(291d) Elucidation of the Mechanism for Ethene Hydrogenation over Single Metal Cation Catalysts: A Combined Modeling and Experimental Study

The increased shale gas
production has potential to decrease cost of energy and secure domestic energy
supply; however, challenges in converting primarily light alkane feedstocks
into higher value energy carriers deters its utilization. This motivates the
development of catalysts for conversion of natural gas to liquids by
dehydrogenation (C-H bond) of alkanes and their subsequent oligomerization (C-C
bond) to higher hydrocarbons. Single metal cations have been shown to
effectively catalyze such reactions. In this work, we examine single metal cation
catalysts supported within metal-organic frameworks (MOFs). MOFs are porous
crystals that can support catalytically active metal cation sites and prevent
their agglomeration through spatial and electronic isolation. In this work, we
examine Mn²⁺, Ni²⁺, Cu²⁺ and Zn²⁺
catalysts supported within on the zirconium oxide/hydroxide nodes of the MOF
NU-1000. We specifically study ethene hydrogenation (H₂ + C₂H₄
à C₂H₆)
as a test reaction. Experimental work from our collaborators ranks the metals Ni²⁺
> Cu²⁺ > Mn²⁺ > Zn²⁺ in terms of
their rates for ethane hydrogenation and suggests the rates have only slight
dependence on temperature over the range 50^oC-200^oC. To
explain the experimental observations, we simulated a branched, dual-site reaction
network. We calculated thermodynamic and kinetic quantities for steps within
the reaction network using density functional theory (DFT) and incorporated those
quantities into a microkinetic model. The two types of sites that we included
were the metal cations themselves, as well as the NU-1000 zirconium oxide/hydroxide
nodes as shown in figure, which we find can act as temporary “storage” sites
for protons. Results from microkinetic modeling, which agree well with
experimental observation, suggest that the reaction mechanism follows multiple branches
in the reaction network, with the preference for the different branches being a
function of temperature as set out for Cu in figure. Kinetic analysis identifies
H-H scission and C-H formation steps to be rate determining. The results
provide an example of the importance of support effects in catalysis and
facilitate the understanding and design of catalysts for natural gas upgrading.