The change of the unit cell dimension of different zeolite types by heating and its influence on supported membrane layers

Microporous and Mesoporous Materials 117 (2009) 10–21

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The change of the unit cell dimension of different zeolite types by heating and its influence on supported membrane layers M. Noack a,*, M. Schneider a, A. Dittmar a, G. Georgi a, J. Caro b a b

Leibniz Institute for Catalysis at the University Rostock, Branch Berlin Richard Willstätter Strasse 12, D-12489 Berlin, Germany Institute of Physical Chemistry and Electrochemistry, Leibniz University of Hannover Callinstr. 3-3A, D-30167 Hannover, Germany

a r t i c l e

i n f o

Article history: Received 10 January 2008 Received in revised form 29 April 2008 Accepted 9 May 2008 Available online 15 May 2008 Keywords: Unit cell dimension MFI LTA MOR FAU crystals Water loading Hydrophilicity Intercrystalline mesopores Zeolite membranes

a b s t r a c t In comparison with the well-known swelling of organic polymer membranes, zeolite membranes have been considered for a long time as relatively stiff. The problem of the different thermal expansion coefficients of the a-Al2O3 support and the zeolite layer was usually reduced on a possible crack formation during the template removal of MFI membranes. However, the crack formation could be avoided by an extremely slow heating rate and by membrane layers formed of small and non-oriented zeolite crystallites. Several papers have shown that the isomorphous incorporation of Al into the MFI structure (silicalite ? ZSM-5) results in an increase of the non-selective intercrystalline transport and – as a consequence – the separation selectivity drops. In accordance with this observation, no shape-selective gas separation is reported for the Al-rich zeolite membranes MOR, FAU and LTA. On the other hand, these membranes allow the highly selective separation of water from organic mixtures due to hydrophilic interactions. In the present paper the change of the unit cell dimension (UCD) for zeolites LTA, FAU, MOR and MFI was studied as a function of temperature and water content using Rietveld refinement. Parallel to the determination of the linear and volume expansion coefficients by in situ-heating XRD, the de-watering and detemplating was studied by thermogravimetry (TGA). A strong UCD-change was found for all Al-rich zeolite types as a result of the de-watering. In contrast, the smallest changes of the UCD in the temperature range 50–450 °C were found for MFI crystals. Nevertheless, also for MFI membrane layers with rising temperature an increasing tension between the expanding a-Al2O3 support and the slightly shrinking membrane layer takes place. Zeolite layers of small crystals with either a random or a preferential crystal orientation relative to the support and an only partial de-watering in the case of Al-rich zeolite layers can minimize the tension in the support-membrane system. Thus, the irreversible formation of macroscopic cracks can be avoided and the formation of small intercrystalline pores in the mesopore region becomes reversible. Ó 2008 Elsevier Inc. All rights reserved.

1. Introduction Molecular sieve membranes are favourable materials for the separation of mixtures by size or interaction. For an ideal molecular sieve membrane, permeation should occur only through the regular intracrystalline pores of the zeolite layer. However, more realistic is a parallel transport through inter and intracrystalline pores giving modified permeation properties with higher fluxes and lower separation factors. One reason for the existence of intercrystalline defect pores can be the insufficient intergrowth of the crystals in the membrane layer [1]. Another reason for intercrystalline defect pores can be the thermal template removal or the complete de-watering of the membrane layer. Already in 1958, Barrer and Meier found for LTA a step-wise increase of the lattice constant * Corresponding author. E-mail addresses: [email protected], [email protected](M.Noack). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.05.013

with increasing water loading [2]. For Ca-exchanged LTA (5A = NaCaA) a contraction of the unit cell dimension (UCD) from a = 2.464 nm for the hydrated to a = 2.400 nm for the de-hydrated zeolite at 400 °C was found. When the dehydrated 5A is further heated, thermal expansion of the u.c. (unit cell) takes place and a value of a = 2.454 nm was found at 800 °C [3]. From the in situ-heating XRD of silicalite it followed that in the low temperature range from 155 to 25 °C the as synthesized MFI showed a volume expansion with 5.3  106 K1 [4]. For the calcined silicalite crystals between 15 and 75 °C the phase change from monoclinic to orthorhombic with a volume expansion of 27.7  106 K1 was observed. However, between 120 and 700 °C a volume contraction with 15.1  106 K1 takes place [5]. At the same time, the first successful preparations of MFI membranes on porous ceramic supports were reported. Dilatometric measurements over the large temperature range between 20 and 1000 °C showed a constant linear thermal expansion coefficient for

M. Noack et al. / Microporous and Mesoporous Materials 117 (2009) 10–21

a-Al2O3 of 8.6  106 K1 [6] and for sintered stainless steel supports of 17  106 K1. This means that the thermal expansion of ceramic and metal supports with increasing temperature is opposite to the volume contraction e.g. a calcined MFI crystal layer. The thermal de-composition of tetrapropylammonium ions (TPA) which are usually used in the MFI synthesis as structure directing agent (SDA), has been studied in detail. If the TPA-containing as synthesized MFI membrane is treated at 300 °C for 24 h in air, only 50% of the maximum N2 flux and no flux for n-butane were measured on this semi-calcined MFI membrane. A temperature of 480 °C was found to be necessary for a complete template removal [7]. From TGA studies of the de-templating of as synthesized MFI powder, a heating rate of 0.3 °C/min to 450 °C and 5 h isothermal treatment at this temperature has been proposed [8]. The template removal as a function of temperature and duration has been systematically studied by Noble and coworkers for ZSM-5 membranes [9], from single and mixture permeation studies the existence of parallel transport paths assigned to intra and intercrystalline permeation was concluded. The application of permporosimetry on microporous materials like ultrafiltration membranes [10] and later on zeolite membranes [11] has become a powerful tool for the diagnostics of intercrystalline defect pores in zeolite membrane layers thus allowing a correlation between membrane synthesis and permeation properties. Not only the presence and the removal of water and template in the as synthesized samples but also the adsorption of guest molecules on the calcined zeolites can cause a structure change. In the case of silicalite, the loading with n-hexane triggers the change from the monoclinic to the orthorhombic symmetry [12]. A high loading with benzene (6–8 molecules per u.c.) causes a pseudotetragonal deformation of the orthorhombic lattice symmetry [13]. In very systematic single and mixture gas permeation studies of n-hexane and 2,2-dimethylbutane [14], methane, n-butane and i-octane [15], the influence of temperature, partial pressure and the order of the adsorption of different co-adsorbed guest molecules on mixture permeation patterns was studied. From the corresponding measurements, a transport model has been proposed which is based on the co-existence of regular zeolite pores and additional intercrystalline pores with specific adsorption affinity for certain mixture components resulting in a ‘‘pore blocking effect” [15]. Recently, the same group [14,15] found that the adsorption of n-hexane causes MFI crystals reversibly to expand and it was concluded that this expansion can reduce the size of non-zeolite intercrystalline defect pores thus increasing the permeation selectivity [16]. The transient permeation was correlated with single crystal XRD data of loaded MFI crystals and the inverted light microscopy of empty as well as loaded MFI single crystals [17]. In co-operation with colleagues from Prague it was found by transient permeation that the permeation of i-octane is strongly influenced by a pre-adsorption of n-hexane but not by that one of benzene. Consequently, transient permeation can be regarded as a modified permporosimetry technique. From single crystal XRD it was found that the u.c. volume was changed by +0.28%, 0.27% and +1.02% when loading the MFI crystals with n-octane, benzene and n-hexane, respectively. The very thorough studies from 1999 [15] show in principle that the size of the intercrystalline non-zeolite pores can be reduced by pre-adsorption of suitable molecules such as n-hydrocarbons [17]. The MFI crystals in a membrane layer will expand when loaded with alkanes such as n-hexane, n-octane and this can lead to a reversible narrowing or even closure of mesopores in the membrane layer thus improving the mixture separation performance. This effect depends on the kind and sorption technique of the pre-adsorbed components [15,18,19]. That is to say that the MFI membranes must not be regarded any longer as stiff and rigid during the permeation process.

11

Very recently, several papers on the change of the unit cell dimension (UCD) of MFI crystals were published. When starting at room temperature, the a-direction of the u.c. continuously contracts with increasing temperature, but for the b- and c-directions between room temperature and 100 °C first an expansion is reported by Bhange et al. [20]. Studying the formation of macroscopic cracks upon temperature increase for MFI layers on a-Al2O3 supports by XRD, it was concluded that MFI layers of large crystals tend to crack formation whereas MFI layers of small crystals reduce the thermal stress by the reversible formation of small intercrystalline mesopores [21]. In a following paper, these authors studied the influence of the heating regime during the template removal by HT-XRD [22]. The u.c. volume of the calcined MFI is found to be smaller in comparison with the template-containing as synthesized one. Further, it was found that the rate of the u.c. contraction is related to the rate of the template de-composition. In XRD studies on hydrated NaA crystals the usual lattice contraction with increasing temperature was found, but in the temperature window between 100 and 150 °C a strong expansion was detected which is by one order of magnitude larger than the expansion of the a-Al2O3 support [23]. Due to the de-watering, for BaY a strong lattice expansion between 50 and 350 °C was found by using XRD [24]. NaY behaves very similarly [25]. The authors of this contribution synthesized MFI membranes of different Si/Al-ratios between 57 and 1 and correlated the permeation studies with the Si/Al-ratio of the MFI membranes [26–28]. It was found that with increasing Al content of the MFI membranes, the contribution of intercrystalline non-zeolite permeation increased. In continuation of these studies, the zeolite types LTA, FAU and MOR were investigated. These membranes are strongly hydrophilic and can perfectly separate water from organic solutions but fail so far in shape-selective separations [1,29]. It is the aim of this paper to study the influence of the Al-content of a zeolite and – thus its hydrophilic properties – on the change of its UCD for the zeolites LTA, FAU, MOR and MFI zeolites by in situ-heating XRD. Starting with hydrated and as synthesized or dried and calcined crystals, the change of the UCD as a result of de-watering, template removal and temperature could be distinguished. The results are correlated with the results of TGA. Our studies are praxis-relevant and simulate the way of a freshly synthesized zeolite membrane until its use in a transport-active, that is to say dried and calcined state for the permeation experiment. By comparing the findings from different zeolite types, the reasons for the experimental difficulties to prepare shape-selective hydrophilic Al-rich zeolite membranes are discussed. 2. Experimental The Si/Al-ratios and crystallographic data of the different zeolite structure types are collected in Table 1. For comparison, corund is included since most of the zeolite membranes are prepared on asymmetric supports, mainly a-Al2O3. The zeolite powders used in this paper were taken from the sediment of the corresponding zeolite membrane syntheses1. After synthesis, the zeolite powders were rinsed with water several times and dried at 100 °C for 5 h. In the case of MFI, the samples were heated with 0.3 °C/min up to 450 °C for the thermal de-composition of the template TPA+, hold for 5 h in air at 450 °C and then slowly cooled during 5 h to room temperature (calcined MFI). The as synthesized template-containing samples were only dried at 100 °C (as synthesized MFI). The calcined and as synthesized samples were

1 In the case of MOR, the commercial product of TRICAT, now Süd-Chemie, was used.

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M. Noack et al. / Microporous and Mesoporous Materials 117 (2009) 10–21

Structure type

Si/Al-ratio

Space group

Symmetry

Unit cell dimension (nm)

LTA FAU MOR

1.0 1.2 6.3

Fm3c Fd3m Cmcm

Cubic Cubic Orthorhombic

MFI

57 or >1000

P21/n

Monocline

Pnma

Orthorhombic

R-3c

Rhombohedral

a = 2.46 a = 2.47 a = 1.81 b = 2.05 c = 0.75 a = 1.988 b = 2.011 c = 1.337 b = 90.67° a = 2.01 b = 1.99 c = 1.34 a = 0.476 b = 1.299

a- Al2O3

saturated with water by storing them above water at room temperature. It was the aim of our measuring program to carry out the in situ-heating XRD measurements with a transient temperature program which is near to the usual de-templating and de-watering procedures but as narrow as possible at adsorption equilibrium. Two different experimental XRD techniques are possible: The simultaneous recording of all signals for 2H between the initial and final signal by an ‘‘imaging plate” in very short times between 3 and 5 min. However, this experimental technique shown in Fig. 1a was not available. We had to use, therefore, the other way, which is the time-consuming scanning of the 2H region by a movable scintillation counter. The recording time is much longer and results from the step distance, the region covered and the time per count (Fig. 1b). For the in situ-heating XRD, the samples were heated with 2 °C/ min in the Bühler-XRD-chamber under 50 ml/min N2 (or air in the case of de-templating). Every 50 °C, the further heating was stopped and a XRD measurement was made under isothermal conditions. The data obtained are attributed to the ‘‘wet” sample. As an example, in the case of recording signals between 10 and 50° with a step size of 0.02° and a recording time of 4 sec at each step, 133 min are necessary. When the recording region is reduced to the characteristic signals in a 10° 2H region (the characteristic 10° region can be different for each zeolite structure), the measuring time can be reduced to 33 min. These two time regimes of the XRD recording were identically used in the TGA. For the two XRD recording times of 133 and 33 min at constant temperature, the mass loss under sweep gas conditions (50 ml N2 or air/min) was determined by TGA for the heating rate of 2°/min and isothermal plateaus every each 50 °C. The TGA program applied for the hydrated FAU and the as synthesized template-containing MFI zeo-

Temperature [°C]

Table 1 Si/Al-ratios and crystallographic data of different zeolite types

x=2.01˚/min

0

200

x=0.83˚/min

400

3 min isoth.

600

x=0.30˚/min

800 1000 Time [min]

1200

33 min isoth.

133 min isoth.

x-ray

low heater

(a) simultaneous measurement (short time)

scintillation counter

sample

low heater

1600

lites are shown in Fig. 2. For comparison, an additional TGA temperature program with a short 3 min isothermal plateau every 50 °C has been added which simulates the ‘‘imaging plate” technique. When the maximum temperature of 450 °C was reached, the temperature was decreased and the corresponding XRD data are attributed to the ‘‘dry” sample. Since the Bühler-XRD-chamber had to be water cooled, below 200 °C we could not avoid a slight water re-adsorption of the sample from water condensed at the walls of the Bühler-chamber, and consequently, the ‘‘dry” samples show due to re-hydration a similar behaviour as the ‘‘wet‘‘ samples in the temperature range below 200 °C. The XRD patterns were recorded as function of heating and drying parameters using a Theta/Theta diffractometer (Seifert/FPM) with Bragg–Brentano geometry and a multilayer mirror for the maximum of the diffracted intensity of Cu Ka radiation. The XRD patterns were scanned with a scintillation counter in the 2H range of 5–45° (step width: 0.02°, 4 sec per step) for the long time measurement during 133 min per temperature step. However, if we record only signals over 10° of the 2H range, the recording time can be reduced to 33 min. For the u.c. data refinement, the software WinXpow (STOE) and the database of Powder Diffraction File (PDF) of International Centre of Diffraction Data (ICDD) was used. The refinement of the lattice constants was done by the program Powder Cell [30], where powder diffraction data derived from single crystal data are adapted to the experimental XRD patterns (Rietveld-technique). The resolution of the XRD detector was <0.06° of the FWHM (full with at half maximum), the Powder Cell program allows a determination of the UCD with ±0.0015 nm. For the TGA measurements of water loaded zeolite crystals the thermo balance type TGA 92 of Setaram was used. The heating rate

high

sample

1400

Fig. 2. Temperature program for the XRD and TGA measurements. The averaged heating rates (2°/min between the plateaus every each 50 °C for different plateau duration) are given.

high

x-ray

500 450 400 350 300 250 200 150 100 50 0

(b) stepwise measurement (long time)

Fig. 1. Schema of the different XRD-diffraction methods.

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M. Noack et al. / Microporous and Mesoporous Materials 117 (2009) 10–21

was 2 °C/min up to 450 °C in a dry nitrogen stream of 50 ml/min or in air for the template removal. Mass loss [wt.%]

time for imaging plate

3. Results and discussion 3.1. Matching of the XRD and TGA experiments Fig. 3 shows the TGA results for the de-watering of the wet zeolites under study. Since the strongly hydrophilic zeolites LTA and FAU lose most of the adsorbed water until 250 °C, a slow heating rate up to 250 °C is recommended. The line at 125 °C indicates the usually applied drying conditions between 100 and 150 °C. For the temperature of 125 °C the remaining water loadings are indicated for the different zeolites. It follows from Fig. 3 that the residual water content correlates with the Si/Al-ratio. In the case of the template-containing MFI crystals, a low heating rate of 0.3 K/min in air was chosen to avoid high internal pressures due to template decomposition. The average heating rates of 2.01, 0.83 and 0.30 °C/min result from the standard heating rate of 2 °C/min between the isothermal plateaus of 3, 33 and 133 min duration, respectively, after each 50 °C (see Fig. 2). This heating regime is identical with the usually applied de-watering and de-templating conditions for zeolites FAU and MFI. Fig. 4 shows the mass changes during the isothermal plateaus for the different recording times of 33 and 133 min. When heating a wet FAU sample to 450 °C, e.g. it looses most of its water (8.5 wt.%) during the 133 min isothermal step at 100 °C. During the further heating, it looses during the 33 min isothermal step at 200 °C slightly more than 1 wt.% and during the 133 min step less than 1 wt.% water. For the template-containing MFI crystals the strongest mass loss of about 8 wt.% takes place at 350 °C. For the isothermal steps at 300 °C and 400 °C only a slight mass loss 61 wt.% is found. The difference of the mass change between the 33 and 133 min isothermal plateaus can be neglected (<1 wt.%). For the FAU zeolites the highest mass loss due to de-watering takes place at 100 °C with 5.5 wt.% during 33 min and 8.5 wt.% during 133 min. For the other isothermal plateaus, the mass losses are of the following order: Dm50°C:Dm150°C:Dm200°C like 4:3:1 and the differences between the isothermal plateau times 33 and 133 min are <1 wt.%. For an assumed ‘‘imaging plate” measuring time of 3 min, the line in Fig. 4 indicates that under these conditions all mass losses are <1 wt.%.

9 8 7 6 5 4 3 2 1 0 0

20

40

FAU 50 °C MFI 300 °C

60 80 Time [min] FAU 100 °C MFI 350 °C

100

FAU 150 °C MFI 400 °C

120

140

FAU 200 °C

Fig. 4. Mass loss detected by TGA during the isothermal plateaus of 3, 33 and 133 min duration, respectively, for different temperatures.

From these findings it can be concluded for the in situ-heating XRD that during  3 min only a slight mass change due to the de-watering or the de-templating occurs and the XRD data allow the exact calculation of the UCD. However, the loading can be far from the thermodynamic adsorption equilibrium.  33 and 133 min major losses of the loading during the isothermal steps of the in situ-XRD can take place. This means that the XRD data collected during 3 min are the most accurate ones. Long measuring times with the scintillation counter are not sharp and reflect changing loadings. As a result, the Rietveld refinement cannot give exact UCD data, especially near to the desorption maxima of water or template. This holds true for FAU and LTA crystals at the 100 °C plateau and for the template-containing MFI at the 350 °C plateau. On the other hand, the slight mass difference of <1 wt.% between 33 and 133 min at temperatures far from the desorption maxima indicates that these data are near to the thermodynamic adsorption equilibrium. The comparison of the UCD data of the water and template containing crystals with those of the calcined ones, i.e. water and template free crystals, shows the influence of the loading on the UCD.

x MFI 1000 = 0.27

0.0 -2.5

-7.5

x MOR = 4.6

x MFI 57= 0.26

-10.0

-15.0

x LTA = 11.27

-12.5 x FAU = 12.36

Δm [wt.%]

-5.0

-17.5 -20.0 -22.5

x = residual loading at 125 °C in wt.%

-25.0 0

50

MFI 1000

100

150

MFI 57

200

250 T [°C]

MOR

300

350

LTA

400

450

500

FAU

Fig. 3. Water desorption of fully at room temperature water-saturated zeolites (100 wt.%) by TGA with a heating rate of 2°/min in 50 ml/min N2. The residual water loadings of the different zeolites at 125 °C are indicated by x.

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M. Noack et al. / Microporous and Mesoporous Materials 117 (2009) 10–21

3.2. UCD of LTA crystals as function of temperature

Table 2 Linear (a) and volume (c = 3a) expansion coefficient of LTA crystals

For hydrated LTA our UCD data of 2.46 nm (Table 1) completely coincide with literature data [31,32]. If hydrated LTA crystals are heated from 25 to 100 °C, a strong contraction followed by a strong expansion between 100 and 150 °C was observed (see samples LTA, wet 33 and LTA, wet 52 from Readman et al. [23] in Fig. 5) due to the removal of the physically bound water. The contraction branch is not observed for sample LTA, wet 133, most probably the water which is responsible for the contraction, has been desorbed during the long in situ-XRD recording time under sweep gas conditions. In the past literature [23], this strong contraction and expansion below 100 °C is not reported since these studies started with a dried zeolite at high temperatures and in these cases water was increasingly adsorbed during cooling. So these measurements represent near-equilibrium data for partially hydrated zeolites. In contrast, we start with a fully hydrated sample at room temperature and observe the UCD changes during the de-watering by heating. In the case of Readman et al. [23] for LTA, wet 52, the in situ-XRD measurements were made during 52 min and our results on LTA, wet 33 coincide with these data very well. The linear and volume expansion coefficients are given in Table 2. Surprising is the strong linear expansion of LTA, wet 133 between 50 and 100 °C with 81.9  106 K1, followed by a continuous contraction. However, the averaged linear expansion over whole temperature range between 50 and 450 °C gives only 3.85  106 K1. From cooling of the now dried LTA sample from 450 to 100 °C, a weak contraction is detected between 450 and 100 °C followed by a weak expansion between 100 and 50 °C. However, the latter effect is due to some re-adsorption of water from the cooled walls of the in situ-XRD chamber (Bühler). Since strong expansions and contractions are found when heating wet LTA crystals, the low temperature range up to 150 °C is critical for the drying of LTA membranes, since the strong expansion and contraction cause a high tension. This tension between the zeolite layer and the continuously expanding a-Al2O3 support can lead to a macroscopic irreversible crack formation.

Zeolite type

Temperature range (°C)

a  106 ½K 1 

c  106 ½K 1 

LTA, wet 133 min

50–100 100–200 200–300 300–450 50–450 25–100 100–150 150–250 250–400 400–450 50–450 25–100 100–150 150–300 300–400 400–450 50–450 450–300 300–200 200–100 100–50

81.86 1.06 6.38 2.00 3.85 74.73 119.70 4.22 10.66 3.42 2.56 47.68 28.61 0.59 3.68 0.38 1.02 2.00 6.38 1.06 9.52a

245.58 3.18 19.14 6.00 11.55 224.19 359.10 12.66 31.98 10.26 7.69 143.04 85.83 1.77 11.04 1.14 12.06 6.00 19.14 3.18 28.6a

3.3. UCD of FAU crystals as function of temperature

LTA, wet 52 min data from Readman et al.[23]

LTA, dry 133 min

a

Data influenced by water re-adsorption.

because of a different Si/Al-ratio. Zeolite FAU is a suitable example to demonstrate the influence of the measuring conditions on the UCD change. Fig. 6 shows the UCD as a function of temperature for the three average heating rates of 2.01, 0.83 and 0.30°/ min (standard heating rate of 2°/min between the isothermal plateaus after each 50 °C for the isothermal plateaus of 3, 33 and 133 min). Roughly, three different regions of the UCD change can be distinguished from Fig. 6 which correlate very well with the de-watering curves shown in Fig. 4: 1. Strong lattice expansion between 50 °C and a first characteristic temperature. 2. Weak lattice expansion between the first and a second characteristic temperature. 3. Weak contraction after the second characteristic temperature.

2.500

2.464 2.462 2.460 2.458 2.456 2.454 2.452 2.450 2.448 2.446 2.444

2.495 2.490 UCD [nm]

UCD [nm]

The usual UCD of hydrated zeolite FAU amounts 2.47 nm [33,34]. Our FAU crystals show a slightly higher UCD of 2.74 nm

LTA, wet 33 min

2.485 2.480 2.475 2.470 2.465

0

100

200

300

400

500

0

50

100

150

200

250

300

350

400

450

500

T [°C]

T [°C] LTA, wet 133

LTA, wet 33

LTA, wet 52

LTA, dry 133

Fig. 5. UCD of LTA crystals as a function of temperature for different isothermal plateaus (XRD recording time). LTA, wet 52 indicates data of Readman et al. [23].

NaX, wet 133 NaY, wet 5.8

NaX, wet 33 NaX, dry 133

Fig. 6. UCD of FAU crystals as function of temperature for different isothermal plateaus (XRD recording time). NaY, wet 5.8 indicates data of Wang et al. [25].

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M. Noack et al. / Microporous and Mesoporous Materials 117 (2009) 10–21

Because of the anisotropy of MOR, it seems to be useful to de = 1/3 (aa+ab+ac) fine a medium linear expansion coefficient a  . For which gives a medium volume expansion coefficient c ¼ 3  a hydrated MOR, the data for the different temperature regions are between 15.25  106 K1 between 50 and 100 °C and 4.42  106 K1 between 200 and 450 °C. This is much higher  for the whole temperature region of 50 to than the average a 450 °C which amounts 8.09  106 K1. Despite this relative low average contraction, remarkable tensions between 50 to 200 °C can be expected due to the water desorption, since the

1.815 a- direction

1.810 UCD [nm]

For the different heating rates, these two characteristic temperatures are shifted (see Table 3). Table 3 gives the linear and volume thermal expansion coefficients for the different steps during the de-hydration in comparison with the average thermal and volume expansion between 50 and 450 °C. The average thermal expansion in the temperature range 50–450 °C amounts to be 10–14.6  106 K1. However, during the heating there can occur extremely high expansions up to almost 100  106 K1 followed by a contraction of up to 2  106 K1 in the high temperature range (average value for all wet samples). The study of the thermal expansion of a hydrated NaY zeolite shows the same three steps [25] and the data of the linear thermal expansion coefficient are very similar to our data for NaX. The shift of 0.015 nm between the UCD-curves (Fig. 6) corresponds to the well-known dependence of the UCD on the Si/Al-ratio of 2.6 for NaY and 1.2 for NaX [35]. Surprising is the strong lattice expansion as a result of the de-watering which is explained by a dislocation of the cations nearer to the negatively charged Al-position and a lattice widening. The slight lattice contraction of the almost dry FAU crystals with increasing temperature corresponds to the results for Al-free [36] and Al-containing zeolite structures [37]. During cooling of the de-hydrated NaX from 450 to 150 °C a slight contraction is observed followed by a slight expansion due to water re-adsorption from condensed water from the walls of the in situ-XRD-Bühler chamber; the same effect was observed by Bhange et al. [20]. The occurrence of a first expansion followed by a contraction for the heating branch can explain the experimental finding that LTA and FAU membranes often de-laminate from disc supports during thermal activation while zeolite layers prepared on the tube side of tubular supports are rather stable. In the latter case the support suppresses the de-lamination of the zeolite layer.

1.805

1.800

1.795 0

100

200

300

MOR, wet

UCD [nm]

MOR is an orthorhombic structure, the u.c. for the three crystallographic directions is given in Table 1 [31,38]. The change of the UCD data for the a-, b- and c-directions as function of temperature are given in Fig. 7. Over the whole temperature region between 50 and 200 °C contraction due to the de-watering is found. However, there are different slopes of the curves in this temperature region. Further, the largest UCD change is found for the b-direction and the smallest one for the c-direction (Table 4).

2.030

2.025

2.020 0

100

200

300

NaX, wet 33 min

NaX, dry 133 min NaY, wet 5.8 min data of Wang et al. [25]

a

500

MOR, dry

0.760

a  106

c  106

½K 1 

½K 1 

50–100 100–300 300–450 50–450 50–150 150–350 350–450 50–450 450–150 150–50 25–75 75–188 188–340 25–340

91.84 18.07 1.92 14.61 48.43 5.38 2.12 14.02 1.76 11.03a 56.75 9.95 1.29 12.87

275.52 54.21 5.76 43.83 145.29 16.14 6.36 42.06 5.28 33.09a 170.25 29.85 3.87 38.61

c- direction

0.755 UCD [nm]

Temperature range (°C)

Data influenced by water re-adsorption.

400

T [°C]

Table 3 Linear (a) and volume (c = 3a) expansion coefficient of FAU crystals

NaX, wet 133 min

MOR, dry

b- direction

MOR, wet

Zeolite type

500

2.040

2.035

3.4. UCD of MOR crystals as function of temperature

400

T [°C]

0.750

0.745

0.740 0

100

200

300

400

500

T [°C] MOR, wet

MOR, dry

Fig. 7. UCD of MOR crystals as function of temperature for the different crystallographic directions.

16

M. Noack et al. / Microporous and Mesoporous Materials 117 (2009) 10–21

Table 4 Linear (a) and volume (c) expansion coefficient of MOR crystals Zeolite type

Temperature range (°C)

MOR, wet

50–100 100–200 200–450 50–450 50–100 100–200 200–450 50–450

MOR, dry

a  106 ½K 1  a

b

c

10.18 15.31 0.03 6.99 0.22 13.96 0.48 4.72

35.57 19.55 11.48 11.48 23.13 13.29 0.97 6.90

0 10.68 1.80 5.81 5.88 8.10 1.60 5.38

a-Al2O3 support shows with +8.6  106 K1 an expansion and wet

2.009

MOR crystals exhibit a maximum shrinkage with 15.25  106 K1. One can find numerous papers on the structure of MOR but no paper addresses the UCD change. During the dehydration of NaMOR from room temperature to 300 °C, a symmetry change from Cmcm to Pbcn was found [39]. In a recent Rietveld refinement, the best fit was obtained for a defect model with c-axis faulting [40]. From a calorimetric study of the dehydration and re-adsorption of water it follows that the cations are not localized in the lattice but interact with the phase of adsorbed water [41,42]. The same authors have shown that structure damage takes place if the dehydration of the Na-MOR takes place above 400 °C under ultra-high vacuum condition whereas the structure remains undamaged until 480 °C when less severe drying condition are applied [43].

2.008

c ¼ 3  a  106 ½K 1 

15.25 15.18 4.42 8.09 9.60 11.78 0.05 5.67

45.75 45.54 13.26 24.27 28.80 35.34 0.15 17.01

a- direction

2.007 UCD [nm]

2.006 2.005 2.004 2.003 2.002 2.001 2.000 1.999 0

100

200

300

400

Silicalite*

MFI 57

MFI 1000 1.9940

b- direction

1.9930 UCD [nm]

1.9920 1.9910 1.9900 1.9890 1.9880 0

100

200

300

400

500

T [°C] MFI 1000

Silicalite*

MFI 57

1.3405 c- direction

1.3400 1.3395 UCD [nm]

3.5.1. UCD of as synthesized MFI At 350 °C about 85 wt.% and at 450 °C about 98 wt.% of the template TPA+ are oxidized during the 133 min isothermal step. Fig. 8 shows the UCD as a function of temperature during the oxidative template removal. For comparison, data of Lassinanti Gualtieri et al. [22] are included. In the temperature region between 250 and 350 °C a contraction of the UCD for all crystallographic directions was found. In comparison with Lassinanti Gualtieri et al. [22], a perfect coincidence with the c-data, the same trend for the b-direction, but a strong difference for the a-direction can be stated. The temperature regions of almost constant UCD change were derived from Fig. 8 and the corresponding data of the linear thermal expansion coefficient a are calculated (Table 5). For the two MFI samples under study, for all directions a strong contraction between 250 and 350 °C was found. For the b- and c-

500

T [°C]

3.5. UCD of MFI crystals as function of temperature All XRD-measurements on MFI were made with a 133 min isothermal plateau. Above 75 °C, MFI shows an orthorhombic structure with UCD data of a = 2.01; b = 1.99 and c = 1.34 nm [44,45]. Silicalite is the Al-free and ZSM-5 the Al-containing MFI structure. There is no definitive threshold between silicalite and ZSM-5 and we define MFI with a Si/Al-ratio > 1000 as silicalite. The Rietveld refinement becomes difficult because of a variety of overlapping signals. Even small deviations in the deconvolution of the peaks cause severe changes in the peak positions thus giving other UCD data. Therefore, differences in the UCD between CuKa scintillation technique and synchrotron radiation technique using imaging plates can be expected.

a  106 ½K 1 

1.3390 1.3385 1.3380 1.3375 1.3370 1.3365 0

100

200

300

400

Silicalite*

MFI 57

500

T [°C] MFI 1000

Fig. 8. UCD change of as synthesized template-containing MFI crystals as function of temperature for the different crystallographic directions. Silicalite* indicates data of Lassinanti Gualtieri et al. [22].

17

M. Noack et al. / Microporous and Mesoporous Materials 117 (2009) 10–21 Table 5 Linear expansion coefficients of as synthesized ZSM-5 and silicalite for the a, b and c directions, respectively

Silicalite [22] MFI 57

MFI 1000

Silicalite [22] MFI 57

MFI 1000

Silicalite [22] MFI 57

a  10

50–250 250–350 350–450 50–450 50–450 50–250 250–350 350–450 50–450 50–200 200–350 350–450 50–450 50–450 50–200 200–300 300–450 50–450 50–200 200–350 350–450 50–450 50–450 50–200 200–350 350–450 50–450

2.24 9.14 0.25 8.22 0.62 3.98 5.98 0.75 7.22 1.00 2.34 0.38 2.51 3.26 2.01 2.81 0.88 3.39 1.99 5.77 0.93 4.67 4.67 1.49 3.73 0.37 2.61

a- direction

Direction

[K1] a-direction

UCD [nm]

MFI 1000

Temperature range (°C)

2.011 2.010 2.009 2.008 2.007 2.006 2.005 0

100

200

300

400

500

T [°C]

b-direction

MFI 57 wet MFI 1000 wet

b

MFI 57 dry MFI 1000 dry

1.993 1.992

c-direction

UCD [nm]

Zeolite type

6

a

b- direction

1.991 1.990 1.989 1.988 1.987 0

100

200

300

400

500

T [°C]

The numbers behind the MFI code denote the Si/Al-ratio.

c

MFI 57 wet

MFI 57 dry

MFI 1000 wet

MFI 1000 dry

1.3405 1.3400

3.5.2. UCD of calcined MFI Fig. 9 gives the UCD data of hydrated and de-hydrated MFI 1000 and MFI 57; the numbers indicate the Si/Al-ratio. In contrast to the hydrophilic Al-containing zeolites, the difference in the UCD between the dry and wet MFI samples of different Si/Al is only slight. For the b- and c-directions there is a discontinuity at 200 °C, where the framework switches from the initial expansion to the following contraction. For the a-direction of the UCD, MFI 1000 exhibit a continuous contraction whereas for the more hydrophilic MFI 57 first expansion until 150 °C is found. Whereas for the a- and c-directions of the UCD a contraction is obtained, the UCD slightly expands in b-direction. However, in the temperature region between 200 and 450 °C, the b-direction of the UCD shrinks. Additionally, the average a data for all MFI samples in the whole temperature region between 50 and 450 °C have been calculated. Temperature regions of constant UCD changes were identified and the corresponding linear expansion coefficients have been calculated (see Table 6). In Fig. 10, the UCD data on MFI 1000 are compared with literature data [20,21]. It is interesting to note that in Lassinanti Gualtieri et al. [21] both silicalite powder as well as silicalite layers on aAl2O3 were studied. Whereas the a- and c-directions of the UCD gave higher data for the powder silicalite in comparison with the supported silicalite layer, for the b-direction it is opposite. Obviously, the orientation of the crystallites in the layer and a mechanical stress between the silicalite layer and the support influence the determined UCD data. In analysis of their stress simulation, Lassinanti Gualtieri et al. [21] came to the conclusion that a less oriented layer with a mixture of differently oriented crystals can compen-

1.3395 UCD [nm]

directions, this contraction starts at 200 °C. The comparison of the thermal expansion coefficients for different zeolites will be given in Section 3.6.

1.3390 1.3385 1.3380 c- direction

1.3375 1.3370 1.3365 0

100

200

300

400

500

T [°C] MFI 57 wet MFI 1000 wet

MFI 57 dry MFI 1000 dry

Fig. 9. UCD change of calcined MFI crystals either hydrated or de-hydrated with temperature for the different crystallographic directions.

sate the thermal stress best. In general our UCD data for MFI 1000 coincide with those of Bhange et al. [20]. Like in Lassinanti Gualtieri et al. [21] for the temperature region 50–300 °C, our cdata of the UCD were by 0.002 nm larger than those of Bhange et al. [20]. Table 7 shows the linear expansion coefficients calculated from temperature regions of constant UCD change.

3.6. Comparison of the linear expansion coefficients and the expansions/contractions of supported zeolite layers with different crystal size For a better comparison of the different zeolite structure types  =(aa+ab+ac)/3. we have defined a medium expansion coefficient a  data are represented in Table 8 and Fig. 11. These a

18

M. Noack et al. / Microporous and Mesoporous Materials 117 (2009) 10–21

Table 6 Linear expansion coefficients of calcined ZSM-5 and silicalite, wet and dry

MFI 1000, wet MFI 57, dry

MFI 1000, dry MFI 57, wet

MFI 1000, wet

MFI 57, dry

MFI 1000, dry

MFI 57, wet

MFI 1000, wet

MFI 57, dry

MFI 1000, dry

a  106

50–150 150–450 50–450 50–450 50–200 200–450 50–450 50–450 50–200 200–450 50–450 50–200 200–450 50–450 50–200 200–450 50–450 50–200 200–450 50–450 50–200 200–450 50–450 50–200 200–450 50–450 50–200 200–450 50–450 50–200 200–450 50–450

3.93 4.37 3.38 5.04 1.96 2.90 2.17 4.48 1.64 0.50 0.11 8.75 2.39 0.89 2,34 0.38 0.50 8.82 2.01 1.29 0.90 0.93 0.60 1.69 2.71 2.07 3.04 1.49 0.36 2.19 2.54 1.72

Direction a-direction

a- direction

2.010 2.009

[K1]

UCD [nm]

MFI 57, wet

Temperature range (°C)

2.008 2.007 2.006 2.005 2.004 2.003 2.002

b-direction

0

100

200

300

400

500

T [°C] MFI 1000

Silicalite*

Silicalite**

1.9920 1.9915 b- direction

1.9910 1.9905 c-direction

UCD [nm]

Zeolite type

2.012 2.011

1.9900 1.9895 1.9890 1.9885 1.9880 1.9875 1.9870 01

00

200

300

400

500

T [°C] MFI 1000

Silicalite*

Silicalite**

1.3405 c- direction

1.3400 Table 7 Comparison of our expansion coefficients of silicalite with literature data

MFI 1000 silicalite [20]

silicalite [21]

MFI 1000

silicalite [20]

silicalite [21]

MFI 1000

silicalite [20]

silicalite [21]

Temperature range (°C)

a  106

50–450 50–100 100–450 50–450 50–200 200–300 300–450 50–450 50–200 200–450 50–450 50–200 200–300 300–450 50–450 50–200 200–300 300–450 50–450 50–200 200–450 50–450 50–150 150–450 50–450 50–300 300–400 400–450 50–450

5.04 21.99 3.98 6.71 0.86 3.31 2.99 1.25 8.75 2.39 0.89 10.39 3.02 0.63 1.38 2.68 0.20 3.64 4.77 1.69 2.71 2.07 6.88 1.72 0.00 0.45 5.65 0.37 5.60

Direction

[K1] a-direction

UCD [nm]

Zeolite type

1.3395 1.3390 1.3385 1.3380 1.3375 1.3370 1.3365 0

100

200

300

400

500

T [°C]

b-direction

MFI 1000

Silicalite*

Silicalite**

Fig. 10. UCD change of silicalite crystals as function of temperature with literature comparison. Silicalite* and Silicalite** indicate data of Bhange et al. [20] and Lassinanti Gualtieri et al. [21], respectively.

c-direction

Whereas zeolites LTA and FAU expand, MOR and MFI contract  value both in the wet and de-watered state upon heating. The a of the a-Al2O3 support determined by high-temperature XRD was found to be 7.53  106 K1 which is slightly lower than 8.6  106 K1 [6]. For the MFI crystals, there is almost no difference between the hydrated and de-hydrated samples MFI 1000 (silicalite) and MFI 57 (ZSM-5). As a result of the thermal template removal by heating the as synthesized MFI in air, a contraction twice that of an already calcined MFI is found.

19

M. Noack et al. / Microporous and Mesoporous Materials 117 (2009) 10–21

70 60 50 40 30

α x10-6[K-1]

20

α−Al2O3

10

MFI 57 dry

MFI1000 dry

MFI 57 wet

MFI 1000 wet

MFI 57 as syn

MFI 1000 as syn

MOR dry

MOR wet

FAU dry

-30

FAU wet

-20

LTA dry

-10

LTA wet

0

-40 -50 max. contraction

max. expansion

50-450°C-range

Fig. 11. Maximum mismatch of the linear expansion and contraction coefficients of the wet and dry zeolites under study upon heating and cooling. For comparison, the average thermal expansion of the zeolites in the whole temperature region between room temperature and 450 °C and that of the a-Al2O3 support are given.

Table 8 Comparison of average linear and volume expansion coefficients for the temperature range 50–450 °C  106 [K1] zeolite type

aa

LTA, wet 133 min LTA, wet 33 min NaA, wet 52 min LTA, dry 133 min NaX, wet 133 min NaX, wet 33 min NaY, wet 5.8 min NaX, dry 133 min MOR, wet 133 min MOR, dry 133 min MFI 1000 as synthesized MFI 57 as synthesized silicalite [22] as synthesized MFI 1000, wet calcined MFI 57, wet calcined MFI 1000, dry calcined MFI 57, dry calcined MFI 1000, wet calcined silicalite, wet calcined [20] silicalite, wet calcined [21] a-Al2O3-support

3.85 2.56 1.02 5.01 14.61 14.02 12.87 10.0 6.99 4.72 8.22 7.22 0.62 5.04 3.38 4.48 2.17 5.04 6.71 1.25 7.36

ab

11.48 6.90 2.51 3.39 3.26 0.89 0.11 1.29 0.50 0.89 1.38 4.77 –

ac

a ¼ aa þa3b þac

c ¼ 3a

5.81 5.38 4.67 2.61 4.67 2.07 0.60 1.72 0.36 2.07 0.0 5.60 7.7

3.85 2.56 1.02 5.01 14.61 14.02 12.87 10.00 8.09 5.67 5.13 4.41 2.44 2.07 0.89 1.64 0.68 2.07 1.78 3.87 7.53

11.55 7.69 3.06 15.03 43.83 42.06 38.61 30.00 24.28 17.0 15.39 13.23 7.32 6.21 2.67 4.92 2.04 6.21 5.34 11.61 22.59

LTA shows in the case of fast heating between 25 and 100 °C a strong contraction followed by a strong expansion during the further de-hydration between 100 and 150 °C (cf. Table 2). A similar behaviour was found by Readman et al. [23]. These strong contractions/expansions can be the key for the insufficient gas separation performance of LTA membranes most probably due to the formation of mesopores during the membrane de-hydration. Surprising is the strong expansion of hydrated FAU between 50 and 150 °C followed by a slight expansion between 150 and 350 °C (cf. Table 3). By the end of the de-hydration a weak contraction is observed. Our finding is supported by studies on NaY [25oder20] and on BaY [24]. For hydrated FAU twice the expansion of the aAl2O3 support is found. This finding corresponds to the experimental observation that the FAU membrane layer often de-laminates

from the support. For FAU the difference between the different TGA and XRD heating programs can be neglected. The difference between the heating programs with 33 min or 133 min isothermal plateaus becomes stronger for LTA, probably due to the kinetic control of the water release from the narrower LTA pores (see Fig. 5). Hydrated MOR shows a contraction of the same order of magnitude as the expansion of the a-Al2O3 supports. The resulting tensions can cause cracks or intercrystalline mesopores. The maximum contraction of hydrated MOR is found between 50 and 200 °C which is double the average contraction for the whole temperature range from 50 to 450 °C. Therefore, a crack formation for a MOR membrane upon de-hydration can not be excluded. The oxidative degradation of the template in the case of as synthesized MFI causes a maximum contraction which is twice the contraction found on calcined MFI as a pure thermal effect. To illustrate the mismatch of the thermal behaviour of zeolite film and support, we have formed the difference in the thermal expansion coefficient between the a-Al2O3 support and the zeolite layer Da = aAl3 O3  aZeolite . Now we can estimate due to Dl = Da  L  DT a difference in the absolute thermal expansion between the a-Al2O3 support and the zeolite layer formed by crystals of the size L. For different L in the zeolite layer, intercrystalline voids between the crystals can be formed (Fig. 12). Fig. 12 a shows the difference Dl of the length changes of the aAl2O3 support and the zeolite layer assuming average linear thermal expansion coefficients between 50 and 450 °C. Fig. 12 b) and c) show the same situation for the maximum expansion and the maximum contraction, respectively. The contraction data are of the order of mesopores, expansion can cause tension. Assuming a crystal size of 10 lm, pores of 20–50 nm can form in the case of a supported LTA layer thus irreversibly damage the membrane. Small silicalite crystals of 1–3 lm show contraction lengths of the order of 5–15 nm which is in accordance with the calculations of Miachon and Dalmon [46]. This consideration shows that a MFI layer of small crystals can form reversibly intercrystalline pores in contrast to MFI layers of large crystals that tend to crack formation [21]. In Sorenson et al. [17] a 1% expansion of MFI crystals was found when they are loaded with n-hexane. Performing a permporosimetry experiment

20

Δ l [nm]

a

M. Noack et al. / Microporous and Mesoporous Materials 117 (2009) 10–21

4. Conclusions

60 40 20 0 -20 -40 -60 -80 -100 -120 -140 0

5

10

15

20

25

crystal size [µm] LTA wet

FAU wet

MOR wet

MFI 1000 as syn

MFI 57 as syn

MFI 1000 wet

MFI 57 wet

Δ l [nm]

b

100 80 60 40 20 0 -20 -40 -60 -80 0

5

10 15 crystal size [µm]

20

LTA wet

FAU wet

MOR wet

MFI 1000 as syn

MFI 57 as syn

MFI 1000 wet

25

MFI 57 wet

c

0

Δ l [nm]

-20

Hydrated zeolite crystals show an irregular thermal expansion and contraction behaviour. In contrast, the most often applied ceramic and metal supports expand continuously with temperature. Drying of hydrophilic zeolites LTA and FAU can result in a large contraction or expansion of the u.c. When drying supported LTA membrane layers, extreme mismatches in the thermal expansion between LTA layer and support (contraction between 50 and 100 °C, expansion between 100 and 150 °C) are found. This mismatch of the thermal expansion seems to be the explanation for the often observed de-lamination of the zeolite layer from a planar support whereas the adherence is better if the zeolite layer is on the core side of a tubular support. When drying FAU, a strong expansion of the u.c. is found which leads to a large mismatch with the support. It seems to be difficult, therefore, to de-hydrate LTA and FAU membranes without damaging them. It is expected that the de-hydration problem would not exist in the case of the full-silica LTA structure ITQ 29 [47,48] or when applying water-free synthesis. Similar, but not so extreme, is the case with MOR. For MOR a strong contraction due to the de-hydration takes place which should cause the formation of major mesopores in a MOR membrane layer. Like membranes LTA and FAU, MOR membranes are suitable, therefore, for the hydrophilic separation but not for the shape-selective gas separation. Template-containing as synthesized MFI shows due to the oxidative template removal twice the contraction of the template-free already calcined MFI. Since there exist temperature regions of expansion and contraction, and the linear thermal expansion coefficients for the different crystallographic directions can be positive or slightly negative, non-oriented MFI layers of small crystals can compensate the thermostress by the reversible formation of mesopores. These pores lower the membrane selectivity but they do not spoil it completely.

-40

References

-60 -80 -100 -120 0

5

10 15 crystal size [µm]

20

LTA wet

FAU wet

MOR wet

MFI 1000 as syn

MFI 57 as syn

MFI 1000 wet

25

MFI 57wet Fig. 12. Length-changes of zeolite crystals in supported zeolite layers on a-Al2O3 assuming (a) averaged linear expansion/contraction, (b) maximum expansion and (c) maximum contraction.

with n-hexane as the condensable component, the authors propose that n-hexane seals the pores created previously by de-watering [17]. If one compares the size of possible intercrystalline pores in the MFI layer with 1 lm crystals on a-Al2O3 which are of the order of 3 nm (see Fig. 12a) with a 1% crystal expansion by n-hexane loading [17], an enlargement of the crystals by 10 nm can take place. This crystal expansion can seal defect pores thus increasing the membrane selectivity. It is recommended, therefore, to use benzene instead of n-hexane in permporosimetry since benzene adsorption does not change the UCD.

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