Copper Nanoparticles Supported on ZIF-8: Comparison of Cu(II) Reduction Processes and Application as Benzyl Alcohol Oxidation Catalysts

We report the synthesis of a stable heterogeneous catalystbasedon copper metal nanoparticles with oxidized surface supported on ZIF-8for the oxidation of benzyl alcohol under mild temperature and usingair as a sustainable oxygen source as well as for the implementationof the tandem "one-pot" catalytic system allowing thesustainable synthesis of benzylidene malononitrile. The influenceof the reduction process applied to form the nanoparticle upon thecatalyst texture and its performances was extensively examined. AfterZIF-8 impregnation with a copper chloride precursor, the reductionof cupric ions into Cu-0 nanoparticles was carried out accordingto two procedures: (i) by soaking the solid into a solution of NaBH4 and (ii) by submitting it to a flow of gaseous H-2 at 340 & DEG;C. The in-depth physicochemical characterization andcomparison of the resulting two types of Cu/ZIF-8 materials revealsignificant differences: the reduction with NaBH4 led tothe formation of 16 nm sized Cu-0 nanoparticles (NP) mainlylocalized on the external surface of the ZIF-8 crystals together withZnO nanocrystallites, while the reduction under H-2 flowresulted in Cu-0 nanoparticles with a mean size of 22 nmembedded within the bulk of ZIF-8 crystals. More, when NaBH4 was used to reduce cupric ions, ZnO particles were highlighted byhigh-resolution microcospy imaging. Formation of ZnO impurities wasconfirmed by the photoluminescence analysis of ZIF-8 after NaBH4 treatment. In contrast, ZnO was not detected on ZIF-8 treatedwith H-2. Both types of Cu-0 NPs supported onZIF-8 were found to be active as catalysts toward the aerobic oxidationof benzyl alcohol under moderate temperature (T <80 & DEG;C) and using air as a sustainable O-2 source. Benzaldehydeyield of 66% and selectivity superior to 90% were obtained with theCu/ZIF-8 catalyst prepared under H-2 flow after 24 h underthese conditions. The same material could be recycled 5 times withoutloss of activity, unlike the catalysts synthesized with NaBH4, as a result of the leaching of the surface copper NPs over theconsecutive catalytic cycles. Finally, the most stable catalyst wassuccessfully implemented in a tandem "one-pot" catalyticsystem associating benzyl alcohol oxidation and Knoevenagel condensationto synthesize benzylidene malononitrile.