Freude D, Ernst H, Wolf I
Abteilung Grenzflächenphysik, Universität Leipzig, Germany.
Solid State Nucl Magn Reson. 1994 Sep;3(5):271-86. doi: 10.1016/0926-2040(94)90003-5.
2H Magic-angle spinning nuclear magnetic resonance (MAS NMR), echo 27Al NMR, two-dimensional (2D) echo 1H MAS NMR and 1H, 27Al and 29Si MAS NMR have been applied to study the acid sites of dehydrated zeolites. The quadrupole coupling constants CQCC determined from 2H MAS NMR spectra of Si-OH-Al sites increase with the framework aluminium content of the zeolites from 208 Hz (H-ZSM-5) to 236 kHz (H-X and H-Y) due to the decreased acid strength of the bridging OH groups. The 27Al signal of the Si-OH-Al sites, which was considered "NMR-invisible" in the past, yields CQCC = 16 MHz for zeolite H-ZSM-5. The majority of non-framework aluminium species could also be observed in dealuminated and dehydrated zeolites H-ZSM-5 giving CQCC approximately 9 mHz. 2D echo 1H MAS NMR spectra yield values of 16-40 ms for the lower limits of the lifetime of hydroxyl species at room temperature. A lifetime of more than 40 ms was obtained from echo 2H MAS NMR spectra for protonated sites giving a signal at ca. 6.5 ppm in partially dealuminated and weakly rehydrated samples.
2H魔角旋转核磁共振（MAS NMR）、回波27Al NMR、二维（2D）回波1H MAS NMR以及1H、27Al和29Si MAS NMR已被用于研究脱水沸石的酸性位点。由Si-OH-Al位点的2H MAS NMR谱确定的四极耦合常数CQCC随着沸石骨架铝含量的增加而增大，从208 Hz（H-ZSM-5）增至236 kHz（H-X和H-Y），这是由于桥连OH基团酸性强度降低所致。Si-OH-Al位点的27Al信号在过去被认为是“NMR不可见的”，对于沸石H-ZSM-5，其CQCC = 16 MHz。在脱铝和脱水的沸石H-ZSM-5中也能观察到大多数非骨架铝物种，其CQCC约为9 mHz。二维回波1H MAS NMR谱给出室温下羟基物种寿命下限的数值为16 - 40 ms。对于部分脱铝和弱再水化样品中在约6.5 ppm处给出信号的质子化位点，从回波2H MAS NMR谱获得的寿命超过40 ms。
