258
TROFIMOV et al.
Initial THI was obtained by the Trofimov reaction pletely restored, however, after its heating in air at
12, 13] from available commercial cyclohexanone 350°С for 5 h.
[
oxime (an intermediate product in caprolactam synꢀ
thesis) and acetylene in a yield up to 97% [14].
Thus, we have developed a new industrially feasible
technology for the synthesis of indole by dehydrogeꢀ
The dehydrogenation of THI is carried out in a nation of available 4,5,6,7ꢀtetrahydroindole (THI)
flowꢀtype vertical steel or quartz tube reactor with using a heterogeneous catalysts, aluminaꢀsupported
external heating. The active zone of the reactor is filled nickel sulfide (0.1–5.0%), at 300–370°С in a solvent
with the catalyst. A solution of THI in benzene or tolꢀ with the use of an inert carrier gas or without it. The
uene is passed downward through a heated (300– technology provides a high yield of indole (up to 96%),
370°С) bed of the catalyst in a flow of inert gas (N2) or a selectivity close to 100% with respect to the target
solvent vapor (without inert gas). Catalyzate vapors are compound, and improved environmental features
condensed in a cooler. The solvent is distilled off and because it can be implemented as lowꢀwaste producꢀ
recovered to the process, crude indole is purified by tion.
recrystallization from hot water. The inert carrier gas
(
nitrogen if used) containing hydrogen is redirected to
the reactor. As hydrogen, which exerts, as a rule, a posꢀ
itive if any effect on the dehydrogenation processes
EXPERIMENTAL
3
A catalyst (47.9 g, 17 cm , 0.44% NiS on
γ
ꢀAl O3,
[
8], accumulates in the inert gas, the latter may be
2
2
directed to separation facility and reused in the proꢀ specific surface of 180–200 m /g, grain size of
3
×
duction cycle after separation of hydrogen. If no carꢀ
5 mm) was placed into a flowꢀtype vertical tube reacꢀ
rier gas is used, evolved hydrogen may be directed to tor as a quartz tube of 12 mm in diameter and 30 cm
other technological processes or compressed and sold long and heated at 350°С in a nitrogen flow (8.0 L/h)
as a commodity product.
for 5 h. A solution of 1.5 g of THI in 15 mL of benzene
was passed through the reactor for 3.5 h in a nitrogen
flow (8.0 L/h). The benzene was distilled off from the
condensate to give 1.41 g of indole containing 2% of
THI and no other admixtures (GLC). The yield of
indole is 96%, selectivity is 98%, mp 53.0°С (from hot
water); lit.: mp 52.5°С [3].
The novelty of the technology consists in the use for
the first time of a catalytic system containing nickel
sulfide as a sole active component. Until now, this catꢀ
alyst has not been not used for the aromatization of
nitrogen heterocycles with saturated substituents.
Common catalysts for these reactions are supported
palladium or rare earth catalysts [8]. It should be
emphasized that, according to general notions, sulfur
compounds are catalyst poisons for heterogeneous
dehydrogenation catalysts based on Ni, Pt, and Pd,
which lose activity even in the presence of traces of
hydrogen sulfide, mercaptans, and sulfides [15].
ACKNOWLEDGMENTS
This work was supported by the Council for Grants
of the resident of the Russian Federation for Support
of Leading Scientific Schools (grant no. NShꢀ
3230.2010.3).
The used catalyst ( ꢀAl O3ꢀsupported nickel sulꢀ
γ
2
fide) was obtained by impregnation of alumina with an
aqueous solution of a nickel salt followed by treatment
an excess of hydrogen sulfide or with an aqueous soluꢀ
tion of alkali metal (or ammonium) sulfide (or hydroꢀ
sulfide).
REFERENCES
1
. Comprehensive Organic Chemistry, Barton, D. and
Ollis, W.D., Eds., Oxford, New York: Pergamon, 1979,
vol. 4. translated under the title Obshchaya orgaꢀ
nicheskaya khimiya, Moscow: Khimiya, 1985, vol. 8.
The best temperature range for the process is 300–
3
70°С. At higher temperatures, side reactions of THI
and indole occur resulting in resinification and
decrease in yield. Below 300°С, dehydrogenation rate
sharply drops.
2
. Sundberg, R.J., The Chemistry of Indoles, San Diego:
Academic, 1997.
3.
Khimicheskaya entsiklopediya (Chemical Encyclopeꢀ
dia), Moscow: Sovetskaya entsiklopediya, 1990, vol. 2,
pp. 456–457.
The concentration of THI in solution supplied to
dehydrogenation is 5–30%. At THI concentration
higher than 30%, the reactions of THI and indole
autocondensation are markedly accelerated, thus
decreasing the yield of the target product. The use of
THI solutions with concentration lower than 5%
requires a larger reactor and auxiliary equipment and,
hence, additional energy input.
4
5
6
. Saxena, V., Shirodkar, V., and Prakash, R., J. Solid
State Electrochem., 2000, no. 4, pp. 231–233.
. Trung, V.Q. and Huyen, D.N., J. Phys.: Conf. Ser.
009, vol. 187, pp. 012058(1–6).
,
2
. Hassanien, R., AlꢀHinai, M., AlꢀSaid, S.A.F., et al.,
ACS Nano, 2010, vol. 4, no. 4, pp. 2149–2159.
The catalyst shows a stable performance during
1
0–15 h (depending on temperature and THI flow
7. Humphrey, G.R. and Kuethe, J.T., Chem. Rev., 2006,
vol. 106, no. 7, pp. 2875–2911.
rate) and then slowly loses activity, which is comꢀ
DOKLADY CHEMISTRY Vol. 434
Part 2
2010