Mesopore-Modified SAPO-18 with Potential Use as Catalyst for the MTO Reaction

Abstract

SAPO-34 silicoaluminophosphates are well known as catalysts for the synthesis of light olefins-ethylene and propylene-from methanol by using the methanol to olefins (MTO) process, first described by Mobil. SAPO-18, which has a microporous framework structure related to but crystallographically distinct from SAPO-34, has been much less studied but promises to be a potential catalyst in the MTO process. The main drawback of these catalysts in this reaction is their rapid deactivation, due to the deposition of heavy carbonaceous products-coke-on the surface of the solid avoiding the access of methanol molecules to the active centres located inside the pores of the SAPO catalysts. We have used different mesoporogen agentsnanoparticulate carbons and chitosan-aiming to generate mesoporosity in the SAPO-18 crystals, in an attempt to improve the accessibility of the reagent to the active centres of SAPO-18 and, in that way, inhibit catalyst deactivation. All the materials prepared in this work present similar framework composition and silicon distribution and the main difference among them is the hierarchical porosity generated by the mesoporogen additives use in the synthesis, as determined by STEM. Using chitosan polymer as a secondary template results in an increase of the external surface, which improved significantly the internal diffusivity enhancing the life time of the catalyst in the MTO process.

SAPO-34 with a low acidity outer layer by epitaxial growth and its improved MTO performance

“…42 The low-silica precursor solution tends to form SAPO-18 with the AEI structure. [43][44][45][46] Even SAPO-5 may be formed in the low enough Si/Al ratio precursor solution. So it has to face the challenge for the one-step synthesis to lower the acid site density.…”

Nanocrystalline SSZ-39 zeolite as an efficient catalyst for the methanol-to-olefin (MTO) process

“…Similar ''non-selective'' deactivation profiles have been previously reported for ZSM-5, 22 SSZ-13, 23 SAPO-34, 10c,13a and SAPO-18. 24 Moreover, comparing selectivities at constant conversion allows the discrimination of the influence of thermodynamical equilibrium of olefins, at 100% conversion of methanol + DME, from the true selectivity of the hydrocarbon pool at lower conversions. 10c Regarding the product selectivities at different methanol conversion values, it can be clearly seen that the SSZ-13 and SSZ-39 materials show different product distributions [see Fig.…”

Combined Ex and In Situ Measurements Elucidate the Dynamics of Retained Species in ZSM‐5 and SAPO‐18 Catalysts Used in the Methanol‐to‐Olefins Reaction

“…As seen, the conversion decreases very rapid for the H‐SAPO‐18 catalyst, indicating that this catalyst undergoes very rapid deactivation in comparison with ZSM‐5 catalysts. This occurs for the strong shape selectivity that the AEI structure imposes to the MTO products, in which only short‐chain aliphatics are able to exit from the pores of this structure [12, 13] …”

“…The typical industrial catalysts are based on SAPO‐34 (CHA structure) zeotypes for the MTO process and ZSM‐5 (MFI structure) zeolites for the MTP process [1, 4–9] . Apart from these materials, there are lots of other proposals in the literature such as SAPO‐18 (AEI structure) zeotypes that exhibit an excellent kinetic performance [10–14] . The selectivity of light olefins, or particularly to propylene, greatly depends on the catalyst properties.…”