Unique mesoporous silicoaluminophosphate assembled from faujasite-type SAPO-37 precursor: A potential catalyst for isomerization

Article abstract of DOI:10.1246/cl.130446

Unique mesoporous silicoaluminophosphate (MESO-SAPO-37) with uniform pores (3 nm) was synthesized for the first time by using a faujasite-type microporous SAPO-37 precursor. MESO-SAPO-37 contains hierarchical mesopores with a microporous secondary building unit. It possesses strong acidity and shows high catalytic activity for the conversion of 1-octene, with the exclusive formation of isomerized products (84%).

Full text of DOI:10.1246/cl.130446

doi:10.1246/cl.130446
1160
Published on the web June 8, 2013
Unique Mesoporous Silicoaluminophosphate Assembled from Faujasite-type SAPO-37 Precursor:
A Potential Catalyst for Isomerization
Rekha Yadav, Arvind Kumar Singh, and Ayyamperumal Sakthivel*
Department of Chemistry, Inorganic Materials and Catalysis Laboratory, University of Delhi, Delhi-110007, India
(Received May 13, 2013; CL-130446; E-mail: asakthivel@chemistry.du.ac.in)
Unique mesoporous silicoaluminophosphate (MESO-SAPO-
37) with uniform pores (3 nm) was synthesized for the first time
by using a faujasite-type microporous SAPO-37 precursor.
MESO-SAPO-37 contains hierarchical mesopores with a micro-
porous secondary building unit. It possesses strong acidity and
shows high catalytic activity for the conversion of 1-octene, with
the exclusive formation of isomerized products (84%).
Concern for natural resources and consideration of environ-
mental impacts are essential for the development of better
catalytic systems for various fine and petrochemical processes.1,2
Zeolite and zeolite-like aluminophosphate materials are of great
interest in this context, as they show good adsorptive and
catalytic properties with flexible frameworks.3 The limitations of
these microporous molecular sieves have been overcome through
Scheme 1. Schematic representation of MESO-SAPO-37 syn-
thesis.
the successful synthesis of mesoporous aluminosilicates such as
the M41s family,4 SBA-15,5 HMS,6 mesoporous titania,7 and
mesoporous aluminophosphate materials.8 Several synthetic
approaches such as hydrothermal and solvent-phase solvent-
evaporation spray drying have been used for mesoporous
materials.9 These mesoporous molecular sieves have opened up
new prospects and excellent opportunities in the area of
heterogeneous catalysis, adsorption, and supports.10-13 However,
in general, their structures collapse at elevated temperatures
because of their amorphous wall properties. The thermal and
hydrothermal stabilities of mesoporous aluminosilicate were
significantly improved through the synthesis of meso-micro-
composites.14,15 Many researchers have endeavored to develop
meso-micro-aluminophosphate by generating mesoporosity on
microporous silicoaluminophosphate (SAPO). Nevertheless, the
resultant hierarchical microporous SAPO does not show any X-
ray mesoporosity.16 Attempts have also been made to convert the
mesoporous silica into meso and micro silicoaluminophosphate
materials, but their intrinsic properties have been similar to those
of mesoporous silica.17 The synthesis of meso and micro
aluminophosphate-based materials is a great challenge and is
still a subject of investigation. On the other hand, isomerized
products of olefins are important feedstocks for various chemical
processes, and SAPO-based molecular sieves have been shown
as promising catalysts.18 In particular, the microporous SAPO-37
faujasite-type molecular sieve is well known as a potential
catalyst for various chemical transformations.19 As far as we
know, there have been no reports on the controlled synthesis of
mesoporous silicoaluminophosphate from microporous structur-
al secondary building units (SBUs) or on its applications.
(a)
(b)
Microporus SAPO-37 synthesized at
Microporous SAPO-37 synthesized at
22 h
6 h
22 h
6 h
-1
γ562 cm
-1
γ562 cm
1600 1400 1200 1000 800 600 400
5
10 15 20 25 30 35
Wavenumber/cm–1
2θ/degree
Figure 1. Nature of SAPO-37 precursors: (a) FTIR and (b) XRD
data.
precursors. These interactions take place at interface between
organic and inorganic phase in solution.10d,10e The resultant
MESO-SAPO-37 possesses both microporous and mesoporous
properties. MESO-SAPO-37 was shown to be a promising
catalyst for the isomerization of 1-octene at ambient temperature
in a liquid-phase medium, with the formation of only the
isomerized product. The microporous SAPO-37 precursor was
synthesized (Supporting Information, SI)20 by using a modifi-
cation of the literature procedure.21 MESO-SAPO-37 was
synthesized from the microporous precursor prepared with a
crystallization time of 6 h in the presence of surfactant solution
(SI).20 The resultant materials were systematically characterized
by various spectroscopic and analytical techniques (SI).20
Fourier transform infrared (FTIR) spectra (Figure 1a) of the
microporous precursors obtained at 6 and 22 h show a broad
vibrational band in the region 530-565 cm¹1, which is character-
istic of the structural secondary building unit of SAPO-37.21,22
Complete crystalline SAPO-37 was obtained after 22 h. The
In the present strategy, a microporous SAPO-37 precursor
was used to assemble mesoporous silicoaluminophosphate
(MESO-SAPO-37), as shown in Scheme 1. The cationic species
resulting from cetyltrimethylammonium bromide (CTAB) inter-
acts electrostatically with the preformed anionic SAPO-37
Chem. Lett. 2013, 42, 1160-1162
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1161
200
150
100
50
(1 0 0)
(1 0 0)
(b)
(a)
(b)
(1 0 0)
(a)
(1 1 0)
(2 0 0)
MESO-SAPO-37-90
MESO-SAPO-37-90
MESO-SAPO-37-50
MESO-SAPO-37-50
2
3
4
5
6
7
8
9
10
2
3
4
5
6
7
8
9
10
0
2θ/degree
2θ/degree
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (p/p )
0
Figure 2. Powder X-ray diffraction patterns of MESO-SAPO-37:
(a) as-synthesized and (b) calcined samples.
Figure 3. N2 sorption isotherms of (a) MESO-SAPO-37-50 and
(b) MESO-SAPO-37-90.
weak XRD reflection observed after 6 h of crystallization
(Figure 1b) indicates that SAPO-37 structural secondary subunit
formation started at this juncture. For the assembly of MESO-
SAPO-37 having micro- and mesoporous properties, the SAPO-
37 precursor obtained at 6 h was utilized.
Table 1. Textural properties of MESO-SAPO-37
Surface area
Pore volume
BJH pore
size
/nm
¹1
¹1
/m2 g
/cm3 g
Samplesa
BET DFT t-plot
BJH DFT HK
MESO-SAPO-37 was synthesized (SI)20 by using the SAPO-
37 precursor in the presence of surfactant solution with a molar
gel composition of (TPA)2O:(TMA)2O:Al2O3:P2O5:SiO2:CTAB:
H2O = 1.0:2.1-2.8:1.0:1.0:0.43:0.45-0.60:226.94 (TPA: tetra-
propylammonium, TMA: tetramethylammonium). The MESO-
SAPO-37 materials prepared at 50 and 90 °C are represented as
MESO-SAPO-37-50 and MESO-SAPO-37-90, respectively. The
powder XRD patterns of the as-synthesized samples (Figure 2)
show well-resolved peaks, which are indexed to the (100), (110),
and (200) reflections of an ordered hexagonal structure.4,8,11 The
powder X-ray diffraction patterns of the calcined samples
(Figure 2) exhibit a broad peak at 2.5-3.5°, indicating the
presence of hierarchical mesostructures.13 Further, the diffraction
patterns in the higher 2ª region from 5 to 40° show weak
reflections around 14, 18, and 24°, confirming the presence of
microporous SAPO-37 units within the system (Figure S1, SI).20
The FTIR spectra of MESO-SAPO-37 (Figure S2, SI)20 retain
the broad band in the region 530-565 cm¹1 attributed to the SBU
of SAPO-37, supporting the fact that MESO-SAPO-37 was
assembled from the microporous precursor.
(a)
(b)
215
458
249
555
14
26
0.16 0.19 0.06
0.28 0.31 0.11
4.1
3.1
a(a) MESO-SAPO-37-50 and (b) MESO-SAPO-37-90.
The nitrogen sorption isotherms of the MESO-SAPO-37
samples are shown in Figure 3. Both microporous and mesopo-
rous features can be seen clearly in both samples. The samples
showed type-IV isotherms (IUPAC)23 with H4-type hysteresis
for MESO-SAPO-37-90, corresponding to materials containing
narrow slit-like pores,23,24 and H2 and H3 types for MESO-
SAPO-37-50, indicating the presence of bottleneck-type meso-
pores.24 The sharp uptake in the isotherms at a relative pressure
(p/p0) of less than 0.01, associated with micropores,23 arises
from the structural SAPO-37 building unit. The textural pro-
perties of MESO-SAPO-37 are summarized in Table 1. In
particular, MESO-SAPO-37-90 shows high surface areas (BET
and DFT) and pore volumes (BJH, DFT, and HK) and uniform
pore-size distributions (Figure S3, SI),20 similarly to hetero-
atom-substituted mesoporous aluminophosphate materials.13,23
It is noteworthy that MESO-SAPO-37 prepared above 90 °C
resulted in the formation of the lamellar phase,25 which
collapsed upon calcination, leading to poor textural properties.
The observed t-plot surface area and HK pore volume clearly
confirmed the presence of a microporous structural subunit on
the domain of MESO-SAPO-37. The relatively low overall
Figure 4. HRTEM images and ED patterns of as-synthesized (a)
MESO-SAPO-37-50, (b) MESO-SAPO-37-90, and calcined (c)
MESO-SAPO-37-50, and (d) MESO-SAPO-37-90.
surface area of MESO-SAPO-37 must be due to the presence of
microporous SBUs on the walls of mesoporous channels.
Figure 4 shows the high-resolution transmission electron
microscopy (HRTEM) images and electron diffraction (ED)
patterns of the as-synthesized and calcined MESO-SAPO-37
samples. Both the as-synthesized and calcined MESO-SAPO-37
samples showed direct evidence for hierarchical mesopores,
consistent with the XRD pattern. The regularly distributed pores
in the range 3-4 nm with short-range order, which can be seen in
the calcined samples, are in agreement with the surface-area
analysis data. The ED pattern of calcined MESO-SAPO-37
further indicates that the walls of the mesopores are crystalline in
nature.
The acidities of the microporous SAPO-37 and MESO-
SAPO-37 were followed by pyridine-FTIR spectroscopy
(Figure S4, SI);20 the spectrum showed a broad vibrational band
between 1440 and 1450 cm¹1, corresponding to pyridine bound
Chem. Lett. 2013, 42, 1160-1162
© 2013 The Chemical Society of Japan
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Table 2. Catalytic performance of MESO-SAPO-37a
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Selectivity
1-Octene
Samplesb
Conv.
/wt %
Linear octene
2- 3- 4-
Iso-
Dimer
4
5
octene product
(a)
(b)
83.4
84.2
16.6
52.8
1.3
23.9 37.2 24.4
24.0 38.2 25.9
41.7 40.3 18.0
33.9 42.1 21.5
14.9
11.9
0
0
0
0
0
0
2.4
0
MESO-SAPO
SAPO-37
No catalyst
6
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0
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49.3c
aReaction conditions: Temp, 200 °C; time, 6 h; catalyst, 200 mg;
b
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8
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20 Experimental section, high-angle powder XRD, FTIR, PSD of MESO-
SAPO-37, and Pyridine FTIR spectra (Figures S1-S4). Supporting Informa-
tion is available electronically on the CSJ-Journal Web site, http://www.
csj.jp/journals/chem-lett/index.html.
The catalytic performances of MESO-SAPO-37 and the
conventional SAPO-37 were tested for the isomerization of
1-octene in a liquid-phase batch reactor (SI);20 the results are
given in Table 2. The major isomerization products in the case
of MESO-SAPO-37 are 2-octene, 3-octene, 4-octene, and iso-
octenes formed by the double-bond shift. In the absence of a
catalyst, the reaction showed only about 1% conversion, which
may be because of the presence of impurities in the reactant
(1-octene, 98%). Microporous SAPO-37 showed around 53%
1-octene conversion with considerable amounts of dimerized
products owing to the reaction occurring on the external surface.
On the other hand, MESO-SAPO-37 showed a better 1-octene
conversion of more than 84%, with selective formation of the
isomerized products. The higher activity of the MESO-SAPO-37
catalyst is thought to be due to the presence of strong acidic sites
and its hierarchical mesoporous nature with microporous
domains, which favors the fast diffussion of reactant molecules.
In summary, the mesoporous silicoaluminophosphate sys-
tem having microporous faujasite-type SAPO-37 building units
has been synthesized for the first time. The resultant MESO-
SAPO-37 possesses mesoporous channels with microporous
wall properties. MESO-SAPO-37 shows exclusively isomerized
products on the conversion of 1-octene under ambient conditions
in the liquid-phase medium.
DST and University of Delhi, India are acknowledged for
their support. RY, AKS also thanks UGC & CSIR for their JRF.
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Chem. Lett. 2013, 42, 1160-1162
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www.csj.jp/journals/chem-lett/