Investigation Of Effective Catalytic Conversion Of Lignin Into Alkylphenols

Lignin is the only renewable aromatic organic carbonaceous source in nature and is expected to be a substitute for fossil resources such as petroleum and coal.Currently,the pulping industries and lignocellulosic fermentation industries produce a large amount of lignin as a waste.The thesis is focused on the catalytic conversion processes of enzymatic hydrolysis lignin(EHL)and Kraft lignin(KL)in ethanol.The goals of the work are to realize the more efficient catalytic valorization of lignin into high value-added alkylphenol products.The selective deoxygenation of guaiacol in supercritical ethanol to produce alkylphenols was investigated using Mo O₃ as a catalyst.The conversion of guaiacol and selectivity of alkylphenols were 99%and 86%,respectively.Mo O₃ showed better catalytic activity in low-carbon alcohols.We further found that catechol is an intermediate of guaiacol conversion,which can be directly alkylated into ethylphenols.A series of higher alkylphenols are obtained via successive substitution of active hydrogen atoms in the?-carbons of ethylphenols with methyl or ethyl groups produced by the solvent.The special Mo O_xC_yH_z phase with Mo⁵⁺formed on the surface of molybdenum oxide was verified to be the active component of the reaction.The activity of molybdenum oxide catalyst for the depolymerization of EHL in supercritical ethanol was studied.Alkylphenols were the main identified aromatic products including ethylphenols,iso-propylphenols and tert-butylphenols.The yield of identified aromatics from EHL conversion in supercritical ethanol at 300 ^oC reached 239 mg/g lignin with 52 wt.%alkylphenols.Ce showed positive effects on the formation of active species and the inhibition of carbon deposition on the catalyst.It is speculated that the dissolved EHL molecules or EHL fragments from non-catalytic ethanolysis can overcome mass transfer limitation and contact effectively with the surface of catalyst.Mo O₃ selectively break the adjacent Ar-O bonds in their structure to form catechol intermediates,which are then converted to small alkylphenols dominantly with alkylation and isomerization.The effective depolymerization of KL was achieved by introducing various sulfates into the molybdenum oxide catalytic system.We found that KL has lower lignin content,larger particle size and molecular weight,less ether bonds as well as limited solubility in ethanol.Fe₂(SO₄)₃ has been verified to be the best co-catalyst for Mo O₃.At 300 ^oC and 6 h,the yield of identified aromatic products from KL was 127mg/g lignin with 73 wt.%alkylphenols.We also characterized the residues and product composition in different reaction systems.Fe₂(SO₄)₃ was not stable and could dissolve in ethanol partly during reaction,which was likely to depolymerize KL preliminarily as a liquid catalyst.The obtained lignin fragments and a small amount of identified primary aromatics continued to be hydrodeoxygenated on the surface of solid molybdenum oxide.The depolymerization of lignin was explored under mild reaction conditons.A novel method of selective conversion of EHL into alkylphenols in a concentrated Zn Cl₂ethanol solution was designed.Zn Cl₂ significantly improved the solubility of EHL in ethanol.The total identified aromatic yield was up to 304 mg/g lignin at 200^oC without H₂ in a 40 wt.%Zn Cl₂ ethanol solution.The fraction of alkylphenols in the identified products was 68 wt.%.In 20 m L 40 wt.%Zn Cl₂ ethanol solution,200-500mg EHL could be converted effectively.The Zn²⁺coordinating with less ethanol molecules in concentrated Zn Cl₂ ethanol solution is able to coordinate with the oxygen atoms possessing lone electron pairs in ether bonds of lignin as well as weaken it.The conversion of lignin model compounds showed that the adjacent C-O bonds and Ar-O-alkyl were able to be broken more easily.The rupture of C-O bonds was realized by formation of-OH.