Metal Catalysts and Reaction Mechanisms in Propylene Epoxidation in Gas-Phase by Molecular Oxygen

Propylene oxide (PO) is an important intermediate chemical and is widely used in chemical, pharmaceutical, food, light industry and other industries. In industry, PO is produced via multiple reaction steps in the liquid phase, using hazardous chlorine or costly organic hydroperoxides as oxidants. Considering the inherent defects of the traditional processes-the chlorohydrin and hydroperoxide process, a new production technology that propylene oxide production via hydrogen peroxide (HPPO) has been developed, but many problems such as the catalyst mass transfer limitation, the catalyst deactivation and regeneration, and the need of a dedicated hydrogen peroxide production plant are encountered. So, a more attractive method for PO production with molecular oxygen by gas-phase is discussed in this paper. The most important new developments for the production of propylene oxide discussed in this paper are concentrated on the supported gold, silver, copper, vanadium, molten salt catalysts and other catalysts that used molecular oxygen as the oxygen source. At the same time, the mechanism involved in the propylene epoxidation such as the Ti active sites mechanism in Au/Ti catalysts, the OMMP (OMMP refers to a ring made up of one oxygen atom, two metal atoms and a propylene unit) intermediate mechanism in silver and copper catalysts and the radical mechanism in silver and other metal oxide catalysts are also mentioned in this paper.