An Efficient and Eco-friendly MoO3–SiO2 Solid Acid Catalyst
1819
Table 4 Reuse of MoO3–SiO2 in the nitration of toluene
Entry
Run
Conversion (%)a
Product distribution (%)a
o/p Ratioa
Ortho
Meta
Para
1
2
3
4
5
Fresh
94
91
90
86
85
54.9
53.5
55.8
55.1
54.2
2.3
2.1
3.1
2.6
2.0
42.8
44.4
41.1
42.3
43.8
1.28
1.20
1.36
1.30
1.24
1st run
2nd run
3rd run
4th run
Reaction conditions: toluene (5 mmol), N2O5 (5 mmol), CH2Cl2 (5 mL), 15 mol% MoO3–SiO2 (9.2 mg, 20 wt% to toluene), 1 h at 0°C
a
Calculated from original GC data
(Table 3, entries 1–4). As seen from the table, nitration of
alkyl aromatic compounds gave high conversion, and the
product distribution showed para-selectivity which mainly
due to the steric effect of the substituted group, and no
nitration of the side-chain was observed (Table 3, entries
5–7). Attempts to nitrate deactivated aromatic rings, such
as nitrobenzene, benzoic acid, acetophenone and benzoni-
trile, using N2O5: substrate molar ratio 1:1 at 30°C for 2 h
without any catalyst resulted in recovery of unreacted
starting material. Fortunately, the nitration of deactivated
substrates showed a positive impact of using the MoO3–
SiO2 as the catalyst, although this protocol showed less
efficiency compared with the application for the alkyl and
halogen aromatics nitration.
when the catalyst was used during five cycles which could
be attributed to the slight leaching of the active species
from the surface due to the use of water for the separation
of the catalyst from the reaction mixture.
4 Conclusion
In conclusion, MoO3–SiO2 catalysts prepared by sol–gel
method were used successfully for nitration of simple
aromatic compounds, and an enhanced regionselectivity
was achieved at the nitration progress. Furthermore, the
MoO3–SiO2 catalysts could be easily separated from the
reaction mixture and reused 5 cycles without significant
loss in its activity. In addition, this procedure avoids using
mixed-acid making the process environmentally friendly.
However, future work on increasing the nitration conver-
sion of deactivated aromatics with this mild reaction sys-
tem would be further studied.
As is shown in the table (Table 3, entries 8–11), under
the reaction conditions employed, the conversion of
nitrobenzene, benzoic acid, acetophenone and benzonitrile
were respectively 7, 15, 13 and 11%. However, it is
interesting to note that nitration of compounds containing a
carbonyl group (e.g. benzoic acid and acetophenone) using
this methodology, which produce increased amounts of the
o-isomer. It is known that nitration of such carbonyl
compounds under non-acidic conditions using NO2/O3
(Kyodai Nitration) afford dramatically different ratio of
regioisomers in comparison with e.g. mixed-acid [36], and
the active nitrating agent is thought to be N2O5 which is
formed in situ from NO2 and O3. It has been speculated that
in the absence of acidic protons the carbonyl group is not
protonated leading to o-nitration whilst the protonated
carbonyl group is meta directing.
Acknowledgments We are grateful for the financial support from
the National Nature Science Foundation of China-Academy of
Engineering Physics (No. 10976014) and Nature Science Foundation
of Jiangsu Province (No. BK2011697).
References
1. Schofield K (1980) Aromatic nitration. Cambridge University
Press, Cambridge
2. Olah GA, Malhotra R, Narang SC (1989) Nitration: methods and
mechanisms. VCH, New York
3. Dagade SP, Waghmode SB, Kadam VS, Dongare MK (2002)
Appl Catal A 226:49
4. Dagade SP, Kadam VS, Dongare MK (2002) Catal Commun 3:67
5. Patil PT, Malshe KM, Dagade SP, Dongare MK (2009) Catal
Commun 4:429
6. Kalbasi RJ, Ghiaci M, Massah AR (2009) Appl Catal A 353:1
7. Parida KM, Pattnayak PK (1997) Catal Lett 47:255
8. Yadav GD, Nair JJ (1999) Catal Lett 62:49
9. Brei VV, Prudius SV, Melezhyk OV (2003) Appl Catal A 239:11
10. Sunajadevi KR, Sugunan S (2005) Catal Commun 6:611
11. Mao W, Ma H, Wang B (2009) J Hazard Mater 167:707
12. Wright OL, Teipel J, Thoennes D (1965) J Org Chem 30:1301
13. Shi M, Cui SC (2002) J Fluoine Chem 113:207
3.3 Catalyst Reusability
The recycling performance of 15 mol% MoO3–SiO2 was
investigated by the nitration of toluene with N2O5 under
the identical conditions (Table 4). In order to regenerate
the catalyst after 1 h reaction time, it was separated by
filtration, washing with water, acetone and diethyl ether,
dried at 120°C for 2 h and reused in next reaction. A
marginal decrease in toluene conversion was observed
123