Beyond Aluminum Loading: How Aluminum Coordination Controls Acidity and Catalytic Performance of Al-SBA-16 in CO2-to-DME Conversion

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 octacoordinated 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.