MgAPSO-34 molecular sieves with various Mg stoichiometries: Synthesis, characterization and catalytic behavior in the direct transformation of chloromethane into light olefins

A series of MgAPSO-34s with various Mg stoichiometries were synthesized, characterized by multiple techniques such as, powder XRD, solid state MAS NMR, DR UV-vis spectrometry, NH3-TPD and FTIR and used as catalysts in the direct conversion of chloromethane into light olefins. XRD results revealed that the unit cell parameters of the CHA structure increased with Mg incorporation. The appearance of some new peaks in the P-31 MAS NMR spectra of MgAPSO-34 indicated new P co-ordination states as a result of the substitution of Al by Mg. Mg incorporation also modified the Si co-ordination states. A DR UV-vis spectroscopic study evidenced that, in comparison with an MgO crystal, the Mg species in the MgAPSO-34s were quite different. These changes in crystal structure and the chemical environment of framework elements created by Mg substitution modified the acidity and catalytic properties. The acidity of SAPO-34 and MgAPSO-34 determined with NH3-TPD and FTIR techniques showed that the amount of acid sites, especially those of strong acidity, is reduced with the increase of Mg incorporation. Direct chloromethane transformation was carried out over SAPO-34 and MgAPSO-34. All the samples were very active and selective catalysts for light olefin production from chloromethane. Compared with SAPO-34, MgAPSO-34 showed an enhanced conversion, longer catalyst life and improved light olefin selectivity. (C) 2008 Elsevier Inc. All rights reserved.