Catalysts for the Vapor-Phase Oxidation of Acetaldehyde
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An apparatus and method for the quantitative comparison of catalysts for the vapor-phase oxidation of acetaldehyde by air to acetic acid are described. The activities of twenty-three catalysts are compared. Attention is directed particularly towards silica catalysts of the xerogel and aerogel types. Pure silica gels are superior to all catalysts studied, with the exception of a silica gel containing 0.19 per cent platinum oxide with which 90 per cent yields of acetic acid were obtained. The effects of temperature on conversions and yields, of space velocity, of the ratio of air to acetaldehyde, and of heat treatment on the activity of the catalyst were investigated.
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H.D.FOSTER AND D.B. KEY^ University of Illinois, Urbana, Ill.
With this previous work in mind, attention has been directed almost entirely towards xerogel and aerogel catalysts, and to silica catalysts in particular. A comparison was made of five different kinds of silica catalysts-xerogels, aerogels, microcrystalline silica, quartz sand, and sintered gels. The effects of sodium ion and of heat treatment on the activity of silica gels were investigated. The effect of catalyst temperature on conversions was determined for a number of catalysts. A silica aerogel containing 0.19 per cent platinum oxide was investigated in some detail; specifically, the effect of catalyst temperature, space velocity, ratio of air to acetaldehyde on conversions, and effect of temperature on the yield of acetic acid were determined.
Apparatus The apparatus developed during this study is shown in Figure 1:
HE control of heterogeneous catalytic oxidations of orThe entire system was made of glass. The various units were ganic compounds in the vapor phase is often difficult. connected with ground-glass joints so that they could be removed This will be obvious when it is realized that there are few for cleaning. Unless the system was absolutely clean, trouble successful industrial examples of such reactions. Although was experienced with polymqrization of the acetaldehyde. a great many studies have been carried out over many years, The acetaldehyde was prepared from distilled paraldehyde containing a drop or two of sulfuric acid. An atmosphere of data for the selection of a successful catalyst are not available. nitrogen prevented oxidation durin preparation. The vacuumA logical attack on this problem is the study of the partial jacketed fractionation column, hf packed with glass beads, oxidation of organic compounds containing an easily oxidizseparated the small amount of paraldehyde rising from flask M . able group such as the carbonyl. The pure aldehyde was condensed in spiral 0 which was surrounded by ice water. From here it passed into the 100-cc. The oxidation of acetaldehyde to acetic acid has been buret, R, about which ice water was circulated by a simple air selected for this investigation because acetaldehyde is one of lift (not shown). the simpler, industrially important compounds that contain After sufficient aldehyde had been collected, the preparation a carbonyl group, and the products of its oxidation are less unit was closed eff, and mercury was run into the buret from the numerous and more e a s i l y determined by analysis than those of a more complex molecule. I n this laboratory Burton (1) studied this oxidation over thirty different catalysts, many of which were familiar in oxidation. A number were prepared in gel form, since this structure has the distinct advantage of offering a large reaction surface. Of the catalysts tested, silica gel was by far the best for conversion to acetic acid. Platinum or vanadium on pumice were inactive a t low temperatures. Aerogels were first described by Kistler (7) in 1932. Kistler, Swann, and Appel (8) pointed out that the aerogel probably has a greater surface area per unit of mass than any other solid suitable as a, catalyst. The first results obtained with aerogel catalysts, announced in 1934 (8),were very promising. Thoria aerogel was shown to be more satisfactory for converting aliphatic acids and esters into ketones than any catalyst known. 1254
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delivery system, P, which contained a capillary, &, of such size as to allow approximately 28 cc. of mercury t o pass in an hour.
This mercury forced an equal volume of aldehyde out through the side arm and into vaporizer D,which was surrounded by water at 45" C. Here the aldehyde was picked up by a stream of air that had passed pressure regulator A , had been dried in calcium chloride tower B , and measured by flowmeter C . The mixture of air and aldehyde left the vaporizer, and passed through the preheatin coil and catalyst chamber, in the electrically heated glycerof thermostat, F . The products from the catalyst chamber then passed successively through ice trap G, solid carbon dioxide-acetone trap H, water scrubber I , acid scrubber J , calcium chloride tube K , and thence into Wesson bulb L containing Ascarite for the determination of carbon dioxide. The line between ice trap G and the catalyst chamber was heated with a Nichrome wire t o prevent condensation of acetic acid. The catalyst chamber shown in Figure 2 was designed to maintain the temperature of the catalyst as near that of the thermostat as possible. Since the reaction was highly exothermic, considerable heat had to be transferred from the catalyst chamber to the liquid in the thermostat. The hot junction of the copper-constantan thermocouple, sealed in with glycerol-litharge cement, was embedded in the body of the catalyst ' / 8 inch below the surface. It was found experimentally that the maximum temperature was read at this point. A portable type potentiometer was used to measure the potential of the thermocouple. The exact amount of acetaldehyde used for a run was determined by reading the mercury level in the buret a t the beginning and end of the run. Before a run was actually begun, the gases were pass'ed over the catalyst.and by-passed around the absorbing equipment until stable conditions were attamed. The progress of the reaction was followed towards a steady state by noting the catalyst temperature. The duration of each run was one hour.
Analysis The condensate in traps G and H was mixed with the solution from scrubber I and diluted to a liter. Acetic acid was determined by titration with 0.1 N sodium hydroxide solution using phenolphthalein as the indicator. The unchanged acetaldehyde was estimated by the bisulfite method described by Parkinson and Wagner (IO). The percentage of aldehyde completely oxidized to carbon dioxide was determined by absorption in a Wesson bulb filled with Ascarite. The carbon dioxide was usually absorbed for about 10 minutes during the middle of the run. A small percentage of the acetaldehyde always condensed to ethyl acetate, usually less than 5 per cent, the exact amount depending on the particular catalyst used. In several runs the condensate from the two traps was distilled, and the fraction boiling between 30" and 100' C. was saponified with sodium hydroxide to determine the ethyl acetate. The immediate liberation of iodine from potassium iodide indicated the prmence of peracetic acid in the liquid products (9). On dilution the peracetic acid rapidly disappeared. Carbon monoxide was shown t o be absent by the iodine pentoxide method (12).
The results of the analysis were expressed as per cent conversions to acetic acid, to carbon dioxide, and to ethyl acetate. Total conversions and yields were also determined. In the runs in which ethyl acetate was determined, a carbon balance accounted for practically 100 per cent of the acetaldehyde vaporized. The space velocity was calculated, considering the total mixture entering the catalyst chamber to be at standard conditions, and is expressed in reciprocal hours.
Catalyst Preparation Most of the catalysts studied were of a gel structure and many were of the recently developed aerogel type. The general methods of preparation and a description of this type of catalyst were reported by Kistler (7,8). A number of the catalysts used in this work were prepared by Kearby (6) who described the details of their preparation and the general method of gel preparation. All the catalysts were screened t o 10-20 mesh and burned out with air for several hours a t 400" to 450" C. unless otherwise specified. 1. PUREXEROGEL FROM SODIUMSILICATE.Equal volumes of 6 N hydrochloric acid and sodium silicate solution
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(specific gravity, 1.15) were' &xed and allowed to gel; this gel was broken into pieces and wpshed for 3 days, first with tap water and then with distilledjwater. The pure hydrogel was dried slowly and screened.
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CATA1 YS7- CHAMBEP N G 2.
2. AEROGELFROM SODIUM SILICATE.Equa1:volumes of 3.7 N hydrochloric acid and sodium silicate solution (specific gravity, 1.17) were cooled with ice, slowly mixed, and placed in an ice box to set. The hydrogel was cut into pieces and thoroughly washed with water. The water was replaced by ethanol and the gel autoclaved to form a n aerogel. 3. PUREAEROGELFROM TETRAETHYL ORTHOSILICATE. Tetraethyl orthosilicate (Carbide and Carbon Chemicals Corporation) was purified by distillation. A solution of 600 cc. of water, 600 cc. of ethanol, and 150 cc. of concentrated hydrochloric acid was cooled to 0" C. and poured with stirring into a cold solution of 1200 cc. of tetraethyl orthosilicate and 600 cc. of ethanol. This solution set to a gel when placed in an oven a t 50" C. The gel was cut into pieces, extracted with ethanol, and a portion autoclaved to form the aerogel. 4. AEROGEL FROM TETRAETHYL ORTHOSILICATE (PARTLY DRIED). A portion of gel 3 was partly dried before autoclaving with ethanol. 5. MICROCRYSTALLINE SILICA. This silica, mined near Cairo, Ill., is usually known as amorphous silica, but an x-ray analysis showed a perfect quartz pattern. The fine powder was moistened with tetraethyl orthosilicate, dried to a cake, and broken into small pieces. 6. OTTAWA QUARTZSAND. Sand from the Ottawa (Illinois) mines, known as standard sand because of its application in cement testing, was used. The 20-30 mesh sample was leached twice with hot hydrochloric acid, thoroughly washed, and dried. 7, 8. SILICA XEROGEL. Equal volumes of 4 N hydrochloric acid and sodium silicate solution (specific gravity, 1.20) were mixed; the hydrogel was washed once, broken into pieces, and dried. 9. SILICAXEROGEL SINTERED. Xerogel 7 was sintered a t 940" C. for 3 hours. 10. XEROGEL SOAKEDIN SODIUMCHLORIDESOLUTION. The pure xerogel from sodium silicate was soaked in a 0.01
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gel in 10 minutes. The gel was extracted and autoclaved with methanol. 22. FERRIC OXIDEAEROGEL(ORGANIC).Ethylene oxide was added to a FeC18.6H20 solution containing a little glycerol. The solution was kept at 0' C. and set in an hour; this gel was extracted and autoclaved with ethanol. 23. ACTIVATEDCHARCOAL.Coconut-shell char was screened and heated to 870" C. for a day in a stream of nitrogen and then cooled to 300" C. Oxygen was passed over it for several hours, and the charcoal was finally allowed to cool in an atmosphere of oxygen. 24. SINTERED CATALYSTS.Equal volumes of 3.7 N hydrochloric acid and sodium silicate (specific gravity, 1.18) were cooled with ice and mixed. The gel formed was allowed to stand overnight, broken into pieces, washed alternately with distilled water and a solution containing 2 per cent ammonium nitrate, and carefully dried to form the xerogel. Portions of this xerogel were heated for 4 hours a t the following temperatures-4O0, 718", 885", and 1010" C.
b
per cent sodium chloride solution and carefully dried; it was unchanged in appearance. 11. AEROGEL SOAKED IN 0.5 PERCENTSODIUM NITRATE SOLUTION. A silica hydrogel was soaked in 0.5 per cent sodium nitrate solution, extracted, and autoclaved with Comparison of Catalyst Activity ethanol. The various catalysts studied are shown in Table I. The 12. AEROGELSOAKEDIN 5 PER CENT SODIUMNITRATE data are based on one run selected from a number made with SOLUTION.Same as 11. each catalyst. They represent the characteristic action of 13. PLATINIZED SILICAXEROGEL. A solution of 675 cc. the catalyst a t a temperature where the yield of acetic acid of water, 675 cc. of ethanol, and 169 cc. of concentrated and conversion are satisfactory. All the runs listed in Table sulfuric acid was cooled to 5" C. and poured into a cold soluI were made with 15 cc, of catalyst, a t a space velocity of tion of 1350 cc. of tetraethyl orthosilicate in 675 cc. of ethanol. about 3500 reciprocal hours and about 50 per cent air in exThis solution was allowed t o stand for 15 hours. Platinic cess of that theoretically required for complete oxidation to chloride (1.82 grams) was dissolved in 20 cc. of water and acetic acid. The 50 per cent excess air was found to give the stirred into this sol., and then 50 cc. of ethylene oxide were best results of any air ratio tried. added. The solution was placed in an oven a t 55" C. and set Additional catalysts tested by Burton (1) were: cupric to a clear transparent gel in 4 hours. It was allowed to stand oxide (metal), aluminum turnings, Pyrex glass beads, pumice, in the oven a t 50" C. for 2 days covered with enough alcohol to keep the surface wet. It gradually turned gray and then black as the platinum oxide precipitated from solution. The gel was broken into TABLEI. COMPARISON OF CATALYSTS~ pieces and extracted as usual. Acid Total ilcHb AcCataConCata- Con14. PLATJNIZED A E R O G E L(ETHANOL).A yerlyst verCOS Acid counted lyst portion of the above gel was extracted and No. Catalyst Grouping sion Temp. sion Yield Yield For autoclaved with ethanol. % OC. % % % % 15. PLATINIZED AEROGEL(PROPANE).Same I. Silica catalysts: 1 A. Xerogel 47.2 125 63.4 11.7 74 93 as 14 but autoclaved with propane. B. Aerogels: 1. From NazSiOa (pure) 51.2 141 68.3 17.9 75 95 2 16. 33 P E R CENT A L U M I N AO N SILICA 94 2. From Si(OEt)4 pure) 46.9 167 64.3 17.4 73 3 XEROGEL.Asilica hydrogel was soaked for 2 3. From Si(0Et)a {.partly dried) 48 2 172 65.7 17.2 73 94 4 77 95 5 C. Microcrystalline silica 29.7 137 36.5 10.5 days in a solution of aluminum sulfate and the D. Ottawa quarts sand 1 142 .. .. .. .. 6 excess drained off. The gel was covered with 11. Gels containing sodium ion: A. Xerogels : ammonium hydroxide, allowed to stand for a 48.8 1. Silica xerogel No. 1 114 61.0 5.8 52 92 7 49.2 99 5.3 87 96 56.5 2. Silica xeroael No. 2 8 day, and then dried as usual. 3. Silica xerogel sintered a t 17. 3.5 PERCENTALUMINA ON SILICAXERO24.1 93 28.3 940' C. 3.6 82 97 9 4. Xerogel soaked i n 0.01% GEL. Same procedure as 16. 155 86.7 17.2 74 94 10 49.3 NaCl soln. B. Aerogels: 18. 2.4 PERCENTALUMINA ON SILICAAERO1. Soaked in 0.5% NRNOa soln. IS.0 21.2 9.4 85 99 11 GEL. Same procedure as 16, but autoclaved with 2. Soaked in 5% NaN03 soln. .. , . No reaction . . . . 12 111. Platinised silica gels: ethanol. 13 38.6 142 43.5 A. Xerogel 9.9 85 98 19. 17 PERCENTPHOSPHORIC ACIDON SILICA 14 83.4 111 70.7 8.9 90 98 B. Aerogel (ethanol) 97 15 60.0 134 6.8 89 c. Aeronel (urovane) 87.5 ._ AEROGEL. Vapors of platinum chloride were IV. Mixed-gel catalysts: passed over pure silica aerogel followed by nitric A. Alumina on silica: Fouled 100 .. .. .. .... 16 1. 33% alumina on xerogel acid fumes to liberate phosphoric acid. Fouled 90 .... .. .. 17 2. 3.5% alumina on xerogel 90 . . .. 18 3 2 4 7 alumina on aerogel Fouled 20. 9 PERCENTKICKELON SILICA AEROGEL. B. ' Phbsptoric acid on silica aerogel 3.8 159 7'6 48 96 19 C. Nickel on silica aerogel 25.7 126 35 5 15:5 72 98 20 A silica hydrogel was soaked with a solution of V. Other gels: nickel tartrate dissolved in ammonium hydroxA. Crz03 aerogel 3.6 138 15.6 15.4 23 21 B. Fez08 aerogel 0.6 110 .. .. . . 90 .. 22 ide, and the gel was extracted and autoclaved VI. Activated coconut charcoal 0.7 140 .. .. . . .. 23 with ethanol. Per cent yields and conversion are based on acetaldehyde. 21. CHROMICOXIDE AEROGEL. A solution b Acetaldehyde accounted for in the products as acetic acid, carbon dioxide, and unof 100 cc. of basic chromium nitrate and 50 cc. changed aldehyde. of animonium acetate was mixed. It set to a ~
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platinum oxide on pumice, vanadium oxide on pumice, aerogels of stannic oxide, aluminum oxide, thorium oxide, titanium oxide, nickel oxide, and pellets of manganous and aluminum acetates. A number of mixed catalysts composed of various oxides incorporated in silica gels were also tested. All of these catalysts were found to be less active for this partial oxidation than silica gel alone. Although manganese oxides are recognized as oxidation catalysts, Burton found that a silica aerogel impregnated with manganese hydroxide was completely 'inactive. The appearance indicated that the gel s t r u c t u r e h a d b e e n destroyed. The first group in Table I consists of silica catalysts in the form of xerogel, aerogel, microcrystalline silica, and quartz sand. I n Figures 3 and 4 the percentage of acetaldehyde converted to acetic acid is plotted against the catalyst temperature, and the curves show the comparative activity of these catalysts. The conversions to acetic acid obtained with the aerogel and xerogel are about the same. Those with the microcrystalline silica are about 60 per cent as high. The activity of the quartz sand was negligible. There are three aerogel catalysts in section B of group I. The first was prepared from sodium silicate and the other two from tetraethyl orthosilicate. The density of the partly dried aerogel was about midway between that of the true aerogels and the xerogels. The conversions and yields obtained with these catalysts are obviously similar. The effect of density is only partly indibated. Further investigation showed that xerogels of an apparent density of 0.8 to 0.9 gram per cc. were low in activity. Density, however, was of no importance when less than 0.6 gram per cc. The activity of the xerogels and aerogels seems to be about the same, in accordance with Burton's results (1). The method of preparation and the treatment during drying were found to be very important. An x-ray analysis of the microcrystalline catalyst gave a perfect pattern of a-quartz. Adsorption measurem'ents made by Kearby (6) showed i t to have a low capacity for water vapor. In spite of the lack of amphorous structure acd of high adsorptive power, both of which were thought highly important if not absolutely necessary for successful catalysis, the conversion over this catalyst was about half of that obtained over the amorphous gel catalysts. The yield was about the same. These results emphasize the high selectivity of silica for this partial oxidation. Quartz sand showed a negligible activity. Group I1 includes data on xerogels and aerogels containing sodium ion. This phase of the investigation was undertaken because Burton's work (1) indicated that small amounts of impurities, presumably the sodium ion, had a beneficial effect on the yields of acetic acid. Gels were therefore prepared from tetraethyl orthosilicate, thus excluding the possibility of the presence of sodium ions. Experiments show that impurities do affect the catalyst's activity by slightly increasing the yield of acetic acid with a corresponding decrease in carbon dioxide. These results are not conclusive because of the marked effect on catalytic activity due to unavoidable differences in the methods of preparing individual catalysts. An impure xerogel similar to No. 1 was sintered by heating it for 3 hours a t 940' C. where sintering was first noticeable. The activity, or percentage conversion, decreased after this treatment. The acid yield was substantially the same, but the carbon dioxide yield had decreased. This introduced the interesting possibility of changing the direction of the oxidation
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by heat treatment of the catalyst. This has been investigated in more detail, and the results appear later in the discussion. Group 11-B includes two aerogels soaked in solutions containing different concentrations of sodium nitrate. In case 1 the activity had definitely diminished, and in case 2, where a greater amount of the salt was present, the activity was eliminated completely. These results indicate the sensitivity of the aerogels' surfaces. There was no evidence of fouling with either of these catalysts. Group I11 contains three platinized silica catalysts. These catalysts gave the highest conversions and acid yields of any studied. At low temperatures the acid yields were about 90 per cent of the theoretical. In order to retain the aerogel structure, only 0.19 per cent platinum oxide was incorporated.
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The xerogel in this group was very dense (apparent density, 0.85 gram per cc.) and it was less active than the less dense catalysts of the same composition. These catalysts were all prepared from the same batch of gel. Group IV contains several mixed-gel catalysts. Three alumina-silica catalysts were tried, and in each case complete
Each of these curves rises to a maximum conversion and then falls off a t higher temperatures. This maximum comes a t different points with the individual catalysts but is usually a t about 145' C . We would expect in an oxidation that the percentage total conversion would be greater a t higher temperatures. Adsorption and activation of the acetaldehyde, hQwever, play the predominant role. At higher temperatures the aldehyde adsorption presumably decreases and the conversions are lower. It is notable that in most cases the percentage of acetaldehyde converted to acid and carbon dioxide is more dependent on the particular catalyst than on the temperature. The yields of acetic acid obtained with three platinized silica gels are shown in Figure 7. The yield is about 90 per cent a t low temperatures and decreases a t higher temperatures since more of the reaction is directed towards carbon dioxide.
Effect of Space Velocity
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fouling occurred within a few minutes. This type of alumina catalyst is well known for its ability to aid polymerization reactions (4), and apparently the adsorbed acetaldehyde molecules underwent condensation and remained on the surface. The active surface of the phosphoric acid catalyst seemed to be merely blanketed since there was no evidence of fouling and very little oxidation. The silica aerogel with 9 per cent nickel was less active than the pure aerogel. This is in agreement with Burton's results on a similar catalyst. The two aerogel catalysts in group V (chromic and ferric oxides) were found to be far inferior to silica gel. Activated coconut charcoal showed some initial activity, but this soon decreased t o a negligible amount. This primary activity was probably due to the reaction with oxygen adsorbed during the period of activation. Silica gel has gained considerable recognition as a catalyst carrier and has been used to catalyze a number of dehydration reactions, but there seems to be no reference in the literature to its application as the primary catalyst in an oxidation process. Its definite superiority in the partial oxidation of acetaldehyde is emphasized when it is recalled that almost fifty catalysts were tried and none, with the exception of the platinum promoted gel, was equal to silica gel alone.
Effect of Temperature A number of runs were made with each catalyst, maintaining the same air ratio and space velocity and varying only the temperature. The general shape of the curves obtained for silica catalysts was very similar. The results with three catalysts are representative of the fourteen studied, and are shown graphically in Figures 4, 5, and 6. The curves for total, acid, and carbon dioxide conversion are plotted against catalyst temperature. The sum of the acid and carbon dioxide conversions does not quite equal the total conversion because there is always some formation of ethyl acetate and also a small loss of aldehyde.
The platinized aerogel, autoclaved with ethanol, was selected for the study of the effect of space velocity on acid yields because it gave especially good yields of acetic acid. A number of runs were made a t a catalyst temperature of 170" C. The space velocity was changed by varying the amount of catalyst from 40 to 7 cc. in order to maintain the same linear velocity of the gases. The results are sl'iown in Figure 8 where the per cent conversions are plotted against the space velocity calculated for the total mixture entering the catalyst chamber a t standard conditions. The units used here for space velocity are reciprocal hours. The acid yield
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in the first run was always slightly low because of the primary activity of the catalyst for carbon dioxide formation. The results were reproducible after operating for an hour or so.
Effect of Air Ratio The effect of the ratio of air to acetaldehyde is shown in Table I1 and Figure 9. Above a ratio of 0.7, further dilution has little effect. This ratio was used for comparing catalysts. This curve is affected by other variablesnamely, catalyst temperature and space velocity, both of which would tend to bend the curve downward.
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effect. Qualitative tests made on the liquid drawn from the trap showed the presence of large amounts of peracetic acid, which reacts with the acetaldehyde present as follows:
TABLE11. EFFECT OF AIR-ACETALDEHYDE RATIO (Runs made with platinized silica aerogel, 24 cc. of catalyst, and a thermostat temperature of 870 C . ) Run No.
On/AcH Ratio
171 172 170 169
0 2
--ConversionAcid Total
0.45 0.71 1.17 1.76
Apid Con- Catalyst Yield sumed Temp.
COS
%
%
%
57.0 69.7 72.8 67.5
66.7 79.9 85.3 79.1
5.3 7.1 6.7 5.8
70
%
86 87 85 85
93 74 45 28
C. 114 109 100 96 O
It might be expected that the carbon dioxide conversion would increase rapidly with the air ratio. This, however, did not occur; the carbon dioxide yields were substantially constant, It was noticed that the catalysts favoring the formation of carbon dioxide are those which will operate a t a low thermostat temperature. Effect of Heat on Silica Catalyst Observations on the sintered silica xerogel (catalyst 9) led to a further investigation of the effect of heat treatment on the activity of a silica catalyst. A batch of pure silica xerogel was divided into four parts. One portion was heated for 4 hours a t each of the following temperatures450", 718", 885", and 1010" C. The activity of these catalysts was tested at a space velocity of 2300 reciprocal hours and a temperature of 136" C. The results are shown in Figure 10 where the percentage conversion to acetic acid is plotted against the temperature of heat treatment. It is evident that
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the percentage conversion decreased rapidly above %O" C. An examination of the catalyst heated to 1010" C. showed that most of the particles had been actually fused into round balls. No such degree of fusion was evident in the other catalysts. The portion of the gel heated to 885", however, contained many particles that were definitely sintered. There had been indications that heat treatment of the catalyst would affect the carbon dioxide formation, but this particular silica catalyst consistently gave good yields of acid. +Theseyields were almost equal to those obtained with the platinized gels-about 88 per cent of the theoretical. An interesting part of the resuIts obtained is the decidedly greater activity of the silica gel catalyst heated at 718" C. This is possibly associated with the removal of a larger percentage of the residual moisture contained in the gel even after being heated a t 450" C. for several hours. The other catalysts studied always burned out a t about 400" C. An x-ray analysis of catalyst 9, heated to 940" C., gave definite evidence of crystallinity, due presumably to the presence of a-cristobalite. Several factors indicate that the secondary reaction of peracetic acid with aldehyde occurs to a much greater extent in the catalyst chamber when operating a t higher temperatures. The liberation of heat by the liquid condensate in the ice trap was noticeable when operating at low temperatures ; a t higher temperatures there was no evidence of this heat
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Space Velocity Reciprocal hr. 1500 2300 3500 5200
CHaCOOOH
+ CHaCHO +2CHaCOOH
The speed of this reaction is known to be affected by the presence of water. When considerable water vapor is f o r m e d in the catalyst c h a m b e r , the reaction occurs largely a t the catalyst surf ace.
Summary Catalysts of the silica gel group were superior to any others studied, with regard to both conversion and yield of acetic acid. A conversion of 50 per cent and an acid yield of 88 per cent were obtained in one pass over an active silica gel. No other catalyst gave more than 9 per cent conversion to acetic acid. A number of mixed catalysts composed of various oxides deposited on and incorporated in silica gels were less active for this partial oxidation than silica gel alone. The gel structure was absolutely essential. Pure silica xerogels and aerogels were similar in activity. Samples of these gel catalysts prepared in approximately the same manner varied in activity. A sample of microcrystalline silica, commonly known as amorphous silica, showed about half the activity of the gel catalysts but gave the same yield of acetic acid. Quartz sand showed a negligible activity. Silica xerogels which were not completely freed of sodium chloride gave better yields of acetic acid and correspondingly less carbon dioxide than carefully purified gels. The addition of small amounts of sodium chloride to a purified xerogel seemed to have no effect on its activity. Fairly large amounts of sodium nitrate as an impurity completely destroyed the activity of the aerogels. A silica aerogel, with 0.19 per cent platinum oxide incorporated as a gel, was the most active catalyst studied and, like the gels with the sodium ion, produced good yields of acid-about 85 to 90 per cent. The total conversion decreased beyond 145-160" C. Both the conversion to carbon dioxide and acid decreased. Similarly, the yields of acid obtained with three platinized silica gels decreased with higher temperatures.
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When the conversions were plotted against the time of contact of the reactants with the catalyst, they increased along a smooth curve. Curves are included for results obtained with a platinized aerogel operated a t 170" C. The conversions to acid over a platinized aerogel increased
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up to the theoretical air ratio. From that point on, they remained substantially constant. Samples of a silica xerogel were heated for 4 hours a t different temperatures, and their catalytic activity was compared graphically. The highest conversion to acetic acid was obtained with the sample heated a t 718” C.
Acknowledgment The authors wish to express their appreciation to Sherlock Swann, Jr., for his helpful suggestions throughout this work, to K. K. Kearby who prepared a number of the catalysts studied, and to 8.T. Gross for making the x-ray study of the microcrystalline silica.
Bibliography (1) Burton, A. A.,Chem. Age (London), 31, 169 (1934). and Houghton, A. C., Am. Chem. J . , 32,43(1904). (2) Clover, A. M.,
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(3) Fajans, E., 2.physik. Chem., 28B,252 (1935). (4) Gayer, F.H., IND. ENC.CHEM., 25,1122 (1933).
(5) Hatcher, W.H., Steacie, E. W. R., and Howland, F., Can. J. Research, 5, 648 (1931);7,149 (1932). (6) Kearby, K. K., Ph.D. Thesis, Univ. Ill., 1937; to be published. (7) Kjstler, S. S., J.Phys. Chem., 36,52 (1932). (8) a s t l e r , S. S., Swann, S., Jr., and Appel, E. G., IND. ENQ. C H ~ M26, . , 388 (1934); Swann, S., Jr., Appel, E . G., and Kistler, 5.S., Ibid., 26,1014 (1934). (9) Marek, L. F.,and Hahn, D. A., “Catalytic Oxidation of Organic Compounds in the Vapor Phase,” A. C. S.Monograph 61,New York, Chemical Catalog Co., 1932. (10) Parkinson, A. E.,and Wagner, E. C., IND. ENQ.CHEM.,Anal. Ed., 6, 433 (1934). (11) Pease, R.N.,J.Am. Chem. Soc., 55,2753 (1933). (12) Vandaveer, F. E.,and Gregg, R. C., IND.ENG. CHEM.,Anal. Ed.,1 , 129 (1929). RECFOIVED July 28, 1937.
Thermodynamics in Hydrocarbon
Research CHARLES L. THOMAS, GUSTAV EGLOFF, AND J. C. MORRELL Universal Oil Products Company, Chicago, Ill.
T
HE more efficient utilization of the natural hydrocarbon resources of the world is one of the major problems which faces the chemist today. The petroleum industry, in particular, is interested in better utilizing crude oil by converting more of it into useful products and in increasing the usefulness of products already being made. This can be done by converting the petroleum by reactions that will give the desired product and by carrying out the reactions under such conditions that the yield of the desired product is a maximum. Thermodynamics can be helpful by pointing out the operating conditions most favorable for the reaction involved. As is well known, thermodynamic calculations give no clue to the rate of a reaction. Since reaction rates are of prime importance in hydrocarbon chemistry, particular care must be used in interpreting thermodynamic calculations applied to hydrocarbon reactions. This has not always been done. For example, Schultze (34) used thermodynamics to calculate a number of equilibrium constants for reactions in the cracking of hydrocarbons and in coal carbonization. These constants have been used to interpret the yields of given products in spite of the fact that the respective reaction rates play an important role in determining the yields from such reactions. Unless reaction rates are considered simultaneously with the thermodynamic data, the novice is likely to infer that the thermodynamic data are the sole controlling factors. For example, Parks (24), in referring to thermodynamic data, concludes: “These facts throw considerable light upon the effect of variation in temperature in determining the character of the products in the oil-cracking process.’’ This conclusion was applied to the known tendency of increased temperatures to favor the formation of aromatic hydrocarbons in this process. In oil-cracking, even if the
standard free energy of each of the reactions is known, it must be remembered that the igcrease in temperature can cause an increase in the rate of the aromatic-forming reaction and thus explain the increased aromatic formation. If thermodynamic calculations are applied only to systems known to be in equilibrium, then such difficulties do not arise. Unfortunately, a t present, uncatalyzed hydrocarbon reactions which are in equilibrium are relatively few in number. Under these circumstances thermodynamics can be of greatest use to the research worker by indicating the most favorable temperature and pressure conditions for the reaction. If the reaction does not have a convenient velocity under these conditions, a catalyst must be sought to promote the reaction velocity. I n this way thermodynamics can be used to eliminate experiments designed to find a catalyst for a reaction which can occur to only a limited extent under a given set of conditions.
Accuracy of Thermodynamic Data on Hydrocarbons So far in this discussion it has been assumed that the data used in making the thermodynamic calculations are accurate. Some of the difficulties involving the accuracy of certain hydrocarbon data will be discussed briefly. [The nomenclature and symbols of Lewis and Randall (21) will be used throughout.] The entropy S , free energy P , heat content or enthalpy H, and heat capacity C, form the foundation for most of the calculations for hydrocarbons. All of these quantities may be evaluated by calorimetric measurements. For the simpler molecules these same quantities may also be calculated from infrared absorption and Raman spectra (IS). In many cases the values obtained by the two methods are
