Biomimetic Catalysts for Oxidation of Veratryl Alcohol, a Lignin Model Compound

Abstract

Abstract:Kraft pulp has to be bleached to eliminate the chromophoric structures, which cause a darkening of the pulp. In Nature, an equivalent role is assumed by ligninolytic enzymes such as lignin peroxidases, manganese peroxidases and laccases. The development of low molecular weight manganese peroxidase mimics may achieve environmentally-safe bleaching catalysts for the industry. Herein we report the synthesis and characterization of six manganese(III) complexes 1-6, incorporating dianionic hexadentate Schiff base ligands (H 2 L 1 -H 2 L 4 ) and different anions. Complex 4, Mn 2 L 2 2 (H 2 O) 2 (DCA) 2 was crystallographically characterized. Complexes 1-4 behave as more efficient mimics of peroxidase in contrast to 5-6. We have studied the use of these complexes as catalysts for the degradation of the lignin model compound veratryl alcohol. The biomimetic catalysts were used in conjunction with chlorine-free inexpensive co-oxidants as dioxygen or hydrogen peroxide. Yields up to 30% of veratryl alcohol conversion to veratraldehyde have been achieved at room temperature in presence of air flow using 0.5% of catalyst.

Synthesis, Characterization, and Catalytic Studies of Mn(III)-Schiff Base-Dicyanamide Complexes: Checking the Rhombicity Effect in Peroxidase Studies

“…As a result of these supramolecular interactions, the Mn⋅ ⋅ ⋅ Mn distances of about 4.96Å are short for monomeric compounds. These types of systems have been reported in literature [15,16] as -acuo dimers. …”

“…ESI (electrospray-ionization) mass spectra registered in methanol show, for complexes 1-3, a peak corresponding are assigned, respectively, to the symmetric and asymmetric dicyanamide modes [16,31].…”

“…Peroxidase activity for complexes 1-3 was tested according to the literature procedure [15,16] by study of the oxidation of 2,2 -azinobis-(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) with H 2 O 2 at ca. pH 6.8 in the presence of the complexes.…”

“…During our search for new metal catalysts containing salen-type ligands, we have observed a better catalytic behaviour when the substrate molecule can be easily coordinated by the complex and this is favoured when the catalyst has either a vacancy in the coordination sphere or a labile ligand [14][15][16][17]. To achieve this, a careful design of the environment around the metal ion is necessary.…”

“…Peroxidases catalyse oxidation of a broad range of substrates by hydrogen hydroxide, so they may be successfully used for different industrial processes since peroxidases are capable of degrading aromatic rings in lignin, dyes, and polycyclic aromatic hydrocarbons [15,16,20,21]. In this sense, dyes are regarded as one of the most important contaminants to be removed from the industrial effluents, and peroxidases arise as a way to treat these pollutants.…”