Preparation Of High Value Chemicals By Catalytic Hydrogenation Of Lignin

With the rapid development of the global economy,the problem of energy shortage has become increasingly prominent.As a potential source of monophenolic high value-added fine chemicals and renewable high-grade fuels,lignin is now directly used as a cheap fuel to be burned directly,causing serious waste of resources and environmental pollution.In this paper,aiming at the conversion and utilization of lignin,an effective catalytic system was applied to the catalytic hydrogenation and depolymerization of lignin.The effects of reaction time,initial hydrogen pressure,metal loading and acidity on the distribution of lignin catalytic hydrogenation depolymerization products were investigated.Moreover,under the optimal conditions,the effects of metal Ru size on the hydrodepolymerization of catalysts prepared by different methods and the possible effects of solvent medium on deep hydrogenation were discussed.The catalysts were characterized by X-Ray Diffraction（XRD）,NH₃-temperature programmed desorption（NH₃-TPD）and Transmission Electron Microscope（TEM）,and the qualitative and quantitative analysis of catalytic hydrogenation products of lignin was carried out by GC-MS.（1）A series of Ru/γ-Al₂O₃ catalysts with different Ru load were prepared by impregnation,which had the advantages of smaller grain size,larger active metal dispersion and stronger acidity.Ru/γ-Al₂O₃catalyzes the lignin cleavage ofβ-O-4 and other linkages to produce monophenol compounds.The optimized conditions for catalytic hydrogenation of lignin in an intermittent stirred reactor are:The initial pressure of hydrogen was 3.0 MPa,the reaction temperature was 250℃,the reaction time was 8h,and the mass fraction of Ru was 5wt.%.Ru/γ-Al₂O₃ catalyzed the conversion of lignin to 76.2%,and the total yield of monophenols reached 4.63%.The HZSM-5 molecular sieve can assist the Ru catalyst to break the C-C bond and the C-O bond,and its unique pore structure may adsorb the fixed intermediate to prevent repolymerization,thereby increasing the yield of the monophenol product.When the mass of HZSM-5 is twice that of the Ru/γ-Al₂O₃ catalyst,the synergistic effect of HZSM-5 is the best,and the yield of monophenols is7.63%.（2）Ru/WO₃-ZrO₂ catalysts with different WO₃ mass fractions were obtained by impregnation method,which have the advantages of smaller grain size and stronger acidity.The added WO₃ can not only provide an oxygen-friendly site,but also affect the process of Zr（OH）₂ crystallizing to form ZrO₂.By changing the tungsten content or the form of the catalyst surface,a catalyst with different surface acidity can be obtained.After optimization of the process conditions,Ru/WO₃-ZrO₂ catalyzed lignin solution under the conditions of hydrogen pressure of 3.0 MPa,reaction temperature of 250℃,reaction time of 8 h,Ru mass fraction of5%and WO₃ mass fraction of 15%.The total yield of polyphenols was5.38%,and the conversion of lignin was 77.1%.The addition of HZSM-5molecular sieve to the Ru/WO₃-ZrO₂ catalytic reaction system synergistically catalyzed,but the catalytic results were not satisfactory.（3）Ru/γ-Al₂O₃ and Ru/WO₃-ZrO₂ catalysts were prepared by impregnation method,ethylene glycol reduction method and sodium borohydride reduction method.As a result,the particle size of the surface metal Ru affected the selectivity of the catalyst to some extent.The catalyst prepared by the impregnation method has better selectivity for guaiacol and 4-methylguaiacol.The catalyst prepared by the ethylene glycol reduction method has poor selectivity,but has the highest total yield of monophenols.The catalyst prepared by the sodium borohydride reduction method has the worst selectivity,and has the smallest total yield of monophenols,but more benzene ring hydrogenation products are formed and the total yield is the largest.The particle size of Ru in the Ru/γ-Al₂O₃ catalyst obtained by different preparation methods:impregnation method（4.30 nm）>ethylene glycol reduction（3.03 nm）>sodium borohydride reduction（2.75 nm）.The particle size of metal Ru in the Ru/WO₃-ZrO₂ catalyst obtained by different preparation methods:impregnation method（6.71 nm）>ethylene glycol reduction（6.03 nm）>sodium borohydride reduction（5.89 nm）.The distribution of lignin catalytic hydrogenation depolymerization products is greatly affected by alcohol solvents.The main products are guaiacol,4-methylguaiacol and4-propyl guaiacol,and lignin in methanol solvent.The total yield of the monophenolic compound obtained by depolymerization was 3.76%,and the total yield in the ethanol solvent reached 4.18%.In addition,more C₁₀-C₁₅5 alkane products and less C₇-C₉ and C₁₅-C₃₀0 alkane products are present in the lignin catalytic hydrogenation product,wherein the total yield of the benzene ring hydrogenation product in the ethanol solvent is at most 2.66%.Therefore,ethanol and Ru/WO₃-ZrO₂ are considered to be a good combination.