Highly efficient hydrogenation of levulinic acid into 2-methyltetrahydrofuran over Ni-Cu/Al2O3-ZrO2 bifunctional catalysts

Article abstract of DOI:10.1039/c8gc02914h

Transformation of biomass or biomass-based feedstocks into fuels or valuable chemicals is of great importance for the sustainable development of our society. Mesoporous Al₂O₃-ZrO₂ with different Al:Zr molar ratios was prepared by the sol-gel method, and Cu-Ni/Al₂O₃-ZrO₂ catalysts were fabricated by the impregnation method. The catalytic performances of the bifunctional catalysts were studied in the selective hydrogenation of levulinic acid (LA) into 2-methyltetrahydrofuran (MTHF). It was demonstrated that both Cu:Ni and Al:Zr ratios affected the selectivity to MTHF significantly. The bimetallic catalysts Cu-Ni/Al₂O₃-ZrO₂ containing 10 wt% Ni and 10 wt% Cu gave 99.8% MTHF selectivity at full conversion of LA when the Al:Zr ratio was 9:1. The outstanding catalytic performance of the catalyst is related to the mesoporous structure, the acidic properties of the support and the synergistic effect between Cu and Ni. The catalyst could be reused five times without a considerable loss of catalytic activity and selectivity.

Full text of DOI:10.1039/c8gc02914h

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Highly efficient hydrogenation of levulinic acid
into 2-methyltetrahydrofuran over Ni–Cu/Al O –
2
3
Cite this: DOI: 10.1039/c8gc02914h
ZrO bifunctional catalysts†
2
a,b
a
a,b
a,b
Zhenbing Xie, Bingfeng Chen, * Haoran Wu, Mingyang Liu,
a,b
a,b
a
a,b
Huizhen Liu,
Jianling Zhang,
Guanying Yang and Buxing Han
*
Transformation of biomass or biomass-based feedstocks into fuels or valuable chemicals is of great
importance for the sustainable development of our society. Mesoporous Al –ZrO with different Al : Zr
molar ratios was prepared by the sol–gel method, and Cu–Ni/Al –ZrO catalysts were fabricated by
2
O
3
2
2
O
3
2
the impregnation method. The catalytic performances of the bifunctional catalysts were studied in the
selective hydrogenation of levulinic acid (LA) into 2-methyltetrahydrofuran (MTHF). It was demonstrated
that both Cu : Ni and Al : Zr ratios affected the selectivity to MTHF significantly. The bimetallic catalysts
Cu–Ni/Al O –ZrO containing 10 wt% Ni and 10 wt% Cu gave 99.8% MTHF selectivity at full conversion
Received 13th September 2018,
Accepted 3rd January 2019
2
3
2
of LA when the Al : Zr ratio was 9 : 1. The outstanding catalytic performance of the catalyst is related to the
mesoporous structure, the acidic properties of the support and the synergistic effect between Cu and Ni.
The catalyst could be reused five times without a considerable loss of catalytic activity and selectivity.
DOI: 10.1039/c8gc02914h
rsc.li/greenchem
fural. In the presence of hydrogen, the hydrodeoxygenation of
Introduction
furfural to MTHF can be achieved over various supported
5
Although fossil resources provide indispensable fuels and noble metals and bimetallic catalysts. The synthesis of MTHF
2
c
chemicals for human society currently, there is an urgent need from LA hydrogenations involves GVL as an intermediate.
to investigate renewable alternatives against deterioration of Although there are many reports on the hydrogenation of LA
6
the environment and the global depletion of fossil resources. to GVL and GVL conversions, the one-pot synthesis of MTHF
Biomass is considered as a promising renewable resource for from LA hydrogenation with high efficiency is still limited.
1
fuel and raw chemical production. Among the bio-platform Homogeneous catalysts, such as Ru catalytic systems, have
molecules, levulinic acid (LA), which can be obtained from been reported in the hydrogenation of levulinic acid into
7
renewable biomass resources, has been used to produce bio- MTHF. Generally, homogeneous catalytic systems suffered
fuels and high value-added products, such as levulinate esters, from air-sensitive organic ligands and difficult catalyst recovery
γ-valerolactone (GVL), 2-methyltetrahydrofuran (MTHF), 1,4- from the reaction mixture. Heterogeneous catalysts have also
2
pentanediol, valeric acid and its esters, and pyrrolidinones.
been used for the reaction. For example, Ru/C has been
MTHF is considered as one of the promising biofuels and reported in the hydrogenation of LA into MTHF with a yield of
8
gasoline additives owing to its high energy density and very 61%. Bifunctional Pt–Mo/H-β catalysts gave 86% yield of
1
b,3
low water solubility.
Moreover, MTHF can be used as a bio- MTHF in the conversion of LA, in which Pt–Mo species pro-
derived solvent as it has a low miscibility with water, a high moted hydrogenation of LA into 1,4-pentanediol, and H-β
4
9
boiling point, and excellent stability.
zeolite catalyzed dehydration of the diol into MTHF. Ni
MTHF can be obtained from the catalytic conversion of bio- (8 wt%)–Cu(72 wt%)/SiO catalyst showed a selectivity of 89%
2
1
0
platform molecules like levulinic acid, γ-valerolactone and fur- for MTHF from LA hydrocyclization. Arias et al. reported that
1
1
Ni–Cu/Al
2 3 2 3
O catalyst gave a yield of MTHF up to 80%. Al O
doped Cu/SiO catalyst achieved 65.2% yield of MTHF under
2
1
2
a
continuous hydrogenation of ethyl levulinate at 250 °C.
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid
and Interface and Thermodynamics, CAS Research/Education Center for Excellence
in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing,
Although heterogeneous catalysts based on non-noble metals
are preferred for the MTHF production, the catalysts usually
suffer from some obvious disadvantages such as low activity
and selectivity. It is highly desirable to develop non-noble
metal catalysts for this reaction with high activity and
selectivity.
1
00190, P. R. China. E-mail: chenbf@iccas.ac.cn, hanbx@iccas.ac.cn
b
School of Chemistry and Chemical Engineering, University of Chinese Academy of
Sciences, Beijing 100049, P.R. China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8gc02914h
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In this work, we prepared Ni–Cu/Al
2
O
3
–ZrO
2
catalysts with with a certain hydrogen pressure for several hours under a stir-
various compositions. The catalytic performances of the cata- ring speed of 600 rpm. After the reaction, product/catalyst sep-
lysts for the hydrogenation of LA into MTHF were investigated. aration was performed by simple filtration. In consecutive
It was shown that Ni–Cu/Al O –ZrO achieved outstanding batch tests, continuous filtration was applied to separate the
2 3 2
catalytic performance in this reaction (MTHF yield: 99.8%). catalyst. The catalyst was recycled into the autoclave with the
The excellent catalytic performance is related to the high feedstock stream. Hot filtration test conditions: 0.1 g substrate,
specific surface area, large pore volume and well-defined 1.5 mL 2-butanol, 20 mg Cu–Ni/Al–Zr(9), and 220 °C/3 MPa H
2
.
acidic properties of the Al O –ZrO supports, and the synergis- The hot filtration test was performed at 220 °C after reaction
2
3
2
tic effect between Ni and Cu also contributed to the enhanced for 30 minutes. The solid catalyst was filtered out immediately
catalytic properties.
under hot conditions and the reaction was continued with the
filtrate under the same conditions.
The products were analyzed by GC and GC-MS. GC was per-
formed by using a GC (Agilent 6820) with a high-temperature
capillary column (HP-5MS, 30 m, 0.25 mm ID) and a flame
ionization detector (FID). The conversion and selectivity values
were determined by GC using phenol as the internal standard.
GC-MS was used to confirm the chemical structures of the
components in the product mixtures. GC-MS was performed
using an Agilent 5977A instrument with a capillary column
Experimental
Materials and chemicals
Aluminium isopropoxide was purchased from J&K Scientific
Ltd (Beijing, China). Levulinic acid (LA) and 2-butanol were
obtained from Aladdin Reagent Limited Company (Shanghai,
China). Zirconium n-butoxide was purchased from Strem
Chemicals, Inc. Nickel(II) nitrate hexahydrate, copper(II) nitrate
trihydrate, nitric acid, phenol and ethanol were purchased
(
HP-5MS, 30 m, 0.25 mm ID).
from Sinopharm Chemical Reagent Co., Ltd, China. Pd(5%)/C, Catalyst characterization
Pt(5%)/C, Ru(5%)/C and ZSM-5 (Si : Al = 50 : 1) were provided
by Alfa Aesar.
XRD patterns were obtained using a Rigaku D/max-2500 X-ray
diffractometer using Cu-Kα radiation. The elemental analysis
was performed using a PerkinElmer Optima 2000 OV induc-
tively coupled plasma-optical emission spectrometer
Catalyst preparation
Al O –ZrO mixed oxide supports were prepared by the sol–gel
2
3
2
(
ICP-OES). The microstructural properties were determined
method. In a typical synthesis for Al–Zr (9), aluminium iso-
propoxide (8 g) and zirconium(IV) n-butoxide (1.56 g) were
added into ethanol (50 ml). The mixture was further stirred for
using a transmission electron microscope ((Hitachi, HT-7700)
which was operated at 120 kV and was equipped with a tung-
sten filament. X-ray photoelectron spectra (XPS) were recorded
on a Thermo Scientific ESCA Lab 250 Xi instrument using 200
W monochromated Al-Kα radiation (1486.6 eV). The binding
energies were referenced to the C 1s line at 284.8 eV from
3
0 min, and nitric acid (mass ratio: 33%, 2.7 mL) was gradually
added until the transparent sol was yielded. After aging for
2 h, the obtained sol was dried at 120 °C for 10 h to form
1
xerogel. After being crushed and sieved to 60–100 mesh, the
xerogel was calcined at 400 °C for 2 h in air. The mixed oxides
were denoted as Al–Zr(x), where x represents the molar ratio of
Al to Zr (x = 1, 3, 6, 7, 8, 9). Al O or ZrO was also prepared by
adventitious carbon. The XPS analysis was conducted under ex
13
situ conditions using
a
UHV-XPS chamber.
Nitrogen
adsorption−desorption analysis was carried out at 77 K using
a NOVA 2200e adsorption analyzer. Before the adsorption ana-
lysis, the samples were degassed under vacuum at 300 °C in
2
3
2
this method.
Catalysts were prepared by the impregnation method.
Certain amounts of metal precursors (Ni(NO ) ·6H O and Cu
the port of the adsorption analyzer. The NH temperature pro-
3
3
2
2
3
grammed desorption (NH -TPD) experiments were carried out
using an Autochem 2920 (Micromeritics) surface analyzer
quipped with a TCD detector.
3 2 2
(NO ) ·3H O) were dissolved in water and stirred for 15 min.
The as-prepared mixed oxide was then added to the solution
and stirred for 12 h at room temperature. Then the suspension
was evaporated under vacuum to remove water. After drying
overnight in an oven at 120 °C, the obtained solid was calcined
at 400 °C for 2 h. Before the activity tests, the catalysts were
Results and discussion
Catalyst characterization
reduced in a tubular furnace under pure H
2
using 5 °C min−
1
heating ramp up to 400 °C, and kept at this temperature for The Al O –ZrO mixed oxides were prepared by the sol–gel
2
3
2
2
h, and then cooled under H flow.
method using aluminum isopropoxide and zirconium n-butox-
ide as precursors. Owing to easy hydrolysis of these precursors,
slow addition of nitric acid is beneficial for the formation of
2
Catalytic evaluations
2
Activity tests were performed in a 10 ml Teflon lined autoclave the sol of the mixed oxides in ethanol. N adsorption–desorp-
with a magnetic stirrer. For a typical run, the autoclave was fed tion analysis was conducted to study the textural properties of
with LA (0.1 g), 2-butanol (1.5 mL), and a certain amount of a the as-prepared supports. As shown in Fig. 1a, the mixed
prereduced catalyst and then sealed and purged using H
times). The reaction was performed at the desired temperature pressure (p/p ) of 0.4–0.9, which are attributed to capillary con-
2
(5 oxides exhibit type-IV curves with hysteresis loops at a relative
0
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showed the largest surface area (628.8 m g−1) and pore
2
3
−1
volume (0.64 cm
g ). High surface area of mesoporous
materials is favorable for facile mass transportation of reac-
tants and products. Fig. 2a shows a TEM image of the porous
structure of the Al O –ZrO mixed oxides. As shown in the
2
3
2
TEM image of the supported bimetallic Ni–Cu catalyst (Fig. 2b
and Fig. S1†), the particle size was in the range of 5–10 nm.
NH3 temperature-programmed desorption (TPD) was con-
ducted to investigate the surface acidity of the Al–Zr mixed
oxides and the single oxides. The desorption profiles of these
oxides after NH3 adsorption are shown in Fig. 3 and the
3
results of desorption peaks and the corresponding NH de-
sorption amount are listed in Table 2. All desorption profiles
contain two peaks of ammonia desorption. The peak centered
at about 107 °C was attributed to weakly acidic sites. The other
peak centered at about 450 °C was ascribed to strongly acidic
sites. The concentration of these weakly acidic sites increased
with increasing the molar ratio of Zr to Al (Table 2). The Al–Zr
(
9) showed the highest concentration of strongly acidic sites
and total acid amount. The acidic properties of the support
may improve catalytic performances for hydrogenation/hydro-
9
,14
genolysis of LA.
X-ray diffraction (XRD) was used to examine the phase
structure of the mixed oxides and their supported catalysts. As
shown in Fig. 4, the XRD patterns of Al O –ZrO calcined at
2 3 2
Fig. 1
2
N adsorption–desorption isotherms (a) and the corresponding
pore size distribution curves (b) of the Al–Zr mixed oxides with different
Al : Zr ratios.
Fig. 2 TEM images of the mesoporous Al–Zr(9) support (a) and the Ni–
Cu/Al–Zr(9) catalysts containing 10 wt% Cu and 10 wt% Ni (b).
Table 1 Porosity properties of Al
2
O
3
–ZrO
2
with various Al : Zr ratios
a
b
c
Surface area
Pore volume
Pore size
(nm)
Al/Zr molar
ratio
2
−1
3
−1
Sample
(m g
)
(cm g
)
Al–Zr(1)
Al–Zr(3)
Al–Zr(6)
Al–Zr(9)
175.5
205.5
471.3
628.8
509.8
0.06
0.22
0.56
0.64
0.63
3.4
3.4
3.8
3.8
3.8
1
3
6
9
Al
O
2 3
—
a
Surface area was calculated by the Brunauer–Emmett–Teller (BET)
b
method. Total pore volume was obtained using the adsorption
0
c
branch of the N
determined from the local maximum of the BJH distribution of pore
diameters obtained in the adsorption branch of the N isotherm.
2
isotherm at P/P = 0.99. Average pore diameter was
2
densation within the mesoporous structure. It can be known
from Table 1 that both the specific surface area and pore
volume increased with the molar ratio of Al to Zr, and the
3
Fig. 3 NH -TPD profiles of the mixed oxide supports prepared by the
average pore diameter ranged from 3.4 to 3.8 nm. Al–Zr(9) sol–gel method.
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Table 2 Acidic properties of porous oxide supports determined by NH
3
-TPD analysis
Peak of strongly
Peak of weakly
acidic sites (°C)
Weakly acidic
NH
(des) (mmol g−
Strongly acidic
NH
(des) (mmol g−1)
Total acidity
(mmol g )
1
−1
Catal.
ZrO
3
)
acidic sites (°C)
3
2
104
109
107
107
0.106
0.891
0.748
0.583
470
439
459
453
1.033
1.770
2.766
2.550
1.139
2.661
3.514
3.133
Al–Zr (1)
Al–Zr (9)
2 3
Al O
(
PDF #85-1326), respectively. The XRD pattern also showed
three shoulder peaks located at 43.4°, 50.5° and 74.3°
(
Fig. 4b), which are located between those for pure Cu and Ni
(
Fig. S2†). These results indicated the formation of Cu–Ni alloy
1
5
in the bimetallic catalyst.
XPS analysis was conducted to characterize the oxidation
state of monometallic and bimetallic catalysts by analyzing the
Cu and Ni 2p peaks (Fig. S3†). Fig. 5a shows the XPS spectra of
the Cu–Ni bimetallic catalysts corresponding to the Ni 2p3/2
region, which shows the presence of three kinds of Ni species:
0
2+
16
Ni (852.5 eV), Ni (853.9 eV), and Ni(OH) (856.4 eV). In the
2
Cu 2p3/2 XPS spectra (Fig. 5b), the reduced Cu containing cata-
lysts showed the intense and broad photoelectron peak
2+
between 930 eV and 937 eV, accompanied by the Cu shakeup
satellite peaks (938–945 eV). Owing to little difference in
0
+
binding energy between Cu and Cu in XPS spectra, Cu LMM
X-ray excited Auger spectroscopy was performed to distinguish
these Cu species (Fig. 5c). The Auger peaks at around 914 eV
+
0
0
+
(
Cu ) and 918 eV (Cu ) confirmed the existence of Cu –Cu
1
7
species.
Levulinic acid conversion
Catalytic hydrogenation of levulinic acid into 2-methyl-tetra-
hydrofuran involves several reaction steps, including hydrogen-
ation of LA into GVL; further ring-opening to 1,4-pentanediol
2
c,18
and final dehydration affords MTHF.
Metal/acid bifunc-
tional catalysts are favorable for the conversion of LA to MTHF,
because the metal can accelerate the hydrogenation and ring-
open reactions, and acidic sites can promote the cyclodehydra-
9
tion process of 1,4-pentanediol. The catalytic properties of the
as-prepared Cu–Ni/Al O –ZrO catalysts were evaluated for this
2
3
2
reaction. The bifunctional catalysts achieved higher perform-
ance than commercial catalysts and other metal/acid catalysts.
The influences of support compositions, active metals, reac-
tion temperature and pressure were studied.
Fig. 6a shows the effect of the mole ratio of Al to Zr on the
catalytic properties. When Cu–Ni/Al–Zr(1) was used, MTHF
selectivity was only 42.3%, and GVL yield was 57.7%. Besides,
the Cu–Ni/Al–Zr(1) showed higher selectivity to MTHF than
Fig. 4 (a) XRD patterns of the Al–Zr(9) mixed oxide calcined at different
temperatures; (b) the Cu–Ni/Al–Zr(9) catalysts containing 10 wt% Cu
and 10 wt% Ni.
low temperatures (400 °C or 700 °C) did not show any obvious that of ZrO (11.9%). Although both supports were prepared by
2
diffraction peak, indicating the amorphous state of supports. the sol–gel method, the mixed oxide had higher concentration
2 3 2
Evan after heat treatment at 900 °C, Al O –ZrO only showed of strongly or weakly acidic sites and higher total acid amount
the weak peak at 2θ = 45.7° and 66.7° which were assigned to than ZrO , which favored the ring-opening/cyclodehydration of
2
1
1c,19
Al
2
O
3
(PDF #85-1326). After supporting Cu–Ni bimetallic nano- GVL into MTHF.
MTHF selectivity increased when the
particles, diffraction peaks at 43.3°, 50.4° and 74.1° were molar ratio of Al to Zr increased from 1 : 0 to 9 : 1. The Cu–Ni/
observed, which correspond to Cu (111), Cu (200) and Cu(311) Al–Zr(9) achieved the highest catalytic selectivity to MTHF
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(
99.8%) at full conversion of LA. Cu–Ni/Al
2
O
3
gave a lower yield
of MTHF (91.1%) than Cu–Ni/Al–Zr(9) (99.8%), which can be
correlated with the lower BET surface area and surface acidity
of the former support. Moreover, the commercial Al
2 3
O
(
neutral) gave GVL as the main product (>90%, Table S1†).
This indicates that acidity of the bifunctional catalysts plays an
important role in the hydrogenolysis of LA into MTHF. The
acidity of the bifunctional Rh–ReOx/C catalyst also plays a
crucial role in the hydrogenolysis of cyclic ethers and
2
0
polyols. From the BET analysis, the specific surface areas
and pore volumes of the mixed oxides increased with the
molar ratio of Al to Zr (Table 1). Thus, both the porous struc-
ture and high surface acidity contributed to the excellent cata-
lytic performance of the Cu–Ni/Al–Zr(9).
The influence of the loading amount of Cu and Ni on the
Al–Zr (9) support was also investigated, and the results are
given in Table 3. As for monometallic catalysts, the Ni catalyst
mainly gave GVL, and the Cu catalyst produced a mixture of
MTHF and GVL. After introducing Ni into a Cu-based catalyst,
the selectivity to MTHF increased first and then decreased
with the ratio of Ni to Cu (entries 1–6). Among them, the cata-
lyst with 10 wt% Cu and 10 wt% Ni gave the best result with a
MTHF selectivity of 99.8%. Catalysts with smaller Cu amounts
gave higher selectivity to GVL, indicating the low catalytic
activity of Ni in GVL hydrogenation. On the other hand, cata-
lysts with larger Cu amounts gave lower selectivity to MTHF,
which can be explained by the lower hydrogenolysis activity of
GVL. Moreover, the physical mixture of monometallic Cu/Al–Zr
(
6
9) and Ni/Al–Zr(9) catalysts gave poor MTHF selectivity (entry
), indicating that the synergistic effect between Cu and Ni in
the Cu–Ni/Al–Zr(9) bimetallic catalyst played a crucial role in
the high selectivity. Carbon-supported noble metal (Pd, Pt, Ru)
catalysts also showed very low selectivity for the desired
product. The commercial HZSM-5 (Si : Al = 50 : 1) zeolite-sup-
ported Cu–Ni catalyst gave GVL as the main product. These
2 3 2
results reveal that the as-prepared Ni–Cu/Al O –ZrO (9) exhibi-
ted superior catalytic properties for the reaction.
The influence of the reaction temperature on the reaction is
shown in Fig. 6b. GVL was obtained as a major product when
the reaction temperature was less than 180 °C. Further
increase of temperature resulted in an increase of MTHF
selectivity, and GVL yield decreased significantly. The catalyst
gave the best selectivity to MTHF (99.8%) at 220 °C. However,
further increasing temperature resulted in the decrease of
MTHF selectivity owing to the generation of by-products like
2
sec-butyl valerate. In addition, the influence of H pressure on
the reaction was also investigated at 220 °C over the Ni–Cu/Al–
Zr(9) (Fig. 6c). Low hydrogen pressure resulted in a high yield
of GVL. MTHF selectivity increased with the increase of the H
Fig. 5 Ni 2p XPS spectra (a), Cu 2p XPS spectra (b) and Cu LMM XAES pressure at the expense of GVL selectivity. The selectivity to
2
spectra (c) of the catalysts. The Cu/Al–Zr(9) contained 20 wt% Cu; the MTHF reached 99.8% when H
contents of Cu and Ni in the Cu–Ni/Al–Zr(1) and Cu–Ni/Al–Zr(9) are
pressure exceeded 3.0 MPa.
2
Low reaction pressure is unfavorable for the ring-opening/
cyclodehydration of GVL to MTHF, leading to the low
selectivity.
1
0 wt%.
To explore the reaction pathway of LA conversion in the
catalytic system, the product distribution as a function of the
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Fig. 6 Effects of the support composition (a), reaction temperature (b), reaction pressure (c) and reaction time (d) on the reaction over Cu–Ni/Al–Zr
9) catalysts containing 10 wt% Cu and 10 wt% Ni. Reaction conditions: 0.1 g LA, 0.02 g Cu–Ni/Al–Zr(9), 1.5 mL 2-butanol (solvent), 3 MPa H
20 °C, 10 h.
(
2
,
2
a
Table 3 Hydrogenolysis of levulinic acid into MTHF with various catalysts
b
Sel. (%)
b
c
Ent.
1
Catalyst
Ni, Cu (wt%)
20, 0
t (h)
Conc. (%)
MTHF
GVL
Others
Ni/Al–Zr(9)
10
100
68.7
100
84.9
100
100
87.4
100
60.0
100
52.6
100
100
100
100
100
1.6
—
1.0
—
64.8
99.8
0.2
92.3
—
86.7
72.8
70.4
99.9
34.2
—
11.7
27.2
f
0.33
2
Cu–Ni/Al–Zr(9)
15, 5
10
28.6
—
1.0
0.2
—
0.33
3
4
Cu–Ni/Al–Zr(9)
Cu–Ni/Al–Zr(9)
12.5, 7.5
10, 10
10
10
0.33
99.8
—
5
6
Cu–Ni/Al–Zr(9)
Cu/Al–Zr(9)
5, 15
10
7.7
16.7
f
0.33
83.3
54.8
74.0
96.7
57.9
89.9
—
0, 20
10
45.2
—
—
26.0
f
0.5
d
7
Ni/Al–Zr(9) + Cu/Al–Zr(9)
Pd/C
Pt/C
Ru/C
20, 0 + 0, 20
10
10
10
10
10
1.6
6.4
10.1
2.6
3.2
1.7
35.7
—
97.4
54.0
e
8
e
9
e
1
1
0
1
Cu–Ni/HZSM-5
10, 10
42.8
a
b
2
Reaction conditions: 0.1 g of LA, 0.02 g catalyst, 1.5 mL 2-butanol (solvent), 3 MPa H , 220 °C, 10 h. Conversion and selectivity were deter-
d
c
mined by GC with phenol as the internal standard. Main other product: sec-butyl pentanoate. Physical mixture of 0.01 g Ni/Al–Zr(9) and 0.01 g
Cu/Al–Zr(9) with 20 wt% metals. The content of the metal in the catalysts was 5 wt%. Main other product: sec-butyl 4-oxopentanoate.
e
f
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reaction time was studied, and the results are presented in active catalytic species leach into the solution. From the XPS
Fig. 6d. The conversion of LA reached 100% over Cu–Ni/Al O – analysis (Fig. S5 and S6†), the surface chemical state of the
2
3
2
ZrO (9) catalysts after 1 h, indicating the high catalytic activity reused catalyst did not change obviously compared with the
of the catalyst. The yield of GVL decreased from 96.6% to 0 fresh catalyst. The slight decrease of catalytic selectivity may be
with the prolongation of the reaction time, suggesting that related to a slight loss of the reused catalyst during the
GVL was the intermediate. The selectivity to other products repeated recovery and reuse cycles.
like sec-butyl valerate was less than 3.5% during the whole
process of the reaction. MTHF yield increased continuously
with the reaction time, and the maximum yield reached 99.8% Conclusion
after 10 h. The reaction pathway was proposed on the basis of
In summary, the Cu–Ni/Al O –ZrO (9) catalyst synthesized by
the results and is shown in Scheme 1. LA was hydrogenated
into GVL that was the intermediate for forming MTHF, which
2
3
2
the combination of the sol–gel method and impregnation
method showed outstanding performance for selective hydro-
genation of LA into MTHF, and the selectivity to MTHF could
be as high as 99.8% at complete conversion of LA. The catalyst
also displayed good recyclability, and catalytic activity and
selectivity did not change considerably after reusing five times.
The excellent catalytic performance of the catalyst can be
attributed mainly to the porous structure, the tunable acidic
properties of the support, and the synergistic effect between
Cu and Ni. In the reaction, LA was first hydrogenated into
GVL, which acted as the reaction intermediate, and then GVL
was further hydrogenated into MTHF. We believe that the
highly efficient catalysts based on non-noble metals have
promising potential for application in the conversion of LA to
MTHF, and some other efficient catalysts for the transform-
ation of biomass based feedstocks into fuels or value-added
chemicals can also be designed by immobilizing multimetallic
particles on acidic supports.
1
1b,12
was similar to that reported by other researchers.
The recyclability of a representative bimetallic catalyst was
also investigated. The catalyst was readily recovered from the
reaction mixture by simple filtration. After rinsing in 2-butanol
and ethanol and drying under vacuum, the recycled catalyst
was used directly in the next cycle. As shown in Fig. 7, Cu–Ni/
Al–Zr(9) could be reused for five catalytic cycles without a con-
siderable loss of catalytic activity. Moreover, a hot filtration
test was performed to verify the heterogeneous nature of the
reaction. After Cu–Ni/Al–Zr(9) was filtered off, the reaction did
not proceed further (Fig. S4†). These results indicate that no
Conflicts of interest
There are no conflicts to declare.
Scheme 1 The possible reaction pathway over Cu–Ni/Al–Zr(9) Acknowledgements
catalysts.
This work was financially supported by the National Key
Research and Development Program of China
2017YFA0403103 and 2017YFA0403003), National Natural
Science Foundation of China (21603235 and 21403248),
Beijing Municipal Science Technology Commission
Z181100004218004), and the Recruitment Program of Global
Youth Experts of China, and Chinese Academy of Sciences
QYZDY-SSW-SLH013).
(
&
(
(
Notes and references
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Fig. 7 Recyclability of Cu–Ni/Al–Zr(9). Reaction conditions: 0.1 g LA,
.02 g Cu–Ni/Al–Zr(9) containing 10 wt% Cu and 10 wt% Ni, 1.5 mL
-butanol (solvent), 3 MPa H , 220 °C, 10 h.
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