Controllable fabrication of single-crystalline, ultrafine and high-silica hierarchical ZSM-5 aggregates: Via solid-like state conversion

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Controllable fabrication of single-crystalline,

ultraﬁne and high-silica hierarchical ZSM-5

aggregates via solid-like state conversion†

Cite this: RSC Adv., 2017, 7, 25605

Hongyao Li,* Yaquan Wang,

Xiao Wang and Shuhai Wang

Fanjun Meng, Fei Gao, Chao Sun, Chunyang Fan,

2 2 3

A series of single-crystalline, superﬁne and high-silica (SiO /Al O > 100) hierarchical ZSM-5 aggregates

(

denoted as HA-ZSM-5) with tuneable crystal morphology and size, and rich intercrystalline

a

mesoporosity, were facilely fabricated within a short reaction time using the commercially available n-

hexyltrimethylammonium bromide (HTAB) as a mesopore template by a solid-like state conversion (SSC)

route; the crystallization of HA-ZSM-5 is induced within 36 h at a HTAB/SiO

assistance of 2% silicalite 1 (S-1) seeds. Key factors (such as SiO /Al ratio, seed/SiO

SiO ratio) in controlling the crystal size and morphology have been systematically investigated. The

2

ratio of 0.05 with the

2

O

3

2

ratio and HTAB/

2

formation process was studied by monitoring the morphology, relative crystallinity and structural

properties of the samples at diﬀerent crystallization times. The preparation of HA-ZSM-5 zeolites may be

a kinetics-controlled dissolution/induction/growth/aggregation process and the possible formation

mechanism was put forward based on experimental results. The obtained samples were characterized by

several techniques and the representative HA-ZSM-5 (HA-167-2S-5H) was evaluated in a methanol to

propylene (MTP) reaction. Compared with the blank sample (denoted as C-ZSM-5) without HTAB

addition and a commercial ZSM-5 zeolite (denoted as C

the highest selectivity towards propylene (40.1% vs. 33.1% and 38.7%) and butylene (22.9% vs. 17.2% and

1.4%), and especially the longest catalytic lifetime (22 h vs. 6 h and 1.5 h). The remarkably enhanced

1

-ZSM-5), the HA-167-2S-5H sample exhibited

2

catalytic performance of HA-167-2S-5H could be attributed to the synergistic eﬀect of the excellent

structural properties, appropriate acidity and good accessibility of acidic sites. In addition, the

introduction of interparticle mesopores not only enhances the utilization of zeolite acid sites by the

enlarged external surface but also partially suppresses side reactions and inhibits coke deposition by

shortening the micropore diﬀusion path lengths, leading to an increased conversion.

Received 7th April 2017

Accepted 8th May 2017

DOI: 10.1039/c7ra03962j

rsc.li/rsc-advances

4

becomes a natural choice. Nonetheless, the low yield in prep-

aration, low thermal stability and diﬃcult separation of super-

1

. Introduction

ZSM-5 zeolites, with MFI-type topology, have been extensively ne nanocrystals from the reaction mixture limit their practical

5

applied as industrial catalysts for various petrochemical application. Another eﬃcient option for solving the above

processes owing to their variable acidity, good shape selectivity mentioned diﬀusion limitations is to construct hierarchically

1

and strong hydrothermal stability. But the relatively narrow structured zeolites (HSZs), coupling a microporous crystalline

individual micropores of conventional ZSM-5 zeolites oen framework with auxiliary mesoporous structure, which is

impose diﬀusion limitations that severely hinder their further considered to be an eﬀective approach stimulating wide

2

6

applications in bulky-molecule-involved process. To address research. For instance, both hard templates, such as carbon

7

8

9

this intractable issue, two main synthetic strategies have been nanotubes, activated carbon, carbon nanobers and so

proposed to reduce the diﬀusion resistance of large molecules templates, such as polymers and surfactants

in recent years. Decreasing the size of zeolite in nanoscale to employed as mesoporogens in the synthesis of HSZs. Combined

shorten the inner diﬀusion path length of bulky substrates with the high cost, energy-intensive and troublesome pre-

preparation of hard templates, so templating method seems

10

11,12

have been

3

a more promising candidate for synthesising HSZs owing to its

Key Laboratory for Green Chemical Technology of the Ministry of Education, School of

inherent advantages of easily tunable mesopores and good

Chemical Engineering & Technology, Tianjin University, Tianjin 300072, P. R. China.

13

compatibility with the synthesis solution. As a very successful

example of HSZs, hierarchical ZSM-5 aggregates (denoted as

HA-ZSM-5) combine dual advantages of both nanosized zeolites

E-mail: lhy0501@163.com; Tel: +86-22-23507881

Electronic supplementary information (ESI) available: Details of sample

†

characterization and results. See DOI: 10.1039/c7ra03962j

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29

and HSZs, such as a shorter diﬀusion path length, a larger homogeneous gels. Recently, we synthesized high-silica HA-

external surface area and more accessible active sites exhibited ZSM-5 (SiO /Al O $ 100) via a SAC method with the aid of

2

2 3

30

in contrast to conventional ZSM-5, while being able to avoid a trace of TPAOH.

high-speed centrifugation due to their larger secondary particle Fortunately, solid-like state conversion (SSC) method has

14

31

size. Therefore, HA-ZSM-5 have aroused great interest for their attracted considerable attention in past years, as it can not

excellent performances and many successful examples of using only avoid these extra steps of SAC method, but also reserve the

seeds or so templates to synthesize HA-ZSM-5 have been re- advantages of SAC method, such as shortening the crystalliza-

ported. It has been reported that silicalite 1 (S-1) seeds have tion time obviously, improving the yield of the products, and

been used to fabricate HA-ZSM-5, however, the obtained HA- reducing the emission of waste liquids, etc. In addition, recent

15

ZSM-5 sample remained still in micron range. Meng and co- progress has demonstrated that a small amount of water is

workers also reported the successful preparation of HA-ZSM-5 necessary for the preparation of HA-ZSM-5 in the solid trans-

2

4

by a seed-induced method with S-1 crystals as the seeds, but formation of the dry gel.

16

only the samples with SiO

More importantly, Ryoo et al. pioneered the use of a new kind of of SACSs and SSC method, this present work aimed to make the

two-in-one” template, combining features of both a primary synthetic process of SAC simplied and this paper reported that

2

/Al

2

O

3

ratio of 50 were reported.

As compared with our previous works and based on the ideas

“

template and a mesogenous template within one molecule, and single-crystalline, ultrane and high-silica HA-ZSM-5 were

successfully fabricated ZSM-5 nanosheets aggregates with directly synthesized via SSC method using commercially avail-

17

intracrystal mesopores. Nevertheless, the synthesis process of able HTAB as the mesoporogen through S-1 seed-induced route.

this bifunctional template involves complex manipulations and Diﬀerent from the previous studies, where LACSs were used as

18

the synthesis of ZSM-5 nanosheets stacks becomes costly.

templates, our research shows that the SACSs and SSC method

19

Most recently, Chen et al. prepared HA-ZSM-5 by seed- are also very eﬀective in the synthesis of high-silica HA-ZSM-5.

inducing via a hydrothermal synthesis method in the pres- For comparison, the blank sample (denoted as C-ZSM-5) was

ence of a long alkyl-chained surfactant (LACS) named cetyl- synthesized without HTAB addition by a similar procedure.

trimethylammonium bromide (CTAB), whereas CTAB oen lead Herein, a very eﬃcient method is developed for the preparation

to the occurrence of phase separation between meso- and of single-crystalline, ultrane and high-silica HA-ZSM-5

micro-phases due to the competition between self-assembly and zeolites. Beneting from the synergistic eﬀect between HTAB

20

zeolite crystallization. Another drawback of using LACSs in the and SSC method, seed/SiO molar ratio is greatly reduced to

2

synthesis of HA-ZSM-5 lies in their poor thermal stability, which 0.02, and interesting smaller nanocrystals size was achieved

21

15

may be decompose at high crystallization temperatures.

compared to what is achieved in literature. This synthesis

Diﬀerent from LACSs, short alkyl-chained surfactants (SACSs), process was rapid and economical and shows potential for

such as n-hexyltrimethylammonium bromide (HTAB), have large-scale production. This catalyst design concept provides

strong coulombic interactions toward the zeolite crystals and a new idea to fabricate high-silica HA-ZSM-5 or other zeolites

good thermal stability under high temperature and high pres- that reveal fast mass transport, which will show a great potential

22

sure conditions. These interactions with nanocrystals may for industrial applications in selective catalytic reactions

hinder their excessive growth and the stacking of the resulting involved bulky molecules. In this paper, key factors (SiO /Al O

2 3

2

23

nano-sized zeolites will form HA-ZSM-5. From an economic ratio, seed/SiO ratio, and HTAB/SiO ratio) in controlling the

2

and synthetic perspective, it prompted us to try using SACSs crystal size and the morphology of the HA-ZSM-5 zeolites during

to synthesize high silica HA-ZSM-5 with controllable synthesis have been investigated. Moreover, a possible forma-

morphologies.

tion mechanism of the HA-ZSM-5 is also proposed based on

In fact, the above-mentioned synthesis routes also have few experimental observations. The catalytic performance of the

inevitable shortcomings, such as procedural complexity (time- representative HA-ZSM-5 (HA-167-2S-5H) were evaluated for the

consumption), economic feasibility (low yield and high cost) MTP reaction to explore the structure–function relationship. In

and environmental issues (generating a large amount of waste addition, the catalytic performances of the HA-167-2S-5H zeolite

24

solvents).

Compared with these direct hydrothermal and the blank sample (denoted as C-ZSM-5) without HTAB

approaches, steam-assisted crystallization (SAC) method is an addition have been compared.

alternative but more favorable choice. SAC has opened up new

possibilities for the synthesis of HA-ZSM-5 and gained more and

2

. Experimental

more attention recently due to its high zeolite yield, few

25

2.1 Materials

amounts of wastes and low template consumption. Huge

eﬀorts have been devoted to synthesize HA-ZSM-5 via SAC The HA-ZSM-5 zeolite was synthesized by using the following

26

27

method. He and Zhang synthesized HA-ZSM-5 via SAC reagents:

28

method. More recently, Deng et al. obtained HA-ZSM-5 via

Tetraethyl orthosilicate (TEOS, 98.0%, Tianjin Kemiou

glycerol-mediated crystallization using a mesoporous steam- Chemical Reagent Co.), colloidal silica (SiO , 40 wt% suspen-

2

treated dry gel as the precursor. However, industrial applica- sion in H O, Qingdao Haiyang Chemical Reagent Co.), sodium

2

tions of SAC method are still limited by the complex manipu- hydroxide (NaOH, 96 wt%, Sinopharm Chemical Reagent Co.,

lations of dry gel preparation because the procedure of Ltd.), sodium aluminate (NaAlO

2 2 3

, 41 wt% Al O , Sinopharm

vaporizing the solvents are necessary for the preparation of Chemical Reagent Co., Ltd.), tetrapropylammonium hydroxide

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(

TPAOH, 25 wt% in H

Sigma-Aldrich), methanol (Tianjin Guangfu Fine Chemical 100/0.6 ¼ 167)-2S-5H, indicating that it was prepared according

Research Institute Co., Ltd.). n-Hexyltrimethylammonium to the formulation of 100 SiO : 0.6 Al O : 7 Na O : 1000

2

O, Shanghai Cairui Chemical Co.), NH

4

NO

3

The sample obtained is denoted as HA-167 (SiO

2

/Al

2

O

3

¼

2

3

2

bromide (HTAB, CP, Tokyo Chemical Industry Co., Ltd.). All H O : 2 S-1 seed : 5 HTAB at a crystallization temperature of

2

ꢀ

reagents were used directly as received and no further purication 120 C and a crystallization time of 48 h.

was conducted.

The C-ZSM-5 zeolite with a molar composition of 100

¼ 167, SiO : 0.6 Al : 7 Na O : 1000 H O : 0 HTAB was crystallized

In addition, a commercial ZSM-5 zeolite (SiO

denoted as C -ZSM-5 and purchased from Nankai Catalyst via SSC method under the same condition of HA-ZSM-5 zeolite

1

2

/Al

2

O

3

2

O

3

2

Plant, Tianjin, China) was used as a reference catalyst to except that no HTAB was added and made a comparison with

investigate the advantages of the HA-ZSM-5 zeolite in MTP the HA-ZSM-5.

reaction.

The inuence of synthesis conditions on resultant HA-ZSM-5

zeolites was studied by changing a parameter while keep the

other parameters unchanged. The variation of the synthesis

parameters is listed in Table S1 (ESI†). The samples prepared

2.2 Synthesis of S-1 seeding suspension

S-1 seeding suspension with a nal molar composition of were dened according to the method as mentioned above.

SiO : 0.24 TPAOH : 24 H O : 4 EtOH (in which EtOH was

produced from the hydrolysis of TEOS) was prepared using

2

32

2.4 Characterization

a method developed previously in our lab. The detailed

synthesis procedure is as follows: 88.67 g of TPAOH solution The textural properties of the obtained samples were evaluated

was completely dissolved in 130.00 g of H O. 97.56 g of TEOS from N adsorption–desorption isotherms measured at a liquid

2

ꢀ

was added dropwise under continuous stirring to the resultant nitrogen temperature of ꢁ196 C on a Micromeritics ASAP 2010

ꢀ

mixture. Aer stirring for 3 h until a clear solution is obtained analyzer. Each sample was degassed at 300 C for 4 h before the

(the complete hydrolysis of TEOS), the mixture was then analysis. The specic surface area (S_B_E_T) was determined by the

transferred into Teon-lined stainless steel autoclaves and Brunauer–Emmett–Teller (BET) equation. The micropore

ꢀ

hydrothermally treated at 105 C for 48 h in an oven to prepare volume (Vmicro) and external surface area (Vext) were calculated

the seed suspension. The obtained S-1 seeding suspension was by the t-plot method and the total pore volume (Vpore) was based

directly used as seeds without any centrifugation or washing on the adsorbed amount of N

2

at p/p

0

¼ 0.99. The mesopore

process.

volume (V_m_e_s_o) was calculated by subtracting V_m_i_c_r_ofrom the

corresponding Vpore. The pore size distribution (PSD) was esti-

mated by employing the Barrett–Joyner–Halenda (BJH) method

from the desorption branch of the isotherm.

2

.3 Preparation of HA-ZSM-5 zeolite

A typical synthesis of the HA-ZSM-5 zeolite is performed as

follows. Firstly, required NaOH, NaAlO

were mixed together with constant stirring, followed by addition at an operating voltage of 3 kV.

The crystal size and morphology were investigated by scan-

2

and deionised water ning electron microscopy (SEM) using a JSM-7800F instrument

of HTAB. Then, diﬀerent amounts of S-1 seeding suspension

Transmission electron microscopy (TEM) images were

was added dropwise to the above clear solution under vigorous collected on a JEM-2100 microscope operating at 200 kV.

stirring. Subsequently, the colloidal silica was added dropwise

The crystalline structures and phase purity were analyzed by

to the above mixture under stirring and agitated for another 3 h X-ray diﬀraction (XRD) in a Rigaku D/Max 2550V diﬀractometer

until a homogeneous gel mixture was obtained. In these steps, using Cu Ka radiation (40 kV, 40 mA), with a scanning angle (2q)

ꢀ

27

ꢀ

ꢁ1

only a small amount of extra water was added except that con- from 5 to 55 at a scanning speed of 8 min

.

2

9

tained in the colloidal silica and S-1 seeding suspension. The

resultant mixture with a molar composition of 100 SiO : x determine the local coordination environments of silicon and

Al : 7 Na O : 1000 H O : y HTAB : z S-1 were transferred into aluminum in the ZSM-5 framework on a Bruker Avance III 500

Si and Al MAS NMR measurements were performed to

2

O

3

2

a 50 mL Teon-lined stainless-steel autoclave and crystallized at MHz spectrometer. The Al(NO

3

)

3

and tetramethylsilane (TMS)

ꢀ

27

29

1

20 C for 48 h in a furnace with autogenous pressure. Aer the were adopted as standard references of

Al and

Si,

33,34

required time, the solution was quenched with cold water to respectively.

room temperature and the precipitated products were collected

The acid amount and strength were tested by NH -temper-

3

by ltration, washed several times with deionised water until ature programmed desorption (NH

3

-TPD) on a Tianjin XQ TP-

sorption, the H-type

ꢀ

neutral pH was reached, dried overnight at 120 C and calcined 5076 Auto TPD system. Prior to NH

3

ꢀ

in the air atmosphere at 550 C for 6 h to remove the organic sample was carefully outgassed at 400 C for 0.5 h under He

ꢀ

template, the crystalline product was achieved. Thereaer, ow to remove the adsorbed moisture, cooled to 100 C and

three consecutive ion exchange reactions of the calcined Na- saturated under NH

form zeolites were carried out at 80 C for 6 h using a 1.0 M purged with He for 2 h to remove the physisorbed NH on the

3

ow for 20 min. Then, the sample was

ꢀ

3

NH NO solution. Lastly, the NH -form zeolites were isolated by surface of the zeolite. Subsequently, NH desorption was carried

4

3

4

3

ꢀ

centrifugation, washed with deionised water twice, dried over- out between 100 and 600 C with a linear heating rate of 10 C

night at 120 C and subsequently calcined at 550 C for 6 h to min

ꢀ

ꢁ1

3

in owing He. The desorbed NH was measured

obtain H-form zeolites.

continuously by means of a thermal conductivity detector

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TCD). The TCD signals were calibrated using 10 mL of NH

Paper

(

3

as compositions of the synthetic solutions and the textural prop-

standard. The number of acid sites was determined from the erties of resultant samples are listed in Table S1 (ESI†).

peak area in their proles. The representative SEM images (low and high magnica-

The acid sites of the samples were also investigated by tions), as shown in Fig. 1A and B, exhibit the crystal size and

pyridine adsorption followed by FTIR spectroscopy. Pyridine morphology of the HA-ZSM-5 zeolite prepared via SSC route.

adsorption-IR (Py-IR) was conducted on a Vertex 70 IR spec- The resultant HA-ZSM-5 zeolite prepared by using HTAB as

ꢁ1

trometer at 4 cm resolution. The sample was pressed into thin

a mesoscale template exhibits well-separated, ellipsoidal

wafer, and then the wafer was placed in an situ cell. The zeolite aggregates with a size of 700–800 nm. Their sponge-like exterior

ꢀ

wafer was degassed for 30 min at 450 C. A spectrum was surface suggests that they are formed by the aggregation of lots

recorded as background aer the wafer was cooled down to of ultrane nanocrystals with a particle size of ꢃ20 nm, which

room temperature. Subsequently, pyridine vapour was intro- are easily distinguishable through the high-magnication SEM

duced into the cell until saturation. The spectra were then image (Fig. 1B). These nanocrystals size was obviously smaller

ꢀ

15

recorded aer evacuation at 200 C for 1 h. The concentrations than that of ZSM-5 aggregates reported in literature.

of Brønsted (B) acid and Lewis (L) acid were calculated from the Comparatively, sample C-ZSM-5 is the one prepared without

HTAB addition, which exhibits a mixture of zeolite crystals and

and the extinction coeﬃcients of 3(B) and 3(L) were 1.88 and amorphous phase (Fig. S2A and B (ESI†)). Zeolite crystals of C-

ꢁ1

peak areas of adsorbed pyridine at around 1540 and 1450 cm

,

ꢁ

1

35

1.42 cm mmol , respectively.

ZSM-5 sample are relatively uniform hexagonal, elongated

Thermogravimetric (TG) analysis was conducted by a SDT600 prismatic shape crystals aggregates with a smoother surface

ꢀ

TGA instrument using a temperature ramp from 30 to 800 C and larger diameter, suggesting that the nanocrystals have

ꢀ

ꢁ1

23

with a heating rate of 10 C min in an air atmosphere.

fused into large crystals without being stabilized by HTAB. In

other words, the C-ZSM-5 zeolite has the typical morphology of

large grains with individual crystal sizes of approximately

ꢃ800 nm (Fig. S2A (ESI†)). High-resolution SEM image Fig. S2B

2

.5 Catalytic performance test

The MTP reaction was carried out at the conditions of 1 atm,

(

ESI†) displays that the C-ZSM-5 structure is quite compact. The

ꢀ

ꢁ1

4

70 C and WHSV (weight hourly space velocity) ¼ 12 h in

primary unit is elongated prismatic shape crystal with sizes of

00–500 nm in length and 20–30 nm in thickness. In addition,

a continuous ow xed-bed quartz-tube reactor (8 mm i.d.)

heated by a furnace.

4

many amorphous materials exist in the C-ZSM-5 sample indi-

cating incomplete crystallization.

In order to further identify the superne nanocrystals

structure, the HA-ZSM-5 zeolites were studied by TEM. The

Before the reaction, the pressed zeolite (0.5 g, 20–40 mesh)

was xed in the middle section of the tube reactor, with two

ends lled with quartz sand. A thermocouple was placed in the

center of the catalyst bed for purpose of monitoring the

temperature. The quartz tube was positioned in a furnace and

aligned ensure that the catalyst was located in the isothermal

Fig. 2A clearly shows that the HA-ZSM-5 zeolites (SiO

2

/Al

2

O

3

¼

167) with rough surface were agglomerated by many superne

nanocrystals with average sizes of about 15 nm. Superne

nanocrystals of aggregates were clearly seen in the picture. The

intercrystal mesoporous distributing homogeneously in the

synthesized HA-ZSM-5 zeolite are directly observed from the

bright area in the TEM image (Fig. 2A). Clear and parallel lattice

fringes spread throughout the entire crystals are visible by the

high-resolution TEM image (Fig. 2B) taken at the edge of the

HA-ZSM-5 sample, indicating that HA-ZSM-5 zeolites are

zone.

ꢀ

Then the catalyst was activated at 470 C for 1 h under N

2



ow. Aer that, the liquid feed (methanol/water molar ratio ¼

1

: 1) was fed into the reactor by a peristaltic pump and purged

2

with N .

All products were analyzed online using a gas chromatog-

raphy (Agilent 7890A) instrument equipped with a ame ioni-

zation detector and a DB-1 capillary column (50 m ꢂ 0.2 mm ꢂ

22

completely crystalline. The dot pattern of selected area elec-

tron diﬀraction (SAED) of the corresponding HA-ZSM-5 zeolite

indicates the single crystalline nature of these nanocrystals

0.5 mm). To ensure that all products were in the gas phase

ꢀ

during the reaction run, the eﬄuent line was heated to 150 C

by a heating belt.

Both methanol and dimethylether (DME) are considered as

the unconverted reactants in the calculation.

(

Fig. 2C), which demonstrates that the introduction of meso-

36,37

pores maintains the single crystal integrity.

Most

3. Results and discussion

In order to explore the structure–function relationship, we used

the SEM, TEM, XRD, N adsorption–desorption, NH -TPD and

Py-IR techniques to characterize selected samples.

2

3

3.1 Morphology observation and control

As seen in Fig. S1A and B (ESI†), the S-1 seeds prepared by the

above mentioned method display an irregular sphere

Fig. 1 Low resolution SEM image (A) and high-resolution SEM image

morphology with the crystal sizes of ca. 170 nm. The (B) of the synthesized HA-ZSM-5 samples.

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ratio of 167, the primary nanocrystals were only ꢃ20 nm in size.

When the SiO /Al O ratio was 200, the primary nanocrystals of

2

2 3

HA-200-2S-5H sample were ꢃ30 nm in size. At SiO /Al O ratio

2

2 3

of 250, the primary nanocrystals of HA-250-2S-5H sample were

ꢃ50 nm in size. Continue to increase the ratio of SiO /Al to

13, the primary nanocrystals of HA-313-2S-5H became more

2

2 3

O

3

larger than HA-250-2S-5H sample (ꢃ100 nm in size). This may

be attributed to the occurrence of particle–particle fusion when

2 2 3

the SiO /Al O ratio of the synthetic gel increases to a certain

value. These results demonstrated that the variation of SiO₂/

Al O ratio in the synthetic gel has a great inuence on the

2

3

morphology and size of resultant zeolites. A reasonable expla-

2 3

nation is that a lower Al-doping content (higher SiO /Al O

2

+

ratio) induces a lower coulomb force between HTA ions and

nanocrystals. The coulomb force is so weak that the synthesis is

similar to conventional hydrothermal synthesis, producing

2 2 3

relatively large nanocrystals when the SiO /Al O ratio is rela-

tively high. On the contrary, ultrane nanocrystals were

produced when the SiO /Al O ratio is relatively low, which is

2

2 3

+

because that the strong coulomb force between HTA ions and

nanocrystals, inhibiting their excessive growth and the stacking

of the ultrane nanocrystals will form HA-ZSM-5. Besides,

Fig. 2 Low resolution TEM image (A) high-resolution TEM image (B)

and the corresponding selected area electron diﬀraction (SAED)

pattern (C) of the synthesized HA-ZSM-5 samples.

23

lower SiO

leading to the generation of more smaller nanocrystals. These

experimental results show that the SiO /Al ratio in the

2 2 3

/Al O ratio means decreased crystallization rate,

26

2

2 3

O

importantly, these ultrane nanocrystals tend to agglomerate synthesis gel is a critical factor in controlling the morphology of

into the larger secondary particles with a loose structure, which HA-ZSM-5.

3

8

may be attributed to their high surface Gibbs free energy. The

abundant intercrystalline mesopores in the HA-ZSM-5 zeolite prepared with diﬀerent seed/SiO

were formed by aggregation of ultrane nanocrystals with sizes SiO in the seeding suspension took up 0.5% (seed/SiO

Fig. S5A–D (ESI†) show the SEM images of the samples

ratio from 0.005 to 0.06, and

ratio ¼

ratio ¼ 0.04)

in the total batch.

2

of 10–20 nm. In view of the low temperature and short crystal- 0.005), 2% (seed/SiO

lization time used in the synthesis (120 C and 48 h, respec- and 6% (seed/SiO

tively), these results indicate the eﬃciency of this method in the SEM observations (Fig. S5A–D (ESI†)) clearly show that all the

preparation of high quality HA-ZSM-5. However, the low samples exhibit ellipsoidal aggregates consisting of superne

magnication and high magnication TEM images (Fig. S3A–C nanocrystals ranging from 20 to 70 nm. As can be seen from the

2

ratio ¼ 0.02), 4% (seed/SiO

2

ꢀ

2

ratio ¼ 0.06) of the SiO

2

(

ESI†)) of C-ZSM-5 sample show that zeolite crystals were SEM images in Fig. S5A–D (ESI†), the aggregates size of the HA-

cohered by many amorphous nanoparticles, which is consistent ZSM-5 decreases from 1000 nm to 850 nm with increasing seed/

with the results obtained from the SEM images. Besides, no SiO

evident mesopores can be seen from the C-ZSM-5 zeolite crys-

tals in the TEM images (Fig. S3A and B (ESI†)).

2

ratio from 0.005 to 0.06.

By adding 0.5% of seeds, the gel of HA-167-0.5S-5H rapidly

crystallizes to large ellipsoidal aggregates with a rough surface

In order to better control the structure of HA-ZSM-5 zeolites, and a particle size of 1000 nm (Fig. S5A, ESI†), indicating that

the inuence of the key synthetic parameters, including SiO

the obtained gel can be quickly converted into HA-ZSM-5 zeolite

Al ratio, seed/SiO ratio and HTAB/SiO

ratio on the when the seed/SiO ratio is only 0.005. The aggregates size of

morphology and structure properties of the resultant zeolite HA-167-2S-5H remains unchanged when the seed amount

was systematically investigated. increases to 2% (Fig. S5B (ESI†), HA-167-2S-5H). When more

2

/

2

O

3

2

Low-magnication SEM images of the HA-ZSM-5 (Fig. S4A–D seeds (4%) are added, zeolite HA-167-4S-5H has a smaller

ESI†)) reveal that all the samples prepared with diﬀerent SiO₂/ aggregates size of about 950 nm (Fig. S5C (ESI†), HA-167-4S-5H).

(

Al

2

O

3

ratios present regular ellipsoidal agglomerates with sizes The aggregates size dramatically decreased from ꢃ950 nm to

of 600–1000 nm. The aggregates sizes of samples HA-167-2S-5H, smaller than 850 nm when the seeds amount further increased

HA-200-2S-5H, HA-250-2S-5H and HA-313-2S-5H are 1000, 800, up to 6%.

600 and 550 nm, respectively. Hence, these SEM images indi-

However, the primary nanocrystals size decreased rst and

then increased with the increasing of the seed/SiO ratio (inset

of Fig. S5A–D, ESI†). The primary nanocrystals size of HA-167-

ratio was

Fig. S4A–D, (ESI†)) clearly show that the primary nanocrystals 0.005. However, the size of the primary nanocrystals decreased

cate that aggregates size of the HA-ZSM-5 decreased as the SiO

2

/

2

Al O ratio used in the synthetic uids increased.

2

3

However, SEM images under high magnication (inset of 0.5S-5H were as large as 70 nm when the seed/SiO

2

size of HA-ZSM-5 became larger with the increase of SiO

ratio. For sample HA-167-2S-5H synthesized with SiO

2

/Al

2

O

3

2

sharply from 70 nm to less than 20 nm with the seed/SiO ratio

increasing from 0.005 to 0.02. As observed in the inset of

2

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Fig. S5A–D,† the smallest nanocrystals size has been obtained in that the self-assembly process gets disturbed. In spite of this,

the case of seed/SiO ¼ 0.02. This may be attributed to the seeds primary nanocrystals sizes of the four samples are much

2

to provide appropriate amount of surfaces to induce the smaller than that of C-ZSM-5 (Fig. S2†). Moreover, when HTAB

nucleation, and then aﬀect the growth and aggregation of was used, the morphology of the synthesized HA-ZSM-5

crystal nucleus, leading to a sharp decrease in primary nano- (Fig. S6A–D (ESI†)) becomes signicantly diﬀerent from that

crystals size of HA-ZSM-5. A further increase of the ratio of seed/ of C-ZSM-5 (Fig. S2 (ESI†)).

SiO

2

up to 0.04 and 0.06, the relatively higher seed/SiO

2

ratio

In light of this, the employed amount of the HTAB has

may cause a high growth rate and the seeds had very little time a major inuence on the crystal size and morphology of resul-

to be dissolved and/or fragmented into small species and then tant HA-ZSM-5 zeolites. It is worth emphasizing that the

15

produce secondary nuclei, and therefore the primary nano- coulomb force between HTAB and nanocrystals must be in

crystals of HA-313-4S-5H and HA-313-6S-5H became more larger a suitable range; if the coulomb force is too strong, the self-

than that of the aforementioned two samples (HA-313-0.5S-5H assembly process of nanocrystals will be aﬀected, if the

and HA-313-2S-5H) (inset of Fig. S5C and D, (ESI†)). There- coulomb force is too weak, it will be unfavourable for zeolites

fore, coordinating the process of growth and nucleation is very crystallization and cannot stabilize the ultrane nanocrystals.

critical for the preparation of HA-ZSM-5. One extreme end (seed/ Therefore, HTAB/SiO

SiO

¼ 0.06) is that the growth process is too fast to produce for the preparation of ultrane HA-ZSM-5 zeolite.

secondary nuclei and the other end (seed/SiO The above results show that the morphology and primary

¼ 0.005) is the

incomplete crystallization of synthetic gel when it progresses nanocrystals size of HA-ZSM-5 zeolite can be controlled by

too slowly. changing the key synthetic parameters of the synthetic solution.

As we all known, the basic behaviors of seeds are to provide

2

ratio in the synthetic gel is very important

2

surface for the synthetic gel and to induce the production of

a large number of secondary nuclei aer their partial dissolu-

3

.2 Textural properties and crystallinity

39–41

tion.

the S-1 seeds (Fig. S1A (ESI†)) and primary nanocrystals of HA- and the corresponding PSD curves of C-ZSM-5 and HA-ZSM-5

ZSM-5 zeolites (Fig. S5B†) conrms the above-mentioned zeolites prepared with diﬀerent SiO /Al ratios. As seen in

In the present study, the obvious diﬀerence between Fig. 3A and B show the N₂adsorption–desorption isotherms

2

2 3

O

results. These nuclei are thought to play a guiding role in the Fig. 3A, the high uptake steps at relatively low pressure (p/p <

0

formation of the target crystalline phases. Consequently, the 0.02) are observed in the N isotherms (Fig. 3A) of HA-ZSM-5,

2

crystallization time is greatly reduced.

In summary, the employed amount of S-1 seeds has an ZSM-5 samples. In addition, all the samples with diﬀerent of

important inuence not only on the aggregates but also on the SiO /Al O ratios exhibit the characteristic features of a combi-

indicating the high microporosity in the single-crystalline HA-

42

2

2 3

primary nanocrystals of the HA-ZSM-5 zeolites.

Fig. S6A–D (ESI†) display the SEM images of the samples was associated with capillary condensation at p/p

prepared with diﬀerent HTAB/SiO ratios. As shown in the SEM taking place in the open mesopores, evidencing that the inter-

images, all the samples exhibit regular ellipsoidal aggregates crystal mesopores have been formed in the HA-ZSM-5 zeolites.

nation of type I + IV isotherms, an obvious hysteresis loop which

0

¼ 0.45–0.99

2

43

whose size varies in the range of 1100–1300 nm and one can see Moreover, these adsorption and desorption isotherms exhibit

the diﬀerence in aggregates size and morphology with the large parallel disposition, further indicating the presence of

44

increase of the HTAB/SiO

ZSM-5 zeolites increased rst and then decreased with the a certain extent, it's easier for the open mesopores to reduce the

increasing of the HTAB/SiO ratio. diﬀusion limitations for large molecules to escape compared to

When the HTAB/SiO ratio was less than 0.05, a mixture of cavities of pores.

amorphous material and HA-ZSM-5 zeolites was obtained It is worthwhile to note that a unusual hysteresis loop in the

Fig. S6A and B (ESI†)). This may be attributed to the strength of N isotherm of the sample HA-250-2S-5H is visible at low rela-

2

ratio. The aggregates size of the HA- open mesopores connected to the external surface. To

2

45

2

(

2

+

coulombic interaction between HTA ions and nanocrystals was tive pressure between 0.10 and 0.30, which is ascribe to a uid-

too weak to stabilize the zeolite nanocrystals. The dandelion- to-crystalline phase transition of the adsorbed nitrogen in

46

like HA-ZSM-5 zeolites were produced when the HTAB/SiO

2

microporosity. This special phenomenon demonstrates that

ratio was above 0.03, which is attributed to the addition of the nanocrystals of the sample HA-250-2S-5H are well crystal-

47

suitable HTAB. Because the HTAB can stabilize the superne lized. The hysteresis loop inseparably linked with uid-to-

nanocrystals by lowering their surface Gibbs free energy, crystalline phase transition gets larger for the high silica HA-

enabling the formation of highly crystalline HA-ZSM-5 zeolites 313-2S-5H zeolite.

23

at short notice.

The PSD curves of samples obtained with diﬀerent SiO₂/

At the relatively low HTAB/SiO ratios from 0.01 to 0.05 (the Al O ratios reveal that the HA-ZSM-5 contained mesopores of

2

2 3

inset of Fig. S6A–C (ESI†)), the primary nanocrystals sizes of HA- a relatively broad size distribution between 3 and 9 nm peaked

ZSM-5 zeolites gradually decrease from 170 nm to 20 nm. With at 3.8 nm. In addition, the mesopore size of the HA-ZSM-5

further increase in HTAB/SiO ratio up to 0.07, the primary zeolite became smaller and narrower with increasing the SiO₂/

2

nanocrystals size of the HA-167-2S-7H became larger than that Al O ratio from 167 to 313 (Fig. 3B).

2

3

of HA-167-2S-5H, which may be because that the strength of

interaction between HTA ions and nanocrystals is so strong nation of type I + IV isotherm, which was associated with the

Comparatively, the C-ZSM-5 sample also displays a combi-

+

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adsorption capacity compared with HA-ZSM-5 zeolites. First of

all, it has a lower V_p_o_r_ethan that of the HA-ZSM-5 zeolites. Also,

2

ꢁ1

2

ꢁ1

the S_B_E_T, S_e_x_tand V

of HA-ZSM-5 (403 m g , 217 m g

micro

3

ꢁ1

2

ꢁ1

and 0.09 cm g ) are higher than that of C-ZSM-5 (311 m g

67 m g and 0.06 cm g ), with the increasing of 30% and

0%, which could be ascribed to the high crystallinity, high

,

2

ꢁ1

3

ꢁ1

1

5

purity and hierarchical structure of HA-ZSM-5. Then again, the

micro of C-ZSM-5 is much lower than HA-ZSM-5 rmly indi-

V

cating that C-ZSM-5 sample may contain amorphous materials.

Fig. 4 shows the powder XRD patterns of C-ZSM-5 and HA-

ZSM-5 zeolites prepared with diﬀerent SiO /Al O ratios from

2

2 3

167 to 313. All the samples exhibit the typical diﬀraction peaks

associated with MFI topology, conrming the successful

synthesis of HA-ZSM-5 zeolites through SSC route. No other

crystalline phases were detected. The relative crystallinity was

ꢀ

calculated by the peak areas between 2q ¼ 7–10 and 22–25

based on the standard sample C-ZSM-5, and the computing

results are summarized in Table S1 (ESI†). The relative crystal-

linity are 107, 108, 109 and 110 for HA-167-2S-5H, HA-200-2S-

5

H, HA-250-2S-5H and HA-313-2S-5H, respectively, based on

the reference crystallinity 100 of C-ZSM-5. Namely, the relative

2 3

crystallinity of HA-ZSM-5 samples increased as the SiO /Al O

2

ratio increased. The relative crystallinity of the C-ZSM-5 is lower

than that of the HA-ZSM-5 zeolites, which is due to their

diﬀerent crystal sizes and morphologies formed by diﬀerent

synthesis conditions. This result is consistent with the SEM and

N sorption studies, demonstrating the incomplete crystalliza-

2

tion of C-ZSM-5.

Fig. S7A and B (ESI†) display the N

isotherms and PSD curves of the HA-ZSM-5 zeolites prepared

with diﬀerent seed/SiO ratios. As shown in Fig. S7A,† all the

2

adsorption–desorption

2

samples exhibit a hybrid of type I and IV isotherm with some

variations in the hysteresis loops. In the isotherms, a sharp

uptake at p/p < 0.02 is associated with microporosity present in

0

the zeolites. All the samples exhibit clear hysteresis loops at

high relative pressure in the range of 0.45–0.99, which mainly

attributes to the capillary condensation in the open mesopores.

Fig. 3

N

2

adsorption isotherms (A) and PSD curves (B) of the HA-ZSM-

/Al ratios.

5

zeolites with diﬀerent SiO

2

2 3

O

capillary condensation at the high relative pressure taking place

in the open mesopores. In addition, C-ZSM-5 shows a low

uptake step in micropore range indicating the presence of

a small quantity of micropores (Vmicro listed in Table S1† only

3

0

.06 g cm ). The PSD curve of C-ZSM-5 demonstrates that the

sample has bi-modal porosity within the range of 3.5–4.4 nm

and 6–90 nm (Fig. 3B). Nevertheless, it is worth noting that the

mesopores or macropores of C-ZSM-5 mainly came from small

sized amorphous materials.

The detailed texture properties of zeolite samples are

depicted in Table S1 (ESI†). As shown in the Table S1 (ESI†), the

increase of the SiO

decrease of both SBET and Vpore, especially for the Vmeso. With

increasing the SiO /Al O ratio from 167 to 313, V decreased

2 2 3

/Al O ratios (from 167 to 313) caused greatly

2

3

meso

3

ꢁ1

3

ꢁ1

from 0.19 cm g to 0.16 cm g (Table S1, ESI†). These results

can be explained by the presence of particle–particle fusion with

2 2 3

the increase of the SiO /Al O ratio, which are well in agreement

with the previous reports. However, C-ZSM-5 has a smaller diﬀerent SiO /Al O ratios.

2 2 3

Fig. 4 XRD patterns of the C-ZSM-5 and HA-ZSM-5 zeolites with

15

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The presence of mesopores is further conrmed by PSD

curves of HA-ZSM-5 samples. As is seen in Fig. S7B,† HA-ZSM-5

zeolites apart from HA-167-6S-5H sample have narrow PSD in

the short range of 3–9 nm. Among these, HA-167-0.5S-5H has

the sharpest and narrowest PSD. In addition, all the samples

exhibit a sharp band centered at ca. 3.8 nm. This band width



rst increased and then decreased with increasing the seed/SiO

2

ratios from 0.005 to 0.04. Moreover, PSD curve of the sample

HA-167-2S-5H exhibits the highest intensity of the band

centered at ca. 3.8 nm. This result may be due to the presence of

the smallest nanocrystals of HA-167-2S-5H sample. Signi-

cantly, HA-167-6S-5H sample exhibits a more intense band

centered at ca. 50 nm in addition to a smaller one at ca. 3.8 nm.

The relative large secondary pores or macropores probably

resulted from the void space between the randomly aggregated

38,48

larger secondary particles.

The textural data are listed in Table S1 (ESI†). With

increasing the seed/SiO ratios from 0.005 to 0.06, the S_e_x_t, V_p_o_r_e

,

2

and V_m_e_s_orst increased and then decreased, and at a seed/SiO₂

ratio of 0.02, the peak values appeared. The HA-167-2S-5H

2

ꢁ1

zeolite exhibits the highest Sext of 201 m

0

g

and Vmeso of

3

ꢁ1

.19 cm g , which can apparently be attributed to its ultrane

nanocrystals with sizes of ꢃ20 nm.

The diﬀerences of texture properties can be attributed to

their diﬀerent morphologies and nanocrystals sizes. The

primary nanocrystals sizes decreased rst and then increased as

the seed/SiO ratios increase; hence the intercrystal mesopores

2

constructed upon the aggregation of ultrane nanocrystals rst

increased and then decreased, which result in the variations of

16

isotherms and the corresponding textural properties.

The XRD patterns of HA-ZSM-5 samples prepared with

diﬀerent seed/SiO ratios from 0.005 to 0.06 are depicted in

2

Fig. S8 (ESI†). These patterns indicate that the high-purity and

high crystallinity HA-ZSM-5 were produced, with no evidence of

the formation of other crystalline phases. The relative crystal-

linity of the resultant HA-ZSM-5 decreases at rst and then

Fig. 5

N

2

adsorption isotherms (A) and PSD curves (B) of the HA-ZSM-

increases as the seed/SiO

2

ratio increasing from 0.005 to 0.06 5 zeolites with diﬀerent HTAB/SiO ratios.

(Table S1, (ESI†)). This result is mainly due to the diﬀerent

nanocrystals sizes of the four samples. The smaller the nano-

crystals size, the lower the degree of relative crystallinity. So the incomplete crystallization. Varying the HTAB/SiO ratio from

2

HA-167-2S-5H sample with the smallest nanocrystals size of 0.03 to 0.07, the PSD of mesoporous broaden rst and then

narrowed. As can be seen, the sample HA-167-2S-3H shows

ꢃ20 nm exhibits the lowest relative crystallinity.

The N adsorption–desorption isotherms and the PSD curves a small and narrow peak in the range of 3.4–4.4 nm centered at

2

of the HA-ZSM-5 zeolites synthesized with diﬀerent HTAB/SiO₂3.8 nm, however, HA-167-2S-5H sample exhibits the higher and

ratios from 0.01 to 0.07 are presented in Fig. 5A and B. All the broader peaks in the range of 3–9 nm centered at around

samples present combined types I and IV isotherm. The high 3.8 nm, reecting its increasing mesoporosity. Like HA-167-2S-

uptake steps in the low relative pressure region of p/p

observed, which indicates that micropores are present. More- peak in the range of 3–5 nm centered at 3.5 nm and the pres-

over, the hysteresis loops at p/p

¼ 0.45–0.99 are considered as ence of mesopores is negligible, which is consistent with its N

capillary condensation in the open mesopores, indicating the adsorption–desorption isotherms.

hierarchical structure of the resultant samples.

As is seen in Table S1 (ESI†), the S_B_E_T, S_e_x_t, V_p_o_r_eand V_m_e_s_oof

0

< 0.01 are 3H, the sample HA-167-2S-7H also shows a small and narrow

0

2

As shown in Fig. 5B, HA-167-2S-1H zeolite shows a bimodal HA-ZSM-5 samples are increases at rst then decreases with

distribution with the porosity around 3.4–4.4 nm and 4.4– increasing the HTAB/SiO ratio from 0.01 to 0.07. In the absence

2

3

0 nm. However, the PSD curves can't reect the real aperture of HTAB, the C-ZSM-5 sample has the lowest S_B_E_Tand V_m_i_c_r_o

distribution when the HTAB/SiO ratio from 0.00 to 0.01, (Table S1 (ESI†)) among the ve samples. With increasing the

because the mesopores of C-ZSM-5 and HA-167-2S-1H partly HTAB/SiO ratio from 0.00 to 0.03, the Vmicro, the SBET are

came from small sized amorphous materials due to their increased and the Vmeso is decreased, which further validate the

2

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incomplete crystallization of C-ZSM-5, HA-167-2S-1H and HA- prepared HA-167-2S-5H was higher than HA-167-2S-1H or HA-

67-2S-3H samples. It is due to the fact that without enough 167-2S-3H. Further increasing the HTAB/SiO ratio from 0.05

1

2

HTAB in the synthesis gel leading to crystallization process to 0.07 saw a progressive decrease in the intensity of the MFI

progresses too slow to completely crystallization of amorphous diﬀraction peaks. This is consistent with the results reported in

49

phase, which are consistent with the SEM images (Fig. S2, S6A the literature. One reasonable explanation is that S-1 seeds

and B†). Moreover, the Vpore of the sample HA-167-2S-5H (0.29 and HTAB are a competitive relationship, not a cooperative

3

ꢁ1

3

ꢁ1

cm g ) was higher than the other two samples (0.23 cm g

relationship under our experimental conditions. Although

3

ꢁ1

for HA-167-2S-1H, 0.24 cm g for HA-167-2S-3H), which was a large amount of mesoporous phase can be produced by add-

attributed to its large mesopore volume. Furthermore, the ing excess HTAB to the synthetic solution, crystallization rate is

sample HA-167-2S-7H has the lower S_B_E_Tand V_p_o_r_ethan that of so slow that cannot ensure the transformation completely from

HA-167-2S-5H, which is because that excessive HTAB inuenced original mesoporous phase to the desired HA-ZSM-5. So, the

the self-assembly process of HA-ZSM-5 zeolites. Thus, the presence of unconvertible mesoporous materials leads to a low

addition of excess HTAB in the synthetic solution is unfav- relatively crystallinity of HA-167-2S-5H sample.

ourable for the formation of HA-ZSM-5 zeolites.

2

7

29

Fig. 6 displays the XRD patterns of HA-ZSM-5 zeolites

prepared with diﬀerent HTAB/SiO ratios from 0 to 0.07, from

3.3

Al and Si MAS NMR analysis

2

In order to study the local coordination environments of silicon

and aluminum and investigate whether all the silicon and

which it could be seen that the typical diﬀraction peaks of the

MFI structure were identied for all the samples. No other

zeolitic phases were observed. Notably, the sample C-ZSM-5

synthesized without HTAB addition has a low crystallinity

despite all other conditions being the same (Fig. 6 and Table S1

ESI†)). On varying the HTAB/SiO ratio from 0.01, 0.03 and 0.05

2

to 0.07, the relative crystallinity of the synthesized HA-ZSM-5

zeolites enhanced rst and then decreased.

2

9

27

aluminum are incorporated in the framework, Si and Al MAS

NMR experiments of the representative HA-ZSM-5 (here, the

sample HA-167-2S-5H was used as a representative) were carried

2

9

out with the spectra as shown in Fig. 7A and B. The MAS Si

NMR spectra of the calcined HA-167-2S-5H zeolite (Fig. 7A)

exhibits a strong resonance with a chemical shi at ꢁ112 ppm,

(

which are associated to Si(OSi) (4Si, 0Al) silica species in the

4

As can be seen from Table S1 (ESI†), the relative crystallinity

of HA-167-2S-1H, HA-167-2S-3H, HA-167-2S-5H and HA-167-2S-

H were 81%, 100%, 107% and 102%, respectively.

HA-167-2S-5H zeolite framework. The weak shoulder peak at

ꢁ

105 ppm can be attributed to (AlO)–Si(OSi)

No signal was detected at ¼ ꢁ100 ppm and ¼ ꢁ92 ppm, indi-

cating that (HO)Si(OSi) (3Si, 1OH) and (HO) Si(OSi) (2Si, 2OH)

silicate species are not present in the HA-167-2S-5H sample.

3

(3Si, 1Al) species.

7

HA-167-2S-5H and HA-167-2S-7H appeared to be highly

3

2

crystalline, whereas HA-167-2S-1H and HA-167-2S-3H samples

were less-crystalline with the signicant presence of amorphous

phase, which can be attributed to the diﬀerence of the coulomb

force between HTAB and nanocrystals at diﬀerent HTAB/SiO

ratios. When the HTAB/SiO ratio is less than 0.05, the HA-167-

S-1H and HA-167-2S-3H samples show low relative crystal-

linity, which could be mainly attributed to their incomplete

crystallization caused by few amounts of HTAB. As the HTAB/

2

SiO ratio was increased to 0.05, the relative crystallinity of the

50

2

7

Al MAS NMR is a very eﬀective technique to investigate the

coordination state of Al species. As shown in Fig. 7B, the MAS

2

7

Al NMR spectrum of calcined HA-167-2S-5H zeolite displays

2

a very strong resonance with a chemical shi at 55 ppm, which

2

is attributed to the tetrahedral-coordinated framework Al with

51

four Al–O–Si bonds. No signal was detected at about 0 ppm,

which indicates that no octahedral extra framework aluminum

existed in the nal product, i.e., all Al atoms have been bonded

52

into the crystal frameworks of the HA-167-2S-5H zeolite. The

absence of extraframework octahedral Al atoms further

conrmed the high crystallinity of the HA-167-2S-5H sample. All

these results demonstrate that the HA-167-2S-5H zeolite

synthesized via SSC route show high crystallinity with a good

distribution of the Al species.

3.4 Surface acidity measurement

The nature and distribution of acid sites are very important for

the catalytic performance of the catalyst. Hence, we compared

the acid sites and acid strength of HA-167-2S-5H and C-ZSM-5

samples. As shown in Fig. 8, both samples exhibit two evident

ꢀ

desorption peaks at 189 C and 400 C representing weak and

strong acid sites, respectively. The TPD peak area represents the

amount of ammonia desorbed from the acid sites of the cata-

lysts, which is proportional to the number of acid sites. Values

of the sample acidity are given in Table S2 (ESI†). As can be seen

from Table S2 (ESI†), both the concentration of weak acid sites

and strong acid sites for HA-167-2S-5H are lower than that of C-

Fig. 6 XRD patterns of the HA-ZSM-5 zeolites synthesized using

diﬀerent HTAB/SiO ratios.

2

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Fig. 8 NH

3

-TPD proﬁles of the HA-ZSM-5 and C-ZSM-5 zeolites.

29

27

Fig. 7

Si MAS NMR (A) and Al (B) spectra of HA-167-2S-5H sample.

ZSM-5. The probable reason for the weak acid strength of HA-

Fig. 9 Py-IR spectra of the HA-167-2S-5H and C-ZSM-5 zeolites.

+

167-2S-5H zeolite is that the presence of HTA partially

inhibits the aluminum from incorporating into the framework

of the zeolite.

3.5 Catalytic performance

The nature of acidity was further characterized by using

pyridine-absorbed FTIR experiments (Fig. 9), in which the In this study, the MTP reaction was selected as a probe reaction

ꢁ

1

vibration band at 1448 cm was associated with Lewis (L) acid to demonstrate the superiority of HA-ZSM-5. The HA-167-2S-5H,

ꢁ

1

1 2 2 3

sites, while the band at 1539 cm was attributed to the vibra- C-ZSM-5 and C -ZSM-5, with the same SiO /Al O ratio (167)

tion of pyridine molecules adsorbed on Brønsted (B) acid sites. were comparatively evaluated under identical conditions. One

The concentrations of B acid and L acid were calculated by the thing to note is that in order to compare the catalysts quickly,

ꢁ

1

ꢁ1

areas of the peaks at 1539 and at 1448 cm , respectively, and the MTP reaction was carried out at high WHSV (12 h ), much

ꢁ

1

17

the computing results are listed in Table S3 (ESI†). As observed higher than that required in industry (about 1 h ). A few

in Fig. 9 and Table S3 (ESI†), the B acid concentration of HA- hours longer of the catalyst lifetime in the evaluation means

1

67-2S-5H sample is higher than that of the C-ZSM-5 sample, much longer catalyst life in the commercial reactors.

while the L acid concentration of HA-167-2S-5H sample is much

Catalytic activity versus time on stream over HA-167-2S-5H, C-

lower than that of the ZSM-5 sample, which can be attributed to ZSM-5 and C -ZSM-5 samples is displayed in Fig. 10. At the

the sample's unique structure framework.

initial reaction stage, all studied catalysts present a high initial

Based on the above results, we can conclude that the addi- activity of approximately 100%, probably due to the availability

1

53

tion of HTAB into the synthetic solution can also adjust the of the most acid sites for methanol. However, as the reaction

concentration of B acid and L acid and the distribution of acid proceeds, all selected catalysts are reducing their catalytic

sites, in addition to stabilizing ultrane nanocrystals.

activity with diﬀerent deactivation rates, which was also likely

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area, high mesopore volume and short diﬀusion path of HA-

167-2S-5H facilitate the removing of primary products and the

coke precursor from the active centers of the catalysts, and then

reduce the secondary reactions for hindering coke deposition.

Earlier studies have proved that the weak acid sites resulted

in the conversion of methanol to DME while the strong acid

sites resulted not only in the conversion of DME (and methanol)

to light olens but also side products (polyolens, aromatic

58,59

compounds and carbon deposits).

Additionally, the strong

acidity also enhances coke formation and catalyst deactiva-

60

tion, whereas the weak acidity could eﬃciently restrain various

59

side-reactions for depressing coke deposition. So, both the

weak acid sites and strong acid sites are of equal importance

and the strong to weak acidity ratio has a signicant impact on

catalytic activity of catalysts in MTP reaction.

Fig. 10 Methanol conversion and selectivity vs. reaction time: T ¼

ꢀ

ꢁ1

4

70 C, P ¼ 1 bar, n (methanol) : n (water) ¼ 1 : 1, WHSV ¼ 12 h

.

The relatively low total acid amount of HA-167-2S-5H sample

(

Fig. 8 and Table S2 (ESI†)) hinders side reactions (alkylation,

isomerization, hydrogen transfer, and aromatization and so on)

attributed to the diﬀerent coverage of acid sites and blockage of on strong acid sites, decreasing coke formation and retarding

54,55

56

the zeolite channels by coke deposit.

Here, the catalytic catalyst deactivation. So the lowered acid strength of the HA-

lifetime of catalyst is set as the running time at which methanol 167-2S-5H sample is responsible for its prominent catalytic

conversion above 95%, which is marked with a dashed line in stability. Again, considering the lowest content of the

+

Fig. 10. As seen, their catalytic lifetime follows 22 h, 6 h and

.5 h with order of HA-167-2S-5H > C-ZSM-5 > C

HA-167-2S-5H maintains prominent catalytic stability and its was suppressed.

catalytic lifetime is nearly 3.7 and 14.7 times longer than that of

Moreover, the enhanced stability of the HA-167-2S-5H can

the C-ZSM-5 and C

-ZSM-5, respectively. However, C-ZSM-5 also be attributed to its excellent accessibility of acid sites.

C

5

compounds produced on the HA-167-2S-5H, it was reason-

1

-ZSM-5. The able to conclude that the formation of coke deposition in zeolite

61

1

exhibits a faster deactivation and presents the shorter lifetime Consequently, secondary reaction, which will eventually lead to

compared to HA-167-2S-5H. Especially, C -ZSM-5 deactivated coking, is largely restrained. So the appropriate total acid

1

very quickly and presents the shortest lifetime among all the amount and superior accessibility are also responsible for its

studied catalysts.

enhanced catalytic lifetime. Hence, weak carbon deposition and

The average selectivities of the products at steady stage are weak sensitivity to more carbon deposits slowed the deactiva-

listed in Table S4 (ESI†). Clearly, in comparison with the C-ZSM- tion of the HA-167-2S-5H, which will be favorable to the oper-

5

and C

tivities toward light olens (C

and propylene (40.1%), while their selectivities toward ethylene located in the micropores of the ZSM-5 and closely related to the

1

-ZSM-5, the HA-167-2S-5H exhibits the highest selec- ation of the long-term methanol conversion.

¼

4

–C

(68.5%)), butylenes (22.9%)

Besides, it is well known that the most B acid sites are

2

+

(

5.5%), C –C alkanes (3.7%) and C hydrocarbons (27.8%) are suppression of the secondary reactions, it thus has unique

1 4 5

62

lowest (Table S4, ESI†), thus resulting in the highest ratio of shape selectivity. So, the HA-167-2S-5H catalyst presents the

propylene/ethylene (P/E ¼ 7.3). The average propylene selec- high propylene selectivity possibly because of its appropriate B

61

tivity of the HA-167-2S-5H sample at steady stage reaches as acidity and B/L ratio, but meanwhile the relatively low total

high as 40.1%, 7.0% and 1.4% higher than that of the C-ZSM-5 acid amount is a critical factor for its long catalytic lifetime

and C

The obvious diﬀerence in the catalytic activities for MTP

reaction over the HA-167-2S-5H, C-ZSM-5 and C

1

-ZSM-5, respectively.

(Tables S2 and S3 (ESI†)).

As a consequence, the synergistic eﬀects of the excellent

-ZSM-5 samples structural properties, appropriate acidity, suitable strong/weak

1

can be explained by their diﬀerent structural properties (PSD acid sites ratio, good accessibility of acidic sites and high

and channel length) and acidic properties (the number, crystallinity contribute to the enhancement of the catalytic

56

strength and accessibility of the acid sites).

Firstly, the superne nanocrystals of well-crystallized HA-

performance.

In contrast, the C-ZSM-5 sample synthesized without HTAB

167-2S-5H prepared with HTAB addition shorten the diﬀusion addition only delivered a lifetime of 6 h under the identical

path of methanol and products, increasing the active sites conditions. As mentioned earlier, detailed characterization

accessibility and reducing the diﬀusional limitations for various have conrmed that (SEM, TEM, XRD and BET analysis) C-ZSM-

products to escape, which could eﬀectively suppress the 5 sample has a low crystallinity and only a portion of this

formation of higher olens, paraﬃns and aromatics and could material possesses small-sized micropores which contain low B

play a more important role in desired products selectivities acid amount. Note that no mesopores can be formed in zeolite

enhancement. In addition, for the HA-167-2S-5H catalyst with crystals of C-ZSM-5 and the mesopores or macropores mainly

intercrystal mesopores, carbon deposition is mainly formed at came from small sized amorphous materials. Moreover, large

57

the outer surface and/or mesopores. Hence, the large surface crystal particles of C-ZSM-5 sample lead to longer diﬀusion

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paths, increasing diﬀusion limitations for various products to

spread out and decreasing the active site accessibility. Hence,

the pure microporosity of C-ZSM-5 zeolite cannot aﬀord the

transport and diﬀusion of bulky substrates within the intra-

zeolitic space, and thereby the reaction-produced products with

larger molecular sizes would be more likely to become trapped

and eventually lead to carbon deposition. So, the solely micro-

porous C-ZSM-5 catalyst is progressively deactivated as the

micropores are lled with coke deposits, or their channels

63

become blocked. Hence, the microporous C-ZSM-5 catalyst is

not capable of enduring higher space velocity for a longer

period of time than the HA-167-2S-5H catalyst.

It is believed that the excellent textural properties are the key

to aﬀord a long catalytic lifetime and a high propylene selec-

tivity in the MTP reaction. Otherwise, the high density of acid

59

sites will accelerate coking, i.e. deactivation. Hence, in this

study, the high value of L acid sites of C-ZSM-5 may bring about

the rapid deactivation. Aiming to investigate this possibility, the

mixture of C-ZSM-5 and mesoporous SiO (denoted as C -ZSM-

2

5

) was tested in MTP under the same conditions as C-ZSM-5 for

comparison in catalytic performance (C-ZSM-5 : SiO

¼ 1 : 1 wt/

wt) (mesoporous SiO : purchased from Jicang nano technology

Co., Ltd., Nanjing, China; this sample shows a high BET specic

2

ꢁ1

surface area (745.34 m g ) consisting of uniform mesopores of

.5 nm without any microporosity; Si/Al atomic ratio (N)). That

2

is, the modication of acid sites by inactive silica may be

eﬀective to prove it. The catalytic activity of C -ZSM-5 sample as

2

a function of time is displayed in Fig. 11A and the average

selectivity of the products at steady stage are listed in Table S4

Fig. 11 Conversion of methanol and selectivity of propylene over

2

zeolites C -ZSM-5 (A) and HA-167-2S-7H (B). Reaction conditions:

ꢁ1

ꢀ

(

ESI†). Unfortunately, the C

than the HA-167-2S-5H zeolite and only slight diﬀerences in

catalytic activity and selectivity were observed between the C

2

-ZSM-5 sample shows worse results WHSV ¼ 12 h , T ¼ 470 C, n(CH

3

OH) : n(H

2

O) ¼ 1 : 1, P ¼ 1 bar.

2

-

ZSM-5 and C-ZSM-5 samples. Compared to the C-ZSM-5 sample,

As we all known, the acidity, including the acid intensity is

¼

C -ZSM-5 shows lower selectivities for light olens (C – viewed as a vital factor in determining catalytic performance in

2

¼

61

C

(59.7% vs. 61.4%)), ethylene (9.9% vs. 11.1%) and propylene the MTP reaction. However, in this case, for the C-ZSM-5

4

(

31.9% vs. 33.1%), while their selectivities toward C –C alkanes sample, the situation is diﬀerent because it is constituted by

1 4

6.1% vs. 5.3%), butylenes (17.9% vs. 17.2%) and bulk zeolite crystals with pure microporosity and amorphous

+

5

C

hydrocarbons (34.6% vs. 33.3%) are higher (Table S4, ESI†), mesoporous particles. As previously noted, the narrow channels

thus resulting in a higher ratio of propylene/ethylene (P/E ¼ of bulk zeolite crystals strongly impose a restriction on the

.2). On the other hand, the catalytic lifetime of C -ZSM-5 did transport of product molecules, leading to an increase of

3

2

not show any changes compared to that of C-ZSM-5 (6 h vs. 6 h), various secondary reactions which may cause relatively easy

suggesting that the lowered acid concentration and strength did coke formation and deactivation of catalysts. Based on the

not slow down the deactivation of the catalysts and the simple experimental results and the above analysis, it is reasonably

dilution of acid sites of C-ZSM-5 with mesoporous SiO₂had speculated that the inferior textural properties (the pure

a negligible impact on the catalytic performance. Hence, the microporosity, low S_B_E_T, low V_m_i_c_r_oand low S_e_x_t), poor accessi-

above experimental evidence converged to prove that the high bility of acidic sites and low relative crystallinity are the main

value of L acid sites could not be responsible for the rapid reasons behind the short lifetime and low target products

deactivation of C-ZSM-5. Taking into account the superior selectivity of C-ZSM-5, rather than the high value of L acid sites.

textural properties of mesoporous SiO

and uniform mesopores), the cause of the lower activity 1.5 h and relatively higher olen (C –C molecules: 66%) and

2

(very high surface area

1

The C -ZSM-5 catalyst exhibits the shortest lifetime of only

¼

2

4

observed for the C -ZSM-5 sample compared to the HA-167-2S- propylene (38.7%) selectivity compared with C-ZSM-5 catalyst.

2

5H catalyst is to be due to its inferior textural properties, i.e. However, it undergoes rapid coking reactions and shows

low Vmicro, poor accessibility of acidic sites and low crystallinity. a sharp deactivation, which can also be explained by its struc-

It is particularly noteworthy that dilution of catalyst with mes- tural properties and acidic properties. In addition to the

+

oporous SiO

2

can eﬀectively improve the heat conduction in the shortened methanol transfer paths, the formation of C₅was

bed, which ensures the steady processing of MTP reaction.

also more or less depressed accordingly (Table S4 (ESI†)).

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64,65

However, the lowest total acid amount and the highest strong to blockage.

The secondary reactions occurred on the acidic

weak acidity ratio (Table S2 (ESI†)) accelerate deactivation of C₁- sites are the main reason for carbon deposition on the catalyst

66

ZSM-5 catalyst. In addition, as clearly shown in Table S2 (ESI†), surface. Thus, it is necessary to investigate their coking

the lowest ratio of B/L and ultra-high L acid amount were also behavior for gaining insight into the dramatic catalytic activity

very unfavourable for the catalytic activity of C

1

-ZSM-5 catalyst and selectivity improvement by the hierarchical pores design.

in MTP reaction. As expected, C -ZSM-5 holding more strong

1

Fig. S9 (ESI†) presents the coke deposition on the deactivated

and less weak acid sites has the shortest lifetime among three

samples.

C

1

-ZSM-5, C-ZSM-5, and HA-167-2S-5H catalysts measured by

TG analysis. As can be seen, HA-167-2S-5H contains 2.73 wt%

In order to prove the reproducibility of the catalytic perfor- coke deposit aer 22 h time on stream, while for C-ZSM-5 and

mance of the HA-ZSM-5 zeolites, we synthesized and evaluated C -ZSM-5 the coke deposit are 1.92 wt% and 1.41 wt% aer 6 h

1

the HA-167-2S-5H several times and the results indicated that and 1.5 h, respectively. The average coking rate on HA-167-2S-

the data in Fig. 10 and Table S4 (ESI†) was reliable. Additionally, 5H is 0.12% per hour, much lower than that of 0.32% and

the catalytic performance over another HA-ZSM-5 zeolite (HA- 0.94% on C-ZSM-5 and C

1

-ZSM-5 catalysts. Accordingly, negli-

167-2S-7H) for MTP reaction was also evaluated under iden- gible coke deposition and less sensitive to the more coke

tical conditions. Catalytic activity versus time on stream over deposits can be helpful for enhancement of the selectivity of

HA-167-2S-7H sample is displayed in Fig. 11B. As seen, like HA- target products (propylene, butylenes and light olens) and

1

67-2S-5H, the HA-167-2S-7H also exhibited prominent stability especially catalytic lifetime.

enhancement and its catalytic lifetime (20 h) is nearly 3.3 and

3.3 times longer than that of the C-ZSM-5 (6 h) and C -ZSM-5

1

3

.6 Formation mechanism of HA-ZSM-5 zeolites

The average selectivities of the products at steady stage are In order to investigate the formation mechanism of HA-ZSM-5

listed in Table S4 (ESI†). Clearly, in comparison with the C-ZSM- prepared via SSC route, the crystallization process was moni-

and C -ZSM-5, the HA-167-2S-7H exhibits the highest selec- tored by characterization of samples obtained at diﬀerent

tivities toward light olens (C

and propylene (40.3%), while their selectivities toward ethylene samples with a molar composition of 100 SiO

(1.5 h), respectively.

5

1

¼

4

2

–C

(68.5%)), butylenes (22.9%) crystallization time of 6, 12, 18, 24, 30 and 36 h. The resultant

: 0.6 Al : 7

5.3%), C –C alkanes (2.9%) and C hydrocarbons (28.6%) are Na O : 1000 H O : 5 HTAB : 2 S-1 were systematically charac-

2

2 3

O

+

5

(

1

4

2

lowest (Table S4, ESI†), thus resulting in the highest ratio of terized by various techniques (including XRD, BET, and SEM).

The XRD patterns of samples prepared at diﬀerent crystal-

propylene/ethylene (P/E ¼ 7.6). The average propylene selec-

tivity of the HA-167-2S-7H sample at steady stage reaches as lization times are depicted in Fig. S10 (ESI†), and the relative

high as 40.3%, 7.2% and 1.6% higher than that of the C-ZSM-5 crystallinity of corresponding samples are listed in Table S5

and C

1

-ZSM-5, respectively.

(ESI†). As shown in Fig. S10 (ESI†), no obvious characteristic

In addition to the similar desired products distribution, the peaks of MFI phase are observed aer crystallizing for 18 h,

catalytic lifetime of HA-167-2S-7H was slightly shorter than the indicating the amorphous nature of the obtained samples.

HA-167-2S-5H catalyst (20 h vs. 22 h), indicating that the addi-

Aer crystallization for 24 h, a few subtle characteristic peaks

tion of an appropriate amount of HTAB to the zeolite synthesis related to the MFI phase begin to appear and the relative crys-

mixture was benecial to the formation of the well-dened HA- tallinity of corresponding sample is merely 61%, which implies

ZSM-5 zeolite with desired structural properties and thus the incomplete crystallization of the raw materials. Also, these

resulting in an enhancement of the catalytic activity and selec- results illustrate that the primary nucleation induction period is

tivity. Meanwhile, excess HTAB was disadvantageous to the relatively long for the preparation of HA-ZSM-5 zeolites.

formation of the highly crystalline HA-ZSM-5 zeolite with hier-

The diﬀraction peak intensity of MFI topology increased

archical pores and hence caused a slightly decrease in catalytic rapidly aer crystallization for 30 h. This result demonstrates

activity. These results, which are again consistent with the that a rapid crystal growth appeared aer the crystallization for



ndings from the SEM, XRD and BET analysis.

As mentioned earlier, higher activity and superior selectivity

24 h.

The synthesized sample exhibits well-resolved diﬀraction

of HA-167-2S-7H should be contributed to by the excellent peaks related to the MFI structure aer crystallization for 36 h.

structural properties, appropriate acidity, suitable strong/weak In spite of this, the diﬀraction peaks intensity of MFI topology

acid sites ratio, good accessibility of acidic sites and high increased gradually when the crystallization time increased

crystallinity (Tables S1–S4 (ESI†)).

from 30 h to 36 h, which suggest that the HA-ZSM-5 zeolite

In conclusion, the improved catalytic stability and propylene gradually grows at a relatively slow rate aer the crystallization

selectivity of HA-167-2S-7H sample further conrm the repro- for 30 h.

ducibility of the catalytic performance. Meanwhile, these

2

N adsorption–desorption measurements were performed to

experimental results indicate the necessity of the combination observe the variation of the structural properties of samples

of HTAB and SSC method for the preparation of highly stable prepared with diﬀerent crystallization times. The N₂adsorp-

MTP catalyst.

Recent studies have proved that catalyst deactivation in the are depicted in Fig. S11 and S12 (ESI†), respectively. The uptake

MTP reaction is mainly caused by carbon deposition, for which at p/p < 0.02 become more and more apparent with the increase

coke deposition could cover the acid sites or make channels of crystallization time (see Fig. S11 (ESI†)), indicating that the

tion–desorption isotherms and the PSD curves of the samples

0

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microporous materials are continuously generated with the aggregates was quite rough, as shown in Fig. 12 (12 h) and inset.

increase of crystallization time. In addition, all samples ob- In addition, a large number of small holes appear in the

tained at diﬀerent crystallization times display a hybrid between external and internal surfaces of the irregular aggregates (see

type I and type IV isotherms, indicating the existence of inter- Fig. 12 (12 h)). Meanwhile, the irregular lumps and seeds dis-

crystal mesopores. Additionally, the hysteresis loop grew appeared completely. These phenomena indicate that the

smaller and it gradually shied to the low pressure region with original mesoporous materials are dissolved into the basic unit

the increase of the crystallization time (Fig. S11 (ESI†)). PSD of ZSM-5 formation. At the same time, the S-1 seeds partly

curves (Fig. S12 (ESI†)) also show that mesopore size decreases dissolved into a small amount of subnanocrystals with MFI

1

with increasing crystallization time from 6 h to 36 h. The above structure.

results conrmed that the original mesoporous materials are

dissolved gradually to form a hierarchical structure.

With the decrease of the irregular aggregates size, more

subnanocrystals are formed by consuming the original meso-

67,68

As shown in Table S5 (ESI†), the S_B_E_Tof the samples porous materials aer crystallization for 18 h.

increases with the crystallization time increasing; the Sext tends

As the time is prolonged to 24 h, a large number of sub-

to increase; the Vmicro of the samples increases gradually, nanocrystals appeared, simultaneously, these subnanocrystals

whereas the Vpore and Vmeso decreases. These results are also are locally rearranged to form a small amount of superne

ascribed to the dissolution of original mesoporous materials nanocrystals. At the same time, the superne nanocrystals were

and thereby the formation of HA-ZSM-5 zeolite. The variation of further aggregated into a small amount of irregular spheres

structural properties with crystallization time is consistent with with rough surface (see Fig. 12 (24 h)). Obviously, more and

the XRD and SEM analyses.

more un-uniform irregular aggregates were obtained by

To clarify the structural evolution of the HA-ZSM-5 zeolites, consuming superne nanocrystals.

the samples morphology during diﬀerent periods of crystalli-

Aer crystallization for 30 h, the sample presents the

zation is captured. As displayed in Fig. 12 and inset, small morphology of sole ellipsoidal aggregates composed of

lumps with irregular shape and some seeds could be observed numerous superne nanocrystals which is similar to that of HA-

aer 6 h crystallization (Fig. 12 (6 h) and inset). As mentioned 167-2S-5H. Notably, a small amount of amorphous materials

above, this result may be attributed to the dissolution of the can be observed (Fig. 12 (30 h)).

original mesoporous materials and the added S-1 seeds were

enwrapped by numerous small sized amorphous materials.

Further extending the crystallization time to 36 h, the ellip-

soidal aggregates size becomes larger and the surface of

At the crystallization time of 12 h, some irregular aggregates aggregates becomes rougher (Fig. 12 (36 h)). The HA-ZSM-5

composed of a large number of superne amorphous nano- zeolite with high purity and high crystallinity was completely

particles with size of about 10 nm and a small amount of sub- synthesized along with the disappearance of amorphous

nanocrystals are visible, and the surface of the irregular materials. Therefore, the proper crystallization time is very

crucial to the formation of HA-ZSM-5 zeolite.

16,19,30,46,69–72

Based on our previous studies

and the above

discussion, the possible formation mechanism of HA-ZSM-5 is

proposed in Scheme 1.

As depicted in Scheme 1, the starting step involves the

preparation of original mesoporous materials using HTAB as

a mesoporous directing agent.

At the beginning of SSC treatment, the original mesoporous

materials dissolved gradually into numerous small sized

amorphous nanoparticles and reorganized under highly alka-

line condition, simultaneously, the S-1 seeds were partially

Fig. 12 The SEM images of HA-ZSM-5 samples at diﬀerent crystalli- Scheme 1 Proposed formation process for the synthesis of HA-ZSM-5

zation time.

zeolites.

25618 | RSC Adv., 2017, 7, 25605–25620

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dissolved into a handful of subnanocrystals. Some irregular bulky molecules and provides a new perspective on the

aggregates were formed by a large number of superne amor- synthesis of HSZs, where previously only large organic mole-

phous nanoparticles with size of about 10 nm.

cules were employed as templates. More importantly, we believe

With the induction of the subnanocrystals and the dissolu- that this approach can also be utilized to design and fabricate

tion of the amorphous silica-alumina gel, a large number of other HSZs (such as BETA, FAU, modernite and zeolite X and Y).

subnanocrystals are formed, meanwhile, these subnanocrystals

are locally rearranged to form superne nanocrystals. At this

Acknowledgements

stage, the induction and dissolution of subnanocrystals occur

1

simultaneously. Moreover, HTAB forms small aggregates at

We acknowledge nancial support from the National Natural

Science Foundation of China (Grant No. 21276183).

ꢀ

relatively lower temperature (120 C) to stabilize the superne

nanocrystals by reducing the surface Gibbs free energy of

23

nanoparticles. These subnanocrystals interacted with HTAB

and the strong coulomb force between them will inhibit the

excessive growth of the crystal.

Notes and references

In the following step, numerous superne nanocrystals

aggregated due to their high Gibbs free energy, grow up grad-

ually and nally constantly consume neighboring raw materials

to form irregular aggregates in small size.

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25620 | RSC Adv., 2017, 7, 25605–25620

Article Doi

Controllable fabrication of single-crystalline, ultrafine and high-silica hierarchical ZSM-5 aggregates: Via solid-like state conversion

DOI: 10.1039/c7ra03962j

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Full text of DOI:10.1039/c7ra03962j

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