Alkali α-MnO2/Na: XMnO2 collaboratively catalyzed ammoxidation-Pinner tandem reaction of aldehydes

Article abstract of DOI:10.1039/c6cy00874g

The tandem reaction is a growing field to yield important advances toward green and sustainable chemistry. Herein, we report a bifunctional manganese oxide catalyst with an interface binding redox phase (α-MnO₂) and a basic phase (Na_xMnO₂). The molar ratio of NaOH/Mn plays a great role in the formation of α-MnO₂/Na_xMnO₂. The sodium cation is essential for the formation of a basic Na_xMnO₂ phase while the potassium cation promotes the formation of a redox-active α-MnO₂ phase. The interface structure of α-MnO₂/Na_xMnO₂ geometrically favors the ammoxidation-Pinner tandem reaction to synthesize imidates in a 58-96% yield from aldehydes. Thus a phase collaborative effect is observed. In the ammoxidation process, the redox cycle of Mn^IV/Mn^III is involved and the lattice oxygen in the α-MnO₂ phase acts as an active oxygen species. The O-H in methanol is activated and dissociated on the basic sites of Na_xMnO₂ to the adsorbed methoxyl species to facilitate the Pinner synthesis. This approach bypasses the conventional synthesis of imidates, which suffer from harsh reaction conditions and the requirement for multiple steps.

Full text of DOI:10.1039/c6cy00874g

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DOI: 10.1039/C6CY00874G
Catalysis Science & Technology
ARTICLE
Alkali α‐MnO2/NaxMnO2 collaboratively catalyzed ammoxidation‐
Pinner tandem reaction of aldehydes
Xiuquan Jia,a,b Jiping Ma,*a Min Wang,a Xiaofang Li,a,b Jin Gaoa and Jie Xu*a
Received 00th January 20xx,
Accepted 00th January 20xx
Tandem reaction is a growing field to yield important advances towards green and sustainable chemistry. We report a
bifunctional manganese oxides catalyst with interface binding redox phase (α‐MnO2) and basic phase (NaxMnO2). The
molar ratio of NaOH/Mn plays great role on the formation of α‐MnO2/NaxMnO2. Sodium cation is essential for formation
of a basic NaxMnO2 phase while potassium cation promotes to form a redox active a‐MnO2 phase. The interface structure
of α‐MnO2/NaxMnO2 geometrically favors the ammoxidation‐Pinner tandem reaction to synthesize imidates with 58‐96%
yields from aldehydes. The phase collaborative effect is observed. In the ammoxidation process, the redox cycle of
MnIV/MnIII is involved and the lattice oxygen in α‐MnO2 phase acts as active oxygen species. The O‐H in methanol is
activated and dissociated on the basic sites of NaxMnO2 to adsorbed methoxyl species to facilitate Pinner synthesis. This
approach bypasses conventional synthesis of imidates suffering from harsh reaction conditions and multiple steps.
DOI: 10.1039/x0xx00000x
www.rsc.org/
two or more distinct functionalities could be located in an
interfacial manner and collaboratively catalyze the tandem
reaction.
1. Introduction
New methodologies to assemble complex synthetic sequences
in a tandem reaction are great impetus to atom economy,
overall process simplicity, as well as biomimetic catalysis.1
Traditionally, more than one catalyst is used to promote two
or more mechanistically distinct reaction steps. The reaction
intermediate has to diffuse from the first catalyst site to the
next. If the distances are too large, concentration gradients
develop and catalytic activity as well as selectivity decreases.2
In comparison to the composite solid catalysts with isolated
components, a nanoscale integrated structure with different
functionalities is a promising candidate for tandem reactions
owing to the close geometry of multicatalytic functions and
favor for collaborative catalysis.3 However, the fabrication of
high catalytic efficiency with nanoscale intimacy of different
functionalities is still challenging owing to the lack of deep
understanding of the physical‐chemical features of the catalyst
and difficulty in accommodating mechanistically different
catalytic cycles under the same reaction condition.4
Scheme 1 The methods for the synthesis of imidates.
Imidate is an important class of nitrogen‐containing motif
in the chemistry and biology.7 It is traditionally obtained
through Pinner synthesis in the presence of hazardous
hydrogen chloride gas, or strong base using nitrile as substrate
(Scheme 1a, b), and was barely reported to be synthesized
though a single tandem reaction from more readily available
aldehydes (Scheme 1c), owing to 1) the easily decomposed or
hydrolyzed properties of imidates at high temperature;8 2) the
generally inert reactivity for ammoxidation at low temperature;
3) the insufficient acid or base catalysts for nitrile addition with
alcohol. We challenge the conventional wisdom of the rules by
fabricating a manganese oxides catalyst with nanoscale
intimacy of redox and base functionalities for ammoxidation
Manganese oxides are characterized by versatile crystal
structures and tunable redox and acid‐base properties.5
Recently, manganese oxides interface was developed by
integrating different structures to form a composite in which
the two components are connected by a structurally
compatible close‐packed oxygen array.6 By tailoring the
composition of the crystal structures of manganese oxides,
a. State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy,
Dalian 116023, P. R. China. E‐mail: xujie@dicp.ac.cn; majiping@dicp.ac.cn.
b. University of the Chinese Academy of Sciences, Beijing 100049, P. R. China.
†Electronic
Supplementary
Information
(ESI)
available:
See
DOI:
10.1039/x0xx00000x
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and nitrile addition, respectively. Very recently, Manganese the total Mn salts (NaOH/Mn) is 1.2, was collected by filtration,
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oxide based octahedral molecular sieves (KMn8O16; OMS‐2) washed repeatedly with distilled watDeOr,I: 1a0n.1d039f/inCa6CllyY00d8r7i4eGd
was found to be efficient catalysts for ammoxidation at low overnight in air at 60 °C, then calcined in air at desired
temperature by our group.9 However, OMS‐2 was not an temperature for 4 h. The preparation procedure of α‐
effective catalyst for the Pinner reaction. A major challenge MnO2/NaxMnO2 with different molar ratio of NaOH/Mn is the
needed in this reasearch is to develop catalytic sites for Pinner same as α‐MnO2/NaxMnO2 with NaOH/Mn molar ratio of 1.2
reaction. To date, no example exists for the heterogeneously except that different amount of NaOH was used.
catalytic addition reaction of nitrile with methanol. Herein, on
the basis of our previous work,9,10 we demonstrate the active
manganese oxides integrating redox phase (α‐MnO2) and basic
2.3 Catalyst characterization
phase (NaxMnO2) for efficiently catalyzing the ammoxidation‐ The X‐ray powder diffraction (XRD) patterns were obtained
Pinner tandem reaction. When α‐MnO2 and NaxMnO2 are using Rigaku D/Max 2500/PC powder diffractometer with Cu
combined in a physical mixture, the initial ammoxidation step Kα radiation (λ = 0.15418 nm) at 40 kV and 200 mA in a
and the further Pinner reaction could not efficiently occur scanning rate of 5 o/min. High‐resolution transmission electron
concurrently. In contrast, the nanoscale co‐existence of the microscopy (HRTEM) was performed with a JEOL JEM‐2100EM
two forms of manganese oxides in a single material was found microscope. The samples were suspended in ethanol with an
to be vital to the success of the tandem reaction. This ultrasonic dispersion for 5−10 min, and then a few droplets of
approach allows for the synthesis of various imidates from the suspension were put on a microgrid carbon polymer
aldehydes under ambient conditions. The phase boundary supported on a copper grid and allowed to dry at room
collaborative effect is observed. The parameters effect on the temperature for TEM observations. CO2 temperature
formation of α‐MnO2/NaxMnO2, the reaction route and the programmed desorption (CO2‐TPD) was conducted using a
activation of substrate as well as intermediates on the α‐ Micromeritics Autochem 2910. The catalyst sample was
MnO2/NaxMnO2 were studied in detail.
pretreated in Ar for 1.5 h at 200 oC. After cooling to 50 oC in Ar,
Pulse CO2 adsorption was performed until peaks are equal or
20 times. The temperature was increased from 50 to 800 oC at
o
a rate of 10 C min‐1 under pure He flow. The evolved gas
2. Experimental
analysis was performed using a quadrupole mass spectrometer
(Ominstar 300). Electron paramagnetic resonance (EPR) was
performed on a Bruker spectrometer in the X‐band at room
temperature with a field modulation of 100 kHz. The
microwave frequency was maintained at 9.401 GHz. The X‐ray
photoelectron spectroscopy (XPS) measurements were
2.1 Materials
All chemicals were of analytical grade and used as received
unless otherwise stated. MeOH, aq. NH3, Fe2O3, Co3O4, NiO,
TiO2, ZrO2, WO3, CuO, KMnO4, MnSO4 and MnCl2 were
purchased from Tianjin Kermel Chemical Reagent Co. Ltd..
Furfural, 5‐methylfurfural, 3,4‐dimethylfurfural, 5‐bromo‐
performed on
a Thermo ESCALAB 250Xi spectrometer
equipped with a monochromated AlKα X‐ray source (hυ =
1486.6 Ev, 15 kV, 10.8 mA). The samples were vacuum dried at
120 °C for 12 h. The charge neutralizer system was used for all
of the analyses. The base pressure was 1 × 10–8 Pa. High
resolution spectra were recorded with 20‐eV pass energy. The
pass energies correspond to the Ag3d5/2 FWHM of 0.65 eV. The
data were acquired with 0.05 eV steps. The binding energy (BE)
furfural,
5‐phenylfurfural,
pyridine‐2‐carboxaldehyde,
pyridine‐4‐carboxaldehyde, 3‐nitrobenzaldehyde, 4‐nitro‐
benzaldehyde, p‐chlorobenzaldehyde, p‐hydroxybenzaldehyde,
anisic aldehyde, cinnamaldehyde, isobutyraldehyde, Nb2O5,
CeO2, MgO and Mn3O4 were obtained from Aladdin Chemistry
Co. Ltd.. V2O5 is domestic reagent. 5‐Iodofurfural, thiophen‐2‐
carboxaldehyde,
4‐bromothiophen‐2‐carboxaldehyde,
2‐
was calibrated to the C1s signal (284.6 eV) as a reference. The
furonitrile and 2‐furamide were purchased from TCI Shanghai.
5‐Ethylfurfural, benzofurfural and γ‐MnO2 were obtained from
Alfa Aesar. 5‐Ethoxymethylfurfural was purchased from Sigma‐
curve fitting procedure was performed using an approximation
based on a combination of the Gaussian and Lorentzian
functions with subtraction of a Shirley‐type background. In situ
Fourier transform infrared spectroscopy (FT‐IR) spectra of
methanol adspecies on catalysts were recorded with a TENSOR
27 spectrometer equipped with an in situ IR cell, which was
connected to a conventional gas flow system. The samples (20‐
30 mg) were pressed into a self‐supporting wafer (20 mm
diameter) and mounted into the IR cell. Adsorption of
methanol was carried out in the following method: The sample
was used without pretreatment. Methanol was fed to the in
situ IR cell under vacuum (< 10‐1 Pa). Then, the IR disk was
purged with N2, and IR measurement was carried out until the
spectrum was stable.
Aldrich. α‐MnO2,11 β‐MnO2,12 δ‐MnO2,13 OMS‐214 and
15
NaxMnO2
were prepared according to the literature
procedures.
2.2 Catalyst preparation
The α‐MnO2/NaxMnO2 catalyst used in this work was prepared
by coprecipitation method. In a typical preparation, the
calculated amounts of KMnO4 (40 mmol) and NaOH (120 mmol)
was dissolved in 100 mL deionized water. The obtained
solution was added into another 100 mL aqueous solution of
Mn(NO3)2 (60 mmol). After stirring at room temperature for 4
h, the resulting solid, where the molar ratio of added NaOH to
2 | J. Name., 2012, 00, 1‐3
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2.4 Determination of H‐ of NaxMnO2
Mn(NO3)2, where the molar ratio of added NaOH to the total
Mn salts (NaOH/Mn) is 1.2. The as‐prepared materials were
View Article Online
DOI: 10.1039/C6CY00874G
The base strength of the sample was detected by a series of
Hammett indicators.16 The indicators included phenolphthalein
thermally treated at different temperatures under air to
modulate the activity of redox and basic sites. The crystal
structure was characterized by X‐ray powder diffraction (XRD)
(Fig. 1A). At low calcination temperature, the crystallinity is
poor and only the peaks attributed to α‐MnO2 (JCPDS#00‐044‐
0141) phase are observed. As calcination temperature
increases, the crystallinity improves. When the calcination
temperature is above 420 oC, the characteristic {002}
diffraction peak of layered NaxMnO2 (JCPDS#00‐027‐0751) at
around 2θ of 15.8o also appears.15 The diffraction peak of
possible segregation of potassium manganate species KxMnO2
(JCPDS#00‐030‐0951) was not observed.
(
H
_ = 9.3), 2,4‐dinitroaniline (
18.4), benzidine ( _ = 22.5), 4‐chloroaniline (
aniline ( _ = 27.0). The sample was transferred to purified
H
_ = 15.0), 4‐nitroaniline (
H_ =
_ = 26.5), and
H
H
H
benzene. Two drops of indicator solution (0.1 wt % indicator
dissolved in benzene) were added to the suspension, and the
color change of the indicator was monitored visually.
2.5 Catalytic activity test
Catalytic reactions were performed in a 20 mL stainless steel
autoclave equipped with a magnetic stirrer, a pressure gauge,
and automatic temperature control apparatus. The reactor
was connected to an oxygen cylinder for reaction pressure. In
a typical experiment, furfural (48.0 mg, 0.5 mmol), aq. NH3
(120 μL, 3 equiv.), MeOH (5 mL) and α‐MnO2/NaxMnO2
prepared with NaOH/Mn molar ratio of 1.2 and calcined at 250
oC (0.1 g) were loaded into the reactor. After sealing and
charging with O2 (0.5 MPa), the autoclave was heated to the
desired temperature (30 oC). After reaction, the autoclave was
cooled. The solution was separated by centrifugation and
analyzed by GC using the internal standard method. A larger
scale transformation of furfural (0.24 g, 2.5 mmol; 5‐fold scale)
was carried out for product isolation. After reaction, the
solution was separated by centrifugation. MeOH was removed
by rotary evaporator and dichloromethane (15 mL) was added.
The resulting solution was washed with brine (3 × 5 mL) and
dried by anhydrous sodium sulfate. The product was obtained
after evaporation of dichloromethane. Other isolated products
were obtained under similar experimental conditions.
Fig. 1 (A) XRD patterns (B) CO2‐TPD profiles of α‐MnO2 (a), α‐MnO2/NaxMnO2
with NaOH/Mn molar ratio of 1.2 calcined at 250 oC (b), 400 oC (c), 420 oC (d),
450 oC (e), 500 oC (f) and NaxMnO2 (g).
The co‐existence of α‐MnO2 and NaxMnO2 is directly
observed by the high‐resolution transmission electron
microscopy (HRTEM) (Fig. 2 and Fig. S7). α‐MnO2 {110} crystal
plane with lattice spacing of 0.69 nm and NaxMnO2 {002}17
crystal plane with lattice spacing of 0.57 nm were observed
and in close proximity which may geometrically favor the
collaborative catalysis in the nanoscale.
2.6 Products analysis
The products were identified by Agilent 6890N GC/5973MS as
well as by comparison with the retention times to
corresponding standards in GC traces. Gas chromatography
measurements were conducted on Agilent 7890A GC with
autosampler and a flame ionization detector. HP‐5 capillary
column (30 m × 530 μm × 1.5 μm) was used for separation of
reaction mixtures. The temperature of the column was initially
kept at 100 oC for 5 min, and then was increased at a rate of 20
o
oC min‐1 to 220 C and kept for 10 min. The conversion of
furfural and yield of corresponding products were evaluated
using 1,3,5‐trimethylbenzene as the internal standard. The
conversion of other substrates and selectivity of corresponding
products were determined based on area normalization
without any purification.
Fig. 2 HRTEM image of α‐MnO2/NaxMnO2 with NaOH/Mn molar ratio
of 1.2 calcined at 450 oC (a), and the corresponding crystal lattice
space of α‐MnO2 {110} (b) and NaxMnO2 {002} (c).
3. Results and discussion
The parameters effect on the formation of α‐
MnO2/NaxMnO2 were further investigated. The molar ratio of
NaOH/Mn is of significance for the formation of α‐
MnO2/NaxMnO2. The co‐existence of α‐MnO2 and NaxMnO2
3.1 Structure of α‐MnO2/NaxMnO2
The α‐MnO2/NaxMnO2 was prepared the same with α‐MnO2
except that NaOH was added in the mixture of KMnO4 and
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was just detected when the molar ratio of NaOH/Mn was more is 20 times more than that of α‐MnO2 (0.3 × 10‐4 mmol m‐2)
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than 1.08. While the co‐existence of α‐MnO2 and Mn2O3 and OMS‐2 (0.4 × 10‐4 mmol m‐2). FurthDeOrmI: 1o0r.e10,3N9/aCxM6CnY0O028w74aGs
(JCPDS#00‐024‐0508) was detected when the molar ratio of found to possess abundant amount of basicity (4.5 × 10‐3 mmol
NaOH/Mn was less than 1.08. Other MnII salts such as MnSO4, m‐2) with strength 9.3 <H‐< 15.0. These results indicate that the
MnCl2, Mn(OAc)2 has no obvious influence on the formation of basicity on the surface of α‐MnO2/NaxMnO2 is ascribed to the
MnO2/NaxMnO2. The co‐existence of α‐MnO2 and Mn2O3 was NaxMnO2 phase. To explore the possible role of potassium
detected when substituting KOH for NaOH. KMnO4 was also cation on the generation of basicity, we synthesized the
replaced with NaMnO4 to prepare the catalyst under the catalyst with KOH instead of NaOH. The obtained manganese
identical condition. Unexpectedly, the crystal strucure has oxides possess little basicity (0.06 × 10‐4 mmol m‐2), indicating
changed completely. The peaks attributed to α‐MnO2 could that the basicity is not caused by potassium cation.
not be obviously detected (Fig. S8), indicating that the
crystalline structures of a‐MnO2 could no longer be stabilized
in the absence of potassium ions. Induced coupling plasma
3.3 Catalytic performance of α‐MnO2/NaxMnO2
elemental analysis results (Table S5) imply that the obtained
manganese oxide is another type of compound, which is quite
different from α‐MnO2/NaxMnO2.
Table 1. Catalytic conversion of furfural over different manganese
oxides.a
Yield (%)
Entry
Catalyst
Conv. (%)
2
3
1b
2
blank
90
‐
‐
> 99
> 99
> 99
> 99
> 99
87
91
67
49
33
86
2
5
α‐MnO2
19
43
17
8
3
β‐MnO2
4
γ‐MnO2
5
δ‐MnO2
6
OMS‐2
2
7
NaxMnO2
8c
9d
10e
α‐MnO2 + NaxMnO2
α‐MnO2 + NaxMnO2
α‐MnO2/NaxMnO2
> 99
> 99
> 99
83
17
11
13
80
89
aReaction conditions: 0.5 mmol furfural, 0.1 g catalyst, 120 μL aq.
NH3 (3 equiv.), 5 mL MeOH, 0.5 MPa O2, 30 oC, 12 h. bOnly aldimine
c
was detected by GC. 0.08 g α‐MnO2 and 0.02 g NaxMnO2 were
physical mixed. d0.1 g α‐MnO2 and 0.12 g NaxMnO2 were physical
mixed. eα‐MnO2/NaxMnO2 refers to α‐MnO2/NaxMnO2 with
NaOH/Mn molar ratio of 1.2 calcined at 250 oC thereafter unless
otherwise stated
.
Initially, we selected biomass‐derived furfural as a
model substrate for ammoxidation‐Pinner tandem reaction.
V2O5, Fe2O3, NiO, Mn3O4, TiO2, Nb2O5, ZrO2, WO3, CeO2,
MgO, and CuO were used as catalyst but nearly no
ammoxidation product was observed (Fig. S10). Previously,
we used OMS‐2 as catalyst for the ammoxidation of 5‐
hydroxymethylfurfural in methanol. The in situ generated
cyano group in the presence of electron‐withdrawing
substituent is exceptionally active to further react with
methanol without additional catalyst.9 Herein, however,
OMS‐2 mainly offers ammoxidation product of 2‐furonitrile
(Table1, entry 6), showing the generally robust property of
unactivated cyano group. Other manganese dioxides, such
as β‐MnO2, δ‐MnO2, γ‐MnO2 and α‐MnO2, give 33%‐91%
yield of 2‐furonitrile, together with a low yield (5%‐43%) of
methyl furan‐2‐carboximidate (Table 1, entries 2‐5). α‐
MnO2 shows the highest yield of 2‐furonitrile (91% yield)
Fig. 3 XRD patterns of manganese oxides calcined at 500 oC with NaOH/Mn
molar ratio of 0 (a), 0.24 (b), 0.60 (c), 0.96 (d), 1.08 (e), 1.2 (f), 1.8 (g), 1.2
and substituting MnSO4 for Mn(NO3)2 (h), 1.2 and substituting MnCl2 for
Mn(NO3)2 (i), and 1.2 and substituting Mn(OAc)2 for Mn(NO3)2 (j) , and 1.2
and substituting KOH for NaOH (k).
3.2 Basicity of α‐MnO2/NaxMnO2
We employed CO2 temperature programmed desorption (CO2‐
TPD) to probe the surface basicity of manganese oxides (Fig.
1B). α‐MnO2/NaxMnO2 with NaOH/Mn molar ratio of 1.2
calcined at different temperatures all show a CO2 desorption
peak at temperature range of 200‐500 oC which is attributed to
the moderate basic sites. The base density of α‐
MnO2/NaxMnO2 calcined at 250 oC is 6.0 × 10‐4 mmol m‐2 which
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obtain a high level of imidate product over the physical
DOI: 10.1039/C6CY00874G
mixture of the catalysts, additional NaxMnO2 should be added
but lowest yield of methyl furan‐2‐carboximidate (5% yield)
(Table 1, entry 2). It is probable that α‐MnO2 possesses
highly active redox sites and few basic sites. NaxMnO2
showed high activity for the addition of 2‐furonitrile with
methanol (Fig. 9B), but nearly no ammoxidation reaction
happened with furfural as initial substrate (Table 1, entry 7).
It could be due to the poor redox properties of NaxMnO2.
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without decreasing the amount of α‐MnO2 (Table 1, entry 9).
In comparison, α‐MnO2/NaxMnO2, with co‐existence of redox
phase of α‐MnO2 and basic phase of NaxMnO2 in an interfacial
manner, shows both high activity and selectivity for the methyl
furan‐2‐carboximidate (Table 1, entry 10). These results
indicate that the nanoscale co‐existence of α‐MnO2 and
NaxMnO2 favors the phase collaborative catalysis.
Fig. 4 Catalytic conversion of furfural over α‐MnO2/NaxMnO2 with different
molar ratio of NaOH/Mn calcined at 500 C. Reaction conditions: 0.5 mmol
o
Fig. 5 Catalytic conversion of furfural over different manganese oxides.
Reaction conditions: 0.5 mmol furfural, 0.1 g catalyst, 120 μL aq. NH3 (3
equiv.), 5 mL MeOH, 0.5 MPa O2, 30 oC, 12 h. α‐MnO2 + NaxMnO2 refers toα‐
MnO2 and NaxMnO2 physical mixed.
furfural, 0.1 g catalyst, 120 μL aq. NH3 (3 equiv.), 5 mL MeOH, 0.5 MPa O2, 30
a
b
oC, 12 h. Substituting MnSO4 for Mn(NO3)2. Substituting MnCl2 for
c
d
Mn(NO3)2. Substituting Mn(OAc)2 for Mn(NO3)2. Substituting KOH for
NaOH.
The tandem synthesis of imidate is very sensitive to
temperature. Amide becomes the major product when the
Then the ammoxidation‐Pinner tandem reactions were
carried out over the different phase‐structured manganese
oxide‐based catalysts presented in Fig. 3, and the catalyst
structure‐activity relationships are shown in Fig. 4. All of these
manganese oxides can truly catalyze the aerobic
ammoxidation reaction at 30 oC. However, the Pinner addition
activities of these samples are quite different. The samples
with the co‐existence of α‐MnO2 and Mn2O3 (NaOH/Mn ratio
less than 1.08) have no addition activities on in situ generated
nitriles, whereas the samples with the co‐existence of α‐MnO2
and NaxMnO2 (NaOH/Mn ratio more than 1.08) show efficient
addition activity. It is only the united α‐MnO2/NaxMnO2 phase
that is responsible for the considerable enhancement in the
redox‐base catalytic activity for the ammoxidation‐Pinner
tandem reaction.
o
temperature is above 90 C. Moreover, the total yields of the
products are also decreased (Table S1, entries 2 and 3).
Furthermore, the physical mixture of α‐MnO2 and
NaxMnO2 were tested for the ammoxidation‐Pinner tandem
reaction (Table 1, entry 8‐9; Fig. 5). As shown in Fig. 5, high
yield of 2‐furonitrile and low yield of methyl furan‐2‐
carboximidate were maintained when NaxMnO2 was the minor
composition. With increasing the molar fraction of NaxMnO2,
the total yield of 2‐furonitrile and methyl furan‐2‐
carboximidate decreased, indicating the ammoxidation
efficiency was decreased. The optimization of molar ratio of α‐
MnO2 to NaxMnO2 gave the best imidate yield of 36%. To
Fig. 6 Catalytic conversion of aryl aldehydes to imidates. Reaction conditions:
0.5 mmol substrate, 0.1 g α‐MnO2/NaxMnO2, 120 μL aq. NH3 (3 equiv.), 5 mL
MeOH, 0.5 MPa O2, 30 oC. The data in bracket is isolated yield.
α‐MnO2/NaxMnO2 is an efficient catalyst for the tandem
synthesis of a wide range of imidates. Furfural derivatives,
thienyl and pyridyl aldehydes were converted to the
corresponding imidates in 66‐96% yields (Fig. 6, entries 1‐12).
Furfural with electron‐withdrawing substituent of Br‐ can
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provide a high level of imidate product in 6 h; even in the case
of using OMS‐2 as catalyst, the corresponding imidate was
effectively generated (Scheme S1). An evident substituent
effect is also observed for phenyl aldehydes. Except for NO2‐
substituted benzaldehydes, nitriles were the major products
for Cl‐, HO‐, H‐, MeO‐ substituted benzaldehydes and non‐
aromatic aldehyde (Fig. 6, entries 13‐20).
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DOI: 10.1039/C6CY00874G
3.4 Kinetic study
The time course of methyl furan‐2‐carboximidate synthesis
over α‐MnO2/NaxMnO2 was investigated (Fig. 7). Firstly,
aldimine was generated and oxidized rapidly to 2‐furonitrile.
The volcano curves for both aldimine and 2‐furonitrile indicate
Fig. 8 The changes of lattice oxygen and MnIII content derived from XPS
that they are reaction intermediates. Methyl furan‐2‐ spectra. (a) fresh α‐MnO2/NaxMnO2, (b) α‐MnO2/NaxMnO2 was treated with
furfural and ammonia in MeOH at 30 oC under N2 atmosphere for 48 h, (c) α‐
carboximidate gradually increased to 94% yields after 30 h.
These results demonstrate that the tandem reaction proceeds
MnO2/NaxMnO2 in (b) was treated at 30 oC under O2 atmosphere for 12 h.
by three consecutive steps: 1) the aldimine formation from
furfural; 2) the ammoxidation of the aldimine group to cyano
group; 3) the imidate formation via the addition reaction of
cyano group with methanol.
The changes of surface oxygen on α‐MnO2/NaxMnO2 were
studied by O1s X‐ray photoelectron spectroscopy (XPS) (Fig.
S12A and Fig. 8). The peaks at 529.5 eV and 531.2 eV are
ascribed to the surface lattice oxygen (Olatt) species and
adsorbed oxygen (Oads) species, respectively.18 Quantitative
analysis shows the content of surface Olatt first decreases from
53% to 39% upon treating with furfural and ammonia in
methanol under N2 atmosphere, and then restores to 49%
after further treatment under O2 atmosphere. These results
demonstrate that ammoxidation step is a Mars–van‐Krevelen
reaction19 where aldimine is oxidized to nitrile, and the lattice
oxygen is consumed and subsequently restored by molecular
oxygen.
The changes of surface lattice oxygen are accompanied by
the variation of Mn valence, which were further investigated
by electron paramagnetic resonance (EPR) characterization
(Fig. S13). A nearly symmetric single line is a feature common
to all MnO2 samples. The broad line width (ΔH) is caused by
MnIII species.20 After α‐MnO2/NaxMnO2 was treated with
furfural and ammonia in methanol under N2 atmosphere, the
ΔH broadens from 241 mT to 318 mT (Fig. S13a, b), and then
returns to 205 mT after further treatment with O2 (Fig. S13c).
These results demonstrate that MnIV was reduced to a
relatively high fraction of MnIII by the substrate under N2
atmosphere, and then MnIII was oxidized back to MnIV by O2,
leading to a low fraction of MnIII. This result is in agreement
with the changes of MnIII content monitored by XPS
characterization (Fig. S12B and Fig. 8).
Fig. 7 Time course of catalytic conversion of furfural to methyl furan‐2‐
carboximidate over α‐MnO2/NaxMnO2 (■ = conversion of 1, ● = yield of
aldimine, ▲ = yield of 2, ▼ = yield of 3. Reaction conditions: 3 mmol furfural,
0.6 g α‐MnO2/NaxMnO2, 720 μL aq. NH3 (3 equiv.), 30 mL MeOH, 0.5 MPa O2,
30 oC.
3.5 Mechanism of ammoxidation‐Pinner tandem reaction
We then studied the reaction mechanism of ammoxidation
step. Only aldimine was obtained without α‐MnO2/NaxMnO2
under 0.5 MPa O2 (Table 1, entry 1). On the other hand, the
reaction can take place without oxygen in the presence of α‐
MnO2/NaxMnO2. The total yields of the products increased
with increasing α‐MnO2/NaxMnO2 amount (Fig. S11). 82% yield
of imidate was obtained with 12 equivalents of α‐
MnO2/NaxMnO217 (Eq.1).
Scheme 2 Kinetic isotopic effects (KIE) for 2‐furonitrile addition with
methanol.
The mechanism of basic sites‐catalyzed addition step was
further investigated. The addition reaction of 2‐furonitrile with
methanol should involve O‐H bond dissociation step. A large
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value of kH kD (1.8) at 30 C indicates that the dissociation of
the O‐H bond in the methanol is the kinetically‐relevant step
for addition reactions (Scheme 2).
In situ CH3OH adsorption Fourier transform infrared
spectroscopy (FT‐IR) characterization was carried out to study
the activation of methanol over α‐MnO2/NaxMnO2. Methanol
on KBr flake shows three C‐O stretching vibrations bands with
P, Q and R branches centering at 1016, 1033 and 1055 cm‐1,
respectively (Fig. 9A).21 By contrast, the Q and P braches
disappear after methanol adsorbing on α‐MnO2/NaxMnO2,
ARTICLE
o
oxygen is consumed. The lattice oxygen is restored by the
/
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DOI: 10.1039/C6CY00874G
molecular oxygen. Methanol is activated by the basic sites on
the NaxMnO2 phase. O‐H is dissociated to form adsorbed
methoxyl species, which attacks the nitrile to form imidate.
The interface structure of α‐MnO2/NaxMnO2 binds the
activated methoxyl and nitrile in close proximity and
geometrically favors the addition reaction.
Conclusions
owing to the restriction effect on the rotation of
C axis by
In summary, the α‐MnO2/NaxMnO2 catalyst with co‐existence
of redox and substantial basicity functionalities in an interface
manner was found to be an efficient and reusable
heterogeneous catalyst for tandem synthesis of imidates with
58‐96% yields from aldehydes and aqueous ammonia in the
presence of methanol under ambient conditions. The molar
ratio of NaOH/Mn plays great role on the formation of α‐
MnO2/NaxMnO2. Sodium cation is essential for formation of a
basic NaxMnO2 phase while potassium cation does not afford
basic phases. On the other hand, potassium cation promotes
to form a redox active a‐MnO2 phase by incorporated into its
tunnel structure while sodium cation can not. Application of
two kinds of alkali metal cations in the synthesis of the
catalysts is the key point for obtaining active catalysts for the
tandem reaction. The interface structure of α‐MnO2/NaxMnO2
geometrically favors the tandem reaction. Ammoxidation of
aldehydes to nitriles happens on the redox phase of α‐MnO2
and subsequent addition of nitrile intermediates with
methanol occurs on the basic sites of NaxMnO2. The phase
collaborative effect is observed.
dissociative adsorption. Furthermore, a red shift of R branch
from 1055 to 1030 cm‐1 is observed, indicating the weakening
of C‐O bond. The addition rate is linearly increased with
surface moderate base density (Fig. 9B). These results
demonstrate that the basic sites in α‐MnO2/NaxMnO2 catalyze
the O‐H bond dissociation and subsequent formation of metal‐
methoxy species.
Fig. 9 (A) FT‐IR spectra of adsorbed MeOH on KBr (a) and α‐
MnO2/NaxMnO2 (b). (B) The dependence of addition reaction rate constant k
on the moderate base density of α‐MnO2/NaxMnO2 calcined at different
temperatures or NaxMnO2. Reaction condition: 1 mmol 2‐furonitrile, 0.05 g
α‐MnO2/NaxMnO2 or NaxMnO2, 10 mL MeOH, 30 oC.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (projects 21233008 and 21303183) and
the “Strategic Priority Research Program‐Climate Change:
Carbon Budget and Related Issues” of the Chinese Academy of
Sciences (XDA05010203).
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Graphical abstract
A bifunctional manganese oxide catalyst with interface binding redox phase (α‐MnO2) and basic phase (NaxMnO2) was
reported for the ammoxidation‐Pinner tandem reaction to synthesize imidates with 58‐96% yields from aldehydes.
This journal is © The Royal Society of Chemistry 20xx
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