Microenvironment Effect Catalysis with Phosphoric Acid-Based Covalent Organic Frameworks

Drawing inspiration from enzymatic catalysis, a phosphoric acid-based covalent organic framework (PA-COF) was engineered for the efficient synthesis of optically pure lactide. PA-COF catalyst features precisely engineered microenvironments within well-defined porous channels decorated with phosphoric acid as the catalytically active sites. Much like enzymatic catalysis, where product selectivity is governed by the protein pocket, PA-COF provides precise microenvironments due to its highly ordered channels and adjustable structures. The phosphoric acids in the channels as catalytically active sites play key roles in directly converting lactic acid monomers into the cyclic dimer lactide. The process effectively avoided oligomerization, achieving an excellent yield of approximately 95%. This approach significantly differs from the traditional two-step strategy. It avoids the use of metal catalysts and high-temperature (∼200 °C) reaction conditions, thus avoiding metal residues and racemization. This microenvironment effect catalysis strategy provides a new pathway for the synthesis of lactide and may be extended to other useful condensation reactions.