Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering, Sydney University, Chemical Engineering Building J01, Sydney, New South Wales 2006, Australia.
Department of Engineering, Macquarie University, Sydney, New South Wales 2109, Australia.
Nat Commun. 2016 Dec 15;7:13820. doi: 10.1038/ncomms13820.
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) 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) by Al-{H} dipolar-mediated correlation two-dimensional NMR experiments at high magnetic field under magic-angle spinning. Both BAS-Al and -Al show a similar acidity to protonate probe molecular ammonia. The quantitative evaluation of H and Al sites demonstrates that BAS-Al co-exists with BAS-Al rather than replaces it, which opens new avenues for strongly enhancing the acidity of these popular solid acids.
沸石和无定形硅铝酸钠（ASA）都提供 Brønsted 酸位（BAS），是化学工业中使用最广泛的固体酸催化剂。人们普遍认为，BAS 仅由沸石中桥接 OH 基团的四配位铝位（Al）或 ASA 表面附近的硅醇组成。在这里，我们通过在高磁场下进行魔角旋转的 Al-{H}偶极子介导相关二维 NMR 实验，在 ASA 中直接观察到了基于五配位铝物种（Al）的新型 BAS。两种 BAS-Al 和 -Al 都表现出相似的质子化探针分子氨的酸度。H 和 Al 位的定量评估表明，BAS-Al 与 BAS-Al 共存，而不是取代它，这为强烈增强这些流行的固体酸的酸度开辟了新途径。
