Accordion-like effect of flexible organic structure-directing agents in the synthesis of ferrierite zeolite

Abstract

Abstract Ferrierite zeolite was synthesized by a co-structure-directing route using a highly flexible diquaternary ammonium cation (N,N′-bis-triethylpentanediyldiammonium) as a structure directing agent in combination with tetramethylammonium as a small co-structure-directing agent. Strong alterations of the 13C NMR bands of the bulky organic dication upon confinement within the ferrierite framework suggest a change of conformation driven by host-guest interactions. A combination of molecular mechanics and DFT calculations of the theoretical 13C NMR chemical shifts allowed to explain the observed differences in the NMR bands of the dication in solution and when hosted in ferrierite: upon confinement, the flexible dication needs to squeeze through the flexible pentyl chain which locates along the 10 MR ferrierite channels in order to host the two bulky triethyl ammonium groups in adjacent intersections with 8 MR channels. Our work suggests that highly flexible cations, usually considered as less convenient organic structure-directing agents because of low specificity, have the advantage of being able to properly fit with different zeolite cell dimensions through an accordion-like effect by squeezing or stretching their flexible chains.