COPPER AS CATALYST FOR THE HYDROGESATIOS O F BENZENE V. N. IPATIEFF, B. B. CORSOS,
AND
I. D. KURBATOV
Research Laboratories, Universal Oil Products Company, Chicago, Illinois Received December 9, 1038 INTRODUCTION
In 1905 Sabatier and Senderens (16) concluded that copper was unable to catalyze the hydrogenation of benzene. Twenty years later, Pease and Purdum (13) reported the hydrogenation of benzene at ordinary pressure in the presence of copper as catalyst. We have reinvestigated this problem and have found that pure copper can just barely hydrogenate benzene at ordinary pressure but that it readily does so under superatmospheric pressure. The authors have found that the hydrogenating activity of copper is so susceptible to impurities that the purity of each sample must be confirmed by spectrographic analysis. Their experiments show that small amounts of nickel, chromium oxide, etc. activate copper sufficiently to enable it to hydrogenate benzene at ordinary pressure. For example, the presence of 0.007 per cent of nickel (1 nickel atom per 13,000 copper atoms) makes it possible for copper to hydrogenate benzene to the extent of 8 per cent in a contact time of 180 sec. With 0.01 per cent and 0.04 per ccnt of nickel, benzene is hydrogenated 21 per cent and 36 per cent, respectively. On the other hand, traces of certain other impurities, as, for example, lead, poison the copper catalyst so that it can not hydrogenate benzene even under superatmospheric pressure. Pease and Purdum apparently assumed that nickel and cobalt were the only pertinent impurities. They reported that their copper catalyst was free from nickel and cobalt, but that it contained a trace of iron. They gave no information as to their method of analysis or as to its accuracy. In order to ascertain whether copper can catalyze the hydrogenation of benzene it was first necessary to prepare pure copper catalyst. Xunierous samples of copper oxide and copper carbonate were analyzed; all were found to be impure. Pure copper preparations were finally obtained by precipitation. The final step in the preparation of copper catalyst was the reduction of copper oxide by hydrogen. The hydrogenating activity of the reduced catalyst was evaluated by passing a mixture of 6 to 7 volumes of hydrogen 589
590
V. N. IPATIEFF, B. B. CORBON, AND I. D. KURBATOV
and 1 volume of benzene over it a t 225OC. and ordinary pressure and determining the amount of cyclohexane in the product. Pure copper catalyst was usually prepared by the decomposition and reduction of basic copper carbonate, which had been precipitated from the nitrate by ammonium carbonate. Spectroscopic examination showed that the reduced copper contained less than 0.005 per cent of nickel, cobalt, iron, lead, or tin, and not more than 0.01 per cent of aluminum, chromium, calcium, or magnesium. Electrolysis, however, revealed the presence of not less than 0.2 per cent of oxygen. Such copper, containing oxygen, catalyzed the hydrogenation of benzene a t ordinary pressure and 225OC. with difficulty (1 per cent hydrogenation in 300 sec. contact time), but it readily hydrogenated benzene a t 350°C. under a pressure of 150 atm. of hydrogen. Pure copper was also prepared by the method of Ipatieff and Werchowsky (11, lo), which consists in the reduction of copper ions by hydrogen under pressure. The original copper sulfate was purified by recrystallization, followed by precipitation with hydrogen sulfide and subsequent conversion of sulfide to sulfate by nitric acid. The metallic copper precipitated by hydrogen analyzed 100.00 f 0.05 per cent pure by electrolysis, and the spectroscope showed it to be as free from other metals as the sample described above. This copper did not hydrogenate benzene a t ordinary pressure and 225OC. or even under 150 atm. of hydrogen a t 350°C. However, a copper preparation obtained by reworking this inactive copper (solution in nitric acid, precipitation as carbonate, decomposition, and reduction) was able to hydrogenate benzene under superatmospheric pressure. In view of the fact that 100.0 per cent copper precipitated by hydrogen in the form of large microcrystals did not hydrogenate benzene, whereas 99.7 per cent copper prepared from basic carbonate, containing oxygen as impurity, and possessing a structure on the borderline between crystalline and colloidal according to its x-ray picture, did hydrogenate benzene, the following questions arise: Is the inactivity of the first type of copper due to insufficient surface area, or is the catalytic activity of the second type of copper due to the activating effect of oxygen and/or water?' The net result of a catalytic process such as the hydrogenation of benzene in the presence of copper is due to the combined effect of many variables. These variables fall into two main divisions: ( 1 ) the experimental conditions of precipitation, of reduction of the oxide, and of hydrogenation of the benzene, and (2) the properties of the catalyst, such as chemical composition, development of internal structure, and stability of lattice. It is only when all of these conditions are favorable that the best results can be expected. 1
See Ipatieff on the rSle of oxides in catalysis (8).
COPPER AS CATALYST
.
591
The system copper-benzene-hydrogen is an excellent one for the study of the variables of hydrogenation catalysis, since the intrinsic hydrogenating ability of copper with regard to benzene is so slight that the importance of these variables is magnified. With an active catalyst such as nickel, which when properly prepared and promoted (9) is able to hydrogenate benzene quantitatively at 50°C. and ordinary pressure in a contact time of 1 sec., it is far more difficult to separate and evaluate the many variables which contribute to hydrogenation catalysis. In this study of the catalytic properties of copper one variable was changed a t a time, the other variables being held constant and favorable for the hydrogenation of benzene. Thus it was possible to determine the separate effects of the variables concerning the preparation and properties of copper hydrogenation catalyst. With a sample of copper oxide which contained about 0.1 per cent of nickel we studied the effect of the time-temperature conditions of reduction. The threshold temperature a t which copper catalyst rapidly loses activity is 350-400°C. The hydrogenating activity of copper catalysts can be estimated by microscopic examination of the surface. The catalytically active surface is a spongy structure of microcrystals with many fissures between the individual crystals. Three types of inactive surface are distinguishable under the microscope: (1) smooth, compact, and without crystal faces; ( 2 ) composed of smooth, round particles; and (3) made up of large microcrystals. Application of the radioactive emanation method made it possible t o correlate the emanating power (and therefore surface) of the copper catalysts with hydrogenating activity. A copper catalyst which contained 3.5 per cent of chromium oxide and whose relative hydrogenating activity was 28 a t 90 sec. contact time was found to possess thirty-nine times as much emanating power as a preparation of pure copper whose relative hydrogenating activity was 0. However, loss in catalytic activity due to heating a t 400°C. could not be directly correlated with the change in emanating power. EXPERIMENTAL
Preparatton and testing of copper catalysts Preparation of copper catalysts. The first copper catalysts, which were prepared by the reduction of C.P. copper oxides and carbonates, hydrogenated benzene a t 225°C. and ordinary pressure to the extent of 30 to 50 per cent, in a contact time of 90 sec. However, these copper catalysts were all contaminated with other metals, sometimes to as high as 0.2 per cent. The most common activating impurity was nickel. Preparation of pure copper catalyst f r o m basic carbonate and hydroxide.
592
V. N . IPATIEFF, B.
E. CORBON, AND I. D. KURBATOV
Precipitation was usually made with ammonium carbonate or hydroxide, the former being preferable. The precipitate was washed once, and the residual ammonium nitrate was removed by heating. Sodium and potassium hydroxide were also tried, but it was difficult to wash out occluded sodium and potassium nitrate, washing being continued until the filtrate gave a negative or very faint test for nitrate with diphenylamine reagent. In a typical preparation 2 gram-molecules of copper nitrate were dissolved in 4000 cc. of warm (distilled) water, and the filtered solution was placed in a 6-gallon earthenware crock, together with an additional 8000 cc. df warm water. To this solution was added, with stirring, a warm, filtered solution of 2 gram-molecules of ammonium carbonate in 4000 cc. of water. After standing for 1 hr. the mixture was filtered by suction. The filter cake was washed on a Buchner funnel with 500 cc. of water and then returned to the crock, where it was stirred with 16,000 cc. of warm water for 15 min. After standing for 1 hr. the solution was filtered. The chemicals used were of the highest reagent grade and their purity was checked, as well as that of the final catalyst. All utensils were washed with nitric acid and rinsed with distilled water just before use. The precipitate was dried at 180-190°C. for 36 hr. in a porcelain dish covered with a watch glass. The dry, fluffy, black powder was pressed into a thin cake (ca. 1.5 mm. thick) in a hydraulic press with a pressure of about 2 tons per square inch. The cake was cut up with a stainless steel knife into granules ranging in size from 6 to 10 mesh. The granules were heated in a stream of nitrogen for 20 hr. a t 4OO0C., and the resulting oxide was reduced in hydrogen. Precipitation of pure copper fromcopper sulfate by hydrogen under pressure. Pure copper was precipitated as follows: A 3515-cc. rotating autoclave of the Ipatieff type, equipped with a glass liner, was charged with 850 CC. of distilled water, 125 g. of copper sulfate pentahydrate, 4 g. of 96 per cent sulfuric acid, and 50 kg. of hydrogen per square centimeter. The bomb was rotated for 12 hr. a t 150OC. The yield of metallic copper was 28.8 g. (90.5 per cent of the theoretical). The filtrate was colorless at first but became blue on standing, owing to oxidation of cuprous to cupric ion by air. The matte of sparkling, mossy copper was washed several times with water and finally with alcohol. Reduction of copper oside. The last step in the preparation of copper catalyst was reduction of copper oxide in a stream of hydrogen. The hydrogen purification train (figure 1) consisted of copper gauze (A) a t 550°C., followed by anhydrous calcium chloride (B), Ascarite (C), and Anhydrone (D). The copper oxide (66.8 g.) was packed into a glass tube (F; 14 mm. inside diameter) and was held in place by plugs of glass wool. In case of shrinkage during reduction the catalyst was again packed by tapping. The conditions of reduction for oxide prepared from precipitated
593
COPPER .4S CATALYST
9 9
8
c
3
ti i;:
594
V. N. IPATIEFF, B. B. CORBON, A N D I. D. KURBATOV
carbonate were 20 hr. a t 225°C. or 15OoC., or 90 hr. a t IOO"C., 99.3-99.8 per cent reduction being obtained. With Kahlbaum granular oxide, which presumably had been calcined, the reduction period was 20 hr. a t 225°C. The standard hydrogen rate (measured a t room temperature) was 2000 cc. per hour per 66.8 g. of copper oxide. The free space of the reduced catalyst was determined after the hydrogenation run by adding water to the charged catalyst tube. Hydrogenation test at superatmospheric pressure. Twenty grams of reduced copper and 50 cc. of benzene were charged to the glass liner of an 850-cc. rotating bomb. The glass liner was closed with a glass stopper equipped with a capillary. The bomb was rotated for 12 hr. a t 350°C. under an initial hydrogen pressure of 100 kg. per square centimeter. The extent of hydrogenation was estimated from the refractive index of the liquid product. Hydrogenation test at ordinary pressure. A mixture of benzene and 6 to 7 volumes of hydrogen was passed over the catalyst a t atmospheric pressure and 225°C. The contact times used with the pure (inactive) catalysts varied from 80 to 440 sec. The activities of the impure (active) catalysts were compared on the basis of their performance a t 90 sec. contact time, as found by interpolation. Four hydrogenation periods (4 to 7 hr. each, depending on the gas rate) were run with each catalyst. Repetition of the original conditions showed that the activity was constant over much longer periods of time than those used in the test. The mixture of hydrogen and benzene was obtained by bubbling hydrogen through benzene contained in a spiral wash bottle (6), the hydrogen-benzene ratio being calculated from the vapor pressure of benzene. Experiment showed that this calculation was justified, inasmuch as the ratios obtained in four experiments a t 30.0"C. ranged from 5.18 to 5.35 (average 5.25), as compared with the theoretical value of 5.33. The liquid catalysate was collected in two (sometimes three) receivers which were cooled in carbon dioxide-trichloroethylene, and sometimes the gas was also collected. The receiver system (figure 1; G and H) and the technique of handling were patterned after the description of Baxter and Hale (2). The first receiver collected about 98.9 per cent of the liquid product, the second 0.8 per cent, and the third, 0.3 per cent. Analysis of hydrogenation product. The composition of the liquid catalysate, a mixture of benzene and cyclohexane, was determined by means of refractive index. The benzene and cyclohexane used in establishing the refractive index-composition curve were originally of C.P. quality and were further purified (15, 7, 3). The curve was plotted from the data2 in table 1. 2 The data of Burrow8 and Lucarini (4) are incorrect, owing to error as to temperature (private communication).
8
typographical
595
COPPER AS CATALYST
Method of culculation. The contact time in seconds a t the temperature and pressure of hydrogenation was calculated as free space divided by gas TABLE 1 Composition and refractive indices of benzene and cyclohezane miztures WEIQaT PER CENT OF
Bensene
I
Cyclohexane
I
7&Em
1.4266 1.4380 1.4510 1 ,4655 1 A824 1.5012
100.00 79.87 59.90
0.00 20.13 40.10 59.78 79.89 100.00
40.22
20.11 0.00 TABLE 2 Typical calculation
Hours on t e s t . . . , , . , . , . . . . . . . , . . . . : . . . . . , . , . , . , . . . . . . . . . . . . , . . . . . . . . . , . Excess pressure a t inlet, inches of water.. . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . Temperature of benzene, "C , , . . . . . . , . . . . . . . . . . . . . . . . . . . . . . , , . . . , . . , . , . Vapor pressure of benzene a t 27.3"C.,mm. of Hg Barometer (corrected), mm. of H g . , . . . . . . . . , . . . . . , , , , . , . , . . . . . , . . . . . . . . Average pressure in system, mm. of H g . . . , . . . . . . . , . . . . . , . . , , . . . , . . . . . . , Hydrogen-benzene volume ratio. . . . , . . . . . . , . , . . . . . . . . . . . . . . . . , . . , . . . . , . . Liquid catalysate: Grams . . , . . , . , , . . , . , . , . , . , . , , . . . , . . . . . . . . . . . . . . . , . . , . . . , . . . , , . nI ,' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Weight composition, 85 per cent benzene and 15 per cent cyclohexane: Benzene, grams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cyclohexane, grams . . , . , . . . . , . . . . . . . . . . . . . . . , . . . , , . . . . . . . . . . . . , . . . Gas volumes a t 225OC. and 745 mm. : Benzene cyclohexane, cc.. . , , . , , . . , . . . , , . . . . . . . . . . . . . , . . . . . , , . , , , , Inlet hydrogen, cc.. . . , . . . . . . . . . . . . , . , . . . . . , ..................... Outlet hydrogen, c c . . ,........ ..................... Average hydrogen, cc , . . . . . . . . .......................... Benzene cyclohexane f average hydrogen, cc.. . . . . . . . . . . . . . . . . . . . . ,
,
, , ,
+
,
+
Cc. of gas (225'C., 745 mm.) per second.. . . . . , . . . . . . . . . . . , . . . , , , , . Contact time, sec. (free space, 19.2 cc.) . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . ,
,
,
4.0 4.6 27.3 105 741 745 6.1 1.200 1 .4870
1.02 0.18 635 3870 3600 3735 4370 0.304 63
rate per second. The gas volume was taken as the sum of the benzene and cyclohexane volumes plus the arithmetical mean of the inlet and outlet volumes of hydrogen. A typical calculation is shown in table 2.
596
V. N . IPATIEFF, B. B. CORSON, .AND I. D. KURBATOV
Hydrogenation of benzene with copper catalysts Hydrogenation at ordinary pressure and 225OC. Copper prepared by reducing Kahlbaum copper oxide (which contained 0.1 per cent of nickel) for 20 hr. a t 225°C. hydrogenated benzene to the extent of 47 per cent in 90 sec. contact time. Pure copper catalysts (table 3, Nos. 1 to 8), prepared from precipitated hydroxides and carbonates and free from other metals but containing 0.2 per cent of oxygen, showed an average hydrogenation of 1 per cent a t TABLE 3 Hydrogenation of benzene with pure copper catalysts* at atmospheric pressure and
NO.
1 2 3
I
886°C.
PRECIPITANT
REDUCTION CONDITIONS
~ o u m
NHiOH NHiOH NHiOH
I
EYDROOENATION OF B E N Z E N E ( P E R CENT) T.) AT DIFFERENTCONTACTTIMES
(c.
T.'c.
Per cent
C. T.
1;
Percent
20
I
225
100 150
20
m
0 0
20
225
0
6
20
7 8
(NHdzCOs Hz Hz
C.T.
0
440
0
5
I
290
2 0 1 2 0 0 20 225
4
9 10
1
1
I
0
0
2
80
320
290 1
'130 150 ~
1,
I
O
0
240 340
290 310
597
COPPER AB CATALYST
pared from Kahlbaum copper oxide hydrogenated benzene quantitatively in a glass-lined bomb in 12 hr. a t 350°C. under an initial hydrogen pressure of 100 kg. per square centimeter. Pure copper catalysts (table 3, Nos. 1 to 8) prepared from precipitated hydroxides and carbonates hydrogenated benzene under similar conditions to the extent of 70 to 97 per cent. Pure copper catalysts (table 3, Nos. 9 to 10) obtained by the reduction of copper ions under pressure did not hydrogenate benzene in 12 hr. a t 350°C. under an initial hydrogen pressure of 100 kg. per square centimeter. Blank runs with benzene in the absence of copper showed that the equipment was completely non-catalytic under the conditions of the experiment. E$ect of temperature-time conditions of reduction of impure copper oxide u p o n hydrogenating activity of reduced catalyst. The study was made with granular (6-to 10-mesh) copper oxide (Kahlbaum-Schering, fur Analyse). BO
5 k 5
%
g *
bo 40
PO
OtO
40
60
nou.es
OF
a0
IW
It0
moucrio,v
FIG.2. Effect of temperature and time conditione of reduction on catalytic activity
The hydrogenating activity of this material was due to the presence of 0.1 per cent of nickel. The oxide was reduced a t dserent temperatures (150", 200°, 225", 300°, 350°, and 4OOoC.) and for different lengths of time (20 to 120 hr.). The hydrogen rate was 2000 cc. per hour (measured a t room temperature), a t which rate the oxide would require about 10 hr. for reduction, assuming complete utilization of the hydrogen. The amount of water produced by the reduction of 66.8 g. of oxide was 14.90 g., as compared with the theoretical value of 15.12 g. for cupric oxide. The water was collected in two receivers, one empty and ice-cooled, the other filled with Anhydrone. The catalytic activity of the reduced catalyst was measured by its ability to hydrogenate benzene a t 225°C. and ordinary pressure in a contact time of 90 sec. Figure 2 shows the effect of time and temperature of reduction upon catalytic activity. Pease and Purdum prepared their catalyst by reducing granular copper
598
V. N. IPATIEFF, B. B. CORBON, AND I. D. KURBATOV
oxide (source not given) in hydrogen for about 50 hr. a t 150°C., followed by 50 hr. a t 2OO0C., and finally several hours a t 300°C. They offered no evidence that this schedule was the optimum. According to our experiments it is evident that low temperature of reduction (e.g., 200°C. versus 400°C.) favors catalytic activity, and prolonged heating (120 hr.) in hydrogen a t 200" and 225°C. has little, if any, detrimental effect upon activity. On the other hand, continued heating in hydrogen at 300" and 350°C. lowers the activity decidedly, and a t 400°C. the catalyst is deactivated almost as much by 20 hr. as by 120 hr. Twenty hours a t 400°C. is more injurious than 120 hr. a t 350°C. Up to the present we have been unable to detect the actual physical change in the catalytic surface that is caused by the temperature and that is responsible for the loss in activity (10, 5, 1). E$ect o j hydrogen rate. Activity also depends upon the hydrogen rate during reduction of the oxide. Increasing the hydrogen rate 3.5fold decreased the activity by more than one-half. A 66.8-g. sample of Kahlbaum granular copper oxide was reduced a t 225°C. (furnace block temperature) for 3 hr. in a hydrogen stream of 7000 cc. per hour (3.5 times the usual rate). The amount of water produced was 14.21 g., as compared with 14.90 g. for the usual reduction (225"C., 20 hr., 2000 cc. of hydrogen per hour). The reduced catalyst hydrogenated benzene 20 per cent, as compared with 47 per cent when the hydrogen rate during reduction of the oxide was 2000 cc. per hour. It might be expected that slow reduction with dilute hydrogen would be advantageous. However, slow reduction (67 hr. a t 225°C.) with a mixture of 1 volume of hydrogen plus 3 volumes of nitrogen (combined gas rate, 2000 cc. per hour per 66.8 g. of cupric oxide) gave the same activity as reduction with hydrogen alone (2000 cc. per hour). Irreversibility of heat deactivation. In the preparation of catalysts for heterogeneous catalysis it is important that the temperature does not exceed a certain threshold value, else the catalytic activity is lowered. The effect is irreversible. Activity is not restored by lowering the temperature. In some cases a catalyst can be reactivated by alternate oxidation and reduction, but it is not alivays easy, as is shown by the following examples. Kahlbaum copper oxide (66.8 g.) was reduced a t 400°C. during 20 hr., 14.16 g. of water being produced. The catalyst hydrogenated benzene 18 per cent. The catalyst was then oxidized in a stream of air. The temperature was raised from 225" to 400°C. during 4 hr. and held a t 400°C. for 20 hr. The temperature was then lowered to 225°C. (in nitrogen) and the catalyst was reduced a t this temperature for 20 hr., 5.46 g. of water being produced. The resulting catalyst hydrogenated benzene 22 per cent. In other words, the effect of oxidation and subsequent reduction was not decidedly beneficial.
599
COPPER AS CATALYST
An unsuccessful attempt was made to obtain a catalyst capable of hydrogenating benzene by alternate oxidation and reduction of copper oxide wire. This oxide wire showed a melted surface under the microscope. The copper oxide wire (Merck reagent grade) was reduced for 20 hr. in hydrogen a t 225°C. and oxidized for 20 hr. in air a t 400°C. About 15 per cent of the copper was oxidized in the oxidation step. Alternate oxidation and reduction was repeated six times. Care was taken that the temperature of the oxide was always lowered to 225°C. before the reduction step was started. Benzene was not hydrogenated by this material. The h a 1 catalyst contained 99.6per cent of copper. Temperature instability of copper catalytic surface. Low temperature is an essential factor in obtainbig an active catalyst by the reduction of copper oxide; for example, 225°C. is decidedly preferable to 400°C. An obvious explanation is that the exothermic heat of the reduction reaction plus the higher initial temperature level (400°C.) is sufficient to injure the catalytic surface. But exothermicity alone cannot account for the extreme TABLE 4 Ezothermidty of copper ozide reduction at 886°C. and at 400°C. REACTION
+
+ HtO
CUO Hr * CU Currr%.-+ CUIO88*C. CUtoo'c.
+ CUIO8I-C.
CALORIEB PER QRAM-MOLE AT 226%.
CALORImB PER
+23.1
+23.4
GRAM-MOW
AT
m'c.
-5.78 -4.61
difference between the reduction temperature of 225°C. and that of 400"C., especially in view of the finding of Pease and Taylor (14) that this reduction is autocatalytic and localized a t the copper-copper oxide interface. It is evident from the data presented in table 4 that the quantity of heat liberated a t 225°C.is able to raise the temperature of copper to its melting point. At both temperatures there is a great excess of heat available for the destruction of catalytic surface. Diminishing the intensity of the exothermal conditions during reduction had no effect upon the activity of the catalyst. For example, reduction of copper oxide a t 400°C. with hydrogen diluted with 3 volumes of nitrogen, the combined gas rate being equal to the usual hydrogen rate (2000 cc. per hour per 66.8 g. of oxide), did not prevent destruction of the catalytic surface. Also, the same deactivation was obtained when an active catalyst (reduced at 225"C.,activity 47 per cent) was heated for 10 hr. in nitrogen at 400°C. Therefore the loss of activity is primarily due to changes which take place in the temperature interval between 225" and 400°C.
600
V. N. IPATIEFF, B. B. CORSON, AND I. D. KURBATOV
Relationship between physical structure and catalytic activity The object was to correlate the catalytic activity of copper catalyst with its physical structure. The external surface was examined microscopically, and information as to the internal structure was obtained by the emanation method. Microscopic study. The sample of Kahlbaum granular copper oxide was a mixture of approximately 10 parts of dull granules and 1 part of granules with sparkling points. These two varieties were separated with the naked eye, and reduced and tested separately. They had the same activity, hydrogenating benzene 47 per cent at ordinary pressure and 225OC. in 90 sec. contact time. But when the oxide granules were separated under the microscope on the basis of surface porosity and the two varieties were reduced and tested separately, it was found that the compact surface type had an activity of only 8 per cent, whereas the open-work surface type had an activity of 60 per cent. By more careful selection under the microscope it was possible to pick out granules with zero activity. On the other hand, from the commercial oxide there could be selected particles possessing. an exceptionally porous surface; aftex seduction in hydrogen at 225OC., these particlea hydrogenated benzene 90 per cent in 90 sec. contact time at 225'C. and atmospheric pressure. When the original copper oxide mixture was reduced in hydrogen at 225'C., the reduced copper was a mixture of about equal parts of salmoncolored and reddish-brown particles. The salmon-colored particles possessed a compact burnished surface and their activity was 16 per cent. The reddish-brown particles possessed an open-work surface and their activity was 58 per cent. In general, microscopic study of reduced copper catalysts prepared under different conditions distinguishes three types of inactive surface and one type of active surface. Inactive surfaces. The first type is a smooth, compact, non-porous surface, without individual crystal faces. This surface (figure 3a) results from the high-temperature reduction of copper oxide, the crystal faces having melted together. The second type is similar to the first, in that it is a compact surface without crystal faces distinguishable under the microscope. It is made up of smooth round particles (figure 3b), the melting process not having proceeded as far as with the first type. There is also another inactive surface of this second type, which is porous. X-ray examination showed the structure to consist of dispersed agglomerates of copper. The third inactive surface is completely different from the other two. It is made up of well-formed copper crystals which measure 0.05 mm. to
h a . 3. Inat.tive s"riar
