Binuclear and mononuclear active sites of porous tin-organic frameworks for selective epimerization of D-glucose into D-mannose

Abstract

Porous tin-organic frameworks (Sn-OFs) are composed of Lewis acid Sn⁴⁺ sites connected via organic linkers by tin-carbon covalent bonds. Although high activity of Sn-OFs for selective epimerization of D-glucose into D-mannose has been reported, the roles of different active sites of the Sn-OFs in catalysis remain under debate. Herein, four Sn-OFs with different aromatic linkers were synthesized and characterized using elemental analysis, low-temperature N₂ adsorption, and SEM, along with ATR-IR, ¹H, ¹³C, and ¹¹⁹Sn MAS NMR, XANES and EXAFS spectroscopies. Comparative studies with organotin model molecules revealed tetraaryltin Ar₄Sn as major structural element of the Sn-OFs with additional sites including dimers Ar₃SnOSnAr₃, alkylaryltin sites ⁿBu_xAr_(4-x)Sn, and SnO₂. Catalytic tests of the respective organotin molecules suggest Ar₄Sn and Ar₃SnOSnAr₃ as active sites for the epimerization, whereas the activity of the latter binuclear sites appears to be superior. In contrast, ⁿBu_xAr_(4-x)Sn alkylaryltin sites and SnO₂ contributed to the formation of side products, converting D-glucose into D-fructose. The Sn-OFs with symmetrical linkers are more structured materials exhibiting higher catalytic activity than the Sn-OFs with asymmetrical linkers. These findings clarify the roles of various Sn sites in Sn-OFs, advancing understanding of their catalytic behavior.