Institute for Integrated Catalysis , Pacific Northwest National Laboratory , Richland , Washington 99354 , United States.
Voiland School of Chemical Engineering and Bioengineering , Washington State University , Pullman , Washington 99163 , United States.
J Am Chem Soc. 2019 Feb 27;141(8):3444-3455. doi: 10.1021/jacs.8b07969. Epub 2019 Feb 15.
The catalytic sites of acidic zeolite are profoundly altered by the presence of water changing the nature of the Brønsted acid site. High-resolution solid-state NMR spectroscopy shows water interacting with zeolite Brønsted acid sites, converting them to hydrated hydronium ions over a wide range of temperature and thermodynamic activity of water. A signal at 9 ppm was observed at loadings of 2-9 water molecules per Brønsted acid site and is assigned to hydrated hydronium ions on the basis of the evolution of the signal with increasing water content, chemical shift calculations, and the direct comparison with HClO in water. The intensity of H-Si cross-polarization signal first increased and then decreased with increasing water chemical potential. This indicates that hydrogen bonds between water molecules and the tetrahedrally coordinated aluminum in the zeolite lattice weaken with the formation of hydronium ion-water clusters and increase the mobility of protons. DFT-based ab initio molecular dynamics studies at multiple temperatures and water concentrations agree well with this interpretation. Above 140 °C, however, fast proton exchange between bridging hydroxyl groups and water occurs even in the presence of only one water molecule per acid site.
沸石的催化活性位在水的存在下会发生深刻变化，从而改变 Brønsted 酸位的性质。高分辨率固态 NMR 光谱表明，水与沸石 Brønsted 酸位相互作用，在很宽的温度和水热力学活度范围内将其转化为水合的质子。在每摩尔 Brønsted 酸位 2-9 个水分子的负载下观察到 9 ppm 处的信号，并基于信号随含水量的增加、化学位移计算以及与水中 HClO 的直接比较，将其分配给水合质子。随着水化学势的增加，H-Si 交叉极化信号的强度先增加后减小。这表明，随着质子与水形成水合离子-水簇，沸石晶格中水分子与四配位铝之间的氢键减弱，质子的迁移率增加。基于密度泛函理论的从头算分子动力学研究在多个温度和水浓度下与这一解释非常吻合。然而，在 140°C 以上，即使在每个酸位只有一个水分子的情况下，桥连羟基和水之间也会发生快速质子交换。
