Microporous and Mesoporous Materials 119 (2009) 217–222

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Microporous and Mesoporous Materials

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Fabrication of silicalite-1 crystals with tunable aspect ratios by microwave-assistedsolvothermal synthesis

Xiaoxin Chen, Wenfu Yan, Xuejing Cao, Jihong Yu *, Ruren XuState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 June 2008Received in revised form 18 October 2008Accepted 21 October 2008Available online 29 October 2008

Keywords:ZeolitesSilicalite-1Microwave radiationSolvothermal synthesisMorphology

1387-1811/$ - see front matter � 2008 Elsevier Inc. Adoi:10.1016/j.micromeso.2008.10.015

* Corresponding author. Tel./fax: +86 431 8516860E-mail address: jihong@jlu.edu.cn (J. Yu).

A new method was developed to finely tune the aspect ratios of silicalite-1 (Si-MFI) crystals. This methodinvolved the utilization of co-solvent of diols and the microwave heating of a reaction system of TEOS–TPAOH–H2O–Diol with the molar ratio of SiO2:TPAOH:EtOH:Diol:H2O = 1.0:0.357:4.0:x:y, where x = 6–9,y = 21.55–45.55. The diols included the ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol(TEG), and tetraethylene glycol (tEG). Under microwave radiation the Si-MFI crystals with tunable sizes,shapes, and aspect ratios were crystallized. An excellent linear correlation between the aspect ratio of theSi-MFI crystals and the ‘‘concentration” of the C/OH, denoted as CC/OH, where C/OH is the ratio of the num-ber of carbon and hydroxyl of the diols of the co-solvent, was observed. On the basis of this linear rela-tionship, the aspect ratios of the well defined Si-MFI crystals could be effectively tuned. The possiblemechanism on how the co-solvent of diols affects the growth kinetics of the Si-MFI crystals is discussed.The resulting products were further characterized by X-ray diffraction (XRD) and scanning electronmicroscopy (SEM).

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1. Introduction

Zeolites, which have periodic three-dimensional framework andwell-defined pore structures, are important due to their wideapplications in catalysis, ion-exchange, chemical separation,adsorption, host/guest chemistry, microelectronic devices, optics,and membranes [1,2]. Studies show that their special propertiesand applications are strongly affected by the morphology of thecrystals [3–7]. Hence, controlling the morphology, i.e. size andshape, of zeolites crystals is of great interest.

Silicalite-1 (Si-MFI) was firstly reported by Flanigen et al. as ahydrophobic but organophilic polymorph of silica zeolite [8]. Thispolymorph has two-dimensional channel systems (straight chan-nels in the b-direction and sinusoidal channels in the a-direction)with 10-rings pore openings, and has high thermal stability.

Si-MFI crystals synthesized with tetrapropylammoniumhydroxide (TPAOH) as the structure directing agent (SDA) underconventional hydrothermal condition have either a characteristiccoffin shape with longer crystal dimension along the c-axis andsmaller ones along the a- and b-axis or hexagonal plate shape. Usu-ally, varying the synthesis conditions such as the temperature, sil-ica/TPA and silica/water ratios, and pH of the solution leads to onlyminor changes in the crystal shape and size. To modify the shapeand size of the Si-MFI crystals, the researchers have tried several

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8.

synthetic strategies. de Vos Burchart et al. reported the synthesisof Si-MFI crystals with longer crystal size along b- than a-axis usingdimer C6 TPA cations ((C3H7)3N+(CH2)6 � N+(C3H7)3) as the SDA [9].Tsapatsis and co-workers used the organic polycations (monomer,dimmer and trimer of the tetrapropylammonium hydroxide(TPAOH)) as crystal shape modifiers to enhance relative growthrates along the desirable b-direction of the Si-MFI crystals and pro-gressively modified the crystal size along the a- and b-axis [3,10].Ban et al. investigated the morphology of the Si-MFI crystals crys-tallized from the hydrothermal synthesis system containing theaqueous mixture suspensions of NPrn

4Br (TPABr)-dispersed silicagel and different amines with a lower pKb value than 4.5 [11]. Withthis method, Si-MFI crystals with either long plate-like or rod-likeshape were obtained by adjusting the basicity of the additionalamine in the starting gel. Qiu and co-workers successfully synthe-sized the large Si-MFI crystals from the system of SiO2–TPABr–NaOH–H2O in the presence of benzene-1,2-diol as a complex agentfor silicon [12]. The crystals synthesized in the presence of benz-ene-1,2-diol have a much larger size and much clearer shape thanthose synthesized in the absence of benzene-1,2-diol. The forma-tion of the silicon-benzene-1,2-diol complex was confirmed bythe 29Si NMR characterization, which was very important for thesynthesis of large single crystals of zeolites. Furthermore, many ef-forts have been focused on manipulating Si-MFI crystals size andmorphology by using microemulsions or reverse micelles [13–15]. In addition to the modification of the synthesis condition, themicrowave-assisted heating was used to modify the morphology

218 X. Chen et al. / Microporous and Mesoporous Materials 119 (2009) 217–222

and size of the Si-MFI crystals [16]. Recently, we successfully con-trolled the formation of the isolated and self-stacked morphologyof the Si-MFI crystals by combining the utilization of the co-solventof alcohols with different dielectric constant and the microwaveheating [17]. On the other hand, because the understanding onthe mechanism governing the crystallization of silica-rich zeolitesis essential for the synthesis of zeolites with desired morphology,the study on the kinetics of crystallization has attracted much moreattention [18–22]. For example, Feoktistova et al. reported theirstudy on the kinetics of crystallization of Si-MFI and found a linearrate of crystals growth of Si-MFI and its dependence on the temper-ature of synthesis [18].

In this work, we present a new synthetic method for the effec-tive control of the aspect ratio of the Si-MFI crystals. The key of thismethod is combining the utilization of co-solvent of diols and themicrowave heating of the synthetic system. By employing thismethod, the Si-MFI crystals with tunable sizes, shapes, and aspectratios were obtained.

2. Experimental section

The reaction solution for the crystallization of Si-MFI crystalswere prepared by mixing tetraethyl orthosilicate (TEOS, 98 wt%),tetrapropylammonium hydroxide (TPAOH, 20 wt%), and variousdiols including ethylene glycol (EG), diethylene glycol (DEG), tri-ethylene glycol (TEG), and tetraethylene glycol (tEG) in the molarratio 1.0 SiO2: 0.357 TPAOH: 4.0 EtOH: x Diols: y H2O, wherex = 6–9, and y = 21.55–45.55 for each batch.

A synthesis solution was prepared by adding aqueous TPAOHsolution to TEOS, followed by the addition of diol under strong agi-tation. The reaction solution was stirred in a sealed vessel at ambi-ent temperature for 48 h for the purpose of aging before it wasloaded into a Teflon autoclave. The crystallization was conductedin a microwave oven (Milestone ETHOS-D) with pre-programmedheating profiles. The microwave synthesis involved a two-stepheating [23,24]. The crystallization temperature, duration, andmaximum heating power were defined as T, t, and P, respectively.First step of the synthesis was conducted at T1 = 80 �C andP1 = 250 W for t1 = 90 min. The second step was conducted atT2 = 180 �C and P2 = 400 W for t2 = 60 min. The product was sepa-rated by centrifugation, washed thoroughly with deionized waterand ethanol, and then dried overnight at 50 �C.

Powder X-ray diffraction (XRD) patterns were recorded on aSiemens D5005 diffractometer with CuKa radiation (k = 1.5418 Å)and all XRD patterns were provided in the Supplementary Material.The scanning electron microscope (SEM) images were taken on aJEOL JSM-6700F field emission scanning electron microscope (FE-SEM).

3. Results and discussion

Microwave is a form of electromagnetic energy. The energy ofthe microwave could be effectively transferred to the target polarmolecules located in a rapidly oscillating electrical field of micro-waves. Compared to the traditional heating, the rapid heating rateand the special heating mechanism of the microwave heating em-ployed may change the reaction mechanism conducted in a micro-wave heating system [25–28]. In the microwave reactionsinvestigated in this study, a series of diols were used as gooddielectric media and special microwave absorbents. The Si-MFIcrystals with various aspect ratios were obtained. It was found thatthe aspect ratio of the resulting Si-MFI crystals can be finely tunedby varying the length of the carbon chain and the concentration ofthe diols, which will be detailedly discussed in the followingsections.

3.1. The type of diols

In this study, various diols were used as co-solvents in themicrowave-assisted solvothermal synthesis of Si-MFI crystals fromthe synthetic system of TEOS–TPAOH–H2O. The diols include EG,DEG, TEG, and tEG. The final gel composition in molar ratio is 1.0SiO2: 0.357 TPAOH: 4.0 EtOH: 7diols: 21.55 H2O. The SEM imagesof the resulting Si-MFI crystals are shown in Fig. 1. The averagelength (L), width (W), thickness (T), and the aspect ratio (L/W andL/W/T) of these Si-MFI crystals are summarized in Table 1. Theaverage length, width, and thickness of the Si-MFI crystals are cal-culated by counting 15 crystals from three high quality SEMimages. The length and width are measured from the same crystal,and the thickness is measured from the other crystals that displaythe thick side in the SEM images.

The results in Table 1 and Fig. 1 show that the well defined Si-MFI crystals become longer, narrower, and thinner when the ratioof the number of carbon and the hydroxyl of the diols (C/OH), aparameter that can roughly reflect the hydrophobic and hydro-philic property of the molecule of the co-solvent, is gradually in-creased. This indicates that the ratio of C/OH could be animportant parameter on controlling the morphology and aspect ra-tio of the Si-MFI crystals. In the presence of the co-solvent of EG (C/OH = 1), the average values of the length, width, and the thicknessof the Si-MFI crystals are 1.05, 0.56, and 0.25 lm, respectively,which give L/W of 1.88 and L/W/T of 7.52, respectively. When theDEG (C/OH = 2) is used as the co-solvent, the average length ofthe crystals is increased to 1.23 lm, whereas the average widthand thickness are decreased to 0.33 and 0.16 lm, respectively,which give L/W of 3.74 and L/W/T of 23.38, respectively. Whenthe co-solvents of TEG and tEG with higher C/OH ratio are used,the average lengths of the Si-MFI crystals appear keeping constant(1.23 lm for DEG vs. 1.20 lm for TEG and 1.19 lm for tEG),whereas the average width and thickness keep decreasing with aless significant trend (width: 0.33 lm for DEG vs. 0.27 lm forTEG and 0.26 lm for tEG; thickness: 0.16 lm for DEG vs.0.13 lm for TEG and 0.11 lm for tEG). The histograms of the L/Wand L/W/T as the function of C/OH ratio are shown in Fig. 2a andb, respectively. The results in Fig. 2 clearly show that the aspect ra-tios of the L/W and L/W/T increase with the increase of the C/OHratio of the diols.

In contrast to the microwave-assisted solvothermal synthesis,the synthesis of all above described batches under conventionalsolvothermal conditions (i.e. heating in a conventional oven at180 �C for 36 h under static condition) gave only single crystalswith hexagonal plate shape with almost same size and aspect ra-tios of L/W and L/W/T (SEM images are shown as Fig. S1 in the Sup-plementary Material). A very similar morphology of the crystalswas obtained when diols were not used as co-sovlent under micro-wave-assisted solvothermal synthesis (one typical SEM image isshown as Fig. S2 in the Supplementary Material). This indicatesthat the special heating mechanism of the microwave affects thegrowth rates of the Si-MFI crystals along different planes. The com-bination of the microwave heating and the utilization of the diolssignificantly changed the growth kinetics of the Si-MFI crystals.Generally, the morphology of a crystal is controlled by the relativegrowth rates of crystal faces and the equilibrium form of crystalwould be such as to minimize the total surface energy for a givenvolume [29]. According to the Bravais–Friedel–Donnay–Harkerlaw, the growth rate along a crystallographic direction is inverselyproportional to the corresponding d spacing [30,31] and the largestfaces are expected to be the faces with the lowest surface energies.Thus, it is expected that the fastest growth direction is alwaysalong the c-axis of the Si-MFI crystals [9]. In this study, the well de-fined Si-MFI crystals become longer, narrower, and thinner whenthe ratio of the C/OH of the diols is gradually increased here. This

Fig. 1. The SEM images of the Si-MFI crystals crystallized from the microwave-assisted solvothermal synthesis system in the presence of the diols: (a) EG, (b) DEG, (c) TEG,and (d) tEG. A schematic identifying the crystal faces is shown on the right part of the figure.

Table 1Influence of the type of diols on the L, W, T, L/W ratio and L/W/T ratio of Si-MFIcrystals.

Diols EG DEG TEG tEG

Ratio of the carbon and hydroxyl of the diols(C/OH)

1 2 3 4

Length (lm) 1.05 1.23 1.20 1.19Width (lm) 0.56 0.33 0.27 0.26Thickness (lm) 0.25 0.16 0.13 0.11L/W 1.88 3.74 4.43 4.56L/W/T 7.52 23.38 34.08 41.45

Gel composition (in molar ratio): SiO2:TPAOH:EtOH:Diols:H2O = 1:0.357:4.0:7:21.55.

X. Chen et al. / Microporous and Mesoporous Materials 119 (2009) 217–222 219

indicates that the molecules of the co-solvent can decrease the sur-face energy of {010} and {100} and that such a capability is in-creased with increase of the C/OH ratio. In our previous studies,we used the alcohols with different dielectric constants as co-sol-vents in the microwave-assisted solvothermal synthesis of Si-MFIcrystals. The alcohols (monols and one diol) with higher dielectricconstant than 18.3 (isopropanol) resulted in the isolated hexagonalplate-like shape of the Si-MFI crystals. However, the monols withlower dielectric constant than 18.3 leaded to the self-stacked Si-MFI crystals connected with chemical bonds. It is speculated thatthe low polarity (dielectric constant) co-solvent may favor the for-

Fig. 2. The histograms of the L/W (a) and L/W/T

mation of abundant Si–OH groups on the {010} faces of the nano-crystals formed at the early stage of the crystallization. These Si-OH groups might survive from the further condensation of the sil-icate species by forming the relative strong hydrogen-bondedinteraction with the co-solvent molecules. In this study, the diolswith various C/OH were used as co-solvent in the microwave-as-sisted solvothermal synthesis of Si-MFI crystals. Compared to theco-solvent of monols in our previous studies, the –O– groups ofthe diols may form relative strong hydrogen-bonded interactionwith the local multi-Si–OH groups. This may allow the adjacent –CH2–CH2– group to have a close contact with the silicon speciesof the framework, which can further decrease the surface energyof the {010} and {100} due to the hydrophobic nature of both spe-cies and thus promote a faster growth rate along the c-directionthan a- and b-directions. The higher dielectric constant of thesediols over 18.3 [32] suggested that the Si–OH groups formed andsurvived at the early stage of the crystallization are not enoughto form the self-stacked poly-crystals, which agrees very well withour previous studies [17].

3.2. The concentration of diols

As described above, the existence of the diols significantly af-fected the growth rate of the crystal planes of Si-MFI crystals.Therefore, it would be of interest to further study the influence

(b) as the function of C/OH ratio of diols.

220 X. Chen et al. / Microporous and Mesoporous Materials 119 (2009) 217–222

of the concentration of the diols (represented by the diol/TEOS ra-tio) on the size and aspect ratio of the Si-MFI crystals. Due to thesimilar influence of the TEG and tEG on the morphology of Si-MFI crystals observed above, the studies were conducted withEG, DEG, and tEG as co-solvents. The molar composition of thestarting mixture is 1.0 SiO2: 0.357 TPAOH: 4.0 EtOH: x diols:21.55 H2O, where x is 6, 7, 8, and 9 for different batch. The crystal-lization conditions and the aging time are kept constant with thosedescribed above. The detailed information for the length, width,thickness, L/W, and L/W/T of the resulting Si-MFI crystals is sum-marized in Table 2. The values of the length, width, and thicknessare calculated by counting 15 crystals from three SEM high qualityimages. The length and width are measured from the same crystal,and the thickness is measured from the other crystals that displaythe thick side in the SEM images.

The results in Table 2 show that the concentration of EG (repre-sented by the EG/TEOS ratio) in the starting gel does not signifi-cantly affect the aspect ratio of L/W and L/W/T of the Si-MFIcrystals. When the EG/TEOS ratio is decreased to 6/1 from 7/1,the Si-MFI crystals with much more reduced length (1.05 lm forx = 7 vs. 0.56 lm for x = 6), width (0.56 lm for x = 7 vs. 0.36 lmfor x = 6), and thickness (0.25 lm for x = 7 vs. 0.16 lm for x = 6)are obtained. However, the aspect ratio of L/W and L/W/T is less af-fected. When the EG/TEOS ratio is increased to 8/1 from 7/1, thelength, width, and thickness of the crystals almost keep constant.Further increasing the EG/TEOS ratio to 9/1 results in the signifi-

Table 2Influence of the concentration of EG, DEG, and tEG on L, W, T, L/W, and L/W/T ratio of Si-M

Diols/TEOS 6 7

EG DEG tEG EG DEG t

Length (lm) 0.56 0.55 1.04 1.05 1.23Width (lm) 0.36 0.35 0.24 0.56 0.33Thickness (lm) 0.16 0.18 0.10 0.25 0.16L/W 1.56 1.55 4.25 1.88 3.74L/W/T 9.75 8.61 42.5 7.52 23.38 4

Reaction composition (in molar ratio): SiO2: TPAOH: EtOH: Diols: H2O = 1: 0.357: 4.0: x

Fig. 3. The correlation between the length, width, or thickness of the Si-MFI crystals an

cant increase of the length and width and slight decrease of thethickness of the Si-MFI crystals.

Compared to the EG, the molecule of DEG has a longer carbonchain, which gives a C/OH ratio of 2. When the DEG/TEOS ratio isdecreased to 6/1 from 7/1, the length of the resulting Si-MFI crys-tals is significantly decreased to 0.55 lm from 1.23 lm, whereasthe width and thickness are kept almost constant (see Table 2 fordetailed information), which leads to a significant drop of the L/W and L/W/T ratio of the Si-MFI crystals to 1.55 and 8.61, respec-tively. Increasing the DEG/TEOS ratio to 8/1 results in the increaseof the length and the slightly decrease of the width and thickness,which leads to a significant increase of the L/W and L/W/T to 4.16and 34.67, respectively. Further increasing the DEG/TEOS ratio to9/1 does not lead to the significant change on the length, width,and thickness of the crystals.

Compared to the co-solvent of EG and DEG, the co-solvent oftEG has a higher C/OH ratio of 4. Decreasing the tEG/TEOS ratioto 6/1 from 7/1 results in a shortened length, whereas the widthand thickness are kept almost constant (see Table 2 for detailedinformation). Increasing the tEG/TEOS ratio to 8/1 and 9/1 doesnot have significant influence on the length, width, and thicknessof the Si-MFI crystals (see Table 2 for detailed information).

The results described above clearly demonstrate that the lengthof the carbon chain of the co-solvent and their concentration in thestarting gel have significant influence on the morphology of theresulting Si-MFI crystals. To get a better understanding on the role

FI crystals.

8 9

EG EG DEG tEG EG DEG tEG

1.19 1.03 1.32 1.29 1.33 1.37 1.320.26 0.52 0.32 0.28 0.63 0.31 0.270.11 0.23 0.12 0.09 0.22 0.13 0.094.56 1.99 4.16 4.58 2.12 4.43 4.831.45 8.65 34.67 50.89 9.64 34.08 53.67

: 21.55, x = 6–9.

d the ‘‘concentration” of C/OH of the diols: (a) length, (b) width, and (c) thickness.

Table 3Influence of the DEG with lower concentration on the L, W, T, L/W, and L/W/T ratio ofSi-MFI crystals.

H2O/TEOS 21.55 22.55 23.55 24.55 29.55 37.55 45.55

H2O/DEG 3.08 3.22 3.36 3.51 4.22 5.36 6.51

Length (lm) 1.23 0.80 0.63 0.61 0.49 0.45 0.46Width (lm) 0.33 0.35 0.39 0.38 0.33 0.32 0.32Thickness (lm) 0.16 0.14 0.17 0.18 0.18 0.16 0.16L/W 3.74 2.34 1.65 1.60 1.50 1.42 1.43L/W/T 23.38 16.71 9.71 8.89 8.33 8.88 8.94

Reaction composition (in molar ratio): SiO2: TPAOH: EtOH: DEG: H2O = 1: 0.357:4.0: 7: y, y = 21.55–45.55.

Fig. 6. The correlation between the aspect ratio of L/W and the ‘‘concentration” ofC/OH of the DEG.

X. Chen et al. / Microporous and Mesoporous Materials 119 (2009) 217–222 221

of the co-solvent played in the microwave-assisted solvothermalsynthesis of Si-MFI crystals, we introduced a new parameter ofC/OH to roughly describe the hydrophobic and hydrophilic prop-erty of the co-solvent and a derived parameter of the ‘‘concentra-tion” of C/OH (denoted as CC/OH), which could include theinformation of the type and the concentration of the diols. TheCC/OH for each diol is calculated by multiplying the molar concen-tration of the diol in the starting gel by its C/OH ratio. The length,width, and thickness of the Si-MFI crystals as the function of the CC/

OH of the diols are plotted in Fig. 3a, b, and c, respectively, and theaspect ratio of L/W and L/W/T as the function of the CC/OH of thediols are plotted in Figs. 4 and 5, respectively.

Fig. 3a shows that the length of the Si-MFI crystals is stronglyaffected by the type of the diols. For each diol, the length of theresulting Si-MFI crystals is increased with increase of the concen-tration in the starting gel. Fig. 3b shows that the width of the Si-MFI crystals is less affected by the type and concentration of thediols except EG. This trend is more clearly observed in Fig. 3c.The thickness of the Si-MFI crystals is kept almost constant regard-less the type and concentration of the diols.

Fig. 4 shows an apparent linear relationship between the aspectratio of L/W of the Si-MFI crystals and the CC/OH of the diols in thestarting gel. The correlation coefficient of R for the linear fit is 0.91.Because the thickness of the Si-MFI crystals is much less affectedby the type and concentration of the diols, a similar linear relation-ship between the aspect ratio of L/W/T of the Si-MFI crystals andthe CC/OH of the diols in the starting gel is observed in Fig. 5 as well.The correlation coefficient of R for the linear fit is 0.96.

Compared to the data of EG and tEG in Figs. 4 and 5, the data ofDEG with low CC/OH has a significant deviation from the fitted lin-ear relationship. A detailed study on the influence of DEG with lowconcentrations on the aspect ratio of the Si-MFI crystals is thusconducted and discussed below.

Fig. 4. The correlation between the aspect ratio of L/W and the ‘‘concentration” ofC/OH of the diols.

Fig. 5. The correlation between the aspect ratio of L/W/T and the ‘‘concentration” ofC/OH of the diols.

3.3. The influence of DEG with low concentration

In previous studies, the H2O/TEOS ratio was fixed at 21.55. Inthe following control experiments, this ratio was set as 21.55,22.55, 23.55, 24.55, 29.55, 37.55, and 45.55, respectively. TheDEG/TEOS ratio was kept at 7/1.

The detailed information on the average length, width, andthickness of the Si-MFI crystals and the resulting L/W and L/W/Tfor each batch are summarized in Table 3. The data in Table 3 showthat the length of the Si-MFI crystals is gradually decreased withincrease of the water content in the starting gel, whereas the widthand thickness are much less affected. The aspect ratios of L/W andL/W/T have a big drop before the H2O/DEG ratio reached 3.36 and isless changed after that. The L/W as the function of the CC/OH of DEGfor all available data is plotted in Fig. 6 and an ‘‘S” shape curve isobtained. The transition point is corresponding to the H2O/DEG ra-tio of 3.22. The data with either higher or lower CC/OH than thatpoint have good linear relationship. The correlation coefficientsof the linear fit for the data with lower and higher CC/OH are 0.93and 0.999, respectively.

4. Conclusions

The co-solvents of diols including EG, DEG, TEG, and tEG areintroduced into the microwave-assisted solvothermal synthesisof Si-MFI crystals. Under the microwave-assisted heating, the typeand concentration of the co-solvent of diols can significantly affectthe length of the resulting Si-MFI crystals, whereas the width andthickness are less affected. A linear correlation between the aspectratios of the Si-MFI crystals and the CC/OH in the starting gel wasobserved. On the basis of this linear relationship, the aspect ratioof the well defined Si-MFI crystals could be effectively tuned. Thestrong dependence of the aspect ratio of the Si-MFI crystals onthe CC/OH of the diols observed in the microwave-assisted solvo-thermal synthesis can not be reproduced in the conventionalsolvothermal synthesis system. The –O– groups of the diols may

222 X. Chen et al. / Microporous and Mesoporous Materials 119 (2009) 217–222

form relative strong hydrogen bonding interaction with the localmulti-Si-OH groups compared with monols. This allows the adja-cent –CH2–CH2– group to have a close contact with the silicon spe-cies of the framework and thus further decreases the surfaceenergy of the {010} and {100}. The strong dependence of the as-pect ratio of the Si-MFI crystals on the CC/OH of the diols can notbe reproduced in the traditional solvothermal synthesis system.This work provides a new way for the controlled crystallizationof zeolites crystals with tunable sizes, shapes, and aspect ratios.

Acknowledgments

This work is supported by the National Natural Science Founda-tion of China and the State Basic Research Projects (2006CB806103and 2007CB936402) of China. W.Y. thanks the support by the Pro-gram for New Century Excellent Talents in University (NCET) andthe Outstanding Youth Fund of Jilin Province (20060120).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.micromeso.2008.10.015.

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