catalysts
Article
Promotive Effect of Sn2+ on Cu0/Cu+ Ratio and
Stability Evolution of Cu/SiO2 Catalyst in the
Hydrogenation of Dimethyl Oxalate
Chuancai Zhang 1 , Denghao Wang 2 and Bin Dai 2, *
1
2
*
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China;
[email protected]
School of Chemistry and Chemical Engineering of Shihezi University, Shihezi 832000, China;
[email protected]
Correspondence: [email protected]; Tel.: +86-993-205-7277
Academic Editor: Rajendra S. Ghadwal
Received: 26 March 2017; Accepted: 14 April 2017; Published: 19 April 2017
Abstract: The influence of Sn2+ doping on the structure and performance of silica supported copper
catalyst was systematically investigated and characterised. Catalytic evaluation showed that the
suitable content of Sn2+ introduced into a Cu/SiO2 catalyst evidently improved the catalytic activity
and stability of ethylene glycol synthesis from dimethyl oxalate. X-ray diffraction and X-ray auger
electron spectroscopy indicated that the Cu0 /Cu+ ratio gradually increased with increasing Sn2+
content, and an appropriate proportion of Cu0 /Cu+ ratio played a very significant role in this reaction.
Transmission electron microscopy revealed that the active copper particles in the Cu-xSn/SiO2 catalyst
were smaller than those of the Cu/SiO2 catalyst. This result may be due to the introduction of Sn2+
species transformed into SnO2 . Furthermore, SnO2 effectively segregated the active copper. These
effects are beneficial in inhibiting the aggregation of copper in the catalysts, thereby improving the
stability of the catalyst and prolonging the life span.
Keywords: Cu-xSn/SiO2 ; dimethyl oxalate; hydrogenation; Cu0 /Cu+ ratio; stability
1. Introduction
Hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG) with coal as raw material
is of industrial interest. This process has also become an important research topic considering
various applications of EG and the growing market outlook. Copper-supported catalysts show
potential catalytic activities for hydrogenation of DMO [1–3]. Copper catalyst exhibits high activity
and selectivity for EG synthesis from DMO. However, the stability of the catalyst remains to be
improved. Several factors, including metal dispersion, Cu0 /Cu+ ratio, metal-support interaction,
active-component sintering and carbon deposition, affect the stability of the catalyst [4–8]. Given the
difficulty of improving the stability with a single copper catalyst, copper is modified by a second
additive (Cr, Ni, B, Zn, Cu, Ag, Co and La), which favours catalytic stability in several recent
studies [9–11]. In spite of some progress in this research direction, further study remains necessary.
In addition, the use of tin additives to improve the stability of copper catalysts for hydrogenation of
DMO to EG is seldom reported.
Sn species is a kind of promoter that improves the dispersion of active metals and stability of the
catalyst [12–15]. Wang et al., reported that the selectivity and stability of isobutane dehydrogenation
can be significantly enhanced by the addition of tin in Ni/SiO2 catalyst [16]. Such improvement is
ascribed to the fact that tin doping can successfully reduce the size of nickel particles, and the presence
of SnOx suppresses the migration of carbon deposition to prolong the lifetime. De Oliveira et al. found
Catalysts 2017, 7, 122; doi:10.3390/catal7040122
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Catalysts 2017, 7, 122
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that a Co-Sn/ZnO2 catalyst with an atomic Sn/Co ratio of 1 exhibits the most desirable activity and
selectivity for the hydrogenation of methyl oleate to oleyl alcohol [17]. Furthermore, the selectivity to
unsaturated alcohol is considerably improved by doping tin to cobalt. These results indicated that
the addition of tin in the catalyst may play a positive role in the hydrogenation of esters to alcohols.
In addition, Sn2+ could play a certain role in adjusting the Cu0 /Cu+ ratio in view of its strong reduction
which is considered to influence the hydrogenation of DMO to EG.
In the present study, we aimed to investigate and explain the effect of Sn2+ content on metal
dispersion, Cu0 /Cu+ ratio and catalytic stability evolution of Cu-xSn/SiO2 catalysts during EG
synthesis from DMO. We synthesised Cu/SiO2 catalysts and impregnated different Sn2+ contents
onto these catalysts to generate Cu-xSn/SiO2 . The Sn2+ content was adjusted over the range 0.3–2%.
Consequently, part of Cu+ was effectively reduced to Cu0 due to the reduction of Sn2+ . Therefore,
we adjusted and optimised the Cu0 /Cu+ ratio. We also embedded Sn species onto the surface
of the catalyst to help separate and partially fix the copper. The catalytic activity and stability of
the catalyst were obviously enhanced with Sn doping when compared with that of the Cu/SiO2
catalyst. Low-temperature N2 adsorption (Brunauer–Emmett–Teller (BET) method), transmission
electron microscopy (TEM), X-ray diffraction (XRD), temperature-programmed reduction (TPR), X-ray
photoelectron spectroscopy (XPS), X-ray auger electron spectroscopy (XAES) and inductively coupled
plasma atomic emission spectrometry (ICP-AES) were used to analyse the structures and properties
of the catalysts. Moreover, we investigated in detail the influence of Sn promoter on the structural
changes and catalytic performance improvements in Cu/SiO2 .
2. Results and Discussion
2.1. Characterisation of the Catalysts
The textural structures and chemical compositions of Cu/SiO2 and Cu-Sn/SiO2 catalysts after
calcination at 450 ◦ C for 5 h were analysed by ICP-AES. The actual amount of copper in the catalyst
was about 11% (Table 1), which may be due in part to the residual copper in the solution. Another
part of the weakly adsorbed copper on silicon dioxide was subsequently washed off. Tin loading was
basically equal to the theoretical value because of the use of the impregnation method without loss of
tin solution.
Table 1. Lists of the chemical compositions and textural features of the reduced Cu/SiO2 and Cu-xSn/SiO2 .
Sample
Cu (wt %)
Sn (wt %)
SBET (m2 ·g−1 )
V pore (cm3 ·g−1 )
Dpore (nm)
Cu/SiO2
Cu-0.3%Sn/SiO2
Cu-0.6%Sn/SiO2
Cu-1.2%Sn/SiO2
Cu-1.8%Sn/SiO2
10.94
10.82
10.86
10.81
10.79
0.29
0.58
1.18
1.82
246.7
244.9
244.3
236.3
221.8
0.67
0.65
0.64
0.62
0.59
10.1
10.2
10.2
10.3
10.5
Figure 1 shows the N2 adsorption–desorption profiles and Barrett–Joyner–Halenda (BJH) pore size
distribution curves of Cu/SiO2 and Cu-xSn/SiO2 catalysts. The N2 adsorption–desorption isotherms
of all the catalysts belonged to type IV. The profiles of the N2 adsorption–desorption and BJH pore
size distribution between Cu/SiO2 and the Cu-xSn/SiO2 catalysts showed no obvious change. This
result indicated that the catalyst retained the type IV isotherms with shape similar to that of the H-type
hysteresis loop.
The SBET was in the range of 221.8–246.7 m2 ·g−1 . The specific surface area of the catalysts decreased
from 246.7 to 221.8 m2 ·g−1 when the Sn content increased from 0.3% to 1.8%. This observation suggested
that a small amount of Sn doping slightly affected the SBET of the catalysts.
Catalysts 2017, 7, 122
3 of 11
Catalysts 2017, 7, 122
3 of 11
Catalysts 2017, 7, 122
3 of 11
(B)
(A)
-1
Cu-0.6%Sn/SiO2
Cu-0.6%Sn/SiO2
Cu-0.3%Sn/SiO2
Cu-0.3%Sn/SiO2
Cu/SiO2
0.2
0.2
Cu-1.8%Sn/SiO2
Cu-1.2%Sn/SiO2
3 -1
Cu-1.2%Sn/SiO2
Cu/SiO2
Cu-1.8%Sn/SiO2
dV/dD
g -1nm
STP)
dV/dD
(cm3(cm
g-1nm
STP)
Cu-1.2%Sn/SiO2
0.0
0.0
(B)
Cu-1.8%Sn/SiO2
Cu-1.8%Sn/SiO2
Quantity Adsorbed (mmol/g)
Quantity Adsorbed (mmol/g)
(A)
Cu-1.2%Sn/SiO2
Cu-0.6%Sn/SiO2
Cu-0.6%Sn/SiO2
Cu-0.3%Sn/SiO2
Cu-0.3%Sn/SiO2
Cu/SiO
2
Cu/SiO2
0.4
0.6
Relative Pressure (p/p0)
0.4
0.6
0.8
1.0
0.8
1.0
10
100
Pore Diameter (nm)
10
100
Pore Diameter (nm)
Relative Pressure (p/p0)
1. N2 adsorption–desorption
profiles
(A)
and Barrett–Joyner–Halenda
(BJH) pore-size
FigureFigure
1. N2 adsorption–desorption
profiles
(A) and
Barrett–Joyner–Halenda
(BJH) pore-size
distribution
2
and
Cu-xSn/SiO
2
catalysts.
distribution
curves
(B)
of
calcined
Cu/SiO
curves
(B) of1.calcined
Cu/SiO2 and Cu-xSn/SiO
Figure
N2 adsorption–desorption
profiles
(A) and Barrett–Joyner–Halenda (BJH) pore-size
2 catalysts.
distribution curves (B) of calcined Cu/SiO2 and Cu-xSn/SiO2 catalysts.
The curve of the BJH pore size distribution showed that the catalyst possessed a double
The curve structure.
of the BJH
sizepore
distribution
showedwas
that the
catalyst
possessed aand
double
mesoporous
Thepore
average
size of the catalysts
the range
of 10.1–10.5
The curve of the BJH
pore size
distribution
showed thatinthe
catalyst
possessednm,
a double
mesoporous
structure.
The
average
porecm
size
the
catalysts
the
rangeincreased
of 10.1–10.5
3·g−1.of
the pore volumes
ranged
within
0.59–0.67
The
average
pore was
size ofinthe
catalyst
with nm,
mesoporous structure. The average pore size of 3the−catalysts
was in the range of 10.1–10.5 nm, and
1 . The average
·
and the
pore volumes
ranged
within
cm
g
pore
size
of
the
catalyst
increased
increasing
tin content,
whereas
the 0.59–0.67
pore volumes
gradually
decreased.
This
effect
may
have
resulted
the pore volumes ranged within 0.59–0.67 cm3·g−1. The average pore size of the catalyst increased with
the partial
tin on thethe
catalyst
andgradually
the slight blocking
of hole
channels.
with from
increasing
tin covering
content,ofwhereas
pore surface
volumes
decreased.
This
effect may have
increasing tin content, whereas the pore volumes gradually decreased. This effect may have resulted
H2-TPR
wereofperformed
order tosurface
study the
reducibility
5% Sn/SiO
, Cu/SiO
2
resulted from
themeasurements
partial covering
tin on theincatalyst
and
the slightofblocking
of2hole
channels.
from the partial covering of tin on the catalyst surface and the slight blocking of hole channels.
and
Cu-xSn/SiO
2 catalysts (Figure 2). For 5% Sn/SiO2 catalyst, TPR showed two weak peaks at around
H2 -TPR
measurements were performed in order to study the reducibility of 5% Sn/SiO2 , Cu/SiO2
2-TPR measurements were performed in order to study the reducibility of 5% Sn/SiO2, Cu/SiO2
400Hand
500 °C, catalysts
which should
be attributed
to
the
reduction
peak of tin
oxides.
It is obvious
thatpeaks
tin
and
Cu-xSn/SiO
(Figure
5%
Sn/SiO
TPR
showed
two
weak
at
22 catalysts (Figure
2 catalyst,
and
Cu-xSn/SiO
2).2).
ForFor
5%reduce
Sn/SiO
2 catalyst,
TPR showed
two
weak
peaks
at
around
oxides
supported
on
silica
are
difficult
to
at
low
temperature.
Two
reduction
peaks
can
be
◦
around
400500
and
500 C, which
should
be attributed
to the reduction
peak
of tinItoxides.
It isthat
obvious
400
and
should
be attributed
to the reduction
peak of tin
oxides.
is obvious
tin
observed
in°C,
thewhich
Cu/SiO
2 catalyst. One sharp and large reduction peak at about 239 °C was attributed
that
tin
oxides
supported
on
silica
are
difficult
to
reduce
at
low
temperature.
Two
reduction
peaks
oxides
on silicaCuO.
are difficult
reduce
at located
low temperature.
reduction
peaks
to thesupported
highly dispersed
Anothertoweak
peak
at around Two
300 °C
was related
to can
the be
◦ C was
canobserved
be
observed
in
the Cu/SiO
One
sharp
large reduction
peak
about
239 be
2 catalyst.
in of
thelarge-particle
Cu/SiO
2 catalyst.
One
sharp
and
largeand
reduction
peak
at about
239atpeaks
°C
wascould
attributed
reduction
CuO
[18,19].
For
Cu-xSn/SiO
2 catalysts,
three
reduction
◦
attributed
to the
highly
dispersed
CuO. Another
weak
peakcorresponded
located
at around
300
was
related
to observed.
the highly
CuO. temperature
Another
weak
peak
located
at around
300
°Creduction
was C
related
to the to
Thedispersed
low and
middle
reduction
peaks
to the
of cupric
thereduction
reduction
of
large-particle
CuO
[18,19].
For
Cu-xSn/SiO
catalysts,
three
reduction
peaks
of large-particle
CuOat[18,19].
Cu-xSn/SiO
2 catalysts,
three reduction
peaks
could
be
oxide, while
the peak detected
400–500For
°C could
be ascribed
of tin oxides.
The
firstcould
2to the reduction
large reduction
peak
of the
Cu-xSn/SiO
2 catalysts
gradually
shifted
to thetolow-temperature
be observed.
observed.
The
low
middle
temperature
reduction
corresponded
to the reduction
The
low
andand
middle
temperature
reduction
peaks peaks
corresponded
the reduction
ofwith
cupric of
increasing
tin
compared
withatthe
Cu/SiO
catalyst.
This
finding
illustrated
that
part
oxide,
whilewhile
the content
peak
at 400–500
°C
could◦2C
be
ascribed
to
the reduction
of tin
oxides.
The
first
cupric
oxide,
the detected
peak
detected
400–500
could
be
ascribed
to
the reduction
ofof
tinthe
oxides.
2+
copper
species
was likely
by Sn2 catalysts
which confirmed
that
Sn2+ doping
was
favorable
for the
reduction
peak
of
thereduced
Cu-xSn/SiO
gradually
shifted
to
the
low-temperature
with
Thelarge
first
large
reduction
peak
of
the Cu-xSn/SiO
catalysts
gradually
shifted
to
the
low-temperature
2
reductiontin
of CuO.
Thecompared
third reduction
peak
at high2 catalyst.
temperature
belongs
to the reduction
of of
tinthe
increasing
content
withwith
the
Cu/SiO
Thisthat
finding
illustrated
that part
with
increasing
tin
content
compared
the
Cu/SiO
2 catalyst. This finding illustrated that part of
oxide
was
considerably
weak
due
to
the
low
content
of
Sn
doping.
2+
2+
2+
2+
copper species was likely reduced by Sn which confirmed that Sn doping was favorable for the
the copper species was likely reduced by Sn which confirmed that Sn doping was favorable for the
reduction
CuO.
Thethird
thirdreduction
reductionpeak
peak at
at high
high temperature
temperature that
of of
tintin
reduction
ofof
CuO.
The
thatbelongs
belongstotothe
thereduction
reduction
239content
℃
oxide
was
considerably
weak
due
to
the
low
of
Sn
doping.
oxide was considerably weak due to the low content of Sn
doping.
Cu-1.8%Sn/SiO
H2 Consumption(a.u)
H Consumption(a.u)
2
Cu-1.2%Sn/SiO2
239℃
Cu-1.8%Sn/SiO2
Cu-0.6%Sn/SiO2
Cu-1.2%Sn/SiO2
2
Cu-0.3%Sn/SiO2
Cu-0.6%Sn/SiO2
Cu/SiO
2
5%Sn/SiO
Cu-0.3%Sn/SiO
2
2
100
200
300
400
Cu/SiO2
500
600
Temperature (℃) 5%Sn/SiO
2
Figure 2. H2-temperature-programmed reduction of 5% Sn/SiO2, Cu/SiO2 and Cu–xSn/SiO2 catalysts
100
200
300
400
500
600
after calcination at 300 °C.
Temperature (℃)
0 and Cu+, all reduced
To2.investigate
the possible Sn2+ doping-induced
phase
changes
between
Cu
Figure
H
-temperature-programmed
reduction
and
Cu–xSn/SiO
22,, Cu/SiO
2 catalysts
Figure
2. 2H
2-temperature-programmed
reductionofof5%
5%Sn/SiO
Sn/SiO
Cu/SiO
22and
Cu–xSn/SiO
2 catalysts
◦ 2 catalysts were analysed by XRD. Four diffraction peaks were observed in
Cu/SiO
2 and Cu-xSn/SiO
after
calcination
atat
300
after
calcination
300 C.
°C.
all of the catalysts (Figure 3). These diffraction peaks of 2θ were located at 21.7°, 36.4°, 43.3° and 50.4°.
0
+
investigate
possible
doping-induced phase
phase changes
reduced
2+2+doping-induced
ToTo
investigate
thethe
possible
SnSn
changes between
betweenCu
Cu0and
andCu
Cu, +all
, all
reduced
Cu/SiO
2 and Cu-xSn/SiO2 catalysts were analysed by XRD. Four diffraction peaks were observed in
Cu/SiO2 and Cu-xSn/SiO2 catalysts were analysed by XRD. Four diffraction peaks were observed in
all of the catalysts (Figure 3). These diffraction peaks of 2θ were located at 21.7°, 36.4°, 43.3° and 50.4°.
all of the catalysts (Figure 3). These diffraction peaks of 2θ were located at 21.7◦ , 36.4◦ , 43.3◦ and 50.4◦ .
Catalysts 2017, 7, 122
Catalysts 2017, 7, 122
4 of 11
4 of 11
◦ exhibited
The first
firstdiffraction
diffractionpeak
peakatat2θ2θ
21.7°
exhibited
amorphous
silica
structure.
position
The
of of
21.7
an an
amorphous
silica
structure.
The The
peakpeak
position
was
◦
◦
was located
the
of attached
36.4°attached
the
Cu
2O phase.
The
two
other
diffraction
peaks
at
the
2θ
of
located
at theat2θ
of2θ
36.4
to thetoCu
O
phase.
The
two
other
diffraction
peaks
at
the
2θ
of
36.4
2
Catalysts
2017,
7,
122
4
of
11
◦ belonged
36.4°43.3
and
43.3° belonged
to metallic
Cu.
The diffraction
peaks
of Cu
Cu2O
were relatively
weak,
and
to metallic
Cu. The
diffraction
peaks of
Cu and
Cu2and
O were
relatively
weak, thereby
thereby
suggesting
that
copper
particles
were
remarkably
small,
and
Cu
species
were
well
dispersed
suggesting
that
copper particles
remarkably
and Cusilica
species
were well
dispersed
on the
The first
diffraction
peak at 2θwere
of 21.7°
exhibited small,
an amorphous
structure.
The peak
position
on the surface
of the supports.
The diffraction
peaks
of cuprous
oxide
evidently
decreased
gradually
surface
of
the
supports.
The
diffraction
peaks
of
cuprous
oxide
evidently
decreased
gradually
was located at the 2θ of 36.4°attached to the Cu2O phase. The two other diffraction peaks at the 2θ of with
with
increasing
amount
of tin
doping,
whereas
the
ofincreased
copper
increased
gradually.
result
increasing
amount
of
tin doping,
whereas
the
of peak
copper
gradually.
This
resultThis
indicated
36.4°
and
43.3°
belonged
to metallic
Cu.
Thepeak
diffraction
peaks
of Cu and
Cu2O were
relatively
weak,
+
0
0
+
+
0
0
+
indicated
that
part
Cu
wastoreduced
towere
Cu remarkably
, and
the ratio
Cu
/Cu
was effectively
adjusted.
No
thereby
suggesting
that copper
particles
small,
andratio
Cu species
were
well dispersed
that
part
of
Cu
wasofreduced
Cu
, and
the
Cu
/Cu
was
effectively
adjusted.
No obvious
2+
obvious
diffraction
peak
was
detected
for
Sn
and
SnO
2
phases,
which
may
be
due
to
the
low
level
on
the
surface
of
the
supports.
The
diffraction
peaks
of
cuprous
oxide
evidently
decreased
gradually
diffraction peak was detected for Sn and SnO2 phases, which may be due to the low level of Snof
2+ doping
with
increasing
amount
of
tin
doping,
whereas
the
peak
of
copper
increased
gradually.
This
result
Sn
in
the
catalysts.
doping in the catalysts.
indicated that part of Cu+ was reduced to Cu0, and the Cu0/Cu+ ratio was effectively adjusted. No
obvious diffraction peak was detected
 for Sn and SnO2 phases, which may be due to the low level of
Sn2+ doping in the catalysts.
 SiO2






Cu
Cu2O

SiO2

Cu2O
 Cu
Cu-1.8%Sn/SiO
2
Cu-1.2%Sn/SiO2

Intensity(a.u)
Intensity(a.u)


Cu-1.8%Sn/SiO
2
Cu-0.6%Sn/SiO
2
Cu-1.2%Sn/SiO2
Cu-0.3%Sn/SiO2
Cu-0.6%Sn/SiO
Cu/SiO
2
2
Cu-0.3%Sn/SiO2
20
Cu/SiO
60
2
40
80
2θ (o)
20
40
60
80
o
2θ ( )
Figure 3. X-ray diffraction patterns
of the reduced
reduced catalysts.
catalysts.
Figure 3. X-ray diffraction patterns of the reduced catalysts.
The TEM
TEM images
images of
of the
the reduced
reduced Cu/SiO
Cu/SiO22 with
The
withand
andwithout
withoutSn
Sndoping
dopingare
are presented
presented in
in Figure
Figure 4.
4.
For all
all the
theThe
catalysts,
the supports
supports
involved
numerous
irregular
spheres
of
~20
nm
diameter,
and
the
TEM images
of the reduced
Cu/SiO
2 with andirregular
without Sn
dopingof
are~20
presented
in Figureand
4. the
For
catalysts,
the
involved
numerous
spheres
nm diameter,
Fornanoparticles
all
the catalysts,
the supports
involved
numerous
irregular
spheres
~20
nmsupports.
diameter,
and average
the The
metallic
nanoparticles
resembled
small
black
spots
scattered
onsurface
theofsurface
of
the supports.
metallic
resembled
small
black
spots
scattered
on the
of the
The
metallic
nanoparticles
resembled
small black spots
scattered
on
theCu-xSn/SiO
surface of the
supports.were
The 4.33,
average
particle
sizes
of
the
metal
nanoparticles
of
the
Cu/SiO
2
and
2
catalysts
particle sizes of the metal nanoparticles of the Cu/SiO2 and Cu-xSn/SiO2 catalysts were 4.33, 3.31,
average
particle
sizesnm.
of the
nanoparticles
of the
the Cu-xSn/SiO
Cu/SiO2 and 2Cu-xSn/SiO
2 catalysts were 4.33,
3.31, 3.13
3.68,
3.13
and nm.
3.21
Themetal
copper
particles
in
catalysts were
smaller than those
3.68,
and
3.21
The copper
particles
in the
Cu-xSn/SiO
2 catalysts were smaller than those of
3.31, 3.68, 3.13 and 3.21 nm. The copper particles in the Cu-xSn/SiO
2 catalysts were smaller than those
2+
2+
of
the
Cu/SiO
2
,
which
may
be
due
to
the
conversion
of
Sn
to
SnO
2
during
immersion.
the Cu/SiO
may be due to the conversion of Sn to SnO2 during immersion. Furthermore,
Furthermore,
2 , which
of the Cu/SiO
2, which may be due to the conversion of Sn2+ to SnO2 during immersion. Furthermore,
tin
oxide
hindered
the
growth and
and aggregation
aggregation of
of active
active copper
copper particles
particles in
in hydrogen
hydrogen reduction
reduction at
tin oxide
hindered
the
growth
tin oxide hindered the growth and aggregation of active copper particles in hydrogen reduction at at
low temperatures.
temperatures.
low
low temperatures.
Figure 4. Cont.
Catalysts 2017, 7, 122
5 of 11
Catalysts 2017, 7, 122
5 of 11
Catalysts 2017, 7, 122
5 of 11
Figure
4. 4.
TheTheTEM
reduced catalysts:
catalysts:(A)(A)Cu/SiO
Cu/SiO
2, (B) Cu-0.3%Sn/SiO2, (C)
Figure
TEMimages
images of
of the
the reduced
2, (B) Cu-0.3%Sn/SiO2, (C)
Figure 4. The TEM images of the reduced catalysts: (A) Cu/SiO2 , (B) Cu-0.3%Sn/SiO2 ,
2
,
(D)
Cu-1.2%Sn/SiO
2
and
(E)
Cu-1.8%Sn/SiO
2
.
Cu-0.6%Sn/SiO
Cu-0.6%Sn/SiO2, (D) Cu-1.2%Sn/SiO2 and (E) Cu-1.8%Sn/SiO2.
(C) Cu-0.6%Sn/SiO2 , (D) Cu-1.2%Sn/SiO2 and (E) Cu-1.8%Sn/SiO2 .
2.2. Catalytic
Activity
andStability
Stability
2.2. Catalytic
Activity
and
2.2. Catalytic Activity and Stability
experiments
DMOhydrogenation
hydrogenation were
outout
to study
the the
effect
of tinofdoping
on
TheThe
experiments
ononDMO
werecarried
carried
to study
effect
tin doping
on
the
catalytic
performance
of
copper
catalyst.
Figure
5
shows
the
comparison
of
DMO
conversion
The
experiments
on
DMO
hydrogenation
were
carried
out
to
study
the
effect
of
tin
doping
on
the catalytic performance of copper catalyst. Figure 5 shows the comparison of DMO conversion
(Figure 5A)
and EG selectivity
(Figure
5B) in Cu/SiO2 and
Cu-xSn/SiO
2 catalysts with
varied conversion
weight
the
catalytic
of copper
catalyst.
shows
the comparison
of DMO
(Figure
5A) performance
and EG selectivity
(Figure
5B) in Figure
Cu/SiO52 and
Cu-xSn/SiO
2 catalysts with varied weight
liquid
hour
space
velocity
(WLHSVs).
The
reaction
conditions
were
2.5
MPa
H
2, 200 °C and H2/DMO
(Figure
EGvelocity
selectivity
(Figure 5B)
inreaction
Cu/SiOconditions
with°C
varied
2 and Cu-xSn/SiO
2 catalysts
liquid 5A)
hourand
space
(WLHSVs).
The
were 2.5
MPa H2, 200
and Hweight
2/DMO
ratio of 90. The catalytic performance exhibited a volcanic-like trend with the increase ◦of tin doping
liquid
hour
space
velocity
(WLHSVs).
The
reaction
conditions
were
2.5
MPa
H
,
200
C
and
2 increase of H
2 /DMO
ratioinofthe
90.copper
The catalytic
performance
exhibited
a
volcanic-like
trend
with
the
tin
doping
catalysts. The Cu-0.6%Sn/SiO2 catalyst showed the most remarkable DMO conversion
ratio
of copper
90. The catalysts.
catalytic performance
exhibited
a volcanic-like
withremarkable
the increase of tin conversion
doping in
in the
Cu-0.6%Sn/SiO
2 catalyst
showedtrend
most
and EG selectivity,
theThe
conversion
and selectivity
of Cu-xSn/SiO
2the
catalyst
with tin contentDMO
higher than
the
copper
catalysts.
The
Cu-0.6%Sn/SiO
catalyst
showed
the
most
remarkable
DMO
conversion
and
2 catalytic of
and 1.2%
EG selectivity,
the conversion
andthe
selectivity
Cu-xSn/SiO
catalyst with tin
content
higher
evidently decreased,
whereas
performance
of2 Cu-1.8%Sn/SiO
2 catalyst
was
the than
EG
selectivity,
the
conversion
and
selectivity
of
Cu-xSn/SiO
catalyst
with
tin
content
higher
than
1.2%
among decreased,
all of the catalysts.
Thisthe
result
may beperformance
due to2excessive
doping, which2 catalyst
covered the
1.2%worst
evidently
whereas
catalytic
of tin
Cu-1.8%Sn/SiO
was the
2+the
evidently
decreased,
whereas
the
catalytic
performance
of
Cu-1.8%Sn/SiO
catalyst
was
worst
2
copper
surface
and
blocked
the
active
site
of
the
copper.
Moreover,
a
large
amount
of
Cu
was
worst among all of the catalysts. This result may be due to excessive tin doping, which covered
the
0 with excessive
2+ so
0 doping,
+. which covered the copper
among
all
of
the
catalysts.
This
result
may
be
due
to
excessive
tin
reduced
to
Cu
Sn
as
to
break
the
balance
of
Cu
and
Cu
copper surface and blocked the active site of the copper. Moreover, a large amount of Cu2+ was
2+
surface
and
active site
large
reduced
to blocked
Cu0 withthe
excessive
Sn2+ofsothe
as copper.
to breakMoreover,
the balancea of
Cu0amount
and Cu+of
. Cu was reduced to
0
2+
0
+
Cu with excessive Sn so as to break the balance of Cu and Cu .
100
90
90
85
Selectivity (%)
95
90
100
80
Selectivity (%)
95
Conversion (%)
(B)
100
(A)
100
Conversion (%)
(A)
70
(B)
90
80
60
85
80
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
70
0.2
-1
WLHSV(h )
80Figure
0.25
60
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
WLHSV(h-1)
5. Comparison of dimethyl oxalate (DMO) conversion (A) and ethylene glycol (EG) selectivity
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
0.50 0.75 1.00 1.25 1.50 1.75 2.00
-1
(B) in Cu/SiO2 andWLHSV(h
Cu-xSn/SiO
) 2 catalysts with varied WLHSVs.
WLHSV(h-1)
2.2
Figure5.5.Comparison
Comparisonofofdimethyl
dimethyloxalate
oxalate(DMO)
(DMO)conversion
conversion(A)
(A)and
andethylene
ethyleneglycol
glycol(EG)
(EG)selectivity
selectivity
Figure
(B)
in
Cu/SiO
2 and Cu-xSn/SiO2 catalysts with varied WLHSVs.
(B) in Cu/SiO2 and Cu-xSn/SiO2 catalysts with varied WLHSVs.
Catalysts
Catalysts 2017,
2017, 7,
7, 122
122
6
6 of
of 11
11
100
100
100
100
98
98
98
98
96
96
96
96
94
94
94
94
92
92
92
92
90
90
88
88
86
86
84
84
90
90
Cu/SiO
Cu/SiO22
Cu/SiO
Cu/SiO22
Cu-0.6%Sn/SiO
Cu-0.6%Sn/SiO22
Cu-0.6%Sn/SiO
Cu-0.6%Sn/SiO22
0
0
20
20
40
40
60
60
80
80
88
88
Selectivity(%)
Selectivity(%)
Conversion
Conversion(%)
(%)
Long-duration
Long-duration experiments
experiments were
were employed
employed to
to investigate
investigate the
the stability
stability of
of the
the catalyst.
catalyst. A
A
Long-duration
experiments
were
employed
to
investigate
the
stability
of
the
catalyst.
comparison of
of the
the DMO
DMO conversion
comparison
conversion and
and EG
EG selectivity
selectivity of
of the
the Cu/SiO
Cu/SiO22 and
and Cu-0.6%Sn/SiO
Cu-0.6%Sn/SiO22 catalysts
catalysts
A comparison
of the DMO
conversion and EG satisfactory
selectivity of the Cu/SiO
2 and Cu-0.6%Sn/SiO
is
The
is shown
shown in
in Figure
Figure 6.
6. Cu-0.6%Sn/SiO
Cu-0.6%Sn/SiO22 exhibited
exhibited satisfactory activity
activity and
and lifetime.
lifetime.
The DMO
DMO was
was2
catalysts is shown
in Figure
6. Cu-0.6%Sn/SiO
satisfactory
activity and lifetime.
The
DMO
2 exhibited
completely
transformed,
the
selectivity
of
EG
was
96%
in
the
Cu-0.6%Sn/SiO
2 catalyst,
and
completely transformed, the selectivity of EG was 96% in the Cu-0.6%Sn/SiO2 catalyst, and no
no
was completely
transformed,
the
selectivity
of EG
was
96% inHowever,
the Cu-0.6%Sn/SiO
and no
2 catalyst,
decreasing
trend
was
observed
after
about
200
h
of
reaction.
the
catalytic
performance
decreasing trend was observed after about 200 h of reaction. However, the catalytic performance of
of
decreasing trend evidently
was observed after
about 200 hThis
of reaction.indicated
However, thea catalytic
performance
of
Cu/SiO
Cu/SiO22 declined
declined evidently after
after 90
90 h
h of
of reaction.
reaction. This result
result indicated that
that a reasonable
reasonable content
content of
of Sn
Sn
Cu/SiO2 declinedthe
evidently after
90 h of reaction. This result indicated that a reasonable content of Sn
doping
doping improves
improves the stability
stability of
of the
the catalyst.
catalyst.
doping improves the stability of the catalyst.
86
86
100
100 120
120 140
140 160
160 180
180
84
84
Time
Time (h)
(h)
The reaction
Figure
6.
experimental
periods
of
Cu/SiO
and
Cu-x0.6%Sn/SiO
Figure 6.
6. Long
Long experimental
experimental periods
periods of
of Cu/SiO
Cu/SiO222 and
and Cu-x0.6%Sn/SiO
Cu-x0.6%Sn/SiO222 catalysts.
catalysts. The
The
reaction
◦ C, H /DMO ratio of 90 and WLHSV of 1 h
−1 .
200 °C,
22,, 200
H
22/DMO ratio of 90 and WLHSV of 1 h−1
.
conditions
were
2.5
MPa
H
conditions were 2.5 MPa H2, 200 °C, H2/DMO ratio of 90 and WLHSV of 1 h−1.
2.3.
Characterisation of
2.3. Characterisation
Characterisation
of the
the Used
Used Catalysts
Catalysts
To
To further
further investigate
investigate the
the reasons
reasons for
for improving
improving the
the activity
activity and
and stability,
stability, the
the XPS
XPS spectra
spectra of
of Sn
Sn
3d
5/2
and
Cu
2p
of
Cu/SiO
2
and
Cu-xSn/SiO
2
catalysts
for
8
h
reaction
were
investigated.
In
Figure
and
Cu
2p
of
Cu/SiO
and
Cu-xSn/SiO
catalysts
for
8
h
reaction
were
investigated.
In
Figure
7,
2
5/2and Cu 2p of Cu/SiO2 2and Cu-xSn/SiO2 catalysts
3d5/2
for 8 h reaction were investigated. In Figure 7,
7,
an
asymmetric
Sn
5/2
peak
at
the
binding
energy
(BE)
of
487
eV
implied
the
existence
of
tin
oxides
3d
peak
at
the
binding
energy
(BE)
of
487
eV
implied
the
existence
of
an asymmetric Sn 3d5/2
5/2 peak at the binding energy (BE) of 487 eV implied the existence of tin oxides
2+ (486.5
4+ (487.3
2+ (486.5
4+ (487.3
in
the
Cu-xSn/SiO
22
catalysts.
the
BEs
of
Sn
eV)
and
Sn
eV)
close
and
the
Cu-xSn/SiO
catalysts.
Although
BEs
eV) were
2+ (486.5
in the Cu-xSn/SiO2 catalysts. Although
Although
thethe
BEs
of of
SnSn
eV)eV)
andand
Sn4+Sn
(487.3
eV) were
were
closeclose
and
difficult
to
distinguish
by
XPS,
we
deduced
that
the
oxidised
tin
species
were
mainly
composed
and
difficult
to
distinguish
by
XPS,
we
deduced
that
the
oxidised
tin
species
were
mainly
composed
difficult to distinguish by XPS, we deduced that the oxidised tin species were mainly composed of
of
4+ because
2+
2+ can 2+
Sn
because
the
between
Sn
SnO
contrast,
SnO
of 4+
Sn
the reaction
between
Sn2+Cu
and
Cu produce
can produce
SnO2 By
easily.
By contrast,
SnO2 is
4+
2+ can
Sn
because
the reaction
reaction
between
Sn2+ and
and
Cu
produce
SnO22 easily.
easily.
By
contrast,
SnO22 is
is difficult
difficult
◦ C and reaction
to
reduce
hydrogen
during
at
at
difficult
reduce
by hydrogen
during reduction
300reaction
at 200 ◦ C.
to
reducetoby
by
hydrogen
during reduction
reduction
at 300
300 °C
°Catand
and
reaction
at 200
200 °C.
°C.
SSS
SSSⅣ
ⅣSS SS
SS5/2
5/2
Cu-1.8%Sn/SiO
Cu-1.8%Sn/SiO22
Intensity(a.u.)
Intensity(a.u.)
Cu-1.2%Sn/SiO
Cu-1.2%Sn/SiO22
Cu-0.6%Sn/SiO
Cu-0.6%Sn/SiO22
Cu-0.3%Sn/SiO
Cu-0.3%Sn/SiO22
487ev
487ev
480
480
485
485
490
490
Binding
Binding Energy(eV)
Energy(eV)
Figure
7. X-ray
X-ray
photoelectron
spectroscopy
(XPS)
spectra
5/2 of Cu/SiO2 and Cu-xSn/SiO2
Figure 7.
X-ray photoelectron
photoelectron spectroscopy
spectroscopy(XPS)
(XPS)spectra
spectraofof
ofSnSn
Sn3d3d
3d
Cu/SiO22 and
andCu-xSn/SiO
Cu-xSn/SiO22
5/25/2ofofCu/SiO
catalysts
catalysts under
under an
an 88 h
h reaction.
reaction.
Catalysts 2017, 7, 122
7 of 11
Catalysts 2017, 7, 122
7 of 11
Figure 88 displays
catalysts under
under an
an 88 h
h
Figure
displays the
the XPS
XPS spectra
spectraof
ofCu
Cu2p
2pofofCu/SiO
Cu/SiO22 and
and Cu-xSn/SiO
Cu-xSn/SiO22 catalysts
0 (about 932.7 eV) and Cu+ (about 932.5 eV) were located near each other.
reaction.
The
BEs
of
Cu
reaction. The BEs of Cu0 (about 932.7 eV) and Cu+ (about 932.5 eV) were located near each other.
0
+ was difficult. Conversely, the peak of Cu0 and Cu+ can be
Hence, dividing
Hence,
dividing the
the peaks
peaks of
of Cu
Cu0 and
and Cu
Cu+ was
difficult. Conversely, the peak of Cu0 and Cu+ can be
distinguished
effectively
by
XAES
with
peak
fitting.
distinguished effectively by XAES with peak fitting.
932.7eV
Cu-1.8%Sn/SiO2
Intensity (a.u.)
Cu-1.2%Sn/SiO2
Cu-0.6%Sn/SiO2
Cu-0.3%Sn/SiO2
Cu/SiO2
930
940
950
960
Binding Energy (eV)
Figure 8. XPS spectra of Cu 2p of Cu/SiO2 and Cu-xSn/SiO2 catalysts under an 8 h reaction.
Figure 8. XPS spectra of Cu 2p of Cu/SiO2 and Cu-xSn/SiO2 catalysts under an 8 h reaction.
Figure 9 presents the Cu LMM XAES profiles of these catalysts after 8 h reaction.
reaction. A wide but
asymmetrical
folding
peak
with
kinetic
energy
ranging
from
907
to
924
eV
was
observed in each
asymmetrical folding peak with kinetic energy ranging from 907
+
catalyst. Two symmetrical peaks located at approximately
approximately 916.2
916.2 and
and 918.6
918.6 eV
eV and
and related
related to
to Cu
Cu+ and
0
0
+ were
werefitted
fittedand
anddistinguished
distinguishedbyby
a peak
separation
software.
areas
of0 and
Cu Cu
and
Cu+
Cu0 were
a peak
separation
software.
TheThe
peakpeak
areas
of Cu
+ to increase with increasing Sn content.
0 and
+ to
were
calculated,
andfound
we found
the proportion
Cu0 Cu
and
Cuincrease
calculated,
and we
the proportion
of Cuof
with increasing Sn content. The
0
+
The0/Cu
Cu+ /Cu
ratios
in the Cu/SiO
catalysts
0.85, 1.17
0.96,and
1.06,
1.17
andresult
1.23.
Cu
ratios in
the Cu/SiO
2 and Cu-xSn/SiO
2 catalysts2 were
0.85,were
0.96, 1.06,
1.23.
This
2 and Cu-xSn/SiO
2+ was reduced
0 because of2+doping Sn2+ , which played
2+
0
This
result
suggested
that
a
part
of
Cu
to
Cu
suggested that a part of Cu was reduced to Cu because of doping Sn , which played an effective
+ ratio. Cu0 is responsible for activate hydrogen and ester,
0 is
an effective
role inthe
regulating
Cu0Cu
/Cu
role
in regulating
Cu0/Cu+the
ratio.
responsible for activate hydrogen and ester, and Cu+ is
+
and Cu is
to be polarized
by C=O
in the
ester
is favourable
forformation
the formation
believed
tobelieved
be polarized
by C=O bonds
inbonds
the ester
and
is and
favourable
for the
and
+ /Cu0 ratio is highly
+
0
and
stabilization
of
the
intermediate
states.
Accordingly,
an
appropriate
Cu
stabilization of the intermediate states. Accordingly, an appropriate Cu /Cu ratio is highly important
+ /Cu0 ratio. Some literatures
+/Cu
important
to theHowever,
reaction. However,
there are different
on
the0 Cu
to
the reaction.
there are different
views onviews
the Cu
ratio.
Some literatures reported
+
0
0 > Cu
reported
that
/Cu > 1 is
for hydrogenation
of but
DMO
EG, butview
the opposite
view is
that
Cu+/Cu
1 is beneficial
forbeneficial
hydrogenation
of DMO to EG,
theto
opposite
is also presented
also
presented
[2,5,20].
In
order
to
explain
this
phenomenon
rationally,
Ma
et
al.
investigated
and
[2,5,20]. In order to explain this phenomenon rationally, Ma et al. investigated and found that there
0
+
0
0 and between
0 can be oxidised
found
that there
is between
a dynamic
Cuester
andhydrogenation.
Cu during ester
Cu
can+ by
be
is
a dynamic
cycle
Cucycle
Cu+ during
Cuhydrogenation.
to Cu
+ by ester, while H +can also reduce
+ into Cu0 in the reaction [21]. LaGrow et al.
0
oxidised
to
Cu
Cu
ester, while H2 can also reduce Cu
2 into Cu in the reaction [21]. LaGrow et al. also observed the
also observed
the interface
of Cu
and via
Cu2switching
O using ESTEM
viaand
switching
and
interface
transition
of Cu andtransition
Cu2O using
ESTEM
hydrogen
oxygenhydrogen
environments
oxygen
It iswhy
reasonable
tocatalytic
explain why
different
catalytic systems
[22].
It isenvironments
reasonable to [22].
explain
different
systems
have different
Cu+/Cu0 have
ratiosdifferent
that are
Cu+ /Cu0 ratios
are favorable
for
reaction;
that may be why
the Cu-0.6%Sn/SiO
favorable
for thethat
reaction;
that may
bethe
why
the Cu-0.6%Sn/SiO
2 catalyst
with the Cu0/Cu
ratio ofwith
1.06
2 +catalyst
0
+
the Cu
/Cu catalytic
ratio of activity
1.06 hasand
the stability
best catalytic
activity
and stability in our catalysts.
has
the best
in our
catalysts.
The TEM images of used Cu/SiO2 (130 h) and Cu-0.6%Sn/SiO2 (200 h) catalysts after long-time
tests are shown in Figure 10. Notably, the average particle size of the monometallic copper catalyst
increased from 4.33 to 7.91 nm and that of Cu-0.6%Sn/SiO2 catalysts increased from 3.68 to 5.48 nm.
This result indicated that tin doping helps prevent the growth of copper particles during the reaction.
Catalysts 2017, 7, 122
8 of 11
0
+
Cu /Cu =1.23
Cu+916.2
Cu0 918.6
e
Catalysts 2017,
2017, 7,
7, 122
122
Catalysts
Intensity (a.u.) Intensity (a.u.)
0
+
Cu /Cu =1.17
of 11
11
88 of
d
+
0
+
=1.06
Cu /Cu =1.23
Cu 916.2
0
Cu 918.6
ec
0
+
Cu 0/Cu +=1.17
Cu /Cu =0.96
db
0
+
Cu0/Cu
/Cu+=1.06
=0.85
Cu
ca
0
+
Cu /Cu912.5
=0.96
910.0
915.0
917.5
920.0
Kinetic Energy (eV)
922.5
b
0
+
Cu /Cu =0.85
a
Figure 9. Curve fitting results of the Cu LMM XAES spectra of catalysts:
(a) Cu/SiO2, (b) Cu0.3%Sn/SiO2, (c) Cu-0.6%Sn/SiO2, (d) Cu-1.2%Sn/SiO2 and (e) Cu-1.8%Sn/SiO2 after 8 h reaction.
910.0
912.5
915.0
917.5
920.0
922.5
Energy
(eV)
The TEM images of used Cu/SiO2 (130Kinetic
h) and
Cu-0.6%Sn/SiO
2 (200 h) catalysts after long-time
tests are shown in Figure 10. Notably, the average particle size of the monometallic copper catalyst
Figure 9. Curve fitting results of the Cu LMM XAES spectra of catalysts: (a) Cu/SiO2 ,
Figurefrom
9. Curve
fitting
results
ofthat
the of
CuCu-0.6%Sn/SiO
LMM XAES spectra
of catalysts: (a) Cu/SiO2, (b) Cuincreased
4.33 to
7.91
and
2 catalysts increased from 3.68 to 5.48 nm.
(b) Cu-0.3%Sn/SiO
, (c)nm
Cu-0.6%Sn/SiO
, (d) Cu-1.2%Sn/SiO
2
2
2 and (e) Cu-1.8%Sn/SiO2 after
2, (c) Cu-0.6%Sn/SiO2, (d) Cu-1.2%Sn/SiO2 and (e) Cu-1.8%Sn/SiO2 after 8 h reaction.
This 0.3%Sn/SiO
result
indicated
that
tin
doping
helps
prevent
the
growth
of
copper
particles during the reaction.
8 h reaction.
The TEM images of used Cu/SiO2 (130 h) and Cu-0.6%Sn/SiO2 (200 h) catalysts after long-time
tests are shown in Figure 10. Notably, the average particle size of the monometallic copper catalyst
increased from 4.33 to 7.91 nm and that of Cu-0.6%Sn/SiO2 catalysts increased from 3.68 to 5.48 nm.
This result indicated that tin doping helps prevent the growth of copper particles during the reaction.
2-used (130 h) and (B) Cu-0.6%Sn/SiO2-used
Figure
TEM
images
of used
catalysts:
(A) Cu/SiO
Figure
10. 10.
TEM
images
of used
catalysts:
(A) Cu/SiO
2 -used (130 h) and (B) Cu-0.6%Sn/SiO2 -used (200 h).
(200 h).
Raman spectroscopy was used to test carbon-coke on the used catalysts. Figure S1 was the Raman
Raman spectroscopy was used to test carbon-coke on the used catalysts. Figure S1 was the
spectrum of the Cu/SiO2 -used catalyst and Cu-0.6%Sn/SiO2 -used catalysts. As can be seen from
Raman spectrum of the Cu/SiO2-used catalyst
and Cu-0.6%Sn/SiO2-used catalysts.
As can be seen
Figure
S1, thereTEM
wereimages
two peaks
at 1350
cm−1(A)
(amorphous
carbon)
and
1580
cm−1 (graphitized
carbon)
2-used
(130 carbon)
h) and
(B)
2-used
used
catalysts:
Cu/SiO
−1 (amorphous
fromFigure
Figure10.
S1, there
wereof
two
peaks
at 1350 cm
andCu-0.6%Sn/SiO
1580 cm−1 (graphitized
in the(200
Cu/SiO
-used
catalyst
[23–25]
which
indicated
that
carbon
deposits
were
formed
in
the
Cu/SiO
2
2
h).the Cu/SiO2-used catalyst [23–25] which indicated that carbon deposits were formed in the
carbon) in
catalyst during the reaction. However, the carbon peaks located at 1350 and 1580 cm−1 could−1not be
Cu/SiO2 catalyst during the reaction. However, the carbon peaks located at 1350 and 1580 cm could
seen Raman
in the Cu-0.6%Sn/SiO
catalysts.
This may be on
duethe
to used
the fact
that theFigure
amount
carbon
spectroscopy was
used
to test carbon-coke
catalysts.
S1ofwas
the
2 -used
not be seen in the Cu-0.6%Sn/SiO
2-used catalysts. This may be due to the fact that the amount of
deposition
was very
or the distribution
of carbon
was very uniform.
Raman
spectroscopic
Raman
spectrum
of small
the Cu/SiO
2-used catalyst
and Cu-0.6%Sn/SiO
2-used
catalysts.
As can beresults
seen
carbon deposition was very small or the distribution of carbon was very uniform. Raman
demonstrated
thewere
doping
ofpeaks
tin was
prevent thecarbon)
formation
carbon
deposition.
from
Figure S1,that
there
two
at beneficial
1350 cm−1to
(amorphous
andof1580
cm−1
(graphitized
spectroscopic results demonstrated that the doping of tin was beneficial to prevent the formation of
carbon) in the Cu/SiO2-used catalyst [23–25] which indicated that carbon deposits were formed in the
carbon
deposition.
3.
Experimental
Section
Cu/SiO
2 catalyst during the reaction. However, the carbon peaks located at 1350 and 1580 cm−1 could
not be seen in the Cu-0.6%Sn/SiO2-used catalysts. This may be due to the fact that the amount of
3.1. Catalyst Preparation
carbon deposition was very small or the distribution of carbon was very uniform. Raman
Cu-xSn/SiO
fabricated through
and subsequent
spectroscopic
results
demonstrated
that theammonia
doping ofevaporation
tin was beneficial
to preventtin
theimpregnation.
formation of
2 was
Silica
sol
(20
nm,
A.R.,
Alfa
Aesar,
London,
UK)
was
used
as
support
material.
Cupric
nitrate and
carbon deposition.
stannous oxide (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were prepared as metal
precursor salts. All samples were produced to achieve a copper content of 12 wt % in the catalyst.
The tin content was changed to investigate the effect of Sn doping. Firstly, 0.4 mol·L−1 Cu(NO3 )2
aqueous solution was prepared in a round-bottomed flask. Ammonia aqueous solution (28 mL) was
Catalysts 2017, 7, 122
9 of 11
dropped into 100 mL of copper nitrate solution with electromagnetic stirring for 30 min. Subsequently,
30 g of 40% silica sol was added to the mixture dropwise. The obtained mixture was successively
stirred at room temperature for 3 h and at 90 ◦ C for another 4 h. A Cu/SiO2 catalyst was prepared after
further filtering, washing, drying and calcined. Secondly, Cu-xSn/SiO2 (x = 0.3, 0.6, 1.2 and 1.8 wt %)
catalysts were prepared by impregnating the promoter Sn2+ on the Cu/SiO2 catalysts. A certain
amount of stannous oxide was dissolved in 10 mL of dilute nitric acid solution, and the pH of the
solution was adjusted to 4–5 by dropping ammonia solution. The required quantities of Cu/SiO2
sample were immersed into the mixture solution at 25 ◦ C for 24 h. The obtained slurry was dried at
120 ◦ C for 10 h and calcined under argon atmosphere at 450 ◦ C for 5 h. The resulting catalysts were
labelled as Cu-xSn/SiO2 , where x refers to the mass content of Sn in the final catalysts.
3.2. Characterisation
ICP-AES was applied to analyse the mass content of active copper and tin additive in the catalysts.
The XRD spectra of catalysts were obtained with a D8 ADVANCE XRD (Bruker Company, Karlsruhe,
Germany) using Cu Kα radiation source at 40 kV to investigate the crystalline phases of the samples.
The microstructure and active metal particle size were examined by TEM (A JEM 2010 TEM, Japan
Electron Optics Laboratory Co. Ltd., Tokyo, Japan) with an acceleration voltage of 200 kV. TEM was
also used to study the metal particle size and dispersion of samples. The XPS spectra were analysed by
an Axis Ultra spectrometer (Shimadzu Corporation, Kyoto, Japan) using monochromatic Mg Kα X-ray
of 1253.6 eV as the radioactive source to measure the valence state and surface content of metallic
elements on the catalysts. All binding energy (BE) values were calibrated according to the BE of carbon
(C 1s = 284.5 eV). XAES was also performed to determine the valence state of copper on the catalyst
surface. A Micromeritics ASAP 2720 instrument (Micromeritics Instrument Corporation, Norcross, GA,
USA) was used to detect the reduction behaviour of Cu and Sn-promoted catalysts. The temperature of
the TPR ranged from 25 to 600 ◦ C at 5 ◦ C·min−1 , and 45 mL/min 10% H2 /Ar was fed into the reactor.
3.3. Activity Measurements
The hydrogenation of DMO was operated in a 400-mm long and 6-mm diameter stainless steel
miniature reactor equipped with a back-pressure control system. For a typical experiment, 0.5 g
of catalysts was loaded into the reactor, and then a certain amount of 5% H2 /Ar atmosphere was
introduced to reduce the catalysts. The reduction temperature was increased from room temperature
to 300 ◦ C with a heating rate of 2.5 ◦ C·min−1 for 4 h. After completing the catalyst reduction process,
the system was cooled to the reaction temperature of 200 ◦ C. Feedstock of pure H2 and 15 wt % DMO
methanol solution were fed into the system. The reactions were conducted at 2.5 MPa, H2 /DMO
molar ratio of 90 and weight hour space velocity of DMO in the range of 0.25–2.0 h−1 . Hydrogenation
products were analysed on a gas chromatograph (Shimadzu GC-9A, Shimadzu Corporation) equipped
with a flame ionisation detector and a Wondacap WAX column (30 m × 0.53 mm × 1.0 µm).
4. Conclusions
This study focused on the effect of Sn2+ doping on the structure and catalytic performance of
Cu/SiO2 catalysts for the hydrogenation of DMO. The introduction of Sn2+ into Cu/SiO2 catalysts
resulted in distinct changes in catalytic performance. The conversion and selectivity of the catalyst
displayed a volcano-like trend with increasing tin content. The optimum amount of tin doping was
about 0.6% of the catalyst. Long-period experimental results showed that the DMO was completely
transformed. The selectivity of EG was above 96% in Cu-0.6%Sn/SiO2 catalyst, and no decreasing
trend was obtained after approximately 200 h. By contrast, the activity and stability of Cu/SiO2
catalyst declined rapidly. This effect may be attributed to the fact that the addition of a suitable amount
of tin could adjust the Cu0 /Cu+ ratio reasonably. Furthermore, the SnO2 that formed during reduction
segregated the active copper and inhibited the aggregation of copper on the catalyst surface. This
phenomenon could be responsible for the increased activity and stability of EG.
Catalysts 2017, 7, 122
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Supplementary Materials: The following are available online at www.mdpi.com/2073-4344/7/4/122/s1,
Figure S1: The Raman spectrum of the Cu/SiO2 -used catalyst and Cu-0.6%Sn/SiO2 -used catalysts.
Author Contributions: Bin Dai conceived and designed the experiments, and provided financial support;
Chuancai Zhang performed the experiments, analyzed the data and wrote the manuscript; Denghao Wang
participated in the operation of experiment and data analysis.
Conflicts of Interest: The authors declare no conflict of interest.
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