Photon-initiated heterogeneous redox couples for methylation of anilines under mild conditions

Article abstract of DOI:10.1039/d0gc01385d

Methylation of anilines has drawn a lot of attention due to their valuable applications and directly using methanol as a methylation reagent is of great advantage. Photon-initiated heterogeneous catalysis of this methylation process meets the requirements of green chemistry. Herein we show that balanced redox zones within carbon nitride supported Pd nanoparticles boost the selectivity of methylation of anilines under mild conditions.

Full text of DOI:10.1039/d0gc01385d

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Photon-initiated heterogeneous redox couples for Methylation of
Anilines under mild conditions
Bing Zhanga*, Hua Gaob, Wei Wangb*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
importance for practical applications as well as mechanism
study.
In order to improve the selectivity of methylation using
Abstract
Methylation of anilines deserves lots of attentions due to their
valuable applications and directly using methanol as methylation
reagent is of great advantages. Photon-initiated heterogeneous
catalysis of this methylation process meets the requirements of
green chemistry. Herein we show that balanced redox zones
within carbon nitride supported Pd nanoparticles boost the
selectivity of methylation of anilines under mild conditions.
methanol, heterogeneous catalysts need oxide the methanol
and reduce corresponding produced imides while avoid other
coupling reactions under light irradiation12. With a moderate
bandgap of 2.7 eV, cheap but stable carbon nitride (CN) has
attracted much attention as sustainble heterogeneous
catalysts
or
supported
substrates
for
artificial
photosynthesis13-18. Metal nanoparticles loaded CN as efficient
Mott-Schottky catalysts were also widely utilized and exhibited
excellent catalytic efficiency due to rectifying effect and
stablization of metal particles via rich amino surface functional
groups of CN especially for photo-initiated reactions19-21. Note
that, CH3OH was common seen as hole sacrifice reagent for
heterogeneous photochemical hydrogen evolution reaction
which means the oxidation of CH3OH can be facile trigered via
photon-initiated holes from semiconductors such as CN and
TiO222,23. Hence, it is reasonable and economic to triger
methylation of anilines via photon-initiated oxidation of
CH3OH on well designed CN based Mott-Schotty catalysts. The
key concern is to fulfill high selectivity of the methylation
process under mild conditions.
Keywords: coupling reaction, photocatalysis, heterogeneous
catalysis, methylation, green chemistry
Introduction
N-Methylamines are pivotal structures in pharmaceuticals,
agrochemicals, dyes, and synthetic intermediates1-3. Among
numerous strategies for catalyzing the reaction, the direct
using methanol as methylation reagent is of great advantages4-
6. The purification can be simplified since methylation process
only produced water as byproduct. Besides, without additional
reagents, the methylation process can allow amines with acid-
sensitive functional groups. However, reported methylation of
anilines often need high temperature or pressure, some
homogeneous catalysts with excellent activity suffer from the
poor reusability7-9. Meanwhile, trigger these reactions by light
irradiation is an efficient way to meet demands of green
chemistry10,11. Till now, facile design of heterogeneous
photocatalysts to catalytic methylation with CH3OH is of great
a) homogeneous metal complex catalyzed N-methylation
>100 oC
[M]
+ CH3OH
R NH
R NH2
strong base
-H2O
b) this work
o
hv
Pd@CN 55 C
+ CH3OH
a. Dr. B. Zhang,
R NH2
R NH
base support
-H2O
International Collaborative Laboratory of 2D Materials for Optoelectronics Science
and Technology of Ministry of Education, Institute of Microscale Optoelectronics,
Shenzhen University, Shenzhen 518060, P. R. China
Email: chemzhb218@126.com
Scheme 1. Approaches for methylation of anilines.
b. Mr. H. Gao, Dr. W. Wang,
Herein, we prepared different metal nanoparticles loaded CN
based Mott-Schotty catalysts through a facile conventional wet
impregnation method. The mesoporous carbon nitride
(mpCN)24,25 with a surface area of 160 m2/g (Figure S1, S2) was
chosen as the semiconductor support. The as-obtained
School of Materials Science and Engineering, Ocean University of China, 238
Songling Road, Qingdao 266100, China
Email: clwang@ouc.edu.cn
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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catalysts were presented as M-x@CN (M means the metal
species such as Pd, Au, while x represents the original mass
ratio of metal species to CN substrate), M-x@C was prepared
following the same conditions used for M-x@CN. Typical
approach in our system for methylation of anilines was
illustrated in Scheme 1. Compared with previous reports7-9,
photocatalytic methylation using CH3OH can be conducted
under lower pressure, lower temperature without other
additives (table S1) whicn are not environment-friendly, and
using basic CN supported heterogeneous catalyst may simplify
the recycle procedure for practical applications.
as conductive support (Figure 1e), Pd nanopartiVcileewsArctiaclne Oanllsinoe
well dispersed.
DOI: 10.1039/D0GC01385D
It is reasonable that Pd loading amount may directly tune the
redox activities of catalysts for photocatalytic methylation
reactions. Acetylacetone combined UV-vis spectra method28
was utilized for detect the exitence of possible formaldehyde
during the photon-initiated methylation process and other
products were determined by GC-MS detection (Figure 2).
Within the first 4 hours, Pd-x@CN and Pd-3@C catalysts
exhibited obvious differences for the methylation process of
anilines. As the Pd loading amount increased, the reductive
activity of as-obtained Pd-x@CN catalysts increased as well.
Without Pd nanoparticles, CN support alone only catalyze the
oxidation process and no methylation product was detected.
Pd-1@CN showed increased conversion efficiency and
selectivity compared with that of CN sample. And Pd-3@CN
catalyst exhibited the best selectivity for the photocatalytic
methylation of anilines. As the Pd loading amount continously
increased, the conversion efficiency and selectivity decreased
greatly, main byproduct was hydrogenated coupling anilines29.
While in the case of Pd-3@C catalyst, neither formaldehyde as
main oxidation product nor methylation products over other
samples were detected.
Figure 1. Morphology of as-obtained Pd-x@CN and Pd-3@C samples. TEM
images of pristine CN (a), Pd-1@CN (b), Pd-3@CN (c), Pd-9@CN (d) and Pd-
3@C respectively.
Different metal species loaded M-3@CN were first valued for
photocatalytic methylation of anilines with CH3OH under
visible light irradiation (Table S2). Pd-3@CN showed best
selectivity and good conversion efficiency. Herein further
characterizations were conducted for this optimized samples.
What’s more, without light irradiation or catalysts, no product
was detected which illustrated the photocatalytic nature of Pd-
3@CN samples. We then changed the Pd loading amount for
optimizing catalytic activities under estimated solar irradiation.
The successful deposition of metallic Pd on the surface of
carbon nitride support was confirmed by transmission electron
microscopy (TEM) and powder X-ray diffraction (XRD)
observation. Pd nanoparticles were well dispersed with
average diameter of 4-6 nm on the carbon nitride for Pd-x@CN
samples (Figure 1) except that Pd-18@CN exhibited severer
aggregation (Figure S3). This can also be clearly illustrated by
HADDF-STEM image and corresponding Pd element mapping
result (Figure S4). Evidence can also be found in XRD spectra,
small peaks near 40° were attributed to Pd (111) peaks15,27
(Figure S5). The accurate loading amount of Pd nanoparticles
were also confirmed via ICP (Inductive Coupled Plasma
Emission Spectrometer) tests (Table S3). When using graphite
Figure 2. Photocatalytic methylation of anilines over different catalysts.
Formaldehyde detection (a, b) and coupling products (c, d) over different
catalysts. (e) Schematic illustration of Pd-x@CN catalysts with different
redox activities. Typical reaction condition: 20mg catalysts, 5 mL CH3OH,
0.2 mmol anilines under estimated solar irradiation for 4 h; for depressing
the formation of formaldehyde, 10% TEOA was added.
The comparison of Pd-3@CN and Pd-3@C catalysts can well
illustrated that the support effect is essential concern for
Mott-Schottky catalysts. Note that Pd nanoparticles can be
well dispersed on both support materials (Figure 1c and 1e).
But the XPS spectra (Figure S6) of Pd 3d presented obvious
shift compared with Pd-3@CN and Pd-3@C which well
illustrated the rectifying effect of mpCN support15,19. More
specificallly, with suitable conduction band edge and valence
2 | J. Name., 2012, 00, 1-3
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band edge, the band gap of CN can include work functions of Therefore, Pd-3@CN catalyst with balanced redox couples can
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DOI: 10.1039/D0GC01385D
most metals. And the injection of electrons from CN to metal fulfill methylation of anilines and CH3OH with high selectivity.
particles can be promoted under the effect of in-built Relatively decreased reduction zone (also considered as
electronic field. With more electrons around, the enhanced oxidation zone) such as Pd-1@CN and pristine CN
corresponding metals would obviously show lower binding catalyst produced more N-methyleneaniline isomer waiting for
energy. Photoluminescence spectra (Figure S7) also further hydrogenation. While catalysts (Pd-9@CN, Pd-18@CN
demonstrated this effect via decreased photoluminescence or Pd-3@C) with relatively enhanced reduction zone led to
intense of Pd-3@CN compared with pristine CN sample. formation of overhydrogenated byproduct such as N-
Decreased photoluminescence intense means enhanced cyclohexylaniline29. Note that adding triethanolamine to
electron-hole separation efficiency30. What’s more, the organic depress the formation of formaldehyde would largely decrease
nature of CN could also stabilize metal nanoparticles for more the formation of methylaniline which also illustrated that the
effective heterogeneous catalysis. Hence Pd-3@CN can serve formation of formaldehyde is one key step for the methylation
coupled redox zones for photocatalytic methylation of anilines. of anilines in our system. Via long time irradiation, Pd-3@CN
The differences of photocatalytic methylation of anilines over can completely convert aniline to methylaniline and
Pd-x@CN with different redox activities help us to further dimethylaniline with high selectivity (99%) within 36 h (Figure
reaveal the possible mechanism of this reaction within the first S11).
4 h. With the increase of Pd contents over Pd-x@CN catalysts, The general applicability of Pd-3@CN for aniline methylation
the amounts of remaining formaldehydes detected showed was also investigated (table 1) via photocatalytic various
almost linear decrease as the corresponding adsorption substituted benzyl anilines with electron-withdrawing or
intensity of UV-vis spectra reflected (Figure 2a, S8). The electron-donating functional groups under standard
amount of N-methyleneaniline exhibited similar trendency conditions. The Pd-3@CN sample generally gave high
with the increase of Pd contents over Pd-x@CN catalysts and conversions (>84%) and selectivity (99%) of these electron-
Pd-3@C catalysts. N-cyclohexylaniline, as another main donating substituted anilines toward corresponding
byproduct, increased when the Pd loading amount reached 9% methylanilines and dimethylanilines within 24h. For anilines
or above. These results well illustrated that CN as main with electron-withdrawing functional groups, Pd-3@CN also
oxidation zones of Pd-x@CN catalysts promoted the formation exhibited relative high activities for methylation except for
of formaldehyde while Pd nanoparticles (with CN enhanced substrate with -CN group wich suffered low conversion. The
Mott-Schottky effect) served as main reduction zones of Pd- substitutions and steric arrangement of functional groups may
x@CN catalysts to fulfill hydrogenation process from N- affect the preadsorption of diverse substrates on the surface
methyleneaniline to methylaniline (Figure 3). Note that, this of Pd-3@CN and finally influence the catalytic activity. The
reaction can be promoted under H2 atmosphere compared presence of the para, meta and ortho isomers of
with Ar atmosphere (Figure S9) while only adsorped active methylanilines with a reactivity trend of para > meta > ortho-
hydrogen species on Pd nanoparticles other than H2 (Figure substituted partially confirmed the steric effects on the
S10)account for the hydrogenation of imines.
photocatlytic aniline methylation process over Pd-3@CN
catalysts. Even the ortho-substituted methyl group showed
typical blocking effect on the methylation reaction, ortho-
substituted methylaniline also achieved high conversion
efficiency (>90%) with excellent selectivity (99%) within 36 h.
Note that heterocyclic anilines such as 3-aminopyridine can
also be oxidized over Pd-3@CN to the corresponding
methylanilines with moderate conversion (>40 %) and high
selectivity (99 %) within 36 h. While without conjugated
aromatic structure, cyclohexylamine could be more easily
catalyzed to corresponding methylanilines with high
conversion (>70 %) and high selectivity (99 %) within 4h. Due
to active self-reactivity of hexylamine, the methylation process
fulfilled with high conversion but low selectivity. As comes to
active functional groups such as chloro- and bromo-,
substrates were also be methylated under Ar atmosphere with
relatively low selectivity. Only substrates with alkynyl or nitro-
functional groups were fully or partial reduced even under
argon atmosphere and hence showed no selectivity for
methylation. We further conducted different types of alcohols
including benzyl and aliphatic alcohols to react with anilines.
All kinds of alcohols can react with anilines to construct C-N
coupling structures under standard conditions although
Figure 3. Proposed methylation process of anilines over Pd-x@CN catalysts
with different redox activities in our system.
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suffered slow reaction efficiency when using aliphatic alcohols Typical synthesis of Pd-x@CN catalysts: 150mgVmieweAsroticpleoOronluines
with more carbon atoms.
carbon nitride powder was dispersed inD4O0I: 1m0.L10d39is/tDil0leGdC0w13a8t5eDr
The optimized Pd-3@CN catalyst also exhibited excellent over night, certain amount of 6 mg/mL Pd2+ solution (PdCl2
stability and reusability for practical applications. Only slight dissolved in 1M aqueous HCl solution) was added in the above
decrease for the activity (Figure S12) of the reused Pd-3@CN dispersion solution overnight to form Pd-x@CN precurer (x
catalyst and quite stable Pd content (Figure S13) were means the mass ratio of Pd to CN). After adjusting the pH to 11
observed after 5 cycles. Note that the selectivity remained via 4M NaOH solution, excess amount of NaBH4 dissolved in 10
99% for each cycle. These results well illustrated the merits of mL distilled water was added to reduce metal species onto
heterogeneous catalysts and indicated great potential of these carbon nitride. As-obtained catalysts were collected after
catalysts for practical and sustainable applications.
centrifugation and washed with water and ethanol followed
dryness at vacuum condition.
Table 1. Photocatalytic activities for selective coupling of different alcohols
and anilines over optimized Pd-3@CN sample.
Acknowledgements
R2
R2
H2 hv
55 o
R1
R1
R1
+
NH2
+
R2-OH
NH
R2
N
C
This work was supported by National Postdoctoral Program for
Innovative Talents (BX20180203), China Postdoctoral Science
Foundation (2018M643176) and National Natural Science
Foundation of China (51572247, 51972289).
a
b
R2=Me
For different anilines
NH2
NH2
O
NH2
2. 24h: C. 99%;
1. 24h: C. 84.5%;
S. 99% (a. 91%)
3. 36h: C. 94.3%;
S. 99% (a. 74.4%)
S. 99% ( b. 51.7%)
NH2
F3C
NH2
F
NH2
Conflicts of interest
There are no conflicts to declare.
4. 36h: C. 90.1%;
S. 99% (a. 85.3%)
5. 24h: C. 88%;
S. 99% (b. 86.7%)
6. 24h: C. 92%;
S. 99% (a. 80.1%)
Cl
NH2
Br
NH2
NC
NH2
8*. 24h: C. 52%;
S. 43% (a. 43%)
7*. 24h: C. 89%;
S. 80% (b. 64%)
9. 24h: C. 67%;
S. 82% (a. 82%)
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