Highly efficient conversion of Kraft lignin into liquid fuels with a Co-Zn-beta zeolite catalyst

Journal Pre-proof Highly efficient conversion of Kraft lignin into liquid fuels with a Co-Zn-beta zeolite catalyst Xiaomeng Dou, Xiao Jiang, Wenzhi Li, Chaofeng Zhu, Qingchuan Liu, Qiang Lu, Xusheng Zheng, Hou-min Chang, Hasan Jameel

PII:

S0926-3373(19)31175-0

DOI:

https://doi.org/10.1016/j.apcatb.2019.118429

Reference:

APCATB 118429

To appear in:

Applied Catalysis B: Environmental

Received Date:

24 April 2019

Revised Date:

31 August 2019

Accepted Date:

11 November 2019

Please cite this article as: Dou X, Jiang X, Li W, Zhu C, Liu Q, Lu Q, Zheng X, Chang H-min, Jameel H, Highly efficient conversion of Kraft lignin into liquid fuels with a Co-Zn-beta zeolite catalyst, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118429

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Highly efficient conversion of Kraft lignin into liquid fuels with a Co-Zn-beta zeolite catalyst

Xiaomeng Doua, Xiao Jiangb, Wenzhi Lia*, Chaofeng Zhua, Qingchuan Liuc, Qiang

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Lud, Xusheng Zhenge*, Hou-min Changb and Hasan Jameelb

Laboratory of Basic Research in Biomass Conversion and Utilization, University of

Science and Technology of China, Hefei 230026, PR China.

Department of Forest Biomaterials, North Carolina State University, Raleigh, NC

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b

c

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27695-8005, USA

School of Biological and Medical Engineering, Hefei University of Technology,

d

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Hefei 230009, PR China.

National Engineering Laboratory for Biomass Power Generation Equipment, North

China Electric Power University, Beijing 102206, PR China. National Synchrotron Radiation Laboratory, University of Science and Technology

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e

of China, Hefei 230029, PR China.

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* Corresponding author.

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E-mail address: [email protected], [email protected].

Graphical abstract

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Highlights: Synergistic effect between Lewis acid sites and hydrogen binding sites.

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Maximally 81 % of monomeric and dimeric degradation products was obtained.

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The HHV of liquid product was increased from 26.0 MJ/kg to 33.3 MJ/kg.

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Co-Zn/Off-Al H-beta exhibits excellent recyclability in lignin depolymerization.

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Abstract

Kraft lignin depolymerization to liquid fuels with high yields is crucial to the

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comprehensive achievement of sustainable and economic feasibility. Herein, we prepared a bimetallic Co-Zn/Off-Al H-beta catalyst through a two-step post synthesis

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method composed of dealumination and metal incorporation. The bifunctional Co-

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Zn/Off-Al H-beta catalyst efficiently converted Kraft lignin to liquid fuels, which was attributable to the synergistic effect of Co hydrogen binding sites and Zn Lewis acid sites on H-beta support. Catalytic hydrogenation with Co:Zn=1:3/Off-Al H-beta catalyst at 320 °C for 24 hours gave the highest yield of petroleum ether soluble product (81%, mainly monomers and dimers). Under these conditions, the liquefied lignin gave a higher heating value of 33.3 MJ/kg, which is a significant increase from 2

26.0 MJ/kg of Kraft lignin. The catalyst stability test showed excellent recyclability. This work provides a paradigm of improving lignin depolymerization efficiency via the combined use of Lewis acid and hydrogenation catalyst.

Keywords: Kraft lignin; synergistic catalyst; lignin degradation; liquid fuels

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1. Introduction

As environmental pollution intensifies, alternatives to fossil fuels are needed to

support long-term energy needs with low impact on overall carbon emissions [1, 2].

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Biomass, the only renewable carbon source in nature, is a potential abundant and

inexpensive sustainable feedstock for fuels [3, 4]. Lignin is 10-35% of lignocellulosic

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biomass by weight, but accounts for 40% of its energy content [5, 6]. Lignin as the

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most abundant aromatic biopolymer on earth that is proving to be a particularly promising resource [3, 7, 8]. It is often presented as a future feedstock for fuels and

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aromatic chemicals [4]. However, because of its high stability, especially for Kraft lignin which is one of the most recalcitrant lignin types, efficient depolymerization

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approaches are lacking, and 98% of the lignin produced by the paper and pulp

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industry and most of which is Kraft lignin generating 40-50 million tons per years, was typically as a waste stream and a low heating value boiler fuel [9-13]. Only 5% of Kraft lignin is effectively utilized in low-value commercial applications [14]. Kraft lignin can be appealing raw materials as they are produced during the processing of lignocellulosic materials and is commercially available in large quantities [15]. Since an increase of excessive stable C-C content (at the expense of cleavable ether bonds) 3

in Kraft lignin lower the potential for the production of chemicals [14]. Hence, developing strategies for Kraft lignin depolymerization is the key to allowing application in liquid fuels and improving the problem of environmental impacts associated with the paper-making process [16]. The main challenges for processing lignin are attributed to its complex threedimensional amorphous structure and labile lignin degradation intermediates that tend

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to repolymerize [17, 18]. For the first challenge, a large proportion of previous

research on lignin valorization, including exploring acid/base-catalyzed hydrolysis

and pre-activation approaches (e.g., by selective pre-oxidation), has been devoted to

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cleavage of C-O and C-C bonds [19-22]. Several approaches to address the second

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challenge have also been attempted, such as the use of trapping agents, including formaldehyde [18], ethanol [23, 24], diols [25], and phenol [26], to suppress the

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condensation of the phenolic intermediates and reduce char formation. One of the

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most popular and efficient strategies to address the second challenge is to rapidly stabilize lignin degradation intermediates by using hydroprocessing before additional

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repolymerization can occur [14, 27]. Combine with these strategies, new types of catalysts have been developed that are multifunctional.

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In recent years, the bifunctional catalysts containing a balance of metal and acid

sites catalytic transform lignin into chemicals and fuels has received strong attention. For example, combining a dual acid-base catalyst S2O82--KNO3/TiO2 and a hydrogenation catalyst Ru/C [13]. This catalyst system produced petroleum ether soluble product at a yield of 67%, composed mainly of monomeric and dimeric 4

degradation products. As the starting Kraft lignin has only 2% solubility in petroleum ether, this high yield is ascribed to the combination of the depolymerization of lignin with the hydrogenation of depolymerized compounds. Jin et al. [28] took a solid acid HTaMoO6 and Rh/C as a mixture to investigate lignin depolymerization. This work also illustrated that compared with only an acid catalyst, a combination of an acid and a hydrogenation catalyst improved the yield of petroleum ether soluble product.

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Similarly, Shu et al. [29] reported on the application of a composite catalyst physically composed of two catalysts, Pd/C and Lewis acid AlCl3, in the

hydrogenolysis of lignin, and obtained a high yield of guaiacol and phenol. These

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results indicate that the combination of bond cleavage and stabilization of the

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intermediates is very important for lignin depolymerization. Compared to reported bifunctional catalysts prepared by physical mixing, it has been confirmed that the

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closely contacted two individual active species have a better effect [30]. Zeolites are

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attractive as supports due to their porous, regular structure and acid tenability [31]. However, one difficulty is that coke/char forms on the zeolites, which will cause

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deactivation of the catalyst. Etching the aluminum out of the zeolite frame using acid could reduce acidity and thereby decrease coke formation [14]. Furthermore, these

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characteristics allow catalysts to be prepared by filling metal precursors into the vacancies left by etched aluminum, thereby meeting the above requirements for active site distribution [32]. Recently, Parsell et al. [33] proposed that zinc played an important role in lignin model compound disassembled by activating ether bonds and had been shown to 5

facilitate removal of the hydroxyl group at the Cγ position of the β-O-4 ether linkage. Klein et al. [34] further described an activation mechanism for the cleavage of the βO-4 ether linkage via Zn2+ through formation of a six-membered ring complex of Zn2+ coordinated to the oxygen atoms at Cα and Cγ of the lignin model compound guaiacylglycerol-β-guaiacyl. Zinc also can enhance the adsorption of lignin degradation products that contain C=O and C-O bonds onto the surface of the catalyst

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and facilitate the hydrogenation and hydrodeoxygenation reaction [35-37]. Thus, it seems more reasonable to use Zn instead of Al in the zeolite catalyst for lignin

depolymerization owing to its proper acidity and adsorption activity. Numerous

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experiments have provided cobalt has high hydrogenation activity [38, 39]. For

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instance, Liu et al. [39], using CoNx@NC to catalyze eugenol conversion, found that cobalt nitrides played an important role in the catalytic hydrodeoxygenation reactions.

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Romero et al. [40] also reported that the addition of Co increased the

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hydrodeoxygenation rate of 2-ethylphenol. In this study, inspired by these studies, we design a bifunctional catalyst by employing the incorporation metal precursors into a

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dealuminated H-beta zeolite framework, of which Zn2+ cations act as Lewis acid and adsorption sites for the depolymerization process while Co species act as hydrogen

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binding sites for the hydrogenation reaction. This preparation method well forms highly dispersed metal embedding, and retains the original pore structure of the zeolite. This study focused on the effect of bifunctional Co-Zn/Off-Al H-beta catalyst on lignin depolymerization to liquid fuels. The catalytic performance was mainly 6

evaluated by the yield of liquid product, the yield of petroleum ether soluble product, and the higher heating value (HHV). A meticulous study on Co-Zn/Off-Al H-beta catalyst revealed that a synergistic effect between the Co and Zn species resulted in the high yield of petroleum ether soluble product.

2. Experimental

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2.1. Materials

The Kraft lignin was produced by Meadwestvaco (WestRock now) at Charleston,

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South Carolina, USA and purchased from WestRock China (Shanghai, China). The elemental composition of Kraft lignin is 62.96 wt% C, 5.82 wt% H, 28.32 wt% O,

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0.72 wt% N, 1.68 wt% S with 2.0 wt% ash. The chemical characteristics of Indulin

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ATTM were described by Chang and co-workers [41]. Cobalt acetate tetrahydrate, cobaltosic oxide, zinc acetate, and zinc oxide were purchased from Sinopharm

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Chemical Reagent Co., Ltd. (Shanghai, China). Commercial H-beta (ID: NKF-625YY) with a Si/Al ratio of 12.5 was purchased from Nankai University Catalyst Co.,

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Ltd (Tianjin, China). Methanol, 1,4-dioxane, ethyl acetate and petroleum ether were of analytical grade and bought from Sinopharm Chemical Reagent Co., Ltd.

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(Shanghai, China). Phenol, ortho-xylene, guaiacol and so forth reagents were purchased from Aladdin Chemical Reagent Co. Ltd. 2.2. Catalyst preparation Commercial H-beta was used to synthesize metal-incorporated beta zeolite catalyst. Pristine H-beta was first treated with 13M HNO3 (20 mL/gzeolite) at 100 °C 7

for 20 hours to obtain a dealuminized beta material (Off-Al H-beta). The treated solids were washed with deionized water (until pH = 7) and recovered by centrifugation. The recovered solids were dried overnight at 80 °C and further ground in a mortar prior to use. Dealuminized beta samples containing cobalt and zinc were synthesized via a post-synthetic method. The catalysts preparing procedure is as follows: 0.5 g of the dealuminized beta zeolite host powder was finely ground with an

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appropriate amount of cobalt acetate tetrahydrate and zinc acetate precursor in a

mortar to achieve an intimate mixture. The total quantity of Co and Zn was kept

constant at 0.844 mmol/gzeolite with different molar ratios of Co and Zn (1:3, 1:1, and

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3:1) investigated. Then the solid mixture was calcined at 550 °C for 5 hours with a

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5 °C/min heating rate under air flow. For reference, Co- and Zn-containing zeolites

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were also prepared using similar procedures. 2.3. Catalyst characterization

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BET surface areas (BET) of samples were measured by isothermal nitrogen adsorption/desorption (-196 °C) on Tristar II 3020M (Micromeritics Instruments,

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American). These catalysts were degassed for more than 10 hours at 300 °C under

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vacuum before N2 adsorption. Powder X-ray diffraction patterns (XRD) of catalysts were recorded on a TTR-

III (Japan) with Cu Kα radiation (λ= 1.54184 Å) from 5◦ to 85◦ at a scanning rate of 8◦/min.

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The silicon, aluminum, cobalt, and zinc contents of samples were analyzed with a PerkinElmer Optima 7300 DV inductively coupled plasma-atomic emission spectrometer (ICP-AES). Transmission electron microscopy (TEM) images and element mapping analysis of Co-Zn containing zeolite catalyst sample were taken on a JEM-2100F FETEM (Japan) with an acceleration voltage of 200 kV.

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X-ray photoelectron spectra (XPS) of the materials were taken on a Thermo

ESCALAB 250Xi with a monochromated Al Ka X-ray source (hv = 1486.6 eV). The binding energies were calibrated using the C 1s peak at 284.6 eV.

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Extended X-ray absorption fine structure spectra (EXAFS measurements). The

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extended X-ray absorption fine structure (EXAFS) spectra were collected at 1W1B station in Beijing Synchrotron Radiation Facility (BSRF). EXAFS measurements at

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the Co K-edge were performed in a transmission mode, using ionization chambers

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with optimized detecting gases to measure the radiation intensity. FTIR spectra of pyridine adsorption (FTIR-pyridine) of the catalysts were

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acquired on a Nicolet is50 FTIR spectrometer. The catalyst samples were activated in the IR cell by heating from ambient temperature to 300 °C under vacuum (1×10-3 Pa)

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and evacuated at this temperature for 1 hour. After cooling to 30 °C, the catalyst was saturated with pyridine vapor and then evacuated at 30 °C for 10 min prior to recording the IR spectra. The spectra were recorded at evacuation temperature in the 1700-1400 cm-1 range by co-addition of 32 scans.

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NH3-Temperature Programmed Desorption (NH3-TPD) measurements were carried out on Automatic Chemical Adsorption Instrument (Quantachrome Instruments, American). 100 mg of catalyst sample was pretreated in a flow of helium (30 mL/min) at 700 °C for 1 hour, and after cooling to 100 °C, then saturated with 5% NH3/He. Subsequently, the excessive, physically adsorbed ammonia was removed by purging with a helium flow rate of 30 mL/min.

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H2-Temperature Programmed Reduction (H2-TPR) studies were also carried out on an Automatic Chemical Adsorption Instrument (Quantachrome Instruments,

American). 100 mg of catalyst sample was pretreated in a flow of argon (30 mL/min)

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at 350 °C for 1 hour, and after cooling to 50 °C, then saturated with 10% H2/Ar.

with a helium flow rate of 30 mL/min.

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Subsequently, the excessive, physically adsorbed hydrogen was removed by purging

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The elemental compositions (C, H, O, N, and S) of Kraft lignin and liquid

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product were obtained using an elemental analyzer (Vario EL III, Elementar Analysensysteme GmbH, Germany).

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X-ray computed tomography (X-ray CT). The X-ray CT video was obtained at the beamline BL07W in the National Synchrotron Radiation Laboratory in Hefei,

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China. The raw data were reconstructed using Mimics 16.0. The movie was made by assembling the static images in sequence. 2.4. Catalytic reaction of lignin Dioxane in the presence of a small amount of water or methanol is a powerful solvent for lignin including Kraft lignin. We select methanol instead of water since 10

methanol is a known hydrogen donor in catalytic hydrogenation [42, 43]. Preliminary studies indicated that methanol along is insufficient and much better results were obtained by supplement of hydrogen pressure. While 4 MPa of hydrogen gave better results than 2 MPa, we selected to use 2 MPa of hydrogen pressure due to the pressure limitation of our reactor. Similar findings have been reported by other researchers [27, 44].

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The catalytic depolymerization of lignin was conducted in a 100 mL stainless steel autoclave reactor equipped with an electromechanical agitator (Anhui Kemi

Machinery Technology Co., Ltd). In a typical run, 0.5 g lignin was dissolved in a

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mixture of 30 mL 1, 4-dioxane and 6 mL methanol under ultrasonication. Then the

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solution and 0.25 g catalyst were loaded into the autoclave. After purging the air in the reactor with H2 three times, the reactor was pressurized with H2 to 2.0 MPa. The

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reactor was heated to the desired reaction temperature with stirring at a rate of 700

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rpm. After the reaction, the reactor was cooled rapidly by submersion in cold water. The contents of the reactor were collected and the residual solid was separated from

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the liquid product by filtration. The residual solid was dried in an oven at 105 °C to a constant weight and weighed. The residual solid included catalyst and lignin

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condensation products (solid product), so the weight of solid product was calculated by subtracting the weight of catalyst from the weight of residual solid. All reactions were performed in triplicates.

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2.5. Products analysis A tiny aliquot of the liquid product was analyzed by GC/MS and GC. The remaining liquid was divided into two equal parts and the solvent was removed from both by rotary evaporation (RE-2000B, Yarong Instrument Company, Shanghai, China) at 40 °C. Then the two parts were weighed individually and the total weight was defined as the weight of liquid product. One evaporated part, half of liquid

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product, was dissolved in 1 mL of acetone and the solution was added dropwise into 200 mL ethyl acetate under constant stirring at a rate of 450 rpm. The mixture was

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filtered with a sand core funnel and the filtrate was evaporated with a rotary

evaporator at 30 °C. The final product was weighed and the weight multiplied by two

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was taken as the weight of ethyl acetate soluble product.

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The other half of the liquid product was dissolved in 1 mL of acetone and the solution added dropwise into 200 mL of petroleum ether. The protocol was the same

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as the precipitation into ethyl acetate as described in the last paragraph. For gas product analysis, the gas phase was collected with airbags and analyzed

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using a gas chromatograph (GC-1960) with two detectors, a TCD for analysis of H2,

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N2, and CO, and an FID for gas hydrocarbons. The monomers in liquid product were identified by GC/MS (Agilent 19091S-433)

equipped with a HP-5MS column (30 m × 250 μm × 0.25 μm) [45]. Quantitative analysis of the monomer products was determined by a GC-2010 gas chromatograph with an FID and a HP-5 column. The oven temperature program increased from 50 °C (held for 3 min) to 250 °C (held for 10 min) at a rate of 10 °C/min. Acetophenone was 12

used as an internal standard for quantitative analysis of the monomer products. There were some unknown peaks in the GC chromatograph. The correcting coefficients of these unknown monomers cannot be defined. Hence, the average correcting coefficient of all identified monomers was applied to unknown monomers [46]. The yields of monomers, liquid product, solid product, gas product, ethyl acetate soluble product and petroleum ether soluble product were obtained according to the

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equations (1)-(7) in Supplementary material. 2.6. Gel permeation chromatography (GPC)

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GPC was used to determine the molecular weights of lignin and depolymerization

products. Prior to analysis, acetylation was accomplished on original Kraft lignin and

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petroleum ether insoluble fractions to enhance lignin’s solubility [47]. GPC analyses

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were performed using a Shimadzu instrument equipped with a UV detector (set at 280 nm) using tetrahydrofuran (THF) as the eluent at a flow rate of 0.7 mL/min at 35 °C.

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A sample concentration of 1 mg/mL and an injection volume of 50 µL were used. Two ultra styragel linear columns linked in series (Styragel HR 1 7.8 × 300 mm and

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Styragel HR 5E 7.8 × 300 mm) were used. A series of monodispersed polystyrene

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standards were used as calibration standards. 2.7. 2D HSQC NMR Two-dimensional (2D) 13C-1H Heteronuclear Single Quantum Coherence

(HSQC) correlation NMR spectra were acquired according to the procedures described by Capanema et al [48]. The lignin samples were dissolved in DMSO-d6

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with the concentration ca. 20% in a special Shigemi NMR. The 2D HSQC NMR spectra were acquired on a Bruker AVANCE 500 MHz spectrometer equipped with a 5 mm double resonance broadband BBI inverse probe using a coupling constant J1 CH of 147 Hz. The HSQC experiment was performed with a Bruker phase-sensitive gradient-edited HSQC pulse sequence ‘hsqcetgpsisp2’.

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3. Results and discussion

3.1. Catalyst characterization

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The crystalline phase and purity of synthesized Co-Zn/Off-Al H-beta zeolites are

shown in Fig. S1. All samples show similar diffraction peaks corresponding to BEA

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topology, indicating that the framework of beta zeolite remained unchanged after

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dealumination and Co-Zn incorporation. In comparison with pristine H-beta, the diffraction peaks of cobalt oxide or zinc oxide were not observed possibly because of

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the good dispersion of Co and Zn throughout the zeolite lattice and the low amount of Co and Zn loadings. The actual weight content of Co and Zn of the Co-Zn/Off-Al H-

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beta catalysts with various ratios of Zn and Co were measured by inductively coupled plasma-atomic emission spectrometer (ICP-AES), which reported that up to 5.4 wt%

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of Zn and Co was incorporated into the lattice of Off-Al H-beta zeolite (Table 1). Dealumination and metal incorporation were observed to be accompanied with lattice contraction/expansion of the zeolite framework structure, visualized by a change in the position of the narrow main diffraction peak (302) around 2θ of 22.50° [49]. As shown in Fig. 1, the significant shift from 22.48° to 22.72° indicates the contraction of 14

the matrix following the dealumination process. This peak decreases to 22.56° after incorporation of Co-Zn into the Off-Al H-beta, indicating the expansion of the beta zeolite structure and suggesting that metals were incorporated into the lattice [50]. Therefore, the analysis of XRD pattern indicates that Co-Zn species were

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incorporated into the vacant H-beta framework resulting from dealumination.

Fig. 1. XRD patterns of selected zeolite samples.

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Fig. S2 displays the N2 adsorption-desorption isotherms for H-beta, Off-Al H-beta

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and Co:Zn=1:3/Off-Al H-beta samples. H-beta shows a typical N2 adsorptiondesorption isotherm. The hysteresis loop of Off-Al H-beta clearly shows the increased

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N2 adsorption at any given relative pressure P/P0 of 0.4 ~1.0, indicating the successful construction of vacancies from H-beta. The hysteresis loop of Co:Zn=1:3/Off-Al H-

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beta is only slightly lower than that of Off-Al H-beta and much higher than that of the H-beta, which indicates good dispersion and incorporation of Co and Zn into the framework of zeolite, while maintaining the porous structure. Table 1 summarizes the physicochemical properties of zeolite samples in this experiment. H-beta sample had a total specific surface area of 469 m2/g with a pore 15

volume of 0.37 cm3/g. After dealumination (Off-Al H-beta), the specific surface area and pore volume increased to 545 m2/g and 0.45 cm3/g, respectively, which indicates successful vacancy formation. On the other hand, loading the Off-Al H-beta with different ratio of Co and Zn resulted in a small decrease of both total specific surface area and pore volume, but both were larger than those of the H-beta. The 1:3 CoZn/Off-Al H-beta had the largest specific surface area and pore volume among the

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Off-Al H-beta loaded with Zn and/or Co in this study. These results clearly confirm that the good dispersion of Co and Zn in Off-Al H-beta lattice vacant by

dealumination and agree with the conclusion obtained by XRD analysis.

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Table 1

Pore volume

Content (wt%)

(m2/g)

(cm3/g)

Si

Al

Co

Zn

H-beta

469

0.37

43.73

3.41

-

-

Off-Al H-beta

545

0.45

46.70

0.048

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SBET

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Sample

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Physicochemical properties of zeolite catalysts under study.

520

0.43

-

-

1.24

4.11

Co:Zn=1:1/H-beta

508

0.41

-

-

2.48

2.71

Co:Zn=3:1/H-beta

494

0.41

-

-

3.47

1.34

Co/H-beta

492

0.41

-

-

4.59

-

Zn/H-beta

496

0.42

-

-

-

5.40

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Co:Zn=1:3/H-beta

The morphology and structure of as-obtained sample were investigated by

transmission electron microscopy (TEM) measurements. No apparent change is observed in the morphology of H-beta support after dealumination and Co-Zn incorporation, which indicates the good stability of support materials and is in good 16

agreement with the result obtained by XRD analysis (Fig. S3). The TEM image in Fig. 2a shows that prepared Co:Zn=1:3/Off-Al H-beta exhibits as aggregations of square nanosheets of ca.120 nm. The high-magnification TEM image in Fig. 2b demonstrates the clear lattice fringes of beta zeolite, which indicates its high crystallinity. The scanning transmission electron microscopy (STEM) image provides a clear view of Co:Zn=1:3/Off-Al H-beta sample (Fig. 2c). The corresponding

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element mapping analysis of Co:Zn=1:3/Off-Al H-beta sample confirms the

homogeneous dispersion of Co and Zn species in as-prepared Co:Zn=1:3/Off-Al Hbeta sample (Fig. 2d). EDS analysis of Co:Zn=1:3/Off-Al H-beta (Fig. S4) reveals

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that the weight contents of Co and Zn are ca. 0.5 wt% and 1.8 wt% in the

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Co:Zn=1:3/Off-Al H-beta respectively, which is lower than the result of ICP analysis

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in Table 1, suggesting that Co and Zn not only exist on the surface of the catalyst.

Fig. 2. (a), (b) TEM images of Co:Zn=1:3/Off-Al H-beta. (c), (d) STEM image of Co:Zn=1:3/Off-Al H-beta with corresponding element mapping analysis. 17

X-ray photoelectron spectroscopy (XPS) measurements were carried out to obtain information about the electronic state of Co and Zn species. In the spectrum of Co:Zn=1:3/Off-Al H-beta sample (Fig. 3a), a broad binding energy peak corresponding to Co 2p3/2 appears between 780-795 eV. Using a Gaussian fitting method, the Co 2p emission spectrum is best fitted with two doublets at 781.5 and 786.0 eV; the latter peak suggesting Co2+ is the major component [51-54]. Fig. 3a

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shows a shoulder near the 2p1/2 peak, suggesting the presence of Co3+ in minor

quantity. EXAFS measurements at the Co-K edge were performed to determine the

atomic structure of Co:Zn=1:3/Off-Al H-beta. The normalized X-ray absorption near-

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edge structure and the corresponding Fourier transform (FT) k3-weighted Co K-edge

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EXAFS curves in R space for the prepared Co-Zn/Off-Al H-beta are shown in Fig. 3c and 3d. Two distinct peaks at 1.46 and 2.45 Å corresponding to the chemical bonds of

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the Co-O split in the FT curves of Co-Zn/Off-Al H-beta confirms the presence of Co-

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O bonds. Thus, Co K-edge spectra further confirmed the presence of both Co2+ and Co3+ bonded to oxygen in all Co-Zn/Off-Al H-beta catalysts.

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Since only Co2+ was used for the preparation of the catalyst, the presence of Co3+ in the catalysts can be rationally explained by oxidation of Co2+ to Co3+ during

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calcining. Since neither XRD nor TEM showed any crystals other than the H-beta zeolite framework, the surface of the Co-Zn/Off-Al H-beta catalyst did not form significant Co3O4 or ZnO crystals. Although K-edge spectra show similarity between the Co-Zn/Off-Al H-beta catalysts and Co3O4, these results only indicate the existence of Co-O bonds. Comparing Co3O4 with Co:Zn=1:3/Off-Al H-beta the binding energy 18

peak appears at 781.5 eV for Co:Zn=1:3/Off-Al H-beta as compared to 779.9 eV for Co3O4 (Fig. 3a). The same upshift was also observed for Zn 2p3/2 (Fig. 3b), which appears at 1022.5 and 1021.3 eV for Co:Zn=1:3/Off-Al H-beta and ZnO, respectively [55, 56]. The similar upshift of the primary binding energies of Co and Zn in the Co:Zn=1:3/Off-Al H-beta XPS spectra points to the existence of Co-O-Si and Zn-O-

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Si bonds instead of Co-O-Co and Zn-O-Zn bonds. Since Si is more electronegative

than either Co or Zn, the Co-O and Zn-O bonds in Co-O-Si and Zn-O-Si would have

higher binding energy than those in Co-O-Co and Zn-O-Zn bonds as shown in Fig. 3a

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and 3b. Considering all results from TEM-EDS, XPS, and EXAFS, it is concluded

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that both elements were well-dispersed throughout and embedded into the Off-Al Hbeta lattice made vacant by dealumination. The adsorbed metals exist mainly in

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oxidized states of Co2+ and Zn2+, as well as a small quantity of Co3+.

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Fig. 3. XPS (a, b) and EXAFS (c, d) characterization of Co-Zn/Off-Al H-beta.

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Acid sites on the surface of Co-Zn/Off-Al H-beta catalysts and the reference

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sample were measured by NH3-TPD (Fig. 4a) and IR spectra of pyridine adsorption (Fig. 4b). As shown in Fig. 4a, a broad NH3 desorption band ranged from 150 °C to

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500 °C is observed in the pattern of H-beta, which indicates that H-beta has weak and strong acid sites on the surface. The total acid content of H-beta through the integral

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peak area is 1.08 mmol/g. Dealumination of H-beta zeolite greatly reduced the

Jo

acidity, causing the decline of total acid sites from 1.08 to 0.40 mmol/g (Table 2). Incorporation of Co into the Off-Al H-beta framework results in a further decrease of acid sites per gram. On the other hand, incorporation of Zn increased total acidity. Thus, incorporation of both Co and Zn in different proportions to the Off-Al H-beta allows for tuning of acidity, as shown in Table 2. The loss of acid sites resulting from the removal of framework Al was partially replenished by the incorporation of Zn. 20

Since Zn is a weaker Lewis acid than Al, the replacement of Al with Zn in the H-beta framework could not achieve the same acidity as the pristine H-beta zeolite. IR spectroscopy following pyridine adsorption was performed to determine the quantity of Brønsted and Lewis acid sites. Generally, the vibration bands at ~1540 cm-1 of a Py-FTIR spectrum are assigned to the signal of Brønsted acid sites, the bands around 1450 cm-1 are attributed to pyridine associated with Lewis acid sites,

ro of

and adsorption at 1490 cm-1 is attributed to pyridine associated with both Lewis and Brønsted acid sites. In addition, all catalysts exhibit a peak at 1600 cm-1 that can be

taken as a measure of Lewis acid strength. Bands at 1640 cm-1 are ascribed to pyridine

-p

adsorbed on Brønsted acid sites [57-60]. As shown in Fig. 4b, the peaks at 1450 cm-1

1

re

and 1600 cm-1 are predominant in all catalysts along with two weak peaks at 1540 cmand 1640 cm-1 that are associated with Brønsted acid sites. These observations

lP

indicate that Lewis acid sites are more prevalent in these catalysts. The amount of

na

Brønsted acid and Lewis acid sites obtained by the integration of peaks at 1540 cm-1 and 1450 cm-1 are summarized in Table 2. The density of Lewis acid sites on the

ur

catalyst surface decreased following dealumination and increases with an increase of Zn content. As previously observed with NH3 absorption, the loss of acid sites

Jo

resulting from the removal of framework Al was partially replenished by the incorporation of Zn. It is clear that the catalyst surface acidity characterized by NH3TPD is similar and consistent with that of Py-IR.

21

ro of

Fig. 4. NH3-TPD (a) and IR- spectra of pyridine adsorption (b) for Co-Zn/Off-Al H-

-p

beta catalysts.

re

Table 2

Acidity of H-beta, Off-Al H-beta, and Co-Zn/Off-Al H-beta catalysts. Total acid sites

Acid sites (μmol Py/g)b

(mmol NH3/g)a

Lewis

Brønsted

1.08

282

116

0.40

168

38

Co/Off-Al H-beta

0.24

181

23

Co:Zn=3:1/Off-Al H-beta

0.43

194

35

Co:Zn=1:1/Off-Al H-beta

0.45

228

41

Co:Zn=1:3/Off-Al H-beta

0.49

243

48

Zn/Off-Al H-beta

0.55

248

47

lP

Catalyst

Jo

ur

Off-Al H-beta

na

H-beta

a

Amount of desorbed ammonia was determined by NH3-TPD.

b

Amount of pyridine was determined by IR spectra of pyridine adsorbed and

outgassed at 30 °C.

22

H2-TPR is a technique that helps identify the reduction behaviors of a reducible metal oxide and estimates the interaction of the active phase with the support [61]. The TPR profiles of the samples are presented in Fig. 5, and the total consumption of H2 for each sample is gathered in Table 3. From Fig. 5, for Co-Zn/Off-Al H-beta catalyst, there are two peaks positioned at the reduction temperatures of approximately 400 °C and 800 °C. The peak positioned at the lower reduction

ro of

temperature of 400 °C is ascribed to the more easily achieved reduction of Co3+ to Co2+, while the peak 800 °C is ascribed to the reduction of Co2+ to Co0, which requires more energy. The reduction ability of the catalyst is affected by the

-p

dispersion of Co species and the interaction between the support material and Co

re

species. Higher dispersed cobalt have stronger interactions with the support material, so the reduction temperature is higher. The fact that the reduction temperature of Co-

lP

Zn/Off-Al H-beta is higher than that of combine Co/Off-Al H-beta indicates that Zn

na

contributes to the dispersion of Co. This conclusion is in line with the BET result which shows that specific surface area of the sample increases with increasing Zn

ur

content. The total consumption of H2 increases gradually with increasing cobalt content. As shown in Fig. 5 and Table 3, catalysts without Co have little sign of

Jo

hydrogen consumption, confirming that Co is the only species in these catalysts having hydrogen binding activity that improves lignin depolymerization. Hydrogenation catalysts such as Co are known to improve lignin degradation in two ways. First, they provide a hydride ion to the oxygen atom of the lignin ether bonds, promoting hydrogenolysis in concert with the Lewis acid. Second, they help to 23

stabilize labile lignin deconstruction intermediates to low molecular weight

-p

Fig. 5. H2-TPR for Co-Zn/Off-Al H-beta catalysts.

ro of

compounds through catalytic hydrogenation.

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Table 3

Active sites of H-beta, Off-Al H-beta, and Co-Zn/Off-Al H-beta catalysts. Active sites (mmol H2/g cat)

lP

Catalysts

Weak

Strong

Total

0.04

0

0.04

0.09

0

0.09

Co/Off-Al H-beta

1.01

2.73

3.74

Co:Zn=3:1/Off-Al H-beta

1.33

1.93

3.26

Co:Zn=1:1/Off-Al H-beta

0.86

1.02

1.88

Co:Zn=1:3/Off-Al H-beta

0.32

0.73

1.05

Zn/Off-Al H-beta

0.03

0

0.03

Jo

ur

Off-Al H-beta

na

H-beta

24

3.2. Catalyst activity testing

3.2.1. Roles of components in catalyst

To understand the roles of Co and Zn species in the Co-Zn/Off-Al H-beta catalyst, the reaction was carried out with different Co:Zn ratio samples as the catalyst individually at 300 °C for 6 hours following the same procedure. Catalytic activity is

ro of

evaluated by the yields of liquid, solid, gas, ethyl acetate soluble, and petroleum ether soluble products. The liquid product represents the degree of lignin depolymerization, whereas the solid product is an indication of lignin condensation. The yield of gas

-p

product, which is the difference between the starting material and the total of liquid

re

and solid products, is generally less than 2% at 300 °C in all cases and hence not considered as a factor. In addition, all liquid product is completely soluble in ethyl

lP

acetate after the evaporation of 1, 4-dioxane and methanol. Another important parameter is the yield of petroleum ether soluble product, which consists mainly of

na

monomers and dimers, and they are potential candidate feedstock for the direct formation of hydrocarbon fuels.

ur

At room temperature, Indulin AT is totally soluble in dioxane/methanol (5:1 v/v)

Jo

mixture, but only 8% and 2% is soluble in ethyl acetate and petroleum ether, respectively. From Fig. 6 it is clear that the depolymerization and condensation of lignin occur simultaneously. In the absence of catalyst, both thermal-catalyzed depolymerization and condensation occur with condensation reactions being predominant. The incorporation of Zn into Off-Al H-beta increases the acidity (Table

25

2) and hence promotes both lignin depolymerization and condensation as judged by the yields of petroleum ether soluble product and solid product, respectively (Fig. 6). On the other hand, the incorporation of Co into Off-Al H-beta decreased the acidity of the catalyst, resulting in less lignin depolymerization and condensation; solid product yield (11.6%) and petroleum ether soluble product yield (30.4%) were both lower. Thus, hydrogen binding sites Co can efficiently decrease the solid product yield by

ro of

stabilizing lignin depolymerization intermediates and Zn can promote lignin

depolymerization. These results clearly demonstrate that hydrogenation is an effective way to minimize lignin condensation [19]. The bimetallic catalysts of Co-Zn/Off-Al

-p

H-beta at different metal loading ratios allow for optimization of the above two

re

opposite reactions. At the Co-Zn ratio of 1:3, a highest yield of petroleum ether soluble product (54.8%) was achieved at 300 °C for 6 hours as compared to 50.8%

lP

and 35.7% at the ratios of 1:1 and 3:1, respectively. Thus, Co:Zn=1:3/Off-Al H-beta

Jo

ur

na

was selected for further investigation in this study.

26

ro of -p re lP

na

Fig. 6. Effect of various catalysts on lignin depolymerization and condensation. Control 1: room temperature; Control 2: reaction conditions without catalyst.

ur

3.2.2. Effect of the catalyst dosage on lignin depolymerization

Jo

The effect of the Co:Zn=1:3/Off-Al H-beta catalyst dosage on lignin depolymerization was also investigated. When the amount of the catalyst reduced progressively from 0.25 to 0.05 g (the amount of lignin remained the same at 0.5 g), the yields of liquid product, petroleum ether soluble product, and identified monomers decreased steadily while the solid product increased as shown in Fig. S5. Petroleum

27

ether soluble product decreased by 50% in the range tested, but solid product only increased by 20% indicating that the catalyst is highly effective in lignin depolymerization. However, the differences are relatively small in the catalyst dosage range from 0.10-0.25 g, hence catalyst dosage higher than 0.25 g was not investigated in this study. In any application of solid catalysts, leaching of catalyst is inevitable. The key

ro of

question is if the amount of leaching is significant enough to affect the cost-

effectiveness of the catalyst. As will be discussed later in the catalyst stability section,

liquid products in Fig. S5 and all data hereafter.

-p

4% of catalyst is leached into liquid product. The value is used to adjust the solid and

re

3.2.3. Effect of reaction temperature on lignin depolymerization

lP

Temperature is a very important factor that influences the degree of lignin depolymerization. In Fig. 7a, with increasing temperature the yields of gas product

na

and solid product increased while the yield of liquid product decreased. The yield of petroleum ether soluble product gradually increased while the yield of ethyl acetate

ur

soluble product decreased with increasing temperature as shown in Fig. 7b. These

Jo

results indicate that temperature plays a dual role in promoting the depolymerization and condensation reactions of lignin. The steady increase in the yield of petroleum ether soluble product with increasing temperature indicates that the catalyst is more effective in stabilizing the depolymerization products of lignin at high temperature. While the yield of petroleum ether soluble product reached 63.2% at 320 °C, the yield

28

of solid product also reached a high amount at 26.5%, suggesting that lignin depolymerization, condensation, and stabilization of the degradation products occur at

na

lP

re

-p

ro of

higher temperature.

Fig. 7. Effect of reaction temperature on lignin depolymerization and product

ur

distribution at 6 h.

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3.2.4. Effect of reaction time on lignin depolymerization

The simultaneous occurrence of lignin depolymerization, condensation, and

stabilization of the degradation products occur at a given temperature prompted us to investigate the effects of reaction time on lignin depolymerization at 320 °C. In Fig. 8a, the formation of solid product was very rapid, reaching 38.3% within one hour. 29

Thereafter, the amount of solid product continued to decrease with time, reaching 10.4% after 24 hours of reaction. The yields of ethyl acetate soluble and petroleum ether soluble products increased steadily with time and reached 87.5% and 81.1%, respectively, after 24 hours (Fig. 8b). Again, the yield of volatile product was negligible even at a prolonged reaction time. The total yield of monomeric products increased rapidly with increasing time of reaction, however monomeric products

ro of

became more complicated. These results clearly indicate that lignin condensation

occurs rapidly, reaching a maximum within one hour. Thereafter, lignin degradation and stabilization of degradation products via hydrogenation take over to give a high

-p

yield of degradation products. The high yield of petroleum ether soluble (81.1% after

re

24 hours) is especially of interest since it consists mostly of monomeric and dimeric degradation products (shown later by GPC data). It also indicates that Co:Zn=1:3/Off-

lP

Al H-beta is a highly efficient catalyst for lignin depolymerization. Thus, it appears

na

that the catalyst serves two functions at high temperature. The catalyst promotes both lignin depolymerization and stabilization of the degradation products. The latter is

ur

supported by the finding that the major monomeric products are methyl-, ethyl- and propyl-guaiacol along with guaiacol (Fig. 10a). These products are no longer able to

Jo

form an electrophile for condensation reactions. To ensure that the solid product, the liquid product and the volatile products

represent the total yield of lignin reaction products, carbon balance was calculated on the reaction products at 320 °C for 24 hours. The liquid product contains 70.14 wt% carbon, 8.98 wt% hydrogen, 19.97 wt% oxygen, 0.44 wt% nitrogen, and 0.37 wt% 30

sulfur with the total carbon recovery of 94.69 wt%. The solid residue (char and catalyst) showed the elemental contents (on dry basis) of 14.08 wt% carbon, 1.68 wt% hydrogen, 0.42 wt% nitrogen, 0.36 wt% sulfur, and 83.46 wt% of others (oxygen plus the elements of catalyst) with the total carbon recovery of 12.67 wt%. Volatile products collected with air bags and analyzed using a gas chromatograph (GC-1960) composed of 0.93 wt% CO, 0.68 wt% CH4, 0.49 wt% C2H4, and 0.10 wt% C3H6 with

ro of

the total carbon recovery of 0.06 wt%. Therefore, the overall carbon recovery was

Jo

ur

na

lP

re

-p

107.42 wt%.

Fig. 8. Effect of reaction time on lignin depolymerization and product distribution at 320 °C. 31

3.2.5. Kinetics of product distribution from lignin depolymerization

The solvent system, dioxane/methanol mixture, is a very powerful solvent for lignin. It dissolves milled wood lignin (MWL) and cellulolytic enzyme lignin (CEL), which have higher weight average molecular weight than the Kraft lignin (Indulin AT) and the latter is totally soluble in the solvent. When close to 40% of Kraft lignin became insoluble after one hour of reaction. There is no plausible explanation other

ro of

than the condensation of lignin under the reaction conditions. Acid-catalyzed

condensation of lignin is well known and it does not take much condensation for Kraft

-p

lignin to reach an insoluble molecular weight.

Lignin condensation reactions mainly occur within the first hour, but lignin

re

depolymerization and stabilization continue as the reaction time was extended for 24

lP

hours at 320 °C (Fig. 8). Lignin condensation was limited at lower temperatures after 6 hours of reaction (Fig. 7). These results prompted us to investigate the influences of

na

reaction time on lignin depolymerization and condensation at three different temperatures as shown in Fig. 9.

ur

The formation of solid product insoluble in the reaction medium is the result of

Jo

condensation reactions of lignin. Condensation reaction products appeared rapidly and reached a maximum amount during the first hour of reaction (Fig. 9a). Temperature has a strong effect on condensation as more solid products were formed at higher temperature. As the reaction time was extended, the amount of solid products decreased gradually. The condensation products are apparently degradable only at high reaction temperature since the amount of solid products remain relatively 32

constant at 240 °C. Furthermore, the solid products were lower at 12 and 24 hours at 320 °C as compared to those at 280 °C, indicating the degradation rate may be much higher at the higher temperature. Thus, the temperature has a strong influence on both the formation and degradation of condensation products. Since only 8% of the starting Kraft lignin is soluble in ethyl acetate, the ethyl acetate soluble product are good indications of the extent of lignin degradation. As

ro of

shown in Fig. 9b, about 60-70% of lignin became soluble in ethyl acetate during the first hour of reaction regardless of the temperatures (240-320 °C), indicating that

degradation of lignin also occurred rapidly during the initial phase of reaction. As the

-p

reaction time increased, the ethyl acetate soluble products continued to increase.

re

However, the effect of temperature was different. At 320 °C, all liquid products were ethyl acetate soluble and their increase depended on the degradation of solid products.

lP

After 24 hours, 87.5% of starting lignin was converted to ethyl acetate soluble

na

product, 10.4% solid product, and 2.1% volatile product. On the other hand, fewer solid products were formed at 240 °C and continued degradation of the liquid

ur

products resulted in reaching a maximal amount of ethyl acetate soluble products (84%) in 6 hours and close to 90% after 24 hours.

Jo

The effects of temperature and time on the petroleum ether soluble products are

of special interest. While only 2% of the starting Kraft lignin is soluble in petroleum ether, this amount increased to 15% and 40% in one hour of reaction at 240 °C and 320 °C, respectively, as shown in Fig. 9b, giving support to the rapid degradation of lignin during the initial phase of the reaction. The petroleum ether soluble products 33

continue to increase with increasing time at all three reaction temperatures, but the strong effects of temperature are obvious (Fig. 9b). At 320 °C, the petroleum ether soluble product yield reached 81% after 24 hours of reaction. All different types of reaction (condensation, degradation, and stabilization of degradation products) are presumably involved. Li et al. [62] demonstrated that both homolytic and acidolytic cleavage of β-ether linkage in lignin model compounds

ro of

occurred at 160 °C in an acidic medium of 0.2 M acetic acid/dioxane (10:1 v/v), so it is conceivable that both homolytic and acidolytic reactions are involved in the

degradation of lignin in the present case. In addition, hydrogenolysis must be also a

-p

part of the degradation reactions, especially at high temperatures. The condensation of

re

lignin and lignin degradation products may involve acid-catalyzed condensation of benzylic carbocations and aromatic nuclei. Besides, radicals formed by the homolytic

lP

cleavage of lignin may couple to form condensed products. Stabilization of the

na

degradation products occurs by deoxygenation and saturation of the side chains via hydrogenation, which is essential in producing petroleum ether soluble products. The

ur

stabilization by hydrogenation appears to require high temperature as high yield of

Jo

petroleum ether soluble product are obtained only at 320 °C.

34

ro of -p re

lP

Fig. 9 Kinetics of product distribution from lignin depolymerization. (a) Distribution

na

of gas, solid, and liquid product: blank column 240 °C, column with grid 280 °C, and column with stripes 320 °C. (b) Yields of the ethyl acetate soluble product (solid

ur

lines) and the petroleum ether soluble product (dotted lines).

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3.3. Characterization of degradation products

3.3.1. Qualitative and quantitative analysis of monomers

Monomers generated from depolymerization of lignin were identified by GC/MS

and quantified by GC. Qualitative analysis results are shown in Fig. 10 and quantitative analysis results are shown in Table S2, S3, S4 and S5. These tables show 35

the distribution and relative proportion of monomer. After reaction at the low temperature of 240 °C for 6 hours, very small amounts of monomers were identified and quantified. Vanillin, isoeugenol and homovanillic acid were the major products. After reaction at 300 °C for 6 hours, a larger amount of monomeric products were formed, with alkyl-substituted guaiacols and homovanillic acid, accounting for about 70% of the identified monomers (Fig. 10a and Table S4). Some methylated products

ro of

of guaiacol, 4-methylguaiacol and 4-ethylguaiacol were also identified, their

formation presumably due to the presence of methanol in the reaction medium as

amounts decrease with increasing temperature.

-p

solvent. Vanillin and isoeugenol are apparently unstable at high temperature as their

re

At the highest temperature of 320 °C, guaiacol, typical hydrogenation products and their methylated products were identified, and homovanillic acid remained the

lP

dominant monomers (Fig. 10b and Table S5). However, the yields of guaiacol,

na

methyl-, ethyl and propyl-guaiacol decreased steadily as reaction time increased from 1 hour to 24 hours, indicating that they were unstable at high temperature.

ur

Furthermore, vanillin, acetoguaiacone and its methyl ether derivative accumulated, suggesting that the catalyst is not effective in hydrogenation of carbonyl groups. The

Jo

yields of unknown monomeric products were estimated by GC. The yields and species of the unknown products increased with increasing reaction time at 320 °C.

36

ro of -p

re

Fig. 10. Qualitative analysis of monomers by GC/MS. (a) 300 °C, 6 h. (b) 320 °C, 24

supplementary material.

lP

h. The different GC/MS temperature programs for (a) and (b) can be found in the

na

3.3.2. Gel permeation analyses of degradation products

ur

Gel permeation chromatography was used to determine the molecular weight

Jo

distributions of degradation products along with the original Kraft lignin (Fig. 11). The liquid product of the reaction was fractionated into petroleum ether soluble (PES) and petroleum ether insoluble/ethyl acetate soluble (PE-I) fractions. In Fig. 11, chromatography of the PE-S / PE-I fractions clearly indicate that the Kraft lignin undergoes extensive degradation under the reaction conditions tested. More importantly, the difference in molecular weights between the degradation products 37

reacted at 240 °C and 320 °C confirms that more lignin degradation occurs at high temperature. The number-average (Mn) and weight-average molecular weight (Mw) of the PE-I fractions at 320 °C are 401 Da and 581 Da, respectively, for 3 hours and 480 Da and 819 Da, respectively, for 12 hours. Thus, the PE-I fractions obtained at 320 °C consist mainly of trimers and low DP oligomers, as opposed to those of the original Kraft lignin and the PE-I fractions obtained at 240 °C. The Mn and Mw of the original

ro of

Kraft lignin are 1699 Da and 9584 Da, respectively and those of the PE-I fractions are 962 Da and 2978 Da, respectively, after 3 hours, and 1058 Da and 2517 Da, respectively, after 12 hours.

-p

Fig. 11b shows chromatographs of the petroleum ether soluble fractions of lignin

re

reacted at 240 °C and 320 °C for 12 hours along with two dimeric lignin model compounds. Both samples have similar distributions; Mn and Mw are around 221-241

lP

Da and 312-330 Da, respectively, with a low polydispersity index of 1.4 in both cases.

na

Regardless of the reaction temperature, the PE-S fractions consist mainly of monomeric and dimeric degradation products, although some trimeric products may

Jo

ur

also be present.

38

ro of

Fig. 11. GPC chromatographs of Kraft lignin and its degradation products at various reaction conditions. PEI = petroleum ether insoluble/ethyl acetate soluble; PES =

re

3.3.3. 2D HSQC NMR spectral analyses

-p

petroleum ether soluble; A and B, dimeric model compounds.

lP

2D HSQC NMR spectral method was used to analyze the chemical structure of the petroleum ether soluble (PE-S) and petroleum ether insoluble/ethyl acetate soluble

na

(PE-I) fractions of the degradation products reacted at 240 °C and 320 °C for 12 hours. Also included is the spectrum of the original Kraft lignin for reference. The

ur

spectra are divided into three regions: the saturated aliphatic hydrocarbon (δC/δH 050/0-3.0 ppm), the oxygenated aliphatic hydrocarbon (δC/δH 50-90/3.0-6.0 ppm) and

Jo

the aromatic hydrocarbon regions (δC/δH 95-135/6.0-8.0 ppm). Fig. 12 shows the spectra of the PE-I fractions obtained at 240 °C and 320 °C (both

at 12 hours) along with the original Kraft lignin. In the saturated aliphatic hydrocarbon region (δC/δH 0-50/0-3.0 ppm), the secoisolariciresinol (E) and dihydroconiferyl alcohol (M) structures present in the original Kraft lignin remain in 39

the PE-I fraction isolated at 240 °C, but disappeared at 320 °C [63]. At the lower temperature of 240 °C, a new signal at δC/δH 24.5-28.5/2.3-2.6 ppm appears and is assigned to the methyl group of the acetoguaiacone. This signal was not found at 320 °C; instead, signals assigned to the methyl group of methyl guaiacol, ethylguaiacol and propylguaiaicol are prominent (δC/δH 14.0-21.0/1.90-2.50 ppm and δC/δH 12.0-17.0/0.8-1.3 ppm). In the oxygenated aliphatic hydrocarbon region (δC/δH

ro of

50-90/3.0-6.0 ppm), the β-O-4’ (A), β-5’ (B) and β-β’ (C) are present in the spectrum of the fraction isolated at 240 °C, but disappear totally in that of the fraction isolated at 320 °C. A new signal appears at δC/δH 50.0-51.5/3.5-3.9 ppm in the spectra of

-p

fractions isolated at both temperatures, which belongs to either the methyl ester of

re

homovanillic acid (9) and/or the methylene group of guaiacylacetone (7). The signal is more prominent at 240 °C than at 320 °C, suggesting that these structures are not

lP

stable at high temperature. In addition, two new signals appear at δC/δH 68-72/3.3-3.7,

na

which are assigned to the methylene group of ethers. These signals are more prominent at 320 °C. The possible origin of these signals may be the incorporation of

ur

dioxane into lignin through homolytic processes. In the aromatic hydrocarbon region (δC/δH 95-135/5.0-8.0 ppm), the original stilbene signal from Kraft lignin (L) remains

Jo

at 240 °C, but totally disappears at 320 °C, suggesting that hydrogenation is not as effective at 240 °C. At 320 °C, the signals of G2, G5 and G6 decrease substantially, indicating substantial condensation reactions occurred at the aromatic rings at high temperature.

40

ro of -p re

lP

Fig. 12. 2D HSQC NMR spectra of petroleum ether insoluble/ethyl acetate soluble fractions.

na

Fig. 13 shows the spectra of the petroleum ether soluble fractions obtained at 240 °C and 320 °C along with the original Kraft lignin. Comparing these spectra, it is

ur

obvious that extensive changes in both sidechains and aromatic structures occurred

Jo

during the reactions, especially at the highest temperature of 320 °C. In the saturated aliphatic hydrocarbon region, almost all sidechains were converted to saturated structures (1, 2 and 3) at 320 °C, the methyl ether of dihydroconiferyl alcohol (5) and acetoguaiacone (6) being the exceptions. At 240 °C, the dihydroconiferyl alcohol (M) homovallinic acid are present in addition to the aforementioned structures. In the oxygenated aliphatic hydrocarbon region, one signal can be attributed to either the 41

methyl ester of homovanillic acid (9) and/or the methylene group of guaiacylacetone (7) (δC/δH 50.0-51.5/3.5-3.9 ppm) and is present at both 240 °C and 320 °C, being more prominent at 240 °C. Also present is the methyl ether of dihydroconiferyl alcohol (5). A strong signal only present in the spectrum of 240 °C at δC/δH 17.5/1.41.9 ppm is that of isoeugenol (4). Like in the petroleum ether insoluble fractions, two strong signals (δC/δH 68-72/3.3-3.7), which may involve the solvent dioxane as

ro of

discussed in connection with Fig. 12, are present only in the spectrum of 320 °C. In the aromatic hydrocarbon region, a decrease in the intensities of G2, G5 and G6

signals are obvious at both 240 °C and 320 °C, likely due to significant condensation

-p

of the aromatic nuclei. Signals that have been attributed to the structures of stilbene

re

(L), carbonyl-conjugated G2 (G’2), and G6 of vanillin and vanillic acid (δC/δH 122124/7.4-7.6 ppm, δC/δH 125-126/7.4-7.6 ppm, respectively, are only present at 240 °C

lP

[64].

na

The petroleum ether soluble fraction is the major product at 320 °C, accounting for 76% of the original weight of lignin, whereas the petroleum ether insoluble

ur

fraction is the major product at 240 °C, accounting for 59% (Fig. 9b). The results obtained from 2D HSQC NMR and GPC clearly indicate that extensive condensation

Jo

and degradation of lignin occur at 320 °C, but only limited condensation and degradation occur at 240 °C.

42

ro of -p re

lP

Fig. 13. 2D HSQC NMR spectra of petroleum ether soluble fractions.

na

3.3.4. Element analysis of liquid product

The elemental compositions of Kraft lignin and liquid product obtained from the

ur

depolymerization of lignin via Co:Zn=1:3/Off-Al H-beta catalyst at 320 °C for 12

Jo

hours are shown in Table 4. The oxygen content (O) decreased significantly with a corresponding increase in carbon (C) and hydrogen (H) after the reaction. The HHV increased from 26.0 MJ/kg to 33.3 MJ/kg with Co:Zn=1:3/Off-Al H-beta catalyst. It is strong evidence that the catalyst Co:Zn=1:3/Off-Al H-beta could convert lignin-toliquid fuels effectively.

43

Table 4 Elemental contents of Kraft lignin and liquid product. Sample

HHVa

Elemental content (wt%) Ash H

O

N

S

MJ/kg

Kraft lignin

62.96

5.82

28.32

0.72

1.68

2.0

26.02

Liquid product

71.41

8.76

19.09

0.44

0.27

0.07

33.30

The higher heating value (HHV) was calculated according to formula: HHV =

ro of

a

C

0.3491C + 1.1783H + 0.1005S - 0.1034O - 0.0151N - 0.0211Ash [65].

-p

3.4. Investigation of the catalyst stability

The recycling ability of Co-Zn/Off-Al H-beta in the lignin-to-liquid fuels

re

conversion was finally investigated and the results are shown in Fig. 14. A five-cycle

lP

conversion reaction experiment was carried out twice at 300 °C for 12 hours using the yield of petroleum ether soluble products as an indicator of the catalytic efficiency. A

na

reduction of petroleum ether soluble product from 59.8% to 40.3% was observed after three cycles, apparently due to deactivation of the Co:Zn=1:3/Off-Al H-beta catalyst

ur

(Fig. 14a). However, the yield of petroleum ether soluble product increased to 54.7% after regeneration of the catalyst by calcination during the fourth cycle. On the fifth

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cycle after the same calcination regeneration, yield increased to 55.2%. These results indicate that the catalytic deactivation primarily due to the deposition of bulky lignin and its degradation product on the surface and within pores of the catalyst. This conclusion is further confirmed by TEM images (Fig. S6a, S6b). The structure of used Co:Zn=1:3/Off-Al H-beta catalyst after being recycled is less regular than the 44

corresponding fresh catalyst, but the lattice fringes are still clear, which illustrates that the crystallinity remained stable after five cycles. Simultaneously, the black floc region surrounding the catalyst is presumably due to the remaining bulky ligninderived molecules that blocked pore channels and covered active sites on the surface of the catalyst. The decreased porosity of catalyst particles was demonstrated by the X-ray CT video (movie c, e, decreased from 10.9% to 8.7%), and it was the main

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reason that the yield of petroleum ether soluble product decreased. In contrast, only a small decrease in the yield of petroleum ether soluble product was measured within

five cycles (calcination at 550 °C in flowing air for 5 hours after every cycle) [66, 67].

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Based on the X-ray CT video analysis (movie d, f, decreased from 10.9% to 9.5%),

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the porosity of Co: Zn=1:3/Off-Al H-beta when calcined after every reaction cycle is only decreased slightly, compared with another method for recycling where the

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catalyst was calcined after every 3rd reaction cycle. Overall, these results demonstrate

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the excellent recyclability of Co:Zn=1:3/Off-Al H-beta for lignin depolymerization.

45

Fig. 14. Recycling tests of Co:Zn=1:3/Off-Al H-beta in lignin depolymerization and corresponding X-ray CT images. (a) Yields of the petroleum ether soluble product, (b, c) Co:Zn=1:3/Off-Al H-beta after five cycles. The catalyst was calcined at 550 °C for 5 h after the 3rd cycle. (d) Yields of the petroleum ether soluble product, (e, f) Co:Zn=1:3/Off-Al H-beta after five cycles. The catalyst was calcined at 550 °C for 5 h after every cycle.

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Another possible catalyst deactivation mechanism is leaching of the active

components during the reaction. Leaching of Co:Zn=1:3/Off-Al H-beta under the

reaction conditions without the presence of lignin was investigated at 300 °C for 6

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hours. The results are shown in Table S6. During the first run, about 4% of catalyst

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leached into the reaction media and about 2% leached in the subsequent two recycling runs. Consequently, the yields of the solid and liquid product were corrected. The

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catalyst leaching with the presence of lignin was also investigated by comparing the

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Co and Zn content in the fresh catalyst and the spent catalyst after the fifth cycle (the catalyst was recalcined at 550 °C for 5 h). The Co content decreased from 1.24% to

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1.09% and the Zn content decreased from 4.11% to 3.64%, respectively. The products distribution is shown in Fig. 15. The gas product remained the same in each cycle.

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Whereas the solid product increased and the liquid product decreased gradually as the increasing cycle. Within the liquid product, the ethyl acetate soluble product also exhibited a decreasing trend but the monomeric product remained approximately the same.

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Fig. 15. Products distribution from lignin depolymerization of recycling tests. The oxidation states of Co or Zn components in the spent catalyst were measured by X-ray photoelectron spectroscopy (XPS) and compared with those of fresh

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catalysts as shown in Fig. S8. The spent catalyst was obtained after the reaction at

300 °C for 12 h, filtered and dried in a vacuum of 30 °C for 12 h. The oxidation state

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of cobalt in the spent catalyst indicated cobalt was not reduced (absence of

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characteristic peak of reduction state (778.2 eV)). The binding energy peaks corresponding to Zn metal (1021.7 eV) and ZnO (~1022 eV) are very close and

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cannot be distinguished in the spectrum (Fig. S8b). However, the Auger peak of Zn metal (495 eV) was not detected in the full survey scan spectra (Fig. S8c). Thus, the

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Zn was also not reduced.

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4. Conclusions

Non-noble bifunctional Co-Zn/Off-Al H-beta zeolite is a highly effective catalyst

for the conversion of Kraft lignin to liquid fuels. The synergistic effect of Co and Zn on H-beta support material significantly enhanced the catalytic activity of bifunctional Co-Zn/Off-Al H-beta catalyst. A maximum yield of 81% petroleum ether soluble 47

product was obtained at 320 °C for 24 hours. This should be the highest yield that has been reported for depolymerization of Kraft lignin to petroleum ether soluble product. Spectral analyses indicated that the original lignin oxygenated sidechains were converted to saturated structures. Besides, under this condition, the HHV increased from 26.0 MJ/kg of Kraft lignin to 33.3 MJ/kg of the liquid product. More importantly, there was no significant activity loss of the catalyst after five cycles,

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indicating excellent recyclability of this catalyst. Considering the economic

practicality and high efficiency, Co-Zn/Off-Al H-beta catalyst may be a cost-effective

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route for converting Kraft lignin into liquid fuels in the future.

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Conflicts of interest

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Acknowledgments

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The authors declare no conflict of interest.

This work was supported by the National Natural Science Foundation of China

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(51676178), “Transformational Technologies for Clean Energy and Demonstration”, Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No.

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XDA 21060101, the National Key Technology R&D Program of China (NO. 2018YFB1501600), and Science and Technological Fund of Anhui Province for Outstanding Youth (1508085J01). The authors appreciate the beamline BL07W in the National Synchrotron Radiation Laboratory (NSRL) for help in X-ray CT characterizations. 48

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version.

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