3693
WATERVAPORDISSOCIATED BY MICROWAVE DISCHARGES AT Low FLOW RATES
Studies of Water Vapor Dissociated by Microwave Discharges at Low Flow Rates.
I.. Effect of Residence Time in the Traversed Volume on Product Yields at Liquid Air Temperature'"
by R. A. Jones, Walter Chan, and M. Venugopalanlb Department of Chemistry, Royal Military College of Canada, Kingston, Ontario, Canada
(Received January 19,1969)
Yields of hydrogen peroxide and evolved oxygen from the products trapped at liquid air temperature from water vapor dissociated by a low pressure microwave discharge at low flow rates were determined as functions of the time elapsing between the dissociation of the vapor and trapping of the dissociated products, using different traversed volumes and inlet pressures. For very small traversed volumes the yields of both Hz02and evolved 02 were negligible at all discharge powers studied, suggesting that either OH was not present in the gases passing out of the discharge or that, at least when H is present, OH radicals do not dimerize in a liquid air-cooled trap to form H202,but react with H to form H20. Both yields increased to a maximum and decreased thereafter with increasing traversed volume (at constant pressure) and increasing discharge pressure (at constant traversed volume), but were not affected markedly by increasing the dischargepower. In any case, the maxima did not coincide and the evolved oxygen to peroxide ratio varied with traversed volume and pressure. However, the ratio was independent of the discharge power at all traversed volumes, thus indicating that the oxygen evolution is not due to decomposition of H20zby impurities carried over from the discharge. Finally, material balances showed that 02 passed through the cold trap only at pressures and traversed volumes higher than those at which the peroxide yield was a maximum, suggesting that 02 is one of the precursors involved in the formation of HzOz and evolved 02.
I. Introduction The influence of residence time of dissociated water vapor in the discharge z0ne2s3and in the reaction volume between the discharge exit and the cold on the yield of hydrogen peroxide has been investigated using different types of discharges and varying either the flow rate, the input pressure, or the volume traversed by the dissociated vapor before trapping. I n general, the results of experiment^^^^ in which the residence time was varied in the discharge zone by varying the flow rate showed that increasing the residence time increased the H20z yield to some extent, the curves showing a maximum. experiment^^-^ in which the traversed volume was varied also showed that the yield of hydrogen peroxide was represented by curves having distinct maxima. I n particular, the latter work showed that a t very low flow rates of water vapor the yield of HzOz was negligible a t small traversed volumes and that it increased with traversed volume to a maximum and then decreased. Another significant product in this system is the oxygen evolved on warming the condensed products. Only in the traversed volume m70rk44 were the yields of evolved 0 2 measured as a function of residence time. I n discharges using internal electrodes4 no correspondence was found between evolved O2 and Hz02 yields for traversed volumes up to 100 ml, but beyond this value the yield of evolved 0 2 followed the HZOZ yield curve. I n electrodeless discharges6p6driven by a
Tesla coil the evolved 02 yield was independent of the traversed volume. It was, therefore, desirable to reinvestigate the product yields as a function of the residence time of the dissociated vapor in the traversed volume. The present paper describes the results of experiments in which the residence time of water vapor dissociated at low flow rates in a microwave discharge was varied by varying either the traversed volume or the pressure, the latter under conditions when the product yield was independent of the discharge power.
11. Apparatus and Procedure The apparatus consisted essentially of a conventional flow system and is shown semidiagrammatically in Figure 1. Two different discharge tubes made of quartz tubing of internal diameters 1.9 cm (D1) and 2.7 cm (1) (a) The research for this paper was supported by the Defence Research Board of Canada, Grant Number 9530-42. (b) To whom all correspondence should be addressed a t the Department of Chemistry, Western Illinoia University, Macomb, Ill. 61465. (2) S. Takahashi, Nippon Kagaku Zasshi, 81,36 (1960). (3) L. I. Nekrasov, N. I. Kobozev, and E. N. Eremin, Vestn. Moak. Univ., S e r I I , Khim., 15,12 (1960);18,7 (1963). (4) R. A. Jones and D. J. McKenney, unpublished work referred to in M. Venugopalan and R. A. Jones, Chem. Rev., 66,135 (1966). See also M.Venugopalan and R. A. Jones, Chemistrg of Dissociated Water Vapor and Related Systema, Interscience Publishers, New York, N.Y., 1968,Chapter 3,pp 89-90. (5) 8. S,Barton and R. A. Jones, Chem. Commun., 406 (1965). (6) S. 5. Barton, F. Groch, 8. E. Lipin, and D. Brittain, J. Chem. SOC. A , 689 (1968). Volume 79, Number 11 November 1969
3694
R. A. JONES,W. CHAN,AND M. VENUGOPALAN
TO GAGE PUMPS VIA COLD TRAP
Figure 1. Apparatus for the study of reactions in dissociated water vapor by the discharge-flow method.
(Dz) were used. The discharge tubes terminated in U-tubes (both 1.7 cm i.d.) made of Pyrex (T1) or quartz (Tz) which were used as cold traps for collecting condensible products, and were pumped by a high-speed mercury diffusion pump. An all-brass cylindrical cavity (CI or Cz, for dimensions see Figure l ) , through which passed the discharge tube, was used together with the Raytheon Microtherm Model CMD-5 (2450 Mc) for exciting the discharge. Pressure in the discharge tube was controlled by the throttling valve V in the by-pass to the pumps and was measured using a Consolidated Engineering Corporation Model 23-105 micromanometer, the gauge G of which .was kept a t 32". Previously deaerated distilled water contained in bulb S was first frozen and then kept immersed in a dewar filled with crushed ice and water. The rate of flow of water vapor into the discharge tube was varied by changing the length and/or diameter of capillary F ; for each capillary the water vapor flow rate per hour was measured by weighing the water that was condensed in T1or Tzcooled with liquid air. I n general, water vapor was first allowed to diffuse through the capillary flowmeter into the discharge tube with only the by-pass to the pumps kept open. The selected microwave power was then applied to the discharge cavity and the discharge initiated using a Tesla coil. Once a steady discharge was obtained, the product trap was cooled with liquid air, its level being kept constant during the course of a run. At the end of an experiment, the water vapor flow and the microwave power supply into the discharge tube were turned off in sequence, the liquid air level raised a few centimeters and the system evacuated. The condensed products in the cold trap were then warmed to ice-water The Journal of Physical Chemistry
temperature and the oxygen evolved was measured. The remaining condensed products were distilled into the smaller weighing trap (Wl or Wz) which could be detached from the system and weighed. The amount of HzOz was determined by titration with ceric sulfate and the amount of water by difference. The weight of water determined by this procedure was only about 10-15 mg and therefore the reported water yields must be of low precision.
111. Results (I) Experiments Using Discharge Tube D1. Two different series of experiments were carried out using a flow rate of 1.9 mmol HzO/hr and a traversed volume of 182 cm3. (a) At a constant water vapor pressure of 0.12 mm Hg, the yields of HzOz (0.45 mmol/hr) and evolved OZ (0.05 mmol/hr) were, within experimental error, not affected by varying the input power in the range 25-75 W. This showed that in the present system the composition of the gas entering the cold trap was independent of the discharge power. (b) At a constant power input of 62.5 W, an increase in discharge pressure increased the yields of HZOZand evolved O2initially by very small amounts to maximum values and afterwards decreased first rapidly and then slowly. A close examination of Figure 2 indicates that the maxima do not coincide and that there is a scatter of points in the falling regions of the curves. Minor changes in pressure (which was controlled during the course of a run at a pre-selected value by the manually operated throttling valve) and flow conditions may have contributed to the scatter of points. Figure 2 also shows that the experimental water yields decreased with the initial increase in pressure to a minimum value
3695
WATERVAPORDISSOCIATED BY MICROWAVE DISCHARGES AT Low FLOW RATES
I
A
n VOLVED 02(XlO)
I
PRESSURE, mrn Hg
Figure 2. Yields of HzOz,evolved 0 2 , and HzO at liquid air temperature as functions of water vapor pressure.
a t a pressure ( ~ 0 . 1 7mm Hg) at which the peroxide yield was a maximum and thereafter increased with increasing pressure. Since material balances indicated that significant oxygen loss from the system occurred only after the maximum in the peroxide yield was attained, theoretical HzO yields were calculated assuming no oxygen loss. The calculated yields (also shown in Figure 2) indicated that oxygen loss beyond the HzOz maximum increased with increasing pressure more or less corresponding with the decreasing peroxide yield. (2) Experiments Using Discharge Tube Dz. Two series of experiments were done at discharge powers of 62.5 W and 100 W, respectively, keeping the input pressure constant at 0.25 mm Hg and using a flow rate of 0.92 mmol HzO/hr. The results are shown in Figure 3. At traversed volumes less than 5 ml, the yields of HzOz and evolved 0 2 were negligible, most of the input water was collected in the cold trap, and small amounts of hard gas (mostly Hz) passed through the trap. It was concluded that either OH was not present in the gases passing out of the discharge zone or that, a t least when H is present, OH radicals do not dimerize in a liquid air-cooled trap to form HZOZ. Although the product yields were slightly dependent on the discharge power used, the nature of their variation with traversed volume was similar a t both discharge powers studied (see Figure 3)-with increasing traversed volume the yields of HzOz and evolved O2 increased first slowly and then rapidly from negligible to maximum values, and thereafter decreased. For both discharge powers, the maximum yields of evolved 0 2 were obtained a t the same traversed volume which was much smaller than the traversed volume a t which peroxide yields were maximum. Water yields decreased with traversed volume to minimum values a t or very near the traversed volume for which HzOz yields were maximum and then increased. The measured water yields were higher than those expected assuming no oxygen loss (see Figure 3). Nevertheless within
0
100
-
/EVOLVED A
t!f
02
Y
I
I
200
300
I
4
400
TRAVERSED VOLUME ml
Figure 3. Yields of HgOz, evolved 02, and HzO at liquid air temperature as functions of the traversed volume. Open and shaded points for data a t 62.5 and 100 W, respectively.
allowable error, the material balances indicated that oxygen loss from the system as 02 occurred only after the maximum H202yield was attained.
IV. Discussion The results from the pressure and traversed volume experiments at 62.5 W interpreted in terms of the residence time’ of the gas in the traversed volume are shown in Figure 4. Other conditions remaining unchanged, with increasing residence time the yields of HzOzand evolved 0 2 (both open points, taken from the traversed volume work a t constant pressure) increased gradually to maximum values and then decreased slowly. This behavior is consistent with a gradual decrease in the concentration of any one or all of the active species in the traversed volume as the residence time is increased, provided the variations in peroxide and evolved 0 2 yields can be accounted for by an increase with residence time in the concentrations of O2 and Hz, i e . , a decrease in the concentrations of 0 and H. Based on this view, the Geib and Harteck8 mechanism for peroxide formation
H
+
Cold Wall 0 2 -+
OzH
(7) For the calculation of the residence timcs, the molar rate of flow through the traversed volume was assumed to vary between the two limits possible: (a) if the gas in the traversed volume was composed of 0 and H only, the molar rate of flow would have been three times the molar rate of input of water vapor while (b) if the gas was composed of HzO molecules only, the molar rate of flow would have been equal to the molar rate of input of water vapor. If the vapor in the traversed volume was composed of H atoms and OH radicals only, naturally the molar rate of flow would have been midway between the above two limits, viz., twice the molar rate of input of water vapor. From these two limiting values of the molar flow rate and the values of the input pressures or traversed volumes, the residence times were calculated. These residence times are, of course, only approximate, but are useful for relative comparisons. (8) K. H. Geib and P. Harteck, Ber., B65,1551 (1932).See also K. H. Geib, J . Chem. Phus., 4,391 (1936).
Volume 78, Number 11
November 1060
3696
R. A. JONES,W. CHAN,AND M. VENUGOPALAN I
I
e
0
PRESSURE, m m Hg
0.5 *
0
Q
I
I
100
200
300
400
TRANSVERSED VOLUME, m l
I
0
o2
E ,\/VOLVED I
4
-1
2-6 4-12 6-18 RESIDENCE TIME IN TRAVERSED VOLUME, sec
Figure 4. Yields of H20z and evolved 02 and evolved 02/8202 ratio (inset) as functions of the two possible limits of residence time in traversed volume. Open and shaded points taken from traversed volume and pressure data for 62.5 W given in Figures 3 and 2, respectively.
together with Ohara’s mechanism9 for evolved 0
+ OzH
OzR
----j.
Warming
H204
2
Cold Wall
-+
H202
H204
+
0 2
(3)
(4)
can explain the present results. That is, the initial increase in H2Oz and evolved 0 2 yields with residence time is due to an increase in the concentration of 0 2 and the subsequent decrease in yields a t longer durations is due to a gradual decrease in H concentration caused by its recombination in the traversed volume to yield H2 or by the reaction with OH to form H2O. It must be noted that the well-known fast reactionlo
0
+ OH --+- + H 0 2
(5)
will certainly contribute to the formation of 0 2 in the traversed volume. On the other hand, the data from pressure work (shaded points in Figure 4) showed that the yield of H2OZincreased rapidly to a maximum value by a small increase in residence time, but fell abruptly thereafter. This was, however, not the case with evolved 0 2 yields which increased with increasing residence time gradually to a maximum and then fell abruptly t o one-half the maximum yield at a residence time a t which the peroxide yield was already less than half its maximum value. It must be noted that an increase in discharge pressure in these experiments increased both the residence time and collision frequency of the particles in the system. If it is assumed that HzO was completely dissociated in the dischargell a t all pressures (-0.1-0.5 mm Hg) studied, then the variation in inlet pressure affected only the processes occurring in the traversed volume. Undoubtedly, both the collision frequency The Journal of Physical Chemistry
Figure 5. Variation of evolved 02/HzO~ratio with traversed volume and pressure. Open and shaded points for data a t 62.5 and 100 W, taken from Figures 2 and 3, respectively.
and residence time of the species in the traversed volume will be increased; an increase in collision frequency naturally increases the recombination rate of the species in the traversed volume. The observed abrupt decrease in H202 and evolved 0 2 yields by a small increase in residence time brought about by increasing pressure seems to be explicable on this basis. Evolved Oxygen to Hydrogen Peroxide Ratio. 02/H202 ratios are meaningful only if the mechanism of the evolution of oxygen is associated with the mechanism of either the formation of (residual) Hz02 by decomposition of hydrogen superoxides or the decomposition of H2Oz itself. The variation of the evolved O2 to peroxide ratios are shown as functions of the traversed volume (Figure 5 ) , pressure (inset, Figure 5), and calculated residence times in the traversed volume (inset, Figure 4). The ratios were independent of the discharge power (see Figure 5 , and also section 111 (la)), suggesting that the evolved oxygen cannot be accounted for by the decomposition of HzO2 by impurities carried over from the discharge tube into the cold trap.12 Neither can impurities originating in the traversed volume be responsible since O2/H2Oz ratios decrease rather than increase with traversed volume. On the other hand, the decreasing ratios with increasing residence time in the traversed volume and the increasing ratios with increasing collision frequency are explicable on the basis of Ohara’s mechanismg (reactions 3 and 4). However, the existence of this compound is hitherto unconfirmed by direct means.13 Other mechanisms for evolved 0 2 such as occlusion, (9) E.Ohara, J . Chem. SOC.Jap., 61,569,657 (1940). (10) F.Kaufman, Progr. Reaction Kinetics, 1, 1 (1961). (11) E.A. Secco, J. Chem. Phys., 23, 1734 (1955). (12) P.A. GiguBre, ICSU (Intern. Council Sci. Unions) Rev., 4, 172 (1962); J. Chem. Phys., 42, 2989 (1965). See also N. Hatta and P. A, GiguBre, Can. J . Chem., 44, 869 (1966);R.A. Jones and M. Venugopalan, Can. J . Chem., 45,2452 (1967). (13) K. Herman and P. A. GiguBre, Can. J . Chem., 46, 2649 (1968). For a review of earlier work, see ref. (4).
3697
DIELECTRIC STUDY OF ESTERS IN BENZENE trapping, and the formation of “complexes” of oxygenI4 with Hz02and/or HzO when codeposited at low temperature from the gas phase are essentially based on evidence from experiments done a t liquid-helium temperature (bp 4.2”K), a t which temperature O2 readily condenses. The likelihood of these mechanisms operat-
Dielectric Study of Esters in Benzene.
ing a t liquid air temperature (bp 81’K) a t which the present experiments were done is remote indeed.I6 (14) R. A. Ruehrwein, J. 8. Hashman, and J. W. Edwards, J . %/a. Chem., 64,1317 (1960). (15) J. Edwards, “Chemical and Physical Studies of Trapped Radicals,’’ in “Formation and Trapping of Free Radicals,“ A. M. Bass and H. P. Broida, Ed., Academic Press, New York, N. Y.,1960, 291. ~
w.
Barrier to Internal Rotation
and Molecular Configuration
by Bal Krishna, Bhartendu Prakash,’ and S. V. Mahadane Department of Chemistry, University of Allahabad, Allahabad, India
(Received January 17, 1969)
Electric dipole moment values at a frequency of 1 Mc/sec and relaxation time values at a frequency of 9.79 kMc/sec for methyl-p-nitrobenzoate, methyl-m-nitrobenzoate, ethyl-m-nitrobenzoate, ethyl-o-nitrobenzoate, methyl-o-aminobenzoate, glycerol triacetate, and glycerol tributyrate have been determined in benzene at 30’. The comparison of the observed and theoretically calculated moment values indicates the absence of intramolecular rotation in these esters except for methyl and ethyl-m-nitrobenzoate molecules. The relaxation time values for these rigid, transplanar esters are expected to be due to the overall rotation of the molecules in the solvent. Discussing the thermodynamic quantities (AB‘* and AH*), a high negative value for the entropy of activation (AS*)has been suggested so that the molecules during activation, are in a state of better order than the initial state. In the case of methyl-o-aminobenzoate, possibility of hydrogen bond formation between the two ortho-substitutedgroups is expected to give this molecule a rigid planar configuration. Introduction Most of the esters derived from a saturated monohydric alcohol and a saturated carboxylic acid, possess a trans-planar configuration,2 and a resonance occurs between the classical (a) and excited (b) structures giving considerable double-bond character to the C-0 bond
0
\\ /
Rz
c-0
/”’
O\
/”‘
cc-0
/
out of the trans plane. A coplanar configuration is expected for the molecules containing the carboxylic group in addition to the general conjugated system^.^ The insertion of an -NOz group, a highly polar one, in the benzene ring, which is symmetric and nonpolar in itself, gives rise to a high value of the electric dipole moment for a nitrobenzene molecule. In the light of above facts, the alkylnitrobenzoate molecules are expected to possess high dipole moments and a rigid planar configuration. The esters derived
R2
(4 (b) and, hence, inhibiting free rotation of 0-RI bond around itm3r4The electric dipole moments of esters have been found to be independent of temperatures over a large range of temperature. The exceptions are methyl and ethyl chloroformatese and carbonates,l where a free rotation or restricted rotation of 0-R1 bond around C-0 bond has been suggested. The dipole moment values for some esters in benzene, reported earlier by the authors,s showed that these molecules may possess a configuration in which C-0-C plane lies considerably
(1) T o whom correspondence is to be addressed a t the Department of Chemistry, Indian Institute of Technology, Kanpur, India. (2) A. Eucken and L.Meyer, Phy8. Z., 30,397 (1929). (3) R. J. B. Marsden and L. E. Sutton, J . Chem. SOC.,1383 (1936). (4) J. M. O’Gorman, W. Shand, and V. Schomaker, J . Amer. Chem. SOC.,72, 4222 (1950). (6) C. T. Zahn, PAys. Z., 33,730 (1932). (6) S. Mizushima and M. Kubo, Bull. Chem. SOC.J a p . , 13, 174 (1938). (7) M. Yasumi, J . Chem. SOC. Jap., 60,1208 (1939). (8) B. Krishna, S. C. Srivastava, and 9. V. Mahadane, Tetrahedron, 23,4801 (1967). (9) G. W. Wheland, “Theory of Resonance,” John Wiley & Sons, Inc., New York, N. Y.,1945,p 92.
Volume 79,Number 11 November 1969
