J. Phys. Chem. 1996, 100, 10771-10778
10771
Conductivity, NMR, Thermal Measurements, and Phase Diagram of the K2S2O7-KHSO4 System K. M. Eriksen,† R. Fehrmann,*,† G. Hatem,‡ M. Gaune-Escard,‡ O. B. Lapina,§ and V. M. Mastikhin§ Chemistry Department A, Technical UniVersity of Denmark, DK-2800 Lyngby, Denmark, Institut UniVersitaire des Syste` mes Thermiques Industriels, UniVersite´ de ProVence, Centre de Saint Je´ roˆ me, AVenue Escadrille Normandie Niemen, 13397 Marseille Cedex 20, France, and BoreskoV Institute of Catalysis, 630090 NoVosibirsk, Russia ReceiVed: December 18, 1995; In Final Form: April 1, 1996X
The phase diagram of the catalytically important K2S2O7-KHSO4 solvent system has been investigated by means of electrochemical, thermal, and spectroscopic methods. The phase diagram exhibits a eutectic at ΧKHSO4 ) 0.94(1) with a temperature of fusion of 205 °C. No compound is formed in the system, but the strong R f β solid-solid transition of K2S2O7, found at 318 °C with ∆Htr ) 21.8 kJ/mol, gives rise to a marked change in the slope of the liquidus curve at this temperature. The experimental phase diagram is in very good accordance with a calculated diagram based on the assumption of an ideal liquid mixture. 39K, 1H, 17O, and 33S NMR measurements on the molten K S O -KHSO mixtures up to 540 °C show that a fast 2 2 7 4 ionic exchange takes place in the melt at all compositions. The conductivities of the solid and molten K2S2O7KHSO4 systems were measured at 13 different compositions in the whole composition range, ΧKHSO4 ) 0-1. For each composition in the temperature range examined , the conductivity of the molten mixtures has been expressed by equations of the form κ ) A(Χ) + B(Χ)(T - 600) + C(Χ)(T - 600)2. The measurements indicate an enhanced molar conductivity of the binary system, probably due to delocalization of the conducting ions compared to the pure molten components.
Introduction The molten salt-gas system K2S2O7/KHSO4/V2O5-SO2/O2/ SO3/CO2/H2O/N2 at around 400 °C is considered a realistic model of the catalyst used for the oxidation of SO2 to SO3 by O2 in a novel “wet” process, developed for the purification of flue gases. The chemistry of the “dry” system, i.e., K2S2O7/ V2O5-SO2/O2/SO3/N2, reflecting the traditional sulfuric acid catalyst, has previously been explored in detail by us.1-5 In both catalytic processes, the active component is the vanadium complex formed in the molten K2S2O7 or in the molten K2S2O7KHSO4 system, respectively. Thus, detailed information about the molten K2S2O7-KHSO4 solvent, including the species formed, their structure, the equilibria governing the melt, and fundamental physicochemical data, seems important for the understanding of the working catalyst for flue gas cleaning. Such investigations have been undertaken by us by means of spectroscopic, thermal, and electrochemical methods. Previous6 Raman and NIR spectroscopic investigations on the molten K2S2O7-KHSO4 have shown that the dominant species most probably present are S2O72-, HSO4-, and H2O. The H2O molecules seem to be strongly associated by hydrogen bonds to the other species of the melt, even at 450 °C. The vapor pressure of H2O appears to control the position of the water sensitive equilibrium 2HSO4- a S2O72- + H2O and, thus, the composition of the melt. The work presented here concerns a reinvestigation of the equilibrium phase diagram of the binary system K2S2O7KHSO4. Our work has proven marked discrepancies from the diagrams previously published 60-70 years ago.7,8 Knowledge of this phase diagram may be useful for the design of new low†
Technical University of Denmark. Universite´ de Provence. § Boreskov Institute of Catalysis. X Abstract published in AdVance ACS Abstracts, May 15, 1996. ‡
S0022-3654(95)03744-0 CCC: $12.00
melting catalysts that are able to operate in the desired temperature range below 400 °C. Four different methods of investigation have been applied for this study, i.e., electrical conductivity, differential enthalpic analysis (DEA), multinuclear NMR, and NIR spectrophotometry at temperatures up to 540°C. These investigations were also carried out to give additional information about the species present in the molten catalyst solvent. Experimental Section Chemicals. The hygroscopic K2S2O7 used was obtained by thermal decomposition of K2S2O8 (Merck, pa) and was kept in sealed ampules until use in the dryboxes, as previously described.9 The KHSO4 used for additions (Merck, Suprapur, 99%) was dried at 110 °C and stored in the drybox. By this procedure, the hygroscopic KHSO4 typically lost mass corresponding to 15 mol % of H2O. Conductivity Measurements. The borosilicate glass cell, with gold electrodes used for measuring the electrical conductivity, has been described in detail previously.10 The cell was filled in the drybox, sealed, placed in the measuring furnace, and regulated to within (0.1 °C, as previously described.11 The composition of the melt was varied by the addition of chemicals (KHSO4 or K2S2O7) to the cell in the drybox by cutting the stem open and resealing it again under nitrogen or in a vacuum. The mixture was mixed well manually by shaking the cell holder for a short while outside the furnace. The resistance of the cell was measured when it was constant. The temperature was lowered in steps of 2-10 °C, and in several cases subcooling was observed, indicated by a sudden jump in the resistance to a much higher value when crystallization occurred. Thereafter, the temperature was gradually raised until the resistance coincided with the previous measurements in the liquid region. The temperature was measured by a calibrated Pt(100) platinum © 1996 American Chemical Society
10772 J. Phys. Chem., Vol. 100, No. 25, 1996 resistance thermometer placed directly at the capillary tube of the conductivity cell. The conductivity was measured by a Radiometer CDM-83 conductivity meter. Different cells were used with cell constants in the range 100-200 cm-1, determined at room temperature in a thermostat using a 0.1 demal KCl standard solution as described in the literature.12 Thermal Investigations. Two thermal techniques were used to measure the temperature or/and the enthalpy change related to fusion or crystallization of melts. The description and capabilities of these techniques have been given in detail elsewhere.13 The same samples were used in both cases. They consisted of about 3 g of material in sealed ampules of borosilicate glass about 1.5 cm in diameter and 2 cm high. Thermal analysis (TA) was carried out in an insulated quartz tube furnace. A Pt/Pt-10% Rh thermocouple was attached onto the bottom of the sample ampule for measurement of temperature. It was checked in a separate experiment with the melting point of lead; temperatures were measured within (1 °C. With this technique, the heating rate was low (i.e., 2 °C/min) so as to make possible the separation of thermal effects occurring at rather close temperatures. This technique, fast and easy to use, has limited sensitivity and was not effective enough in the composition range where the thermal effects were either small or overlapping. Differential enthalpic analysis (DEA) then was used and carried out in a Calvet microcalorimeter. The large number of thermocouples in the two calorimetric cells (several hundreds) and the very slow heating rate (10 °C/h) controlled by a linear temperature programmer resulted in high-quality thermograms, with a very well defined base line up to 1000 °C and separation of thermal effects occurring at very close temperatures. Although the same experiment would be 12 times more timeconsuming with DEA, compared to thermal analysis it is easy to calculate that the sensitivity is more than 60 times larger and the results obtained are more reliable. NMR Measurements. The 39K, 33S, and 17O NMR spectra were measured on a Bruker MSL-400 spectrometer, while 1H NMR spectra were recorded on a Bruker CXP-300 spectrometer. The measurements were performed on natural isotope-containing samples, i.e., containing 93.26% 39K, 0.76% 33S, 0.039% 17O, and 99.99% 1H. The measurements at high temperatures, i.e., up to 600 °C, were performed by a homemade high-temperature probe head shown in Figure 1. It consisted of a saddle silver rf coil for samples placed vertically. Heat transfer to the magnet and shim system was minimized by thermal insulation with highly porous silica of low heat conductivity. This type of probe can be used on NMR spectrometers with a narrow-bore (89 mm) superconducting magnet. The sample heating is performed by a vertical quartz heater with chromel wire. To avoid temperature gradients inside of the heater, a maximum ratio of heater to sample length should be obtainedsca. 10 in our case. Nevertheless, a temperature gradient is expected. The temperature of the bottom of the sample was measured with an accuracy of 2 °C by a chromel-alumel thermocouple placed 2 mm below the sample. Apart from the usual experimental difficulties in measuring the true intensities of lines by FT-NMR, the high-temperature NMR experiments involve additional problems and may be subject to a number of errors. For this reason, our NMR data should be considered as qualitative and not used for detailed quantitative considerations. The 39K NMR spectra were measured at a frequency of 18.67 MHz, a pulse width of 50 µs, and a delay of 0.15 s between the pulses in a 50 kHz frequency range. The chemical shifts were
Eriksen et al.
Figure 1. High-temperature NMR probe head: 1, sample; 2, rf coil; 3, thermocouple; 4, heating device; 5, thermal insulation. Top: cross section.
referred relatively to the signal of a 0.1 M aqueous solution of KCl at room temperature. The 33S spectra were measured at 30.3 MHz in a frequency range of 30 kHz, with a pulse width of 50 µs and a delay of 150 ms between pulses. The accumulation number of free induction decay was from 2 × 104 to 8 × 104. Chemical shifts were measured relative to 70% sulfuric acid measured at room temperature. The 17O spectra were measured at 54.2 MHz in a frequency range of 50 kHz, with a pulse width 50 µs and a delay of 150 ms between pulses. The accumulation number was 104. The chemical shifts were measured relative to water at room temperature. The 1H NMR MAS spectra were measured at a magnetic field of 7.04 T, a resonance frequency of 300.066 MHz in a frequency range of 50 kHz, a pulse width of 5 µs, and a delay of 1 s between pulses. The number of signal accumulations was 103. The chemical shifts were measured relative to tetramethylsilane (TMS). Before the measurements, the sealed tubes with the salt mixtures were opened in a glove box with Ar atmosphere, ground in a mortar, and compressed in a quartz rotor, which was closed with a Teflon cap. The rotation frequency was about 3 kHz. 1H NMR static spectra were measured in a frequency range of 50 kHz, a pulse width of 10 µs, and a delay of 1 s between pulses. The number of signal accumulations was 500. The chemical shifts were measured relative to the signal of TMS at room temperature. The samples of around 3 g were contained in sealed quartz ampules. Spectrophotometry. Fused quartz cells (Ultrasil, Helma, Germany) with optical path lengths of 0.2 cm were used. The cells were loaded with the K2S2O7-KHSO4 mixtures in the drybox and closed before they were transferred to a specially
K2S2O7-KHSO4 Solvent System
J. Phys. Chem., Vol. 100, No. 25, 1996 10773
designed furnace for spectrophotometric measurements, using a Cary 14R spectrometer as previously described.6 General Considerations Conductivity Measurements. The conductivity κ is given by -E(κ)/RT
κ ) A(κ)e
(1)
where A(κ) is the preexponential factor, E(κ) is the activation energy for ionic migration, and R and T have their usual meanings. The experimental molar conductivity of the melt, at a given composition and temperature, is given by
Λexptl ) κVM
(2)
where κ and VM are the measured conductivity and the molar volume of the melt, respectively, calculated from the composition and the previously measured densities of the K2S2O7KHSO4 system.14 As mentioned previously,1,10 to discuss possible complex formation in the melt, Λexptl should be compared to Λcalcd obtained from the following equation:15
Λcalcd ) Χ12Λ1 + Χ22Λ2 + 2Χ1Χ2Λ1 (Λ1 < Λ2) (3) where X1 and X2 are the mole fractions and Λ1 and Λ2 the molar conductivities (with Λ1 < Λ2) of the two components, respectively. It should be noted that this equation previously1,10,16 contained misprints of the first two terms, i.e., X1 and X2 were given in the first degree, but all calculations were done by use of the correct equation. Results and Discussion Phase Diagram of the K2S2O7-KHSO4 System. The conductivity of the solid and molten mixtures of the K2S2O7KHSO4 binary system was measured at 13 different compositions in the mole fraction range ΧKHSO4 ) 0-1. The measured conductivities are plotted as ln(κ) vs 1/T in Figure 2. As found recently17,18 for the M2S2O7-V2O5 systems (M ) Cs or 80% K + 20% Na), a marked change in the conductivity of the mixture is found at the phase transition temperature. This is in accordance with eq 1, whereby plots of -ln(κ) vs 1/T should give rise to straight lines of different slopes in the liquid and the solid regions especially since E(κ) will be different for the two regions. In the intermediate solid + liquid region, the curve will deviate from these two lines. However, for the mixtures rich in KHSO4, the transition from the two-phase solid + liquid region to the liquid region does not give rise to a marked change. Thus, no liquidus temperature could be found for ΧKHSO4 ) 0.8 and 0.9, where only one transition is observed, corresponding to the fusion of the eutectic mixture. This solid to solid + liquid transition was very marked in all cases, but unfortunately the temperature could not be lowered sufficiently to observe this transition in the K2S2O7rich region, i.e., for ΧKHSO4 ) 0-0.4, due to the increased risk of breaking the cell. The transition temperatures are listed in Table 1. DEA and TA measurements have been carried out on 12 compositions over the whole composition range of the K2S2O7KHSO4 system. In Figure 3, the thermograms of four of the measurements are shown as examples. A solid-solid transition in KHSO4 was found at 178 °C, with the related enthalpy ∆Htr ) 2.11 kJ/mol. Two rather close transitions are reported in the literature19 at 164 and 181 °C, with heats of transition of
Figure 2. Electric conductivities in the K2S2O7-KHSO4 system: -ln(κ) vs 1/T for the following compositions. ΧKHSO4: A, 0.0000; B, 0.1002; C, 0.1997; D, 0.3003; E, 0.3833; F, 0.5002; G, 0.5300; H, 0.6001; I, 0.6660; J, 0.7005; K, 0.7976; L, 0.9046; M, 1.0000. For clarity, the data [except those of pure K2S2O7 (A)] are offset on the ordinate by the specified values. Open circles indicate subcooling, and arrows indicate the liquidus temperatures.
2.06 and 0.40 kJ/mol, respectively. The sum of these heats corresponds rather well to our value for the single peak observed at 178 °C. Earlier20,21 it also appeared that only one small peak was observed close to the melting point. The temperature and enthalpy of fusion were also determined as 208 °C and ∆Hfus ) 16.6 kJ/mol (not reported earlier), respectively. The melting point deviates somewhat from 215 °C found by the conductivity measurements and 214-218 °C given in the literature8 for the melting point of KHSO4 in a closed ampule. Since the broad peak in the thermogram for KHSO4 may include prefusion effects, the estimated fusion temperature might be too low. Thus, the value of 215 °C from the conductivity measurements is considered more correct. For K2S2O7, a melting point of 417420 °C and an enthalpy of fusion of 21.2 kJ/mol were found, in good agreement with our previous measurements9 of 419 °C and 19.9-20.4 kJ/mol, respectively. In addition, two solidsolid phase transitions have been found at 200 and 318 °C, which are also in good agreement with earlier measurements.8,20,22 The transition at 318 °C corresponds to a structural change from the β form to the R form of K2S2O722 and that at 200 °C corresponds to the change from the γ to the β form.8 The enthalpy change related to the R T β transition was measured in the present work to be ∆Htr ) 21.8 kJ/mol; this valuesnot previously reportedshas about the same magnitude as the enthalpy of fusion. The transition temperatures found from the thermal methods are also given in Table 1. The heat capacities of the pure compounds, K2S2O7 and KHSO4, have been measured by differential scanning calorimetry (DSC). The apparatus used was a DSC 121 from Setaram.
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TABLE 1: Conductivity, Differential Enthalpic Analysis, and Thermal Analysis on the K2S2O7-KHSO4 System: Temperatures (°C) of Transtionsa solid phase transition 1 ΧKHSO4
DEA
0.0000 0.0992 0.1002 0.1997 0.2499 0.3003 0.3833 0.3963 0.4920 0.5002 0.5300 0.5996 0.6001 0.6562 0.6660 0.7005 0.7030 0.7500 0.7976 0.8457 0.8500 0.9046 1.0000
200
a
TA
fusion of eutecticb
transition 2 DEA
TA
318 324
337
cond
DEA
liquid temperature TA
cond
DEA
TA
420
417 399
412
199 397 380
324
320
201
367 340 327
203
201 203
207
308
319 309
308 308 204
205
301 306
202 208 207
285 291 289
175
204 202
204
290 270
283
205 224* 241*
204 201
171 206 178
215
208
Values indicated by an * are less certain due to the small thermal effect. NIR meaurements: ΧKHSO4 ) 0.94(1) and Tfus ) 207 °C. b
Figure 3. Differential enthalpic analysis of the K2S2O7-KHSO4 system for the following compositions. ΧKHSO4: A, 1.0000; B, 0.7000; C, 0.2500; D, 0.0000. Transition temperatures are marked by arrows. For convenience, the thermograms have been offset on the temperature axis.
TABLE 2: Heat Capacities, Cp, of K2S2O7 and KHSO4 compound
temperature range (°C)
Cp (J mol-1 °C-1)
K2S2O7 (β) K2S2O7 (R) K2S2O7 (l) KHSO4 (β) KHSO4 (R) KHSO4 (l)
25-318 318-419 419-450 25-178 178-215 215-300
183(1) + 0.177(7)t 260(4) 267(2) 88.6(8) 97(1) 136.7(9)
The mass of the samples, contained in sealed quartz cells, was around 300 mg, and the rate of heating was 1 °C/min. The results are shown in Table 2. The 39K NMR spectra of a sample with the composition ΧKHSO4 ) 0.7 measured in the temperature range 195-355 °C are shown in Figure 4. Below around 200 °C, no signal is seen, presumably due to line broadening in the solid state. In the
Figure 4. Integrated 39K NMR spectra of the K2S2O7-KHSO4 system, ΧKHSO4 ) 0.7000, at different temperatures. Recorded spectra are inserted.
range 200-300 °C, a rather sharp line appears at -18.8 ppm with a gradually increasing intensity (i.e., obtained by integration of the peak) up to around 310 °C. Thereafter the intensity remains constant, indicating that the sample is completely melted. However, in accordance with the Curie law, a decrease of about 10% of the intensity is expected from the increase of the temperature from 300 to 350 °C. This is not observed probably due to a comparable experimental error. In the twophase region the chemical shift is independent of the temperature. Thus, the mobility of K+ seems high and the association with the anions appears low, independent of the large composition variation in the melt in this region. The signal intensity should be proportional to the fraction of liquid phase at the measured temperature. However, a closer analysis and comparison with the phase diagram are not justified due to the rather large experimental error in the NMR intensities. Nevertheless,
K2S2O7-KHSO4 Solvent System
Figure 5. Phase diagram of the K2S2O7-KHSO4 system obtained from conductance (9), DEA ([), TA (2), and NIR (b) measurements. Open symbols indicate the melting point of the eutectic. The liquidus line and the composition and melting temperature of the eutectic mixture are calculated by assuming ideality of the binary system.
the observed transition temperatures agree well with those obtained from other methods. The composition of the eutectic of the K2S2O7-KHSO4 system could not be found accurately by the conductivity or thermal methods. However, a marked change in the absorption spectra (due to O-H vibrations) in the near-infrared region (NIR) due to a change in melt composition has previously been observed.6 Thus, a mixture of K2S2O7 and KHSO4 with ΧKHSO4 ) 0.80 was equilibrated in a furnace at a temperature slightly above the melting point of the eutectic at 205 °C. The solidliquid mixture was contained in a cell with a sintered glass frit as previously described,3 which allowed filtration of the melt by turning the furnace vertical and applying dry N2 of around 2 atm on the mixture. The molten part of the mixture could then be collected in the ampule below the glass frit and transferred to the optical cuvette in the drybox. Thereafter the absorption spectrum was recorded in the range 500-2500 nm at 260 °C. Similar spectra were recorded of four samples with known composition in the range ΧKHSO4 ) 0.8-1.0. From a plot of the absorption at the peak at 2200 nm vs the melt composition, the eutectic mixture was found to have the composition ΧKHSO4 ) 0.94 ( 0.01. The phase diagram depicted in Figure 5 shows considerable variations above ΧKHSO4 ) 0.45 from the diagram of Hagisawa and Takai,7 which was obtained from differential enthalpic analysis only. The diagram of Cambi and Bozza8 also deviates dramatically from ours, i.e., up to 55 °C lower liquidus temperatures were found by these authors, probably because their DTA measurements were performed by cooling, leading to large subcooling before crystallization. Apparently a peritectic is present at ΧKHSO4 ) 0.45, but this would imply that a compound should be formed in the range ΧKHSO4 ) 0-0.45. However, this is not the case since the eutectic mixture is formed over the whole composition range. Furthermore, the 1H NMR MAS spectra of K2S2O7-KHSO4 solidified mixtures in the range ΧKHSO4 ) 0.33 -1 (an example is shown in Figure 8) confirms that solid KHSO4 is formed at
J. Phys. Chem., Vol. 100, No. 25, 1996 10775 all measured compositions and that no new 1H lines due to a possible compound formed between K2S2O7 and KHSO4 appear in the spectra. In addition, no such compound has previously been reported in the literature. Finally, thermodynamic calculations show that the heat of the R f β transition at 318 °C for K2S2O7 is so large that the liquidus curve in the vicinity of the temperature, corresponding to ΧKHSO4 ) 0.45 in the phase diagram, will exhibit a marked change in the slope. Thus the liquidus curve shown in Figure 5 of the K2S2O7-KHSO4 system has been calculated from the Gibbs energies of the two pure components. The Gibbs energy, G, has been calculated by taking into account the temperature and enthalpy of transition (R f β, s f l) and the heat capacity (Table 2) of each compound. The reference of this G function is deduced from the stable element reference system (SER). That means that for each temperature we have calculated the difference Gø(T) - GSER(T), where Gø(T) is the value of G for the compound at T in the phase ø (R, β, ..., l) and GSER(T) ) HSER(298.15) TSSER(298.15). HSER and SSER reflect the values of the enthalpy and entropy of the compound in the stable phase at T ) 298.15 K, respectively. For the calculation the solution is assumed to be ideal. The calculated eutectic composition and melting point are ΧKHSO4 ) 0.92 and 205 °C, respectively. All transition lines in the phase diagram presented in Figure 5 are drawn from the calculations. A very good agreement between the calculated phase transitions and the experimental data is observed. Ongoing calorimetric measurements of the heat of liquid-liquid mixing of the K2S2O7-KHSO4 system point to a maximum value below 1 kJ/mol, in good accordance with the assumption of ideality of the system. The Liquid K2S2O7-KHSO4 System. For the measured conductivities of the 13 different compositions displayed in Figure 2, the data in the liquid region have been expressed by polynomials of the form κ ) A(Χ) + B(Χ)(T - 600) + C(Χ)(T - 600)2, where X is the molar fraction of KHSO4, T is the temperature in kelvin, and 600 K is a reference temperature. This reference temperature is chosen since it is the intermediate temperature of the liquid range measured for all compositions [however, for molten K2S2O7 (mp ) 419 °C) this temperature is chosen as 450 °C]. This empirical fitting is considered more correct than applying eq 1 since the conducting properties of the melt, and thus the activation energy E(κ) for ionic migration, most probably change in the temperature range investigated. This is due to a change in the composition of the melt, e.g., in accordance with a shift of the temperature sensitive equilibrium
2HSO4- a S2O72- + H2O
(4)
as previously described.6 The polynomials and the standard deviations for the different compositions of the melt are given in Table 3. To discuss the variation in the conductivity with the composition of the melt, κ has been calculated by extrapolation of the data in Table 3 at the intermediate measuring temperature, i.e., around 330 °C (600 K), as shown in Figure 6A. For K2S2O7 the polynomial has been calculated from the data of the earlier work10 since more measurements were performed in the liquid region. κ increases with the composition, ΧKHSO4, i.e., by increasing the concentration of H+ in the melt. By applying eq 2, the experimental molar conductivities, Λexptl, of the melts of different compositions could then be obtained. The theoretical values, Λcalcd, could be found from eq 3 by use of the experimental molar conductivities of 9.782 and 20.029 Ω-1 cm2 mol-1, which can be obtained from the data in Table 3 at 330 °C for K2S2O7 and KHSO4, respectively. Thereafter, as before1,10 the deviation between the experimental and calculated
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TABLE 3: Coefficients for Empirical Equationsa for the Specific Conductivity of Different Compositions, ΧKHSO4, of the Moltenb K2S2O7-KHSO4 System ΧKHSO4
A(Χ) (Ω-1 cm-1)
B(Χ) (10-3 Ω-1 cm-1 deg-1)
C(Χ) (10-6 Ω-1 cm-1 deg-2)
SD (Ω-1 cm-1)
0.0000c 0.1002 0.1997 0.3003 0.3833 0.5002 0.5300 0.6001 0.6660 0.7005 0.7976 0.9046 1.0000
0.296 325 0.096 379 0.108 557 0.125 383 0.132 229 0.145 546 0.147 885 0.161 477 0.176 829 0.181 473 0.226 358 0.246 941 0.291 763
2.0050 1.5449 1.5784 1.5366 1.5808 1.6124 1.6589 1.6731 1.7894 1.8413 2.2763 1.8692 2.0638
1.9281 2.7545 2.4860 3.9959 3.4893 3.4850 3.3214 1.9963 3.8211 7.2937 6.1816 2.4798 2.2142
0.001 06 0.000 95 0.000 64 0.001 36 0.000 56 0.000 28 0.000 33 0.000 83 0.000 27 0.001 31 0.001 11 0.002 91 0.001 42
a κ ) A(Χ) + B(Χ)(T - 600) + C(Χ)(T - 600)2, T in kelvin. Consult Table 1 and Figure 4 for the temperature range. c κ ) A(Χ) + B(Χ)(t - 450) + C(Χ)(t - 450)2, t in celsius. b
Figure 6. (A) Conductivity isotherm at 600 K for the K2S2O7-KHSO4 system. The curve fit corresponds to the polynomial κ ) 0.163 + 0.0318X + 0.0895X2. (B) Deviation (%) between the experimental and calculated molar conductivities of the molten K2S2O7-KHSO4 system at 600 K. The dashed curves indicate data for the extrapolated liquid state.
molar conductivities could be calculated and plotted vs the composition, as shown in Figure 6B. A large positive deviation is observed, as previously found for binary systems of the simple eutectic type,15 having a maximum possibly at ΧKHSO4 ) 0.33. This indicates that for the mixture of 2K2S2O7‚KHSO4 the conducting ions, primarily H+ (judging from the much lower molar conductivity of K2S2O7 than KHSO4), are less localized than in molten KHSO4. This may imply H-bonding of HSO4to neighboring S2O72- ions, where a maximum liberation of the proton from HSO4- takes place at the molar ratio S2O72-: HSO4- ) 2:1, i.e., at ΧKHSO4 ) 0.33. The 39K NMR spectra at 470 °C of six different molten K2S2O7-KHSO4 mixtures over the whole composition range ΧKHSO4 ) 0-1 are presented in Figure 7A. All spectra exhibit a narrow single line with a width of about 50 Hz. The chemical shift was found to be -18.8 ppm for all the samples. The 39K nuclei have the nuclear spin I ) 3/2, resulting in an electric quadrupole moment. This usually complicates the 39K NMR spectra of solids and broadens the resonance line due to short spin-lattice relaxation, as observed for the spectra in Figure 4 below the melting temperature. The range of chemical shifts for 39K resonance is usually about 100 ppm. The observation of a narrow single line, with the same chemical shift in the
melts of different composition, suggests an identical chemical state of K. Thus, dissociation of both KHSO4 and K2S2O7 seems to take place with the formation of K+ cations with very weak ion association and high mobility in both types of melts. The 33S isotope (I ) 3/2) has a relatively large electric quadrupole moment, and therefore 33S NMR spectra of solutions usually are rather broad due to rapid spin-lattice relaxation. Due to low sensitivity and relatively large line widths, the recording of 33S NMR spectra needs a long time for accumulation. The 33S NMR spectra of the melts, in the composition range ΧKHSO4 ) 0.40-1 at 400 °C, are shown in Figure 7B. A single line is observed with a chemical shift that seems to change from -6 to -3 ppm by increasing the K2S2O7 content. It is close to that found for concentrated sulfuric acid. This indicates, firstly, that 33S NMR chemical shifts of HSO4- and S2O72anions are close to each other and, secondly, that rapid exchange reactions take place in the melt, leading to averaged NMR lines. The 17O NMR spectra at 470 °C of six samples, in the range ΧKHSO4 ) 0-1, are illustrated in Figure 7C. In all spectra, a single resonance line was observed with a line width of about 200 Hz. No other resonances were detected. The chemical shifts varied from 163 ppm for molten KHSO4 to 184 ppm for molten K2S2O7. The 17O isotope has a very low natural abundance (0.04%) and nuclear spin I ) 5/2. Therefore, the registration of 17O NMR spectra from nonenriched samples usually needs a long time for accumulation. The range of chemical shifts of 17O NMR spectra in general spans more than 1000 ppmsthe shift is very sensitive to chemical bonding. Although the chemical shifts of KHSO4 and K2S2O7 differ by about 20 ppm, the molten mixtures of the compounds show only an averaged line, indicating a rapid exchange between the different species in the melt. In agreement with previous data,23 no separate lines from bridging and terminal oxygens in the pyrosulfate anions have been detected, most probably due to equilibria that include a fast dissociation and recombination of the pyrosulfate anion, e.g., S2O72- a SO42- + SO3. The rate of this process is probably more than 103 Hz as estimated from the expected difference in chemical shifts of bridging and terminal oxygens. Similar considerations can be put forward for the hydrogen sulfate anion, where the fast proton exchange reaction HSO4- f SO42- + H+ f HSO4- can account for the observation of only one 17O NMR line in molten KHSO4. No other signals, e.g., from H2O, have been detected in the 17O NMR spectra. This can be due either to a small concentration or a rapid equilibrium process (e.g., 2HSO4- f S2O72- + H2O f 2HSO4-), which averages the signals from water oxygen with those from the hydrogen sulfate and pyrosulfate oxygens. 1H NMR spectra of four samples, with the compositions ΧKHSO4 ) 0.099, 0.300, 0.698, and 1, have been recorded in the temperature range 20-520 °C. The 1H NMR spectrum of KHSO4 at room temperature, shown in Figure 8, exhibits a broad asymmetric line with a line width of about 10 kHz and a maximum around 12 ppm. An increase in the temperature to 180 °C makes the line narrower, with a chemical shift of 14 ppm. This value is very close to δiso ) 13.5 found from the magic angle spinning spectra of the solid sample at 20 °C. At 190 °C, two lines can be seen simultaneously in the spectrum. Along with a broad component, a narrow line with δ ) 9.7 appears in the spectrum. At higher temperatures the broad line disapears completely and only a single line of about 1 kHz width remains in the spectrum. Its chemical shift decreases to δ ) 7.5 by an increase in the temperature to 520 °C. The δ vs T dependencies, shown in Figure 9, are similar for the four samples of different compositions. At temperatures that are close to the melting point of the eutectic mixture, δ is close to 9.7 for all
K2S2O7-KHSO4 Solvent System
J. Phys. Chem., Vol. 100, No. 25, 1996 10777
Figure 7. NMR spectra of the molten K2S2O7-KHSO4 system: (A) 39K spectra at 470 °C for different compositions in the range KHSO4 (1) to K2S2O7 (6); (B) 33S spectra at 400 °C for different compositions in the range KHSO4 (1) to ΧKHSO4 ) 0.40 (6); (C) 17O spectra at 470 °C for different compositions in the range KHSO4 (1) to K2S2O7 (6).
Figure 9. 1H chemical shifts of the K2S2O7-KHSO4 system in the temperature range 20-520 °C for the following compositions: KHSO4 (O), ΧKHSO4 ) 0.698 (b), ΧKHSO4 ) 0.300 (0), ΧKHSO4 ) 0.099 (9). Partly filled symbols indicate coinciding data points. The chemical shift of 13.5 ppm for all samples at 20 °C (*) is based on the MAS NMR spectra of Figure 5.
Figure 8. 1H NMR static and MAS spectra of KHSO4 in the temperature range 20-520 °C: 1 20 °C (MAS); 2, 20 °C; 3, 180 °C; 4, 190 °C; 5, 270 °C; 6, 400 °C; 7, 440 °C.
samples. This value decreases to around 7.5 ppm and does not depend much on the temperature above 400 °C. The change in the chemical shift due to temperature corresponds to a shift in the equilibrium 2HSO4- a S2O72- + H2O from the left to the right. For ΧKHSO4 ) 0.099, the amount of molten eutectic
is very small and the 1H chemical shift follows the trend of the other samples only above the liquidus temperature of around 390 °C. The large values of the 1H NMR chemical shifts are typical for H-bonded compounds and were reported previously for trihydrogen selenites.24 From a correlation between the isotropic value of the chemical shift and the oxygen-oxygen distance of the O-H-O bridge found in ref 24, a distance of 2.53 Å could be estimated for the solid samples at all compositions. This is in good agreement with crystallographic data for KHSO4, where O-H-O distances were found25,26 to be in the range 2.58-2.68 Å. An increase in the temperature up to about 175 °C does not affect the chemical shift. The decrease in the line width is caused by the increased motion of atoms in the crystal lattice as a premelting effect. Above 175 °C a sharp decrease in δ to 9.7 is observed, possibly due to the β to R transition of solid KHSO4. The fusion of the samples above 205 °C results in the breaking of hydrogen bonds, as evidenced by the further
10778 J. Phys. Chem., Vol. 100, No. 25, 1996 decrease in the chemical shift value and the disappearance of the broad line, e.g., see Figure 8. The chemical shift of around 9.7 ppm, observed for all samples in the low-temperature range, can be ascribed to the formation of the HSO4- species. The relatively large width of the line could be due to the formation of relatively large agglomerates (or summation of many states) in the melt, e.g., formed by H-bonding. Still not identified species or agglomerates, which include protons in their structure, are formed at temperatures higher than 400 °C for all compositions since R chemical shift of 7.5 ppm is found for all samples. These species are probably in equilibrium with water, since it is well-known that the evolution of water increases with temperature. The chemical shift of bulk water at room temperature is 4.5 ppm and it decreases by increase in temperature, due to the breaking of hydrogen bonds, down to a value of ∼0 ppm typical for water vapor. Therefore, the line at 7.5 ppm cannot be ascribed to water. The formation of water in the KHSO4-K2S2O7 melts nevertheless has been detected by Raman spectroscopy.6 The absence of a separate NMR signal from water molecules indicates a fast (on the NMR scale) exchange of protons between water and HSO4- and/or other species containing protons in their structure with frequencies νexch > 3 × 103 Hz (ν0 ) 300 × 1 000 000 Hz, resonance frequency for 1H). The exchange frequency, however, is less than around 1012 Hz (laser frequency), whereby separate bands from H2O, S2O72-, and HSO4- species could be observed in the Raman spectra.6 The possibility of further dissociation of HSO4- to H+ and SO42- can be ruled out since the chemical shift of the isolated proton is about 30 ppm.27 Furthermore, the previous Raman spectra revealed bands that could be ascribed to S2O72-, HSO4-, and H2O in the whole composition range of the K2S2O7-KHSO4 system at 200-450 °C. Thus, no bands of HS2O7-, H2SO4, or SO42-, for example, could be detected. Therefore, the jumping of the proton from HSO4- to the surrounding S2O72- ions is most probably accompanied by a cleavage of the S-O-S bridge and the reformation of HSO4and S2O72- ions, as illustrated by the following equation:
| OsSO3‚‚‚Hs | OsSO3‚‚‚SO3s | OsSO3‚‚‚Hs | OsSO3 SO3s (HSO4-) (S2O72-) (HSO4-) (5) Fast synchronized exchange of this type in the molten K2S2O7-KHSO4 system, with a rate between 103 and 1012 Hz, can explain the observed averaged 1H, 17O, and 33S NMR spectra and the previous Raman spectra, indicating lifetimes of the individual species HSO4-, S2O72-, and H2O long enough to be detected. Also, the relative increase in the molar electrical conductivity by adding K2S2O7 to KHSO4 can be explained by this picture of the melt, leading to increased delocalization of the proton.
Eriksen et al. Acknowledgment. This investigation has been supported by the EEC programs Human Capital and Mobility (Contract No. ERBCHBGCT 920129), Brite Euram II (Contract No. BRE2.CT93.0447), and INTAS (Contract No. 93-3244 ). Further support is acknowledged from the Danish Natural Science Foundation, the French Ministry of Foreign Affairs, and the Russian Foundation for Fundamental Investigations (Grant No. 95-03-08365a). References and Notes (1) Folkmann, G. E.; Hatem, G.; Fehrmann, R.; Gaune-Escard, M.; Bjerrum, N. J. Inorg. Chem. 1993, 32, 1559. (2) Nielsen, K.; Fehrmann, R.; Eriksen, K. M. Inorg. Chem. 1993, 23, 4714. (3) Boghosian, S.; Fehrmann, R.; Bjerrum, N. J.; Papatheodorou, G. N. J. Catal. 1989, 119, 121. (4) Bal’zhinimaev, B. S.; Ivanov, A. A.; Lapina, O. B.; Mastikhin, V. M.; Zamaraev, K. I. Faraday Discuss. Chem. Soc. 1989, 87, 133. (5) Eriksen, K. M.; Karydis, D. A.; Boghosian, S.; Fehrmann, R. J. Catal. 1995, 155, 32. (6) Fehrmann, R.; Hansen, N. H.; Bjerrum, N. J. Inorg. Chem. 1983, 22, 4009. (7) Hagisawa, T.; Takai, T. Sci. Papers Inst. Phys. Chem. Soc. 1937, 31, 677. (8) Cambi, L.; Bozza, G. Ann. Chim. Appl., 1923, 13, 221. (9) Hansen, N. H.; Fehrmann, R.; Bjerrum, N. J. Inorg. Chem. 1982, 21, 744. (10) Hatem, G.; Fehrmann, R.; Gaune-Escard, M.; Bjerrum, N. J. J. Phys. Chem. 1987, 91, 195. (11) Andreasen, H. A.; Bjerrum, N. J.; Foverskov, C. E. ReV. Sci. Instrum. 1977, 48, 1340. (12) Jones, G.; Bradshaw, B. C. J. Am. Chem. Soc. 1933, 55, 1780. (13) Gaune-Escard, M. In Molten Salt Techniques; Gale, R., Lovering, D. G., Eds.; Plenum Press: New York, 1991; Chapter 5. (14) Hansen, N. H.; Bjerrum, N. J. J. Chem Eng. Data 1981, 26, 13. (15) Delimarskii, Iu. K., Markov, B. F., Eds. Electrochemistry of Fused Salts (English ed.); The Sigma Press: Washington, DC, 1961. (16) Hatem, G.; Fehrmann, R.; Gaune-Escard, M. Thermochim. Acta 1994, 243, 63. (17) Folkmann, G. E.; Hatem, G.; Fehrmann, R.; Gaune-Escard, M.; Bjerrum, N. J. Inorg Chem. 1991, 30, 4057. (18) Karydis, D. A.; Boghosian, S.; Fehrmann, R. J. Catal. 1994, 145, 312. (19) Bridgman, P. W. Proc. Am. Acad. 1917, 52, 119. (20) Meehan, B. J.; Tariq, S. A.; Hill, J. O. J. Thermal Anal. 1977, 12, 235. (21) Tahoon, K. K. Acta Phys. Polon. 1994, 86, 349. (22) Ha¨hle, S.; Meisel, A. Z. Chem. 1968, 8, 278. (23) Mastikhin, V. M.; Lapina, O. B.; Simonova, L. G. React. Kinet. Catal. Lett. 1984, 26, 431. (24) Rosenberger, H.; Scheler, G.; Moskvitch, Yu. N. Magn. Reson. Chem. 1989, 27, 50. (25) Loopstra, L. H.; MacGillavry, C. H. Acta Crystallogr., 1958, 11, 249. (26) Cotton, F. H.; Frenz, B. A.; Hunter, D. L. Acta Crystallogr. 1975, B31, 302. (27) Pople, J. A.; Scheiner, W. G.; Bernstein, H. J. High Resolution Nuclear Magnetic Resonance; McGraw-Hill Book Company Inc.: New York, 1959.
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