PdGa/TiO2 an efficient heterogeneous catalyst for direct methylation of: N -methylaniline with CO2/H2

Article abstract of DOI:10.1039/c6ra22089d

An effective sustainable heterogeneous PdGa/TiO₂ catalyst was prepared for direct methylation of N-methylaniline with CO₂/H₂ to N,N-dimethylaniline. The PdGa/TiO₂ catalyst exhibited 98% conversion and 94% selectivity under the reaction conditions (180 °C, 5 MPa H₂, 5 MPa CO₂, 10 h), which are one of the best results reported to date for the heterogeneous N-methylation with CO₂ as methylation reagent over the supported metal catalysts. The PdGa/TiO₂ catalyst was well characterized using TEM, TPR, XPS, CO-adsorption IR studies and high-pressure in situ FTIR studies. It was confirmed that PdGa bimetallic alloy nanoparticles were formed and highly dispersed on the TiO₂ support, and the electron-deficient Pd derived from the interaction with Ga could activate CO₂ and transform it to formic acid, which is the key step for the discussed methylation, and high activity was obtained over the PdGa/TiO₂ catalyst. The activating properties of the PdGa/TiO₂ catalyst will open a new route for transfer and utilization of CO₂ in the fields of energy and environmental fields.

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Article
PdGa/TiO an efficient heterogeneous catalyst for direct
2
methylation of N-methylaniline with CO /H2
2
a,b,c
b,c*
b,c
b,c
b,c
b,c,d
b,c,d
Xinluona Su , Weiwei Lin , Haiyang Cheng , Chao Zhang , Yan Li , Tong Liu , Bin Zhang
,
b,c,d
a*
b,c*
Received 00th January 20xx,
Accepted 00th January 20xx
Qifan Wu , Xiujuan Yu and Fengyu Zhao
An effective sustainable heterogeneous catalyst of PdGa/TiO
CO /H to N, N-dimethylaniline. The PdGa/TiO catalyst exhibited 98% conversion and 94% selectivity under the reaction
conditions (180 C, 5 MPa H , 10 h), which are ones of the best results reported up to now for the
heterogeneous N-methylation with CO as methylation reagent over the supported metal catalysts. The PdGa/TiO catalyst
was well characterized by using TEM, TPR, XPS, CO-adsorption IR and high-pressure in situ FTIR. It was confirmed that
PdGa bimetallic alloy nanoparticles were formed and highly dispersed on the TiO support, and the electron-deficient Pd
derived from the interaction with Ga could activate CO and transferred it into formic acid, which is the key step for the
present methylation, and so the high activity was obtained over the PdGa/TiO catalyst. The activating properties of
in the fields of energy and environmental
2
was prepared for direct methylation of N-methylaniline with
2
2
2
o
2
, 5 MPa CO
2
2
2
2
DOI: 10.1039/x0xx00000x
www.rsc.org/
2
2
PdGa/TiO
fields.
2 2
catalyst will open a new route for transfer and utilization of CO
2
with CO and hydrosilane reductant. Shortly afterwards, a more
green and effective method for methylation of amines with
homogeneous Ru-based catalysts was reported by Beller et al. [11],
they used CO as methyl reagent and H as reducing agent to
2 2
replace hydrosilances, in which water was generated as a sole by-
product. Moreover, heterogeneous catalyst was studied for the N-
Introduction
The N-methylation of amines is an important reaction in the organic
synthesis and chemical industry, as it can produce pharmaceuticals,
agrochemicals, dyes, perfumes, and key intermediates[1,2].
Therefore, to develop efficient and green methylation process has
2 2
methylation reaction with CO and H . That can settle the problems
2
attracted much attention. Recently, CO as green and renewable
of homogeneous catalyst such as the inability to reuse, difficult
catalyst/product separation, and the necessity of additives (ligands,
carbon resources was used for the N-methylation reaction to
replace the conventional methylation reagent, such as methyl
iodide and diazomethane and some progresses have been achieved
acids, and salts). It was reported that CuAlO
effective for the methylation of amines with CO
high yield of product about 96% was achieved[12, 13] . In addition,
a metal supported heterogeneous catalyst of Pt-MoO /TiO was
reported to be effective for N-metylation with CO and H by
x
, PdCuZrO
x
were
and H , and the
2
[
3-7]. However, the application of CO
synthesis is extremely difficult because of its thermodynamic
stability. Therefore, exploring suitable active catalyst to activate CO
2
as a raw material in chemical
2
x
2
2
is an important key issue in the utilization and transfer of CO to the
2
2
2
Shimizu et al [14], in which they discussed the possible
intermediates and proposed a plausible reaction mechanism, that
formic acid was firstly formed by Pt-catalyzed hydrogenation of
value-added chemicals and fuel or energy compounds, and it is a
big challenging mission to the researchers. In 2013, Cantat et al. [6]
reported for the first time that N-methylation reaction can be
2
CO , then reacts with amine to give N-methylformanilide, and
performed with using CO as methyl reagent in the presence of Zinc
2
finally undergoes Pt-catalyzed hydrogenation to give tertiary amine.
These is the first report for the reaction mechanism of N-
methylation with CO and H , it provides important information and
2 2
salts catalyst and hyrosilanes reluctant. And then, Leitner et al. [8]
and Beller et al. [9] independently reported the methylation of
2
amines with CO in the presence of homogeneous Ru complex
reference for the further study of this N-methylation reaction
system. As inspired by this result, we pay our attention to prepare
an efficient catalyst with activity in activating CO and transferring it
2
catalysts and hydrosilane. Subsequently, Dyson et al. [10] reported
a metal-free homogeneous catalyst for the methylation of amines
to formic acid as well as activity in hydrogenation of N-
methylformanilide, the intermediate of methylation. This, we
a
Department of Environmental Science and Engineering, Heilongjiang
University, Harbin, 150080, PR China
herein prepared
a
PdGa/TiO
2
catalyst for the N-methylation
b
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of reaction of N-methylaniline with CO
2
and H because that PdGa was
2
Applied Chemistry, Chinese Academy of Sciences (CAS), Changchun 130022 reported to be active for the hydrogenation of CO , and Ga O
3
2
2
P.R. China
c
could promote the activity of Pd/SiO
from CO reduction [15, 16]. The PdGa/TiO
higher activity compared to the Pd/TiO . The reaction mechanism
2
in the production of methanol
Laboratory of Green Chemistry and Process, Changchun Institute of Applied,
hemistry, CAS, Changchun 130022 P.R. China
2
2
catalyst presented
d
University of Chinese Academy of Sciences, Beijing 100049c,
Corresponding authors, E-mail: fyzhao@ciac.ac.cn; yuxiujuan@hlju.edu.cn; was discussed in detail by using the CO-adsorption IR and high-
2
linwei@ciac.ac.cn
pressure in situ FTIR spectra combining with the results of control
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experiments. It was confirmed that PdGa bimetallic alloy the peak areas of the corresponding lines using a Shirley type
nanoparticles were formed and highly dispersed on the TiO
support, and the electron-deficient Pd derived from the interaction
with Ga could activate CO
is the key step for the present methylation, the high activity was spectrometer equipped with
obtained over the PdGa/TiO catalyst compared to the Pd/TiO experiment, all the catalysts were reduced in situ with 10% H
2
2
background and empirical cross-section factors for XPS.
In situ diffuse reflectance infrared Fourier-transformed (DRIFT)
2
and transferred it into formic acid, which spectra of CO adsorption were collected using a Nicolet iS50
MCT detector. Before the
-Ar
70 ml min ) at 200 C for 0.5 h. Then, the flowing gas was switched
a
2
2
.
-1
o
(
-1
to He (70 ml min ) at the same temperature to remove the residual
o
H and Ar. After that, the sample was cooled down to 30 C and a
2
background spectrum was collected. This was automatically
subtracted from the measured spectra. Subsequently, the sample
Experimental Section
-1
was exposed to 10% CO-He at a flow rate of 70 ml min for about
0 min. Finally, the sample was purged by He for another 15 min
and the spectra were recorded. In all the cases the spectra were
Catalyst preparation
All chemicals used in this experiment were analytical grade and
used without further purification. TiO (P25) was used as support
3
2
o
-1
taken at 30 C, with a resolution of 4 cm and cumulative 32 scans.
In situ high-pressure FTIR spectra were performed in a high-
pressure reaction cell with ZnSe windows using a Nicolet iS50
spectrometer equipped with a MCT detector. After loading the
and PdCl
2 3 3 2
and Ga(NO ) as metal precursors. Pd/TiO was prepared
by deposition-precipitation (DP) using Na CO as the precipitating
agent. In a typical procedure, the support TiO
aqueous solution containing PdCl , and the suspension was put into
a water bath maintained at 35 C. Then, the Na
mol/L) was added dropwise into the suspension until the pH value
was adjusted to 10.0 under vigorous stirring. After the DP
procedure, the precipitate was collected and washed with water,
2
3
2
was added to an
2
o
sample, the cell was purged with H for a few times to remove the
CO
3
solution (1
2
2
o
2
air and heated to 200 C. H was further introduced until the
pressure reached 2 MPa and maintained at this temperature for 30
o
min. Then, the cell was cooled to 30 C, purged again with
2 2
atmospheric H a few times, and introduced with 2 MPa H and 4
and then vacuum-dried, and finally calcined under a flow of air at
o
MPa CO and a background spectrum was collected. Then, the cell
3
50 C for 4 h.
2
o
was heated to 180 C and maintained at this temperature for 30
The Ga-modified Pd/TiO
precipitation method. The support TiO
solution containing a given amount of PdCl
following procedure was the same as that of Pd/TiO
content was varied with Ga/Pd mole ratio of 1/1, 3/1 and 6/1. The
as-prepared catalysts were labeled as PdGa/TiO (1), PdGa/TiO (3),
2
catalysts were prepared by co-
was added to an aqueous
and Ga(NO , the
. The Ga
o
min. After that, the cell was cooled down to 30 C and the spectra
2
were collected. The gases in the cell were released to ambient
pressure and the tested sample was purged by He for 5min and
FTIR spectra were further collected.
2
3 3
)
2
2
2
Activity test
2
PdGa/TiO (6) (1, 3 and 6 in parenthesis refers to the mole ratio of
2 2
The N-Methylation of N-Methylaniline with CO /H was carried out
Ga/Pd). Pd content in all the catalysts was about 3 wt.% as
examined by inductively coupled plasma-optical emission
spectrometry (ICP).
o
in a 50 ml stainless steel autoclave reactor at 180 C. Prior to the
o
reaction, all the catalysts were reduced in a quartz tube at 400 C
with a flow of hydrogen (30 mL/min) for 2 h. Certain amounts of N-
methylaniline (1 mmol), solvent (n-octane 2 ml) and catalysts (metal
Catalyst characterization
0
.3 mol%) were added into the reactor, and the reactor was sealed
and flushed with 1 MPa H at least three times to remove the air.
Then, the reactor was heated up to 180 C in an oil bath and
introduced with 5 MPa H first and then 5 MPa CO with a high-
Powder XRD was performed using a Bruker D8 Advance X-ray
diffract meter with a Cu Ka source at 40 kV and 40 mA, at 2θ from
2
o
o
o
o
1
0 to 90 with 4 /min speed.
TEM was carried out in a JEOL JEM-2010 instrument at an
2
2
pressure liquid pump. The reaction was started with continuously
stirring. When the reaction was finished, the reactor was cooled
down to room temperature in ice-water bath and then vented mix
gas to ambient pressure. The liquid products were analyzed with a
gas chromatograph (Shimadzu GC-2010, Rtx-5 capillary column)
accelerating voltage of 200 kV. High-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM) images
and element analysis mapping were collected on a Tecnai G2 F30
transmission electron microscope operating at 200 kV (FEI
company). The STEM, element analysis mapping and EDS line
scanning were determined by Titan 80-300 Cs-corrected TEM with
resolution 0.08 nm at 300 kV.
using
a flame ionization detector and identified by gas
chromatography/mass spectrometry (GC/MS, Agilent 5890). The GC
results were obtained using an internal standard method and
biphenyl as standard substrate. The conversion and selectivity were
calculated using the following equations.
Hydrogen temperature-programmed reduction (H
experiments were performed using a Micromeritics AutoChem II
920 Automated Catalyst Characterization System. Prior to the
experiment, 50 mg sample was loaded into a U-shaped quartz
reactor and pretreated with Ar at 150 ◦C for 30 min to remove MAꢀconversionꢀ(%) =ꢀꢁ1 −
adsorbed water. After cooling to 50 ◦C, 10 vol% H /Ar mixed gas
was passed into the sample, and then heated to 800 ◦C at a ramping
2
-TPR)
2
ꢂꢃꢄꢅꢆꢇꢃꢇꢀꢈꢄꢄꢉꢊꢆꢋꢀꢉꢌꢀꢍꢎ
ꢑ × 100% (1)
ꢏꢆꢆꢅꢋꢅꢈꢐꢀꢈꢄꢄꢉꢆꢋꢀꢉꢌꢀꢍꢎ
2
ꢎꢄꢉꢊꢆꢋꢀꢉꢌꢀꢒꢍꢎꢀꢓꢂꢉꢇꢊꢔꢃꢇ
DMAꢀselectivityꢀ(%) =
ꢀ× 100% (2)
o
-1
ꢕꢊꢄꢀꢈꢄꢄꢉꢊꢆꢋꢀꢉꢌꢀꢈꢐꢐꢀꢋꢖꢃꢀꢓꢂꢉꢇꢊꢔꢋꢗ
rate of 10 C min . Hydrogen consumption was determined using a
thermal conduction detector (TCD).
X-ray photoelectron spectroscopy (XPS) (VG Microtech 3000
Multilab) were measured the electronic properties of Pd and Ga on
the surface of the Pd/TiO
2 2
and PdGa/TiO catalysts. The C1s peak at
284.6 eV arising from adventitious carbon was used as reference.
This reference gives binding energy values with a precision of ±0.02
eV. The surface composition of the samples was determined from
2
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Fig. 2. TEM images of catalysts (a) Pd/TiO
catalysts.
2 2
, (b) PdGa/TiO (6)
Results and Discussion
Characterization of Pd/TiO and PdGa/TiO catalysts
2
2
TiO
2
2
supported Pd catalyst (Pd/TiO ) and Ga-modified Pd catalysts
(
PdGa/TiO ) were prepared by deposition-precipitation, and the
2
powder XRD patterns of precursors are shown in Fig.1. All the
samples displayed the characteristic diffraction of the typical TiO2,
but no any diffraction peaks were found for Pd and Ga or their
metallic compounds, which should be ascribed to the low metal
loadings or the fine dispersion of the Pd and Ga species on the
surface of TiO2.
Fig. 3. STEM and corresponding elemental mapping images of
PdGa/TiO (6) catalyst.
2
Fig. 4. (a) STEM-HAADF image of the PdGa/TiO (6) catalyst; (b) X-
2
Fig.1. XRD patterns of (a) Pd/TiO
TiO (3), and (d) PdGa/TiO (6).
2
, (b) PdGa/ TiO
2
(1), (c) PdGa/ EDS line scanning spectra of the selected metal particles.
2
2
The TPR experiments were performed to figure out the redox
The dispersion of Pd or PdGa particles on the TiO
characterized by using TEM and element analysis mapping with line negative peak centred at about 70 C is corresponding to β-PdH
scanning. As shown in Fig. 2, both Pd and PdGa nanoparticles were decomposition and a peak at around 220 C is ascribed to the
2
2
were properties of the catalysts. For the Pd/TiO catalyst (Fig. 5a), a
o
x
o
2
+
well dispersed on the support of TiO
2
and Pd/TiO
2
and PdGa/TiO
2
reduction of Pd having strong interaction with TiO support, and
2
o
catalysts have the similar distribution with an average metal the peak at around 512 C is ascribed to the reduction of TiO
particle size around 3 nm, indicating that addition of Ga did not (Ti →Ti ) and this process is promoted by the presence of
affect Pd distribution. STEM images and EDX elements mapping for dispersed metal crystallites [17]. As for the PdGa/TiO catalysts (Fig.
2
4
+
3+
2
o
the PdGa/TiO
has the similar distribution on the surface of TiO
deposit with Pd together. It was further confirmed by a STEM image that of Pd/TiO
2
(6) catalyst in Fig. 3 (b and c) showed that Pd and Ga 4b-d), the similar negative peak centred at about 70 C for the β-
, indicating Ga PdH decomposition was also observed, but it is much smaller than
catalyst, suggesting that the presence of Ga has an
2
x
2
and the EDX line profile of the Pd-L and Ga-L signals across a single influence on the reduction behaviour. The peak presented at
o
particle as shown in Fig. 4. It is clear there is no segregation of Pd around 83 C and its intensity or area increased with the content of
and Ga and they are coherent together very well, which suggested Ga, which is assigned to the reduction of the Pd species
o
the formation of PdGa bimetallic compounds or PdGa nanoalloys on neighboured to Ga. Moreover, a peak presented at around 288 C,
the surface of TiO
FTIR later.
2
. That was further demonstrated by XPS and CO- and it moved slightly to the higher temperature with increasing of
Ga content, which is attributed to the reduction of Ga species
adjacent to Pd, suggesting the interaction between Pd species and
Ga species was stronger and the presence of Pd could accelerate
the reduction of gallium oxide moieties [18]. It was reported that
pure Ga O would not undergo reduction in hydrogen until 900°C
2
3
o
[
19]. The peak at around 512 C for the reduction of TiO
2
was not
found in the PdGa/TiO catalysts, it is assumed that the presence of
2
2
Ga reduced the interaction of Pd and TiO .
To further certify the interactions among Pd, Ga and TiO , the
2
surface properties and the electronic states of Pd and Ga were
analyzed by XPS and the results are shown in Fig. 6. For Pd/TiO2
catalyst, the Pd 3d spectra showed two main peaks at the binding
energy around 334.4 eV and 339.8 eV, they are in accordance with
0
the literature values for Pd 3d5/2 and Pd 3d3/2 of Pd , indicating that
Pd exists mainly as metallic phase in the Pd/TiO catalyst [20]. As for
2
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0
2
PdGa/TiO (1) catalyst, the binding energy of Pd 3d5/2 presented at
3
34.6 eV, moving 0.2 eV to the higher binding energy compared to
that of Pd/TiO , and it moved much more with increasing content of
Ga, about 0.3 eV and 0.4 eV shift for PdGa/TiO (3) and
PdGa/TiO (6), respectively. That suggested the presence of Ga leads
2
2
2
Pd to be more positive as the electron transfer from Pd to Ga
species. The above results clearly revealed the stronger interaction
between Pd and Ga, and the electronic environment of Pd was
modified with the addition of Ga, which is correspondence to the
shift of the binding energy of Ga 3d spectra also. For the Ga
2
O
3
/TiO
2
Fig. 6. A. Pd 3d XPS spectra of catalysts (a) Pd/TiO , (b) PdGa/
2
catalysts, Ga 3d showed two peaks at 20.0 eV and 21.6 eV, ascribing TiO (1), (c) PdGa/ TiO (3), and (d) PdGa/ TiO (6); B. Ga 3d XPS
2
2
2
+
to Ga and Ga , respectively [21, 22]. This indicated that Ga is spectra of catalysts(a) Ga/TiO , (b) Pd/TiO , (c) PdGa/ TiO (1)
3+
2
2
2
o
hardly reduced at 400 C. For the PdGa/TiO
2
2
catalysts, Ga 3d and (d) PdGa/ TiO (3).
showed three peaks at 18.9 eV, 19.9 eV and 21.5 eV, ascribing to
0
+
3+
Ga , Ga and Ga , respectively [23, 24]. This indicated that a part of Table 1 Results for the methylation of N-methyl aniline over
3
+
0
Ga was partially reduced to Ga during the reduction In the different catalysts.
a
presence of Pd. Meanwhile, it is obvious, with the increase of Ga
content, the Ga peaks shift to the lower binding energies (18.5 eV
Entry Catalyst
Conv.
Sel. (%)
0
（%）
DMA MFA AN
FA
to 21.4 eV), that suggested again the strong electron interaction
between Pd and Ga species for the formation of the PdGa bimetallic
compounds, in which Pd is electron deficient and Ga electron
enriched. Which are accordance to literature, that Haghofer et al.
1
Pd/TiO
2
34.7
8.0
4.6
9.3
78.1
2
3
4
PdGa/TiO
PdGa/TiO
PdGa/TiO
2
2
2
(1)
(3)
(6)
46.9
64.3
92.9
72.0 3.4
83.1 2.5
90.3 4.2
23.7
14.4
9.3
0.8
-
-
reported Pd
Pd/Ga [25], and Collins et al. certified a series of Pd
and Pd Ga bimetallic alloy compounds formed under the reduction
temperature above 300 C on the surface of Pd/Ga
present work, the XRD for the PdGa/TiO catalysts was also Catalyst 0.3mol % Pd to MA, 180 C, 8 h. N,N-dimethylaniline
measured as seen in Fig. 1, but we did not find the diffractions of Pd (DMA), N-methylformanilide (MFA), Aniline(AN) , N-
2
Ga bimetallic alloy could form on the surface of
5
6
PdGa/TiO (10) 88.2
95.0 0.4
4.65
8.2
-
2
2
O
3
5
Ga , Pd Ga
2
5
3
,
Ga
2
O
3
/TiO
2
13.1
31.9 59.3
0.6
2
o
2
3
2 2
O [26]. In the Reaction conditions: MA 1mmol, CO 5 MPa, H 5 MPa, Octane 2ml,
o
a
2
or PdGa, except for the diffraction of TiO
2
, due to their low loading.
phenylformamide (FA).
Fig. 7 shows the change of the conversion and products
selectivity with reaction time over PdGa/TiO2 (6) catalyst. The
conversion of MA was 40% at a reaction running for 2 h, and
reached 98% at a reaction time of 10 h. With prolonging reaction
time, the selectivity of DMA increased from 78% to 94%, and the
selectivity of MFA decreased slightly, indicating MFA is an
intermediate and it could be transferred to the final product of
DMA. The activity of the present PdGa/TiO2 catalyst has been
compared with several catalysts including homogeneous and
heterogeneous ones as reported recently for the direct methylation
of N-methylaniline with CO
PdGa/TiO is comparable to the best one as it gave the higher yield
of the desired product of DMA. In addition, the efficient of the
present PdGa/TiO catalyst was checked for several amines with
different structures (Table S2), it shows that PdGa/TiO was active
for all the selected substrates, but the reactivity and efficiency
depend on the molecular structure of the amines. The cyclic amines
gave a higher yield, by comparison, the aliphatic amines showed
less reactivity under the similar reaction conditions.
2 2
/H (Table S1). It is clearly, the
2
Fig. 5. H -TPR profiles of (a) Pd/TiO , (b) PdGa/ TiO (1), (c)
2
2
2
PdGa/ TiO
2
(3), and (d) PdGa/TiO
2
(6).
and PdGa/TiO
The catalytic performances of Pd/TiO and PdGa/TiO catalysts
2
2
Catalytic performances of Pd/TiO
2
2
2
2
have been checked and compared for the methylation of N-
methylaniline (MA) to N, N-dimethyl aniline (DMA) with CO and H .
2
2
As shown in Table 1, the conversion of MA was 34.7% and the
For the present methylation of N-methylaniline with CO /H , the
2
2
selectivity of DMA was 8% over the Pd/TiO catalyst. In comparison,
2
2
catalysis mechanism of PdGa/TiO , both the intermediate and rate-
2
the addition of a tiny amount Ga species to the Pd/TiO catalyst
controlling step are the important and attractive issues. To reveal
the catalytic mechanism and reaction pathway, herein, we collected
the results of several control experiments combining with in situ
could improve the conversion of MA and the selectivity of DMA
obviously, and with increasing of Ga content, the conversion
increased and reached a maximum value of 92.9% over the
CO-IR spectroscopy analysis of Pd/TiO
was reported that the N-methylaniline was carbonylated with
CO /H firstly to generate N-methylformanilide intermediate and it
then was quickly hydrogenated into N-methyl aniline[13, 27]. The
catalytic activity of Pd/TiO and PdGa/TiO in the above two
2 2
and PdGa/TiO catalysts. It
PdGa/TiO
then decreased to 88.2% with further increasing of Ga over the
PdGa/TiO (10), and the Ga /TiO catalyst gave poor activity
entry 7). It is evident that the addition of Ga species could improve
2
(6) (with a Ga:Pd mole ratio of 6). While, the conversion
2
2
2
2
O
3
2
(
2
2
the catalytic performance of Pd largely by electron modification via
the formation of PdGa bimetallic alloy compounds on the surface of
possible reaction steps of the intermediate of N-methylformanilide
formation and the hydrogenation reaction were checked and
2
TiO .
4
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discussed. As the results list in Table 2, the hydrogenation of N-
methylformanilide was a faster step, it could reach a 99.6%
species. [28] When released off H
min, the absorption band at 1614 cm disappeared on the surface
of Pd/TiO catalyst (Fig. 8B a). For the PdGa/TiO catalysts,
however, the absorption bands at about 1284 cm and 1624cm
2 2
and CO with purging He for 5
-1
conversion and a 77.6% selectivity within 2 h over the PdGa/TiO
catalyst. By contrast, the Pd/TiO was less active irrespective to the
presence or absence of CO , and the N-methylformanilide could
transfer to N-methylaniline by a broken of C-N bond, but which can
2
(6)
2
2
-
1
-1
2
-1
-1
2
disappeared either, while the bands at 1387 cm , 1579 cm and
-
1
1978 cm still existed (Fig. 8B b, c). These results indicated that
be reduced over the PdGa/TiO
reaction conditions.
2
catalyst under the N-methylation
the formate and CO species have been formed on the surface of
o
PdGa/TiO
2
catalysts upon exposure in H
2
and CO
2
at 180 C, and
which are very less and weak on the Pd/TiO
2
catalyst.
Table 3 Results for N-methylaniline methylation with different
carbon resources
Catalyst
Carbon
Conv.
Sel.(%)
resource
（%）
DMA
93
MFA
0.3
AN
6.8
FA
-
PdGa/TiO
2
formic acid
methanol
60
8.8
24
76.6
0.9
10
-
70.7 5.3
CO/0.6 MPa
formic acid
CO/0.6 MPa
12.7
98.2
13.5
0.2
22.1
0.5
-
Pd/TiO
2
95.3
69.4
3.8
11.7
8.6
Reaction conditions: MA 1mmol, Catalyst 0.3 mol % Pd to MA,
o
5 MPa, Octane 2 ml, 180 C, 8 h.
Fig. 7. Variation of conversion and selectivity with reaction
time in the methylation of N-methylaniline to N, N-dimethyl
H
2
2
aniline over PdGa/TiO (6) catalyst.
2
To examine the surface adsorption properties of Pd/TiO and
PdGa/TiO catalysts, the in situ CO-IR spectroscopy was collected
and recorded. It is well known that CO adsorption on Pd-based
2
Table 2 Catalytic performance for N-Methylformanilide
hydrogenation over different catalysts.
catalysts generally appeared in two main regions: the first one at
Entry
Catalyst
Conv.
%)
Sel. (%)
-1
2
100–2030 cm is named as linearly adsorbed CO band and the
(
-1
other around 2000–1800 cm range is attributed to the bridged
adsorbed CO species. [30-33] The in situ DRIFT spectra of CO
adsorption for the catalysts with different Ga contents are shown in
MA DMA
AN
FA
1
2
3
4
5
PdGa/TiO (1)
98
38.1 55.5
13.4 77.4
69.3 16.0
34.2 54.3
6
0.5
-
2
Fig. 9. For the Pd/TiO
bridge-bonded CO presented at about 2090 cm and 1988 cm
respectively. For the PdGa/TiO (6) catalyst, these bands appeared
2
catalyst, the vibration bands of the linear and
PdGa/TiO
2
(6)
99.6
33.9
100
56.4
9.1
3.9
-1
-1
Pd/TiO2
11
2
-1
-1
PdGa/TiO
2
(6)
11.2 0.3
2.4 30.7
at 2085 cm and 1978 cm respectively. Obviously, it has a
-1
a
2
significant red shift with about 10 cm for the PdGa/TiO (6)
Pd/TiO
2
63
3.9
2
compared to Pd/TiO . Generally, the red-shift can be explained by
Reaction conditions: MFA 1mmol, CO
Octane 2 ml, Catalyst 0.3mol % Pd to MA, 180 C, 2 h. a:
without CO
2 2
5 MPa, H 5 MPa,
geometric and electronic effects[25, 34]. In the present work, the
neighboring Ga atoms increased the electronic density of Pd and
changed its adsorption properties. It is also found that the ratio of
linear to bridge adsorption increased with increasing of Ga content.
Both the red-shift and the changes in adsorption intensity of CO
indicated the electronic interaction between Pd and Ga as the PdGa
o
2
Then, the N-methylation of methylaniline was carried out by
using methanol, formic acid or CO as the methyl source because
they are general produced from CO
results shown in Table 3, the formic acid is effective, with it the
higher conversion was obtained with both the Pd/TiO and
PdGa/TiO catalysts, while the CO and methanol was less active.
Moreover, the conversion was much higher over the Pd/TiO
2
hydrogenation[6, 28]. As the
nanoalloy formed on the TiO
by the PdGa nanoparticles on the surface of PdGa/TiO
formed formic acid.
2
support. Thus, the CO
2
was activated
catalyst and
2
2
2
2
catalyst in the presence of formic acid, indicating that the
formation of formic acid is the key step in the present methylation
reaction with CO
active and efficient for the production of formic acid compared
with Pd/TiO catalyst, which was discussed and confirmed herein
by using the in situ high-pressure FTIR spectra under the reaction
conditions, the results are shown in Fig. 8. For Pd/TiO catalyst
Fig. 8A a), the band at 1614 cm was assigned to carbonate types
species and physisorbed water on the support. [29, 30] For both
the PdGa/TiO (1) and PdGa/TiO (6) catalysts, the bands at 1284
cm and 1624 cm were assigned to carbonate type species, the
2 2 2
/H , and the PdGa/TiO was assumed to be more
2
2
-1
(
2
2
-1
-1
Fig.8. High-pressure FTIR absorption spectra of CO
2
and H
2
-1
-1
-1
bands at around 1360 cm , 1387 cm and 1579 cm were
adsorption on A: (a) Pd/TiO , (b) PdGa/TiO (1) and (c) PdGa/TiO
2
2
2
(6).
10,21
-1
formate species
, and the band at 1978 cm was the CO
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(
CO
2
: 4 MPa and H
2 2 2
: 2 MPa) B: (a) Pd/TiO , (b) PdGa/ TiO (1) and (c)
Notes and references
PdGa/ TiO
2
(6) ( After gas escape)
1
2
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Fig.9. In situ Fourier-transformed infrared spectra of CO adsorption
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and transfer it with H to formic acid which is the key step for the
2
present methylation. The excellent catalytic performance of the
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2 2
attract much attention of researchers in the fields of green and
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Acknowledgements
2
The authors gratefully acknowledge the financial support from 30. H. Yoshida, S. Narisawa, S.-i. Fujita, R. Liu and M. Arai, Phys.
the NSFC (21273222, 21603212, 21311140166), and the MOST Chem. Chem. Phys., 2012, 14, 4724-4733.
(
2016YFA0602901) of China.
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6
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Graph abstract
PdGa/TiO an efficient heterogeneous catalyst for direct methylation of
2
N-methylaniline with CO /H2
2
Xinluona Su, Weiwei Lin, Haiyang Cheng, Chao Zhang, Yan Li, Tong Liu, Bin Zhang Qifan Wu, Xiujuan Yu and
,
Fengyu Zhao
2
PdGa/TiO showed excellent catalytic performance in the methylation of amine with
CO /H as PdGa bimetallic particles formed on the surface of TiO , which could
2
2
2
activate CO
2
to form formic acid, the important and active methylation reagent.