Technology Type

Introduction

Propylene oxide (PO) is produced commercially by several fundamentally different routes that have evolved over the past 70 years.

• The historical chlorohydrin process, introduced in the 1950s, was the first industrial PO-only route and is still operated at some integrated chlorine sites, despite environmental drawbacks such as large salt effluents and significant chlorine usage.
• To address these issues, indirect oxidation methods using organic hydroperoxides were developed and can be divided into two main categories:
  • Co-Product Processes (PO-SM/SMPO and TBA Methods): The original methods, such as the PO-SM/SMPO (using ethylbenzene hydroperoxide) and TBA (using tert-butyl hydroperoxide) routes, produce PO alongside valuable co-products like styrene monomer or tert-butanol. These processes captured a significant share of global capacity by the early 2000s.
  • Cumene Method (POC): Introduced in the mid 2000s, the cumene-based process uses cumene hydroperoxide to epoxidize propylene, with cumene recycled in a closed loop. This method produces PO as the sole product, eliminating reliance on co-product markets and further reducing environmental impact.
• In parallel with the development of the cumene method, direct oxidation approaches—most notably the hydrogen peroxide to propylene oxide (HPPO) process—were also introduced. These routes aim to produce PO only, minimizing or eliminating chlorinated and organic-peroxide by-products entirely, and represent the latest advances in sustainable PO manufacturing, which are reviewed here below.
  
  

Key Direct Oxidation Routes and Chemistry

Three principal direct oxidation processes produce propylene oxide (PO) by reacting propylene with oxygen and/or hydrogen, without preformed organic hydroperoxides or chlorohydrin intermediates. A complementary route uses hydrogen peroxide generated via the anthraquinone process as the oxidant.

1. Hydrogen Peroxide (by Anthraquinone) Route

Process Description

This route employs hydrogen peroxide produced via the anthraquinone process as the oxidizing agent for propylene epoxidation. The hydrogen peroxide is generated separately through the well-established anthraquinone oxidation (AO) process, also known as Riedel-Pfleiderer process.

Chemistry and Mechanism

• Main Reaction: C₃H₆ + H₂O₂ → PO + H₂O
• Catalyst: TS-1 (Titanium Silicalite-1) zeolite in fixed-bed configuration
• Solvent: Methanol is used as the reaction medium
  
  

Anthraquinone Process for H₂O₂ Production

The hydrogen peroxide is produced through a cyclic process involving:

1. Hydrogenation: 2-alkylanthraquinone is hydrogenated using Pd catalyst at 40-50°C under 2-4 bar H2 pressure to form anthrahydroquinone
2. Oxidation: Anthrahydroquinone is oxidized with air at 30-60°C to regenerate anthraquinone and produce H2O2
3. Extraction: H₂O₂ is extracted with water to produce 30-50% aqueous solution
   
   

Process Conditions

• Temperature: < 90°C
• Pressure: Up to 30 bar
• Selectivity: 94-99% toward PO
• By-product: Only water

Technology Developers

• Dow (Eni)/BASF/Solvay/Degussa
• Evonik/thyssenkrupp Uhde (joint development)
• Sinopec (100,000 t/y unit commissioned 2014)

Commercial Implementation

The HPPO (Hydrogen Peroxide to Propylene Oxide) process has been successfully commercialized with multiple world-scale plants:

• SK plant in South Korea (2008, expanded to 130,000 t/y)
• BASF Verbund site in Belgium (300,000 t/y)
• Sadara Chemical Co. in Saudi Arabia (390,000 t/y)
• Multiple plants in China

2. In-Situ Hydrogen Peroxide Route

Process Description

This route generates hydrogen peroxide in situ from hydrogen and oxygen in the same reactor where propylene epoxidation occurs, eliminating the need for separate H₂O₂ production and handling

Chemistry and Mechanism

• Overall Reaction: C₃H₆ + H₂ + O₂ → PO + H₂O
  
  
   
• Two-step mechanism:
  1. H₂ + ½ O₂ → H₂O₂ (catalyzed by metal nanoparticles)
  2. C₃H₆ + H₂O₂ → PO + H₂O (catalyzed by TS-1 sites)

Catalyst System

• Bifunctional catalyst: Pd/TS-1 (Palladium supported on TS-1 zeolite)
• Metal function: Pd nanoparticles catalyze H₂O₂ synthesis from H₂ and O₂
• Acid function: TS-1 framework Ti sites catalyze propylene epoxidation
• Solvent: Methanol

Process Conditions

• Temperature: 25-80°C
• Pressure: 2.5-8.5 bar
• Reaction medium: Methanol solution/slurry system

Advantages and Limitations

• Advantages: Single-step operation; eliminates H₂O₂ storage and transport risks; only water by-product
• Limitations: Low per-pass productivity; catalyst deactivation by water and peroxide; complex slurry handling; requires precise control of H₂/O₂ ratios

Technology Developers

• LyondellBasell, BASF
• Various research groups developing non-noble metal alternatives (Ni/TS-1)

3. Vapor-Phase Direct Oxidation Route

Process Description

This represents the most direct approach, involving gas-phase reaction of propylene with oxygen (with or without hydrogen) over solid catalysts to produce PO directly.

Chemistry and Mechanism

• Primary Reaction: C₃H₆ + ½ O₂ → PO
• Alternative Reaction: C₃H₆ + H₂ + O₂ → PO + H₂O
• Competing Reactions: Deep oxidation to CO₂, H₂; formation of acetaldehyde, acrolein

Catalyst Systems

• Au/TS-1: Gold nanoparticles supported on TS-1 zeolite
• Ag/CaCO3: Silver supported on calcium carbonate with various promoters
• Other systems: Bimetallic formulations, Cu-based catalysts

Process Conditions

• Temperature: 180-350°C (varies by catalyst)
• Pressure: Atmospheric to moderate pressures
• Feed composition: Precise control of propylene/oxygen ratios required
• Conversion: Typically < 10% per pass to minimize deep oxidation

Performance Characteristics

• Selectivity: Generally lower than H₂O₂-based routes (< 50% typical)
• Conversion: Limited to low per-pass values to avoid combustion
• Catalyst stability: Short lifetimes due to sintering and coke formation
• By-products: Water, CO₂, aldehydes

Technology Developers

• Dow, BASF, LyondellBasell, Nippon Shokubai
• Academic research groups working on improved catalyst formulations

Technical Challenges

• Catalyst deactivation: Rapid sintering of active metal sites
• Selectivity limitations: Competing deep oxidation reactions
• Safety concerns: Explosive H₂/O₂ mixtures in some variants
• Process intensification: Requires high recycle rates due to low per-pass conversion

Comparative Summary

Current Status and Outlook

• The hydrogen peroxide route has achieved commercial success and represents the most advanced direct oxidation technology, with multiple world-scale plants operational.
• The in-situ hydrogen peroxide route remains under development, with challenges related to catalyst stability and process intensification.
• The vapor phase direct oxidation route continues to be primarily a research focus, with ongoing efforts to improve selectivity and catalyst lifetime through advanced catalyst design and process optimization

References

1. Junpei T SUJI et al., Development of New Propylene Oxide Process, R&D Report SUMITOMO KAGAKU, vol. 2006-I
2. HPPO Technology - Evonik
3. Propylene oxide - The clean Evonik-Uhde HPPO technology - thyssenkrupp uhde
4. White Paper -Utilizing online chemical analysis to optimize propylene oxide production - Metrohm
5. The new HPPO Process for Propylene Oxide: From Joint Development to Worldscale Production
6. Peter Bassler et al., Anthraquinone Process for Production of Hydrogen Peroxide, Chem. Eng. Trans., Vol. 21, 2010, DOI: 10.3303/CET1021096
7. Emilia A. Carbonio et al., Adjusting the Chemical Reactivity of Oxygen for Propylene Epoxidation on Silver by Rational Design: The Use of an Oxyanion and Cl, ACS Catal. 2023, 13, 9, 5906–5913, DOI: 10.1021/acscatal.3c00297
8. Jun 21, 2023 - Introduction to Sinopec HPPO process & Vinyl Acetate process to maximize carbon efficiency [Conference presentation slides] - Sinopec Shanghai Engineering Co., Ltd.
9. Jimey Yang et al., Review and perspectives on TS-1 catalyzed propylene epoxidation, iScience 2024 Feb 1;27(3):109064. DOI: 10.1016/j.isci.2024.109064
10. Matias Alvear et al., Study of the Product Distribution in the Epoxidation of Propylene over TS-1 Catalyst in a Trickle-Bed Reactor, Ind. Eng. Chem. Res. 2021, 60, 6, 2430–2438, DOI: 10.1021/acs.iecr.0c06150.
11. Jeremy Arvay, thesis posted on 26^th Apr 2022, Reaction kinetics of direct gas-phase propylene epoxidation on Au/TS-1 catalysts, Purdue University.