路易斯酸
沸石
化学
Atom(片上系统)
结晶学
共振(粒子物理)
核磁共振波谱
分子
傅里叶变换红外光谱
物理化学
催化作用
立体化学
物理
有机化学
原子物理学
量子力学
计算机科学
嵌入式系统
作者
Libor Kobera,Jiřı́ Dědeček,Petr Klein,Edyta Tábor,Jiřı́ Brus,Anna V. Fishchuk,Štěpán Sklenák
摘要
Framework AlFR Lewis sites represent a substantial portion of active sites in H-BEA zeolite catalysts activated at low temperatures. We studied their nature by 27Al WURST-QCPMG nuclear magnetic resonance (NMR) and proposed a plausible mechanism of their formation based on periodic density functional theory calculations constrained by 1H MAS, 27Al WURST-QCPMG, and 29Si MAS NMR experiments and FTIR measurements. Our results show that the electron-pair acceptor of AlFR Lewis sites corresponds to an AlTRI atom tricoordinated to the zeolite framework, which adsorbs a water molecule. This AlTRI-OH2 complex is reflected in 27Al NMR resonance with δiso = 70 ± 5 ppm and CQ = 13 ± 2 MHz. In addition, the AlTRI atom with adsorbed acetonitrile-d3 (the probe of AlFR Lewis sites in FTIR spectroscopy) exhibits a similar 27Al NMR resonance. We suggest that these AlFR Lewis sites are formed from Al-OH-Si-O-Si-O-Si-OH-Al sequences located in 12-rings (i.e., close unpaired Al atoms).
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