CO2 hydrogenation to alcohols over highly dispersed Co/SiO2 catalysts derived from acetate

Article abstract of DOI:10.1246/cl.2001.904

Highly dispersed Co(A)/SiO₂ catalysts were prepared from acetate, and used for CO₂ hydrogenation to alcohols under the reaction conditions of T = 493 K, P = 2.1MPa, H₂/CO₂ = 3/1, and GHSV = 2000 h^-1. Catalytic activity was greatly improved by adding a small amount of Ir to the catalyst, and methanol (ca. 20%) was formed, in contrast with Co(N)/SiO₂ catalyst derived from nitrate. Ethanol selectivity was increased up to 8% by adding Na salt to the Ir/Co(A)/SiO₂ catalyst.

Full text of DOI:10.1246/cl.2001.904

904
Chemistry Letters 2001
CO2 Hydrogenation to Alcohols
over Highly Dispersed Co/SiO2 Catalysts Derived from Acetate
Kiyomi Okabe,* Hidekazu Yamada,† Takaaki Hanaoka, Takehiko Matsuzaki, Hironori Arakawa, and Yoshimoto Abe†
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central #5, Tsukuba, Ibaraki 305-8565
†Science University of Tokyo, Noda, Chiba 278-8510
(Received June 12, 2001; CL-010549)
Highly dispersed Co(A)/SiO2 catalysts were prepared from
tried to use for CO2 hydrogenation to oxygenates.
acetate, and used for CO2 hydrogenation to alcohols under the
reaction conditions of T = 493 K, P = 2.1MPa, H2/CO2 = 3/1,
and GHSV = 2000 h–1. Catalytic activity was greatly improved
by adding a small amount of Ir to the catalyst, and methanol
(ca. 20%) was formed, in contrast with Co(N)/SiO2 catalyst
derived from nitrate. Ethanol selectivity was increased up to
8% by adding Na salt to the Ir/Co(A)/SiO2 catalyst.
Silica gel (Fuji–Davison #57, 16–32 mesh size, S = 260
m2g–1, Vp= 1.2 cm3g–1, Dp= 18 nm) was impregnated with
aqueous solution of cobalt acetate or nitrate by the incipient
wetness method to form 5 wt%-Co/SiO2 catalysts (denoted as
Co(A)/SiO2 and Co(N)/SiO2, respectively). Alkali and alkaline
earth metal (M) acetate were added by co-impregnation. After
pre-treatment in H2 flow at 573 K for 3 h, Ir4(CO)12 was added
to Co–M/SiO2 by the vapor phase deposition method at 433 K
in vacuo. After reducing the Ir/Co–M/SiO2 catalysts at 723 K
for 3 h, CO2 hydrogenation was carried out under reaction con-
ditions of T = 473–508 K, P = 2.1 MPa, H2/CO2 = 3/1, GHSV =
2000 h–1. Effluent gas from the fixed bed reactor was analyzed
by on-line gas chromatography.
The reaction results at 493 K were summarized in Table 1.
The amounts of the additives were Ir/Co/M = 0.13 / 1.0 / 0.67
in atomic ratio. Since almost no deactivation was observed, the
results at time-on-stream of t = 4–6 h were listed. The conven-
tional catalyst Co(N)/SiO2 showed relatively high activity with-
out additives, and the main product was CH4, as commonly rec-
ognized.8 In contrast, the catalyst derived from acetate without
additives, Co(A)/SiO2, showed almost negligible activity. The
results of XPS and H2 chemisorption measurement indicated
that Co in Co(N)/SiO2 could be reduced to metallic state, Co0,
while that Co in Co(A)/SiO2 was hardly reduced. However, by
adding Ir to Co(A)/SiO2 (Ir/Co = 0.13 in atomic ratio), CO2
conversion was greatly improved, and MeOH was formed.
Since Ir/SiO2 itself did not show any activity in CO2 hydro-
genation, the improvement is thought to be a synergistic effect
between Ir and Co(A)/SiO2. The TPR result indicated that
Ir/Co(A)/SiO2 was easily reduced as low as 540 K, while that
Co(A)/SiO2 was not reduced below 1070 K. The XPS results
showed that %-reduction to Co0 (or 100 × Co0/(Co0+Co2+)) on
Ir/Co(A)/SiO2 surface was %-Co0 = 75%, which was almost
comparable to Co(N)/SiO2 surface, %-Co0 = 73%, as shown in
Catalytic hydrogenation is one of the most promising ways
of CO2 fixation from view points of the efficiency and the
diversified products.1 Methanol (MeOH) synthesis by CO2
hydrogenation has been extensively studied over mainly Cu-
based catalysts, and even a bench scale test is now in progress.2
On the contrary, only several investigations have been reported
on the direct conversion of CO2 to ethanol (EtOH). Okamoto et
al.3 and Inui et al.4 developed combined catalyst systems based
on K/Fe–Cu–ZnO(–Al2O3), respectively, and high STY of
EtOH was obtained. The concept of the catalyst design is
reported to be the combination of C–C bond formation catalysts
and catalysts for oxygenate formation. Rh-based catalysts were
reported by Kusama et al.5 Rh is known as the typical catalyst
species for oxygenate production from synthesis gas (CO/H2
mixture). The concept of the catalyst design is the modification
of the electronic state of Rh, resulting in application to CO2
hydrogenation. Ru-based homogeneous catalyst system was
also reported by Tominaga et al.6 The noble metal catalysts are
financially limited for wide application in industrial scale.
Cobalt is well known as typical catalyst for Fischer–Tropsch
(F–T) synthesis to form hydrocarbons. However, it is reported
that some oxygenates are also formed in F–T reaction, when Co
is highly dispersed.7 The catalysts for CO hydrogenation is
generally applicable to CO2 hydrogenation, too. Thus, Co/SiO2
catalysts with high-dispersion were prepared from acetate, and
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
905
were increased, respectively. Since conversion was increased
without significant change in EtOH selectivity at a higher tem-
perature below 523 K, the highest STY of EtOH up to 7.9 g
dm–3 h–1 was obtained over Ir/Co(A)–Na2O/SiO2 catalyst at 508
K.
References and Notes
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