Aluminium Salt of Phosphomolybdic Acid Fabricated by Nanocasting Strategy: An Efficient System for Selective Oxidation of Benzyl Alcohols

Abstract

Preparation of $AlPMo_{12}O_{40}$ (AlPMo) salts, supported on mesostructured SBA-15 silica, by the reaction deposition strategy causes the formation of isolated AlPMo nanocrystals inside the nanotubular channels. The remarkable characteristic of the SBA-15 structure is that all the cylindrical pores are connected by some small channels. This makes the whole pore system in SBA-15 three-dimensional. We have used 2D hexagonal SBA-15 silicas as hard templates for the nanofabrication of AlPMo salt nanocrystal. The oxidation of alcohols occurs effectively and selectively with $H_2O_2$ as the oxidant. AlPMo salt nanocrystal was used as the catalyst.

INTRODUCTION

High-surface area inorganic materials have attracted widespread attention in diverse areas such as heterogeneous catalysis,1 adsorption,2 gas sensing,3 energy storage,4 drug delivery5 biomedical applications,6 electrochemistry, 7 etc. The application of the nanocasting technique to the fabrication of inorganic compounds implies that the fabrication of these products takes place in the nanospaces provided by the pores of a porous solid (hard template). After the synthesis of the material, the template framework is selectively removed and the inorganic product is obtained. Basically, the nanocasting route comprises three steps: i) Infiltration of the porosity of the template with a solution containing the precursors of the inorganic compound; ii) Heat treatment under a controlled atmosphere of the impregnated template to convert the infiltrated precursor into the inorganic material and iii) removal of the template framework by dissolution (i.e. silica) or by oxidation at high temperatures (i.e. carbon). Due to the fact that the synthesis takes place in a confined nanospace, the sintering of the particles is restricted and the preparation of highsurface area materials (nanostructures or nanoparticles) is achieved. In this way, numerous inorganic compounds of high-surface area can be obtained, although high-temperatures are required to synthesize them. Moreover, this synthetic strategy clearly suggests that the structure of the synthesized inorganic compounds can be tailored depending on the pore characteristics of the selected template.8 Polyoxometalaes (POMs) belong to a large class of nanosized metal-oxygen cluster anions. The chemistry of polyoxometalates (heteropoly acids (HPAs) and heteropoly salts), started by Berzelius back in 1826, has now reached maturity. But it is still a rapidly developing field interconnected with many disciplines.9 Catalysis by polyoxometalates has attracted much attention because of their strong acidity and redox properties. An additional attractive aspect of polyoxometalates in catalysis is their inherent stability toward oxygen donors.10

Oxidation of alcohols into the corresponding aldehydes and ketones is one of the most fundamental transformations in organic synthesis. Usually, the oxidation of benzylic alcohols has been carried out using oxidants such as chromium (VI) trioxide,11 nitric acid,12 dimethyl sulfoxide/HBr,13 and hypervalent iodine compounds.14 In recent years, replacement of toxic oxidants in organic reactions has become of high priority in environmentally benign chemistry. Among other reagents, hydrogen peroxide is a cheap and easily available oxidizing reagent, and it is considered as the most desirable oxidant in terms of environmental promoted oxidation of benzylic alcohols.

Scheme 1.

As part of a continuing effort to understand catalytic properties of heteropoly acids, (HPAs),15-18 herein, we wish to report H2O2/AlPMo12O40 (AlPMo) salt nanocrystal system for the aerobic oxidative of alcohols (Scheme 1).

 

EXPERIMENTAL

All materials were commercial reagent grade. Infrared spectra (400-4000 cm-1) were recorded from KBr pellets on a Nicolet Impact 400 D spectrometer. 1H NMR spectra were carried out with a Bruker-Avance AQS 300 MHZ. The melting points were determined using an electrothermal digital melting point apparatus and were uncorrected. Reaction courses and product mixtures were monitored by thin layer chromatography. The X-ray powdered diffraction patterns were performed on a Bruker-D8 advance with automatic control. The patterns were run with monochromatic Cu Kα (1.5406 Å) radiation with a scan rate of 2° min-1. The micrographs were recorded using a SEM (HITACHI COM-S-4200) operated at an accelerating voltage of 30 kV. Thermogravimetic analysis was carried out on a Labsys-Setaram TGA-DSC instrument in flowing N2 with a heating rate of 20 ℃ min-1 up to 1000 ℃.

SBA-15

SBA-15 was first synthesized by Zhao et al. in 1998,19 using amphiphilic triblock copolymers, EO20PO70EO20 (Pluronic P123), as template. In a typical synthesis, 1.00 g of P123 was dissolved in a mixture of HCl/H2O (30.00 g of 2 M HCl to 7.50 g H2O). After dissolution 2.08 g of TEOS or 1.96 g of sodium silicate was added. The slurry was hydrothermally treated at 100 ℃ for 48 h after stirring at 40 ℃ for 16 h. The product was filtered off and dried at 800 ℃ for 10 h.

AlPMo/SBA

The CsHPMo was inserted in the SBA-15 silica matrix by the two-step reaction deposition method. The parent AlPMo sample was obtained by dispersing the surfactant free SBA-15 (0.5 g) in 10 ml of n-propanol followed by an addition of 0.20 g of Aluminium carbonate (Aldrich). The contents were stirred for 4 h, filtered and dried under vacuum. The dry solids were treated for 12 h with a solution containing an excess of H3PMo12O40 dissolved in n-propanol under continuous stirring, after which they were filtered and washed with an excess of n-propanol. The resulting solids were dried at 383 K for 2 h and calcined at 573 K for another 2 h.

Bulk-AlPMo

Preparation of aluminium dodecatungstophosphate (AlPW12O40) was reported in 1982 by Ono by the reaction of aluminium nitrate and dodecatungstophosphoric acid in a quantitative yield.20 We prepared aluminium dodecamolybdophosphate (AlPMo12O40, Denoted as AlPMo hereafter) by the addition of aluminium nitrate or by aluminium carbonate to the aqueous solution of molybdatophosphoric acid, which, on complete evaporation of water, gave the desired compound as yellow (AlPMo) powders in a quantitative yield.

AlPMo Nanoparticles

The silica matrix was gradually removed from the AlPMo/SBA materials by a treatment with H2O. AlPMo/SBA composite 0.50 g was continuously stirred in 20 ml of H2O for 10 min at room temperature. The AlPMo (aq) was recovered by decantation after centrifugation (7000 rpm) and finally the material was evacuated at 333 K.

Oxidation of Benzylic Alcohols, General Procedure

A 25-ml round bottomed flask with 2 ml of CH3CN equipped with a magnetic stirrer and reflux condenser was charged with 0.01 mmol catalyst and 5 mmol aqueous hydrogen peroxide (30%). The mixture was stirred and then 1 mmol alcohol was added. The biphasic mixture was stirred at 90 ℃ for the required time. Progress of the reaction was followed by the aliquots withdrawn directly from the reaction mixture analysed by GC using internal standard. After completion of the reaction, the mixture was treated with a 10% sodium hydrogen sulfite solution to decompose the unreacted hydrogen peroxide and then with 10% sodium hydroxide. The product was extracted with n-butyl-ether. The pure product was obtained by distillation or silica gel column chromatography (hexane/ethyl acetate, 10/1).

 

RESULTS AND DISCUSSION

Characterization of the Catalyst

The prepared catalyst was characterized by infrared spectroscopy (FT-IR), X-ray diffraction (XRD), SEM, EDS, TEM, TG and atomic absorption spectroscopy techniques.

FT-IR spectra have proved to be a powerful technique for studying surface interaction between HPA and organic and inorganic supports. The infrared spectrum of the bulk-AlPMo exhibited strong vibrations at 1063 νas(P-Oa), 964 νas(W-Oa), 884 νas(W-O-W), and 787 cm-1 νas(W-Oc-W) respectively (Fig. 1a), while that of AlPMo nanocasts (Fig. 1c) exhibited them at 1065 νas(P-Oa), 963 νas(WOa), 912 νas(W-O-W), and 781 cm-1 νas(W-Oc-W) cm-1. Fig. 1b represents the FT-IR transmission spectra of calcined AlPMo/SBA-15. It shows a typical infrared spectrum of silica21 with bands assigned at 1634 cm-1 with a broad band at 1070 cm-1. The FT-IR spectra of AlPMo/SBA-15 indicate that most of characteristic bands of the parent Keggin structure, which could be found in HPA fingerprint region (1250-500 cm-1), are not shown or do not appear in the same assignable position of the bands corresponding to SiO2 host material (Fig. 1b).

Fig. 1.FTIR spectra of (a) bulk-AlPMo12O40; (b) AlPMo12O40/SBA-15; (c) AlPMo12O40 nanocast.

X-ray powder analysis is widely used to study the structure of heteropoly complexes. The XRD patterns of the asprepared AlPMo12O40/SBA-15 composite and the AlPMo12O40 nanocasts are shown in Fig. 2. The XRD peaks for the AlPMo phase inside the AlPMo/SBA-15 composites (Fig. 1a) and AlPMo nanocasts (Fig. 2b) could be attributed to bcc cubic structure, which was commonly associated with the pure alkaline heteropoly salts.22 Thus, the removal of the silica matrix did not affect the crystal structure. The wide amorphous halo centered at 2θ=23° in the spectra of SBA-15 silica in as-prepared composites completely disappeared for nanocasted materials.

Fig. 2.XRD patterns of (a) AlPMo12O40/SBA-15; (b) AlPMo12O40 nanocast.

Fig. 3.Uv-vis spectra of AlPMo12O40 nanocasts.

Fig. 3. shows the Uv-vis spectra of AlPMo12 nanocast. Two main absorptions are present in the bulk-PMo12 spectrum: the first is centered at 255 nm, and is attributed to the oxygen-tungsten charge-transfer absorption band for Keggin anions.23 The second broad absorption in the bulk-H3PMo12O40 is centered at 360 with a shoulder at 345 nm. For the encapsulated AlPMo12 nanocast, these bands are clearly observed in 230 and 310 nm, respectively.

The thermal stabilit 1000 ℃ (Fig. 4). The TGA curve indicates that the weight loss of AlPMo can be divided into two steps. The first weight loss is 13.907% from 41 to 123℃, involving the release of three lattice water molecules. The second step beginning about 701-943 ℃ (10.693%) belongs to the decomposition of POM and release WO3.

Fig. 4.TG analysis of AlPMo12O40 nanoparticles.

Fig. 5.EDS measurements carried out during SEM observations.

The AlPMo nanoparticles have been found to be high purity by EDS measurements. EDS results (Fig. 5) indicated that the as-prepared samples contained Al, Mo, P and O elements. The percentage of silica matrix removed from the AlPMo/SBA composite is about 99%. The good distribution of all elements analyzed is in agreement with the atomic adsorption analysis, neutron activation analysis (NAA) and ICP results with deviation in the range of ±0.09. These analyses indicate that the value of Al/Mo ratio is 3 which agrees well with calculated formula AlPMo12O40.

The SEM picture of CsHPMo is shown in Fig. 6. AlPMo exhibited nearly the dense aggregate of rock-like morphology. There is no definite shape shown for any particles but the edges appear to be spherical.

Fig. 6.SEM micrographs of the sample AlPMo12O40 nanoparticles.

TEM

Fig. 7 shows the TEM images of PW12/MCF and AlPMo12O40 nanoparticles. The sample for the TEM measurement was suspended in ethanol and supported on a carbon coated copper grid. Isolated 5-9 nm nanoparticles of AlPMo/SBA-15 were clearly observed by TEM (Fig. 7a) in the curved nanotublar channels inside the silica microfibers.

Fig. 7.TEM micrographs of (a) AlPMo12O40/SBA-15; (b) AlPMo12O40 nanocasts.

Physical Properties of SBA-15, bulk- AlPMo, AlPMo/BA-15 and nanocasted AlPMo materials are listed in Table 1. As expected, the BET surface areas, total pore volumes and mesopore sizes of SBA-1522 material decreased from 873 to 299 m2g-1, from 1.4 to 0.37 cm3g-1, and from 6.4 to 4.9 nm, respectively, after the functionalization with AlPMo. These changes reflect that part of the mesopore volume in the SBA-15 matrix is filled with AlPMo nanocrystals, resulting in pore diameters that are less than that of silica channels.

Table 1.aTotal pore volume measured at p/p0 = 0.99.

Oxidation of Benzylic Alcohols

The catalytic activity of the prepared catalyst was tested using Benzyl alcohols as reference alcohol. Oxidation was carried out with H2O2/Urea (UHP) as an oxidant and in the presence of catalytic amounts of AlPMo nanocast. The optimum conditions used for the oxidation of benzyl alcohol by this catalytic system was catalyst, oxidant, and substrate in a mol ratio of 1:15:0.03, respectively (Table 2). In the catalytic reactions the choice of solvent is crucial. The influence of the various solvent on the yield of the reaction was investigated using benzyl alcohol as the substrate. From these studies it was concluded that CH3CN was the most favorable solvent (Table 2). The performance of the AlPMo/SBA-15 composite and AlPMo (bulk) are shown in Table 2. It is important that both removal of silica and nanocasting synthesis caused the increase of reactivity (Table 2, entries 15, 16).

Table 2.aReaction condition:benzyl alcohol (1 mmol),catalyst (0.03 mmol), H2O2/Urea (15 mmol) under reflux conditions after 35 min. bIsolated yield. ccatalyst (0.02 mmol) was used. dcatalyst (0.025 mmol) was used. eH2O2/Urea (12 mmol) was used. fThe bulk-AlPMo (0.03 mmol) was used as catalyst. gThe AlPMo/SBA-15 (0.03 mmol) was used as catalyst.

To study the scope of this procedure the oxidation of other alcohols was next studied (Table 3). To test the role of electron influence of phenyl substituents on the efficiency of oxygenation, 4-MeO-benzyl alcohol and 4-NO2-benzyl alcohol were exposed to the oxidation system. These two substrates obtained 95% and 60% of conversions, respectively, after 30 min. The electron-withdrawing nitro group reduced the reactivity of benzyl alcohol toward oxidation, whereas the methoxy-group on the paraposition of the phenyl ring increased the tendency of benzyl alcohol oxidation (Table 3).

Table 3.aReaction conditions: benzyl alcohol (1 mmol), catalyst (0.03 mmol) , H2O2/Urea (15 mmol), CH3CN (2 ml) under reflux conditions. bIsolated yield. cAll products were identified by comparison with authentic sample (mp, IR, NMR).

The recovery and reusability of the catalyst were investigated. We have noticed that after the addition of CHCl3 to the reaction mixture, this catalyst can be easily recovered quantitatively by simple filtration. The wet catalyst was recycled (the nature of the recovered catalysts was followed by NAA, XRD and FT-IR spectra) and no appreciable change in activity was noticed after three cycles.

 

CONCLUSIONS

The elimination by HF of the silica matrix from the composites occurred by the two-step reaction deposition of AlPMo12O40 (AlPMo) salt nanocrystal. We have used 2D hexagonal SBA-15 silica as templates for the nanofabrication of AlPMo nanoparticles. We have developed an efficient strategy for aerobic oxidation of benzylic alcohols using AlPMo nanoparticles, as an eco-friendly, inexpensive and efficient catalyst. This reaction provides a new environmentally friendly route to the conversion of alcoholic functions to carbonyl groups. Aldehydes do not undergo further oxidation to carboxylic acids. The advantages of this catalytic system is mild reaction conditions, short reaction times, moderate to good product yields, easy preparation of the catalysts, non-toxicity of the catalysts, simple and clean work-up of the desired products.