Synergism between Molybdenum and Lanthanum in the Disproportionation of Hydrogen Peroxide into Singlet Oxygen Joos Wahlen, † Dirk E. De Vos, † Marijke H. Groothaert, † Ve ´ ronique Nardello, ‡ Jean-Marie Aubry, ‡ Paul L. Alsters, § and Pierre A. Jacobs* ,† Centre for Surface Chemistry and Catalysis, Katholieke UniVersiteit LeuVen, 3001 HeVerlee, Belgium, LCOM, Equipe de Recherches “Oxydation et Formulation”, 59652 VilleneuVe d’Ascq Cedex, France, and DSM Pharma Chemicals, AdVanced Synthesis, Catalysis and DeVelopment, 6160 MD Geleen, The Netherlands Received July 14, 2005; E-mail: pierre.jacobs@biw.kuleuven.be Reaction of H 2 O 2 with certain metal ions produces peroxo-metal intermediates capable of releasing molecular oxygen in the singlet ( 1 Δ g ) state. 1 In contrast to triplet ( 3 Σ g - ) state oxygen, singlet oxygen ( 1 O 2 ) is a reactive yet highly selective oxidant for the peroxidation of olefinic compounds. 2 In particular, molybdate (MoO 4 2- ) is an efficient catalyst for the disproportionation of H 2 O 2 into 1 O 2 . Both the mechanism 3 and the synthetic utility 4 have been investigated. In aqueous alkaline media, Mo and H 2 O 2 form a series of mononuclear peroxo species, [MoO 4-n (O 2 ) n ] 2- (n ) 1-4). Of these, especially oxotriperoxo-Mo (n ) 3), releases 1 O 2 at a high rate, while tetraperoxo-Mo is far more stable: This means that the reaction rate decreases at high [H 2 O 2 ]; hence, fast disproportionation requires gradual addition of H 2 O 2 over time. The H 2 O 2 concentration not only affects the rate but also the selectivity of the reaction; at high [H 2 O 2 ] direct oxygen-atom transfer from tetraperoxo-Mo becomes competitive with 1 O 2 formation, resulting in the formation of unwanted epoxide products. 5 To overcome these drawbacks, we reasoned that by adding other peroxo-forming metals to Mo, the stability of the peroxo-Mo species might be influenced in such a way that the dependence of the rate and selectivity on [H 2 O 2 ] is altered and that higher catalytic activities are attainable. Various peroxo-forming metals were screened as cocatalysts for Na 2 MoO 4 in the disproportionation of H 2 O 2 at high initial [H 2 O 2 ] (3 M in methanol, Mo/H 2 O 2 ) 1/2000). Addition of lanthanum- (III) nitrate, another 1 O 2 -generating catalyst, 5 led to a strong increase of the overall activity compared to the monometallic Mo or La catalysts (Figure 1). The combination of other peroxo-forming metals with Mo or La gave no or only small rate enhancements (Figure S1, Supporting Information). Control experiments estab- lished the essential role of both Mo and La in the improved catalytic turnover (Figure S1). Similar rate enhancements were observed in other solvents such as water and acetonitrile. Even under nonop- timized conditions (Mo/La/NaOH ) 1/4/20), the overall turnover frequency (TOF, based on Mo+La) is 219 h -1 . In the same conditions, the individual TOF for Mo and La are 4 and 31 h -1 , respectively. The low activity of Mo is due to the high [H 2 O 2 ], promoting the formation of stable tetraperoxo-Mo. The maximum activity of Mo under optimum conditions of low [H 2 O 2 ] has been reported to be 36 h -1 . 3 The initial rates of H 2 O 2 disproportionation were determined for different Mo/La/NaOH combinations, keeping the total amount of metal at 0.1 mmol. As shown in Figure 2A, at constant [NaOH], the reaction rate strongly depends on the Mo/La ratio, an optimum being observed at a Mo/La molar ratio of 1/4. These data clearly show a synergistic effect between Mo and La. 6 The amount of NaOH also has a profound influence on the activity of Mo and La. This was studied by varying [NaOH] at a constant Mo/La ratio of 1/4 (Figure 2B). A well-defined maximum is observed at a NaOH/ La ratio of 3/1. Under optimized conditions (Mo/La/NaOH ) 1/4/ 12), the overall TOF amounts to 546 h -1 . Most likely, the NaOH/La optimum is related to the hydrolysis of La 3+ , yielding partially (NaOH/La < 3/1) or fully hydrolyzed La species, La(OH) 3 . 7 In systems with only La, it is believed that the insoluble La hydroxide is catalytically active, while partially hydrolyzed La shows much lower activity. 5 The Mo/La ratio of † Katholieke Universiteit Leuven. ‡ LCOM. § DSM Pharma Chemicals. MoO 4 2- + nH 2 O 2 a MoO 4-n (O 2 ) n 2- + nH 2 O MoO 4-n (O 2 ) n 2- 9 8 n ) 2-4 1 O 2 + MoO 6-n (O 2 ) n-2 2- Figure 1. H2O2 disproportionation catalyzed by Mo (2), La (0), and the combined action of Mo and La (b). Conditions: 0.1 mmol Mo, 0.1 mmol La, or 0.02 mmol Mo and 0.08 mmol La, and 0.4 mmol NaOH and 40 mmol H2O2 (50 wt %) were stirred (700 rpm) at 25 °C in 10 mL of methanol. H2O2 determination by cerimetry. Figure 2. Influence of the La/Mo (A) and NaOH/La (B) ratios on the rate of H2O2 disproportionation. Conditions A: x mmol La, (0.1 - x) mmol Mo, 0.4 mmol NaOH and 40 mmol H2O2 were stirred (700 rpm) at 25 °C in 10 mL of methanol. B: 0.08 mmol La, 0.02 mmol Mo, and x mmol NaOH. Published on Web 11/16/2005 17166 9 J. AM. CHEM. SOC. 2005, 127, 17166-17167 10.1021/ja0547026 CCC: $30.25 © 2005 American Chemical Society