Available online at www.sciencedirect.com
Microporous and Mesoporous Materials 109 (2008) 58–65
www.elsevier.com/locate/micromeso
Phase-selective crystallization of cobalt-incorporated
aluminophosphate molecular sieves with large
pore by microwave irradiation
Sung Hwa Jhung
a,*
, Taihuan Jin
a,b
, Young Ho Kim b, Jong-San Chang
a,*
a
Research Center for Nanocatalysts, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yusung, Daejeon
305-600, Republic of Korea
b
Department of Fine Chemicals Engineering and Chemistry, ChungNam National University, Daejeon 305-764, Republic of Korea
Received 8 December 2006; received in revised form 26 March 2007; accepted 12 April 2007
Available online 29 April 2007
Abstract
Several cobalt aluminophosphate (CoAPO) molecular sieves are synthesized hydrothermally under microwave irradiation and conventional electric heating. Relatively unstable CoAPO molecular sieves with large pore can be obtained preferentially with microwave
synthesis due to the rapid crystallization involved in the microwave method. For example, the VFI (composed of 18 membered rings;
18 MR) and AFI molecular sieves (12 MR), rather than AFI (12 MR) and AEI (8 MR) molecular sieves, respectively, can be selectively
produced under the microwave irradiation. On the contrary, from the same reactant gel, more stable CoAPO molecular sieves with relatively small pore are obtained by conventional hydrothermal synthesis for long crystallization time. The AFI and VFI are transformed
into AEI and AFI, respectively, with the increase of reaction time because the relative stability of former CoAPOs is less than that of
latter CoAPOs under the reaction conditions adopted in this study. The present results suggest that the pore size is a more important to
explain the relative stability of CoAPO molecular sieves than the framework density.
Ó 2007 Elsevier Inc. All rights reserved.
Keywords: Phase selectivity; Microwave heating; Phase transition; Large pore molecular sieve; Aluminophosphate; Cobalt
1. Introduction
Nanoporous materials including zeolites and aluminophosphate molecular sieves (AlPO) [1,2] are widely used
in catalysis and separation, and are still being developed
for new applications [3]. VFI molecular sieve (VFI) including VPI-5, CoVPI-5 and FeVPI-5 is one of the molecular
sieves containing extra-large pore (pore size of 1.27 nm)
composed of 18 membered rings (18 MR) [4] and has
attracted much interest not only for fundamental understanding but also for applications aimed at large molecules
*
Corresponding authors.
E-mail addresses: sung@krict.re.kr (S.H. Jhung), jschang@krict.re.kr
(J.-S. Chang).
1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2007.04.031
[5]. AFI type molecular sieve (AFI) [1] such as AlPO-5,
SAPO-5 and CoAPO-5 with one-dimensional channel of
0.73 nm (delineated by 12 MR) has been widely studied.
AEI type molecular sieves (AEI) [6] such as CoAPO-18
and SAPO-18 with small channel of 0.38 nm (delineated
by 8 MR) have been investigated as catalysts for the ‘methanol to olefin’ (MTO) process or oxidation reactions [7].
The VFI and AFI molecular sieves have hexagonal structure, whereas the AEI has monoclinic symmetry [8].
Metal-containing molecular sieves such as cobalt-, titanium-, iron- and vanadium-incorporated molecular sieves
are very interesting since they show remarkable performances in the redox-, photo- and acid-catalysis [9–12].
Especially, cobalt-containing molecular sieves are useful
catalysts in the oxyfunctionalization of hydrocarbons with
air or oxygen [13].
S.H. Jhung et al. / Microporous and Mesoporous Materials 109 (2008) 58–65
So far, nanoporous materials have been synthesized
mainly by hydrothermal crystallization using conventional
electric heating. Since the pioneering works [14], it has been
reported that synthesis of zeolites and AlPO by microwave
heating has many advantages such as fast crystallization
[14–16], narrow particle size distribution [17,18] and facile
morphology control [19]. Malinger et al. have recently
shown that the octahedral molecular sieve materials
(OMS-1 and OMS-2) synthesized by microwave methods
exhibit superior properties such as stability, crystallinity,
morphology and even catalytic activity compared with
those of OMS-1 and OMS-2 synthesized by conventional
methods [20]. Moreover, the synthesis of an inorganic
material by using microwave method is very efficient to
analyze the effect of reaction conditions [21] because of
the fast crystallization of the method.
However, to the best of our knowledge, phase selective
synthesis of a structure by using microwave heating has
not received much attention. We have reported that
SAPO-5, rather than SAPO-34, can be synthesized preferentially by microwave heating [22]. The selective crystallization of SAPO-5 has been explained in terms of a
kinetic and thermodynamic effect; SAPO-5 forms at an
early stage of reaction and is transformed into a more stable phase, SAPO-34 with the increase of reaction time [22].
Nanoporous materials can be utilized as a carrier of
nanomaterials by using well-defined pore structures. For
instance, an AFI molecular sieve is one of the smallest carriers for carbon nanotubes [23]. The VFI is capable of confining nanomaterials such as buckyball (C60) in the large
channel [24]. The C60 in the VFI exhibits anomalous luminescence due to a confinement effect [24]. Moreover, molecular sieves with large pore should open new catalysis in the
fields of petrochemistry and life-science [5]. It has been
reported that the phase obtained by hydrothermal syntheses depends more on the kinetics rather than on the relative
stability or thermodynamics [25,26]. Several structures
59
coexist under some conditions since the obtained phase
can be changed by subtle reaction conditions and the heat
of formation differs only slightly with crystal structures
[26]. For example, AFI may coexist with AEL or AEI
[27–29], and the template di-n-propylamine leads to the
simultaneous crystallization of VFI, AEL and APC
(AlPO4-H3, 8 MR) structures [30]. Therefore, phase selective crystallization of pure aluminophosphate type nanoporous materials, especially with large pore, is important
from the standpoint of applications and characterization.
Herein, we report the phase selectivity or selective crystallization of CoAPOs with large pore by microwave heating from a reactant gel which would produce CoAPOs with
smaller pore by conventional hydrothermal synthesis. The
preferential crystallization of AFI and VFI over AEI and
AFI, respectively, will be discussed. In particular, the relative stability and inter-conversion of AFI and AEI (and of
VFI and AFI) in the reaction condition are described.
2. Experimental
Cobalt-containing VFI, AFI and AEI molecular sieves
were synthesized from pseudoboehmite (Catapal A, Vista),
phosphoric acid (85 wt%, Aldrich), Co(OAc)2 Æ 4H2O
(Aldrich, 98%) and deionized water by following the
method [31,32] reported earlier at relatively low temperature. Diisopropylethylamine (DIPEA) and triethylamine
(TEA) were used as template molecules for AFI/AEI and
VFI/AFI, respectively. Pseudoboehmite was added to a
diluted phosphoric acid solution, and stirred until a white
uniform gel was obtained. Dissolved cobalt acetate was
added to the aluminophosphate gel and a template was
added successively to the gel, which was stirred to a uniform reaction mixture. The reactant compositions for
AFI/AEI and VFI/AFI were Al2O3:1.05P2O5:0.08CoO:
1.7DIPEA:50H2O (pH 9.5) and Al2O3:1.05P2O5:0.08CoO:
Table 1
Reaction conditions and results for the synthesis of CoAPO molecular sieves
Sample
no.
A
B
C
D
E
F
G
H
I
J
a
b
c
d
e
Reaction conditionsa
Reaction results
Molar composition
Heating
methodb
Time
(h)
Temperature
(°C)
Phase yield (%)
Composition (Co/
(Al + P + Co), at.%)
SBETc
Al2O3:1.05P2O5:0.08CoO:1.7DIPEA:50H2O
Al2O3:1.05P2O5:0.08CoO:1.7DIPEA:50H2O
Al2O3:1.05P2O5:0.08CoO:1.7DIPEA:50H2O
Al2O3:1.05P2O5:0.08CoO:1.7DIPEA:50H2O
Al2O3:1.05P2O5:0.08CoO:1.7DIPEA:50H2O
Al2O3:1.05P2O5:0.08CoO:1.7DIPEA:50H2O
Al2O3:1.05P2O5:0.08CoO:1.7DIPEA:50H2O
Al2O3:1.05P2O5:0.08CoO:1.7DIPEA:50H2O
Al2O3:1.05P2O5:0.08CoO:1.0TEA:50H2O
Al2O3:1.05P2O5:0.08CoO:1.0TEA:50H2O
MW
MW
MW
MW
CE
CE
CE
CE
MW
CE
0.5
1
1.5
2
24
48
96
120
3
24
165
165
165
165
165
165
165
165
130
130
AFI
AFI
AFI
AFI
AFI + AEI
AFI + AEI
AEIe
AEIe
VFI
AFI
NDd
ND
2.2
2.3
ND
ND
2.2
2.4
1.2
2.4
ND
ND
ND
210
ND
ND
ND
560
490
297
The pH of the reactant gel is 9.5 (for A–H) or 5.5 (for I and J).
MW: microwave heating; CE: conventional electric heating.
BET surface area (m2/g).
ND: Not determined.
Samples G and H are slightly contaminated with CoAPO-34 (CHA structure).
45
60
77
85
10 + 28
4 + 53
80
85
72
80
ID
76412
Title
Phase-selectivecrystallizationofcobalt-incorporatedaluminophosphatemolecularsieveswithlargeporeby
microwaveirradiation
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