Production of Phenol-rich Monomers from Kraft Lignin

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Biofuels and Biomass

Production of Phenol-rich Monomers from Kraft Lignin Hydrothermolysates in Basic-subcritical Water over MoO3/SBA-15 Catalyst Masud Rana, Mohammad Nazrul Islam, Ashutosh Agarwal, Golam Taki, Seong Jae Park, Shin Dong, Young Tae Jo, and Jeong-Hun Park Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02616 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Production of Phenol-rich Monomers from Kraft Lignin Hydrothermolysates in Basicsubcritical Water over MoO3/SBA-15 Catalyst Masud Rana,a Mohammad Nazrul Islam,b Ashutosh Agarwal,a Golam Taki,a Seong-Jae Park,a Shin Dong,a Young-Tae Jo,a Jeong-Hun Park.a* a

Department of Environment & Energy Engineering, Chonnam National University, Gwangju, 61186, South Korea. b

Department of Soil Science, University of Saskatchewan, Saskatoon, SK, S7N 5A8, Canada.

Corresponding author: Tel: 82-62-530-1855, Fax: 82-62-530-1859. E-mail: [email protected]

0000-0002-0661-0059

Abstract MoO3 catalyst supported on synthesized mesoporous SBA-15 matrix was employed in the hydrothermolysis of Kraft lignin in basic-subcritical water system. Experiments were carried out in a batch reactor at 350 °C for 1h under N2 atmosphere using two different solvent systems of sub-H2O and 0.5% NaOH. At 10 % (w/w) of synthesized MoO3/SBA-15 catalyst with lignin, the highest total crude bio-oil yield (~56.40 wt.%) and lowest char formation (~8.60 wt.%) were observed in basic medium, in comparison to the subcritical water system. The GC-MS analysis of the produced bio-oil indicated that the major products in the crude bio-oil were phenol-rich monomers such as guaiacols (19.50 %), alkylguaiacols (24.83 %), catechol (26.76 %), and alkyl-catechols (17.82 %) under subcritical water condition. Alternatively, catechols (42.07 %) and alkyl-catechols (34.59 %) were the primary products in the basic solvent system with the presence of MoO3/SBA-15 catalyst. Additionally, monophenols (5.0%) were also produced as a result of the demethoxylation of guaiacol or hydrodeoxygenation of catechol. MoO3/SBA15 catalyst significantly reduced the oxygen-carbon ratio (O/C) from 0.72 (in original Kraft lignin) to 0.29 (in basic-subcritical water system). Decreased oxygen and increased carbon and hydrogen contents enhanced the calorific value of the produced bio-oil, whereby a higher heating value (HHV) of 32.00

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MJ/Kg was achieved. The synthesized catalysts were characterized through BET, XPS, ICP-OES, and HR-TEM analyses. Keywords: Kraft lignin; Hydrothermal liquefaction; Subcritical water; Phenol-rich monomers; MoO3/SBA-15 1. Introduction In recent years, rapid urbanization and the depletion of conventional fossil fuels have guided researchers towards find an alternative resources to meet the current worldwide energy demand.1 Researchers have recently become interested in biomass conversion to fuels and value-added chemicals.2 Lignocellulosic biomass is being explored worldwide as a non-edible renewable resource for the production of bio-oil due to its low cost, ubiquity, and carbon neutrality.3 Lignocellulose is primarily composed of three main constituents namely cellulose, hemicellulose, and lignin.4 Lignin is a highly branched three-dimensional amorphous polymer consisting of different methoxylated phenylpropanoid building blocks.5 The most important of these building blocks are guaiacol alcohol, p-coumaryl alcohol, and syringyl alcohol. These are linked by various bonds such as β-O-4, β-1, β-5, 5-5, α-O-4, 4-O-5, dibenzodioxocin, and β-β.6 The structure and physiochemical properties of lignin are complicated and vary with different biomass sources. Based on the sulfur content, lignin is generally divided into two categories.7 The first category includes sulfur-containing lignin such as lignosulfonates, and Kraft lignin. The second category involves sulfurfree lignin such as organosolv lignin, soda lignin, steam-explosion lignin, hydrolysis lignin, and bioethanol lignin. For a long time, Kraft lignin has been produced on a large scale as bulk waste by the pulp and paper industry. Over the past few decades, several countries have been isolating Kraft lignin from black liquor, due to environmental regulations.8 Typically, in the pulp and paper industry, Kraft lignin is isolated from liquor using NaOH and Na2S solutions. One of the main problems during Kraft lignin extraction process is that the structure is strongly attracted by the hydrogen sulfide ions (HS-) which leads to a sulfur2


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enriched structure.9, 10 The presence of sulfur imposes an additional difficulty during catalytic valorization of Kraft lignin. Therefore, the efficient conversion of Kraft lignin still remains a big challenge. Despite all of the challenges associated with Kraft lignin, such as high sulfur (S), nitrogen (N), and ash contents, there has been a considerable amount of interest towards finding new techniques for the production of phenol-rich aromatics compounds. This is because of its abundance, high heating value, and unique aromatic properties. Recently,

a

number

of

insightful

lignin

conversion

strategies,

including

hydrolysis,11-13

hydrocracking/hydrogenolysis,14-17 ionic liquid-assisted method,18, 19 hydrothermal liquefaction,20, 21 and chemical degradation involving acid-base and metal catalyzed reactions22 have been investigated for the production of bio-oil. Of these strategies, hydrothermal liquefaction has attracted much attraction over the past few years.23 This process is considered as one of the most promising technologies for bio-oil production from biomass. In the hydrothermal liquefaction process, different types of solvents are used at near critical conditions that suppress char formation while producing large amounts of liquid products.23 Additionally, the HTL process retains more than 70% of the feedstocks carbon in the produced bio-oil 1. More recently,24-28 lignin degradation into low molecular weight compounds have been carried out in hydrothermal liquefaction under sub- and supercritical conditions. Although, lignin depolymerization in sub- and supercritical water yields high amounts of liquid products, the molecular weight of ligninderived products as well as the reaction time still remains high.2 Very recently, several attempts have been developed to produce substantial amount of low molecular weight phenolic compounds in HTL process. Wang et al.29 have studied hydrothermal treatment of lignin in the absence of catalyst at 310 °C for 0.5h and obtained 3.17% of mono-phenolic compounds. From this study, it was inferred that water at subcritical conditions could depolymerize lignin to a certain amount. Thring et al.30 investigated the effects of time and temperature on Alcell lignin catalyzed by NaOH and found catechol, guaiacol, and syringol as the major depolymerization products. Toledano et al.31 observed 3


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that base catalyst hindered the repolymerization of lignin-derived reactive monomers and oligomers, thus improving bio-oil yields. Miller et al.12 investigated the alkaline (KOH) hydrolysis of Alcell lignin in supercritical ethanol/methanol, and concluded that the lignin depolymerization reaction was favored by the presence of strong bases. Several years later, the same group12 studied the alkaline depolymerization of lignin using water as a solvent and concluded that alkali concentration was the most important factor which leads to lignin depolymerization. Moreover, product analysis from model compound depolymerization revealed that the phenyl ether bonds were effectively broken during base catalyzed depolymerization reaction. Therefore, lignin depolymerization at subcritical conditions over a basic medium is an excellent route for producing mono-aromatic phenolic compounds. This is attributed to the co-catalytic activity of subcritical water toward the cleavage of aryl-alkyl (β-O-4) ether bonds in the lignin polymer structure. To date, very few works have been reported in the literature concerning Kraft lignin depolymerization under basic-subcritical water conditions over metal catalyst.32, 33 While a series of alkali catalysts, such as NaOH, Na2CO3, KOH, and metal catalyst including nickel, palladium, platinum, cobalt, and ruthenium etc., have been investigated to depolymerize organosolv lignin to low molecular weight products rather than Kraft lignin.2 Furtherer, most of the existing work suffered from one or more drawbacks, such as low yield, harsh reaction conditions, high heavy fraction yield, char and tar formation.2, 32, 34 Nowadays, SBA15 has become the most widely-used support, possessing good thermal stability owing to its large pore size, good mechanical strength, highly ordered 2D hexagonal structure, thick pore wall, and high surface area.35 In lignin valorization, SBA-15 is well-known as efficient supports matrix for reducing char formation and β-O-4 bond cleavage.36 It was also reported that during lignin depolymerization, SBA-15 can effectively suppressed the repolymerization of reactive intermediates due to its well-ordered pore structure and large pore size. In comparison to SBA-15, SiO2 possesses lower specific surface area and smaller pore size.37 Therefore, SBA-15 has a better catalytic activity over silica. In a recent study,38 it was 4


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investigated that large pore size of SBA-15 support material has a positive effect on the suppression of the char formation. Furthermore, the deposition of metals in SBA-15 support improve the depolymerization efficiency and saturate the instable species. Therefore, in this study, SBA-15 was chosen as support matrix for lignin depolymerization. Undoubtedly, different metal catalysts with various supports have been employed as effective catalysts for lignin degradation. Based on this premise, various molybdenumbased catalyst such as Mo2C, MoS2, Mo2N/Al2O3, MoO2, MoO3/CeO2, MoOX/NC, MoOX/CNT, HTaMoO6, and MoO3 have emerged as effectual catalysts for biomass conversion.39-46 For instance, Mo2N/Al2O3 catalyst has been applied for the ethanolysis of lignin to low-molecular weight products whereby satisfactory results was obtained.41 Compared with the other molybdenum-based catalysts, MoO3 catalyst is regarded as a promising catalyst due to its low-cost, earth-abundance, and good catalytic activity towards hydrodeoxygenation of different lignin derived model compounds under nitrogen atmosphere.47 Moreover, MoO3 is highly selective for the cleavage of C-O bond linkages. Our previous study of HTL of Kraft lignin in a batch reactor48 investigated the effect of time (0-60 min), temperature (200-350 °C), and lignin to water ratio (1:10-1:80) in subcritical water without catalyst. Since the effect of MoO3/SBA-15 catalyst under basic conditions for Kraft lignin depolymerization still remains unknown. Therefore, in this study, we explored the potential of MoO3/SBA-15 catalyst under water-alkali sub-critical conditions for Kraft lignin conversion to monoaromatic phenolic monomers. The workup procedure including isolation, and identification of most abundant phenolic monomers is also discussed. 2. Materials and methods 2.1. Materials and analytical methods All solvents and chemicals, including Kraft lignin (CAS no. 8068-05-1), ethyl acetate (CH3COOC2H5), diethyl ether (C2H5OC2H5), acetone (CH3COCH3), high grade sodium hydroxide (NaOH), sulfuric acid (H2SO4), hydrochloric acid (HCl), ammonium molybdate ((NH4)6Mo7O24·4H2O), and Pluronic 123 TEOS 5


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were purchased from Sigma-Aldrich (USA, ≥99.5%). Deionized water was prepared using an aqua MAXTM Young Lin instrument (South Korea). High purity N2 (≥99.5%) purchased from Dae-Deok Gas co. Ltd. (South Korea) was used to remove air from the reactor. All of the chemicals were used without further purifications. The bio-oil composition was measured using GC-MS (Agilent 6890) through GCFID quantification (Agilent Technologies, U.S.A). Elemental analysis (C, H, S, and N composition) of the bio-oil was performed by employing a vario MARCO cube/elementer (Germany). Mo content in the support material was estimated by ICP-OES, PerkinElmer Optima 8300, US. 3. Experimental 3.1. Mesoporous silica (SBA-15) preparation Mesoporous silica SBA-15 support was prepared based on the method found in the literature.49 Typically, Pluronic 123 (4.0 g, Aldrich, ≥99.5%) was dissolved in 2M HCl solution (120.0 mL) and de-ionized water (30.0 mL) in a round bottom flask and stirred at room temperature for 0.5h. To the above solution, TEOS (8.5g, Aldrich, ≥99.5%) was added while vigorously mixing the solution for 1min, which was followed by moderate stirring at 40 °C for two days. The solution was transferred into Teflon-lined autoclave and then heated at 100 °C without stirring for 24h. The solution was then filtrated and washed with deionized water. The solid residue thus obtained was dried in an oven at 100 °C for 12 h. Finally, the residue was calcined in a muffle furnace for 6h under static air conditions at 550 °C (10 °C/min). 3.2. Catalyst preparation Mesoporous silica (SBA-15) supported molybdenum catalyst (MoO3/SBA-15) was synthesized by employing an incipient impregnation method. Briefly, 4.00g of SBA-15 was dispersed in 100 mL of deionized water and sonicated for 0.5h in order to obtain stable colloidal solution. To the above colloidal solution, a sufficient amount of ammonium molybdate (NH4)6Mo7O24.4H2O was loaded under constant stirring so as to obtain 3 wt.% molybdenum metal catalyst. The mixture was then dried by slow evaporation at 60-70 °C for 2h under constant stirring and followed by long term drying at 100 °C for 12h. The dried precursor was finally calcined in a muffle furnace at 550 °C (10 °C/min) for 5 h under static air 6


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condition. In order to confirm the Mo loading onto the SBA-15 support, ICP-OES (inductively coupled plasma-optical emission spectroscopy) was performed and it was found that the Mo content in the support was ca. 3.95 wt.%. 3.3. Hydrothermal treatment of Kraft lignin The hydrothermal depolymerization reaction of Kraft lignin was performed in a 300 mL batch reactor (Hastelloy-C-276 HR-8300) equipped with a reactor controller Parr 4848. The reaction mixture was prepared by dissolving 5.0g of lignin in 150 mL of solvent (H2O or 0.5% NaOH solution) with or without 10 % (w/w) MoO3/SBA-15 catalyst loading. In order to create an inert atmosphere in the reaction vessel, the reactor was purged twice with high-purity N2 and initially sealed at 0MPa. The reactor contents were heated at subcritical conditions and the reactor was kept untouched at 350°C for 1h at a constant stirring speed of 214 rpm. After the reaction completion, the reactor was cooled instantly to room temperature by immersing the lower part of the reactor into an ice-water bath. Finally, after releasing the inside pressure through the relief valve, the reactor was opened for bio-oil collection. 3.4. Lignin depolymerized products separation technique and analysis In this study, lignin depolymerization products were categorized into three categories vis; bio-oil, char, and (gases + others). The gaseous products formed during lignin depolymerization were not collected and analyzed. Therefore, after the reaction was completed, the gaseous products were safely released through the relief valve. Figure 1 shows the schematic of bio-oil isolation technique. The reaction mixture was acidified with 37% HCl solution so as to obtain a pH of ~1.5. Subsequently, the liquid products were separated by adding 150 mL ethyl acetate to the above mixture and agitated at a speed of 300 rpm for 20 min in order to completely extract the liquid part from the solid residue. The solution was then centrifuged and filtered. The ethyl acetate part containing crude oil was recovered from the aqueous phase by employing a separating funnel. Finally, the ethyl acetate soluble products were extracted by using a rotary evaporator. On the other side, the obtained solid residue was then washed with tetrahydrofuran (THF) at several times. The filtrate (THF soluble part) was subjected to rotary evaporation in order to 7


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retrieve the unconverted lignin. Whereas, the filter cake (THF insoluble part) was dried in an oven at 70°C for 6h and weighed. In order to determine the composition of crude oil, it was analyzed by GC-MS equipped with a HP-5MS column (30 m x 0.25 mm x 0.25 µm). The GC-MS was programmed at the same conditions as stated in our previous work.48 Typically, the oven temperature was set at 40 °C for 2 min, followed by further heating to 170 °C with a ramping rate of 10 °C/min for 5 min, then even further heating to 250°C with the same ramping rate for 10 min, and finally held at 310 °C with a constant ramping rate for 10 min. In order to determine the product quantification, the relative response factor was measured as follows:50 Response factor RF =

Relative response factor = Concentration X % =

!"

peak area'( 1 × × concentration)* RRF peak area)*

where, X = the known pure compound, IS = internal standard, and X % = an unknown compound. The various products formed in the reaction vessel during depolymerization reaction were calculated by the following equations: Total bio-crude oil yield (wt.%) = Char (wt.%) =

-./ 0 /12 32452 2 × -./ 0 2 2.

100

-./ 0 78 32452 394 : )2 ;./ 0 213 × -./ 0 2 2.

Unreacted lignin (wt.%) =

-./ 0 78 32452 943 × -./ 0 2 2.

100

100

Gas + others (wt.%) = 100 – (Biocrude oil yield + Char + Unreacted lignin). 4. Results and discussion 4.1 Properties and composition of the Kraft lignin

8


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The proximate and ultimate analysis results of Kraft lignin are provided in Table 1. The C, H, N, S, and O (cal.) compositions of Kraft lignin were found to be 53.68%, 4.95%, 0.52%, 2.05%, and 38.8% respectively. In a previous study,51 it has been reported that Kraft lignin has lower carbon (C) and hydrogen (H) contents and higher nitrogen (N), oxygen (O), and sulfur (S) contents than organosolv lignin (with C, H, N, O, and S contents of 64.3%, 6.7%, 0.3%, 27.8%, and 0%, respectively). It was because, Kraft lignin is generally extracted from black liquor in Kraft pulping process. This process mainly focusses on obtaining high quality cellulose from lignocellulosic biomass upon extensive use of inorganic acid, base or salt.52 As a result, the Kraft lignin structure is heavily modified under these harsh processing conditions. The modification is occurred by the cleavage of α- and β- aryl ether bonds.9 Consequently, Kraft lignin loses some part of α- and β- aryl ether fractions. In addition, during pulping process, Kraft lignin structure is also strongly attracted by the hydrogen sulfide ions.9 On the other hand, organosolv lignin is extracted by using organic solvents such as methanol, ethanol, acetic acid or mixture of methanol-water/ethanolwater.53 In these processes, the α- and β- aryl ether fractions remain unaltered. Therefore, Kraft lignin has relatively lower HHVs (due to higher O/C ratio and lower H/C ratio) in comparison with organosolv lignin. 4.2 Physicochemical properties of MoO3/SBA-15 catalysts 4.2.1 BET data analysis: The textural properties of MoO3/SBA-15 catalyst and mesoporous support matrix (SBA-15) were measured by a BELCAT-A instrument using N2 adsorption at liquid nitrogen temperature. The pore size distribution and pore volume were calculated by desorption profiles of the isotherms using the BJH method. The textural properties of SBA-15 and MoO3/SBA-15 as well as Mo content in the synthesized catalyst are listed in Table 2., and the N2 adsorption-desorption isotherms are given in Figure 2. Initially, the support matrix showed a high specific surface area of 630.33 m2/g with a pore volume of 0.90 cm3/g and an average pore diameter of 5.74 nm. 9


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As expected, when molybdenum was incorporated onto SBA-15 supports, both the SBET value and the total pore volume were decreased and reached at the minimal values of 432.64 m2/g and 0.78 cm3/g, respectively. The deposition of MoO3 on the surface matrix via incipient impregnation decreased the surface area while still retaining the matrix pore structure to a certain extent. The reduction in surface area indicated the deposition of MoO3 particles inside the support matrix (SBA-15) channel. In a recent study,36 it was reported that the deposition of metals/metal species on the SBA-15 support has a positive effect on the reduction of char formation during lignin depolymerization. That was because the partial replacement of silicon ions with metals/metal species in the SBA-15 skeleton could decrease the acidity while increasing the activity of SBA-15 support. In another report,54 it was investigated that during Kraft lignin depolymerization, the repolymerization of reactive species could be significantly promoted over strong catalyst. As expected, in the SBA-15 support matrix, the deposition of MoO3 species might moderate the acidity of SBA-15 which in turn could promote the cleavage of β-O-4 and α-O-4 ether bonds in lignin to mono-aromatic units. Therefore, it was expected that MoO3/SBA-15 could exhibit better catalytic activity than SBA-15 during Kraft lignin depolymerization. The nitrogen adsorption-desorption isotherms (see Figure 2) were examined in order to understand the pore structure of the support materials. The adsorption-desorption isotherms of SBA-15 and MoO3/SBA15 catalyst showed a type iv isotherm with H1 hysteresis loop, which is a typical characteristic of a mesoporous structure of SBA-15, as per IUPAC.55 N2 adsorption-desorption isotherms evidently supports the decrease in the average pore size of the support matrix due to void blockage upon MoO3 loading onto SBA-15 matrix. 4.2.2 XPS data analysis: To determine the phases formed during catalyst synthesis, X-ray photoelectron spectroscopy (XPS) analysis was performed with a Perkin-Elmer PHI-1600 spectrometer using monochromatic Mg Kα radiation. The X-ray Photoelectron Spectrum (XPS) of Si (2p), O(1s), and Mo (3d) are shown in Figure (3a-3d). The identification of oxidation state of Mo (3d5/2 and 3d3/2) in the synthesized catalyst is 10


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primarily based on the binding energies (BE) spin-orbit components. Figure 3d shows the Mo spin-orbit components (3d5/2, 3d3/2). The peaks identified at 232.73 eV and 235.71 eV, corresponding to Mo (VI) 3d5/2 and Mo (VI) 3d3/2 respectively, were associated with the spin orbit splitting of Mo (VI).56, 57 The shift in binding energy from lower to higher energy states was attributed towards strong interactions between hydroxyl groups of silica matrix and the molybdenum species. It has been reported that the stronger the interaction between Mo and the support matrix, the higher the binding energy.58 The higher binding energy of Mo is related to oxidized form i.e., Mo VI species.59 On the other side, the principal and shake-up satellite peaks for O1s is in the range of 532.02-534.29 eV, indicating different forms of oxygen with Mo and Si in the synthesized catalyst (see Figure 3c). The Si (2p) binding energy (BE) at 103.82 eV, and O (1s) binding energy (BE) from 532.02 eV to 534.29 eV are assigned to the O-Si-O and O-Si from SBA-15 phase, respectively.60 The XPS results of Mo (3d), O (1s), and Si (2p) samples confirms the presence of Mo in +6 oxidation states onto the support material (SBA-15). The binding energy (BE) values of Mo 3d ranging from 232.73-235.71 eV are characteristics of the molybdenum oxide (MoO3) phase.61, 62 The BE of Mo 3d spin-orbit is very similar to MoO3. These results are also similar for the Mo oxo-species in its highest oxidation state (VI).63 Atomic weight analysis reveals that Molybdenum (VI) oxide (MoO3) loading onto SBA-15 support matrix was about 3 wt.%. 4.2.3 HR-TEM data analysis: In order to gather further information about the synthesized catalyst, HR-TEM images were taken using a HR-TEM micrograph. The HR-TEM images of SBA-15 and MoO3/SBA-15 catalyst is shown in Figure 4a. Lots of parallel channels which look like the pores of the support matrix (SBA-15) are easily seen in Figure 4a. From Figure 4b, it is evident that during surface modifications of SBA-15, the parallel channels were retained even after the introduction of MoO3. All of the above analytical evidences support the successful synthesis of MoO3/SBA-15 catalyst.

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4.2.4 ICP-OES data analysis: To estimate the deposition/content of Mo in the support matrix, inductively coupled plasma-optical emission spectroscopy (ICP-OES) was investigated by following aqua regia (HNO3 + HCl; 1:3 ratio) method. It was observed that the element Mo (Molybdenum) was well anchored to the SBA-15 support. After impregnating the SBA-15 support with Mo species, the metal loading (Mo) was found to be ca. 3.95 wt.% on the supporting material (SBA-15). 4.3 Performance of MoO3/SBA-15 catalyst over lignin depolymerization In order to understand the catalytic efficiency of MoO3/SBA-15 catalyst for Kraft lignin depolymerization, experiments were performed at different reaction conditions. The products obtained from Kraft lignin degradation at different experimental conditions are listed in Table 3. At the outset, reaction was carried out in subcritical water in the absence of catalyst at 350 °C for 1h under hydrothermal liquefaction. As a green solvent, water was used as the reaction medium. Moreover, the hydrothermal liquefaction process exhibits water at subcritical (water at 100–374 °C and 22.1MPa) phase. Under this condition, water possesses significantly different physical properties, including in term of density, values of dielectric constant, pH, and ion product.64 The dielectric constant of water decreases with increase in temperature, for instance: water exhibits dielectric constants of 78 and 21 at 25 °C and 300 °C, respectively.65 Low dielectric constant increases the ion product (Kw) of water under subcritical conditions which leads to an increase in the concentrations of H+ and OH− ions in the reaction medium. The increases in the concentrations of H+ and OH− ions enhance the yield of acid/base catalyzed reactions.23, 65 Under the aforementioned experimental conditions, crude bio-oil yields of approximately 44.60 wt.% (sum of aqueous phase oil and solid phase oil) was obtained. The GC-MS data of the crude oil indicated the presence of high molecular weight compounds. When the synthesized MoO3/SBA-15 catalyst was used in the above reaction medium under subcritical conditions, the crude bio-oil yields increased by only 2.43 wt.%. While the lignin derived products still had high molecular weight fragments with higher amount of unconverted lignin and char formation (sum of 12


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unconverted lignin and char formation accounted for ~19.60 wt.%) (see Table 3). Finally, the catalytic activity of MoO3/SBA-15 for lignin degradation under alkaline conditions was performed. Recently, base catalyzed depolymerization has emerged as an efficient pathway for delignification.11 This is attributed towards higher solubility of lignin in alkaline medium than normal water, which in turn, facilitates the depolymerization reaction. High solubility of lignin in reaction medium favors high yields of fragmented products from catalytic lignin depolymerization. In the present study, 0.5% NaOH aqueous solution was used to depolymerize lignin under the same reaction conditions as used previously, but without the catalyst. In this case, the crude oil yield increased to about 45.70 wt.% compared with subcritical water but was still lower than that obtained for 10 % (w/w) MoO3/SBA-15 loadings at the same conditions. Upon using 10 % (w/w) MoO3/SBA-15 catalyst, the crude oil yield increased up to ~56.40 wt.%, whereas the yield of char formation and unconverted lignin were significantly decreased from 12.60 and 8.40 wt.% (in Sub-H2O) to ~8.60 and 4.20 wt.% (in basic sub-H2O), respectively (see Table 3 or Figure 5). The highest bio-oil yield using 0.5% NaOH solution may be due to the hydrolysis and hydrogenolysis effect of MoO3/SBA-15 catalyst. It is expected that MoO3/SBA-15 catalyst accelerates the base catalyzed hydrolysis reaction of lignin. As a result, the total bio-oil yield was augmented. The support matrix SBA15 could also have a positive effect in reducing the char formation during lignin depolymerization. This was due to its large pore size and well-ordered structure that enhanced the activity of MoO3 catalyst which in turn suppressed the repolymerization of reactive monomers. 4.4 Effect of different reaction conditions on lignin depolymerization The solvent plays a vital role in lignin depolymerization reactions. As can be seen in Figure 5, the lignin depolymerization was greatly influenced by MoO3/SBA-15 catalyst at two different solvents. When the reaction was carried out in subcritical water at 350 °C for 1h, only 44.60 % yield was achieved. The lower crude oil yield was due to the poor solubility of Kraft lignin in subcritical water. In the alkaline medium, the degree of lignin solubility increased significantly, which accelerated the percentage conversion of 13


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lignin into low molecular weight compounds. In this report, the addition of base (0.5% NaOH) in water as solvent medium was to enhance the degree of depolymerization efficiency of the catalyst. It has been reported that water is more efficient in basic medium for the depolymerization of lignin than other solvents, such as methanol, ethanol, 1,4 dioxane, and tetrahydrofuran etc.66 Additionally,11,

51

the

alkalinity of the reaction medium has significant impact on lignin depolymerization. When the depolymerization reaction was performed in subcritical water at pH 6.99 (before reaction) without catalyst, the yield of crude oil was 44.60 wt.% with high char formation (about 12.60 wt.%) at pH 6.44 (after reaction) (see Table 3). But, in the presence of MoO3/SBA-15 catalyst, the initial pH slightly decreased from 6.99 to 5.38. (before reaction) and after completing the reaction the pH was 5.12 with ~ 47.03 wt.% crude oil yields and ~11.80 wt.% char formation. The decrease in initial pH was attributed as MoO3/SBA-15 possesses acidic sites,36 therefore, its addition to the lignin solution, the pH dropped from 6.99 to 5.38. The further decrease in pH value from 5.38 to 5.12 was due to the fact that upon completing the depolymerization, phenolic compounds were produced which are slightly acidic properties. Alternatively, when the reactor was fed with 0.5% NaOH solution to achieve high pH value (~11.85) (before reaction), the crude oil yield of 45.70 wt.% at pH 8.70 (after reaction) was obtained which was higher than that obtained employing subcritical water. This clearly indicated that an increase in pH value had a positive effect on the lignin depolymerization reaction. When MoO3/SBA-15 catalyst was inserted into the medium, the initial pH of the system (~11.85) slightly decreased to 11.33. Upon completion of the reaction, bio-oil yield of ~56.40 wt.% with minimum char formation ~8.60 wt.% at pH 8.10 (after reaction) was achieved. From the obtained results, it was found that MoO3/SBA-15 catalyst plays vital role for making the solution pH ~ 8.10, at which the reactive monomers get stabilized. In a previous study,67 it has been reported that pH plays vital role for reducing the char formation during Kraft lignin depolymerization, for instance, high pH favors minimum char formation (about 8.1-8.9, after reaction).

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Besides solvents and pH, temperature also plays an important role during delignification employing hydrothermal liquefaction process. In this study, all the experiments were performed at 350 °C. This is because, in our previous study,48 the highest yield of crude oil (~ 44.6 wt.%) along with lowest amount of solid residue (~ 27.4 wt.%) was obtained at 350 °C. As can be seen in Table 3, in the current study, the maximum amount of crude oil was found (~56.40 wt.%) with minimum unconverted lignin and char formation (~ 4.20 and 8.60 wt.%) under alkaline medium in the presence of MoO3/SBA-15 catalyst. This is attributed towards the cleavage of C-O-C ether bonds by hydrolysis and hydrodeoxygenation effect of MoO3/SBA-15 catalyst during hydrothermal conversion of Kraft lignin under high temperature and alkaline conditions. 4.5. Bio-oil Characterization The elemental composition of crude bio-oil derived from Kraft lignin depolymerization is shown in Table 4. It can be seen from the elemental analysis that crude bio-oil possess high hydrogen (H) and carbon (C) contents while, low sulfur (S) and nitrogen (N) contents compared with original lignin (see Table 1 & Table 4). The H/C and O/C ratios of original Kraft lignin were 0.09 % and 0.72 %, respectively, whereas, the HHV (higher heating value) was accounted for at 19.05 MJ/Kg. The H/C ratio of bio-oil obtained from hydrothermal treatment of lignin in sub-H2O increased to 0.10 % while O/C ratio decreased to 0.32%, resulting in an overall increase of HHV to 29.87 MJ/kg compared with original lignin. Additionally, the sulfur (S) and nitrogen (N) contents decreased from 2.05% and 0.52% (in Kraft lignin) to 0.74 % and 0.38% (in sub-H2O, without catalyst) respectively. With the introduction of MoO3/SBA-15 catalyst into sub-H2O, the O/C ratio, sulfur and nitrogen contents further decreased to 0.31%, 0.32% and 0.21% respectively followed by an increase in HHV’s to 30.09 MJ/Kg. This clearly indicates that the catalyst had a positive effect on lignin depolymerization by accelerating the hydrolysis reaction. The O/C and H/C ratios of the bio-oil were also affected by changing the solvent system, for instance, the O/C ratio of the bio-oil decreased from 0.31% (in sub-H2O, with catalyst) to 0.29% (in alkaline solution, with catalyst), resulting in further increment of HHV from 30.09 to 32.00 MJ/Kg. This suggest that 15


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MoO3/SBA-15 catalyst further enhanced hydrolysis and hydrogenolysis reactions under basic medium during lignin depolymerization. Overall, the combined effect of MoO3/SBA-15 and 0.5% NaOH solution, increased the carbon and hydrogen contents in the crude bio-oil to 71.18% and 7.97% respectively, whereas, the nitrogen and sulfur contents were reduced from 0.52% and 2.06% (in original lignin) to 0.11% and 0.03%, respectively. After catalytic reaction, the oxygen (O) content in the original lignin was significantly reduced from 38.8% to 20.7% which in turn, improved the HHV from 19.05 MJ/Kg to 32.00 MJ/Kg. High HHV value indicated that better quality of bio-oil was produced after catalytic depolymerization of lignin in alkaline medium. The HHV’s of crude bio-oil obtained from different reaction conditions were calculated by using Dulong’s Formula:68

HHV (MJ/Kg) =

.D × ;. %G ABB

In order to identify monoaromatic phenolic compounds, the isolated crude oil was analyzed by GC–MS. It was found that MoO3/SBA-15 catalyst not only increased the bio-oil yields, but also reduced the char formation with base-catalyzed hydrolysis and demethylation reactions during lignin depolymerization reaction. Furthermore, the GC-MS analysis inferred that the produced crude bio-oil contained lower molecular weight phenolic monomers such as mono-phenols, alkyl-phenols, guaiacols, alkyl-guaiacols, catechol, and alkyl-catechol, along with traces of other compounds. 4.6 Analysis of Monomer Yields (GC-MS) Monomers produced from lignin depolymerization were quantified by GC-MS. Table 5 shows the products (by relative % peak area) obtained upon hydrothermal treatment of Kraft lignin at different experimental conditions. The results suggest that different kinds of monoaromatic phenolic monomers were produced at different experimental conditions. The monomeric components present in the bio-oil mainly consisted of substituted phenols, including methyl-, ethyl-, and methoxy-, as substituted groups in 16


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the benzene core unit with guaiacol and catechol monomers being the major compounds (see Table 6). In our previous study,48 it was reported that low temperature (up to 300°C), favors guaiacol production while other phenolic monomers were produced in lower amount. Additionally, upon increasing the temperature from 200 °C to 350 °C, catechol type monomers were produced in a large amount. Furthermore, it was also observed that some compounds such as homovallinic acid, acetovanillone, vanillin, and syringaldehyde were enriched at lower temperature. In this work, MoO3/SBA-15 catalyst significantly increased the production of substituted phenolic monomers in comparison with control experiments (without catalyst) at 350 °C temperature. This indicates that MoO3/SBA-15 catalyst actively favored C-OC bond cleavage while subsequently transforming the cleaved components into phenolic monomers. When the experiments were carried out in sub-H2O, guaiacol and catechol type monomers were produced in higher amount than that of other phenolic monomers (see Figure 6). It is also noticeable that the percentage of guaiacol type monomers decreased with increase in the percentage of catechol type monomers (see Figure 6 & Table 6). This could be attributed towards hydrogenolysis or demethylation of guaiacol during depolymerization of lignin in the presence of synthesized catalyst. From Figure 6, it is evident that both the introduction of MoO3/SBA-15 catalyst and 0.5% NaOH solution had positive impacts for the formation of low molecular weight phenolic compounds. However, the addition of 0.5% NaOH had little influence on the production of monomeric compounds. Figure 7 shows plausible reaction mechanism for the formation of different phenolic monomers. As can be seen from Figure 6, the yield of monophenol was higher in basic medium (about 5.00% relative peak area) in the presence of MoO3/SBA-15 catalyst. Monophenol could be formed either from demethoxylation of guaiacol or demethylation of guaiacol via catechol as an intermediate (see Figure 7). On the other hand, substituted monophenols such as 2-and 4-methyl phenols were only formed in both solvent medium in the presence of catalyst (see Table 6). Moreover, in basic medium, the percent yield of

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these monoaromatic phenolic monomers was slightly higher than that under sub-H2O conditions. The substituted monophenol may be the alkylated product of phenol. The maximum yield of guaiacol was achieved in sub-H2O (~19.50% relative peak area) without catalyst, whereas the minimum yield was observed in basic medium (~2.41% relative peak area) with catalyst. Alternatively, the yield of catechol was higher than other monomers under all reaction conditions (see Figure 8). Alkyl-guaiacols such as 6-methyl-guaiacol, 4-methyl-guaiacol, 4-propyl-guaiacol and 4-ethoxymethylguaiacol, in sub-H2O with or without catalyst were produced in a large amount in comparison with that formed in basic medium at 350 °C. In contrast, the yields of alkyl-catechols including 3-methyl-catechols, 4-methyl-catechols and 4-ethyl -catechol were high in basic medium with or without the catalyst (see Table 6). Among alkyl-catechols, 3-methyl-catechol (total 2.99 % relative peak area) were only found in sub-H2O conditions. Whereas, the production of 4-methyl-catechols and 4-ethyl-catechols were observed in both solvent medium with a gradual increase of the yield from 10.35 % to 27.02 % (see Figure 8). The product namely 1,4 Benzenediol, 2-methyl was also observed only in basic medium with or without the catalyst. GC-MS analysis of the liquid fraction demonstrated that the major components were primarily substituted monoaromatic phenolic monomers. In addition to guaiacols and catechol type compounds, small amount of other compounds such as ethyl homovanillate, phenanthrene, and 1-methyl-7-(1-methylethyl) were also produced only in sub-H2O without catalyst. 5. Conclusions Kraft lignin was converted to low molecular weight phenolic monomers in an environmentally benign HTL process using alkaline water as a solvent medium. MoO3 supported on SBA-15 has been applied for screening its catalytic efficiency for Kraft lignin depolymerization under hydrothermal treatment. It was observed that guaiacol and catechol were the most abundant phenolic monomers in sub-H2O, whereas 18


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catechol was the dominant monoaromatic phenolic monomers in basic-subcritical water at 350 °C. It is believed that in base-catalyzed reaction, MoO3 plays vital role in minimizing lignin repolymerization while reducing char formation, thus enabling the formation of low molecular weight compounds. As expected, the synthesized catalyst not only reduced the oxygen content but also improved the hydrolysis and hydrogenolysis process while producing high bio-oil yields (~56.40 wt.%) enriched with low molecular weight phenolic monomers. Most importantly, under the present catalyst, the char formation was reduced to ~8.60 %wt. Further, the HHV of the bio-oil increased from 19.05 MJ/kg for Kraft lignin to 32.00 MJ/kg for the depolymerization products. Acknowledgment This work was supported by a grant (NRF-2016R1A2B4008115) from the National Research Foundation of the Republic of Korea.

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(32) Nguyen, T. D. H.; Maschietti, M.; Belkheiri, T.; Åmand, L.-E.; Theliander, H.; Vamling, L.; Olausson, L.; Andersson, S.-I. Catalytic depolymerisation and conversion of Kraft lignin into liquid products using near-critical water. J. Supercrit. Fluids 2014, 86, 67-75. (33) Karagöz, S.; Bhaskar, T.; Muto, A.; Sakata, Y.; Oshiki, T.; Kishimoto, T. Low-temperature catalytic hydrothermal treatment of wood biomass: analysis of liquid products. Chem. Eng. J. 2005, 108 (1), 127137. (34) Azadi, P.; Inderwildi, O. R.; Farnood, R.; King, D. A. Liquid fuels, hydrogen and chemicals from lignin: A critical review. Renewable Sustainable Energy Rev. 2013, 21, 506-523. (35) Ganiyu, S. A.; Alhooshani, K.; Ali, S. A. Single-pot synthesis of Ti-SBA-15-NiMo hydrodesulfurization catalysts: Role of calcination temperature on dispersion and activity. Appl. Catal. B Environ. 2017, 203, 428-441. (36) Chen, P.; Zhang, Q.; Shu, R.; Xu, Y.; Ma, L.; Wang, T. Catalytic depolymerization of the hydrolyzed lignin over mesoporous catalysts. Bioresour. Technol. 2017, 226, 125-131. (37) Gonçalves, V. O. O.; Ciotonea, C.; Arrii-Clacens, S.; Guignard, N.; Roudaut, C.; Rousseau, J.; Clacens, J.-M.; Royer, S.; Richard, F. Effect of the support on the hydrodeoxygenation of m-cresol over molybdenum oxide based catalysts. Appl. Catal. B Environ. 2017, 214, 57-66. (38) Ma, Z.; Troussard, E.; van Bokhoven, J. A. Controlling the selectivity to chemicals from lignin via catalytic fast pyrolysis. Appl. Catal. A Gen. 2012, 423-424, 130-136. (39) Grilc, M.; Likozar, B.; Levec, J. Hydrodeoxygenation and hydrocracking of solvolysed lignocellulosic biomass by oxide, reduced and sulphide form of NiMo, Ni, Mo and Pd catalysts. Appl. Catal. B Environ. 2014, 150-151, 275-287. (40) Veryasov, G.; Grilc, M.; Likozar, B.; Jesih, A. Hydrodeoxygenation of liquefied biomass on urchinlike MoS2. Catal. Commun. 2014, 46, 183-186.

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(41) Chen, M.; Hao, W.; Ma, R.; Ma, X.; Yang, L.; Yan, F.; Cui, K.; Chen, H.; Li, Y. Catalytic ethanolysis of Kraft lignin to small-molecular liquid products over an alumina supported molybdenum nitride catalyst. Catal. Today 2017, 298, 9-15. (42) Chen, M.; Ma, X.; Ma, R.; Wen, Z.; Yan, F.; Cui, K.; Chen, H.; Li, Y. Ethanolysis of Kraft Lignin over a Reduction-Modified MoO3 Catalyst. Ind. Eng. Chem. Res. 2017, 56 (47), 14025-14033. (43) Grilc, M.; Veryasov, G.; Likozar, B.; Jesih, A.; Levec, J. Hydrodeoxygenation of solvolysed lignocellulosic biomass by unsupported MoS2, MoO2, Mo2C and WS2 catalysts. Appl. Catal. B Environ. 2015, 163, 467-477. (44) Xu, L.; Han, Z.; Zhang, Y.; Fu, Y. In situ synthesis of molybdenum oxide@N-doped carbon from biomass for selective vapor phase hydrodeoxygenation of lignin-derived phenols under H2 atmosphere. RSC Adv. 2016, 6 (110), 108217-108228. (45) Xiao, L.-P.; Wang, S.; Li, H.; Li, Z.; Shi, Z.-J.; Xiao, L.; Sun, R.-C.; Fang, Y.; Song, G. Catalytic Hydrogenolysis of Lignins into Phenolic Compounds over Carbon Nanotube Supported Molybdenum Oxide. ACS Catal. 2017, 7 (11), 7535-7542. (46) Jin, L.; Li, W.; Liu, Q.; Wang, J.; Zhu, Y.; Xu, Z.; Wei, X.; Zhang, Q. Liquefaction of kraft lignin over the composite catalyst HTaMoO6 and Rh/C in dioxane-water system. Fuel Process. Technol. 2018, 178, 62-70. (47) Cui, K.; Yang, L.; Ma, Z.; Yan, F.; Wu, K.; Sang, Y.; Chen, H.; Li, Y. Selective conversion of guaiacol to substituted alkylphenols in supercritical ethanol over MoO3. Appl. Catal. B Environ. 2017, 219, 592-602. (48) Islam, M. N.; Taki, G.; Rana, M.; Park, J.-H. Yield of Phenolic Monomers from Lignin Hydrothermolysis in Subcritical Water System. Ind. Eng. Chem. Res. 2018, 57 (14), 4779-4784. (49) Yang, Y.; Lv, G.; Deng, L.; Lu, B.; Li, J.; Zhang, J.; Shi, J.; Du, S. Renewable aromatic production through hydrodeoxygenation of model bio-oil over mesoporous Ni/SBA-15 and Co/SBA-15. Microporous Mesoporous Mater. 2017, 250, 47-54. 24


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(50) Singh, S. K.; Nandeshwar, K.; Ekhe, J. D. Thermochemical lignin depolymerization and conversion to aromatics in subcritical methanol: effects of catalytic conditions. New J. Chem. 2016, 40 (4), 36773685. (51) Limarta, S. O.; Ha, J.-M.; Park, Y.-K.; Lee, H.; Suh, D. J.; Jae, J. Efficient depolymerization of lignin in supercritical ethanol by a combination of metal and base catalysts. J. Ind. Eng. Chem. 2018, 57, 45-54. (52) Zhu, J. Y.; Pan, X.; Zalesny, R. S. Pretreatment of woody biomass for biofuel production: energy efficiency, technologies, and recalcitrance. Appl. Microb. Biotechnol. 2010, 87 (3), 847-857. (53) Xu, C.; Ferdosian, F. Structure and Properties of Lignin. In Conversion of Lignin into Bio-Based Chemicals and Materials, Springer Berlin Heidelberg: Berlin, Heidelberg, 2017; pp 1-12. (54) Shu, R.; Long, J.; Yuan, Z.; Zhang, Q.; Wang, T.; Wang, C.; Ma, L. Efficient and product-controlled depolymerization of lignin oriented by metal chloride cooperated with Pd/C. Bioresour. Technol. 2015, 179, 84-90. (55) Meynen, V.; Cool, P.; Vansant, E. F. Synthesis of siliceous materials with micro- and mesoporosity. Microporous Mesoporous Mater. 2007, 104 (1), 26-38. (56) Li, Z.; Gao, L.; Zheng, S. Investigation of the dispersion of MoO3 onto the support of mesoporous silica MCM-41. Appl. Catal. A Gen. 2002, 236 (1), 163-171. (57) Jaroszewska, K.; Masalska, A.; Marek, D.; Grzechowiak, J. R.; Zemska, A. Effect of support composition on the activity of Pt and PtMo catalysts in the conversion of n-hexadecane. Catal. Today 2014, 223, 76-86. (58) Montesinos-Castellanos, A.; Zepeda, T. A.; Pawelec, B.; Fierro, J. L. G.; de los Reyes, J. A. Preparation, Characterization, and Performance of Alumina-Supported Nanostructured Mo−Phosphide Systems. Chem. Mater. 2007, 19 (23), 5627-5636.

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(59) Cordova, A.; Blanchard, P.; Lancelot, C.; Frémy, G.; Lamonier, C. Probing the Nature of the Active Phase of Molybdenum-Supported Catalysts for the Direct Synthesis of Methylmercaptan from Syngas and H2S. ACS Catal. 2015, 5 (5), 2966-2981. (60) Boufaden, N.; Akkari, R.; Pawelec, B.; Fierro, J. L. G.; Said Zina, M.; Ghorbel, A. Dehydrogenation of methylcyclohexane to toluene over partially reduced Mo–SiO2 catalysts. Appl. Catal. A Gen. 2015, 502, 329-339. (61) Anwar, M.; Hogarth, C. A.; Bulpett, R. An XPS study of amorphous MoO3/SiO films deposited by co-evaporation. J. Mater. Sci. 1990, 25 (3), 1784-1788. (62) A., T. Practical surface analysis, 2nd edn., vol I, auger and X‐ray photoelectron spectroscopy. Edited by D. Briggs & M. P. Seah, John Wiley, New York, 1990; pp 657. (63) Grim, S. O.; Matienzo, L. J. X-ray photoelectron spectroscopy of inorganic and organometallic compounds of molybdenum. Inorg. Chem. 1975, 14 (5), 1014-1018. (64) Yang, S.; Yuan, T.-Q.; Li, M.-F.; Sun, R.-C. Hydrothermal degradation of lignin: Products analysis for phenol formaldehyde adhesive synthesis. Int. J. Biol. Macromol. 2015, 72, 54-62. (65) Akiya, N.; Savage, P. E. Roles of Water for Chemical Reactions in High-Temperature Water. Chem. Rev. 2002, 102 (8), 2725-2750. (66) Ouyang, X.; Ruan, T.; Qiu, X. Effect of solvent on hydrothermal oxidation depolymerization of lignin for the production of monophenolic compounds. Fuel Process. Technol. 2016, 144, 181-185. (67) Belkheiri, T.; Mattsson, C.; Andersson, S.-I.; Olausson, L.; Åmand, L.-E.; Theliander, H.; Vamling, L. Effect of pH on Kraft Lignin Depolymerisation in Subcritical Water. Energy Fuels 2016, 30 (6), 49164924. (68) Theegala, C. S.; Midgett, J. S. Hydrothermal liquefaction of separated dairy manure for production of bio-oils with simultaneous waste treatment. Bioresour. Technol. 2012, 107, 456-463.

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List of Figures

Figure 1. Lignin depolymerization products separation technique.

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Figure 2. N2 adsorption-desorption isotherms of support SBA-15 (a) and synthesized catalyst MoO3/SBA15 (b)

Figure 3. XPS spectra of MoO3/SBA-15 (a); Si 2p1/2 (b); O1s (c); Mo 3d (5/2, 3/2) (d)

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Figure 4. HR-TEM images of SBA-15 (a) and MoO3 supported SBA-15 (b)

Figure 5. Overall yields of lignin degradation products at different experimental conditions.

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Energy & Fuels

Figure 6. Yield of grouped products at different experimental conditions.

Figure 7. Plausible reaction mechanism for different monomers formation.

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Figure 8. Major monoaromatics phenolic monomers produced from lignin fragmentation over MoO3/SBA-15 and 0.5% NaOH solvent at 350 °C for 1h.

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Energy & Fuels

List of Tables Table 1. Proximate and ultimate analysis of original Kraft lignin.

Proximate analysis Ultimate analysis wt.%

HHV (MJ/Kg)

wt.%

MC

FC

Ash

C

H

N

S

O (cal.)

O/C

H/C 19.05

5.67

-

3.85

53.68

4.95

0.52

2.05

38.80

32


0.72

0.09

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Table 2. Textural properties of SBA-15 support, MoO3/SBA-15 catalyst and Molybdenum content of the catalyst.

Catalyst

SBET (m2/g)

Average pore diameter

Mo content by ICP

(cm /g)

(nm)

(wt.%)

Total pore volume 3

SBA-15

630.33

0.90

5.74

N/A

MoO3/SBA-15

432.64

0.78

5.52

3.95

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Energy & Fuels

Table 3. Results and different experimental conditions for Kraft lignin depolymerization reactions with or without catalyst. Water No catalyst Lignin feed (g)

0.5% NaOH

MoO3/SBA-15

No catalyst

MoO3/SBA-15

5.00

5.00

5.00

5.00

Catalyst loading(g)

-

0.50

-

0.50

Mixture pH (before)

6.99

5.38

11.85

11.33

Mixture pH (after reaction)

6.44

5.12

8.70

8.18

Bio-oil (g)

2.23

2.35

2.28

2.82

Char (g)

0.63

0.59

0.62

0.43

Unreacted lignin (g)

0.42

0.39

0.41

0.21

Yield of bio-oil (%)

44.60

47.03

45.70

56.40

Yield of char (%)

12.60

11.80

12.40

8.60

Yield of unreacted lignin (%)

8.40

7.80

8.20

4.20

Gases + others (%)

34.37

33.29

33.75

30.84

Other experimental condition: 0.5g catalyst, 350 °C, 1h, N2 atmosphere.

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Table 4. Elemental analysis and HHV (MJ/Kg) values for lignin derived bio-oil at different experimental conditions with or without catalyst. S Entry

Catalyst

C (%)

H (%)

O (%)

N (%)

HHV O/C

H/C

(%)

(MJ/Kg)

1

NC-W

69.35

7.09

22.43

0.38

0.74

0.32

0.10

29.87

2

MoO3/SBA-15-W

70.45

6.95

22.07

0.21

0.32

0.31

0.10

30.09

3

NC-NaOH

71.23

6.93

21.55

0.14

0.14

0.30

0.09

30.40

71.18

7.97

20.71

0.11

0.03

0.29

0.11

32.00

MoO3/SBA-154 NaOH Other experimental condition: 5.0g lignin, 0.5g catalyst, 350 °C, 1h, N2 atmosphere

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Table 5. Yields of the products (by relative % peak area) obtained from Kraft lignin depolymerization over two solvents with or without catalyst. Subcritical water

0.5% NaOH

No catalyst 1.98

No catalyst 3.08

RT (min) 7.152

Phenol

8.367

Phenol, 2-methyl-

-

0.78

-

1.26

8.696

Phenol, 4-methyl-

-

1.19

-

1.99

8.940

Phenol, 2-methoxy-

19.50

17.54

10.331

2-Methoxy-6-methylphenol

1.17

1.00

-

-

10.537

2-Methoxy-4-methylphenol

7.02

8.25

-

-

10.600

1,2-Benzenediol

26.76

28.83

35.16

42.07

11.499

1,4-Benzenediol, 2-methyl-

-

-

7.39

8.19

11.505


0.94

2.05

-

-

11.790

Phenol, 4-ethyl-2-methoxy-

9.20

9.70

-

-

11.907


10.35

10.36

24.98

27.02

12.621

1,3-Benzenediol, 4-ethyl-

-

0.78

2.53

-

12.774

1,3-Benzenediol, 4,5-dimethyl-

-

-

3.85

2.94

12.996

Phenol, 2-methoxy-4-propyl-

3.81

3.50

-

-

13.176

1,2-Benzenediol, 4-ethyl-

6.53

5.57

7.97

7.57

14.355

(S)-(+)-5-sec-Butyl-2-pyrimidinol

2.04

1.91

-

-

15.090

Ethyl homovanillate

-

3.05

-

-

15.096

2-methoxy-4-propyl-phenol

1.64

-

-

-

16.756

Phenol, 4-(ethoxymethyl)-2-methoxy-

1.99

-

-

-

1.54

-

-

-

94.49

97.40

95.39

98.44

26.604 Total

Compound name

Phenanthrene, methylethyl)-

1-methyl-7-(1-

MoO3/SBA15 2.89

Other experimental condition: 5.0g lignin, 0.5g catalyst, 350 °C, 1h, N2 atmosphere.

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10.42

MoO3/SBA15 5.00

2.41

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Table 6. Yield of phenolic monomers (by relative % peak area) found in the bio-oil. Subcritical water MoO3/ No SBAcatalyst 15

Groups

Compounds Name

Monophenol

Phenol

1.98

Phenol, 2-methylAlkyl-phenols

Guaiacol

Alkyl-guaiacols

Catechol

Alkyl-catechols

0.5% NaOH No catalyst

MoO3/SBA15

2.89

3.08

5.00

-

0.78

-

1.26

Phenol, 4-methyl-

-

1.19

-

1.99

Total

-

1.97

-

3.25

Phenol, 2-methoxy-

19.50

17.54

10.42

2.41

2-Methoxy-6methylphenol

1.17

1.00

-

-

2-Methoxy-4methylphenol

7.02

8.25

-

-

Phenol, 4-ethyl-2methoxy-

9.20

9.70

-

-

3.81

3.50

-

-

1.99

-

-

-

-

-

Phenol, 2-methoxy-4propylPhenol, 4-(ethoxymethyl)2-methoxy2-methoxy-4-propylphenol Total

1.64

-

24.83

22.45

1,2-Benzenediol

26.76

28.83

35.16

42.07

1,2-Benzenediol, 3methyl-

0.94

2.05

-

-

1,2-Benzenediol, 4methyl-

10.35

10.36

24.98

27.02

1,2-Benzenediol, 4-ethylTotal

6.53 17.82

5.57 17.98

7.97 32.95

7.57 34.59

Other experimental condition: 5.0g lignin, 0.5g catalyst, 350 °C, 1h, N2 atmosphere.

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Production of Phenol-rich Monomers from Kraft Lignin

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