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Ethene oxidation over Ag-Al2O3: a transient technique approach.
cabalysisToday,10 (1991)‘397-400 Ekvier SciencepublishersB.V.,Amsterdam
ETHENE OXIDATIONOVER Ag-+O;
397
A TRANSIENTTECHNIQUEAlTROACH.
D. LAW and M. BOWKER_
LeverhulmeCentre for InnovativeCatalysisand Surface ScienceIRC, Universityof Liverpool,P.O. box 147, Liverpool,L69 33X, UK.
Over the years there has been considerabledebateover the details of the mechanismand kinetics of ethene epoxidation Ill. At one time it was thought to be a molecularly chemisorbedspecies of oxygen which was responsiblefor the selectiveroute to ethene oxide [21. More recently however, evidence generally points to atomic oxygen species bemg involvedin both selectiveand non-selective&xnbustive~routes [3-51. The work at Liverpool has been concerned with examining the nature of chemisorbedoxygen on a variety of silver surfaces, including supported catalysts &I, silver powder 171and at promoter e&xts on this adsorption 181. This work extends these studies to conditions under which ethene epoxidationactuallyoccursand utilisesan atmosphericpressuremicroreactorwhich is run under transient flow conditions;this will be describedin detail elsewhere. The experiment involves a silver catalyst supported on an alpha-ahnnina (12% by weightAg) preparedby the incipientwetnesstechniqte &sing a 1.2OMsolutionof AgNC& indeionisedwater).~Of~s~t~y~wasl~d~in~~erea~r~~~~~at~K, and was then thermallydecomposedup to a temperatureof 723K. The transientflow experimentsare carriedout in the followingway. The flowsare mass flowcontrolledto give a 10% 0, in He mix whichpassescontinuouslyover the catalyst PeriodicaUythen (every 60 seconds)a pulse of ethene is injectedinto the flow via a computercont&led samplingvalve (0.5ans volume&An exampleof the resultsof a seriesof such pulsesis shownin Figure 1; theseare car&d out at a temperatureof 513K, which is closeto the optimum yield of ethene oxide. Note that the conversionat this temperatureis very low (
393
produced. The highest selectivity is obtained about the same temperature at which the industrial production is carried out. The selectivity is also at the same level as reported by others for unpromoted catalysts 19-111. On single crystals, Campbell found similar selectivities J121, though Lambert consistently reports much lower values [51. It must
be noted
that in
previous studies using thermal desorption
methods,
we have
found that catalysts prepared in this
expose almost way exclusively (111) type planes [6J.
200
300
These show kinetics for oxygen adsorption
which
identical to A&II)
are nearly D31, with a
sticking probability of lo-6 at 300K and an adsorption
400
500
Time @eoonds) Fig. 1. Diagram showing formation of C,H,O, CO, and HrO from pulses of 0.5cm3CrH, (in 10% O,:He) at 523K.
activation
energy of 14kJmol-1. Only one atomic state of oxygen was found for low pressure dosing, with a peak at 55OK, though dosing at pressures >l torr and temperatures >45OK showed increased uptake of oxygen indicating that
Li
.
0
some bulk oxidation had taken
. . .
..a--.-l-.-l
.
2 3 4 1 NumberdDCEDoses
.
7.1
5
place [14]. It is noticeable that the epoxide
production
decrease
at about
begins the
to
same
Fig. 2. Plot of qH,O, CO, peak areas and ethene oxide selectivity versus number of DCE doses.
temperature at which oxygen desorption from the surface is occurring at a significant rate. High oxygen coverages appear to be associated with reasonable selectivity, whereas low coverages lead to combustion; this is a point which has b
referred to elsewhere Jl, 111,
and is discussed further below. Using this methodology, an investigation was carried out into the effects of chlorine deposition onto the catalyst surface upon the course of the reactlon. Organochlorlne compounds are
incorporated into commercial processes in
ord~~~p~ve~e
~l~~~~e~~de.
Thus 1,2~~0~~)
wasused as the
chlorinating agent. The DCE was added by manual injection into the feed to the reactor in a dosewlse fashion. Each dose corresponded to 0.05an3 of a 40~1HeDCE mixture, and the results of such additions are shown in Figure 2. The additive has little effect on the yield of ethene oxide, but severely reduces the combustion reaction, so dramatically improving the selectivity up to the >80% region. These findings are very similar to those reported by Campbell on Ag(111) obtained in a high pressure cell attached to a UHV machine [121;the results are not the same for the much rougher surface, the (110) plane [12,151. This lends further support to claims that single crystal work can glve us a greater insight into the details of catalytic reactions which are carried out on much more finely dispersed materials under more demanding conditions.
We were not able to directly determine the
coverage of the catalyst with chlorine atoms, but by comparison with the single crystal data, it can be estimated. From this, dose one corresponds with a coverage of about 0.4 monolayers on the metal component of the catalyst. This means that, since the number of molecules in each DCE dose is approximately 4 monolayers (based on TPD estimates of the metal area given in early work 161,which may be high compared with a high pressure treated sample), the reaction probability of the DCE with the total surface is around 0.1. It is tempting to draw a similarity between thebehaviour of chlorine and oxygen in that low coverages of both result in low selectivity, but high coverages result in good selectivity. Of course, the high oxygen coverages would not be expected to give as high a selectivity as for the halogen, since it is itself also a non-selective reactant. Also site size restriction, as proposed by Campbell and Paffett 1151would seem to be important in determining high efficiency production, though not the sole factor. Obviously, on these grounds, any additive which does not show island growth characteristic should show primary beneficial effect on selectivity, modification) will also play a role.
but secondary phenomena (such as electronic
In this short communication the effects of other
additives (such as promoters) cannot be reported, but these effects together with a more detailed presentation of this work will be given elsewhere. The main aim here has been to show that meaningful results can be obtained in pulsed flow experiments and transiently the reaction can be observed to take place with efficiencies similar to those reported from steady state reactors. Additionally, more detailed kinetic determinations can be obtained by line shape analysis and this will also be reported elsewhere. Supported silver catalysts are composed of the close packed (111) faces and show almost identical behaviour to the single crystal surface.
400
ACKNOWLEDGEMENTS The authors would like to express their gratitude to ICI C&P Ltd. for their provision of a studentship to DL, and to Leybold for the loan of the mass spectrometer system.
REFERENCES 1. For a review, see for instance, R. van Santen and H.P. Kuipers, Adv.Cataiysis 35(1987)265. 2. H. Voge and C. Adams, Adv.Catalysis 17(1%7)151. 3. C. Backx, J. Mooihuysen, P. Geenen and R van Santen, J.Catalysis 7209811364. 4. R van Santen and C.P. de Groot, JCatalysis 9809861530. 5. R Grant, C. Harbach, RM. Lambert and S.A.Tan, J.Chem.Soc. Faraday 183(1987)2035. 6. M. Dean and M. Bowker, Appl.Surf.Sci. 35(1988/89)27. 7. M. Bowker, P. Pudney and G. Roberts, J.Chem.Soc. Faraday 185098912635. 8. M. Dean and M. Bowker, J.Catalysis 115(1989)138. 9. S. Seyedmonir, J. Plischke, M. A. Vannice and H. Young, J.Cataiysis 1230990)X23. 10. E.Force and A.T. Be& J.Catalysis 40(1975)356. 11. L. Akella and H. Lee, J.Catalysis 99(1984)465. 12 C.T. Campbell, J.Catalysis 99W86128. 13. C.T. Campbell, SurfSci. 157(1985)43. 14. M. Dean, A. McKee and M. Bowker, Surf.Sci. 211/212(1989)1061. 15. C.T. Campbell and M.T. Paffett, Appl.Surf.Sci. 190984128.
ETHENE OXIDATIONOVER Ag-+O;
397
A TRANSIENTTECHNIQUEAlTROACH.
D. LAW and M. BOWKER_
LeverhulmeCentre for InnovativeCatalysisand Surface ScienceIRC, Universityof Liverpool,P.O. box 147, Liverpool,L69 33X, UK.
Over the years there has been considerabledebateover the details of the mechanismand kinetics of ethene epoxidation Ill. At one time it was thought to be a molecularly chemisorbedspecies of oxygen which was responsiblefor the selectiveroute to ethene oxide [21. More recently however, evidence generally points to atomic oxygen species bemg involvedin both selectiveand non-selective&xnbustive~routes [3-51. The work at Liverpool has been concerned with examining the nature of chemisorbedoxygen on a variety of silver surfaces, including supported catalysts &I, silver powder 171and at promoter e&xts on this adsorption 181. This work extends these studies to conditions under which ethene epoxidationactuallyoccursand utilisesan atmosphericpressuremicroreactorwhich is run under transient flow conditions;this will be describedin detail elsewhere. The experiment involves a silver catalyst supported on an alpha-ahnnina (12% by weightAg) preparedby the incipientwetnesstechniqte &sing a 1.2OMsolutionof AgNC& indeionisedwater).~Of~s~t~y~wasl~d~in~~erea~r~~~~~at~K, and was then thermallydecomposedup to a temperatureof 723K. The transientflow experimentsare carriedout in the followingway. The flowsare mass flowcontrolledto give a 10% 0, in He mix whichpassescontinuouslyover the catalyst PeriodicaUythen (every 60 seconds)a pulse of ethene is injectedinto the flow via a computercont&led samplingvalve (0.5ans volume&An exampleof the resultsof a seriesof such pulsesis shownin Figure 1; theseare car&d out at a temperatureof 513K, which is closeto the optimum yield of ethene oxide. Note that the conversionat this temperatureis very low (
393
produced. The highest selectivity is obtained about the same temperature at which the industrial production is carried out. The selectivity is also at the same level as reported by others for unpromoted catalysts 19-111. On single crystals, Campbell found similar selectivities J121, though Lambert consistently reports much lower values [51. It must
be noted
that in
previous studies using thermal desorption
methods,
we have
found that catalysts prepared in this
expose almost way exclusively (111) type planes [6J.
200
300
These show kinetics for oxygen adsorption
which
identical to A&II)
are nearly D31, with a
sticking probability of lo-6 at 300K and an adsorption
400
500
Time @eoonds) Fig. 1. Diagram showing formation of C,H,O, CO, and HrO from pulses of 0.5cm3CrH, (in 10% O,:He) at 523K.
activation
energy of 14kJmol-1. Only one atomic state of oxygen was found for low pressure dosing, with a peak at 55OK, though dosing at pressures >l torr and temperatures >45OK showed increased uptake of oxygen indicating that
Li
.
0
some bulk oxidation had taken
. . .
..a--.-l-.-l
.
2 3 4 1 NumberdDCEDoses
.
7.1
5
place [14]. It is noticeable that the epoxide
production
decrease
at about
begins the
to
same
Fig. 2. Plot of qH,O, CO, peak areas and ethene oxide selectivity versus number of DCE doses.
temperature at which oxygen desorption from the surface is occurring at a significant rate. High oxygen coverages appear to be associated with reasonable selectivity, whereas low coverages lead to combustion; this is a point which has b
referred to elsewhere Jl, 111,
and is discussed further below. Using this methodology, an investigation was carried out into the effects of chlorine deposition onto the catalyst surface upon the course of the reactlon. Organochlorlne compounds are
incorporated into commercial processes in
ord~~~p~ve~e
~l~~~~e~~de.
Thus 1,2~~0~~)
wasused as the
chlorinating agent. The DCE was added by manual injection into the feed to the reactor in a dosewlse fashion. Each dose corresponded to 0.05an3 of a 40~1HeDCE mixture, and the results of such additions are shown in Figure 2. The additive has little effect on the yield of ethene oxide, but severely reduces the combustion reaction, so dramatically improving the selectivity up to the >80% region. These findings are very similar to those reported by Campbell on Ag(111) obtained in a high pressure cell attached to a UHV machine [121;the results are not the same for the much rougher surface, the (110) plane [12,151. This lends further support to claims that single crystal work can glve us a greater insight into the details of catalytic reactions which are carried out on much more finely dispersed materials under more demanding conditions.
We were not able to directly determine the
coverage of the catalyst with chlorine atoms, but by comparison with the single crystal data, it can be estimated. From this, dose one corresponds with a coverage of about 0.4 monolayers on the metal component of the catalyst. This means that, since the number of molecules in each DCE dose is approximately 4 monolayers (based on TPD estimates of the metal area given in early work 161,which may be high compared with a high pressure treated sample), the reaction probability of the DCE with the total surface is around 0.1. It is tempting to draw a similarity between thebehaviour of chlorine and oxygen in that low coverages of both result in low selectivity, but high coverages result in good selectivity. Of course, the high oxygen coverages would not be expected to give as high a selectivity as for the halogen, since it is itself also a non-selective reactant. Also site size restriction, as proposed by Campbell and Paffett 1151would seem to be important in determining high efficiency production, though not the sole factor. Obviously, on these grounds, any additive which does not show island growth characteristic should show primary beneficial effect on selectivity, modification) will also play a role.
but secondary phenomena (such as electronic
In this short communication the effects of other
additives (such as promoters) cannot be reported, but these effects together with a more detailed presentation of this work will be given elsewhere. The main aim here has been to show that meaningful results can be obtained in pulsed flow experiments and transiently the reaction can be observed to take place with efficiencies similar to those reported from steady state reactors. Additionally, more detailed kinetic determinations can be obtained by line shape analysis and this will also be reported elsewhere. Supported silver catalysts are composed of the close packed (111) faces and show almost identical behaviour to the single crystal surface.
400
ACKNOWLEDGEMENTS The authors would like to express their gratitude to ICI C&P Ltd. for their provision of a studentship to DL, and to Leybold for the loan of the mass spectrometer system.
REFERENCES 1. For a review, see for instance, R. van Santen and H.P. Kuipers, Adv.Cataiysis 35(1987)265. 2. H. Voge and C. Adams, Adv.Catalysis 17(1%7)151. 3. C. Backx, J. Mooihuysen, P. Geenen and R van Santen, J.Catalysis 7209811364. 4. R van Santen and C.P. de Groot, JCatalysis 9809861530. 5. R Grant, C. Harbach, RM. Lambert and S.A.Tan, J.Chem.Soc. Faraday 183(1987)2035. 6. M. Dean and M. Bowker, Appl.Surf.Sci. 35(1988/89)27. 7. M. Bowker, P. Pudney and G. Roberts, J.Chem.Soc. Faraday 185098912635. 8. M. Dean and M. Bowker, J.Catalysis 115(1989)138. 9. S. Seyedmonir, J. Plischke, M. A. Vannice and H. Young, J.Cataiysis 1230990)X23. 10. E.Force and A.T. Be& J.Catalysis 40(1975)356. 11. L. Akella and H. Lee, J.Catalysis 99(1984)465. 12 C.T. Campbell, J.Catalysis 99W86128. 13. C.T. Campbell, SurfSci. 157(1985)43. 14. M. Dean, A. McKee and M. Bowker, Surf.Sci. 211/212(1989)1061. 15. C.T. Campbell and M.T. Paffett, Appl.Surf.Sci. 190984128.