A Study On Catalytic Conversion Of Furfural To Alkyl Levulinates In Near-critical Alcohols Without Hydrogen

With the increasing consumption of coal,petroleum and other non-renewable resources,the development of lignocellulose,the most abundant biomass resources in the world,has become a hot research topic.Hemicellulose,as a significant component of lignocellulose,can be used for the production of a high value-added chemical:alkyl levulinates,which have been widely used.Hemicellulose can be generally converted to furfural through acid hydrolysis.Thus,synthesis of alkyl levulinate from furfural has attracted great attention.Traditionally,alkyl levulinates are generally produced from furfural by two-step reactions:catalytic hydrogenation and alcoholysis.But it is restricted remarkably due to high catalyst costs,high hydrogen consumption and bad catalytic performance.Synthesis of alkyl levulinate from furfural is restricted due to its long reaction path,numerous by-products and low product yields.As a result,the screening and development of catalysts,which show good stability and high catalytic activity for the conversion of furfural to alkyl levulinates is of great significance.Anew low-cost and hydrogen-free method for the the preparation of alkyl levulinates from furfural is established.The paper systematically studies one-pot synthesis of alkyl levulinate from furfural,inlcuding:preparing the catalysts;evaluating the catalysts;exploring the reaction mechanism of the transfer hydrogenation,pathway of alkyl levulinate from furfural and catalystic mechanism;optimizing the catalytic conditions;evaluating recycling performance.The main contents are generalized as follows:Firstly,the Cu-Pd/Al2O3 catalysts with different Cu,Pd loading are synthesized using coprecipitation method and applied for the transfer hydrogenation of furfural in near-critical isopropanol.The catalysts are characterized by XRD,N2-BET and TEM.The influences of catalyst loading amount,furfural loading amount,reaction temperature and reaction time on the transfer hydrogenation of furfural are investigated.Results show that with the Cu load decrease and Pd load increase,the yield of furfuryl alcohol increases firstly,and then decreases,18%Cu-2%Pd/Al2O3 bimetallic catalyst has the best effect,superior than Pd/Al2O3 and Cu/Al2O3.The optimized conditions are obtained:with 100 mg of furfural,25 mg of 18%Cu-2%Pd/Al2O3,5 mL of isopropanol,furfural conversion is 100%,and furfuryl alcohol yield is 86.3%at 190?for 3 h.Moreover,catalysts still maintain good catalytic activity after 4 times recycling use.Secondly,in order to further reduce the catalyst costs,Cu/MgO-Al2O3 are developed and applied on the transfer hydrogenation of furfural in near-critical isopropanol.Compared with Cu-Pd/Al2O3,Cu-Ni/Al2O3,Cu/Al2O3,Pd/Al2O3 and Ni/Al2O3,Cu/MgO-Al2O3 shows the best catalytic performance.With the increase of loading amount of Cu,the yield of furfuryl alcohol increases firstly and then decreases.The optimized conditions are obtained:with 100 mg of furfural,25 mg of 20%Cu/MgO-Al2O3,5 mL of isopropanol,furfural conversion is 100%,and furfuryl alcohol yield is 89.3%at 210?for 1 h.Catalysts still maintain good catalytic activity after 4 times recycling use.Then,SBA-15 mesoporous molecular sieve supported different metals are prepared by a hydrothermal method.The self-synthesized catalysts,showing hydrogen transfer and acid catalytic activity,are applied for one-pot synthesis of methyl levulinate from furfural in near-critical methanol without added hydrogen.The effects of different metals supported on SBA-15,Si/Zr molar ratios,catalyst loadings,methanol loadings,reaction temperatures and reaction times are investigated and the optimized conditions are obtained:with 100 mg of furfural,50 mg of Zr-SBA-15-10 and 7 mL of methanol,furfural conversion is 100%,and methyl levulinate yield is 36.3%at 270?for 10 h.With Si/Zr increase,the conversion of furfural decreases,the yield of methyl levulinate increases firstly and then decreases.Zr-SBA-15 catalysts are characterized by XRD,N2-BET,FT-IR,TEM.Catalysts still maintain good catalytic activity after 5 times recycling use.The catalystic mechanisms are also demonstrated and the possible reaction pathway on the one-pot production of methyl levulinate from furfural over Zr-SBA-15 mesoporous zeolites is proposed,pathwayl:Firstly,furfural is converted to?-Methoxy-2-furanethanol,?-Methoxy-2-furanethanol is then converted to methyl levulinate.Pathway2:Firstly,furfural is converted to furfuryl alcohol,then furfuryl alcohol is converted to 2-Furfuryl methyl ether,2-Furftuyl methyl ether is converted to methyl levulinate at last.In the reaction system,two pathways exist at the same time.Finally,a method on one-pot synthesis of ethyl levulinate from furfural in near-critical ethanol without added hydrogen is established,and the optimized conditions are obtained:with 100 mg of furfural,50 mg of Zr-SBA-15-10 and 7 mL of ethanol,furfural conversion was 100%,and ethyl levulinate yield was 47.4%at 220?for 3 h.Catalysts still maintain good catalytic activity after 5 times recycling use.Compared with methyl levulinate,ethyl levulinate has more moderate reaction temperature,shorter reaction time,and higher yield.The catalytic mechanisms are also demonstrated and the possible reaction pathway on the one-pot production of ethyl levulinate from furfural over Zr-SBA-15 mesoporous zeolites is proposed,pathway1:Firstly,Furfural is converted to 2-(Diethoxymethyl)furan,2-(Diethoxymethyl)furan is then converted to ethyl levulinate.Pathway2:Firstly,Furfural is converted to furfuryl alcohol,furfuryl alcohol is then converted to 2-(Ethoxymethyl)furan,2-(Ethoxymethyl)furan is converted to methyl levulinate at last.