Technology Type

Introduction

Direct gas-phase oxidation of propylene with molecular oxygen to produce propylene oxide (PO) represents the conceptually simplest and most atom-efficient PO route — requiring only propylene and oxygen as feedstocks, with water as the sole by-product and no co-feed hydrocarbon, no separately manufactured oxidant, and no liquid solvent. It is therefore the ultimate target of PO process development from both an economic and a sustainability perspective.

Despite decades of research by major industrial and academic groups, no gas-phase direct oxidation PO process has yet been commercialized. The fundamental barrier is selectivity: the conditions that activate molecular oxygen for epoxidation also favor complete combustion of propylene to CO2 and H2O, as well as partial oxidation to acrolein and acetaldehyde. This contrasts sharply with the analogous ethylene oxide process, where silver catalysts achieve 80–90% selectivity at world scale — a performance level that has never been approached with propylene due to the greater reactivity of its allylic C–H bonds relative to the vinyl C=C bond.

Principle

The two principal reaction pathways are:

Oxygen-only route:

C₃H₆ + 12 O₂ → C₃H₆O (PO)

Hydrogen-assisted route:

C₃H₆ + H₂ + O₂ → C₃H₆O (PO) + H₂O

In the hydrogen-assisted variant, hydrogen does not reduce propylene but maintains the catalyst surface in an epoxidation-active state by removing strongly adsorbed oxygen species that would otherwise over-oxidize propylene. The net effect is a significant improvement in PO selectivity at the cost of hydrogen consumption.

Competing reactions (all reducing PO yield):

• C3H6 + 9/2 O2 → 3 CO2 + 3 H2O (deep combustion — dominant side reaction)

• C3H6 + O2 → CH3CHO (acetaldehyde) + HCHO (formaldehyde)

• C3H6 + O2 → CH2=CHCHO (acrolein)

Process Conditions

• Temperature: 180–350°C (varies by catalyst system)

• Pressure: atmospheric to moderate

• Reactor type: fixed-bed; high recycle rates required

• Feed: precise control of propylene/oxygen ratios required to stay outside flammable limits; propylene conversion kept <10–12% per pass to limit deep oxidation

Catalyst Systems

Three principal catalyst families have been investigated:

• Au/TS-1 (gold nanoparticles on TS-1 zeolite): the most extensively studied and highest-performing system; gold nanoparticles <5 nm activate H2 (hydrogen-assisted variant) to generate surface peroxo species that epoxidize propylene at adjacent TS-1 Ti sites; best reported performance ~58% PO selectivity at 12% propylene conversion per pass; limited by gold sintering and TS-1 deactivation by water accumulation on Ti sites

• Ag/CaCO3 with promoters (alkali metals, oxyanions, Cl?): silver catalyzes direct O2-based oxidation by analogy with ethylene oxide chemistry; promoters modulate oxygen binding energy and suppress allylic C–H activation; recent work by Carbonio et al. (2023) on oxyanion and Cl promoters has shown renewed selectivity improvements

• Bimetallic and Cu-based systems: Au–Pd/TiO2, Cu/SiO2, and other formulations under academic investigation; generally inferior to Au/TS-1 in current reported performance

Performance

Technical Challenges

• Catalyst deactivation: sintering of gold or silver nanoparticles at reaction temperature; water from combustion side reactions accelerates TS-1 Ti site hydrolysis

• Selectivity ceiling: allylic C–H bonds in propylene are thermodynamically more reactive than the C=C bond under oxidizing conditions, making it inherently difficult to arrest oxidation at the epoxide stage

• Safety: propylene/oxygen mixtures require operation outside the explosive envelope; the hydrogen-assisted variant introduces an additional H2/O2 flammability constraint requiring precise feed ratio control

• Process intensification: low per-pass conversion (<12%) demands high recycle compression costs and large reactor volumes per unit PO capacity

Technology Developers

Dow, BASF, LyondellBasell, Nippon Shokubai; Purdue University, Fritz Haber Institute, University of Utrecht (academic research)

Commercial status: research stage — no commercial deployment to date

References

1. Tsuji J. et al. (2006). Development of New Propylene Oxide Process. Sumitomo Kagaku.
2. Arvay J. (2022). Reaction kinetics of direct gas-phase propylene epoxidation on Au/TS-1 catalysts. Thesis, Purdue University.
3. Carbonio E.A. et al. (2023). Adjusting the Chemical Reactivity of Oxygen for Propylene Epoxidation on Silver by Rational Design: The Use of an Oxyanion and Cl. ACS Catal., 13(9), 5906–5913. DOI: 10.1021/acscatal.3c00297.
4. Yang J. et al. (2024). Review and perspectives on TS-1 catalyzed propylene epoxidation. iScience, 27(3), 109064. DOI: 10.1016/j.isci.2024.109064.
5. Alvear M. et al. (2021). Study of the Product Distribution in the Epoxidation of Propylene over TS-1 Catalyst in a Trickle-Bed Reactor. Ind. Eng. Chem. Res., 60(6), 2430–2438. DOI: 10.1021/acs.iecr.0c06150.
6. Kalyoncu S. (2012). Catalytic Reaction of Propylene to Propylene Oxide on Various Catalysts. Thesis, Middle East Technical University.