Depolymerization of lignin over Ni-Pd bimetallic catalyst using

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Biofuels and Biomass

Depolymerization of lignin over Ni-Pd bimetallic catalyst using isopropanol as in situ hydrogen source Bingxing Jiang, Jun Hu, Yiheng Qiao, Xiaoxiang Jiang, and Ping Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01976 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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Depolymerization of lignin over Ni-Pd bimetallic catalyst using isopropanol as in situ hydrogen source Bingxing Jiang, Jun Hu*, Yiheng Qiao, Xiaoxiang Jiang*, Ping Lu Engineering Laboratory for Energy System Process Conversion & Emission Control Technology of Jiangsu Province, School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing, 210042, China Abstract Performance of Ni-Pd bimetallic catalysts on depolymerization of lignin into monomeric phenols was investigated using isoproponal as in situ hydrogen source. The performance of catalysts and the effects of temperature and starting lignins were extensively examined. Ni50Pd50/SBA 15 exhibited the best performance in depolymerization of cellulolytic enzyme corn stalk lignin (CECL), affording the highest total monophenols yield of 8.14wt% which was 1.96 times that of Ni/SBA 15 and 1.44 times that of Pd/SBA 15. For depolymerization of Acid-extracted birch lignin (ABL), 18.52wt% monophenols were obtained over Ni50Pd50/SBA 15 at 245°C for 8 h, and 4-propylsyringol had 8.89 wt% yield with 47.97% selectivity. The physicochemical characteristics of the catalysts were elucidated with SEM, HR-TEM, nitrogen adsorption/desorption, XRD, and XPS. The bimetallic catalyst had the characteristics of narrow distribution of metal particles with small size, well-retained hexagonal pore structure of SBA 15, higher dispersion of Pd, and more Pd0 and Ni0 as compared with monometallic catalysts. The properties of bimetallic catalyst provide more contact opportunities between the abundant active centers and lignin fragments, 1 ACS Paragon Plus Environment

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and therefore significantly promote the hydrogenolysis process to produce monomeric phenols. Keywords: lignin; depolymerization; catalytic transfer hydrogenolysis; nickel; palladium.

1. Introduction Biomass is the only renewable resource of organic carbon on earth, possessing a great potential to replace fossil resources.1, 2 Lignocellulosic, the main nonfood component of biomass, is comprised of cellulose, hemicellulose, and lignin.3, 4 In the pulping and cellulosic ethanol processes, carbohydrates are used, leaving lignin as a waste byproduct. Lignin is a polymer of phenylpropane units linked with C-O and C-C bonds. By cleaving these bonds, lignin can be converted into high value-added monophenolic compounds.5, 6 Hydrogenolysis is one of the most promising lignin valorization routes because of the high-yield aromatic monomers and mild reaction conditions.3 Although a number of studies have been conducted, hydrogenolysis of lignin into monophenols with high yields, high selectivities and low costs still represents a challenge. Previous studies on lignin hydrogenolysis mainly focus on the development of catalysts. Noble metals (Pt, Rh, Ru, Ir, and Pd) exhibit excellent performances in lignin depolymerization. 7-14 Nevertheless, these catalysts have the limitations of low abundance and high price. Non-noble metals (Ni, Cu, and Mo) which are abundant on earth have also been

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applied in recent decades.15-22 However, lignin depolymerization over non-noble metal catalysts requires severe conditions, and the monomer yields are low. Recently, some researchers moved to the bimetallic catalysts which likely show enhanced performances as compared with their parent metals.23 Yan’s group initiated the synthesis of Ni-based bimetallic catalysts, and hydrogenolysis of organosolv lignin over the bimetallic nanoparticles with H2 produced more monomers than those over monometallic catalysts.24, 25 Cai et al studied the hydrogenolysis of lignin model dimers over PdNi bimetallic catalysts, reporting that the β-O-4 linkage can be selectively cleaved over PdNi/ZrO2 using NaBH4 as the hydrogen source.26 Zhai et al. studied the depolymerization of organosolv birch lignin over Ni-Fe supported on activated carbon, and 23.2wt% monomers were obtained in total with H2.27 Kim et al evaluated the depolymerization of soda lignin over RuNi bimetallic catalysts, and 12.7 wt% monomeric phenols were obtained over Ru0.6Ni0.4/SBA-15 at 350°C with H2.28 These excellent studies demonstrated that Ni-based bimetallic catalysts are promising alternatives to expensive noble monometallic catalysts for lignin depolymerization. Hydrogen is essential for lignin hydrogenolysis. As the most widely used hydrogen donor, molecular hydrogen requires extra energy input and high equipment investment. Molecular hydrogen could also promote the undesired competitive hydrogenation of aromatic rings.29 Reductive organic molecules, such as alcohols and formic acid, can also act as hydrogen source for catalytic transfer conversion of biomass.30 Among the various organic molecules, isopropanol is one of the best


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choices due to its ability to donate hydrogen and high solubility of lignin.29 Ferrini and Rinaldi reported that the lignin fraction in biomass can be efficiently converted to nonpyrolytic lignin bio-oil rich in monomers, using isopropanol as the hydrogen donor in the presence of Raney Ni.31 Liu et al. reported that using isopropanol as the hydrogen donor, lignosulfonate can be converted into 11.61 wt% phenolic monomers over Raney-Ni.32 Paone et al found that benzyl phenyl ether can be completely hydrogenolyzed into phenol and toluene, using isopropanol as the hydrogen donor over Pd/Fe3O4.33 Mauriello et al. demonstrated that C−O bonds in various lignin dimers can be successfully cleaved using isopropanol as the hydrogen donor over Pd/Ni catalysts.34 Despite of these efforts, knowledge on catalytic transfer hydrogenolysis of real lignin with isopropanol over Ni-based bimetallic catalysts remains limited. Herein, using isopropanol as the hydrogen donor, catalytic transfer hydrogenolysis of lignin was studied over Ni-Pd bimetallic and monometallic catalysts supported on SBA-15. SBA-15 was used as the support material due to its mesoporous structure, high surface area, and relative high hydrothermal stabilities.28 The influences of catalyst, reaction temperature, and starting lignin were studied. Then, physicochemical characteristics of the catalysts were elucidated with scanning electron microscope (SEM), high resolution transmission electron microscopy (HR-TEM), nitrogen adsorption/desorption, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Finally, a possible mechanism of bimetallic catalyst on lignin


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depolymerization was discussed.

2. Materials and Methods 2.1. Materials Cellulolytic enzyme corn stalk lignin (CECL) was obtained from Yanghai Co. (Shandong, China) as the byproduct from a cellulosic ethanol biorefinery process. Acid-extracted birch lignin (ABL) was prepared according to the procedures reported elsewhere.35 2-(2', 6'-dimethoxyphenoxy)-1-phenylethanol was synthesized according to the reported process. 25, 36 Palladium(II) chloride (PdCl2, Aladdin), nickel(II) chloride hexahydrate (NiCl2·6H2O, Aladdin), Pd/C (5% metal loading, Aladdin), sodium borohydride (NaBH4, Aladdin), guaiacol (Sigma-Aldrich), 4-methylguaiacol (Aladdin), 4-ethylphenol (Aladdin), 4-ethylguaiacol (Aladdin), syringol (SigmaAldrich), syringaldehyde (Aladdin), and 2-bromoacetophenone (Aladdin) were used as-received. Mesoporous silica SBA-15 was purchased from Nanjing XFNANO Materials Tech Co. Ltd. Other chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd and used as received.

2.2 Catalyst Preparation PdCl2 and NiCl2·6H2O were employed as the metal precursors. The catalysts were prepared by incipient wetness impregnation, keeping the total metal amount constant at 0.68 mmol metal (Ni+Pd) per 1 g SBA 15 with different Ni/Pd molar ratios. These


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catalysts with different molar ratios of Ni and Pd were marked as Ni/SBA 15 (Ni : Pd = 100 : 0), Ni75Pd25/SBA 15 (Ni : Pd = 75 : 25), Ni50Pd50/SBA 15 (Ni : Pd = 50 : 50), Ni25Pd75/SBA 15 (Ni : Pd = 25 : 75), and Pd/SBA 15 (Ni : Pd = 0 : 100). SBA-15 was dried at 80℃ in a vacuum oven before the impregnation. The typical preparing procedure is as follows: the precursors of PdCl2 and/or NiCl2 with different molar ratios (PdxNi100-x, x=0, 25, 50, 75, and 100) were dissolved in water with 0.1M HCl and then dropwise added into SBA-15. The solid mixture was dried at 80°C overnight to remove water and calcined at 300°C for 6 h in air with a heating rate of 5°C/min. Then the calcined solids were reduced using a freshly prepared aqueous solution of NaBH4 under vigorous stirring for 10 min. The reduced catalysts were then washed with water to neutral and dried in a vacuum oven. 2.3 Conversion of Lignin and Lignin Dimer Depolymerization of lignin was conducted in a 50 mL high-pressure autoclave (SLM50). Briefly, the reactor was loaded with 0.2 g of lignin, 0.1 g of catalyst, 20 mL of isopropanol, and 10 mL of deionized water. Any trace of air in the reactor was driven out by fluxing nitrogen 5 times. Then, the reactor was pressurized with nitrogen to 0.5 MPa and electrically heated to the desired temperature (200−280°C). After reaction under 600 rpm for 8 h, the reactor was cooled by an electric fan to 100°C and then quenched with water to room temperature. One quarter of the product mixture was taken, and the reaction solvents (including isopropanol and water) was removed at 60°C under reduced pressure in a rotary evaporator. The remained residue was


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extracted with diethyl ether (3×10 mL). The organic soluble fraction was collected and concentrated by removing diethyl ether to obtain a viscous oil. Then 1 mL ethanol with n-dodecane as the internal standard was added to dissolve the oil product and the mixture was analyzed by GC MS and GC FID. At least twice runs were carried out for each condition, and the average values were used for analysis. Hydrogenolysis of lignin dimer was also conducted in the 50 mL high-pressure autoclave following a similar process to lignin depolymerization. Briefly, the reactor was loaded with 0.05 g of 2-(2', 6'-dimethoxyphenoxy)-1-phenylethanol, 0.05 g of catalyst, 20 mL of isopropanol, and 10 mL of deionized water. After the air was driven out, the reactor was pressurized with nitrogen to 0.5 MPa and electrically heated to the desired temperature (245 and 265°C). After reaction under 600 rpm for 8 h, the reactor was cooled by an electric fan to 100°C and then quenched with water to room temperature. Then 1 mL liquid product and 1 mL ethanol with n-dodecane as the internal standard was combined and analyzed by GC MS and GC FID. 2.4 Characterization Methods Scanning electron microscope (SEM) images were captured with a JEOL JSM-6300 SEM system. High resolution transmission electron microscopy (HR-TEM) images were taken on a JEOL 2100F operated at 200 kV to observe atomic structure of catalysts. Nitrogen adsorption/desorption for the catalysts was carried out on an Autosorb-iQ 2 instrument (Quantachrome, USA) using nitrogen as the adsorbate at 196℃. Brunauer-Emmett-Teller (BET) theory was used to determine the surface area.


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The pore volumn was obtained using the adsorption amount at the relative pressure of 0.995. X-ray diffraction was carried out on a D/max 2500 VL/PC X-ray diffractometer using Cu Kα radiation (40 kV, 200 mA). The low-angle XRD patterns were collected from 0.5 ° to 5 ° at 0.5 °/min, while the wide-angle XRD patterns were collected from 5° to 85° at 5 °/min. X-ray photoelectron spectroscopy (XPS) was carried out on a ESCALAB 250xi, using Al Kα radiation (1486.6 eV) at a pass energy of 30 eV under ultra high vacuum(∼10−8 mbar). The binding energies were calibrated using adventitious carbon (C1s peak at 284.8 eV). The main monophenolic compounds were determined with a GC/MS system (Agilent 5977B MSD) equipped with a RESTEK VMS capillary column (30 m× 0.25 mm× 1.4 μm). Helium was used as the carrier gas. The oven temperature was programmed from 50°C (3 min) to 150°C with a 25°C/min heating rate and then to 230°C (15 min) with a 6°C/min heating rate. Products were identified according to NIST MS database and reported results.37 Yields of the main monophenolic compounds were quantified with external standard method using standard compounds assisted with a GC/FID system (Shimadzu GC2014) equipped with a AT.SE 54 capillary column with the helium of 1.0 mL/min. For products without commercial standard compounds available, the response factor of compounds with similar structure were applied. Mass yields of monomeric phenols (Yi) were calculated as Yi=Mi/Mlignin×100%

(1)

Where Mi is the mass of product i, and Mlignin is the mass of starting lignin.


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Selevtivities of the monomeric phenols (Si) were calculated as, Si=Mi/Mtotal×100%

(2)

Where Mtotal is the total mass of 18 monomeric phenols detected by GC MS.

3. Results and Discussion 3.1 Catalytic Depolymerization of Lignin CECL was used to testify the performance of the catalysts and the effects of temperature on depolymerization. 18 main monophenols were identified as listed in Table 1. The products can be categorized into three groups as phenol-type (H), guaiacol-type (G), and syringol-type (S). Table 1 Main monophenols from depolymerization of lignin Label

Compounds

Label

Compounds

Label

Compounds

H1

4-Methylphenol

G4

4-Vinylguaiacol

S1

Syringol

H2

4-Ethylphenol

G5

4-Propylguaiacol

S2

4-Ethylsyringol

H3

4-Vinylphenol

G6

4-Propenylguaiacol

S3

4-Propylsyringol

G1

Guaiacol

G7

Vanillin

S4

4-Propenylsyringol

G2

4-Methylguaiacol

G8

Acetovanillone

S5

Acetosyringone

G3

4-Ethylguaiacol

G9

Guaiacylacetone

S6

Syringylacetone

3.1.1 Effect of catalyst


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Figure 1. Yields of main monophenols for CECL depolymerization over different catalysts at 220°C for 8 h Yields of main monophenols for CECL depolymerization over different catalysts are shown in Figure 1. Ni/SBA 15 achieved 4.15wt% monophenols in total at 220°C. 4-Vinylphenol (H3) and 4-vinylguaiacol (G4) had high yields of 1.07wt% and 0.32wt%, respectively. H3 and G4 were mainly derived from the disrupture of ferulates (FA) and p-coumarates (pCA) rich in grass lignin.35 Phenolic compounds with unsaturated groups on side chains, like acetovanillone (G8), 4-propenylsyringol (S4), and acetosyringone (S5), could also be identified, indicating the weak hydrogenation ability of Ni/SBA 15 in the presence of in situ hydrogen. When Pd/SBA 15 was present, the total yield of monophenols reached 5.65wt%. 4-Propylsyringol (S3) had the highest yield of 2.39wt% with 42.33% selectivity. 4Ethylphenol (H2) and 4-ethylguaiacol (G3) followed with 0.91wt% yield and 0.41wt% yield, respectively. The total selectivity towards above three compounds reached 65.73%. Phenolic compounds with unsaturated side chains were not detected. The 10 ACS Paragon Plus Environment

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yields of main monophenols over commercial Pd/C were close to the values over Pd/SBA 15. Obviously, Pd/SBA 15 showed higher activity than Ni/SBA 15. Cai et al. studied the β-O-4 linkages hydrogenolysis over various Pd–Ni monometallic and bimetallic nanoparticles using NaBH4 as hydrogen source, reporting that Pd1Ni8/ZrO2 performed the best for cleaving the β-O-4 bonds.26 Kim et al. evaluated the performances of various Ru-Ni monometallic and bimetallic catalysts using H2 as hydrogen source, and Ru0.6Ni0.4/SBA-15 afforded the most monomeric phenols.28 Consistent with these studies, using isopropanol as in situ hydrogen source, Ni-Pd/SBA-15 catalysts exhibited higher activity than the parent monometallic catalysts. 26, 28 Ni50Pd50/SBA 15 afforded the highest total monophenols yield of 8.14wt%, and this value was 1.96 times that of Ni/SBA 15 and 1.44 times that of Pd/SBA 15. Over Ni50Pd50/SBA 15, S3 had the highest yield of 2.10wt%, and H2 followed with 1.24wt%. The high activity of Ni-Pd bimetallic catalyst indicates the existence of synergistic effect between the two metals.25, 28, 34 3.1.2 Effect of temperature


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Figure 2. Yields of main monophenols for CECL depolymerization over Pd50Ni50/SBA-15 against reaction temperature for 8 h Temperature is a very important factor in lignin depolymerization 37. The depolymerization of CECL over Ni50Pd50/SBA 15 which performed the best activity was carried out at 200−285°C to investigate the effect of temperature. The yields of main monophenols against reaction temperature are shown in Figure 2. At 200°C, the total yield of main monophenols was 6.31wt%. S3 had the highest yield of 1.96 wt%, indicating that β-O-4 linkages with S-type phenylpropane units could be cleaved easily at low temperature.37 Figure 3 shows the total-ion chromatogram (TIC) of the liquid product of CECL depolymerization over Pd50Ni50/SBA-15 at 245°C for 8 h. The peak at 6.94 min is n-dodecane which was used as the internal standard. At 245°C, the total yield of main monophenols reached 10.68wt%. H2 had the highest yield of 2.33wt%, indicating that cleavage of ferulates in grass lignin was extensively promoted at this temperature. In general, high temperature means more energy for cleavage of stronger bonds, resulting in the increased yields of monophenols.27 However, as the 12 ACS Paragon Plus Environment

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temperature was further increased to 265−285°C, the total yield of main monophenols decreased slightly to 10.24−10.62wt%. With the temperature increasing from 200°C to 285°C , S-type products, especially S2 and S3 increased, showing that the cleavage of β-O-4 bonds linking to S-type phenylpropane units was promoted at high temperature. However, products like H2, G3, S5, and S1 had the maximum yields around 245°C, which could be due to condensation to form solid char and/or hydrogenation to form hydrogenated products at high temperature.27, 38 Further work on the exact reason for the decrease of certain monomers was ongoing.

Figure 3. The total-ion chromatogram (TIC) of lignin (CECL and ABL) depolymerization product over Pd50Ni50/SBA-15 at 245°C for 8 h

3.1.3 Effect of starting lignin Lignin is a polymer of phenylpropane units linked by C-O and C-C bonds, and cleavage of these bonds produce monophenols. The monophenol yields are related to 13 ACS Paragon Plus Environment

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many factors, including catalysts, reaction conditions and the various starting lignin materials. These factors varied significantly in the published work, leading to the monophenol yields from isolated lignin ranging greatly from 4wt% to 30wt%.3, 32, 39, 40 To study the effect of starting lignin, the depolymerization of ABL was also tested over Pd50Ni50/SBA-15 at 245°C and 265°C for 8 h. The TIC of the liquid product at 245°C is included in Figure 3. Over Pd50Ni50/SBA-15, the total yield of monophenols from ABL reached 18.52wt% at 245°C and 19.36 wt% at 265°C. At 245°C, the yield of S3 over Pd50Ni50/SBA-15 reached 8.89 wt% with 47.97% selectivity. Under the mild conditions in present work, the monophenols were mainly produced by cleavage of C-O bonds. The higher monomer yields of ABL than that of CECL was mainly ascribed to the abundant C-O bonds (36 β−O−4 bonds per 100 phenylpropane units in ABL), which has been revealed by HSQC in our published work.35 CECL and ABL showed different products distribution. CECL mainly afforded 4-ethylphenol and 4ethylguaiacol which were derived from ferulates and p-coumarates rich in grass lignin. While ABL produced 4-propylsyringol and 4-ethylsyringol as the main products, consistent with the abundant syringyl subunits in birch lignin.35

3.1.4 Reusability of Pd50Ni50/SBA-15 The recyclability of Pd50Ni50/SBA-15 which showed the highest conversion efficiency was tested on the depolymerization of ABL at 245°C. It has been reported that biochar formed during the depolymerization process would deposit on the surface


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of catalyst, resulting in the decrease of accessible active sites. 16, 28 Because these biochar could not be effectively separated through conventional solvent extraction, the collected solid fraction, including used catalyst and biochar, was firstly burnt in air at 500°C for 2 h, and then reduced before each test of recyclability. The total yield of monophenols decreased slightly to 17.06wt% in the second run. Overall, the total yield of monophenols was still up to 16.23wt% after four runs (see Table S1), indicating that the catalyst is highly reusable, consistent with the reported results on Pd-Ni bimetal catalyst.26, 34

3.2 Catalytic Hydrogenolysis of Lignin Dimer The activity of Pd50Ni50/SBA-15 was further tested on 2-(2', 6'dimethoxyphenoxy)-1-phenylethanol, a typical β-O-4 model dimer of lignin.25, 41 Using isopropanol as the hydrogen source, the model dimer was converted into 55.46wt% syringol and 42.47 wt% acetophene at 245°C, confirming that β-O-4 bonds could be effectively cleaved in the present condition. Yan et al. reported that the same dimer was converted into 59% aromatic monomers, 13% hydrogenated monomers and 28% hydrogenated dimers over Ni85Ru15 nanoparticles with 1 MPa H2.25 Notably, the aromatic rings retained completely in the products in the present work, indicating that hydrogenation of aromatic rings was effectively inhibited in the present catalytic transfer hydrogenolysis system.


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3.3 Physicochemical Properties of Catalysts. To probe the structure-reactivity relationship in catalytic depolymerization of lignin, characterization of the catalysts was conducted.42

Figure 4. HRTEM microphotographs and metal particle size distribution of Ni/SBA15 (a, b), Pd50Ni50/SBA-15 (c, d) and Pd/SBA-15 (e, f) Figure S1 shows the SEM images of the catalysts. All catalysts present a wheatlike characteristic morphology of the SBA 15.43 No specific description of Ni or Pd particles could be defined in the images. Figure 4 shows the HRTEM images of the catalysts. A well-ordered mesoporous channel structure of SBA-15 retained in the catalysts, indicating that introducing metal species in the channels did not affect the 16 ACS Paragon Plus Environment

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framework of the mesoporous materials.44 The Ni particles in Ni/SBA-15 exhibited a wide size distribution between 1.50 nm and 4.50 nm, with a mean diameter of 3.41 nm. The mean size of Pd particles in Pd/SBA-15 was 3.06 nm which was lower than that of Ni. In Pd50Ni50/SBA-15, the metal particles distributed in two narrow size ranges around 2.25 nm and 3.75 nm, respectively, resulting in the lowest mean diameter of 2.71 nm. The narrow distribution of metal particles with small size of Pd50Ni50/SBA-15 can provide more contact opportunities between the active sites and substrates, which means higher catalytic efficiency.45


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Figure 5. The N2 adsorption /desorption isotherm curves (a) and pore width distribution (b) of catalysts Table 2 Porous characteristics of catalysts Samples

BET surface

Pore volume

Average

pore

area (m2/g)

(cm3/g)

diameter (nm)

SBA-15

435.23

0.64

5.90

Ni/SBA-15

399.32

0.67

6.70

Pd50Ni50/SBA-15

379.28

0.62

6.49

Pd/SBA-15

346.08

0.58

6.65

The N2 adsorption /desorption isotherm curves and pore width distribution of the catalysts are displayed in Figure 5, and the porous characteristics are shown in Table 2. All catalysts and SBA 15 showed a typical IV isotherm with H1 hysteresis loop at the relative pressure p/p0=0.55-0.75, indicating that a well-defined hexagonal pore structure remained after the impregnation of metals.28 As compared to SBA 15, the


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total pore volume and the BET surface area of catalysts were decreased, probably due to the blockage of the pore system by some loaded metal particles. Incorporation of active metal species inside the internal pore channels would benefit the orderly growth of particle with smaller size.44 Generally, the well-remained hexagonal pore structure of the SBA 15 can provide more contact opportunities between the active sites and substrates.

Figure 6. Small- angle (a) and wide-angle (b) XRD patterns of catalysts Figure 6 shows the small- and wide-angle XRD patterns of the catalysts. Three


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well-resolved peaks for (1 0 0), (1 1 0) and (2 0 0) reflection planes appeared in all the small-angle XRD patterns, corresponding to the hexagonal mesoporous structure of SBA-15 (see Figure 6a).43 The intensity of (1 0 0) plane was decreased after introducing metals. The addition of metals also resulted in the shift of the three peaks to lower angles, corresponding to an increase of lattice parameter a0 from 9.81 nm for SBA 15 to 10.63 nm, 10.59 nm, and 10.67 nm for Ni/SBA-15, Pd50Ni50/SBA-15 and Pd/SBA-15, respectively. The decreased intensity and the shift of three peaks to low angles indicated that the metal particles were well loaded inside the well-preserved mesopores, as reported elsewhere. 46 Obviously, introducing metals did not damage the ordered hexagonal mesoporous structure of SBA-15.28 The broad diffraction peak at 2θ=15–30o in wide-angle XRD patterns was attributed to the amorphous silica of SBA-15 (see Figure 6b).28 No peak for metallic Ni and NiO could be identified in Ni/SBA-15 and Pd50Ni50/SBA-15, suggesting that the particles were well-dispersed.34 The monometallic Pd/SBA-15 displayed four diffraction peaks at 2θ =40.36°, 45.68°, 68.40°, 81.90° for (111), (200), (220), (311) corresponding to metallic Pd (JCPDS file: 46-1043), and four peaks at 2θ =33.2°, 42°, 55°, 61° for (101), (110), (112), (200) corresponding to PdO (JCPDS file: 41-1107).26, 44 In the pattern of Pd50Ni50/SBA-15, Pd and PdO showed broad peaks with low intensity. The XRD patterns showed that the Pd in bimetallic catalysts exhibited a higher dispersion than monometallic catalysts.


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Figure 7. XPS spectra of Ni 2p and Pd 3d for Ni/SBA-15 (a), Pd50Ni50/SBA-15 (b, c) and Pd/SBA-15 (d) XPS was carried out to further identify the surface chemical states. XPS spectra of Ni 2p and Pd 3d for Ni/SBA15 (a), Pd50Ni50/SBA15 (b, c) and Pd/SBA15 (d) are shown in Figure 7. The Ni 2p3/2 region (845 eV – 869 eV) and Pd 3d5/2 region (330.00 eV – 338.5 eV) with higher peak intensities were used for analysis. The Ni 2p spectrum of Ni/SBA-15 (see Figure 7a) reveals obvious existence of NiO and Ni(OH)2 at 856.59 eV.47 The peak of metallic Ni around 852.00 eV could hardly be identified for Ni/SBA-15, consistent with reported results. Due to the oxophilic nature 21 ACS Paragon Plus Environment

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of Ni, the surface bound nickel atoms are easily to be oxidized when exposed to oxygen.26, 28, 47 However, in Ni 2p spectrum of Pd50Ni50/SBA15 (see Figure 7b), a small diffraction peak of metallic Ni appeared at 852.05 eV, indicating that the presence of Pd might help to alleviate the oxidation of metallic Ni. The Pd 3d spectrum of Pd/SBA15 (see Figure 7d) reveals metallic Pd (Pd0) at the binding energy peaks of 335.42 eV (Pd 3d5/2) and Pd2+ at 336.3 eV. For the Pd 3d spectrum of Pd50Ni50/SBA15 (see Figure 7c) , peaks for Pd0 and Pd2+ were also identified. Notably, an obvious shift of Pd 3d to higher binding energy (about 0.1–0.4 eV) were observed for Pd50Ni50/SBA15, suggesting that the electronic structure of Pd was modified by Ni.26 The relative contents of Pd and Ni species on the catalyst surface were further quantitatively compared using the areas of the characteristic peaks, and the results were listed in Table 3. Pd50Ni50/SBA-15 had more Pd0 than Pd/SBA15. 19.33% Ni species in Pd50Ni50/SBA-15 was retained as Ni0. The higher relative content of Pd0 and Ni0 in Pd50Ni50/SBA-15 as compared with monometallic catalysts suggest that the interaction between the bimetals might facilitate the reduction of NiO and PdO or alleviate the oxidation of reduced metals. Table 3 The relative contents of Pd and Ni species in catalysts (%) Samples

Pd0

PdO

Ni0

NiO/Ni(OH)2

Ni/SBA-15

0.00

0.00

0.00

100.00

Pd50Ni50/SBA-15

74.81

25.19

19.34

80.66

Pd/SBA-15

65.40

34.60

0.00

0.00


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3.4 Possible Mechanism of Bimetallic Catalyst on Lignin Depolymerization. As revealed above, the bimetallic Pd50Ni50/SBA-15 catalyst possessed the following structural features: (1) narrow distribution of metal particles with small size; (2) wellremained hexagonal pore structure of the SBA 15; (3) a higher dispersion of Pd than monometallic catalysts; (4) more Pd0 and Ni0 as compared with monometallic catalysts. Lignin is a polymer composed of phenylpropane units connected with C-O bonds and C-C bond. Various mechanisms have been proposed for lignin depolymerization to produce monomeric phenols.3, 38, 40 Generally, the mechanisms comprise a chemisorption of lignin fragments on the active sites on metal interface, bond breaking, and eventual desorption.48 In the present system, monomeric phenols were mainly formed by the hydrogenolysis of C-O bonds, like β-O-4 bonds, ferulates esters and p-coumarates esters, while cleavage of C-C bonds with higher dissociation energy hardly happen. Hydrogen for the hydrogenolysis is provided by the in situ Pd-H formed from isopropanol contacting with the active sites.8, 29


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Figure 8. A possible mechanism of bimetallic catalyst on transfer hydrogenolysis of lignin Figure 8 presents a possible mechanism of bimetallic catalyst on transfer hydrogenolysis of lignin. Adsorption is the first step to all heterogeneous catalyst reactions. The primary requirement of an efficient lignin depolymerization is that both the lignin fragments and isopropanol should be closely adsorbed and activated on catalytic active sites at the molecular level.38, 49 In the bimetallic catalyst system, isopropanol could easily transform to H and acetone over active sites on Pd, and the continuous spillover of H from Pd to Ni would result in abundant active hydrogen over the catalyst surface of Ni-Pd catalyst system.8, 23 Then, the chemisorption of real lignin, a three-dimensional macromolecular compounds, should be the limited step. As compared with monometallic catalysts, the bimetallic catalyst particles with more uniformly dispersed Pd0 and Ni0 on the larger surface of SBA-15 can provide more contact opportunities between the abundant active centers and substrates, which significantly promote the hydrogenolysis process.26

4. Conclusion The bimetallic Ni-Pd catalysts exhibited higher activity than the parent monometallic catalysts in lignin depolymerization. The bimetallic particles with more uniformly dispersed Pd0 and Ni0 on the larger surface of SBA-15 can provide more contact opportunities between the abundant active centers and substrates. Among the


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various bimetallic catalysts, Ni50Pd50/SBA 15 afforded the highest total monophenols yield of 8.14wt% for deplymerization of CECL at 220oC after 8h. The total yield of monophenols from ABL over Ni50Pd50/SBA 15 reached 18.52wt% at 245°C. Replacement of partial costly Pd with inexpensive nickel could greatly enhance the catalytic transfer hydrogenolysis of lignin to monophenols, while substantially lowering the catalyst material cost. Furthermore, the catalytic transfer hydrogenolysis process using isopropanol as the hydrogen donor without molecular hydrogen significantly lower the energy input and equipment investment.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. SEM images of Ni/SBA-15, Pd50Ni50/SBA-15 and Pd/SBA-15 (Figure S1), Yields of main monophenols for ABL depolymerization over reused Pd50Ni50/SBA15 (Table S1).

Author information Corresponding authors *E-mail: [email protected] (Jun Hu) *E-mail: [email protected] (Xiaoxiang Jiang) ORCID


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Jun Hu: 0000-0001-9250-8853 Notes The authors declare no competing financial interest.

Acknowledgements The authors greatly acknowledge the funding support from the projects supported by the National Natural Science Foundation of China ( 51706110) and Jiangsu Key Lab of Biomass Energy and Materials (Grant JSBEM201918).

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Depolymerization of lignin over Ni-Pd bimetallic catalyst using

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