Activated carbon aerogel supported copper catalysts for the hydrogenation of methyl acetate to ethanol: Effect of KOH activation

Article abstract of DOI:10.1039/c9nj01434a

Methyl acetate (MA) hydrogenation is crucial for indirect ethanol synthesis through syngas (CO + H₂). In our work, activated carbon aerogel supported copper (Cu-ACA) catalysts have been prepared by a conventional impregnation method. The surface area and functional groups of the ACA soared after KOH activation. The highest surface area achieved for the ACA was 2562 m² g^-1. The anchoring effect of micropores and external oxygen-containing groups (OCGs) significantly enhanced the metal-support interaction in catalysts, facilitating the high dispersion of Cu and an enhancement in surface Cu⁺ species, both of which improved the catalytic activity of catalysts. Cu-ACA-A4 showed the most outstanding catalytic performance, with a MA conversion of 95.2% and an ethanol selectivity of 62.2%, close to the carbon equilibrium selectivity of 66.7%.

Full text of DOI:10.1039/c9nj01434a

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Activated carbon aerogels supported copper catalysts for the
hydrogenation of methyl acetate to ethanol: Effect of KOH
activation
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaoxiong Houa, Jinxian Zhaoa, Junjie Liub, Yahong Hana, Yongli Peia and Jun Rena*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Methyl acetate (MA) hydrogenation is crucial in the indirect ethanol synthesis through syngas (CO+H2). In our work,
activated carbon aerogels supported copper (Cu-ACAs) catalysts have been prepared by a conventional impregnation
method. The surface area and functional groups of the ACA soared after KOH activation. The highest surface area of the
ACA achieved was 2562 m2g-1. The anchoring effect of micropores and external oxygen-containing groups (OCGs)
significantly enhanced the metal-support interaction in catalysts, facilitating the high dispersion of Cu and an
enhancement in surface Cu+ species, both of which improved catalytic activity of catalysts. Cu-ACAs-A4 showed the most
outstanding catalytic performance, with a MA conversion of 95.2% and an ethanol selectivity of 62.2%, close to the carbon
equilibrium
selectivity
of
66.7%.
investment cost, and high energy consumption10–12. In recent
years, the indirect ethanol synthesis by two successive
processes, dimethyl ether (DME) carbonylation and methyl
acetate (MA) hydrogenation, have been proposed3,13–17. As
Compared with the hydrogenation of acetic acid, the
hydrogenation of MA has outstanding advantages in terms of
the cost of catalysts, the conversion rate, and the equipment
Introduction
The production of ethanol has become a hot topic around
the world because it can be widely used as an alternative
energy source, a hydrogen carrier, and a chemical feedstock.
In particular, ethanol can also simultaneously be blended with
gasoline and holds great promise due to its environmentally
friendly nature1–6. Currently, two main methods have been
developed for the production of ethanol: fermentation and
ethylene hydration. However, fermentation of edible grain
crops and ethylene hydration face great challenges owing to
the negative effect on global food supplies and the shrinking
worldwide oil resource crisis, respectively2, 7. As a result, it is
critical to explore alternative processes for ethanol production.
It is known that synthetic gas (syngas) can be produced from
extensive ingredients, such as biomass and coal, and has a
wide range of applications in producing quantities of
chemicals, including ethanol8, 9. However, the direct ethanol
production by syngas is hampered by difficulties, such as low
conversion and low selectivity, abundant byproducts, high
requirement18, 19
.
Copper catalysts supported on silica are well-suited for
the hydrogenation of esters, showing high activation in the
hydrogenolysis of C-O or C=O bonds, but much lower
stimulation for C-C cleavage20–23. In a previous study, the
catalytic mechanism that synergistic effect of Cu0 and Cu+ has
been shown to improve the catalytic hydrogenation activity,
whereby Cu0 promotes H2 splitting and Cu+ adsorbs and
stimulates CH3O- or CH3CO-24. However, Cu0 nanoparticles
toward to transfer and agglomerate and Cu+ species are apt to
reduce during the reaction. Therefore, it is crucial to develop
copper catalysts with high stability and high dispersion for the
hydrogenation of MA.
It is noted that carbon nanotubes (CNTs) had been used to
prepare supported copper catalysts for gas-phase catalytic
hydrogenation of MA25, 26. However, only a relatively low MA
conversion (22.5%) and low ethanol selectivity (27.6%) were
thereby acquired. Owing to its particular superiorities, such as,
low density, high surface area, fine pore size, three-
dimensional nano-networks, and outstanding electrical
a Key Laboratory of Coal Science and Technology, Taiyuan University of Technology,
Ministry of Education and Shanxi Province, Taiyuan 030024, China. Mailing address
for correspondence: No. 79 Yingze West Street, Taiyuan 030024, China.
E-mail address: renjun@tyut.edu.cn; Tel/Fax: +86 351 6018598.
conductivity27–29
, carbon aerogel is considered to be a
b Division of Nanoscale Measurement and Advanced Materials, National Institute of
Metrology, No. 18, Bei San Huan Dong Lu, Chaoyang Dist, Beijing 100029, China
*Corresponding author. Mailing address for correspondence: No. 79 Yingze West
Street, Taiyuan 030024, China. Tel/Fax: +86 351 6018598.
prospective support for catalysts. By virtue of its unique
properties and surface chemistry, carbon aerogels represent a
promising materials for performance in catalyst support30.
E-mail address: renjun@tyut.edu.cn (J. Ren).
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In the present work, ACAs was synthesized by a sol–gel
method and then subjected to KOH activation at various Catalytic activity evaluation.
concentrations. The obtained materials were then used to
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The catalytic performance was checked through a micro
reaction device. Samples (0.5 g in all cases) were placed
between plugs of quartz wool in the reactor. For each catalyst,
tests were performed with fresh samples, heating from
ambient temperature to 330 °C. After pre-heating, MA was
blended with hydrogen, then fed into the reactor to initiate
prepare supported copper catalysts through
a
wet
impregnation method, and these were tested for their efficacy
in the hydrogenation of MA. The influence of KOH activation
on the hydrogenation activity had been studied in terms of
structural properties, surface chemical groups, and dispersion
and valence state of copper of the Cu-ACAs catalysts. Our
results may encourage readers to design and investigate other
related carbon material-based catalysts with favorable
performances for the hydrogenation of methyl acetate.
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the reaction. The products were tested on
a
gas
chromatograph (GC-950, Shanghai Haixin, China) fitted with a
flame-ionization detector (FID).
Conversions and products selectivity were acquired by the
following equations:
푎푚표푢푛푡 표푓 푀퐴 푎푓푡푒푟 푟푒푎푐푡푖표푛(푚표푙)
Conversion (%) = 1 ―
× 100%
(1)
(
)
Experiment
Catalysts preparation
푡표푡푎푙 푎푚표푢푛푡 표푓 푀퐴 푖푛 푡ℎ푒 푓푒푒푑(푚표푙)
퐴푚표푢푛푡 표푓 푐푎푟푏표푛 표푓 푒푡ℎ푎푛표푙 (푚표푙)
Selectivity (%) =
푇표푡푎푙 푎푚표푢푛푡 표푓 carbon of 푀퐴 푐표푛푣푒푟푡푒푑 (푚표푙 ) × 100% (2)
Preparation of carbon-silica aerogel (CSA). Carbon-silica aerogel
was synthesized by a sol–gel process through resorcinol,
formaldehyde and TEOS. Resorcinol (1.5 g) was dissolved in a
mixture of acetone (8.5 mL), TEOS (5.5 mL), and APTES (640
L). After the addition of formaldehyde (37%, 3.2 g), the mixed
solution was transferred to a polypropylene container and left
undisturbed at 50°C for 6 h. The container was tightly packed
to avoid solvent loss. The polymer was then washed
thoroughly using acetone and kept at 60 °C for 10 h in a hot
dryer. This established material was carbonized at 900 °C for
60-minutes in nitrogen. The as-synthesized sample was
designated as CSA.
Characterization of the catalysts
The texture properties of each sample was observed by
transmission electron microscopy (TEM, JEOL, JEM-2100F,
Tokyo, Japan) operating at 200 keV. The catalysts were
ultrasonically dispersed in ethanol solvent and this suspension
was then dropped on a Cu grid and was allowed to dry before
measurement. Actual loading of Cu obtained by atomic
absorption spectrometer (AAS, AA240FS, VARIAN). XRD was
performed through a D/max 2500 diffractometer employing
Cu-Kα radiation in a region of 10–80o. The specific surface
areas were tested by N2 adsorption-desorption data obtained
at P/P0 = 0.04–0.32 using the Brunauer–Emmett–Teller (BET)
method (Micromeritics Instrument Co., GA, USA). The nitrogen
at P/P0 =0.99 was used to evaluate the gross pore volume. To
characterize the functional species in the samples, Fourier-
transform infrared (FTIR) measurement was performed in the
range from 650 to 4000 cm−1 with a KBr beam splitter. The
content of the surface OCGs on the surface of the ACA was
equipped by the Boehm’s titration method. N2O chemisorption
on the catalysts was determined to reckon the copper
dispersion (DCu %) and copper surface area (SCu, m2g-1Cu). XPS
was measured on a 250Xi electron spectrometer (Al-Kα X-ray).
Etching of silica templates. To etch silica, the obtained CSA was
immersed in NaOH liquor (1 M) at 80 °C for 12 h and then
collected by filtration. The step was repeated every 12 hours
to ensure the total removal of silica. The obtained aerogel was
rinsed thoroughly, and then dried overnight in 80 °C oven. This
material was carbon aerogel, designated as CAs.
Activation of CAs with KOH. Activated carbon aerogels (ACAs)
was obtained by the following procedure. CAs was pre-treated
with 0.5 m aqueous HNO3 at 80 °C for 6 h then flushed out,
and dried overnight at 100 °C. Subsequently, the pre-treated
CAs was activated by mixing with solid KOH in various
proportions (2/1, 4/1, and 6/1, w/w), and carbonizing at 750 °C
(3 °C/min) in N2 atmosphere. The resulting materials were
flushed out with HCl solution (1 M) and deionized water for
several times. The achieved materials were designated as
ACAs-A2, ACAs-A4, and ACAs-A6, respectively.
Cu-ACAs preparation. The Cu-ACAs catalysts with Cu loading of
30 % were synthesized through a conventional impregnation
method. In brief, ACAs or CAs (1 g) was immersed in copper (II)
nitrate solution (40 mL) at a certain concentration, and the
mixture was ultrasonicated for several minutes and then
vigorously stirred under ambient temperature for 8 h.
Thereafter, each precursor was reduced in H2 atmosphere at
350 °C for 2 h. The catalysts are defined as Cu-CAs, Cu-ACAs–
A2, Cu-ACAs-A4, and Cu-ACAs-A6.
Table 1 Hydrogenation of MA over Cu-ACAs catalysts.
Selectivity
MA
(%)
Samples
Conversion
(%)
EtOH
MeOH
EAC
Else
Cu-CAs
Cu-ACAs-A2
Cu-ACAs-A4
Cu-ACAs-A6
Cu-CNTs31
77.4
89.5
96.7
79.5
19.5
35.0
61.1
61.5
62.2
59.5
26.6
32.5
32.9
30.2
30.5
34.1
57.1
54.7
0.4
3.5
5.6
4.8
4.7
6.0
-
2.6
0.4
16.3
11.9
Cu-inside-CNTs32
0.9
Reaction conditions: T = 330 ºC, P = 2.0 MPa, H2/MA = 60, mcat = 0.5 g,
LHSV = 2.5 h-1, reaction time was 10 h.
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a
CAs
1600
1400
1200
1000
800
600
400
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0
c
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1400
ACAs-A2
ACAs-A4
ACAs-A6
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Cu-ACAs-A4
Cu-ACAs-A6
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0
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0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
d
Relative Pressure (P/P0)
Relative pressure (P/P0)
0.2
0.1
0.0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
b
CAs
Cu-CAs
ACAs-2
ACAs-4
ACAs-6
Cu-ACAs-A2
Cu-ACAs-A4
Cu-ACAs-A6
Pore width (nm) 10
1
5
10
Pore Width (nm)
Figure 1 N2 adsorption-desorption isotherms and pore size distribution of supports (a, b) and Cu-ACAs catalysts (c, d).
TPR test was actualized on an AutoChenFinsore-3010
apparatus (Zhejiang FINETEC).
The catalytic performances of Cu-CAs and Cu-ACAs
catalysts in the hydrogenation of MA were evaluated under
conditions of 2.0 MPa, 330 °C, H2/MA = 60, and LHSV = 2.5 h-1,
and the results are revealed in Table 1. The Cu-CAs catalyst
displayed high MA conversion (77.4%) and ethanol selectivity
(61.1%) and a methanol selectivity of about 33%. Besides, 0.4%
ethyl acetate and 5.6% other hydrocarbons were obtained.
After activation by KOH, crude stock conversion and ethanol
selectivity were incrementally influenced by varied KOH/CAs
mass ratio, reaching maximum values (96.7 and 62.2%,
respectively) when the ratio was 4, beyond which a distinct
decrease was observed. In general, the MA conversion and
EtOH selectivity followed the order Cu-ACAs-A4 > Cu-ACAs-A2
> Cu-CAs > Cu-ACAs-A6. Apparently, the catalytic capability
over the Cu-ACAs catalysts were influenced by the amount of
KOH used during the activation process, and were significantly
enhanced when the amount of KOH was within an appropriate
range. It is worth noting that the two Cu-CNTs catalysts
exhibited lower catalytic activity on the hydrogenation of MA.
The MA conversion of Cu-ACAs was nearly three and five times
as high as those of Cu-inside-CNTs and Cu-CNTs. The carbon
aerogel has significant advantage compared with CNT support.
Results and discussion
Catalytic activity
Table 2 Physicochemical and textural properties of ACAs
supports and Cu-ACAs catalysts.
Sb
micr
Sa
Vc
Vd
micro
BET
total
KOH/
CAs
De
o
Samples
(m2*
g-1)
(cm3*g-
(cm3*g-
(m2*
(nm)
1
1
)
)
g-1)
979
CAs
0
0
2
1279
516
1.47
0.91
1.79
0.57
0.23
0.80
1.82
1.92
1.82
Cu-CAs
ACAs-A2
335
1849
1570
Cu-ACAs-
A2
2
4
4
6
6
716
2562
695
518
2193
553
1.00
2.33
0.86
2.29
1.73
0.33
0.96
0.30
0.94
0.60
1.92
1.76
1.84
1.80
1.84
ACAs-A4
Characterization of the samples
Cu-ACAs-
A4
BET. As shown in Figure 1, the isotherms of the supports were
of type I on the basis of the Brunauer–Emmett–Teller
classification, without obvious hysteresis loop. The isotherms
indicated a microporous structure of the samples33, as is
apparent from the data in Table 2. For each support, a
trimodal distribution was observed in the pore size distribution
curve provided by the Horvath–Kawazoe method in Figure 1.
Abundant micropores were formed upon the removal of the
silica from the CSA and in calcination procedure, during which
the organic components were purged at high temperature.
ACAs-A6
2497
1358
2186
936
Cu-ACAs-
A6
a Total specific surface area
b Specific surface area of micropores
c Total pore volume
d Pore volume of micropores
e Average pore diameter calculated from adsorption branch by DFT method
As indicated by the results in Figure 1 and Table 2, the N2
uptakes of the ACAs samples were significantly increased after
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-1
3600 cm
-1
2250 cm
CAs
It can be seen that the isotherms of all catalysts
maintained type I on the basis of BET classification after Cu
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ACAs-2
loading. However, the specific surface areas and whole pore
volumes decreased significantly for all the catalysts (shown in
Table 2), compared with the corresponding supports. The
highest specific surface areas of ACAs-A4 support is 4 times
higher than that of Cu-ACAs-A4, indicating that part of Cu
particles could reside at the opening micropore over ACAs
support38.
ACAs-4
ACAs-6
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FTIR spectroscopy. The Infrared optical transmittance spectra of
CAs supports after treatment by KOH were exhibited in Figure
2. The broad band in =3600–3440 cm-1 can be distributed to
OH stretching vibrations corresponding to free and associated
hydroxyl groups, respectively, which may be associated with
carboxyl, phenol groups and adsorbed water. Additionally,
hydrogen bonds also offered assistance39. The absorption
spectrum at  = 2850 and 2918 cm-1 belonged to the stretching
vibrations of methylene and methyl group, respectively, and
that at  = 1614 cm-1 can be apportioned to an aromatic ring
stretching vibration. The band at  = 1475 cm-1 can be
allocated to a CH2 simple harmonic vibration and CH3 scissor
vibration. The CH2-O-CH2 vibration emerged at 1055 cm-1,
indicating the formation of cross bond between aromatic rings
during condensation polymerization40.
Distinct differences in stretching vibrations were
observed between CAs and ACAs. The absorption peaks
occurred at =1501 and 1435 cm-1 only seen for the ACA
samples, due to C=C stretching vibrations of aromatic rings,
suggesting the benzol rings remained intact after high-
temperature carbonization. The absorption peak occurred at
2250 cm-1 only seen for the ACA samples can be accredited to
the =C-H telescopic vibration of R2C=CHR. The presence of
these organic functional groups indicated the appearance of a
gel reticulated structure during the procedure of
polycondensation-polymerization. Molecules were interlinked
through the bonds of -CH2 and -CH2-O-CH241, generating a
continuous criss-cross interconnected structure when the
water was eliminated, which facilitated ion transfer.
4000
3000
2000
1000
-1
Wavenumber (cm
)
Figure 2 FTIR spectra of ACAs supports.
chemical activation by KOH over the full range of relative
pressures. The inset in Figure 1 also showed the increased
amount of micropores with an increment in mass ratio of KOH
and CAs, suggesting the formation of new pores and a
widening of existing pores by KOH34-36. The specific surface
areas and whole pore volumes (Table 2) multiplied with raised
mass ratio, beyond which no obvious change was observed.
The maximum surface area of ACAs-A4 reached up to 2562
m2g-1, which was twice as high as that of the original CAs (1279
m2g-1), indicating a more developed pore structure of ACAs
than the primary CAs. The expansible specific surface area and
incremental porosity contributed to higher dispersion of Cu
nanoparticles, improving the catalytic performance compared
with that of the original CAs37.
Boehm’s titration and XPS. Boehm titration has been used to
quantify surface OCGs on carbon materials42. The
concentrations of the superficial functional groups on the CAs
and ACA were summarized in Table 3. The contents of surface
Table 3 Amount of OCGs of Cu-ACAs catalysts.
Carboxyl group
(mmol/g)
Lacton group
(mmol/g)
Total
group
Samples
Phenol group (mmol/g)
Basic group (mmol/g)
(mmol/g)
Cu-CAs
0.01
0.10
0.15
0.17
0.02
0.06
0.10
0.14
0.01
0.12
0.17
0.20
0.01
0.10
0.15
0.12
0.05
0.38
0.57
0.63
Cu-ACAs-A2
Cu-ACAs-A4
Cu-ACAs-A6
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Cu-ACAs-A6
290 295
280
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528
530
532
534
536
538
Binding Energy (eV)
Binding energy (eV)
100
200
300
400
500
Figure 3 XPS spectra of C 1s (a) and (b) O 1s of Cu-ACAs catalysts.
Temperature (℃ )
Figure 5 H2-TPR profiles of as-prepared Cu-ACAs catalysts.
groups increase from 0.05 to 0.63 mmol/g during increasing
KOH/CAs values. This result, along with the FTIR results,
comparatively large Cu nanoparticles form over the catalyst.
Therefore, it implied that the support with appropriate
chemical activation were more beneficial to enhance
Cu
Cu2O
Cu-CAs
a
b
Cu-ACAs-A2
0.30
0.15
0.00
0.30
0.15
0.00
Cu-ACAs-A4
Cu-ACAs-A6
10
20
30
10 20 30 40 50
Partical size (nm)
Partical size(nm)
d
c
0.30
0.15
0.00
10
20
30
40
50
60
70
80
2θ (° )
0.15
0.00
Figure 4 XRD pattern of as-prepared Cu-ACAs catalysts
s of Cu-ACAs catalysts.
20 30 40 50 60 70
Partical size(nm)
10
20
30
Partical size(nm)
indicated that the quantity of external OCGs was greatly
Figure 6 TEM images of Cu-CAs (a), Cu-ACAs-A2 (b), Cu-ACAs-A4
(c) and Cu-ACAs-A6 (d) catalysts.
influenced by the KOH values.
XPS patterns of both the original and activated CA
samples showed characteristic peaks in the C 1s and O 1s
regions. The C 1s pattern for CAs in Figure 3(a) featured a
characteristic peak at 284.6 eV, attributable to sp2 carbon
atoms. Besides, peaks arose in 286.3, 287.6 and 289.4 eV were
corresponding to C–O, C=O, and O=C–OH, respectively. The
feature occurred at around 290.4 eV can be assigned as a p-p*
shake-up satellite peak43, 44. The three peaks at 531.2, 533.2,
and 535.5 eV in the O 1s pattern in Figure 3(b) also
demonstrated the existence of C–O, C=O, and O=C–OH,
respectively45, 46. The O 1s/C 1s peak area ratio (Table 4)
increased with increasing amounts of KOH, indicating that
element content was influenced by the indirect effects from
the KOH/CAs mass ratio.
XRD. In Figure 4, sharp diffraction peaks appeared at 43.3°,
50.5°, and 74.2° were vested in the characteristic (111), (200),
and (220) facets of metallic Cu (JCPDS no. 65-9026),
respectively, suggesting that Cu2+ was reduced to Cu0 by H2.
The peak at 2θ=36.4° demonstrated the presence of Cu2O
(JCPDS no. 05-0667), probably arising from incomplete
reduction of Cu2+. This confirmed that the active species on the
supports were identical. The diffraction peak in Cu-ACAs-A6
was sharper than that of the other samples, suggesting that
dispersion of the active species.
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H2-TPR. H2-TPR figures of Cu-ACAs catalysts were exhibited in carbon-supported Ru catalysts47. Gong et al. also VoiebwseArrtviceledOntlhinee
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DOI: 10.1039/C9NJ01434A
Figure 5. In every specimen, three main obvious peaks were existence of OCGs was favorable to the Ru dispersion .
observed. The first peak at around 150 °C corresponded to the Additionally, the presence of abundant micropores in ACAs
dispersed CuO. The second reductive peak centred at 225 °C is was a further important factor for high dispersion of Cu. As
in virtue of the reduction of crystalline CuO and the partial indicated in Figure 1, the amount of micropores achieved the
reduction of Cu2+ to Cu+. The last peak centred at 250 °C was maximum at KOH/CAs of 4, which greatly contributed to
caused by the reduction of Cu+ to Cu0, implying a strong decreasing the copper particle size. Kowalczyk et al. concluded
interaction of Cu+ species and support. Notably, KOH that pores with diameter <3 nm, acting as anchor sites,
treatment of the CAs brought about a shift of the reduction facilitated the formation of fine Ru particles over carbon49.
peak of Cu species, and the reduction peak of CuO transferred Hence, the synergistic effect between increased specific
to higher temperature. The results indicated that the surface area and more abundant exterior OCGs favored the
appropriate OCGs can strengthen the copper-support dispersion of copper species.
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interaction, thus forming smaller Cu particles.
H2-N2O titration. N2O titration examination were performed to
TEM. The TEM images in Figure 6 showed that copper particles confirm the dispersion of Cu, and the consequential data were
sizes were sized down from 40 to 11 nm, indicating Cu presented in Table 5. It was notable that the dispersion of Cu
increased incrementally with increasing KOH/CAs value, and
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
a
b
Cu-CAs
Cu-CAs
Cu-ACAs-A2
Cu-ACAs-A2
Cu-ACAs-A4
Cu-ACAs-A6
Cu-ACAs-A4
Cu-ACAs-A6
Conversion of Cu-CAs
Selectivity of Cu-CAs
Conversion of Cu-ACAs-A4
Selectivity of Cu-ACAs-A4
928
930
932
934
936
938
905
910
915
920
925
Kinetic energy (eV)
Binding energy(eV)
0
10 20 30 40 50 60 70 80 90 100
Stream on time (h)
Figure 7 Cu 2p3/2 XPS (a) and Cu LMM Auger (b) spectra of Cu-
ACAs catalysts.
Figure 8 Long-term stability tests of Cu-CAs and Cu-ACAs-
A4 at T = 330 ºC, P = 2.0 MPa and H2/MA = 60.
uniformly dispersed on the CAs and ACAs supports. The
average Cu particle size progressively decreased with
increasing KOH/CAs ratios, reaching a minimum at a ratio of 4,
but then increased at a ratio of 6.
On the one hand, the presence of OCGs resulted in an
enhancive dispersion of Cu particles and a lessened trend in
their size. Some investigator discovered the positive effect of
exterior functional groups on support on the dispersity of Ru in
achieved a maximal value of 26.4% with a ratio of 4, beyond
which a decrease was observed. This tendency was generally
consistent with the TEM images. Accordingly, the calculated
copper surface areas in the reduced catalysts increased
incrementally, reaching a maximum at a KOH/CA ratio of 4, but
decreased on further increasing the ratio to 6. Therefore, Cu-
Table 4 Surface elemental composition of Cu-ACAs catalysts from XPS analysis.
C 1s
O 1s
Samples
O/C
Peak 1
Peak 2
Peak 3
Peak 4
Peak 1
Peak 2
Peak 3
(284.6 eV)
(286.3 eV)
(287.6 eV)
(289.4 eV)
(531.2 eV)
(533.2 eV)
(535.5 eV)
Cu-CAs
52.8
47.7
45.1
44.6
14.8
17.4
27.9
24.3
11.4
12.4
13.6
13.6
20.8
22.3
13.2
17.2
0.2
0.3
0.4
0.4
39.6
61.1
21.1
75.6
42.4
24.1
28.9
12.9
20.9
14.7
49.9
11.4
Cu-ACAs-A2
Cu-ACAs-A4
Cu-ACAs-A6
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DOI: 10.1039/C9NJ01434A
a
b
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0.10
0.05
0.00
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Partical size (nm)
Figure 9 TEM images of the used Cu-CAs (a) and used Cu-ACAs-A4 (b) catalysts.
ACAs-A4 displayed the optimal catalytic performance due to its Stability of Cu-CAs and Cu-ACAs-A4 catalysts for MA
greatest dispersion and largest copper specific surface area.
hydrogenation. Time on stream of 100 h for catalytic
Table 5 Physicochemical properties of Cu-ACAs catalysts.
Cu loadinga
Dispersionb
Area of Cu LMMd (%)
Cu surface areab
(m2/g-Cat)
Cu particle sizec
(nm)
Samples
(%)
Cu0
42.8
27.2
25.3
29.1
Cu+
(%)
Cu-CAs
30.0
27.8
30.6
28.3
10.3
11.4
13.9
10.6
69.7
77.1
94.0
71.7
25
15
11
21
57.2
72.8
74.7
70.9
Cu-ACAs-A2
Cu-ACAs-A4
Cu-ACAs-A6
a Determined by AAS.
b Calculated from N2O titration.
c Observed by TEM analysis.
d
Intensity
ratio
of
Cu+
and
Cu0
to
(Cu+
+
Cu0)
obtained
by
deconvolution
of
Cu
LMM
XAES
spectra.
XPS. XPS spectrum was obtained to judge the surface chemical performance was performed to certify stabilities of the Cu-CAs
states of the catalysts, as revealed in Figure. 7. Two Cu 2p3/2 and Cu-ACAs-A4 catalysts at 330°C, 2 MPa and H2/MA=60.
spectra position at 934.4 and 932.3 eV were ascribed to Both MA conversion and ethanol selectivity were displayed in
bivalent copper and (Cu+ and Cu0), respectively (Figure 7(a)). Figure 8. The Cu-CAs and Cu-ACAs-A4 catalysts exhibited
To distinguish Cu+ and Cu0 50–53, Cu Auger spectra of the decrease tendency in MA conversion and EtOH selectivity in
catalysts were obtained and revealed in Figure 7(b), in which different extents. The MA conversion and EtOH selectivity for
wide and symmetric peaks centered at 914.6 and 917.5 eV the Cu-ACAs-A4 catalyst decreased 35.7 and 18.4%,
were allocated to Cu+ and Cu0, respectively54. It was worth respectively, which is only half of the corresponding values of
noting that the proportion of Cu+/ (Cu0+Cu+) ratio increased 60.5 and 35.7% for the Cu-CAs until the reaction time of 100 h.
incrementally, reaching
KOH/CAs of 4, but decreased on further increasing the stability for MA hydrogenation than the Cu-CAs catalyst.
KOH/CAs ratio (Table 5). Thus, the distributions of superficial TEM images of the used Cu-CAs and used Cu-ACAs-A4
a maximum value of 74.7% at It is demonstrated that the Cu-ACAs-A4 possessed far superior
Cu+ and Cu0 were consumedly affected by the mass ratio of catalysts were displayed in Figure 9. It is obvious that there
KOH/CAs. These results illustrated that treatment of the Cu- was severe agglomeration of copper particles in the used Cu-
CAs catalyst with KOH was an effective means of controlling CAs catalyst after reaction for 100 h. While in the used Cu-
the distributions of surface Cu+ and Cu0.
ACAs-A4 catalyst, the average particle size was ~33 nm (far
smaller than that in the used Cu-CAs), although the
aggregation could be observed.
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As displayed in Figure 10 and Table 6, for both the used Cu-ACAs–A4. From the XPS signal, the higher bindViniegw AernticelergOynlionef
+
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DOI: 10.1039/C9NJ01434A
Cu-CAs and used Cu-ACAs-A4, the peaks at 934.4eV Cu than that for a bulk sample , indicating the existence of a
corresponding to Cu2+ species significantly weakened in strong Cu+-support interaction, which would favor formation
comparison with those in fresh catalyst, indicating Cu species of Cu+ during catalysts reduction. TPR trace (Figure 5) also
were apt to be reduced during the MA hydrogenation reaction. showed that the reduction peaks of copper supported on ACAs
However, the proportion of Cu+/(Cu0+Cu+) ratio in the used Cu- catalysts were transferred to an elevated temperature
ACAs-A4 was higher than that in Cu-CAs. This result compared with that of Cu-CAs by the vigorous interaction
demonstrated the positive influence of micropores and surface force in Cu+ and supports.
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OCGs on the stability of both Cu particle size and valence state
Firstly, the forceful interaction force in Cu+ species and
of Cu species, which greatly improved the stability of catalyst.
supports originated primarily from the anchoring function of
Discussion
As revealed in Table 1, Cu-ACAs-A4 catalyst displayed the
micropores61, as mentioned above. Additionally, OCGs also
greatly contributed to strengthening the interaction between
Cu+ descriptions and activated supports. As depicted in Table
3, the total amount of OCGs increased with increasing KOH/CA
ratios, as proven by Boehm’s titration. Our previous study
based on DFT calculation62 also demonstrated that C=O groups
significantly enhanced the binding energy between copper
clusters and activated supports. Consequently, both abundant
micropores and a high density of functional groups in ACAs-A4
can heighten the copper particles dispersity and increase the
number of superficial Cu+ groups, further improving the
hydrogenation activity. Our results showed that adoption of
chemical activation of CAs in copper-based catalysts effectively
controlled dispersion of Cu and distribution of surface Cu+.
optimal catalytic activity in the MA hydrogenation. Generally,
the catalytic performances of Cu catalysts had a bearing on
their Cu dispersion and Cu+/ (Cu++Cu0) ratio55. In this work,
carbon aerogels with abundant micropores and high specific
surface areas were achieved through activation with KOH. The
highest BET surface area of 2562 m2 g-1 permitted admirable
dispersion of copper nanoparticles, further improved the
catalytic performance35. Additionally, as anchor sites, the
appearance of a crowd of micropores in support greatly
enhanced the dispersion of Cu particles56, as verified by TEM
analysis (Figure 6) and N2O titration (Table 5). As indicated in
Table 2, accompanied with a diminution of surface areas, the
micropores volumes of the ACAs supports decreased sharply
after loading copper component. In particular, for Cu-ACAs-A4,
the micropores volume was only one-third of that for ACAs-A4,
indicating that part of Cu species could reside at the micropore
opening over ACAs support38. Moreover, FTIR analysis showed
a band due to organic functional groups at =2550 cm-1, the
amount of which was highly dependent on the KOH/CA ratio
according to Boehm titration and XPS analyses. The increase in
the amount of surface functional groups also contributed to
improving Cu dispersion and further enhancing catalytic
activity.
Conclusions
Cu nanoparticles supported on activated carbon aerogels
(ACAs) have been prepared by a convenient wet chemical
approach and applied for ethanol preparation through gas-
phase MA hydrogenation. KOH activation greatly heightened
the surface area, the number of micropores, and the content
of functional groups on carbon aerogels supports. The highest
surface area of ACAs was achieved up to 2562 m2 g-1 when the
mass ratio of KOH/CAs was 4. Both the dispersion of Cu and
proportion of Cu+ were significantly enhanced in the Cu-ACAs
compared with Cu-CAs owing to the strengthened
interreaction between Cu species and the ACAs supports
originated from micropores and functional groups. Cu-ACAs-4
As reported in the literature57, 58, for ester hydrogenation,
the synergistic effect in Cu0 with Cu+ was of critical importance
to catalytic activity in ester hydrogenation. Cu0 species
promoted the splitting of hydrogen, and Cu+ species can
adsorb and stimulate the CH3O and the acyl group59. As
revealed in Table 1 and 5, the activity of Cu catalyst increased
with increasing proportions of Cu+, and the highest MA
conversion was achieved at a Cu+/(Cu++Cu0) ratio of 74.7% in
Table 6. Chemical state of copper in the used Cu-ACAs-A4 and
used Cu-CAs catalysts.
Area of Cu LMMa (%)
Samples
Cu0
60.5
51.7
Cu+
39.5
48.3
Used Cu-CAs
Used Cu-ACAs-A4
a Intensity ratio of Cu+ and Cu0 to (Cu+ + Cu0) obtained by deconvolution of
Cu LMM XAES spectra.
Figure 10 XPS spectra of Cu 2p3/2 (a) and Cu LMM (b) of
the used Cu-CAs and used Cu-ACAs-A4.
8 | J. Name., 2012, 00, 1-3
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catalyst displayed the most admirable catalytic performance,
giving a MA conversion (96.7%) and an ethanol selectivity
(62.2%), which can be ascribed to its highest Cu dispersion of
13.9% and Cu+ proportion of 74.7%. Considering the facile
procedure and inexpensive materials, such chemical
modification of carbon materials can develop new possibilities
for the rational invention and fabrication of related catalysts
showing excellent performances.
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Acknowledgements
This work has been supported by a grant from the
National Natural Science Foundation of China (21776194,
21606159 and 21808154), and Key Research and Development
Program of Shanxi Province (201703D121022-1 and
201803D121039).
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DOI: 10.1039/C9NJ01434A
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DOI: 10.1039/C9NJ01434A
Graphic abstract
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