Low temperature hydrogenation of α-angelica lactone on silica supported Pd-NiO catalysts with synergistic effect

Article abstract of DOI:10.1039/c6ra13374f

The hydrogenation of α-angelica lactone (α-AL) was achieved under mild conditions on silica supported Pd-NiO catalysts. NiO and palladium were sequentially loaded on silica by wet-impregnation and deposition-reduction, respectively. First a series of NiO/SiO₂ supports with varying Ni contents were prepared by a wet-impregnation method with Ni(NO₃)₂ as the precursor followed by calcination in air. Then a minute amount of palladium (0.2 wt%) was loaded by a deposition-reduction method using NaBH₄ as a reducing reagent. The Pd-NiO catalysts were characterized by nitrogen adsorption, XRD, H₂-TPR, XPS and TEM. The NiO were heterogeneously dispersed on silica with particle sizes ranging from 10 to 50 nm, whereas Pd was finely loaded with a diameter less than 5 nm. Nanoscale intimacy between Pd and NiO was noticed by HRTEM, resulting in high catalytic activity in liquid phase hydrogenation of α-angelica lactone to γ-valero lactone (GVL) under mild conditions. 0.2Pd-9.9NiO/SiO₂ showed the best activity among all the catalysts investigated, with 82% conversion and 100% selectivity to GVL within several minutes at 30 °C and 0.3-1 MPa H₂ pressure.

Full text of DOI:10.1039/c6ra13374f

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Low temperature hydrogenation of a-angelica
lactone on silica supported Pd–NiO catalysts with
synergistic effect†
Cite this: RSC Adv., 2016, 6, 65377
*
Pei Zhang, Qingqing Yuan, Li Chen, Teng Xue, Yejun Guan and Peng Wu
The hydrogenation of a-angelica lactone (a-AL) was achieved under mild conditions on silica supported
Pd–NiO catalysts. NiO and palladium were sequentially loaded on silica by wet-impregnation and
deposition–reduction, respectively. First a series of NiO/SiO2 supports with varying Ni contents were
prepared by a wet-impregnation method with Ni(NO3)2 as the precursor followed by calcination in air.
Then a minute amount of palladium (0.2 wt%) was loaded by a deposition–reduction method using
NaBH4 as a reducing reagent. The Pd–NiO catalysts were characterized by nitrogen adsorption, XRD,
H2-TPR, XPS and TEM. The NiO were heterogeneously dispersed on silica with particle sizes ranging
from 10 to 50 nm, whereas Pd was finely loaded with a diameter less than 5 nm. Nanoscale intimacy
between Pd and NiO was noticed by HRTEM, resulting in high catalytic activity in liquid phase
hydrogenation of a-angelica lactone to g-valero lactone (GVL) under mild conditions. 0.2Pd–9.9NiO/
SiO2 showed the best activity among all the catalysts investigated, with 82% conversion and 100%
selectivity to GVL within several minutes at 30 ꢀC and 0.3–1 MPa H2 pressure.
Received 23rd May 2016
Accepted 30th June 2016
DOI: 10.1039/c6ra13374f
www.rsc.org/advances
Pd–NiO@SiO2 nanocatalysts illustrated an excellent catalytic
performance for catalytic p-chloronitrobenzene (p-CNB) hydro-
Introduction
genation (full conversion, 82.7% selectivity of p-chloroaniline).
The conversion of p-CNB over different catalyst followed the
trend of Pd–NiO@SiO2 > Pd@SiO2 > PdNi@SiO2. Synergistic
effect between nickel or nickel oxide and palladium has been
proposed to explain the unique catalytic behavior of these
catalysts.12,18 Inspired by these studies, we herein reported the
hydrogenation performance of the Pd–NiO catalyst with minute
amount of Pd (0.2 wt%).
a-AL has been dened as a new potential platform molecule
which can be produced from lignocellulosic materials levulinic
acid (LA) by dehydration (Scheme 1).22 As a perfume or food
additive, a-AL is widely used in the food and tobacco industry.
Moreover, it can serve as a fuel blending agent or a precursor
for the preparation of g-valerolactone (GVL), 2-methyltetrahy-
drofuran (2-MTHF) and levulinic acid esters. GVL is such a plat-
form chemical that offers tremendous exibility in downstream
applications.23 It has been successfully used for the production of
ionic liquids,24 butenes,25 1,4-pentanediols and alkanes,26 etc.
There have been extensive studies about the hydrogenation LA
and/or ethyl levulinate (EL) to produce GVL.27–34 Compared with
As a kind of promising composite material, bimetal catalysts
have been intensively studied in recent years due to the well-
known synergistic effect which will not be present in corre-
sponding monometallic catalysts.1–8 From a practical point of
view, the use of a bimetal catalyst is an interesting strategy as it
offers many possibilities such as an increase of catalytic activity
and tuning of the selectivity, especially as observed in upgrad-
ing biomass to chemicals and fuels.9–13 Precious metals, such as
palladium, have been widely used in catalytic hydrogenation
processes. When combined with a second transition metal,
such as nickel, enhanced reactivity and/or selectivity can be
achieved in many hydrogenation reactions.14–21 Nakagawa and
co-workers14 reported the hydrogenation of 5-hydroxymethyl-2-
furaldehyde by Ni–Pd bimetallic catalysts supported on silica
(Ni/Pd ¼ 7). The yield of 2,5-bis(hydroxymethyl)tetrahydrofuran
reached 96% over bimetallic catalysts, which was more active
than commercial RANEY® Ni and more selective than Pd/C.
Yang et al.16 have introduced a new procedure for the reduc-
tion of aromatic nitro compounds into amines with Pd/NiO-MC
(Ni-doped monolithic carbon aerogels) catalysts under ambient
conditions (25 ꢀC, 1 atm H2). Recently, Liu et al.18 reported that
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Chemical Engineering, East China Normal University, North
Zhongshan Road 3663, 200062, Shanghai, China. E-mail: yjguan@chem.ecnu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra13374f
Scheme 1 The synthesis of GVL from levulinic acid via a-angelica
lactone as an intermediate.
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LA or EL, a-AL can be hydrogenated smoothly under mild
The power X-ray diffraction (XRD) patterns were collected on
conditions over both Pd and Ru catalysts.22,35 Zhang et al. have a Rigaku Ultima IV X-ray diffractometer using Cu Ka radiation
reported the conversion of a-AL to GVL at 60 ꢀC and 4.0 MPa H2 (l ¼ 1.5405 A) operated at 35 kV and 25 mA. Transmission
ꢀ
pressure using 10 wt% Pd/C catalyst in a series of ionic liquids. electron microscopy (TEM) images were taken on a FEI Tecnai
Among these ionic liquids, [Bmim]PF6 showed the best perfor- G2 F30 microscope operating at 300 kV.
mance on the selectivity of GVL (99.8%). However, some ionic
Temperature programmed reduction (TPR) measurements
liquids are in solid phase at room temperature and difficult to were carried out in TP-5080 (Tianjin Xianquan, China) equipped
form a homogeneous solution with the reactant as a result of low with a thermal conductivity detector (TCD). The calcined
conversion. In another study, Al-Shaal et al.35 found that the GVL samples were treated at 100 ꢀC for 30 min under He stream (25
yield exceeded 97% with full conversion of a-AL under moderate mL minꢁ1) for 1 h in order to remove the physisorbed water.
ꢀ
reaction conditions (50 C, 24 bar H2 pressure for 1 h or room Then the samples were cooled down to 25 ꢀC in the same
ꢁ1
temperature, 500 mL minꢁ1 H2-ow rate for 24 h) over Ru/C atmosphere, and were heated up to 630 C at 10 C min in
ꢀ
ꢀ
catalyst. These results indicated that noble metals (Pd or Ru a 5% (v/v) hydrogen/nitrogen ow.
species) are able to saturate C]C bonds easily under mild
The X-ray photoelectron spectroscopy (XPS) was recorded on
reaction conditions. Herein, we showed that fast hydrogenation an Escalab 250xi spectrometer, using a standard Al Ka X-ray
of a-AL can be achieved on a silica supported Pd–NiO catalyst source (300 W) and analyzer pass energy of 30.0 eV. All
with minute amount of Pd (0.2 wt%) under mild conditions.
binding energies were referenced to the adventitious C 1s line at
284.6 eV.
Experimental
Preparation of catalysts
Catalytic tests
The catalytic hydrogenation was conducted in a Teon-lined (60
mL) steel batch reactor. 25 mg of catalyst was loaded into the
reactor along with 9.8 mL of H2O and 0.2 mL of substrate. No
pretreatment on the catalysts was conducted prior to reaction.
The reactor was purged with H2 for ve times and then pres-
surized with 0.3 MPa H2. The mixture was stirred in a water-bath
at 30 C. The products were diluted with ethanol and analyzed
with ame ionization detector (FID) and capillary column of DB-
FFAP (30 m length and 0.25 mm internal diameter).
NiO-bulk. NiO-bulk was prepared by calcination of the
Ni(NO3)2$6H2O at 500 C for 1 h in air.
ꢀ
xNiO–SiO2. xNiO–SiO2 (x ¼ 0.26, 2, 9.9, 19, 29 and 40
determined by ICP, wt%) samples were prepared by wet-
ꢀ
impregnation method followed by calcination at 500 C in air
for 1 h. The SiO2 used was Evonik Degussa Aerosil® 200 with
ꢀ
surface area of 208 m2 gꢁ1
.
Supported Pd catalysts. Supported Pd catalysts (ꢂ0.2 wt%,
determined by ICP) were prepared by a typical deposition–
precipitation–reduction method.36 The support (0.3 g) was
dispersed in H2O (60 mL) with stirring. A specied amount of Results and discussion
H2PdCl4 aqueous solution (21.512 gPd Lꢁ1) was added to the
mixture and stirred for 3 h. The nal pH value of the
Characterization of supported catalysts
The textural properties of the supported catalysts are shown in
suspension was adjusted to 10 by adding NaOH solution (1
M). Then, NaBH4 aqueous solution (NaBH4/Pd ¼ 10, molar
ratio) was added into the suspension and the mixture was
stirred for another 30 min allowing for the full reduction of
Pd2+ species. Thus obtained catalyst was dried at 110 ꢀC
overnight. The sample was denoted as Pd–xNiO–SiO2 (x
stands for Ni loading).
Table 1. The surface area of SiO2 is 208 m2 gꢁ1. As can be seen,
deposition of 0.2 wt% Pd caused a slight decrease in the specic
surface (from 208 to 192 m2 gꢁ1). With the NiO loading
increased (from 0.26 to 19.9 wt%), the surface area reduced
substantially (from 197 to 149 m2 gꢁ1). These can be explained
by the introduction of large amount of NiO particles into the
SiO2 pores.
Fig. 1 shows the XRD patterns of Pd–xNiO/SiO2. The
diffraction peaks located at 37, 43, 63, 75, and 79ꢀ are assigned
to the NiO phase [JCPDS 44-1159]. The intensity of these peaks
Catalyst characterization
Nitrogen adsorption–desorption isotherms at ꢁ196 ꢀC were
obtained on
a BELSORP-Max equipment. Prior to the
measurement, the samples were rst degassed at 150 ꢀC under
vacuum for 6 h. Specic surface areas (SSA) were calculated by
the Brunauer–Emmett–Teller (BET) method using ve relative
pressure points in the interval of 0.05–0.30. The pore size
distribution was obtained by the Barrett–Joyner–Halenda (BJH)
model applied to the adsorption isotherm.
The Pd loading was quantied by inductively coupled
plasma (ICP) on a Thermo IRIS Intrepid II XSP atomic emission
spectrometer. About 25 mg catalysts were digested using 5 mL
of aqua regia. The obtained solutions were diluted with deion-
ized water to the desired Pd concentration.
Table 1 Textural properties of supports and supported Pd catalysts
Catalysts
SSA (m2 gꢁ1
)
Pore volume (cm3 gꢁ1
)
SiO2
NiO
Pd–SiO2
Pd–0.26NiO/SiO2
Pd–2NiO/SiO2
Pd–9.9NiO/SiO2
Pd–19NiO/SiO2
208
5.5
0.8
0.078
0.65
0.64
0.67
0.55
0.44
192
197
183
166
149
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Fig. 1 XRD patterns of Pd–xNiO–SiO2 catalysts.
became weakened with the decrease of NiO loading. In all cases,
no Pd diffraction peaks were observed, meaning that Pd nano-
particles were highly dispersed on metal supports or the
diffraction intensity was beyond the detection limit at very low
Pd loading (0.2 wt%).
Fig. 2 shows the TPR proles of the NiO/SiO2 and Pd–xNiO/
SiO2 samples. The reduction prole of crystalline NiO prepared
by calcining Ni(NO3)2$6H2O at 500 ꢀC showed temperature
maximum (Tmax) at 408 ꢀC. This result is in agreement with
earlier reports.37–41 9.9NiO/SiO2 and 2NiO/SiO2 presented broad
ꢀ
reduction peak with Tmax of 388 and 418 C, respectively. The
Fig. 3 X-ray photoelectron spectra fitting of (a) Ni 2p3/2 and (b) Pd 3d
of Pd–9.9NiO/SiO2.
lower Tmax of 9.9NiO/SiO2 is likely assigned to those easily
reducible nickel oxide particles with large crystallites or those
weakly interacting with the suppoꢀrt.37 Pd–2NiO/SiO2 and Pd–
ꢀ
nanoparticle in air.46 The accurate distribution of Pd and NiO
on Pd–9.9NiO/SiO2 was further examined by HRTEM, high-
angle annular dark eld scanning tunneling electron micros-
copy (HAADF-STEM) (Fig. 4). From Fig. 4a and b one can see
that irregular spherical particles were well dispersed on the SiO2
support. HRTEM images reveal the presence of two types of
lattice spacing due to Pd nanoparticles: 0.22 and 0.19 nm
assigned to Pd (111) and Pd (200) plane, respectively.18,47,48
These Pd nanoparticles were found to be closely attached on
a (220) surface of NiO particles with lattice spacing about 0.14–
0.15 nm.16 To get more insight on the spatial distribution of Pd
9.9NiO/SiO2 had their Tmax at 371 C and 381 C, respectively.
This result reveals that the presence of trace amount of Pd
species enhanced the reduction of NiO. Moreover, the negative
peak due to palladium b-hydride phase as commonly observed
in H2-TPR prole of Pd/SiO2 (Fig. S2†) disappeared on Pd–NiO
catalysts. This phenomenon certainly suggests the strong
interaction between Pd and NiO, as has been reported earlier on
PdFe catalysts.42
We studied by XPS the surface Pd and Ni species for Pd–
9.9NiO/SiO2. The Pd 3d and Ni 3p spectra were tted and the
results are shown in Fig. 3. NiO species were clearly found in
Pd–9.9NiO/SiO2 according to the binding energies located at
854.7, 856.3 and 861.1 eV (Fig. 3a).43–45 Oxidized Pd2+ (Fig. 3b)
species were observed for this sample (binding energies of 337.2
and 342.0 eV), which is likely due to the exposure of Pd
Fig. 4 TEM images of (a) high resolution and (b) low resolution of Pd–
Fig. 2 H2-TPR profiles of NiO-bulk, 2NiO/SiO2, 9.9NiO/SiO2, 0.2Pd– 9.9NiO–SiO2; (c) Pd–9.9NiO–SiO2-reused; (d) HAADF-STEM-EDX
2NiO/SiO2 and 0.2Pd–9.9NiO/SiO2 catalysts.
mapping images of the Pd–9.9NiO/SiO2.
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and NiO, HAADF-STEM along with high resolution elemental particles very well so that more Pd–NiO active sites were
mapping was undertaken. The mapping results indicated that provided. Further increase of NiO content did not lead to
NiO (shown by green dots) and Pd (shown by yellow dots) improved a-AL conversion (Table 2, Entry 7).
located in exactly the same zone. These results lead to the
To clarify the role of Pd–NiO in facilitating a-AL hydroge-
conclusion that palladium closely interacted with NiO in a big nation, we thus performed a set of control experiments. First we
chance, which is consistent with the TPR results.
tested the hydrogenation activity of physical mixture of Pd/SiO2
and 9.9NiO/SiO2 (each in 15 mg catalyst). The conversion of a-
AL was 12% aer 1 h with GVL selectivity of 70% (Table 2, Entry
9). This result showed that a nanoscale intimacy of Pd and NiO
was the prerequisite to achieve the high hydrogenation activity
of a-AL. Second we pre-reduced the Pd–9.9NiO/SiO2 in H2 ow
and tested its hydrogenation activity (Table 2, Entry 10). The
results clearly suggested that the reduction of Pd–NiO led to
dramatic drop of a-AL conversion (27%). This unexpected result
prompted us to test the catalytic activity of Pd–Ni/SiO2 catalyst
in this reaction.
A series of Ni/SiO2 supports were prepared (see ESI†) by
reducing the NiO/SiO2 supports in owing H2 and then Pd was
loaded via the same deposition–reduction method as Pd–NiO/
SiO2 catalysts. We compared the turnover frequencies (TOF,
based on a-AL conversion per Pd weight) of Pd nanoparticles
supported on NiO/SiO2 and Ni/SiO2 (Fig. 5). It should be noted
that in order to achieve a reliable TOF, the conversion of a-AL
Catalytic activities of supported Pd catalysts
Table 2 summarizes the catalytic performance of NiO/SiO2, Pd/
SiO2, and Pd–NiO/SiO2 catalysts in the hydrogenation of a-AL
under 0.3 or 1 MPa H2 pressure. For comparison, SiO2 (Table 2,
Entry 1) and NiO–SiO2 (Table 2, Entry 2) were tested and both
supports did not show any catalytic activity under 0.3 MPa H2.
Increasing the H2 pressure to 1 MPa led to the production of
minute amount of b-AL (Table 2, Entry 3) due to the isomeri-
zation of a-AL catalyzed by acid sites. It is likely that certain
amount of acid sites related to Ni–OH species might be formed
at high H2 pressure, which favors the isomerization reaction
pathway. Nevertheless, NiO–SiO2 was not active at all for the
hydrogenation of a-AL under mild conditions. When 0.2 wt%
Pd was supported on SiO2, the a-AL conversion aer 1 h was
29% with GVL selectivity being 95% (Table 2, Entry 4). This
result demonstrates that Pd species were very active for the
hydrogenation of C]C bond in a-AL molecule even supported
on an inert support such as SiO2. When supported on an active
support, namely bulk NiO, the a-AL conversion was 76% aer
0.5 h with enhanced GVL selectivity as 100% (Table 2, Entry 5).
This result points to the synergy between Pd and NiO in
hydrogenation reactions, which is most likely related to the
structure modication of Pd metal by the addition of NiO.
Due to the low surface area (5.5 m2 gꢁ1) and high cost of bulk
NiO, we next tested the activity of Pd supported on NiO–SiO2
with high surface area and low Ni content. The result (Table 2,
Entry 6) showed that for Pd–9.9NiO/SiO2, the conversion of a-AL
aer 30 min and selectivity of GVL was 82% and 100%,
respectively. This reactivity was equal to or slightly higher than
Pd supported on bulk NiO. As expected, the inert SiO2 with
Fig. 5 TOF values, defined as molGVL (molPd min)ꢁ1, of Pd–xNiO/SiO2
relatively larger specic surface area could disperse NiO and Pd–xNi/SiO2 in a-AL hydrogenation.
Table 2 Catalytic activity of supported Pd catalysts in a-AL hydrogenationa
Sel.%
GVL
Entry
Catalyst
Time (min)
H2 pressure (MPa)
Conv. (%)
b-AL
1
2
3
4
5
6
7
8
9
10
SiO2
60
60
60
60
30
30
30
30
60
30
0.3
0.3
1
0.3
0.3
0.3
0.3
0.3
0.3
0.3
—
—
1
29
76
82
78
20
12
27
—
—
—
95
100
100
100
100
70
—
—
100
5
—
—
—
—
30
—
9.9NiO–SiO2
9.9NiO–SiO2
Pd–SiO2
Pd–NiO-bulk
Pd–9.9NiO/SiO2
Pd–40NiO/SiO2
Pd–40Ni/SiO2
NiO/SiO2 + Pd/SiO2
b
c
Pre-reduced-Pd–9.9NiO/SiO2
100
a
Reaction conditions: catalyst (25 mg), H2O (9.8 mL), a-AL (0.2 mL), 30 ꢀC. b Physical mixture: 15 mg of NiO/SiO2 and 15 mg of Pd/SiO2. c Catalyst
was pretreated with H2 at 200 ꢀC for 10 min, cooled down to room temperature under N2 ow, and then the reaction solution was added
immediately.
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has been controlled to be below or close to 30%. For catalysts showed very stable reactivity in the rst ve runs and starts to
with high Ni loading, for instance Pd–40Ni/SiO2, the a-AL deactivate in the next run. TEM image of the reused catalyst
conversion was 20% aer 1 h (Table 2, Entry 8), which was much (Fig. 4c) suggested no signicant aggregation of Pd nano-
lower than that of Pd–40NiO/SiO2 (78% aer 0.5 h, Table 2, particles. Elemental analysis (ICP) of the reused catalyst
Entry 7). Metallic Ni signicantly suppressed the hydrogenation revealed that no leaching of Pd occurred (not shown) either.
activity of Pd nanoparticles supported thereof. Surprisingly, no However, we noticed that the catalyst particles became badly
signicant difference in TOF between Pd–2Ni/SiO2 and Pd– dispersed in the aqueous solution aer 6 runs, leading to strong
2NiO/SiO2 was observed. At this stage we cannot give a con- diffusion limitations of substrates. Compared with Pd–9.9NiO/
vincible explanation for this catalysis behavior. It might because SiO2, the selectivity of GVL on Pd/SiO2 started decreasing
the inuence of metal Ni on the hydrogenation activity of Pd is already in the second run and decreased from 95% to 66% in
rather weaker comparing with that of surface NiO when Ni the 6th run, which implying the hydrogenation activity loss of Pd
loading is low.
in hydrogenation.
Based on these results, we try to explain the pathways of a-AL
hydrogenation by taking into account of previous reports.22,35 At
the initial stage of the reaction, isomerization of a-AL to b-AL
Conclusions
might occur (Table 2, Entry 3). Then, concerted adsorption of We demonstrated that the hydrogenation of a-AL to GVL can be
C]C bond of a(and/or b)-AL and hydrogen takes place on successfully performed under mild conditions over a series of
neighbored Pd atoms and NiO surface. Aer the saturation of Pd–xNiO/SiO2 a with very low Pd loading (0.2 wt%). Amongst the
C]C bonds by nearby hydrogen atoms, the formed GVL catalysts investigated, Pd–9.9NiO–SiO2 was the most active one
molecules desorbs to the liquid phase. It should be noted that and it could be reused at least six times without Pd leaching.
another possible reaction mechanism cannot be excluded, Pd–O–Ni species formed by the intimate interaction between Pd
wherein NiO acts as the adsorption sites of a-AL and Pd acti- and NiO tend to be the active site on Pd–xNiO/SiO2 catalysts in
vates hydrogen. The hydrogenation is then accomplished on H2 activation and the following C]C saturation. The hydroge-
NiO via the H-spillover mechanism. Additionally, we tried to nation rate with respect to the a-AL and hydrogen pressure is
reveal the reaction kinetic by testing the hydrogenation nearly zero order and rst order, respectively. Our results
performance of Pd–9.9NiO/SiO2 catalyst under different exper- suggest that it is possible to achieve very high hydrogenation
iment conditions (Table S1†). The results showed that the activity while using minute amount of Pd with the aid of
hydrogenation rate was nearly zero order in lactone and rst a second cheap transition metal oxide in the conversion of
order in hydrogen. This indicated that the Pd–NiO surface was biomass-derived oxygenates.
largely covered with unsaturated molecules, whereas hydrogen
was only weakly adsorbed.
Acknowledgements
We acknowledge the nancial support from the National
Natural Science Foundation of China (Grants No. 21203065 and
21533002).
Catalyst reusability
The reusability of Pd–9.9NiO/SiO2 and Pd/SiO2 catalyst was
evaluated through six repeated reactions. Aer the rst run, the
catalyst was separated through centrifugation, thoroughly
washed and then dried at 60 ꢀC before the next use. The activity
in terms of a-AL conversion and GVL selectivity is shown in
Fig. 6. It can be clearly seen that Pd–9.9NiO/SiO2 catalyst
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Fig. 6 Recycle study of Pd–9.9NiO/SiO2 (Conv. (-) and sel. to GVL
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65382 | RSC Adv., 2016, 6, 65377–65382
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