Thermal Degradation of Kraft Lignin in Tetralin

Abstract

WA 98195 and from the U.S. Forest Products Laboratory, Madison, WI 53705 Keywords Kraft lignin Lignin model compounds Hydrocracking Phenols Tetralin Schlüsselwörter (Sachgebiete) Kraftlignin Lignin-Modellverbindungen Hydro-Verkrackung Phenole Tetralin Thermal Degradation of Kraft Lignin in Tetralin SummaryHydrocracking of kraft lignin (Indulin-AT) and of model compounds p-ethylguaiacol and dehydrodihydrodiisoeugenol (DDDI) was studied with tetralin äs hydrogen donor in the temperature ränge 375 to 400° C. It was found that aliphatic oxygen functions reacted most rapidly. The aromatic methoxyl groups underwent either methyl-oxygen or ring-oxygen cleavage forming catechol and phenol groups, respectively. The former process is more prevalent at higher temperatures. Side-chain cleavages resulted in the general lowering of molecular weight and formation of monomeric phenols. Careful analysis of the latter products suggested that these cleavages occurred mostly between the a-and ß-carbon atoms but cleavages between the aromatic ring and the -carbon also occurred. Upon prolonged hydrocracking, guaiacols and catechols disappeared while a mixture of phenol, cresols and ethyl phenols was relatively resistant towards further conversions. Thermischer Abbau von Kraftlignin in TetralinZusammenfassung Die Hydro-Verkrackung von Kraftlignin (Indulin AT) und von Modellverbindungen wie p-Äthyl-guajakol und Dehydrodihydro-Diisoeugenol (DDDI) wurde mit Tetralin als Wasserstoff-Donator im Temperaturbereich von 375 bis 400° C untersucht. Dabei wurde festgestellt, daß die Sauerstofftragenden Seitenketten sehr schnell reagieren. Die aromatischen Methoxylgruppen unterliegen entweder Methyl-Sauerstoff-oder Ring-Sauerstoff-Spaltung, wobei Catechol-bzw. PhenolGruppen gebildet werden. Der erstgenannte Prozeß läuft vorzugsweise bei höheren Temperaturen ab. Infolge der Seitenketten-Spaltung erniedrigt sich allgemein das Molekulargewicht, und es entstehen monomere Phenole. Genaue Analysen der letztgenannten Produkte weisen darauf hin, daß diese Spaltungen meistens zwischen dem a-und /3-Kohlenstoffatom vorkommen, aber auch zwischen dem aromatischen Ring und dem «-Kohlenstoff. Bei längerer Verkrackung verschwinden die Guajakol-und Catechol-Verbindungen, während eine Mischung von Phenol, Kresol und Äthylpheriolen relativ resistent gegen weitere Umwandlungen war.

Heterogeneously catalyzed lignin depolymerization

“…Oxidizing routes are generally undesirable due to radical formation which can result in partial repolymerization, whereas hydrogenolysis protocols minimize lignin repolymerization and promote C-O bond cleavage by quenching and recombination of radicals, although the addition of molecular hydrogen can result in fully hydrogenated cyclic hydrocarbons of relatively low commercial value. Hydrogen-donating solvents able to generate/transfer hydrogen have, thus, also shown promise, e.g., Connors et al report hydrogen transfer from ethanol, tetralin, and formic acid during lignin hydrocracking [5], while Kleinert et al reported a novel solvolysis method employing high-pressure thermal treatment of lignin in the presence of ethanol as solvent and formic acid as hydrogen-donor, the latter decomposing to liberate CO 2 and hydrogen [3]. Glycerol [6] and isopropanol [7] are also promising hydrogen-donor solvents.…”

Liquid Fuels from Lignins: Annual Report

“…The maximum yield of soluble phenolics was found to be approximately 35% , which compares well with 37% fotmd by Connors et al (1980) from kraft lignin. While the total yields of phenolics from these two studies were similar, the detailed composi tion of the phenolic compotmds showed some differences.…”

“…While the total yields of phenolics from these two studies were similar, the detailed composi tion of the phenolic compotmds showed some differences. High yields of p-methylphenol were reported by Connors et al (1980). Schultz, Preto, Pittman, and Goldstein (1982) In later work, (Inventa 1956b;Giesen 1959), the scale of the process was increased from about the 200 g level up to approximately 10 kg of 1 ignin per run.…”

Lignin Degradation Processes and the Purification of Valuable Products

“…These reductive reaction characteristics of hydrogenolysis tend to reduce or remove functionality in lignin ultimately leading to simpler phenols [2]. Therefore, hydrogenolysis is a promising method for the production of bulk phenolic chemicals from lignin [2,5,142]. A large number of heterogeneous metal-based catalysts (e.g., Ni, Rh, Ni-Mo, Co-Mo and Pd) and a significant smaller number of homogeneous catalysts have been combined with the hydrogenolysis of different lignin models, technical lignins or lignocellulosic biomass [2,141,143].…”