Experimental spectroscopic high-temperature high-pressure techniques for studying liquid and supercritical fluids

Vibrational Spectroscopy 35 (2004) 97–101

Experimental spectroscopic high-temperature high-pressure techniques for studying liquid and supercritical fluids Yuri E. Gorbatya, Galina V. Bondarenkoa, Eleni Venardoub, Stephen J. Barlowb, Eduardo Garcia-Verdugob, Martyn Poliakoffb,* a

Institute of Experimental Mineralogy, Russian Academy of. Sciences, Chernogolovka Moscow Region, 142432 Moscow, Russia b School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK Received 10 October 2003; received in revised form 1 December 2003; accepted 1 December 2003 Available online 3 February 2004

Abstract The purpose of this paper is to share our many years of experience in designing and using high-temperature, high-pressure (HTHP) devices for studying the behavior of fluids and chemical reactions at elevated temperatures and pressures. The HTHP cells for IR and Raman spectroscopic techniques described in the paper are unique and can be used at pressures of up to 1500 bar and temperatures up to 550 8C. However, the most attractive feature of these designs is that they provide high accuracy of measurements, in most cases even higher than that achieved with standard cells at ambient conditions. The performance of these IR and Raman HTHP spectroscopic cells is demonstrated using some representative spectra. # 2003 Elsevier B.V. All rights reserved. Keywords: Vibrational spectroscopy; High pressure; High temperature; Supercritical fluids

1. Introduction Apart from the need to improve our fundamental understanding of the structure and properties of liquids and supercritical fluids, high-temperature high-pressure (HTHP) studies of liquids and supercritical fluids have been greatly stimulated by the recent achievements in environmentfriendly technologies [1–5]. A supercritical fluid is defined as ‘‘any substance the temperature and pressure of which are higher than the critical values, and which has a density close to or higher than its critical density’’ [1]. The role of vibrational spectroscopy to gain an insight into these systems is crucial and there have been numerous attempts to construct suitable HTHP devices for this purpose [6,7]. In this short paper, we describe some examples of how spectroscopic cells can be effectively used for studying the behavior of fluids and chemical reactions at high temperatures and pressures. The HTHP equipment created over the past few years in our laboratories covers the range of wavelengths from UV to mid-IR and the common range of the Raman shift [8–13]. * Corresponding author. Tel.: þ44-115-951-3520; fax: þ44-115-951-3058. E-mail addresses: [email protected] (Y.E. Gorbaty), [email protected] (M. Poliakoff).

0924-2031/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2003.12.002

The equipment provides high accuracy of measurements up to 500–550 8C and pressures of up to 1500 bar.

2. HTHP cell for IR and FTIR spectroscopy The most commonly used HTHP IR cells have fixed pathlengths, which are determined by the spacers used in each case, and usually thin gaskets are used as well to provide effective sealing [14,15]. Cells of this type have certain shortcomings. For example, it is very difficult to make a spacer if the required pathlength should be very small. Under high temperatures and pressures the body of any spectroscopic cell is subject to thermal and mechanical deformations, which can essentially exceed the thickness of a very thin spacer. The cells with fixed pathlengths cannot be used with only one size of spacer in the full range of temperatures and pressures needed to be explored. Because of a strong change in density or due to a chemical reaction, the concentration of a substance under study may change significantly during the experiment. In this case, the experiment should be interrupted and the cell disassembled to insert another spacer, depending on the required intensity of the signal under question. The disassembling procedure can seriously affect the reproducibility of the results and the

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corrections on the numerous factors influencing the accuracy of measurements are very difficult and often impossible to introduce. Another type of HTHP IR cell has been designed to provide the possibility of a changeable pathlength [8]. The pathlength can be varied in situ during a run at high pressures and temperatures. As will be shown below, such a cell allows one to overcome the difficulties mentioned above and to avoid most common and specific errors of intensity measurements. We describe here such a HTHP cell with changeable pathlength designed to study highly IR-absorbing substances like water and carbon dioxide in a wide range of densities. The importance of such experiments is growing rapidly due to the fast development of supercritical technologies [16,17]. The unique feature of this cell is the possibility of changing smoothly and with high precision the separation between windows, directly during a run at high temperature and pressure. The prototype of this cell has been described elsewhere [11]. The present design shown in Fig. 1 differs radically from the prototype. The drive mechanism has been greatly improved, so that it allows one to change the pathlength manually, despite the force that draws the windows apart, which can reach 1.5 t. In the new design, the whole body of the cell moves about the window mounting; the latter is fixed in the pillar of the drive mechanism. This facilitates measuring small shifts of the movable window. The new cell can work both in static and in flow mode.

Finally, there are no caps on the sapphire windows in the new design; this allows one to diminish the internal diameter of the cell and hence reach a higher pressure. The preliminary sealing is achieved by pressing the windows against each other with the aid of the drive mechanism. As pressure is applied, the separation between the windows becomes larger because of deformation of the parts of the cell under stress. Then, the principle of unsupported area comes into effect. All the sealings in the cell are made of GraFlex (elastic graphite) and they are self-sealing, except the gland, in which movable window mounting (WM2 in Fig. 1) travels. Perhaps the most difficult aspect of HTHP IR absorption spectroscopy is to obtain precise values of pathlengths. The pathlengths are usually very small, sometimes much smaller than the deformation of the parts of cell under stress which occurs due to thermal expansion. A special measuring technique has been developed to avoid common errors and to obtain accurate values of pathlengths. After temperature and pressure are stabilized, we obtain two spectra—one at high transmittance (in the FTIR technique this is a background spectrum) and another at lower transmittance. It is not possible to know the absolute value of pathlengths but we can measure the difference, D, between the pathlengths quite accurately. Then the spectra of absorption coefficient, k, can be calculated as the difference of absorbances divided by D and the molar concentration. However, to measure small difference in the pathlengths is itself not an easy task. We have developed the following method to enhance the accuracy of measurements, demonstrated in Fig. 2. First, the absorbance as a function of the window shift has to be found at a chosen wavenumber. To obtain the value of D, an optical scale has been attached to the cell body. A measuring microscope with a spiral scale is used to measure the shift, d, of the scale, with an accuracy of 0.5 mm. It is possible then to obtain two spectra with suitable maximum absorbances and calculate the difference

iso - propanol 350 ˚C, 300 bar

∆ = δ1 δ2

1.5

δ1

A1

iso-propanol 350 ˚C

1.0

0.5

A 7 . 7 -1 ) 0 -9 m .9 0 c 80 356 1 δ= at (

A2 δ2

100-1000 bar

Absorbance

Absorbance, A

2.0

4000 3500 3000 2500 Wavenumber / cm-1

0.0 Fig. 1. HTHP IR cell. MT, movable thermocouple; IN1 and IN2, inserts; NL, nut with lugs; LS, lead screw; TB, thrust bearing; PI, pillar of the drive mechanism; L1 and L2, levers; LI, link; AP, arm of pillar; WM1, fixed window mounting; WM2, movable window mounting; FL, flange; SW, sapphire window; GS, GraFlex sealing; PV, pivot.

0

20 40 60 80 100 120 140 160

Window travel, δ / µ Fig. 2. Method for determining pathlength difference, D, for spectra of iso-propanol obtained at 350 8C at different pressures. Absorbance measurements were made at 3560 cm1.

Y.E. Gorbaty et al. / Vibrational Spectroscopy 35 (2004) 97–101

k x 10-3/ cm2 mol-1

120 1000 bar, 3 mol.% HDO in H2O 100

(a)

80 60 40 20

99

1000 bar, 1.1 M NaCl in HDO/H2O

20 ˚C 50 ˚C 100 ˚C 150 ˚C 200 ˚C 250 ˚C 300 ˚C 350 ˚C 400 ˚C 450 ˚C 500 ˚C

20 ˚C 50 ˚C 100 ˚C 150 ˚C 200 ˚C 250 ˚C 300 ˚C 350 ˚C 400 ˚C 450 ˚C 500 ˚C

(b)

0 2800

2600 2400 2200 Wavenumber / cm-1

2800

2600 2400 2200 Wavenumber / cm-1

Fig. 3. Absorption spectra of nOD HDO in water and aqueous solution of NaCl obtained at a constant pressure of 1000 bar in the temperature range 20–500 8C.

Peak position / cm-1

2650

1000 bar 2600

2550 HDO NaCl solution

2500

Integrated Intensity

250 HDO NaCl solution

200 150 100 50 0

100 200 300 400 500

Temperature /˚C Fig. 4. The temperature behavior of the peak position and intensity of the band nOD HDO in a 5% aqueous solution of NaCl, and pure water, at 1000 bar and up to 500 8C.

Absorption / a.u.

of pathlengths, using the equation shown in Fig. 2. If the absorbance of a particular band is too high, one can choose any point on the slope of the band with a lower absorbance. The cell performance can be illustrated by a study of an aqueous solution of NaCl. Fig. 3a shows spectra of HDO in H2O (3 mol%), while Fig. 3b shows spectra of 1.1 M NaCl solution in the same mixture. It might be assumed that about 25% of water molecules form hydration shells around ions. It could be therefore expected that the addition of an electrolyte had to change essentially the character of hydrogen bonding in the solvent and hence the position, shape and intensity of the nOD HDO should be quite different for pure water and for a NaCl solution. It was surprising, however, to discover that no essential differences could be found between the spectra of the solvent and solution, as shown in Fig. 3. So, only a very detailed analysis of the shape and intensity of the observed bands can help to understand whether the ions of the dissolved salt influence the character of hydrogen bonding in water. Indeed, the difference in the integrated intensities and positions of nOD HDO band for the, pure solvent and for the solution shown in Fig. 4 turned out, to be very small. However, these characteristics are very sensitive to the extent of hydrogen bonding, so we can say that, most likely, hydrogen bonding is slightly stronger in water at low temperatures, while at high temperatures it is stronger in the solution. The experienced reader can easily estimate the accuracy of the described technique. Another example of the use of our HTHP cell with FTIR is a preliminary attempt to see how high temperatures and pressures influence the character of hydrogen bonding in acetic acid. Acetic acid is known as a substance with very strong hydrogen bonds between molecules. It is assumed that at least at low density hydrogen bonding results in forming stable cyclic dimers, but the acid crystallizes in long chains. Fig. 5 shows spectra of acetic acid in the region of OH stretching region obtained at isobaric heating up to 350 8C, at a pressure of 300 bar (higher than the critical value by a factor of five), in batch. The spectra were collected with a Perkin-Elmer (Model 2000) FTIR spectrometer and they are quite complicated and difficult to interpret.

Acetic acid P = 300 bar 250 ˚C 300 ˚C 350 ˚C

4000

3500

23 ˚C 50 ˚C 100 ˚C 150 ˚C 200 ˚C

3000

2500

2000

Wavenumber / cm-1 Fig. 5. FTIR spectra of acetic acid obtained during isobaric heating at 300 bar, from room temperature to 350 8C.

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The narrow bands on the low energy side of the broad continuum belong, most likely, to C–H vibrational modes [18]. It is also clear that the spectra reveal a weakening of hydrogen bonds as the temperature rises, as manifested by the blue shift of the broad O–H band by about 230 cm1. The spectra seem to change monotonically up to 300 8C. In principle, acetic acid should be the most stable and hence the most inert of all aliphatic carboxylic acids, because of the electron-donating CH3 group that strengthens the C–C bond [19]. However, at 350 8C a radical change occurs. A new strong band at 2340 cm1 appears, which can be assigned to CO2 [20]. This undoubtedly indicates the onset of acetic acid decomposition. It is interesting that hydrogen bonding in acetic acid is so strong that the decomposition takes place prior to the complete breaking of hydrogen bonds. We have also recently designed another HTHP cell with a changeable pathlength that can be used for NIR spectroscopy. The cell has been described in detail elsewhere [12].

3. HTHP cell for Raman spectroscopy

Unlike the Renishaw instrument, the Almega spectrometer (Thermo Nicolet) has a separate sample compartment that is used independently of the microscope. The cell was installed on the XYZ motorized stage in the compartment, using a simple adapter. However, in some cases, the cell had to be aligned manually, which could be a time-consuming procedure, but still the use of the cell directly with the spectrometer was also quite effective. An example demonstrating the performance of the cell involves the collection of spectra of aqueous solutions with the multi-atomic anion like [NO3], revealing strong lines in Zn[NO3 ]2 aq. sol., 1000 bar

20 200 300 350 400 1850 cm-1 450 e tur era mp Te

/˚ C

An advantage of Raman spectroscopy over the complementary technique of IR absorption is that the materials most frequently used for optical windows, such as sapphire or diamond, are transparent over the whole region of vibrational modes and hence they do not interfere with the spectra of the analyte. Raman spectroscopy has been used previously in conjunction with HTHP equipment [21–27], but most cells are based on the classic 908 geometry, where the scattered light is collected normal to the incident laser beam. A back-scattered, or 1808 geometry, simplifies the design of an HTHP cell and increases reliability at high pressures, since only one window is required. Here, a miniature HTHP Raman cell is briefly described, which can be used either with a microscope (Fig. 6a) or directly with a Raman spectrometer. The details of the cell and its sealing mechanism can be found elsewhere [13]. Fig. 6b shows the principle design of the cell. The cell is provided with a single sapphire window, 6 mm in diameter, with an unsupported area of 3.5 mm in diameter, which is sufficient for working with modern, high-sensitivity Raman spectrometers. All parts of the cell are made of the same heat and corrosion resistant Ni–Cr alloy that the IR cell, described previously, is made. The Raman cell has been tested for several hours at 580 8C and 1800 bar. It was used with two different commercial Raman spectrometers. Firstly, with the Leika DMLM microscope, attached to a Renishaw RM1000 Raman System (Fig. 6a). In this case the laser beam is directed into the cell through the standard Renishaw angular accessory with a working distance of 60 mm fixed onto the nosepiece of microscope, in place of an objective. The cell is mounted on the microscope stage, and by viewing through the microscope it is very easy to move the cell using the XYZ adjustments and hence choose the optimum position of focus.

Fig. 6. (a) The Raman cell installed on a Renishaw Raman microscope. RC, Raman cell; MN, microscope nosepiece; AA, angular accessory; HD, holder; TS, telescopic light shield; MS, microscope stage. (b) Side view of the miniature HTHP Raman cell. IN1 and IN2, inlets; GY, guiding yoke; EC, cell body; SG; shallow groove for thermocouple; TW, thermocouple well; GS, GraFlex sealing; FL, flange.

4000 3000 2000 1000 Raman shift / cm-1

0

Fig. 7. Raman spectra of 5 mol% aqueous solution of Zn[NO3]2 obtained during the isobaric heating at 1000 bar, from 20 to 450 8C.

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the Raman spectra. This experiment was aimed at the understanding of the effect that various ions have on the structure of supercritical water. Water becomes a supercritical fluid at 374 8C and 221 bar. In this experiment, the spectra of 5 mol% Zn(NO3)2 in water were obtained at a constant pressure of 1000 bar and temperatures up to 450 8C. As shown in Fig. 7, at the near-critical temperature of 350 8C and supercritical temperature of 400 8C the spectrum of Zn(NO3)2 becomes very weak and at a further temperature rise, up to 450 8C, it disappears completely. In the spectra obtained at 400 and 450 8C a new weak band near 1850 cm1 can be observed, that can be attributed to the NO molecule. So, it may be assumed that the [NO3] anion becomes unstable at supercritical temperatures.

Acknowledgements Support from the Royal Society, the EPSRC (Grant GR/ N06892), the EU Marie Curie Research Programme and the Russian Basic Research Foundation (Grants 03-05-64332 and 03-03-32950) is greatly appreciated. We thank Prof. M.W. George, Dr. K. Whiston and Mr. W.B. Thomas for their help and Invista Process Technologies for financial support. We are very grateful to Messrs. M. Dellar, M. Guyler, J. Whalley, R. Wilson, P. Fields and K. Hind for technical assistance.

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