Study On Hydrodeoxygenation Of Lignin Derived M-cresol Over Ni,Pd And Pt Catalysts

Conversion of waste bio-resources?for example, agricultural and forest residue, municipal organic solid wastes, and so on? into biofuels can reduce the dependency on fossil fuels, reduce environmental problems such as carbon dioxide emissions, and improve the sustainability of human society. As a major fraction of biomass, the lignin could be depolymerized to phenolic coupounds. Hydrogenation and hydrodeoxygenation reactions can convert phenolics into value added chemicals and fuels. In this work, meta-cresol?m-cresol? was selected as a phenolic model compound. Hydrodeoxygenation of m-cresol was investigated on supported transition metal catalysts?Ni/SiO₂, Pt/SiO₂, Pd/SiO₂?, at atmospheric pressure and mild temperature?250-400??.The catalysts were prepared by impregnation method, and were characterized by N₂ physisorption, X-ray diffraction?XRD? and transmission electron microscope?TEM?. The characterization results showed that the SiO₂ support has a specific surface area of about 200 m2/g. Loading of metals on the support resulted in slight decrease in surface area. Supported Pd and Pt catalysts showed better dispersion than Ni catalyst.The catalytic performance of Ni, Pd and Pt catalysts at 250? was firstly studied. The major reaction pathway for hydrodeoxygenaiton of m-cresol is similar on all tested catalysts. O can only be removed through direct deoxygenation?DDO? pathway. Because of the lack of acid active sites, the hydrogenation products 3-methyl-cyclohexanone and 3-methylcyclohexanol can only go back to m-cresol through dehydrogenation reaction but can not be dehydrated. Compared to Pd and Pt catalyst, Ni catalyst shows demethylation and hydrogenolysis activity while its deoxygenation activity is much lower than the noble metal catalysts?Pd and Pt?.The reaction data 250? was fitted with pseudo first-order model, the rate constants of each reaction step was therefore obtained. Comparison of Ni, Pd and Pt catalysts, Pd and Pt catalysts have similar catalytic activity for hydrodeoxygenation of m-cresol at 250?. The initial conversion rate and deoxygenation rate of the two noble catalysts are about 5 times and 20 times more than the Ni catalyst. Comparing to Pd and Pt, Ni catalyst has a stronger preference for hydrogenation of the aromatic ring and is more selective for hydrogenation products 3-methyl-cyclohexanone and 3-methylcyclohexanol. The selectivity of alcohol/ketone follows the order Ni/SiO₂>Pt/SiO₂?Pd/SiO₂. Toluene is the major deoxygenation product of the hydrodeoxygenation of m-cresol at 250?. The reaction catalyzed by Pt/SiO₂ yields more cycloalkane than that by Pd/SiO₂ and Ni/SiO₂ because of its higher hydrogenation activity of toluene.The reaction temperature has significant impact on both activity and selectivity. Increased reaction temperature suppressed the hydrogenation activity but promoted the hydrodeoxygenation and hydrogenolysis activity. At 400?, the major prduct is toluene of hydrodeoxygenation on Pd and Pt catalysts, while the major product is methane of hydrogenolysis on Ni catalyst. Ni/SiO₂ catalyzes hydrogenolysis to methane and Pd/SiO₂ deactivates quickly at 400?, while Pt/SiO₂ still shows good catalytic performance.