comprehensive inorganic chemistry ii || zeolite nanoparticles

18
5.10 Zeolite Nanoparticles S Mintova, Universite ´ de Caen, Caen, France E-P Ng, Universiti Sains Malaysia, Penang, Malaysia ã 2013 Elsevier Ltd. All rights reserved. 5.10.1 Introduction 286 5.10.2 Syntheses of Zeolite Nanoparticles 286 5.10.2.1 Precursor Suspensions 286 5.10.2.1.1 Templated synthesis approach 286 5.10.2.1.2 Template-free synthesis approach 289 5.10.2.1.3 Seed-induced synthesis 290 5.10.2.1.4 Multistep synthesis approach 291 5.10.2.1.5 Ionothermal synthesis 291 5.10.2.2 Types of Heating 292 5.10.2.2.1 Microwave-assisted synthesis 292 5.10.2.2.2 Microchannel-assisted synthesis 293 5.10.2.3 Confined Space Synthesis 293 5.10.2.3.1 Reverse microemulsion synthesis 294 5.10.2.4 Other Methods 295 5.10.2.4.1 Direct-conversion synthesis approach 295 5.10.2.4.2 Centrifugation-assisted grinding 295 5.10.2.4.3 Laser-induced fragmentation method 295 5.10.2.5 Separation of Zeolite Nanocrystals 296 5.10.3 Applications of Zeolite Nanoparticles 297 5.10.3.1 Zeolite Membranes 297 5.10.3.2 Optical and Other Devices 297 5.10.3.3 Biological and Medical Applications 299 5.10.4 Conclusion 300 Acknowledgment 300 References 301 Abbreviations C 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 MEL 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 Comprehensive Inorganic Chemistry II http://dx.doi.org/10.1016/B978-0-08-097774-4.00512-X 285

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Page 1: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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/B978-0-08-09777

MEL 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

285
Page 2: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

286 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.

Page 3: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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

Page 4: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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

Page 5: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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.

Page 6: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

(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

Page 7: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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.

Page 8: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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).

Page 9: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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.

Page 10: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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

Page 11: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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.

Page 12: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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

Page 13: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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

Page 14: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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.

Page 15: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

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

Page 16: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

• 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.

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Zeolite Nanoparticles 301

References

1. Binns, C. Introduction to Nanoscience and Nanotechnology. John Wiley & Sons:New York, 2010.

2. Ramsden, J. Applied Nanotechnology: The Conversion of Research Results toProducts. Elsevier Inc.: New York, 2009.

3. Mathur, S.; Singh, M. Nanostructural Materials and Nanotechnology. John Wiley& Sons: New York, 2007.

4. Barrer, D. W. Zeolite Molecular Sieves. Wiley-Interscience: New York, 1974.5. Tosheva, L.; Valtchev, V. Chem. Mater. 2005, 17, 2494.6. Larsen, S. C. J. Phys. Chem. C 2007, 111, 18464.7. Lew, C. M.; Cai, R.; Yan, Y. Acc. Chem. Res. 2010, 43, 210.8. Kumar, S.; Wang, Z.; Penn, R. L.; Tsapatsis, M. J. Am. Chem. Soc. 2008, 130,

17284.9. Yoo, W. C.; Kumar, S.; Penn, R. L.; Tsapatsis, M.; Stein, A. J. Am. Chem. Soc.

2009, 131, 12377.10. Lee, P.-S.; Zhang, X.; Stoeger, J. A.; Malek, A.; Fan, W.; Kumar, S.; Yoo, W. C.;

Hashimi, S. A.; Penn, R. L.; Stein, A.; Tsapatsis, M. J. Am. Chem. Soc. 2011, 133,493.

11. Hould, N. D.; Kumar, S.; Tsapatsis, M.; Nikolakis, V.; Lobo, R. F. Langmuir 2010,26, 1260.

12. Huang, L.; Wang, Z.; Sun, J.; Miao, L.; Li, Q.; Yan, Y.; Zhao, D. J. Am. Chem. Soc.2000, 122, 3530.

13. Wang, H.; Wang, Z.; Huang, L.; Mitra, A.; Yan, Y. Langmuir 2001, 17, 2572.14. Lew, C. M.; Liu, Y.; Day, B.; Kloster, G. M.; Tiznado, H.; Sun, M.; Zaera, F.;

Wang, J.; Yan, Y. Langmuir 2009, 25, 5039.15. Li, Q.; Creaser, D.; Sterte, J. Stud. Surf. Sci. Catal. 2001, 135, 140.16. Wang, Z.; Hedlund, J.; Sterte, J. Micropor. Mesopor. Mater. 2002, 52, 191.17. Kobler, J.; Abrevaya, H.; Mintova, S.; Bein, T. J. Phys. Chem. C 2008, 112,

14274.18. Moller, K.; Yilmaz, B.; Jacubinas, R. M.; Muller, U.; Bein, T. J. Am. Chem. Soc.

2011, 133, 5284.19. Biemmi, E.; Bein, T. Langmuir 2008, 24, 11196.20. Valtchev, V. P.; Tosheva, L.; Bozhilov, K. N. Langmuir 2005, 21, 10724.21. Li, Q.; Mihailova, B.; Creaser, D.; Sterte, J. Micropor. Mesopor. Mater. 2001, 43,

51.22. Mintova, S.; Valtchev, V.; Micropor, V. Micropor. Mesopor. Mater. 2002, 52,

171.23. Holzl, M.; Mintova, S.; Bein, T. Stud. Surf. Sci. Catal. 2005, 158, 11.24. Mintova, S.; Fieres, B.; Bein, T. Stud. Surf. Sci. Catal. 2002, 142, 223.25. Mintova, S.; Valtchev, V. Microporous Molecular Sieves as Colloids. In Zeolites:

From Model Materials to Industrial Catalysts; Cejka, J., Perez-Pariente, J.,Roth, W. J., Eds.; Research Signpost: India, 2007; p 63.

26. Mintova, S. Collect. Czech. Chem. Commun. 2003, 68, 2032.27. Bein, T.; Mintova, S. Advanced Applications of Zeolites. In Zeolites and Ordered

Mesoporous Materials, Progress and Prospects; Cejka, J., van Bekkum, H., Eds.;Elsevier: Amsterdam, 2005; p 263.

28. Larlus, O.; Mintova, S.; Wilson, S. T.; Willis, R. R.; Abrevaya, H.; Bein, T.Micropor. Mesopor. Mater. 2011, 142, 17.

29. Mintova, S.; Olson, N.; Valtchev, V.; Bein, T. Science 1999, 283, 958.30. Mintova, S.; Olson, N.; Bein, T. Angew. Chem. Int. Ed. 1999, 38, 3201.31. Ng, E.-P.; Chateigner, D.; Bein, T.; Valtchev, V.; Mintova, S. Science 2012, 335,

70.32. Knagge, K.; Johnson, M.; Grassian, V. H.; Larsen, S. C. Langmuir 2006, 22,

11077.33. Tokay, B.; Karvan, O.; Senatalar, A. E. Micropor. Mesopor. Mater. 2010, 131, 230.34. van Heyden, H.; Mintova, S.; Bein, T. J. Mater. Chem. 2006, 16, 514.35. Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am.

Chem. Soc. 1982, 104, 1146.36. Sharma, P.; Rajaram, P.; Tomar, R. J. Colloid. Interf. Sci. 2008, 325, 547.37. Chu, C. T. W.; Kuehl, G. H.; Lago, R. M.; Chang, D. D. J. Catal. 1985, 93, 451.38. de Moor, P. P. E. A.; Beelen, T. P. M.; van Santen, R. A. J. Phys. Chem. B 1999,

103, 1639.39. Persson, A. E.; Shoeman, B. J.; Sterte, J.; Ottersted, J. E. Zeolites 1994, 14, 557.40. Schoeman, B. J. Micropor. Mesopor. Mater. 1998, 22, 9.41. Burkett, S. L.; Davis, M. E. Chem. Mater. 1995, 7, 920.42. Dokter, W. H.; Garderen, H. F.; Beelen, T. P. M.; Santen, R. A.; Bras, W. Angew.

Chem. Int. Ed. 1995, 34, 73.43. Kecht, J.; Mintova, S.; Bein, T. Chem. Mater. 2007, 19, 1203.44. Kecht, J.; Mintova, S.; Bein, T. Langmuir 2008, 24, 4310.45. Zhan, B. Z.; White, M. A.; Lumsden, M.; Neuhaus, J. M.; Robertson, K. N.;

Cameron, T. S.; Gharghouri, M. Chem. Mater. 2002, 14, 3636.

46. Valtchev, V. P.; Bozhilov, K. N. J. Phys. Chem. B 2004, 108, 15587.47. Fan, W.; Morozumi, K.; Kimura, R.; Toshiyuki, T.; Yokoi, T.; Okubo, T. Langmuir

2008, 24, 6952.48. Ren, N.; Bronic, J.; Subotic, B.; Lv, X.-C.; Yang, Z.-J.; Tang, Y. Micropor.

Mesopor. Mater. 2011, 139, 197.49. Larlus, O.; Tosheva, L.; Holzl, M.; Mintova, S.; Metzger, T.; Valtchev, V. P. Stud.

Surf. Sci. Catal. 2005, 158, 367.50. Meng, X.; Zhang, Y.; Meng, M.; Pang, W. In Proceedings of the 9th International

Zeolite Conference, Montreal 1992; von Ballmoos, R., et al. Eds.; Butterworth-Heinemann, London, 1993; p 297.

51. Yao, J.; Wang, H.; Ratinac, K. R.; Ringer, S. P. Chem. Mater. 2006, 18, 1394.52. Jawor, A.; Jeong, B. H.; Hoek, E. M. V. J. Nanopart. Res. 2009, 11, 1795.53. Brar, T.; France, P.; Smirniotis, P. G. Ind. Eng. Chem. Res. 2001, 40, 1133.54. Vaughan, D. E. W. U.S. Patent 4,534,947 A 19850813, 198555. Majano, G.; Darwiche, A.; Mintova, S.; Valtchev, V. Ind. & Engineer. Chem. Res.

2009, 48, 7084.56. Majano, G.; Delmotte, L.; Valtchev, V.; Mintova, S. Chem. Mater. 2009, 21, 4184.57. Hincapie, B. O.; Garces, L. J.; Zhang, Q.; Sacco, A.; Suib, S. L. Micropor.

Mesopor. Mater. 2004, 67, 19.58. Serrano, D. P.; Aguado, J.; Escola, J. M.; Rodriguez, J. M.; Peral, A. Chem. Mater.

2006, 18, 2462.59. Serrano, D. P.; Aguado, J.; Rodrıguez, J. M.; Peral, A. Stud. Surf. Sci. Catal.

2007, 170, 282.60. Hu, Y.; Zhang, Y.; Tang, Y. Chem. Commun. 2010, 46, 3875.61. Iyoki, K.; Kamimura, Y.; Itabashi, K.; Shimojima, A.; Okubo, T. Chem. Lett. 2010,

39, 730.62. Hincapie, B. O.; Garces, L. J.; Zhang, Q.; Sacco, A.; Suib, S. L. Micropor.

Mesopor. Mater. 2004, 67, 19.63. Serrano, D. P.; Aguado, J.; Escola, J. M.; Rodriguez, J. M.; Peral, A. Chem. Mater.

2006, 18, 2462.64. Serrano, D. P.; Aguado, J.; Rodrıguez, J. M.; Peral, A. Stud. Surf. Sci. Catal.

2007, 170, 282.65. Hu, Y.; Zhang, Y.; Tang, Y. Chem. Commun. 2010, 46, 3875.66. Song, W.; Grassian, V. H.; Larsen, S. C. Chem. Commun. 2005, 295.67. Larlus, O.; Mintova, S.; Bein, T. Micropor. Mesopor. Mater. 2006, 96, 405.68. Ng, E.-P.; Delmotte, L.; Mintova, S. Green Chem. 2008, 10, 1043.69. Welton, T. Chem. Rev. 1999, 99, 2071.70. Cooper, E. R.; Andrews, C. D.; Wheatley, P. S.; Webb, P. B.; Wormald, P.;

Morris, R. E. Nature 2004, 430, 1012.71. Parnham, E. R.; Morris, R. E. Acc. Chem. Res. 2007, 40, 1005.72. Harrison, W. T. A. Inorg. Chem. Commun. 2007, 10, 833.73. Cho, S. D.; Park, H. J. Colloid Interface Sci. 2011, 357, 46.74. Lian, J.; Ma, J.; Duan, X.; Kim, T.; Li, H.; Zheng, W. Chem. Commun. 2010,

2650.75. Yin, S.; Luo, Z.; Xia, J.; Li, H. J. Phys. Chem. Solid 2010, 71, 1785.76. Liu, X.; Zhao, J.; Sun, Y.; Song, K.; Yu, Y.; Du, C.; Kong, X.; Zhang, H. Chem.

Commun. 2009, 6628.77. Hayakawa, Y.; Nonoguchi, Y.; Wu, H.-P.; Diau, E. W.-G.; Nakashima, T.; Kawai, T.

J. Mater. Chem. 2011, 21, 8849.78. Zhang, X.; Chen, H.; Zheng, M.; Liu, J.; Cao, J. Chem. Lett. 2007, 36, 1498.79. Ng, E.-P.; Sekhon, S. S.; Mintova, S. Chem. Commun. 2009, 1661.80. Ng, E.-P.; Itani, L.; Sekhon, S. S.; Mintova, S. Chem. Eur. J. 2010, 16, 12890.81. Han, L.; Wang, Y.; Li, C. AIChE J. 2008, 54, 280.82. Slangen, P. M.; Jansen, J. C.; van Bekkum, H. Micropor. Mater. 1997, 9, 259.83. Tompsett, G. A.; Conner, W. C.; Yngvesson, K. S. Chemphyschem 2006, 7, 296.84. Mintova, S.; Mo, S.; Bein, T. Chem. Mater. 1998, 10, 4030.85. Lin, S.; Li, J.; Sharma, R. P.; Yu, J.; Xu, R. Top. Catal. 2010, 53, 1304.86. Hu, Y.; Liu, C.; Zhang, Y.; Ren, N.; Tang, Y. Micropor. Mesopor. Mater. 2009,

119, 306.87. Jahnisch, K.; Hessel, V.; Lowe, H.; Baerns, M. Angew. Chem. Int. Ed. 2004, 43, 406.88. Pan, Y.; Yao, J.; Zhang, L.; Xu, N. Ind. Eng. Chem. Res. 2009, 48, 8471.89. Ju, J.; Zeng, C.; Zhang, L.; Xu, N. Chem. Eng. J. 2006, 116, 115.90. Pan, Y.; Ju, M.; Yao, J.; Zhang, L.; Xu, N. Chem. Commun. 2009, 7233.91. Pan, Y.-C.; Yao, J.-F.; Zhang, L.-X.; Ju, J.-X.; Wang, H.-T.; Xu, N.-P. Chem. Eng.

Technol. 2009, 32, 732.92. Madsen, C.; Jacobsen, C. J. H. Chem. Commun. 1999, 673.93. Persson, A. E.; Shoeman, B. J.; Sterte, J.; Ottersted, J. E. Zeolites 1994, 14, 557.94. Schmidt, I.; Madsen, C.; Jacobsen, C. J. H. Inorg. Chem. 2000, 39, 2279.95. Jacobsen, C. J. H.; Madsen, C.; Janssens, T. V. W.; Jakobsen, H. J.; Skibsted, J.

Micropor. Mesopor. Mater. 2000, 39, 393.96. Huu, C. P.; Wine, G.; Tessonnier, J. P.; Ledoux, M. J.; Rigolet, S.; Marichal, C.

Carbon 2004, 42, 1941.

Page 18: Comprehensive Inorganic Chemistry II || Zeolite Nanoparticles

302 Zeolite Nanoparticles

97. Tang, K.; Wang, Y. G.; Song, L. J.; Duan, L. H.; Zhang, X. T.; Sun, Z. L. Mater.Lett. 2006, 60, 2158.

98. Wang, B.; Ma, H. Z.; Shi, Q. Z. Chin. Chem. Lett. 2002, 13, 385.99. Wang, H. T.; Holmberg, B. A.; Yan, Y. S. J. Am. Chem. Soc. 2003,

125, 9928.100. Li, D.; Ratinac, K. R.; Ringer, S. P.; Wang, H. Micropor. Mesopor. Mater. 2008,

116, 416.101. Yang, X.; Feng, Y.; Tian, G.; Du, Y.; Ge, X.; Di, Y.; Zhang, Y.; Sun, B.; Xiao, F.-S.

Angew. Chem. Int. Ed. 2005, 44, 2563.102. Carr, C. S.; Shantz, D. F. Micropor. Mesopor. Mater. 2005, 85, 284.103. Axnanda, S.; Shantz, D. F. Micropor. Mesopor. Mater. 2005, 84, 236.104. Zhang, J.; Yan, W.; Ding, H.; Liu, Y.; Tang, K.; Yu, J.; Xu, R. Stud. Surf. Sci. Catal.

2007, 170, 475.105. Lin, J.-C.; Yates, M. Z. Langmuir 2005, 21, 2117.106. Carr, C. S.; Shantz, D. F. Chem. Mater. 2005, 17, 6192.107. Naskar, M. K.; Kundu, D.; Chatterjee, M. Mater. Lett. 2011, 65, 436.108. Zhang, Y.; Jin, C. J. Solid State Chem. 2011, 184, 1.109. Zhang, J.; Yan, W.; Ding, H.; Liu, Y.; Tang, K.; Yu, J.; Xu, R. Stud. Surf. Sci. Catal.

2007, 170, 282.110. Reyes, P. Y.; Espinosa, J. A.; Trevino, M. E.; Saade, H.; Lopez, R. G. J. Nanomater.

2010, 1.111. Faghihian, H.; Godazandeha, N. J. Porous Mater. 2007, 170, 282.112. Kamali, M.; Vaezifar, S.; Kolahduzan, H.; Malekpour, A.; Abdi, M. R. Powder

Technol. 2009, 189, 52.113. Honda, K.; Yashiki, A.; Itakura, M.; Ide, Y.; Sadakane, M.; Sano, T. Micropor.

Mesopor. Mater. 2011, 142, 161.114. Kong, C.; Tsuru, T. Chem. Eng. Process 2010, 49, 809.115. Wakihara, T.; Ichikawa, R.; Tatami, J.; Endo, A.; Yoshida, K.; Sasaki, Y.;

Komeya, K.; Meguro, T. Cryst. Growth Des. 2011, 11, 955.116. Kharitonov, A. S.; Fenelonov, V. B.; Voskresenskaya, T. P.; Rudina, N. A.;

Molchanov, V. V.; Plyasova, L. M.; Panov, G. I. Zeolites 1995, 15, 253.117. Mafune, F.; Kohno, J. Y.; Takeda, Y. J. Phys. Chem. B 2001, 106, 7575.118. Mafune, F.; Kohno, J. Y.; Takeda, Y. J. Phys. Chem. B 2002, 106, 8555.119. Kawasaki, M.; Masuda, K. J. Phys. Chem. B 2005, 109, 9379.120. Nichols, W. T.; Kodaira, T.; Sasaki, Y.; Shimizu, Y.; Sasaki, T.; Koshizaki, N. J.

Phys. Chem. B 2006, 110, 83.121. Kobler, J.; Abrevaya, H.; Mintova, S.; Bein, T. J. Phys. Chem. C 2008, 112, 14274.122. Maurer, T.; Kraushaar-Czarnetzki, B. Helv. Chim. Acta 2001, 84, 2550.123. Bein, T. Chem. Mater. 1996, 8, 1636.124. Lai, Z.; Bonila, G.; Diaz, I.; Nery, J. G.; Sujaoti, K.; Amat, M. A.; Kokkoli, E.;

Terasaki, O.; Thompson, R. W.; Tsapatsis, M.; Vlachos, D. G. Science 2003, 300,456.

125. Yoo, W. C.; Kumar, S.; Penn, R. L.; Tsapatsis, M.; Stein, A. J. Am. Chem. Soc.2009, 131, 12377.

126. Chen, Y.; Zhu, G.; Peng, Y.; Yao, X.; Qiu, S. Micropor. Mesopor. Mater. 2009,124, 8.

127. Ghoroghchian, F.; Aghabozorg, H.; Farhadi, F.; Kazemian, H. Chem. Eng.Technol. 2010, 33, 2066.

128. Li, D.; Zhu, H. Y.; Ratinac, K. R.; Ringer, S. P.; Wang, H. Micropor. Mesopor.Mater. 2009, 126, 14.

129. Holmberg, B. A.; Wang, X.; Yan, Y. J. Membrane. Sci. 2008, 320, 86.130. Choi, J.; Jeong, H.-K.; Snyder, M. A.; Stoeger, J. A.; Masel, R. I.; Tsapatsis, M.

Science 2009, 325, 590.131. Kuzniatsova, T. A.; Mottern, M. L.; Chiu, W. V.; Kim, Y.; Dutta, P. K.; Verweij, H.

Adv. Funct. Mater. 2008, 18, 952.132. Tavolaro, A.; Drioli, E. Adv. Mater. 1999, 11, 975.133. Caro, J.; Noack, M.; Kolsch, P.; Schafer, R.Micropor. Mesopor. Mater. 2000, 38, 3.134. Chiang, A. S. T.; Chao, K. J. Phys. Chem. Solids 2001, 62, 1899.135. Noack, M.; Kolsch, P.; Schafer, R.; Toussaint, P.; Caro, J. Chem. Eng. Technol.

2002, 25, 221.136. Coronas, J.; Santamaria, J. Top. Catal. 2004, 29, 29.137. Nair, S.; Tsapatsis, M. In Handbook of Zeolite Science and Technology;

Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker: New York,2003; p 869.

138. Urbitzondo, M. A.; Pina, M. P.; Santamaria, J. Gas sensing with silicon-basednanoporous solids. In Ordered Porous Materials; Valtchev, V., Mintova, S.,Tsapatsis, M., Eds.; Elsevier: Amsterdam, 2009; p 387.

139. Mintova, S.; Bein, T. Micropor. Mesopor. Mater. 2001, 50, 159.140. Leite, E.; Naydenova, I.; Mintova, S.; Leclercq, L.; Toal, V. Appl. Optics. 2010, 49,

3652.141. Leite, E.; Naydenova, I.; Pandey, N.; Babeva, T.; Majano, G.; Mintova, S.; Toal, V.

J. Opt. Pure Appl. Opt. 2009, 11, 1.142. Naydenova, I.; Sherif, H.; Mintova, S.; Martin, S.; Toal, V. Proc. SPIE 2006,

6252, 1.143. Miller, R. D. Science 1999, 286, 421.144. Wang, Z.; Wang, H.; Mitra, A.; Huang, L.; Yan, Y. Adv. Mater. 2001, 13, 746.145. Wang, Z.; Wang, H.; Mitra, A.; Huang, L.; Yan, Y. Adv. Mater. 2001, 13, 746.146. Li, S.; Li, Z.; Yan, Y. Adv. Mater. 2003, 15, 1528.147. Chen, C.-H.; Li, S.-Y.; Chiang, A. S. T.; Wu, A. T.; Sun, Y. S. Solar Energ. Mater.

Solar Cell 2011, 95, 1694.148. Chiang, A. S. T.; Wong, L. J.; Li, S. Y.; Cheng, S. L.; Lee, C. C.; Chen, K. L.;

Chen, S. M.; Lee, Y. J. Stud. Surf. Sci. Catal. 2007, 170, 1583.149. Minkowski, C.; Pansu, R.; Takano, M.; Calzaferri, G. Adv. Funct. Mater. 2006,

16, 273.150. Calzaferri, G.; Huber, S.; Maas, H.; Minkowski, C. Angew. Angew. Chem. Int. Ed.

Engl. 2003, 42, 3732.151. Calzaferri, G.; Pauchard, M.; Maas, H.; Huber, S.; Khatyr, A.; Schaafsma, T. J.

Mater. Chem. 2002, 12, 1.152. Majano, G.; Mintova, S.; Bein, T.; Klapotke, T. M. Adv. Mater. 2006, 18, 2440.153. Majano, G.; Mintova, S.; Bein, T.; Klapotke, T. M. J. Phys. Chem. 2007, 111, 6694.154. Mintova, S.; De Waele, V.; Schmidhammer, U.; Riedle, E.; Bein, T. Angew. Chem.

Int. Ed. 2003, 42, 1611.155. Mintova, S.; De Waele, V.; Holzl, M.; Schmidhammer, U.; Mihailova, B.; Riedle, E.;

Bein, T. J. Phys. Chem. A 2004, 108, 10640.156. Meinershagen, J. L.; Bein, T. Adv. Mater. 2001, 13, 208.157. Kecht, J.; Tahri, Z.; De Waele, V.; Mostafavi, M.; Mintova, S.; Bein, T. Chem.

Mater. 2006, 18, 3373.158. Yordanov, I.; Knoerr, R.; De Waele, V.; Bazin, P.; Thomas, S.; Rivallan, M.;

Lakiss, L.; Metzger, T. H.; Mintova, S. J. Phys. Chem. C 2010, 114, 20974.159. Tahri, Z.; Luchez, F.; Yordanov, I.; Poizat, O.; Moissette, A.; Valtchev, V.;

Mintova, S.; Mostafavi, M.; De Waele, V. Res. Chem. Intermed. 2009, 35, 379.160. Baba, T.; Ono, Y. Zeolites 1987, 7, 292.161. Jacobs, P. A.; Uytterhoeven, J. B.; Beyer, H. K. J. Chem. Soc. Chem. Commun.

1977, 128.162. Calzaferri, G.; Hugues, S.; Hugentobler, T.; Sulzberger, B. J. Photochem. 1984,

26, 108.163. Ozin, G. A.; Hugues, F. J. Phys. Chem. 1982, 86, 5174.164. Sato, S.; Yu-u, Y.; Yahiro, H.; Mizuno, N.; Iwamoto, M. Appl. Catal. 1991, 70, L1.165. Aoyama, N.; Yoshida, K.; Abe, A.; Miyadera, T. Catal. Lett. 1997, 43, 249.166. Danilczuk, M.; Dlugopolska, K.; Ruman, T.; Pogocki, D. Mini Rev. Med. Chem.

2008, 8, 1407.167. Bresinska, I.; Balkus, K. J. J. Phys. Chem. 1994, 98, 12989.168. Zhang, Y. H.; Wang, X. Y.; Shan, W.; Wu, B. Y.; Fan, H. Z.; Yu, X. J.; Tang, Y.;

Yang, P. Y. Angew. Chem. Int. 2005, 117, 621.169. Yu, T.; Zhang, Y. H.; You, C. P.; Zhuang, J. H.; Wang, B.; Liu, B. H.; Kang, Y. J.;

Tang, Y. Chem. Eur. J. 2006, 12, 1137.170. Zhang, Y.; Liu, Y.; Kong, J.; Yang, P.; Tang, Y.; Liu, B. Small 2006, 2, 1170.171. Xu, F.; Wang, Y. J.; Wang, X. D.; Zhang, Y. H.; Tang, Y.; Yang, P. Y. Adv. Mater.

2003, 15, 1751.172. Shan, W.; Yu, T.; Wang, B.; Hu, J.; Zhang, Y.; Wang, X.; Tang, Y. Chem. Mater.

2006, 18, 3169.173. Arruebo, M.; Pacheco, R. F.; Irusta, S.; Arbiol, J.; Ibarra, M. R.; Santamaria, J.

Nanotechnol. 2006, 17, 4057.174. Andersson, L. I. M.; Eriksson, H. Scand. J. Immunol. 2007, 66, 52.175. Kihara, T.; Zhang, Y.; Hu, Y.; Mao, Q.; Tang, Y.; Miyake, J. J. Biosci. Bioeng.

2011, 111, 725.176. Bhattacharyaa, K.; Nahaa, P. C.; Naydenova, I.; Mintova, S.; Byrne, H. J. Toxicol.

Lett. 2012, 215, 151.