CATALYTIC AND THERMAL DECOMPOSITION OF ISOPROPYL METHYL FLUOROPHOS PH0 N A T E R . W. BAlER Aeronutronic Division, Philco-Ford Gorp., Newport Beach, Calif. S. W. WELLER
Department of Chemical Engineering, State University of Yew York at Buffalo, Buffalo, N.Y.
Decomposition products of isopropyl methyl fluorophosphonate (Sarin or GB) vapor in air at temperatures between 300" and 400" C. in the absence of catalyst are exclusively propylene and methyl phosphonofluoridic acid, presumably arising from an intramolecular dealkylation reaction. Catalytic decomposition of GB vapor over platinized alumina results initially in stoichiometric amounts of the oxidation products C02, HF, H20, and H3P04. As the activity for catalytic oxidation declines, the decomposition over the catalyst shifts to dealkylation, as in the absence of catalyst. The steady-state rate of dealkylation is very much greater in the presence of catalyst than either in its absence or in the presence of unplatinized alumina. Catalytic dealkylation has been followed for periods of operation corresponding to the complete conversion of an amount of GB equal to four times the weight of catalyst, with no evidence of deactivation.
measures against the possible use of toxic gases in warfare require filters which can remain in service for extended periods of time and protect personnel from continuous as well as intermittent attack. Catalytic converters are being investigated as a possible alternative to conventional charcoal filters for this purpose. This paper presents results from a study of one toxic organophosphonate, isopropyl methyl fluorophosphonate (Sarin or GB), and points out possible application of the decomposition process to other poisons in the same class. The studies were made in flow trains at atmospheric pressure and in the temperature range of 333' to 400' C. Inlet concentrations between 1000 and 3000 p.p.m. by weight were found to provide maximum analytical precision and reasonable times to bring the system to steady state.
t
EFEKSIVE
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Experimental
A flow system of conventional design was used to study the decomposition of GB vapor. Methods of experimentation were very similar to those previously described (Graven et al., 1966). Cylinder gases are passed through a purification train, and a small portion is passed through a GB saturator before rejoining the main stream entering the reactor. Gas flow rate is controlled by precision needle valves and measured by rotameters. Only Nz enters the saturator. I n either air or Nz runs, only the main gas stream passes through a preheater before entering the reactor. Effluent gas samples are analyzed by gas chromatography or infrared absorption spectroscopy. An all-Teflon sample valve was employed (Graven and Harmon, 1965), and Teflon lines were used to connect the valve to the reactor effluent line and to the chromatographic column. Figure 1 indicates construction details of the saturator. All of the phosphorus agents are subject to thermal decomposition if they are kept at elevated temperatures for extended periods. To minimize the extent of decomposition of agent in the saturator, a gas lift principle was incorporated in the saturator. A small Nz bleed stream, introduced near the base of the saturator, carries liquid slugs to the vaporization section, which is packed with cotton surgical gauze. Only this packed vaporization section is heated and thermostated. The main 380
l&EC PROCESS D E S I G N A N D DEVELOPMENT
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Figure 1 . mixtures
PROVISIONS FOR
ELECTRICAL HEATING
Saturator for generating inlet agent-gas
Nz stream passes through a helical preheater and enters the saturator at the base of the packed section. The GB used in this work was obtained from Edgewood Arsenal; it typically contained 97% GB, 0.6y0 n-tributylamine, and 0.7y0difluoromethyl phosphonate. Some of the catalytic runs were made using 927, GB which was later distilled; the purity was increased to 98%. Infrared spectra and
N a O H titrations were essentially identical for the distilled product and Edgewood's 97% material. Catalysts were prepared by impregnating 35/65-mesh (0.31-mm. average particle diameter) A1203 with chloroplatinic acid and drying to remove excess water and hydrochloric acid. Adsorption measurements (BET) showed the resulting catalyst to have a specific area of 64 sq. meters per gram. The development of this catalyst has been described (Graven et a/., 1966). Gas samples and liquid condensates were analyzed with an infrared spectrophotometer (Perkin-Elmer Model 221 for quantitative work > 30 p.p,m,; Infracord Model 137B for qualitative analyses of toxic liquids). Quantitative analyses below 30 p.p.m. were performed with a gas chromatograph using a 20-foot, 0.019-inch i.d. capillary column coated with Carbowax 4000 or a silica gel-packed column. Hydrogen flame or thermal conductivity detectors are used with either column to achieve versatility (CO, COz, hydrocarbons, GB, etc.) and sensitivity (-1 p.p.m.). Among the various wet chemical analyses used, p H titration of the completely hydrolyzed liquids has been the most informative. Sampling is by far the most important factor affecting sensitivity and reproducibility. Reaction products from the decomposition of phosphonates are condensed in all parts of the analytical train downstream of the reactor and adsorb significant amounts of volatile components. I n addition, precise temperature control of all parts of the system and assurance of steady-state conditions are essential for meaningful measurements.
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RESIDENCE TIME, SECONDS
Figure 2. Decomposition of GB in borosilicate glass tube at 325" C. in NZat 1 atm.
Results and Discussion
"Uncatalyzed" Conversion. A control run with glass beads showed that surface reactions accounted for about 3oy0 of the total decomposition of GB vapor in an empty reactor at 400" C. Although this wall effect is insignificant in comparison to the activity of the catalyst beds used, its consideration is essential for proper interpretation of results obtained without added catalyst. Further characterization of the activation energy of glass catalysis is needed to assess the wall effect at other temperatures. T h e over-all conversion of GB at atmospheric pressure appears to obey first-order kinetics. This is illustrated by Figure 2, which is a plot of log C,/C us. residence time for a number of runs made at 325" C . The effect of 2.6 volume yo of water vapor in either air or nitrogen (not shown) was found to increase the first-order specific rate constant by 27% under these conditions. A similar semilog plot for runs made in several gas atmospheres at 395" C. is shown in Figure 3. Of interest here are the facts that the over-all rate of GB conversion is essentially the same in N Pas in air, and the rate is increased 13% by the addition of 2.6 volume % of H 2 0vapor to the feed. I t is possible that the apparent increase in rate when water vapor is present follows from an increase in residence time for that portion of the stream which comes in contact with the walls of the reactor. As the water content of the influent is increased, a larger mass of adsorbed water would be present and provide a medium in which GB can equilibrate with the gas phase. This partitioning would effectively retard a portion of the GB and result in higher over-all conversions. An Arrhenius plot for runs made in dry air at a number of temperatures is shown in Figure 4. The least squares line corresponds to an activation energy of 23 kcal. per mole. This value cannot be accepted as well established, however, in view of the scatter of the data points. Although measurements of the rate constants during the steady-state portion of any run were reproduced within +27& determinations made from
0 %
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z
0 !3 z
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014 0!6 0!8 RESIDENCE TIME, SECONDS
012
l!O
Figure 3. Decomposition of GB in borosilicate glass tube at 395" C. and 1 atm.
different runs conducted under the same conditions showed typical variations of f10%. Gas chromatographic analysis of the effluent gas from a n "uncatalyzed" conversion run has indicated the presence of only propylene and unconverted GB. No CO2 has been found in runs made in air. To obtain further information on condensable products, several runs were made in which the reactor effluent was first passed through a liquid N Z trap. The condensate, after subsequent addition of solvent (CCld or acetone), was examined by infrared. Figure 5 shows a typical infrared spectrum from one of these experiments, in which CC14 was used as the solvent. The principal absorption peaks observed in such a condensate spectra are compared with those given by VOL. 6
NO. 3
JULY 1967
381
c lo 1 .Y
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1.65 x
(333%)
RECIPROCAL
1;55 x
TEMPERATURE,
Figure 4. Arrhenius plot of air at 1 atm.
GB
1.50 x (393OC) OK-’
decomposition in
FREQUENCY, CM-’ 1400
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k
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10 11 WAVELENGTH, MICRONS
9
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Figure 5. Infrared spectrum o f condensate from decomposition run
13
GB
Bouck and Goldenson (1965) for a number of possible reaction products in Table I. Only unreacted GB and “fluor” acid (methyl phosphonofluoridic acid) are shown to be present. There is no evidence of the disproportionation products methylphosphonic difluoride (“difluoride”) or diisopropyl methyl phosphonate (DIMP), or for the hydrolysis product isopropyl methyl phosphonate (“isopropoxy” acid). Another sample of condensate frozen at -195’ C. was progressively vaporized, and the volatile fractions were found to be propylene with a total of less than 100 p.p.m. by volume of methane, ethylene, ethane, and propane. This work has established, therefore, that the homogeneous vapor phase decomposition goes cleanly to propylene and fluor acid. Catalytic Conversion. The catalytic decomposition was studied during a 113-hour run using Pt-Al203 catalyst. The inlet GB-air stream was maintained a t a fixed composition of 2550 mg. of GB per cubic meter (25’ C., 1 atm.) and passed through a 2-inch (50-cc., 39.5 grams) bed at a flow rate of 10 382
I & E C P R O C E S S DESIGN A N D DEVELOPMENT
Table 1.
Principal Infrared Peaks of GB and Possible Reaction Products Principal Peaks Compound Wavelengths Intensity Observed S GB 7.60 Yes S Yes 7.83 Yes vs 9.91 S Yes 10.85 Yes S 11.93 MS Yes Fluor acid 7.59 S 8.00 Yes Yes 9.73 Yes 10.88 Missing Difluoride 7.48 Yes 7.62 Missing 10,60 Yes 10.86 8.05 S Yes DIMP 9.90 S ? 10.17 vs Missing 7.61 MS Yes Isopropoxy acid 8.33 S Missing 8.75 MS ? 9.02 MS Missing NlO.00 vs Missing 11.01 MS Yes
liters per minute (25’ C., 1 atm.). This resulted in a residence time at 400’ C. (based on bulk bed volume) of 0.13 second. Results of effluent analyses are shown in Figure 6. Because of a shortage of high purity agent (99f % GB), commercial material (97.3% GB and 0,6y0 tributylamine) was used after 67 hours of run time. No change in the infrared or gas chromatographic analyses of effluent from the commercial agent could be detected after the pure agent was depleted. At the beginning of the run, complete oxidation of the agent is shown (Figure 6) by the near-stoichiometric values of COn and the high initial temperature rise. Although COS measurements were not made before 15 hours, the extent of oxidation can be inferred from the lack of propylene during this period and the fact that the methyl group is held by phosphorus under conditions when propylene is removed. Similarly, HF (measured as SiF4from reaction with glass) is at its maximum value during the first 15 hours of the run. After about 12 hours propylene appears and reaches the stoichiometric level after 28 hours. This nearly coincides with a decrease in COZ and the establishment of a minimum bed temperature rise. The absence of methane, propane, methanol, 2-propanol, and heptane was established by gas chromatography using a silica gel column and a hydrogen flame detector. Temperature rise near the top of the bed was followed with a thermocouple positioned in a thermal well along the axis of the bed. Several times during the run a complete axial temperature profile was measured. These profiles, which were measured by sliding the thermocouple along the axial thermowell, are shown in Figure 7. Since the total travel through the 2inch bed required several hours, errors in locating the peak are probably very small. The temperature profile could be reproduced within 1 2 ’ C. when traversing the length of the bed from either direction. This indicates that convection was not displacing the peak significantly. The passage of the temperature peak through the bed with time correlates well with the decrease in the extent of oxidation. The peak axial temperature is initially 41’ C. higher than the inlet gas temperature but then decreases with time (and position) as it moves through the bed. As expected, when the endothermic 0 the temdealkylation mechanism predominates ( ~ 2 hours), C.). If GB at a concentration of perature rise is small (-5’
2550 mg. per cu. meter in air were oxidized adiabatically, the temperature rise would be 84’ C., based on a heat of combustion of 700 kcal. per mole a t 400’ C. The heat of combustion for GB was calculated by summing the heats of combustion for propylene and fluor acid and the endothermic heat of dealkylation of GB as estimated from bond energies. Values used for these quantities are shown in the following tabulation:
60 50
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N
It em AHoomb. propylene gas AHeomb. fluor acid AHddk.
GB
Value, Kcal./Mole - 492 -219
+IO
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Hougen et al., 1943 Masi, 1960; NBS, 1955 Cottrell, 1958
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Figure
The measured temperature rise is half of the adiabatic change and appears reasonable when account is made for heat losses. If no oxidation were taking place, the endothermic dealkylation of 2550 mg. of GB per cu. meter would cause a temperature drop of only 1’ C., which is again consistent with the small measured temperature rise, during the latter part of the run, probably due to temperature gradients along the axis of the furnace. The rate at which the peak temperature rise moves down the bed can be used to estimate a maximum value for the initial number of active sites available for oxidation. The rate of change in peak location, L, (in cubic centimeters of bed traversed per hour) is plotted in Figure 8 against time and extrapolated to the start of the run. This gives the velocity expected in virgin catalyst when sites downstream from the wave have not been in contact with GB or its products of decomposition. T h e number of initial sites, So (sites per cubic centimeter of bed), multiplied by the rate at which the bed is being “used,” @J0, must be equal to or less (for the case of multiple uses of each site) than the rate at which GB molecules are entering the bed, C,F (molecules per hour). Expressed mathematically the equivalent statement becomes: where
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Catalytic conversion of
I 410 370
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So 2 C0F/(LP], D I S T A N C E F R O M TOP OF B E D , INCHES
C, = GB concentration, molecules per cu. meter F = influent rate, cu. meters per hour Using the extrapolated value of 14 for Jo!,( from Figure 8, So 2 6.6 X loz1sites per cc. For 1 gram of A1203 containing 1 weight yo Pt, the number of GB molecules which are converted by each atom of platinum was calculated to be 300. A similar calculation was made in which the number of GB molecules converted by each [OP2]ion on the surface of the A1203 particles (using a surface area of 64 sq. meters for 1 gram of A1203 having a hexagonal close-packed structure and an interatomic spacing for [ O + ] of 2.8 A.) was found to be 9.6. Although these types of “turnover” numbers are not definitive in themselves, they are instructive in comparing catalyst effectiveness during the oxidation portion of various runs. A viscous condensate near the sampling valve was removed and its spectrum (top) was compared to that for reagent 85% H3P04 (bottom) in Figure 9. In the top spectrum, the peak at 7.58 microns is characteristic of the C-H distortion of P-CHI groups. This peak along with the absorption near 10 and 13 microns and the absence of a peak at 11.7 microns (P-F) suggest the presence of methylphosphonic acid or condensed derivatives. The spectrum of 85% phosphoric acid (bottom) was prepared by the same technique; it is very similar except for the absence of peaks a t 7.58 (P-CH3) and 13.15 microns, and slight changes in slopes near 3 and 10.5. After 105 hours of continuous run time, 160 grams of GB had been converted by 39.5 grams of catalyst-initially by
D I S T A N C E F R O M TOP OF BED, INCHES
Figure 7. Temperature profiles along the axis of the catalyst bed
E W
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RUN TIME, HOURS
Figure 8. Movement of temperature peak through bed during oxidation of GB VOL 6
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383
oxidation and subsequently by dealkylation. The apparent discrepancy between propylene measured by G C a t 28 hours (Figure 6) and that which would be obtained by dealkylating all of the GB (1 18% of theory) is primarily due to SiF4 which elutes along with propylene. Later in the run (70 hours) when the SiF4 has dropped significantly, the product peak is 829 mg. of propylene per cu. meter of 108% or theory. An additional contribution to this peak may be from difluor. T o confirm the fact that oxygen does not take part in this stage of agent decomposition, the oxygen flow entering the catalyst bed was replaced with nitrogen. For 8 hours under these conditions no change in effluent composition could be discerned. After this period of time (113 hours of run time) a total of 175 grams of GB had been converted. The catalyst bed was examined after the run and found to have increased in weight by 17.4 grams (44%) and to be partially agglomerated near the bottom. The particles were similar in color to that of the starting material. No significant increase in the 130-mm. Hg pressure drop through the bed was encountered during the run. Analysis of the used catalyst by total phosphorus and acid titration before and after water leaching showed that only about half of the material attributed to the weight increase could be removed by Soxhlet extraction for a week. X-ray diffraction patterns (Figure 10) were more diffuse after the catalyst was used for GB conversion and subsequently leached. Although this implies reduction in crystallite size, surface area measurements by NZ adsorption h
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63.4 HOURS ON STREAM. IXATTENUATION.
WAVELENGTH. MICRONS 85 PERCENT PHOSPHORIC ACID. IX ATTENUATION.
Figure 9.
Infrared spectra of viscous condensate lop.
Bottom.
From decomposed GB From phosphoric acid
T W I C E GLANCING A N G L E
showed a significant decrease from 64 to 45 sq. meters per gram. The used catalyst thus has the following characteristics: Leaching with water is incomplete even after a week. 41y0 of the weight increase of the bed was accounted for as HsP04 by titration. 66% of the weight increase of the bed was accounted for as Hap04 by phosphorus analysis. Crystallinity is decreased (x-ray). Surface area is decreased (BET). A possible explanation for these observations is that reaction of product Hap04 with A1203 gives relatively noncrystalline Alp04 which fills some of the pores.
Decomposition of GB over Al2O3. During the last 80 hours of the catalytic run described above, platinum appeared to have lost its activity with respect to carrying out oxidation. Decomposition of GB was complete, however, during this period and apparently a steady state had been reached during which dealkylation of GB to fluor acid and propylene was the sole conversion process. T o determine if either platinum or hydrochloric acid, which is retained by the alumina during thermal decomposition of HzPtC16, played an active role in the dealkylation phase, a run was made without passing the alumina through the platinum impregnation step. A catalyst bed made up of 35/65-mesh A1203, but without platinum, was used to decompose GB vapor under the same conditions of temperature and residence time used in the catalyst run described previously. The average agent concentration was 13% higher (2880 instead of 2550 mg. per cu. meter). The effluent contained 202 mg. per cu. meter of C 0 2 (theory = 3620 for complete conversion of carbon entering the reactor as GB) after the first hour and was almost free of COZ (
