The mechanism of CO coupling reaction to form dimethyl oxalate over Pd/α-Al2O3

Article abstract of DOI:10.1016/j.molcata.2009.08.018

The purpose of this study is to investigate the mechanism of CO coupling reaction to form dimethyl oxalate (DMO) over Pd/α-Al₂O₃ by using in situ infrared spectroscopy. The formation of DMO is initiated by methyl nitrite (MN) adsorpt

Full text of DOI:10.1016/j.molcata.2009.08.018

Journal of Molecular Catalysis A: Chemical 314 (2009) 63–70
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
The mechanism of CO coupling reaction to form dimethyl oxalate over
Pd/␣-Al2O3
Yang Ji∗, Gang Liu, Wei Li, Wende Xiao
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 May 2009
Received in revised form 19 August 2009
Accepted 20 August 2009
Available online 27 August 2009
The purpose of this study is to investigate the mechanism of CO coupling reaction to form dimethyl
oxalate (DMO) over Pd/␣-Al2O3 by using in situ infrared spectroscopy. The formation of DMO is initiated
by methyl nitrite (MN) adsorption and CO adsorption over Pd/␣-Al2O3. Two intermediates, ON–Pd–OCH3
and ON–Pd–COOCH3, were identified. The formation of DMO proceeds via coupling two ON–Pd–COOCH3
intermediates. CO adsorption generates linearly bonded CO and bridge bonded CO over Pd/␣-Al2O3, but
only bridge bonded CO participates in the reaction. Moreover, the ratio of ON–Pd–OCH3 to bridge bonded
CO over Pd/␣-Al2O3 surface has significant influence on the formations of by-products (methyl formate
(MF) and dimethyl carbonate (DMC)).
Keywords:
Dimethyl oxalate
Methyl nitrite
CO
© 2009 Elsevier B.V. All rights reserved.
Pd/␣-Al2O3
FTIR
Mechanism
1. Introduction
insertion on ON–Pd–OR would give ON–Pd–COOR. Repeating the
formation of the ON–Pd–COOR leads to a Pd(COOR)2 intermediate
In the past decades, the synthesis of dimethyl oxalate (DMO)
by gas-phase oxidative coupling reaction of CO and methyl nitrite
(MN) over palladium catalysts has attracted great interest of
researchers over the world [1–9], because one of the most interest-
ing aspects of DMO synthesis is related to the possibility of DMO
hydrogenation to ethylene glycol (EG) [10–13]. Now, the industri-
alization of the new EG synthesis technology is at preparatory stage
in China. The present study is a part of a research program of EG
synthesis from syngas in which DMO is utilized as intermediate. To
continue a fundamental understanding of the catalytic reaction of
CO coupling to form DMO over palladium catalyst, the mechanism
of the reaction was investigated by using FTIR in present study. The
oxidative coupling reaction of CO and MN is described as follows:
which yields the oxalate. Besides, they proposed that a double car-
bonylation pathway that a second CO insertion on ON–Pd–COOR
would give ON–Pd–CO(COOR) is possible.
Subsequently, Uchiumi et al. [3] found that: (1) the catalytically
active species for the formation of oxalate are Pd0. (2) The addition
of RONO oxidizes Pd0 to Pd2+. (3) RONO dissociative adsorption pro-
vides alkoxylpalladium (Pd–OR) for the DMO formation, because
alcohol has no effect on the rate of the DMO formation.
Gao et al. [6] studied a coupling reaction of CO and ethyl nitrite
(EN) to form diethyl oxalate by using XPS and in situ DRIRS. They
identified two kinds of binding energy of Pd2+ in the reaction
process, and inferred that the two kinds of Pd2+ represents two
intermediates, Pd(OR)2 generated from oxidative addition of EN
and Pd(COOR)2 generated from CO insertion, respectively.
According to these literatures, it could be concluded that: (1)
the addition of alkyl nitrite causes Pd0 to be oxidized to Pd2+; (2)
the alkyl nitrite adsorption generates the first intermediate, but the
mode of the first intermediate (ON–Pd–OR or Pd(OR)2) is uniden-
tified; (3) at least two kinds of intermediates exist in the reaction
process; (4) it is still uncertain that the formation of di-alkyl oxalate
proceeds via coupling two acyl groups or double carbonylation.
Moreover, the role of adsorbed CO in the mechanism of the
reaction was not investigated in pervious studies [3,6,14–16]. Typ-
ically, there are two kinds of adsorbed CO, linearly bonded CO and
bridge bonded CO, over the palladium catalysts, when CO adsorbs
on Pd [17–21]. Two literatures [4,22] are referred to the influence
2CO + 2CH3ONO = (COOCH3)2 + 2NO
(1)
The mechanism of the formation of di-alkyl oxalate via oxidative
coupling reaction has been studied by several research groups since
1980s [3,6,14–16]. Waller [14] proposed a apeculative mechanism
for the formation of di-alkyl oxalate that the oxidative addition
of alkyl nitrite was followed by a intramolecular coupling reac-
tion. They speculated that the oxidative addition of alkyl methyl
(RONO) to Pd0 would generate an intermediate ON–Pd–OR, and CO
∗
Corresponding author.
E-mail address: matthew j@mail.ecust.edu.cn (Y. Ji).
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.08.018

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Y. Ji et al. / Journal of Molecular Catalysis A: Chemical 314 (2009) 63–70
of adsorbed CO on the reaction. It is reported that enhancement of
CO adsorption over catalysts could promote catalysts performance.
However, which kind of adsorbed CO takes part in the coupling
reaction has not been reported in literatures [4,22]. In fact, verifying
which kind of adsorbed CO participates in the reaction is significant
to catalysts preparation; because the ratio of bridge bonded CO to
linearly bonded CO could change with changing Pd cluster size [21].
To gain detailed information on the pathways of the coupling
reaction of CO and MN to form DMO, the present study was per-
formed by using in situ FTIR under unsteady state conditions.
2. Experimental
2.1. Preparation of catalysts
Pd/␣-Al2O3 catalysts containing 0.1 wt% Pd were prepared by
impregnation of ␣-Al2O3 (BET area 16.58 m2/g) with acidified (with
HNO3, pH 1.5) aqueous solutions of Pd(NO3)2. Water contained in
samples was gradually removed by heating at 353 K. Finally sam-
ples were calcined in static air at 773 K for 5 h. Catalysts were
reduced by hydrogen (25 mol% H2) for 5 h at 473 K before using.
Fig. 1. Spectra of DMC (408 K, 1 atm). (a) Gaseous DMC and (b) gaseous DMC with
the presence of Pd/␣-Al2O3 outgassed by N2 for 10 min.
Esters have two characteristic absorbance in FTIR spectra aris-
ing from the C O and C–O– groups [23]. According to literatures
[23,24], DMO has two bands assigned to C O vibrations at 1780
and 1752 cm−1and two bands assigned to C–O– vibrations at 1208
and 1158 cm−1. Since the C O stretching vibrations for DMC and
MF are coincident with those of DMO, it is impossible to identify the
presence of DMC and MF by inspection of the C O stretching por-
tion of the spectrum. But, the band at 1298 cm−1 assigned to C–O
vibration in DMC could be taken as evidence to identify the pres-
ence of DMC in the spectra of in situ experiments. Although C–O
stretching vibrations for MF are partially coincident with those of
DMO, bands ranging from 1158 to 1221 cm−1 assigned to C–O vibra-
tion in MF could also be taken as evidence to identify the presence
of MF, because the shape of bands assigned to C–O vibrations in MF
is very unique from those of DMC and DMO.
2.2. FTIR experiments
In situ FTIR experiments were carried out using 10 mg of cata-
lyst pressed in to a 12-mm diameter pellet. The pellet was placed
into an in situ IR cell, and spectra were recorded by using a Nicolet
Protégé 460 Fourier transmission spectrometer with a resolution
of 4 cm−1 and 10 scans per spectrum. Prior to adsorbate exposure,
the catalysts had been heated in N2 flow at 493 K for 14 h. In a typi-
cal experiment, the temperature was reduced to 408 K prior to the
introduction of MN (MN:N2 = 1:9) or CO (CO:N2 = 2:8). All flow rates
were approximately 30 cm3/min. Once the MN or CO adsorption
equilibrium was attained, the stream was switched to a mix flow
containing CO, MN, and N2 (MN:CO:N2 = 1:2:7) to start the cou-
pling reaction. Once the reaction started, IR spectra were recorded
with reaction time until the reaction attained steady state. All FTIR
experiments were performed under the conditions of 408 K and
1 atm.
3.2. IR spectra of adsorbed MN and CO
Spectra of the MN adsorption on Pd/␣-Al2O3 are shown in
Fig. 3. For gaseous MN, as Fig. 3(a) shows, the bands at 1625
and 1680 cm−1 are attributed to the N O vibration. The bands at
2960 and 2844 cm−1 are assigned to CH3− asymmetric stretching
vibrations and symmetric vibration, and the bands of the CH3−
deformation modes are at 1443 and 1388 cm−1. Fig. 3(b and c)
shows the MN adsorption on Pd/␣-Al2O3. Comparing with the
spectra of gaseous MN, the bands at 1060 cm−1 assigned to C–O
vibration obviously increased. The bands at 1060 cm−1 can be
3. Results and discussion
3.1. IR spectra of DMC and MF
Dimethyl carbonate (DMC) and methyl formate (MF) are by-
products of the CO coupling reaction to form DMO. Infrared spectra
of DMC and MF with the presence of Pd/␣-Al2O3 were recorded to
aid the interpretation of the spectra observed under in situ con-
dition. Spectra of gaseous DMC and DMC with the presence of
Pd/␣-Al2O3 at 408 K are shown in Fig. 1. Bands at 2971, 2869 and
1463 cm−1 are due to the anti-symmetric stretching, symmetric
stretching, and deforming C–H vibrations of the CH3− in DMC.
Moreover, bands at 1780 and 1767 cm−1 are attributable to C
O
stretching vibrations in DMC. Besides, a strong band at 1298 cm−1
can be attributed to the C–O stretching frequencies [23].
Spectra of MF are presented in Fig. 2. Bands at 2946 and
1450 cm−1 are attributed to stretching and deforming C–H vibra-
tions of the CH3 in MF. Three bands ranging from 1740 to 1767 cm−1
are due to C O stretching vibrations in MF. Bands from 1158 to
1221 cm−1 could be attributed to C–O–C vibrations in MF. Accord-
ing to spectra in Figs. 1 and 2, it is discovered that spectra of gaseous
by-products are the same as spectra of by-products with the pres-
ence of catalysts. Consequently, adsorptions of DMC and MF do not
occur on Pd/␣-Al2O3 under the conditions of 408 K and 1 atm.
Fig. 2. Spectra of MF (408 K, 1 atm). (a) Gaseous MF and (b) gaseous MF with the
presence of Pd/␣-Al2O3 outgassed by N2 for 10 min.

Y. Ji et al. / Journal of Molecular Catalysis A: Chemical 314 (2009) 63–70
65
attributed to surface methoxide species [4]. Besides, the bands at
1875 cm−1 assigned to gaseous NO were observed in Fig. 3(a–c),
but the absorbance of the bands at 1875 cm−1 was fairly weak.
It is known [25,26] that when MN adsorbed on Pd/Al2O3, MN
dissociative adsorption could occur to form surface methoxide
species. Since the CH3O–NO bond (about 42 kcal/mol in gas phase
[1,27]) is weak, the cleavage of CH3O–NO bond is believed to be
the first step for the chemisorption of MN over Pd/␣-Al2O3. Many
researchers [15,16] believed that alkyl nitrite adsorption resulted in
di-alkyl oxide species, RO–Pd–OR, on Pd surface. If the MN adsorp-
tion on Pd resulted in MeO–Pd–OMe species, the bands assigned
to gaseous or adsorbed NO could be observed besides the bands at
1060 cm−1 attributed to methoxide in Fig. 3. However, adsorbed
NO (1753 cm−1 [28]) was not observed in Fig. 3, and the bands
assigned to gaseous NO were attributed to trace NO contained in
feed gas (by comparing with spectra of gaseous MN). Consequently,
the experimental results indicate that the adsorption of MN over
Pd/␣-Al2O3 may not occur in a pattern in which MN decomposes
to form di-methoxide species and NO. Although it is lack of litera-
tures on MN adsorption on Pd catalysts, MN adsorptions on other
transition metals have been reported [26,29–33]. Gomes and Illas
[33] studied MN adsorption on Au by means of density functional
calculations, and they found that the most favorable adsorption
geometry for MN on Au(1 1 1) is the bridge-bonded central oxygen
atom. Pressley et al. [31] studied the MN adsorption on Ag(1 1 1),
and they also gained similar conclusion. Considering Au, Ag, and Pd
are transition metals, it is possible that the MN adsorption on Pd
also generates ON–Pd–OMe as an intermediate participating in the
CO coupling reaction. The further discussion on ON–Pd–OMe will
be given in next section.
Fig. 3. Spectra of MN (408 K, 1 atm). (a) Gaseous MN, (b) MN adsorption on Pd/␣-
Al2O3 and (c) MN adsorption on Pd/␣-Al2O3 outgassed by N2.
vibration, and the bands at 1875 cm−1 attributed to gaseous NO
appeared and were increasing, as shown in Figs. 5(1–4) and 7(1–4).
The decreasing of the bands at 1060 cm−1 indicates that CO reacts
with MN adsorption species to form surface acyl groups. The bands
ranging from 1150 to 1230 cm−1 could be assigned to C–O–C vibra-
tion of surface acyl groups (−COOMe). Besides, according to spectra
of MF in Fig. 2, the bands at 1767, 1754, 1742 and 1182 cm−1 are
attributed to gaseous MF.
Subsequently, when bands ranging from 1150 to 1230 cm−1
increased to a certain level, the bands at 1767, 1208 and 1158 cm−1
attributed to DMO appeared and increased, and eventually replaced
the bands assigned to MF and surface acyl groups in the spec-
tra, as shown in Fig. 5(5 and 6). Furthermore, it is discovered that
the bands at 1298 cm−1 appeared and increased when the bands
assigned to DMO appeared. Comparing with standard spectra of
DMO [24] and spectra of DMC in Fig. 1, it is believed that the bands at
1298 cm−1 could be attributed to the C–O–C vibration of DMC. After
the tenth minute, the reaction and the spectrum attained steady
state. According to Fig. 5 an appearance sequence which is surface
methoxide, surface acyl groups and MF, and DMC and DMO can be
identified in in situ IR spectra.
Fig. 4 illustrates the spectra of the CO adsorption on Pd/Al2O3
catalysts. Two CO adsorbed species on palladium were observed.
The bands at about 1950 cm−1 were attributed to bridge bonded CO.
And the bands at 2082 cm−1 assigned to linearly bonded CO were
observed after removing gaseous CO by N2 displacement, as shown
in Fig. 4(b–d). Moreover, it is found that bridge bonded CO is less
mobile and bonded more tightly than linearly bonded CO, because
the band at 2082 cm−1 is weaker than the band below 1950 cm−1
in N2 flow, which is in good agreement with literatures [16,20,34].
3.3. In situ IR study of CO coupling reaction
3.3.1. CO coupling reaction after MN pre-adsorption over
Pd/˛-Al2O3
The rate of CO coupling reaction to DMO is too fast to identify
intermediates by simply performing in situ FTIR experiment. So,
slowing down the rate of the reaction is necessary for studying the
reaction mechanism by using FTIR.
It has been known that the coupling reaction is initiated by
adsorptions of CO and MN. Reducing the surface concentration of
reactants adsorption species could make the rate of the DMO for-
mation slow. Consequently, conducting the CO coupling reaction
over a catalysts surface which has been occupied by MN adsorp-
tion species provides an opportunity to slow down the rate of DMO
synthesis.
Figs. 5 and 6 demonstrate spectra of the CO coupling reaction
after MN pre-adsorption. Fig. 5(1) presents spectra of the MN pre-
adsorption over Pd/␣-Al2O3 before reaction, which is the same as
the spectra in Fig. 3(b). The MN adsorption resulted in dominated
surface methoxide species as the first intermediates on Pd surface
because the band at 1060 cm−1 was obviously observed as shown
in Fig. 6(1). After conducting the reaction, during the first 5 min, the
bands at 1060 cm−1 sharply decreased, as shown in Fig. 6; more-
over the bands at 2082 and 2052 cm−1 assigned to linearly bonded
CO, the bands at 1767, 1754 and 1742 cm−1 assigned to C O vibra-
tion, the bands ranging from 1150 to 1230 cm−1 assigned to C–O–C
Fig. 4. Spectra of CO adsorption on Pd/␣-Al2O3 (408 K, 1 atm). (a) CO adsorbed
on catalyst, (b) outgassed by N2 for 3 min, (c) outgassed by N2 for 11 min and (d)
outgassed by N2 for 30 min.

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Y. Ji et al. / Journal of Molecular Catalysis A: Chemical 314 (2009) 63–70
Fig. 5. Spectra of CO coupling reaction after MN pre-adsorption with reaction time, range from 1000 to 3000 cm−1. (1) 0 min, (2) 1 min, (3) 3 min, (4) 5 min, (5) 6 min and (6)
10 min.
MN dissociative adsorption on Pd is the first step of the CO cou-
pling reaction. The dissociative adsorption generates an adsorbed
species of MN adsorption as the first intermediate participating in
the coupling reaction. And other intermediates participating in the
reaction are derived from CO insertion on the first intermediate. So,
it is significant to identify the first intermediate for investigating the
mechanism of the coupling reaction.
and ON–Pd–OCH3 [3,14]. If the species of MN adsorption was
Pd(OCH3)2, MF could not appear in the appearance sequence.
Because the reaction between Pd(OCH3)2 and adsorbed CO
would generate CH3O–Pd–COOCH3 which could more probably
derive DMC than MF. Consequently, considering the appear-
ance sequence above-mentioned, IR spectra of MN adsorption
in Fig. 3, and literatures [31,33] on MN adsorption on other
transition metals, it is believed that the first intermediate is
ON–Pd–OCH3.
It is reported that there are two speculative modes of the sur-
face species of MN adsorption, which are Pd(OCH3)2 [6,15,16]
Fig. 6. Spectra of CO coupling reaction after MN pre-adsorption with reaction time, range from 700 to 1100 cm−1. (1) 0 min, (2) 1 min, (3) 3 min, (4) 5 min, (5) 6 min and (6)
10 min.

Y. Ji et al. / Journal of Molecular Catalysis A: Chemical 314 (2009) 63–70
67
Fig. 7. CO adsorption segment of Fig. 5. (1) 0 min, (2) 1 min, (3) 3 min, (4) 5 min, (5) 6 min and (6) 10 min.
Reactions involving in Fig. 5 are described as reactions (2)–(8).
Fig. 5(1–4) illustrates a process that adsorbed CO inserts on the
ON–Pd–OCH3 to form ON–Pd–COOCH3 followed by the partial
decomposition of ON–Pd–COOCH3 to form MF and NO. The pro-
cess is described as reactions (2)–(4). The proton participating in
reaction (5) could be supplied from partial thermal decomposition
of MN [25]:
Subsequently, as shown in Fig. 5(5 and 6), with increasing con-
centration of ON–Pd–COOCH3 the rates of reactions (5) and (6)
increased and bands assigned to DMO were observed in spectra.
Since competition from reaction (5), reaction (4) was suppressed
when the concentration of ON–Pd–COOCH3 over catalysts surface
increased to a certain level:
(2)
(3)
(4)
(5)
(6)
In Fig. 5(4–6), it is also observed the bands at 1298 cm−1 attributed
to the C–O–C vibration of DMC accompanying with the bands
assigned to DMO. The formation of DMC is caused by the reac-
tion between ON–Pd–OCH3 and ON–Pd–COOCH3. The formation
is described as reactions (7) and (8). Reaction (7) is parallel to reac-
tion (5). It has been known that DMC is the major by-product of
the coupling reaction, and MF can be detected under the condition
of low CO partial pressure. That is in good agreement with the IR
experimental results:
(7)
(8)
A special phenomenon that linearly bonded CO appeared prior to
bridge bonded CO when the coupling reaction was conducted after
MN pre-adsorption is noteworthy, as shown in Figs. 5 and 7. The
Fig. 8. Absorbance variety of the band at 2080 cm−1 assigned to linearly bonded CO
with reaction time.

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Y. Ji et al. / Journal of Molecular Catalysis A: Chemical 314 (2009) 63–70
Fig. 9. Spectra of CO coupling reaction after CO pre-adsorption with reaction time. (1) 0 min, (2) 1 min, (3) 2 min, (4) 3 min, (5) 4 min, (6) 5 min and (7) 30 min.
bands at 2080 and 2050 cm−1 are attributed to linearly bonded CO,
and the bands at 1960 cm−1 are attributed to bridge bonded CO in
spectra. According to Figs. 5, 7 and 8, linearly bonded CO appeared
prior to bridge bonded CO after the reaction occurred, and it was lin-
early increasing during the first 5 min. In the same time, the bands
assigned to ON–Pd–COOMe also increased. The bands assigned to
bridge bonded CO was not observed until the time when the bands
assigned to C–O–C vibration in ON–Pd–COOMe disappeared and
the bands assigned to C–O–C vibration in DMO appeared in the
spectra.
However, it has been well known that bridge bonded CO appears
prior to linearly bonded CO when CO adsorbed on Pd. In general,
bridge bonded CO is less mobile and bonded more tightly than lin-
early bonded CO. Ladas et al. [17] studied the adsorption of CO on
palladium particles supported on Al2O3. They found that bridge
bonded CO was filled at exposures of a few Langmuirs and it des-
orbed at temperatures above 400 K. Linearly bonded CO was filled
at higher exposures and it desorbed already at temperatures below
400 K. Based on their work, it can be easily concluded that bridge
bonded CO appears prior to linearly bonded CO when CO adsorbed
Fig. 10. CO adsorption segment of Fig. 9. (1) 0 min, (2) 1 min, (3) 2 min, (4) 3 min, (5) 4 min and (6) 5 min.

Y. Ji et al. / Journal of Molecular Catalysis A: Chemical 314 (2009) 63–70
69
3.3.2. CO coupling reaction after CO pre-adsorption over
Pd/˛-Al2O3
Spectra of the CO coupling reaction after CO pre-adsorption with
reaction time are presented in Fig. 9. CO pre-adsorption resulted in
linearly bonded CO and bridge bonded CO over catalysts surface,
as shown in Fig. 9(1). When the coupling reaction occurred, it was
immediately observed the bands assigned to DMO without bands
attributed to acyl groups or MF. Meanwhile, a sharp decrease of
bridge bonded CO was also observed, as shown in Figs. 10 and 11.
Moreover, it is found that the reaction under the conditions of CO
pre-adsorption reached steady state more quickly than the one
under the conditions of MN pre-adsorption. In Fig. 5, it took more
than 10 min. But it took 5 min in Fig. 9.
As Fig. 11 shows, the absorbance of bridge bonded CO decreased
sharp when the coupling reaction occurred. Subsequently, it gradu-
ally reached a steady level. The sharp decrease suggests that bridge
bonded CO over catalysts surface reacted with adsorbed MN when
the coupling reaction occurred. Consequently, combining the sharp
decrease of bridge bonded CO with the linear increase of linearly
bonded CO above-mentioned, it is also evident that bridge bonded
CO rather than linearly bonded CO takes part in the coupling reac-
tion.
Fig. 11. Absorbance variety of the band at 1960 cm−1 assigned to bridge bonded CO
with reaction time.
Furthermore, according to spectra in Figs. 5 and 9, when the cou-
pling reaction occurred with low surface concentration of bridge
bonded CO, formations of MF, DMC, and DMO were observed. But
when the surface concentration of bridge bonded CO was on a high
level, MF formation was not observed, and the bands assigned to
DMC also became weak. This means that bridge bonded CO has a
significant influence on the selectivity of DMO. This is also in good
agreement with our kinetic study results that MF and DMC could
be detected when the ratio of MN to CO is very high, but MF and
DMC as by-product are hardly detected when the ratio is very low.
on Pd. In addition, Xu et al. [34] studied CO adsorption on Pd/Al2O3
by FTIR. And they also observed that bridged bonded CO appeared
prior to linearly bonded CO on Pd/Al2O3.
The conflict between facts observed in present study and con-
clusions of literatures [20,34] reveals that bridge bonded CO
rather than linearly bonded CO takes part in the reaction. When
the reaction began, because of abundant ON–Pd–OCH3 over cat-
alysts surface before the coupling reaction, the formation of
NO–Pd–COOMe (reaction (3)) was the dominated reaction over cat-
alysts. So, the rate of disappearance of bridge bonded CO was much
faster than its rate of appearance. Meanwhile, since linearly bonded
CO did not participate in the reaction, its rate of appearance was
much faster than its rate of disappearance. Consequently, it was
observed a linear increase of linearly bonded CO in Fig. 6, and bands
assigned to ON–Pd–COOMe in Fig. 5 during the first 5 min. As the
coupling reaction undergoing, when the appearance rates of bridge
bonded CO gradually caught up with its disappearance rates, bridge
bonded CO were observed in 5 min.
4. Conclusions
Two intermediates were identified in present study. The first
intermediate is generated from MN adsorption on Pd catalyst, and
the second one is caused by CO insertion on the first one. Combining
literatures [31,32] with experimental facts, the first intermedi-
ate is ON–Pd–OMe. The second intermediate is ON–Pd–COOMe.
Besides the two observed intermediates, the third intermediate,
Pd(COOMe)2 may exist, its existence has been identified in litera-
ture [35].
Moreover, in present study, it is firstly discovered that although
CO adsorption generates linearly and bridge bonded CO over cata-
lysts surface, bridge bonded CO solely takes part in the reaction.
Furthermore, it is found that the low coverage of bridge bonded
CO over catalysts surface leads to the formations of by-products
(MF and DMC). But the formations of by-products are suppressed
at high coverage of bridge bonded CO.
Finally, the mechanism of CO coupling reaction to form DMO is
proposed. The coupling reaction is initiated by reactants adsorp-
tions to form ON–Pd–OMe and bridge bonded CO. Once the
ON–Pd–OMe is formed, bridge bonded CO inserts on the inter-
mediate to form ON–Pd–COOMe. Repeating the formation of the
ON–Pd–COOMe leads to a Pd(COOMe)2 intermediate which, upon
reductive elimination, yields DMO. It was observed ON–Pd–OMe
and ON–Pd–COOMe in FTIR spectra, but ON–Pd–CO(CO2Me) was
not observed. Consequently, the formation of DMO proceeds via
coupling of two acyl groups rather than double carbonylation. The
mechanism for CO coupling reaction to DMO is described in Fig. 12.
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Fig. 12. The mechanism of CO coupling reaction to DMO.
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