Jungsuttiwong Siriporn, Limtrakul Jumras, Truong Thanh N
Henry Eyring Center for Theoretical Chemistry, Chemistry Department, University of Utah, 315 S 1400 E, Room 2020, Salt Lake City, Utah 84112, USA.
J Phys Chem B. 2005 Jul 14;109(27):13342-51. doi: 10.1021/jp045021k.
Modes of adsorption of water dimer on H-ZSM-5 and H-Faujasite (H-FAU) zeolites have been investigated by a quantum embedded cluster approach, using the hybrid B3LYP density functional theory. The results indicate that there are two possible adsorption pathways, namely the stepwise process where only one water binds strongly to the (-O)3-Al-O(H) tetrahedral unit while the other weakly binds to the zeolite framework and the concerted process where both water molecules form a large ring of hydrogen-bonding network with the Brønsted proton and an oxygen framework. With inclusion of the effects of the Madelung potential from the extended zeolite framework, for adsorption on H-ZSM-5 zeolite, both the neutral and ion-pair complexes exist with adsorption energies of -15.13 and -14.73 kcal/mol, respectively. For adsorption on the H-FAU, only the ion-pair complex exists with the adsorption energy of -14.63 kcal/mol. Our results indicate that adsorption properties depend not only on the acidity of the Brønsted acidic site but also on the topology of the zeolite framework, such as on the spatial confinement effects which lead to very different adsorption structures for the ion-pair complexes in H-ZSM-5 and H-FAU, even though their adsorption energies are quite similar. Our calculated vibrational spectra for these ion-pair complexes support previous experimental IR interpretations.
采用量子嵌入簇方法，运用杂化B3LYP密度泛函理论，研究了水二聚体在H-ZSM-5和八面沸石（H-FAU）分子筛上的吸附模式。结果表明，存在两种可能的吸附途径，即分步过程，其中只有一个水分子与（-O）3-Al-O（H）四面体单元强烈结合，而另一个水分子与沸石骨架弱结合；以及协同过程，其中两个水分子与布朗斯特质子和氧骨架形成一个大的氢键网络环。考虑到来自扩展沸石骨架的马德隆势的影响，对于在H-ZSM-5分子筛上的吸附，中性和离子对络合物均存在，吸附能分别为-15.13和-14.73 kcal/mol。对于在H-FAU上的吸附，仅存在离子对络合物，吸附能为-14.63 kcal/mol。我们的结果表明，吸附性质不仅取决于布朗斯特酸性位点的酸度，还取决于沸石骨架的拓扑结构，例如空间限制效应，这导致H-ZSM-5和H-FAU中离子对络合物的吸附结构非常不同，尽管它们的吸附能相当相似。我们计算得到的这些离子对络合物的振动光谱支持了先前的实验红外解释。
