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 (H2L1-H2L4) and different anions. Complex 4, Mn2L22(H2O)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.
Highlights
The predominant chemical pulping process used nowadays is the Kraft process, which, at high temperatures and under forceful alkaline conditions, is able to remove large amounts of lignin from the cellulose fiber by reductive depolymerisation with sulfide [1]
The objective of pulping and bleaching of wood is the selective removal of lignin without degrading the polysaccharides, and the removal of colored structures, which are originally present in the wood pulp or have been formed during the pulping process
Synthetic models for manganese peroxidase have been developed [7] from polydentate ligands [8,9] or porphyrins [10,11]
Summary
The predominant chemical pulping process used nowadays is the Kraft process, which, at high temperatures and under forceful alkaline conditions, is able to remove large amounts of lignin from the cellulose fiber by reductive depolymerisation with sulfide [1]. Synthetic models for manganese peroxidase have been developed [7] from polydentate ligands [8,9] or porphyrins [10,11]. In our search of biomimetic models for peroxidase, we have reported active manganese complexes involving tetradentate ONNO Schiff bases, and the influence of the geometry around the manganese ion on peroxidase activity has been studied by us [12,13,14]. Dimeric complexes can be achieved using the appropriate ligands, for instance polydentate Schiff bases with both inner and outer compartments. We report six Mn(III) complexes obtained from the reaction between different manganese salts and the Schiff base ligands (see Figure 1): N,N'-bis(3-methoxy-5-bromo-salicylidene) propane-1,2-diamine (H2L1); N,N'-bis(3-methoxysalicylidene)ethylenediamine (H2L2); N,N'-bis (3-methoxy-5-bromo-salicylidene)propane-1,3-diamine (H2L3) and N,N'-bis(3-methoxysalicylidene)2,2'-dimethylpropane-1,3-diamine (H2L4). The possibility of evaluating catalyst properties of 1–6 for pulp-bleaching purposes has been studied using veratryl alcohol (VA), which can be considered as a model compound for lignin substructures [16]
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