S. Li et al. / Catalysis Communications 61 (2015) 92–96
93
Scheme 1. Reaction sequence of IA to MGBL.
2. Experimental
caused by the oxygen-containing functional groups produced by the
oxidation treatment with HNO3 and NaClO [24].
2.1. Active carbon pretreatment
3.2. FT-IR and NH3-TPD analyses
A commercially available wooden active carbon (AC, denoted as C0)
with BET surface area of 1600 m2/g was pretreated with 15% HNO3 at 75
°C for 3 h, and then washed with distilled water until the pH of the fil-
trate was neutral and dried in vacuum at 100 °C (denoted as CN). The
AC was also treated with 10% NaClO at room temperature for 24 h,
washed with distilled water and dried (denoted as CCl). Then CN was
further treated with NaClO of four different concentrations at room
temperature for 24 h. For convenience, they are hereafter called C1, C2,
C3 and C4, where 1, 2, 3 and 4 represented the successively increasing
concentrations of NaClO used in the pretreatment. The detailed proce-
dure was provided in the Supporting information (SI).
It could be obviously seen from the FT-IR spectra of the catalysts
(Fig. 1) that the characteristic peaks of \COOH at 1708 cm−1
,
1520 cm−1 and 1328 cm−1 existed in the catalysts with support
pretreated with HNO3, which indicated that the acidic functional groups
of AC were primarily introduced by HNO3 pretreatment. The NH3-TPD
results (Table 1 and Fig. S2) showed the Pd/CN catalyst possessed the
highest acid density (2.32 mmol NH3/g), while Pd/CCl showed the low-
est acid density (0.39 mmol NH3/g). The acid densities of Pd/C1, Pd/C2,
Pd/C3 and Pd/C4 were all around 1.70 mmol NH3/g, a notable decrease
compared with Pd/CN. These results indicated that the surface acidity
of the catalyst was influenced by HNO3 and NaClO for their oxidability
and acidity–basicity.
2.2. Catalyst preparation and characterization
Oxidation treatment could introduce oxygen-containing functional
groups to the surface of active carbon [23–28], such as \COOH, \OH
and \COOR, and such groups generally enhanced the interaction be-
tween the support and metal nanoparticles, which could bring about
fine dispersion of metal nanoparticles and thus improve the catalytic
activity [25]. On the other hand, pretreatment with HNO3 was an effi-
cient way to increase the surface acidity [24,28]. Herein, oxidation treat-
ment with proper concentration of HNO3 was mainly to increase the
surface oxygen-functional groups in order to improve the dispersion
of metal and improve the activity of the catalysts indirectly. However,
high concentration HNO3 was not suitable for its forceful oxidability
[21,24,25]. In this study, we selected low concentration HNO3 and
NaClO with low oxidability and basicity to treat active carbon to create
oxygen-rich AC surface environment and proper acidity, for the purpose
of obtaining suitable catalysts for the IA hydrogenation to MGBL.
The 3 wt.% Pd/C catalysts were prepared by deposition–precipitation
using NaHCO3 as precipitant and reduced with HCHO and were charac-
terized by XRD, BET, TEM, FT-IR and NH3-TPD to study their physical–
chemical properties. The detailed preparation procedures and the char-
acterization techniques were described in the SI.
2.3. Catalytic hydrogenation reaction
Catalytic hydrogenation of IA was performed in a 50 ml autoclave.
After 1 wt.% IA aqueous solution and 1.5 mol% Pd catalysts were intro-
duced into the reactor, the reactor was purged with H2 for five times
and then pressurized to the desired pressure at room temperature.
After reaction, unconsumed IA was detected by high performance liquid
chromatography (HPLC) and products were quantified by gas chroma-
tography (GC). The products were also analyzed by GC or LC coupled
with a mass spectrometer (MS). All experiments were performed in
duplicate.
3.3. Results of XRD and TEM
As shown in Fig. 2, the catalysts exhibited characteristic diffraction
peaks of carbon's crystalline plane (111) around 2θ = 26.6° (JCPDS
No. 75-2078), and the peaks at 2θ = 40°, 46.5° and 68.2° were ascribed
to the characteristic diffraction peaks of palladium. The weak peaks of
Pd indicated that Pd particles were finely dispersed on AC. Small and
uniform Pd particles were also confirmed by the TEM results (Figs. 3
and S3), which were favored to improve the activities of catalysts. As
shown in Table 1, the Pd particle sizes of the catalysts Pd/C1, Pd/C2,
Pd/C3 and Pd/C4 were smaller than 3 nm, while Pd particle sizes of Pd/
C0, Pd/CCl and Pd/CN were 4.97 nm, 3.46 nm and 3.22 nm, respectively.
The results suggested that different AC pretreatment influenced Pd par-
ticle size. Copretreating with HNO3 and NaClO led to the decrease of Pd
particle size, which was due to the changes of the facial environment of
AC.
3. Results and discussion
3.1. BET analysis
As shown in Table 1, the SBET of Pd/C0, Pd/CCl, Pd/CN were 1591.7 m2/g,
1418.7 m2/g and 1011.1 m2/g respectively, while the SBET of Pd/C1, Pd/C2,
Pd/C3 and Pd/C4 were all below 900 m2/g. As revealed in the TEM images
of carbon supports (Fig. S1), the AC structures were altered by the pre-
treatments, which could be attributed to the oxidability of HNO3 and
NaClO [21–24]. Table 1 showed that pretreatments affected pore size dis-
tribution greatly. As the oxidation strength increased, the pore volume
and pore size decreased, which could be ascribed to the pore closure