Direct observation of catalyst behaviour under real working conditions with X-ray diffraction: Comparing SAPO-18 and SAPO-34 methanol to olefin catalysts

• Published in: Journal of catalysis 2011-04, Vol.279 (2), p.397-402
• Main Authors: Wragg, David S., Akporiaye, Duncan, Fjellvåg, Helmer
• Format: Article
• Language: English
• Subjects: aromatic compounds; Catalysis; Catalysts; Chemistry; Comparative studies; Diffraction; Exact sciences and technology; General and physical chemistry; Heterogeneous catalysis; hydrocarbons; Ion-exchange; methanol; Methanol to olefins; olefin; Surface physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; working conditions; X-ray diffraction; X-rays; Zeolites; Zeolites: preparations and properties
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2011.02.011

The closely related silicoaluminophosphate framework catalysts SAPO-18 and SAPO-34 are shown to behave very differently during the methanol to olefin conversion. SAPO-18 shows less expansion despite larger coke molecules being present. [Display omitted] ► Much smaller expansion in SAPO-18 than in SAPO-34 during the MTO process. ► No clear end point to the expansion of SAPO-18, fitting with extended MTO activity. ► Larger quantities of coke in SAPO-18 do not give rise to greater expansion. We have made direct observations of the behaviour of the silicoaluminophosphate framework of the SAPO-18 catalyst under working conditions in the methanol to olefin conversion process. Time-resolved synchrotron powder X-ray diffraction data for the process at 450 °C show that the unit cell of the catalyst expands by around 0.9%, significantly less than the structurally similar SAPO-34. The smaller expansion is explained in terms of the cage volume and the rigidity of the structure compared with SAPO-34. Expansion is caused by the build-up of “hydrocarbon pool” intermediates in the cages. The lack of expansion and hence slower build-up of large hydrocarbons is linked to the slower deactivation of SAPO-18 compared with that of SAPO-34. The mechanism for suppressing the formation of the large polycyclic aromatics associated with the deactivation of SAPO MTO catalysts remains open.