Ind. Eng. Chem. Res. 1998, 37, 1755-1760
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Direct Observation of H2O2 during Alcohol Oxidation by O2 in Supercritical Water Eric Croiset and Steven F. Rice* Combustion Research Facility, Sandia National Laboratories, MS9052, P.O. Box 969, Livermore, California 94551-0969
This paper presents the direct quantitative measurement of the presence of hydrogen peroxide as an intermediate produced during the oxidation of methanol, ethanol, and 1-propanol by oxygen in supercritical water. The concentration of H2O2 is measured as a function of reaction time in an optically accessible, high-pressure flow reactor by monitoring the intensity of a strong resonance in the Raman spectrum at 874 cm-1 that is characteristic of the O-O vibrational stretch. Transient concentrations of H2O2 as high as 0.0065 mol/L (0.1% mole fraction) are measured during the oxidation of ethanol at feed mole fractions of 0.4% at 430 °C and 24.5 MPa. Significantly lower transient concentrations of H2O2 are observed during methanol oxidation. The experimentally observed transient H2O2 concentration during methanol oxidation is compared to that predicted by two similar elementary reaction mechanisms and found to be distinctly lower than the models predict. Introduction Hydrogen peroxide plays an important role during the oxidation of methanol by oxygen in supercritical water (Alkam et al., 1996; Rice et al., 1996; Dagaut et al., 1996; Brock et al., 1996), and by analogy it is likely that it is also formed during the oxidation of other simple alcohols. It is formed during the early stages of oxidation through the hydrogen abstraction reactions
ROH + •HO2 w RO• + H2O2
(1)
H-ROH + •HO2 w •ROH + H2O2
(2)
Sensitivity analysis of elementary reaction mechanisms shows that it is the most important species determining the overall rate of conversion. H2O2 functions as a relatively unreactive reservoir of oxidizer and controls the rate of conversion through its thermal decomposition by metering the supply of OH radicals to the system. Recently, the rate constant of H2O2 thermal decomposition in supercritical water was measured experimentally (Croiset et al., 1997). However, no data have been reported concerning direct measurement of hydrogen peroxide during oxidation experiments in supercritical water. This research note presents the quantitative direct observation of H2O2 by Raman spectroscopy during ethanol, methanol, and 1-propanol oxidation by oxygen in supercritical water. Experimentally observed transient H2O2 concentration during methanol oxidation is compared to that predicted by two similar elementary reaction mechanisms. Experimental Section Apparatus. The experiments described in the present paper were conducted in Sandia’s Supercritical Flow Reactor (SFR), described in detail elsewhere (Hanush et al., 1995; Rice et al., 1996). In these experiments, * Corresponding author. Telephone: (925) 294-1353. Fax: (925) 294-2276. E-mail:
[email protected].
however, the reactor configuration has been slightly modified (Croiset et al., 1997). The organic materials to be oxidized, here ethanol, methanol, or 1-propanol, and hydrogen peroxide in the calibration measurements, are not preheated in a separate feed line to avoid premature pyrolysis of these feeds. Rather, they are injected directly at a mixing point through an HPLC pump into a preheated single-phase supercritical mixture of water and oxygen. The injection is achieved using a 0.025 cm (0.010 in.) capillary tube inserted into the large bore tube of the reactor in a counterflow configuration. About 4 in. of the capillary tubing is surrounded by supercritical water at the experimental temperature. The spectroscopic cell is constructed of Inconel 625 and is fitted with sapphire windows. It is installed into the 1.42 cm (9/16 in.) o.d., 0.477 cm (3/16 in.) i.d., highpressure flow reactor. The optical cell can be inserted at any point along the length of the 3 m reactor. To generate the Raman scattering signal, the probe volume in the cell is excited with the 514.5 nm line of an argon ion laser operating at a nominal power of 5 W. By varying flow rate and cell position, a range of reaction times can be monitored. Hydrogen Peroxide Calibration and Measurement. The Raman spectrum of hydrogen peroxide is known to have a very strong resonance at 863.5 cm-1 in the gas phase and at ≈880 cm-1 in the liquid phase, corresponding to the O-O symmetric stretching vibration (Gigue`re and Srinivasan, 1974). In the present work, the H2O2 peak is detected at 884 cm-1 in the liquid phase at ambient conditions and at 874 cm-1 in the supercritical phase, as shown in Figure 1, and is used to identify the presence of H2O2 as a transient intermediate. Previous experiments that monitored the reactivity of methanol (Rice et al., 1996) and 2-propanol (Hunter et al., 1996) established the utility of Raman scattering intensity to measure the stable species concentration with a detection limit of less than 0.001 mol/L, depending on the specific species. However, to obtain quantitative concentration data, the intensity of the identifying
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1756 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998
decomposition and the decomposition on the reactor’s surface in this specific apparatus. In this way, previous nonspectroscopic measurements can be used to place the Raman results on an absolute concentration scale. This global first-order rate constant is expressed by
kg (s-1) ) 103.5(0.2 exp((-46 ( 2 kJ/mol)/RT) T < 374 °C, P ) 24.5 MPa (3)
Figure 1. Raman spectra of hydrogen peroxide at 24.5 MPa. (a) Aqueous liquid phase at 200, 250, 300, and 360 °C. For each temperature, the initial H2O2 mole fraction at the mixing point was 0.53 wt %. (b) Raman spectra of H2O2 in the supercritical phase at 390, 410, and 430 °C obtained with higher feed concentrations than were used in a.
Raman feature must be calibrated to a reference concentration. This is easily done for stable species by simply feeding a known concentration and measuring the integrated intensity of a Raman band to relate a measured counts per second to moles per liter. The principal difficulty in calibrating hydrogen peroxide at temperatures near and above the critical temperature of water (374 °C) is its rapid homogeneous thermal decomposition in the reactor and catalytic decomposition in the system’s injector (Croiset et al., 1997). Because of this decomposition, it is difficult to know the hydrogen peroxide concentration at the optical cell at temperatures above 370 °C with adequate precision to use the measurements for concentration calibration purposes. In addition, hydrogen peroxide thermal decomposition is sufficiently fast that no H2O2 can be detected with the Raman methods at temperatures above 430 °C when injected directly in this experimental apparatus. To address this problem, H2O2 calibration was done in the liquid phase at 24.5 MPa at temperatures ranging between 200 and 360 °C. At these conditions, no significant decomposition occurs in the injector, permitting the initial hydrogen peroxide concentration to be known with precision at the mixing point. The hydrogen peroxide concentration at the optical cell is then determined from kinetic calculations as follows. The distance from the mixing point to the optical cell and the total flow rate are used to calculate the residence time, and thus the hydrogen peroxide concentration at the optical cell is determined by using the experimental data for the H2O2 thermal decomposition rate determined by Croiset et al. (1997). This global rate takes into account both the homogeneous thermal
such that, given an initial feed concentration and a known reaction time to the spectroscopic detection point, the hydrogen peroxide Raman scattering cross-section can be determined and an integrated intensity-toconcentration ratio can be established for the spectroscopic system. For this paper we have reexamined the Arrhenius parameters obtained from the data in Croiset et al. (1997) by weighting the H2O2 decomposition rates for the
