Beyond the Si/Al ratio: structure–acidity correlation in mesostructured Al-SBA-16 catalysts for the one-pot CO₂-to-DME conversion

Introduction
Nowadays, research is increasingly focusing on green fuels produced from captured CO₂ (e-fuels). One of the most promising candidates is dimethyl ether (DME), a substitute for diesel fuel. DME can be synthesized from CO₂ via two consecutive reactions: the first, catalyzed by Cu-based reduction catalysts, involves the reduction of CO₂ to methanol; the second, promoted by solid acid catalysts, is the dehydration of methanol to DME. In this work, three mesostructured aluminosilicates (Al-SBA-16) with three different Si/Al ratios (10, 15, and 20) are presented as methanol dehydration catalysts for the one pot-CO₂-to-DME conversion. The catalysts have been tested and characterized with a particular focus on the correlation of their structural properties with their acid features and their catalytic performance.

Methods
The Al-SBA-16 samples were obtained with an Evaporation-Induced Self-Assembly (EISA) method using a silicon alkoxide (TEOS) and aluminum chloride as precursors. The samples were studied with a wide range of techniques to determine their structural, textural, morphological, and acid properties, and evaluated to determine their catalytic performance.

Results and discussion
Catalytic tests reveal an increased activity with higher Al content, in agreement with pyridine-FTIR acid site characterization, which shows a moderate increase in acid site number with decreasing Si/Al ratios; however, the trend is less pronounced than expected based merely on the Si/Al ratio. To investigate this finding, ²⁷Al and ²⁹Si solid-state NMR were employed to gather molecular-level insights into the structure–acidity relationship. The ²⁷Al-SS-NMR spectra reveal the presence of both tetrahedral (framework and Al₂O₃-derived) and octahedral Al species. A higher Si/Al ratio enhances aluminum incorporation into the framework, while higher aluminum content favors the formation of extra-framework Al₂O₃. These findings highlight the critical role of Al coordination and distribution in tuning acidity and catalytic activity.