Lignin fragmentation over magnetically recyclable composite Co@Nb2O5@Fe3O4 catalysts

• Xwt%Co@Nb2O5@Fe3O4 - multifunctional and magnetic catalysts for lignin fragmentation.
• Cobalt nanoparticles were highly dispersed on the external surface of niobia.
• Bifunctional catalysts with acid sites and finely dispersed cobalt nano-particles.
• A cobalt loading of 4 wt% is optimal for the fragmentation of lignin.
• The reaction occurs at temperatures as low as 180 °C.
• The investigated catalysts were completely recyclable.

Recoverable catalysts for lignin fragmentation were prepared as Xwt%Co@Nb2O5@Fe3O4 composites having magnetite as an inner core. A shell of niobia covering the magnetic nanoparticles (leading to intermediate Nb2O5@Fe3O4 composite) was deposed from a solution of ammonium niobate (V) oxalate complex by precipitation with NH3. Finally, the deposition of cobalt following the deposition–precipitation method generated the Xwt%Co3O4@Nb2O5@Fe3O4 composites that were reduced with hydrogen to Xwt%Co@Nb2O5@Fe3O4. Catalysts with loading of cobalt in the range X = 1–20 were prepared using this procedure. The different steps of the preparation were followed using various techniques such as surface area measurements, XRD, Raman and NH3-DRIFT spectroscopy, XPS, Mössbauer and TEM. On this basis it was concluded that the synthesized composites exhibit both Lewis and Brønsted acid sites associated with the niobia shell and contain finely dispersed cobalt nanoparticles. The fragmentation of lignin occurred on the constituent parts of this composite, i.e. Fe3O4, Nb2O5, Nb2O5@Fe3O4 but with lower performances. The addition of cobalt (Co@Nb2O5@Fe3O4 catalysts) led to a complete fragmentation of lignin where the dominant fragments were those containing C20–C28 molecules. This catalytic behavior is explained on the capability of niobia to catalyze the acidic hydrolysis of the β-O-4′ bounds and of Co to break the C–C bonds via hydrogenolysis. The optimization of the catalyst composition indicated a loading of 4 wt% as optimal. Working at 180 °C and 10 atm H2 this catalyst allowed a conversion of 53% leading to a mixture containing over 96% in C20–C28 and C29–C37 fragments. The investigated catalysts were completely recyclable as it was showed in six successive cycles. No leaching of the elements included in the composition was determined by ICP-OES.

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