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