11114
J. Phys. Chem. B 1999, 103, 11114-11123
Molybdate- and Tungstate-Exchanged Layered Double Hydroxides as Catalysts for 1O2 Formation: Characterization of Reactive Oxygen Species and a Critical Evaluation of 1O2 Detection Methods Bert F. Sels, Dirk E. De Vos, Piet J. Grobet, Fre´ de´ ric Pierard,† F. Kirsch-De Mesmaeker,† and Pierre A. Jacobs* Centre for Surface Chemistry and Catalysis, Katholieke UniVersiteit LeuVen, Kardinaal Mercierlaan 92, 3001 HeVerlee, Belgium ReceiVed: June 30, 1999; In Final Form: September 18, 1999
Layered double hydroxides (LDHs) with the formula [Mg0.7Al0.3(OH)2](NO3)0.3 were partially exchanged with MoO42- and WO42- and were used as heterogeneous catalysts for the decomposition of H2O2 into singlet molecular oxygen (1O2). The oxometalate anions are present on the LDH as monomeric, tetrahedral MO42anions. With LDH-MoO42- and H2O2, 1O2 is by far the prevailing reactive oxygen species. EPR trapping detected a minor amount of free OH• radicals, but these do not appreciably contribute to product formation in olefin oxygenation. LDH-WO42- transforms H2O2 into 1O2 about 4 times slower than the Mo catalyst, and additionally effects mono-oxygen transfer to electron-rich substrates such as amines and olefins. Only for alkenes with very low β values, the Schenck hydroperoxidation dominates over the epoxidation in the W-catalyzed reactions. While most tests for 1O2 detection were designed for photosensitized reactions, the applicability of these methods to dark catalytic reactions is not always clear. Therefore, a series of common 1O detection methods was critically evaluated in the LDH-WO 2- or LDH-MoO 2- + H O reactions. 2 4 4 2 2 Isomer distributions in the olefin hydroperoxidation and detection of NIR luminescence are the most reliable and sensitive methods. Quenching or trapping methods that involve amines should not be used when the catalyst also effects mono-oxygenation.
Introduction Dioxygen in its first excited singlet state (1∆g) is a regioand stereoselective reagent for the preparation of hydroperoxides, allylic alcohols, endoperoxides, diepoxides, or even for ring-opening reactions.1 Practical use requires the availability of singlet oxygen (1O2) sources that can generate high 1O2 fluxes without causing side reactions, such as light-induced radical reactions or chlorinations in the case of the Kasha-Khan reagent (OCl-/H2O2). A promising alternative is therefore the controlled, nonradical disproportionation of H2O2,
2H2O2 f 2H2O + 1O2
(1)
The acceleration of this reaction by a series of dissolved mineral compounds has been documented in much detail, and procedures for homogeneous catalytic oxygenation have been proposed.2,3 We have recently reported that the use of a molybdateexchanged layered double hydroxide (LDH) as a catalyst for reaction 1 has several advantages.4 First, the LDH with its high anion exchange capacity has a strong affinity for the molybdate, which ensures true heterogeneity of the molybdate.5 Second, as high substrate concentrations can be used in a salt-free reaction solvent, substrate oxygenation is favored over solvent quenching of 1O2. Finally, heterogeneous catalysis has obvious advantages for workup and catalyst reuse. * Corresponding author. Phone: 32 16 32 1465. Fax: 32 16 32 1998. E-mail:
[email protected]. † Physical Organic Chemistry CP 160/08, UL Bruxelles, 1000 Bruxelles, Belgium.
The present full paper presents detailed proof of the generation of 1O2 from H2O2 in the presence of LDH-MoO42- and LDH-WO42-. First, in a thorough characterization of the materials, it will be demonstrated that the metal speciation on these LDHs is different from that on pillared LDHs, which have been used in epoxidation catalysis.6 Next, the MoO42- and WO42- exchanged LDHs are compared in their ability to transform H2O2 into 1O2 or other reactive oxygen species. Typical tests for detection of 1O2 include trapping experiments, spectroscopy, and reactions with organic substrates, such as mono-olefins, dienes, or aromatics. Such tests are frequently employed in the dye-sensitized generation of 1O2 but have rarely been applied in heterogeneously catalyzed 1O2 generation. Therefore, the results with the LDH catalysts will be used to critically evaluate the scope or the limitations of the various methods for 1O2 detection. Experimental Section Materials. R-Terpinene, rubrene, 1,3-diphenylisobenzofuran, N-tert-butyl-R-phenylnitrone, 2,6-di-tert-butylphenol, cyclohexene, Mg(NO3)2‚6H2O, and Na2WO4‚2H2O were purchased from Acros. 1,4-Dimethylnaphthalene, 2,2,6,6-tetramethylpiperidine, TEMPO, 1-methylcyclopentene, 1-methylcyclohexene, cyclopentene, 2,3,4-trimethyl-2-pentene, 2,3-dimethyl-2-butene, Na2MoO4‚2H2O, and KO2 were purchased from Aldrich. 2Methyl-2-pentene and Al(NO3)3‚9H2O were from Fluka, 2-methyl2-butene from Alfa, and 1-methylcycloheptene from ICN. All products were of the highest commercial grade and were used as such. Only DABCO (Fluka) was recrystallized twice from acetone before use.
10.1021/jp992236z CCC: $18.00 © 1999 American Chemical Society Published on Web 11/30/1999
Catalysts for 1O2 Formation Catalyst Preparation. LDH preparation was based on literature procedures.5 LDH-NO3- was prepared by coprecipitation of Mg and Al nitrate salts. In a 1 L three-neck roundbottom flask, 100 mL of deionized and boiled water was brought to pH 10 ( 0.2 with 1 N NaOH. Next, 120 mL of 0.3 M Al(NO3)3‚6H2O and 120 mL of 0.7 M Mg(NO3)2‚6H2O were simultaneously added (60 mL/h for each) under N2 atmosphere at room temperature. The pH of the slurry was maintained at 10 ( 0.2 by addition of 1 M NaOH. After addition of the nitrate salts, the suspension was stirred for 24 h at ambient temperature. The white precipitate formed was washed, centrifuged four times, and dried by lyophilization (yield ∼90% based on Mg added). For ion exchange, 1.5 g of the LDH-NO3- was suspended in 150 mL of a 1.875 mM aqueous Na2WO4‚2H2O or Na2MoO4‚2H2O solution and stirred for 12 h at ambient temperature under N2 atmosphere. The pH of the final suspension was 9.3 ( 0.2. The final solid products (LDH-MoO42and LDH-WO42-) were obtained by repeated centrifugationwashing cycles, eventually followed by lyophilization. Reaction Procedures. LDH-MoO42- or LDH-WO42catalysts contain 0.187 mmol Mo or W per gram of lyophilized and ambient air rehydrated solid. Homogeneous reactions were performed with Na2MoO4‚2H2O or Na2WO4‚2H2O in 0.01 N NaOH in a 15:85 volumetric ratio of water and methanol. Comparisons between heterogeneous and homogeneous catalysts were made for equal total concentrations of Mo or W. Reactions were generally performed with stirring at room temperature. Yields are calculated as (moles of desired product formed)/ (moles of starting product initially present). Selectivity is defined as (moles of desired product formed)/(moles of starting product converted). Decomposition of H2O2. Reactions were conducted with 0.15 g catalyst and 0.25 M H2O2 in 10 mL of MeOH. H2O2 was determined by cerimetry.7 EPR Detection of O2•-, OH•, and 1O2. Reactions were conducted with 3.75 mM catalyst and 0.8 M H2O2 in 1.5 mL MeOH. For O2•- detection, reaction suspensions were quickly frozen at 130 K after 10 min of reaction. For 1O2 trapping, TEMP was added to the reaction in a 50 mM concentration; after 30 min, the suspension was centrifuged and the spectrum of the supernatant was recorded at room temperature in a flat quartz cell. Radicals were trapped by adding 0.1 M N-tert-butylR-phenylnitrone (PtBN) to the reaction; after 30 min, 4 g of water and 2 g of toluene were added to extract the adducts into the toluene layer. Spectra were recorded at room temperature in standard cylindrical EPR tubes. Peroxidation of r-Terpinene. The reaction was run with 62.5 mM R-terpinene, 2.5 mM Mo or W, and 330 mM H2O2 in 4 mL MeOH, or on a larger scale, with 75 mM R-terpinene, 2.8 mM Mo, and 230 mM H2O2 (450 µL) in 20 mL MeOH. After 2.5 h, when the suspension had changed color from red to yellow, another portion of 450 µL of H2O2 was added. After consumption of a third portion of H2O2, R-terpinene was converted for over 99.5%. The product was isolated by addition of 50 mL water, followed by three extractions with 50 mL diethyl ether. The collected extracts were washed with 20 mL H2O and dried over Na2SO4, yielding an oil with following 13C NMR lines (CD3OD): δ 138.0, 134.5, 81.7, 76.1, 33.9, 30.9, 27.1, 22.1, 18.0, 17.9 ppm.8 Formation of Other Endoperoxides. Rubrene: 1 mM rubrene, 0.225 mM Mo or W catalyst, 39 mM H2O2, 100 mL dioxane. 1,3-Diphenylisobenzofuran (DPIBF): 0.82 mM substrate, 0.6 mM catalyst, 29 mM H2O2, 100 mL THF. 1,4Dimethylnaphthalene (DMN; reaction in a thermostatic bath at
J. Phys. Chem. B, Vol. 103, No. 50, 1999 11115 20 °C): 42.7 mM DMN, 22.5 mM catalyst, 1.5 M H2O2 (added in three portions), 10 mL MeOH. For spectrophotometry, samples were sufficiently diluted with the organic solvent and quickly centrifuged. After completion of the reaction, white reaction products were isolated from the reactions with rubrene and DPIBF, by addition of 100 mL water and extraction into CHCl3 (50 mL). Endoperoxide of rubrene: characteristic 13C resonance at 84.7 ppm (C-O).9 1,2-Dibenzoyl benzene (from DPIBF): 13C δ 196.7, 140.8, 138.1, 133.7, 131.3, 130.4, 130.3, 129.2 ppm.10 Hydroperoxidation of Olefins. Reactions were performed with 3 mmol olefin, 0.06 g of LDH catalyst, and 4 mL of MeOH (or 5 mL for 2,3,4-trimethyl-2-pentene and 1-methyl-1-cycloheptene). After 20 min of stirring, the reaction was started by addition of a portion of 1.65 mmol H2O2. Four subsequent portions of H2O2 were added with intervals of 1.5 h, for Mo, or 6 h, for W reactions. After complete H2O2 consumption, reaction mixtures were analyzed as such or after reduction of the hydroperoxides to the corresponding alcohols with tributylphosphine or Na2SO3. Product distributions were determined with 0.1 M olefin, 2.8 mM catalyst, and 0.9 M H2O2 in MeOH. Competitive reactions were performed in identical conditions, with 0.05 M 2-methyl2-pentene (MP) and 0.05 M of a second olefin A. Relative reactivities are given by
kAr kMP r
)
log(1 - [AO2]t/[A]0) log(1 - [MPO2]t/[MP]0)
(2)
with [AO2]t and [MPO2]t the concentrations at time t of the hydroperoxides formed from A and MP, and [A]0 and [MP]0 the initial olefin concentrations.11 Reference oxidations with H2O2/OCl- were performed with solutions of olefin (0.1 M) and H2O2 (0.9 M) at 10 °C in 10 mL of MeOH. A 0.9 mL amount of 1.05 M NaOCl was added dropwise below the surface under vigorous stirring over 15 min. Prior to GC analysis, NaCl was removed by centrifugation. The solvent deuterium effect was studied with 0.5 M cyclohexene, 1.87 mM catalyst, and 0.61 M H2O2 in 2 mL of MeOH or MeOH-d4. Samples were taken after 5 h. Alternatively, 0.25 M 2,3-dimethyl-2-butene, 2.5 mM catalyst, and 0.29 M H2O2 were used; in this case, samples were taken after 1 h. The effect of quenchers was studied with 0.375 M 2-methyl2-pentene in 4 mL of MeOH, to which DABCO, NaN3, or 2,6di-tert-butylphenol was added (0-0.08 M). Reactions were started by addition of 2.5 mM catalyst and 0.55 M H2O2. NIR Luminescence was detected from cuvettes with 3.8 mM catalyst and 450 mM H2O2 in 1.5 mL of MeOH or MeOH-d4. As a reference, 1O2 was generated photochemically with the sensitizer Ru(bpy)32+. Emission spectra were recorded with an Edinburgh Instruments FL/FS 900 instrument, coupled to a liquid N2 cooled Ge detector (North Coast EO-817). The light going to the detector was passed through a 110 Hz chopping wheel (Bentham 218). The detector signal was filtered by a muon filter (North Coast 829 B) and was processed by a lockin amplifier (Bentham 225). For each sample, 10 scans were accumulated. Instrumentation. For X-ray powder diffraction, a Siemens D5000matic with Ni-filtered Cu KR radiation was used (40 kV, 50 mA). Infrared and Raman spectra were recorded with a Nicolet FT-IR 730 and a Bruker IFS200, respectively. Raman excitation was at 1062 nm (Nd:YAG, 50 mW); 28 scans were accumulated in 180° backscattering geometry. BET surface areas were obtained in an Coulter Omnisorp 100 CX apparatus from
11116 J. Phys. Chem. B, Vol. 103, No. 50, 1999
Sels et al.
TABLE 1: Physicochemical Characterization of the LDH Materials bulk analysis Mg/Al M/Al microanalysisc Mg/Al M/Al weight loss (%) in TGA 270 °C cell dimensions (Å) a0 c0 layer charge density (e+/Å2)d BET area (m2 g-1)
LDH-NO3-
LDH-MoO42- LDH-WO42-
2.30 (2.33)a
2.28 0.056 (0.056)b
2.31 0.055
2.25
2.23 0.054
2.28 0.057
11 (
