double-bond shift isomerization predominates at low temperatures (150°C) on untreated alumina, but some skeletal isomerization does occur a t higher temperatures (above 316°C). When the' alumina is treated with HC1, skeletal isomerization was found to occur a t lower temperatures and to a greater degree. It has been suggested by both Peri (1966) and Myers (1971) that the HC1 reacts with both hydroxyl groups and oxygen in the alumina. These very interesting studies are not in a form or at conditions that are directly comparable to the neodymium oxide tests. Accordingly, a low-soda y-alumina was tested at conditions directly comparable with the neodymium oxide data in both untreated and HCl treated states. The HC1 treatment was conducted in the same manner as for neodymium oxide. Results are summarized in Table III. As expected, skeletal isomerization is greatly enhanced by HC1 treatment. A comparison of the results for both oxides as reported in Tables 1-111 is instructive. As approximated by total conversion a t 400-405"C, the activities of both untreated alumina and neodymium oxide are about the same. Both decline with time, but the decline for neodymium oxide is more rapid. cis- and trans-2-pentene are the primary products, but trans-2-pentene is less strongly favored on neodymium oxide. Treatment with HC1, as expected, also causes skeletal isomerization activity on alumina. An increase in overall isomerization activity occurred in the HC1-treated alumina just as in the HC1-treated neodymium oxide, but the increase was greater. Comparison of the two acid-treated catalysts for the most nearly equal conversions a t 400405°C (64% for Ndz03 and 79% for A1203) indicates similar cis and trans selectivities.
Conclusions This preliminary study adequately demonstrates a further important parallel between alumina and rare earth oxides. Skeletal isomerization activity and higher intrinsic activity are induced in both by HC1 treatment. Thus, the acid-catalyst functions of alumina, so important to processes such as reforming and hydrocracking, can also be created on neodymium oxide. In both cases operating a t higher severities will increase the production of all the isomers, and the observed differences in selectivities should be more pronounced. Literature Cited API Research Project 44, "Data on Hydrocarbons and Related Compounds, Tables of Physical and Thermodynamic Properties." Texas A&M Press, College Station, Texas, 1952. Berg. L., Summer, G. L.. Montgomery, C. W., Ind. Eng. Chem., 38, 734 (1946), Ciapetta, F. G., Wallace, D. N., Catal. Rev., 5, 67 (1971). Gianetti, J. P., Mcllvried, H. G., Sebulsky, R . T., Ind. Eng. Chem.. Process Des. Develop., 9, 473 (1970). Harrison. D. P., Hall, J. W., Rase, H. F., Ind. Eng. Chem., 57, 18 (1965). Hay, R . G., Montgomery, C. W., Coull, J.. Ind. Eng. Chem.. 37, 355 (1945) Maclver, D. S., Tobin, H. H., Barth, R . T.. J. Catal., 3, 502 (1964). Myers, J. W., Ind. €ng. Chem., Prod. Res. Develop., 10, 200 (1971). Oblad, A. G., Messenger, J. V., Brown, H. J . . Ind. Eng. Chem., 39, 1462 (1947). Peri, J . B., J . Phys. Chern., 70, 1482 (1966). Tanaka. N., Ogasawara, S.. J . Catal., 16, 164 (1970). Tosun, G..Ph.D. Dissertation, The University of Texas at Austin, Austin. Texas, 1971 Tosun, G., Rase, H. F., Ind. Eng. Chem., Prod. Res. Develop., 11, 249 (1972). Welper. A. T., Master's Thesis, The University of Texas at Austin, Austin, Texas, 1972. Received for review December 26,1973 Accepted May 8, 1974
Catalytic Decomposition of Halogenated Hydrocarbons over Hopcalite Catalyst James K. Musick* and Frederick W. Williams Chemistry Division, Naval Research Laboratory, Washington, D. C. 20375
Nineteen halogenated hydrocarbons mixed with air were exposed to hopcalite catalyst in a laboratoryscale catalytic burner under operating conditions used in submarine burners. In addition to testing at the typical submarine burner temperatures of 305 and 315"C, most of the 19 compounds were also tested at 360 and 415°C. Degree of decomposition was measured by analyzing the burner influent and effluent for the halogenated hydrocarbons by gas chromatography. To estimate the amount of decomposition products (free halogens, halogen acids, and phosgene) of the hydrocarbons, detector tubes and detector paper were used. These detector devices revealed slight degrees of decomposition not detectable by the gas chromatograph. The 19 compounds were arbitrarily classified into two groups: a group of seven in which no decomposition at 305 or 315°C was detectable by gas chromatography and a group of 12 in which the degree of decomposition at 305 or 315°C (1.5 to 100%) was measurable by gas chromatography.
Purification of air containing a wide variety of low-concentration contaminants has assumed increasing importance in recent years largely because of the need for men to spend relatively long periods of time in closed habitats. Different types of systems (Carhart, 1965; Ramskill, 1962) have been devised to remove different classes of contaminants from the air: electrostatic precipitators remove
aerosols, various types of scrubbers using either liquid or solid absorbents remove COZ, and activated carbon beds absorb many gaseous contaminants. For low-molecularweight compounds such as Hz, CO, and CH4, which are not absorbed by activated carbon, catalytic oxidation is the most useful removal method. Catalytic oxidation converts these compounds to their oxidation products, COz Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 3, 1974
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and HzO which can then be readily removed by other processes, whereas there is no suitable process for removing the original contaminants. Catalysts used in the oxidation process, in general, consist of either precious metals or oxides of heavy metals. Hopcalite is a widely used catalyst for this purpose and has been studied by other investigators (Christian and Johnson, 1965). It is a coprecipitate of the oxides of copper and manganese. Although hopcalite has been used successfully for many years to oxidize CO and HZ in nuclear submarines, it does have some undesirable characteristics. One problem associated with the use of hopcalite catalyst for the removal of certain contaminants from air is due to the usual presence in the gas stream of other contaminants which are oxidized to compounds not so readily handled as COz and HzO. Relatively innocuous compounds can be completely or partially oxidized to products which are more toxic and/or corrosive than are the original materials (Saunders, 1967). An example of this type of compound is the class of halogenated hydrocarbons which generally produce highly toxic halogen acids, such as HCl, when they are decomposed by hopcalite. Halogenated hydrocarbons, which have many useful properties (e.g., excellent solvent power, high volatility, and low flammability) are widely used as solvents for adhesives, paint removers, cleaning fluids, degreasing solutions, pressure-can propellants, refrigerants, fire extinguishants, and for other purposes. Consequently, these compounds commonly contaminate closed and semi-closed atmospheres as a result of leaking equipment, outgassing materials, and escaping of volatiles during the use of supply materials and performance of maintenance operations. Even in cases where catalytic combustion has been used to purify air containing low concentrations of halogenated hydrocarbons there has been severe corrosion of metal parts of combustion equipment by the acid products (Hama and Curley, 1965). This has generated a need for knowledge of the catalytic combustion of these compounds. With proper knowledge, halogenated hydrocarbon compounds could be selected to satisfy specific needs and a t the same time to minimize the production of toxic and corrosive products in closed atmospheres. As a step toward gaining this knowledge, a continuing series of studies of the catalytic combustion of halogenated hydrocarbons has been carried out over the past few years. During this time 19 different compounds of this type have been investigated.
Experimental Section Catalyst. The hopcalite catalyst, obtained from Mine Safety Appliances Corp., is a coprecipitate of copper and manganese oxides which has been analyzed as 78.3 wt % MnOz, 13.1 w t % CuO, and 7.9 wt % ignition loss. The ignition weight loss is probably water or chemisorbed gases. The catalyst is about 6 mesh size and has an apparent density of 1g/cm3 and a surface area of 150 m2/g. Apparatus. Two catalytic burners were used in this study. The first burner was a 28-in. length of ll/s-in. i.d. stainless steel tubing mounted vertically and containing a preheater in the lower section and a reactor (catalyst bed) in the upper section. Except for size, this burner duplicated the operating conditions of a 500-cfm burner used in nuclear submarines. Experience with this first burner led to the design of an improved burner which is similar to the first burner in size and operating conditions. The improved burner has been adequately described elsewhere (Musick, et al., 1972). 176
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Halogenated Hydrocarbons. The various halogenated hydrocarbons investigated were generally 99 mol % pure or better, according to manufacturers' statements, and were used as received. Exceptions were trichloroethylene which was distilled to remove traces of two lower-boiling contaminants before it was used and vinylidene chloride which was used a t a purity of about 97%. Procedure. The gas for the burner was prepared from compressed air taken from the laboratory supply line. This air was purified (Williams and Eaton, 1974), humidified to about 50% r.h. a t room temperature and pressure, and mixed with the halogenated hydrocarbons (Musick, et al., 1972) before being admitted to the burner. All the bubblers, tubes, valves, and other components to which the contaminated burner air was exposed prior to analysis were constructed of either glass or stainless steel. In general, the burner was brought to temperature while operating on humid air. The flow of contaminant was then admitted to the inlet air and time was allowed for concentrations, flow rates, and temperature to approach equilibrium before taking samples for analysis. The rate of sample withdrawal was held to less than 1% of the main stream flow to prevent significant disturbance of the air flow in the reactor. The length of a run varied somewhat depending on the length of time required to reach equilibrium and to make reproducible measurements but was generally 4-6 hr. Operating Conditions. According to the literature (Anderson, 1968), catalyst bed diameter in an integral fixedbed flow-type catalytic reactor should be a t least six times and bed depth a t least 30 times the effective catalyst particle diameter to keep the bed edge and end effects suitably small. Since the hopcalite catalyst had an effective diameter of Ys in. and the catalyst bed was 1%in. in diameter and 5 in. deep, the bed was in the suitable size range. The operating conditions for the experimental work were selected first to duplicate those which extensive work had shown to be optimum for hopcalite as it was used in a submarine burner (Ramskill, 1962). The purpose here was to determine the performance the catalyst would provide in its intended use. Conditions of higher catalyst temperature and higher contaminant concentrations than normally found in submarine atmospheres were used in additional work. This was done to provide a more severe test for the catalyst bed, to get a better measure of the resistance of the contaminants to catalytic decomposition, and in some cases to ease analytical requirements. The submarine burner operates a t a catalyst temperature of 600 f 25°F (316 f 14"C), an inlet air humidity of about 50% r.h., and a space velocity of 21,000 hr-l. This space velocity is provided by an air flow rate of 1 cfm/in.2 of cross-sectional area of the 5 in. deep catalyst bed. The space velocity and flow rate specified are for the air a t room temperature and have higher values in the catalyst bed a t 316°C. The laboratory burner has the same catalyst bed depth and used the same space velocity and the same inlet air humidity as the submarine burner. However, four different temperatures (305, 315, 360, and 415°C) were used in the study of the 19 hydrocarbons in the laboratory burner. The lower two of these temperatures are in the 316 f 14°C range used in the submarine burner. Most concentrations of the halogenated hydrocarbons were in the 50150 ppm range but concentrations as low as 10 ppm and as high as 600 ppm were used. Analyses. The stabilities of the different halogenated hydrocarbons studied were determined by analyzing the
burner inlet and exhaust air to determine both the loss of compound in the burner and the appearance of decomposition products in the burner exhaust. The analyses to determine loss of compound in the burner were made by a Beckman Model GC-2A gas chromatograph. To detect the appearance of decomposition products in the burner exhaust, detector paper and detector tubes were used. These are more sensitive detectors of incipient decomposition than is the gas chromatograph. One advantage of using these devices is due to the greater ease of detecting a small amount of a new substance (the decomposition product) than in detecting a small loss from a relatively large amount of substance (the halogenated hydrocarbon). Thus, the detector devices were particularly useful in proving either the absence of or the occurrence of a slight amount of decomposition which the gas chromatograph could not reveal. However, the detection of decomposition products by detector tubes and paper was considered to be a qualitative determination only. One reason for this is that some of the acid formed in the burner is known to be retained by the catalyst and cannot be measured quantitatively by analyses of the burner exhaust. Another reason is that the detector devices, though very sensitive, do not have a high degree of precision. In the presence of available hydrogen (from water vapor in the burner influent or formed in the combustion process) possible decomposition products of the halogenated hydrocarbons include the halogen acids: hydrogen fluoride (HF), hydrogen chloride (HCl), and hydrogen bromide (HBr); the free halogens: fluorine (Fz), chlorine (Clz), and bromine (Brz); and the carbonyl halides: carbonyl fluoride (COFz), carbonyl chloride or phosgene (COClZ), and carbonyl bromide (COBrZ). The burner exhaust was analyzed for all these potential breakdown products except COBr2, COF2, and Fz, which were excluded because of analytical difficulties. The acids were analyzed by use of the NRL Strong-Acid Vapor Detector (Williams, et al., 1965). This is a device for exposing detector paper which is sensitive to the vapors of strong acids but unresponsive to weak acids. This detector paper was calibrated over the range of concentration of 0.2-40 ppm. In addition, a detector tube (Drager CH 303) specific for H F and calibrated over the range of concentration of 0.5-7.5 ppm was available and was used for that acid. Total Cl2 and Brz were indicated by a detector tube, Drager CH 243, which was calibrated for use with concentrations of 0.2 to 3 ppm. A detector tube (Drager CH 283) specific for COClz in the concentration range of 0.25 to 15 ppm was used to detect phosgene. Results Stability of Halogenated Hydrocarbons. The 19 halogenated hydrocarbons exposed to hopcalite in the benchscale burner have been arbitrarily classified into two groups according to their stabilities: those resistant to hopcalite-catalyzed decomposition and those susceptible to hopcalite-catalyzed decomposition. The seven compounds classed as resistant to the action of hopcalite are listed in Table I with the results of analyses made to determine the extent of their decomposition by exposure to the catalyst. It can be seen that no detectable loss, by gas chromatographic measurements, of any of the compounds listed was observed upon exposure to the hopcalite catalyst at a temperature of 305 or 315°C. In two cases, measurable loss of halogenated hydrocarbon was evident at 360°C. Small amounts of acid resulting from slight decomposition of certain of the compounds were detected a t 305 or 315°C and all compounds formed some acid a t 360
and/or 415°C. The more stable compounds yielded only a trace of acid. In the case of CBrF3, slight decomposition a t 360°C was confirmed by the appearance of 2-3 ppm of Brz in the burner exhaust when the concentration entering the burner was increased to 600 ppm. The 12 halocarbons classified a s susceptible to hopcalitecatalyzed decomposition are listed in Table I1 with the results of their analyses. A substantial percentage (1.5-100%) of all of these compounds were decomposed by exposure to hopcalite a t 315°C or lower temperatures. The most stable (R-113) of the compounds in Table II, which decomposed to the extent of 1.5% in one pass over the hopcalite a t 305"C, was 26% decomposed by exposure to the catalyst a t 415°C. As expected, significant amounts of acid were detected in the burner effluent when these compounds were passing through the burner. Free Clz and/or Brz, however, were detected as decomposition products of only three (R-133a, R-12B1, and chloroform) of the eight compounds for which the burner exhaust was analyzed. As mentioned above, the amounts of breakdown products detected in the burner exhaust are not considered quantitative measurements of the extent of decomposition. Phosgene. The burner effluent from all 19 halocarbons was tested for phosgene. Only R-12B1 gave any evidence of the production of phosgene and even in this case the evidence was minimal. With 100 ppm of R-12B1 in the burner influent, one measurement produced a color change in the detector tube which was barely discernable. Several other similar measurements were made and all gave negative results. The conclusion is that if phosgene was produced from any of the compounds studied it was in concentrations no higher than the threshold value of the detector tube (about 0.1 ppm). The absence of phosgene in the decomposition products from the burner is of obvious importance in inhabited spaces. Discussion Correlation of Data. An attempt was made to correlate the measured catalytic stabilities of the 19 halogenated hydrocarbons with the values of other properties of these compounds found in the literature. For this purpose, our values for their stabilities over hopcalite were compared to previously reported values for their thermal stabilities (Williams, 1972), heats of formation, and bond-dissociation energies. This comparison for the eight methane derivatives is summarized in Table 111. It can be seen that the catalytic stabilities did not vary with any of the three properties in any orderly or consistent way. Thus it is not possible to predict the approximate catalytic stability of a new halogenated hydrocarbon by comparing either its bond energy, thermal stability, or heat of formation with those of the 19 halogenated hydrocarbons for which the catalvtic stabilities have now been measured. Effect of Molecular Structure. I t can be seen in Tables I and I1 that the five compounds having the highest rate of decomposition were the only ones studied which contained no fluorine in their molecules, and in addition, all contained either a hydrogen atom or a double bond. Further, none of the more stable molecules listed in Table I contained either a hydrogen atom or a double bond and all contained fluorine. This suggests that the stability of the molecules was increased by the presence of fluorine, the absence of hydrogen, and the absence of a double bond. It was expected that the presence of a double bond would decrease the stability of a molecule. However, the first three compounds listed in Table I1 did not follow the same pattern. Here it is seen that the measured stability of CHClzF is greater than that of either CC13F or CHClFZ. Catalyst Deactivation. It has been shown that the efInd. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 3, 1974
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Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 3, 1974
Table 111. Comparison of the Catalytic Stabilities of Methane Derivatives with Their Thermal Stabilities, Bond Dissociation Energies, and Heats of Formation" Thermal stability
Catalytic stability
Bond dissociation energy
Heat of formation
5Note: The compounds listed in each column are arranged in the order of the value of their properties. The compound having the highest value is at the top of the column.
fectiveness of a n oxidation catalyst can be adversely affected by exposure to certain compounds. Compounds of the halogens and of sulfur are particularly effective as deactivators of hopcalite (Musick, et al., 1972). For example, when exposed to thiophene (C4H4S), hopcalite gradually loses its activity and converts less of the compound as the time of exposure increases. Simultaneously, the hopcalite suffers a drastic loss of activity toward CO. The loss of activity can be partially restored by treating the catalyst with cool humid contaminant-free air but some of the loss is permanent. Further, i t has been shown that the activity of hopcalite toward' some compounds is less when two are exposed simultaneously than it is when they are exposed separately (Musick, et al., 1972). This effect varies for different compounds and in some cases is nonexistent or immeasurably low. The mechanism of reactions of CO and halocarbons on hopcalite is not well understood. Our interest has been focused on the macro-effects of the catalyst on various compounds rather than the micro-events happening on the surface of the catalyst.
Summary and Conclusions Of 19 halogenated hydrocarbons studied, seven were not decomposed by hopcalite catalyst a t 305 or 315°C to an extent detectable by a gas chromatograph and were classified as resistant to hopcalite-catalyzed decomposition. Twelve of the compounds studied were decomposed by hopcalite a t 305 or 315°C to the extent of 1.5 to 100% and were classified as susceptible to hopcalite-catalyzed decomposition. Exposure of the 19 halogenated hydrocarbons to hopcalite a t temperatures of 305 to 415°C produced no phosgene in concentrations higher than the limit of detection (about 0.1 ppm) of the detector tubes used for the analyses. A trace of phosgene was detected in only one measurement (as 100 ppm of R-12B1 was passing through the burner). The stabilities of the 19 halogenated hydrocarbons over hopcalite do not correlate with the thermal stabilities, bond-dissociation energies, or heats of formation in a way which would permit the data to be used to predict the catalytic stabilities of other compounds of this class. The presence of fluorine and the absence of both hydrogen and a double bond in the molecule of a halogenated hydrocarbon increases its stability in the presence of hopcalite a t elevated temperatures.
Acknowledgment The authors are indebted to R. H. Gammon for doing the experimental work on four of the compounds reported. The work was funded by the Naval Ship Systems Command, United States Navy.
Literature Cited Anderson, R. 8.."Experimental Methods in Catalytic Research," p 8, Academic Press, New York, N. Y., 1968. Carhart, H. W., Underwater Technology Conference, The American Society of Mechanical Engineers, New London, Conn., May 5-7. 1965. Christian, J. G., Johnson, J. E., lnt. J. Air Water Pollut., 9 , 1 (1965). Hama. G. M.. Curley, L. C.. AirEng., 7 (4), 38 (1965). Musick, J. K.. Thomas, F. S., Johnson, J. E., lnd. Eng. Chern., Process Des. Develop., 11, 350 (1972). Ramskill, E. A., SOC.Aut. Eng. Trans., 70, 350 (1962). Saunders, R. A.. Arch. Environ. Health, 14, 380 (1967). Williams, D. D., Johnson, E. T., Miller, R. R., NRL Report 6332, Naval Research Laboratory, Washington, D. C. 20375 (Oct 14, 1965). Williams, F. W., Anal. Chern., 44, 1317 (1972). Williams. F. W., Eaton, H. G., Anal. Chern., 46, 179 (1974).
Received for review January 10,1974 Accepted June 10,1974
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