Enzyme-like Enantioselectivity in GTM Chiral Zeolite Catalysts upon Preactivation of Ge Sites

Extra-large-pore Ge-containing GTM chiral zeolite catalysts have recently proved useful asymmetric catalysts, with chirality emerging from their chiral confined nanospace. However, so far these exceptional materials have suffered from low framework stability in the presence of water and moderate catalytic enantioselectivity in the ring-opening of chiral trans-stilbene oxide with 1-butanol used as a test reaction. Here, we report that these chiral zeolite catalysts can be easily stabilized upon exposure of the calcined material to 1-butanol, providing stability against water and, most importantly, prompting a preactivation of the chiral active sites that boosts their enantioselective properties, reaching unprecedented enantiomeric excesses up to 88% where one enantiomer reacts 16 times more than the other. A range of physicochemical studies, including in situ Fourier transform infrared (FTIR) and X-ray absorption spectroscopy, indicates that framework Ge sites increase their coordination environment upon interaction with 1-butanol molecules, which after a thermal treatment above 100 °C remain irreversibly bound to Ge as a consequence of a condensation and dehydration reaction, providing a route to easily functionalize these materials. These preactivated GTM asymmetric catalysts act similarly to enzymes by controlling the confinement of the chiral reactants in particular orientations through coordination with Ge and development of H-bonds with nearby hydroxyl groups, thus attaining enantioselective catalytic activities close to those reached by enzymatic systems but with the crucial advantage associated with heterogeneous catalysts and, notably, the possibility of preparing both enantiomeric versions of the catalyst by using an easily accessible alkaloid.