Modeling Catalytic Steps on Extra-Framework Metal Centers in Zeolites. A Case Study on Ethylene Dimerization

In
a case study of organometallic catalytic reactions, this work benchmarks
density functional theory calculations on zeolite-supported transition
metal complexes. Elementary steps of ethylene dimerization and hydrogenation
reactions involving the complex [Rh­(C2H4)2(H2)]+, supported on faujasite, were
examined by comparing explicit QM (quantum mechanics) cluster models
as well as QM/MM (molecular mechanics) embedded models to plane-wave
periodic models as reference. Two QM cluster models, 1T and 5T where
T refers to tetrahedral units of zeolite, as well as four QM/MM cluster
models were explored. For the MM region, the UFF force field was found
preferable to the semiempirical method PM6. The embedded cluster models
reproduce barriers of C–C and C–H bond formation with
deviations from the reference of at most 10 kJ mol–1. With variations of similar size, the effect of embedding on the
energetics of the reactions under study is moderate, likely because
of the small nonpolar reactants. For elucidating such catalytic reactions
at transition metal species in zeolites, cluster models appear equally
well-suited as periodic models but computationally advantageous.

在有机金属催化反应的案例研究中，本工作针对负载于沸石的过渡金属配合物的密度泛函理论（density functional theory）计算开展基准测试。研究以负载于八面沸石（faujasite）的配合物[Rh(C₂H₄)₂(H₂)]⁺所参与的乙烯二聚与氢化反应的基元步骤为对象，通过将显式量子力学（quantum mechanics，QM）团簇模型以及QM/MM（分子力学，molecular mechanics）嵌入模型与作为参照的平面波周期性模型进行对比分析。本次共探索了两种QM团簇模型（1T与5T，其中T指代沸石的四面体结构单元）以及四种QM/MM团簇模型。针对MM区域，研究发现通用力场（universal force field，UFF）的表现优于半经验方法PM6。嵌入型团簇模型可较好重现C-C键与C-H键形成过程的能垒，其与参照值的最大偏差不超过10 kJ·mol⁻¹。在偏差幅度相近的情况下，嵌入处理对本次研究中反应能量学的影响较为温和，这一现象大概率源于反应物为非极性小分子。若要阐明沸石中过渡金属物种上的此类催化反应机理，团簇模型与周期性模型的适用性相当，但在计算成本上更具优势。