NaOH modified WO3/SiO2 catalysts for propylene production from 2-butene and ethylene metathesis

A WO3/SiO2 catalyst is used in industry to produce propylene from 2-butene and ethylene metathesis. Catalysts with various WO3 loading (4% to 10%) were prepared by impregnation and tested for the metathesis of ethene and trans-2-butene. Ion exchange of NaOH onto the WO3/SiO2 catalyst was used to mitigate the acidity of the catalysts in a controlled way. At low WO3 loading, the treatment with large amounts of NaOH resulted in a significant decrease in metathesis activity concomitant with significant W leaching and marked structural changes (XRD, Raman). At higher WO3 loading (6% to 10%), the treatment with NaOH mainly resulted in a decrease in acidity. FT-IR experiments after adsorption of pyridine showed that the Lewis acidic sites were poisoned by sodium. Nevertheless, the metathesis activity remained constant after the NaOH treatment. This suggested that the remaining acidity on the catalyst was enough to ensure the efficient formation of the carbene active sites. Interestingly, Na poisoning resulted in some modification of the selectivity. The mitigation of acidity was shown to favor propene selectivity over the formation of isomerization products (cis-2-butene, 1-butene, etc.). Moreover, treatment with NaOH led to a shorter induction period and reduced coke formation on the WO3/SiO2 catalyst.