Flexible Al Coordination with H<sub>2</sub>O Explaining the Deviation of Strong Acid Amount from the Framework Al Content in Al-Rich SSZ-13
Yue Ma, Jiancai Ding, Letong Yang, Xiaodong Wu, Yang Gao, Rui Ran, Duan Weng
Abstract
To explain the deviation between the amount of strong Brønsted acid sites determined by NH 3 temperature-programmed desorption (NH 3 -TPD) and the amount of framework Al detected by magic angle spinning nuclear magnetic resonance (MAS NMR) in Al-rich SSZ-13, H-form, and NH 4 -form of zeolites, as well as those saturated by gaseous NH 3 and exchanged by Co 2+, were carefully characterized. Density functional theory (DFT) calculations were adopted to investigate the influence of the Si/Al ratio on the changes in Al coordination. It was revealed that the reversible hydration and hydrolysis of tetrahedrally coordinated framework Al transformed strong Brønsted acid sites to weaker ones, but these Al species still remained in the zeolite framework even though they were converted into octahedrally coordinated moieties. The changes in the Al coordination environment could be fully reversed by low-temperature ( e . g ., 50 °C) NH 3 saturation and Co 2+ ion exchange. Paired Al atoms were more vulnerable to hydration and hydrolysis compared with isolated Al atoms due to the higher positive charge density of the former atoms, so Al-rich SSZ-13 exhibited a more pronounced deviation in the strong acid amount. The Co 2+ ion exchange combined with NH 3 -TPD was suggested for quantifying the total amount of strong acid sites and ion-exchange sites in Al-rich SSZ-13.