Catalytic transfer hydrodeoxygenation of guaiacol to phenol with formic acid over N-doped carbon encapsulated non-noble alloy catalysts

Catalytic transfer hydrodeoxygenation of lignin-derived guaiacol using formic acid (FA) as a hydrogen donor is a sustainable and secure way to obtain value-added phenol. In this work, we prepared N-doped carbon encapsulated CoNi and FeCoNi nanoparticles (CoNi@NC and FeCoNi@NC) for this reaction and found that NC shells rather than the alloy cores are the active sites. Ultraviolet photoelectron spectroscopy (UPS) results and Density Functional Theory (DFT) calculations suggested that Mott-Schottky heterostructures were constructed in CoNi@NC and FeCoNi@NC, leading to the spontaneous electron transfer from alloy cores with smaller work functions to NC shells. DFT calculations also confirm that the number of electrons transfer from alloy cores to NC shells with 1.46 a.u. and 1.59 a.u. for CoNi@NC and FeCoNi@NC, respectively. The increased electron density on NC shells improved the absorption strength of reactants and the intermediate, thereby reducing the energy barriers for the dehydrogenation of FA and hydrodeoxygenation of guaiacol. FeCoNi@NC, due to its higher surface electron density, exhibited better catalytic activity than that of CoNi@NC, 93.4% conversion of guaiacol and 87.3% selectivity to phenol can be achieved at 260 °C within 12 h, which is even better than commercially available Pd/C catalyst. The mechanistic studies revealed that guaiacol is first converted into catechol via the demethylation and hydrolysis, then to phenol via hydrogenolysis over FeCoNi@NC with the aid of FA. Moreover, the magnetically separatable FeCoNi@NC possessed high catalytic stability because NC shells protect alloy cores from the acidic solution.