The Preparation of Calcium Superoxide from Calcium Peroxide Diperoxyhydrate E. Vernon Ballou,” Peter C. Wood, and LeRoy A. Spitze Department of Chemistry, San Jose State University, San Jose, California 95 192
Theodore Wydeven NASA Ames Research Center, Moffefi Field, California 94035
There is interest in solid materials containing a high percentage of stored oxygen for use in emergency breathing apparatus for miners and as auxiliary oxygen sources for astronauts. In theory, the amount of available oxygen in calcium superoxide, Ca(02)2is higher than in potassium superoxide, KO*, and its availability during use should be unhindered by the formation of a low melting and hydrous coating. The decomposition of solid calcium peroxide diperoxyhydrate, Ca02.2H202has been studied, using an apparatus which allows good control of the critical reaction parameters. Samples have been prepared showing apparent superoxide contents in excess of those previously reported and higher than the theoretical 58.4 YO expected from a disproportionation reaction.
Introduction There is interest in the preparation of high purity calcium superoxide as an oxygen source for breathing apparatus because both the available oxygen and the capacity for carbon dioxide removal, per unit weight of superoxide, are higher than that of a number of other chemical oxygen sources. In addition, the most widely used chemical source, potassium superoxide, has been subject to problems with utilization efficiency due to formation of a hydrous coating on the reaction products. It has been claimed that improved canister design (Perry and Wagner, 1973) and mixing the reagent with a drying agent such as molecular sieves (Buban and Gray, 1974) alleviates the KO2 “caking” problem, but an alternative course is the development of a reactant which has higher oxygen yield and carbon dioxide removal capacity and less liklihood of a problem with surface coatings. This has led to efforts by Petrocelli and Capotosto (1965) to make high purity calcium superoxide by mixing and vacuum drying calcium hydroxide and hydrogen peroxide, and to efforts by Wydeven and Michaels (1974) to make high purity calcium superoxide by freeze drying the product of the same reaction. However, the largest continuous effort on calcium superoxide preparations has been reported by Vol’nov and co-workers. These prior efforts produced calcium superoxide of 30-55% purity, but the higher purity yields did not appear to be reproducible and reaction conditions were either imprecisely described or incompletely controlled. Previous Work Vol’nov et al. (1956) first reported preparation of calcium superoxide, Ca(O&, in attempts to verify the experiments of Traube and Schulze (1921) with calcium peroxide octahydrate, Ca02-8H20,and hydrogen peroxide, H202. A yield containing 2-5% Ca(O& by this method was confirmed. The next effort was the direct drying or decomposition of calcium peroxide diperoxyhydrate, Ca02-2H202. Yields containing up to 16% Ca(02)2 were reported (Vol’nov et al., 1957) after decomposing the diperoxyhydrate for 2 h (7.2 ks) a t 50 “ C (323 K ) and 10 000 pmHg (1.3 kPa). In subsequent work (Vol’nov and Chamova, 1960) the reactant was spread over a larger surface and yields containing up to 45% Ca(02)2were reported, with 1 g of calcium peroxide diperoxyhydrate covering more than 900 cm2 and decomposing at 50 “C (323 K) and 180
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10 000-pmHg (1.3 kPa) pressure for 100 min (6.0 ks). A yield containing as high as 54% calcium superoxide was reported (Vol’nov et al., 1959a) from the decomposition of the diperoxyhydrate for 100 min (6.0 ks) at 50 “C (323 K ) and 10 000 pmHg (1.3 kPa), with 1 g of reactant spread over 1800 cm2. The 54% purity was not reproducible, and most superoxide yields were in the 2040% purity range. Vol’nov and Shatunina reported (1966) that the best conditions found previously were 50 “C (323 K), 10 000 pmHg (1.3 kPa), and 1800 cm2/g-to yield a product containing 40% Ca(02)Z. However, a higher vacuum was tested, and a product containing 55.4% Ca(02)2 was found by decomposing Ca02.2H202 at 40 “C (313 K ) for 60 min (3.6 ks), with 100 cm2/g and a pressure of 6 pmHg (0.8 WmHg) (13 pPa) the Pa). In a lower pressure experiment Ca(O2)2 purity was lower. Vol’nov and Shatunina pointed out that 55.40/0was close to the 58.4% maximum predicted for the postulated equimolar disproportionation of Ca02.2H202. No further work on this reaction has been noted, although yields containing u p to 44% calcium superoxide were reported (Latysheva et al., 1974) for the exchange reaction between calcium nitrate and tetramethylammonium superoxide in liquid ammonia. Although the experiments of Vol’nov and co-workers covered a wide range of conditions, a number of possibly critical parameters were not clearly defined. These included (a) the configuration of the reaction chamber with respect to sample position and vacuum pumping and temperature measurement and control units, (b) the variability of the temperature a t the site of the reaction (Le., a t the sample), (c) the variability of the pressure during the reaction and the type and accuracy of the pressure measurement device, (d) the particle size and uniformity of spreading of the sample over the area of the reaction dish, (e)the purity of the reactant, (f) the evacuation pumping speed, and (g) the techniques for cessation of the reaction and the techniques for subsequent transfer of the reaction products to either a storage chamber or analytical apparatus. From both the information given in the literature and the lack of information on experimental details, it seemed likely that prior experiments were not done with the type of instrumentation, control, and continuous readout described in this work. Although Vol’nov and Shatunina (1966) had eventually assumed that yields approaching the equimolar dispropor-
----- ELECTRICAL CONNECTIONS Figure I. Reaction chamber and auxiliary equipment for disproportionation of CaOT2Hz02
tionation of Ca02-2H20~ were the maximum attainable (58.4% Ca(O&), they had earlier proposed (Vol’nov et al., 1957) a free-radical mechanism which would allow yields up to 100%. An analogous process and starting material had been used in earlier work on the disproportionation of K202.2H~02by Kazarnovskii and Neiding (1952). They obtained, with very little effort a t optimizing reaction conditions, KO2 yields in the 90% range. The removal of evolved water from the decomposing Ca02.2H202, before it had an opportunity to back-react with Ca(O2)sformed in the reaction, became the primary rationale of the experiments described here. An apparatus was designed and constructed which allowed the initial reactant sample to be spread out uniformly and uniformly heated or cooled while evolved oxygen was evacuated and evolved water vapor was removed by a cold finger filled with liquid nitrogen. Experimental Section Materials Preparation. The first step in Ca(O2)zsynthesis was the preparation of the peroxide octahydrate, Ca02.8H20 from calcium chloride hexahydrate, CaC12.6H20. This was done by a standard method (Makarov and Grigor’eva, 1959), and it was found that dehydration over 40% sulfuric acid solution brought the octahydrate to the 99% purity level. Next, 90% hydrogen peroxide was prepared by a standard distillation method (Schmeisser, 1963) and reacted with calcium peroxide octahydrate to make calcium peroxide diperoxyhydrate. As the tests proceeded, it was found that the peroxide octahydrate could most efficiently be obtained in -100% purity by drying in a 4 X IO-” m:j/min flow of COS free air a t 26-27% relative humidity a t ambient temperature for 1.5 h (5.4 ks). A significant improvement in the steps for the preparation of C a 0 ~ 2 H ~ 0without 2, excess peroxide or extraneous water, resulted from the finding that dispersing the prepared material in dry 2-propanol a t -17 “C (256 K) apparently removed excess H2On and HzO. The dispersion was filtered and
dried in a flow of COS-free, controlled humidity air, to yield calcium peroxide diperoxyhydrate powder with the theoretical ratio of calcium to active oxygen. The diperoxyhydrate powder samples were stored a t liquid nitrogen temperature until they were used in the disproportionation experiments. Apparatus and Procedure. Preliminary superambient temperature disproportionation experiments were carried out in a modified vacuum desiccator, with the calcium peroxide diperoxyhydrate powder spread on a glass dish under a liquid nitrogen cold finger and heated with a hot plate. There was difficulty in obtaining reproducible purities of superoxide with the same apparent reaction conditions, and a new reaction chamber was constructed from a conical pipe cross (Corning Glass Works, Corning, N.Y.) of -10 cm pipe ID. This chamber allowed faster evacuation, controlled radiant heating and conductive cooling, and connections to continuous temperature and pressure recording instrumentation. Figure 1 is a schematic of the reaction chamber and supporting equipment, as finally evolved in the experiments. Figure 2 is a photograph of the reaction chamber. The left port was used to transfer samples, in conjunction with a dry nitrogen purged portable glove bag. For superambient temperature disproportionations, the radiant heat lamp responded to a thermocouple junction buried in the reactant powder, while another thermocouple junction was connected to a strip chart recorder. The end plate of the right port held a silvered glass liquid nitrogen reservoir outside the chamber and an unsilvered glass cold finger in a hollowed configuration inside the reaction chamber. This end plate also had a 2.2-cm opening, leading to a liquid nitrogen cold trap and a two-stage rotary Model D150 (Precision Scientific Co., Chicago, Ill.) vacuum pump. A Model 941-6001 Cryopump (VariadVacuum Division, Palo Alto, Calif.) was installed in the vacuum line for the subambient disproportionation experiments. This gave the system an effective m:j/s a t Il-KmHg pressure (1.5 pumping speed of 4.0 X Pa). The sample dish platform was also a cooling table, with Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 2, 1977
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Figure 2. Conical glass pipe cross reaction chamber for dispropm tionation of CaOyZHrOy
connections to a Masterline 2095 (Forma Scientific Co., Marietta, Ohio) constant-temperature refrigerator bath. The sample itself (calcium peroxide diperoxyhydrate powder) was spread on either a 9-cm glass Petri dish or a gold-plated copper dish of similar diameter (for suhamhient temperatures). A feed-through in the bottom end plate connected to a Baratron capacitance manometer Model 145 BHS-10 pressure sensor head (system error 1.2% of pressure reading in range > I pmHg), which was electrically connected to a Baratron Model 170M-6A pressure meter (MKS Instrumenk Inc., Burlington, Vt.). During the initial experiments at higher pressures a small, high velocity D.C. fan was placed in the left arm of the glass cross to increase circulation of evolved gases past the cold finger. With the use of either the controlled radiant heating lamp or the cpoling table, the temperature of a reacting sample could he controlled in a range of ca. -15 to 95 "C (258-368 K), as recorded to fl".The thermocouple junction sensed the temperature in a discreet volume element of the powder sample and was relatively insensitive to reaction temperatures at other points in the powder sample. The pressure record, however, was extremely sensitive to transient reactions from any part of the powder sample, and the tracing of the pressure record was used to monitor the smoothness of the decompositon. In the procedure for superambient temperature experiments a 0.4-g sample of 2-propanol washed Ca022HzOz was passed through a Tyler no. 48 stainless steel sieve (0.30-mm opening) onto the glass Petri dish in such a way as to obtain an even layer of powder over the surface of the dish. The sample dish was then placed on the platform, the two thermocouples were buried in the powder, and the chamber was pumped down to a residual pressure of 20-30 pmHg (2.7-4.0 Pa). Liquid nitrogen was then added to the cold finger and the chamber was pumped to a residual pressure of 4-8 pmHg (0.5-1.1 Pa). The heat lamp was then switched on and the sample was brought to the preset reaction temperature. At the end of the reaction period, the heat lamp was switched off and the chamber was hack-filled to atmospheric pressure with dry 182
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nitrogen. The sample dish was then removed from the left port of the reaction chamber into the glove bag and placed in a small desiccator. The desiccator was transferred to a dryhox where the sample was loaded into vials for analysis. In the suhamhient temperature experiments the CaOz. 2H202 sample was spread on the gold-plated copper dish instead of the Petri dish. A thin layer of silicone stopcock grease containing an equal part of 400 mesh silver powder was used between the dish and the cooling tahle to ensure good heat transfer. One thermocouple was buried in the sample powder, while the other was placed in a small well in the dish. The reaction chamber was pumped down to 10 pmHg (1.3 Pa) with the roughing pump, and then to 2 pmHg (0.3 Pa) with the Cryopump. At this pressure the liquid nitrogen was added to the cold finger and the flow of coolant through the cooling tahle was initiated. At the end of the reaction period, the coolant temperature was raised to 20 "C (293 K) and the warmed sample was transferred to the drybox. Analytical Method. A precise analytical determination of the proportion of calcium superoxide in prepared samples was needed to monitor preparation schemes and conditions, in order to know if reaction conditions could he reproduced and to know if variations in conditions or apparatus configuration improved the yield. The method of Seyh and Kleinherg (19511, which has also been used by Vol'nov and Latysheva (1959h), was adopted. In this method the sample was decomposed by acetic acid in diethyl phthalate, which released oxygen from the superoxide hut not from the peroxide. The evolved oxygen was measured volumetrically and compared with the amount evolved by iron chloride in hydrochloric acid. The peroxide oxygen content was obtained from the difference or from the titration of the acidified (3 N HsP04) residue with potassium permanganate. The appropriate equations are Ca(Oz)z + ZCH:,COOH
=
Ca(CHaC00)z + H202
+0 2
(1)
for the acetic acid/diethyl phthalate decomposition,
+ 2HCl (with FeCl3) = CaCIZ + H20 + ,%Oz (2) CaOz + 2HC1 (with FeCla) = CaC12 + HzO + KO2 (3)
Ca(02)Z
Ca02.2H202
+ 2HC1 (with FeC13) = CaClz
+ 3Hz0 + 3/20,
65
(4)
for the iron chloride/hydrochloric acid decomposition, and 5H202
+ 2Mn04- + 6H+ = 2Mn2+ + 8 H z 0 + 502
N
I
60
(5)
for the permanganate titration. A comparison of eq 1 and 2 shows that, when the sample contains only superoxide and inert material, the acetic acid decomposition should yield exactly 213 of the quantity of oxygen released by the FeC13/HCl decomposition. If any peroxide is present in the sample, the acetic acid decomposition should yield
