Study On The Influence Of Alkali/alkaline Earth Metals On The Pyrolysis Of Hemicellulose And Lignin

Efficient conversion of biomass is one of the important technology routes to achieve China’s carbon-neutrality selected chemicals get before 2060 and enhance energy security capability.As an efficient and low-cost technique,fast pyrolysis of biomass can provide high yield of bio-oil（up to 75%）.However,the alkali/alkaline earth metals（AAEMs）,which are naturally existed in biomass,have a negative effect on the yields and quality of bio-oil.The current literature mainly focuses on the effect of AAEMs on biomass and cellulose pyrolysis,while the effect of AAEMs on hemicellulose and lignin pyrolysis and reaction mechanism-is less studied.In this paper,the effects of AAEMs on the pyrolysis characteristics and reaction mechanism of xylan and lignin were investigated deeply,which can provide theoretical basis and technical guidance to produce high-quality bio-oil efficiently through biomass pyrolysis technique.Firstly,hydrochloric acid was used to remove the AAEMs contained in xylan and alkali lignin,and then the demineralized samples were doped with alkali/alkaline earth metal salts.Secondly,the pyrolysis characteristics and chemical kinetics of xylan and lignin were studied by TG-MS-FTIR,Py-GC/MS and fixed-bed experiments,and compared with the pyrolysis characteristics demineralized cellulose further.Thirdly,Different factors including AAEMs type,doping amount and anion type doped in xylan and alkali lignin were investigated by the same method.Finally,the online monitoring and analysis on the evolution of the main functional groups of xylan and lignin in the process of various xylan and alkali lignin samples,were carried out by using the in-situ Fourier transform infrared spectrometer（DRIFTS）combined with 2D-PCIS,to reveal its microscopic reaction mechanism.The experimental results show that the effect of AAEMs on xylan mainly lies in the ring-opening reaction of pyran ring,which increases the activation energy of the whole pyrolysis reaction.First,compared with demineralized xylan,the ring-opening reaction of xylan was advanced to 300°C by the doping of AAEMs,and the yield of H₂O,CO₂,CO and small molecular compounds such as acetic acid and hydroxyacetone were promoted,while the production rate of dehydrated sugar,furfural and other ring compounds was reduced.In addition,the catalytic effect of alkali earth metals（Mg,Ca）was weaker than that of alkali metals（K,Na）.The doping amount of AAEMs was positively correlated with the pyrolysis behavior of xylan.Besides,the anions of AAEMs have a few effects on the pyrolysis of xylan.Compared with KNO₃ and KAc,KCl-doped xylan releases its volatile peaks earlier and produces fewer permanent gas and more solid products.Finally,the combination of DRIFTS with 2D-PCIS showed that,the cleavage of the side chain-CH₂ and the ring-opening reaction of pyranose in the presence of AAEMs are earlier than the dehydration reaction of hydroxyl/hydrogen bond.The ring-opening reaction of C-O in xylose matrix was earlier than the bond formation reaction of C=C and C=O under the action of AAEMs.Lignin pyrolysis experiments showed that the existence of AAEMs promoted the further break of the ether bond and the side chain of the demineralized alkali lignin,and lowered the activation energy of the pyrolysis reaction.First,compared with the demineralized alkali lignin,the addition of AAEMs resulted in the break of the inter-lignin ether bond,the increase of the aromatic ring products and the side chain of the monomers,which resulted in more permanent gases such as H₂O,CO_2,CO,CH₄ and light aldehydes and ketoacids.In addition,the yields of 2-methylphenol,2-hydroxybenzaldehyde,and phenol increased with the addition of alkali metals,while decreased with the addition of alkaline earth metals,it is speculated that alkali metals promote the break of side chains of aromatic rings（such as methoxy group）to form simpler monomers,whereas alkaline earth metals have the opposite effect.The yield of phenol,2-methoxy,2-methoxy-4-vinylphenol,1,2-dihydroxybenzene and phenol increased with the increase of doping amount of AAEMs,while that of vanillin,furfural and 2-methoxy-1-decreased.Finally,the results of DRIFTS combined with 2D-PCIS showed that the addition of AAEMs could destroy the internal hydroxyl/hydrogen bond connection of lignin,improve the reactivity of free hydroxyl group,and promote the break of the initial bonding bond.AAEMs promotes the removal of side chain and unstable functional groups such as hydroxyl group,phenyl ether group and part of methoxy group,and affects the recombination of benzene group.