(703h) Inorganometallic Catalyst Design: Alkane Metathesis Catalysis in Nu-1000 MOFs Functionalized with Transition Metals

Mid-weight (C₃-C₈) n-alkanes are petrochemical raw materials that are available in large quantities from oil and gas reserves. They are also produced in Fischer-Tropsch catalysis, which is the major route for the conversion of natural gas and coal to liquid fuels. It is desirable to convert mid-weight n-alkanes into higher-value molecules. Alkane metathesis, in which alkane molecules are transformed into higher and lower homologs, is a potential method for achieving such conversion.

The majority of single-site alkane metathesis catalysts are metal hydrides on amorphous SiO₂ and Al₂O₃. In each case, the metal plays dual roles: first, dehydrogenation of alkanes to alkenes, and second, perform alkene metathesis to form C–C bonds. The present work considers using a metal–organic framework (MOF), in particular NU-1000 (molecular formula [Zr₆(μ₃–O)₄(μ₃–OH)₄(OH)₄(OH₂)₄](TBAPy)₂, H₄TBAPy = 1,3,6,8-tetrakis(p-benzoic-acid)pyrene), as a support for new catalyst development. Because of their tunable surface structures, MOFs are ideal for rational heterogeneous catalyst design based on structure–reactivity relationships.

We used Kohn–Sham density functional calculations with the M06-L exchange-correlation functional for computational screening of NU-1000 functionalized with various transition metals, in particular screening for propane metathesis. We considered eight transition metals (V, Nb, Ta, Cr, Mo, W, Mn, Re) along with three variations of the NU-1000 framework: fully hydrated NU-1000, partially dehydrated NU-1000, and NU-1000 decorated with yttrium oxide. The activation energies for C–H bond activation and [2+2] cycloaddition were identified as descriptors of catalytic activity of different catalysts. Using the Sabatier principle, we constructed volcano plots to evaluate the expected efficacy of the trial catalysts. This identified vanadium-functionalized, yttrium oxide-decorated NU-1000 (VYO_x@NU-1000) as the most suitable catalyst among the trial catalysts for the propane metathesis. The complete reaction cycle for VYO_x@NU-1000 catalyst was computed and the free energy of activation is predicted to be 37.8 kcal/mol. This relatively high activation energy obtained by using a dual-role single-site catalyst indicates that the overall performance of the catalyst is compromised in order to achieve a balance between different reactions. We conclude that future developments in alkane metathesis catalysis should focus on dual-site catalysts and/or tandem systems.