Catalytic Conversion of Organosolv Lignins to Phenolic Monomers in

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Catalytic Conversion of Organosolv Lignins to Phenolic Monomers in Different Organic Solvents and The Effect of Operating Conditions on Yield with MIBK Wanwitoo Wanmolee, Navadol Laosiripojana, Pornlada Daorattanachai, Lalehvash Moghaddam, Jorge Rencoret, José C. Carlos del Río, and William O. S. Doherty ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02721 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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ACS Sustainable Chemistry & Engineering

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Catalytic Conversion of Organosolv Lignins to

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Phenolic Monomers in Different Organic Solvents

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and The Effect of Operating Conditions on Yield

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with MIBK

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Wanwitoo Wanmolee, † Navadol Laosiripojana,†,‡ Pornlada Daorattanachai,† Lalehvash

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Moghaddam, § Jorge Rencoret,∥ José C del Río,∥ and William O. S. Doherty*, § †

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University of Technology Thonburi, Prachauthit Road, Bangmod, Bangkok, 10140 Thailand ‡

BIOTEC-JGSEE Integrative Biorefinery Laboratory, Innovation Cluster 2 Building, 113

Thailand Science Park, Phahonyothin Road, Khlong Luang, Pathumthani 12120, Thailand §

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The Joint Graduate School of Energy and Environment (JGSEE), King Mongkut’s

Centre for Tropical Crops and Biocommodities, Queensland University of Technology (QUT), GPO Box 2432, 2 George St., Brisbane, Australia

∥

Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS), CSIC, Av. Reina

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Mercedes, 10, 41012-Seville, Spain

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*Corresponding author

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Postal address: GPO Box 2432, 2 George St, Brisbane, QLD 4001, Australia.

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Tel.: +61 7 3138 1245; fax: +61 7 3138 4132.

1 ACS Paragon Plus Environment

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*Corresponding author’s E-mail: [email protected] (W.O.S. Doherty)

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KEYWORDS: Lignin, catalyst, depolymerization, zeolites, phenolic monomers, MIBK

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solvent

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ABSTRACT

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Catalytic depolymerization of organosolv lignin to phenolic monomers with zeolites was

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investigated under various operating conditions. H-USY (Si/Al molar ratio = 5) outperformed

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H-USY with Si/Al ratios of 50 and 250, H-BEA, H-ZSM5 and fumed SiO2, to produce the

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highest phenolic monomer yield from a commercial organosolv lignin in methanol at 300 °C

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for 1 h. It was then further investigated in the presence of acetone, ethyl acetate, methanol

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and methyl isobutyl ketone (MIBK) on the depolymerization of organosolv bagasse lignin

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(BGL). The total highest phenolic monomer yield of 10.6 wt% was achieved with MIBK at

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350 °C for 1 h with a catalyst loading of 10 wt%. A final total phenolic monomer yield of

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19.4 wt% was obtained with an initial H2 pressure of 2 MPa under similar processing

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conditions. The main phenolic monomers obtained are guaiacol (7.9 wt%), 4-ethylphenol (6.0

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wt%) and phenol (3.4 wt%). The solvent properties were used to account for the differences

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in phenolic monomer yields obtained with the different organic solvents.

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INTRODUCTION

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Increasing consumption of fossil fuels for production of transportation fuels, chemicals and

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materials leads to a number of environmental issues like air pollution and emission of

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greenhouse gases. The utilization of lignocellulosics biomass as a renewable feedstock for the

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production of these products will minimize these issues.1 Lignin represents a major

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component of lignocellulosics and is comprised of three aromatic alcohols, p-coumaryl

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alcohol, coniferyl alcohol, and sinapyl alcohol, linked together by C-O-C and C-C bonds.2

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The composition and the proportion of the lignin substructures vary considerably from

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species to species, as well as the type and quantity of linkages in the polymer and the number

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of methoxy groups present on the aromatic ring.3 These differences in chemical structures,

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and the extraction and purification processes4 impact on lignin conversion to valuable

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chemicals and products.5 In recent years, organosolv pulping process has been widely used

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for the development of biofuels and biochemical technologies. The most commonly used

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solvents are ethanol, acetic acid, formic acid, and peroxyorganic acids.6 The quality of lignins

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generated from organosolv process is superior to other lignins isolated from other extraction

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processes, and the latter lignins are generally of lower molecular weight and have higher

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proportions of reactive sites required for derivation.7 Hence, organosolv lignin has the

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potential to be used for the production of oxygenated aromatics, phenolic resins,

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polyurethanes, polyesters, and carbon fibers.8

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Selective thermochemical depolymerization of lignins to aromatics and fuel additives is

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one of the most challenging research areas because of the diversity of ether and condensed

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linkages such as β-O-4, β-β, β-5, 4-O-5, and dihydrobenzofuran. The heterogeneity in

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aromatic structures and linkages lead to char formation, re-polymerization of products and

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low product yield with poor selectivity.9 Fortunately, the β-O-4 is by far the most

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predominant linkage between the lignin substructures and studies have shown with the use of



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catalysts, solvents and reaction conditions how lignins can be selectively cleaved to reduce

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the multitude of products and increase product yield.

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Zeolite-based catalysts have been shown to possess acid sites which catalyze cracking,

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dehydrogenation, dealkylation, and isomerization and hydrogenolysis of lignins.9 Generally,

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decreasing the Si/Al ratios in zeolites typically increases the acid concentration and acid

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strength. Ma et al.10 using catalytic fast pyrolysis investigated zeolite catalysts with different

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acidity and pore size for the conversion of lignin to phenolics. The catalyst properties played

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key roles in product selectivity and yield. The H-USY catalyst with the largest pore size and

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lowest Si/Al gave the highest aromatic yield. The study conducted by Song et al.

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HZSM-5 showed that the rate of methylcyclohexane conversion to liquid hydrocarbons

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increased with increased Al concentration. Besides the composition of the catalyst, the

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solvent type has been shown to affect lignin depolymerization, reduce char formation and

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minimize repolymerization reactions. Different solvents including water, methanol, 1,4-

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dioxane, tetrahydrofuran, ethanol, and their mixtures were investigated by hydrothermal

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oxidation depolymerization of lignin for the production of monophenolic compounds by

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Ouyang et al.12. They found that up to 17.9% yield of monomeric compounds with the

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highest selectivity of syringyl derived compounds was obtained in 1:1 (v/v) of methanol-

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water compared to the pure solvents under the optimal condition at 150 °C for 60 min. Other

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studies13 in which a methanol-water mixture was used doubled the syringol yield from

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eucalyptus lignin than when water was used alone. Wu et al.14 studied depolymerization of

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lignin at 280 °C for 4 h over SiO2-Al2O3 in various organic solvents – methanol, ethanol,

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isopropanol and THF. Among the solvents, ethanol was found to be the most efficient in

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terms of phenolic monomer yield and char suppression. Similarly, Chen et al.15 who reported

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that ethanol showed the best performance to reduce char formation and gave the maximum

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yield of 21.9% monomers under H2 pressure of 1 MPa at 300 °C for 4 h over Ni/Al-SBA-

11

on


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15(20). Also Wang and Rinaldi,16 found that in lignin conversion methanol when compared

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to ethanol and butanol gave the highest proportion of phenols in the presence of Raney Ni,

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while in another study Ercodia et al.17 found that acetone was the most reactive to break

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down lignin to phenolic monomer compared to methanol and ethanol. So, the effectiveness of

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a solvent to breakdown lignin may depend on the operating conditions.

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Methyl isobutyl ketone (MIBK) is one of the most studied organic solvent for the

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pretreatment and fractionation of biomass and the lignins derived from MIBK-based

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processes have been depolymerized to low molecular weight products.13, 18 However, there is

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limited information on the use of MIBK as a solvent in lignin depolymerization using zeolite

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catalysts, though there are some studies with the lignin model compound, 2-phenoxy-1-

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phenethanol.198 The role of MIBK in the formation of phenolic monomers during the

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liquefaction process, relative to other commonly used solvents is not totally understood. In

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the present study, we have studied the depolymerization of bagasse lignin (BGL) to phenolic

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monomers, isolated by ethyl acetate-ethanol-water pulping process, using the zeolite catalyst

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H-USY in MIBK and other solvents (methanol, acetone and ethyl acetate) for comparison.

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The catalyst selection process was carried out with commercial organosolv lignin (COL) in

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methanol. The processing parameters examined for the selected catalyst were catalyst

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loading, solvent type and reaction conditions (temperature and H2 pressure). The resulting

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phenolic monomers were determined by gas chromatography-mass spectrometer (GC-MS)

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and gas chromatography-flame ionization detector (GC-FID), while the types of gases

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produced with the different solvents were determined by GC-MS. The solubility of BGL in

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the various organic solvents was also carried out as well as the molecular weight of the

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soluble fractions derived from these organic solvents. It was hypothesized that the data will

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provide some evidence on the differences in performance among the solvents in lignin

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depolymerization.



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EXPERIMENTAL SECTION

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Materials. Commercial organosolv lignin (COL) was obtained from Chemical Point UG,

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Germany, whilst bagasse lignin (BGL) was prepared according to a procedure described

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previously.20 Briefly, bagasse (10 % (w/v)) was pulped in a ternary mixture (79% (v/v)) of

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ethyl acetate:ethanol:water (32:25:43) with 21% (v/v) of formic acid at 164 °C for 45 min

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with initial pressure of 20 bars N2. The lignin was recovered from the organic solvent by

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filtration, solvent evaporation and drying to constant weight at 45 °C.

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The basic zeolite catalysts, H-USY (Si/Al molar ratio = 5, 50 and 250) and H-BEA (Si/Al

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molar ratio = 250) were purchased from TOSOH, Japan. H-ZSM5 (Si/Al molar ratio = 15)

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was purchased from Zeolyst International, USA. Fumed silica was obtained from Sigma-

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Aldrich (St. Louis, MO, USA). Standards of aromatic monomers i.e., phenol, 2,6-

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dimethoxyphenol, vanillin, 4-ethylphenol, and 4-ethylguaiacol were purchased from Sigma-

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Aldrich (St. Louis, MO, USA). For guaiacol and p-cresol were obtained from Tokyo

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chemical industry Co. Ltd. (Japan). All the chemicals and reagents were used as received.

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The compositions of COL and BGL (Tables S1, S2, S3 and S4, and Figure S1) were

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analyzed using a variety of standard analytical techniques such as Klason lignin, elemental

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analysis, Gel permeation chromatography (GPC), and Py-GC/MS. The amounts of Klason

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lignin are 91.7% and 89.9% for COL and BGL respectively. The Mw and Mn for COL are

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1,424 and 1,133 respectively, while those for BGL are 3,053 and 1,814 in that order. The Py-

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GC/MS analysis showed that the lignin substructures for COL contain 15.4 %, 45.7 % and

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38.9 % of H, G and S in that order, while BGL contains 36.9 %, 30.8% and 32.3 % of H, G,

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and S respectively.

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Lignin depolymerization. A known amount of lignin (0.0875 g) was mixed in 5 mL

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solvent. Depolymerization was carried out in a stainless steel tubular reactor (1/2 inch O.D.

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and 10 cm, length). Prior to depolymerization reaction, the reactor was purged with nitrogen 6 ACS Paragon Plus Environment

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and shook in vertical dimension at 40 rpm. Different catalysts, catalyst loadings, solvents and

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initial H2 pressures were applied depending on the experiment requirement. After reaction,

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the reactor was quenched in a water bath to stop the reaction immediately. Experiments were

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conducted in triplicate.

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Sample characterization after depolymerization. After cooling down the reactor, the

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reaction mixture was processed as illustrated in Figure 1. Briefly, the liquid fraction was

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separated from solid fraction (include char, unconverted lignin, and catalyst) by

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centrifugation at 5000 rpm for 10-15 min. Subsequently, the liquid fraction was then

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analyzed by GC-FID. A portion of the liquid fraction was dried to remove the solvent and to

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determine its weight gravimetrically. The solid fraction was dried at 60 °C and then weighed

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and the weight of the char/unconverted lignin fraction was calculated by difference from the

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weight of the catalyst.

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Product characterization. Phenolic products were identified by GC-MS. The GC-MS

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system was equipped with an Agilent CP-Sil 5 CB capillary column (60 m x 0.32 mm x 1.00

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µm; Agilent, USA). The temperature program starting from 50 °C for 5 min, then was rose up

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at a rate of 10 °C.min-1 to 120 °C and held for 5 min. Then, heated up to 280 °C at a rate of

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10 °C.min-1 and kept at this temperature for 10 min. The final temperature was held at 300 °C

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for 10 min at the same heating rate. Identification of compounds was performed by

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comparison of MS data to the NIST (National Institute of Standards and Technology) library.

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The main phenolic monomers showing a relatively high concentration in the liquid product

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were further quantitated by using external standard method on GC-FID (Shimadzu GC-2014,

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Japan). The quantitative analysis was carried out by a GC-FID (Shimadzu GC-2014, Japan)

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equipped with a PetrocolTM DH 50.2 fused silica capillary column (50 m x 0.2 mm x 0.5 µm;

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Supelco, USA) with flame ionization detector (FID). The column was initially kept at 50 °C

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for 5 min, then was heated at a rate of 10 °C.min-1 to 120 °C, and maintained for 5 min. After



Page 8 of 39

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that heated up to 280 °C at 10 °C.min-1 and kept at this temperature for 15 min. The phenolic

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products were quantified using the calibration curves of a number of lignin depolymerization

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products (i.e., guaiacol, phenol, 2,6-dimethoxyphenol, p-cresol, vanillin, 4-ethylphenol, and

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4-ethylguaiacol).

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The yields of phenolic monomers, liquid fraction, and char/unconverted lignin were calculated using the following equations:

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 weight of phenolic monomers (g) Monomers yield in liquid fraction (%) =100  weight of liquid fraction (g)

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 weight of liquid (g)   Liquid fraction yield (%) =100  weight of initial lignin (g) 

  

(1)

(2)

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 weight of solid fraction (g) - weight of catalyst (g) Char/unconverted lignin yield (%) =100 weight of initial lignin (g) 

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2D HSQC NMR. 2D heteronuclear single quantum coherence (HSQC) NMR spectra were

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collected to determine the changes in lignin structures mainly focused on depolymerized

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products. The sample (20-30 mg) was dissolved in 1 mL of dimethyl sulfoxide-d6 (DMSO-

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d6) and transferred to an NMR tube (a diameter 5 of mm). The 1H and

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HSQC NMR spectra were recorded at 25 °C on Bruker AVIII HD 400 MHz NMR

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spectrometer (Agilent, US) fitted with a cryogenically cooled 5 mm TCI gradient probe (cold

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1

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dimension, respectively. A total of 1024 complex points were collected for the 1H dimension

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with a 1.5 s recycle delay. A total of 64 transient at 256 time increments were recorded in the

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13

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point (δC/δH 39.5/2.5). The HSQC spectra were acquired using a standard Bruker pulse

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sequence “‘hsqcetgpsisp2.2” under the following conditions: spectra were acquired from 10

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to 0 ppm in F2 (1H) using 200 ms, and an interscan delay of 1 s, and from 165 to 0 ppm in F1

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(13C) using 256 increments of 32 scans with a total acquisition time of 10 h 20 min. 1ȷC-H of

H and

13

  

(3)

13

C correlation 2D

C channels). The spectral widths were 5 kHz and 20 kHz for the 1H and

13

C

C dimension. The central solvent peak was used as an internal chemical shift reference


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145 Hz was used. HSQC data processing and plots were carried out using ACD/NMR

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processing software, with automatic phase and baseline correction. 2D NMR cross-signals

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were assigned according to the literature.21-22Gases determination. Gas compounds were

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identified by GC-MS using an Agilent 6890 Series Gas Chromatograph and a HP 5973 mass

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spectrometer detector, employing helium as the carrier gas. The installed column was an

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Agilent HP-5MS capillary column (30m x 0.25mm x 0.25µm; Agilent, USA).The GC oven

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was heated to 40 °C and held for 10 min. Compounds were identified by means of the Wiley

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library-HP G1035A and NIST library of mass spectra and subsets-HP G1033A (a criteria

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quality value >80% was used).

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Solubility of BGL in the solvents. Lignin solubility test procedures have been described

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according to Klamrassamee et al. 23. The organic solvents including methanol, acetone, ethyl

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acetate, and MIBK used for the lignin solubility test were purchased from Sigma-Aldrich

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(Australia). Dried BGL lignin (1 g) sample was dissolved in 50 mL of each solvent. The

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solubility test was conducted at room temperature (25 °C) with a magnetic stirring speed of

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100 rpm for 18 h. After this time, the lignin solution was filtered and dried for 48 h in a

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vacuum oven at 45 °C. The solubility was expressed as g lignin/100 mL solvent.

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Polystyrene sulfonate equivalent molecular weight of the soluble fraction by GPC.

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After the test of lignin solubility, the recovered lignin soluble fractions were analyzed by Gel

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permeation chromatography (GPC) to determine the molecular weight distribution of these

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lignins. The samples were dissolved in tetrahydrofuran (THF) at a concentration of 1-2

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mg.mL-1 and filtered through a 0.45 µm Teflon syringe filter. A 100 µL injection volume was

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used. The instrument used for this analysis incorporated a GPC Water Breeze system model

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151 with an isocratic HPLC pump. Eluted fractions were detected with UV and refractive

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index Water model 2414 detector. Three Phenomenex phenogel columns (500, 104 and 106

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Å porosity; 5 µm bead size) were used for size exclusive separation. The mobile phase was



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THF, and introduced at a 1 mL. min-1 flow rate at 30 °C. Sodium polystyrene sulphonate

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standards with molecular weights 1530, 4950, 16600, and 34700 g.mol-1 were used to prepare

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a standard calibration curve. Lignin weight average molecular weight (Mw) and number

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average molecular weight (Mn) were calculated after comparison with standards.

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Klason lignin determination. Lignin samples were added with 72% H2SO4 in pressure

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tubes. The tubes were then immersed in a water bath at 30°C for 1-2 h. The hydrolyzed lignin

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samples were then diluted to 4% through the addition of water and autoclaved at 121°C for 1

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h. The samples were filtered with porcelain crucibles to remove any solids and the liquid

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fraction was analyzed by high performance liquid chromatography (HPLC) for glucose,

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xylose and arabinose. The solid fraction comprised of acid insoluble lignin and the acid-

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insoluble ash was placed in a muffle furnace at 575 ± 25°C for 3 h. After cooling, the

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remaining acid-insoluble ash was weighted and calculated as a percentage of the original dry

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weight of sample.24

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Elemental analysis. The elemental composition by ultimate analysis in terms of carbon,

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hydrogen, nitrogen, and sulfur (CHNS) content of organosolv lignin is essential for study

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lignin quality. The test was carried out by an Elemental Analyzer CHN-S 628 (LECO Corp.).

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Lignin samples were dried at 60°C in vacuum evaporator with 20 mbar to remove moisture.

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Then, lignin sample (100 mg) was encapsulated in the container to determine carbon,

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hydrogen, and nitrogen in sample. For sulfur analysis, lignin sample (200 mg) was placed

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into ceramic boat furnace. This was incineration at 1,350 °C using sulfur IR cells detect the

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amount of sulfur.

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Py-GC/MS. Py-GC/MS was used to characterize chemical composition of the lignin used

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in this study. Pyrolysis of samples (COL and BGL lignins) (ca. 0.1 mg) was performed at 500

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ºC in an EGA/PY-3030D micro-furnace pyrolyzer (Frontier Laboratories Ltd., Fukushima,

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Japan) connected to a GC 7820A (Agilent Technologies, Inc., Santa Clara, CA) and an


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Agilent 5975 mass-selective detector (EI at 70 eV). The column used was a 30 m x 0.25 mm

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i.d., 0.25 µm film thickness, DB-1701 (J&W Scientific, Folsom, CA). The oven temperature

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was programmed from 50 °C (1 min) to 100 at 20 °C.min-1 and then to 280 °C (5 min) at 6

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°C.min-1. Helium was the carrier gas (1 mL.min-1). Identification of the released compounds

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was made by comparison of their mass spectra with those of the Wiley and NIST libraries

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and with those reported in the literature25-26 and, when possible, by comparison with the

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retention times and mass spectra of authentic standards. Molar peak areas were calculated for

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each lignin degradation product released, the summed areas were normalized, and the data for

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two replicates were averaged and expressed as percentages.

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Surface area, pore volume, and pore diameter analysis of catalyst. The specific surface

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area and porosity of catalysts were characterized by Brunauer-Emmett-Teller (BET) by using

245

nitrogen adsorption/desorption of the samples by nitrogen sorption isotherms at -196 °C of in

246

a Belsorp-max Bel Japan equipment .Prior to the measurements, the fresh samples were

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degassed at 150 °C for 3 h.

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EDX analysis for carbon content of catalyst. Fresh and spent catalysts after

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depolymerization reaction were gold coated and analyzed with a JEOL 7001F FESEM at

250

15kV with an Oxford X-Max 80 mm2 SDD EDS detector using chemical standards for semi-

251

quantification.

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RESULTS AND DISCUSSION

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Depolymerization of COL by different catalysts. Zeolite-derived solid acid catalysts of

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the types H-USY, H-BEA, and H-ZSM5, with different Si/Al ratios from 5 to 250, and fumed

256

SiO2 were evaluated for the depolymerization of COL at a catalyst loading of 1 wt% (catalyst

257

mass/lignin mass). Depolymerization of COL was also conducted with 1 wt% of H2SO4 as a

258

homogenous acid catalyst and also without the use of a catalyst. The depolymerization

259

reactions were carried out in stainless steel tubular reactors at 300 ºC for 1 h in methanol with

260

an atmospheric initial pressure. Figure 2 shows the monomeric phenolic yields with the

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different catalysts. Among the zeolite catalysts, H-USY (Si/Al = 5) gave the highest yield of

262

the phenolic monomers (2.5 wt%) and the highest selectivity for 4-ethylphenol, 4-

263

ethylguaiacol, guaiacol, syringol and phenol. Most of the other catalysts gave lower total

264

yield of the main phenolic monomers (1.25-2.3 wt%) but produced relatively higher

265

proportions of vanillin. The catalyst fumed SiO2 gave the lowest yield, even lower than the

266

depolymerization carried out without a catalyst. The homogeneous catalyst H2SO4 performed

267

better than some of the catalysts, an indication that acidity of the solution assists in the

268

cleavage of the C-O-C and C-C bonds in lignin. Since a stainless steel reactor was used in the

269

depolymerization reaction, its surface may be an active hydrogenation metal catalyst25,

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particularly in the case where aqueous H2SO4 is used. An aqueous H2SO4 solution is more

271

effective to etch the metal surface of the reactor compared to the non-aqueous heterogeneous

272

catalysts used in the present study. It should therefore be noted that the role of the reactor as a

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catalyst is small because of the solvent medium as well as the operating conditions of

274

pressure (20 bar) and reaction time (1 h) which are less severe to a pressure of 50 bar and

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reaction time of 24 h reported by Mondo et al.27 In Mondo et al.27 study, it was clearly

276

demonstrated that a stainless steel reactor acted as deoxygenation catalyst.


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Table 1 gives the physical properties of the solid catalysts used in this study. H-USY (Si/Al

278

= 5) zeolite has the next highest BET surface area, but has the highest pore volume, pore

279

diameter, and total acid sites (i.e., Bronsted and Lewis acids). While these combined

280

properties resulted in the highest phenolic monomer yield, it is worth noting that the H-BEA

281

catalyst with the second lowest surface area, pore volume and pore diameter, and with 1/6th of

282

the total acid sites as H-USY (Si/Al = 5), gave the second highest phenolic monomer yield.

283

So, it is likely that additional factors such as the topography, mass transfer, surface structures

284

and morphology of the catalyst play vital roles in the electronic properties of the catalyst

285

which enhance the overall effectiveness of the depolymerization process to phenolic

286

derivatives.28

287

Depolymerization of BGL-Effect of catalyst loading. As the H-USY (Si/Al = 5) gave the

288

highest yield of the main phenolic monomers, it was used in all subsequent depolymerization

289

reactions with BGL in methanol. Figure 3 shows the results of depolymerization of BGL in

290

methanol at different catalyst loading. The yield of the liquid fraction is highest at 1 wt%

291

catalyst loading (58.3 wt%), whereas the char/solid residue is highest without the use of a

292

catalyst (8.6 wt%). However, the 10 wt% catalyst loading gave the lowest amount (49.7 wt%)

293

of the liquid fraction (Figure 3A).

294

The yield of the main phenolic components in the liquid fraction is given in Figure 3B. 4-

295

ethylphenol, 4-ethylguaiacol, phenol, syringol, and guaiacol are the main products similar to

296

the results obtained with COL, although no vanillin or p-cresol were detected in any

297

quantifiable amount. The differences in the proportions of the compounds in COL and BGL

298

are associated with the differences, to a large extent, in the proportions of the lignin

299

substructures. The Py/GC-MS analysis of lignins as shown in Table 2 and Figure 4 show that

300

COL contains 15.4 % of H-type, 45.7 % G-type and 38.9 % S-type, while BGL contains 36.9

301

% H-type, 30.8 % G-type and 32.3 S-type. As COL contains a higher proportion of the G-



Page 14 of 39

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type substructures, the proportions of the phenolic monomers originating from them are

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higher than that of BGL.29 The corollary to this is that BGL with a higher proportion of H-

304

type lignin produced a higher proportion of 4-ethylphenol . As the main phenolic monomers

305

detected are derived from H-type and G-type substructures, it is apparent from the present

306

study, the S-type substructures were not selectively depolymerized.

307

Figure 3B shows that increasing proportion of phenol and 4-ethyguacicol is formed by

308

increasing the catalyst loading from 0-5 wt%. A similar trend is observed with 4-ethylphenol

309

up to 10 wt% catalyst loading. Besides, increasing the catalyst loading to 10 wt%, resulted in

310

the maximum yield of total phenolic monomers (3.7 wt%).

311

Depolymerization of BGL-Effect of solvent system. Figure 5 shows the effect of four

312

solvents on the depolymerization of BGL at 300 °C with a catalyst loading of 10 wt% and

313

working at atmospheric initial pressure. As shown in Figure 5A, acetone has the highest

314

conversion to the liquid fraction (54.8 wt%), followed by ethyl acetate, methanol and MIBK

315

in that order. Moreover, the highest yields of char/unconverted lignin was observed with the

316

use of MIBK and ethyl acetate respectively. This may be related to their inability to readily

317

supply hydrogen in-situ to reduce polymerization reactions. For gaseous products, the GC-

318

MS identified trace amounts of oxygen, nitrogen, carbon dioxide, and the solvent.

319

Interestingly, MIBK gave the highest yield of the main phenolic monomers (5.0 wt%)

320

followed by acetone, ethyl acetate and methanol in that order (Figure 5B). The solvent

321

polarity index from non-polar to polar for these solvents is in the following order MIBK >

322

ethyl acetate > methanol ~ acetone. This does not explain the phenolic monomer results

323

neither does the solubility of the solvents in lignin as the results from the most soluble to the

324

least is in the following order acetone > methanol > ethyl acetate > MIBK (Table 3). The

325

solvents, except methanol do contain a carbonyl group, and hence are expected to encourage


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lignin self-aggregation. It is plausible therefore that this will allow more intimate contact

327

between lignin molecules and the catalyst and enhance the depolymerization process.

328

Figure 6 shows the molecular weight distribution of the lignin soluble fractions obtained

329

with the different solvents. Although, the profile of the molecular weight distribution is

330

similar for each fraction, the molecular weight is in the following increasing order MIBK

Catalytic Conversion of Organosolv Lignins to Phenolic Monomers in

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