| Lignin is a kind of complex natural aromatic biopolymers which accounts for about 30%of the earth s total organic carbon,it is one of the renewable resources that can be used in the production of the aromatic chemicals.Lignin depolymerization is a important way to achieve high value aromatic chemicals or low molecular chemical raw materials.In many studies,supercritical depolymerization is considered to be a highly efficient technology used for biomass depolymerization to achieve oil and biological chemicals.We can adjust the depolymerization reaction conditions so that we can selective control target phase content in the product through regulation of supercritical fluid temperature,pressure and the reaction time,we can also join different catalysts to take orientation of lignin depolymerization.Lignin of supercritical depolymerization alternative petrochemical resources to product low molecular substances has caused people ’s attention mainly until recent years because of the diversification and structure complexity of lignin hinders the lignin depolymerization.Ethanol pulping lignin is considered to be the most close to the natural structure of lignin which not bring in metal,sulfur,chlorine and other functional groups,it can reflect the natural lignin form which has important guiding significance for the the study of lignin supercritical depolymerization mechanisms.The development of efficient catalyst with inhibiting condensation,hydrogenation,suitable acid and alkali is the key for lignin supercritical depolymerization.The noble metals and transition metal doped catalysts are wide used in the research of lignin depolymerization,metal doped hydrotalcite catalyst has attracted much attention because of its special layered structure.Hydrotalcite compound is a kind of layered compounds,its chemical composition of layer can be controlled and interlayer anionic has exchangeability,based on these characteristics,we can introduc in appropriate element into hydrotalcite layer board used as catalyst,we can also achieve mixed metal oxides catalysts after further roasting.Based on the above background,the research content of this paper includes the following three aspects:(1)Ethanol pulping lignin preparationLignin preparation use the method of autocatalytic cypress ethanol pulping,its condition was 60%ethanol concentration,liquid ratio 1:10,cooking temperature set at 205℃,heating time 90 min,holding time 150 min.Extraction of rough lignin from black liquor and then processing to achieve pure ethanol pulping lignin.The ethanol pulping lignin is characterized by FT-IR.(2)Catalyst preparationA series of catalysts with various ratios of n(Cu2++Mg2+)/n(Al3+)and n(Mg2+)/n(Cu2+)hydrotalcite-like were prepared by chemical co-precipitation.The properties of catalysts were characterized by XRD,FT-IR and TGA.The best ratios of n(Cu2++Mg2+)/n(Al3+),n(Mg2+)/n(Cu2+)and ionic content were obtained.The catalyst of hydrotalcite-like with best ionic ratio of n(Cu2+)/n(Mg2+)/n(Al3+)=3:12:5 was roasted at different temperatures.The results of XRD and SEM showed that the metal oxide calcined obtained by roasting at 460℃has a good layer structure.A series of Co-doped and Ni-doped catalysts with various ratios of n(Mg2+)/n(Co2+)and n(Mg2+)/n(Ni2+)hydrotalcite-like were prepared in the condition of n(Co2++Mg2+)/n(Al3+)=3 and n(Ni2++Mg2+)/n(Al3+)=3 by chemical co-precipitation.The properties of catalysts were characterized by XRD,FT-IR and TGA.The properties of catalysts were characterized by XRD after roasting at 460℃.(3)Ethanol pulping lignin supercritical ethanol catalytic depolymerizationLignin catalytic depolymerization reaction using a 100 ml supercritical reactor,the reaction conditions are catalyst 1 g,lignin 2 g,anhydrous ethanol 40 ml,vacuum then heat to preset temperature,cooling to room temperature immediately after a certain period of time by flowing water.Lignin depolymerization products were analysed by GC-MS,FT-IR,NMR,it is concluded that different metal doped catalysts,different metal doping amount of catalysts,different reaction temperature and different reaction time have different effects on the lignin supercritical depolymerization.The experiment results show that:(1)The benzene unit of cypress ethanol pulping lignin is syringyl ring mainly,following by guaiacyl ring.(2)The preparation condition of Cu-doped hydrotalcite is 2<n(Cu2++Mg2+)/n(Al3+)<4 and n(Mg2+)/n(Cu2+)≥2.When n(Cu2++Mg2+)/n(Al3+)=3 and n(Mg2+)/n(Cu2+)=4,the copper doped hydrotalcite has highest crystallinity and best order of layered structure.Meanwhile,the copper doped hydrotalcite contains a relatively large amount of copper.The metal oxide obtained by calcining the Cu-doped hydrotalcite at 460℃ has a good layer structure.The thermal decomposition process of Cu-,Co-or Ni-doped hydrotalcite is only lose physical adsorbed water,crystal water and interlayer water molecules when the temperature lower than 200℃.The-OH,CO32-and other anion in the form of H2O,CO2 and corresponding form loss when the temperature at 250～650℃,respectively.The layered structure of hydrotalcite is completely destroyed at this time.The CO32-insert between the layers is not really free,the CO32-and the water molecules between the layers have a strong hydrogen-bond interaction.(3)Lignin depolymerization products complex,composed by containing benzene ring,esters,alcohols,and other compounds.The main reaction is esterification reaction and alkylation reaction.The solvent of ethanol acts as the end capping agent and hydrogen donor.Different metal doped catalysts,different metal doping amount catalyst,different reaction temperature,different reaction time have different effect on cypress ethanol pulping lignin supercritical ethanol catalytic depolymerization,all kinds of product content have taken place great changes with the change of the reaction conditions. |
