Efficient Synthesis of Furfuryl Alcohol and 2-Methylfuran from Furfural over Mineral-Derived Cu/ZnO Catalysts

Article abstract of DOI:10.1002/cctc.201700279

Two kinds of typical mineral-derived Cu/ZnO catalysts consisting of aurichalcite and zincian malachite were introduced for furfural hydrogenation to furfuryl alcohol (FOL) and 2-methylfuran (2-MF) in a fixed-bed reactor. Under proper reaction conditions, high yields of FOL (above 99 %) and 2-MF (94.5 %) could be obtained over the aurichalcite Cu/ZnO catalyst (AC-CZ), whereas the best yield of 2-MF was only 76.9 % (0.5 h^?1) over the zincian malachite Cu/ZnO catalyst (ZM-CZ). The normalized productivity of 2-MF was 43.5 mol kg_Cu ^?1 h^?1 and 17.4 mol kg_Cu ^?1 h^?1 for AC-CZ and ZM-CZ, respectively (LHSV=1.5 h^?1). The catalysts were characterized by XRD analysis, Raman spectra, CO IR spectroscopy, H₂ temperature-programmed reduction, N₂O titration, NH₃ temperature-programmed desorption, and X-ray photoelectron spectroscopy. The far better performance of AC-CZ in furfural hydrogenation was ascribed to its higher dispersion of copper species, superior copper surface area, better surface acidity distribution, and stronger Cu⁰–ZnO synergy. In addition, the surface acidity of the catalysts seemed to have a higher influence on 2-MF production than the Cu surface area, but the optimal balance of both factors still needs to be investigated systematically.

Full text of DOI:10.1002/cctc.201700279

Accepted Article
Title: Efficient synthesis of furfuryl alcohol and 2-methylfuran from
furfural over mineral derived Cu/ZnO catalysts
Authors: Xiaohai Yang, Xiaomin Xiang, Hongmei Chen, Hongyan
Zheng, Yong-Wang Li, and Yulei Zhu
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To be cited as: ChemCatChem 10.1002/cctc.201700279
Link to VoR: http://dx.doi.org/10.1002/cctc.201700279
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10.1002/cctc.201700279
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Efficient synthesis of furfuryl alcohol and 2-methylfuran from
furfural over mineral derived Cu/ZnO catalysts
Xiaohai Yang[a,b], Xiaomin Xiang[a,b], Hongmei Chen[c], Hongyan Zheng[c], Yong-Wang Li[a,c] and Yulei
Zhu*[a,c]
Abstract: Two kinds of typical mineral derived Cu/ZnO catalysts
consisted of aurichalcite and zincian malachite were introduced for
furfural hydrogenation to furfuryl alcohol and 2-methylfuran via a
fixed-bed reactor. Under the proper reaction conditions, high yields
of furfuryl alcohol (above 99%) and 2-methylfuran (94.5%) could be
obtained over the aurichalcite Cu/ZnO catalyst (AC-CZ), while the
best yield of 2-methylfuran was only 76.9% (0.5h-1) over the zincian
malachite Cu/ZnO catalyst (ZM-CZ). The normalized productivity of
2-MF was 43.5mol•kgCu-1h-1 and 17.4mol•kgCu-1h-1 for AC-CZ and
ZM-CZ respectively (LHSV=1.5h-1). Catalysts were characterized via
XRD, Raman spectra, CO-IR, H2-TPR, N2O- titration, NH3-TPD and
XPS. The much more excellent performance for furfural
hydrogenation of AC-CZ was due to its higher dispersion of copper
species, superior copper surface area, better surface acidity
distribution and stronger Cu0-ZnO synergy. In addition, surface
acidity of the catalysts seemed to contribute more obviously to 2-MF
production than Cu surface area, but the optimized balance of them
needed to be investigated systematically.
FFA to FOL, 2-MF etc., such as Cu/Cr, Cu/SiO2, and
Cu/ZnO(Al2O3).6-10 Generally, FOL could be obtained under mild
reaction conditions (about 120 C, ambient pressure) over these
o
copper based catalysts. However, harsh reaction conditions
were required to improve the yield of 2-MF. For the first time,
Adkins et al.11 reported the reduction of furfural in the liquid
phase at 100 °C and 10–15 MPa pressure (H2) over a Cu/Cr
catalyst but only a low yield of 2-MF was obtained. After that, a
good yield of 2-MF was reported (88.2%) by Wu et al.12 via a
solvated metal atom impregnation Cu-Cr/γ-Al2O3 catalyst.
Recently, Dong et al.13 reported a Cu/SiO2 catalyst derived from
copper phyllosilicate could achieve excellent 2-MF yield of
95.5% under relatively milder reaction conditions (200°C,
ambient pressure, 0.5h-1). Although the Cu/Cr and Cu/SiO2
catalysts had good performance for hydrogenation of FFA, the
poisonousness of Cr and the NH3 pollution of Cu/SiO2 catalyst
(prepared via ammonia evaporation13) hindered their further
application in industry. Thus, it is desirable but more challenging
to design an efficient and environment-friendly catalyst for the
hydrogenation of FFA to FOL and 2-MF under mild conditions.
Cu/ZnO based catalysts had been widely studied in many
chemical processes. As their high catalytic efficiency for C=O
and C-O bonds hydrogenation and good environmental
friendliness, mineral derived Cu/ZnO catalysts were widely
applied for the industrial processes such as methanol-synthesis
and water-gas shift reactions.14-16 The hydrogenation activity
and selectivity of aldehyde group (furfural molecule, scheme 1),
included C=O and C-O bonds reduction, should be expected
excellent as well. The major precursors of these Cu/ZnO
catalysts were confirmed as aurichalcite and zincian malachite,
and the precursor phases strongly influenced the activity of the
final catalyst.17 Although the preferred precursor phase for these
reactions was usually regarded as zincian malachite,18,19 the
better precursor phase for other reactions was ambiguous yet.
Introduction
As the shortage of fossil energy, biomass is more and more
attractive because of its potential to displace non-renewable
fossil resources in the production of chemicals and liquid
transportation fuels.1 Furfural (FFA), one of the important
biomass-derived platform compounds, can be transformed to
various of products with high added value, such as furfuryl
alcohol,
2-methylfuran,
tetrahydrofurfuryl
alcohol,
2-
methyltetrahydrofuran, 1,2-pentanediol and 1,5-pentanediol
etc.(as seen in Scheme 1). Furfuryl alcohol (FOL) and 2-
methylfuran (2-MF) are ones of the most significant down-
stream products and can be widely applied in chemical
industries. FOL is an important fine chemical intermediate, which
can be used to produce farm chemicals, resins, binders etc.;2, 3
2-MF has a good fuel performance (octane value =103) and
could be an excellent substitute or additive agent of fossil fuels. 4
The inexpensive Cu-based catalysts were widely used for
C=O and C-O bonds hydrogenation because of their good
catalysis performance and low price.5 In the previous work,
many Cu-based catalysts had been applied in hydrogenation of
[a]
Xiaohai Yang, Xiaomin Xiang, Yong-Wang Li and Yulei Zhu
State Key Laboratory of Coal Conversion, Institute of Coal
Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR
China.
Scheme 1. Down-stream products of furfural hydrogenation
E-mail: zhuyulei@sxicc.ac.cn.
Xiaohai Yang, Xiaomin Xiang
To obtain these active catalyst precursors, the well-
[b]
[c]
acknowledged
preparation
method
is
conventional
University of Chinese Academy of Sciences, Beijing 100049, PR
China.
Hongmei Chen, Hongyan Zheng, Yulei Zhu, Yong-Wang Li
Synfuels China Co. Ltd, Beijing, 101407, PR China.
coprecipitation by modulation molar ratios of Cu and Zn.20 For
aurichalcite (AC) phase, the Cu/(Cu+Zn) (mol/mol) was
controlled around 40% and for zincian malachite (ZM), the value
was usually above 65%.20 The industrial Cu/ZnO based catalyst
is usually characterized copper rich (70%, wt.%). However,
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10.1002/cctc.201700279
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copper nitrate, as the most frequently-used copper resource of
the catalyst via the coprecipitation, is more expensive than zinc
nitrate (zinc resource). A better economic effectiveness would
be obtained if the copper cost could be reduced and the catalytic
performance could be maintained.
In this work, we employed two typical mineral derived
Cu/ZnO catalysts mainly consisted of aurichalcite and zincian
malachite respectively on furfural hydrogenation to FOL and 2-
MF. A high yield of furfuryl alcohol (above 99%) could be
obtained at 120°C, and a steady 94.5% yield of 2-methylfuran
could be achieved at 200°C over the aurichalcite derived
catalyst. Interestingly, the lower copper cost catalyst performed
a better catalytic activity compared with the copper rich one. The
different hydrogenotion performances of these two catalysts
were discussed based on various characterizations such as
XRD, Raman spectra, CO-IR, H2-TPR, N2O-titration, NH3-TPD
and XPS.
2θ=43.3° and 50.4° could be attributed to Cu(111) and Cu(200)
respectively (PDF# 04-0836). All the remainder peaks could be
assigned to ZnO described above. The enhanced diffraction
peaks of ZnO between 2θ=55° to 70° of calcined ZM-CZ catalyst
after reduction (seen in Figure 2a and b), which had be reported
by Simon A. et al.17 before, might imply the divorce of ZnO from
Cu species. This should influence the interaction between
copper species and ZnO finally. The intensities of CuO(111) for
both calcined catalysts are similar, but the Cu(111) diffraction
peak of ZM-CZ is distinctly intensive. The dispersion of copper
species should be markedly different after reduction.
Results and Discussion
As shown in Figure 1(a), the evident peaks positioned at
2θ=13.0°(400), 2θ=24.2°(610), 2θ=27.4°(511), 2θ=30.9°(420),
2θ=34.3°(620) etc. of the synthesized-1 sample are assigned to
the characteristic peaks of pure aurichalcite (Cu,Zn)5(CO3)2(OH)6
(PDF# 17-0743). Calculated XRD patterns of malachite are
depicted in Figure1(b), 2θ=14.8°(020), 17.6°(120), 24.1°(220),
31.2°(20−1), 32.2°(21−1) and 35.6°(240) etc. are assigned to
pure malachite Cu2(CO3)(OH)2 (PDF# 41-1390). The XRD
patterns of synthesized-2 are well coincide with the calculated
one. While, the diffraction peaks at 31.2° (20−1) and 32.2°
(21−1) shift toward higher angles (32.1° and 33.0°) indicating the
substitution of Cu2+ with Zn2+ in the lattice (change of M-O bonds
while M=Cu or Zn), implying the crystalline phase was changed
from malachite to zincian malachite (Cu,Zn)2(CO3)(OH)2 finally.20
The residual peaks at 2θ=13.0° and 2θ=27.4° implied the slight
existence of aurichalcite in synthesized-2. The structures of the
synthesized samples could be demonstrated by Raman spectra
shown in Figure 1(c) as well. The band at 1070 cm−1
2−
corresponds to the CO3
symmetric stretching mode of
Figure 1. XRD (a, b) and Raman patterns (c) of the synthesized precursors.
aurichalcite while the peaks at 1100 cm−1, 1370 cm−1 and 1500
cm−1 can be assigned to zincian malachite phase.21-23 Based on
the results of XRD patterns and Raman spectra, the synthesized
samples were verified to be the presupposed precursors mainly
consisted of aurichalcite and zincian malachite respectively.
Structure transformations of calcined and reduced catalysts
were investigated via XRD patterns. As displayed in Figure 2(a),
oxide composites containing an intimate mixture of CuO and
ZnO species are the main products of the calcined catalysts.
The evident peaks positioned at 2θ=31.8°(100), 34.4°(002),
36.3°(101), 47.5°(102) etc. can be assigned to ZnO (PDF# 36-
1451). The visibly peaks at 2θ=35.6°(002), 38.7°(111) are
reflections of CuO (PDF# 45-0937). The rest of undefined peaks
could be assigned to residual carbonates. 24,25 Figure 2(b) shows
XRD patterns of reduced catalysts. The peaks located at
Figure 2. XRD patterns of calcined (a) and reduced (b) samples.
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Table 1 The main physiochemical and structural properties of the Cu/ZnO catalysts
[b]
[c]
[c]
[d]
[d]
[d]
Cat.
Cu loading[a]
(wt%)
SBET
dCuO
dCu
dCu
DCu
(%)
SCu
Total acidity [e]
Calcined/Reduced
Nominal acidity [e]
Calcined/Reduced
(m2g-1)
(nm)
(nm)
(nm)
(m2gcat
)
-1
(mmolNH3/gcat
)
AC-CZ
ZM-CZ
39.3
61.1
80.9
83.7
7.8
8.5
8.0
7.0
14.4
6.9
38.2
28.5
1.35/0.38
48.0/13.5
100.0/7.9
13.5
14.5
2.81/0.22
[a] Determined using ICP-OES.[b] Determined by N2 adsorption experiments, calcined samples. [c] The crystalline size(dCuO/Cu) were
calculated using the CuO(111) reflection at 38.7° and Cu(111) at 43.3° based on the Scherrer equation. [d] The average size of Cu
particles(dCu), dispersion(DCu) and copper surface area (SCu) were determined by N2O titration measurements.[e] The acidity of catalysts
was determined via NH3-TPD.
The main physiochemical properties of the catalysts were
further investigated via various experiments. As exhibited in
Table 1, the calcined AC-CZ and ZM-CZ possessed similar BET
surface area (80.9m2g-1 and 83.7m2g-1). The crystalline sizes of
CuO were 7.8nm and 8.5nm for AC-CZ and ZM-CZ respectively.
It might be concluded that the mineral derived oxide composites
had the similar surface physical properties in spite of the
different Cu loadings. The crystalline size of Cu was calculated
via XRD patterns (Figure 2b) and determined by N2O titration
measurements as well. The size of Cu was 8.0nm and 13.5nm
for AC-CZ and ZM-CZ respectively according to the calculated
results based on the Scherrer equation, implying much smaller
Figure 3. CO-IR spectra of the reduced catalysts (a), the profiles displayed
from outside to inside are respectively purged with He for 1, 5, 10min; H2-TPR
Cu nanoparticles for the former catalyst. The results determined
by N2O titration measurements were close to that of XRD,
confirmed the reliability of the calculated copper size. As
suggested by the observations of XRD, ZnO of calcined ZM-CZ
divorced of from Cu species after reduction. The distinctly
enlarged size of Cu nanoparticles of ZM-CZ comparing with
CuO confirmed this as well. The agglomeration of Cu species for
Cu rich ZM-CZ catalyst was suggested to be much more
acutely, which was supposed to be mainly caused by the low
H ttig(134°C) temperature of Cu. 26
N2O titration measurements show that the copper
dispersion of AC-CZ was twice as much as ZM-CZ (14.4% vs.
6.9%). The dispersion of copper species could be also
qualitatively investigated by CO-IR based on the strong
adsorptive action between CO and copper species.27 According
to Figure 3(a), strong CO absorbed peaks could be detected
located at around 2100 cm-1 on both of the samples.27,28
However, the peak intensity of AC-CZ was about 30 times as
much as ZM-CZ, implied that AC-CZ was provided with a much
better dispersion of copper species and more superficial copper
sites.27,28 Furthermore, H2-TPR was performed to study the
reducibility of the catalysts. As shown in Figure 3(b), both of the
catalysts presented an asymmetric reduction peak in the range
of 100-250°C. ZM-CZ displayed a higher reduced temperature at
about 212 °C comparing with AC-CZ (about 200°C). It had been
confirmed that higher dispersion Cu was easily to be
reduced.13,29,30 The superior dispersion of Cu species of AC-CZ
could be supported distinctly according to the H2-TPR results,
which was coincide well with N2O titration measurements and
CO-IR.
curves of the catalysts (b).
of catalysts.13,31 In this work, surface acidity of the catalysts was
investigated through NH3-TPD experiments. The nominal acidity
of calcined and reduced catalysts was calculated based on NH3-
TPD profiles described in Figure 4 and exhibited in Table 1.
Nanoparticles contain a number of crystal defects. The defects
of the metal oxides usually result in the existence of Cu2+ and
Zn2+ on the surface. It is acknowledged that lewis acidity is
correlated to the electron configuration of the positive ions. The
higher ability to accepting electrons, the stronger lewis acidity it
has. Therefore, Cu2+ (3d9) should be a stronger lewis acid center
than Zn2+ (3d10) because of its unoccupied d orbit. As shown in
Figure 4(a), both of the calcined catalysts performed strong
acidity. The nominal surface acidity for calcined ZM-CZ was
twice as much as AC-CZ, which should be mainly because of its
much more CuO species (strong lewis acid center: Cu2+). After
reduction, the nominal surface acidity of ZM-CZ was drastically
decreased from 100.0 to 7.9, while the value of reduced AC-CZ
was dropped from 48.0 to 13.5. The transformation of CuO to
metallic Cu should be the primary reason of the surface acidity
decline. However, amphoteric oxide ZnO could contribute a
number of surface acidity sites yet. The higher surface acidity of
reduced AC-CZ than ZM-CZ (13.5 vs. 7.9) could be assigned to
its more ZnO species. Generally speaking, three kinds of
surface acid sites of Cu/ZnO catalysts could be classified based
on the desorption temperature of NH3: weak acid sites (140–
180°C), medium acid sites (200–240°C) and strong acid sites
(>310°C).33,34 Yeojin Jeong et al.33,34 reported a positively
relation between the amount of strong acid sites and the
performance in methanol synthesis from syngas over Cu/ZnO
catalysts. According to Figure 4(a) and (b), more strong acid
sites of calcined AC-CZ were retained after reduction in spite of
the drastically decline of the total surface acidity. The amount of
The surface acidity of catalysts was regarded as a critical
role in determining the product distribution in furfural
hydrogenation.13,31,32 It was suggested that the capacity of
hydrogenolysis for C-O bonds was related to the surface acidity
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strong acid sites of reduced AC-CZ was calculated about 1.7
times as much as ZM-CZ based on Figure 4(b).
X-ray photoelectron spectroscopy (XPS) was used to
characterize the Cu oxidation state. The down-shift at 933.7eV
and the absence of “shakeup” peaks at 940-945eV compared
be obtained over ZM-CZ at 120°C. The conversions of FFA at
100°C over AC-CZ and ZM-CZ were 84.5% and 70.0%
respectively, implied that AC-CZ performed better activity for
FFA hydrogenation to FOL. According to Table 2, almost
completed conversions (>99.5%) of FFA could be achieved as
Figure 4. NH3-TPD patterns of calcined(a) and reduced(b) samples.
with CuO in Cu 2p spectra (Figure 5a) indicated that the Cu2+
was completely reduced to metallic copper (Cu0) and/or Cu+ for
reduced AC-CZ and ZM-CZ catalysts.35,36 Noteworthily, the
visible higher Cu 2p binding energy (BE) of AC-CZ implied that
its electronic state was different to ZM-CZ. The change of
copper electronic state was supposed to be the critical evidence
of the existence of Cu-ZnO synergy.37 AES spectra (Figure 5b)
showed that CuLMM kinetic energy (KE) of ZM-CZ was mainly
located at 919.0 ± 0.2eV, which could be assigned to Cu0.35,38
The CuLMM KE of AC-CZ was 0.4-0.5eV lower than ZM-CZ,
indicating its more obvious electronic perturbation between
copper and ZnO (partial reduced ZnO species on the interface).
The partial reducibility of ZnO could be confirmed by the multi-
peaks of AES spectra of ZnLMM (Figure 5c). The prominent BE
peak located at 498.2eV could be attributed to Zn2+; the visible
BE peak located at about 495.2eV had been suggested as the
evidence of partial reduced Zn2+ species (Znσ+, 0<σ<2) on the
interface of Cu-ZnO.39 The change of the valence state from
Zn2+ to Zn0 was observed more pronounced for AC-ZM (seen in
Fig 5c), implying the more obvious electronic perturbation
between copper and reduced oxides ZnO, as mentioned before.
The electronic perturbation between copper and reduced ZnO
could be further revealed from XPS of O 1s. Figure 5(d) shows
two oxygen species, the hydroxyl group on the ZnO surface
(531.6eV) and the crystal lattice oxygen in ZnO (530.2eV).35,40
The BE shift of the lattice oxygen in ZM-CZ was unconspicuous
comparing with pure ZnO (530.2eV35,40), while the value of AC-
CZ (530.5eV) was visible higher than that of ZM-AC, suggesting
the more obvious electron transfer from ZnO to Cu on the Cu-
ZnO interface. It could be speculated that the electron transfer
from ZnO to Cu could result in variation/deficiency of lattice
oxygen, and leaded to the generation of the strong acidity sites
finally. According to Figure 5b, the absence evidence of Cu+
(KE=916.7eV35) should mean that the strong acidity sites of the
catalysts were mainly contributed by these Cu0-ZnO sites. The
more strong acidity sites of AC-CZ determined by NH3-TPD
coincided well with XPS and AES results.
Figure 5. XPS and AES patterns of reduced catalysts. (a) XPS of Cu 2p;
(b) AES of CuLMM; (c) AES of ZnLMM; (d) XPS of O 1s.
Table 2 Furfural hydrogenation over Cu/ZnO catalysts under different
temperatures [a]
T
(°C)
LHSV
(h-1)
Conv.
(%)
Sel.(%)
FOL
Cat.
2-MF
Others [b]
ZM-CZ
100
120
0.5
0.5
70.0
99.5
99.7
99.5
0.0
0.1
0.3
0.5
160
200
100
120
160
200
0.5
0.5
0.5
0.5
0.5
0.5
99.7
99.7
84.5
99.7
99.9
100.0
84.7
14.4
99.5
92.6
49.0
0.1
12.9
76.9
0.2
2.4
8.7
0.3
1.1
7.0
27.2
AC-CZ
6.3
44.0
72.7
[a] Conditions: 4.8g catalyst, ambient pressure, pure furfural feed, after
24h time on stream (products of the first 16h were collected and
discarded); Carbon balances
>
95%. [b] Others mainly include
tetrahydrofurfuryl alcohol, γ-valerolactone, 1,2-pentanediol etc.
Table 3 Furfuryl alcohol hydrogenation over Cu/ZnO catalysts under
different temperatures [a]
T
(°C)
120
LHSV
(h-1)
0.5
Conv.
(%)
1.4
Sel.(%)
2-MF
99.5
TOF
Cat.
Others [b]
0.5.
ZM-CZ
10.2
n.d.
n.d.
75.8
n.d.
n.d.
160
200
120
160
200
0.5
0.5
0.5
0.5
0.5
12.5
23.7
13.9
54.1
88.1
98.9
96.5
98.8
89.3
81.4
1.1
3.5
AC-CZ
1.2
10.7
18.6
[a] Conditions: 4.8g catalyst, ambient pressure, pure furfuryl alcohol feed, after
24h time on stream (products of the first 16h were collected and discarded);
Carbon balances > 96%. [b] Others mainly include tetrahydrofurfuryl alcohol,
1,2-pentanediol etc. n.d.: not determined.
the reaction temperature was improved above 120°C over both
of the catalysts. AC-CZ performed a better 2-MF selectivity,
6.3% yield of 2-MF could be obtained at 120°C, while the value
As shown in Table 2, both of the catalysts exhibited good
activity for furfural hydrogenation under the mild reaction
conditions. Both of the catalysts performed excellent FOL
selectivity (>99%) bellow 120°C. Nearly 99% yield of FOL could
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Table 4 Furfural hydrogenation over Cu/ZnO catalysts under different LHSV[a]
Catalyst
Temperature
(°C)
LHSV
(h-1)
Conv.
(%)
Sel.(%)
FOL
Productivity of 2-MF[c]
2-MF
Others [b]
mol·kgCat-1h-1/mol·kgCu-1h-1
200
200
200
200
200
200
0.5
1.0
1.5
0.5
1.0
1.5
99.7
14.4
24.7
36.6
0.1
76.9
67.8
59.1
72.7
80.3
94.5
8.7
4.6/7.6
ZM-CZ
99.4
7.5
8.1/13.3
10.6/17.4
4.6/11.8
9.7/24.7
17.1/43.5
99.3
4.3
AC-CZ
100.0
100.0
100.0
27.2
19.4
5.0
0.3
0.5
[a] Conditions: 4.8g catalyst, ambient pressure, pure furfural feed, after 24h time on stream (products of the first 16h were collected and
discarded); Carbon balances > 95%. [b] Others mainly include tetrahydrofurfuryl alcohol, γ-valerolactone, 1,2-pentanediol etc. [c]
Productivity of 2-MF normalized by catalyst mass and copper mass.
over ZM-CZ was neglectful. As shown in Scheme 1,
hydrogenation of FFA to FOL and then 2-MF was actually a
consecutive reaction. It was well-known that the increase of
reaction temperature intensified the hydrogenolysis of C-O
bonds of FOL over copper based catalysts.13,31,41 The yields of
2-MF was improved as expected over both of the catalysts as
the reaction temperature increased. Further increase of
temperature above 200°C usually resulted in intensified ring-
opening reactions.41 As the ring-opening reactions should take
placed under higher activation energy, the much more
intensified yields of by-products over AC-CZ should be attributed
to its higher catalytic activity for FFA hydrogenation. According
to Table 2, AC-CZ seemed to be much more efficient for 2-MF
production from FFA. To further investigate hydrogenolysis of
C-O bonds over the different catalysts, the intermediate product
FOL was feed as the reactant instead of FFA. As shown in Table
3, conversions of FOL were increased as the reaction
temperature was improved from 120°C to 200°C over both of the
catalysts. The FOL conversion over AC-CZ was 13.9% at 120°C,
but the value over ZM-CZ was only 1.4%. As exhibited in Table
3, the activity of AC-CZ for FOL hydrogenation should be
observably higher than ZM-CZ under the experimental
conditions. In addition, products over both catalysts were almost
2-MF at low reaction temperature. As the improvement of
temperature, selectivity of by-products was also increased as
expected. The slight discrepancy of 2-MF yield under the same
conditions but disparate reactant feed (FFA and FOL, seen in
Table 2 and Table 3) might be due to the different adsorption
between C-O/C=O and the surface of the catalysts.
Table 4 shows the LHSV test (FFA feed) over the catalysts
at 200°C . As the increase of LHSV, the selectivity of 2-MF over
ZM-CZ was dropped from 76.9% to 59.1%, on the other hand,
the selectivity over AC-CZ was increased from 72.7% to 94.5%.
According to Table 4, the reduction of 2-MF selectivity as the
increase of LHSV over ZM-CZ was mainly attributed to the
incomplete conversion of FOL. However, the improvement of
LHSV restrained the ring ring-opening reactions over AC-CZ
catalyst and increased the yield of 2-MF.
Stabilities of 2-MF productivity over the catalysts were
forthright investigated under the harsh conditions (200°C,
LHSV=1.5 h−1). According to Figure 6(a), 2-MF yield over ZM-CZ
catalyst at 160°C was dropped from 45% to 23% as the
persistent feed of FFA; while the yields were almost hold at 55%
over AC-CZ. As described in Figure 6(b), 2-MF yield over AC-CZ
at 200°C could be stabilized above 90%; on the contrary, the
yield of 2-MF over ZM-CZ catalyst under the same reaction
conditions was decreased from initial 70% to 55% as the time on
stream. As shown in Table 3, AC-CZ showed higher
productivities of 2-MF normalized by catalyst mass especially
under high LHSV. Furthermore, AC-CZ performed much higher
productivities of 2-MF normalized by copper mass than ZM-CZ
Figure 6. 2-MF yield at 160°C (a) and 200°C (b) for AC-CZ and ZM-CZ
catalysts under LHSV=1.5h-1 as a function of time on steam. Conversions of
FFA were closed to 100%.
(e.g. AC-CZ: 43.5 mol·kgCu-1h-1; ZM-CZ: 17.4 mol·kgCu-1h-1,
200°C and LHSV=1.5 h−1), implying the much more excellent
efficiency of copper for the former catalyst.
Table 5 Comparisons of the investigated parameters[a]
Parameters
ZM-CZ
AC-CZ
1.71
Surface acidity
Cu surface area
TOF[b] (FOL to 2-MF)
FOL conversion[c]
1
1
1
1
1.35
7.43
9.93
[a] Relative magnitudes and calculated based on the values exhibited above.
[b],[c] Determined at 120°C (Table 3).
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10.1002/cctc.201700279
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(AC) and zincian malachite (ZM) were prepared via modified
coprecipitated method described by Bettina Bems et al 20. For AC, two
kinds of aqueous solution were prepared before the precipitation, solution
A: Cu(NO3)2 (0.4 M) and Zn(NO3)2 (0.6 M) and solution B: 0.5 M Na2CO3.
Solution A and B was simultaneously introduced into a vessel in a water
bath of 65°C by peristaltic pump under vigorous stirring, pH of the
suspension was controlled at 6.5~7 by adjusting the adding rates. After
the precipitation, the suspension was aged for 60 min in the mother liquor
at the same conditions and then filtered, washed by deionized water for 5
times, followed by dried at 100°C for 12h. For ZM, the mixed solution A
was consisted of 0.7 M Cu(NO3)2 and 0.3 M Zn(NO3)2, solution B was
1.2M Na2CO3. All of the dried precursors were calcined at 300°C for 3h in
air atmosphere.
The contributions of surface acidity and copper surface area
to the catalytic performance for 2-MF production had not been
identified so far. As the comparisons of the investigated
parameters shown in Table 5, the FOL conversion and TOF over
AC-CZ were both markedly higher than ZM-CZ. It might be
speculated that surface acidity of the catalysts contributed more
obviously according to the higher value (1.71 vs. 1.35) in theses
catalysts. However, a synergistic function of them should not be
neglectful and there might be an optimized balance between
them. In conclusion, AC-CZ showed a better activity for FFA
hydrogenation, especially for hydrogenolysis of C-O bonds
compared with ZM-CZ. The higher dispersion of copper species
and better copper surface areas were suggested to be
significant factors influencing catalytic activity of Cu-based
catalysts.31,42 The excellent activity of AC-CZ could be attributed
to its superior dispersion of copper species and copper surface
area. The better C-O bonds hydrogenolysis activity of AC-CZ
could mainly be assigned to its more acidity sites and strong
acidity sites confirmed by NH3-TPD.13,31,32 Cu0-ZnO sites had
been suggested to be the main catalytic active site of the
Cu/ZnO catalyst for levulinic acid hydrogenation.35 The better
Cu0-ZnO synergy of AC-CZ was demonstrated via XPS and AES
spectra and should be beneficial to improving the catalytic
performance.
Characterization of catalysts. XRD of precursors and catalysts were
conducted on a Rigaku D/max-2000 diffractometer with Cu Kα radiation
operating at 40 kV. Raman patterns were recorded with a LabRAM
HR800 system equipped with a CCD detector at room temperature. The
325nm of the He–Cd laser was used as the exciting source with a power
of 30MW. BET surface areas were measured by N2 physical adsorption
experiments. CO-IR spectra of powder samples were recorded with a
Vertex 70 (Bruker) FT-IR spectrophotometer in the range of 4000−400
cm−1 at a resolution of 4 cm−1. Cu surface area and H2 temperature-
programmed reduction (H2-TPR) were both performed on a Micromeritics
ASAP 2920 instrument equipped with a TCD and the former was
determined by N2O titration experiments. Surface acidity of the catalysts
was determined by temperature-programmed desorption of ammonia
(NH3-TPD) using a Micromeritics ASAP 2920 instrument and a mass
spectrometric detector. The in situ X-ray photoelectron spectroscopy
(XPS) and Auger electron spectroscopy (AES) experiments were
performed on a VG MiltiLab 2000 spectrometer. All of the samples were
reduced in a H2/Ar (or N2) (10%, v/v) flow in the experiments if necessary.
Conclusions
We synthesized two typical mineral derived Cu/ZnO
catalysts mainly consisted of aurichalcite and zincian malachite
via coprecipitation method. The excellent activity for furfural
hydrogenation to produce furfuryl alcohol and 2-methylfuran
over mineral derived Cu/ZnO catalysts had been demonstrated
in a fixed-bed reactor. Under the proper reaction conditions, high
yields of furfuryl alcohol (above 99%) and 2-methylfuran (94.5%)
could be obtained. The catalyst stem from aurichalcite was
evaluated to have much better activity for furfural hydrogenation,
especially for 2-MF production compared with zincian malachite.
The much more excellent performance for furfural hydrogenation
of aurichalcite Cu/ZnO catalyst was due to its higher dispersion
of copper species, superior copper surface area, better surface
acidity distribution and stronger Cu0-ZnO synergy. In addition,
surface acidity of the catalysts seemed to contribute more
obviously to 2-MF production than Cu surface area in theses
catalysts, but the optimized balance of them needed to be
investigated systematically. As the most frequently-used
precursors of widely utilized industrial Cu/ZnO based catalysts,
aurichalcite and zincian malachite, were contrastively
investigated in this work. As a result, the nearly 40% lower
copper cost catalyst derived from aurichalcite was suggested to
be much more efficient for furfural hydrogenation to furfuryl
alcohol and 2-methylfuran.
Evaluation of catalysts. Catalytic performance of the catalysts was
carried out in a vertical tubular fixed-bed reactor with a length of 500 mm
and i.d. of 12 mm which was filled with quartz sands. Catalysts were
shaped with a pressure of 40 MPa and crushed to 20-40 mesh before the
evaluation. Catalysts were loaded in the central section of the reactor
and reduced in a H2/N2(10:90, v/v) flow at 250°C for 120 min in situ
before the reaction. After the reduction, the gas flow was switched to
pure hydrogen. Liquid furfural/furfuryl alcohol was introduced in
a
preheater (heated to 100ºC) connected above the reactor by a peristaltic
pump and mixed with H2 simultaneously. The H2 flow with a high speed
(about 160 ml/min controlled by a flowmeter) acted as both carrier gas
and reactant. The well mixed reactants arrived at catalysts bed and then
the catalysis took placed. On the bottom of the reactor, the product tank
equipped with a condensation system was connected. The reaction take
placed in ambient pressure at different temperatures, the products were
collected and analyzed by using a GC instrument with an FID. The
conversion and selectivity/yield were calculated on basis of the following
Experimental Section
Preparation of catalysts. Typical precursors consisted of aurichalcite
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10.1002/cctc.201700279
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FULL PAPER
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10.1002/cctc.201700279
ChemCatChem
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FULL PAPER
Cu/ZnO catalyst derived from less
copper cost aurichalcite showed much
more excellent performance for FFA
hydrogenation compared with zincian
malachite.
Xiaohai Yang, Xiaomin Xiang, Hongmei
Chen, Hongyan Zheng, Yong-Wang Li
and Yulei Zhu*
Page No. – Page No.
Efficient synthesis of furfuryl alcohol
and 2-methylfuran from furfural over
mineral derived Cu/ZnO catalysts
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