Conversion of Dimeric Diaryl Ethers over SiO2- and HZSM5-Supported Pd and Ru Catalysts: A Focus on the Role of the Metal and Acidity

Abstract

The effect of metal and support acidity on the hydroconversion of dimeric aryl ethers, used as model molecules for lignin, is still under debate, both in terms of hydrogenolysis (cleavage of the ether bond) and formation of by-products (coupling of aromatic monomers to dimers by alkylation reaction). Their role is investigated here in the conversion of three typical molecules representative of the α-O-4, β-O-4, and 4-O-5 ether linkages of lignin, respectively, benzyl phenyl ether (BPE), phenethoxybenzene (PEB), and diphenyl ether (DPE), at 503 K, under 18 bar of H2 in decalin. Ru- and Pd-based catalysts were synthesized on non-acidic SiO2 and on acidic HZSM5. Under these reaction conditions, the conversion of the ethers over the bare supports was observed in the presence of acidic sites; the effect decreased as the ether bond strength increased. The results also suggest that the product distribution is directly affected both by the support acidity and by the oxophilicity of Ru. Alkylated products from isomerization reactions, which are reported to be formed only over acidic sites, were also produced on the surface of the Ru nanoparticles.

The Role of Metal and Acid Sites in the Liquid‐phase Hydrodeoxygenation of Lignin‐derived Phenolic Dimers

“…[117] However, in this case, the hydrogenation of phenol after CÀ O cleavage was not observed. Rafael et al, [112] evaluated the effect of Pd and Ru supported on SiO 2 for BPE conversion at 230 °C and 18 bar of H 2 in decalin as solvent. Phenol and toluene were formed on both catalysts; however, alkylated products were also observed over Ru/SiO 2 .…”

“…The role of metal in the conversion of PEB was studied by Rafael et al [112] over Pd and Ru-based catalysts supported over SiO 2 . Pd/SiO 2 yielded only products from partial and full hydrogenation of PEB aromatic rings, while in the presence of Ru/SiO 2 , ethylbenzene, ethylcyclohexane, phenol, and benzene were also detected.…”

“…Ru and Pd supported over HZSM5 were used by Rafael et al [112] for the conversion of PEB (230 °C, 18 bar H 2 ) in decalin. Regarding the product distribution, the authors observed that besides ethylbenzene and phenol (products from PEB hydrogenolysis), the presence of palladium favored the partial and full hydrogenation of PEB aromatic rings, while alkylated products were mainly formed over Ru/HZSM5.…”