Catalytic hydroxylation of phenol using immobilized late transition metal salicylaldimine complexes

https://doi.org/10.1016/j.molcata.2006.11.031Get rights and content

Abstract

Immobilized late transition metal complexes based on salicylaldimine ligands were used as catalysts in the wet air hydroxylation of phenol in the presence of hydrogen peroxide. Three metals, viz. cobalt, nickel and copper, and two different kinds of supports, namely, amorphous silica and mesoporous MCM-41, were employed. The effect of pH on the hydroxylation process was also investigated. It was observed that the regioselectivity of the hydroxylation process strongly depends on the kind of support employed.

Graphical abstract

Late transition metal complexes of salicylaldimine ligands immobilized on amorphous silica and MCM-41 were used as catalysts in the hydroxylation of phenol in the presence of hydrogen peroxide. Product selectivity was shown to be highly dependent on the type of support employed.

Introduction

Catechol and hydroquinone have a very wide range of applications, ranging from pharmaceuticals to polymers, photographic and agrochemical materials. Wet air oxidation of phenol is a well-known synthetic route to produce these dihydroxybenzenes. Most of the common techniques for wet oxidation of phenol involve the use of hydrogen peroxide as an oxidant. The advantage of hydrogen peroxide over other potential oxidants is its relative stability. In addition, hydrogen peroxide is a relatively strong oxidant.

Examples of homogeneous processes known to produce dihydroxybenzenes are the Rhône-Poulenc process [1], where the oxidation of phenol is catalyzed by strong mineral acids, or the Hamilton process [2], where Fenton reagent is used as a catalyst. These processes, being homogeneous in nature, face difficulties with continuous operation, catalyst recovery and separation. The use of heterogeneous catalysts can eliminate such difficulties. Titanium silicate (TS-1), which is a titanium containing zeolite, is one of such heterogeneous catalysts [3]. One of the advantages of TS-1 is higher para selectivity, which is attributed to the shape selectivity of the catalyst [4], [5], [6], [7], [8], [9], [10]. Another advantage of this catalyst is the higher phenol conversion and the low amount of tar formation. However, the difficult preparative method for TS-1 makes it a less desirable option for phenol hydroxylation. Several other heterogeneous systems have been tried for the hydroxylation of phenol. These include metals encapsulated within zeolites [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21] and metals encapsulated in pillared clays [22]. Several metals, such as iron, cobalt, nickel and copper have been tried as catalysts in the phenol hydroxylation processes. All of the above-mentioned heterogeneous systems involve the physical absorption of metal salts or metal complexes on the inorganic support.

In this paper we report on salicylaldimine complexes of cobalt, nickel and copper chemically immobilized on two different kinds of materials, namely, amorphous silica and mesoporous MCM-41. These systems were evaluated as catalysts in phenol hydroxylation over a range of pH values. The effect of the nature of the support on catalyst performance was also evaluated.

Section snippets

Materials

All the chemicals were procured from Sigma–Aldrich Ltd. unless otherwise mentioned. The solvents ethanol, dichloromethane and toluene were procured from Kimix Ltd., South Africa and were dried using appropriate drying agents. Amorphous silica gel (Davisil, Grade 710) was procured from Sigma–Aldrich. MCM-41 was synthesized using a reported method [23] employing tetraethylorthosilicate as the starting material and cetyltrimethylammonium bromide as the template.

Techniques

1H NMR spectra were recorded in CDCl3

Result and discussion

In the current study, the effect of two different types of support systems were investigated, viz. amorphous silica (Davisil 710) and mesoporous MCM-41 using three different metal salicylaldimine complexes, namely, cobalt, nickel and copper (Table 1, Scheme 1). Liquid phase hydroxylations of phenol were carried out in conjunction with H2O2 at a controlled pH (2–6 range). The pH of the reaction medium was controlled by using appropriate buffer solutions.

Conclusions

Salicylaldiminato complexes of cobalt, nickel and copper were immobilized onto amorphous silica and mesoporous MCM-41 and their efficacies towards hydroxylation of phenol were examined. The MCM-41 supported catalysts have higher activity for phenol conversion than the amorphous silica systems probably because of the higher surface area of the support, resulting in a higher concentration of catalytically active sites. The amorphous silica immobilized catalysts are more sensitive to pH changes in

Acknowledgements

SR acknowledges National Research Foundation, South Africa for both grant holder and free-standing postdoctoral fellowships. Financial support from the WRC of South Africa and the Research Committee of the University of the Western Cape is greatly acknowledged. Dr. Remi Bucher, Ithemba Laboratories, South Africa is also acknowledged for the XRD measurements.

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