Catalyst selectivity is critical for reducing waste and increasing process efficiency, making it critical to understand how catalyst structure is connected to activity and selectivity. In porous materials, the pore itself can impact the formation of certain products based upon steric constraints. Microporous materials like zeolites are particularly adept at this and these so-called shape selectivities have been largely studied in Brønsted acid zeolites. Yet, we currently lack similar insights for Lewis acid zeolites. A primary issue is that only a handful of zeolites have been made into Lewis acid zeolites despite 256 frameworks and many Lewis acid metals. Sn zeolites are promising Lewis acid zeolites with excellent conversion for epoxide ring opening (ERO) reactions. The selectivity in this reaction is key, yielding a terminal ether or terminal alcohol. With different use cases (polyether polyols for terminal ethers and epoxide oligomerization for terminal alcohols), it is important to understand the causes for selectivity so that they may be tuned.
Specific Lewis acid zeolites, Sn-Beta and Sn-MFI, are both highly selective for the terminal ether when the epoxide is epichlorohydrin. As the bulkiness of the epoxide increases however, Sn-Beta becomes more selective for the terminal alcohol and Sn-MFI becomes unselective. These selectivity differences could be attributed to the different pore/ring sizes of the two frameworks, but without other materials to test out this hypothesis, the shape selectivity is hard to identify. In our ongoing work, we present a synthesis for a novel Lewis acid zeolite, Sn-FER. This zeolite is tested using similar reactions as Sn-Beta and Sn-MFI to demonstrate that the ring size affects the selectivity. Overall, the work adds a critical dimension to control the reaction selectivity.
