Morphology control of self-stacked silicalite-1 crystals using microwave-assisted solvothermal synthesis

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Microporous and Mesoporous Materials 104 (2007) 296–304

Morphology control of self-stacked silicalite-1 crystals usingmicrowave-assisted solvothermal synthesis

Xiaoxin Chen a, Wenfu Yan a, Wanling Shen b, Jihong Yu a,*, Xuejing Cao a, Ruren Xu a

a State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR Chinab State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics,

The Chinese Academy of Sciences, Wuhan 430071, PR China

Received 19 October 2006; received in revised form 5 February 2007; accepted 16 May 2007Available online 24 May 2007

Abstract

Morphology control of silicalite-1 (Si-MFI) crystals was achieved by employing microwave-assisted solvothermal synthesis in thereaction system TEOS–TPAOH-alcohol using dipolar alcohols as a co-solvent. The influences of various experimental parameters, suchas dielectric parameters of the alcohols, aging time, and the amount of the alcohols in the reaction mixture on the morphology of theresulting Si-MFI crystals were investigated. Our studies show that self-stacked Si-MFI crystals could be prepared by using alcohols withlow dielectric constants as a co-solvent with short aging time and high concentration in the reaction mixture under microwave radiation.The resulting products were further characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and 29Si MASNMR analyses.� 2007 Elsevier Inc. All rights reserved.

Keywords: Zeolites; MFI; Microwave radiation; Solvothermal synthesis; Morphology

1. Introduction

Zeolites have been widely used as catalysts, adsorbents,ion exchangers, and microreactors in petroleum refiningand fine chemical industry [1–4]. Studies show that the cat-alytic performance, adsorption, separation and ionexchange properties of zeolites are strongly affected bytheir size and morphologies [5–9]. Many emerging applica-tions of zeolite materials require precise control of size andmorphology [9–11], and therefore, the development of syn-thetic strategies for the control of crystal size and morphol-ogy of zeolites is increasingly important.

Microwave (MW) dielectric heating has been widelyused in many recent chemical reactions, such as organicand inorganic synthesis, selective sorptions, oxidation–reductions, and polymerizations among many other pro-cesses [12,13]. Microwave-assisted synthesis is generally

1387-1811/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2007.05.015

* Corresponding author. Tel./fax: +86 431 85168608.E-mail address: [email protected] (J. Yu).

much faster, cleaner and more energy efficient than theconventional methods [14,15]. Recently, MW-assistedhydrothermal synthesis has been applied for the synthesisof a number of zeolites, such as zeolite A (LTA), faujasite(FAU), sodalite (SOD), analcime (ANA), beta (BEA),ZSM-5 (MFI), silicalite-1 (Si-MFI), AlPO4-5 (AFI), VPI-5 (VFI), and cloverite (-CLO), etc. [16–23]. This methodcan efficiently control the morphology of zeolites [24,25].

Recently, Hwang et al. reported MW-assisted fabrica-tion of Ti-MFI zeolite crystals with fibrous morphology,which exhibited enhanced shape-selective adsorption ofxylene isomers and might have potential applications inmicroreactors and separation technology [26]. The stackingof crystals under MW radiation appears to be related to thedipole moment strength of the Ti–O bonds, which arestrongly activated by MW absorption and undergo con-densation reactions to form Ti–O–Ti and/or Ti–O–Sibonds between crystals.

In this work, by adding alcohols with low dielectricconstant to the reaction solution TEOS–TPA, self-stacked

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Table 1The summary of the synthetic parameters and morphologies of theresulting Si-MFI crystals

Sampleno.

Co-solvent Dielectricconstant

Morphology ofproduct

1 Ethyleneglycol

37.0 Isolated single crystals

2 Methanol 32.6 Isolated single crystals3 Ethanol 24.3 Isolated single crystals4 1-Propanol 20.1 Stacked + isolated5 Isopropanol 18.3 Stacked crystals6 n-Butanol 17.8 Stacked crystals7 Hexanol 13.3 Stacked crystals

Aging time: 24 h; reaction composition (in molar ratio): TPAOH:SiO2:Alcohol:EtOH:H2O = 0.357:1:8:4:21; Microwave condition: t1 =90 min, T1 = 80 �C, P1 = 250 W; t2 = 60 min, T2 = 180 �C, P2 = 400 W.

X. Chen et al. / Microporous and Mesoporous Materials 104 (2007) 296–304 297

Si-MFI crystals could be successfully prepared. The syn-thetic parameters, such as dielectric parameters of the alco-hols, aging time, and the amount of alcohols in the reactionsolution on the morphology of the resulting Si-MFI crys-tals have been investigated.

2. Experimental section

The reaction solutions for the crystallization of Si-MFIwere prepared by mixing tetraethyl orthosilicate (TEOS,98 wt%), tetrapropylammonium hydroxide (TPAOH,16.5 wt%), and various alcohols including ethylene glycol,methanol, ethanol, 1-propanol, isopropanol, n-butanol,and hexanol in the molar ratio 1.0 SiO2:0.357 TPAOH:xalcohol:4.0 EtOH:1H2O, where x is 2, 4, or 8 for eachbatch.

A synthesis solution was prepared by adding aqueoustetrapropylammonium hydroxide solution to tetraethylorthosilicate, and then co-solvent was added dropwise tothe above solution followed by strong agitation. The reac-tion solution was stirred in a sealed vessel at ambient tem-perature for different period for the purpose of aging beforeit was loaded into a Teflon autoclave. The crystallizationwas conducted in a microwave oven (Milestone ETHOS-D) with pre-programmed heating profiles. The microwavesynthesis was typically conducted in two steps. For eachstep, the synthetic conditions (temperature, duration, max-imum power) were defined as (T1, t1,P1) and (T2, t2,P2),respectively. The product was separated by centrifugation,washed thoroughly with deionized water and ethanol, andthen dried overnight at 80 �C.

Powder X-ray diffraction (XRD) patterns were recordedon a Siemens D5005 diffractometer with CuKa radiation(k = 1.5418 A). The scanning electron microscope (SEM)images were taken on a JEOL JSM-6700F scanning elec-tron microscope. Solid-State NMR experiments were per-formed on a Varian Infinity-plus 300 spectrometeroperating at a magnetic field strength of 9.4 T. The reso-nance frequency at this field strength was 59.6 MHz for29Si. Single-pulse 29Si MAS NMR experiments with 1Hdecoupling were performed with a 90� pluse length of4.4 ls, a 100 s pulse delay and a spinning rate of 4 kHz.All the 29Si MAS spectra were collected by using the same7.5 mm probe. TMS (0 ppm) was used as external chemicalshift standard for 29Si.

3. Results and discussion

Microwave is a form of electromagnetic energy. Theenergy of the microwave could be effectively transferredto the target polar molecules located in the rapidly oscillat-ing electrical field of microwaves. In microwave reactioninvestigated in this study, the dipolar alcohols act as boththe solvents and good microwave absorbents. The self-stacked Si-MFI crystals were solvothermally synthesizedunder microwave radiation in the presence of differentdipolar alcohols as a co-solvent. The morphology of the

resulting Si-MFI crystals were affected by various experi-mental parameters such as the dielectric constant of thealcohol co-solvent, the aging time, and the amount ofthe alcohol co-solvents, which will be discussed in detailbelow.

3.1. The polarity of alcohols

In this study, alcohols with different polarities repre-sented by dielectric constant EN

T , such as methanol, ethanol,ethylene glycol, 1-propanol, isopropanol, n-butanol, andhexanol were added to the starting mixture TPAOH–TEOS, with a molar ratio of 0.357:1:8 for TPAOH:TEOS:alcohol. Detailed information on the syntheticparameters and resulting crystal morphologies are summa-rized in Table 1.

The starting mixture was strongly stirred for 24 h for thepurpose of aging before it was loaded into a Teflon auto-clave. The synthesis involved a two-step heating [27,28].The first step was conducted at T1 = 80 �C andP1 = 250 W for t1 = 90 min. The second step was con-ducted at T2 = 180 �C and P2 = 400 W for t2 = 60 min. Itwas found that the polarity of alcohols significantly affectsthe morphology of the resulting Si-MFI crystals. The alco-hols with relatively high polarity (dielectric constant)resulted in isolated single crystals of Si-MFI, while alcoholswith relative low polarity gave self-stacked Si-MFI crystals.As shown in Fig. 1a–c, when methanol, ethanol, and ethyl-ene glycol with relative high dielectric constants were usedas the co-solvent, the crystal shape is well defined. Furtherdecrease in the dielectric constants of the co-solventresulted in the loss of well defined shape of the Si-MFIcrystals, as well as the stacking of individual crystals(Fig. 1d–g). For instance, the clear angular hexagonalshape of the crystals tends to become rounder and unclear,and the individual crystals are partially stacked when 1-propanol was used as co-solvent (Fig. 1d). The well definedangular hexagonal shape of the crystals is totally lost andself-stacked Si-MFI crystals formed when isopropanol, n-butanol, and hexanol with lower dielectric constants than

298 X. Chen et al. / Microporous and Mesoporous Materials 104 (2007) 296–304

1-propanol were used as co-solvent (Fig. 1e–g). The crys-tals are stacked on top of one another along their b direc-tion to form a self-stacked morphology. The ‘‘fiber’’ cannotbe destroyed even by long time and strong ultrasonication,which indicates that strong chemical bonds might existbetween the individual crystals. This speculation is furthersupported by the 29Si MAS NMR analyses, which will bediscussed later. The powder X-ray diffraction patterns ofthe resulting products show characteristic peaks for theMFI structure without any impurities.

Above results imply a strong correlation between thepolarity (dielectric constant) of the co-solvent of alcoholsand the shape and stacking manner of the resulting Si-MFI crystals. Considering the fact that the self-stackingphenomenon of crystals is related to the condensation ofsurface Si–OH bonds among individual crystals at the earlystage, it is speculated that low polarity (dielectric constant)alcohol solvents might favor the formation of abundant Si–OH groups in the precursor species and on the surface ofthe nanocrystals formed at the early stage of the MW-crys-tallization. The Si-OH groups of individual crystals mightundergo further condensation to form self-staked crystals.

Fig. 1. SEM images of Si-MFI crystals crystallized by using different alcoholmethanol, (c), ethanol (d) 1-propanol, (e) isopropanol, (f) n-butanol and (g) h

3.2. The aging time

Aging time is an important factor affecting the size andshape of the particles in the synthesis of zeolites becausestirring can affect the existing states and distribution of var-ious silica precursor species [29–32]. To investigate theinfluence of aging time on the shape and stacking mannerof Si-MFI crystals, three synthetic systems were studiedin which methanol, ethanol, and isopropanol with signifi-cantly different dielectric constants were used as a co-sol-vent, respectively. The crystallization conditions for thesethree systems are identical to those described above, whilethe aging time was set as 1 h and 72 h, respectively. Thedetailed synthetic parameters and the morphologies ofthe resulting crystals, along with the results obtained byaging for 24 h as described above are summarized in Table2. The SEM images of the Si-MFI crystals obtained withaging time of 1 h and 72 h are shown in Fig. 2.

In the presence of methanol or ethanol, which havedielectric constant of 32.6 and 24.3, respectively, shorten-ing or increasing the aging time resulted in isolated singlecrystals. Notably, the prolonged aging time did affect the

co-solvents under microwave radiation conditions: (a) ethylene glycol, (b)exanol.

Fig. 1 (continued)

Table 2The summary of the synthetic parameters and the morphologies of theresulting Si-MFI crystals

Sampleno.


Aging time(h)


1 Methanol 32.6 1 Isolated singlecrystals



4 Ethanol 24.3 1 Isolated singlecrystals



7 Isopropanol 18.3 0.5 Stacked crystals8 Isopropanol 18.3 1 Stacked crystals9 Isopropanol 18.3 24 Stacked crystals

10 Isopropanol 18.3 72 Isolated singlecrystals

Reaction composition (in molar ratio): TPAOH:SiO2:Alcohol:EtOH:H2O = 0.357:1:8:4:21; Microwave condition: t1 = 90 min, T1 = 80 �C,P1 = 250 W; t2 = 60 min, T2 = 180 �C, P2 = 400 W.


shape of the crystals. The isolated single crystals of Si-MFIwith clear angular hexagonal shape crystallized from thestarting mixture aged for 1 h (Fig. 2a and c). When theaging time was prolonged to 72 h, the isolated single crys-tals of Si-MFI with rounder and less clear angular hexago-nal shape crystallized from the starting mixture (Fig. 2band d). In the case of co-solvent isopropanol, which hasdielectric constant of 18.3, prolongation of the aging timesignificantly changed the stacking manner of the resultingcrystals. As shown above, the self-stacked crystals crystal-lized from the starting mixture aged for 24 h (Fig. 1e).Decreasing the aging time to 1 h (Fig. 2e) gave stackedcrystals, while prolongation of the aging time to 72 h(Fig. 2f) resulted in the formation of isolated single crystalswith round shape. This suggested that prolonged agingtime might reduce the number of Si–OH groups in the pre-cursor species leading to the formation of isolated MFIcrystals. Thus, it is believed that a shorter aging time mightfavor the formation of self-stacked Si-MFI crystals. Asexpected, further reducing the aging time to 0.5 h resultedin highly stacked crystals on top of one another along their(010) face to form stacked morphology of Si-MFI(Fig. 2g). It is noteworthy that the number of the stacked

Fig. 2. SEM images if Si-MFI crystals by using different alcohols as a co-solvent and with different aging time: (a) methanol (1 h), (b) methanol (72 h), (c)ethanol (1 h), (d) ethanol (72 h), (e) isopropanol (1 h), (f) isopropanol (72 h) and (g) isopropanol (0.5 h).


crystals for each ‘‘fiber’’ of Si-MFI increases with thedecrease of aging time. The Q4/Q3 ratios calculated fromthe 29Si MAS NMR spectra of the Si-MFI-MW shown inFig. 3 for the products aged for 0.5 h and 72 h are 4.6and 4.2, respectively, which suggest that the surface hydro-xyl groups of the former individual crystals have a higherdegree of condensation than those of the later individualcrystals.

3.3. The amount of isopropanol co-solvent in the starting

mixture

As described above, the use of alcohols with low dielec-tric constants as a co-solvent resulted in the self-stackedmorphology of Si-MFI crystals. Among these alcohols, iso-propanol can lead to the formation of both stacked andisolated crystals of Si-MFI depending on the aging time.Therefore, it would be of interest to study the detailed syn-thetic conditions on the formation of self-stacked morphol-ogy of Si-MFI crystals. The following study focuses on theinfluence of the amount of the isopropanol on the forma-tion of self-stacked morphology of Si-MFI crystals.

The molar composition of the starting mixture is 1.0SiO2:0.357 TPAOH:x alcohol:4.0 EtOH:21H2O, where x

is 2, 4, or 8 for different batch. The aging time is fixed at0.5 h. The crystallization conditions are identical to thosedescribed above. When the ratio of isopropanol/TEOS is2:1, the product contains isolated single crystals with roundplate shape and clear surface as shown in Fig. 4a. Furtherincreasing the ratio of isopropanol/TEOS to 4:1 did notaffect the morphology of the resulting isolated crystals(Fig. 4b). However, when the ratio of isopropanol/TEOSreaches 8:1, the crystals started to stack on top of oneanother along their b direction to form self-stacked mor-phology (Fig. 2g).

In contrast to the microwave-assisted solvothermal syn-thesis, the synthesis of all above described experimentsunder conventional solvothermal conditions (i.e. heatingin oven) gave only isolated Si-MFI crystals and the corre-sponding SEM images are shown in Fig. 5. The detailedsynthetic parameters and the morphology of the resultingcrystals are summarized in Table 3. In addition, it wasfound that the self-stacked Si-MFI crystals could not beformed without the addition of alcohol co-solvent with a

Fig. 3. 29Si MAS NMR spectra for Si-MFI by using isopropanol under MW radiation condition: (a) aging time: 72 h and (b) aging time: 0.5 h.

Fig. 2 (continued)


relative low dielectric constant even under the microwaveconditions in our synthetic system. This indicates that theutilization of alcohols with relative low dielectric constantsas a co-solvent under MW radiation is necessary for theformation of self-stacked morphology of the resulting Si-MFI crystals in our synthetic system.

On the basis of above synthetic results, it is believed thatthe microwave radiation is a critical factor for the forma-tion of self-stacked morphology of the Si-MFI crystals.In the system TEOS–TPAOH-alcohol under microwaveradiation, the utilization of the alcohols with relative lowdielectric constants as a co-solvent with appropriate

Fig. 4. SEM images of Si-MFI crystals crystallized by using isopropanol as a co-solvent with different alcohol/TEOS molar ratios: (a)isopropanol:TEOS = 2:1 and (b) isopropanol:TEOS = 4:1.

Fig. 5. SEM images of Si-MFI crystals crystallized by using different alcohol co-solvents under conventional solvothermal conditions: (a) ethylene glycol,(b) methanol, (c), ethanol (d) 1-propanol, (e) isopropanol, (f) n-butanol and (g) hexanol.


concentration and short aging time favors the formation ofself-stacked Si-MFI crystals. Previous work by Hwanget al. on the fabrication of Ti-MFI zeolite crystals with

fibrous morphology by microwave-assisted synthesisemphasized that the addition of Ti is necessary for the for-mation of fibrous Ti-MFI crystals, and that self-stacked

Table 3The summary of the synthetic parameters and the morphology of theresulting crystals

Samplenumber



1 Ethyleneglycol

37.0 Isolated singlecrystals

2 Methanol 32.6 Isolated singlecrystals

3 Ethanol 24.3 Isolated singlecrystals

4 1-Propanol 20.1 Isolated singlecrystals

5 Isopropanol 18.3 Isolated singlecrystals

6 n-Butanol 17.8 Isolated singlecrystals

Gel composition in mole: TPAOH:SiO2:Alco-hol:EtOH:H2O = 0.357:1:8:4:21; aging/stirring time = 24 h; Conventionalcondition : t = 36 h T = 180 �C.

Fig. 5 (continued)


fibrous Si-MFI crystals would not be formed under theirsynthetic conditions [26]. They attributed the stacking ofcrystals under MW radiation to the dipole momentstrength of the Ti–O bonds, which are strongly activated

by MW absorption and undergo condensation reactionsto form Ti–O–Ti and/or Ti–O–Si bonds between crystals.However, in our studies, Si-MFI crystals with self-stackedmorphology could be prepared in the presence of alcoholswith low dielectric constants as a co-solvent under micro-wave condition without the addition of Ti or other metalsources. Therefore, we believe that the self-stacking of theMFI crystals under microwave conditions is related to, infact the rapid condensation of abundant surface T–OHgroups among individual crystals, which might be inducedby the addition of Ti or the use of alcohols with low dielec-tric constants as a co-solvent, or perhaps other syntheticparameters. Further investigation of the influence of alco-hol co-solvents on the states of Si species existing in thestarting mixture by 29Si NMR is undergoing, which mightprovide important clue for the precise control of crystalgrowth process.

4. Conclusions

Isolated or self-stacked crystals of silicalite-1 (Si-MFI)were prepared in the presence of alcohols with differentdielectric constants as a co-solvent via microwave-assisted


solvothermal synthesis approach. Under microwave radia-tion, the utilization of alcohols with low dielectric con-stants as a co-solvent with appropriate concentration andshort aging time of the starting mixture can result in theformation of self-stacked morphology of the Si-MFI crys-tals. The low polarity (dielectric constant) co-solvent mayfavor the formation of abundant Si–OH groups in the pre-cursor species and on the surface of the nanocrystalsformed at the early stage of the crystallization which mightundergo further condensation to form self-stacked crystalsdue to the rapid crystallization under microwave condition.Further understanding of the formation mechanism of self-stacked crystals by 29Si NMR spectroscopic studies will beimportant for the precise control of crystal growth process.

Acknowledgements

This work is supported by the National Natural ScienceFoundation of China and the State Basic Research Project(2006CB806103) of China.

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Morphology control of self-stacked silicalite-1 crystals using microwave-assisted solvothermal synthesis