Nitration of aromatic compounds on solid catalysts

Article abstract of DOI:10.1081/SCC-100000197

o-Xylene, phenol and toluene were nitrated with 100% nitric acid on MoO₃/SiO₂, WO₃/SiO₂, TiO₂/SiO₂, and TiO₂-WO₃/SiO₂ systems. Phenol and toluene were nitrated with yields higher than 90%, and the 10% and 15% MoO₃/SiO₂ catalysts were most active in the nitration of o-xylene. The most active catalysts exhibited the para-position selectivity of nitration.

Full text of DOI:10.1081/SCC-100000197

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NITRATION OF AROMATIC COMPOUNDS ON SOLID
CATALYSTS
Tomasz Miłczak a , Jarosław Jacniacki a , Janusz Zawadzki a , Monika Malesa a
Wincenty Skupiński b
&
a Division of Highenergetic Materials, Faculty of Chemistry , Warsaw University of
Technology (Politechnika) , Noakowskiego, 3, Warszawa, 00-664, Poland
b Division of Highenergetic Materials, Faculty of Chemistry , Warsaw University of
Technology (Politechnika) , Noakowskiego, 3, Warszawa, 00-664, Poland
Published online: 16 Aug 2006.
To cite this article: Tomasz Miłczak , Jarosław Jacniacki , Janusz Zawadzki , Monika Malesa & Wincenty Skupiński (2001)
NITRATION OF AROMATIC COMPOUNDS ON SOLID CATALYSTS, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 31:2, 173-187, DOI: 10.1081/SCC-100000197
To link to this article: http://dx.doi.org/10.1081/SCC-100000197
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SYNTHETIC COMMUNICATIONS, 31(2), 173–187 (2001)
NITRATION OF AROMATIC COMPOUNDS
ON SOLID CATALYSTS
✡
✡
Tomasz Milczak, Jaroslaw Jacniacki, Janusz Zawadzki,
Monika Malesa, and Wincenty Skupin´ski∗
Division of Highenergetic Materials, Faculty of Chemistry,
Warsaw University of Technology (Politechnika),
Noakowskiego 3, 00-664 Warszawa, Poland
ABSTRACT
o-Xylene, phenol and toluene were nitrated with 100% nitric
acid on MoO3/SiO2, WO3/SiO2, TiO2/SiO2, and TiO2–WO3/SiO2
systems. Phenol and toluene were nitrated with yields higher than
90%, and the 10% and 15% MoO3/SiO2 catalysts were most active
in the nitration of o-xylene. The most active catalysts exhibited the
para-position selectivity of nitration.
INTRODUCTION
The idea of the use of the “solid” nitric acid and a “solid” sulphuric acid in
nitration reactionshasitsrootsin theattemptsto obtainsystems in whichseparation
of the product from the catalyst is easy so that the product is free of the catalyst.
Moreover, in the case of substituted phenyl rings, the para substitution should be
a preferred course of the nitration reaction in these systems. Examples of such
∗ To whom correspondence should be addressed.
173
Copyright ꢀ 2001 by Marcel Dekker, Inc.
www.dekker.com
C

✡
174
MILCZAK ET AL.
systems are in particular 65% HNO3/SiO2 (1) and H2SO4/SiO2 (2,3) systems.
Here silica gel is a neutral (pKa = 7) if we consider the acid/basic properties—it
doesn’t exhibit strong acidic or basic sites (4) and should not activate the acids
deposited. It is of interest therefore to modify the solid support so as to make the
acid centres formed capable of activating nitric acid, for example, just as it does
the sulphuric acid in the nitrating mixture.
According to the literature data, such modifications that yield acid centres
on the silica gel surface involve doping of silica with titanium, molybdenum, or
tungsten ions (5–15). Such systems were obtained in our studies in the nitration of
toluene, o-xylene, and phenol using 100% HNO3 and 65% HNO3. Results of the
studies on these reactions are presented in this paper.
EXPERIMENTAL
Preparation of Catalysts
Preparation of Silica
The solid support for the catalyst studied is silica gel SiO2 (supplied by
Matwy). Grain size range: 0.5–0.2 mm and 1.0–0.5 mm. Silica was purified by a
c
repeated extraction by boiling in concentrated hydrochloric acid to remove trace
metals, followed by repeated extraction with boiling water to a pH ∼7. The silica
thus prepared was dried for 24 h in an oven at 110◦C.
Preparation of Catalysts (Examples)
HNO3/SiO2 catalyst was prepared thus (1):
60 g SiO2 is mixed with 140 mL of 8M HNO3 for 2 h at room temperature.
On filtering it is dried in the open air. The HNO3 content is determined by titration
of the filtrate with an 0.1M NaOH. The amount of the nitric acid adsorbed on
silica equals the difference between the amount of the acid used and that of the
acid present in the filtrate.
15% TiO2/SiO2
19.9 mL 15% TiCl3 solution in 10% HCl (Merck) is applied to 8.5 g SiO2.
It is then dried in a dryer at 110◦C for 2 h, then calcined at 600◦C in an air stream
for 18 h to obtain TiO2 on the surface of the carrier.

AROMATIC COMPOUNDS
15% WO3/SiO2
175
6.23 g (NH4)2 WO4 ∗ 8H2O (pure-POCh Gliwice) is dissolved in 15 mL 3%
H2O2 at room temperature. The solution thus prepared is applied to 8.5 g SiO2 and
dried at 110◦C for 24 h in a dryer. It is then calcined at 600◦C in a stream of air
for 18 h to obtain WO3 on the carrier surface.
15% V2O5/SiO2
1.92 g NH4VO3 (pure-POCh Gliwice) is dissolved in 15 mL 3% H2O2 at
room temperature. The resultant solution is applied to 8.5 g SiO2 and, on drying at
110◦C for 24 h in a dryer, it is calcined at 250◦C for 18 h to obtain V2O5 deposited
on the carrier surface.
15% MoO3/SiO2
1.83 g (NH4)6Mo7O24*4H2O (pure-POCh Gliwice) is dissolved in 15 mL
3% H2O2 at room temperature. The solution thus prepared is applied to 8.5 g SiO2
and, on drying at 110◦C for 24 h in a dryer, it is calcined at 550◦C for 18 h to
obtain MoO3 deposited on the carrier surface.
15% TiO2–WO3/SiO2
0.83 g (NH4)2WO4*8H2O (pure-POCh Gliwice) is dissolved in 15 mL 3%
H2O2 at room temperature. The solution thus prepared is applied to 9.8 g of the
previously prepared catalyst 15% TiO2/SiO2 and dried at 110◦C for 24 h in a dryer.
It is then calcined at 600◦C in a stream of air for 18 h to obtain WO3 on the carrier
surface.
Physicochemical Studies of the Catalysts
Determination of Acidity of the Catalysts Prepared
Hirschler indicators were used to determine the acidity of the catalysts pre-
pared, which are employed to determine selectively the strength of the Bro¨nsted
type (proton) acid centres and are correlated with the acid strength corresponding
to a given concentration of sulphuric acid. This allows us to assign an analogous
nitrating strength to the system H2SO4/HNO3 and solid acid/HNO3, when both
acids, the solid acid and the sulphuric acid of a given concentration, cause the color
of an indicator to change.
The results obtained indicate that all the catalysts studied have Bro¨ensted
centres of an HR strength corresponding to a 50% H2SO4 concentration. Only the

✡
176
MILCZAK ET AL.
Table 1. Properties of the Catalysts Used
Color
Corresponded H2SO4
Hirschler Indicator∗ h
pKa
Concentration
Acid Form
Basic Form
Benzyl alcohol
Diphenylcarbinol
Triphenylcarbinol
−18.1
−13.3
−6.63
93
77
50
yellow
yellow
yellow
colorless
colorless
colorless
∗The indicators used were in the form of 0.1% solutions in toluene (POCh Gliwice).
catalysts 10% and 15% MoO3/SiO2 have the centres of a higher acidity strength
HR corresponding to an acidity strength of a 77% H2SO4.
Determination of the HNO3 Concentration in CCl4 Solutions
over the Catalysts Studied
The HNO3 concentration in CCl4 solutions over the catalysts examined
were studied for the amounts of the solvent, catalyst, and the nitric acid used in the
nitration study by adding the acid to the carbon tetrachloride over the catalyst. After
5 min the solvent was filtered off, which was then extracted with water (10 mL).
The presence of acids in the extract was detected against phenolphthalein.
The study performed for all the catalysts prepared showed no acids present
in the extract.
Nitration Reaction
The nitration reactions were carried out on the basis of the literature data
(16) initially under water-free conditions.
o-Xylene and Toluene Nitration
Immediately before the reaction the catalyst was heated for 1.5 h in a stream
of air at 250◦C and subsequently cooled in dry argon.
Into a 100-mL three-neck flask equipped with a magnetic stirrer and a drop-
ping funnel are introduced 10 mL of dry CCl4, 10 g of the catalyst, and 0.43 g
(6.8 mmol) of 100% HNO3. In the dropping funnel o-xylene (0.42 g–4.0 mmol)
or toluene (0.37 g–4.0 mmol) and CCl4 (17 mL) are placed. On the dropwise

AROMATIC COMPOUNDS
177
addition of the funnel content (for ca. 10 min), the reaction is carried out at room
temperature for 2.5 h. Then the catalyst is filtered off and washed with some CCl4,
which was added to the filtrate. The CCl4 solution was washed with a few por-
tions of aqueous NaHCO3, then several times with water, dried with MgSO4, and
examined by GLC.
Phenol Nitration
The catalyst is introduced into 8M HNO3 at a weight ratio of 3:7 and stirred
for 2 h at room temperature. On filtration the catalyst is dried in the air. The content
of the nitric acid adsorbed on silica is determined by titration of the filtrate with
an NaOH solution.
The acid used in the reaction is taken in a 110% excess of the theoretical
amount needed.
The catalyst (1 g) with the adsorbed HNO3 and CH2Cl2 (45 mL) were placed
in the flask. Then 0.18 g (2.0 mmol) phenol was added. The reaction is allowed to
proceed for 3 min at room temperature. On completion of the reaction the catalyst
is filtered off and washed with a little CH2Cl2, which was added to the filtrate. The
reaction products are analyzed by GLC.
Other reaction conditions are presented as a footnote of the tables.
Analysis of the Reaction Products
For the determination of the conversion of substrates and composition of the
nitration products, gas liquid partition chromatograph was used.
In the analysis of nitro-o-xylenes and nitrophenols, we used a CHROM-5
gas chromatograph (with a CI-100 computing integrator) and a chromatographic
column 10% OV-101/chromosorb PAW DMCS.
In the analysis of mono- and dinitrotoluenes, we used a Perkin-Elmer Auto
System XL instrument and a PE-5 chromatographic column.
Results
o-Xylene Nitration
The nitration of o-xylene using a 100% nitric acid was carried out with or
without catalytic systems of a content of 5, 10, and 15% by wt. of individual metal
oxides, i.e. MoO3, WO3, TiO2, and V2O5 on silica. The TiO2/SiO2 system with
its variety enriched with 2% by wt. of WO3 was examined.

✡
178
MILCZAK ET AL.
The nitration reaction occurring in the presence of HNO3 alone affords the
nitro-derivatives in an 80% yield. By isomers, the yield is slightly higher for
3-nitro-o-xylene (52%), whereas for 4-nitro-o-xylene, it is 48%.
The use of the MoO3/SiO2 catalyst increases the reaction yield, which in-
creases with the molybdenum trioxide on silica (15% MoO3–98% yield). The
catalyst also affects the formation of the 4-nitro-o-xylene derivative in an over-
whelming amount.
The WO3/SiO2 system acts more as an inhibitor in the o-xylene nitration
reaction. As little as 5% of WO3 reduces the nitration yield, compared with the
non-catalyzed reaction, from 80% to 66%. More tungsten oxide on silica causes
a further drop in reaction yield. Here, the 3-nitro-o-xylene is obtained in a higher
yield.
Also, the TiO2/SiO2 catalyst makes the yield of the o-xylene nitration
reaction drop in comparison with the yield of the reaction occurring without the
catalyst. Here, however, a rise in the yield with the amount of the titanium diox-
ide supported on silica is observed for the nitroderivatives. At 15% of TiO2, less
4-nitro-o-xylene isomer (41%) is found to form, whereas the 3-nitro-isomer was
produced in greater quantity.
The enrichment of the TiO2/SiO2 system in WO3 results, in that the reaction
for 15% TiO2–2% WO3/SiO2 proceeds analogously as the nitration by means of the
nitric acid alone, both as concerns yield and composition of products. At a greater
TiO2 content, a drop in the reaction yield and the formation of equal amounts of
both nitro-o-xylene isomers are observed.
Table 2. Nitration of o-Xylene on Solid Catalysts
% by wt. of Metal
Oxide on SiO2
Yield
[%]
3-nitro-
o-xylene [%]
4-nitro-
o-xylene [%]
Isomer 3- to
Isomer 4-Ratio
no catalyst
5% MoO3
10% MoO3
15% MoO3
5% WO3
10% WO3
15% WO3
5% TiO2
10% TiO2
80
59
90
98
66
55
21
21
36
39
80
78
73
16
52
52
43
44
49
52
53
52
52
59
48
50
50
56
48
48
57
56
51
48
47
48
48
41
52
50
50
44
0.92
0.92
1.33
1.27
1.04
0.92
0.89
0.92
0.92
0.96
1.18
1.00
1.00
0.79
15% TiO2
5% TiO2–2% WO3
10% TiO2–2% WO3
15% TiO2–2% WO3
15% V2O5

AROMATIC COMPOUNDS
179
The V2O5/SiO2 system fails as an o-xylene nitration catalyst, as it reduces
the reaction yield from 80% (no catalyst) to 16%. Here 3-nitro derivative forms in
prevalence.
The nitration of o-xylene under these conditions afforded mononitroderiva-
tives only. Activity of the catalytic systems (based on the nitration reaction yield)
declines in the following order: MoO3 > 100% HNO3 > TiO2–WO3 > WO3 >
TiO2 > V2O5.
For the MoO3/SiO2 and V2O5/SiO2 systems, the o-xylene nitration was
carried out at elevated temperatures, 40◦C and 60◦C. The results obtained are
shown in Table 3.
o-Xylene nitration using 100% nitric acid at temperatures 20◦ and 40◦C es-
sentiallydoesnotaffecttheo-xyleneconversion, whichisca. 70%. At60◦Cthecon-
version drops to 54%. Selectivity of the reaction to obtain 3- and 4-nitro-o-xylene
is independent of the temperature. In the products invariably the 3-nitroisomer is in
excess, although the para/ortho ratio increases from 0.92 at 20◦C to 0.96 at 60◦C.
The raised temperature also affords o-nitrotoluene as a result of ypso-nitration (5).
The V2O5/SiO2 system at 20◦C affords a very low yield. Only if the tem-
perature is raised to 40◦ and 60◦C does the yield attain 50–60%. If the product
obtained at 20◦C contains 3-nitro-o-xylene in excess, at 40◦C concentrations of
both isomers are the same. Besides these derivatives, o-nitrotoluene is produced
also in a ca. 1.5% amount.
The use of the MoO3/SiO2 catalyst containing 15% MoO3 allows us to obtain
as much as 99% yield in o-xylene nitration at 20◦C, whereas raising temperatures to
40◦ and 60◦C yields a complete conversion of the substrate, with a rising selectivity
to 4-nitroisomer (para/ortho = 1.4 at 60◦C) and to ypso nitration products, i.e. o-
nitrotoluene.
Another variable of the o-xylene nitration reaction studied was the effect of
the amount of the catalyst used.
If less than 7 g of the 15% MoO3/SiO2 catalyst is used, the reaction yield
decreases significantly, although the reaction continues to proceed on the catalyst
Table 3. o-Xylene Nitration on Solid Supported Catalysts at Various Temperatures
No Catalyst
15% V2O5/SiO2
15% MoO3/SiO2
Reaction
Temperature
20◦C 40◦C 60◦C 20◦C 40◦C 60◦C 20◦C 40◦C 60◦C
Conversion [%]
o-NT [%]
4-NOX [%]
3-NOX [%]
4/3-nitro
70
–
34
36
0.92
77
3.5
33
35
0.94
54
4.0
24.5
25.5
0.96
16.5
–
7.0
9.0
0.77
61.5
1.5
30.5
30.0
1.02
51.5
1.5
25.5
25.0
1.02
99
7.0
52
40.0
1.30
100
8.5
51.5
40.5
1.28
100
8.5
53.5
38.0
1.40

✡
180
MILCZAK ET AL.
Table 4. o-Xylene Nitration Catalyzed with Variable Amounts of 15%
MoO3/SiO2 Catalyst
Catalyst [g]
1 g
3 g
5 g
7 g
10 g
Conversion [%]
o-NT [%]
71.5
2.0
71.5
2.0
71.5
2.0
92
3.0
99
7.0
4-NOX [%]
3-NOX [%]
4-nitro/3-nitro
37.5
32.0
1.17
39.5
30.0
1.32
41.5
28.0
1.48
50.0
39.0
1.28
52.0
40.0
1.30
surface. Evidence of this may be the excess of the 4-nitro-o-xylene derivatives in
the nitration products. A fall in the quantity of the side product, o-nitrotoluene,
that forms there is also observed.
For the reaction conducted with 1 g of the 15% MoO3/SiO2 catalyst at 60◦C,
the reaction time was extended from 2.5 to 6 h. The nitration yield obtained was
78% (71.5% at 20◦C). Longer reaction time does not significantly affect the yield
or composition of the products; however, a rise in o-nitrotoluene formed was noted.
Attempts were made to utilize the same catalyst for o-xylene nitration. The
catalyst used was 15% MoO3/SiO2. The yield slightly decreases following each
successive reaction. The repeated use of the catalyst shifts selectivity of the nitra-
tion process; relatively more 3-nitroderivative is produced, yet the 4-nitro-o-xylene
isomerstillprevailsintheproducts. ResultsoftheexperimentsareshowninTable5.
Phenol Nitration
The same catalytic systems were used in the reaction of nitration of phenol
with 65% nitric acid. A very high activity of all the catalysts used was noted to lead
to a reaction yield of over 90%. For the WO3/SiO2 system, very high selectivity
of the nitration reaction at para position was observed to obtain p-nitrophenol in a
yield higher than 90% in the products. The use of the TiO2/SiO2 catalyst produces
a mere ca. 49% of the isomer.
Table 5. o-Xylene Nitration with the 15% MoO3/SiO2 Catalyst Used Repeatedly
Reaction I
Reaction II
Reaction III
Reaction IV
Conversion [%]
o-NT [%]
99
8.5
88.5
4.0
79.5
1.0
74.0
1.0
4-NOX [%]
3-NOX [%]
4-nitro/3-nitro
51.0
39.0
1.29
48.0
36.0
1.15
41.0
37.0
1.12
38.0
35.0
1.09

AROMATIC COMPOUNDS
Table 6. Phenol Nitration on Supported Catalysts
181
% by wt.
Oxide on SiO2
Yield
[%]
o-nitrophenol
[%]
p-nitrophenol
[%]
Para to Ortho
Isomer Ratio
5% WO3
10% WO3
15% WO3
5% MoO3
10% MoO3
15% MoO3
5% TiO2
10% TiO2
97
92
99
99
93
98
90
90
91
93
92
95
9
10
8
91
90
92
85
78
67
49
48
49
77
79
51
10.11
9.00
11.50
5.67
3.55
2.03
0.96
0.92
0.96
3.35
3.76
1.04
15
22
33
51
52
51
23
21
49
15% TiO2
5% TiO2–2% WO3
10% TiO2–2% WO3
15% TiO2–2% WO3
The results achieved also show that the p-nitrophenol content in the ni-
tration product decline with the rise in content of the individual metal oxides
(i.e. MoO3/SiO2, TiO2/SiO2, TiO2–WO3/SiO2), supported on silica. The reaction
performed resulted merely in mononitro-derivatives of phenol.
The systems studied behaved very similarly concerning the yield of the
nitro-derivative obtained. They differ merely in the selectivity of para-nitration.
The highest selectivity was found in the 5–15% WO3/SiO2 system yielding
in the products over 90% of p-nitrophenol. A slightly lower selectivity is displayed
by the molybdenum catalyst, from 85% to 67% of para-isomer and, in this case, a
drop in selectivity is observed in favor of para-nitration, with the MoO3 supported
on silica. The TiO2–WO3 behaves analogously.
The TiO2/SiO2 catalyst shows the same selectivity for the ortho- and para-
nitration, irrespective of the TiO2 content on the silica support. The isomers are
obtained in nearly the same yield.
The selectivity order can be arranged thus: WO3 > MoO3 > TiO2–WO3 >
TiO2.
Toluene Nitration
Catalyst, 15% by wt. of MoO3 supported on silica, was studied in the reaction
of toluene nitration using 100% nitric acid.
Initially the effect of rising temperature of the reaction on the yield and
selectivity of toluene nitration were investigated. At as low a temperature as
20◦C, toluene becomes completely converted to afford respective nitro-derivatives

✡
182
MILCZAK ET AL.
Table 7. Toluene Nitration using a 15% by wt. MoO3 Silica-supported Catalyst∗
No Catalyst
40◦C
15% MoO3/SiO2
Reaction Temperature
20◦C
60◦C
20◦C
40◦C
60◦C
Conversion [%]
Nitrobenzene [%]
o-nitrotolune [%]
m-nitrotoluene [%]
p-nitrotoluene [%]
p/o ratio
83
87
1.54
53.2
–
32.3
0.61
83.5
1.96
48.7
–
32.9
0.68
100
–
55
–
45
0.82
100
–
51.2
–
48.8
0.95
100
–
50.15
–
49.85
0.99
0.27
49.6
2.2
31.1
0.62
∗Reaction conditions: HNO3/toluene = 1.5:1, 10 g catalyst, 22 ml CCl4, reaction time 2.5 h.
(Table 7), when, for reactions without catalyst, only about 80% conversion of
toluene is observed. Increase of temperature increases the p/o ratio in both cases.
This ratio is always higher for the catalyzed reactions. Any ypso-product was
obtained when the catalyst was used.
In the nitration reactions performed merely mononitrotoluene derivatives
were obtained. Subsequently, the effect of the catalyst on the formation of toluene
nitro-derivatives containing more than one nitrogroup, was studied. A greater ex-
cess of nitric acid was used to achieve this goal. The results are compared in
Tables 8–10.
An attempt to make dinitrotoluene in a 100% yield for a slight stoichiometric
excess of HNO3 at an HNO3 to toluene mole ratio of 3:1 failed, both when the
Table 8. Toluene Nitration with a 15% MoO3/SiO2 Catalyst for a HNO3 to Toluene Mole
Ratio of 3:1∗
No Catalyst 15% MoO3/SiO2 15% MoO3/SiO2 15% MoO3/SiO2
T = 20◦C
T = 20◦C
T = 20◦C
T = 60◦C
Time = 24 h
Time = 2.5 h
Time = 24 h
Time = 6 h
Conversion [%]
Nitrobenzene [%]
o-nitrotoluene [%]
p-nitrotoluene [%]
p/o ratio
Dinitrotoluenes
2,6-DNT [%]
2,4-DNT [%]
100
–
100
–
100
1.23
100
0.88
39.04
40.56
1.03
16.8
20.16
79.84
33.36
30.51
0.91
17.84
23.53
76.47
39.95
43.90
1.10
14.38
22.9
77.1
26.38
37.21
1.41
32.44
25.61
74.39
∗Reaction conditions: Amount of the catalyst was 10 g, 22 ml CCl4.

AROMATIC COMPOUNDS
183
Table 9. Toluene Nitration with a 15% MoO3/SiO2 Catalyst for a HNO3 to Toluene
Mole Ratio of 8:1∗
15%
15%
15%
No Catalysts
Time = 24 h
MoO3/SiO2
Time = 24 h
MoO3/SiO2
Time = 48 h
MoO3/SiO2
Time = 72 h
Conversion [%]
o-nitrotoluene [%]
p-nitrotoluene [%]
p/o ratio
100
100
1.48
9.66
100
–
2.77
–
100
–
2.68
–
26.87
23.89
0.88
6.54
Dinitrotoluenes
2,6-DNT [%]
2,4-DNT [%]
2,6/2,4 ratio
45.17
23.61
76.39
0.31
87.46
29.31
70.69
0.41
96.64
25.79
74.21
0.34
97.31
23.61
76.39
0.39
∗Reaction conditions: Amount of the catalyst was 10 g, 22 ml CCl4, t = 20◦C.
nitric acid was used alone, and in the present of the 15% MoO3/SiO2 catalyst.
After 24 h of the reaction, dinitrotoluenes were obtained in a yield of only 27%
when HNO3 was used alone, and in 33% with a catalyst present in the reaction
environment. Raised temperature failed to give any desirable results.
An increased HNO3 excess in relation to toluene as 8:1 for the nitration with
HNO3 alone affords an increased yield of dinitroderivatives, the amount for the
catalyzed system being twice the amount for the non-catalyzed reaction.
The nitration reaction with the catalyst present in the reacting system is
rather slow and after at least 48 h toluene converted to its dinitroderivatives attains
over 90%.
Table 10. Toluene Nitration with a 15% MoO3/SiO2 Catalyst for an HNO3 to Toluene
Mole Ratio of 16:1∗
No Catalyst 15% MoO3/SiO2 15% MoO3/SiO2 15% MoO3/SiO2
Time = 2 h
Time = 0.5 h
Time = 1 h
Time = 2 h
Conversion [%]
o-nitrotoluene [%]
p-nitrotoluene [%]
p/o ratio
100
100
0.56
0.56
–
100
0.40
0.40
–
100
0.12
0.12
–
18.98
14.50
0.71
Dinitrotoluenes
2,6-DNT [%]
2,4-DNT [%]
2,6/2,4 ratio
61.24
23.39
76.61
0.30
99.44
22.86
77.14
0.29
99.60
20.91
79.09
0.26
99.88
21.43
78.57
0.27
∗Reaction conditions: Amount of the catalyst was 10 g, 22 ml CCl4, t = 20◦C.

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MILCZAK ET AL.
In increased mole ratio to 16:1 leads to an abrupt rise in the nitration re-
action, affording nearly 100% conversion of toluene to its dinitroderivatives in
as short a time as 0.5h. Under these conditions, the uncatalyzed reaction affords
dinitrotoluene in a yield of 61.24% after 2 h.
The results obtained indicate that both an increased HNO3/toluene mole ratio
and the presence of MoO3/SiO2 catalyst have a favorable effect on the toluene
dinitration. A major component of the products obtained is 2,4-dinitrotoluene.
As a component of the reaction mixture with other dinitrotoluenes, it is formed
invariably in a constant yield independent of the HNO3/toluene mole ratio. About
threefold more of the 2,4-dinitrotoluene is formed, compared with the 2,6- dini-
troderivative.
Discussion
If the nitration of o-xylene with 100% HNO3 for a HNO3/o-xylene ratio
equalling 1.5:1 is assumed as reference for the same process by carried out in
the presence of catalytic systems studied, the catalysts may be divided into three
groups:
1. 10% and 15% MoO3/SiO2 catalysts, in the presence of which the nitra-
tion process proceeds in the respective yields of 90% and 98%, which
is higher than when nitrating with neat 100% HNO3 and with a higher
selectivity towards nitration at position 4-, determined as a ratio of the
amount of isomers 4- to that of isomer 3- of mononitro-o-xylene, the
value of which is above 1.2;
2. 5% MoO3/SiO2, 5–15% TiO2–2% WO3/SiO2 catalysts, in the presence
of which the nitration proceeds with yields above 65%, which is less
than for the nitration with 100% HNO3 only, yet with the selectivities
towards nitration at position 4- than for the acid used alone;
3. 10–15% WO3/SiO2, 5–15% TiO2/SiO2 and 15% V2O5/SiO2, in the pres-
ence of which the nitration with 100% HNO3 proceeds with yields below
55% and the selectivities towards nitration at position 4- that are the same
or even lower than for the nitration with 100% HNO3 only.
The catalysts of the first group mentioned are the only ones that have on their
surface the proton acidity centres of a strength corresponding to the concentration
of the hydrogen cations in H2SO4 of at least 77%, and it is those that are most
likely responsible for the generation of the NO+2 cations from the activated nitric
acid adsorbed on the surface of those catalysts. This effect is stronger here than in
the case of the neat 100% HNO3. The nitronium cations here are adsorbed on the
catalyst surface and it is where the nitration reaction proceeds in the first place,

AROMATIC COMPOUNDS
185
just as for the reactions of nitration in the presence of solid acids (the para effect)
and H3PO4/P2O5 system described in the literature (5).
On the surface of the second group catalysts, there occur only the proton
acidity centres corresponding to the proton cation concentration in a 50% H2SO4,
which, just as in the 50% H2SO4/HNO3 system, are not capable of producing the
nitrating agents of an efficiency observed for 100% HNO3. A higher selectivity
towards nitration at position 4- indicates that the nitration reaction in the presence
of those catalysts proceeds to a perceptible degree, on the catalyst surface.
The catalysts of the third group, which significantly impair yield of the nitra-
tion with 100% HNO3, also possess the proton centres of a strength corresponding
to that of the hydrogen cations in 50% H2SO4. Here, however, the selectivity to-
wards the nitration at position 4- is the same as, or even lower than, for the nitration
with 100% HNO3 only. This suggests that the nitration reaction proceeds in the
presence of these catalysts in a solvent, of which the HNO3, originally adsorbed
on the surface, desorbs. The reaction in a solvent occurs at a much slower rate than
it does on the catalyst surface.
Despite the fact that in a solvent decanted from above the grains of the
catalysts studied, no perceptible amounts of nitric acid could be detected, it is
quite likely that it is desorbed in particular in the presence of the substrates and
reacts with them immediately. The process should occur with a relatively higher
intensity for the third group of the catalysts studied.
When the o-xylene nitration reaction temperature is raised from 20◦ to 60◦C,
the reaction yield decreases when conducted with 100% HNO3. Selectivity of the
nitration at position 4-, on the other hand, increases and the products of ypso-
nitration of o-toluene are observed to form.
If the nitration process is conducted in the presence of a 15% MoO3/SiO2,
the temperature rise causes the reaction yield to increase to 100%, the selectivity
towards the nitration at position 4-, but also causes an increase in ypso-nitration
affording 8.5% o-nitrotoluene in the reaction products, that is twice as much as
previously. This indicates that raising the nitration temperature, as well as the
presenceofthemolybdenumcatalyst, favorstheformationofxylenecationradicals
to a greater degree at the expense of the Wieland complexes (5), the nitration of
which proceeds at positions with an increased unpaired electron density (5), in
the case of o-xylene in the position 4-, twice bigger than in the position 3-. It also
increases the nitration at position 4- additionally to the effect of the catalyst surface.
A relatively high ratio of the 4- to 3- isomers obtained from nitroxylenes
demonstrates that the nitration reaction in these systems proceeds primarily on
the catalyst surface regardless of its amount. The nitration reaction conducted
against 1 g of the catalyst at 60◦C where higher yields were observed to 78%,
and a higher selectivity towards ypso-nitration with no change in selectivity to-
wards the 3- and 4- isomers, as compared to the reaction run at 20◦C, suggest
that the differences in yields for the systems with 1–10 g of the catalyst during

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MILCZAK ET AL.
the nitration at 20◦C are due to thermal effects. For the catalyst amounts of 10
and 7 g, local overheating in the catalyst bulk are likely to occur, resulting in a
higher catalyst activity and its ypso-nitration selectivity. For the systems with a
decreased catalyst amount, the catalyst concentration in the suspension is lower,
which results in a better heat exchange that prevents overheating. This results in a
reduced nitration rate, decreased ypso-nitration selectivity, and even an observed
rise in para-nitration selectivity. Already for the catalyst amount of 1 g a relatively
greater effect than on the supported catalyst is observed in the nitration reaction,
which manifests itself in a respectively lower value of the isomer 4- to isomer
3- ratio.
A subsequent use of the same catalyst, on its washing with carbon tetrachlo-
ride after each reaction, leads to its gradual deactivation. This effect results in a
reduced amount of the products whose formation is ascribed to the reaction occur-
ring on the catalyst surface, notably a decrease in the isomer 4- content and in the
ypso-nitration products, at nearly unaltered content of isomer 3- in the products.
This suggests that the deactivated catalyst does not adsorb or activate nitric acid,
causing the nitration process to occur in the solvent.
Toluene nitration using 100% HNO3 in the presence of 15% MoO3/SiO2
catalyst over a temperature range of 20◦–60◦C proceeds with a 100% conversion,
which is ca. 20% higher than in the nitration without the catalyst. A rise in the
reaction temperature in either case results in an increased amount of the isomer
4- formed in the products, the effect being relatively more pronounced for the
catalysed reaction. Nearly 1.5 times greater amount of this isomer for the cat-
alyzed reaction is evidence that the reaction here goes primarily on the catalyst
surface.
Attempts made to obtain dinitrotoluenes were successful when the HNO3/
toluene ratio was 16:1. Their yield of 99.44% was obtained just after 0.5 h of the
reaction course for the catalyzed reaction. For the uncatalyzed reaction, a yield
of 61.24% of these products is attained after 2 h of the reaction course. A further
extension of the reaction in the presence of a catalyst up to 2 h affords a relatively
greater amount of 2,4-dinitrotoluene. This suggests that here occurs the reaction
of isomerization of the nitro group from the kinetically favored ortho-position to
the thermodynamically more stable para-position.
The results demonstrate that the catalyst used in any of the cases studied
generates more nitrating agents than are produced in the neat 100% HNO3. This
is favored by increasing the HNO3/toluene ratio.
A 100% yield in the nitration of phenol using 65% HNO3 in the presence of
the catalysts that contain oxides of molybdenum, tungsten, and titanium supported
on silica gel in analogous conditions as described in the literature (3) for the
65%HNO3/SiO2, for which the yield was 90%, showed that each of the catalysts
studied generates and activates the nitrating agents in a more effective way than
does nitric acid on silica alone. This is accountable by a higher acidity of these

AROMATIC COMPOUNDS
187
catalysts, due to the presence of the oxide modifiers used. The modifiers in the case
of molybdenum oxide, and even more for tungsten oxide, yielded an exceedingly
high selectivity of p-nitrophenol, which was above 90%. This is probably due to
the superficial structures of these oxides that make up entities of a “tower-like”
structure (17) with the proton centres at their base. It is likely that it is where
the nitrating agents are generated and only the para-position of phenol can access
them.
The nitrating agents in the systems studied should be the nitronium cations
for the systems where the nitric acid of a suitable concentration is used and the
nitrosyl cations for the systems with dilute acids. In either case the presence of the
proton centres should favor their formation.
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