Maag Alex R, Tompsett Geoffrey A, Tam Jason, Ang Cheen Aik, Azimi Gisele, Carl Alexander D, Huang Xinlei, Smith Luis J, Grimm Ronald L, Bond Jesse Q, Timko Michael T
Department of Chemical Engineering at Worcester Polytechnic Institute, 100 Institute Road, Worcester, Massachusetts 01609, USA.
Phys Chem Chem Phys. 2019 Aug 28;21(32):17880-17892. doi: 10.1039/c9cp01490j. Epub 2019 Aug 5.
Zeolites have recently attracted attention for upgrading renewable resources in the presence of liquid water phases; however, the stability of zeolites in the presence of liquid-phase water is not completely understood. Accordingly, the stability of the ZSM-5 framework and its acid sites was studied in the presence of water at temperatures ranging from 250 to 450 °C and at pressures sufficient to maintain a liquid or liquid-like state (25 MPa). Treated samples were analyzed for framework degradation and Al content and coordination using a variety of complementary techniques, including X-ray diffraction, electron microscopy, N sorption, Al and Si NMR spectroscopy, and several different types of infrared spectroscopy. These analyses indicate that the ZSM-5 framework retains >80% crystallinity at all conditions, and that 300-400 °C are the most aggressive. Decrystallization appears to initiate primarily at crystal surfaces and share many characteristics in common with alkali promoted desilication. Liquid water treatment promotes ZSM-5 dealumination, following a mechanism analogous to that observed under steaming conditions: initiation by Al-O hydrolysis, Al migration to the surface, and finally deposition as extra framework Al or possibly complete dissolution under some conditions. As with the framework, dealumination is most aggressive at 300-400 °C. Several models were evaluated to capture the non-Arrhenius effect of temperature on decrystallization and dealumination, the most successful of which included temperature dependent values of the water auto-ionization constant. These results can help interpretation of previous studies on ZSM-5 catalysis in hot liquid water and suggest future approaches to extend catalyst lifetime.
近年来，沸石在液相水存在的情况下用于升级可再生资源引起了人们的关注；然而，沸石在液相水存在下的稳定性尚未完全明了。因此，研究了ZSM-5骨架及其酸性位点在250至450℃的温度以及足以维持液态或类液态状态（25MPa）的压力下与水共存时的稳定性。使用多种互补技术对处理后的样品进行骨架降解、铝含量及配位情况分析，这些技术包括X射线衍射、电子显微镜、氮吸附、铝和硅核磁共振光谱以及几种不同类型的红外光谱。这些分析表明，ZSM-5骨架在所有条件下均保持>80%的结晶度，且300 - 400℃时最为剧烈。脱晶似乎主要在晶体表面开始，并且与碱促进脱硅有许多共同特征。液态水处理促进了ZSM-5的脱铝，其机理类似于在蒸汽条件下观察到的情况：由铝 - 氧水解引发，铝迁移至表面，最终在某些条件下以骨架外铝的形式沉积或可能完全溶解。与骨架情况一样，脱铝在300 - 400℃时最为剧烈。评估了几种模型以捕捉温度对脱晶和脱铝的非阿累尼乌斯效应，其中最成功的模型包括水自电离常数的温度相关值。这些结果有助于解释先前关于ZSM-5在热液态水中催化作用的研究，并为延长催化剂寿命提出未来的方法。
