One of the most remarkable properties of zeolites and zeolite-like aluminophospate materials is their thermal behaviour (i.e., stability, phase transformations, rate, temperature and number of dehydration/ rehydration cycles) which is of crucial importance in the wide industrial applications of these materials (e.g. adsorbers, catalysts, molecular sieves). A number of factors contribute to the macroscopically observable thermal effects such as evolution of H2O and encapsulated organic species, variation in unit-cell volume and structural breakdown or modification [1,2]. These effects, which modify the pore and channel geometry, affect the adsorption and diffusion of molecules in zeolites and microporous AlPO4 frameworks, and consequently the adsorption, molecular sieving and catalytic properties of these materials. However, for many zeolite-type materials, detailed and accurate information on their response to heating is still missing and sometimes, even when available, controversial or unreliable. The present investigation strikes to give an exhaustive picture of the structural modifications upon removal of water in metavariscite (AlPO4·2H2O), by means of an in situ temperature-resolved powder diffraction study using synchrotron radiation. This Al-phosphate commonly occurs as a weathering product of phosphatic rocks as well as concretions due to phosphatization of kaolin during weathering. It bears a great importance for soil science, since its formation strongly reduces the effectiveness of phosphate fertilizers. The metavariscite structure consists of a three-dimensional framework in which the tetrahedral units link to one another by shared corner-oxygen atoms, and their formulae are designated as T(1)T(2)O4, where T(1) represents the trivalent atom and T(2) is P. We have performed time-resolved dehydration experiments on metavariscite powders at the ID22 beamline (ESRF, Grenoble), using a fixed X-ray wavelength of 0.400031(1)Å. X-ray diffraction patterns were recorded in the 0.5–19.5 2θ range and the crystal structure evolution was continuously monitored through 30 Rietveld structure refinements from 30° to 800°C. TG/DTA curves on a fraction of the same sample contained in an open alumina crucible were measured in air using a Netzsch STA 409 simultaneous TG/DTA thermoanalyser. Temperature range and heating rate were from room temperature (RT) to 900°C and 10°/min, respectively. The results of the above studies indicate that: (i) from room temperature to 150°C metavariscite (monoclinic dimorph of AlPO4 · 2H2O) appears as the main phase, (ii) in the range of 170°–250°C metavariscite starts to disappear giving rise to α-berlinite (trigonal form of AlPO4) that is the stable phase up to about 610°C, and (iii) above the latter temperature the structure adopts the more stable configuration of the tetragonal β-berlinite polymorph. The occurrence of a transient metastable phase has been also detected.  Alberti A. and Martucci A. (2011): Reconstructive phase transitions in microporous materials: Rules and factors affecting them. Microporous and Mesoporous Materials. 141, 192-198.  Cruciani G. (2006): Zeolites upon heating: Factors governing their thermal stability and structural changes. Journal of Physics and Chemistry of Solids. 67, 1973-1994.
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|Titolo:||New insights on the response to heating of metavariscite: an in situ synchrotron powder diffraction study|
|Data di pubblicazione:||2016|
|Appare nelle tipologie:||04.2 Contributi in atti di convegno (in Volume)|