Immobilized methyltrioxo rhenium (MTO)/H2O2 systems for the oxidation of lignin and lignin model compounds

doi:10.1016/j.bmc.2006.03.046

Bioorganic & Medicinal Chemistry

Volume 14, Issue 15, 1 August 2006, Pages 5292-5302

https://doi.org/10.1016/j.bmc.2006.03.046 Get rights and content

Abstract

A convenient and efficient application of heterogeneous methylrhenium trioxide (MTO) systems for the selective oxidation of lignin model compounds and lignins is reported. Environmental friendly and low-cost H₂O₂ was used as the oxygen atom donor. Overall, the data presented and discussed in this paper point toward the conclusion that the immobilized heterogeneous catalytic systems based on H₂O₂/and MTO catalysts are able to extensively oxidize both phenolic and non-phenolic, monomeric, and dimeric, lignin model compounds. Condensed diphenylmethane models were found also extensively oxidized. Technical lignins, such as hydrolytic sugar cane lignin (SCL) and red spruce kraft lignin (RSL), displayed oxidative activity with immobilized MTO catalytic systems. After oxidation, these lignins displayed the formation of more soluble lignin fragments with a high degree of degradation as indicated by the lower contents of aliphatic and condensed OH groups, and the higher amounts of carboxylic acid moieties. Our data indicate that immobilized MTO catalytic systems are significant potential candidates for the development of alternative totally chlorine-free delignification processes and environmental sustainable lignin selective modification reactions.

Graphical abstract

Heterogeneous MTO catalysts are able to extensively oxidize lignin model compounds and showed significant potential for H₂O₂ delignification processes and environmentally sustainable lignin selective modification reactions.

Introduction

In the paper production processes, environmental concerns have prompted us to design pulping and bleaching sequences avoiding the use of chlorinated compounds. Totally chlorine-free (TCF) processes have been developed, as, for example, by the use of oxygen, hydrogen peroxide (H₂O₂), and ozone as primary oxidants.¹ However, their major drawback consists in a lack of selectivity in the oxidation of lignin, which leads to the partial degradation of the cellulose contained in pulps, and ultimately in a lower final product yield. The lack of selectivity is due fundamentally to the formation of radical intermediates, such as hydroxyl radicals, that are able to attack both cellulose and lignin.² Selective catalytic processes based on a concerted oxygen atom transfer from environmental friendly H₂O₂ might solve these problems.

A novel catalyst potentially useful for this purpose is methyltrioxorhenium (VII) (MeReO₃, MTO).³ MTO in combination with H₂O₂ has become in recent years an important catalyst for a variety of synthetic transformations, such as oxidation of olefins,⁴ alkynes,⁵ sulfur compounds,⁶ phosphines,⁷ Bayer–Villiger rearrangement,⁸ and oxidation of C–H bonds.⁹ Accordingly with this high reactivity, MTO is able to catalyze the oxidation of aromatic derivatives.¹⁰

Irrespective of the substrate to be oxidized, the reaction proceeds through the formation of a mono-peroxo [MeRe(O₂)] (A) and a bis-peroxo [MeRe(O₂)₂] complex. These reactive intermediates transfer one oxygen atom to the substrate by a concerted mechanism avoiding the formation of any radical species.⁴

With the aim to design novel delignification and bleaching processes, we recently investigated the reactivity of MTO in the oxidation of lignins and lignin model compounds with H₂O₂ as the primary oxidant.¹¹

MTO showed to be a powerful and efficient catalyst for the oxidation of phenolic and non-phenolic lignin model compounds representative of the main bonding pattern present within native lignins, affording both side-chain oxidations and aromatic ring cleavage reactions. Diphenylmethane models, that are usually recalcitrant to oxidation, were also found to be extensively degraded mainly via cleavage of their interunit methylene linkages. MTO was also able to catalyze the extensive degradation of selected technical lignins, increasing their degree of solubility and reducing their content in condensed subunits.

The possibility for alternative environmentally sustainable delignification processes requires the use of clean oxidants such as hydrogen peroxide coupled with techniques for the recovery and reuse of the catalysts. From this point of view immobilized catalysts present the advantage of an easy recovery and possibility of reuse and a low environmental impact. In an effort to develop more versatile and environmental friendly heterogeneous catalysts, we described the preparation of novel rhenium compounds of general formula (polymer)_f/(MTO)_g (the f/g quotient expresses the ratio by weight of the two components) by heterogenization of MTO on easily available and low-cost polymeric support, poly(4-vinylpyridine) or polystyrene,¹² applying the ‘mediator’ concept¹³ and the microencapsulation technique.¹⁴

All the novel MTO compounds were characterized by FT-IR, scanning electron microscopy (SEM), and wide-angle X-ray diffraction (WAXS) analyses.¹² To the best of our knowledge, apart from silica supported MTO complexes,¹⁵ NaY zeolite/MTO supercage system,¹⁶ and a niobia supported MTO compound,¹⁷ no further data are available in the literature about heterogeneous MTO catalysts. These polymer/MTO catalysts have already proved as efficient and selective systems for the epoxidation of simple olefins,¹⁸ and for the oxidation of substituted phenol and anisole derivatives.¹⁹

The structures of poly(4-vinylpyridine)/MTO and polystyrene/MTO catalysts I–IV employed, namely poly(4-vinylpyridine) 2% and 25% cross-linked (with divinylbenzene)/MTO (PVP-2%/MTO I and PVP-25%/MTO II, respectively), poly(4-vinylpyridine-N-oxide) 2% cross-linked/MTO (PVPN-2%/MTO III), and microencapsulated polystyrene 2% cross-linked/MTO (PS-2%/MTO IV), are schematically represented in Figure 1.

We report here that catalysts I–IV can be efficiently used for the selective oxidation of lignins and lignin model compounds with H₂O₂ as environmentally friendly oxidant.

We selected an array of monomeric and dimeric lignin model compounds resembling the main bonding patterns in native and technical lignins, and studied their reactivity with supported MTO catalysts by characterization of the main oxidation products in the presence of H₂O₂. Our attention was next turned to more complex lignin polymers, hydrolytic sugar cane lignin (SCL) and red spruce kraft lignin (RSL), that are representative examples of widely diffused para-hydroxyphenyl-guaiacyl, and guaiacyl lignins. Their oxidation was analyzed by means of advanced ³¹P NMR techniques that allow the quantitative determination of all labile OH groups on the polymer—that is, aliphatic, different kinds of phenolic OH groups, and carboxylic acids—after phosphitylation of the sample.20, 21

Section snippets

Oxidation of lignin model compounds

The reactivity of aromatic model compounds resembling the most representative lignin bonding patterns is of pivotal interest in order to rationalize the oxidative behavior of this biopolymer. Thus, an array of monomeric and dimeric, phenolic and non-phenolic lignin model compounds was carefully selected for study in order to clarify the reactivity of the various lignin subunits with H₂O₂ and catalysts I–IV.

Vanillyl alcohol 1, veratryl alcohol 6,

Conclusions

Immobilized MTO catalytic systems I–IV were found to be efficient catalysts for the oxidation of phenolic and non-phenolic lignin model compounds with environmental benign H₂O₂ as the oxygen atom donor. Exceptional reactivity toward monomeric and dimeric, phenolic and non-phenolic lignin model compounds was apparent as displayed with alkyl side chains being subjected to oxidation and fragmentation reactions as well as aromatic moieties being hydroxylated, demethylated and oxidative ring-opening

Experimental

¹H NMR, ¹³C NMR, and ³¹P NMR spectra were recorded on a Bruker AM 400 or Bruker 200 spectrometer. Mass spectroscopy (MS) was performed with a GC Shimadzu GC-17A and a mass-selective detector QP 6000. All solvents were of ACS reagent grade and were redistilled and dried according to standard procedures. Chromatographic purifications were performed on columns packed with Merck silica gel 60, 230–400 mesh for flash technique. Thin-layer chromatography was carried out using Merck platten Kieselgel

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Lignin is a waste by-product of bio-refineries and paper-pulp industries. It has an attractive potential to produce numerous valuable chemicals due to its highly aromatic character. At present, large amount of lignin is burnt as a source of energy due to lack of suitable efficient lignin valorisation processes. The challenge exists in handling its complex heterogeneous structure and bond breaking at selective locations. The production of high value chemicals/petrochemical feedstocks will improve the economic viability of a bio-refinery. Oxidative depolymerization is a promising way to produce functional compounds from lignin. The aim of the current review is to present the novel methodologies currently used in the area of lignin oxidative depolymerization including effect of temperature, residence time, solvent, oxidizing agents, homogeneous and heterogeneous catalysis etc. It aims to present an insight into the structure of lignin and its breakdown mechanism.

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Immobilized methyltrioxo rhenium (MTO)/H2O2 systems for the oxidation of lignin and lignin model compounds

Abstract

Graphical abstract

Introduction

Section snippets

Oxidation of lignin model compounds

Conclusions

Experimental

J. Org. Chem.

J. Mol. Catal. A: Chem.

J. Am. Chem. Soc.

Tetrahedron Lett.

Tetrahedron Lett.

Bioorg. Med. Chem.

J. Org. Chem.

J. Org. Chem.

Angew. Chem., Int. Ed. Engl.

Inorg. Chem.

J. Am. Chem. Soc.

J. Pulp Pap. Sci.

Bioorg. Med. Chem.

J. Chem. Soc. C

Holzforshung

Angew. Chem., Int. Ed. Engl.

Angew. Chem., Int. Ed. Engl.

J. Mol. Catal.

Tetrahedron

Catal. Today

J. Mol. Catal.

Tetrahedron Lett.

Immobilized methyltrioxo rhenium (MTO)/H₂O₂ systems for the oxidation of lignin and lignin model compounds