Details
| Original language | English |
|---|---|
| Pages (from-to) | 10-21 |
| Number of pages | 12 |
| Journal | Microporous and Mesoporous Materials |
| Volume | 117 |
| Issue number | 1-2 |
| Early online date | 15 May 2008 |
| Publication status | Published - 1 Jan 2009 |
Abstract
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 α-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 de-templating 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 α-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.
Keywords
- FAU crystals, Hydrophilicity, Intercrystalline mesopores, LTA, MFI, MOR, Unit cell dimension, Water loading, Zeolite membranes
ASJC Scopus subject areas
- Chemistry(all)
- Materials Science(all)
- Physics and Astronomy(all)
- Condensed Matter Physics
- Engineering(all)
- Mechanics of Materials
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In: Microporous and Mesoporous Materials, Vol. 117, No. 1-2, 01.01.2009, p. 10-21.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - The change of the unit cell dimension of different zeolite types by heating and its influence on supported membrane layers
AU - Noack, M.
AU - Schneider, M.
AU - Dittmar, A.
AU - Georgi, G.
AU - Caro, J.
PY - 2009/1/1
Y1 - 2009/1/1
N2 - 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 α-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 de-templating 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 α-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.
AB - 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 α-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 de-templating 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 α-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.
KW - FAU crystals
KW - Hydrophilicity
KW - Intercrystalline mesopores
KW - LTA
KW - MFI
KW - MOR
KW - Unit cell dimension
KW - Water loading
KW - Zeolite membranes
UR - http://www.scopus.com/inward/record.url?scp=55749104201&partnerID=8YFLogxK
U2 - 10.1016/j.micromeso.2008.05.013
DO - 10.1016/j.micromeso.2008.05.013
M3 - Article
AN - SCOPUS:55749104201
VL - 117
SP - 10
EP - 21
JO - Microporous and Mesoporous Materials
JF - Microporous and Mesoporous Materials
SN - 1387-1811
IS - 1-2
ER -