Confined space of a nickel-triazole metal-organic framework responsible for high product selectivity and enantiospecific yield of lactic acid converted from sugar in a water-based system

Enantioselective production of lactic acid from xylose sugar in the constrained pore space of the Ni-triazole metal-organic framework (MOF) is demonstrated. A robust hydrophilic structure with a confined nano-pocket is constructed from the [Ni3(mu 3-OH)(Tz)3(OH)2(H2O)4]n secondary building unit (SBU). At elevated temperature, the de-coordination of water and hydroxide species on the Ni node can generate accessible an open metal site (OMS) possessing Lewis acidicity, acting as catalytic center for the catalysis of xylose to lactic acid. Density functional theory (DFT) calculations suggests that an enantiospecific yield of lactic acid is realizable by a preferential interaction between any pair of SBUs and the interlocked trans-pyruvaldehyde intermediate. This work highlights the unique spatial and chemical environment of MOFs as an advantageous platform for specific processes in catalysis. Hydrophilic Ni-MOF performs a crucial role in upgrading natural sugar to value-added lactic acid in aqueous media. At elevated temperature, open metal sites with Lewis acid attributes, responsible for the catalytic centers, can be simultaneously generated on Ni nodes. The more stable trans-pyruvaldehyde interlocked within the confined pore pocket determines the preferential conversion to l-lactic acid as the final product.