Synthesis of orderly nanoporous aluminophosphate and zirconium phosphate materials and their catalytic applications

Article abstract of DOI:10.1039/c4cc02614d

Amorphous alumino phosphate (AP) and zirconium phosphate (ZP) materials possessing an ordered nanoporosity have been successfully synthesized by a hydrothermal method using a P123 block co-polymer as the structure directing agent. The materials exhibited excellent catalytic activity towards selective alkylation of phenol with cyclohexanol, where AP showed as high as 100% selectivity to produce the industrially important O-alkylation product, while the corresponding ZP selectively produced a C-alkylation product (93% selectivity). This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02614d

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Synthesis of orderly nanoporous
aluminophosphate and zirconium phosphate
materials and their catalytic applications†
Cite this: Chem. Commun., 2014,
50, 6232
Received 9th April 2014,
Accepted 22nd April 2014
Peta Sreenivasulu,ab Nagabhatla Viswanadham,*ab Trymbkesh Sharmab and
B. Sreedharc
DOI: 10.1039/c4cc02614d
www.rsc.org/chemcomm
Amorphous alumino phosphate (AP) and zirconium phosphate (ZP) the narrow pores in such materials also cause severe mass transfer
materials possessing an ordered nanoporosity have been successfully limitations. Here, the amorphous materials provide alternate
synthesized by a hydrothermal method using a P123 block co-polymer solution for creation of large size pores (nano-pores 4 2 nm)
as the structure directing agent. The materials exhibited excellent cata- but, obtaining ordered nano-pores in amorphous materials is
lytic activity towards selective alkylation of phenol with cyclohexanol, critical yet important in catalysis and organic synthesis.
where AP showed as high as 100% selectivity to produce the indust-
Metal phosphates are one of such important materials that need
rially important O-alkylation product, while the corresponding ZP to be addressed for the creation of nano-pores as the applications
selectively produced a C-alkylation product (93% selectivity).
of many open-framework and layered metal phosphates with varied
compositions and properties have been constrained by the lack of
Porosity is an important feature of materials that plays a vital large surface areas and the dominance of small pores or interlayer
role in facilitating molecular level interactions of reactants with spaces.7–10 Two synthesis approaches are reported in the literature
the active sites and governs their suitability for various applica- for these materials. The first approach involves the direct synthesis
tions. Nano-pores, referring to the pores of nanometer range, are of nano-porous metal phosphates via surfactant templating,11–19
recently gaining importance due to their suitability for accom- but the material obtained in this method suffers from the draw-
modating bulky molecular transformations. Porous materials backs of lower surface area and pore volume essential for catalytic
should have a narrow pore size distribution which is critical applications. The second approach involves the dispersion of metal
for size-specific applications and a readily tunable pore size phosphate guest on nano-porous silica host (possessing large
allowing flexibility for host–guest interactions and the materials surface areas and ordered nanopores).20–22 However, it is difficult
should possess high thermal, chemical and mechanical stabilities, to finely control the guest–host interaction to obtain homogenous
with high surface area and large pore volume.1 The materials composition and structure via wetness impregnation in this
should also have appropriate particle size and morphology.
method. Though the synthesis of zirconium phosphates by using
The ordered framework structures of crystalline aluminosilicate a P123 block copolymer has been reported, most of the methods
and aluminophosphate materials possessing a uniform (narrow) involve complicated procedures and longer synthesis times are
pore size distribution have been successfully applied for size- required to obtain the material.23,24 These deficiencies prompted
specific adsorption, molecular sieving, host–guest chemistry and us to develop methodologies for the synthesis of metal phosphates
shape-selective catalysis.2–6 However, the pore size in such crystal- with large surface areas and large pores which were successfully
line materials (zeolites) is limited to B1.5 nm thus excluding size explored for the industrially important alkylation of phenol with
specific processes involving large molecules such as heavy oil cyclohexanol. Further, the nature of metal (Al or Zr) is observed to
conversion in petroleum refining, macromolecule transformations influence the selective alkylation at the oxygen or the carbon atom
in drug and fine chemical applications, separation processes of the phenol group to produce the desired product.
and for support applications to host large molecules. Further,
Herein we report a simple method for the synthesis of
zirconium phosphate and alumino phosphate materials by using
a P123 block copolymer as an organic template. The typical
synthesis method involves the admixing of 6.102 g of P123,
2.12 g of 1 N HCl, 12.21 g of ammoniumdihydrogenphosphate,
75 g of methanol followed by stirring at 60 1C for 2 h until the
formation of a clear solution (mixture A). Similarly 28.26 g of
a AcSIR-Indian Institute of Petroleum, Dehradun, India
b Catalysis and Conversion Processes Division, Indian Institute of Petroleum,
Council of Scientific and Industrial Research, Dehradun-248005, India.
E-mail: nvish@iip.res.in; Fax: +91-135-2525702; Tel: +91-135-2525856
c CSIR-Indian Institute of Chemical Technology, India. E-mail: sreedharb@iict.res.in
† Electronic supplementary information (ESI) available: Experimental details,
wide angle XRD, EDX, SEM images, IR spectra, etc. See DOI: 10.1039/c4cc02614d zirconium acetylacetonate and 75 g of methanol were admixed
6232 | Chem. Commun., 2014, 50, 6232--6235
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and heated at 45 1C under stirring for obtaining a clear solution
(mixture B). Then mixture A was added slowly to mixture B with
continuous stirring (for 30 minutes) to form a precipitate. The
resultant mixture was treated inside the Teflon-lined autoclave at
150 1C for 24 h. At the end of the treatment, the compound
was washed and collected by filtration, dried and calcined at
500 1C for 5 h. The same procedure is followed for the synthesis of
aluminophosphate with the only difference being the use of
aluminoisopropoxide in place of zirconium acetylacetonate. The
resultant materials are denoted as ZP (zirconium phosphate) and
AP (aluminophosphate) respectively.
The presence of Al, Zr, P and O samples was confirmed from
EDX analysis (ESI,† Fig. S1). Both AP and ZP samples exhibited
similar weight loss patterns in TGA (ESI,† Fig. S2), where a
major weight loss of B17–26 wt% was related to the degrada-
tion of P123-polymer was observed. The minor weight losses of
1.8–4.3 wt% were also observed at higher temperature range of
400–500 1C in these samples, which indicates the continuous
dehydrogenation and carbonization processes of the polymer
species. These results indicate that the copolymer templates
can be decomposed by calcination at 500 1C for 5 h under N2.25
The low-angle XRD patterns (ESI,† Fig. S3) of AP and ZP samples
reveal the presence of mesopores in these materials.26 A wide angle
XRD of AP and ZP shows that both the samples are amorphous in
nature (ESI,† Fig. S4). The SEM (ESI,† Fig. S5) and TEM images
(Fig. 1) reveal the porous nature of the samples. The enlarged
portion given in Fig. 1C and D indicates the uniformly distributed
pores. Such a porous material is expected to exhibit high surface area
and porosity. The porosity of the AP and ZP samples measured by N2
adsorption–desorption measurements (Fig. 2) further confirms the
presence of mesopores. The adsorption–desorption isotherm also
represents mixed types of isotherms (combination of types I, II
and IV). The isotherm indicates the occurrence of well-defined
capillary condensation at a relative pressure (P/Po) of 0.5–1.0. The
BJH pore size distribution curve of AP and ZP samples (Fig. 2, inset)
reveals the presence of mesopores. However, the shape of isotherms
is different for two samples this may be due to the difference in the
Fig. 2 N2 adsorption–desorption isotherm, and BJH pore size distribution
of AP and ZP materials (inset).
Table 1 Textual properties of AP and ZP samples
BET surface
area (m2 gÀ1
Total pore volume
Mean pore
diameter (nm)
Samples
)
(cm3 gÀ1
)
AP
ZP
370
188
1.17
0.51
12.68
10.92
interaction of the chemical ingredients and the distribution of the
organic template with Al and Zr. This is aÀls1o reflected in the
difference in the BET surface area (370 m2 g and 180 m2 gÀ1
)
and pore volume (1.17 cm3 gÀ1 and 0.51 cm3 gÀ1) of the AP and ZP
samples (Table 1). The relatively low surface area and pore volume
observed in ZP can be ascribed to the involvement of heavy metal Zr
and the higher density of the material. Both the samples exhibited
similar BJH pore size distribution patterns with the majority of pores
having a diameter of 5 nm (peak maxima in Fig. 3) revealing the for-
mation of ordered nano-size pores in these two amorphous materials.
The acidity patterns measured by TPD (Fig. 3) indicate the
presence of two NH3 desorption peaks in both the samples: one
Fig. 1 A, C and B, D are TEM images of AP, and ZP samples respectively.
Fig. 3 TPD spectra of the AP and ZP samples.
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compounds,29 whereas the C-alkylated products are used as impor-
tant intermediates in the preparation of dyestuffs, drugs, printing
inks, wire enamels, rubber chemicals, petroleum additives, poly-
mer additives, resins and so on.30 Hence, the selective production
of each alkylated product is quite challenging in these reaction
processes. The concept of green chemistry and its applications in
synthetic organic chemistry has been emerging as major route for
the development of clean chemical synthesis. A green chemical
process for sustainable development always involves considerations
such as the use of cost-effective catalysts without any additives and
employing a solvent-free system.31 In alkylation of phenol with
cyclohexanol very few catalysts have been used, such as the cation-
exchange resin KU-2, H-zeolite and other conventional Lewis and
Brønsted acid catalysts.32 But these catalysts produce many side
products, and the process is time consuming and consumes a large
amount of solvent that gets wasted. Hence, the development of an
environmentally benign catalyst that works in solvent-free alkyl-
ation, yet exhibits efficient activity is a challenging task for modern
Fig. 4 Pyridine FT-IR XRD of the AP and ZP samples.
broad peak centered at 150 1C and the other around 400 1C organic synthesis. The AP and ZP samples possessing nanoporosity
representing the weak and strong acid sites respectively. The intensity along with acidity are expected to exhibit promising catalytic activity
of the strong acidity peak is relatively higher in the case of ZP, which towards this reaction. In the present study we have conducted
indicates the creation of strong acid sites via the interaction of Zr with reaction studies on these two nanoporous materials for the selec-
P in this material. The nature of acidity is further characterized using tive alkylation of phenol with cyclohexanol (ESI†). The influence of
Pyridine FT-IR spectroscopy (Fig. 4). Usually, the IR bands appearing the reaction temperature and reaction time was studied to under-
at 1540–1548 cmÀ1 and 1445–1460 cmÀ1 are related to Brønsted (B) stand the catalyst performance. By varying the reaction temperature
and Lewis (L) acid sites respectively. The IR patterns given in Fig. 4 from 50 to 150 1C, a gradual improvement in conversion of cyclo-
clearly indicate the presence of both Brønsted and Lewis acid hexanol (48 to 68%) and selectivity of O-alkylated product (100%) is
sites in both AP and ZP samples. However, a higher intensity of observed for the AP sample (Table 2, entries 1 to 3). The ZP sample
both Brønsted (at B1540) and Lewis (B1445) acid sites was also exhibited a similar trend in conversion of cyclohexanol (82 to
observed in the sample ZP when compared to those in AP. A band 100%) and selectivity of the C-alkylated product (88 to 93%) for a
at B1490 related to the combination of both Brønsted and Lewis constant reaction time of 4 h (Table 2, entries 1 to 3). However,
acid sites is also observed in both the samples.27
further increase in the reaction temperature (175 1C) did not cause
Catalytic alkylation of substituted phenols with cyclohexanol is any significant effect on the conversion values of both the catalysts
one of the most important industrial organic reactions.28 These (Table 2, entries 4 for AP and 4 for ZP). By varying the reaction time
reactions yield both O- and C-alkylated products having com- from 1 to 4 h, a gradual improvement in conversion of cyclohexanol
mercial utility. The O-alkylated products are promising perfumery (39 to 68%) and selectivity of O-alkylated product (100%) is
Table 2 Performance of catalysts in phenol alkylation reaction
Product selectivity (%)
Entry
Catalyst
AP
Temperature (1C)
Time (h)
Cyclohexanol conversion (%)
O-alkylation
C-alkylation
1
2
3
4
5
6
7
8
50
100
150
175
150
150
150
150
4
4
4
4
1
2
3
6
48
56
68
66
39
47
53
69
100
100
100
100
100
100
100
100
—
—
—
—
—
—
—
—
1
2
3
4
5
6
7
8
ZP
50
100
150
175
150
150
150
150
4
4
4
4
1
2
3
6
82
87
100
99
62
73
12
9
7
88
91
93
94
80
86
89
94
6
20
14
11
6
87
99
Phenol = 1 mole, cyclohexanol = 1 mole, catalyst = 0.5 g.
6234 | Chem. Commun., 2014, 50, 6232--6235
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observed for the AP sample (Table 2, entries 5 to 7 and 3). The ZP Notes and reference
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