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doi.org/10.1002/cctc.202001870
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based on Mn, Cu, Co, or Fe because of their tunable redox
properties and abundant availability.[6]
Table 1. Aerobic oxidation of benzylamine over a series of nanostructured
TiO2 based catalysts.[a]
Nanostructured TiO2 is a promising material for a number of
catalytic applications because of its unique structural, morpho-
logical, electrical, and redox properties. TiO2 based nanomate-
rials have been widely used in photocatalytic applications
including visible light-driven selective imines synthesis with
excellent yields at mild reaction conditions.[7] However, their
applications in thermochemical organic synthesis were rarely
investigated. Under this scrutiny, we noticed an interesting
work published by Klitgaard et al., which highlighted the
catalytic role of TiO2 based materials for cyclohexylamine
oxidation.[8] The structure-activity properties of nanostructured
TiO2 could be significantly modified by tuning its shape
(nanotube, nanorod, nanoparticle, etc.).[9] The homogeneous
dispersion of transition metal oxide nanoparticles (e.g., MnOx)
on shape-controlled TiO2 may provide synergistic interface
structures with enriched properties for selective catalysis. With
this motivation, we developed a promising heterostructured
catalyst, consisting of highly dispersed Mn-oxide nanoparticles
on TiO2 nanotubes, for solvent- and base-free oxidation of
aromatic amines using 5 vol% O2/Ar as the oxidant. Various
analytical techniques, such as powder XRD, BET surface area,
TEM, STEM-EDX, XPS, and H2-TPR are used to understand
structural, textural, redox, and morphology properties of MnOx/
TiO2 nanocatalysts. This work not only reports a promising
catalytic application of TiO2 based nanomaterials for the
synthesis of imine compounds at industrially important con-
ditions, but also provides valuable insights for selective
oxidation reactions.
Entry Catalyst
Amine conv. [%] Selectivity [%]
Imine Nitrile Oxime
1
2
3
4
Blank
9.2
99.9
97 0.1
0.1
TiO2-NT400
TiO2-NT500
TiO2-Aldrich
Aeroxide P25
MnOx/TiO2-NT400
38.3
34.6
36.5
18.4
95.6
2.8
2.3
25.3
11.9
97.6
73.8
20.8
99.9
98.7
99.9
99.5
99.9
96.2
94.3
0.1
0.9
5.2
0.1
1.3
0.1
0.5
0.1
3.8
5.7
5[b]
6[c]
7
MnOx/TiO2-Aldrich 57.8
8
MnOx/TiO2-NT500
MnOx/TiO2-NT400
MnOx/TiO2-NT400
MnOx/CeO2
62.8
57.3
54.2
79.2
81.7
9[d]
10[e]
11[f]
12[g]
Mn2O3
[a] Reaction conditions: 4 mmol benzylamine, 50 mg catalyst, 5 vol% O2/Ar
(oxidant), 5 bar initial pressure, 6 h reaction time, 120 C reaction
temperature, and no solvent/base. [b] Various non-identified products
were seen in the GC. [c] 10 wt% Mn with respect to TiO2. [d] 5 wt% Mn. [e]
25 mg catalyst. [f] 10 wt% Mn with respect to CeO2 nanorods. [g] Catalyst
amount: maintained the same mol% of Mn in Mn2O3 as in the 50 mg of
MnOx/TiO2-NT400 catalyst.
°
Irrespective of the catalysts and the reaction conditions used, a
high selectivity to imine product was obtained in this study. In
the case of MnOx/TiO2-Aldrich catalyst calcined at 400 C
°
(entry 7, Table 1), only a 57.8% benzylamine conversion was
obtained, which is very low compared to that of MnOx/TiO2-
NT400 catalyst (entry 6, Table 1). It clearly indicates the
favorable role of shape-controlled TiO2 nanotubes in improving
the catalytic activity of MnOx/TiO2 materials in benzylamine
oxidation. This could be due to the synergistic catalytic effect of
TiO2 nanotube and MnOx nanoparticle, which is discussed in the
later section. Benzylamine conversion was decreased from
95.6% to 62.8% when the calcination temperature of MnOx/
Results and Discussion
°
Catalytic activity studies
TiO2-NT catalyst increases from 400 to 500 C (entry 8, Table 1).
Similarly, low conversions of benzylamine were obtained for
low Mn content (entry 9, Table 1) and low catalyst amount
(entry 10, Table 1). It indicates the necessity of using a sufficient
amount of Mn-oxide to obtain higher conversions in benzyl-
amine oxidation. Low amine conversions were obtained over
MnOx/CeO2 nanorods[3a] (entry 11, Table 1) and Mn2O3 (entry 12,
Table 2), indicating the promising activity of MnOx/TiO2 nano-
tube catalyst in benzylamine oxidation at solvent-free con-
ditions.
Benzylamine oxidation as a model reaction was evaluated over
a series of TiO2 based catalysts and the results are shown in
Table 1. This reaction typically follows two pathways, resulting
in dibenzylimine and benzonitrile as major products as shown
in Scheme 1.[2c] Initially, we performed a blank reaction (entry 1,
Table 1), and only a 9.2% amine conversion was obtained. In
the case of both TiO2-NT400 and TiO2-NT500 nanomaterials
(entries 2 & 3, Table 1), amine conversion was increased to 34–
38% with a high selectivity to imine (~97%), indicating the
catalytic role of TiO2 nanomaterials in benzylamine oxidation. In
contrast, very low imine selectivity was found over commercial
TiO2 catalysts, with considerable amounts of oxime (entries 4 &
5, Table 1). It obviously indicates the promising role of nano-
structured TiO2 catalysts (Figure 3) to achieve improved imine
selectivity in benzylamine oxidation.
The effect of reaction temperature, reaction time, and
reaction pressure on benzylamine oxidation was studied with
the best MnOx/TiO2-NT400 catalyst. High imine selectivity
(99.9%) was obtained at all reaction conditions (Figure 1 and
Figure S1, Supporting Information). Amine conversion was
increased from 28.7% at 90 C to 39.4% at 100 C (Figure 1a).
Afterwards, amine conversion was significantly increased to
64.2 and 95.6% at 110 and 120 C, respectively. Water, one of
the by-products in benzylamine oxidation (Scheme 1), is
considered to be a key factor for catalyst deactivation by
blocking active sites. When the benzylamine oxidation is carried
out at higher than 100 C, the in-situ generated water would be
mostly in vapor phase state in the autoclave reactor vessel. As a
°
°
°
Afterwards, we explored the catalytic activity of MnOx
(10 wt% metal) deposited on TiO2-NT400, TiO2-NT500, and
commercial TiO2 for benzylamine oxidation at similar reaction
conditions used for pristine TiO2 samples. Among the catalysts
tested, the MnOx/TiO2-NT400 catalyst showed the best perform-
ance with a 95.6% benzylamine conversion (entry 6, Table 1).
°
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