Complex relationship between SAPO framework topology, content and distribution of Si and catalytic behaviour in the MTO reaction

Three small-pore silicoaluminophosphates containing relatively large cavities in their structure (LEV, LTA and SAV) have been hydrothermally synthesized with various silicon concentrations. The effect of silicon content and distribution on both the physicochemical and catalytic properties of the SAPO molecular sieves was studied. Remarkable differences in Si incorporation were found, showing that the topological features and the structure directing agent employed in each case play an important role, controlling not just the Si location in the framework but also the amount of Si incorporated. In all the cases, the formation of Si islands, which are associated with stronger acid sites, was favoured by the use of higher concentrations of the Si source in the synthesis gel. However, it has been found that the framework topology can have a greater influence than acidity on the catalytic behaviour of SAPO materials in the MTO transformation.