Selective N-Methylation of N-Methylaniline with CO2 and H2 over TiO2-Supported PdZn Catalyst

Article abstract of DOI:10.1021/acscatal.9b04677

A series of Pd-ZnO/TiO₂, Pd/TiO₂, and Pd/ZnO catalysts were synthesized and investigated for N-methylation of N-methylaniline (MA) to N,N-dimethylaniline (DMA) with CO₂ and H₂. A high performance was observed with a Pd-ZnO/TiO₂ catalyst, with 99.9% DMA selectivity at 94% MA conversion. By contrast, both Pd/TiO₂ and Pd/ZnO were less active and/or selective. The catalytic performance of Pd-ZnO/TiO₂ largely depended on reduction temperature and ZnO loading. The rates for MA conversion (rate_MA) and DMA production (rate_DMA) increased linearly with the amount of PdZn alloy formed. The reaction was likely to take place via intermediates of N-methylformanilide (MFA) and formate. Formate was produced through the reduction of CO₂ with H₂ as confirmed by in situ diffuse reflectance Fourier transform infrared spectroscopy and then added to MA producing MFA, and finally, MFA was subsequently adsorbed and hydrogenated to DMA. All these steps were promoted by the PdZn alloy. The hydrogenation of MFA to DMA was much faster than the N-methylation of MA to MFA; DMA was stable, so the selectivity to DMA was almost 100% over the Pd-ZnO/TiO₂ catalyst.

Full text of DOI:10.1021/acscatal.9b04677

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Article
Selective N-Methylation of N-Methylaniline with
CO2 and H2 over TiO2-Supported PdZn Catalyst
Weiwei Lin, Haiyang Cheng, Qifan Wu, Chao Zhang, Masahiko Arai, and Fengyu Zhao
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b04677 • Publication Date (Web): 11 Feb 2020
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Selective N-Methylation of N-Methylaniline with
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CO and H over TiO -Supported PdZn Catalyst
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Weiwei Lin , Haiyang Cheng * , Qifan Wu , Chao Zhang , Masahiko Arai , Fengyu
Zhao *,†,‡
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State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China
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Jilin Province Key Laboratory of Green Chemistry and Process, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China
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KEYWORDS: PdZn alloy; N-methylation; CO ; N-methylaniline; N, N-dimethylaniline
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ABSTRACT: A series of Pd-ZnO/TiO , Pd/TiO , and Pd/ZnO catalysts were synthesized and
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investigated for N-methylation of N-methylaniline (MA) to N,N-dimethylaniline (DMA) with
CO and H . A high performance was observed with a Pd-ZnO/TiO catalyst, with 99.9% DMA
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14 selectivity at 94% MA conversion. By contrast, both Pd/TiO and Pd/ZnO were less active
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and/or selective. The catalytic performance of Pd-ZnO/TiO largely depended on reduction
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temperature and ZnO loading. The rates for MA conversion (rate ) and DMA production
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17 (rateDMA) increased linearly with the amount of PdZn alloy formed. The reaction was likely to
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take place via intermediates of N-methylformanilide (MFA) and formate. Formate was produced
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through the reduction of CO with H as confirmed by in situ diffuse reflectance Fourier-
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transform infrared spectroscopy, and then added to MA producing MFA and finally MFA was
subsequently adsorbed and hydrogenated to DMA. All these steps were promoted by the PdZn
alloy. The hydrogenation of MFA to DMA was much faster than the N-methylation of MA to
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MFA; DMA was stable, so the selectivity to DMA was almost 100% over Pd-ZnO/TiO catalyst.
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INTRODUCTION
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The application of CO as a nontoxic, renewable, and abundant carbon source is of raising
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interest and great importance for the synthesis of highly valuable chemicals.1-4 However, the
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utilization of CO as a feedstock in chemical industry is retarded because of its thermodynamic
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stability and low energy. Therefore, classical transformations of CO usually use high-energy
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molecules such as epoxide, aziridine, alkyne, and others as starting materials.5, 6 In this respect,
only a small number of industrial processes have been realized for CO utilization, such as the
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production of urea, salicylic acid, and carbonates.7-9 In recent years, great developments to
15 transform CO into methanol, higher alcohols, methane, light alkanes, formates, and CO have
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been achieved using different catalysts.3, 10-12 In addition to these transformations, one of the
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important advances in this research area is the utilization of CO as a methylation reagent for the
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18 production of N-methylated compounds.13, 14 The N-methylation of amines to produce methyl-
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substituted amines, which can produce pharmaceuticals, agrochemicals, dyes, perfumes, and so
on, is an important reaction in the organic synthesis and chemical industry. Traditionally, methyl
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iodide, formaldehyde, and formic acid derivatives are used as methylation reagents. In industry,
methyl-substituted amines are produced by N-methylation of amines with methanol as a
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methylation reagent. Direct N-methylation of amines with CO and H will be an attractive
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alternative to those processes, for which these methylation reagents are principally produced
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from CO . Up to now, various effective homogeneous and heterogeneous catalysts for the N-
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methylation reaction have been reported.17, 18 An important development in the N-methylation of
amines was reported, in which the direct N-methylation of amines used CO as C1 source,
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molecular hydrogen as reducing agent, and homogeneous Ru complex catalysts.13, 19-21 It is
demonstrated for the first time that with a readily available ruthenium complex and a
commercially available ligand, various kinds of secondary and primary amines were successfully
transformed into the desired tertiary amines and secondary amines with good to excellent yields
10 using silanes and CO2.13 Klankermayer and co-workers showed that the [Ru(triphos)(tmm)]
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complex mixed with appropriate amounts of organic acids was effective in the catalytic N-
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13 mono- or dimethylated products were obtained with 70% to 99% yields. Beller et al. extended
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the scope of this reaction to a wide variety of substrates, including aliphatic and aromatic amines,
and primary and secondary amines, which were methylated by an in situ formed catalyst system
of triphos ligand, Ru (III) precursor, and either LiCl or acid additives. The consecutive
methylation reaction proceeds with these formamide (major) and methanol (minor) intermediates
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formed during the reaction. However, homogeneous catalysts suffer from several problems of
difficult catalyst/product separation, the necessity of additives (ligands, acids, and salts), and
20 difficulty to reuse. Therefore, effective multiphase catalysis is needed to be developed. At
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They extended the substrates to nitrobenzene and aromatic nitrile derivatives, in this system,
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however, a large amount of 38 mol% Cu with respect to amine was required. Then, an active
Pd/CuZrO catalyst was found for the N-methylation of amines under mild reaction conditions.
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N-Methyl or N,N-dimethyl amines with various structures can be synthesized with yields of up to
97%.23 Shimizu et al. reported the solvent-free N-methylation of aliphatic and aromatic
secondary amines with CO and H by Pt-MoO /TiO catalyst and proposed a plausible reaction
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mechanism, that formic acid, first formed by hydrogenation of CO , reacts with amine to give N-
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methylformanilide, which undergoes hydrogenation to give tertiary amine. In addition, it has
been demonstrated that Au/Al O is a highly active heterogeneous catalyst, which can be used in
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10 the N-methylation of aromatic and aliphatic amines to their corresponding tertiary amines with
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CO and H in excellent yields. It was reported that a heterogeneous Cu/CeO catalyst showed
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high selectivity for the synthesis of N-methylaniline from N-methylation of aniline with CO and
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Recently, PdZn alloy has been used and examined for many reactions such as hydrogenation,
dehydrogenation, and steam reforming.27-30 In recent years, it has been found that the supported
Pd catalysts are active for CO hydrogenation and the supports or promoters used determine the
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selectivity to the product (CO or methanol). Liang and co-workers proposed that PdZn alloy,
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rather than metallic Pd, was active sites for methanol formation. Huang et al. reported that both
Pd modified by ZnO islands (partially reduced ZnO ) and PdZn alloy were active for CO
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stabilizing the formate intermediate and gave high activity for CO hydrogenation to methanol.
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Very recently, we have found that PdGa/TiO is an efficient catalyst for the direct N-methylation
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the target reaction.35 As motivated by the above-mentioned results, we have studied the
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possibility of PdZn alloy catalysts for the direct N-methylation of MA with CO and H .
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disadvantages of low surface area and poor structural stability. In the present work, therefore,
TiO was employed as a support to disperse PdZn alloy particles well and Pd-ZnO/TiO samples
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were prepared by deposition-precipitation method. The catalysts were prepared under different
conditions (reduction temperature, ZnO loading) and tested for the N-methylation of MA to N,N-
dimethylaniline (DMA) in n-octane. The catalysts prepared were characterized by several
methods including X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray
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hydrogen temperature-programmed reduction (H -TPR) and possible reaction intermediates were
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results were used to tailor an active catalyst for the selective production of DMA from MA and
discuss the significance of PdZn alloy for the MA N-methylation and possible reaction pathways
concerned.
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EXPERIMENTAL SECTION
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Chemicals. TiO (P25) was purchased from Acros Organics and used as obtained. PdCl ,
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biphenyl, and 4-methoxy-, 4-methyl-, 2-methyl-, 4-chloro-, 4-fluoro-N-methylaniline were
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purchased from Chemical Reagent Sinopharm Group Co. Ltd., which were analytical grade and
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used without further purification. H and CO with purity of > 99.999% were purchased from
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Catalyst preparation. TiO was used as support and PdCl and Zn(NO ) •6H O were used as
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metal precursors. (1) Pd/TiO catalyst was prepared by deposition-precipitation using Na CO as
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precipitating agent. In a typical procedure, 1 g TiO and 0.3 g Na CO were added to distilled
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water (40 mL) and the resulting suspension was put into a water bath maintained at 35 °C. Then,
a certain amount of PdCl solution (10 mg/mL) was added drop by drop into the suspension
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under vigorous stirring. The mixture obtained was further stirred for 2 h and the precipitate
formed was collected, washed with water several times, dried at 100 °C overnight, calcined in air
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3.0 wt.% and the catalyst so prepared will be named as Pd/TiO -400 in the following. (2) TiO -
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supported PdZn alloy catalyst was prepared by step precipitation method. First, ZnO/TiO was
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aqueous solution containing a given amount of Zn(NO ) •6H O and the suspension was put into
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a water bath maintained at 60 °C. Then, a Na CO solution (0.5 mol/L) was added drop by drop
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into the suspension under vigorous stirring until the pH value became to 10.0. The precipitate
formed was collected, washed with water several times, and dried overnight. The sample so
obtained was calcined in air at 350 °C for 4 h. Then, Pd was loaded to the ZnO/TiO by the same
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15% in weight. The sample was reduced at 100, 250, or 400 °C for 2 h. In the following, the
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ZnO was prepared and Pd was loaded thereon by the same procedures as described above, which
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Characterization. The loading of Pd in the catalysts were examined and confirmed by
inductively coupled plasma-optical emission spectroscopy (ICP-MS), and their actual contents
were very close to their introduced ones within 97 ± 2%. The crystal phases of the catalysts
prepared were measured by powder X-ray diffraction (XRD) using a Bruker D8 Advance X-ray
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scan speed of 4 /min. The metal particle sizes were determined by transmission electron
microscopy (TEM) on a JEOL JEM-2010 instrument at an accelerating voltage of 200 kV.
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flow of 10 vol% H /Ar mixed gas using a Micromeritics AutoChem II 2920 Automated Catalyst
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Characterization System with a ramping rate of 10 °C min . Hydrogen consumption was
determined using a thermal conductivity detector (TCD). X-ray photoelectron spectroscopy
(XPS) and X-ray excited Auger electron spectroscopy (XAES) (VG Microtech 3000 Multilab)
were used to examine the electronic properties of Pd and Zn on the catalysts. The C1s peak at
284.8 eV arising from adventitious carbon was used as reference. This reference gives binding
20 energy values with a precision of ± 0.05 eV. The surface composition of different species was
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determined from the peak areas of the corresponding deconvolution using a Shirley type
background and empirical cross-section factors for XPS. Thermogravimetric analysis (TGA)
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air flow, from room temperature to 800 °C. In order to examine possible species produced from
CO and H , in situ diffuse reflectance Fourier-transform infrared spectroscopy (DR-FTIR) was
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0
H several times to remove the air and heated to a certain temperature (catalyst reduction
2
temperature as mentioned above). H was further introduced until the pressure reached 2 MPa
2
and this temperature was kept for 30 min. Then, the cell was cooled to 50 °C, purged again with
atmospheric H for a few times, and a background spectrum was collected. Subsequently, the cell
2
was introduced with 2 MPa H and 1 MPa CO , heated to 180 °C, and maintained for 30 min.
2
2
10 After that, the cell was cooled to 50 °C, the gases in the cell were released to ambient pressure,
1
1
2
and the FTIR spectra were collected. The MFA adsorption on catalysts was conducted as
follows: Pd/TiO -400 or Pd-ZnO(7.5%)/TiO -400 and KBr (with a weight ratio of 3 : 100) were
1
2
2
13 ground and compressed into a tablet, loaded on the porous SiO griddle of glass steamer, and
2
36
1
1
1
1
1
1
4
5
6
7
8
9
then placed into a 50 mL Teflon-vessel, in which 2 mL MFA was added at the bottom. Then,
the Teflon vessel was sealed in an autoclave and then moved to the oven for adsorption at the
temperature of 80 °C for 2 h. Finally, the autoclave was cooled to room temperature, the sample
−
1
tablet was transferred to the IR cell, and He was passed at 10 mL min for 0, 1, 5, 10 min at
room temperature, respectively.
Activity test. Those different Pd catalysts prepared were tested for the N-methylation of MA
20 with CO and H in a 50 mL stainless steel autoclave reactor at 180 °C. All the samples were
2
2
2
1
2
pre-reduced in a quartz tube at different temperatures with a flowing H (30 mL/min) for 2 h. A
2
2
certain amount of catalyst, 2 mL n-octane (solvent), and 1 mmol MA were added into the reactor
23 and it was sealed. Then, the reactor was flushed with H five times to remove the air and placed
2
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in an oil bath at 180 °C for 15 min. Then, H (4.5 MPa) and CO (1.5 MPa) were introduced into
2
2
2
the reactor and the reaction was started under stirring. After the reaction finished, the reactor was
cooled in an ice-water bath and the gas was vented to ambient pressure. The influence of internal
and external diffusion was examined by changing the size (mesh) of catalyst granules and the
8
9
1
1
1
1
1
1
1
1
1
1
2
2
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0
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0
1
2
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0
1
2
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8
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2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
stirring speed for Pd-ZnO(7.5%)/TiO -400 (Table S1 and S2). Under the reaction conditions
2
used (1200 rpm, 180 mesh), the mass transport limitation was eliminated and all experiments
were carried out under reaction - controlled conditions. The liquid products were collected and
analyzed by gas chromatograph (Shimadzu GC-2010, Rtx-5 capillary column) using a flame
ionization detector and identified by gas chromatography/mass spectrometry (GC/MS, Agilent
10 5890). The GC results were obtained using an internal standard method with biphenyl as
1
1
standard substrate. The conversion and selectivity were calculated using the following equations:
1
2
(1)
1
3
(2)
1
1
1
4
5
6
For recycling experiment using a selected catalyst (most active one), the catalyst was collected
by decantation after the first run, washed with ethanol and then n-octane several times each, and
reused for the next run. To examine the possibility of metal leaching, the amount of metal in the
17 catalyst after repeated runs was measured. The used catalyst was washed with ethanol, dried and
1
8
9
calcined in air at 550 °C for 2 h, and then dissolved in aqua regia and the amount of dissolved Pd
species was measured by ICP-MS.
1
20
2
1
RESULTS AND DISCUSSION
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Catalyst characterization. The XRD patterns of the catalysts prepared are shown in Figure 1
For all the supported Pd catalysts, no diffraction peaks of Pd species were found and this may be
due to the lower Pd loading (3 wt.%) and/or small Pd particle sizes. In addition, no obvious
0
1
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diffraction peaks related to ZnO and Zn species were detected for Pd-ZnO(7.5%)/TiO -400
2
(Figure 1b), suggesting ZnO and/or Zn should be finely dispersed or amorphous. For Pd-
ZnO(15%)/TiO -400 containing a larger amount of ZnO and a physical mixture of Pd/TiO +ZnO
2
2
(7.5% in weight) (Figure 1c, 1d), diffraction peaks were observed at about 31.8º and 34.5º
corresponding to the (1 0 0) and (0 0 2) planes of ZnO. The Pd/ZnO-400 and ZnO samples
showed strong diffraction peaks of ZnO (Figure 1e, 1f) (JSCPDS 65-3411). In order to confirm
10 the formation of PdZn alloy, a TiO (P25) - supported sample with larger Pd (9 wt.%) and ZnO
2
1
1
2
(22.5 wt.%) loadings was prepared; however, the corresponding XRD pattern of PdZn alloy was
not distinguished since, probably, the dispersion of the PdZn alloy was high through the
1
13 deposition-precipitation and/or the pattern was overlapped by that of rutile (Figure S1I).
1
1
1
1
1
1
4
5
6
7
8
9
However, when anatase TiO was used as support and the catalyst prepared by impregnation
2
with the high Pd (9 wt.%) and ZnO (22.5 wt.%) loadings, the diffraction peaks corresponding to
o
o
the (1 1 1) and (2 0 0) planes of PdZn alloy at 41.2 and 44.2 were observed, which are different
0
o
o
33, 37
from the (1 1 1) and (2 0 0) planes of Pd at 40.3 and 46.7 (Figure S1II).
These results
indicated that the PdZn alloy could be formed on TiO at the reduction temperature of 400 °C.
2
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Figure 1. XRD patterns of (a) Pd/TiO -400, (b) Pd-ZnO(7.5%)/TiO -400, (c) Pd-
2
2
3
4
ZnO(15%)/TiO -400, (d) Pd/TiO +ZnO(7.5%)-400, (e) Pd/ZnO-400, and (f) ZnO.
2 2
5
6
7
8
9
The state of dispersion of metal (Pd or PdZn) particles was examined by TEM for selected
samples on different supports. As shown in Figures. 2 and S2, the metal particles were well
dispersed on the supports. The average metal particle size was 3.8, 2.2, 2.3, 3.6, 2.6, 2.8, and 3.4
nm for Pd/TiO -400, Pd-ZnO(7.5%)/TiO -100, Pd-ZnO(7.5%)/TiO -400, Pd/ZnO-400,
2
2
2
Pd/TiO +ZnO (7.5%)-100, Pd/TiO +ZnO(7.5%)-400, and Pd-ZnO(1%)/TiO -400, respectively.
2
2
2
10 It is indicated that the Pd species (Pd and PdZn) were more highly dispersed in Pd-
11
12
13
ZnO(7.5%)/TiO -100, Pd-ZnO(7.5%)/TiO -400, and Pd-ZnO(1%)/TiO -400 than in Pd/ZnO-400
2 2 2
and Pd/TiO -400, and so the addition of ZnO to TiO improved the dispersion of Pd species (Pd
2
2
and PdZn).The physical mixture of Pd/TiO with ZnO also improved the dispersion of Pd species
2
14 after reduction (2.6 nm for Pd/TiO +ZnO (7.5%)-100, 2.8 nm for Pd/TiO +ZnO(7.5%)-400 vs.
2
2
1
5
6
3.8 nm for Pd/TiO -400). In addition, the lattice signals corresponding to PdZn (1 1 1) and PdZn
2
1
(2 0 0) could be observed for Pd-ZnO(22.5%)/TiO -400 with 9 wt.% Pd loading (Figure S3), for
2
17 which the average metal particle size was large (4.1 nm) and the lattice signals could be easy to
18
find. These results indicated again that the PdZn alloy was formed on TiO2.
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Particle size (nm)
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d
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0
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10
5
0
2
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1
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Particle size (nm)
Particle size (nm)
1
2
Figure 2. TEM of (a) Pd/TiO -400, (b) Pd-ZnO(7.5%)/TiO -400, (c) Pd/ZnO-400 and (d)
2
2
3
4
5
Pd/TiO +ZnO(7.5%)-400 with the corresponding histograms of the particle size distribution. The
2
white scale bar of the micrographs is 20 nm.
6
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0
The redox properties of the three samples, Pd/TiO , Pd-ZnO(7.5%)/TiO , and Pd/ZnO, were
2
2
characterized by H -TPR (Figure 3). For Pd/TiO , a negative peak was observed at 78 °C
2
2
3
5
attributable to β-PdH decomposition. A peak at about 227 °C was due to the reduction of TiO
x
2
2
+
having strong interaction with Pd . A small peak centered at 394 °C was ascribed to the partial
1
reduction of TiO , which was accelerated by the presence of Pd species.38 For Pd-
2
11 ZnO(7.5%)/TiO , a similar negative peak appeared at around 76 °C; however, it was smaller than
2
1
2
3
that of the Pd/TiO catalyst, indicating that the addition of ZnO influenced the redox properties.
2
1
A peak centered at 96 °C was attributed to the reduction of Pd species and ZnO ones close to Pd.
14 Another peak was observed at around 350 °C, probably due to the partial reduction of TiO2,
1
5
6
which was lower by 44 °C than that of Pd/TiO . It is thus indicated that the formation of PdZn
2
1
alloy and the higher dispersion of Pd species may favor the reduction of TiO . In addition, for the
2
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Pd/ZnO catalyst, there were a negative peak at about 70 °C and two reduction peaks around 93
and 420 °C. The peak at about 93 °C may be ascribed to the reduction of Pd species and ZnO
neighboring to Pd and the peak around 420 °C to the reduction of isolated ZnO species. It was
previously reported that pure ZnO showed no reduction peak at temperatures lower than 600
2
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0
3
9
°C. Hence, the presence of Pd and β-PdH could accelerate the reduction of ZnO to form PdZn
x
alloy, which was confirmed by XPS (shown later).
8
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Figure 3. H -TPR results of (a) Pd/TiO , (b) Pd-ZnO(7.5%)/TiO , and (c) Pd/ZnO samples.
2
2
2
1
0
11
The supported Pd catalysts were subjected to XPS measurement to analyze the surface
properties and valence state of Pd and Zn species. The XPS results obtained are shown in Figure
4. Generally, three Pd species different in the chemical state can be detected to exist, being Pd0,
1
2
3
1
2
+ 40
14 PdZn alloy, and Pd . Table 1 gives the relative amounts of these Pd species in the catalyst
1
5
6
samples examined, in addition to the binding energy values. For Pd/TiO -400 catalyst, two peaks
2
0
2+
1
presented at 334.7 and 336.8 eV, which were Pd and Pd 3d 5/2, respectively (Figure 4Ia). For
17 Pd-ZnO (7.5%)/TiO reduced at 100 °C, two peaks were observed at 335.8 and 337.4 eV
2
2+
1
8
attributable to Pd in PdZn alloy and Pd , respectively, indicating the formation of PdZn alloy
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(Figure 4Ib). It was reported that the PdZn alloy formation already started at a temperature as
low as 100 °C in Pd/ZnO.37, 41 When the same catalyst of Pd-ZnO(7.5%)/TiO was reduced at a
2
higher temperature of 400 °C, it also showed two main peaks at 335.8 and 337.4 eV (Figure
4Ic), similar to that reduced at 100 °C. At the same time, the content of PdZn alloy in Pd-
0
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ZnO(7.5%)/TiO increased from 0.57 to 0.65 when the reduction temperature was raised from
2
100 to 400 °C (entries 2, 3). For Pd/ZnO-400, the binding energies at 335.8 and 337.4 eV were
2
+
also assigned to Pd in PdZn alloy and Pd , respectively (Figure 4Id). Besides, the surface
properties of a physical mixture of Pd/TiO and ZnO (7.5% in weight) were also examined.
2
When the physical mixture was reduced at 100 °C, three peaks appeared at 335.1, 335.9, and
0
2+
10 337.4 eV, which were attributed to Pd , Pd in PdZn alloy and Pd , respectively (Figure 4Ie).
0
1
1
2
The binding energy of Pd 3d5/2 was larger by 0.4 eV compared to that of Pd/TiO -400. This
2
1
suggested that there occurred electron transfer and strong interaction between Pd and ZnO, and
13 the electronic environment of Pd on TiO was modified by physical mixing with ZnO after
2
1
1
1
1
1
1
4
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reduction. When the physical mixture of Pd/TiO and ZnO was ground together and reduced at
2
100 °C, the PdZn alloy could be formed at the surface but the amount of PdZn alloy of
Pd/TiO +ZnO(7.5%)-100 (0.22) is small than that of Pd-ZnO(7.5%)/TiO -100 (0.57) (entries 2,
2
2
5). When the same physical mixture was reduced at a higher temperature of 400 °C, it also
0
2+
showed three peaks attributed to Pd , PdZn alloy, and Pd (Figure 4If), with a larger PdZn alloy
content of 0.42 compared to the one of mixture reduced at 100 °C (entries 5, 6). Besides, at the
20 smaller ZnO loading of 1.0%, the Pd-ZnO(1%)/TiO -400 showed a lower PdZn alloy content of
2
2
1
2
0.12 (Figure 4Ig and entry 7). Furthermore, the chemical state of Zn in the samples was
analyzed in detail by Zn LMM Auger peaks (Figure 4II), which were more sensitive than the Zn
2
23 2p XPS. All the samples showed a peak at a kinetic energy of 988 eV, which was attributed to
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ZnO.42, 43 Meanwhile, a shoulder at 991.2 eV was previously ascribed to metallic zinc species,
which also demonstrated the formation of PdZn alloy by reduction in the presence of ZnO.44
1
2
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9
1
1
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0
2+
I Pd
PdZn
Pd 3d
g
Zn
Z0n LMM
II
2+
Pd
Zn
f
f
e
e
d
c
d
c
b
a
b
332
336
340
344
980 984 988 992 996
Kinetic Energy (eV)
Binding Energy (eV)
4
5
Figure 4. (I) Pd 3d XPS and (II) Zn LMM XAES results of (a) Pd/TiO -400, (b) Pd-
2
6
7
8
ZnO(7.5%)/TiO -100, (c) Pd-ZnO(7.5%)/TiO -400, (d) Pd/ZnO-400, (e) Pd/TiO +ZnO(7.5%)-
2
2
2
100, (f) Pd/TiO +ZnO(7.5%)-400, and (g) Pd-ZnO(1%)/TiO -400.
2
2
9
0
Table 1. Binding energy values of supported Pd species and their relative quantities in different
1
catalysts
Binding energy of Pd
species (eV)
Relative amount of Pd
Sample a
species (-) b
Entry
Pd0
334.7
--
PdZn
--
Pd2+
Pd0
0.67
--
PdZn Pd2+
1
2
3
Pd/TiO -400 (a)
336.8
--
0.33
0.43
0.35
2
Pd-ZnO(7.5%)/TiO -100 (b)
335.8 337.4
335.8 337.4
0.57
0.65
2
Pd-ZnO(7.5%)/TiO -400 (c)
--
--
2
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Pd/ZnO-400 (d)
--
335.8 337.4
--
0.54
0.22
0.42
0.12
0.46
0.17
0.20
0.16
Pd/TiO +ZnO(7.5%)-100 (e) 335.1 335.9 337.5
0.61
0.38
0.72
2
Pd/TiO +ZnO(7.5%)-400 (f) 335.1 335.9 337.4
2
0
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Pd-ZnO(1%)/TiO -400 (g)
335.3 336.1 337.5
2
a
1
2
3
Letters in parentheses correspond to the results of Figure 4.
b
0
2+
Relative to the total amount of Pd , PdZn, and Pd .
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Catalytic performance. The catalytic performances of the Pd-based catalysts were
investigated and compared for the N-methylation of N-methylaniline (MA) with CO and H .
2
2
Under the conditions used, N-methylformanilide (MFA), N,N-dimethylaniline (DMA), aniline
(AN), N-phenylformamide (NPFA), N,N-dimethylcyclohexylamine, and other condensation
products were detected and the product distribution depended on the catalysts used (Scheme 1).
10
1
1
Scheme 1. N-methylation of N-methylaniline (MA) to N,N-dimethylaniline (DMA) with CO2
12 and H2.
1
3
1
4
5
Table 2 shows the conversion, the product selectivity, and the reaction rate for various
1
supported Pd catalysts. The selectivity was compared at iso-conversion. The reaction rate of the
16 transformation of MA, rate , and that of the production of DMA, rateDMA, were collected at low
MA
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conversion levels under reaction-controlled conditions. First, the catalysts reduced at 400 °C will
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be compared. The rateMA was 0.21 mmol gcat.-1 h-1, the selectivity to DMA was 19.7%, and the
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major product was MFA over Pd/TiO -400 catalyst (entry 1). In comparison, the addition of a
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0
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small amount of ZnO (1 wt.%) could improve the DMA selectivity from 19.7% to 78.5% (entry
2). When the mass fraction of ZnO was increased to 3.75%, both the rateMA and the selectivity to
DMA were significantly enhanced, reaching 1.91 mmol gcat.-1 h-1 and 97.3%, respectively (entry
3). With a further increase in the ZnO content to 7.5% and 15%, the rateMA increased to about
2.3 mmol gcat.-1 h-1 and the DMA selectivity changed slightly (entries 4, 5). While, the Pd/ZnO
was less active than Pd-ZnO/TiO -400, but it was selective to DMA (entry 6). This may be due
2
10 to that the metal particle size of Pd/ZnO-400 was larger than that of Pd-ZnO(7.5%)/TiO -400 as
2
1
1
2
shown in Figure 2b, c. It is thus indicated that the addition of ZnO to Pd/TiO improves the
2
1
catalytic performance, with respect to both the rateDMA and the DMA selectivity. Then, the
13 influence of reduction temperature was studied using a Pd-ZnO(7.5%)/TiO sample. A rate of
2
MA
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1
1
1
1
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1.85 mmol gcat.-1 h-1 was obtained with a DMA selectivity of 98.5% for the sample reduced at
100 °C (entry 7). When the reduction temperature was raised to 250 °C, the rateMA increased a
little to 1.91 (entry 8), which should be ascribed to an increased amount of PdZn alloy formed at
higher reduction temperature.37 In order to understand the role of ZnO in the catalyst, the
catalytic performance of a physical mixture of Pd/TiO and ZnO (7.5% in weight) was also
2
examined. When the mixture was used without reduction before reaction (entry 9), the rateDMA
20 and the selectivity to DMA were almost the same as obtained with Pd/TiO -400 for 4 h (entry 1).
2
2
1
2
For a long reaction time of 24 h, the conversion of MA was 24.9% with Pd/TiO -400+ZnO
2
2
(7.5%), which was almost the same (26.9%) as obtained with Pd/TiO -400 (Table S3, entries 1,
2
23 9), whereas the selectivity to DMA was larger (71.1%) due to the formation of some PdZn alloy
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in the reaction atmosphere at the reaction temperature of 180 °C for a long reaction time. For the
mixture reduced at 100 and 400 °C, the rateDMA increased to 0.81 and 1.25 mmol gcat.-1 h-1,
respectively (entries 10, 11). For both the cases, the selectivity to DMA was higher than 93%.
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This may be due to the formation of more PdZn alloy when the mixture was reduced in H . It is
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evident that the addition of ZnO improves the catalytic performance of Pd by electronic
modification and/or the formation of PdZn alloy compounds on the surface of TiO . The
2
conversion and the selectivity were also compared at the same reaction time between these
catalysts (Table S3). The change of the conversion is similar with the rateDMA at a wide range
transformation of MA. The selectivity was found to change little with conversion except the
10 physical mixture of Pd/TiO and ZnO (7.5% in weight) without reduction before reaction; for
2
1
1
2
example, Pd/TiO -400 was MFA – selective (entry 1 in Table 2 and Table S3) while Pd-
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1
ZnO(7.5%)/TiO -400 was DMA – selective (entry 4 in Table 2 and Table S3). For the latter
2
13 catalyst, the selectivity to DMA was close to 100% even from the start of reaction until to nearly
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4
5
completion (94% conversion) (Figure 5).
1
16 Table 2. Results of N-methylation of MA with CO and H over various supported Pd catalysts
2
2
b
RateMA c RateDMA c
Selectivity (%)
Time Conv.
Entry
Catalyst a
(mmol
gcat.-1 h-1)
0.21
(mmol
(
h)
(%)
MFA DMA AN
gcat.-1 h-1)
0.04
1d
2
Pd/TiO -400 (a)
4
2
8.5
50.5
15.1
2.4
19.7
78.5
97.3
16.3
6.4
2
Pd-ZnO(1%)/TiO -400 (g)
10.8
12.8
0.54
0.42
2
3
Pd-ZnO(3.75%)/TiO -400
0.67
0.3
1.91
1.86
2
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Pd-ZnO(7.5%)/TiO -400 (c)
0.67
0.67
1
15.6
15.4
11.8
12.4
12.8
8.9
1.5
3.4
--
98.4
93.4
97.1
98.5
99.4
19.3
93.7
95.6
0.1
2.6
2.9
--
2.33
2.30
1.18
1.85
1.91
0.22
0.81
1.25
2.29
2.15
1.15
1.82
1.90
0.04
0.76
1.19
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Pd-ZnO(15%)/TiO -400
2
Pd/ZnO-400 (d)
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Pd-ZnO(7.5%)/TiO -100 (b)
0.67
0.67
4
1.5
0.3
57.3
2.1
1.8
2
8
Pd-ZnO(7.5%)/TiO -250
0.3
23.4
4.2
2.5
2
9 e
Pd/TiO -400+ZnO (7.5%)
2
1
0 e
1 e
Pd/TiO +ZnO (7.5%)-100 (e) 1.75
14.2
12.5
2
1
Pd/TiO +ZnO (7.5%)-400 (f)
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4
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0
Reaction conditions: Catalyst 0.1 g, MA 1 mmol, CO 1.5 MPa, H 4.5 MPa, n-octane 2 mL, 180
2 2
°C.
a
All include Pd in 3 wt.-%. Figures in parentheses indicate the content of ZnO in wt.-%.
In mole%.
b
c
The reaction rates, rateMA and rateDMA, were calculated by the moles of MA converted and
DMA produced per gram catalyst per hour, respectively.
d
The other byproducts were N-phenylformamide (NPFA), N,N-dimethylcyclohexylamine and
condensation products.
e
Physical mixture. Pd/TiO 0.1 g, ZnO 0.0081 g.
2
1
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Figure 5. Changes of conversion () and DMA selectivity () of N-methylation of MA over
3
4
5
Pd-ZnO(7.5%)/TiO -400. Reaction conditions: Catalyst 0.1 g, MA 1 mmol, CO 1.5 MPa, H 4.5
2
2
2
MPa, n-octane 2 mL, 180 °C.
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From the above-mentioned results (characterization and reaction), the authors assume that Pd
and PdZn are catalytically active sites for the target conversion of MA to DMA. The metal
particles on TiO in Pd-ZnO/TiO catalyst include Pd and Zn species and these two species are
2
2
exposed on their surface. The surface exposes a few kinds of Pd species different in the chemical
0
2+
1
1
1
state, Pd (metallic), PdZn (alloy), and Pd . The former two Pd species are catalytically active.
The total moles of exposed Pd atoms in PdZn alloy per gram catalyst (MPdZn) and those in Pd
metal and PdZn alloy per gram catalyst (MPd+PdZn) were calculated using XPS and TEM results
13 and the calculated results are summarized in Table S4. The rateMA and the rateDMA are also
1
4
5
summarized in Table S4. The relationship of the selectivity with the ratio of MPdZn/MPd+PdZn is
shown in Figure 6(I). The DMA selectivity over the monometallic Pd catalyst is low but it is
1
16 significantly enhanced by the addition of Zn species. A small amount of Zn added has a large
1
7
8
impact on the selective transformation of MA to DMA. The nature of the exposed Pd atoms
should change the DMA - selective by the presence of Zn, the selectivity being of > 95% at the
1
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ratio of MPdZn/MPd+PdZn > 0.26. The rateMA and rateDMA also increase with MPdZn (Pd in PdZn)
(Figure 6(II)), indicating that the Pd atoms in PdZn alloy are more active for the target reaction
than those in Pd metal, so PdZn alloy being the main active species.
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Figure 6. (I) DMA selectivity () as a function of the ratio of MPdZn to MPd+PdZn, and (II) rateMA
7
8
9
() and rateDMA () as a function PdZn alloy content (MPdZn). (a) Pd/TiO -400, (b) Pd-
2
ZnO(7.5%)/TiO -100, (c) Pd-ZnO(7.5%)/TiO -400, (d) Pd/ZnO-400, (e) Pd/TiO +ZnO(7.5%)-
2
2
2
100, (f) Pd/TiO +ZnO(7.5%)-400, and (g) Pd-ZnO(1%)/TiO -400. M is the total moles of
PdZn
2
2
10 exposed Pd atoms in PdZn alloy in the per gram catalyst. MPd+PdZn is the total moles of exposed
1
1
2
Pd atoms in Pd metal and PdZn alloy per gram catalyst. The calculation procedures for MPdZn and
MPd+PdZn are described in detail in Supporting information.
1
13
1
1
1
1
1
4
5
6
7
8
To understand the influence of substitution in the aromatic ring, the N-methylation of various
substituted secondary anilines was investigated (Table 3). Compared with no substituted
secondary aniline of MA, when electron-donating 4-methoxy- and 4-methyl- substituted
secondary anilines were used as reactant, the conversion was a little higher and the selectivity
changed slightly (> 98.6%) (Entries 1-3). However, when electron-donating 2-methyl-N-
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methylaniline was used, the conversion (30.1%) and the selectivity to substituted N,N-
dimethylaniline (72.9%) were much lower (Entries 1,3,4). When the electron-withdrawing 4-
chloro and 4-fluoro substituted N-methylanilines were used, the selectivity changed slightly but
the conversion was decreased seriously (Entries 1,5,6). These results indicated that the reactivity
and the selectivity were correlated with not only the electron density of the substituted group, but
also the substituent position on the aromatic ring.19, 20 Besides, the catalytic performance of the
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present Pd-ZnO(7.5%)/TiO -400 is compared with other heterogeneous catalysts reported in the
2
literature (Table S5). The yield of DMA over Pd-ZnO(7.5%)/TiO -400 is comparable to the best
2
result with the bimetallic Pd-based catalysts of PdGa/TiO and PdCuZrOx catalysts, but it is a
2
10 little lower than the monometallic Re/TiO and Au/Al O catalysts.
2
2
3
1
1
2
1
Table 3. Pd-ZnO(7.5%)/TiO -400 catalyzed N-methylation of substituted aromatic amines with
2
13 CO and H
2
2
Entry
1
Substrate
Product
Conv. (%)
87.0
Sel. (%)
100
2
93.0
98.6
3
4
91.0
30.1
99.8
72.9
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60.4
94.3
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Reaction conditions: Pd-ZnO(7.5%)/TiO -400 0.1 g, substrate 1 mmol, CO 1.5 MPa, H 4.5
2 2 2
MPa, n-octane 2 mL, 180 °C, 24 h.
Finally, the catalyst recyclability was examined with the most active Pd-ZnO(7.5%)/TiO -400
2
catalyst using short and long reaction times (small and larger conversions) for each run (See
experimental section for the recycling test). When the catalyst was recycled in a short reaction
time of 0.67 h (Figure 7a), the conversion and rate did not change so much for the four runs.
When a longer reaction time of 24 h was used, the conversion reached around 85% and the
selectivity of DMA was above 99.5% for the first three runs (Figure 7b). For the fourth run,
10 however, the conversion decreased to 59% while the high DMA selectivity remained unchanged.
1
1
2
The decrease in the conversion should result from carbon deposition on and sintering of
dispersed metal particles. The amount of carbon deposited in the catalyst after the fourth run was
1
13 about 46 wt.% to the total weight of the catalyst according to TGA (Figure S4). In addition, the
1
1
1
4
5
6
average metal particle size increased to 3.0 nm compared with the fresh catalyst of 2.3 nm
(Figure 2b, Figure S2d). The loading of Pd (2.9 wt.%) in the Pd-ZnO(7.5%)/TiO -400 catalyst
2
was not found to change after used four times, indicating that no Pd leaching occured during the
17 reaction. The carbon deposition and metal sintering occurred gradually during the repeated runs
1
8
9
in a longer reaction time of 24 h for each, causing the activity loss for the fourth run. However,
the nature of exposed Pd species in PdZn is unlikely to be influenced by these two undesired
1
20 phenomena and so the high DMA selectivity does not change (Figure 7b).
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Figure 7. Recycling results for N-methylation of MA with CO and H by Pd-ZnO(7.5%)/TiO -
2
2
2
3
4
5
6
7
8
9
400. Reaction conditions: Catalyst 0.1 g, MA 1 mmol, CO 1.5 MPa, H 4.5 MPa, n-octane 2
2 2
mL, 180 °C, (a) 0.67 h, (b) 24 h.
Possible reaction pathways The reaction mechanisms of the N-methylation of MA on PdZn
.
were examined and discussed. First, 13CO is used instead of CO to study the possible
2
2
methylation process (Table 4). In the absence of CO , the conversion of MA was only 0.5%, and
2
the N-methylation reaction did not happen (entry 1). The conversion of MA and the selectivity to
13
10 DMA with CO is lower than those with CO (entries 2, 3). The intensity of molecular ion peak
2
2
13
1
1
2
of m/z = 122 increased obviously and that of m/z = 120 disappears when CO was used,
2
1
3
1
indicating that the product of DMA was C labelled (Figure S5). These results showed that
1
3
13 either CO or CO was the N-methylation reagent. It has been reported that the formation of
2
2
35
1
1
1
4
5
6
formate is the key step in the methylation reaction with CO and H . Lee et al. reported that
2 2
formic acid was obtained via CO hydrogenation over PdNi alloy on a carbon nanotube-graphene
2
4
5
support. To investigate the intermediate species formed under the present reaction conditions,
17 the Pd/TiO -400 and Pd-ZnO(7.5%)/TiO -400 catalysts were exposed to CO and H at 180 °C
2
2
2
2
1
8
9
and then subjected to DR-FTIR measurements to examine chemical species present on them.
1
Second, the DR-FTIR spectra collected are shown in Figure 8. There are no observable
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absorption bands for Pd/TiO -400 catalyst in the ranges of 1100 - 2100 and 2700 - 3100 cm
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(Figure 8a). However, for Pd-ZnO(7.5%)/TiO catalysts reduced at 100 and 400 °C, two
2
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-
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-
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46
-1
species. Moreover, the bands at 2873 and 2945 cm are assigned to C-H stretching vibration
modes, further indicating the formation of formate species from CO and H over Pd-
2
2
ZnO(7.5%/TiO )-100, -400 catalysts (Figure 8b, 8c). Meanwhile, the absorption bands are also
2
-1
observed at 1595, 2873, and 2945 cm for the physical mixture of Pd/TiO and ZnO (7.5%)
2
reduced at 400 °C (Figure 8d), although these are much weaker compared to those of Pd-
ZnO(7.5%)/TiO catalysts. However, there is no absorption band detected for the unreduced
2
10 physical mixture (Figure 8e). As mentioned above from XPS results, PdZn alloy was formed for
1
1
2
the Pd-ZnO(7.5%)/TiO and the physical mixture when reduced. Therefore, the above results
2
1
show that the formate species was formed during the N-methylation of MA with CO and H on
2
2
13 PdZn alloy, and the stronger intensity of DR-FTIR spectra indicated the larger amount of the
1
1
1
1
1
1
4
5
6
7
8
9
formate species formed with the larger PdZn alloy content. In addition, methanol could be
3
2-34
formed in the hydrogenation of CO over PdZn alloy in the literature,
and methanol was also
2
an N-methylation reagent in the N-methylation of MA. However, methanol or methoxy species
was not observed from DR-FTIR spectra; the C-O stretching vibration in the ranges of 1250 -
-1
1050 cm could not be found (Figure 8A). Furthermore, when methanol was used as an
alkylation reagent, the conversion was only 22.7% with a selectivity to DMA of 99.6%, which
20 was much lower than that when CO was used (87% conversion with 100% DMA selectivity)
2
2
1
2
(Table S6). These results indicated that methanol or methoxy species was not the N-methylation
2
reagent or the intermediate in the present N-methylation of MA with CO over Pd-
2
23 ZnO(7.5%)/TiO -400.
2
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Table 4. The N-methylation of MA with CO and CO over Pd-ZnO(7.5%)/TiO -400
2
2
2
Sel. (%)
Entry
CO2
Conv. (%)
MFA
--
DMA
--
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1a
2b
3b
--
0.5
1.5MPa CO2
68.0
49.0
--
99.4
83.2
13
1.5 MPa CO
16.8
2
2
3
4
5
Reaction conditions: Pd-ZnO(7.5%)/TiO -400 0.1 g, MA 1 mmol, H 4.5 MPa, n-octane 2 mL,
2 2
a
b
180 °C, 12 h. The main product was AN via C-N bond cleavage. The gas chromatography and
the mass spectrometry spectra were given in Figure S5.
6
7
Figure 8. In situ DR-FTIR spectra for (a) Pd/TiO -400, (b) Pd-ZnO(7.5%/TiO )-100, (c) Pd-
2
2
8
9
0
ZnO(7.5%)/TiO -400, (d) Pd/TiO +ZnO (7.5%)-400, and (e) Pd/TiO -400+ZnO (7.5%). FTIR
2 2 2
spectra were collected under ambient conditions after the exposure to CO (1 MPa) and H (2
2
2
1
MPa) at 180 °C for 30 min.
11
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1
The MFA adsorption on the Pd/TiO -400 and Pd-ZnO(7.5%)/TiO -400 catalysts was also
2
2
-1
2
measured by FTIR (Figure 9). The band at 1678 cm is assigned to C=O stretching vibration.
6
7
-
1
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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9
The bands at 1497, 1596 cm are assigned to C-C stretching vibration of aromatic ring. There is
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
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3
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5
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7
8
9
0
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0
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8
9
0
no C-N stretching vibration of MFA. For Pd/TiO -400, when He gas was passed through the
2
sample, the peaks did not change but the intensity became weak. After exposed to He stream for
5 min, the adsorption of MFA was very weak. But, for Pd-ZnO(7.5%)/TiO -400, the C=O
2
-1
-1
stretching vibration red shifted from 1678 to 1665 cm and a new band at 1616 cm appeared
after 10 min. These results indicate the existence of interactions between MFA and Pd-
ZnO(7.5%)/TiO -400. The adsorption of MFA through C=O on the surface of Pd-
2
10 ZnO(7.5%)/TiO -400 is strong, but the adsorption of MFA on Pd /TiO -400 is weak.
2
2
1
1
1
2
3
1
Figure 9. FTIR of the adsorption of MFA on Pd/TiO -400 and Pd-ZnO(7.5%)/TiO -400. (a)
2
2
1
1
1
4
5
6
MFA on pure KBr tablet. After the adsorption of MFA on catalyst and KBr tablet, the catalyst
sample was exposed to He stream for (b) 0 min, (c) 1 min, (d) 5 min, (e) 10 min.
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To discuss the reaction pathways involved in the N-methylation of MA using CO and H as
2
2
the alkylation reagent, we made the following control reaction experiments. Our previous work
declared that MA was carbonylated with CO and H first to MFA intermediate and then it was
2
2
0
1
2
3
4
5
6
7
8
9
0
1
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0
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0
35
quickly hydrogenated to DMA. It was also reported that N-formamide was the intermediate
2
0
during N-methylation of aniline (AN) with CO and H with Ru catalyst. Shi et al. reported that
2
2
N-formamides were obtained under relatively mild reaction conditions using PdCu/ZrO as
2
catalyst.23 In the present work, therefore, the activity of selected Pd/TiO -400 and Pd-
2
ZnO(7.5%)/TiO -400 catalysts was checked with MFA as reactant. Table 5 shows the results for
2
the reaction of MFA and H in the absence and presence of CO . When CO was absent, MFA
2
2
2
10 was almost consumed in 2 h (99.7% conversion), giving DMA (38.4%), MA (60.9%), and AN
1
1
2
(0.7%) with Pd-ZnO(7.5%)/TiO (entry 1). When 1.5 MPa CO was added to the reaction
2 2
1
system, the conversion of MFA changed slightly (98.0%), but the selectivity to DMA increased
13 to 48% (entry 3). For Pd/TiO , on the contrary, it was less active regardless of the presence and
2
1
1
1
1
1
1
4
5
6
7
8
9
absence of CO , the MFA mainly transferred to MA and NPFA, and the selectivity to DMA was
2
lower than 4% (entries 4, 6). The rate of hydrogenation of MFA to DMA over Pd-
-1
-1
ZnO(7.5%)/TiO -400 (19.7 mmol g
h ) was much higher than that over Pd/TiO -400 (0.27
2
2
cat.
mmol gcat.-1 h-1) (entries 3, 5). That is, the former catalyst is even more active for the
hydrogenation of MFA to DMA than the latter one.
2
0
Table 5. Catalytic performance of different catalysts for MFA hydrogenation
2
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5
Selectivity (%)
RateMFA
(mmol
RateDMA
(mmol
CO2
Time Conv.
Entry
Catalyst
6
(
MPa)
(h)
(%)
MA DMA AN NPFA
7
8
9
gcat.-1 h-1) a gcat.-1 h-1) a
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
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2
2
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4
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0
1
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9
0
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9
0
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0
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0
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0
1
2
Pd-ZnO(7.5%)/TiO -400
--
2
99.7 60.9 38.4
13.1 49.7 45.1
0.7
5.2
--
--
--
--
2
Pd-ZnO(7.5%)/TiO -400
1.5
0.1
43.7
19.7
2
3
Pd-ZnO(7.5%)/TiO -400
1.5
2
98.0 50.5 48.0
1.2
1.5
0.3
--
--
2
4
5
6
Pd/TiO -400
--
2
0.5
2
26.2 78.1
15.0 43.9
37.7 56.8
3.7
2.7
2.1
16.7
--
10.0
--
--
0.27
--
2
Pd/TiO -400
1.5
1.5
11.8 41.6
3.1 38.0
2
Pd/TiO -400
2
1
2
3
4
Reaction conditions: Catalyst 0.03 g, MFA 1 mmol, H 4.5 MPa, n-octane 2 mL, 180 °C.
2
a
The reaction rates of rateMFA and rateDMA were calculated by the moles of MFA converted and
DMA produced per gram catalyst per hour, respectively.
5
6
7
8
9
0
1
2
The activity of selected Pd/TiO -400 and Pd-ZnO(7.5%)/TiO -400 catalysts was also checked
2
2
with DMA as reactant (Table 6). Over Pd/TiO -400, DMA was transformed to AN, the
2
selectivity to AN is 100%, and the C-N bond cleavage rate of DMA to AN (1.18 mmol gcat.-1 h-1)
was much higher than the N-methylation rate of MA to DMA (0.04 mmol gcat.-1 h-1) (entry 1 in
Table 2 and Table 6). Over Pd-ZnO(7.5%)/TiO -400, rate
is much lower than that over
2
DMA’
1
Pd/TiO -400 (Table 6, entries 1, 2). Furthermore, the C-N bond cleavage rate of DMA to AN
2
1
1
and MA (0.06 mmol gcat.-1 h-1) is much lower than the N-methylation rate of MA to DMA (2.29
mmol gcat.-1 h ) over Pd-ZnO(7.5%)/TiO -400 (Table 6, entry 2; Table 2, entry 4).
-1
2
13
1
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