Tools for Prescreening the Most Active Sites on Ir and Rh Clusters toward C-H Bond Cleavage of Ethane: NBO Charges and Wiberg Bond Indexes.

Yingbin Ge,Ashley Noble,Morgan Schimelfenig,Anna Le,Phuc Q Nguyen,Kollin Trujillo,Gregory J Marquino

doi:10.1021/acsomega.9b02813

Abstract

B3LYP calculations were carried out to study the insertion of iridium (Ir) and rhodium (Rh) clusters into a C–H bond of ethane, which is often the rate-limiting step of the catalytic cycle of oxidative dehydrogenation of ethane. Our previous research on Ir catalysis correlates the diffusivity of the lowest unoccupied molecular orbital of the Ir clusters and the relative activities of the various catalytic sites. The drawback of this research is that the molecular orbital visualization is qualitative rather than quantitative. Therefore, in this study on C–H bond activation by the Ir and Rh clusters, we conducted analyses of natural bond orbital (NBO) charges and Wiberg bond indexes (WBIs), both of which are not only quantitative but also independent of the basis sets. We found strong correlation between the NBO charges, the WBIs, and the relative activities of the various catalytic sites on the Ir and Rh clusters. Analyses of the NBO charges and the WBIs provide a fast and reliable means of prescreening the most active sites on the Ir and Rh clusters and potentially on other similar transition-metal clusters that activate the C–H bonds of ethane and other light alkanes.

Highlights

Catalyzed dehydrogenation of alkanes is an important petroleum industrial process
Transition-metal (TM) catalysts used for the dehydrogenation of ethane and other light alkanes have been extensively studied both experimentally and theoretically.[3−29] The energy consumption for converting alkanes to alkenes is tremendous because it takes more than 400 kJ/mol of energy to break a C−H bond
In the Results and Discussion section, we present the strong correlations between the natural bond orbital (NBO) charges, the Wiberg bond indexes (WBIs), and the catalytic ability of the various sites on the Ir and Rh clusters

Summary

Introduction

Catalyzed dehydrogenation of alkanes is an important petroleum industrial process. Approximately 134 million tons of ethene (ethylene) and 94 million tons of propene (propylene) were produced in 2014.1,2 These numbers are expected to increase continuously as the consumption of ethene and propene is expected to increase continuously. Platinum (Pt) atomic clusters were found to have an extraordinary ability to break C−H bonds at lower temperatures by reducing the energy barrier to ∼20 kJ/mol, both in the gas phase and on a metal oxide surface.[6,7] This is a breakthrough toward the catalysis of the ODH of alkanes. Iridium (Ir), rhodium (Rh), and palladium (Pd) atomic clusters have shown tremendous potential toward activating the C−H bonds of ethane and propane, both in the gas phase and on metal oxide surfaces.[12−19] studies show that small TM clusters such as Ir3 migrate on metal oxide surfaces.[32] Apart from enhancing the catalytic ability of these TM clusters, preventing their migration, coalescence, and deactivation are important. One strategy we proposed in a previous study was to find catalytically active Ir clusters with large enough surfaces that can bond strongly to their support to hinder migration.[16]

Methods

Results

Conclusion