J. Phys. Chem. 1996, 100, 7455-7462
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Spectroscopy of Hydrothermal Reactions. 1. The CO2-H2O System and Kinetics of Urea Decomposition in an FTIR Spectroscopy Flow Reactor Cell Operable to 725 K and 335 bar M. L. Kieke, J. W. Schoppelrei, and T. B. Brill* Department of Chemistry, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: April 4, 1995; In Final Form: October 27, 1995X
A combined microflow reactor and short-path-length spectroscopy cell along with the accompanying process controls are described to obtain real-time, in situ transmission IR spectra of reaction components of aqueous solutions up to 725 K and 335 bar. Quantitation of the spectra was required to obtain kinetics and equilibrium constants. The extinction coefficient of CO2 in H2O at 275 bar was found to increase monotonically from 1.52 × 106 at 298 K to 2.26 × 106 cm2 mol-1 at 573 K. Also, CO2 dissolved in H2O was rotationally quenched on the IR time scale below 375 K but progressed into rotational diffusion around 625 K and finally essentially free rotation above 700 K. The kinetics and pathway of hydrothermolysis of urea to CO2 and NH3 were determined directly from spectral data at 473-573 K. Good agreement was obtained between experimental and calculated concentration-time data by using a reaction model consisting of (NH2)2CO f NH4+ + OCN- and NH4+ + OCN- + H2O f CO2 + 2NH3. The Arrhenius parameters for the first-order reaction are Ea ) 84.2 kJ mol-1 and ln A (s-1) ) 17.5, and for latter pseudo-second-order reaction are Ea ) 58.5 kJ mol-1 and ln A (L mol-1 s-1) ) 17.1. The global rate of formation of CO2 without the kinetic model is first-order and has different Arrhenius parameters. As part of this study, the species of 0.1 m (NH4)2CO3 equilibrium were determined at 298-650 K and 275 bar. The equilibrium shifted from the hydrolyzed ionic components at lower temperature to the neutral CO2, NH3, and H2O components at higher temperature. Therefore, the (NH4)2CO3 equilibrium does not influence the kinetic model of urea above about 475 K.
I. Introduction Kinetics and mechanisms of reactions in H2O below 373 K are among the most widely studied areas of chemistry.1,2 Conversely, determination of reaction kinetics and mechanisms in H2O at hydrothermal conditions (e.g., above about 493 K) is a leaner subject, although extensive thermochemical information is available.3 Most spectroscopic studies under hydrothermal conditions are of H2O itself or of nonreacting electrolytes and nonelectrolytes dissolved in H2O, as opposed to chemical reactions. Many barriers confront real-time, in situ spectral measurements on hydrothermal reactions. First, relatively passive cellwall materials are desirable because H2O at high temperature and pressure can be corrosive, especially when inorganic ions are also present.4-6 Diamond or sapphire windows enable optical access during spectrokinetic measurements. Second, although several studies are available,7-25 the relationships among peak height, area, line shape, concentration, and molecular environment are not well-known and are essential for kinetic analyses based on spectral data. Directly measured global reaction rates have only recently begun to appear.24,25 Third, real-time measurement and control of the temperature, pressure, and flow rate require considerable effort, partly because of the limited number of materials and devices suitable for use with H2O at high pressure and temperature and partly because of the intrinsic physical properties of hydrothermal and supercritical H2O. The phase diagram for pure H2O at high temperature and pressure is well characterized,26 including the vapor-liquid critical point at 647 K and 221 bar. Around the critical point, spectroscopy and chemical processes may be difficult to interpret because of the nonlinear, rapidly changing physicochemical * Correspondence author. X Abstract published in AdVance ACS Abstracts, April 1, 1996.
S0022-3654(95)00964-6 CCC: $12.00
properties.27 In the critical point region, the isothermal compressibility and specific heat of H2O approach infinity. Temperature and density gradients introduce anomalous scattering of radiation (critical opalescence) which affects the spectral data. Chemical equilibrium may develop comparatively slowly because diffusion controls the rate of mixing. Any of these effects alone complicates interpretation of chemical processes and spectroscopy. Taken together, these phenomena are an exhortation to spectroscopists to be cautious in the pressure-temperature region close to or at the critical point. On the other hand, applications of hydrothermal H2O can be envisioned in the subcritical and supercritical regions of the phase diagram. Hydrothermal synthesis may be aided by the high solubility of organic compounds. An increasingly acknowledged thrust is the use of oxidative hydrothermal technology to destroy toxic waste.28-30 For direct study of chemical processes in H2O at high temperature, a microflow reactor, which is also a precision IR spectroscopy cell, was built for operation at constant, chosen conditions in the ranges of 293-725 K, 1-335 bar, and 0.052.5 mL/min flow rate. A detailed description of the flow reactor and operating procedure is provided here because this is the first article on this subject. The experiences may be useful to accelerate future developments. IR absorption by CO2 in H2O was analyzed because CO2 is a major product of hydrothermal reactions in which the reactants contain carbon. Large changes in the spectrum of CO2 were induced by different temperatures. This information is essential for determining the kinetics and pathways of decomposition of urea in the hydrothermal environment. Urea solutions are important to understand because urea is the chief product of nitrogen metabolism in mammals, is a fertilizer component, and is a component of numerous commercial products. The (NH4)2CO3 equilibrium was investigated spectrally to define the © 1996 American Chemical Society
7456 J. Phys. Chem., Vol. 100, No. 18, 1996
Figure 1. Schematic diagram of the flow system for aqueous solutions at high temperature and pressure. ANSI symbols are used.
behavior of the thermodynamically stable final products of urea under hydrothermal conditions. This article is the first of a series in which IR and Raman spectroscopy will be used to construct the foundation for reactions of simple organic and inorganic molecules in the hydrothermal environment. Reaction schemes of more complicated molecules can be assembled and understood from this information. II. Experimental Considerations The control of the feed solution, flow rate, and pressure is described first, followed by the IR spectroscopy cell in which the reaction occurs. When studying reactions in H2O up to 725 K, compounds which produce a solid residue that could plug the flow cell and transfer lines must be avoided. Therefore, preliminary tests are conducted with an inexpensive batch reactor and fluidized sand bath.31 A. Flow System. A schematic diagram of the flow system and controls is shown in Figure 1. The essential features are discussed below in sequence from the sample reservoir to the waste ballast tank. A 300 mL sample reservoir (Autoclave Engineers), which can be stirred, heated, and pressurized with Ar gas was maintained at conditions such that no reaction occurred. All transfer lines (0.3175 cm o.d. × 0.155 cm i.d. type 316 stainless steel (SS) tubing) and subsequent components were maintained at the reservoir temperature. The solution was pumped from the reservoir against the back pressure on the system by using a dual piston HPLC pump (LDC Analytical). A 5 µm sintered metal filter was placed before the pump to remove any particles. Flow rates were set by the pump, which was calibrated by weighing the amount of fluid that passed through the back-pressure regulator (vide infra) in a specific time increment. The pressure was determined by using a digital melt pressure transducer (Omega Engineering) which was calibrated independently with a Bourdon gauge. The fluid then flowed into the reactor spectroscopy cell which was positioned in the sample compartment of the spectrometer. The temperature of the cell was set so that the fluid reached the desired temperature and reactions occurred in the spectral observation volume. Details of this cell are described in Section II.B. Natural convection from the exit line of the cell to the surroundings cooled the solution. A manifold of tubes arranged in parallel enabled six aliquots, each containing 3.5 mL of fluid from a different reaction temperatures, to be collected for post reaction analysis if desired. Each trap was a 5 cm length of 0.635 cm o.d. × 0.493 cm i.d. 316 SS tubing straddled by two
Kieke et al. ball valves (Whitey Severe Service). Postreaction determination of the products can be obtained by such techniques as GC or GC-MS. The rate of filling of these tubes was established by following the visible absorption spectrum of a KMnO4 solution as a function of time at a given flow rate. Because plug flow does not exist in the tubes, some mixing of the water flush and the KMnO4 solution occurred. Approximately 25 min was required to fill a tube with 80% of the initial KMnO4 concentration at a flow rate of 0.5 mL/min. The in-line calorimetric flow meter (Rheotherm Instruments) nonintrusively measured the rate of the effluent flow to 0.01 mL/min accuracy. Independent determination of the flow rate as described earlier was necessary to establish the exact rate. The in-line flow meter was essential to ensure that the fluid was moving as desired. A 35 µm sintered metal filter protected the flow meter from any solid particles. Steady flow without trapped gas voids was achieved by remachining the fittings where necessary and by pumping against gravity. The waste effluent was stored in a 1000 mL ballast tank (Whitey) rated for 670 bar. A check-valve was inserted between the flow meter and the ballast tank to prevent high-pressure back-flow in the event of a rapid pressure drop upstream from the ballast tank. Initially, pressure was applied to the ballast tank and, in effect, all of the fluid after the pump by regulation of a 400 bar tank of Ar. A back-pressure regulator (Circle Seal Controls) and a bleed-and-lock valve arrangement were both incorporated to maintain constant pressure ((1 bar) on the flowing system. Temperature indication, recording, and control (TIRC) at (0.5 K were achieved at important stages (autoclave, transfer lines, and cell) by using PID controllers (Omega Engineering). The outputs of these controllers, the pressure transducer, and flow meter can be transferred through an RS485 serial port into a PC so that real-time monitoring and a permanent record of the pressure, flow rate, and temperature were available. B. Flow Reactor-Infrared Spectroscopy Cell. The central component of the flow system is the reactor spectroscopy cell wherein the reaction was induced and monitored. The intention was to step the temperature of the solution from the cell inlet temperature to the desired reaction temperature in the time required for the fluid to reach the cell windows. The most suitable cell wall materials appear to be noble metals and alloys, such as Au, Pt, Pt/Ir, and Pt/Rh. Ta has also been used.16 Highstrength Ni superalloys, such as Hastelloy C-276 and Inconel 625, dissolve selectively6 but are useful if slow corrosion can be tolerated. Type IIa diamond is currently the only suitable window material for broad-range optical access in the IR region. Sapphire is transparent throughout a large segment of the UV, visible, and near-IR region but is opaque below about 1800 cm-1. Absorption by H2O at 3000-3600 cm-1 closed the window on the short-wavelength side. In the future we will describe a combined IR spectroscopy cell-flow reactor constructed of Pt/Ir alloy with diamond wafer windows. A detailed heat-transfer and fluid mechanics model has been developed for this cell. However, several technical difficulties impede its routine use. Once these problems are overcome, this cell can be used generally to determine rates and pathways of hydrothermal reactions. The IR spectra of the (NH4)2CO3 equilibrium described later in this article were obtained with this Pt/Irdiamond cell. Figure 2 shows the IR spectroscopy cell-flow reactor employed for the majority of spectral and kinetic measurements described herein. It was constructed from 316 SS and sapphire and was designed similarly to the Pt/Ir cell. Very slow corrosion was observed. Corrosion characteristics of 316 SS are described
Spectroscopy of Hydrothermal Reactions. 1
J. Phys. Chem., Vol. 100, No. 18, 1996 7457
Figure 2. Side and end view of the transmission IR spectroscopy cell designed for operation up to 725 K and 335 bar.
in more detail in the companion article.32 Because the spectral window of sapphire is limited in the mid-IR region, reactions must consist of species having IR absorbances in the window. In this cell design, a pair of circular 0° single-crystal sapphire disks (0.476 cm diameter × 0.239 cm thick) were recessed in the steel heater block and end flange as shown. Sealing of the cell was achieved by placing a gold foil washer between the steel block and the sapphire and between the sapphire windows. The block and flange were drawn together by five hardenedsteel bolts. The gold foil spacer separating the sapphire windows contained a slot which accepted the inlet and exit feed tubes and defined the flow path past the windows. The thickness of the gold spacer roughly defined the path length of the sample cavity. A pathlength of b ) 30 µm was sought but was determined to (1 µm in all spectral measurements from the interference fringe pattern by eq 1. The index of refraction, η,
b ) n/2η(ν1 - ν2)
(1)
for H2O was used by accounting for the pressure, temperature, and wavelength;33 n is the number of fringe maxima counted; ν1 and ν2 are the wavenumber values of the maxima of the first and last n value. Typically, the maximum available value of n was 8-10. No perceptible change in b ((1 µm) occurred during heating or pressurization. To enhance heat transfer and control of the operating parameters, tight tolerances ((0.001 cm) were maintained during construction of this cell. Heating of the cell was accomplished with five 3.8 cm long × 0.635 cm diameter 70 W cartridge heaters (Whiddet) which were inserted in the alternating pattern with the hardened-steel bolts. K-type thermocouples (0.8 mm) were inserted into fine holes drilled into the SS block near the windows and the exit tube. By the use of the PID controllers, the temperature of the fluid was known to (1 K as it flowed past the windows. The IR spectral change of pure H2O as the vapor-liquid phase boundary was crossed provided an in situ indication of the temperature accuracy. A Nicolet 60SX FTIR spectrometer employing an MCT detector was used for all spectral measurements. Holes having a cone angle of 16° were machined as entrance and exit light ports of the cell. This angle is several degrees larger than the cone angle of the IR beam and was chosen to minimize blockage of the beam by the SS block. The beam waist at the focal point is about 3 mm so that the diameter of the hole cut in the center of the Au foil spacer shown in Figure 2 was 3.17 ( 0.02 mm. The assembled cell was sandwiched between two calcium silicate insulation blocks into which conical optical ports and the cell cavity had been cut. The surface of the blocks was covered with ceramic cement to reduce dust. The insulation
block was clamped to an xyz translation stage mounted to the floor of the sample compartment of the spectrometer. The cell windows were positioned at the focal point of the IR beam by maximizing the interferogram. Flexible flanges were attached to the optical ports of the spectrometer to butt against the insulation block to permit purging of atmospheric CO2 and H2O from the beam path. Once installed and operating, we found that keeping the cell pressurized and heated to at least 425 K extended the life of the seals from days to months. C. Operation of the System. Initially, the entire flow track was filled with 18 MΩ H2O that was filtered at 0.2 µm and sparged with Ar for 30 min. Such sparging assured that no CO2 was detectable in this H2O. Water was pumped at a rate of 2-5 mL/min for about 1 h against gravity while gradually increasing the pressure to the desired level of 275 bar. During pressurization, the valves were tapped to dislodge any trapped bubbles. These procedures ensured steady fluid flow. To prevent gaseous cavities from developing in the fluid, the system was flushed thoroughly between studies with about 200 mL of H2O without interrupting the stream. Background IR spectra of pure water were recorded at the same spectrometer settings, temperatures, and pressures that were used for the sample solutions. The sample solution was introduced into the system at 1-2 mL/min without interuption of the steady flow. IR spectra were recorded until the spectrum was constant (20-30 mL) to ensure complete flushing. The temperature of the cell was increased, and the flow rate was set to the desired value. Once stabilized (e10 min), 32 spectra (2 spectra/s, 4 cm-1 resolution) were summed in one destination file. Each file was rationed against the spectrum of pure H2O under the same conditions. Stepping of the cell to higher or lower temperatures and changing the flow rate enabled spectra to be recorded for a wide variety of conditions on fresh samples from the reservoir. The optimum range of flow rates was 0.05-2.5 mL/min. The residence times at these flow rates were 412-1.67 s based on the calculated volume of the inlet tube and observation cavity of the cell in Figure 2 of 0.0604 cm3. As mentioned above, any volume changes induced by pressure and temperature were below experimental detection (
