| Fossil resources are the main source of energy,materials and chemicals in industrial societies.However,excessive exploitation of non-renewable fossil resources will not only lead to resource depletion,but also cause serious greenhouse gas effects.The 21st century is a turning point in the return of ecological civilization in human history.For the sustainable development of society,humans must find renewable alternative resources and disengage from the dependence on fossil resources.Scientists turned their attention to biomass,the richest renewable resource in nature.Lignin,as one of the main constituents in lignocellulose,accounts for about 15%-30%.It is mainly composed of three phenylpropanoid derivatives:p-hydroxyphenyl(H),guaiacyl(G)and syringyl(S)monomer-type polymerization.Aromatic structure of lignin makes it be a renewable resource for the production of aromatic chemicals.Organic solvent pulping is the preferred method for green pulping in the future.The organosolv lignin produced by organic solvent pulping is the ideal raw material closest to natural lignin.Microwave has the advantages of accelerating the reaction rate of reaction.Microwave radiation produces thermal effects by inducing strong polar chemical bond vibrations,and rise the temperature fast,and improve the breakage of chemical bonds.In this paper,we first optimized the organic pulping process of moso bamboo,characterized the structure of organosolv lignin,and carried out a systematic study on the preparation of aromatic compounds by microwave-assisted catalytic depolymerization of organosolv lignin from moso bamboo,and obtained the following main results:1.Extraction and Characterization of High Purity Organosolv LigninUsing moso bamboo as raw material,organosolv lignin was extracted under different reaction conditions.The effects of reaction temperature and reaction time on lignin yield and lignin molecular weight were investigated.When the solid-liquid ratio was 1:8,the ethanol mass fraction was 50%,and the reaction time was 2 hours,the temperature was used as a variable,ie,120?-140?-160?-180?-200?-220?.The experimental results show that the effect of temperature on lignin extraction is very obvious.When the temperature is 120?,the lignin yield is lower than 16%,while as the temperature increased,the lignin yield increased first and then decreased,which is attributed to the high temperature leading to lignin repolymerization.It was shown that lignin molecules were degraded to varying degrees during lignin extraction,indicating that high temperature has a great influence on lignin extraction experiments.The depolymerization and repolymerization of lignin occur simultaneously in a high temperature environment.High temperatures not only promote the breakage of chemical bonds in lignin molecules,but also provide an environment that can rebind between chemical bonds.When solid-liquid ratio 1:8,ethanol mass fraction 50%,reaction temperature 180?,and reaction time as a variable,ie 1-2-3-4-5-6 hours.The experimental results show that the effect of reaction time on lignin extraction experiment is also relatively large,indicating that long reaction time would not helpful for lignin extraction experiments.Unfavorable,resulting in changes in the molecular structure of lignin.Therefore,the optimum reaction conditions for organosolv lignin extraction were as follows:solid-liquid ratio 1:8,ethanol mass fraction 50%,reaction temperature 180?,reaction time 3h,and maximum lignin yield 49.1%.The molecular structure of lignin was detected by FT-IR,TG-DTG and NMR,and compared with the molecular structure of alkaline lignin,the former chemical structure was closer to the lignin standard sample.The results of 13C NMR and 1H NMR spectra showed that the organosolv lignin molecular contains Hp,Hy,methoxy protons,C-a,C-?,?-O-4,?-?,?-5',aryl and aliphatic chemical bonds.At the same time,it is also observed that the S-type,G-type and H-type basic structural units in its chemical structure are all contained.The results of thermogravimetric analysis showed that the weight loss of alkali lignin and organosolv lignin occurred mainly in the second stage,which indicated that the main components of the two lignins were high-temperature degradable compounds.However,alkali lignin started to degrade at 200?,and its Y%char(Y%char=TGultimate/Original sample weight)was 63.1%,indicating a high content of high temperature non-degradable compounds.The organosolv lignin began to degrade at 50?,and its Y%char(Y%char=TGuitimate/original sample weight)was 35.2%,indicating that the high temperature degradable compound content in the organosolv lignin structure is high.This indicates that organosolv lignin has high chemical activity and is relatively easy to be depolymerized.2.Microwave-Assisted Acid Catalytic Depolymerization of Organosolv Lignin0.5 g of organosolv lignin as raw material,11.7 ml of ethanol as solvent,0.3 ml of sulfuric acid(H2SO4),hydrochloric acid(HC1),phosphoric acid(H3PO4),and formic acid(HCOOH)(2.5%of the total volume)as a catalyst(its concentrations were 3 mol/L,6 mol/L,2 mol/L,and 6 mol/L,respectively,the H+concentration in each reaction system was the same),under mild conditions(reaction at 160? for 30 minutes,microwave power Less than 80w)to depolymerize organosolv lignin and explore the acid catalytic depolymerization of lignin.The depolymerization products were detected and characterized by SEM,TG-DTG,FT-IR and HLPC.The liquefaction degrees of H2SO4,HCl,H3PO4,and HCOOH catalytic reactions were 87.2%,96.1%,92.7%,and 83.3%,respectively,which was greater than the liquefaction degree of non-catalytic reaction group(74.2%).This is because the addition of acid clearly promotes the fragmentation of ether bonds(?-O-4 and 4-?-5)in lignin molecules.The catalytic effect of H2SO4 was most prominent among the four acid catalysts.The molecular weight distribution of the depolymerized product showed that the weight average molecular weight(1268)and number average molecular weight(375)of the non-catalyzed reaction group were higher than the molecular weight of the acid-catalyzed reaction,while the Mw value(1020)and Mn value(310)of the H2SO4 catalyzed reaction were lower than those of the other acid-catalyzed reactions.On the other hand,the experimental results show that dense hydrogen bonds in lignin increase its stability,but the use of excellent hydrogen bond acceptors such as Cl-in HCl catalyzed reaction systems,which are highly electronegative and widely available for biomass dissolution,greatly improves its transformation and liquefaction(96.1%).The catalytic ability of formic acid was significantly lower than the other three inorganic acids tested in this study.However,formic acid is widely used for lignin depolymerization as a hydrogen donor because of its role in increasing liquefaction yield and preventing repolymerization of reaction intermediates.However,the addition of phosphoric acid can lead to the repolymerization of lignin after being heated.From the TG-DTG results,it can be seen that the phosphoric acid-catalyzed reactions exhibit quite different thermodynamic behavior and decompose between 100? and 500?.Its Y%char(Y%char=TGultimate/Original sample weight)47.9%is much larger than the other samples,indicating a high content of high temperature non-degradable compounds.Unlike other samples,the sulfuric acid-catalyzed reaction was thermally decomposed between 30? and 130? and presented a faster rate of thermal decomposition,which indicated that the decomposition products were mainly composed of water and volatile compounds.The microstructure of residual lignin was observed in scanning electron microscopy(SEM).It was found that the lignin residue after phosphoric acid-catalyzed reaction was huge in size,and the smooth and sharp and rigid cracks also demonstrated its repolymerization.From the FT-IR spectra,it can be observed that the C-O stretching vibration at 1217 cm-1(C-O belonging to syringyl group(S))and the C-O stretching vibration at 1268 cm-1(C-O belonging to guaiacyl group(G))both weaken.Combined with the results of other analytical means,it can be inferred that the attenuated C-O stretching vibrations indicate C-O bonds severed in the reaction.In addition to this,the reduction of hydroxyl and methoxy groups in lignin molecules is also demonstrated by the reduction of O-H stretching at 3423 cm-1 and methoxy C-H stretching at 2937 cm-1,which can be inferred for C-O bond fragmentation.3.Preparation of Biocar Metal Salt Composite Catalyst Ni-600/Fe-800 and Its Effect in Microwave-Assisted Catalytic Depolymerization of Organosolv LigninIn the first step,four different biochar metal salt composite catalysts were prepared.60 g of bamboo powder was first soaked in 0.3 mol/L of a 300 mL solution of Ni(NO3)2·6H2O and Fe(NO3)3.9H2O,and then the mixture was kept at 105? for 12 hours.Afterwards,bamboo charcoal containing metal nitrate was heated in a tube furnace in an inert gas(e.g.,N2)atmosphere at different temperatures(600? and 800? for 3 hours).Solid residues were collected as catalysts for bamboo biochar loading and annotated as "Fe-600","Fe-800","Ni-600" and "Ni-800",respectively.The second step,characterization of catalystsThe catalyst was characterized by specific surface area analyzer(BET)and X-ray diffractometer(XRD).The BET results showed that the SBET,Vt,and Vmec of biochar,as well as the average pore size,were largely dependent on the preparation temperature and the type of metal ion(Ni2+ or Fe3+).When the catalyst preparation temperature is increased,the specific surface area,total pore volume,and micropore volume of the catalyst are all reduced.For example,under the same nitrate conditions,the specific surface area of biochar Fe-600 and Ni-600 prepared at a lower temperature of 600?(418.716 m2/g and 394.445 m2/g),total pore volume(0.347 cm3/g and 0.235 cm3/g),micropore volume(0.2193 cm3/g and 0.2074 cm3/g)greater than the specific surface area of biochar Fe-800 and Ni-800 prepared at a higher temperature of 800?(338.469 m2/g and 271.570 m2/g),total pore volume(0.302 cm3/g and 0.173 cm3/g),micropore volume(0.1791 cm3/g and 0.1452 cm3/g).However,the average pore size of biochar increases with increasing temperature.The average pore size of biochar Fe-600 is 3.708 nm,while that of biochar Fe-800 is 4.099 nm.Similarly,the average pore sizes of biochar Ni-600 and Ni-800 were 2.628 nm and 2.861 nm,respectively.However,different nitrates at the same temperature can also affect the structure of biochar.The biochar with nickel nitrate as the soaking solution,its SBET,Vt and Vmec,and average pore size are generally smaller than those with iron nitrate as the soaking solution.It is worth noting that the specific surface area,total pore volume and micropore volume of biochar Ni-800 are relatively small.XRD observed the crystal structure of biochar.Two typical biochar bands were observed at 2??22° and 45°,corresponding to graphite(002)and(101)bands,respectively.(002)bands indicate a low crystallinity ordering of biochar,which contains amorphous carbon and aliphatic side chains.For Ni-800 and Fe-800,the intensities of graphite(002)at 2??22.79°and graphite(101)at 2??45.2° become stronger,indicating that when the temperature increases,the peak position of the cellulosic graphite microcrystals shifts to a high angle,resulting in an increase in peak intensity and peak area.Interestingly,there are two distinct peaks around 52.81° and 77.36° corresponding to the(102)and(440)planes of Ni-800,indicating that the biochar decomposes with increasing temperature,the diffraction peaks of the d002 crystal plane of the cellulosic graphite grains are enhanced,and the crystallinity also increases.In the third step,microwave-assisted biochars catalyze depolymerization of organosolv lignin.The reaction conditions were as follows:ethanol as solvent(20 mL),formic acid as hydrogen supply and providing acidic environment(20 mL),biochar Ni-600,Ni-800,Fe-600 and Fe-800 as catalysts(0.5 g),organosolv lignin is raw material(0.5 g).The experimients are carried in a microwave reactor(power 650W)for 30 minutes and rapidly heated to 180?.In addition to this,non-catalytic depolymerization experiments were performed for comparison study.The experimental conditions were ethanol 20mL,formic acid 20mL,lignin raw material 0.5g,reaction time of 30 minutes,reaction temperature of 140?-160?-180?-200? in a microwave reactor(power of 650W)?The experimental results showed that in the non-catalytic depolymerization experiment,the yield of bio-oil significantly increased when the temperature was increased from 140? to 180?,reaching a maximum yield of 76.1%at 180?,and the lignin residue was 22.08%at this temperature,while the amount of coke was 1.9%.The bio-oil yield gradually decreased to 69.6%when the temperature was 200?.Furthermore,carbonization of lignin became more and more pronounced with increasing temperature.At 200?,the yield of coke increased to 6.9%,indicating that part of coke formation could be attributed to repolymerization at high temperature.In the biochar catalysis experiment,Ni-600 showed excellent catalysis,the yield of the bio-oil increased to 88%,the lignin residue decreased to 10.9%,and only 1%of the coke.Among the other three biocar catalysts,the bio oil yield of Fe-800 catalyst reaction was slightly increased,which was 79.06%,lignin residue was 17.7%,and coke was 3%.The bio oil yield of Fe-600 and Ni-800 catalyzed reactions was 66.53%and 68.56%,respectively,and the amount of coke was 3.06%and 1.1%,respectively.Quantitative analysis of bio-oil using matrix-assisted laser desorption mass spectrometry(MALDI-TOF MS)detected 13 major peaks with molecular weights ranging from 160 m/z to 500 m/z of 164 m/z,172 m/z,188 m/z,212 m/z,228 m/z,234 m/z,288 m/z,296 m/z,316 m/z,336 m/z,402 m/z,432 m/z,and 446 m/z,respectively.Moreover,each peak has a different peak area.In the non-catalyzed bio-oil,the relative peak area corresponding to these 13 peaks(the proportion of monomer and dimer in bio-oil)was 63.3%.Whereas,the relative total peak area of the catalyzed reactions(Fe-600,Fe-800 and Ni-600)increased.Among them,the relative peak areas of biochar Fe-600 and Fe-800 were 73%and 72%,respectively,while the relative peak areas of biochar Ni-600 catalytic reaction reached 80.4%.It is noteworthy that the MALDI-TOF MS spectrum of the Ni-600 catalyzed reaction showed the largest peak areas for monomers and dimers of 228 m/z,234 m/z,288 m/z,296m/z,446 m/z.It can be seen that the catalytic ability of the biochar catalyst Ni-600 is very significant,which is attributed to the porous structure of the nickel-based catalyst,which increases the catalyst surface area and improves the catalytic activity.Moreover,nickel catalyst is also a hydrogenating catalyst,which is conducive to the breaking of C-C bond in lignin.Whereas,interestingly,the total relative peak area of the Ni-800 catalyzed reaction was only 58%.This is because the specific surface area,total pore volume,and micropore volume of the Ni-800 biochar catalyst are very small,losing the advantages of nickel-based biochar,resulting in reduced catalytic activity.The coke(residue)was analyzed by gel permeation(GPC).In the data of Mw and Mn of residue,Mw and Mn without catalytic reaction were 1390 and 697.However,Mn decreased after biochar catalytic reaction.The Mn of the residue after catalytic reaction of biochar Fe-600,Fe-800,Ni-600 and Ni-800 were 1307,1289,1158 and 1388,respectively.It can be seen that the molecular weight of the residue of biochar Ni-600 reaction is the smallest.This indicates that the catalyst has a good depolymerization effect,which is consistent with other analytical results.According to Fourier infrared spectroscopy,it can be observed that not only the O-H stretching at 3423 cm-1,the methoxy C-H stretching at 2937 cm-1,belonging to syringyl(S)in the catalyst Ni-600 and Fe-800 depolymerization products.The C-O stretching vibration and the C-O stretching belonging to guaiacyl(G)are all weakened,but also the C=C stretching belonging to the carbon bond is significantly weakened.Combined with the results of other analytical means,it can be inferred that this experiment not only broke the C-O bond in lignin,but also broken C=C bond.This is rooted in the acidic environment provided by formic acid that promotes the fragmentation of ether bonds(?-O-4 and 4-0-5)in lignin molecules,whereas the biochar nickel-based catalyst Ni-600 and the iron-based catalyst Fe-800 promote the fragmentation of the C=C bond. |
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