Isosorbide Production from Sorbitol over Heterogeneous Acid Catalysts: Screening and Kinetic Study

The catalytic performance of two types of heterogeneous acid catalysts—sulfonic acid-functionalized materials and aluminum containing zeolites,—in the dehydration of sorbitol to isosorbide, in solventless and autogenous pressure conditions, has been studied. Catalysts screening evidenced strong differences between sulfonic acid-based materials and acid zeolites in terms of catalytic performance. Whereas sulfonic materials, such as Amberlyst-70 and SBA-15-Pr-SO₃H, showed a very high catalytic activity, zeolites with beta structure evidenced good catalytic performance together with minimized promotion of side reactions (production of non-desired sorbitans, humins, etc.). Kinetic studies performed at different temperatures, adjusting to a Langmuir–Hinshelwood type model, allowed correlating the physicochemical properties of the acid materials with their catalytic performance in sorbitol dehydration. Thus, the analysis of initial selectivity through kinetic constants comparison indicated that commercial beta zeolite with a Si/Al ratio of 19 is the most selective catalyst for the production of isosorbide, though following a slower kinetics than the sulfonic materials. Furthermore, an equivalent hierarchical beta zeolite has been synthesised and evaluated, resulting in a slight improvement of the catalytic performance, in terms of both yield and selectivity to isosorbide. This improvement is attributed to the superior textural properties.