Ionothermal synthesis of FeAPO-5 in the presence of phosphorous acid

Article abstract of DOI:10.1039/c5ce01973g

The ionothermal crystallization of FeAPO-5 molecular sieves in the presence of phosphorous acid (H₃PO₃) has been investigated. The use of H₃PO₃enabled the formation of a metastable intermediate phase (FeNKX-2) that transforms into a more open-framework crystalline phase (FeAPO-5). The initial raw materials dissolved rapidly in the presence of the [bdmim]Cl polar ionic liquid, and the addition of the Fe³⁺salt resulted in the crystallization of the FeNKX-2 intermediate. At this stage, the [bdmim]⁺cation did not play the role of a pore filler for the FeNKX-2 crystalline structure. Consecutive phase transformation from the FeNKX-2 to the FeAPO-5 phase occurred under prolonged ionothermal treatment, and during this stage, the tetrahedral Fe³⁺species was found to not only participate in the construction of the FeAPO-5 framework but also act as an intermediary electron-transfer medium. The fast crystallization of FeAPO-5 was explained by the presence of Fe³⁺as an intermediate electron-transfer medium promoting the fast release of phosphorus nutrients (P⁵⁺) from the phosphite (P³⁺) reservoir that were further required for the crystallization of the FeAPO-5 molecular sieves. The use of ionic liquids as dual solvents and templates in combination with H₃PO₃as an alternative phosphorus source thus opens the possibility to synthesize other microporous materials via a phase transformation approach.

Full text of DOI:10.1039/c5ce01973g

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Ionothermal synthesis of FeAPO-5 in the presence
of phosphorous acid
Cite this: DOI: 10.1039/c5ce01973g
a
a
b
b
a
Eng-Poh Ng,* Jia-Pei Ghoy, Hussein Awala, Aurélie Vicente, Rohana Adnan,
c
b
Tau Chuan Ling and Svetlana Mintova
The ionothermal crystallization of FeAPO-5 molecular sieves in the presence of phosphorous acid (H
has been investigated. The use of H PO enabled the formation of a metastable intermediate phase
FeNKX-2) that transforms into a more open-framework crystalline phase (FeAPO-5). The initial raw mate-
3 3
PO )
3
3
(
3
+
rials dissolved rapidly in the presence of the [bdmim]Cl polar ionic liquid, and the addition of the Fe salt
+
resulted in the crystallization of the FeNKX-2 intermediate. At this stage, the [bdmim] cation did not play
the role of a pore filler for the FeNKX-2 crystalline structure. Consecutive phase transformation from the
FeNKX-2 to the FeAPO-5 phase occurred under prolonged ionothermal treatment, and during this stage,
3
+
the tetrahedral Fe species was found to not only participate in the construction of the FeAPO-5 frame-
work but also act as an intermediary electron-transfer medium. The fast crystallization of FeAPO-5 was
3
+
Received 9th October 2015,
Accepted 24th November 2015
explained by the presence of Fe as an intermediate electron-transfer medium promoting the fast release
5+
3+
of phosphorus nutrients (P ) from the phosphite (P ) reservoir that were further required for the crystalli-
zation of the FeAPO-5 molecular sieves. The use of ionic liquids as dual solvents and templates in combi-
DOI: 10.1039/c5ce01973g
3 3
nation with H PO as an alternative phosphorus source thus opens the possibility to synthesize other
www.rsc.org/crystengcomm
microporous materials via a phase transformation approach.
intermediate (180 °C, 6 days), whereas this intermediate phase
1
. Introduction
8
is not observed in the H PO -containing route. The transfor-
3
4
The discovery of microporous aluminophosphates (AlPO-n) is
considered as a breakthrough in zeolite science since they
mation of microporous aluminophosphates including SAPO-46,
AlPO-47, AlPO-41, SAPO-31, AlPO-11, AlPO-5 and AlPO-CJ2
from NKX-2 has been observed but it requires long crystalliza-
tion time (7–16 days) and high temperature (ca. 200 °C) due to
1
,2
3
have wide applications as adsorbents,
catalysts, mem-
4
5
6,7
branes, sensors, etc. Typically, these zeotype materials are
prepared under hydrothermal conditions where phosphoric
acid (H PO ) is used as a source of phosphorus.
5+
the slow release of phosphorus nutrients (P ) that are required
8,9,12
for the crystallization and growth of aluminophosphates.
3
4
A new route for the hydrothermal synthesis of alumino-
phosphate materials based on phosphorous acid (H PO ) as a
Recently, ionothermal synthesis using ionic liquids as sol-
vents and templates has been developed to prepare alumino-
3
3
8
,9
13–16
source has also been reported.
H PO is a moderately
phosphate molecular sieves.
The synthesis can be
3
3
strong dibasic acid (pKa1 = 1.80, pKa2 = 6.15) with the struc-
tural formula HPOIJOH)2. The HPOIJOH)2 species usually
performed in either open or closed vessels. Furthermore, by
using the ionothermal synthesis method and phosphorous
acid (H PO ), NKX-2 can be obtained within 3 h, which is
1
0
exists in equilibrium with a minor tautomer PIJOH) where
3
3
3
3
+
the phosphorus ion exists in the P state. H
PO
3 3
was found
faster than the time needed for the hydrothermal approach
1
7
to behave differently from H PO during the crystallization
(6 days).
3
4
of aluminophosphite and/or aluminophosphate molecular
sieves. In the synthesis process using H PO as the phos-
3 3
phorus source, NKX-2 is formed as an aluminophosphite
Isomorphous substitution of transition metals into the
framework sites of aluminophosphate molecular sieves
(MeAPO-n) is a useful tool for the modification of the proper-
ties of such materials, enabling the study of their crystalliza-
1
1
1
8,19
20,21
tion process
and possible applications.
However, the
a
School of Chemical Sciences, Universiti Sains Malaysia, 11800 USM, Penang,
effect of transition metals on the crystallization process of
Malaysia. E-mail: epng@usm.my; Tel: +6046534021
AlPO-5 via the ionothermal and H PO -containing route is still
b
3
3
Laboratoire Catalyse & Spectrochimie, CNRS-ENSICAEN, Université de Caen,
unknown, and hence it is worth further exploring.
France
c
In this work, the ionothermal crystallization of FeAPO-5
using phosphorous acid (H PO ) as the phosphorus source is
Institute of Biological Sciences, Faculty of Science, University of Malaya, Kuala
Lumpur, Malaysia
3
3
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reported. The formation process of FeAPO-5 during the
ionothermal synthesis was monitored by microscopic and
spectroscopic methods. A possible mechanism for the growth
of FeAPO-5 crystals was proposed based on the experimental
results.
for the solid products after different crystallization time
periods (20 min, 1 day, 3 days and 5 days).
Characterization
Powder XRD patterns were recorded on a PANalytical X'Pert
PRO diffractometer with Cu K
α
radiation (λ = 0.15418 nm, 40
mA, 45 kV, a step size of 0.02° and a scan speed of 0.2°
2
. Experimental
−
1
min ). The morphology of the powder samples was analyzed
by using a Leo Supra 50VP field emission scanning electron
microscope (FESEM) operating at 30 kV. The FTIR spectra of
the samples were obtained using the KBr pellet technique in
Synthesis of [bdmim]Cl
The [bdmim]Cl ionic molten salt was prepared by using the
modified synthesis procedure from ref. 22: 1,2-
dimethylimidazole (45.00 g, 96%, Merck) was dissolved in
ethanol (40 mL, 99.7%, QRëC) in a 250 mL round bottom
flask. Benzyl chloride (86 mL, 99%, Merck) was then added
to the mixture. The resulting solution was refluxed at 100 °C
for 6 h. The round bottom flask was cooled down and placed
in an ice bath to allow white crystals to precipitate out
−
1
the range of 400–2800 cm . The UV-vis spectra of the solids
were measured in the spectral range of 200–500 nm using a
2
7
31
Varian Cary® 100 UV-vis-NIR spectrophotometer. Al and
P
solid-state MAS NMR measurements were conducted on a
Bruker Ultrashield 500 spectrometer at MAS frequencies
2
7
31
between 5 and 15 kHz. The Al and P MAS NMR spectra
were obtained with 8000 scans by single-pulse excitation
using π/12 (0.6 μs) and π/2 (3 μs) pulses, respectively. Simi-
(Scheme 1). The white crystals were purified and washed with
acetone prior to drying in an oven at 100 °C overnight. The
pure ionic molten salt was kept in a tight polypropylene bot-
1
3
larly,
C solid-state CP/MAS NMR measurements were
tle and stored in a desiccator. A product yield of 86.4% was
conducted too (spinning speed of 14.5 kHz at a magnetic
field of 9.4 T with a resonance frequency of 104.26 MHz
using the single-pulse mode at 1 ms contact time).
1
obtained. H NMR (400 MHz, ppm, D O) appeared as follows:
2
3
δ = 2.48 (3H, imidazole N–C(CH )–N), 3.70 (3H, imidazole N–
CH ), 4.70 (2H, imidazole N–CH –C H ), 7.22 (1H, imidazole
3
2
6
5
N–CHCH–N), 7.24 (1H, imidazole N–CHCH–N) and 7.28–
3
. Results and discussion
−
1
7
.41 (5H, phenyl). FT-IR (KBr disk, cm ): 1036 (imidazolium,
C–N), 1452 and 1638 (aromatic CC), 1530 (imidazolium,
CN), 3497 (O–H stretching). Anal. calcd for C H N Cl: C,
XRD analysis
1
2
15
2
Fig. 1 shows the XRD patterns of the products. The solid was
found to be amorphous after 10 minutes of treatment as indi-
cated by the absence of reflections in the X-ray diffractogram.
Several diffraction peaks at 2θ = 12.98°, 25.96°, 29.09° and
6
4.72%; H, 6.79%; N, 12.58%; Cl, 15.92%; found: C, 65.03%;
H, 6.66%; N, 11.55%; Cl, 16.77%.
3
4.17° appear after 20 min. These peaks are assigned to
Ionothermal synthesis of FeAPO-5
8
FeNKX-2, an aluminophosphite intermediate. As compared
to NKX-2 aluminophosphite (Fig. 1f), the peaks are slightly
right-shifted due to the incorporation of Fe into the NKX-2
framework. It is worth mentioning here that the incorporation
of Fe has shortened the crystallization time of NKX-2 to 20
min, which is much faster than those observed for the prod-
ucts prepared using ionothermal (170 °C, 5 h) and hydrother-
The synthesis of FeAPO-5 was carried out in a 50 mL Teflon-
lined stainless steel autoclave. Prior to the synthesis, H
3
+
3
PO
3
(0.421 g, 85%, Sigma-Aldrich) and FeCl ·6H O (0.0178 g, 98%,
3
2
3
+
Merck) were first dissolved in distilled water (0.0743 g). Alu-
minum isopropoxide (0.416 g, 98%, Sigma-Aldrich) and
[bdmim]Cl ionic molten salt (8.89 g) were then introduced
8
,17
into the mixture and the autoclave was sealed prior to
heating at 170 °C for 10 min. The final gel molar composi-
mal (195 °C, 6 days) methods reported previously.
attempt to use FeIJNO ) instead of FeCl as the Fe source
An
3
+
3 3 3
tion of FeAPO-5 was IL : Al : P : Fe : H
2
O = 80 : 2 : 6 : 0.13 : 11.5.
was also made in order to study the effect of the salt used,
and fully crystalline FeNKX-2 was obtained within similar time.
After 1 day of heating, several small peaks at 2θ = 7.46°,
19.77°, 21.15° and 22.47°, which correspond to FeAPO-5,
The reactor vessel was cooled down and the solid product
was washed carefully with double distilled water before dry-
ing at 80 °C overnight. A similar procedure was also adopted
Scheme 1 Synthesis of [bdmim]Cl ionic molten salt.
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Fig. 1 XRD patterns of solid products after (a) 10 min, (b) 20 min, (c) 1
day, (d) 3 days and (e) 5 days of ionothermal synthesis. The XRD
pattern of pure NKX-2 aluminophosphite is shown in (f).
emerge. After extending the crystallization time up to 3 days,
the FeAPO-5 phase becomes dominant and full transforma-
tion of the FeNKX-2 ferroaluminophosphite intermediate into
FeAPO-5 ferroaluminophosphate was achieved within 5 days.
The synthesis was also carried out without adding the
3
FeCl salt. It was observed that the crystallization time for
obtaining the NKX-2 aluminophosphite intermediate was
extended to 3 h and this phase was stable with further
increase of the heating time, i.e. the AlPO-5 crystalline phase
was not detected even after 20 days of ionothermal treatment.
3 4 3 3
An attempt to use H PO instead of H PO for the synthesis
of FeAPO-5 was also made, and no trace of FeNKX-2 was
observed under the same gel composition and crystallization
conditions.
SEM microscopy study
3 3
The crystallization of FeAPO-5 in an ionic liquid with H PO
was monitored by SEM analysis. The initial amorphous solid
material (10 min) is composed of nanoparticles (ca. 120 nm)
with irregular shapes (Fig. 2a and b). The nanoparticles tend
to agglomerate to form bulk entities with micrometer sizes.
The morphology of the bulk solid changes after 20 min; the
FeNKX-2 crystals with more pronounced structural features
start to form (Fig. 2c and d). As shown in Fig. 2c, the FeNKX-
2
crystals (size of ca. 0.6–2.5 μm) show a novel rounded
rectangle-like morphology with a rough surface, which is dif-
ferent from the hydrothermally and ionothermally synthe-
Fig. 2 SEM images of solid products after (a, b) 10 min, (c, d) 20 min,
(e, f) 1 day, (g, h) 3 days and (i, j) 5 days of ionothermal synthesis at
different magnifications.
8
,17
sized NKX-2 crystals.
This could be due to the absence of
fluoride ions and the use of a different ionic liquid ([bdmim]
Cl) as a solvent and a template during the ionothermal
synthesis.
After 1 day of crystallization, the rounded rectangle-like
FeNKX-2 crystals containing FeAPO-5 as a minor phase
agglomerate with each other and form big spherical particles
with a size of ca. 13.8 μm (Fig. 2e and f). With prolongation
of the crystallization time up to 3 days, big spherical particles
(ca. 11.5 μm) containing both the FeNKX-2 intermediate and
FeAPO-5 crystalline solids are observed (Fig. 2g and h). As is
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shown, these FeNKX-2 intermediate crystals slowly dissolve
in the ionic liquid, and FeNKX-2 crystals with a smooth sur-
face and rectangle-like morphology appear and become
embedded within the big spherical entities (Fig. 2g and h).
Furthermore, the FeAPO-5 phase is also observed in the big
spherical solid, arranging as stacking layered hexagonal thin
flakes. Thus, the change in the morphology of the crystals
reveals that phase transformation takes place in the ionic liq-
uid medium, which is in line with the XRD observation
(Fig. 1d). The FeNKX-2 phase is completely consumed and
pure FeAPO-5 crystals are formed after 5 days of ionothermal
treatment (Fig. 2i and j). As can be seen in the micrograph,
the resulting crystals have sizes of about 13.0 μm with stack-
ing layered hexagonal thin flakes growing in rounded
hexagonal-shaped crystals.
Fig. 3 UV-visible spectra of solid products after (a) 10 min, (b) 20 min,
(c) 1 day, (d) 3 days and (e) 5 days of ionothermal synthesis.
ICP-AES elemental analysis
The elemental compositions of the solids measured by ICP-
3
+
AES are shown in Table 1. The analysis shows that Fe cat-
ions participate in the early stage of the crystallization pro-
cess. Initially, 1.53 mol% Fe is found in the powder solid,
and the Fe content keeps increasing with increasing
ionothermal synthesis time. After 20 min, a P/(Al + Fe) ratio
of 1.51 is obtained when the FeNKX-2 solid is formed which
3+
UV-vis spectroscopy study reveals that Fe
participates
actively in the construction of the NKX-2 framework. Upon
increasing the heating time, the band at 218 nm slowly disap-
pears. These iron species are gradually converted to tetrahe-
dral framework iron species, where a strong shoulder band at
88 nm slowly appears (Fig. 3e). Furthermore, a weak band at
68 nm also emerges and it can be attributed to the octahe-
2
4
1
7
corresponds to the P/Al ratio of pure NKX-2 (1.50). The
3
+
amount of Fe incorporated into the crystalline framework
continues to increase where 3.23 mol% Fe is recorded for the
FeNKX-2/FeAPO-5 mixture after 3 days. After 5 days, pure
FeAPO-5 with a P/(Al + Fe) ratio of 0.99 is measured. The final
FeAPO-5 contains 3.39 mol% Fe, which is higher than the
amount of Fe incorporated into the AFI framework using a
3+
25
dral Fe species.
Infrared spectroscopy analysis
A complementary FTIR spectroscopy study was performed to
monitor the crystallization kinetic process of the solid. As
can be seen in Fig. 4a, only several IR bands are observed for
2
3
hydrothermal approach (3.0 mol% Fe).
the amorphous sample (10 min) at 2432, 1096, 1017 and 625
UV-visible spectroscopy analysis
−1
cm
which correspond to the P–H stretching mode of
3
+
32−
−
The incorporation of Fe into the solid entities was further
investigated by UV-visible spectroscopy. Initially, the amor-
phous sample which underwent 10 min of heating exhibits
two bands at 218 nm and 260 nm (Fig. 3a). The sharp absorp-
2 4
HPO and the symmetrical vibrations of PO , P–O and TO
(T = Al, P or Fe), respectively. After ionothermal heating for
20 min, the solid exhibited an IR spectrum identical to that
of pure NKX-2 where the above described bands are slightly
3
+
−1 17
tion band at 218 nm is attributed to finely dispersed Fe in
the framework position, whereas the band at 260 nm is due
shifted to 2469, 1217, 1139, 1036, 588 and 549 cm . This
indicates that Fe is incorporated into the NKX-2 material. No
3
+
−1
to the ligand-to-metal charge transfer (LMCT) band of Fe in
peak is observed at 1400–1550 cm indicating that no
2
4
+
tetrahedral positions. The peak intensities increase signifi-
cantly as FeNKX-2 is formed after 20 min (Fig. 3b). Thus, the
[bdmim] cation is chemically bound to the amorphous and
crystalline FeNKX-2 solids (inset of Fig. 4a and b).
Table 1 Framework compositions of the solid samples
Atomic ratios
Crystallization
P/(Al + Fe)
ratio
Fe/(Al + P + Fe)
ratio (mol%)
Phase(s)
time
Al
P
Fe
Amorphous
FeNKX-2
FeNKX-2, FeAPO-5*
FeNKX-2, FeAPO-5
FeAPO-5
10 min
20 min
1 day
3 days
5 days
0.611
0.619
0.670
0.826
0.938
1.000
1.000
1.000
1.000
1.000
0.025
0.038
0.051
0.061
0.068
1.57
1.52
1.39
1.13
0.99
1.53
2.29
2.96
3.23
3.39
*Trace amount.
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Fig. 4 IR spectra of solid products after (a) 10 min, (b) 20 min, (c) 1
day, (d) 3 days and (e) 5 days of ionothermal synthesis.
The IR spectra especially at the fingerprint region (1300–
−
1
4
1
00 cm ) change with further heating. The IR signals at
−
1
+
426, 1459 and 1544 cm , which correspond to the [bdmim]
cation, slowly appear (inset of Fig. 4c–e). Furthermore, the IR
−
1
bands at 1000–1250 cm change slowly and a strong band
−
1
with shoulders at 1108 and 1229 cm , corresponding to
external linkage asymmetrical stretching and internal T–O–T
(T = Al, P, Fe) tetrahedral asymmetrical stretching, respec-
tively, is detected after 5 days. Furthermore, the signals at
−
1
1
036 and 2469 cm
which are attributed to the P–H
27
31
2−
Fig. 5 (A) Al and (B) P MAS NMR spectra of solid products after (a)
0 min, (b) 20 min, (c) 1 day, (d) 3 days and (e) 5 days of ionothermal
synthesis.
stretching and P–H deformation modes of HPO
3
, respec-
1
tively, slowly disappear with heating time. Concurrently, four
characteristic bands of the AFI framework (480, 547, 637 and
−
1 26
7
40 cm ) start to appear after 3 days, showing that phase
transformation to the AFI framework structure is taking
place, which agrees with the XRD and SEM results. The four
characteristic bands of the AFI framework become more pro-
found as heating of the solid is further prolonged to 5 days.
The solid exhibits an IR spectral pattern similar to that of the
pure AFI phase indicating that fully crystalline FeAPO-5 has
been obtained (Fig. 4e).
2
8,29
dissolved and reacted in the ionic liquid after 10 min.
When the solid is heated for 20 min, the peak at −9.7 ppm
splits into two peaks at −14.8 and −7.5 ppm (Fig. 5A(b)). The
first intense peak at −14.8 ppm is due to octahedral AlO spe-
6
2−
cies, which share three Al–O corners with three HPO
3
tetra-
octahe-
dron. The second band at −7.5 ppm can be assigned to
Al IJHPO ] species, which are the basic building units of
hedra and three Al–O corners with one adjacent AlO
6
8
[
2
3 3
)
3
1
27
P and Al MAS NMR spectroscopy analyses
the FeNKX-2 material. These two signals, however, slowly
The evolution of the crystalline structure during the amor-
phous–intermediate–crystal transformation was monitored as
decrease and shift downfield (−14.8 → −14.5 ppm and −7.5 →
−6.2
ppm)
with
increasing
crystallization
time
2
7
31
a function of time by Al and P MAS NMR spectroscopy.
(Fig. 5A(c and d)). This indicates that the matrix suffers from
a loss of lattice periodicity due to distorted chemical bonds
in Al sites and the presence of the AFI crystalline phase. In
addition, a new band emerges at 39.9 ppm which is attrib-
uted to the tetrahedral Al species in the AFI-type frame-
2
7
31
Fig. 5 shows the Al and P MAS NMR spectra of the solid
samples after ionothermal treatment for various times. Ini-
tially, two broad signals at −9.7 and 7.3 ppm in the Al MAS
NMR spectrum are observed which are assigned to the octa-
hedral and pentahedral aluminum species due to
27
2
7
work. The pentahedral Al signal, on the other hand, is
2
7
AlIJMO)
4
IJOH
2
)
2
and AlIJMO)
4
IJOH
2
) (M = P, Fe), respectively
shifted from 7.6 ppm to 6.1 ppm. The changes in the Al
MAS NMR spectra are in good agreement with the changes
observed through XRD and SEM analyses at the macroscopic
2
7
(Fig. 5A(a)). No signals are detected at 1.7 and 61.5 ppm
indicating that aluminum isopropoxide has completely
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level. After 5 days, the peaks in the range of −20 to 0 ppm are
completely disrupted, producing a broad band at −7.8 ppm.
The signal at 38.9 ppm shifts gradually to 38.5 ppm and
becomes dominant, which suggests that the tetrahedral Al is
in a different chemical environment in the fully crystalline
FeAPO-5 sample (Fig. 5A(e)). In addition, a significant chemi-
cal shift of tetrahedrally coordinated Al is also observed when
NMR spectroscopy. The results reveal that no carbon peaks
are detected in the solid after 20 min of ionothermal heating,
indicating that no [bdmim] cation is confined in the micro-
+
pores of the FeNKX-2 sample (Fig. 6a). Thus, it can be con-
+
cluded that [bdmim] cations do not play the role of pore-
filling agents during the formation of the FeNKX-2 frame-
work structure. As the heating time was extended to 3 days,
where the AFI phase started to form (Fig. 1), some small sig-
3
+
Fe is incorporated into the framework of AlPO-5; the chemi-
3
0
+
cal shift of as-synthesized AlPO-5 is 36.0 ppm.
nals corresponding to [bdmim] cations started to appear
3
1
The P MAS NMR spectra of the samples are shown in
Fig. 5B. A broad signal at −4.6 ppm was detected in the sam-
ple at the beginning of the ionothermal synthesis (10 min)
(Fig. 6b). The signals at 128.68 and 123.31 ppm are assigned
to the C5 and C4 atoms in the imidazole ring, and the reso-
nance peaks at 36.65 and 9.41 ppm represent the C8 and C7
atoms of alkyl groups attached to the imidazole ring (inset of
(
Fig. 5B(a)). This peak, which is assigned to PIJOH) IJOM) (M
2 2
1
3
+
=
Al, Fe), is relatively broad and not well resolved. This indi-
Fig. 6). Thus, the C NMR results imply that [bdmim] cat-
ions start to be confined in the micropores of the FeAPO-5
sample, which is consistent with the IR results (inset of
cates a disordered environment around the P site for the
sample, which is in agreement with the amorphous character
of the XRD pattern (see Fig. 1). Upon ionothermal heating for
1
3
+
Fig. 4). The C signals of [bdmim] occluded in the AFI
framework become profound as heating of the solid is fur-
ther extended to 5 days, and fully crystalline FeAPO-5 is
formed (Fig. 6c). As is shown, all the signals are present thus
2
0
min, these local environments evolve to become
PIJH)IJOAl tetrahedral species where an upfield shift to
)
Oct 3
−
13.6 ppm is recorded (Fig. 5B(b)). The signal is sharp due to
the symmetry of the tetrahedral phosphite species in FeNKX-
+
indicating that the [bdmim] molecules are preserved upon
2
. The peak in the region from 0 to −19.0 ppm can also be
occlusion in the AFI crystals. Thus, the NMR spectroscopy
data confirm that the [bdmim] cations act as pore-filling
agents which direct the formation of FeAPO-5 during the
ionothermal treatment.
+
assigned to the lower condensation degree of phosphorus
species, which is in line with the IR spectroscopy data for the
FeNKX-2 samples proving the existence of the P–H bond.
3
1,32
After 1 day of crystallization, the peak at −13.6 ppm is shifted
to −14.6 ppm and is broadened as well. This is due to the dis-
tortion of the FeNKX-2 structure, in addition to the change of
the chemical environment of P in the matrix during the
ionothermal process. A small peak at −26.7 ppm is also
observed which can be ascribed to P sites connected to both
tetrahedral and octahedral Al sites, which are present in the
Growth mechanism of FeAPO-5
Classically, the AFI-type framework is templated by amines or
quaternary ammonium salts under hydrothermal conditions
3
6
using H PO as a phosphorus source. H PO is generally
3
4
3
3
described with the structural formula HPOIJOH)2 in which
this species exists in equilibrium with a PIJOH) tautomer.
Because of the rapid interconversion, these tautomers alter
2
7
AFI crystalline solid. Thus, the NMR study reveals that
phase transformation from FeNKX-2 to FeAPO-5 takes place.
The peak shifting and broadening remain and the intensity
of the peak at −28.6 ppm, which corresponds to tetrahedral P
species in FeAPO-5, increases after 3 days. After 5 days of
heating, a single peak at −29.3 ppm indicating the complete
3
3
+
the charge of P species in the NKX-2 and AFI frameworks (P
5
+
3 3
↔ P ). Thus, the use of H PO enables exploration of the
2
2
transformation to FeAPO-5 is detected. As is shown, the
peak is sharp and this reveals that only one distinguishable
type of tetrahedral phosphorus is present in the AFI structure.
3+
Fe substitution in the AlPO-5 framework can also be
3
1
proven by P MAS NMR spectroscopy besides observing the
low-field chemical shift of the peak of tetrahedrally coordi-
2
7
33–35
nated Al in the Al MAS NMR spectrum (Fig. 5A(e)).
Due
3
1
to the strong dipolar coupling between P and paramagnetic
3
+
Fe species, intense sidebands are usually observed in the
MAS spectra. It can be seen that intense sidebands are
observed even at the very early stage of ionothermal synthesis
Fig. 5). Thus, the P MAS NMR spectroscopy data are in agree-
ment with those of UV-vis spectroscopy, i.e. Fe participates
31
(
3
+
actively in the crystallization process of the FeAPO material.
1
3
C MAS NMR spectroscopy analysis
+
The [bdmim] cations confined in the micropores of the
13
Fig. 6
C MAS NMR spectra of solid products after (a) 10 min, (b) 3
1
3
FeNKX-2 and FeAPO-5 samples were studied using C MAS
days and (c) 5 days of ionothermal synthesis.
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metastable intermediate phase with a novel open-framework
crystalline phase and unique framework properties. H PO as
a phosphorus source, however, is seldom used in the synthe-
where the construction of the NKX-2 framework structure is
3+
achieved merely in 20 min in the presence of Fe under
ionothermal heating (Stage III). During the crystallization
3
3
8
,9,12
3+
sis of AlPO-n materials neither in hydrothermal
nor in
ionothermal pathways. On the other hand, it was reported
that the dibasic H PO acid (pKa1 = 1.80, pKa2 = 6.15) has
stronger acidity compared to H PO (pK = 2.12, pKa2 = 7.21,
process, Fe species actively participate in the isomorphous
1
7
incorporation in the NKX-2 framework. After 20 min of
ionothermal synthesis, 2.29 mol% Fe is incorporated into the
framework of NKX-2. On the other hand, the Al species
existing in FeNKX-2 are in octahedral form, and it is
3
3
3
4
a1
pKa3 = 12.67), and its acidity and the oxidation state of P spe-
2−
cies also govern the selectivity to the crystalline phase prod-
connected to HPO
3
6
tetrahedra and AlO octahedra; whereas,
9
,10
uct during the crystallization process.
The ionothermal synthesis pathway of FeAPO-5 via the
PO -containing route is shown in Fig. 7. During the initial
0 min of heating, the process involves the mixing of raw alu-
minum isopropoxide, H PO and FeCl3 in the [bdmim]Cl
the P species appear in the form of PIJH)IJOAlOct)3 tetrahedral
species. No [bdmim]Cl is confined in the pores of FeNKX-2
as revealed by FTIR spectroscopy, indicating that the
H
3
3
+
1
[bdmim] cation does not function as a pore-filling agent dur-
ing the construction of the FeNKX-2 crystalline structure. As
can be seen in the SEM image (Fig. 2b), the FeNKX-2 crystals
(ca. 1.4–2.5 μm) exhibit a novel rounded rectangle-like mor-
phology with a rough surface.
After 1 day of ionothermal treatment, the FeNKX-2 crystals
agglomerated with each other, forming big spherical particles
(ca. 13.8 μm) (Stage IV). In addition, the peaks (1544, 1463
3
3
ionic liquid, followed by successive dissolution (Stage I). As
2
7
revealed by
completely hydrolyzes within 10 min in the polar [bdmim]Cl
ionic liquid, and then further reacts with H PO to form
AlIJMO) IJOH ) (M = P, Fe) monomers and oligomers (Stage
Al MAS NMR, aluminum isopropoxide
3
3
4
2 2
II). These intermediates are composed of H–P–O, P–O–Al, P–
O–Fe and Al–O–Fe units which are present, along with free
−
1
+
and 1425 cm ) corresponding to [bdmim] appear in the IR
spectrum of the solid sample (Fig. 4d). Thus, this indicates
that the inorganic monomers and oligomers slowly enfold
3
+
phosphite and Fe species as well, in the mixture.
3
+
In the absence of Fe , the intermediates slowly undergo
reorganization and the NKX-2 structure is formed after 5 h of
ionothermal heating or 6 days of hydrothermal treatment.
The crystallization rate, however, is significantly enhanced
+
the [bdmim] cations through van der Waals interactions to
2
7
form AFI micropores. This observation is supported by the
XRD study where the beginning of phase transformation to
3 3
Fig. 7 The ionothermal crystallization pathway of FeAPO-5 through the H PO -containing route.
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the AFI crystalline phase is revealed. During this period of 4. Conclusions
time, the iron species in FeNKX-2 are gradually converted to
A detailed investigation of the ionothermal synthesis of
FeAPO-5 molecular sieves in the presence of H PO has been
tetrahedral framework iron species. Furthermore, the alumi-
num species, which is octahedrally coordinated in FeNKX-2,
is partially converted into tetrahedrally coordinated Al in the
AFI framework. The phosphorus species are present in meta-
3
3
reported. The results show that initially raw chemicals dis-
solve rapidly in the [bdmim]Cl polar ionic liquid. By adding
3
+
2
−
the Fe salt to the reaction mixture, the FeNKX-2 intermedi-
ate is crystallized within 20 min instead of 3 h of heating.
phosphite units where slightly distorted HPO
tetrahedral
3
3−
units are slowly converted to PO
tetrahedral units via
4
+
3
+
5+
The study also reveals that the [bdmim] cation does not
changing the oxidation state of P to P .
function as a pore-filling agent during the construction of the
FeNKX-2 crystalline structure. The phase transformation from
FeNKX-2 to FeAPO-5 starts after 1 day of heating. During this
From these results, we believe that the transformation of
2
−
−1
pseudo-tetrahedral HPO
species (−7.7 kcal mol ) to tetra-
3
3−
−1
hedral PO
tion energy.
4
species (−12.0 kcal mol ) requires high activa-
3
+
3
7,38
stage, the tetrahedral Fe species was found to not only par-
ticipate in the construction of the NKX-2/AFI framework but
also act as an intermediary electron-transfer medium. As a
This is the reason why no transformation of
3
+
5+
phosphorus species (P → P ) was observed in the absence
of Fe . However, the activation energy of the transformation
of pseudo-tetrahedral HPO3 species to tetrahedral PO
species is significantly reduced with the addition of Fe . The
UV-vis spectroscopy data reveal that Fe initially exists in the
form of a ligand-to-metal charge transfer (LMCT) complex.
3+
3
+
5+
2−
3−
result, the oxidation of phosphite (P ) to phosphate (P )
was enhanced resulting in the fast release of phosphorus
4
3
+
5
+
3
+
nutrients (P ) that are required for the crystallization and
+
growth of FeAPO-5. Furthermore, confinement of [bdmim]
3+
cations by the anionic ferroaluminophosphate intermediate
is also observed along the phase transformation process.
Fully crystalline FeAPO-5 was obtained after 5 days of
ionothermal synthesis.
Thus, the use of ionic liquids as dual solvents and tem-
3 3
plates in combination with H PO as an alternative phospho-
rus source offers the possibility of preparing FeAPO micropo-
rous materials via the phase transformation method. The
highly porous FeAPO samples with insufficient thermal sta-
bility are not used in catalysis yet. However, considerable
efforts in improving the thermal stability of the FeAPO sam-
ples to explore further their catalytic properties are in progress.
This tetrahedral Fe species can act as an intermediary
electron-transfer medium allowing the change of the oxida-
3
+
5+
3+
tion state from P to P , and the tetrahedral Fe species
can even take part in the isomorphous incorporation in both
the NKX-2 and AFI frameworks.
During the FeAPO-5 nucleation process, the dissolved
FeNKX-2 particles aggregate and form nuclei, which serve as
the crystallization centre of FeAPO-5. Following the rapid
nucleation process, the FeAPO-5 nuclei grow larger with time
and form the AFI molecular sieve structure. The phase trans-
formation continues to take place after 3 days (Stage V). This
phenomenon can be proven by the XRD study where the
peaks corresponding to the AFI phase become dominant
Acknowledgements
(
Fig. 1c and d). Furthermore, the IR peak intensity of
+
[bdmim] becomes stronger (Fig. 4c and d) indicating that
Financial support from RUI (1001/PKIMIA/811264) and USM
Short-term (304/PKIMIA/6313047) Grants is acknowledged. J.
P. Ghoy would also like to thank the MyBrain and USM Fel-
lowship for the scholarship provided.
+
more and more [bdmim] molecules are interacting and con-
fined inside the micropores of FeAPO-5. The pseudo-
tetrahedral HPO
2−
species still co-exist with the tetrahedral
3
3−
PO
4
species in the FeNKX-2/FeAPO-5 intermediates. This
transformation thus may lead to the formation of highly
strained tetrahedral T-atom sites present in the framework
and play the key role in the formation of FeAPO-5. Further-
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