2 ,501 8
I
I
I
0-
-6
0
21.4 atmospheres
- 14.6 atmospheres
A - 7 . 8 atmospheres V - 4 . 4 atmospheres
6 - 1.0
CONVERSION,
Figure 7. Methane production from 3Hz conversion
atmospheres
Y
+ 1 CO gas as a function of system pressure and
by the empirical equation, r = kpB,1.33pc0-0.*3. The catalytic process was inhibited by high concentrations of CO, and actiimpaired by the use Of l H z vity ’vas at least 1CO gas. The present paper demonstrates that catalysts containing 0.5% ruthenium on a high surface area alumina have adequate activity for synthesis, and their hydrocarbon yield was 140 grams per cu. meter compared with 35 grams per cu. meter for Pichler’s unsupported ruthenium catalyst a t the same pressure. Although ruthenium is expensive ($1.93 per gram), it is less which has been expensive than platinum ($2.57 per gram) (8), used in commercial catalysts for refining petroleum, oxidation of ammonia, and hydrogenation of fats.
+
literature Cited (1) Anderson, R. B.3 Hall, W. K., Krieg, A , , Seligman, B., J . Am. Chem. Sac. 71, 183 (1949). (2) ..\nderson, R. B,, Krieg, A,. Seligman, B., O’Neill, 5%’. E,, Ind. Eng. Chem. 39, 1548 (1947). (3) Bienstock, D., Field, J. H., Forney, A. J., Demski, R. J., E. s. Bur. ,I?ines, Rept. Invest. 5841 (1961). (4) Fischer, F., Pichler, H., Brennstoj-Chem. 14, 306 (1933). (5) Karn, F. S., Shultz, J. F., Anderson, R. B., J . Phys. Chem. 64, 446 (1960). (6) Pichler, H., Adban. Catalysis 4 , 289-92 (1952). (7) Pichler, H.. Buffleb, H., Brennstoj-Chem. 21, 273 (1940). (8) u. s. Bur. Mines, “Minerals Yearbook,” Vol. I. pp. 899-911, 1963. RECEIVED for review July 15, 1965 ACCEPTED October 8, 1965 Trade names used for information only, and endorsement by the U. S.Bureau of Mines is not implied.
RHENIUM A S A CATALYST IN HYDROCARBON REF0 R M I NG REACTIONS A N G U S U . B L A C K H A M , R O B E R T R . B E I S H L I N E , A N D L A V A U N S. M E R R I L L , J R . Department of Chemistry, Brigham Young University, Provo, Utah
appeared recently in which rhenium catalysts discussed (7). The use of rhenium heptasulfide as a catalyst in the hydrogenation of some organic compounds has shown that sulfur compounds d o not destroy the activity of the catalyst ( 2 ) . A rhenium catalyst pretreated with hydrogen sulfide was effective in the reforming of gasolines ( 3 ) . T h e purpose of this study was to determine the extent to which rhenium catalysts would effect the dehydrocyclization of n-heptane and whether or not the catalyst prepared from rhenium heptasulfide was different from those prepared from rhenium heptoxide or perrhenates. These rhenium catalysts were also compared with platinum and molybdenum catalysts. REVIEW
A were
Experimental
Apparatus a n d Procedure. The apparatus, shown schematically in Figure 1, was designed so that hydrogen saturated with hydrocarbon passed through the catalyst in a heated reactor. The product steam was analyzed by either gas chromatographic or ultraviolet spectrophotometric methods. The apparatus comprised three stages : the hydrocarbonhydrogen bubbler fitted with a gas dispersion tube, the heated reactor, and the product traps. Hydrogen from a commercial cylinder was dried by passing through phosphorus pentoxide and silica gel. T h e hydrogen, in sequence, passed through a flowmeter, the bubbler, and the reactor tube. The reactor tube was twice the length of the furnace, so that as the temperature of the furnace increased, the catalyst bed remained cool VOL. 4
NO.
4
DECEMBER 1965
269
Rhenium catalysts, prepared b y reduction o f rhenium heptasulfide, rhenium heptoxide, or perrhenic acid on an alumina support, promote the dehydrocyclization of n-heptane to toluene, the hydrocracking reaction of n-heptane to lower molecular weight alkanes, and a combination of these reactions, resulting in the conversion of n-heptane t o benzene. For example, in one run at 550' C. and a contact time of 1.4 seconds, nheptane in a stream of hydrogen was converted on a weight basis to 66% lower molecular weight alkanes, 23% to toluene, and 4% to benzene. Data are presented to show the way in which these reforming reactions vary as a function of reaction temperature, temperature of catalyst reduction, contact time, and nature of the catalyst. Platinum and molybdenum catalysts are included for comparison.
in part of the reactor tube extending below the furnace. As the furnace attained the desired temperature, the reactor tube was lifted into place, permitting rapid heating and reduction of the catalyst in the steam of hydrogen. This technique was used when rhenium heptoxide was deposited on the support to avoid possible loss of the volatile RezOi before reduction started. The bubbler was bypassed during the reduction of the catalyst. T h e bubbler was placed in a constant temperature bath. When this bath was operated above room temperature it was necessary to heat a coil of Sichrome wire wound around the glass tubing from the bubbler to the reactor to prevent condensation of the hydrocarbon. The extent of the reaction was determined by: (1) analysis of the aromatic content by absorption a t 262 mp with a Beckman DU ultraviolet spectrophotometer and (2) analysis of liquid condensate and product gas stream with either a FisherGulf Partitioner or Beckman GC-2 gas chromatograph. When Method 1 was used, the exit gas was bubbled through nheptane in trap G, Figure 1, during the time of sampling. Before and after sampling the exit gas was bubbled through the adjacent trap. Thus the pressure within the system was essentially constant before, during, and after sampling. The contents of the sampling trap were drained into a volumetric flask and diluted to the mark with n-heptane. The absorbance of this solution was determined a t 262 mp. The aromatic content was assumed to have been toluene. Comparison with a calibration curve of toluene in n-heptane gave a measure of the extent of dehydrocyclization. Because benzene was also present in the aromatic fraction, Method 1 was not precisely quantitative, but it did permit a rather rapid semiquantitative study of reaction variables. I n Method 2 the quantitative results were based upon the assumption that the relative areas of the peaks gave the relative weight percentage of the components (4).
chamber was 90 =t2%. Two separate experimental trains were shown to give duplicate results within acceptable limits. Contact Time. The contact times reported here represent the average time of contact of the gaseous mixture with the catalyst bed as calculated from the void volume of the catalyst bed and the rate of flow of the hydrocarbon-hydrogen mixture considered as a n ideal gas. The temperature of the bubbler was controlled to give a 10 to 1 molar ratio of hydrogen to hydrocarbon. Contact times of 0.50, 1.00, and 1.50 seconds correspond to weight hourly space velocities (WHSV) of 0.40, 0.20, and 0.13, respectively. Preparation of Catalysts. Three methods were used in preparation of the rhenium catalysts. The alumina support was wet with a solution of Re2Oi in acetone, the acetone evaporated, and the solid reduced in hydrogen (Method A). Potassium perrhenate suspended in water was treated with H2S to precipitate Re& in the presence of alumina, and the solid was filtered, dried, and reduced in hydrogen (Method B). A solution of perrhenic acid in water was evaporated in the presence of alumina and the solid reduced in hydrogen (Method C). The platinum catalysts were prepared by evaporating solutions of chloroplatinic acid in the presence of alumina and reducing the solid in hydrogen. The molybdenum catalysts \\.ere prepared similarly from aqueous solutions of molybdic acid. Results and Discussion
Data are given in Table I for the catalytic reforming of nheptane over a catalyst of 570 rhenium on alumina, prepared via Re& and reduced a t 500' C. (Method B). I n this experiment the exit gas passed through traps held a t about -80' C. The liquid and gaseous products were determined by Method 2. Methane, ethane, propane, benzene, and toluene were
The efficiency of the hydrocarbon vaporization chamber (bubbler) was 89 i 3% of that calculated based on the vapor pressure of heptane a t 26' C. The efficiency of the sampling
Catalytic Reforming of n-Heptane over 5% Rhenium (from Re2&) on Alumina with Hydrogen as Carrier Gas and a t a Contact Time of 1.4 Seconds Catalyst age, hoursa 0 1.7 2.2 3.1 3.4 3.8 4.1 4.7 Reaction temp., C. 500 500 550 550 500 500 550 600
Table 1.
Product Composition, Weight
Methane Ethane Propane XI
xz
Benzene Toluene n-Heptane Dehydrocyclization Hydrocracking Combination (benzene) Recovered
47 13
27
9 7 1 1
14
3 19
4 75 1
20 0.05
19
1
4 4 16 14 17 64 5 14 0.27
21 27 15 6
24 28 15 7
...
...
20
270
23
9
17 19 15 10 6 4 24 5
6 3 3 5 21 20 25 6 3 4 Mole yo n-Heptane Reacting 23 22 27 26 62 64 66 71 6 5 4 4 4 5 7 3 Ratio of Dehydrocyclization to Hydrocracking 0.35 0.31 0.44 0.41 Mole
0
70
14 21 16
15
Catalystprepared by Method B and reduced at 500' C. f o r 1.5 hours.
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
18 28 15 5 2 3 24 4
46 30 7
27 65 4 4
14 81 4
0.42
1
3 12 1
1
0.17
7*Benzene in Aromatic Fraction 15
18
16
13
22
I
""t
i
70
B
C
n
60 D
.- 5 0 :: 'y
G
E
40
0
0
H I
30 2?
+ 20 Figure 1 . A.
B. C. D.
Drying tube Flowmeter Manometer Bubbler
Diagram of apparatus
E.
Reactor tube F. Fisher microcomburtion furnace G. Sampling trap H. Bypass trap
identified. Two compounds (or mixtures of compounds), XI and X p , although not identified, had retention times corresponding to butanes or pentanes. The dehydrocyclization of n-heptane to toluene is a significant although not the principal reaction. Hydrocracking accounts for more of the products than dehydrocyclization. For example, a t 550' C. and a t a contact time of 1.4 seconds, 66yc of the n-heptane is converted to low molecular weight alkanes, 23yc is converted to toluene, 4% to benzene, and 7% is recovered. The benzene formed may result from the hydrocracking of toluene or from the dehydrocyclization of a Cg fragment. Either way, benzene results from a combination of the dehydrocyclization and hydrocracking reactions. The ratio of toluene to the summation of the saturated hydrocarbons (excluding n-heptane) may be interpreted as the approximate ratio of dehydrocyclization to hydrocracking. This ratio was low (0.05) as the n-heptane and hydrogen first passed over the catalyst, but increzsed to 0.27 after almost 2 hours. The temperature was then raised to 550' C., with the ratio increasing slightly to 0.35. For about 1 hour the reaction proceeded uniformly (columns 3 and 4). As the temperature dropped to 500' C. the conditions prevailing a t first were not re-established; the ratio of dehydrocyclization to hydrocracking increased to 0.44. As the temperature again was raised to 550' C., the ratio as indicated in column 7 was slightly higher than that in columns 3 and 4. With a further increase in temperature to 600' C., the ratio of dehydrocyclization to hydrocracking dropped to 0.17. These observations may be interpreted using the principles of bifunctional catalysis ( 5 ) . The low ratio of dehydrocyclization to hydrocracking a t the start may be due to incomplete reduction of the catalyst, with the acidic sites of the alumina support effecting most of the reaction-viz., cracking. As the metallic rhenium sites develop, hydrogenation-dehydrogenation activity of the catalyst increases. Both metallic and acidic sites become effective and a greater extent of dehydrocyclization is observed. With a further increase in temperature carbonaceous residues may reduce the number of active metallic sites and the dehydrocyclization decreases. Figures 2, 3, 4, and 5 summarize the study of reaction variables with the technique of rapid sampling (Method 1).
I
IO 0 Temperature
of R e a c t o r ,
OC
Figure 2. Conversion of n-heptane to toluene as a function of temperature Contact time. 1 .O second Cotalysts. A. 5% rhenium on alumina B. 1 % platinum on alumina C. 2% molybdenum on alumina D. Alumino
701 60
400 Temperature
5 00 600 of Reoctor OC
Figure 3. Conversion of n-heptane to toluene as a function of temperature Contact time. 1 .O second 5% rhenium on alumina Catalyst. A. First run (same as A of Figure 2 ) B . Second run made after 20 grams of n-heptane in hy. drogen had passed through reactor a t 500' C. C. Third run made after 20 grams of n-heptane in hydrogen h a d passed through reactor a t 550' C.
VOL. 4
NO. 4
DECEMBER
1965
271
The curves of Figure 2 are representative of experimental runs with selected catalyst preparations of platinum, rhenium, and molybdenum. The platinum catalyst appears more selective for dehydrocyclization than either the rhenium catalyst or the molybdenum catalyst. T h e extent of dehydrocyclization is over 80% for the 1% platinum-on-alumina catalyst in the temperature range 500' to 600' C. and about 60% for the 5% rhenium-on-alumina catalyst in this same temperature range. Because the acidic nature and the surface area of the support were not determined for these catalysts, the comparison is of only qualitative significance. Subsequent runs with this same rhenium catalyst (Figure 3) show a decrease in the activity of the catalyst. Perhaps this is due to partial coverage of the rhenium by carbonaceous residues, as has been observed with platinum catalysts ( 5 ) . Figure 4 shows a difference in dehydrocyclization activity resulting from a difference in the temperature of reduction of the catalyst. When the temperature of reduction was 400' C., the dehydrocyclization activity was less than a t 500' C. The two catalysts used here were from the same preparation (Method A). Probably the reduction a t 400' C. was not complete. A subsequent run with fraction B gave about the same activity as did fraction A after two runs. This activity is represented by curve C of Figure 3. The 57, rhenium-on-alumina catalysts prepared by Methods B and C gave results similar to those of the catalyst prepared by Method A. Figure 5 compares the first runs made on each catalyst (curves A , B , and C). T h e catalyst prepared from rhenium heptasulfide has about the same activity as those prepared from rhenium heptoxide and perrhenic acid. Subsequent runs with catalysts prepared from Regs7 and Re20, show reduced activity of the catalysts (curves D and E, Figure 5 ) . Although a subsequent run through the same temperature cycle was not made with the catalyst prepared from H R e 0 4 , a n extended run with this catalyst a t 500' C. in which 25 ml. of heptane passed through the system gave 13 mole yG conversion to toluene. The activity of the rhenium catalysts for dehydrocyclization does not reach a steady state immediately. Table I shows that within the first l1/2 hours the toluene increased from 4 to 17 mole YG. Figures 3 and 5 show that in the temperature range 500' to 550' C. the extent of toluene formation fluctuated during the first few hours and then attained a relatively steady state with approximately 20 mole yo conversion to toluene. Therefore, the data of Figures 2, 3, and 4 probably do not represent steady-state conditions. Figure 6 shows the extent of the reaction as a function of average contact time of the gas stream with the catalyst bed.
Table II. Composition of Aromatic Hydrocarbon Fraction Mole 7,Toluene in Benzene- Toluene Fraction at Contact Time of -___ T:mp., 0.35 0 . 7 7 . 4 Catalyst C . sed. see. see. Hydrocarbon Heptane 770 ReonA1203 550 79 65 76 600 48 5% Re on A1208 550 77 75 600 83 75 1% Re on A1203 600 90 85 70 1% Pt onA1203 550 79 81 76 600 58 46 2 Methylcyclohexane 27, Re on A1203 550 81 82 600 82 80 17~ PtonA1203 550 52 10 0
272
l&EC
P R O D U C T RESEARCH A N D D E V E L O P M E N T
70 60 E
0 .c
50
," 40 E
0
u
30
400 Temperature
500 600 of Reactor OC.
Figure 4. Conversion of n-heptane to toluene as a function of temperature Contact time o f 1 .O second over separate fractions of same 5% rhenium on alumina cotolyst preparation A. Initial temperature of reduction, 500' C. (same as A of Figure 2) 8. Initial temperature of reduction, 40OoC.
400 Temperature
500 of
600 Reactor
"C Figure 5. Conversion of n-heptane to toluene as a function of temperature Contact time. 1 .O second Catalyst. 5% rhenium on alumina First run after reduction: A. Prepared from Reno? (same as A of Figures 2 and 3) B. Prepared from Ret& C. Prepared from HReOa Subsequent run: D. Prepared from Rep07 (same as C of Figure 3) E. Prepared from Re&, second run
more benzene than toluene. Although less pronounced, the same trend \vas observed with the rhenium catalysts (48y0 toluene a t GOO0 C. and 1.4 seconds). Methylcyclohexane with the platinum catalyst showed a decrease in the toluene-benzene ratio as a function of contact times a t 550' C. lt'ith the rhenium catalyst this decrease was not observed. Conclusions
I
I
I
I I.o
I
I.5 Contact Time , sec, Figure 6. Conversion of n-heptane to toluene as a function of contact time
0.5
A. 6.
At 500" C. over 5% Re on A1203 At 550' C. over 0.1 70Re on A1203
Because of the leveling of the curves with increasing contact time and the relatively small amount of heptane remaining after 1.4 seconds ('Table I), the contact time used most often with the rapid sampling technique was 1.Osecond. Benzene accounted for 13 to 23 mole yo of the aromatic hydrocarbon fraction in Table I . Some additional data on the composition of the aromatic hydrocarbon fraction are given in Table 11. The toluene in this fraction under most conditions was between 70 and 90 mole yc,but under some extreme conditions this value dccreased. Higher temperatures and longer contact times with the platinum catalyst resulted in
Rhenium catalysts show activity in the dehydrocyclization 3 f n-heptane to toluene. Other reforming reactions also occur. Platinum is more selective for dehydrocyclization than rhenium. Rhenium is cheaper than platinum (7), but a higher percentage of rhenium is needed on the alumina support. These observations suggest that rhenium may merit further consideration in the catalytic reforming of hydrocarbons. Acknowledgment
Appreciation is gratefully expressed to the Kennecott Copper Corp. for the financial support of this work, and to H. Smith Broadbent for permitting this work to begin as part of the rhenium research program he supervised a t Brigham Young University. literature Cited
(1) Blom, R. H.: Kollonitsch, Valerie, Kline, C. H., Ind. Eng. Chem. 54, 16 (1962). (2) Broadbent, H. S., Slaugh, L. H.? Jarvis, H. L., J . A m . Chem. Soc. 76, 1519 (1954). (3) Minachev. Kh. &I.. Rvashentseva. M. A,, Ittest. Akad. AVauk k W R . Otdel. Khzm. AVauk. 1961. 107: C . A . 55. 16978e (1961). (4) Nunez, L. J., ~rmstrong.'LV. H., Cogiweil, H: w.,' Anal. Chem. 29, 1164 (1957). (5) Sinfelt, J . €I., A h a n . Chem. Eng. 5, 64-9 (1964). RECEIVED for review April 5, 1965 ACCEPTEDOctober 8, 1965
HYDROGENATION OF BORON TRICHLORIDE TO D ICH LOR0 BO R A N E A New Route to Diborane J A W A D H . M U R I B , D A V I D H O R V I T Z , A N D CHARLES A .
B O N E C U T T E R
U. S. Industrial Chemicals Co., Division of lVationul Distillers and Chemical Cor$, , Cincinnati, Ohio Dichloroborane, BHC12, has been synthesized in quantitative yields by the thermal hydrogenation of boron trichloride. The various parameters affecting the reaction of boron trichloride with hydrogen have been investigated and the rates of the reverse reaction have a I s 0 been determined. Dichloroborane disproportionates quantitatively to diborane and boron trichloride under suitable conditions. Equilibrium studies were made of the disproportionation.
methods for the synthesis of chloroboraries have been described : the reaction of gaseous boron trichloride with hydrogen in a n electric discharge a t low pressure (73); the reaction of boron trichloride with hydrogen in the presence of reactive metals as halogen acceptors (5, 7) ; the reaction of gaseous diboron tetrachloride with hydrogen ( 7 d , 15j ; the reaction of diborane with boron trichloride in ethers to prodr1c.e the corresponding chloroborane-etherates (2, 3, 7 7 ) ; and the EVERAL
reaction of boron trichloride and diborane in the absence of solvent (6, 9, 72). I n 1958 a new process was developed in this laboratory in which boron trichloride is hydrogenated to dichloroborane simply by reaction with hydrogen a t elevated temperatures in the gas phase ( 7 , 70). T h e process does not employ metals or metal hydrides as halogen acceptors. Thermal energy is used rather than an electric discharge. Yields are essentially VOL. 4
NO. 4
DECEMBER 1 9 6 5
273
