sustainability/ Industry perspective green chemistry

FRANZ SCHMIDT, MAIK BERNHARD, HEIKO MORELL, MATTHIAS PASCALY* *Corresponding author Evonik Industries AG, Advanced Intermediates – Innovation, Rodenbacher Chaussee 4, 63457 Hanau-Wolfgang, Germany

Matthias Pascaly HPPO Process Technology A novel route to propylene oxide without coproducts

KEYWORDS: HPPO process, propylene oxide, hydrogen peroxide, titanium silicalite.

The common industrial technologies for the conversion of propylene to propylene oxide have been Abstract compared with a special focus on the direct oxidation using hydrogen peroxide. The HPPO process is an economically and ecologically superior technology since there are no market dependencies of other coproducts and water is the only waste product. The catalyst used in this process is a partly titanium substituted silica based zeolite called TS-1. The article summarizes the most important information concerning the HPPO process.

INTRODUCTION approximately 7 Mt/a in the year 2010 at a production capacity of approximately 8 Mt/a. Based on a total The oxidation of organic compounds is of vital importance market growth of around 5 % per year, the expected for the chemical industry. Besides basic oxidation reactions values for demand and capacity in 2015 are nearly such as bleaching processes (e.g. paper or laundry) also at 9 and 10 Mt/a respectively. The growing markets are in oxidation reactions of chemicals are important: Epoxides Asia and potentially in the Middle East (1). - especially ethylene and propylene oxide – are among the There are several industrial routes to produce propylene major chemicals. Replacement of traditionally used halide- oxide, of which the chlorohydrin process (CH) is the based oxidants (chlorine) by hydrogen peroxide provided a oldest one (2). Other indirect oxidation processes new route to the desired oxidized products. Past demand for coupled with coproducts are propylene oxide / styrene hydrogen peroxide was due to the replacement of a monomer (PO/SM) and propylene oxide / methyl chlorine bleaching step in the paper industry or the tert-butyl ether (PO/MTBE) (3). Newer technologies are introduction of percarbonates as bleaching agent in based on an oxidation without a coproduct. One of detergents. Today, the newly developed HPPO (hydrogen- these technologies is the propylene oxide cumene peroxide-to-propylene-oxide) process is one of the largest (PO/CU) process developed by Sumitomo (4). However, consumers of hydrogen peroxide for the epoxidation of the most promising way is the oxidation of propylene with propylene yielding propylene oxide on a titanium doped zeolite without any coproducts. Also in this case, the oxidative potential of hydrogen peroxide allows the replacement of traditionally used chlorine-based oxidants enabling a novel environmentally more benign process. This article provides an overview on the HPPO technology which is now state of the art for the industrial production of propylene oxide. The article touches the catalyst TS-1, the propylene oxide reaction as well as the process conditions. Furthermore, a future perspective is given, based on the market situation and in comparison to other processes.

PROPYLENE OXIDE MARKETS AND PRODUCTION PROCESSES

Figure 1. Development of PO-technologies (* data based on (1) Propylene oxide ranges on place eleven of all organic and Evonik’s own estimates). chemicals being produced with a total demand of

Chimica Oggi - Chemistry Today - vol. 32(2) March/April 2014 31 hydrogen peroxide (HPPO), independently developed by Evonik/TKIS (ThyssenKrupp Industrial Solutions AG) and BASF/Dow Chemical, respectively. Figure 1 shows the percentaged PO production capacity of the different processes in the past and an estimated future trend (1). In recent years, the PO production technology is observed to shift away from the formerly standard chlorohydrin route. This shift takes place in favor of the HPPO process. However, the majority of the propylene oxide is currently still produced via the Chlorohydrin (CH) route (Figure 2). This process is performed generally in two-steps. In the first step, intermediately generated hypochlorous acid reacts with propylene resulting in two kinds of propylene chlorohydrins. These chlorohydrins are subsequently dehydrochlorinated by calcium hydroxide or sodium hydroxide. Beside the aspired propylene oxide 2.1 tons

CaCl2 and 0.1 tons 1,2-dichloropropane are obtained as byproducts per ton propylene oxide. This is the main disadvantage of this process. For process optimization Figure 2. Reactions of the industrial relevant propylene oxide Ca(OH)2 can be replaced by NaOH. Subsequently the producing processes. generated NaCl is converted to NaOH and Cl2 via electrolysis. This step reduces the salt load of the waste water, but increases investment and production costs achieved by Shell´s SMPO process using a heterogeneous due to additional power consumption and required TiO2/SiO2 catalyst offering a more efficient catalyst purification of NaCl prior electrolysis (2). separation in the epoxidation step (6). An alternative is the so called Lummus process using Due to the above mentioned drawbacks, intensive tert-butyl hypochlorite and water to form tert-butanol research was performed to develop coproduct free routes and the propylene chlorohydrins (5). These chlorohydrins for the production of propylene oxide. For example, in the are converted to propylene oxide using NaOH and the Bayer-Degussa-process perpropionic acid is used as resulting NaCl is electrolyzed to NaOH and Cl2. Therefore, oxidation agent (6). This process offers a high selectivity to a total recycling to build up the required HOCl-carrier propylene oxide and an efficient recycling of propionic

(tert-butyl hypochlorite) is possible. The drawback is the acid. But a high price of H2O2 at the time of development slower formation rate of propylene chlorohydrins using prevented the commercialization of this process. tert-butyl hypochlorite. The breakthrough regarding a direct oxidation and The other industrially used propylene epoxidation therefore a coproduct-free method was achieved by ENI processes can be divided into coproduct-producing (PO/ in the 1980s (7). Using a novel titanium silicalite-1 (TS-1) SM and PO/MTBE) and coproduct-free (PO/CU and HPPO) catalyst the direct oxidation of propylene with hydrogen processes and offer the opportunity of being chlorine free peroxide was enabled without further oxidation agents (8). (Figure 2). In the PO/SM and the PO/MTBE routes a Evonik and TKIS improved this process by developing a precursor is used which is oxidized by readily available air special TS-1 catalyst quality using an optimized process or molecular oxygen. The intermediate hydroperoxide technology. On this basis, the HPPO process could be transfers the oxygen to the propylene resulting in developed and finally commercialized. This coproduct- propylene oxide and a primary coproduct, which is usually free process offers high specific propylene oxide yields, an alcohol. The challenge using the coproduct routes is to resulting in low feedstock consumptions. A long catalyst achieve a high selectivity for PO and receive an lifetime is achieved by moderate reaction conditions additional benefit from selling the coproduct. which are enabled by the high-performance TS-1 During the last few years several processes were catalyst. Since the HPPO process is a stand-alone- developed using acetaldehyde, isobutane, isopentane, technology, the product is independent from offering cyclohexane, ethyl benzene and cumene as precursors coproducts on the market. Contrary to the chlorohydrin leading to different secondary coproducts (Table 1) (6). route the HPPO-process enables an environmentally However, all of these processes suffer from the need to friendly production due to a totally closed solvent and process the coproduct, preferentially by obtaining a credit for supplementing it in the PO production costs. Therefore, only the PO/SM and PO/MTBE process yielding styrene monomer and methyl tert-butyl ether as coproduct are currently economically feasible (1). Nevertheless, high amounts of styrene (690 kta) and MTBE (830 kta) produced as coproducts in a world scale PO-plant (300 kta) need to be traded and the risk of an oversupply with Table 1. Summary of possible precursors used for propylene oxide production via styrene and MTBE could reduce the efficiency indirect epoxidation routes with coproduct. of such processes. An optimization was

32 Chimica Oggi - Chemistry Today - vol. 32(2) March/April 2014 feedstock cycle and the complete absence of chlorine. interconnected channel system of straight and sinusoidal Compared with the other state-of-the-art technologies, channels with pore diameters of 5.1 – 5.6 Å (8). Important the HPPO-process offers lower investment costs and is the avoidance of non-tetrahedral coordinated titania energy consumption. Furthermore, an additional benefit (TiO2) which is supposed to promote side reactions. can be achieved by recovering valuable byproducts like The ratio of framework incorporated titanium to extra- propylene glycol which is obtained in the range framework species of titanium is a crucial factor for the of 30 kg/t propylene oxide. catalytic performance of the catalyst and can be All these improvements meet the standard of a modern tailored via the synthesis route. Therefore, the choice of sustainable process for propylene oxide and lead to the raw materials and the right parameters during catalyst startup of the first commercial plant of its kind at SKC in synthesis are very important. A typical TS-1 synthesis starts

Ulsan (Korea) under license of Evonik/TKIS with a starting with the dissolution of a silica (SiO2) and a titanium capacity of 100 kta (9). Several months later a further source in the presence of an organic template. The HPPO-plant in Antwerp using the BASF/Dow technology obtained white powder is removed from the mother went on stream (10). The expansion of the HPPO plant in liquor, dried and calcined. The most common templates Ulsan to 130 kta, Dow’s announcement for a further HPPO used are TPAOH (8) and TPABr (18,19). It is shown in plant in Saudi Arabia (11) after the startup of a plant several publications that the catalytic activity is a operated by Dow and SCM in Thailand (12) and the setup function of decreasing crystal size (20). The crystal size of a further HPPO plant in Jilin (China) by Evonik/TKIS (13) itself usually depends on the ratio of template to silicon. underline the potential of this modern process. However, on an industrial scale it is important to minimize A further coproduct-free process in commercial the use or to reduce the costs of the template, since the operation is the Sumitomo process. This process uses zeolite template is one of the cost driving factors during cumene hydroperoxide as epoxidation intermediate, the synthesis. which is obtained upon oxidation of cumene. Propylene Another important aspect in the TS-1 synthesis is the purity oxide and cumylalcohol are obtained in the epoxidation of the crystallographic phases. Hasenzahl et al. describe stage. The latter is hydrogenated to cumene enabling a in their patent the production of TS-1 from pyrogenic complete recycling (4). mixed oxides produced via the aerosil process (21). Since the mixture of silicon and titanium is already present in Future Trends of propylene oxide production the raw material, these pyrogenic mixed oxides result in Despite the existing processes the further development of highly phase pure materials with increased catalytic direct oxidation processes is ongoing. The use of N2O as activity. However, the zeolite synthesis is only one crucial oxidation agent for propylene is under intense discussion part during catalyst synthesis. The powder needs to be (14). Realizing this route would enable the use of a designed for an application in fixed bed reactors. The coproduct, obtained during the production of adipic physical and chemical properties of these formed acid. However, further optimization is necessary due to catalyst particles have a considerable effect on the the limited catalyst performance resulting in a low catalytic performance in the HPPO process. selectivity and short catalyst lifetime. Additionally, the local availability of N2O can prevent this process from being economically realized. REACTION A further possibility is the oxidation of propylene using oxygen/hydrogen mixtures. This process can be performed There are several factors, having an impact on the using bifunctional precious metal heterogeneous catalysts catalytic performance. The activation energy of the such as Au/TiO2 or Au/TS-1 (15). Due to the in-situ epoxidation is with 26kJ/mol considerably high (22). generation of hydrogen peroxide this method cannot be Therefore, a certain temperature has to be applied to classified as a direct oxidation of propylene. achieve the productive conversion. Typical reaction The direct oxidation of propylene using oxygen is one of temperatures range between 0–60 °C (23). On the other the major challenges in heterogeneous catalysis. Due to side the epoxidation is an exothermic reaction the huge activation energy (497 kJ/mol) of O2 generating high temperatures (propylene oxide dissociation and the high affinity of monooxygen to the formation enthalpy: ‑123kJ/mol) (24) which lower the hydrogen atoms in allylic position, acrolein is propylene oxide selectivity due to side reactions. The preferentially formed during oxidation of propylene. ring-opening of propylene oxide to glycol or glycol ethers Therefore, the direct oxidation of propylene is much is the major side reaction (2). more difficult than ethylene oxidation, which is already Haas et al. reported an optimized balance between H2O2 established on an industrial scale. conversion and propylene oxide selectivity using a fixed bed reactor. Running the process at ambient temperatures, they could obtain almost full conversion THE CATALYST while keeping the selectivity very high (25). Other reactor designs offer an intermediate external cooling to prevent The catalyst used for the HPPO reaction is a titanium excessive side product formation (26-28). Also the reaction silicalite-1 zeolite with framework type MFI. In the pressure for the conversion of propylene to propylene structure Si-atoms are substituted by Ti-atoms forming the oxide is crucial for the reaction rate. On the one hand a catalytically active TiO4-centers (16). However, the high pressure increases the solubility of gaseous propylene amount of titanium, which can be inserted into the in the solvent, on the other hand propylene is being framework, is limited to about 3 wt.% TiO2 (17). liquefied. Therefore, the reaction system either comprises a TS-1 comprises a microporous two-dimensional gas-liquid-solid phase system or a liquid-liquid-solid phase

Chimica Oggi - Chemistry Today - vol. 32(2) March/April 2014 33 and there are several approaches known in the literature to restore the activity of the catalyst (38-40). One approach is the thermal treatment of the deactivated catalyst at elevated temperatures in the presence of an oxidizing atmosphere (40). The residuals are broken down and removed from the catalyst. Another possibility is the regeneration using liquids at ambient temperatures Figure 3. Schematic scheme of Evonik/TKIS’s HPPO process. to restore the catalytic activity (39). system with a propylene rich phase and a solvent rich PROCESS DESIGN phase. It is obvious that the concentration of propylene in the liquid phase has a direct influence on the reaction The literature describes several reactor concepts for the rate. The HPPO technology usually operates at pressures HPPO process (25-28). For laboratory and catalyst above 15 bar (23). research purposes, batch reactors proved to be the most The solvent used for the epoxidation of propylene has a suitable solution. Upon scale up, packed bed reactors, fundamental impact on the reaction rate. An OH-group trickle bed reactors and heat exchanger reactors are is mandatory for formation of the five-membered ring, commonly used. The Evonik/TKIS technology employs a which is mechanistically necessary for the propylene trickle bed reactor operated at appropriate reaction oxide production (29). Clerici et al. showed that using temperature (25). Besides a highly selective catalyst, an methanol (being the smallest alcohol) results in the efficient reaction temperature control is essential to highest activity for the epoxidation (30). suppress side reactions and to ensure a high PO By using methanol not only higher conversion rates could selectivity. be achieved, but also an increased selectivity towards A simplified process flow diagram with all reaction and propylene oxide (31). Corma et al. showed that the purification steps is shown in Figure 3 (41). The reactor is polarity of the solvent is also very important (32). While fed with H2O2, propylene and a solvent. After the using a more hydrophilic Ti-Beta catalyst, aprotic solvents reaction the residual propylene is separated in a like acetonitrile are superior, whereas using a consecutive flash and purge gas system. The remaining hydrophobic TS-1, protic solvents like methanol are the product stream is lead to a preseparation unit, where solvents of choice. However, water as the smallest protic an enriched propylene oxide containing stream is solvent is not suitable since the solubility of propylene is separated from a water solvent mixture. The obtained very low and the concentration of propylene at the product stream is further purified via a propylene titanium center in the channels of TS-1 would be stripper and a distillation column. The resulting insufficient. propylene oxide is very pure (polymer grade). The Intense research has been carried out to use basic or remaining solvent/water mixture is purified and non-basic additives to improve the selectivity (33-35). The recycled. The recycled propylene as well as, the basic additives are poisoning the acid sites of the TS-1, purified solvent is lead back to the reaction mixture moderating their activity. Furthermore unwanted side enabling integrated solvent recycling leaving only reactions such as ring opening of propylene oxide are water as a byproduct (42). diminished. The decomposition of hydrogen peroxide to oxygen and water is another side reaction. Due to higher temperatures in the catalyst bed, especially hot spots, CONCLUSION hydrogen peroxide tends to decompose (36, 37). It has also been reported in the literature that non-tetrahedral Propylene oxide ranges on place eleven of all organic coordinated species of titanium enhance the chemicals produced worldwide. It is one of the most catalytically decomposition of hydrogen peroxide (37). important epoxides currently used in industry. To serve The deactivation of the catalyst accompanied by the loss these market needs several industrial routes to produce of catalytic activity is a challenge in heterogeneous propylene oxide – with and without coproducts – have catalysis. The main reason for the deactivation of the HPPO been commercially established in the past. From the catalyst is caused by pore blockage due to side product coproduct producing processes only the PO/SM and formation. However, the blockage of active sites is reversible PO/MTBE process yielding styrene monomer

34 Chimica Oggi - Chemistry Today - vol. 32(2) March/April 2014 and methyl tert-butyl ether as coproduct are currently 21. Hasenzahl S., Mangold H., Roland E., Scholz M., Thiele G., US economically feasible. Patent No. 5919430 (1999). The HPPO process is an economically and ecologically 22. Shin S.B., Chadwick D., Ind. Eng. Chem. Res., 49, state-of-the-art technology being coproduct-free and 8125-8134 (2010). with water as the only waste product. The heart of the 23. Russo V., Tesser R., Santacesaria E., Di Serio M., Ind. Eng. HPPO process is the TS-1 catalyst system showing high Chem. Res., 52, 1168-1178 (2013). 24. Cox J.D., Pilcher D., Academic Press, New York, 1-636, (1970). activity, improved selectivity for propylene oxide and low 25. Haas T., Hofen W., Sauer J., Thiele G., US Patent No. 6600055 H O decomposition. Excellent process control of the 2 2 (2003) catalyst manufacturing is the vital necessity for creating 26. Hofen W., Thiele G., US Patent No. 6610865 (2003). this high performing catalyst system. 27. Strickler G.R., Quarderer G.J.Jr., Lindner J.P., US Patent No. 7273941 (2007). 28. Jubin J.C. Jr., Danner J.B., US Patent No. 5849937 (1998). REFERENCES 29. Clerici M.G., Domine M.E., Oxidation Reactions catalyzed by Transition-Metal-Substituted Zeolites, Chapter 2, 1. Nexant’s CHEMSYSTEMS, Propylene Oxide, PERP Report in Liquid Phase Oxidation via Heterogeneous Catalysis, 07-2012, (2013). Edited by Clerici M.G., Kholdeeva O.A., John Wiley & Sons, 2. Winnacker-Küchler: Chemische Technik, 5th Edition, Edited Inc., Hoboken, New Jersey, USA (2013). by Dittmeyer R., Keim W.,Kreysa G., Oberholz A., Wiley-VCH, 30. Clerici M.G., Bellussi G., Romano U., J.Catal., 129, Weinheim, Germany (2005). 159-167 (1991). 3. Kollar J., DE 1468012 (1962). 4. Development of New Propylene Oxide Process: http://www.sumitomo-chem.co.jp/english/rd/report/theses/ Readers interested in a full list of references docs/20060100_ely.pdf (2006) (accessed Jan 2014) are invited to visit our website at www.teknoscienze.com 5. Gelbein A.P., Kwon J.T., US Patent No. 4008133 (1977). 6. Industrial Organic Chemistry 3rd Edition, Edited by Weissermehl, Arpe, Wiley-VCH, Weinheim, 13th13tH InternAtIonALInternatIonal Germany (1997). ConferenCe on MICroreACtIonMICroreaCtIon 7. Neri C., Anfossi B., Esposito A., teCHnoLoGyteChnology EP 0100119 (1986). 8. Taramasso M., Perego G., Notari B. US Patent No. 4410501 (1983). June 23-25 • 2014 • Budapest • Hungary 9. http://corporate.evonik.com/en/ Budapest university of technology and economics • Building Q media/archive/pages/news-details. aspx?newsid=15734 (accessed Flow Chemistry Society cordially invites you Jan 2014) MaIn topICS oF IMret13 to join 13th InternatIonal ConFerenCe 10. http://www.dow.com/ on MICroreaCtIon teChnology / IMret13 propyleneoxide/news/20090305a. Fundamentals: fluidics, mixing, mass & heat transfer that will take place in the historic city of htm (accessed Jan 2014). Process data acquisition, kinetics and chemical Budapest, hungary in June 23–25, 2014. analysis; materials aspects, micro- and 11. Alperowicz N., IHS Chemical Week nanostructures and micro- and nanoparticles (2013). The aim of the IMRET series is to strengthen the Flow chemistry 12. http://www.dow.com/ bridge between micro-process technology and Catalysis polyurethane/ flow chemistry, and help their integration into everyday practices throughout the world by Multipurpose flow systems: micro-, meso- or news/2012/20120104a.htm delivering the latest knowledge and making it miniscale flow synthesis (accessed Jan 2014). available for the entire micro-process technology Process optimization and intensification 13. http://corporate.evonik.com/en/ and chemistry communities. Flow plants − process design & control media/search/pages/news-details. Fine & commodity chemical synthesis aspx?newsid=22539 (accessed Jan 2014). plenary SpeakerS Advanced material synthesis 14. Duma V.; Hoenicke D., J. Catal., 191, Energy, biomass conversion and thermal systems 93–104 (2000). Food, personal care, and other applications 15. Hayashi T., Tanaka K., Haruta M., J. Catal., 178, 566–575 (1998). keynote SpeakerS 16. Bellussi G., Carati A, Clerici M.G., Maddinelli G., Millini R., J. Catal, C. olIver kappe volker heSSel 133, 220-230 (1992). University of Graz, Eindhoven University of 17. Millini R., Massara E.P., Perego G., Austria Technology, The Netherlands

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