Polymer-supported Mo(Ⅵ) Alkene Epoxidation Catalysts

- Activity, Catalyst Structure and Support Morphology

D. C. Sherrington and S. Leinonen

Dept. of Pure and Applied Chemistry, Univ. of Strathclyde, UK

Over the last few years a range of highly active and selective polymer-supported Mo(Ⅵ) alkene epoxidation catalysts have been developed in the laboratory of one of the present authors with the potential to replace the homogeneous Mo(Ⅵ) catalyst systems currently used industrially. In each case the polymer-bound catalyst is used with t-butylhydroperoxide (TBHP) as the mono-oxygen source. The earliest examples employed polystyrene- and polymethacrylate-based resins to which specific ligand structures e.g. aminomethylpyridine (PSHPAMP) were attached but later species have involved highly thermo-oxidatively stable resins such as polybenzimidazole (PBI) and polyimides as the support. In the case of polybenzimidazole resin no separate ligand was introduced onto the resin matrix, instead the benzimidazole residues of the backbone appear to act as the sites for co-ordination of the Mo(Ⅵ) centres. The PBI.Mo and other polymer imidazole-containing catalysts also differ from the polystyrene- and polymethacrylate-bound chelating ligand Mo complex catalysts in that the latter have to be pre-treated to form active catalysts by oxidation with TBHP prior to use as alkene epoxidation catalysts. In the absence of this pre-treatment this group of polymer-based catalysts can display long induction periods before becoming active and selective catalysts. In contrast, in the liquid phase epoxidation of propene PBI.Mo requires no pre-treatment and is a highly active, selective, stable and long lived catalyst as made.

PBI	PSHPAMP

Why these differences arise has so far not been clear and indeed a detailed knowledge of the molecular structure of the catalytic species, and of the morphology of the polymer support, has been essentially absent. Thanks to a large international collaborative effort we are now able to say much more about these systems, which provide an interesting comparison with closely related alumina-supported Mo-based alkene epoxidation catalysts.

Catalyst Molecular Structure

Treatment of PBI and PSHPAMP resins with MoO2(acac)2 followed by exhaustive washings generates the polymer catalysts PBI.Mo and PSHPAMP.Mo □as made□ samples. Activation of each with TBHP yields the corresponding □activated□ catalysts. Elemental composition suggests a nominal metal/ligand ratio of ∼1/2 and ∼1/1 respectively for the above; and FTIR spectra show the presence of Mo=O, and Mo-O-Mo bonds in all four samples. Electron paramagnetic resonance (EPR) spectra show that the content of Mo(Ⅴ) is very low in all cases (10-3∼10-4 %) and that the overwhelming content of Mo is in the +Ⅵ oxidation state. This is confirmed by X-ray photoelectron spectroscopy (XPS) which also indicates the binding energies of the Mo 3d5/2 and Mo 3d3/2 states in the □activated□ complexes to be ∼232.5eV and ∼235.6eV respectively, very similar to the data for Mo in MoO2(acac)2.

X-ray absorption fine structure analysis(EXAFS), and systematic modelling of the spectra obtained using structurally viable guesses, suggest that both the PBI.Mo □as made□ and □activated□ samples contain Mo species with an average structure(1). In contrast the PSHPAMP.Mo species □as made□ has an average structure(2), similar to (1), which changes rapidly to the oligomeric form (3) on activation with TBHP. These structural differences in part account for the different catalytic behaviour.

( 1 )		( 2 )

( 3 )

(bond lengths in Å)

Polymer-support morphology

Transmission electron microscopic (TEM) studies show the PBI.Mo □as made□ and □activated□ samples to be essentially identical. Likewise the PSHPAMP.Mo □as made□ and □activated□ species. However, there is a marked difference in morphology between the PBI and PSHPAMP-based species. The PBI has a remarkable population of large cells or voids ∼1-2□ in diameter, heavily interconnected by a second well-developed system of pores ∼0.05□ in diameter. The latter appears to lie between a fused mass of discrete rather uniform polymer globules each typically ∼0.1□ in size. A Mo K□ X-ray map shows the Mo to be evenly distributed throughout this latter system of pores. With the PSHPAMP-based species the morphology is typical of a □macroporous□ polystyrene resin. There are no large voids or cells, but nevertheless there is a clear porous texture, somewhat less developed than that of the PBI-based samples and possibly somewhat finer.

N2 sorption/desorption porosimetry and Hg intrusion porosimetry tend to conform the above visual morphological characteristics. The N2 BET surface areas are ∼35m2g-1 and ∼15m2g-1 respectively for the PSHPAMP and PBI-based species, and the corresponding dry pore volumes(from Hg intrusion) ∼0.4 and 1.5□g-1 respectively. In both cases the micropore content is very limited. Interestingly the PSHPAMP and PBI-based species sorb ∼ 0.4 and 1.5□ of toluene g-1 resin respectively, which correlates extremely well with the dry pore volume data from Hg intrusion.

Overall therefore the PBI-based species have a much more open pore structure than the PSHPAMP-based catalysts and this almost certainly makes a second important contribution to the high and immediate catalytic activity displayed by these species.

Acknowledgement

We acknowledge receipt of a Research Studentship for SL from The Neste Oy Foundation, and Resources for the EPSRC to complete the EXAFS studies at the Daresbury Laboratoy., UK. DCS is also grateful to The British Council and KOSEF for exchange funds, allowing participation at this conference.

References

1. D.C. Sherrington and S. Simpson, J. Catalysis, 131, 115 (1911).

2. D.C. Sherrington and S. Simpson, Reactive Polymer, 19, 13 (1993).

3. M.M. Miller and D.C. Sherrington, J. Chem. Soc., Perkin Ⅱ, 2091 (1994).

4. M.M. Miller and D.C. Sherrington, J. Catalysis, 152, 368 and 377 (1995).

5. J.H. Ahn and D.C. Sherrington, J. Chem. Soc., Chem. Comm., 643 (1996).

6. S. Leinonen, D.C. Sherrington, A. Sneddon, D. McLoughlin, J. Corker, C. Canevali, F. Marazzoni, G. van Alboda, J. Reedijk and S.D.N. Spratt, J. Catalysis, submitted.

7. Imamura, H. Sasaki, M. Shono and H. Kasai, J. Catalysis, 177, 72 (1998).