Propylene Epoxidation using Molecular Oxygen over Copper- and Silver-Based Catalysts: A Review

Abstract

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 Lord of the Chemical Rings: Catalytic Synthesis of Important Industrial Epoxide Compounds

“…In this context, another chemical that needs great attention is propylene. Propylene is obtained from the cracking of hydrocarbons and it can react with oxygen, in the presence of catalysts, to obtain different compounds [29]. Among these, propylene oxide is one of the most interesting since it is used in several applications and in the production of industrial materials.…”

“…Among these, propylene oxide is one of the most interesting since it is used in several applications and in the production of industrial materials. In particular, it counts, among its uses, the production of polyurethane foams, polyols, propylene glycols and propylene glycol ethers and by 2025, the annual request of propylene oxide will be over 20 Mt [29][30][31].…”

“…In the second aforementioned synthesis route, hydroperoxide is formed and it is used for the formation of propylene oxide using Mo-based homogeneous catalysts or Ti on silica as heterogeneous catalysts [30,32]. Other possible methods can be used for this purpose: the cumene process (CHP) has the advantage that the byproduct can be transformed to cumene through dehydration and hydrogenation [29] while another one consists in the use of hydrogen peroxide to obtain peracids that are the oxidizing agents to give the propylene oxide. However, the negative aspect of this process is the low selectivity [30].…”

Directing transition metal-based oxygen-functionalization catalysis

“…Total and partial oxidation of propene generates propylene oxides with multiple C3 oxygenates, like acrolein, propanal, 1-propanol, isopropanol, and acetone. 107 However, to date, no reported catalyst is being used industrially. Haruta et al were the rst to report the potential of nanometer-sized gold clusters dispersed on TiO 2 as an active and highly selective catalyst for direct epoxidation of propene with O 2 .…”

Direct propylene epoxidation with oxygen enabled by photo-electro-heterogeneous catalytic system

“…Its production exceeds 10 million tons per year and demand for PO is gradually increasing 2,3 . Most industrial PO has been produced using either a chlorohydrin or hydroperoxide process 4 . However, they require additional complex puri cation steps due to the generation of large amounts of coproducts and pose environmental risks due to the use of toxic chemicals and large productions of wastewater.…”