J. Xu et al.
Molecular Catalysis 503 (2021) 111440
•
acylperoxy radical [26] or NO2 radical [27]. The strongly electrophilic
PINO radical involves the transformations of substrates to the desired
product via H-abstraction [18, 28,29,30], electrophilic addition [19],
etc. to promote the related transformations. It is Ishii and his co-workers
who firstly developed NHPI/Co(II) as catalysts for selective oxidation of
toluene in acetic acid and achieved a high degree of transformation of
toluene at room temperature [31]. However, the predominating product
from toluene in Ishii and coworkers’ researches was benzoic acid and the
selectivity to the desired benzaldehyde was only ca. 3%.
2. Experimental
2.1. Preparation of catalysts
2.1.1. Synthesis of N,N-dihydroxypyromellitimide (NDHPI)
Hydroxylamine hydrochloride (40 mmol) and triethylamine (Et3N,
40 mmol) were dissolved in 120 ml of ethanol. After a thorough stirring
(the solid was dissolved completely), 1,2,4,5-benzenetetracarboxylic
anhydride (PMDA) (20 mmol) was added to the above ethanol solu-
tion. The solution was refluxed at 80 ◦C for 8 h, and then deionized water
(200 mL) was introduced into the solution. The resulting suspend was
filtered and the obtained yellow powder was dried at 50 ◦C in static air.
The synthetic route is shown in Fig. 1 and the prepared yellow powder
was referred to as N,N-dihydroxypyromellitimide (NDHPI).
The newest breakthrough in selective oxidation of toluene to benz-
aldehyde was made by Pappo’s group [32]. They found that the pres-
ence of hexafluoropropan-2-ol (HFIP) markedly enhanced the selectivity
to benzaldehyde even at a high toluene conversion using the homoge-
neous NHPI/Co(II) catalysts. The surprising effect of HFIP on the se-
lective production of benzaldehyde from toluene was attributed to the
formation of intermolecular hydrogen bonds between HFIP and the
aimed benzaldehyde based on the NMR analysis and theroretical
calculation [32]. It was found that the hydrogen bonds formed enhanced
2.1.2. Synthesis of NHPI-GPTMS-SiO2
The synthesized sample NDHPI (4 mmol) and 3-(glycidoxypropyl)
trimethoxysilane (GPTMS) (12 mmol) were added into ethyl acetate
(120 mL) under a strict stirring. The reaction mixture was refluxed for 24
h at 78 ◦C in the atmosphere of nitrogen. Then, the solvent was removed
by rotary evaporation and the solid was washed successively with
ethanol and dichloromethane. The synthetic route is shown in Fig. 2 and
the intermediate synthesized was referred to as NHPI-GPTMS.
The synthesized intermediate NHPI-GPTMS (1 mmol) and commer-
cial SiO2 (silica gel for column chromatography, 1 g) was dispersed in
toluene under a strict stirring. The prepared suspend was refluxed at 100
◦C for 24 h in the atmosphere of nitrogen. Then, the resulting suspend
was filtered and the resulting solid was washed using a mixed solution of
diethyl ether and chloroform (1 : 1). The synthetic route is shown in
Fig. 3 and the catalyst synthesized was referred to as NHPI-GPTMS-SiO2.
–
the bond dissociation energy (BDE) of aldehydic C H bond and thus
inhibited its successive cleavage and the formation of the over-oxidized
product benzoic acid.
The combination of NHPI/Co(II) with HFIP was praised as “a sur-
prisingly simple and remarkably elegant solution to the partial oxidation
of toluene to benzaldehyde” [33]. However, a difficulty in the separa-
tion, recovery and reuse of the homogeneous NHPI and cobaltous ions
will be encountered when the route is considered for possible industrial
production [34]. Moreover, NHPI is easily decomposed under reaction
conditions and the resulting trace of by-products will lead to a
contamination of the desired products and thus more work has to be
devoted to their purification [16]. A reliable immoblization of the ho-
mogeneous catalyst NHPI will facilitate its separation, recovery and
reuse and inhibit its decomposition to some extent. Thus, the remaining
question is how to attach the NHPI molecules to the suitable supports
with an excellent textural properties and to realize a high catalytic ac-
tivity for direct oxidation of toluene to benzaldehyde using molecular
oxygen.
2.1.3. Synthesis of SiO2-GPTMS-NHPI
The SiO2-GPTMS-NHPI catalyst was synthesized at the same reaction
conditions, but an opposite order adopted in comparison with the
catalyst NHPI-GPTMS-SiO2.
Firstly, GPTMS was attached onto the surface of the commercial SiO2
by a condensation reaction between the silanol of the substrate and the
methoxyl of GPTMS, and the prepared intermediate was referred to as
SiO2-GPTMS (Fig. 4).
Impregnation is a convenient method to prepare the immobilized
organic catalysts, while it often suffers from a fatal defect: the leaching
of the active components. Hermans et al. tried the immobilization of
NHPI on silica support by impregnation [35], and the obtained catalysts
were applied to the solvent-free oxidation of cyclohexane. The immo-
bilized NHPI was well adsorbed on support and there no apparent
leaching observed under reaction conditions due to a low polarity of
cyclohexane. Therefore, the prepared NHPI/SiO2 catalysts exhibited
high and steady catalytic activity for the selective oxidation of cyclo-
hexane. However, similar results were not observed when toluene was
used as the raw material for catalytic transformation on the anchored
NHPI/SiO2 catalysts prepared by impregnation [36]. As reported by
Zhou et al., the immobilized NHPI/SBA-15 catalyst prepared by
impregnation presented a sharp decrease in catalytic activity when it
was reused in the toluene oxidation [36]. Covalent anchoring was
believed more reliable for the immobilization of organic catalysts [37].
It was found that the immobilized NHPI catalysts synthesized by the
binding of amide or ester bonds possessed repeatable catalytic activity in
some systems [37,38,39], but the grafting bonds were easily hydrolyzed
in the presence of acids [40,41].
Then, the covalent attachment proceeded between the synthesized
intermediate SiO2-GPTMS and the prepared NDHPI by the formation of
– –
O N bond. The resulting catalyst was referred to as SiO2-GPTMS-
a C
NHPI (Fig. 5).
2.2. Characterization of catalysts
FT-IR spectra were recorded on a TENSOR 27 spectrometer using a
self-supporting wafer diluted by KBr with a sample concentration of 1%.
1H NMR and/or 13C NMR spectra of the intermediate GPTMS-NHPI and
the catalysts SiO2-GPTMS-NHPI and NHPI-GPTMS-SiO2 were measured
at room temperature on a solid NMR spectrometer (Bruker, Avance III,
400 W Hz). LC–MS (Therm, LCQ Deca XP MAX) was adopted to analyze
the reaction solution and the purity of the synthesized NDHPI. Elemental
analyses were performed on a Vario EL Cube element analyzer. Ther-
mogravimetric analyses were carried out in a temperature range 30–800
◦C with a scan rate of 10 ◦C/min under a N2 atmosphere. The grafting
densities of the synthesized catalysts were determined by their weight
loss at a temperature range from 240 to 800 ◦C, which is shown in Eqn.
1. Low temperature N2 adsorption-desorption was operated at 77 K on a
Micromeritics ASAP 2010 micropore analysis system. XPS measure-
ments were performed on an ESCALAB electron spectrometer.
In this work, immobilized NHPI/SiO2 catalysts were prepared by
– –
O N bonds, and the resulting catalysts
covalent grafting in term of C
were characterized to analyze their structural and textural properties,
and further used to catalyze the direct oxidation of toluene to benzal-
dehyde in HFIP. The properties and catalytic performance of the cata-
lysts were discovered, and the possible mechanism of selective oxidation
of toluene to benzaldehyde was probed.
D = (L × 1000)/Mw
(1)
D: Grafting density (mmol/gcatalyst
)
L: Weight loss from 240 to 800 ◦C (wt. %)
Mw: Relative molecular weight of the grafted organic moiety (392)
2