Reduction of Cofed Carbon Dioxide Modifies the Local Coordination Environment of Zeolite-Supported, Atomically Dispersed Chromium to Promote Ethane Dehydrogenation

The reduction of CO2 is known to promote increased
alkene
yields from alkane dehydrogenations when the reactions are cocatalyzed.
The mechanism of this promotion is not understood in the context of
catalyst active-site environments because CO2 is amphoteric,
and even general aspects of the chemistry, including the significance
of competing side reactions, differ significantly across catalysts.
Atomically dispersed chromium cations stabilized in highly siliceous
MFI zeolite are shown here to enable the study of the role of parallel
CO2 reduction during ethylene-selective ethane dehydrogenation.
Based on infrared spectroscopy and X-ray absorption spectroscopy data
interpreted through calculations using density functional theory (DFT),
the synthesized catalyst contains atomically dispersed Cr cations
stabilized by silanol nests in micropores. Reactor studies show that
cofeeding CO2 increases stable ethylene-selective ethane
dehydrogenation rates over a wide range of partial pressures. Operando X-ray absorption near-edge structure (XANES) and
extended X-ray absorption fine-structure (EXAFS) spectra indicate
that during reaction at 650 °C the Cr cations maintain a nominal
2+ charge and a total Cr–O coordination number of approximately
2. However, CO2 reduction induces a change, correlated
with the CO2 partial pressure, in the population of two distinct Cr–O scattering paths. This indicates that
the promotional effect of parallel CO2 reduction can be
attributed to a subtle change in Cr–O bond lengths in the local
coordination environment of the active site. These insights are made
possible by simultaneously fitting multiple EXAFS spectra recorded
in different reaction conditions; this novel procedure is expected
to be generally applicable for interpreting operando catalysis EXAFS data.