Highly selective cleavage C–O ether bond of lignin model compounds over Ni/CaO–H-ZSM-5 in ethanol
Herein, 2-(2-methoxyphenoxy)-1-phenylethanol (β-O-4), 2-methoxyphenyl anisole (α-O-4) and 4-phenoxyphenol (4-O-5) were selected as typical lignin model compounds. Given the effectiveness of traditional acid–base catalysts for lignin depolymerisation, a novel Ni/CaO–H-ZSM-5(60) catalyst was prepared to investigate the difficulty level of C–O bond of three model compounds cleavage in ethanol. It was observed that Ni/CaO–H-ZSM-5(60) had prominent performance on the C–O bond cleavage at very mild conditions (140 °C, 1 MPa H2). Among them, the C–O bond of α-O-4 and β-O-4 could be completely cleaved within 60 min. Although the C–O bond of 4-O-5 had high bond energy, 41.2% of conversion was occurred in 60 min. The introduction of CaO could regulate the acidity of H-ZSM-5 to enhance the ability to break C–O bonds. Moreover, the possible pathways of C–O ether bonds in three lignin model compounds cleavage were proposed in order to selectively obtain target products from the raw lignin degradation.Electronic supplementary materialThe online version of this article (10.1186/s13065-019-0557-z) contains supplementary material, which is available to authorized users.
Introduction<!><!>Introduction<!>Materials<!>Preparation of lignin model compounds<!>Catalyst synthesis<!>Characterization of catalyst<!>Catalytic tests and product analysis<!><!>Characterization of the catalysts<!>Catalyst screening<!><!>Catalyst screening<!><!>Hydrogenolysis of β-O-4 model compound<!><!>Hydrogenolysis of α-O-4 model compound<!><!>Conclusion<!>
<p>As one of the most abundant renewable energy sources today, lignocellulosic biomass, which can generate zero-carbon fuels and green fine chemical, has the advantages of widely distributed, renewable and clean [1]. Hemicellulose (20–35%), cellulose (35–50%), and lignin (10–25%) are mainly three components in lignocellulosic biomass. Among them, extensive efforts have been devoted to convert hemicellulose and cellulose into fine chemicals and fuels for a long time [2–4].</p><!><p>Representative structure of native lignin and three lignin model compounds for the β-O-4, α-O-4, and 4-O-5 linkages</p><!><p>In this study, 2-(2-methoxyphenoxy)-1-phenylethanol, 2-methoxyphenyl anisole, 4-phenoxyphenol were used as lignin model compounds for the β-O-4, α-O-4 and 4-O-5 bonds, respectively. We selected H-ZSM-5 zeolite with Si/Al2 ratio of 60, and modified it by incorporating the metal Ni (Ni loading = 45 wt%) and CaO with the deposition–precipitation (DP) method to prepare Ni/CaO–H-ZSM-5(60) catalyst. The efficiency of the cleavage of C–O bond in three typical lignin model compounds was investigated. Moreover, according to the results, the possible pathways of C–O bonds cleavage of lignin model compounds were proposed, respectively.</p><!><p>Chemicals and reagents were received from commercial suppliers: Ni (NO3)2·6H2O (Tianjin Kai-Chemical Reagent Co. Ltd), Ca (NO3)2·4H2O (Aladdin), NaOH (Sinopharm Chemical Reagent Co. Ltd), Na2CO3 (Sinopharm Chemical Reagent Co. Ltd), 4-O-5 lignin model compound (Aladdin), β-O-4 lignin model compound and α-O-4 lignin model compound were prepared in our laboratory, as seen below.</p><p>H-ZSM-5 zeolite with Si/Al2 ratio of 60 from Nan Jin Huang Ma was used for the preparation of the catalyst involved in the experiment. In addition, H-ZSM-5(60) zeolite was used after calcination at 550 °C for 6 h and all chemicals were of analytical grade and used without any purification.</p><!><p>2-(2-Methoxyphenoxy)-1-phenylethanol (β-O-4) was synthesized in a two-step process [32]. 2-Bromoacetophenone (50 mmol, 9.9 g), guaiacol (50 mmol, 6.2 g) and K2CO3 (137 mmol, 19 g) were added to a 100 mL flask. Stirred it for 5 min after adding 50 mL of acetonitrile, then, added KI (1.2 mmol, 0.2 g) and stirred it continued for 10 min. Next, the reaction mixture was stirred at reflux temperature for 24 h before recrystallized from ethanol. Then, the obtained compound was dissolved in the mixture of THF:H2O (5:1) (60 mL), and sodium borohydride (37 mmol, 1.4 g) was added batch-wise, next, the mixture was stirred at room temperature overnight. After the reaction, the mixture was diluted with dilute hydrochloric acid until no bubbles were formed. Later, 80 mL of ethyl acetate was added thereto, and the organic phase was washed three times with 30 mL of saturated brine. β-O-4 model compound obtained after solvent evaporation (shown in Additional file 1: Figure S1).</p><p>The preparation processes of 2-methoxyphenyl anisole (α-O-4) and 1-methoxy-2-phenylethoxybenzene were the same as that of β-O-4, and only the reactant materials were different. 2-methoxyphenyl anisole (α-O-4) was prepared from 2-bromoethylbenzene (50 mmol, 9.2 g) and guaiacol (50 mmol, 6.2 g). Preparation of 1-methoxy-2-phenylethoxybenzene from benzyl bromide (50 mmol, 8.6 g) and guaiacol (50 mmol, 6.2 g). In addition, the structure and purity of the model compounds prepared were confirmed by the H1NMR spectrum (shown in Additional file 1: Figure S1).</p><!><p>The nickel and calcium oxide nanoparticles were introduced into the zeolites using deposition–precipitation (DP) method, a typical synthetic method, and the specific procedure was carried out as follows [33].</p><p>0.30 g of Ca (NO3)2·4H2O (Ca loading = 5 wt%) and 2.23 g of Ni (NO3)2·6H2O (Ni loading = 45 wt%) were dissolved in deionized water, denoted as solution A. The mixture of NaOH and Na2CO3 was prepared at a concentration of 0.25 mol/L and 0.8 mol/L, respectively, called solution B. In another 300 mL beaker, 0.48 g of H-ZSM-5(60) and 50 mL of deionized water were introduced under fast stirring at 60 °C (solution C). In the next step, solution B was added dropwise to the beaker C, until the pH of the solution in the beaker C was maintained between 10 and 11. Then, solution A was added slowly to the beaker C under vigorous stirring and light green precipitation was observed. Throughout the precipitation process, the pH of the solution in the beaker C was kept it around 10 in order to precipitation of all metal cations. After that, the substance was stirred at 60 °C for 16 h before dried overnight at 80 °C and calcined in a flow of air at 400 °C for 5 h (heat rate 1 °C/min). Next, the freshly synthesized material was reduced in a flow of H2 at 570 °C for 2.5 h (flow rate: 40–45 mL/min) and passivated in a flowing 5% O2/N2 (flow rate: 7–8 mL/min) for 12 h at room temperature. The catalysts were prepared by a specific procedure shown in Additional file 2: Figure S2. In order to explore the physicochemical properties of Ni/CaO–H-ZSM-5(60), we prepared CaO–H-ZSM-5(60) and Ni/H-ZSM-5(60) catalyst at the same way.</p><!><p>Prior to the reactions, detailed characterizations of the fresh or used catalyst were carried out. X-Ray diffraction (XRD). The powder XRD patterns were recorded on a Dmax2200PC (Rigaku) diffractometer with a Cu Kα1 radiation source (λ = 0.1540 nm), operating at 40 kV and 40 mA. Transmission electron microscopy (TEM). The TEM images were taken with a JEOL model JEM 2010 EX microscope instrument, and the accelerating voltage was 200 kV. Brunauer–Emmett–Teller (BET). The N2 adsorption–desorption experiments were performed at 77 K using Micromeritics ASAP2010 surface area Analyzer. The specific surface area, pore volume and pore size distribution were obtained using BET, N2 adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) methods. Fourier transform infrared spectroscopy (FT-IR). The acid sites in supports were determined from FT-IR on a Thermo Nicolet 380. The sample was pretreated in a vacuum at 500 °C for 1 h before adsorption of pyridine for 30 min at room temperature, and then desorbed at 150 °C, 250 °C and 350 °C for 30 min. Determination of leaching content of Ca and Ni by inductively coupled plasma atomic emission spectrometry (ICP-AES).</p><!><p>In a typical reaction, 0.61 g β-O-4 lignin model compound (2.5 mmol) and 0.102 g active catalyst were added to 30 mL ethanol, and loaded into a 50 mL stainless steel batch reactor. Then, the autoclave was flushed with H2 three times before filled 1 MPa H2 and the reaction were conducted at 100–250 °C with the stirring speed of 700 rpm (detailed steps as shown in Additional file 3: Figure S3). After the reaction, the reactor was placed in ice water and cooled to room temperature, then, the liquid products in the reactor were collected for further analysis.</p><p>The reaction products were qualitatively analysis by GC–MS (Agilent 19091S-433, HP-5 ms, 30 m × 250 μm × 0.25 μm), and the temperature setting program was as follows: the temperature of the injector and the detector were 260 °C and 270 °C, respectively. The initial temperature of the GC column oven was 50 °C kept for 5 min. Then the temperature was increased to 100 °C at the rate of 10 °C/min and retained for 5 min, followed by an increase to 300 °C (30 °C/min) kept for 4 min. Quantitatively analysis using GC-FID (GC-2014C, Wondacap-5, 30 m × 250 μm × 0.25 μm) without further dilution. The temperature setting program was as follows: the initial temperature of the GC column oven was 60 °C kept for 1 min, then the temperature was increased to 100 °C at the rate of 2 °C/min and retained for 1 min, followed by an increase to 260 °C (10 °C/min) kept for 2 min (Additional file 3: Figure S3). In addition, calibration the concentration of the products using the external standard, and the products of 2-methoxyphenyl anisole and 4-phenoxyphenol conversion were conducted as the same way. The conversion of model compound, as well as the yield and the selectivity of the products were calculated based on the following equation, respectively:1\documentclass[12pt]{minimal}
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				egin{document}$$Conversion\left( \% 
ight) \, = {	ext{ n}}_{	ext{a}} /{	ext{n}}_{	ext{b}} * 100\%$$\end{document}Conversion%=na/nb∗100%
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				egin{document}$$Molar \, yield \, of \, product \, i\left( \% 
ight) \, = {	ext{ n}}_{i} /{	ext{n}}_{	ext{b}} * 100\%$$\end{document}Molaryieldofproducti%=ni/nb∗100%
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				egin{document}$$Selectivity \, of \, product\;i\left( \% 
ight) \, = {	ext{ n}}_{i} /{	ext{n}}_{	ext{c}} * 100\%$$\end{document}Selectivityofproducti%=ni/nc∗100%where na and nb were the moles of the model compound consumed and the model compound initially added, respectively. ni and nc were deemed as the moles of product i and the total product, respectively. In the above calculation, 1 mol of reactant could produce 2 mol of monomer products.</p><!><p>a N2 adsorption–desorption isotherms and b size distributions of original and modified H-ZSM-5(60) catalysts</p><p>The physicochemical properties of H-ZSM-5 and Ni catalysts</p><p>XRD pattern of different catalysts. (a) H-ZSM-5(60), (b) CaO–H-ZSM-5(60), (c) Ni/CaO–H-ZSM-5(60) and (d) Ni/H-ZSM-5(60)</p><p>TEM images and EDX mapping of different catalysts. TEM images of a H-ZSM-5(60), b CaO–H-ZSM-5(60), c Ni–H-ZSM-5(60) and d Ni/CaO–H-ZSM-5(60). e EDX mapping of Ni/CaO–H-ZSM-5(60)</p><p>TEM images (a, b) and EDX mapping (c) of the used Ni/CaO–H-ZSM-5(60) catalyst</p><p>TEM images and metal particle size distributions of Ni/CaO–H-ZSM-5(60). a Before reaction, b after one cycle and c after three cycles</p><!><p>The stability of the catalyst was determined by measuring the concentration of the metal in the liquid phase product using ICP-AES. In the first cycle, the concentrations of Ca and Ni in the liquid phase were 0.13 and 0.08 mg L−1, respectively. After the second cycle, the concentrations of Ca and Ni were 0.12 and 0.04 mg L−1, respectively. It experienced the last cycle, the concentrations of Ca and Ni were 0.18 and 0.12 mg L−1, respectively. The leaching concentrations of Ca and Ni were extremely low in three cycles, it explained why the Ni/CaO–H-ZSM-5(60) catalyst was still active for three cycles.</p><!><p>2-(2-Methoxyphenoxy)-1-phenylethanol containing alkyl-aryl-ether linkages, was used as the reactant for testing the activity of catalyst, because it was a kind of representative β-O-4 lignin model compound and was most abundant in native lignin. Ethanol was selected as reaction solvent for the lignin model compounds conversion. Because the alcohol molecule could be used as a nucleophilic reagent in the C–O cracking process [38, 39], and the products dissolved in ethanol could stably maintained without separation and condensation even at a harsh condition. However, subsequent experiments had found that H2 provided hydrogen source during the reaction, which was essential for the cleavage of C–O ether bond.</p><!><p>The main product distributions of the conversion of β-O-4, 4-O-5, α-O-4 and 1-phenyl ethanol over different catalyst</p><p>Reaction conditions: the amount of the reactant (β-O-4, 1-phenyl ethanol, 4-O-5 and α-O-4) was 2.5 mmol, mreactant:mcatalyst = 6:1, ethanol (30 mL), 140 °C, 1 MPa H2, 90 min, stirring at 700 rpm</p><!><p>Similarly, the conversion and product distributions of 4-O-5 and α-O-4 model compounds over different catalysts was also investigated (Table 2). It was found that the conversions of 4-O-5 and α-O-4 model compounds over H-ZSM-5(60) and CaO–H-ZSM-5(60) were extremely low. Other than this, the product distributions of 4-O-5 model compounds over Ni/CaO–H-ZSM-5(60) were also different from that over Ni/H-ZSM-5(60). There was no doubt that over Ni/CaO–H-ZSM-5(60), about 47% of the 4-O-5 model compounds had been converted mainly producing phenol and cyclohexanol with the selectivity of 66.1% and 20.6%, respectively. However, over Ni/H-ZSM-5(60), about 26% of 4-O-5 model compounds was converted, as well as benzene and phenol were deemed as the main products with the selectivity of 65.5% and 22.9%, respectively (Table 2). According to the results, the addition of CaO was beneficial to the conversion of 4-O-5 model compound. The C–O ether bond of 4-O-5 model compounds had different break sites over Ni/CaO–H-ZSM-5(60) and Ni/H-ZSM-5(60), resulting in a change in product distributions. The product distributions of α-O-4 conversion over these two catalysts [Ni/H-ZSM-5(60), Ni/CaO–H-ZSM-5(60)] was consistent (Table 2), the reason may be that the bond energy of the C–O ether bond in α-O-4 model compounds was lower, and it was easily cleaved during the hydrogenation of the metal Ni. It was well known that the acidity of H-ZSM-5 was beneficial to the hydrodeoxygenation reaction of biomass [40, 41]. However, it could be seen that Ni/H-ZSM-5 had quite low ability on breaking ether bond. The introduction of CaO significantly enhanced the cleavage of ether bonds, which would more favorable for lignin degradation to small molecular weight compounds. Therefore, we selected Ni/CaO–H-ZSM-5(60) and focused on the catalytic conversion of three typical lignin model compounds over it.</p><!><p>The main product distributions of β-O-4 conversion at different temperature over Ni/CaO–H-ZSM-5(60)</p><p>Reaction conditions: β-O-4 (0.61 g, 2.5 mmol), Ni/CaO–H-ZSM-5(60) (0.102 g), ethanol (30 mL), 140 °C, 1 MPa H2, 60 min, stirring at 700 rpm</p><p>a The product distributions for the conversion of 2-(2-methoxyphenoxy)-1-phenylethanol (β-O-4) over Ni/CaO–H-ZSM-5(60) as a function of time. Reaction conditions: 2-(2-methoxyphenoxy)-1-phenylethanol (β-O-4) (0.61 g, 2.5 mmol), Ni/CaO–H-ZSM-5(60) (0.102 g), ethanol (30 mL), 140 °C, 1 MPa H2. b Two different reaction pathways of 2-(2-methoxyphenoxy)-1-phenylethanol (β-O-4) conversion over Ni/CaO-H-ZSM-5(60)</p><p>The product distributions for the conversions of a 1-phenylethanol, b guaiacol, and c 1-methoxy-2-phenylethoxybenzene over Ni/CaO-H-ZSM-5(60) as a function of time. Reaction conditions: reactant (2.5 mmol), Ni/CaO-H-ZSM-5(60) (0.102 g), ethanol (30 mL), 140 °C, 1 MPa H2, stirring at 700 rpm</p><!><p>Combining with the results of Figs. 7a and 8, we deduced the corresponding reaction pathways about the conversion of 2-(2-methoxyphenoxy)-1-phenylethanol (β-O-4) over Ni/CaO–H-ZSM-5(60) which were presented in Fig. 7b. The conversion of β-O-4 mainly followed two parallel competitive pathways over Ni/CaO–H-ZSM-5(60) catalyst. The first one (major) was that the C–O bond of β-O-4 was selectively cleaved by initial hydrogenolysis to produce 1-phenylethanol and guaiacol. Since 1-phenylethanol over Ni/CaO–H-ZSM-5(60) catalyst was more easily converted than guaiacol (Fig. 8), the catalyst preferentially converts 1-phenylethanol to ethylbenzene (major) and acetylcyclohexane (minor) through two parallel reactions (hydrogenolysis and hydrogenation), whereas the selectivity of the products of guaiacol was less than 4%. Another pathway was that the formation of 1-methoxy-2-phenylethoxybenzene by dehydration of β-O-4, then, followed by hydrogenolysis to form ethylbenzene and guaiacol.</p><!><p>The product distributions for the conversion of 2-(2-methoxyphenoxy)-1-phenylethanol (β-O-4) over Ni/CaO–H-ZSM-5(60) under different H2 pressures. Reaction conditions: 2-(2-methoxyphenoxy)-1-phenylethanol (β-O-4) (0.61 g, 2.5 mmol), Ni/CaO–H-ZSM-5(60) (0.102 g), ethanol (30 mL), 140 °C, stirring at 700 rpm</p><p>a The product distributions for the conversion of 2-methoxyphenyl anisole (α-O-4) over Ni/CaO–H-ZSM-5(60) as a function of time. Reaction conditions: 4-phenoxyphenol (2.5 mmol, 0.535 g), Ni/CaO–H-ZSM-5(60) (0.036 g), ethanol (30 mL), 140 °C, 1 MPa H2, stirring at 700 rpm. b Reaction pathway of 2-methoxyphenyl anisole (α-O-4) conversion over Ni/CaO–H-ZSM-5(60)</p><!><p>During the reaction, toluene and guaiacol were resulted from hydrogenolysis of the C–O bond of 2-methoxyphenyl anisole (α-O-4). It indicated that a very low catalyst concentration could effectively break the C–O bond of 2-methoxyphenyl anisole (α-O-4) by hydrogenolysis, but no further conversion products of guaiacol were found. There were two reasons to explain this phenomenon. On the one hand, guaiacol was quietly stable and difficult to hydrogenate or hydrogenolysis, on the other hand, the catalyst with low concentration so that not sufficient for guaiacol fully converted. As above, the reaction pathway of the conversion of 2-methoxyphenyl anisole (α-O-4) was dominated through hydrogenolysis to generate toluene and guaiacol (Fig. 10b). After that, if the catalyst was sufficient, there would be a certain degree of hydrogenolysis of guaiacol to produce phenol (Fig. 8b).</p><!><p>The product distributions for the conversion of 2-methoxyphenyl anisole (α-O-4) over Ni/CaO–H-ZSM-5(60) under different H2 pressures. Reaction conditions: 2-methoxyphenyl anisole (α-O-4) (0.535 g, 2.5 mmol), Ni/CaO–H-ZSM-5(60) (0.036 g), ethanol (30 mL), 140 °C, stirring at 700 rpm</p><p>a The product distributions for the conversions of 4-phenoxyphenol (4-O-5) over Ni/CaO–H-ZSM-5(60) as a function of time. Reaction conditions: 4-phenoxyphenol (2.5 mmol, 0.465 g), Ni/CaO–H-ZSM-5(60) (0.0775 g), ethanol (30 mL), 140 °C, 1 MPa H2, stirring at 700 rpm. b Two different reaction pathways of 4-phenoxyphenol (4-O-5) conversion over Ni/CaO–H-ZSM-5(60)</p><p>The product distributions for the conversion of a phenol and b hydroquinone over Ni/CaO–H-ZSM-5(60) as a function of time. Reaction conditions: phenol (2.5 mmol) or hydroquinone (2.5 mmol), Ni/CaO-H-ZSM-5(60) (0.0775 g), ethanol (30 mL), 140 °C, 1 MPa H2, stirring at 700 rpm</p><p>Recycling tests of Ni/CaO–H-ZSM-5(60) catalyst. Reaction conditions: β-O-4 (0.61 g, 2.5 mmol), Ni/CaO–H-ZSM-5(60) (0.102 g), ethanol (30 mL), 140 °C, 1 MPa H2, stirring at 700 rpm</p><!><p>The C–O ether bonds in the β-O-4, α-O-4 and 4-O-5 model compounds are typical and abundant in native lignin. In this study, these ether bonds were selectivity cleaved by a novel Ni/CaO–H-ZSM-5(60) catalyst to produce small molecular compounds in ethanol under very mild conditions (140 °C, 1 MPa H2). The C–O bond of α-O-4 was cleaved by direct hydrogenolysis. However, the C–O bonds of β-O-4 and 4-O-5 were cleaved via two parallel reactions, but the ways of breaking C–O bonds were different. Moreover, H2 was indispensable for the hydrogenolysis and hydrogenation, and low H2 pressure favored hydrogenolysis, while high H2 pressure favored hydrogenation.</p><!><p>Additional file 1: Figure S1. The H1NMR spectrums of three lignin model compound. (a) 2-(2-Methoxyphenoxy)-1-phenylethanol (β-O-4), (b) 2-methoxyphenyl anisole (α-O-4), and (c) 2-methoxy-1-phenylethoxybenzene.</p><p>Additional file 2: Figure S2. Specific preparation process of Ni/CaO-H-ZSM-5(60) catalyst.</p><p>Additional file 3: Figure S3. Specific reaction process of β-O-4 conversion over Ni/CaO-H-ZSM-5(60) catalyst and the main analytical approaches.</p><p>2-(2-methoxyphenoxy)-1-phenylethanol</p><p>2-methoxyphenylanisole</p><p>4-phenoxyphenol</p><p>sinapyl alcohol</p><p>coniferyl alcohol</p><p>p-coumaryl alcohol</p><p>hydrotalcite</p><p>methanol to aromatic hydrocarbons</p><p>deposition–precipitation</p><p>X-ray diffraction</p><p>transmission electron microscopy</p><p>Brunauer–Emmett–Teller</p><p>Barrett–Joyner–Halenda</p><p>Fourier transform infrared spectroscopy</p><p>inductively coupled plasma atomic emission spectrometry</p>