3 Morphology Control of MFI Zeolite Particles (of Size Less than 100 µm)

5.3 Morphology Control of MFI Zeolite Particles (of Size Less than 100 µm)



So the selectivity and effectiveness need to be compromised by optimization of the

crystal size and shape [37, 52].

Even though more than 130 different zeolites have been discovered so far, only

a few are available as industrial catalysts. The crystal morphology of the materials

possessing a three-dimensional channel system, for example, FAU- and LTA-type

zeolites, is not expected to have a great impact on their properties. In contrast,

the performance of materials with mono- or bidimensional channel systems might

be strongly affected by the morphology of the crystals. Among them, we prefer to

introduce morphological synthesis of zeolites having MFI-type framework.

MFI-type zeolite possesses an anisotropic framework with two intersecting

10-ring channels. The straight channels are parallel to b axis and the zigzag

channels with an estimated pore opening of 0.51 nm × 0.55 nm are parallel

to the a axis (Figure 5.8). The b channels and a channels are interconnected

with each other, so diffusion along the c direction is also possible. It has been

known that MFI-type zeolites with various crystal shapes can be prepared, such

as spherical, hexagonal twined disk, rodlike, and so on [53–68]. In the synthesis

of Al-MFI, the various factors influencing the dimensions along each axis of

the crystal have been investigated systematically [69]. The dependence of metal

cations, structure-directing agents, chemical source, and compositions has been

summarized by Singh and Dutta [70].

5.3.1

Dependence of Structure-Directing Agents (SDAs)



The typical structure-directing agent (SDA) for MFI-type zeolite is the TPA cation.

Instead of TPA, synthesis of MFI zeolite in the presence of dC6 (Figure 5.9) has been

reported in several studies [72, 73]. The characteristic crystal shape of TPA-Si-MFI



0.54 × 0.56 nm



b

0.51 × 0.55 nm



a



[h0h]



c

Figure 5.8



Pore structure of the AL-MFI [71].



139



140



5 Morphological Synthesis of Zeolites



c



c

TPA



a

(a)



b

5 µm

dC7

N+



N+



(b)



1 µm

dC6

N+



N+



(c)



5 µm



tC6

(d)



1 µm



N+



N+



N+



Figure 5.9 SEM images of Si-MFI: (a) pill- or coffin-shaped

crystals using TPA; (b) octagonal shaped crystals with twin

intergrowths using dC7; (c) leaf-shaped crystals from dC6;

(d) b-elongated leaf-shaped (or platelike) crystals from

tC6 [74].



is hexagonal prismatic, more commonly referred to as a coffin shaped, with the

order of crystal dimensions Lc > La > Lb (where Li indicates crystal size along i

axis). Tsapatsis et al. controlled the order of crystal dimensions to Lc > La = Lb with

dimer of TPA (dC6, Figure 5.9) and to Lc > Lb > La with trimer tC6 [74].

The morphological changes for Si-MFI zeolites with amine additives and TPABr

have been reported [75]. The TPABr-containing crystals are rather elongated

(a × b × c = 80 × 40 × 20 µm3 ), whereas the crystals containing TPA and DPA

were smaller (30 × 25 × 20 µm3 and 6 × 5 × 4 µm3 , respectively) and isometric in

shape (Figure 5.10).

Al-MFI crystals have been synthesized into cubic crystals in pyrrolidine-containing hydrous gels with uneven size and the diameter ranging from 0.5 to 4 µm [76].

By varying the TPABr content, Si-MFIs have been grown in a rodlike shape.

Because fewer nuclei are formed at lower TPABr concentrations, the volume

of the individual crystallites was inversely proportional to the initial TPABr

concentration [77].

Si/TPA ratios have varied with values of 10, 24, and 48 [78]. With a ratio of 10,

tablet-shaped crystals were formed with knobs at the top and bottom; for a ratio

of 24, the crystals had a similar shape with sharp corners and were significantly

larger. The larger size was a reflection of lower TPA content and reduced rate of



5.3 Morphology Control of MFI Zeolite Particles (of Size Less than 100 µm)



(a)



(b)



25 µm



141



(c)



25 µm



Figure 5.10 SEM images of MFI-type zeolites prepared with

structure directing agents: (a) TPABr; (b) tripropyl amine;

and (c) dipropyl amine.



nucleation. With a Si/TPA ratio of 48, the size and shape remained the same as in

24, but there appeared to be a solid phase growing on the surface of the crystals.

5.3.2

Dependence on Alkali-Metal Cations



The morphology of AL-MFI was found to be dependent on the presence of

alkali-metal ions [79]. Li and Na zeolites consisted of spheroidal 2–5 and 8−15 µm

crystal aggregates of very small platelet-like units, respectively. K, Rb, and Cs zeolites

consisted of twins of rounded (K, Rb) or sharp-edged crystals (Cs). (NH4 )-Al-MFI

consisted of large lath-shaped, well-developed, and double-terminated single

crystals.

The (Li, Na)-, Na-, and (Na, K)-Al-MFI zeolites have spherical or egg-shaped

polycrystallites [80], and similar morphology of the Al-MFI zeolites for Na and K

have been observed [81].

Morphology of Al-MFI synthesized from Na, K-TPA depended on the relative

ratio of the alkali-metal cations. With both Na and K cations present at a ratio of

K/(K + Na) = 0.75, large crystal aggregates were obtained in the range of 5−10 µm

[82, 83].

Here, some of the batch compositions were studied, that is, xNa2 O/8TPABr/

100SiO2 /1000H2 O and xTPA2 O/(8 – 2x)TPABr/100SiO2 /1000H2 O, where x varies

from 0.5 to 4.0. As the alkalinity of the reaction mixture was reduced from x = 4

to x = 0.5, the aspect ratio (length/width) of the crystals increased from 0.9 to 6.7.

Both nucleation and crystallization occurred more rapidly in the presence of Na+ .

Synthesis of Al-MFI in glycerol solvent has been reported, and the morphology of

the crystals was found to be hexagonal columns [84].

Addition of Li2 O in the synthesis of zeolite TPA-Al-MFI with (NH4 )2 O/Al2 O3

= 38 produces unusually uniform, large, lath-shaped crystals of Al-MFI about

140 ± 10 µm in length [85].



1 µm



142



5 Morphological Synthesis of Zeolites



5.4

Morphological Synthesis by MW



Zeolites can be effectively and rapidly synthesized by using microwave heating

techniques [86]. The advantages in the microwave synthesis of zeolites are homogeneous nucleation, fast synthesis by rapid heat-up time, selective activation of

the reaction mixture by microwaves, phase selective synthesis by fine-tuning of

synthesis conditions, uniform particle, size-facile morphology control, fabrication

of small crystallites and enhancement of crystallinity, and so on [87].

5.4.1

Examples of MW Dependency



AFI-type molecular sieves Aluminophosphate five such as AlPO-5 and SAPO-5

have one-dimensional channels with micropores (0.73 nm) and they have been

synthesized with various morphologies under microwave irradiation [88–91]. The

morphologies were controlled by varying the reaction conditions and addition of

extra components such as fluoride and silica (Figure 5.11). Rodlike crystals (with

aspect ratio of about 40) were obtained with the addition of fluoride and increase

of template and water concentrations. The platelike crystals (with an aspect ratio

of about 0.2) were synthesized in an alkaline condition with the addition of an

appropriate concentration of silica sol. In this case, it was supposed that silica

might hinder the crystal growth in the c direction and the fluoride ion might retard

the nucleation rate.

Most recently, Xu et al. controlled the morphology Si-MFI crystals (Figure 5.12)

from a microwave-assisted solvothermal synthesis system in the presence of diols,

that is, ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), and

tetraethylene glycol (tEG) [92]. Under microwave radiation, the Si-MFI crystals

with tunable sizes, shapes, and aspect ratios were crystallized.



(a)



(b)



5 µm



Acc.V Spot Magn Det WD

10.0 kV 3.0 5000× SE 10.1



5 µm



20 µm

Acc.V Spot Magn Det WD

10.0 kV 3.0 1000× SE 4.9



Figure 5.11 SEM images of typical AFI molecular sieves:

(a) platelike crystal and (b) rodlike crystal. White scale bar

corresponds to 10 µm [87].



20 µm



5.4 Morphological Synthesis by MW



143



c



a



b



General



SEI



5.0kV ×18,000 1 µm WD 5.8mm



(a)



General



SEI



5.0kV ×19,000



1 µm WD 5.9mm



(b)



L



W



T

General



SEI



5.0kV ×18,000



1 µm



WD 5.8mm



(c)



General



SEI



5.0kV



×18,000 1 µm WD 5.8mm



(d)



Figure 5.12 The SEM images of the

Si-MFI crystals crystallized from the

microwave-assisted solvothermal synthesis

system in the presence of the diols: (a) EG,

(b) DEG, (c) TEG, and (d) tEG. A schematic



identifying the crystal faces is shown on the

right part of the figure. Gel composition (in

molar ratio), SiO : TPAOH : EtOH : diols :

H2 O = 1 : 0.357 : 4.0 : 7 : 21.55 [92].



5.4.2

Morphological Fabrication by MW



Fabrication of nanostructured zeolites has attracted much attention in order to

(i) optimize zeolite performance (no pore blocking and zeolite diluting binding

additives are present), easy handling, and attrition resistance; (ii) minimize diffusion limitations with the secondary larger pores; and (iii) apply to nonconventional

applications, such as guest encapsulation, bioseparation, enzyme immobilization,

and so on [33]. By applying various templates or nanotechniques, zeolites could

be fabricated into membranes and films, biomimic or hierarchical structures, and

micro-/mesoporous materials [33]. Among them, fabrication of nanoporous materials using a chemical glue can be used for implementing nanoscopic or microscopic

arrays of these materials. So far, there have been a few reports on the utilization

of chemical glues such as inorganic glue [93], nano-glue [94], and organic covalent

linkers [95]. Recently, we reported microwave fabrication of zeolites directly from

the synthetic solution and proposed the incorporation of a transition metal as

nano-glue [96].



144



5 Morphological Synthesis of Zeolites



In the microwave synthesis of Ti-MFI zeolite (TS-1), the surface titanol groups

behave as inorganic glue and will stack Ti-MFI crystals to a fibrous morphology. This

technique could be expanded to the fabrication of zeolite films and zeolite coatings

[96]. Pure and metal-incorporated MFI crystals were synthesized by microwave

heating. These samples will be denoted as M-MFI-MW, where M stands for the

incorporated metal (Ti, Fe, Zr, and Sn) and MW for the microwave condition;

metal-free MFI by the microwave synthesis will be denoted as Si-MFI-MW [96].

The microwave induces a dramatic change in the morphology depending on the

composition. Si-MFI-MW and Ti-MFI-CH show the characteristic hockey-puck-like

crystals of submicrometer sizes with well-developed, large (010) faces (Figure 5.13a

and b). The microwave syntheses of metal (Ti, Sn) incorporated systems produced

similar primary crystals, but in this case the crystals were all stacked on top of each

other along their (010) direction (b axis) to form a wormlike or fibrous morphology

(Figure 5.13c and d). This stacking is sufficiently robust so as not to be destroyed

by a sonication treatment for more than 1 h, indicating that this morphology is

not a result of simple aggregation but of strong chemical bonding between the

crystals. This fibrous morphology persists as long as there is incorporated Ti, whose

concentration is kept in the range of Si/Ti = 70−230. When the concentration of

Ti is increased (Si/Ti ≤ 50), the product is composed of isolated ellipsoidal crystals

(a)



(b)



1 µm



(c)



1 µm



(d)



2 µm



Figure 5.13 SEM images of (a) Si-MFI-MW, (b)

Ti-MFI-CH (Si/Ti = 70), (c) Ti-MFI-MW(Si/Ti = 70), and

(d) Sn-MFI-MW(Si/Sn = 70) [96].



5 µm



5.4 Morphological Synthesis by MW



(not shown). This is probably because the high concentration of Ti interferes with

the crystal growth mechanism and the lack of flat surfaces does not allow crystal

stacking. The crystals with other incorporated metals (Fe, Zr, and Sn) also show

the fibrous morphology.

High-resolution TEM image and ED patterns were used to observe the closely

connected boundary between crystals (Figure 5.14). The connected parts have

well-crystallized structures. We assume that these crystallized parts are mostly

connected with the straight channels of each other except some edge parts, which

allows the formation of mesopores. Although it is not clear how the incorporated

metals induce the stacking of crystals under microwave conditions, it appears to

be related to the magnitude of the local dipole moment of the M–O bonds that

might be originated from the differences in the electronegativities. Electrically

insulating materials absorb microwave energy through the oscillation of dipoles,

and the magnitude of absorption increases with the increase of the dipole moment.

The magnitude of a dipole moment is mainly determined by the difference in

the electronegativities ( χ) of the two bonded atoms. Therefore, the Ti–O bond

( χ = 2.18 according to the Allred–Rochow scheme) [91] is a better microwave

absorber than the Si–O bond ( χ = 1.76). The Ti–O bonds on the surface

are strongly activated by microwave absorption and can undergo condensation

reactions to form Ti–O–Ti and/or Ti–O–Si bonds between crystals. The same

explanation applies to the Fe-MFI-MW and Zr-MFI-MW zeolites because of the

large χ values for Fe–O and Zr–O bonds. In the case of the Sn-MFI-MW, the

χ value of the Sn–O bond (1.78) is rather small, close to that of Si–O bond.

However, because Sn is large in size, the valence electron density of the Sn–O

bond is shifted to the O side, making this bond more polar than estimated by the

χ value alone, namely, the homopolar contribution to the dipole moment [96],

and the above explanation of microwave absorption by polar bonds can be applied

to the Sn-MFI case. Further work is needed to fully understand the role of the

incorporated metals in the stacking of crystals under the microwave condition.

1

2

1



5

6



2



3

4

1 µm



6



Figure 5.14



5



4



3



HRTEM images and ED patterns of Ti-MFI-MW [87].



145



146



5 Morphological Synthesis of Zeolites



(a)

1 µm



(b)



(d)



14

(c)



12



1 µm



(b)



10.0 µm



10.0 µm



(c)

1 µm



(d)



Stacking layer



10

8



(a)



6

4

2



1 µm



0

360

10.0 µm



10.0 µm



480



600



720



Microwave power (W)



Figure 5.15 SEM images and average number of stacking

layers of stacked Ti-MFI-MW synthesized under different

microwave powers: (a) 360 W, (b) 480 W, (c) 600 W, and

(d) 720 W [87].



Microwave power was controlled and varied for figuring out the formation

of nanostacked Ti-MFI zeolite [97]. FE-SEM images of Ti-MFI zeolites prepared

at different powers are given in Figure 5.15. All the samples show stacked

morphologies. The crystals are all stacked on top of each other along their (010)

direction to form a wormlike or fibrous morphology. And the average number of

stacking layers increased from 7 to 13 as the microwave power was increased from

360 to 720 W. From the above discussion, we see that microwave can strongly

affect the nano stacking process. This is because a higher power will give more

condensation for the dehydration between hydroxyl groups on the crystal surface.

5.4.3

Formation Scheme of Stacked Morphology



In this study, the formation of the stacked morphology was observed more clearly

through the SEM images of Ti-MFI depending on different microwave irradiation

times [96]. At the first stage of the microwave irradiation (Figure 5.16a–c), small

zeolite seeds grew up to uniform crystals; after 40 min they started to form

the stacked morphology and the number of stacks kept increasing till 60 min

(Figure 5.16d–f). In the microwave synthesis, the silica precursor would be

crystallized to uniform, hockey-puck-shaped morphology during the first synthesis

step. Under prolonged irradiation, those small crystals adhered together along the

b orientation to form a stacked morphology. The surface Ti-OH groups seemed to

be activated by microwaves and accelerate the condensation reaction between the

OH groups on the surface of the crystals (Figure 5.17).

MW irradiation induces a three-dimensional stacking of zeolite particles with

opal-like morphology through bimetal incorporation [98]. Microwave synthesis of

Al- and Ti-bimetal incorporated MFI zeolite ((Al, Ti)-MFI) gives both fibrous and

arrayed morphologies. Uniform zeolite crystals were stacked to form long lines and



5.4 Morphological Synthesis by MW



(a)



(b)



(c)



(d)



(e)



(f)



Figure 5.16 SEM of Ti-MFI with different microwave irradiation times: (a) 10 min, (b) 20 min, (c) 30 min, (d) 40 min,

(e) 50 min, and (f) 60 min [87].



MW



165 °C

15 min



165 °C

30 min



Nano-sol of TS-1

Hockey puck shape



165 °C

90 min



165 °C

60 min



Ti



Si



Ti



Si



OH OH OH OH

OH OH OH OH

Nanoglue

Si Ti Si Ti



Figure 5.17



Formation scheme of stacked Ti-MFI [87].



those lines were arrayed to form the three-dimensional opal structure (Figure 5.18).

The submicrometer-sized pucklike crystals also were stacked face to face of the

(010) plane (Figure 5.18c). Microwave synthesis of bimetal-incorporated systems

produced the primary crystals, which were stacked on top of each other along

their (010) direction to form long lines (Figure 5.18a–c). The void spaces in the

nanoarrayed materials were observed through the SEM image of carbon replicas

(Figure 5.18d).

Xu and coworkers synthesized silicalite-1 (Si-MFI) crystals by applying

microwave-assisted solvothermal heating (Figure 5.19). Even without the metal

as a nano-glue, the zeolite particles could be stacked into fibrous morphology



147



148



5 Morphological Synthesis of Zeolites



(a)



(b)



104674



15.0 kV



× 3.00 K 6.00 µm



104673



(c)



× 7.00 K 2.57 µm



15.0 kV



(d)



104672 15.0 kV



× 7.00 K 2.57 µm



602926



× 30.0 K 600 nm



15.0 kV



Figure 5.18 SEM images of nanoarrayed (Al, Ti)-MFI:

(a) cross view, (b) top, (c) side, and (d) carbon replica.



(a)



General



(b)



SEI



5.0 kV



×7.000



1 µm



WD 7.9mm



(c)



General



General



SEI



5.0 kV



×18.000



1 µm



WD 6.0mm



(d)



SEI



5.0 kV



×30.000



100nm



WD 2.8mm



General



(f)



(e)



General



SEI



5.0 kV



×25.000



1 µm



WD 2.8mm



General



SEI



5.0 kV



(g)



SEI



5.0 kV



×33.000



100nm



WD 2.8mm



General



SEI



Figure 5.19 SEM images of Si-MFI crystals crystallized

using different alcohol cosolvents under microwave radiation conditions: (a) ethylene glycol (x = 37), (b) methanol

(x = 32.6), (c) ethanol (x = 24.3), (d) 1-propanol (x = 20.1),

(e) isopropanol (x = 18.3), (f) n-butanol (x = 17.8), and (g)

hexanol (x = 13.3). x is the dielectric constant [99].



5.0 kV



×33.000 100nm



WD 5.9mm



×19.000



1 µm



WD 7.9mm



5.5 Summary and Outlook



by controlling the dipolar cosolvents [99]. The low polarity (dielectric constant)

of the cosolvent may favor the formation of abundant Si-OH groups in the

precursor species and, on the surface of the nanocrystals formed at the early stage

of the crystallization, might undergo further condensation to form self-stacked

crystals due to the rapid crystallization under microwave conditions. The crystals

are stacked on top of each other along their b direction to form a self-stacked

morphology. The ‘‘fiber’’ cannot be destroyed even by long-time and strong

ultrasonication, which indicates that strong chemical bonds might exist between

the individual crystals.



5.5

Summary and Outlook



The representative techniques for the growth of large single crystals of zeolites are

reviewed, especially for typical zeolites such as FAU, LTA, or MFI. To prepare perfect

zeolite particles, we should understand the mechanism of nuclei formation and the

crystal growth process properly in order to critically control the process. Morphology

of large zeolite crystals reflects the expanded shape of their building units.

For fine-tuning the growth, the morphology of small zeolite particles was studied

properly. Various additives, for example, alkali cations, alcohols, and amines, were

investigated. Different alkali-metal cations result in distinct morphologies; a mixed

cationic system will provide uniform, large, lath-shaped crystals (about 140 µm).

The use of different structure-directing molecules changes the crystal morphology,

as noted with the results in the case of various amines. Hydroxide ion content

can alter the morphology significantly. Even for a particular ion, such as TPA, the

amount used can alter the morphology. The crystallites tend to be larger at lower

template concentrations, presumably because of the formation of fewer nuclei.

Morphologies of crystals from mixed solvents or nonaqueous media are distinct

from comparable compositions in an aqueous medium.

Microwaves can control the morphology of zeolites by utilizing the precursor

solutions including lossy components such as metals and various organic and

inorganic additives. They have played as the absorption sites of microwave energy,

which were termed nano-glues, for the fabrication of oriented fiber morphology.

Such preferred orientations were found to be helpful in the selective adsorption,

transportation, or diffusion of longer molecules and could provide advanced

shape-selective catalysis.

Morphological synthesis of zeolite crystals is still an expanding area, and the

preparation of high-quality zeolite crystals will continue to be of great importance. In

order to maximize the performance of zeolite catalysts, it is important to understand

the crystallographic organization within zeolite crystallites, particularly regarding

access to the variously structured pores. Special attention is paid to the study of

well-shaped zeolite crystals by utilizing in situ microspectroscopic techniques [100].

Unique insight into diffusion, intergrowth structure, and catalysis can be achieved

by applying newly developed technology. It is the way to understand and design



149