Journal of Catalysis 211, 244–251 (2002) doi:10.1006/jcat.2002.3723 On the Role of Oxygen in the Liquid-Phase Aerobic Oxidation of Alcohols on Palladium Csilla Keresszegi, Thomas Burgi, ¨ Tamas Mallat, and Alfons Baiker1 Laboratory of Technical Chemistry, Swiss Federal Institute of Technology (ETH), Honggerberg¨ HCI, CH-8093 Zurich, Switzerland Received April 9, 2002; revised June 7, 2002; accepted June 7, 2002 out by replacing oxygen with a hydrogen acceptor such as The mechanism of alcohol oxidation was investigated using the quinone, nitrobenzene, or an olefin (8). This approach has conversion of cinnamyl alcohol (1) over Pd-based catalysts as a been applied recently in the transfer dehydrogenation of sensitive test reaction. Studies in a slurry reactor revealed that de- allylic and aromatic alcohols on Pd using a cheap olefin as hydrogenation and oxidative dehydrogenation of 1 follow the same a hydrogen acceptor (9, 10). reaction pathways independent of the presence or absence of oxy- In situ catalyst potential measurements provided fur- gen and reaction conditions. Hydrogenation and hydrogenolysis ther support to the dehydrogenation mechanism and to side reactions indicated the presence of hydrogen on the metal sur- the secondary role of oxygen. Muller ¨ and Schwabe demon- face during reactions. Catalyst deactivation in Ar is attributed to decarbonylation reactions and site blocking by CO. Introduction strated for the first time that during oxidation of ethanol or of molecular oxygen induced a dramatic enhancement of alcohol 2-propanol the potential of Pt was in the “hydrogen region”; conversion rate by a factor of up to 285 due to oxidative removal of i.e., the metal was partially covered by hydrogen despite the CO. Strong adsorption of CO on Pd/Al2O3 and its rapid removal by presence of gaseous oxygen (11, 12). Some more examples oxygen were corroborated by in situ ATR-IR spectroscopy. All these of this situation can be found in the recent literature observations conform to a model according to which oxidation of (13–15). 1 follows the classical dehydrogenation mechanism, and the key However, an increasing number of reports contradict this role of oxygen is the continuous oxidative removal of CO and other concept and suggest that oxidation of some alcohols pro- degradation products from the active sites. This oxidative cleaning ceeds on partially oxygen-covered metal surface and the of the metal surface allows a high rate of alcohol dehydrogenation actual concentration of adsorbed hydrogen during reaction even when the oxidation of the co-product hydrogen is slow and seems to be negligible (16–20). These observations can- incomplete. It is likely that the observed effects are not limited to the oxidation of 1 on Pd, and regeneration of the active sites by not easily be interpreted by the classical dehydrogenation oxygen generally plays an important role during aerobic oxidation mechanism. A feasible interpretation could be an oxidative of alcohols on platinum metals. c 2002 Elsevier Science (USA) dehydrogenation mechanism in which the adsorbed oxygen Key Words: oxidation; dehydrogenation; palladium; cinnamyl al- or OH species play a crucial role. The rate-determining el- cohol; deactivation; decarbonylation; CO poisoning; ATR-IR spec- ementary step might involve direct interaction of the ad- troscopy. sorbed oxidizing species with the adsorbed reactant or its partially dehydrogenated intermediate (3, 7). Several kinetic studies indicated a Langmuir– INTRODUCTION Hinshelwood-type behavior: a surface reaction between the adsorbed reactant and the dissociatively adsorbed oxygen Platinum-group metals (particularly Pt and Pd) and gold in the rate-determining step (see, e.g., (21, 22)). In recent are active catalysts in the liquid-phase oxidation of alco- studies Markusse et al. suggested that oxygen-assisted hols, diols, and carbohydrates with molecular oxygen un- dehydrogenation would be the rate-determining step der close to ambient conditions (1–7). A dehydrogenation (20, 23). However, kinetic analysis requires working under mechanism of the reaction is mostly accepted, assuming conditions free from mass-transport limitations, which that the role of oxygen is to oxidize the co-product hydro- conditions lead to a rapid catalyst deactivation due to gen formed in the rate-determining step and thus shift the “overoxidation” of the metal surface (24, 25)—an unpleas- equilibrium toward the carbonyl compound. The secondary ant obstacle in determining the real initial rate. It was importance of oxygen in this mechanism is supported by also shown that multiple steady states may complicate the Wieland’s early observation that the reaction can be carried kinetic analysis (26, 27). We can conclude that the apparent general agreement 1 To whom correspondence should be addressed. Fax: +41 1 632 11 63. in the oxidative dehydrogenation mechanism of alcohol E-mail: [email protected]. oxidation includes contradictory opinions concerning the 244 0021-9517/02 $35.00 c 2002 Elsevier Science (USA) All rights reserved. LIQUID-PHASE AEROBIC OXIDATION OF ALCOHOLS 245 real nature of elementary steps and the role of oxidizing in 5 ml toluene. Periodically withdrawn samples were an- species. Some years ago we proposed the blocking of active alyzed by GC (Thermo Quest Trace 2000, equipped with sites by strongly adsorbed alcohol degradation products as an HP-FFAP capillary column and an FID). Products were a possible explanation for this contradiction (18, 19, 28, 29). identified by GC-MS and GC analysis of the corresponding Oxidative removal of these impurities accelerates the target authentic samples. reaction; thus, adsorbed oxygen apparently plays an active (direct) role in the reaction mechanism. ATR-IR Spectroscopy This question is addressed here involving catalytic and in The transformation of cinnamyl alcohol (1)onPdwas situ spectroscopic methods. The model reaction is the trans- studied by in situ attenuated total reflection (ATR) infrared formation of an allylic alcohol, trans-cinnamyl alcohol. Aer- spectroscopy (32). A catalyst layer on the internal reflection obic oxidation of allylic alcohols on Pt- and Pd-based cata- element was used for this purpose. A slurry of 10 mg 5 wt% lysts under appropriate conditions affords sometimes Pd/Al O in 5 ml toluene was stirred for 30 min. Then a α,β 2 3 excellent yields to -unsaturated aldehydes (29, 30). In ZnSe internal reflection element (IRE, 45◦,50× 20 × 2 mm, the absence of oxygen the selectivity is low due to side re- KOMLAS) was covered by dropping the slurry on its one actions in which the reactant alcohol acts as a hydrogen side. The solvent was allowed to evaporate and the pro- acceptor (10). The high reactivity of reactant and product cedure was repeated three times. After drying for several offers the possibility to demonstrate the importance of com- hours in air the catalyst layer was ready for use. The cata- peting side reactions, and to clarify the role of oxygen in lyst exposed to the reaction mixture, a film of 3.1-cm2 area, these reaction pathways. amounted to 1–1.5 mg. ATR spectra were recorded using a home-built stainless- steel flowthrough cell with a total volume of 0.077 ml (33). EXPERIMENTAL The cell was heated to 60◦C by a thermostat during the mea- Materials surements. After assembling, the cell was mounted onto an ATR attachment (OPTISPEC) within the Fourier trans- The following commercial catalysts were used for alcohol form IR spectrometer (Bruker, IFS-66/S) equipped with oxidation: 5 wt% Pd/Al2O3 (Johnson Matthey 324; mean an MCT detector. At the entrance and exit of the IRE the Pd particle size, 3.4 nm determined by TEM; calculated IR beam was collimated to minimize signal contributions dispersion, 0.34 (31)), 5 wt% Pt/Al2O3 (Engelhard 4759; from the O-ring region. Liquid (toluene or a toluenic so- mean Pt particle size, 2.9 nm determined by TEM; calcu- lution of 1) was flowed over the catalyst sample at a rate lated dispersion: 0.40), and 4 wt% Pd–1 wt% Pt–5 wt% of 0.5 ml min−1 by a peristaltic pump (ISMATEC Reglo Bi/C (Degussa, CEF 196 XRA/W). trans-Cinnamyl alco- 100) located behind the cell. Liquid was provided from two hol (Acros, 98%) was purified by recrystallization from separate glass bubble tanks where the liquids could be sat- 1 petroleum ether (99.3% by H NMR and GC). Ethylene urated with gases. The flow from the two tanks was con- + glycol diacetate (Aldrich, 99 %, internal standard) and trolled by a pneumatically activated three-way Teflon valve > . toluene (J. T. Baker, 99 5%) were used as received. Gases (PARKER PV-1-2324). After the sample compartment of were of 99.999% purity (PANGAS). the spectrometer was purged with dry air for about 1 h, neat toluene was admitted to the sample for 20 min. Then the Catalytic Studies catalyst was reduced in situ by admitting toluene saturated The reactions were performed in a 150-ml flat-bottomed with H2 for 10 min. After purging with Ar-saturated toluene glass reactor equipped with gas inlet, reflux condenser, ther- for 10 min the catalyst was oxidized by flowing toluene satu- mometer, inlet for taking samples, and magnetic stirring. rated with air. Then the toluenic solution of cinnamyl alco- According to the general procedure, 0.10 g catalyst was pre- hol was pumped over the pretreated catalyst and the spectra were recorded. reduced in situ by H2 for 10 min in 25 ml toluene, purged with N2 for 5 min, and reoxidized by air for 30 min. The aim of this reduction–oxidation cycle was to activate the cata- RESULTS lyst stored in air but to avoid the presence of any hydrogen Reaction Network when introducing the reactant. The catalyst prereduction temperature was lower by 20–25◦C than the temperature Preliminary studies revealed a complex reaction net- of the alcohol oxidation reaction.