Vol. 26, No. 22 (2014)
Direct Hydroxylation of p-Xylene to 2,5-Xylenol with Hydroxylamine 7529
TABLE-1
EFFECT OF REACTION MEDIUM ON THE CATALYTIC HYDROXYLATION OF p-XYLENE
Si (%)
Entry
Reaction media (g:g:g )
XXylene (%)
2,5-xylenol
-
Othersa
-
1
2
3
4
5
6
7
[HSO3-bmim][CF3SO3] (12 g)
0
[HSO3-bmim][CF3SO3]-H2O (6:8)
0.3
3.1
3.6
5.9
6.4
8.2
37.2
62.8
6.3
[HSO3-bmim][CF3SO3]-CH3COOH-H2O (2:10:4)
[HSO3-bmim][CF3SO3]-CH3COOH-H2O (3:10:4)
[HSO3-bmim][CF3SO3]-CH3COOH-H2O (4:10:4)
[HSO3-bmim][CF3SO3]-CH3COOH-H2O (5:10:4)
[HSO3-bmim][CF3SO3]-CH3COOH-H2O (6:10:4)
93.7
93.8
6.2
98.3
1.7
88.4
11.6
90.6
9.4
a2,5-xylidine and tetramethylbiphenyl. Reaction conditions: 0.32 g (NH4)6Mo7O24·4H2O catalyst, 12.2 mmol (NH2OH)2·H2SO4, 20 mmol p-xylene,
90 °C, 4 h
TABLE-2
EFFECT OF THE KIND OF IONIC LIQUIDS ON THE CATALYTIC HYDROXYLATION OF p-XYLENE
Si (%)
Entry
Reaction mediaa
XXylene (%)
2,5-xylenol
Othersb
8
9
10
11
12
[HSO3-bmim][H2PO4]-CH3COOH-H2O
[HSO3-bpim][CF3SO3]-CH3COOH-H2O
[HSO3-beim][CF3SO3]-CH3COOH-H2O
[HSO3-bmim][CF3SO3]-CH3COOH-H2O
[HSO3-btma][CF3SO3]-CH3COOH-H2O
1.7
3.2
4.4
5.9
4.9
91.6
92.0
93.2
98.3
88.3
8.4
8.0
6.8
1.7
11.7
aWeight ratio of ionic Liquids, organic acid and water is 4:10:4. b2,5-xylidine and tetramethylbiphenyl. Reaction conditions: 0.32 g
(NH4)6Mo7O24·4H2O catalyst, 12.2 mmol (NH2OH)2·H2SO4, 20 mmol p-xylene, 18 g reaction media, 90 °C, 4 h
[HSO3-bmim][CF3SO3] (Table-1). But no reaction is observed.
And pure ionic liquids did not give complete dissolution of
hydroxylamine sulfate. Considering hydroxylamine salts is
entirely soluble in water, [HSO3-bmim][CF3SO3]-H2O solution
was used as reaction media. However, the conversion of
p-xylene is nearly zero (0.3 %), revealing no activity of the Mo
catalyst in this aqueous solution.
[HSO3-bmim][CF3SO3]-CH3COOH-H2O. Better result is
observed in [HSO3-bmim][CF3SO3]-CH3COOH-H2O solvent
system. Therefore, the [HSO3-bmim][CF3SO3]-CH3COOH-H2O
solution with a weight ratio of 4:10:4 was chosen as a reaction
media. Then an ionic liquids/molybdenum catalytic system,
i.e., combination of the above media with (NH4)6Mo7O24·4H2O
catalyst, was selected as the ILs/Mo catalytic system.
Previous studies showed that H2SO4-CH3COOH-H2O me-
dium with a weight ratio of 2:10:4 is favorable for the hy-
droxylation9. Consequently, a weight ratio of 2:10:4 [HSO3-
bmim][CF3SO3]-CH3COOH-H2O solution was used as a reac-
tion media (Entry 3). Fortunately, the desired reaction can be
achieved. And p-xylene conversion increases to 3.1 % with
93.7 % selectivity for 2,5-xylenol. Hence subsequent studies
were conducted in various [HSO3-bmim][CF3SO3]-
CH3COOH-H2O solvents containing different amount of ionic
liquids. With increasing ionic liquids content, p-xylene con-
version increases steadily. While 2,5-xylenol selectivity in-
creases first and then decreases. Better result is obtained at a
[HSO3-bmim][CF3SO3]:CH3COOH:H2O weight ratio of
4:10:4. The conversion of p-xylene and the selectivity for 2,5-
xylenol are 5.9 and 98.3 %, respectively.
Optimization of reaction conditions: As the above ILs/
Mo catalytic system showed better activity, the catalytic system
was selected for further investigation. Figs. 3 and 4 display
the effect of reaction temperature and time on p-xylene
hydroxylation. With the temperature rise from 50 to 95 °C, p-
xylene conversion as well as 2,5-xylenol selectivity increases
first, passing through a maximum at 90 °C and then decreases.
Among the reaction time, higher conversion of p-xylene can
be obtained by prolonging reaction time. However, p-xylene
conversion increases slightly when the time exceeded 4 h.
Therefore, the optimum reaction temperature and time are
90 °C and 4 h, respectively.
Various amounts of Mo catalyst and hydroxylamine were
also examined. The results are showed in Figs. 5 and 6. With
increasing catalyst dosage, the selectivity of 2,5-xylenol
increases steadily and then keep around 96 %. For p-xylene
conversion, a bell-shaped dependence on catalyst amount is
observed. High yield of 2,5-xylenol is obtained at 0.2-0.5 g of
catalyst and the molar ratio of Mo to p-xylene is 1:7-18.As for
the influence of hydroxylamine, p-xylene conversion increases
steadily from 1 to 9.9 % with the variation of hydroxylamine
dose from 0.8 to 4.9 g. However, 2,5-xylenol selectivity first
keep around at 98 % and then decreases to 80 %. Better results
can be achieved at 2 g (12.2 mmol) of hydroxylamine sulfate,
i.e. the molar ratio of NH2OH to p-xylene is 1.2:1.
To further investigate the effect of ionic liquids, several
kinds of ionic liquids were selected to form acidic medium,
while the weight ratio of ILs:CH3COOH:H2O was keep at
4:10:4. Table-2 gives the results. Interestingly, p-xylene
conversion and 2,5-xylenol selectivity increase first and then
decrease with increase the acidity of the ionic liquids. A small
amount of 2,5-xylenol is obtained in [HSO3-bmim][H2PO4]-
CH3COOH-H2O solvent, probably due to the weak acidity of
[HSO3-bmim][H2PO4] as demonstrated by UV-visible spectro-
scopy. Subsequently an increase in p-xylene conversion as well
as 2,5-xylenol selectivity, is observed as the hydroxylation
was performed in the following solutions, [HSO3-bpim][CF3SO3]-
CH3COOH-H2O, [HSO3-beim][CF3SO3]-CH3COOH-H2O and
Reusability of the ILs/Mo catalytic system: To investi-
gate the possibility of reusing the ILs/Mo catalytic system, a
series of recycle experiments were conducted. And the results