(621bu) A Modeling Approach for MOF-Encapsulated Metal Catalysts and Application to Butane Oxidation

Designing catalysts with high selectivity is one of the grand challenges in catalysis, and it is becoming increasingly important to the chemical industry.  Highly selective processes, where the majority of the reactants are converted into the desired products, reduce waste and minimize the need for energetically intensive separation processes. They are, therefore, tremendously important in the development of “green” and economic processes for the production of commodity chemicals.  However, tailoring the structure of catalysts so the reaction pathway of choice is favored can be a difficult task, even more so when the undesired products are more stable.

Nature, in the form of enzymes, presents a working principle for highly selective catalysts, in which the orientation of reactants is controlled as they reach the catalytic sites so that only specific reactant regions (e.g. the terminal end of an n-butane molecule) can undergo catalysis. However, effectively translating these principles to the design of heterogeneous catalysts remains a challenge.  In recent promising work, Lu et al. [1] developed a strategy for enshrouding catalytic nanoparticles (NP) with porous shells made of metal-organic frameworks (MOFs), which can be envisioned to control the motion of reactants in so-called “NP@MOF” systems.  Indeed, Lu et al.[1] showed that hydrogenation proceeded for n-hexene, but not for the bulkier cyclooctene. Furthermore, Stephenson et al. [2] demonstrated regioselective catalytic activity of these systems by hydrogenating trans-1,3-hexadiene to 3-hexene in a Pt@ZIF-8 system.

These findings suggest that NP@MOF systems may potentially be used to carry out more challenging tasks such as regioselective oxidation reactions.  Given the overwhelmingly large number of MOF structures and NP compositions, as well as combinations thereof, molecular simulation can be a valuable tool to accelerate the identification of the most promising NP@MOF catalyst designs.  A major challenge in modeling reactions at the NP/MOF interface includes the large number of atoms typically needed to construct a chemically meaningful model of the system.  In principle, this problem could be solved using QM/MM methods, but unknowns about the exact structure of the NP/MOF interface structure (which is bound to change from MOF to MOF) of these newly explored NP@MOF systems, among other factors, also hinder the efficient construction of appropriate simulation models.

 To overcome this problem, we propose a conceptually simple approach that can be used to explore how changes in the NP composition affect the energetics of a given reaction under sterically constrained conditions, as would occur at a NP/MOF interface.  In this approach, two size- and shape-tunable rings composed of helium atoms placed on top of the catalyst surface mimic the steric constraints that would arise from a given MOF pore. This model can be implemented with standard DFT programs.  Here we demonstrate the suitability of the model to sterically constraint the interaction of reactants, intermediates, and products with catalytic palladium surfaces. We apply this model to investigate the energetics of reactions relevant to the oxidation of n-butane to n-butanol (and 2-butanol) on clean and oxygen-covered palladium (111) and (100) surfaces under sterically constrained (and unconstrained) conditions.  We find that the relative favorability (as given by reaction energies) of the investigated reactions can be affected by the presence of steric constraints, oxygen coverage, and the crystallography of the surface.  For instance, we find that the oxygen coverage on Pd(111) alters the favorability of the desired C₄H₉* + O* à C₄H₉O* + * reaction over the undesired reaction C₄H₉* + O* à C₄H₈* + O*.  Based on our findings we propose design strategies for NP@MOF catalysts.

[1] Lu, G.; Li, S.; Guo, Z.; Farha, O. K.; Hauser, B. G.; Qi, X.; Wang, Y.; Wang, X.; Han, S.; Liu, X.; DuChene, J. S.; Zhang, H.; Zhang, Q.; Chen, X.; Ma, J.; Loo, S. C. J.; Wei, W. D.; Yang, Y.; Hupp, J. T.; Huo, F. Nat. Chem., 4, 310 (2012).

[2] Stephenson, C. J.; Hupp, J. T.; Farha, O. K. Inorg. Chem. Front., 2, 448, (2015)