Cyclohexanone is an important organic compound in the manufacture of nylon. Cyclohexanone is produced by the selective oxidation of cyclohexane in air over cobalt catalysts ^{[1, 2, 3, 4]}. Alternatively, cyclohexanone can also be synthesized by the selective hydrogenation of phenol, but this requires high temperature and gives several byproducts such as cyclohexanol and cyclohexane ^{[2, 3]}. Ways to increase the selectivity to cyclohexanone have been much studied in recent years ^{[5, 6, 7]}. Several catalysts including Pd ^{[8, 9, 10, 11, 12]}, Pt ^{[13, 14]}, Rh ^[15], and Ni ^{[16, 17]} catalysts were reported frequently, and Pd-based catalysts are favored for this reaction for its high activity and selectivity. At the same time, many supports, including NaY zeolite ^[1], carbon ^[18], hydrophilic carbon ^[19], MgO ^[20], Fe₂O₃ ^[20], mesoporous CeO₂ ^[21], HZSM-5 ^[22], metal organic frameworks (MOFs) ^{[23, 24]}, SiO₂, γ-Al₂O₃ ^{[25, 26]}, and mpg-C₃N₄ ^{[5, 27]}, have been used as the support of Pd. The product distribution depends on both the active metal and support, and a co-added acid or acidity of the catalyst can improve the conversion of phenol ^{[1, 28]}. However, achieving a high selectivity of cyclohexanone (> 90%) at high phenol conversion (> 80%) with a single catalyst remains challenging.

An ion exchange resinis a porous, insoluble matrix with a high surface area. It is widely used in catalysis for its strong acidity. The Amberlyst-45 resin (A-45) is a new macroporous polymer designed for use at high temperature. It is made of polystyrene sulfonate and its concentration of acid sites can reach 2.95 eq/kg. In this work, Pd nanoparticles (NPs) were loaded on A-45 by a facile routine, and this Pd/A-45 catalyst was used in the selective hydrogenation of phenol to cyclohexanone in water.

Commercial resin Amberlyst-45 (A-45, Dow Chemical Company) was purchased from Sigma Co. (China Branch). It was washed with distilled water 3 times and dried in vacuum at 50 °C for 12 h before use. First, 1.0 g of A-45 was immersed in an aqueous solution of PdCl₂ (30 mL, containing a controlled amount of Pd) under stirring at 50 °C for 24 h. Then, the solid particles were separated by filtration, washed with a mixed solution of ethanol/water (1:1 by volume) until free of Cl^-, and further dried under vacuum at 50 °C for 12 h. Finally, the dried solid was pretreated in a flow of H₂ at 120 °C for 1.0 h before the catalytic reaction. These catalysts were denoted as Pd(x)/A-45, where x wt% is the loading of Pd.

As references, Fe₂O₃, SiO₂, ZnO, MgO, Al₂O₃, and active carbon (AC) supported Pd catalysts were also prepared by an impregnation method described elsewhere ^[29]. Before impregnation, the support was first calcined at 500 °C in N₂ flow for 4 h and then impregnated with an aqueous solution of PdCl₂ with equal weight ratio of Pd and support. The precursor was dried in N₂ at 110 °C overnight followed by calcination in N₂ at 500 °C for 4 h. Before the catalytic reaction, the catalyst was reduced in H₂ at 120 °C for 1 h.

The loading of Pd on the above catalysts was checked by inductively coupled plasma atomic emission spectroscopy (ICP, Plasma-Spec-II spectrometer). The results are summarized in Table 1.

Table 1 Physical properties of the catalysts. Catalyst Pd loading a Particle size of Pd b Surface area c (wt%) (nm) (m2/g) Pd/Fe3O4 2.5 5.5 9.2 Pd/SiO2 2.7 7.4 289.3 Pd/ZnO 2.8 10.1 14.7 Pd/MgO 2.8 8.0 39.4 Pd/Al2O3 2.6 4.5 156.2 Pd/AC 2.9 6.6 1396.0 Pd(0.9)/A-45 0.9 9.0 40.3 Pd(2.7)/A-45 2.7 12.0 39.8 Pd(4.5)/A-45 4.5 14.5 38.0 a Measured by ICP. b Calculated using Scherrer's equation. c Measured by N2 adsorption.

Table 1 Physical properties of the catalysts.

X-ray diffraction (XRD) patterns were performed on a Rigaku D/MAX 2550/PCdiffractometer (18 kW) at 40 kV and 100 mA with Cu K_α radiation (λ = 1.5406 Å) in the range of 5°-80°. The surface area of the catalysts was measured by N₂ adsorption using an ASAP 2010 analyzer (Micromeritics) after pretreatment at 100 °C for 4 h under vacuum. X-ray photoelectron spectra (XPS) were recorded on a Perkin-Elmer PHI ESCA System. The X-ray source was an Mg standard anode (1253.6 eV) at 12 kV and 300 W. Transmission electron microscopy (TEM) images were obtained using an accelerating voltage of 200 kV (TEM, JEOL-2010F).

The hydrogenation of phenol was carried out in a 50 mL custom designed stainless steel autoclave with a Teflon inner layer. In a typical reaction, a controlled amount of catalyst was dispersed in 20 mL aqueous solution of phenol. Then, the reactor was sealed, purged with purified hydrogen 5 times, and pressurized to the desired pressure. The reactor was heated in an oil bath and stirred with a magnetic stirrer (MAG-NEO, RV-06M, Japan). After reaction, the solid catalyst was separated by centrifugation. The liquid reaction mixture was analyzed by a gas chromatograph (HP 5890, USA) with a 30 m capillary column (HP-5) using a flame ionization detector. All products were confirmed by GC-MS (Agilent 6890-5973N). For each successive use, the catalyst was washed with water three times and dried under vacuum at 40 °C for 6 h. The conversion of phenol and selectivity for cyclohexanone (and cyclohexanol) were calculated as:

Conversion = (phenol_added-mol - phenol_remain-mol)
/phenol_added-mol × 100%

Selectivity = cyclohexanone_formed-mol/(phenol_added-mol
- phenol_remain-mol) × 100%

Figure 1 shows the XRD patterns of the pristine A-45 resin and Pd/A-45 catalysts with different loadings of Pd. Only a broad peak of amorphous carbon appeared with A-45. Beside the peak of amorphous carbon, four characteristic diffraction peaks of Pd were detected at 40.0°, 46.5°, 68.1°, and 82.1° with all the Pd/A-45 catalysts, which corresponded to the (111), (200), (220), and (311) crystalline planes of face centered cubic of Pd (JCPDS 46-1043). The crystalline size of Pd was calculated from the half-width of the Pd(111) peak using the Scherrer equation. The results are summarized in Table 1. The particle size of Pd on A-45 increased from 9.0 nm (in Pd(0.9)/A-45) to 14.5 nm (in Pd(4.5)/A-45) with increasing Pd loading.

Fig. 1. XRD patterns of A-45 (1), Pd(0.9)/A-45 (2), Pd(2.7)/A-45 (3), and Pd(4.5)/A-45 (4).

Figure 2 shows typical TEMimages and the histogram of the Pd particle size distribution of the Pd(2.7)/A-45 catalyst. Pd particles were dispersed mainly in the macropore channels of A-45, and the particles of Pd were in the range of 5-20 nm. The calculated mean particle size of the counted particles was 12.2 nm, which agreed with the XRD results.

Figure 2 shows typical TEMimages and the histogram of the Pd particle size distribution of the Pd(2.7)/A-45 catalyst. Pd particles were dispersed mainly in the macropore channels of A-45, and the particles of Pd were in the range of 5-20 nm. The calculated mean particle size of the counted particles was 12.2 nm, which agreed with the XRD results.

Fig. 2. (a, b) TEM images of Pd(2.7)/A-45; (c) Histogram of Pd particle size distribution in Pd(2.7)/A-45; (d) High resolution TEM image of Pd nanoparticles in Pd(2.7)/A-45.

Figure 3 presents the XPSspectra of pristine A-45 and the Pd(2.7)/A-45 catalyst. The binding energy and surface content of C, S, O, and Pd are summarized in Table 2. The binding energy confirmed that plentiful -SO₃H (168.9 eV) groups existed on the surface of pristine A-45 ^{[30, 31]}. When Pd was loaded, the surface composition of A-45 remained stable, and mainly metallic Pd was formed after reduction ^[32].

Table 2 Surface composition of the Pd(2.7)/A-45 catalyst. Sample Binding energy (eV) Surface content (wt%) Pd C S Pd C S O A-45 — 284.40 168.89 — 79.38 5.13 15.49 Pd(2.7)/A-45 335.74 340.81 284.32 168.45 3.98 76.30 4.92 14.80

Table 2 Surface composition of the Pd(2.7)/A-45 catalyst.

Fig. 3. XPS spectra of A-45 and Pd(2.7)/A-45 (a), and the Pd 3d in Pd(2.7)/A-45 (b).

First, the selective hydrogenation of phenol to cyclohexanone was performed over the different catalysts in water at 100 °C for 3 h (Table 3). All the tested catalysts were active for this reaction. Cyclohexanone and cyclohexanol were the only reaction products observed over the entire range of conditions studied. The conversion of phenol increased in the order of Pd/Fe₃O₄, Pd/SiO₂, Pd/ZnO, Pd/MgO, and Pd/Al₂O₃. Only mainly the byproduct cyclohexanol was formed over Pd/MgO, Pd/ZnO, and Pd/Fe₃O₄. On the other hand, Pd/AC and Pd(2.7)/A-45 were more active than the oxide supported Pd catalysts. Phenol was converted completely over Pd(2.7)/A-45 even for 0.5 h, and the selectivity for cyclohexanone reached 89.0%. These results suggested that the ion exchanged resin (A-45) was a good support for the selective hydrogenation of phenol to cyclohexanone.

Table 3 Hydrogenation of phenol over the different catalysts. Entry Catalyst Conversion Selectivity (%) (%) Cyclohexanone Cyclohexanol 1 blank 0 — — 2 Pd/Fe3O4 14.7 32.2 67.8 3 Pd/SiO2 26.6 >99.0 — 4 Pd/ZnO 27.7 26.1 74.0 5 Pd/MgO 35.0 17.7 82.3 6 Pd/Al2O3 87.5 65.1 34.9 7 a Pd/AC 69.1 62.4 37.6 8 a Pd(2.7)/A-45 100.0 89.0 11.0 9 A-45 0 — — Reaction conditions: phenol 0.5 mmol, water 20 mL, phenol/Pd ratio 8.0, 100 °C, 3 h, H2 1.0 MPa, 1000 r/min.

Table 3 Hydrogenation of phenol over the different catalysts.

The good performance of Pd(2.7)/A-45 was confirmed under various reaction conditions (Table 4 and Fig. 4). The conversion of phenol increased quickly from 23.8% (at 40 °C) to 100% (at 100 °C) with rising temperature, while the selectivity for cyclohexanone only decreased slightly from 92.4% to 89.0% (Table 4). At the same time, it was confirmed that a low pressure was more favorable for the formation of cyclohexanone and the selectivity for cyclohexanone reached 95.6% at 0.2 MPa. More importantly, the selectivity for cyclohexanone remained higher than 89.0% in a wide range of phenol/Pd ratio in the feed at 100 °C and 1.0 MPa (Fig. 4). The performance of the Pd(2.7)/A-45 catalyst was lower than that of Pd/C + AlCl₃ ^[1] and mpg-C₃N₄ supported Pd ^[27], but it was better than those of active carbon, hydrophilic carbon, and Al₂O₃ supported Pd catalysts ^[19], and the selectivity for cyclohexanone was higher than that of Pd/C ^[33].

Table 4 Performance of Pd(2.7)/A-45 at different temperatures and pressures. Temperature (°C) H2 pressure (MPa) Conversion (%) Selectivity (%) Cyclohexanone Cyclohexanol 40 1.0 23.8 92.4 7.6 60 1.0 60.2 91.0 9.0 80 1.0 84.5 90.0 10.0 100 1.0 100.0 89.0 11.0 100 0.5 52.3 91.8 8.2 100 0.2 21.7 95.6 4.4 Reaction conditions: phenol 0.5 mmol, water 20 mL, phenol/Pd ratio 8.0, 1000 r/min, 0.5 h.

Table 4 Performance of Pd(2.7)/A-45 at different temperatures and pressures.

Fig. 4. Performance of Pd(2.7)/A-45 for different phenol/Pd ratios in the feed. Reaction conditions: 100 °C, H2 1.0 MPa, 0.5 h.

The published work ^{[1, 21, 22, 23, 24, 25, 26, 27, 28]} suggested that the selectivity for cyclohexanone in phenol hydrogenation depended on the adsorption mode of phenol, and the nonplanar form of adsorbed phenol was more favorable for the formation of cyclohexanone than a coplanar form of adsorbed molecule ^[21]. Wang et al. ^[27] further disclosed that the acid sites on the surface of the catalyst helped the nonplanar form adsorption of phenol through the hydroxy group forming strong O−H···π interactions. Therefore, we think that the good performance of Pd(2.7)/A-45 can be attributed to its strong acidity (2.95 mmol/g in A-45). At the same time, the acid sites also enrich the electron density of Pd, enhance the desorption of phenoxy species ^[28], and inhibit further hydrogenation of the intermediate product ^[1].

The recycle experiments also indicated that Pd/A-45 showed a better performance in reusability (Fig. 5). Pd(2.7)/A-45 could be reused at least 6 times without significant loss of activity, maintaining at least 46.6% conversion, which is a prerequisite for practical application. It is important to highlight that the selectivity for cyclohexanone remained high (about 90%) and comparable to that of the fresh catalyst. XRD analysis of a spent Pd(2.7)/A-45 catalyst indicated that the particle size of Pd was increased slightly from 12.0 to 13.6 nm (Fig. 6), and the decreased conversion of phenol can be attributed to the sintering of Pd. Another possible reason for activity loss may be due to the swelling of the polymer resin in the polar solvent and/or reactants ^[34], or because the resin beads were gradually mechanically damaged by being vigorously stirred with a magnetic stirrer bar during the reaction ^[35].

Fig. 5. Recycling of Pd(2.7)/A-45 for phenol hydrogenation. Reaction conditions: 0.24 g Pd(2.7)/A-45 in the first run (no further addition), 0.5 mmol phenol in each run, 60 °C, H2 1.0 MPa, 0.5 h.

Fig. 6. XRD patterns of Pd(2.7)/A-45 before (1) and after (2) six recycles.

As Pd(2.7)/A-45 with a similar loading amount of Pd exhibited the best performance among the tested catalysts (Table 3), a series of Pd/A-45 catalysts with different Pd loadings were prepared and compared for the hydrogenation of phenol to cyclohexanone at 60 °C. The crystallite sizes of Pd in these catalysts were calculated using the half-width of the Pd(111) peak and the Scherrer equation and were summarized in Table 5. At a constant phenol/Pd ratio in feed, the conversion of phenol decreased from 100% over Pd(0.1)/A-45 (5.0 nm sized Pd NPs) to 44.5% over Pd(4.5)/A-45 (14.5 nm sized Pd NPs), but the selectivity for cyclohexanone reached a maximum value (91.0%) over Pd(2.7)/A-45 (12.0 nm sized Pd NPs). At the same time, the turnover frequency (TOF) on the basis of surface Pd atoms reached 116-118 h^-1 over the catalysts with a particle size of Pd in 9-13 nm (in Pd(0.9)/A-45, Pd(1.8)/A-45, and Pd(2.7)/A-45). This volcano shaped TOF versus Pd particle size curve and the higher selectivity for cyclohexanone over Pd(2.7)/A-45 indicated that the formation of cyclohexanone was a structure sensitive reaction, and Pd particles of 12-14 nm were better for their high selectivity.

Table 5 Performance of Pd/A-45 with different Pd loadings. Pd loading (wt%) Size (nm) Conversion (%) Selectivity (%) D a (%) TOF b (h-1) Cyclohexa- none Cyclohexa- nol 0.1 5.0 100.0 6.3 93.7 19.4 80 0.3 6.3 95.5 21.5 78.5 15.4 96 0.6 7.7 90.3 42.2 57.8 12.6 111 0.9 9.0 82.5 65.7 34.3 10.8 118 1.8 10.9 67.1 77.6 22.4 8.9 116 2.7 12.0 60.2 91.0 9.0 8.1 116 3.6 13.6 50.0 90.2 10.8 7.2 108 4.5 14.5 44.5 89.8 10.2 6.7 103 Reaction conditions: phenol 0.5 mmol, water 20 mL, phenol/Pd ratio 8.0, 60 °C, 0.5 h, H2 1.0 MPa, 1000 r/min. a Dispersion of Pd calculated as D = 0.97/particle size in nm. b Turnover frequency defined as (converted phenol molecules at 0.5 h)/((number of surface Pd atoms) x (reaction time, h)).

Table 5 Performance of Pd/A-45 with different Pd loadings.

Pd NPs were deposited on A-45 resin by a convenient method. The obtained Pd/A-45 catalyst was highly active, selective, and stable for the hydrogenation of phenol to cyclohexanone in water under mild conditions (40-100 °C, 0.2-1 MPa). The selectivity for cyclohexanone remained higher than 89% even at complete conversion of phenol. A low pressure is more favorable for the formation of cyclohexanone. These results showed the potential of Pd/A-45 for practical application.