Organic Salts and Merrifield Resin Supported [PM12O40]3− (M = Mo or W) as Catalysts for Adipic Acid Synthesis

Abstract

Even though recent reports describe a very fast AA production under high pressure and high temperatures conditions [15,16,17,18,19], using membrane reactors [20] or microwave conditions [21], our research goal is to apply user-friendly and safe protocols (lower temperatures avoiding the H2O2 decomposition, atmospheric pressure, one step reaction, recoverable catalyst), and to compare them with the classical protocols used in industrial processes. The replacement of the substrates (CHA, CHK) used in industrial processes by other starting substrates (CH, cyclohexane oxide CHO, 1,2-cyclohexanediol CHD), is guided by several economical parameters (substrate price, target yield, availability, time cost analysis, etc.). [...]CH is largely available by benzene hydrogenation [43,44,45] or by cyclohexane dehydrogenation [46,47,48] and, therefore, less expensive and more promising. Since CHD can be obtained by the oxidation of CH, a more direct and selective AA “one-pot” production from CH is of great interest. 31P Nuclear Magnetic Resonance (NMR) Spectroscopy The 31P NMR spectra of C4-Mo and C6-Mo salts in the solution exhibit a characteristic peak at −4.15 ppm, while those of C4-W and C6-W salts show a peak at −15.5 ppm, in accordance with the literature data [64,65].

1. Introduction

Adipic acid (AA) is one of the reagents used for the production of nylon-6,6, a polymer that exhibits high mechanical strength, rigidity, and good thermal and chemical stability [1]. Most industrial processes for the AA production (2.3 Mtons/year) involve nitric acid oxidation of cyclohexanol (CHA) and/or cyclohexanone (CHK) [2,3]. This inexpensive acid, used in several industrial processes [4], however, leads to pollution, producing unwanted waste and various nitrogen oxides (NO_x), including ca. 400 ktons of the greenhouse gas N₂O per year [5,6]. Although this emission can be significantly reduced using a thermal decomposition process [7,8], current academic and industrial process developments focus on cleaner technologies [2,3,9,10,11]. One possible alternative route for AA production is the selective oxidation of cyclohexene (CH) with hydrogen peroxide (H₂O₂) [12,13] or molecular oxygen (O₂) [14] as preferred oxidants (Scheme 1). Even though recent reports describe a very fast AA production under high pressure and high temperatures conditions [15,16,17,18,19], using membrane reactors [20] or microwave conditions [21], our research goal is to apply user-friendly and safe protocols (lower temperatures avoiding the H₂O₂ decomposition, atmospheric pressure, one step reaction, recoverable catalyst), and to compare them with the classical protocols used in industrial processes.

Heteropolyoxometalates—inorganic compounds with general chemical formula [X_nM_pO_q]^z−, M being commonly Mo or W, and X being usually a main group element (Si, P, B)—were described as suitable for catalyst design at the atomic and molecular levels [22]. Among all structures, the one corresponding to the discrete assembly of 12 M around one X, with a general formula [XMo₁₂O₄₀]ⁿ⁻ (called Keggin), is very stable. Different isomers of Keggin polyoxometalates (POMs) can exist, according to the arrangement of the metal oxides around the heteroatom. Keggin-type POMs are efficient catalysts for liquid phase oxidation [23], due to their stability towards oxygen donors and their ability to incorporate different transition metals [24]. Immobilization of POMs through electrostatic interactions in insoluble polymer resins can lead to easier separation and recovery of the molecular catalysts [25,26]. Other interesting supports are SiO₂ [27], mesoporous silica (SBA) [28,29] or certain macroporous resins [30]. Among the resins, the low-cost and relatively robust Merrifield resins (MR), originally developed for the solid-state synthesis of peptides [31], have the ability to undergo facile functionalization [25]. Even though many transition metal catalysts have been supported on MR for catalytic purposes [25,32,33], there is no report, to the best of our knowledge, on the application of such ionically supported POM-based catalysts for AA production.

The AA production from a mixture of CHA and CHK (so-called KA “ketone-alcohol” oil [34]) has previously been described using various homogeneous catalysts such as H₂WO₄ [35] or Keggin-based POMs, i.e., M_xPMo₁₂O₄₀ (M = Ni, Co) [36], H_3−2xCo_xPMo₁₂O₄₀ [37], or Y_3−2xNi_xPMo₁₂O₄₀ (Y = H or NH₄, x = 0.25–1.5) [38]. With (C₄H₉N)₃[PMo₁₂O₄₀], AA (18% yield) and CHK were produced from CHA, while (C₄H₉N)₃[PW₁₂O₄₀] produced CHK only [39]. The replacement of the substrates (CHA, CHK) used in industrial processes by other starting substrates (CH, cyclohexane oxide CHO, 1,2-cyclohexanediol CHD), is guided by several economical parameters (substrate price, target yield, availability, time cost analysis, etc.). An attractive starting material, CH, was successfully converted to AA by aqueous H₂O₂ using Na₂WO₄ as a catalyst and the [CH₃(n-C₈H₁₇)₃N]HSO₄ phase transfer agent [40]. The oxidation of trans-CHD is another suitable pathway for AA production [3,10,41,42] providing the highest AA yields but CHD is the most expensive reagent among the listed substrates. On the other hand, CH is largely available by benzene hydrogenation [43,44,45] or by cyclohexane dehydrogenation [46,47,48] and, therefore, less expensive and more promising. Since CHD can be obtained by the oxidation of CH, a more direct and selective AA “one-pot” production from CH is of great interest.

Different heterogeneous catalysts based on calcined POMs [49], mesoporous solids [50], Ti-MMM-2 [51], Ce-SBA-15 [51], TAPO-5 [52], and Ti-AlSBA-15 [53,54] were also tested for direct CH oxidation, the AA yield being improved by the stepwise addition of H₂O₂ to the reaction mixture. The catalytic mechanism for the CH oxidation into AA through CHD was clarified with TAPO-5 as a catalyst [52], showing that trans-CHD was formed from cyclohexene oxide (CHO) while the cis-CHD formation followed a radical mechanism. The CH oxidation to AA was also tested in CH₃CN using TBHP (tert-butylhydroperoxide) in decane as oxidant and Ti-AlSBA-15 as a catalyst [54]. Some literature data and reaction conditions with CH, CHO, CHD, CK, and CHA substrates are listed in Table 1.

In the present contribution, we report the preparation and characterization of new molecular and resin-supported Mo- and W-based Keggin-type catalysts and their catalytic performance for the production of AA starting from CH. The main objective of the presented research was to evaluate a greener process, avoiding the use of organic solvent, using a new family of grafted catalysts and minimizing the catalyst loading. Different starting substrates for AA production were tested, in order to gain a better insight into the oxidation mechanism, especially by comparison between molecular and grafted catalysts.

2. Results and Discussion 2.1. Synthesis and Characterization of the Prepared Catalysts

2.1.1. Synthesis

Four molecular catalysts, (Scheme 2, left), were straightforwardly prepared by the reaction of the corresponding ionic liquid (1-hexyl-3-methylimidazolium bromide or 1-butyl-3-methylimidazolium bromide) and phosphomolybdic or phosphotungstic acid. The four isolated complexes are composed by a [PM₁₂O₄₀]³⁻ (M = Mo or W) surrounded by three alkylmethylimidazolium cations.

The synthesis of the polymer-supported catalysts (Scheme 2, right) was achieved as shown in Scheme 3. Merrifield resins were used as polymeric supports. The resins were modified to obtain two different types of imidazolium-functionalized supports (MR₄ or MR₁₂) in two steps (see Scheme 3). The first step consisted of the addition of 2-methylimidazole in the presence of a base to the Merrifield resin, leading to the MR_Im [59]. The imidazole-functionalized intermediate MR_Im was further treated in the second step with the appropriate alkyl bromide to obtain the desired imidazolium moieties (MR₄ and MR₁₂). The different alkyl chain lengths were expected to increase the lipophilicity of the catalytic object, enhancing the catalyst compatibility with the apolar substrate relatively to the water present in the reaction media since carried by the oxidant (H₂O₂ in water). In the last step, the POMs ([PMo₁₂O₄₀]³⁻ or [PW₁₂O₄₀]³⁻) were then immobilized by simple ion exchange adding the corresponding heteropolyacids (H₃[PMo₁₂O₄₀] or H₃[PW₁₂O₄₀]). All the synthesized catalysts were characterized by FTIR and elemental analysis.

2.1.2. Characterization

• Infrared (IR) Spectroscopy

The IR spectra of the molecular catalysts C_n-M (n-4,6; M-Mo, W) were assigned according to the literature data [60,61,62,63]. The main characteristic features of the Keggin structure were observed at 1078 cm⁻¹ (ν_as P−O_a), 970 cm⁻¹ (ν_as M−O_d), 885 cm⁻¹ (ν_a M−O_d−M), and 771–777 cm⁻¹ (ν_as M−O_c−M) for both M (W and Mo). For the catalysts grafted on Merrifield resins MR_n-M (n = 4, 12; M-Mo, W), the IR analysis showed the characteristics bands of the 2-methylimidazolium cation, 1420–1490 cm⁻¹. The final POM-containing resins exhibit characteristic IR bands at 1060, 955, 880, and 795–802 cm⁻¹ for MR₄-Mo and MR₁₂-Mo, and at 1080, 975, 890, and 795–810 cm⁻¹ for MR₄-W and MR₁₂-W, corresponding to the Keggin anions (Figure 1 shows the specific area corresponding to the POM backbone for C₆-Mo, MR₁₂-Mo and the corresponding starting H₃[PMo₁₂O₄₀], proving that the POM structure remains unchanged when grafted).

• ³¹P Nuclear Magnetic Resonance (NMR) Spectroscopy

The ³¹P NMR spectra of C₄-Mo and C₆-Mo salts in the solution exhibit a characteristic peak at −4.15 ppm, while those of C₄-W and C₆-W salts show a peak at −15.5 ppm, in accordance with the literature data [64,65]. In the solid-state ³¹P NMR spectrum of the Mo-grafted catalysts (Figure 2), the resonance is shifted to −3.5 ppm in case of MR₄-Mo, while two resonances at −1.9 and −3.5 ppm are observed for MR₁₂-Mo. Likewise, in the case of the W catalysts (Figure 3), one resonance is observed for MR₄-W (at −15.1 ppm) and two for MR₁₂-W (at −12.4 and −15.2 ppm). The similar behavior of both POMs seems to depend on the length of the imidazolium substituent and not on the quality of the Keggin POM precursor (the ³¹P NMR in D₂O of the commercial Mo and W precursors displayed only one signal at −3.9 and −15.3 ppm, respectively). The observed behavior indicates the presence of different cation-dependent interactions at the resin surface, the two simultaneously observed species possibly corresponding to species located in two different sites (i.e., closer to or farther from the imidazolium moiety) [66,67].

• Elemental and Thermogravimetric Analyses

The EA data of the grafted resins show that 32% of Cl from the –CH₂Cl pending functions has been replaced by the 2-methylimidazolium function.

The metal loadings were determined from TG analyses. The residue after thermal treatment was identified as P₂O₅ and MO₃ (M = Mo or W), see Table 2. Figure 4 shows an illustrative example of the TG curve for MR₄-W.

2.2. Catalysis

The transformation of CH to AA is reported to involve olefin epoxidation, alcohol oxidations, and Baeyer-Villiger oxidation (Scheme 4) [52].

For this reason, we studied the AA formation from different starting compounds (CH, CHO, CHD), using molecular and grafted catalysts. CHO and CHD were used as substrates since they are intermediates in the formation of AA from CH. The formation of the CHD intermediate has also been monitored when starting from CH or CHO (see results in Table 3).

• Swelling Tests

In order to verify whether all the MR-supported POMs are accessible to the substrates under organic solvent-free conditions, swelling tests were performed on non-treated MR. Heating the MR in the presence of CH or CHO led to a mass increase by ca. 2% in the case of CH and 8% in the case of CHO, indicating substrate incorporation into the polymer pores and, therefore, catalyst accessibility without organic solvent. This feature is of great importance since it justifies the use of MR as a support in the catalytically solvent-free processes.

• Classical Conditions

Initial tests were performed with the molecular catalysts C_n-M (n = 4 or 6, M = Mo or W; 0.025 mol% vs. substrate), i.e., a 40 times lower catalyst loading than that reported for Na₂WO₄ [40]. All the catalysts were insoluble in the reaction media, but became partially soluble as the reactions evolved, better solubilization was observed for the W-based ones.

• CH as the Substrate

After 60 min, the CH conversion was between 7 and 10% (analyzed by GC) with a low selectivity towards CHO (3%). After 6.5 h, all the molecular catalysts provided a 5–8% selectivity towards CHO with a CH conversion between 12 and 18%. No CHD was observed in the organic layer. The slow conversion is certainly due to the low catalyst loading and to the mild reaction conditions compared to those described in the reference study [35]. After 24 h, however, reasonable conversions were achieved and CHO was no longer detected by GC whereas CHD was present (see Table 1). AA was isolated as a solid in relatively good yields. As stated in several mechanistic studies [52], the species unidentified by GC might correspond to the product of Baeyer Villiger oxidation (see Scheme 4). The observation that CHO is formed and then consumed indicates that this is involved in the AA formation mechanism, via the conversion to CHD.

The homogeneous C_n-W catalysts were more active than the corresponding C_n-Mo, with AA isolated yields ca. 10% higher on average (see Table 1). Control experiments without catalyst or in the presence of the commercial MR yielded only a very low amount of cis and trans-CHD (<7%) after 24 h. The shorter carbon chain on the imidazolium cation afforded better results. The same trend was previously observed for the same reaction with (Hgly)₃[PM₁₂O₄₀] (M = Mo, W; gly = glycine) [68] or for the cyclooctene epoxidation with alkylpyridinium salts of [PMo₁₂O₄₀]³⁻ [61]. The stirred biphasic reaction medium appeared as an emulsion, possibly stabilized by the solid POM salt (Pickering-type) [69,70,71,72].

For the grafted version of the catalysts, the W-based were again more active, despite the lower W content in the case of MR₁₂-W than for MR₁₂-Mo (see Table 1). In contrast to the molecular catalysts, the longer chain on the imidazolium cation led to greater activity. This could be linked to a better “solubilization” of the substrate near the catalyst active site.

• CHO as the Substrate

Complete substrate conversion was observed in this case, with the molecular or the grafted catalysts, producing AA after one day in 21–47% isolated yields together with a relatively high amount of trans-CHD (Table 1). This experiment shows that the limiting factor is not the formation of CHD, as seen in other studies [52]. The molecular catalysts (especially those based on W) were more selective towards AA than the grafted ones. It seems that the transformation of CHD to AA is quicker in the presence of the molecular catalysts, since less CHD intermediate accumulates.

• CHD as the Substrate

When trans-CHD was used as the starting substrate, the reaction produced AA in higher isolated yields (59–74%) than when starting from CH within the same time period. 2-Hydroxycyclohexanone, a known intermediate within the postulated oxidation pathway (Scheme 4), [52] was also observed by GC but not quantified. The C_n-M molecular catalysts are highly active (>99% conversion), more than the grafted ones for which the measured conversion is between 71 and 95%. In all the reactions, the W-based catalysts show slightly better performances in terms of activity and AA yields.

• Recycling

Catalyst recovery and recyclability were investigated for the most active grafted catalyst, MR₁₂-W, and CH as the starting substrate. The IR spectrum of the isolated MR₁₂-W after the first experiment exhibited no change, except for the presence of additional vibrations attributed to CHD. After the 2nd run, the CH conversion was 54% and the AA yield was 29%. The lower activity after reuse could result from the grafted catalyst inhibition by the presence of CHD within the resin.

• CHK/CHA as the Substrate

In order to mimic a greener industrial process (replacing the traditionally used and environmentally harmful oxidants) the most active molecular catalyst C₄-W (0.025% loading) was also tested for the oxidation of CHK and CHA by H₂O₂ (Scheme 5), leading after 24 h to isolated AA yields of 41% and 35% respectively. These results show that C₄-W is less active than H₂WO₄ and certain Dawson POMs (58 and 59% with 0.5–2% cat loading with CHA as the substrate) [73], quite similar to (NH₄)_xA_yPMo₁₂O₄₀ (Aⁿ⁺ = Sb³⁺, Bi³⁺, Sn²⁺) species with CHK as the substrate and better with CHA with a lower POM/substrate ratio [74] but more active than other described POMs (see Table 1), and, therefore, also seems promising for the solvent-free oxidation of CHK and CHA [35].

3. Materials and Methods 3.1. Materials

1-Hexyl-3-methylimidazolium bromide (Aldrich, Saint Louis, MO, USA), H₃PMo₁₂O₄₀·26H₂O (99.9% Merck, Kennallsburg, NJ, USA), H₃PW₁₂O₄₀·15H₂O (purum, Aldrich), 1-methylimidazole (Aldrich), bromobutane (Aldrich), bromododecane (Aldrich), Merrifield resin (Aldrich), EtOH (Aldrich), DMF (Aldrich), dichloromethane (Aldrich), Acetone (Aldrich), Cyclohexene (Aldrich), 35% H₂O₂ (Acros, Geel, Belgium) were used as received

3.2. Physical Methods

Elemental analyses (EA) were provided by the Analytical Services Laboratory of LCC, Toulouse. The thermogravimetric analyses (TGA) were performed on a SETARAM TGA 92-16.18 thermal analyzer. The sample was placed into an alumina crucible and heated at 0.83 K·s⁻¹ in a reconstituted air flow from 25 °C to 600 °C. An empty crucible was used as a reference. The POM percentage in the grafted species was calculated from the percentage of residue measured in the TGA traces divided by the molar mass of PM₁₂O_38.5.

Infrared spectra were recorded using the ATR technique with a Perkin Elmer FTIR/FIR 400 spectrometer.

A Bruker Avance DPX-300 spectrometer was used to record ³¹P NMR spectra in the solution at 121.5 MHz. All solid-state ³¹P NMR experiments were recorded on a Bruker Avance 400 spectrometer equipped with a 3.2 mm probe. Samples were spun at 16 kHz at the magic angle using ZrO₂ rotors. For ³¹P MAS single pulse experiments, small flip angle (~30°) were used with recycling delays of 20 s. ³¹P-CP/MAS spectra were recorded with a recycle delay of 3 s and contact times of 2 ms. ³¹P chemical shifts were referenced to an external 85% H₃PO₄ sample.

All the catalytic reactions were followed by gas chromatography on an Agilent 6890 A chromatograph equipped with FID detector, a HP5-MS capillary column (30 m × 0.25 mm × 0.25 μm) and automatic sampling, or on a Fisons GC 8000 chromatograph equipped with FID detector and with a SPB-5 capillary column (30 m × 0.32 mm × 0.25 μm).

The GC parameters were quantified with authentic samples of the reactants and products. The CH conversion, the CHD formation, and the CHO conversion/formation were calculated from calibration curves (r² = 0.999) relative to an internal standard (acetophenone). Acetophenone and CHD are present both in the organic and in the water phase at the end of the catalytic experiments. Therefore, extraction with AcOEt was carried out before the GC analysis. The AA yield was calculated from the isolated mass recovered from the filtered reaction mixture at the end of the reaction.

3.3. Preparative Part

• Molecular Catalysts

1-Hexyl-3-methylimidazolium bromide (1.65 mmol, 0.41 g) was mixed with aqueous H₃PM₁₂O₄₀ (M = Mo or W) (0.54 mmol, 1.0 g H₃PMo₁₂O₄₀ or 1.55 g H₃PW₁₂O₄₀) and stirred for 2 h. Yellow (C₆-Mo) or white (C₆-W) solids precipitated immediately. The solid was filtered, washed with acetone and ether and dried in air.

The preparation of the C₄-M (M = Mo or W) catalysts involved the synthesis of the corresponding ionic liquid [75]. 1-methylimidazole (0.05 mol, 4.1 g) and bromobutane (0.05 mol, 6.85 g) were stirred at 70 °C for 24 h in EtOH (20 mL). After the reaction, EtOH was removed by evaporation under reduced pressure. The formation of the corresponding ionic liquid BMImBr was confirmed by IR [76]. BMImBr (4.4 g, 0.02 mol) was directly mixed with aqueous H₃PM₁₂O₄₀ (M = Mo or W) (6.67 × 10^‒3 mol) and stirred for 2 h. A yellow (C₄-Mo) or white solid (C₄-W) precipitated immediately. The solid was filtered, washed with acetone and ether and dried in air.

C₄-Mo: Yield: 10.3 g (69%). EA: calculated for C₂₄H₄₅Mo₁₂N₆O₄₀P (%), C: 12.9, H: 2.0, N: 3.6, found (%), C: 12.9, H: 1.9, N: 3.7. IR-ATR (cm⁻¹): 1060, 945, 867, 763, 732. ³¹P NMR (ppm): −4.15.

C₄-W: Yield: 18.6 g (85%). EA: calculated for C₂₄H₄₅W₁₂N₆O₄₀P (%), C: 8.7, H: 1.4, N: 2.6, found (%), C: 8.9, H: 1.5, N: 2.7. IR-ATR (cm⁻¹): 1053, 945, 870, 775, 722. ³¹P NMR (ppm): −15.5.

C₆-Mo: Yield: 11.4 g (74%). EA: calculated for C₃₀H₅₇Mo₁₂N₆O₄₀P (%), C: 15.5, H: 2.4, N: 3.6, found (%), C: 16.0, H: 2.3, N: 3.8. IR-ATR (cm⁻¹): 1078, 970, 885, 771, 732. ³¹P NMR (ppm): −4.15.

C₆-W: Yield: 17.6 g (78%). EA: calculated for C₃₀H₅₇Mo₁₂N₆O₄₀P (%), C: 10.7, H: 1.7, N: 2.5, found (%), C: 10.8, H: 1.6, N: 2.5. IR-ATR (cm⁻¹): 1072, 969, 885, 777, 726. ³¹P NMR (ppm): −15.5.

• Grafted Catalysts

The Merrifield resin (1.4–1.6 mmol Cl/g, 5.6–6.4 mmol, 4 g), 2-methylimidazole (0.05 mol, 4 g), and Na₂CO₃ (0.01 mol, 1.3 g) were stirred in DMF (20 mL) at 80 °C during 24 h [77]. After washing the resin with dichloromethane (to remove the organic compounds) and then in water until neutral pH, the resin was dried in air. This resin was then placed with 5 mL bromobutane (MR₄-) or bromododecane (MR₁₂-) and stirred for 72 h at 80 °C. The resin was then washed with dichloromethane, ethanol, and acetone. Finally, it was treated with 5 mL of aqueous solution of H₃PMo₁₂O₄₀ (5 × 10⁻⁵ mol, 0.09 g) or H₃PW₁₂O₄₀ (5 × 10⁻⁵ mol, 0.15 g). After filtration and washing with distilled water and acetone, around 4 g of light yellow resins (MR₄-Mo, MR₄-W, MR₁₂-Mo, MR₁₂-W) were obtained.

MR₄-Mo: EA: found (%), C: 59.5, H: 6.8, N: 5.0. TGA (residue) 11.9%. Mo loading (μmol POM/g of polymer): 66.7. IR-ATR (cm⁻¹): 1060, 953, 878, and 802. ³¹P NMR (ppm): −3.5.

MR₄-W EA: found (%), C: 55.9, H: 6.0, N: 4.5. TGA (residue) 3.5%. W loading (μmol POM/g of polymer): 12.3. IR-ATR (cm⁻¹): 1079, 975, 894, and 811. ³¹P NMR (ppm): −15.1.

MR₁₂-Mo EA: found (%), C: 40.9, H: 4.1, N: 4.3. TGA (residue) 9.99%. Mo loading (μmol POM/g of polymer): 55.6. IR-ATR (cm⁻¹): 1060, 952, 876, and 794. ³¹P NMR (ppm): −1.9, −3.5.

MR₁₂-W EA: found (%), C: 30.0, H: 3.0, N: 3.5. TGA (residue) 5.3%. W loading (μmol POM/g of polymer): 18.9. IR-ATR (cm⁻¹): 1078, 974, 893, and 797. ³¹P NMR (ppm): −12.4, −15.2.

3.4. Swelling Test for MR

The MR (0.5 g) was placed in a glass test tube with stopper and CH or CHO was added. The reaction mixture was heated at 60 °C for 6 h. The mixture was filtered and the recovered resin was dried in air and weighted.

3.5. General Procedure for Catalytic Oxidation to AA

3.5.1. CH Oxidation (Procedure A)

A round-bottom flask equipped with a magnetic stirring bar and a reflux condenser was charged with CH (80 mmol, 8 mL), acetophenone (3.5 mmol, 0.4 mL), and catalyst (0.02 mmol of molecular catalyst or 0.06 g of grafted catalyst). After the stirred reaction mixture reached 60 °C, H₂O₂ (320 mmol, 31.1 g, 35%) was added and stirring was continued for 72 h. The final mixture was filtered and the filtrate was left to stand in the refrigerator for one day. The resulting white precipitate was filtered and identified as AA by IR, NMR, and DSC.

3.5.2. CHO Oxidation (Procedure B)

Similar to procedure A, with a reaction time of 24 h and 2 eq of H₂O₂ (160 mmol, 35%).

3.5.3. CHD Oxidation (Procedure C)

Similar to procedure A, with a reaction time of 72 h and 2 eq of H₂O₂ (160 mmol, 35%).

3.5.4. Recycling of the Resins

The resins were isolated at the end of the catalytic reaction, washed with diethylether at room temperature, and dried under ambient air.

3.5.5. CHK or CHA Oxidation (Procedure D)

A round-bottom flask equipped with a magnetic stirring bar and a reflux condenser was charged with cyclohexanone (80 mmol, 7.85 g) or cyclohexanol (80 mmol, 8.0 g) and 10 mL of water. Acetophenone (3.5 mmol, 0.4 mL) was added as an internal standard and the catalyst (0.02 mmol) was added. After the stirred reaction mixture reached 80 °C, H₂O₂ (320 mmol, 31.1 g, 35%) was added and stirring was continued for 24 h. The homogeneous solution was allowed to stand at 0 °C for 12 h, and the resulting precipitate was separated by filtration and washed with 20 mL of cold water. The product was dried in a vacuum to give AA as a solid.

4. Conclusions

Immobilization of the Keggin phosphomolybdate and phosphotungstate on functionalized Merrifield resins has led to the development of new heterogeneous catalysts for AA production in a one-step procedure from CH. Different intermediates were detected and the key steps could be determined by running comparative oxidation tests form each one of the possible intermediates. The substrate itself is able to swell the MR allowing access to all catalytic sites without the use of the classical swelling organic solvents. This atom economical methodology obeys a few of the green chemistry principles (greener oxidant, no phase transfer catalysts, no organic solvents added (safer reagents and atom economy), minimal catalyst loading and recyclability). The developed protocol is straightforward, safe, and environmentally benign and can be extended to a variety of other POM-based catalyzed oxidations.

Author Contributions

Conceptualization, J.P. and D.A.; methodology, J.P. and D.A.; validation, J.P., D.A. and R.P.; formal analysis and investigation, J.P., D.A.; writing-Original draft preparation, J.P., D.A.; writing-Review and editing, J.P., D.A. and R.P.

Funding

The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7 2007-2013) as the project "Diligent search for chemical bio-sources: Solvent-free homogeneous and heterogeneous oxidation processes catalyzed by polyoxometalates", under grant agreement n°291823 Marie Curie FP7-PEOPLE-2011-COFUND (The new International Fellowship Mobility Programme for Experienced Researchers in Croatia-NEWFELPRO).

Acknowledgments

The authors acknowledge LCC-CNRS for elemental analyses and especially. Y. Coppel for the NMR and the Department of Chemistry of IUT at Castres for the facilities in synthesis, catalysis, and TGA.

Conflicts of Interest

The authors declare no conflict of interest.

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

References

1. Viers, V.; Brendt, D. Polymer Data Handbook; Oxford University Press: Oxford, UK, 1999; p. 189.

2. Van de Vyver, S.; Roman-Leshkov, Y. Emerging catalytic processes for the production of adipic acid. Catal. Sci. Technol. 2013, 3, 1465-1479. [Green Version]

3. Cavani, F.; Alini, S. Synthesis of Adipic Acid: On the Way to More Sustainable Production in Sustainable Industrial Processes; Cavani, F., Centi, G., Perathoner, S., Trifiró, F., Eds.; WILEY-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2009; pp. 367-425. ISBN 978-3-527-31552-9.

4. Buchner, W.; Schliebs, R.; Winter, G.; Buchel, K.H. Industrielle Anorganische Chemie, 2nd ed.; VCH Weinheim: Weinheim, Germany, 1986.

5. Kroeze, C.; Mosier, A. New estimates for Emissions of Nitrous Oxide. In Non-CO2 Greenhouse Gases: Scientific Understanding, Control and Implementation; van Ham, J.M., Baede, A.P., Meyer, L.A., Ybema, R., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1999; pp. 45-64. ISBN 978-0-7923-6199-2.

6. Hermans, I.; Spier, S.E.; Neuenschwander, U.; Turra, N.; Baiker, A. Selective oxidation catalysis: Opportunities and challenges. Top. Catal. 2009, 52, 1162-1174.

7. Shimizu, A.; Tanaka, K.; Fujimori, M. Abatement technologies for N2O emissions in the adipic acid industry. Chemosphere Glob. Chang. Sci. 2000, 2, 425-434.

8. Reimer, R.A.; Slaten, C.S.; Seapan, M.; Lower, M.W.; Tomlinson, P.E. Abatement of N2O emissions produced in the adipic acid industry. Environ. Prog. 1994, 13, 134-137.

9. Bart, J.C.J.; Cavallaro, S. Transiting from Adipic Acid to Bioadipic Acid. 1, Petroleum-Based Processes. Ind. Eng. Chem. Res. 2015, 54, 1-46.

10. Cavani, F. Catalytic selective oxidation faces the sustainability challenge: Turning points, objectives reached, old approaches revisited and solutions still requiring further investigation. J. Chem. Technol. Biotechnol. 2010, 85, 1175-1183.

11. Brégeault, J.-M. Transition-metal complexes for liquid-phase catalytic oxidation: Some aspects of industrial reactions and of emerging technologies. Dalton Trans. 2003, 3289-3302.

12. Jones, C.W. Applications of Hydrogen Peroxide and Derivatives; Royal Society of Chemistry: Cambridge, UK, 1999; ISBN 978-0-85404-536-5.

13. Strukul, G. (Ed.) Catalytic Oxidations with Hydrogen Peroxide as Oxidant; Kluwer Academic: Dordrecht, The Netherlands, 1992; ISBN 978-0-7923-1771-5.

14. Sheldon, R.A.; Kochi, J.K. Metal Catalyzed Oxidations of Organic Compounds; Academic Press: New York, NY, USA, 1981; ISBN 978-0-12-639380-4.

15. Damm, M.; Gutmann, B.; Kappe, C.O. Continuous-Flow Synthesis of Adipic Acid from Cyclohexene Using Hydrogen Peroxide in High-Temperature Explosive Regimes. ChemSusChem 2013, 6, 978-982.

16. Shang, M.; Noel, T.; Su, Y.; Hessel, V. High Pressure Direct Synthesis of Adipic Acid from Cyclohexene and Hydrogen Peroxide via Capillary Microreactors. Ind. Eng. Chem. Res. 2016, 55, 2669-2676.

17. Vural-Gursel, I.; Wang, Q.; Noel, T.; Hessel, V.; Tinge, J.T. Improving Energy Efficiency of Process of Direct Adipic Acid Synthesis in Flow Using Pinch Analysis. Ind. Eng. Chem. Res. 2013, 52, 7827-7835.

18. Shang, M.; Noël, T.; Wang, Q.; Su, Y.; Miyabayashi, K.; Hessel, V.; Hasebe, S. 2- and 3-Stage temperature ramping for the direct synthesis of adipic acid in micro-flow packed-bed reactors. Chem. Eng. J. 2015, 260, 454-462.

19. Wen, Y.; Wang, X.; Wei, H.; Li, B.; Jin, P.; Li, L. A large-scale continuous-flow process for the production of adipic acid via catalytic oxidation of cyclohexene with H2O2. Green Chem. 2012, 14, 2868-2875.

20. Buonomenna, M.G.; Golemme, G.; De Santo, M.P.; Drioli, E. Direct Oxidation of Cyclohexene with Inert Polymeric Membrane Reactor. Org. Process Res. Dev. 2010, 14, 252-258.

21. Freitag, J.; Nüchter, M.; Ondruschka, B. Oxidation of styrene and cyclohexene under microwave conditions. Green Chem. 2003, 5, 291-295.

22. Misono, M. Unique acid catalysis of heteropoly compounds(heteropolyoxometalates) in the solid state. Chem. Commun. 2001, 1141-1152.

23. Kholdeeva, O.A.; Maksimchuk, N.V.; Maksimo, G.M. Polyoxometalate-based heterogeneous catalysts for liquid phase selective oxidations: Comparison of different strategies. Catal. Today 2010, 157, 107-113.

24. Mizuno, N.; Misono, M. Heterogeneous Catalysis. Chem. Rev. 1998, 98, 199-218.

25. Gupta, K.C.; Sutar, A.K.; Lin, C.-C. Polymer-supported Schiff base complexes in oxidation reactions. Coord. Chem. Rev. 2009, 253, 1926-1946.

26. McNamara, C.A.; Dixon, M.J.; Bradley, M. Recoverable Catalysts and Reagents Using Recyclable Polystyrene-Based Supports. Chem. Rev. 2002, 102, 3275-3300.

27. Mizuno, N.; Yamaguchi, K.; Kamata, K. Molecular design of polyoxometalate-based compounds for environmentally-friendly functional group transformations: From molecular catalysts to heterogeneous catalysts. Catal. Surv. Asia 2011, 15, 68-79.

28. Bentaleb, F.; Makrygenni, O.; Brouri, D.; Coelho Diogo, C.; Mehdi, A.; Proust, A.; Launay, F.; Villanneau, R. Efficiency of Polyoxometalate-Based Mesoporous Hybrids as Covalently Anchored Catalysts. Inorg. Chem. 2015, 54, 7607-7616.

29. Cheng, C.-Y.; Lin, K.-J.; Prasad, M.R.; Fu, S.-J.; Chang, S.-Y.; Shyu, S.-G.; Sheu, H.-S.; Chen, C.-H.; Chuang, C.-H.; Lin, M.-T. Synthesis of a reusable oxotungsten-containing SBA-15 mesoporous catalyst for the organic solvent-free conversion of cyclohexene to adipic acid. Catal. Commun. 2007, 8, 1060-1064.

30. Xiao, Y.; Chen, D.; Ma, N.; Hou, Z.Y.; Hu, M.B.; Wang, C.H.; Wang, W. Covalent immobilization of a polyoxometalate in a porous polymer matrix: A heterogeneous catalyst towards sustainability. RSC Adv. 2013, 3, 21544-21551.

31. Merrifield, R.B. Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide. J. Am. Chem. Soc. 1963, 85, 2149-2154.

32. Maurya, M.R. Catalytic Applications of Polymer-Supported Molybdenum Complexes in Organic Transformations. Curr. Org. Chem. 2012, 16, 73-88.

33. Da Silva, J.A.L.; Fraústo da Silva, J.J.R.; Pombeiro, A.J.L. Oxovanadium complexes in catalytic oxidations. Coord. Chem. Rev. 2011, 255, 2232-2248.

34. Alini, S.; Babini, P. The Industrial Oxidation of KA Oil to Adipic Acid. In Handbook of Advanced Methods and Processes in Oxidation Catalysis; Imperial College Press: London, UK, 2014; pp. 320-333.

35. Usui, Y.; Sato, K. A green method of adipic acid synthesis: Organic solvent- and halide-free oxidation of cycloalkanones with 30% hydrogen peroxide. Green Chem. 2003, 5, 373-375.

36. Mazari, T.; Benadji, S.; Tahar, A.; Dermeche, L.; Rabia, C. Liquid-Phase Synthesis of Adipic Acid Using Keggin-Type Phosphomolybdates Catalysts. J. Mater. Sci. Eng. B 2013, 3, 146-152.

37. Benadji, S.; Mazari, T.; Dermeche, L.; Salhi, N.; Cadot, E.; Rabia, C. Clean Alternative for Adipic Acid Synthesis Via Liquid-Phase Oxidation of Cyclohexanone and Cyclohexanol Over H3-2xCoxPMo12O40 Catalysts with Hydrogen Peroxide. Catal. Lett. 2013, 143, 749-755.

38. Tahar, A.; Benadji, S.; Mazari, T.; Dermeche, L.; Roch-Marchal, C.; Rabia, C. Preparation, Characterization and Reactivity of Keggin Type Phosphomolybdates, H3-2xNixPMo12O40 and (NH4)3-2xNixPMo12O40, for Adipic Acid Synthesis. Catal. Lett. 2015, 145, 569-575.

39. Nomiya, K.; Miwa, M.; Sugaya, Y. Catalysis by heteropolyacid-VII. catalytic oxidation of cyclohexanol by dodecamolybdate. Polyhedron 1984, 3, 607-610.

40. Sato, K.; Aoki, M.; Noyori, R. A "Green" Route to Adipic Acid: Direct Oxidation of Cyclohexenes with 30 Percent Hydrogen Peroxide. Science 1998, 281, 1646-1647.

41. Cavani, F.; Teles, J.H. Sustainability in catalytic oxidation: An alternative approach or a structural evolution? ChemSusChem 2009, 2, 508-534.

42. Schindler, G.P.; Bartl, P.; Hoelderich, W.F. Oxidative cleavage of cyclohexane derivatives over titanium-containing Y zeolites. Appl. Catal. A 1998, 166, 267-279.

43. Wang, W.; Liu, H.; Ding, G.; Zhang, P.; Wu, T.; Jiang, T.; Han, B. Ru-Cd/Bentonite for the Partial Hydrogenation of Benzene: A Catalyst without Additives. ChemCatChem 2012, 4, 1836-1843.

44. Schwab, F.; Lucas, M.; Claus, P. Ruthenium-catalyzed selective hydrogenation of benzene to cyclohexene in the presence of an ionic liquid. Angew. Chem. Int. Ed. 2011, 50, 10453-10456.

45. Liu, H.; Jiang, T.; Han, B.; Liang, S.; Wang, W.; Wu, T.; Yang, G. Highly selective benzene hydrogenation to cyclohexene over supported Ru catalyst without additives. Green Chem. 2011, 13, 1106-1109.

46. Jin, M.; Cheng, Z.-M. Oxidative Dehydrogenation of Cyclohexane to Cyclohexene over Mg-V-O Catalysts. Catal. Lett. 2009, 131, 266-278.

47. Patcas, F.; Patcas, F.C. Reaction pathways and kinetics of the gas-phase oxidation of cyclohexane on NiO/γ-Al2O3 catalyst. Catal. Today 2006, 117, 253-258.

48. Zhu, H.; Ge, Q.; Li, W.; Liu, X.; Xu, H. Study of Mn-based Catalysts for Oxidative Dehydrogenation of Cyclohexane to Cyclohexene. Catal. Lett. 2005, 105, 29-33. [Green Version]

49. Soares, J.C.S.; Gonçalves, A.H.A.; Zotin, F.M.Z.; Raddi de Araújo, L.R.; Gaspar, A.B. Cyclohexene to adipic acid synthesis using heterogeneous polyoxometalate catalysts. Mol. Catal. 2018, 458, 223-229.

50. Bohström, Z.; Rico-Lattes, I.; Holmberg, K. Oxidation of cyclohexene into adipic acid in aqueous dispersions of mesoporous oxides with built-in catalytical sites. Green Chem. 2010, 12, 1861-1869.

51. Timofeeva, M.N.; Kholdeeva, O.A.; Jhung, S.H.; Chang, J.-S. Titanium and cerium-containing mesoporous silicate materials as catalysts for oxidative cleavage of cyclohexene with H2O2: A comparative study of catalytic activity and stability. Appl. Catal. A 2008, 345, 195-200.

52. Lee, S.O.; Raja, R.; Harris, K.D.M.; Tomas, J.M.; Johnson, B.F.J.; Sankar, G. Mechanistic Insights into the Conversion of Cyclohexene to Adipic Acid by H2O2 in the Presence of a TAPO-5 Catalyst. Angew. Chem. Int. Ed. 2003, 42, 1520-1523.

53. Lapisardi, G.; Chiker, F.; Launay, F.; Nogier, J.P.; Bonardet, J.L. Preparation, characterisation and catalytic activity of new bifunctional Ti-AlSBA15 materials. Application to a "one-pot" green synthesis of adipic acid from cyclohexene and organic hydroperoxides. Microporous Mesoporous Mater. 2005, 78, 289-295.

54. Lapisardi, G.; Chiker, F.; Launay, F.; Nogier, J.P.; Bonardet, J.L. A "one-pot" synthesis of adipic acid from cyclohexene under mild conditions with new bifunctional Ti-AlSBA mesostructured catalysts. Catal. Commun. 2004, 5, 277-281.

55. Gui, J.; Liu, D.; Cong, X.; Zhang, X.; Jiang, H.; Hu, Z.; Sun, Z. Clean synthesis of adipic acid by direct oxidation of cyclohexene with H2O2 catalysed by Na2WO4·2H2O and acidic ionic liquids. J. Chem. Res. 2005, 8, 520-522.

56. Wang, B.; Zhang, Z.; Zhang, X.; Sun, S.; Wu, L.; Xing, R. Efficient and convenient oxidation of cyclohexene to adipic acid with H2O2 catalyzed by H2WO4 in acidic ionic liquids. Chem. Pap. 2018, 72, 643-649.

57. Lin, Z.-P.; Wan, L. Synthesis of Adipic Acid Oxidized by H2O2-Silicotungstic Acid Under Ultrasonication. Asian J. Chem. 2013, 25, 6008-6010.

58. Chiker, F.; Launay, F.; Nogier, J.P.; Bonardet, J.L. Green epoxidation on Ti-mesoporous catalysts. Environ. Chem. Lett. 2003, 1, 117-120.

59. Santini, R.; Griffith, M.C.; Qi, M. A measure of solvent effects on swelling of resins for solid phase organic synthesis. Tetrahedron Lett. 1998, 39, 8951-8954.

60. Rocchiccioli-Deltcheff, C.; Fournier, M.; Franck, R.; Thouvenot, R. Vibrational investigations of polyoxometalates. 2. Evidence for anion-anion interactions in molybdenum(VI) and tungsten(VI) compounds related to the Keggin structure. Inorg. Chem. 1983, 22, 207-216.

61. Rocchiccioli-Deltcheff, C.; Fournier, M. Catalysis by polyoxometalates. Part 3.-Influence of vanadium(V) on the thermal stability of 12-metallophosphoric acids from in situ infrared studies. J. Chem. Soc. Faraday Trans. 1991, 87, 3913-3920.

62. Aouissi, A.; Abdullah Al-Othman, Z.; Al-Anezi, H. Reactivity of Heteropolymolybdates and Heteropolytungstates in the Cationic Polymerization of Styrene. Molecules 2010, 15, 3319-3328. [Green Version]

63. Nakashima, T.; Kimizuka, N. Interfacial Synthesis of Hollow TiO2 Microspheres in Ionic Liquids. J. Am. Chem. Soc. 2003, 125, 6386-6387.

64. Ganapathy, S.; Fournier, M.; Paul, J.F.; Delevoye, L.; Guelton, M.; Amoureux, J.P. Location of Protons in Anhydrous Keggin Heteropolyacids H3PMo12O40 and H3PW12O40 by 1H{31P}/31P{1H} REDOR NMR and DFT Quantum Chemical Calculations. J. Am. Chem. Soc. 2002, 26, 7821-7828.

65. Chen, Y.; Tan, R.; Zheng, W.; Zhang, Y.; Zhao, G.; Yin, D. Dendritic phosphotungstate hybrids efficiently catalyze the selective oxidation of alcohols with H2O2. Catal. Sci. Technol. 2014, 4, 4084-4092.

66. Chang, T. NMR characterization of the supported 12-heteropoly acids. J. Chem. Soc. Faraday Trans. 1995, 91, 375-379.

67. Edwards, J.C.; Thiel, C.Y.; Benac, B.; Knifton, J.F. Solid-state NMR and FT-IR investigation of 12-tungstophosphoric acid on TiO2. Catal. Lett. 1998, 51, 77-83.

68. Ren, S.; Xie, Z.; Cao, L.; Xie, X.; Qin, G.; Wang, J. Clean synthesis of adipic acid catalyzed by complexes derived from heteropoly acid and glycine. Catal. Commun. 2009, 10, 464-467.

69. Guérin, B.; Mesquita Fernandez, D.; Daran, J.-C.; Agustin, D.; Poli, R. Investigation of induction times, activity, selectivity, interface and mass transport in solvent-free epoxidation by H2O2 and TBHP: A study with organic salts of the [PMo12O40]3- anion. New J. Chem. 2013, 37, 3466-3475.

70. Leclercq, L.; Mouret, A.; Proust, A.; Schmitt, V.; Bauduin, P.; Aubry, J.M.; Nardello-Rataj, V. Pickering Emulsion Stabilized by Catalytic Polyoxometalate Nanoparticles: A New Effective Medium for Oxidation Reactions. Chem.-Eur. J. 2012, 18, 14352-14358.

71. Yang, B.; Leclercq, L.; Schmitt, V.; Pera-Titus, M.; Nardello-Rataj, V. Colloidal tectonics for tandem synergistic Pickering interfacial catalysis: Oxidative cleavage of cyclohexene oxide into adipic acid. Chem. Sci. 2019, 10, 501-507.

72. Wei, X.-R.; Liu, J.; Yang, Y.; Deng, L. A general approach towards efficient catalysis in Pickering emulsions stabilized by amphiphilic RGO-Silica hybrid materials. RSC Adv. 2014, 4, 35744-35749.

73. Moudjahed, M.; Dermeche, L.; Benadji, S.; Mazari, T.; Rabia, C. Dawson-type polyoxometalates as green catalysts for adipic acid synthesis. J. Mol. Catal. A Chem. 2016, 414, 72-77.

74. Mouheb, L.; Dermeche, L.; Mazari, T.; Benadji, S.; Essayem, N.; Rabia, C. Clean Adipic Acid Synthesis from Liquid-Phase Oxidation of Cyclohexanone and Cyclohexanol Using (NH4)xAyPMo12O40 (A: Sb, Sn, Bi) Mixed Heteropolysalts and Hydrogen Peroxide in Free Solvent. Catal. Lett. 2018, 148, 612-620.

75. Huddleston, J.G.; Visser, A.E.; Reichert, W.M.; Willauer, H.D.; Broker, G.A.; Rogers, R.D. Characterization and comparison of hydrophilic and hydrophobic room temperature ionic liquids incorporating the imidazolium cation. Green Chem. 2001, 3, 156-164.

76. Cha, S.; Ao, M.; Sung, W.; Moon, B.; Ahlström, B.; Johansson, P.; Ouchi, Y.; Kim, D. Structures of ionic liquid-water mixtures investigated by IR and NMR spectroscopy. Phys. Chem. Chem. Phys. 2014, 16, 9591-9601.

77. Poli, E.; Clacens, J.-M.; Pouilloux, Y. Synthesis of peroxophosphotungstate immobilized onto polymeric support as heterogeneous catalyst for the epoxidation of unsaturated fatty esters. Catal. Today 2011, 164, 429-435.