Indirect oxidation of propylene

Indirect oxidation of propylene is an important route for propylene oxide production that proceeds in two reaction steps. The first step is the formation of a peroxide from alkanes, aldehydes, or adds by oxidation with air or oxygen. The second reaction step is the epoxidation of propylene to PO by oxygen transfer from the peroxide with formation of water, alcohol, or acid. The catalytic oxidation of propylene with organic hydroperoxides is nowadays a successful commercial production route (51% of world capacity). Two organic hydroperoxides dominate the processes (i) a process using isobutane (peroxide tert-butyl hydroperoxide, co-product tert-butyl alcohol), which accounts for 15% of the world capacity and (ii) a process using ethylbenzene (peroxide ethylbenzene hydroperoxide, co-product styrene) that accounts for 33% of the world capacity. The process via isobutane is presented by  

The yield of propylene oxide is about 94% and approximately 2.2 mol of the co-product tert-butanol is produced per mol of propylene oxide. From this ratio it becomes immediately understandable that it is essential for an economic indirect propylene oxidation process to find a good market for the coupling product, here tert-butanol. For the isobutane hydroperoxidation reaction propylene is converted with pure oxygen at 120-140 °C, applying pressures of 25-35 bar. The non-catalyzed reaction takes places in the liquid-phase and acetone is formed as a minor by-product. The subsequent epoxidation is carried out in the liquid phase at 110-135 °C under 40-50 bar pressure in five consecutive reactors. The reaction is catalyzed by a homogeneous molybdenum naphthenate catalyst. The co-product tert-butanol can be dehydrated and is afterwards converted into methyl tert-butyl ether (MTBE), an important fuel additive for lead-free gasoline. 

The indirect propylene oxidation process via ethylbenzene hydroperoxide (Halcon process) is displayed in Eq. (6.12.12). Ethylbenzene, obtained by the acid-catalyzed Friedel-Crafts alkylation of benzene with ethylene, is converted with air into ethylbenzene hydroperoxide. The hydroperoxide epoxidizes propylene and generates the co-product a-phenylethanol that is later dehydrated to styrene. Styrene is a major industrial chemical used mainly as monomer for polymers such as polystyrene or styrene-containing copolymers  

The yield of PO in the Halcon process is in the range 87-91% and more than 2 t of the co-product styrene are generated for each produced ton of propylene oxide. The investment costs for the ethylbenzene process are higher than for the tert-butanol process, because of the isolation and purification demands for polymer-grade styrene, figure 6.12.6 shows the plant design for an indirect propylene oxidation process via ethylbenzene hydroperoxide. 

In the peroxidation reactor ethylbenzene is converted with air at 146 °C and 2 bar to form a 12-14 wt% solution of ethylbenzene hydroperoxide in ethylbenzene. The reaction takes place in the liquid phase and conversion is limited to 10% for safety reasons. The reactor is a bubble tray reactor with nine separate reaction zones. To avoid decomposition of the formed peroxide the temperature is reduced from 146 °C to 132 °C over the trays. In the epoxidation reactor the reaction solution is mixed with a homogeneous molybdenum naphthenate catalyst. Epoxidation of propylene in the liquid phase is carried out at 100-130 °C and 1-35 bar. The crude product stream (containing PO, unreacted propylene, a-phenylethanol, acetophenone, and other impurities) is sent to the recycle column to remove propylene. The catalyst can be removed by an aqueous alkali wash and phase separation. The crude PO, obtained as head stream in the crude PO column, is purified by distillations. The unconverted reactant ethylbenzene can be recycled in the second recycle column. The bottom stream containing a-phenylethanol is sent to the dehydration reactor. The vapor-phase dehydration of a-phenylethanol to styrene takes place over a titanium/alumina oxide catalyst at 200-280 °C and 0.35 bar (conversion 85%, selectivity 95%). 

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This process, which involves the indirect oxidation of propylene, developed initially by Sohio and Ugine, and then by various Japanese companies such as Asahi, Osaka Gas, Showa Denka anti Toyo Koatsu, is only used industrially today in Japan and Mexico. The main reasons restricting its commercialization are its low total yield compared with the direct oxidation of propylene, and the by-production in large quantities of sulfuric wastes that can be converted to ammonium sulfate, amounting to nearly 2 t/t of acrylate. 

Table 6.12.1 gives an overview of the world capacity of PO production by processes in 2005. The two major processes are the chlorohydrin process (which accounts for 46% of the world capacity, 2005) and indirect oxidation of propylene oxide (51%). 

Compared to EO, propylene oxide (PO) is less reactive and less hazardous. PO is mainly used for the production of polyether, polyols, polyurethane, glycols, and ethers. Direct oxidation of propylene with air or pure oxygen is not efficient, and PO is produced either by the chlorohydrin process (46% share) or by indirect oxidation. Indirect oxidation of propylene proceeds in two steps. The first step is the formation of a peroxide from iso-butane or ethylbenzene by oxidation with air/oxy-gen (peroxides tert-butyl hydroperoxide and ethylbenzene hydroperoxide, respectively). The second step is the catalytic epoxidation of propylene to propylene oxide by oxygen transfer from the peroxide. In future, oxidation processes based on H2O2 will probably also play an important role. In 2008, the first commercial plant of this kind went on stream. 

Ammonia is used in the fibers and plastic industry as the source of nitrogen for the production of caprolactam, the monomer for nylon 6. Oxidation of propylene with ammonia gives acrylonitrile (qv), used for the manufacture of acryHc fibers, resins, and elastomers. Hexamethylenetetramine (HMTA), produced from ammonia and formaldehyde, is used in the manufacture of phenoHc thermosetting resins (see Phenolic resins). Toluene 2,4-cHisocyanate (TDI), employed in the production of polyurethane foam, indirectly consumes ammonia because nitric acid is a raw material in the TDI manufacturing process (see Amines Isocyanates). Urea, which is produced from ammonia, is used in the manufacture of urea—formaldehyde synthetic resins (see Amino resins). Melamine is produced by polymerization of dicyanodiamine and high pressure, high temperature pyrolysis of urea, both in the presence of ammonia (see Cyanamides). 

Write the equation for the indirect oxidation of ethylbenzene to propylene oxide and styrene. 

There is another route to propylene oxide and to styrene not shown on the figure. Indirect oxidation of isobucane or of ethylbenzene just doesn t fit neatly on the chart, but both are commercial processes, the former yielding PO and TBA, the latter giving PO and styrene. 

Epoxidation with hydroperoxides is the basis for the large-scale indirect production of propylene oxide by a process that has been called the Oxirane or Halcon processes. Early work was reported by Smith in a patent issued in 1956 [457], which described soluble heteropoly acids containing transition metals such as chromium, molybdenum, and tungsten that could be employed as homogeneous catalysts for the reaction of olefins with organic hydroperoxides and hydrogen peroxide. 

Isopropyl alcohol (IPA) has been called the first petrochemical. Both historically and today, it is prepared by sulfuric acid-mediated indirect hydration of propylene (see equations below and Fig. 22.28). Originally it was the source of most of the acetone used in the world. Now, this route must compete with acetone derived from the cumene oxidation process, in which cumene is converted to equimolar amounts of phenol and acetone. Between 1959 and 1978 the amount of acetone derived from IPA dehydrogenation declined from 80 percent to 34 percent, and the amount of IPA used for this purpose declined from 47 percent in 1978 to 12 percent in 1990. 

Synthesis. The total aimual production of PO in the United States in 1993 was 1.77 biUion kg (57) and is expected to climb to 1.95 biUion kg with the addition of the Texaco plant (Table 1). There are two principal processes for producing PO, the chlorohydrin process favored by The Dow Chemical Company and indirect oxidation used by Arco and soon Texaco. Molybdenum catalysts are used commercially in indirect oxidation (58—61). Capacity data for PO production are shown in Table 1 (see Propylene oxide). 

Isopropyl Alcohol. Propylene may be easily hydrolyzed to isopropyl alcohol. Eady commercial processes involved the use of sulfuric acid in an indirect process (100). The disadvantage was the need to reconcentrate the sulfuric acid after hydrolysis. Direct catalytic hydration of propylene to 2-propanol followed commercialization of the sulfuric acid process and eliniinated the need for acid reconcentration, thus reducing corrosion problems, energy use, and air pollution by SO2 and organic sulfur compounds. Gas-phase hydration takes place over supported oxides of tungsten at 540 K and 25... 

Propylene Glycol. Propylene glycol, the second largest use of propylene oxide, is produced by hydrolysis of the oxide with water. Propylene glycol has very low toxicity and is, therefore, used direcdy in foods, pharmaceuticals (qv), and cosmetics, and indirectly in packaging materials (qv). Propylene glycol also finds use as an intermediate for numerous chemicals, in hydrauhc fluids (qv), in heat-transfer fluids (antifreeze), and in many other apphcations (273). 

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Epoxidation of olefins Propylene -> Propylene oxide U.S., Europe, Japan - Indirect oxidation via BrO, or cio-... 

This indirect oxidation route takes two steps. In the first, a hydrocarbon, such as iso butane or ethylbenzene, is oxidized. The source of the oxygen is air. The reaction takes place just by mixing the ingredients and heating them to 250-300°F at 50 psi, producing a hydroperoxide. In the second step, the oxidized hydrocarbon reacts with propylene in a liquid phase and in the presence of a metal catalyst at 175-225°F and 550 psi to produce PO yields of better than 90%. The process flow is shown in Figure 11—3. 

The formation of epoxides is synthetically a very important transformation. The indirect epoxidation of olefins (see Eq. 7) in the presence of electrogenerated chlorine (or bromine) [95] is a commercial process in which chlorine is recycled and not part of the product. The products such as propylene oxide are key intermediates in many further chemical processes. 

The indirect electrochemical generation of propylene oxide via propylene chloro- or bromohydrin using anodically generated hypochlorite or hypobromite has been studied very intensively. The reason is the lack of a technically useful process for the synthesis of propylene oxide by way of heterogeneous catalysis. The propylene halohydrins are saponified using the cathodically generated sodium hydroxide (Eqs. (42)-(47)) (Table 4. No. 12-15)... 

In a %iies of papers Korovina and Entelis have proposed the zwittaionic polymerization of THF initiated by BF3 in the presence of propylene oxide ). Unfortunately, the collected evidence is of indirect nature. Recently, according to Entelis ), the H-NMR and F-NMR methods have confirmed the earlier proposals )... 

Table 7.4 gives economic data on the synthesis of propylene oxide by chlorohydrin processes and by indirect oxidation passing through r-butyl hydroperoxide. 

All the styrene monomer (bpi.ou = 145-2°C, propylene oxide Some attempts have been made to extract styrene from pyrolysis Cj- gasolines (Stex process by Toray, described in Section 4.2.5), but they have not culminated in commercial plants. 

Styrene is manufactured nearly entirely by the direct dehydrogenation of ethylbenzene. Smaller amounts are obtained indirectly, as a co-product, from the production of propylene oxide by the Oxirane and Shell technologies, industrialized in the United States, the Netherlands and Spain, and whose essential intermediate step is the formation of ethylbenzene hydroperoxide, or from the production of aniline, by a technique developed in the USSR, which combines the highly exothermic hydrogenation of nitrobenzene with the highly endothermic dehydrogenation of ethylbenzene. 

Only industrial producers (e.g. Dow) with a highly integrated and cost competitive supply chain of chlorine-caustic soda (through production from caustic soda by NaCl electrolysis) to provide chlorine for the chlorohydrin reactor and sodium hydroxide for the dehydrochlorination step can operate chlorohydrin units for propylene oxide production competitively with indirect oxidation units. 

Evonik and Uhde have also developed a HPPO process, which was commercialized in 2008 in Ulsan, South Korea (100 000 t a ). The indirect oxidation takes place at increased pressure and temperatures below 100 °C with the solvent methanol. The reaction is catalyzed by a titanium-silicate catalyst in a solid-bed reactor that is special due to its p,-reactor characteristic in one dimension. Using this new reactor type it is possible to improve isothermicity and to avoid disadvantageous concentration profiles. Figure 6.12.7 shows the pilot plant reactor of this new propylene oxidation process. Yields of 95% (relating to propylene) and 90% (relating to H2O2) are obtained in the Evonik/Uhde process. 

There have been a number of cell designs tested for this reaction. Undivided cells using sodium bromide electrolyte have been tried (see, for example. Ref. 29). These have had electrode shapes for in-ceU propylene absorption into the electrolyte. The chief advantages of the electrochemical route to propylene oxide are elimination of the need for chlorine and lime, as well as avoidance of calcium chloride disposal (see Calcium compounds, calcium CHLORIDE Lime and limestone). An indirect electrochemical approach meeting these same objectives employs the chlorine produced at the anode of a membrane cell for preparing the propylene chlorohydrin external to the electrolysis system. The caustic made at the cathode is used to convert the chlorohydrin to propylene oxide, reforming a NaCl solution which is recycled. Attractive economics are claimed for this combined chlor-alkali electrolysis and propylene oxide manufacture (135). 

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