Reports: G5

46131-G5 Ab Initio Study of Support Effects in the Direct Oxidation of Propylene to Propylene Oxide

Randall Meyer, University of Illinois (Chicago)

1.Introduction

Propylene oxide (PO) is one highest production commodity chemicals The current industrial methods of producing propylene oxide are either not profitable or are environmental unfriendly due to the production of either chlorinated or peroxycarboxylic acid waste1. However, despite the considerable research effort, there is no current industrial scale direct partial oxidation process for propylene epoxidation using molecular oxygen over a heterogeneous catalyst. If novel active and selective catalysts could be developed, their potentially economic and environmental impact can be considerable. Recently direct partial oxidation routes from propylene to propylene oxide have been developed using Au/TS-1 or AuPd/TiO2 catalysts by co-feeding hydrogen.2-4 The leading hypothesis for this reaction mechanism is that hydrogen and oxygen first react to produce hydrogen peroxide which then reacts with propylene to produce the epoxide5.  Density functional theory calculations from Staykov et al. have given some indication of the reaction mechanism for hydrogenation of oxygen to produce hydrogen peroxide6. However, those authors did not consider hydrogen assisted dissociation of oxygen in their calculations. Therefore we have attempted to extend their work examining hydrogen peroxide formation on Pd(111) and Pt(111) surfaces.

2. Results

Calculations were performed using the VASP code, a plane wave periodic supercell density functional theory software package,7 to explore potential pathways for the formation of hydrogen peroxide over the Pd(111), and Pt (111) surfaces.  The following reaction steps must be considered:

H2 + *  ↔  H2*                                   
H2* + * ↔  2 H*                                
O2 + * ↔  O2*                                   
O2* + * ↔  2 O*                                 
O2* + H* ↔  OOH*  + *                   
OOH* +  H* ↔ HOOH* +*               
HOOH* ↔  HOOH + *                     
OOH* + * ↔ OH* + O*                   
HOOH* + * ↔ OH* + OH*               
Frequently the first two steps are combined in a single step as the dissociative adsorption of hydrogen. On the other hand, the dissociative adsorption of oxygen is not desirable in the formation of hydrogen peroxide as oxygen atoms will react with hydrogen to produce water instead. We find that the barrier for the hydrogenation of O2 on Pd(111) is 0.90 eV. This barrier is higher than that for O2 dissociation (0.63 eV) on the same surface. Furthermore, the barrier for OOH dissociation into an oxygen atom and a hydroxyl group is even lower (0.47 eV). Therefore unpromoted palladium catalysts should not be expected to be good hydrogen peroxide catalysts in agreement with literature results8.  On Pt(111) the barriers for the hydrogenation steps in the mechanism are lower than on Pd(111). However, the barriers for OOH and H2O2 are not sufficiently high enough to prevent decomposition. Interestingly, in the presence of sub-surface hydrogen, the barrier to the dissociation of oxygen rises to 1.14 eV while the barrier for the hydrogenation of O2 drops to 0.68 eV.
In the previous year, we had calculated the pathways for propylene epoxidation on an Ag3 cluster. We found that the lowest energy pathway for propylene oxide formation from atomic oxygen and propene involved a barrier of 1.15 eV (not including the 2.5 eV barrier for O2 dissociation). On the Ag(111) surface, we find that the barrier for O2 dissociation is now lowered to 0.72 eV.  Propylene epoxidation then occurs exothermically (−0.83 eV) with a barrier of 0.71 eV in agreement with the results of Lambert et al.9. The presence of sub-surface oxygen and its role in epoxidation catalysis has long been debated10-12. We have examined the epoxidation of propylene on an Ag(111) surface with 0.25 ML of subsurface oxygen. In the presence of subsurface oxygen, the barrier to propylene epoxidation plummets to 0.20 eV.

Previous work from Haruta involving Au/TiO2 catalysts has revealed large differences between anatase (which is active) and rutile (which is not)13. Therefore although this grant is now officially complete, work examining supported Ag clusters on TiO2 will be pursued by undergraduate researchers at UIC under Prof. Meyer’s supervision.
References

[1]        K. Weissermel, H.J. Arpe, Industrial Organic Chemistry,4th ed.,2003, Weinheim, Germany: Wiley-VCH.

[2]        J. Edwards, P. Landon, A.F. Carley, G.J. Hutchings, J Mater Res 22 (2007) 831-837.

[3]        A.K. Sinha, S. Seelan, S. Tsubota, M. Haruta, Top Catal 29 (2004) 95-102.

[4]        T.A. Nijhuis, M. Makkee, J.A. Moulijn, B.M. Weckhuysen, Ind. Eng. Chem. Res. 45 (2006) 3447-3459.

[5]        B. Taylor, J. Lauterbach, G.E. Blau, W.N. Delgass, J Catal 242 (2006) 142-152.

[6]        A. Staykov, T. Kamachi, T. Ishihara, K. Yoshizawa, J Phys Chem C 112 (2008) 19501-19505.

[7]        G. Kresse, J. Furthmuller, Phys Rev B 54 (1996) 11169-11186.

[8]        Q.S. Liu, J.C. Bauer, R.E. Schaak, J.H. Lunsford, Angew Chem Int Edit 47 (2008) 6221-6224.

[9]        D. Torres, N. Lopez, F. Illas, R.M. Lambert, Angew Chem Int Edit 46 (2007) 2055-2058.

[10]      Y. Xu, J. Greeley, M. Mavrikakis, J Am Chem Soc 127 (2005) 12823-12827.

[11]      J.G. Serafin, A.C. Liu, S.R. Seyedmonir, J Mol. Catal.  131 (1998) 157-168.

[12]      V.I. Bukhtiyarov, M. Havecker, V.V. Kaichev, A. Knop-Gericke, R.W. Mayer, R. Schlogl, Catal Lett 74 (2001) 121-125.

[13]      M. Haruta, B.S. Uphade, S. Tsubota, A. Miyamoto, Res Chem Intermediat 24 (1998) 329-336.