Cr-doped magnetite for the catalytic radical amination of C(sp3)–H in toluene with NH3 and H2O2

Article abstract of DOI:10.1007/s00706-018-2140-z

Abstract: The direct catalytic amination of C(sp³)–H is difficult to achieve but significant. The compound of Fe_2.35Cr_0.65O₄, prepared by the oxidation–precipitation method, was characterized by X-ray diffractio

Full text of DOI:10.1007/s00706-018-2140-z

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ORIGINAL PAPER
Cr-doped magnetite for the catalytic radical amination of C(sp3)–H
in toluene with NH3 and H2O2
Chuan-Jun Yue1 Xu-Dong Hu1 Li-Ping Gu1 Bao-Liang Liu1
•
•
•
Received: 6 April 2017 / Accepted: 1 January 2018
Ó Springer-Verlag GmbH Austria, part of Springer Nature 2018
Abstract
The direct catalytic amination of C(sp3)–H is difficult to achieve but significant. The compound of Fe2.35Cr0.65O4, prepared
by the oxidation–precipitation method, was characterized by X-ray diffraction, scanning electron microscopy, Brunauer–
Emmett–Teller, X-ray photoelectron spectroscopy, and inductively coupled plasma spectrometry, and used to catalyze the
decomposition of hydrogen peroxide to hydroxyl-free radical, which sequentially reacted with ammonia and toluene to
produce benzylamine. Results showed that Fe2.35Cr0.65O4 with high Cr content, particularly the outside surface, could well
better catalyze hydrogen peroxide to hydroxyl-free radical and its transformation. Under the conditions of pH\ 6, 30 °C,
and the NH3 amount, maintaining the match of one hydroxyl-free radical one by the process control, toluene in the other
phase of the reaction system was converted into benzylamine with 30% yield. The catalytic radical amination of C(sp3)–H
in toluene was achieved without any oriented group.
Graphical abstract
FeSO4 + CrCl3
Oxidation-precipitation
Cr-Fe3O4
NH2
NH3 + H2O2
Keywords C(sp3)–H activation Á Radicals Á Cr-doped magnetite Á Catalytic amination Á Metals Á Heterogeneous catalysis
Introduction
bonds, which broadens the synthetic approach to organic
synthesis. Especially for the construction of C–N bond in a
practical molecule, it is well known that the C–N bond
exists in the molecular structure of drugs and natural
products with biological activity [4, 5]. Some of which
were synthesized by the amination of C–H bond [6, 7].
Thus, the C–N bond formation from the C–H functional-
ization has become one of the most important methods
derived from the potential of green sustainable develop-
ment. Research showed that many methods were adapted to
the different types of C–H bond activation including
C(sp)–H, C(sp2)–H, and C(sp3)–H in substrate, focusing on
In recent decades, the functionalization of catalytic C–H
bond activation in organic molecule has attracted more and
more attention [1–3], because the C–H functionalization
can result in the formation of C–O, C–S, C–N, and C–F
& Chuan-Jun Yue
ywangjung@163.com
1
Department of Chemical Engineering, School of
Mathematics and Chemical Engineering, Changzhou Institute
of Technology, Changzhou, China
123

C.-J. Yue et al.
the catalytic activity and selectivity [8]. For C(sp)–H and
C(sp2)–H activation, the former was easily practicable like
the typical Sonogashira reaction and the amination by the
catalysts of Pd and Rh [9–11]. The latter was processing for
the development of catalytic amination system containing
the catalysts of Cu, Rh, Ag, Fe, and Co [12–16], rendering
the comprehensive review [17]. Moreover, for the C(sp3)–
H activation, report is creasing in Fe, Pd, and Ni catalytic
system [18–20]. However, many difficulties are still wait-
ing for solution. One of those is the limit of activated
substrate structure, always needing to assist the C–H bond
activation with the oriented groups in substrate or reagent,
such as the ortho-oriented group of aromatic nucleus [21],
specific reagents such as the guidance of organic azides
[22] and one amino group [23]. Fortunately, recent studies
have developed few systems for the direct catalytic func-
tionalization of C(sp3)–H bond under the conditions of the
non-oriented group covering the catalytic system of Pd for
the C–C coupling in benzyl [24] and Cu for the amination
of heteroaromatics [25]. However, no reports are available
using cheap inorganic ammonia as nitrogen resource to
construct C–N bond by the catalytic amination of C(sp3)–H
bond. The reason is attributed to both the nonpolar C(sp3)–
H bond and weak nucleophilicity in ammonia compared
with organic amine, causing difficult reaction between
both. As mentioned previously, the traditional method of
C–N formation from the beginning of alkane is always
performed through several step reactions, presenting the
low overall yield. Recently, the free radical activation of
C–H bond by the catalysts of Fe, Ni, and Co presents a new
highlight in this area [26–28]. Therefore, the reaction
between C(sp3)–H and ammonia by the free radical cou-
pling becomes possible. This study reports the Cr-doped
magnetite for the catalytic decomposition of H2O2 to
induce the free radical reaction towards C(sp3)–H
amination.
proved in the following description. According to the
above data, x value in formula of Fe3-xCrxO4 was
0.65. This value was the molecular composition of
Fe2.35Cr0.65O4. In contrast to that in the literature [29], the
high content of Cr in the catalyst was successfully obtained
in this study. This outcome was conductive to the
decomposition of hydrogen peroxide to hydroxyl radical on
account of the amount of hydroxyl radical with the relevant
Cr content in the catalyst [29]. However, the high Cr
content went against the stability of original Fe3O4 lattice.
Therefore, it was calculated that some Cr probably was
enriched in the surface of the catalyst, which was helpful to
the homolysis of hydrogen peroxide to hydroxyl radical.
For comparison, Fe3O4 powder was prepared by the
same method as Fe2.35Cr0.65O4. The X-ray diffraction
(XRD) results are shown in Fig. 1, and they both exhibited
some peaks at the double theta degree including small
angle diffraction of 18.2°, 30.0°, 35.4°, 43.2°, 53.5°, 57.0°,
62.7°, and 74.3°, well indexed to Fe3O4 crystalline
(JCPDSI-1111). Moreover, some of those peaks slightly
became broad, presenting that partial particles became
amorphous, probably deriving from Cr enriched in the
surface of Fe3O4 lattice. The broadening of the XRD peak
occurs at 35.6° after Cr hybridized Fe3O4, revealing the
incorporation of Cr into the Fe3O4 structure, that is Cr
doped the Fe3O4. Some Fe was substituted by Cr in the
Fe3O4 lattice to a certain extent [29]. Nevertheless, the
original Fe3O4 lattice was still maintained; in other words,
new crystal lattice was not found. However, the surface
nature of Fe2.35Cr0.65O4 covering surface area and elec-
tronic property should be changed because of the different
atomic radius between Cr and Fe. This change was not
unlimited; otherwise, it would have led to the destruction
of the original lattice. In fact, the change from Cr into the
Fe3O4 lattice may improve the catalytic performance for
H2O2 decomposition. In addition, the only diffraction peak
at 31.7° was attributed to NaCl crystal. This result was
Results and discussion
Catalyst texture property
Fe2.35Cr0.65O4
NaCl
The compound obtained from the above-mentioned
preparation process was of the Fe3-xCrxO4 formula. Here,
the composition of the catalyst was determined by chemi-
cal and spectral methods. The results from inductively
coupled plasma spectrometry (ICP) analysis showed that
the content of Fe and Cr was 57.1 and 14.6% in the catalyst
sample, respectively. This value was consistent with that of
the chemical analysis by titration. Moreover, the trace
chloride, attributed to sodium chloride adsorbed in the
catalyst surface, was qualitatively determined based on the
chemical titration analysis. This result would be also
Fe3O4
10
20
30
40
50
60
70
80
2θ/°
Fig. 1 XRD patterns for Fe3O4 and Fe2.35Cr0.65O4
123

Cr-doped magnetite for the catalytic radical amination of C(sp3)–H in toluene with NH3 and H2O2
consistent with the outcome on the composition analysis
mentioned above.
g similar to that of Langmuir with the same trend. The
average pore size also reduced to 2.8 nm at approximately
14-fold decrease, suggesting that the pore volume and the
dispersion of the particles enhanced, which was consistent
with the SEM results. As revealed and mutually confirmed
by XRD, the texture properties of the catalyst were
improved once Cr entered and modified the Fe3O4. Those
properties most presumably resulted from the high Cr
content and more distributions in the outside surface of
Fe2.35Cr0.65O4.
As shown in Fig. 2, the scanning electron microscopy
(SEM) images of Fe2.35Cr0.65O4 reflected its morphology
and microstructure. From the Fig. 2a, the particles of the
catalyst showed uniform distribution with nearly uniform
size of approximately 15 lm diameters, attributing to the
excellent preparation process. Inevitably, large particles
also existed in local region as shown in Fig. 2b, wherein
many small particles were close but with no agglomeration
trend. In other words, the particles blossom like cotton,
further indicating the loosely folded porous texture as
shown in Fig. 2c, d, that obviously increased the surface
area of the catalyst. The dispersion uniformity of the cat-
alyst was prior to that of Fe3O4 prepared by another method
[31] and Fe3O4 containing the distinguish Cr content [30],
which was closely related to the strictly conditional control
during the preparation process. The catalyst with the cotton
feature texture enhanced the surface for strengthening the
catalytic decomposition of hydrogen peroxide.
Further investigation on the composition and element
chemical states of Fe2.35Cr0.65O4 was carried out by X-ray
photoelectron spectroscopy (XPS). The obtained curves are
presented in Fig. 3. As shown in Fig. 3a, the full spectra of
Fe2.35Cr0.65O4 clearly showed that the catalyst contained
Fe, Cr, O, Na, and Cl elements, corresponding to the
individual peaks at 709, 588, 524, 1075, and 199 eV,
respectively. From the Fig. 3b, c, the related peaks at 587.1
and 577.3 eV displayed the presence of Cr3? with the Cr
2p3/2 core level, typically being attributed to trivalent
chromium [32]. The Fe 2p3/2 at binding energy between
Brunauer–Emmett–Teller (BET) gas sorptometry was
conducted to examine the porous nature of the
Fe2.35Cr0.65O4 and Fe3O4. Table 1 shows the results from
the N2 adsorption/desorption isotherm analysis, and the
porous difference from both was obviously observed. In the
relative pressure range, Fe2.35Cr0.65O4 generally exhibited
a larger adsorption amount of nitrogen than that of Fe3O4.
This result indicates the increase of surface area from
Fe3O4 to Fe2.35Cr0.65O4 and inferring the texture change
after Cr-doped Fe3O4. Compared with Fe3O4, the surface
area of Fe2.35Cr0.65O4 BET increased to 89.3 from 32.2 m2/
725.9 and 715.2 eV was assigned to Fe2? and Fe3?
,
respectively. This result occurred in the Fe2.35Cr0.65O4,
presenting the Fe2? predominance based on the reported
method in the literature [33]. In addition, the XPS fig-
ures approximately gave a Cr/Fe ratio of 1.16 in the cata-
lyst surface, which was much higher than that of 0.28 in
Fe2.35Cr0.65O4 bulk phase. This result demonstrates the
location of high Cr content in Fe2.35Cr0.65O4, and con-
firmed the result obtained in XRD. In other words, many Cr
was enriched in the surface of Fe2.35Cr0.65O4. This finding
meant that high Cr content provided a structural foundation
for the catalytic decomposition of hydrogen peroxide to
hydroxyl radical.
Catalytic reactions
The C(sp3)–H bond is of low bond energy and weak
polarity compared with the C(sp2)–H in toluene. Therefore,
it was very difficult to implement the selective cleavage of
three C(sp3)–H bonds on methyl by the general transfor-
mation method based on the polarity of chemical bond.
Naturally, the radical transformation of the C(sp3)–H bond
was an alternative. This method was an ideal way to obtain
benzylamine from directly reacting between toluene and
ammonia with two hydrogen atoms taken off, which was
really called green process. In fact, to directly remove them
was almost impossible to date. Thus, the catalytic con-
version of toluene and ammonia into benzylamine was
chosen to be performed indirectly. Amino radical was
rapidly produced by the reaction of ammonia and hydroxyl
radical [34], hydroxyl radical also selectively collided with
toluene toward benzylamine product. However,
Fig. 2 SEM images of Fe2.35Cr0.65O4
123

C.-J. Yue et al.
Table 1 Texture properties of
Fe3O4 and Fe2.35Cr0.65O4
Sample
Surface area/m2/g
Average pore volume/cm3/g
Average pore size/nm
BET
Langmuir
Fe3O4
32.2
89.3
49.3
0.14
0.49
39.7
2.8
Fe2.35Cr0.65O4
173.6
experiments showed that the control of the reaction con-
ditions was crucial, especially the pH value and the ratio of
hydrogen peroxide and ammonia in the reaction system if
the desired product was obtained.
(a)
12
10
Na1s
8
6
4
2
0
The temperature was a variable at the range of wide
scope for the catalytic reaction among the generally opti-
mized parameters. The optimal results were listed in
Table 2. The yield of benzylamine obtained by
Fe2.35Cr0.65O4 catalysis to toluene was higher than that in
Fe3O4 under the equal conditions, which was 30 vs 8% in
30 °C. The reason was that a premise of the reaction to
proceed was a hydroxyl radical. However, with the
increase of temperature, the yield remarkably decreased.
Unfortunately, the yield decreased to trace amount when
the temperature increased to 60 °C because hydroxyl rad-
ical was of high energy with activity, probably resulting in
rapid annihilation at a great extent before it entered the
toluene phase. As the reaction producing amino radical was
feasible considering the energy [35], any products was
hardly observed besides benzylamine based on GC–MS
analysis. It was meant that the catalytic selectivity may be
improved, due to the catalytic process of free radical relay
and the produced free radical of benzylamine with rela-
tively stability based on its structure analysis to avoid its
further transformation. Although the yield of catalytic
toluene to benzylamine was low, the catalytic system
presented the direct amination of C(sp3)–H bond with no
other group oriented, as well as with the absence of noble
metal.
Fe2p
Cr2p
O1s
NaKII
Cl2p
1200
1000
800
600
B.E./eV
400
200
0
(b)
250
240
230
220
210
200
190
180
170
Cr2p3/2
600 595 590 585 580 575 570 565 560
B.E./eV
(c)
In view of the results above and reported in the litera-
tures [29, 36, 37], the proposed process of the catalytic
amination is described in Fig. 4. Fe(II) in the catalyst
catalyzed the decomposition of hydrogen peroxide to
hydroxyl radical, and was oxidized to Fe(III) at the same
time. Then, hydroxyl radical was involved in both reactions
with ammonia to amino radical and the reduction of Fe(III)
to Fe(II). Chromium assisted the catalytic process with the
change of the valence state during the period [26]. More-
over, as reported in the literature [30], Cr in Fe2.35Cr0.65O4
played a role in stabilizing the generation of hydroxyl
radical. Amino radical then quickly entered the toluene
phase. Subsequently, the reaction occurred between amino
radical and toluene to produce benzylamine. Obviously, the
catalytic process was compositive. The reaction path was
also dependent on the reaction conditions. To make the
catalytic reaction successful, the interferential factors
46
44
42
40
38
36
34
32
30
28
Fe2p1/2
Fe2p3/2
740 735 730 725 720 715 710 705 700
B.E./eV
Fig. 3 a XPS survey spectrum of Fe2.35Cr0.65O4, b Fe2p photoelec-
tron spectra from Fe2.35Cr0.65O4, c Cr2p photoelectron spectra from
Fe2.35Cr0.65O4
123

Cr-doped magnetite for the catalytic radical amination of C(sp3)–H in toluene with NH3 and H2O2
Table 2 Catalytic performance of toluene amination with H2O2 and NH3
Reaction conditions: 0.2 g catalyst, 5 cm3 toluene, 10 cm3 30% H2O2, 5 cm3 28% NH3, 15 min, pH \ 6 with the relation to the dropping
velocity of ammonia
traditional multistep production. Moreover, prior to the
activation of C(sp3)–H with the oriented group by the
catalyst containing the precious metal in any aspects, the
method here was of the academic significant and exhibits
potential application for industry. As the catalytic reaction
system was compositive, it is necessary to further study
that the improved design of catalytic system and the opti-
mization of multiphase reaction process.
Experimental
Fig. 4 Proposed process for the catalytic amination of toluene by
Fe2.35Cr0.65O4
All the used reagents were analytical grade and without
further purification before use, and the purity of nitrogen
should be kept away as much as possible. Oxygen from the
was 99.999%.
reaction was also generated, which mainly influenced the
performance of Fe catalysis. Thus, oxygen must be
Preparation and characterization of catalyst
immediately removed by the nitrogen flow. In addition, the
decreasing ammonia with alkalinity must instantaneously
The improved catalyst was prepared by oxidation–precip-
react with hydroxyl radical in the acid system. Otherwise,
itation method [29, 30] as follows: 6.5 g FeSO4Á7H2O and
the desired reaction would impossibly occur. Moreover,
1.7 g CrCl3Á6H2O were slowly dissolved in 30 cm3 dilute
kinetic study on the catalytic system should also be per-
HCl solution under a stream of nitrogen, and a small drop
formed to understand clearly the reaction process.
of hydrazine was added. Then the 37% concentrated
hydrochloric acid was quantificationally introduced to
adjust pH value below 2 in the solution. The solution was
Conclusion
heated to 90 °C with stirring for 3 h. Subsequently, the
35 cm3 mixed solution of 4 mol/dm3 NaOH and 0.90 mol/
Fe2.35Cr0.65O4 with high Cr content and enriched in the
dm3 NaNO3 with equal volume was dropwise added to the
surface was obtained and catalyzed the tandem reactions in
hot solution with vigorous stirring. After half an hour, the
this study. The catalyst can well decompose catalytic
solution was naturally cooled to room temperature. Finally,
hydrogen peroxide into hydroxyl radical. The latter also
the particle was collected by centrifugal separation, washed
quickly forced ammonia to produce amino radical, which
directly reacted with C(sp3)–H in toluene to obtain ben-
with distilled water, and dried in a vacuum; up to 2.4 g of
gray powder was obtained. At the same time, the Fe3O4
C–N bond formation via the transformation of C(sp3)–H
powder was also prepared using the same method without
zylamine product. In matching the reaction conditions, the
CrCl3Á6H2O.
and H2N–H in one pot was achieved. The yield of the
The catalysts were characterized by the undermentioned
product varied with temperature in the special conditions.
technologies. The chemical composition of the catalyst was
Although the best yield of 30% was low, the process was
obtained by chemical analysis, and the contents of Fe and
green without any organic waste compared with that in the
123

C.-J. Yue et al.
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Procedure for catalytic radical amination
of toluene
The catalytic amination of toluene was performed in a
100 cm3 three flasks using a magnetic stirrer under nitro-
gen atmosphere. A typical test was presented below, and
0.2 g of the catalyst and 5 cm3 toluene were added to the
flask with a stirrer. Then, 10 cm3 hydrochloric acid solu-
tion with a pH value of 3–4 was slowly introduced with
stirring. Afterwards, the mixed liquid was heated to the
desired temperature. The reaction was in process when
10 cm3 30% H2O2 solution and 5 cm3 28% ammonia
solution were timely dropped, respectively, maintaining
slightly the pH value of less than 6. The reaction was fin-
ished when H2O2 was used up. After cooling, the alkali
solution was added to obtain the liquid alkalinity. After the
complete stratification, a drop of the upper liquid was taken
for gas chromatography-mass spectrometry (GC–MS)
(GCQ, Thermo Finnigan Company). The remainder of
upper liquid was treated with hydrochloric acid, and
extracted by water to obtain the product solution. Then, the
solution was washed enough using NaOH liquid. Finally,
the procedure was followed by concentration, filtration,
and drying to obtain pure benzylamine product with 30%
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