Efficient Conversion of Pine Wood Lignin to Phenol

Article abstract of DOI:10.1002/cssc.202000485

Obtaining chemical building blocks from biomass is attractive for meeting sustainability targets. Herein, an effective approach was developed to convert the lignin part of woody biomass into phenol, which is a valuable base chemical. Monomeric alkylmethoxyphenols were obtained from pinewood, rich in guaiacol-type lignin, through Pt/C-catalyzed reductive depolymerization. In a second step, an optimized MoP/SiO₂ catalyst was used to selectively remove methoxy groups in these lignin monomers to generate 4-alkylphenols, which were then dealkylated by zeolite-catalyzed transalkylation to a benzene stream. The overall yield of phenol based on the initial lignin content in pinewood was 9.6 mol %.

Full text of DOI:10.1002/cssc.202000485

Accepted Article
Title: Efficient Conversion of Woody Biomass to Phenol
Authors: Xianhong Ouyang, Xiaoming Huang, Michael Boot, and Emiel
Hensen
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To be cited as: ChemSusChem 10.1002/cssc.202000485
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10.1002/cssc.202000485
ChemSusChem
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Efficient Conversion of Pine Wood Lignin to Phenol
Xianhong Ouyang,[a] Xiaoming Huang,*[a] † Michael D. Boot,[b] and Emiel J. M. Hensen*[a]
[a]
X. Ouang, Dr. X. Huang, Prof. E. J. M. Hensen
Laboratory of Inorganic Materials and Catalysis
Department of Chemical Engineering and Chemistry
Eindhoven University of Technology
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
E-mail: xiaoming.huang@ptgeindhoven.nl; e.j.m.hensen@tue.nl
Dr. M. D. Boot
[b]
Combustion Technology
Department of Chemical Engineering and Chemistry
Eindhoven University of Technology
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
Current address: Polymer Technology Group Eindhoven (PTG/e) B.V., P.O. Box 6284, 5600 HG Eindhoven, the Netherlands
†
Supporting information for this article is given via a link at the end of the document.
Abstract: Obtaining chemical building blocks from biomass is
attractive in order to meet sustainability targets. Herein we report an
effective approach to convert the lignin part of woody biomass into
phenol, which is valuable base chemical. Monomeric
alkylmethoxyphenols are obtained from pinewood, rich in guaiacol-
type lignin, through Pt/C-catalyzed reductive depolymerization. In a
second step an optimized MoP/SiO2 catalyst is used to selectively
remove methoxy groups in these lignin monomers to generate 4-
to benzene using a zeolite catalyst. The limited phenol yield is
due to the low efficiency of the demethoxylation step, while the
use of gold also raises concerns about the scale-up of this
process.
Earth-abundant metals (e.g. Ni, Mo, Fe), which can replace
noble metals, and their corresponding oxide, carbide, sulfide,
phosphide compounds have been reported as potential
candidates for the deoxygenation of biomass-derived
molecules.[8] Among them, transition metal phosphides show
promising properties in the selective removal of oxygen in lignin-
derived molecules without hydrogenation of the aromatic ring.
For example, 80% of a gaseous guaiacol feed could be
converted using a supported Ni2P catalyst with benzene (60%)
and phenol (30%) as main products.[9] In this work, we explored
the potential of metal phosphides in the bifunctional upgrading of
2-methoxy-4-propylphenol to phenol as a model reaction. Our
approach was to first optimize the transition metal phosphide
towards optimum HDMeO of 2-methoxyl-4-propylphenol to 4-
propylphenol. MoP/SiO2 was identified as the most promising
metal phosphide, because the product yield was 88 mol%
without touching the aromatic ring. The optimization of this
catalyst in the bifunctional conversion of 2-methoxyl-4-
propylphenol to phenol led to a phenol yield close to 90 mol% at
350 °C at a high weight hourly space velocity of 40 h-1. The
optimized system and conditions were then applied to convert a
LFP-derived lignin oil from pinewood into phenol. This strategy
opens a new avenue in the valorization of biomass towards bulk
chemicals, contributing to a greener chemical industry.
We first optimized the HDMeO function by comparing FeP/SiO2,
CoP/SiO2, Ni2P/SiO2, WP/SiO2 and MoP/SiO2 using 2-methoxy-
4-propylphenol as a model compound. The metal phosphides
were prepared by a two-step wetness impregnation method of
appropriate metal salts and diammonium hydrogen phosphate,
followed by reduction at 700 °C aiming at the conversion of the
metal oxide precursor to the corresponding phosphide
(supporting information section 2.1). Silica is a preferred support
for metal phosphides.[10] Elemental analysis confirms proper
metal and phosphorus loadings and metal-to-phosphorus ratios
(Table S1). The formation of metal phosphides is evidenced by
XRD (Figure S1) and XPS (Figure S2).
alkylphenols, which are dealkylated by
a
zeolite-catalyzed
transalkylation to a benzene stream. The overall yield of phenol
based on the initial lignin content in pinewood is 9.6 mol%.
Lignocellulosic biomass holds great promise as a source of
sustainable chemicals replacing fossil resources. Lignin is the
largest renewable source of aromatics, which explains the
substantial interest from the chemical industry.[1] Many different
methods have already been explored to depolymerize the
recalcitrant polyphenolic network of lignin into aromatic
monomers.[2] Among these, the lignin-first process (LFP) stands
out in terms of the high yield of monomers, in the form of
alkylmethoxyphenols, using optimized catalysts to cleave the
linkages that bind lignin fragments to hemicellulose and lignin
intralinkages.[3]
As
the
market
demand
for
these
alkylmethoxyphenols represents a small size, it is desirable to
convert them into bulk chemical building blocks.[4] For instance,
Song et al. described an approach to convert 2-methoxy-4-
propylphenol to teraphthalic acid through demethoxylation,
carbonylation and oxidation.[5] Other than terapthalic acid, the
removal of the methoxy functionalities and alkyl side groups can
give access to phenol, which is a valuable intermediate in the
manufacture of agro-chemicals, detergents, plastics, medicine,
dyes and plasticizers.[6] We earlier explored a combination of
hydrodemethoxylation (HDMeO) and transalkylation of the alkyl
groups to a benzene co-reactant in order to obtain phenol from
the same lignin model compound in a one-step approach.[7] Full
conversion of 2-methoxy-4-propylphenol, representative for
guaiacol-type alkylmethoxylphenols obtained from LFP, was
achieved with
a reasonable phenol yield of 60%. The
bifunctional catalytic approach combined hydrodemethoxylation
catalysed by Au/TiO2 and transalkylation of the propyl side group
1
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10.1002/cssc.202000485
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compared to the other catalysts as judged from CO uptake
measurements (Table S1). In contrast to the other metal
phosphides, TEM shows a high MoP dispersion (Figure S4). The
high dispersion of MoP can be caused by the metal weight
loading and the mobility of metal phase in reduction process
(Table S2).[12],[13] As earlier work indicated that MoO3 is also a
potential catalyst for the deoxygenation of lignin monomers,[14]
we compared MoP/SiO2 to MoO3/SiO2 in our model HDMeO
reaction. Although MoO3/SiO2 can also convert 2-methoxy-4-
propylphenol to the desired reaction product, MoP/SiO2 is much
more active and produces less by-products. These results
highlight the unique ability of MoP/SiO2 for the selective removal
of the methoxy group of 2-methoxy-4-propylphenol by HDMeO.
Obtaining phenol from the demethoxylated intermediate requires
the removal of the alkyl group for which we explored
transalkylation to another aromatic molecule. Zeolites are
shape-selective
catalysts
for
various
kinds
of
alkylation/dealkylations reactions.[15] These catalysts can also be
involved in transalkylation, allowing a shift from alkyl groups
from one aromatic molecule to another.[16] We exploit this latter
property by using benzene as a solvent, which is present in
excess to the lignin monomer. In addition to the formation of
valuable phenol, this approach also increases the value of the
benzene stream. Alkylated benzenes are more valuable than
benzene in the chemical industry. We selected acidic HZSM-5
with Si/Al 15 as a transalkylation catalyst. In a typical reaction, a
mixture of 100 mg passivated MoP/SiO2 (particle size 75-200
µm) and 100 mg of HZSM-5 (particle size 300-500 µm) were
mixed and placed in a stainless-steel fixed-bed reactor (Figure
2a). Figure 2b (left) shows the results of catalytic tests using a
feed of 2-methoxy-4-propylphenol and benzene (1:20 molar
ratio) at 350 °C and 90 bar H2 at WHSV 40 h-1. The combined
demethoxylation by MoP/SiO2 and transalkylation by zeolite
resulted in a promising phenol yield of 83% after 1 hour. The
propyl chain is mainly transferred to benzene yielding n-
propylbenzene and cumene. These products have particular
value in the current context as they can be converted to phenol
by oxidation.[17] Toluene and xylenes were other reaction
products, which are most likely obtained by alkylation of
benzene with methanol derived from removal of the methoxy
group as well as isomerization reactions of cumene and n-
Figure 1. Conversion of 2-methoxyl-4-propyl phenol demethoxylation and
product distribution over different metal phosphide catalysts: FeP/SiO2,
CoP/SiO2, Ni2P/SiO2, WP/SiO2, MoP/SiO2 and MoO3/SiO2 catalyst.
Pretreatment conditions: 100 mg of catalyst is reduced in 100 ml/min H2 at
450 °C for 1 hour. Reaction conditions: 5 mol% of 2-methoxyl-4-propylphenol
in benzene, 350 °C, 90 bar, gas flow rate 30 ml/min H2, weight hourly space
velocity (WHSV) 80 h-1.
The reduced catalysts were used to demethoxylate 2-methoxy-
4-propylphenol at 350 °C under 90 bar of H2 pressure using
benzene as the solvent (Figure 1).[7],[11] The desired reaction is
the removal of the methoxy group from the model reactant,
which yields 4-propylphenol and methanol. Removal of the
phenolic hydroxyl group is more difficult and would lead to the
formation of propylbenzene, while aromatic ring hydrogenation is
another possible side-reaction yielding propylcyclohexane. The
reactant as well as the aromatic products can also be ring-
alkylated by methanol. The lowest product yield is obtained with
the incompletely reduced Fe-based catalyst. CoP/SiO2 converts
the first hour 84% of the reactant with a 45% yield of 4-
propylphenol, 18% of methylated 4-propylphenols and 6% of n-
propylbenzene. Although the product distribution is promising
and aromaticity is retained, this catalyst suffers from severe
deactivation. A higher and more stable conversion is attained
with Ni2P/SiO2 as the catalyst. However, a drawback of this
catalyst is that hydroxyl group removal and aromatic ring
hydrogenation proceeded at substantial rates, leading to the
formation of n-propylcyclohexane. The higher aromatic ring
hydrogenation rate also leads to hydrogenation of the solvent,
which is highly undesired (Figure S3). WP/SiO2 shows a
reasonable 4-propylphenol yield of 40 mol% with a relatively
small amount of methylated 4-propylphenols by-product.
However, the conversion rate was low with this catalyst. The
best performance was obtained with the MoP/SiO2 catalyst,
propylbenzene.
Furthermore,
the
observation
of
diphenylmethane (3 mol%) points to dimer formation, which can
slowly accumulate and then result in the coke formation (Figure
S5, S6). Propane (1 mol%) was found dissolved in benzene and
also a minor amount of methane can be formed as the gaseous
products due to the propyl guaiacol demethylation catalyzed by
HZSM-5.[7] Although the initial phenol yield was high during the
first hour, the catalyst deactivated as evident from the slowly
decreasing phenol yield. This is clearly due to a loss of the
transalkylation activity, because the 4-propylphenol yield
increases with time on stream (Figure 2b left). A similar trend is
observed among benzene-derived products in which the
propylbenzene yield became lower with time on stream (Figure
S7).
To investigate the origin of this undesired activity loss, the used
HZSM-5 zeolite was separated from the catalyst mixture by
sieving (different particle sizes were used for the two catalyst
components) and analysis by XPS (Figure 2a). XPS shows that
the used zeolite contains phosphate species as evident from the
P 2p signal at 134.9 eV (Figure 2c). Importantly, the zeolite
enabling
a
near complete conversion of 2-methoxy-4-
propylphenol with an 88 mol% yield of 4-propylphenol.
Remarkably, this catalyst can remove the methoxy group without
touching the phenolic hydroxyl group or the aromatic ring. The
major by-products are methylated 4-propylphenols (10 mol%).
Besides the nearly quantitative yield of alkylphenols, the catalyst
also exhibited good stability during 6 h of reaction. While the
desirable selectivity can be correlated to the intrinsic catalytic
properties of MoP, the high activity can be related to the
significantly higher dispersion of the MoP phase on silica
2
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Figure 2. One-pot conversion of 2-methoxy-4-propylphenol to phenol in a fixed-bed flow reactor, separation of the two catalytic components of the used catalysts
and XPS analysis of used HZSM-5 (a); conversion and product yield using HZSM-5 with passivated MoP/SiO2 (left in b) and non-passivated MoP/SiO2 (right in b).
Pretreatment: reduction of 100 mg MoP/SiO2 and 100 mg HZSM-5 at 450 °C for 1 hour. Reaction conditions: 5 mol% of 2-methoxyl-4-propyl phenol in benzene,
350 °C, 90 bar, H2 flow rate 30 ml/min, weight hourly space velocity (WHSV) 40 h-1. P 2p (c) and Mo 3d (d) XPS spectra of used HZSM-5 catalyst.
component does not contain molybdenum nor other (reduced)
phosphide species (Figure 2d). Thus, we can firmly conclude
that unreduced phosphate left over from the preparation
migrated from the MoP/SiO2 component to HZSM-5 during the
ongoing reaction. Although phosphate is regarded as a promoter
for the hydrothermal stability of HZSM-5, it strongly interacts with
Brønsted acid sites of zeolites, lowering the total acidity and thus
the activity.[18] This deactivation mechanism is confirmed by the
poor performance (lower conversion and phenol yield) of a
phosphate-modified HZSM-5 in comparison to a P-free HZSM-5
zeolite in 4-propylphenol transalkylation (Figure S8). Thus, we
can firmly conclude that the zeolite acid sites are deactivated by
phosphate species migrating from the MoP/SiO2 to HZSM-5. We
considered that these phosphate species can originate from the
passivation procedure of the MoP phase during the reduction
step and possibly the further exposure in ambient air when
loading the reactor. In order to verify this, we developed a
protocol in which the reduced MoP/SiO2 was directly mixed with
the zeolite catalyst and loaded into the reactor in a nitrogen-
flushed glovebox. Figure 2b (right) show that this approach
yielded a more stable catalyst system for phenol production. A
high and stable phenol yield of ~80% was obtained with cresol
as the main by-product. The alkylation of benzene in this
modified approach was also confirmed and the yield of alkylated
products with time on stream was also stable (Figure S7). Thus,
preventing MoP oxidation, which would otherwise generate
phosphate species, is a key factor in obtaining a stable catalyst
for a high phenol yield.
catalysts was evaluated in combination with HZSM-5 for the
conversion of 2-methoxy-4-propylphenol to phenol. The
conversion in these tests was complete and the phenol yield
was very high with typical cresol yields of about 10 mol% (Figure
S12). There was no observable deactivation during these
measurements, as also confirmed by the near constant yield of
n-propylbenzene and cumene (Figure S13). Despite the minor
effect on the conversion, Figure 3 shows that a higher reduction
temperature results in a higher phenol yield. The higher degree
of MoP formation and the slightly lower amount of residual
phosphate can contribute to these improvements. XPS analysis
of used MoP/SiO2 catalysts shows that the Mo oxidation state
did not change during the reaction (Figure S14). Nevertheless,
we still observed a small loss of phosphate species from the
In recognizing the importance of avoiding phosphate, we
optimized the preparation of MoP by varying the reduction
temperature required to obtain the phosphide from Mo-oxide and
phosphate precursors in the 600-900°C range (Figure S9-S11).
The catalytic performance of these differently reduced MoP/SiO2
Figure 3. Influence of reduction temperature on phosphide formation (blue), 2-
methoxy-4-propylphenol conversion (black) and phenol yield (red, average
yield during 6 h reaction).
3
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Figure 4. Schematic representation of the three-step approach developed in this work to obtain phenol from woody biomass.
decrease of the corresponding XPS
P
2p feature. The
In summary, we developed a novel catalytic approach for
valorization of lignin monomers obtainable from different types of
wood by LFP. In a relevant example, lignin in pinewood is
phosphorus content on used zeolite was very high (0.7 wt%) in
the experiment in which MoP/SiO2 was reduced at 600 °C and
lowest in the experiment in which reduction was carried out at
900 °C (0.4 wt%) (Figure S15, S16).
The above results show the promise of using a combination of
MoP/SiO2 and HZSM-5 zeolite in converting lignin-derived
guaiacol-type compounds in high yield to phenol. We also
evaluated the stability of this catalyst combination in a 48 hours
reaction (Table S3). The phenol yield was found to slowly
decrease with time due to the coke formation in used HZSM-5.
After catalyst regeneration at 700 ºC for 2 hours in a hydrogen
atmosphere, the phenol yield was 85% again. The utility of this
converted to phenol using
a
combination of reductive
depolymerization of in planta lignin followed by combined
hydrodemethoxylation and transalkylation. The upgrading of the
carbohydrate pulp to value-added products via different
strategies has already been explored before.[20] The novelty of
our approach is the use of relatively cheap MoP/SiO2 to catalyse
hydrodemethoxylation. We demonstrated that the formation of
MoP species is not only important for a high demethoxylation
activity, but also essential to limit the amount of remaining
phosphate precursor to stabilize the performance. A too high
residual phosphate yield leads to slow deactivation of the zeolite
component due to migration to Brønsted acid sites. Conceptually,
an advantage of the described approach is the benefit of
integrating the described novel biobased process in existing
chemical processes for phenol production.
catalyst combination in converting
a
syringol-type 2,6-
dimethoxy-4-propylphenol, in separate demethoxylation and
transalkylation reactions showed that the phenol yield in this
case was much lower at 31 mol% with a non-closed mass
balance, likely due to oligomerization of intermediates (Table S4
and S5). Although further investigations are needed to more
efficiently convert syringol-type intermediates coming from
hardwood biomass, we decided to evaluate the potential of our
approach on a real lignin oil derived from pinewood by LFP
(Figure 4). We choose pinewood, because this kind of softwood
is rich in guaiacol-type building blocks. For the LFP, pinewood
sawdust was subjected to a reaction at 230 °C and 30 bar H2
using a Pt/C catalyst in a methanol-water mixture (molar
methanol-to-water ratio ½).[19] All β-O-4 lignin interlinkages in the
original biomass were cleaved as confirmed by HSQC NMR
(Figure S19). Gel permeation chromatography evidenced the
formation of lignin monomers with only a small amount of
relatively low-molecular-weight lignin fragments being dissolved
in benzene (Figure S20). The total monomer yield was 15 wt%
of which 78% was 2-methoxy-4-propylphenol. Other lignin
monomers were guaiacol, methylguaiacol and ethylguaiacol.
The lignin monomers in this oil were then further converted by
our combination of MoP/SiO2 and HZSM-5 catalysts (Figure
S21). Not only 4-propylguaiacol, but also ethylguaiacol and
methylguaiacol were converted to phenol, showing that our
approach is broadly able to convert substituted guaiacols.
Importantly, full conversion of these compounds in the lignin oil
and stable phenol yield support our conclusion that the dual-
catalyst is stable for at least 6 h. Based on the initial lignin
content in pinewood, we determined a phenol yield of 9.6 mol%.
Acknowledgements
Xianhong Ouyang thanks the China Scholarship Council for the
financial support. The authors thank Adelheid Elemans Mehring
for the ICP analysis and Jiadong Zhu, Jan Wiesfeld, Arno van
Hoof and Rim van de Poll for acquiring TEM images.
Keywords: biomass • phenol • lignin-first • demethoxylation •
transalkylation
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Entry for the Table of Contents
Phenol is obtained from renewable woody biomass in a promising yield through lignin reductive depolymerization, demethoxylation
and transalkylation.
6
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