleation, which is closely related to a solubility change in the scCO2 process. Thus, the solubility of these complexes in scCO2 is important not only for the success of the process, but also for microstructure design. However, solubility data for noble metal complexes are scarce; in addition, the solubility data have to be obtained experimentally. Furthermore, difficulties exist in the measurements for noble metal complexes. Two major methods are frequently used for solubility measurements of solid substances in scCO2 : the dynamic flow method and the spectroscopic method. The dynamic flow method includes the extraction of the solute from the pressurized vessel, followed by a quantitative analysis such as weighing [3,4]. The dynamic flow method, however, has a couple of difficulties when employed with metal complexes. It requires a considerable amount of sample to achieve a solubility equilibrium before the analysis. This point can be a significant technical barrier for expensive solutes such as noble metal com-

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Nomenclature C2 M1 N S2 S2,calc T Vloop y2

moler concentration of component 2 molecular weight of component 1 (mol/L) Number of data experimental weight concentration of component 2 (g/L) calculated weight concentration of component 2 (g/L) temperature (K) Volume of sample loop on scCO2 system mole fraction solubility of component 2

Greek letter density of component 1 (g/L)

ρ1

plexes. Additionally, a blockage in the pipes of the extraction system frequently takes place during the measurement of the metal complex. Many metal complexes are sensitive, and easily decomposed to metal/metal oxide by hydrolysis, heating and irradiation. A careful operation is required to escape this problem. The spectroscopic method provides another approach to solubility measurements in this system [3,5], and works with both flow and batch systems. The spectroscopic method with a batch system is preferred for an expensive sample because the required sample amount is small. Since a batch spectroscopic measurement is conducted in a closed system, the hydrolysis of substances around the outlet can be avoided. However, when we use the spectroscopic method, we have to consider the change of molar adsorptivity (ε) and the spectrum shift occurring with the change of scCO2 density [5–7]. A few published reports mention how to avoid these difficulties [5,8]. However, we have to be careful when we apply the spectroscopic method to the solubility measurement of organic metal complexes, because adsorptivity and spectrum changes on such complexes may be more sensitive to solvent circumstances than those for organic compounds. Drastic changes in the spectrum due to removal or exchange of ligands are possible in metal complexes, and need to be considered during the measurements. Ashraf-Khorassani et al. [9] developed an useful method for solubility measurements using direct injection of a scCO2 solution into a high pressure liquid chromatography (HPLC) eluent. In this method, the substances employed should have a higher solubility in the HPLC eluent than in scCO2 , a condition that is fulfilled for most substances. We believe this method has many advantages, including the following. First, HPLC analysis is highly sensitive and needs a quite small amount of samples. Since the scCO2 solution is directly introduced into the pressurized HPLC eluent, fewer blockages in the piping is expected. All the injected scCO2 can be dissolved into the HPLC eluent before detection; thus, the scCO2 density and pressure basically do not affect the analysis—in other words, the analysis is independent of molar adsorptivity changes. If the substances have already been measured by HPLC, the experimental conditions can be considered as analytical conditions for the scCO2 system. Since

there are huge numbers of previous HPLC analyses, a variety of substances are suited to this method. In addition, it can be applied to mixtures, and can detect each substance separately. Thus, the HPLC method shows promise for monitoring scCO2 systems. An HPLC analysis, however, is subject to the problem of noise from CO2 bubbling. The pressure of the eluent for a conventional HPLC system is lower than that of scCO2 ; therefore, the injected scCO2 would bubble in the eluent. These bubbles cause noise on the chromatogram. In the case of a UV–vis detector, the noise is observed at all wavelengths, since the bubbles scatter the light. The use of a backpressure regulator on the HPLC channel is partially effective, but is not sufficient. In this work, we have used the HPLC method to conduct solubility measurement of precious metal acetylacetonates, including those of Pt, Pd, Ru, Rh and Ag, familiar to us from previous investigations of nano-functional materials made using these novel metals [10]. The ␤-diketonato complex is a popular metal chelate, soluble in scCO2 , and is employed in many applications. Due to the variety of central metals and analogous compounds with modified ligand structures, it is important to collect solubility data for many acetylacetonates for process control and to design suitable ligands for nano-material processing. The solubility measurements for the acetylacetonates could provide a check of the utility of the HPLC measurement system. Additionally, we have tried to develop an approach to eliminate the noise from the scCO2 injection. We used the Chrastil model [11] to evaluate the measured solubility data based on the relationship between solubility and scCO2 density. 2. Experimental 2.1. Materials Table 1 is the list of reagents used in this work. All reagents were purchased from Aldrich. They were used without further purification and were treated under an inert nitrogen atmosphere. HPLC grade methanol (MeOH, 99.8%, Wako), dichloroethane (CH2 Cl2 , 99.8%, Wako), hexane (C6 H12 , 99.8%, Wako) and acetonitrile (AN, 99.8%, Wako) and purified water were used as HPLC eluents. A CO2 sample of 99.995% purity (Showa Tansan) was used after dehydration by passing it through a zeolite column. 2.2. High-pressure equipment The high-pressure flow/circulation system for this work (Fig. 1) was designed with reference to Ashraf-Khorassani’s et al. [9]. The metal complex was placed in the small sample column (1 cm3 ) (8), along with 1 mm glass beads. Then, the scCO2 was loaded at the prescribed pressure (8–30 MPa) and temperature (313 K) by means of a high-pressure pump (1) and back pressure regulator (14). Next, valves 2 and 13 were closed, and the scCO2 was circulated with a pump (4) so that to achieve a solubility equilibrium. The system was heated with a silicone rubber heater and a temperature controller (Chino DB-1000). The temperature was measured around the sample loop (9) with an RTD probe and a digital thermometer (Advantest TR2114).

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Table 1 Reagents used in this work Reagents

Abbreviation

Purity (%)

Comments/reference

Anthracene Copper(II)(acetylacetonate) Palladium(II)(acetylacetonate) Platinum(II)(acetylacetonate) Rhodium(III)(acetylacetonate) Ruthenium(III)(acetylacetonate) Silver(I)(acetylacetonate)

Cu(II)(acac)2 Pd(II)(acac)2 Pt(II)(acac)2 Rh(III)(acac)3 Ru(III)(acac)3 Ag(I)(acac)

99.99 99.99 99.98 99.99 97 97 98

For verification of NP-HPLC [5,17,18] For verification of RP-HPLC [19] [10,20,21] [10,20]

The temperature was controlled within ±0.5 K. The pressure was measured with a pressure transducer (Kyowa Electric Instruments, PG-500KU) and an indicator (Kyowa, WGA-710A). The pressure transducer was factory-calibrated with an accuracy of ±0.2% of full scale. The equilibrium time before starting analysis had been optimized previously and was defined as 1 h in this study. We found that a longer equilibrium time did not yield significant differences in solubility. The equilibrium was checked by actual periodical analyses until the measured values were constant for three successive analyses. A small amount of the sample/scCO2 solution was directly injected into the HPLC eluent through the inner-volume sampler (2 ␮L, Valco) (9). Line (A) and (B) were used for washing out the remaining HPLC eluent in both the sample loop and piping 9–11 after each injection. The scCO2 system was flushed out whenever the experimental conditions were changed.

2.3. HPLC setup and analysis The HPLC system was made up of a degasser (Shimazu DGU-12A), a high-pressure HPLC pump (Shimazu LC-10AD), and a manual injector for the reference to construct a calibration curve (Rheodyne 7000 with 2 ␮L sample loop), a column oven and UV–vis detector (Shimazu SPD-10A). In some measurements, a multi-channel spectrometer (JASCO MD-910) was used. A calibration curve for each sample dissolved in the eluent was established before starting analysis with the scCO2 solution. The injected sample/scCO2 solution was mixed and dissolved into the HPLC eluent, and then detected at the detector after passing through the analytical column. The optimized HPLC eluents, analytical columns and conditions for each sample are summarized in Table 2. Both normal phase HPLC (NP-HPLC) and reverse phase HPLC (RP-HPLC) were used for this work. The measurements were taken at least five times for each point, and the average after removing the maximum and minimum values was used for the solubility calculations. The following equation, including the molar concentration of the sample C2 derived from the analytical curve on the HPLC, defines the solubility y2 : y2 =

Fig. 1. Schematic diagram of the scCO2 system in this work. (1) CO2 pump, (2) CO2 inlet valve, (3) valve for circulation loop, (4) circulation pump, (5) heating loop, (6) pressure gauge, (7) 6-port valve for sample column, (8) sample column, (9) 6-port valve with sample loop (2 ␮L), (10) and (11) valve for washing, (12) 6-port valve for washing, (13) CO2 outlet valve, (14) back pressure regulator, (15) constant temperature chamber. The pipework from A to B is for washing the sample loop.

C2 Vloop C2 = (ρ1 /M1 )Vloop + C2 Vloop ρ1 /M1 + C2

(1)

Here, Vloop is the volume of the sample loop, ρ1 the density of the scCO2 at the measurement temperature and pressure, and M1 is the molecular weight of CO2 . Because the solubility of the metal complexes in this work is not high (y2,exp < 10−3 ), we assumed that the volume of the solute in the sample loop was negligible. The density of CO2 , ρ1 , was calculated from the equation of state developed by Span and Wagner [12]. They reported uncertainties of between ±0.03 and ±0.05% in the density up to 30 MPa and temperatures of 523 K. These uncertainties are considerably smaller than the reproducibility of an HPLC measurement; thus, the errors in y2 in this work are the same as those in the reproducibility of a C2 analysis. The relative standard deviation (R.S.D.) of y2 for each result is shown in Table 3. 2.4. Solubility correlations Correlation of the solubility data were conducted with the Chrastil model [11]. This correlation has been recognized as

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Table 2 Analytical conditions for HPLC Reagents

Phase

Analytical condition Eluent

Anthrathene Cu(II)(acac)2 Pd(II)(acac)2

NP RP NP RP NP RP RP NP

Pt(II)(acac)2 Rh(III)(acac)3 Ru(III)(acac)3 Ag(I)(acac)

Column

Solvent

Comp. (vol. ratio)

CH2 Cl2 /MeOH/hexane AN CH2 Cl2 /MeOH/hexane AN/H2 O CH2 Cl2 /MeOH/hexane AN/H2 O AN/H2 O CH2 Cl2 /MeOH/hexane

45/45/10 100 45/45/10 50/50 45/45/10 50/50 50/50 45/45/10

Table 3 Solubility of metal acetylacetonate (2) in supercritical carbon dioxide (1) at 313 K P (MPa) Pd(II)(acac)2 (NP)

y2 × 107

R.S.D. (%)

10.5 13.0 15.8 18.0 20.5 23.1 25.4 27.6

93.0 213 241 294 453 469 472 473

4.7 3.8 1.2 0.8 3.9 0.1 3.7 3.9

Pd(II)(acac)2 (RP)

10.0 12.5 15.0 17.5 20.0 25.0 30.0

64.5 240 297 336 424 495 575

4.3 1.4 7.5 3.6 3.8 7.1 4.9

Pt(II)(acac)2

10.5 13.1 15.5 17.5 20.7 24.7 29.0

163 218 246 295 379 376 371

2.7 0.4 1.7 0.6 3.9 1.2 1.9

Rh(III)(acac)3

10.0 15.0 20.0 24.9 29.8

332 626 808 959 1030

16.0 2.3 4.5 0.3 2.2

10.0 15.0 20.0 24.5 30.0

170 591 781 921 950

2.6 4.9 0.1 0.7 0.5

Ru(III)(acac)3

Ag(I)(acac)

10.3 15.0 20.2 25.0 30.0

0.89 1.20 3.62 3.16 3.46

51.2 16.1 5.0 3.9 3.0

Silica (InertSil 150A-5) ODS(InertSil ODS-3V) Silica (InertSil 150A-5) ODS(InertSil ODS-3V) Silica (InertSil 150A-5) ODS(InertSil ODS-3V) ODS(InertSil ODS-3V) Silica (InertSil 150A-5)

an effective method for many solutes in supercritical carbon dioxide. It is based on the hypothesis that one molecule of a solute A associates with k molecules of a solvent B to form one molecule of a solvato-complex ABk in equilibrium with the system. The definition of the equilibrium constant in terms of thermodynamic considerations leads to the following expression for the solubility:  ln S2 = k ln ρ1 +

a T +b

 (2)

where S2 is the weight concentration of the solute in the supercritical phase; ρ1 the density of the scCO2 solution, and k the association number; a the depends on the heat of solvation and the heat of vaporization of the solute; and b is the depends on the molecular weight of the species. Parameters k, a and b are adjusted to conform to experimental solubility data. As the solubility of metal compounds in this work is low, we assume that the effect of solute solid on the scCO2 solution density is negligible, and that the density of the solution is the same as that of pure scCO2 . Using the experimental data, S2 and ρ1 , we optimized k and (a/T + b) in Eq. (1) to minimize the error. The quality of all of the data correlation is quantified by the average absolute relative deviation (AARD), defined as follows:   1  N  S2 − S2,calc  AARD = (3)   × 100 N S2 where N is the number of data points, and S2,calc is the calculated solubilities used. 3. Results and discussions 3.1. Noise peak from scCO2 bubbling Fig. 2(a) shows a typical chromatogram when scCO2 (without any solute) is injected to the HPLC system. The height/area of this noise peak depend on both the composition of the eluent and the pressure of scCO2 . The noise peak can overlap the sample peak, especially using NP-HPLC due to its poor peak separation performance.

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[14]. We assume that dichloromethane could have a kinetic difficulty in mixing with scCO2 due to its high density and viscosity. We therefore assume that a solvent with both a high affinity for scCO2 and with a similar density/viscosity would effectively suppress the noise. This concept would be helpful for measuring new substances. If the solute has already been investigated by HPLC, it is then easy to modify the eluent for the scCO2 –HPLC system. The effect of scCO2 noise on RP-HPLC is generally larger than that in NP-HPLC due to a lower affinity of scCO2 for the hydrophilic eluent. However, the effect of noise can be eliminated by optimization of the peak separation in many cases. Fig. 2(c) shows an example. RP-HPLC has been used for analysis of metal complexes including acetylacetonates [16], but some metal complexes are easily hydrolyzed or decomposed in the hydrophilic solution. Thus using a combination of (1) NPHPLC with a scCO2 -philic additional solvent and (2) RP-HPLC can cover a variety of metal complexes. In the following work we chose the method most suitable depending on nature of the complexes.

Fig. 2. Effect of scCO2 injection on chromatogram and its avoidance. (a) Typical chromatograph of scCO2 using NP-HPLC (CH2 Cl2 and MeOH eluent). (b) Typical NP-HPLC chromatogram with a scCO2 -philic solvent (hexane). (c) Typical RP-HPLC chromatogram. All chromatograms were taken for a Pd(II)(acac)2 sample at 313 K and a scCO2 pressure of 20 MPa.

We found that the addition of a solvent with a high affinity to scCO2 to the eluent was effective to decrease the noise. Fig. 2(b) shows typical chromatogram of such a measurement. Adding hexane to the eluent can depress the scCO2 peak until it almost disappears with little effect on the peak shape and retention time. A similar effect was observed when AN was added to an ethanol-based eluent. This was probably due to repression of the scCO2 bubbling and to improved dissolution in the eluent thanks to the increased affinity of the solvent for scCO2 . The phase diagram of the binary systems CO2 and hexane [13] and CO2 and AN [14] indicates a higher affinity of these solvents for CO2 than methanol [15] and ethanol [15]. However, dichloromethane, which does not effectively eliminate the noise (see Fig. 2(a)), shows a phase diagram similar to that of hexane

3.2. Verification of the measurement system Verification of the measurement process was conducted by measuring the solubility of anthracene for NP-HPLC and Cu(II)(acac)2 for RP-HPLC at 313 K, 8–30 MPa. Solubility data at the same temperature by Ngo et al. [5] Hampson [17] and Anitescu and Tavlarides for anthracene [18] and by Lagalante et al. for Cu(II)(acac)2 [19] were used as references. Fig. 3 shows the results. The datum for Cu(II)(acac)2 taken by extraction with a similar experimental setup to our previous work [20] is also shown. The HPLC results for anthracene confirm that our system provides reasonable solubility data. The HPLC results for Cu(II)(acac)2 are also closely in accord with those previously reported. These data had been collected within 2 days for 7 points of each sample. The amount of sample in the column was 0.1 g for both samples and it covered all the measurement. The initial sample amount was fixed at 0.1 g for the following measurements.

Fig. 3. Verification of the measurement system: solubility y2 of (a) anthracene and (b) Cu(II)(acac)2 in scCO2 at 313 K, 8–30 MPa. Reference data were taken from Refs. [5,17,18] for (a) and Ref. [19] for (b).

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Fig. 4. (a) Typical chromatogram of Pd(II)(acac)2 using NP-HPLC (CH2 Cl2 /MeOH/hexane (45:45:10)). A chromatogram used to make calibration curve (without scCO2 ) is shown as well for comparison. (b) Typical chromatogram of Pd(II)(acac)2 using RP-HPLC(AN/H2 O (50:50)). (c) Solubility of Pd(II)(acac)2 obtained here compared with data obtained in our previous work by dynamic flow method (Tsuruta et al.) [20] and in literature [21].

Fig. 5. (a) Typical chromatogram of Pt(II)(acac)2 using NP-HPLC (CH2 Cl2 /MeOH/hexane (45:45:10)). (b) Typical chromatogram of Pt(II)(acac)2 using RPHPLC(AN/H2 O (50:50)). (c) Solubility of Pt(II)(acac)2 obtained here compared with data obtained in our previous work by dynamic flow method (Tsuruta et al.) [20].

3.3. Solubility of noble metal acetylacetonates Solubility data obtained in this work are summarized in Table 3 together with the R.S.D. of the results. 3.3.1. Palladium(II) and platinum(II) acetylacetonates Fig. 4(a) and (b) shows a typical chromatogram for Pd(II)(acac)2 . The sample peak was clearly observed on both NP- and RP-HPLC without being disturbed by the scCO2 noise peak. The relationship between solubility and pressure is shown in Fig. 4(c). The data measured by extraction and weighing in our previous work by Tsuruta et al. [20], and recent data by Aschenbrenner et al. [21] are also shown. The y2 value is located between y2 = 10−5 and 10−4 and it is of the same order as that

obtained by extraction. At lower pressures the HPLC data show a difference of about 12–30% between the NP and RP results, but they agreed with each other at higher pressures. Fig. 5(a) and (b) shows typical chromatograms for Pt(II)(acac)2 . The Pt(II)(acac)2 is difficult to measure by RPHPLC because the sample and scCO2 peak overlap even when the analytical conditions are optimized. Thus, NP-HPLC was used in this case. Fig. 5(c) shows the solubility results. The data shows good agreement with those from extraction and weighing [20]. 3.3.2. Rhodium(III) and ruthenium(III) acetylacetonates These trivalent acetylacetonates were measured by RPHPLC. Fig. 6(a) and (b) shows typical chromatograms and

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Fig. 6. (a) Typical chromatogram of Ru(III)(acac)3 using RP-HPLC(AN/H2 O (50:50)) (b) Typical chromatogram of Rh(III)(acac)3 using RP-HPLC(AN/H2 O (50:50)). (c) Solubility y2 of Ru(III)(acac)3 and Rh(III)(acac)3 .

Fig. 7. (a) Typical chromatogram of Ag(I)(acac) using NP-HPLC(CH2 Cl2 /MeOH/hexane (45:45:10)) (b) Solubility y2 of Ag(I)(acac) obtained by HPLC with data obtained by extraction (at 30 MPa only).

Fig. 6(c) shows the pressure dependence of the solubilities. The peak separation was good for both samples. The solubility y2 of these acetylacetonates were located between y2 = 10−5 and 10−4 , larger values than those of divalent metal compounds. Larger differences in retention times than those seen with metal acetylacetonates and some tailing on the peak even after optimization of the conditions were observed.

tion with similar experimental procedure to our previous work [20] is also shown. The solubility y2 was located in the 10−7 to 10−6 region. Even though the error in the data is higher than in the case of di- or trivalent acetylacetonates, the signal peak was large enough to detect. Since HPLC is well suited for the analysis of low-concentration substances, our system will be useful for substances with quite low solubility.

3.3.3. Silver(I) acetylacetonate Only one report of the solubility of a monovalent metal acetylacetonate (lithium (I) acetylacetonate) by Saito et al. [22] exists and the solubility quoted was quite low. We tried to evaluate the solubility of Ag(I)(acac). This compound was reported to decompose during extraction [21] but we were able to measure the solubility successfully. In this case NP-HPLC was effective. Fig. 7 shows a typical chromatogram and the solubility of Ag(I)(acac). The solubility datum (at 30 MPa only) by extrac-

3.4. Correlations and comparison with other acetylacetonates The results of solubility data correlations are shown in Fig. 8. They are categorized by the valence of the central metal. Solubility data of acetylacetonates found in the literature, namely for iron(III) [23,24], chromium(III) [19], copper(II) [19], nickel(II) [21] at 313 K and lithium(I) at 333 K [22] are plotted for reference. The AARD values and the solubility constants k and

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Fig. 8. Results of correlation of solubility data for noble metal acetylacetonates. Correlation curve is shown as a solid line. Solubility data of other acetylacetonates found in the literature; Fe(III) [23,24], Cr(III) [19], Cu(II) [19], Ni(II) [21] at 313 K and Li(I) at 333 K [22] are also shown.

(a + T/b) in Eq. (2) are listed in Table 4. A linear relationship of the ln ρ − ln S plot was observed for all samples in this work, as has been found for other acetylacetonates. The correlation curve and the measured data showed good agreement except for Ag(I)(acac). At lower pressures and at lower concentrations of solute, the larger error could be included, probably due to the sensitivity the scCO2 density to changes in pressure. The solubility of noble metal acetylacetonates depended on the valence of the central metal. The Rh(III)(acac)2 and Ru(III)(acac)2 compounds exhibit higher solubilities than Pd(II)(acac)2 and Pt(II)(acac)2 . The monovalent Ag(I)(acac) shows quite low solubility. As mentioned in the literature [22,25], the solubility mainly depends on how well the central metal ion is protected by its ligands. Our results followed that

Table 4 Average absolute relative deviation (AARD) and solubility constants of constants of noble metal acetylacetonates at 313 K in the Eq. (2) ln S = k ln ρ + (a + T/b) Acetylacetonate

AARD (%)

Number of data

k

(a + T/b)

Pd(II)(acac)2 (NP) Pd(II)(acac)2 (RP) Pt(II)(acac)2 Rh(III)(acac)3 Ru(III)(acac)3 Ag(I)(acac)

7.5 10.8 7.0 2.0 8.4 19.6

8 7 7 5 5 5

6.5 6.7 4.0 4.1 5.9 5.5

−45.3 −46.6 −28.1 −28.2 −39.9 −43.8

trend. The Ag(I)(acac) shows the lowest solubility ever reported for an acetylacetonate, presumably since the coverage of the central metal by a single ligand is insufficient. These solubility data and the k values are located in a reasonable range compared with those of other acetylacetonates in the literature. Differences of solubility and the k value within the same valence group are clearly observed (e.g. lower solubility of Cu(II)(acac)2 than other divalent metal group) but the responsible parameters (possibly element number or molecular weight) have not yet been identified. Additional solubility data of other metal acetylacetonates and further investigation of the molecular structures are needed. 4. Conclusions Solubility measurements of Pd, Pt, Rh, Ru and Ag acetylacetonates using a combination of a high-pressure system and HPLC were investigated at 313 K and 10–30 MPa. The HPLC system was optimized to minimize the noise from scCO2 injection, and their effectiveness can be traced to (1) the use of additional solvent with a high affinity for scCO2 for NP-HPLC and (2) the use of RP-HPLC. Solubility measurements could be conducted with shorter experimental times and smaller amounts of samples (less than 0.1 g) than by the standard dynamic flow method. The solubility data measured here shows good agreement with those found by the dynamic flow method. The solubility y2 depended on the valence of the central metal and was of the order 10−5

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to 10−4 for Ru(III)(acac)3 and Rh(III)(acac)3 , and 10−5 for Pd(II)(acac)2 and Pt(II)(acac)2 , and 10−7 for Ag(I)(acac). Data correlation was conducted using the Chrastil model and good agreements were confirmed except for Ag(I)(acac). These solubility measurements are easier and faster than the dynamic flow measurement and are promising for the screening of reagents for nano-material processing. We are aware of the problem that the variability between measurements of this work is higher than that of the HPLC analysis, probably caused by pressure instability in the small scCO2 circulation loop. Further improvements to create a useful solubility measurement system are under investigation. Acknowledgements This work has been supported by New Energy and Industrial Technology Development Organization (NEDO), Japan. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.supflu.2007.11.002. References [1] Y. Zhang, C. Erkey, Preparation of supported metallic nanoparticles using supercritical fluids: a review, J. Supercrit. Fluids 38 (2006) 252–267. [2] F. Cansell, C. Aymonier, A. Loppinet-Serani, Review on materials science and supercritical fluids, Curr. Opin. Solid State Mater. Sci. 7 (2003) 331–340. [3] R. Dohrn, G. Brunner, High-pressure fluid-phase equilibria: experimental methods and systems investigated (1988–1993), Fluid Phase Equilibr. 106 (1995) 213–282. [4] F.P. Lucien, N.R. Foster, Solubilities of solid mixtures in supercritical carbon dioxide: a review, J. Supercrit. Fluids 17 (2000) 111–134. [5] T.T. Ngo, D. Bush, C.A. Eckert, C.L. Liotta, Spectroscopic measurement of solid solubility in supercritical fluids, AIChE J. 47 (2001) 2566–2572. [6] J.K. Rice, E.D. Niemeyer, F.V. Bright, Evidence for density-dependent changes in solute molar absorptivities in supercritical CO2 : impact on solubility determination practices, Anal. Chem. 67 (1995) 4354–4357. [7] H. Inomata, Y. Yagi, M. Saito, S. Saito, Density dependence of the molar absorption coefficient – application of the beer-lambert law to supercritical CO2 – naphthalene mixture, J. Supercrit. Fluids 6 (1993) 237–240. [8] T. Sako, T. Kunie, S. Takahashi, K. Moribe, Solubility measurement in supercritical CO2 using high-pressure UV/visible adsorption spectroscopy., in: Proceedings of the International Symposium on Supercritical Fluids, 2006. [9] M. Ashraf-Khorassani, M.T. Combs, L.T. Taylor, Solubility of metal chelates and their extraction from an aqueous solution via supercritical CO2 , Talanta 44 (1997) 755–763.

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