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

Abstract

To track dehydration behavior of cavansite, Ca(VO)(Si 4 O 10 )·4H 2 O [space group Pnma, a = 9.6329(2), b = 13.6606(2), c = 9.7949(2) Å, V = 1288.92(4) Å 3 ] single-crystal X-ray diffraction data on a crystal from Wagholi quarry, Poona district (India) were collected up to 400 °C in steps of 25 °C up to 250 °C and in steps of 50 °C between 250 and 400 °C. The structure of cavansite is characterized by layers of silicate tetrahedra connected by V 4+ O 5 square pyramids. This way a porous framework structure is formed with Ca and H 2 O as extraframework occupants. At room temperature, the hydrogen bond system was analyzed. Ca is eightfold coordinated by four bonds to O of the framework structure and four bonds to H 2 O molecules. H 2 O linked to Ca is hydrogen bonded to the framework and also to adjacent H 2 O molecules. The dehydration in cavansite proceeds in four steps. At 75 °C, H 2 O at O9 was completely expelled leading to 3 H 2 O pfu with only minor impact on framework distortion and contraction [V = 1282.73(3) Å 3 ]. The Ca coordination declined from originally eightfold to sevenfold and H 2 O at O7 displayed positional disorder. At 175 °C, the split O7 sites approached the former O9 position. In addition, the sum of the three split positions O7, O7a, and O7b decreased to 50% occupancy yielding 2 H 2 O pfu accompanied by a strong decrease in volume [V = 1206.89(8) Å 3 ]. The Ca coordination was further reduced from sevenfold to sixfold. At 350 °C, H 2 O at O8 was released leading to a formula with 1 H 2 O pfu causing additional structural contraction (V = 1156(11) Å 3 ). At this temperature, Ca adopted fivefold coordination and O7 rearranged to disordered positions closer to the original O9 H 2 O site.At 400 °C, cavansite lost crystallinity but the VO 2+ characteristic blue color was preserved. Stepwise removal of water is discussed on the basis of literature data reporting differential thermal analyses, differential thermo-gravimetry experiments and temperature dependent IR spectra in the range of OH stretching vibrations.

Dehydration of microporous vanadosilicates: the case of VSH-13Na

“…The dimorphism of cavansite and pentagonite is based on the different linkages in the silicate layer: in cavansite the tetrahedral chains are joined into fourfold and eightfold rings, whereas in pentagonite only sixfold rings are formed (Evans, 1973). In cavansite, the water removal proceeds in four steps preserving the same space group Pnma with only minor framework distortion and contraction (Danisi et al, 2012). Pentagonite undergoes phase transitions to a monoclinic structure at 448 K and to an orthorhombic structure at 498 K with a decrease of the normalized unit-cell volume by 21% (Danisi et al, 2013).…”

“…In contrast to VSH-13Na, the dehydration behaviour of cavansite was Scanning electron microscope (SEM) images of VSH-13Na crystals: (a) crystals covered by unreacted silica particles and (b) a larger crystal after vacuum filtration and washing. extremely well characterized by ex situ single-crystal X-ray diffraction (Rinaldi et al, 1975), in situ single-crystal X-ray diffraction (Danisi et al, 2012) and in situ synchrotron powder diffraction under dynamic conditions (Martucci et al, 2016;Fisch et al, 2016Fisch et al, , 2017.…”

Introduction to the special issue on mineralogical crystallography

“…In their contribution to the issue, Danisi and Armbruster (2018) report on a step-wise dehydration behaviour of VSH-13Na, a framework vanadosilicate structurally related to the minerals cavansite and pentagonite. By analogy with the latter two (Danisi et al 2012(Danisi et al , 2013, the authors observe the formation of new low and dehydrated phases, in which each new dehydrated state corresponds to a new structure type and a new conformation state of an octahedral-tetrahedral framework.…”

Continuous dehydration of cavansite under dynamic conditions: an in situ synchrotron powder-diffraction study

“…However, the manuscript lacks originality and aim and it is also sloppy prepared (1). The paper should be rejected because the quality of the synchrotron data is by far not sufficient (2) to allow any comparison with X-ray single-crystal data of a very recent paper by Danisi et al (2012). In addition, if the authors aim in their manuscript for kinetic aspects of dehydration of cavansite, experiments with only a single heating rate (3) of 5K/min are insufficient for a meaningful interpretation.…”

“…In general, temperature dependent powder synchrotron diffraction is a useful method to track dehydration of a zeolite if single crystals are not available or kinetic aspects are systematically investigated. However, in case of cavansite such an approach is not useful because temperature dependent X-ray data have previously demonstrated (Danisi et al, 2012) that at elevated temperature the H2O distribution is strongly disordered. The limited spatial resolution of powder synchrotron data (2) does obviously not allow to detect or to model such disordered arrangements.…”