Thermal expansion studies of silicalite-2 molecular sieves of MEL(ZSM-11) topology

D. S. Bhange • Veda Ramaswamy

Published online: 8 May 2011

� Springer Science+Business Media, LLC 2011

Abstract Thermal expansion behaviour of siliceous poly-

morph of MEL type molecular sieves i.e. silicalite-2 is inves-

tigated by high temperature X-ray diffraction technique in the

temperature range 298–773 K. The negative lattice ther-

mal expansion coefficients, aa = -3.252 9 10-6 and ac =

-6.436 9 10-6 K-1 in the temperature range 298–773 K for

silicalite-2 samples were observed. The thermal expansion

behavior of silicalite-2 is anisotropic, with the relative

strength of contraction along ‘c’ axis is more than that along

‘a’ and ‘b’ axes. NTE seen over a temperature range 298-

773 K could be associated with transverse vibrations of

bridging oxygen atoms in the structure, which results in an

apparent shortening of the Si–Si nonbonding distances.

Keywords Microporous materials � X-ray diffraction �Thermal expansion

1 Introduction

ZSM-5 (MFI topology) is considered as one end-member

of polytypes of the pentasil zeolite family, while the other

end-member of this family is ZSM-11 (MEL topology)

zeolite. The framework structure of ZSM-11 zeolite shows

higher symmetry than that of ZSM-5 [1]. The clear dis-

cussion of the framework structures of both the polytypes

is reported by Treacy et al. [2]. Zeolites with MEL

topology have been synthesized and studied in details by

using NMR and X-ray diffraction techniques [3–7]. The

MEL zeolites act as catalysts for the transformation of light

paraffins into aromatic hydrocarbons at higher tempera-

tures [8–10]. To understand the properties of zeolites at

operating conditions, one has to understand the crystal

structure at the reaction temperatures, because both the

structure and property are intimately related to each other.

As these materials have been used as catalysts in various

catalytic reactions, it is necessary to study the thermal

expansion behavior of these materials within their opera-

tional regenerative temperature range (298–773 K).

Microporous crystalline zeolites and molecular sieves viz.,

pure silica zeolites ITQ-1, ITQ-3, and SSZ-23 [11], ITQ-4

[12], faujasite [13], ZSM-5 [14–16] Silicalite-1 and met-

allosilicate of MFI family [17–20], and AlPO-5 [14] and

AlPO-17 [21], are amongst the materials which are known

to show negative thermal expansion (NTE) on heating. The

vibrational modes of bridging Si–O–Si/Al are held

responsible for the NTE behaviour exhibited by the zeolitic

materials.

Research interest is increasing day by day in the mate-

rials exhibiting NTE. Materials that display NTE are being

investigated both because of their technological interest

and scientific curiosity. No MEL types of molecular sieve

materials are studied for their high temperature behavior

till to date, while the other end member of the pentasil

family i.e. MFI is reported to exhibit negative thermal

expansion when heated. In the present work we have car-

ried out the high temperature X-ray diffraction (HT-XRD)

studies on the silicalite-2 molecular sieves having MEL

topology first time in order to study its thermal expansion

behaviour. The results are correlated with the thermo-

gravimetric data and compared with the thermal expansion

coefficient of siliceous zeolites reported in literature.

D. S. Bhange (&)

Department of Chemistry, Shivaji University, Kolhapur 416004,

India

e-mail: bhangeds@yahoo.co.in

V. Ramaswamy

Chemical Physics Laboratory, Central Leather Research

Institute, Chennai 600020, India

123

J Porous Mater (2012) 19:301–305

DOI 10.1007/s10934-011-9476-7

2 Experimental

Synthesis of the silicalite-2 molecular sieve under study

was carried out by using the procedure described elsewhere

[22]. Obtained powder was filtered, washed, dried at 80 �C

and calcined at 550 �C in air (*8 h) to remove template

inside the pores. The phase purity of the prepared sample

was checked using powder X-ray diffraction technique.

High temperature X-ray diffraction (HTXRD) patterns in

air were collected on the Philips X’Pert Pro 3040/60 dif-

fractometer equipped with Anton Parr HTK 1600 attach-

ment. A small amount of silicon (NIST a = 5.4311 A) as

an internal standard was added to the sample. The sample

thickness was *0.5 mm which was mounted on a platinum

strip (cavity), which serves as the sample stage as well as

the heating element. A Pt/Rh–13% thermocouple spot-

welded to the bottom of the stage was used for measuring

the temperature. a-Al2O3 standard (NIST, Gaithersberg,

USA) was used for the calibration of the high temperature

stage. The HTXRD patterns for silicalite-2 sample in the

temperature range 298–773 K were collected in the 2hregion 5�–60� in the continuous mode with a step size of

0.0167 and a time 20 s/step using Ni filtered Cu Ka radi-

ation (k = 1.5406 A) and X’celerator as detector with

Bragg–Brentano geometry. Diffraction patterns were col-

lected at every 50 K interval. A heating rate of 10 K min-1

and a soak time of 10 min were applied. The optics used in

the incident beam (primary) were 0.04 radian soller slit, ��divergence slit and 10 mm mask, the and 0.02 radian soller

slit in the secondary beam path. The information regarding

the unit cell parameters were extracted from the collected

HTXRD patterns using the full profile fitting method. The

thermal expansion coefficient along the crystallographic

directions a and c were calculated using the formulae

aa = Da/(T-RT)aRT and ac = Dc/(T-RT)cRT respectively,

the lattice or volume thermal expansion coefficient was

calculated using the formula aV = DV/(T-RT)VRT where T

and RT are the typical temperature of the scan and room

temperature respectively, Da, Dc and DV are the differences

in the values of the respective unit cell parameters of the

scans at T and RT. In addition, the thermogravimetric (TG)

measurements were performed on Metller Toledo Star sys-

tem at the rate of 10 K min-1 from 300 to 773 K.

3 Results and discussion

XRD pattern (not shown here) of calcined silicalite-2

confirmed the sample to be single-phase material of MEL

topology without any impurity phases. Figure 1 shows the

multiple plot of HTXRD patterns of silicalite-2 in the

temperature range 298–773 K at intervals of 50 K from

323 to 773 K. Peaks appearing at 2h = 39.75� and 46.35�

are the (111) and (002) reflection of Pt sample holder

respectively. The Fig. 2 shows the expanded areas of the

high-temperature X-ray diffraction patterns at typical

temperatures 298 and 773 K in the 2h region 22�–25�which give a greater visual indication of the changes that

we have observed in the silicalite-2. It can be seen that the

diffraction lines are shifted to higher 2h values with

increasing temperature indicating the contraction of the

10 20 30 40 50 60

0

2000

4000

6000

8000

∗∗

Cou

nts

2 Theta (deg)

Fig. 1 Powder XRD patterns of silicalite-2 recorded during in situ

heating under static air at intervals of 50 K. The bottom trace is the

pattern measured at room temperature before heating. X-ray peaks

from the platinum sample holder are marked by asterisks and Si

standard by filled circle

22 23 24 25

0

2500

5000

Cou

nts

2 Theta (deg)

298 K 773 K

Fig. 2 Magnified view of the high-temperature X-ray diffraction

patterns at typical temperatures 298 and 773 K in the 2h region

22�–25�

302 J Porous Mater (2012) 19:301–305

123

lattice. Effect of temperature on unit cell parameters a, c

and the unit cell volume V of silicalite-2 are shown in

Fig. 3. The error bars shown are according to estimated

standard deviation (esd) from the full profile refinement.

The thermal expansion coefficients calculated for different

temperature ranges are given in Table 1. The values in

Table 1 clearly show that there is negative thermal

expansion along the ‘c’ axis within temperature range

298–773 K. The material exhibits a positive thermal

expansion along ‘a’ axis (aa = 0.443 9 10-6 K-1) in the

temperature range 298–523 K, yet the overall thermal

expansion along ‘a’ axis is negative (aa = -3.252 9

10-6 K-1) in the temperature range 298–773 K. The initial

expansion can be explained on the basis of the unfolding of

polyhedrons comprising the structure, which may attain the

saturation at 523 K and after that it may contract due to the

transverse thermal vibration of the bridging oxygen atoms

in the Si–O–Si bonds. The initial thermal expansion

behaviour is correlated to the dehydration of physisorbed

water in the pores. Thermogravimetric (TG) data (Fig. 4)

reveals that *3.5% of the original weight of a specimen is

lost at temperatures below 373 K, over this initial tem-

perature range, the observed XRD data shows gradual

changes in unit cell parameters ‘a’ and ‘c’. Although there

is only marginal change in unit cell volume from 298 to

323 K, the values of ‘a’ and ‘c’ are modified considerably.

20.04

20.05

20.06

20.07

'a' Å

T, K

(a)

300 400 500 600 700 800

13.37

13.38

13.39

13.40

13.41

'c' Å

T, K

(b)

5368

5376

5384

5392

5400

'V' Å

3

T, K

(c)

300 400 500 600 700 800

300 400 500 600 700 800

Fig. 3 a–c Variation of unit

cell parameters a, c and V as

function of Temperature for

silicalite-2 sample

Table 1 The thermal expansion coefficients calculated for different

temperature ranges

Temperature (K) aa (10-6 K-1) ac (10-6 K-1) aV (10-6 K-1)

298 – – –

323 11.96 -26.844 -3.77

373 4.651 -10.936 -1.136

423 1.993 -6.562 -2.592

473 1.139 -5.113 -2.825

523 0.443 -5.302 -4.494

573 -0.724 -5.423 -6.895

623 -1.073 -6.195 -8.404

673 -1.727 -6.164 -9.649

723 -2.579 -5.965 -11.101

773 -3.252 -6.436 -12.954300 400 500 600 700 800

96

98

100

Wei

ght

loss

(%

)

Temperature (K)

Fig. 4 Thermogravimetric (TG) curve of silicalite-2

J Porous Mater (2012) 19:301–305 303

123

The length of ‘a’ axis was increased by 0.007 A while the

value of ‘c’ was decreased by 0.01 A. The similar thermal

expansion behavior was observed for HZSM-5 during

thermally induced departure of crystal water in H-ZSM-5

[23]. When dehydrated HZSM-5 was studied for thermal

expansion behavior it revealed the total negative thermal

expansion behavior in the same temperature range.

Therefore, it is reasonable to rationalize the remarkable

change of thermal expansion behavior occurring in silica-

lite-2, between 298 and 323 K, as a function of thermally

induced physisorbed water departure (dehydration). In the

temperature range 298–773 K, the material exhibits overall

negative thermal expansion. The thermal expansion coeffi-

cients along ‘a’ and ‘c’ axis are aa = -3.25 9 10-6 K-1,

ac = -6.44 9 10-6 K-1 respectively, and the volume

thermal expansion coefficient is aV = -12.95 9 10-6 K-1

(Table 1). The negative thermal expansion exhibited by this

material is due to the transverse thermal vibration of the

bridging oxygen atom. The contraction of the calcined sili-

calite-2 material may originate from the availability of the

empty space in the framework structure to accommodate the

changes occurring in the structure at elevated temperature.

Most of the zeolitic materials studied so far in the lit-

erature for their negative thermal expansion behavior, have

been characterized by the presence of ‘‘rigid unit modes’’

where local flexibility resulted from oxygen hinges linking

corner-sharing, rigid tetrahedral silicate building units

(TO4). The concerted rotation of rigid TO4 units about their

linked corners is dominant over other structural distortions,

leading to the shrinkage of the structure induced by the

change in temperature. Thus the TO4 tetrahedra essentially

remain intact and the required changes are accommodated

by the transverse thermal vibrations of T–O–T bonds. The

same mechanism could be used to explain the negative

thermal expansion exhibited by silicaite-2 in the studied

temperature range. It is worthwhile to compare the thermal

expansion coefficients of silicalite-2 with that of other

siliceous zeolites reported in literature, which will allow us

to correlate the effect of channel dimensionality on the

thermal expansion behavior of all silica zeolites. The val-

ues of thermal expansion coefficients of various all silica

zeolites studied so far in the literature are listed in Table 2.

The direct comparison of the ‘a’ values of silicalite-2 with

those of reported zeolites is not straight forward as the

different temperature ranges are employed by different

authors in their studies. It should also be noted that the

negative thermal expansion observed by Park et al. does

not occur over the full temperature range studied [14] and

ITQ-4 [12] and FAU [13] are studied under the cryogenic

conditions i.e. below room temperature. Compressibility of

zeolitic framework is to some extent, revealed by the

possible variation in the framework density of the structure,

which depends on the composition and pore volume. The

framework density is calculated using the unit cell volume

(from XRD data) of the silicalite-2 and silicalite-1 and

given in Table 2. The data in Table 2 reveal that the

compressibility of silicalite-2 is larger than the silicalite-1

and reflected in framework density value. On the other

hand the compressibility of silicalite-2 is matching well

with compressibility shown by other high silica zeolites

such as ITQ-1, ITQ-3, ITQ-4, ITQ-9 [24], SSZ-23 and

FAU, irrespective to their framework density and channel

dimensions. The quality of the present data was not enough

good for the full structural analysis by using the Rietveld

refinement of the PXRD patterns collected at high tem-

peratures. So we suggest further detailed structural analysis

of the silicalite-2 molecular sieve using high resolution

PXRD data in order to get the insights into the changes

occurring in channel dimensions and T–O–T angles as a

function of temperatures. Further the silicalite-2 molecular

sieve can be used (by adding the calculated amount) to

make the composite materials of interest having desired

thermal expansion properties e.g. composites with zero

thermal expansion.

Table 2 The thermal expansion coefficients reported for variety of siliceous zeolites

Material Thermal expansion coefficient (10-6 K-1) Temperature range (K) Framework density Reference

ITQ-1 -4.23 -4.23 -3.21 -12.1 323–773 16.6 [11]

ITQ-3 -0.29 -2.06 -10.1 -11.4 323–823 16.2 [11]

SSZ-23 -6.09 -3.21 -0.73 -10.3 323–773 16.7 [11]

ITQ-4 -11.5 -7.47 7.19 -9.1 95–510 17.0 [12]

CHA -8.24 -8.24 -13.3 -28.5 293–873 15.4 [12]

ITQ-7 -2.28 -2.28 -1.05 -5.6 473–873 15.4 [24]

ITQ-9 -5.58 -2.37 -2.19 -10.0 293–873 17.2 [24]

FAU -4.2 -4.2 -4.2 -12.6 25–573 13.5 [13]

Silicalite-1 (MFI) -5.7 -0.5 -0.6 -6.8 298–1,023 17.9 [17]

Silicalite-2 (MEL) -3.25 -3.25 -6.44 -12.95 298–773 17.7 This work

304 J Porous Mater (2012) 19:301–305

123

4 Conclusions

The HTXRD studies were carried out on silicalite-2 molec-

ular sieves in the temperature range 298–773 K. Silicalite-2

exhibits anisotropic negative thermal expansion on heating.

The strength of the NTE is higher along the ‘c’ direction. The

overall lattice thermal expansion coefficient in the temper-

ature range studied is aV = -12.95 9 10-6 K-1.

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