Metal-organic frameworks as effective catalysts for glycerol acetalization: morphology and functionality

Glycerol is a major by-product generated in large quantities by the biodiesel industry, and its valorisation is essential to promote the usage of this relatively green energy resource. Conversion into fuel additives as solketal via a solvent-free acetalization reaction is one of the most sustainable possible ways to valorise glycerol. The key to the success of this process is the design of an active, selective and robust catalyst. In this work, we investigated how the framework topology and Brønsted acid functionality of different MOF materials can influence their catalytic performance. We selected the microporous UiO-66(Zr) and mesoporous MIL-101(Cr) archetypes and functionalized them with both carboxylic (-COOH)₂ and sulfonic (-SO₃H) acid groups. This work reports the first use of acid-functionalized MIL-101(Cr) for the solvent-free production of the fuel additive solketal from glycerol, and the first study to directly compare both MOF families under identical conditions. Reaction parameters were optimized under sustainable, solvent-free conditions. Both UiO-66(Zr)-(COOH)₂ and MIL-101(Cr)-SO₃H demonstrated remarkable catalytic activity (>85% conversion and >98% selectivity to solketal after 1 h) and, crucially, maintained exceptional structural stability with no observable loss of activity over ten consecutive catalytic cycles. This study establishes a crucial structure-property relationship, validating both specific MOF functionalities for highly effective and reusable heterogeneous catalysts.