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

## TL;DR

This study shows how the coordination of aluminum in a catalyst affects its ability to convert CO2 into dimethyl ether.

## Contribution

The study reveals that aluminum coordination and distribution, not just its quantity, control catalytic performance in CO2-to-DME conversion.

## Key findings

- Higher Al content does not proportionally increase catalytic activity due to framework and extra-framework Al species.
- Framework Al incorporation increases with higher Si/Al ratios, while excess Al forms amorphous Al2O3.
- Bronsted acidity increases only moderately with Al content, while Lewis acidity persists at high temperatures.

## Abstract

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.