Yields Of Monomeric Products Research Articles

Selective Depolymerization of Lignin: Assessment of Yields of Monomeric Products

A method is developed to assess the limits of selective depolymerization, that is, splitting of aryl ether bonds to yield monomeric aromatic products, of native and technical lignins. The method and respective formula is based on measuring the difference between a sum of aryl ether bonds (α-О-4 and β-О-4) and sum of 5-5 and 4-O-5 bonds. The values calculated based on the proposed formula are correlated with published experimental data on catalytic degradation of lignin. In all cases, wood species and experimental conditions notwithstanding, the yields of monomeric aromatic products are found to be close to theoretically calculated for both softwood (23%) and hardwood (51%). Notice that in the actual experiments the yield in hardwood is twice as much as in softwood. In technical lignins (kraft, organosolv) the yields of monomeric products are lowed due to splitting of α-О-4 and β-О-4 bonds during pulping. The developed method is helpful in evaluation of technologies aiming to bring value-added products from lignin through selective degradation.

Decomposition of lignin model compounds by Lewis acid catalysts in water and ethanol

The conversion of benzyl phenyl ether, diphenyl ether, diphenyl methane and biphenyl as representative model compounds for α–O4, 5–O4, α 1 (methylene bridges) and 5–5′ lignin linkages was investigated. We compared the use of metal chlorides and acetates. The reactions were studied in sub- and supercritical water and supercritical ethanol between 300 and 400°C. At low temperature in water, Lewis acids mainly catalyzed condensation of hydrolysis products of the dimeric model compounds. At higher temperature, mono-aromatic products were formed. The yield of monomeric products was higher in ethanol than in water. The preference for ethanol is due to extensive alkylation of the mono-aromatic products, which inhibits their condensation into larger products. The highest yields of deoxygenated mono-aromatics were obtained using Lewis acid catalysts at 400°C in supercritical ethanol. The preferred Lewis acid catalysts were Fe, Cu, Ni and Al chlorides.

Oxidation of Lignin Using Aqueous Polyoxometalates in the Presence of Alcohols

A novel approach has been developed in order to use Kraft lignin as a renewable resource for the production of chemicals. The concept is based on the use of polyoxometalates as reversible oxidants and on the use of radical scavengers, which prevent lignin fragments from repolymerizing. The oxidation of Kraft lignin, which is a potential source of functionalized phenols, by H3PMo12O40 in water yields a relatively small amount of monomeric species detected by GC-MS. The addition of methanol to the reaction resulted in an increase in the yield of monomeric products by a factor of up to 15. Vanillin and methyl vanillate are the main products obtained, in a maximum yield of 5 wt % based on dry Kraft lignin. Methanol plays a decisive role in the prevention of repolymerization by reducing lignin-lignin condensation reactions. Furthermore, it is proposed that methanol generates small amounts of .CH3 and CH3O. radicals through the acid-catalyzed formation of dimethyl ether which couple with lignin fragments.

Chemical Recycling, Kinetics, and Thermodynamics of Alkaline Depolymerization of Waste Polyesters

Depolymerization of waste poly (ethylene terephthalate) and poly (butylene terephthalate) were undertaken in various alkaline solutions at different reaction parameters at 1 atm in a batch operation. Results were evaluated in terms of weight loss of both the waste polyesters as well as products obtained during depolymerization of polyesters. Poly (ethylene terephthalate) (PET) showed more weight loss as compared to that of poly (butylene terephthalate) (PBT) in various potassium hydroxide (KOH) solutions. Methanolic alkaline solution has showed greater reaction effect as compared to those of aqueous as well as ethanolic solutions for both the waste polyesters. Reaction products composed of ethylene glycol (EG)/1,4-butanediol (BD) and terephthalic acid (TPA) that were analyzed and confirmed. Yields of monomeric products (TPA and EG/BD) were almost equal to those of polyester conversion. To accelerate the rate of reaction cyclohexylamine (CHA), tetrahydrofurane (THF), and dimethyl sulfoxide (DMSO) were introduced during depolymerization of polyesters. A kinetic model is developed and experimental data are fitted with it for validity of the model. The model shows good agreement with the experimental data. Thermodynamics are also undertaken to estimate the rate constants, equilibrium constant, Gibbs free energy, enthalpy, and entropy of reaction that is required to transfer laboratory data through the pilot plant for commercialization. Dependence of rate constant on reaction temperature was correlated by Arrhenius plot that was used to determine activation energy and Arrhenius constant.