SAPO-34 molecular sieves have been synthesized with different structure directing agents. Although materials with the same framework structure (CHA type) are obtained in all cases, they possess different physicochemical properties, especially textural parameters and crystal size. These catalysts have been tested in the MTO process. All the samples exhibited high activity and selectivity to short chain olefins at the initial stages of the reaction, but they deactivate rapidly with time on stream, especially at high space velocity. It has been observed an important influence of the external surface, crystal size and acidity on the activity, selectivity and lifetime of the different samples. Thus, the sample synthesized with tetraethylammonium hydroxide as structure directing agent rendered the best catalytic performance owing to its higher external surface, smaller crystal size and higher acidity.
Microwave mediated synthesis produced SAPO-34 nanocrystals with increased catalyst lifetime in the methanol-to-olefin reaction owing to their plate-like crystal shape.
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.
The MTO process produces lower olefins from natural gas or coal via methanol, providing an interesting route to obtain valuable petrochemicals from carbon sources alternative to petroleum. Small-pore silicoaluminophosphate SAPO-34 has been proven an efficient catalyst for the MTO process, showing exceptionally high selectivity to lower olefins. However, these catalysts undergo rapid deactivation due to deposition of high molecular weight hydrocarbons on the pore entrances, which completely blocks the internal channels of the SAPO-34 crystals. In the present work, we explore the use of very dilute zeotype precursor solutions to reduce the size of the crystals of these materials, improving greatly their catalytic behaviour in the MTO process by increasing the catalyst life time.
A molecular-mechanics computational study is performed in order to analyze the arrangement of (1R,2S)-(-)-ephedrine molecules within the 12-MR channels of the AFI aluminophosphate microporous framework and the influence on the spatial distribution of dopants embedded in the tetrahedral network. Results showed that ephedrine molecules arrange exclusively as dimers by π-π stacking of the aromatic rings within the AFI channels. Interestingly, the asymmetric nature of ephedrine and the presence of H-bond-forming groups (NH2 and OH) involve a preferential orientation where consecutive dimers within the channels are rotated by an angle of +30°; this is driven by the establishment of inter-dimer H-bonds. This preferential orientation leads to the development of a supramolecular enantiomerically-pure helicoidal (chiral) arrangement of ephedrine dimers. In addition, the computational results demonstrate that the particular molecular structure of ephedrine imparts a strong trend to attract negative charges to the vicinity of the NH2(+) positively-charged groups. Hence divalent dopants such as Mg, whose replacement by trivalent Al in the aluminophosphate network involves the generation of a negative charge, will tend to locate close to the NH2(+) molecular groups, suggesting that an imprinting of the organic arrangement to the spatial distribution of dopants would be feasible. Combined with the trend of ephedrine to arrange in a helicoidal fashion, an enantiomerically-pure helicoidal distribution of dopants would be expected, thus inducing a new type of chirality in microporous materials.
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