Propylene oxide (PO) is a versatile chemical, mainly used in the synthesis of polyurethane plastics. Propylene epoxidation using molecular oxygen could replace the tedious current synthesis protocols, which use expensive H 2 O 2 or organic peroxides as oxidants. This review focuses on the propylene epoxidation reaction using molecular oxygen in the gas phase over copper-and silver-based catalysts. Silver is a proven and industrially used ethylene epoxidation catalyst. However, it initiates allylic hydrogen stripping (AHS) in propylene epoxidation, shifting the selectivity toward unwanted acrolein and total oxidation. Nevertheless, silver has been extensively studied to determine if AHS could be mitigated by targeted active site design and various doping strategies. Copper-based catalysts have been less extensively studied but have been experimentally proved as well as theoretically confirmed that their PO selectivity is on par with that of silver. In this review, different catalyst modification strategies have been analyzed and the achieved improvements discussed. Theoretical approaches aimed at understanding the mechanism and predicting catalytic performance on the basis of electronic states (density functional theory calculations) are also reviewed. We conclude with a future outlook on how the current state of the art knowledge of active site modification and reaction engineering approaches could leverage the PO selectivity toward industrial requirements, thus enabling a breakthrough in gas-phase propylene epoxidation using O 2 .
The
methane activation and methane dry reforming reactions were
studied and compared over 4 wt % Ni/CeO2 and 4 wt % Ni/CeZrO2 (containing 20 wt % Zr) catalysts. Upon the incorporation
of Zr into the ceria support, the catalyst exhibited a significantly
improved activity and H2 selectivity. To understand the
effects of the Zr dopant on Ni and CeO2 during the dry
reforming of methane (DRM) reaction and to probe the structure–reactivity
relationship underlying the enhanced catalytic performance of the
mixed-oxide system, in situ time-resolved X-ray diffraction (TR-XRD),
X-ray absorption fine structure (XAFS), and ambient-pressure X-ray
photoelectron spectroscopy (AP-XPS) were employed to characterize
the catalysts under reaction conditions. TR-XRD and AP-XPS indicate
that ceria–zirconia supported Ni (Ni/CeZrO2) is
of higher reducibility than the pure ceria supported Ni (Ni/CeO2) upon the reaction with pure CH4 or for the methane
dry reforming reaction. The active state of Ni/CeZrO2 under
optimum DRM conditions (700 °C) was identified as Ni0, Ce3+/Ce4+, and Zr4+. The particle
size of both nickel and the ceria support under reaction conditions
was analyzed by Rietveld refinement and extended XAFS fitting. Zr
in the ceria support prevents particle sintering and maintains small
particle sizes for both metallic nickel and the partially reduced
ceria support under reaction conditions through a stronger metal–support
interaction. Additionally, Zr prevents Ni migration from the surface
into ceria forming a Ce1–x
Ni
x
O2–y
solid
solution, which is seen in Ni/CeO2, thus helping to preserve
the active Ni0 on the Ni/CeZrO2 surface.
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