Temperature-induced transformations in CoAPO-34 molecular sieve: A combined in situ X-ray diffraction and FTIR study

Thermally induced processes of CoAPO-34, an aluminophosphate molecular sieve with chabasite-type structure, synthesized in the presence of morpholine as a structure-directing agent and HF as a mineralizing agent, have been studied by in situ X-ray synchrotron powder diffraction augmented with Fourier transform (FT) IR analysis. A time-resolved experiment was performed using a translating imaging plate system. At room temperature, the structure refinement by full-profile Rietveld analysis showed P-1 symmetry and the presence of one Al site with sixfold coordination. At around 400°C, both fluorine and morpholine are lost, and the four-connected chabazite (CHA)-type topology is restored. Notwithstanding the metrically rhombohedral values of the cell parameters, the symmetry remains triclinic P-1. Inhomogeneous dealumination of the framework begins at 725°C, accompanied by a strong triclinization of the unit cell and followed by the collapse of the structure above 775°C. The insertion of cobalt ions within the CHA framework was monitored by FTIR spectroscopy, which showed that bridged Co²⁺-O(H)-P hydroxyls are present after morpholine removal.