Crack formation in alpha-alumina supported MFI zeolite membranes studied by in situ high temperature synchrotron powder diffraction

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

Accurate measurement of the thermal expansion of MFI zeolite membranes by in situ HTXRPD

Template removal by calcination of MFI type membranes is often accompanied by crack formatìon. The thermal behavior of MFI type membranes, synthesized with and without masking, was studied to understand the mechanìsm. Masking prevents growth of zeolite in thè interior of the support during membrane synthesis. Rietveld refìnements of powder diffraction data collected in situ at high temperature allowed to accurately determine the change in thermal expansion of thè MFI film and thè porous a-alumìna support. During heating, a relatively large contraction of the celi volume during template removal occurred in thè zeolite powder and in thè film of thè membrane prepared with masking. The much smaller decrease in thè 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 thè membrane and thè support

Crack formation in a-alumina supported MFI zeolite membranes studied by in situ high temperature synchrotron powder diffraction

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

Porous Inorganic Membranes for CO 2

INGENIERIE+MP