| 1. |
- Gualtieri, Magdalena Lassinantti, et al.
(författare)
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Crack formation in α-alumina supported MFI zeolite membranes studied by in situ high temperature synchrotron powder diffraction
- 2007
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Ingår i: Journal of Membrane Science. - : Elsevier BV. - 0376-7388 .- 1873-3123. ; 290:1-2, s. 95-104
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Tidskriftsartikel (refereegranskat)abstract
- Cracks are frequently formed in α-alumina supported MFI membranes during calcination. To better understand crack formation, in situ powder diffraction data were collected during calcination of a type of MFI membrane (ca. 1800 nm thick) which is known to crack reproducibly. In addition, data for MFI powder and a blank support were also collected. Both a synchrotron radiation facility and an in-house instrument were used. The unit cell parameters were determined with the Rietveld method, and the strain in the direction perpendicular to the film surface was calculated for the film as well as for the support. The microstrain in the support was also estimated. Based on the results obtained here, a model for crack formation in this type of MFI membrane was proposed. The lack of cracks in other types of MFI membranes (ca. 500 nm) prepared in our laboratory is also explained by the model. In thicker MFI films, the crystals are well intergrown. During heating, the MFI crystals contract and the α-alumina support expands. Consequently, a thermal stress develops in the composite which eventually leads to formation of cracks in the film and structural defects in the support. In thinner films, the crystals are less well intergrown and the thermal expansion mismatch leads to opening of grain boundaries rather than cracks.
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| 2. |
- Lassinantti-Gualtieri, Magdalena, et al.
(författare)
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Accurate measurement of the thermal expansion of MFI zeolite membranes by in situ HTXRPD
- 2004
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Ingår i: Recent advances in the science and technology of zeolites and related materials. Part A. - Amsterdam : Elsevier. - 0444518274 ; , s. 703-709
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Konferensbidrag (refereegranskat)abstract
- Template removal by calcination of MFI type membranes is often accompanied by crack formation. The thermal behavior of MFI type membranes, synthesized with and without masking, was studied to understand the mechanism. Masking prevents growth of zeolite in the interior of the Support during membrane synthesis. Rietveld refinements of powder diffraction data collected in situ at high temperature allowed to accurately determine the change in thermal expansion of the MFI film and the porous alpha-alumina support. During heating, a relatively large contraction of the cell volume during template removal occurred in the zeolite powder and in the film of the membrane prepared with masking. The much smaller decrease in the non-masked sample indicates that this membrane is under stress during heating and as a consequence, cracks are formed. The stress imposed in the membrane prepared without masking may be due to the opposite thermal behavior of the Substrate in combination with strong bonds between the membrane and the support.
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| 3. |
- Lassinantti Gualtieri, Magdalena, et al.
(författare)
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The influence of heating rate on template removal in silicalite-1: An in situ HT-XRPD study
- 2006
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Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 89:1-3, s. 1-8
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Tidskriftsartikel (refereegranskat)abstract
- The effect of heating rate on thermal behavior of TPA-silicalite-1 during calcination and the reaction kinetics for TPA decomposition were investigated. The cell parameters of the TPA-silicalite-1 during the heating cycles were determined with the aid of high temperature X-ray diffraction data and the Rietveld method. The template decomposition is accompanied by a large contraction of the unit cell. The unit cell dimensions during template removal are not affected significantly by the heating rate. Consequently, the rate of contraction is approximately proportional to the heating rate. The intensity of some diffraction peaks changes during heating, especially the 101/011 and the 200/020 peaks. The intensity change of those peaks shows the same dependence with temperature as the TPA occupancy, indicating that these parameters are related. An analysis of the kinetics for TPA decomposition based on the intensity change of the 101/011 and the 200/020 peaks was performed. The apparent activation energy (Ea) of the template decomposition in silicalite-1 determined with the Kissinger and the Flynn–Wall–Ozawa methods was 138 (±25) and 138 (±29) kJ mol−1, respectively. The reaction order, determined with the method of Kennedy and Clark, was close to 0.5 indicating that the rate-limiting step is mono-dimensional diffusion. Ea was 140 (±30) kJ mol−1, in good agreement with the results obtained with the other methods.With the results presented here, it is possible to discuss possible effects of the heating rate on the crack formation frequently observed in zeolite membranes during calcination.
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