In order to develop selective catalytic processes for the production of aromatics from lignin, a complete understanding of the molecular aspects of the basic chemistry and reactivity of aromatic ethers is crucial. Diphenyl ether (DPE), Phenethyl phenyl ether (PPE) and Benzyl phenyl ether (BPE) are the simplest model molecules of 4-O-5, β-O-4 and α-O-4 lignin linkages. In this context, a bimetallic Pd/Ni catalyst (with a nominal palladium loading of 3 wt%) has been prepared by the co-precipitazion technique with the aim to test its catalytic activity in the cleavage of the C-O bond of DPE, PPE and BPE under catalytic transfer hydrogenolysis (CTH) conditions using 2-propanol as H-donor. CTH reactions have gained increasing attention as an efficient alternative to the direct use of molecular hydrogen. For comparison, a monometallic Ni catalyst was prepared with the similar procedure as above and tested under the same reaction conditions. The physico-chemical characterization of all investigated catalysts was performed through H2-TPR, TEM, XRD and XPS analysis. Using Pd/Ni as catalyst and 2-propanol as H-source, under batch conditions, an appreciable DPE (0.1 M) conversion was achieved after 90 min at a temperature as low as 180 °C. The conversion of DPE increases by rising the reaction temperature and it is fully converted at 240 °C. On the other hand, the metallic Ni catalyst was found to be less active. At the same time, a lower tendency in aromatic ring hydrogenation is observed in presence of palladium as co-metal (Figure 1). The catalytic tests on all possible reaction intermediates clearly show that, under CTH condition, the etheric C-O bond of DPE is previously cleaved via the hydrogenolysis route into benzene and phenol. In a successive reaction, phenol is hydrogenated into cyclohexanol. CTH of PPE and BPE were also investigated. Catalytic tests show that higher conversions and aromatic yields can be easily obtainable under mild reaction conditions as a consequence of their lower bond dissociation energy (β-O-4 = 289 kJ/mol and α-O-4 = 218 kJ/mol) with respect to the C–O bond of DPE.

Catalytic transfer hydrogenolysis of aromatic ethers promoted by the bimetallic Pd/Ni system

255th National Meeting (2018)

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