Polyoxometalate-Based Amphiphilic Catalysts for Selective Oxidation

Please wait while the data is being loaded.. Hide Menu Back .... Get another CAPTCHA Get an audio CAPTCHA Get an image CAPTCHA Help. ReCaptcha ...
0 downloads 0 Views 4MB Size
Download PDF
Article pubs.acs.org/IECR

Polyoxometalate-Based Amphiphilic Catalysts for Selective Oxidation of Benzyl Alcohol with Hydrogen Peroxide under Organic Solvent-Free Conditions Li Jing, Jing Shi, Fumin Zhang,* Yijun Zhong, and Weidong Zhu* Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, 321004 Jinhua, People’s Republic of China S Supporting Information *

ABSTRACT: A series of polyoxometalate (POM)-based amphiphilic catalysts were prepared via functionalization of the Vcontaining Keggin POM H4PMo11VO40 by cationic surfactants with different carbon-chain lengths. These prepared catalysts were systematically characterized by Fourier transform infrared (FT-IR), 1H nuclear magnetic resonance (NMR), thermogravimetric (TG), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption, and X-ray diffraction (XRD) techniques as well as by the elemental analysis. Their catalytic activities were evaluated in the selective oxidation of benzyl alcohol to benzaldehyde by H2O2 under organic solvent-free conditions. Among the catalysts investigated, the amphiphilic (ODA)4PMo11VO40 (ODA: octadecylmethylammonium) shows the highest catalytic efficiency for the selective oxidation. The high activity and selectivity of the prepared (ODA)4PMo11VO40 are probably related to its amphiphilic property. A maximum conversion of benzyl alcohol is 60.6% with a selectivity of 99% for benzaldehyde under the optimized reaction conditions over (ODA)4PMo11VO40, which offers excellent reusability, confirmed by the recycling of the used catalyst.

1. INTRODUCTION Benzaldehyde is an important starting chemical for manufacturing odorants, flavors, and pharmaceutical intermediates. The conventional processes for the production of benzaldehyde include the hydrolysis of benzyl chloride and the liquid−phase oxidation of toluene.1,2 In the former process, a trace amount of chlorine inevitably exists in the product benzaldehyde, and, in the latter process, the selectivity for benzaldehyde is rather low. In order to produce chlorine-free benzaldehyde to meet practical requirements for perfumery and the pharmaceutical industry, the vapor-phase oxidation of benzyl alcohol has been investigated.3−7 The disadvantage of the vapor-phase oxidation of benzyl alcohol is associated with high reaction temperatures required, resulting in high energy consumption, low selectivity for benzaldehyde, and deactivation of the catalysts used.3,8 Therefore, the liquid-phase oxidation of benzyl alcohol to benzaldehyde is more preferable, from an application point of view, if it could be of low energy consumption, high selectivity for the desired product, and preventing catalyst deactivation. In the liquid-phase oxidation of organic compounds, H2O2 is widely used as oxidant, because it is environmentally friendly, quite inexpensive, readily available, easy to handle, and gives water as the only byproduct.9 Therefore, in the past decade, many efforts have been devoted to the selective oxidation of benzyl alcohol by H2O2 over solid catalysts, such as tetraalkylpyridinium octamolybdate,10 nano-γ-Fe2O3,11 Fe(Cu)containing coordination polymers,12 hydrotalcites,13 MCM-41 immobilized Cr(salen),14 tetraazamacrocycle complexes of Cu(II) and Ni(II) encapsulated in zeolite-Y,15 Ti-SBA-15,16 alkali-treated ZSM-5 zeolite,17 Co-metalloporphyrin supported on silica,18 and Au catalyst.19 However, in general, both high conversion of benzyl alcohol and high selectivity for © XXXX American Chemical Society

benzaldehyde over these reported catalysts cannot be satisfied simultaneously. Polyoxometalates (POMs) are a large family of anionic metal−oxygen clusters of early transition metals, which have stimulated current research activities in the fields of catalysis, materials science, and medicine, because their chemical properties such as redox potentials and acidities can be finely tuned by choosing the constituent elements and countercations.20−24 POMs have been revealed to be very active in the oxidation of benzyl alcohol to benzaldehyde with H2O2.25−30 However, the oxidation is often carried out in the presence of chlorinated hydrocarbons, which are environmentally unfriendly, as solvents in order to promote the reactants fully dissolved.26,27 Moreover, pure POMs utilized in their bulk form have some drawbacks such as difficulty for catalyst recovery and reuse, because they are dissolved in reaction media. Therefore, the immobilization of POMs onto porous supports becomes the commonly used method to heterogenize POM catalysts.31−33 However, supported POM catalysts usually show low activities due to mass-transfer limitations and leaching of active species and/or solvent dependence during reactions.34,35 Apparently, development on highly active and stable POMbased heterogeneous catalysts under organic solvent-free conditions is desirably required. Modification of POMs with organic units has been applied as an effective strategy to achieve POM-based hybrid catalysts with improved catalytic activity and convenient catalyst recovery and reuse. In this context, different organic groups, Received: March 4, 2013 Revised: June 26, 2013 Accepted: July 8, 2013

A

dx.doi.org/10.1021/ie4007112 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

such as ionic liquids,36,37 organometallic complexes,38 organic polymers,39 et al., have been utilized. Actually, the solubility and oxidizing capability of the obtained organic POM salt catalysts could be significantly adjusted by the introduction of the organic moieties, leading to an improved catalytic activity and convenient catalyst reusability, compared with the bulk POMs.40−45 For example, Ikegami and co-workers46 developed a temperature responsive catalyst comprising poly(N-alkylacrylamide) polymer and phosphotungstic acid anions (PW12O403−), showing a high activity in the oxidation of various alcohols by H2O2 due to the formation of emulsion system while the catalyst could be easily precipitated from the reaction mixture upon cooling after the addition of diethyl ether. Hou and co-workers47 found that an ionic liquid derivative of POM catalyst could be obtained by exchanging the protons of the POM with alkylimidazoles, and this derivative could be used as a reaction-induced phase-separation catalyst for olefin epoxidation with H2O2. Wang and coworkers48,49 found that POM-based ionic hybrids prepared by combining amino-functionalized organic cations with PW12O403− anions were highly efficient for the oxidation of alcohols with H2O2. Functionalization of phosphotungstic acids by surfactants with long carbon chains has been studied in terms of synthesis, structure, and morphology for the purpose to enhance their activities in catalysis.50−56 However, there have been few reports on V-containing POMs like H4PMo11VO40 functionalized by surfactants with different carbon-chain lengths to tune their catalytic properties up to now. Because H4PMo11VO40 possesses a stronger oxidation capability than other POMs due to the fact that the V species act as active centers for oxidation22 and has been utilized as an effective oxidation catalyst in many types of catalytic reactions,57−61 in the current work, we select H4PMo11VO40 as the research subject and deal with the preparation and characterization of H4PMo11VO40-based heterogeneous catalysts and their application in the liquidphase oxidation of benzyl alcohol to benzaldehyde under organic solvent-free conditions. Scheme 1 illustrates our preparation protocol. The cationic surfactant octadecyldimethylammonium chloride (ODACl in abbreviation) is attached onto H4PMo11VO40 to prepare (ODA)4PMo11VO40 where ODA with a long alkyl chain would be expected to be capable

of encapsulating the relatively less polar reactant benzyl alcohol and of releasing the more polar product benzaldehyde, because their dielectric constants are different.62 Thus, the ODA in the catalyst could act as a dynamic trap to enhance the probability of an interaction between the substrate and the catalytic center, resulting in the enhanced catalytic activity for the selective oxidation of benzyl alcohol to benzaldehyde.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All reagents with AR purity (analytical reagent grade) were purchased and used as received without further purification. ODACl, dioctadecyldimethylammonium chloride (DODACl), dodecyltrimethylammonium chloride (DDACl), and hexadecyltrimethylammonium chloride (HDACl) were purchased from Sigma-Aldrich. Molybdenum trioxide, vanadium pentoxide, phosphoric acid (85 wt % aqueous solution), N,N-dimethylformamide, n-butanol, chloroform, benzyl alcohol, tetrabutylammonium chloride (TBACl), phosphomolybdic acid (H3PMo12O40), and hydrogen peroxide (30 wt % aqueous solution) were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water with resistivity larger than 18.2 MΩ was obtained from Millipore Milli-Q ultrapure water purification system. 2.2. Catalyst Preparation. The molybdovanadophosphoric heteropolyacid H4PMo11VO40 was prepared according to the procedure reported in the literature31 as follows: 15.8 g of MoO3 and 0.91 g of V2O5 were added into 250 mL of deionized water and the suspension was then heated up to 100 °C under stirring. Afterward, 1.15 g of H3PO4 (85 wt.% aqueous solution) was added into the above−prepared suspension. After the suspension became clear and transparent, it was cooled to room temperature. Finally, the water in the solution was evaporated in a rotary evaporator at 50 °C and an orange solid was collected, into which a suitable amount of deionized water was added for the purification of H4PMo11VO40 powder by recrystallization. For the preparation of (ODA)4PMo11VO40, a solution containing H4PMo11VO40 (1.78 g) and deionized water (10 mL) was dropped into the solution of ODACl (1.40 g) and chloroform (10 mL) under magnetic stirring. Upon addition of the entire POM aqueous solution, the yellow precipitate (ODA)4PMo11VO40 was formed. After 1 h under stirring, the solid was filtered, washed with deionized water twice, and dried in a vacuum desiccator at 110 °C. Then 0.1 g of the dried (ODA) 4 PMo 11 VO 40 powder, which is referred to as (ODA)4PMo11VO40-P, was dissolved in 10 mL of N,Ndimethylformamide under stirring. Afterward, n-butanol (2 mL) was slowly dropped into the above-prepared solution so that a yellow precipitate was formed. After 5 min under stirring, the solid was filtered, washed with deionized water twice, and dried in a vacuum desiccator at 110 °C. Finally, the catalyst (ODA)4PMo11VO40 with a flowerlike structure was obtained. The other catalysts containing surfactant cations with different carbon-chain lengths, DODA, DDA, HDA, and TBA, referred to as (DODA)4PMo11VO40, (DDA)4PMo11VO40, (HDA)4PMo11VO40, and (TBA)4PMo11VO40, respectively, were prepared by the same method as aforementioned for the preparation of (ODA)4PMo11VO40. For the preparation of (ODA)3PMo12O40, except for the starting POM H3PMo12O40 instead of H4PMo11VO40, the preparation procedure was the same as that for the preparation of (ODA)4PMo11VO40. 2.3. Characterization. The FT-IR spectra of the prepared catalysts were collected on a Nicolet NEXUS670 Fourier

Scheme 1. Illustration for Preparing a Flowerlike POMBased Amphiphilic Catalyst

B

dx.doi.org/10.1021/ie4007112 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

Table 1. BET Specific Area and Chemical Composition of the Catalysts Investigated content of elementa (wt %) −1

SBET (m g )

C

H

N

Mo

V

(DDA)4PMo11VO40

1.25

(HDA)4PMo11VO40

1.12

(ODA)4PMo11VO40

0.82

(DODA)4PMo11VO40

0.68

(ODA)4PMo11VO40b (TBA)4PMo11VO40

0.79 0.76 1.07

5.16 (5.09) 5.90 (5.81) 6.19 (6.12) 8.22 (8.10) 6.21 5.31 (5.28) 6.08 (6.03)

2.12 (2.08) 1.89 (1.92) 1.88 (1.85) 1.39 (1.41) 1.89 2.02 (2.04) 1.84 (1.82)

39.29 (39.22) 36.34 (36.20) 34.89 (34.86) 26.68 (26.51) 34.91 38.46 (38.42) 37.51 (37.47)

1.87 (1.89) 1.82 (1.75) 1.71 (1.68) 1.21 (1.28) 1.69 1.84 (1.85)

(ODA)3PMo12O40

26.89 (26.78) 31.42 (31.31) 33.26 (33.32) 45.96 (45.85) 33.48 28.01 (27.98) 32.88 (32.83)

catalyst

a

2

Data in parentheses represent the theoretical content of the element in the catalyst. bUsed catalyst after the fourth run reaction.

Figure 1. FT-IR spectra of the catalysts investigated.

transform infrared spectrophotometer in KBr disks at room temperature. The X-ray powder diffraction (XRD) patterns were obtained on a Philips PW3040/60 diffractometer, using Cu Kα radiation (λ = 0.1541 nm) in a scanning range of 1.5− 40° at a scanning rate of 1°/min. The 1H NMR measurements were obtained using a Bruker AVANCE III 400 WB spectrometer. The thermogravimetric analyses (TGA) were performed on a NETZSCH STA 449C thermogravimetric analyzer under N2 atmosphere at a heating rate of 10 °C min−1. The scanning electron microscope (SEM) observations were performed on a Hitachi S-4800 apparatus equipped with a field emission gun. The acceleration voltage was set to 5 kV. The sample was stuck on the observation platform and sprayed with gold vapor under high vacuum for about 20 s. The transmission electron microscopy (TEM) observations were carried out on a JEOL JEM-1200 working at 300 kV. The sample was diluted in

ethanol to give a 1:5 volume ratio and sonicated for 10 min. The ethanol slurry was then dropped onto a Cu grid covered with a thin film of carbon. The chemical composition of all the prepared catalysts was determined by an IRIS Intrepid II XSP inductively coupling plasma−atomic emission spectrometer (ICP-AES) in combination with a CHN elemental analyzer (Vario EL III). The BET surface areas of the samples were determined by the adsorption isotherms of N2 at −196 °C using a Micromeritics ASAP 2020 instrument. The samples were outgassed under vacuum at 150 °C for 10 h, prior to the adsorption measurements. The concentration of H2O2 in its aqueous solution was determined iodometrically prior to use in the oxidation reaction. Self-decomposition of H2O2 as a function of time was monitored by measuring the volume of oxygen evolved. The adsorption capability of benzyl alcohol on the amphiphilic catalyst was calculated through the weight C

dx.doi.org/10.1021/ie4007112 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

change of the catalyst before and after adsorption of benzyl alcohol, following a similar procedure described by Xiao and co-workers. 6 3 In a typical run for adsorption on (ODA) 4 PMo 1 1 VO 4 0 , for instance, 0.03 mmol of (ODA)4PMo11VO40 was added into 60 mmol of benzyl alcohol under vigorous stirring, and the adsorption temperature was kept at 90 °C. After expected intervals, (ODA)4PMo11VO40 in the mixture was separated by centrifugation, weighing, and calculation of the adsorption capacity. 2.4. Catalytic Oxidation of Benzyl Alcohol. The liquidphase catalytic oxidation of benzyl alcohol was carried out in a two-necked round-bottom flask (25 mL) connected with a reflux condenser and a thermometer. In a typical reaction, benzyl alcohol (60 mmol) and catalyst (0.03 mmol) were added to the glass flask. The reaction was initiated by adding H2O2 solution (50 mmol) with vigorous stirring. The typical reaction temperature and time were 90 °C and 6 h, respectively. The reaction mixture was sampled periodically. The sampled mixture was then centrifuged to remove the solid catalyst and the liquid was analyzed by a gas chromatography (Agilent 6820) equipped with an flame ionization detector (FID) and a capillary column (DB-5, 30 m × 0.45 mm × 0.42 μm). The same set of the experiment on the catalytic oxidation of benzyl alcohol with hydrogen peroxide was repeated at least three times and the standard deviation was then calculated. Additionally, the reaction conditions with respect to temperature, time, and the amount of catalyst and H2O2 were optimized. In order to test the catalytic recyclability of (ODA)4PMo11VO40, after the reaction, the catalyst was separated by filtration, washed with ethanol, and dried in a vacuum desiccator at 120 °C for 5 h, and the recovered catalyst was then reused in the next run.

Figure 2. TGA curves of (DDA)4PMo11VO40 (a), (HDA)4PMo11VO40 (b), (ODA)4PMo11VO40 (c), and (DODA)4PMo11VO40 (d) with a heating rate of 10 °C min−1 in an nitrogen flow of 40 mL min−1.

TEM image demonstrates that the assembly is composed of the amphiphilic building blocks arranged into lamellar patterns (Figure 4d). For the other POM-based self-assemblies, although the different structures can be observed by SEM, they all reveal lamellar patterns by the TEM observations (see Figures S2−4 in the Supporting Information). The small-angle XRD patterns show a typical feature of the layered structure for the different catalystssee Figure 5and the estimated layer spacing for (ODA)4PMo11VO40 is about 3.4 nm. On the basis of the above results, it can be concluded that the amphiphilic catalysts were successfully prepared. 3.2. Catalytic Oxidation of Benzyl Alcohol. The liquidphase catalytic oxidation of benzyl alcohol to benzaldehyde by H2O2 was conducted, and the results are summarized in Table 2. In all cases benzaldehyde was detected as the main product and benzyl acid as the only byproduct with very limited amount was formed under the applied reaction conditions. A low conversion of benzyl alcohol and a low selectivity for benzaldehyde were observed in the presence of H3PMo12O40 (entry 1) while the conversion of benzyl alcohol was increased up to 28.2% with a selectivity of 90.0% for benzaldehyde in the presence of H4PMo11VO40 (entry 2), implying that the V species in the catalyst are essential to catalyze the oxidation. In the presence of H4PMo11VO40, the reactant benzyl alcohol cannot be completely dissolved in the aqueous medium, leading to the formation of an oil/water biphasic reaction system, resulting in low catalytic activity. Differently, over (ODA)4PMo11VO40, the conversion of benzyl alcohol was as high as 60.6% with a selectivity of 99.0% for benzaldehyde (entry 3), although ODACl itself was inactive in the oxidation (entry 4), and much superior to (ODA)3PMo12O40 (entry 5). When (ODA)4PMo11VO40-P was used as the catalyst, the conversion of benzyl alcohol was 58.9% (entry 6), slightly lower than that over (ODA)4PMo11VO40 with a flowerlike structure (entry 3). Interestingly, when ODACl and H4PMo11VO40 were added into the reaction system simultaneously, almost the same conversion of benzyl alcohol (entry 7) as that over (ODA)4PMo11VO40-P (entry 6) could be achieved. The higher activity of (ODA)4PMo11VO40 with a flowerlike structure is obvious due to its amphiphilic characteristics, i.e., the hydrophobic organic segments in the catalyst possessing the enhanced encapsulation of the relatively less polar benzyl alcohol and the promoted release of the more polar benzaldehyde as well as the inorganic segments POM in the catalyst accessing the oxidant H2O2 to form active per-POM

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The prepared catalysts have rather low BET specific surface areas, approximately 1 m2 g−1 (see Table 1), because the organic segments inserted into the Keggin structure of H4PMo11VO40 could block the channels of the POM. The characteristic vibrations attributable to the surfactant cations at 2920 (vas CH2), 2855 (vs CH2), and 1480 cm−1 (δ CH2), and to PMo11VO404− anion at 1060 (νas POa), 950 (νas MoOd), 875 (νas MoObMo), and 795 cm−1 (νas Mo O c Mo) appear in the FT-IR sp ectra of (DDA)4PMo11VO40, (HDA)4PMo11VO40, (ODA)4PMo11VO40, (DODA)4PMo11VO40, and (TBA)4PMo11VO40, respectively, shown in Figure 1 and Figure S1 in the Supporting Information, preliminarily confirming the feasibility for the construction of the catalysts. Furthermore, the elemental analysis results in the different catalysts confirming their chemical compositions as the theoretical ones; see Table 1. Additionally, the TGA results show an obvious weight loss ranging from 36 to 56 wt % in the temperature range between 100 and 450 °C (Figure 2), in agreement with the calculated values based on the postulated formulas. The 1H NMR spectra in Figure 3 further verify the formation of the organic surfactant−POM amphiphilic complex via electrostatic interactions in terms of the slight shifts of the resonance signals of N−CH3 and N−CH2 protons from their original positions in the pure surfactants.53 The SEM characterization indicates that (ODA)4PMo11VO40 self-assembles and forms a flowerlike structure having length on a micrometer scale, as shown in Figure 4a−c. Moreover, the D

dx.doi.org/10.1021/ie4007112 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

Figure 3. 1H NMR spectra of the catalysts investigated.

Table 2. Selective Oxidation of Benzyl Alcohol to Benzaldehyde with H2O2 in the Presence of the Catalysts Investigateda entry 1 2 3 4 5 6 7 8 9 10 11

catalyst H3PMo12O40 H4PMo11VO40 (ODA)4PMo11VO40 ODACl (ODA)3PMo12O40 (ODA)4PMo11VO40-P (ODACl + H4PMo11VO40)e (DODA)4PMo11VO40 (DDA)4PMo11VO40 (HDA)4PMo11VO40 (TBA)4PMo11VO40

converb (%) 19.1 28.2 60.6 0 36.5 58.9 57.1 53.3 51.6 56.3 28.9

± 1.9 ± 1.8 ± 1.5 ± ± ± ± ± ± ±

1.7 1.6 1.7 1.8 1.8 1.8 1.2

d

selc (%) 88.6 90.0 99.0 0 94.5 99.0 98.9 99.0 98.0 99.0 91.1

± 1.6d ± 1.4 ± 0.8 ± ± ± ± ± ± ±

1.1 0.8 0.9 0.9 1.0 0.9 1.4

a

Figure 4. SEM images with different scale bars (a−c) and TEM image (d) of (ODA)4PMo11VO40.

Reaction conditions: 0.03 mmol of catalyst, 60 mmol of benzyl alcohol, and 50 mmol of H2O2 at 90 °C for 6 h. bConversion of benzyl alcohol. cSelectivity for benzaldehyde. dStandard deviation. eA 0.12 mmol portion of ODACl and 0.03 mmol of H4PMo11VO40 were simultaneously added into the reaction system.

species for the oxidation,50−52 although the reactant benzyl alcohol cannot be completely dissolved in the aqueous medium. The comparably lower activities for (ODA)4PMo11VO40-P and (ODACl + H4PMo11VO40) than that of (ODA)4PMo11VO40 are probably due to the arrangement of the surfactant alkyl chains and Keggin anions, thus leading to their less amphiphilic properties. For comparison, the other catalysts containing surfactant cations with different carbon-chain lengths were also applied in the liquid-phase oxidation of benzyl alcohol. However, although the cationic surfactant DODA containing dual long alkyl chains could also enhance encapsulation of the organic substrate molecules, it hinders the interaction of H2O2 with POM to form active per-POM species due to the limited space available, resulting in the lower activity of (DO-

Figure 5. Small−angle XRD patterns of (DDA)4PMo11VO40 (a), (HDA) 4 PMo 11 VO 40 (b), (ODA) 4 PMo 11 VO 40 (c), and (DODA)4PMo11VO40 (d). E

dx.doi.org/10.1021/ie4007112 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

DA)4PMo11VO40 (entry 8), compared to (ODA)4PMo11VO40. On the other hand, the catalysts (DDA)4PMo11VO40 and (HDA)4PMo11VO4 containing surfactant cations with shorter alkyl chains also showed a comparably lower catalytic activity (entries 9 and 10) than (ODA)4PMo11VO40, probably ascribed to the relatively lower encapsulation of the reactant benzyl alcohol and the comparably high catalytic activity to catalyze the decomposition of H2O2 (see below). For the controlled catalyst (TBA)4PMo11VO40, over which a low conversion of 28.9% was obtained (entry 11), which is almost the same as that over H4PMo11VO40, obviously due to the lack of an amphiphilic characteristic for TBA cations in (TBA)4PMo11VO40.27 In order to clarify the difference in the catalytic activity of the amphiphilic catalysts with different alkyl chain lengths in the organic segments, the adsorption kinetics of benzyl alcohol in the catalysts were investigated, and these results are summarized in Figure 6. It was found that the adsorbed

consumed by the self-decomposition accounted for 57.8% of its initial amount within 6 h in the presence of H4PMo11VO40. However, the amounts of H2O2 consumed by the selfdecomposition were 36.5%, 32.2%, 28.1%, and 26.5% of its initial amount in the presence of (DDA)4PMo11VO40, (HDA) 4 PMo 11 VO 40 , (ODA) 4 PMo 11 VO 40 , and (DODA)4PMo11VO40, respectively, under the same conditions. The lowest activity of (DODA)4PMo11VO40 to catalyze the decomposition of the oxidant H2O2 is probably due to the steric hindrance of dual alkyl chains in the catalyst that suppress the interaction of H2O2 with the POM to form active per-POM species. Furthermore, the encapsulation of the organic substrate benzyl alcohol could increase with increasing the alkyl chain length in the catalysts while the interaction of H2O2 with the POM to form active per-POM species might decrease with increasing the alkyl chain length, and in the current case, the optimized organic segment is ODA, i.e., (ODA)4PMo11VO40 shows the best catalytic performance in the selective oxidation of benzyl alcohol. 3.3. Optimization of Reaction Conditions. Figure 8 shows the conversion of benzyl alcohol and the selectivity for

Figure 6. Adsorption uptakes of benzyl alcohol in (TBA)4PMo11VO40 (a ) , ( D D A ) 4 PM o 1 1 V O 4 0 ( b ) , (H D A ) 4 P M o 1 1 V O 4 0 ( c ), (ODA)4PMo11VO40 (d), and (DODA)4PMo11VO40 (e) at 90 °C. Figure 8. Conversion of benzyl alcohol and selectivity for benzaldehyde as a function of reaction time in the selective oxidation of benzyl alcohol under organic solvent-free conditions over H4PMo11VO40 (a) and (ODA)4PMo11VO40 (b). Reaction conditions: 0.03 mmol of catalyst, 60 mmol of benzyl alcohol, and 50 mmol of H2O2 at 90 °C.

amount of benzyl alcohol in the amphiphilic catalyst increased with increasing the alkyl chain length of the organic segment in the catalyst. Additionally, benzyl alcohol was hardly adsorbed in (TBA)4PMo11VO40, as expected. On the other hand, the self− decomposition of H2O2 under the reaction conditions but excluding benzyl alcohol was also investigated, and these results are shown in Figure 7. It was found that the amount of H2O2

benzaldehyde as a function of reaction time in the selective oxidation of benzyl alcohol under organic solvent-free conditions over H4PMO11VO40 and (ODA)4PMo11VO40, respectively. Over (ODA)4PMo11VO40, the conversion of benzyl alcohol continuously increased up to 60.6% in the reaction of 6 h and then remained almost constant, and the selectivity for benzaldehyde slightly decreased from 100% to 99.0%. Therefore, the optimized reaction time is 6 h for the selective oxidation of benzyl alcohol over (ODA)4PMo11VO40. In comparison, a lower benzyl alcohol conversion of 28.2% with a lower selecivity of 90.0% for benzaldehyde was obtained in the same reaction period over H4PMO11VO40. The low activity of H4PMO11VO40 is due to the limited contact between catalyst and benzyl alcohol under the organic solvent−free conditions, as mentioned above, while the low selectivity for benzaldehyde is owing to the fact that the desired product benzaldehyde can be further oxidized to benzoic acid. Figure 9 shows the conversion of benzyl alcohol and the selectivity for benzaldehyde as a function of reaction temperature over (ODA)4PMo11VO40. It can be seen that the maximum conversion of benzyl alcohol is 60.6% with a

Figure 7. Self-decomposition of H2O2 as a function of reaction time in the presence of H4PMo11VO40 (a), (DDA)4PMo11VO40 (b), (HDA) 4 PMo 11 VO 40 (c), (ODA) 4 PMo 11 VO40 (d), and (DODA)4PMo11VO40 (e). Reaction conditions: 0.03 mmol of catalyst and 50 mmol of H2O2 at 90 °C for 6 h. F

dx.doi.org/10.1021/ie4007112 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

Figure 9. Conversion of benzyl alcohol and selectivity for benzaldehyde as a function of reaction temperature in the selective oxidation of benzyl alcohol under organic solvent-free conditions over (ODA)4PMo11VO40. Reaction conditions: 0.03 mmol of catalyst, 60 mmol of benzyl alcohol, and 50 mmol of H2O2 for 6 h.

Figure 11. Conversion of benzyl alcohol and selectivity for benzaldehyde as a function of the molar ratio of hydrogen peroxide to benzyl alcohol in the selective oxidation of benzyl alcohol under organic solvent-free conditions over (ODA)4PMo11VO40. Reaction conditions: 0.03 mmol of catalyst at 90 °C for 6 h.

selectivity of 99.0% for benzaldehyde at 90 °C. At higher temperatures, the conversion of benzyl alcohol decreases with increasing temperature. This could be due to the fact that at higher temperatures, the decomposition of the oxidant H2O2, which is a competitive reaction for the oxidation of benzyl alcohol,45 also proceeds faster. Figure 10 represents the conversion of benzyl alcohol and the selectivity for benzaldehyde as a function of the catalyst

conversion increased with increasing the molar ratio and reached a maximum conversion of 60.6% at a H2O2:benzyl alcohol molar ratio of 0.83:1. No obvious increase in the conversion of benzyl alcohol was observed with further increasing the molar ratio. 3.4. Catalyst Reusability. (ODA)4PMo11VO40 was not dissolved in the reaction medium, and in order to gain insight into the factors crucial for its stability, the catalytic cycles were performed. Figure 12 displays these results. A benzyl alcohol

Figure 10. Conversion of benzyl alcohol and selectivity for benzaldehyde as a function of the amount of (ODA)4PMo11VO40 in the selective oxidation of benzyl alcohol under organic solvent-free conditions. Reaction conditions: 60 mmol of benzyl alcohol and 50 mmol of H2O2 at 90 °C for 6 h.

Figure 12. Reusability of (ODA)4PMo11VO4 in the selective oxidation of benzyl alcohol with H2O2. Reaction conditions: 0.03 mmol of catalyst, 60 mmol of benzyl alcohol, and 50 mmol of H2O2 at 90 °C for 6 h.

amount used in the selective oxidation over (ODA)4PMo11VO40. A negligible conversion of benzyl alcohol (