1130
Ind. Eng. Chem. Res. 1990,29, 1130-1136
Copper-Catalyzed Autoxidation of Carbohydrazide: Kinetics and Mechanism David R. Cosper* and David J. Kowalski Nalco Chemical Company, One Nalco Center, Naperuille, Illinois 60563-1198
Carbohydrazide (CHZ) reduces dissolved oxygen t o H202and probably H20 in the presence of Cu(I1) salts a t alkaline pH. Hydrazine is also produced. The rate of the reaction increases with increasing pH. The reduction is a chain reaction initiated by a transformation of the known Cun(CHZ), complex and obeys first-order kinetics, i.e., -d[02]/dt = k ' [ 0 2 ] ,for a substantial portion of the reaction. The reaction is complex in that hydrazine competes with CHZ for Cu(1I). In more concentrated solutions, autoxidation produces a condensation dimer, 1,2-hydrazinedicarbonic dihydrazide, and higher homologues. Feed water systems for boilers suffer from corrosion, which is largely caused by dissolved oxygen. Mechanical deaerators normally remove most of the dissolved gases. However, it has been common practice to feed chemical scavengers to destroy all but the final traces of oxygen after mechanical deaeration. Industrial oxygen scavengers are typically simple reducing agents that react with dioxygen in the feed water environment where the temperature is typically 50-90 "C and pH is 9.0-9.5. Sodium sulfite, isoascorbic acid, hydroquinone, and dialkylhydroxylamine have all been used industrially. Hydrazine has been used extensively as well. It has now been replaced in many boiler feed water systems by carbohydrazide (carbonic dihydrazide), a solid and less hazardous derivative (Slovinsky, 1981). Carbohydrazide has proven to be a rapidly reacting scavenger, particularly in the presence of copper-containing metals, and to promote the formation of protective passive layers of magnetite oil steel. The reactions of most scavengers with dissolved oxygen have been studied and reported. Such is not the case for carbohydrazide or for hydrazides generally. The literature contains very little pertaining to the autoxidation of hydrazides other than some recent reports of preparative acylations stemming from this reaction (Attanasi and Serra-Zanetti, 1980; Tsuji et al., 1980). Reactions with other simple oxidants are known, however (Keim et al., 1950; Feher and Linke, 1966; Smith, 1966). In one particularly interesting example, 1,2-diacylhydrazines are formed during the mild oxidation of hydrazides (Smith, 1966). The redox chemistry of carbohydrazide itself is virtually unreported. The present study was undertaken to identify the products and define the kinetics of the autoxidation of carbohydrazide. This knowledge will be useful in applying carbohydrazide as an oxygen scavenger and metal passivator and in designing new molecules for the same use.
Experimental Section Materials. All chemicals were of the highest purity obtainable. Cupric chloride dihydrate was obtained from Fisher Scientific, and its solutions were standardized iodimetrically (Meites, 1952). Diethylenetriaminepentacetic acid pentasodium salt (DTPAeNa,) was obtained from Dow Chemical Co. Carbohydrazide (I). A high-purity industrial grade of carbohydrazide was recrystallized from water-ethanol as follows. The material was dissolved in a minimum volume of water a t room temperature and filtered with Celite to crystal clarity. Dry ethanol (1.5 vol) was added and the solution chilled to induce crystallization. The crystals were dried under a nitrogen stream and redissolved in a minimum volume of water at room temperature. This solution 0888-5885/90/ 2629-1130$02.50/ 0
was filtered into 7 vol of ethanol. The crystals were dried under a nitrogen stream and finally at 78 "C and 1 mm over Pz05: mp 153-155 "C (154 "C in Borsche et al., (1929)). Anal. Calcd for CH&O: C, 13.3; H, 6.7; N, 62.2. Found: C, 13.3; H, 6.1; N, 62.5. NMR (H,O) 6 164.2 (s). UV (H20)featureless absorption at X < 205 nm. Assay (KIO, consumption): 100.1 f 0.5%. There is no evidence for a hemihydrate as has been reported elsewhere (Krivis et al., 1963). 1,2-HydrazinedicarboxylicDihydrazide (or Bicarbamic Acid Dihydrazide) (11). This dimer was isolated from an aged moist sample of carbohydrazide by two recrystallizations from hot watel-ethanol(6040). Hot filtration removed insoluble oligomer: mp 191-195 "C (196 OC in Stoll6 (1910)). 13C NMR (H20) 6 156.3 (s). UV (H20)broad shoulder at h 205 nm ( E = 5400). Anal. Calcd for C,H8N6o2: C, 16.2; H, 5.4; N, 56.7. Found: C, 15.7; H, 4.9; N, 57.8. Carbohydrazide Oligomer. A white amorphous material precipitated from an aqueous carbohydrazide solution on standing several weeks. The solid was collected, washed with water, and dried in a vacuum oven: mp 230 "C (dec). Anal. Calcd for C3HI0N8O3:C, 17.5; H, 4.8; N, 54.4. Found: C, 17.6; H, 4.2; N, 52.6. Kinetic Experiments. Deoxygenation reactions were carried out at 32 f 0.1 "C in a completely filled Pyrex flask equipped with pH electrode, thermometer, oxygen electrode, and septum-sealed injection port. The concentration of dissolved oxygen was followed with an Orion Model 97-08 oxygen electrode, a polarographic device consisting of membrane-covered silver electrodes. The reaction medium was 0.025 M NaHC0, adjusted to the desired pH with aqueous HC1 or NaOH and saturated with air. Continuous stirring was achieved with a 1-in. Teflon-coated magnetic bar a t 450 rpm. Each kinetic experiment was performed at least in triplicate. Most reactions were followed until no further decrease in oxygen concentration could be detected. Product Analyses. Experiments to determine the composition of reaction mixtures were carried out in the above-described apparatus. Reactions were stopped a t varying degrees of deoxygenation by injection of DTPA at a 10-fold molar excess (relative to total copper). Hydrazine was determined colorimetrically a t 470 nm after reaction with p-(dimethy1amino)benzaldehyde(Watt and Chrisp, 1952). Hydrogen peroxide was determined at 420 nm as the complex with TiOSO, (Rynasiewicz, 1954). Total residual reductant was determined by consumption of KIO, in 2 N aqueous H2S04. Additional experiments were carried out in open beakers without bicarbonate buffer (which masks the UV absorption of 1,2-hydrazinedicarbonic dihydrazide) and changes 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1131
2"1 2.0
OL 1000
2000 TIME
3000
4000
ISECl
Figure 1. Products of autoxidation of CHZ a t 32 OC and pH 7.9 vs time: [CuC12] = 0.393 X 10".
followed by UV at X 205 nm. Further analyses of products were carried out by liquid chromatography and by ion chromatography.
Results Autoxidation Products. We have observed that solid carbohydrazide (CHZ) (I) forms amorphous material on aging. The same is seen in concentrated (ca.10%) aqueous solutions. In addition, a crystalline solid of reduced solubility is found. We have identified this latter material as 1,2-hydrazinedicarbonicdihydrazide (HDD) (11)based
0 0
2
0.oY
0.0
I1
on elemental and spectroscopic analyses as well as melting point comparison with published values (see Experimental Section). The amorphous material appears to be the next higher homologue containing three carbonyls and four hydrazine moieties or a related structure. The formation of these "condensation" products in solution can be stopped by excluding oxygen or retarded markedly by addition of complexing agents (DTPA, salicylaldoxime, EDTA). Dry carbohydrazide can be stored without decomposition in a desiccator after careful recrystallization and drying. Carbohydrazide was allowed to react with dioxygen in air-saturated buffer at pH 7.9 in the presence of cupric chloride at 32 OC. The reaction was quenched at various degrees of completion by addition of chelant. Hydrogen peroxide and hydrazine were determined colorimetrically. Total residual reductant was determined by consumption of potassium iodate with corrections for hydrazine and hydrogen peroxide. The net remaining reductant (in equivalents/liter) was multiplied by 0.125 in order to generate the curves in Figures 1, 2, and 4. This factor converts equivalents/liter to molarity of reductant assuming that all residual reductant is carbohydrazide (8 equiv/mol). There is no increase in oxygen concentration on addition of EDTA, indicating that there is no significant amount of a Cu-O2 adduct present. In the presence of Cu(I1) salts, carbohydrazide reduces dissolved dioxygen rapidly at pH 7.9. About the same rates of autoxidation were obtained with either CuS04or CuC12. The latter salt was used exclusively in the kinetic and product studies reported here. In the absence of Cu(II), no reaction occurs as measured by the disappearance of dioxygen. The chlorides of Fe(III), Ni(II), Mn(II), and
.)'
1.0--
:i
.;F .....
.__...I
4
i
0.5
1.5
1 .O [ (0210
-
1021 tl
2.0
2.5
x loo00
Figure 3. Product/oxidant stoichiometry for autoxidation of CHZ at various [CuC12].
Co(I1) are not catalytic under these conditions. After a brief (ca. 4 s) induction period, dioxygen disappears rapidly in the initial stage of the reaction with formation of hydrogen peroxide and hydrazine as shown in Figures 1 and 2. This rapid phase is followed by a much slower reaction. The length of the induction period is independent of stirring speed and the presence of light and is too long to be due solely to a delay in electrode response. The sum of the product concentrations at time t ([N2H4],+ [H202],)is approximately equal to the change in oxygen concentration ([O2lO- [O,],) over much of the reaction (see Figure 3). This equivalency persists until about 1 mol of dioxygen has been reduced per mole of carbohydrazide initially present. At greater extents of reaction, [N2H4],+ [H202],< [O2lO- [O,],, apparently as the result of secondary reactions. It is also noteworthy that at low total concentration of copper ([Cu(II)] = 0.393 X the ratio of product concentrations ([N2H4],/[H202],) remains relatively constant (ca. 0.5) throughout the reaction. This suggests that about one-third of the dioxygen consumed during the rapid phase of the reaction is reduced directly to water (or to some other undetected species) without passing through the partially reduced state as free hydrogen peroxide. The formation of H202 during the initial reaction must be a parallel process that may share a partial mechanistic pathway. A t the higher copper concentration (0.480 X 10"' M), on the other hand, the product ratio increases from an initial value of 0.5 as the autoxidation proceeds, apparently as a result of Cu-catalyzed secondary reactions.
1132 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1 4-
Table I. Rate Constants for Cu(I1)-CatalyzedAutoxidation of Carbohydrazide at Various [O& at pH 7.9 and 32 "C: [CHZl,, = 0.777 X 1st order
I. C"j
0.0393 0.0393 0.0393 0.0393 0.480 0.480 0.480 0.480 I 0 . 0
I__c_
io0
200 TIME
300
ISECI
Figure 4. Formation of N2H4 from CHZ; aerobic vs anaerobic conditions: [CuC12] = 1.07 X lo-'
That hydrazine is produced aerobically is illustrated in Figure 4. In the absence of dioxygen, carbohydrazide and Cu(I1) produce only minor amounts of hydrazine during the time scale of the aerobic experiments. The final reaction solution is turbid. This insoluble product has not been examined. It is probably a reduced copper species (Campbell and Grzeskowiak, 1976). The Cu(I1)-catalyzed autoxidation (in air-saturated pH 7.9 buffer) of hydrazine is likewise very slow (see Table V). The autoxidation was carried out in bicarbonate-free air-saturated water, which had been adjusted to pH 9.1 with NaOH, and changes in composition followed by UV (see Experimental Section). The initial concentration of carbohydrazide was 1.30 X lo4 M in all cases and that of M in CuCl, set at 3.93 X lo+, 3.93 X lo*, and 4.80 X the three conditions studied. At the two higher concentrations of Cu, which correspond to the conditions used in the following kinetic experiments, no evidence for HDD was visible in the UV at any stage of the reaction. On the other hand, substantial quantities of HDD are indicated by UV absorption at 205 nm in the very slow reaction obtained with [CuCl,] = 3.93 X 10-8. We estimate that over half of the initial carbohydrazide is converted to dimer (HDD) under these latter conditions. Kinetic Experiments. The kinetics of the Cu(I1)catalyzed autoxidation of carbohydrazide (CHZ) in airsaturated 0.025 M NaHC03 have been studied over a concentration range [CHZIo = (0.162-3.81) X lo4 and [ C U ] , ~=. (0.0393-1.07) X lo4 and pH 7.27-9.85 at 32 "C. The reaction was followed by measuring the concentration of dissolved dioxygen polarographically. Reactions in many cases were followed to a point where no further change in [O,] could be detected. The complete [O,] vs time curves do not conform to simple first- or second-order integrated kinetic expressions. An initial rapid reaction is followed by a much slower one. A simple scheme of competitive, consecutive first-order reactions is also not completely satisfactory. Plots of log ([02]0/[02]t) vs time, however, are linear over extents of reaction that depend on initial reaction conditions. In general, high ratios of catalyst to reductant ( [CU],~: [CHZ],) result in first-order kinetics to high extents of reaction that approach 1mol of oxygen reduced per initial mole of carbohydrazide. The rate constant (k') for the first-order portion of the reaction was determined as the slope of the linear portion of the log ([02]o/[Oz],)vs time plot, i.e., log ([021o/[Ozlt) = k'(t - t o )
0.77 1.30 1.88 2.39 0.743 1.31 1.86 2.47
0.659 (f0.030) 0.651 (f0.075) 0.535 (f0.043) 0.645 (10.040) 6.72 (f0.43) 4.55 (f0.15) 3.50 (f0.22) 3.21 (f0.15)
0.3 0.35 0.4 0.4 0.8 0.9 0.8 1.o
a In 0.025 M aqueous NaHCO, adjusted to pH 7.9. * k' = fintorder rate constant for -d[02J/dt = k'[02]. 'f95% confidence limits. dApproximate duration of linear portion of log ([02]o/[02]) vs time curve, expressed in moles of O2 reduced per mole of CHZ present initially.
Table 11. Rate Constants and Stoichiometries for Cu(I1)-CatalyzedAutoxidation of Carbohydrazide ([CHZ], = 1.30 X lo-') at pH 7.9 and 9.4 and 32 O C at Various Total [CUl0 1st order total A[021/d A[O,l/ i o 4 [ c ~ C i , l ~ k: s-l x loZc lCHZln ICHZln' pH 7.9 0.0393 0.869 (f0.044) 0.2 1.68 (10.04) 0.0944 1.67 (fO.09) 0.5 1.72 (f0.08) 0.157 2.45 (f0.25) 0.6 1.70 (f0.02) 0.315 4.43 (f0.22) 0.8 1.62 (f0.04) 0.480 4.96 (f0.17) 0.9 1.53 (fO.O1) 0.961 5.44 (10.27) 0.8 1.07 5.92 (f0.73) 0.8 0.00393 0.0138 0.0393 0.0787 0.267
pH 9.4 0.574 (f0.062) 2.05 (f0.26) 4.66 (f0.19) 1.80 (f0.78) 12.6 (f1.2)
0.3 0.5 0.75 0.85 0.9
"In air-saturated ([O2lO= 2.4 X lo4) 0.025 M NaHC03 pH adjusted with HCl or NaOH. bTotal [CUI. 'For -d[O,]/dt = k'[O,], with f95% confidence limits. dSee Table I, note d. "Total moles of O2consumed per mole of CHZ initially present.
where to is the correction for the induction period. There is a positive correlation between k'and the duration (in terms of moles of O2 consumed) of first-order behavior. Effect of [02]0. The initial concentration of dioxygen ([O2lO)was varied by purging a portion of the reaction medium with nitrogen before introduction of reductant and catalyst. Reaction conditions and rate constants are given in Table I. A t the lower reactant ratio ([CUI,,: [CHZI0 = 0.0506), k' is relatively independent of [O2lO, = 0.618) k'dewhile at the higher ratio ([Cu],,:[CHZ], creases with increasing [02],,. The extent of first-order kinetics is relatively insensitive to [O2lOwithin the concentration range studied. Effect of [CUI,,. First-order rate constants were determined for varying [CUI,, in air-saturated 0.025 M NaHC03 a t pH 7.9 and 9.4 and 32 "C. [CHZIowas held constant at 1.30 X M. These data are presented in Table 11. The extent of the first-order character in the [O,] vs time plots increases with increasing [CUI,, up to a maximum of about 0.9 mol of dioxygen reacted per initial mole of CHZ. The total amount of dioxygen reduced per mole of carbohydrazide decreases somewhat with increasing [ C U ] , ~A plot of log k'vs log [CUI,, is linear at lower values of [ C U ]with ~ slope of 0.78 (Figure 5). At higher [CU],~,the slope falls off from this value. This behavior is typical of a rapid equilibrium before the rate-controlling step of the reaction mechanism.
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1133
0.100:
Y
4
m
0 010-
I
.
m
lCuC121 -0.393.-5 icuci21- 0 . 4 n o e - 4 ICuC121-0.9811-4
m
~
0.OOli
0.000001
I
0.000010
0.000100
0,00100
I
0.001~’ 0.00001
0.00010
[CUC121
0.00100
[CHZI 0
Figure 5. Autoxidation of CHZ at 32 “C and pH 7.9 k’vs [CuC12]. Table 111. Rate Constants for Cu(I1)-Catalyzed Autoxidation of Carbohydrazide at pH 7.9 and 32 “C: Effect of Varying [CHZIoat Various [Cu(II)l” 1st order total A[O,l/ A[O,l/ 10‘[CHZ]ob k’, s-’ X 10’’ [CHZ]$ [CHZlo8 [CuCll] = 0.0393 X lo-‘ 0.3i2 (h6.063) 0.4 0.324 1.97 (h0.02) 0.645 (h0.040) 0.4 0.777 1.68 (h0.04) 0.869 (h0.044) 0.25 1.30 0.890 (h0.070) 0.2 1.91 1.08 (h0.08) 0.2 2.86 1.28 (h0.19) 0.15 3.81 X
0.162 0.240 0.324 0.400 0.481 0.600 0.777 1.00 1.30 1.91 2.86 3.81
[CuC12] = 0.480 0.246 (h0.017) 0.471 (h0.038) 0.856 (hO.090) 1.21 (hO.11) 1.68 (h0.45) 1.99 (10.30) 3.21 (10.15) 3.82 (10.18) 4.96 (h0.17) 8.05 (10.62) 11.3 (h1.5) 12.8 (h1.6) [CuC12] = 0.961 0.230 (h0.027) 0.777 (h0.094) 1.60 (h0.03) 3.46 (h0.15) 5.44 (10.27)
X
0.200 0.400 0.600 1.00 1.30
lo4 0.75 0.8 0.9 0.9 1.0 0.9 1.0 0.8 0.9 1.0 0.8 0.6
1.91 (h0.10) 1.79 (hO.11) 1.70 (10.15) 1.73 (h0.04) 1.53 (hO.01)
lo-‘ 0.6 0.6 0.6 0.8 0.8
Figure 6. Autoxidation of CHZ at 32 “C and pH 7.9; k’vs [CHZ]@
Oo0.
0 6
,D 0.OOli
0.0000001
o I
0.0000010
0.0000100
0.000100
[Cu I C H Z I 21 o
Figure 7. Autoxidation of CHZ at 32 OC and pH 7.9: k’vs [Cu(CHZ),I,. Table IV. Rate Constants for Cu(I1)-Catalyzed Autoxidation of Carbohydrazide at 32 “ C Effect of pH ([CHZ], = 1.30 X lo4: [Cu(II)] = 0.0393 X 1O4Y 1st order total A[02l/ A[O,l/ PH k ’, s-l X lo2 [CHZIOC [CHZlod 7.27 0.309 (h0.044) 0.3 7.90 0.869 (10.044) 0.3 1.68 (hO.06) 8.40 1.77 (10.08) 0.4 1.77 (h0.03) 8.87 3.04 (hO.10) 0.5 9.40 4.66 (hO.19) 0.8 1.70 (h0.06) 9.85 5.87 (h0.44) 0.8 1.67 (10.05)
‘In air-saturated ([O2lO 2.4 X lo-’) 0.025 M NaHCOS pH adjusted with HC1 or NaOH. bTotal [Cu]. ‘For -d[Oz]/dt = k’[02], with h95% confidence limits. See Table I, note d. e Total moles of O2 consumed per mole of CHZ initially present.
OIn air-saturated ([O2lO= 2.4 X lo-‘) 0.025 M NaHC02 pH adjusted with HC1 or NaOH. *For -d[02]/dt = k’[02], with h95% confidence limits. ‘See Table I, note d. dTotal moles of 02 consumed per mole of CHZ initially present.
Effect of [CHZIo. First-order rate constants were determined at various [CHZIoat pH 7.9 and 32 OC (see Table 111). Plots of log k’ vs log [CHZIo are linear at lower with slopes of 1.8 (see reactant ratios ( [CHZIo:[Cu],,) Figure 6). At higher values of [CHZ],,, the slope falls off from this value, indicating again that an equilibrium step precedes the rate-controlling one. At the lowest [Cu],, (0.0393 X 10”’ M) studied, the plot is not linear and its slope is everywhere less than one. For the linear portions of these plots, the rate constants are k’= 8.2 X 10S[CHZIols (for [CUI,, = 0.480 X lo4) and k’ = 4.5 X 10S[CHZ]01*8 (for [CUI,, = 0.961 X lo4). Although the exponential terms for the dependence of the rate on [CHZjo and [CUI,, are not integers, their relative values suggest a rate dependence on a 21 CHZ-Cu complex. Indeed, the curve of the k’vs concentration plots is typical of the saturation effect due to complex formation
before the rate-determining step. In fact, a plot of log k’ vs log [ C U ~ ~ ( C H Z (calculated )~+]~ from the published values of K1and K2)is quite linear with a slope of 0.87 and y intercept of 2.9 (see Figure 7). Effect of pH. Kinetic experimentswere performed over the pH range 7.27-9.85 at [CHZIo = 1.30 X lo4 and [CUI,,, = 0.0393 X 10“‘ at 32 OC (Table IV). The firstorder rate constants increase smoothly with increasing pH. The extent of the first-order kinetics also increases with pH, but the overall stoichiometry remains unchanged. Effect of Inhibitors. Since many autoxidations are free-radical chain processes and subject to inhibition or retardation, several potential inhibitors were tested. pBenzoquinone, tetranitromethane, and 3-mercapto-lpropanol (as well as chelants) were found to inhibit the Cu(I1)-catalyzed autoxidation of carbohydrazide. Conventional antioxidants such as p-methoxyphenol and 2,6-
1134 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 Table V. Rate Constants and Stoichiometries for Cu(I1)-Catalyzed Autoxidation of Carbohydrazide in the Presence of Hydrazine at pH 7.9 and 32 "C" 1st order total
0.500
0
0.400
1.89 (f0.18)
0.68
1.46 (f0.05)
0.500
0..500
0.400
1.47 (10.25)
0.75
2.14 (f0. 17I
0.650 1.00
1.30 0
0.157 0.200
0.379' 2.47 (f0.36)
0.60
1.00
0,200
0.200
1.83 (f0.28)
0.55
1.78 (f0.12)
1.00
0.500
0.200
1.26 (f0.25)
0.42
1.90
100
1.00
0.200
0.846 (f0.20)
0.29
(f0.05) 1.97
1.00 1.30
2.00
0.214 (f0.071) 2.45 (f0.25)
0.27
0
0.200 0.157
1.30 1.30
0.655 1.30
0.157 0.157
0.91gC 0.464c
>3.3 1.68 (fO.07)
(*0.13)
0.60
1.70 (f0.02)
>1.80 >1.79
air-saturated ([O2I0I 2.4 X lo4) 0.025 M NaHCO, pH adjusted with HCI. bFor -d[O,]/dt = k'[O,], with f95% confidence limits. Single determination. See Table I, note d. e Total moles of O2 consumed per mole of CHZ originally present.
di-tert-butylphenol were without effect. p-Benzoquinone was the most active of the inhibitors studied. Its main effect was to lengthen the induction period with only a small reduction in k ', For example, while the autoxidation of carbohydrazide at [CHZ], = 2.50 X and [CU(II)],~ = 0.100 X exhibits k'= 0.0147 s-l (zt0.0013) and to= 4.0 s, the presence of p-benzoquinone (0.100 X M) reduces k'slightly (0.0132 s-* f 0.0007) and increases to to 26.3 s. These data suggest that the autoxidation is a free-radical chain process that involves the intermediacy of reactive species at steady state. Secondary Reactions. The complexity of the [O,] vs time curves led us to examine various reactions involving the products of carbohydrazide autoxidation. Secondary reactions that consume or produce dioxygen or that inhibit or catalyze the primary reaction could account for the observed complexity a t greater extents of reaction. Hydrazine (0.984 X lo4 M) and hydrogen peroxide (2.00 X lo4 M) were allowed to react in partially deaerated 0.025 M NaHCO, at pH 7.9 and 32 "C for 72 s in the presence M). About 65% of the of CuCl, ([CUI,, = 1.97 X hydrazine and 84% of the hydrogen peroxide were consumed. An increase in [O,] was observed, consistent with about 14% disproportionation of the H202present initially. In the absence of hydrazine, however, hydrogen peroxide disproportionates a t a much slower rate under these conditions (ca. 2% disproportionation during the same time). Indications of these reactions are visible in Figures 1-3. They are probably of little significance in the initial rapid phase of dioxygen reduction by carbohydrazide but must be of importance in the later stages of reaction. The observed formation of 0, during the reaction of H20zand N2H4 could account for some decrease in overall stoichiometry in the autoxidation of carbohydrazide. A further complication exists in the decreased rate of carbohydrazide autoxidation in the presence of hydrazine (Table V), This effect probably results from competition for Cu(I1) between hydrazine ( K = 4.7 X lo6 (Banerjea and Singh, 1967)) and carbohydrazide ( K , = 8.3 X lo4, K 2 = 1.1 X lo4 (Campi et al., 1964)) and may account for the decrease in k'in the later stages of reaction. This simple explanation does not adequately account for all of the
observations, however. The computed values of [Cu"(CHZ)22+]0 in the presence of hydrazine yield predicted values of k' that are much lower than experimental ones. We also observe that, in the presence of carbohydrazide, hydrazine reduces oxygen, while in its absence hydrazine is virtually inert toward autoxidation at pH 7.9 and 32 "C. This effect is most easily seen in the higher stoichiometries for mixtures of carbohydrazide and hydrazine as compared to carbohydrazide alone. We conclude, therefore, that the total rate of oxygen disappearance in the presence of both reducing agents is the sum of Cu"(CHZ)?+-catalyzed autoxidations of both carbohydrazide and hydrazine. Because of the possibility that 1,2-hydrazinedicarbonic dihydrazide (11) could form even during these dilute autoxidation experiments, the Cu(I1)-catalyzed autoxidation of this dimer was investigated briefly. The reaction is very slow relative to that of carbohydrazide and is typical of simple hydrazides (Cosper, 1983). For example, the autoxidation of I1 (1.30 X M) in the presence of 4.80 X M CuCl, (pH 7.9, 32 "C) exhibits a first-order rate constant of 1.7 X s-l, about 2900 times slower than carbohydrazide. Therefore, the dimer can be considered to be inert on the time scale of carbohydrazide autoxidation at pH 7.9. If it is formed during the low concentration autoxidation of carbohydrazide (ca. 1 x IO4 M), the total amount of dioxygen consumed would decrease since 1 mol of unoxidized carbohydrazide is eliminated for each mole of dimer formed.
Discussion The copper(I1)-catalyzedautoxidation of carbohydrazide occurs rapidly even at 32 "C. The reaction was followed in this study by the disappearance of oxygen, which was virtually complete in a few minutes a t most. The only products that we detect are hydrazine and hydrogen peroxide. We assume by implication that water is also formed since the amount of hydrogen peroxide accounts for only part of the oxygen consumed. We have demonstrated that the autoxidation of carbohydrazide is complex. Even the initial reactions, which produce hydrazine and hydrogen peroxide, are not simple. However, we are able to kinetically analyze the reaction to a great extent (ca. 50%) under most conditions. We have demonstrated the following characteristics of the reaction: 1. The reaction exhibits an induction period that can be lengthened with benzoquinone, a known free-radical inhibitor. 2. The reaction proceeds rapidly for a time (ca. 50% O2 consumption) and then slows appreciably. 3. In its initial rapid phase, the reaction is pseudo first order; i.e., log ( [ 0 2 ] 0 / [ 0 2 ] t ) = k'(t - to) where to is the induction period and k ' is nearly proportional (0.9 power) to the initial concentration of the Cu(CHZ)22+complex. 4. There is no experimental evidence for a reversible Cu-O2 complex as an intermediate. 5. The products of the reaction (H20z,NzH4) react with one another at a rate slow enough to let them accumulate. 6. Hydrazine retards the reaction. However, its autoxidation is apparently catalyzed by the same species that catalyzes CHZ autoxidation. If we had resorted to the common initial rate method of analyzing the kinetics of complex reactions, we might have concluded that this was a simple second-order reaction of Cu(CHZ)p and O2 Examination of the entire [O,] vs time curve, however, shows that this is not the case. At
Ind. Eng. Chem. Res., Vol. 29, NO. 7, 1990 1135 least in its initial phases (ca. 50%),the reaction is first order and first order with respect to [O,]. It is the firstorder rate constant that is nearly proportional to the initial concentration of Cu-CHZ complex ([Cun(CHZ)t+lO).We can explain this result only by postulating that a reactive intermediate derived from Cu"(CHZ)*+ is present at steady-state concentration during the initial part (ca. 50%) of the reaction. It is this intermediate that undergoes an apparent bimolecular (but kinetically first-order) reaction with OP The mathematics of such steady-state reaction models are generally complex. Some simple examples have been worked out by Benson (1952). In general, his analysis shows that steady-state concentrations are reached at low total extents of reactions when the rate constant for consumption of intermediate is much larger than the rate constant for its formation. The induction period and retardation by benzoquinone observed in this study show that we are dealing with a regenerative free-radical chain reaction. We can only speculate about the nature of the initiating and chain carrier species. However, benzoquinone is known to quench superoxide anion rapidly (Sellers and Simic, 1976). The reduction of dioxygen in water is generally believed to yield initially the superoxide anion (02-)which disproportionates rapidly to dioxygen and H202and/or H20 depending on pH (Sawyer and Nanni, 1981). At the nearly neutral pH where most of this study was conducted, both disproportionations must occur to some extent. We suggest that a closely related derivative of Cu"(CHZ)*+ initiates the reaction in the same manner that some metal-containing enzymes initiate nonenzymatic regenerative autoxidations of organic substrates (Yokota and Yamazaki, 1977). The active species (CuN,) in this scheme most likely contains copper in a square-planar geometry and generates superoxide anion from dioxygen. The autoxidation of substrate then proceeds by a classic regenerative chain mechanism (Mill and Hendry, 1980). How this might occur is shown in the following reaction sequence: Cu2++ 2CHZ
2
Cu"(CHZ)$+
+ OH-
+ + + 02
RNHNH2 RNNHz RNHNH2
CU"(CHZ),~+
CuN,
-
CuN,
02-
02- RNNH2 + H02I11
O2
HOz'
RN=NH
+ H02'
RNNH2 + H202
In this scheme, the chain carrier is superoxide or its conjugate acid. Applying the customary steady-state treatment to the RNNH, radical (111) yields the observed pseudo-first-order kinetics. To explain the formation of 2 mol of hydrogen peroxide per mole of hydrazine, the following scheme (steps 1-3) could be invoked, with step 2 being faster than step 1; i.e., NHPNHC(=O)NHNH* + 02 NH=NC(=O)NHNH, + Hz02 (1) IV NH=NC(=O)NHNH, + 0 2 + H2O NH2NHz + Nz + COP + H202 (2) NHzNH2 + 2H202 N2 + 4H20 (3) This scheme accounts for all of the oxygen consumed provided that one-third of the hydrazine formed in step 2 is consumed in step 3 in the initial portion of the reaction -+
-
-
and that all three steps are catalyzed by Cu(I1). In that case, step 3 would have to proceed at about the same rate (or somewhat slower) as steps 1and 2 in order for hydrogen peroxide and hydrazine to accumulate. In a qualitative sense, this analysis is consistent with the observation that - [O,],) decreases from the ratio ([N2H4J,+ [H202J,/([02Jo unity as the reaction proceeds past its initial fast phase. Several other reaction schemes could be devised to account for the stoichiometry. For example, if intermediate IV partitions between reaction with dioxygen (step 2) and Michael reaction with CHZ to form an unstable tetrazane (V) that is further oxidized (with O2 or Hz02)to yield N2 and dimer (11) (steps 4 and 5), the accumulation of H202 IV + CHZ -,
NH,NHC(=O)NHNHNHNHC(=O)NHNH, (4)
V V + H202 N2 + 2H2O + I1 (5) and N2H4 can be explained as long as dimer accumulates. However, since we do not detect any dimer except at very low copper concentration, we find this explanation unlikely. Coupling of two molecules of IV would yield a 1,4-diacyltetrazene, which could decompose directly to dinitrogen and dimer (11). Many autoxidations of organics are catalyzed by transition metals. The reaction of oxygen with ascorbic acid, for example, is very sensitive to copper. Although that reaction has been the subject of numerous studies, the exact nature of copper's role remains uncertain (Mushran and Agrawal, 1977). This is typical of many such catalyzed autoxidations. It is of special interest, therefore, that the autoxidation of carbohydrazide is apparently catalyzed by a specific Cu(I1)-CHZ species about which something is known. The soluble Cun(CHZ)2+complex has been shown to have a planar CuN, structure (Campbell and Grzeskowiak, 1976). It is unstable in the presence of halide counterions and is reported to decompose to nitrogen and other materials including a Cu(1) species under unspecified conditions. As was stated above, however, our data do not shed much light on the nature of the initiating species. We can say that the Cu-CHZ system is complex and apparently initiates the autoxidation of carbohydrazide, which proceeds by a regenerative free-radical chain mechanism. The complexity of the reaction increases as reaction products accumulate. Although hydrazine is oxidized in the presence of Cu(CHZ)2+,it also competes for copper and thus retards the overall deoxygenation reaction. Any coupling to form the dimer (11) would likewise result in slower and lower total oxygen consumption since the dimer reacts only slowly. Although the present study has been carried out at a lower temperature than is encountered in feed water systems, our results should be useful in understanding the industrial situation. The dependence of the reaction on copper can be explained on the basis of the above mechanism. Because of its high thermal conductivity and ease of fabrication, copper alloys are used extensively in deaerators, heat exchangers, and other components of the feed water system. Electrochemical corrosion of these metals releases enough Cu(I1) to catalyze the reduction of oxygen by carbohydrazide. In fact, complexation of Cu(I1) by carbohydrazide would thermodynamically favor dissolution of copper as would traces of ammonia or oxygen. The passivation observed during the use of carbohydrazide is more difficult to explain. Hydrogen peroxide has been applied as a pretreatment to induce passivation in boilers, and perhaps, also as an intermediate, it plays a role in the production of magnetite during the oxygenscavenging reaction (Freier, 1977). It is also of interest to
Ind. Eng. Chem. Res. 1990,29, 1136-1142
1136
note that hydrazine, a recognized high-temperature oxygen scavenger, is produced by oxidation of its parent compound and not by hydrolysis. The principal reason for developing carbohydrazide as a scavenger was to reduce the risk of handling hydrazine in industrial environments. That objective has been realized. The increased reactivity and passivation, however, show that carbohydrazide is not just a safe form of hydrazine. These properties do not result from in situ release of hydrazine but are characteristic of carbohydrazide itself. The observed accumulation of hydrazine is apparently not material to the overall result. Registry No. I, 497-18-7; 11, 1617-13-6; Cu, 7440-50-8; 02, 7782-44-7; water, 7732-18-5; hydrazine, 302-01-2.
Literature Cited Attanasi, 0.;Serra-Zanetti, F. Effect of metal ions in organic synthesis; VII. Conversion of acylhydrazines and N-acyl-N'-tosylhydrazines to carboxylic acids and esters in the presence of copper(I1) chloride. Synthesis 1980, 314-315. Banerjea, D.; Singh, I. P. Hydrazine complexes of some divalent metal ions in aqueous solutions. 2. Anorg. Allg. Chem. 1967,349, 213-219. Benson, S. W. The induction period in chain reactions. J . Chem. Phys. 1952,20, 1605-1612. Borsche, W.; Muller, W.; Bodenstein, C. A. Relation between quinonehydrazones and p-hydroxyazo compounds. VII. Aliphatic aromatic dihydroxy4,4'-disazo compounds Justus Liebigs Ann. Chem. 1929,475,120-131. Campbell, M. J. M.; Grzeskowiak, R. An epr study of some complexes formed by interaction between copper(I1) oxyacid salts and carbohydrazide. Inorg. Nucl. Chem. Lett. 1976, 12, 545-549. Campi, E.; Ostacoli, G.; Vanni, A.; Casorati, E. Complexes of carbohydrazide with metallic ions in aqueous solution. Ric. Sci., Parte 2: Sez. A 1964, 6, 341-356. Cosper, D. R. (Nalco Chemical Co.) Unpublished data on autoxidation rates of hydrazides of aliphatic acids, 1983. Feher, F.; Linke, K. H. Concerning the oxidation of urea, semicarbazide and carbohydrazide with sodium hypochlorite. J. Prakt. Chem. 1966, 32, 190-197. Freier, R. K. Corrosion protection of steam power boilers with hydrogen peroxide. Energie 1977,29, 294-296.
Keim, G. L.; Henry, R. A.; Smith, G. B. L. The oxidation of di- and triaminoguanidine with potassium iodate. J . Am. Chem. SOC. 1950, 72, 4944-4946. Krivis, A. F.; Gazda, E. S.; Supp, G. R.; Kippur, P. Coulometric determination of carboxylic acid hydrazides. Anal. Chem. 1963, 35, 1955-1957. Meites, L. Iodometric determination of copper. Anal. Chem. 1952, 24, 1618-1620. Mill, T.; Hendry, D. G. Kinetics and mechanisms of free radical oxidation of alkanes and olefins in the liquid phase. In Comprehensive Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1980; Vol. 16 (Liquid-phase Oxidation). Mushran, S. P.; Agrawal, M. C. Mechanistic studies on the oxidation of ascorbic acid. J . Sci. Ind. Res. 1977, 36 (6), 274-283. Rynasiewicz, J. Hydrogen peroxide determination in the presence of chromate. Anal. Chem. 1954,26, 355-358. Sawyer, D. T.; Nanni, E. J. Redox chemistry of dioxygen species and their chemical reactivity. In Oxygen and Oxy-radicals in Chemistry and Biology, Rodgers, M. A. J., Powers, E. L., Eds.; Academic: New York, 1981. Sellers, R. M.; Simic, M. G. Pulse radiolysis study of the reactions of some reduced metal ions with molecular oxygen in aqueous solution. J . Am. Chem. SOC.1976, 98, 6145-6150. Slovinsky, M. Boiler additives for oxygen scavenging. .- - US. Patent 4,269,717, 1981. Smith. P. A. S. The Chemistrv of ODen-Chain Organic Nitrogen Compounds;W. A. Benjamin, Inc.; New York, 1966: Vol. 11, p i 8 5 and references cited therein. Stoll6, R. Hydrazidicarboxylic hydrazide. Ber. Dtsch. Chem. Ges. 1910, 43, 2464-2470. Tsuji, J.; Nagashima, T.; Qui, N. T.; Takayanagi, H. Facile oxidative conversion of hydrazides of carboxylic acids to corresponding acids, esters and amides using copper compounds. Tetrahedron 1980,36, 1311-1315. Watt, G. W.; Chrisp, J. D. A spectrophotometric method for the determination of hydrazine. Anal. Chem. 1952,24, 2006-2008. Yokota, K.; Yamazaki, I. Analysis and computer simulation of aerobic oxidation of reduced nicotinamide adenine dinucleotide catalyzed by horseradish peroxidase. Biochemistry 1977, 16, 1913-1920.
Received for reuiew July 25, 1989 Accepted January 31, 1990
Methanol Oxidation over Nonprecious Transition Metal Oxide Catalysts Umit S. Ozkan,* Richard F. Kueller,+and Edgar Moctezuma Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210
Methanol oxidation over nonprecious transition metal oxide catalysts was studied in a gradientless external recycle reactor. The catalysts (oxides of Cr, Mn, Fe, Co, Ni, Cu), which were prepared by using the incipient wetness technique, were supported on 1/8-in. -y-Al,O, tablets. The catalysts were characterized by using BET surface area measurement, X-ray diffraction, laser Raman spectroscopy, scanning electron microscopy, and energy dispersive X-ray analysis techniques. All the catalysts exhibited similar activities for methanol conversion, but the Cu catalyst was found to be considerably more selective to COz. The order of the reaction was 1.2 with respect to methanol concentration and appeared to range between 0.5 and 0 with respect to oxygen concentration. As the use of alcohol-fueled vehicles grows, effective catalytic control of emissions from these vehicles is becoming increasingly important. Gasoline-powered vehicles typically emit unburned fuel, CO, and a mixture of C2-C, hydrocarbons along with various cyclic compounds (Goodrich, 1982). The catalysts used to control these emissions generally contain a precious metal (Pt, Pd, Rh, Ag) supported on a ceramic monolith. The emissions from pure-alcohol-fueled vehicles contain the unburned fuel and CO, but the number of partially oxidized fuel derivatives
* To whom correspondence
should be addressed.
t Present address: Dow Chemical, Midland,
MI 48674.
0888-5885/90/2629-1136$02.50/0
are much smaller compared to gasoline combustion. Since complex hydrocarbons, sulfur, and lead are absent from the exhaust, nonprecious transition metal oxide catalysts can offer an effective and inexpensive alternative. Compared to the considerable work done on gasoline engine emission control (Stein et al., 1960; Yao and Kummer, 1973, 1977; Severino and Laine, 1983; Saverino et al., 19861, the number of studies on exhaust emission control from alcohol-fueled vehicles is much fewer. Most of the earlier work has been on either ethanol oxidation (Yao, 1984; McCabe and Mitchell, 1983, 1984; Gonzales and Nagai, 1985) or on aldehyde oxidation (McCabe and McCready, 1984; McCabe and Mitchell, 0 1990 American Chemical Society
