Propylene epoxidation liquid-phase

Key Words Propylene, Propylene oxide. Liquid-phase epoxidation, Semibatch reactor. Supercritical CO2, Gas-phase epoxidation, Pd/TS-1 catalyst, Au/Ti02 catalyst, Au/Ti-Si02 catalyst, Ag catalyst, Mesoporous... 

The latest industrial strides in PO synthesis are liquid phase epoxidation of propylene with H2O2 over TS... 

Hydronium ion, 14 23 Hydroperoxidates, 18 411 Hydroperoxide process, for propylene oxide manufacture, 20 798, 801-806 Hydroperoxides, 14 281, 290-291 18 427-436 alkylation of, 18 445 a-oxygen-substituted, 18 448-460 chemical properties of, 18 430 433 decomposition of, 14 279 18 431-432 liquid-phase epoxidation with, 10 656 physical properties of, 18 427-430 preparation by autoxidation, 18 434 synthesis of, 18 433-435 Hydrophile-lipophile balance system,... 

Heterolytic liquid-phase oxidation processes are more recent than homolytic ones. The two major applications are the Wacker process for oxidation of ethylene to acetaldehyde by air, catalyzed by PdCl2-CuCl2 systems,98 and the Arco oxirane" or Shell process100 for epoxidation of propylene by f-butyl or ethylbenzene hydroperoxide catalyzed by molybdenum or titanium complexes. These heterolytic reactions require less drastic conditions than the homolytic ones... 

Gritter, R. /., and E. C. Sabatino Free-Radical Chemistry of Cyclic Ethers. VII. Ultraviolet Photolysis of Epoxides and Propylene Sulfide in the Liquid Phase. J. org. Chem. 29, 1965 (1964). 

Rouchaud and co-workers492 494 studied the liquid phase oxidation of propylene in the presence of insoluble silver, molybdenum, tungsten, and vanadium catalysts. Moderate yields of propylene oxide were obtained in the presence of molybdenum catalysts. These reactions almost certainly proceed via the initial formation of alkyl hydroperoxides, followed by epoxidation of the propylene by a Mo(VI)-hydroperoxide complex (see preceding section). 

The first step is oxidation of EB to form EB hydroperoxide. The oxidation is carried out in the liquid phase with a target EB conversion of approximately 13%. Although higher conversions are attractive from an EB recovery and recycle standpoint, there is a significant disadvantage because the EB hydroperoxide selectivity declines sharply. The second step is epoxidation of propylene to form propylene oxide product and 1-phenylethanol. In the last step, the 1-phenylethanol is dehydrated to styrene and water. The dehydrated reaction mixture is typically stripped of light components and rerun in a styrene column to remove heavy by-products, resulting in a purified styrene product. 

A subsequent step in the sequence is the formation of Ti-hydroperoxo species from the reaction of H2O2 and tetrahedral Ti sites, more likely Ti tripodal sites. This Ti-hydroperoxo species has been proposed as an intermediate in the gas-phase epoxidation of propylene by analogy with the well-known chemistry for oxidations in the liquid phase with H2O2 and TS-1 [105,106,401,446]. In gas-phase reactions, hydroperoxide species have been inferred by D2 isotopic experiments [431] and detected by ex situ INS [432] and in situ UV-vis measurements [433]. Other species in this simplified sequence include adsorbed propylene on a Ti-hydroperoxo site and adsorbed PO on a Ti tripodal site. Desorption of PO and water results in the original Ti species, which closes the catalytic cycle. 

The liquid-phase epoxidation of propylene with O2-H2 mixtures was first reported by Sato and Miyake of Tosoh Corp., who used BuOH as a solvent and Pd as the catalytic metal [10]. In liquid-phase epoxidation, TS-1 is usually used as a catalyst support for Pd because it exhibits the best performance for the epoxidation of propylene with H2O2 [13]. When the reaction is conducted at temperatures below 323 K, the selectivity to PO is above 90%, whereas the conversion of propylene is not sufficiently high and below 4%. 

While industry has selected liquid-phase H202-based technology for imminent PO plants [11], due to handling issues and cost associated with making H2O2, a direct gas-phase propylene epoxidation process, analogous to the commercial ethylene oxide (EO) process [19,20], has long been desired [21]. While ethylene... 

The subsequent step, the epoxidation of propylene by EBHP, is carried out in the liquid phase over a proprietary heterogeneous catalyst, to produce crude PO and MPC. The feed to the reactors consists of make-up and recycle propylene and EBHP in EB. The reaction train consists of a number of adiabatic fixed bed reactors with interstage cooling. Deactivated catalyst is replaced, incinerated to remove residual hydrocarbons, and dispersed in a landfill. 

Shell subsequently developed a heterogeneous, silica-supported titania catalyst [11,12] which forms the basis of the commercial process for the epoxidation of propylene with ethylbenzene hydroperoxide. The co-product alcohol is dehydrated, in a separate step, to styrene. Ti(IV)Si02 was the first truly heterogeneous epoxidation catalyst useful for continuous operation in the liquid phase. 

Among such oxidations, note that liquid-phase oxidations of solid paraffins in the presence of heterogeneous and colloidal forms of manganese are accompanied by a substantial increase (compared with homogeneous catalysis) in acid yield [3]. The effectiveness of n-paraffin oxidations by Co(III) macrocomplexes is high, but the selectivity is low the ratio between fatty acids, esters, ketones and alcohols is 3 3 3 1. Liquid-phase oxidations of paraffins proceed in the presence of Cu(II) and Mn(II) complexes boimd with copolymers of vinyl ether, P-pinene and maleic anhydride (Amberlite IRS-50) [130]. Oxidations of both linear and cyclic olefins have been studied more intensively. Oxidations of linear olefins proceed by a free-radical mechanism the accumulation of epoxides, ROOH, RCHO, ketones and RCOOH in the course of the reaction testifies to the chain character of these reactions. The main requirement for these processes is selectivity non-catalytic oxidation of propylene (at 423 K) results in the formation of more than 20 products. Acrylic acid is obtained by oxidation of propylene (in water at 338 K) in the presence of catalyst by two steps at first to acrolein, then to the acid with a selectivity up to 91%. Oxidation of ethylene by oxygen at 383 K in acetic acid in... 

C-C bond formation reactions) Propylene epoxidation in liquid phase The older synthesis TS-1 (Enichem) BASF/Dow and Degussa-Evonik/Headwaters [67-69]... 

A liquid phase heterogeneous propylene epoxidation process has been described, using 13X Linde zeolite impregnated with ammonium... 

An issue which deserves further mention is the environmentally fiiendly nature of TS-l/H Oj system. It involves the use of a safe silica based catalyst, titanium silicalite, and a reagent, hydrogen peroxide, which yields water as the coproduct. This holds for the in situ route illustrated in Scheme I and also for the epoxidation of propylene with preformed hydrogen peroxide, either used as an aqueous solution (72) or extracted by means of die epoxidation solvent (Scheme 11). Hazardous chemicals, such as chlorine, performic or other organic peracids, are not required in the process. The disposal of chlorinated salts or the recycle of brine (chloroydrin process) and any possible burden resulting from the coproduction of odier chemicals (styrene and r-butanol in the hydroperoxide route) are eliminated. The liquid phase oxidation of isobutane and ethylbenzene with air under pressure and at high temperature, to produce... 

Liquid phase selective oxidations of propylene over TS-1 have also been achieved [153, 154] but while Laufer and Hoelderich extended their TS-l/propylene work to include epoxidations of styrene and pinene over a Ti-MCM-41-based catalyst [155], in situ epoxidation reactions of more complex alkenes than propylene remain poorly documented and have yet to attain the industrially relevant success of TS-1-based catalysts in selective oxidations of even simple alkenes. 

A radical pathway going through the allylic hydroperoxide to give allyl alcohol and propylene oxide cannot be the only pathway involved since the maximum epoxide selectivity would be only 50% by this route. DeRuiter [501] concludes that such a pathway is a minor one since allyl alcohol was stable under reaction conditions and its incorporation into oxidation reactions led to no increase in epoxide yields. Even a route such as (299)-(306) would not appear to give selectivities as high as are currently being reported in some systems [504]. Soviet workers describe the direct liquid phase oxidation of propylene to propylene oxide in 89% selectivity at 15% conversion [504], equation (308). If indeed radical pathways are involved. 

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. 

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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