Selective oxidation of 1- and 2-propanol with molecular oxygen by noble metal catalysis in ‘‘supercritical’’ carbon dioxide Roger Gla¨ser*, Rouven Jos and Jo¨rg Williardt Institute of Chemical Technology, University of Stuttgart, D-70550 Stuttgart, Germany E-mail: roger.glaeser@po.uni-stuttgart.de The selective oxidation of 1- and 2-propanol by molecular oxygen over supported platinum catalysts was investigated in ‘‘supercritical’’ carbon dioxide as an environmentally benign andsafe reaction medium. The reaction occurs exclusively to acetone or propionic aldehyde and propionic acid in a single-phase region at 100–190bar and at a mild temperature (40  C). Compared to conversions in aqueous solution, catalyst stability is significantly enhanced in ‘‘supercritical’’ carbon dioxide and depends on the oxygen concentration in the reaction medium. Thus, at least a fourfold higher substrate/catalyst ratio than with water as a solvent can be used. Platinum catalysts with nanoporous silica (MCM-41, silicalite-1) as a support are also active for the oxidation of 2-propanol in ‘‘supercritical’’ carbon dioxide. KEY WORDS: heterogeneous catalysis; selective oxidation; noble metal; deactivation; supercritical carbon dioxide 1. Introduction The selective oxidation of alcohols to the corre- sponding ketones or aldehydes is an important step in the synthesis and manufacture of fine chemicals. Con- ventionally, this reaction is carried out with permanga- nate or dichromate salts as stoichiometric oxidants leading to heavy-metal-contaminated waste streams. Research efforts have, therefore, been aimed at devel- oping ‘‘greener’’ routes for alcohol oxidation based on homogeneous or heterogeneous catalysis using mole- cular oxygen or air as the favored oxidizing agent [1,2]. Among those routes is the use of supported noble metal catalysts, which are active at mild temperatures, highly selective, e.g. in the oxidation of polyols, and can be used with a wide variety of substrates and solvents [3], above all water [3–5]. However, the deactivation of the noble metal catalysts and the consequently high cata- lyst/substrate ratio needed to achieve appreciable con- version as well as the water-assisted promotion of overoxidation of the primarily formed aldehydes to acids have so far prevented their industrial application. A particularly attractive solvent for heterogeneously catalyzed oxidations is ‘‘supercritical’’ carbon dioxide [6–10]. The term ‘‘supercritical’’ refers to a dense multi- component, but single, homogeneous reaction phase. As pointed out interalia by Jenzer etal. [11], a critical point can only be unambiguously defined for a single-com- ponent fluid. ‘‘Supercritical’’ carbon dioxide is an environmentally friendly reaction medium that is readily available at low cost and allows the facile separation of products, typically by simple expansion to ambient pressure. Among the specific advantages of ‘‘super- critical’’ carbon dioxide as a solvent for catalytic oxi- dations are its chemical inertness towards oxidizing agents, thus avoiding explosion risks commonly involved with the use of organic solvents, its ability to dissolve water-insoluble alcohols and its complete mis- cibility with oxygen or air over a wide range of tem- peratures. Thus, limitations of the reaction rate by oxygen mass transfer from the gas to the liquid phase can also be avoided. It has been shown recently that the oxidation of a broad spectrum of alcohols over sup- ported noble metal catalysts can be successfully achieved in ‘‘supercritical’’ carbon dioxide with high conversion and selectivities at temperatures above 65  C without apparent deactivation of the catalyst [11–14]. The alco- hol conversions that could be achieved in a flow-type reactor were mainly limited by the efficiency of heat removal from the catalyst bed [13]. The aim of the present study is to further investigate the range of applicability of noble metal-catalyzed alcohol oxidation by molecular oxygen in ‘‘super- critical’’ carbon dioxide. Particularly, a lower reaction temperature, closer to the critical point of carbon dioxide ðT c ¼ 31:1  C, p c ¼ 73:8bar), was chosen to fully utilize the strong pressure dependence of the fluid properties such as density and viscosity in the ‘‘near- critical’’ region ðT r ¼ 1:0–1.1 [6,7]), here 40  C or T r ¼ 1:03. The oxidative dehydrogenation of 2-propa- nol (IPOH) to acetone (AC) and water over supported platinum catalysts [15] was chosen as a simple model reaction enabling the prediction of the high-pressure phase behavior of the reaction mixture. For comparison, the conversion of 1-propanol (1-PrOH) with oxygen over the same catalyst was also included. Special emphasis was placed on the stability of the catalyst towards deactivation as compared to conversions in *To whom correspondence should be addressed. Topics in Catalysis Vol. 22, Nos. 1/2, January 2003 (# 2003) 31 1022-5528/03/0100-0031/0 # 2003 Plenum Publishing Corporation