Table 3 Phenol hydroxylation using H2O2 at 60 °C in water with different
catalysts after 6 h
When the acid strength is too high, the products desorption is
probably limited and the catalyst is deactivated faster; hence
relative catalytic activity is lower.
Furthermore, when more Sn was incorporated, the catalytic
activity decreased. This indicated that not all Sn introduced was
active. Some of Sn was in the form of Sn6+ or SnO2. Both of
which are not active for the phenol hydroxylation.
Conversion (%)
Selectivity (%)
HQa + CTb
Catalyst
Phenol
H2O2
BQc
1. TS-1
2. Sn–S-1
50.0
1.4
99.9
35.0
99.9
99.5
94.4
99.4
37.6
97.7
95.3
99.1
98.8
99.2
98.4
88.8
2.3
4.7
0.9
1.2
0.8
Non-framework incorporation of Sn by impregnation, exp.6,
gave a catalyst with a lower performance than TS-1 or Ti–Sn–S-
1 [1], which contains framework Sn atoms. The reason for this
is because the active Sn sites are only on the external surface
and some of them are in the non-active form. When Ti is
impregnated on Sn–S-1 (exp.7), a non-framework catalyst, its
activity is lower than TS-1. This is due to Ti sites that are only
on the external surface. Some of them are in the non-reactive
form such as TiO2 or anatase, which is not active for the phenol
hydroxylation but is active for hydrogen peroxide decomposi-
tion.
In summary, Ti–Sn–S-1 opens up an alternative for the
phenol hydroxylation with H2O2 to dihydroxybenzenes. The
highest product selectivity to hydroquinone and catechol was
achieved with water as a solvent. Sn incorporated in TS-1
framework promoted greater catalytic activity for phenol
hydroxylation in comparison to TS-1 due to crystal morphology
and acidity optimization and/or modification. The framework of
Ti or Sn incorporation is critical for the phenol hydroxylation.
Gratefully acknowledgments are forwarded to The Petroleum
and Petrochemical College, Chulalongkorn University, Thai-
land, and UOP LLC, USA. Special thanks are given to Jaime G.
Moscoso and Gregory J. Lewis for catalyst preparation. The
assistance of UOP LLC’s analytical staff is also greatly
appreciated.
3. Ti–Sn–S-1 [1]
4. Ti–Sn–S-1 [2]
5. Ti–Sn–S-1 [3]
6. Sn on TS-1
7. Ti on Sn–S-1
50.0
46.8
38.6
47.5
5.0
1.6
11.2
a Hydroquinone; b Catechol; c Benzoquinone
We tested the synthesized catalysts in water under the same
standard conditions. According to Table 3, Ti–Sn–S-1 [1] and
TS-1 show similar final conversions to products. Nevertheless,
the kinetic study of TS-1 and Ti–Sn–S-1 [1] (Fig. 1) shows that
the Ti–Sn–Silicalite [1] converted phenol to dihydroxy com-
pounds faster than TS-1. The initial reaction rate is 6.4 3 1028
and 8.1 3 1028 mol m22 s21 for TS-1 and Ti–Sn–S-1 [1],
respectively. This means that the small amount of Sn incorpora-
tion results in 26% higher activity than TS-1. However, only Sn
incorporation generated a silicalite catalyst with much less
activity than TS-1 (Table 3, exp. 2).
When a small amount of Sn was incorporated (exp. 4 and 5),
two main catalyst properties were modified; smaller crystal size
and higher acidity (see Table 1). Generally, the liquid phase
zeolite catalyzed reactions are controlled by the diffusion step;
the smaller crystal size catalyst gives the higher catalytic
activity.6 However, when Sn incorporation was increased, the
increased acidity has more effect on the phenol conversion than
the decreasing crystal size.
The acidity of Ti–Sn–S-1s increased with the increasing of
Sn incorporation. The NH3 adsorption data (Table 1) indicate
that increasing of Sn incorporation increased the acid strength
of the catalyst. Sn incorporation may alter the reaction
mechanism by changing the adsorption of phenol and phenol
hydroxylation products thereby increasing the phenol conver-
sion.13,14 The small increase in acidity has the benefit for of the
faster adsorption of phenol, and results in a faster reaction.
Notes and references
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Fig. 1 Phenol conversion at 60 °C vs. time.
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