Novel Nonprecious Metal Loading Multi-Metal Oxide Catalysts for

Jun 21, 2019 - In this work, a low-cost silicon-based multi-metal oxide sphere (glass bead, .... the Barrett–Joyner–Halenda method, and the total ...
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Article Cite This: Energy Fuels 2019, 33, 6491−6500

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Novel Nonprecious Metal Loading Multi-Metal Oxide Catalysts for Lignin Depolymerization Lei Pu, Xing Wang, Qiping Cao, Bingyang Liu, Huan Liu, Ying Han, Guangwei Sun, Yao Li,* and Jinghui Zhou* Liaoning Province Key Laboratory of Pulp and Papermaking Engineering, Dalian Polytechnic University, Dalian 116034, Liaoning, China

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S Supporting Information *

ABSTRACT: In this work, a low-cost silicon-based multi-metal oxide sphere (glass bead, GB) and its derivatives were used as novel catalysts to depolymerize poplar lignin. The surface morphology (flaky structure) and specific surface area of GB could be effectively changed by the subcritical water treatment. The treated GBs were further loaded with nonprecious metals (Ni and Mn) to obtain GB derivatives. Based on the results of product analysis, it was found that the introduction of nonprecious metals could effectively prevent the departure of the methoxy group from the aromatic hydrocarbon structure, thus reducing lignin polycondensation (Ni: 8.8 wt % solid residue and 3.5 wt % char, Mn: 5.1 wt % solid residue and 1.7 wt % char) and increasing the yield of monomer products (Ni: 13.4 wt %, Mn: 15.5 wt %). Furthermore, GB derivatives could be recycled to catalyze depolymerization of lignin, and the monomeric product yield remained above 12 wt %. the position-4 carbon in the benzene ring of another unit.13−15 Among the successful approaches of lignin depolymerization, hydrothermal treatment has been considered a green and efficient technology. This approach is typically carried out at high temperature and high pressure. Under these conditions, lignin molecules undergo both depolymerization (the chemical bonds in the lignin structure are broken to produce lowmolecular-mass aromatic products) and polycondensation processes (new chemical bonds are formed between lignin molecules to produce lignin with larger molecular weight, even coke).16,17 To increase the yield of low-molecular-mass aromatic products, the polycondensation process of lignin molecules should be inhibited.18−20 Therefore, various catalysts are added to the hydrothermal process to improve the depolymerization effect and inhibit the polycondensation process. Although precious metal catalysts (such as Pt) and organic acid catalysts can effectively convert lignin into a variety of low-molecular-mass aromatic products, these catalysts are unsuitable for industrial production because of their high cost and nonreusability.21−25 As a kind of recyclable lignin depolymerization catalyst, the solid acid catalyst is still unable to be used in the practical application of lignin depolymerization because of their low specific surface area and pore size (catalytic sites in catalyst pores are unable to contact lignin).26,27 In addition, the pores of solid acid catalysts are easily blocked by coke in the process of catalysis, which weakens its catalytic ability in the process of reuse.28−31 In this work, we discussed the feasibility of using a glass bead (GB, silicon-based multi-metal oxide spheres) and its derivatives as novel catalysts to depolymerize lignin. After subcritical water treatment, the porous structure with high

1. INTRODUCTION Fossil fuel resources (coal and petroleum) supply over 95% of fuels and chemical products, which are the foundation of human industrial production.1 However, with the soaring prices and gradual depletion of coal and petroleum, the production of fuels and chemical products from nonrenewable fossil fuel resources has been a severe challenge.2 Therefore, an enormous amount of effort has gone into the biorefinery, using renewable resources (nonfood plant biomass including cellulose, hemicellulose, and lignin) to produce fuels and chemical products.3 Nowadays, biorefinery research always focuses on the utilization of carbohydrate (cellulose and hemicellulose), such as cellulosic ethanol, but lignin is still considered a by-product and burns as a low-value-added fuel.4,5 Lignin is the second most abundant biomacromolecule, and only next to cellulose. In the pulp and paper industry, only a small fraction (about 2%) of the 50 million tons of lignin produced annually is effectively utilized, and most of the lignin is burned or discharged into rivers.6 These factors highlight the necessity and inevitability of high-value applications of lignin. As one of the few renewable biomacromolecules with aromatic structures, lignin could provide low-molecular-mass aromatic products (such as benzene, toluene, and phenol derivatives) to directly incorporate into the downstream industrial processing by depolymerization.7 However, low monomeric product yield, high coke formation, and expensive catalysts still limit the application of lignin in industry. Lignin consists primarily of sinapyl (3,5-dimethoxy-4-hydroxycinnamyl) (S), coniferyl (3-methoxy-4-hydroxycinnamyl) (G), and pcoumaryl (4-hydroxycinnamyl) alcohol (H) units.8,9 The major linking modes of the units are C−C (β-1, β−β, 5−5) bonds and C−O−C (β-O-4, α-O-4, 4-O-5) bonds, especially β-O-4, which is a principal target for the depolymerization of lignin.10−12 The β-O-4 bond is formed between the β-carbon (the second carbon) in the aliphatic side chain of a unit and © 2019 American Chemical Society

Received: April 18, 2019 Revised: June 18, 2019 Published: June 21, 2019 6491

DOI: 10.1021/acs.energyfuels.9b01218 Energy Fuels 2019, 33, 6491−6500

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Energy & Fuels Scheme 1. Separation Flow Chart of Lignin Depolymerization Products

specific surface area (194.8 m2/g) appeared on the surface of the GB, which allowed it to load more active metals and increased catalytic sites. The treated GB was used to load nonprecious metals (Ni and Mn) to obtain GB derivatives. The catalytic activities of GB and GB derivatives were verified by depolymerization of poplar lignin (ethanol extraction). Based on the analysis of the yield and chemical structure of monomer products, solid residue and char, we found that the GB treated with subcritical water could improve the lignin depolymerization efficiency, but it could not inhibit the polycondensation of lignin (16.6 wt % solid residue and 7.6 wt % char). The GB derivatives with Ni or Mn element could effectively prevent the departure of the methoxy group from the aromatic hydrocarbon structure, thus reducing lignin polycondensation (Ni: 8.8 wt % solid residue and 3.5 wt % char, Mn: 5.1 wt % solid residue and 1.7 wt % char) and increasing the yield of monomer products (Ni: 13.4 wt %, Mn: 15.5 wt %). Furthermore, GB derivatives could be recycled to catalyze depolymerization of lignin, and the monomeric product yield remained above 12 wt %.

2.3. Preparation of GB Catalysts. The GBs (10 g) and deionized water (100 mL) were placed in a 150 mL tank reactor. Then, this tank reactor was sealed and gradually heated to 300 °C. The pressure of the tank reactor increased from the atmospheric pressure to 8 MPa (subcritical state of water). The subcritical state was maintained for 60 min, and then the reaction system was cooled to room temperature quickly.34 The solid substances were filtered and washed three times with deionized water. The resulting porous GBs were marked as GB-60, and the number represented the time of subcritical water treatment. The GB-60 (5 g) was added to Ni(NO3)2· 6H2O and Mn(NO3)2·6H2O solution (5 mL, 0.1 g/mL), respectively. The mixture was shaken for 24 h at room temperature. Then, the solid substances were filtered and washed three times with deionized water. The resulting GB-60 loaded with metal ions (Ni2+ or Mn2+) were calcined at 400 °C for 6 h in a muffle furnace to obtain GB-60Ni and GB-60-Mn eventually.35 2.4. Characterization of the Catalyst. Scanning electron microscopy (SEM) was performed on a JEOL JSM 7800 electron microscope with a primary electron energy of 5.0 kV. Energy dispersive X-ray spectroscopy (EDS) was recorded with Oxford INCA at an electron energy of 15 kV. Nitrogen adsorption−desorption isotherms were measured at 77 K on a Micromeritics ASAP 2020 PLUS HD88 surface area and porosity analyzer. The specific surface area was calculated from the adsorption branches in the relative pressure range of 0.05−0.25 mg. The pore diameter and the pore size distribution were calculated from the desorption branches using the Barrett−Joyner−Halenda method, and the total pore volume was evaluated at a relative pressure of about 0.99 Pa. 2.5. Catalytic Activity Measurements. 50 mL of stainless steel SS316 reactor (Bin Hai Zheng Xin Instrument Factory) was used to study the catalytic activity of GB-60, GB-60-Ni, and GB-60-Mn for lignin depolymerization. The catalysts (250 mg), lignin (500 mg), water (10 mL), and ethanol (10 mL) were placed in an autoclave. After leak testing, the autoclave was sealed and heated to 300 °C under continuous stirring at 600 rpm for 6 h. The separation process of the reaction products is shown in Scheme 1. After the reaction, the mixture (including liquid and residue) was collected and diluted with 60 mL of water. Then, the mixture was acidified by adding HCl solution (5 mL, 12.27 mol/L, final pH = 1) to precipitate the unconverted lignin and the repolymerized lignin. After being aged for 30 min, the resulting mixture was filtered over a filter paper with 0.45 μm pore size. The filtrate was collected and extracted with ethyl acetate. The organic phase extraction was combined and characterized

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. A glass bead (GB) was obtained from Hebei Chiye Corporation. Ni(NO3)2·6H2O and Mn(NO3)2· 6H2O (99.0%) were obtained from Sinopharm Chemical Reagent Co., Ltd. Poplar was purchased from a local manufacturer and was then shattered into wood sawdust. Hydrochloric acid (36−38% HCl), ethanol (99.7%), ethyl acetate (99.5% EAC), tetrahydrofuran (99.0% THF), and sodium hydroxide (96.0% NaOH) were purchased from Beijing Chemical Plant. All commercial reagents were of analytical grade and used without further purification. 2.2. Preparation of Poplar Lignin. In this study, the poplar lignin was obtained by ethanol pulping. The poplar sawdust (80 g) and water/ethanol (2:3, v/v, 800 mL) were mixed, and then the mixture was placed in an autoclave. This autoclave was incubated at 200 °C for 120 min.32 The obtained mixture was filtered and washed three times with deionized water. The filtrate was collected and diluted with 2400 mL of water. Then, the mixture was acidified by adding HCl solution (5 mL, 12.27 mol/L, final pH = 1) to precipitate lignin.33 The precipitated lignin was filtered and freeze-dried to obtain poplar ethanol lignin (PEL). 6492

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Figure 1. Surface morphologies of (a) GB, (b) GB-60, (c) GB-60-Ni, and (d) GB-60-Mn.

yield x (wt %) ÅÄÅ areax / ECNx ÅÅ (mass n ‐ dodecane (mg)/50) × area ÅÅÅ n ‐ dodecane / 12 = ÅÅÅ ÅÅ 500 (mg) ÅÅ ÅÅÇ

as the monomeric products of lignin depolymerization. The solid residue (filter cake) was washed with excess THF to retrieve the unconverted lignin. The collected THF solution was concentrated by vacuum distillation at 60 °C to obtain THF-soluble lignin residue (TLR). The remaining residue (insoluble in THF) was considered to be a mixture of coke and repolymerized lignin. To distinguish the yield of repolymerized lignin, the excess NaOH solution (4 wt %) was used to dissolve and retrieve the repolymerized lignin. The resulting mixture was filtered over a 250 mL sand star filter (the filter cake was the coke and catalysts). Then, the solution of HCl (12.27 mol/mL) was added dropwise to the collected filtrate (until pH = 1 was reached, some black solid substances were precipitated). After being aged for 120 min, the solid substances were separated by centrifugation and dried in a vacuum oven at 60 °C for 4 h to obtain the NaOH-soluble lignin residue (NLR). 2.6. Analysis of Lignin Products. The liquid phase products were analyzed by a Bruker SCION TQTM GC-MS system equipped with a DDZ-5 column (60 m × 0.25 mm × 0.25 mm) and a flame ionization detector together with a mass spectrometer detector.36 One milliliter of n-dodecane (as the internal standard) was added to the liquid phase products. Identification of products was achieved based on a search of the MS spectra with the NIST11 and NIST11 s MS libraries. These products were further divided into three groups, namely the p-hydroxyphenyl monomer derivative (HP), guaiacyl monomer-derived phenolic (GP), and phenolic monomer-derived syringyl (SP), according to the type and location of the substituent.37 The influence factors were calculated by the ECN, and their mass yield was determined by dividing its mass (calculated by referencing the internal standard) by 500 mg (the mass of starting organosolv lignin).38 The yields of monomers, lignin residue, condensation-type lignin, and char (in wt %) were calculated using eqs 1−5.39

(

ÑÉ

) × MW ÑÑÑÑÑÑÑÑ x

ÑÑ ÑÑ ÑÑ ÑÖ

ÄÅ ÉÑ ÅÅ w(NaOH solubles + coke) ÑÑÑÑ Å Å lignin conv. (wt%) = ÅÅ1 − ÑÑ × 100 ÅÅÇ ÑÑÖ w(initial lignin) (2) (1)

× 100

THF soluble products(wt %) =

w(THF solubles) × 100 w(initial lignin)

NaOH soluble products (wt %) =

(3)

w(NaOH solubles) × 100 w(initial lignin)

ÄÅ É ÅÅ w(NaOH insolubles) − w(catalyst added) ÑÑÑ Å ÑÑ Å coke(wt %) = ÅÅ ÑÑ ÅÅÇ ÑÑÖ w(initial lignin) × 100

(4)

(5)

Gel permeation chromatography (GPC) analysis was performed using a Shimadzu apparatus equipped with a Shodex GPC KD-804 column and a refractive index detector (RID-10A). The dry acetylated lignin was dissolved in THF for 4 h (the concentration was 10 mg/mL).40 The sample was filtered using a 0.22 μm filter membrane before injection. Analysis was carried out using THF as the eluent. The thermal properties of the lignin were studied by thermogravimetric analysis (TGA, TA instruments Q500 TGA). The sample was heated 6493

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Figure 2. EDS spectra of (a) GB, (b) GB-60, (c) GB-60-Ni, and (d) GB-60-Mn.

Figure 3. Pore size distributions of (a) GB, (b) GB-60, (c) GB-60-Ni, and (d) GB-60-Mn. from 30 to 700 °C at a heating rate of 10 °C/min under a dry nitrogen atmosphere.41 The carbon, hydrogen, and oxygen contents of the lignin were quantitatively determined by the elemental analysis (Elementar Vario MICRO cube Elemental Analyzer, CHNS mode). Two-dimensional HSQC NMR spectra were recorded on a Bruker AVANCE 400 MHz spectrometer, and the data were acquired by 80 mg of the sample dissolved in 0.6 mL of DMSO-d6.42

spherical with a smooth surface (Figure 1a). The surface morphology of the GB-60 was covered with a layer of flaky structure (Figure 1b). The interior of GB still maintained a complete and smooth structure (solid glass sphere) after the subcritical water treatment. The results indicated that the subcritical water could effectively etch the surface of the GB without affecting its internal structure. The flaky structure could effectively increase the loading capacity of metals on GB60. After loading the Ni or Mn metal, the GB-60-Ni and GB60-Mn still maintained their original flaky morphologies, which was of positive significance for increasing their catalytic sites (Figure 1c,d). The EDS was measured to confirm the content of elements on the surface of the GB, GB-60, GB-60-Ni, and GB-60-Mn.

3. RESULTS AND DISCUSSION 3.1. Characteristics of GB Catalysts. The surface morphologies of GB, GB-60, GB-60-Ni, and GB-60-Mn were characterized using high-resolution SEM (Figure 1). The surface morphologies revealed the surface influence mechanism of the subcritical water treatment. The untreated GB was 6494

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Energy & Fuels Table 1. Yields of Monomers, Lignin Residues, and Char catalyst

solvent

monomers (wt %)

TLR (wt %)

NLR (wt %)

char (wt %)

GB-60 GB-60-Ni GB-60-Mn

EtOH−water EtOH−water EtOH−water

11.4 ± 0.6 13.4 ± 0.5 15.5 ± 0.3

62.5 ± 0.5 70.1 ± 0.6 75.3 ± 0.4

16.6 ± 0.8 8.8 ± 0.5 5.1 ± 0.7

7.6 ± 0.5 3.5 ± 0.7 1.7 ± 0.5

As shown in Figure 2 and Table S1, the GB, GB-60, GB-60-Ni, and GB-60-Mn all contained Si, Na, Ca, O, K, Mg, and Al elements. Moreover, the subcritical water treatment increased the contents of Mg and Al elements on the surface of GB-60. After loading the Ni or Mn metal, the contents of Ni and Mn elements on the surface increased from zero to 6.8 and 7.8 wt %, respectively. The inductively coupled plasma (ICP) results (Table S2) showed that the Ni content of GB-60-Ni was 3.6 wt %, and the Mn content of GB-60-Mn was 4.1 wt %. The ICP results were the analysis of the entire catalyst, the EDS results were only analyzed for the surface-active sites of the catalyst. The difference between ICP and EDS results indicated that a large amount of Ni and Mn ions were successfully loaded on the surface of GB-60. At the same time, the X-ray diffraction (XRD) patterns of GB, GB-60, and GB-120 are presented in the Supporting Information (Figure S2). The XRD patterns (Figure S2) showed that GB had significant differences compared to GB-60, GB-60-Ni, and GB-60-Mn catalysts in crystal structures. It was demonstrated that the metal ions of the surface layer undergo a process of dissolution and recrystallization under treatment in a subcritical environment. It also proves that GB-60 can effectively adsorb metal ions in solution and fix it on the surface of scales. The pore size distributions of GB, GB-60, GB-60-Ni, and GB-60-Mn are shown in Figure 3. Their specific surface areas, pore volumes, and mean pore sizes are listed in Table S3. The specific surface areas of GB, GB-60, GB-60-Ni, and GB-60-Mn were 2.1, 194.8, 156.7, and 164.7 m2/g, respectively. The pore volumes of GB-60, GB-60-Ni, and GB-60-Mn were 0.26, 0.23, and 0.24 cm3/g, respectively. The average pore diameters of GB-60, GB-60-Ni, and GB-60-Mn were 5.36, 4.99, and 5.12 nm, respectively. For the GB, there were no pores on the surface. After the subcritical water treatment, a large number of mesopores were formed on the surfaces of GB-60. Moreover, the specific surface area of GB-60 significantly increased (from 2.1 to 194.8 m2/g). The large specific surface effectively increased the contact opportunities of the lignin and catalysts, thereby improving the lignin depolymerization efficiency. After loading the Ni and Mn metals, the GB-60-Ni and GB-60-Mn still maintained their original mesopores, pore volume, and average pore size. Although the specific surface areas of GB-60Ni and GB-60-Mn were reduced, 156.7 and 164.7 m2/g were still larger than that of GB. 3.2. Identification and Quantification of Products. PEL was used to verify the catalytic activities of GB, GB-60, GB-60-Ni, and GB-60-Mn. Lignin conversion was determined by the monomeric product yield and TLR yield. The role of ethanol in the catalytic system was the capping agent. Ethanol prevented the polycondensation of lignin molecules, thereby reducing the formation of the solid residue and char. However, the resistance−polycondensation efficiency of the ethanol/ water system without catalysts was unsatisfactory, there were still a large number of the solid residue and char (Table S4). The introduction of GB-60 and its derivatives could effectively increase the resistance−polycondensation efficiency, thus increasing the yield of monomer products. As shown in

Table 1, the monomeric product yields for the GB-60, GB-60Ni, and GB-60-Mn catalytic systems were 11.4, 13.4, and 15.5 wt %, respectively. In addition, the GB-60, GB-60-Ni, and GB60-Mn prevented the polycondensation of lignin molecules, thereby reducing the formation of the solid residue (16.6, 8.8, and 5.1 wt %, respectively) and char (7.6, 3.5, and 1.7 wt %, respectively). It was noteworthy that the metal-loaded catalyst (GB-60-Ni and GB-60-Mn) system could obtain a high monomeric product yield and less residue, especially the GB60-Mn catalytic system. Furthermore, the species of monomeric products was more than no-catalyst system (Figures 4

Figure 4. Product distribution under different reaction conditions (a) at 300 °C for 6 h in water−ethanol with GB-60, (b) at 300 °C for 6 h in water−ethanol with GB-60-Ni, (c) at 300 °C for 6 h in water− ethanol with GB-60-Mn, (d) depolymerization route of poplar in ethanol-aqueous solution catalyzed by GB-60, GB-60-Ni, and GB-60Mn.

and S3, S4). Among the monomeric products, guaiacol phenol (GP), syringe phenol (SP), and p-coumarol phenol (HP) were predominant, and no deoxygenated aromatics, hydrogenated cyclic, and branched-chain hydrocarbons were formed. This result indicated that these catalysts could effectively depolymerize lignin, because of the fact that the catalysts could not only break the ether bond structure between lignin but could also have strong destructive capacity on the carbon−carbon bond of lignin. As shown in Figure 4, compared with the GB60 catalytic system, GP-type monomeric product yield increased significantly in the GB-60-Ni and GB-60-Mn catalytic systems. The yields of 3,4-dimethoxyphenol, catechol, 1,2-dimethoxybenzene, and 1,2,4-trimethoxybenzene in the monomer product formed were 0.22, 0.20, 0.09, and 0.59 wt %. It was worth noting that 3,4-dimethoxyphenol and catechol were more prone to the lignin repolymerization than 1,2dimethoxybenzene and 1,2,4-trimethoxybenzene (phenolic hydroxyl groups were very easy to form new chemical bonds with other active chemical groups at high temperature and high 6495

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Figure 5. Thermogravimetric curves and derivative thermogravimetry curves of PEL, GB-60-TLR, GB-60-Ni-TLR, GB-60-Mn-TLR, GB-60-NLR, GB-60-Ni-NLR, and GB-60-Mn-NLR.

Figure 6. Two-dimensional HSQC NMR spectra of the PEL, GB-60-TLR, GB-60-Ni-TLR, and GB-60-Mn-TLR.

pressure). Interestingly, in GB-60-Ni and GB-60-Mn catalytic systems, 3,4-dimethoxyphenol and catechol did not appear, and the yields of 1,2-dimethoxybenzene (0.41 and 0.43 wt %) and 1,2,4-trimethoxybenzene (0.83 and 1.16 wt %) were significantly increased. This indicated that the introduction of the Ni and Mn elements could effectively prevent the departure of the methoxy group in 1,2-dimethoxybenzene and 1,2,4-trimethoxybenzene, thus reducing the variety of monomer products, and the yield of solid residue and char. The catalytic mechanism diagram of GB-60 and GB-60

derivatives is shown in Figure 4d. To further prove this depolymerization mechanism, the straw ethanol lignin (SEL) was used to verify the catalytic activities of GB, GB-60, GB-60Ni, and GB-60-Mn. As shown in Figure S4, the yields of resorcinol and 2-methoxy-phenol were 0.1 and 2.34 wt %, respectively, in the GB-60 catalytic system. Interestingly, in GB-60-Ni and GB-60-Mn catalytic systems, resorcinol disappeared in the products, and the yield of 2-methoxyphenol was 2.41 and 2.53 wt %, respectively. This result also 6496

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Figure 7. Surface morphologies of the reused glass bead catalyst (a) GB-60 for the third time, (b) GB-60-Ni for the third time, and (c) GB-60-Mn for the third time.

side chain region (0−90/0−6.0 ppm) provided a signal for the major chemical bonds between the structural units of the lignin, such as the β-O-4 aryl ether bond, methoxy group, methyl group, and methylene group.45,46 For example, the correlation signals of the α and γ positions on β-O-4 in the Atype structural unit were δC/δH 72.3/4.8 and 60.1/3.7 ppm, respectively.47 The correlation signal of the β position on β-O4 in the S-type structure was δC/δH 86.0/4.11 ppm and the correlation signal of CH3/CH2/CH was δC/δH 11.21−38.53/ 0.47−2.88 ppm. The aromatic ring region (90−135/6−8.0 ppm) mainly provided structural features of basic structural units such as syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H). The carbon signal peaks at the 2nd and 6th positions of the S-type structural unit were at δC/δH 104.2/6.72. The carbon signal peaks at the 2nd, 5th, and 6th positions of the Gtype structural unit appeared at δC/δH 111.4/6.96, 114.8/6.71, and 119.8/6.72, respectively. As shown in Figure 6a, the signal of methoxy and α signal of met of β-O-4(A) in the PEL side chain region were at δC/δH 56.3/3.77, 72.3/4.89, and 60.3/ 3.72 ppm, respectively. The signals of the β position of the βO-4 structure formed by the G-type and S-type units were transferred to δC/δH 84.2/4.33 and 86.7/4.15 ppm, respectively. The signal peaks of the β-position (Aβ(G) and Aβ(S)) of the β-O-4 structure formed by the G-type and S-type structural units were transferred to δC/δH 84.3/4.24 and 87.1/ 4.17 ppm, respectively. In addition to the typical aryl ether bond and methoxy signal, the C−C bond structure in PEL is also shown in Figure 6a, such as the β−β linkage (B, resin alcohol) and the β-5 linkage (C, phenylcoumaran). Semiquantitative analysis of the major linkages in PEL by 2DHSQC spectra revealed that the contents of β-O-4, β−β, and β-5 were 33.3, 6.5, and 7.7%, respectively. In the aromatic region of the PEL (Figure 6d), the signal peaks of the basic structural units of G, S, and H were quite obvious. The S2/6 and S′2/6 of S-type condensed structural units were located at δC/δH 105/6.5 and 107/7.25 ppm, respectively. The G2, G5, and G6 of G-type structural units were located at δC/δH 112.5/ 7.0, 115.5/6.75, and 120.5/6.8 ppm, respectively. The PB2,6 type structure was mainly located at δC/δH 131.4/7.72. In addition, the contents of G, S, and PB type structural units and the S/G values were calculated by a semi-quantitative method using 2D-HSQC, which were 59.1, 36.3, 4.6, and 1.6%, respectively. It was proved that the main structural units were G and S in PEL. Compared with PEL, the β-O-4 structure of

conformed to the above catalytic mechanism of GB-60 and GB-60 derivatives. 3.3. Identification and Quantification of Lignin Residue. Previous research studies had correlated the chemical structure with the thermal properties of lignin and its derivatives. The TG and derivative thermogravimetry (DTG) of the lignin residues could reflect the catalytic effects of GB-60, GB-60-Ni, and GB-60-Mn on the chemical structures of lignin in the process of depolymerization, because of the fact that the thermal weightlessness of lignin was closely related to the type of functional groups and the connection mode of structural units. As shown in Figure 5, weightlessness below 100 °C was considered the volatilization of water, and the main thermal weightlessness of lignin and its derivatives occurred at 100−600 °C. The thermal weight loss at 100−350 °C was caused by the fracture of the ether bond (β-O-4 ether bond) in the lignin chemical structure.43 The thermal weightlessness temperatures of the lignin side chains and the aromatic rings (C−C bond cleavage) were 350−400 and 400− 600 °C, respectively.44 The DTG curve of TLR had a multimodal distribution (the main weight loss and weight loss rate was about 100−200 °C), which proved that TLR was mainly composed of ether-bonded structural units (Figure 5a). Compared with other lignin residues, the GB-60-Mn-TLR had the largest weight loss (40 wt %) at 100−200 °C. This indicated that the GB-60-Mn effectively interrupted the C−C bonds in the original lignin, and the remaining structure (ether-bonded structural units) was easier to be depolymerized into the monomeric products. Moreover, GB-60-TLR, GB-60Ni-TLR, and GB-60-Mn-TLR had less weight loss in the temperature range of 200−400 °C, and there was no obvious weight loss phenomenon after 400 °C. As shown in Figure 5b, the DTG curve of NLR was similar to that of PEL. The Thermal weightlessness peaks of GB-60-NLR, GB-60-Ni-NLR, and GB-60-Ni-NLR appeared at 340, 280, and 280 °C, respectively. The low weightlessness temperature implied the low degree of lignin polycondensation. Moreover, the weight loss of NLR obtained by GB-60 catalysis was the largest (78 wt %) compared with GB-60-Ni and GB-60-Mn catalytic systems, indicating the highest degree of polycondensation for GB-60NLR. This result suggested that GB-60-Ni and GB-60-Mn effectively inhibited the polycondensation reaction. The 2D-HSQC spectrum of lignin was usually divided into two parts: a side chain region and an aromatic ring region. The 6497

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Energy & Fuels Table 2. Monomer Product Yield Catalyzed by GB-60, GB-60-Ni, and GB-60-Mn after the Third Cycle catalyst catalyst cycle times monomers (wt %)

GB-60 1 11.2 ± 0.2

2 11.4 ± 0.1

3 11.3 ± 0.1

GB-60-Ni 1 12.9 ± 0.1

2 12.8 ± 0.2

3 13.2 ± 0.2

GB-60-Mn 1 14.3 ± 0.2

2 13.4 ± 0.4

3 14.1 ± 0.2

Figure 8. Product distribution following the reaction of the reused glass bead catalyst. (a) GB-60 for the third time, (b) GB-60-Ni for the third time, and (c) GB-60-Mn for the third time.

GB-60-Mn showed that the Ni and Mn contents on the surface were 6.78 and 7.76 wt %, respectively. Based on the results of monomeric product yield, solid residue and char, it was found that the surface morphologies and active metals on the GB surface could effectively prevent the departure of the methoxy group from the aromatic hydrocarbon structure, thus reducing lignin polycondensation (Ni: 8.8 wt % solid residue and 3.5 wt % char, Mn: 5.1 wt % solid residue and 1.7 wt % char) and increasing the yield of monomer products (Ni: 13.4 wt %, Mn: 15.5 wt %). Furthermore, GB derivatives could be recycled to catalyze depolymerization of lignin, and the monomeric product yield remained above 12 wt %. This is a novel strategy to prepare low-cost and high-quality nonprecious metal catalysts, which have great potential applications in lignin depolymerization.

TLR significantly reduced, while the signal peak of CH3/CH2/ CH obviously increased. The Aα, Aβ, Cα, and Cβ signal peaks of TLR disappeared, but the signals of the Aγ structure aryl ether bond still existed. In addition, there were still strong methoxy (OMe) signals in the side chain regions of TLR. As shown in Figure 6e−h, the G-type and S-type units remained in the TLR. It was worth noting that the G-type lignin structure signal peak of the aromatic region of GB-60-Mn-TLR was significantly lower than that of other TLR, especially the G2 type condensed lignin structure disappeared. It was proved that GB-60-Mn inhibited the lignin polycondensation. Mw distribution of PEL and TLR is shown in Figure S7. Elemental compositions of PEL, TLR, and NLR are listed in Table S7. The 2D-HSQC spectrum of NLR is shown in Figure S8. 3.4. Catalyst Recovery. After depolymerization, GB-60, GB-60-Ni, and GB-60-Mn were collected and soaked in a 4 wt % NaOH solution for 24 h. The surface morphologies of GB60, GB-60-Ni, and GB-60-Mn after the third catalytic cycle are shown in Figure 7. The GB-60, GB-60-Ni, and GB-60-Mn still maintained a complete spherical structure with a flaky structure on the surface. The surface element contents of GB-60, GB-60Ni, and GB-60-Mn after the third catalytic cycle are listed in Table S8. The GB-60, GB-60-Ni, and GB-60-Mn still maintained a high content of active Mg, Ni, and Mn elements on the surface (Figure S9). As shown in Table 2, with GB-60, GB-60-Ni, and GB-60-Mn as catalysts for the third cycle, the yields of the monomer products were 11, 12, and 14 wt %, respectively (significance analysis: there was no significant difference between the results). In addition, the distributions of monomer products were also consistent with that of the first catalysis (Figure 8). It proved that the GB-60 and its derivatives had excellent stability and could be repeatedly used to catalyze the depolymerization of lignin.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.9b01218. Surface element composition of GB, GB-60, GB-60−Ni, and GB-60-Mn; ICP and Brunauer−Emmett−Teller results of GB, GB-60, GB-60-Ni, and GB-60-Mn; product distribution of lignin after depolymerization in a catalyst-free system and a GB catalyst system; monomer yield of SEL depolymerized in different catalyst systems; SEM images of the internal morphologies of GB-60; XRD spectra of GB, GB-60, GB-60-Ni, and GB-60-Mn; product distribution under different reaction conditions at 300 °C for 6 h in water−ethanol without a catalyst, at 300 °C for 6 h in water−ethanol with the GB, at 300 °C for 6 h in water−ethanol with GB-60-Mg, and at 300 °C for 6 h in water−ethanol with GB-60-Al; Mw distribution of SEL; 2D-HSQC spectrum of SEL; Mw distribution of PEL, TLR-60-TLR, GB-60Ni-TLR, and GB-60-Mn-TLR; 2D-HSQC spectrum of NLR (PDF)

4. CONCLUSIONS In this work, we discussed the feasibility of using a siliconbased multi-metal oxide sphere (GB) and its derivatives as novel catalysts to depolymerize lignin. The subcritical water treatment effectively changed the surface morphology and specific surface area of the GB. The treated GB was used to load nonprecious metals (Ni and Mn) to obtain GB derivatives. The EDS characterization of the GB-60-Ni and 6498

DOI: 10.1021/acs.energyfuels.9b01218 Energy Fuels 2019, 33, 6491−6500

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.L.). *E-mail: [email protected] (J.Z.). ORCID

Xing Wang: 0000-0002-1539-5741 Yao Li: 0000-0003-4784-2785 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (Nos. 31170554, 31770635, and 31800498).

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