March 2023

Special Focus: Petrochemicals Technology

Efficient and robust hydrogen peroxide propylene oxide (HPPO) process

Propylene oxide (PO) is an important organic building block in industry and is used to manufacture polyether polyols, unsaturated resins, nonionic surfactants and carbonates.1,2,3

Xia, C., Lin, M., Zhu, B., Peng, X, Luo, Y., Shu, W., Research Institute of Petroleum Processing, SINOPEC; Li, H., Wang, W., Changling Petrochemical Co. Ltd., SINOPEC

Propylene oxide (PO) is an important organic building block in industry and is used to manufacture polyether polyols, unsaturated resins, nonionic surfactants and carbonates.1,2,3 In 2020, the global PO market was more than 10 MMt—China's consumption was estimated at 3.5 MMt. However, more than 50% of PO in China is still prepared by the classic chlorohydrin method, which produces serious environmental pollution (with wastewater and waste residues of 40 t/t–80 t/t PO and 2 t /t PO, respectively) and equipment corrosion problems due to the use of hazard chlorine (Cl2) and solid base as raw materials.4,5,6

Although co-oxidation processes (including PO/SM, PO/TBA and PO/CHP processes) eliminate the generation of wastes, they still generally suffer from complex processing, high investment cost, restriction by the co-product market and a stringent technology monopoly by international companies.7,8 Comparatively, the direct epoxidation of propylene with hydrogen peroxide (the authors’ company’s HPPO process) catalyzed by TS-1 zeolite under mild conditions shows promising and positive advantages—with high atomic efficiency and almost without hazardous waste formation—and is attracting attention in various countries.9

However, this process is rather difficult at large scale due to the high reactivity of the PO product and the poor mass diffusion property of epoxidation catalysts, causing serious byproduct formation and rapid coke deposition inside confined zeolite micropores during HPPO processing, therefore limiting the commercial application process. As a result, before 2010, only two international companies (BASF/Dow and Evonik/Uhde) held this novel and efficient HPPO technology at industrial scale.

Consequently, to master this technology, the authors’ company began to develop its HPPO process and corresponding catalysts on a small scale in 2000, and constructed the first Chinese 100,000-tpy industrial HPPO unit at the Changling branch in 2014. This novel HPPO process features a long catalyst lifetime, excellent catalytic performance and low material and energy cost,10 providing a green alternative pathway to produce PO in China to meet strong downstream market demand while supporting environmental protection and carbon neutralization. This article details the development evolution and characteristics of the HPPO catalyst, fixed-bed reactor design, and novel process and combined technologies, while demonstrating the technology’s advantages.

Overview of the HPPO process

In this industrial HPPO approach, two independent processes are involved—a 150,000-tpy hydrogen peroxide (H2O2 ) (50% concentration in an aqueous solution) synthesis process and a 100,000-tpy HPPO process—due to the short transport radius and explosion risk of the aqueous H2O2 solution.11 H2O2 is generally synthesized from hydrogen (H2) and oxygen (O2) over Pd-based catalysts via a conventional anthraquinone oxidation (AO) method within fixed-bed reactors.12

Six main systems are involved with the HPPO process: a propylene epoxidation system, a propylene separation system, a propylene purification system, a PO separation system, a PO refining system, and a methanol separation/purification system, as shown in FIG. 1. The propylene epoxidation system serves as the heart of the HPPO process, in which propylene directly reacts with H2O2 to create PO under the effects of framework Ti sites of TS-1 zeolite and methanol solvent at relative low temperatures (around 20°C–50°C). After separation and purification, both methanol and propylene are reinjected into the propylene epoxidation reactor, while PO is collected as a high value-added product. Additionally, the effluent is removed from the bottom of the methanol distillation column and further treated by combined treatment methods.

FIG. 1. A flow chart of the authors’ company’s HPPO process.

For more than 20 yr, the authors’ company has focused on catalyst development, as well as the small-scale, pilot-scale, side-line and industrial demonstration tests of the HPPO process, as illustrated in FIG. 2. From 2010–2012, to optimize reaction parameters and investigate the stability of the HPPO catalyst, a 1,000-tpy HPPO process pilot plant was constructed and evaluated at the company’s Changling Branch—this pilot plant has run smoothly for more than 6,000 hr without any decrease in H2O2 conversion and PO selectivity. Following this achievement, the first Chinese 100,000-tpy industrial HPPO industrial plant was built and run successfully for first time at the end of 2014. After that, to meet the high PO product demand dictated by the Chinese national standard published in 2015 (GB/T 14491-2015) and to improve process efficiency, the HPPO technology was updated by intensifying the inhibition and distillation removal of low-molecular side products, resulting in a final PO purity of > 99.97% and wide applications in many areas. In April 2018, this device completed 72 hr of full load operation calibration, and the key parameters were superior to design values and reference technologies.

FIG. 2. The development evolution of the HPPO process.

In November 2018, the HPPO process passed its technical appraisal, meaning the successful commercialization of the technology. In 2020, the first Chinese self-created 300,000-tpy HPPO technology package was developed and approved. To date, four sets of 300,000-tpy commercial HPPO processes have been licensed in China, while five HPPO technology transfers are under negotiation and will be licensed in the near future. These units are anticipated to begin operations in 2024, promoting environmentally-friendly and sustainable bulk PO production in China and, to some extent, globally. To continue technological advantages, the second-generation HPPO technology at the authors’ company, based on the catalyst design and reaction processing innovation, is under development with the goal of a 1,000-tpy pilot to be built at the end of 2023.

The HPPO catalyst

Catalyst plays a vital role in the industrial HPPO process—not unlike a chip within a computer operating system—because high catalytic activity, PO selectivity and catalyst stability, and a long running time are required to achieve optimum economic and operational benefits.13 However, both H2O2 and PO product are highly reactive, so side reactions like H2O2 decomposition, deep oxidation, PO ring opening and polymerization can easily occur within confined catalyst channels, decreasing catalytic activity, selectivity and lifetime.14

To overcome these problems, researchers at the authors’ company have explored active component TS-1 zeolite synthesis and catalyst shaping technologies since 2000. It was determined that both high mass transfer performance and inhibiting H2O2 decomposition are vital for promising HPPO performance. Consequently, a novel surface silicon-rich hollow titanium silicate (SR-HTS) zeolite and the corresponding shaped catalyst (referred to as the proprietary catalysta) with sufficient mechanical strength and catalytic activity were initially developed, exhibiting improved mass diffusion and catalytic performance than reference catalysts and, therefore, supporting the industrialization and smooth extended operation of the integrated 100,000-tpy HPPO process.

Synthesis of SR-HTS zeolite as an active component

The classic TS-1 zeolite synthesized by the EniChem method is a poor framework for Ti incorporation and mass diffusion property, leading to low H2O2 utilization efficiency, high byproduct selectivity and fast coke deposition in the microporous channels during the HPPO process.

To enhance the accessibility and intrinsic activity of active sites, SR-HTS zeolite was initially synthesized by combining controlled hydrothermal crystallization and post synthesis methods developed by researchers from the authors’ company.15,16 In the first step, surface silicon-rich TS-1 (SR-TS-1) zeolite was prepared by slowing down the hydrolysis and crystallization rate of Si precursors in the presence of organic template [tetrapropylammonium hydroxide (TPAOH)] at high temperatures under hydrothermal conditions.

Then, the calcined SR-TS-1 was post treated through an in-situ dissolution-recrystallization process in the presence of an aqueous TPAOH solution under hydrothermal conditions, thus with the formation of SR-HTS zeolite. It is confirmed that abundant hollow cavities (intracrystalline mesopores and/or macropores) are highly dispersed within SR-HTS zeolite particles, as shown in FIG. 3; therefore, the diffusion coefficient of SR-HTS (9.41 ± 0.20 × 10-10 m2s-1) is far higher than that of SR-TS-1 zeolite (8.04 ± 0.16 × 10-10 m2s-1), using rigid cyclohexane as a probe molecule.

FIG. 3. TEM images of SR-TS-1 and SR-HTS zeolites: (a, left) SR-TS-1 zeolite and (b, right) SR-HTS zeolite.

It has been confirmed that hollow cavities can significantly shorten the residence time within zeolite particles and so favor the mass transfer of reactant and product molecules (FIG. 4). It is useful to restrict the etherification and polymerization of PO to propylene glycol ether, propylene glycol and polymers, thus concurrently improving PO selectivity and catalyst lifetime. In addition, much more tetrahedral framework Ti species are generated by intensifying the condensation of Ti-OH and Si-OH groups during post treatment. More importantly, the ratio of Ti and Si of bulk SR-HTS zeolite is higher than that of the zeolite’s external surface, which means it reduces the direct contact of H2O2 molecules and the framework Ti species, thus increasing H2O2 utilization efficiency. Consequently, SR-HTS shows far better catalytic performance than TS-1 in both laboratory and industrial tests, as shown in FIG. 5. The SR-HTS zeolite active component was commercially manufactured by using industrial 12-m3 autoclaves by the authors’ company. TABLE 1 shows the pore volume, surface area and self-diffusion coefficients for SR-TS-1 and SR-HTS.

FIG. 4. The main epoxidation reaction and side reactions, including H2O2 decomposition, ring-opening and polymerization of PO product in the HPPO process.
FIG. 5. Catalytic evaluation of SR-TS-1 and SR-HTS tablet catalysts in the HPPO process at the same conditions in small scale, with a catalyst weight of 20 g in the micro-reactor, a reaction temperature of 35°C, and a propylene pressure of 2.0 MPa.

Manufacture of proprietary catalysta

The HPPO process is normally operated in a fixed-bed reactor due to the strong exothermic properties of the propylene epoxidation reaction, with a reaction heat of ~210 KJ/mol PO formation.17 Therefore, SR-HTS zeolite must be shaped by mixing with binders, carriers and modification agents under the effect of mechanical force, with the generation of industrial epoxidation catalyst, which is referred to as the proprietary catalysta (FIG. 6). This catalyst possesses controllable morphology and size, excellent mechanical strength and good coke formation resistance. As shown in FIG. 7, through a low-temperature nitrogen adsorption-desorption method, it is confirmed that two hysteresis loops are in the P/P0 range of 0.45–0.8 and 0.8–1, respectively. These loops are attributed to hollow cavities within zeolite particles and intercrystalline mesopores of 10 nm–30 nm in size, respectively, suggesting excellent mass diffusion performance. Additionally, to inhibit the influence of extra acidic centers from binders and carriers on PO selectivity, specific auxiliaries were added during catalyst molding processing, thus only few weak acid and base sites, originated from tetrahedral framework Ti centers, are discovered in the proprietary catalysta.

FIG. 6. An image of aggregated zeolite morphology and intercrystalline mesopores of the proprietary catalysta at high magnification.
FIG. 7. Low-temperature N2 adsorption-desorption curves and pore size distribution (inserted image) of the proprietary catalysta.

The HPPO reactor and process

The authors’ company’s HPPO process consists of catalyst application and regeneration systems, an epoxidation reactor and corresponding controlling system, a reaction operation process, separation and refining technologies, and safety, environmental protection and energy-saving technologies.

The propylene epoxidation reactor and reaction engineering system play an ultra-important role in an industrial plant—the heart of the entire industrial process. In principle, from the viewpoint of reaction engineering, propylene epoxidation can be carried out in both a fixed-bed reactor and a slurry-bed reactor. The advantages of a slurry-bed reactor are in the great flexibility of catalyst regeneration and high heat removal efficiency; however, the PO selectivity is difficult to maintain at a high level due to the strong backmixing effect and non-uniform residence time distribution of mixed flow operation.1,2,4

Therefore, it is optimal to carry out the HPPO reaction by using a fixed-bed reactor because plug flow benefits from high catalyst activity and PO selectivity at the same time, owing to the low back-mixing effect. The major challenges of a fixed-bed reactor involve quick heat removal, high reactants distribution and intrinsic safety, as a high-concentration H2O2 solution is a dangerous chemical regent and sensitive to temperature for fast decomposition and even explosion.

A novel domestic epoxidation reactor of a unique inner structure and feed controlling system was developed and manufactured by the authors’ company. In this reactor, all reactants can be homogenously mixed in a short time and uniformly injected into every reaction tube through a specific feed unit structure under optimal flow parameters, avoiding the existence of hot spots in the reaction zone and reducing side reactions. Conversely, the reaction heat can be quickly removed by a specific heat exchange system, ensuring the propylene epoxidation reaction is precisely carried out at low temperature, and with low energy cost and high safety.

Moreover, based on pilot-test experimental results, increasing the reaction pressure and solvent amount enhances the reaction rate and PO selectivity due to the increase of propylene solubility in methanol—this does cause high energy consumption on methanol solvent distillation separation and purification for recycling. Due to an activation energy of propylene epoxidation that is less than the side reaction, rising reaction temperature favors high H2O2 conversion but has a negative impact on PO selectivity. Additionally, a short contact time of reactants and catalyst helps high PO selectivity when high H2O2 conversion must be maintained.

It is vital to flexibly alter the reaction parameters and catalyst feature, as the apparent performance of catalyst is influenced by coking and pore blocking by large organic polymers. In the end, the optimized reaction parameters and controlling methods were achieved via numerous experimental tests at pilot scale by using the proprietary catalysta, a reaction temperature of 40°C–50°C, and a turnover frequency of 30.9 mmol.g-1h-1–32.2 mmol.g-1h-1.10

Other key practical factors, such as capital investment, machining feasibility and reactor bed pressure drop were also considered and investigated. Consequently, a novel and advanced multi-tube, fixed-bed reactor of 100,000-tpy PO production scale with optimized structure parameters to remove epoxidation reaction heat rapidly by cool intermediates was developed and evaluated by the authors’ company to ensure the intrinsic safety of the HPPO reactor during industrial processing.

Notably, in the interest of best industrial practices and in addition to catalytic performance, energy consumption, safety and environmental protection issues of the process were also considered—they are all closely correlated to the sustainability, technical economy and market competitiveness of this HPPO process.

A novel comprehensive heat utilization technology was developed with high-energy cascade utilization efficiency. For example, the distillation of methanol from water requires high energy consumption at high temperatures, so the outlet evaporator (usually > 100°C) serves as the heat source to drive other low-temperature unit operations, such as propylene separation and PO purification. To further enhance process efficiency, the wastewater treatment system cooperates with the fluid catalytic cracking unit (FCCU)—the most important part of a petroleum refinery—using chlorine-free effluent of high-chemical oxygen demand (COD) value as the FCC process terminator.

Notably, these organic byproducts in wastewater (i.e., polyether polyol, propylene glycol, propylene glycol ether) can be transformed into valuable chemical intermediates, such as ethylene and propylene, under the thermal effects at high temperature. Conversely, these organic byproducts can also be converted to methane via an anaerobic fermentation approach. Obviously, these waste effluents can not only be treated easily and efficiently, but can also produce useful products, thus further improving the technical economy of this process.

At the end of 2014, based on the reactor and processes mentioned here, the first set of Chinese independently-developed 100,000-tpy HPPO industrial plant using the proprietary catalysta was built and successfully operated for the first time by the authors’ company at Changling Branch in Yueyang, Human province (FIG. 8).

FIG. 8. The first Chinese-created 100,000-tpy HPPO industrial demonstration unit began operations in 2014.

The plant boasts H2O2 conversion and PO selectivity of 97.6% and 97.8%, respectively, based on a 72-hr continuous full-load calibration test in 2018—this exceeds the referenced HPPO processes and achieves the initially designed goals. Additionally, compared to reference technologies, the propylene consumption and energy cost of this process are also reduced by 2.1% and 11.9%, respectively, showing the desired economic advantages as a result of the relatively high price of propylene and H2O2 in the market.

The general PO product purity is > 99.97% (and even higher to 99.99% when dictated by the market), which is much greater than the China national standard of premium grade PO product (purity ≥ 99.95%, GB/T 14491-2015). The content of aldehydes and water impurities are < 0.003 wt% and < 0.015 wt%, respectively, which are far lower than those of the national standard level, as shown in TABLE 2.

The final PO product has been used in many PO downstream areas, such as polyether polyols, surfactants, oilfield demulsifiers and other products. In particular, when it is used in polyether polyols and the polyurethane industry, the final products are colorless and present only a slight odor, making them more popular. As shown in FIG. 9, to date, the proprietary catalysta in the 100,000-tpy HPPO unit has run smoothly for > 38,000 hr while showing almost the same excellent catalytic performance as that of fresh catalyst in the initial stage.

FIG. 9. The running characteristics of the industrial 100,000-tpy HPPO plant.

Although a variety of harsh reaction conditions have been experienced, there is no difference in texture properties and Ti coordination between the fresh and used proprietary catalysta in accordance with multiple characterization methods.

In August 2020, the first Chinese self-developed 300,000-tpy HPPO technology package, with several process innovations, was developed by the authors’ company. Owing to the significant economic and technological advantages of this scale, four sets of the 300,000-tpy HPPO process have been commercially licensed; five additional HPPO technology transfers are predicted to be completed by the end of 2023. To further optimize and update HPPO processes, the second-generation HPPO technology, which involves both catalyst and process route sustainable innovations, is being designed—a pilot test at 1,000-tpy scale will be constructed and put into operation at the end of 2023.


To provide an efficient alternative PO synthesis method to overcome the drawbacks of a traditional chlorohydrin route, the first Chinese self-developed 100,000-tpy HPPO integrated technology has been developed by researchers at the authors’ company combining several innovations, including a SR-HTS zeolite active component, a proprietary catalysta and corresponding shaping technology, an epoxidation reactor and process design and safety investigation, a heat integration technology, and wastewater treatment in collaboration with the refinery FCCU. This HPPO process exhibits superior advantages in high catalytic performance, catalyst lifetime, raw material and energy consumption, product purity, and economic and social benefits.

Based on further comprehensive innovations, a 300,000-tpy HPPO technology package and the second generation of the HPPO process have been developed—the 300,000-tpy HPPO technology has been licensed to four commercial units, and will be licensed to more than three in the near future. HP



This work was financially supported by the National Basic Research Program of China (973 Program, 2006CB202508), China Petrochemical Corporation (SINOPEC Group 413108, 409085, 411058, 412091, 413023, 413025, 413068, 414046 and 419019).



a Sinopec’s HPO-1 catalyst and



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The Authors

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