Structured nanoporous-gold/Al-fiber: Galvanic deposition preparation and reactivity for the oxidative coupling of methanol to methyl formate

Article abstract of DOI:10.1039/c3gc42561d

Microfibrous-structured nanoporous gold (NPG) on Al-fiber catalysts, obtained by galvanic deposition onto a 3D network using 50 μm Al-fiber, is cost-effective (low Au-loading, e.g., 1 wt%), highly active/selective (>90% selectivity at >50% conversion) and stable (at least 300 h without NPG sintering) for oxidative coupling of methanol to methyl formate. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3gc42561d

Green Chemistry
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Structured nanoporous-gold/Al-fiber: galvanic
deposition preparation and reactivity for the
oxidative coupling of methanol to methyl formate†
Cite this: Green Chem., 2014, 16,
992
2
Received 17th December 2013,
Accepted 18th March 2014
Qiaofei Zhang, Yakun Li, Li Zhang, Li Chen, Ye Liu and Yong Lu*
DOI: 10.1039/c3gc42561d
www.rsc.org/greenchem
Microfibrous-structured nanoporous gold (NPG) on Al-fiber cata- consideration because of the strong exothermicity of the OCM
lysts, obtained by galvanic deposition onto a 3D network using process. The metal oxide catalysts as well as HPAs are not
5
0 μm Al-fiber, is cost-effective (low Au-loading, e.g., 1 wt%), active and/or selective enough from the industrial standpoint,
highly active/selective (>90% selectivity at >50% conversion) and and their poor thermal conductivity will cause heat transfer
stable (at least 300 h without NPG sintering) for oxidative coupling limitations. Graphene-supported Ag–Pd nanocatalysts, with a
of methanol to methyl formate.
good heat conductivity, are reported to show excellent low-
temperature activity and selectivity, delivering 90% conversion
and 100% selectivity at 70 °C.
6
Methyl formate is an important commodity chemical, used as
a solvent, an environmentally friendly blowing agent and pre-
cursors for formic acid, formamide, and dimethyl formamide.
The current industrial process for methyl formate production
is based on the homogeneous carbonylation of methanol in
the presence of sodium methoxide (a caustic liquid base) cata-
Recently, nanoporous gold (NPG) has attracted increasing
attention because of its wide range of applications including
1
7
8
9
catalysis, cell imaging, energy storage/transformation, and
10
the development of chemical sensors. Fabricated by selec-
tively leaching silver from Ag–Au alloys, NPG possesses high
thermal/electrical conductivity, three-dimensional nanostruc-
ture, and much larger characteristic lengths both in ligament
lyst, using ultra-highly purified CO as a feedgas (H O- and
2
2
CO -free). This method is generally linked to higher risk for
2
environmental issues and faces serious problems in product
separation, obviously conflicting with the concept of green
11
and nanopore diameters. Owing to an abundance of surface
defects and residual Ag, the bulk NPG exhibits excellent cataly-
3
chemistry. There is thus an urgent need to develop a greener
7,12
tic performance in some important reaction processes
such
method, aimed at using solid catalysts under heterogeneous
conditions.
as the OCM to MF. Most notably, the bulk NPG provides high
resistance to sintering compared to the Au-NP catalysts, which
In this context, the oxidative coupling of methanol (OCM)
to methyl formate (MF) is attractive as a simple and environ-
mentally benign process. In addition, the flourishing of the
methanol economy generates a further impetus to explore the
OCM process, which is superior to other routes for methanol
conversion and utilization regarding the extension of the
methanol industrial chain. Several catalysts, including non-
7
is an important consideration for practical applications.
However, the high cost of the bulk NPG hampers its practical
application as a catalyst for the OCM process.
In this communication, we present a cost-effective micro-
fibrous-structured NPG on the Al-fiber catalyst system for the
OCM to MF. The NPG/Al-fiber catalysts are obtained on the
macro-scale by galvanically depositing Au–Ag alloy onto a thin-
sheet 3-dimensional (3D) network structure using 50 μm Al-
fiber followed by Ag-leaching. A methanol conversion of >50%
is achieved with a MF selectivity of >90% at 150 °C, using a
feedgas of MeOH/O /N = 2/1/17 (10 vol% methanol). The cata-
noble metal oxides (such as SnO
2
–Mo
2
O
3
, V
2 5 2
O –TiO and
4
5
Fe O /SiO ), heteropoly acids (HPAs) and noble metal
2
3
2
6
systems, have been extensively investigated in OCM reactions.
However, their practical applications are particularly challen-
ging. Besides concerns on the catalytic performance, the
thermal conductivity of the catalysts is also an important
2
2
lyst is stable for at least 300 h without NPG sintering. Gold
loading can be reduced down to 1 wt% without any unaccepta-
ble reactivity decline. Leaching of Ag is essential for the OCM
selectivity of the NPG/Al-fiber catalysts, by nature, playing a key
role in constructing the 3D nanopore structure and tuning the
surface O species.
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of
Chemistry, East China Normal University, Shanghai 200062, P.R. China.
E-mail: ylu@chem.ecnu.edu.cn; Fax: +86 21 62233424; Tel: +86 21 62233424
Fig. 1 shows the preparation strategy and the structural
features from the micro- to macro-scale of representative NPG/
†
Electronic supplementary information (ESI) available: Supplementary text for
TOF calculation; Fig. S1, Table S1. See DOI: 10.1039/c3gc42561d
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as Al-fibers are wetted by HAuCl
4
or AgNO
3
aqueous solution
at room temperature, due to the large electrode potential
3
+
0
−
0
4
difference between the Al /Al (−1.6 V) and [AuCl ] /Au
(
+
0
13
1.0 V) or Ag /Ag (0.8 V) pairs. Alloying of Au–Ag is facile, which
can proceed by calcining the samples at 300 °C (Fig. 1c-3).
After that, gold contents in the Au–Ag alloys are measured by
ICP-AES, for all the resulting Au–Ag/Al-fiber samples, to be
∼45 wt%. This meets the criteria on Au content in Au–Ag alloy
for getting a nanopore structure by Ag-leaching, since no
porosity evolution can be observed as the Au content is higher
than 50 wt% for the Au–Ag alloy system according to Bischoff
1
4,15
et al.
Subsequently, the Au–Ag/Al-fiber sample is immersed into a
6
5% HNO aqueous solution to selectively leach Ag from Au–
3
Ag alloy for the desired time length (e.g., 24 h) at room temp-
erature (Fig. 1c-4) while passivation of the Al-fiber surface auto-
matically proceeds, thereby avoiding the dissolving of Al-fiber.
As a result, NPG/Al-fiber catalysts are obtained with a well-
preserved thin-sheet structure (Fig. 1d). Clearly, for the repre-
Fig. 1 Galvanic deposition (GD) preparation and structural features sentative NPG-7/Al-fiber catalyst (Au: 7.1 wt%, Table 1), a very
from the micro- to macro-scale. Photograph (a) and SEM image (b) of
the Al-fiber substrate. (c) Preparation of schematic illustration (stepwise
fiber (Fig. 1e). The high magnitude SEM image shows that the
galvanic deposition of Au and Ag at the Au/Ag weight ratio of 45/55;
uniform NPG shell (∼5 μm thick) is formed along with the Al-
supported NPG exhibits a 3D nanopore structure almost the
alloying is at 300 °C in He for 2 h; Ag leaching at room temperature for
1
1,16
24 h, using 65 wt% HNO
3
solution). Photograph (d), SEM (e), and N
2
same as the reported bulk NPG
(Fig. 1e-3). Notably, when
adsorption–desorption isothermal and Barrett–Joyner–Halenda (BJH) reducing the Au loading to or lower than 3 wt%, NPG islands
mesopore size (f) of the representative NPG-7/Al-fiber catalyst.
are formed instead of the uniform shell (Fig. S1, ESI†).
The NPG-7/Al-fiber presents a N
1
2
-BET specific surface area of
6 m g and shows a mesopore feature evidenced by the step
at P/P of 0.4–0.6 on the N adsorption–desorption isothermal
2
−1
Table 1 The OCM to MF, catalysed by NPG/Al-fiber catalysts
0
2
with a Barrett–Joyner–Halenda (BJH) mesopore size of 4–8 nm
Fig. 1f).
We initially investigated the effect of Au loading on the cata-
a
b
c
c
d
[
Au]/[Ag]
S
BET
S
NPG
X
S
TOF
(s
(
(m g−1) (m g−1) (%) (%)
2
2
−1
)
Catalyst
(wt%)
e
e
NPG-7/Al-fiber 7.1/0.6 (5.6/6.8) 16
NPG-4/Al-fiber 3.9/0.3 (2.8/4.5)
NPG-2/Al-fiber 1.9/0.2 (1.2/2.7)
14
7
7
6
7
50
32
15 100
8
50
94
95
0.4
0.9
0.9
1.1
0.2
lytic performance of NPG/Al-fiber in the OCM to MF (Table 1).
At 140 °C, the MF selectivity (>90%) is almost independent of
the Au loading but not the methanol conversion. The NPG-7/
Al-fiber catalyst delivers the highest methanol conversion of
50%, outperforming the NPG-1/Al-fiber by at least 5-fold. Note
that all the catalysts provide a TOF of 0.4–1.1 s− , indicating
9
9
8
7
NPG-1/Al-fiber 1.0/0.1 (0.9/1.7)
100
91
f
g
e,g
Bulk NPG
99/1 (30/70)
a
Data in and outside the parentheses are the Au-/Ag-loading in NPG/
Al-fiber samples before and after Ag leaching for 24 h, determined by
1
inductively coupled plasma-atomic emission spectrometry (ICP-AES). almost equivalent intrinsic activity. This observation is not sur-
b
S
NPG: surface area of NPG, assumed to be the mesopore surface area.
c
prising because of the similarity in nanostructure and compo-
Methanol conversion (X) and MF selectivity (S) at 140 °C (0.2 g Cat.,
−
1
−1
sition of NPG in all the cases (Table 1, Fig. 1 and Fig. S1†). Yet,
MeOH/O
2 2
/N
= 2/1/17 (10 vol% methanol), GHSV = 5000 mL gcat.
h ).
d
−1 −1
−1
Turnover frequency (TOF, molMF molsurf.-Au
s ) based on MF yield TOF for the bulk NPG was obtained to be 0.2 s under identi-
e
and surface Au (ESI for details). CO
2
is a dominant byproduct and
f
cal reaction conditions.
CH
nitric acid (HNO
diluted with 25–50 mesh quartz sand (0.15 g).
2
O is trace. Bulk NPG is prepared by etching Ag–Au alloys in conc.
g
Fig. 2 shows the temperature dependence of OCM conver-
sion and selectivity for the representative NPG/Al-fiber cata-
lysts. Similar conversion and selectivity evolution behaviours
are observed for the temperature in all cases. The NPG-7/
3
, 65 wt%) by free corrosion. Using 0.05 g bulk NPG
Al-fiber catalysts. The specific surface area, Au loading, and Al-fiber offers higher conversion than the other ones in the whole
activity/selectivity of the NPG/Al-fiber samples are collected in temperature range tested. The methanol conversion shows a
Table 1. A microfibrous structure consisting of 10 vol% 50 μm dramatic increase with increasing the temperature to 160 °C,
aluminium-fiber and 90 vol% void volume, with a thin-sheet and then almost remains constant with further increasing the
form and characteristic irregular 3D networks (Fig. 1a and 1b), temperature to 200 °C. The MF selectivity shows reverse evolu-
is employed as a substrate. Stepwise deposition of Au and Ag tion behaviour along with the increase of temperature.
onto the Al-fiber is realized with the aid of a galvanic exchange Notably, for OCM using NPG/Al-fiber, high Au loading is
reaction (denoted galvanic deposition [GD]; Fig. 1c-1 and 1c-2). required for achieving low-temperature activity while an ele-
Note that the GD processes can automatically proceed as soon vated reaction temperature is needed for low-Au-loading catalysts
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Fig. 2 Methanol conversion (a) and methyl formate selectivity (b) for
temperature-dependent OCM over NPG/Al-fiber catalysts with varied Au
loadings of 1 wt% (■), 2 wt% (□) and 4 wt% (●), 7 wt% (○). Reaction con-
= 2/1/17, GHSV = 5000 mL gcat.−
1
h
−1
.
ditions: 0.2 g Cat., MeOH/O
2
/N
2
Fig. 3 (a) Methanol conversion and MF selectivity for the OCM to MF
2
vs. time on stream using NPG-2/Al-fiber catalysts (0.2 g Cat., MeOH/O /
= 2/1/17 (10 vol% methanol), 170 °C, GHSV = 5000 mL gcat.−
1
h ).
−1
N
2
SEM images of NPG-2/Al-fiber: (b) after 300 h on stream and (c) fresh.
to delivery good reactivity. At 100 °C, the NPG-7/Al-fiber is
capable of achieving ∼100% MF selectivity with ∼25% metha-
nol conversion of a feed of 10% methanol at a GHSV of
tion. A very low ΔT (between the bed and reactor external
walls) of ∼15 °C was observed throughout the entire 300 h test.
Isothermal operation is not only a safety consideration but a
very important feature that benefits activity and selectivity
000 mL gcat.−
1
h
−1
. With increasing the temperature to
5
1
80 °C, a high methanol conversion of 57% is obtainable over
this catalyst but the MF selectivity declines to 85% (intermedi-
ate CH O accounts for 10% and undesired combustion to CO
2
2
17
maintenance.
accounts for the remaining 5%). Over the NPG-1/Al-fiber (Au:
only 1 wt%), a conversion of 35% can still be obtained with an
MF selectivity of 85% at 160 °C. In addition, two Ag-7/Al-fiber
and Au-7/Al-fiber catalysts are also prepared by the GD method
and are tested in the OCM reaction process. Both of them,
with the bulk-like Ag and Au on the Al-fibers, are inactive for
the OCM to MF, strongly confirming that the nanoporous
As previously noted, the activity and selectivity for bulk
16,18
NPG are very sensitive to the Ag residual content.
Whether
the chemical feature exhibits the tuning ability for the reacti-
vity of the NPG/Al-fiber catalysts and to what extent, however,
is not clear. To gain insight into it, we investigate the OCM on
the samples exhibiting the same morphology but varied Ag
residual contents tuned from 13 at% (almost cannot be
reduced further by prolonging the leaching time in the present
work) to 38 at% (by ICP-AES) for NPG-1/Al-fiber by controlling
the extent of Ag leaching. As shown in Fig. 4a, both selectivity
and activity (especially at middle range temperatures) are
dependent on the Ag residual content. In the temperature
range from 140 to 180 °C, the selectivity to MF declines visibly
with increasing the Ag residual content from 13 at% to 25 at%
and then remains almost unchanged from 25 at% to 38 at%
1
6
structure is essential to the OCM reaction process.
As previously noted, the bulk NPG is active (10% conver-
sion) and selective (100% MF selectivity) even at room temp-
erature while a high conversion of 60% is obtainable with 97%
1
6
MF selectivity only at an elevated temperature of 80 °C. In
addition, during a 17-day testing at 60 °C, the bulk NPG cata-
lyst shows a continuous decline in conversion with a rate of
1
6
∼
6% per 24 h. More recently, the Au2.0–Pd1.0/graphene cata-
lyst is reported to be active and stable for methanol conversion
6
(85–90%) with 100% selectivity at 70 °C for 48 h running.
However, these catalysts are far away from practical appli-
cations because of the high cost, the low methanol concen-
tration (1.0–6.4 vol%) and/or the challenge of regeneration.
Besides the significant reduction of gold loading, our NPG/
Al-fiber catalysts demonstrate superior stability over the above
mentioned catalyst systems. A long-term test on the NPG-2/Al-
fiber catalyst for the OCM to MF is carried out at 170 °C using
−
1
−1
a GHSV of 5000 mL gcat.
00 h, showing excellent conversion (∼45%) and selectivity
∼85%) maintenance throughout the entire testing (Fig. 3a).
h . The catalyst is stable for at least
3
(
Not surprisingly, the NPG for the used catalyst still preserves
its 3D nanostructure very well (Fig. 3b) with a surface area of
2
−1
8
m g , being almost unchanged compared to the fresh cata-
Fig. 4 (a) Methanol conversion and MF selectivity for the OCM to MF
vs. Ag residual contents for NPG-1/Al-fiber catalysts (0.2 g Cat., MeOH/
lyst (Fig. 3c). Another advantage of this catalyst is that its excel-
lent heat conductivity can rapidly dissipate a large quantity of
−1
−1
O
2
/N
2
= 2/1/17 (10 vol% methanol, GHSV = 5000 mL gcat.
h ).
reaction heat from such a strongly exothermic oxidation reac- (b) O
2
-TPD profiles of the catalyst samples after the reaction.
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+
while the methanol conversion decreases continuously. The exchange reaction of Ag with Al, by impregnating the Au/
higher Ag residual content causes a loss of selectivity bigger Al-fiber with an aqueous solution containing the specified
than the lower Ag content as the reaction temperature amount of AgNO
increases: [97% (140 °C) → 66% (200 °C)] for 38 at% Ag vs. Au and Ag deposited onto the Al-fiber is performed by calcin-
100% (140 °C) → 80% (200 °C)] for 13 at% Ag. ing the samples at 300 °C under an inert helium atmosphere
Temperature-programmed desorption (TPD) experiments for 2 hours to give Au–Ag/Al-fiber. At last, the Au–Ag/Al-fiber
are performed and dual-peak profiles are obtained on all the samples are immersed into a 65% HNO aqueous solution to
samples with varied Ag residues with almost equivalent peak selectively leach Ag from Au–Ag alloy for the desired time
areas (i.e., an equivalent amount of desorbed O ) (Fig. 4b, length (e.g., 24 h if not specified) at room temperature while
3
at room temperature. After that, alloying of
[
3
2
Table S1†). Most interestingly, as the Ag residual content the passivation of the Al-fiber surface proceeds automatically,
decreases, the low-temperature O -TPD peak (centred at thereby avoiding the dissolving of Al-fiber.
2
∼340 °C) increases slightly while the high-temperature peak
(centred at 435 °C) shows a downward trend (Fig. 4b,
Characterization
Table S1†). By correlating the findings with the above reaction
results, it is rational to infer that surface O species desorbed at
The NPG/Al-fiber catalysts are characterized using a scanning
electron microscope (SEM, Hitachi S-4800). The specific
surface area is determined from a N2 adsorption isotherm
using standard Brunauer–Emmett–Teller (BET) theory on a
Quantachrome Autosorb-3B instrument. The pore size distri-
bution and the total pore volume are determined using the
BJH method. Temperature programmed desorption (TPD) was
performed on a Quantachrome ChemBET 3000 chemisorption
apparatus with a TCD and an online mass spectrometer
∼340 °C are selectively active for the OCM reaction and the
ones desorbed at ∼435 °C are inactive at low reaction tempera-
ture (e.g., 140 °C) but active for deep oxidation at high reaction
temperature (e.g., 200 °C).
Conclusions
(
Proline Dycor, AMETEK Process Instrument, USA). The
Cost-effective, highly active/selective and stable NPG on Al-
fiber catalysts are developed for the OCM to MF. The NPG/Al-
fiber catalysts are obtainable by galvanically depositing Au–Ag
alloy onto a 3D network structure using 50 μm Al-fiber fol-
lowed by Ag-leaching. Gold loading can be reduced down to
amounts of Au and Ag for the NPG/Al-fiber catalysts are quanti-
tatively determined by inductively coupled plasma atomic
emission spectrometry (ICP-AES) on a USA Thermo IRIS
Intrepid IIXSP ICP spectrometer.
1
wt% without an unacceptable reactivity decline. The catalyst
Reactivity tests
is stable for at least 300 h without association of NPG sinter-
ing. Lowering the Ag residual content is essential for higher The gas-phase selective oxidation of methanol on these cata-
selectivity of OCM, by nature, decreasing the amount of un- lysts with molecular oxygen was performed on a fixed-bed
selective surface O species. We expect that our findings will pave quartz tube reactor (600 mm length by 16 mm inner diameter)
the way for innovative applications of NPG in the near future.
under atmospheric pressure. Circular chips (16.1 mm dia-
meter) of the microfibrous-structured NPG/Al-fiber catalysts
are punched down from their large sheet sample and packed
layer-up-layer into the tube reactor. The catalyst used in testing
experiments weighs 0.2 g. Notice that the 0.1 mm larger dia-
meter than the inner diameter of the tubular reactor was deli-
Experimental section
Catalyst preparation
The thin-sheet 3-dimensional (3D) sinter-locked network struc- berately retained to avoid the appearance of a gap between the
ture using 50 µm Al-fiber is employed as a support, which was reactor wall and the edges of the catalyst chips, thereby pre-
purchased from Shanghai Xincai Screen Manufacturing Co. venting gas bypass. A gaseous mixture of MeOH/O
2 2
/N (2/1/17;
Ltd. Prior to stepwise galvanic deposition, the microfibrous- 10 vol% methanol) is employed as feedstock. Such feed is
−
1
structured 50 µm Al-fiber substrate is cleaned by immersing it obtained by controlling oxygen (oxidant, 5 mL min ) and
in a sodium hydroxide (1 wt% NaOH) aqueous solution at nitrogen (diluted gas, 85 mL min− ) to pass through a metha-
1
room temperature for removing the Al
2
O
3
film.
nol vapor saturator prior to feeding into the catalytic bed. Two
Gold and silver are stepwise deposited onto the Al-fiber calibrated mass flow controllers are used to control the oxygen
surface of the microfibrous substrate with the aid of a galvanic and nitrogen gases. The methanol vapor saturator is immersed
exchange reaction. First, gold is galvanically deposited onto into a water bath precisely controlled at 20 °C. The product
the cleaned Al-fiber by impregnating it with an aqueous solu- effluent, as well as the methanol concentration in the feed
4
tion containing the specified amount of HAuCl at room temp- mixture, is analyzed online by an HP 6850 gas chromatography
erature. The solution gradually turns from a bright yellow chromatograph equipped with a thermal conductivity detector
color to colorless within 30 min. The resulting sample is (TCD) connected to Porapak Q and MS 5A parallel capillary
washed with distilled water until there is no chloride ion to column (DIKMA). The gas pipeline and sampling 6-way valve
obtain the Au/Al-fiber sample. Subsequently, silver is placed between the reactor outlet and the GC injector are heated to
on the Au/Al-fiber sample again with the aid of a galvanic ensure that the effluent is completely evaporated.
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This work was funded by the “973 program” (2011CB201403)
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2
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