Insights into the Microwave-Assisted Mild Deconstruction of Lignin

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Insights into the microwave-assisted mild deconstruction of lignin feedstocks using NiO-containing ZSM-5 zeolites Jelena Milovanovic, Nevenka Rajic, Antonio A. Romero, Hangkong Li, Kaimin Shih, Roman Tschentscher, and Rafael Luque ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00825 • Publication Date (Web): 03 Jun 2016 Downloaded from http://pubs.acs.org on June 5, 2016

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ACS Sustainable Chemistry & Engineering

Insights into the microwave-assisted mild deconstruction of lignin feedstocks using NiOcontaining ZSM-5 zeolites Jelena Milovanović,# Nevenka Rajić,ǁ Antonio A. Romero,† Hangkong Li,‡ Kaimin Shih, ‡ Roman Tschentscher,§ and Rafael Luque*,† #

Innovation Centre of the Faculty of Technology and Metallurgy, University of Belgrade,

Karnegijeva 4, 11120 Belgrade, Serbia. E-mail: [email protected] ǁ

Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade,

Serbia. E-mail: [email protected] †

Departamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales,Edificio

Marie Curie (C-3), Ctra Nnal IV, Km 396, E-14014 Córdoba, Spain. E-mail: [email protected] *

E-mail: [email protected]

‡

Department of Civil Engineering, University of Hong Kong, Pokfulam Road, Hong Kong. E-

mail: §

SINTEF, Forskningsveien 1, 0314 Oslo, Norway. E-mail:[email protected]

ACS Paragon Plus Environment

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ABSTRACT: The depolymerisation of BioligninTM (BL), Eucalyptus (EL) and Hardwood lignins (HL) to simple aromatics was studied by a microwave assisted approach using NiO/H-ZSM-5 zeolites as catalysts. The catalysts prepared by mechanochemical dry milling (MCDM) method contained bunsenite nano NiO particles with an average size of 67 µm in concentration of 14.8 wt. % NiO. Yields and composition of the obtained bio-oils are highly dependent on lignin type as well as on the content of NiO. The highest bio-oil yield (about 20 wt. %) was obtained using 3.5 wt. % NiO and HL as the feedstock. A number of relevant findings based depolymerisation experiments are provided which pointed to different monomeric products (mostly S-derived) that could be produced in different proportions depending on the source of investigated lignin and content of the NiO at H-ZSM-5.

KEYWORDS: Lignin, Microwave, Depolymerisation, Zeolite, Nickel oxide


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INTRODUCTION Lignin is one of the most abundant bio-polymers composed of up to three different monolignol monomers, methoxylated to various degrees including p-coumaryl, coniferyl and sinapyl alcohols (Scheme 1).1-3Coniferyl alcohol occurs in all lignin types, being a dominant in conifers (softwoods), syringyl units are present in deciduous (hardwoods) up to 40% while coumaryl alcohol is present in grasses and agricultural crops. The monomers are connected through a variety of alkyl, aryl and ether linkages likely via oxidative cross-coupling reactions2 providing a particular recalcitrance to lignin.

Scheme 1. The three main precursors of lignin. Lignin depolymerisation is a highly challenging task and numerous efforts have been attempted in order to establish technologies capable to deconstruct lignin into useful chemicals.3 Chemical, biochemical and thermal treatment have been usually considered.1,3-5 Microwave (MW) heating has also be shown as a promising method.6,7 MW provides heat at the molecular level which prevents formation of undesired products.8 Since the MW effectiveness depends on the microwave absorption ability, the biomass due its low MW absorption characteristics should be mixed with an effective MW receptor.9 Previous studies of this research group proved that


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lignin can be partially deconstructed by MW under mild reaction conditions, using hydrogendonor solvents, and in the presence of a suitable catalyst to a variety of phenolic products.10

Scheme 2. Targeting C-C and C-O bond cleavage in lignin: the depolymerisation strategy. Reproduced by permission of Elsevier from reference 16. For the lignin depolymerisation two different types of linkages and several types of reactions are essential. Hydrogenolysis of α- and β aryl alkyl and aryl-aryl ether linkages (Scheme 2) is most important and requires a metal-containing catalyst.1,3,11 The beta-type linkages (β-O-4, arylalkyl ether bonds) seem to be the weakest bonds in C-O ether linkages in comparison to the highly stable aryl-aryl ether bonds.12,13 Comparatively, both aryl-aryl and aryl-alkyl C-C linkages are generally more stable and can only be efficiently cleaved under very particular and often harsh conditions (e.g. 400 °C, base catalysis).12-14 Moreover, acid sites present in the catalyst offers an additional possibility of dealkylation and/or deacylation-related reactions10-15, which in combination with acidolytic aryl ether bond cleavage could ultimately lead to very simple aromatic compounds including syringaldehyde, vanillin, guaiacol and related chemicals. Nature


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and strength of the acidity needs to be carefully controlled in order to prevent secondary lignin self-condensation reactions. Noble metals such as Rh, Pt and Pd are widely used in the transformation of lignin model compounds.15-17 Also, Cu has been shown to promote a near-complete conversion of lignin.18 Until very recently, the promising potential of Ni in the C-O bond cleavage was overlooked.16,19-22 Li et al. performed the direct catalytic conversion of raw woody biomass (up to 46.5% yield based on lignin) into monophenols without any pretreatment step using carbon supported Ni-W2C catalyst.16 The catalyst exhibited a comparable activity to that of noble metals, paving the way to a further development of Ni-based materials for the lignin depolymerisation. In the light of these premises, a carefully and rationally design of a bifunctional catalyst combining metal (efficient for the C-O bond cleavage) and acid sites (for dealkylation and related deacylation reactions) seem to plausibly provide the required prerequisites for the lignin depolymerisation. H-ZSM-5 is a promising catalyst due its high surface area, high hydrothermal stability, shape selectivity (which controls conversion of biomass to aromatics) and combination of Brönsted (preferentially) and Lewis acid sites. Accordingly, H-ZSM-5 was used in this study for the preparation of a bifunctional catalyst using a mechanochemical approach for which we have previously found to improve the catalytic activities of zeotype catalysts by creating novel active sites and increasing the external surface area of the catalysts.23 Mechanochemically prepared NiO/ZSM-5 catalysts were tested in lignin deconstruction under MW heating and mild reaction conditions.


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EXPERIMENTAL SECTION Materials. Three different lignins were used in this work. The EL and HL were supplied by Innventia AB, Stockholm, while BL was kindly donated by CIMV, France comprising a mix of linear oligomers with low molecular weight of high purity (91%), behaving exactly like phenolic polymers obtained from petrochemistry (http://www.cimv.fr/products/11-.html). Nickel nitrate hexahydrate (Aldrich, p.a.) and H-ZSM-5 (Si/Al=30, Zeolyst International) were used for the preparation of the catalysts. The zeolite was calcined at 600 °C (24 h) prior to the modification and use in the catalytic reaction. Catalyst preparation. The catalysts were prepared using a previously reported mechanochemical dry milling (MCDM) method.11 Zeolite samples were grinded with an appropriate amount of Ni(NO3)2 x 6H2O to reach the desired Ni content (up to 5 wt. %) in a 125 mL reaction chamber of a Retsch PM-100 planetary ball mill under optimum milling conditions (350 rpm, 10 min) using 10 mm stainless-steel balls. The obtained material was then calcined at 400 °C under air for 4 h to remove the excess of unreacted and/or physisorbed nickel salt and activated via reduction in a flow of hydrogen/helium (20 mL min-1) at 400 °C for 4 h. Ni contents were selected based on previous studies of our research group in which Ni contents between 2-4% provided the optimum of catalytic activity for lignin depolymerisation.10,11 Other materials were prepared with Ni contents between those covered in this manuscript (not shown) but were unable to yield additional improvements in textural/structural activity properties. Catalyst characterization. Powder X-ray diffraction (PXRD) patterns were collected at room temperature on a Bruker D8 Discover in the 2θ range from 10 to 80°. Qualitative PXRD analysis were conducted using the Diffrac.EvaV3.1 software.24


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N2 adsorption measurements were carried out at 77 K using an ASAP 2010 volumetric adsorption analyzer from Micrometrics. The samples were outgassed for 24 h at 130 °C under vacuum (p < 10-2 Pa) and subsequently analyzed. The linear part of the BET relationship (relative pressures between 0.05 and 0.22) was used for the determination of the specific surface area. Nickel content in the prepared catalyst was measured by Inductively Coupled Plasma with Mass Spectrometry (ICP/MS). The solid sample was dissolved using a mixture of HF-HNO3HCl (1:1:1) and subsequently analyzed by ICP at the Central Facilities for Research (SCAI) from Universidad de Cordoba. The morphologies and structures of Ni-containing zeolites were also examined by Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM) and selected-area Electron Diffraction (SAED) experiments, using a FEI Tecnai G2 20 S-TWIN Scanning Transmission Electron Microscope. The elemental composition was also checked using energy dispersive spectroscopy (EDS) and compared to ICP results. Lignin

characterization.

Diffuse

Reflectance

Infrared

Fourier-Transform

(DRIFT)

experiments were conducted in an ABB MB3000 Spectrometer equipped with an Attenuated Total Reflectance (ATR) module. Attenuated total reflectance infrared (FTIR-ATR) spectra of the lignin samples were recorded using ABB MB3000 spectrometer equipped with an ATR PIKE MIRacleTM sampling device containing diamond/ZnSe crystal. Besides, for powdered samples an extra accessory plate with a conic awl was used which required only a few milligrams without any previous sample preparation. Spectra were acquired and then processed with the Horizon MBTM software. The spectra were scanned at room temperature in absorbance


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mode over the wave number range of 4000–400 cm-1, with a resolution of 8 cm-1 and 256 scans. Materials were dried at 100 °C for 3 h prior to measurement. 13

C NMR experiments were performed using a Bruker Avance III spectrometer at 11.7 T (500

MHz proton resonance frequency) with a 3.2 mm triple resonance MAS probehead at room temperature. The MAS rate was 20 kHz in all experiments. 1H-13C cross-polarization (CP) spectra was acquired with 12000 scans, a recycle delay of 5 s, and a Hartmann-Hahn contact time of 1500 µs. 13C chemical shifts were referenced to adamantane by the substitution method where the -CH2- peak in adamantane was set to be at 38.48 ppm.25 TG-MS data were obtained using a SDT Q600 simultaneous TGA-DTA instrument (TA instruments) coupled with a Hiden HPR-20/QIC mass spectrometer. For the TG-MS analysis about 10 mg of each lignin sample was heated up to 800 °C (10 °C min-1) in an open Al2O3 crucible, under nitrogen flow (100 ml min-1). Microwave-assisted depolymerisation. Depolymerisation reactions were conducted in a microwave reaction system from Milestone ETHOS-1 at 400 W and 180 °C for 1 h. Formic acid (FA) was used as hydrogen-donor and solvent in the reaction. The microwave vessel was filled with a 1/1 lignin/catalyst ratio and a solid/liquid ratio of 1/12.5. A blank test (without catalyst) as well as a test with the parent ZSM-5 zeolite were carried out as control for all experiments. Products separation and characterization. Products were separated using the procedure illustrated on Scheme 3. 15 wt. % NaOH was firstly added to the obtained mixture after microwave-assisted depolymerisation, stirred for 30 min and the liquid was then separated by filtration. Such liquid phase was subsequently acidified to pH 1-2 by the addition of conc. HCl, and ethyl acetate was then added until the solvent was colorless. In the next step, Na2SO4 was


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added to remove any water remaining and then separated by filtration. Finally, ethyl acetate was evaporated to obtain the phenolic bio-oil (Scheme 3).

Scheme 3. Schematic representation of the separation procedure for bio-oil recovery. The yield of the bio-oil was calculated gravimetrically as referred to the initial lignin content and the identity of the products was confirmed by GC MS. Identification of the components in the obtained bio-oil was performed using a GC (7890A)-MS (5975D inert MSD with Triple-Axis Detector) Agilent equipped with a capillary column HP-5MS [(5%-phenyl)-methylpolysiloxane, 60 m x 0.32 mm]. The temperature programme started at 50 °C and followed by heating to 120 °C, 280 and 300 °C at 10 °C min-1. During the programme, heating was held at 120 °C for 5 min, at 280 °C (8 min) and at 300 °C (2 min). Helium was used as a carrier gas. RESULTS AND DISCUSSION Catalyst characterization. The PXRD patterns show that the milling did not significantly affect the crystallinity of the ZSM-5 lattice (Figure 1). The Ni-containing zeolites display additional diffractions at 2Θ= 37.2, 43.3. 62.9 and 75.4° which intensities increase with the increase of the Ni content. A qualitative PXRD analysis indicated that the diffractions


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correspond to bunsenite (NiO) phase. Selected characteristics of the catalysts are summarized in Table 1.

Figure 1. PXRD patterns of H-ZSM-5 and NiO/H-ZSM-5 (corresponding to NiO3.5 %/H-ZSM-5). Diffractions of bunsenite NiO have been marked with an asterisk. Table 1. Selected properties of the catalysts used in this study. NiO

Surface Area

Pore volume

Pore diameter

(wt. %)

(m2 g-1)

(mL g-1)

(nm)

-

330

0.24

2.90

1.0

322

0.27

3.37

NiO3.5%/H-ZSM-5

3.5

330

0.29

3.50

NiO5%/H-ZSM-5

4.8

325

0.28

3.45

Catalyst

Si/Al

H-ZSM-5 NiO2%/H-ZSM-5 30

Calcined H-ZSM-5 exhibited a surface area of 330 m2 g-1, with a pore volume of 0.24 mL g-1 and a pore diameter of 2.9 nm, which are the average values for textural properties of typical ZSM-5.26 Nevertheless, the formation of NiO upon milling did not significantly affect the lattice even in the case of relatively high NiO quantities (i.e. 4.8 wt. %). The actual Ni content was in


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good agreement with the predicted Ni content except for NiO2%/H-ZSM-5 for which an unusual low metal content was obtained.

Figure 2. TEM micrographs of NiO3.5%/H-ZSM-5. a) NiO nanocrystals as dark spots, b) SAED pattern of NiO along the [112] zone axis, c) ZSM-5 phase and d) HRTEM image. The inset in d) shows the corresponding SAED pattern of ZSM-5 along the [100] zone axis. The catalyst structure was further studied by a TEM analysis. TEM micrographs of a representative material (NiO3.5%/H-ZSM-5) have been depicted in Figure 2. These confirmed the presence of NiO nanoparticles that can be clearly visualized in TEM micrographs as dark cubic/quadrilateral spots (Figure 2a). Their size was found to vary from 30 to 200 nm, with an average of 67 nm and the presence of some large aggregates. Figure 2b illustrates the SAED pattern along the [112] zone axis of an individual NiO particle (selected from Figure 2a), indicating the single and highly crystalline structure of NiO particles. Figure 2c also depicts a well crystalline ZSM-5 in the Ni-containing product confirming preserved ZSM-5 structure after the milling treatment. HRTEM images (Figure 2d) show that the measured periodic fringe


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spacing of 1.146 nm agrees well with the d-spacing corresponding to the {011} plane of orthorhombic ZSM-5. These results additionally confirmed that the ZSM-5 lattice is not affected by NiO deposition under milling. Besides, the inset in Figure 2d presents the corresponding SAED pattern of ZSM-5 along the [100] zone axis, further confirming the crystallinity of ZSM5. The TEM data for NiO2%/H-ZSM-5 and NiO5%/H-ZSM-5 (not shown) displayed similar information as for NiO3.5%/ZSM-5, without any remarkable changes in the particle size (6667 nm) or the ZSM-5 structure.

Figure 3. FTIR spectra of Biolignin, hardwood and softwood lignin. a) Spectral range from 4000 to 500 cm-1 and b) magnified region from 1800 to 800 cm-1. Lignin characterization. DRIFT spectra (Figure 3) indicate that there are slight differences in lignin structures. All samples display the absorption bands at similar wavenumbers in the range between 4000 to 2000 cm-1 corresponding to O-H stretching vibration of hydroxyl groups (at about 3400 cm-1) and C-H stretching vibration from methyl and methylene groups (around 2900 cm-1). In the fingerprint region from 1700 to 900 cm-1, the spectra of EL and HL are rather


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similar whereas the spectrum of BL significantly differs. Only BL displays strong absorption bands at 1709 and 1651 cm-1 attributed to the C=O stretching non-conjugated to the aromatic ring. The bands at 1605 and 1512 cm-1 present in all three spectra correspond to aromatic skeletal vibrations, whereas the bands at 1458 and 1423 cm-1 are attributed to C-H deformation vibrations.27 The strong band at 1327 cm-1 (evidenced only in the spectra of HL and EL) corresponds to the stretching vibration of C-O which is attached to syringyl (S-) rings. The relative intensity of this band differs for HL and EL, being stronger for EL sample. It is noticeable that the band corresponding to aromatic skeletal vibration centered at 1122 cm-1 in the spectrum of BL is shifted to lower wavenumbers for both EL and HL. Finally, all three samples display a vibration band at 1030 cm-1 which can be assigned to the C-O deformation belonging to primary alcohols.28 The band is more intense for BL than for the HL and EL samples. In order to get a deeper insight into the presence of functional groups in lignin samples 13C NMR analysis was performed. The spectra are shown in Fig. 4, while signal assignments are

Figure 4. 13C NMR spectrum of BL, HL and EL.


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given in Table 2. Table 2. 13C NMR Chemical shift assignment for BL, HL and EL.29,30

BL

HL

EL Assignment

Examples

ppm 20.4

-

-

29.9

29.9

-

37.2

37.0

38.4

55.9

55.6

55.6

C in Ar-OCH3

73.8

70.6

73.9

C-α in G type β-O-4 units

-

82.6

81.4

C-β in G type β-O-4 units

-

105.5

106.0

C-2/C-6, S with α-CO

115.2

114.5

114.2

C-5 in G units

-

133.7

133.6

CH3 and CH2 in saturated aliphatic chain

C-1, S non-etherified; C-1, G non-etherified 147.4

147.8

147.6

C-3, G units

171.5

175.3

176.4

COOH groups

13

C NMR spectra exhibited several distinct peaks in the aliphatic (0-95.8 ppm), aromatic (95.8-

166.5 ppm) and carboxyl (166.5-215.0 ppm) regions. The main differences in the aliphatic region relate to peak intensity of aliphatic C-C bonds and C-α and C-β in G type β-O-4 units. The first one is more pronounced for BL with respect to that of HL and EL. In contrast, the second one is more intense for HL and BL as compared to EL. Differences are also observed in


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the aromatic region. The main signal assigned to C-2/C-6, S with α-CO cannot be observed in the BL spectrum. Also, the peak at around 115 ppm (corresponding to C-5 in G units) is more pronounced in the BL spectrum as compared to that present in HL and EL. The aromatic region of HL and EL contains a broad peak at around 133 ppm which is not evidenced in the BL spectrum. Finally, the peak at around 147 ppm is more intense in HL and EL with respect to BL. Considering the intensity of the peak corresponding to carboxyl region it could be concluded that the BL contains more carboxyl bonds then in EL and HL. Taking into account complexity of lignin structures, TG/DTG analysis was performed to provide additional insights into structural features of the three types of lignin, expecting that both thermal degradation behavior and hydrogenolysis could be affected by lignin origin (Figure 5).

Figure 5. a) TG and b) DTG curves of BL, HL and EL samples. The thermal decomposition of lignin generally proceeds in two steps: from 50 to 200 °C and 200 to 650 °C. The first weight loss up to 200 °C corresponds to water release. This weight loss differs for the samples: BL and EL lost about 12 and 16 wt. %, respectively, whereas HL lost only 5 wt. %. The weight loss is accompanied with two DTG maxima at 52 and 103 °C (BL), 54 and 161 °C (HL) and 56 and 192 °C (EL) indicating a non-continuous water loss. The second


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weight loss up to 650 °C also exhibits different pattern for the different lignin samples. The BL lost 53 wt. % accompanied with a broad maximum in the DTG curve (at least two maxima centered at about 250 and 361 °C are evident). HL lost about 60 wt. % with two well separated maxima at 250 and 370 °C. Finally, EL lost 47.8 wt. % with DTG maxima at higher temperatures (355 and 450 °C) indicating a higher thermal stability of this sample. The residue at 650 °C also differs being (in wt. %) 31.5 (BL), 33.0 (HL) and 37.2 (EL). All presented results clearly show that the studied lignin samples exhibited different structural features and composition.

Microwave-assisted depolymerisation of lignin. The catalytic activity of the NiO-containing H-ZSM-5 was subsequently tested in the depolymerisation of HL. Table 3 summarizes the yields of bio-oil, clearly pointing out the highest bio-oil yield obtained using NiO3.5%/H-ZSM-5 as compared to typical yields obtained for H-ZSM-5 and related NiO/H-ZSM-5 catalysts with different NiO contents (2-5%). The blank run in the absence of a catalyst provided about 5% of bio-oil due to acidolytic cleavage of lignin by formic acid as has been previously reported by our group.10,11,31 Table 3 also shows that H-ZSM-5 and NiO2%/H-ZSM-5 gave similar bio-oil yields indicating that low contents of NiO in the materials do not seem to provide any significant improvements (no synergetic catalytic effect) as compared to the use of H-ZSM-5. Interestingly, a further increase of NiO to 5 wt. % and above was also proved to be detrimental in terms of bio-oil yield. These results are in line with previous results by the group for the case of Ni5%AlSBA catalysts.11 The results of GC-MS analysis of the bio-oils show the nature and concentration of the isolated simple phenolic monomers (Table 4), with the main quantified compounds in the oil depicted on


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Scheme 4. A number of aromatic oligomers were also observed to be present in the bio-oil but are of low interest for quantification (results not shown).

Table 3. The influence of NiO content on bio-oil yield obtained from HL depolymerisation. Bio-oil Catalyst (wt. %) None

Insights into the Microwave-Assisted Mild Deconstruction of Lignin

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