Structural engineering of Zr-based metal-organic framework catalysts for optimized biofuel additives production

This study reports the first investigation of the systematic introduction of defects in functionalized UiO-66 structures and its effect on their catalytic activities. Indeed, fifteen UiO-66-based MOFs were synthesized and used as catalysts in the esterification reaction of butyric acid in presence of butanol to produce the novel green biofuel additive, butyl butyrate. The samples included three different structures, UiO-66, UiO-66(COOH)₂, and UiO-66(NH₂), and five different modulation synthesis conditions for each structure yielding a total of fifteen samples. The increase in the modulator acidity or concentration significantly increased the number of missing linkers per cluster, surface area, and pore volume of the three structures. This leads to the increase in the conversion to butyl butyrate to levels higher than those previously achieved with similar systems. The improved conversion in each structure was attributed to the missing-linker defective sites which act as catalytic centers for the activation of butyric acid. However, using organic linkers with uncoordinated Brønsted acid sites boosted the catalytic activity of UiO-66(COOH)₂ to levels higher than both counterparts even for lower number of defects, surface area, and pore size. The catalytic reaction was thus proved to be occurring utilizing both the defective sites and the carboxylic functional groups as catalytic centers and a reaction mechanism is proposed based on this assumption. Finally, the choice of the organic linker for the engineering of the MOFs' structure in catalytic applications offers abundant possibilities especially if coupled with the systematic increase in defective sites to overcome challenging diffusion limitations.