Systematic study of the crystallization process of CrAPO-5 using in situ high resolution X-ray diffraction

Abstract

Chromium substituted nano-porous aluminophosphate (CrAPO-5) was investigated to discuss the effect of Cr³⁺ ions on the crystallization kinetics and crystal structure parameters by in situ time-resolved high resolution X-ray powder diffraction (HRXRPD). Results showed that the introduction of Cr³⁺ into pure AlPO₄-5 influenced both the crystallization velocity and structural parameters of the resultant AFI. The presence of Cr³⁺ ions can severely retard the crystallization of AFI. The more Cr³⁺ ions in the gels, the stronger inhibitory effect would be. A kinetic analysis of the crystallization of CrAPO-5 was performed. A fair linear relationship between the activation energy and Cr³⁺ content was concluded. The calculated Avrami exponent n was in the range of 1–1.4, implying a similar 1D growth mechanism to that of AlPO₄-5. The extracted structural parameters showed obvious crystal expansion, which might be caused by the trapping of Cr³⁺ ions in the framework. However, this influence on the structural parameters was limited. The a (=b) parameters increased to a maximum with chromium content, which is similar to the influence on the crystal growing velocity. The grain size variation of AFI differed from other metal substituted aluminophosphate molecular sieves, which implied a different state of chromium in the AlPO₄-5 framework. The time-resolved HRXRPD proves to be a direct and efficient method to reveal the influence of Cr³⁺ ions on the crystallization and shows indirect evidence on the state of Cr³⁺ in the crystalized CrAPO-5.