Cu-substituted molecular sieves as liquid phase oxidation catalysts

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Abstract

Cu(II)-substituted molecular sieves were found to be efficient catalysts in liquid phase oxidation of aromatic compounds. CuAPO-5 was an efficient catalyst in hydroxylation of alkyl-substituted benzenes. For benzene itself as the reactant, a phenol selectivity of near 100% could be obtained using hydrogen peroxide as the oxidant at mild reaction conditions. Acetonitrile was the only effective solvent for this one-step oxidation of benzene to phenol. With the increase in steric hindrance of alkyl substitution on benzene, the oxidation reaction was driven toward benzylic oxidation instead of ring hydroxylation. However, the selectivities of the ring hydroxylation products were higher than those usually obtained in homogeneous radical reactions. On the other hand, Cu(II)-substituted MCM-41 of relatively larger pores was effective in catalyzing the oxidation of trimethylphenol to trimethylbenzoquinone at mild reaction conditions. The latter is an intermediate in manufacture of vitamin E. The optimal activity was obtained when MCM-41 was concomitantly substituted with small amounts of Cu(II) and Al(III). With acetonitrile as solvent, hydrogen peroxide and tert-butyl hydrogen peroxide were effective oxidants, but with benzaldehyde as solvent, oxygen gas was also an effective oxidant.

Introduction

The substitution of hetero-elements into the framework of molecular sieves was usually reported to create acidic sites, while the incorporation of transition metal elements, such as Ti, V, Cr, Mn, Fe and Co, endows sites for redox reactions. Ti-substituted molecular sieves, such as TS-1, Ti-beta and Ti-MCM-41, have attracted most of the attention. They were reported to have remarkable redox catalytic activities, particularly in oxidation reactions with hydrogen peroxide as oxidant at mild reaction temperatures [1], [2], [3], [4], [5]. In contrast, very limited number of reports deals with Cu-substituted molecular sieves. In this paper, Cu(II)-substituted molecular sieves were prepared by hydrothermal reactions and the materials were examined as catalysts in liquid phase oxidation of aromatic compounds.

The hydroxylation of aromatic hydrocarbons to phenols and phenol derivatives with H2O2 was reported to proceed readily over Ti- and V-substituted microporous zeolites of MFI and MEL crystalline structures [6], [7]. Phenol is an important compound for industry and pharmaceuticals. The annual world production in 1996 had reached 4.9×106 tons [8]. Although a direct oxidation process of benzene to phenol would be the most economical route, until now only the indirect manufacturing processes have been operated. The reason resides in that the oxidation capability of phenol is higher than that of benzene and the selectivity of one-step oxidation of benzene into phenol is generally not high. For example, liquid phase oxidation of benzene using Ti- or V-containing zeolites as catalysts and hydrogen peroxide as the oxidants also generated some deep oxidation side products such as p-benzoquinone and hydroquinone [9], [10]. In the present study, a near 100% selectivity and 30% yield of phenol could be achieved with Cu(II)-substituted AlPO4-5 under proper reaction condition.

Trimethylbenzoquinone (TMBQ) is a key intermediate in making vitamin E [11]. The latter is expected to have a constantly increasing demand in pharmaceutical and food industry in the coming decade [12]. In the literature, various catalysts and oxidants have been declared in the oxidation of trimethylphenol (TMP) to TMBQ [13], [14], [15], [16], [17], [18], [19], [20], [21]. Most of these systems were homogeneous phase reactions, and the process may be improved by using stable heterogeneous catalysts to achieve easier work-up. Toward this direction, a carbon supported heteropoly anion (HPA) was examined as a catalyst [22]. However, leaching of HPA during the reaction was the drawback. In the present study, we found Cu(II)-substituted MCM-41 to be an efficient catalyst for this reaction.

Section snippets

Synthesis of Cu(II)-substituted AlPO4-5 molecular sieve

The material was prepared by hydrothermal method. The molar composition in the synthesis gel was Al:P:Cu = 1:1:0.01–0.04. 5.5 g of pseudo-boehmite (73.5% Al2O3, Condea Vista, Catapal® A Alumina) was hydrolyzed with 15 g of H2O. Phosphoric acid (9.2 g, 85% Janssen) was diluted with 10 g of water and added to the Al source under stirring. After adding desired amount of Cu(NO3)2·3H2O (Hanawa) in 5 g of water, 4.2 g triethylamine (Janssen) was added. The precursor gel was transferred to an

CuAPO-5

Pure AlPO4-5 (with the structure type code AFI) crystalline phase was obtained when Cu/(Al+P) molar ratio of up to 2% was incorporated into the synthesis gel of AlPO4-5. Table 1 summarizes the BET surface area and the chemical composition of the samples determined by ICP-AES. Generally speaking, the metal contents in the crystals are proportional to but lower than those added into the gels. As the metal content increases, the difference between the amount added in the gel and that incorporated

Acknowledgements

Financial supports from National Science Council and China Petroleum Corporation, Taiwan are gratefully acknowledged. The authors also thank Condea Vista, Inc. for free supply of pseudo-boehmite. The assistance of Chih-Chang Chen and Yu-Jen Lin in TPR studies is also acknowledged.

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