Catalytic Effect of Metals on Paraffin d
Hydrocarbons CHARLES L. THOMAS, GUSTAV EGLOFF, AND JACQUE C. MORRELL Universal Oil Products Company, Riverside, Ill.
As catalysts, metals are potentially capable of fostering carbon-carbon scission or cracking, dehydrogenation to form olefins, and complete decomposition to carbon and hydrogen. From the available data there is no metal that sponsors catalytic cracking or carbon-carbon scission. There is some evidence to indicate that copper and palladium are capable of sponsoring dehydrogenation to form olefins, although copper is comparatively inert unless specially prepared, and palladium has a tendency to promote carbon formation. Iron, cobalt, and nickel are powerful catalysts fostering the complete decomposition of paraffin hydrocarbons to carbon and hydrogen. I t is possible that this catalytic reaction is not noticed in commercial cracking equipment because there is enough sulfur present in the oil to poison this catalytic action of the metal.
B
OTH the natural gas and the petroleum industries desire to utilize hydrocarbon resources to the fullest possible extent. Catalysts are now widely used in hydrocarbon chemistry and are destined to play a still greater role. Because metals are readily obtainable, i t is desirable to know just how the hydrocarbons react in the presence of metals. This information is also useful in selecting the materials of construction for plants for processing hydrocarbons. It is unsatisfactory to use metal tubes for a reaction if the metal is catalytic and causes undesirable reactions. A study of the available information shows that one of the commonest effects of metals is to bring about complete decomposition of paraffin hydrocarbons into carbon and hydrogen, Less common are metals which cause dehydrogenation of the paraffin to give hydrogen and the corresponding olefin. There is no clear case of a metal that catalyzes the cracking reaction-i. e., breaking of the carbon-carbon bond. Many of the metals are without effect and the degree of effectiveness varies considerably with different metals. Important factors are the method of preparation of the metal, the form and extent of the surface, and internal structure. Considerable care must be exercised in determining both the qualitative and quantitative catalytic effect and certain criteria must be considered in their evaluation.
Criteria for Judging Catalytic Effect THERMAL EFFECTS. In general, the standards of zero catalytic effect for the paraffin hydrocarbons are glass, porcelain, and quartz. T o study the effect of metals, they may be put in such inert tubes in the form of powder, pellets, or turnings, or the tube may be made from the metal itself. I n all these cases thermal effects occur that give rise to phenomena which may be “apparent” or false catalytic effects. Heat transfer conditions may also be changed when tubes are filled with powder, pellets, or turnings, owing to increased turbulence. This results in a greater conversion under a given set of conditions or milder conditions (lower temperature and/or shorter reaction time) for a given conversion, as the excellent work of Cambron and his associates (8, 9) has shown. These effects are similar to those caused by a catalyst, although the turbulence may be caused by inert material. Substitution of a metal tube for a glass or silica tube of similar dimensions in the same furnace changes the heat distribution. The metal is a better conductor of heat than glass or silica so it can better supply the heat to an endothermic reaction. It is therefore desirable to determine when there is a true catalytic effect. This may be done from an examination of the reaction products. If all the reaction products in the presence of metal are formed in the same ratio as in the purely thermal reaction, the metal may be classed as noncatalytic. This criterion is based on the fact that all paraffin hydrocarbons, with the possible exception of methane and ethane, undergo more than one primary reaction, and on the assumption that no heterogeneous catalyst will accelerate two or more of the primary reactions of the paraffins to the same degree. I n all the cases which have been carefully studied the true catalyst has some selectivity, favoring one reaction or one type of reaction more than another. SURFACE.It is a general rule in heterogeneous catalysis that the greater the surface, the more active the catalyst for a given material. This is sufficient to explain why one worker can use a tube made from a given metal and find no catalytic effect, whereas another worker with the same metal in the form of powder reports a powerful catalytic effect. There may be nothing inconsistent about these reports. It merely means that in judging the catalytic effects, i t is of prime importance to know the form of the catalyst used in the test. The fact that many catalysts POISONS AND PROMOTERS. are subject to poisons is well known, and therefore care must be used in determining the catalytic effect of a given material. For example, it is not good practice to leave a nickel tube around a laboratory for days or weeks where it is exposed to hydrogen sulfide, and then use it in a test with methane and report that it is catalytically inert when the same tube 1090
SEPTEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
+
CnHzn+ 2 % C,Hzn (olefin) Ha (dehydrogenation) C,H2, + z -,C, - m H ~ ( nm) + 2 C,H2, (cracking) C,HZn+ nC (n l)Hz (complete decomposition)
-
scrupulously clean would be so active that it would soon become plugged with carbon from catalyzing the reaction, CHr S C
1091
+ 2Hi
+ +
(1) (2) (3)
+
Thus a metal favoring reaction 1 to a greater extent than is known to occur thermally is classed as a dehydrogenation catalyst, etc.
It would also be undesirable to employ methane or natural
gas which contained hydrogen sulfide. Since the poisons for a given catalyst are seldom known in advance, it is imperative Group I that materials and technique both be directed to exclude all Of the five alkali metals (Table I) only lithium and potasthe poisons possible. sium have been studied, but no catalytic effects have been Just as catalysts are affected by poisons, they are also reported. affected by promoters. I n general it is not possible to know a priori whether a substance will be a poison, promoter, or inert when added to a catalyst. I n reporting the catalytic reactivity of a TABLE I. GROUPI metal, it is preferable that the unpromoted acCatalytic Properties and TemperaCatalyst tivity be given if possible. I n some cases this Referenoes ture, C. Reactions Ocourring Hydrocarbon Form is difficult, for unsuspected substances may be LITHIUM acting as promoters. This may be illustrated 850 CHI Liquid No effect reptd. (44) by the report (93)that butane may be heated POTABSIUM 850 to 1400" C. in the presence of a metal withCH4 Vapor No effect reptd. (44) COPPER out decomposition, whereas a t a much lower Noncatalytic 875 C H1 Tube temperature (about 400")it can be quantitatively C + HZ 910 CHI Powder 900-1000 Noncatalytio CHI Wire dehydrogenated in the presence of the same Not given C + Hi CH4 Not given metal and a trace of water vapor (cf. also I ) . 450 CZH6 4- Ha On MgO 600 CeH6 Tube CARBON. One of the most common observa500-900 CzHe Tube 990a tions made in experimental work on the thermal CZH6 Gauze 550-950 CeHeCaHa Tube reactions of paraffin (and other) hydrocarbons 580-975 CzHe-CsHe -, Gauze 350-400 is that the reaction vessel becomes coated with CsHs Granules No effect reptd. ($0) 550 CaHs On pumice Dehydrogenation ( 6 5) carbon. I n the case of glass and silica this 600-700 CaHa Tube Noncatalytic (68) 625-1010 CSHI-CIHIO Noncatalytic (97) Gauze . . deposit is in the form of a thin lustrous film. 575 n-C4Hlo On pumice Mostly CzH4 -k CeH6 (65) This film forms comparatively rapidly on a clean Natural gas Tube Noncatalytic (12) 525-565 n-CeHl4 surface but much more slowly or even stops No evidence of catalysis (la) 496-570 n-CaHia Cu-lined iron once the surface is covered. I n most cases the 2,5-dimethylhexane 492-576 tube SILVER reactions in a clean tube are different from those 650-1050 Noncatalytic Wire CHI in a carbon-coated tube. There is some eviNoncatalytic 550-950 CeHsCaHs Foil dence that the carbon film acts as an inhiQOLD Noncatalvtio lifij 550-950 CzHeCaHs Foil -, bitor (78) for the reactions although it is equally 0 I n this experiment a carbon rod was electrically heated to 990' C. but was located axially likely that glass and silica are not absolutely in a water-cooled tube. The copper gauze was between the heated rod and the cooled tube. noncatalytic, and these surfaces themselves cause the formation of this carbon film. Metal surfaces are also subject to this carbon More work has been done in the presence of copper than formation. Therefore when testing catalytic activity of the any other metal in the group. Copper is a mild catalyst for metal i t is essential that tests be completed before an apprecithe complete decomposition of the hydrocarbon into carbon1 able part of the surface is carbon-coated. Similarly, in seekand hydrogen. A film of carbon is formed on the surface of ing a noncatalytic metal for reaction equipment, a metal that the metal. If the metal is in the form of a tube, these films does not cause carbon formation is preferred. Since no such peel off from time to time and remove some of the copper. metal has yet been found, one which forms a tightly adhering A new film is formed to take the place of that which peeled off. deposit is preferred (31)if the carbon deposit ceases with the This process continues until the tube is corroded away (1%'). formation of the film. I n the cases where a light, fluffy, Many workers have reported copper to be noncatalytic. sootlike carbon is formed, this does not adhere to the walls It seems likely that i t may have a slight activity for dehydroand much of it may be carried out of the tube with other genation which results in the formation of olefins. This has reaction products. This might appear advantageous for been shown specially with propane although the same catalyst keeping the tube free from carbon; however, this type of with butane was reported to favor cracking (63). carbon contains more or less of the metal of the tube so that If gold and silver have any activity it is probably small, the tube is eventually corroded. Such carbon apparently although tests should be made on specially prepared samples exerts some catalytic effect but the extent has not been with large, potentially catalytically active surfaces before definitely determined, this conclusion is final, O
\-
'
\-
I
i
Discussion of Data In presenting the data, the various metals have been divided according to the groups of the Periodic Table. Alloys are included in a separate table. The form of the metal (tube, powder, gauze, etc.), the hydrocarbon studied, the temperature range covered, and the catalyzed reaction in so far as it could be determined, are shown. I n attempting to explain these reactions, particular attention has been paid to the possible primary reactions of the paraffin:
Group I1 Magnesium (Table 11) favors the complete decomposition of the paraffin hydrocarbons to give carbon and hydrogen. Magnesium carbides are also reaction products. The carbides may be formed directly from the hydrocarbon and magnesium without the carbon appearing in the free state as an intermediate product. Since magnesium carbide can also be formed directly from the elements, one cannot definitely choose between these two possibilities.
INDUSTRIAL AND ENGl:NEERING CHEMISTRY
1092
Group V
Calcium has a mild tendency to favor complete decomposition of paraffin hydrocarbons. It seems likely that a more detailed investigation will reveal the presence of carbides. Zinc is inactive under the conditions studied. Cadmium and mercury have not been studied under conditions which would reveal their catalytic properties as liquids or solids; as vapors they are said to favor the formation of liquids from methane (94, 96).
Paraffin reactions in the presence of the metallic members of this group have not been reported. In view of the possibility of vanadium, columbium, and tantalum forming stable carbides, one would predict that these carbides would be formed with the liberation of hydrogen from paraffin hydrocarbons. Arsenic, antimony, and bismuth, since they have no such affinity for carbon, would not be expected to catalyze the complete decomposition reaction.
TABLE 11. GROUPI1 Catalyst Form
Hydrocarbon
Groups VI and VI1
Catalytic Properties and Reactions Ocourring
Temperature, C.
Chromium has not been studied as a catalyst with paraffin hydrocarbons although the writers' experience has shown that chrome-iron tubes substantially reduce carbon formation in commercial cracking units. I n the massive state both molybdenum and tungsten are practically noncatalytic. I n the powdered form, both catalyze the formation of carbon and hydrogen with the formation of the corresponding carbides (Table IV). Tellurium is reported to favor olefin formation, especially in the presence of iodine. It appears possible for tellurium to act as a hydrogen carrier in dehydrogenation according to the following scheme:
Reference
MAQNEBIUM
Powder Filings Powder Powder Powder Powder
CH4 CHI CH4 CZH6 n-CnHn n-CsHis
C Hz, MgzCs, MgCz Ckrbon C Hz C Hz carbides C : Hz, MggCs, MgCz C, Hz, MgzCs
!00-780
Red" "Red" "Red" 515-761 500-830
+ +
[%I
(60)
CALCIUM
Turnings
CzHn-CaHs
Weakly catalytic for C Hz
(16)
No effect a t 25-40 kg./ sq. cm. pressure
(42)
+
550-800 ZINC
CH4
Dust Liquid Vapor
+ vapor
465-518 CADMIUM
C H4 CH4
400-1100 Liquids 850 No effect reptd.
(96)
+
(44)
MERCURY
Vapor
CHI
(94)
Liquids
400-1100
Groups 111 and IV Aluminum favors complete decomposition; the metal unites with the carbon to form carbides (Table 111). Thallium and indium are also in this group and are worthy of testing although no results have so far been reported. TABLE111. GROUPSI11 AND IV Catalyst and Form AI dust AI dust Si lump Ti lump Sn, liquid in boat Ce, not given
Catalytic Properties Temperaand Referture, ' C. Reactions Occurring ences 660 C 4- HI; AhCs 660 H I ; Al&s C Noncatalytic 550-950 Noncatalytic 550-950 iooo Noncatalytic ~... No effect reptd. 850
Hydrocarbon CH4 CZH6 CzHeCsHs CzHn-CaHs CH4 CH4
+
Silicon, titanium, and tin are stated to be noncatalytic although silicon and titanium have not been studied in the proper form to detect slight activities if they were present. Tin did not act catalytically with methane. TABLEIV. GROUPS VI Catalyst Form
Hydrocarbon
Temperature, C.
AND
VI1
Catalytic Properties and Reactions Oocurring
References
MOLYBDENUM
Powder
CHI
700-800
Wire Wire
CH4 C2HvCsHs
900-1100 550-950
Carbides ( 7 4 % C in carbide) No effect reptd. Noncatalytic
(37) (85, 26) (16)
TUNQBTEN
Powder Powder Wire Wire Wire Not given Rods
CHI CHa CH4 CH4 CHI CH4 CzHsCsHs
On pumice
CsHs
800-900 1000
900-1150 1000-2300 2200-3000 850 550-950
+ + +
Hz tungsten carbide Hz free C No effect reptd. tungsten carbide Hz No catalytic effect reptd. No effect reptd. Noncatalytic
TELLURIUM
IZ + catalyst favors ole-
550
fin formation
(86)
M. A . . . .N.O-A . .N ..E-R-F-:
Powder Not given Lumps
CHa CHI CzHe-CsHs
600-900 850 550-950
+
VOL. 31, NO. 9
Hz carbides No effeot reptd. Noncatalytic
(38)
%!,'
I
+ +
C,Hm + z Te + CnHtn H2Te H2Te+ H2 Te Tellurium under these circumstances fulfills the requirements of a dehydrogenation catalyst according to the intermediate compound theory. Manganese in powder form unites with the carbon of the paraffins to form carbides. I n the massive state the activity is practically zero.
Group VI11 This group includes the metallic elements with the most powerful catalytic properties so far reported for metals. Iron, cobalt, and nickel all strongly promote the formation of carbon and hydrogen. There is little doubt in the reported literature (Table V) that nickel is a more effective catalyst than iron. Of the pair, cobalt and nickel, it is not possible to say which is the more active. Both are suitable for bringing about the equilibrium, CHa F', C 2Hz
+
in the 300-700" C. temperature range. Our experience with a mixture of liquid paraffins shows that cobalt is the more active. The catalytic carbon formed in these reactions has characteristic properties; it is light, fluffy, sootlike material and contains some of the metal. A piece of 150-mesh nickel gauze was almost completely disintegrated by a few liters of propane a t 800" C. (20). Usually, with even a moderately active catalyst the carbon deposit plugs the tube in a short time. The catalytic effect of iron is important not only in metal equipment but also for refractory reaction equipment. It has been shown that firebrick which contain considerable iron compounds disintegrate in the presence of methane a t 800" C. The methane reduces the iron compounds to form active spots where carbon is deposited in increasing amounts so that the firebrick ruptures and ultimately disintegrates. The deposits consist of a strongly magnetic iron compound, carbon, and iron carbide (66). What is the mechanism of this carbon-forming reaction? Even in the simplest case (methane) are we to imagine that the hydrogens are all simultaneously removed? If not simultaneously, then what are the intermediates? The answers to these questions are not complete but data are available which have a definite bearing on the subject.
INDUSTRIAL AND ENGINEERING CHEMISTRY
SEPTEMBER, 1939 TABLE V. Catalyst Form
Hydrocarbon
is the intermediate. On this basis, in the case of methane, the reaction can be written,
GROUPVI11
Temperature, O C.
Catalytic Properties and Reactions Occurring
IRON
On pumice Wire Powder Tube Wire Powder Tube O n clay or coke
Wire
CH4 C H4 C H4 CHI CHI CHI CHI CHI C H4
“Reduced iron” Not given Turmnas Gauze and strips Gauze Powder Gauze Gauze Gauze Tube Tube
CHI C HI “Coal Las” CiH.&Hs CzHsCaHs CaHs CaHs CaHs CaHs-C4Hio CsHa-C.rHlo n-C4Hm
Tube
iso-C*Hlo
550-600
On pumice
CH4
310-740
On olav
CHI
470-620
On coke or cIay Not given On MgO On kieselguhr Strips
CHI 1000-1200 CHI Not given CzHs Hz 184-255 CzHe HZ 255-280 CzHaCaHa 550-950
350-900 650-800 700-800 875-1100 900-1100 910 950-1050 1000-1200
Incandesernt
Nit&en Not given 1100-1200 550-950 580-876 760 760 830 580-975 Ca. 600 500-600
COBALT
+ +
CH4
References
+ + + + + + +
CHI CHI
320-390 340-680
Reduced Powder Powder Reduced Powder
CH4 CHI CHI CHI C + 2Ha
420-500 470-620 500 500 500-510
+
++ Hz Hz + Hz C + Hi C + Ha C + Hz No effect C + Ha C + Hz C + Hz
C C C
C3HS F? C3H6 .. (98)
yo) SO)
Practicallv noncatalytic Practically noncatalytic ~
.+ Hz equilik rium, below 668 , CoaC C + Hz eauilibrium C + H1 C + Hz 2CH4 2CH4 C + Hz
CH4 CH4 CH4 CHI CH4 CH4 CHI C H4 CH4 CH4 CHI CZH6 CzHe CZH6
Gauze On kieselguhr Powder
CzHe-CaHa CaHs Hz CaHa
550-950 157-184 200-405
Powder Gauze
C8Ha CaHa
780 800-830
600-700 600-780 6.50 .-. 800 850 900 1000-1200 1000-1500
+ Hz +
Not given Not given Not given 157-184 325 500-600
Gause On chamotte Tube
C~HI-C~HIO 510-700 CaH4-CcHlo 800-700
HID
605
Tube
iso-C4Hn
600
Tube and gauze
Natural gas
700-880
Reduced
n-CsH1z
320
Reduced powder Reduced on pumice
n-CsH1z n-Ci6Har
350-400 350-600
+ HZ
The propene was not isolated; the investigators assumed that it reacted as rapidly as formed in the reaction: CaHe + CHI HI 2C
(16)
+ +
($0) (97) (13)
This assumption can be justified on the basis of the observation that under comparable conditions olefins decompose to carbon and hydrogen more readily than the corresponding paraffins in the presence of iron and nickel (41). Obviously the key to this problem is the isolation of the intermediate olefin reaction products. A step in the direction of indicating the mechanism of these reactions is to put another substance into the system to unite with the intermediate. This has been done with both ethane and propane in the presence of nickel catalysts (67,68) and ethane in the presence of cobalt (84)with hydrogen as the added substance. Ethane reacts :
(41)
(41)
C
(48, 71)
g3
(94) (84)
(84)
(16)
CzHs
+ Hz +2CH4
and propane reacts similarly: om.)
On silica gel and on alumina On asbestos Powder ~. On pumice Powder Deposit on bulb On clay or coke Tube On kieselguhr Powder Not kiven On kieselguhr Powder On pumice
++
+
ZM +? MzC 2H1 M,C 8 z M C
where M is iron, cobalt, or nickel. This whole picture probably has superimposed on it the system of solid solutions of metal, carbide, and carbon. I n the case of propane, Frey and Smith (30) were of the opinion that in the presence of nickel one of the primary reactions was:
C Hz, FeaC C Hz Hz C Hz C No effect Hz C C Hz C Hz C HI, FeaC
NICKEL
Powder Powder
1093
CrHs
(49)
+ ++ + ++ +
From a study of the reaction rates and the temperature coefficients of these reactions, it was concluded (57): “In the presence of excess hydrogen, the ethane undergoes a dissociative adsorption to methyls which are converted quantitatively with adsorbed hydrogen to methane. With deficiency of hydrogen the activated adsorption of ethane continues beyond the stage of formation of adsorbed methyls a t the surface and the dissociative adsorption proceeds via CH2 and CH to C with simultaneous formation of adsorbed atomic hydrogen.” On this basis the nickel and cobalt catalysts can be regarded as cracking catalysts since they operate to break a carbon-tocarbon bond. Calculations from the same work (67) indicate that the apparent activation energy required to break the carbon-carbon bond is greater than that for the carbonhydrogen bond on the nickel surface studied. By this concept, if one is to utilize this catalytic cracking property of the metal with any degree of selectivity, then the catalytic dehydrogenating properties must be selectively poisoned. A scheme which was thought (58) to represent the reactions of the lower paraffins on nickel or cobalt surfaces shows both radicals and olefins as intermediate products; the hydrogen atoms formed are intentionally omitted from the diagram:
C Hz Hz C HZ C C Hz C Hz Hz C C Hz No effect C Hz C 4- Hz HZ C 2CH4 C HI, CHa 10-sec contact time CZHP Hz’ 400: sec., C ,Hz, kH4 C Hz CzHe CHI 2CHa c; (CaHe Hz) 4CH4 2Hz
+
+ +
++
+
+
+ 2C + C 4- Hz + CH4
+
+ +
1 sample, C HZ CH4; 2 samples in-
active C Hz C Hz Practically nonoata,-.L:-
+ +
($0) , ,
(97) (65)
,, I \
ly LilV
(4’1
+,
(41)
Practically noncatalytic C Hz, the C contamed Ni C544, CZHB,C3H8 L
+
C, Hz, CH4 Gas CHq 4- Hz. lihuid slightly ole: finic; considerable naphthenes and aromatics
(18
.
+ Hz +CrHe 4-CH4
_,
$8
I
I
($8)
Carbides have been isolated in the case of each metal-iron (6, 72), cobalt ( 7 l ) , and nickel (70). Further, in the case of nickel the carbide can be isolated only below 480” C . although 4 the catalyst does not seem to change in activity in this range (70). Similarly, cobalt carbide can be isolated only below ’ 668“ C. (71). These observations are consistent with the intermediate compound theory of catalysis where the carbide i
1
While this scheme is attractive from some points of view, it must be remembered that the presence of the radicals on the surface of the catalyst has not been proved. Further, the dehydrogenation of a paraffin hydrocarbon to the corresponding olefin in the presence of such catalysts has been only incompletely demonstrated. The above system leaves much
I
1094
INDUSTRIAL AND ENGINEERING CHEMISTRY
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SEPTEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
to be desired as far as explaining actually what is taking place on such catalysts. By using a much simpler system, palladium and hydrogen, it is possible to obtain an analogy that is quite useful in arriving a t a more detailed hypothesis to explain a number of the observations made in studying the hydrocarbon systems. I n the case of palladium and hydrogen there is considerable evidence to indicate that the absorbed hydrogen is completely dissociated into protons and electrons (13, 66). The energy required for such a dissociation is 31.4 electron-volts or 721,900 calories per mole (1 e. v. = 23,054 calories per mole). This energy requirement has been considered and, by a careful analysis of the processes occurring, has been met (28). Briefly, the protons and electrons in the process are not free but are associated with the palladium. The elcctron work function (the minimum potential required to separate an electron from the metal) of palladium is 5 e. v. (IQ), and since
1095
Following the above analogy to hydrogen in palladium, a part of it can be furnished by the work function of the electrons. The electron work function of the metals under consideration are: 7-Fe = 4.77 e. v. (76) Co (cube) 4.25 e. v. (11) Co (hexagon) = 4.12 e. v. Ni = 5.02 e. v. (87, 33) =I
For purposes of approximation, the value of 5 e. v. will be used as the work function of the electron. Again following the hydrogen in palladium analogy, the protons have a work function. This value is not available but the value 11 e. v. will be assumed as part of this hypothesis. This is the same value that has been assigned to the proton work function for palladium. The additional assumption is made that the hydrocarbon positive ions also have a work function of 11 e. v. (This value actually may be somewhat more or less than 11 e. v.) On the basis of these assumptions there are 16 e. v. available in each decomposition step indicated for the methane and ethane schemes. These figures are given TABLEVI. IONIZATION ER'ERGIES USEDIN MAKINGCALCULA- as approximations to indicate that the energy required and TIOKS FOR REACTIONS IN TABLE VI1 the energy available are sufficiently close in magnitude to Energy Required make the ionization hypothesis tenable under these conditions. Ionization Electron-Volts References Without resort to such quantitative figures, the hypothesis H -f H + 13.5 (77) merely says that the catalysts iron, cobalt, and nickel act as CH4 + CH4' 13.1 (77) CHa -+ CHa+ 10 (av.) (39,7 7 ) if they were hydrocarbon solvents which are capable of ionizCHz -+ CH;+ 12.0 (773 CH -+ f.H .14.0 (av.) ing the hydrocarbons and that the hydrocarbon reactions 11.2 c-fc occur through the ionic state. CZH6 + CZH6' 11.6 (89) Using methane as an example, the first step in the reaction is CzHa -+ CzHs+ 9.8 (39) C Z H-+ ~ CZH4+ 10.8 (47) pictured as the reversible adsorption or solution of methane CzHs -+ CzHa+ 10.9 (39,47) C2H2 + CzH:+ 11.2 (80) on the catalyst surface. A part of the methane then ionizes. CzH + CzH 13.9 (av.) (39) The ions undergo the decomposition reactions of the types cz -+c2+ 16.5 (47) that have been illustrated in Table VII. As soon as any hydrogen ions are formed, they are pictured as being in equitwo electrons are involved this will furnish 10 e. v. Similarly, a work function of the positive ion (proton) of about 11 e. v. has been proposed (28); since there are two of them, this TABLE VII. PROPOSED REACTION TYPESBY WHICHPARAFFINS furnishes about 22 e. v. or a total with the electrons of about MAYREACTCATALYTICALLY ON IRON,COBALT, OR NICKEL I 32 e. v. which is the amount required to ionize and dissociate Energy Required I Electron-Volts the hydrogen (28). In explaining the large work function of Methane, Scheme 1 the proton in palladium, it has been postulated that the proton becomes associated with a large number of practically free CH4 S CH4+ + E 13 1 CHn"$ CHs+ + H + + E 14 9 electrons (i. e , the electrons that are responsible for electrical CHa+ S CHz+ + H + + E 19 l conduction) in the metal with the result that an electron CHI++ CH+ + H + + E 19 o cloud is formed around the proton. This is analogous to the CH++C+ + H + + E 14 2 ion cluster or cloud postulated by the Debye-Huc' el theory -11 2 C+ + E - = C H + + E - S H -13 5 for ions in solution in water. The proton work function is 2H S Ha - 4 4 then the separation of the proton from this electron cloud XC + C, (graphite) - 5 4 (28). In addition, there is some evidence (87) to indicate Methane, Scheme 2 that the proton unites with the palladium to give PdHf. A small part, 0.3 e. v., of the work function energy is required CHI e CHa + H ++ E 18 0 CHa 2 CHz + H + + E 16 6 to decompose this compound. C H z e C H + H + + E16 5 There is evidence to indicate that a similar situation exists CH e C + H + + E 16 5 with other metals, particularly iron and nickel (28, 29) and Other steps as in scheme l probably- cobalt a t elevated temperatures. The same reaEthane, Scheme 1 soning with appropriate modification can be applied to hydroCzHe e CzHa+ + E 11 6 carbon ions. By this analogy the hydrocarbon in the metalCzHs e CzHs+ + H + + E 14 6 lic catalyst surface would become dissociated and ionized CzHs+ S CzHa+ + H + + E 17 O just as hydrogen seems to be in palladium. The reactions CzH4" S CzHa+ + H + + E 16 7 CzHs+ S CzHz' + H + + E 17 l occurring would be the reactions of hydrocarbon ions, proCzHz++ C z H + + H + + E 19 7 tons, and electrons. There are numerous ways in which hyCzH+ S Cz+ + H + + E 19 9 drocarbon ions might react. For the present it will suffice Cz+ + E - e Cz -16 5 to illustrate two of these for each methane and ethane. By H + + E-*H -13 5 nC? e C, (graphite) About - 5 3 using the energies of the various states of methane (7'7) and 2H F? Hz - 4 4 ethane (SQ),together with the various ionization potentials given in Table VI, it is possible to estimate the energy reEthane, Scheme 2 quired for each postulated reaction step in the reaction 11 6 CZH6 S CZHB"+ E schemes (Table VII). 12 2 CZH6' e ~ C H S + + EFor the reaction types illustrated in Table VII, it is neces(Cf. methane, scheme l, for CHs+ reactions.) sary for the indicated energy to come from somewhere.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1096
librium with unionized hydrogen atoms and hydrogen molecules. I n turn the hydrogen molecule in the catalyst is pictured as being in equilibrium with hydrogen molecules in the gas phase. On account of their size the hydrocarbon ions are pictured as remaining on the surface of the catalyst; the protons and electrons are pictured as being able to move throughout the metal. At the end of the decomposition the carbon ions are pictured as being in equilibrium with carbon atoms, which in turn are in equilibrium with solid carbonfirst in solution in the metal (a part of it may be in the form of carbides), and finally the solid carbon reaction product. No ion is pictured as leaving the catalyst. Since the final reaction products are electrically neutral, no large work function energy must be supplied to release the final products from the catalyst. The energy of neutralization (deionization) of the carbon ions and the protons is regarded as sufficient to separate these ions and the requisite electrons from the catalyst. This whole process is just the reversal of the ionization steps for the starting material. Methane can be synthesized from carbon and hydrogen by means of these catalysts. The above hypothesis is regarded as applying equally well to this reaction. In other words, the entire reaction system is reversible. Just as any other reversible reaction, the direction of the reaction is dependent on the temperature and the concentrations of initial and final products. Besides giving an insight into the possible reactions of hydrocarbons in the presence of these catalysts, the above hypothesis is able to explain a number of experimental observations: 1. The reactions
are known (66) to occur in the temperature range 138” to 255” C. on activated nickel. The first has an energy of activation of about 19,000 calories and the second about 28,000 calories per mole. The original workers (66) said: “The conclusion is reached that the activated adsorption of the hydrocarbon is essentially a dissociative adsorption.” On this and other evidence they concluded that the dissociation was to hydrocarbon radicals, methyl, methylene, etc., and hydrogen atoms. The proposed ionic hypothesis seems to fit the observations quite as well as the radical theory. 2. The second scheme for ethane gives a hint as to why ethane is not synthesized from the elements in the presence of these catalysts. If any activation energies involved are comparable the reaction removing methyl ions by the route, CHs+
+ H+ CHI+ + E- (energy change = -14.9 + EC2H6+(energy change
greatest concentration of electrons (i. e., the largest number of electrons per cubic centimeter) would be expected to form the densest electron cloud and have the highest positive-ion work function. These should be the metals having small atomic volumes accompanied by several valence electrons (28). On this basis polyvalent elements having the largest electron work functions and the smallest atomic volumes should be the best catalysts. Table VI11 lists a few of the elements in order of increasing atomic volumes. Nickel, cobalt, and iron head the list in the order named. Copper comes next in the list. It will be recalled from Table I that copper has a mild tendency to decompose the paraffin hydrocarbons completely. This is a significant parallel, for in most other respects copper is quite different from the iron, cobalt, and nickel group. This hypothesis would predict that the element rhodium would have a tendency completely to decompose the hydrocarbons into carbon and hydrogen. Thorium and barium would not be catalysts for these reactions.
ELECTRON WORKFUNCTIONS AND ATOMICVOLSOME POLYVALENT ELEMENTS
TABLE VIII.
UMES OF
Element Nickel Cobalt Iron Copper Rhodium Palladium
Atomic Volume 6.6 6.8 7.1 7.1 8.3 8.8
Electron Work Function ElectronVolts 5.02 4.2 4.75
4.07 4.58 4.9s
Electron Function Work Element Platinum Tungsten Molybdenum Tantalum Thorium Barium
Atomic Volume 9.2 9.8 10.7 10.9 20.7 36.2
ElectronVolts
6 30 4.75 4.15 4.15 2.6 2 07
The hypothesis has served to correlate a number of observations; it has explained several others and it has served to make predictions which only future experiments can verify or disprove. So far, consideration of the hypothesis has been restricted to the paraffin hydrocarbons. It seems apparent that the general principle of reaction by ionization under similar conditions is equally applicable to other hydrocarbon types.
Palladium and Platinum Palladium favors the complete decomposition of methane into carbon and hydrogen. This is in agreement with the preceding hypothesis. I n the case of ethane the hydrogen-ethylene equilibrium occurs. This is the uncatalyzed thermal reaction of ethane and hence cannot be regarded as evidence that palladium is a dehydrogenation catalyst. I n the case of the higher paraffins the evidence is conflicting.
e. v.)
would take preference over the reaction, 2CHs+
VOL. 31, NO. 9
TABLE IX. GROUPVI11 Catalyst Form
Hydrocarbon
Temperature, C.
= -12.2 e. v.)
At this point it should be mentioned that there is nothing critical about ethane scheme 2. A whole series of schemes may be considered wherein the carbon-carbon break occurs a t any of the steps in ethane scheme 1. 3. To be a catalyst for these reactions, the catalyst must have work functions for electrons and positive ions, the sum of which is equal to about 12-16 e. v. The elements iron, cobalt, and nickel have electron work functions among the highest known. As for the positive-ion work functions there is no direct evidence. On the basis of the electron cloud explanation of the positive-ion work furction, metals having the
Catalytic Pro erties and Reactions 8courring
References
PALLADIUM
“Black” Wire Supported On asbestos O n asbestos Black” “Black” “Black” “Black” “Black”
CH4 CH4
CZHE
CaHs CsHs iso-CsHs n-CsHir n-CeH14 n-C.iHie n-CsHls
1
250 “Red” 510 568 700 300 300 300
++
C Hz C Hz CZHEF? CZH4 HZ Slight activity for C formation Noncatalytio Hz and unsatd. hydrooarbons None
(100)
H2 and unsatd. hydrocarbons
(81, 8 2 )
+
(99)
(14)
g;)
(80) (81, 8 2 )
PLATINUM
Tube Ribbon Filament
CHI CHI CHI
Not given Wire Strips Filament Foil On asbestos On pumice
CH4 CH4 CZHE CzHe CzHsCaHs CaHs C3Ha
+ +
1000-1500 HZ C. only slightly catalytic 1100-1278 HZ C : part of C was dissolved in P t CHZ radicals; 1100-1400 1100-12b0° C., HZ 1200-1400°, Hz, H, C H Z Not given r. -c w. Not given 800-1200
1100-1400 550-950 568 700
+
74)
34)
SEPTEMBER, 1939
INDUSTRIAL AND ENGINEEHING CHEMISTRY
1097
TABLE X. ALLOYS Cat a1ys t Form
Catalyst Composition
Brass ;I$43
Cyteel
Hydrocarbon CHI CH4 C H4 CHI CzHa CaH6
Temperature,, C. 600-700 600-700 910 1050 777-855 9900
Gauze Turnings Tube Tube
C2HsCsHs CaHs CsHs CsHs
550-950 555-880 555-585 600-700
1 8 7 Cr-8% Ni steel Niogrome Ca-Si Steel 28% Cr steel KA2 stainless steel CaCz Monel metal Sic 18% Cr-8% Ni steel
Tube Gauze Granules Gauze Tube baffles Tube Granules Gauze Granules Tube baffles
CSHB CsHs CsHs CsHs CaHs CsHs CaHs CaHa CaHs CaHa
653-816 690 700 800-842 800-850 815 830 840 805-977
Monel metal
Tube
n-CaH~o
500-600
C r - 8 g N/ steel Cr-8 Ni steel
Tube Tube
n-C~Hla n-C~Hlo
525-550 600-650
KA2 stainless steel 2 8 7 Cr steel 18% Cr-8% Ni steel Monel metal
Tube Tube Tube Tube
n-CaHla n-C4Hio n-CdHlo iSO-CaHlo
775-825 800-842
886-904
Cr-8% Ni steel Cr-8% Ni steel
Tube Tube
iso-CaHio iso-C~Hlo
550 650
Tube Tube Tube
iso-C4Hio Natural gas Natural gas
800-825 630-1040 670-750
r-8'7 Cr-8
Ni steel Ni steel
+
+
KA2 stainless steel Steel Nlonel metal
++ baffles baffles
800
500-600
Catalytic Properties and Reactions Occurring
Referenoe
Cu gauze Hz above 8OOo C Noncatalvtic Noncatal$tic (50 kg./sq. ,om.) Noncatalytic after poisoning 7 kg./sq. om.) (atm. Noncatalytic Noncatalytio Slight dehydrogenation Hz C Noncatalytic Noncatalytic No catalysis reptd. C Hz No catalysis reptd. Ha; C contained Slight for C Cr Ni 50O0, noncatalytio; GOO', C HZ Noncatalytic (50 kg./sq. ,om.) Noncatalytic after poisoning 7 kg./sq. om.) (atm. Noncata1yt;c Noncatalytio Ha Slight fcr C 500", noncatalytic; GOO", C Hz Noncatalytic (50 kg./sq. om.) C Ha (atm.. 7 kg./s,q. om.); noncatalytio after poisoning Noncatalytic Hz Aromatics; slight for C Aromatics; slight for C Hz; some NiaC formed
+
+
+
+
+
+
+
+
+
+
+
+
++
26% Cr-1.5% Mn-0.970 Si steel Tube Natural ga8 750-1050. No catalysis reptd. (7) 20% Cr-9% Ni steel Tube Natural gas 750-1050 No catalysis reptd. (7) Calorized" steel Tube Natural gas 760-1050 No catalysis reptd. (7) Natural gas 800-900 Nichrome Tube Noncatalytic (64) 6 I n this experiment a carbon rod heated t o 990' C. was located coaxially in a water.-cooled tube: the bronze gauze was placed between the rod and the tube.
One worker reports the formation of hydrogen and unsaturated hydrocarbons from Cg, Ca, C,, and Cg paraffins (81, 82). As yet the corresponding olefin hydrocarbons have not been identified as products, and the reaction is unconfirmed. Another worker has found palladium to be without effect under similar conditions with n-hexane (100)as Table IX shows. Platinum seems to be less active than palladium. If any reaction is favored, it is complete decomposition. Some of the carbon so formed dissolves in or combines with the platinum. Alloys Of the alloys for which data are available (Table X) those containing iron, cobalt, or nickel show the most pronounced catalytic effects. The most active seems to be Monel metal (60 Ni, 33 Cu, 7 Fe). Next is 18 chrome-8 nickel steel, although here attention must be called to inconsistencies in the experiment. Egloff, Thomas, and Linn previously reported (21) that 18 chrome-8 nickel steel tubes were very active in causing the complete decomposition of propane and butanes into carbon and hydrogen. After the activity of the tubes was poisoned with hydrogen sulfide, it was possible to study the uncatalyzed thermal reactions of propane and the butanes. One possible explanation of the inconsistencies in the reports on the activity of 18 chrome-8 nickel steel is that the metal has been poisoned either during fabrication or storage, The other alloys are either noncatalytic or only slightly active. Many of these materials are therefore satisfactory for constructing equipment for hydrocarbon reactions. Particular attention is called to the ability of chromium to decrease and destroy the activity of iron (28 per cent chrome steel) and nickel (Nichrome) when present in alloys to considerable extents.
Commercial Petroleum Cracking in Metal Equipment From the data presented in Tables V and X it is clear that iron and its simple alloys have powerful ca,talytic action on paraffin hydrocarbons, giving complete decomposition to carbon and hydrogen. Many of the crude oils and their fractions contain considerable amounts of paraffin hydrocarbons; yet they can be satisfactorily cracked in iron and steel equipment with no evidence of the complete decomposition to the sootlike catalytic carbon. As a matter of fact, a tube in a cracking coil builds up a hard, dense liner of carbon which is in direct contrast to the catalytic carbon. Thus in the laboratory when pure compounds are used, one result is obtained, while in commercial practice a different result is obtained with a mixture. It is natural to attribute the difference to some of the other components of the mixture. In view of our experience (21) of poisoning the catalytic activity of 18 chrome-8 nickel steel with hydrogen sulfide, we are inclined to the theory that the sulfur compounds in commercial charging stocks form a film of inert iron sulfide on the interior of the tube and thus permit the deposition of the hard, dense carbon liner. This theory finds some support in the well-known observation that sulfur compounds are powerful poisons for iron, nickel, and cobalt catalysts for other reactions. Sulfur compounds are not the only possible poisons for this activity of iron. Both steam and carbon dioxide can react with iron to form a strongly adhering film of Fe304 that seems to prevent further activity of the iron (36). Thus it seems quite likely that petroleum cracking in steel equipment is possible because some constituent of the petroleum acts on the iron to destroy its catalytic activity and prevent the formation of the catalytic carbon that would be expected from the results obtained with pure compounds
INDUSTRIAL AND ENGI NEERING CHEMISTRY
1098
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I
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