Negative thermal expansion in hydrated HZSM-5 orthorhombic zeolite

Orthorhombic HZSM-5 zeolite, in hydrated form and with the silica/alumina molar ratio of 30, exhibits a complex thermal expansion behavior over the temperature range 40-840degreesC. Strong negative average overall linear coefficient of thermal expansion (alpha = -9.0 x 10(-6) K-1) was found in this region. Thermal expansion behavior of HZSM-5 is anisotropic, with the a and c axes contracting more than the b axis. Five different thermal expansion regions could be established over the overall studied temperature range. The first region, between 40 and 80degreesC, demonstrated positive average thermal expansion with alpha = 6.19 x 10(-6) K-1. The second one, identified between 80 and 440degreesC, displays negative average thermal expansion with alpha = -7.38 x 10(-6) K-1. The next region between 440 and 520degreesC shows a small positive average thermal expansion with alpha = 3.89 x 10(-6) K-1. The fourth region has a strong negative average thermal expansion with alpha = -6.22 x 10(-6) K-1, while the last one exhibits a very strong negative average thermal expansion with alpha = -25.75 x 10(-6) K-1. It could be concluded that the release of H2O molecules does not provoke negative average thermal expansion of HZSM-5. One can ascribe the negative thermal expansion behavior of orthorhombic HZSM-5 from 80 to 440degreesC mainly to the action of H3O+ species. The thermal expansion behavior in the range between 440 and 520degreesC could be correlated to the processes of dehydroxylation. Above 520degreesC the strong negative thermal expansion seems to be related to the intrinsic mechanism of negative thermal expansion, i.e., transverse vibrations of the bridging oxygen atoms between two rigid polyhedrons. (C) 2004 Elsevier Inc. All rights reserved.