Heterogeneous oxidation of aromatic compounds catalyzed by metallophthalocyanine functionalized silicas

Abstract

Controlled procedures for the covalent anchoring of iron tetrasulfophthalocyanine onto amino-modified silicas have been developed to fix the complex either in a monomer or dimer form. Usually considered as a catalytically inert form, dimeric µ-oxo iron tetrasulfophthalocyanine grafted onto amino-modified silica or MCM-41 is shown to be a selective catalyst for the oxidation of 2-methylnaphthalene to 2-methylnaphthaquinone (vitamin K₃) and of 2,3,6-trimethylphenol to trimethylbenzoquinone.

Catalytic methods involving environmentally friendly oxidants are needed to perform selective oxidation of alkylaromatics. For example, 2-methylnaphthoquinone (Vitamin K₃) is still prepared by stoichiometric oxidation of 2-methylnaphthalene (2MN) by CrO₃ in H₂SO₄, thus leading to severe environmental problems. As an alternative, several homogeneous catalytic methods have been developed Oxidation of aromatic compounds to quinones is a multistep reaction and the yields are rarely high because of coupling and over-oxidation reactions. Vitamin K₃ is obtained by 2MN oxidation with CrO₃–H₂SO₄ in 38–60% yields. A 46% yield of vitamin K₃ was reported in 2MN oxidation by the CH₃ReO₃–85% H₂O₂ system Recently, iron tetrasulfophthalocyanine (FePcS) has been shown to be an active catalyst in the oxidative degradation of chlorinated phenol and in the homogeneous oxidation of condensed aromatics The fixation of active catalysts onto appropriate supports is highly desirable to provide a high catalyst stability as well as facile recovery and recycling. Another reason to immobilize homogeneous complexes is that homogeneous solutions of complexes often contain several species, for example, monomers and dimers, while only some species, usually monomeric ones, are catalytically active. For example, among the several monomer and dimer forms of FePcS in aqueous solutions, monomeric FePcS has been proposed to be the catalytically active complex in the oxidative degradation of trichlorophenol Consequently, by using appropriate methods one can prepare a heterogeneous catalyst supporting only the catalytically active form of the complex. We report here the controlled covalent anchoring of phthalocyanine complexes onto mesoporous MCM-41 silica (Scheme 1) and the catalytic properties of these hybrid materials in the oxidation of 2-methylnaphthalene and 2,3,6-trimethylphenol. Recently discovered mesoporous silicas having large ordered hexagonal channels with diameters from 15 to 100 Å(MCM-41) and high surface areas (above 700 g^-1) are promising supports for different types of catalysts

MCM-41 was prepared as previously describe and modified with 3-aminopropyltriethoxysilane to obtain 0.5 mmol of NH₂ groups per gram of material. The modification was proven by the decrease of the silanol signals Q² (-91 ppm) and Q³ (-100 ppm) and the appearance of a signal characteristic of (SiO)₃Si–CH₂ groups (-60 ppm) in the ²⁹Si MAS NMR spectrum. ¹³C CP MAS NMR shows aminopropyl carbon signals at 5.5, 18.9 and 40.3 ppm. Textural characteristics of some solid materials are listed in Table 1. A decrease of specific surface area from 860 to 698 g^-1 and average pore diameter from 40 to 36 Åas well as a decrease of the mesoporous volume from 1.30 to 1.04 cm³ g^-1 also indicates a successful modification of MCM-41.

FePcS was converted to iron tetrasulfochlorophthalocyanine, FePc(SO₂Cl)₄, by treatment with SOCl₂ or PCl₅. For covalent anchoring of FePc(SO₂Cl)₄ onto amino-modified MCM-41 two strategies were used in order to fix the phthalocyanine complex either as a monomer or as a dimer. We used UV-vis spectroscopy to identify these species.