Impact of cationic surfactant chain length during SAPO-11 molecular sieve synthesis on structure, acidity, and n-octane isomerization to di-methyl hexanes

The structure, acidity, and n-octane di-branched isomerization of SAPO-11 synthesized with surfactants of different chain lengths in water–propanol system were investigated. The results showed that with the increasing chain length of surfactants, the crystallite size of SAPO-11 firstly decreased and then increased, but the external surface area and the active B acid sites of SAPO-11 firstly increased and then decreased. The smallest crystallite, the highest external surface area, and the most B acid sites were obtained simultaneously with the surfactant of 12 carbon atoms. The isomerization results of n-octane indicated that among the series Pt/SAPO-11 catalysts, the catalyst with DoTAB-directed SAPO-11 as support had the highest selectivity to di-branched isomers and the lowest cracking selectivity due to the largest external surface area that created more active sites to promote the formation of di-branched isomers by the key-lock mechanism and improve the diffusion of intermediates and products for reducing cracking reactions.

Graphical abstractWith the increasing chain length of surfactants, the crystallite size of SAPO-11 firstly deceases and then increases, while the external surface area shows a reverse variation. The small-sized SAPO-11-based catalyst has superior di-branched isomer selectivity due to more active sites at pore mouths produced from the higher external surface area.Figure optionsDownload full-size imageDownload high-quality image (104 K)Download as PowerPoint slideHighlights► DoTAB with 12 carbons produces SAPO-11 with smaller size and more external surface. ► The high external surface area with small crystallites creates more pore mouths. ► The small-sized SAPO-11-based catalyst has superior di-branched isomer selectivity.