In situ DRIFTS study of CO coupling to dimethyl oxalate over structured Al-fiber@ns-AlOOH@Pd catalyst

Article abstract of DOI:10.1016/j.jcat.2016.09.031

Pd-catalyzed CO coupling to dimethyl oxalate (DMO) process has been commercialized but its reaction mechanism is still open for debate between COCOOCH₃^? and COOCH₃^? (^?, a surface site). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies were performed to clarify such controversy on a high-performance Al-fiber@ns-AlOOH@Pd catalyst. Intermediate species consisting of stretching vibrations of C[dbnd]O and C[sbnd]O, and rocking vibration of CH₃ was indeed captured, which could be assigned to either COCOOCH₃^? or COOCH₃^?. To make discrimination between COCOOCH₃^? and COOCH₃^?, methyl oxalyl chloride (CH₃OCOCOCl) and methyl chloroformate (CH₃OCOCl) were employed to form COCOOCH₃^? and COOCH₃^? species on the catalyst surface for DRIFTS analyses. Interestingly, the characteristic bands of the as-observed intermediate species in the real reaction matched the obtained COCOOCH₃^? species from dissociative adsorption of CH₃OCOCOCl. A double carbonylation reaction pathway was thus confirmed for the CO coupling to DMO, i.e., consecutive insertion of two CO molecules into OCH₃^? to form COCOOCH₃^? followed by combining OCH₃^? to yield DMO (CH₃OCOCOOCH₃).

Full text of DOI:10.1016/j.jcat.2016.09.031

Journal of Catalysis 344 (2016) 173–183
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
In situ DRIFTS study of CO coupling to dimethyl oxalate over structured
Al-fiber@ns-AlOOH@Pd catalyst
⇑
Chunzheng Wang, Pengjing Chen, Yakun Li, Guofeng Zhao, Ye Liu, Yong Lu
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Pd-catalyzed CO coupling to dimethyl oxalate (DMO) process has been commercialized but its reaction
Received 2 June 2016
Revised 27 August 2016
Accepted 25 September 2016
⁄
⁄
⁄
mechanism is still open for debate between COCOOCH and COOCH ( , a surface site). In situ diffuse
3 3
reflectance infrared Fourier transform spectroscopy (DRIFTS) studies were performed to clarify such con-
troversy on a high-performance Al-fiber@ns-AlOOH@Pd catalyst. Intermediate species consisting of
stretching vibrations of C@O and CAO, and rocking vibration of CH
3
was indeed captured, which could
⁄
⁄
⁄
be assigned to either COCOOCH
3
or COOCH
OCOCOCl) and methyl chloroformate (CH
species on the catalyst surface for DRIFTS analyses. Interestingly, the
3
. To make discrimination between COCOOCH
3
and
Keywords:
Mechanism
⁄
COOCH
3
, methyl oxalyl chloride (CH
3
3
OCOCl) were employed
⁄
⁄
to form COCOOCH
3
and COOCH
3
DRIFTS
characteristic bands of the as-observed intermediate species in the real reaction matched the obtained
Structured catalyst
Palladium catalysis
Methyl oxalyl chloride
Dimethyl oxalate
⁄
COCOOCH
3
species from dissociative adsorption of CH
3
OCOCOCl. A double carbonylation reaction path-
way was thus confirmed for the CO coupling to DMO, i.e., consecutive insertion of two CO molecules into
⁄
⁄
⁄
OCH to form COCOOCH3 followed by combining OCH to yield DMO (CH OCOCOOCH ).
3
3
3
3
Ó 2016 Elsevier Inc. All rights reserved.
1
. Introduction
2CH
1/2O
ronmentally benign route.
In the past decades, the effects of supports (such as
3
ONO + H
2
O. Clearly, the overall reaction (2CO + 2CH
3
OH
+
2
? (COOCH
3
)
2
+ H O) leads to a chemical-looping and envi-
2
Green and sustainable production of commodity chemicals and
liquid fuels based on C1 chemistry via syngas (a mixture of CO and
) from abundant sources (such as biomass, natural gas and coal)
is an appearing alternative to the petrochemical-based routes [1–
]. In particular, dimethyl oxalate (DMO) is a versatile platform
2 3
a-Al O ,
H
2
boehmite, MgO, ZnO, activated carbon, carbon nanofiber, and so
on) [19–23], Pd shape/size [14,15] and promoters (e.g., Fe and
Ce) [24–25] have been intensively investigated to promote the cat-
alytic performance of Pd-based catalysts for the CO coupling to
DMO. Up to date, this process has been successfully industrialized
[14,26], but the reaction mechanism is still open for debate [19,27].
Based on experience, Waller [28] speculated a possible intramolec-
ular coupling mechanism for the first time that consists of two
steps: (1) initial oxidative addition of alkyl methyl (RONO) to Pd0
to form ON-Pd-OR and (2) subsequent CO insertion into ON-Pd-
OR to generate ON-Pd-COOR, which couples with another ON-Pd-
COOR to yield the oxalate (ROOC-COOR). Meanwhile, the authors
also proposed another double carbonylation pathway that involves
consecutive insertion of CO into ON-Pd-COOR to give ON-Pd-
COCOOR followed by coupling with ON-Pd-OR to form the oxalate
3
chemical for producing oxalic acid, ethanol, oxamide, vitamin B1,
dyes, pharmaceuticals, etc. [4–8], and its hydrogenation to ethy-
lene glycol has been successfully developed and industrialized
[
9–11]. The traditional production of DMO mainly depends on
the esterification between oxalic acid and methanol using inor-
ganic acid catalysts (e.g., H SO ), which causes serious equipment
2
4
corrosion and environmental pollution, obviously conflicting the
basic concept of green chemistry [12,13]. Therefore, the gas-
phase CO oxidative coupling to DMO, a process converting inor-
ganic C1 to organic C2, has been attracting ever-growing interest
due to the moderate reaction conditions, atom economy and inde-
pendence on oil [14–18]. This process involves two major chemical
0
reactions with methyl nitrite (MN: CH
catalyzed CO coupling: 2CO + 2CH ONO ? (COOCH
2) MN regeneration without catalyst: 2NO + 2CH
3
ONO) as a key agent: (1) Pd-
+ 2NO; and
OH + 1/2O
[28]. Uchiumi et al. [19] conjectured that Pd is the catalytic active
3
3
)
2
site and high NO partial pressure retards the DMO formation. Fur-
thermore, they also inferred that the RONO plays a key role in oxi-
(
3
2
?
0
2+
dizing Pd to Pd to generate ON-Pd-OR thereby producing alkoxyl
species for the oxalate formation. The X-ray photoelectron spec-
troscopy results by Gao et al. [29] also evidenced that the sole
⇑
Corresponding author.
E-mail address: ylu@chem.ecnu.edu.cn (Y. Lu).
http://dx.doi.org/10.1016/j.jcat.2016.09.031
021-9517/Ó 2016 Elsevier Inc. All rights reserved.
0

1
74
C. Wang et al. / Journal of Catalysis 344 (2016) 173–183
2+
0
0
Pd on pre-reduced catalyst is transformed into Pd /Pd after the
RONO adsorption; in the same work, it was further inferred that
work structure consisting of 10 vol% 60-lm Al-fiber and 90 vol%
voidage was utilized as the substrate. The Al-fiber felt was ultra-
sonically degreased in pure acetone and etched in sodium hydrox-
ide (0.1 wt%) aqueous solution. Then the pre-treated Al-fiber felt
was placed in steam flow at 120 °C for 6 h and dried at the same
2
+
two types of Pd observed in the reaction process represent two
possible intermediates, i.e., RO-Pd-OR and/or Pd(COOR) . Ji et al.
30] used in situ infrared (IR) spectroscopy to investigate the reac-
2
[
tion mechanism and concluded that RONO adsorption results in
the formation of ON-Pd-OR, and the intermediate of ON-Pd-
COOR is identified on the basis of its similar characteristic bands
2
temperature for 2 h in N flow to form a uniform shell of
honeycomb-like boehmite nanosheets (ns-AlOOH) along with the
Al-fiber core (denoted as Al-fiber@ns-AlOOH) [23,35]. The as-
prepared Al-fiber@ns-AlOOH felt was impregnated with a toluene
solution of palladium acetate by an incipient wetness impregna-
tion method, subsequently dried at 100 °C for 2 h and calcined in
static air at 300 °C for 2 h to obtain the product catalyst denoted
as Al-fiber@ns-AlOOH@Pd (Pd loading of 0.25 wt%). The Pd disper-
sion of this catalyst was determined to be 54% by CO pulse
chemisorption and the average Pd particle size was about 1.8 nm
observed by TEM images [35].
3
with the methyl formate (HCOOCH ). They also noted that only
bridged CO participates in the CO coupling reaction while linear
CO is unavailable for the oxalate formation [30]. In terms of the
reaction kinetics, Meng et al. [31,32] speculated that the surface
reaction between adsorbed CO and alkoxyl is the sole rate-
determining step, and NO has a suppression effect on the DMO for-
mation due to the reversible reaction between RONO and alkoxyl/
NO. Recently, Li et al. [27] employed density functional theory
(
DFT) to investigate three possible pathways of CO coupling reac-
tion and showed that: (1) the pathway of intramolecular coupling
is unfeasible due to the very high barrier of CAC coupling; (2) the
OCACO pathway (i.e., combination of two CO) can reduce the steric
hindrance but the OCCO species is readily decomposed and only
exists when CO is pre-adsorbed on the catalyst surface; and (3)
the double carbonylation pathway seems to be preferred owing
to the low barrier.
2.3. Catalytic reaction tests
Catalytic reactions were performed in a fixed-bed quartz tubu-
lar reactor (8-mm inner diameter) under atmospheric pressure.
The MN was synthesized by dropping H
aqueous solution of sodium nitrite and methanol, collected into a
steel cylinder and subsequently mixed with high-purity N to
obtain a gas mixture of MN/N of 1/7.6 (vol/vol, similarly here-
inafter). Four calibrated mass flow meters were used to control
the H , N , CO and the MN/N gas mixture, respectively. Typically,
catalyst of 0.200 g was subjected to in situ reduction in H at 150 °C
for 2 h, flushed with N at 200 °C for 1 h and cooled down to the
desired temperature in N . In order to investigate the transient
effects of the presence of CH OCOCOCl and CH OCOCl on the CO
coupling reaction, either CH OCOCOCl or CH OCOCl of 2 L was
2 4
SO (50 vol%) into an
2
The above-gathered mechanisms of CO coupling to DMO reac-
tion can be briefly summarized as follows. It could be generally
accepted that the reactant RONO oxidizes Pd0 to Pd , but the
formed intermediate species in this step remains controversial
among Pd-OR [27], ON-Pd-OR [19,30] and RO-Pd-OR [29,31]. In
particular, the intramolecular coupling mechanism (i.e., two Pd-
COOR couplings to form oxalate) has been generally accepted;
however, this pathway is mainly based on experience and the
key intermediate species is not directly identified by rigorous evi-
dence and experiments [27,29,30].
2
2+
2
2
2
2
2
2
3
3
3
3
l
injected into a bottle (30 mL; as evaporator) connected with the
feed gas line when reaction attained steady-state under appointed
conditions, and subsequently was introduced by feed gas flow into
the fixed-bed reactor. The bottle temperature was controlled at
Therefore, it is particularly desirable to reach a consensus on
this controversial issue. To accomplish this goal, in situ diffuse
reflectance infrared Fourier transform spectroscopy (DRIFTS),
which can offer accurate and detailed information without special
catalyst pre-treatment and use of inert substances [33,34], was
employed for mechanistic study of the CO coupling to DMO over
our previously reported high-performance and low Pd-loading
Al-fiber@ns-AlOOH@Pd catalyst [35]. The intermediate species of
3
25 °C for CH OCOCOCl, and ꢀ20 °C for CH
3
OCOCl, to obtain their
comparable saturated vapor pressure. The composition of the feed
gas and effluent gas was monitored by an online gas chro-
matograph (GC) equipped with a thermal conductivity detector
(TCD) connected to a ShinCarbon ST packed column (DIKMA) and
a flame ionization detector (FID) connected to an Innowax PEG-
20M capillary column (HP). The main by-products were identified
including dimethyl carbonate (DMC), methyl formate and metha-
nol. The calculations of CO conversion, MN conversion, DMO selec-
⁄
⁄
COCOOCH
3
( , a surface site) was directly observed in both
steady-state CO coupling reaction and dynamic MN pulse experi-
⁄
ments with pre-adsorbed CO. More notably, COCOOCH
3
rather
⁄
than COOCH
3
was clearly identified as the key intermediate species
OCOCOCl) and methyl
OCOCl) probed DRIFTS experiments. A double
carbonylation reaction pathway was also discussed deeply.
tivity, and space time yield of DMO (STYDMO) and DMC (STYDMC
were available in the Supporting Information.
Notably, our low Pd-loading Al-fiber@ns-AlOOH@Pd catalyst,
with high permeability and high thermal conductivity [35], deliv-
ered catalytic performance comparable to the commercial Pd/
)
with the aid of methyl oxalyl chloride (CH
chloroformate (CH
3
3
Al
2 3
O catalyst (Ube Industries, Japan) [36], showing a potential
2
. Experimental
industrial application (Table S1 in the Supporting Information).
2.1. Chemicals and materials
2.4. Catalyst characterizations
Palladium acetate (Shanghai Jiuling Chemical Co., Ltd., China),
In situ DRIFTS experiments were performed using a Bruker Ten-
sor 27 spectrometer equipped with a liquid N cooled mercury-
2
cadmium-telluride (MCT) detector and a Harrick Scientific HVC-
DRP-4 reaction cell fitted with ZnSe windows. Four calibrated mass
methyl oxalyl chloride (CH
ical Co., Ltd., China), methyl chloroformate (CH
Chemical Reagent Co., Ltd., China) and thin-sheet Al-fiber felt
Shanghai Xincai Net-structured Material Co., Ltd., China) were
all used as received.
3
OCOCOCl, Shanghai Macklin Biochem-
3
OCOCl, Sinopharm
(
flow meters were used to control the H
mixture, respectively.
2 2 2
, N , CO and the MN/N gas
Typically, the sample of 0.020 g without diluents was placed
into the DRIFTS cell, pre-reduced in H at 150 °C for 2 h, flushed
with N at 200 °C for 1 h and cooled down to the desired temper-
ature in N for taking a reference spectrum. First, for MN and CO
adsorption experiments, the sample was exposed to the gas mix-
2
.2. Catalyst preparation
2
2
The catalyst was prepared as described in detail elsewhere
2
[
23,35]. A thin-sheet Al-fiber felt with three-dimensional (3D) net-

C. Wang et al. / Journal of Catalysis 344 (2016) 173–183
175
ture of MN/N
2
(1/7.6) or pure CO at 35 °C for 0.5 h and purged in N
2
attributed to gaseous N
2
O and NO
2
that were likely derived from
at that temperature for 1 h. The DRIFTS spectra were subsequently
collected by accumulating 128 scans in temperature range from 35
to 200 °C. Second, for the transient CO coupling reaction, the gas
the disproportionation of gaseous NO after long time storage in
our feed-gas cylinder. Fig. 1C shows the DMC spectra at 35 and
150 °C with the assignable bands as follows: both of the bands at
ꢀ1
mixture of CO/N
2
(1/7.6) was introduced into the DRIFTS cell for
2961 and 2922 cm were assigned to
3
mas(CH ), caused by the fun-
0
N
.5 h at 150 °C, followed by switching to the gas mixture of MN/
damental transitions [39,40]; the bands at 2854, 1780 and 1768,
ꢀ1
2
(1/7.6), and simultaneously, the DRIFTS spectra were continu-
1456, and 1298 cm
(C@O), CH deformation (d) and
Fig. 1D, the bands of solid DMO (35 °C) at 1738, 1440, 1209 and
respectively corresponded to
s 3
m (CH ), m
ꢀ1
ously collected for 0.5 h at a rate of 8 spectra min (accumulating
4
N
3
m(CAO) [41]. As shown in
scans for each spectrum); commutatively, the gas mixture of MN/
(1/7.6) was introduced into the DRIFTS cell for 0.5 h at 150 °C
(1/7.6), and
ꢀ
1
2
1160 cm were respectively ascribed to
and rocking vibration of CH ) [42]. Notably, however, the spec-
trum of gaseous DMO was different from solid counterpart
(Fig. 1D): at 150 °C (gaseous DMO), two bands of (C@O) at 1775
3
m(C@O), d(CH ), m(CAO)
followed by switching to the gas mixture of CO/N
2
3
(c
simultaneously, the DRIFTS spectra were collected as above. To
avoid the interference of overlapped bands, the DRIFTS difference
spectrum was acquired by subtracting the appointed spectrum
from the spectrum after switch with OMNIC software, if necessary
m
ꢀ1
ꢀ1
and 1757 cm and the new band at 1321 cm appeared as a
result of the breaking of the centrosymmetric d symmetry [43];
ꢀ1
[
37]. Third, for the steady-state CO coupling reaction, the sample
the bands of gaseous DMO at 2966 and 2860 cm corresponding
to as(CH ) and (CH ) were also observed. Obviously, DRIFTS
spectra of the target product DMO and by-product DMC were dif-
ferent in (C@O), (CAO) and (CH ), which could be used to iden-
was exposed to the gas mixture of CO/MN/N (1.4/1/7.6) for 0.5 h
2
m
3
m
s
3
at each temperature of 10, 30, 50, 70, 90, 110, 130 and 150 °C,
and the DRIFTS spectra were respectively collected by accumulat-
ing 128 scans at given temperature before and after the purge with
m
m
c
3
tify their presence in the following sections.
N
2
for 1 h. Fourth, for the dynamic MN pulse experiments, the sam-
ple was exposed to the gas mixture of CO/N (1/7.6) for 0.5 h at 50
or 100 °C and purged in N at that temperature for 1 h. Subse-
quently, the gas mixture of MN/N (1/7.6) was pulsed into the
DRIFTS cell with each pulse volume of 40 L for total 25 pulses,
and the DRIFTS spectra were collected by accumulating 128 scans
after each pulse at 50 or 100 °C; commutatively, for the dynamic
CO pulse experiments, the sample was exposed to the gas mixture
2
3.2. DRIFTS spectra of MN and CO adsorbed on the Al-fiber@ns-
AlOOH@Pd catalyst
2
2
l
In the CO coupling to DMO process, chemisorption of MN onto
the Pd catalyst has been widely considered as the first step but the
dissociative form of MN is still under discussion [29–31]. Some
authors guessed that the dissociative adsorption of alkyl nitrite
(RONO) can form di-alkoxy species (RO-Pd-OR) [29,31], whereas
other authors took it for granted that RO-Pd-ON is generated
because gaseous or adsorbed NO is unobservable in the IR experi-
ment [30]. Initially, catalytic decomposition of MN was investi-
gated on our Al-fiber@ns-AlOOH@Pd catalyst in a fixed-bed
of MN/N
ature for 1 h. Subsequently, the gas mixture of CO/N
pulsed into the DRIFTS cell, and the DRIFTS spectra were collected
as above. Fifth, for the CH OCOCOCl and CH OCOCl adsorption
experiments, CH OCOCOCl or CH OCOCl of 2 L was injected into
a bottle (30 mL; as evaporator) connected with the gas line, and
subsequently introduced by N carrier gas into the DRIFTS cell of
5 °C. The bottle temperature was controlled at 25 °C for CH
OCOCOCl, and ꢀ20 °C for CH OCOCl, to obtain their comparable
saturated vapor pressure. After CH OCOCOCl or CH OCOCl adsorp-
tion, the sample was purged in N at 15 °C for 1 h and the DRIFTS
2
(1/7.6) for 0.5 h at 50 °C and purged in N
2
at that temper-
2
(1/7.6) was
3
3
3
3
l
reactor using
a
MN/N
2
(1/7.6) gas mixture at
a GHSV of
ꢀ
1
ꢀ1
2
10,400 L kg
h
. Conversion of MN increased dramatically from
1
3
-
8.2% to 99.7% with increasing the reaction temperature from 100
to 140 °C (Fig. S1 in the Supporting Information), indicating the
high reactivity of MN as a result of largely weakened CH O-NO
3
band when adsorbed on the Pd catalyst [44,45]. Subsequently,
we employed in situ DRIFTS to examine MN adsorption on the
Al-fiber@ns-AlOOH@Pd catalyst, and the corresponding spectra
3
3
3
2
spectra were collected by accumulating 128 scans in temperature
range from 15 to 150 °C. In all cases, the flow rate of gas to the
ꢀ
1
DRIFTS cell was maintained at 50 mL min . In addition, the spec-
are shown in Fig. 2. MN was adsorbed on the catalyst at 35 °C fol-
ꢀ1
ꢀ1
tra were all collected from 4000 to 600 cm at a resolution of
lowed by N
2
purge, and the bands at 2947 cm
3
(mas(CH )),
ꢀ1
ꢀ1
ꢀ1
4
cm
.
2826 cm
(m
s
(CH
3
)) and 1035 cm
(m(CAO)) could be clearly
observed (Fig. 2: spectrum a). These observed bands were attribu-
⁄
table to the CH
3
O
species from the dissociatively adsorbed MN
O-
3
. Results and discussion
which is labile and readily broken due to the very weak CH
NO band (176 kJ mol ) especially in the presence of catalyst
[46]. With the temperature increase in the DRIFTS cell (Fig. 2: spec-
3
ꢀ1
3.1. DRIFTS spectra of MN, NO, DMO and DMC
tra b–d), the CH
3
O⁄ species remained observable until 100 °C but
Prior to mapping out the reaction mechanism of CO coupling to
disappeared at 150 °C with its transformation into methanol and
DMO, the DRIFTS spectra of MN, NO, DMO and DMC, which would
be used as reference spectra in the following sections, were ini-
tially recorded at 35 and 150 °C in the absence of catalyst, with
the results as shown in Fig. 1. The first to be noted was that at
methyl formate [47], being consistent with the catalytic decompo-
⁄
sition of MN as above (Fig. S1). The bands ascribed to NO
ꢀ1
(1753 cm ) [48] or gaseous NO were undetectable, but more
ꢀ
1
interestingly, the bands at ꢁ1605 cm as well as at 1410 and
ꢀ1
3
5 °C, MN and NO exist in the gaseous state but DMC and DMO
1330 cm
corresponding to the coordinated alumina bound
in solid state while at 150 °C they are all in the gaseous state. As
nitrates and nitrites [49,50] were observed instead (Fig. 2). Com-
bining this information with the fact that MN could not be directly
adsorbed onto the pure Al-fiber@ns-AlOOH support (Fig. S2), the
observed alumina coordinated nitrates and nitrites could be well
shown in Fig. 1A, the characteristic bands of MN could be definitely
ꢀ1
identified [38]: the bands at 2961 and 2841 cm were respec-
tively assigned to CH asymmetric ( as) and symmetric stretching
vibration ); the bands at 1687–1611 cm
3
m
ꢀ
1
⁄
(m
s
,
1054 and
95 cm , 820 and 801 cm , and 612 cm corresponded to
N@O), (CAO), (NAO) and bending vibration of OAN@O, respec-
explained by considering that NO species derived from MN disso-
ꢀ
1
ꢀ1
ꢀ1
9
(
m
ciation might migrate from Pd to alumina [48,51]. Note that the
formed nitrates and nitrites made little impact on the catalytic per-
formance of Al-fiber@ns-AlOOH@Pd in the real reaction since the
catalyst could provide pleasurable activity/selectivity maintenance
throughout the entire 200 h test elsewhere [35]. Furthermore, as
m
m
ꢀ1
tively. As shown in Fig. 1B, the bands at 1912 and 1845 cm were
the characteristic bands of gaseous NO, and the bands at 2238 and
ꢀ
1
ꢀ1
2
208 cm as well as at 1630 and 1599 cm were respectively

1
76
C. Wang et al. / Journal of Catalysis 344 (2016) 173–183
MN
687-1611
801
20
NO
845
(
A)
(B)
8
6
12
1
912
1
1
9
95
1
630
599
2
961
2
238
1
2841
2208
1
054
o
o
150 C
1
50 C
o
o
3
5 C
35 C
3000
2000
1000
3000
2000
1000
-
1
-1
Wavenumber (cm )
Wavenumber (cm )
DMC
(D)
DMO
(
C)
1160
209
1
298
1
1
757
2
961
922
854
1775
1
768
1
2
1780
1321
440
456
2966
2
1
o
1
50 C
2860
o
1
50 C
1
738
o
o
3
5 C
35 C
3000
2000
1000
3000
2000
1000
-
1
-1
Wavenumber (cm )
Wavenumber (cm )
Fig. 1. DRIFTS spectra of (A) MN, (B) NO, (C) DMC and (D) DMO at 35 and 150 °C in the absence of catalyst.
Adsorption of CO at 35 °C was also investigated on the Al-
fiber@ns-AlOOH@Pd catalyst as well as the desorption behavior
(
C-O)
(
CH3)
at elevated temperature using in situ DRIFTS, with the results as
1
410
ꢀ
1
shown in Fig. 3. The bands appeared at 2050 cm and at 1973,
1
330
1308
1
524
ꢀ1
1
942 and 1885 cm were typically assigned to the linear CO and
2
947
2
1605
bridged CO, respectively [53,54]. The increase in the temperature
from 35 to 200 °C progressively attenuated both linear and bridged
CO, but interestingly, the band area of the bridged CO was much
larger than the counterpart of linear CO in the whole temperature
range (e.g., area ratio of bridged/linear CO of 12.5 at 35 °C and 28.6
at 100 °C); in particular, the linear CO was scarcely detectable at
1
035
826
d
c
b
a
1
50 °C, indicating CO would not be adsorbed in a linear manner
on the catalyst at P150 °C. Given that the co-adsorption (i.e.,
Langmuir-Hinshelwood (L-H) mechanism) of MN and CO is neces-
sary for the titled reaction [27,30,31], it was rationally inferred that
3
000
2000
1000
-
1
Wavenumber (cm )
Fig. 2. DRIFTS spectra of MN adsorbed on Al-fiber@ns-AlOOH@Pd at 35 °C (a) and
the evolution behavior at different temperatures: (b) 50 °C, (c) 100 °C and (d)
1
50 °C. The pre-reduced sample was exposed to the MN/N
2
(1/7.6, vol/vol) gas
1
973
mixture at 35 °C and subsequently purged with N at 35 °C for 1 h.
2
1
942
1885
2050
the temperature was increased to 100 °C, the bands of alumina
ꢀ1
a
bound nitrates at 1605, 1524 and 1308 cm
were increased
remarkably associated with the decline of nitrites at 1410 and
b
ꢀ1
c
1
330 cm (Fig. 2: spectrum c). The formation of nitrates was pos-
d
e
sibly caused by the oxidation of nitrites with oxygen (created by
dissociation of NO) bonded to Pd or the disproportionation (i.e.,
decomposition) of unstable nitrites [51,52]. It is thus reasonable
to infer from the above results that adsorption of MN on the Pd cat-
2
100
2000
1900
1800
⁄
⁄
-1
alyst readily leads to the formation of OCH
3
(i.e., 2 + CH
3
ONO ?
Wavenumber (cm )
⁄
⁄
NO + OCH
3
) as the first intermediate participating in the subse-
quent coupling reaction, and the generated NO desorbs directly
i.e., NO ? + NO) or partially migrates to the support to form
⁄
Fig. 3. DRIFTS spectra of CO adsorbed on Al-fiber@ns-AlOOH@Pd at 35 °C (a) and
the desorption behavior at different temperatures: (b) 50 °C, (c) 100 °C, (d) 150 °C
and (e) 200 °C. The pre-reduced sample was exposed to pure CO at 35 °C and
⁄
⁄
(
nitrates or nitrites.
2
subsequently purged with N at 35 °C for 1 h.

C. Wang et al. / Journal of Catalysis 344 (2016) 173–183
177
the bridged CO would make predominant contribution to DMO for-
mation at <150 °C, and even entire contribution at P150 °C. This
was in accordance with the well-investigated results that only
bridged CO participates in the CO coupling reaction but the linear
CO is unavailable [30,35].
length up to 0.38 min after switching to CO/N
2
(Fig. 5A). With fur-
ther prolonged time, the bands of DMO and DMC gradually
⁄
declined due to the shrink of OCH
3
species on the catalyst surface
(Fig. 4: spectra d–h and Fig. 5A) while the bands of bridged CO
ꢀ1
(2000–1800 cm ) increased gradually (Fig. 4). Unfortunately, the
recognizable bands assigned to the intermediate species were not
observed although the DRIFTS spectra were collected as continu-
ously as possible (Fig. 4).
3.3. In situ DRIFTS of CO coupling to DMO on the Al-fiber@ns-
AlOOH@Pd catalyst
Commutatively, CO was pre-adsorbed at 150 °C followed by
2 2
switching from CO/N (1/7.6) to MN/N (1/7.6) gas mixture, and
the DRIFTS spectra against time since switch are collected in
3
.3.1. Transient CO coupling reaction after MN or CO pre-adsorption
In view of the L-H mechanism for CO coupling reaction
Fig. 6. To eliminate the interference of methyl formate bands at
[
27,30,35], reducing the reactant coverage on the catalyst surface
ꢀ1
ꢀ1
1
1
775–1736 cm
790–1750 cm
(Fig. 6A), the DRIFTS difference spectra at
are also collected in Fig. 6B. Contrary to the
provides an opportunity to slow down the reaction rate. Thus,
the intermediate species might be captured during the transient
reaction after MN or CO pre-adsorption. On the Al-fiber@ns-
AlOOH@Pd catalyst, MN was pre-adsorbed at 150 °C followed by
observations in spectra a–c (Fig. 4), the bands of DMO [43] at
ꢀ1
1
1
757, 1321, 1209 and 1160 cm rather than those of DMC at
ꢀ1
780 and 1298 cm first appeared with the introduction of gas-
switching from MN/N
2
(1/7.6) to CO/N
2
(1/7.6) gas mixture, and
ꢀ1
eous MN at 1464–1422 and 1687–1611 cm (Fig. 6A and B: spec-
tra a, b). Clearly, the band area of DMO increased much faster than
that of DMC along with the continuous introduction of MN for time
length from 0 to 0.88 min (Fig. 5B and 6A and B: spectra a–e), indi-
cating that the high coverage of CO species (2000–1800 cm ) on
the catalyst surface favored the formation of DMO rather than
the DRIFTS difference spectra against time since switch are col-
lected in Fig. 4. To eliminate the interference of strong bands of
gaseous MN, the DRIFTS difference spectrum was acquired by sub-
tracting the spectrum recorded before switch from the spectrum
⁄
ꢀ1
after switch with OMNIC software. Just after switching to CO/N
2
(
1/7.6) the spectrum exhibited an almost straight line (Fig. 4: spec-
DMC. In accordance, it was reasonable to infer that after switching
trum a); after CO/N
0
while the bands of DMC at 1780 cm
2
(1/7.6) passing through the DRIFTS cell for
⁄
to MN/N
2
(1/7.6) the produced OCH
3
from MN reacted with the
ꢀ1
.25 min, the gaseous CO (2171 and 2116 cm ) was observed
⁄
⁄
predominant CO to form COOCH
3
, which might couple with
to yield DMO, or might consecutively react with
ꢀ1
ꢀ1
(m(C@O)) and 1298 cm
⁄
another COOCH
3
(m
(CAO)) appeared simultaneously (Fig. 4: spectrum b). As previ-
⁄
⁄
⁄
CO to form COCOOCH
3
followed by subsequent OCH
3
insertion
ously noted, the MN pre-adsorption could give birth to a high cov-
erage of CH
however, the by-product DMC rather than the target product
DMO was first formed upon the introduction of gaseous CO. In
accordance, it was reasonable to infer that the CO species with a
low coverage on the catalyst surface could insert into the predom-
inant CH
another CH
tion of DMC rather than DMO at a low coverage of CO . After
0
increased remarkably; meanwhile, DMO was also observed follow-
ing the DMC formation due to the appearance of the bands at 1757,
⁄
to yield DMO. With the consumption of CO , the band area of
DMO declined progressively, conversely but more interestingly,
the DMC area increased continuously and reached a maximum,
and then gradually decreased (Fig. 5B and 6A and B: spectra f–i).
These above results could be explained by considering that high
CO coverage benefited the formation of DMO whereas low CO
coverage favored the formation of DMC as stated above. Moreover,
when CO (2000–1800 cm ) was completely exhausted, MN
would catalytically decompose into methyl formate at 1775–
⁄
3
O
species on the catalyst surface. Interestingly,
⁄
⁄
⁄
⁄
⁄
3
O species to produce COOCH
3
which could couple with
⁄
3
O to generate DMC, indicating the preferential forma-
⁄
ꢀ1
⁄
ꢀ1
ꢀ1
.38 min, the bands of DMC at 1780 cm and 1298 cm were
ꢀ1
ꢀ1
1
736 and 1221–1151 cm
,
NO at 1912 and 1845 cm
Fig. 6A and C: spectra h–j), and methanol [55] at 1028 cm (not
ꢀ1
(
shown).
ꢀ1
1
209 and 1160 cm that were respectively attributed to
CAO) and (CH ) of DMO (Fig. 4: spectrum c). The band area of
DMC increased more remarkably than that of DMO with time
m(C@O), m
3
Clearly, MN was ascertained to catalytically dissociate into CH -
(
c
3
⁄
⁄
⁄
O , and the produced CH
DMO if with a high CO coverage but to form DMC if with a low CO
3
O subsequently reacted with CO to form
⁄
⁄
⁄
coverage. It could be inferred that the CO species inserted into
⁄
⁄
⁄
CH
CH
3
O
O
species to generate COOCH
to yield DMC (steps 1–3). However, the DMO formation
3
which coupled with another
3
2
171
pathway still remained ambiguous between the intramolecular
coupling (steps 4 and 5) and the double carbonylation (steps 6–
2
116
1780
2
000-1800
1
298
1
757
1209
⁄
⁄
8
), since the key intermediate species of COOCH
3
or COCOOCH
3
1
160
h
g
was unobservable.
ꢂ
ꢂ
ꢂ
CO þ OCH ! COOCH
ð1Þ
ð2Þ
ð3Þ
3
3
f
e
ꢂ
ꢂ
ꢂ
COOCH þ OCH ! CH
3
OCOOCH
3
3
3
d
c
ꢂ
CH
3
OCOOCH ! ꢂ þ CH
3 3
OCOOCH
3
b
a
The intramolecular coupling pathway was described as follows:
ꢂ
ꢂ
ꢂ
COOCH þ COOCH ! ꢂ þ ðCOOCH
3
Þ
ð4Þ
3
3
2
2200
2000
1800
1300
1200
-
1
Wavenumber (cm )
ꢂ
ðCOOCH
3
Þ ! ꢂ þ ðCOOCH
3
Þ2
ð5Þ
2
Fig. 4. Dynamic changes of DRIFTS difference spectra on Al-fiber@ns-AlOOH@Pd
after switching from MN/N (1/7.6, vol/vol) to CO/N (1/7.6, vol/vol) gas mixture at
50 °C as a function of time: (a) 0 min, (b) 0.25 min, (c) 0.38 min, (d) 0.63 min, (e)
.00 min, (f) 2.25 min, (g) 5.00 min and (h) 25.00 min. The DRIFTS difference
The double carbonylation pathway was described as follows:
2
2
ꢂ
ꢂ
ꢂ
1
1
CO þ COOCH ! COCOOCH þ ꢂ
ð6Þ
ð7Þ
3
3
spectrum was acquired by subtracting the spectrum recorded before switching to
CO/N from the spectrum after switch with the OMNIC software.
ꢂ
ꢂ
ꢂ
COCOOCH þ OCH ! ꢂ þ ðCOOCH Þ
3
3
3
2
2

1
78
C. Wang et al. / Journal of Catalysis 344 (2016) 173–183
(A)
(B)
DMC
DMC
DMO
DMO
4
0
.88 min
0.38 min
0
2
4
0
2
Time (min)
Time (min)
ꢀ
1
ꢀ1
Fig. 5. The band areas of DMO at 1160 cm and DMC at 1298 cm from the DRIFTS spectra in Fig. 4 after switching from MN/N
2
(1/7.6, vol/vol) to CO/N
2
(1/7.6, vol/vol) gas
mixture (A) and in Fig. 6 after switching from CO/N
2
(1/7.6, vol/vol) to MN/N
2
(1/7.6, vol/vol) gas mixture (B) as a function of time.
(
A)
j
a
(B)
1757
1
687-1611
1
209
1780
1
775-1736
1221-1151
1160
1
464-1422
1
912
1
1321
2
171
2
845
j 1298
116
i
h
1
785
Wavenumber (cm )
1912
1770
1755
-1
g
f
(C)
j
e
d
c
a
2
000-1800
b
a
2200
2000
1800
1600
1400
1200
2000-1800
-1
2000
1950
1900
1850
Wavenumber (cm )
-1
Wavenumber (cm )
Fig. 6. Dynamic changes of (A) DRIFTS spectra in the range of 2220–1120 cm 1, (B) DRIFTS difference spectra in the range of 1790–1750 cmꢀ1, and (C) DRIFTS spectra in the
ꢀ
ꢀ
1
2 2
range of 2000–1850 cm on the CO-preadsorbed Al-fiber@ns-AlOOH@Pd after switching from CO/N (1/7.6, vol/vol) to MN/N (1/7.6, vol/vol) gas mixture at 150 °C as a
function of time: (a) 0 min, (b) 0.25 min, (c) 0.38 min, (d) 0.63 min, (e) 0.88 min, (f) 1.63 min, (g) 2.25 min, (h) 3.50 min, (i) 5.00 min and (j) 25.00 min. The DRIFTS difference
spectrum (B) was acquired by subtracting the spectrum recorded at 30.00 min after switch from the corresponding spectrum with the OMNIC software.
ꢂ
ðCOOCH
3
Þ ! ꢂ þ ðCOOCH
3
Þ2
ð8Þ
with elevating the temperature from 30 to 70 °C and then
improved sharply to 62.3% with further to 150 °C for a feed flow
2
ꢀ
1
ꢀ1
of CO/MN/N
2
(1.4/1/7.6) at a GHSV of 12,000 L kg
h
(Fig. S3).
3.3.2. Steady-state CO coupling reaction and dynamic MN pulse
Subsequently, in order to capture the surface species, after
experiments after CO pre-adsorption
exposing to CO/MN/N gas mixture from 10 to 150 °C, the catalyst
2
In heterogeneous catalysis, it is difficult to capture the interme-
diate species commonly due to their poor thermo-stability and
short lifetimes [56,57]. Decreasing the reaction temperature is a
viable way to avoid their thermal-decomposition and prolong their
lifetimes so as to make the intermediate species captured possibly.
Fig. 7 shows in situ DRIFTS spectra recorded on the Al-fiber@ns-
AlOOH@Pd catalyst exposed to probe molecules in temperature
range from 10 °C to 150 °C. In the case of exposing the catalyst to
was purged thoroughly with pure N and then the DRIFTS spectra
2
⁄
were recorded (Fig. 7B–D). Apparently, the bridged CO (2000–
ꢀ
1
⁄
ꢀ1
1800 cm ) and produced OCH (1035 cm ) from MN dissociation
3
predominated on the catalyst surface at 10 °C as well as the coor-
ꢀ1
dinated alumina bound nitrites [49,50] (1423 and 1340 cm
)
(Fig. 7B: spectrum a). Interestingly, several unknown bands at
ꢀ1
1772, 1742, 1223 and 1165 cm appeared at 30 °C and their inten-
sities rose gradually with increasing the temperature to 70 °C
(Fig. 7B–D: spectra b–d). Based on the band assignment of DMO
a feed flow of CO/MN/N
well as bridged CO (2000–1800 cm ) were all observed at 10,
2
(1.4/1/7.6), the gaseous CO and MN as
⁄
ꢀ1
ꢀ1
(1766, 1209 and 1160 cm , Fig. 1D), such unknown bands were
ꢀ
1
3
0 and 50 °C (Fig. 7A: spectra a–c); DMO product started to be
reasonably assignable to
m
(C@O, 1772 and 1742 cm ),
m
(CAO,
ꢀ
1
ꢀ1
formed at 70 °C evidenced by appearance of two characteristic
1223 cm ) and
c
(CH , 1165 cm ), and thus the corresponding
species was probably to be the intermediate of COOCH or
COCOOCH , which came from the reaction between OCH and
CO (Fig. 7B–D: spectra b–d). With further increase in the temper-
ature to P90 °C, the spectra were quite different from the counter-
part at 70 °C, showing that the DMO bands (1766, 1209 and
3
ꢀ
1
⁄
bands at 1766 and 1160 cm (Fig. 7A: spectrum d); DMO forma-
tion was improved dramatically with further increase in the tem-
perature to 150 °C, accompanied by observation of DMC
3
⁄
⁄
3
3
⁄
ꢀ1
ꢀ1
(
1298 cm ) and gaseous NO (1912 cm ) (Fig. 7A: spectra e–h).
Indeed, such temperature-dependent DRIFTS spectra evolution as
shown in Fig. 7A was consistent with the fixed-bed reaction
results, i.e., the CO conversion increased slowly from 0.1% to 0.4%
ꢀ1
1160 cm ) were apparently detected associated with the disap-

C. Wang et al. / Journal of Catalysis 344 (2016) 173–183
179
(
A)
1160
209
298
321
(B)
1
766
1165
1209
1
766
1
1
772
1160
1742
1423
444
1
1223
1
1035
1
1
912
h
g
1
516 1340
h
g
f
f
e
e
d
c
d
c
b 2000-1800
a
b
a
2000-1800
2100
1800
1500
1200
2100
1800
1500
1200
-1
-1
Wavenumber (cm )
Wavenumber (cm )
(
C)
h
(CH3)
(D)
h
(C=O)
1742
1772
1766
(
C-O)
1209
1
160
1
165
a
a
1
223
1230
1200
1170
1140 1800
1775
1750
1725
-1
-1
Wavenumber (cm )
Wavenumber (cm )
Fig. 7. DRIFTS spectra of (A) the steady-state CO coupling reaction on Al-fiber@ns-AlOOH@Pd and (B: 2125–1075 cmꢀ1, C: 1250–1140 cm and D: 1800–1720 cm ) the
surface species after subsequent N purge at different temperatures: (a) 10 °C, (b) 30 °C, (c) 50 °C, (d) 70 °C, (e) 90 °C, (f) 110 °C, (g) 130 °C and (h) 150 °C. After the pre-
reduced sample was exposed to the gas mixture of CO/MN/N (1.4/1/7.6, vol/vol/vol) at appointed temperature for 0.5 h, the spectra were recorded and collected in (A);
for 1 h and then the spectra were recorded and collected in (B–D). The intensities of spectra (g) and (h) in (B–D) were reduced
ꢀ1
ꢀ1
2
2
subsequently, the catalyst was purged with N
three times.
2
ꢀ
1
ꢀ1
⁄
3
pearance of abovementioned unknown bands (1772, 1742, 1223
1224 cm to
m
(CAO), and 1165 cm to
c
(CH
3
)) of COOCH
or
ꢀ1
⁄
and 1165 cm ; Fig. 7B–D: spectra d–h). The ‘‘appearance-disap
pearance” transformation of these unknown bands indicated that
COCOOCH
3
intermediate species appeared at once, indicating that
⁄
⁄
the formed OCH
generate the intermediate (Fig. 8A: spectrum b). Along with the
MN pulse, the bands (1769, 1741, 1224 and 1165 cm ) increased
gradually while the bridged CO was consumed bit by bit (Fig. 8A:
spectra c–h), suggesting the continuous formation of COOCH
3
from MN pulse coupled with pre-adsorbed CO to
⁄
⁄
the correspondingly assigned COOCH or COCOOCH might be
3
3
ꢀ1
the key intermediate species for selective formation of DMO at
P90 °C. It should be pointed out that DMO bands could still be
⁄
⁄
observable even experiencing thorough N
2
purge, because of visi-
3
or
⁄
ble DMO condensation on the ZnSe window of DRIFTS cell as a
result of much higher DMO boiling point (164 °C) than the window
temperature close to room condition. Note that the condensed
DMO was possibly amorphous with accidental orientation of mole-
COCOOCH
3
species. Such observation in above MN pulse experi-
ment at 50 °C was highly consistent with the counterpart in the
steady-state CO coupling reaction at 30–70 °C (Figs. 7B and 8A).
Moreover, the intermediate species bands at 1769, 1741, 1224
ꢀ1
ꢀ1
cules, which made the
m
(C@O) band (1766 cm ) of DMO during
and 1165 cm became undetectable (Fig. 8B) at 100 °C because
the steady-state CO coupling reaction different from that
it was completely converted into DMO at such high temperature
and carried away by the carrier gas, being in good agreement with
the results in the steady-state CO coupling reaction above 90 °C as
noted previously (Fig. 7B). Commutatively, CO gas was pulsed on
the MN-preadsorbed Al-fiber@ns-AlOOH@Pd catalyst at 50 °C
(Fig. S4 in the Supporting Information). Not surprisingly, the inter-
ꢀ1
(
1738 cm ) of the polycrystalline DMO in Fig. 1D, due to the
breaking of the centrosymmetric d symmetry [43]. In addition,
ꢀ1
the bands at 1516 and 1444 cm
were detected at or above
9
0 °C, being assigned to the coordinated alumina bound nitrates
as previously stated [49,50].
⁄
⁄
⁄
In order to further confirm the formation of COOCH
3
or
mediate species of COOCH
in Fig. 8A was continuously formed as evidenced by the appear-
3
or COCOOCH same as the one detected
3
⁄
COCOOCH
3
observed in the steady-state CO coupling reaction as
ꢀ
1
described above, the dynamic MN pulse experiments were carried
out on the CO-preadsorbed Al-fiber@ns-AlOOH@Pd catalyst at 50
and 100 °C, with the in situ DRIFTS spectra as shown in Fig. 8. Typ-
ically, the pre-reduced catalyst was exposed to CO and thoroughly
ance and growth of bands at 1769, 1741, 1224 and ꢁ1165 cm
along with the CO pulses, undoubtedly indicating that the bridged
CO from CO pulse reacted with the pre-adsorbed OCH . The
⁄
⁄
3
dynamic MN and CO pulse experiments indicated that both
adsorbed CO and adsorbed MN could participate in the CO coupling
reaction (Fig. 8A and Fig. S4 in the Supporting Information), which
indirectly evidenced that the titled reaction followed an L-H type
reaction mechanism [27,30,31].
2
purged in N , and subsequently MN gas was pulsed. As shown in
⁄
ꢀ1
spectrum a (Fig. 8A), the bridged CO (2000–1800 cm ) predomi-
nated on the surface of CO-preadsorbed catalyst at 50 °C. When
MN/N
2
(1/7.6) gas mixture was pulsed into the DRIFTS cell at
ꢀ
1
5
0 °C, the bands (1769 and 1741 cm
assigned to m(C@O),

1
80
C. Wang et al. / Journal of Catalysis 344 (2016) 173–183
(
A)
(B)
1
769
741
(
C=O)
Not detected
224
(
C-O)
1
(
CH3)
2
000-1800
1
1
769
1
165
1
741
2
000-1800
1224
h
g
1
165
h
g
f
e
f
e
d
c
d
c
b
b
a
a
2
100 1950 1800
1300 1200 1100
2100 1950 1800
1300 1200 1100
-
1
-1
Wavenumber (cm )
Wavenumber (cm )
Fig. 8. Dynamic changes of DRIFTS spectra on Al-fiber@ns-AlOOH@Pd pre-adsorbed with CO at 50 °C (A) and 100 °C (B) as a function of MN pulse number: (a) 0, (b) 1, (c) 3, (d)
, (e) 9, (f) 13, (g) 17 and (h) 25. The pre-reduced catalyst was exposed to the CO/N (1/7.6, vol/vol) at 50 °C or 100 °C and then purged in N for 1 h at that temperature, and
finally the MN/N (1/7.6, vol/vol) gas mixture was pulsed into the DRIFTS cell at 50 or 100 °C.
5
2
2
2
⁄
⁄
3
3
CH
.4. Discrimination between COOCH
3
and COCOOCH
: DRIFTS of
CH
way to obtain the COOCH
Fig. 9 shows the DRIFTS spectra of the CH
3 3
OCOCl and CH OCOCOCl on Pd catalyst should be a feasible
⁄
⁄
3
3
OCOCl and CH
3
OCOCOCl probe molecule
3
and COCOOCH
species.
3
OCOCl and CH -
3
To the best of our knowledge, no literature has reported the
OCOCOCl adsorbed on the Al-fiber@ns-AlOOH support and Al-
fiber@ns-AlOOH@Pd catalyst at 15 °C, as well as the
temperature-dependent evolution behavior from 15 to 150 °C.
⁄
⁄
direct capture of the intermediate species of COOCH
3
or COCOOCH
3
by IR techniques for the CO coupling reaction, so it is an appealing
yet challenging conundrum to definitely discriminate them.
Although we successfully observed the intermediate species by
ꢀ1
The bands at 1744, 1308, 1215 and 1168 cm were attributed to
the adsorbed CH OCOCl on the Al-fiber@ns-AlOOH support at
15 °C and 1778, 1753, 1209 and 1171 cm
(Fig. 9A and B: spectrum a). The bands of CH
3
ꢀ1
both steady-state CO coupling reaction and dynamic MN pulse
3
to CH OCOCOCl
⁄
experiments (Figs. 7 and 8), it was still uncertain between COOCH
3
3
OCOCl and CH -
3
⁄
3
and COCOOCH . Fortunately, methyl chloroformate (CH
3
OCOCl)
OCOCOCl adsorbed on the pre-reduced catalyst were obviously dif-
ferent from the counterparts on the catalyst support (Fig. 9A and B:
spectra a, b) because of Pd-catalyzed dissociative adsorption: 1764,
and methyl oxalyl chloride (CH
to their similar structures to COOCH
Note the following facts: the band of
3
OCOCOCl) crossed our mind due
⁄
⁄
3
3
and COCOOCH
(CACl) is at 840 cm for
OCOCOCl [59]; the bond
length of CACl (0.17 nm) is obviously longer than that of CAO
, respectively.
ꢀ
1
ꢀ1
m
1215 and 1161 cm to
m
(C@O),
m
(CAO) and
c
(CH
3
) for dissociative
ꢀ1
ꢀ1
CH
3
OCOCl [58] and 800 cm
for CH
3
adsorption of CH
to (C@O), (CAO) and
spectrum b). The (C@O) for both the dissociatively adsorbed CH
OCOCl and CH OCOCOCl exhibited a marked red-shift compared to
that of their gaseous state (1764 vs. 1795 cm
3
OCOCl; and 1769 and 1745, 1227, and 1171 cm
m
m
c
(CH ) for CH
3
3
OCOCOCl (Fig. 9A and B:
(
[
0.14 nm), C@O (0.12 nm) and CAH (0.11 nm) in CH
60,61]; CACl (0.18 nm) is longer than that of CAO (0.15 nm),
OCOCOCl [62]; CACl
3
OCOCl
m
3
-
3
ꢀ
1
C@O (0.12 nm) and CAH (0.11 nm) in CH
3
3
for CH OCOCl;
ꢀ1
bond would be further weakened when adsorbed on the active
metal surface [44]. Therefore, the low-temperature adsorption of
1769 and 1745 vs. 1789 cm for CH
spectrum b and Fig. S5), suggesting the generation of COOCH
3
OCOCOCl) (Fig. 9A and B:
⁄
3
(
A)
(B)
(
C=O)
(
CH3)
(CH3)
(C-O)
(
C=O)
(
C-O)
1
7691745
1764
i
1161
1
215
1
i
1753
1750
1227
1
1
778
1171
209
h
g
f
1744
1
308
168
h
g
f
e
e
d
d
c
b
a
c
b
a
1
850
1800
1750
1300
1200
1100 1850
1800
1300
1200
1100
-
1
-1
Wavenumber (cm
)
Wavenumber (cm )
Fig. 9. DRIFTS spectra of (A) CH
3
OCOCl and (B) CH
3
OCOCOCl adsorbed on the Al-fiber@ns-AlOOH support and Al-fiber@ns-AlOOH@Pd catalyst: CH
3
OCOCl and CH
3
OCOCOCl
adsorbed on (a) the support and (b) the catalyst at 15 °C, and their thermal evolution behavior on the catalyst at different temperatures of (c) 35 °C, (d) 50 °C, (e) 65 °C, (f)
5 °C, (g) 100 °C, (h) 125 °C and (i) 150 °C. Note: CH OCOCOCl or CH OCOCl of 2 L was injected into a bottle (30 mL; as evaporator) connected with the gas line and was
subsequently introduced by N carrier gas into the DRIFTS cell of 15 °C. The bottle temperature was controlled at 25 °C for CH OCOCOCl and at -20 °C for CH OCOCl, to obtain
their comparable saturated vapor pressure.
7
3
3
l
2
3
3

C. Wang et al. / Journal of Catalysis 344 (2016) 173–183
181
⁄
⁄
3
and COCOOCH
3
. The bands of COOCH
remained until the temper-
a competitive L-H mechanism [63,64], with the apparent activation
energy of 94 kJ mol (Fig. S6; detailed calculations in the Support-
⁄
ꢀ1
ature was increased to 75 °C, indicating that the COOCH
3
species
was stable below this temperature (Fig. 9A: spectra c–i), while
ing Information). The reaction order of 0.83 with respect to MN
suggested the reaction rate had a positive relationship with the
partial pressure of MN, possibly due to its direct effect on the
⁄
the COCOOCH
3
until to 100 °C (Fig. 9B: spectra c–i). So, it was
⁄
not surprising that the intermediate species of COOCH
3
or
⁄
⁄
⁄
COCOOCH
3
could be captured at 30–70 °C in the steady-state CO
rate-determining step between OCH and COCOOCH (step 7) as
3
3
coupling reaction and dynamic MN pulse experiments at 50 °C
proposed above. With a low CO conversion below 8% at 110 °C dur-
ing the kinetic study, CO with reaction order of ꢀ1.11 showed a
suppression effect on the reaction rate, indicating that there was
a competitive adsorption between CO and MN on the catalyst sur-
face sites [63,64]. Moreover, the estimated apparent activation
(
Figs. 7 and 8).
⁄
⁄
3
Most notably, the as-formed COOCH and COCOOCH from CH -
3
3
3
OCOCl and CH OCOCOCl dissociatively adsorbed on the Al-
fiber@ns-AlOOH@Pd catalyst respectively had one and two bands
ꢀ
1
of
m
3
(C@O), and the
) for COCOOCH
m
(CAO) intensity was stronger than that of
c
energy of 94 kJ mol on the Al-fiber@ns-AlOOH@Pd catalyst was
⁄
⁄
ꢀ1
(
CH
3
but weaker for COOCH
3
(Fig. 9A and B: spec-
consistent with that of 87 kJ mol [35] on the conventional Pd/
tra b–e). Such differences in their bands allowed us to make a clear
assignment of the bands detected in both steady-state CO coupling
reaction and dynamic MN pulse experiments. As shown in Table 1
and Figs. 7–9, clearly, the detected intermediate species in both
steady-state reaction and dynamic MN pulse experiments had
2 3
a-Al O catalyst, which indirectly implied that the reaction path-
way was similar on the above two catalysts.
On the basis of above in situ DRIFTS experiments and kinetic
study, the catalytic mechanism of the CO coupling to DMO over
the Al-fiber@ns-AlOOH@Pd catalyst could be reasonably proposed
as shown in Scheme 1. First, one reactant MN is dissociated on Pd
two bands of
m
(C@O) with obviously stronger intensity of
m
⁄
⁄
⁄
⁄
(
CAO) than that of
c
3
(CH ), being consistent with the COCOOCH
3
to generate NO and OCH , and the NO species desorbs directly or
3
formed from dissociatively adsorbed CH
3
OCOCOCl rather than
partially migrates to the support to form nitrates and nitrites. The
other reactant CO is adsorbed to mainly form bridged CO species,
⁄
⁄
the COOCH
sion that the as-confirmed COCOOCH
3
from CH
3
OCOCl. It is thus rational to draw a conclu-
⁄
⁄
⁄
⁄
3
rather than COOCH
3
is the
which can insert into OCH
3
to generate COOCH
3
followed by con-
⁄
⁄
⁄
key intermediate species for the CO coupling to DMO reaction.
secutive CO insertion into COOCH
CO coverage on the catalyst surface; subsequently, the COCOOCH
3
to form COCOOCH
3
at a high
⁄
⁄
3
⁄
⁄
reacts with OCH
3
to obtain CH
3
OCOCOOCH
3
, which easily desorbs
3.5. Catalytic mechanism of CO coupling to DMO
⁄
to produce the target product DMO. In addition, at a low CO cov-
⁄
⁄
⁄
3
The key intermediate species of COCOOCH has been verified
through elaborately designed in situ DRIFTS experiments as above.
Based on the following two facts: (1) in the steady-state CO cou-
erage, the COOCH
product DMC.
3
is prone to react with OCH to form the by-
3
To further validate the proposed mechanism, the effects of CH
OCOCl and CH OCOCOCl on the CO coupling reaction over the Al-
fiber@ns-AlOOH@Pd catalyst were investigated by introducing
3
-
⁄
⁄
3
pling reaction at 30–70 °C, abundant COCOOCH
3
rather than OCH
3
⁄
and COOCH
3
was observed on the catalyst surface (Fig. 7); (2) in
them into the fixed-bed reactor. Considering that the CH
3
OCOCOCl
the dynamic MN pulse experiments at 50 °C, the amount of
⁄
⁄
and CH
3
OCOCl could respectively produce CH
3
OCOCO and CH
3
-
COCOOCH
3
increased gradually on the CO-preadsorbed catalyst
⁄
⁄
OCO as previously stated, the OCH
3
species derived from the MN
along with the pulse number (Fig. 8); it was thus reasonably
⁄
would react with them respectively to generate DMO (CH
OCOCOOCH ) and DMC (CH OCOOCH ) highly selectively. The
dynamic changes of STYDMO and STYDMC were monitored by GC
along with the introduction of CH OCOCl or CH OCOCOCl into
the fixed-bed reactor, with the results collected in Table 2. As
3
-
inferred that the formation of COCOOCH
3
was fast, while the con-
⁄
3
3
3
sumption of COCOOCH
3
(step 7) was likely the rate-determining
step for the CO coupling reaction in view of the kinetics of consec-
utive pathway for DMO formation. Meanwhile, the double car-
bonylation pathway (i.e., consecutive insertion of CO into
3
3
ꢀ1
ꢀ1
expected, at 150 °C the STYDMO increased from 0 to 0.7 g gcat.
with the introduction of CH OCOCOCl into the MN/N (1/7.6) gas
mixture and the STYDMC to 0.4 g gcat. OCOCl (Table 2,
entries 1, 2). More interestingly, the introduction of CH OCOCl in
pure N at 150 °C could not generate DMO, indicating that the
intramolecular coupling pathway (steps 4 and 5) seemed unfeasible
Table 2, entry 3). Moreover, when the CO coupling reaction reached
steady state for a feed gas of CO/MN/N (1.4/1/7.6) at 100 °C and a
, the STYDMO was increased by 86% (i.e.,
h
⁄
⁄
⁄
COOCH to form COCOOCH followed by reacting with OCH to
3
3
3
3
2
generate DMO) was confirmed for the CO coupling reaction but
the intramolecular coupling (i.e., the coupling of two COOCH
ꢀ1
ꢀ1
h
with CH
3
⁄
3
to
3
generate DMO) seemed unfeasible. This was consistent with the
well-investigated results by DFT calculations: the double carbony-
lation pathway performed better with the highest barrier (CAC
2
(
ꢀ1
coupling step) of 118.5 kJ mol while the intramolecular coupling
2
was extremely difficult due to the highest barrier (CAC coupling
ꢀ
1
1
ꢀ1
ꢀ1
ꢀ
1
GHSV of 12,000 L kg
h
step) of 155.6 kJ mol as well as larger electrostatic interaction
and steric effects [27].
ꢀ
from 2.9 to 5.4 g gcat.
feed gas while by 45% with CH
⁄
3
Table 2, entry 4). Now that the consumption of COCOOCH (step 7)
h
) with introducing CH
3
OCOCOCl into the
ꢀ
1
ꢀ1
3
OCOCl (i.e., from 2.9 to 4.2 g gcat.
h
)
In order to clarify the catalytic mechanism, the reaction kinetics
of CO coupling to DMO was tentatively investigated. The rate equa-
tion on our Al-fiber@ns-AlOOH@Pd catalyst was established as
(
was the rate-determining step as proposed above, the DMO forma-
⁄
7
0.83
ꢀ1.11
tion rate mainly relied on the concentrations of COCOOCH
3
and
ꢀr
CO = 4.3 ꢃ 10 ꢃ exp(ꢀ94,000/RT) ꢃ PMN
ꢃ PCO
, indicating
Table 1
Bands of the intermediate species observed in DRIFTS experiments.
(C@O) (cm 1
ꢀ
)
m
(CAO) (cm
ꢀ1
)
c
(CH
) (cm )
ꢀ1
Experiment
Species
m
3
Steady-state reactiona
Dynamic MN pulseb
Unknown
1772/1742
1769/1741
1764
1223
1224
1215
1227
1165
1165
1161
1171
Unknown
c
⁄
CH
CH
3
OCOCl dissociation
COOCH
3
OCOCOCl dissociationc
COCOOCH
⁄
1769/1745
3
3
a
b
c
DRIFTS spectra of steady-state CO coupling reaction at 30–70 °C in Fig. 7.
DRIFTS spectra of dynamic MN pulse experiment at 50 °C in Fig. 8.
DRIFTS spectra of CH OCOCl and CH OCOCOCl probe molecules at 50 °C in Fig. 9.
3 3

1
82
C. Wang et al. / Journal of Catalysis 344 (2016) 173–183
Scheme 1. Proposed mechanism of Pd-catalyzed CO oxidative coupling to DMO, complied with a double carbonylation pathway.
Table 2
3 3
Effects of CH OCOCl and CH
OCOCOCl on the CO coupling reaction.a
GHSV (L kg 1
ꢀ
h
ꢀ1
)
Probe moleculeb
Timec (min)
STYd (g gcat.
ꢀ1 ꢀ1
h
)
Entry
CO/MN/N
2
(vol/vol/vol)
T (°C)
DMO
DMC
1
2
3
0/1/7.6
0/1/7.6
0/0/1
150
150
150
10,400
10,400
–
CH
3
CH
3
CH
3
OCOCOCl
OCOCl
OCOCl
1
1
1
0.7
0
0
0
0.4
0
4
5
6
a
1.4/1/7.6
100
100
100
12,000
12,000
12,000
–
CH
CH
0
1
1
2.9
4.2 (45%)
5.4 (86%)
0.1
0.2
0.2
3
OCOCl
OCOCOCl
3
1/1/8
–
CH
CH
0
1
1
4.5
5.1 (13%)
6.0 (33%)
0.1
0.1
0.1
3
OCOCl
OCOCOCl
3
0.6/1/8.4
–
CH
CH
0
1
1
6.1
6.2 (2%)
7.2 (18%)
0.2
0.2
0.2
3
OCOCl
OCOCOCl
3
Reaction conditions: 0.200 g catalyst, 0.1 MPa. The sample was pre-reduced in H
2
at 150 °C and purged in N
2
at 200 °C.
b
3 3
CH OCOCOCl or CH OCOCl of 2 lL was injected into a bottle (30 mL; as evaporator) connected with the feed gas line and was subsequently introduced by feed gas flow
3 3
into the fixed-bed reactor at 100 or 150 °C. The bottle temperature was controlled at 25 °C for CH OCOCOCl and at -20 °C for CH OCOCl, to obtain their comparable saturated
vapor pressure.
c
The time length after probe molecule introduction.
The space time yield (STY, detailed calculations in the Supporting Information). The number in parentheses showed the STY percentage increment (e.g., 45% for CH OCOCl
3
d
with CO/MN/N
2
of 1.4/1/7.6), which was calculated based on the STY at 0 min.
⁄
⁄
OCH
3
(rate equation: ꢀrDMO = k ꢃ C(COCOOCH
3
⁄) ꢃ C(OCH
3
)). The
nitrite pulse experiments at 50 °C. The as-captured intermediate
⁄
⁄
⁄
COCOOCH
3
could be directly produced from CH
3
OCOCOCl dissocia-
species is clearly identified to be COCOOCH
comparing the DRIFTS spectrum with that of the COCOOCH
3
( , a surface site) by
⁄
⁄
tion, but CH
3
OCO from CH
3
OCOCl had to experience one insertion
through the step 6 and thus the STYDMO
OCOCl (86%
vs. 45%; Table 2, entry 4). With decreasing CO content in the feed
gas, the improvement of STYDMO by CH OCOCOCl was more remark-
able compared to that by CH OCOCl (i.e., 86% vs. 45% for 14% CO, 33%
3
spe-
⁄
⁄
of CO to form COCOOCH
3
cies formed from the dissociatively adsorbed methyl oxalyl chlo-
promotion of CH
3
OCOCOCl was higher than that of CH
3
ride (CH
intermediate COCOOCH
3
OCOCOCl). On the basis of the identification of such key
⁄
3
species, the CO coupling to DMO reaction
3
is believed to abide by a double carbonylation pathway as follows:
⁄
3
(1) the reactant methyl nitrite (CH
3
ONO) dissociates into NO and
⁄
⁄
⁄
⁄
vs. 13% for 10% CO and 18% vs. 2% for 6% CO; Table 2, entries 4–6).
These results further confirmed that COCOOCH
mediate species for the DMO formation. Moreover, in the cases of
varying the CO concentration in the feed gas, introduction of either
OCH
3
and the bridged CO inserts into OCH
3
to form COOCH
3
; and
⁄
⁄
⁄
3
was the key inter-
(2) another bridged CO consecutively inserts into COOCH
COCOOCH
3
to form
to yield DMO, and the
⁄
⁄
3
followed by coupling with OCH
3
⁄
rate-determining step is likely the coupling of COCOOCH
3
with
⁄
CH
3
OCOCOCl or CH
3
OCOCl almost did not affect the STYDMC, most
OCH
3
. In addition, low CO coverage on the catalyst surface facili-
ꢀ1
⁄
⁄
3
likely due to the highest DMC formation barrier of 198.3 kJ mol
than that of DMO of 118.5 kJ mol (Table 2, entries 4–6) [27].
tates the reaction of COOCH
dimethyl carbonate.
3
and OCH to form the by-product
ꢀ
1
4
. Conclusions
Using in situ diffuse reflectance infrared Fourier transform spec-
Acknowledgments
This work was funded by the NSF of China (21473057,
U1462129, 21273075, 21076083) and the ‘‘973 program”
(2011CB201403) from the MOST of China. Prof. Dr. Chen Zhao
and her group members at the East China Normal University are
troscopy (DRIFTS), the vital intermediate in the CO coupling to
dimethyl oxalate (DMO) reaction is directly captured in both
steady-state CO coupling reaction at 30–70 °C and dynamic methyl

C. Wang et al. / Journal of Catalysis 344 (2016) 173–183
[24] X. Zhao, Q. Lin, W. Xiao, Appl. Catal. A 284 (2005) 253.
183
greatly acknowledged for their help in the DRIFTS experiments. Dr.
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Xinping Wu at the East China University of Science and Technology
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