(32c) Combined Quantum Mechanical and Molecular Mechanical Method for Catalyst Design on the Nu-1000 Metal-Organic Framework

Metal-organic frameworks (MOFs) have high internal surface area and tailorable nodes and organic ligands, and for these reasons they are very promising for heterogeneous catalysis, hydrogen storage, and gas separation. Fully quantum chemical investigations of these systems are usually quite demanding in terms of time and computational resources; therefore we have developed a combined quantum mechanical and molecular mechanical (QM/MM) method to investigate these systems.

Our method is illustrated by calculations on MOF NU-1000 [Zr₆(μ₃-O)₈(O)₈(H)₁₆(TBAPy)₂], which is a superb candidate for use as a catalytic support for various reactions (e.g., alkane metathesis and ethylene dimerization). We study the relative energetics for various proton topologies of the NU-1000 node. Because of the high partial charges on the atoms of the highly polar bonds in this structure, this provides a challenging test of the adequacy of the QM/MM boundary treatment. In our QM/MM calculations, we use a cluster model of the node [Zr₆(μ₃-O)₈(O)₈(H)₁₆(PhCOO)₈] with eight phenyl rings (88 atoms) treated classically while the other atoms (62 atoms) are treated quantum mechanically. Two kinds of link atoms (in particular H atoms and tuned F atoms) are used to cap the dangling bonds in the QM subsystem, and six kinds of charge modification schemes are considered for the treatment of boundary charge. By comparing with full QM calculations, we found that the QM/MM method can give remarkably accurate results with a mean unsigned error of only 2 kJ mol^-1 in the relative energies of seven proton topologies.

To facilitate the design of MOF-based catalysts, it can be desirable to further reduce the number of atoms in the QM subsystem. To do this for NU-1000 we have to cut through two types of polar bonds (Zr-O and O-Zr) in the same calculation. We are developing a new method based on a tuned F link atom to treat this situation.