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 natural gas to liquids converison 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 -&gt; C2H6)as test reaction.Experimental work from our collaborators ranks the metals Ni2+ &gt; Cu2+ &gt; Mn2+ &gt; Zn2+ in terms of rates for ethene hydrogenation and suggests the rates have only slight dependence on temperature over the range 50oC-200oC.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,that agree well with experimental observation,suggest that the reaction mechanism follows multiple branches in reaction network, with preference for different branches being a function of temperature as an example 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.

Elucidation of the mechanism for ethene hydrogenation over single metal cation catalysts: Combined modeling and experimental study

2019 Fall National Meeting

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