Two-stage surfactant-assisted crystallization for enhancing SAPO-11 acidity to improve n-octane di-branched isomerization

Silicoaluminophosphate (SAPO-11) samples with enhanced acidity were synthesized by two-stage crystallization in the presence of dodecyltrimethylammonium bromide (DoTAB), in which the Si source was added after the aluminophosphate framework was formed. The structure, elemental composition, acidity, Si environment, and n-octane di-branched isomerization of the as-synthesized SAPO-11 with small crystallites were investigated. The results showed that compared with the conventional one-stage crystallization, the two-stage crystallization produced more Si borders in the resulting SAPO-11. Among the SAPO-11 samples synthesized by two-stage crystallization, the SAPO-11 with the optimal DoTAB content (mol(DoTAB)/mol(SiO2) = 0.015) had the most Si(nAl) (0 < n < 4) species with moderate and strong Brønsted acidity. The catalyst based on the small crystallite SAPO-11 containing a greater number of moderate and strong Brønsted acid sites exhibited superior performance for n-octane di-branched isomerization. This two-stage DoTAB-assisted crystallization opens a new route to enhancing the SAPO-11 acidity and can be extended to the other SAPO molecular sieves.

A suitable DoTAB content in two-stage crystallization promotes the formation of small Si domains with more Si border environments. The corresponding SAPO-11-based catalyst with more Si(nAl) (0 < n < 4) environments has superior di-branched isomer selectivity due to greater numbers of moderate and strong B acid sites at the external surface of SAPO-11.Figure optionsDownload high-quality image (260 K)Download as PowerPoint slideHighlights
► Optimal DoTAB content in two-stage synthesis gives SAPO-11 more Si borders.
► SAPO-11 with more Si borders has more active sites at the external surface.
► The Pt/SAPO-11 with more active sites has superior di-branched isomer selectivity.