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F. Zhang et al. / Applied Catalysis A: General 531 (2017) 60–68
Fig. 4. The calculated BET surface area and XPS analysis for the as-prepared 12%Cr-PKU-5: (A) BET surface area calculated by N2 adsorption-desorption isotherm; (B) XPS
analysis for Cr 2p binding energy.
was observed. GC–MS was used to identify its molecular struc-
ture; however, no reasonable molecular structure has matched by
searching NIST mass spectrum databases (See Fig. S5 in support-
ing information). Therefore, a further measurement by UPLC–MS
was performed to determine its molecular weight, the analytical
results indicated this organic compound has a molecular weight of
about 192.3 (See Fig. S6 in supporting information). Such a result
has excluded the possibility of cyclohexanone oxime, which is in
generally the main product for the ammoximation of cyclohex-
anone.
Although the accurate molecular structure hasn’t been identi-
fied by means of chromatograph-mass spectrometry technology,
however, a well-crystallized single crystal has been grown after a
careful free-evaporation method at room temperature. A qualified
single crystal was selected for the X-ray diffraction analysis and its
structure was determined. This unknown crystal structure has been
identified as a monoclinic structure (P21/n) with lattice parame-
ters a = 5.1594(2), b = 13.5667(6), c = 7.9305(3) Å and ˇ = 93.834(4)◦.
Two independent cyclohexanone azine molecules (C12H20N2) have
been included in each unit cell, and the hydrogen bonds between
N and H atoms have interlinked each azine molecules to form a
3D framework structure. The structural determination by single-
crystal diffraction gives an identical conclusion with the results
obtained with UPLC–MS measurement, and thus inclusively evi-
dences the formation of cyclohaxanone azine. Detailed structural
and refinement parameters can also be found in Fig. S8 and Tables
1–3 of supporting information.
bly results from the promotion effect of an active intermediate
O* species, which was generated in parallel with the transforma-
tion from CH3CN into acetamide in the presence of H2O2 oxidant
and Cr-PKU-5 catalyst. Actually, as an important byproduct, large
amounts of acetamide have been detected, and the concentration of
The blank experiment also shows that CH3CN has a weak
self-catalytic capacity towards ketone ammoximation without Cr-
PKU-5 catalyst, and the yield of ketazine is very low compared
with using Cr-PKU-5 (Fig. 5A). In some previous literatures, some
authors also believed acetamide has functioned as catalytic species
for ketone ammoximation to azine [33,34]. Based on the above anal-
ysis, a reasonable conclusion can be drawn that the ammoximation
of ketone is probably accelerated cooperatively by an important
acetamide, which was discussed detailed in reaction mechanism
section.
As the catalytic sites, Cr amounts containing in the PKU-5 frame-
work will presumably influence the yield of ketazine. As shown in
Fig. 5B, the yield of ketazine changed largely and was not propor-
tional to the incorporated Cr amount in PKU-5. The highest yield
was achieved over 10%Cr-PKU-5 catalyst. In other words, an excess
of Cr sites has a negative effect on the yield of ketazine probably
due to accelerating the decomposition of H2O2 into O2 and thus
decreasing the catalytic efficiency. It was worth noting that 10%Cr-
PKU-1 can’t afford to suffer from alkali corrosion and thus exhibit
an undesirable catalytic activity.
Our investigation also shows reaction rates were significantly
influenced by reaction temperature (as shown in Fig. 5C). A remark-
able increase in ketazine yield was observed at 313 K compared
to other temperatures. It must be noted that, a further increase
in temperature has a detrimental influence on azine yields. Com-
monly speaking, a higher reaction temperature is usually favorable
to the transformation of reactants. However, the decomposition of
temperatures, thus results into a significant decrease of the yield.
The chemical equivalent of H2O2 or NH3·H2O to substrate also
has a key influence on the yield of ketazine, an appropriate amount
of NH3·H2O or H2O2 will effectively minimize the production of
other by-products (Fig. 5D and E). In the present study, the simi-
lar tendency was obtained both using NH3·H2O and H2O2, i.e. the
yield of ketazine firstly becomes larger with increasing the ini-
tial amount, and then reaches the maximum value at 2.0 equiv. of
NH3·H2O and 1.5 equiv. of H2O2, respectively. Further increasing
3.3. Catalytic activity
In order to find the optimal operational conditions, catalytic
ammoximation reactions have been performed with the differ-
ent parameters such as catalyst types, solvent medium, chemical
equivalent and reaction temperature. Fig. 5 gives the corresponding
results along with the overall reaction scheme.
Under the given reaction conditions, some different sol-
vents including acetonitrile (CH3CN), dimethyl sulfoxide (DMSO),
tetrahydrofuran (THF), ter-butyl alcohol (TBA), pyridine and
methanol were used as reaction medium, the obtained results and
the overall reaction scheme were shown in Fig. 5A. The results show
that CH3CN solvent is the optimal reaction medium and the maxi-
mum yield for azine was acquired; however, it is much surprising
that only traces of azine have been detected in all of solvents except
CH3CN. Based on the comprehensive analysis, it can be reasonably
drawn that such an enormous difference in yield most proba-