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Nitrate occlusion studies in Y zeolite and in a clay pillaredwith aluminium oxide
Ana Carvalho a, Jo~aao Pires a,*, Patr�ııcia Veloso a, Manuel Machado a,M. Brotas de Carvalho a, Jo~aao Rocha b
a Department of Chemistry and Biochemistry, University of Lisbon, Faculty of Sciences, Campo Grande, 1749-016 Lisbon, Portugalb Department of Chemistry, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal
Received 15 July 2002; received in revised form 7 October 2002; accepted 25 November 2002
Abstract
The occlusion of nitrates is a subject that is important, for instance, in soil amendment, as far as the control of the
release of nitrates to the environment is concerned. In this work, the occlusion of molten nitrates was studied in the
sodium form of zeolite Y and, to our knowledge, for the first time, in a pillared clay. The samples were characterized by
different techniques, such as low-temperature nitrogen adsorption, X-ray diffraction, differential scanning calorimetry,
FTIR, 23Na MAS NMR and 15N MAS NMR. From the different techniques evidence was obtained that supports the
existence of occluded nitrates in the studied samples. Both materials, i.e., zeolite Y and the clay from Wyoming pillared
with aluminum oxide, seem to have a potential in the release of nitrates for use in soil amendment.
� 2002 Elsevier Science Inc. All rights reserved.
Keywords: Nitrates occlusion; Pillared-clays; Zeolite Y
1. Introduction
It is widely recognized that the increase in the
levels of nitrates and nitrites in drinking water has
become a major problem [1]. A large contributionto this problem has been given by the excessive,
and sometimes uncontrolled, use of fertilizers,
particularly in modern commercial farming [2],
which are leached into surface and ground waters.
The in vivo transformation of nitrates into nitrites
can cause, for instance, methahaemoglobinaemia
in newborn children, and the formation of nitros-
amines in the intestine can also cause serious dis-
eases [3]. In fact, the reduction of the amount of
nitrates that can enter the human food chain is
now an interdisciplinary problem to which, besidessoil scientists, material scientists, chemists and bio-
chemists can contribute. In this sense, the leaching
of fertilizers in soils [4], and the use of natural
zeolites, such as clinoptilonite, as water purifiers or
as soil amendments [5], are subjects that have been
studied. Another possibility is the catalytic hy-
drogenation of nitrates and nitrites to nitrogen,
using metal supported catalysts [6].The role of microporous materials, such as zeo-
lites, in soil amendment consisted mainly in the
slow release of NHþ4 or Kþ by these ion exchangers.
* Corresponding author. Tel.: +351-21-750-08-98; fax: +351-
21-750-09-79.
E-mail address: [email protected] (J. Pires).
1387-1811/02/$ - see front matter � 2002 Elsevier Science Inc. All rights reserved.
doi:10.1016/S1387-1811(02)00625-X
Microporous and Mesoporous Materials 58 (2003) 163–173
www.elsevier.com/locate/micromeso
More recently, the occlusion of KNO3 or NH4NO3
in natural zeolites, by molten salt treatments [7–9],
has opened a new route for potential slow-release
fertilizers, mainly having in mind that other types
of microporous materials with different pore
openings and pore volumes can also be of interestin this field. On such class of materials are pillared
clays (PILCs) which are obtained by the interca-
lation of swelling clays, usually montmorillonites,
with large oxygen containing cations, followed by
calcination at 350–500 �C. In this way, a material
with permanent porosity is obtained the structure
of which has galleries with pillars, which can be
visualized as a car parking, although the distribu-tion of the pillars is, to a certain degree, at random.
These relatively new solids are mainly microporous
materials, that is, the pore widths are mainly lower
than 2 nm [10] although a considerable amount of
mesopores (pore widths between 2 and 50 nm [10])
can also be present, depending on the pillaring
method and the starting clay [11].
Different authors have reviewed the methodsfor the preparation of PILCs, and their adsorption
and catalytic properties [12–14]. Montmorillonite
is the most frequently selected clay for pillaring
due to the favorable characteristics of expand-
ability, cation-exchange capacity and availability.
The pillars are most often from aluminum oxide,
not only because the chemistry of formation of
aluminum polyoxocations in solution is well de-scribed but also due to the fact that these materials
were initially thought of as potential cracking
catalysts, for which the acid properties of the
Al2O3 pillars are important [15]. In spite of the
attempts to increase the thermal stability of PILCs
(see, for instance, Ref. [16]) the initial objective of
using these materials as cracking catalysts was
compromised. However, other potential applica-tions for pillared clays have been tried, commonly
comparing the efficiency of the pillared clays with
zeolitic materials, not only as catalysts, as reviewed
recently [14] but also as adsorbents [17–20].
The occlusion of potassium or ammonium ni-
trates was studied in natural zeolites such as
chabazite, erionite, phillipsite or clinoptilolite [7,8]
or in synthetic zeolites such as zeolite A [21], thatis, in zeolites that have relatively small and me-
dium pores [22], although the occlusion of am-
monium nitrate was also studied in zeolites with
larger pores [9].
In this work, a clay from Wyoming, pillared
with aluminum oxide and extensively character-
ized in previous studies [23–25] was selected for the
occlusion experiments. While with pillared claysthere is always some uncertainty concerning the
distance between the pillars, the height of the
galleries can be addressed with some precision
from the basal spacing determined by X-ray dif-
fraction. The PILC used in this work was char-
acterized by the value of 0.87 nm for the height of
the free space in the pores and a minimum di-
mension between the pillars of 0.78 nm, as sug-gested by adsorption studies made with probe
molecules with different dimensions [23,25]. In
fact, this solid has an open structure that can be
compared with the structure of large-pore zeolites,
as zeolite Y which has the faujasite structure with
pore openings of 0.74 nm [22]. This synthetic
material is produced in large amounts since it is
the basis of the cracking catalysts, but as far as weknow, it was not used previously in studies on
nitrate occlusion, while zeolite X was already
studied but only in the occlusion of ammonium
nitrate [9]. On the other hand, to our knowledge,
the occlusion of nitrates in pillared clays was never
studied before.
2. Experimental
2.1. Materials
The sodium form of zeolite Y from Aldrich (lot
01511LN) was used as received. A Na, Ca mont-
morillonite from Wyoming ‘‘Volcay SPV-200’’ was
obtained from the American Colloid Company(Arlington Heights, Illinois). The clay was dis-
persed in water and placed in glass containers, the
fraction with the spherical equivalent diameter <2
lm was then obtained (Stoke�s law). Some relevant
parameters that characterize this parent clay are
displayed in Table 1.
The pillared clay (Al-PILC) was obtained as
follows: an oligomer solution was prepared fromAlCl3 and NaOH 0.2 M with a ratio OH=Al ¼ 2,
aged 2 h at 60 �C after which the pH was increased
164 A. Carvalho et al. / Microporous and Mesoporous Materials 58 (2003) 163–173
to 6. This solution was then added dropwise to anaqueous suspension of clay (2.5 g/500 cm3) at 80
�C under stirring, refluxed (3 h) and kept overnight
at 25 �C. After centrifugation and washing in a
dialysis tube until the conductivity was less than 1
mSm�1 the solid was freeze-dried and calcined at
350 �C, under dried air, with a ramp of 1 �C/min,
the final temperature being kept for 2 h. The spe-
cific surface area, the micopore volume and d001 forthe pillared clay are given in Table 1.
To obtain the occluded samples, the nitrates,
KNO3 (BDH, 99%) or NH4NO3 (Merck, 99%),
and the adsorbent (adsorbent ¼ Y or Al-PILC)
were dried overnight at 50 �C. The desired
amounts of salt and adsorbent used in the occlu-
sion experiment were mixed in a quartz mortar, in
order to obtain a homogeneous sample.Two proportions KNO3:adsorbent were ini-
tially studied: 4:1, following the literature for the
occlusion in natural zeolites [7], and 2:1. From the
preliminary results (shown below) it was con-
cluded that the proportion of 2:1 was sufficient to
produce occluded solids and, therefore, this was
the proportion used in the present work, since no
significant differences were observed when largeramounts of nitrate were used. The mixture (typi-
cally, 2 g of KNO3 þ 1 g of zeolite or Al-PILC)
was placed in an Al2O3 boat and kept in a tubular
oven for 4 h at 350 �C, a temperature slightly
above the melting point of KNO3 (334 �C). After
this treatment, the samples were washed at ambi-
ent temperature following two alternative methods
(described in more detail below). The washingswere always performed in a Buchner funnel using
water purified by the Millipore.
Preliminary studies of occlusion with NH4NO3
were also made, but only for the Y zeolite. The
methodology followed to prepare to samples with
nitrate:adsorbent proportions of 2:1 and 4:1 was
similar to that described above for the occlusion of
KNO3, with the exception of the thermal treat-
ment. In this case, due to the lower melting
point of NH4NO3 (169 �C), the temperature was
185 �C.
For the NMR studies, the samples of Y zeolitewith KNO3 were prepared as described above but
an enriched form of potassium nitrate was used
(Aldrich, 99 at.% 15N).
2.2. Characterization
Nitrogen (Air Liquide, 99.995%) adsorption
isotherms at )196 �C were measured in conven-tional volumetric installations, equipped with
pressure transducers from Datametrics model
600A (USA) or from Shaevitz model P724-0004
(England) and a system composed of a combina-
tion of rotary/oil diffusion pumps. The reproduc-
ibility of the experiments with such installations
was within 5%. Before the measurements, the
samples, with and without occluded nitrate, wereoutgassed at 120 �C for 2.5 h. Adsorbed amounts
are expressed by weight of the outgassed samples.
Infrared spectra were obtained at room tem-
perature in a Perkin Elmer model 1725X or in a
Hitachi 250-50, using KBr pellets, prepared from
mixtures of 300 mg of potassium bromide and 3
mg of sample.
X-ray diffractograms were recorded in a PhilipsPX1820 instrument using CuKa radiation. In the
case of the pillared clay the diffractograms were
obtained using oriented mounts, prepared as de-
scribed elsewhere [23]. Diffractograms were also
made for NaY, without nitrates, as obtained, after
calcination at 350 �C and after calcination and
washing, as well as in the materials submitted to
the different treatments with the nitrates. The di-fractograms were always very similar confirming
the structural integrity of the solids after the
treatments with the nitrates.
Differential scanning calorimetry (DSC) exper-
iments were performed in an apparatus from
Setaram, model 111 (France) in a flow of dry he-
lium (Air Liquide, 99.995%). The sensitivity for
the DSC signal was 10 lW.All NMR spectra were recorded on a Bruker
Avance 400 wide-bore (9.4 T) spectrometer. 15N
Table 1
Specific surface areas (ABET), micropore volumes (w0), obtained
from N2 adsorption isotherms at )196 �C, and d001 values for
the parent clay and the PILC
ABET (m2/g) w0 (cm3/g) d001 (nm)
Parent clay 67 0.009 1.27
Pillared clay 350 0.130 1.83
A. Carvalho et al. / Microporous and Mesoporous Materials 58 (2003) 163–173 165
MAS NMR spectra were recorded with high-
power 1H decoupling, with a spinning rate of 4.5–5
kHz, a 3 ls pulse (flip angle 45�) and a recycle
delay of 60 s. 23Na solid-state NMR spectra were
recorded at 105.81 MHz, short (0.6 ls, equivalent
to 15�) rf pulses, a recycle delay of 2 s and aspinning rate of 14 kHz. Chemical shifts are quo-
ted in ppm from aqueous 1 M NaCl. The triple-
quantum 23Na MAS NMR spectra were recorded
using the z-filter three-pulse sequence [26]. The
lengths of the first and second hard pulses were 3.6
and 1.4 ls, respectively. The length of the third
soft pulse (t1 ¼ 10 kHz) was 12.5 ls. The MAS
rate was tR ¼ 14 kHz. For NaY and K15NO3-treated zeolite NaY calcined at 350 �C, 90 data
points were acquired in the t1 dimension in incre-
ments of 1=mR ¼ 71:4 ls. With the K15NO3-treated
zeolite NaY calcined at 350 �C and washed 200
data points were acquired in the t1 dimension in
increments of 10 ls. The recycle delay was 2 s. The
ppm scale of the sheared spectra was referenced to
t0 in the t2 domain and to 3.78 t0 in the t1 domain(reference 1 M aqueous NaCl). As far as we know,23Na and 15N MAS NMR were employed in the
context of nitrate occlusion in zeolites for the first
time in this work.
3. Results and discussion
3.1. NH4NO3 occlusion
In this work, the occlusion of ammonium ni-
trate was only attempted in the Y zeolite for the
2:1 and 4:1 nitrate:adsorbent ratios, as mentioned
previously. After the thermal treatment the solids
were washed with 70 cm3 of water per g of solid.
The total volume of water was divided into smallfractions that were added sequentially to the solid.
This washing procedure will henceforth be desig-
nated as method A.
The efficiency of the occlusion process was
evaluated through the nitrogen adsorption iso-
therms at )196 �C that allow an estimation of the
portion of the initial void volume that remained
occluded. The obtained isotherms are shown inFig. 1 for the Y zeolite and the samples submitted
to the thermal treatment with ammonium nitrate
for the NH4NO3:Y ratios of 2:1 and 4:1. It is
clearly seen in Fig. 1 that the three nitrogen iso-
therms are almost superimposed, which indicates
that, with the experimental procedure used the
occlusion of NH4NO3 in the Y zeolite occurredonly up to a very limited extent. This situation is
rather different from what occurs in other zeolites
as, for instance, in erionite [7] where a signifi-
cant decrease in the adsorption capacity of N2
was observed for samples occluded with a 4:1
nitrate:zeolite ratio and washed in similar con-
ditions. The difference between the behavior of
erionite and Y zeolite towards the occlusion ofNH4NO3 (about 0.4 nm in diameter [27]) is most
probably related with the differences in the struc-
ture, particularly in the pore openings, which are
0:36 � 0:51 nm for erionite and 0.74 nm for Y
zeolite [28]. Therefore, it can be stated that the
very open structure of this zeolite does not retain
the molten salt, which is leached to a large extent
during the washing, a situation which is similar towhat was found before, for instance, with 13X
zeolite [9]. Due to this result, and because the
structure of the pillared clay used in the present
work is expected to be at least as open as that of
zeolite Y, the occlusion of NH4NO3 in PILCs was
not made and, therefore, only the occlusion of
KNO3 was considered in the remaining study.
0
2
4
6
8
10
0 0.2 0.4 0.6 0.8 1
p/p 0
na(m
mo
l/g)
initial
2:1-A
4:1-A
Fig. 1. Nitrogen adsorption isotherms at )196 �C in the initial
Y zeolite and in the samples submitted to thermal treatment
with NH4NO3 with the nitrate:zeolite 2:1 and 4:1 ratio washed
by method A (see text).
166 A. Carvalho et al. / Microporous and Mesoporous Materials 58 (2003) 163–173
3.2. KNO3 occlusion
3.2.1. Nitrogen adsorption isotherms
Using the same methodology followed with
ammonium nitrate, particularly in what concernsthe process of washing (method A), the occlusion
of potassium nitrate was firstly studied in the Y
zeolite, for the 2:1 and 4:1 KNO3:Y zeolite ratios.
The samples obtained after the thermal treatment
and washing with 70 cm3 of water per g of solid are
designated by 4:1-A and 2:1-A. The nitrogen ad-
sorption isotherms obtained on the samples trea-
ted with KNO3, which are presented in Fig. 2,have a quite different pattern from that of the
isotherms displayed in Fig. 1. These results reveal
a significant reduction of the micropore volume of
the zeolite after treatment with molten KNO3 and
clearly suggest that the use of a large proportion of
this salt, as in the 4:1 ratio, does not represent a
clear advantage on the extent of occlusion. On the
other hand, it must be emphasized that, accordingto the solubility of KNO3, 10 cm3 of water would
be enough to dissolve about 7 g of this nitrate [29].
To quantify the micropore volumes of the sam-
ples, the Dubinin–Radushkevich equation (DR),
which is based in Polanyi�s concept of character-
istic curve, was used [30]. This equation can be
tested in the linear form: logðwÞ ¼ logðw0Þ � D log2
(p=p0), where w is the amount adsorbed at the re-
spective relative pressure p=p0, expressed in liquid
volume (cm3/g) after converting the amounts ad-
sorbed in mmol/g, through the millimolar volume.
The constant D can be related with parameters
that depend both on the solid and on the adsor-bate, and the value of w0 is the micropore volume.
The reduction in the micropore volumes, estimated
from the DR equation, verified in the samples
from the 4:1-A and the 2:1-A ratios, to 30% and
21% of the initial value, respectively, are not suf-
ficiently different to justify the use of the larger
amount of potassium nitrate. In this way, only the
samples obtained with the 2:1 ratio were hence-forth studied in more detail. In particular, it was
checked how the process of washing could influ-
ence the properties of the final product and can
give some indications on the leaching of the ni-
trate. So, to clarify this aspect, besides the washing
method quoted above (method A) another wash-
ing procedure (method B) was used. In this
method small quantities of water were also addedsequentially, but between the additions the sam-
ples were dried overnight at 50 �C. The fractions of
water used were such that the samples were wa-
shed until the proportions 5, 15, 30, 50 and 70 cm3
of water per g of sample were obtained. To dis-
tinguish the samples prepared following this
washing procedure the proportion of cm3 of water
per g of solid used is quoted in parentheses.The nitrogen isotherms obtained on samples
washed following method B are also depicted in
Fig. 2, and the micropore volumes estimated from
the DR equation are shown in Table 2. Comparing
the results obtained in samples 2:1-A and 2:1-B(70)
it is evident that when the water addition is dis-
continuous and the samples are dried in between
(method B), much more nitrate is released. Nev-ertheless, as revealed by the value of the micropore
volume of 2:1-B(70) sample (Table 2), a significant
part of the potassium nitrate must still be retained
in the structure. It is interesting to note that the
sample labelled 2:1-B(15), although washed with a
much smaller amount of total water contains less
KNO3 than the 2:1-A sample, as shown by the
respective values of the micropore volumes (Table2). From these results one can conclude that, in
fact, the washing procedure used is determining
0
2
4
6
8
10
0 0.2 0.4 0.6 0.8 1
p/p 0
na(m
mol
/g)
initial
2:1-B(70)
2:1-B(15)
2:1-A
4:1-A
Fig. 2. Nitrogen adsorption isotherms at )196 �C in the initial
zeolite and in the samples submitted to occlusion with KNO3
with the 4:1 and 2:1 nitrate:zeolite ratio, and washed by
methods A and B (see text).
A. Carvalho et al. / Microporous and Mesoporous Materials 58 (2003) 163–173 167
for the nitrate leaching process: when the solid is
left to dry between the additions of water, higher
amounts of KNO3 are removed.
For the pillared clay, only the relation 2:1 (ni-
trate:Al-PILC) was studied and the samples were
washed only according to method B. The nitrogen
adsorption isotherms, obtained in the initial Al-
PILC and in representative samples, are shown inFig. 3. A decrease in the adsorbed amounts of
nitrogen in the samples with KNO3 was registered
which, as in the case of Y zeolite, is dependent on
the volume of water used in the washing. The
micropore volume is recovered when the occluded
Al-PILC is washed with a proportion of 70 cm3/g
(Table 2). Nevertheless, a significant difference in
the shape of the isotherms of the ‘‘initial’’ and ofthe treated sample was noticed (Fig. 3), the former
being steeper after the relative pressure around 0.2.
However, this is most probably not related with
the amount of KNO3 but with the dissimilar way
employed to prepare the samples. In fact, after the
pillaring process the initial PILC was freeze-dried,
and the samples after the treatments with KNO3
were oven-dried. It is known that freeze drying
preserves the ‘‘house-of-cards’’ structure of the
clay, which develops a secondary porosity, while
oven drying tends to optimize the face-to-faceaggregation of the clay particles, precluding the
formation of the secondary porosity mentioned
[31], which is responsible for the upwards devia-
tion of the adsorbed amounts in the isotherms.
3.2.2. Differential scanning calorimetry
The samples were also characterized by DSC
since, in principle, this technique can be informa-tive on the process of occlusion. The DSC curves
between 25 and 350 �C for zeolite Y and the pil-
lared clay after different treatments with potassium
nitrate are shown in Fig. 4. For comparison, the
results for pure KNO3 and for the other initial
materials are also given. In curve for pure KNO3,
the two expected peaks for the phase transition
and melting were observed at 133 and 335 �C, re-spectively. The curves for the initial Y zeolite and
Al-PILC (b and f, respectively) also have their
usual shape, that is, a broad peak is noticed, with a
maximum between 100 and 200 �C, which can be
attributed to the loss of water. This peak is larger
for the zeolite than for the pillared clay since, in
the former, there exists not only physisorbed water
but also water that interacts with the exchangeablecharge compensating sodium cations and is, there-
fore, more firmly bounded. For the system
KNO3:Y zeolite (2:1), before (physical mixture,
curve c) and after heating (curve d), a reduction in
the onset temperatures of the phase transition and
Table 2
Micropore volume (w0) obtained from the nitrogen adsorption isotherms )196 �C, using the DR equation, in Y zeolite, PILC and
samples occluded with KNO3 and washed following methods A and B (see text)
Zeolite
w0 (cm3/g) Initial 4:1-A 2:1-A 2:1-B(70) 2:1-B(15)
0.330 0.068 0.100 0.200 0.119
PILC
w0 (cm3/g) Initial 2:1-B(30) 2:1-B(50) 2:1-B(70)
0.130 0.031 0.063 0.126
For samples prepared according to method B the proportion of cm3 of water per g of solid used is given in parentheses.
0
2
4
6
0 0.2 0.4 0.6 0.8 1
p/p 0
na(m
mol
/g)
initial
B(70)
B(50)
B(30)
Fig. 3. Nitrogen adsorption isotherms at )196 �C in the initial
Wyoming pillared clay and in the samples submitted to occlu-
sion with KNO3 with the 2:1 nitrate:PILC ratio and washed
according method B (see text). In parentheses the proportion of
ml of water per g of solid used is given.
168 A. Carvalho et al. / Microporous and Mesoporous Materials 58 (2003) 163–173
the melting point is always observed (Table 3),
together with the enlargement of the latter peak.
This could suggest the existence of a broad range
of interactions between the nitrate and the zeolitic
structure. After washing (curve e) the onset tem-
peratures of the phase transition increase, the peak
lower to melting becomes narrow, and the shape is
more alike to that recorded for pure KNO3 but,
even in this sample, the onset temperatures areclearly lower to that of the pure nitrate. In the case
of the pillared clay based samples, the onset tem-
peratures (Table 3), either for the phase transition
or the melting process, are closer to the values for
pure KNO3. Comparing the curves in the pillared
clay and in the Y zeolite the peaks are narrower in
the PILC, particularly the one related with the
melting. For both materials, the initial peak re-lated with the loss of water (between 100 and 200
�C) is much flatter in the solids with KNO3, a
feature that can be attributed to the substitution of
the water molecules in the void volume by KNO3,
that is, to the occlusion of the nitrate.
3.2.3. Infrared spectra
Selected infrared spectra are given in Fig. 5.Pure KNO3 exhibits a band at 1390 cm�1 assigned
to the asymmetric stretching mode of the nitrate
ion [32]. The spectra of the parent Y zeolite shows
an intense band near 1050 cm�1 which is due to
internal vibrations of the structural tetrahedra
25 125 225 325
T (ºC)
Hea
tFlo
w(a
.u.)
d)
a)
b)
c)
e)
f)
g)
h)
i)
Exo
Fig. 4. DSC curves between 25 and 350 �C for: (a) pure KNO3;
(b) initial Y zeolite; (c) physical mixture KNO3:Y zeolite (2:1);
(d) after heating the mixture KNO3:Y zeolite (2:1); (e) after
washing the heated mixture KNO3:Y zeolite (2:1); (f) initial
PILC; (g) physical mixture KNO3:PILC (2:1); (h) after heating
the mixture KNO3/PILC (2:1); (i) after washing the heated
mixture KNO3:PILC (2:1).
Table 3
Onset temperatures (in �C) for the transformations observed in
the DSC curves in Fig. 4 for pure KNO3 and KNO3 in zeolite Y
or in the PILC (2:1 nitrate:adsorbent ratio), after different
treatments
Last
treatment
Phase
transition
Melting
KNO3 – 133 335
KNO3 þY zeolite Mixture 129 282
KNO3 þY zeolite Heating 120 284
KNO3 þY zeolite Washing 132 322
KNO3 þPILC Mixture 130 332
KNO3 þPILC Heating 131 333
KNO3 þPILC Washing 132 333
Values in the literature for the phase transition and the melting
point of KNO3 are 129 and 334 �C, respectively [28].
Tran
smita
nce
(a.u
.)
pure KNO3
parent Y zeolite
Y/KNO3
PILC/KNO3
2000 1500 1000
ν (cm-1
)
Fig. 5. Infrared spectra, between 500 and 2000 cm�1 for, the
pure KNO3 parent Y zeolite and zeolite Y and the PILC after
occlusion of KNO3.
A. Carvalho et al. / Microporous and Mesoporous Materials 58 (2003) 163–173 169
[33]. This band also appears in the spectra of
the initial PILC (not shown), where it is due to the
vibration of tetra-coordinated silicon [34]. The
curves for occluded KNO3 in zeolite Y and
the PILC (Fig. 5) show an enlarged band at 1390
cm�1 the enlargement being, most probably due tothe interactions developed between the nitrate
group and the internal microporous structure of
the zeolite and the PILC. The spectrum of the
parent Y zeolite also shows a band around 1640
cm�1 which is due to physisorbed water. The in-
tensity of this band decreases when the micropore
volume is, at least partially, occupied by KNO3.
Both the above mentioned features, that is, theenlargement of the 1390 cm�1 band and the si-
multaneous decrease in the intensity of the 1640
cm�1 band, are consistent with the occlusion of
KNO3 in the structure of the zeolite and the pil-
lared clay.
As an attempt to ascertain the process of the
leaching of occluded KNO3, Fig. 6 was con-
structed. In this figure the absorbance of the ni-trate band (1390 cm�1)––Anitrate––was normalized
against the absorbance of the structural band near
1050 cm�1, of the zeolite or the PILC––
Azeolite or PILC––and plotted against the volume of
water per g of solid used to leach the nitrate. From
this representation it is clear that, as expected, the
normalized absorbance decreases as the volume of
water increases, nevertheless, the release of nitrate
from the occluded zeolite is easier than from the
occluded PILC, which is a feature that can favor
the PILCs if these materials are to be considered in
soil amendment.
3.2.4. NMR
The 27Al MAS NMR spectra of all zeolitic
materials studied (not shown) are very similar to
the (typical) spectrum of the parent NaY zeolite,
confirming the structural integrity of the frame-
work of the samples, which is in line with the DRX
results mentioned above. The 23Na MAS NMR
spectra of the parent and calcined NaY zeolite and
the potassium nitrate-treated materials are shownin Fig. 7. The parent NaY zeolite, the materi-
als obtained after (i) calcination at 350 �C (not
shown), and (ii) calcination and washing with
water give similar spectra. The sample treated with
potassium nitrate (and calcined at 350 �C) exhibits
a very different spectrum, containing several peaks.
Washing this sample significantly changes the 23Na
MAS NMR spectrum, only a single broad reso-
0
0.4
0.8
1.2
1.6
0 20 40 60 80cm3 of washing water / g of sample
PILC
Zeolite Y
Ani
trat
e/A
zeol
ite o
r PI
LC
Fig. 6. Relation between the absorbances of the nitrate band
(Anitrate), normalized against the structural band of the zeolite or
PILC (Azeolite or PILC), and the volume of water used to leach the
KNO3.
δ
Fig. 7. Single-quantum 23Na MAS NMR spectra of the as-
prepared and the calcined (350 �C) and washed zeolite NaY,
and the K15NO3-treated zeolite NaY calcined at 350 �C and
washed.
170 A. Carvalho et al. / Microporous and Mesoporous Materials 58 (2003) 163–173
nance being observed. Because 23Na is a half-
integer quadrupolar (I ¼ 3=2) nucleus the (central
transition) NMR lines are broadened by the sec-
ond-order quadrupole interaction, and the infor-
mation deriving from the (single-quantum) MAS
NMR spectrum is very limited. We have, thus,recorded 23Na triple-quantum (3Q) MAS NMR
spectra of selected materials, shown in Fig. 8. The
parent NaY zeolite exhibits at least two peaks, one
of which (S1) corresponds to a distributed Na site.
The sample treated with potassium nitrate (not
washed) displays three types of Na sites: a strong,
broad, distributed peak S1; a faint peak S2; and a
second-order quadrupole powder doublet S3. S2and S3 may be due to a small amount of an, as yet,
unidentified phase (or phases) containing sodium
which, however, has not been detected by powder
XRD. Washing this sample yields a different
spectrum, displaying two broad peaks assigned to
very distributed Na sites. It is clear from these
experiments that treating zeolite NaY with potas-
sium nitrate produces major changes in the localenvironments of the extra-framework Na cations.
This may be due to both a change in the popula-
tions of the different types of Na sites and to the
presence of nitrate and potassium ions occluded in
the pores. Moreover, washing with water also
seems to alter the nature and distribution of zeo-
litic Na species (peak S1) and removes (impurity)
peaks S2 and S3.The occlusion of potassium nitrate in zeolite
NaY is further supported by 15N MAS NMR
spectroscopy (Fig. 9). Unwashed potassium ni-
trate-treated NaY zeolite gives a main peak at 0.6
ppm and a second resonance at )2.6 ppm. The
former is assigned to bulk 15N- enriched KNO3,
since this solid resonates at 0.6 ppm, and wash-
ing the sample greatly decreases its intensity. Thewashed sample exhibits a small peak at 0.5 ppm
(with a high-frequency shoulder at �0.6 ppm) and
a main peak at )1.9 ppm. All 15N NMR reso-
nances are typical of nitrate ions. The small peak
at 0.5 ppm may be due to surface nitrate ions,
while the )1.9 ppm is assigned to occluded nitrate
species. Since the main 15N NMR resonance shifts
from )2.6 to )1.9 ppm, washing with waterslightly changes the local environment of the ni-
trate ions.
Fig. 8. Triple-quantum 23Na MAS NMR spectra of as-pre-
pared zeolite NaY- and K15NO3-treated zeolite NaY calcined at
350 �C and washed.
A. Carvalho et al. / Microporous and Mesoporous Materials 58 (2003) 163–173 171
4. Conclusions
The occlusion process of molten nitrates in
a clay from Wyoming, pillared with aluminumoxide, was studied in parallel with a synthetic zeo-
lite type Y. Firstly it was shown that, in the case of
ammonium nitrate, occlusion occurred only to a
very limited extent in the open structure of the
zeolite Y, contrary to what is described in the lit-
erature for natural zeolites with smaller pores than
those of zeolite Y but in line with the results from
literature obtained with 13X zeolite.Considering the occlusion of potassium nitrate
this was achieved in the Y zeolite and in the pil-
lared clay with a departure mixture with a 2:1 ni-
trate:PILC or zeolite ratio. Evidence for the
occlusion was obtained by different techniques,
such as nitrogen adsorption data at low tempera-
ture, DSC or infrared spectroscopy. Nitrogen ad-
sorption showed that the micropore volumes ofthe occluded samples are lower than those found
for the initial materials. This reduction is related
with the amount of water used to leach the nitrate
after treatment with the molten salt and with the
leaching method used. In fact, the nitrate removal
is increased when, between the additions of water,
the samples are left to dry. By DSC it was possible
to follow, to a certain extent, the process of oc-
clusion and it was shown that, particularly in the
case of the more constrained structure, of zeolite
Y, the occlusion process affects both the temper-ature of the phase transition and the melting point
of KNO3. The DSC and infrared results also
showed the decrease of water in the micropore
structures after treatment with the molten salt as
the result of the occlusion process. For the Y
zeolite it was shown by 23Na MAS NMR and 23Na
triple-quantum (3Q) MAS NMR that treatment
with molten potassium nitrate clearly changes thelocal environments of the exchangeable sodium
cations. Moreover, besides a peak related with the
bulk 15N-enriched KNO3 a second peak was ob-
served by 15N MAS NMR spectroscopy. Both
observations clearly support the occlusion of po-
tassium nitrate in the microporous structure of
zeolite Y.
An attempt to approach the process of leachingof the occluded KNO3, was made in a semi-
quantitative way by infrared spectroscopy nor-
malizing the absorbance of the bands due to the
nitrate against the structural vibration bands. This
showed that pillared clays present a slower release
of KNO3 to the water than Y zeolite. Both mate-
rials, the Y zeolite and the pillared clay,
are good candidates for further studies in largerscales considering their potential use in soil
amendment.
Acknowledgements
This work was partially funded by the Fundac�~aao
para a Cieencia e Tecnologia through Centro deCieencias Moleculares e Materiais.
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