Dehydration and high temperature behavior of zeolite ferrierite: in situ synchrotron X-ray powder diffraction study

Zeolite dehydration has been widely studied because the sorptive and catalytic properties of these materials are profoundly influenced by processes which occur at relatively high temperature (HT). The knowledge of the structural modifications induced by HT and the definition of the stability fields of these materials is fundamental to assure their persistence and effectiveness in technological applications. Here we discuss the thermal stability and dehydration dynamics of the natural zeolite ferrierite. A sample from Monastir [(Na0.56K1.19Mg2.02Ca0.52Sr0.14)(Al6.89Si29.04)O72·17.86H2O; a=19.2241(3)Å; b=14.1563(2)Å; c=7.5106(1)Å, V=2043.95(7)Å3] was gradually heated and investigated by thermogravimetric (TG) analysis and in situ synchrotron X-ray powder diffraction. TG analysis shows that water release starts from the very early stages of heating and is complete at 600°C. The results of the structural refinements performed by Rietveld method up to 670°C in both Immm and Pnnm s.g. did not reveal any significant differences, hence only the data obtained in the topological Immm s.g. are discussed. By continuously monitoring the thermal behavior, it is evident that ferrierite belongs to the group of zeolites that do not undergo neither phase transitions nor significant modifications of the framework upon dehydration. Upon heating to 670°C ferrierite from Monastir behaves as a noncollapsible structure displaying only a slight contraction of the unit cell volume (-3%). Moreover, the cell parameter reductions are anisotropic, more marked for a (Delta a= -1.6%) than for b and c axes (Delta b= -0.76%; Delta c= -0.70%). This anisotropic response to heating is interpreted as due to the presence, in ferrierite framework, of five-membered ring chains of SiO4 tetrahedra that confer to the structure a higher rigidity along b and c. Upon dehydration we observe: i) the gradual water loss - starting from the molecules hosted in the 10MR channel - is almost complete at 670°C, in good agreement with the TG data; ii) as a consequence of the decreased water coordination, Mg and K migrate from their original positions moving from the center of the channel towards the walls, to find a better coordination with the framework oxygen atoms. Beyond providing information on the thermal stability and heating behavior of natural ferrierite, the results of this work allow a comparison with the dehydration kinetics and mechanisms of the corresponding synthetic phases, contributing in identifying the role played by framework and extraframework composition on the high-temperature behavior of porous materials with FER topology. Moreover, the information on the thermal behavior of natural ferrierite can be exploited to predict the energetic performances of analogous synthetic counterparts, namely “zeosil-electrolyte” systems, under non ambient conditions.