Study of Mn incorporation into SAPO framework: Synthesis, characterization and catalysis in chloromethane conversion to light olefins

Abstract

MnAPSO-34 molecular sieve has been synthesized with triethylamine as the template, characterized with XRD, XRF, 31P, 27Al and 29Si NMR and FT-IR techniques and compared with SAPO-34. The template decomposition and removal have been investigated with TG–DTG–DSC coupled with mass spectrometer. Mn incorporation generates a negligible difference on the chemical shift in 31P and 27Al MAS NMR, while an effect on the intensity of resonance peaks is revealed. 29Si MAS NMR study has demonstrated that Mn incorporation favors the Si island formation, which may give rise to the stronger acidic sites. The thermal analysis (TG–DSC) on template removal in a diluted oxygen atmosphere, leading to the formation of CO 2, NO and H 2O, showed, besides a low temperature endothermic weight loss due to the desorption of water, two weight losses (200–400 and 400–600 °C) for SAPO-34 and MnAPSO-34, suggesting two different chemical location environments of template molecules in these two molecular sieves. The quantity of template removed at higher temperature range is much higher in MnAPSO-34, indicating stronger template–framework interaction and stronger acidity after calcination. The acid difference caused by Mn incorporation has also been evidenced by ammonia adsorption evaluated by FT-IR. Chloromethane transformation was carried out over MnAPSO-34 and SAPO-34 and the catalytic performance showed that both molecular sieves are very active and selective catalyst for light olefins production. MnAPSO-34 demonstrated higher activity and light olefins selectivity.