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Microporous and Mesoporous Materials 65 (2003) 137–143

Energetics of a nanophase zeolite independent of particle size

Qinghua Li, Sanyuan Yang, Alexandra Navrotsky *

Thermochemistry Facility and NEAT ORU, University of California at Davis, Davis, CA 95616-8779, USA

Received 22 April 2003; received in revised form 22 April 2003; accepted 1 July 2003

Abstract

Silicalite-1 nanocrystals were synthesized with different sizes (40, 95 and 180 nm) from clear solutions and the or-

ganic structure directing agent removed by calcination. X-ray diffraction, scanning electron microscopy, nitrogen ad-

sorption, Fourier transform infrared spectroscopy and thermogravimetric analysis were used for characterization.

High-temperature solution calorimetry using lead borate (2PbO ÆB2O3) solvent at 974 K measured the enthalpies

(DHtran) relative to quartz at 298 K. The DHtran values are 8.2 ± 0.4 kJ/mol (40 nm), 8.3 ± 0.3 kJ/mol (95 nm) and

8.0± 0.5 kJ/mol (180 nm). These values are essentially the same as DHtran for microsized MFI crystals, 8.0 ± 0.8 kJ/mol

[J. Phys. Chem. B 104 (2000) 10001]. Thus the particle size has no effect on the framework energetics of silicalite. This

unusual phenomenon, which is different from that of condensed nanocrystalline metal oxides, is discussed in terms of

internal surface area and surface energy of zeolites.

� 2003 Elsevier Inc. All rights reserved.

Keywords: Silicalite-1; Nanocrystalline zeolite; Enthalpy of transition; Surface area

1. Introduction

The reduced size and large surface area in

nanocrystalline materials enable unique applica-

tions in microelectronics, nanocomposites, ultra-

thin oxide films, and other devices [1]. In general,

nanocrystalline materials have excess heat capa-

city, energy and entropy relative to their corre-

sponding bulk phases because of their large surfacearea and surface energy [2]. Since energetics of

nanocrystalline materials provide insight into their

thermal stability, which needs to be known for

rational and efficient processing, substantial re-

search has been focused recently on understanding

* Corresponding author. Tel.: +1-530-752-3252; fax: +1-530-

752-9307.

E-mail address: anavrotsky@ucdavis.edu (A. Navrotsky).

1387-1811/$ - see front matter � 2003 Elsevier Inc. All rights reserve

doi:10.1016/S1387-1811(03)00480-3

the energetic stability of nanocrystalline materialsby directly measuring thermochemical data. For

instance, using high-temperature oxide melt drop

solution calorimetry of nanocrystalline Al2O3, it

was found that the enthalpies of the nanophase

Al2O3 relative to coarse grained corundum in-

creased with the decrease of particle size and c-Al2O3 (the phase observed for nanosized particles)

was more stable in enthalpy than nanophase a-Al2O3 (where a-Al2O3 (corundum) was the macro-

crystalline thermodynamically stable phase) [3,4].

The enthalpy of TiO2 polymorphs (rutile, anatase,

and brookite) was also studied by the same cal-

orimetric method [5]. The closely balanced ener-

getics of TiO2 polymorphs with respect to bulk

rutile directly confirmed the crossover in stability

of nanophase polymorphs inferred by Zhang andBanfield [6].

d.

138 Q. Li et al. / Microporous and Mesoporous Materials 65 (2003) 137–143

Nanophase zeolitic materials are attracting in-

tense attention because of their unique properties.

For example, decreasing the particle size of zeolites

from micrometer to nanometer scale reduces mass-

and heat-transfer resistance in catalytic and ad-

sorptive processes [7]. Many studies dealing withthe synthesis, characterization and application of

nanosized zeolites have been done during the last

decade. However, little attention has been paid to

their surface energy. Recently, Moloy et al. [8] cor-

related the energetic metastabilities of high-silica

zeolites to a surface energy term that originates

from the large internal surfaces of these materials.

By means of Cerius2 molecular simulation soft-ware, they found that the enthalpy of anhydrous

silica zeolites could be correlated linearly to their

internal surface area (ISA). Many zeolitic silicas

are similar on energy and there is little thermo-

dynamic driving force for the interconversion

among the various micro- and mesoporous poly-

morphs of a given composition [9–11]. Zeolite

synthesis often produces micrometer sized crystals;however, a broad size distribution is often seen

[12]. In addition, specialized synthesis procedures

can produce nanometer sized crystals in a narrow

size range, perhaps by the aggregation of pre-

existing clusters or protoparticles [13]. Given the

large effect of external surface area (ESA) on the

energetics and polymorphism of dense metal ox-

ides such as alumina and titania, it is important toask whether similar effects are seen in microporous

materials such as silicalite.

In this work, nanocrystalline silicalite-1 (a crys-

talline microporous polymorph of silicon dioxide

with MFI framework topology [14]) with different

particle size have been synthesized from clear so-

lutions. High-temperature oxide melt solution cal-

orimetry is used to measure enthalpies relative toquartz. The results and their implications will be

useful to understand the effect of particle size on

energetics in microporous materials.

2. Experimental

2.1. Synthesis procedure

The synthesis of nanocrystalline silicalite-1 was

performed following a procedure reported earlier

[15,16]. Briefly, synthesis solutions with molar com-

position of xTPAOH : 25SiO2 : 480H2O : 100EtOH

(x ¼ 4; 7; 9) were used, where TPAOH was tetra-

propylammonium hydroxide and EtOH was eth-

anol. When lower TPAOH was used, i.e., x ¼ 4 or

7, a clear homogeneous solution was obtained byhydrolyzing tetraethoxy silane (TEOS) with

TPAOH solution at room temperature for 1 day

under stirring. For x ¼ 9, the synthesis solution

was aged at room temperature for 13 months to

produce smaller crystals. All synthesis mixtures

were heat-treated in a polypropylene reactor sub-

merged in a preheated bath maintained at 371 K.

After crystallization for 20 h, silicalite-1 solidswere obtained by centrifuging samples with a rel-

ative centrifugal force of 60,000 g for 2 h. The

liquid phase was decanted, and the solid phase

was redispersed in distilled water by ultrasound

treatment. This process was repeated four times to

remove all the unreacted materials. After centri-

fugation, samples were dried in air at room tem-

perature. These are called as-synthesized samples.Such as-synthesized samples were calcined in a

flow of nitrogen gas at 873 K for 2 h and, subse-

quently, in air for 10 h at the same temperature.

All calcined samples were redispersed in distilled

water for 24 h by ultrasound treatment, then dried

at room temperature.

2.2. Characterization

Prior to and after calcination, phase purity and

crystallinity were determined by X-ray powder

diffraction (XRD) on an Inel X-ray diffractometer

(XRG 3000) using Ni-filtered CuKa radiation

and operated at 30 kV and 30 mA. Data were

collected in the 2h range 5–45�. Particle size was

estimated by scanning electron microscopy (SEM)(Philips XL30 with a LaB6 emission source). Ni-

trogen adsorption measurements were performed

on as-synthesized and calcined samples with a

Micromeritics ASAP 2010 surface area analyzer.

As-synthesized samples were outgassed at room

temperature for 24 h prior to BET analysis.

Calcined samples were outgassed at 523 K over-

night prior to analysis. Fourier transform infraredspectroscopy (FTIR) (Bruker spectrometer, Equi-

nox 55) and thermogravimeric analysis (TGA)

Q. Li et al. / Microporous and Mesoporous Materials 65 (2003) 137–143 139

(Netzsch STA 449) were carried out to examine

whether the organic structure directing agent

(SDA) had been completely removed.

2.3. High-temperature drop solution calorimetry

All thermochemical measurements were per-

formed using a Tian–Calvet twin microcalorimeter

described in detail by Navrotsky [17]. High-tem-

perature oxide melt solution calorimetry with lead

borate (2PbO ÆB2O3) at 974 K was employed to

measure the enthalpies of drop solution (DHds). A

sample pellet weighing �15 mg was dropped from

room temperature into the solvent in the hot cal-orimeter. The enthalpy measured includes the heat

associated with heating the sample from room

temperature to 974 K plus its enthalpy of solution.

Silicalite-1 is hydrophobic. However, since even a

small amount of adsorbed water can strongly af-

fect the measured enthalpies, a special procedure

was carried out to prevent samples from being re-

hydrated. All calcined samples were first weighedinto �15 mg pellets in air and placed into small but

deep Pyrex vials. After overnight dehydration at

573 K, the vials with the sample were rapidly

transferred from the furnace to a desiccator, then

moved to a water-free glove box where the pellet

was accurately weighed. Right before the calori-

metric experiment, the sealed vial with the pellet

was taken out of the glove box, and the pellet wasdropped into the calorimeter. The total exposure

time to air was less than 5 s. All calorimetric ex-

periments were carried out under a flowing argon

atmosphere with a flow rate of �40 cm3/min.

3. Results and discussion

3.1. Characterization

The influence of TPAOH content and aging

time on the final particle size with the molar com-

position of xTPAOH : 25SiO2 : 480H2O :100EtOH

has been studied, where x is between 4 and 9.

The final particle size decreased with increasing

TPAOH content (x ¼ 4–7) and aging time. Inagreement with the previous findings [18,19], Fig.

1(a) and (b) show that the final particle size de-

creases from 180 nm to 95 nm when the x value ofTPAOH increases from 4 to 7. After extending

aging time to more than one year, small crystals

with a diameter of 40 nm were obtained when xequals 9, as shown in Fig. 1(c). Further increasing

TPAOH content (x > 7) did not lead to smallercrystals at the similar aging time. Therefore, 40 nm

silicalite-1 crystals may be the smallest achievable

from the molar composition of xTPAOH : 25SiO2 :

480H2O : 100EtOH (x ¼ 3–13). A similar result

was reported in the literature [20], where 55 nm

silicalite-1 particles were synthesized with a molar

composition of 3TPAOH : 25SiO2 : 390H2O at low

temperature (<308 K) with a very long synthesistime (40 months). Fig. 1 also shows the corre-

sponding calcined samples (see Fig. 1(a0), (b0), and

(c0)). There is no evidence for aggregation of small

particles and no obvious change of crystal topol-

ogy and crystal size after calcination, in agreement

with earlier work [21]. XRD patterns of the as-

synthesized and calcined samples are shown in Fig.

2. Compared to diffraction patterns of microme-ter-sized silicalite-1, the diffraction peaks of both

as-synthesized and calcined samples are broadened

[14], and intensity and crystallinity decrease with

decreasing particle size. The peaks of the 40 nm

silicalite-1 are broadened more after calcination,

which may be linked to framework deformation

upon removal of the SDA. BET surface areas are

listed in Table 1. It is assumed that no internalsurface area is vacated by simply drying the as-

synthesized samples at room temperature since

tetrapropylammonium cations (TPAþ) are fully

occluded in the channels. Thus, the data measured

from the as-synthesized samples are considered as

an estimate of the ESA of nanocrystalline silica-

lite-1. As expected, the surface area increases with

decrease in particle size. For smaller crystals (40nm), the ESA is 95 m2/g, while for larger crystals

(180 nm), it is 24 m2/g. It should be pointed out

that the ESA data from the as-synthesized samples

somewhat depend on the outgassing time. Longer

time leads to a slightly larger surface area, prob-

ably because some of TPAþ/H2O sitting close to

the surface of particles can be pulled out by pro-

longed outgassing. No difference in BET surfacearea values is observed between 24 and 36 h of

outgassing. Thus, all the as-synthesized samples

Fig. 1. SEM micrographs of nanocrystalline silicalite-1 synthesized with a molar composition of xTPAOH : 25SiO2 : 480H2O :

100EtOH (a) as-prepared sample with x ¼ 4, (a0) calcined sample of (a), (b) as-prepared sample with x ¼ 7, (b0) calcined sample of (b),

(c) as-prepared form with x ¼ 9 after aging 13 months, (c0) calcined sample of (c).

140 Q. Li et al. / Microporous and Mesoporous Materials 65 (2003) 137–143

have been outgassed for 24 h in this work. On

the basis of a framework density of 17.9 SiO2 units

per nm3 and assuming spherical particles, the

theoretical ESA is calculated, as seen in Table 1.

These values are very close to the ESA obtained by

adsorption. BET surface areas of the calcined

samples are also listed in Table 1. The data ob-tained are considered to be the total surface area

(TSA). The ISA, determined by the difference be-

tween the TSA and the ESA, are similar for all

samples, in the range of 400–430 m2/g. All the

calcined samples have no infrared absorption

peaks in the region 2900–3000 cm�1, confirming

the absence of SDA. TGA profiles of the cal-

cined samples indicate no weight loss from 473

to 723 K. These results are consistent with the

complete removal of SDA and the presence of

negligible H2O.

3.2. Enthalpies of transformation (quartz! zeo-

lite), (DHtrans)

The difference between the heat of drop solution

for quartz and silicalite-1 represents the enthalpy

of transition for the reaction quartzfi silicalite-1

at 298 K. The enthalpies of transition at 298 K

5 10 15 20 25 30 35 40 45

2 theta/degree

5 10 15 20 25 30 35 40 45

(a)As-synthesized form

40 nm silicalite-1

95 nm silicalite-1

180 nm silicalite-1

Inte

nsity

(lin

ear

scal

e)

2 theta/degree

40 nm silicalite-1

95 nm silicalite-1

180 nm silicalite-1

(b)Calcined form

Inte

nsity

(lin

ear s

cale

)

Fig. 2. X-ray diffraction patterns of nanocrystalline silicalite-1

synthesized with a molar composition of xTPAOH :

25SiO2 : 480H2O : 100EtOH (a) as-prepared forms, (b) calcined

forms.

Table 1

Surface area data from BET measurement and calculation

Silicalite-1

with different

particle size

(nm)

BET surface area (m2/g) Calculated

surface

area (m2/g)

ESA (as-

prepared

form)

TSA (calcined

form)

ESA

40 95± 1.2 504± 15.1 85

95 39± 0.4 456± 12.9 34

180 24± 0.2 460± 13.7 18

ESA¼ external surface area; TSA¼ total surface area.

Table 2

Measured enthalpies of drop solution (DHds) and enthalpies of

transition relative to quartz (DHtran) at 298 K for three calcined

samples

Samples DHds

(kJ/mol)

DHtran

(kJ/mol)

40 nm silicalite-1 30.9± 0.3 8.2 ± 0.4

95 nm silicalite-1 30.8± 0.1 8.3 ± 0.3

180 nm silicalite-1 31.1± 0.4 8.0 ± 0.5

Large crystals (MFI/OH)a 31.5± 0.2 8.0 ± 0.8

aDHds and DHtrans values are from Ref. [9].

Q. Li et al. / Microporous and Mesoporous Materials 65 (2003) 137–143 141

relative to quartz are calculated by the following

thermodynamic cycle:

from which the enthalpies of transition of silicalite-

1 are computed by

DHtran ¼ DH1 � DHds;

where DH1 ¼ 39.1 ± 0.3 (kJ/mol) [22]; DHds ¼mea-

sured enthalpy of drop solution.

The measured enthalpies of drop solution

(DHds) of three samples and the calculated en-thalpies of transition (DHtran) are presented in

Table 2. The DHds and DHtran values for all three

samples are the same within experimental error.

SiO2 (quartz, 298 K)fi SiO2 (sol, 974 K) DH1

silicalite-1 (s, 298 K)fi SiO2 (sol, 974 K) DHds

SiO2 (quartz, 298 K)fisilicalite-1 (s, 298 K) DHtran

The measured DHtran value for micrometer sizedpure-silica MFI is 8.0 ± 0.8 kJ/mol [9,23] which is

the same as that of our three nanosized samples.

Thus, the enthalpy of the pure-silica MFI zeolite

(silicalite-1) does not depend on the particle size,

that is, the ESA, at least for particles P 40 nm.

The ratio of ISA to ESA is 4.1 (40 nm); 10.6 (95

nm), and 18.1 (180 nm), indicating the ISA dom-

inates the ESA and the TSA even for nanosizedzeolites. From the viewpoint of energetics, the

large ISA of zeolite requires a small surface energy

to make the system energetically accessible. Moloy

et al. have calculated the internal surface enthalpy

for a series of pure-silica zeolites to be 0.093±

0.009 J/m2 [8], which is more similar to the external

surface energy (ESE) of amorphous silica than that

of quartz, the latter being significantly higher [23].If this surface enthalpy reflects the energetics of

both internal and external surfaces, the ESE of

silica zeolites can be also estimated to be 0.093±

0.009 J/m2. Table 3 presents external surface en-

ergies for various oxides. Compared to zeolites, the

Table 3

Surface energies and excess enthalpy of 40 nm particles relative to bulk for various oxides

Oxides ESE (J/m2) Excess enthalpy of 40 nm

particle relative to bulk (kJ/mol)

Ref.

SiO2 (zeolite) 0.093± 0.009 0.53± 0.01 [8]

a-Al2O3 2.6 ± 0.2 10.0± 0.4 [3]

c-Al2O3 1.7 ± 0.1 6.5 ± 0.3 [3]

TiO2 (rutile) 2.2 ± 0.2 6.2 ± 0.3 [5]

TiO2 (anatase) 1.0 ± 0.2 3.1 ± 0.2 [5]

TiO2 (brookite) 0.4 ± 0.1 1.2 ± 0.1 [5]

ESE¼ external surface enthalpy.

142 Q. Li et al. / Microporous and Mesoporous Materials 65 (2003) 137–143

ESE is considerably larger for oxides which do notform framework structures with low density. Table

3 also lists the excess enthalpy of 40 nm particles

relative to bulk. For 40 nm a-Al2O3 (which is

unstable relative to 40 nm c-Al2O3) the excess

enthalpy relative to bulk is as large as 10 kJ/mol.

Even for 40 nm brookite (the stable polymorph at

the nanometer scale [5]), the excess enthalpy is 1.2

kJ/mol. For 40 nm silicalite-1, the ESA of 95 m2/gyields an excess enthalpy relative to bulk of only

0.53 kJ/mol, which is within the experimental error

of DHtran. This indicates that the effect of particle

size on energetics is too small to measure for silica

zeolites, at least for P 40 nm particles, because of

small surface energy.

These observations lead to several inferences of

general applicability. (1) The existence of a largenumber of polymorphs with microporous struc-

tures requires them to be similar in energy and not

too much higher than dense frameworks. This re-

quires a small ISA. If ISE and ESE are similar, this

implies small ESE and little dependence of energy

on particle size for microporous materials in gen-

eral. (2) Because of the small surface energy,

crossovers in stability of microporous material as afunction of particle size are unlikely in contrast to

the behavior of dense polymorphs. (3) Silica zeo-

lites having different structures (variation of den-

sity of a factor of two) have many similar entropies

at 298 K [24]. Although the heat capacity of a

given zeolite has not been measured as a function

of particle size to obtain a surface entropy, we

expect the difference in S0 to be small, especially inview of the above. Thus, the thermodynamic

driving force for coarsening, DG ¼ DH � TDS, isexpected to be very small. The latter may explain

why it is hard to grow large zeolite crystals. If

similar trends hold for other types of microporousmaterials, for example manganese oxides, it can be

predicted that the enthalpy of formation of

nanosized microporous MnO2 will be similar to

that of micrometer sized MnO2. Many MnO2

phases are nanosized and do not coarsen readily,

consistent with the arguments above. A low sur-

face energy (for both ESA and ISA) makes it

possible to synthesize and maintain nanophaseporous MnO2, which has potential applications in

electrochemical and catalytic fields [25].

4. Conclusion

Pure-silica MFI zeolites (silicalite-1) with par-

ticle size of 40, 95, and 180 nm were synthesizedfrom clear solutions by varying TPAOH content

and aging time. SEM images showed that there

was no aggregation of small particles and no ob-

vious change of crystal topology and particle size

after thermal calcination. High-temperature drop

solution calorimetry was used to measure the

enthalpy relative to quartz (DHtrans) at 298 K to

study the effect of particle size on energetics. Themeasured DHtrans value for all three nanosized

samples is the same within experimental error and

the same as that of micrometer sized pure-silica

MFI. This behavior is different from that of more

dense metal oxides, which show significant in-

crease in energy with decreasing particle size. This

unusual independence of particle size and ener-

getics for zeolites is associated with a large ISA,which dominates the ESA and the TSA even for

nanosized zeolites, and with small surface energy.

This behavior may be general for other zeolites

and microporous materials.

Q. Li et al. / Microporous and Mesoporous Materials 65 (2003) 137–143 143

Acknowledgement

This work was supported by the National Sci-

ence Foundation, Grant DMR-01-01391.

References

[1] A.N. Goldstein, Handbook of Nanophase Materials,

Marcel Decker, New York, 1977.

[2] A. Navrotsky, Reviews in Mineralogy and Geochemistry,

Nanoparticles and the Environment, 44, 2001, p. 73.

[3] J.M. McHale, A. Auroux, A.J. Perrotta, A. Navrotsky,

Science 277 (1997) 788.

[4] J.M. McHale, A. Navrotsky, A.J. Perrotta, J. Phys. Chem.

101 (1997) 603.

[5] M.R. Ranade, A. Navrotsky, H.Z. Zhang, J.F. Banfield,

S.H. Elder, A. Zaban, P.H. Borse, S.K. Kulkarni, G.S.

Doran, H.J. Whitfield, PNAS 99 (2002) 6476.

[6] H.Z. Zhang, J.F. Banfield, J. Phys. Chem. B 104 (2000)

3481.

[7] J. Weitkamp, L. Puppe, Catalysis and Zeolites (Funda-

mentals and Applications), Springer-Verlag, Berlin, Hei-

delberg, 1999.

[8] E.C. Moloy, L.P. Davila, J.F. Shackelford, A. Navrotsky,

Micropor. Mesopor. Mater. 54 (2002) 1.

[9] P.M. Piccione, C. Laberty, S. Yang, M.A. Camblor, A.

Navrotsky, M.E. Davis, J. Phys. Chem. B 104 (2000)

10001.

[10] A. Navrotsky, I. Petrovic, Y. Hu, C.Y. Chen, M.E. Davis,

Micropor. Mater. 4 (1995) 95.

[11] Y. Hu, A. Navrotsky, C.Y. Chen, M.E. Davis, Chem.

Mater. 7 (1995) 1816.

[12] R.M. Barrer, Hydrothermal Chemistry of Zeolites, Aca-

demic Press, New York, 1982.

[13] C.E.A. Kirschhock, R. Ravishankar, F. Verspeurt, P.J.

Grobet, P.A. Jacobs, J.A. Martens, J. Phys. Chem. B 103

(1999) 4965.

[14] E.M. Flanigen, J.M. Bennet, R.W. Grose, J.P. Cohen, R.L.

Patton, R.M. Kirchner, J.V. Smith, Nature 271 (1978) 512.

[15] B.J. Schoeman, J. Sterte, J.-E. Otterstedt, Zeolites 14

(1994) 568.

[16] B.J. Schoeman, J. Sterte, J.-E. Otterstedt, J. Porous Mater.

1 (1995) 185.

[17] A. Navrotsky, Phys. Chem. Miner. 24 (1997) 3.

[18] A.E. Persson, B.J. Schoeman, J. Sterte, J.-E. Otterstedt,

Zeolites 14 (1994) 557.

[19] Q. Li, B. Mihailova, D. Creaser, J. Sterte, Micropor.

Mesopor. Mater. 43 (2000) 51.

[20] R.W. Corkery, B.W. Ninham, Zeolites 18 (1997) 379.

[21] R. Ravishankar, C. Kirschhock, B.J. Schoeman, P. Va-

noppen, P.J. Grobet, S. Storck, W.F. Maier, J.A. Martens,

F.C. De Schryver, P.A. Jacobs, J. Phys. Chem. B 102

(1998) 2633.

[22] I. Kiseleva, A. Navrotsky, I.A. Belitsky, B.A. Fursenko,

Am. Mineral 81 (1996) 668.

[23] I. Petrovic, A. Navrotsky, M.E. Davis, S.I. Zones, Chem.

Mater. 5 (1993) 1805.

[24] P.M. Piccione, B.F. Woodfield, J. Boerio-Goates, A.

Navrotsky, M.E. Davis, J. Phys. Chem. B 105 (2001)

6025.

[25] S.L. Brock, N. Duan, Z.R. Tian, O. Giraldo, H. Zhou, S.L.

Suib, Chem. Mater. 10 (1998) 2619.