Department of Engineering, Macquarie University, Sydney, New South Wales 2109, Australia.
Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zürich, Hönggerberg, HCI, Zurich CH-8093, Switzerland.
Acc Chem Res. 2020 Nov 17;53(11):2648-2658. doi: 10.1021/acs.accounts.0c00459. Epub 2020 Oct 22.
Silica-alumina catalysts, including zeolites and amorphous silica-aluminas (ASAs), are among the most widely used solid acid catalysts and supports to produce petrochemicals, fine chemicals, and renewable energy. The coordination, distribution, and interactions of aluminum in ASAs have an enormous impact on their acidic properties and catalytic performance. Unsaturated tetracoordinated aluminum (Al) species are commonly accepted as the key sites in generating catalytically active Brønsted acid sites (BASs) in silica-alumina catalysts. Extensive efforts focus on increasing the concentration of Al as the main route to enhance their Brønsted acidity for efficient catalysis. However, increasing the Al concentration either weakens the acid strength in zeolites or lowers Brønsted acidity in ASAs at high Al/Si ratios, impeding acidity enhancement of these popular catalysts."Pentacoordinated aluminum (Al) species" are potential unsaturated Al species like Al but rarely observed in silica-aluminas, and thus, are widely considered unavailable for BAS formation or surface reactions. In this Account, we will describe novel strategies for the controlled synthesis of Al-enriched ASAs using flame-spray pyrolysis (FSP) techniques and highlight the contribution of Al species in acidity enhancement, together with their structure-activity relationship in the conversion of biomass-derived compounds into valuable chemicals. Using various and advanced 2D solid-state NMR (SSNMR) experiments, the studies of the acidic properties and local structure of Al-enriched ASAs reveal that Al species can highly populate on ASA surfaces, promote BASs formation, and facilitate adaptable tuning of BASs from moderate to zeolitic strength by synergy with neighboring Al sites. Moreover, the BASs with enhanced acidity can work jointly with surface Lewis acid sites or metal active species for bifunctional catalysis on Al-enriched ASAs. Compared to zeolites, these Al-enriched ASAs are highly active in acid-catalyzed biomass conversion, including alcohol dehydration and sugar conversion reactions, as well as in promoting the performance of supported metal catalysts in chemoselective hydrogenation of aromatic ketones. These new insights provide a state-of-the-art strategy for strongly enhancing the acidity of these popular silica-alumina catalysts, which offers an interesting potential for a wide range of acid and multifunctional catalysis.
硅铝酸盐催化剂，包括沸石和无定形硅铝酸盐（ASAs），是应用最广泛的固体酸催化剂和载体之一，可用于生产石化产品、精细化学品和可再生能源。ASAs 中铝的配位、分布和相互作用对其酸性性质和催化性能有巨大影响。不饱和四配位铝（Al）物种通常被认为是在硅铝酸盐催化剂中产生催化活性的 Brønsted 酸位（BASs）的关键位。人们广泛关注通过提高 Al 浓度来增强其 Brønsted 酸性，从而提高催化效率。然而，在高 Al/Si 比下，提高 Al 浓度要么削弱沸石中的酸强度，要么降低 ASAs 的 Brønsted 酸性，阻碍了这些常用催化剂的酸性增强。“五配位铝（Al）物种”是潜在的不饱和 Al 物种，类似于 Al，但在硅铝酸盐中很少观察到，因此，被广泛认为无法形成 BAS 或表面反应。在本综述中，我们将描述使用火焰喷雾热解法（FSP）技术可控合成富铝 ASAs 的新策略，并强调 Al 物种在酸性增强方面的贡献，以及它们在将生物质衍生化合物转化为有价值化学品的结构-活性关系。通过使用各种二维固态 NMR（SSNMR）实验，对富铝 ASAs 的酸性性质和局部结构的研究表明，Al 物种可以高度富集在 ASA 表面上，促进 BASs 的形成，并通过与相邻 Al 位的协同作用，灵活调节 BASs 从中等强度到沸石强度。此外，具有增强酸性的 BASs 可以与表面 Lewis 酸位或金属活性物种共同作用，在富铝 ASAs 上进行双功能催化。与沸石相比，这些富铝 ASAs 在酸催化的生物质转化反应中，包括醇脱水和糖转化反应，以及在促进负载金属催化剂在芳族酮的选择性加氢反应中的性能方面，具有更高的活性。这些新的见解为强烈增强这些常用硅铝酸盐催化剂的酸性提供了一种最先进的策略，为广泛的酸催化和多功能催化提供了有趣的潜力。
