A1P0 4-A1 2O 3 catalysts with low alumina content : I. Structural and textural characterization of catalysts obtained with aqueous ammonia

Abstract

A series of amorphous AlPO 4-Al 2O 3 (APA1-A) catalysts were prepared to contain various A1PO 4/Al 2O 3 weight ratios (Al 2O 3: 5–15 wt.%). The effect of the weight ratio and calcination temperature (at 773–1273 K, for 3 h) on the structure and texture of the resultant catalysts were determined. The samples were characterized by nitrogen adsorption-desorption, thermogravimetric analysis, differential thermal analysis. X-ray diffraction, high resolution 27Al and 31P MAS NMR and diffuse reflectance infrared Fourier transform spectroscopy. The presence of Al 2O 3 greatly affected the surface properties of AlPO 4 upon heating, exerting a protective effect against sintering, and delaying the crystallization process. The thermal stability of the texture of the APAl-A systems was found to be considerably improved, in comparison with the AlPO 4 catalyst, by the stabilization of the amorphous structure in the APA1-A systems, which was found to be fully retained, in the presence of Al 2O 3, upon calcination up to 1073 K. Calcination at 1273 K develops AlPO 4 crystallinity in the α-cristobalite polymorph (which is greater as the Al 2O 3 content increases) but AlPO 4 inhibits both the crystallization of the γ-Al 2O 3 phase and the γ- to α-Al 2O 3 phase transition. In particular, incorporation of 5 wt.-% Al 2O 3 resulted in an APAl-A system with remarkable thermal stability, after calcination, over the entire range of temperatures studied (773–1273 K). Amorphous APA1-A systems contain both tetrahedral and octahedral aluminum (their ratio varies with the Al 2O 3 content and the calcination temperature) while phosphorus always remains in P(OAl) 4 environments. After calcination at 1273 K, only tetrahedral aluminum was found. Two types of aluminum surface hydroxyl groups (at 3786 and 3728 cm −1) and only one type of phosphorus surface hydroxyl group (at 3670 cm −1) are found in the DRIFT spectra. Furthermore, bands at 1130 and 478 cm −1 are found in the skeletal region. These are due to the triply degenerate P-O stretching vibration v 3 mode of tetrahedral (PO 4) 3− and to the triply degenerate O-P-O bending variations v 4 mode of (PO 4) 3− tetrahedra, respectively. A band at 714 cm −1, due to the stretching vibrations of Al-O bonds in combination with P-O bonds, is also found. Therefore the diffuse reflectance infrared Fourier transform spectra substantiated the structural changes in the samples in agreement with XRD and 27Al and 31P MAS NMR.