HZSM-5 catalysts

Journal of the Energy Institute xxx (xxxx) xxx

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Hydrodeoxygenation of lignin model compounds to alkanes over PdeNi/HZSM-5 catalysts Yun-Peng Zhao a, c, *, Fa-Peng Wu a, Qing-Lu Song a, Xing Fan a, Li-Jun Jin b, Rui-Yu Wang c, Jing-Pei Cao a, **, Xian-Yong Wei a a

Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China University of Mining & Technology, Xuzhou, 221116, Jiangsu, China State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China c Low Carbon Energy Institute, China University of Mining & Technology, Xuzhou, 221008, Jiangsu, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 January 2019 Received in revised form 6 August 2019 Accepted 28 August 2019 Available online xxx

In this work, a bimetallic catalyst of Pd and Ni supported on HZSM-5 was designed and evaluated in the hydrodeoxygenation of model compounds representing various CeO bonds in lignin. The effects of temperature, holding time, and initial H2 pressure on the catalytic performance of the PdeNi/HZSM-5 catalyst were investigated. The results indicated that the PdeNi/HZSM-5 catalyst exhibited superior catalytic activity towards the conversion of the model compounds to alkanes compared to its monometallic counterparts. Direct cleavage of the CeO bond in DPE was achieved under low H2 pressure, mainly producing benzene and minor amounts of phenol and cyclohexane, while at high H2 pressure, the reaction pathway was altered towards hydrogenation of the benzene ring, followed by then cleavage of the CeO bond to afford only cyclohexane. Moreover, the PdeNi/HZSM-5 catalyst showed high activities when applying other substrates including anisole, veratrole, guaiacol, benzyloxybenzene, phenethyl phenyl ether, and dibenzyl ether. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.

Keywords: Hydrodeoxygenation Lignin PdeNi/HZSM-5 CeO bond Alkanes

1. Introduction In view of increasing world energy demand and fast depletion of petroleum reserves, extensive efforts have been dedicated to finding alternative sources, such as coal, oil shale and biomass [1,2]. As the only abundant source of non-fossil-based renewable feedstock, the efficient upgrading of lignocellulosic biomass to fuels and bio-based chemicals has been widely studied in recent years [3e5]. Currently, cellulose and hemicellulose can be efficiently converted to value-added chemicals and biofuels via various technologies, while lignin is relatively intractable. Unlike the two above mentioned components, lignin is the only large-volume renewable feedstock consisting of an aromatic skeleton in nature, which is suitable for producing fuels and aromatic chemicals [6]. Utilization of lignin with high efficiency not only contributes to addressing the petroleum crisis, but also relieves the solid waste disposal problem, as most of them from current biorefinery processes are viewed as waste streams. As one of the three major components of lignocellulosic biomass, lignin makes up 15e30 wt% of its mass and contains approximately 40% of its energy content. Methoxylated phenylpropane units connected by CeO and CeC bonds constitute the aromatic skeleton of lignin [7]. Through thermochemical processes, lignin can be transformed into a liquid mixture product, which is mainly composed of phenolic chemicals and can be used as a potential transportation fuel. However, the product is characterized by a high oxygen content, which leads to poor chemical and thermal stability, high viscosity, low heating value and corrosiveness [8,9]. Therefore, a deoxygenation process is required to solve these disadvantages and obtain alkanes as high-grade fuels.

* Corresponding author. Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China University of Mining & Technology, Xuzhou, 221116, Jiangsu, China. ** Corresponding author. E-mail addresses: [email protected] (Y.-P. Zhao), [email protected] (J.-P. Cao). https://doi.org/10.1016/j.joei.2019.08.002 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.

Please cite this article as: Y.-P. Zhao et al., Hydrodeoxygenation of lignin model compounds to alkanes over PdeNi/HZSM-5 catalysts, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.002

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Among various chemical conversion approaches for oxygen removal, catalytic hydrodeoxygenation (HDO) is a promising option, which has been applied to study lignin and its model compounds in depth, and significant progress has been made in recent decades [10,11]. Metal catalysts including precious metals and noneprecious metals have been widely investigated as key factors in the HDO process. It is well known that catalysts based on precious metals (Pd, Rh, Ru and Pt) are efficient for HDO of phenols [12e15], while most of these catalysts generally exhibit poor selectivity. Additionally, the low-abundance and high price of precious metals limit large-scale application. Nonprecious metal catalysts, such as Fe [16], Mo [17,18], Co [19] and especially Ni [20e26], have been widely used as potential replacements for noble metal catalysts. Raney Ni, Ni/SiO2, Ni/HZM-5, Ni/AC and other Ni-based catalysts have been tested for HDO of lignin-derived phenols. Tieuli et al. [25] investigated the HDO of isoeugenol over Ni-SBA-15 catalyst. They found the Ni particles in Ni-SBA-15 with the size of 20 nm were located outside SBA-15 promoting the HDO of isoeugenol. Alda-Onggar [26] compared the catalytic activity of Ir/ZrO2 and Ni/ZrO2 in the HDO of three different phenolic model compounds, isoeugenol, guaiacol and vanillin. Although Ni catalysts are often characterized by excellent selectivity in the HDO of the CeO linkage, further improvements of Ni-based catalysts are still ongoing due to their limited activity and poor dispersion as Ni nanoclusters tend to agglomerate [27]. In recent studies, bimetallic catalysts have attracted considerable attentions due to their unique properties and superior catalytic activity to the corresponding monometallic catalysts. For example, Ni-M (M ¼ Rh, Pd and Ru) catalysts exhibited remarkable activity for b-O-4 ether linkage hydrogenolysis [28], and these bimetallic catalysts were proved to be superior to the corresponding single-metal counterparts. An ordered mesoporous carbon (OMC) supported PdeFe catalyst was applied to the catalytic hydrogenolysis of benzyl phenyl ether and gave a 74.3% yield of aromatics at 250  C under 10 bar H2, which enhanced the selectivity of Pd/OMC and the activity of Fe/OMC [29]. Recently, bimetallic PdeNi nanoparticles supported on ZrO2 were prepared for efficient hydrogenolysis of b-O-4 ether linkages to a mixture aromatics and phenolics [30]. In addition to active components, the performance of catalysts is greatly influenced by the support materials. Acidic supports were widely applied to the HDO of phenols owing to the deoxygenation ability. Pt/HeY, Ru/HZSM-5, Ru/H-Beta, and Ni/HZSM-5 have been reported to be active for HDO of lignin-derived phenolic monomers/dimers [31e33]. Generally, the hydrogenation activity is controlled by the metal components, and the support provides acid sites to boost the deoxygenation process [34]. In the present work, encouraged by the aforementioned studies, a bimetallic system and acid sites were combined to prepare bimetallic PdeNi catalysts by loading Pd and Ni on HZSM-5. Our focus is on converting different phenols derived from lignin into alkanes by the HDO process. Diphenyl ether (DPE) was used as a benchmark model compound to investigate the effects of reaction parameters such as H2 pressure, temperature and reaction time. Furthermore, possible reaction pathways of DPE under different conditions were proposed. Additionally, the catalytic HDO activity of PdeNi/HZSM-5 on other lignin-derived phenolic compounds was evaluated. 2. Experimental section 2.1. Materials All chemicals are analytical reagent and obtained commercially. Diphenyl ether, anisole, veratrole, guaiacol, 4-methoxyphenol, phenylethyl phenyl ether, benzyl phenyl ether, dibenzyl ether, n-hexane, SiO2, Al2O3 and PdCl2 were purchased from Aladdin Chemicals Co., Ltd. Nickel nitrate hexahydrate (Ni(NO3)2,6H2O, 97%) was provided from Jien Nickel Co., Ltd. HZSM5 (Si/Al ¼ 50) were purchased from the Catalyst Plant of Nankai University. The commercially available n-hexane was purified by distillation prior to use, and the supports (SiO2, Al2O3 and HZSM-5) were calcined in an air atmosphere at 550  C for 4 h before usage. 2.2. Preparation of catalysts Co-impregnation method was applied to prepare the PdeNi catalysts, and the composition of the bimetallic nanoparticles was controlled by adjusting the ratio of the metal precursors. Taking 1Pde10Ni/HZSM-5 as an example, in a typical procedure, PdCl2 (0.3523 g) was dissolved in hydrochloric acid (20 mL) in a flask and stirred at 200 rpm for 20 min to afford solution A. Additionally, Ni(NO3)2,6H2O (0.4955 g) was dissolved in 2 mL of distilled water to afford solution B. Then solution A (2.84 mL) and solution B were mixed, and 0.89 g of HZSM-5 was added into the mixture with continuous magnetic stirring at ambient temperature overnight. Next, a rotary evaporator was used to remove water and the obtained solid sample was dried at 110  C overnight. The catalyst precursor was calcined at 460  C for 4 h in air (heating rate: 1  C,min1), and finally reduced under flowing H2 (120 mL, min1) at 460  C for 4 h (heating rate: 1  C,min1). The obtained catalyst was denoted as xPd-yNi/HZSM-5, where x and y refer to the Pd and Ni mass percentage, respectively. The preparation procedure of the monometallic catalyst was similar to that of the PdeNi/HZSM-5 catalyst. 2.3. Catalyst characterization N2 adsorption-desorption measurements were performed using a Quantachrome Autosorb-IQ2-MP-XR gas adsorption analyzer. Metal loadings were analyzed by an Agilent 213e7900 ICP-MS (inductively coupled plasma mass spectrometry) system. Solid samples were dissolved in a mixture of HCl and HNO3 (volume ratio ¼ 1:3), followed by water dilution. The crystalline properties of the catalysts were investigated using a Bruker D8 ADVANCE X-ray diffractometer, with Cu-Ka (l ¼ 0.5142 nm) radiation operating at 40 kV/30 mA in the two theta (2q) range of 10e70 with a step size of 0.02 . X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Thermo Fisher Escalab 250Xi spectrometer with an Al-Ka X-ray source (hn ¼ 1486.6 eV), and the binding energy of the C 1s (284.8 eV) was set as the standard for correction. Temperature programmed desorption of ammonia (NH3-TPD) was measured on a ChemStar Pulsar TPR/TPD chemisorption apparatus equipped with a thermal conductivity detector. Scanning electron microscope (SEM) analysis was conducted using a Merlin Zeiss microscopy (FEI Quanta TM 250). Transmission electron microscopy (TEM) was performed using a FEI Tecnai G2 F20 transmission microscope operating at 200 kV. Thermogravimetric analyses of the fresh and spent catalysts were performed to observe the presence of coke on the spent catalysts using a Mettler TOLEDO DSC1 thermal analyzer under an air atmosphere with a heating rate of 10  C/min. Please cite this article as: Y.-P. Zhao et al., Hydrodeoxygenation of lignin model compounds to alkanes over PdeNi/HZSM-5 catalysts, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.002

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2.4. Catalytic tests The catalytic experiments were carried out in a 100 mL stainless steel autoclave reactor. Typically, solvent (20 mL), catalyst (20 mg) and reactant (1 mmol) were placed into the reactor. After flushing the reactor with high-purity nitrogen (99.999%) for three times and charging with high-purity hydrogen (99.999%) to a set pressure at room temperature, reactions were conducted at the desired temperature for the prescribed period of time with a stirring rate of 200 rpm. After each reaction, the reactor was cooled to ambient temperature and depressurized. The liquid mixture was extracted by ethyl acetate and subsequently analyzed by an Agilent 7890/5973 gas chromatography-mass spectrometry (GC-MS). Dodecane was used as an internal standard to calibrate the reactants and products for quantitative analysis. Each experiment was repeated at least three times to ensure the repeatability of the data. The calculation of mass balance requires to analysis the composition of solid, liquid and gas products, while the solid products deposited on the surface of spent catalyst are difficult to separate and analysis. In this work, the analysis of gaseous products by GC showed that only a small amount of methane, ethane and propane were detected in the HDO of lignin model compounds. Luo et al. [35] pointed out 98% of carbon was preserved in the liquid product, only a trace amount of methane was detected in the gas product, and no aromatic compounds was formed on the spent catalysts during the HDO of lignin-derived substituted phenols. Therefore, in current work, the selectivity of each component was based on the liquid products but did not take into account the influence of solid and gas products. Conversion (%) ¼ (moles of substrate reacted/moles of substrate supplied)  100% Selectivity (%) ¼ (moles of each component/moles of substrate reacted)  100%

2.5. Computational details All the quantum chemical calculations were carried out with the Gaussian 09 program [36]. Density functional theory B3LYP with the 631G (d) basis set was used for geometry optimizations and frequency calculations [37]. No imaginary frequencies confirmed that the optimized structures are minima structures. Single-point energies were computed at the mPW2PLYP/cc-pVTZ level of theory on the optimized structures [38]. 3. Results and discussion 3.1. Characterization of the catalysts Fig. 1 shows the textural properties of the catalysts and the HZSM-5 support. All samples displayed type-IV isotherms with type-H4 hysteresis loops, implying the existence of microporous and mesoporous structures. The pore size distribution and detailed textural properties of the materials are summarized in Fig. S1 and Table S1. As expected, the pore size distribution was within the range of 1.6e2.3 nm. The pore volume and surface area of the support were higher than those of the catalysts due to metal loading. However, the average pore diameter of all samples was almost identical, suggesting that all catalysts retained the pore characteristics of HZSM-5. The metal loadings of the catalysts determined by ICP-MS analyses are listed in Table S1. The obtained metal loadings were consistent with the intended values (Table S1), implying the high efficiency of the impregnation process.

Fig. 1. N2 adsorption/desorption isotherms of HZSM-5 and the reduced catalysts.

Please cite this article as: Y.-P. Zhao et al., Hydrodeoxygenation of lignin model compounds to alkanes over PdeNi/HZSM-5 catalysts, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.002

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Fig. 2. XRD patterns of HZSM-5 and the reduced catalysts.

Fig. 2 shows the XRD patterns of the support and reduced catalysts. All samples exhibited typical peaks of the MFI framework structure corresponding to the HZSM-5 zeolite, implying that the zeolite framework structure was well preserved after metal loading. The catalysts containing nickel displayed peaks at 2q ¼ 44.5 and 51.8 , which were assigned to the Ni (111) and (200) lattice planes, respectively. In addition, no diffraction peaks of palladium were observed in the supported palladium catalysts due to its low metal loading on the support. XPS spectroscopy was performed to analyze the surface chemical composition and valence state of the metal elements of Pd/HZSM-5, Ni/ HZSM-5 and 1Pde10Ni/HZSM-5. The global XPS spectra of the as-prepared catalysts presented in Fig. S2 show that C, Si, O, Pd and Ni were the dominant species, and the atomic densities of the main element on the catalyst surface are displayed in Table S2. The surface ratio of Ni/ Pd was determined to be 18:1, which was higher than the result from ICP-MS, suggesting that Ni was probably surface-enriched. The Pd 3d core-level and Ni 2p core-level spectra are shown in Fig. 3. The Pd peaks in Pd/HZSM-5 at binding energies of 335.2 eV and 340.5 eV were ascribed to Pd (0) 3d5/2 and Pd (0) 3d3/2 [39]. Pd remained in the metallic Pd (0) state for PdeNi/HZSM-5 after Ni addition, although the binding energy increased by 0.3 eV. The Ni spectral feature of the Ni/HZSM-5 catalyst consisted of Ni hydroxide (880.1 eV and 861.9 eV) and oxide (873.8 eV and 856.1 eV) as well as metallic Ni (852.9 eV) [40]. Typically, the multi-electron excitation of nickel species leads to a complex Ni 2p3/2 spectrum, in which main peaks are adjacent to intense satellite signals [41]. The reason for the presence of NiO and Ni(OH)2 is that the nickel atoms on the catalyst surface were oxidized to hydroxide and oxide when exposed to oxygen and moisture in air [42]. It should be noted that there were no obvious Ni oxide and hydroxide diffraction peaks in the XRD patterns, which indicated that the structures of Ni oxide and hydroxide were amorphous. Because the XPS signals were derived from the sample surface, the peak of metallic Ni was rather weak. In the case of the PdeNi/HZSM-5 catalyst, the binding energies of NiO, Ni(OH)2 and metallic Ni all decreased relative to the Pd-free catalyst. The increased binding energy of Pd species and decreased binding energy of Ni species was due to electron transfer from Pd to Ni, resulting in electron-deficient Pd and electron-enriched Ni atoms. As reported by Luo et al., electron-enriched Ni atoms can increase the strength of the interaction between Ni and Pd [43], which is good for the HDO process. To investigate the morphologies of the catalyst, the PdeNi/HZSM-5 catalyst was directly observed by SEM and TEM. As shown in Fig. 4a, the PdeNi/HZSM-5 catalyst had a rough surface due to metal particle loading. The TEM image and EDS spectrum indicated that Ni and Pd particles were well dispersed, which could provide more contact surface area between the substrates and active sites. It can be seen from Fig. 4b that some metal particles are agglomerated, which is caused by the preparation process [44]. The average size of nanoparticles was approximately 17 nm, which was responsible for hydrogenation [45]. The surface acidity of the supported catalyst plays a critical role in the HDO process. Acid sites can facilitate the adsorption of reactants and intermediates, and catalyze dehydration and cracking reactions that occur during the HDO process [46]. In the present work, the total acidity of different supported catalysts was determined by NH3-TPD analysis, and the acidity distribution was obtained by fitting TPD curves to Gaussian lines. On the basis of desorption temperature, three areas were used representing weak (T < 300  C), medium (300  C < T < 500  C) and strong (T > 500  C) acidity [47,48]. As shown in Fig. 5, the distribution of acid sites in all samples is obviously different. There was only one broad weak desorption band ranging from 110  C to 650  C in the pattern of the PdeNi/Al2O3 catalyst and PdeNi/SiO2 catalyst, while two desorption peaks observed for the PdeNi/HZSM-5 catalyst were located in the weak and medium acid range. The quantity of acid sites in PdeNi/HZSM-5 is much higher than that in PdeNi/SiO2 and PdeNi/Al2O3 (Table 1). For PdeNi/HZSM-5, the medium acid sites predominated. A previous study has shown that the HDO of DPE requires medium acidic sites [49]. With the introduction of medium acidic sites, the catalytic activity of PdeNi/ HZSM-5 for HDO of DPE is enhanced.

Please cite this article as: Y.-P. Zhao et al., Hydrodeoxygenation of lignin model compounds to alkanes over PdeNi/HZSM-5 catalysts, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.002

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(a)

335.2 340.5

(b)

Pd/HZSM-5

340.8

Pd-Ni/HZSM-5

Intensity (a.u.)

Intensity (a.u.)

880.1

335.5

5

856.1 873.8

Ni/HZSM-5

861.9

852.9

855.8 Pd-Ni/HZSM-5 879.6

873.1

861.1

852.6

345

340

335

Binding energy (eV) of Pd 3d

330

880

875

870

865

860

855

850

845

Binding energy (eV) of Ni 2p

Fig. 3. XPS spectra of (a) Pd 3d and (b) Ni 2p regions in catalysts (Pd/HZSM-5, Ni/HZSM-5, and PdeNi/HZSM-5).

Fig. 4. SEM image (a), TEM image (b) and EDS spectrum of PdeNi/HZSM-5.

Fig. 5. TPD profile of NH3 adsorbed on the calcined PdeNi catalysts.

Please cite this article as: Y.-P. Zhao et al., Hydrodeoxygenation of lignin model compounds to alkanes over PdeNi/HZSM-5 catalysts, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.002

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Table 1 Acid sites distribution for PdeNi catalysts. Acid site concentrations (mmol NH3 , g1)

Sample

PdeNi/SiO2 PdeNi/Al2O3 PdeNi/HZSM-5

Weak <300  C

Medium 300e500  C

Strong >500  C

Total

230 154 465

224 119 873

325 20 89

772 293 1427

3.2. Catalytic conversion of diphenyl ether 3.2.1. Catalytic activity As a representative of the 4-O-5 linkage, DPE was selected as the benchmark model compound to characterize the catalytic activities of several catalysts due to its high CeO bond dissociation enthalpies [50]. Table 2 illustrates the conversion and product selectivity over various catalysts. The blank support HZSM-5 showed no catalytic activity. After 1 wt% of Pd was loaded on HZSM-5, the conversion was enhanced to 70.4% and most of the benzene rings were saturated, yielding considerable amounts of undesired ethers (24.2% dicyclohexyl ether and 11.8% cyclohexyl phenyl ether). On the other hand, 10Ni/HZSM-5 displayed negligible catalytic activity under the same conditions. The above results implied that a catalyst system containing individual Ni or Pd species had a poor efficiency for HDO of DPE. However, when 1Pde5Ni/ HZSM-5 was applied in this reaction, although the conversion of DPE was 63.4%, the selectivity of cyclohexane dramatically increased to more than 99.5%. These phenomena revealed that there are synergetic effects between Pd and Ni, which could enhance the selectivity towards cyclohexane. To further confirm this hypothesis, 1Pd/HZSM-5 and 10Ni/HZSM-5 were mechanically mixed and used as the catalyst, but the catalytic performance was not promoted compared to that in 1Pde5Ni/HZSM-5. In addition to the metallic sites, the composition of loaded metals greatly affects the catalytic performance of catalysts; thus, an optimal PdeNi ratio is critical for the catalytic system [51]. Table 2 (entries 5e7) displays the effect of the PdeNi ratio on the catalytic performance. The higher percentage of Ni resulted in more activity when the Pd loading remained constant at 1 wt%, and a PdeNi ratio of 1:10 was found to be optimal. Furthermore, to investigate the influence of supports on the HDO of DPE, 1Pde10Ni/SiO2 and 1Pde10Ni/Al2O3 catalysts were prepared. As shown in Table 2 (entries 8 and 9), the SiO2 and Al2O3 support catalysts showed lower conversion than the HZSM-5-supported catalysts, and the products profiles were quite complex. Apart from cyclohexane, many O-containing products such as cyclohexanol, cyclohexyl phenyl ether and dicyclohexyl ether were detected. It should be noted that no O-containing products were observed in the reaction product mixture when HZSM-5 was used as the support. The NH3-TPD results proved that stronger acidity of the support was necessary for catalytic HDO performance. 3.2.2. Optimization of reaction conditions Because of the high activity for HDO of DPE, 1Pde10Ni/HZSM-5 was selected to explore the optimal reaction conditions. Reactions at different temperatures were carried out, as displayed in Table 3. When the reaction was 120  C, the conversion of DPE was only 11.1% and cyclohexanol, dicyclohexyl ether and cyclohexyl phenyl ether, which could be intermediates, were detected. As the reaction temperature increased from 120 to 220  C, the conversion of DPE increased from 11.1% to 100%. Simultaneously, the selectivity to cyclohexane increased from 78.6% to 100%, while the O-containing product amounts rapidly decreased until they disappeared. Obviously, a higher reaction temperature was favorable for converting DPE into cyclohexane, which means that the HDO of DPE is sensitive to temperature. It has been reported that a low reaction temperature is conducive to creating spillover hydrogen on the catalyst surface, which is favorable for the hydrogenation of benzene rings in producing hydrogenation products (dicyclohexyl ether and cyclohexyl phenyl) [52]. As shown in Table 4, the product distribution and conversion depended heavily on the H2 pressure. The reaction conducted under low H2 pressure (5 bar) mainly produced benzene, cyclohexane and a minor amount of phenol, while the direct hydrogenation products of DPE were hardly detected. As the H2 pressure increased from 5 bar to 20 bar, the conversion of DPE increased from 54.4% to 100%, but the selectivity of benzene decreased due to further hydrogenation of the benzene ring forming cyclohexane. The maximum yield of cyclohexane was achieved with a H2 pressure of 20 bar.

Table 2 HDO of DPE over different catalysts. Entry

Catalyst

Conv. /%

1 2 3 4 5 6 7 8 9

HZSM-5 10Ni/HZSM-5 1Pd/HZSM-5 1Pd/HZSM-5 þ10Ni/HZSM-5 1Pde5Ni/HZSM-5 1Pde8Ni/HZSM-5 1Pde10Ni/HZSM-5 1Pde10Ni/SiO2 1Pde10Ni/Al2O3

Trace Trace 70.4 64.1 63.4 86.4 >99.9 54.4 34.2

Selectivity/%

61.3 69.1 >99.5 >99.5 100 24.2 32.7

2.7

22.8 23.4

24.2 11.7

11.8 19.2

35.3 28.0

17.7 15.9

Reaction conditions: DPE 1 mmol; catalyst 0.02 g; n-hexane 20 mL; H2 pressure 2 MPa; 220  C; 2 h.

Please cite this article as: Y.-P. Zhao et al., Hydrodeoxygenation of lignin model compounds to alkanes over PdeNi/HZSM-5 catalysts, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.002

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Table 3 The effect of temperature on DPE conversion. Entry

Temp./oC

Conv./%

Selectivity/%

1 2 3 4 5 6

120 140 160 180 200 220

11.1 17.1 33.2 49.9 70.9 100

78.6 92.5 95.4 97.1 100 100

5.9 1.6 1.4 1.1

10.3 2.2 1.3 1.2

4.8 3.4 1.2

Reaction conditions: DPE 1 mmol, catalyst 0.02 g, n-hexane 20 mL, H2 pressure 2 MPa, 2 h.

3.3. Reaction routes of DPE conversion over 1Pde10Ni/HZSM-5 Based on the above results and many published works, the products from the HDO of DPE are greatly affected by H2 pressure. To determine the detailed pathway, herein, variation in product selectivity as a function of DPE conversion was investigated at different H2 pressures. It was reported that low H2 pressure has a negative effect on DPE conversion [53]. Fig. 6a shows the product selectively for the HDO of DPE as a function of DPE conversion under 0.5 MPa H2. At the early stage, benzene, phenol and cyclohexane were the major products, whereas the selectivity of benzene and phenol decreased and the selectivity of cyclohexane increased with increasing conversion. During the reaction process, some cyclohexene, cyclohexanone and cyclohexanol could be detected. The absence of direct hydrogenation products of DPE indicates that DPE undergoes cleavage of the CeO bond instead of first undergoing hydrogenation of the aromatic ring and then cleavage of the CeO bond. Equimolar phenol and benzene can be obtained from direct cleavage of DPE, which can be further converted into cyclohexane. In the present work, the selectivity of cyclohexane was higher than the total selectivity of cyclohexane and phenol, suggesting that the transformation of benzene and phenol into cyclohexane is faster than the cleavage of DPE. It can be seen from Fig. 6a that the phenol selectivity dramatically decreased to 0, while the benzene selectivity decreased more slowly. As previous study reported, the presence of DPE inhibits the conversion of benzene to cyclohexane [33]. Fig. 6b shows the product selectivity for the HDO of DPE as a function of DPE conversion under 2 MPa H2. With the increase of H2 pressure from 0.5 MPa to 2 MPa, the reaction products changed markedly. In addition to benzene, cyclohexane and phenol, cyclohexanol and direct DPE hydrogenation products such as dicyclohexyl ether and cyclohexyl phenyl ether were detected. It is easy to conclude that H2 pressure has a significant effect on the DPE conversion pathway. At the early stage, cyclohexane, benzene, cyclohexanol and cyclohexyl phenyl ether were the main products. With increasing DPE conversion, the selectivity of cyclohexane increased, while the selectivity of other products decreasesd, and only cyclohexane was detected when the conversion reached to 100%, suggesting a complex reaction pathway for HDO of DPE, i.e., hydrogenation, dehydroxylation and cracking. Cyclohexyl phenyl ether was an important intermediate product in the reaction process. There are three pathways for cyclohexyl phenyl ether: (1) direct hydrogenation to form dicyclohexyl ether; (2) cleavage of the CaryO bond to form benzene and cyclohexanol; and (3) cleavage of the Calk-O bond to form phenol and cyclohexane. The low selectivity of dicyclohexyl ether suggested that direct hydrogenation of cyclohexyl phenyl ether was a minor pathway. There are two types of CeO bonds in cyclohexyl phenyl ether (Cary-O bond and Calk-O bond), and the bond dissociation enthalpy of the Calk-O bond (277.76 kJ mol1) is lower than that of the Cary-O bond (435.52 kJ mol1); therefore, the Cary-O bond is more stable and the Calk-O bond is easier to cleave. The spin density population and singly occupied molecular orbitals of the phenyl and phenoxy radicals are shown in Fig. S3. In the phenyl radical, the unpaired electron is localized at the radical site, while the unpaired electron of the phenoxy radical can delocalize into the p orbital of the benzene ring [49]. Therefore, phenoxy radicals are more stable than phenyl radicals. In a word, cleavage of the Calk-O bond to form phenol and cyclohexane is the major pathway, while cleavage of the Cary-O bond forming benzene and cyclohexanol is the minor pathway. The relatively low selectivity of phenol is because phenol can be hydrogenated to cyclohexanol, which can be further converted into cyclohexane. On the basis of the obtained experimental results and quantum chemical calculations, the proposed reaction routes for the HDO of DPE over 1Pde10Ni/HZSM-5 are shown in Scheme 1. Typically, direct cleavage of the CeO bond instead of hydrogenation of the benzene rings is favored under low H2 pressure, forming phenol and benzene. Phenol can be transformed into cyclohexane via cyclohexanol as an intermediate through a dehydroxylation step and benzene can be hydrogenated to cyclohexane. However, in addition to direct cleavage of the CeO bond of DPE, DPE can be hydrogenated to dicyclohexyl ether and cyclohexyl phenyl ether under high H2 pressure, followed by the cleavage of CeO bonds to form phenol and cyclohexanol. Subsequently, the intermolecular dehydration reaction of cyclohexanol will happen to form cyclohexane in the presence of an acidic support.

3.4. HDO of various phenolic monomers/dimers The HDO of other model compounds was also tested over the 1Pde10Ni/HZSM-5 catalyst. Since lignin is a polymer of methoxylated phenylpropane units, anisole, veratrole, guaiacol and 4-methoxyphenol were selected as representative bio-derived phenolic monomers. As displayed in Table 5, anisole, veratrole and 4-methoxyphenol were converted to cyclohexane with substrate conversions >90% and cyclohexane selectivities >95% after 2 h. However, guaiacol conversion was only 51.4% after 2 h. Extending the reaction time to 4 h, guaiacol could be totally converted to cyclohexane. The reason for this phenomenon was the formation of hydrogen bonds between the hydroxyl group and the methoxy group of guaiacol, which stabilized the molecule and restricted the HDO process [35]. Notably, compared with other catalysts applied in the above systems, the reaction temperature and pressure for PdeNi/HZSM-5 are 220  C and 2 MPa, respectively, which Please cite this article as: Y.-P. Zhao et al., Hydrodeoxygenation of lignin model compounds to alkanes over PdeNi/HZSM-5 catalysts, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.002

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Table 4 The effect of H2 pressure on DPE conversion. Entry

H2 pressure/bar

Conv. /%

Selectivity/%

1 2 3 4 5

1a 2a 5 10 20

28.9 42.4 54.4 67.2 100

65.7 52.2 35.1 5.1

15.8 10.1 8.4

18.1 37.1 56.3 94.7 100

Reaction conditions: DPE 1 mmol, catalyst 0.02 g, n-hexane 20 mL, 220  C, 2 h. a Reaction atmosphere: 5% hydrogen and 95% argon.

(a) 100

(b) 100

Phenol Benzene Cyclohexane

80

Selectivity (%)

Selectivity (%)

80

60

40

20

Cyclohexane

Benzene

Cyclohexanol

Phenol

Cyclohexyl phenyl ether Dicyclohexyl ether

60

40

20

0

0 30

40

50

60

70

80

90

100

20

40

60

80

100

DPE Conversion(%)

DPE Conversion (%)

Fig. 6. Product distributions for DPE HDO as a function of conversion. Reaction conditions: DPE, 1 mmol; , n-hexane, 20 mL; 220  C; (a) catalyst 0.02 g; 0.5 MPa H2; (b) catalyst 0.01 g; 2 MPa H2.

OH

High H2 pressure

minor pathway

O

OH O

Low H2 pressure minor pathway OH O

major pathway

OH

OH OH

Scheme 1. Proposed pathways of DPE conversion over the 1Pde10Ni/HZSM-5 catalyst under different H2 pressures.

Please cite this article as: Y.-P. Zhao et al., Hydrodeoxygenation of lignin model compounds to alkanes over PdeNi/HZSM-5 catalysts, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.002

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Table 5 HDO of lignin-derived compounds over 1Pde10Ni/HZSM-5. Entry

Substrate

Conv./%

1

91.6

2

93.6

3

51.4

4a

>99.9

5

>99.9

6

>99.9

7

>99.9

8

99.1

9

>99.9

Selectivity/%

95.4

97.3

95.2

>99.9

>99.9

>99.9

29.4

44.7

48.6

51.2

2.7

17.1

95.6 Reaction conditions: Substrate, 1 mmol; catalyst, 0.02 g; n-hexane, 20 mL; H2 pressure, 2 MPa; 220  C; 2 h. a Reaction time 4 h.

is lower than in the cases of Raney NiNafion/SiO2 (300  C, 4 MPa) [24], Ru/copolymerH3PO4 (240  C, 4 MPa) [54], Pd/CeH3PO4 (250  C, 5 MPa) [55], Pt/AC (280  C, 4 MPa) [56], Ni/HZSM-5 (250  C, 5 MPa) [25] and Pt/HY (250  C, 4 MPa) [57]. The most abundant CeO bonds in lignin are b-O-4, a-O-4 and 4-O-5 linkages [58]. To explore the application scope of the 1Pde10Ni/ HZSM-5 catalyst, model compounds representing these CeO linkages were used as substrates under the same conditions. From Table 5, diphenyl ether (4-O-5) was completely converted into cyclohexane. By contrast, previously explored catalysts such as Ni/NbAC [52], Ni/ MFA@AIL [59] and Ru/HZSM-5 [12] required higher H2 pressure (3e5 MPa) and longer reaction time (4 h or above) for efficient HDO of DPE to cyclohexane, again showing that PdeNi/HZSM-5 is a more efficient catalyst. For HDO of phenylethyl phenyl ether (b-O-4), >99% substrate conversion was obtained and the products were cyclohexane and ethylcyclohexane. These results were inconsistent with the catalyst system of Ru/H-Beta [33], in which cyclohexane, ethylcyclohexane, cyclohexanol and bicycloalkanes were obtained as major products. In contrast to diphenyl ether and phenylethyl phenyl ether, HDO of benzyl phenyl ether (a-O-4) resulted in quite different product distributions. Typically, cyclohexane (29.4%), methylcyclohexane (44.7%), toluene (2.7%) and cyclohexanol (17.1%) were detected as major products. The relatively high percentage of cyclohexanol was attributed to its low dehydroxylation rate. Methylcyclohexane was the major product when dibenzyl ether was chosen as the reactant, indicating that the catalyst was also active for the HDO of the aryl alkyl CeO bond. The above results regarding HDO of various phenolic monomers/dimers suggest that the 1Pde10Ni/HZSM-5 catalyst can be applied for converting phenols into cycloalkanes under mild conditions. 3.5. Recyclability of the catalyst The reusability of the 1Pde10Ni/HZSM-5 catalyst was tested using DPE as the substrate under the abovementioned conditions (Table 5). After the reaction was complete, the catalyst was separated from the liquid by filtration and washed with n-hexane for three times. Then, the catalyst was directly reused for the next run; the results are shown in Fig. 7. The DPE conversion slightly decreased from 95.3% in the first run to 84.6% in the third run. However, the DPE conversion dramatically decreased to 32.8% in the fourth run. To investigate the reason of catalyst deactivation, the spent catalyst after the fourth run was analyzed by TEM. After four cycles, the metal particles on the surface of the Please cite this article as: Y.-P. Zhao et al., Hydrodeoxygenation of lignin model compounds to alkanes over PdeNi/HZSM-5 catalysts, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.002

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DPE Conversion (%)

100

95.3

90.2

84.6

80

60

40

32.8

20

0 1

2

3

Number of recycles

4

Fig. 7. Recycling tests of the 1Pde10Ni/HZSM-5 catalyst.

catalyst were agglomerated, leading to larger particle size and lower dispersion, which might be the reason for the degraded performance of the catalyst. As shown in Fig. S5, the weight loss from 40 to 900  C for the fresh and spent 1Pde10Ni/HZSM-5 catalysts after fourth run in the HDO of DPE are 7.31% and 10.54%, respectively. The difference of weight loss especially in the temperature range of 200e400  C between the fresh and spend catalysts can be attributed to the coke and oligomers accumulated on the catalyst surface [25,26]. The accumulation of coke and oligomers on the surface of catalyst also possibly caused catalyst deactivation. 4. Conclusion In the present work, well dispersed PdeNi nanoparticles supported on HZSM-5 were synthesized via a co-impregnation method and characterized by N2 adsorption-desorption, ICP-MS, XRD, XPS, TEM, SEM and NH3-TPD. The bimetallic PdeNi/HZSM-5 catalyst showed superior properties relative to monometallic catalysts, and a PdeNi ratio of 1:10 was found to be optimal. The acidity of the support also had a significant effect on the catalytic performance, and PdeNi supported on HZSM-5 exhibited higher catalytic activity than that supported on SiO2 and Al2O3. Direct cleavage of the CeO bond in DPE was achieved at low H2 pressure mainly producing benzene and minor amounts of phenol, followed by hydrogenation and dehydration to form cyclohexane, while hydrogenation of the benzene ring in DPE followed by cleavage of CeO bonds could be achieved at high H2 pressure. The catalyst also showed high activity in the HDO of other model compounds containing various CeO bonds. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (China University of Mining and Technology, 2019XKQYMS49) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.joei.2019.08.002. References [1] S. Vasireddy, B. Morreale, A. Cugini, C. Song, J.J. Spivey, Clean liquid fuels from direct coal liquefaction: chemistry, catalysis, technological status and challenges, Energy Environ. Sci. 4 (2011) 311e345. [2] G.W. Huber, S. Iborra, A. Corma, Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering, Chem. Rev. 106 (2006) 4044e4098. [3] C.Z. Li, X.C. Zhao, A.Q. Wang, G.W. Huber, T. Zhang, Catalytic transformation of lignin for the production of chemicals and fuels, Chem. Rev. 115 (2015) 11559e11624. [4] T.R. Carlson, Y.T. Cheng, J. Jae, G.W. Huber, Production of green aromatics and olefins by catalytic fast pyrolysis of wood sawdust, Energy Environ. Sci. 4 (2011) 145e161. [5] J.N. Chheda, G.W. Huber, J.A. Dumesic, Liquid-phase catalytic processing of biomass-derived oxygenated hydrocarbons to fuels and chemicals, Angew. Chem. Int. Ed. 46 (2007) 7164e7183. [6] Y.X. Zhai, C. Li, G.Y. Xu, Y.F. Ma, X.H. Liu, Y. Zhang, Depolymerization of lignin via a non-precious NieFe alloy catalyst supported on activated carbon, Green Chem. 19 (2017) 1895e1903. [7] J. Zakzeski, P.C.A. Bruijnincx, A.L. Jongerius, B.M. Weckhuysen, The catalytic valorization of lignin for the production of renewable chemicals, Chem. Rev. 110 (2010) 3552e3599. [8] M.P. Pandey, C.S. Kim, Lignin depolymerization and conversion: a review of thermochemical methods, Chem. Eng. Technol. 34 (2011) 29e41. [9] X.Z. Zhang, X. Chen, S.H. Jin, Z.J. Peng, C.H. Liang, Ni/Al2O3 catalysts derived from layered double hydroxide and their applications in hydrodeoxygenation of anisole, Chem. Select 1 (2016) 577e584. [10] T. Parsell, S. Yohe, J. Degenstein, T. Jarrell, I. Klein, E. Gencer, B. Hewetson, M. Hurt, J.I. Kim, H. Choudhari, B. Saha, R. Meilan, N. Mosier, F. Ribeiro, W.N. Delgass, C. Chapple, H.I. Kentt€ amaa, R. Agrawal, M.M. Abu-Omar, A synergistic biorefinery based on catalytic conversion of lignin prior to cellulose starting from lignocellulosic biomass, Green Chem. 17 (2015) 1492e1499.

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