Study on the catalytic properties of heteroatom-incorporated aluminophosphate molecular sieve, $AlPO_4$-5

Abstract

Heteroatom-incorporated aluminophosphate molecular sieves, FAPO-5 and MnAPO-5 were synthesized using Fe(II) and Mn(II), and TEAOH as a templating agent. They were characterized by using various physico-chemical methods to elucidate the incorporation of metal ions into $AlPO_4$-5 framework. Also a comparison is made on the nature of the iron site in FAPO-5 with that of Fe-silicate ZSM-5. In as-synthesized FAPO-5, iron ions were located in the framework position as Fe(II) as well as Fe(III) ions. The resonance absorption around g=4.3 in EPR spectra and a band around 425nm in PA spectra indicated the presence of octahedral Fe(III) ions in as-synthesized FAPO-5. The dehydration of as-synthesized FAPO-5 resulted in a decrease in the intensity of the g=4.3 signal in EPR as well as 425nm band in PAS. These Fe(III) ions seemed to be located in the framework position having pseudooctahedral symmetry as was observed for Al(III) ions in AlPO4 molecular sieves. The results of the thermal analysis showed one peak around 570-620K associated with the decomposition of TEAOH in $ALPO_4$-5, whereas in FAPO-5 an additional peak was observed at about 710K due to the decomposition of charge balancing $TEA^+$ ions. The intensity, of the peak corresponding to the decomposition of $TEA^+$ increased with increasing iron content. The thermal decomposition patterns of template in FAPO-5 was consistent with the results of DTG and DSC giving acid-actalyzed products such as aromatic compounds in the same temperature range where additional DTG and DSC peak was observed. The template seemed to play a role in placing Fe (II) ions in the framework, incorporating Fe(II) ions in place of Al(III) ions in the form of Fe(II) $TEA^+$ After the calcination, PA spectra exhibited four well resolved d-d transition bands at about 375,410,435 and 480nm of Fe(III) ions having tetrahedral symmetry similar to the spectrum of Fe(III) ions in as-synthesized iron-silicate ZSM-5, suggesting the oxidation of frame...