(621as) Computational Design of Highly Selective Transition Metal Catalysts Encapsulated By Metal-Organic Frameworks for Butane Oxidation to 1-Butanol
Catalysts are one of the most important technologies in society today, with catalytic processes accounting for nearly 20% of the US GDP. A key focus of catalysis research is designing catalysts that convert feedstocks into higher value products. One of the greatest challenges is selectivity, i.e., production of a desired product over an undesired one, and this is particularly challenging when the desired product is less thermodynamically stable. For example, the oxidation of butane to 1-butanol is an important reaction in the pharmaceuticals and specialty chemicals industries, and selectivity is challenging for two reasons: 1) CO2 and H2O are significantly more stable than butanol, and 2) even 2-butanol is more stable than 1-butanol, since the secondary carbon is more reactive than the primary. Thus designing a catalyst for production of 1-butanol requires designing precisely tuned sites that 1) activate the C-H bonds of an alkane without over-dehydrogenating, 2) favor oxidation without overoxidizing, and 3) exclusively target the primary carbon of butane. The aim of this project is to design a metal nanoparticle catalyst encapsulated within a metal-organic framework (MOF) for this purpose. MOFs are porous crystalline solids comprised of metal-based nodes connected by organic “linker” molecules. The appropriate MOF for this system has small pores that force the surface/molecule interaction to occur at the molecule’s terminus. The appropriate metal nanoparticle optimally balances dehydrogenation, hydrogenation, and oxidation processes. In this work, we use molecular simulations to map out several possible pathways for 2 C4H10 + O2 = 2 C4H9OH on oxygen-covered transition metal catalyst surfaces, using “featureless” rings comprised of He to simulate the steric restrictions imposed by the MOF pores. Our results suggest that the reaction proceeds via an “oxygen assisted” mechanism, that oxygen activation is rate limiting, and that, even in MOFs where the pores have diameters that are similar to the kinetic diameter of butane, formation of 2-butanol is possible. Hence, even in this encapsulated catalyst system, selectivity is a significant challenge. We conclude by providing predictions about which materials will ultimately promote butane selective oxidation to 1-butanol.