Nitrate occlusion studies in Y zeolite and in a clay pillared with aluminium oxide

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

mail to: [email protected]

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).


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


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).


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)


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.


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.


[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.


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.


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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Documents

Nitrate occlusion studies in Y zeolite and in a clay pillared with aluminium oxide