Propylene oxide (PO), as the third largest propene derivative ranking behind polypropylene and acrylonitrile, is widely used in light industry, pharmaceutical and textile industries. Compared with traditional chlorohydrin and hydroperoxide processes, direct propene epoxidation with hydrogen and oxygen to synthesize PO has the advantages of being green, simple and profitable. It is therefore a current worldwide research hotspot. Herein, recent progress is reviewed with the purpose of solving remaining problems such as poor stability and low activity. Moreover, the effect of surface and structural properties of Ti-containing supports together with the morphologic and electronic effect of gold nanoparticles on catalytic activity and stability are introduced. The reaction mechanism and deactivation mechanism are further discussed. The influences of additives such as Cs, Ag, Pd, Pt, Ge, surface alkylation, ionic liquid and nitrogen doping on catalytic performance are summarized. The existing problems in direct propene epoxidation with H₂ and O₂ are also analyzed. In the end, possible solutions and directions from the aspects of selecting supports and improving catalytic performance are remarked.

Contents
1 Introduction
2 Effect of support properties
2.1 Different Ti-containing supports
2.2 Hydrophobicity of supports
2.3 Si/Ti molar ratio
3 Effect of properties of gold nanoparticles
3.1 Gold nanoparticle size
3.2 Gold active sites
3.3 Gold deposition location
4 Effect of promoters
5 Conclusion