Hydrogenation of Phenol to Cyclohexanone in Aqueous Phase on WO3 Modified Ni/ZrO2 Catalyst

Ni/ZrO2 (Ni/Zr) and Ni/WO3-ZrO2 (Ni/xWZr, x denotes WO3/ZrO2 mass ratios) were prepared by the impregnation-direct reduction method and tested for the aqueous phase hydrogenation of phenol to cyclohexanone in an autoclave reactor. It has been found that the Ni-W alloy forms in Ni/xWZr, and a charge transfer occurs from Ni to W. The presence of W species promotes the Ni dispersion and increases the amounts of acid sites and spilt-over hydrogen species. This leads to higher hydrogenation and direct deoxygenation activity of Ni/xWZr than that of Ni/Zr. In addition, the W6+ and W4+ species, acted as Lewis acidic sites, may stabilize cyclohexanone and the hydrogenation of cyclohexanone to cyclohexanol is inhibited. Under suitable condition, the phenol conversion and the cyclohexanone selectivity reach 93.1 and 90.6% on Ni/0.8WZr, respectively.


Introduction
The utilization of biomass has caught great attention due to its renewability.Lignocellulose is abundant on earth, and it can be converted to bio-oil through thermal cracking and liquefaction.Lignin-based bio-oil contains amounts of phenolic compounds (such as anisole, phenol, guaiacol) [1].However, bio-oil contains amounts of water (20 -30 wt%) [2][3][4], and it is high cost and difficult to separate phenolic compounds from water.One feasible strategy is to convert the phenolic compounds to chemicals and fuels via hydrogenation in aqueous phase.
Cyclohexanone is important intermediate in synthesis of nylon 6 and nylon 66 and also widely used as industrial solvents.One of its production processes is the selective hydrogenation of phenol.To achieve this, noble metals (Pd [5], Pt [5], Ru [6]) and non-noble metals (Ni and Co) catalysts have been explored [7,8].Usually, Pd-based catalysts show good performance even at mild condition.For instance, Pd/C gives the phenol conversion of 96% and the cyclohexanone selectivity of 95% at 80 °C and 0.1 MPa H 2 with hexane as solvent [9].Pd@HMSNs efficiently catalyze hydrogenation of phenol and m-cresol to cyclohexanone derivatives with ≥ 98.3% selectivity at ≥ 99.0% conversions at 55 °C, ambient H 2 pressure and water media [10].A nearly full conversion of phenol with high selectivity (> 99.0%) to cyclohexanone can be achieved on TiO 2 nanowires supported Pd at 50 °C and 5.0 bar H 2 in water [11] Although noble metal catalysts possess high activity, the high-cost limits the wide industrial application.Low-cost metallic Ni is very active for hydrogenation and very attractive for the industrial application.However, the Ni-based catalysts has not been heavily investigated for the hydrogenation of phenol to cyclohexanone [12,13].In addition, over-hydrogenation on metallic Ni gives rise to the conversion of cyclohexanone to cyclohexanol [14].Therefore, it is significant to develop the nickelbased catalysts with high activity and selectivity under mild condition.
It has been found that the property (e.g., oxygen defects and acidic sites) of the supports plays an important role in the selective hydrogenation of phenol.It has been reported that, on TiO 2 nanowires supported Pd catalysts, Pd sites adsorb and activate hydrogen, while the Lewis acidic sites due to Ti 4+ species participate in the activation of phenol, and Ti 3+ and O vacancies (the Lewis basic sites) are preferable for the desorption of cyclohexanone [15].ZrO 2 is reducible and high hydrothermally stable.Its surface can be reduced to Zr 3+ , especially when contacting with metals.The activated hydrogen species on metals spill over to ZrO 2 surface.The reduction of ZrO 2 surface generates oxygen vacancies, and the synergistic effect between metal sites and oxygen vacancies can promote hydrogenation [16].It has been found that WO 3 can provide Lewis acid centers, and the enhancement of Lewis acidity is conducive to generating more oxygen vacancies and improving the hydrogenation of phenols [17].Interestingly, Lewis acidic sites can stabilize cyclohexanone and hamper its over-hydrogenation [18].Base the above, it is worth investigating the performance of WO 3 -ZrO 2 supported Ni for hydrogenation of phenol to cyclohexanone.
In this work, Ni/ZrO 2 and Ni/WO 3 -ZrO 2 was prepared, and the effect of W content on the catalyst structure and performance for hydrogenation of phenol in aqueous phase were investigated.It has been found that the presence of WO 3 enhances the conversion of phenol and the selectivity to cyclohexanone, which reach 93.1 and 90.6% on Ni/ WO 3 -ZrO 2 , respectively.The relationship between catalyst structure and reactivity has been explored.

Catalyst Preparation
ZrO 2 supported Ni (Ni/Zr) and WO 3 -ZrO 2 (WZr) supported Ni (Ni/xWZr, x denotes the WO 3 /ZrO 2 mass ratio) were prepared by the incipient impregnation-reduction method.Zr(OH) 4 was calcined at 500 °C for 3 h to give ZrO 2 .WZr was prepared by incipiently impregnating Zr(OH) 4 with an aqueous solution of (NH 4 ) 6 W 7 O 24 •6H 2 O, followed by drying at 120 °C for 12 h and calcination at 500 °C for 3 h.ZrO 2 and xWZr were incipiently impregnated with the aqueous solution of Ni(NO 3 ) 2 .After drying at 120 °C for 12 h, the resulting samples were reduced by a H 2 flow at 500 °C for 2 h to get Ni/Zr and Ni/xWZr, which were passivated by 0.5% O 2 / N 2 at room temperature for 4 h before exposure to air.In the as-prepared catalysts, the nominal Ni contents were all set as 10 wt%.In the Ni/Zr catalyst, the nominal Ni/Zr atomic ratio is 0.23/1.In the Ni/0.38WZr and Ni/0.8WZr catalysts, the Ni/Zr/W atomic ratio are 0.31/0.19/1and 0.41/0.41/1,respectively.The dried sample was also directly performed for H 2 -TPR.

Catalyst Characterization
H 2 -TPR was performed on a home-made apparatus.50 mg catalyst precursor dried at 120 °C was loaded in a quartz U-tube reactor (4 mm in diameter) and reduced by a 10 vol% H 2 /N 2 flow (60 mL/min) at a heating rate of 10 °C/ min.A cold trap containing silica gel was set between the reactor and the thermal conductivity detector (TCD) to remove water.The hydrogen consumption was recorded by TCD.The H 2 consumption was calculated using CuO as external standard.
X-ray diffraction (XRD) was recorded on a Bruker D8 Focus polycrystalline powder diffractometer.X-ray photoelectron spectroscopy (XPS) was carried out on a PHI 5000 VersaProbe with Al Kα radiation (1486.6 eV).Before the XPS test, the sample was treated with Ar + sputtering.Binding energies were calibrated by adventitious carbon (C1s at 284.8 eV).NH 3 -TPD and H 2 -TPD were carried out on a TPDRO instrument (TP-5080, Tianjin Xianquan Co., Ltd.).N 2 sorption was carried out on a Quantachrom Quadra Sorb SI type physical adsorption instrument.

Catalytic Test
The performance of Ni/Zr and Ni/xWZr catalysts in the aqueous phase hydrogenation of phenol was evaluated on an autoclave (100 mL, Beijing Century Senlong Experimental Apparatus Co., Ltd).6 g of deionized water, 5 g of phenol (0.053 mol), and 0.5 g of catalyst were loaded into the reactor.The sealed reactor was purged with 4.0 MPa N 2 three times to replace air and then filled with 1.0 MPa N 2 (the internal standard of the gas phase product) and H 2 to the designated pressure.And then the reactor was heated to the reaction temperature under the stirring of 500 rpm.After the reaction, the reactor was cooled to about 50 °C, and the gaseous product was collected with an air bag.And n-octane was used to extract organic compounds in the liquid products, which was qualitatively analyzed on a SP-3420 gas chromatography (GC) equipped with a flame ionization detector (FID) and a DB-1 capillary column (60 m × 0.32 mm × 3.0 μm), and ethylbenzene was adopted as an internal standard.
The phenol conversion (X) and the selectivity to product i containing 6 carbon atoms were calculated as follows.
where n 0 and n denote the moles of initial and unreacted phenol, respectively; n i denotes the mole of product i.

H 2 -TPR
Figure 1 shows the H 2 -TPR profiles of the Ni/Zr and Ni/xWZr samples dried at 120 °C.For all the samples, the strong peak appeared between 200 − 350 °C involves the decomposition of Ni(NO 3 ) 2 •Ni(OH) 2 •2H 2 O to NiO and the following reduction of NiO to Ni [19][20][21][22].This is confirmed by the H 2 consumption.The actual H 2 consumptions are about 2.4 × 10 −3 mol/g for Ni/Zr and Ni/xWZr, higher than the nominal ones (1.7 × 10 −3 mol/g) due to NiO.For Ni/xWZr, the peaks at 600 -700 and 750 -900 °C are attributed to the reductions of WO 3 to WO x and WO x to W [20][21][22], respectively.Compared with that on Ni/Zr, the decomposition of Ni(NO 3 ) 2 •Ni(OH) 2 •2H 2 O and the reduction of NiO occur at higher temperatures on Ni/xWZr, which is more obvious with higher WO 3 content.This indicate that there is an interaction between WO 3 and Ni species.

XRD
Figure 2 shows the XRD patterns of the supports and their supported Ni catalysts after reduction at 500 °C.For all the samples, the diffraction peaks at 2θ = 30.27 crystal plane (PDF#37-1484), respectively.Clearly, all the m-ZrO 2 and t-ZrO 2 phases exist when metallic Ni and WO 3 are supported, while the addition of WO 3 to ZrO 2 decreases m-ZrO 2 and increases t-ZrO 2 , reflected by the increased relative peak strength of m-ZrO 2 and t-ZrO 2 .For Ni/Zr and Ni/xWZr, the diffraction peaks at 2θ = 44.5, 51.9 and 76.4° are attributed to metallic Ni (111), ( 200) and (220) crystal planes (PDF#04-0850).Based on Ni (111) crystal plane, the metallic Ni crystallites are estimated as 20, 17 and 6 nm in Ni/Zr, Ni/0.38WZr and Ni/0.8WZr, respectively.That is, the presence of WO 3 promotes the dispersion of Ni, attributed to the interaction between Ni and W species as indicated by H 2 -TPR.WO 3 , whose diffraction peaks are at 2θ = 23.1, 23.6, 24.4,28.4,33.6 and 41.9° (PDF#32-1395), exists in xWZr, and they are more obvious for 0.8WZr with higher WO 3 content.Therefore, the dispersion of WO 3 is reduced with increasing WO 3 amount.No obvious diffraction peaks due to WO 3 phase are visible for Ni/0.38WZr and Ni/0.8WZr, while obvious diffraction peaks due to metal W appear at 2θ = 40.3,58.3 and 73.2° (attributed to W (110), ( 200) and (211) crystal planes (PDF#04-0806), respectively).Different from Ni/0.38WZr, Ni/0.8WZr also contains WO 2 phase (2θ = 25.8, 36.8, and 52.9°corresponding to (002, 011 and − 220) crystal planes (PDF#32-1293) respectively).Therefore, WO 3 are reduced to W and WO 2 at 500 °C.According to H 2 -TPR results (Fig. 1), WO 3 cannot be reduced at 500 °C, while its reduction can be accelerated by the spilt-over hydrogen from the metallic Ni [23].In addition, with the increase of WO 3 content, the diffraction peak due to Ni (111) shifts to low angle, while the diffraction peak due to W(110) shifts to high angle.This indicates that W or Ni atoms respectively enter into the metallic Ni or W lattice forming Ni-W alloy [24].Table 1 shows the atomic ratios of different elements on the catalyst surface.With the increase of W content, the surface Ni/Zr and Zr 3+ /Zr 4+ atomic ratios increase.The former further indicates that the addition of W is beneficial to the dispersion of Ni on the catalyst surface, consistent with the XRD results (Fig. 2), while the latter means that more oxygen vacancies are generated.Reasonably, high Ni dispersion can enhance the contact interface area between Ni and ZrO 2 , which is beneficial to the reduction of ZrO 2 and create more oxygen vacancies.In addition, the proportions of W 0 and W 4+ species are higher on Ni/0.8WZr than on Ni/0.38WZr.This is also related to higher Ni dispersion on Ni/0.8WZr.

NH 3 -TPD
Figure 4 shows the NH 3 -TPD curves of Ni/Zr and Ni/xWZr.All the catalysts give a broad NH 3 desorption peak at 100 -400 °C, indicating that there are weak and medium strong acid sites on the catalyst surface [35,36].The acid amounts of Ni/Zr, Ni-0.38WZr and Ni-0.8WZr are 182, 213 and 681 µmol/g, respectively (Table 2).Clearly, the presence of W species generates new acid sites, which increase with increasing the WO 3 content.The new acid sites are related to W 2+ and W 4+ (Lewis acidic sites) and W-OH groups (Brönsted acidic sites) [37].

H 2 -TPD
Figure 5 presents the H 2 -TPD profiles of Ni/Zr and Ni/ xWZr.Usually, the desorption peak below 400 °C is ascribed the desorption Ni-H species, while that above 400 °C is attributed to the desorption of spilt-over hydrogen on the support [19,38].With the increase of W content, the desorption peak area of Ni-H species increases.This is due to the increased Ni dispersion as indicated by XRD and XPS results (Fig. 2; Table 2).There is no obvious spilt-over hydrogen desorption peak on Ni/Zr, while there is a large amount of spilt-over hydrogen on Ni-xWZr.
The existence of spilt-over hydrogen is related to W-OH group [39].

N 2 Sorption
Figure 6 shows the N 2 adsorption-desorption isotherms of Ni/Zr and Ni/xWZr.All catalysts show obvious type IV isotherms and H 2 type hysteresis loops, indicating that all catalysts contained mesopores.As indicated in Table 2, the BET specific surface areas of Ni/Zr, Ni/0.38WZr and Ni/0.8WZr are 43.8,56.6, and 59.7 m 2 /g, respectively.The introduction of WO 3 enhances the surface areas of catalyst, while has no obvious effect on the pore volume and average pore diameter.

Performance of Ni/Zr and Ni/xWZr
As shown in Fig. 7, the phenol conversion on Ni/Zr is 84.9%, and the selectivity to cyclohexanone and cyclohexanol are 25.9 and 34.3% respectively.The total selectivity to benzene, cyclohexene and cyclohexane is below 1%.In contrast, As indicated above, the introduction of WO 3 promotes the conversion of phenol, and obviously enhances the selectivities to cyclohexanone and benzene and reduces the selectivity to cyclohexanol.This is more remarkable for Ni/0.8WZr containing more amount of WO 3 .First, as indicated by XRD, XPS and H 2 -TPD, the presence of W species is conducive to the dispersion of Ni, and it is reasonable that more exposed Ni sites promote the conversion of phenol.In addition, the spilt-over hydrogen can also provide secondary active sites for the hydrogenation.Second, XPS results show that the presence of WO x gives rise to more oxygen vacancies (i.e., higher Zr 3+ /Zr 4+ ).Consequently, cyclohexanone is easily desorption and its over-hydrogenation is suppressed [15].Third, XRD and XPS demonstrates that Ni-W alloy forms in Ni/xWZr, while Ni-W alloy may promote the direct deoxygenation (DDO) of phenol to benzene [40].This may be due to the synergy between Ni and W. Finally, the increased amounts of acidic sites slightly promote the dehydration of cyclohexanol, which may be suppressed by water solvent.To confirm this, the hydrogenation of phenol on Ni/0.8WZr in water and n-octane is compared (Fig. 8).Interestingly, the conversion of phenol (96.2%) in water is higher than that

X conversion
Fig. 8 Hydrogenation of phenol on Ni/0.8WZr in water and n-octane Reaction condition: 250 °C, 3 MPa, 6 g water or n-octane, 5 g phenol, 0.5 g catalyst, 60 min (74%) in n-octane solvent.This indicates that water may be involved in the reaction via hydrogen exchange, which accelerates the hydrogenation [13].The selectivity to cyclohexanone and cyclohexanol (50.3 and 25.9%, respectively) in water are also higher than those (42.3 and 18.2%, respectively) in n-octane, while the selectivity to cyclohexene and cyclohexane (1.19 and 1.27%, respectively) in water is lower than those (4.3 and 13.2%, respectively) in n-octane.Therefore, water as solvent is unbeneficial to dehydration of cyclohexanol.

Effect of Reaction Conditions on Performance of Ni/0.8WZr
Figure 9 shows the effect of reaction temperature on performance of Ni/0.8WZr.With raising the temperature from 150 to 350 °C, the phenol conversion first increases and then decreases, and reaches − 96.4% at 200 and 250 °C.From the viewpoint of kinetics, increasing the reaction temperature accelerates the reaction rate.However, the phenol hydrogenation and hydrodeoxygenation are both exothermic, too high temperature is unfavorable for the conversion of phenol due to the thermodynamic equilibrium limit.Therefore, there is a suitable temperature with high phenol conversion.With raising temperature, the main products are cyclohexanone and cyclohexanol, and their selectivities tend to decrease, while the selectivities to benzene, cyclohexane and cyclohexene increase.At 200 °C, the selectivity to cyclohexanone is the highest (67.3%) and that of cyclohexanol is 21.4%.Particularly, at 350 °C, the main product is cyclohexane with the selectivity of − 50.1%, and the next one is cyclohexene with the selectivity of 17%.This indicates that raising temperature promotes the hydrogenation of cyclohexanone to cyclohexanol, and then cyclohexanol undergoes dehydration-hydrogenation to produce to cyclohexane.
Figure 10 shows the effect of H 2 pressure on performance of Ni/0.8WZr at 200 °C.With increasing the H 2 pressure from 1.5 to 4.0 MPa, the conversion of phenol is all above 95%, the selectivity to cyclohexanone decreases from 85.3 to 15.4%, while the selectivity to cyclohexanol increases from 7.1 to 73.8%.In addition, the selectivity to benzene maintains − 1%, and there are trace other products.Thus, the increased H 2 pressure favors the hydrogenation of cyclohexanone to cyclohexanol.
Figure 11 shows the performance of Ni/0.8WZr at different reaction time.With prolonging the reaction time from 30 to 150 min, the conversion of phenol increases from 93.1 to 97.5%, while the selectivity to cyclohexanone decreases from 90.6 to 52.5%, and the selectivity to cyclohexanol increases from 6.48 to 34.5%.In addition, the selectivity to cyclohexane increases from 0.17 to 8.2%.Clearly, prolonging reaction time leads to the hydrogenation of cyclohexanone to cyclohexanol, followed by the dehydration-hydrogenation to produce cyclohexane.In addition, the selectivity of benzene is always below 2.0%.Based on above, the conversion of phenol on Ni/0.8ZrW mainly undergoes following pathway: phenol → cyclohexanone → cyclohexanol → cyclohexene → cyclohexane.The direct deoxygenation of phenol to benzene is very minor.Based above, under the condition of 200 °C, 1.5 MPa and reaction for 30 min, the Ni/0.8WZrcatalyst gives the best performance with the phenol conversion and cyclohexanone selectivity of 93.1 and 90.6%, respectively.

Conclusions
Compared with Ni-Zr, Ni-xWZr exhibits higher activity and selectivity of cyclohexanone and benzene in the hydrogenation of phenol, which is mainly due to the Ni-W alloy formed in Ni-xWZr.The addition of W could improve the dispersion of Ni and increase the amounts of acid sites and spilt-over hydrogen species.Additionally, acted as the Lewis acid sites, the W 6+ and W 4+ species might stabilize cyclohexanone and the hydrogenation of cyclohexanone to cyclohexanol is suppressed.Under suitable reaction conditions (200 °C, 1.5 MPa and reaction for 30 min), the phenol conversion and cyclohexanone selectivity on Ni-0.8WZr are 93.1 and 90.6%, respectively, that is, the cyclohexanone yield reaches 84.3%.The hydrogenation of phenol in aqueous phase on Ni-Zr and Ni-xWZr catalysts mainly generates cyclohexanone and cyclohexanol through the HYD route.

Table 1
Surface atomic ratio of Ni/Zr and Ni/xWZr determined by XPS

Table 2
Properties of Ni/Zr and Ni/xWZr