Direct Synthesis of Hydrogen and Dimethoxylmethane from Methanol on Copper/Silica Catalysts with Optimal Cu+/Cu0 Sites

Article abstract of DOI:10.1002/cctc.201701416

Hydrogen is an important sustainable resource, and here we report a catalytic route for the direct production of hydrogen (with a purity of 95 %) and dimethoxylmethane (DMM) from supercritical methanol over a Cu/SiO₂ catalyst prepared by deposition–precipitation with ammonia (DPA) at 240°C in a one-pot process. The procedure starts with methanol dehydrogenation to hydrogen and formaldehyde at the interface of the Cu₂O?SiO₂–Cu⁰ particle mixture, and subsequently, the formaldehyde intermediate condenses with substantial methanol to form DMM in the liquid phase. The Cu⁰, CuO, and chrysocolla species are inactive for hydrogen generation from methanol, and the intrinsic active species for methanol decomposition is the Cu₂O?SiO₂–Cu⁰ nanoparticle interface, which is produced from the hydrogen reduction of Cu₂Si₂O₅(OH)₂ or from the methanol reduction of Cu?O?Si moieties. A correlation between the structure and activity on reduced Cu/SiO₂ (DPA) suggested that only Cu⁰ was not active, but the combined Cu⁰ and Cu⁺ sites with interfaces on SiO₂ with an optimal Cu⁺/Cu⁰ ratio of 1.56 were highly active for methanol dehydrogenation and subsequent condensation steps. The developed new catalytic system offers a facile and atom-economical way to generate pure hydrogen (almost CO free) from liquid methanol that can be used in fuel cell and hydrogen-involved biomass reactions.

Full text of DOI:10.1002/cctc.201701416

Accepted Article
Title: Direct Synthesis of Hydrogen and Dimethoxylmethane from
Methanol on Cu/SiO2 Catalysts with Optimal Cu+/Cu0 Sites
Authors: Chen Zhao, Liubi Wu, and Bolong Li
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To be cited as: ChemCatChem 10.1002/cctc.201701416
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ChemCatChem
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FULL PAPER
Direct Synthesis of Hydrogen and Dimethoxylmethane from
+
0
Methanol on Cu/SiO Catalysts with Optimal Cu /Cu Sites
2
a
a
a
Liubi Wu, Bolong Li and Chen Zhao*
Abstract: Hydrogen is an important sustainable resource, and here
we report a catalytic route for direct production of hydrogen (purity
attains 95%) and dimethoxylmethane (DMM) from supercritical
is also enhanced, but the corresponding selectivity is decreased
[
12]
. Moreover, the addition of Cr causes serious environmental
pollution.
Concerning on methanol decomposition to hydrogen and
carbon oxide, it is proposed that the active species can be the
2
methanol over Cu/SiO catalyst prepared by deposition precipitation
with ammonia (DPA) at 240⁰C in a one-pot process. The procedure
0
+
2+
starts with methanol dehydrogenation to hydrogen and formaldehyde
copper species of Cu , or Cu , or Cu , or with Cu species
0
[14]
over the interface of Cu
2
O·SiO
2
and Cu particle mixture, and
combinations in methanol decomposition
. But an agreement
subsequently the formaldehyde intermediate condenses with
substantial methanol to form DMM in the liquid phase. The species
concerning on the true Cu active species into the process has
[
15]
not reached. For example, Domokosa et al.
pointed out that
0
of Cu , CuO, or chrysocolla species are shown to be inactive for
CuO prepared by melt-quenching method was the active
species in the reaction of dehydrogenation of methanol to methyl
formate, which was proven by in-situ spectroscopic
hydrogen generation from methanol, and the intrinsic active species
0
for methanol decomposition is the Cu
2
O·SiO
2
/ Cu nanoparticles with
[
16, 17]
0
interfaces, which is produced from hydrogen-reduction of
Cu Si (OH) or from methanol reduction of Cu-O-Si moieties. A
correlation between structure-activity on reduced Cu/SiO (DPA)
characterization. Guerreiro et al.
species as the active species in dehydrogenation of methanol to
methyl formate with Cu/SiO catalyst prepared by the ion-
exchanged method. Tsoncheva et al.
were three kinds of surface Cu species onto Cu/activated
recognized the Cu
2
O
2 5
2
2
2
0
0
[18]
suggested that sole Cu was not active, but the combined Cu and
reported that there
+
+
0
Cu sites with interfaces on SiO
2
with an optimal Cu /Cu ratio of
2
+
1.56 were highly active for methanol dehydrogenation and
carbon: 1) Cu ions only linked with diverse surface functional
2
+
subsequent condensation steps. The developed new catalytic
system offers a facile and atom-economic way for generating pure
hydrogen (almost carbon oxide free) from liquid methanol, which can
be feasibly used into fuel cell and hydrogen involved biomass
reactions.
groups of activated carbon; 2) Cu ions both linked with various
surface functional groups of activated carbon and with
themselves; 3) particles of CuO. These three species can be
active species for methanol dehydrogenation. Later, Tsoncheva
[
19]
0
2+
et. al
methanol decomposition over the uniform and well dispersed Cu
particles loaded on mesoporous SiO support at high
revealed that the addition of
were active species for hydrogen
reported that Cu and Cu were both active species of
2
[
20]
temperatures. Fisher and Bell
metallic Cu on ZrO /SiO
Introduction
2
2
generation from methanol under mild condition.
It should be addressed that methanol is decomposed to H
and CO at relative high temperatures, and the molar ratio of H
Hydrogen is a green and environmentally friendly resource, and
the increasing production of methanol from cheap natural gas
2
2
and/or coal reserves makes it potential as a source of clean H
2
and CO in gaseous products follows the stoichiometric value of
the reaction (H : CO = 2). Therein, a separation procedure
via decomposition if carbon oxides can be eliminated.
Supported copper catalysts have been extensively studied for
hydrogen generation from methanol decomposition. For
example, CuO/ZnO catalysts are found to be effective for
methanol decomposition, as well as for reverse synthesis of
2
should be conducted on the gas mixture to obtained pure
hydrogen. Here, we report a catalytic process for production of
hydrogen from methanol with a high hydrogen purity (exceeds
9
5%) over Cu/SiO
ammonia (DPA), and in the liquid phase dimethoxylmethane
DMM) is also obtained as the exclusive product from methanol
2
prepared by deposition precipitation with
[1-6]
methanol from syn-gas
. CuO/ZnO catalysts are active in
initial activity tests of methanol decomposition, but their lifetimes
are quite short when ZnO is reduced to Zn and then
incorporated into the Cu lattice to form Cu-Zn alloy during the
(
condensation with the formaldehyde intermediate. Another
address is paid for exploring and comparing the active species
on three Cu/SiO catalysts synthesized by different methods for
2
[
7-11]
working reaction
. Later the additives of Ba, Mn or Si into
CuO/ZnO catalysts remarkably improve the activities of
methanol decomposition.
[
7, 12]
methanol decomposition
. When Ni is added into CuO/ZnO
catalyst, the activity and stability of the catalyst are highly
improved since Ni can distinctly impede the formation of Cu-Zn
alloy . The stability over Cu-Cr catalysts without Zn additives
Results and Discussion
[
13]
Catalytic performances of methanol decomposition over
2
three Cu/SiO samples
[
a]
L. Wu, B. Li, Prof. C. Zhao.
Shanghai Key Laboratory of Green Chemistry and Chemical Pro-
cesses, School of Chemistry and Molecular Engineering
East China Normal University
North Zhongshan Road 3663, Shanghai, 200062, China
E-mail: czhao@chem.ecnu.edu.cn (C. Zhao)
In the present work, three synthesis methods were used to
synthesize Cu/SiO catalysts, i.e., deposition-precipitation with
ammonia (Cu/SiO -DPA) and with urea (Cu/SiO -DPU), together
with the impregnation approach (Cu/SiO -IM), for catalyzing
methanol decomposition. To compare the activities of methanol
2
2
2
2
Supporting information for this article is given via a link at the end of
the document.
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FULL PAPER
a
2
Table 1. The catalytic performances of three Cu/SiO catalysts used in methanol dehydrogenation.
b
c
Catalyst
Gas content (%)
H/Cgas phase
r
initial
r
average
H
2
CO
14.4
9.2
CH
0.1
1.1
0.5
0
4
CO
1.3
1.0
1.7
4.0
1.9
1.0
2
(mol/mol)
(MPa/h)
(MPa/h)
Calcined Cu/SiO
2
-DPA
84.2
88.8
88.5
80.0
94.2
94.8
10.6
15.8
15.4
8.0
2.8
2.8
1.0
1.0
0.2
0.8
1.15
0.85
0.05
0.13
0.08
0.15
Reduced Cu/SiO
2
-DPA
Calcined Cu/SiO
2
-DPU
9.2
Reduced Cu/SiO
2
-DPU
16.0
3.8
Calcined Cu/SiO
2
-IM
0.1
0.4
32.1
36.1
Reduced Cu/SiO
2
-IM
3.9
a
2 2
Reaction conditions: methanol (40 mL), Cu/SiO (0.3 g), 240 ˚C, N (2 MPa, ambient temperature), 4 h, stirring at 400 rpm.
b
c
The initial rates were recorded within 0.5 h at 240 ˚C.
The average rates were recorded within 4.0 h at 240 ˚C.
a.
b.
c.
14
Cu/SiO - DPA
decomposition, three Cu/SiO
2
catalysts (after air-calcination or
2
1
3
hydrogen-reduction treatments) were then tested. In the blank
test without catalyst, the vapor pressure of methanol at 240 ˚C
12
reached 9.5 MPa (with 2 MPa N
During reactions, the variations for pressures (including
methanol, N , and in situ produced gases) were recorded using
three Cu/SiO catalysts as a function of time, as displayed in
Figs. 1a-1c. The highest pressure (13.6 MPa) during methanol
decomposition was achieved on calcined Cu/SiO -DPA after 4 h
Fig. 1d). Sequentially, the slightly lower pressures (around 13.0
2
at ambient temperature).
1
1
1
0
9
2
calcined
reduced
2
0
1
2
3
4
Time (h)
2
14
Cu/SiO - DPU
calcined
reduced
2
(
1
1
3
2
2
MPa) were attained with reduced Cu/SiO -DPA after 4 h,
respectively. The initial rates within 0.5 h for gas production
exceled 2.5 MPa/h over calcined and reduced Cu/SiO
2
-DPA
11
(
Table 1). If comparing the average rates for methanol
1
0
9
dehydrogenation in 4 h, such rates on calcined Cu/SiO
2
-DPA
(
1.15 MPa/h) and on reduced Cu/SiO
samples were also much higher than those on Cu/SiO
Cu/SiO -DPU samples (0.05-0.15 MPa/h) at 240 ˚C (see Table
).
The pressures after reactions were also recorded at ambient
temperatures over different Cu/SiO samples (Fig. 1d). Calcined
and reduced Cu/SiO -DPA produced about additional 2.6 and
.1 MPa gases after reaction at the ambient temperature,
respectively. However, calcined and reduced Cu/SiO -DPU and
Cu/SiO -IM only generated around 0.1-0.5 MPa gas. The order
for the rates and amounts of generated gases was listed as
2
-DPA (0.85 MPa/h)
0
1
2
3
4
2
-IM and
Time (h)
14
2
Cu/SiO - IM
calcined
reduced
2
1
1
1
1
1
3
2
1
0
9
2
2
2
2
2
0
1
2
3
4
Time (h)
follows: calcined Cu/SiO
reduced Cu/SiO -IM
Cu/SiO -DPU ≈ calcined Cu/SiO
The produced gases from methanol decomposition included a
mixture of H , CO, CH , and CO as detected by GC (Fig. S1).
Shown from Table 1, it can be seen that highly-pure hydrogen
above 80% content) was produced on these Cu/SiO catalysts.
The gases products included H (nearly 95%) together with 3.8%
CO, 0.1% CH , and 1.9% CO , were obtained with Cu/SiO -IM,
and reduced Cu/SiO -DPA also produced hydrogen with a purity
2
-DPA > reduced Cu/SiO
calcined Cu/SiO -DPU
-IM.
2
-DPA >>
reduced
16
14 13.6
Reaction pressure at 240 C
Pressure after reaction at r.t.
d.
2
≈
2
≈
1
2.9
1
2
0
8
6
4
2
0
2
2
1
0.0 10.1 10.1
1
9.7
vapor pressure of methanol
at 240 C with 2 MPa N
2
2
4
2
4.6
4.1
(
2
2.5
2.5
2.1
2.5
plunge N2 at room temperature
2
-
IM
-
IM
4
2
2
2- DPA
2- DPA
-
DPU
- DPU
2
2
2
2
Cu/SiO
Cu/SiO
2
Cu/SiO
Cu/SiO
Cu/SiO
Cu/SiO
calcined
reduced
of 89%. It was noteworthy that molar ratio of H/C (8.0 - 36) in
gaseous products (Table 1) highly exceled the theoretical value
calcined
reduced
calcined
reduced
Figure 1. The recorded pressure changes (a, b, c) during methanol reaction
over diverse Cu/SiO catalysts as a function of time. Reaction conditions:
methanol (40 mL), Cu/SiO (0.3 g), 240 ˚C, N (2 MPa, ambient temperature),
h, stirring at 400 rpm. The recorded pressures (d) for methanol
decomposition using different Cu/SiO catalysts. Reaction conditions:
methanol (40 mL), Cu/SiO (0.3 g), 240 ˚C, N (2 MPa, ambient temperature),
h, stirring at 400 rpm.
(
H/C = 4) for direct decomposition to 2 mol H
2
and 1 mol CO. It
2
is indicated that methanol dehydrogenation occurs in a different
2
2
way compared to generation of CO and H
discussed in the later section.
2
, which will be
4
2
2
2
4
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It can be summarized that calcined and reduced Cu/SiO
2
-
05-0667] due to incomplete reduction. With urea as precipitant
DPA show the superior hydrogen generation with rates of 0.75 –
.15 MPa/h. By contrast, the other Cu/SiO -IM and Cu/SiO
dried Cu/SiO -DPU sample exhibited characteristic diffraction of
3
Cu (OH) NO [JCPDS: 15-0014], subsequently it was changed
2
1
2
2
-
2
3
DPU samples are almost inactive (average rates: 0.05 – 0.15
MPa/h). Hydrogen with high purity (> 80%) was produced from
into CuO particles after calcination by air, and then turned into
Cu particles after reduction by hydrogen. The XRD patterns of
0
methanol decomposition by these Cu/SiO
values of H/C in gaseous phase were 8.0-36, highly excelling
2
catalysts, while the
calcined and reduced Cu/SiO
were quite similar. It can be apparently concluded that the
dominant component of calcined Cu/SiO samples is CuO, and
after reduction, the main specie in Cu/SiO -IM and Cu/SiO -DPU
is metallic Cu, while in the reduced Cu/SiO -DPA, the main
2 2
-IM and Cu/SiO -DPU samples
the theoretical values of
decomposition to CO and
4
towards direct methanol
In order to explain these
2
H
2
.
2
2
observations and find the intrinsic Cu active species for
methanol decomposition, we performed an extensive and
2
0
species are Cu /Cu
2
O·SiO
2
.
intensive catalyst characterization on these Cu/SiO
subsequently.
2
samples

Cu
(b)
(111)
Cu
 CuO
(
a) Cu/SiO -DPA
Cu/SiO -IM

2
(
111)
2
CuO


Cu O
2
(200)

(
111)
(200)
Comparative catalyst characterization of three Cu/SiO
samples
2


reduced
reduced
calcined
(220)

(
11-1) (111)




(
11-3)
Table 2. The data of physic-chemical properties of SiO
2
and three Cu/SiO
2
calcined (
110)
(20-2) (020)



(31-1)




catalysts




(
202) (220)
Catalyst
Cu
2
-1
3
-1
Pore
S
BET (m ∙g )
Vpore (cm ∙g )
1
0
20
2
30
40
50
60
70
10
20
30
40
50 60 70
2 Theta (degree)
loading
Diameter
(nm)
Theta (degree)
(
-
wt.%)
Micro
0
Meso
211
Micro
0
Meso
0.85
(111)
(
c) Cu/SiO -DP

2
Cu
CuO
SiO
Calcined
Cu/SiO -DPA
Reduced
Cu/SiO -DPA
Calcined
Cu/SiO -DPU
Reduced
Cu/SiO -DPU
Calcined
Cu/SiO -IM
Reduced
Cu/SiO -IM
2
38

(
200)

reduced
2
55
55
51
51
48
48
64
14
15
23
18
19
207
47
89
74
78
74
0.03
0.01
0.01
0.02
0.01
0.01
0.30
0.22
0.51
0.43
0.45
0.40
5.0
15
20
18
19
18
(11-1)
(111)


(
20-2) (11-3)
calcined (110)
2
(
11-2) (020) (220)



202)(31-1)


(
2
1
0
20 30 40 50 60 70
Theta (degree)
2
Figure 2. XRD patterns of three Cu/SiO
2
catalysts with different states. (a)
-DPU.
2
Cu/SiO -DPA, (b) Cu/SiO -IM, and (c) Cu/SiO
2
2
2
2
2
Particle sizes of Cu in calcined and reduced Cu/SiO catalysts
were calculated based on XRD patterns by Scherrer formula, as
2
shown in Table S1. Generally, the particle sizes of CuO and
0
metallic Cu particles in calcined and reduced Cu/SiO
2
-DPA
The Cu loadings of Cu/SiO
2
-DPA, Cu/SiO
2
-DPU, and Cu/SiO
2
-
samples were much smaller than corresponding Cu/SiO
samples prepared by IM and DPU methods. In the calcined
Cu/SiO -DPA, particle sizes of CuO (111) and Cu O (111) were
shown to be ca. 11 and 11 nm, and that of Cu (111) was around
6 nm in reduced Cu/SiO -DPA sample. In comparison, the
particle sizes of CuO (111) of calcined Cu/SiO -DPU and
Cu/SiO -IM were 27 and 25 nm, respectively, and those of
Cu(111) of reduced Cu/SiO -DPU and Cu/SiO -IM were 38 and
6 nm. So it is inferred that the quite small particle sizes of Cu
oxides and metallic Cu are obtained on the calcined and
reduced Cu/SiO -DPA samples.
TEM image of the calcined Cu/SiO
2
IM samples were comparable at 55%, 51%, and 48%,
respectively, as determined from ICP measurements.
nanosized SiO (around 20 nm) was adopted as the support.
The N sorption results (Fig. S2 and Table 2) showed that
micropore and mesopore surface areas of calcined Cu/SiO
A
2
2
2
2
1
2
2
-
2
-1
DPA were 64 and 207 m ∙g , respectively, which were much
2
2
-1
larger than those of Cu/SiO
Cu/SiO -IM (18/19 and 78/74 m ∙g ). On the contrary, the pore
diameter of calcined Cu/SiO -DPA was 5.0 nm, while other two
Cu/SiO catalysts had the average pore diameters at 15-20 nm.
The SiO support showed a H1-type hysteresis loop (Figure
S2d), implying its uniform sphere structure. In comparison, a H3-
type hysteresis loop (Figure S2a) on calcined Cu/SiO -DPA
2
-DPU (15/23 and 89/74 m ∙g ) and
2
2
-1
2
2
2
3
2
2
2
2
2
-DPA sample (Fig. S3a), it
was clearly observed that there were substantial lamellar copper
phyllosilicate, as well as some small well-dispersed CuO
particles. After reduction, the copper phyllosilicate structure
2
suggested a specific lamer structure which may be formed
during the DPA synthesis..
Fig. 2 displayed XRD patterns of dried, calcined, and reduced
0
disappeared and CuO particles were turned to be Cu particles.
By contrast, calcined and reduced Cu/SiO
2 2
-DPU and Cu/SiO -IM
three Cu/SiO
precipitate the pattern of dried Cu/SiO
calcined Cu/SiO -DPA, with CuO particles [JCPDS: 48-1548]
being the main species. After hydrogen reduction, the main
2
catalysts. As shown in Fig. 2a, using ammonia as
2
-DPA was same as
did not show copper phyllosilicate structure, and the reduced
0
Cu particle sizes on Cu/SiO
larger than Cu/SiO -DPA (6.8 ± 2.2 nm) and Cu/SiO
.3 nm) (Fig. 3). The size trend data from TEM images were
2
-IM (16.1 ± 6.0 nm) were much
2
2
2
-DPU (3.5 ±
0
0
2 2
species were Cu [JCPDS: 04-0836] and Cu O·SiO [JCPDS:
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ChemCatChem
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consistent to those results obtained from XRD patterns (Table
S1).
strength vibration of –OH functional groups. In dried and
2
calcined Cu/SiO -DPA, copper phyllosilicate was obviously
-1
observed by the characteristic single δOH band at 667 cm as
normalized to the integrated intensity of the symmetric ν-SiO
0
0
0
0
0
0
0
.30
(a)
(b)
.25 D = 6.8
-1
band of amorphous silica at 802 cm , as designated as the label
.20
.15
.10
.05
.00
[
21]
of I667/I802
.
In addition, such peak disappeared in this reduced
sample due to the disappearance of copper phyllosilicate (as
shown in Fig. S3b). In the Cu/SiO samples prepared by IM and
2
DPU methods, such specific peaks for single δOH band were
absent.
4
5
6
7
8
9
Diameter (nm)
Shown from the The H
reduced three Cu/SiO catalysts are shown at Figs. 4e-4f. With
respect to the calcined samples, the peaks at temperatures of
83 to 290 ˚C were assigned to the reduction peaks of bulk CuO
nanoparticles. In analyzing of calcined Cu/SiO -DPA, the
2
-TPR profiles of the calcined and
0.30
0.25
0.20
0.15
0.10
0.05
0.00
(c)
(d)
2
D = 3.5
2
2
reduction peak of 255 ˚C was attributed to reduction of copper
phyllosilicate, which was consistent to the resutls in TEM and IR.
In comparison, the peak at 269 ˚C of calcined Cu/SiO -IM was
2
2.5
3.0
3.5
4.0
4.5
Diameter (nm)
assigned to reduction of the Cu-O-Si structure.
The Cu XPS and Cu LMM analyses were carried out to
elucidate the chemical states of Cu (Fig. 5 and Table S2). In Fig.
0.30
0.25
0.20
0.15
0.10
0.05
0.00
(
e)
(f)
D = 16.1
5a, three calcined Cu/SiO
2
samples all showed strong peaks at
binding energy (BE) of 935.4 and 942.9 eV, assigned to Cu 2p3/2
2
+ [23]
and 2p→3d satellite (*) Cu . The peaks of kinetic energy of
16.5 eV on calcined Cu/SiO -DPU and Cu/SiO -IM in Cu LMM
9
2
2
2
+
spectrum (Fig. 5b) were also attributed to Cu , in line with the
results obtained from Cu XPS. Meanwhile, the left-shift peak of
1
1 12 13 14 15 16 17 18 19 20 21 22 23
Diameter (nm)
Figure 3. TEM images and particle size distributions of (a) and (b) reduced
calcined Cu/SiO
probably due to the presence of copper (Cu ) phyllosilicate, and
by contrast the main species on Cu/SiO -IM and Cu/SiO -DPU
were CuO nanoparticles. After hydrogen-reduction, the intensity
2
-DPA (915.8 eV) compared to 916.5 eV,
2
+
Cu/SiO
2 2 2
-DPA, (c) and (d) reduced Cu/SiO -DPU, (e) and (f) reduced Cu/SiO -
IM samples.
2
2
4
00
00
2+
Cu/SiO -DPA
2
2
of Cu satellite peaks in XPS spectra of reduced Cu/SiO were
(
a) Cu/SiO - DPA
(d)
2
2
83 C
2+
+
3
strongly reduced since the reduction processes of Cu to Cu or
SiO

256 C
2+
0
2+
reduced
200
Cu to Cu . Shown from Fig. 5c, no Cu satellite peaks (BE:
42.8 eV) was observed in reduced Cu/SiO -DPU and Cu/SiO
9
2
2
-
100
2
+
calcined
IM samples, indicating Cu species were completely reduced to
0
0
Cu nanoparticles in these two samples. The low intensity at the
4
000
3000
2000
11000
0
100 200 300 400 500 600
-
2
peak of 942.8 eV (Fig. 5c) on Cu/SiO -DPA (Fig. 5c) suggested
Wavenumber (cm )
Temperature (C)
4
3
2
1
00
00
00
00
0
that such sample was not totally reduced, probably due to the
presence of copper (Cu ) phyllosilicate residue. In addition, the
Cu/SiO
89 C
2
-DPU
(
b) Cu/SiO - DPU
2
(e)
2+
2
peak at BE of around 932.5 eV of XPS spectra may be attributed
reduced
calcined
0
+
to Cu or Cu species.
3
64 C
Cu 2p
35.4
(b)
(d)
calcined samples
Cu LMM
(
a)
calcined samples
9
15.8
942.9*
9
Cu/SiO -DPA
2
Cu/SiO -DPA
4
000
3000
2000
1000
0
100 200 300 400 500 600
Temperature/C
2
-
1
Wavenumber (cm )
916.5
Cu/SiO
2
-IM
Cu/SiO -DPU
Cu/SiO -DPU
2
4
00
00
00
00
0
2
(
c) Cu/SiO - IM
2
(f)
3
2
1
269 C
290 C
Cu/SiO -IM
2
Cu/SiO -IM
2
reduced
calcined
960
950
940
930
9
05
910
Kinetic Energy(eV)
915
920
925
Binding Energy (eV)
(c)
reduced samples
reduced samples
Cu LMM
Cu 2p
32.5
9
-100
Cu/SiO
2
-DPA
Cu O
2
Cu/SiO -DPA
942.8
4
000
3000
Wavenumber (cm )
Figure 4. IR spectra of (a) Cu/SiO
2000
1000
0
100 200 300 400 500 600
Temperature/C
2
-
1
0
Cu
2
-DPA, (b) Cu/SiO
2
-DPU, (c) Cu/SiO
2
-IM.
Cu/SiO -DPU
Cu/SiO -DPU
2
2
TPR-H profiles of (d) Cu/SiO -DPA, (e) Cu/SiO
2
2
2
-DPU, and (f) Cu/SiO
2
-IM.
0
Cu
Cu/SiO -IM
Cu/SiO -IM
2
2
9
60
950
940
930
905
910
915
920
925
From IR spectra of Cu/SiO
be observed that the peak at 3460 cm was attributed to
2
catalysts (see Figs. 4a-4c), it can
Binding Energy (eV)
Kinetic Energy(eV)
-1
Figure 5. Cu XPS (a and c) and XAES (b and d) spectra of three
Cu/SiO catalysts.
2
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+
0
To distinguish the Cu and Cu species on three reduced
Cu/SiO samples, we further conducted the Cu LMM x ray auger
electron spectroscopy (XAES) at kinetic energy range of 905 to
introduced, and part of SiO
2
was dissolved in ammonia solution
2
as well. During the refluxing at 60 ˚C, the species of
Cu Si (OH) , and Cu-O-Si were formed after the interaction of
Cu and dissolved SiO , as detected by the XRD and IR. The
species CuO (black species) was probably originated from
dehydration of Cu(OH) . Copper phyllosilicate still existed in
calcined Cu/SiO (as evidenced by XRD and TEM analysis).
After hydrogen-reduction of Cu/SiO -DPA, Cu Si (OH) was
O•SiO particles (as confirmed
2
2
O
5
2
0
9
25 eV (Fig. 5d). The peaks of 911.6 eV (assigned to Cu ) and
of 915.7 eV (assigned to Cu ) were shown for reduced Cu/SiO -
2
DPA on XAES spectra, with the Cu /Cu ratio of 1.56. In
comparison, only 917.3 eV (assigned to Cu ) was detected on
-DPU and Cu/SiO
Such XAES results are in agreement with the XPS and XRD
analysis.
x
+
+
0
2
0
2
reduced Cu/SiO
2
2
-IM samples (Table S2).
2
2
2
O
5
2
0
turned into a mixture of Cu /Cu
2
2
0
by XRD and XPS analysis), whereas CuO became to be Cu
particles (see Fig. 6).
Exploring the active species of three Cu/SiO
catalyzing methanol dehydrogenation
2
samples for
While in comparison, the colors of aqueous Cu solution kept
blue upon the addition of urea and SiO
2
support (refluxing at 90
˚
C). The color was turned to be black (CuO species alike) after
0
air-calcination and was eventually changed to red (Cu species
alike) after hydrogen-reduction. The information from XRD
Cu/SiO
2
-DPA
Cu/SiO
2
-IM
Cu/SiO
2
-DPU
Cu 2 + +NH
•H
O+SiO
Cu 2 + +SiO2
Stirring at room temperature
Cu 2 +
+CO(NH ) +SiO
3
2
2
2
2
2
patterns indicated that dried Cu/SiO
Cu (OH) NO because the precipitation agent urea contained
OH . Then CuO particles were formed from Cu (OH) NO after
calcination, which was reduced to Cu after hydrogen reduction.
Concerning on the Cu/SiO -IM sample, the colors kept blue
from impregnating aqueous Cu(NO solution into SiO , and
2
-DPU showed species of
Stirring at room temperature
90 ˚C reflux
60 ˚C dried
2
-
3
3
Cu(NH
3
)
42+
Cu (OH) NO
2
3
3
Cu2+
attached on the surface of SiO2
2
3
3
60 ˚C reflux, dried
60 ˚C dried
0
Ion-exchanged Cu-O-Si,
Copper phyllosilicate,
CuO
450 ˚C calcination
2 3 3
Cu (OH) NO
2
CuO
450 ˚C calcination
450 ˚C calcination
)
3
2
2
CuO
Cu
Ion-exchanged Cu-O-Si,
Copper phyllosilicate,
CuO
450 ˚C reduction
they became black after air-calcination and then turned to red
after hydrogen-reduction, as the color variations were similar to
450 ˚C reduction
Cu
Cu
4
50 ˚C reduction
DP procedure with urea. During the impregnation process, some
Cu species were attached on the surface of SiO . After
2
Cu
Cu
2+
2
O
calcination, CuO species was formed. With reduction by
0
Figure 6. Illumination of Cu species variations during three Cu/SiO
2
synthesis
hydrogen, CuO species was completely reduced to Cu . The
process.
changes
of
Cu
species
during
synthesis
and
calcination/reduction-treatments with three preparation methods
are compiled in Fig. 6.
Abundant Cu Si O (OH) structure was observed in Cu/SiO -
2 2 5 2 2
DPA sample. To verify the function of copper phyllosilicate, a
hydrothermal (HT) method was used to prepare copper
3 2 3 2 4 2
phyllosilicate, i.e., mixing Cu(NO ) , NH ·H O, NH Cl, and SiO
together, and then reacted at 120 ˚C for 3 h in Teflon autoclave.
The obtained blue solid as precursor was then evidenced by
XRD, IR, and TEM measurements (Fig. 7, Figs. S5, S6), and the
During the synthesis, it was observed that the colors of
aqueous Cu solution varied, indicating that different Cu species
may be formed within specific periods (see Fig. S4 and Fig. 6).
For example, the original color of aqueous Cu(NO
3
)
2
solution
O was added,
ions may be formed. Then the SiO
(
blue) was changed into bluish violet as NH
3
·H
2
2
+
indicating the Cu(NH
3
)
4
2
carrier was introduced and such slurry kept stirring at 60 ˚C.
After 1 h, the color of solid (white) altered to black (similar to the
color of CuO). It remained black even after sequential air-
calcination and hydrogen-reduction. Based on the color changes
and the former characterization results, it can be inferred that
copper phyllosilicate structures were identified in dried and
0
calcined Cu/SiO
2
2 2
samples. After reduction, Cu O•SiO /Cu
species were observed from hydrothermally prepared Cu/SiO
HT.
2
-
2
4
+
initially Cu(NH
3
)
3 2
ions were generated upon NH ·H O was
Table 3. The catalytic performances of hydrothermally prepared Cu/SiO
2
catalysts (main component: copper phyllosilicate) used in methanol
a
decomposition.
b
Catalysts
Surface Species
Gas content (%)
H/Cgas phase
Increased pressure
at r.t. (MPa)
r
average
H
-
2
CO
-
CH
-
4
CO
-
2
(mol/mol)
(MPa/h)
-
Dried Sample
CuO·SiO
2
·nH
2
O
-
0
Calcined Sample
CuO·SiO
2
·(n-x)H
2
O
-
-
-
-
-
0
-
0
Reduced Sample
Cu /Cu
2
O·SiO
2
87.3
12.2
0.1
0.4
6.8
4.1
5.4
a
Reaction conditions: methanol (40 mL), hydrothermally prepared Cu/SiO
2
(0.3 g), 240 ˚C, N
2
(2 MPa, ambient temperature), 4 h, stirring at 400 rpm.
b
The initial rates for increased gas pressure were recorded within 0.5 h at 240 ˚C.
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
 Cu O
2
(
a)
 Chrysocolla
Cu
(b)

CuO Cu
catalyze rapid methanol decomposition. However, the test with
Cu was shown to be inactive (Table 1), demonstrating that
CuSiO ·(n-x)H O
(
111)
3
2
(
111)
0
reduced
calcined


reduced
calcined
SiO
0
single Cu without Cu
2
O•SiO
Si (OH)
precursors of Cu sites for methanol decomposition. The intrinsic
active species are confirmed to be Cu O•SiO /Cu species
2
did not work in the reaction.
(
132) (023)
(
130)



(360)(362)
OH


In conclusion, Cu
O
5
and Cu-O-Si are both active
2
2
2
dried
1
0
20 30 40 50 60 70
Theta (degree)
2500 2000 1500 1000-
500
2
2
1
2
Wavenumber (cm )
generated from in-situ reduction of Cu-O-Si moieties by
methanol, or from subsequent air-calcination and hydrogen-
reduction of chrysocolla. This also explains the reason why
(
d) reduced sample
(
c) calcined sample
2
nCuSiO ∙ (n-x)H O
3 2
2
calcined and reduced Cu/SiO -DPA samples are both shown to
be quite active for methanol decomposition.
Exploration the reaction steps for decomposition of
methanol to hydrogen and dimethoxy methane
5
0 nm
Figure 7. Characterizations of Cu/SiO
2
-HT catalyst: (a) XRD patterns; (b) FT-
IR spectra; (c, d) TEM images of calcined and reduced samples.
2
Catalyzed by these Cu/SiO samples in the selected catalytic
system, the produced hydrogen from methanol showed a high
purity (> 80%) (Table 4). The obtained H/C ratios (Table 4) are
much higher than the theoretical values (H/C=4), indicating that
some carbon elements are transferred into their corresponding
products in liquid phase. The liquid products were then collected
and analyzed by GC coupled with MS, as shown in Fig. S8.
Highly selective dimethoxy methane (DMM, close to 100%
selectivity) and trace amounts of formaldehyde were detected,
1
1.0
calcined Cu/SiO -HT
2
1
1
0.5
0.0
dried Cu/SiO -HT
2
9
.5
.0
.5
.0
and the gained data over calcined and reduced Cu/SiO
samples prepared by three methods were compiled in Table 4.
In the former report, HCOOCH was frequently observed over
, but here in our case, DMM rather than
was capture in the liquid phase. DMM, also called
2
9
8
8
3
[17]
Cu catalysts
HCOOCH
3
methylal, is a kind of important chemical materials applied as
solvent in cosmetics, pharmaceuticals, households, rubber
industry and paints, etc. Another application is used as gasoline
0
50
100
Time (min.)
150
200
and diesel additives to greatly improve combustion
Figure 8. The recorded pressure changes for methanol decomposition over
Cu/SiO prepared by the hydrothermal method after drying and calcination.
[22]
characteristics.
The presence of HCHO indicated that CH
and HCHO over Cu O·SiO
and Cu (Equation 1), and then the formed HCHO condensed
with substantial CH OH to form DMM (Equation 2). As a minor
part, CH OH was also decomposed to 2 mole H and 1 mole CO
over Cu/SiO samples in the gas phase (Equation 3), and then
CO reacted with H to form CH and H O with such catalytic
system (Equation 4). This explains the reasons for the high
purity of H in the gas phase from methanol decomposition, and
3
OH
2
was majorly dehydrogenated to H
2
2
2
Reaction conditions: methanol (40 mL), catalysts (0.2 g), 240 °C, N
stirring at 400 rpm.
2
(2.0 MPa),
0
3
3
2
The activity tests showed that the Cu/SiO
2
–HT after
2
calcination and reduction was very active with an initial rate for
2
4
2
-1
producing hydrogen excelling 5.4 MPaH2·h , while copper
phyllosilicate treated by drying (Cu Si (OH) ) or calcination
Cu Si (OH) ) or direct hydrogen-reduction (Cu ) conducted
poor catalytic performances (with rates lower than 1.0 MPaH2·h )
Table 3, Fig. 8). These experiments combined with the catalyst
2
2
O
5
2
2
0
(
2
2
O
5
2
in the liquid phase the high selectivity of DMM from methanol.
-1
(
Table 4. Liquid phase product distributions from methanol
0
characterization clearly indicated that single centers of Cu after
a
2
dehydrogenation with diverse Cu/SiO catalysts.
2
+
reduction, Cu after calcination, or chrysocolla precursor itself
were not active. The true active species was therefore
reasonably attributed to the uniform dispersed small
Catalysts
Compound selectivity in the liquid phase (%)
HCHO
DMM
Calcined Cu/SiO
2
-DPA
99.8
99.6
99.1
98.9
99.5
99.2
0.2
0.4
0.9
1.1
0.5
0.8
Cu
hydrogen-reduction of chrysocolla.
2
Apart from reduced Cu/SiO -DPA, calcined Cu/SiO -DPA also
2 2
O•SiO /Cu species mixture after air-calcination and
Reduced Cu/SiO
2
-DPA
Calcined Cu/SiO
Reduced Cu/SiO
2
-DPU
2
2
-DPU
showed a high activity in methanol decomposition (Fig. 1). In
principle, the Cu-O-Si moieties can be gradually reduced by
Calcined Cu/SiO
2
-IM
2
Reduced Cu/SiO -IM
methanol to a mixture of Cu
in-situ formed highly-dispersed Cu
which were proved by the XRD patterns of calcined Cu/SiO
DPA after reaction with methanol (Fig. S7), are attributed to
2
O•SiO
2
and Cu during reaction. The
a
Reaction conditions: methanol (40 mL), Cu/SiO
at 400 rpm.
2
(0.3 g), 240 ˚C, 4 h, stirring
2
O•SiO and Cu nanoparticles,
2
2
-
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The reaction network may include the following steps:
Conclusions
An atom-efficient method for direct production of hydrogen and
dimethoxylmethane (DMM) from methanol is reported over
0
Cu/SiO
2
catalyst with Cu
2
O·SiO
2
/Cu species in a one-pot
process. The overall reaction starts with initial methanol
dehydrogenation to hydrogen and formaldehyde over
0
Cu
2
O·SiO
2
/Cu particles, and subsequently, formaldehyde reacts
with abundant methanol to form DMM in the liquid phase. In the
0
Furthermore, we tested the catalytic performances of reduced
2
gas phase, the purity of H can attain 95%. The species of Cu ,
Cu/SiO
2
-DPA sample at temperature ranges of 180 to 240 ⁰C.
CuO, or chrysocolla species are shown to be inactive for
hydrogen generation from methanol, and the intrinsic active
As shown in Fig. 9, the catalyst was shown to be inactive at 180
and 200 ⁰C. When the temperature was increased to 220 ⁰C, the
increased pressure attained 0.8 MPa at ambient temperature
with the hydrogen content of 85%. At the temperature of 240 ˚C,
species for methanol decomposition is the adjacent species of
0
Cu
2
O·SiO
2
and Cu with interfaces, which is produced from
hydrogen-reduction of Cu
2
Si
2
O
5
(OH)
2
or from methanol
2
.1 MPa gas with 88.4% H
2
was generated. The presence of
reduction of Cu-O-Si moieties. This developed new approach
directly converts of liquid methanol to DMM and hydrogen, and
the fast formed hydrogen (almost carbon oxides free) can be
wildly applied into fuel cell and hydrogen-involved biomass
reactions.
supercritical methanol (critical point: 239 ˚C) leads to a co-
existence of gas and liquid, and thus, it is beneficial for
interaction and accessibility of methanol molecules towards the
Cu active species.
Therefore, the mechanism occurred in the selected catalytic
liquid phase can be drawn, that is, methanol is selectively
0
dehydrogenated to HCHO and H
2
over Cu
2
O·SiO
2
/Cu species
Experimental Section
at 240 ⁰C as the major route, and then HCHO reacts with
substantial CH OH in the liquid phase to form DMM. The
conventional route of methanol decomposition (1 mol) for
producing 2 mol H and 1 mol CO only occurs as the minor
pathway. Here the developed new route on dehydrogenation of
methanol for producing H and HCHO as major products leads
to the H/C ratio much higher than expected molar ratio (4/1) in
the gas phase. It should be addressed that such reaction on
methanol for directly forming hydrogen and DMM is highly atom-
economic, almost fully incorporating the elements of C, H, and O
into products.
3
Preparation of Cu catalyst by deposition-precipitation using
ammonia as precipitant (Cu/SiO
A typical example for synthesis of Cu/SiO
Firstly, Cu(NO ) ∙3H O (8.47 g) was dissolved in deionized water (350
2
-DPA)
2
2
-DPA was described here.
3
2
2
mL). The pH value of the copper solution was adjusted to 9 by ammonia.
After that, the mixture with SiO (2.0 g) was heated up to 60 °C and kept
2
2
reflux condensation. After 10 h, the suspension was cooled to ambient
temperature, followed by filtration and by washing with distilled water
several times. Then it was dried at 60 °C overnight. Afterwards, the solid
was dried at 60 °C overnight, calcined in flowing air at 450 °C for 4 h
-1
-1
(
heating rate: 2 °C∙min , flowing velocity:100 mL∙min ) and reduced in
-
1
flowing hydrogen at 450 °C for 4 h (heating rate: 2 °C∙min , flowing
-
1
2.5
2.0
1.5
1.0
0.5
0.0
100
80
60
40
20
0
velocity:100 mL∙min ).
Preparation of Cu catalyst by deposition-precipitation using urea as
precipitant (Cu/SiO
A typical catalyst Cu/SiO
solution of Cu(NO ∙3H
into two parts. 200 mL was suspended with 2 g SiO
heated to 70 °C. The rest (50 mL) was dissolved with urea (6.3 g)
solution B), then drop-wise added into the former suspension. Next, the
2
-DPU)
-DPU was synthesized as follows. An aqueous
O (0.10 M, 350 mL) was prepared and divided
(solution A) and
2
3
)
2
2
2
(
mixture was heated up to 90 °C and kept reflux condensation for 10 h.
After cooling down to ambient temperature, the catalyst precursor was
filtered and washed by distilled water several times. Finally, the solid was
dried at 60 °C overnight, calcined in flowing air at 450 °C for 4 h (heating
1
80
200
220
240
-1
-1
rate: 2 °C∙min , flowing velocity:100 mL∙min ) and reduced in flowing
-
1
Reaction Temperature (C)
hydrogen at 450 °C for 4 h (heating rate: 2 °C∙min , flowing velocity:100
-
1
mL∙min ).
Figure 9. The recorded increased pressure at ambient temperature and
hydrogen content on methanol dehydrogenation over reduced Cu/SiO
DPA sample at reaction temperatures from 180 to 240 C. Reaction
(2 MPa, ambient
2
-
2
Preparation of Cu catalysts by impregnation method (Cu/SiO -IM)
⁰
A typical Cu/SiO
Cu(NO ∙3H O (3.80 g) was dissolved in distilled water (10 mL). Then
the copper precursor solutions was well-dropped onto SiO (2.0 g), which
2
-IM was synthesized by impregnation method. Firstly,
conditions: methanol (40 mL), catalyst (0.3 g), N
temperature), 4 h, stirring at 400 rpm.
2
3
)
2
2
2
kept magnetic stirring at ambient temperature to dry out the water. Finally,
the solid was dried at 60 °C overnight, calcined in flowing air at 450 °C
-
1
-1
for 4 h (heating rate: 2 °C∙min , flowing velocity:100 mL∙min ) and
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-
1
reduced in flowing hydrogen at 450 °C for 4 h (heating rate: 2 °C∙min
flowing velocity:100 mL∙min ).
,
Acknowledgements
-
1
We acknowledge the financial support from the National Key
Research and Development Program of China (Grant No.
Preparation of copper phyllosilicate by hydrothermal synthesis
Cu/SiO -HT)
A typical Cu/SiO -HS was prepared as follows. Deionized water (140 mL)
was added with Cu(NO ∙3H O, NH Cl, NH ∙H O and a nano sized SiO
2
(
2
2016YFB0701100), the Recruitment Program of Global Young
2
Experts in China, and National Natural Science Foundation of
China (Grant No. 21573075).
3
)
2
2
4
3
2
in order. The solution was stirred for half an hour at ambient temperature,
then dispersed with ultrasonic for half an hour. Afterwards, the mixture
was transferred to a Teflon autoclave (200 mL) and heated to 120 °C
with rotation for 3 h. After the autoclave was cooled to ambient
temperature, the solid was filtered and washed by distilled water several
times. Finally, the solid was dried at 60 °C overnight, calcined in flowing
Keywords: Hydrogen • Methanol composition • Cu-based
Catalysts • Dimethoxylmethane
-
1
air at 450 °C for 4 h (heating rate: 2 °C∙min , flowing velocity:100
-
1
mL∙min ) and reduced in flowing hydrogen at 450 °C for 4 h (heating
-
1
-1
rate: 2 °C∙min , flowing velocity:100 mL∙min ).
References:
Catalyst Characterization
[
1]
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Cu contents were calculated by data from Inductively coupled plasma
atomic emission spectroscopy (ICP–AES) performed on a Thermo IRIS
Intrepid II XSP emission spectrometer after dissolving the catalyst in the
[2]
[3]
K. Klier, V. Chatikavanu, R. Herman, G. Simmons, J. Catal., 1982, 74,
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adsorption measurements were carried out at 77 K on a
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method. Powder X-ray diffraction (XRD) patterns were detected to
confirm the structure and crystal size by Rigaku Ultima IV X-ray
diffractometer utilizing Cu-Kα radiation (λ = 1.5405 Å) operated at 35 kV
and 25 mA. The IR spectra (IR) were collected by Nicolet Nexus 670 FT-
[4]
[5]
[6]
[7]
[8]
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-1
IR spectrometer in absorbance mode at a spectral resolution of 2 cm
using KBr technique. Hydrogen temperature programmed reduction
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[11] W. H. Cheng, Appl. Catal. A: General, 1995, 130, 13-30.
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Appl. Catal. A: Gen, 1998, 170, 215-224.
(
-
1
instrument by at the ramp rate of 10 K min . Scanning electron
microscopy (SEM) were performed on the Hitachi S-4800 microscope. To
illuminate crystal morphology and size, transmission electron microscopy
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237, 171-180.
2
(TEM) images were obtained by the FEI Tecnai G F30 microscope
working at 300 kV. X-ray photoelectron spectroscopy (XPS) were
performed with Al Kα (hν = 1486.6 eV) radiation on a Thermo Scientific
K-Alpha spectrometer. Charging effects were corrected by using the C 1s
peak due to adventitious carbon with EB fixed at 284.6 eV.
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[
[
[
[
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Catalytic measurements
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A
typical test for producing highly pure hydrogen from methanol
decomposition was carried out as follows: Cu/SiO catalyst (0.1 g) and
methanol (40 mL) were added into a batch autoclave (70 mL). The
reactor was then flushed with N at ambient temperature for three times
to remove the residual air. After evacuating nitrogen, it was heated up to
40 °C. The reaction was conducted for 4 h at a stirring speed of 450
2
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2
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Catal. B: Environ., 2014, 147, 684-697.
2
rpm. After cooling down to ambient temperature, liquid products were
analyzed on a gas chromatograph equipped with a mass spectrometer
[
20] I. A. Fisher and Al. T. Bell. J. Catal., 1999, 184, 357-376.
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[
(
GC-MS, Shimadzu QP-2010 Ultra) with a Rtx-5Sil MS capillary column
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(30 m×0.25 mm×0.25 μm). Analysis for gaseous products was performed
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on a GC (Techcomp 7900) equipped with thermal conductivity detector
TCD) and columns (TDX-01: 30 cm×3 mm, TDX-01: 2 m×3 mm), as well
(
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as flame ionization detector (FID) and a HP-PLOT Q capillary column (50
m×0.53 mm×25 μm).
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ChemCatChem
10.1002/cctc.201701416
FULL PAPER
Text for Table of Contents
Liubi Wu, Bolong Li, Chen Zhao*
CH OCH OCH
3
2
3
H2
Page No. – Page No.
HCHO
H2
Condensation
Direct Synthesis of Hydrogen and
Dimethoxylmethane from Methanol
Cu
Dehydrogenation
on Cu/SiO
Cu /Cu Sites
2
Catalysts with Optimal
Cu
+
0
CH OH
Cu
3
CH OH
3
Cu
Cu O-SiO
2
2
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