comprehensive inorganic chemistry ii || zeolite nanoparticles
TRANSCRIPT
Co
5.10 Zeolite NanoparticlesS Mintova, Universite de Caen, Caen, FranceE-P Ng, Universiti Sains Malaysia, Penang, Malaysia
ã 2013 Elsevier Ltd. All rights reserved.
5.10.1 Introduction 2865.10.2 Syntheses of Zeolite Nanoparticles 2865.10.2.1 Precursor Suspensions 2865.10.2.1.1 Templated synthesis approach 2865.10.2.1.2 Template-free synthesis approach 2895.10.2.1.3 Seed-induced synthesis 2905.10.2.1.4 Multistep synthesis approach 2915.10.2.1.5 Ionothermal synthesis 2915.10.2.2 Types of Heating 2925.10.2.2.1 Microwave-assisted synthesis 2925.10.2.2.2 Microchannel-assisted synthesis 2935.10.2.3 Confined Space Synthesis 2935.10.2.3.1 Reverse microemulsion synthesis 2945.10.2.4 Other Methods 2955.10.2.4.1 Direct-conversion synthesis approach 2955.10.2.4.2 Centrifugation-assisted grinding 2955.10.2.4.3 Laser-induced fragmentation method 2955.10.2.5 Separation of Zeolite Nanocrystals 2965.10.3 Applications of Zeolite Nanoparticles 2975.10.3.1 Zeolite Membranes 2975.10.3.2 Optical and Other Devices 2975.10.3.3 Biological and Medical Applications 2995.10.4 Conclusion 300Acknowledgment 300References 301
AbbreviationsC Degree Celsius
AEI Aluminophosphate number eighteen
AFI Aluminophosphate number five
AlPO-5 Aluminophosphate number five
AlPO-18 Aluminophosphate number eighteen
BEA Beta
CHA Chabazite
CTAB Cetyltrimethylammonium bromide
DLS Dynamic light scattering
EMT Elf Mulhouse two
FAU Faujasite
GIS Gismondine
HRTEM High-resolution transmission electron
microscopy
IL Ionic liquid
IR Infrared
LTA Linde type A
MFI Mobil number five
mprehensive Inor
ganic Chemistry II http://dx.doi.org/10.1016/B978-0-08-09777MEL Mobil number eleven
MnAlPO-5 Manganoaluminophosphate number five
MOR Mordenite
MTW Mobil twelve
nm Nanometer
NMR Nuclear magnetic resonance
OFF Offretite
RCF Relative centrifugal force
rpm Rotation per minute
SAPO-5 Silicoaluminophosphate number five
SAPO-34 Silicoaluminophosphate number thirty four
SDA Structure-directing agent
SOD Sodalite
TEOS Tetraethyl orthosilicate
TS-1 Titanosilicate number one
wt.% Weight percent
XRD X-ray diffraction
ZSM-5 Zeolite socony material number five
4-4.00512-X
285286 Zeolite Nanoparticles
5.10.1 Introduction
Nanoscale science and engineering provide unique under-
standing and control of matter and mostly on a fundamental
level. In particular, the nanosized particles have been fascinat-
ing the world of science due to their unique properties and use
in diverse fields including catalysis, photography, photonics,
electronics, labeling, imaging, sensing, etc.1–3 Inorganic nano-
particles, in particular zeolite materials, are the focus of many
researchers due to their diverse framework-type structures con-
taining one-, two-, or three-dimensional channel systems
(pores) whose dimension is on the order of the molecular
size.4 Hence, these materials with nanosized dimensions have
been considered in wide-ranging applications such as photon-
ics, sensors, electronic and optical detection systems, therapeu-
tics, diagnostics, photovoltaics, and catalysis.
In addition to the conventional zeolites, their nanosized
counterparts with a size in the range of 5–1000 nm have
attracted considerable attention during the last two de-
cades.5–33 Although the chemical composition and the
framework-type structure of zeolites are important, even
more vital are the size and shape of the nanoparticles which
determine their surface/colloidal properties. Different mor-
phologies and sizes of the zeolite nanocrystals can result
from fine-tuning of the synthesis parameters such as initial
gel composition, type of precursor materials, heating time
and type, and temperature of preparation and postsynthesis
treatment in order to alter the nucleation and crystal growth
processes.20–24 The reduction in particle size frommicrometers
to the nanometer scale leads to substantial changes in their
properties and thus different performances even in traditional
applications are expected. Therefore, the reduction in the size
of zeolite crystals to the range of some unit cells is expected to
provide materials with completely new properties. Moreover,
the significance of these nanosized materials is mainly related
with the emerging area of applications that goes far beyond
traditional separation and catalytic processes. The possibility to
obtain stable colloidal suspensions of microporous particles
capable of processing on different surfaces by rapid techniques
is also of great importance for their advanced applications.
The development of versatile methods for the preparation
of zeolite nanocrystals with defined structure, size, stability,
and morphology has attracted significant attention during the
last two decades. Some of the recent developments in the
synthesis, characterization, and application of zeolite nano-
crystals have been summarized in the review papers.5–7,25–27
This chapter aims to give a vivid look on the use of various
techniques for synthesis of zeolite nanocrystals with controlled
size, stability, morphology, and possibility for increased crys-
talline yield and scale-up processes. The most recent applica-
tions of the zeolite nanocrystals different from conventional
zeolites are presented.
5.10.2 Syntheses of Zeolite Nanoparticles
The zeolite syntheses are performed in closed reacting systems,
where the high super-saturation leads to spontaneous nucle-
ation and controlled crystallization process. Upon such
conditions directing the crystal size, that is, the nutrient pool
is limited, and thus after the exhausting a building component
the growth process would stop. Hence, the formation of zeolite
nanocrystals requires conditions that favor the nucleation over
crystal growth.
The main approaches applied for the synthesis of nanosized
molecular sieves are (1) synthesis from clear precursor suspen-
sions in the presence of organic template, (2) low-temperature
syntheses from highly alkaline organic-template-free hydro-
gels, and (3) other methods including ionothermal, seed-
induced, confined space synthesis, etc.
In order to prepare zeolite nanocrystals, the type of the
precursor suspensions (Section 5.10.2.1), type of heating
(Section 5.10.2.2), confined space effect (Section 5.10.2.3),
and other physicochemical parameters (Section 5.10.2.4) have
to be considered. Once the zeolite nanoparticles are prepared
with desired size, structure, andmorphology, the emphasis is on
their stabilization in colloidal suspensions, to prevent further
agglomeration and coalescence (Section 5.10.2.5).
5.10.2.1 Precursor Suspensions
5.10.2.1.1 Templated synthesis approachThe templated synthesis approach is considered as the most
common method for preparation of zeolite nanocrystals. The
zeolite nanocrystals are synthesized via hydrothermal treat-
ment of clear aqueous suspensions at moderate temperatures
(30–120 �C). Clear precursor suspensions have been used to
synthesize several nanocrystalline materials with faujasite
(FAU)-, Mobil number five (MFI)-, Mobil number eleven
(MEL)-, sodalite (SOD)-, gismondine (GIS)-, Linde type A
(LTA)-, Beta (BEA)-, aluminophosphate number eighteen
(AEI)-, and chabazite (CHA)-type framework structures.5–24
Organic templates (or structure-directing agents, SDAs) in
most of these syntheses play the structure-directing role and
provide the high alkalinity and high super-saturation level
which is needed for crystallization of nanocrystals. The organic
templates with different size, shape, and hydrophilicity influ-
ence the position of tetrahedrally coordinated Al and Si atoms in
the zeolite framework, and lead to the formation of crystals with
various type structures. On the other hand, several amines can
direct the synthesis of the same structure, while one type organic
amine can also template many different structures. For example,
tetraethylammonium cations (TEAþ) can template nanosized
zeolites such as BEA, ZSM-5 (MFI), AlPO-5 (AFI), SAPO-5
(AFI), AlPO-18 (AEI), and SAPO-34 (CHA).
The organic templates mostly used for preparation of zeo-
lite nanocrystals are summarized in Table 1. In addition to the
mostly used tetraalkylammonium (TAA) cations, some amines
are also used as co-templates or SDAs.
In general, within a given range of molecular compositions,
an increase of the SDA induces a rather narrow particle-size
distribution of smaller crystals by hindering the agglomeration
of the precursor and crystalline particles. However, alkali cat-
ions used as counterbalancing ions for the crystalline structure
cannot be used that liberally as their influence in the crystalline
phase obtained is far greater due to the different interactions
during the early stages of crystallization.20 Normally, the large
amount of organic templates is used not only to control the
crystal size but also to obtain stable crystalline suspensions.
Table 1 Organic templates used for the preparation of nanosizedzeolites
Templates Crystalline phase
Tetramethyl ammonium, TMAþ LTA, FAU, SOD, GIS, OFFTetraethyl ammonium, TEAþ AEI, AFI, CHA, BEATetrapropyl ammonium, TPAþ MFI, AFITetrabutyl ammonium, TBAþ AFI, MELIsopropyl amine, i-Pr2NH AEL1-Ethyl-2,3-dimethylimidazolium, edmimþ AFI4,40-Trimethylenebis(N-methyl,N-benzylpiperidinium), TMP2þ
BEA
Table 2 Synthesis of silicalite-1 nanocrystals
Zeolite T (�C) Time (h) Crystal size (nm)
Silicalite-1a 60 240 2060 288 2560 360 4070 240 5880 96 80
aPrecursor suspension: 9 TPAOH:0.16 NaOH:25 Si:495 H2O:100 EtOH.
Zeolite Nanoparticles 287
The as-prepared precursor suspensions with excess of organic
templates are treated under mild conditions (syntheses temper-
ature in most of the cases do not exceed 120 �C) so that the
nucleation occurs more readily than crystal growth.9,11,20,29–32
The high super-saturation is also important to provide concen-
trated precursor mixtures containing soluble species as depoly-
merized and reactive as possible. This is generally accomplished
through lowering the synthesis temperature that favors nucle-
ation over crystal growth. On the other hand, these conditions
result in slow nucleation and crystal growth and, thus, a
prolonged synthesis time and reduced crystalline yields are ob-
served in most of the syntheses carried out in highly super-
saturated precursor suspensions.
Besides, the hydrothermal process in clear precursor sus-
pensions can be systematically modified in order to have more
control over the size of the nanocrystals by changing the reac-
tion parameters such as template concentration/type, temper-
ature, aging, heating time, and source of reactants (silica,
alumina, or phosphate). The ratio between the organic tem-
plates and inorganic reactants allows additional control of
crystalline size; that is, the high ratio results in the formation
of more nuclei that subsequently result in the formation of
zeolite nanocrystals.33 Besides, the choice of template also
strongly influences the size of crystals. For instance, in the
synthesis of AlPO-n materials, amines usually produce
micrometer-sized crystals, whereas quaternary ammonium
salts lead to the formation of AlPO-n nanocrystals.5,34,35
Synthesis of zeolite nanocrystals is generally accomplished
through lowering the synthesis temperature and prolonging
the synthesis time. An example of how the synthesis tempera-
ture and treatment time affect the crystallite size of silicalite-1 is
given Table 2. As can be seen, the size of the crystals increased
from 20 to 40 nm by systematically increasing the synthesis
time at a constant synthesis temperature (60 �C). On the other
hand, the crystal size increased more significantly at higher
temperatures (70–80 �C), that is, from 20 to 80 nm.6
Furthermore, aging under ambient condition has a pro-
nounced effect on the subsequent crystallization process. Dur-
ing aging, structural rearrangement in the precursor suspensions
occurs, which leads to the formation of zeolite nuclei. As a
result, the crystal size, induction period, and crystallization
time decrease upon lengthening of the aging process.
Another parameter that plays a significant role in the syn-
theses of zeolite nanoparticles is the type of heating applied to
the clear precursor suspensions. The synthesis of zeolite nano-
crystals can be performed in conventional air-driven ovens,
through reflux, and in microwave ovens. The general trend in
the syntheses of zeolite crystals using the three heating systems
is shown in Figure 1. Generally, microwave irradiation pro-
vides the fastest crystallization rate and the smallest crystal size
in comparison to those prepared using reflux and conventional
heating. On the other hand, the synthesis through microwave
irradiation, reflux, and hydrothermal treatment under opti-
mized conditions renders particles with desired morphologies,
high purity, and colloidal stability. An example of how the size
of zeolites can be changed by replacing the slow conventional
heating with fast and short heating under microwave irradiation
is shown in Figure 1. Zeolite nanocrystals with spheroidal,
square, plate, orthorhombic crystals, and spheres can be synthe-
sized via optimizing the crystallization conditions.
Precursors such as alkoxysilanes and metal alcoholates pro-
vide reactants in molecular form which favor the synthesis of
microporous materials. The uses of different sources of reac-
tants (silica, alumina, and phosphate) play a significant role in
the nucleation and crystallization processes, and therefore they
influence the growth and morphology of the final material
(Figure 2). The silica source can influence different aspects of
zeolite crystallization and it leads to changes in the properties
of the final product. Some of the important parameters de-
scribing the process of zeolite crystallization such as the nucle-
ation and crystallization rates depend on the dissolution of the
silica precursors. The fragile silicate intermediates released dur-
ing the process of the silica source dissolution play an impor-
tant role in the zeolite formation as well. Besides the effects on
the formation of a particular zeolite, the silica source can
influence the particle size and shape of the crystals.36–38 The
impurities introduced by the silica in the starting system can
also affect the properties of the zeolite. The substitution of
tetraethyl orthosilicate (TEOS) by colloidal silica in the synthe-
sis of nanosized silicalite-1 was found to prolong the duration
of the nucleation period.21 The nucleation rate is faster for
silicalite-1 when TEOS is used as a silica source compared to
amorphous silica (colloidal or fume silica). Depending on the
silica source employed, the size of the silicalite-1 nanocrystals
increases in the following order TEOS (15 nm)!Cab-O-Sil
(25 nm)!Ludox LS 30 (50 nm).
Several microporous materials have been prepared in the
form of colloidal suspensions with narrow particle-size
distribution.39,40 The synthesis of zeolite crystals with equal
particle radius requires a homogeneous distribution of the
viable nuclei in the system. Therefore, the homogeneity of
the starting system and simultaneity of the events leading to
the formation of precursor gel particles and their transforma-
tion into crystalline zeolitic material is of primary importance.
In order to obtain such homogeneous starting systems, abun-
dant amounts of TAA hydroxides and water are employed. On
Beta zeolite
Hydrothermal
Reflux
Microwave
Min Days
Siz
e (n
m)
Time
Conventional heating
Magn301x lar417
50 μm
Microwave heating
Reflux heating
Figure 1 Trends in the change of zeolite dimension and morphology using different methods of heating (microwave, reflux, and conventional heating).
(a) (b) (c)
Figure 2 SEM micrographs of silicalite-1 nanocrystals synthesized using (a) TEOS, (b) Ludox LS-30, and (c) Cab-O-Sil as silica source.Scale bar: 500 nm. From Fig. 6 in Mintova, S.; V. Valtchev, V. Micropor. Mesopor. Mater. 2002, 55, 171.
Size (nm)
5 10 15 20 252θ (degree)
30 35
100 nm
Inte
nsity
(a.u
.)
Figure 3 Characterization of LTL zeolite nanocrystals with XRD,HRTEM, and DLS. Modified from Fig. 2c in Wong, J. -T.; Ng, E.-P.; Adam,F. J. Am. Cer. Soc. 2012, 95, 805–808.
288 Zeolite Nanoparticles
the other hand, the content of alkaline cations is very limited.
All these factors together with the careful choice of the reac-
tants allow the stabilization of ‘clear’ starting mixtures where
only discrete gel particles are present.38,41,42
Once the zeolite crystals are prepared with the desired size
and shape, the emphasis is on isolating a nonagglomerated
form of the particles and also on their characterization mainly
in liquid form as well as nanosized powder. Common
parameters used for characterization of zeolite nanocrystals
are diameter, colloidal stability, crystallinity, porosity, surface
chemistry, etc. Very often, the standard techniques used for
characterization of inorganic crystalline materials cannot pro-
vide conclusive information on the crystallization process of
zeolite nanocrystals. Therefore, the primary particles (diameter
smaller than 10 nm), their aggregation, and further transfor-
mation into zeolite crystallites in most cases can be character-
ized by complementary techniques such as spectroscopy
(infrared (IR), Raman, and nuclear magnetic resonance
(NMR)), small-angle x-ray diffraction (XRD), high-resolution
transmission electron microscopy (HRTEM), and dynamic
light scattering (DLS).5–24,29–33 In Figure 3, DLS, HRTEM,
and XRD data for Linde Type L (LTL)-type zeolite nanocrystals
are shown.
The templating concept for preparation of zeolite nanocrys-
tals is further developed for other organic additives based on
metal complexes, which allow to form the zeolite and at the
same time to introduce desired metals with possible applica-
tions in catalysis.43,44 Metal–amine complexes [M(NH3)4]2þ
are used as templates for zeolite nanocrystals as they carry a
high positive charge density and interact with the anionic
Zeolite Nanoparticles 289
silicate species. Besides, they have different shapes (square,
planar, or linear), which are not common for classic quater-
nary ammonium templates. The use of metal complexes was
recently exemplified in the preparation of Edingtonite (EDI)-
type zeolite nanocrystals using [M(NH3)4]2þ, where M is Cu,
Pd, Pt. It is observed that the square-planar Pd and Pt amine
complexes having the same geometry reinforce their role as
templates and also co-template the formation of EDI- and
FAU-type zeolites.43 The Pd and Pt complexes also lead to a
fast nucleation in the precursor aluminosilicate suspensions,
resulting in an exceptionally small crystalline particle with size
below 20 nm. The copper–amine complexes act as templates at
temperatures lower than 100 �C, thus avoiding the decompo-
sition of thermally unstable complexes.
In summary, the templated synthesis approach offers sev-
eral advantages, which include the (1) possibility to control
overall particle size, (2) colloidal stability, (3) morphology,
and (4) surface reactivity. Nevertheless, this approach is not
easily scalable due to the use of a large amount of expensive
and in some cases toxic organic templates. Besides, the crystal-
line yield from the templated clear precursor suspensions is
usually very low (<10%).
5.10.2.1.2 Template-free synthesis approachThe synthesis of zeolite nanocrystals from template-free pre-
cursor suspensions is highly desired since it opens an alterna-
tive route for preparation of nanosized materials. This
approach, however, could be applied only in the synthesis of
low-silica zeolites crystallizing from highly reactive precursor
gels without employing organic SDAs.
In the template-free synthesis, the alkali metal cations such
as Naþ and Kþ act as SDAs. In these precursor suspensions, the
type of cations strongly affects the nucleation and crystalliza-
tion process. For example, a potassium aluminosilicate zeolite
(zeolite L) is formed when Kþ is used in abundant amount,
while the high concentration of Naþ provokes the formation of
zeolite Y.
(a) (b)
(d) (e)
Figure 4 HRTEM images of low-silica zeolite nanocrystals prepared without oScale bar: 100 nm. (c) From Hu, Y.; Liu, C.; Zhang, Y.; Ren, N.; Tang, Y. Micr
Typically, the synthesis of zeolite nanocrystals in template-
free suspensions is performed at a relatively low temperature
(<100 �C).31,45–52 After completion of the crystallization reac-
tion, the nanocrystals with diverse particle-size distribution
and high yield (>80%) are obtained. A substantial reduction
in the synthesis time combined with full conversion of the
initial amorphous system into zeolite nanomaterial under
low temperature has been developed for several zeolite struc-
tures including LTL, FAU, EMT, SOD, MFI, and LTA
(Figure 4).31,45–52
Various silica sources with different specific surface areas,
impurities, and abilities to dissolve in alkaline mixtures are
employed. The vigorous mixing of the alkaline silicate and
aluminate solutions produced a precursor gel where all com-
ponents are expected to be homogeneously distributed. Usu-
ally the lowest possible temperature for a particular zeolite is
employed in order to favor the nucleation over the growth and
thus smaller crystals to be obtained. This example illustrates
the utility of the clear solutions in the investigation of different
populations of precursor particles involved in the crystalliza-
tion process. Contrary to the conventional gel systems, where a
large diversity of (alumino) silicates species is usually present,
the initially clear solutions contain a limited number of well-
defined discrete amorphous precursor particles.
The T (Si, Al) elements are introduced in the reaction mix-
ture as easily dissolvable sources. Most often, amorphous hy-
droxides, hydrous oxides, or related solids are employed. These
solids may be introduced in different physical states, for in-
stance, precipitated gels, ground glasses, volcanic ashes, colloi-
dal suspensions, or fumed silicas. Primary reactants are also
used to prepare such solids. Among them, the ones most often
used are alkaline silicate solutions, halides, silicon fluoro com-
plexes or alkoxides, aluminum salts, or aluminate solutions.
The silica content in the zeolite framework determines the
basic characteristics of the zeolite, namely, thermal stability,
hydrophilic–hydrophobic properties, amount and distribution
of the active sites, ion-exchange properties, etc. Pure siliceous
(c)
(f)
rganic templates (a) LTA, (b) FAU, (c) LTL, (d) EMT, (e) MFI, and (f) SOD.opor. Mesopor. Mater. 2009, 119, 306–314.
(a)
(b)(c) (d)
100 nm 200 nm
500 nmGrowth ofaggregrate
Single seed growth
Growth ofaggregrate
Single seed growth
20 nm
Figure 5 SEM micrograph of (a) MFI-type nanocrystals used forseeding of initial gels, (b) a sketch of a seed aggregate, and (c, d) theresult of the secondary growth of a monocrystalline particle and apolycrystalline aggregate exemplified by TEM images, respectively.Modified from Majano, G.; Darwiche, A.; Mintova, S.; Valtchev, V. Ind. &Engineer. Chem. Res. 2009, 48, 7084.
290 Zeolite Nanoparticles
zeolitic materials have a neutral framework. Isomorphous sub-
stitution of Si4þ with Al3þ introduces a negative charge in the
zeolite framework, which is compensated by the cations located
in the channels and voids of the structure. The level of this
substitution (the Si/Al ratio) determines the density of the active
sites and the ion-exchange capacity of the particular zeolite.
Numerous ways for reducing the mean particle size of
zeolite with LTA-type structure, keeping the economic impera-
tives in mind, have been presented.53 A study of the effect of
type of heating, postsynthesis treatment, and ultrasonication
on the size reduction in LTA crystals has been explored. It was
found that the application of ultrasonication did not lead to a
decrease in particle size, while the crystal morphology has
changed only. However, the microwave radiation is leading
to a narrow particle-size distribution of the zeolite with smaller
mean diameter. It is apparent that the rate of gel dissolution,
the distribution of the germ nuclei, and the crystallization time
play a significant role in determining the particle-size distribu-
tion and size of the crystals. Agitation promotes the formation
of larger crystals, although the type of agitation, namely, stir-
ring versus ultrasonication leads to completely different mor-
phologies. Aging leads to a substantial decrease in the spread of
particle size, which can be seen as a consequence of the relatively
low energy of nucleation. The template-free approach is opti-
mized to obtain nanosized LTA-type crystals.20 The high alka-
linity (high Na2O and/or low H2O) of the precursor suspension
is able to reduce the crystallization time from 2 weeks to 3 days.
Another zeolite, NaX, with controlled particle size from 20 to
800 nm at 60 �C, is synthesized from template-free precursor
suspensions.45 It is shown that the silica source and the hydro-
thermal conditions including crystallization temperature and
agitation govern the crystallization rate of the zeolite nanocrys-
tals. Additionally, the crystallization temperature has a pro-
nounced effect on the ultimate zeolite crystal size. Highly
crystalline zeolites A and X were obtained within 3 and
21 days, respectively.46,52 Both zeolites have nanosized particles;
however, the morphological appearance differs substantially.
Single zeolite A crystals (80–250 nm) were obtained, whereas
zeolite X crystallized in the form of spherical aggregates
(200–300 nm) built of very small 20–40 nm crystallites. The
relatively rapid transformation of the amorphous aluminosili-
cate species into zeolite under room temperature conditions was
achieved by a fine-tuning of all parameters affecting the crystal-
lization kinetics. It is also found that the postsynthesis ultrasonic
treatment has disintegrated the loosely attached particles and
provided a product with a relatively narrower particle-size dis-
tribution. The fraction of the silica converted into zeolite nano-
crystals of A and X was 75% and 83%, respectively.
In conclusion, the synthesis of zeolites from organic-
template-free precursors is an alternative approach for prepa-
ration of nanosized crystals. This method is environment
friendly since the synthesis does not employ any harmful
organic templates. Moreover, the absence of organic template
implies that no high-temperature calcination step is required
for opening up the pore system for the intended applications,
and thus this method is considered beneficial for both the
environment and the scaling-up through reducing the produc-
tion cost and reducing the use of harmful chemicals. However,
stable colloidal suspensions of these nanocrystals can be
prepared under additional treatment.
5.10.2.1.3 Seed-induced synthesisThe use of preformed zeolite seeds as nucleation centers in the
synthesis has a long history.54,55 Ideally, the advantage of this
method is that the nucleation of the desired phase is stimulated
and the growth process results in nanosized zeolites with a
narrow particle size and desired morphology. This method is
used to suppress the formation of undesired phases and enable
the intergrowth of different crystalline phases.56,57 The main
importance of this approach is the complete elimination of
organic templates with calcined or noncalcined seed crystals
and the formation of zeolite nanoparticles with high yields.
Successful preparation of MFI and BEA nanoparticles using
seeding method has been reported by several groups.48,58–60 In
all cases, zeolite seeds (0.1–10 wt%) were used as nuclei for
further crystal growth. For example, Al-rich zeolite BEA with Si/
Al ratio as low as 3.9 is synthesized starting from the precursor
suspension without organic SDA, but in the presence of zeolite
seeds. In some cases, the zeolite seeds were partially dissolved
and the amorphous entities nucleate and grow with the help of
these preformed seeds. The product yields are above 80% and
the resulting nanocrystals do not require calcination to open
the zeolite porosity, and, thus, the aggregation between zeolite
particles is avoided during the postsynthesis treatment. Be-
sides, the Mobil Twelve (MTW)61 and Mordenite (MOR)62
nanocrystals have also been made at moderate temperature
(160–180 �C) using the seed-induced approach.
In some cases, the zeolite nanocrystals synthesized by seed-
ing of an organic-template-free initial gel are aggregated in
comparison to the particles synthesized from clear suspension
rich in organic templates. The high aggregation level is most
probably due to the nanoseeds, which are polycrystalline ag-
gregates built of much smaller crystallites. Secondary growth of
closely situated nanocrystallites leads to the formation of com-
plex aggregates larger in size in respect to the seeding particles
(Figure 5). Consequently, a part of the product does not be-
have as colloidal matter, although the size of the individual
crystals is in the nanometric range.
Besides, functionalization of the zeolite seeds to obtain
zeolite nanocrystals with enhanced surface area and porosity
has been developed.63,64 This method is based on perturbing
Zeolite Nanoparticles 291
the growth of the zeolite crystals by functionalization with the
hydrophobic organosilane group in order to hinder and pre-
vent their further agglomeration, according to the following
steps: (1) formation of the zeolite nuclei in synthesis gel during
the precrystallization step, (2) functionalization of the zeolite
seeds by reaction with organosilanes, which form a protective
organic barrier against aggregation, and (3) crystallization to
complete the zeolitization of the functionalized seeds. In ad-
dition, the organosilane can also influence the stability of tetra-
hedrally coordinated Al species during the nucleation process,
and thereby impact the crystalline process of nanozeolites.65 The
nanoproduct obtained by this method consists of ultra-small
zeolite nanocrystals (<50 nm) having an additional porosity in
the meso-/macropore regions generated by the presence of the
silanization agents (Figure 6). This synthesis approach also
opens new ways for the development of environmentally
friendly synthesis procedure for preparation of Al-rich zeolite
Beta and ZSM-5 without organic template.
5.10.2.1.4 Multistep synthesis approachMultistep templated synthesis is another method for prepara-
tion of nanosized zeolites with high crystalline yield.66–68 The
basis of this concept is the reuse of nonreacted chemicals
separated from the crystalline suspensions and subjected to
further crystallization with or without addition of chemicals.
The reuse of clear precursor suspensions for the multistep
synthesis approach is rational since (1) a low amount of che-
micals are consumed and (2) only zeolite nanocrystals are
recovered by centrifugation, so that very small zeolite crystals
(10 nm or less) are still present in the clear suspension. This
approach is applied for the preparation of colloidal NaY and
silicalite-1 nanocrystals.66,67 The cumulative product yield is
increased from 4% to 44% after several synthesis cycles. Fi-
nally, the crystallization process is terminated since the
TEAOH
(a)
Multi
H2O
AI(OiPr)3
H3PO4
Multivs. singlesynthesis
25 g
Single
12
21
1 3
13
Experimental
Figure 7 Total consumption of chemical reagents for the synthesis of nano(a) laboratory (‘experimental’) and (b) industry (‘calculated’) scales production1043–1048.
Amorphousnanoparticles
Functionalizedwith organosilanes a
Figure 6 Schematic diagram of the synthesis of zeolite nanocrystals with enFrom Serrano, D. P.; Aguado, J.; Escola, J. M.; Rodriguez, J. M.; Peral, A. Ch
precursor suspension is discarded after several cycles when
the silica and alumina nutrients are insufficient in the precur-
sor suspensions for further syntheses.
It is very important to note that the zeolite nanocrystals
prepared from several synthesis cycles have the same proper-
ties, that is, chemical composition, colloidal stability, particle
size, morphology, and porosity.
This approach is also considered environmentally benign
since the nonreacted chemicals are reused and thus the ap-
proach is considered beneficial for both the environment and
the scaling-up through reducing the production cost and dis-
posal of chemical waste. Besides, the conventional chemical
process for preparation of nanosized microporous molecular
sieves, which is based on the use of large amounts of toxic
reactants (amine/ammonium salt and phosphoric acid) and
volatile solvents (ethanol and methanol), is optimized by
applying the multistep synthesis approach.
The multistep synthesis approach is also applied for prepa-
ration of nanosized aluminophosphates.68 Unlike the alumino-
silicate system, the multistep synthesis can proceed many times
with a minimal chemical compensation of the reacting mixture
after recovering the crystalline nanoparticles from each step.
Thus, almost complete consumption of the organic templates
and inorganic species without disposing harmful reagents to the
environments, and making possible the scaling-up process of
nanocrystalline zeotype materials, is reported (Figure 7).
5.10.2.1.5 Ionothermal synthesisIonic liquids (ILs) as green solvents have shown great promise
as an attractive alternative or replacement to conventional
volatile organic solvents, attributed to the following distinct
features, that is, negligible vapor pressure, better thermal sta-
bility, tunable hydrophilibility/hydrophobicity, ease of recir-
culation, and manipulation.69
TEAOH(b)
Estimated
Multi
H2O
H3PO4
AI(OiPr)3
Multivs. singlesynthesis
1 ton
Single
14
31
1 13
15
crystalline AlPO-18 by single- and multistep synthesis approaches in. From Fig. 5 in Ng, E.-P.; Delmotte, L.; Mintova, S. Green Chem. 2008, 10,
Crystallization ofmorphous particles
Zeolitenanocrystals
hanced textural properties synthesized from organo-functionalized seeds.em. Mater. 2006, 18, 2462.
Nucleation of MnAIPO-550 h
Crystal growth atintermediate surface
70 h
Homogenization5 min
Dissolution5 h
Formation of intermediates25 h
Additional crystal growth from surface to core
80 h
Liberation ofdiscrete
nanocrystals90 h
ILIL
IL
IL
IL
ILILIL
ILAIO4
–
PO4+
PO4+
PO4+
AIO4–
AIO4–
AIO4–
MnO2
Mn2O3
IL
IL
IL
ILIL
IL
IL
ILIL
IL
ILIL
ILIL
IL
IL
IL
H
H
H
H
H
H
H
H HH
HH
HH HH
H
H
H
HH
HH
H
H
HH
H
C
C
C
C
C C
C
N
N
C
C
C C
C
C C
N
N
H
CC
C C C
C
C
N
N
H
H
O
O
TO
O
HHH
H
N
N
H
H
H
HH
H
H
HH
C C
C
C
CC
C
HH
HH
HH
HH
IL
IL
IL
IL
IL
IL
IL
ILILIL
IL
IL
ILIL
IL
IL
IL
IL
IL
ILIL
IL
IL
ILIL
IL
IL
Mixing Induction
Crystallization
IL
T = AI, P, Mn
Figure 8 Schematic illustration of the crystallization pathway of MnAlPO-5 nanocrystals under ionothermal conditions. From Scheme 1 in Ng, E.-P.;Itani, L.; Sekhon, S. S.; Mintova, S. Chem. Eur. J. 2010, 16, 12890–12897.
(b)(a)
(c) (d)
Figure 9 Zeolite A crystals synthesized under (a) conventional and (b)microwave heating, and AlPO-18 nanoparticles prepared using (c)conventional and (d) microwave treatments. Scale bar: 1 mm.
292 Zeolite Nanoparticles
Recently, the ILs are applied for the preparation of zeotype
nanocrystals and inorganic–organic hybrids materials.70–72
The so-called ionothermal technique uses an IL or eutectic
mixture as reaction solvent and as SDA. The most important
features of this strategy are the use of green IL instead of
harmful quaternary ammonium hydroxides, and the synthesis
can be performed in an open vessel instead of using autoclave
due to the low vapor pressure of ILs. Since then, many studies
have been reported on ionothermal synthesis of metal oxide
nanoparticles.73–77 The synthesis of nanocrystalline zeolite
materials in ILs, however, is still a new field, which has
emerged over the last several years. A novel hexagonal AlPO
molecular sieve with a crystal thickness of 170 nm was first
prepared in the presence of HF.78 Later, discrete nanosized
MnAlPO-5 crystals using 1-ethyl-2,3-dimethylimidazolium
bromide ([edmim]Br) as IL media were synthesized.79,80 Com-
pared with conventional organic templates, the ILs are more
environmentally benign and recyclable.81 An example of the
crystallization pathway of MnAlPO nanocrystals is presented
schematically in Figure 8. This approach can contribute to
green and sustainable chemical synthesis and production of
zeolite nanocrystals by improving product yield and so pro-
ducing less waste, and avoiding environment pollution.
5.10.2.2 Types of Heating
5.10.2.2.1 Microwave-assisted synthesisAs compared to the conventional techniques, the microwave-
assisted hydrothermal method provides an efficient way for the
synthesis of various zeolite nanocrystals and rational control
on their particle-size distribution, yield, phase purity, and
morphology. In the conventional heating reactors (auto-
claves), the time needed for heating the zeolite synthesis sus-
pensions to a specified temperature is long due to the slow
heating rate. During this heating period, nucleation and
crystallization processes coexist, resulting very often in a
broad particle-size distribution of products.82,83 By contrast,
the microwave heating causes internal heating of the precursor
suspension and very often a significant change of the kinetics
and selectivity of ongoing reactions is observed. Mainly, lower
temperatures and shorter time for crystallization of the nano-
sized zeolites are observed in the systems subjected to micro-
wave heating compared to those under conventional heating.
The energy transfer to the precursors is achieved through the
interaction of the microwaves with water or other compounds
with high dielectric constant or large dipole moments.83
Various zeolite nanoparticles such as AEI,34 AFI,84 CHA,85
LTA,52,86 MFI,86 EMT,31 and BEA86 have been prepared by
microwave-assisted hydrothermal treatment (Figure 9).
Zeolite Nanoparticles 293
Among them, a rapid synthesis of silicalite-1, BEA, ZSM-5, LTL,
and LTA nanocrystals by using microwave heating is
achieved.86 A comparison of conventional and microwave hy-
drothermally synthesized zeolite A nanocrystals indicates that
the conventional crystals are larger and highly nonuniform in
shape with secondary crystals apparently growing from pri-
mary crystals. By contrast, the particle-size distribution of
microwave-synthesized zeolites is visually smaller and more
uniform than the conventionally heated zeolites.
Synthesis of zeotype nanocrystals (AlPO-18, SAPO-34)
with narrow particle-size distribution under microwave radia-
tion has been reported.34,85 The chemical compositions of the
starting sols and the microwave hydrothermal synthesis con-
ditions exhibit synergic effect and lead to the formation of
nanosized crystals. The microwave synthesis is also reported
to produce zeolite nanocrystals with unique morphology.34 It
was also shown that the AlPO-18 nanoparticles are growing as
thin elongated hexagons, which are different from the conven-
tional square plate-shaped AlPO-18 crystals.
The effective synthesis of ultra-small (6–15 nm) crystals of
the large-pore zeolite EMT-type zeolite with very high yield
from template-free colloidal precursors at low temperature
(30 �C) for 4 min under microwave irradiation and for 36 h
under conventional heating is reported (Figure 10).31 From an
environmental perspective, the synthesis of EMT zeolite is
extremely attractive as the nanocrystals can be easily synthe-
sized at very high yield at near-ambient temperature without
using any organic templates. This is suggesting that scale-up of
an energy-efficient synthesis would be easily feasible. These
nanoscale EMT materials offer exciting opportunities for both
fundamental study and potential industrial applications.
5.10.2.2.2 Microchannel-assisted synthesisSimilarly, fast heating microchannel reactors are used to syn-
thesize zeolite nanocrystals with a small mean particle size and
(b)(a)
(c)
10 nm 500 nm
500 nm20 nm
(d)
Figure 10 HRTEM pictures of EMT nanocrystals synthesized in (a, b)conventional oven and (c, d) microwave oven. From Fig. 4 in Ng, E.-P.;Chateigner, D.; Bein, T.; Valtchev, V.; Mintova, S. Science 2012, 335, 70.
narrow particle-size distribution. Unlike normal conventional
heating, the microreactor with continuous operation has fast
heat and mass-transfer features, which aid in the synthesis of
uniform nanoparticles.87 It was shown that the crystallization
rate of the zeolite nanocrystals conducted in the segmented
flow microreactor is comparable to that in the microwave-
heated reactor.88–91
A schematic diagram of the microfluidic system for the
synthesis of zeolite nanocrystals in microchannel reactors is
shown in Figure 11. The application of microchannel reactors
for production of zeolite A and silicalite-1 nanocrystals is
reported.88–91 These reactors are suitable for continuous syn-
thesis of nanosized zeolites and show great advantages for
controlling both the reaction conditions and the crystal prop-
erties. However, one of the main drawbacks is that the micro-
channel reactor always suffers from channel blockage due to
the viscous aluminosilicate precursor suspensions commonly
used for zeolite synthesis.
5.10.2.3 Confined Space Synthesis
The confined space synthesis method is applied for prepara-
tion of highly dispersible zeolite nanocrystals.92 This method
involves the crystallization of the zeolite inside a matrix with
controlled porosity. Thus, the crystals cannot grow larger than
the pores of the used matrix.
Confined space synthesis has several advantages in compar-
ison with other methods. First, it is possible to predict the
maximal crystal size of the resulting zeolites by choosing an
appropriate porous matrix. Second, the crystallization of zeo-
lite nanocrystals in confined space proceeds until complete
consumption of the precursor suspension. Third, the zeolite
crystals can be isolated simply by calcinations at a sufficiently
high temperature to ensure a complete combustion of the inert
matrix. Moreover, the recovery of the zeolite is simple since
no filtration (high-speed centrifugation) for the nanocrystals
is involved which is a needed step in the syntheses of small
crystals. Finally, this method is highly reproducible and can
be used to prepare a wide range of zeolites and possibly
other materials.
To date, a variety of space confiners such as carbonblacks,92–95
carbon nanotubes,96,97 starch,98 gelling polymer,99,100 and poly-
mer spheres101 have been utilized to confine the crystallization
of zeolite nanocrystals. A summary of the confiner, zeolite
nanocrystals synthesized within this matrix with sizes in the
range of 10–300 nm is displayed in Table 3.
Liquidparaffin
Nanozeolites
Nanodroplets
Liquidparaffin
Synthesissolution
Heating bath
Figure 11 Synthesis of zeolite nanocrystals in microchannel fluidicsystem.
Table 3 Zeolite nanoparticles synthesized in confined space
Confiner Zeolite nanoparticles Size (nm)
Carbon black Na-ZSM-5, Silicalite-1, Beta, NaY, NaA, Sodalite, LTL 13–30, 55–63, 10–15, 100, 230–240, 37–50, 30Carbon nanotubes NaZSM-5, NaY 20–30Starch NaY 50–100Polymer hydrogels NaA, NaX 20–180, 10–100Polymer spheres ZSM-5, Beta, TS-1, MORa, Aa, Xa, Ya, La 300, 90, 220
aCrystallite size not reported.
Colloidal zeolite suspension
Heating
Cooling down
Zeolite nanocrystals ingelling polymer solution
Zeolite nucleationand growth
Hydrothermaltreatment
Precursor gel trappedinside hydrogel
Precursor gel withgelling polymer
Purificationand re-dispersion
Figure 12 Schematic representation of template-free zeolite synthesis using thermoreversible polymer hydrogels.
294 Zeolite Nanoparticles
A slightly modified approach involves the application of
thermoreversible polymer hydrogel as confined space for limita-
tion of crystal growth of low-silica zeolites (Figure 12).99 In
particular, the polymer hydrogels are of interest as they are con-
sidered a soft space confinement additive. The three-dimensional
pores of the polymer hydrogels can be adjusted and can serve as
microreactors or nanoreactors for controlling the zeolite growth.
Unlike natural starch polymer and other carbon confine spacers,
the thermoreversible gelling polymers can be readily removed
after the synthesis by simplewashing and the zeolite nanocrystals
obtained are readily redispersed in various solvents.
Besides, polymer spheres have been used in the preparation
of zeolite nanoparticles. These confined voids can be simply
adjusted by the solid content and diameter of the polymer
spheres. The adjustable confined voids formed by polymer
spheres potentially serve as micro- or nanoreactors for control
growth of zeolite nanocrystals. So far, ZSM-5, A, X, Y, L, MOR,
Beta, and TS-1 nanocrystals have been synthesized using this
approach.101
5.10.2.3.1 Reverse microemulsion synthesisThe reverse microemulsion or water-in-oil microemulsion syn-
thesis approach consists of aqueous domains (termed reverse
micelles) dispersed in a continuous oil phase solvent, so that a
variety of reactants can be introduced into the nanometer-sized
aqueous domains (Figure 13). By the reaction confined within
the reverse micelles, nanostructure materials with controllable
particle size, shape, and higher colloidal stability are obtained.
Recently, reverse microemulsions have been extensively
applied in the synthesis of nanocrystalline solids.102–107
Various parameters including initial synthesis composition,
microemulsion composition, surfactant identity, and electro-
lyte that affect the morphology and size of silicalite-1 crystals
are investigated.108 It was observed that the formation of zeolite
nanocrystals is mainly dependent on the type of surfactants.
Microemulsions formed with a straight-chained surfactant
(cetyltrimethylammonium bromide (CTAB)) lead to amor-
phous silica, whereas microemulsions formed with a branched
surfactant lead to the formation of silicalite-1 nanocrystals.
The morphology of silicalite-1 nanocrystals is well defined.
These results also indicate that the silicalite-1 nanoparticles do
not nucleate in the microemulsion, but rather nucleate and
grow heterogeneously after amorphous silica particles in the
microemulsion phase are formed.
Zeolite nanoparticles with the MFI-type structure using
a nonionic emulsion system are synthesized.108 The results
Mixing andheating
Zeolite precursor
Oil phase
Emulsion system
Crystallization
Purification
Magnify
Aging
Hydrophilichead
Zeolitenanocrystal
Zeolitenanocrystals
Hydrophobictail
Figure 13 Schematic illustration of zeolite formation process in the microemulsion system.
Zeolite Nanoparticles 295
reveal that the emulsion system allows rapid crystallization of
ZSM-5 in comparison with the conventional hydrothermal
synthesis. Zeolite A nanocrystals with narrow particle-size
distribution (100–120 nm) and sphere-shaped morphology
are prepared in the presence of a cationic microemulsion.109
The microemulsion system has been proven to be able to
accelerate the crystallization process of the LTA nanoparticles.
However, an increase in the concentration of the surfactant
and co-surfactant results in a decrease in the crystallinity of
the LTA-type zeolite.
In spite of the desirable characteristics of zeolite nanoparti-
cles, reverse microemulsions have the drawback of relatively
low crystalline yield.110 In addition, this method uses excess of
solvents to stabilize the microemulsion system and compli-
cates the recycling of the solvents. The high consumption of
solvents not only causes detrimental effects to human health
but also requires significant waste disposal. This is a very
important issue when scale-up of this approach is considered.
5.10.2.4 Other Methods
5.10.2.4.1 Direct-conversion synthesis approachNatural rocks such as bentonite, clinoptilolite, stilbite, erionite,
and kaolinite are minerals applied as precursors for preparation
of zeolite nanocrystals. Prior to the synthesis, the minerals are
treated with acid to remove impurities before dissolving the
aluminosilicate residue in concentrated alkaline solutions. The
conversion of natural bentonite to zeolite Y nanocrystals has
been reported.111 Also nanosized zeolite A is successfully
obtained from natural clinoptilolite via direct conversion.112
The direct conversion demonstrated to be simple, and the pro-
duction cost is low in comparison to the conventional templat-
ing approach. Thismethod is also used for preparing zeolite BEA
nanocrystals starting from FAU zeolite under hydrothermal
treatment with and without SDA.113 The crystallization rate of
BEA zeolite is significantly enhanced and also demonstrated the
possibility of synthesizing other template-free zeolite nanocrys-
tals. Nevertheless, the zeolite nanocrystals derived from natural
minerals rocks have more impurities and therefore they are not
useful for some applications including semiconductor industry,
medical, and pharmaceutical applications.
5.10.2.4.2 Centrifugation-assisted grindingThe centrifugation-assisted grinding method is considered ef-
fortless and convenient, and permits the preparation of zeolites
with tunable morphology and fairly narrow size distribution.
This method is applied in the production of MFI and FAU
nanocrystalline solids without organic templates.114 In a typi-
cal procedure, a known amount of micrometer-sized zeolites
dispersed in water are mechanically treated to smaller crystals.
The nanocrystals with various sizes are separated using centri-
fugation, and a very high crystalline yield (above 85%) is
achieved. However, the nanocrystals might face low mechani-
cal stability and some of the zeolite nanocrystals are destroyed
during the grinding treatment. This approach is improved by
combining a bead mill grinding and recrystallization, and
demonstrated for the preparation of nanozeolite A free of
organic templates.115 Conventional grinding methods such as
ball milling and planetary ball milling downsize the zeolites
but the destruction of the outer zeolite framework may cause
pore blocking, and this impedes the desirable properties of the
zeolite.116 Therefore, a milder grinding method is required to
prevent the degradation of crystallinity, for example, amorphi-
zation and/or formation of dislocations, by the use of small
beads (30–500 mm in diameter). The damaged part as a result
of bead milling treatment, however, can be subjected to recrys-
tallization and repaired using a dilute aluminosilicate solution
(Figure 14). Under these conditions, the poorly crystalline parts
of the milled zeolite particles are more easily dissolved than the
crystalline parts, and tend to be recrystallized into zeolites.
5.10.2.4.3 Laser-induced fragmentation methodLaser-induced fragmentation of micron-sized into nanosized
particles has been developed to reduce the dimensions of
colloidal metal particles.117–119 Recently, this approach has
been applied in the preparation of LTA nanocrystals.120 The
laser-induced fracture method is valuable because it can pro-
duce nanoparticles directly and rapidly from many commer-
cially available zeolites. In addition, no organic SDAs are
involved in the synthesis, thus avoiding the calcination proce-
dure. However, the appropriate wavelength and laser energy
density have to be used in order to reduce the possible damage
to the zeolite crystalline structure during the fragmentation.
Milling/grinding
Micronsized LTA
Crystallinephase
Amorphousphase Highly crystalline
nanosized LTA
Recrystallization
50–200 nm3–4 μm
50–200 nm
Figure 14 Schematic illustration describing the fabrication process of zeolite nanocrystals by bead milling and postmilling recrystallization.
High
−30 +300Zeta potential, mV
Rat
e of
floc
cula
tion
Figure 15 Variation of zeta potential values of zeolite suspensions.
296 Zeolite Nanoparticles
5.10.2.5 Separation of Zeolite Nanocrystals
Special attention is paid to the stability of zeolite nanocrystals
during purification process. The colloidal suspensions contain-
ing nanocrystalline zeolites are purified by (1) high-speed
centrifugation, (2) membrane filtration, or (3) coagulation.
Typically, after the hydrothermal treatment of the precursor
suspension, the crystalline product is subjected to a series of
high-speed centrifugation steps (for instance, 1 h at 47.800 g
relative centrifugal force (RCF) or 25000 rpm) and subse-
quently redispersed in water under sonication (1–2 h in ice
bath) until the pH of the crystalline purified suspension reaches
about 7.5–9. Alternatively, colloidal suspensions containing ze-
olite nanoparticles are purified by dialysis using a dialysis tubing
device with different sizes. The advantage of this method is that
no additional agglomeration of the particles is provoked as it is
observed often during the centrifugation treatment. In addition,
the purified and stabilized aqueous suspensions of zeolite nano-
particles keep their original solid concentrations. At pH of the
colloidal suspensions in the range of 9–10, the concentration of
solid particles can be varied from 1 to 10 wt%. However, during
the prolonged dialysis, the unreacted organic template present
in the colloidal suspension can be removed almost completely
and then the zeolite nanoparticles will become unstable.
Therefore, postsynthesis treatment has to be considered leading
to disintegration of the particles and keeping as discrete in the
colloidal suspension.
Coagulation is the third process used to precipitate out the
zeolite nanoparticles of colloidal suspensions. The coagulation
of the zeolite nanoparticles can be carried out by mixing two
oppositely charged compounds. Both compounds may be par-
tially or completely precipitated. This approach also can be
accomplished with boiling of the zeolite suspensions. This
reduces the charge on the particles and ultimately they settle
down to form a precipitate where the particles stay discrete and
do not agglomerate irreversibly.
Different electrolytes are used for coagulation of zeolite
nanocrystals; these electrolytes have different coagulation
values, and the smaller the coagulation value of the electrolyte,
the larger is its coagulating power. The coagulation of zeolite
nanoparticles is governed by two factors: (1) ions carrying
charge opposite to that of the zeolite particles are effective in
bringing about coagulation and (2) coagulation power of an
electrolyte is directly proportional to the valency of its ions.
The zeolite nanoparticles can be coagulated by mutual
precipitation, repeated dialysis, and heating. This is performed
by adding the electrolyte and by choosing a suitable solvent.
For instance, the as-prepared zeolite nanoparticles (BEA and
MFI) can be treated with an aqueous solution of diluted am-
monium chloride, and subsequently washed with ethanol to
prevent further irreversible agglomeration.121
The stability of the zeolite nanocrystals in different solvents
is of significant importance for their applications. The stability
is usually governed by intermolecular interactions such as
electrostatic, van der Waals, and London forces, between the
particles and the solvents.122 The stability of colloidal suspen-
sions is determined by measuring the zeta potential values at
constant pH and constant solid concentration (Figure 15).
This refers to the electrostatic potential generated by the accu-
mulation of ions at the surface of a colloidal particle that is
organized into an electrical double layer, consisting of a stern
layer and a diffuse layer. Flocculates and/or aggregates are
formed if the coulomb interactions are lower than the van
der Waals forces between the particles, and finally the sedi-
mentation of particles also causes a substantial increase in the
zeta potential value.
The stability of zeolite nanoparticles is described by their
resistance to coalescence and aggregation. Two or more small
particles fusing together and form a single large particle that is
described as coalescence. The essential feature of the coales-
cence is the fact that the total surface area is reduced. Aggrega-
tion is a process by which small particles clump together
(aggregates) but do not fuse into new entities. There is no
Esterificationwith alcohols
Aggregation of zeolite nanoparticlesdue to hydrophilic interactions
Stabilization of zeolite nanoparticlesdue of hydrophobic surfaces
OH
OH
OHO
H OH
OH
OH
OH
OH
OH
OH
OH OH
OHOH OH
OH
OH
OH OH
OH
OH
OH
OH
OH
OH
OH
OH
OR
OR
OR
RO
RO
RO
OROR
HO
HO
HO
HO
RO
OROR
RO
RO
RO
OROR
OR
OR
OROR
OR
RO
RORO
RO
RORO
RORO ORO
R OR
Figure 16 Effect of chemical posttreatment on the stability of zeolite nanocrystals.
Zeolite Nanoparticles 297
reduction of the surface area, although certain surface sites may
be blocked at the points at which the smaller particles touch.
The term coagulation is also used to describe the process of
aggregation; colloids stable against coagulation and coales-
cence are considered kinetically stable. The stability of the
colloids is described by the extent of which small particles
remain uniformly distributed through the sample. The stability
of zeolite nanoparticles depends on their size, solid concentra-
tion, surface charge, and type of solvent.
The effect of surface chemical treatment on the degree of
aggregation of colloidal particles is important. An esterification
step of the surface of zeolite nanoparticles can be performed
after purification in alcoholic media, which has resulted in the
formation of ‘estersil’ and hydrophobization of the particles
according to the following scheme:
SiOH +Zeolite surface
ROH ÆAlcohol
SiOR +Zeolite coveredby estersil
H2O
This procedure strongly disaggregates and stabilizes the par-
ticles in alcohols such as ethanol, iso-propanol, and n-butanol at
certain pH of the suspensions (Figure 16). The posttreated
particles are between 10 and 150 nm, while the nontreated
particles are between 200 and 500 nm. A redispersion of the
posttreated particles in toluene or benzyl alcohol leads to a
completely clear solution, indicating the presence of very small
particles. This indicates that the aggregation level depends not
only on the effect of silanol groups at the surface of particles, but
also on the type of solvents. The disaggregation–reaggregation
process of the esterified particles is a reversible process and
depends on the surface chemistry of the crystals.28
5.10.3 Applications of Zeolite Nanoparticles
The possible green mass production of zeolite nanocrystals
provides excellent opportunities for applications in catalysis,
adsorption and separations involving larger molecules, and for
designing thin films, membranes, and nanoscale devices.
5.10.3.1 Zeolite Membranes
Over the last two decades, the development of zeolite-based
membranes has attracted considerable research efforts. The
supported membrane based on zeolite nanocrystals was first
reported in the 1990s. Since then, many types of zeolite
membranes (LTA, MFI, LTL, SOD, BEA, AFI, and MWW) de-
rived from zeolite nanoparticles have been prepared.19,123–130
Zeolite membranes are used for gas–vapor and liquid–liquid
separation. Both groups are important from industrial and
environmental points of view. For instance, the CO2 separation
from different gas mixtures is among the most serious environ-
mental problems nowadays. Consequently, numerous studies
have been performed to extract hydrogen from H2/N2, H2/O2,
H2/CH4, and multicomponent gas mixtures. Liquid–liquid
separation on zeolite membranes is a promising alternative to
such an energy-intensive process as the distillation. Different
alcohol–water (methanol, ethanol, propanol, butanol, etc.) mix-
tures have been subjected to separation by zeolite membranes.
Additionally, sodalite–polyimide membranes with good hy-
drogen permeability and selectivity over nitrogen at the same
testing temperature have been prepared.128 In addition, crystals
with controlled orientation have been shown to exhibit new
properties, since their porous framework is capable of orienting
and restricting rotation of the adsorbed guest molecules.126 A
significant improvement in the separation of p-/o-xylenemixture
in c-oriented MFI membrane compared with the conventionally
calcined membranes has been reported.130 Also, the oriented
zeolite membrane prepared via secondary growth gave excellent
cumulative O2 permeance.131 A number of comprehensive
reviews devoted to the preparation and characterization of
zeolite-based membranes have been published.123,132–137
The great expectations related to zeolites as separation
media are based on their selectivity, long-term stability at
high temperature, resistance to harsh environments, resistance
to high-pressure drop, inertness of microbiological degrada-
tion, and easy cleanability and catalytic activation.
The preparation of zeolite membranes, in particular on a
large scale, remains a challenge. The major disadvantages of
zeolite membranes are poor processability and mechanical
stability, and very often poor fluxes and technical difficulties
related to sealing at high temperature. Substantially higher
costs of zeolite membranes are also an obstacle to their large
industrial utilization. Obviously, the processing methods will
have to be further developed in order to ensure an industrial
application of supported zeolite membranes.
5.10.3.2 Optical and Other Devices
There has been a considerable amount of work toward the
development of other applications for zeolite nanocrystals
298 Zeolite Nanoparticles
such as optical devices, films with low dielectric constant, and
gas/liquid sensors. A comprehensive knowledge for prepara-
tion of zeolite crystals with desired shape, size, morphology,
and crystalline structure and their subsequent assembly in
thin-to-thick films is of great help in the design of these novel
applications. Moreover, the mechanical and thermal stability
and physicochemical properties of the zeolites assembled in
the films are of significant importance.27
Many fabrication methods are used in the production of
zeolite-based chemical sensors. Factors that must be considered
when selecting the production technique include their expense,
purity, porosity, reliability, and reproducibility. Common tech-
niques for preparation of sensors include (1) screen printing,
(2) sol–gel techniques, (3) dip and spin coatings, and (4) direct
growth with and without preseeding of the substrates. Indeed,
the completely controlled zeolite structures, fine-tuned chemical
composition, and ion-exchange capacity make them attractive
as true shape-selective compounds.138 Considerable progress
has already been reached in this field by assembling zeolites in
thin films either by controlled attachment on self-assembled
monolayers followed by growth or by different patterning
techniques.
The zeolite-based chemical sensors are divided into two
groups depending on the respective role of the molecular
sieves. The zeolite can act as a main functional element,
which is the case for sensor principles relying directly on con-
ductive, adsorptive, or catalytic properties of one specific type
molecular sieve and its interaction with the analytes.
The second group includes devices where the zeolites are sup-
plementary or secondary elements.
High sensitivity, good reversibility, and long life of zeolite-
based sensors were demonstrated for detection of hydrocar-
bons and water at low concentrations. Such materials are of
interest in biochemical and sensing systems.19,84,139 Besides,
optical sensors based on acrylamide photopolymer doped with
zeolite nanoparticles (BEA, LTA, and AEI) have also been de-
veloped.140,141 These holographic sensors have shown a higher
dynamic range, a higher ultimate refractive index modulation,
and a lower level of shrinkage.142 The ultimate modulation of
the optical refractive index (n1) is caused by polymerization of
the monomer and its conversion into polymer, density
50 100 1500
1
2
3
4
Ref
ract
ive
ind
ex m
odul
atio
n �
103
Time, s(a) (b
1.2 wt. % 40 nm1.2 wt. % 60 nm0 wt. %
Laser offIntensity 5 mW cm–2
Figure 17 (a) Refractive index modulation of films with thickness of 40 mm(b) Angular selectivity curves for nondoped (gray) and doped (black) photopo
variation due to concentration-driven monomer diffusion
from dark to bright fringe areas, density variation due to
concentration-driven short/mobile polymer chain diffusion
from bright to dark fringe areas, and spatial patterning of the
zeolite nanosized particles. The modulation of the optical
refractive index is observable only if the refractive index of
the nanoparticles is significantly different from that of the
photopolymer matrix. The results shown in Figure 17 reveal
that an increase in the refractive index modulation is observed
in the nanocomposite layers doped with MFI nanoparticles. In
addition to the improved dynamic range, a lower level of
shrinkage in the zeolite-doped photopolymer was observed
(Figure 17). This is of vital importance for the zeolite-doped
photopolymer layers in applications such as holographic data
storage. The level of shrinkage in the photosensitive nanocom-
posites is studied by recording holographic transmission
slanted gratings in samples containing different concentrations
of zeolite nanoparticles. The shift in the position of the angular
selectivity curve due to shrinkage in nondoped and doped
photopolymers is shown in Figure 17. A substantial decrease
from 1.07% to 0.1% after adding the zeolite nanoparticles to
the photopolymer is demonstrated. Moreover, dependence of
the level of shrinkage on the film thickness of the layers was
observed, that is, the percentage shrinkage increases as the
sample thickness decreases. In all cases, the shrinkage of the
layers containing zeolite nanoparticles was significantly lower
than in the pure photopolymer samples.
Many organic and inorganic polymer materials have been
considered potential candidates for low-k dielectrics. The or-
ganic polymers (e.g., highly fluorinated alkane derivatives), for
example, could have k below 2.2, but they suffer from low
thermal stability and thermal conductivity.143 For porous
inorganic-based materials (e.g., sol–gel silica), they offer a tun-
able k-value, but its low mechanical strength, wide pore-size
distribution, and hydrophilicity have been cited as concerns.143
The pure siliceous zeolite films, however, possess many distin-
guished features, such as high mechanical strength, good heat
conductivity, and small uniform pore size (<3 nm), which
make them a very promising candidate as low-k dielectric for
future-generation microprocessors. Thin films with a k-value in
the range of 1.8–2.1 are prepared by spin coating or secondary
–1.0 –0.5 0.0 0.5 1.00.0
0.2
0.4
0.6
0.8
1.0DopedNondoped
Sig
nal (
a.u.
)
Angle (degrees)
0.1% 1.07 %
)
doped with 1.2 wt% of zeolite nanoparticles with a size of 40 and 60 nm;lymer with nanocrystals with a size of 60 nm.
Zeolite Nanoparticles 299
growth of pure-silica MFI and MEL nanocrystals.144,145 Porogen
(such as g-cyclodextrin) is incorporated in the amorphous silica
phase between nanocrystals to reduce k-value to the ultra-low-k
range (<2.0).146 Additionally, the silica zeolite films can be
silylated to increase the hydrophobicity.
The preparation of antireflection coating on solar glass
support using zeolite nanocrystals has been reported.147,148
The antireflection coatings are very thin and the existence of
40–70-nm nanoparticles in the coating is not a serious prob-
lem as long as it does not affect the transparency. A very strong
film for solar glasses is prepared using crystalline microporous
aluminosilicate nanoparticles.
Attempts to build an artificial antenna by enclosing dyes
inside a microporous material have been demonstrated.149–151
Nanosized LTL crystals are used as a host for different lumines-
cent dyes organized in the one-dimensional channels of
the zeolite.
The reduction in sensitivity of high energetic materials has
been an essential subject of interest since the discovery of
energetic materials and it continues to be important nowadays.
Desensitizing these compounds is of great importance not only
to enhance their safe use, but also to make them less dangerous
in fields such as production of standards for analytical
purposes and the detection of explosive device. In this regard,
MFI-type nanosized zeolite with three-dimensional channel
network is used as a host for stabilizing a high-density energetic
material (Fox-7).152,153 The immobilized energetic material in
the zeolite nanocrystals showed high stability (�100 �C above
the explosive temperature), which opens up new possibilities
for preparation of safe standards and safe manipulations.
Novel nanostructured functional materials for applications
in the area of ultraviolet (UV) filtering, sensing, and molecular
switching by encapsulation of photophysical and photochem-
ical active guests in the zeolite nanocrystals have been pre-
pared.154–156 The nanosized zeolites stabilized in suspensions
and thin films are promising for the development of host/guest
ultrafast switching systems since on a picosecond (10–12) time-
scale a functional guest molecule may behave in a zeolite host
like in free solution.
In addition to the incorporation of organic guest molecules,
the stabilization of metal (Me¼Cu, Pd, Pt, etc.) and semicon-
ducting (SC¼CdS, ZnS, PbS, etc.) clusters in nanosized zeo-
lites has attracted considerable attention.157,158 On the one
hand, the size and shape of the Me and SC clusters are deter-
mined by the dimensions of the pores of the zeolites, and, on
the other hand, once they are hosted they do not agglomerate
and keep their unique properties. The metal and semiconduc-
tor clusters stabilized in zeolites are important for the develop-
ment of new materials with remarkable catalytic, optical, or
electronic properties.159 In addition, the Me clusters stabilized
in zeolites are proven to be good catalysts for disproportion-
ation of ethylbenzene,160 photochemical/thermal cleavage of
water to H2 and O2,161 photo-oxygen production from
water,162 photodimerization of alkanes,163 and selective reduc-
tion of NO by ethylene164 and ethanol.165
5.10.3.3 Biological and Medical Applications
During the last few decades, zeolite nanocrystals have been
considered for medical use due to their properties and stability
in biological environment.166 The vast diversity of the zeolite
nanoparticles makes them very interesting as selective sorbent,
antioxidant, UV, and antimicrobial and antiviral protection.
Based on their unique properties, they are applied in processes
including (1) absorption of unpleasant odors, (2) skin cleaning,
(3) protection from harmful emission and microorganisms, (4)
protection from rheumatic problems, (5) favoring of healing
process in skin wounds, and (6) regulating of the pH of skin.
Zeolite nanocrystals have been investigated as drug carriers,
positive magnetic resonance imaging (MRI) agents, enzyme-
immobilizing carriers, etc. Several diagnostic pharmaceuticals
for MRI based on zeolite nanocrystals have been investigated.
Zeolite nanoparticles modified with gadolinium(III), Gd3þ,are used as effective positive contrast agents.167 The zeolite
nanocrystals function by shortening the proton relaxation
times of water and tissue, which results in image brightening.
The immobilization of biomolecules onto the surface of
zeolite nanocrystals has been the focus of intense activity
in biotechnology and biomedicine. Especially, amine groups
in the proteins can be bound onto the surface of zeolite
particles and can be stabilized electrostatically. In addition,
zeolite nanocrystals show great promise as substrates for
immobilizing proteins due to their small size, sufficient func-
tional groups for further grafting or attachment, special surface
hydrophilic–hydrophobic microregion distribution, stable col-
loidal properties, and high level of adsorption capacity.
Nanozeolites are used as good enzyme-immobilizing car-
riers for biosensing and as substances in the enrichment and
identification of low-abundance peptides/proteins.168,169 For
example, LTL nanocrystals have shown to be a promising
trypsin-immobilizing carriers and have been patterned in poly
(methyl methacrylate) (PMMA) microfluidic channels for fabri-
cating an enzyme microreactor.170 The resulting trypsin micro-
reactor not only achieves the efficient digestion of proteins at a
low concentration in a very short reaction time but also shows
good stability and universality for protein identification. There-
fore, the LTL has been shown to have a great potential in
automated high-throughput analysis using a parallel-channel
microchip platform for proteomic analysis integrated with the
developed separation and identification procedures.
Nanosized zeolites are also used in the purification of pro-
tein on the basis of electrostatic or hydrophobic interaction
between the sought-after biomolecules and zeolites. A success-
ful separation of the selenoprotein-P (Se-P) in mouse plasma
through their chelation with Co2þ-exchanged nanozeolite/
diatomite composites has been reported.171 It was found
that, after immobilizing transition-metal ions in zeolite crys-
tals, the transition-metal-ion immobilized zeolites would se-
lectively adsorb the histidine-rich domains in the targeted
protein molecules.
Nanocrystalline zeolites have been modified and used as
unique ‘magnetic zeolites’. The zeolite nanocrystals combined
with superparamagnetic magnetite (Fe3O4) nanoparticles
demonstrated an excellent adsorption separation of enzymes
and a good biocatalytic performance.172 Zeolite nanocompo-
sites consisting of magnetite and FAU zeolite are used to store
and release substantial amounts of doxorubicin, an anticancer
antibiotic belonging to the tetracycline group. In this form of
drug delivery, an external or internal magnetic field can be used
to direct drug delivery particles to the proximity of the tumor
• Separation and catalysis
• Storage materials: heat, hydrogen, methane
• Membranes: gas and liquids
• Sensors: acoustic, electrochemical, optical
• Films: dielectric layers, anticorrosion and antireflective coatings
• Hosts for clusters and organics: photoconductors, semiconductors, supercapacitors
• Biology and medicine: selective sorbent, antioxidant, antimicrobial and antiviral protection, drug carrier
Figure 18 Advanced applications of zeolite nanocrystals.
300 Zeolite Nanoparticles
cells, thus enabling significant reduction of the necessary dose
of medication and minimizing the side effects.173
Moreover, recent papers showed that nanozeolite particles
possessed marvelous adsorption and immobilization cap-
ability for biomolecules due to their unique surface prop-
erty,169,170 which make them promising for delivery and
stabilization of protein medicine. Nanosized FAU zeolite had
also been applied to follow endosomal acidification and pro-
teolysis for the investigation of the endocytosing mechanisms
of human peripheral dendritic cells owing to their high ad-
sorption capacity for various biomolecules.174 It was reported
that low concentration of zeolite particles (13 mg ml�1)
showed nontoxicity in exposure to the cells for 3 days. In
summary, the toxicities of various zeolite nanoparticles were
studied and the results indicate that toxicities of nanozeolites
are relative to their size, composition, and shape. The spherical
pure-silica nanozeolites display a nontoxic effect, but the
aluminum-containing zeolite nanoparticles show a dose-
dependent toxicity. More importantly, the toxic nanozeolites
seem to induce cell necrosis rather than cell apoptosis by
demolishing the cell membranes.175,176 Thus, the spherical
pure-silica zeolites nanoparticles are potentially useful for
medical purposes.
In conclusion, the design and synthesis of nanosized zeo-
lites with diverse compositions, structures, and their further
processing to macroscopic constructs are important for enlarg-
ing their technological perspectives and also they are interest-
ing from a fundamental point of view. The reduction of particle
size from the micrometer to the nanometer scale leads to
substantial changes in the properties of the materials, which
have an impact on the performance of the zeolite nanoparticles
even in the traditional applications. The possibility to obtain
stable suspensions of zeolite nanoparticles able to be processed
on different surfaces by rapid techniques is also of great im-
portance for the advanced application of colloidal molecular
sieves.
Some of the important advanced applications of zeolite
nanoparticles are presented in Figure 18.
5.10.4 Conclusion
This chapter gives a general view on the different approaches
for preparation of zeolite nanoparticles. The emphasis is on the
various approaches for preparation of zeolite nanocrystals
including templated synthesis, template-free, ionothermal,
seed-induced, and confined space synthesis. Other methods
including microemulsion, direct conversion, and multistep
crystallization of zeolite nanoparticles are presented. Depend-
ing on the conditions of synthesis, discrete nanoparticles can
be obtained with variable morphologies and particle sizes.
Most of the zeolite nanocrystals for basic research and used
in advanced applications are prepared using the templated
synthesis approach. This approach results in zeolite nanocrys-
tals with monomodal particle-size distribution and high sta-
bility in suspensions. However, low crystalline yield and high
consumption of organic template are the main concerns.
Therefore, the zeolite nanocrystals are considered in areas
where very small amounts are required including preparation
of coatings for sensor application, optics, drug delivery sys-
tems, solar cells, medicine, etc. Additionally, the organic-free
systems provide nanocrystals with high yield; however, the
very fine colloidal particles are not stable and tend to agglom-
erate with time. Additional modification of their surfaces is
required to prevent further agglomeration and enable further
applications.
The studies on the toxicity of zeolite nanocrystals with
different sizes, compositions, and shapes are expected to fur-
ther direct their applications in biomedicine and pharmaceu-
tical industry. For related chapters in this Comprehensive, we
refer to Chapters 4.05, 5.05, 5.06, 7.04, and 7.10.
Acknowledgment
The financial support from MEET INTEREG EC project is
acknowledged.
Zeolite Nanoparticles 301
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