| With the depletion of traditional fossil energy and the increasingly serious environmental problems,the utilization of renewable biomass resources has attracted more and more attention.Lignin is the second largest biomass energy after cellulose,and it is also the only non-petroleum resource that can provide renewable aromatic compounds in nature.However,due to its complex structure and stable chemical properties,its conversion and utilization is quite difficult.Thus,it is an important way to convert it into high-valued aryl oxy-compounds by chemical methods.Perovskite has the advantages of good chemical stability,high mechanical strength and strong ability of transferring oxygen ions and electrons.In this paper,a series of doped LaFeO3 perovskite samples were synthesized and characterized by XRD,SEM,BET and XPS.The oxygen-release and regeneration performance of these samples were investigated.Their abilities of catalyzing bagasse lignin(BL)pyrolysis were evaluated in a fixed-bed reactor.The catalytic pyrolysis kinetics was calculated,and gaseous,liquid and solid products were analyzed.The mechanism of catalyzing BL pyrolysis to produce aryl oxy-compounds was proposed.The main results are as follows:1.A series of LaTixFe1-xO3(LTF-SS-x)samples with cubic crystal phase were synthesized by the solid-state method.The maximum crystallinity was 85.02%when x=0.2,it’s compact,the specific surface area was only 0.55 m2·g-1,there are La2O3 and Fe2O3 hetero phases in it.By optimizing the synthesis method,the porous LaTixFe1-xO3(LTF-SG-x)samples were synthetized by the sol-gel method,the specific surface area can be 12.95 m2·g-1,and there is no La2O3 and Fe2O3 hetero phase generated.The optimum synthesis conditions were:EDTA-H3Cit-NH3·H2O was used as the complexing system,the optimum doping amount x=0.2,nEDTA:nH3Cit:ntotal metal=1:1.6:1,the pH value of the solution was 8.5;the best calcination condition was that roasting at 800℃ for 6 h.2.The characterization results showed that the BL grains are ellipsoid,and its particle size was 15～20μm.The pyrolysis process could be divided into three stages:(i)90～200℃,the weight loss rate was 2.5 wt.%,which was mainly due to the evaporation of water adsorbed;(ii)200～477℃,the weight loss rate was 66 wt.%.At this stage,the ether bonds and C-C bonds in BL were cracked to form small-molecule gases and aryl oxy-compounds,this stage was the main pyrolysis stage of BL;(iii)above 477℃,the weight loss was about 6.5 wt.%.In this stage,aromatic rings condensed,and the pyrolysis of BL was basically completed.With the addition of LTF-SG-0.2-800 and LTF-SS-0.2,the final temperature of the first pyrolysis stage was basically unchanged;the final temperature of the second pyrolysis stage was reduced by 49℃ and 25℃ respectively.3.The catalytic pyrolysis performance of BL was evaluated in a micro fixed-bed reactor.The experimental conditions were as follows:perovskite and BL were mixed evenly with the mass ratio of 1:3.The flow rate of the carrier gas,N2,was 100 mL·min-1.The temperature was raised from room temperature to 600℃ at the rate of 10℃·min-1 and kept for 2 h.With the addition of LTF-SG-0.2,the contents of CO2,CO and CnHm(n=2～4.m=2n+2 or m=2n)in the gaseous products were reduced by 8.0%,30.7%and 0.3%,respectively,and the content of CH4 was increased by 30.4%.It was concluded that LTF-SG-0.2 can inhibit decarboxylation and decarbonylation and promote the methyl side chain to break.The yield of liquid product increased from 16.46%to 22.17%,and the selectivity of aromatic oxygen increased from 62.49%to 74.05%,indicating that the addition of LTF-SG-0.2-800 can promote more oxy-containing functional groups go into the liquid product.4.B-site doped LaFeO3,LaB’0.2Fe0.8O3(B’=Fe,Cu,Al,Ti)was synthesized,which had cubic crystal phase and porous structure.The specific surface area of LaTi0.2Fe0.8O3 was the largest,12.95 m2·g-1.Compared with the pyrolysis of BL,the addition of LaB’0.2Fe0.8O3(B’=Fe,Cu,Al,Ti)can promote BL pyrolysis,and LaCu0.2Fe0.8O3 showed the best catalytic effect.Compared with the pyrolysis of BL catalyzed by LaFeO3,with the addition of LaCu0.2Fe0.8O3,the total selectivity of guaiacols and syringols increased by 13.3%.That is to say with the addition of LaCu0.2Fe0.8O3,some chemical bonds in BL can be broken to form aryl oxy-compounds.After five catalysis-regeneration cycles,LaCu0.2Fe0.8O3 still showed satisfactory structural and catalytic stability.5.A-site doped LaFeO3,La0.8A’0.2FeO3(A’=La,Ca,Sr,Ba)was synthesized,which had cubic crystal phase and porous structure.The specific surface area of La0.8Sr0.2FeO3 was the largest,12.29 m2·g-1.Compared with the pyrolysis of BL,the addition of La0.8A’0.2FeO3(A’=La,Ca,Sr,Ba)can promote BL pyrolysis,and La0.8Sr0.2FeO3 showed the best catalytic effect.Compared with the pyrolysis of BL catalyzed by LaFeO3,with the addition of La0.8Sr0.2FeO3,the yield of liquid products increased by 25.9%,the total selectivity of guaiacols and syringols increased by 17.4%.After five catalysis-regeneration cycles,La0.8Sr0.2FeO3 still showed satisfactory structural and catalytic stability.6.The kinetic parameters of the pyrolysis processes were calculated by the Coats-Redfern integral method.The results showed that BL pyrolysis catalyzed by LaB’0.2Fe0.8O3(B’=Fe,Cu,Al,Ti)and La0.8A’0.2FeO3(A’=La,Ca,Sr,Ba)conformed to the second-order kinetic law in the temperature range of 200～357℃,R2 was larger than 0.99.When LaCu0.2Fe0.8O3 was used to catalyze BL pyrolysis,the activation energy E was 60.37 KJ·mol-1,the pre-exponential factor A was 540.4 s-1,and the apparent kinetic equation was dα/dt=540.4×e-(60370/8.314T)(1-α)2.When La0.8Sr0.2FeO3 was used to catalyze BL pyrolysis,the activation energy E was 55.26 KJ·mol-1,the pre-exponential factor A was 495.2 s-1,and the apparent kinetic equation was dα/dt=495.2×e-(55260/8.314T)(1-α)~2. |
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