Microporous and Mesoporous Materials 84 (2005) 343–352 www.elsevier.com/locate/micromeso
An anionic photo-sensitizer intercalated in a layered double hydroxide: Preparation, characterization and photo-oxidation efficiency T. Pigot, J.C. Dupin, H. Martinez, C. Cantau, M. Simon, S. Lacombe
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Laboratoire de Chimie The´orique et de Physico-Chimie Mole´culaire, UMR CNRS 5624, FR 2606, Faculte´ des Sciences, B.P. 1155, 64013 PAU cedex, France Received 23 February 2005; received in revised form 26 May 2005; accepted 30 May 2005 Available online 27 July 2005
Abstract The synthesis and complete characterization of layered double hydroxides (LDHs) where 4-benzoyl benzoate is either intercalated or adsorbed has been achieved and their photo-sensitizing efficiency for the oxidation of di-n-butylsulfide has been compared. XRD data are noticeably different for the two materials and the presence of an intercalated new phase is obvious for the first material. Elemental analysis and thermogravimetric analysis (TGA) results indicate that intercalated LDH contains much more benzoyl benzoate than adsorbed LDH, while TGA further suggests stronger interactions between the host and the organic anion in the former case. The Fourier transform/infra red (FT/IR) spectrum of the intercalated sample shows no modification of the carbonyl vibration of the benzoyl benzoate moiety upon intercalation within LDH, whereas the diffuse reflectance UV (DRUV) spectrum is strongly modified relative to that of the adsorbed sample. Both intercalated and adsorbed LDHs proved to be efficient and selective sensitizers for the photo-oxidation of di-n-butylsulfide in oxygenated acetonitrile solution. The intercalated photo-sensitizer was efficiently recycled for three successive runs. According to X-ray photoelectron spectroscopy (XPS) analysis of the material after irradiation, the major drawback of these supported sensitizers is the partial replacement of the benzoate anion within the LDH by sulfate and sulfonate anions arising from sulfide oxidation. 2005 Elsevier Inc. All rights reserved. Keywords: Layered double hydroxide; Intercalation; Benzophenone photo-sensitization; Sulfide photo-oxidation
1. Introduction Layered double hydroxides (LDHs) are a class of layered compounds (hydrotalcite-like compounds or anioIII nic clays), with the general formula ½MII 1x Mx xþ n II III ðOHÞ2 ; Ax=n , yH2O] (with M = Mg, M = Al and An ¼ CO2 3 in the present work). The LDHs adopt a hexagonal layered structure derived from that of brucite (Mg(OH)2), by substituting a fraction of the Mg2+ ions
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Corresponding author. Tel.: +33 5 59 40 75 79; fax: +33 5 59 40 74
51. E-mail address:
[email protected] (S. Lacombe). 1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.05.044
by Al3+ ions. This imparts a positive charge to the layers and requires the presence of anions An x=n in the interlayer space. These anions may be either inorganic (CO2 3 , NO , . . .) [1], or organic [2]. When the intercalated or3 ganic anion has some chromophore activity, the hybrid organic–inorganic material may display photo-physical or photo-chemical properties which have been extensively investigated for different applications. For example, photo-chemical reactions of unsaturated carboxylates intercalated in LDHs include cis–trans isomerization and cycloaddition [3–5]. In a recent paper, the photo-chemical hydrogen abstraction of 4-benzoyl benzoate ions from aliphatic carboxylates ions in LDH interlayer was studied and yielded 1:1 adducts [6], while
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a Norrish type II reaction of aromatic ketocarboxylates intercalated in LDH was shown to be dependant on the conformational flexibility of the ion imposed by the interlayer space [7]. In another example, an anionic porphyrin, Co(II)TPP, intercalated in MgAFe(II) LDH was recently shown to undergo a photo-reversible electron transfer reaction [8]. An extensive review of photophysical and photo-chemical processes in various clay minerals has recently been published [9]. However, sensitized photo-oxidation reactions with intercalated photo-sensitizers have only been reported for cationic methylene blue in hectorite and laponite [10] or in bentonite, and for Rose Bengal in LDH [11,12]. Fe(III) porphyrins and Co(II) phthalocyanines immobilized in LDH were claimed as efficient catalysts for the oxidation reaction of cyclohexane in the presence of H2O2 [13] and of mercaptans, respectively [14], without photo-chemical activation. As benzophenone (in organic solution) [15] or its derivative, 4-benzoyl benzoic acid, 4-BBH (in aqueous solution) [16] are known to be efficient photo-oxidation sensitizers, we undertook the study of the photo-sensitizing properties of 4-benzoyl benzoic acid based materials. Besides electron transfer sensitizers deposited or included in silica networks, recently shown to be photo-active materials for gas-phase depollution [17], we indeed discovered that 4-benzoyl benzoate anion intercalated in LDH was efficient for the photo-sensitized oxidation of di-n-butylsulfide (DBS) in acetonitrile solution [18]. We compare in the following the efficiency and selectivity of 4-benzoyl benzoic acid (4-BBH) and of 4-benzoyl benzoate (4-BB) adsorbed or intercalated in LDH for di-n-butyl sulfide photo-oxidation in acetonitrile. We also paid special attention to the evolution of the photo-sensitizing material after irradiation. The pure parent LDH (HT0) and the intercalated LDH (HT14BB) were prepared by direct synthesis, while 4-benzoyl benzoate adsorbed onto LDH (HT2-4BB) was prepared by equilibrating HT0 with an aqueous basic solution of 4-benzoyl benzoate sodium salt.
2. Experimental Acetonitrile (SdS, HPLC quality) was used as received without any drying treatment (water content around 800 ppm i.e., 4.5 · 102 M). 4-benzoyl benzoic acid, di-n-butylsulfide (DBS), magnesium nitrate (Mg(NO3)2 Æ 6H2O) and aluminium nitrate (Al(NO3)3 Æ 9H2O) were purchased from Aldrich and were used as received. 2.1. LDHs characterization methods Powder XRD patterns were recorded on an INEL diffractometer using a curved position-sensitive detector
(INEL CPS 120) calibrated with Na2Ca3Al2F14 as standard. The monochromatic radiation applied was Cu Ka ˚ ) from a long fine focus Cu tube operating at (1.5406 A 40 kV and 35 mA. Scans were performed over the 2h range from 5 to 115. Accurate unit cell parameters were determined by a least squares refinement from data collected by the diffractometer. Basal spacing distances for the different samples analyzed were determined from the position of the d(0 0 3) reflection. The thermal behaviour of the LDHs was determined from thermogravimetric experiments carried out on a TGA model 2950 (TA Instruments) using about 5 mg of sample, from 303 to 873 K (4 K min1), under a nitrogen flow. XPS analyses were performed on a Kratos Axis Ultra photoelectron spectrometer with a magnetic immersion lens to increase the solid angle of photoelectron collection from small analysis areas and to minimize the aberrations of the electron optics. A monochromatic and focused Al Ka radiation (1486.6 eV, spot dimensions of 700 lm · 300 lm) was operated at 450 W under a residual pressure of 5 · 109 mbar. The spectrometer was calibrated using the photoemission lines of Au (Au4f7/ 2 = 83.9 eV, with reference to the Fermi level) and Cu (Cu2p3/2 = 932.5 eV); for the Au4f7/2 line, the full width at half maximum (FWHM) was 0.86 eV in the recording conditions. Charge effects were compensated by the use of a charge neutralization system (low energy electrons [typically 1.85 eV]) which had the unique ability to provide consistent charge compensation. All the neutralizer parameters remained constant during analysis. Peaks were then shifted to align adventitious carbon C1s photoemission to 284.6 eV binding energy. High resolution regions were acquired at constant pass energy of 40 eV. The XPS signals were analyzed by using a least squares algorithm and a non-linear baseline. The fitting peaks of the experimental curves were defined by a combination of Gaussian (70%) and Lorentzian (30%) distributions. All the samples were ground prior to analysis to minimize effects due to the surface texture. It is worth noting that powder XRD has been used to explore the bulk of the material while information about the near surface has been be elucidated using the XPS. Elemental analyses were made at the Service Central danalyse du CNRS, Vernaison. Diffuse reflectance UV (DRUV) spectra of the LDHs were recorded on a Varian Cary 5 spectrometer, in a home-made powder-cell located on the sample port of a diffuse reflectance integration sphere (Varian). Corrected reflectance spectra relative to a pressed PTFE standard spectrum were transformed to the Kubelka– Munk remission function F(R): F ðRÞ ¼
ð1 RÞ2 k ec ¼ ¼ S S 2R
ð1Þ
T. Pigot et al. / Microporous and Mesoporous Materials 84 (2005) 343–352
where S stands for the scattering coefficient (depending on the size and the form of the particles), e for the molar absorption coefficient of the chromophore and c for its molar concentration. The Kubelka–Munk transformation thus converts a reflectance spectrum into a spectrum similar to a conventional absorbance spectrum for solution samples [19]. Owing to the different particle sizes of the samples under investigation, no quantitative comparison of the Kubelka–Munk spectra was attempted in the following study and no conclusion may be drawn on the relative intensities. 2.2. Synthesis of HT0, HT1-4BB and HT2-4BB HT0 was prepared by direct synthesis from Mg(NO3)2 Æ 6H2O and Al(NO3)3 Æ 9H2O according to a known procedure [20]. About 100 ml of a water (treated by inverse osmosis) solution of Mg(NO3)2 Æ 6H2O and Al(NO3)3 Æ 9H2O (0.1 mol L1) in a Mg/Al ratio of 2 (HT1-4BB) or 2.5 (HT11-4BB) and 100 ml of a NaOH solution (2 mol L1) for HT0 or of a 4-BB (0.11 mol L1) and NaOH (2 mol L1) solution for HT1-4BB and HT11-4BB were added drop wise while carefully controlling the pH between 8 and 10. The slurry was stirred for 12 h at 60 C, then filtered and washed thoroughly with hot water. The filter cake was then dried at 353 K for 15 h. HT2-4BB was prepared by stirring for 30 h 1 g of HT0 in 100 ml of a 6 · 102 M aqueous solution of 4BBH in the presence of excess NaOH. 2.3. Solution photo-oxidation Photo-chemical experiments (150 min except when otherwise stated) were carried out by external irradiation in a Rayonet photo-reactor with four RPR ˚ lamps. The reacting mixtures (10 ml of a 3500 A 1.2 · 102 M solution of di-n-butylsulfide in acetonitrile containing 4-benzoyl benzoic acid 5 · 103 M or 30 mg of HT1-4BB or HT2-4BB and cyclododecane as internal standard) were stirred and continuously bubbled with oxygen through a mass flow-meter (2 cm3 min1) during irradiation. After filtration, product analysis was performed by Gas Chromatography (VARIAN 3900 with a FID detector, l5 m CP-Sil-5 Chrompack column, i.d.: 0.25 mm, coating 0.25 lm) or by GC–MS (HP 5973, SPB35 column, l: 60 m, i.d.: 0.23 mm), while the amount of 4-benzoyl benzoate released in solution was checked by UV spectroscopy. Acidic products were extracted by adding to 1 cm3 of the CH3CN oxidized solution, 2 cm3 of water and 2 cm3 of di-chloromethane. Acid titrations of the aqueous phase were performed by ion exchange chromatography (IEC) on a Dionex DX-20 exchange ion chromatograph equipped with a column AS9-HC (4 mm), and operating in the suppressed conductivity mode.
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3. Results 3.1. Synthesis and characterization of intercalated and adsorbed LDHs Parent LDH HT0 and its intercalated HT1-4BB and HT11-4BB derivatives, differing by the Mg/Al ratio (Table 1) were prepared by direct synthesis at controlled pH, while HT2-4BB was obtained by adsorbing 4-benzoyl benzoate onto HT0. The elemental analysis results and the molecular formulae obtained for these different materials are given in Table 1. These data indicate that the Mg/Al ratio has only a minor influence on the formula of the materials HT1-4BB and HT11-4BB. As the following structural data are very close for both materials, they will only be described for HT1-4BB. It is also obvious that the LDHs prepared by direct synthesis in the presence of an aqueous solution of 4-benzoyl benzoate contain much more 4-benzoyl benzoate (1.9 and 1.8 mmol g1 for HT1-4BB and HT11-4BB, respectively) than HT2-4BB (0.14 mmol g1) prepared by adsorption of 4-benzoyl benzoate onto HT0. The amount of nitrates is obviously lower for the parent LDH, HT0, and the adsorbed LDH, HT2-4BB, (
