Enhancing the selectivity of Pare-dihydroxybenzene in hollow titanium silicalite zeolite catalyzed phenol hydroxylation by introducing acid-base sites

Article abstract of DOI:10.1016/j.catcom.2016.03.011

A facile and effective method for enhancing the selectivity of Pare-dihydroxybenzene in phenol hydroxylation has been developed by introducing acid-base sites (MgO-Al₂O₃ binary oxide) to the micropores of hollow titanium silicalite (

Full text of DOI:10.1016/j.catcom.2016.03.011

Catalysis Communications 80 (2016) 49–52
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Enhancing the selectivity of Pare-dihydroxybenzene in hollow titanium
silicalite zeolite catalyzed phenol hydroxylation by introducing
acid–base sites
a,b
b
b
b
b
Changjiu Xia , Lihua Long , Bin Zhu , Min Lin , Xingtian Shu
a
Department of Materials and Environmental Chemistry, Stockholm University, Stockholm SE-106 91, Sweden
State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile and effective method for enhancing the selectivity of Pare-dihydroxybenzene in phenol hydroxylation
Received 30 November 2015
Received in revised form 16 March 2016
Accepted 22 March 2016
has been developed by introducing acid–base sites (MgO–Al
2 3
O binary oxide) to the micropores of hollow
titanium silicalite (HTS, Ti/Si = 25) zeolite. For MgO–Al modified HTS zeolites, the biggest ratio of para- to
2 3
O
ortho-dihydroxybenzene is over 2, while that is close to 1 for conventional HTS zeolite. The high pare-
dihydroxybenzene selectivity is ascribed to the steric hindrance and the synergistic effect between acid–base
sites of mixed oxide and tetrahedral framework Ti species in HTS zeolite.
Available online 4 April 2016
Keywords:
Phenol hydroxylation
HTS zeolite
©
2016 Elsevier B.V. All rights reserved.
Para-dihydroxybenzene
Acidic–basic sites
1
. Introduction
hydroquinone can be directly produced by TS-1 catalyzed route, with-
out the formation of hazard pollutants and wastes. And it is worthy to
note that the ratio of para- to ortho-dihydroxybenzene (Sp/o) is usually
close to 1. Nevertheless, conventional TS-1 zeolite usually suffers the
poor catalytic activity and reproducibility, owing to the mismatch
of hydrolysis rate between Si and Ti species during hydrothermal
synthesis. To improve it catalytic performance, hollow titanium
silicate (HTS) zeolite has been synthesized by a dissolution-reassembly
treatment of conventional TS-1 zeolite [17–19]. Up to now, HTS zeolite
has been commercially produced, and widely applied in several oxi-
dation processes, such as epoxidation of propylene, cyclohexanone
ammoximation and phenol hydroxylation at SINOPEC, China.
From the viewpoint of industrial application, pare-dihydroxybenzene
is more expensive and of even larger requirement than the other one in
global chemical market [20]. Thus, it is of great interest to increase the
selectivity of hydroquinone based on tuning the property of TS-1 zeolite
catalyst in this route. To obtain this goal, great efforts have been devoted
to the modification of TS-1 zeolite, such as tuning the external surface or
decreasing the pore mouth diameter by using chemical vapor deposition
(CVD) and other methods [21,22]. Unfortunately, these treatments don't
work well in enhancing the selectivity of pare-dihydroxybenzene, and
there is still no efficient method developed to tune the Sp/o of phenol
hydroxylation by modifying the property of TS-1 zeolite.
Phenol hydroxylation is of ultra-importance for the commercial
production of catechol (ortho-dihydroxybenzene) and hydroquinone
pare-dihydroxybenzene), which are widely applied in many areas,
(
e.g. agrichemical, photography chemicals, antioxidants, polymerization
inhibitors, etc. [1,2]. In conventional routes, dihydroxybenzenes are
commercially produced by several complex processes, including
(i) the oxidation of aniline and diisopropylbenzene; (ii) sulfurization
of benzene and hydrolysis of benzene sulfonate [3]. Although the selec-
tivity of target product in conventional approaches is very high, they are
still far beyond to be as ideal choices in industry, due to some serious
economic and environmental issues. In order to overcome these draw-
backs, heterogeneous catalysts, such as metal oxide [4,5], transition
metal substituted molecular sieves [6–9], hydrotalcite-based com-
pounds [10,11], metal incorporated mesoporous silica [12,13] and
heteropoly-compounds [14], have been introduced to phenol hydroxyl-
ation reaction. Apparently, heterogeneous catalysts are stable, cheap,
available, reusable, and easy to be separated/recovered from solvent.
Among these solid catalysts, TS-1 zeolite presents relative excellent cat-
alytic performance and stability in phenol hydroxylation with 30 wt.%
2 2
aqueous H O solution as oxidant under mild conditions, as proposed
and scaled up by Enichem in 1980s [15,16]. Both catechol and
Herein, we report a novel and effective method for maximizing
pare-dihydroxybenzene selectivity in HTS zeolite catalyzed phenol
hydroxylation route, by loading the acid–base sites (MgO–Al
2 3
O binary
E-mail address: xiachangjiu@gmail.com (C. Xia).
oxide) inside the micropores at the same time. The preparation of
http://dx.doi.org/10.1016/j.catcom.2016.03.011
1566-7367/© 2016 Elsevier B.V. All rights reserved.

50
C. Xia et al. / Catalysis Communications 80 (2016) 49–52
xMgO–yAl
2
O
3
-HTS zeolite is based on the incipient wetness impregnation
2 3
(around 5.4 Å). Thus, it indicates that MgO–Al O coats the internal
method, which is facile to be operated in both laboratory and industry.
surface of HTS zeolite by forming a thin binary oxide layer, making
the pore size become narrow. Therefore, pare-dihydroxybenzene
molecule can diffuse more flexibly than ortho-dihydroxybenzene
inside the modified HTS zeolite, due to the steric hindrance effect.
2
. Experimental
2
.1. Catalyst preparation
2 3
The quantified pore and structure properties of xMgO–yAl O -HTS
zeolites determined by BET method is shown in Table 1. We can see
that the specific surface area and pore volume, especially for the micro-
pore volume, of Mg–Al modified HTS zeolite are dramatically decreasing
as a function of the increase of metal oxide content loaded. It suggests
that metal oxide narrows the pore mouth of HTS zeolite, which is in
good agreement with the characterization of TEM and pore size distri-
bution. Thus, we propose that the metal oxide is majorly located in
the micropores and blocked them, causing the micropore size becomes
smaller. Furthermore, the acid and base properties of Mg–Al modified
HTS zeolite are determined by probe molecule temperature pro-
HTS zeolite was commercially produced by SINOPEC via following
the published literatures [18,23]. Mg–Al modified HTS zeolites are
prepared by using incipient wetness impregnation, as following
3 2 2 3 2
steps: (i) amount of Mg(NO ) ·6H O and AlCl ·9H O solid salts
were dissolved in water; (ii) 6 g HTS zeolite was added into the
Mg–Al containing solution at 40 °C with continuous stirring for 4 h;
(
iii) and then the mixture was put into 110 °C oven for 12 h to re-
move the water; (iv) the dried powder was calcined at 550 °C for 6 h.
The final product was labeled as xMgO–yAl -HTS zeolite, while the
to HTS zeolite in
2 3
O
x and y stand for the weight ratio of MgO and Al
2
O
3
grammed desorption (TPD) method. As shown in Fig. 2, NH
CO -TPD spectra are provided to illustrate the features of acid and
base sites in 0.09MgO–0.05Al -HTS zeolite, respectively. It is well
known that, TS-1 zeolite has only very weak Lewis acid property and
no basic sites, without adsorption peak in the NH -TPD spectrum.
Therefore, we can infer that both acid and base sites, with different
strength distribution, are derived from the MgO–Al binary oxide.
They are attributed to the isomorphous substitution of Mg ions by
3
-TPD and
the starting gel material, respectively.
2
2 3
O
2
.2. Characterization method
Powder X-ray diffraction (PXRD) analysis was operatedby a
PANalytical powder diffractometer equipped with a Cu Kα radiation,
λ = 1.54178 Å, under the following conditions: beam voltage of
3
2 3
O
2
+
3
+
4
0 kV, dwell time 500 s, and 2θ range 5° – 80°. The N
2
physisorption
Al
ions, with the formation of Mg–O–Al bonds, as illustrated in
isotherm was measured on a Micromeritics AS-6B apparatus, using
conventional BET and BJH methods to quantify the surface areas
and pore volume of zeolite samples. Prior to analyses, the samples
were heated to a constant weight under vacuum (10 Pa) at 300 °C
for 6 h. Transmission electron microscope images were taken on a
JEM-2100 microscope.
Fig. 3. Interestingly, K. Kaneda et al. pointed out that the cooperation
of acid- and basic-sites at neighbor location can effectively promote
the cycloaddition reaction between CO and epoxides [24]. Therefore,
2
three kinds of active sites are existed in Mg–Al modified HTS zeolite,
i.e. redox sites (tetrahedral framework Ti species), acid sites and base
sites.
−
1
As shown in Table 2, the phenol conversion (XPH) of xMgO–yAl
HTS zeolite is not as high as that of original HTS zeolite, which is
decreasing continuously along with the increase of MgO–Al oxide
2 3
O -
2
.3. Catalytic evaluation
2 3
O
Phenol hydroxylation reaction was carried out in the three necked
loaded inside the micropores of HTS zeolite, owing to the steric limita-
flask reactor, with continuous heating and magnetic stirring. For each
reactor, 0.63 g HTS zeolite, 12.5 g phenol and 10 ml acetone were
mixed together homogeneously in the flask. And the reactor was fixed
in the stirring apparatus, and heated to 80 °C. Then, 4.98 g 30 wt.%
tion. However, the Sp/o of Mg–Al modified zeolite is higher than that
of HTS zeolite. For example, 0.09MgO–0.10Al O -HTS zeolite in entry
2 3
4, the SP/O is over 2 (the ⊿Sp/o is about 102%), while the XPH is about
13.80% (RXPH is 53.3%). It is demonstrated that the XPH and SP/O can be
H
2
O
2
solution was injected in this reactor. Two hours later, small
tuned by changing the content of MgO–Al
optimized catalyst composition (0.03MgO–0.02Al O
2 3
20.8% and the SP/O ratio 1.67.
2
O
3
mixed oxide. Under
-HTS), the XPH is
amount of reaction mixture was picked up from the reactor, and
analyzed by Agilent-6890 GC equipment, which is of HP-5 column
and hydrogen frame detector.
Seen from entry 2 and 3 of Table 2, the ⊿Sp/o of 0.09MgO-HTS zeo-
lite is larger than that of 0.09Al -HTS zeolite under the same reaction
2 3
O
3
. Results and discussion
conditions. The reducing of XPH is caused by the steric hindrance effect
inside the micropores, which is attributed to the narrowing of micro-
pore mouth. The narrow pore diameter benefits the shape selectivity
in phenol hydroxylation, due to hydroquinone of smaller molecular
diameter than catechol. Moreover, it is well known that MgO and
Fig. 1 (a) and (b) depict the TEM images of HTS and 0.09MgO–
0
2 3 2 3
.05Al O -HTS zeolites. Similar as HTS zeolite, 0.09MgO–0.05Al O -
HTS is still of abundant intracrystalline cavities, which are in favor of
the mass diffusion of reagents and products inside zeolite crystal.
Moreover, there is no apparently large MgO–Al O particles observed
2 3
in the TEM image of Mg–Al modified HTS zeolite. It is suggested that
the Mg and Al containing species are highly dispersed in the internal
and external surface of HTS zeolite. Thus, we can confirm that the
mixed MgO–Al O precursors can be homogeneously introduced via
2 3
incipient wetness impregnation method. Consequently, there is no
new diffraction peak detected in the PXRD pattern (at the region between
Al
the basicity is of greater impact than acidity on tuning the selectivity
of phenol hydroxylation. However, MgO–Al mixed oxide modified
2 3
O are of basicity and acidity, respectively. Therefore, we can infer
2 3
O
HTS zeolites present much higher ⊿Sp/o than Mg- and Al-oxide solely
modified HTS zeolites, even the metal oxide amount is very small
(entry 9–10). It is inferred that there is a synergistic effect between
acid and base sites on maximizing the ⊿Sp/o of hydroxylation reaction
2
+
[24,25]. When comparing entry 6 and 8, it suggests that Mg (basic)
sites are in favor of higher Sp/o value than Al3 ions under the similar
phenol conversion level (about 16%). Thus, we infer that steric limita-
tion and synergistic effect between redox and acid–base sites take a
more major role than conversion effect in enhancing para-product
+
5
° and 50°) of Mg–Al modified HTS zeolite, as shown in Fig. 1(c). Further-
more, the structure and texture properties of Mg–Al modified HTS zeolite
have been identified by using low temperature N adsorption–desorption
2
isotherm, as shown in Fig. 1. (d). It is observed that Mg–Al modified HTS
zeolite also has the hysteresis loop from P/Po = 0.45–1, which is in good
agreement with the hollow cavities illustrated in TEM image. Particularly,
it is worthwhile to note that the micropore size distribution of 0.09MgO–
2 3
selectivity. Consequently, to explain the effect of MgO–Al O oxide, a
possible mechanism has been proposed, according to the catalytic and
spectroscopic results, as illustrated in Fig. 3. Firstly, in principle, phenol
0
.05Al
2
O
3
-HTS (around 5.0 Å) calculated by Horvath–Kawazoe (HK)
molecules are absorbed by MgO–Al
2
O
3
oxide via the interactions
between Al3 (acidic) sites and O atoms in phenol molecules through
+
model for slit-shaped micropores, is more narrow than that of HTS zeolite

C. Xia et al. / Catalysis Communications 80 (2016) 49–52
51
Fig. 1. (a) TEM image of HTS zeolite; (b) TEM image of 0.09MgO–0.05Al
2
O
3
2 3 2
-HTS zeolite crystals; (c) XRD patterns of HTS and xMgO–yAl O -HTS zeolites; (d) low temperature N
adsorption isotherm and pore size distribution (insert, calculated by HK model for slit-shaped micropores) of HTS and 0.09MgO–0.05Al
2 3
O –HTS zeolites.
the acceptor-donator of lone electron pairs, and/or between the basic
O) sites of binary oxide and H atoms in phenol molecules via hydrogen
bonding. Obviously, the Ti–OOH species, produced by the dehydration
reaction between tetrahedral framework Ti–OH group and H mole-
cules, are much more preferentially to attack the para-C atoms
Fig. 3a) than ortho-C atoms (Fig. 3b) in phenol molecule, owing to
the steric limitations inside the narrow micropores of zeolite. As a result,
this selectivity of pare-dihydroxybenzene becomes higher than that of
original HTS zeolite.
4. Conclusion
(
In summary, we have developed a facile and effective strategy to
maximize the selectivity of pare-dihydroxybenzene in HTS zeolite cata-
lyzed phenol hydroxylation reaction, via introducing acid–base sites
into the channels of zeolite. The superior para-product selectivity is
mainly attributed to the steric hindrance and the synergistic effect be-
2 2
O
(
tween the acid–base sites in MgO–Al
framework Ti species in HTS zeolite. Therefore, this study provides a
2 3
O mixed oxide and tetrahedral
Table 1
2 3
The BET analysis results of HTS and xMgO–yAl O -HTS zeolites.
Sample
HTS
S
BET/(m2 g−1
)
S
z
a/(m2 g−1
)
V
pore/(cm3 g−1
)
V
micro/(cm3 g−1
)
V )
meso/(cm3 g−1
434
389
344
393
357
300
0.332
0.301
0.287
0.177
0.162
0.138
0.155
0.139
0.149
0.03MgO–0.02Al –HTS
2
O
3
0.09MgO–0.05Al O –HTS
2 3
a
Sz stands for the internal surface area of Mg–Al modified HTS zeolite.

52
C. Xia et al. / Catalysis Communications 80 (2016) 49–52
Table 2
2 3
Phenol hydroxylation evaluation of HTS and xMgO–yAl O -HTS zeolites.
aRXPH, %
S
p/o
b
⊿Sp/o, %
Entry
Catalysts
X
PH, %
1
2
3
4
5
6
7
8
9
HTS
0.09MgO-HTS
0.09Al O -HTS
2 3
0.09MgO–0.10Al
0.09MgO–0.05Al
0.07MgO–0.04Al
0.05MgO–0.09Al
0.04MgO–0.07Al
0.03MgO–0.02Al
0.02MgO–0.01Al
25.9
19.2
19.9
13.8
16.5
16.1
18.1
16.7
20.8
22.2
100
0.99
1.31
1.12
2.01
1.62
1.68
1.38
1.42
1.67
1.31
0
32.3
13.1
102
63.6
69.7
39.4
43.4
68.7
32.3
74.1
74.8
53.3
63.7
62.2
69.9
64.5
80.3
85.7
2
O
2
O
2
O
2
O
2
O
2
O
2
O
3
-HTS
3
-HTS
3
-HTS
3
-HTS
3
-HTS
3
-HTS
3
-HTS
1
0
a
2 3
RXPH is the relative activity of the xMgO–yAl O -HTS zeolite, which represents the ratio
of phenol conversion of metal modified HTS zeolite to that of the original HTS zeolite.
b
⊿
2 3
Sp/o represents the change of Sp/o of the xMgO–yAl O -HTS zeolite, when comparing
with the Sp/o of original HTS zeolite.
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2 3
Fig. 3. Schematic mechanism of phenol hydroxylation catalyzed by xMgO–yAl O -HTS
zeolites.