Synthesis and catalytic applications of hierarchical mesoporous AlPO 4/ZnAlPO4 for direct hydroxylation of benzene to phenol using hydrogen peroxide

Article abstract of DOI:10.1039/c3ta00113j

Amorphous hierarchical mesoporous AlPO₄ and ZnAlPO₄ materials have been successfully synthesized for the first time by a simple physical mixing method, where tetrapropyl ammonium bromide acts as both a template and structure directing agent. The materials exhibited excellent catalytic activity for the production of phenol from benzene (99% conversion with 85% selectivity).

Full text of DOI:10.1039/c3ta00113j

Journal of
Materials Chemistry A
View Article Online
View Journal | View Issue
COMMUNICATION
Synthesis and catalytic applications of hierarchical
mesoporous AlPO4/ZnAlPO4 for direct hydroxylation of
benzene to phenol using hydrogen peroxide†
Cite this: J. Mater. Chem. A, 2013, 1,
3268
Received 9th January 2013
Accepted 25th January 2013
*
Peta Sreenivasulu, Devaki Nandan, Manoj Kumar and Nagabhatla Viswanadham
DOI: 10.1039/c3ta00113j
www.rsc.org/MaterialsA
Amorphous hierarchical mesoporous AlPO4 and ZnAlPO4 materials reactants in the active sites and governs their suitability for
have been successfully synthesized for the first time by a simple various applications. Recently, hierarchical porous materials
physical mixing method, where tetrapropyl ammonium bromide acts have been observed to exhibit superior catalytic and adsorp-
as both a template and structure directing agent. The materials tion12 properties due to the presence of multiple sizes of pore
exhibited excellent catalytic activity for the production of phenol channels and their inter-connectivity, making them suitable for
from benzene (99% conversion with 85% selectivity).
the free accessibility of the reactant molecules, and their facile
diffusion which is responsible for the activity and stability of the
materials for catalytic applications. The hierarchically porous
aluminophosphate materials are also synthesized by using
small amines and bulky organosilane surfactants.13 However,
most of the synthesis methods are time consuming and involve
multiple steps. Hence, the synthesis of amorphous hierarchi-
cally porous AlPO4/ZnAlPO4 materials through a simple one-
step method using a single organic template is attempted in the
present study. The resultant materials with hierarchical
porosity are applied to the industrially important reaction of
benzene to phenol in a single-step direct hydroxylation using
hydrogen peroxide.
Phenol is one of the most important industrial chemicals
that is used in dyes, resins and pharmaceuticals etc. The
industrial process produces 90% of phenol through a three step
reaction system.14 The direct conversion of benzene to phenol is
potentially an area of great interest and is still regarded as one
of the 10 most difficult challenges in catalysis,15 as the phenol
selectivity is limited due to the higher oxidation reactivity of
phenol when compared to benzene.16 The direct hydroxylation
of benzene to phenol is being investigated using different
oxidizing agents such as N2O,17 H2O2,18 NH3 + O2,19 air + CO,20
molecular O2 (ref. 21) and a mixture of H2 + O2.22 However, the
gas phase hydroxylation has some major drawbacks due to
rapid deactivation of the catalyst by coke deposition.23 Transi-
tion metal doped AlPO4 is known for its selective oxidation of
hydrocarbons,24 but it gives a benzene conversion of only 28%
in the presence of oxidants such as molecular O2, N2O and H2O2
(ESI Table S2†).
The synthesis of mesoporous aluminophosphate materials has
received great attention due to their wide range of potential
applications in catalysis and adsorption.1,2 The surfactant
templating method is most popular for the synthesis of lamellar
or hexagonal mesoporous AlPO4.3 However, the main drawback
of this method lies in the structural collapse of the material
upon removal of the surfactant by calcination.4,5 Another
drawback is obtaining porous AlPO4 with an adjustable P/Al
ratio close to or above 1 : 1,3a,6,7 which limits the acidic property
of the material required for catalytic applications.8,9 The acidity
and catalytic properties of AlPO4 materials are also controlled by
introducing a heteroatom into the structure of AlPO4. Divalent
metal ions such as Mg2+, Mn2+, Zn2+, and Co2+ etc. are incor-
porated into AlPO4 to substitute a small amount of framework
negative charges,10 and the materials possess additional
features, due to well-separated active-site centers, allowing
them to act as efficient solid catalysts for many reactions.11 It is
interesting to study the possibility of synthesizing AlPO4 and
MAlPO4 materials that are structurally stable even aer the
removal of the structure directing agent, and which possess the
desired acidity and catalytic properties which are suitable for
various catalytic applications.
Porosity is another important feature of the materials that
plays a vital role in facilitating molecular level interactions of
Catalysis and Conversion Processes Division, CSIR-Indian Institute of Petroleum,
Dehradun-248005, India. E-mail: nvish@iip.res.in; Fax: +91-135-2525702; Tel: +91-
135-2525856
Herein we report a simple physical mixing method for the
synthesis of hierarchical mesoporous AlPO4 and ZnAlPO4
materials by using a single organic template, TPABr. The
† Electronic supplementary information (ESI) available: Experimental details, low
angle XRD, EDX, TPD, SEM, IR spectra, reaction table. See DOI:
10.1039/c3ta00113j
3268 | J. Mater. Chem. A, 2013, 1, 3268–3271
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Communication
Journal of Materials Chemistry A
material exhibited excellent catalytic activity in the direct
hydroxylation of benzene for the selective production of phenol.
To the best of our knowledge, the present study is the rst of its
kind in terms of (1) the synthesis of hierarchical mesoporous
ZnAlPO4 and AlPO4, (2) a simple solvent-free synthesis method
using a single organic template, TPABr, by adopting physical
mixing, and (3) showing promising catalytic activity of ZnAlPO4,
achieving 85% selectivity for phenol at as high as 99% benzene
conversion.
In a typical synthesis method, the amorphous mesoporous
ZnAlPO4 materials were synthesized by admixing 4 g of tetra-
methyl ammonium hydroxide pentahydrate 25 wt% in meth-
anol (TMAHP), 6 g of aluminum isopropoxide, 4 g of tetrapropyl
ammonium bromide (TPABr), 3.2 g of ammonium dihy-
drophosphate and 0.97 g of zinc nitrate to obtain a paste, fol-
lowed by treatment of the resultant mixture at 150 ꢀC in an oven
for 24 h. At the end of the treatment, the compound was washed
with an ample amount of ethanol and calcined at 500 ꢀC for 5 h.
The zinc samples are denoted by ZAC and the aluminium
sample denoted by APC. Before calcination the samples are
denoted by ZAS and APS and aer reaction the samples are
denoted by ZAU and APU.
The presence of Zn, Al, P and O in the synthesized, calcined
and used samples was conrmed by energy-dispersive X-ray
spectroscopy (EDX) analysis (ESI Fig. S1†). The wide angle X-ray
diffraction (XRD) patterns of as-synthesized AlPO4 and ZnAlPO4
show that both samples are crystalline in nature (Fig. 1A),
but are both converted to an amorphous material upon calci-
nation (Fig. 1B). This may be due to the removal of the structure-
governing organic template during the high temperature
calcination.
The scanning electron microscopy (SEM) images (Fig. S2†)
indicate that the materials possess layer-like structures, while
the transmission electron microscopy (TEM) images (Fig. 2)
reveal a sponge-like structure with uniformly distributed pores.
The materials retained the textural properties even aer the
reaction (Fig. 2E and F), which supports the reaction stability of
the material. Such a porous material is expected to exhibit high
surface area and porosity. The low-angle XRD patterns (ESI
Fig. S3†) of the synthesized and calcined samples of AlPO4 and
ZnAlPO4 also reveal the presence of mesopores in these mate-
rials.25 The porosity of the APC and ZAC samples measured by
N2 adsorption–desorption measurements (Fig. 3A) further
conrms the presence of mesopores with a wide range of pore
Fig. 2 (A, C and E) TEM images of as-synthesized, calcined and used AlPO4
materials respectively, and (B, D and F) TEM images of as-synthesized, calcined
and used ZnAlPO4 materials respectively.
sizes. The adsorption–desorption isotherm also represents
mixed types of isotherms (a combination of type I, II and IV). A
steep increase occurs in the curve at a relative pressure of 0.01,
indicating the lling of the micropores, something that is
commonly observed for larger mesopores and provides direct
evidence for the presence of worm-like mesopores in these
samples. The isotherm indicates the occurrence of well-dened
capillary condensation at a relative pressure (P/Po) of 0.01–1.0.
The BJH pore size distribution curve of APC and ZAC (Fig. 3B)
reveals the presence of micropores as well as mesopores with
various pore diameters. The mesopores are distributed from a
2–100 nm pore diameter with the major population of pores
having a diameter of 10–50 nm (Fig. 3B), revealing the hierar-
chical nature of the mesopores. The formation of the pores with
Fig. 1 Wide angle XRD patterns of AlPO4 and ZnAlPO4 materials (A) before and
Fig. 3 (A) N2 adsorption–desorption isotherms, and (B) BJH pore size distribu-
(B) after calcination.
tion, of AlPO4 and ZnAlPO4 materials.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. A, 2013, 1, 3268–3271 | 3269

View Article Online
Journal of Materials Chemistry A
Communication
different pore diameters obtained in the present study may be method. Such a high enhancement in catalyst performance of
due to the heterogeneous distribution and aggregate size of the AlPO4 in the presence of Zn clearly shows the vital role of Zn in
template which occur in the physical mixing method, where the the benzene hydroxylation reaction. However, both the catalysts
calcination of the corresponding materials expels the template exhibited a common phenomenon of high phenol selectivity
to form pores of different sizes. In spite of similarities in the and reusability, which may be due to the facile diffusion facil-
hierarchical porous nature, there is a distinct difference in the itated by the hierarchical mesopores of the catalysts. To the best
loop conguration and pore size distribution patterns of AlPO4 of our knowledge, this is the highest conversion of benzene ever
and ZnAlPO4 which may be due to the difference in the removal reported for this reaction on AlPO4/ZnAlPO4 materials and
patterns of the organic template from these materials during proves the role of Zn in selective hydroxylation reactions.
the calcination. This is also reected in the difference in the Further, the ZnAlPO4 of the present study exhibited superior
BET surface area (126.7 m2 gÀ1 and 109.6 m2 gÀ1) and pore performance to that of the reported catalysts in the literature
volume (0.23 cm3 gÀ1 and 0.16 cm3 gÀ1) of the AlPO4 and (ESI Table S2†). Moreover, the 100% selectivity to phenol
ZnAlPO4 samples respectively (ESI Table S1†). The AlPO4 and obtained on the AlPO4 catalyst reveals the selective hydroxyl-
ZnAlPO4 materials exhibiting hierarchical porosity aer calci- ation ability of the catalyst for benzene hydroxylation without
nation at 500 ꢀC conrms the high temperature thermal encouraging further hydroxylation of phenol. A reference
stability, while retaining its porosity. This is a novel property experiment was also conducted in the absence of the catalyst,
and has not been reported so far.
The Fourier transform infrared spectroscopy (FT-IR) spectra conrms the catalytic role of the AlPO4/ZnAlPO4 materials.
(ESI Fig. S4†) of the samples show a broad band at 3540 cmÀ1
In summary, the present study provides a simple and novel
using only H2O2, in which no reaction was observed, which
,
which is related to the presence of the surface hydroxyl groups method for the synthesis of hierarchical mesoporous ZnAlPO4
associated with phosphorus that is perturbed by a hydrogen through a solvent-free, single organic template method. The
bridge bond from a surface hydroxyl band. The other two bands material exhibited promising benzene conversion for the effi-
at 1110 cmÀ1 and 468 cmÀ1 may be due to triply degenerate P–O cient production of phenol by the selective hydroxylation of
stretching vibrations and triply degenerate O–P–O bending benzene. Furthermore, the material shows its reusability with
3À
vibrations of tetrahedral PO4
respectively.26 The acidity an excellent catalytic performance even aer three reaction
patterns measured by temperature-programmed desorption cycles (Table 1). The subject opens up a new property of the
(TPD) (ESI Fig. S5†) indicate the presence of two NH3 desorption metal AlPO4 materials as suitable catalysts for selective oxida-
peaks in both samples; one broad peak centered at 150 ꢀC and tion reactions, and has scope in the improvement of the cata-
ꢀ
the other at around 400 C, representing the weak and strong lytic activity through the optimization of the synthesis
acid sites respectively. However, the high temperature desorp- procedure of ZnAlPO4 for expansion of its applications to other
tion peak is shied to the higher temperature side in the ZAC selective hydroxylation reactions.
sample, indicating the creation of strong acid sites by the Zn.
The authors are thankful to the director, IIP, for his
The AlPO4 and ZnAlPO4 possessing high surface area, hier- encouragement. PS and DN acknowledge CSIR, New Delhi for
archical mesoporosity and strong acidity are expected to exhibit awarding a fellowship. We are thankful to the XRD, IR, and gas
promising catalytic activity and in the present study are chromatography–mass spectrometry (GC/MS) groups at IIP for
explored in the selective hydroxylation of benzene (Table 1). The analysis.
AlPO4 sample exhibits a lower benzene conversion of around
13%, with 100% selectivity to phenol. It is interesting to see that
the conversion increases to 99% in the case of the ZnAlPO4
Notes and references
sample. However, a small amount (15%) of 1,4 benzoquinone
by-product (product B) is obtained along with the phenol
(product A) (85%) in this case. The acidity patterns of NH3–TPD
(ESI Fig. S5†) suggest the increased acidity as the reason for the
enhanced reaction performance of ZnAlPO4 when compared to
the corresponding AlPO4 catalyst synthesized by a similar
1 J. Lu, K. T. Ranjit, P. Rungrojchaipan and L. Kevan, J. Phys.
Chem. B, 2005, 109, 9284.
2 (a) P. Selvam and S. K. Mohapatra, J. Catal., 2006, 238, 88; (b)
T. Kimura, Chem. Mater., 2005, 17, 337.
3 (a) T. Kimura, Microporous Mesoporous Mater., 2005, 77, 97;
(b) M. Tiemann and M. Froba, Chem. Mater., 2001, 13, 3211.
4 (a) F. Schuth, Chem. Mater., 2001, 13, 3184; (b) M. Tiemann,
M. Schulz, C. Jager and M. Froba, Chem. Mater., 2001, 13,
2885.
Table 1 Performance of catalysts in benzene hydroxylation
5 L. Wang, B. Tian, F. Jie, X. Liu, H. Yong, C. Yu, B. Tu and
D. Zhao, Microporous Mesoporous Mater., 2004, 67, 123.
6 (a) Y. Z. Khimyak and J. Klinowski, Phys. Chem. Chem. Phys.,
2000, 2, 5275; (b) Z. Luan, D. Zhao, H. He, J. Klinowski and
L. Kevan, J. Phys. Chem. B, 1998, 102, 1250.
7 J. E. Haskouri, C. Guillem, J. Latorre, A. Beltram, D. Beltran
and P. Amoros, Chem. Mater., 2004, 16, 4359.
8 F. M. Bautista, J. M. Campelo, G. D. Luan, J. M. Marinas,
R. A. Quiros and A. A. Romero, Appl. Catal., A, 2003, 243, 93.
Product
selectivity (%)
Catalyst
Conversion
A
B
Benzene hydroxylation
AlPO4
AlPO4 (reused)
ZnAlPO4
13
11
99
85
100
100
85
—
—
15
12
ZnAlPO4 (reused)
88
3270 | J. Mater. Chem. A, 2013, 1, 3268–3271
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Communication
Journal of Materials Chemistry A
9 X. Li, W. Zhang, G. Liu, L. Jiang, X. Zhu, C. Pan, D. Jiang and 19 R. Bal, M. Tada, T. Sasaki and Y. Iwasawa, Angew. Chem., Int.
A. Tang, React. Kinet. Catal. Lett., 2003, 79, 365. Ed., 2006, 45, 448.
10 M. Hartmann and L. Kevan, Res. Chem. Intermed., 2002, 28, 20 M. Tani, T. Sakamoto, S. Mita, S. Sakaguchi and Y. Ishii,
625. Angew. Chem., Int. Ed., 2005, 44, 2586.
11 (a) J. M. Thomas and R. Raja, Acc. Chem. Res., 2008, 41, 708; 21 (a) Y. Liu, K. Murata and M. Inaba, J. Mol. Catal. A: Chem.,
(b) M. Hartmann and L. Kevan, Chem. Rev., 1999, 99, 635; (c)
J. M. Thomas and R. Raja, Microporous Mesoporous Mater.,
2006, 256, 247; (b) T. D. Bui, A. Kimura, S. Ikeda and
M. Matsumura, J. Am. Chem. Soc., 2010, 132, 8453.
2007, 105, 5; (d) J. M. Thomas, R. Raja and D. W. Lewis, 22 W. Laufer and W. F. Hoelderich, Chem. Commun., 2002,
Angew. Chem., Int. Ed., 2005, 44, 6456. 1684.
12 (a) F.-S. Xiao, L. F. Wang and C. Y. Yin, Angew. Chem., Int. Ed., 23 (a) E. J. M. Hensen, Q. Zhu and R. A. Van santen, J. Catal.,
2006, 45, 3090; (b) T. D. Tang, C. Y. Yin, L. F. Wang and
F.-S. Xiao, J. Catal., 2008, 257, 125.
13 M. Choi, R. Srivastava and R. Ryoo, Chem. Commun., 2006,
4380.
2003, 220, 260; (b) S. Niwa, M. Eswaramoorthy, J. Nair,
A. Raj, N. Itoh, H. Shoji, T. Namba and F. Mizukami,
Science, 2002, 295, 105; (c) R. Dittmeyer and L. Bortolotto,
Appl. Catal., A, 2011, 391, 311; (d) Y. Guo, X. Zhang,
H. Zou, H. Liu, J. Wang and K. L. Yeung, Chem. Commun.,
2009, 5898; (e) R. Molinari, T. Poerio and P. Argurio,
Desalination, 2009, 241, 22.
´
´
14 A. Conde, M. M. Dıaz-Requejo and P. J. Perez, Chem.
Commun., 2011, 47, 8154.
15 B. Cornil and W. A. Herrmann, J. Catal., 2003, 216, 23.
16 J. R. L. Smith and R. O. C. Norman, J. Am. Chem. Soc., 1963, 24 R. A. Sheldon and J. K. Kochi, Metal Catalyzed Oxidations of
85, 2897. Organic Compounds, Academic Press, New York, 1981.
17 (a) A. J. J. Koekkoek, H. Xin, Q. Yang, C. Li and 25 Z. Niu, S. Kabisatpathy, J. He, L. A. Lee, J. Rong, L. Yang,
E. J. M. Hensen, Microporous Mesoporous Mater., 2011, 145,
172; (b) N. R. Shiju, S. Fiddy, O. Sonntag, M. Stockenhuber
and G. Sanker, Chem. Commun., 2006, 4955.
G. Sikha, B. N. Popov, T. S. Emrick, T. P. Russell and
Q. Wang, Nano Res., 2009, 2, 474.
26 (a) K. Mtalsi, T. Jei, M. Montes and S. Tayane, J. Chem.
Technol. Biotechnol., 2001, 76, 128; (b) J. M. Campelo,
M. Jaraba, D. Luna, R. Luque, J. M. Marinas and
A. A. Romero, Chem. Mater., 2003, 15, 3352.
18 (a) J. Chen, S. Gaoa and J. Xu, Catal. Commun., 2008, 9, 728;
(b) J. Chen, J. Li, Y. Zhang and S. Gao, Res. Chem. Intermed.,
2010, 36, 959.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. A, 2013, 1, 3268–3271 | 3271