Microporous and Mesoporous Materials 131 (2010) 45–50

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Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate /micromeso

Quantitative correlation between morphology of silicalite-1 crystals anddielectric constants of solvents

Xiaoxin Chen, Wenfu Yan *, Xuejing Cao, 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 3 August 2009Received in revised form 25 September2009Accepted 27 November 2009Available online 2 December 2009

Keywords:ZeolitesSilicalite-1Microwave radiationMixed solventsMorphology

1387-1811/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.micromeso.2009.11.039

* Corresponding author. Tel./fax: +86 431 8516860E-mail address: yanw@jlu.edu.cn (W. Yan).

The quantitative correlation between morphology of silicalite-1 (Si-MFI) crystals and dielectric constantsof solvents for the reaction system of TEOS–TPAOH-alcohol under microwave heating was investigated.The dielectric constant of the solvent was finely tuned by mixing water and additional alcohols accordingto the equation of em ¼ U1e1 þU2e2 þ � � � þUnen. The length and fraction of Si-MFI crystals with self-stacked morphology as a function of dielectric constant was investigated, and the linear correlationwas found. The self-stacked crystals were obtained when the dielectric constant of the mixed solventwas lower than 41.0, while only fully isolated crystals were formed when the dielectric constant washigher than 45.0. The regularities observed in the microwave-assisted solvothermal synthesis systemwere not observed in the conventional oven-heated solvothermal synthesis system.

� 2009 Elsevier Inc. All rights reserved.

1. Introduction

Due to their unique properties, such as large surface areas, welldefined channel systems, and controllable densities of the activesites [1–4], zeolites have been widely used in catalysis, adsorption,chemical separation, ion-exchange, and host/guest chemistry.Studies show that their unique properties and applications arestrongly affected by the morphology of the crystals, e.g. the shape,size, and aggregation state of the crystals [5–9]. Therefore, devel-opment of new synthetic strategies for controlling the crystal mor-phology of zeolite materials is becoming increasingly important.

Morphology of the zeolite crystals can be determined by (i) rel-ative rates of growth of discrete crystals in different directions, (ii)aggregation of primary (discrete) particles (crystals) and/or de-aggregation of such formed aggregates, and (iii) combination of(i) and (ii). In the past decades, several methods have been devel-oped for morphology control of zeolites [9–45]. These methods canbe summarized as follows: (1) Modifying the synthesis recipe; forexample, alkalinity of reaction mixture not only considerably influ-ences the morphology of ZSM-5 and zeolite A, but also influencesthe aggregation of zeolite A crystals [11–15]. Changing Si/Al ratioof the reaction mixture considerably influences the morphologyof zeolite A [16,17], omega [18] and ZSM-5 [19]. The concentrationof organic template also has a great influence on the morphology ofZSM-5 crystals as well [20]. (2) Introducing hetero-atom to the

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9.

zeolite framework; Introduction of the hetero-atom into theframework of zeolites results in the aberration of the crystal sur-face from the form they should be. Typical examples include met-allosilicate (M-MFI) [9,21], Ni/Cu-ZSM-5 [22], and Fe-substitutedAl-MOR [23], etc. In these cases, the morphology of hetero-atomdoped crystals is significantly different from their regular shape.(3) Modifying the shape of internal organic template [20,24,25];for example, by using monomer, dimmer and trimer of the tetra-propylammonium hydroxide (TPAOH) as the structure directingagents, Tsapatsis and co-workers progressively modified the shapeof the Si-MFI crystals from the well-known morphology with thelongest crystal dimension along the c-axis to the crystals withthe longest dimension along the a- and b-axis [24]. They inter-preted that relative crystal size along the a- and b-axis in termsof how well the internal templates fit in the straight (b-axis) versusthe sinusoidal (a-axis) channels [25]. (4) Modifying the chemicalor physical properties of the environment of crystallization; forexample, the additives of inorganic cations or co-solvent, whichcan significantly change the chemical properties of the environ-ment of zeolite nuclei, were used to modify the crystal size, shape,and growth behavior of silicalite-1 crystals and the aspect ratio ofzeolite-LTL [26–29]. The confined space such as hydrogel, porouscarbon, and microemulsion (reverse micelle) were used to synthe-size nanosized zeolites [30–38]. (5) Applying microwave heating;Compared to the traditional heating method, microwave heatingprovided many advantages on the synthesis of zeolites with newmorphologies [39–43]. (6) Changing physical factors; besides thechemical factors described above, the physical factors such as time

46 X. Chen et al. / Microporous and Mesoporous Materials 131 (2010) 45–50

and temperature of crystallization can also influence the crystalmorphology of zeolites [13,18,44,45]. In our previous studies, wecombined method 4 and 5 to control the shape, size, aspect ratio,and aggregation state of the Si-MFI crystals [46,47]. Different fromthe results reported in the literature [9], we found that the additionof the second solvent can affect the aggregate state of the Si-MFIcrystals under microwave heating condition without using theadditive of Ti [46]. The possible mechanism was investigated aswell [46]. Later, we found that the second solvent of diols can sig-nificantly affect the aspect ratios of the Si-MFI crystals [47].

In this study, on the basis of the equation of em ¼ U1e1þU2e2 þ � � � þUnen (U is the volume fraction of the component)[48], we finely tuned the dielectric constant of the solvent by usingco-solvent of alcohols and systematically investigated the quanti-tative correlation between the length and fraction of Si-MFI crys-tals with self-stacked morphology and the dielectric constant ofthe solvent under microwave heating condition.

2. Experimental section

The reaction solutions for the crystallization of Si-MFI crystalswere prepared by mixing tetraethyl orthosilicate (TEOS, 98 wt.%),

Scheme 1. Scheme of the morphology and aggregation control of the Si-MFIcrystals (DC: dielectric constant).

Table 1The summary of the synthesis parameters and aggregation of the resulting Si-MFI crystals

Sample No. Molar ratio EG/iPrOH

ema Aggregation state Average length (lm)c Fr

1 0:8 38.5 Stacked crystals 1.95 102 1:7 39.7 Stacked crystals 1.57 103 2:6 41.0 Stacked crystals 1.20 104 3:5 42.3 Stacked + isolated 0.93 265 4:4 43.6 Stacked + isolated 0.64 196 5:3 45.0 Isolated 0 07 6:2 46.5 Isolated 0 08 7:1 48.0 Isolated 0 09 8:0 49.6 Isolated 0 0

Aging time: 48 h; sol composition in mol TPAOH:SiO2:EtOH:iPrOH:Y:H2O = 0.357:1:4:xP = 400 W.

a em is the dielectric constant of the mixed solvents including iPrOH,em ¼ Ui

PrOHeiPrOH þUEGeEG þUH2OeH2O þUEtOHeEtOH.

b em is the dielectric constant of the mixed solvents including iPrOH, n-Bem ¼ Ui

PrOHeiPrOH þUn-BuOHen-BuOH þUH2 OeH2 O þUEtOHeEtOH.

c The average length is calculated by measuring the self-stacked Si-MFI crystals withd The fraction parameter is calculated by the ratio of the sum of the self-stacked Si-MF

the SEM images.

tetrapropylammonium hydroxide (TPAOH, 16.5 wt.%), and mixtureof the two different alcohols. The molar ratio of the resulting sol is1.0 SiO2:4.0 EtOH:0.357 TPAOH:x X:y Y:21 H2O (X = isopropyl alco-hol, Y = n-butanol or ethylene glycol, ethanol is generated by thehydrolysis of TEOS), where x and y are integers and the sum of xand y is 8.

Typically, aqueous tetrapropylammonium hydroxide solutionwas dropwise added to tetraethyl orthosilicate followed by thealcohols under strong agitation. The resulting sol was strongly stir-red in a sealed vessel at ambient temperature for 48 h for the pur-pose of aging before it was loaded into a Teflon autoclave. Thecrystallization was conducted in a microwave oven (MilestoneETHOS-D) with pre-programmed heating profiles. For the usedcrystallization temperature (T = 180 �C) and corresponding crystal-lization pressure (P = 400 W), the time of crystallization, t was10 min in all the syntheses. The product was separated by centrifu-gation, washed thoroughly with deionized water and ethanol, andthen dried overnight at 50 �C.

Powder X-ray diffraction (XRD) patterns were recorded on aSiemens D5005 diffractometer with CuKa radiation (k = 1.5418 Å).The scanning electron microscope (SEM) images were taken on aJEOL JSM-6700F scanning electron microscope. The powder X-raydiffraction patterns of the resulting products show characteristicpeaks of MFI structure without any impurities (not shown).

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 the rapidly oscillating electrical field of micro-waves and the efficiency of the energy transfer (e.g. heating rate)depends on the dielectric constant of the molecules. A higherdielectric constant of a molecule corresponds to a higher heatingrate [49]. In the synthesis of zeolites, thus, the dielectric constantof the solvent will significantly affect the formation and crystalliza-tion behavior of the crystalline product [39,42]. In this study, thedielectric constant of the solvent used in the synthesis of Si-MFIcrystals under microwave heating is finely tuned by adding theco-solvents to the synthesis system according to the equation ofem ¼ U1e1 þU2e2 þ � � � þUnen. The dependence of the morphologyand aggregation state of the resulting Si-MFI crystals on the dielec-tric constant of the solvent is systematically investigated and theresults are shown in Scheme 1 (taking into account of our previousresults published in [46,47]).

.

n-BuOH/iPrOH

actiond (%) emb Aggregation state Average length (lm)c Fractiond (%)

0 38.5 Stacked crystals 1.95 1000 38.2 Stacked crystals 2.48 1000 38.0 Stacked crystals 1.79 100.80 37.7 Stacked crystals 2.73 100.55 37.4 Stacked crystals 2.60 100

37.2 Stacked crystals 2.15 10036.9 Stacked crystals 2.12 10036.7 Stacked crystals 2.18 10036.4 Stacked crystals 2.03 100

:y:21 (Y denotes as EG or n-Butanol); microwave condition: t = 10 min, T = 180 �C,

EG, ethanol, and water. The calculation is based on the equation of

uOH, ethanol, and water. The calculation is based on the equation of

appropriate position (angle) in the SEM images.I crystals to the total number of Si-MFI crystals with appropriate position (angle) in

X. Chen et al. / Microporous and Mesoporous Materials 131 (2010) 45–50 47

The isopropanol (iPrOH, dielectric constant e: 18.3) is selectedas the main co-solvent in tuning the dielectric constant of the sol-vent and the second co-solvent of alcohol are ethylene glycol (EG,dielectric constant e: 37.0) or n-butanol (n-BuOH, dielectric con-stant e: 17.8). The molar composition of the starting mixture is1.0 SiO2:4.0 EtOH:0.357 TPAOH:x iPrOH:y EG (or n-BuOH):21H2O, where x and y are integers and the sum of x and y is 8 foreach batch. The detailed information for the synthesis parameters,the corresponding dielectric constant (em) of the mixed solvents,the resulting morphologies and aggregation state of the Si-MFIcrystals and average length (L) and fraction of the self-stacked

Fig. 1. SEM images of the Si-MFI crystals crystallized by using iPrOH and EG as mixed co-s6:2; (g) 7:1; (h) 8:0.

crystals are summarized in Table 1. Their SEM images are shownin Figs. 1 and 2, respectively. The dielectric constant of EG is 37.0,which is higher than that of iPrOH (18.3) and lower than that ofH2O (80.0). Thus, the dielectric constant of the solvent containingH2O, EG, and iPrOH can be finely tuned from 18.3 to 80.0 accord-ing to the equation of em ¼ U1e1 þU2e2 þ � � � þUnen. When EG isused as second co-solvent and the ratio of the EG to iPrOH variesfrom 0:8 to 8:0, dielectric constant of the solvent increases fromem = 38.5 to 49.6 (see Table 1). When the ratio of the EG to iPrOHis 0:8, the dielectric constant of the solvent (em) is 38.5, theresulting crystals are self-stacked morphology and their SEM

olvents with different ratios of EG/iPrOH: (a) 1:7; (b) 2:6; (c) 3:5; (d) 4:4; (e) 5:3; (f)

Fig. 2. SEM images of the Si-MFI crystals crystallized by using iPrOH and n-BuOH as mixed co-solvents with different ratios of n-BuOH/iPrOH: (a) 1:7; (b) 2:6; (c) 3:5; (d) 4:4;(e) 5:3; (f) 6:2; (g) 7:1; (h) 8:0.

48 X. Chen et al. / Microporous and Mesoporous Materials 131 (2010) 45–50

image was achieved as characterized and well described in ourprevious publication [50]. With increasing the ratio of EG to iPrOHfrom 0:8 to 1:7 and 2:6, the dielectric constants of the resultingsolvent are modified to 39.7 and 41.0, respectively, their SEMimages are shown in Fig. 1a and b. Further increasing the ratioof EG to iPrOH to 3:5 and 4:4 (41.0 < em < 45), self-stacked andindividual crystals co-exist (Fig. 1c and d). Both fraction andlength of the self-stacked features decrease with increasingdielectric constant of solvent (see Table 1). Finally, for em > 45only isolated Si-MFI crystals are present in the product of crystal-lization. The SEM images of the resulting Si-MFI crystals obtained

from the solvents ratio of EG to iPrOH to 5:3, 6:2, 7:1, and 8:0 areshown in Fig. 1e–h.

The dielectric constant of n-BuOH (17.8) is lower than that ofiPrOH (18.3). It is expected that the self-stacked aggregation stateof the resulting Si-MFI crystals will be obtained when the n-BuOHis used as co-solvent with or without the presence of iPrOH. With-out the presence of iPrOH, the co-solvent of n-BuOH can lead to thestacked aggregation of the Si-MFI crystals under microwave heat-ing, which has been previously proved by us [46]. In this study,the n-BuOH is used as the second co-solvent in addition the co-sol-vent of iPrOH. The ratios of the n-BuOH/iPrOH are set as 0:8, 1:7,

Fig. 4. The correlation between the dielectric constants of mixed solvents (em) andthe fraction of resulting self-stacked Si-MFI crystals.

X. Chen et al. / Microporous and Mesoporous Materials 131 (2010) 45–50 49

2:6, 3:5, 4:4, 5:3, 6:2, 7:1, and 8:0, respectively. The dielectric con-stants of the resulting solvent are modified in the range from 38.5to 36.4. The detailed information for the synthesis parameters, thecorresponding dielectric constant of the mixed solvents, the result-ing morphologies and aggregation state of the Si-MFI crystals andaverage length and fraction of the self-stacked crystals are summa-rized in Table 1. Fig. 2a–h show the SEM images of the resultingclosely self-stacked Si-MFI crystals. This is constant with the regu-larity described above. Above results clearly indicated that it is astrong correlation between the dielectric constant of the solventsand stacked manner of the resulting Si-MFI crystals. The self-stacked Si-MFI crystals can be obtained when the dielectric con-stant of solvent is less than 41.0. To check if there is quantitativecorrelation between the average length of the self-stacked crystalsand the corresponding dielectric constant of mixed solvents, weplot the average length as the function of dielectric constants inFig. 3. In Fig. 3, the linear relationship (or trend) between the aver-age length of self-stacked Si-MFI crystals and the dielectric con-stants of mixed solvents is observed. In the range em = 46 toem = 38.5, a linear relationship between em and the length ofstacked crystals is evident when EG is used as the second co-sol-vent. However, the relationship between em and the length ofstacked crystals is more complex in the em range 38.5 to 36.4 whenn-BuOH is used as the second co-solvent. The length of stackedcrystals increases from about 1.95 to about 2.73 lm when em de-creases from 38.5 to 37.7 and then decreases slightly with furtherdecrease of em. Thus, the formation of the isolated or self-stackedaggregation state of Si-MFI crystals obtained under microwaveheating conditions is mainly controlled by dielectric constant ofthe solvent, but also the chemical properties of co-solvent canprobably influence the relationship between em and the length ofstacked crystals as well as between em and the fraction of stackedcrystals. According to this relationship, higher dielectric constantof the solvent results in the shorter self-stacked Si-MFI crystalsand finally isolated crystals, while lower dielectric constant ofthe solvent leads to the longer of self-stacked Si-MFI crystals. Withthe help of this linear relationship, the length of the aggregated Si-MFI crystals can be controlled by tuning the dielectric constant ofthe solvent.

On the basis of the results described above, we can concludethat the isolated or self-stacked aggregation state of the Si-MFIcrystals crystallized under microwave heating condition is con-trolled by the dielectric constant of the solvent. The critical dielec-tric constants of the solvent for getting these two aggregation

Fig. 3. The correlation between the dielectric constants of mixed solvents (em) andthe length of resulting self-stacked Si-MFI crystals.

states are 45.0 and 41.0 (see Table 1 for details), respectively. Tomore clearly describe the dependence of the aggregation state ofthe Si-MFI crystals on the dielectric constant of the solvent, we plotthe fraction of self-stacked Si-MFI crystals as the function of dielec-tric constants in Fig. 4. According to Fig. 4, only stacked crystals arepresent in the product for em < 41 and that only isolated crystalsare present in the product for em > 45. The self-stacked and isolatedSi-MFI crystals may co-exist when em of the solvent is between45.0 and 41.0. Thus, the aggregation state of the Si-MFI crystalscan be controlled by careful selecting the dielectric constant ofthe solvent via mixing different co-solvent according to the equa-tion of em ¼ U1e1 þU2e2 þ � � � þUnen.

In contrast to the microwave-assisted solvothermal synthesis,the synthesis of all above described experiments under conven-tional solvothermal conditions (i.e. heating in oven) gave only iso-lated Si-MFI crystals, which indicates that the special heatingmechanism of microwave plays an important role in the regularitydescribed in this work.

4. Conclusions

Together with the microwave heating, the dielectric constant ofthe solvent has significantly influence on the morphology andaggregation of the Si-MFI crystals. The studies on the averagelength and fraction of the self-stacked Si-MFI crystals as the func-tion of the dielectric constant of the mixed solvent show that thesolvent with a dielectric constant of lower than 41.0 or higher than45.0 will result in the self-stacked crystals or the fully isolatedcrystals, respectively. There is a linear correlation between theaverage length of self-stacked Si-MFI crystals and the correspond-ing dielectric constant of the mixed solvent. The aggregation stateof the Si-MFI crystals can be controlled by careful selecting thedielectric constant of the solvent via mixing different co-solventaccording to the equation of em ¼ U1e1 þU2e2 þ � � � þUnen. How-ever, the complex relationship between em and the length of thestacked crystals when n-BuOH is used as the second co-solventsuggests that the formation of the isolated or self-stacked aggrega-tion state of Si-MFI crystals obtained is mainly controlled bydielectric constant of the solvent, but also the chemical propertiesof co-solvent can probably play an important role as well. The reg-ularities described in the microwave-assisted solvothermal syn-thesis system were not observed in the conventional oven-heated solvothermal synthesis system.

50 X. Chen et al. / Microporous and Mesoporous Materials 131 (2010) 45–50

Acknowledgements

This work is supported by the National Natural Science Founda-tion of China and the State Basic Research Projects(2006CB806103) of China. W. Y. thanks the support by the Programfor New Century Excellent Talents in University (NCET) and the Sci-entific Research Foundation for the Returned Overseas ChineseScholars, State Education Ministry.

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