Supported β-Mo2C on Carbon Materials for Kraft Lignin

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Cite This: Ind. Eng. Chem. Res. 2019, 58, 12602−12610

Supported β‑Mo2C on Carbon Materials for Kraft Lignin Decomposition into Aromatic Monomers in Ethanol Kejing Wu,† Junbo Wang,‡ Yingming Zhu,† Xueting Wang,‡ Chunyan Yang,‡ Yingying Liu,† Changjun Liu,‡ Houfang Lu,*,†,‡ Bin Liang,†,‡ and Yongdan Li§,∥ †

Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu 610207, China School of Chemical Engineering, Sichuan University, Chengdu 610065, China § Department of Chemical and Metallurgical Engineering, School of Chemical Engineering, Aalto University, Kemistintie 1, Espoo, P.O. Box 16100, FI-00076 Aalto, Finland ∥ Department of Catalysis Science and Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, China

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‡

S Supporting Information *

ABSTRACT: The β-Mo2C on different carbon supports was prepared conveniently via direct H2 reduction or self-reduction for catalytic conversion of Kraft lignin into aromatic hydrocarbons, aromatic alcohols, and aromatic aldehydes. The β-Mo2C on macroporous carbon (MC) with large pore size exhibits the highest activity with aromatic yield of 0.543 g/g Kraft lignin at 280 °C for 3 h, resulting from the accessibility of lignin fragments to β-Mo2C. Mo, MoO2, and MoO3 possess relatively low activity. Higher calcination temperature is required to obtain β-Mo2C over graphite (Gr) support than MC and activated carbon (AC), and small grain size of Gr-supported catalysts is beneficial for high activity. The β-Mo2C on AC exhibits the highest selectivity to aromatic hydrocarbons, which are attributed to highly dispersed βMo2C in micropores. over nano-Ni catalysts.10 This oxidation process is difficult to be controlled due to the intense H2O2 oxidation. Different with the complex conversion strategies, a one-step conversion using α-MoC1−x catalysts with simple and hydrogen-free operations was introduced by Li et al.2 Kraft lignin was almost completely decomposed into benzene derivatives, esters, and other organics in ethanol at 280 °C. Other Mo-based catalysts, such as elementary molybdenum,11 molybdenum oxides,12 and molybdenum nitrides,13 also possess high catalytic activity, but the α-MoC1−x catalysts exhibit the highest activity among the various molybdenum species. The selected MgAlOx support and Cu promoter are beneficial for the activity and stability of α-MoC1−x catalysts.14 The molybdenum(V) ethoxide and radicals in supercritical ethanol contribute to the decomposition of Kraft lignin and formation of esters.15 Different species of molybdenum carbides can be prepared using controlled methods, among which both α-MoC1−x and β-Mo2C exhibit high catalytic activity on C−C and C−O scission and preferred electrochemical properties.16−18 Also βMo2C can be easily prepared via one-step carburization with carbon sources.16 Considering the complex preparation of α-

1. INTRODUCTION Lignin is one of the most abundant components in lignocellulose and regarded as important renewable benzene resource. The paper and pulp industry releases more than 130 million tons of lignin annually,1 and typical Kraft lignin from alkaline pulp technology contains high amounts of lignin, polysaccharides, organic acids, and inorganics.2 However, the application of Kraft lignin is limited. Most of the lignin is used as low-grade fuel, and less than 5% of the lignin as additives or other chemicals.3,4 Because of the high content of lignin, efficient conversion of Kraft lignin to benzene derivatives and other organics shows dramatic potential for the utilization of this renewable lignin resources.5 The linkages in native lignin consist β-O-4 with occurrence varying between 45% and 84% for different wood biomass.1 Substantial investigations were carried out for efficiently decomposition of β-O-4, such as formaldehyde pretreatment,6 preoxidation,7,8 and methylation.9 However, the linkages in Kraft lignin differs from the native lignin due to the pulp process. C−C linkages between monomers and/or carbohydrates, carboxylic acid groups, and thiol groups mainly exist in Kraft lignin,5 and the occurrence of β-O-4 is significantly smaller.1 Thus, the conversion strategies and catalysts for Kraft lignin are distinguished from native lignin. A harsh preoxidation process with H2O2 was introduced for Kraft lignin decomposition to produce aromatic monomers © 2019 American Chemical Society

Received: Revised: Accepted: Published: 12602

April 2, 2019 June 20, 2019 June 24, 2019 June 24, 2019 DOI: 10.1021/acs.iecr.9b01807 Ind. Eng. Chem. Res. 2019, 58, 12602−12610

Article

Industrial & Engineering Chemistry Research MoC1−x with H2 prereduction and CH4/H2 carburization,2 there is room for improvement on catalysts for efficient conversion of Kraft lignin. Thus, β-Mo2C with simple and lowcost preparation process exhibits more potential for industry application, which is seldom reported for Kraft lignin decomposition. Supported molybdenum carbide on carbon materials exhibits enhanced performance on catalytic activity and electrochemical properties. Surface areas, porous structure, and surface functional group of the carbon supports show significant influence on the performance.19 In addition, the location of active species on carbon supports also has effect on catalytic activity.20 Therefore, the activity of β-Mo2C supported on different carbon supports should be further discussed for better understanding the decomposition of Kraft lignin. In this work, one-step carburization of (NH4)6Mo7O24 in H2 or N2 atmosphere was applied to prepare β-Mo2C with high activity for the conversion of Kraft lignin to aromatic monomers. Carbon materials with different chemical and porous properties, including activated carbon (AC), macroporous carbon (MC), and graphite (Gr), were used to study the effect of supports on the molybdenum species and their activities. The decomposition products of Kraft lignin were analyzed to understand the relationship between the activity and β-Mo2C on various supports.

Cu Kα radiation (40 kV, 30 mA) with a scanning step of 1.8°/ min from 5 to 90°. The peak of β-Mo2C (101) was fitted, and crystallite sizes were calculated through Debye−Scherrer equations.22 X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Fisher Escalab 250Xi spectrometer with Mg Kα radiation. All binding energies were corrected using C 1s line of graphite at 284.4 eV. Raman shifts were obtained over Thermal Fisher DXR with 455 nm laser to study the structure of carbon support. The spectra was collected from 200 to 2600 cm−1 with a resolution of 0.964 cm−1. Fourier-transform infrared spectroscopy (FT-IR) was carried out at Thermal Fisher Is50 from 600 to 1800 cm−1 with a resolution of 0.2 cm−1. Morphological structures were identified using scanning electronic microscopy (SEM) of Hitachi Regulus 8230 operated at 5 kV. The elemental contents were analyzed through the equipped energy dispersive spectrometer (EDS) operated at 15 kV with a scale of 100 μm. The Mo contents over AC and MC supports were identified based on thermogravimetric (TG) analysis over Netzsch STA 449 F3 Jupiter. About 10 mg of sample was placed in the furnace covered by a metal cover with a small hole. The temperature increased from 50 to 800 °C at a heating rate of 10 °C/min under 50 mL/min air condition. The samples were completely oxidized to MoO3 at 700 °C, resulting in weight loss platform, and the Mo content was calculated according to the weight loss at 700 °C. The Mo contents of catalysts supported on AC and Gr were analyzed via Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). About 0.05 g solid sample was added in Teflon tube with 5 mL concentrated nitric acid and 1 mL hydrofluoric acid. The tube was sealed and heated at 190 °C for 10 h. Subsequently, the solution was diluted to 50 mL with deionized water, and 1 mL of diluted solution was further diluted to 50 mL. The test was carried out over Agilent ICPOES730 using external standard for concentration determination. N2 absorption/desorption isotherms were carried out over ASAP 2460 (Micromeritics) at 77 K. The samples were pretreated in vacuum at 300 °C. The surface areas and pore volumes are measured via BET and BJH equations according to isotherms. 2.4. Kraft Lignin Decomposition. Decomposition of Kraft lignin was carried out in Hastelloy batch autoclave (100 mL, Parr). In a typical experiment, 0.4 g of Kraft lignin and 0.2 g of catalyst was carefully added in the reactor in case it adhered to the wall. Then, 40 mL of ethanol was added with stirring. After the autoclave was sealed directly (in ambient air condition), the reactor was heated at 5 °C/min to 280 °C and kept for 3 h under 400 r/min stirring. When the reactor was cooled to the ambient temperature, the pressure was released and the product solution was collected by filtration using a 0.4 μm film. The solid residual was washed with ethanol and dried, and the liquid was collected together with product solution to a total volume of 60 mL. Subsequently, about 0.05 g of p-cresol was added as internal standard for gas chromatography−mass spectrometry (GC−MS) analysis. The collected product solution was rotary evaporated in vacuum at 40 °C for 1 h to obtain the liquid product. The solid residue, consisting of catalyst and remained lignin, was dried in an oven and

2. MATERIALS AND METHODS 2.1. Materials. The wheat straw Kraft lignin (typical contents in Table S1) was provided by Shandong Tralin Paper Co. Ltd. and stored in the absence of moisture.2 AC and Gr were purchased from Chengdu Kelong Chemical Reagent and Huayuan Co. Lit., respectively. The MC was synthesized via a hard template method (see the Supporting Information).21 AR reagent grade ammonium molybdate, ethanol and other chemicals, including p-cresol, toluene, 1,3,5-trimethylbenzene, and benzyl alcohol were purchased from Chengdu Kelong Chemical Reagent. The p-xylene (AR) was purchased from Aladdin. 2.2. Catalyst Preparation. The β-Mo2 C catalysts supported on different carbon materials were prepared via impregnation method followed by direct H2 reduction and selfreduction. Typically, 2.371 g of ammonium molybdate was dissolved in 6 mL of water, and 3 g of carbon materials, AC and Gr, were added under stirring. In the case for macroporous carbon (MC), 0.237 g of ammonium molybdate was dissolved in 3 mL of water, and 0.3 g of carbon material was added. The solution and carbon materials were well mixed and stored at ambient conditions for 12 h. Subsequently, the mixture was dried at 60 °C in vacuum for 12 h and calcined at certain temperature under 50 mL/min H2 or 100 mL/min N2 flow. The ammonium molybdate was decomposed to Mo oxides, which were reduced by H2, and the reduced Mo reacts with carbon supports to form β-Mo2C. Meanwhile, under N2 atmosphere the self-reduction occurs that Mo oxides directly react with carbon supports to form β-Mo2C. The catalysts were labeled according to the supports, calcination temperature and atmosphere. For example, AC-700H2 referred to molybdenum catalysts supported on AC and calcined at 700 °C in H2 atmosphere. 2.3. Catalyst Characterization Methods. X-ray diffraction (XRD) patterns were measured on a DX-2700 diffractometer (Haoyuan Instrument, Dandong, China) using 12603

DOI: 10.1021/acs.iecr.9b01807 Ind. Eng. Chem. Res. 2019, 58, 12602−12610

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Industrial & Engineering Chemistry Research

Figure 1. XRD patterns (a and b) and Raman shifts (c and d) of molybdenum catalysts on different carbon supports. Crystal phases are observed with graphite (▼), β-Mo2C (●), Mo (■), and MoO2 (⧫). D and G refer to different types of graphite carbon.

Table 1. Crystal Parameters on the β-Mo2C (101) Facet, XPS Analysis for Different Catalysts, and ICP (TG) Analysis for Mo Contents Mo speciesa (at. %)

total Mo contents (wt %)

catalysts

2θ (deg)

crystal size (nm)

Mo2C

Mo4+

Mo6+/5+

XPSb

ICP (TG)c

MC-700H2 AC-700H2 Gr-800N2 Gr-900N2 Gr-1000N2

39.5 39.2 39.5 39.5 39.5

16 9 28 29 36

2 11 31 19 24

0 12 23 12 17

98 76 46 69 59

24 24 25 28 22

(38) 43(39) 33 32 36

a

The Mo species refer to the atom contents of different types of Mo species in total Mo according to XPS peak isolation. bOnly C and Mo are considered in XPS analysis. cThe data in brackets was calculated via TG analysis.

3. RESULTS AND DISCUSSION 3.1. Characterization. The XRD patterns of molybdenum catalysts on different carbon supports are shown in Figure 1a. Significant β-Mo2C crystal (PDF 11-0680) is formed on AC and MC supports, and typical Mo crystal (PDF 04−0809) and Gr crystal (PDF 08-0415) are observed on Gr support. Under H2 condition, the Mo species first decompose to MoO3 at low temperature, and subsequently, MoO3 is reduced and carburized by H2 and carbon support, respectively, to obtain β-Mo2C.17 However, highly crystallized Gr is more stable than AC and MC and difficult to react with Mo to form β-Mo2C at 700 °C, which resulting in elementary Mo for Gr-700H2. Despite of the stability of Gr crystal, weak reduction of MoO3 to MoO2 (PDF 65-1273) is observed at 700 °C in Figure 1b. Significant solid−solid reaction between Gr and Mo species occurs to form highly crystallized β-Mo2C at higher calcination temperature of 800, 900, and 1000 °C under N2 atmosphere, and the grain size of β-Mo2C crystal increases with the calcination temperature (Table 1). The grain size in Table 1 also shows that Gr benefit the growth of β-Mo2C crystal because the grain size of β-Mo2C on AC calcined in H2 at 800 °C is smaller (Figure S2). The inert atmosphere is required for Gr to form β-Mo2C because partial oxygen in calcination atmosphere would results in MoO3 when calcined at 1000 °C (Figure S2). Thus, β-Mo2C on Gr could be formed at higher temperature under N2 condition, while β-Mo2C on AC and MC is easily prepared at lower temperature under H2 condition. The Raman shifts of the carbon supports show graphite carbon with G (in-plane vibration of sp2) and D (defects and disorder) peaks20 for MC and AC. The Gr possesses narrow G peak, indicating the highly crystallized graphite structure. The high content of sp2 carbon results in much stable Gr, while the

weighed. The product yield (Yi) and lignin conversion (X) were calculated as follows Yi =

mi × 100% m0

m − mcat zyz ji X = jjj1 − R zz × 100% j z m0 k {

(1)

(2)

where mi is the weight of product i, m0 is the initial weight of Kraft lignin, mR is the weight of solid residue, and mcat is the weight of catalyst. Then, 5 μL of product solution was used for product identification and qualitative analysis in Shimadzu GCMSQP2010 SE system with DB-5MS UI column (30 m length and 0.25 mm diameter). The inlet and interfacial temperature were both 250 °C, and the oven temperature was maintained at an initial temperature of 40 °C for 1 min and then heated to 70 °C at a heating rate of 5 °C/min, and finally heated to 250 °C at a heating rate of 10 °C/min. The temperature of ion source was 200 °C, and solvent delay was set as 2.5 min. The mass spectra of product peaks were matched using NIST 11(s) database for product identification, and the MS signals were used for qualitative analysis using p-cresol as internal standard. The correction equations were observed using standard substances for quantification (Table S2 and Figure S1), and product yield of each component was defined as grams per gram of Kraft lignin. This article aims at the aromatics and esters products, and the yields of other products are not corrected. The repeatability result shows that decomposition experiments can be repeated with errors less than 4%. 12604

DOI: 10.1021/acs.iecr.9b01807 Ind. Eng. Chem. Res. 2019, 58, 12602−12610

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Industrial & Engineering Chemistry Research defects and disorder lead to higher reactivity of MC and AC. This is the main reason that β-Mo2C is formed with MC and AC supports rather than Gr at 700 °C. In addition, the increased peak intensity ratio of ID/IG after supporting Mo species (Figure 1c) suggests that reduction process under H2 atmosphere would destroy the graphite structure to produce more defects. The Raman shifts of the Gr supported β-Mo2C calcined under N2 atmosphere exhibit similar G and D peaks with original Gr support as shown in Figure 1d. This result indicates that the graphite structure remains when calcined under N2 atmosphere. The wide peaks arranging from 500 to 1000 cm−1 are mainly assigned to β-Mo2C for catalysts calcined at temperatures from 800 to 1000 °C. Differently, the Raman peaks of β-Mo2C with trace carbon (Figure S3) are strong and narrow. Thus, the Raman signal of Mo species in the supported catalysts is significantly influenced by carbon supports. The XPS results in Figure 2a show that peak couples of Mo 3d5/2 and 3d3/2 assigned to Mo6+/5+ (Mo2O5 and/or MoO3),

highly reactive, which results in the sufficient surface oxidation for MC-700H2 catalyst. The morphological structure of catalysts on different supports is shown in Figure 3. Similar to literature,21 MC-

Figure 2. XPS of Mo 3d (a) and O 1s (b) for different β-Mo2C catalysts on carbon supports.

Figure 3. Morphological structure of MC-700H2 (a−c), AC-700H2 (e), and Gr-900N2 (g), and Mo distribution mapping of related catalysts (d, f, and h for MC-700H2, AC-700H2, and Gr-900N2, respectively).

Mo4+, and Mo2C are presented in Figure 2, and β-Mo2C is observed near 228.2 eV for Mo 3d5/2.23,24 Despite of the bulk β-Mo2C phase (Figure 1), high Mo oxide peaks in XPS results indicate that the surface β-Mo2C catalyst is oxidized to Mo oxides or other species when exposed in air,25 while bulk phase remains the β-Mo2C crystal. The XPS of O 1s in Figure 2b shows that three types of O atoms, namely Mo oxides, Mo2C(OH)x (−OH terminated), and C−OH species in carbon supports.20,23,26 The Mo2C(OH)x mainly refers to the oxygen on surface β-Mo2C species. The high content of surface O in Mo oxides confirms the surface oxidation of βMo2C in air. The Mo atom contents of different β-Mo2C catalysts are listed in Table 1. MC-700H2 exhibits the highest surface Mo oxides, and only 2 at. % of surface Mo is identified as β-Mo2C. The different surface oxidation is highly related to the different properties of carbon supports. The Raman shifts in Figure 1c show that MC exhibits the highest ID/IG value, namely the highest defects. Meanwhile, the FT-IR curves of different carbon supports in Figure S4 show significant peaks near 1565 and 1092 cm−1, which are assigned to carbon− oxygen bonds of CO and −C−OH, respectively.27 Obviously, MC exhibits the highest surface −OH species, and high content of surface defects on carbon supports is accompanied by high surface −OH. The defects and −OH are

700H2 exhibits ordered macropores of about 200 nm with shell thickness of about 10 nm, and mesopores are observed within macropores (Figure 3a−c). The macropores in the particles are stacked layer by layer and connected by the mesopores, and the particles remain the ordered structure during β-Mo2C loading (Figure S5). The SEM of used MC-700H2 catalyst shows that porous structure is similarly with fresh catalyst (Figure S6), indicating that the macroporous structure is stable in ethanol condition. The Mo distribution in Figure 3d shows that β-Mo2C is dispersed in the macroporous structure of MC. The AC-700H2 exhibits irregular shape and particle surface similarly with AC support (Figures 3e and S4), and the Mo species are almost dispersed uniformly in AC support (Figure 3f). Interestingly, the smooth edge of Gr sheet is turned to be rough after β-Mo2C loading, while the smooth surface remains except for the covered individual particles (Figures 3g and S4). The Mo distribution results (Figure 3h) indicate that Mo species mainly locate at the edge of Gr particles. Thus, β-Mo2C would be formed by reactions between MoOx and carbon at the edge of (002) facet for Gr, which allows the growth of βMo2C into larger crystal grains (Table 1). These results indicate that Mo species are highly dispersed on AC support and easily separated from Gr carbon. Meanwhile, Mo species 12605

DOI: 10.1021/acs.iecr.9b01807 Ind. Eng. Chem. Res. 2019, 58, 12602−12610

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Industrial & Engineering Chemistry Research

Figure 4. N2 absorption (solid symbol) and desorption (hollow symbol) isotherms of MC and MC supported catalyst (a), AC and AC supported catalyst (b), and Gr and Gr supported catalyst (c). The insert figures are normalized pore size distributions from 2 to 100 nm with black lines for carbon support and red lines for related catalysts.

Table 2. Porous Properties and Mo Contents of β-Mo2C Catalysts over Different Carbon Supports

are tend to locate in the hollow of macroporous structure of MC. The Mo contents in different catalysts are calculated according to different methods (Table 1). The XPS analysis mainly focuses on the surface properties, while ICP-OES and TG analysis obtain the bulk Mo content. The Mo contents of MC-700H2 and AC-700H2 are similar to each other but significantly higher than that of Gr-supported catalysts. The high Mo contents of MC-700H2 and AC-700H2 is mainly attributed to the release of carbon through hydrogenation of surface carbon species with H2 to CH4 under H2 atmosphere.28 Meanwhile, the Mo content differences in ICP-OES and XPS analysis for AC-700H2 are much larger than that for Grsupported catalysts, indicating that Mo species could be distributed inside AC particles but on the Gr surface. The SEM-EDS results in Figure 3 confirm that Mo species on Gr support are located on the surface or edge of Gr. The similar differences in bulk and surface Mo species for MC-700H2 and AC-700H2 catalysts suggest that Mo would also locate inside MC particles with macroporous struture. In addition, the Mo contents of Gr-supported catalysts calcined at different temperature are similar to an average value of 34 wt % for bulk Mo content. The N2 absorption/desorption isotherms and pore size distributions of carbon supports and catalysts are shown in Figure 4. The isotherms of MC and MC-700H2 in Figure 4a are classified as Type II according to IUPAC standard, indicating the macroporous structures of the materials.29 The Type H3 hysteresis loops at higher p/p0 value show the platelike particles with interparticle adsorption,22,30 which confirms the ordered macropores of MC-700H2 (Figure 3). The pore size distribution of MC exhibits a wide distribution from 2 to 100 nm. When β-Mo2C was loaded on MC, the distribution less than 10 nm decreases significantly and the larger distribution decreases slightly, indicating the increased pore size. This result is in accordance with the average pore size in Table 2. Meanwhile, the BET surfaces and pore volumes decrease significantly after β-Mo2C loading. Therefore, the thin shell of MC would react with Mo oxides to destroy micro- and mesoporous structure. Differently, Type I isotherms and H4 hysteresis loops are observed for AC and AC-700H2 (Figure 4b), indicating structure of micropores. The pore size mainly distributes

Vpore (cm3/g)

a

catalysts

SBET (m /g)

pores >1.7 nm

micropores

Dpa (nm)

MC-700H2 MC AC-700H2 AC Gr-900N2 Gr

359 885 255 1378 855 665

0.719 1.141 0.112 0.410 0.037 0.021

0.034 0.158 0.028 0.250 0.410 0.328

10.4 8.01 3.78 3.20 5.06 6.15

2

Dp refers to average pore size of pores ranging from 2 to 300 nm.

between 2 and 10 nm. As indicated that β-Mo2C locates in the bulk phase of AC support (Figure 3), the β-Mo2C inserted into bulk carbon could destroy the porous structure. Thus, significant decrease in surface areas and pore volumes is observed after β-Mo2C loading on AC (Table 2). The isotherms and hysteresis loops of Gr and Gr-900N2 show large absorption in micropores and little absorption at high p/ p0 value (Figure 4c), which is confirmed by the smooth surface of Gr (Figure S5). The increase in surface areas and microporous volumes after loading of β-Mo2C would be resulted from the reaction between carbon support and Mo oxides to obtain rough surface (Figure S5). 3.2. Catalytic Performance. The catalytic performance of different catalysts is shown in Table 3. Without any catalyst, conversion of Kraft lignin is high, but product yields is relatively low, which agrees with literature that Kraft lignin solid is converted into soluble large fragments in ethanol.2 The molecular weight distributions in Figure S7 show large fraction with molecular weight larger than 1000 when no catalyst is used. The large molecules confirm the fragments dissolved in ethanol. The blank experiment without Kraft lignin over AC700H2 show little aromatics product, indicating that aromatics product is assigned to Kraft lignin rather than ethanol solvent.15 However, the slightly higher total product yield suggests that ethanol could be converted over AC-700H2 catalyst. Over AC-700H2 and MC-700H2 catalysts, the conversion of Kraft lignin is only slightly higher than that without catalyst, but the product yields are significantly higher, indicating the high catalytic performance of β-Mo2C for fragment decomposition. The low product yield of Gr-700H2 is 12606

DOI: 10.1021/acs.iecr.9b01807 Ind. Eng. Chem. Res. 2019, 58, 12602−12610

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Industrial & Engineering Chemistry Research Table 3. Catalytic Performance of Kraft Lignin over Different Catalysts at 280 °C for 3 h product yield (g/g Kraft lignin)b catalysts

X (%)

aromatic hydrocarbons

aromatic alcohols

aromatic aldehydes

esters

total

none AC-700H2 (blank)a MC-700H2 AC-700H2 Gr-700H2 Gr-800N2

48.0

0.011 0.002 0.140 0.171 0.028 0.121

0.011 0.000 0.275 0.177 0.030 0.256

0.000 0.000 0.128 0.079 0.019 0.078

0.051 0.060 0.646 0.607 0.146 0.337

0.074 0.131 1.431 1.257 0.232 0.916

64.2 52.7 57.1 64.4

a

The blank experiment was carried out without Kraft lignin under the same condition. bOther minor components without benzene rings are hydrocarbons, alcohols, ethers, and aldehydes.

resulted from the formation of Mo rather than β-Mo2C on the Gr support. The Gr-800N2 with β-Mo2C crystal exhibits higher aromatic yields than Gr-700H2, indicating the advantages of βMo2C. Considering the ash content of 36.4 wt %, the Kraft lignin could be completely converted over MC-700H2 catalyst. Aromatic yields over MC-700H2 catalyst, as well as ester and total yields, are higher than that over AC-700H2 and Gr800N2, indicating the advantages of MC support. As indicated in literature, acidity of catalysts exhibits influence on lignin conversion.31 The NH3-TPD curves of different catalyst in Figure S8 show that AC-700H2 exhibits the largest amount of acid sites, while MC-700H2 exhibits the lowest. However, the activity seems to be independent with acid properties, due to the basicity of Kraft lignin with a pH of about 11 in aqueous solution. The detailed products are shown in Figure S9, and typical aromatics, esters, and alcohols are identified according to GCMS. Herein, the major aromatic components are listed in Table 4, which could be classified as aromatic hydrocarbons, aromatic alcohols, and aromatic aldehydes (Table 3). Clearly, the selectivity to aromatic components is different between MC-700H2 and AC-700H2. The large fragments from decomposition of Kraft lignin are catalyzed to obtain the three types of aromatic monomers through C−C and C−O cleavage. Meanwhile, esters are formed between ethanol and oxygenated compounds (such as alcohols, aldehydes, and acids from linkages and acid branches of lignin) with hydrogen radical involved, and the carbon number increases via Guerbet reaction with ethanol.15 The experiments show that H2 (Table S3 and Figure S10) is produced from dehydrogenation of ethanol solvent over β-Mo2C.32 Assuming 1.5 MPa of H2 product (Table S3) and 60 mL free volume at 25 °C, about 1.6 g of ethanol are dehydrogenated and converted to other compounds over AC-700H2. Considering weight loss of esterification and GCMS analysis, it is reasonable that total product yields are larger than 1. Additionally, aromatic alcohols and aldehydes could suffer from hydrogenation reactions to obtain aromatic hydrocarbons and alcohols (Scheme 1),33,34 respectively, which could be confirmed by the reactions at different time and temperature in Figure 5. All aromatic compounds increase at short reaction time, and aromatic aldehydes are gradually converted to alcohols after reaction time of 2 h. The conversion of alcohols to hydrocarbons is difficult to observe because hydrodeoxygenation of alcohols to hydrocarbons is difficult than hydrogenation of aldehydes to alcohols.35 The production yields at different temperature in Figure 5b also suggest that aromatic aldehydes could be converted to alcohols. Meanwhile, higher aromatic hydrocarbons are obtained at 300 °C for 3 h than those at 280 °C for 6 h, but similar aromatic alcohols and

Table 4. Product Yields over Different Catalysts at 280 °C for 3 h According to GCMS Analysis

a

The isomers with the same functional groups but different positions on benzene ring are listed in the same raw, including 1-methyl-4propyl-benzene, 1-ethyl-2,3-dimethyl-benzene, and 4-methyl-benzenemethanol, 4-methyl-benzaldehyde, which cannot be distinguished directly via GCMS. The p- and o-xylene are identified using standard substances. bThe components with yield higher than 0.005 g/g lignin are listed in the table.

lower aromatic aldehydes are observed under the former condition. This result suggests that aromatic alcohols could be converted into hydrocarbons when carried out at higher temperature of 300 °C with higher hydrogenation activity. 12607

DOI: 10.1021/acs.iecr.9b01807 Ind. Eng. Chem. Res. 2019, 58, 12602−12610

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Industrial & Engineering Chemistry Research

Because of the stable structure of Gr, high temperature calcination is required to prepare β-Mo2C species on Gr support. The Gr-supported catalysts calcined under different atmosphere and at different temperature exhibit distinguished activities, as shown in Figure 6. The β-Mo2C on Gr support are

Scheme 1. Brief Conversion Pathway of Kraft Lignin in Ethanol over β-Mo2C on Different Carbon Supportsa

a

R′ stands for the alkane branch with different numbers and positions. The red and blue arrows refer to fragment decomposition and hydrogenation, respectively. Figure 6. Product yield of Gr-supported catalysts calcined at different temperature under different atmosphere (pure H2, pure N2, and partial air). The crystal phases of the related catalysts are marked over bars.

Considering the stepwise conversion of Kraft lignin in Scheme 1, the accessibility of fragments to β-Mo2C is important for high performance. The surface areas and Mo contents of AC-700H2 and MC-700H2 (Table 2) are mainly similar to each other, but the β-Mo2C grain size of the former is smaller, which benefits the exposure of surface active sites. However, the MC-700H2 catalyst is more active, which is mainly attributed to the larger pore size of MC-700H2. According to the literature, the molecular weight of fragments mainly ranges from 700 to 1400, indicating fragments with about 4 to 8 monomers,2 and molecule size of fragments could be estimated as about 0.7 to 0.9 nm,36 compared with monomer size of about 0.45 nm. Thus, accessibility of fragments to β-Mo2C in the pores (3.78 nm) of AC would be poorer than monomers. However, the pore size of MC700H2 is much larger than the fragment size, resulting in better accessibility of fragments to β-Mo2C. Additionally, the better accessibility of monomers to β-Mo2C in the smaller pores of AC contributes to the advantages of smaller grain size. Therefore, the MC-700H2 with large pore size exhibits higher total aromatic yield, but AC-700H2 possesses higher yield of and selectivity to aromatic hydrocarbons (Table S3).

much more active than Mo, MoO2, and MoO3 crystals. With the increase in calcination temperature from 800 to 1000 °C, the grain size of β-Mo2C gradually increases, while the Mo contents and surface properties are similar according to XPS and ICP-OES results. Therefore, small grain size of β-Mo2C is beneficial for high activity due to exposed surface active sites. In addition, the relatively low activity of bulk Mo oxides indicates that the surface Mo oxides on β-Mo2C particles (Figure 2) can scarcely influence the catalytic activity. Different with other Mo species or other carbon supports, the aromatic yields over β-Mo2C on Gr are higher than esters, and the higher selectivity to aromatics is beneficial for purification of aromatic products. Compared with AC-700H2 catalysts, Gr-supported β-Mo2C catalysts possess relatively high surface areas and micropores, but slightly lower Mo contents (Table 1). The aromatic yield over Gr-800N2 is still higher than that over AC-700H2 despite the disadvantages of grain size and Mo content for Gr-800N2.

Figure 5. Product yield from Kraft lignin over AC-700H2 catalyst at 280 °C for different reaction time (a) and at different temperature for 3 h (b). 12608

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Industrial & Engineering Chemistry Research This result could also be explained via accessibility of fragments to β-Mo2C (Scheme 1). The SEM results in Figure 3 show significant boundaries between β-Mo2C and Gr support, and β-Mo2C is located on the surface or edges of Gr particles. Thus, the accessibility of lignin fragments to βMo2C is seldom influenced by the micropores in Gr particles, resulting in high aromatic yield. In addition, the selectivity to aromatic hydrocarbons for Gr-supported catalysts are smaller than that for AC-700H2, but similar to that for MC-700H2 (Table S3). The isolated β-Mo2C on Gr surface or edges can hardly provide enough micropores for hydrogenation of aromatic alcohols, thus, resulting in low selectivity to aromatic hydrocarbons.

ACKNOWLEDGMENTS

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REFERENCES

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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.iecr.9b01807. Preparation of MC, the properties of Kraft lignin, correction equations and standard curves of different substances, total aromatic yield and selectivity to different aromatic compounds, XRD patterns of βMo2C supported on AC and Gr, XRD (a) and Raman shifts (b) of β-Mo2C, FT-IR spectra of different carbon supports and catalysts, morphological structure of carbon supports and catalysts, morphological structure of used MC-700H2 catalyst, NH3-TPD curves of different catalysts, the total-ion chromatogram (TIC) of the liquid products and the related molecules and product yields (PDF)

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This work is supported by the Program of National Natural Science Foundation of China (No. 21808148), the Key Program of National Natural Science Foundation of China (No. 21336008), and the Fundamental Research Funds for the Central Universities (2018SCU12002). We would like to thank the Institute of New Energy and Low-Carbon Technology, Sichuan University, for SEM images and XRD analysis.

4. CONCLUSION The active β-Mo2C catalysts were conveniently prepared via direct H2 reduction or self-reduction at high temperature. Kraft lignin is efficiently converted over β-Mo2C to aromatic hydrocarbons, aromatic alcohols, aromatic aldehydes, and other compounds, and hydrogenation of aromatic alcohols and aldehydes to aromatic hydrocarbons are observed. The activity of Mo, MoO2, and MoO3, is relatively low. The highest aromatics yield of 0.543 g/g Kraft lignin is obtained over βMo2C on MC support with large pore size, which is attributed to accessibility of lignin fragments to β-Mo2C. Higher calcination temperature is required to obtain β-Mo2C over Gr support than MC and AC, and the activity of Gr-supported catalysts decreases with grain size. AC-700H2 catalyst exhibits the highest selectivity to aromatic hydrocarbons, which are attributed to highly dispersed β-Mo2C in micropores.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yingming Zhu: 0000-0002-4926-8847 Yingying Liu: 0000-0002-2106-6418 Houfang Lu: 0000-0003-3537-2480 Bin Liang: 0000-0003-2942-4686 Yongdan Li: 0000-0002-0430-9879 Notes

The authors declare no competing financial interest. 12609

DOI: 10.1021/acs.iecr.9b01807 Ind. Eng. Chem. Res. 2019, 58, 12602−12610

Article

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