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Progress in Catalyst for Ethylene Oxide (EO) Production and

Epoxidation Mechanism

Katsumi NakashiroProcess 2 Laboratory, Petrochemicals Research Center, Mitsubishi Chemical

Corporation, 1, Toho-cho, Yokkaich-city, Mie, Japan

AbstractReactivity of adsorbed O2 molecule and O atom adsorbed on Ag surface for

ethylene oxide (EO) formation and complete oxidation were examined with densityfunctional (DF) method. Ag clusters of Ag3 to Ag10, of which surfaces wereconstructed similar to Ag (111) facet, were modeled for EO catalyst. DF studies have

shown that not only the size of the clusters, but also the configuration of the Ag atomsdecisively effects on HOMO level of the clusters and, thus energy of O2 adsorption,indicating that both parameters are important for an accurate simulation of O2adsorption.

Investigation of reactivity of adsorbed O2molecule and O atom has shown that EOis formed by both of the O species. In order to understand complete oxidation route,H-migration and H-abstraction leading to acetaldehyde, radical intermediate,respectively, were examined. It has been proven that energy of transition state for theH-migration is lower than that for the H-abstraction in oxidation by both the O species,

suggesting the H-migration route as complete oxidation.It is interesting to note that reactivity of adsorbed O2molecule has been proven tosuppress complete oxidation because the energy of transition state for completeoxidation is much higher by ca. 28 kcal/mol than that for epoxidation. On the otherhand, adsorbed O atom has given rise to EO formation slightly more than or almost asmuch as complete oxidation, since EO formation is little easier by ca. 4 kcal/mol. Inconclusion, EO selectivity of O2molecule adsorbed on Ag surface is remarkably high,while O atom is slightly selective.

Finally, an upper limit of EO selectivity (85.7% on a C2H4basis, corresponding to50% on an O2basis) under a mechanism, in which adsorbed O2molecule and O atomlead to epoxidation, combustion, respectively, is supposed to be invalid even by eitheroxygen species.

Introduction

Ethylene oxide (EO) has been an important chemical substance until now. Atpresent EO is produced with the amount of ca. 12 million tons per year. A severalpercent growth rate of an EO demand has been continuing until now and been predictedsuccessively in future. More than a half of EO produced is hydrated into ethyleneglycol (EG) for a resource of polyester resin (i.e. fiber, film and bottle), of which a

major product is polyethylene telephtalate (PET). Other than polyester, EO is


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80

90

100

consumed for an intermediate of surfactant, antifreeze, etc. In some decades, ademand of polyester resin, especially fiber in Asia, is expected to grow successively infuture and, thus EO as an intermediate.

A higher selectivity of ethylene for EO has been eager, since a cost of EOproduction is strongly dependent on that of ethylene consumed. A selectivity for EO issummarized in Fig. 1.

40

50

60

70

1920 1940 1960 1980 2000

C2H4selectivity

forEO/%

ChlorohydrineDirect oxidationHigh s elect ive ca t.Upper limit ?

Figure 1. Improvement of EO catalystperformance

In direct oxidation of ethylene, an improvement of EO selectivity has been carriedout by process developments, then by catalyst developments as follows:

1) Deposition of highly dispersed Ag particles (Shell, at 1972)12) Addition of Cs for silver modification (Shell, at 1976)23) Surface-area enlargement of support and its modification (Nippon

Shokubai since 1983,3later Mitsubishi Chemical,4recently Shell)4) Addition of Re, W, etc. to Ag-Cs component for high selective catalyst

(Shell since 1988,4later Mitsubishi Chemical5)

It is found that a current EO selectivity has become superior to that in chlorohydrinmethod, and that Shell had played an important role in development of EO catalyst. At

present, the highest selectivity reported in patent seems higher than an upper limit,which used to be predicted by a mechanism, in which adsorbed O2 molecule and Oatom exclusively lead to epoxidation and complete oxidation, respectively, which isabbreviated to the adsorbed O2 mechanism. In general, the adsorbed O2mechanismhas prevailed since studies by Kilty et al. in 1970s.6,7 The adsorbed O2mechanism,however, seems controversial in these days. On the other hand, epoxidation byadsorbed O atom has now been suggested, which is abbreviated to the adsorbed O

mechanism. The reason is that the upper limit of EO selectivity is unlikely to be true,


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as shown in Fig. 1, and that O2 molecule adsorbed on Ag surface is much lower instability than O atom, which is demonstrated with TPD and LEED,8,9and STM.10

From viewpoint of theoretical chemistry, several reports have been published withthe extended Hckel,11,12LCAO-X ,13Hartree-Fock (HF) + MP3,14GVB-CI15,16anddensity functional (DF) method.17,18 However, a comparison between energetics in EOformation and complete oxidation has been made in few reports. Reactivity ofadsorbed O atom and O2 molecule as an oxidant has rarely been understood by adifference in energetics of both the oxidations.

Carter and Goddard III evaluated energetics in both epoxidation and completeoxidation on GVB-CI level,15,16but an oxidant of ethylene was only adsorbed O atom.It was concluded that an energy barrier in H-radical abstraction followed by completeoxidation was so high that ethylene oxidation should be selective for EO formation.

Nakatsuji et al. investigated in detail reactivity of adsorbed O2molecule and O atom

with HF + MP2 and SAC-CI method using their original model, dipped adcluster model(DAM) consisting of Ag2 cluster.

19-21 However, it was difficult to describe with theDAM complete oxidation route, in which dynamic electron transfer is allowed torepresent the positively or negatively charged state by both oxygen species.

The objective of this work is to evaluate reactivity of adsorbed O2molecule and Oatom to ethylene into EO formation and complete oxidation with a DF method, in orderto clarify reaction mechanisms of both oxidations. On EO catalysts Ag clusters fromAg3to Ag10, of which surfaces are constructed similar to Ag (111) face, were modeled,since Ag (111) facet is mainly generated in industrial EO catalysts, in which Ag

particles are from sub-microns to microns in diameter. For detailed studies of EOformation and complete oxidation, a Ag5 cluster was applied to EO catalyst. Forcomplete oxidation, two routes, i.e. acetaldehyde formation via H-migration and vinylradical intermediate formation via H-abstraction were evaluated.

Details of Calculation Method

A DF method used in this work is the hybrid of HF and DF methods.22,23 Thismethod is implemented in Gaussian 98 program. Parameterization is the onesuggested by Pople et al.,23 i.e., 0.2, 0.8, and 0.72 for the HF, Slater,24 and Becke

exchange functionals,

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and 0.19 and 0.81 for the Vosko-Wilk-Nusair

26,27

andLee-Yang-Parr28 correlation functionals. The Los Alamos effective core potentials(ECP) is used for the 1s to 4p electrons of Ag atoms along with the correspondingvalence basis sets.29-31 For H, C, and O atoms, Dunning-Huzinaga full double zeta(D95) basis set is used, (10s5p)/[3s2p] and (4s)/[2s], respectively.32

The DF method calculations are performed is as follows: At first, the structures oflocal minima and transition states are optimized using the basis sets just mentioned.Then, at the equilibrium structures, the energies are reevaluated with a larger basis set,in which the diffuse s and p functions of = 0.059 and the polarization d functions of = 2.704 and 0.535 are added on the O atoms, as used by Nakatsuji et al.19-21Throughout this work, the structure of Ag clusters is fixed, and the structure and relative


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orientation of ethylene and oxygen with respect to the cluster is optimized.The stabilization energy was defined as follows: E = {E(Ag cluster) + E(C2H4) + x E(O2or O) } - E(combined system),where x is 1 or 1/2 depending on oxygen molecule or atom, respectively. Positive

value in E means exothermic adsorption or reaction, i.e. thermodynamically easystep to proceed.

Results and Discussion

1.1. Adsorption of O2Molecule on AgnClusters (n=3-10)

Eight Agnclusters with n = 3 to 10 were constructed, and twelve configurations ofO2 adsorption on them were studied in energetics. Their optimized structures aresummarized in Fig. 2. Dependence of O2adsorption energy on Ag number of the Agn

clusters is presented in Fig. 3. It seems that the larger number of Ag in the clusters

gives the smaller O2 adsorption energy, though the dependence is not always strict.The reason of this complicated behavior is because energy of O2 adsorption is

dependent on HOMO energy level of the clusters, rather than the cluster size. That is,HOMO level results not only from the Ag number in the clusters, but also from theconfiguration of Ag atoms and the spin multiplicity of the cluster electronic states,which depends on electron number (even or odd), since a Ag atom is d10s1valence inelectron configuration.

0

510

15

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2 3 4 5 6 7 8 9 10 11

Ag Number in a cluster

Energyo

fO2Adsorption/kcal/mol

0

510

15

20

25

30

35

40

45

-5 -4.5 -4 -3.5 -3

HOMO / eV

Energyo

fO2Adsorption/kcal/m

Figure 4. Dependence of the O2

adsorption energy on the HOMO level

Figure 3. Dependences of O2adsorption

ener on A number of the A clusters

Dependence of the O2adsorption energy on the HOMO level is plotted in Fig. 4.It shows that the higher HOMO level leads to the higher energy of O2 adsorption,indicating electron transfer from the clusters to adsorbed O2molecule. For example,adsorbed O2 molecule on a Ag5 cluster is negatively charged totally at -0.580e (ca.

-0.29e in each O atom). In conclusion, not only the size but also the spin multiplicity


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and the atomic configuration of model clusters should be taken care of in order toinvestigate O2adsorption systems.

A model cluster is requested to have enough area to represent both epoxidation andcomplete oxidation, where O2molecule or O atom and C2H4are coadsorbed and reactmutually. Therefore, a Ag5cluster with lozenge configuration was selected for furtherinvestigation. A HOMO level and O2 adsorption energy with the Ag5 cluster have

been calculated to be -3.62 eV and 15.7 kcal/mol, while -4.74 eV and 16-25 kcal/moldetermined with UPS,33 adsorption measurements,6 respectively. Thus, our valuesobtained with DF calculation are in good agreement with those measuredexperimentally. Discussion on reaction mechanisms is focused on comparison

between two competing reactions.

1.2. Oxidation by Adsorbed O2Molecule on Ag (111) Surface

It is reported that adsorption of O2and C2H4on Ag surface takes place as follows34

:1) O2is adsorbed on Ag surface at room temperature,2) C2H4is not adsorbed on clean Ag surface, but does on Ag surface after

O2adsorption.On the other hand, adsorbed O2molecule on Ag surface has been known to be so

reactive to dissociate for two of adsorbed O atoms even at room temperature, which ismuch lower than a temperature of 200-300 in EO formation, as published with TPDand LEED,8,9and STM.10

According to above observations reported, oxidation by adsorbed O2molecule isassumed to occur after O

2adsorption and, then O

2-C

2H

4coadsorption on Ag surface, in

the Rideal-Elay mechanism. Structures in O2, O2/C2H4and EO adsorption states wereoptimized, and then transition states between both the states were pursued. Thus, twotransition states (TS1, TS2) were determined, showing that the above reaction scheme isreasonable. The energy diagram obtained is presented in Fig. 5. It is found thatO2-C2H4coadsorption is less stable than O2adsorption, and that the first transition state(TS1) between them exists, suggesting that O2-C2H4 coadsorption is a process withactivation energy. This phenomenon is consistent with experimental results.34 It isnoticeable that the next transition state (TS2) is the least stable in the EO formationroute, while the first transition state (TS1) has similar stability to reactant state.


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0

15.7

-1.6

4.3

-5.2

33.529.2

-28.9

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-30

-20

-10

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Ag + O2 Ag-O2 TS1 Ag-O2-

C2H4

TS2 Product

Formationenergy/kcal/mol

EO

acetaldehyde(CO2/H2O)

TS2 for EO

1.481

1.515 1.855

TS2 for acetaldehyde

1.379 1.489

1.332

1.377

1.531

Figure 5. Energy diagram in epoxidation and complete oxidation by adsorbed O2

On the other hand, for studies of complete oxidation route, H-migration for

acetaldehyde and H-abstraction for radical intermediate, the corresponding transition

state (TS2) was searched, since we assumethat the TS1 is common for all the routes.

It is found that H-migration is slightly more plausible than the H-abstraction bycomparison of both TS energies. The structure and energy for H-migration into

acetaldehyde (TS2) as complete oxidation route are superimposed in Fig. 5. The TS2

is surprisingly much more unstable than TS2, indicating that the first O atom of

adsorbed O2 should bring about EO formation in high selectivity, in consistent with

results of Nakatsuji et al.19-21 Large difference (23.7 kcal/mol) in activation energy for

EO formation and complete oxidation means that the first atom of O2may give almost

100% in EO selectivity.

After TS2 for EO and H-migration for acetaldehyde, large stabilization energies

account for change in the heat of formation (from C2H4to EO or acetaldehyde) and the

adsorption energy by O atom (13.0 kcal/mol), which is remained after consumption of

the first O atom of adsorbed O2molecule.

1.3. Oxidation by Adsorbed O Atom on Ag (111) Surface

Oxidation by adsorbed O atom seems to occur with the Redeal-Eley mechanism

similar to that by adsorbed O2molecule, discussed in a previous section. Generation

of adsorbed O atom on Ag surface is supposed to results from either dissociative

adsorption of O2 molecule or consumption of the first O atom from adsorbed O2

molecule. Optimization of structures in O, O-C2H4 and EO adsorption states was

performed and a transition state before EO formation was pursued. The energetics in


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0

13

19.8

2.2

18.1 16.2

-1.5

45.9

-30

-20

-10

0

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Ag + 1/2 O2 Ag-O Ag-O-C2H4 TS Product

Formationenergy/kcal/mol

EO

acetaldehyde(CO2/H2O)

1.488 1.294

1.331 1.470

TS for acetaldehyde

Figure 6. Energy diagram in epoxidation and complete oxidation by adsorbed O

atom

epoxidation by adsorbed O atom are summarized in Fig. 6.On the other hand, complete oxidation routes by adsorbed O atom H-migration for

acetaldehyde and H-abstraction for radical intermediate were also estimated, similar to

those by adsorbed O2molecule. Corresponding transition states were obtained. It isfound that adsorbed O atom gives H-migration proceeds slightly more easily thanH-abstraction, which is the same tendency as adsorbed O2 molecule. Energetics forcomplete oxidation is presented also in Fig. 6, accompanying with those for epoxidation.Formation energy of adsorbed O atom from Ag cluster and 1/2 O2 molecule iscalculated to be 13.0 kcal/mol. As shown in Fig. 6, both transition states forepoxidaiton (TS) and complete oxidation (TS) have the highest energies on the path.It is important that difference between energies of both transition states is only 3.7kcal/mol and much smaller than that with adsorbed O2molecule, suggesting that EO

selectivity of adsorbed O atom is much lower than that of O2molecule, though adsorbedO atom is still selective to some extent. This result is consistent with reports byNakatsuji et al.19-21 And the reason why our results are different from those of Carterand Goddard III seems that their complete oxidation route is not H-migration butH-radical abstraction.

Here, consider relationship between difference in activation energies forepoxidation and combustion, and selectivity for EO. For example, if the differencewould be nothing in oxidation by adsorbed O atom, EO selectivity should be 50%,corresponding to 85.7% on a C2H4basis due to a following equation:


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H2C CH2

O

H2C CH2 O2+ CO2+7 6 2 H2O+ 26

Therefore, selectivity of adsorbed O atom is supposed to be slightly higher than85.7%, since adsorbed O atom is slightly selective by ca. 4 kcal/mol. Selectivity of

adsorbed O2molecule is much higher than 85.7%, since the first O atom of adsorbed O2molecule seems nearly 100%, while the second O atom has the same selectivity asadsorbed O atom. It is concluded that selectivity of O should be higher than 85.7% ona C2H4basis, even under the adsorbed O or O2 mechanism. This seems to disagreeagainst actual selectivity of EO catalyst, as observed in Fig. 1. It is supposed thatfurther investigation is necessary in order to clarify this point.

Conclusion

It is concluded that epoxidation into EO is reasonably brought about by both O2

molecule and O atom adsorbed on Ag surface, and that complete oxidation which issupposed not H-abstraction but H-migration into acetaldehyde, radical intermediate,respectively, is ascribed to adsorbed O atom only. In case of adsorbed O2mechanismepoxidation of C2H4may precede dissociative adsorption of O2molecule.

It is, however, unclear which oxygen species are responsible for EO formation atthe present study. The reason is partly because EO selectivity observed in actual EOcatalysts is lower than that expected by this theoretical study, i.e. higher than 85.7%. Itseems that lower selectivity of actual EO catalysts imply epoxidation by adsorbed Oatom, since the difference between activation energy of epoxidation and combustion is

small, but that activation energy observed in O2-C2H4 coadsorption suggests theadsorbed O2 mechanism. According to our results, an upper limit of EO selectivity(85.7% on a C2H4 basis), which is expected by the adsorbed O2 mechanism, maybenegligible. We have found that further studies are necessary to clarify which oxygenspecies bring about epoxidation. Many kinds of elements like alkaline as a dopant andhalogen compounds as a reaction moderator, which has been utilized in industrialEO/EG process, may control these reaction routes.

Finally, it is pointed out that consideration of modeled clusters and understanding ofall reaction routes proposed, e.g. epoxidation, combustion (via H-migration,H-abstraction), are important for investigation of reaction mechanisms. For this

purpose, DF method is found to be useful.

Acknowledgement

This work has been collaborated with Hisayoshi Kobayashi, Professor of KurashikiUniversity of Science and the Arts, Japan. I thank Professor Kobayashi so much foraccomplishing difficult DF calculation, discussing reaction mechanisms.


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