Study On Metal Catalyzed Biomass Pyrolysis For Preparation Of High-value Chemicals

Lignocellulosic biomass can be converted into bio-oil,biochar,and gas through rapid pyrolysis,and the liquid bio-oil contains hundreds of organic compounds,which has broad application prospects in fuel heating,transportation,and platform compound preparation.The bio-oil obtained by direct pyrolysis is usually with high acidity,oxygen content,and water content,making it hard for combustion.The complex composition and low enrichment degree of various compounds also make it hard for subsequent refining.These problems restrict the large-scale application of biomass rapid pyrolysis liquefaction technology.By optimizing the pyrolysis process and controlling the pyrolysis reaction path,the quality of bio-oil can be effectively improved and the enrichment degree of high-value chemicals in bio-oil can be increased.The key to realizing the directional regulation of biomass pyrolysis products is to clarify the mechanism of biomass pyrolysis reaction and to develop suitable catalysts.The abundant,cheap,and easily available metals are widely used in the catalytic pyrolysis of biomass.However,due to the complexity of the biomass pyrolysis reaction,it is difficult to meet the demand for practical application only by the single catalytic action of metal function.In order to improve the enrichment of high value-added chemicals in bio-oil,this paper studied the regulation mechanism of biomass pyrolysis catalyzed by metals and analyzed the effects of different metals on the distribution of biomass pyrolysis products and the pyrolysis reaction process.The catalytic mechanism of ammonium phosphate for selective pyrolysis of biomass to produce furfural and levoglucone was investigated simultaneously.Based on the above results,the potassium nitrogen phosphorus doped carbon-based catalyst was successfully prepared by treating the distillation residue with metal salt and ammonium phosphate,and its application in biomass catalytic pyrolysis was preliminarily investigated.1.Catalytic mechanism of common metals on biomass pyrolysisRice husk was washed by acetic acid to remove the inherent alkali and alkaline earth metals(AAEMs),and then was loaded with Na+,K+,Ca2+,and Mg2+by impregnation.The catalytic effects of different metals on biomass pyrolysis were studied by TG,Py-GC/MS,and Py-SVUV-PIMS(Pyrolyzer connected with synchrotron vacuum ultraviolet photoionization mass spectrometry).The results showed that the inherent AAEMs in biomass could promote the pyrolysis of cellulose and enhance the carbonization reaction of pyrolysis residue.Under the effects of the nherent AAEMs in biomass,the generation of phenols,acids,aldehydes,ketones,and cyclopentanones was promoted while the generation of anhydrosugars and furans was inhibited.Alkali metals(Na+and K+)could facilitate the pyrolysis of cellulose at low temperature,enhance the fragmentation of biomass components and increase the relative content of phenols,acids and aldehydes in the pyrolysis products.Compared with alkali metals,alkaline earth metals(Ca2+ and Mg2+)with stronger Lewis acidity are more favorable for depolymerization and dehydration of cellulose to produce anhydrosugars.Moreover,The stronger dehydration capacity of alkaline earth metals promotes the polymerization of phenolic products through the shrinkage reforming of methoxy groups,inhibiting the production of phenolics,especially methoxyphenols.Py-SVUV-PIMS observed that the time for different types of products reaching the maximum generation rate in the pyrolysis process of biomass is different.In the early pyrolysis stage,the generation of acids,aldehydes,aldehydes,phenols,and anhydrosugars was dominant,while in the late pyrolysis stage,the generation of furan and cyclopentanones was dominant.Inherent AAEMs could promote the pyrolysis of biomass components in the early pyrolysis stage,which is conducive to the precipitation of acid,aldehyde,ketone,and phenolic products.In the late pyrolysis stage,the secondary pyrolysis of oligomers or pyrolysis products was promoted by inherent AAEMs.Alkali metals could accelerate the reaction process significantly.Alkali metals can promote the fracture of ether bond between phenylpropane units and the secondary thermal cracking of big molecule phenolic products and pyrolytic oligomers,leading to an increase of phenol and m-cresol.Alkaline earth metals could also induce the secondary dehydration of anhydrosugars into small molecules such as furfural during pyrolysis.Most of the furfural is directly generated from hemicellulose decomposition in the early pyrolysis stage,while a small part comes from the secondary conversion of anhydrosugars in the late pyrolysis stage.Zn2+ was selected as the representative to further explore the catalysis mechanism of metals with Lewis acidity on biomass pyrolysis reaction.The acid-washed rice husk was loaded with Zn2+ by impregnation.The mechanism of catalytic pyrolysis of biomass by Zn2+was studied by Py-GC/MS and Py-SVUV-PIMS.It was found that Zn2+ could promote the dehydration and cyclization reaction in the pyrolysis process of biomass,promoting the generation of acids,furans,and anhydrosugars,but suppressing the generation of phenols,aldehydes,and ketones.Under the action of Zn2+,the yields of phenolic products decreased,indicating Zn2+could inhibit the decomposition of lignin,The yields of furfural and 1-hydroxy-3,6-dioxy dicyclo-octane[3.2.1]-2-one were significantly increased,and the yields of 4-vinylphenol were significantly decreased.Zn2+advanced the time for the maximum production rate of furan and cyclopentanone products.The higher the Zn2+loading,the greater the advancement,The Lewis acidity of Zn2+could promote the fracture of glycosidic bond,strengthen the transformation of hemicellulose to furfural in the early pyrolysis stage,and reduce the transformation of cellulose to anhydrosugar as well as other dehydrated products in the late pyrolysis stage.2.Mechanism of selective pyrolysis of biomass for the preparation of furfural and levogluconeRice husks were loaded with different doses of NH4H2PO4 by wet impregnation.The pyrolysis experiments were carried out by TG-FTIR and Py-GC/MS to study the catalytic mechanism of ammonium phosphate on the pyrolysis of biomass and its regulation mechanism on the production of furfural and levoglucone.The results showed that NH4H2PO4 could provided additional hydrogen radical,which facilitates glycosidic bond rupture to release monomer sugar,andlowered the activation energy needed for the initial decomposition of cellulose.Under the action of NH4H2PO4,the dehydration reaction was enhanced as well as the formation of unsaturated functional groups and oxygen-containing heterocyclic was promoted,strengthening the release of C=O group contained compounds and carbohydrates below 300?.Simultaneously,NH4H2PO4 enhanced the dehydration condensation between methoxy groups and inhibited the formation of phenolic products.NH4H2PO4 showed good selectivity for both furfural and levoglucosenone.When the samples were pyrolysis at 550?,NH4H2PO4 increased the relative peak area of furfural from 2.5%to 16.5%,and increased the relative peak area of levoglucone from undetectable to 23.3%.With the increased ADP dose,the yield and relative content of furfural and levoglucosenone increased firstly and then kept stable.With the increased pyrolysis temperature,the yield of furfural showed a growth trend at 350??450? and then kept stable,while the yield of levoglucosenone increased to the maximum at 400? and then decreased.The maximum relative peak area of levoglucosenone at 400? was 34.28%,at which temperature the sum of the relative peak area of levoglucosenone and furfural also reached the maximum of 52.23%.3.Preparation and application of potassium nitrogen phosphorus doped carbon-based catalyst on biomass catalysis pyrolysisPotassium nitrogen phosphorus doped carbon-based catalyst AC-NP-K was prepared by treating the bio-oil distillation residue with NH4H2PO4 and CH3COOK.The effects of catalyst dosage and pyrolysis temperature on the yield distribution of three-phase products and the composition distribution of liquid products were investigated by means of a fixed-bed experimental device.The experimental results showed that nitrogen and phosphorus doping of biomass carbon with NH4H2PO4 was conducive to pore growth.K+modification effectively removed oxygen-containing functional groups and reduced the oxygen content on the surface of activated carbon.The catalytic effects of AC-NP-K on biomass pyrolysis are influenced by the catalyst dose.The catalytic effects of low dose AC-NP-K(biomass:catalyst=5:1)are mainly to promote the breaking of the connection bonds between the unit structures of each component,which was beneficial to the generation of phenolic products.When the samples was pyrolyzed at 550?,under the effect of low dose AC-NP-K,the relative peak area of 4-vinylphenol increased from 4.74%to 8.31%.The catalytic effects of medium dose AC-NP-K(biomass:catalyst=2:1 and 1:1)are mainly to enhance the dehydration effect which was conducive to the formation of acid and furan products,and the water content of liquid products increased.The catalytic effects of high dose AC-NP-K(biomass:catalyst=1:2)were mainly to promote the fragmentation of both biomass components and pyrolysis products,and to enhance the carbonization of oligomers,which improves the yield of solid and gas products as well as reduces the yield of liquid products.From 450? to 750?,the low dose of AC-NP-K increased the yield of liquid products.The moisture content and acid content of liquid products were reduced and the content of phenolic productswas increased.The higher the pyrolysis temperature,the more significant the catalytic effects.The proportion of 4-vinylphenol in all phenolic products reached the highest of 50.32%at 650?,and the yield of 4-vinylphenol maintained a high level at this temperature.Therefore,the pyrolysis temperature of 650? and the catalyst/biomass mass ratio of 1/5 can be used as the optimal conditions for the preparation of 4-vinylphenol by catalytic pyrolysis of biomass by AC-NP-K.In this paper,the catalytic mechanism of Na+,K+,Ca2+,Mg2+ and Zn2+ on biomass pyrolysis was studied.The changes of the time-evolution of pyrolysis reaction under the effects of these metal ions were investigated for the first time by synchrotron radiation photoionization mass spectrometry.Besides,the regulation mechanism of ammonium phosphate on the selective pyrolysis of biomass for the preparation of furfural and levoglucone was studied and the optimal conditions for the production of levoglucone and furfural were proposed.Based on the above research results,the Potassium nitrogen phosphorus doped carbon-based catalyst AC-NP-K was prepared by treating the bio-oil distillation residue with NH4H2PO4 and CH3COOK.,and the regulation mechanism of AC-NP-K on biomass pyrolysis reaction was explored.Under the action of AC-NP-K,the proportion of 4-vinyl phenol in phenol products was significantly increased.