Journal of Catalysis 211, 134–143 (2002) doi:10.1006/jcat.2002.3711 Mesoporous (Cr)MCM-41: A Mild and Efficient Heterogeneous Catalyst for Selective Oxidation of Cyclohexane A. Sakthivel and P. Selvam 1 Department of Chemistry, Indian Institute of Technology—Bombay, Powai, Mumbai 400 076, India Received February 25, 2002; revised May 27, 2002; accepted June 5, 2002 Liquid-phase oxidation of cyclohexane was carried out at mod- erate reaction conditions over mesoporous (Cr)MCM-41 molecu- lar sieves using acetic acid as solvent, hydrogen peroxide as oxi- dant, and methyl ethyl ketone as initiator. Under these conditions, the catalysts showed high substrate conversion and excellent prod- uct selectivity. Although the activity of the catalyst decreased af- ter the first recycle, owing to leaching of nonframework chromium ions, it, however, remained nearly the same thereafter. This obser- vation was further confirmed by washing experiments where the nonframework chromium ions were removed upon ammonium ac- etate treatment or under reaction conditions. Further, the washed catalyst also showed a activity similar to that of the recycled cata- lyst. The recycled or washed (Cr)MCM-41, therefore, behaves truly as a heterogeneous catalyst, and thus it forms the first example of a nonleaching chromium-based catalyst reported so far. The use of a stronger oxidizing agent, e.g., tertiary butyl hydroperoxide, re- sulted in the formation of cyclohexanone as the major product. On the other hand, the use of solvents like methanol, dioxan, and ace- tone showed lower conversion. Likewise, the use of initiators such as acetone, cyclohexanone, and acetaldehyde also resulted in lower activity. c  2002 Elsevier Science (USA) Key Words: cyclohexane; cyclohexanol; cyclohexanone; (Cr)MCM-41; oxidation; mesoporous molecular sieves. INTRODUCTION Selective oxidation reactions using heterogeneous cata- lysts are of growing importance for the modern chemical industry. The oxidation products of cyclohexane, viz., cyclo- hexanol and cyclohexanone, are important intermediates in the production of adipic acid and caprolactam, which are used in the manufacture of nylon-6 and nylon-66 poly- mers (1–4). In addition, they are also used as solvents for lacquers, shellacs, and varnishes as well as stabilizers and homogenizers for soaps and synthetic detergent emulsions. Furthermore, cyclohexanol ester, viz., cyclohexyl phthalate, is widely used as a plasticizer as well as in the surface-coating industry. Other uses of cyclohexanone are as starting mate- rial in the synthesis of insecticides, herbicides, and pharma- ceuticals. In general, both cyclohexanol and cyclohexanone 1 To whom correspondence should be addressed. Fax: +91-22-576-7152/ 572-3480. E-mail: selvam@iitb.ac.in. are produced on an industrial scale by the oxidation of cy- clohexane or hydrogenation of phenol (1–5). In the early 1940s, Du Pont developed a process in which cyclohexane was oxidized in the presence of air to cyclohexanol and cyclohexanone using cobalt napthenate or cobalt acetate as catalyst (1, 4). In this process, several by-products, viz., mono- and dicarboxylic acids, esters, aldehydes, ketones, and other oxygenated materials, were generated. Later, in the 1950s, Scientific Design (now Halcon International) de- veloped a new process where anhydrous meta-boric acid was added as a slurry in the oxidation vessel. This led to the formation of borate ester of cyclohexanol (3, 5), which prevents over oxidation (ring-opening products). The re- sulting cyclohexyl ester is subsequently hydrolyzed to cy- clohexanol. In comparison to the former, the latter process showed good yield of cyclohexanol, which, however, re- quires high investment and a high operating cost to recover and recycle the boric acid. On the other hand, for the laboratory-scale reaction ex- tensive literature is available on the selective oxidation of cyclohexane using a variety of transition metal compounds in stoichiometric amounts or as homogeneous catalysts (6–15). In the cases of the latter, the use of initiators, e.g., cy- clohexanone (3), cyclohexylperoxide (3), methyl ethyl ke- tone (10), and acetaldehyde (16), shortens the induction period as well as enhances the catalytic activity. However, owing to the limitations of these soluble (homogeneous) catalysts, viz., catalyst separation from the product and the disposal of solid/liquid wastes, which pose serious problems to the environment, in recent years attention has been fo- cused on the development of transition metal-based hetero- geneous catalysts with oxygen or peroxides as nonpolluting oxidants (6, 7, 17–25). However, in most cases, extreme re- action conditions such as high pressure (2 MPa) and high temperature (450 K) in conjunction with low activity make the process less attractive. In addition, leaching of active metal ions has often been observed under the reaction con- ditions (20, 25, 26). Hence, the oxidation of cyclohexane over heterogeneous catalysts under mild/moderate condi- tions is a topic of great interest. In this regard, chromium- containing mesoporous MCM-41 molecular sieves show promise for certain oxidation reactions (21, 27–30). In 0021-9517/02 $35.00 c  2002 Elsevier Science (USA) All rights reserved. 134