Sn-modified Zr-UiO-66 metal–organic frameworks for dihydroxyacetone conversion into lactic acid†

Dihydroxyacetone is a valuable waste-valorization compound and is a main intermediate in lactic acid production from different recyclable sources. In the present work, Sn incorporation into a Zr-based metal–organic framework is used to increase its acidity and facilitate dihydroxyacetone conversion to lactic acid through the minimization of side reactions. Three series of Sn-modified metal–organic frameworks with different synthetic conditions and Zr/Sn molar ratios are prepared and characterized by low-temperature nitrogen adsorption, XRD, XRF, TGA, XPS, and FTIR spectra of the adsorbed CO, and NH₃-TPD methods. The nature of the Sn species depends on both the Sn precursor and the amount of water in the liquid phase during the synthesis, allowing the control of SnCl_x hydrolysis. Sn addition leads to an increase in the acidity of the MOF and mostly depends on the nature of the Sn species rather than Sn loading. Insertion of a tin species directly into the UiO-66 structure results in an almost quantitative yield of the target products. The material retains its structure after catalysis, but the deposition of the polycondensation products on the MOF surface slightly decreases the selectivity at consecutive runs.