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Ind. Eng. Chem. Res. 2006, 45, 3447-3459 3447 REVIEWS The Production of Propene Oxide: Catalytic Processes and Recent Developments T. Alexander Nijhuis,*,† Michiel Makkee,‡ Jacob A. Moulijn,‡ and Bert M. Weckhuysen† Department for Inorganic Chemistry and Catalysis, Faculty of Science, Utrecht UniVersity, Sorbonnelaan 16, 3584 CA Utrecht, The Netherl
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  REVIEWS The Production of Propene Oxide: Catalytic Processes and Recent Developments T. Alexander Nijhuis,* ,† Michiel Makkee, ‡ Jacob A. Moulijn, ‡ and Bert M. Weckhuysen †  Department for Inorganic Chemistry and Catalysis, Faculty of Science, Utrecht Uni V  ersity, Sorbonnelaan 16,3584 CA Utrecht, The Netherlands, and Department for Reactor and Catalysis Engineering, Faculty of Applied Sciences, Delft Uni V  ersity of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Propene oxide, which is one of the major commodity chemicals used in chemical industry, desperately requiresa new process for its production, because of the disadvantages that are encountered with the currently availableprocesses. This paper discusses the existing processes used for the production of propene oxide s thechlorohydrin and hydroperoxide processes s and their advantages and disadvantages. Furthermore, the newprocesses and catalysts under development for the propene oxide production are discussed, as well as thechallenges that are still limiting the applications of some of those prospects. The most important newdevelopments for the production of propene oxide discussed in this paper are: the hydrogen peroxidecombination process, the ethene oxide alike silver catalysts, the molten salt systems, and the gold - titaniacatalyst systems. I. Introduction Propene oxide, which is also known as propylene oxide,methyloxirane, or 1,2-epoxypropane, is one of the most import-ant starting materials in the chemical industry. The productionof propene oxide consumes over 10% of all propene produced. 1,2 In 1999, the total production for propene oxide amounted to ∼ 5.8 million tons per year. 3 This market is annually growingby  ∼ 4% - 5%. 1,3 The major application of propene oxide is inthe production of polyether polyols (65%), which are mainlyused for the production of (polyurethane) foams. The secondand third largest applications are in the production of propeneglycol (30%) and propene glycol ethers (4%), respectively. 4 Propene glycols are mainly used in the production of polyesters,whereas propene glycol ethers are primarily used as solvents.Propene oxide is currently produced using two different typesof commercial processes: the chlorohydrin process and thehydroperoxide process. In 1999, the production capacity wasdistributed evenly between these two processes; however,because of the environmental impacts of the chlorohydrinprocess, the most recently built plants are all using hydroper-oxide process technologies. However, a disadvantage of thehydroperoxide processes is the production, in a fixed ratio, of a coproduct (either styrene or  tert  -butyl alcohol, depending onwhich variant of the hydroperoxide process is applied). Becausethese co-products are produced in a volume that is  ∼ 3 timeslarger than that of propene oxide, the economy of the processis primarily dominated by the market of the co-product. A majorresearch effort has been made in the development of alternativedirect epoxidation processes for the production of propene oxide.The aim has been to develop a process for the direct gas-phaseoxidation, similar to the direct epoxidation of ethene. However,the selectivity of the catalysts that have been developed for thedirect epoxidation using oxygen or air is, by far, insufficient toresult in a viable process (usually  < 30%, with the remainingportion of the propene being converted to carbon dioxide). Olindeveloped a process for the direct epoxidation of propene usingmolten salt “catalysis”, claiming promising results of 65%selectivity to propene oxide at 15% propene conversion. 5 However, this process is not yet commercially applied. Analternative to using alkyl-hydroperoxides, which are used indehydroperoxide processes, is hydrogen peroxide. Especially,when TS-1 (titanium silicalite-1) is used as a catalyst, this allowsfor the possibility of a very selective (95%) and hydrogenperoxide-efficient production of propene oxide. 6 However, themajor problem for the commercialization of this process is thefact that, on a molar basis, propene oxide and hydrogen peroxidehave comparable market values, making it impossible to runthe process profitably at this time. For other epoxides used infine chemistry, the use of hydrogen peroxide or organicperoxides is more favorable, because the cost of these oxidantsis much smaller, compared to the product value. The disadvan-tage of the high cost of hydrogen peroxide for the propeneepoxidation can be solved by the in situ production of thehydrogen peroxide, which is a process that is currently underconstruction by Dow - BASF. 7 Another route toward the production of propene oxide underdevelopment is based on propene epoxidation, using a mixtureof hydrogen and oxygen over a gold - titania catalyst. AfterHaruta and co-workers 8 discovered this system almost a decadeago, many research groups performed work on this catalyst.Although the selectivity for propene oxide is very high, the lowconversion and hydrogen efficiency still need to be improved.Also, the mode of operation of the catalyst is still unclear.This paper will discuss the aforementioned propene epoxi-dation processes that are in use and those under development,and the paper examines the possibilities for alternative epoxi-dation routes. II. Current Processes1. Chlorohydrin Process.  The synthesis of ethane oxide andpropene oxide using the chlorohydrin route was first described * To whom correspondence should be addressed. Phone:  + 31-30-2537763. Fax:  + 31-30-2511027. E-mail: x.nijhuis@chem.uu.nl. † Utrecht University. ‡ Delft University of Technology. 3447  Ind. Eng. Chem. Res.  2006,  45,  3447 - 3459 10.1021/ie0513090 CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 04/04/2006  in 1859 by Wurtz. 9 In this reaction, the alkene reacts withhypochlorous acid (HOCl) to produce the chlorohydrin. Thehypochlorous acid is produced in situ by an equilibrium reactionof the acid with water and chlorine. The chlorohydrin isthereafter dehydrochlorinated, using aqueous potassium hy-droxide to produce the epoxide. 10 The conversion of chlorohy-drins to epoxides is performed by an adaptation of the Wilkinsonsynthesis for ethers. 11 This route has long been the main processfor producing both ethene oxide and propene oxide. In the1940s, the process began to be phased out for the etheneepoxidation, because of the development of a more-efficientdirect epoxidation process using a silver catalyst. After thatintroduction, many ethene epoxidation plants that were usingthe chlorohydrin process were converted for the epoxidation of propene. The process is still applied for the propene epoxidation;however, at the moment, it is gradually being replaced by themore environmentally friendly hydroperoxide processes.Figure 1 schematically demonstrates the chlorohydrin process.The two reaction steps in the production of propene chlorohydrinin the first reactor (chlorohydrination) are production of thepropene chloronium complex in the first reaction step,followed by a reaction with water to produce two propenechlorohydrin isomers.The selectivity of these reactions to the chlorohydrin isomersis  ∼ 90% - 95%. The byproducts formed are primarily 1,2-dichloropropane (from the gas-phase reaction of propene withchlorine) and smaller quantities of dichloropropanols (producedfrom allyl chloride, which is also formed in the gas phase fromthe reaction between propene and chlorine), as well as dichloro-isopropyl ethers (from the reaction of the chloronium complexwith propene chlorohydrin). The chlorohydrination is usuallyperformed in a bubble column reactor at a pressure of 1.5 barand temperature of 323 K. Because of the corrosive nature of the reaction mixture, the use of rubber-, plastic-, or brick-linedequipment is necessary.In the epoxidation reactor, the dehydrochlorination of propenechlorohydrin occurs using a base (usually calcium hydroxyde).The propene oxide is steam-stripped from this reactor, to preventbase-catalyzed hydrolysis of the product. The dehydrochlori-nation is performed in the same column where the products arestripped from the wastewater stream (1 bar, 373 K). Thechlorinated hydrocarbons that are produced ultimately residein the propene oxide stream and must be removed. The brineleaving the bottom of the reactor contains some propene glycols,because the hydrolysis of propene oxide cannot be completelyavoided. These glycols and small amounts of other hydrocarbonspresent must be removed biologically. Subsequently, the brineis discharged, because the calcium chloride in the stream hasno commercial value. This is one of the major disadvantagesof the chlorohydrin process, because the amount of brine (5%CaCl 2 ) produced is usually  ∼ 40 times larger than the amountof propene oxide produced and it is extremely difficult to removeall hydrocarbons from this wastewater stream. Reuse of thecalcium chloride is not economically feasible, because of itslow commercial value. Alternatively, sodium hydroxide can beused instead of calcium hydroxide. This has an advantage inthat the sodium chloride produced can then be used in theproduction of chlorine, which can be discharged easier orrecycled in the first step of the process. 4,12 Therefore, thismodification to the process is able to remove the environmentalproblem of this process, to a large extent.The raw propene oxide stream must be purified by distillationfrom the water and chlorinated hydrocarbons. In this separation,again, care must be taken that the propene oxide is nothydrolyzed to propene glycol. The relatively large amount of 1,2-dichloropropane (up to 10%) is obtained as a second“product” from the separation section. Because this compoundhas very little usage, it not only causes a loss in yield, but alsocreates a disposal problem. 9 Alternatively, however, it is alsopossible to recycle the chlorinated propanes to propane orpropene, 13 which is a very effective way to improve theattractiveness of the process and reduce the environmentalimpact.The two disposal problems (brine and chlorinated byproducts)are the main reason that no new chlorohydrin plants are builtand old plants are closed down instead of being modernized.Only the large-scale plants ( > 100 000 tons/yr) are expected toremain operational for a longer period, because they are oftenintegrated with chlorine production plants. 2. Hydroperoxide Processes.  Hydroperoxide processes arebased on the peroxidation of an alkane to an alkyl-hydroper-oxide. These alkyl-hydroperoxides then react with propene,producing propene oxide and an alcohol. A characteristic of these processes is that, besides propene oxide, a coproduct isproduced in a fixed ratio, usually 2 - 4 times the amount of propene oxide produced. Currently, two variants of this processare applied commercially. The first is the propene oxide - styrenemonomer (PO - SM, also commonly abbreviated as SMPO)process ( ∼ 60% of the hydroperoxide plants use this version). 14 - 16 In this process, ethylbenzene is oxidized to ethylbenzenehydroperoxide, which reacts with propene to produce propeneoxide and  R  -phenyl ethanol. The  R  -phenyl ethanol is thendehydrated to produce styrene. The second process in use is Figure 1.  Schematic representation of the chlorohydrin process for theproduction of propene oxide. 3448  Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006  the propene oxide - tert  -butyl alcohol (PO - TBA) process. 17,18 In this process, isobutane is oxidized to  tert  -butyl hydroperoxide(TBHP), which reacts with propene to produce propene oxideand  tert  -butyl alcohol. This can be dehydrated to isobutene orconverted directly with methanol to methyl- tert  -butyl ether(MTBE). Although other combination processes are possible,no others have been applied so far. Other possibilities include,for example, acetaldehyde to acetic acid, 2-propanol to acetone,isopentane (via  tert  -pentyl alcohol) to isoprene, cumene (viadimethylphenyl methanol) to R  -methylstyrene, and cyclohexene(via cyclohexanol) to cyclohexanone. Characteristics of thehydroperoxide processes are that they are selective and producefar less waste than the chlorohydrin process. However, the majordisadvantage of the hydroperoxide processes is that a fixedamount of coproduct is always produced. Because the marketsfor propene oxide and the coproducts are not linked, a problemcould arise, should the demand for one of the products collapse.Since the use of MTBE as a fuel additive is becoming lessfavorable (especially in the United States), the latest plants thathave been built using a hydroperoxide process are all of thePO - SM type.Figure 2 schematically demonstrates the PO - SM process.The basic principle of the PO-TBA process is similar to that of the PO - SM process, so both processes are discussed simulta-neously. The first reactor converts the ethylbenzene or isobutanenoncatalytically to its corresponding hydroperoxide by directliquid-phase oxidation, using oxygen or air. The oxidation isusually performed in a bubble column at 400 K and 30 bar whenisobutane is used, or 423 K and 2 bar in the case of ethylbenzene. The reaction equation of this reaction in the PO - SM process isA disadvantage of the processes using isobutane is that arelatively large fraction of the TBHP that is produced im-mediately decomposes to TBA, thus reducing the ratio of propene oxide to coproduct. The unreacted hydrocarbons areremoved and recycled. The hydroperoxide stream is sent to asecond reactor, where it catalytically reacts with propene toproduce propene oxide and an alcohol. The temperature in thisreactor is ∼ 373 K at 30 bar pressure. The reactor used for theepoxidation is usually a compartmentalized reactor with stagedpropene feeding. The total conversion in the reactors is > 95%(of the hydroperoxide) at  > 95% selectivity to propene oxide,and the only byproduct produced is acetone. Reaction 5 givesthe epoxidation reaction in the PO - SM process:Most processes use either a homogeneous tungsten, molybde-num, or vanadium catalyst or a heterogeneous titanium-basedcatalyst to catalyze the epoxidation reaction. 3,19 The disposalof a homogeneous catalyst causes a waste/separation problem.After the reactor, the propene and propene oxide are removedconsecutively from the liquid stream. In case of the PO - SMprocess, the remaining stream still contains some unreactedethylbenzene, which then is removed and recycled. The alcoholstream can then be dehydrated to produce styrene or isobutene,or, for the PO - TBA process, the  tert- butyl alcohol can be useddirectly. An important side reaction that can occur in thisdehydration is the oligomerization of the styrene produced,which results in a loss of catalyst activity and reduces thecatalyst lifetime. 20 A process that is very closely related to these two hydro-peroxide processes is operated by Sumitomo Chemical. 21 In thisprocess, cumene is oxidized to its hydroperoxide, which is usedto produce propene oxide. The alcohol produced is subsequentlyconverted back to cumene over a copper - chromium oxidecatalyst to be reused in the process. The advantage of thisprocess is that cumene is easier to hydroperoxidate (more stable)than styrene and that no coproduct is being produced. Propeneoxide selectivities of 99% can be obtained, whereas 95% of the peroxide is used for the oxidation. 3. Ethene-Epoxidation-Type Silver Catalysts. 3.1. EtheneEpoxidation Process.  Since the 1940s, all newly built plants Figure 2.  Schematic representation of the propene oxide - styrene monomer (PO - SM) process for the production of propene oxide. Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006  3449  for the production of ethene oxide have been based on a directoxidation process using a silver catalyst. 22 A fixed-bed reactoris used to epoxidize ethene at 500 K and 30 bar. A typicalconversion in the reactor is ∼ 10%, to prevent a further oxidationof the ethene oxide that is produced and to simplify the problemscaused by the exothermicity of the reaction. This relatively lowconversion explains why the use of pure oxygen is usuallypreferred instead of air, because this simplifies the separationsnecessary for the recycle stream. The catalyst used in the processis a 10 - 20 wt % silver catalyst supported on low-surface-area( < 1 m 2  /g) R  -alumina that contains several promoters. A typicalscanning electron microscopy (SEM) image of silver on analumina catalyst, similar to that used for the epoxidation of ethene, is shown in Figure 3. The most important promoter onthe catalyst itself is an alkali metal, which is used to reduce thecatalyst acidity. Cesium chloride is often added to facilitate thedesorption of the epoxide. 23 The selectivity of the reaction isusually ∼ 90%, with the remainder of the ethene being convertedto carbon dioxide. Recently, this selectivity has been graduallyincreasing. 24 3.2. Silver Catalyst.  The ethane epoxidation silver catalysthas a high metal loading on a low-surface-area support, typically R  -alumina. The use of unsupported silver as a catalyst is alsopossible; however, this has a tendency to sinter at the reactiontemperature. Other possible support materials include inertmaterials such as silicon carbide (which is also used com-mercially), glass wool, and quartz. All support materials havea low surface area, because microporous materials generallyyield poor results, which is due to the occurrence of heatproblems inside the particles and further oxidation of etheneoxide produced in the pores. 22 Originally, the idea existed that the epoxidation of etheneover silver occurred by molecular oxygen, producing etheneoxide and atomic oxygen on the catalyst. The atomic oxygenthen could only be removed from the catalyst via the completeoxidation of ethene. This model resulted in a maximumobtainable selectivity of 86%. This seemed to be confirmed bythe fact that higher selectivities had never been reached.However, during the last 20 years, this idea started to changeand atomic oxygen has now been confirmed to be the activespecies in all recent studies. One of the first studies to clearlyidentify atomic oxygen as the active species was research thatwas conducted by van Santen and de Groot, 25 who studied theinitial reaction rates for silver surfaces precovered by eitheratomic or molecular oxygen. In that study, the surfaces that wereprecovered with atomic oxygen had significantly higher etheneoxide production rates than the surfaces that were precoveredwith molecular oxygen.An explanation for the unique characteristics of silver in theepoxidation of ethene is found in its ability to dissociativelyadsorb oxygen, which is weakly bound at high coverages. 26 If oxygen is unable to dissociate, the epoxidation will not occur.When the oxygen - metal bond is too strong, the formation of an epoxide is thermodynamically impossible. Oxygen adsorbedon silver is also able to activate the C - H bond of ethene,explaining the selectivities that were  < 100%. However, thisreaction is slower than the epoxidation reaction and the sitesmost active for this reaction can be blocked using promoters.The generally accepted reaction scheme for the epoxidationof ethene over a silver catalyst is represented in Figure 4. Thisreaction scheme indicates that the complete oxidation occursas both parallel and sequential reactions. The oxidation rate of the epoxide ( k  3 ) is very small, compared to reactions 1 and 2. 27 The reactions are zero order in oxygen and first order in ethene,within the operating window of the process. When the samesilver catalyst is applied in the epoxidation of propene, thereaction scheme in Figure 4 is also determined to be valid andidentical reaction orders are found; however, the selectivitytoward propene oxide is very low (usually  < 10%). The rateconstant for the complete oxidation of propene oxide is againextremely low and negligible. The low selectivity for propeneoxide is primarily caused by the high rate of the completeoxidation of propene, which is almost a factor of 10 faster thanthe complete oxidation of ethene. On the other hand, theepoxidation reaction of propene is almost a factor of 10 slowerthan the epoxidation reaction of ethene. The next paragraphdiscusses the differences in reactivity of propene and etheneon a molecular scale to explain these differences in reactivity. 3.3. Reaction Mechanism in the Ethene and PropeneEpoxidation.  Carter and Goddard 28 presented a comprehensiveoxyradical mechanism to explain the differences in reactivityof ethene and propene in both the epoxidation and the completecombustion. This mechanism is based on valence-bond calcula-tions on the thermodynamics of the different reaction steps andis verified by experimental results. The active species is assumedto be a surface atomic oxyradical, which is adsorbed atomicoxygen with its unpaired electron pointing away from thecatalyst surface. This is in contrast to the di- σ  oxide-type oxygen,which has both electrons bonded to the silver surface. Thethermodynamic calculations suggest that the oxyradical typeoxygen will only form at higher oxygen surface coverages. Thisis consistent with reported higher epoxidation selectivities athigher oxygen surface coverages. 26 This increasing selectivityof the silver catalyst at an increasing oxygen coverage wasexplained by Lambert et al. 29 by a decreasing valence chargedensity. The most common promoter for the reaction, chlorine,has a similar electronic effect on the adsorbed atomic oxygen:chlorine (or another halogen) adsorbed on silver decreases thevalence charge on oxygen adatoms, thereby favoring O-insertioninto the C d C bond, rather than C - H cleavage followed bycombustion. Figure 3.  Typical scanning electron microscopy (SEM) image of silver(18 wt %) on an R  -alumina catalyst, such as that used in ethene epoxidationprocesses. The silver particles are the light-colored particles visible in thepicture. (Picture taken as a mixture between backscattered electrons (30%)and secondary electrons (70%), to get an optimal contrast between silverand alumina.) Figure 4.  Reaction scheme for the epoxidation and combustion of ethene. 3450  Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006
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