Asian Journal of Chemistry; Vol. 25, No. 6 (2013), 3511-3512
NOTE
Metal Ion-Exchanged Bentonites as Useful Solid Catalysts for Regioselective Nitration of Toluene
1
1
H. ZHANG1,2,*, X. PENG and S. ZHAO
1Department of Chemistry, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, P.R. China
2School of Physics and Chemistry, Henan Polytechnic University, Henan 454003, P.R. China
*Corresponding author: Tel/Fax: +86 25 84315520; E-mail: xhpeng@mail.njust.edu.cn
(Received: 20 February 2012;
Accepted: 28 December 2012)
AJC-12630
Metal ion-exchanged bentonites were found to be efficient catalysts in regioselective nitration of toluene when using nitric acid as
nitrating agent. Fifteen metal ion-exchanged bentonites were tested as catalysts toward a para position considerably. Out of a range of
catalysts tried silver-supported bentonite appeared of higher regioselectivity of ortho-para ratio of 0.64 in an acceptable yield of 45 %.
The possible recovery and reuse of catalyst suggest that system would be commercially viable.
Key Words: Bentonite, Toluene, Nitrotoluene, Regioselective nitration.
In recent years many selective aromatics nitrations have
been carried out with the help of solid or solid-supported
catalysts1. Smith and Fry2 have used inorganic solids such as
aluminum or proton exchanged large port modenite to catalyze
nitration reactions of alkylbenzenes utilizing benzoyl nitrate
as the nitrating agent. The reaction is highly para selective
giving 67 % of 4-nitrotoluene isomer for toluene nitration in
almost quantitative yield, but the nitrating agent is inconvenient
and carbon tetrachloride is needed as solvent. Toluene has also
been successfully nitrated with copper nitrate supported on
montmorillonite clay3. Moreover the method affords high para
selectivity only under conditions of high dilution and a long
reaction periods (120 h). Unfortunately, therefore, none of the
methods which exhibit para regioselectivity are attractive for
large-scale application. However, with the progress in surface
science and the improved understanding of heterogeneous
catalysis, the availability of modified solids now still provided
opportunities for clean nitration4-9. It is suggested that the clay
should be a promising catalyst which could meet desirable
'enviro-economics'. This account describes our efforts involving
the use of ion-exchanged bentonite in the field of clean nitration
of toluene.
The metal ion-exchanged bentonite catalysts used for
nitration of toluene were prepared. To 100 mL of a stirred
aqueous solution (l.0 mol/L) of corresponding metal salts in
deionized water, 10 g of Na-based commercial bentonite were
added. Stirring was maintained under reflux temperature for
8 h. The clay suspension was centrifugated and the supernatant
solution was discarded and the washing cycles with deionized
water were repeated until disappearance of anion ions from
metal salt. The collected metal ion exchanged bentonite, after
drying overnight in an oven at l30 ºC, was finely ground to
pass a 100 mesh screen in a mortar. The powdery clay was
calcined for 8 h at 300 ºC prior to use.
Followed by nitration of toluene from the following
procedure, i.e., a mixture of toluene (10.0 mL), acetic anhy-
dride (8.0 mL), carbon tetrachloride (40 mL) and clay catalysts
(2.5 g) was magnetically stirred. Nitric acid (10.0 mL, d =
1.4) was added to carbon tetrachloride (20 mL) solvent existing
in Dean-Stark trap. The mixture was heated at reflux for 24 h.
After cooling, the resulting mixture was filtered and the filtrate
was then washed with 5 % sodium bicarbonate solution. The
organic layer was adjusted to pH 6-7 with deionized water,
subsequently concentrated and analyzed by gas-liquid chroma-
tography.
All of the starting materials were chemical purity grade
or above. The analyses of toluene and its nitro isomers were
carried out by gas-liquid chromatography with a Shimadzu
GC-2014C gas chromatography equipped with a hydrogen
flame ionization detector and with a 30 m × 0.25 mm i.d. OV-
101 glass capillary column with nitrogen as a carrier gas using
n-dodecane as internal standard.
We chose the mononitration of toluene (Scheme-I) using
nitric acid as nitrating agent during development of the method.
The reaction was initiated by acetic anhydride-carbon tetra-
chloride extraction of the nitrating species from the aqueous
phase in a Dean-Stark trap5. Metal ion-exchanged bentonites
were easily prepared from commercial sodium based bentonite