Hydrogen in Organic Solvents - ACS Publications - American

Article pubs.acs.org/jced

Determination of Gas Concentrations in Liquids by Nuclear Magnetic Resonance: Hydrogen in Organic Solvents Mario Baseda Krüger, Carmen Selle, Detlef Heller, and Wolfgang Baumann* Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Straße 29A, 18059 Rostock, Germany ABSTRACT: An important parameter for the calculating of rate constants or for process enhancement, both for sciences and engineering, is the knowledge of gas concentrations of gas−liquid systems. We introduce the method SelPULCON for the determination of gas concentrations in liquids for NMR active substances (e.g., hydrogen) by means of NMR spectroscopy. This method is based on the “principle of reciprocity” and correlates the absolute signal intensity of the sample with that of an external reference. As a selective excitation is applied, this method is independent of whether the solvent is deuterated or not. In this way one can measure the gas concentration in almost all solutions and utilize all of the advantages of the NMR spectroscopy (e.g., for in situ or OPERANDO investigations). To demonstrate this method, we determined the hydrogen saturation concentrations in eight organic solvents at 25 °C and 0.101 MPa total pressure, for example, in dichloromethane (c = 1.51 mmol·dm−3, b = 1.15 mmol·kg−1) and in 2,2,2-trifluoroethanol (c = 3.28 mmol·dm−3, b = 2.39 mmol·kg−1).

■

■

INTRODUCTION The knowledge of gas concentrations, for example, hydrogen, in liquids is very important, for example for the interpretation of the kinetics of reactions where gases are reactants, for the calculating of rate constants, and also for process optimizations in the industry. Usually, saturation concentrations (solubilities) are determined by indirect methods based on changes of pressure or volume. But such methods are associated with great experimental effort. This is one reason for gaps in the gas solubilities database,1 for example of hydrogen solubilities in halogenated or deuterated solvents. A further disadvantage of these indirect methods is that they are impractical for gas concentration determinations in reaction solutions during a reaction (in situ). In recent years, quantification by nuclear magnetic resonance (qNMR) is becoming more and more popular and favorite,2 because a qNMR measurement is in principle simple and also has the beneficial property that the signal strength is direct proportional to the number of nuclei in the considered volume element contributing to the signal, so that quantification by NMR automatically delivers the molar concentration, which directly can be incorporated in kinetic calculations. Therefore, the aim of this work was to develop a method to simplify the measuring of gas concentrations in solvents/solutions by the use of NMR methods, which allows to quantify gases in a simple, fast, and reliable way, even in reaction solutions in situ and also without deuterated solvents. With this method we determine the H 2 saturation concentrations (solubilities) in methanol, benzene, methylbenzene, dichloromethane, 1,2-dichloroethane, 2,2,2-trifluoroethanol, propan-2-ol, and ethanol at 25 °C and 0.101 MPa total pressure and compare these, if available, to published data. © 2012 American Chemical Society

METHOD DEVELOPMENT Hydrogen in thermal equilibrium consists of ortho- and parahydrogen (two nuclear spin isomers in a ratio of 75:25 with different total spin), of which only ortho-hydrogen is NMRactive. However, this has no effect on the NMR measurement itself (as long as you measure “thermal hydrogen”), because quantum-mechanical calculations from Canet et al.3 show that the intensity of the detected hydrogen signal represents the entire hydrogen. Furthermore, quantification by NMR is, like many other methods, a relative procedure and therefore needs a reference with known concentration. Thus, the way of referencing becomes important. Adding a reference compound (internal standard) to the sample could influence the gas solubility and thus falsify the result. Alternatively, if the reference is enclosed in a capillary tube inside the sample, this influence can be avoided, but this might either affect the magnetic field homogeneity in the transverse plane and thus the quality of the spectra, or it might lead to small volume variations.4 Such effects would produce additional errors. Quite apart of that, it is often difficult to find a reference compound that resonates in an empty region of the spectrum. For that reasons, a method published by Wider and Dreier as PULCON5 seemed very suitable. This is based on the determination of absolute signal intensities (principle of reciprocity) by using external standards of known concentrations, that is, a sequential rather than a simultaneous measurement of sample and reference. Thus, we obtain the unknown concentration cU with the equation Received: January 6, 2012 Accepted: April 27, 2012 Published: May 11, 2012 1737

dx.doi.org/10.1021/je2013582 | J. Chem. Eng. Data 2012, 57, 1737−1744

Journal of Chemical & Engineering Data

Article

Figure 1. Saturated solution of hydrogen in methanol: A, spectrum acquired with unselective (rectangular) pulses. B, spectrum acquired with shaped pulses selective for the hydrogen resonance.

Figure 2. Preprocessing steps (y axis: arbitrary units). Left column: saturated solution of hydrogen in methanol. Right column: methanol without hydrogen. A, Fourier transform 1H NMR spectrum after multiplication of the FID with an exponential function with a line broadening factor of 1 Hz. B, phase-corrected 1H NMR spectrum. C, (partial) baseline correction. The rungs in the left spectrum are marked. D, signal integrated according to the procedure described in the text. ° SUTU Θ360 U nR

(1)

parameter cR is equivalent to the known concentration of a reference sample, the parameter S stands for the observed signal

where f stands for a factor encompassing variations in signal intensities if different experimental schemes are used for the measurements of the samples U and the references R. The

strength, T for the sample temperature in Kelvin, Θ360 ° for the duration of the 360° rf pulse, and n for the number of transients.

c U = fc R

° SR TR Θ360 R nU

1738



Article

weak gas signal with a moderate signal-to-noise ratio (S/N). According to a publication from Rischel about the fundamentals of peak integration,7 the accuracy of signal integrals can be improved by considering the S/N and setting of optimal integration limits, because, within a peak, the S/N as a function of integration limits passes through a maximum. This is beneficial mainly for weak signals, like our hydrogen signals. The NMR signals usually exhibit Lorentzian bandshapes, so the best S/N can be obtained by setting the integration limits at ± 1.392β, with the line width β, which corresponds to 60.3 % of the total peak volume with the most effective information.7 The resulting integral must, for the concentration calculation, be correlated with the signal strength of an external reference, which therefore has to be integrated completely analogously. In our investigation, all integrations were done without any further correction for slope and bias. Now, we demonstrate the preprocessing for a selected sample, keeping in mind that, however, this procedure is software-specific. In Figure 2 you can see an example spectrum for every step. The left column shows spectra of a saturated solution of hydrogen in methanol and the right column spectra of pure methanol (the latter prove that with our method it is indeed possible to generate spectra completely devoid of baseline distortion): (a) The raw data must be Fourier-transformed, and it is advisible here to combine this step with a multiplication of the raw data with a decaying exponential function with appropriate line broadening factor. After that, your 1H NMR spectrum may look like Figure 2A. (b) Then you must accurately correct the signal phase (Figure 2B). (c) and also the baseline (Figure 2C). As only the close environment of the hydrogen signal is taken into account for the evaluation, not the baseline of the entire spectrum must be corrected. You can see the limits of the actually corrected region as little “rungs” in Figure 2C (marked with cycles and arrows). Both phase and baseline correction must be done with utmost care, because they have a major impact on the integral value.2 (d) The last step is the signal integration. As described above, it is very advantageous to integrate only 60.3 % of the total peak volume. So you determine the peak maximum and the line width β of the signal (for the sample in Figure 2D left of the maximum is at 4.587 ppm, and the width at half height is 0.0038 ppm; for the spectrum at the right we use the same parameters). The integration limits then result from the signaling center ± 1.392β, the factor specified by Rischel.7 Remember that it is mandatory to use absolute integral values as they are provided by the software (of course this integral has to be scaled by the number of chemically equivalent nuclei in the molecule that contribute to the integrated signal). The accuracy of the calculated gas concentration is essentially dependent on the used references with their uncertainties. Therefore, we prepared four reference solutions consisting of maleic acid ((2Z)-but-2-enedioic acid) in D2O or DMSO-d6 (dimethyl sulfoxide-d6), respectively. Maleic acid is recommended as a primary standard for quantitative NMR, also by commercial suppliers, despite the fact that its aqueous solutions are not stable long-term8 and must therefore be measured quickly. Table 2 shows an overview of the purity, conditioning, and supplier. The concentrations were obtained gravimetrically; the density of the respective pure solvent was used for

Preferentially, experimental schemes for all measurements should be the same. Then (if for samples and references onepulse experiments with the signal acquisition immediately following the rf pulse, no presaturation and identical parameters, in particular receiver gain, are used) the factor f is equal to unity, and eq 1 can be simplified to cU = cR

° SU Θ360 U ° SR Θ360 R

(2)

An important aspect of our work was that the gas concentration determinations should be in step with actual chemical/preparative practice, and the solvents must be undeuterated. This approach also eliminates any influence of the isotopes on the gas solubilities. But this means for the NMR measurements that one can expect weak analyte and strong solvent signals, which will lead to distortion in the acquired spectra and make a meaningful integration of the H2 signal impossible (Figure 1A). For other spectroscopic methods, a common workaround for that problem is to record a reference spectrum with zero analyte concentration and to consider this as “background” which is subtracted from the spectrum of interest. This approach is not feasible for NMR due to the extreme sensitivity of band shapes and positions on sample composition and minute variations in experimental conditions (see ref 6, for example) which preclude such a subtraction. To solve this problem, we use selective pulses that are specific for the H2 resonance in the solution rather than rectangular pulses as described.5 This means that the H2 signals now become strong compared to the solvents' signals, like Figure 1B shows. But the concentration calculation (eq 2) requires now that the nucleus excitation of the H2 and the reference is done the same way. Therefore, the references must also be measured with selective pulses. For the differentiation to the primary method we call this SelPULCON (Selective Pulse Length based Concentration determination). So, based on the PULCON5 method, we propose the procedure SelPULCON as follows. Carrying out the measurement itself is, however, device- and software-dependent: (1) Lock (only if deuterium is present) and shim the sample. (2) Tune the probe to the basic transmitter frequency of the spectrometer and to the impedance of minimal reflected power. (3) Measure the exact 360° pulse length for the unselective pulse (this value physically characterizes the system spectrometer/probe/sample and will be entered in eq 1/ 2) and determine the resonance frequency of interest. (4) Measure the power level for selective pulse with appropriate shape and pulse length for the specific signal (on-resonance). (5) Measure the spectrum with the appropriate selective pulse; a signal-to-noise ratio of at least 100 is desirable. (6) Process (EM with appropriate factor, FT), phase- and baseline-correct, and integrate the signal. (7) Get stored reference 360° pulse length and reference integral, which was measured and determined from the reference sample before in the same way (steps 1 to 6). (8) Calculate the gas concentration in the solution using eq 1 or 2, as appropriate. In case of hydrogen the concentrations of the gas in solutions are very low. Only the usage of selective pulses makes a meaningful integration possible; nevertheless, we can expect a 1739



Article

Table 1. Overview of the Important Data for the Reference Solutions

a

According to the manufacturer. bWeast, R. C. CRC Handbook of Chemistry and Physics, 58th ed.; CRC Press: Cleveland, 1977−1978. cMass determination by using Mettler Toledo AT261 DeltaRange; the specified error was determined by “Type B evaluation” in virtue of the manufacturer’s data.

Table 2. Overview of Supplier, Purity, and Conditioning of All Used Chemicals

a

Purity in mass fraction according to the manufacturer’s instructions. The method itself provides a control option on the purity of the used solvent/ solution which will be indicated by impurity peaks at the NMR spectrum. Regarding the used solvents, no impurity peaks were found in the unselective proton spectrum.

calculations. Practical tests with concentrations exceeding those of the reference solutions have shown that the density difference was below 1 %. Nevertheless, at the possible sources of error, we calculated, besides the mass uncertainty, with a density error of 1 %, to reflect this. Table 1 compiles all reference values inclusive of uncertainties. The possible error of the calculated gas concentration was determined by the propagation of uncertainty (all calculated uncertainties on a confidence level of 95 %) and contains the errors of the reference concentrations (about 1 % of each

concentration), the precision of the measurements determined from the standard deviation (for sample and reference, see below), and the uncertainty of the measuring volume caused by using different individuals of the sample tubes (although of the same type). The latter one was determined with 1.1 %. Now, each sample was measured 10 times by NMR, and the calculated error for each gas sample was, independently from the used reference, always about 3 %. The temperature at the sample preparation has an error of ± 1 °C and was not considered in the calculated uncertainty. 1740



Article

the acquisition time and the dwell time were always the same at 6.84 s and 208.8 μs, respectively, the same shape for the selective pulse (Gauss with the size of 1024 data points and with a truncation of 1 %), and a constant pulse length for the selective pulse of 80 ms. The acquisition (except for the individually determined 90° pulses and the actual frequency for the selective pulses with their corresponding power levels) and processing parameters for the reference and the “gas” samples were the same. Table 3 shows a listing of longitudinal nuclear spin relaxation times T1 of maleic acid (CH proton) and hydrogen in the

For every system (solvent), at least three solutions/samples were prepared independently, and 10 spectra were taken for every sample, which were processed and integrated as described below. The 10 integral values were then combined to their mean value (with its associated standard deviation), and c(H2) was calculated using eq 2 with all four reference values; therefore we got four concentration values per sample, each with its uncertainty of about 3 %. By combining these four single values, we determined the concentration in each sample by calculating the arithmetic mean and its corresponding uncertainty (by propagation of uncertainty it is reduced to about 2 %). So we get three or more (depending on the number of samples) concentration ranges per solvent.

■

Table 3. Longitudinal Relaxation Time of Maleic Acid and Hydrogen in Different Solvents

MATERIALS, PROCEDURE, AND PARAMETERS Table 2 gives an overview in terms of the supplier, purity, and conditioning. Essentially, all solvents for the determination of hydrogen concentrations were kept under anaerobic conditions, that is, under inert gas argon (to exclude possible oxidations) and without H2O. The samples with the hydrogen solutions were always saturated. To prepare such saturated solutions, we use a Schlenk vessel with a magnetic stirrer bar and with a special nozzle for the NMR tube (equipped with a J. Young valve) at the top. This apparatus with the J. Young tube was connected to the gas distributor and a bubble counter and was six times evacuated/purged with hydrogen. Four mL of the undeuterated solvent as well as 0.2 mL of the deuterated solvent was in the apparatus injected by using a syringe. As stated above, for the measurement itself a deuterated solvent is not essential, but we used this small amount for a more convenient operation by locking the sample. The solvent sample was degassed by three freeze−pump−thaw cycles. After the solvent was liquid again, hydrogen was pressurized (about 0.15 MPa), and the apparatus was thermally equilibrated in a (25 ± 1) °C thermostat. Then the magnetic stirrer was switched on. The bubble counter was opened (this causes pressure compensation against ambient pressure) while still stirring the sample for about 0.50 h. We did not note any correlation between observed variation of ambient pressure (in the order of 1.5 %) and detected hydrogen concentration because this is within the uncertainty of the method and thus negligible. Preliminary tests with methanol and saturation times of (0.25, 0.50, and 1.00) h showed that already after 0.25 h stirring the solution was saturated; nevertheless every solution was magnetically stirred for about half an hour. The bubble counter and Schlenk were closed, and the hydrogen-saturated solution was transferred (within the thermostat) into the J. Young tube. This tube was filled almost completely so that no gas phase is present. At least, the J. Young tube was closed and dismounted from the Schlenk vessel and was ready for the NMR measurement. All NMR measurements were done in 5 mm precision J. Young NMR tubes (528-JY-7, Wilmad Labglass, USA) on a BRUKER Avance 400 spectrometer using a Quattro Nucleus [1H, 13C, 31P, 29Si] probe, which was properly tuned and matched each time. The integrals of the different spectra were measured and calibrated using standard NMR software (i.e., Topspin version 1.3, BRUKER, Germany). For both unselective and selective pulses we used standard pulse programs from BRUKER. The following parameters were used for all measurements: time domain size 32 768 data points, receiver gain 2, scan number 32, measuring temperature (25.20 ± 0.05) °C, sweep width 2394.64 Hz which means that

solution

T1/s

reference

maleic acid in D2O maleic acid in DMSO-d6 hydrogen in methanol-d4 hydrogen in benzene hydrogen in benzene-d6 hydrogen in methylbenzene-d8 hydrogen in dichloromethane hydrogen in 2,2,2-trifluoroethanol hydrogen in 1,2-dichloroethane hydrogen in propan-2-ol hydrogen in propan-2-ol-d8 hydrogen in ethanol hydrogen in ethanol-d6

5.0a 2.4a 1.7b 1.4c 1.4c 1.3c 1.6a 1.5a 1.9a 0.7a 0.7b 1.1a 1.0b

this this 10 9 9 9 this this this this 10 this 10

work work

work work work work work

At a field of 9.4 T (400 MHz), at 298 K by using the inversion− recovery experiment with selective pulses. bAt a field of 4.7 T (200 MHz), at 298 K. cAt a field of 11.7 T (500 MHz), at 300 K. a

relevant solvents. In accordance with Turro et al.,9 the deuteration degree of the solvent has no significant influence on the nuclear relaxation of H2; this suggests the dominance of an intramolecular relaxation mechanism. The T1 times for the hydrogen samples were always < 2 s and for the references always < 6 s. To exclude interferences from different longitudinal relaxation times we used interscan delays of 10 s for the H2 samples and of 30 s for the references which guarantee more than 99 % recovery of signals. Prior to Fourier transformation the data were multiplied with a decaying exponential function (line broadening factor of 1 Hz) and zero filled. The spectra were manually carefully phased, baseline-corrected, and integrated as described above.

■

RESULTS AND DISCUSSION First, the hydrogen concentrations in solvents, where published data are available, were determined to validate our method. The published hydrogen solubilities were expressed in various ways (concentration, mole fraction, Henry's constant, Ostwald coefficient, Bunsen coefficient) and always refer to a pure hydrogen atmosphere. Since our method delivers molarities at 0.101 MPa total pressure (hydrogen plus solvent vapor pressure), the literature values had to be converted. Wherever it was necessary for this conversion, the density and the molar mass of the solvent were used (see Table 4 for solvent specific data). The following graphic representations give such converted data of “effective hydrogen concentrations” under an atmosphere consisting of hydrogen and solvent vapor. Figure 3 shows the graphical comparison of hydrogen saturation concentrations of different saturated solutions with 1741



Article

published by Tsuji et al.20 at 30 °C are said to be similar to those from Brunner.19 Figure 3B shows hydrogen concentrations of saturated methanol solutions. The reference values vary with about 6 %, at which our measured values are in this range. Figure 3C shows measured values of hydrogen concentrations compared with reference values of saturated benzene solutions. For this solvent, our data represent the lower border of expectation values. Regarding the saturated hydrogen solution of 1,2-dichloroethane, only one reference value is available, and this is clearly below the measured values, as can be seen in Figure 3D. The measured values are very close together, and the average of these concentrations is about 2.08 mmol·dm−3. The reference values for hydrogen dissolved in ethanol and propan-2-ol scatter widely. A possible reason for that could be water in the solvent. Choudhary and Chaudhari investigated the influence of water and temperature with the result that at all temperatures the solubility of H2 in methanol decreases with increasing the amount of water in the sample.24 Delmas et al. conducted similar experiments with ethanol and stated that the hydrogen solubility rather be influenced by water presence than by temperature difference (temperature range 298 K to 323 K).31 Therefore, all our solvents were kept under argon, without H2O, as already mentioned. Figure 3E shows the results for saturated ethanol solutions; they cover the range 2.89 mmol·dm−3 to 3.37 mmol·dm−3, which means a difference of more than 14 %. Our measured values vary less than 1 % and are all at the top of the reference range with an average value of 3.28 mmol·dm−3. Figure 3F finally shows the reference and measured values of hydrogen in saturated propan-2-ol solutions. As mentioned above, the reference values vary from 2.88 mmol·dm−3 to 3.36 mmol·dm−3, which is very strong. The measured values are exactly within their 3.01 mmol·dm−3 to 3.14 mmol·dm−3 range. The juxtaposition of the measured values with the reference values at the different systems shows that the hydrogen solubilities can be determined reliably and with a reasonable uncertainty. Figure 4A,B shows the measured hydrogen concentrations of saturated dichloromethane and 2,2,2-trifluoroethanol solutions,

Table 4. Physical Data for the Used Solvents at (25 ± 1) °C and (0.101 ± 0.002) MPa ρa solvent methanol ethanol propan-2-ol 2,2,2-trifluoroethanol benzene toluene dichloromethane 1,2-dichloroethane

g·cm

pa −3

11

0.79 0.7911 0.7811 1.3712 0.8711 0.8611 1.3213 1.2511

−3

10

MPa 14

16.4 7.914 5.914 9.515 12.714 3.814 57.316 10.614

M17 g·mol−1 32.04 46.07 60.10 100.04 78.11 92.14 84.93 98.96

a

Due to the generally low solubility of hydrogen in organic solvents, density or vapor pressure of the pure solvents can be used for conversion calculations.

Figure 3. Graphical comparison of saturation concentrations of hydrogen solutions in different solvents at (25 ± 1) °C and (0.101 ± 0.002) MPa; black ■, this work; gray ■ with black border, ref 1; ×, ref 18; black ●, ref 19; black ▲, ref 21; black ◆, ref 22; gray ●, ref 23; △, ref 24; □, ref 25; ○, ref 26; ◊, ref 27; gray ● with black border, ref 28; gray ◆ with black border, ref 29; gray ▲ with black border, ref 30; , ref 31; gray ■, ref 32; gray ◆, ref 33; gray ▲, ref 34.

published data at 25 °C and 0.101 MPa total pressure. All our data are given with their uncertainty which is represented by error bars. The respective uncertainties of the published data, if available, are also being represented by error bars, but unfortunately they were always given without any confidence level. Figure 3A shows H2 concentrations in methylbenzene, and it is obvious that the measured hydrogen concentrations are in good agreement with the published data with one exception, the data reported by Zhou et al.,18 which are much lower. Zhou et al.18 compared their determined hydrogen concentrations with data from Brunner19 with the result that their data show a systematic negative derivation. Zhou et al.18 argue that the denoted temperature by Brunner19 is incorrect, because data

Figure 4. Graphical comparison of saturation concentrations of hydrogen solutions in different solvents at (25 ± 1) °C and (0.101 ± 0.002) MPa.

respectively. For both solvents reference values are unavailable to the best of our knowledge. The saturation concentration of hydrogen in dichloromethane averages at 1.51 mmol·dm−3, and the single values fluctuate around with less than 2 %. The saturation concentration of hydrogen in 2,2,2-trifluoroethanol averages at 3.28 mmol·dm−3, with about 3 % scattering. Comparing the hydrogen concentration values of the alcohols, it appears that the values of methanol (3.32 mmol·dm−3), ethanol (3.28 mmol·dm−3), and 2,2,2-trifluoroethanol (3.28 1742



Article

mmol·dm−3) are close together, but the hydrogen concentration in a saturated propan-2-ol solution is with 3.08 mmol·dm−3 significantly lower. Furthermore, the influence of isotopes on the solubility of hydrogen at the system methanol/methanol-d4 was investigated but not detected: Up to the normal scattering the measured values show neither significant differences nor any trend as Figure 5 shows.

undeuterated solution and even at low gas concentrations, without the hassle of such an extrapolation. This is realized by using pulses which are selective for the gas resonance. Therefore, no interference with other signals, as it may occur with solvent signal suppression techniques, has to be considered. Since the gas signals are typically weak, it is advantageous to minimize integration inaccuracies by using integration limits.7 The gas concentration can be determined based on an external referencing (eq 2) which avoids the danger of a falsification of the gas solubility. The uncertainty of the calculated gas concentration is, depending of the number of samples, usually about or even smaller than 3 % with a confidence level of about 95 % for the system investigated (hydrogen in organic solvents). An additional benefit to use qNMR for the direct gas quantification in solution is, besides the simple handling and the direct proportionality of the signal strength to the concentration, the supporting information you can get by NMR. A simple change of the NMR experiment allows us to check the purity of the solvent or to monitor and analyze other components in the solution, for example, reaction partners, simultaneously with the gas concentration determination. Such experiments are currently under way. Possible extensions of the method include the use of a gas inlet and circulation device36 to monitor the gas concentration in situ during a reaction or under changing conditions (pressure, sample composition). Thus, SelPULCON is a smart method which uses NMR spectroscopy for the quantification of gases for sciences (e.g., investigation of catalytic reactions) as well as for engineering with the decisive advantage that qNMR directly leads to molar concentrations.

Figure 5. Graphical comparison of saturation concentrations of the hydrogen solutions: methanol-h4 vs methanol-d4 at (25 ± 1) °C and (0.101 ± 0.002) MPa.

The final data (Table 5) for the saturated solutions at 25 °C and 0.101 MPa total pressure result from combining the Table 5. Effective H2 Concentrations/Molalities of Saturated Solvents at (25 ± 1) °C and (0.101 ± 0.002) MPa solvent methanol ethanol propan-2-ol 2,2,2-trifluoroethanol benzene toluene dichloromethane 1,2-dichloroethane

c(H2)

b(H2)

mmol·dm−3

mmol·kg−1

3.33 3.28 3.07 3.28 2.46 2.82 1.51 2.08

± ± ± ± ± ± ± ±

0.13 0.09 0.14 0.18 0.11 0.09 0.06 0.05

4.23 4.18 3.93 2.39 2.82 3.27 1.15 1.67

± ± ± ± ± ± ± ±

■

0.17 0.13 0.20 0.14 0.14 0.10 0.05 0.05

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +49 (0)381 1281 51 201. Funding

Financial support by the Leibniz-Gemeinschaft (Pakt für Forschung and Innovation) and by Evonik Industries AG, Marl, is gratefully acknowledged.

concentration ranges determined for all samples of each solvent system (Figures 3 to 5) by calculating the average of the single concentration (mean) values ± the difference to the highest and the lowest limit in percent. The originally obtained molarities c (eq 2) are also converted to molalities b (see Table 4 for the solvent data).

Notes

The authors declare no competing financial interest. E-mail: [email protected], [email protected], [email protected].

■

■

ACKNOWLEDGMENTS We thank Dr. Jochen K. Lehmann, University of Rostock (Institute of Chemistry), for carrying out density determinations on aqueous solutions of maleic acid. Dr. U. Marx and E. Humpfer (both Bruker BioSpin) are thanked for helpful discussions.

CONCLUSION In this work we have shown that gas concentrations can be easily and accurately measured by NMR spectroscopy. Dyson et al. carried out similar experiments with unselective pulses before.35 They determined hydrogen concentrations in ionic and other liquids, and they had exactly the problems described above (low hydrogen concentration and weak signals at atmospheric pressure, the peak corresponding to hydrogen could not be properly resolved and integrated relative to the solvent peaks) and tried to overcome these by measuring at elevated hydrogen pressure (10.1 MPa). The spectra were evaluated by curve-fitting techniques, the solubility values for other pressures were calculated with the assumption of a linear pressure-concentration relation. With the method described here, “SelPULCON,” which is a modification of the published method PULCON,5 it is possible to measure concentrations in solutions of any gas producing a NMR signal, even in different

■

REFERENCES

(1) Young, C. L. IUPAC Solubility Series 5/6, Hydrogen and Deuterium; Pergamon Press: Oxford, 1981. (2) Malz, F.; Jancke, H. Validation of quantitative NMR. J. Pharm. Biomed. Anal. 2005, 38, 813−823. (3) Canet, D.; Aroulanda, C.; Mutzenhardt, P.; Aime, S.; Gobetto, R.; Reineri, F. Para-Hydrogen Enrichment and Hyperpolarization. Concepts Magn. Reson. Part A 2006, 28A, 321−330. (4) Mo, H.; Harwood, J.; Zhang, S.; Xue, Y.; Santini, R.; Raftery, D. “R:” A quantitative measure of NMR signal receiving efficiency. J. Magn. Reson. 2009, 200, 239−244.

1743



Article

(5) Wider, G.; Dreier, L. Measuring Protein Concentrations by NMR Spectroscopy. J. Am. Chem. Soc. 2006, 128, 2571−2576. (6) Guo, L.; Sprenger, P.; Garland, M. A combination of spectral realignment and BTEM for the estimation of pure component NMR spectra from multi-component non-reactive and reactive systems. Anal. Chim. Acta 2008, 608, 45−55. (7) Rischel, C. Fundamentals of Peak Integration. J. Magn. Reson., Ser. A 1995, 116, 255−258. (8) Jones, N. D.; Meessen, P.; Losehand, U.; Patrick, B. O.; James, B. R. Platinum(II) and Palladium(II) Complexes of Bisphosphine Ligands Bearing o-N,N-Dimethylanilinyl Substituents: A Hint of Catalytic Olefin Hydration. Inorg. Chem. 2005, 44, 3290−3298. (9) Sartori, E.; Ruzzi, M.; Turro, N. J.; Decatur, J. D.; Doetschman, D. C.; Lawler, R. G.; Buchachenko, A. L.; Murata, Y.; Komatsu, K. Nuclear Relaxation of H2 and H2@C60 in Organic Solvents. J. Am. Chem. Soc. 2006, 128, 14752−14753. (10) Aroulanda, C.; Starovoytova, L.; Canet, D. Longitudinal Nuclear Spin Relaxation of Ortho- and Para-Hydrogen Dissolved in Organic Solvents. J. Phys. Chem. A 2007, 111, 10615−10624. (11) Timmermans, J. Physico-Chemical Constants of Pure Organic Compounds; Elsevier Publishing Company, Inc.: Nijmegen, 1950. (12) Palepu, R.; Clarke, J. Viscosities and densities of 2,2,2trifluoroethanol + water at various temperatures. Thermochim. Acta 1989, 156, 359−363. (13) Comelli, F.; Francesconi, R. Excess Molar Enthalpy of Binary Mixtures of Diethyl Carbonate + Nine Chloroalkanes at 298.15 K and Atmospheric Pressure. J. Chem. Eng. Data 1995, 40, 509−511. (14) Jordan, T. E. Vapor Pressure of Organic Compounds; Interscience Publishers, Inc.: New York, 1954. (15) Cooney, A.; Morcom, K. W. Thermodynamic behaviour of mixtures containing fluoroalcohols I. (Water + 2,2,2-trifluoroethanol). J. Chem. Thermodyn. 1988, 20, 735−741. (16) Nath, J. Thermodynamic behaviour of binary systems of 1,4dioxane with 1,2-dichloroethane, dichloromethane, trichloroethene, tetrachloroethene and cyclohexane. Vapour pressures and excess molar Gibbs energies. J. Chem. Soc., Faraday Trans. 1991, 87, 1345−1350. (17) Wieser, M. E.; Coplen, T. B. Atomic weights of the elements 2009 (IUPAC Technical Report). Pure Appl. Chem. 2011, 45, 175− 178. (18) Zhou, Z.; Cheng, Z.; Yang, D.; Zhou, X.; Yuan, W. Solubility of Hydrogen in Pyrolysis Gasoline. J. Chem. Eng. Data 2006, 51, 972− 976. (19) Brunner, E. Solubility of Hydrogen in 10 Organic Solvents at 298.15, 323.15, and 373.15 K. J. Chem. Eng. Data 1985, 30, 269−273. (20) Tsuji, T.; Shinya, Y.; Hiaki, T.; Itoh, N. Hydrogen solubility in a chemical hydrogen storage medium, aromatic hydrocarbon, cyclic hydrocarbon, and their mixture for fuel cell systems. Fluid Phase Equilib. 2005, 228−229, 499−503. (21) Brückel, N.; Kim, J. I. Gibbs Free Energies of Solute Solvent Interactions for He, Ne, Ar, Kr, Xe, H2, O2, N2, CH4, SF6, C2H4, CO2 and C2H2 in Various Solvents: Comparison of Theoretical Prediction with Experiments. Z. Phys. Chem. Neue Folge 1981, 126, 133−150. (22) Schaffer, S. K.; Prausnitz, J. M. Correlation of Hydrogen Solubilities in Nonpolar Solvents Based on Scaled-Particle Theory. AIChE J. 1981, 27, 844−848. (23) Saylor, J. H.; Battino, R. The Solubilities of the Rare Gases in Some Simple Benzene Derivatives. J. Phys. Chem. 1958, 62, 1334− 1337. (24) Choudhary, V. R.; Chaudhari, S. K. Solubility of Hydrogen in Methanol Containing Reaction Components for Hydrogenation of pNitrotoluene. Indian J. Technol. 1984, 22, 156−158. (25) Radhakrishnan, K.; Ramachandran, P. A.; Brahme, P. H.; Chaudhari, R. V. Solubility of Hydrogen in Methanol, Nitrobenzene, and Their Mixtures. Experimental Data and Correlation. J. Chem. Eng. Data 1983, 28, 1−4. (26) Katayama, T.; Nitta, T. J. Solubilities of Hydrogen and Nitrogen in Alcohols and n-Hexane. J. Chem. Eng. Data 1976, 21, 194−196.

(27) Gjaldbæk, J. Ch. The Solubility of Hydrogen, Oxygen, and Carbon Monoxide in Some Non-Polar Solvents. Acta Chem. Scand. 1952, 6, 623−633. (28) Kruyer, S.; Nobel, A. P. P. Solubility of Hydrogen in Benzene, Cyclohexane, Decalin, Phenol and Cyclohexanol. Recl. J. R. Neth. Chem. Soc. 1961, 80, 1145−1156. (29) Cook, M. W.; Hanson, D. N.; Alder, B. J. Solubility of Hydrogen and Deuterium in Nonpolar Solvents. J. Chem. Phys. 1957, 26, 748− 751. (30) Waters, J. A.; Mortimer, G. A.; Clements, H. E. Solubility of Some Light Hydrocarbons and Hydrogen in Some Organic Solvents. J. Chem. Eng. Data 1970, 15, 174−176. (31) Purwanto; Deshpande, R. M.; Chaudhari, R. V.; Delmas, H. Solubility of Hydrogen, Carbon Monoxide, and 1-Octene in Various Solvents and Solvent Mixtures. J. Chem. Eng. Data 1996, 41, 1414− 1417. (32) Maxted, E. B.; Moon, C. H. The Temperature Coefficient of the Solubility of Hydrogen in Organic Solvents. Trans. Faraday Soc. 1936, 32, 769−775. (33) Brunner, E. Solubility of Hydrogen in Alcohols. Ber. Bunsen-Ges. Phys. Chem. 1979, 83, 715−721. (34) Frohlich, K.; Tauch, E. J.; Hogan, J. J.; Peer, A. A. Solubilities of gases in liquids at high pressure. Ind. Eng. Chem. 1931, 23, 548−550. (35) Dyson, P. J.; Laurenczy, G.; Ohlin, C. A.; Vallance, J.; Welton, T. Determination of hydrogen concentration in ionic liquids and the effect (or lack of) on rates of hydrogenation. Chem. Commun. 2003, 2418−2419. (36) Selent, D.; Baumann, W.; Börner, A. (Leibniz-Institut für Katalyse e. V. an der Universität Rostock): Gaseinleitungs- und -zirkulationsvorrichtung zur Verfolgung von Reaktionen in flüssiger Phase unter Beteiligung gasförmiger Reaktanden unter Normal- und Hochdruck mittels Kernresonanzspektroskopie (Druck-NMR-Spektroskopie) unter stationären Bedingungen. German Patent DE 103 33 143 B4, August 7, 2008.

■

NOTE ADDED IN PROOF Meanwhile, we became aware that there are solubility data for hydrogen in dichloromethane (Shirono, K.; Morimatsu, T.; Takemura, F. J. Chem. Eng. Data 2008, 53, 1867−1871). They were, however, determined under very different conditions and thus cannot be compared directly to our results. For 2,2,2trifluoroethanol, a saturation concentration of c = 3.4 mmol·dm−3 can be derived from published data (Mainar, A. M.; Pardo, J.; Royo, F. M.; López, M. C.; Urieta, J. S. J. Solution Chem. 1996, 25, 589−595) which is close to our value.

1744


Hydrogen in Organic Solvents - ACS Publications - American

Recommend Documents