Efficient production of methanol and diols via the hydrogenation of cyclic carbonates using copper-silica nanocomposite catalysts

Article abstract of DOI:10.1039/c5gc00810g

Copper-silica nanocomposite catalysts with a uniform Cu dispersion prepared by a precipitation-gel method have been found to be highly efficient in the heterogeneous catalytic hydrogenation of CO₂-derived cyclic carbonates, providing an indirect but practical approach for the transformation of CO₂ to methanol with the co-production of diols under relatively mild conditions. The catalysts possessed remarkable stability in both batch and fixed-bed continuous flow reactors especially after promotion with B₂O₃. The reaction was found to depend sensitively on the Cu particle size, the surface acidity-basicity and the Cu valence of the catalysts. The synergetic effect between balanced Cu⁰ and Cu⁺ sites was considered to play a critical role in attaining high yields of methanol and diols.

Full text of DOI:10.1039/c5gc00810g

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ARTICLE
Efficient Production of Methanol and Diols via Hydrogenation of
Cyclic Carbonates Using Copper-silica Nanocomposite Catalyst
Hailong Liu,†a, d Zhiwei Huang,†a Zhaobin Han,b Kuiling Ding,*b Haichao Liu,c Chungu Xia,*a Jing
Chen*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Copperꢀsilica nanocomposite catalysts with uniform Cu dispersion prepared by precipitationꢀgel method have been
disclosed to be highly efficient in the heterogeneous catalysis of hydrogenation of CO2ꢀderived cyclic carbonates, providing
an indirect but practical approach for the transformation of CO2 to methanol with the coꢀproduction of diols under relatively
mild conditions. The catalysts possessed remarkable stability in both batch and fixedꢀbed continuous flow reactors
especially after promotion with B2O3. The reaction was found to depend sensitively on the Cu particle sizes, the surface
acidꢀbasicity and Cu valence of the catalysts. The synergetic effect between balanced Cu0 and Cu+ sites was considered to
play a critical role for attaining high yields of methanol and diols.
www.rsc.org/
reaction conditions (e.g. 523 K, 5 MPa) are usually required for
these heterogeneous processes to obtain satisfactory methanol yield.
Introduction
With an increasing utilization of carbonꢀrich fossilꢀbased resources,
the concentration of carbon dioxide in the atmosphere has been
significantly increased in the past century, causing a globally
environmental concern.1,2 The catalytic transformation of cheap and
renewable CO2 to chemicals and fuels is highly attractive although
the chemical utilization of CO2 would not solve the problem of this
global challenge.3ꢀ5 Accordingly, in addition to all concepts reducing
the CO2 output, the utilization of CO2 as C1 feedstock for carbon
recycling becomes increasingly important. Among the various
products derived from CO2, methanol is particularly desirable, since
it can be used not only as a fuel substitute, but also as a versatile
chemical intermediate on the basis of “methanol economy”
concept.2,6 From this point of view, the hydrogenation of CO2 to
CH3OH has attracted extensive investigations with the focus on
Cu/Znꢀbased multiꢀcomponent catalysts.2,7ꢀ9 However, due to the
thermodynamically stable and kinetically inert nature of CO2
molecule, high activation energy barriers have to be overcome for
the cleavage of the C=O bonds in CO2,4,7,10 and thus relatively harsh
O
C
3 H2
Cat. 1
3 MeOH
OMe
Cat.
Milstein11 : >99% yield
MeO
O
373 K, 1 MPa
MeOH
CO2
2
3 H2
O
OH
OH
+
MeOH
O
O
413 K, 5 MPa
Ding12: >99% yield
tBu
H
N
H
PR2
CO
P
N
H
tBu
Ru
Ru
Cl
R = alkyl or aryl
P
N
R2
CO
Cat. 1
Cat. 2
Scheme 1. Indirect method for transformation of CO2 to methanol
via hydrogenation of organic carbonates under mild conditions.
The catalytic hydrogenation of organic carbonates has provided an
alternative approach for the indirect transformation of CO2 to
methanol, since the organic carbonates can be synthesized directly
by reaction of CO2 with alcohol or epoxides (Scheme 1).5,10ꢀ12
However, the hydrogenation of C=O group in carbonates is
particularly difficult due to the resonance stabilization effect of two
adjacent alkoxy groups.13,14 Thus, only a few homogeneous catalyst
systems have been reported for the hydrogenation of organic
carbonates.11,12,15ꢀ17 Milstein and coworkers pioneered this reaction
process and reported the first hydrogenation of dimethyl carbonate
recently with the catalysis of
a homogeneous dearomatized
PNN/RuII pincer complex (Cat. 1), affording methanol with a TON
up to 4,400 under mild conditions (383 K and 5 MPa H2).11 More
recently, Ding and coworkers12 realized the hydrogenation of cyclic
ethylene carbonate (EC), a readily available industrial chemical from
CO2 and ethylene epoxide, using a PNP/RuII pincer complex (Cat. 2)
as homogeneous catalyst, affording two important bulk chemicals of
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methanol and ethylene glycol (EG), with excellent selectivity and solution of ZnO (20 wt%, 15ꢀ20 nm), Al2O3 (20 wt%, 10 nm), TiO2
high TON (up to 87,000), at 423 K and 5 MPa H2. Despite the high (15 wt%, 15 nm) and ZrO2 (40 wt%, 20ꢀ30 nm) were purchased from
efficiency and mild reaction conditions of these homogeneous Xuancheng Jingrui New Material Co., LTD. NH3 (99.9%), CO2
processes,11,12,15ꢀ17 their practical application would be hindered to (99.9%), 5%N2OꢀN2, 5%H2ꢀAr, 20% H2ꢀN2 and He (99.999%) were
some extent due to the inevitable difficulty associated with the supplied from Lanzhou Lanmei Cryogenic Products Co., LTD. The
separation of the alcohol products from the catalysts and the possible colloidal aqueous solution of MgO, Mg3AlOx, Zn3AlOx, were
dehydrogenation of methanol and ethylene glycol in the presence of prepared by precipitating the nitrates solution of Mg(NO3)2 or the
Ruꢀcatalyst.12 Accordingly, it is highly desirable to develop a mixture of Al(NO3)3 and Mg(NO3)2 or Zn(NO3)2 with NaOH
heterogeneous catalyst system for the hydrogenation of cyclic solution. 4ꢀButylꢀ1,3ꢀdioxolanꢀ2ꢀone and 4ꢀphenylꢀ1,3ꢀdioxolanꢀ2ꢀ
carbonates under mild conditions with ease of catalyst separation one were synthesized from 1,2ꢀepoxyhenane and styrene oxide with
and recycling. Very recently, Li and coworkers attempted the CO2, respectively, according to the literature28.
hydrogenation of EC over
a heterogeneous copperꢀchromite
nanocatalyst at 453 K and 5 MPa H2,18 and moderate selectivity
(60%) was observed for the formation of methanol albeit good
selectivity for production of EG (93%), demonstrating the feasibility
for the hydrogenation of organic carbonates to methanol and EG
over heterogeneous catalysts. Nonetheless, this study still suffers
from low catalytic efficiency and the use of environmentally
unfriendly Crꢀcontaining catalyst. Therefore, it is highly desirable to
develop Crꢀfree nonꢀnoble metal based heterogeneous catalysts for
the efficient hydrogenation of organic carbonates.
Catalyst preparation
5%Ru/C catalyst was prepared by incipient wetness impregnation
(IM) of C with an aqueous solution containing the calculated amount
of RuCl3 at room temperature, followed by drying at 393 K in air
and calcined at 723 K in N2 flow for 3 h. Similarly, the SiO2
supported 10 wt% NiO, 10 wt% Co3O4 and 10 wt% CuO were
prepared except for calcined at 723 K in air for 3 h. 5 wt% WO3 and
5
wt% K2O modified 10%CuꢀSiO2ꢀPG were prepared by
We have previously reported that the copperꢀsilica
nanocomposites prepared by precipitationꢀgel (PG) method showed
uniform Cu dispersion and high thermal stability, as well as high
activity and selectivity in the hydrogenolysis of polyols to glycols.19ꢀ
21 Its high efficiency in hydrogenation of C=O and CꢀO promoted us
to investigate the possibility for the use of this type Crꢀfree
heterogeneous catalysts for the hydrogenation of organic carbonates.
Although the Cuꢀsilica catalyst has been extensively studied in C=O
hydrogenation,22ꢀ27 its application in carbonates hydrogenation is still
rare. To our delight, the CuꢀSiO2ꢀPG catalysts do act as highly
efficient and selective heterogeneous catalyst for the hydrogenation
of cyclic carbonates, affording methanol and EG in high yields
(>97%) in both batch and fixedꢀbed continuous flow reactor under
relatively mild conditions. The reaction was found to depend
sensitively on the Cu particle sizes, the surface acidꢀbasicity and Cu
valence of the catalysts. The synergetic effect between Cu0 and Cu+
was also disclosed to play a critical role for attaining high yields of
methanol and diols.
impregnation of the calcined 10%CuOꢀSiO2ꢀPG sample (prepared by
the PG method described below) with aqueous solution of
ammonium tungstate or KNO3, followed by drying and calcining
under the same conditions for 10%CuO/SiO2ꢀIM. Similarly, 1 wt%
B2O3 incorporated 70%CuꢀSiO2ꢀPG catalyst was prepared by
impregnation of the 823 K calcined 70%CuOꢀSiO2ꢀPG sample with
an aqueous solution of H3BO3.
The samples with 10 wt% CuO dispersed by different oxides
(MgO, ZnO, Zn3AlOx, Mg3AlOx, Al2O3, TiO2 and ZrO2) and CuOꢀ
SiO2 with CuO loadings from 10 to 90 wt% were prepared by a
precipitationꢀgel (PG) method as described in our previous
reports19,20. Briefly, an aqueous solution of NaOH (4 mol Lꢀ1) was
dropped in a Cu(NO3)2 (0.5 mol Lꢀ1) solution to form Cu(OH)2
microparticles at a constant rate under vigorous stirring at 303 K
until pH >11. Next, a calculated amount of colloidal aqueous
solution of SiO2, ZnO, Al2O3, TiO2, ZrO2, MgO, Zn3AlOx, and
Mg3AlOx was added to the solution of Cu(OH)2 to form a gel. After
aging at 303 K for 4 h (for CuꢀSiO2 aged at 373 K), the slurry of the
gel was filtrated and washed with hot distilled water thoroughly.
Afterwards, the samples were dried at 393 K for 12 h and then
calcined at 723 K under air for 3 h. The calcined samples prepared
by IM and PG methods were marked as ω%MO/supportꢀIM and
ω%MOꢀsupportꢀPG, respectively, where ω represents the nominal
Experimental section
Materials
All reagents were of analytical grade, tetrahydrofuran (THF) and
metal oxide loading and
M represents the metal, and the
1,4ꢀdioxane were distilled from sodium benzophenone ketyl. corresponding reduced catalysts were marked as ω%M/supportꢀIM
5%Pd/C was purchased from Fluka. The copper chromite catalyst and ω%MꢀsupportꢀPG.
was purchased from Yingkou Tianyuan Chemical Industry Research
Institute Co., LTD. SiO2 powder (350ꢀ410 m2 gꢀ1), ethylene
Catalyst characterization
carbonate (99%), propylene carbonate (99%), glycerol carbonate
(99%), 4ꢀethylꢀ1,3ꢀdioxolanꢀ2ꢀone (99%), 1,3ꢀdioxanꢀ2ꢀone (99%),
dimethyl carbonate (99%), 1,2ꢀepoxyhenane (96%), styrene oxide
(98%) and pꢀxylene (99%) were purchased from Alfa Aesar.
Colloidal SiO2 aqueous solution (25.0 wt%, ammonium stabilized
type, pH~8.5ꢀ10.5, particle size 10ꢀ20 nm) was purchased from
Qingdao Haiyang Chemical Co., China. Active Carbon (C, AR) was
supplied from Tianjin Fengyue Chemical. The colloidal aqueous
The Xꢀray powder diffraction (XRD) was carried out on a
PANalyticalX’pert Pro Diffractometer using nickel filtered Cu Kα
radiation at a voltage and current of 40 kV and 30 mA, respectively.
The in situ XRD measurements were also conducted on the same
instrument equipped with a XRKꢀ900 highꢀtemperature chamber.
For selected samples, patterns were obtained during in situ reduction
in 5% H2ꢀAr at a flow rate of 40 mL minꢀ1 up to 773 K. In these
2 | Green Chem., 2015, 00, 1-3
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ꢥ
ꢙ. ꢚ
ꢀꢁꢂ
experiments the temperature was ramped between measurements at a
heating rate of 10 K minꢀ1 with a 2ꢀh pause at each temperature
before recording the pattern.
ꢃ
ꢄ
ꢧꢁꢂ ꢉꢌ =
≈
ꢃꢩꢄ
ꢛꢁꢂ × ꢨꢁꢂ
Where Nav: Avogadro constant = 6.02 × 1023 (molꢀ1); MCu: relative
Xꢀray photoelectron spectra (XPS) were obtained using an atomic mass = 63.5 (g molꢀ1); ρCu: Cu density = 8.92 (g cmꢀ3).
ESCALAB250xi spectrometer equipped with a monochromatized Al
Kα Xꢀray radiation source (hν = 1486.6 eV) with 20 eV pass energy.
For the in situ experiments, the sample was reduced in the
pretreatment chamber at the given temperature for 2 h with 5% H2ꢀ
Ar at a flow rate of 40 mL minꢀ1. After cooling to room temperature,
the pretreatment chamber was vacuumized to 10ꢀ3 Pa, and then the
reduced catalyst was transferred to the transfer chamber (10ꢀ6 Pa)
and further to analytic chamber (10ꢀ9 Pa) for XPS measurement
without exposure to air. All binding energies were calibrated using
the C1s peak at 284.8 eV as the reference. The XPS peaks were
analyzed using a Shirleyꢀtype background and a nonlinear leastꢀ
Catalytic performance
The discontinuous cyclic carbonates hydrogenation reaction was
carried out in a stainless steel autoclave (100 mL) at a stirring speed
of 800 rpm. Prior to the reaction, the calcined samples were preꢀ
reduced at 573 K or other temperatures in 20% H2ꢀN2 for 3 h. In a
typical experiment, 40 g of 10 wt% cycle carbonates in THF solution
and the reduced catalyst was introduced into the autoclave. After
flushing with H2 three times, the autoclave was pressurized with H2
to 6.0 MPa, and then heated to 433 K, which was kept steady during
the reaction. The reactant and liquid products with pꢀxylene as the
internal standard were analyzed by gas chromatograph (Agilent
7820A GC) with a SEꢀ54 capillary column (50 m × 0.25 mm × 0.25
ꢀm) or a PONA capillary column (50 m × 0.20 mm × 0.50 ꢀm). The
gas products were determined on a SP6800A chromatograph
equipped with a TDXꢀ01 packed column. Products were also
identified on an Agilent 7890A/5975C GC–MS with a HPꢀ5MS
column. The detected liquid products were methanol, ethylene
glycol, methyl formate, 2ꢀhydroxyethyl formate and ethanol; gas
products contained CO and CO2. The conversions of cyclic
carbonates and selectivities to products were calculated as following:
mol of reactant charged ꢀ mol of reactant left
square fitting of the experimental data based on
Gaussian/Lorentzian peak shape.
a mixed
The BET surface area measurements were performed on a
Micromeritics Tristar 3020 instrument at liquid nitrogen
temperature. Prior to measurements, the samples were degassed at
573 for h. Transmission electron microscopic (TEM)
K
4
investigations were carried out using
microscope at 200 kV.
a JEM2010 electron
NH3ꢀTPD and CO2ꢀTPD were carried out on a unit DAXꢀ7000
instrument (Huasi Technology Co., Ltd, China). Before each
measurement, the catalyst samples were pretreated at 473 K under
He flow for 1 h and reduced at 573 K with 5% H2ꢀAr for 2 h. For
CO2ꢀTPD, the reduced catalysts were exposed to a CO2 flow (40 mL
minꢀ1) at 313 K for 1 h, and then heated linearly from 313 to 973 K
at a rate of 10 K min−1 in a He flow (40 mL minꢀ1). The desorbed
CO2 was detected by a thermal conductivity detector (TCD), and the
TCD signals were calibrated by a given volume of CO2. NH3ꢀTPD
for Cu catalysts was similar to the CO2ꢀTPD procedures with the
saturation adsorption of NH3 at 373 K for 1 h.
The Cu dispersion was determined by dissociative N2O
adsorption–H2ꢀTPR reverse titration.21,29 After pretreated in a He
flow at 473 K, the samples (containing ~10 mg Cu) were reduced
with 5% H2ꢀAr at a flow rate of 40 mL minꢀ1 and the temperature
was increased from room temperature to 773 K at a ramping rate of
5 K minꢀ1 (denoted as total TPR, containing the reduction of bulk
and surface Cu species). After cooling to 323 K, the reduced samples
were exposed to 5% N2OꢀN2 (40 mL minꢀ1) for 1 h. Afterwards, the
resulting surface oxidized samples underwent the second TPR run at
the ramping rate of 5 K minꢀ1 from 323 to 623 K (denoted as surface
TPR). By assuming spherical shape of the copper particles,
1.46×1019 Cu atoms per m2 and a molar stoichiometry N2O/Cus = 0.5
(Cus: the Cu atoms on the surface), the Cu dispersions (DCu), Cu0
surface area (SCu) and mean Cu particle size (dCu) for the Cu–SiO2
catalysts were calculated by the Equation (1), (2) and (3),
respectively.
Conversion (%) =
× 100%
mol of reactant charged
mol of a product generated
Selectivity (%) =
× 100%
mol of reactant charged ꢀ mol of reactant left
The continuous ethylene carbonate hydrogenation reaction was
performed in a vertical fixedꢀbed reactor (36 cm long and 0.9 cm
inner diameter). Typically, 3.0 g of catalyst (40–60 meshes) was
loaded into a stainless steel tubular reactor with the thermocouple
inserted into the catalyst bed for better control of the actual
pretreatment and reaction temperature. Catalyst activation was
performed at 573K for 3 h under 20% H2ꢀN2 flow. After cooling to
the reaction temperature, 10 wt% ethylene carbonate in 1,4ꢀdioxane
and H2 were fed into the reactor at a H2 to ethylene carbonate molar
ratio of 60 : 1 and a system pressure of 3 MPa. The reaction
temperature and the roomꢀtemperature weight hour space velocity
(WHSV) of ethylene carbonate were set at 433 K and 0.2 hꢀ1,
respectively. The products collected in the condenser were analyzed
offline as indicated above.
Results and discussion
Table
1 shows catalytic performance of a wide variety of
heterogeneous catalysts in the hydrogenation of EC at 433 K and 6
MPa H2. Among the traditional hydrogenation catalysts (i.e.
supported Co, Ni, Cu, Pd and Ru prepared by impregnation method)
investigated, Cu/SiO2 was more active and selective in the formation
of both methanol and EG (entries 1ꢀ5). The 10%Cu/SiO2ꢀIM catalyst
gave a 69% EC conversion with 57% and 97% selectivity to
methanol and EG, respectively, after reaction for 10 h (entry 5).
Considering the poor stability and dispersion of Cu in Cu/SiO2ꢀIM, a
ꢅ × ꢆꢅ ꢇꢈꢉꢊꢋꢌꢍꢎꢏꢈꢉ ꢃꢊꢋꢐꢑꢒꢓꢔ ꢕꢖꢗꢄ
ꢃ
ꢄ
ꢀꢁꢂ % =
ꢆꢅ ꢇꢈꢉꢊꢋꢌꢍꢎꢏꢈꢉ ꢃꢎꢈꢎꢒꢘ ꢕꢖꢗꢄ
× ꢙꢚꢚ%
ꢃꢙꢄ
Dꢁꢂ × ꢠꢡꢢ
ꢣꢁꢂ × ꢙ. ꢤꢥ × ꢙꢚꢙꢦ
ꢛꢁꢂ ꢜꢌꢅ ꢝꢞꢙꢇꢋꢟ =
≈ ꢥꢤꢦ × ꢀꢁꢂ ꢃꢅꢄ
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series of CuꢀSiO2 catalysts with Cu loadings of 10 to 90 wt% were calcined at different temperatures (623ꢀ1023 K). An attempt to
synthesized by precipitationꢀgel (PG) method and investigated for correlate the intrinsic activities and methanol selectivities with Cu
EC hydrogenation (entries 6ꢀ10). Notably, the CuꢀSiO2ꢀPG catalysts particle sizes (Figure 1A) and Cu surface area (Figure 1B) is shown
with uniform Cu particles distribution (for detailed characterizations in Figure 1. The intrinsic activity, i.e. turnover frequency (TOF),
see Table 2 and reference 21) exhibited much higher activity (>98% normalized by the methanol generating per Cu site estimated by N2O
conversions) than that of the catalyst prepared by IM method and per time, increased sharply from 7.2 hꢀ1 to a maximum of 21 hꢀ1 with
also a commercial CuꢀCr catalyst (entry 20). The selectivity for the increasing Cu particle size from 4.6 to 9.9 nm, and then dropped to
formation of methanol in the presence of CuꢀSiO2ꢀPG catalysts was 9.7 hꢀ1 by further increasing Cu size to 23.8 nm. Similarly, the
improved gradually with the increase of Cu content and reached the methanol selectivities passed a maximum of 95% at 8.1 nm from
maximum value of 95% in the case of 70%CuꢀSiO2ꢀPG. On the 59% at 4.6 nm and then decreased to 71% at 23.8 nm. These findings
other hand, the selectivity for the production of EG kept almost reveal a structureꢀsensitive character of the title reaction and a
constant (97ꢀ98%) for the CuꢀSiO2ꢀPG catalysts. The superior superior reactivity of Cu particles with sizes of around 8ꢀ10 nm,
activity and selectivity of CuꢀSiO2ꢀPG catalysts might be attributed probably originated by that the activation of C=O bond in EC
to the specific activity of Cu in CꢀO and C=O bonds requires stepꢀedge sites in which molecules are activated through
hydrogenation19,30,31 and the uniformly dispersed copper particles by multiple contact with several surface atoms.32 Because of such stepꢀ
SiO2 nanoparticles with high loading of active Cu species.
sites are geometrically not possible for particle below a particular
size, their relative probability at the edge of a particle shows a
maximum as a function of particle size. In addition, the variation of
Cu particle size would also influence the copperꢀsilica interfacial
sites, the electronic properties and surface Cu0ꢀCu+ species, which
govern the catalytic reactivity as well.33,34 For more quantitative
conclusions about the structure sensitivity of the reaction, further
studies would be needed. The TOFs and methanol selectivities
increased steadily with elevating of Cu surface area and passed
maximums of 21 hꢀ1 and 95% at 65.5 m2 gꢀ1 and 79.8 m2 gꢀ1,
respectively, then declined upon further increasing the Cu surface
area (Figure 1B). This finding implies that the hydrogenation of EC
to methanol is not governed solely by metallic Cu sites, rather the
concurrence of many factors, such as Cu sizes mentioned above, the
synergistic effects of Cu0ꢀCu+ sites and surface acidꢀbasicity
discussed below.
Table 1. Conversion and selectivity in hydrogenation of EC on
different supported catalysts. a
Selectivity (%)
Entry
Catalyst
Conversion (%)
Methanol EG
1
2
5%Pd/C
38
58
21
26
4
94
94
97
98
97
97
98
98
97
98
98
97
98
98
96
98
96
98
97
97
5%Ru/CꢀIM
3
10%Ni/SiO2ꢀIM
10%Co/SiO2ꢀIM
10%Cu/SiO2ꢀIM
10%CuꢀSiO2ꢀPG
30%CuꢀSiO2ꢀPG b
50%CuꢀSiO2ꢀPG b
70%CuꢀSiO2ꢀPG b
90%CuꢀSiO2ꢀPG b
10%CuꢀTiO2ꢀPG
10%CuꢀZrO2ꢀPG
10%CuꢀMgOꢀPG
10%CuꢀZnOꢀPG
10%CuꢀAl2O3ꢀPG
10%CuꢀZn3AlOxꢀPG
10%CuꢀMg3AlOxꢀPG
10%CuꢀSiO2ꢀPGꢀW
10%CuꢀSiO2ꢀPGꢀK
28
4
29
8
5
69
57
68
78
85
95
87
40
58
26
51
50
58
43
56
53
64
6
>99
>99
>99
>99
98
7
25
20
15
10
5
100
80
60
40
20
0
8
(A)
9
10
11
12
13
14
15
16
17
18
19
20
71
81
46
70
5
10
15
20
25
Cu particle size (nm)
66
100
75
(B)
90
80
70
60
50
40
30
20
10
0
25
58
20
15
10
5
79
82
b
CuCr2O4
64
a
Reaction conditions: 40 g 10 wt% EC in THF solution, 3.26 g oxidic
catalysts were used prior activation in H2, 6 MPa H2, 433 K, 10 h. For
b
more detail selectivities to other byproducts see Table S1 in SI.
20
40
60
80
100
120
140
160
Cu0 surface area (m2 gꢀ1)
The same amount of Cu was used as 10%Cu-SiO2-PG catalyst.
Figure 1. Dependence of turnover frequencies and methanol
selectivities in EC hydrogenation at ca. 30% conversion on Cu
particle sizes (A) and Cu0 surface area (B) for Cu–SiO2-PG catalysts
prepared with different Cu loadings and 70%Cu-SiO2-PG with
Table 2 illustrates the physicoꢀchemical properties and catalytic
performance of the CuꢀSiO2 with different Cu loadings (10ꢀ90%)
prepared by PG method calcined at 723 K and 70%CuꢀSiO2ꢀPG
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different calcined temperatures. Reaction conditions: 40 g 10 wt%
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Table 2. Structures and catalytic performances of the Cu-SiO2 with different Cu loadings prepared by PG method calcined at 723
K and 70%Cu-SiO2-PG calcined at different temperatures for EC hydrogenation. a
b
d
e
Selectivity (%)
Methanol EG
SBET
SCu
dCu
Catalyst
DCu c (%)
TOF f (hꢀ1)
(m2 gꢀ1)
(m2 gꢀ1Cu) (nm)
10%CuꢀSiO2ꢀ723
30%CuꢀSiO2ꢀ723
50%CuꢀSiO2ꢀ723
70%CuꢀSiO2ꢀ723
70%CuꢀSiO2ꢀ623
70%CuꢀSiO2ꢀ823
70%CuꢀSiO2ꢀ873
70%CuꢀSiO2ꢀ923
70%CuꢀSiO2ꢀ973
70%CuꢀSiO2ꢀ1023
90%CuꢀSiO2ꢀ723
211
259
280
232
248
196
158
146
124
34
21.8
18.5
16.1
13.4
17.5
12.3
11.1
10.1
7.0
141.4
120.0
104.4
86.9
4.6
5.4
7.2
8.5
59
70
81
90
82
95
90
86
77
71
80
93
92
92
91
92
94
93
92
94
94
92
6.2
12.8
16.2
11.3
19.7
20.1
21.0
13.6
9.7
7.5
113.5
79.8
5.7
8.1
72.1
9.0
65.5
9.9
45.4
14.2
23.8
10.6
4.2
27.2
167
9.4
61.0
18.6
a Reaction condition: 40 g 10 wt% ethylene carbonate in THF solution, 0.16-0.67 g Cu, 6.0 MPa H2, 433 K, 2 h, ~30% conversion. b
BET method. c Cu dispersion obtained from dissociative N2O adsorption. d Cu0 surface area obtained from dissociative N2O
adsorption. e Mean Cu particle size calculated from dissociative N2O adsorption. f TOF is defined as the methanol generating per
Cu site per time.
10%CuꢀAl2O3ꢀPG < 10%CuꢀTiO2ꢀPG < 10%CuꢀMg3AlOxꢀPG <
10%CuꢀMgOꢀPG. Interestingly, correlation the amounts of acidic
and basic sites of the catalysts with their catalytic activities showed
that both EC conversion and methanol selectivity declined almost
linearly with the increase of the amounts of their acidic and basic
sites (Figure 2). In comparison, there is no clear correlation between
catalytic activities and the amounts of acidic or basic concentration
of the catalysts (Figure S2 in SI). It suggests that the significant
presence of acidic and basic sites is unfavourable for efficient
production of methanol from EC. This finding is further confirmed
by the comparative experiments on CuꢀSiO2 modified with 5 wt%
WO3 and 5 wt% K2O (entries 13ꢀ14), which increased the amounts
of the strong acidic and basic sites of CuꢀSiO2ꢀPG (Table 3 and
Table S2 in SI), respectively, but largely suppressed the formation of
methanol. Presumably, excessive basicity or acidity of the catalyst
might block the catalytic sites by strongly adsorbed reactants on the
EC in THF solution, 0.16-0.67 g Cu, 6.0 MPa H2, 433 K, 2 h.
To study the effect of surface acidityꢀbasicity on the catalytic
performance of this type of nanocomposite catalysts, the Cu
component was dispersed on a variety of oxide supports including
SiO2, TiO2, ZrO2, Al2O3, MgO, ZnO, Zn3AlOx and Mg3AlOx by PG
method. As can be seen from Table 1 (entries 1 and 6ꢀ12), the
activity and selectivity for methanol production varied significantly
although the Cu particle sizes were controlled in a narrow range of
4.1ꢀ5.4 nm (Table 3), indicating the critical role of the supports on
the catalytic performance. The acidity and basicity of the catalysts
were measured by NH3ꢀTPD and CO2ꢀTPD, respectively, and the
results were compiled in Table 3, Table S2 and Figure S1 in
Supporting Information (SI). The total amounts of acidic and basic
sites of the Cu catalysts increased in the order 10%Cuꢀ SiO2ꢀPG <
10%CuꢀZrO2ꢀPG < 10%CuꢀZn3AlOxꢀPG < 10%CuꢀZnOꢀPG <
Table 3. The textural properties of different oxides dispersed Cu catalysts.
a
a
c
d
SBET
dPore
dCu
SCu
Total acidic and basic
Catalysts
DCu b (%)
(m2 gꢀ1) (nm)
(nm) (m2 gꢀ1 Cu)
sites e (ꢀmol gꢀ1)
10%CuꢀSiO2ꢀPG
10%CuꢀMgOꢀPG
10%CuꢀZnOꢀPG
211
132
128
185
216
128
58
13.7
27.2
23.4
15.6
16.6
18.7
25.0
19.8
11.8
12.9
23.0
24.5
23.1
22.3
24.1
21.9
18.6
22.3
24.1
23.6
4.3
4.1
4.3
4.5
4.1
4.6
5.4
4.5
4.1
4.2
149.3
159.1
150.0
144.8
156.5
142.2
120.8
144.8
156.5
153.2
75.9
425.6
251.6
209.7
325.3
265.0
306.2
169.1
182.6
148.5
10%CuꢀZn3AlOxꢀPG
10%CuꢀMg3AlOxꢀPG
10%CuꢀAl2O3ꢀPG
10%CuꢀTiO2ꢀPG
10%CuꢀZrO2ꢀPG
10%CuꢀSiO2ꢀPGꢀW
10%CuꢀSiO2ꢀPGꢀK
130
191
185
a BET method. b Cu dispersion obtained from dissociative N2O adsorption. c Mean Cu particle size calculated from dissociative
N2O adsorption. d Cu0 surface area obtained from dissociative N2O adsorption. e Calculated from NH3-TPD and CO2-TPD, for detail
see Table S1.
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one hand; and induce the side reactions such as decomposition of the energy of Cu 2p3/2 to 932.7.eV after reduction at 473ꢀ773 K show
reaction intermediates, most probably 2ꢀhydroxyethylformate12,16, to that copper species in these catalysts were reduced to Cu0 or/and
EG and CO/CO2 on the other hand, both of which likely deteriorate Cu+. Both Auger kinetic energy peaks of Cu LMM at 916.1 and
the EC conversion and methanol selectivity. When these oxide 918.4 eV corresponding to Cu+ and Cu0, respectively, were observed
supports were solely used as catalysts, however, almost no methanol for the reduced catalysts (Figure 4B).19,35 The intensity ratio of
could be detected, revealing the indispensable role of metal in EC surface Cu0/(Cu+ + Cu0) goes up gradually from 73.9% to 97.9% as
hydrogenation. Pure Cu metal presented a rather low conversion the reduction temperature was raised from 473 to 773 K, showing
(26%) and methanol selectivity (8%) in EC hydrogenation, showing the increased amount of Cu0 on the catalyst surface at higher
the importance of metalꢀsupport interaction in EC hydrogenation. reduction temperature. Note that a small amount of Cu+ was still
Although the metalꢀsupport interaction resulted in the different observed even when the catalyst was reduced at a higher temperature
reducibility of Cu on various supports, as the H2ꢀTPR results shown of 773 K, which may be formed by the partial reduction of copper
in Figure S3, and may influence their catalytic activities, the acidityꢀ phyllosilicate (the presence of which in the calcined sample was
basicity of the catalysts is inferred to play a more profound effect on confirmed by FTIR result (Figure S4 in SI)).25,26
their catalytic performances. Obviously, SiO2 is the most preferable
100
100
support among the above oxides examined, probably due to its weak
acidity and basicity properties, as well as proper interaction with the
active metal. Previous investigations on ester hydrogenations also
show that SiO2 is a preferable support for Cu catalysts.22,24ꢀ26
(A)
80
60
40
20
0
80
60
40
20
0
100
90
80
70
60
50
40
30
20
Conversion
Methanol selectivity
R2= 0.94
No Reduction 473
573
673
773
Reduction temperature (K)
Cu0
Cu2O
Crystallite
Size (nm)
CuO
(B)
R2= 0.91
773 K
8.8
673 K
8.2
8.0
573 K
473 K
50
100
150
200
250
300
350
400
450
Concentration of acid and basic sites (umol gꢀ1
)
Figure 2. The effect of acidic and basic concentration of different
metal oxides dispersed Cu catalysts prepared by PG method on
catalytic properties for EC hydrogenation. Reaction conditions: 40 g
10 wt% EC in THF solution, 0.32 g Cu, 6.0 MPa H2, 433 K, 10 h.
No Reduction
20
30
40
50
60
70
80
2 Theta (degree)
Figure 3. Conversion and selectivity to methanol as a function of
reduction temperature of 70%Cu-SiO2-PG catalyst (A) and the in situ
XRD profiles of 70%Cu-SiO2-PG catalyst reduced at different
temperatures (B). Reaction conditions: 40 g 10 wt% EC in THF
solution, 0.32 g Cu, 6 MPa H2, 433 K, 10 h.
To gain insights into the effect of the copper valence state on EC
hydrogenation and to study the catalytic active site, the impact of
reduction temperature on the catalytic performance of 70%CuꢀSiO2ꢀ
PG was examined. The catalyst was first calcined at 723 K and then
reduced at different temperatures. As shown in Figure 3A, the EC
conversion and methanol selectivity increased sharply from 10 and
32% using the nonꢀreduced catalyst to >99 and 95% with the catalyst
reduced at 573 K, and then dropped to 41 and 68% when the catalyst
was hydrogenated at 773 K, respectively. Such a strong effect of
catalyst reduction temperature on the catalytic performance is
probably not caused by the Cu particle size since the crystallite size
of Cu0 has been maintained in a very narrow range of 8.0ꢀ8.8 nm
with the increase of reduction temperature from 573 to 773 K
(Figure 3B), but by their different surface compositions and
chemical states.
Correlation of the surface properties of 70%CuꢀSiO2ꢀPG reduced
at different temperatures with the corresponding catalytic
performances, the maximum of both EC conversion and methanol
selectivity obtained at a surface Cu0/(Cu+ + Cu0) ratio of 78.2% (573
K reduced catalyst) demonstrates that an appropriate surface
Cu0/Cu+ ratio was required for optimum hydrogenation activity. This
finding is in agreement with the volcanic variation of the TOFs of
the CuꢀSiO2ꢀPG catalysts with increasing Cu0 surface area (Figure
1B), inferring a Cu0ꢀCu+ twoꢀsite mechanism for EC hydrogenation.
It has been suggested that Cu0 species dissociatively adsorbs H2 and
facilitate the hydrogenation and/or hydrogenolysis reactions; while
Cu+ sites function as electrophilic or Lewis acidic sites to polarize
Since XRD would only provide information on the bulk catalysts,
in situ XPS and XAES were carried out to determine the surface
properties of the reduced catalysts. As can be seen from Figure 4A,
the disappearance of the satellite peak and the decrease of binding
the carbonyl groups via the electron lone pair in oxygen.36,37
A
recent study by Scotti et al.38 reported that the Lewis acidity of
6 | Green Chem., 2015, 00, 1-3
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ARTICLE
CuO/SiO2 catalyst was profoundly increased by the reduction of selectivity to methanol and EG, respectively. Additionally, the Cu
dispersed CuO phase to metallic state. In addition, due to the higher catalysts were also active and selective for hydrogenation of
electron affinity of silicon, the electropositive copper species (Cu+), different cyclic carbonates to form methanol and the corresponding
which are strongly interacting with the surface of SiO2, are reported diols with high yields (84~97%) (Table 4, entries 1ꢀ7). Moreover,
to with strong Lewis acidity.39 In our condition, in situ XRD less reactive linear dimethyl carbonate can also be hydrogenated to
reduction results showed the presence of solely dispersed Cu0 species methanol with 76% selectivity at 93% conversion (Table 4, entry
with crystalline sizes lower than 9 nm after reduction at 573ꢀ773 K 8).16,17
(Figure 3B), while in situ XPS reduction proved the presence of Cu+
(originated from partial reduction of copper phyllosilicate with
acid concentration
-1
(umol g )
773K
25.6
strong metalꢀsupport interaction in the calcined sample (Figure S4 in
673K
SI)) in the reduced 70%CuꢀSiO2ꢀPG catalyst with reduction
32.8
temperatures even up to 773K (Figure 4). Thus, it is likely that the
Lewis acidity of dispersed Cu0 was enhanced by the presence of
573K
57.1
proper amount of Cu+, as evidenced from the maximum of acidity of
473K
28.9
the 70%CuꢀSiO2ꢀPG catalyst reduced at 573 K (Figure 5), which in
NoReduction
turn promoted the reactivity of the ester group in EC. Therefore, the
synergistic cooperation of Cu0 and Cu+ sites with optimized Cu0/Cu+
distributions is suggested to play a critical role for the efficient
400
500
600
700
800
900
Temperature (K)
hydrogenation of EC to methanol and EG, which is in line with
previous studies on dimethyl oxalate hydrogenation.25,37
Figure 5. NH3ꢀTPD patterns of 70%CuꢀSiO2ꢀPG catalyst reduced at
different temperatures.
(A)
Cu2p3/2 932.7 eV
952.5 eV
Cu2p1/2
Table 4. Hydrogenation of different carbonates in the presence of
70%Cu-SiO2-PG catalyst calcined at 823 K and reduced at 573 K. a
773 K
673 K
573 K
Selectivity (%)
Methanol Diols
Conversion
(%)
Entry
1
Substrate
O
>99
>99
97
98
O
O
473 K
satellite peak
satellite peak
No reduction
933.9 eV
952.6 eV
2
3
97
97
98
98
965
960
955
950
945
940
935
930
925
Binding Energy (eV)
Cu0
Cu0+Cu+
Cu0
Cu+
(%)
918.4 eV
916.1 eV
(B)
>99
>99
97
773 K
97.9
673 K
573 K
89.9
4
97
89
98
78.2
73.9
473 K
5 c
29 (70) b
Cu2+
917.4 eV
No Reduction
925
920
915
910
905
6 c
97
87
96
Kinetic Energy (eV)
Figure 4. The in situ Cu 2p XPS spectra (A), Cu LMM Auger kinetic
energy profiles (B) of 70%Cu-SiO2-PG catalyst reduced at different
temperatures.
7
>99
93
98
76
>99
ꢀ
O
The reaction parameters, including the reaction temperature, H2
pressure, substrate concentration and solvent were optimized in
order to obtain high yield of methanol and EG (Table S3). The
optimal selectivity of methanol and EG (both 97%) at >99% EC
conversion was achieved over 70%CuꢀSiO2ꢀPG calcined at 823 K
and reduced at 573 K (the most selective catalyst showed in Table 2)
8 d
O
O
CH3 CH3
a
Reaction conditions: 40 g 10 wt% substrate in THF solution, 0.32 g
b
Cu, 6.0 MPa H2, 433 K, 10 h. The data in parenthesis was the
c
d
selectivity to glycerol; 15 h; 20 h, the selectivity to methanol
under relatively mild conditions (at 433 K, 6 MPa H2) in THF for 10 after deducting the contribution of two methoxyl groups.
h. Noticeably, even in the absence of any solvent, pure EC can be
efficiently hydrogenated with >99% conversion and 98 and 97%
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into the autoclave together with fresh reactant. The yields of
methanol maintained steady at 95ꢀ97% with ~100% conversion in
the first 6 runs (Figure 6A). No significant decline of methanol
selectivity was observed for the catalyst at ~50% conversion in 6
recycles (Table S4 in SI). These findings show the good stability of
the catalyst during EC hydrogenation, which could be ascribed to the
inclusion of SiO2 nanoparticles between Cu nanoparticles 27 and the
presence of stable Cu+ 19,26,31. Thereafter, a continuous decline of
methanol yield due to the gradual agglomeration of Cu nanoparticles
was found. XRD characterization showed that the Cu0 crystallite size
of the catalyst after 10 cycles significantly sintered to 19.2 nm from
8.0 nm of the fresh catalyst (Figure 7). This finding was also verified
by TEM images of the used catalyst, as the 70%CuꢀSiO2ꢀPG catalyst
after 10 cycles showed the presence of many larger Cu particles with
20ꢀ30 nm in diameter (Figure 8B), whereas uniformly dispersed Cu
particles with diameters of 6ꢀ10 nm were seen for the freshly
reduced catalyst (Figure 8A).
In order to alleviate the sintering of Cu particles, 70%CuꢀSiO2ꢀPG
was doped with B2O3 (1 wt%), as reported previously.35,40,41 The
incorporation of B2O3 greatly enhanced the stability of the catalyst
without obvious influence on the acidꢀbasicity (Table S1 in SI) as
well as the of catalytic activity of the catalyst. As shown in Figure
6A, the methanol yields remained essentially unchanged (over 95%)
during recycling experiments of up to 10 cycles, in contrast to the
undoped catalyst. XRD and TEM characterizations also confirmed
the high structure stability of the B2O3ꢀdopped CuꢀSiO2 catalyst, as
only slight increase of Cu0 crystallite size to 9.5 nm (Figure 7B) and
no obvious agglomeration of Cu was seen for this catalyst after 10
cycles (Figure 8C). By using a fixedꢀbed flow reactor, we reported
the first example that methanol and EG, the two bulk chemicals, can
be continuously produced by the efficient hydrogenation of EC
under milder conditions of 433 K and 3 MPa H2. No appreciable
decrease of methanol and EG yields (both >95%) was observed after
80 h of time onꢀstream (Figure 6B). Further research is undergoing
to concretely study the effect of B2O3 on CuꢀSiO2 in carbonate
hydrogenation.
(A)
70%CuꢀSiO2ꢀPG
70%CuꢀSiO2ꢀPGꢀB
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
Number of cycles
100
80
60
40
20
0
(B)
Methanol
Ethylene glycol
10
20
30
40
50
60
70
80
Time (h)
Figure 6. The recycling use of 70%Cu-SiO2-PG and 70%Cu-SiO2-PG-B
(both calcined at 823 K and reduced at 573 K) in batch reactor (A)
and time on-stream reaction of 70%Cu-SiO2-PG-B in a fixed bed
reactor (B). Reaction conditions for batch reaction: 40 g 10 wt% EC
in THF solution, 0.32 g Cu, 6 MPa H2, 433 K, 10 h; for fixed bed
reaction: WHSV of 0.20 h-1, hydrogen to EC mol ratio 60:1, 3 MPa
H2, 433 K.
▽
Cu0
▽
crystallite
size (nm)
▽
▽
(C)
19.2
(B)
9.5
(A)
8.0
30
40
50
60
70
80
2 Theta (degree)
Figure 7. The XRD profiles of (A) freshly reduced 70%Cu-SiO2-PG; (B)
70%Cu-SiO2-PG-B after 10 cycles; (C) 70%Cu-SiO2-PG after 10 cycles.
Both catalysts were calcined at 823 K and reduced at 573 K.
Figure 8. The TEM images of (A) freshly reduced 70%Cu-SiO2-PG; (B)
70%Cu-SiO2-PG after 10 cycles; (C) 70%Cu-SiO2-PG-B after 10 cycles.
Both catalysts were calcined at 823 K and reduced at 573 K.
To investigate the recyclable ability of 70%CuꢀSiO2ꢀPG, the used
catalyst was centrifuged and washed with THF, and then recharged
8 | Green Chem., 2015, 00, 1-3
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ARTICLE
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In summary, we have demonstrated a facile and highly efficient
route for the synthesis of methanol and diols from CO2ꢀderived
organic carbonates using a simple CuꢀSiO2 nanocomposite
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conditions presents promising applications in the sustainable
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The authors gratefully acknowledge the technical assistance
provided by Ms. Li He and Ms. Jiamei Liu for XRD and XPS
measurements and the financial support from the National Natural
Science Foundation of China (Grant No. 21133011, 21203221,
21473224) and the Natural Science Foundation of Gansu province
(1308RJZA281).
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DOI: 10.1039/C5GC00810G
ARTICLE
Green Chemistry
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Green Chemistry
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DOI: 10.1039/C5GC00810G
Graphical and Textual Abstract
Copper-silica nanocomposite catalysts have been
disclosed to be highly efficient and stable in the
heterogeneous hydrogenation of CO2-derived cyclic
carbonates to methanol with the co-production of diols
under relatively mild conditions.