Selective hydrogenation of benzene to cyclohexadiene and cyclohexene by lanthanide precipitates obtained from Eu or Yb metal solutions in liquid ammonia

Article abstract of DOI:10.1246/cl.2001.450

Lanthanide precipitates obtained by decomposition of Eu or Yb metal solutions in liquid ammonia were active at 393 K for the partial liquid-phase hydrogenation of benzene to cyclohexene with high selectivity (≥90%). When the reaction temperature decreased to 323 K, cyclohexadienes were preferentially formed. The reducing behavior was markedly enhanced by the addition of ammonia to the reaction system.

Full text of DOI:10.1246/cl.2001.450

450
Chemistry Letters 2001
Selective Hydrogenation of Benzene to Cyclohexadiene and Cyclohexene by
Lanthanide Precipitates Obtained from Eu or Yb Metal Solutions in Liquid Ammonia
Hayao Imamura,* Kouzi Nishimura, Kenji Sumioki, Mitoshi Fujimoto, and Yoshihisa Sakata
Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University,
2-16-1 Tokiwadai, Ube 755-8611
(Received February 1, 2001; CL-010095)
Lanthanide precipitates obtained by decomposition of Eu or
99.9%; Santoku Co.) under an atmosphere of dry nitrogen. The
reactor was briefly evacuated, cooled by a dry ice/methanol bath,
and then, about 1350 cm3 of purified ammonia gas was liquefied.
The Eu and Yb metal readily dissolve in liquid ammonia to yield
a blue homogeneous solution containing the divalent metal
cations and ammoniated electrons.5 On standing at 273 K for 1
or 12 h, the solutions of Eu and Yb decomposed with the forma-
tion of a precipitate. Ammonia was subsequently removed from
the reactor leaving powdery lanthanide precipitates.
Yb metal solutions in liquid ammonia were active at 393 K for
the partial liquid-phase hydrogenation of benzene to cyclohexene
with high selectivity (≥90%). When the reaction temperature
decreased to 323 K, cyclohexadienes were preferentially formed.
The reducing behavior was markedly enhanced by the addition of
ammonia to the reaction system.
Synthetically and industrially it is of great interest to partially
hydrogenate benzene. With benzene rings it is usually difficult to
stop the reaction after only one or two bonds have been reduced,
since olefins are more easily reduced than aromatic rings. A num-
ber of reagents and catalysts have been reported for the partial
hydrogenation of benzene.1–3 However, scarcely any studies have
been reported with the hydrogenation of benzene using lan-
thanides. Recently we have developed methods for the prepara-
tion of new catalytic materials containing lanthanides and have
demonstrated that they exhibit specific catalytic behavior.4 In this
paper, we report unexpected and interesting properties of lan-
thanide precipitates obtained upon decomposition of liquid ammo-
nia solutions of Eu or Yb metal for the partial liquid-phase hydro-
genation of benzene to cyclohexadiene and cyclohexene.
Ammonia gas (Iwatani Ind. Ltd.) was purified through a cal-
cium oxide column and subsequently through a sodium hydroxide
column. Benzene was of research purity and further purified
twice by distillation in the presence of sodium wire. In a typical
preparation of lanthanide precipitates, in a 30-cm3 stainless steel
reactor were placed measured amounts of Eu or Yb metals (0.5 g;
Following the preparation of the lanthanide precipitates in the
stainless steel reactor, benzene (2 cm3; 22.5 mmol) was immedi-
ately placed in this reactor, subsequently hydrogen was intro-
duced. The liquid-phase hydrogenation was usually carried out
for 3 h under a hydrogen pressure of 3 MPa. The lanthanide pre-
cipitates thus obtained were active at 393 K for the partial hydro-
genation of benzene to cyclohexene, simultaneously yielding trace
amounts of cyclohexadiene (Table 1). The selectivity toward
cyclohexene was more than ca. 90%; the lanthanides were more
selective than reported previously.6,7 When the reaction tempera-
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
451
tures decreased to 323 K, the formation of cyclohexadiene rather
than cyclohexene were predominant. Cyclohexadiene was formed
with a 1,4-/1,3-isomer ratio of 1–4, depending on the reaction con-
ditions applied. Cyclohexane was scarcely formed at 323 K.
The reaction times were prolonged, but the benzene conver-
sion was not so raised. This is partly due to that the precipitates
changed in quality during the reaction and that the hydrogenation
consists of processes proceeding stoichiometrically and catalyti-
cally. The europium and ytterbium exhibited similar reducing
properties, but the ytterbium was superior to the europium in
activity.
The reducing behavior of the lanthanide precipitates marked-
ly depended upon the conditions separating out from the Eu or Yb
metal solutions in liquid ammonia during the preparation process.
It has been reported that the liquid ammonia solutions of lan-
thanide metals decompose through metal, amide or imide to final-
ly convert into nitride.8 To obtain information as to the structural
changes of precipitates, XRD powder patterns were measured
without exposure to air. XRD of precipitates obtained when the
Yb metal in liquid ammonia was permitted to stand for 1 h,
showed the existence of metallic Yb with additional diffraction
peaks at 2θ = 29.8, 34.5, 49.7 and 59.0°, while on standing for 12
h, the Yb precipitates (No. 2) consisted primarily of diamide and
triamide. The crystallite sizes for the Yb metallic phase were
estimated as ca. 12–15 nm from XRD.
The lanthanide precipitates acted more effectively in the pres-
ence of ammonia, although the mechanism of its action in the
reducing process was not appreciably proved yet. As shown in
Table 2, the benzene conversion and the selectivity were enhanced
in the presence of ammonia. Upon addition of ammonia gas of
1000 cm3 to the reaction system, the benzene conversion
increased by a factor of about 2 (Nos. 1 and 10).
Moreover, the reaction results of the lanthanide precipitates
as prepared and otherwise were quite different (Tables 1 and 2).
The precipitates (Nos. 1, 4, 7 and 12) were used more efficiently
when subjected to the reaction immediately after the preparation,
in contrast to the precipitates (Nos. 3, 6, 8 and 13) kept once in an
atmosphere of dry nitrogen after preparation, respectively. The
"stale" lanthanides (Nos. 3, 6 and 8) exhibited very low activity
for the hydrogenation of benzene with hydrogen. A very impor-
tant characteristic of such precipitates is that upon addition of
ammonia the reducing power was restored to some extent. Even
if hydrogen was not used, benzene was reduced with ammonia
(No. 14); the transfer hydrogenation of benzene with ammonia
occurred. It has been shown that the lanthanides promote the cat-
alytic transfer hydrogenation with ammonia as a hydrogen
source.9 That the ammonia directly participates in the hydrogena-
tion is likely responsible for the selective formation of cyclohex-
ene or cyclohexadiene. For the hydrogenation of benzene using
both hydrogen and ammonia, the conversion of benzene was natu-
rally higher than that for the hydrogenation with ammonia.
The hydrogenation of benzene to cyclohexadiene is known as
the Birch reaction: when benzene is reduced by sodium in liquid
ammonia, usually in the presence of alcohol, 1,4-addition of
hydrogen takes place to yield cyclohexadiene.10 The reduction is
known as homogeneously dissolving metal reductions in which
direct electron transfer from the Na metal to the benzene ring with
subsequent protonation by alcohol is involved. Unlike the Birch
reduction, the present system was heterogeneous, and besides, no
alcohol was added to the reaction system. As described above,
the addition of ammonia to the reaction system led to the
increased conversion of benzene, in which ammonia acted as a
promoter, as well as a hydrogen donor. Thus, it is understandable
on the basis of the comparison of Nos. 14 and 15 that the ammo-
nia promotes the lanthanide precipitates, resulting in an increase in
the benzene conversion for the hydrogenation by hydrogen.
References
1
2
A. W. Weitkamp, Adv. Catal., 18, 1 (1968).
E. L. Muetterties and J. R. Bleeke, Acc. Chem. Res., 12, 324
(1979).
3
4
R. L. Burwell, Jr., Chemtracts: Inorg. Chem., 5, 330 (1993).
H. Imamura, “The Metal and Alloys (Prepared Utilizing Liquid
Ammonia Solutions) in Catalysis II,” in “Handbook on the Physics
and Chemistry of Rare Earths,” ed. by K. A. Gschneidner, Jr. and
L. Eyring, North-Holland, Amsterdam (2000), Vol. 29, p. 45.
C. Thompson, “Electrons in Liquid Ammonia,” Clarendon Press,
Oxford (1976).
5
6
H. Nagahara, M. Ono, and Y. Fukuoka, Stud. Surf. Sci. Catal., 92,
375 (1995).
7
8
F. Doebert and J. Gaube, Catal. Lett., 31, 431 (1995).
K. Howell and L. L. Pytlewski, J. Less-Common Met., 19, 399
(1969).
9
H. Imamura, Y. Miura, K. Fujita, Y. Sakata, and S. Tsuchiya, J.
Mol. Catal. A: Chem., 140, 81 (1999).
10 A. A. Akhrem, I. G. Reshotova and Yu. A. Titov, “Birch
Reduction of Aromatic Compounds,” Plenum, New York (1972).