Temperature and humidity dependent investigations on paulingite

Paulingite (PAU-Ca), a rare, natural small-pore zeolite was dehydrated by cooling in a flow of dry N-2 gas and the structural changes of the partly dehydrated phase were analysed at 100 K using synchrotron diffraction. The LT-dehydration led to a symmetry reduction from space group Im (3) over barm to I (4) over bar 3m accompanied by a cell volume decrease of 3.7% to cell constant 34.681(1) angstrom. The refinement converged with residuals of 6.07% (R1) and 16.46% (wR2). Structural changes affect the d8r-units (double eight-rings) with elliptical deformations of the single 8-rings aligned perpendicular to each other and the alpha-cage (great rhombicuboctahedron) with a tetrahedral compression. The openings of the main channel system of the complex framework structure were reduced by 0.87 angstrom. The dehydration starts with hydrated Ca cations inside the medium-sized plg-cages accompanied by cation migrations. The study documents the high sensitivity of zeolites to changes in air humidity and the importance of understanding thermally induced and dehydration-induced transformations in zeolite materials.