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

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.