Solans-Monfort Xavier, Sodupe Mariona, Branchadell Vicenç, Sauer Joachim, Orlando Roberto, Ugliengo Piero
Departament de Quimica, Universitat Autonoma de Barcelona, Bellaterra 08193, Spain.
J Phys Chem B. 2005 Mar 3;109(8):3539-45. doi: 10.1021/jp045531e.
The adsorption of NH(3) and H(2)O in acidic chabazite has been studied with the B3LYP method within the cluster approach (5T, 48T clusters) and the periodic approach adopting a Si/Al = 11/1 chabazite and a basis set of polarized double-zeta quality. The 5T cluster has been treated fully ab initio at the B3LYP level whereas the 48T cluster has been treated with the ONIOM2 scheme using B3LYP as the high level of theory and the MNDO, AM1, and HF/3-21G methods as low levels of theory. Periodic calculations show that the adsorption of NH(3) in acidic chabazite takes place through an ion pair (NH(4)(+)-CHA(-)) structure, the computed adsorption energy being -32 kcal/mol. The adsorption of H(2)O leads to a hydrogen bonded (H(2)O-HCHA) complex with the computed adsorption energy of -20 kcal/mol. All ONIOM combinations provide similar structures to those obtained with periodic calculations. Adsorption energies, however, are sensitive to the low level used, especially for NH(3). The ONIOM B3LYP:HF/3-21G method is the one that provides more satisfactory results. Present results show that, for larger zeolites, the ONIOM scheme can be successfully applied while drastically reducing the cost of a fully ab initio treatment.
采用B3LYP方法，在簇模型（5T、48T簇）和周期性模型下，使用硅铝比为11/1的菱沸石以及极化双ζ基组，研究了酸性菱沸石中NH₃和H₂O的吸附情况。5T簇在B3LYP水平下进行了全从头算处理，而48T簇则采用ONIOM2方法，以B3LYP作为高水平理论，MNDO、AM1和HF/3 - 21G方法作为低水平理论进行处理。周期性计算表明，酸性菱沸石中NH₃的吸附通过离子对（NH₄⁺ - CHA⁻）结构发生，计算得到的吸附能为 - 32 kcal/mol。H₂O的吸附形成氢键（H₂O - HCHA）复合物，计算得到的吸附能为 - 20 kcal/mol。所有ONIOM组合得到的结构与周期性计算得到的结构相似。然而，吸附能对所使用的低水平理论敏感，尤其是对于NH₃。ONIOM B3LYP:HF/3 - 21G方法给出的结果更令人满意。目前的结果表明，对于更大的沸石，ONIOM方法可以成功应用，同时大幅降低全从头算处理的成本。
