Advanced Propellant Chemistry - ACS Publications

18 Synthesis of OF2 by Electrolysis of Wet HF J O H N A. D O N O H U E , T H O M A S D. N E V I T T , and ALEX Z L E T Z

Downloaded by CORNELL UNIV on September 28, 2016 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0054.ch018

Research and Development Department, American Oil Co., Whiting, Ind.

The electrolysis of wet hydrogen fluoride produces a mixture of oxygen difluoride, ozone, and oxygen at the anode. The effects of electrolysis variables on product yields and anode life were studied at water concentrations from 0.2 to 7.8%. Methods were also developed for analyzing the anode products by gas chromatography and for measuring the water content of the hydrogen fluoride in situ by infrared spectroscopy. Brief, periodic interruptions of the cell current during electrolysis give consistent yields of oxygen difluoride at current efficiencies of at least 45% for several hours. No loss of anode material was detected. Potentially, this direct electrolytic synthesis of oxygen difluoride is less expensive and more efficient than the conventional two-step synthesis in which fluorine produced by electrolysis reacts with aqueous alkali.

^ V x y g e n difluoride ( O F ) is an attractive oxidizer for many fuels ( I I , 16), especially hydrocarbons, because it provides the optimum O / F ratio for hydrocarbon oxidation. Since it is denser than the equimolar 0 - F mixture ( F l o x ) , it should be easier to handle and should perform better. However, O F has been expensive to make because the usual preparation from F plus base (18) converts half the F to F~~. C o n sequently, the less attractive but lower cost Flox mixtures have received more attention. A better synthesis for O F would remove this obstacle and justify a more thorough investigation of its performance. While O F was first identified i n the electrolysis of wet H F i n 1927 (7, 8, 13, 14), this process was not suggested for O F production until 1955 (10, 15). Yields of 6 0 % O F have been claimed for electrolysis of H F containing from 1-20% water ( 9 ) . However, we find that O F yields vary with the water concentration. In a study to determine whether the O F radical is an intermediate in the electrolysis of wet H F , we have found conditions that ensure 2

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1 92

Holzmann; Advanced Propellant Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

18.

DONOHUE ET A L .

Synthesis of OF

g

193

consistent and satisfactory yields of O F . The effects of water, time, and current interruption on product yields and oh anode surface are presented. Analyses developed for this work are also presented. 2


Experimental Equipment. The electrolysis cell is shown i n Figure 1. It is a 300-ml. K e l - F cup equipped w i t h a stainless steel cap and a Teflon gasket. (The l i q u i d level can be observed through the translucent K e l - F . ) The cap contains: a thermocouple w e l l and electrode leads; separate ports for introducing electrolyte and water, replacing anodes, and flushing w i t h helium; separate lines for circulating the electrolyte to an infrared cell and for passing the gaseous products through a dry ice (—78° C . ) con denser and a N a F scrubber to remove the last traces of H F . D u r i n g the electrolysis, the cell is immersed i n an ice bath. το G X .

ΓΝ°

p

- 7 8 ° C REFLUX CONDENSER

Figure 1.

HF electrolysis cell

The power supply is an Electro Products model D-612T. F o r auto matic interrupted operation, it is connected to the electrolysis cell through a mercury relay operated by a Flexopulse timer (Eagle Signal C o r p . ) . Voltages are read at the power supply and corrected for I R drop i n the leads to thé cell. Procedure. Incremental amounts of water were added to 250 m l . of H F containing 1 mole % of K F , the solution was electrolyzed w i t h nickel anodes, and the gaseous products were analyzed periodically. I n


1 94

ADVANCED PROPELLANT CHEMISTRY

later experiments water was added continually to replace that lost b y electrolysis. Current density was followed continually. F o r study of anode surface deposits and weight changes, the anode was removed, rinsed with H F to remove K F , and dried at reduced pressure. I n our earlier runs without continuous infrared analysis, the cell temperature was 0 ° - 3 ° C . However, when the electrolyte was circu lated for infrared analysis, the cell temperature rose to 10°-15° C . Gas Analysis. Analysis of gaseous products—H , 0 , 0 , O F , and possibly F —has been improved periodically. A t first the gaseous prod ucts were passed through K I solution, and 0 and O F were estimated from the amounts of I and F~~ produced. Gas chromatography on silica gel {4,6) was used next. O u r latest setup is temperature programmed as shown schematically i n Figure 2. The apparatus uses only dry, degreased metals and fluorocarbon plastics passivated with O F and O 3 . The 6-inch column length is a compromise to minimize 0 decomposi tion and still permit separation of H and 0 at a convenient temperature. 2

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VACUUM GAUGE

2

HELIUM SUPPLY

Θ

I VENT

VACUUM _ PUMP

Je-COLSJ

1 !

ί

SAMPLE VALVE

VENT BUBBLE COUNTERS

POWER SUPPLY

FLOW CONTROLLERS

TO VENT —« VIA BUBBLE COUNTER

[RECORDER

SURGE TANKS

τ

DETECTOf

[_ CONJAÏCR [INDICATOR GASES FOR STANDARDIZATION

^ LABORATORY DRYING TOWER

GASES FROM ELECTROLYSIS CELL

DEWAR FOR LIQUID N

Figure 2.

ROTA METER. 2

Low temperature programmed gas chromatograph

I n a typical analysis the columns are cooled to about —75° C . b y dry air precooled by liquid N . Then the gas sample is injected into one column b y a Perkin-Elmer sampling valve; the second column is for reference. After sampling is complete, the liquid N is removed, and the air flow is continued for about 5 minutes or long enough to warm the columns to about —10° C . and remove the 0 . Yields, as percent of current, are calculated from the gas chromatographic, amperage, sample volume, and total gas flow measurements. This analysis has limitations. The thermal conductivity detector has a low sensitivity for H ; yet high concentrations cannot be allowed be cause response is not linear. Consequently, the flow of helium through the electrolysis cell must be adjusted to keep the H concentration i n the sensitive range. 2

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18.

Synthesis of OF

DONOHUE E T A L .

195

9

E v e n after prolonged passivation, the components of the system still react with F so that only qualitative trace peaks are obtained. However, F production occurs outside the usual range of conditions for O F syn thesis. Thus, the F analysis is less important. Because 0 decomposes readily (17), the true 0 yields may be higher than found, and the 0 yields may be correspondingly lower. Determination and C o n t r o l of Water Concentration. The water content of the electrolyte is monitored continuously by infrared absorp tion at 1.95/4 (12) as shown schematically i n Figure 3. The motor-driven syringe (Figure 3) is used to add water to the electrolyte and thus to maintain a constant water concentration d i n i n g electrolysis. A diaphragm pump ( a l l parts that contact the electrolyte are Teflon except the Hastelloy C balls i n the check valves) circulates the electrolyte to an Infracord through F E P Teflon or K e l - F tubing. The cells are made of F E P tubing compressed to a thickness of about 2 mm. between C a F plates. H o w ever, gradual fogging of the tubing and the C a F plates causes a slow shift i n base line so the usual absorbance vs. concentration calibration cannot be used. Instead, a "compensated" transmittance (T ) vs. con centration is calculated from the absorption at 1.12/4 where H 0 does not absorb: 2

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2

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c

2

T

e

=

%T 1.95/x of electrolyte vs. screen %T 1.12M of electrolyte vs. air

Recently, a K e l - F cell without C a F plates has given little if any fogging. T o increase the sensitivity, a metal screen is placed at the widest aperture of the reference beam to balance the instrument to near full-scale reading when the cell containing the dry electrolyte is i n the same beam. K F absorbs at 1.95/A but d i d not interfere because its concentration was held constant. W h i l e hydrogen peroxide is a possible electrolysis product, its concentration never exceeded 0.005 mole % . A d d e d concentrations of 0.05 mole % had a negligible effect on the 1.95/x absorption. 2

» RELOAD SUPPLY FOR SYRINGE

FOR

ELECTROLYSIS CELL

Figure 3.

Arrangement for continuous water control and analysis by infrared

Results The effect of water concentration on the product distribution is shown i n Figure 4. Electrolysis was continuous, and water was added i n crementally. As water increased beyond about 0.5%, the O F yield 2



1 96


Figure 4.

Yields at changing water concentration. KF» 1 mole %, volts = 7.0-7.3, temperature = 2°-3°C.

dropped very rapidly and then leveled out at 7-10% O F . Ozone i n creased as O F decreased and appeared to pass through a broad maxi mum. Oxygen was constant at 45-50%. Current efficiency for H and total anode gas decreased as water increased, possibly because the cathode was depolarized by dissolved anode products. Figure 5 shows three sets of data for O F yields. Curve A is the same run shown i n Figure 4. Curve Β is also a continuous r u n except that the water concentration was high initially and decreased as water was consumed. Curve C is a run i n w h i c h electrolysis was stopped after each sample had been taken for gas analysis, and water was added be fore electrolysis was resumed. The yield of O F was not the same at a given water concentration, but depended on the manner of operation. A l l three curves show a maximum i n O F yield at less than 1 mole % followed by a decrease as water increases. However, interrupting the 2

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DONOHUE ET AL.

197

Synthesis of OF

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current increased the O F yields, at least at intermediate water concen trations. Figure 6 (run 1) shows the effect of time on O F yield during con tinuous electrolysis at 0.56 mole % H 0 . A fairly constant ( 3 5 - 3 6 % ) yield was obtained for about 3 hours, and then a sharp decline occurred. There was no break i n the current density that might indicate an anode surface change. However the 3-hour plateau was not reproducible be cause i n the next run (run 2) the O F yield fell rapidly from the start. Nevertheless, interrupting current again gave higher O F yields. Thus, although run 1 was shut down at 2 0 % O F yield, run 2 started at about 3 5 % . Also, the system showed no permanent effect from a r u n that 2

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2

60 CURRENT

VOLTS

CONTINUOUS

7.0-7.3

— CONTINUOUS

5.8-6.2

l Λ I ! \ «/ \ ^ t'

0F

—

2

—

\.

0

INTERRUPTED

2.0

4.0

5.6-5.9

6.0

8.0

MOLE % H 0 2

Figure 5. OF* yields at changing water concentra tion. KF » I mole %, temperature = 0°-3°C.

CONDITIONS KF = 1 MOLE % VOLTS = 6.0-6.1 TEMP = 12.15*C HzO = 0.56 MOLE % RUN 1 • RUN 2 •

A N O D E TOTAL

Ο ο

ο

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320

TIME (MIN)

Figure 6.

Continuous electrolysis


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lasted many hours and ended with a low O F yield. The 0 and 0 showed slight increases w i t h time while H was reasonably constant at 85-90%. Since anode total is less than H , some unidentified anode prod ucts are possible. The consistent pattern i n which off-on operation gives higher O F yields suggested operating with programmed interruption. I n Figure 7 a continuous run is compared with two interrupted rims at water levels that bracket the continuous run. Both interrupted runs had higher and more constant O F yields. 2

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Figure 7.

MOLE % HtO

INTERRUPTION RATE, SEC/SEC

0.56 0.62 0.32

CONTINUOUS 1*0

160 TIME(MIN)

200

920

Interrupted electrolysis. KF = 1 mole %, volts = 6.0-6.1, temperature = 10°-14°C.

In addition to yields, current density and anode life are also i m portant i n evaluating an electrochemical synthesis. Although the current density should drop as water (a strong electrolyte i n H F ) is consumed, it does not always do so. Instead, for the first 15-30 minutes of electrol ysis it increases i n both continuous and interrupted electrolysis. This may be caused by a breakdown i n a resistive anode coating. Once a maximum current is reached, the current density remains constant; how ever, it drops as the last few tenths percent of water are consumed. Also, high water levels ( > 3 % ) cause low current densities. The current density maximum was at 0.5-1.0 mole % water. Nickel anodes lose weight during continuous electrolysis. The loss is large at low water concentration but drops to near zero at higher water (Table I ) . The nature of the anode surface depends on the water concentration. A t 0.2% H 0 , where little or no F is made, the deposit is soft, thin, and 2

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Synthesis of OF,

DONOHUE E T A L .

Table I

Volts

Faradays/ sq, cm*

5.0-8.2 7.6 7.6

0.0132 0.0324 0.0670

Mole % HiO

0.2 0.2-0.5 0.5-1.5

%

Current

Weight Loss % of Current

6.0 20-47 25-10

5.0 1.0 0.01

OF, Yield

adherent and contains only N i F . W i t h interrupted operation no weight was lost at comparable water levels and faradays. However, lower volt ages and improved H 0 control may have also contributed to anode stability. W h i l e the times here are short ( 10-30 hours ), the data indicate that anode life should be long. 2


2

Discussion The anode is a key component i n the electrolysis. T h e surface i n fluences product formation and at the same time is influenced b y elec trolysis reactions. T h e restoration of high O F yields as excess H 0 is electrolyzed away ( Figure 5, curve Β ) indicates that the surface is formed reversibly. T h e nickel-nickel fluoride anode is unique and essential to O F formation. Copper and aluminum passivate completely and require very high voltages; platinum disintegrates rapidly ( J ) . Only 0 , no 0 or O F , was found with these metals. T h e nature of changes i n the nickel-nickel fluoride anode surface, such as occur during start-up, is still uncertain. Several possibilities exist—i.e., mechanical break-up of the film, different forms of N i F (α, β, y) ( θ ) , or mixed oxide-fluoride films. Speculation (2) on electrochemical fluorination considers free F as an unlikely intermediate. O u r data also eliminate this route to O F . I n cases where F is found i n the products, adding H 0 does not increase O F yields until the current is interrupted. Another possible route to O F is fluorination of water b y K N i F or K s N i F i n the film. However, this path is unlikely because K N i F and K N i F react with water to give o n l y 0 (5). Formation of 0 suggests oxygen atoms as an intermediate. Further more, increase i n 0 yield when O F yield drops suggests that oxygen atoms are also i n intermediate i n O F formation and being diverted from O F to 0 formation. However, only nickel of several metals tested gives 0 and O F although 0 was present with the other metals. I f 0 also originates from oxygen atoms, then an added condition on the nickel/nickel fluoride anode surface, lacking on other metal surfaces, con tributes to 0 and O F formation. Further study may reveal the nature of intermediates and how the nickel/nickel fluoride anode contributes to 0 and O F formation. 2

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The overall current yield of O F and the conversion of H F to O F are higher with the one-step electrolysis than w i t h the two-step process i n which F reacts with base ( Table II ). 2

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Table II Current Conversion Efficiency of HF to for OF OF t

2

60% OT


tit

»% 8

„

2F, + 20H- —

» F ;

dect. HF + H 0

> OF, + 2F- + H 0

30%

2

30%

40%

2

2F + 40H> 0 + 4F- + 2H 0 > OF + O, + 0 + H 45% 2

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dect.

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100%

Lower cost O F should result from a development of this electrolysis. The constant O F yields obtained w i t h current interruptions show we are approaching the consistent operation necessary for pilot plant synthesis or detailed mechanism studies. O u r present study provides the neoessary analysis, control techniques, and yield data and indicates areas for further possible improvements. However, achieving maximum yields w i l l require study of more variables than we have surveyed here. The interruption technique may also have applications i n preparing other hypofluorite or fluorine compounds b y H F electrolysis. Indeed, any electrolysis i n which electrode surface changes are important may benefit from interrupted operation. 2

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Acknowledgment Research reported i n this publication was supported by the Advanced Research Project Agency through the U . S. A r m y Research Office, Durham, N . C . under contract DA-31-124-ARO ( D ) -78. This report also discloses proprietary information owned b y the American O i l Co., and its use by others is prohibited except as may be provided by the contract. W e thank W . A . W i l s o n for suggesting the study of wet H F elec trolysis and J . Markovich, F . S. Jones, and R. R. Hopkins for help i n various parts of the program. Literature Cited (1) American O i l Co., Quart. Repts., Contract No. DA-31-124-ARO(D)-78, A R P A Order 402 (1963). (2) Burdon, J., Tatlow, J. C., Advan. Fluorine Chem. 1, 160, (1960). (3) Clifford, A . F., Tulumello, A . C., J. Chem. Eng. Data 8, 425 (1963). (4) Cook, G. A . et al., ADVAN. CHEM. SER. 21, 44 (1959). (5) Cotton, F. Α., Prog. Inorg. Chem. 2, 200 (1960). (6) Donohue, J. Α., Wilson, W . Α., U.S. Patent 3,134,656 (May 26, 1964). (7) Downing, R. C. et al., Ind. Eng. Chem. 39, 259 (1947). (8) Emeleus, H., J. Chem. Soc. 1942, 441.


18.

DONOHUE ET AL.

Synthesis of OF

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(9) Engelbrecht, Α., Nachbaur, E . , Monatsh. Chem. 90, 367 (1959). (10) Grabb, W. T., U.S. Patent 2,716,632 (Aug. 30, 1955). (11) Haberman, E . G., Chem. Eng. Prog. 60, 72 (1964). (12) Hyman, H. H., Kilpatrick, M . , Katz, J . J., J. Am. Chem. Soc. 79, 3668 (13) L e Beau, P. and Damiens, Α., Compt. Rend. 185, 652 (1927). (14) Simons, J . H., et al., J. Electrochem. Soc. 95, 47 (1959). (15) Schmidt, H., Schmidt, H. D., Z. Anorg. Chem. 279, 289 (1955). (16) Streng, A . G., Chem. Rev. 63, 607 (1963). (17) Thorp, C . E . , "Bibliography of Ozone Technology," Vol. 1, Armour Re search Foundation, Chicago, 1954. (18) Yost, D . M., Cady, G. H., Inorg. Syn. 1, 109 (1939). Downloaded by CORNELL UNIV on September 28, 2016 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0054.ch018

RECEIVED April 19, 1965.


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