Efficient and controllable alcoholysis of Kraft lignin catalyzed by porous zeolite-supported nickel-copper catalyst

Bioresource Technology 276 (2019) 310–317

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Efficient and controllable alcoholysis of Kraft lignin catalyzed by porous zeolite-supported nickel-copper catalyst

T

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Liping Kong, Chunze Liu, Ji Gao, Yuanyuan Wang , Liyi Dai Shanghai Key Laboratory of Green Chemistry and Green Process, College of Chemistry and Molecular Engineering, East China Normal University, No. 500 Dongchuan Road, Shanghai 200241, People’s Republic of China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Kraft lignin Alcoholysis Ni-Cu/H-Beta Catalyst structure Cycloalkanes

The alcoholysis of Kraft lignin was catalyzed by bimetallic Ni-Cu supported on H-Beta, HZSM-5, MAS-7, MCM-41 and SAPO-11 zeolite materials in isopropanol solvent. Results showed that a higher bio-oil yield of 98.80 wt% and monomer yield of 50.83 wt% without obvious char were achieved at 330 °C for 3 h over Ni-Cu/H-Beta catalyst. Isopropanol was found to be more effective in H2 generation and facilitated to the hydrodeoxygenation of lignin-derived phenolic compounds. Moreover, the composition of liquid products was also influenced by the acidity and pore structure of catalyst. The superior cycloalkanes yield was produced over Ni-Cu/H-Beta with larger pore size and more acidity. In contrast, a large number of cyclic ketones/alcohols and alkanes were obtained over other zeolites supported catalysts with smaller pore size and less acid content. Besides, the temperature, time and solvent effect on the lignin depolymerization were also researched.

1. Introduction The use of lignocellulosic biomass as a feedstock for the production of fuels and high-value chemicals has attracted extensive attention (Shuai et al., 2016). Lignocellulosic biomass, which consists of cellulose, hemicellulose and lignin, is the most renewable, abundant and cleanest form of earth-based biomass. Although most attention has been focused on the conversion of the cellulosic and hemicellulose parts of

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biomass, the full exploitation and utilization of lignin is essential for making lignocellulosic biorefineries profitable. High volumes of lignin produced in the conventional pulp mills are not fully utilized due to its complicated three-dimensional amorphous polymer structure. The complex structure of lignin arises from three precursors (guaiacyl alcohol (G), syringyl alcohol (S) and p-coumaryl alcohol (H)) linked by CeO and CeC bonds, among which CeOeC ether linkages occupy 67–75% (Li et al., 2015). Hence, lignin has been shown promising to be

Corresponding author. E-mail address: [email protected] (Y. Wang).

https://doi.org/10.1016/j.biortech.2019.01.015 Received 8 November 2018; Received in revised form 3 January 2019; Accepted 4 January 2019 Available online 06 January 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.

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catalysts pore structure and acidity on the monomer yield, char yield and product distribution. Moreover, the recyclability of the optimal NiCu/H-Beta catalyst was also observed. Furthermore, our research has greatly assisted in gaining insight into the method of acquiring more high value-added chemicals in supercritical isopropanol system. In addition, the effect of temperature, time and solvent were also discussed in detail.

used as a resource for the production of valuable chemicals, for instance, phenols, cyclic ketones, cycloalkanes, alkanes and some other compounds. The depolymerization of lignin into value-added chemicals has already been investigated by various processes such as hydrocracking, pyrolysis, hydrogenolysis, oxidation, and hydrolysis (Das et al., 2018). Among them, catalytic hydrogenolysis process is effective for removing oxygen from bio-oil and improving its fuel quality. Many of the previous reports have stated the effectiveness of bifunctional catalysts, containing a balance of metal and acid sites, in hydrodeoxygenation of lignin-derived phenolic compounds (Kärkäs, 2017). The active hydrogenation ability of noble metal like Pt, Pd, Au-Ni, Rh-Co have been applied in depth (Chen et al., 2018; Yang et al., 2016). However, high price and deficiency of these noble metals limit their application in the industry. As feasible substitutes, numerous non-noble transition metals, such as W, Fe, Ni-Mo, Ni-Fe have been reported (Ji et al., 2018). In addition, Ni-Cu bimetallic catalysts were synthesized for hydrodeoxygenation of guaiacol into transportation fuels. The higher yield of cyclohexane was obtained not only related to the better hydrogenation ability of metals but also the outstanding dehydration, hydrogenolysis and hydrocracking performance of acid support (Ambursa et al., 2017). By comparison, the relevance of many metal oxide supports, such as Al2O3, SiO2, CeO2 and NbOPO4 for lignin hydrogenolysis has been blocked due to their limited specific surface and weak acidic properties (Shao et al., 2017; Wang et al., 2016). Moreover, the absence of larger surface and suitable acidity in the support will also increase the formation of serious char during lignin depolymerization processes. Long et al. reported that the formation of residual solid in the process of pine lignin depolymerization over MgO was mainly caused by the repolymerization of unsaturated products (Long et al., 2014). Luterbacher’s group has made great efforts in inhibiting repolymerization and increasing monomer yields. Formaldehyde and other stabilizers were introduced into the biomass pretreatment, and then some active groups of macromolecular lignin were passivated in the subsequent hydrogenolysis process (Lan et al., 2018). However, the depolymerization process is relatively complex and the use of a large number of organic reagents has a great impact on the environment pollution. Zeolites as an ideal support is favorable to enhance the monomer yield and suppress the repolymerization for catalytic upgrading of lignocellulose due to the larger surface area, well-ordered pore structure and certain acidity. Valla et al. synthesized Ni/HZSM-5 catalyst for the pyrolysis of biomass, where the synthetic bifunctional catalyst was favorable to obtain substantial monoaromatics, and solid product was reduced drastically. Chen et al. also reported the synthesis of Ni/AlSBA-15 and its application in eliminating the char formation during the deconstruction of hydrolyzed lignin (Chen et al., 2017). However, to the best of our knowledge, relatively little attention has been devoted to the effect of catalyst structure on the distribution of liquid products during the hydrodeoxygenation process of lignocellulosic-derived phenol monomers. Solvent is also an important factor in lignin deconstruction process. Hydrogen donor solvents have coordination function on promoting hydrodeoxygenation and reducing char formation in the catalytic conversion of lignin to fine chemicals (Feng et al., 2017). Alcohols, such as methanol, ethanol, isopropanol and ethylene glycol, act as nucleophilic reagent for cleaving C-O-C linkages. Rinaldi et al. researched various alcohols as solvent for the catalytic depolymerization of lignin model compounds at 300 °C over a Ni-base catalyst. The results showed that isopropanol was the optimal solvent because of its good transfer hydrogenation properties. Besides, enhanced solubility of lignin was also beneficial to suppress the formation of solid residue. In this contribution, the effect of Ni-Cu bifunctional catalysts including Ni-Cu/H-Beta, Ni-Cu/HZSM-5, Ni-Cu/MAS-7, Ni-Cu/MCM-41 and Ni-Cu/SAPO-11 on the catalytic depolymerization of kraft lignin (KL) in supercritical isopropanol were investigated systematically. The purpose of this paper is to determine the influence of bifunctional

2. Materials and methods 2.1. Materials The KL was purchased from Sinopharm Chemical Reagent Co. Ltd. The samples were dried overnight at 100 °C in the oven before use. Nickel (II) nitrate hexahydrate, copper (II) nitrate hydrate, urea, ammonium hydroxide (28–30% NH3), sodium hydroxide, sodium aluminate, fumed silica, ammonia (Ⅰ) chloride, phosphoric acid (85% H3PO4), ethanol, isopropanol, tetrahydrofuran (THF) and dichloromethane (DCM) were also purchased from Sinopharm Chemical Reagent Co. Ltd. Hexadecyltrimethylammonium bromide (CTAB), tetraethyl orthosilicate (TEOS), fumed silica, tetraethyl ammonium hydroxide (20% TEAOH), polyethylene oxide-polypropylene oxide-polyethylene oxide (P123) and poly dimethyl diallyl ammonium chloride (PDDA) were purchased from Sigma Aldrich. Aluminium isopropoxide, Tetraethyl ammonium hydroxide (TPAOH), acidic silica gel (23.4% SiO2), dipropylamine were purchased from Aladdin. AR reagent grade solvents and chemicals including guaiacol, cyclohexanol, methyl cyclohexane, pentadecanoic acid, tetradecane, dodecane were purchased from Alfa Aesra. Standard gases (H2, CO, CO2, CH4, C2H4, C2H6, C3H8) were purchased from Shanghai Pujiang gas Co., Ltd. 2.2. Catalyst preparation The Ni-Cu bimetal supported on H-Beta, HZSM-5, MAS-7, MCM-41 and SAPO-11 zeolite were prepared by a deposition-precipitation method. The catalyst was prepared to obtain Ni and Cu nominal loadings of 20.0 wt% and 20.0 wt%, corresponding to an equimolar loading of 3.40 mmol/gcat. and 3.12 mmol/gcat., respectively. Typically, the predetermined amount of Ni and Cu precursors solutions were mixed in the deionized water (200 mL), and 0.2 g support was suspended with mixture solution under stirring severely and then heated to 70 °C. A certain amount of urea dissolved in 50 mL water, and added drop wise to the former support suspension. After that, the mixture was maintained at 90 °C with stirring for 10 h. After the liquid temperature dropped to room temperature, the solid was filtered, washed and dried at 100 °C overnight. Finally, the catalyst was calcined at 400 °C for 4 h, and reduced at 550 °C for 4 h in 80 vol% H2/N2 flow (heating rate: 2 °C/ min) before investing the catalytic reaction. 2.3. Catalyst characterization Powder X-ray diffraction (XRD) patterns of catalysts were characterized by Bruker D8 Discover with Cu Kα from 10° to 80° with a scan speed 10°/min. The morphology of as-synthesized catalysts was recorded on scanning electron microscopy (SEM, Philips XL 30) and transmission electron microscopy (TEM, JEOL 2100F). N2 adsorptiondesorption isotherms of the catalysts were performed on a BELSORPmax apparatus. Surface area (SBET) was measured using the Brunauer, Emmett and Teller (BET) method. The pore size were calculated using the desorption branch of the isotherm according to the Barrett-JoynerHalenda (BJH) method. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) measurements were obtained on the Thermo Scientific iCAP 6300 instrument. Thermogravimetry-Differenital Thermal Analysis (TG-DTA) analysis was carried out with NETZSCH 409 PC/PG analyzer using 10–30 mg of samples and a heating rate of 10 °C/min under air atmosphere. 311

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2.4. Catalytic depolymerization of kraft lignin

2.5. Products analysis

The catalytic hydrotreatment reaction was performed in stainlesssteel batch autoclave equipped with an overhead stirrer. Typically, the reactor was filled with a suspension of 0.1 g lignin and 0.04 g catalysts in 12 mL isopropanol. The reactor was sealed and purged with N2 5–6 times to remove air. Afterward, the autoclave was heated up to a desired temperature of 543–623 K at a stirring speed of 600 rpm. After the reaction, the reactor was cooled to room temperature rapidly in a water bath. The as-prepared products were in the form of gas, liquid, and solid (include catalyst particles and char), respectively. A series of product separation steps were formed as follows. Gas products were firstly collected with a gas pack. Then the liquid/solid mixture was collected by washing the reactor with 50 mL DCM and separated by vacuum filtration with a pre-weighted filter membrane. For the obtained liquid products, one was volatile product which would go through qualitative and quantitative analysis, the other was oligomers which was detected by GPC and elemental analysis to obtain the molecular weight and element composition. After that, the filter solid fraction was washed with THF several times to remove the unreactive lignin. Finally, the filter cake was dried at 100 °C overnight and weighted. The conversion rate of lignin, the yields of bio-oil, monomer, residue, and the selectivity of gas product were calculated using the following Eqs. (1)–(6).

The gas phase sample was analyzed by Agilent 7900 GC equipped with thermal conductivity detector (TCD). In order to get the component of gaseous products, helium and hydrogen were selected as carrier gases during the chromatographic separation to analyze the H2 and carbon-containing gases. The molar percentage composition of the gas products were calculated using peak normalization method. The liquid fraction qualification was performed on a GC-MS instrument (Agilent 7890 A GC/5975C MS detector) equipped with an HP-5 capillary column (30 m × 0.25 mm × 0.32 mm). The column was initially kept at 45 °C for 2 min, and then ramped up to 150 °C at a rate of 5 °C min−1 and held for 3 min. Afterwards, raised to the final temperature 280 °C at the same rate and maintained for another 2 min. In addition, quantification was determined by using an average relative response factor per component group with n-dodecane as the internal standard. The molecular weights of liquid fraction was conducted by a Shimadzu apparatus equipped with two columns connected in series (Mixed-C and Mixed-D, polymer Laboratories) and a UV/vis detector at 254 nm. Analyses were performed at 35 °C using THF as eluent with a flow rate of 1 mL/min. Infrared diffuse reflection spectroscopy (FTIR-ATR) were measured on a Bruker VERTEX 70 spectrometer with additional PKE ATR (ZnSe crystal) internal reflection instrument, using DCM as sampling background. The component (wt.%) of C, H, N in lignin liquefaction fraction was measured by an element analyzer. Oxygen content in the bio-oil was calculated by difference using 100% minus C%, H% and N%. The heating value of liquid products was calculated using the Dulong forHeatingva mula (Yang et al., 2016): lue(MJ / kg ) = 0.335 × [C] + 1.423 × [H] − O. 154 × [O] − 0.145 × [N], where C, H, N, O was the contents of each element.

Xlig . (wt %) = 100 −

Ybio − oil (wt %) =

Sx =

Mb =

(1)

Wl × 100 Wp

Ymonomer (wt %) =

Ychar (wt %) =

Wu × 100 Wp

(2)

Wm × 100 Wp

(3)

Wc × 100 Wp

3. Results and discussion 3.1. Depolymerization of lignin in isopropanol system

(4)

Vx × 100 Vt

3.1.1. Catalytic depolymerization of kraft lignin and analysis of the volatile products It was founded that the deconstruction of KL generated four main products: gas products, volatile products, non-volatile products (namely the oligomer) and the char. GC-MS was used to identify the volatile products obtained from bio-oil. The catalytic depolymerization of KL was conducted at 330 °C for 3 h (Table 2) in isopropanol solvent in order to research the effect of catalyst composition on the KL liquefaction regularities. All catalysts exhibited favorable aspects for lignin conversion, such as higher monomer yields, oil yields and lower solid residues. Simplying heating KL without a catalyst resulted in only 70.75% lignin conversion and 15.09 wt% of monomer yield, and most of the KL was converted to char (27.01 wt%), indicating that the KL was

(5)

Wu + Wl + Wc × 100 Wp

(6)

Xlig: the conversion of lignin; Ybio-oil: the liquefaction degree of lignin; Ymonomer: the yieid of monomer; Ychar: the yield of residue; Sx: the selectivity of gas product; Wu: the weight of unreacted lignin; Wp: the weight of parent lignin; Wl: the weight of bio-oil; Wm: the weight of monomer; Wc: the weight of char; Vx: The volume of a certain component of the gas product; Vt: Total volumes of all gases in the gaseous product; Mb: Mass balance. Table 2 The effect of Ni-Cu catalysts on lignin hydrogenolysis and product distribution. Entry

1 2 3b 4b 5b 6 7 8 9 a b

Catalyst

None Ni-Cu/H-Beta Ni-Cu/H-Beta(1) Ni-Cu/H-Beta(2) Ni-Cu/H-Beta(5) Ni-Cu/HZSM-5 Ni-Cu/MAS-7 Ni-Cu/MCM-41 Ni-Cu/SAPO-11

Yield of monomera (wt.%) Aromatics

Cyclic ketones/alcohols

Cycloalkanes

Alkanes

Total

12.42 0 2.79 4.34 7.03 6.99 6.16 6.29 8.03

0 0 0.38 0.26 1.37 2.98 26.23 25.15 2.98

0 40.39 37.75 35.96 30.58 2.87 6.32 2.36 2.01

0 9.01 7.37 4.67 1.05 27.83 2.43 1.24 4.99

15.09 50.83 49.76 48.06 43.69 41.72 44.09 41.32 23.09

Xlig.(wt.%)

Ybio-oil(wt.%)

Ychar(wt.%)

Mass balance (wt.%)

70.75 98.03 97.25 96.79 91.96 90.50 94.82 93.96 85.43

47.10 98.80 97.69 96.08 90.07 88.76 95.20 93.32 85.74

27.01 0.04 0.53 1.08 7.79 7.86 2.35 3.04 8.02

92.11 96.99 97.34 93.49 93.08 96.59 98.34 96.03 94.87

Detected by GC 7890, where n-dodecane was used as interal standard. The catalytic performance of catalyst after repeated 1,2,5 times, respectively. Reaction condition: 0.1 g KL, 0.04 g catalysts, 12 mL isopropanol, 330 °C, 3 h. 312

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Table 1 Textural properties of different catalysts. Entry

1 2f 3f 4f 5 6 7 8 a b c d e f

Catalysts

Ni-Cu/H-Beta Ni-Cu/H-Beta(1) Ni-Cu/H-Beta(2) Ni-Cu/H-Beta(5) Ni-Cu/HZSM-5 Ni-Cu/MAS-7 Ni-Cu/MCM-41 Ni-Cu/SAPO-11

Aciditya (mmol/g)

1.30 1.25 1.22 0.98 1.02 0.86 0.54 0.61

SBETb (m2/g)

439.75 436.06 430.85 403.89 301.75 347.76 303.78 106.89

Smeso (m2/g)

Vtotal (cm3/g)

328.56 326.29 324.59 298.76 121.69 340.01 234.59 54.87

0.61 0.59 0.58 0.49 0.43 0.56 0.31 0.19

Vmesoc (cm3/g)

0.55 0.53 0.50 0.43 0.37 0.54 0.28 0.13

Pore sized (nm)

6.71 6.70 6.68 6.03 5.34 5.75 5.67 5.08

Metal loadinge (mmol/g) Ni

Cu

2.56 2.54 2.53 2.54 2.55 2.57 2.56 2.58

2.34 2.32 2.34 2.33 2.31 2.30 2.33 2.31

Determined by NH3-TPD. Multi-point BET method. Total pore volume at P/P0 = 0.99 minus the micropore volume. From BJH analysis. Detected by ICP-AES. The structural changes of the catalyst after repeated 1,2,5 times, respectively.

the adequate hydrogen content coupled with hydrodeoxygenation of phenolic compounds boosted the conversion of the reactants and the selectivity of the cycloalkanes. Meanwhile, the production of cyclopentanes may be due to carbon skeleton isomerization. This was also in accordance with the elemental analysis and GPC results, the bio-oil with lower O/C ratios and smaller molecular weight was obtained. In contrast, Ni-Cu/MAS-7 and Ni-Cu/MCM-41 were inert to obtain more cycloalkanes yield, the yield of cyclic ketones/alcohols was as high as 26.23 wt% and 25.15 wt%, respectively. The results indicated that the lack of strong Brønsted acid was not favorable for the dehydration of cyclic alcohols and their substituents in the in-situ hydrodeoxygenation reaction. Recently Kong and co-workers described the important catalytic routes for the conversion of guaiacol in reductive environments, and the relevant reaction mechanism were: (i) removing of methoxy groups and hydrogenation of benzene-ring of reactant and (ii) a series of sequential dehydration, hydrogenolysis, and hydrogenation steps assistant with acid sites (Kong et al., 2015). Thus, tuning the nature of the acidic property could control the selectivity of the process and yield of desired products. Besides, the product yield might also be influenced by the pore structure of catalysts. Especially, alkanes (27.83 wt%) instead of cycloalkanes were the predominant components of the liquid products in the presence of Ni-Cu/HZSM-5. Moreover, a large number of aromatics (6.99 wt%) and cyclic ketones/alcohols (2.98 wt%) were still detected. This should be due to the fact that oxygen-containing aromatics and other intermediates suffered from steric hindrance through the small channels of Ni-Cu/HZSM-5 catalysts (5.34 nm) and their further hydrodeoxygenation was therefore slow. By comparison, a certain number of aromatics and cyclic ketones/alcohols were also obtained in the liquid product over Ni-Cu/SAPO-11, the yield of cycloalkanes and alkanes was only 2.01 wt% and 4.99 wt%, respectively. The above results further confirmed that the lack of larger pore diameter and more acidity was not conducive to the conversion of substituted phenols into cycloalkanes. Simultaneously, some oxygencontaining chain compounds have also been detected in GC-MS results. In order to better study the formation process of oxygen-containing chain compounds, blank experiment was added without lignin (Table 3). The results showed that many kinds of alkanes (C12–C24) , aliphatic acid/ester compounds (C16–C18) and linear ketones/alcohols (C6–C12) were produced in the optimal reaction condition at 330 °C for 3 h. More concretely, the produce of alkanes and aliphatic acids/esters suggested that the polymerization occurred among active intermediates such as propene and acetaldehyde. Ma et al. also reported that the thermal decomposition of isopropanol could be converted into propene, acetone and acetaldehyde by dehydration, dehydrogenation and demethanation reaction (Ma et al., 2014). Moreover, the linear ketones/ alcohols were likely obtained mainly due to the acetone dimerization

not effectively depolymerized (Table 2, entry 1). However, the Ni-Cu bimetal supported on different zeolite supports has been shown to be very active in catalytic lignin alcoholysis process. The superior KL conversion of exceeding 90% was observed for H-Beta, MAS-7, MCM-41 supported Ni-Cu catalysts. The larger mesoporous surface area of the support was not only conducive to the dispersion of the catalyst, but also beneficial to the conversion of lignin-derived oxygenates (Table 1). The highest oil yield was gained for the Ni-Cu/H-Beta (98.80 wt%), followed closely by the Ni-Cu/MAS-7 (95.20 wt%) > Ni-Cu/MCM-41 (93.32 wt%) > Ni-Cu/HZSM-5 (88.76 wt%) > Ni-Cu/SAPO-11 (85.74 wt%) under higher lignin conversion. The trend of oil yield was positively correlated with the pore size of the catalyst. It was indicated that the larger pore size of catalyst would allow larger species to fit in and undergo consecutive reaction during lignin depolymerization reaction. Jae et al. investigated the effect of zeolite pore size on the conversion of glucose to aromatics. The results indicated that most of the aromatic compounds were obtained inside the medium or large pores of zeolites (Jae et al., 2011). What’s more, the resulting solid residue was also reduced more or less, ranging from 8.02 wt% for the Ni-Cu/SAPO-11 to 0.04 wt% for the Ni-Cu/H-Beta. Li et al. claimed that a large number of intermediates which generated from biomass pyrolysis could inevitably undergo repolymerization to form the coke under high temperature (Li et al., 2007). However, large pore size zeolite like H-USY, which enabled adsorption and further reaction of larger intermediates molecules, produced small amounts of coke in contrast to the small pore size zeolite such as HZSM-5 (Xu et al., 2017). Therefore, moderate surface area and ordered pore structure also had a good stabilization effect on the intermediates obtained from lignin depolymerization and inhibited the formation of char. The product compositions, for the various catalysts of our research, were also summarized in Table 2. To simply the analysis, the GC-detected main products were classified into five groups, i.e., aromatics, cyclic ketones/alcohols, cycloalkanes, alkanes and oxygenic-chain compounds. It seemed that the acidity of catalysts could change the reaction pathway in hydrodeoxygenation process, thus affecting the product distribution. Typically, Ni-Cu/H-Beta has been shown to be very active in enhancing the yields towards cycloalkanes in bio-oil. The yield of substituted cycloalkanes (including alkyl cyclopentanes and cyclohexanes) produced from the hydrodeoxygenation of substantial phenolic compounds increased to 40.39 wt%. Interestingly, the yield to cycloalkanes was relevant with the acidity of catalyst. It was also clearly observed that the acidity, especially the specific strong Brønsted acid sites on Ni-Cu/H-Beta facilitated the intermediate cyclic alcohols and their derivatives could be quantitatively dehydrated to cycloalkanes. Besides, the existence of moderate acid sites also contributed to the efficient in-situ hydrogen production from the isopropanol. Hence, 313

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Table 3 Component analysis (mg) of liquid products catalyzed by different catalysts without Kraft lignin in isopropanol solvent. Entry

Catalyst

Alkanes (C12–C24)

Aliphatic acids/esters (C16–C18)

Aliphatic ketones/alcohols (C6–C12)

1 2 3a 4a 5a 6 7 8 9

None Ni-Cu/H-Beta Ni-Cu/H-Beta(1) Ni-Cu/H-Beta(2) Ni-Cu/H-Beta(5) Ni-Cu/HZSM-5 Ni-Cu/MAS-7 Ni-Cu/MCM-41 Ni-Cu/SAPO-11

13.59 4.81 5.02 4.35 5.21 8.72 4.39 3.25 6.30

18.52 1.40 1.37 1.29 2.03 2.37 5.61 5.33 4.97

0 15.92 15.87 15.80 15.03 14.78 14.96 14.89 14.58

Reaction condition: 0.04 g catalyst, 12 mL isopropanol, 330 °C, 3 h. a The catalytic performance of catalyst after repeated 1,2,5 times, respectively.

through an aldol condensation reaction. Comparatively speaking, more linear alcohols/ketones were received over Ni-Cu/ H-Beta, which attested that the dehydrogenation of isopropanol was promoted significantly than other catalysts. Taking all the factors into consideration, Ni-Cu/H-Beta should be the optimal catalyst for the valorization of lignin in isopropanol solvent. To estimate the reusability of the catalyst in supercritical isopropanol system, the optimal Ni-Cu/H-Beta was cyclically used in lignin depolymerization without any treatment. The experiments were also summarized in Table 2. The first two cycles tests gave similar lignin conversion from a mass fraction of 97.25% to 96.79%. Regarding the oil yield, no significant decrease after two consecutive tests was observed. However, a significant lignin conversion and oil yield decrease was detected after the fifth test. In order to better explore the reason of catalyst deactivation, the SEM and TEM images of used Ni-Cu/H-Beta was observed. The results clearly revealed that the size of metal particle increased slightly with progressive cyclic numbers. And then, the XRD patterns for the spent Ni-Cu/H-Beta showed the diffraction peaks intensity decreased somewhat. Based on the analysis of TG results, we speculated that the catalyst surface may be covered by carbon deposition. What’s more, the drop in the conversion of lignin was accompanied by the change of liquid product distribution in the recyclability tests. From Table 2, we also noticed that the yield of cycloalkanes and alkanes declined while the yield of aromatics and cyclic ketones/alcohols increased after each cycle. This could be assigned to the reduction of specific surface area, pore volume and acid content of the catalysts would prevent the progressively hydrodeoxygenation of lignin-derived phenolic compounds. Oregui-Bengoechea et al. also described the recyclability of the HNiMo-SAL catalyst in the lignin depolymerization reaction. A slight oil yield decrease was detected after the first test, which was assigned to carbon deposition on catalyst surface (Oregui-Bengoechea et al., 2017). Fortunately, serious loss of active component was not found in the process of recycling, which further showed that the main reason for catalyst deactivation was increasing particle size and carbon deposition of catalyst.

Table 4 Element composition, heating value of kraft lignin and liquid product over different catalysts. Entry

1 2 3 4 5 6 7

Catalyst

KL None Ni-Cu/H-Beta Ni-Cu/HZSM-5 Ni-Cu/MAS-7 Ni-Cu/MCM-41 Ni-Cu/SAPO-11

Element content (wt.%)

HHV

C

H

O

N

O/C

H/C

(MJ/kg)

47.68 69.07 78.39 75.97 76.47 74.81 72.24

5.11 7.21 9.84 9.58 8.76 8.66 8.91

47.12 22.95 11.58 14.17 14.48 16.34 18.40

0.09 0.77 0.19 0.28 0.29 0.19 0.45

0.99 0.33 0.15 0.19 0.19 0.22 0.25

0.11 0.10 0.13 0.13 0.11 0.12 0.12

16.22 29.76 38.45 36.86 35.82 34.85 33.98

Reaction condition: 0.1 g KL, 0.04 g catalyst, 12 mL isopropanol, 330 °C, 3 h.

and strong Brønsted acid site of Ni-Cu/H-Beta bifunctional catalyst. In contrast, the heating value of the oligomers obtained by Ni-Cu/SAPO11 was only 33.98 MJ/kg. This further indicated that smaller surface area and pore size of the catalyst were not conducive to the production of high heating value bio-oil (Zhou et al., 2016). To further investigate the effect of the catalyst on the chemical properties of the oligomer product, the gel permeation chromatograms (GPC) was carried out to characterize the molecular weight distribution of bio-oil. Direct hydrogenolysis of lignin and the peaks within a small retained volume indicated liquid products had a broad molecular weight distribution. We speculated that lignin was not completely depolymerized into monomers in the absence of catalyst (Kloekhorst and Heeres, 2016). Comparatively speaking, the peak appeared in a larger retained volume and the molecular weight distribution becomed narrower in the presence of catalyst. The largest retention volume with NiCu/H-Beta demonstrated that the oligomer obtained had the lowest molecular weight. It was probably because the large fragments of lignin could be stabilized in the well-ordered pore and further converted into the small molecular compounds. What’s more, the molecular weight distribution of bio-oil produced in the presence of Ni-Cu/MAS-7 and NiCu/MCM-41was distinctly lower than that of Ni-Cu/HZSM-5, this observation further illustrated that larger pore structure was beneficial to the conversion of lignin to small molecular compounds (Cattelan et al., 2017). Moreover, in the presence of Ni-Cu/SAPO-11, the larger molecular weight distribution was in line with the lower monomer yield and higher yield of char, the aggravation of repolymerization would be appeared in the small pore, which presented an inhibitory effect on lignin depolymerization. FTIR-ATR was further used to measure the structure change of lignin depolymerization oligomer. It could be found that the absorption peak of phenolic hydroxyl groups (1276 cm−1), aryl aldehydes (1715–1695 cm−1), aromatic (1606 cm−1, 1501 cm−1) was observed in the absence of catalyst, which clarified that the product contained a large number of aromatic compounds. However, the structure of oligomers changed significantly after introducing different catalysts.

3.1.2. Analysis of the nonvolatile products The elemental analysis was used to determine the main elemental composition of original KL and oligomers after depolymerization over different catalysts. As displayed in Table 4, the KL material contained 47.68 wt% of carbon, 5.11 wt% of hydrogen and 47.12 wt% of oxygen. The oxygen content of the bio-oil obtained using Ni-Cu/H-Beta (11.58 wt%), Ni-Cu/HZSM-5 (14.17 wt%), Ni-Cu/MAS-7 (14.48 wt%), Ni-Cu/MCM-41 (16.34 wt%), Ni-Cu/SAPO-11 (18.40 wt%) was much lower than that of without catalyst. It showed that the introduction of catalyst could enhance the ability of hydrodeoxygenation in the process of lignin depolymerization. In particular, the O/C atomic ratio decreased and heating value increased evidently in the presence of Ni-Cu/ H-Beta, it could be ascribed to the excellent hydrogenation capability 314

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Fig. 2. Effect of reaction temperature on the lignin hydrogenolysis. Fig. 1. The analysis of gaseous product obtained by different catalysts.

99.78%) over Ni-Cu/H-Beta. Wherein, the amount of residue also sharply decreased from 8.98 wt% to 0.02 wt%. The liquid yields increase remarkably accompanied by nearly sustained lignin conversions, suggested that enhanced temperatures was beneficial for the cleavage of the lignin-derived phenolic oligomers to much smaller fragments (Yuan et al., 2016). Besides, the supercritical isopropanol also exhibited better hydrogen transfer property and promoted the occurrence of hydrogenolysis of lignin further under higher temperature (Luo et al., 2018). However, when the reaction temperature increased to 350 °C, the content of char improved to some extent. It was contributed to that the repolymerization and cross-linking reaction between reactive sites of phenolic and the active aldehyde groups could occur at higher temperature (Zhang et al., 2017). Strikingly, an increase of monomer yield was accompanied by elevation of reaction temperature, proportional to a cycloalkane mixture yield from 3.65 wt% to 42.08 wt%. It was attributed to the hydrogenolysis, dehydration and hydrogenation process of phenolic monomers being enhanced over Ni-Cu/H-Beta under higher temperature. Reaction time also had a pronounced effect on the conversion of lignin and yield of monomers (Fig. 3). The conversion of lignin gradually increased from 82.35% to 99.90% as the reaction time increased from 1 to 5 h. Specially, the conversion of lignin had reached to 98.03% after 3 h, which confirmed that the complete deconstruction of lignin mainly occurred during the initial 3 h. Yet the appearance of a small amount of char further indicated that repolymerization of the phenolic

Especially, the sharp strength decrease of hydroxyl (3430 cm−1) could be attributed to the hydrodeoxygenation of phenolic compounds. To be specific, the remarkably existing peak of 2970 cm−1 of bio-oil was attributed to the absorption peak of cycloalkane skeletal vibrations (NiCu/H-Beta). Besides, the characteristic absorption peak of Ar-O structure (1215 and 1110 cm−1) was obviously decreased and it indicated that the eOCH3 bond was effectively cleaved. Furthermore, 1385 cm−1 and 1365 cm−1 were mainly due to saturated aliphatic hydrocarbon acquired by Ni-Cu/HZSM-5. The strength increase of 1660 cm−1 and 1715 cm−1 could be owning to the existence of unsaturated cyclic ketones, as shown in Ni-Cu/MAS and Ni-Cu/MCM-41. However, some aromatic ethers (1097 cm−1, 1260 cm−1) were also found in bio-oil obtained by Ni-Cu/SAPO-11, this was also in good agreement with the GCeMS results, where a certain number of guaiacols and syringyl were detected. 3.1.3. Effect of the catalyst on the gaseous product distribution The gaseous products composition in the in-situ catalytic upgrading of lignin, for the various catalysts in our study, was summarized in Fig. 1. The gas was mainly composed of H2, CO, CO2 and CH4, and small numbers of olefins (< 0.5 wt%), such as C2H4 and C3H6, were also detected. The formation of CO, CO2 and CH4 was due to various hydrocarbon conversion reactions, such as decarbonylation, decarboxylation and demethylation (Qi et al., 2017). In comparison to the noncatalytic experiments, all the catalysts increased the gaseous product yields somewhat. Evidently, the more hydrogen content (83.50 mmol) was obtained in the presence of Ni-Cu/H-Beta, it was advantageous to undergo consecutive hydrodeoxygenation reaction (Liu et al., 2016). Specially, more CH4 and CO were acquired over Ni-Cu/MAS-7 and NiCu/MCM-41, which were probably the results of efficiently cracking of eCH3 and eCHO groups. Instead, only 38.32 mmol H2 was gained by SAPO-11 supported catalyst, which was also unfavorable for the conversion of alkoxyphenols to cycloalkanes. Similar result was observed by Kim et al. during lignin catalytic pyrolysis in a bench-scale pyrolysis system. They also found that the acid sites of the catalyst would have a great effect on the production of H2 (Kim et al., 2017). 3.1.4. Effect of reaction temperature, time and solvent The influence of reaction temperature and time were investigated on the catalytic conversion of KL in isopropanol over Ni-Cu/H-Beta catalyst. It could be seen that the conversion and liquid yield of lignin were significantly influenced by the temperature (Fig. 2). For instance, varying the temperature from 270 °C to 350 °C led to an increase in the liquid yield from 85.37 wt% (conv.: 57.58%) to 99.98 wt% (conv.:

Fig. 3. Effect of reaction time on lignin depolymerization and product distribution. 315

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an effect on the alcoholysis of lignin. Isopropanol was also found to be more efficient in suppressing repolymerization than other solvent. Acknowledgments This work was finally supported by the key project of Shanghai Science and Technology Committee (No. 14231200300) and Shanghai Key Laboratory of Green Chemistry and Chemical Processes. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.01.015. References Ambursa, M.M., Sudarsanam, P., Voon, L.H., Hamid, S.B.A., Bhargava, S.K., 2017. Bimetallic Cu-Ni catalysts supported on MCM-41 and Ti-MCM-41 porous materials for hydrodeoxygenation of lignin model compound into transportation fuels. Fuel Process. Technol. 162, 87–97. Cattelan, L., Yuen, A.K.L., Lui, M., Masters, A.F., Selva, M., Perosa, A., Maschmeyer, T., 2017. Renewable aromatics from kraft lignin with molybdenum-based catalysts. ChemCatChem 9, 2717–2726. Chen, H., Lu, Q., Yi, C., Yang, B., Qi, S., 2018. Design of bimetallic Rh-M catalysts for N 2 O decomposition: From DFT calculation to experimental study. Mol. Catal. 446, 1–9. Chen, P., Zhang, Q., Shu, R., Xu, Y., Ma, L., Wang, T., 2017. Catalytic depolymerization of the hydrolyzed lignin over mesoporous catalysts. Bioresour. Technol. 226, 125–131. Das, L., Li, M., Stevens, J.C., Li, W., Pu, Y., Ragauskas, A.J., Shi, J., 2018. Characterization and catalytic transfer hydrogenolysis of deep eutectic solvent extracted sorghum lignin to phenolic compounds. ACS Sustain. Chem. Eng. 6, 10408–10420. Feng, J., Hse, C.Y., Yang, Z., Wang, K., Jiang, J., Xu, J., 2017. Liquid phase in situ hydrodeoxygenation of biomass-derived phenolic compounds to hydrocarbons over bifunctional catalysts. Appl. Catal. A 542, 163–173. Jae, J., Tompsett, G.A., Foster, A.J., Hammond, K.D., Auerbach, S.M., Lobo, R.F., Huber, G.W., 2011. Investigation into the shape selectivity of zeolite catalysts for biomass conversion. J. Catal. 279 (2), 257–268. Ji, J., Guo, H., Li, C., Qi, Z., Zhang, B., Dai, T., Jiang, M., Ren, C., Wang, A., Zhang, T., 2018. Tungsten-based bimetallic catalysts for selective cleavage of lignin C–O bonds. ChemCatChem 10 (2), 415–421. Kärkäs, M.D., 2017. Lignin hydrogenolysis: improving lignin disassembly through formaldehyde stabilization. ChemSusChem 10, 2111–2115. Kim, Y.M., Jae, J., Kim, B.S., Hong, Y., Jung, S.C., Park, Y.K., 2017. Catalytic co-pyrolysis of torrefied yellow poplar and high-density polyethylene using microporous HZSM-5 and mesoporous Al-MCM-41 catalysts. Energy Convers. Manage. 149, 966–973. Kloekhorst, A., Heeres, E., 2016. Catalytic hydrotreatment of Alcell lignin fractions using a Ru/C catalyst. Catal. Sci. Technol. 6, 7053–7067. Kong, J., He, M., Lercher, J.A., Zhao, C., 2015. Direct production of naphthenes and paraffins from lignin. Chem. Commun. 51, 17580–17583. Lahive, C.W., Deuss, P.J., Lancefield, C.S., Sun, Z., Cordes, D.B., Young, C.M., Tran, F., Slawin, A.M., de Vries, J.G., Kamer, P.C., 2016. Advanced model compounds for understanding acid-catalyzed lignin depolymerization: identification of renewable aromatics and a lignin-derived solvent. JACS 138, 8900–8911. Lan, W., Amiri, M.T., Hunston, C.M., Luterbacher, J.S., 2018. Protection group effects during α, γ-diol lignin stabilization promote high-selectivity monomer production. Angew. Chem. 57, 1356–1360. Li, C., Zhao, X., Wang, A., Huber, G.W., Zhang, T., 2015. Catalytic transformation of lignin for the production of chemicals and fuels. Chem. Rev. 115, 11559–11624. Li, J., Henriksson, G., Gellerstedt, G., 2007. Lignin depolymerization/repolymerization and its critical role for delignification of aspen wood by steam explosion. Bioresour. Technol. 98, 3061–3068. Liu, H., Zhang, H., Shi, L., Hai, X., Ye, J., 2016. 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Fig. 4. Effect of solvent system on the lignin depolymerization.

dimer and oligomer could be strengthened for a long time (Lahive et al., 2016). Interestingly, it could also be seen that the composition of bio-oil tended to simplify after 2 h. The hydrogenated cyclic ketones/alcohols disappeared with the extension of time. The content of cycloalkanes and alkanes occupied more than 95% of the product, indicating that the phenolic monomers and derivatives have completed the deep hydrodeoxygenation process. In addition to the reaction temperature and time, solvent system had a great influence on the hydrogenolysis performance and product distribution. Fig. 4 exhibited the lignin hydrogenolysis results catalyzed by Ni-Cu/H-Beta in different solvents. Only 78.78 wt% liquefaction yield and 20.57 wt% monomer yield were obtained, most of the lignin was transformed into oligomers and residue due to the poor solubility of lignin fragments in glycol. By comparison, ethanol and methanol showed a better activity than glycol. Although the lignin conversion and oil yield were nearly the same, the monomer yield in methanol reached to 44.71 wt%. The best performance on the lignin depolymerization process was exhibited in isopropanol. Especially, Highest conversion and monomer yield were achieved, which was ascribed to the better hydrogen transfer performance of isopropanol. Additionally, different alcohol solvents also had a greater effect on the composition of liquid products. More concretely, oxygenic-chain compound occupied the main components of liquid products and nearly 10.12 wt% oxygenrich polymers were produced in glycol system. The observation proved that glycol could undergo intermolecular dehydration to produce higher alcohols (Matson et al., 2011). Then 7.93 wt% yield of alkylphenolics was obtained in ethanol solvent, which further indicated that the oxygen-containing phenolic compounds could not achieve a deep hydrodeoxygenation process due to the lack of hydrogen supply. Next, the content of cycloalkanes (9.03 wt%) and alkanes (15.31 wt%) in methanol was much higher than that in ethanol solvent. Instead, the highest yield of cycloalkanes was observed in isopropanol system, which further indicated that good hydrogen supply ability of isopropanol was also beneficial to the hydrodeoxygenation of lignin-derived phenol monomers.

4. Conclusions The efficient depolymerization of kraft lignin was performed over Ni-Cu/H-Beta catalyst. Both Ni-Cu/H-Beta catalyst and isopropanol played important roles in promoting the lignin depolymerization and reducing char yield. The distribution of liquid products was greatly influenced by pore structure and acidity of the catalyst. The highest yield (40.39 wt%) of cycloalkanes was obtained over Ni-Cu/H-Beta catalyst. Reaction conditions such as temperature, time and solvent also had 316

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Shao, Y., Xia, Q., Lin, D., Liu, X., Han, X., Parker, S.F., Cheng, Y., Daemen, L.L., Ramirezcuesta, A.J., Yang, S., 2017. Selective production of arenes via direct lignin upgrading over a niobium-based catalyst. Nat. Commun. 8, 16104. Shuai, L., Amiri, M.T., Questell-Santiago, Y.M., Heroguel, F., Li, Y., Kim, H., Meilan, R., Chapple, C., Ralph, J., Luterbacher, J.S., 2016. Formaldehyde stabilization facilitates lignin monomer production during biomass depolymerization. Science 354, 329–333. Wang, W., Wu, K., Liu, P., Li, L., Yang, Y., Wang, Y., 2016. Hydrodeoxygenation of p–cresol over Pt/Al2O3 catalyst promoted by ZrO2, CeO2 and CeO2–ZrO2. Ind. Eng. Chem. Res. 55, 7598–7603. Xu, L., Yao, Q., Zhang, Y., Fu, Y., 2017. Integrated production of aromatic amines and Ndoped carbon from lignin via ex situ catalytic fast pyrolysis in the presence of ammonia over zeolites. ACS Sustain. Chem. Eng. 5, 2960–2969.

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