Study of structure–performance relationships in Meerwein–Ponndorf–Verley reduction of crotonaldehyde on several magnesium and zirconium-based systems

Several magnesia and zirconia systems were synthesized through the sol–gel process (calcination temperature in the 175–600 °C range) and tested for liquid and gas-phase Meerwein–Ponndorf–Verley reduction of crotonaldehyde with 2-propanol. In the liquid phase, only zirconia systems were active, probably because carbonates and water “poison” active sites in basic magnesia. Moreover, the more surface OH groups were present in zirconia solids, the higher the activity exhibited. As far as reactions in the gas phase are concerned, both zirconia and magnesia solids were active, the latter exhibiting higher conversions at the same reaction temperatures. Furthermore, for MgO solids selectivity to crotyl alcohol increases with the reaction temperature which suggests that either new active sites were “in situ” created or the existing ones were unblocked and made accessible to the reactants. All in all, selectivities to crotyl alcohol of ca. 62% at 16% conversion were obtained for more active systems ZrO2-200 and ZrO2-250 in the liquid phase whereas values above 85% at 50% conversion were achieved on MgO solids in the gas-phase.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (135 K)Download as PowerPoint slideHighlights► For MPV reaction in the liquid phase, the more surface OH groups in zirconia the higher the activity since 2-propanol anchors to the catalyst surface through such groups. ► MgO is not active in the liquid phase, probably because of the poisoning/blocking of active sites by carbonates. ► In the gas phase, both MgO and ZrO2 solids are active, the former exhibiting higher conversions. ► The fact that selectivity to crotyl alcohol increases with the temperature, suggests the “in situ” generation of active sites in magnesia or that some existing ones are then made accessible because of the cleaning of the catalyst surface. ► Both Mg2+–O2− pair sites and surface OH groups seems to be important in magnesia.