Ind, Eng. Chem. Prod. Res. Dev. 1986, 25, 367-372
Alumina from Coal Wastes through the Formation of Aluminum Nitride by Carbothermal Reduction under Nitrogen Hong Yong Sohn' and Donna D. Harbuckt Department of Metallurgy and Metallurgical Englneerlng, Unlverslv of Utah, Salt Lake Clty, Utah 84 1 12- 1183
A novel method for extracting alumina from coal wastes has been investigated. When the aluminosilicate com-
ponents contained in coal wastes are reduced with carbon under a nitrogen atmosphere, they are converted to aluminum nitride and silicon carbide. The carbothermal reaction was carried out in the temperature range 1623-1873 K with various amounts of carbon and for various lengths of time. The effect of particle size was also studied. Up to 98% of the alumina originally contained in the coal waste was converted to aluminum nitride. The aluminum nitride formed is optimally leached in 1 M sodium hydroxide at 100 OC. Leaching for 2 h dissolves approximately 70% of aluminum nitride as sodium aluminate. Ammonia is a valuable byproduct of leaching.
Introduction Currently, the Bayer process, using bauxite ore, accounts for nearly all of the alumina production in the world. The United States imports over 90% of the bauxite it uses. To avoid becoming dependent upon foreign imports, the US. must find alternative resources and technologies. Many other aluminum-bearing minerals including alunite, anorthosite, halloysite, and kaolinite are abundant. Their usefulness is limited by economics and the lack of appropriate technology in processing them. Several innovative processes using non-bauxite sources have been analyzed and reviewed by the US. Bureau of Mines (Bengtson, 1979). Although some methods appear promising, none have been used beyond the pilot-plant stage because of high production costs. More domestic coal has been mined due to the increasing costs for foreign fuels. With this increase in coal production comes an increase in coal wastes-the overburden material and preparation-plant tailings. These coal wastes cause ever-increasing disposal problems. Yet coal wastes contain a significant fuel value and may prove to be a valuable source of alumina. To solve both problems simultaneously, a process using coal wastes to produce alumina has been investigated. This is attractive in that waste containing 10-30% alumina and 40-70% silica can be used as a resource. The coal content present in the wastes helps reduce the alumina and silica under a nitrogen atmosphere to form aluminum nitride and silicon carbide. A similar reaction scheme for the synthesis of ammonia using bauxite ore was originally patented by Serpek (1907, 1911). The aluminum nitride is then leached in a caustic solution in the form of sodium aluminate, which can subsequently be treated to produce alumina. Silicon carbide and ammonia are obtained as byproducts. The following reactions are involved: A1203 3C N2 = 2A1N + 3CO (1)
+
+
Si02 + 3C = S i c 3sio2 + 6C
+ 2CO
+ 2N2 = Si3N, + 6CO A1N + NaOH + H 2 0 = NaA102 + NH3
(2) (3)
(4) Reactions 1-3 are examples of reactions between two solids proceeding through gaseous intermediates. Sohn Formerly Donna M. Dickson. Present address: Salt Lake City Research Center, U.S. Bureau of Mines, Salt Lake City, UT 84108.
Table I. Coal Waste Analyses preparation plant A1203 18.0% SlOZ 52.6% Fez03 2.6% CaO 1.7% MgO 1.5% C 14.0%
overburden 17.3% 63.0%
and Szekely (1973) have formulated a theory to determine important reaction mechanisms for such systems. Thermodynamic Considerations When carried out at atmospheric pressure, the direct reduction of many metal oxides with solid carbon takes place through gaseous intermediates in the following steps: MeO, nCO = Me + nC02 (5)
+ nC + nCO2 = 2nCO
(6) MeO, + nC = Me + nCO (7) Several examples are given in the literature supporting this type of mechanism. Reduction occurs when the CO2/COratio in the gas phase falls below the equilibrium value for reaction 5. Carbon can be interpreted as either a COz getter or a CO generator and acts to keep the pco / p c o ratio low enough to make the reduction possible. alumina can be reduced only with great difficulty at high temperatures with carbon alone. Therefore, it is proposed to form aluminum nitride by having both carbon and nitrogen present. Free-energy consideration in the A1203/C/N2system indicates that aluminum nitride can be formed at a substantially lower temperature than aluminum (Dickson, 1982). Experimental Section Preparation-plant and overburden coal wastes from the New Mexico York Canyon strip mine were obtained for use in this study. The preparation-plant material was chosen for the experiments because of its higher carbon content and Al/Si ratio. Analyses are given in Table I. Unless otherwise specified, coal wastes samples were ground to -200 mesh for the experiment. The carbon content of this particular waste is not enough for reactions 1-3 to proceed to their stoichiometric completion. Carbon in the form of Carbon Black Raven 2000 of -200 mesh size was added to the coal wastes for this study. In industrial operation, coal would be preferred
Ql96-4321/86/1225-Q367~0~.50/0 0 1986 American Chemical Society
368
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
Table 11. Production Analysis sample condition 1450 "C, 2 h 1550 "C, 3 h mixture of knowns
AlN 22.7 32.4 40.0
dissolved in HF" Sic 45.0 17.5 44.0
quantitative X-ray diffraction AlN Sic Si,N4 24.4 50.4 25.3 30.9 8.0 55.4 40.0 40.0 20.0
Si3N4 30.0 50.0 16.0
Analyzed by atomic absorption spectrometer. Impurities excluded to make total A1N
because of its availability and lower price. However, it would also introduce more impurities into the system. Two high-temperature vertical furnaces were used. One of these, a Lindberg single-zone tube furnace, Model 45543, with a mullite tube, was capable of reaching a temperature of 1450 OC. A closed system was maintained by attaching a Pyrex extension tube with a removable cover on the top of the mullite tube and inserting a rubber stopper with a gas inlet into the bottom of the tube. The Pyrex cover also served to hold the sample chain and to simplify insertion and removal of the sample. A porcelain crucible sample holder was suspended into the tube with Kanthal A wire. Kanthal A was chosen after wires of platinum, tungsten, and molybdenum failed in this high-temperature nitrogen atmosphere. Another vertical tube furnace using Kanthal Super 33 heating elements was built to reach temperature greater than 1450 "C. The tube and crucibles used were made of alumina. Tungsten wire, covered with an alumina shield, was used to hang the sample because Kanthal A could not withstand the high temperatures. The leaching system consisted of a hot plate, Teflon beaker, and condensation tube. Teflon was used because at high temperatures caustic solutions leach silica from the glassware. Further details of the experimental equipment used in this work can be found in Dickson (1982). Initial experiments showed the formation of three principal products: aluminum nitride, silicon carbide, and silicon nitride. Because of its corrosion and thermal-shock resistance, silicon carbide has found extensive use in industry. Silicon nitride is also receiving growing interest as a ceramic material for high-temperature applications (Clauss, 1969). Depending on the desired product, silicon nitride can be transformed to silicon carbide according to the following reaction at 1400 "C: Si3N, 3C = 3SiC + 2Nz (8)
+
Analyses were done by X-ray diffraction using a Norelco diffractometer with Cu Ka radiation. X-ray diffraction is adaptable to quantitative analysis by comparing peak intensities. However, because of differences in the absorption coefficients, the intensities must be correlated with correction factors. These factors can be determined by comparing known standards. Relatively pure samples of aluminum nitride, silicon nitride, and silicon carbide were obtained. Different proportions were mixed and X-rayed to develop standard calibration curves such as shown in Figure 1. Other curves for AlN/SiC and Si,N,/SiC were also obtained. By the use of these curves the relative amounts of products were determined. Hoggard (1978) used a similar technique in determining alumina content in SIALON. This provides a quick method to determine relative amounts of products without having to perform a chemical analysis on each product. Some results obtained by X-ray diffraction were verified by chemical analysis, as shown in Table 11.
Results Effect of Carbon Addition. When the coal-waste sample was heated in a nitrogen atmosphere at 1450 "C,
+ Sic + Si3N4 = 100%.
7 0
00
I
2 0
4.0
80
eo
100
120
140
R A T I O OF X - R A Y D I F F R A C T I O N PEAK HEIGHTS AIN/S13N4
Figure 1. X-ray diffraction calibration curve used for quantitative analysis of reaction product. Table 111. Effect of Carbon Addition" relative % of added productsc conversion,d 70 carbon,b 7 0 A1N Si3N4 A1N Sic Sic Si3N, 10 33.8 99.0 16.9 49.3 19.7 66.6 20 31.2 22.0 46.8 93.7 25.6 63.2 50 15.8 10.3 47.4 74.0 98.2 11.9 100 11.4 2.0 34.2 86.6 98.0 1.9 "Reacted for 2 h at 1500 "C. *Percent of coal waste sample. CImpuritiesexcluded to make total A1N + S i c + Si3N4 = 100%. dPercent of original amount in coal waste converted to product.
only mullite and silicon carbide were formed with no aluminum nitride. Because of the insufficient amount of carbon in the coal-waste sample, additional carbon had to be added to produce aluminum nitride. The effect of the added amount of carbon was examined by varying the amount of carbon black mixed with the coal wastes. Each sample was reacted for 2 h at 1500 "C under a nitrogen atmosphere. The results are given in Table 111. As the amount of carbon increased, so did the amount of silicon carbide formed, while the amounts of aluminum nitride and silicon nitride formed decreased. Too much carbon decreases the rates of production of aluminum nitride and silicon nitride by increasing the length of the diffusion path for nitrogen. Therefore, the remaining experiments were performed with 10% excess carbon. This compensates for any reducible oxide impurities that may be present in the coal wastes and also assures that enough carbon exists for the reduction of alumina and silica. Effect of Mixing. In the proposed mechanism the rate-controlling steps are a chain reaction: A1203
+ nCO = AlzO, + nCOz C + co, = 2co
Carbon monoxide and carbon dioxide gases must travel between the surfaces of alumina and carbon particles. For a rapid reaction, the gases must be able to transport back and forth quickly. Thorough mixing of the carbon black with the waste material reduces the distances between the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 369
Table IV. Effect of MixinP relative % of productsb sample AlN Sic SiaNa layers 18.2 63.8 18.0 mixed 33.2 16.9 49.3
conversion,C % AlN Sic SisNa 54.7 99.7
24.3 66.6
74.2 19.7
Same as a, c, and d, respectively, in Table 111.
Table V. Effect of Particle Size" products,b % mesh A1N Sic Si,N, -200
100 X 200 65 X 100 +65 a,b,c Same
30.7 24.3 21.7 21.4
23.6 23.2 30.1 36.9
45.7 52.6 48.2 41.7
conversion: % AlN Sic SLN, 92.2 73.0 65.2 64.3
27.4 27.0 35.0 42.9
61.8 71.1 64.9 56.4
as a, c, and d , respectively, in Table 111.
particles and enhances the reaction rate. If the carbon is not well mixed, the gases have to diffuse over larger distances. Consequently, the local partial pressure of COP rises, slowing the overall reaction. To confirm these assumptions, two samples were prepared with different degrees of mixing. The first was arranged with alternate layers of coal wastes and carbon black. The second was vigorously shaken for 20 min in a plastic container. Both were reacted for 2 h at 1500 "C. As seen in Table IV, the better-mixed sample shows a greater amount of aluminum nitride and silicon nitride formed, indicating a higher reaction rate. When the carbon and waste material are separated by layers, the reaction rate is slow. Although this information does not eliminate other possible reaction mechanisms (Le., true solid-solid contact), it is consistent with the proposed mechanism of a solid-solid reaction proceeding through gaseous intermediates. Surface Area Effect. For a gas-solid reaction it is generally true that the overall reaction rate depends on the surface area of the reactants. A large surface area means more sites available for reactions to occur and thereby increases the reaction rate. To test this, experiments were performed in which the particle size of the coal-wastes material was varied. Four different size fractions of wastes were used: -200, 100 X 200,65 X 100, and -65 mesh. Table V lists the results. As the particle size increases, the formation of aluminum
nitride and silicon nitride decreases, indicating that the reaction rate decreases with increasing particle size. Effect of Temperature. The reactions involved are endothermic. At 1300 "C no aluminum nitride formed, but silicon carbide and silicon nitride were detected. When the temperature was increased to 1350 "C, aluminum nitride appeared. This temperature is somewhat lower than other investigators claimed is necessary for the reaction. Perhaps the amorphous nature of the aluminosilicates in coal wastes makes the formation of aluminum nitride more favorable. As indicated in Table VI, increasing temperature enhances the formation of aluminum nitride to a point. However, if the temperature is too high or the reaction time too long, the yield drops off. It has also been found that high temperatures promote the formation of carbides (Stroup, 1965). Kohlmeyer and Lundquist (1949) found carbon and alumina to form aluminum carbide at temperatures as low as 1560 "C. Stroup (1965) noted that overheating causes the material to fuse, thus destroying porosity and causing nitrification to cease. A more recent patent (Clair, 1962) states that in the alumina-nitrogen-carbon system, overheating causes volatilization which may cause sintering, thus impairing the quality of the end product. From the data, it appears that the optimal temperature range is 1450-1500 "C, for 1.5-3 h. At these conditions, approximately 95% of the alumina originally present in the coal waste is converted to aluminum nitride. Effect of Pelletizing. Pellets are easier to handle than powder and promote better heat transfer and diffusion of gases. To test the effect of pelletizing, samples were pressed into pellets of 1.8-cm diameter and height under 3.4 X lo4 kPa pressure. Figure 2 shows the effect of pelletizing vs. nonpelletizing at 1450 and 1500 "C. For the lower temperature pelletizing enhances the reaction. At 1500 "C pelletizing caused the end product to partially sinter: the longer the pellet was reacted, the more difficult it became to remove, analyze, and leach. Sintering limits access by the reactant gas, thus markedly reducing the overall rate of reaction. Leaching. The success of this process rests on the ability to leach the aluminum nitride to obtain alumina and ammonia. To obtain the optimal leaching condition, several leaching experiments were performed on the reaction products produced after 2 h in the furnace at 1500
Table VI. Effect of Temperature time, h
temp, "C
AlN
1
1500 1550 1600 1350 1450 1500 1550 1400 1500 1550 1400 1450 1500 1550 1350 1400 1450 1350 1450
15.3 18.7 29.6 27.4 32.7 30.5 29.2 21.0 30.1 24.0 27.5 30.1 27.5 21.8 25.7 24.7 30.8 26.5 30.9
2
3 4
6 10
"vbSameas c and d , respectively, in Table 111.
relative % of productsa Sic Si3N4 52.6 58.4 22.5 44.9 46.8 19.1 26.8 31.4 13.0 17.8 31.2 13.0 14.2 10.3 11.2 27.4 44.0 9.3 22.8
32.1 22.9 47.9 27.7 20.5 50.4 44.0 47.5 56.1 58.2 41.3 34.6 58.2 67.9 63.1 47.9 25.2 64.2 46.3
AlN
conversion,* % Sic
Si3N4
45.9 56.1 88.9 82.3 98.2 91.6 87.7 63.0 90.4 72.0 92.6 90.4 82.9 65.5 77.2 74.2 92.5 79.6 92.8
51.2 67.9 26.2 52.2 54.4 22.2 31.2 36.5 15.1 20.7 36.3 41.0 16.5 12.0 13.0 31.9 51.2 10.8 26.5
43.4 30.9 64.8 37.4 27.7 68.1 59.5 64.3 76.0 78.6 55.8 46.8 78.6 86.8 85.3 64.7 34.1 86.8 62.6
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
370
100 0
Y
POWDER
A 1
00
I 4 0
2 0
00
I
I
8 0
80
I I00
12 0
T I M E ( h o u r s ) I N FURNACE 100 0
eo 0
80
z
0
2
u)
c;L
eo0
= o
=
>
w -
= 0 0 I-
5 0 -
gz
Y
40.
400
2I 4-
L
0 Y
E2 u-i
n. Y
30.
=I 0 -
20 0
o -a w
2 0 .
c w
=s
Y O
00 00
I O
2 0
4 0
3 0
Y S w n
6 0
10.
a
T I M E (hours) I N FURNACE
Figure 2. Effect of pelletizing: (top) 1450 O C (percent of original amount in coal waste converted to product; (bottom) 1500 OC (percent of original amount in coal waste converted to product).
I
0
I
I
I
coo'
/ I
I
/
\
A,
30
AI
2z
100
Y O
a
I
00 00
I
/
2 0 0 c
10
/
I
u-,
20 0
400
00 I
#
I
eo o
eo o
100 0
T E M P E R A T U R E ('C)
Figure 3. Effect of temperature on the dissolution of reaction products (1M NaOH, 2 h).
"C. It was found that the dissolution of A1N was rapid in the first 20 min and continued slowly up to 2 h when it essentially leveled off (Dickson, 1982). The effect of the temperature on leaching is shown in Figure 3. These experiments were completed by using a 1M caustic concentration and leaching for 2 h the higher the temperature, the better the recovery of alumina. The best results are obtained when the solution is boiling. It is of interest that the dissolution of iron and silica is rather insensitive to temperature. Figure 4 summarizes the effect of different caustic concentrations when leaching for 2 h at the boiling temperature. As seen, a concentration of approximately 1 M gives the best results. When the caustic concentration is increased, more silicon and iron compounds are leached into solution, while the recovery of aluminum decreases. High-caustic concentrations cause the formation of insoluble aluminum compounds. A number of aluminasilica-sodium compounds are insoluble in alkaline solutions.
I
.o
2 .o
3.0
4 .O
6 0
T I M E (hours) IN FURNACE
Figure 5. Leaching of reaction products: (top) 1450 "C; (bottom) 1550 OC.
These include A1203.Si02,3A1203-Si02,Na20-A1203.4Si02, and Na20.Al2O3.2SiOp Other combinations are also possible. All further leaching experiments were carried out by using 1 M NaOH solution at the boiling point for 2 h. The results shown in Figure 5 indicate that the leachability goes through a maximum with increasing reaction time in the furnace. Even the maximum recovery of A1N under optimal conditions of 1500 "C for 1.5-2 h is only about 70%, indicating that the aluminum nitride formed is not completely leached. To investigate the solubility of aluminum nitride, a relatively pure sample was obtained from Chem-Tech Laboratories. When this was leached at optimal conditions, 99% decomposed. This demonstrates that aluminum nitride dissolves easily in caustic solutions. However, commercial aluminum nitride is produced by heating powdered aluminum in nitrogen with a catalyst at 700 "C (Edwards et ai., 1930), a much lower temperature than in this study.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, NO. 2,
C o a l W u s t e t Carbon
7
2oloB
1986 371
A120J+3C+N2*ZAiN t3CO 95% Conversion t o A I N
i n NOOH Solution
I O k g / m J Co(OHI2 Solution
Y Calcination
1200'Cc, I - 1 5 h
R e d u c t i o n - Grade Alumina
Figure 7.
0
1
2
3
I
A
4
6
8
Co(OH)z ADDITION, k g l d
Figure 6.
Desilication
of
sodium aluminate solution.
In the 19209, Alcoa Industries performed extensive leaching experiments on aluminum nitride produced via reaction 1 (Stroup, 1964). Because of the refractory behavior of the nitride, many difficulties were encountered. Studies showed that the crystalline variety of aluminum nitride formed at these high temperatures was much more difficult to decompose than anticipated. Long and Foster (1959) produced high-purity aluminum nitride by striking a dc arc between two high-purity aluminum electrodes in nitrogen atmosphere. An unexpected finding was the extreme chemical inertness of the aluminum nitride formed at these very high temperatures. These and other findings led Taylor and Lenie (1960) to conclude that aluminum nitride prepared at high temperatures is relatively inert. Some of the aluminum nitride from Chem-Tech was heated at 1500 OC for 2 h under a nitrogen atmosphere. Leaching under optimal conditions recovered only 85% of the alumina, verifying that crystalline aluminum nitride formed at high temperatures is more inert and difficult to leach. This helps to explain the low recovery of alumina in the leaching process. Precipitation. Once aluminum nitride is digested, a relatively pure form of aluminum hydroxide must be precipitated to obtain alumina. Therefore, iron and silica impurities must be removed from the solution. Iron removal was relatively easy under most desilication conditions. However, the traditional Bayer method of desilication proved ineffective. Alcoa found that for the aluminum nitride process silica goes into solution and cannot be precipitated as a conventional desilication product (Stroup, 1964). The researchers concluded that a separate and expensive desilication process would be necessary for a good recovery of alumina. Since that time, however, more work has been done on desilicating sodium aluminate solutions. A method proposed by Noworyta (1981) was used with promising results. The pH of the solution was adjusted to 12.5 with hydrochloric acid. The temperature was then raised to 96 OC. A given amount of a 10 kg/m3 solution of calcium hydroxide was added to leach liquor, and the entire solution was left to desilicate for 1 h. The results are shown in Figure 6. Virtually complete removal of silica was achieved with the addition of 1.3 kg of Ca(OH),/m3 of aluminate solution, with only approximately 10% of the alumina lost to the calcium aluminosilicate precipitate. However, when too much calcium hydroxide was added,
Proposed flow sheet.
alumina was totally precipitated out of solution. Care must be taken to add the correct amount of desilicant. With iron and silica removed, aluminum hydroxide was precipitated by bubbling carbon dioxide gas through the alumina-rich solution according to This precipitate was washed with warm water to dissolve any precipitated salts and calcined in a platinum crucible at a temperature of 1200 "C to remove the waters of hydration. X-ray diffraction analysis showed alumina to be the final product.
Conclusions It has been shown that it is possible to obtain alumina by the carbothermal reduction of coal wastes. Figure 7 shows a proposed process flow sheet with yields of alumina at the various stages when optimum operation conditions are maintained. As seen, the overall conversion of alumina from coal wastes is about 60%. The conversion of alumina to aluminum nitride is relatively complete, as are the desilication and calcination steps. It appears that the greatest loss in recovery occurs in the leaching process. Only up to 70% of the aluminum nitride formed is leached. Because of the refractory behavior of aluminum nitride formed at high temperatures, perhaps high-pressure and high-temperature leaching should be experimented with to increase the recovery. Once this major problem is solved, more effort could be spent in improving the other stages of the process. Acknowledgment We express our sincere gratitude to the late Dr.Ivan B. Cutler for drawing our attention to the work of Serpek (1907, 1911). Registry No. A1203,1344-28-1; AlN, 24304-00-5.
Literature Cited Bengtson, K. B. "A Technolcglcai Comparison of Six Processes for the Production of Reduction Grade Alumina from Non-Bauxite Raw Materials"; Metallurgical Society of AIME: New York. 1979; Paper Selection LM-7914. Clair, J. U S . Patent 3 032 398, 1962. Clauss, J. Enoineer 's Guide to High - Temperature Meterials ; Addison-Wesley: Reading, MA, 1969; p 245. Dickson. D. M. "Alumina from Coal Wastes through the Formation of Alumlnum Nltrlde": M.S. Thesis. Universitv of Utah. 1982. Hoggard. D. "Chemical and Mineraicgi6al Effects of Impurities on the Formation and Sinterability of SIALON"; M.S. Thesis, University of Utah, 1978. Kohimeyer, E.; Lundquist, S. Z . Anorg. A/@. Cbem. 1949, 260, 208. Long, 0.; Foster, L. J . Am. Ceram. SOC. 1959, 42(2), 53. Noworyta, A. Hydrometallurgy 1981, 7, 99. Serpek, 0. U S . Patent 867615, 1907. Serpek, 0. U.S. Patent 987 408, I91 I. Sohn, H. Y.; Szekely, J. Chem. Eng. Sci. 1973, 28, 1789.
Ind. Eng. Chem. Prod. Res. Dev. 1986,2 5 , 372-375
372
Stroup, P. T. Trans. Metall. SOC. AIM€ 1964, 230, 365. Taylor, K.; Lenie, C. J . Nectrochem. SOC. 1980, 107(4),308.
Received for review September 25, 1985 Accepted December 12, 1985
This material is based upon work supported in part by the Department of the Interior under Grant Nos. G1105089 and
G1115493. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the Department of the Interior. This work was also supported in part by an Alcoa Foundation Science Support grant and the State of Utah Mineral Leasing Fund. D. D. Harbuck (then D. M. Dickson) received a Domestic Mining and Mineral and Mineral Fuels Conservation Fellowship during this work.
Reactions of Bromine and Cyclohexene in Aqueous Media. Selectivity of Bromohydrin vs. Dibromo Adduct Formation Jacob Zablcky’ and Mordechal Nutkovltch The Institutes for Applied Research, Ben-Gurion University of the Negev, Beer-Sheva 84 1 10, Israel
Water may be used as the dispersing medium for olefin bromination. Olefin suspensions in water would yield mainly the dibromo adduct when treated with neat liquid bromine, or bromohydrin when treated with aqueous bromine solutions. These general results may be modified by both additives and operational procedures of practical significance. When cyclohexene is added to aqueous bromine solutions, the ratio of bromohydrin to dibromo adduct in the final product depends on the olefin feed rate. A slow feed rate leads to high dibromo adduct yields, while a fast feed rate gives mainly bromohydrin. Additives in the aqueous solution, such as sodium bromide, hydrogen bromide, or perchloric acid, modify the bromohydrin to dibromo adduct ratio of the final product. At low additive concentration a rise in bromohydrin yield, which is strongest for sodium bromide, is observed.
Introduction The reaction of olefins with bromine is an electrophilic addition to a carbon-carbon double bond. The mechanism of the process has been amply investigated (March, 1977 and reviews mentioned there). The mechanism consists of two main reaction steps as follows: (a) production of a cationic reactive intermediate Br2
+
I I C=C I 1
slow
1
Br - C I
1
(41
1
C1
Br
+
( 1 )
3
The structure of this carbonium-ion intermediate mainly depends on the nature of the starting olefin and may involve cyclic bromonium ions (Bethel and Gold, 1967; Freeman, 1975; Bienvenue-Goetz et al., 1982). (b) product achievement by nucleophilic attack
4
1
1
1
where Nu: may be a bromide ion, a solvent molecule, another nucleophile present in solution, or a nucleophilic moiety present in the reactive intermediate. The overall reaction rate of simple olefins with bromine is very fast and might be considered to be instantaneous when a practical synthesis is carried out. When bromine undergoes the addition reaction t o an olefin in homogeneous aqueous systems, the expected result is production of bromohydrin, as illustrated in reaction 3 for cyclohexene (1). The practical use of such systems is limited because of the low water solubility of olefins, The main alternative reaction path for 1 in the presence of bromine would be the production of a dibromo adduct (3), as shown in reaction 4.
When the process is carried out on a practical scale, the cyclohexene-bromine-water system ceases to be homogeneous and the reaction paths become complicated by additional factors. For example, Swarts (1937) obtained a 2:3 ratio of 1.7 for 1 reacting with “bromine water”, while Zabicky (1971) found that reaction 3 proceeds quantitatively for bromine-hydrogen bromide aqueous solutions in spite of added bromide ions which should, in principle, promote reaction 4. In the present work we found that the selectivity of reaction 3 vs. reaction 4 could be changed from one extreme to the other with the aid of additives and operational factors; thus, either bromohydrin 2 or dibromo adduct 3 could be synthesized by using water as the dispersant. Why should water be used for this type of reaction? First, it is a reagent if one is interested in bromohydrin synthesis. On the other hand, even if dibromo adducts are the desired products, water may offer some advantages over organic solvents should a dispersing medium be needed for the reaction: (a) Economic aspects involve both solvent price and the ease with which products may be separated from the dispersant. Thus, water may be separated in a settler followed by drying of the organic phase,
0196-4321/86/1225-0372$01.50/00 1986 American Chemical Society
