Journal of Molecular Catalysis A: Chemical 208 (2004) 195–202 Metalloporphyrins immobilized on motmorillonite as biomimetic catalysts in the oxidation of lignin model compounds Claudia Crestini ∗ , Alessandra Pastorini, Pietro Tagliatesta Dipartimentodi Scienze e Tecnologie Chimiche Università di Tor Vergata,Via della Ricerca Scientifica, 00133 Rome, Italy Received 8 April 2003; received in revised form 8 April 2003; accepted 13 July 2003 Abstract Biomimetic catalysts such as metalloporphyrins, which can yield highly oxidized metallo-oxo species, have been used as lignin peroxidase models. However, natural porphyrins are unstable under catalytic oxidation conditions due to their self-destruction or to the formation of inactive -oxo complexes. An alternative approach to the preparation of robust catalysts consists in the immobilization of metalloporphyrins on supports that mimic the polypeptide envelope, which protects the catalytic center of natural enzymes. We report here the use of manganese meso-tetrakis(tetramethylpyridinio)porphyrin pentaacetate immobilized on the smectite clay montmorillonite as a suprabiotic catalyst for the oxidation of lignin model compounds. © 2003 Elsevier B.V. All rights reserved. Keywords: Bioinorganic chemistry; Supported catalysis; Oxidation; Porphyrins; Clays; Biomimetic synthesis 1. Introduction The best characterized lignin degrading fungus is the ba- sidiomycete Phanerochaete chrisosporium [1]. In 1983, an extracellular lignin peroxidase (ligninase, LiP) was isolated from ligninolytic cultures of this microorganism [2]. In the presence of hydrogen peroxide, the active center of LiP performs a one-electron oxidation of the lignin aromatic moieties [3]. The catalytic cycle consists in a two electron oxidation of Fe(III) protoporphyrin IX (high spin) to give a highly reactive oxo-iron(IV) protoporphyrin IX -cation radical, the LiP I complex (LiP compound I) [4,5]. The LiP compound I is then reduced to the initial state by two dif- ferent one-electron reductions by the substrates [6]. LiP is a fragile enzyme. When exposed to an excess of hydrogen per- oxide (more than 20 equivalents), it is subject to inactivation by overoxidation, and gives the inactive form, LiP III [7,8]. Lignin itself can be oxidized by LiP in the presence of lim- iting amounts of hydrogen peroxide with different degrees of success [9–12]. In principle, the use of an enzymatic sys- tem for delignification is not economically convenient with respect to simpler catalysts, due to the costs of purification. ∗ Corresponding author. Tel.: +39-06-7259-4734; fax: +39-06-7259-4754. E-mail address: crestini@uniroma2.it (C. Crestini). Moreover, since LiP is sensitive to hydrogen peroxide ex- cess, its practical utilization in pulp and paper is difficult to develop. Hence the need for the design of suitable ligninase models resistant to peroxide inactivation. These biomimetic systems are also helpful for the understanding the mecha- nisms of complex lignin degradation. Natural metalloporphyrins suffer from the major disad- vantage of being unstable in the presence of excess oxi- dants. Their lability is due either to self-destruction or to the formation of inactive -oxo complexes [13,14]. The study of biomimetic systems has thus focused toward the development of synthetic metalloporphyrins more resistant to degradative oxidation. Synthetic metalloporphyrins have been used as biomimetic lignin peroxidase models, and their potentiality for lignin degradation has been a subject of several studies [13–17]. When synthetic metalloporphyrins are used as biomimetic catalysts in the presence of hydrogen peroxide, several side reactions can occur. The peroxidic bond can undergo ho- molytic scission to yield Fe(IV)-OH and hydroxyl radical in a Fenton like fashion. This reactivity is more significant in the presence of iron complexes and hydrogen peroxide as oxygen donor. A second molecule of peroxide may re- act with the metal oxo complex in a catalase like fashion to yield the formation of H 2 O and O 2 , and ultimately the degradation of the active oxidant species. The metallo-oxo 1381-1169/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2003.07.015