Levulinic Acid Production: Comparative Assessment of Al-Rich Ordered Mesoporous Silica and Microporous Zeolite

Abstract: In this work, the formation of levulinic acid (LA) as one of the top-twelve chemical building blocks from glucose was studied. In particular, the formulations of heterogeneous acid catalysts based on SBA-15, MCM-41 mesoporous silica was carried out and their performance in catalytic conversion of glucose to LA were assessed and compared with commercial H-Beta-25 (SiO₂/Al₂O₃ = 25) microporous zeolite. The high surface area, suitable porosity, balanced acid sites were considered as the main factors of a proper catalytic performance. Thus, essential modification of mesoporous SBA-15 and MCM-41 materials was carried out by introducing Al in their structures for Lewis acid sites improvement. Alumina was introduced to SBA-15 by post synthesis evaporation impregnation method while it was embedded inside the MCM-41 mesoporous material during the synthesis. In addition, Brønsted acidity was introduced via post-synthesis grafting of sulfonic acid groups. The textural and morphological features and acidity of the materials were investigated using N₂ physisorption, SEM, EDX, TEM, XRD and pyridine-FTIR. All catalysts were tested for aqueous glucose conversion in an autoclave at 180 °C. Al-MCM-SO₃H has shown the best performance with 54% of LA yield after 4 h reaction. According to Py-FTIR introduction of alumina and sulfonic acid groups improved weak and medium Lewis and Brønsted acid sites. However, the Brønsted to Lewis acid site ratio (B/L) was higher for Al-MCM-SO₃H compared to SBA-Al-SO₃H leading the reaction pathway to LA. H-Beta-25 zeolite displayed a poor performance because of harsh medium and strong acid sites catalyzing humins formation. Graphical Abstract: [Figure not available: see fulltext.].