J. Phys. Chem. 1995, 99, 6857-6863
6857
Kinetics of Hydrogen Formation from Formaldehyde in Basic Aqueous Solutions S. Kapoor, F. A. Barnabas, M. C. Sauer, Jr.,* D. Meisel, and C. D. Jonah Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 Received: May 20, 1994; In Final Form: January 27, 1995@
A phenomenological description of dihydrogen generation from solutions of formaldehyde and glyoxylate in basic aqueous solutions at high ionic strength is presented. Effective rate constants and activation energies for the HZ production reactions of these two substrates are given. The effect of formaldehyde and NaOH concentrations on the rate of HZ generation was studied at 23, 45,and 60 "C. The reaction is first order in formaldehyde and approaches second order in [OH-] at low [HCHO]. At high ionic strength the experimental results can be modeled by a reaction mechanism that includes the Cannizzaro reaction in competition with the hydrogen production reaction. To obtain acceptable agreement of the experimental observations with the computational modeling, one needs to invoke a reaction of hydroxide ions with the two deprotonated forms of methylene glycol (the hydrated formaldehyde). The quantitative description of this reaction is important in safety issues of stored high level liquid nuclear waste.
Introduction The generation of potentially flammable mixtures of H2 and N20 in highly alkaline nuclear waste solutions has become of concem in recent years.'q2 Some of the mixed radioactive wastes in temporary storage tanks are known to generate mixtures of gases including H2, N20, N2, and NH3. Of particular concem are the potentially flammable mixtures of Hz and NzO that are released periodically. We have determined the role of radiation in the production of H2 and the other gases in the waste tanks3 as part of a program to develop a better understanding of the processes that lead to the buildup of these hazardous gas mixtures. In this context we have studied the kinetics of hydrogen generation from basic aqueous solutions of formaldehyde and glyoxylate. It is known that the radiolytic degradation products of EDTA and HEDTA include formaldehyde and glyoxylate. It has been known for some time that formaldehyde generates hydrogen in aqueous solutions at high alkaline concentration^.^ Recently, it has been shown that formaldehyde and glyoxylate can quantitatively generate Hz in aqueous solutions at very high base concentration^.^ This is in spite of the fact that the H2 generation reaction is in competition with the Cannizzaro reaction.6 It is commonly believed that the mechanism of the Cannizzaro reaction is hydride transfer from the singly and doubly ionized forms of the aldehyde-hydrate (methylene glycol) to another aldehyde molecule.' The mechanism for the H2 generation reaction has been proposed to utilize one water p r ~ t o n we ; ~ have presented evidence elsewhere that this is true for glyoxylate but only partially for formaldehyde.8 Experimental Section NaOH, NaNO3, NaN02, glyoxylic acid, and formaldehyde (37 wt % in aqueous solution containing 10-15 wt % of methanol) from Aldrich were used as received. NaA102 was from ICN Biomedicals, Inc.; concentrations were corrected for NaOH and H20 found as impurities by analysis of the aluminate. It was verified that no H2 is generated from aqueous solutions containing up to 15 wt % methanol at the base concentrations used in this study. For glyoxylate, a neutralized solution of glyoxylic acid was used. The gas measurements were done by
gas chromatography using a thermal conductivity detector (argon carrier gas) and a molecular sieve 5A column at 23 "C; details are given el~ewhere.~ Solutions were degassed by bubbling with argon at room temperature in modified Erlenmeyer vessels equipped with septa for gas bubbling and gas sample withdrawal. The vessels were put into a thermostated water bath held at the desired temperature. To initiate the reaction, formaldehyde or glyoxylate was injected into the flasks after the solutions were equilibrated at the desired temperature for half an hour. Gas samples were withdrawn for chromatographic analysis at various time intervals following the injection. For measurements of the rate from formaldehyde, the time of initiation of the reaction is in some cases somewhat uncertain because the rate of H2 generation during injection and mixing of the aldehyde is not negligible. Equilibrium between the solution and the gas phase was maintained by stimng. The concentration of H2 in the liquid phase was calculated from the ratio of volumes of the two phases, the volume of sample withdrawn for analysis, and the result of the gas chromatographic analysis. The possibility of leakage of gases to or from the vessels during long thermal reaction times was checked in control experiments. The results indicated that the loss of H2 by leakage in our experiments at the longest times used is less than =lo%. The maximum leakage of air into the vessel resulted in a concentration of 02 in the liquid of less than 2% of the concentration that would result from 1 atm of air in equilibrium with the solution. From determinations of the effect of 0 2 on the rate of thermal production of H2 (vida infra), the effect of 0 2 leakage on HZ production rates is negligible even at the longest times used. Results and Discussion Formaldehyde in Aqueous Solutions. The rates of HZ generation from formaldehyde were measured in solutions containing HCHO and NaOH. Figure 1 shows the effect of [HCHO] on the rates of thermal generation of H2 at 60 "C. If, during the time span of Figure 1, the consumption of HCHO by the Cannizzaro reaction were negligible, the slopes of the lines in Figure 1 would be the initial rates and could be expressed in terms of eq 1, where n and m are the orders of
* Author to whom correspondence @
should be addressed. Abstract published in Advance ACS Abstracts, April 15, 1995.
0022-365419512099-6857$09.00/0
0 1995 American Chemical Society
6858 J. Phys. Chem., Vol. 99, No. 18, 1995
Kapoor et al.
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Time, minutes Figure 1. Rate of H2 generation at 60 "C from 2.3 M NaOH solution at various concentrations of HCHO: circles, 0.13 M; triangles, 0.065 M; squares, 0.0325 M. TABLE 1: HZProduction vs [NaOH] and [HCHO] in Aqueous Solution and in Simulated Waste Solution [NaOH] [HCHO] temp initial rate x lo8 kbs x lo* (M-l s-') ("C) (M min-') (M) (M) No Additional Additives 1.o 5 x 10-3 60 2.00 6.7 2.3 4.0 1.o 2.3 4.0 2.3 2.3 2.3 2.3 2.3
5 x 10-3 5 x 10-3 5 x 10-4 5 x 10-4 5 x 10-4 0.0325 0.065
0.13 0.065 0.065
60 60 60 60 60 60 60 60 21 45
6.50 17.0 0.20 0.77 2.21 24.7 55.2 91.0 2.5 14.0
9.00 13.93 6.7 10.00 18.4 5.51 6.15 5.01 0.28 1.56
Solution P: Contains 2.79 M NaN03, 2.22 M NaN02, and 0.86 M NaA102 2.3 2.3 2.3 2.3 2.3 3.0 4.0
0.065 0.065 0.0325 0.065 0.13 0.065 0.065
23 45 60 60 60 60 60
1.9 26.2 66.2 85.2 126 162 167
0.21 2.92 14.8 9.50 7.02 13.9 10.7
reaction in the corresponding reactant. The initial rate increases upon increasing the concentration of HCHO, as can be seen in Figure 1. A similar dependence was observed for variation in [OH-]. The initial rates of H2 formation obtained at various concentrations of NaOH and HCHO are compiled in Table 1. However, [HCHO] should decrease appreciably during the given time period, based on the rate constants for the Cannizzaro reactions derived later for simulated waste solutions, which predicts that the initial rate plots should show some curvature. Therefore, the fact that the plots of Figures 1 and 2 appear to be linear is probably due to limited experimental precision. Note that the analogous experimental results for formaldehyde in simulated waste solutions, as well as the simulated results (which
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Time, minutes Figure 2. Rate of H2 generation from 0.065 M HCHO and 2.3 M NaOH solution at three temperatures: circles, 60 "C; triangles, 45 "C; squares, 21 "C. give a good fit to the experimental results), are also approximately linear in this time regime. This is true despite the fact that more than half of the formaldehyde should be consumed during 100 min at 0.13 M HCHO and 2.3 M OH-. Figure 2 shows the effect of temperature on the rate of thermal generation of H2 from HCHO-containing solutions at 2.3 M NaOH. Assuming that the rates determined from the slopes of the lines of Figure 2 are defined by eq 1 with n = m = 1, effective rate constants (kobs) were calculated from eq 1 using the rates obtained from the slopes of the lines shown in Figure 2, for [HCHO] = 0.065 M and [NaOH] = 2.3 M. Because the reaction is not first order in [OH-] (see below), this treatment will yield an activation energy that is slightly dependent on [OH-]. The activation energy obtained (Figure 3) for 2.3 M OH- and 0.065 M HCHO is 65 kJ/mol (15.5 kcavmol). Also, the initial rates are expected to be low due to consumption of HCHO. This effect should be greater the higher the temperature, which means that the slopes of the lines in Figure 3 are lower limits; hence, 65 kJ/mol is a lower limit. The amount of H2 generated vs time at various concentrations of NaOH and HCHO is shown in Figures 4 and 5 for times extending up to a few days. The conversion ratios of HCHO to H2, from the [H2] measured from these figures at long times after initiation of the thermal reaction, are summarized in Table 2. The conversion ratio increases upon decreasing HCHO concentration and upon increasing [OH-]. This is the expected effect because the Cannizzaro reaction is second order in formaldehyde, whereas the hydrogen evolution is first order in the aldehyde. Similar observations were reported by Ashby and co-workers5 at much higher base concentrations. Shown in Table 1 are the observed rate constants, kobs, calculated from eq 1 assuming n = m = 1. If the latter assumption holds, and if the consumption of HCHO had no
Hydrogen Formation from Formaldehyde in Basic Solutions
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J. Phys. Chem., Vol. 99, No. 18, 1995 6859
Formaldehyde in Aqueous Base
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(UT) x103, K Figure 3. Arrhenius plots for the Hz generation reaction in solutions containing 2.3 M NaOH: squares, formaldehydein solution P; triangles, formaldehyde in water; circles, glyoxylate in solution P.
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Figure 5. Generation of HZat 60 "C from an aqueous solution of 2.3 M NaOH and several concentrations of HCHO: squares, 0.0005 M; triangles, 0.005 M; diamonds, 0.0325 M; circles, 0.065 M; asterisks, 0.13 M.
TABLE 2: Hz Production vs [NaOH] and [HCHO] in Aqueous Solution at 60 "C P (h) [HCHOI (M) [NaOHl (M) 100{ [HzI/[HCHOIO}" 46.6 50.0 52.6 52.6 52.6 52.6 92.8
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effect on the initial rate determination, then a constant kobs should be obtained. The deviations of kobs from constancy are likely to be related to the consumption of HCHO; for both aqueous solutions and solution P, the values of kobs tend to increase as the initial [HCHO] decreases. This is consistent with the fact that the disappearance of HCHO by the Cannizzaro reactions,
0.065 0.0325 0.005 0.005 0.005 0.0005 0.0005
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during the period of about 100 min used to measure the initial rates, becomes relatively less important as [HCHO] decreases, due to the bimolecular nature of the Cannizzaro reaction. Thus, the variations in initial rate with [HCHO] can be explained by consumption of HCHO, and a value of m > 1 is not indicated. For [HCHO] from 0.5 to 5 mM, the consumption of HCHO is relatively unimportant during the measurement of initial rates. Figure 6 shows the initial rates under these conditions, and it is seen that the rate is higher than first order in [NaOH]; Le., n > 1. Also shown in Figure 6 are the initial rates of H2 formation is a function of the activity of OH-. The activities were obtained from the 1iteratu1-e.~Although the dependence on activity leads to a smaller n than the dependence on concentration, the value of n in eq 1 is still > 1. Analysis of these results gives, using [OH-], n = 1.72 at 0.5 mM OH- and n = 1.53 at 5 mM OH-. Using the activity of OH-, we obtain n = 1.40 at 0.5 mh4 OH- and n = 1.25 at 5 mM OH-. The order of the reaction may approach 2 as the interference from the Cannizzaro reaction is minimized. Polymeric Formaldehyde. To examine the possibility that polymers of HCHO, paraformaldehyde o r trioxane, are present in the original source of HCHO and generate H2, the following tests were conducted. The source of formaldehyde was a 37%
Kapoor et al.
6860 J. Phys. Chem., Vol. 99, No. 18, 1995
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