Meeting Program — October 2017
Anisha Chakrabarti — Student Speaker
Operando Molecular Spectroscopy & Catalysis Laboratory
Department of Chemical and Biomolecular Engineering
Lehigh University, Bethlehem, PA 18015 USA
Abstract — The olefin metathesis reaction was commercialized in the late 1960s to produce ethylene and 2-butene from propylene in the Phillips Triolefin Process. The reverse reaction, however, is currently desired due to a global propylene shortage caused by the shift to lighter feedstocks derived from shale gas fracking. Heterogeneous supported MoOx/Al2O3 catalysts are employed for olefin metathesis in the Shell Higher Olefin Process (SHOP) that operates between room temperature and ~200°C.
To probe the molecular details of the supported MoOx/Al2O3 catalysts, a modern in situ spectroscopy approach was undertaken. In situ UV-vis measurements (Eg values) confirmed the presence of isolated and oligomeric MoOx surface sites, with the latter increasing with molybdena loading. In situ Raman spectroscopy revealed that at low loadings of molybdena (1 Mo atoms/nm2), only isolated dioxo (O=)2MoO2 surface sites are present. As the molybdena loading is increased (1–4.6 Mo atoms/nm2), oligomeric mono-oxo O=MoO4 surface sites co-exist with the isolated dioxo (O=)2MoO2 surface sites. Above monolayer loadings (>4.6 Mo atoms/nm2), crystalline MoO3 nanoparticles are also present. In situ IR indicates that the isolated dioxo MoO4 sites are anchored at more basic HO-μ1-AlIV surface hydroxyls, while the surface oligomeric mono-oxo sites are anchored to more acidic HO-μ1/3-AlV/VI surface hydroxyls. Propylene metathesis at reaction conditions suggest that the isolated dioxo (O=)2MoO2 surface site may still be present after activation of the mono-oxo surface sites with propylene. In situ UV-vis during propylene metathesis indicates that Mo+6 sites are dominant during propylene metathesis due to the presence of unreduced surface dioxo Mo+6O4 sites and re-oxidation of reduced Mo+4 sites by propylene back to Mo+6=CH2 and Mo+6=CHCH3 reaction intermediates. The surface chemistry was chemically probed by C3H6-TPSR that initially formed oxygenated products (CH3CHO, H2CO, CH3COCH3, H2O and CO/CO2) during catalyst activation. The reactivity of the activated catalysts to butene progressively increased with molybdena loading, indicating that the oligomeric mono-oxo MoOx sites are much more active than isolated dioxo MoO4 sites for olefin metathesis. The crystalline MoO3 nanoparticles, however, were found to be inactive for metathesis. This presentation will address the fundamental molecular and structural details of the supported MoOx/Al2O3 catalysts during propylene metathesis and establish their structure-activity relationships.