J. Phys. Chem. A 2000, 104, 53-58
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Kinetics of Deuterium Exchange on Resorcinol in D2O at High Pressure and High Temperature Shi Bai, Bruce J. Palmer, and Clement R. Yonker* EnVironmental and Energy Sciences DiVision, Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed: April 12, 1999; In Final Form: August 23, 1999
The kinetics of deuteration of resorcinol in pure D2O were studied for the first time using a flow-through capillary tubular reactor with on-line, proton, and deuterium NMR detection at high temperatures and high pressure. The global rate constants for hydrogen/deuterium (H/D) exchange were determined from temperatures of 200-450 °C (723 K) at a pressure of ∼400 bar (the critical temperature and pressure of water are 374.2 °C and 218.3 bar, respectively). The H/D exchange rate in resorcinol (1,3-dihydroxybenzene) under these extreme conditions was determined using proton NMR as a function of the resorcinol residence time in a capillary tubular reactor, which also served as a high-pressure NMR cell. The 1H and 2H NMR results indicate that H/D exchange in resorcinol for the ring protons was observed at temperatures as low as 200 °C. The kinetics of H/D exchange in resorcinol and the activation energy was extracted from the experimental 1H NMR data.
Introduction In subcritical and supercritical water the hydrothermal oxidation of many organic compounds is enhanced due to the increased reactivity of the substrate with water and the enhanced solubility of nonpolar organic compounds at these high temperatures. The increased reactivity of organic compounds in subcritical water shows potential for the oxidative destruction of toxic and organic wastes.1,2 Because of the enhanced miscibility of many nonpolar organic compounds with supercritical water, this solvent may prove to be a new reaction medium for organic chemistry.3,4 Hydrogen/deuterium (H/D) exchange of some extremely weak organic acids in subcritical and supercritical water has been investigated.5-7 Yao and Evilia5 estimated the equilibrium constant for H/D exchange of benzene with OD- to be 3 orders of magnitude higher at 400 °C than at 25 °C in water. The hydrogen in the ortho position was found to be slightly more acidic than those in the meta or para positions for the substituted benzene compounds of fluorobenzene, 1,2-diphenylhydrazine, and nitrobenzene, when reacted under basic conditions in D2O at 400 °C.5 However, Yao and Evilia encountered difficulties when attempting to obtain quantitative chemical kinetic data for these reactions due to the large experimental errors in determining temperature, pressure, and heating time when using a batch reactor. Kuhlmann et al.6 reported the first-order global rate constant (1.47 × 10-4 s-1) for H/D exchange of the R-methyl protons in pinacolone at 225 °C and 25.4 bar using a batch reactor. In situ, real time Raman spectroscopy has been used in the kinetic investigations of the oxidation of methane,8 methanol,9 and water-gas shift reactions10 in supercritical water. Fiberoptic11 and other Raman spectroscopic methods12 have been applied to examine the rates of chemical reactions in supercritical water. Other techniques, such as fluorescence, UV* Corresponding author. E-mail:
[email protected].
vis, and IR, have also been used to characterize reactions in subcritical and supercritical water.11 Recently, a review of reactions at supercritical water conditions has been published by Savage.13 NMR has not been widely used for in situ investigations of reactions in supercritical water; its main use has been to monitor batch reaction products after quenching.6,14 There is one report of the in situ monitoring by NMR of hydrogen exchange in supercritical water.15 The use of NMR detection for the in situ study of chemical kinetics generally requires that the NMR data acquisition time for a spectrum be much shorter than the reaction half-life. With flow-through NMR,16-18 it is possible to investigate chemical reactions provided the equilibrium Boltzmann distribution of nuclei spin states is not violated by rapid flow rates. Applications and technical concerns using flow NMR have been previously discussed.19-21 We describe our experimental effort using a novel combination of a high-pressure, high-temperature flow-through capillary tubular reactor with on-line NMR detection for the investigation of reaction kinetics in subcritical and supercritical water. Hightemperature, high-pressure 1H NMR allowed the measurement of peak area, and therefore concentration of reactant as a function of reactor residence time. 2H NMR was used to monitor the extent of the deuteration reaction under similar conditions. We assume that steady-state concentration conditions are rapidly achieved in the heated portion of the capillary due to the very low thermal impedance of the capillary reactor wall (∼90 µm thick), which should contribute to the rapid heating of the solution. Of course, this is dependent on the flow rate through the capillary, but over the flow range studied, we believe this assumption to be valid. Using this high-temperature, highpressure, microvolume flowing NMR technique, the kinetics of H/D exchange of resorcinol in subcritical and supercritical water were studied. The global rate constant for H/D exchange was determined by the disappearance of the R-hydrogens in resorcinol (reactant) under these extreme conditions. The kinetic data for the resorcinol H/D exchange reaction in pure D2O and
10.1021/jp991192r CCC: $19.00 © 2000 American Chemical Society Published on Web 12/09/1999
54 J. Phys. Chem. A, Vol. 104, No. 1, 2000
Bai et al.
Figure 1. Schematic diagram of the apparatus used for the highpressure, high-temperature flow-through capillary tubular reactor with on-line NMR detection.
the difference in the reactivity between the R- and β-hydrogens in resorcinol are discussed. Experimental Section Sample Preparation. Resorcinol (Aldrich Chemical Co., Inc., 99+%) was used without further purification. To deoxygenate the solution, D2O (Cambridge Isotope Laboratories) was boiled for 5 min under dry nitrogen and a 20% (w/w) (∼2.2 M) resorcinol-D2O solution was prepared and purged using dry nitrogen for 10 min. Flow-Through NMR. A schematic of the experimental system is shown in Figure 1. This system includes a highpressure syringe pump (ISCO 260D), a tubular piston separator (HIP), a fused-silica capillary NMR cell (180 µm i.d. by 360 µm o.d., Polymicro Technologies, Inc), and back-pressure regulators (Upchurch Scientific). In practice, the resorcinolD2O solution was loaded into one side of the piston separator and the other side was filled with water and connected to the syringe pump. The fused silica capillary was bent in a U shape and placed in a 10 mm glass NMR tube so that the bent end did not extend below the bottom edge of the NMR sensitive region along the z-axis. One end of the capillary tube was connected to the solution outlet of the piston separator, and the other end of the capillary was connected to the back-pressure regulators. When the high-pressure syringe pump was operated in a constant volumetric flow rate mode, the sample pressure was controlled by the back-pressure regulators. Five backpressure regulators (∼80 bar each) were used in series so that the sample pressure was maintained between 380 and 400 bar depending on the flow rate. The residence time (tR) of the resorcinol in the heated region of the capillary can be determined as
tR ) Vcap(F′/MR)
(1)
where F′ is the density in the heated region, Vcap is the volume of the capillary within the NMR detection region, and MR is the mass flow rate of the solution in the heated reactor. The difficulty is accurately determining the mass flow rate of the solution. Since, the mass flow rate is constant throughout all temperature regions of the capillary, one can write a similar equation for residence time at room temperature. Equating the two mass flow rates, allows one to express the residence time of resorcinol in the heated region in terms of the volumetric flow rate at room temperature as
tR ) (F′/F)(Vcap/fr)
(1′)
Figure 2. Plot of the natural logarithm of the rates for the thermal decomposition of AIBN from ref 24 (b) versus reciprocal temperature compared to the experimental rate constants (O) determined by the flow-through NMR technique.
Here F is the solution density at room temperature and fr is the volumetric flow rate of the high-pressure pump at room temperature. A step profile is assumed for the temperature gradient in the reactor region. Using the inner diameter (180 ( 6 µm) of the capillary and twice the length of the NMR receiving coil (3.90 cm), the volume of the NMR detection region was estimated to be ∼9.8 × 10-4 mL (0.98 µL). The density at high temperature was approximated using the density of pure D2O at the experimental temperature and pressure.22 A standard reaction was chosen for initial investigation with the capillary flow-through reactor using the well-characterized thermal decomposition of 2,2′-azobis(isobutyronitrile) (AIBN), a radical initiator. The thermal decomposition of this molecule was investigated in the same manner as outlined in the Experimental Section for resorcinol. The temperature range investigated was 120-150 °C at ∼190 bar, in this case using perdeuterated benzene as the solvent. The first-order rate constants were determined at four different temperatures and from an Arrhenius plot of the data, an A factor of 1.57 × 1018 s-1 (ln A ) 41.9 ( 3.2) and an activation energy (Ea) of 35.3 ( 2.6 kcal/mol were obtained. The values reported in the literature using standard techniques for AIBN are A ) 1.58 × 1015 s-1 and Ea ) 30.8 kcal/mol.23,24 At 120 °C the rate constant for thermal decomposition of AIBN is 0.0193 s-1.24 The value determined using the flow-through NMR technique is 0.0375 ( 0.0118 s-1. Figure 2 is a plot of the natural logarithm of the experimental rates for AIBN compared with the literature values versus reciprocal temperature.24 The two sets of data are comparable. The derivations of the concentration profile in the NMR reactor cell will be discussed in the next section. The accuracy of the solute residence time in the heated region also depends on the volumetric flow rate of the pump. A system calibration was performed at room temperature and a pressure of 400 bar by collecting and weighing the water eluted from the capillary at specific flow rates over a set time interval. We found a 1:1 relationship between the pump volumetric flow rate and the measured volumetric flow rate over the range of interest. NMR Measurements. The NMR data for the AIBN thermal degradation experiments was obtained on a Varian VXR 300 NMR spectrometer, while the resorcinol-D2O H/D exchange experiments were performed on a Chemagnetics CMX-300 wide-bore spectrometer operating at 299.3 and 46.13 MHz for proton and deuterium detection, respectively. A high-temperature, high-resolution, specially built broad band 10 mm NMR probe (Doty Scientific, Inc.) was used for the D2O experiments.
Deuterium Exchange on Resorcinol
J. Phys. Chem. A, Vol. 104, No. 1, 2000 55
The resolution was 8-10 Hz for proton and 4-8 Hz for deuterium in the resorcinol-D2O solution in the capillary cell at room temperature, which generally improved at higher temperatures due to a decrease in viscosity of the resorcinolD2O solution. The z-axis temperature profile for the hightemperature probe was determined. The temperature at various positions in the NMR sensing region was measured using a calibrated thermocouple along the z-axis at 325 °C. A temperature deviation of (5 °C was noted in this calibration. The relative reproducibility in the temperature setting of the probe is estimated to be about (2 °C for low (250 °C) temperatures, respectively. The calculated Reynolds number for the conditions investigated in the capillary was
