be recycled, and (iv) the catalysts that are involved are usually
readily accessible and not sensitive.11 Transfer hydrogenation
has recently been employed for the hydrogenation of carbonyl
compounds such as furfural to furfuryl alcohol using metal
oxides, including zeolite beta, Ru/RuO2/C, MgO, Al2O3, ZrO2,
ZnO, SiO2, and Mg-Al, as catalysts.10,12-15 However, the catal-
ytic performance of these catalysts have been relatively low
(their selectivities were below 90%).
(M = Ce, Y, Gd, and Yb) until the solution reached a pH of 12.
After mixing, the obtained white dispersion was transferred to
a Teflon bottle and placed in a stainless steel autoclave, which
was sealed and maintained at 423 K for 24 hours. After cool-
ing the autoclave in an ice bucket, the white precipitate was
filtered and washed with hot deionized water until the filtrate
reached a pH of 7. The precipitates were dried under vacuum
for 12 hours at room temperature to provide M(OH)3. The as-
synthesized M(OH)3 was calcined under air at the desired
temperature (923 K) for 5 hours with a heating rate of 5
K.min¹1. The calcined sample was then heat treated under H2
at 673 K, and this oxide was defined as M2O3*_C923_HT673
(*M2O3 = Y2O3, Gd2O3, and Yb2O3; MO2 = CeO2). Commer-
cial Mg(OH)2 was calcined at 973 K for 5 hours and then
subjected to H2 treatment at 673 K, and this oxide was defined
as MgO_C923_HT673.
2.3 TGA-DTA Experiment. Thermogravimetric analysis
(TGA) and differential thermal analysis (DTA) were conducted
with a Rigaku TG-8120 ThermoPlus Evo instrument. Pow-
dered samples were placed in a platinum holder that rested on
ring-type differential thermocouples. N2 was used as the carrier
gas at a flow rate of 250 mL/min, and α-alumina was used as
the reference material. The heating rate was 5 K.min¹1, and the
temperature range was 298-1173 K. For determining the basic
sites, the catalyst was heated at 423 K for 30 minutes under
vacuum. The CO2 gas was introduced at room temperature
and aged for 24 hours. Then the samples were analyzed using
TGA. The basic sites were calculated by calculating the dif-
ferent mass of the fresh catalyst and the treated catalyst with
CO2.
Lanthanum oxide (La2O3) has high thermal stability and a
high k-value, it is relatively inexpensive compared to other rare
earth oxides, and it can perform a host of catalytic reac-
tions.16,17 La2O3 can easily react with CO2 gas in the atmos-
phere due to its basicity.18 In addition, La2O3 is hygroscopic
and can easily be converted to La(OH)3 under air.17 Therefore,
reactions that involve La2O3 and storage of La2O3 must be
carried out under anhydrous conditions to preserve the integrity
of the La2O3. La2O3 has been used for hydrogenation reactions
both as bulk La2O3 and as supported La2O3.19,20 The effects
of heat treatment and the atmosphere during the preparation
of the La2O3 have been reported.21 Heat treatment influences
the catalytic performance of calcined La2O3, and due to the
presence of surface O2¹ ions and oxygen vacancies on the sur-
face La2O3, the optimum yield of furfural was achieved when
the calcination temperature was 923 K, and the process was
conducted under air. Hydrogen treatment of calcined La2O3
increased the selectivity for furfuryl alcohol due to increasing
interactions between furfural and the Lewis acidic sites.21 In
the current report, selective hydrogenations of carbonyl com-
pounds with 2-propanol using La2O3 were studied in detail and
performed with remarkably high selectivity.
2.4 XRD Experiment. Powdered X-ray diffraction patterns
of the samples were recorded on a Rigaku MiniFlex 600
diffractometer using Cu Kα radiation (40 kV, 40 mA). Scans
were performed over the 2θ range from 10° to 80°.
2.5 BET Experiment. The specific surface areas were
determined from nitrogen adsorption-desorption isotherms at
liquid nitrogen temperature by using a Belsorp Max system
with Brunauer-Emmett-Teller (BET) methods. Before analysis,
all samples were pre-treated at 423 K for 2 hours to remove
physisorbed water and gases.
2. Experimental
2.1 Materials.
The following materials were used to
synthesize the catalyst: La(NO3)3 (La(NO3)6¢6H2O, Wako),
LaCl3 (LaCl3, Wako), commercial La2O3 (La2O3, Wako), com-
mercial La(OH)3 (La(OH)3, Aldrich), Ce(NO3)3¢6H2O (Wako),
Y(NO3)3¢nH2O (Wako), Gd(NO3)3¢6H2O, Mg(OH)2 (Wako),
and Yb(NO3)3¢nH2O (Wako). Furfural, furfuryl alcohol, 2-
propanol, methanol, ethanol, 1-propanol, 2-butanol, and 1-4
dioxane were purchased from Wako.
2.2 Preparation of the Catalyst. In a typical procedure,
aqueous NaOH (3 M) was added to 0.3 M aqueous La(NO3)3
until the mixture reached a pH of 12. After mixing, the
obtained white dispersion was transferred to a Teflon bottle and
placed in a stainless steel autoclave, which was sealed and
maintained at 423 K for 24 hours. After cooling the autoclave
in an ice bucket, the white precipitate was filtered and washed
with hot deionized water until the filtrate reached a pH of 7.
The precipitate was dried under vacuum for 12 hours at room
temperature to afford La(OH)3. The as-synthesized La(OH)3
was calcined under air at the desired temperature (923 K) for
5 hours with a heating rate of 5 K.min¹1, and this oxide was
defined as La2O3_C923. The calcined sample was then heat
treated under H2 at 673 K, and this oxide was defined as
La2O3_C923_HT673 (HT673 refers to hydrogen treatment of
the calcined catalyst at 673 K). The residual Na on the catalyst
was 0.01%(w/w) determined using ICP.
2.6 NH3-TPD. NH3-TPD measurements of La2O3 were
carried out in a flow apparatus with a thermal conductivity
detector (TCD) detector by using a Belsorp Max system. Prior
to analysis, the samples were pre-treated under flowing He at
873 K for 30 minutes to remove physisorbed water and gases.
NH3 adsorption was determined at 373 K for 30 minutes with a
5% NH3/He mixture. After purging with flowing He for 15
minutes, desorption of the adsorbed NH3 was measured during
heating to 1223 K. The NH3-TPD measurements of La(OH)3
were determined using acid/base titration. Samples were pre-
treated under flowing He at 523 K for 60 minutes, and NH3
adsorption was determined using the same procedure as was
used for La2O3. The desorbed NH3 was trapped using an acidic
solution, and then the solution was titrated with an NaOH
solution.
2.7 FTIR Experiment. Fourier transform infrared (FT-IR)
spectra were recorded on a HORIBA FF-720 FREEXACT-II
spectrophotometer in the range of 4000-500 cm¹1. Pellet were
prepared by mixing the powdered solid with KBr in a 6:1
A similar method was used to synthesize metal oxides.
Aqueous NaOH (3 M) was added to 0.3 M aqueous M(NO3)3
© 2018 The Chemical Society of Japan