Highly efficient conversion of biomass-derived levulinic acid into γ-valerolactone over Ni/MgO catalyst

Article abstract of DOI:10.1039/c5ra12548k

Hydrogenation of levulinic acid (LA) was investigated with several supported Ni catalysts for the production of γ-valerolactone (GVL). The catalysts have been characterized with different techniques (XRD, N₂ adsorption-desorption, TPR, ICP-AES, TEM). Under optimal reaction conditions of 150 °C, 1.0 MPa and 20 mL i-PrOH for 2 h, the highest selectivity (93.3%) of GVL was obtained. The productivity reached 0.32 mol_GVL g_metal^-1 h^-1, which was highest compared with all reported Ni-based catalysts.

Full text of DOI:10.1039/c5ra12548k

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Highly Efficient Conversion of Biomass-derived
Levulinic Acid into γ-Valerolactone over Ni/MgO
Catalyst
Cite this: DOI: 10.1039/x0xx00000x
Jinkun Lv , Zeming Rong* , Yong Wang , Jinghai Xiu, Yue Wang , Jingping Qu
Received 00th January 2012,
Accepted 00th January 2012
Hydrogenation of levulinic acid (LA) was investigated with several supported Ni catalysts for
DOI: 10.1039/x0xx00000x
the production of
γꢀvalerolactone (GVL). The catalysts have been characterized with different
techniques (XRD, N2 adsorptionꢀdesorption, TPR, ICPꢀAES, TEM). Under the optimal
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reaction conditions of 150 °C, 1.0 MPa and 20 mL iꢀPrOH for 2 h, the highest selectivity
(93.3%) of GVL was obtained. The productivity reached 0.32 molGVLgmetalꢀ1hꢀ1, which was
highest compared with all reported Niꢀbased catalysts.
Keywords: Levulinic acid;
γ
ꢀValerolactone; Nickel; MgO; Hydrogenation
O
1) H
O
2
Introduction
OH
H O
2
OH
2) HCOOH
O
LA
Furfural
O
With the fossil energy diminishing, biomass has been identified
as a sustainable resource1ꢀ3 for alternative fuels and addedꢀvalue
chemicals4, decreasing the dependence upon fossil resources
and reducing the emissions of greenhouse gas5, 6. Considering
the chemical complexity of the feedstock and end product,
biomass would typically convert to some intermediates ꢀ often
referred as platform chemicals ꢀ for downstream processes7, 8
Levulinic acid (LA), being obtainable by the acid catalyzed
degradation of biomass9, is an important platform chemical due
to its high reactivity and extensive uses in fine chemical10, 11
Moreover, LA can be converted into many valuable
derivatives12ꢀ14, in which
ꢀvalerolactone (GVL) has caught
most attention in recent studies15, 16. GVL is renewable, safe to
store and can be employed in many fields, for example, as a
kind of liquid fuel, food additive, solvent and as an intermediate
for the synthesis of fine chemicals16.
Generally, GVL could be obtained by the reduction of LA or
its esters (Scheme 1), which generated from the esterification of
LA and several lowꢀcarbon alcohols, such as ethanol17,
butanol18, secꢀbutyl alcohol19, etc. The economical efficiency to
be taken into account in the design of reaction pathway, it
seems more favourable by the direct reduction of LA to GVL,
which was commonly conducted with H2 gas as a reductant.
Recently, several groups explored formic acid (FA) as a
hydrogen source instead of external H2 gas20ꢀ23, since FA is a
byꢀproduct of LA production from C6 sugars. However, the use
of FA entails several unresolved disadvantages, such as the
need for precious metal, homogeneous catalysts, and/or harsh
Lignocellulose
O
ROH
OR
O
GVL
O
ROH
O
OH
1) H
2
O
2) HCOOR
HMF
Levulinate ester
Scheme 1 Reduction of LA and its esters to γꢀvalerolactone
.
Comparatively speaking, the hydrogenation of LA to GVL is
especially suited to the commercialꢀscale manufacturing of
GVL by using heterogeneous catalytic systems24. Group
Ⅷ
.
metals, notably ruthenium16,
25ꢀ32, have been shown to
21,
facilitate the hydrogenation step using molecular H2. Manzer et
al. compared several noble metals for LA hydrogenation under
the same reaction conditions. Results showed that the catalytic
γ
activity order was as follows: Ru > Ir > Rh
≈
Pd > Pt > Re33.
summarized the development of
11
Palkovits et al.
heterogeneous catalysts for the targeted conversion of LA to
GVL and found the similar results that Ruꢀbased catalysts
performed best with the relatively highest productivity.
Although high yield of GVL could be obtained with Ruꢀ
based catalysts, the financial cost associated with precious
metal is detrimental to the production of high volume and
relatively low value liquid transportation fuels. So the cheap
and efficient catalysts should be developed to replace precious
metal catalysts. Hwang et al. 34 used the nano composite 5 wt.%
Cu/SiO2 for the vaporꢀphase hydroꢀdehydration of LA and
35, 36
99.9% yield of GVL was obtained at 260 °C. Yan et al.
used Cuꢀcatalyst for the hydrogenation of LA. However, the
harsh reaction conditions (200 °C and 7 MPa H2) still remained
unsatisfied.
reaction conditions et al. 18
.
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Niꢀbased catalysts are widely used in both industry and Ni/SupportꢀN. The number stands for Ni loadings, ‘Support’
academia owing to their low initial cost and high activity37. In stands for all the oxide support being used.
the 1940s, Raney Ni was first employed for the conversion of
The catalytic hydrogenation reaction was conducted in a
LA and 94% yield of GVL was obtained at 200 °C and 5ꢀ6 stainlessꢀsteel autoclave (75 mL capacity). Typically, LA (2 or
MPa H227. Recently, improved work was still underway with 4 mmol), solvent (20 mL) and catalyst (0.01 g) were directly
Raney Ni in order to reduce the reaction conditions. However, added into the reactor, and then it was sealed and purged with
the results are far from satisfactory due to low conversion N2 and H2 for 3 times respectively.
efficiency (<20%)31 or complex reaction systems38. Rao etc.
The products were quantitatively analysed by GC (Techcomp
employed several supported Ni catalysts for vapour phase GC 7890F, FID detector, DBꢀFFAP (30 m×250 ꢁm×0.25 ꢁm)
hydrocyclisation of LA39. Ni/SiO2 showed better activity at 250 capillary column) with calibrated area normalization method
°C with good dispersion, but the productivity was and qualitatively analysed by GCꢀMS (Agilent 5979C).
comparatively lower with 0.028 molGVLgmetalꢀ1hꢀ1. In the former
Catalyst characterization
study, we present an efficient technology for LA hydrogenation
to GVL over Raney Ni prepared from meltꢀquenching NiꢀAl
alloys40. Excellent 97.4% GVL yield was obtained under the
XRD
Xꢀray diffraction (XRD) patterns for all samples were recorded
mild reaction conditions. In order to further reduce the dosage
on a Rigaku D/maxꢀ2400 diffractometer (Cu Kα/λ = 0.1541
of Ni metal, we employed supported Ni catalysts for LA
nm/40 kV/40 mA). The sample was scanned from 10 ° to 90 °
hydrogenation to increase the catalytic efficiency. Ni/MgO
provides excellent 69.9% GVL yield under the mild reaction
conditions and the productivity reached 0.32 molGVLgmetalꢀ1hꢀ1,
with a rate of 10 °/min.
TEM
which was the highest compared with all reported Niꢀbased
Transmission electron microscopy (TEM) images were
catalysts24, 27, 31, 37, 38, 40
.
obtained with an accelerating voltage of 300 kV (TEM, FEI
TF30).
Experimental section
XPS
Materials and methods
XPS experiment was performed on an Escalab 250Xi
spectrometer of Thermo Scientific with an Al anode using Kα
All the reagents used in this study were analytical grade and
used without further purification. NiSO4 (Tianjin Guangfu Fine
Chemical Research Institute), Ni(Ac)2, NiCl2 (ChengDu Kelong
Chemical Co.,Ltd), Ni(NO3)2 (Tianjin Damao Chemical
Reagent Factory). NaOH, KOH, Na2CO3, NH3·H2O,
(NH4)2CO3 (Guangdong Xilong Chemical Co.,Ltd), CO(NH2)2
(Tianjin Guangfu Fine Chemical Research Institute). SiO2
(Degussa Aerosil 200), Al2O3 (Tianjin Chemical Research &
Design Institute), TiO2 (Degussa P25), ZnO (Guangdong
Xilong Chemical Co.,Ltd), MgO (Guangdong Xilong Chemical
Co.,Ltd) and ZrO2 (Sinopharm Chemical Reagent Co.,Ltd).
The supported nickel catalysts were prepared according to
the depositionꢀprecipitation (DP) method41ꢀ43 and the
experimental procedure is as follows. Firstly, the support was
added into an aqueous solution containing nickel salt (0.017
M). Then the suspension was heated to 50 °C [90 °C for
CO(NH2)2 system] in constant stirring for 1 h and a little silica
sol was added into the suspension for another 1 h. The
precipitant solution (0.017 M) was dripped into the above
mixture. After a given time of depositionꢀprecipitation, the
suspension was cooled to 25 °C and then filtered. The sample
was washed with distilled water to remove the possible
adsorbed ions and dried at 60 °C for 24 h. The dried precursor
was heated to 500 °C at a heating rate of 7.5 °C/min under N2
and reduced for 4 h in 20 mL/min flow of H2. The catalyst was
cooled to room temperature under N2 and gradually exposed to
air in 10 min for further usage. The catalysts prepared using
nickel sulfate, nickel acetate, nickel chloride, and nickel nitrate
were named as Ni/SupportꢀS, Ni/SupportꢀA, Ni/SupportꢀCl and
(
hv = 1486.6 eV) radiation. To reduce differential surface
charging effects, an electron flood gun was employed. Binding
energies were corrected for charge effects by reference to C 1
peak at 284.6 eV.
s
N2 adsorptionꢀdesorption
The N2 adsorption–desorption isotherms were obtained by
using a 3Hꢀ2000PS1 sorption system (Beishide InstrumentꢀST
(Beijing) Co., Ltd.). Firstly, the sample was dried in vacuum at
473 K for 6 h. The N2 isotherms were carried out at 77 K. The
specific surface areas were calculated from the isotherms using
the BrunauerꢀEmmettꢀTeller (BET) method. The pore
distribution and the cumulative volumes of pores were obtained
by the BarrettꢀJoynerꢀHalenda (BJH) method from the
desorption branches of the isotherms.
ICPꢀAES
The sample was dissolved in aqua regia (9 mL HCl and 3 mL
HNO3 dilute to 100 mL) and then test the concentration of Ni
on an Optimi2000DV.
H2ꢀTPR
H2ꢀTPR was conducted in TPꢀ5076 (Tianjin Xianquan Industry
and Trading Co., Ltd.). Firstly, the Sample (0.05 g) was heated
to 100 °C in N2 and kept for 3 h. After being cooled to room
temperature, N2 was replaced by 5% H2/Ar and the sample was
reheated to 900 °C at a rate of 10 °C/min.
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temperature lower than 400 °C was denoted as TL, 400ꢀ750 °C
as TM and 750ꢀ850 °C as TH. First, the TL peak at 228.4 °C is
assumed to represent the reduction of NiO located on the
surface50ꢀ52, which has been ascertained in XPS measurement.
The TM peaks are usually assigned to the reduction of Ni2+ ions
in the outermost layer or subꢀsurface layers of MgO lattice,
while the TH peaks can be attributed to the reduction of Ni2+
ions located deep in the MgO lattice39, 51ꢀ54. In the literature, it
is reported that such NiO seemed to be influenced by MgO and
the NiOꢀMgO solid solution formed.
Generally, the presence of TM and TH peaks is a clear symbol
that NiO has undergone a progressive diffusion into the MgO
lattice to form NiOꢀMgO solid solution49. According to
researches, the Ni2+ ions located in the NiOꢀMgO solid solution
were difficult to be reduced and only a small amount of NiO
can be reduced to metallic nickel52, 55. And the more NiOꢀMgO
solid solution the catalyst had, the less NiO can be reduced to
metallic status at low temperature, which would have an
negative influence on the catalytic activity of Ni/MgO.
Therefore, to avoid the formation of NiOꢀMgO solid solution49,
we chose 500 °C to reduce 44Ni/MgO precursor in a flow of H2
in this study.
Results and discussion
Characterization of catalysts
XPS measurements for Ni/MgO were performed as shown in
Fig. 1. Generally, the binding energy of metallic Ni0 2p level
was corresponded to 851 eV ± 0.5, which not being detected in
the XPS analysis. It indicates the absence of metallic Ni0
which might be caused by the surface oxidation of Ni particles
in air due to its high activity. As a result, the binding energy of
Ni 2p3/2 level in the NiO status was detected at 855.7 eV45 and
its satellite peak was displayed at 861.8 eV46, in agreement with
the reported results47. The catalyst had a higher Ni 2p3/2 binding
energy relative to pure NiO species (854.5 eV)48, indicating that
electron transfer took place from Ni to MgO49.
44
,
Support effects on LA hydrogenation
We initiated the hydrogenation of LA by screening six supports
and results are summarized in Table 1. All Niꢀbased catalysts
were prepared by the same DP method and the actual loading
was measured by ICP for each catalyst. To make direct
comparisons, the same reaction conditions were employed in all
experiments. The product distribution is as follows. GVL was
obtained as the main product by the hydrogenation of LA and
subsequent cyclization. The main byꢀproduct was isopropyl
levulinate (Ester), which was produced by the esterification of
Fig. 1 Ni2p XPS of the Ni/MgO catalyst
iꢀPrOH and levulinic acid. Pseudoꢀlevulinic acid (PꢀLA) was an
intermediate generating from the isomerization of LA and could
be further converted into GVL.
According to the reported results56, it could induce the solid
acid when using Al2O3, SiO2, TiO2 and ZrO2 as supports.
Taking both the acid strength and catalytic activity into
account, we can conclude that the activities of supportedꢀNi
catalysts seem to be related to their acid strength, i.e. higher
acid strength of the catalysts resulting in higher conversion. The
order of catalytic activity is as follows: Ni/Al2O3>
Ni/TiO2>Ni/SiO2≈Ni/ZrO2. However, it provides comparatively
higher activities when using ZnO and MgO, which were usually
regarded as solid bases56 and could promote the esterification
process57, 58. The conversions with Ni/ZnO and Ni/MgO
catalysts were 41.3% and 58.1% respectively.
Fig. 2 H2-TPR profiles of the catalysts
Furthermore, the support was directly used for LA
hydrogenation in order to investigate the support effect (Table
2). Results showed that the acid supports (Al2O3, TiO2, SiO2,
ZrO2) have little influence over the reaction, but the base
supports (ZnO, MgO) could obviously promote the
The H2ꢀTPR profile for (a) MgO support, (b) 0Ni/MgO and
(c) 44Ni/MgO is shown in Fig. 2. The blank test for MgO was
carried out according to the preparation procedure of catalysts
(The only difference is aqueous solution containing no nickel
salt.) and the corresponding sample was denoted as 0Ni/MgO.
MgO support and 0Ni/MgO almost do not show any hydrogen
consumption under the measurement conditions. Comparatively
speaking, 44Ni/MgO provides three TPR peaks: the peak at
esterification of
iꢀPrOH and levulinic acid, which was similar
with the intramolecular esterification of
γꢀhydroxyvaleric acid.
Based on the reaction mechanism, it could be concluded that Ni
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mainly provided the active sites for LA hydrogenation and ZnO acid. Considering the selectivity of GVL, Ni/MgO exhibited
(MgO) promoted the esterification process of
γꢀhydroxyvaleric comparatively higher reactivity than Ni/ZnO in this study.
Table 1 Hydrogenation of LA with supported Ni catalysts
Sel./%
Cat.
aActual loadings/%
Conv./%
bProductivity/molGVLgmetalꢀ1hꢀ1
GVL
58.9
85.8
76.3
73.8
57.6
92.4
Ester
9.1
4.3
7.4
8.2
PꢀLA
29.9
7.3
14.0
11.8
6.8
Others
2.1
2.6
2.3
6.2
Ni/SiO2
Ni/Al2O3
Ni/TiO2
Ni/ZrO2
Ni/ZnO
Ni/MgO
47.89
45.95
43.63
39.03
48.19
47.48
19.7
37.2
25.7
19.5
41.3
58.1
0.024
0.069
0.045
0.037
0.049
0.11
33.2
2.4
2.4
2.3
2.9
Reaction Conditions: 0.2302 g LA, 20 mL iꢀPrOH, 0.01 g catalyst, 150 °C, 2.0 MPa, 120 min, the nominal loading of each sample is 66wt%. a. Detedted by
ICPꢀAES. b.Productivity = n (LA)* Yeild /[ m (Ni)/ t ].
Table 2 Hydrogenation of LA with blank supports
Pore
Sel./%
GVL
3.0
2.5
3.0
2.0
4.5
0.5
2.0
Supports
Surface area(m2/g)
Conv./%
V (cm3/g)
ꢀ
0.68
1.49
0.23
0.04
0.05
0.11
D (nm)
ꢀ
9.86
19.78
12.15
19.58
16.89
15.15
Ester
60.6
60.0
60.6
36.7
59.1
84.1
71.5
PꢀLA
36.4
37.5
36.4
63.3
34.1
15.5
26.5
Blank
SiO2
Al2O3
TiO2
ZrO2
ZnO
ꢀ
3.3
4.0
3.3
4.9
4.4
202.6
213.0
51.4
5.9
7.9
18.6
20.7
15.1
MgO
Reaction Conditions: 0.2302 g LA, 20 mL iꢀPrOH, 0.01 g catalyst, 150 °C, 2.0 MPa, 120 min
Effects of Ni loadings
Based on the above study about support effect, we concluded crystalline sizes of Ni were estimated to be about 4.4 nm
that Ni provided the reactive sites for hydrogenation and MgO according to the 44Ni/MgO sample.
promoted the esterification process. It seems that there is a
balance between these two steps. So we investigated various
loadings of Ni/MgO (22~66 wt.%) for LA hydrogenation to
GVL (Table 3). By increasing the loading of Ni, the conversion
of LA firstly increased, and then decreased. When the nominal
loading was 44%, the conversion reached the maximum value
of 79.3% and the productivity was 0.32 molGVLgꢀ1hꢀ1. Those
phenomena are related to the status of Ni in the composite.
Xꢀray diffraction (XRD) patterns of MgO and Ni/MgO
catalysts with different loadings were shown in Fig. 3. The
fresh MgO gave the typical diffraction peaks at 2θ = 36.8 °,
42.8 °, 62.2 °, 74.6 °, 78.5 ° and 2θ = 18.5 °, 37.9 °, indicating
the crystalline phase of most MgO (JCPDS card No. 77ꢀ2364)
and a little Mg(OH)2 (JCPDS card No. 44ꢀ1482), respectively.
It showed that the sharp peaks for MgO turned much weaker
Fig. 3 XRD patterns of MgO and Ni/MgO catalysts with different loadings
and even partly disappeared, indicating the crystalline structure
was severely destroyed in the process. Three diffraction peaks
The TEM images of 44Ni/MgO were shown in Fig. 4.1~4.2.
Judging from the surface, Ni particles uniformly decorated on
MgO support with less aggregation (Fig. 3). However, it seems
difficult to definitely differentiate MgO and Ni possibly due to
a strong interaction between metal and support. Furthermore,
the image is magnified until a higher resolution in order to get
much finer details. Based on Fig. 4.2, the 0.20 nm and 0.18 nm
lattice spacing could be assigned to (111) and (200) lattice
planes in metallic Ni, which were also obtained in XRD
patterns. For MgO support, the crystalline structure was partly
destroyed during the preparation of catalysts, but the residual
at 42.8 °, 62.2 ° and 78.5 ° were assigned to the (200), (220)
and (222) facets of MgO respectively. After loading Ni
particles, another three peaks appeared at 2θ = 44.5 °, 51.8 °
and 76.4 °, with respect to (111), (200) and (220) facets of Ni
(JCPDS card No. 70ꢀ0989) lattice reflection. The broad peak
shape suggested the amorphous structure or small size of Ni
particles. As the loadings of Ni increased, the peaks for MgO
got weak, but got strong for Ni. This could be attributed to
more Ni covering the surface of MgO matrix and destroying its
crystalline structure. Using Scherrer’s equation, the average
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MgO crystal could also be observed by TEM images. The phase is spread on the surface of MgO support in an amorphous
lattice distances at 0.21 nm and 0.24 nm were ascribed to (200) or highly dispersed form. The strong interaction between Ni
and (111) planes of MgO respectively, which were also and MgO might stabilize the small Ni particles and prevent
obtained in the characterization of XRD. The good agreement their aggregation. On the other hand, as Ni content increasing,
between the results of TEM and XRD in crystally phase the LA hydrogenation to
excludes the occurrence of new phase induced by interaction of but subsequent dehydration of
the support and reduced metal. slowed down. Take two process into account, an appropriate Ni
For Ni/MgO catalysts, XRD analysis showed the presence of loading was necessary in this study. A detailed study on the
γꢀhydroxyvaleric acid was accelerated
γꢀhydroxyvaleric acid was
Ni metallic phase with a weak signal. It indicated that the Ni
mechanism of catalytic process is under way.
Fig. 4.1 TEM image of 44Ni/MgO catalyst
Fig. 4.2 TEM image of 44Ni/MgO catalyst
Table 3 Hydrogenation of LA to GVL with Ni/MgO
Sel./%
Cat.
Conv./%
Productivity/molGVLgmetalꢀ1hꢀ1
GVL
Ester
PꢀLA
Others
0Ni/MgO
22Ni/MgO
33Ni/MgO
44Ni/MgO
55Ni/MgO
66Ni/MgO
18.2
26.4
45.9
79.3
55.4
30.9
1.2
85.2
39.0
15.7
4.8
7.2
13.6
13.6
17.0
7.0
3.3
4.9
0
ꢀ
41.7
73.6
88.1
84.3
72.5
2.3
3.7
3.7
3.6
3.6
0.10
0.20
0.32
0.17
0.068
10.4
Reaction Conditions: 0.4604 g LA, 20 mL iꢀPrOH, 0.01 g catalyst, 150 °C, 1.0 MPa, 120 min
Effects of precursor salts
or chlorine occupying the active sites of Ni, leading to the
catalyst poisoning59ꢀ63. In addition, when the Ni(Ac)2 solution
was used to impregnate into MgO, the mixture was too thick to
be stirred, which might decrease the metal dispersion and the
reaction activity. It appeared that the catalyst derived from
Ni(NO3)2 precursor salt had the highest activity in the control
experiments and the productivity was 0.29 molGVLgꢀ1hꢀ1 with
93.3% selectivity to GVL.
The precursor salts including NiSO4, Ni(Ac)2, NiCl2 and
Ni(NO3)2 were investigated for the preparation of Ni/MgO
catalyst. Results in Table 4 indicated that the actual loadings
were
fairly
similar
according
to
ICP
analysis
(31.58%~33.91%), but the catalytic performance was quite
different from each other. It should be noted that the catalyst
provides low activity and selectivity when using NiSO4 or
NiCl2 as precursor salts. This may be due to the residual sulfur
Table 4 Effects of precursor salts for LA hydrogenation
Sel./%
PꢀLA
Productivity
/molGVLgmetalꢀ1hꢀ1
0.0094
Cat.
aActual loadings/%
Conv./%
Ester
44.6
14.6
40.2
1.5
GVL
17.5
71.1
13.2
93.3
Others
1.1
3.3
0.5
4.4
Ni/MgOꢀS
Ni/MgOꢀA
Ni/MgOꢀC
Ni/MgOꢀN
33.06
31.58
33.91
31.80
17.7
39.8
18.9
100
36.2
11.3
45.5
0.8
0.090
0.0074
0.29
Reaction Conditions: 0.2302 g LA, 20 mL iꢀPrOH, 0.01 g catalyst, 150 °C, 2.0 MPa, 120 min, the nominal loading of each sample is 44wt%. a. Detedted by
ICPꢀAES.
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heating, the precipitation of Ni(OH)2 would be formed with
more uniform dispersion. So it provided a higher activity with
Effects of precipitants
To prepare Ni/MgO catalyst, MgO was firstly suspended in an 53.8% conversion (Entry 4). Similar with NH3•H2O, it also
aqueous Ni(NO3)2 solution. Then a precipitant being dissolved showed good performance with CO(NH2)2 as a precipitant.
+
into H2O was dropwise added into the mixture to transfer the
When CO(NH2)2 was regarded as a precipitant, NH4 , CO2
Ni2+ ions in the liquid phase onto the surface of MgO. and OHꢀ generated by the hydrolysis of urea in neutral or basic
Generally, a moderate alkaline environment is preferential for solution66. There were some responding peaks of
the dispersion of Ni(OH)2 precipitants. Herein, several Mg5(CO3)4(OH)2•4H2O [(110), (011) and (310)] and Mg(OH)2
frequentlyꢀused bases as a key factor were investigated for the [(001), (011), (012) and (110)]. Other peaks were well
precipitation process and results were summarized in Table 5.
consistent with the literature results for Ni2(OH)2CO3•4H2O
The XRD patterns of Ni/MgO precursors preparaed with (JCPDS card No. 38ꢀ0714). As a result, the basic nickel
different precipitants were shown in Fig. 5. The result with carbonate was obtained and it could effectively afford
NaOH or KOH as a precipitant is similar with the patterns of agglomeration of sheetꢀlike or unꢀregular particles67.
Mg(OH)2 (JCPDS card No. 86ꢀ0441). Comparatively, the peaks
The XRD pattern of Ni/MgO [(NH4)2CO3] showed that it
of Ni(OH)2 (JCPDS card No. 73ꢀ1520) were overlapped with also provided the basic nickel carbonate using (NH4)2CO3 as a
that of Mg(OH)2. The peak at 2θ=33.0 ° may contain two precipitant to react with Ni(NO3)2. After being reduced by H2,
phases taking peak intensity into account. So Ni2+ ions could be Ni/MgO [(NH4)2CO3] catalyst provided highest 79.3%
completely converted into Ni(OH)2 when using NaOH or KOH conversion and 88.1% selectivity to GVL. Furthermore, the
as precipitants (Entry 1 and 2). Obviously, it provided lower XRD patterns of Ni/MgO catalysts with different precipitants
activity, which might be caused by the migration and were shown in new Fig. S7. It indicated that the Ni particles are
aggregation of Ni particles when Ni(OH)2 was reduced by H2 well dispersed on supports and particles size of Ni prepared by
flow at 500 °C, exceeding to the melting point of Ni(OH)2.
NH3•H2O, CO(NH2)2 and (NH4)2CO3 are smaller than other
When Na2CO3 was used as a precipitant, Ni(OH)2 and NiCO3 sample. As a result, Ni/MgO with (NH4)2CO3 precipitant
might be obtained due to the coexists of OHꢀ and CO3 in the showed highest activity and seletivity to GVL.
2ꢀ
solution. Two diffraction peaks at 15.3 ° and 30.8 ° were
assigned
to
the
(011)
and
(310)
facets
of
Mg5(CO3)4(OH)2•4H2O (JCPDS card No. 25ꢀ0513). Two
diffraction peaks at 33.5 ° and 41.1 ° were assigned to the (110)
and (300) facets of 3Ni(OH)2•2H2O (JCPDS card No. 22ꢀ0444).
There were no peaks of NiCO3, which is consistent with other
research64. According to reports, when pH was higher than 10,
NiCO3 turned into Ni(OH)2. In this study, pH was 11.26 for
Na2CO3 solution. In combination with the calculation results
(Listed in Supporting information), we found a reaction for
transformation of precipitation continuously happened: NiCO3
+ 2OHꢀ
Ni(OH)2 + CO32ꢀ. So Ni(OH)2 was obtained at last
and it also provided low activity using Na2CO3 as a precipitant.
When NH3•H2O was added into the suspension containing
MgO and Ni(NO3)2 solution, [Ni(NH3)x]2+ complex was firstly
formed as an intermediate phase and it would impregnate the
cavity of MgO65. With the decomposition of NH3•H2O upon
Fig. 5 XRD patterns of Ni/MgO precursors with different precipitants, The
nominal loading is 44wt%
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Table 5 Hydrogenation of LA to GVL on Ni/MgO prepared with different precipitants
Sel./%
Entry
Precipitantsa
pH
Conv./%
Ester
22.0
35.5
25.9
8.7
GVL
61.3
46.1
58.1
80.5
88.1
81.5
PꢀLA
12.1
15.9
13.3
7.1
Others
2.8
2.4
2.7
3.5
1
2
3
4
5
6
NaOH
KOH
12.97
13.30
11.26
8.87
9.20
9.02
32.3
24.5
30.1
53.8
79.3
58.9
Na2CO3
NH3·H2O
(NH4)2CO3
4.8
8.5
3.3
3.7
b
CO(NH2)2
6.3
3.7
Reaction Conditions: 0.4604 g LA, 20 mL iꢀPrOH, 0.01 g catalyst, 150 °C, 1.0 MPa, 120 min, the nominal loading of each sample is 44wt%. a:
n(Precipitant):n(Ni2+)=2; b: n(Precipitant) :n(Ni2+)=3
Optimization of reaction conditions and the reaction pathway
Employing Ni/MgO catalyst, effect of temperature, pressure
and solvents were investigated (Fig. 6ꢀ8). The optimal reaction
conditions were 150 °C, 1.0 MPa H2, which were much milder
34, 36, 37, 68
comparing with the reported results33,
and the
catalyst’s dosage was significantly reduced simultaneously.
In this study, ꢀPrOH and 1,4ꢀdioxane appeared to be more
appropriate than MeOH, EtOH and H2O as solvents. When
there were acetyl group of LA, catalyst and ꢀPrOH, the
i
i
MeerweinꢀPonndorfꢀVerley (MPV) reaction might happen as
reported69, 70. According to the references71, 72, MgO can be
used as the catalyst for the MPV reaction. So a contrast test was
designed with nitrogen replacing hydrogen. 23.7% conversion
and 54.9% selectivity to GVL were obtained in Table S2.
When H2 is absence, isopropyl alcohol provides a hydrogen
source and it is converted into acetone. Because acetone
couldn't be removed out in the closed reactor, there is a
reversible hydride transfer from alcoholate to carbonyl
acceptor. It is not enough for iꢀPrOH to afford all the
Fig. 6 The effect of solvents with Ni/MgO (0.2302 g LA, 20 mL i-PrOH, 0.01 g
catalyst, 150 °C, 2.0 MPa, 120 min.)
hydrogenation of LA to GVL. As a whole,
double roles not only solvent but also hydrogen donator. So
when ꢀPrOH is solvent, we got the highest activity.
iꢀPrOH acted as
i
Under optimal reaction conditions, the reaction process was
monitored by GC and the relationships of components
concentration versus reaction time were shown in Figure 9.
Based on the above results and reported references11, 24, 68, 73, a
reaction pathway was proposed in Scheme 2. Theoretically,
GVL was obtained by the hydrogenation of LA to
γꢀ
hydroxyvaleric acid (1-b) and subsequent dehydration of 1-b. In
this study, 1-b was not be detected according to the analysis
method by Omar Ali Abdelrahman73, which might be attributed
to the quick intramolecular esterification of 1-b under the
reaction conditions. Pseudoꢀlevulinic acid (1-d) was an
intermediate generating from the cyclization of LA.
Fig. 7 The effect of temperature with Ni/MgO (0.2302 g LA, 20 mL i-PrOH, 0.01 g
catalyst, 2.0 MPa, 120 min.)
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Conclusions
In summary, several supported Ni catalysts were prepared
according to the depositionꢀprecipitation (DP) method. Various
factors for catalyst preparation were systematically investigated
to obtain the optimal preparation conditions with [Ni(NO3)2] as
a precursor salt and [(NH4)2CO3] as a precipitant. iꢀPrOH acted
as double roles not only solvent but also hydrogen donator,
which accelerating the reaction rate. As a result, 44Ni/MgO
provided excellent 69.9% GVL yield under the optimal reaction
conditions. The productivity reached 0.32 molGVLgmetalꢀ1hꢀ1,
which was highest compared with all reported Niꢀbased
catalysts. However, the catalyst durability was still far from
satisfaction and the improved work remains underway in our
group.
Fig. 8 The effect of Pressure with Ni/MgO (0.2302 g LA, 20 mL i-PrOH, 0.01 g
Acknowledgements
catalyst, 150 °C, 120 min.)
This work was financially supported by the National Natural
Science Foundation of China (21206015), Specialized Research
Fund for the Doctoral Program of Higher Education
(20120041120022), the Fundamental Research Funds for the
Central Universities (DUT13LK29).
Notes and references
a
State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, Dalian University of Technology, Dalian 116024, P. R.
China. Tel: 86 411 84986242; Eꢀmail: zeming@dlut.edu.cn
†
Footnotes should appear here. These might include comments
relevant to but not central to the matter under discussion, limited
experimental and spectral data, and crystallographic data.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
Fig. 9 The concentration-time plots for the hydrogenation of LA with Ni/MgO
(0.4604 g LA, 20 mL i-PrOH, 0.01 g catalyst, 150 °C, 1.0 Mpa, the nominal loading
is 44wt%.)
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