ISBN 9783843926089

978-3-8439-2608-9, Reihe Organische Chemie

Jakob Mottweiler
Transition metal-catalyzed oxidative cleavage of lignin and lignin β-O-4 model compounds

178 Seiten, Dissertation Rheinisch-Westfälische Technische Hochschule Aachen (2015), Softcover, A5

Zusammenfassung / Abstract

The oxidative cleavage of lignin with transition metal catalysts was investigated. In the majority of these studies erythro 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol (1) was employed as model substrate. A superior protocol for the synthesis of 1 and other diastereomerically pure 1,3-dilignol model compounds was developed. Two iron catalyzed homogeneous reaction systems were studied. Nonheme iron catalysts displayed high activity for the conversion of 1 but afforded low selectivity for the formation of cleavage products. FeCl3-derived catalysts also showed high activity but were significantly more selective than the tested nonheme iron catalysts. 2-Methoxyphenol and veratraldehyde were the main products in 42% and 35% yield. The reaction likely involves the formation of methyl radicals that are generated from dimethyl sulfoxide and H2O2. Furthermore, heterogeneous transition metal catalysts were screened. Supported gold nanoparticles displayed distinctly different activity for the cleavage of 1 depending on the support and the reduction method. The most active catalyst in the oxidative cleavage was 1-phenylethanol reduced gold on hydrotalcite. In reactions performed in dimethyl carbonate it was observed that basic supports could function as catalysts for the base-catalyzed cleavage of 1. Transition metal-containing hydrotalcite-like catalysts were employed in the cleavage of lignin model compounds and different lignin samples. Copper-vanadium hydrotalcite-like catalysts (HTc-Cu-V) showed high activity and good selectivity for the cleavage of 1. Veratric acid and veratraldehyde were the main products in a combined yield of over 70%. Leaching experiments with HTc-Cu-V revealed that it acts to a significant degree as dispenser of catalytically active homogeneous copper and vanadium species that are continuously released. Lignin cleavage studies with organosolv and kraft lignin showed that HTc-Cu-V cleaved the different lignin samples to dimer and trimer size products. V(acac)3 and Cu(NO3)2·3H2O were tested as homogeneous vanadium and copper sources. V(acac)3/Cu(NO3)2·3H2O showed high activity and good selectivity for the cleavage of 1 affording veratric acid and veratraldehyde in a combined yield of over 80%. Studies with organosolv and kraft lignin showed that V(acac)3/Cu(NO3)2·3H2O displays similar activity as HTc-Cu-V for the cleavage of lignin furnishing dimer and trimer size products.