Beyond Aluminum Loading: How Aluminum Coordination Controls Acidity and Catalytic Performance of Al-SBA-16 in...

Mesostructured aluminosilicates (Al-SBA-16) with Si/Al molar ratios of 10, 15, and 20 were synthesized and evaluated as methanol dehydration catalysts for the one-pot conversion of CO₂ to dimethyl ether (DME). Increasing the Al content was expected to enhance activity by generating additional acid sites. Although catalytic tests confirmed higher DME selectivity at lower Si/Al (higher Al content), the gain was modest relative to the nominal increase in Al amount, motivating a closer examination of Al incorporation and its contribution to Bro̷nsted acidity. To address this, ²⁷Al and ²⁹Si solid-state NMR were combined with pyridine adsorption FT-IR. ²⁷Al NMR resolved framework tetrahedral Al alongside extra-framework penta- and octa-coordinated species. Higher Si/Al ratios favored framework incorporation, whereas increased Al loading promoted segregation as amorphous Al₂O₃. ²⁹Si MAS/CP-MAS supported partial framework substitution (subtle Q⁴ shift) together with a slight increase in Q³/Q² (silanol/Si-O-Al) contributions. FTIR corroborated these findings, showing only a moderate increase in the amount of Bro̷nsted sites with decreasing Si/Al and a greater persistence of Lewis sites at high temperature. This work demonstrates that catalytic performance in CO₂-to-DME conversion is controlled not only by the nominal Al content of Al-SBA-16 but also by the coordination and distribution of Al species between framework and extra-framework environments, establishing a direct structure-acidity-activity relationship that guides the design of more efficient aluminosilicate catalysts.