In situ dehydration behavior of zeolite-like pentagonite: A single-crystal X-ray study

Abstract

The structural modifications upon heating of pentagonite, Ca(VO)(Si4O10)·4H2O (space group Ccm21, a=10.3708(2), b=14.0643(2), c=8.97810(10)Å, V=1309.53(3)Å3) were investigated by in situ temperature dependent single-crystal X-ray structure refinements. Diffraction data of a sample from Poona district (India) have been measured in steps of 25 up to 250°C and in steps of 50°C between 250 and 400°C. Pentagonite has a porous framework structure made up by layers of silicate tetrahedra connected by V4+O5 square pyramids. Ca and H2O molecules are extraframework occupants.Room temperature diffraction data allowed refinement of H positions. The hydrogen-bond system links the extraframework occupants to the silicate layers and also interconnects the H2O molecules located inside the channels. Ca is seven-fold coordinated forming four bonds to O of the tetrahedral framework and three bonds to extraframework H2O. The H2O molecule at O9 showing a high displacement parameter is not bonded to Ca. The dehydration in pentagonite proceeds in three steps.At 100°C the H2O molecule at O8 was released while O9 moved towards Ca. As a consequence the displacement parameter of H2O at O9 halved compared to that at room temperature. The unit-cell volume decreased to 1287.33(3)Å3 leading to a formula with 3H2O per formula unit (pfu). Ca remained seven-fold coordinated.At 175°C Ca(VO)(Si4O10)·3H2O transformed into a new phase with 1H2O molecule pfu characterized by doubling of the c axis and the monoclinic space group Pn. Severe bending of specific TOT angles led to contraction of the porous three-dimensional framework. In addition, H2O at O9 was expelled while H2O at O7 approached a position in the center of the channel. The normalized volume decreased to 1069.44(9)Å3. The Ca coordination reduced from seven- to six-fold.At 225°C a new anhydrous phase with space group Pna21 but without doubling of c had formed. Release of H2O at O7 caused additional contraction of TOT angles and volume reduction (V=1036.31(9)Å3). Ca adopted five-fold coordination. During heating excursion up to 400°C this anhydrous phase remained preserved. Between room temperature and 225°C the unit-cell volume decreased by 21% due to dehydration. The dehydration steps compare well with the thermo-gravimetric data reported in the literature.