Propylene oxide oxidation— polymeric residue

The oxidation of propylene oxide on porous polycrystalline Ag films supported on stabilized zirconia was studied in a CSTR at temperatures between 240 and 400°C and atmospheric total pressure. The technique of solid electrolyte potentiometry (SEP) was used to monitor the chemical potential of oxygen adsorbed on the catalyst surface. The steady state kinetic and potentiometric results are consistent with a Langmuir-Hinshelwood mechanism. However over a wide range of temperature and gaseous composition both the reaction rate and the surface oxygen activity were found to exhibit self-sustained isothermal oscillations. The limit cycles can be understood assuming that adsorbed propylene oxide undergoes both oxidation to CO2 and H2O as well as conversion to an adsorbed polymeric residue. A dynamic model based on the above assumption explains qualitatively the experimental observations. 

There is evidence for isomerization of chemisorbed propylene oxide to acrolein on silver and for surface polymer formation on metal oxide catalysts (11,12). Formation of a surface polymeric structure has also been observed during propylene oxidation on silver (13). It appears likely that the rate oscillations are related to the ability of chemisorbed propylene oxide to form relatively stable polymeric structures. Thus chemisorbed monomer could account for the steady state kinetics discussed above whereas the superimposed fluctuations on the rate could originate from periodic formation and combustion of surface polymeric residues. 

In an attempt to prepare polycyclopentadiene which would be stable in toluene solution, the polymer was hydrogenated over a platinum oxide catalyst in a Parr bomb immediately after the completion of the polymerization reaction. Infrared analysis indicated the presence of residual unsaturation and the polymer became insolubilized on standing. An attempted copolymerization of cyclo-pentadiene with propylene gave a product whose infrared spectrum indicated the presence of C-methyl groups but which was still insoluble in toluene. No attempt was made to determine whether copolymerization had occurred. 

Different diluent-phase polypropylenes showed similar behaviour but the proportion of affected powder particles varies. Clearly these local variations would tend to disappear in the processing of the polymer to produce stabilised pellets. Variations in oxidation rates of individual particles were also found with gas-phase polymerized propylenes (Figure 11), in this case the catalyst residues could not be seen. 

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