Study On Conversion Of Bio-oil To Light Components In-situ Catalyzed By Biochar-based Catalyst

Rapid pyrolysis is a method of thermochemical conversion of biomass,and the yield of pyrolysis oil obtained in the gentle pyrolysis process can reach 70%.However,primary biomass pyrolysis oil is limited in its application field due to its complex composition,high oxygen content,poor stability,etc.Therefore,upgrading process is needed regarding the target of obtaining the high-quality liquid fuel.Catalytic pyrolysis is an effective way to promote the conversion of heavy bio-oil from deoxidation to light bio-oil,and the biochar as a catalyst for the upgrading of biomass pyrolysis oil has attracted extensive attention from researchers.In this study,the pine sawdust was used as raw material to focus on the key scientific issues of in-situ catalytic cracking of biomass pyrolysis oil on biochar with multiple active sites.Firstly,the interaction between biochar and volatiles was studied through model compounds,and the critical role of typical biochar surface functional groups in the catalytic process was clarified.Then,based on the physical and chemical structure regulation of biochar was taken as the core,and the synergistic mechanism of multiple active sites of char-based catalyst was revealed by analyzing the conversion pathway of volatiles.Finally,the deactivation mechanism and regeneration methods of char-based catalysts were studied based on the evolution in the catalytic process,and a light-weight pyrolysis system for biomass pyrolysis oil rich in phenolic compounds was established.The main results are as follows:（1）The interaction mechanism between functionalized char-based catalysts and volatiles was investigated by the decomposition experiments using Py-GC/MS.The results indicated that the conversion of volatile model compound benzyl phenyl ether（BPE）was only 0.1%in the absence of catalyst,while the functionalized carbon nanotubes（CNTs）as a catalyst significantly promoted the decomposition of BPE,and the conversion was 55.1～70.2%.Compared with hydroxylated carbon nanotubes（CNT-OH）and carboxylated carbon nanotubes（CNT-COOH）,aminated carbon nanotubes（CNT-NH₂）showed better catalytic activity,mainly attributed to the hydrogen bonding between BPE and CNTs,and the influence ofπ-πstacking effect on the density distribution of C-O bond electron cloud in BPE.As a typical pyrolysis product of guaiacol glycerol-β-guaiacol ether（G-β-GE）,guaiacol has a relative content of more than 35%.Under the action of CNT-COOH and CNT-OH,it was promoted the cleavage of the methoxy group on the benzene ring to generate phenol,simple aromatic compounds such as 2-methylphenolm,which was attributed to the fact that the reduction of the dissociation energy of C_β-O by CNTs,thus promoting the rupture of chemical bonds.（2）Py-GC/MS was used to investigate the interaction between the modified biochar catalyst and pyrolytic volatile BPE,and to reveal the catalytic mechanism of different active sites in the real biochar during the interaction.The results showed that the activity of biochar prepared at low temperature was mainly attributed to the surface oxygen-containing functional groups.With the increase of the preparation temperature from 400°C（N400）to 1000°C（N1000）,the catalytic activity of the biochar decreased,and the conversion decreased from 17.63%to 7.79%.When the acid-washed N1000 was used as a catalyst,the conversion of BPE recovered to 17.27%,mainly due to the active sites of the carbon structure（such as small aromatic rings）occupied by alkali metals and alkaline earth metals（AAEMs）were re-released after acid washing.The BPE conversion reached 100%and only phenol and toluene were found in the product when the carbon structure and pore structure of biochar were modified by steam activation.The catalytic activity of biochar can also be significantly improved when loaded with iron,especially when loaded with10%and 20%iron over N1000,all conversion of BPE can also be achieved.When different biochar was used as the carrier,the occurrence form of iron was obviously different,and Fe_xO_ywas dominant in N400 whileα-Fe₂O₃ was dominant in N1000.The synergistic effect of the structure of biochar and iron greatly promoted the high selectivity of phenol and toluene in the catalytic reforming of BPE and its derived volatiles.（3）The distribution and conversion mechanism of pyrolysis products of real bio-oil catalyzed by real biochar were further investigated in a second-order fixed-bed quartz reactor.It can be seen that when the reaction temperature was 400℃,the pyrolysis products were ketones,phenols,acids and a small amount of esters,aldehydes,furans and other compounds.However,when the reaction temperature rose to 600℃,all the ketones,acids,aldehydes and esters generated in the non-catalytic pyrolysis disappeared,and only phenols and aromatic hydrocarbons were detected in the pyrolysis products.With the introduction of biochar-based catalysts,the content of phenol in phenolic substances in the catalytic reaction product increased from 76%to 97%,that is,catalytic cracking enhanced the dissociation of side chains of alkylphenol or alkoxyphenol from aromatic compounds.Surface oxygen-containing functional groups and pore structure were key factors to enhance the catalytic activity.During the catalytic pyrolysis,the active center contributed to improve the pyrolysis reaction rate and promote high selectivity for phenolic or aromatic substances.（4）The catalytic pyrolysis mechanism of pine sawdust by activated char supported iron catalyst and the catalytic reforming after regeneration of CO₂ and H₂O were investigated in a second-order fixed-bed quartz reactor.The study found that when the volatiles from biomass pyrolysis were subjected to the activated char-supported iron catalyst,all the original phenolic substances were converted to toluene,and a small amount of o-xylene,dimer naphthalene and1,3-xylene,indicating that the introduction of iron led to the change of the active site in the catalyst.The analysis showed that iron could promote the formation of lattice defects in carbon matrix and new active sites in crystal layer.The hydrodeoxygenation process of volatiles was enhanced by the interaction between iron and activated char-based carrier.However,the iron-containing char-based catalyst regenerated by CO₂ or H₂O could convert all the pyrolysis volatiles into phenolic substances.（5）The continuous catalytic pyrolysis of activated char-based catalyst was enhanced by introducing metal iron and surface active groups.The pyrolysis pathway of light bio-oil and the catalytic capacity of the catalyst in continuous use were investigated.The results showed that the relative content of phenolic compounds produced by the activated char-supported iron catalyst was still higher than 90%after five consecutive pyrolysis reactions,while the selectivity to phenol was reduced from 80%to 30%.And after it was regenerated by CO₂,its catalytic activity can be restored,all the products were converted into phenolic substances,and the selectivity to phenol was up to 87%.The activated char-based catalysts promoted the interaction between the phenolic intermediates adsorbed on char carrier and the oxygen species,thereby forming more phenolic compounds.