Effective hydrogenation of carbonates to produce methanol over a ternary Cu/Zn/Al catalyst

Article abstract of DOI:10.1039/d0ra00347f

Methanol synthesized from carbonate hydrogenation is of great importance for CO₂ utilization indirectly. Herein, a series of Cu/Zn/Al heterogeneous catalysts were prepared by co-precipitation with a synchronous aging step, and were applied for hydrogenation of diethyl carbonate (DEC) to produce methanol. Furthermore, the catalysts were characterized by physicochemical methods, such as N₂ adsorption, ICP-OES, N₂O titration, SEM, TEM, XRD, H₂-TPR and XPS in detail. Higher copper concentration led to a higher ratio of bulk CuOx species in the calcined samples, which resulted in different copper species distribution after the reduction process. Structure activity relationship analysis indicated that the balance of surface Cu⁰ and Cu⁺ species influenced the formation rate of methanol. A higher proportion of Cu⁺ to (Cu⁺ + Cu⁰) was conductive to methanol formation, while excessive Cu⁰ site density played a negative influence on the methanol synthesized from DEC. Cu/Zn/Al with a 45.2% weight fraction of copper showed better performance with a total methanol formation rate of 131.0 mg g_cat.^-1 h^-1. The reaction temperature and reaction time could obviously affect the reaction performance and the results suggested that 200 °C and 6 h were suitable. Furthermore, the long-term stability and activity of the catalyst was also studied on a fixed bed, and the yield of total methanol reached to 88.5% and the selectivity of total methanol gradually decreased to 74.0% within 200 h, which could be attributed to the detrimental influence derived from the increase of Cu⁰. The reaction pathways involved in the hydrogenation of DEC process were proposed. The substance scope was also extended to other carbonates and the catalyst exhibited superior catalytic performance toward linear carbonates. This work provided insights into carbonate hydrogenation over an effective Cu/Zn/Al catalyst, which could be utilized into upgrading CO₂ indirectly to produce commodity methanol under relatively mild reaction conditions.

Full text of DOI:10.1039/d0ra00347f

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Effective hydrogenation of carbonates to produce
methanol over a ternary Cu/Zn/Al catalyst†
Cite this: RSC Adv., 2020, 10, 13083
abcdf
Jiachen Li,abcd Liguo Wang,
Xiang Hui,abcdg Chanjuan Zhang,abcd Yan Cao,ab
*
Shuang Xu,ab Peng Heab and Huiquan Li
abcde
*
Methanol synthesized from carbonate hydrogenation is of great importance for CO2 utilization indirectly.
Herein,
a series of Cu/Zn/Al heterogeneous catalysts were prepared by co-precipitation with
a synchronous aging step, and were applied for hydrogenation of diethyl carbonate (DEC) to produce
methanol. Furthermore, the catalysts were characterized by physicochemical methods, such as N2
adsorption, ICP-OES, N2O titration, SEM, TEM, XRD, H2-TPR and XPS in detail. Higher copper
concentration led to a higher ratio of bulk CuOx species in the calcined samples, which resulted in
different copper species distribution after the reduction process. Structure activity relationship analysis
indicated that the balance of surface Cu0 and Cu+ species influenced the formation rate of methanol. A
higher proportion of Cu+ to (Cu+ + Cu0) was conductive to methanol formation, while excessive Cu0
site density played a negative influence on the methanol synthesized from DEC. Cu/Zn/Al with a 45.2%
weight fraction of copper showed better performance with a total methanol formation rate of 131.0 mg
h
ꢀ1. The reaction temperature and reaction time could obviously affect the reaction performance
ꢀ1
gcat.
and the results suggested that 200 ꢁC and 6 h were suitable. Furthermore, the long-term stability and
activity of the catalyst was also studied on a fixed bed, and the yield of total methanol reached to 88.5%
and the selectivity of total methanol gradually decreased to 74.0% within 200 h, which could be
attributed to the detrimental influence derived from the increase of Cu0. The reaction pathways involved
in the hydrogenation of DEC process were proposed. The substance scope was also extended to other
carbonates and the catalyst exhibited superior catalytic performance toward linear carbonates. This work
provided insights into carbonate hydrogenation over an effective Cu/Zn/Al catalyst, which could be
utilized into upgrading CO2 indirectly to produce commodity methanol under relatively mild reaction
conditions.
Received 13th January 2020
Accepted 20th March 2020
DOI: 10.1039/d0ra00347f
rsc.li/rsc-advances
could reduce carbon emission and global warming, as well as
providing a long-term solution for the depletion of global
reserves of fossil fuels.1–4 So far, a series of high value-added
1. Introduction
Chemical utilization of greenhouse gas CO2 as a feedstock to
make value-added chemicals, materials, and transportation fuel
chemicals from CO2 chemical conversion has been achieved,
including cyclic carbonates, methanol, formic acid, alkyl
carbamates, salicylic acid, dimethyl carbonate (DMC) and so
on.5,6 Among them, methanol (CH3OH) is a widely applied bulk
chemical and key feedstock for industrial chemicals, which can
be further transformed into alternative high molecular weight
liquid fuels.7 In this regard, hydrogenation of CO2 into meth-
anol is a promising, efficient, and economical technique for
CO2 utilization.8–10 Nevertheless, the reduction/activation of
CO2 into useful liquid products is challenging due to the limi-
tation of thermodynamics and kinetics.11,12 Carbonates, which
can be produced from CO2 and alcohols, are reported could be
further catalytic hydrogenated to produce methanol under
certain circumstances.13–16 Thus, carbonates can be deemed as
a bridge between CO2 and methanol to realize indirect hydro-
genation of CO2.17–19 As shown in Scheme 1, this route is atom
economy and consistent with the green chemistry concept.
aCAS Key Laboratory of Green Process and Engineering, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing, 100190, China. E-mail: lgwang@
ipe.ac.cn
bNational Engineering Laboratory for Hydrometallurgical Cleaner Production
Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing,
100190, China
cSino-Danish College, University of Chinese Academy of Sciences, Beijing, 100049,
China
dSino-Danish Center for Education and Research, University of Chinese Academy of
Sciences, Beijing, 10049, China
eSchool of Chemical Engineering, University of Chinese Academy of Sciences, Beijing,
10049, China
fDalian National Laboratory for Clean Energy, Dalian, 116023, China
gDepartment of Chemical and Biochemical Engineering, Technical University of
Denmark, Lyngby 2800 Kgs, Denmark
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra00347f
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including N2 physisorption, ICP-OES, N2O titration, XRD, H2-
TPR, TEM, EDX-mapping, XPS and XAES. Furthermore, the
effects of copper concentration, reaction temperature, and
reaction time were also investigated. The structure–activity
relationship was studied, and the reaction mechanism was
proposed. Stability and catalytic performance of catalyst for
a long period were also investigated on the xed bed. At last, Cu/
Zn/Al catalyst was also tested on other carbonates.
Scheme
hydrogenation.
1 Synthesis of methanol via CO2-derived carbonates
2. Experimental
2.1 Catalyst preparation
Until now, several groups did researches on this route, and
focused on the different catalyst systems. For example, Ding
et al.20 proposed the route of ethylene carbonate (EC) hydroge-
nation to co-produce methanol and ethylene glycol, and
a homogeneous Ru-based catalyst was applied in this system
with nearly full conversion of EC, and selectivity of EG as well as
MeOH exceeded 99% at 40 ꢁC. Beller et al.21 used homogeneous
catalyst Co(BF4)2 combined with tridentate phosphine ligands
to catalyze carbonates hydrogenation under relatively mild
conditions. However, homogeneous catalysts must face the
difficulties of separation, reuse, and stability industrially. For
heterogeneous catalysts, copper-based catalysts are widely used
in selective hydrogenation reactions since they are excellent
catalytic active sites for C–O bonds hydrogenation (forming
alcohols) and relatively inactive for C–C bond hydrogenolysis
(forming low molar weight chemicals like CO2, CO and
methane).22
Until now, Cu/HMS, CuCr2O4, Cu/CeO2 had been reported to
catalyze carbonates hydrogenation effectively.23–26 Cu/SiO2 was
reported to catalyse dimethyl carbonate (DMC) hydrogenation
to methanol, and high copper dispersion and the synergetic
effect between balanced Cu0 and Cu+ sites contribute to the
yield of methanol.26 Dai et al.25 applied Cu/CeO2 nanomaterial
in the hydrogenation of diethyl carbonate (DEC) to synthesize
methanol, and conrmed the activity of Cu/CeO2 was inu-
enced by the shape/crystal planes of CeO2. Furtherly, they
prepared different Cu/CeO2 nanorod catalysts with various
copper concentration aꢀn1d achieved relatively high space-time
yield of 8.4 mmol gcat. hꢀ1, which was due to a high Cu0
surface area, the moderate ratio of Cu+/Cu0, as well as sufficient
surface oxygen vacancies associated with Ce3+.27
In recent years, there has been continued interest in the
ternary Cu/Zn/Al catalyst. Industrially, Cu/Zn/Al catalysts have
been utilized for methanol synthesis from syngas at typical
reaction conditions of 230–280 ꢁC and 5–10 MPa.28–30 What's
more, Cu/Zn/Al catalyst is a kind of environmentally friendly
catalyst with high efficiency in direct CO2 hydrogenation
because of excellent activity for hydrogenation of C]O
bond.31,32 Hence, the development of effective Cu/Zn/Al catalyst
for hydrogenation of carbonate is highly desired both in
fundamental research and industrial application.
In this work, ternary Cu/Zn/Al heterogeneous catalysts were
prepared by co-precipitation method and were applied in the
hydrogenation of DEC into methanol. Moreover, the physico-
chemical properties as well as the structures of Cu/Zn/Al cata-
lysts were systematically characterized by various techniques,
Typically, ternary Cu/Zn/Al was prepared by co-precipitation
method using Na2CO3 as a precipitant. In a typical procedure,
1 mol Lꢀ1 Na2CO3 aqueous solution was added to 1 mol Lꢀ1
mixed metallic aqueous solution of Cu(NO3)2$3H2O, Zn(NO3)2-
$6H2O, and Al(NO3)2$9H2O with desired molar ratio of Cu–Zn–
Al. Precipitation was carried out with synchronous aging step at
70 ꢁC until pH of the solution achieved to 9, aer which the
mixture was cooled down to room temperature with stirring.
Then, the resulting suspension was recovered by ltration and
washed to neutral, then dried at 110 ꢁC overnight and calcined
at 500 ꢁC for 5 h in the air. The catalyst was nally obtained by
reduction at 500 ꢁC for 3 h in the atmosphere of 10 vol% H2/N2.
The Zn/Al ratio was xed at 1/1 in molar ratio. In this work,
a series of Cu/Zn/Al catalysts were prepared with different
copper molar ratios of 0.5 : 1 : 1, 1 : 1 : 1, 2 : 1 : 1, 3 : 1 : 1,
4 : 1 : 1, which are denoted as Cu-0.5, Cu-1, Cu-2, Cu-3, Cu-4,
respectively. We marked the samples in different preparation
stages as Cu/Zn/Al-D (dry precursors), Cu/Zn/Al–C (samples
aer calcination) and Cu/Zn/Al catalyst (samples aer
reduction).
2.2 Catalyst characterization
The element contents of catalysts were identied by inductively
coupled plasma optical emission spectrometry (ICP-OES) on
Agilent ICP-OES730. N2 physical adsorption measurements
were carried using a Quantachrome Autosorb-1 at ꢀ196 ꢁC. The
BET surface areas were performed via the Brunauer–Emmett–
Teller (BET) model. The total absorbed pore volumes were
received from the nitrogen absorbed volume with a relative
pressure of 0.99. Initial estimation of pore size distribution was
obtained by Barrett, Joyner and Halenda (BJH) methods based
on the isotherm desorption branch. Scanning electron micros-
copy (SEM) images were measured on a HITACHI SU8020
microscopy. Transmission electron microscopy (TEM), high-
resolution transmission electron microscopy (HRTEM) and
energy-dispersive X-ray (EDX) images were obtained on eld-
emission transmission electron microscopy (JEOL, JEM-2100F)
using an acceleration voltage of 200 kV. Pre-treatment method
was to disperse samples in ethanol ultrasonically for 30 min at
ambient temperature. X-ray diffraction (XRD) patterns of
samples were recorded in the 2q range of 10–90ꢁ on a PAN-
alytical Empyrean diffractometer with Cu Ka radiation (l ¼
0.15406 nm). For the X-ray photoelectron spectroscopy (XPS)
and X-ray Auger electron spectroscopy (XAES), the powder
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catalysts are taken from the sealing bottle. Samples to be tested capillary column and an AOC-20i automatic sampler using p-
are then pressed into pieces and glued to the sample stage, aer xylene (20 mL) as the internal standard. Identication of the
24 hours in a vacuum, they are performed on a Thermo scalable liquid products was studied using GCMS (Shimadzu-QP2020).
250XI system with an Al Ka radiation source (hn ¼ 1486.6 eV) to The gas samples were analyzed via GC-2014 (Shimadzu Ltd.)
investigate the surface chemical states of samples. equipped with an FID detector and two TCD detectors with
Temperature-programmed reduction (TPR) was performed to seven packed columns. The conversion of DEC, selectivity/yield
give further insight into the reducibility of the catalysts. The H2- of detected MeOH were determined by the data from GC-2014
TPR proles were conducted with a Quantachrome Chembet and calculated through the following eqn (3)–(5). In this reac-
pulsar TPR/TPD instrument. The TPR proles were obtained tion, a liquid by-product of ethyl methyl carbonate (EMC) was
with 5 vol% H2/Ar ow (40 mL minꢀ1). The temperature was produced through methanol reacting with DEC, and the
increased from 30 to 750 ꢁC at a rate of 10 ꢁC minꢀ1. The copper amount of total methanol produced is the sum of EMC and
dispersity was determined by applying the nitrous oxide methanol detected which were calculated through eqn (6)–(9).
chemisorption method and the reactions are described as eqn
n0;DEC ꢀ nDEC
a ꢂ np-xylene
Conv:DEC
¼
ꢂ 100% ¼ 100% ꢀ
(3)
(1). The catalyst was rstly reduced in a 10 vol% H2 in argon for
1 h at 450 ꢁC. The samples were cooled at room temperature for
1 hour and then a stream of 10 vol% N2O/Ar gas was fed into the
reactor 40 ꢁC and the effluent gas was analyzed by a thermal
conductivity detector (TCD). Nitrogen evolved by the reaction
was derived via peak area from the TCD signal. A Cu : N2O ¼
2 : 1 chemisorption stoichiometry was assumed. SCu is the
surface area of exposed surface copper per gram catalyst, and it
was calculated by eqn (2).
n0;DEC
n0;DEC
nMeOH
b ꢂ np-xylene
Sel:MeOH
¼
ꢂ 100% ¼
ꢂ 100%
n0;DEC ꢀ nDEC
n0;DEC ꢀ a ꢂ np-xylene
(4)
nMeOH
b ꢂ np-xylene
ꢂ 100% ¼ ꢂ 100%
n0;DEC n0;DEC
YieldMeOH
¼
(5)
nEMC
c ꢂ np-xylene
Sel:EMC
¼
ꢂ 100% ¼
ꢂ 100%
n0;DEC ꢀ nDEC
n0;DEC ꢀ a ꢂ np-xylene
2Cu(s) + N2O(g) / Cu2O(s) + N2(g)
(1)
(2)
(6)
À
Á
nCuNA
SCu m2gꢀ1
¼
NCuWCat:
nEMC
c ꢂ np-xylene
YieldEMC
¼
ꢂ 100% ¼
ꢂ 100%
(7)
where nCu is the number of exposed copper atoms, NA is the
Avogadro constant equalling to 6.02 ꢂ 1023 molꢀ1, NCu is the
number of copper atoms per square meter (1.4 ꢂ 1019), and
WCat. is the weight of catalyst.
n0;DEC
n0;DEC
Sel.total ¼ Sel.MeOH + Sel.EMC
(8)
(9)
Yieldtotal ¼ YieldMeOH + YieldEMC
2.3 Catalyst performance
where, n0,DEC is the initial mole of DEC, nDEC is the residual
mole of DEC, nMeOH is the mole of MeOH detected. nEMC is the
mole of EMC. a, b, c are the reaction results calculated and
analysed by GC-2014. np-xylene is the mole number of p-xylene
added to the reaction liquid. Furthermore, the activity of cata-
lysts was dened for evaluating the catalytic performance based
on the product of methanol and EMC. The physical signicance
of “Activity” is the formation of methanol perꢀh1our by gram
catalyst, as a result of this, the unit is mg gcat. hꢀ1 and the
calculation formula is shown in eqn (10) and (11).
The DEC hydrogenation was studied in a 50 mL autoclave. In
addition, a xed-bed reactor was also utilized to investigate the
stability and activity of catalysts over a long time. In a typical
experiment, 10 mmol DEC and 20 wt% catalyst based on DEC,
10 mL tetrahydrofuran and 20 mL p-xylene was added into the
reactor. The reactor was then sealed and purged with N2 ve
times. Aer that, 5 MPa H2 were charged into the reactor at
room temperature, and then the reactor was heated to desired
temperature and hold for desired time with vigorous mechan-
ical stirring of 600 rpm. Aer the reaction, the reactor was
placed in an icy water bath and the residual gas was released
gradually. For the long-period test, 0.5 g catalyst (40–60 mesh)
was packed into a stainless-steel tubular reactor (11 mm in
inner diameter, 500 mm in length) with a thermocouple inser-
ted into the catalyst bed. And quartz wool was placed on both
sides of the catalyst bed. A solution of 10 wt% DEC in tetrahy-
drofuran (THF) and 2.5 Mpa H2 were fed into the xed bed at
a H2/DEC molar ratio of 200. The reaction was carried out at
200 ꢁC for 200 h, with the DEC liquid hourly space velocity
(LHSV) of 0.2 hꢀ1. Aer reaction, the used catalysts were dried
aer rinsing with ethanol and were then kept under seal for
further characterizations.
mmethanol
Methanol formation rate ¼
(10)
mcat: ꢂ t
MMeOH ꢂ nTotal
Total MeOH formation rate ¼
mcat: ꢂ t
MMeOH ꢂ ðnMeOH þ nEMC
mcat: ꢂ t
Þ
#
¼
(11)
where, mcat. is the mass weight of the catalyst with the unit of g,
MMeOH is the mole weight of methanol and t is the reaction
time, h.
The liquid mixture was analyzed by GC-2014 (Shimadzu Ltd.)
equipped with a ame ionization detector (FID), a DB-FFAP
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3. Results and discussion
3.1 General characterization
Fig. S1† illustrates N2-physisorption isotherms and the corre-
sponding pore size distributions from the adsorption branches
of the catalysts. All curves of Cu-0.5, Cu-1, Cu-2, Cu-3, Cu-4 were
of type IV and had mesoporous structure (2 nm < pore diameter
< 50 nm).33 For these samples, the H3 type hysteresis loop
(according to IUPAC classication) was observed, which was
related to non-rigid aggregates of plate-like particles.27,34,35 In
addition, with the decreasing of Cu content, the condensation
occurred at relatively low pressure. Fig. S1(b)† shows BJH pore
size distribution curves from the adsorption branch of the
isotherm. Two peaks were observed, of which the rst peak
showed a broad pore size band in the pore size diameter of 2–
10 nm. The second one revealed a relatively low-intensity band
in the range of 10–100 nm. Table 1 lists the catalyst composi-
tion, textural property and specic copper surface area for each
catalyst in this work. BET surface area of Cu/Zn/Al catalysts
decreased from 74.4 to 34.6 m2 gꢀ1 with copper content rising.
Pore volumes of the catalysts were also followed the same trend,
which decreased from 0.32 to 0.15 cm3 gꢀ1. What's more,
copper dispersion degree of Cu/Zn/Al catalysts were also
determined by N2O titration in Table 1. The dispersion of Cu
decreased gradually from 9.6% to 3.8% with the increment of
copper concentration from 19.5% (Cu-0.5) to 72.8% (Cu-4).
To study the catalyst structure at different preparation stages, X-
ray diffraction patterns was analyzed systematically. The XRD
patterns of the ve dried precursors Cu-0.5, Cu-1, Cu-2, Cu-3 and
Fig. 1 XRD patterns of (a) Cu/Zn/Al-D; (b) Cu/Zn/Al catalysts (after
reduction).
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Cu-4 are illustrated in Fig. 1(a). For the curve of Cu-0.5, the sharp
peaks of hydrotalcite phase (Zn(OH)2) at diffraction angles 2q ¼
11.6ꢁ, 23.3ꢁ, and 34.5ꢁ with good crystallinity were detected, which
corresponded to the basal planes (003), (006), and (009), respec-
tively.36 When the copper concentration increased to 45.2% (Cu-2),
aurichalcite phase (Zn,Cu)5(OH)6(CO3)2, were detected at 2q ¼
13.1ꢁ, 34.2ꢁ, and 36.7ꢁ, which were considered more probability to
form interface between copper species and Zn/Al oxide
supports.37,38 It is worth mentioning that the peak intensity of
aurichalcite phase in Cu-2, Cu-3 and Cu-4 increased with copper
content increasing, indicating a more intimate contact between
copper species and Zn/Al oxide supports with higher copper
content. The peak at 2q ¼ 25.9ꢁ is corresponding to the gerhardtite
phase of copper hydroxide nitrate (Cu2(OH)3NO3)39, which further
decomposed to CuO in the calcination step.40 XRD patterns of
calcined samples are shown in Fig. S2(a).† Aer calcination, the
peaks related to hydroxycarbonates in the dry precursors dis-
appeared, but new characteristic peaks of metal oxides were
detected. The peaks at 2q ¼ 31.8ꢁ, 34.4ꢁ, 36.3ꢁ,56.6ꢁ and 62.9ꢁ are
attributed to ZnO with crystalline plane of (1 0 0), (0 0 2), (1 0 1), (1 1
0) and (1 0 3), respectively. The peaks at 35.6ꢁ, 38.8ꢁ, 48.7ꢁ, 58.3ꢁ,
61.2ꢁ, 65.9ꢁ and 68.2ꢁ are ascribed to CuO. Crystallization of CuO
and ZnO are present in all ve samples, while no crystal phases
containing aluminum are detected. Aer calcination, aluminum
tends to remain in a mixed oxide phase, which is amorphous state
at the relatively lower calcination temperature of 500 ꢁC.41 Fig. 1(b)
Fig. 3 HRTEM images of samples (a) Cu/Zn/Al–C of Cu-2; (b) Cu-2
catalyst.
shows the XRD patterns of catalysts aer reduction with various
copper contents. In comparison with XRD patterns before reduc-
tion, the samples showed obvious differences. Diffraction peaks of
Cu were shown at 43.3ꢁ, 50.3ꢁ and 73.9ꢁ, belonging to the (111),
(200), and (220) planes, respectively.42,43 When the mass fraction of
copper increased, intensities of (111), (200), and (220) diffraction
peaks of Cu were enhanced, indicating that the crystallinity of Cu
increased. The possible reason is that the reduction of CuO to Cu is
an exothermic reaction with DH ¼ ꢀ80.8 kJ molꢀ1, so copper
sintering occured in the reduction process to a certain degree.44
The morphological evolution for different preparation stages of
Cu-2 are illustrated in Fig. 2. The sample of dry precursor showed
a signicant lamellar-like structure (Fig. 2 (a) and (b)). Aer
calcination, morphology of the sample altered from lamellar to
spherical. Compared with calcination samples, the morphology of
Cu-2 didn't change much aer reduction. The surface geometry of
the Cu-2 catalyst was further studied by HRTEM (Fig. 3). Before
reduction, copper species with a crystal plane of (ꢀ1 1 1) could be
observed (Fig. 3(a)), which is in accordance with the results in XRD
patterns. In addition, ZnO (1 0 1), Cu0 (1 1 1), (2 2 0), (2 0 0) can be
observed in reduced Cu-2 catalyst aer reduction. These results
veried that the formation of interface between Cu and Zn/Al
oxide, which enable an intimate contact between Cu species and
oxide matrix.45 EDX elemental mapping in Fig. S3† illustrated that
Cu, Zn and Al atoms are all homogeneously dispersed on Cu-2
Fig. 2 SEM & HRTEM images of Cu-2 catalyst in different preparation
stages, (a) and (b) Cu/Zn/Al-D, (c) and (d) Cu/Zn/Al–C, (e) and (f) Cu-2
catalyst (after reduction).
Fig. 4 TPR profiles of Cu/Zn/Al–C with different copper content.
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catalyst. TEM imagines of Cu/Zn/Al catalyst with different copper consistent with XRD curves in Fig. 1(a) that the dry precursors
content in Fig. S4† shows the average particle diameters of Cu/Zn/ for these three samples showed the enhanced intensities of
Al catalysts increased from 7.2 to 8.6 nm with copper content aurichalcite phase. It is reasonable that more intimate metal
increasing.
contact led to lower onset temperature. The TPR results sug-
H2-TPR was used to study the reduction behavior of Cu/Zn/ gested that higher copper concentration resulted in higher ratio
Al–C. Fig. 4 shows the TPR proles of calcined catalysts for of bulk CuO in the calcined samples, and the percentage of
different Cu contents, exhibiting a broad reduction peak in the dispersed CuO and bulk CuO might inuence the reduction
range of 450–600 K. Since ZnO and Al2O3 could not be reduced process, as well as the ratio of Cu+/(Cu+ + Cu0).
under certain experimental conditions,46 these proles are
XPS-XAES was measured to conrm the chemical states of
described to CuO reduction in the CuO/ZnO/Al2O3 phase. samples. The results were shown in Fig. 5 and 6, and the rele-
Generally, the pure CuO was reduced at around 750 K, which vant quantitative data were summarized in Table 2. For all
was far over these main peaks.47 Therefore, it was inferred that calcined samples, the XPS peaks at around 933.4 eV binding
the involvement of Zn and Al facilitated the dispersion of copper energy corresponding to Cu 2p3/2 and the shake-up of satellite
species and hence reduced the reduction temperature. The peaks indicated that copper state was +2, which was consistent
reduction of Cu2+ to Cu+ and Cu+ to Cu0 are considered as two with XRD results. Aer the reduction process, the photoelectron
steps in this reduction process.48 To gain more insight into the peak of Cu 2p3/2 at about 932.6 eV towards lower binding energy
results of TPR, all proles were deconvoluted into two Gaussian and the absence of the satellite peaks suggested that Cu2+
peaks. According to literatures,33,49 the peaks a and b could be species had been reduced to metallic Cu0 and/or Cu+. To better
ascribed to the reduction of different CuO phases: the low- distinguish Cu0 and Cu+, the X-ray induced Auger spectrum of
temperature peak (peak a) was the surface CuO species, which Cu LMM was also investigated. The peak positions, as well as
is highly dispersed and inuenced by ZnO, the high- the values of Cu+% derived from the deconvolution were
temperature peak (peak b) belonged to isolated copper oxide summarized in Table 2. The asymmetric and broad peaks of Cu/
(bulk CuO). Table 3 illustrates that the peak positions and the Zn/Al catalysts were deconvoluted into two overlapping Cu LMM
area proportion of these two peaks. Accordingly, it is seen from Auger kinetic energy peaks at around 917.0 eV and 918.8 eV,
Table 3 that the ratio of dispersed CuO decreased with copper representing Cu+ and Cu0, respectively.
concentration increasing, which was in good accordance with
trends of Cu dispersion degree determined by N2O titration in not be excluded completely, it usually required high temperature
Table 1 and resulting in the difficulty in CuO reduction.
in oxygen atmosphere for effective oxidation of Cu0 to Cu+.50–52 In
Although the ex situ re-oxidation of Cu and Cu2O in air could
In addition, the onset reduction temperature for Cu-2, Cu-3 this work, as also veried by XPS results, there are not detectable
and Cu-4 samples decreased gradually, the result of which is in CuO species in Cu/Zn/Al catalysts because of the absence of the
characteristic satellite peaks. Therefore, the state of copper kept
stable essentially during the characterization. As shown in Table 2,
the ratio of Cu+/(Cu+ + Cu0) rst rose and then decreased with the
Fig. 6 Cu LMM XAES spectra of Cu/Zn/Al catalysts with different
Fig. 5 Cu 2p XP spectra of Cu/Zn/Al–C (a), and Cu/Zn/Al catalysts (b). copper contents.
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Table 2 H2-TPR data of all prepared Cu/Zn/Al–C
TPR peak position (temperature/K) and area
proportion (%)
Entry
Catalyst
Peak a
Peak b
1
2
3
4
5
Cu-0.5
Cu-1
Cu-2
Cu-3
Cu-4
533.8 K
539.4 K
548.9 K
539.4 K
534.2 K
69.9%
562.3 K
565.2 K
570.5 K
576.2 K
572.7 K
30.1%
33.1%
53.0%
58.7%
60.0%
66.9%
47.0%
41.3%
40.0%
increase of copper concentration. The binding energy values of Zn
(2p1/2) and Zn (2p3/2) were around 1046 eV and 1023 eV, respec-
tively, which were closed to the energy peaks (1044.8 eV and 1021.8
eV) of ZnO.53 Moreover, there was an energy difference of 23 eV
between Zn (2p1/2) and Zn (2p3/2) peaks, which is equivalent to that
of ZnO.54 In addition, the spectrum recorded Al 2p peaks with the
binding energy from 74.1–74.4 eV,55,56 which are characteristic for
Al3+ ions in Al2O3. The XPS results showed a good agreement with
the corresponding XRD analysis.
3.2 Catalytic performance of Cu/Zn/Al catalysts
The catalytic performance of Cu/Zn/Al catalysts with different
Cu concentration for the hydrogenation of DEC was performed
under 200 ꢁC for 6 h, and the results were summarized in Table
1. Increase in the copper concentration led to an increase rstly
and then decrease in DEC conversion. The selectivity of meth-
anol and methanol formation rate reached to the maximum on Fig. 7 The effect of reaction temperature on the hydrogenation of
DEC for 6 h, (a) methanol, EMC, and DEC, (b) total methanol, based on
the dosage of 10 mmol DEC, 5 MPa H2, 10 mL THF and 0.24 g Cu-2
catalyst (20 wt% DEC).
the Cu-2 catalyst (45.2%). For all the Cu/Zn/Al catalysts, meth-
anol and ethanol were the main liquid products with the small
amount of ethyl methyl carbonate (EMC). EMC is a by-product
in heterogeneously catalyzed DEC hydrogenation probably
resulting from transesterication of produced methanol and
unreacted DEC. This transesterication was veried over Cu/Zn/
Al catalyst (Cu-2) at 200 ꢁC as well as on Zn/Al catalyst in Scheme
S1,† which indicated that Zn/Al could facilitate the trans-
esterication of methanol and DEC. For the gas products, CO2
and CO could be detected (Fig. S6†). The majority of gas prod-
ucts was CO2 which was considered to be produced through
maximum of 95.6 mg gcat.ꢀ1 hꢀ1 at the highest Cu+/(Cu+ + Cu0)
ratio of 0.69 among the catalysts tested. The percentage of Cu+
increased in the order of Cu-4 < Cu-0.5 < Cu-1 < Cu-3 < Cu-2 as
shown in Table 2, the order of which is the same as the meth-
anol formation rate and methanol selectivity. Hence, it could
conceivably be hypothesised that the catalytic activity is highly
related to the ratio of Cu+/(Cu+ + Cu0).
hydrolysis reaction between DEC and a small amount of water
The inuence of reaction temperature on the catalytic
activity was carried out in the range of 180–220 ꢁC at 6 h, shown
contained in the solvent,57 which could further react with H2 to
form CO.26 Notably, the methanol formation rate reached to the
Table 3 XPS-XAES data of Cu/Zn/Al catalysts
Binding energy
Binding energy (eV)
(eV)
Cu+/(Cu+ +
Entry
Catalysts
Cu wt%
Cu 2p3/2
Zn 2p1/2
Zn 2p3/2
Al 2p
Cu+
Cu0
Cu0) (molar ratio)
1
2
3
4
5
Cu-0.5
Cu-1
Cu-2
Cu-3
Cu-4
19.5
35.9
45.2
53.5
72.8
933.4
933.3
932.9
932.7
932.6
1046.1
1046.4
1046.0
1046.1
1045.5
1023.2
1023.2
1022.9
1023.1
1022.6
74.1
74.2
74.4
74.4
73.4
917.3
916.9
916.8
916.8
916.9
919.3
918.8
918.7
918.7
918.7
0.45
0.56
0.69
0.57
0.45
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then decreased, accompanied by elevating reaction tempera-
ture. Noteworthy to say, since part of the methanol was further
transesteried with DEC to produce by-product EMC, the total
methanol yield was dened as the summary of detected meth-
anol and EMC produced from methanol. And the trends of total
methanol yield/selectivity/formation rate in Fig. 7(b) showed the
same trends with the selectivity/yield of methanol and EMC in
Fig. 7(a). All of them showed the highest data at the temperature
ꢁ
ꢁ
of 200 C, because the reaction temperature exceeding 200 C
might promote DEC hydrolysis, which caused a decrease of
yield and selectivity of methanol, as well as total methanol. The
effect of reaction time on the catalytic performance was also
investigated and the results was shown in Fig. 8, the conversion
of DEC increased gradually from 68.4% to 89.0% with reaction
time increasing from 3 to 7 h. The selectivity and yield of
methanol increased to 52.5% and 45.2% at 6 h but declined
slightly when the reaction time further prolonged to 7 h. The
total methanol selectivity and yield also increased gradually to
maximum values at 6 h, 71.9% and 61.9%, respectively. The
ꢁ
results in Fig. 7 and 8 implied that 200 C and 6 h is an opti-
mized temperature and a favourable reaction time, respectively.
3.3 Stability of Cu/Zn/Al catalyst
It is crucial for industrial application to study the long-term
stability and activity of the catalyst. Fig. 9 and S5† plots the
catalytic activity as a function of reaction time for the most
efficient Cu-2 catalyst for DEC hydrogenation. It is obviously
seen that DEC conversion stayed at around 95.0% over a 200 h
reaction time, while the total methanol yield/selectivity was over
Fig. 8 The effect of reaction time at 200 ꢁC on the hydrogenation of
DEC, (a) methanol, EMC and DEC, (b) total methanol, based on the
dosage of 10 mmol DEC, 5 MPa H2, 10 mL THF and 0.24 g Cu-2
catalyst (20 wt% DEC).
in Fig. 7. It is found that the reaction temperature had
a signicant effect on the performance of the catalyst, and the
higher of the reaction temperature, the more favourable of the
DEC conversion. The DEC conversion could reach to the highest
ꢁ
of 97.8% when the temperature increased to 220 C. However,
selectivity and yield of methanol and EMC rstly increased and
Fig. 9 Hydrogenation performance of Cu-2 catalyst as a function of
time on stream. Temperature: 200 ꢁC, liquid hour space velocity Fig. 10 (a) XRD and (b) XPS-XAES of used Cu-2 catalyst after running
(LHSV) ꢃ 0.2 hꢀ1, H2/DEC ¼ 200, 2.5 MPa.
200 h.
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Table 4 The scope of carbonates for methanol synthesisa
Conversion
(%)
MeOH selectivity
(%)
MeOH yield
(%)
Entry
1
Substrate
Time (h)
8
100
76.8
76.8
2
3
8
8
98.9
99.4
49.1
41.2
48.6
38.9
4
8
6
82.7
98.7
15.2
7.0
12.6
6.9
5
a
Reaction conditions: the hydrogenation of carbonates are achieved at 200 ꢁC, the dosage of carbonates is 10 mmol, in the solvent of 10 mL THF
and 20 wt% Cu-2 catalyst (based on the carbonates).
80.0% before the rst 40 h, which showed good performance.27 more, the synergistic cooperation of Cu0 and Cu+ sites, formed
However, both methanol and total methanol selectivity/yield through CuO reduction, played a key role for the high perfor-
declined slightly within 200 h. The decline of selectivity/yield mance of Cu/Zn/Al catalysts in the hydrogenation process.
might be explained by the fact that part of Cu+ was reduced
Fig. 11 shows the ratio of Cu+/(Cu+ + Cu0) and specic surface
by the H2 during the hydrogenation process, leading to the area SCu as a function of copper concentration. It could be
decrease of Cu+/(Cu+ + Cu0) ratio. This result was further veried found that the Cu+/(Cu+ + Cu0) showed a volcanic trend as
by comparing Cu LMM XAES spectra of Cu-2 catalyst before and a function of Cu concentration. The formation of stabilized Cu+
aer 200 h of reaction (Fig. 10) that, the ratio of Cu+/(Cu+ + Cu0) is considered to be related to the interface between copper
decreased from 0.69 to 0.65.
species and metal oxide support, while the formation of Cu0 is
considered to be originated from the bulk CuO.59–61 At the
beginning of copper loading increase, Cu+/(Cu+ + Cu0) values
initially increased because of the increase of relative interface
area between Cu and Zn/Al species, thus resulting in the
formation of more Cu+–O–Zn/Al like structures at the inter-
face.62 Aer the copper concentration reached to a certain
3.4 Performance of Cu-2 on different scope of the carbonates
The applicability for a variety of symmetrical and/or asymmet-
rical carbonates, e.g., DMC, ethyl methyl carbonate, EMC,
dibutyl carbonate (DBC), ethylene carbonate (EC), propylene
carbonate (PC) over Cu-2 catalyst was surveyed at 200 ꢁC and
5 MPa H2 initial pressure. Table 4 provides the summary
statistics for the corresponding results.
For the hydrogenation of DMC, 76.8% yield of methanol
could be obtained with 100% DMC conversion within 8 h, entry
1. For the hydrogenation of other carbonates linear, EMC and
DBC, 48.6% and 38.9% methanol yield were obtained, respec-
tively, and both of the conversion exceeded 98.9%. However, Cu-
2 did not perform well on cyclic carbonates, i.e. EC and PC
(entries 4 and 5), which was possibly caused by decomposition
of cyclic carbonates in the presence of Zn/AlOx.58
3.5 Mechanism of the hydrogenation of DEC over Cu/Zn/Al
catalysts
Above all, it is considered that Cu in the Cu/Zn/Al catalyst played
a vital role in methanol synthesis from DEC hydrogenation.
Without Cu, the binary catalyst Zn/Al can't promote the
Fig. 11 SCu (m2 gcat.ꢀ1) and Cu+/(Cu+ + Cu0) as a function of Cu
production of methanol from DEC and H2 (Scheme S1†). What's concentration.
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degree, the ratio of bulk CuO continued to increase (Table 2 methanol over Cu/Zn/Al, and the process is shown in Fig. 12.
peak b area proportion) but the relative interface area between The whole process includes three steps, rstly, Cu+ species and
copper species and Zn/Al oxides decreased. Therefore, the ZnO adsorbs the ester groups of DEC. At the same time, H2 is
proportion of Cu+ decreased from 0.69 to 0.45. As a result, the absorbed and splitted into H atoms on Cu0 species. H atoms,
ratio of Cu+/(Cu+ + Cu0) showed a volcanic trend as a function of which are then transferred to ZnO surface and trapped at the
Cu concentration. The conclusion had been come up with in surface, further migrate to other Cu particles, and react with the
catalytic performance part that the catalytic activity is highly absorbed DEC to form ethyl formate with one molecular of
related to the ratio of Cu+ sites. Higher Cu+/(Cu+ + Cu0) ratio led ethanol released at the surface of the catalyst (step 1); then, the
to higher methanol formation rate. While, the correlation in formed ethyl formate continued react with H atoms to form
Fig. 11 demonstrates the optimal methanol formation rate on formaldehyde with another molecular of ethanol released (step
the Cu/Zn/Al catalysts lies not only in Cu+, but also in Cu0. 2); at last, formaldehyde was hydrogenated by H atoms into
However, excessive Cu0 sites density would play a negative methanol (step 3). Meanwhile, Zn/Al binary catalyst promoted
inuence on the methanol synthesis, which meant a balanced the formation of EMC through transesterication reaction of
distribution of surface Cu0 and Cu+ species is considered to be methanol and DEC.
an essential factor to obtain outstanding performance in the
hydrogenation of carbonates.
4. Conclusions
Studies showed that Cu+ played as a function of adsorbing
and stabilizing the intermediate products in the reaction and In this work, a series of Cu/Zn/Al catalysts with various copper
acts as an electrophilic or Lewis acid site to polarize the C]O concentrations were prepared by the co-precipitation method
bond through an electron lone pair on the oxygen, thereby with synchronous aging step, which were applied in DEC
increasing the reactivity of the ester group.63,64 While Cu0 hydrogenation. All catalysts possessed disordered mesoporous
species played as a function of absorption and activation of H2 structures. In the preparation process, the morphology of
via splitting hydrogen, which provided H atoms for reactant/ samples altered from lamellar to spherical. Copper species of
intermediates.65 It should be noted that, excessive Cu0 sites lead Cu+ and Cu0 were considered as active sites, and they had
to more H2 splitting, which will be transferred via spillover from synergistic effect on the hydrogenation of DEC. The highest
metallic Cu particles to ZnO surface.57 This meant the function Cu+/(Cu+ + Cu0) value led to the best performance of catalyst
of ZnO, acting as electronic promoter to absorb ester groups,32,62 (Cu-2). However, excessive Cu0 sites resulted in the decline of
were suppressed. Thus, the excessive hydrogen spillover DEC conversion, since hydrogen spillover between Cu and ZnO
between Cu and ZnO led to the insufficient absorption coverage inuenced the absorption of carbonates/intermediate.
of the carbonates or reaction intermediate, leading the Aluminum functioned as a support in the form of Al2O3. ZnO
conversion of DEC decreased. Also, HRTEM results had veried played the role of physical spacer between copper species and
the formation of interface between Cu and Zn/Al oxide in this helped dispersing the Cu phase during the catalyst preparation,
paper, which is contribute to the high dispersion of Cu. In thus brought more intimate interface contact between Cu and
addition, Al2O3 was observed in XPS result and was considered Zn/Al oxide.
as promoter to enhance the long-term stability, besides, the
At the optimized temperature of 200 ꢁC and time of 4 h, Cu-2
addition of Al has been reported to enhance the BET surface catalyst with 45.2% mass fraction exhibited 86.1% DEC
area and Cu dispersion as well as restrain copper particles conversion and 71.9% total methanol selectivity with 131.0 mg
sintering.62 In this paper, we think Al2O3 mainly played a sup- gcat.ꢀ1 hꢀ1 total methanol formation rate under 5 MPa. Time-on-
porting role and worked as a structure promoter.57
stream experiments on a xed bed showed that the conversion
Based on our experimental results and analysis, a possible of DEC kept around 95.0% with a small declining selectivity of
mechanism was proposed for DEC hydrogenation into methanol over a 200 h reaction time at 200 ꢁC and 2.5 MPa H2,
and thus Cu-2 catalyst provided a long-term stability. This work
provided a facile Cu/Zn/Al catalyst, which could be utilized into
indirectly upgrading CO2 to produce commodity methanol
under relatively mild reaction conditions.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors acknowledge the National Natural Science Foun-
dation of China (21576272), “Transformational Technologies
for Clean Energy and Demonstration”, Strategic Priority
Research Program of the Chinese Academy of Sciences, Grant
No. XDA 21030600. We also acknowledge the project from
Fig. 12 The possible mechanism of DEC hydrogenation to produce
methanol.
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Science and Technology Service Network Initiative, Chinese 24 C. Lian, F. Ren, Y. Liu, G. Zhao, Y. Ji, H. Rong, W. Jia, L. Ma,
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