Enhanced Selectivity of Phenol Hydrogenation in Low-Pressure CO2

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Enhanced Selectivity of Phenol Hydrogenation in Low-Pressure CO2 over Supported Pd Catalysts Tianzhu Liu, Hu Zhou, Bingbing Han, Yongbing Gu, Suiqin Li, Jian Zheng, Xing Zhong, Gui-Lin Zhuang, and Jian-guo Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02974 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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ACS Sustainable Chemistry & Engineering

Enhanced Selectivity of Phenol Hydrogenation in Low-Pressure CO2 over Supported Pd Catalysts Tianzhu Liu, Hu Zhou, Bingbing Han, Yongbing Gu, Suiqin Li, Jian Zheng, Xing Zhong, Gui-Lin Zhuang, Jian-guo Wang*

Institute of Industrial Catalysis, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China.

*Corresponding author:

E-mail: [email protected]., Fax: +86-571-88871037.

Tel: +86-571-88871037.

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ABSTRACT Selective hydrogenation of phenol to cyclohexanone is an important process in both chemical industry and renewable feedstock processing. However, direct hydrogenation of phenol to cyclohexanone under mild condition over catalysts with high reactivity, selectivity, facile preparation is still a challenge. In the present study, we report that 99% conversion and 99% selectivity can be achieved over as-prepared Pd/γ-Al2O3 catalyst under the medium of low-pressure CO2 (0.05–0.2 MPa) and H2O at 373 K. According to experiment results, ab initio calculations and in situ high-pressure FTIR measurements indicated enhanced selectivity of cyclohexanone in low-pressure CO2; this result originated from the molecular interaction between cyclohexanone and CO2 and can prevent the further hydrogenation of cyclohexanone. Notably, enhancement of selectivity to cyclohexanone in low-pressure CO2 was also achieved by using commercial Pd/γ-Al2O3 and Pd/C catalysts. KEYWORDS: Phenol hydrogenation, Low-pressure CO2, Cyclohexanone, Pd

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INTRODUCTION

Cyclohexanone is an important precursor to caprolactam and adipic acid for manufacture of nylon-6 and nylon-66, respectively.1 Industrial production of cyclohexanone normally occurs by oxidation of cyclohexane2-3 or phenol hydrogenation.4-6 Hydrogenation of phenol is a desired strategy as the traditional route consumes more energy and yields low amounts of cyclohexanone (˂ 10%).5 Hence, significant works were devoted to this area; phenol hydrogenation can be efficiently catalyzed by supported Pd (such as Pd/C and Pd/MgO)7-9 or Pt10 catalysts in vapor phase at high temperatures. This process can also be performed in liquid phase over multiple-supported noble metal catalysts (Pd11-20 and Ru21-22) at relatively mild conditions. However, use of such materials in practical production is not extensively applied owing to complicated preparation of catalysts and harsh reaction conditions. Considering of this perspective, designing an effective catalyst system or developing an alternative reaction medium23 are viable methods for phenol hydrogenation over conventional metal catalysts. A remarkable example was the work Han24 and co-workers, who achieved both high rates of phenol conversion and selectivity to cyclohexanone by using commercial Pd/C catalyst through addition Lewis acids, such as AlCl3 and ZnCl2, which worked in synergy with cyclohexanone to further inhibit cyclohexanone hydrogenation in dichloromethane or compressed CO2 (6–7 MPa); however, this method remained unsatisfactory due to high-pressure CO2 conditions. Hence, effective reaction medium must be developed for such systems.

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Components of reaction medium affect synthetic reactions,25-26 whereas previous hydrogenation studies demonstrated that selective hydrogenation of benzaldehyde,27 cinnamaldehyde,27-28 nitrobenzene,29 maleic anhydride30 and chloronitrobenzene31-32 over metal (Pt, Ru, Ni, and Pd) catalysts can be enhanced in compressed CO2 medium or supercritical CO2. Enhanced product selectivity in pressurized CO2 reaction medium was also investigated for the phenol hydrogenation over Rh33-34 or Pd35-36 catalysts at 323 K, but one drawback of high-pressure conditions is easily deactivation of Rh or Pd catalysts due to formation and adsorption of CO from H2 and CO2. Considering the abovementioned disadvantages, effective medium at low-pressure CO2 must be developed for hydrogenation reactions. Most recently, Zhao37 and Shirai38 reported that chemoselective hydrogenation of aromatic nitro compounds and acetophenone can be achieved in a H2O and low pressure CO2 medium (0.8–3.0 MPa), respectively. Thus, scientific interest centers on exploring the high selectivity to cyclohexanone for phenol hydrogenation in low-pressure CO2. In the present study, we report that enhance selectivity of cyclohexanone can be possibly achieved over the as-prepared Pd/γ-Al2O3 catalyst in the medium of H2O and low-pressure CO2 (0.05–0.2 MPa, this is defined as the range of low-pressure CO2 in the present work). Using commercial Pd/γ-Al2O3 and Pd/C catalysts also contributes to achieving such goals under similar reaction conditions. To investigate the influence of low-pressure CO2 on hydrogenation of phenol, reactions were conducted in the absence and the presence of low-pressure CO2 or in the presence of pressured Ar, or solvent-free conditions,

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respectively. In situ high-pressure Fourier transform infrared spectroscopy (FTIR) and ab initio calculations were employed to study the molecular interaction between cyclohexanone and CO2. Results further elucidate the possible mechanism of phenol hydrogenation in low-pressure CO2 over Pd catalysts.

EXPERIMENTAL AND COMPUTATIONAL SECTION

Materials Phenol, cyclohexanone, cyclohexanol, ethyl acetate, and ammonium hydroxide (NH3•H2O) were purchased from Sigma-Aldrich and used as received. Commercially available 5 wt.% Pd/C and 5 wt.% Pd/γ-Al2O3 catalyst (Alfa Aesar China) were used without further treatments. γ-Al2O3 was obtained from Alfa Aesar (China), and palladium(II) acetate (Pd(O2CCH3)2, ≥ 99.9%) was purchased from NOTECH (China). All chemicals were used as received and without further purification. CO2 (99.99%), H2 (99.99%), and Ar (99.99%) were provided by Pujiang Special Gases Company (Shanghai, China). Pressure cylinders were equipped with precisely control devices (± 0.025 MPa).

Synthesis of Pd/γ-Al2O3 catalyst Pd/γ-Al2O3 catalyst was prepared by incipient wetness impregnation method, γ-Al2O3 was treated with 5% nitric acid, then filtered and dried at 323 K overnight. In a typical experiment, 1000 mg of pretreated γ-Al2O3 and 106 mg of Pd(O2CCH3)2 were placed in a 100 mL round bottom flask, 10 mL water was added dropwise into 5



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the round bottom flask under gentle agitation. Then, the flask was heated in oil-bath at 333 K for 12 h. The sample was evaporated with rotary evaporator, after washing several times and vacuum drying, it was immersed again in NH3•H2O immersion for 30 mins and calcined at 523 K overnight. Finally, reduced in flowing hydrogen at 523 K for 3 h with 2 K min-1 ramp rate, and hydrogen flow rate of 30 mL min-1.

Characterization of catalysts X-ray diffraction (XRD) analysis of pretreated γ-Al2O3 and fresh or used catalysts were conducted by a XPERT-PRO X-ray diffractometer with a Cu Kα radiation operating at 40 kV and 40 mA. Diffraction pattern were scanned using the 2θ angle from 10°to 80°at a sweep rate of 4° min-1. Prepared catalysts were characterized by Tecnai G2 F30 S-Twin electron microscope equipped with an energy-dispersive X-ray spectrometer (EDX) operating at 300 kV. Samples for transmission electron microscopy (TEM) analyses were prepared by adding a droplet of ethanol solution onto a copper grid. Particle size analysis of catalysts was determined by randomly approximating 200 particles on TEM images. Hydrogen pulse chemisorption measurements was performed to test Pd dispersion of Pd/γ-Al2O3. Dispersion of Pd NPs was calculated on the basis of the equation17: D (%) =

2Mn mW

× 100 (1)

It was calculated using average H2 : Pd adsorption stoichiometry factor (Pd / H2 molar ratio) = 2, M = atomic mass of Pd (106.4 g mol-1), n = cumulative quantity of

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chemisorbed H2 (mol), m = mass of the sample (g), W = the actual loading of Pd in the sample was analyzed by ICP-MS, and test results were used as reference. X-ray photoelectron spectra (XPS) were determined using Kratos AXIS Ultra DLD X-ray photoelectron spectrometer. Figures for Brunauer–Emmett–Teller (BET) specific surface areas were measured on Micromeritics ASAP2460 analyzer. Pd catalysts and reaction solution were analyzed in a PerkinElmer DRC-e inductively coupled plasma-mass spectrum (ICP-MS).

Hydrogenation Reactions Phenol hydrogenation was carried out in a 30 mL autoclave was equipped with three one-way valves (two inlet valves and an outlet valve) using water as a solvent. The autoclave was charged with 500 mg phenol, 15 mL water and 50 mg Pd catalysts. The autoclave connected to pressure cylinders were equipped with a pressure transducer (FOXBORO/ICT, Model 93), which could be accurate to ± 0.025 MPa. The autoclave was purged with pure hydrogen for five times, and then desired hydrogen pressure was introduced to the reactor, followed by the introduction of CO2 to the desired pressure with a high-pressure gas pump (Sup-B-5) connected to another one-way inlet valve, finally, the autoclave was heated to 373 K with an intelligent temperature control furnace, and stirred with magnetic stirrer. At the end of reaction, the autoclave was cooled with cold water. Products were extracted by ethyl acetate, and the catalysts were collected for catalytic cycle. The mixture was analyzed by gas chromatograph (GC, Fuli 9790II) equipped with a flame ionization detector and an

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AT.SE–54 column. For comparison, cyclohexanone was hydrogenated under the same conditions. In low-pressure CO2 reaction medium, gas compositions were analyzed by GC equipped with a thermal conductivity detector and packed column (TDX–01) during the reaction.

In situ high-pressure FTIR measurements High-pressure FTIR was used to investigate the interaction between CO2 and cyclohexanone. Thermo Fisher Nicolet IS50 with mercury cadmium telluride (MCT– A) detector and a 2.0 cm3 customized spectrum in situ cell was equipped with pressure and temperature control devices (Tuosi instrument, Xiamen, China), respectively. In a typical experiment, 30 µL cyclohexanone was added to the cell after using pure CO2 at the corresponding pressure as background. Then, expected pressure value of CO2 and/or H2 was applied to the cell, which was then heated to 373 K. FTIR spectra were recorded on spectrometer. Each sample was recorded with 64 scans at an effective resolution of 4 cm-1. The in-situ FTIR measurements were used to examine the competitive adsorption of cyclohexanone and CO2 on the Pd/γ-Al2O3. In a typical experiment: a 30 mg of the catalyst sample was prepared to catalyst pellet. The catalyst was reduced in the cell by flowing H2 at 473 K for 2 h. Then the cell was cooled to 373 K and purged with Ar for several times, and the spectra were collected as background. A certain amount of cyclohexanone was introduced into the cell with a syringe by injected into the flowing Ar. Finally, CO2 was introduced at desired pressure for 1 h. Each FTIR spectra was

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collected after the cell was purged with Ar a few times for 15 min.

Computational Methods Ab initio calculations were performed using Gaussian 09 program. Preliminary geometry optimizations were carried out at the Hartree–Fock level using 6-31G. More exact calculations of geometry optimizations and vibrational frequencies were performed at Wb97XD level to include empirical dispersion and long range corrections, and 6-311++G (d,p) basis set was employed.

RESULTS AND DISCUSSION

Characterization of Catalysts Figure 1a displays N2 sorption isotherm and Barrett–Joyner–Halenda (BJH) mesopore size distribution plots of γ-Al2O3 and fresh Pd/γ-Al2O3. Both samples showed type IV curves, suggesting a mesoporous structure. Pore size distributions indicated that mesopores before and after Pd support are still around 10 nm. However, compared with γ-Al2O3 support, BET surface area of Pd/γ-Al2O3 decreased slightly (Table 1); this result can be attributed to plugging of Pd nanoparticles (NPs) in the pores of support. XRD analysis (Figure 1b) also revealed high-similarity diffraction patterns of γ-Al2O3 and fresh Pd/γ-Al2O3. Thus, these faint changes revealed that supported Pd particle size should be small, as further demonstrated by TEM observation.

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Figure 1. a) N2 sorption isotherms and BJH mesopore size distribution plots of γ-Al2O3 and Pd/γ-Al2O3, respectively. b) Powder XRD patterns of γ-Al2O3, fresh Pd/γ-Al2O3. TEM images and relevant particle size distributions revealed microscopic morphology of fresh Pd/γ-Al2O3, as shown in Figure 2. Average size of Pd NPs of fresh Pd/γ-Al2O3 measured 2.32 nm (as mentioned in experimental section), coinciding with results of H2 pulse chemisorption (Pd dispersion: 45% and particle size: 2.44 nm), and detailed result is given in Figure S1. Commercial Pd/γ-Al2O3 and Pd/C catalysts were compared; average size of their Pd NPs reached 5.22 nm (Figures S2a and S2b) and 4.3 nm (Figures S2c and S2d), respectively. These results elucidate that as-prepared Pd/γ-Al2O3 featured small size and high dispersion over γ-Al2O3. Surface chemical property of Pd/γ-Al2O3 were further studied by XPS and results are shown in Figure 3. XPS spectrum of Pd 3d of Pd/γ-Al2O3 can be decomposed into two predominant peaks with 3d5/2 and 3d3/2. The Pd 3d5/2 peak at 335.2 eV is related to Pd0 (metallic palladium), whereas the Pd 3d5/2 peak at 337.2 eV is attributed to Pd2+ (palladium oxide).39-40 Data indicate that metallic Pd0 was formed as the major phase of Pd for Pd/γ-Al2O3 (70%), whereas Pd2+ accounted 30%; these findings were similar 10


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to those reported in literature.15

Figure 2. a) TEM image and b) particle-size distribution of fresh Pd/γ-Al2O3.

Figure 3. XPS curve of Pd 3d in fresh Pd/γ-Al2O3 catalyst. Catalyst evaluation

Under conditions adopted in the present work, products of phenol hydrogenation only included cyclohexanol and cyclohexanone; other by-products were not detected. The as-prepared Pd/γ-Al2O3 was tested for phenol hydrogenation in water under various conditions. First, we investigated whether the selectivity of cyclohexanone

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can be enhanced at lower hydrogen partial pressure (< 0.05 MPa), and the results show that selectivity to cyclohexanone was not promoted (entries 1-4 in Table S1). Then, as expected, selectivity to cyclohexanone was enhanced by introducing different pressure CO2 (from 0.05 MPa to 0.2 MPa) to aqueous phase, while other conditions remained unchanged, Figure 4a presents the results. However, the same findings were not achieved when CO2 was replaced by argon (entries 3 and 6 in Table S1). On the other hand, the selectivity of cyclohexanone has also been enhanced with different degrees in low-pressure CO2 over commercial Pd/γ-Al2O3 and Pd/C under similar conditions (Figures 4b and 4c). Hence, low-pressure CO2 may play an important role in enhancing selectivity to cyclohexanone in phenol hydrogenation over Pd catalysts.

Figure 4. Influence of CO2 pressure on the selectivity of cyclohexanone over a) Pd/γ-Al2O3, b) Pd/γ-Al2O3-com., c) Pd/C-com., respectively. We noted that the present Pd/γ-Al2O3 catalyst exhibited better catalytic activity and higher selectivity to cyclohexanone in low-pressure CO2 (0.05–0.2 MPa,) compared with commercial Pd/γ-Al2O3 and Pd/C catalysts (detailed data in Table S1). Therefore, we summarized the main physicochemical properties of Pd catalysts were compiled in Table 1. Similarly, the as-prepared Pd/γ-Al2O3 catalyst features higher practical loading, smaller size of Pd NPs and higher degree of dispersion, which can 12


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expose more active sites. Previous studies also reported that Pd NPs < 4 nm with the number of surface atoms may enhance hydrogen perforation,41 leading to significantly activity of Pd/γ-Al2O3 catalyst. Pore size of Pd/γ-Al2O3 was larger (8.5 nm) than the present commercial catalysts (entries 2, 5, and 6 in Table 1); such properties may facilitate diffusion of reactants and products,42 thereby decreasing mass transfer resistance.36 Table 1. Textural properties of the Pd-supported catalysts. BJH BET surface Pore Dispersion g Entry Catalyst area (%) diameter (m2g-1) (nm) 1 γ-Al2O3 232 9.4 a 2 Pd/γ-Al2O3 215 8.5 47 e 3 Pd/γ-Al2O3 194 8.4 47 f 4 Pd/γ-Al2O3 151 8.2 44 5 Pd/γ-Al2O3-com. b 137 4.4 21 c 6 Pd/C-com. 951 2.8 26

Metal loading d (wt.%) 4.7 4.3 4.2 3.4 3.5

a

. Fresh as-prepared Pd/γ-Al2O3, b commercial Pd/γ-Al2O3, c commercial Pd/C.

e

. and f after fifth used Pd/γ-Al2O3, reaction conditions for e: phenol (0.5 g), H2O (15

mL), catalyst (0.05 g), H2 (0.05 MPa), CO2 (0.05 MPa), 373 K, 5 h, stirred at 600 rpm, the reaction conditions of f were the same as e, except CO2 was not involved. d

. The actual loading of the Pd was analyzed by ICP-MS.

g

. Dispersion = 1.1/d, where dispersion is the metal dispersion and d the average metal

particle size.43

Influence of low-pressure CO2 on phenol hydrogenation We noted that phenol conversion and product selectivity depended on pressure of H2 and CO2 mixture and catalysts used (entries 5–40 in Table S1). Thereby, influence 13



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of low-pressure CO2 on reactions was investigated with the most effective Pd/γ-Al2O3 catalyst at optimal conditions. Phenol hydrogenation was carried out in different systems, that is, in the absence and presence of low-pressure CO2 or presence of Ar instead of CO2 in water or under solvent-free conditions, respectively. Figure 5 shows evolution of conversion and product species over time during phenol hydrogenation in four reaction mediums. Yields of cyclohexanone were all higher than 99% in 30 min but decreased with different degrees of increased conversion. Hence, cyclohexanone is a reaction intermediate.11-12, 36 In the absence of low-pressure CO2, selectivity to cyclohexanone reached 84% (Figure 5a) at the end of reaction, whereas in the presence of low-pressure CO2, the reaction was also completed at the same time, but selectivity of cyclohexanone remained above 98% (Figure 5b). To further examine the effects of low-pressure CO2, Ar was used instead of CO2 for the reaction under identical conditions. Ar posed no effects on conversion and product selectivity (Figure 5c). In the presence and absence of low-pressure CO2, additional reactions were run under solvent-free conditions; selectivity of cyclohexanone was also attained in low-pressure CO2 reaction medium but not in the absence of CO2 (Figure 5d and Figure S3). However, both of reaction rate was slower than that in water, and the conversion of phenol tends to reach its equilibrium gradually in the latter period. Wang12 reported that hydrogenation of phenol in water involves proton transfer; thus, the presence of water can significantly reduce activation energy of processes, as elucidated by density functional theory (DFT) calculations and isotope tracing experiments.

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Scheme 1 describes pathway of phenol hydrogenation.34,

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Early stage of the

reaction presented simultaneous formation of cyclohexanol and cyclohexanone. (Routes I and II of Scheme 1, respectively). Reaction rate of route I is much faster than that of route II because of intermediate cyclohexanone,24 which was sacrificed to enhance selectivity to cyclohexanol in later stages of the reaction (Route III). To conclude, the presence of low-pressure CO2 inhibited direct hydrogenation of cyclohexanone to cyclohexanol. However, low-pressure CO2 negatively affected conversion of phenol and more serious in solvent-free conditions. Figure S10 displays influence of CO2 pressure measuring 0 MPa to 1.0 MPa on phenol hydrogenation. Phenol conversion slightly changed with CO2 pressure from 0 MPa to 0.2 MPa. However, in the region of high-pressure CO2 (0.3–1.0 MPa), phenol conversion rapidly dropped to 25% first before leveling off. Selectivity of cyclohexanone remained above 98%. To further confirm results, cyclohexanone was hydrogenated under the same conditions, indicating that this compound can be easily hydrogenated to cyclohexanol without CO2. Almost no hydrogenation to cyclohexanol was noted in low-pressure CO2 for 5 h or after extending reaction time (Table S2 provides detailed results). We explored the possible reasons for the obtained results: First, CO2 was almost not adsorbed on Pd NPs according to a previous report of our group.45 The in-situ FTIR measurements were made to examine the competitive adsorption between cyclohexanone and CO2 on Pd/γ-Al2O3 catalyst, as shown in Figure S4. Figure S4 shows that the relatively stronger adsorption peak of ν(C=O) at about 1736 cm-1 in the

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presence of cyclohexanone and CO2 (Figure S4(2-4)), indicating that cyclohexanone is preferentially adsorbed on Pd than CO2. Pressure of reaction system is much lower than the critical pressure of CO2 in reaction at 373 K,46 thus, dissolution of reaction species in CO2 phase was improbable. Second, CO was reported to form from mixture of CO2 and H2 over Pd catalysts; CO causes catalyst deactivation.47 The gas composition in the reactors was analyzed by GC before and after phenol hydrogenation under various conditions, and the results were shown in Figure S5. The results indicate that no CO2 was transformed into CO except that the amount of hydrogen was significantly reduced after the reaction. Third, one of positive factors may be effects of dissolved CO2 in aqueous phase on pH of reaction medium,37, 48 Greiner49 reported that pH as a function of CO2 partial pressure, that is the higher the CO2 pressure is, the lower the pH-values are. The effects of solution pH on phenol hydrogenation was investigated by adding acid into the reaction solution (detail data in Table S3), and they were summarized: the Pd/γ-Al2O3 catalyst gradually lost its activity with the pH of reaction solution decreases. It is noted that the selectivity of cyclohexanone is slightly changed after complete conversion of phenol. Finally, ICP analysis suggested that the leaching of Pd was detected during reactions in high-pressure CO2 (0.3–1.0 MPa), and concentration of Pd in filtrate increased with CO2 pressure (Table S4). From above results, indicating that the acidic nature of the reaction medium and leaching of Pd were responsible for deactivated catalysts in high-pressure CO2.

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Figure 5. Kinetics of phenol hydrogenation in four different reaction mediums: a) H2 and H2O, b) H2, CO2 and H2O, c) H2, Ar and H2O, d) H2 and CO2. Reaction conditions: 50 mg Pd/γ-Al2O3, 5.32 mmol phenol, 15 mL water, 373 K, PH2=0.05 MPa, PCO2=0.05 MPa or PAr=0.05 MPa.

Scheme 1. Possible Reaction Pathways for Phenol Hydrogenation over Pd catalyst. 17



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Ab initio calculations and in situ FTIR were performed to investigate the interaction between cyclohexanone and CO2. In CO2 molecules, the two oxygen atoms are negatively charged (δ-), whereas the intermediate carbon atom is positively charged (δ+).27 Studies reported that carbonyl groups of aldehydes, and ester compounds interact through Lewis acid – Lewis base (LA−LB) interaction with CO2 molecules.50 Wallen50 and co-workers reported a C−H…O interaction of CO2 molecules with carbonyl groups through a C−H…O hydrogen bond attached to an α-carbon atom. The optimized geometries for complex of cyclohexanone with CO2 is given in Figure 6, and Table 2 records changes in interaction configurations. Results revealed that the C=O bond length of CO2 involved in a C−H…O interaction is longer than “free” C=O bond (L1 > L2) and bond length of cyclohexanone carbonyl also increased. Calculated stretching vibration frequency of ν(C=O) in cyclohexanone red-shifted to 6.0 cm-1, which can be attributed to LA−LB interaction. Stretching vibration frequency of ν(C=O) in CO2 molecule was detected by in situ FTIR in the absence and the presence of cyclohexanone under low-pressure CO2 at 373 K. Results are presented in Figure 7b. Asymmetric stretching vibration peak (red line) of pure CO2 were measured at 2338 (P-branch) and 2359 cm-1 (R-branch), but enhanced absorption of shoulder peak (purple line) was noted in the presence of cyclohexanone. The C−H…O hydrogen bonding interaction between cyclohexanone and CO2 is similar to conventional X−H…Y (X, Y = N, O, F) hydrogen bond effects, which reduces bond force constant of the C=O bond of CO2, resulting in enhanced absorption peak of C=O bond, and increased peak width with increasing CO2 pressure. 18


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These results provide strong evidence for the presence of the weak C−H…O interaction between cyclohexanone and CO2.50 Changes in carbonyl absorption peaks due to interactions of cyclohexanone with CO2 were recorded at the different pressures of CO2. Figure 7a presents that ν(C=O) FTIR spectra of gaseous cyclohexanone blue-shifted at 1736.4 cm-1 compared with the liquid state (1714 cm-1). For spectra of carbonyl absorption measured in CO2, however, the ν(C=O) band was red-shifted (1.5−4.0 cm-1) with CO2 pressures ranging from 0.05 MPa to 0.2 MPa at 373 K. However, the ν(C=O) band red-shifted did not happen when CO2 was replaced by Ar under the same conditions. The dotted line shows results in the presence of 0.05 MPa H2, which only posed marginal effects.28 This red-shift exhibits direct evidence for LA−LB interactions between carbonyl of cyclohexanone and CO2.34 These changes agree with calculation results.

Figure 6. Optimized structures (Wb97XD/6-311++G(d,p)) for the interaction geometries of the cyclohexanone-CO2 complex. Table 2. Geometric parameters of the cyclohexanone complex formed with CO2 for interaction configurations, calculated at the Wb97XD/6-311++G(d,p). (see Figure 6) a 19



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a

Molecular species

A (deg)

C6H10O

-

-

-

CO2

180

1.180

C6H10O-CO2

177.2

1.189

L1 (Å)

L2 (Å)

νas(cm-1)

∆ν

C=O

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∆L

(mÅ)

C=O

-

(cm-1) 0

0

1.180

2464.63

-

-

1.173

2468.28

6.0

2.42

. The abbreviations are used: Angle A is respective angle O=C=O; L1 and L2 are bond

lengths (C=O) of the “free” and “complexed” bonds of CO2, νas: the stretching vibration of free CO2 and C6H10O-CO2. ∆νC=O (represent the change in the frequency of the stretching vibration of cyclohexanone carbonyl group; ∆LC=O is the change in the carbonyl bond length).

Figure 7. a) FTIR spectra of cyclohexanone (CYC).1: liquid; 2: gaseous; 3: in 0.05 MPa CO2; 4: in 0.1 MPa CO2; 5: in 0.2 MPa CO2; 6: in 0.05 MPa H2 and 0.05 MPa CO2 (dotted line); 7: in 0.05 MPa Ar; 8: in 0.1 MPa Ar; 9: in 0.2 MPa Ar. Spectra 2−9 were measured at 373 K expect 1at 298 K. b) FTIR spectra of pure CO2 and mixture of CO2 and cyclohexanone. 1−2: in 0.05 MPa CO2; 3−5: in 0.05 MPa, 0.1 MPa and 0.2 MPa CO2 with cyclohexanone, respectively; spectra 2−5 were measured at 373 K expect 1 at 298 K.

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Stability of catalysts Recycling experiments were performed for five runs to investigate stability of catalysts in the presence and absence of low-pressure CO2. Comparing with absence of low-pressure CO2 conditions (Figure 8a), the as-prepared Pd/γ-Al2O3 catalyst was highly reused and the conversion of phenol (> 98%) did not drop significantly with the selectivity to cyclohexanone unchanged (98%) after five runs in low-pressure CO2 (Figure 8b). For commercial Pd/γ-Al2O3 and Pd/C catalysts, the presence of low-pressure CO2 compared with the absence of the same (Figures 8c and 8e) also presented high selectivity toward cyclohexanone even when catalytic activity gradually decreased (Figures 8d and 8f), respectively. In the present work, changes in morphology of the as-prepared Pd/γ-Al2O3 were observed by TEM after catalytic cycles. In the above reaction medium, average Pd NPs sizes (2.5 nm) did not change after recycling (Figure S6); this result agrees with that for fresh samples (2.4 nm) as shown in Figure 2. Thus, aggregation of Pd NPs could be neglected in both of reaction mediums. Since the leaching of active component of supported catalyst usually relates with deactivation. The possibility of Pd leaching during reactions in the presence and absence of low-pressure CO2 was also analyzed by ICP−MS. After the first to fifth runs of reaction, Pd was not detected in both of filtrate due to lower concentration than detection limit (0.1 ppb). This finding indicates that leaching of Pd was also not the main factor for the decreased catalytic selectivity in the absence of low-pressure CO2. Whereafter, XPS was used to investigate electronic state of Pd on the surface of 21



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used Pd/γ-Al2O3 catalysts, and results are shown in Figure S9. According to corresponding characteristics of Pd0 and Pd2+ binding energy,51 atomic ratios of Pd2+/Pd0 reached 0.49 and 0.51 for fifth usage of Pd/γ-Al2O3 in the absence and presence of low-pressure CO2, respectively. Moreover, no significant change was observed in texture of used Pd catalysts (entries 3 and 4 in Table 1, Figure S8a). In conclusion, there are no distinct differences of used catalyst between H2 and H2-CO2 reaction medium. As for a significant drop in selectivity during recycling was conducted without CO2, it was speculated that slight change in the physical properties of Pd/γ-Al2O3 catalyst surface during recycling might lead cyclohexanone to leave difficultly from the catalyst surface, and then cyclohexanone was further hydrogenation. Thus, the presence of low-pressure CO2 should be main factor for remained selectivity of cyclohexanone after successive uses. For commercial Pd/C-com. and Pd/γ-Al2O3-com. catalysts, changes in morphology and structure of the catalysts were observed by TEM and BET test after catalytic cycles, the results were presented in Figure S7, Figures S8 b and c, respectively. The Pd NPs of two commercial Pd catalysts obviously aggregated, and leads to average particle size has increased. After recycling, the BET surface areas of the Pd/C-com. and Pd/γ-Al2O3-com. are both decreased nearly 50%, it might result from the damage to the pore structure. The Pd catalysts after fifth run were analyzed by ICP-MS and proved that the leach of Pd was maximal (detail data in Table S5). It can be concluded that an obvious drop in activity and selectivity for commercial Pd catalysts during recycling is attributed to the block of pores, aggregation and leaching of Pd.

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Figure 8. a), c), and e) Catalytic cycle were performed in the presence of H2 over Pd/γ-Al2O3, Pd/γ-Al2O3-com., Pd/C-com., respectively. b), d), and f). Catalytic cycles were performed in the presence of CO2 over Pd/γ-Al2O3, Pd/γ-Al2O3-com., Pd/C-com., respectively. Reaction conditions: (see Table S1).

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Possible mechanism Based on the above experimental results and theoretical calculations, we summarized the most possible reaction mechanism for hydrogenation of phenol over Pd catalysts in Scheme 2. Hydrogen molecules were initially activated by Pd species. Simultaneously, phenol molecules were easily absorbed on γ-Al2O3 or carbon support by formation of H-bridge19, and aromatic ring of phenol favored adsorption through mixed σ-π interaction on Pd surface, with hydrogen atoms tilting upward on Pd surface (Scheme 2a).52-53 Then, adsorbed phenol was hydrogenated to cyclohexanone by the attack of two dissociated hydrogen atoms (this process also involves partial hydrogenation of phenol to cyclohexenol and its rapid isomerization to cyclohexanone).24 As cyclohexanone left the Pd surface, LA−LB interaction occurred between CO2 and cyclohexanone carbonyl (Scheme 2b), and a cooperative C−H…O interaction transpired with hydrogen atoms attached to the α-carbon atom (this interaction must be weaker due to steric hindrance from cyclohexanone ring).27 These two interactions promoted the cyclohexanone to leave rapidly from the Pd surface to avoid excess hydrogenation of cyclohexanone (Scheme 2c). Finally, cyclohexanone is replaced by new phenol molecules with higher adsorption capacity.54

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Scheme 2. Proposed Mechanism for Phenol Hydrogenation in Low-pressure CO2 over Pd Catalysts.

CONCLUSIONS In the present work, enhanced selectivity of cyclohexanone (99%) and conversion (99%) in a low-pressure CO2 and H2O medium was achieved over as-prepared Pd/γ-Al2O3 at mild conditions. The as-prepared Pd/γ-Al2O3 catalyst exhibited excellent stability and was used of five times without losing its activity and product selectivity in low-pressure CO2. Moreover, the enhancement of selectivity to cyclohexanone in low-pressure CO2 can be also achieved by using commercial Pd/γ-Al2O3 and Pd/C catalysts. The possible mechanism of enhanced selectivity to cyclohexanone is the cooperative C−H…O interaction along with LA−LB interaction

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between CO2 and cyclohexanone through a hydrogen atom attached to the α-carbon atom, this interaction can prevent further hydrogenation of cyclohexanone. Our present work may provide an effective strategy for practical production of fine chemical products in low-pressure CO2 medium.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX. Table for showing component details, TEM images, XPS spectra, in situ FTIR spectra, particle-size distribution of the Pd catalysts, additional spectra of gas chromatogram, and H2 pulse chemisorption profiles.

AUTHOR INFORMATION

Corresponding Authors *E-mail:[email protected].

Fax: +86-571-88871037. Tel: +86-571-88871037.

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS

This work was supported by National Natural Science Foundation of China (NSFC-21625604 and 21671172).

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For Table of Contents Use Only TOC graphic:

Synopsis: The Pd/γ-Al2O3 catalysts have 99% conversion and 99% selectivity for hydrogenation of phenol to cyclohexanone under the medium of low-pressure CO2 (0.05–0.2 MPa) and H2O at 373 K.

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Enhanced Selectivity of Phenol Hydrogenation in Low-Pressure CO2

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