Brønsted acid sites based on penta-coordinated aluminum species

Zeolites and amorphous silica-alumina (ASA), which both provide Brønsted acid sites (BASs), are the most extensively used solid acid catalysts in the chemical industry. It is widely believed that BASs consist only of tetra-coordinated aluminum sites (Al^IV) with bridging OH groups in zeolites or nearby silanols on ASA surfaces. Here we report the direct observation in ASA of a new type of BAS based on penta-coordinated aluminum species (Al^V) by ²⁷Al-{¹H} dipolar-mediated correlation two-dimensional NMR experiments at high magnetic field under magic-angle spinning. Both BAS-Al^IV and -Al^V show a similar acidity to protonate probe molecular ammonia. The quantitative evaluation of ¹H and ²⁷Al sites demonstrates that BAS-Al^V co-exists with BAS-Al^IV rather than replaces it, which opens new avenues for strongly enhancing the acidity of these popular solid acids.