A highly efficient Cu/AlOOH catalyst obtained by in situ reduction: Catalytic transfer hydrogenation of ML into Γ-GVL

Article abstract of DOI:10.1016/j.mcat.2019.01.033

Catalytic transfer hydrogenation (CTH) of carbonyl compounds is considered as one of the most promising processes in the synthesis of fuels and chemicals. In this work, we propose a one-step strategy for catalyst preparation and CTH. Using the strategy, the production of γ-valerolactone (γ-GVL) was performed with isopropanol (2-PrOH) as solvent over in situ reduced nano-Cu/AlOOH catalyst from Cu₂(OH)₂CO₃/AlOOH and the optimal reaction conditions for γ-GVL are 180 °C for 5 h using the in situ reduced catalyst with Cu/Al molar ratio 3/1 (90.51% yields of γ-GVL). Furthermore, it has been confirmed by different characterization methods (such as: SEM, TEM, XPS, etc.) that the catalyst is heterogeneous and exhibits high catalytic activity and stability which is attributed to the stability of the zero-valent copper in the catalyst and the nanosized particles of the catalyst. In addition, the catalysts also show general applicability to other carbonyl compounds.

Full text of DOI:10.1016/j.mcat.2019.01.033

Molecular Catalysis 467 (2019) 52–60
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
A highly efficient Cu/AlOOH catalyst obtained by in situ reduction: Catalytic
transfer hydrogenation of ML into γ-GVL
a,1
a,1
a
a
b
c
a
Mingwei Ma , Hui Liu , Jingjie Cao , Pan Hou , Jiahui Huang , Xingliang Xu , Huijuan Yue ,
a,⁎
a
Ge Tian , Shouhua Feng
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
a
b
c
College of Chemistry and Material Science, Shandong Agricultural University, Taian 271018, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Catalytic transfer hydrogenation (CTH) of carbonyl compounds is considered as one of the most promising
processes in the synthesis of fuels and chemicals. In this work, we propose a one-step strategy for catalyst
preparation and CTH. Using the strategy, the production of γ-valerolactone (γ-GVL) was performed with iso-
In situ reduced Cu/AlOOH
Catalytic transfer hydrogenation
Nanocatalyst
Carbonyl compounds
Zero-Valent copper
propanol (2-PrOH) as solvent over in situ reduced nano-Cu/AlOOH catalyst from Cu
2 2 3
(OH) CO /AlOOH and the
optimal reaction conditions for γ-GVL are 180 °C for 5 h using the in situ reduced catalyst with Cu/Al molar ratio
3
/1 (90.51% yields of γ-GVL). Furthermore, it has been confirmed by different characterization methods (such
as: SEM, TEM, XPS, etc.) that the catalyst is heterogeneous and exhibits high catalytic activity and stability
which is attributed to the stability of the zero-valent copper in the catalyst and the nanosized particles of the
catalyst. In addition, the catalysts also show general applicability to other carbonyl compounds.
1. Introduction
has been regarded as a high-efficiency route to synthesize γ-GVL from
LA or its esters by using different alcohols as hydrogen source with
On account of limited natural sources and high demand, selective
various catalysts such as metal, [27,28]metal complexes [2,25,29],
metal oxides or hydroxides [26,30,31],various zeolites [32], etc. In
these catalysts, copper as a non-precious metal, combined with metal
oxides, plays an important role in the synthesis of γ-GVL. Among these
catalysts, Cu° or metal oxides supported Cu° as catalyst has been ex-
tensively studied and the relevant research results are summarized in
Table S1. C. V. Rode et al. and Fu et al. researched copper-based cat-
conversion of biomass to fuels and chemicals is important in the rapidly
developing human society. [1,2] Among different methods for the
conversion of biomass, catalytic routes are especially important and
many biomass-derived chemicals and fuels can be produced by care-
fully designing catalytic systems [3]. γ-Valerolactone (γ-GVL), one of
the most promising platforms for the sustainable products of fuels and
high value-added chemicals [4,5], can be synthesized by hydro-
genolysis of levulinic acid (LA) or its esters from lignocellulosic bio-
mass.
alysts for hydrogenation of LA into γ-GVL with H
[33,34]Huang et al. studied CTH reaction of EL into γ-GVL using al-
cohol as a hydrogen source over the Cu-Ni/Al catalyst. [27] Lin
et al. demonstrated for the first time that the catalytic hydrogenation of
ML to γ-GVL in methanol with in situ reduced Cu or Cu/Cr from
CuO or CuO/Cr as catalyst, which simplified the reaction by com-
2
as a hydrogen source.
2 3
O
There are two different paths in the reactions. The first path is to use
a series of metals (such as Pd, [6,7] Ru [8,9], Rh [10], Pt [11,12], Re
13], Ni [14–16], Co [17–19], Mo [20,21] and Cu [22,23]) as catalyst
2 3
O
[
2 3
O
in the presence of molecular hydrogen (H ). [24,25] And the other one
is accomplished under mild conditions [26] by using alcohols or formic
acid (FA) and transition metal compounds instead of molecular hy-
2
bining GVL production and catalyst synthesis into one step. [35,36] In
the reaction, as a substitute for hydrogen, methanol reduces CuO to Cu°
above 200 °C, and it also catalyzes the synthesis of γ-GVL as a hydrogen
source.
Some researches show that secondary alcohols have a stronger hy-
drogen supply capacity than primary alcohols at low temperatures.
drogen (H
respectively.
Recently, in the second path, the catalytic transfer hydrogenation
2
) and precious metals as hydrogen sources and catalysts,
(
CTH) reaction, as a potential replacement to molecular hydrogen (H
2
),
2 2 3
[4,37] And basic copper carbonate (Cu (OH) CO ) is the compound
⁎
Corresponding author:
E-mail address: tiange@jlu.edu.cn (G. Tian).
These authors contributed equally.
1
https://doi.org/10.1016/j.mcat.2019.01.033
Received 25 August 2018; Received in revised form 27 December 2018; Accepted 25 January 2019
2468-8231/ © 2019 Published by Elsevier B.V.

M. Ma et al.
Molecular Catalysis 467 (2019) 52–60
Scheme 1. Schematic for the in situ formation of the Cu/AlOOH catalyst with 2-PrOH as solvent (A). The possible reaction sequence for the conversion of ML into
GVL, 1, 4-PDO and 1-PAO over the in situ reduced Cu/AlOOH catalyst (B).
most easily reduced by secondary alcohols among all copper sources
below 200 °C (Scheme S1). Therefore, in this study, nano-Cu/AlOOH
2.3. CTH reaction and sample analysis
catalysts, prepared by in situ reduction of Cu
2
(OH)
2
CO
3
/AlOOH
CTH reaction of ML was performed without stirring in a steel alloy
autoclave (Fe-Cr-Ni alloy, GB1220-92) with an internal volume of
35 ML. Typically, carbonyl compounds (0.67 mmol), solvents (20 mL),
and catalyst (0.1 g) were charged into the reactor, which was then
sealed and heated to a designed temperature (140–220 °C) for an in-
tended reaction time (1–24 h). After the reaction, the autoclave was
taken out and cooled to ambient temperature.
(
Scheme 1A), efficiently participated in the CTH reaction from ML to γ-
GVL with 2-PrOH as solvent (Scheme 1B). The high activity and sta-
bility of the catalyst are attributable to the stability of the zero-valent
copper in the catalyst and the nanosized particles of the catalyst, which
were proved by different characterization methods, such as: SEM, TEM,
XPS, etc.
Identification of liquid products in the reaction mixture was
achieved by the TRACE ISQ GC–MS (Thermo Scientific Co,
TR–WAX–MS column 30.0 m × 320 μm × 0.25 μm). The temperature
program started at 60 °C for 1 min, then increased from 60 °C to 230 °C
at a rate of 15 °C /min and held for 2 min.
2
. Experimental
2.1. Materials
Levulinate esters, γ-valerolactone, 1,4-pentanediol,1-pentanol, fur-
fural, furfuryl alcohol, 5-hydroxymethylfurfural, 2,5-bis(hydro-
xymethyl)furan, acetophenone, α-phenylethyl alcohol, cyclohexanone,
cyclohexanol, Citral, Citrate, cinnamaldehyde and cinnamyl alcohol
were purchased from TCI Chemical Reagent Company (Shanghai,
2.4. Catalyst characterization
X-ray diffraction (XRD) was carried out via a Rigaku D/max-2550
diffractometer using Cu-Kα radiation. The wavelength of X-ray source
was set at 0.154 nm, and the step width was 0.02° with a scanning
speed of 6° per minute between two theta of 10° and 80°. Fourier
transform infrared (FT-IR) spectra were recorded between 400 to 4000
3 2 2 2 3 3 2 2 3
China). Cu(NO ) ·6H O, Al (NO ) ·9H O, Na CO and all solvents were
purchased from Sinopharm Chemical Reagent Company (Shanghai,
China). All chemicals were used without further purification.
−
1
cm
by using Bruker VERTEXV 80 V spectrometer. The thermogravi-
metric (TG) analysis was performed using a thermogravimetric analysis
system (TG: PerkinElmer Instruments) under air atmosphere at the
heating rate of 10 °C min . Brunauer -Emmett-Teller (BET) surface
area was measured on an Asap 2420 surface adsorption apparatus. The
elemental compositions of catalysts were determined by inductively
coupled plasma (ICP) on an Optima 3300 DV (PerkinElmer
2
.2. Catalyst preparation
−
1
The Cu
were prepared by the co-precipitation method. Briefly, Cu(NO
15 mmol) and Al (NO ·9H O (5 mmol) were dissolved in 40 mL
deionized water and were stirred at ambient temperature for 15 min.
With vigorous stirring, 1 M Na CO solvent was dropwise added until
2
(OH)
2
CO
3
/AlOOH catalyst with Cu/Al molar ratio of 3/1
3
)
2
·6H O
2
(
2
3
)
3
2
3 2
Instruments). NH -TPD and CO -TPD (temperature-programmed deso-
2
3
rption) were conducted on a Micromeritics AutoChem II2920 auto-
mated chemisorption analyzer to assess the surface acidity and basicity
of catalysts, respectively. X-ray photoelectron spectroscopy (XPS)
measurements were performed using ESCALAB250 photoelectron
spectrometer equipped with a charge neutralizer and an Mg Kα X-ray
source. The morphology of the samples was investigated by transmis-
sion electron microscopy (TEM; Tecnai F20) and scanning electron
microscopy (SEM; Jeol JSM-6700 F).
pH = 8. After being aged at room temperature for 5 h, the precipitate
was separated by filtration and washed with deionized water several
times. The precipitate obtained was dried overnight in the oven at 80 °C
and then calcined at 200 °C for 5 h. Other Cu/Al molar ratios of
Cu
in the same method.
CuO/Al for in situ reduction to produce Cu/Al
by baking the dried sample at 500 °C in an air atmosphere and hy-
drogen-reduced Cu/Al catalyst was prepared by calcined CuO/
Al at 500 °C in a hydrogen atmosphere.
2 2 3
(OH) CO /AlOOH catalyst (0, 1/5, 1/3, 1/1, 5/1, ∞) were prepared
2
O
3
2 3
O was prepared
2 3
O
2 3
O
53

M. Ma et al.
Molecular Catalysis 467 (2019) 52–60
Fig. 1. The characterization of catalysts with
different Cu/Al molar ratios. XRD patterns of
2 2 3
Cu (OH) CO /AlOOH with different Cu/Al
molar ratios (A). XRD patterns of in situ re-
duced catalysts with different Cu/Al molar ra-
tios after the reaction (B). XPS spectra of
Cu
alysts with 3/1 ratio of Cu/Al (C). FT-IR
spectra of Cu (OH) CO /AlOOH with different
Cu/Al molar ratios (D).
2 2 3
(OH) CO /AlOOH and in situ reduced cat-
2
2
3
3
. Results and discussion
.1. Characterization of catalysts
Cu (OH) CO /AlOOH catalysts were designed with Cu/Al molar
ML into γ-GVL. Apart from this, the XPS analysis further proved this
view (Fig. 1C). For Cu 2p3/2 and Cu 2p1/2, in situ reduced samples have
3
2 2 3
lower binding energy than Cu (OH) CO /AlOOH, demonstrating that
2
+
+
Cu species were reduced to Cu or Cu species after the CTH reaction.
2+
2
2
3
Meanwhile, Cu satellite peak becomes weaker, apparently confirmed
that more Cu2+ species were reduced to Cu or Cu+ species. In addition,
the XPS peaks of Cu 2p3/2 were deconvoluted to explore Cu species in
ratios of 0, 1:3, 1:1, 3:1 and ∞ (100% copper). And ICP results de-
monstrate that the real Cu/Al molar ratios is consistent with the theo-
retical value (Table S2). In a series of Cu
with different Cu/Al molar ratios, the surface areas, pore volume and
pore size of the catalysts were determined by BET, and the results reveal
that the catalysts have a rich pore structure. Furthermore, surface area
of the catalysts decreases as the Cu/Al molar ratio increases. (Table S3
and Figure S1 A).
2+
2
(OH)
2
CO
3
/AlOOH catalysts
different valence states (Figure S2), and Cu
is obviously reduced to
Cu° or Cu+ after the reaction. The fact that Cu/AlOOH can be prepared
by in situ reduced method in the process of CTH reaction is proved.
The FT-IR analysis (Fig. 1D) clearly reveals the characteristic ab-
sorption peaks of Cu
[38,39], the peaks at 3416 and 3343 cm
OeH stretching modes. The two vibration peaks appearing at 1508 and
2
(OH)
2
CO
3
and AlOOH. For the Cu
2
(OH)
2 3
CO
−1
are assigned to the (Cu)
XRD patterns of Cu
ratios are presented in Fig. 1A. When the Cu/Al molar ratio is 0 and 1/
, XRD patterns only indicate the faint characteristic peak of AlOOH
2θ = 14.5°, 28.2°, 38.3°, 48.9° and 64.9°. PDF, 21–1307). With the
increase of Cu/Al ratio from 1/1 to ∞, the characteristic peaks of
Cu (OH) CO (2θ = 14.8°, 17.5°, 24.0°, 31.2°,35.6° and 54.8°. PDF,
1–1390) strengthens, while the peak of AlOOH becomes weak or
disappears due to its low crystallinity or low content, which is also the
reason that no Cu (OH) CO peak exists in the pattern of the catalyst
2 2 3
(OH) CO /AlOOH with different Cu/Al molar
-
1
2-
1401 cm arise from the asymmetric stretching vibration of CO
3
. The
-
1
3
(
peaks at 1055 and 880 cm can be attributed to the symmetric
stretching and out of plane bending vibrations of CO
The peaks at 588 and 531 cm are assigned to the Cu-O stretching
modes. For the AlOOH, [40–42] the two peaks at 3297 and 3091 cm
are assigned to the symmetrical and asymmetrical stretching vibrations
2-
3
, respectively.
-
1
-
1
2
2
3
4
-
1
of (Al)O-H groups respectively. The peak at 1381 cm is attributed to
the OH bending vibration. The absorption peaks at 1068 and 1162 cm
-
1
2
2
3
with 1/3 ratio of Cu/Al. In Fig. 1B, XRD patterns of in situ reduced
catalysts with different molar ratios after the reaction are recorded and
metallic Cu is acquired. Except the catalyst with 0 ratio of Cu/Al, all
catalysts show the characteristic peaks of metallic Cu (2θ = 43.2°, 50.4°
and 74.1°. PDF, 04-0836). At the same time, it can be observed that as
the Cu/Al proportion increases, the intensity of Cu peak increases. And
no obvious diffraction peaks of AlOOH was observed in the XRD pat-
terns except for the catalyst with 0 ratio of Cu/Al, attributed to its low
crystallinity or low content. Therefore, we can draw preliminary con-
clusions that Cu/AlOOH is generated by in situ reduction of
correspond to the symmetric and asymmetric bending vibrations of
AleOH, respectively. And the three characteristic peaks at 762, 641,
-
1
and 480 cm were ascribed to the vibration mode of Al-O. The above
results verify that the composition of catalyst is Cu
AlOOH.
2 2 3
(OH) CO and
From TG/DTA analysis (Figure S1B), the temperature range of NH
TPD and CO -TPD tests are determined to be between 100 °C and 310 °C
(Figure S1 C–F). Figure S1C presents the NH -TPD profiles with various
3
-
2
3
Cu/Al molar ratio samples, which indicates that all samples exhibit a
medium strong acidic site at around 300 °C, especially high Cu/Al
molar ratio catalysts (3/1 and ∞). After the in situ reduction, the
2 2 3
Cu (OH) CO /AlOOH after catalytic transfer hydrogenation (CTH) of
54

M. Ma et al.
Molecular Catalysis 467 (2019) 52–60
2 2 3 2 2 3
Fig. 2. The characterization of the Cu (OH) CO /AlOOH and the Cu/AlOOH. SEM-EDS elemental mapping images of the Cu (OH) CO /AlOOH with a Cu/Al molar
ratio of 3/1 (A).SEM-EDS elemental mapping images (B), SEM images (C) and TEM image (D) of the Cu/AlOOH with a Cu/Al molar ratio of 3/1.
catalyst maintains medium strong acid sites (300 °C). The results from
CO -TPD analysis is similar to the results of NH -TPD analysis, there are
large amounts of basic sites in the Cu/AlOOH, which meet the need for
acidic and basic sites of the catalysts in the CTH reaction of LA or its
esters.
Table 1
CTH reaction of ML with isopropanol over various Cu/Al molar ratio catalysts
and related catalysts.
2
3
Microstructure of samples was further monitored by SEM, TEM and
HRTEM. From the Fig. 2A and B, we can conclude that the distribution
of O, Al and Cu elements in the sample is very uniform before and after
the in situ reduction. In particular, the Cu/ AlOOH after in situ re-
duction have more blue bright spots than before the reaction, which is
due to the reduction of O element resulted by the decomposition of
Entry Cu/Al
molar
N
Cu
Catalyst
Conv
(%)
GVL
1,4-PDO
(mmol)
Yield (%) Yield (%)
ratio
1[a]
0
0
0
3.02
0.17
0
Cu
2
(OH)
2
CO
3
and the in situ reduced Cu accumulated on the surface of
2
3
0
1/5
0
0.24
AlOOH
33.02
89.33
26.56
81.89
0
1.85
the particles. In addition, the catalyst shows a good dispersion of O, Al
and Cu elements (Figure S3), and an average particle size of nano-Cu/
AlOOH after in situ reduction is about 30–40 nm (Fig. 2C and D).
Further analysis, The HRTEM image highlights a typical metallic Cu
with a lattice space of 0.208 nm, which is assigned to the (111) plane of
metallic Cu and a typical AlOOH with a lattice space of 0.234 nm,
which is assigned to the (031) plane of AlOOH. Similarly, the fact that
Cu/AlOOH can be prepared by in situ reduced method in the process of
CTH reaction is also confirmed.
Cu
AlOOH
Cu (OH)
AlOOH
Cu (OH)
AlOOH
Cu (OH)
AlOOH
Cu (OH)
AlOOH
2
(OH)
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
/
/
/
/
/
4
5
6
7
8
1/3
1/1
3/1
5/1
0.34
0.58
0.76
0.81
2
90.26
93.14
96.07
91.65
84.48
86.48
90.51
84.64
2.71
3.22
3.47
2.16
2
2
2
∞
3/1
——
0.76
Cu (OH) CO
2
2
3
30.56
75.26
11.02
47.95
0
0.77
[
b]
9
Cu
2
(OH)
2
CO
3
/
AlOOH
Uncalcined
CuO/Al
c]
1
1
0[
1
3/1
3/1
——
1
85.39
> 99
(89.86) (82.22)
88.17 77.95
81.89
77.45
2.33
3
.2. Catalytic activity of the various copper - aluminum catalysts for the
[d]
2
O
3
14.56
(2.42)
3.86
CTH of ML
(0.76)
1
2[e]
3/1
1.23
2 3 2
Cu/Al O (H )
In our research, ML was used as a model reactant to study the ac-
Conditions: catalyst(0.1 g), substrate(0.67 mmol), 2-PrOH (20 mL), 180 °C, 5 h.
a] Reaction without any catalyst.[b] Physical mixture of Cu (OH) CO and
AlOOH with a Cu/Al molar ratio of 3/1.[c] Cu (OH) CO /Al(OH)
cined at 200 °C.[d] The data in parentheses is 0.063 g CuO/Al
tivity of various copper - aluminum catalysts for the production of γ-
GVL through the CTH reaction. No reaction happened in the absence of
any catalyst (Table 1, entry 1). With the Cu/Al molar ratio of the cat-
alyst increasing from 0 to ∞, the conversion of ML and the yield of the
products all become volcano-like (Table 1, entry 2–8), and the best
catalytic performance is exhibited with Cu/Al molar ratio 3/1. The
yields of γ-GVL and 1,4-PDO at this time were 90.51% and 3.47% at
[
2
2
3
2
2
3
3
is not cal-
2 3
O that is cal-
2
+
cined at 500 °C for 5 h (0.76 mmol Cu ).[e] CuO/Al
atmosphere at 500 °C for 5 h.
2 3 2
O is reduced under H
account of the large amount of copper (1 mmol), the in situ reduced Cu/
Al catalyst displayed a very good catalytic effect (> 99% conversion
1
80 °C with 5 h, respectively. This phenomenon can be seen more
clearly from the 3D figure (Figure S4). Both the physically mixed
Cu (OH) CO /AlOOH catalyst and the uncalcined catalyst showed
2 3
O
of ML and 77.45% yield of GVL). Further, the amount of the catalyst
was reduced to containing 0.76 mmol of copper, which is the same as
the number of moles of copper in the Cu (OH) CO /AlOOH catalyst
2 2 3
with Cu/Al molar ratio 3/1, and their catalytic activities under the
same conditions were compared. The results showed that the catalytic
2
2
3
lower catalytic activity, and the yields of γ-GVL were 47.95% and 81%,
respectively (Table 1, entry 9–10). The former was caused by in-
homogeneous mixing of Cu
2
(OH)
2
CO
3
and AlOOH, and the latter was
affected by the support of Al(OH)
3
in the catalyst, possibly due to the
activity of in situ reduced Cu/Al
AlOOH catalyst with the same molar number of copper (Table 1, entry 6
and 11). This is because Cu (OH) CO is easier to be reduced to zero
2 3
O catalyst is lower than that of Cu/
lower amount of catalytically active sites than AlOOH in the same mass
of catalyst. Finally, we compared the in situ reduced of Cu/AlOOH and
the in situ reduced Cu/Al O in the CTH of ML (Table 1, entry 11). On
2 3
2
2
3
55

M. Ma et al.
Molecular Catalysis 467 (2019) 52–60
2 3 2 3 2 3
Fig. 3. XPS analysis of the in situ reduced Cu/AlOOH (A), in situ reduced Cu/Al O (B), hydrogen-reduced Cu/Al O (C) and spent hydrogen-reduced Cu/Al O (D).
valence copper than CuO in a shorter time (1 h and 3 h), which is
proved by XRD (Figure S5 A and Figure S5C). By comparing the in situ
reduced catalyst with the hydrogen-reduced catalyst (Table 1, entry
3.3. Effect of reaction temperature and time
The influence of reaction temperature and time on the production of
γ-GVL, 1,4-PDO and 1-PAO from ML was studied, and all experiments
were conducted at 140 °C, 160 °C, 180 °C, 200 °C and 220 °C with the
reaction time in the range of 1 to 24 h, respectively. It can be seen that
the effect of temperature and time on the reaction is remarkable. From
Fig. 4A–E, we can see that as the temperature increases, the reaction
time to achieve the highest yield of γ-GVL is significantly reduced (from
24 h at 140 °C to 1 h at 220 °C). Comprehensive temperature and time
factors, the optimal reaction condition of γ-GVL is 180 °C and 5 h. In
addition, the optimal reaction condition of 1, 4-PDO, the further pro-
duct of γ-GVL, is 200 °C for 5 h with 28.36% yield. The reaction tem-
perature and time are continuously increased (220 °C and 24 h), the
reaction will further generate 1-PAO with the best yield of 9.27%,
which possibly due to the hydrogenolysis of 1, 4-PDO.
2 3
12), it is found that the hydrogen-reduced Cu/Al O has lower catalytic
activity, which is attributed to the agglutination of the particles of the
catalyst (Figure S6).
From ICP results (Table S2) and XRD patterns (Figure S5B) of the
hydrogen-reduced Cu/Al
reduction, the actual Cu/Al molar ratio is in line with the theoretical
value and CuO/Al can be reduced by in situ reduction as expected.
From the SEM images of in situ reduced Cu/Al (Figure S6 A) and
hydrogen-reduced Cu/Al (Figure S6B), we can clearly observe that
2 3 2 3
O and CuO/Al O before and after in situ
2 3
O
2 3
O
2 3
O
the in situ reduced catalyst has good dispersibility while the hydrogen-
reduced catalyst particles tend to aggregate, which may be the reason
for its low catalytic activity. To explore further the influence of the
chemical state of the metal on the reaction, XPS analysis of the various
catalysts was accomplished (Fig. 3). The main peak of Cu 2p3/2 at
Through further XRD and XPS analysis, we can infer the effect of
time on in situ reduction, and then the impact on the reaction. From
Figure S5C, the Cu (OH) CO in the samples was partially reduced to
2 2 3
+
9
32.4 eV corresponded to the Cu°/Cu species because the binding
+
energies of Cu° and Cu in Cu 2p3/2 were extremely close. The peak at
9
34.2 eV, together with its satellite peak around 944 eV, was attributed
metallic Cu at 1 h. As time increases, the samples were completely re-
duced to Cu after 3 h. Further integration Figure S5D, the phenomenon
that the peak positions of Cu 2p3/2 and Cu 2p1/2 shift toward low
2
+
to the Cu
species. [43,44] Comparing Fig. 3A, B, and C, the decon-
volution of the Cu 2p3/2 peak was determined to be about 1.62:1, 2.35:1
and 0.91:1 for Cu°/Cu : Cu , respectively, indicating that the in situ
reduced Cu/AlOOH and Cu/Al
hydrogen-reduced Cu/Al
and therefore have a better catalytic pe+r-
formance. As a result of reducibility of 2-PrOH, the quantity of Cu°/Cu
species increased in spent hydrogen-reduced Cu/Al after the reac-
+
2+
binding energy with the increase of time indicates that more and more
+
2+
is reduced to Cu+ or Cu° over time and the trend of smaller sa-
2
O
3
has more Cu°/Cu species than the
Cu
2
O
3
tellite peaks of Cu 2p3/2 also confirms this view. Therefore, it can be
demonstrated that the time influences the degree of in situ catalyst
synthesis, which in turn affects the CTH reaction of ML.
2 3
O
tion (Fig. 3D). From the XPS analysis, the conclusion was drawn that
high catalytic activity of the in situ reduced catalysts was caused by
high concentration of low chemical state in Cu species, the result was
achieved by the in situ reduction of 2-PrOH as a hydrogen source.
3
.4. Effect of the catalyst dosage
Effect of the catalyst dosage on the CTH reaction of ML was re-
searched (Fig. 4F). It was evident that both the conversion of ML and
56

M. Ma et al.
Molecular Catalysis 467 (2019) 52–60
Fig. 4. Effect of different reaction conditions on the CTH reaction of ML. Effect of different reaction times on the CTH reaction of ML at 140 °C (A), 160 °C (B), 180 °C
C), 200 °C (D) and 220 °C (E). Reaction condition: catalyst (0.1 g), substrate (0.67 mmol), 2-PrOH (20 ml). Effect of different amount of Cu (OH) CO /AlOOH on the
CTH reaction of ML (F). Reaction condition: substrate (0.67 mmol), 2-PrOH (20 mL), 180 °C, 5 h.
(
2
2
3
the yield of 1,4-PDO increased with increasing amounts of the catalyst.
At 0.15 g, the conversion of ML reached 100%, and at 0.20 g, the 1, 4-
PDO yield reached a high value (12.72%), whereas the yield of γ-GVL
began to decline after an optimal yield (90.51%) of 0.1 gwas attribu-
table to the further conversion of γ-GVL to 1, 4-PDO. Therefore, 0.1 g
was chosen as the optimum amount of catalyst.
donors in the CTH reaction owing to the difficult β-hydride removing
after they shaped into alkoxy species on the surface of the catalyst. [45]
Similarly, tertiary alcohols were also weak H-donors and failed to
produce the corresponding carbonyl compounds because there were no
hydrogens close to the hydroxyl group, which did not make the tertiary
alcohols. As a result, hydrogen is not available for the CTH reaction,
resulting in an inefficient CTH reaction with t-BuOH as the H-donors
(
Table 2, entry 6). It follows that the primary alcohols and tertiary al-
3
.5. Scope of substrate and alcohols for production of γ-GVL over
cohols are negative hydrogen donors for the CTH reaction. On the
contrary, the secondary alcohols shew better performances, the CTH
reaction with 2-BuOH gave a similar effect to that with 2-PrOH, ob-
tained 89.37% and 90.51% yields of γ-GVL, respectively (Table 2, en-
tries7 and 8). Besides, CyOH was also an better H-donor for the CTH
reaction, obtained 40.92% yield of γ-GVL. These results further confirm
Cu (OH) CO /AlOOH
2
2
3
With the optimal reaction conditions obtained from above, the
substrate and solvent scope of the CTH reaction are explored as re-
vealed in Table 2. The CTH reaction with primary alcohols as the H-
donors provided low yield of γ-GVL, which was similar to many pre-
vious works (Table 2, entry 1–5). [4,37] Primary alcohols were poor H-
the aforementioned CTH reaction mechanism with secondary alcohols
as hydrogen donors.9
We also studied the catalytic performance of the Cu
AlOOH for various substrates. Ethyl levulinate (EL) and butyl levulinate
BL) were favourably converted to γ-GVL with 90.12% and 91.14%
yield, respectively. However, the yield of γ-GVL achieved a very low
yield (17.80%) when LA is subjected to the CTH reaction. This may be
attributed to the fact that the acidity of LA causes LA to react with the
AlOOH carrier, reducing the activity of the catalyst.
2 2 3
(OH) CO /
Table 2
2 2 3
Scope of substrate and alcohols for production of γ-GVL over Cu (OH) CO /
AlOOH.
(
Entry
Substrate
Alcohol
Conv
%)
GVL
Yield (%)
(
1
2
3
4
5
6
7
8
9
ML
ML
ML
ML
ML
ML
ML
ML
ML
LA
MeOH
EtOH
15.77
14.81
18.99
32.27
54.12
39.42
96.07
97.81
49.82
> 99
98.71
98.03
0.39
0.12
1.57
2.20
0.86
1-PrOH
1-BuOH
3.6. Reusability and heterogeneity of the catalyst
[
a]
BnOH
t-BuOH[
b]
0.47
In addition to high catalytic activity, stability is also extremely
important for heterogeneous catalysts. Therefore, reusability and het-
erogeneity were also investigated. In each cycle, the in situ reduced Cu/
AlOOH catalyst was recovered by filtration, washed with fresh 2-PrOH
2-PrOH
2-BuOH
CyOH[
90.51
89.37
40.92
17.80
90.12
91.14
c]
10
11
12
2-PrOH
2-PrOH
2-PrOH
EL
BL
(
3 times), and then reused in next run. The results showed there was no
obvious decrease in conversion and yield after five cycles, indicating
that the catalyst is very stable (Fig. 5A). Leaching test of the catalyst
was also explored (Fig. 5B). The reaction was stopped after 2 h, the
catalyst was filtered out, and the reaction was then continued under the
Conditions: catalyst(0.1 g), substrate(0.67 mmol), Alcohol (20 mL), 180 ℃, 5 h.
a] BnOH stands for benzyl alcohol. [b] t-BuOH stands for T-butanol. [c] CyOH
stands for Cyclohexanol.
[
57

M. Ma et al.
Molecular Catalysis 467 (2019) 52–60
Fig. 5. Reusability (A) and leaching test (B) of the catalyst for the CTH reaction of ML. Reaction condition: catalyst (0.0631 g), substrate (0.67 mmol), 2-PrOH
20 mL), 180 °C, 5 h.
(
same conditions to examine whether the yield of γ-GVL continued to
increase without the Cu/AlOOH catalyst. Clearly, the yield of γ-GVL
after the removal of the catalyst no longer increased, indicating that the
catalyst is heterogeneous. Meanwhile, trace of copper ions (0.11 ppm)
in the solution after the reaction also indicated that the catalyst was
heterogeneous and stable.
the yield of γ-GVL? The answer is got from XPS analysis (Fig. 6D). After
+
five cycles, for Cu 2p3/2, that the ratio of the peak areas of Cu°/Cu
species to Cu2+ species significantly increased and that the satellite
peaks disappeared indicated that there is more Cu° active sites in the
spent catalyst. The emergence of more and more of Cu° active sites
guarantees the stability of catalytic performance.
The stability of the catalyst is further analyzed by SEM, TEM and
XPS. By comparing with SEM image of the spent catalyst after one cycle
3.7. CTH reaction of carbonyl compounds over the Cu
2 2 3
(OH) CO /AlOOH
(
Fig. 2C), the particle size of the catalyst became a bit larger after five
times (Fig. 6A), which can be more clearly observed in the TEM image.
The particle size of the spent catalyst was approximately 35 nm and
Inspired by the excellent catalytic performance of the prepared
catalysts for the conversion of ML, we sought the possible results of the
CTH reaction of other carbonyl compounds using the in situ reduced
5
0 nm after one cycle and five cycles, respectively (Fig. 6B and C). In
addition, the crystallinity of Cu became stronger after five cycles
Figure S7). Why these phenomena do not cause a significant drop in
Cu/AlOOH catalyst from Cu
2 2 3
(OH) CO /AlOOH (Table 3). The results
(
exhibited good catalytic activity on the examined compounds. In
Fig. 6. SEM image of the in situ reduced Cu/AlOOH after five cycles (A), TEM image of the in situ reduced Cu/AlOOH after one cycle (B), TEM image of the in situ
reduced Cu/AlOOH after five cycles (C), XPS analysis of the in situ reduced Cu/AlOOH after five cycles (D).
58

M. Ma et al.
Molecular Catalysis 467 (2019) 52–60
different carbonyl compounds. Overall, due to its advantages of low
cost, high efficiency, good stability and simple preparation, etc., the
Table 3
CTH reaction of carbonyl compounds over the Cu
2
(OH)
2
CO
3
/AlOOH.
2 2 3
Cu (OH) CO /AlOOH catalyst holds a promising potential in catalyzing
the CTH reaction of biomass.
Entry Reactant
1[a]
Main product Temperature Time Conv GVL Yield(%)
(
℃)
(h)
3
(%)
180
180
180
> 99 95.07
> 99 98.66
> 99 98.86
Acknowledgements
2[b]
10
7
This work was supported by grants from the National Natural
Science Foundation of China (No. 21771086), Science and Technology
Research Program of the Education Department of Jilin Province
3[c]
(
JJKH20170778KJ), and Natural Science Foundation of Jilin Province,
China (20180101293JC).
4[d]
160
160
9
> 99 89.09
> 99 90.21
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at https://doi.org/10.1016/j.mcat.2019.01.033.
5[e]
12
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