Bifunctional Molybdenum Polyoxometalates for the Combined Hydrodeoxygenation and Alkylation of Lignin-Derived Model Phenolics

Article abstract of DOI:10.1002/cssc.201700297

Reductive catalytic fractionation of biomass has recently emerged as a powerful lignin extraction and depolymerization method to produce monomeric aromatic oxygenates in high yields. Here, bifunctional molybdenum-based polyoxometalates supported on titania (POM/TiO₂) are shown to promote tandem hydrodeoxygenation (HDO) and alkylation reactions, converting lignin-derived oxygenated aromatics into alkylated benzenes and alkylated phenols in high yields. In particular, anisole and 4-propylguaiacol were used as model compounds for this gas-phase study using a packed-bed flow reactor. For anisole, 30 % selectivity for alkylated aromatic compounds (54 % C-alkylation of the methoxy groups by methyl balance) with an overall 72 % selectivity for HDO at 82 % anisole conversion was observed over H₃PMo₁₂O₄₀/TiO₂ at 7 h on stream. Under similar conditions, 4-propylguaiacol was mainly converted into 4-propylphenol and alkylated 4-propylphenols with a selectivity to alkylated 4-propylphenols of 42 % (77 % C-alkylation) with a total HDO selectivity to 4-propylbenzene and alkylated 4-propylbenzenes of 4 % at 92 % conversion (7 h on stream). Higher catalyst loadings pushed the 4-propylguaiacol conversion to 100 % and resulted in a higher selectivity to propylbenzene of 41 %, alkylated aromatics of 21 % and alkylated phenols of 17 % (51 % C-alkylation). The reactivity studies coupled with catalyst characterization revealed that Lewis acid sites act synergistically with neighboring Br?nsted acid sites to simultaneously promote alkylation and hydrodeoxygenation activity. A reaction mechanism is proposed involving activation of the ether bond on a Lewis acid site, followed by methyl transfer and C-alkylation. Mo-based POMs represent a versatile catalytic platform to simultaneously upgrade lignin-derived oxygenated aromatics into alkylated arenes.

Full text of DOI:10.1002/cssc.201700297

Accepted Article
Title: Bifunctional Molybdenum Polyoxometalates for the Combined
Hydrodeoxygenation and Alkylation of Lignin-derived Model
Phenolics
Authors: Eric Michael Anderson, Anthony Crisci, Karthick Murugappan,
and Yuriy Roman-Leshkov
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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the VoR from the journal website shown below when it is published
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To be cited as: ChemSusChem 10.1002/cssc.201700297
Link to VoR: http://dx.doi.org/10.1002/cssc.201700297
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10.1002/cssc.201700297
ChemSusChem
FULL PAPER
Bifunctional Molybdenum Polyoxometalates for the Combined
Hydrodeoxygenation and Alkylation of Lignin-derived Model
Phenolics
Eric Michael Anderson‡, Anthony Crisci‡, Karthick Murugappan, and Yuriy Román-Leshkov*
‡Contributed equally to this work
because it contains highly recalcitrant C-C linkages that render
Abstract: Reductive catalytic fractionation of biomass has
the material unsuitable for selective depolymerization to
recently emerged as
a
powerful lignin extraction and
monomers.9-10
depolymerization method to produce monomeric aromatic
oxygenates in high yields. Here, bifunctional molybdenum-based
polyoxometalates supported on titania (POM/TiO2) are shown to
promote tandem hydrodeoxygenation (HDO) and alkylation
reactions, converting lignin-derived oxygenated aromatics into
alkylated benzenes and alkylated phenols in high yields. In
particular, anisole and 4-propylguaiacol were used as model
compounds for this gas-phase study using a packed bed flow
reactor. For anisole, 30% selectivity for alkylated aromatic
compounds (54% C-alkylation of the methoxy groups by methyl
balance) with an overall 72% selectivity for HDO at 82% anisole
conversion was observed over H3PMo12O40/TiO2 at 7 h on stream.
Under similar conditions, 4-propylguaiacol was mainly converted
Recently, reductive catalytic fractionation (RCF) has
emerged as a promising method to extract and selectively
depolymerize lignin into monomers and dimers from raw biomass
feedstock.11-16 RCF is typically performed in the presence of protic
solvents at temperatures ranging from 453-523 K using supported
transition metal catalysts (e.g., ruthenium, palladium or nickel)
and hydrogen gas or other hydrogen transfer agents (e.g.,
methanol, isopropyl alcohol or formic acid) as reductants.17-19 The
RCF process generates two highly valuable streams:
a
carbohydrate rich solid residue (>90% carbohydrate retention)
and a lignin rich bio-oil (>70% lignin extraction) that contains high
concentrations of monomeric aromatic species. Notably, the RCF
of hardwoods (e.g., birch and poplar) have been shown to
generate total yields of 4-propylguaiacol and 4-propylsyringol as
high as 40-50 wt%.11, 20
Unfortunately, these lignin-derived monomers cannot be
used directly as fuel additives due to their high boiling points
(>541 K) resulting from their high oxygen content (20-25 wt%).21-
22 The energy density and volatility of 4-propylguaiacol and 4-
propylsyringol can be improved by deoxygenation. In particular,
hydrodeoxygenation (HDO) is an effective reductive method for
cleaving aromatic C-O bonds via hydrogenolysis.
into 4-propylphenol and alkylated 4-propylphenols with
a
selectivity to alkylated 4-propylphenols of 42% (77% C-alkylation)
with a total HDO selectivity to 4-propylbenzene and alkylated
4-propylbenzenes of 4% at 92% conversion (7 h on stream).
Higher catalyst loadings increased the 4-propylguaiacol
conversion to 100% and resulted in a selectivity values to
propylbenzene of 41%, alkylated aromatics of 21% and alkylated
phenols of 17% (51 % C alkylation). The reactivity studies coupled
with catalyst characterization revealed that Lewis acid sites act
synergistically with neighboring Brønsted acid sites to
simultaneously promote alkylation and hydrodeoxygenation
activity. A reaction mechanism is proposed involving activation of
the ether bond on a Lewis acid site, followed by methyl transfer
and C-alkylation. Mo-based POMs represent a versatile catalytic
platform to simultaneously upgrade lignin-derived oxygenated
aromatics into alkylated arenes.
Many different materials have been studied for the HDO of
aromatic compounds, such as late transition group metals, metal
oxides and metal carbides/phosphides. Supported late transition
metal catalysts have been extensively investigated for the HDO
of phenolic model compounds, including eugenol, guaiacol and
phenol.23-30 Typically, these studies are performed in batch
reactions with high hydrogen pressures (30-100 bar) and
temperatures (423-573 K) with reaction times ranging from 1-8 h.
Under these conditions, the oxygenated aromatic compounds are
saturated to cyclohexanols and then deoxygenated either by HDO
or through an acid catalyzed dehydration to yield alkanes.
Recently, Wang et al. has also performed HDO on lignin oil
produced through the reductive fractionation of poplar wood.31 In
this study, nitrogen doped ordered mesoporous carbon supported
PtCo nanoparticles converted lignin oil into numerous different
alkanes at 100 bar of H2 (at temperature), 573 K and a reaction
time of 12 h. Therefore, many materials exhibit high activity for C-
O bond hydrogenation, but show poor selectivity for C-O bond
cleavage in the presence of C-C double bonds and aromatic
bonds. Furthermore, very few materials are capable of directly
activating and deoxygenating the aromatic C-O bond of phenolic
compounds.
Ideally, the HDO of lignin derived compounds would yield
alkylated aromatics thereby preserving an important feature of
lignin while simultaneously reducing hydrogen consumption.
Typically, when HDO is performed on 4-propylguaicol and 4-
propylsyringol, the carbon contained in the methoxy groups on
these compounds will be lost as methane. Directly alkylating this
carbon onto the aromatic ring simultaneously while removing
oxygen will effectively increase both the energy density and the
atom economy of the reactions by producing alkylated aromatics
while reducing methane production (illustrated in scheme 1 with
anisole as a model compound).
Introduction
Lignocellulosic biomass, composed primarily of cellulose,
hemicellulose and lignin, has widely been touted as a source for
renewable chemicals and fuels.1-3 Hemicellulose and cellulose
are both polysaccharides, while lignin is an aromatic polymer built
by oxygenated aromatic monomers linked with a variety of C-O
and C-C bonds.4-5 Although lignin theoretically could generate
valuable aromatic chemicals and fuel additives, technical
challenges around its selective depolymerization and upgrading
have prevented its widespread use as a renewable feedstock.
Currently, biorefineries target optimal sugar utilization to
maximize bioethanol production. The first step in lignocellulosic
biomass processing involves removing lignin with chemical
processes.6-8 These pretreatments significantly alter the chemical
structure of native lignin and generate large streams of technical
lignin that is burned for process heating. Technical lignin is burned
Anderson. E. M.; Crisci, A. Murugappan, K. Román-Leshkov, Y.*
Department of Chemical Engineering
Massachusetts Institute of technology
25 Ames ST Cambridge, MA 02139
E-mail: yroman@mit.edu
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/cssc.201700297
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Results and Discussion
The reaction profile of anisole over the bifunctional
POM/TiO2 catalyst is shown in Figure 1A. Four types of products
are generated based on the combination of HDO and alkylation
reactions with the following distribution in C mole %: 44%
benzene, 12% alkylated aromatics, 12% alkylated oxygenates
and 4% methane (shown in Figure 1B). Additionally, minimal
aromatic hydrogenation was observed with the only detectable
product being cyclohexene at 0.16 C mol %. This product
distribution was observed at 7 h on stream and an anisole
conversion of 82%. The selectivity for HDO and alkylation
products was 72% and 30%, respectively. The alkylated
aromatics ranged from toluene up to pentamethylbenzene with
the yield of each alkylated product being inversely proportional to
the number of methyl groups on the aromatic ring. The yield of
alkylated oxygenates mirrors the product distribution of the
alkylated aromatics, as observed in the inset of Figure 1B. The
alkylated oxygenates can exist as either phenolic alkylated
oxygenates (e.g., cresol) or as alkylated phenolic ethers (e.g.,
methylanisole). The total alkylated oxygenates comprised mainly
phenolic compounds (ca. 70%). A mole balance on the methyl
groups was used to track the amount of alkylation, given that a
calculation based on carbon mole % does not allow the
quantitation of the actual degree of alkylation. The mole balance
of the anisole-derived methyl groups shows that 54% of the
methyl groups were alkylated onto the aromatic ring and the
remaining 46% was converted to methane.
The reactivity profile for anisole conversion presented in
Figure 1A shows that a 3 h transient period is needed to achieve
steady-state and that the catalyst loses only 5% of this activity
after 16 h on stream. We note that the alkylation selectivity was
relatively constant throughout the reaction and that the decrease
in anisole conversion was solely due to a decrease in HDO activity,
as evidenced by the corresponding decrease in benzene yield.
The mass balance based on condensable liquids and methane
over a 16 h reaction was 86% given that some of the more volatile
products escaped the condenser over the 16 h experiment.
For 4-propylguaiacol, when an experiment was performed
with 100 mg of 10% POM/TiO2 (0.26 h mass averaged contact
time), the conversion remained above 90% over the course of the
16 h experiment (shown in Figure 1C). Specifically, at 7 h on
stream, the 4-propylguaiacol conversion was 92% with an
alkylation selectivity of 42%. The mole balance around the
methoxy group of 4-propylguaiacol showed that 77% of methyl
groups were alkylated onto the aromatic rings. The HDO
selectivity of 4-propylguaiacol to fully deoxygenated products was
only 4%, but the HDO selectivity to 4-propylphenols and alkylated
4-propylphenols was 93%. The alkylated products follow the
same trend as that observed for anisole (shown in Figure 1D).
Additionally, no 4-propylcatechol was observed, indicating that
complete deoxygenation of the methoxy group occurred during
the reaction. The catalyst clearly promoted deoxygenation of the
weaker methoxy group of 4-propylguaiacol as opposed to the
stronger phenolic group, as evidenced by the low yield of
propylbenzene and alkylated propylbenzenes.
Scheme 1. Possible reaction pathways for anisole conversion.
Performing coupled HDO/alkylation reactions requires
bifunctional catalysts that contain both redox and Brønsted acid
sites. For example, CoMo sulfide supported on γ-alumina has
been used to catalyze coupled HDO/alkylation of anisole at 573
K.32-33 While this catalyst has an alkylation selectivity of 28%, it
shows a low HDO selectivity of 30% that results in alkyl phenols
as the primary products. In contrast, 0.2 wt% Pt on an acidic Beta
zeolite converted anisole into alkylated aromatics at 673 K with a
selectivity of ca. 50%.34 Higher platinum loadings resulted in less
deactivation.35 Unfortunately, the catalyst lost 50% of its activity
by 4 h on stream. Alkylated phenols can also be produced with
bare zeolites.36-37 Molybdenum carbide—a material featuring
redox sites and tunable Brønsted acidity38-40—has been shown to
be a highly active HDO catalyst, but with low selectivity for
alkylation (<1%),41-42 while nickel phosphides have shown
improved selectivity for alkylation approaching 15%.43 We
recently demonstrated that molybdenum oxide is highly active for
HDO and alkylation, but suffers from over-reduction, phase
changes and eventual deactivation on stream at temperatures
above 623 K.44-45 Further studies showed that the stability of the
catalyst improved when the active molybdenum oxide was
supported on TiO2 or ZrO2.46 Ceria-zirconia catalysts also show
high activity for the demethoxylation of guaiacol to phenol 80-90%
with moderate yields of cresols 6-10%.47
One alternative to molybdenum oxide for HDO/alkylation
reactions are molybdenum-based polyoxometalates (POMs),
which have similar redox properties to the oxide, but also contain
strong Brønsted acid sites. Keggin type POMs have the common
structure where twelve metal oxide clusters consisting of Mo or W
surround a central heteroatom such as P or Si. The identity of the
central atoms defines the valence of the POM.48-52 Metal
substitutions (e.g, V or Nb) can be made into the external oxide
to alter the chemical composition and, consequently, the strength
and number of Brønsted acid sites, the redox properties and the
thermal stability of the POM.48 This tunability was exploited for W
and Mo based POMs to alter the deoxygenation rates of ketone
and carboxylate functional groups by substituting different metals
into the external oxide shell.53-55 POMs offer a highly customizable
scaffolding to construct the specific active sites to modulate the
dual active site motifs required to promote tandem HDO and
alkylation reactions; yet, these materials, have seldom been
studied in the context of biomass upgrading.
Herein, we demonstrate that Mo-based Keggin POMs with
the formula H3PMo12O40 supported on titania (POM/TiO2) are
effective catalysts to upgrade lignin-derived compounds via
coupled HDO/alkylation reactions. The conversion of anisole to
alkylated aromatics was investigated to understand the origins of
HDO and alkylation reactivity, as well as the resulting product
distribution. In addition, 4-propylguaiacol, a direct product from
the RCF of lignin, was also deoxygenated to alkylated phenols.
Reactivity studies using ethylphenol and ethylbenzene co-feeds
with anisole were combined with X-ray photoelectron and infrared
spectroscopy to discriminate the types of catalytic sites
responsible for reactivity and to propose a plausible mechanism.
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10.1002/cssc.201700297
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Alkylated oxygenates
Methane
Phenol
30%
21%
A
B
Methane
Anisole
Alkylated oxygenates
Benzene
Alkylated aromatics
Cresol
5%
4%
60
40
20
0
Methylanisole
Dimethylphenol
Dimethylanisole
Trimethylphenol
Phenol
40%
Alkylated aromatics
Alkylated aromatics
17%
60%
13%
Alkylated oxygenates
Anisole
Toluene
Xylene
6%
4%
Trimethylbe6n0z%ene
Tetramethylbenzene
Pentamethylbenzene
Benzene
2
4
6
8
10
12
14
16
0
10
20
30
40
50
ToS (h)
C mole %
4-Propylguaiacol
Methane
D
C
Alkylated 4-propylphenols
65%
Methane
4-Propylguaiacol
Alkylated oxygenates
Propylbenzene
60
Alkylated aromatics
Propylphenol
23%
8%
3%
Methyl 4-propylphenol
40
20
0
Dimethyl 4-propylphenol
Trimethyl 4-propylphenol
Tetramethyl 4-propylphenol
Alkylated aromatics
Alkylated 4-propylphenols
4-Propylphenol
2
4
6
8
10
12
14
16
0
10
20
30
40
50
ToS (h)
C mole %
Alkylated 4-propylphenols
Methane
F
E
60
Methane
4-Propylguaiacol
Alkylated oxygenates
Propylbenzene
86%
Alkylated aromatics
Propylphenol
14%
17%
4-Propylphenol
Alkylated 4-propylphenols
Alkylated propylbenzenes
Propylbenzene
Methyl 4-propylphenol
Dimethyl 4-propylphenol
Alkylated propylbenzenes
40
20
0
80%
3%
Methyl propylbenzene
Dimethyl propylbenzene
Trimethyl propylbenzene
2
4
6
8
10
12
14
16
0
10
20
30
40
50
C mole %
ToS (h)
Figure 1. A) Yield (C mole %) for the reaction of anisole (200 μL/hr) over 300 mg of 10% POM/TiO2 at 593 K, 1.01 bar (70 mL/min) H2 as a function of time on
stream. B) Product distribution of binned species at 7 h on stream for the reaction of anisole over POM/TiO2. C) Yield (C mole %) for the reaction of 4-propylguaiacol
over 100 mg of 10% POM/TiO2 at 593K, 1.01 bar (17.5 mL/min H2 and 52.5 mL/min N2) as a function of time of stream. D) Product distribution of binned species at
7 h on stream during the reaction of 4-propylguaiacol over 100 mg of POM/TiO2 . E) Yield (C mole %) for the reaction of 4-propylguaiacol over 300 mg of 10%
POM/TiO2 at 593K, 1.01 bar (17.5 mL/min H2 and 52.5 mL/min N2) as a function of time of stream. F) Product distribution of binned species at 7 h on stream during
the reaction of 4-propylguaiacol over 300 mg of POM/TiO2. 4-propylguaiacol was fed as a 5 wt% solution in toluene at 900 μL/h.
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In order to tailor the product distribution to produce
The former may either desorb from the catalyst surface or
undergo further reduction to form benzene and water. In pathway
B, the methyl group of the phenolic ether transfers to an adjacent
Brønsted acid site resulting in the production of phenol and an
electron deficient surface bound methyl species. Alternatively,
benzene and methanol may be formed via the hydrogenolysis of
the aromatic C-O bond. Methanol can form a reactive methoxy
species upon interacting with an adjacent Brønsted acid site
leading to either intramolecular (to the adjacent adsorbed phenol)
or intermolecular (to another adsorbed oxygenated aromatic
molecule) C-alkylation. Similarly to pathway A, the oxygenated
aromatic substrate can desorb from the catalyst surface leaving a
vacancy that can be occupied by another oxygenate susceptible
to the same alkylation and deoxygenation, leading ultimately to
the production of multi-alkylated aromatics. Regardless of the
catalytic pathway followed, the acid sites are always regenerated
completely deoxygenated products, such as propylbenzene and
alkylated propylbenzenes, the experiment with 4-propylguaiacol
was repeated with a longer mass-averaged contact time of 0.77
h. Under these conditions, 100% of the 4-propylguaiacol feed was
converted
mainly
into
propylbenzene
and
alkylated
propylbenzenes (Figure 1E). Specifically, at 7 h on stream, 40, 21,
and 17% yields of propylbenzene, alkylated propylbenzenes and
alkylated 4-propylphenol were obtained, respectively. Taken
together, these results translate to a 61% yield of completely
deoxygenated products with a 38% yield for alkylation (51% of
methyl groups were alkylated by mole balance). These data
demonstrate that the catalyst is capable of activating and
deoxygenating the phenolic functional group at longer contact
times. Interestingly, when the space velocity was decreased, the
alkylation selectivity decreased from 42 % to 38% and the product
distribution of alkylated products was altered (shown in Figure 1F). to close the catalytic cycle.
At high conversions, mono and di alkylation were favored,
whereas at a low HDO conversions, up to tetra alkylation was
observed. These results imply that that the oxygen on the
aromatic ring influences alkylation rates and selectivity and are
further supported by the slightly lower selectivity for alkylation and
higher degree of multi-alkylated products observed for anisole
when compared to 4-propylguaiacol at high HDO conversions.
We posit that the differences in product distribution and yield are
likely due to the additional hydroxyl group present in 4-
propylguaiacol. Finally, the rate of phenolic bond cleavage
decreased more rapidly when compared to methoxy group
cleavage, suggesting a higher deactivation rate when full HDO is
performed. Over 16 h, the yield of propylbenzene decreased from
56 % to 36% with a mirrored increase in the yield of 4-
propylphenol.
X-ray photoelectron spectroscopy (XPS) was employed
before and after reaction with anisole to investigate the formation
of catalytic active sites under reducing conditions. The initial
oxidation state of Mo in the as-prepared, unsupported POM is
MoVI. Upon supporting it on TiO2, some of the molybdenum
becomes partially reduced to MoV. Indeed, TiO2 is
a
photosensitive semiconductor that promotes electrons into the
conduction band and reduces the POM. The post reaction sample
contains a mixture of MoIV and MoV oxidation states after 16 h on
stream. This mixed oxidation state is required to form the Lewis
acid vacancies in the POM lattice, as illustrated in Figure 2.
In Scheme 2, we have outlined a reverse Mars-Van
Krevelen reaction pathway to yield the main components detailed
57-58
in Figure 1B.54,
Specifically, a Lewis acidic vacancy is
generated by reducing the catalyst surface with H2.59 Next,
anisole binds onto this oxophilic site through its oxygen atom,
giving way to two plausible pathways. In pathway A, the
coordinated anisole can undergo hydrogenolysis to produce
phenol and methane.
POM Initial
54:46:0
Brønsted Acid
Lewis Acid
POM Reduced
21:25:54
27:28:45
Vacancy Formation
A
Substrate Adsorption
POM Spent
B
240 238 236 234 232 230 228 226
Binding Energy (eV)
Figure 2. X-ray photoelectron spectroscopy of three supported POMs. POM
Initial is fresh POM/TiO2. POM reduced is POM/TiO2 after 4 hours of reduction
at 593 K with hydrogen gas. POM spent is POM/TiO2 after 16 hours of reaction
with 200 μL/hr anisole at 593 K in hydrogen gas (70 mL/min). The numbers in
the bottom right denote the distribution of oxidation states from MoVI to MoIV. The
strucutres located on the right of the XPS data illustrate potential vacancies that
could exist in the reduced POM.
Scheme 2. Proposed reaction network for coupled HDO and alkylation reaction
on the surface of POM/TiO2.
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10.1002/cssc.201700297
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In addition, a sample reduced in H2 at 593 K for 4 h in the absence
of the feed contained a higher proportion of MoIV than that
observed post-reaction after 16 h on stream. We hypothesize that
the redox process is altered when an oxidant (oxygenated feed)
is present that changes the proportion of MoVI and MoV species.
To gain further insight into the nature of the catalytic sites,
the acid sites on the material were characterized by ammonia
TPD and pyridine adsorption coupled with FT-IR. The acid sites
of POM/TiO2 and TiO2 quantified by NH3-TPD showed values of
338 and 178 µmol NH3 g-1, respectively (see Table 1). To isolate
the role of Lewis acid sites on the reaction network, Brønsted acid
protons were exchanged with Na ions to generate a NaPOM
showed two distinct chemical shifts observed at -3.2 and -1.6 ppm
corresponding to molybdenum Keggin POM and the lacunary
POM derivative, respectively.52, 65 Notably, the fresh POM/TiO2
materials featured these two resonances and also exhibited an
additional resonance at -4.8 ppm, which is attributed to the
reduced POM state observed by XPS (Figure 4). Both the
POM/TiO2 sample reduced under H2 and the spent catalyst did
not contain the resonances associated with the lacunary and
Keggin POM, but instead showed highly shielded phosphorus
resonances at -6.8 ppm and -13.2 ppm. The presence of such
highly shielded signals implies that a phosphomolybdic polyanion
remains
intact
during
the
reduction/reaction.
This
material featuring a reduced acid site count of 71 μmol NH3 g-1
phosphomolybdic species was extracted from the catalysts into
an aqueous solution and precipitated as a solid. The PXRD
pattern for this species is similar to that obtained for the as
synthesized POM (Figure S2), which is strong evidence
suggesting that the POM architecture in a reduced state is
preserved on the catalyst surface. These results would not have
been observed if the POM had decomposed under reaction
conditions into water insoluble molybdenum oxide and a surface
phosphate species.
.
FT-IR spectra of pyridine adsorbed onto unsupported POM
showed bands corresponding to Lewis acid sites at 1605 and
1448 cm-1 (Figure 3).60-61 Additional bands were also observed at
1587 cm-1 and 1487 cm-1 corresponding to the oxidative
breakdown of pyridine to carboxylate species.62 Interestingly, no
bands associated to pyridine bound to Brønsted acid sites (1636
cm-1 or 1542 cm-1) were observed.63-64 The band at 1440 cm-1 was
associated to hydrogen bonded pyridine.62 We hypothesize that
this effect is likely due to solid-state ion exchange with the KBr
used to produce the pellets for analysis. Upon reducing the
materials under flowing H2 at 583 K for 2 h, the vibration related
to Lewis acidity shifted from 1605 to 1607 cm-1 and a new intense,
broad band appeared at 1402 cm-1. We attribute this band to the
binding of a pyridine molecule between two adjacent Lewis acid
sites (designated as the MoV vacancy in Figure 2), which would
result in lower frequency vibrational modes as a result of the
coordination to two metal sites.
Reduced POM/TiO2
Post Reaction POM/TiO2
Table 1. Physicochemical properties of various supported POMs and TiO2.
Surface area was measured by N2 adsorption and acid sites were quantified
with NH3 TPD.
POM/TiO2
Sample
Surface Area
NH3 Adsorption
(m2/g)
6
(μmol/g)
POM
-
Unsupported POM
TiO2
56.3
48.1
43.9
178
338
71
-2
-4
-6
-8
-10
-12
-14
10% POM/TiO2
10% NaPOM/TiO2
 ppm
Figure 4. Solution phase 31P NMR of POMs in D2O. In these samples, the POM
catalyst was extracted off of the surface of the support with D2O acidified with
phosphoric acid. Reduced POM is POM/TiO2 after 4 hours of reduction at 593
K with hydrogen gas. Post reaction POM is POM/TiO2 after 16 hours of reaction
with 200 μL/hr anisole at 593 K in hydrogen gas (70 mL/min).
Control reactions were performed to gain insight into the
reaction mechanism (Figure S3). First, a reaction in the absence
of hydrogen over POM/TiO2 resulted in an anisole conversion of
<5%, implying that hydrogen is critical towards generating
reaction sites capable of performing HDO/alkylation reactions.
The experiment without hydrogen was repeated with a 30 mole %
water co-feed yielding a 10% anisole conversion into phenol as
the primary product. The low C-O bond cleavage rates imply that
acid catalyzed hydrolysis is not responsible for the high anisole
conversion observed with hydrogen gas. Note that while the
catalyst innately possesses Lewis and Brønsted acid sites, these
sites did not result in significant turnovers in the absence of
hydrogen. To directly investigate the role of the Brønsted acidity,
anisole was also reacted over NaPOM/TiO2 under regular
reaction conditions. The NaPOM/TiO2 featured an anisole
conversion of 18%, and primarily produced benzene. Less than 1
C mole % of the products were alkylated, thus showing that HDO
activity dominates in the absence of Brønsted acidity.
Experiments performed with the bare TiO2 support in flowing
hydrogen resulted in anisole conversion values < 10 %. The
POM Reduced
POM
1700 1650 1600 1550 1500 1450 1400 1350 1300
Wavenumber (cm-1)
Figure 3. FT-IR spectra of pyridine chemisorbed onto unsupported POM and
the unsupported POM after reduction. Vertical dashed lines indicate the position
of adsorption peaks. In situ reductions were performed at 583 K with pure
hydrogen (20 mL/min) for 2 h.
Analysis of spent catalysts confirmed that the POM
structure exists as a highly, reduced phosphomolybdic species.
Specifically, the 31P NMR spectra of the unsupported POM
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10.1002/cssc.201700297
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reactor effluent contained ca. 6 C mole % phenol, 2 C mole %
benzene and alkylated aromatics, and 2 C mole % of the
alkylation product, methylanisole. Although TiO2 is known to
possess both Lewis and Brønsted acid sites, these data show that
the support does not promote HDO/alkylation to a significant
extent.
Conclusions
Molybdenum based POMs are highly active bi-functional
catalysts for coupled HDO and alkylation of lignin model
compounds. Specifically, these catalysts convert anisole into a
range of alkylated arenes with alkylation and HDO selectivity
values of 30% and 72%, respectively, at 82% conversion after 7
h on stream. 4-Propylguaiacol, a product from the RCF of lignin,
was also deoxygenated over POM/TiO2. Under the conditions
tested, a 4-propylguaiacol conversion of 92% was observed with
an alkylation selectivity of 42% to alkylated phenols. A decrease
<10% in catalyst activity was observed over 16 h for both
reactants. Additionally, a high conversion reaction using 4-
propylguaiacol generated a 40 % yield of propylbenzene, 21 %
yield of alkylated propylbenzenes and a 17 % yield of alkylated 4-
propylphenol. Finally, a potential reaction mechanism for which
HDO and alkylation occurs was postulated. The mechanism is
based on catalyst characterization such as XPS, FT-IR pyridine
adsorption, and additional reactivity studies with multi component
feeds. Insights into the active sites of the catalyst now allow for
further catalyst optimization through metal substitutions to
modulate the Brønsted acidity and Lewis acidity of the catalyst.
Ultimately, balancing these two functionalities will further improve
the HDO/alkylation selectivity of the catalyst.
It was observed in the 4-propylguaiacol runs that the
alkylation rate and the product distribution are impacted by the
degree of deoxygenation. To study this effect, anisole was co-fed
into the reactor with either an equimolar amount of ethylbenzene
or ethylphenol while keeping the total molar flow of oxygenates
constant (Figure 5). When compared to a pure anisole feed base
case, an ethylbenzene co-feed resulted in a decrease in the
anisole alkylation selectivity from 54% to 47 % at similar
conversions of 81% and 96%, respectively. Interestingly, although
no ethylbenzene alkylation products (e.g., ethyltoluene) were
observed, co-feeding ethylphenol yielded mixed alkylation
products, such as methylethylphenol. The addition of ethyl phenol
also resulted in a lower conversion of the anisole (75%). Given
that mixed alkylation occurs with ethylphenol, but not with
ethylbenzene, we hypothesize that the presence of oxygen is
essential for the alkylation reaction to occur. This observation is
consistent with the 4-propylguaiacol results that resulted in the
highest yield of alkylated products when the HDO conversion was
the lowest. Furthermore, when the HDO conversion was
increased, the reaction favored mono and di alkylation products
because the oxygen was removed before additional alkylation
reactions could occur. We posit that the presence of oxygen in the
molecular structure either allows for binding to the surface for the
reaction to occur or provides electron density to promote
electrophilic aromatic substitution. Additionally, the formation of
mixed alkylation products supports the proposed alkylation
pathway in Scheme 2, since alkyl groups
Experimental Section
Catalyst Synthesis. Molybdenum oxide (99.5%, Sigma
Aldrich) was added to deionized water to a concentration of 0.13
g/mL. The slurry was added to a 250 mL two neck, round bottom
flask with a magnetic stir bar. The slurry was then heated to 373
K and allowed to reflux. Next, 2 g (1.5 equivalents per mole of
molybdenum) of 85 wt% phosphoric acid (Sigma Aldrich) was
quickly added to the slurry and the slurry was allowed to reflux for
36 h. The yellow-green solution was then filtered to remove
residual molybdenum oxide. The resulting POM was recovered
from the aqueous solution by evaporation. Given the low surface
area of the POM, it was supported on nano-sized titanium oxide
(99.5%, Sigma Aldrich, primarily Anatase) by dissolving the solid
POM in water and adding the solution dropwise to the support
while mixing vigorously. The water was removed by drying in a
393 K oven overnight. The nominal POM/TiO2 catalyst loading
was 10 wt%. The identity of the material was confirmed by 31P
nuclear magnetic resonance (NMR), powder X-ray diffraction
(PXRD) and infrared (IR) spectroscopy.51, 56 The sodium form of
the POM, denoted NaPOM, was prepared by ion exchange with
a Na Amberlyst (Sigma Aldrich) resin. The ion exchange was
performed by dissolving the POM in DI water and mixing it with
an amount of resin equivalent to a fivefold molar excess of Na for
24 hours. This procedure was repeated twice to achieve full Na
exchange.
Methane
Mixed Alkylation
Methyl Alkylation
100
90
80
44
47
70
60
50
18
40
30
53
20
38
10
0
Reactivity Studies. The vapor phase reactions were
performed in a packed bed reactor. The reactor was constructed
from a 0.95 cm OD (0.089 cm wall thickness) stainless steel tube
with a welded lip in the center. The reactor was mounted inside a
single stage furnace (850W/115V, Applied Systems Series 3210).
The temperature of the bed was controlled by a thermocouple
(Omega, model TJ36-CAXL-116u) positioned downstream of the
bed and connected to a temperature controller (Digi-sense, model
68900-10). The catalyst bed was supported on a glass wool plug.
The bed consisted of two inert segments of α-alumina (100-200
mesh) on the top and bottom of the catalyst bed. The catalyst bed
was made by homogenously distributing 300 mg for reactions with
anisole or 100 mg for reactions with 4-propylguaiacol of 10%
Ethylphenol Ethylbenzene
Figure 5. Mole balance performed around the methoxy group of anisole with
different co-feeds. The methyl alkylation bin is defined as the mole percent of
methoxy groups that went into alkylating anisole. The mixed alkylation bin
signifies the fraction of methoxy groups that alkylated the co-feed such as
methylethylbenzene and methylethylphenol. The experiment with ethyl benzene
had 5% unreacted methoxy groups whereas the experiment performed with
ethyl phenol had 30% unreacted methoxy species.
are leaving the anisole and coupling onto a different oxygenate.
Ultimately, the two reactions are highly coupled because the yield
of alkylated products is low with no HDO activity, but also is
reduced at high HDO conversion.
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10.1002/cssc.201700297
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POM/TiO2 (100-140 mesh) in α-alumina (100-200 mesh) for a
total bed weight of 1g. Anisole was introduced into the reactor via
a syringe pump (Harvard Apparatus, model 703005) at a flow rate
of 200 μL/h. These experiments were performed at atmospheric
pressure and at a gas flow rate of 70 mL/min (90/1 H2/anisole
molar ratio, equivalent to a mass-based contact time of 0.15 h).
Experiments with 4-propylguaiacol used toluene as a solvent.
Specifically, a solution of 5 wt% 4-propylguaiacol in toluene was
fed to the reactor at a flow rate of 900 uL/h. To account for the
reduced 4-propylguaiacol partial pressure, the hydrogen partial
pressure was adjusted to maintain a constant H2/Oxygenate
molar ratio of 90 (mass-averaged contact time based on total
mass loadings of 0.26 h or 0.77 h for long contact time
experiments) by flowing 17.5 mL H2/min (Airgas, 99.999%) with a
balance of N2 (Airgas, 99.999%) with a total gas flowrate of 70
mL/min. Reaction conditions were chosen to operate in the
absence of mass transfer limitations.
The reactor effluent was transferred from the reactor to a
Gas Chromatograph-Mass Spectrometer (GC-MS, Agilent 7890
A, Agilent 5975 C) using heated lines at ca. 523 K to avoid
condensation. The reaction products were monitored and
quantified in real time with a Flame Ionization Detector (GC-FID,
Agilent Technologies, model 7890A) and a DB-5 column (Agilent
30 m x 0.25 mm id, 0.25 μm). The conditions used in the analysis
were as follows: the gas flow rate was 1.0 mL/min helium (Airgas,
99.999%) or 1.4 mL/min hydrogen in the case of 4-propylguaiacol,
the injector temperature was 523 K, the detector temperature was
573 K and the split ratio was 1:25. The oven temperature ramp
started at 308 K and was held for 2 min and was then ramped to
453 K at a rate of 10 K/min for anisole. The final oven temperature
2 cm−1 resolution in the 4000–400 cm−1 range. Unsupported POM
was used for these studies due to the low mass loading and poor
compressibility of the titania supported catalyst. Unsupported
POM was pressed into wafers with KBr as a diluent (10 % w/w
POM). The wafers were loaded into
a high temperature
transmission cell (Harrick Scientific) with ZnSe windows. The
samples were dried at 373 K for 12 hours under vacuum
(<0.01 Pa, dynamic vacuum, Edwards’ T-Station 75 Turbopump).
After taking a reference spectrum, the sample was dosed with
pyridine (Sigma Aldrich, anhydrous 99.8%) at room temperature
which can adsorb to both Lewis and Brønsted acid sites. The
pyridine was dosed by administering pulses of vaporized pyridine
at room temperature from a liquid pyridine sample. The pyridine
was degassed before adsorption by a freeze-pump-thaw cycle
performed at 77 K. The vacuum was then reestablished to remove
gas phase and physisorbed pyridine. The resulting vibrational
frequencies were used for acid site speciation. A second set of
experiments was performed under reducing conditions.
Specifically, fresh wafers were prepared and dried as before, and
the sample was then reduced with H2 at 583 K with a gas flow rate
of 20 mL/min for 2 hours. The sample was cooled to room
temperature and pyridine dosed onto the sample. The excess
pyridine was removed by exposing the sample to vacuum before
acquiring a spectrum.
Ammonia temperature programmed desorption (TPD) was
performed using a U-tube reactor setup coupled to a Hiden
Analytical HPR20 mass spectrometer. The U-tube reactor
consisted of a quartz U-tube mounted in an insulated single-zone
furnace (550W/115V, Carbolite GTF 11/50/150B).
A
thermocouple (Omega, model TJ36-CAXL-116u) was mounted
within the U-tube slightly above the sample bed, and was
connected to a temperature controller (Digi-Sense, model 68900-
10). The bed consisted of 300-400 mg (22-38 mesh) of pelletized
catalyst. Next, the catalyst was dried under flowing 1% argon in
helium (100 mL/min) at 373 K and then exposed to several pulses
of the ammonia, totaling ca. 10 mL at STP. The samples were
heated to 823 K (10 K/min) and held for 30 min at that
temperature and the effluent gas containing desorbed ammonia
was monitored by mass spectroscopy (tracking the m/z = 17
fragment). The MS response was calibrated for each ammonia
pulse prior to every analysis.
was increased to 553
K
for the experiment utilizing
4-propylguaiacol. The products were simultaneously identified
using a mass selective detector (MSD, Agilent Technologies,
model 5975C). The mass balance was calculated by quantifying
both the gas and liquid phase products. Methane was quantified
by the GC-FID and liquid products were condensed in an ethanol
bath maintained at 273 K. The liquid phase products were
analyzed by GC-FID with an external standard, dodecane. The
water in the liquid phase was titrated by Karl-Fischer analysis
(Mettler-Toledo, 5131128813).
X-ray photoelectron spectroscopy (XPS) samples were
prepared in a stainless steel U-tube reactor 0.95 cm OD (0.089
cm wall thickness) equipped with two valves located at the inlet
and outlet, which allowed the reactor to be sealed. The reactor
was heated in an insulated single-zone furnace (550W/115V,
Carbolite GTF 11/50/150B). A thermocouple (Omega, model
TJ36-CAXL-116u) was mounted inside the furnace at the same
height as the catalyst bed and controlled with a temperature
controller (Digi-Sense, model 68900-10). The reaction effluent
was monitored by GC-MS. The liquid feed was introduced through
a gas saturator located upstream of the reactor. The reactor was
packed with 140 mg (100-140 mesh) for these experiments. This
was done to maintain a similar anisole conversion to reactivity
studies. Once the reaction was completed, the reactor was
purged with nitrogen and sealed via the two valves and
transferred to a glove box where the XPS samples were prepared.
XPS samples were prepared with 10 wt % niobium oxide (99.99%,
Sigma-Aldrich) as a standard. The binding energies were
corrected to 207.4 eV (Nb 3d5/2). The samples were transferred to
the instrument using a portable transfer vessel, thereby ensuring
that samples were not exposed to air. XPS spectra were acquired
The following definitions were used to evaluate experimental data
and values are based on GC detectable species:
Alkylated products refer to any alkylated aromatics or alkylated
aromatic oxygenates
퐶 푚표푙푒푠 표푓 푟푒푎푐푡푎푛푡 푐표푛푠푢푚푒푑
퐶표푛푣푒푟푠ꢀ표푛 =
퐶 푚표푙푒푠 표푓 푟푒푎푐푡푎푛푡 푓푒푑
푀푒푡ℎ푦푙 푏푎푙푎푛푐푒
푚표푙푒푠 표푓 푚푒푡ℎ표푥푦 푔푟표푢푝푠 푎푙푘푦푙푎푡푒푑
=
푚표푙푒푠 표푓 푚푒푡ℎ표푥푦 푔푟표푢푝푠 푎푙푘푦푙푎푡푒푑 + 푚표푙푒푠 표푓 푚푒푡ℎ푎푛푒
퐶 푚표푙푒푠 표푓 푎푙푘푦푙푎푡푒푑 푝푟표푑푢푐푡푠
퐴푙푘푦푙푎푡ꢀ표푛 푆푒푙푒푐푡ꢀ푣ꢀ푡푦 =
퐶 푚표푙푒푠 표푓 푟푒푎푐푡푎푛푡 푐표푛푠푢푚푒푑
퐶 푚표푙푒푠 표푓 푓푢푙푙푦 푑푒표푥푦푔푒푛푎푡푒푑 푝푟표푑푢푐푡푠
퐻퐷푂 푆푒푙푒푐푡ꢀ푣ꢀ푡푦 =
퐶 푚표푙푒푠 표푓 푟푒푎푐푡푎푛푡 푐표푛푠푢푚푒푑
Catalyst Characterization. Fourier transform infrared (FT-IR)
on
a PHI Versaprobe II equipped with a multichannel
spectra were collected with
a
Bruker Vertex 70
hemispherical analyzer and aluminum anode X-ray source
operating at 100 W with a 100 mm beam scanned over a 1.4 mm
spectrophotometer equipped with an Hg–Cd–Te (MCT) detector.
Each spectrum was recorded by averaging 64 scans at
line across the sample surface.
A
dual-beam charge
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10.1002/cssc.201700297
ChemSusChem
FULL PAPER
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of 1.2 eV and Argon ion beam energy of 10 eV. A 7-point Shirley
background correction was then applied to the Mo 3d XPS spectra
after charge correction. The composition of Mo oxidation states
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of 3:2 for Mo 3d5/2 – Mo 3d3/2, and (3) Equal full width at half
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Entry for the Table of Contents
FULL PAPER
Anderson, E. M., Crisci, A. Murugappan,
K., Román-Leshkov, Y.*
Brønsted Acid
Hydrodeoxygenation & Alkylation
Lewis Acid
Page No. – Page No.
Bifunctional Molybdenum
Polyoxometalates for the Combined
Hydrodeoxygenation and Alkylation
of Lignin-derived Model Phenolics
593 K
1 atm
Hydrodeoxygenation Only
Gas Phase
Bifunctional molybdenum based polyoxometalates containing both Brønsted and Lewis acid sites catalyze the combined
hydrodeoxygenation and alkylation of lignin derived model phenolics to alkylated benzenes.
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