CONICET | Buscador de Institutos y Recursos Humanos

Benzaldehyde (B) isan important intermediate widely used in pharmaceutical, dye, food, perfumeryand agrochemical industries. Traditional synthesis of aldehydes via alcohol oxidationimplies methods involving toxic homogeneous catalysts such aschromium(VI)-based reactants or nitric acid and inorganic oxidants (H2O2,dimethyl sulfoxide, permanganates), with the consequent environmental concernsbecause of disposal issues. The use of solid catalysts and O2 overcomethe problems associated with traditional processes, the catalyst may berecovered and reused. Both noble and non-noble metal catalysts have been testedin oxidation reactions. In recent years, much attention has been paid to MnCO3because of its accessible cost and abundance in nature. Thus, MnCO3, MnO2, Mn2O3 and Mn3O4have beenshown to be active and selective to aldehyde formation by alcohol oxidationbecause their redox properties. The catalytic activity of these materials isusually improved by adding promoters such as ZnO, CeO2, CuO and Fe2O3. It was previously reported that the promoter natureas well as catalyst synthesis method can strongly influence the solid particle size and the surface, structural and redox properties of the Mn species. In this work, we focus on the synthesis andcharacterization of Zn-modified MnCO3 solids. The evolution of thesolid physicochemical properties such as Mn oxidation state changes and thedevelopment of different surface and tridimensional Mn and/or Zncontaining species upon increasing the Zn content was elucidated. The knowledge acquired from solid characterization allowedus to interpret results obtained during selective liquid-phase oxidation ofbenzyl alcohol (BA) using O2 as oxidant.Chemical, textural, structural and surfaceproperties of Zn-modified MnCO3 catalysts were thoroughlyinvestigated and showed that strongly depend on the Zn content (x).Impregnation of MnCO3 with increasing Zn contents from 1.0 to 15.4wt.% gave rise to an increment in the surface area andpore volume due to the generation of porosity during the catalystpreparation steps. A further increment of x to 22.6wt.% caused adecrease in the textural parameter values due to a partial porous blockage caused by formation of large ZnO crystallites.Structural characterization by XRD and TPN2showed that solids investigated consist of different crystalline phasescontaining Mn and/or Zn species. In these phases Mn oxidation state changesfrom 2+ toward higher values (3+ and 4+) as x increases entailing thesubsequent transformation of MnCO3 toward ZnMn2O4 spineland MnOx oxides. Furthermore, the Reitveld refinement methodallowed to determine that contributions of MnCO3, ZnO, MnOxand ZnMn2O4 spinel phases also varies with Zn content.The ZnMn2O4, ZnO and MnOxphase fractions increased in detriment of rhodochrosite (MnCO3). XPSanalysis confirmed that as Zn loading increases on xZnMn samples, Mn2+specie of MnCO3 is transformed into Mn3+ and Mn4+ species. The application of xZnMn in the conversion of BA to B using O2 as oxidantshowed that all solidsresulted active and selective during BA oxidationat 373 K and atmospheric pressure. Contrarily, pure ZnO and MnCO3were unable to convert BA. For xZnMn solids, an optimum Zn content of 15.4 wt.% generates anew catalytically active structural composition where the synergisticinteraction between Zn2+ and Mn4+gives rise to high B yields in good agreement with the postulated Mars-vanKrevelen mechanism. The highest B yield (~70%) was obtained on 15.4ZnMn after 6h of reaction. The reuse of this sample confirmed the efficiency of the mildregeneration treatment in air at 493K, that restores the B yield to a valuesimilar to that achieved on the fresh catalyst. Characterization of Zn-modifiedMnCO3 solids with Zn content between 1.0-22.6 wt.% Zn showed thatthese samples consist of different crystalline phases containing Mn and/or Znspecies. The superior catalytic performance of 15.4ZnMn catalyst during the benzyl alcohol oxidation wasattributed to a synergistic interaction of both metals (Zn and Mn).

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