Placing catalytic active sites within the framework of zeolites and molecular sieves at specific positions is a desired synthetic strategy to alter catalytic pathways and improve the catalytic reactivity toward target chemical products. There is continuing efforts to discover ways to prepare zeolites with heteroatoms ordered at certain tetrahedrally coordinated sites (T-sites) within the framework. Using boron (B) as the heteroatom in the synthesis can be particularly advantageous for producing such molecular sieve catalysts with controlled siting because (i) the smaller radius of B can lead to the substitution at specific T-sites and (ii) framework B atoms can be replaced by aluminum (Al) via post-synthesis treatments, enabling the creation of zeolites with T-site selective Brønsted acid sites.
This study focuses on testing the hypothesis that B ordering within the molecular sieve framework can be influenced by the structure of organic cations due to the charge interaction with the borate anion present in hydrothermal conditions. Here, we design two structurally related bicyclic organic structure-directing agents (OSDAs) with the quaternary ammonium moiety installed at different parts of the molecule.
Powder X-ray Diffraction and Nuclear Magnetic Resonance (NMR) analyses show that both OSDAs crystallize borosilicate molecular sieves with MWW-type framework topology while exhibiting distinct distribution of framework-incorporated B sites. Extensive characterizations involving 1H-11B heteronuclear correlation NMR elucidate that the difference in B siting is the result of altered positions of positively charged quaternary ammonium moiety of the OSDA. Additionally, post-synthetic treatment experiments demonstrate that framework B within the prepared molecular sieves can be replaced by Al. Collectively, these results offer insights into the rational design of OSDAs for preparation of zeolitic catalysts with purposeful alteration of active site positioning.
