Hydrodeoxygenation of Meta-Cresol On Pt/SiO2 and Pt/Hbeta

Catalytic conversion of lignin derived meta-cresol (3-methylphenol) was investigated over Pt/SiO2 , HBeta and Pt/HBeta catalysts.  Over Pt/SiO2 , 3-methyl-cyclohexanone , 3-methyl-cyclohexanol and toluene are obtained at 300oC. But at 400oC , only toluene was obtained.  Acid site on HBeta catalyzes methyl transfer reactions (isomerization and transalkylation) , yielding in cresol isomers , dimethylphenol (xylenol) isomers and phenol.  Bifunctional catalyst Pt/HBeta catalyzes methyl transfer reactions and hydrodeoxygenation reactions in parallel.  The turn over frequency for hydrodeoxygenation of meta-cresol is 3 times higher over Pt/HBeta than over Pt/SiO2 , highlighting the significant role of acid sites in hydrodeoxygenation of lignin derived phenolics.  A reaction pathway has been proposed: m-cresol tautomerizes to an unstable ketone intermediate (3-methyl-3 ,5-cyclohexadienone).  Hydrogenation of the C=C bonds of this intermediate produces 3-methyl-cyclohexanone and 3-methyl-cyclohexanol , which are only observed at 300oC and in the absence of acid sites.  Over Pt/HBeta at 400oC , dehydration of 3-methyl-cyclohexanol followed by dehydrogenation results in toluene production.  The interesting question is why toluene observed in the absence of acid sites and at low temperatures.  The answer is that toluene can also derive from the ketone intermediate (3-methyl-3 ,5-cyclohexadienone) by hydrogenation of the carbonyl group that produces a very reactive unsaturated alcohol (3-methyl-3 ,5-cyclohexadienol) , which can be easily dehydrated to toluene.