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Table 4 (Continued)
Product distribution (%)
Entry
15
Substrate
2/6-
3/5-
4-
Selectivity (ortho : para)
Conv. (%)
12.3
8.8
90.2
1.0
0.1d
a
b
c
Aromatic (0.08mol), 67% nitric acid (1.05 equiv.), Si–PW 1.0 g, 50 uC, 3 h. Ratio of 3-/4-. Some o- and m- xylene is over-nitrated or
d
oxidized Ratio of 2-/4-.
wt.%, conversion will gradually decease, but no obvious change in
the selectivity is observed. This can be attributed to the formation
of bulk [Bmim]3PW12O40 on the support surface. The surface area
will reduce significantly because of agglomeration and the bulk
itself, which results in fewer opportunities for contact between the
catalysts and substrates.
However, because its concentration is too low and even below the
limits of spectroscopic determination in solution, this strength is
usually defined as the effective concentration of nitric acid or
sulfuric acid in waste acid in industry. Low nitration strength not
only leads to a low conversion but also shows poor performance in
selectivity. Although a high nitration strength improves both
aspects, it brings unwanted byproducts (mainly oxidized products)
at the same time, which means a waste of raw materials and a rise
in economic cost.
The catalytic activity and thermal stability of different
amphiphilic salts were also monitored, which allowed us to
determine the stability of the compound. The effect of thermal
treatment on the amphiphilic salts by thermogravimetric analysis
and IR experiments has been reported by Rao et al.20 In their work,
there was a sharp loss of weight at about 400 uC, which suggests
that the organic part (Bmim cation) begins to decompose. Then,
the complete collapse of the Keggin structure to form a metal
oxide phase was observed at 600 uC in TGA. As can be seen in
Fig. 3, the conversion and selectivity of supported amphiphilic
salts shows only a small drop after calcination at 200 uC. For
Encouraged by the remarkable results obtained under the
above reaction conditions, and in order to show the generality and
scope of this new protocol, we applied this catalyst in the nitration
of various substituted aromatics. The results obtained are
summarized in Table 4. In the nitration of alkylbenzene, the
same phenomenon is observed as in that of toluene: a higher
conversion and para-selectivity than using mixed acid. A little
difference is that the products of the para-position increase
significantly with the steric effect of the substituted group. If there
is another activated group such as –CH3 on the benzene ring, all
the substrates will be nitrated with some of them being over-
nitrated or oxidized. When a nitro-group is introduced, the
conversion will decrease sharply. The conversions of halobenzene
are lower than that of toluene, since the electron induction caused
by halogen atoms leads to a lower cloud density. The nitration of
chlorobenzene demonstrates that the para position is more active
than the ortho position, affording significantly higher para-
selectivity with a 0.29 ratio of ortho to para. When another
deactivation group is introduced, the conversion became worse,
reaching no more than 6%. For some substituted aromatics that
have no other products, their conversions are improved in this
nitration system more or less. Another praiseworthy advantage of
the system was that mononitro-products were predominant
during the nitration process. Di-nitration and multi-nitration can
only be seen with two highly activated substrates.
32
42
PW12O40 and SiW12O40 structure salts, this drop becomes
more serious when the calcination temperature reaches 300 uC,
while the phenomenon occurs at 400 uC for PMo12O4032. This is
closely related to the thermal stability of their Keggin structure.
Although the Bmim cation does not strongly decompose at 300 uC,
32
42
PW12O40 and SiW12O40 can not be sustained and will lose
their secondary structure at this temperature. This problem will
become more serious with the decomposition of the organic part
32
when calcinated at 400 uC. The PMo12O40 ion suffers both
transformations mentioned above at 400 uC. For all three
supported materials, most structures and the catalytic activity will
be destroyed above 500 uC.
Several reaction factors are optimized, as shown in Table 3. It
can be found that both conversion and selectivity increase with
increasing amount of catalyst, yet this improvement becomes
inconspicuous when the amount is over 1.0 g. A higher
temperature gives a higher conversion. On the other hand, more
byproducts such as dinitrotoluene are also generated. The reaction
time has little influence on the selectivity and slow increments in
conversion are witnessed when it is prolonged to 5 h. The
nitrification strength relates to the concentration of nitronium
The recyclability of the catalytic system was also examined and
the results with SiO2–[Bmim]3PW12O40 as an example are shown in
Fig. 4. The untreated catalyst is directly used in the next step
without purification, and the treated catalyst was heated at 100 uC
for 2 h each time before being used. After four cycles, the
+
ions (NO2 ), which is the real electrophilic species in nitration.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 2197–2202 | 2201