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Highly stable Nb2O5-Al2O3 composites supported Pt catalysts for hydrodeoxygenation of diphenyl ether Weixiang Guan, Xiao Chen, Shaohua Jin, Chuang Li, Chi-Wing Tsang, and Changhai Liang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03736 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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Industrial & Engineering Chemistry Research

Highly stable Nb2O5-Al2O3 composites supported Pt catalysts for hydrodeoxygenation of diphenyl ether Weixiang Guan, Xiao Chen, Shaohua Jin, Chuang Li, Chi-Wing Tsang, Changhai Liang*

Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116023, China

* To whom correspondence should be addressed: Fax: + 86-411-84986353; E-mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT Various TiO2, SiO2, Al2O3, and Nb2O5-Al2O3 supported Pt catalysts had been prepared by urea precipitation method for catalytic hydrodeoxygenation (HDO) of diphenyl ether (DPE) as a 4-O-5 aryl-ether lignin model compound. The selectivity toward deoxygenated product cyclohexane increased obviously with Nb2O5·nH2O decorated, owing to the significant promotion effect of NbOx species and acid sites on C-O bond cleavage. At higher pressure (3.0 MPa H2), DPE underwent a HYD route, while direct hydrogenolysis route occurred at low pressure (0.1 MPa H2). In addition, the reaction rate constants and activation energies were obtained in the temperature range from 160 to 220 °C. Based on the Arrhenius law, the activation energy for the cleavage of the C-O bond in DPE was calculated to be 91.22 kJ/mol. It was noteworthy

that

the

Pt/20Nb-Al2O3

showed

higher

stability

than

Pt/Al2O3

for

hydrodeoxygenation of diphenyl ether, which can be attributed to its water-tolerant Lewis acid sites. Keywords: Nb2O5-Al2O3; Pt catalysts; Diphenyl ether, Hydrodeoxygenation, Kinetic.

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1. INTRODUCTION As the fossil fuel being continuously depleted, searching for alternative renewable energy sources has been very active for quite some time. Bio-oil which is produced by pyrolysis or liquefaction of lignocellulosic biomass is considered as a promising and sustainable energy source1. However, it is infeasible to direct utilize bio-oil as a transportation fuel due to various drawbacks such as high viscosity, low heating value, thermal instability, incompatibility with conventional fuels, and being corrosive to machines2-5. Due to the high oxygen content, phenolic bio-oil, consisting of phenolic molecules like phenol, guaiacol, syringol, and their derivatives requires upgrading to reduce the oxygen content and produce high value-added chemicals and alkane fuels6. Catalytic hydrodeoxygenation (HDO) of lignin-derived compounds is considered as the most effective strategy to upgrade bio-oil and produce transportation fuels and chemicals1, 7-8. Since the lignin structure is complex and variable, extensive research and development efforts have been focused on the HDO of lignin model compounds such as phenol, anisole and guaiacol6, 9-10

. Diphenyl ether (DPE) is a typical representative of the 4-O-5 linkage model compound with

a bond dissociation energy (BDE) of 330 kJ/mol, which is widely used to characterize the activity for newly developed catalytic materials11. Considerable efforts have been devoted to removing the oxygen and converting lignin into useful chemicals and high energy fuels using different catalysts. Sulfided CoMo catalysts had been employed in catalytic HDO of lignin-derived12. However, it is reported that the low content of sulfur (＜0.1 wt%) in the bio-oil makes the unstable structure of the sulfide catalyst 3. -3-



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Moreover, the obtained products are easily contaminated by sulfide catalysts during HDO process. Nickel-based catalysts have been widely studied in HDO of DPE, such as Ni/SiO2 13, Ni/ZrO2, Ni/Al2O3, Ni/Al2O3-KF, Ni/SBA-15, and Ni/Al-SBA-1514. In addition, bimetallic MMo (M=Fe, Co, Ni, Pt, Re, Rh etc.) catalysts were utilized in the hydrotreating of DPE15. Nevertheless, the activity of these transition metal catalysts is rather low, thereby, high content of metal and high temperature were required for the HDO process9. Noble metals are well known to be active at lower temperature, thus possibly to resist coke formation and prevent deactivation2. So far a wide range of noble metal catalysts such as Ru/SZ16, Ru/H-Beta9, Rh/H-Beta9, Pt/C17, Pd NPs18 et al. have been used to conduct HDO of DPE. Besides, Pt was found to provide superior deoxygenation ability and stability, compared with Pd, Ru and Rh19. Formation of water during the deoxygenation leads to the development of water-tolerant solid catalysts in the upgrading of lignin-derived compounds. Niobic acid (Nb2O5·nH2O), as a potential solid Lewis acid with both stability and activity in water, is an amorphous metal oxide and mainly composed of distorted octahedra NbO6 and tetrahedra NbO4 units20. Lewis acidity has been found in all of the supported niobium oxide systems, while Brønsted acid sites have only been detected in niobia supported on alumina and silica21. Until now, niobic acid (Nb2O5·nH2O) has been studied as a Lewis acid catalyst for the dehydration of glucose to 5-HMF in water20. Foo and co-workers examined the dehydration of glycerol over niobic acid catalysts22. Sugi et al. have reported that niobic acid exhibited the highest activity among zeolites and ion-exchange resins in the hydrolysis of phenyloxirane in water23. Barrios discussed the HDO of phenol over Nb2O5·H2O supported Pd catalyst24. Recently, Ru/Nb2O5 was employed in -4-


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upgrading lignin because of high durability and high selectivity to arene25. Therefore, Nb2O5·H2O with a good thermal stability has the potential applications in biomass conversion. Herein, the activity of HDO of DPE over TiO2, SiO2 and γ-Al2O3 supported Pt catalysts have been investigated. Afterward, γ-Al2O3 was selected as support and decorated with niobic acid (Nb2O5·nH2O) in order to improve the catalytic activity and deoxygenation ability for the C-O bond cleavage in DPE under the specific conditions. The effects of niobic acid incorporation, temperature and pressure on the HDO reaction were investigated and kinetics analysis was used to further study the reaction mechanism. In addition, the stability of Pt/Nb-Al2O3 catalyst has been investigated thoroughly.

2. Experimental Section 2.1 Catalyst Preparation Niobic acid deposited on Al2O3 was synthesized by ammonia precipitation method with niobium pentachloride as the niobium source. During a typical preparation procedure of 20 wt% Nb2O5·nH2O-Al2O3 (denoted as 20Nb-Al2O3), 1.0 g Al2O3 and certain amounts of niobium pentachloride were added into ethanol with stirring continuously. Then, the ammonia was added dropwise into the white mixture, the mixture was vigorously stirred at 80 °C for 2 h. After the precipitation was completed, the solid was filtered and washed with deionized water repeatedly until the filtrate was neutral. Subsequently, the solid was dried at 80 °C overnight. 0.5 wt% Pt nanoparticles supported on TiO2, SiO2, Al2O3, and 20Nb-Al2O3 catalysts were prepared by urea precipitation method. TiO2, SiO2 and Al2O3 were used as received and -5-



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pelletized to sizes below 80 meshes. In a typical preparation, appropriate amounts of urea was put into the H2PtCl6·6H2O aqueous solution to maintain a pH of 9.5~10.0, then added 1.0 g support and stirred at 95 °C for 24 h. The as-prepared sample was washed with distilled water, dried at 80 °C overnight and then calcined under air flow at 400 °C for 3 h before use.

2.2. Characterization Method The content of Pt was measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES). Nitrogen adsorption–desorption isotherms of the powdered samples were measured at 77 K using an Autosorb iQ automated gas sorption analyzer. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. Prior to the measurements, the samples were outgassed in vacuum at 200 °C for 12 h. XRD patterns of the powdered samples were recorded at room temperature in a D/MAX-2400 diffractometer using a Cu Kα radiation (λ=1.5418 Å), operated at 40 KV and 100 mA. Scanning Electron Microscopy (SEM) was performed by tungsten filament scanning electron microscopy (SEM, Nova NanoSEM 450 from FEI Co.) with X-ray microanalysis was used to obtain morphological observations of the surface of sample in the oxidized state, mapping of the elements was carried out to determine the distribution on the surface of the catalyst. NH3-TPD was used to probe the surface acidity of the samples. The reduced catalysts were degassed in He at 500 °C for 1 h, then NH3 was vaporized at 125 °C and pulsed to the reduced samples (0.1 g) until the samples were saturated. After the saturated catalysts were flushed in He for 40 min, the catalysts were heated to 480 °C at 10 °C/min. Pyridine adsorption-IR was carried out by an EQUIOX-55 Fourier transform infrared spectrometer (Bruker Corp) with a resolution of 4 cm-1. A self–supporting thin -6-


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wafer (15 mm diameter) sample (10 mg) was degassed at 400 °C for 35 min under vacuum, and the IR spectrum was recorded at 50 °C as a background. Pyridine adsorption was carried out at 50 °C for 10 min to reach adsorption equilibrium, and then, the IR spectrum was measured at 150 °C. The acid amount of Brønsted and Lewis acidic sites was calculated based on Emeis1 reported. The formulas were as follows: 1.88 1 1.42 = 2 =

C = concentration (mmol/g-catalyst) I A (B, L) = integrated absorbance of B or L band (cm-1) R= radius of catalyst disk (cm) W= weight of disk (mg)

2.3 Catalyst Evaluation Catalytic HDO of DPE was carried out in a stainless steel fixed-bed continuous flow reactor at various reaction temperatures (160 ~ 300 °C) and pressures (0.1 ~ 3.0 MPa). A selected 0.05 g catalysts diluted in 5 mL quartz (60 ~ 80 meshes) was placed in the middle of the reactor between two pillows of quartz wool and in situ reduced by H2 (40 mL/min, 99.999%) at 300 °C for 2 h before HDO experiments. The liquid reactants are composed of 5.0 wt% DPE in n-decane and 2.0 wt% n-dodecane as an internal standard for gas chromatography analysis, which were pumped into the fixed-bed reactor at the different flow rates (0.05~0.70 mL/min). When the reaction reached steady state, the products were collected at different space time, defined as the ratio between the mass of catalyst (g) and the flow rate of the substrate (g/min). The products -7-



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were analyzed by gas chromatograph (Bruker GC 450, FID, FFAP capillary column 30m×0.32mm×0.25µm) and identified by an Agilent 7890B GC (HP-5, 30m×0.32mm×0.25µm) with 5977A MSD. The carbon balance was checked in each run, and was found to be in the range (100 ± 5) %. The following definitions are used to quantitate experimental data: =

! " #$%&' (' % $#)$') #'!* + ! " #$%&' (' % $#)$') (',*)

0 1 2 342 =

× 100%

! " #$%&' (' ,%+*#) ( ! " #$%&' (' % $#)$') #'!* +

! " #$%&' (' ,%+*#) (

5 1 342 =

! " #$%&' (' % $#)$') (',*)

× 100%

(3) × 100%

(4) (5)

3. Results and Discussion 3.1. Characterization of Catalysts Table 1 summarizes the metal loading, BET specific surface area, pore volume, and average pore diameter of various supported catalysts. For all catalysts, the actual metal loading is almost close to the theoretical value of 0.5 wt%. The specific surface area of the Pt/Al2O3 catalyst slightly decreases with 20 wt% Nb2O5·nH2O decorated, as well as pore volume and pore diameter (N2 adsorption-desorption isotherms and pore size distribution see Figure S1). Figure 1 presents the XRD patterns for studying the crystal phase change of niobic acid species with the increase in calcination temperature from 400 °C to 700 °C under air for 3 h over 20Nb/Al2O3. The TT-phase of Nb2O5 is clearly identified with the peaks at 2θ = 22.6°, 28.3°, 36.8°, 46.3°, 50.7°, and 55.2°, corresponding to lattice plane (001), (180), (200), (181), (002), (380) and the (182) of hexagonal crystalline Nb2O5 (JCPDS 30-0873)26. Meanwhile, the peaks at -8-


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36.8°, 46.3°, and 66.8° can be attributed to Al2O3. When the calcination temperature is beyond 500 °C, the peaks due to Nb2O5 become sharper, indicating that hydrated niobic acid has been transformed from amorphous into crystalline Nb2O5. It is obvious that the hydrated niobic acid exhibits good thermal stability below 500 °C. In addition, it is reported that the acid sites of niobic acid weakened when the calcination temperature is above 500 °C21. Therefore, the calcination temperature for Pt/20Nb-Al2O3 samples was chosen at 400 °C in order to guarantee that Nb2O5·nH2O remains amorphous and acidity. SEM microscopy and corresponding element mapping of O, Al, Pt and Nb for Pt/20Nb-Al2O3 are shown in Figure S2. It can be observed that the catalyst exhibits amorphous structure, and the Pt and Nb elements are uniformly dispersed on the catalyst surface. Nevertheless, almost no Pt nanoparticles is found in TEM and STEM images of sample due to very low content of Pt, which can be proved by ICP-OES. Predicting the activity of deoxygenation based on the acidity indicating that C-O bond cleavage occurs on acid sites1, 22, 27. Thus, the surface acidity of the Al2O3 and the Nb2O5·nH2O modified samples were investigated and characterized by NH3-TPD profiles, as shown in Figure 2(A). It can be seen that two main NH3 desorption peaks centered at 230 °C (identified as a weak acid sites) and 340 °C (identified as a medium acid sites) for 20Nb-Al2O3 while only one broad peak centered at 230 °C for Al2O328. Table 2 lists the uptakes of NH3 per gram of catalysts, which clearly reflects the total acid sites of 20Nb-Al2O3 is more than Al2O3. It is reported that the area of the peaks represented the concentration of acid sites on the catalysts, and the peak temperature point corresponded with the overall acid strength of catalysts29.Hence, the observations indicate -9-



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that the concentration and strength of acidic sites increases as the Nb2O5·nH2O loaded. To gain further information about the Brønsted and Lewis acid sites of Al2O3 and 20Nb-Al2O3 samples, infrared spectrometry was employed by using pyridine as a probe absorption molecule9, as shown in Figure 2(B) and Table 2. The completely infrared spectra of pyridine was also presented in Figure S3. As Mendes et al.

30

reported, the bands appear at

1440~1460 cm-1 are assignable to adsorbed pyridine on Lewis acid sites (LAS), while the band associated with pyridine adsorbed on Brønsted acid sites (BAS) appears at 1535~1550 cm-1. Additionally, the band appears at 1491 cm-1 belongs to Lewis and Brønsted acid sites. Accordingly, only small part of the spectra range was meaningful. Table 2 lists the amount of different acid sites, calculated using the method reported by Emeis31. Apparently, the concentration of the Lewis acid sites was greatly enhanced while the Brønsted acid sites slightly increased, which are unambiguously confirmed by Figure 2(B) and Table 2. This result is well in accordance with the NH3-TPD observation. In addition, Brønsted acid sites are detected in niobia supported on Al2O3, which is attributed to the formation of bridging hydroxyl groups (Nb-OH-Al)30.

3.2 HDO of DPE 3.2.1 Effect of Supports Initial experiments were focused on the investigation of the activity of several supported Pt catalysts for the HDO of DPE. Table 3 shows the conversion and product selectivity over different catalysts. The products from HDO of DPE in all experiments were identified by GC and GC-MS, including cyclohexane (CHN), cyclohexanol (CHL), benzene (BEN), cyclohexene - 10 -


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(CHE), cyclohexyl cyclohexyl ether (CCE), cyclohexyl phenyl ether (CPE) and phenol (PHE). As presented in Table 2, the Pt/Al2O3 catalyst exhibits the highest conversion (99.3%) and cyclohexane selectivity (31.6%). The conversion of DPE and selectivity of CHN decreased in the order Pt/Al2O3＞Pt/TiO2＞Pt/SiO2. This could be due to the stronger acidity of Al2O3 than TiO2 and SiO2. This is consistent with previous results reported by He et al32. Therefore, in the following study, Al2O3 is selected as support and modified with niobic acid to further understand the performance of HDO of DPE. 3.2.2 Effect of niobic acid on C-O bond cleavage To evaluate the HDO reactivity of the niobic acid modified catalyst, Pt/20Nb-Al2O3 was prepared and employed for the HDO of DPE, the results are listed in Table 3. Although the catalytic activity decreases from 99.3% (Pt/Al2O3) to 89.5% (Pt/20Nb-Al2O3), the selectivity to CHN increases from 31.6% to 54% significantly. Table 1 shows that there is a slightly decrease in surface area, pore volume and average pore diameter. Typically, the surface area of the Pt/Al2O3 declined from 261m2/g to 245 m2/g after niobic acid incorporated. It is well established that niobia is a typical SMSI (strong metal-support interaction) oxide, and niobia is considered as a reducible support24, 33. Thus, the decrease in activity for Pt/20Nb-Al2O3 catalyst could be a result from the partial coverage of Pt surface with NbOx species, which was formed under reduction at 300 °C33 instead of changing in physical properties . The high selectivity to CHN observed for Pt/20Nb-Al2O3 catalyst mainly due to of two reasons: (i) the increased amount of acid sites are beneficial to cleaving of the C-O aryl ether, as has been proved by NH3-TPD and Py-IR; (ii) the formation of unique NbOx significantly promotes cleavage the C-O bond in the - 11 -



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CCE, owing to the bond strength of Nb-O that reduces the C-O bond dissociation barriers and thus favors C-O bond cleavage24, 34. This phenomenon is similar to the previous results reported on NbOx species in the HDO and dehydration process24, 34-36. Figure 3 shows the products distribution in HDO of DPE over Pt/Al2O3 and Pt/20Nb-Al2O3 catalysts at 200 °C and 3.0 MPa H2. For these two samples, the first step of DPE HDO reaction in this study probably involved the addition of 3 moles of hydrogen to the single aromatic ring to generate a product of CPE. After that, CPE was further hydrogenated to saturated CCE as the major route. Meanwhile, minor amount of CPE could be converted to BEN and CHL via the hydrogenolysis of CAR-O bond. The produced CCE can undergo further hydrogenolysis reactions to form CHN and CHL with a molar ratio of 1:1. Importantly, the selectivity of CHL has not significantly changed as the function of the W/F increased, indicating that dehydration of CHL occurs during the reaction, as shown in Figure 3. CHN is the target products, which are important high value-added chemicals as well as liquid fuel components. Overall, the major pathway is primary hydrogenation/secondary deoxygenation (HYD) route37. The proposed reaction network for the cleavage of DPE over Pt/20Nb-Al2O3 is depicted in Scheme 1. 3.2.3 Effect of reaction temperature Reaction temperature acts as an essential role in the hydrodeoxygenation process. Herein, the effect of reaction temperature on the hydrodeoxygenation of DPE over the Pt/20Nb-Al2O3 catalyst is evaluated. The results shown in Figure 4 revealed that the thermal effect has a positive impact on the catalytic activity. Typically, DPE conversion increases from 26.9% at 160 °C to 100% at 240 °C. Simultaneously, the selectivity of CHN sharply increases from 16.7% to 99.2%, - 12 -


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while the selectivity of CPE gradually decreases until CPE disappeared. Obviously, the increasing reaction temperature significantly boosted the transformation of DPE and the rate of producing O-free product CHN, which means aromatic hydrogenation process and C-O bond dissociation are sensitive to temperature. The diphenyl ether achieved complete hydrogenation at 220 °C. Further increase in the reaction temperature to 240 °C, the intermediate products CCE and CHL entirely convert into CHN. 3.2.4 Effect of reaction pressure Apart from the reaction temperature, the H2 pressure is another critical factor for HDO of DPE. Figure 5 shows the H2 pressure dependence of conversion and selectivity of main products in HDO of DPE over Pt/20Nb-Al2O3. The conversion of DPE increased continuously from 37.6% to 75.5% with H2 pressure increasing from 0.5 to 3.0 MPa. Moreover, the selectivity of CCE constantly increases with H2 pressure increase, while the selectivity of BEN and PHE decline. These results revealed that H2 pressure plays a crucial role in changing the reaction pathways: direct hydrogenolysis of DPE was achieved at low H2 pressure while aromatic ring first was hydrogenated and then was deoxygenated at high H2 pressure. Since DPE was transformed into BEN via hydrogenolysis of the C-O bond at low pressure, hence, details were discussed following aim to obtain BEN. Much work so far has been focused on producing aromatic hydrocarbons, which are important components for conventional fuel additives, solvents, as starting blocks for plastics, rubber, and fiber, among others38. Generally, aromatic hydrocarbons are obtained at low H2 pressure and high temperature8, 10, 27. As shown in Figure 6 (A), the reaction conducted under 0.1 MPa H2 at different temperatures mainly produced - 13 -



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CHN, BEN, PHE, and minor amount of CPE and CHE. For the conversion of DPE, the selectivity of BEN gradually increased from 8.2% and 22.5% to 49.8% and 63.6% with the temperature increased from 200 °C to 300 °C, respectively. Conversely, the selectivity of CHN and PHE decreased from 39.1% and 30.7% to 23.6% and 12.8%, respectively. The absence of CCE indicates that the DPE double aromatic ring hydrogenation is unrealistic in this case. When the temperature was below 240 °C, a part of DPE went through single aromatic ring hydrogenated, followed by hydrogenolysis of CPE to CHN and PHE. Furthermore, when the temperature was higher than 240 °C, negligible CPE (Yield%＜1%) was obtained, indicating that hydrogenolysis of DPE to BEN and PHE was the only reasonable reaction pathway under this condition. These results clearly suggest that high temperature leads to more BEN. In summary, the route of hydrogenolysis is highly favored at high temperature and low H2 pressure. To figure out the detailed reaction pathway, Pt/20Nb-Al2O3 catalyst was severed to explore the hydrodeoxygenation of DPE at 0.1 MPa and 300 °C. As shown in Figure 6 (B), the major products are CHN, BEN and PHE, while CHL, CHE and CPE are detected as minor products (Denoted as others, yield＜1%). It is observed that DPE was transformed into BEN and PHE according to hydrogenolysis process. And then, the intermediate product PHE undergoes further direct hydrogenolysis of C-O into BEN. Since the selectivity of CHL and CHE is rather low, the direct hydrogenation of PHE should be a minor pathway compared with hydrogenolysis. To clearly understand the reaction pathway, PHE was served as a probe molecule at the same conditions. Indeed, the results agreed with our presumed reaction pathway (See Figure S4). The results are consistent with what Boullosa-Eiras and co-workers reported2. Therefore, a proposed - 14 -


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reaction pathway of hydrodeoxygenation of DPE over Pt/20Nb-Al2O3 at high temperature and low H2 pressure was presented in Scheme 1. Two possible explanations for the high selectivity to BEN over Pt/20Nb-Al2O3 under the studied conditions are as follows: (i) the coverage of benzene molecule decreases as the reaction temperature increases, which significantly suppressed further hydrogenation; (ii) under lower H2 pressure, the adsorption of hydrogen on the catalysts is weaker which lead to lesser number of hydrogen atoms being activated. This is consistent with the observation by Lee et al 39. In summary, the calcination temperature for Pt/20Nb-Al2O3 samples was chosen at 400 °C in order to guarantee that Nb2O5·nH2O remains amorphous and acidity (as confirmed by XRD patterns and SEM images). In addition, the specific surface area of the catalyst decorated with 20 wt% Nb2O5·nH2O has not been changed significantly. Although the conversion of HDO of DPE decreased about 10% after niobic acid modified due to the partial coverage of Pt surface with NbOx species, the deoxygenation ability and stability notably increased because of the formation of water tolerance solid acid Nb2O5·nH2O and the increased amount of acid sites (determined by NH3-TPD and Py-IR results). 3.2.5 Kinetic of HDO of DPE The kinetic analysis of the HDO of DPE was conducted at 160 ~ 220 °C by changing the W/F (1.9 ~ 15.2 min), and Pt/20Nb-Al2O3 was employed in the reaction. In this work, the H2 pressure is invariable and can be considered as a constant. As a result, the only variable is the concentration of DPE. It is widely reported that kinetic in terms of rate constants is pseudo-first-order in the organic reactant27, 40-41. Accordingly, a pseudo-first-order model was - 15 -



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employed for the HDO of DPE to investigate the reaction rate constant and the activation energy of the reaction was studied by the Arrhenius equation. All equations were presented below: ( =

+67 = >

+689:,< ×

= ?( @AB

(6)

( = @AB,C × D(

(7)

@AB = @AB,C 1 − D(

(8)

( =

+ F7 = >

+689:,< ×

= ?( 1 − D( I

−ln1 − D( = ?( @AB,C × J B

ln KL@M = − OPN +

(9) (10) (11)

In the above equations, ki represents the rate of production of each product, CDPE is the concentration of DPE in reaction, CDPE, 0 is the initial concentration of DPE before reaction, Xi represents the DPE conversion, KHDO represents the rate of reaction constant. Data showing the conversion of DPE as a function of W/F in a typical run are presented in Figure 7 (A). The data all fall on the straight line that pass through the origin indicating that the overall reaction is pseudo-first-order. In addition, the slopes of the lines are used to determine the rates of the reactions. As shown in Figure 7 (A), the rates of DPE conversion at 160 ~ 220 °C were found to be 5.49×10-4 , 2.08×10-3, 3.69×10-3 and 6.56×10-3 min-1), respectively, which increased with increasing temperature. Obviously, the increase of temperature promotes the transformation of the reactant DPE. Based on the Arrhenius law, the activation energy for the cleavage of the C-O band in DPE was calculated to be 91.22 kJ/mol for the catalytic system that was used, as determined in Figure 7 (B). This result is close to that calculated by He et al. 13. Owing to the very small amount of BEN (Selectivity ＜5%) and absence of PHE, CHN, - 16 -


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CHL, CCE and CPE were considered as main products. Table 4 illustrated the apparent rate constants for products at different temperatures and were used to determine values of the apparent activation energies, which were calculated according to the Arrhenius law (Eq. 11). Obviously, the rates of generating each product increase with increasing temperature. As calculated in Table 3, the activation energies of each product were 119.9 (CHN), 84.5(CHL), 84.4 (CCE) and 41.0 (CPE) kJ/mol, respectively. Therefore, the lowest activation energy is associated with the reactions leading to the formation of CPE, which also verified that CPE was the primary product. 3.2.6 Stability The stability tests with time on stream were performed over Pt/Al2O3 and Pt/20Nb-Al2O3 catalysts for hydrodeoxygenation of DPE, as shown in Figure 8. The main products were consistently CHN, CHL and CCE, with minor amounts of CPE and negligible quantities of BEN and PHE (selectivity ＜1%). Clearly, the activity of Pt/Al2O3 gradually decreased as a function of time on stream. After 26 h the conversion and the selectivity of CHN had declined ca. 33% and 8%, respectively. However, the Pt/20Nb-Al2O3 catalyst exhibits good stability with only ca. 7% declined in HDO activity and almost no decrease in CHN selectivity. This is attributed to the water-tolerant nature of Nb2O5·nH2O solid Lewis acid catalyst 20. It is confirmed that most of the NbO4 tetrahedra form NbO4-H2O adducts under H2O, and Lewis acid sites is observed even on hydrated Nb2O5 ·nH2O 20. Promisingly, this advantage is conducive to employ Nb2O5·nH2O into direct pyrolysis or hydrolysis of biomass into biodiesel. Hence, the catalytic activity and deoxygenation activity were stable. - 17 -



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4. Conclusion A series of Pt supported on TiO2, SiO2, Al2O3, and 20Nb-Al2O3 catalysts were developed for hydrodeoxygenation of DPE in a fixed bed reactor. The effects of modified Nb2O5·nH2O, reaction temperature and hydrogen pressure on the catalytic performance were investigated. The results shed light on that DPE underwent a primary hydrogenation/secondary deoxygenation route at mild conditions (200 °C, 3.0 MPa H2) over Pt/Nb-Al2O3 catalyst, while direct hydrogenolysis route occurred at high temperature and low pressure (300 °C, 0.1 MPa H2). In addition, the kinetics results demonstrated that the single aromatic hydrogenation is the first step. More importantly, the Pt/20Nb-Al2O3 catalyst greatly improved the reaction stability compared with the Pt/Al2O3 catalyst after the reaction was run for 26 h in the fixed-bed reactor, which is due to the water-tolerant nature of Nb2O5·nH2O.

Acknowledgments We gratefully acknowledge the financial support provided by the National Key Research & Development Program of China (2016YFB0600305).

Supporting Information N2 adsorption/desorption isotherms and pore size distributions for different catalysts; SEM image and elements mapping over Pt/20Nb-Al2O3; Infrared spectra of pyridine adsorbed on Al2O3 and 20Nb-Al2O3; The conversion of PHE, the product distribution, and reaction pathway for the cleavage of PHE over Pt/20Nb-Al2O3. This material is available free of charge via the Internet at http://pubs.acs.org. - 18 -


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References (1) Song, W.; Liu, Y.; Baráth, E.; Zhao, C.; Lercher, J. A., Synergistic effects of Ni and acid sites for hydrogenation and C–O bond cleavage of substituted phenols. Green Chem. 2015, 17, 1204. (2) Boullosa-Eiras, S.; Lødeng, R.; Bergem, H.; Stöcker, M.; Hannevold, L.; Blekkan, E. A., Catalytic hydrodeoxygenation (HDO) of phenol over supported molybdenum carbide, nitride, phosphide and oxide catalysts. Catal. Today 2014, 223, 44. (3) Lu, M.; Zhu, J.; Li, M.; Shan, Y.; He, M.; Song, C., TiO2-Modified Pd/SiO2 for Catalytic Hydrodeoxygenation of Guaiacol. Energy Fuels 2016, 30, 6671. (4) Wang, W.; Wu, K.; Liu, P.; Li, L.; Yang, Y.; Wang, Y., Hydrodeoxygenation of p-Cresol over Pt/Al2O3 Catalyst Promoted by ZrO2, CeO2, and CeO2–ZrO2. Ind. Eng. Chem. Res. 2016, 55, 7598. (5) Wang, W.; Yang, Y.; Luo, H.; Peng, H.; Wang, F., Effect of La on Ni–W–B Amorphous Catalysts in Hydrodeoxygenation of Phenol. Ind. Eng. Chem. Res. 2011, 50, 10936. (6) Zhang, W.; Chen, J.; Liu, R.; Wang, S.; Chen, L.; Li, K., Hydrodeoxygenation of lignin-derived phenolic monomers and dimers to alkane fuels over bifunctional zeolite-supported metal catalysts. ACS Sustainable Chem. Eng. 2014, 2, 683. (7) Feng, J.; Hse, C.-y.; Yang, Z.; Wang, K.; Jiang, J.; Xu, J., Liquid phase in situ hydrodeoxygenation of biomass-derived phenolic compounds to hydrocarbons over bifunctional catalysts. Appl. Catal. A 2017, 542, 163 (8) Prasomsri, T.; Nimmanwudipong, T.; Román-Leshkov, Y., Effective hydrodeoxygenation of biomass-derived oxygenates into unsaturated hydrocarbons by MoO3 using low H2 pressures. - 19 -



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(18) Chen, L.; Xin, J.; Ni, L.; Dong, H.; Yan, D.; Lu, X.; Zhang, S., Conversion of lignin model compounds under mild conditions in pseudo-homogeneous systems. Green Chem. 2016, 18, 2341. (19) Gao, D.; Schweitzer, C.; Hwang, H. T.; Varma, A., Conversion of Guaiacol on Noble Metal Catalysts: Reaction Performance and Deactivation Studies. Ind. Eng. Chem. Res. 2014, 53, 18658. (20) Nakajima, K.; Baba, Y.; Noma, R.; Kitano, M.; Kondo, J. N.; Hayashi, S.; Hara, M., Nb2O5.nH2O as a heterogeneous catalyst with water-tolerant Lewis acid sites. J. Am. Chem. Soc. 2011, 133, 4224. (21) Nowak, I.; Ziolek, M., Niobium Compounds: Preparation, Characterization, and Application in Heterogeneous Catalysis. Chem. Rev. 1999, 99, 3603. (22) Foo, G. S.; Wei, D.; Sholl, D. S.; Sievers, C., Role of Lewis and Brønsted Acid Sites in the Dehydration of Glycerol over Niobia. ACS Catal. 2014, 4, 3180. (23) Hanaoka, T.A.; Takeuchi, K.; Matsuzaki, T.; Sugi, Y., Niobic acid as a solid acid catalyst for ring-opening reactions of phenyloxirane. Catal. Today 1990, 8, 123. (24) Barrios, A. M.; Teles, C. A.; de Souza, P. M.; Rabelo-Neto, R. C.; Jacobs, G.; Davis, B. H.; Borges, L. E. P.; Noronha, F. B., Hydrodeoxygenation of phenol over niobia supported Pd catalyst. Catal. Today 2017, https://doi.org/10.1016/j.cattod.2017.03.034. (25) Shao, Y.; Xia, Q.; Dong, L.; Liu, X.; Han, X.; Parker, S. F.; Cheng, Y.; Daemen, L. L.; Ramirez-Cuesta, A. J.; Yang, S.; Wang, Y., Selective production of arenes via direct lignin upgrading over a niobium-based catalyst. Nat. Commun. 2017, 8, 16104. - 21 -



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Table 1. Textural properties and Pt Loadings of supported Pt catalysts. Pt a

SbBET

Vb

db

(wt %)

(m2/g)

(cm3/g)

(nm)

Pt/TiO2

0.51

37

0.28

30.8

Pt/SiO2

0.44

302

1.44

19.0

Pt/Al2O3

0.41

261

0.45

6.9

Pt/20Nb-Al2O3

0.40

245

0.39

6.4

Catalyst

a

As determined by the ICP-OES. Specific surface area (SBET), pore volume (V) and average pore diameter (d) as determined by N2 adsorption-desorption isotherms at 77 K.

b

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Table 2. Density of the acid sites measured by NH3-TPD and pyridine-FT-IR. Total acidity a

Brønsted acid b

Lewis acid b

mmol/g-catalyst

mmol/g-catalyst

mmol/g-catalyst

Al2O3

0.05568

0

0.08482

20Nb-Al2O3

0.08399

0.00219

0.21095

Sample

a

As determined by the NH3-TPD .

b

As determined by Py-IR method.

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Table 3. Catalytic hydrodeoxygenation of DPE over various supported Pt catalysts at W/F=27.3 min. Catalyst

Conv. (%)

Sel. (%)

Pt/TiO2

93.4

13.6

0.4

3.8

81.5

0.6

Pt/SiO2

74.8

9.0

1.0

8.9

71.5

9.7

Pt/Al2O3

99.3

31.6

11.9

0

56.4

0

Pt/20Nb-Al2O3

89.5

54.0

13.3

0.5

32.2

0

Reaction conditions: 0.05 g catalyst, 200 °C, 3.0 MPa H2, H2/oil=300.

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Table 4. Pseudo-first-order rate constants and apparent activation energies for reactions in the network representing DPE conversion in the presence of H2 and Pt/20Nb-Al2O3 under different temperatures. Product

Cyclohexane (CHN)

Cyclohexanol (CHL)

Cyclohexyl cyclohexyl ether

T, (K)

ki, L /(g of DPE ×min)

433

4.27×10-5

453

1.18×10-4

473

5.10×10

-4

493

2.42×10-3

433

4.85×10-5

453

1.24×10-4

473

3.09×10

-4

493

8.56×10-4

433

4.86×10-5

453

1.38×10-4

473

3.26×10

-4

493

8.71×10-4

433

2.33×10-4

453

4.43×10-4

473

8.17×10

-4

493

8.74×10-4

Ea, ( kJ/mol )

119.9

84.5

84.4 (CCE)

Cyclohexyl phenyl ether (CPE)

Reaction conditions: 0.05 g catalyst, 3.0 MPa H2, H2/oil=300.

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41.0


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Figure and scheme captions Scheme 1 Proposed reaction pathway for hydrodeoxygenation of DPE over Pt/20Nb-Al2O3. Figure 1 X-ray diffraction patterns of 20 wt% Nb2O5·nH2O-Al2O3 calcined at different temperature for 3 h. Figure 2 (A):NH3-TPD profiles for Al2O3 and 20Nb-Al2O3; (B): Infrared spectra of pyridine adsorbed on Al2O3 and 20Nb-Al2O3 (LAS: Lewis acid sites; BAS: Brønsted acid sites) Figure 3 Products distribution of the HDO of DPE over (A) Pt/Al2O3 and (B) Pt/20Nb-Al2O3. Reaction conditions: 0.05 g catalyst, 200 °C, 3.0 MPa H2, H2/oil=300. Figure 4 Effect of temperature on DPE conversion and products selectivity over Pt/20Nb-Al2O3. Reaction conditions: W/F = 15.2 min, 0.05 g catalyst, 3.0 MPa H2, H2/oil=300. Figure 5 Variation of the conversion of HDO of DPE and selectivity toward main products under different H2 pressure. Reaction conditions: W/F = 15.2 min, 0.05 g catalyst, T=200°C, H2/oil=300. Figure 6 (A) Influence of temperature on conversion of DPE over Pt/20Nb-Al2O3 at W/F=15.2 min, 0.1MPa; (B) Variation of the conversion of DPE and the product distributions with W/F over Pt/20Nb-Al2O3 at 0.1MPa, 300°C. Reaction conditions: 0.05g catalyst, 300°C, 0.1MPa, H2/oil=300.) Figure 7 (A) Conversion of DPE at various temperatures; (B) Arrhenius plot recored at various temperatures. Reaction conditions: 0.05 g catalyst, 3.0 MPa H2, H2/oil=300. Figure 8 Effect of time on stream of DPE conversion and products selectivity over (A): Pt/Al2O3 and (B): Pt/20Nb-Al2O3. Reaction conditions: W/F=15.2min, 0.05 g catalyst 200°C, 3.0 MPa H2, H2/oil=300.

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Scheme 1 Proposed reaction pathway for hydrodeoxygenation of DPE over Pt/20Nb-Al2O3.

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Figure 1 X-ray diffraction patterns of 20 wt% Nb2O5·nH2O-Al2O3 calcined at different temperature for 3 h.

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Figure 2 (A):NH3-TPD profiles for Al2O3 and 20Nb-Al2O3; (B): Infrared spectra of pyridine adsorbed on Al2O3 and 20Nb-Al2O3 (LAS: Lewis acid sites; BAS: Brønsted acid sites)

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Figure 3 Products distribution of HDO of DPE over (A) Pt/Al2O3 and (B) Pt/20Nb-Al2O3. Reaction conditions: 0.05 g catalyst, 200 °C, 3.0 MPa H2, H2/oil=300.

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Figure 4 Effect of temperature on DPE conversion and products selectivity over Pt/20Nb-Al2O3. Reaction conditions: W/F = 15.2 min, 0.05 g catalyst, 3.0 MPa H2, H2/oil=300.

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Figure 5 Variation of the conversion of HDO of DPE and selectivity toward main products under different H2 pressure. Reaction conditions: W/F = 15.2 min, 0.05 g catalyst, T=200°C, H2/oil=300.

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Figure 6 (A) Influence of temperature on conversion of DPE over Pt/20Nb-Al2O3 at W/F=15.2 min, 0.1MPa; (B) Variation of the conversion of DPE and the product distributions with W/F over Pt/20Nb-Al2O3 at 0.1MPa, 300°C. Reaction conditions: 0.05g catalyst, 300°C, 0.1MPa, H2/oil=300.

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Figure 7 (A) Conversion of DPE at various temperatures; (B) Arrhenius plot recorded at various temperatures. Reaction conditions: 0.05 g catalyst, 3.0 MPa H2, H2/oil=300.

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Figure 8. Effect of time on stream on DPE conversion and products selectivity over (A): Pt/Al2O3 and (B): Pt/20Nb-Al2O3. Reaction conditions: W/F=15.2min, 0.05 g catalyst 200°C, 3.0 MPa H2, H2/oil=300.

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For Table of Contents Only

Catalytic hydrodeoxygenation of diphenyl ether over highly stable Pt/20Nb-Al2O3 has been investigated in this work. The selectivity of deoxygenation product cyclohexane (CHN) increased significantly with Nb2O5·nH2O incorporated, owing to promotion effect of NbOx species and acid sites on C-O bond cleavage.

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Highly Stable Nb2O - ACS Publications - American

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