Enhanced performance in vapor o-methylation of hydroxybenzene over a noval kind of mesoporous rare earth phosphate

Article abstract of DOI:10.1524/zpch.2010.5516

Mesoporous rare earth phosphates (MREPs) have been prepared by template method. The mesoporous structure of these materials has been characterized by XRD and TEM measurement. The catalytic performance of these materials in vapor O-methylation of hydroxybenzene has been investigated. These MREPs show higher performance in O-methylation of hydroxybenzene than the materials prepared without template. The excellent performance of these MREPs could be ascribed to their porous structure and much more amount of active sites. by Oldenbourg Wissenschaftsverlag, Mu?nchen.

Full text of DOI:10.1524/zpch.2010.5516

Z. Phys. Chem. 224 (2010) 857–864 . DOI 10.1524.zpch.2010.5516
© by Oldenbourg Wissenschaftsverlag, München
Enhanced Performance in Vapor O-Methylation
of Hydroxybenzene Over a Noval Kind of
Mesoporous Rare Earth Phosphate
1
By Zhen Zhang1, Qingbo Liu2, Xiangguang Yang3, , Xiangping Zhang , and
Suojiang Zhang
*
1,**
1
Key Laboratory of Green Process and Engineering, Institute of Process Engineering,
Chinese Academy of Sciences, Beijing, China
College of Science, Nanjing Forestry University, Nanjing, China
Laboratory of Green Chemistry and Process, Changchun Institute of Applied Chemistry,
2
3
Chinese Academy of Sciences, Changchun, China
(Received September 7, 2009; accepted April 9, 2010)
Mesoporous . Rare Earth Phosphate . O-Methylation . Hydroxybenzene
Mesoporous rare earth phosphates (MREPs) have been prepared by template method. The meso-
porous structure of these materials has been characterized by XRD and TEM measurement.
The catalytic performance of these materials in vapor O-methylation of hydroxybenzene has
been investigated. These MREPs show higher performance in O-methylation of hydroxybenzene
than the materials prepared without template. The excellent performance of these MREPs could
be ascribed to their porous structure and much more amount of active sites.
1. Introduction
Porous materials have some good characters, such as high surface area, controlla-
ble adsorption properties and designable morphologies, thus, porous material are
potential candidates for catalysis, adsorption and hosts of nanomaterial synthesis,
etc[1]. For mesoporous materials, they possess larger channels(2~50 nm) than
zeolites, holding more promise for the catalytic application to the direct activa-
tion of large organic molecule or for supporting active sites bearing large organic
functionalities[2]. In recent years, much attention has been devoted to the synthe-
sis and characterization of rare earth phosphates, because of their practical or
*
Corresponding author. E-mail: xgyang@ciac.jl.cn
**
Corresponding author. E-mail: sjzhang@home.ipe.ac.cn
Unauthenticated
Download Date | 3/29/16 5:05 PM

858
Z. Zhang et al.
potential applications in optical materials (including laser, phosphors, anti-UV
materials), sensors, ceramic materials, heat-resistant materials and catalysis mate-
rials, etc[3–13]. For these reasons, it is anticipated to the synthesis and applica-
tion of mesoporous rare earth phosphates(MREPs). However, to our knowledge,
there is still little research in mesoporous rare earth phosphates, which might be
caused by the reasons that the prepared mesoporous rare earth phosphates are
thermally unstable, or the application of mesoporous rare earth phosphates is still
limited.
O-alkylation of hydroxybenzene is a series of reactions, such as O-alkylation
of phenol with methanol for the synthesis of anisole, O-alkylation of catechol
with methanol for the synthesis of guaiacol, etc. The products of O-alkylation
reactions, such as anisole and guaiacol, are important intermediates in the fields
of flavouring agents, fragrances, dye, agricultural chemicals and pharmaceuti-
cals[14, 15]. Traditionally, these kinds of products were synthesized by William-
son ether synthesis, which require corrosive reagents such as dimethyl sulfate or
alkyl halide in the presence of stoichometric amount of sodium hydroxide as the
homogeneous catalyst[16]. The use of corrosive reagents and homogeneous cata-
lyst will cause environmental problems and produce a large amount of wastes.
To overcome these problems, various solid catalysts have been used to finish
this process, such as clays, heteropoly acids, sulphate-doped metal oxides, metal
and mixed metal oxides, metal phosphates and zeolites, etc[16–31], since this
route is more economical and more environmentally friendly for industrial appli-
cations. It was concluded that the strong acid catalysts favored ring alkylation,
yet the weak acid or base catalysts can help O-alkylation[32]. However, the
performance of solid catalysts is still not satisfactory for their low conversion
and short lifetime.
In this study, a novel kind of rare earth phosphate with a mesoporous micro-
structure has been prepared by template method. Catalytic performance of this
kind of rare earth phosphate in vapor O-methylation of hydroxybenzene has been
investigated, and the effect of catalyst structure of these MREPs for the excellent
performance has also been discussed.
2. Experimental
The preparation of MREPs is very similar to the preparation of mesoporous
metal oxides invented by Yang et al[33]. For a typical preparation, 0.005mol of
Rare earth(III) (La, Eu, Ho, Yb, Er) oxide was dissolved in 3.5g of concentrated
HCl(36%), then the solution was heated to remove water and all other volatiles.
The resulting solid was diluted in ethanol(60ml) and the template
HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H (designated as P123, 0.84g)
was added. Then the result mixture was stirred to be a clear solution and precipi-
tated by the addition of 1.17g (0.01mol) of concentrated H3PO4(85%). Then the
mixture was heated at 40oC overnight to evaporate ethanol slowly and then it
Unauthenticated
Download Date | 3/29/16 5:05 PM

Enhanced Performance in Vapor O-Methylation of …
859
was dried at 60oC for an additional 2hrs. Finally, the resulting solid was calcined
in air from room temperature to 550oC with a heating rate of 1oC.min and
kept at 550oC for 6hrs to remove the template. The final product obtained was
mesoporous rare earth phosphate. For comparison, the rare earth phosphates
(REPs) without the addition of template were prepared by precipitation method.
The preparation method was as follows: 0.005mol of Rare earth(III) (La, Eu,
Ho, Yb, Er) oxide was dissolved in 3.5g of concentrated HCl(36%). Then the
solution was heated to remove water and all other volatiles. The resulting solid
was diluted in water(60ml) and 1.17g(0.01mol) concentrated H3PO4(85%)was
added. The resulting mixture was stirred to uniformity for 1h. Then NH3
·
H2O(10%)was added to adjust the pH at 4~5. The resulting mixture was aged
o
overnight and was recovered by filtering off, washing, and dried at 110 C for
12 h. Finally, the resulting solid was calcined at 550oC for 3 h. All samples were
crushed and sieved to 20~40 mesh before be used in O-methylation reaction.
The powder X-ray diffraction (XRD) data were obtained by using an x-ray
diffractometer (D.MAXIIB, Rigahu), operating at 40 kV and 10 mA, using Cu K
radiation combined with Nickel filter. Transmission electron microscope (TEM)
images were obtained using a JEOL JEM-2010 microscope operating at 200 kV.
The O-alkylation of hydroxybenzene was carried out in a fixed bed glass
reactor with an internal diameter of 8mm. Before the reaction, the catalyst was
pretreated at 350ºC for 3h in flowing dry N2 stream. Then a mixture of hydroxy-
benzene and methanol(hydroxybenzene : methanol = 1:4(mol)) was fed to the
reactor by a syringe pump at a space velocity(WHSV) of 1hK1 under N2
flow(10ml.min). The reaction effluent were collected in an ice-trap and analyzed
by GC equipped with a SE-60 capillary column. Before the data were obtained,
reactions were maintained for a period of 1h at each temperature to ensure
steady-state conditions.
3. Results and discussion
P123 is a poly(alkylene oxide) block copolymer structure non-ion surfactant,
which possesses larger molecular configuration than most of other surfactants. It
was utilized as a versatile template to synthesize mesoporous metal oxide and
mixed metal oxides for the first time by Yang et al[33]. In this study, MREPs
were also synthesized by using P123 as template. Fig. 1 shows the low-angle
powder XRD patterns of MREPs calcined at 550oC. From Fig. 1, every trace of
MREPs has a peak in 0.6~0.8° range, it indicates that mesoporous structure was
preserved in the calcined MREPs. However, this structure was relatively disor-
dered because the peaks were not clear except the (100) reflection. It can also
Unauthenticated
Download Date | 3/29/16 5:05 PM

860
Z. Zhang et al.
Fig. 1. Low angle powder XRD patterns of MREPs (a) LaPO4 (b) EuPO4 (c) YbPO4 (d) HoPO4
(e) ErPO4.
be confirmed by the transmission electron microscopy(TEM) images obtained
from sample of EuPO4(Fig. 2).
Fig.2a, b show that EuPO4 prepared with P123 has a mesoporous structure.
In contrast EuPO4 prepared without P123 has amorphous structure as shown in
Fig. 2 c, d. It can be concluded that the template P123 played an important role
for the mesoporous structure formation. At the same time, it can be seen from
the TEM graphs that the shape of MREP was appeared to be disordered worm-
hole-like structure, which could be due to the effect of synthesis conditions and
the raw materials. Anyway, the novel kind of MREPs materials were synthesized
by the template method and the correlation fields of application were expected.
The application of this kind of MREPs in O-alkylation of hydroxybenzene
was investigated. Table 1 summarized the catalytic performance of MREPs in
the reaction of O-alkylation of phenol with methanol. From Table 1, MREPs
show perfect selectivity for anisole at low reaction temperature, and the highest
selectivity of anisole(100%) was obtained at 250~275oC. With the increasing of
reaction temperature, the alkylation of ring and the selectivity of cresole in-
creased. However the conversion of phenol increased with the increasing of reac-
tion temperature till about 325oC, and then decreased with the continual increas-
ing of reaction temperature. It could be attributed to the isomerization of anisole
was enhanced at high reaction temperature.
The results of O-alkylation of catechol with methanol catalyzed by MREPs
are presented in Table 2. From Table 2, MREPs also show perfect selectivity for
guaiacol at low reaction temperature and the highest selectivity of guaiacol is
nearly 100%. With the increasing of reaction temperature, O-alkylation of the
other hydroxyl and alkylation of ring, the selectivity of veratrole and C-methyl-
ated products increased. But the conversion of catechol increased with the in-
Unauthenticated
Download Date | 3/29/16 5:05 PM

Enhanced Performance in Vapor O-Methylation of …
861
Fig. 2. TEM image of EuPO4 (a) and (b) EuPO4 prepared with P123 (c) and (d) EuPO4 prepared
without P123.
Table 1. Performance of O-alkylation of phenol with methanol Catalyzed by MREPs.
Catalyst
Reaction temper- Conversion
Selectivity(%)
ature(oC)
(%)
anisole
100
100
92.5
67
o-cresole
(m+p)-cresole
EuPO4
250
275
300
325
350
250
275
300
325
350
9.4
0
0
25
0
0
58.7
74.7
74
7.5
19.2
18.7
0
0
13.8
35.8
0
22.4
100
97.1
96.8
89.3
60
HoPO4
21.3
39.7
62.8
73.3
67.6
2.9
3.2
10.7
22.2
0
0
0
17.8
Reaction conditions: WHSV = 1 hK1, phenol: methanol = 1:4 (mol), N2 = 10mL · minK1
creasing of reaction temperature till about 325oC, then decreased with the contin-
ual increasing of reaction temperature. This result is very similar to the result of
O-alkylation of phenol with methanol.
Unauthenticated
Download Date | 3/29/16 5:05 PM

862
Z. Zhang et al.
Table 2. Performance of O-alkylation of catechol with methanol Catalyzed by MREPs.
Catalyst
EuPO4
Reaction tempera- Conversion
Selectivity(%)
Guaiacol
98.2
ture(oC)
(%)
Veratrole
0.5
C-methylated
250
275
300
325
350
250
275
300
325
350
56.4
89.1
99.2
100
1.3
3.3
6.1
12.4
15
92.3
4.4
83.4
9.7
66.7
12.5
9.3
92.8
65.4
97.2
100
67.7
LaPO4
99.1
0
0.7
0.4
0.3
2.9
6.7
98.9
0.7
98.2
1.5
100
92.4
4
87.9
81.3
6.3
Reaction conditions: WHSV = 1 hK1, catechol: methanol = 1:4 (mol), N2 = 10mL · minK1
Many researchers have discussed the effect of active sites in O-alkylation of
hydroxybenzene, however, the studies about the effect of the structure of catalyst
is limited. In fact, the active sites of catalyst can be affected by the catalyst
structure in the same catalyst composition. In the foregoing part, we have charac-
terized the structure of mesoporous EuPO4 and the corresponding REP by TEM
measurement. For understanding the novel MREPs materials, the comparison of
the performance of mesoporous catalysts with the catalysts prepared without
templates has been studied (Fig.3 and Fig. 4). These two figures show that the
yield of anisole and guaiacol are obviously higher for MREPs than those for the
corresponding material prepared without P123 at low reaction temperature. Thus,
the higher catalytic performance of mesoporous materials would be ascribed to
their porous structure and much more amount of catalytic active sites. As we
known, many reactions show low activity at low temperature. In order to obtain
higher activity, higher reaction temperature is required. However, the higher
reaction temperature will result in the production of more byproducts and forma-
tion of cokes, consequently accelerating the deactivation process of the catalysts.
These MREPs presented higher conversion and selectivity at low temperature,
as a result, they would be potential prior materials in catalysis.
4. Conclusion
Mesoporous rare earth phosphates have been synthesized by using P123 as tem-
plate. The catalytic performance of this kind of MREP in O-methylation of hy-
droxybenzene has been studied. These MREPs exhibit higher catalytic perform-
ance in O-methylation of hydroxybenzene than the materials prepared without
template at low temperature. The higher performance of these MREPs would be
ascribed to their porous structure and much more amount of active sites. The
Unauthenticated
Download Date | 3/29/16 5:05 PM

Enhanced Performance in Vapor O-Methylation of …
863
Fig. 3. Comparison of performance of MREPs with REPs in vapor O-methylation of phenol
(a) EuPO4 (mesoporous) (b) EuPO4 (c) HoPO4 (mesoporous) (d) HoPO4.
Fig. 4. Comparison of performance of MREPs with REPs in vapor O-methylation of catechol
(a) EuPO4 (mesoporous) (b) EuPO4 (c) LaPO4 (mesoporous) (d) LaPO4.
characterization and applications of this kind of material are still under investiga-
tion.
Unauthenticated
Download Date | 3/29/16 5:05 PM

864
Z. Zhang et al.
Acknowledgement
We thank the Ministry of Science and Technology of China (973 program
2009CB219900) the National Natural Science Foundation (20803075) and China
Postdoctoral Science Foundation (20090450583) for the support of this research.
References
1. A. Dong, N. Ren, Y. Tang, Y. Wang, Y. Zhang, W. Hua, Z. Gao, J. Am. Chem.
Soc. 125 (2003) 4976.
2. G. Liu, M. Jia, Z. Zhou, L. Wang, W. Zhang, D. Jiang, J. colloid interf. sci. 302
(2006) 278.
3. S. Lucas, E. Champion, D. Bregiroux, D. Bernache-Assollant, F. Audubert, J. Solid
State Chem. 177 (2004) 1302.
4. Y. Takita, K. Sano, T. Muraya, H. Nishiguchi, N. Kawata, M. Ito, T. Akbay, T.
Ishihara, Appl. Catal. A: Gen. 170 (1998) 23.
5. R. Rao, J. Electrochem. Soc. 150 (2003) H165.
6. V. Mehta, G. Aka, A. Dawar, A. Mansingh, Opt. Mater. 12 (1999) 53.
7. N. Imanaka, T. Masui, H. Hirai, G. Adachi, Chem. Mater. 15 (2003) 2289.
8. D. Bregiroux, S. Lucas, E. Champion, F. Audubert, D. Bernache-Assollant, J. Eur.
Ceram. Soc. 26 (2006) 279.
9. L. Ho, H. Nishiguchi, K. Nagaoka, Y. Takita, Mater. Chem. Phys. 97 (2006) 494.
10. F. Meiser, C. Cortez, F. Caruso, Angew. Chem. Int. Edit. 43 (2004) 5954.
11. Q. Liu, Z. Zhang, Y. Du, J. Li, X. Yang, Catal. Lett. 127 (2009) 419.
12. Z. Zhang, J. Li, X. Yang, Catal. Lett. 118 (2007) 300.
13. Y. Du, Y. Xiong, J. Li, X. Yang, J. Mol. Catal. A: Chem. 298 (2009) 12.
14. G. Dorothea ,E. Barbara, Ullmann's encyclopedia of industrial chemistry, Phenol
Derivatives, Vol, A19, Weinheim, VCH.Verlagsgesellschaft (1991) p. 354.
15. G. Wu, X. Wang, B. Chen, J. Li, N. Zhao, W. Wei, Y. Sun, Appl. Catal. A: Gen.
329 (2007) 106.
16. R. Bal, S. Sivasanker, Appl. Catal. A: Gen. 246 (2003) 373.
17. V. Durgakumari, S. Narayanan, L. Guczi, Catal. Lett. 5 (1990) 377.
18. V. Rao, K. Chary, V. Durgakumari, S. Narayanan, Appl. Catal. 61 (1990) 89.
19. C. Bezouhanova, M. Al-Zihari, Appl. Catal. A: Gen. 83 (1992) 45.
20. J. Xu, A. Yan, Q. Xu, React. Kinet. Catal. L. 62 (1997) 71.
21. M. Samolada, E. Grigoriadou, Z. Kiparissides, I. Vasalos, J. Catal. 152 (1995) 52.
22. G. Sarala Devi, D. Giridhar, B. Reddy, J. Mol. Catal. A: Chem. 181 (2002) 173.
23. S. Kirumakki, N. Nagaraju, K. Chary, S. Narayanan, J. Catal. 221 (2004) 549.
24. X. Li, W. Zhang, G. Liu, L. Jiang, X. Zhu, C. Pan, D. Jiang, A. Tang, React. Kinet.
Catal. L. 79 (2003) 365.
25. Y. Fu, T. Baba, Y. Ono, Appl. Catal. A: Gen. 166 (1998) 419.
26. S. Porchet, L. Kiwi-Minsker, R. Doepper, A. Renken, Chem. Eng. Sci. 51 (1996)
2933.
27. R. Bal, B. Tope, S. Sivasanker, J. Mol. Catal. A: Chem. 181 (2002) 161.
28. V. Vishwanathan, K. Raghavan, S. Ndou, L. Sikhwivhilu, N. Plint, N. Coville, Chem.
Commun. 2001 (2001) 893.
29. Z. Fu, Y. Yu, D. Yin, Y. Xu, H. Liu, H. Liao, Q. Xu, F. Tan, J. Wang, J. Mol.
Catal. A: Chem. 232 (2005) 69.
30. G. Liu, Z. Wang, M. Jia, X. Zou, X. Zhu, W. Zhang, D. Jiang, J. Phys. Chem. B.
110 (2006) 16953.
31. X. Zhu, X. Li, X. Zou, Y. Wang, M. Jia, W. Zhang, Catal. Commun. 7 (2006) 579.
32. C. Dartt, M. Davis, Catal. Today. 19 (1994) 151.
33. P. Yang, D. Zhao, D. Margolese, B. Chmelka, G. Stucky, Nature. 396 (1998) 152.
Unauthenticated
Download Date | 3/29/16 5:05 PM