Hydrogenation of lower alkenes and conjugated diene catalyzed by Ga 2O3

Article abstract of DOI:10.1016/j.cplett.2012.04.052

Hydrogenation of lower alkenes such as ethylene, propylene, and 1-butene proceeded over gallium oxide (Ga₂O₃). The rate of the hydrogenation was faster in the order of ethylene ? propylene > 1-butene over Ga₂O₃. Ga₂O₃ also showed the activity for hydrogenation of 1,3-butadiene where 1,4-addition of hydrogen to 1,3-butadiene mainly took place to give 2-butenes as main products. The formation rate of hydrogenated products depended on the amounts of H₂ and alkenes adsorbed on Ga₂O₃. This implies that the hydrogenation proceeds on the basis of the Langmuir-Hinshelwood mechanism.

Full text of DOI:10.1016/j.cplett.2012.04.052

Chemical Physics Letters 539–540 (2012) 79–82
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Chemical Physics Letters
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Hydrogenation of lower alkenes and conjugated diene catalyzed by Ga2O3
a
Tetsuya Shishido a, , Hirotaka Kuno , Kentaro Teramura a,b, Tsunehiro Tanaka a,
⇑
⇑
a Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan
b Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Saitama 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrogenation of lower alkenes such as ethylene, propylene, and 1-butene proceeded over gallium oxide
(Ga2O3). The rate of the hydrogenation was faster in the order of ethylene ꢀ propylene > 1-butene over
Ga2O3. Ga2O3 also showed the activity for hydrogenation of 1,3-butadiene where 1,4-addition of hydro-
gen to 1,3-butadiene mainly took place to give 2-butenes as main products. The formation rate of hydro-
genated products depended on the amounts of H2 and alkenes adsorbed on Ga2O3. This implies that the
hydrogenation proceeds on the basis of the Langmuir–Hinshelwood mechanism.
Received 13 March 2012
In final form 26 April 2012
Available online 4 May 2012
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
via heterolytic adsorption of hydrogen molecule [21,22]. Pan
et al. also reported that hydrogen is dissociatively adsorbed to form
Activation of molecular hydrogen on solid surface is one of the
most fundamental elementary steps in various chemical reactions
such as hydrogenation of carbon–carbon unsaturated bond. Hydro-
genation of alkenes and conjugated dienes catalyzed by various
metals (platinum group metals, Ni and Co etc.) and transition me-
tal oxides such as Cr2O3 [1], TiO2, V2O5, MnO, Fe2O3, Co3O4 [2], ZnO
[3–6], alkaline earth metal oxides (MgO, CaO, BaO, and SrO) [7–9],
La2O3 [10,11], ZrO2 [2,12–17], Al2O3 [18] and ThO2 [11,19] have
been investigated. Kokes and co-workers investigated the interac-
tion of alkenes with molecular hydrogen on ZnO, and reported
heterolytic cleavages of H2 and C–H bond [3–6]. Hattori and co-
workers reported that hydrogen molecule is adsorbed on heteroge-
neous basic catalysts such as alkaline earth metal oxides and La2O3
to form H+ and Hꢁ [7–11]. It has been generally applicable partic-
ipation of heterolytically dissociated H+ and Hꢁ in the hydrogena-
tion [20]. In addition to hydrogenation of alkenes, dissociatively
adsorbed H+ (MꢁOH) and Hꢁ (MꢁH) also hydrogenate CO on
MgO, La2O3, ZrO2, and ThO2 [20]. On the other hand, Kondo and
co-workers reported that dissociatively adsorbed hydrogen form-
ing Zr–H and Zr–OH species on the ZrO2 did not react with gaseous
ethylene [17]. They proposed that H2 was activated on (or in the
vicinity of) the site on which ethylene was already preadsorbed
H+ and Hꢁ on Ga2O3 by using FT-IR and DFT calculations [23]. Re-
cently, we found that Ga2O3 also showed activity for the photocat-
alytic reduction of CO2 in the presence of H2 as a reductant [24,25].
CO was selectively produced at room temperature and ambient
pressure under photo-irradiation. The amount of evolved CO de-
pended not only on the amounts of CO2 but also on the amounts
of H2 adsorbed on Ga2O3, which indicates that chemisorbed species
of both CO2 and H2 are involved in the photocatalytic reduction of
CO2. FT-IR spectra showed that CO2 adsorbed to form bidentate and
monodentate bicarbonate and H2 adsorbed dissociatively to form
Ga–Hꢁ and Ga–OH+. We proposed that the dissociatively-adsorbed
hydrogen on Ga2O3 reduced the monodentate bicarbonate to the
bidentate formate under photoirradiation. The bidentate formate,
which was an intermediate in the photocatalytic reduction,
decomposed to CO. Hydrogenation of other molecules such as al-
kenes and dienes with molecular hydrogen is expected to take
place over Ga2O3, because, on the surface of Ga2O3, Ga–Hꢁ and
Ga–OH+ is generated from molecular hydrogen as well as MgO
and these species (Ga–Hꢁ and Ga–OH+) showed activity for the
photoreduction of CO. However, up to now, there is no report on
the hydrogenation of alkenes and dienes over Ga2O3.
In this study, we report that hydrogenation of various lower
alkenes and diene such as ethylene, propylene, 1-butene and
1,3-butadiene with molecular hydrogen takes place over Ga2O3.
This is the first report of hydrogenations over Ga2O3.
as a
p-bonded species and preadsorbed p-bonded ethylene was
found to be hydrogenated by gaseous H2 to ‘side-on’ adsorbed eth-
ane followed by a transformation into the more stable ‘end-on’ ad-
sorbed ethane.
On the basis of FT-IR spectra, Collins et al. have reported that
dissociatively-adsorbed hydrogen atoms (Ga–Hꢁ and Ga–OH+)
are found to be formed on the surface of gallium oxide (Ga2O3)
2. Experimental
Ga2O3 was synthesized by adding aqueous solution of
Ga(NO3)3ꢂxH2O (Koujundo Chemical Laboratory Co. Ltd., 99.999%)
to 25 mass% NH3 aqueous solution to produce Ga(OH)3. The precip-
itated gel was filtered under vacuum and washed with distilled
⇑
Corresponding authors. Fax: +81 75 383 2561.
E-mail addresses: shishido@moleng.kyoto-u.ac.jp (T. Shishido), tanakat@mo-
leng.kyoto-u.ac.jp (T. Tanaka).
0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cplett.2012.04.052

80
T. Shishido et al. / Chemical Physics Letters 539–540 (2012) 79–82
water. The gel was then dried at 353 K for 24 h, followed by calci-
nation at 1073 K for 6 h. The calcined sample was assigned to
b-Ga2O3 phase (distorted hexagonal close-packed structure) based
on X-ray diffraction (XRD) patterns. The Brunauer–Emmett–Teller
10
8
ethylene
propylene
1-butene
(BET) specific surface area was 17 m2 gꢁ1
.
The hydrogenations of lower alkenes and 1,3-butadiene with
molecular hydrogen were carried out in a closed recirculation reac-
tor connected to a vacuum line having a dead volume of 180.3 ml,
0.5 g of a Ga2O3 catalyst was used. Prior to reaction, Ga2O3 was
evacuated for 0.5 h at 773 K and pretreated with 8 kPa of O2 for
1 h at 773 K, followed by evacuation for 15 min at the same tem-
perature. H2 was purified by passage through 4A molecular sieves
at 77 K. Lower alkenes (ethylene, propylene, and 1-butene) and
1,3-butadiene was purified by vacuum distillation at 77 K. Pre-
6
4
2
mixed gas of alkene (100
lmol) or 1,3-butadiene with H2
(100 mol) was circulated over Ga2O3. The reaction temperature
l
0
of hydrogenation of lower alkenes was 373 K. Hydrogenation of
1,3-butadine was examined at 473 K. The products were analyzed
using an on-line gas chromatograph (TCD, Shimadzu GC-8A)
equipped with a column packed with Unicarbon A-400, and He
was used as the carrier gas.
0
1
2
3
4
5
Reaction time / h
8
6
4
2
0
3. Results and discussion
Figure 1 shows the time dependence of the amount of alkanes
in hydrogenation of lower alkenes with H2 and the initial
formation rate of alkanes over Ga2O3. The lower alkenes were
hydrogenated to the corresponding alkanes. The rate of the
hydrogenation was faster in the order of ethylene ꢀ propylene >
1-butene over Ga2O3. This order is well accorded with those over
heterogeneous basic catalysts such as MgO and CaO [7–9].
Figure 2 represents the time dependence of the amount of bu-
tenes and n-butane in the hydrogenation of a conjugated diene
(1,3-butadiene) with H2. 1,3-Butadiene was mainly hydrogenated
to trans and cis-2-butens and a trace amount of n-butane was de-
tected. This result indicates that the hydrogenation rate of mono-
enes is much different from that of conjugated diene, in other
words, conjugated diene (1,3-butadiene) undergo hydrogenation
much faster than monoenes (butenes). This result also indicates
that 1,4-addition of hydrogen atoms took place over Ga2O3 in con-
trast to 1,2-addition which is commonly observed for conventional
hydrogenation catalysts such as transition metals and transition
metal oxides [7–9].
etylene
propylene
1-butene
Figure 1. The time dependence of the amount of alkanes in the hydrogenation of
lower alkenes with H2 and the initial formation rate of alkanes over Ga2O3. Reaction
temperature: 373 K, alkene (100 lmol) and H2 (100 lmol).
Hattori and co-workers reported that the hydrogenation over
heterogeneous basic catalysts like alkaline earth oxides has charac-
teristic features which distinguish basic catalysts from conven-
tional metallic hydrogenation catalysts [7–11,20,26]. In the cases
of alkaline earth metal oxides, 1,3-butadiene undergoes hydroge-
nation at 273 K, and the products of diene hydrogenation consist
exclusively of monoenes with no alkanes being formed at 273 K.
Hydrogenation of 1,3-butadiene mainly yields 2-butenes. These
characteristic features of basic catalysts are basically similar to
those of Ga2O3. Hattori and co-workers demonstrated that MgO
was active and highly selective for the hydrogenation of 1,3-buta-
diene to cis-2-butene, during which H2 maintained its molecular
identity by using NMR and mass spectra. Both H atoms in a H2 mol-
ecule are incorporated into one hydrogenated molecule [7–9]. On
the basis of above features in hydrogenation of alkenes over basic
catalysts, they proposed the reaction mechanism of hydrogenation
of 1,3-butadiene in which anionic intermediates are involved. In
this proposed mechanism, at first, hydrogen adsorbed dissociative-
ly on the surface of metal oxide to form H+ and Hꢁ. Then, Hꢁ at-
tacks 1,3-butadiene to form the allyl anion of the trans-form
which takes place either interconversion to form cis-allyl anion
or addition of H+ to form butenes. Since the electron density of
trans-2-butene
cis-2-butene
1-butene
butane
1,3-butadiene
100
80
60
40
20
0
30
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
Reaction time / h
Figure 2. The time dependence of the amount of butenes and butane in the
hydrogenation of 1,3-butadiene with H2. Reaction temperature: 473 K, alkene
(100 lmol) and H2 (100 lmol).

T. Shishido et al. / Chemical Physics Letters 539–540 (2012) 79–82
81
the allyl anions is the highest on the terminal C atom, the posi-
8
6
4
2
0
30
25
20
15
10
5
tively charged H+ selectively adds to the terminal C atom to com-
plete 1,4-addition of H atoms to yield 2-butene. Because alkyl
anions are less stable than allyl anions, a large difference in the
reactivity between dienes and monoenes results from difficulty
of alkyl anion formation compared to allyl anion formation. A sim-
ilar feature in 1,3-butadiene hydrogenation over Ga2O3 and alka-
line earth metal oxides suggests that both anionic intermediates
and heterolytically dissociative-adsorbed hydrogen (H+ and Hꢁ) in-
volved in the reaction mechanism of 1,3-butadiene hydrogenation
over Ga2O3 as well as MgO (Scheme 1). However, a detailed reac-
tion mechanism is now under investigation and will be reported
in the near future.
The variations of the activity of Ga2O3 with the evacuation tem-
perature for the hydrogenation of ethylene and the isomerization
of 1-butene were measured to examine the nature of the active
sites on Ga2O3 (Figure 3). The highest activity for hydrogenation
of ethylene was observed after evacuation at 773 K. On the other
hand, in the case of isomerization of 1-butene, the optimum evac-
uation temperature was 1173 K. This value was remarkably higher
than those for the isomerization of 1-butene over MgO which is a
typical basic catalyst. It was reported that the activities for hydro-
genation of ethylene over MgO, CaO, SrO, and BaO reached the
maxima for values of evacuation temperatures at 1373, 1073,
1273, and 1270 K, respectively [7–9]. These values were consider-
ably higher than those for the isomerization of 1-butene which
were 873, 873, 1073 and 1073 K, respectively. These temperatures
are also higher than 773–873 K for the esterification [27], the poly-
merization [28] and the H2–D2 exchange reaction [29]. These facts
strongly suggest that the active sites for hydrogenation of alkenes
over alkaline earth metal oxides are different from those for isom-
erization of 1-butene, esterification, polymerization or H2–D2 ex-
change reaction. It has been believed that the active sites for
hydrogenation on alkaline earth oxides are metal cation–O2ꢁ ion
0
473
673
873
1073
1273
Pretreatment temperature / K
Figure 3. Effect of evacuation temperature of Ga2O3 on the activities in hydroge-
nation of ethylene (d) and isomerization of 1-butene (s). Reaction temperature:
473 K for hydrogenation of ethylene, 343 K for isomerization of 1-butene, Compo-
sition of reactants: ethylene (100
ethylene, 1-butene (100 mol) for isomerization of 1-butene.
lmol) and H2 (100 lmol) for hydrogenation of
l
at 773 K which was the temperature of the highest activity for
the hydrogenation of ethylene. In the case of MgO, CO2 completely
poisoned the active site for both isomerization of 1-butene and
hydrogenation of ethylene. In contrast, in the case of Ga2O3, CO2 re-
duced only about a half of the activity for isomerization of 1-bu-
tene and 20% of the activity for hydrogenation of ethylene
(Figure S1). These results indicate that the hydrogenation on
Ga2O3 proceeds on different sites from those for isomerization
and that basic sites on Ga2O3 was not active sites for hydrogena-
tion of ethylene.
pairs having low coordination number like Mg23cþ–O2ꢁ pairs (c de-
3c
Figure 4 shows the dependence of adsorbed amount of ethylene
and hydrogen over b-Ga2O3 against evacuation temperature. For
both ethylene and hydrogen, the adsorbed amounts increased with
a rise in evacuation temperature. In the case of hydrogen, the larg-
est adsorbed amount was observed at 673 K and maintained up to
notes the coordination number of metal cation or oxygen anion),
because such highly unsaturated sites are formed by the evacua-
tion and removal of carbon dioxide at high temperatures
[7–9,30,31].
In the case of Ga2O3, the optimum evacuation temperatures for
hydrogenation of ethylene and isomerization of 1-butene were dif-
ferent as well as alkaline earth metal oxides. On the other hand, the
optimum evacuation temperature for hydrogenation of ethylene
was considerable lower than that for isomerization of 1-butene.
Moreover, even after the evacuation at 1173 K, the activity for
isomerization of 1-butene on Ga2O3 was much lower than that of
MgO evacuated at 873 K. Moreover, Ga2O3 did not show any
detectable activity for isomerization of 1-butene when evacuated
20
ethylene
H2
15
CH2
CH2D
CH2D
+D-
+D+
10
5
H
C
H
C
H
C
CH
CH
CH
H2C
DH2C
H2C
D-
Ga
D+
O
+D+
H
H
H
H
C
C
C
C
H2C
DH2C
CH2D
CH2D
0
D2
473
673
873
1073
Pretreatment temperature / K
Ga
O
Figure 4. Effect of evacuation temperature of Ga2O3 on the amount of adsorbed H2
(d) and etylene (s). The amount of hydrogen adsorption was measured at the
reaction temperature (373 K).
Scheme 1. Plausible reaction mechanism of hydrogenation of 1.3-butadiene with
D2 over Ga2O3.

82
T. Shishido et al. / Chemical Physics Letters 539–540 (2012) 79–82
873 K. On the other hand, in the case of ethylene, the adsorbed
amount reached a maximum around 773 K. Therefore, a rela-
tively-large amount of hydrogen and ethylene adsorbed over
Ga2O3 evacuated at 773 K. According to Figures 3 and 4, the initial
rate of ethylene hydrogenation depends on the amount of ad-
sorbed hydrogen and ethylene. This implies that the hydrogenation
of ethylene would proceed on the basis of the typical Langmuir–
Hinshelwood mechanism. We have found that the adsorption iso-
therm of H2 on b-Ga2O3 after the adsorption of ethylene is incon-
sistent with the conventional adsorption isotherm of H2 on b-
Ga2O3. Therefore, at present, we assume that a part of hydrogen
adsorbed competitively on Ga2O3 with lower alkanes. The adsorp-
tion behaviors of lower alkenes and H2 and the conformation of the
adsorbed lower alkenes and H2 species on b-Ga2O3 are now under
investigation.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cplett.2012.
04.052.
References
[1] R.L. Burwell, A.B. Littlewood, M. Cardew, G. Pass, C.T.H. Stoddart, J. Am. Chem.
Soc. 82 (1960) 6272.
[2] D.L. Harrison, D. Nicholls, H. Steiner, J. Catal. 7 (1967) 359.
[3] A.L. Dent, R.J. Kokes, J. Phys. Chem. 73 (1969) 3781.
[4] A.L. Dent, R.J. Kokes, J. Phys. Chem. 73 (1969) 3772.
[5] A.L. Dent, R.J. Kokes, J. Phys. Chem. 74 (1970) 3653.
[6] R.J. Kokes, A.L. Dent, Adv. Catal. 22 (1972) 1.
[7] H. Hattori, Y. Tanaka, K. Tanabe, Chem. Lett. (1975) 659.
[8] H. Hattori, Y. Tanaka, K. Tanabe, J. Am. Chem. Soc. 98 (1976) 4652.
[9] Y. Tanaka, H. Hattori, K. Tanabe, Chem. Lett. (1976) 37.
[10] Y. Imizu, K. Sato, H. Hattori, J. Catal. 76 (1982) 65.
[11] Y. Imizu, H. Hattori, K. Tanabe, J. Chem. Soc.-Chem. Commun. (1978) 1091.
[12] Y. Nakano, T. Yamaguchi, K. Tanabe, J. Catal. 80 (1983) 307.
[13] J. Kondo, Y. Sakata, K. Domen, K. Maruya, T. Onishi, J. Chem. Soc.-Faraday Trans.
8 (6) (1990) 397.
[14] J. Kondo, K. Domen, K. Maruya, T. Onishi, J. Chem. Soc.-Faraday Trans. 8 (6)
(1990) 3021.
[15] J. Kondo, K. Domen, K. Maruya, T. Onishi, J. Chem. Soc.-Faraday Trans. 8 (6)
(1990) 3665.
[16] J. Kondo, K. Domen, K. Maruya, T. Onishi, Chem. Phys. Lett. 188 (1992) 443.
[17] J. Kondo, K. Domen, K. Maruya, T. Onishi, J. Chem. Soc.-Faraday Trans. 8 (8)
(1992) 2095.
4. Conclusion
Hydrogenation of lower alkenes (ethylene, propylene, and
1-butene) to the corresponding alkanes proceeded over b-Ga2O3.
The rate of the hydrogenation was faster in the order of ethyl-
ene ꢀ propylene > 1-butene over Ga2O3. Ga2O3 also showed the
activity for hydrogenation of 1,3-butadiene and 1,4-addition of
hydrogen to 1,3-butadiene mainly took place to give 2-butenes
as main product. The highest activity for hydrogenation of ethylene
on Ga2O3 was observed after evacuation at 773 K, whereas the
highest activity for isomerization of 1-butene was observed after
evacuation at 1173 K. The active sites for hydrogenation on
Ga2O3 are different from those for isomerization and basic sites
of Ga2O3 is not active for hydrogenation. The initial rate of ethylene
hydrogenation depends on the amounts of chemisorbed hydrogen
and ethylene, which indicates that the hydrogenation of ethylene
on Ga2O3 involves two adsorbed species, that is, H2 and ethylene.
This implies that the hydrogenation proceeds on the basis of the
Langmuir–Hinshelwood mechanism.
[18] J.H. Sinfelt, J. Phys. Chem. 68 (1964) 232.
[19] Y. Imizu, H. Hattori, K. Tanabe, T. Kondo, Bull. Chem. Soc. Jpn. 52 (1979) 2189.
[20] G.W. Wang, H. Hattori, J. Chem. Soc.-Faraday Trans. I (80) (1984) 1039.
[21] S.E. Collins, M.A. Baltanas, A.L. Bonivardi, Langmuir 21 (2005) 962.
[22] S.E. Collins, M.A. Baltanas, J.L.G. Fierro, A.L. Bonivardi, J. Catal. 211 (2002) 252.
[23] Y.X. Pan, D.H. Mei, C.J. Liu, Q.F. Ge, J. Phys. Chem. C 115 (2011) 10140.
[24] K. Teramura, H. Tsuneoka, T. Shishido, T. Tanaka, Chem. Phys. Lett. 467 (2008)
191.
[25] H. Tsuneoka, K. Teramura, T. Shishido, T. Tanaka, J. Phys. Chem. C 114 (2010)
8892.
[26] Y. Imizu, K. Tanabe, H. Hattori, J. Catal. 56 (1979) 303.
[27] K. Tanabe, K. Saito, J. Catal. 35 (1974) 247.
[28] T. Iizuka, H. Hattori, Y. Ohno, J. Sohma, K. Tanabe, J. Catal. 22 (1971) 130.
[29] M. Boudart, A.B. Walters, A. Delbouil, E.G. Derouane, V. Indovina, J. Am. Chem.
Soc. 94 (1972) 6622.
[30] H. Hattori, Chem. Rev. 95 (1995) 537.
[31] H. Hattori, Appl. Catal. A-Gen. 222 (2001) 247.
Acknowledgement
A part of this work was supported by the Research Seeds Quest
Program (Lower Carbon Society (2010)) from the Japan Science and
Technology Agency (JST).