Spectroscopic Investigation of Thermochemical Depolymerization of Lignin Model Compounds in the Presence of Novel Liquidlike Nanoparticle Organic Hybrid Solvents for Efficient Biomass Valorization

Article abstract of DOI:10.1021/acs.oprd.8b00282

This study reports the thermochemical transformation of lignin model compounds using nanoparticle organic hybrid materials (NOHMs). NOHMs have recently been developed as an emerging class of self-suspended nanoparticle solvent systems created by ionically or covalently grafting organic oligomers or polymers (canopy) onto surface-modified inorganic nanoparticles (core). Because NOHMs exhibit negligible vapor pressure with the ability to tailor physicochemical properties, they could be a promising catalytic solvent for the lignin thermochemical conversion process. The thermochemical conversion of lignin model compounds was achieved with the synthesized NOHM at an elevated temperature of 473 K, and the results were compared with the case of the ionic liquid [EMIM][ESO₄]. The fractured moieties of the lignin model compounds were qualitatively identified by ATR FT-IR and 2D COSY NMR spectroscopies. The results indicated that the NOHM decomposed the C-O and/or C-C bonds of lignin model chemicals more efficiently than [EMIM][ESO₄].

Full text of DOI:10.1021/acs.oprd.8b00282

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Full Paper
Spectroscopic Investigation of Thermochemical Depolymerization
of Lignin Model Compounds in the presence of Novel Liquid-like
Nanoparticle Organic Hybrid Solvents for Efficient Biomass Valorization
Seokyoon Moon, Yunseok Lee, Soyoung Choi, Sujin Hong,
Seungin Lee, Ah-Hyung Alissa Park, and Youngjune Park
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.8b00282 • Publication Date (Web): 23 Oct 2018
Downloaded from http://pubs.acs.org on October 24, 2018
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Spectroscopic Investigation of Thermochemical
Depolymerization of Lignin Model Compounds in
the presence of Novel Liquid-like Nanoparticle
Organic Hybrid Solvents for Efficient Biomass
Valorization
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Seokyoon Moon,† Yunseok Lee,† Soyoung Choi,† Sujin Hong,† Seungin Lee,† Ah-Hyung A.
Park,‡,⁋ and Youngjune Park*,†
† School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and
Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
‡ Department of Chemical Engineering, Columbia University, New York, NY 10027, USA
⁋ Lenfest Center for Sustainable Energy, Columbia University, New York, NY 10027, USA
Y.P.: young@gist.ac.kr
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Table of Contents
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ABSTRACT
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This study reports the thermochemical transformation of lignin model compounds using
nanoparticle organic hybrid materials (NOHMs). NOHMs have recently been developed as an
emerging class of self-suspended nanoparticle solvent systems created by ionically or covalently
grafting organic oligomers or polymers (canopy) onto surface-modified inorganic nanoparticles
(core). Because NOHMs exhibit negligible vapor pressure with the ability to tailor
physicochemical properties, they could be a promising catalytic solvent for lignin thermochemical
conversion process. The thermochemical conversion of lignin model compounds was achieved
with the synthesized NOHM at an elevated temperature of 473 K, and the results were compared
with the case of ionic liquid, [EMIM][ESO4]. The fractured moieties of the lignin model
compounds were qualitatively identified by ATR FT-IR and 2D COSY NMR spectroscopies. The
results indicated that the use of NOHM can decompose the C–O and/or C–C bonds of lignin model
chemicals more efficiently than the case of [EMIM][ESO4].
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Keywords: Lignin, NOHMs, Thermochemical Decomposition, NMR, Ionic Liquid
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1. INTRODUCTION
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Significant efforts across the globe have been made up to now to reduce greenhouse gas
(GHG) emissions and to mitigate atmospheric GHG level. Among the GHGs, CO2 has the most
critical role in global warming, and it is mainly caused by the use of fossil fuels such as petroleum
based oil, coal, and natural gas. In this regard, there are two plausible strategies to cope with the
climate change triggered by CO2 − first, CO2 produced from large point sources such as fossil fuel
power plants and industrial sites can be captured and stored so not to enter the atmosphere by using
carbon capture and storage (CCS) technologies; second, CO2 emissions can be prevented by fuel
switching from fossil fuels to renewable energy resources such biomasses, wind power, solar
energy, etc. For most of human history, particularly, biomasses have been used as an important
energy resource. Biomasses are considered as an approximately carbon neutral energy resource
because photosynthesis involves the uptake of CO2 from the atmosphere with the storing solar
energy. Furthermore, the total atmospheric CO2 inventory could become negative when combined
with CCS.1 For this reason, biomasses are expected to replace a significant portion of petroleum-
based energy consumption in the near future in the context of current energy and environmental
concerns.2-4
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Recently, lignocellulosic biomasses have received considerable attention because they
could be a promising alternative to fossil resources for value-added chemical intermediates as well
as fuels.5-12 Lignocellulosic biomasses are the most abundant renewable resource in the world.
They are generally composed of semi-crystalline cellulose (38−50%), amorphous hemicellulose
(23−32%), and amorphous lignin (15−25%).13-15 Among the components, lignin – a highly
branched hydrophobic heteropolymer mainly providing mechanical strength to the plant cell walls
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and containing up to 40% of the energy content of lignocellulosic biomasses – has significant
potential as a renewable feedstock for the production of industrial biofuel and useful aromatic
compounds.7, 8, 9, 10, 16, 17 Despite the promising potential, however, valorization of lignin via
selective conversion has been a challenging problem due to its heterogeneous, non-hydrolysable
cross-linked structure making it recalcitrant to hydrolysis in lignin valorization. Lignin has
complex polymeric structures arising from dehydrogenative polymerization of synapyl alcohol,
coniferyl alcohol, and p-coumaryl alcohol (Figure 1).18 The aromatic subunits of the polymeric
lignin structure include syringyl type (4-alkyl-2,5-dimethoxyphenol), p-hydroxyphenyl type,
coniferyl alcohol fragment (4-(3-hydroxy-1-propenyl)-2-methoxyphenol), guaiacyl type (4-alkyl-
2-methoxyphenol), and phenylcoumaran19,20 and the subunits are linked via interunit linkages such
as β–O–4, α–O–4, β–5, β–β, 4–O–5, 5–5, and β–1, where the β–O–4 ether linkage is the most
common representing 50% of all interunit bonds.15,21 To break down the complexly integrated
subunits to transform it to value-added products, various reactions including hydrolysis,22, 23
catalytic cracking,24-26 reduction,27-30 and oxidation31-33 can be adopted. As an alternative approach
to convert lignin into the value-added aromatic compounds, ionic liquids (ILs) have recently been
explored.15 Because ILs exhibit a low or negligible volatility with the ability to tailor
physicochemical properties, various applications including lignin extraction from lignocellulosic
biomasses34-38 as well as catalytic,39 separations,40 and biotechnology applications41 have been
investigated.
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Here, we report the thermochemical transformation of lignin model compounds with an
organic−inorganic hybrid solvent, otherwise known as nanoparticle organic hybrid materials
(NOHMs). NOHMs have recently been developed as an emerging class of self-suspended
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nanoparticle solvent systems created by ionically or covalently grafting organic oligomers or
polymers (canopy) onto surface-modified inorganic nanoparticles (core).42-52 NOHMs exhibit
promising potential as a green solvent, particularly for energy and environmental applications,
providing environmentally benign and highly tunable properties with a negligible vapor pressure
at relatively high working temperatures.44 Versatile combinations of an organic canopy and
inorganic core species offer unique physicochemical properties, and thus, it is potentially
applicable to a variety of areas including reaction solvents, CO2 capture media, thermal
management materials, electrolytes, magnetic fluids, and lubricants.44, 45, 49, 51, 53, 54, 55 As an analog
to ILs, we synthesized NOHM with a surface-modified SiO2 nanocore (anion) and a functionalized
polyetheramine (cation), and the effects of the NOHM on the thermochemical conversion of lignin
model compounds were investigated.
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2. EXPERIMENTAL SECTION
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2.1. Chemicals
To explore the thermochemical conversion reactions, two lignin model compounds, 2-
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phenoxyethanol (99%, Sigma-Aldrich Co., USA), and benzyl phenyl ether (98%, Sigma-Aldrich
Co., USA) were used without further purification. As reaction solvents, ethyl alcohol (EtOH,
99.9%) was purchased from Daejung Chemicals & Metals Co., LTD (Republic of Korea), and tert-
butyl alcohol (TBA, 99%) was supplied from Alfa Aesar (USA). A 1-ethyl-3-methylimidazolium
ethyl sulfate ([EMIM][ESO4], 95%) was also purchased from Sigma Aldrich Co. (USA).
2.2. Synthesis of the Nanoparticle Organic Hybrid Materials
The NOHM was synthesized based on previously reported methods.44, 46, 55 Briefly, 3.05 g
of a 7 nm silica colloidal suspension (Sigma-Aldrich Co., USA) was added to deionized water to
prepare a 3 wt% silica suspension. The solution was mixed with a 6 wt% solution of 3-
(trihydroxysilyl)-1-propane sulfonic acid (Gelest Inc., USA). The acidity of the surface-modified
SiO2 nanoparticle suspension was adjusted to pH 5 with a 1 M NaOH solution. Next, the mixture
was heated to 70 ºC in a heating bath (B-100, BUCHI Labortechnik AG, Switzerland) for 24 hours
to complete the reaction. During the incubation, the mixture was stirred vigorously. After that, the
excess silane was removed by dialysis (3.5k MWCO, Pierce Biotechnology Inc., USA) against
deionized water for 48 hours. The dialyzed solution was then added to the cation exchange column
(Dowex HCR-W2, Sigma-Aldrich Co., USA) to remove the sodium ions and to protonate the
sulfonate groups of the functionalized silica nanoparticles.
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Next, a polyetheramine (Jeffamine 2070, Mw. ~ 2000, Huntsman Co., USA) was used as
a polymeric precursor for the NOHM used in this study. A 20 wt% of the polymer solution was
added dropwise to the functionalized silica nanoparticle suspension while monitoring the pH. The
addition of the Jeffamine 2070 solution was done until reaching the equivalent point to obtain the
highest grafting density. Finally, the NOHM was dried in a vacuum oven at 35 ºC for 48 hours.
Figure 2 shows the schematic structure of the synthesized NOHM.
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2.3. Thermochemical Depolymerization of the Lignin Model Compounds
A high-pressure, bolted closure type magnetic drive stirred reactor made of 316-stainless
steel with a working internal volume of 300 ml was used (M-Series, REXO Engineering Inc.,
Republic of Korea) for the thermochemical conversion of lignin model compounds. The reaction
temperature was controlled with PID (proportional-integral-derivative) electrical heating and
cooling arrangements.
The synthesized NOHM was used as a catalytic solvent for the thermochemical conversion
of lignin model compounds. The [EMIM][ESO4] was also used as a catalytic solvent for
comparison. EtOH and TBA were used as a proton source. The detailed experimental sets are
tabulated in Table 1. A 20 ml glass vial was filled with 10 ml of solution included a lignin model
chemical, catalytic solvent, and proton source. The vial was inserted into the high-pressure reactor.
After tightening the reactor with bolts, the residual air inside the reactor was evacuated by flushing
with N2 several times. Finally, the high-pressure reactor was pressurized up to 20 bar with a syringe
pump (500D, Teledyne Isco Inc., USA). After stabilization, the reactor was heated to 473 K for 30
minutes. The thermochemical conversion reaction was carried out at 473 K for 2 hours, followed
by cooling for 30 minutes to 293 K for the analyses.
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2.4. Spectroscopic Analyses
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A Fourier-transform infrared (FT-IR) spectrometer (Spectrum Two, PerkinElmer Inc.,
USA) equipped with a deuterated triglycine sulfate (DTGS) detector, and an attenuated total
reflectance (ATR) accessory (single reflection ZnSe crystal) was used. All spectra were recorded
with a resolution of 2 cm−1 in the 4000 – 500 cm−1 wavenumber region with 8 scans at room
temperature.
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A 400 MHz nuclear magnetic resonance (NMR) spectrometer (AVANCE III HD 400,
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Bruker Co., Germany) located at Chonnam National University was used. All 2D H−1H
correlation spectroscopy (COSY) NMR spectra were recorded in the Fourier-Transfrom mode
using dimethyl sulfoxide-d6 (DMSO-d6, 99.9 atom% D, Sigma-Aldrich Co., USA) as the solvent
and tetramethylsilane (TMS) as an internal standard.
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3. RESULTS AND DISCUSSION
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In general, lignin − a waste woody resource − has a high energy content because its
structure is based on aromatic hydrocarbons. Accordingly, it can be converted to useful chemicals
by depolymerization into various substituents. However, the structural complexity and diversity
make it difficult to use lignin efficiently. For better utilization, several approaches to break down
the major linkages such as C−O−C and C=O have been investigated with oxidation,56, 57
hydrolysis,58, 59 thermochemical depolymerization,60, 61 and hydrothermolysis.62, 63 Recently, ILs
were used as an effective catalytic solvent, particularly for the cleavage of the C–O ether bond of
the β–O–4 linkage.64, 65 ILs consist of relatively small inorganic anion and large organic cations
that can act as a high performance reaction media as a liquid phase under 573 – 673 K.66 Due to
their non-volatile and high thermal stability, ILs could accelerate the cracking of the C–O ether
bond of the β–O–4 linkage by thermochemical depolymerization.
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Thus, we explored the thermochemical depolymerization of lignin model compounds, 2-
phenoxyethanol and benzyl phenyl ether, using reaction solvents that were mixtures of proton
sources (EtOH or TBA) and catalytic liquids ([EMIM][ESO4] or NOHM). To compare the effect
of primary and tertiary alcohols as a proton source, EtOH and TBA were chosen. The
depolymerization performance of the catalytic solvents was also compared focusing on an IL
(([EMIM][ESO4]) and the synthesized NOHM. The thermal stability of the NOHM used in this
study was previously investigated by Lin et al., and the depolymerization was conducted below its
decomposition temperature.44
For the preliminary analysis of the depolymerized products by the thermochemical
conversion reactions, first, the ATR FT-IR spectra of the products for samples #1 − #6 were
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obtained shown in Figure 3. Compared to the pristine samples (before reaction), the
thermochemical conversion reactions caused significant changes over a wide range of the FT-IR
spectra. The characteristic bands of 2-phenoxyethanol and benzyl phenyl ether in the range of 1300
– 1000 cm–1, which correspond to the vibration of the phenyl C–O stretching, disappeared after
the thermochemical depolymerization (Figure 3 (a), (c), (d), (e), and (f)). The intensities of bands
in region 800 – 600 cm–1 were changed, where the vibrations of the aromatic C–H bonds, were
also observed shown in Figure 3 (c) – (f). Consequently, the ATR FT-IR results clearly show the
cleavage of the C–O bonds, and this would be evidence showing the change in the substitution of
2-phenoxyethanol and benzyl phenyl ether in the case of samples #1 − #6 by the thermochemical
depolymerization.
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For an in-depth qualitative analysis of samples #1 − #6, 2D H−1H COSY NMR
spectroscopy was employed, and the decomposed products were analyzed shown in Figure 4 – 9.
Figure 4 and 5 show the 2D COSY NMR spectra of the decomposed products from samples #1
and #2, respectively. As shown in Figure 4, most of the products remained as EtOH and 2-
phenoxyethanol, only a small amount of phenol was observed. In the case of sample #2, similar to
sample #1, the benzyl phenyl ether and EtOH were remained, phenol and toluene were additionally
observed in Figure 5. The cross peaks ((f1 (ppm), f2 (ppm))) at (1.06, 3.45) and (3.45, 4.41)
identified as EtOH; (3.72, 3.97), (3.72, 4.88) and (~ 6.83, ~ 6.97) were assigned to 2-
phenoxyethanol; the cross peaks at (~ 6.87, ~ 6.95) and (~ 7.23, ~ 7.30) were assigned as phenol
and the cross peaks at (~ 6.89, ~ 6.93) and (~ 6.97, ~ 7.03) were identified as toluene which were
decomposed from 2-phenoxyethanol or benzyl phenyl ether. The proposed depolymerization
reaction scheme is shown in Scheme 1.
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To explore the effects of the type of alcohol, TBA, a tertiary alcohol, was selected as a
proton source, and the thermochemical decomposition reactions were conducted with
[EMIM][ESO4]. The 2D COSY NMR spectra of the decomposed products from sample #3 and #4
are given in Figure 6 and 7, respectively. In the case of sample #3, more diverse forms of aromatic
hydrocarbons such as phenol (scheme 2), benzene (~ 7.26, ~ 7.42), (~ 7.33, ~ 7.49) (Scheme 3
(a)), and anisole (~ 6.74, ~ 6.79), (~ 7.12, ~ 7.18) (Scheme 4 (a)) were produced from the
decomposition of 2-phenoxyethanol. Interestingly, EtOH (Scheme 3(a)) and methanol (3.50, 4.27)
were also observed, even though TBA was used as a proton source. The cross peaks at (1.05, 3.32)
and (1.12, 4.28) assigned as an [EMIM][ESO4] was observed. It is speculated that the formation
of phenol, benzene, anisole and methanol can be synthesized at elevated temperature and pressure
conditions. In the case of sample #4, the peak patterns, which were assigned to phenol (Scheme
2), and toluene (Scheme 3 (b)), were similar to the case of sample #2 (Figure 5). The
[EMIM][ESO4] was also observed, however, what was different, EtOH, benzene (Scheme 3 (b))
and phenylmethanol (4.52, 5.24) (Scheme 4 (b)) were produced from the depolymerization of
benzyl phenyl ether.
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When compared to the cases of EtOH, the use of a tertiary alcohol, TBA, seemed to
produce more diverse forms of aromatic hydrocarbon compounds. This implies that TBA has a
significant role as a proton source in the thermochemical depolymerization of 2-phenoxyethanol
and benzyl phenyl ether exhibiting a better ability than that of the primary alcohol, EtOH. As
illustrated in Scheme 5, it is speculated that a tertiary carbocation or hydroxide ion generated from
TBA could be stabilized by the ionic species of [EMIM][ESO4] resulting in more effectively
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promoting the formation of diverse moieties cleaving the C–O and/or C–C bonds of the lignin
model compounds.
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To examine the potential application of the NOHM for lignin valorization, the synthesized
NOHM were adopted as a catalytic solvent, and the thermochemical reactions were carried out for
2-phenoxyethanol and benzyl phenyl ether using TBA as a proton source. The 2D COSY NMR
results are shown in Figure. 8 and 9. As seen in Figure 8, the peak patterns were assigned as EtOH
(Scheme 3 (a)), phenol (Scheme 2), benzene (Scheme 3 (a)), anisole (Scheme 4 (b)) were similar
to the case of sample #4. What was different, however, the amount of aromatic hydrocarbons such
as phenol, benzene, and anisole, which were decomposed from 2-phenoxyethanol was relatively
larger than sample #4. This phenomenon indicates the NOHM can decompose the ether bond in
the aromatic compounds more selectively than the case of [EMIM][ESO4] adaptation.
Figure 9 shows the thermochemical conversion result of sample #6. The result indicates
that EtOH, phenol (Scheme 2), benzene (Scheme 3(b)), toluene (Scheme 4 (b)), phenylmethanol
(Scheme 4 (b)) were observed. Interestingly, the benzaldehyde (7.59, 7.71), (7.59, (7.91), and (7.59,
8.13) and 2-benzyl phenol (3.72, 7.39) were produced from benzyl phenyl ether. The conversion
of benzyl phenyl ether to benzaldehyde67 and 2-benzyl phenol68 has been reportedly supported by
hydrolysis and/or hydrothermal treatment. It is speculated that the NOHM stabilized tertiary
carbocation or hydroxide ion more effectively than [EMIM][ESO4] to promote the cleavage of C–
O and/or C–C bonds in the lignin model compounds.
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4. CONCLUSIONS
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This study examined the thermochemical depolymerizations of 2-phenoxyethanol and
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benzyl phenyl ether by using proton sources and catalytic solvents. ATR FT-IR and 2D H−1H
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COSY NMR spectra revealed that both lignin model compounds were successfully decomposed
to aromatic hydrocarbon intermediates such as phenol, benzene, and toluene under the reaction
condition of 20 bar in a N2 environment at 473 K. The results revealed that the use of a tertiary
alcohol, TBA, enhanced the thermochemical decomposition of the lignin model compounds
producing diverse aromatic hydrocarbons compared to the case of the primary alcohol, EtOH. The
results also revealed that the synthesized NOHM could be an effective catalytic solvent cracking
valuable compounds such as benzaldehyde and 2-benzyl phenol which were not observed in the
case of [EMIM][ESO4]. Also, the results obviously indicated that the NOHM can decompose the
C–O and/or C–C bonds of lignin model chemical more selectively than the case of [EMIM][ESO4].
As a designer solvent, the NOHM could be a promising catalytic solvent for lignin valorization by
optimal designing of an organic canopy and inorganic catalytic nanocore species. Because only
qualitative analyses were done in this study, further studies that include quantitative approaches
are needed to fully utilize the potential applications of NOHM in lignin valorization.
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FIGURES AND SCHEMES
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Figure 1. Lignin monolignols: (a) p-Coumaryl alcohol (H), (b) Coniferyl alcohol (G), (c) Sinapyl
alcohol (S).
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Figure 2. (a) Synthesis and schematic structure of the NOHM with ionically tethered amine
terminated polymeric canopy species, and (b) photo image.
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Figure 3. ATR FT-IR spectra of the thermochemical depolymerization of (a) 2-phenoxyethanol +
[EMIM][ESO4] + EtOH, (b) benzyl phenyl ether + [EMIM][ESO4] + EtOH, (c) 2-phenoxyethanol
+ [EMIM][ESO4] + TBA, (d) benzyl phenyl ether + [EMIM][ESO4] + TBA, (e) 2-phenoxyethanol
+ NOHM + TBA, and (f) benzyl phenyl ether + NOHM + TBA.
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Figure 4. 2D COSY NMR spectra of the thermochemical conversion of 2-phenoxyethanol +
[EMIM][ESO4] + EtOH.
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Figure 5. 2D COSY NMR spectra of the thermochemical conversion of benzyl phenyl ether +
[EMIM][ESO4] + EtOH.
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Figure 6. 2D COSY NMR spectra of the thermochemical conversion of 2-phenoxyethanol +
[EMIM][ESO4] + TBA.
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Figure 7. 2D COSY NMR spectra of the thermochemical conversion of benzyl phenyl ether +
[EMIM][ESO4] + TBA.
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Figure 8. 2D COSY NMR spectra of the thermochemical conversion of 2-phenoxyethanol +
NOHM + TBA.
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Figure 9. 2D COSY NMR spectra of the thermochemical conversion of benzyl phenyl ether +
NOHM + TBA.
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Scheme 1. Proposed ether cleavage mechanism of 2-phenoxyethanol and benzyl phenyl ether
using [EMIM][ESO4] and EtOH.
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Scheme 2. Proposed ether cleavage mechanisms of 2-phenoxyethanol and benzyl phenyl ether
using [EMIM][ESO4] or NOHM with TBA.
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Scheme 3. (a) Proposed ether cleavage mechanisms of 2-phenoxyethanol for the formation of
phenol, benzene, and ethanol. (b) Proposed decomposition mechanism of benzene, toluene, phenol
in the [EMIM][ESO4] or NOHM with TBA.
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Scheme 4. Proposed decomposition mechanisms of 2-phenoxyethenaol for the formation of (a)
anisole and (b) phenylmethanol reacted with hydroxide ion produced from TBA.
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Scheme 5. Proposed stabilization mechanisms of the catalytic solvents; the tertiary carbocation
and hydroxide ion produced from TBA were induced (a) [EMIM][ESO4], and (b) NOHM.
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Table 1. Experimental sets of the thermochemical conversion of lignin model compounds
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examined in this study.
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Sample
#1
Lignin model compound (g)
2-Phenoxyethanol (0.138 g)
Benzyl phenyl ether (0.184 g)
2-Phenoxyethanol (0.138 g)
Benzyl phenyl ether (0.184 g)
2-Phenoxyethanol (0.138 g)
Benzyl phenyl ether (0.184 g)
Catalytic solvent (g)
[EMIM][ESO4] (0.236 g)
[EMIM][ESO4] (0.236 g)
[EMIM][ESO4] (0.236 g)
[EMIM][ESO4] (0.236 g)
NOHM (0.75 g)
Proton source solvent
EtOH (7.89 g)
EtOH (7.89 g)
TBA (7.81 g)
#2
#3
#4
TBA (7.81 g)
#5
TBA (7.81 g)
#6
NOHM (0.75 g)
TBA (7.81 g)
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