Elucidating the reactivity of methoxyphenol positional isomers towards hydrogen-transfer reactions by ATR-IR spectroscopy of the liquid-solid interface of RANEY Ni

Article abstract of DOI:10.1039/c8cy00491a

In the valorisation of lignin, the application of catalytic hydrogen transfer reactions (e.g. in catalytic upstream biorefining or lignin-first biorefining) has brought a renewed interest in the fundamental understanding of hydrogen-transfer processes in the defunctionalisation of lignin-derived phenolics. In this report, we address fundamental questions underlining the distinct reactivity patterns of positional isomers of guaiacol towards H-transfer reactions in the presence of RANEY Ni and 2-PrOH (solvent and H-donor). We studied the relationship between reactivity patterns of 2-, 3- and 4-methoxyphenols and their interactions at the liquid-solid interface of RANEY Ni as probed by attenuated total reflection infrared (ATR-IR) spectroscopy. Regarding the reactivity patterns, 2-methoxyphenol or guaiacol is predominantly converted into cyclohexanol through a sequence of reactions including demethoxylation of 2-methoxyphenol to phenol followed by hydrogenation of phenol to cyclohexanol. By contrast, for the conversion of the two non-lignin related positional isomers, the corresponding 3- and 4-methoxycyclohexanols are the major reaction products. The ATR-IR spectra of the liquid-solid interface of RANEY Ni revealed that the adsorbed 2-methoxyphenol assumes a parallel orientation to the catalyst surface, which allows a strong interaction between the methoxy C-O bond and the surface. Conversely, the adsorption of 3- or 4-methoxyphenol leads to a tilted surface complex in which the methoxy C-O bond establishes no interaction with the catalyst. These observations are also corroborated by a smaller activation entropy found for the conversion of 2-methoxyphenol relative to those of the other two positional isomers.

Full text of DOI:10.1039/c8cy00491a

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Catalysis and Science Technology
ARTICLE
Elucidating the reactivity of methoxyphenol positional isomers
towards hydrogen-transfer reactions by ATR-IR spectroscopy of
the liquid-solid interface of RANEY® Ni
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
c
a
DOI: 10.1039/x0xx00000x
Ilton Barros Daltro de Castro, Inês Graça, Laura Rodríguez-García, Marco Kennema,
,
b
,c
Roberto Rinaldi* and Fabian Meemken*
www.rsc.org/
In the valorisation of lignin, the application of catalytic hydrogen transfer reactions (e.g. in catalytic upstream biorefining
or lignin-first biorefining) has brought a renewed interest in the fundamental understanding of hydrogen-transfer
processes in the defunctionalisation of lignin-derived phenolics. In this report, we address fundamental questions
underlining the distinct reactivity patterns of positional isomers of guaiacol towards H-transfer reactions in the presence of
RANEY® Ni and 2-PrOH (solvent and H-donor). We studied the relationship between reactivity patterns of 2-, 3- and 4-
methoxyphenols and their interactions at the liquid-solid interface of RANEY® Ni as probed by attenuated total reflection
infrared (ATR-IR) spectroscopy. Regarding the reactivity patterns, 2-methoxyphenol or guaiacol is predominantly
converted into cyclohexanol through a sequence of reactions including demethoxylation of 2-methoxyphenol to phenol
followed by hydrogenation of phenol to cyclohexanol. By contrast, for the conversion of the two non-lignin related
positional isomers, the corresponding 3- and 4-methoxycyclohexanols are the major reaction products. The ATR-IR spectra
of the liquid-solid interface of RANEY® Ni revealed that the adsorbed 2-methoxyphenol assumes a parallel orientation to
the catalyst surface, which allows a strong interaction between the methoxy C—O bond and the surface. Conversely, the
adsorption of 3- or 4-methoxyphenol leads to a tilted surface complex in which the methoxy C—O bond establishes no
interaction with the catalyst. These observations are also corroborated by a smaller activation entropy found for the
conversion
of
2-methoxyphenol
relative
to
those
of
the
other
two
positional
isomers.
pathways (e.g. hydride transfer or direct hydrogen transfer
a.k.a. Meerwein−Ponndorf−Verley mechanism). Suggested by
the considerable changes in product distributions for H-
transfer reactions compared to regular hydrogenations
catalysed by Raney® Ni, a different occupation of the catalyst
surface by reactants and intermediates seems to occur under
Introduction
Catalytic hydrogen transfer reactions (e.g., hydrogenation
HYD’, hydrogenolysis ‘HYL’, demethoxylation ‘DMO’, and
hydrodeoxygenation ‘HDO’) have recently emerged as a
‘
1
-8
promising pathway for biomass valorisation.
From the
5
,6
H-lean conditions such those found for H-transfer reactions.
catalytic upstream biorefining, in which a well-defined pulp
and depolymerised lignin stream are obtained, to the
H-transfer reactions have been performed on various
biomass-derived substrates (furans, alcohols, acids, phenols,
and aromatics) over several Pd, Pt, Ru, Au and Ni-based
4
,9-12
downstream processing of these fractions,
H-transfer
reactions in the presence of a liquid organic H-donor have
demonstrated several advantages over conventional
4
-7,15-22
heterogeneous catalysts.
Currently, H-transfer HDO of
5
,6,13
phenolic compounds has attracted much attention owing to its
potential as part of strategies for the conversion of lignin into
hydrogenation in the presence of molecular hydrogen.
use of liquid organic H-donors instead of H enables a
considerable degree of control over the reaction regarding the
The
2
6
,7,23
high-value products.
Despite the refractory nature of some
5
,14
phenolic molecules to undergo hydrogenation under H
2
selectivity for given products.
2
Compared to H , organic H-
24,25
pressure,
promising results have been found for HDO of
donors can transfer H-species to a substrate by varied
biophenols over Raney® Ni under unusual low-severity
conditions (80-120°C), employing 2-propanol as an H-donor
6
and solvent. Moreover, when compared to the conventional
reductive catalytic processes, Raney® Ni shows the ability to
controllably defunctionalise aromatic compounds through
6
stepwise H-transfer reactions. Shown in Scheme 1 is the
defunctionalisation pathway of 2-methoxyphenol in the
presence of Raney® Ni and 2-propanol (H-donor).
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applied to 1,2-dimethoxybenzene and 2-ethoxyphenol. Finally,
the interactions of the isomers adsorbed on Raney® Ni at the
catalytic solid-liquid interface were examined by ATR-IR
spectroscopy. The surface spectra reveal key features
distinguishing the adsorption geometry of 2-methoxyphenol
compared to the other two isomers. Overall, this paper aims at
providing fundamental knowledge for the development of
process options directed towards the chemo-catalytic
funnelling of lignin-derived phenols into target platform
chemicals for the monetisation of lignin streams.
Results and discussion
Scheme 1. Reactions proposed for the conversion of 2-methoxyphenol in the
presence of Raney® Ni/2-PrOH.
6
a
Reactivity profile of the positional isomers of methoxyphenol
Figure 1 presents the evolution of the reaction mixture
composition as a function of time for the experiments
performed on the positional isomers of methoxyphenol in the
presence of Raney® Ni and 2-PrOH (solvent and H-donor). To
better evaluate the selectivity towards hydrogenation or
hydrogenolysis, the experiments were first performed at 80°C
(Figure 1A, 1B and 1C). The conversion of 2-methoxyphenol is
faster than the transformation of the other two positional
isomers. Furthermore, 2-methoxyphenol is mainly converted
to phenol via DMO. In sequence, the hydrogenation of phenol
into cyclohexanol confirms phenol as the key intermediate in
this process (Figure 1A). By contrast, the conversion of the
other positional isomers of methoxyphenol rendered the
corresponding methoxycyclohexanol counterparts as the
major components of the reaction mixture (at 2h, 45-50%,
Figure 1B and 1C). These products stem from the direct
substrate saturation. Indeed, these reactions show only a
slight formation of phenol throughout the reaction duration
The conversion of 2-methoxyphenol involves first a DMO
step, which is followed by the HYD of the phenolic
intermediate to the corresponding cyclohexanol. By H-transfer
reactions, 2-methoxycyclohexanol is obtained as the minor
6
product. However, it is the primary product from
2
4,26
hydrogenation under H
2
pressure.
Notably, with Raney®
Ni/2-PrOH system, 2-methoxyphenol undergoes DMO more
easily than dehydroxylation of phenol. This reactivity pattern is
quite surprising considering that both C—O bonds in C
OCH and C -OH present similar high values of dissociation
enthalpy (385.6 and 448.4 kJ mol , respectively).
6 5
H -
3
6 5
H
-1
11
Studies using either the meta- or para-isomer as model
compounds have been rather scarce in the literature, most
likely because these structures are not primarily found in lignin
1
0
itself. However, differences in the steric constraints of the
methoxy group in the various structural isomers could
influence their adsorption on the catalyst surface and
significantly affect the reaction, providing critical information
on the mechanism underlining the H-transfer demethoxylation
(Figure 1B and 1C). Cyclohexanol arises at the beginning of the
reaction, and its concentration gradually increases to 25 mol%,
in both cases, after 2h.
2
3,27-31
of lignin-derived phenols.
Therefore, a systematic study
At 120°C, full conversion of all methoxyphenols is achieved
in less than 20 min (Figure 1E-F). The reactivity profile
observed for the substrates in the experiments confirms that
DMO in the case of 2-methoxyphenol conversion is much more
favourable than the substrate saturation to the 2-
methoxycyclohexanol. Again, for the other positional isomers,
the mixtures of the corresponding methoxycyclohexanol
isomer and cyclohexanol (~60:40, mol/mol) was obtained. In
these cases, this finding demonstrates that DMO and HYD
proceed by parallel, competing pathways. Nonetheless, HYD is
slightly more favourable than DMO when starting from the 3-
or 4-methoxyphenol. Finally, no cyclohexanone derivatives
were detected in both experiment sets, indicating that the
preferential H-donor is 2-propanol (i.e. the solvent). This
observation is expected, since 2-PrOH (solvent and H-donor) is
present in a large excess of 40 equivalents relative to the
substrate. Hence, the reaction is set to be pseudo-zero order
regarding 2-PrOH.
of the three substituted methoxyphenols seems to be crucial
to understanding the H-transfer processes catalysed by the
Raney® Ni/2-PrOH system.
In this report, we address the relationship between the
reactivity of the different methoxyphenol positional isomers
during H-transfer reactions on RANEY® Ni and their
preferential adsorption geometry on the complex surface of
this catalyst. The results and discussion are organised as
follows. In the first section, the reactivity patterns for the
positional isomers of methoxyphenol towards H-transfer
reactions in the presence of RANEY® Ni and 2-PrOH (H-donor
and solvent) were analysed based on the evolution of the
product
hydrogenation of methoxyphenol isomers was carried out
under H pressure in 2-methyltetrahydrofuran (a non-H-donor
distribution.
Subsequently,
conventional
2
solvent). Differences in the product distribution between
conventional hydrogenation and H-transfer reaction were
studied. Furthermore, to further examine the distinct reactivity
pattern of 2-methoxyphenol, H-transfer reactions were also
2
| J. Name., 2012, 00, 1-3
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T = 80 °C
A)D) 100
B)E) 100
C)F) 100
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
0
20
40
60
80
100 120
0
20
40
60
80
100 120
0
20
40
60
80
100 120
Time / min
Time / min
Time / min
T = 120 °C
D)a) 100
E)b) 100
F)c) 100
90
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
OH
O
OH
O
80
70
60
50
40
30
20
10
0
0
20
40
60
80
100 120
0
20
40
60
80
100 120
0
20
40
60
80
100 120
Time / min
Time / min
Time / min
Figure 1. Distribution of compounds for the reaction at 80 and 120°C of A and D) 2-methoxyphenol, B and E) 3-methoxyphenol and C and F) 4-methoxyphenol. Reactions
6c
conditions: substrate (1.8 mmol), RANEY® Ni (1g washed with 3 × 1 mL 2-PrOH), 2-PrOH (6 mL), 2h.
To obtain further information on the kinetics of the hydrogenation in an H-donor solvent, a synergism between the
conversion of methoxyphenol positional isomers, a kinetic hydrogenation
description of the reaction network for the conversion of
Table 1. Kinetic parameters based on the Arrhenius equation for the H-transfer
isomers was developed, by fitting global rate expressions to
conversion of 2-methoxyphenol (2-MeOPh), 3-methoxyphenol (3-MeOPh) and 4-
the considered reaction pathways HYD and DMO of Scheme 1. methoxyphenol (4-MeOPh).
To determine the reaction order, experiments were performed
Paramete-r1s
2-MeOPh
74.6 ± 3.3
3-MeOPh
79.7 ± 2.6
4-MeOPh
78.9 ± 3.6
with varying concentrations of the substrate at 80°C
Supporting information, Figure S1). The conversion of the
a
E (kJ mol )
(
-1
9
9
9
A (min )
(2.2 ± 0.1) ×10
(8.6 ± 0.4) ×10
(6.5 ± 0.4) ×10
positional isomers was found to be zeroth-order regarding the
substrate concentration. This finding indicates that an Eley-
Rideal mechanism is not observed in the H-transfer conversion
of methoxyphenols. Otherwise, the substrate must be
adsorbed on the surface to undergo the H-transfer process.
Moreover, Arrhenius plots were obtained based on the
measured rate at different temperatures (Supporting
information, Figure S2). The Arrhenius plots indicated the
activation energies of 74.6 ± 3.3, 79.7 ± 2.6 and 78.9 ± 3.6 kJ
routes (via H2 gas and H-transfer) was observed in the
3
4
conversion of diphenyl ether. Therefore, to examine whether
there exist changes in product distribution in conventional
hydrogenation and H-transfer reactions, experiments were
carried out on the methoxyphenol positional isomers under H2
pressure in 2-methyltetrahydrofuran, a non-H-donor solvent.
Interestingly, for all the substrates, the experiments
-1
mol
for 2-methoxyphenol, 3-methoxyphenol and 4-
performed under pressure of H showed much higher yields of
2
methoxyphenol, respectively (Table 1).
methoxycyclohexanols when compared to H-transfer reactions
under similar conditions (Table 2). This pattern is especially
evident for the 2-methoxyphenol as almost similar amounts of
cyclohexanol (54%) and 2-methoxycyclohexanol (46%) in the
Hydrogenation under H
2
pressure
In conventional hydrogenations under H
2
pressure, RANEY®
Ni typically exhibits higher activity than in transfer
hydrogenation employing an organic molecule as an H-
reaction under H pressure were formed. In contrast, by H-
2
32-34
transfer, cyclohexanol was by far the dominant product (95%).
These results suggest that the overall contribution of the HYD
donor.
In addition, when performing conventional
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to the final product distribution is more significant in reactions adsorption of phenol on RANEY® Ni in the presence of
3
8
conducted under H
2
pressure. However, for the conversion of cyclohexane as the solvent at 40°C. This study revealed key
2-methoxyphenol, DMO also appears to be an important features of co-adsorption of phenols, alcohols and polyols on
pathway regardless of the hydrogen source. This observation the complex RANEY® Ni surface. Furthermore, our previous
raises the question on how the presence of the substituent in study also confirmed the adsorption of phenol occurring
ortho-position could cause steric and electronic constraints, parallelly to the catalyst surface at the solid-liquid interface.
interfering with the substrate adsorption on the catalyst
surface.
Gas-phase studies on a Ni(111) surface have shown that
the required to cleave the O—H bond of phenol is small
compared to the energy required for its initial
23,36,39
H-transfer reactions of 1,2-dimethoxybenzene and 2-
ethoxyphenol
adsorption.
Moreover, gas-phase intramolecular
structural dynamics investigations on the methoxyphenol
To shed light on the role of substitution in the ortho position, positional isomers showed that the rotational barrier of the
H-transfer reactions were applied to 1,2-dimethoxybenzene OH substituent in 2-methoxyphenol is significantly higher than
4
0
and 2-ethoxyphenol, using the same experimental conditions. that of 3- and 4-methoxyphenol in their ground states. The
For 1,2-dimethoxybenzene, no significant transformation high rotational barrier for the 2-methoxyphenol was found to
occurred at 80°C, only with a very slight amount of substrate be the result of an intramolecular hydrogen-bond established
being converted into anisole (Figure 2A), in agreement with between the OH group and the methoxy group, resulting in a
6
previous research. In the case of 2-ethoxyphenol (Figure 2B), rotation of the methoxy group out of the plane of the ring by
4
0
despite the reaction being slower than that on 2- up to 13.5°.
methoxyphenol, the same reactivity pattern was observed. To provide in-depth insight into the impact of the position
Initially, the cleavage of the ethoxyl group occurs, followed by of the methoxyl substituent on the adsorption geometries, and
phenol saturation, yielding cyclohexanol.
their relationships with the selectivity towards H-transfer
DMO, we examined the adsorption of the positional isomers at
the catalytic solid-liquid interface employing ATR-IR
spectroscopy.
Table 2. Results of hydrogenation of 2-methoxyphenol (2-MeOPh), 3-methoxyphenol
(
3-MeOPh) and 4-methoxyphenol (4-MeOPh) under H
transfer results. Reactions conditions: substrate (1.8 mmol), Raney® Ni (1 g), 2-MeTHF
6 mL), H (40 bar pressure), 120°C, 2h.
2
pressure. Comparison with H-
(
2
In Figure 3 the spectra of phenol (A) and methoxyphenol
positional isomers (B, C, and D) dissolved in the solvent
cyclohexane (displayed as grey traces) and adsorbed on
RANEY® Ni in the presence of the solvent cyclohexane (shown
Positional Conversion
Cyclohexanol
(%)
Methoxycyclohexanol
isomers (%)
isomer
(%)
H-transfer
H
2
54
11
5
H-transfer
H
2
46
89
95
o
as bold black traces) at 40 C are presented. Compared to the
2
3
4
-MeOPh
-MeOPh
-MeOPh
100
100
100
95
40
45
5
spectra of dissolved phenols, which were recorded in the
absence of a catalyst at the same liquid-phase concentration (1
mM solutions), several vibrational modes are red-shifted and
show different intensities, indicating the participation of the
corresponding molecular moieties in the surface interaction(s).
In this manner, the surface spectra contain information on the
molecular orientation of the detected adsorbates on the
60
55
A
a
)
)
100
B)c) 100
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
41
paramagnetic metal catalyst and the interaction of the
phenols with RANEY® Ni.
Supported by DFT gas-phase calculations, the vibrational
modes of the four phenolic compounds were assigned.
Detailed assignment of the IR absorption bands is provided in
Tables S1-S4 in the Supporting Information. For phenol, the
assignment obtained is in excellent agreement with the results
0
20
40
60
80
100 120
0
20
40
60
80
100 120
Time / min
Time / min
4
2
Figure 2. Distribution of compounds for the reaction with A) 1,2-dimethoxybenzene, reported by Michalska et al. The adsorption mode of the
and B) 2-ethoxyphenol in the presence of Raney® Ni/2-PrOH at 80°C, for 2h.
methoxyphenol positional isomers was characterised based on
the vibrational modes highlighted in Figure 3, which are the
ATR-IR spectroscopy
stretching vibrations of the alcohol function (ν(OH)), of the
aromatic ring (ν(CC)arom), of the C—O bonds involving
aromatic C atoms (ν(CO)arom) and of the C—O bond involving
the terminal methyl group (ν(CO)aliphat), as well as the in-
plane bending of the aromatic CH groups δ(CH). Due to the
rigidity of these aromatic molecules, more than one molecular
bond may contribute significantly to a dipole moment change
and had to be considered for the assignment. Furthermore,
except for the ν(OH) mode, the dipole moment changes of
these major vibrational modes are all oriented along the plane
The pieces of evidence presented in the previous sections
demonstrated the DMO pathway to be favoured over HYD
when the methoxy group is located at the ortho position,
compared to the meta and para positions. In several
experimental and computational studies, the adsorptions of
23,31,35,36
phenol
28-31,37
on Ni and 2-methoxyphenol
on other
transition metals were investigated. Both the chemisorption of
phenol and 2-methoxyphenol on RANEY® Ni were proposed to
involve the aromatic ring. Recently, we have studied the
4
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of the molecule in the gas-phase, but the molecular scaffold formation of the corresponding methoxyphenolates at the
may be slightly distorted upon adsorption.
catalytic solid-liquid interface.
To understand the selectivity of the catalytic systems
towards C—O bond cleavage, it is particularly crucial to gain
information on the orientation of the C—O methoxy bond,
relative to the surface, of the adsorbed methoxyphenols.
4
1
According to the surface selection rule, IR signals belonging
to a dipole moment change of a bond oriented parallel to
paramagnetic RANEY® Ni would be invisible. Regarding the
adsorption of 2-methoxyphenol on RANEY® Ni, the surface
spectrum (Figure 3B) shows distinguished spectral features not
only in the ν(OH) absorption, but also in the mid-frequency
-1
region (1850 – 950 cm ). Similar to the surface spectrum of
phenol, the ν(C=C)arom bands were strongly attenuated upon
adsorption onto the RANEY® Ni, with the ν(C=C)arom band at
-
1
1
493 cm still discernible. In addition, the absorption modes of
the C—O bond stretch vibrations were almost fully attenuated.
-1
The spectral features at 1285, 1255 and 1212 cm were hardly
discerned. Owing to the absence of a C—O stretch vibration,
the adsorbed 2-methoxyphenol has to adopt a predominant
surface configuration characterised by rather flat adsorption of
the aromatic ring with a nearly parallel orientation of the
methoxy group to the surface. Such an adsorption
configuration enables stronger interaction between the
methoxy C—O bond and the catalyst surface, which can
explain the higher affinity of the 2-methoxy substituent
towards DMO.
By contrast, the detection of intense signals for ν(CC)arom
and ν(CO)arom in Figure 3C and 3D revealed that the aromatic
ring system and the two in-plane C—O bonds of meta- and
para-methoxyphenolate surface species are considerably
tilted. In the ν(CO)arom stretch region in Figure 3D, the rather
broad absorption band of the dissolved 4-methoxyphenol
Figure 3. ATR-IR surface spectra (bold black) of phenol (A), 2-methoxyphenol (B), 3-
-1
centred at 1235 cm was split into two distinct peaks at 1227
methoxyphenol (C) and 4-methoxyphenol (D) adsorbed from a 1 mM cyclohexane
-1
solution onto RANEY® Ni at 40°C. For the sake of comparison, the spectra of dissolved and 1218 cm upon adsorption. Based on our gas-phase DFT
phenol and methoxyphenols recorded in the absence of RANEY® Ni with the same
calculations, these two bands cannot be assigned
solutions are also included (grey, scaled by 0.5). The background spectrum was
unambiguously to two distinct C—O bonds. However,
recorded in the presence of the solvent cyclohexane before admission of the
considering the formation of a surface phenolate, it can be
methoxyphenols.
inferred that the phenoxy C—O bond of adsorbed 4-
A key distinguishing feature for the surface interaction of methoxyphenolate is weakened and we assign the band at
-1
the four phenols was observed in the higher frequency region 1218 cm to the phenolate C—O bond. On the other hand, the
-1
-1
of the spectra (4000 – 2800 cm ). Adsorbed phenol (Figure 3A) higher-frequency band at 1227 cm could be attributed to the
and 2-methoxyphenol (Figure 3B) gave rise to a broad ν(OH) methoxy C—O stretch. The negligible red-shift of this
absorption band revealing that the alcohol moieity of these vibrational mode indicates that the strength of this C—O bond
detected surface species was weakened, but still remained at is rather unaffected by the adsorption onto the catalyst, and
the catalytic solid-liquid interface. In contrast, 3- accordingly, the para-methoxy substituent would be located
methoxyphenol (Figure 3C) and 4-methoxyphenol (Figure 3D) away from the surface. As seen in Figure 3C, the ν(CC)arom
-
1
featured a broad ν(OH) absorption band in dissolution, but not absorption band of adsorbed 3-methoxyphenol at 1285 cm ,
in the presence of RANEY® Ni. In liquid solution, both 3- and 4- which can specifically be assigned to the methoxy C—O bond,
methoxyphenols form intermolecular hydrogen-bonds was not shifted compared to dissolved 3-methoxyphenol.
involving the phenolic and methoxy group. Nonetheless, the Therefore, we propose that, in the predominant surface
intermolecular H-bonding is broken upon adsorption on orientation of 3-methoxyphenol and 4-methoxyphenol, the
RANEY® Ni. The adsorption of 3-methoxyphenol and 4- methoxy bond is most of the time not interacting with the
methoxyphenol on RANEY® Ni was evident from the catalyst. This feature explains the low selectivity towards C—O
-
absorption bands in the mid-frequency region (1850 – 950 cm bond cleavage observed for these two substrates.
1
), and the absence of any ν(OH) vibration indicates the
Based on the spectroscopic surface characterisation, we
propose the predominant adsorption geometries depicted in
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Scheme 2. In addition to the interpretation above, the aliphatic Conclusions
C—O bond must be oriented rather parallel to the surface for
We have demonstrated that the interaction established
between methoxyphenol positional isomers and RANEY® Ni
determines their reactivity pattern towards H-transfer
reactions. In fact, 2-methoxyphenol predominantly undergoes
demethoxylation. On the other hand, for 3- and 4-
methoxyphenols, the aromatic ring hydrogenation is the
dominant reaction. Particularly for 2-methoxyphenol, the DMO
preference over HYD is very interesting as it also deviates from
the typical reactivity pattern found in conventional
all methoxyphenols, for the DMO to take place.
2 2
hydrogenation under H pressure. Apparently, under H
pressure, the planar adsorption through the aromatic ring is
less favoured, reducing the population of phenolic species with
the methoxy group orientated parallelly to the catalyst
surface. As a result, DMO becomes less dominant leading to a
product mixture of cyclohexanol and 2-methylcyclohexanol
(
54:46). For meta- and para-isomers, tilted adsorption of
methoxyphenol via the formation of a phenolate surface
species prevents the alignment of the C—O bond of the
methoxyl group parallel to the surface, thus hindering the
DMO pathway.
The current findings also present an alternative
explanation to previously reported enol-keto tautomerisation
Scheme 2. Proposed adsorption of phenol (A), ortho-methoxyphenol (B), meta-
methoxyphenol (C) and para-methoxyphenol (D) on RANEY® Ni from the liquid
solution.
4
4-46
mechanism,
rationalising the preference of 2-methoxyphenol to undergo
DMO rather than direct phenol dehydroxylation despite the
similar bond dissociation enthalpy for both C—O bonds in
Further evidence on the features of the adsorption
complex of 2-methoxyphenol distinguishing from those of the
other two positional isomers was obtained by revisiting the
kinetic data (Table 3). The transition-state enthalpies and
entropies were determined for the conversion of the
positional isomers, by applying the Eyring-Polanyi equation to
the reaction rates obtained at different temperatures
6 5 3 6 5
C H -OCH and C H -OH. This means that this tautomeric
3
mechanism, in which cleavage of C(sp )-O bonds prevails over
2
the partial hydrogenation of the ring and the direct C(sp )-O
cleavage due to the lower activation energy required, does not
need to be primarily invoked to elucidate the product
distribution for each positional isomer of methoxyphenol
obtained from H-transfer reactions.
(Supporting Information, Figure S3). Notably, the predominant
adsorption geometries inferred from the features of ATR-IR
spectra agree with the transition-state entropies determined
for the conversion of the methoxyphenol positional isomers. In
fact, while enthalpies of the transition states are similar for all
substrates (as also previously noticed for the energy of
activation), entropy estimated for the 2-methoxyphenol is
slightly smaller than those for the other two positional
isomers. This observation is consistent with a compact surface
complex of 2-methoxyphenol parallel to the catalyst surface.
Indeed, when a molecule adsorbs onto a surface in a more
planar geometry, a decrease in the entropy greater than that
for tilted geometries should be expected, owing to a
In a broader context, this fundamental knowledge on how
to direct product selectivity based on
a
molecular
understanding of interactions between substrate and catalyst
surface is of extreme importance for the biorefinery concept.
In fact, it contributes to the design of catalysts for the target
production of value-added chemicals in which the problem
posed by the formation of (complex) product mixtures must be
alleviated, to improve the atom-economy of the overall
process chains. Ultimately, as the transfer-demethoxylation
o
takes place under low severity conditions (i.e. 80-120 C,
autogenous pressure) and employs a non-expensive catalyst
(
RANEY® Ni), in-depth understanding of mechanisms
4
3
considerable reduction in the molecular motion.
underlining the product selectivity contributes to the further
elaboration of chemocatalytic strategies for the funnelling of
lignin products (e.g. S and G → H-derived products, where ‘S’,
Table 3. Kinetic parameters based on the Eyring–Polanyi equation for the H-transfer
conversion of 2-methoxyphenol (2-MeOPh), 3-methoxyphenol (3-MeOPh) and 4- ‘G’ and ‘H’ stand for syringyl, guaiacyl and coumaryl alcohol-
methoxyphenol (4-MeOPh).
derived products, respectively) to the corresponding
6
demethoxylated cyclohexanols.
Parameters
2-MeOPh
71.7±3.3
-75.4±6.0
3-MeOPh
76.9±2.6
-64.2±3.7
4-MeOPh
76.1±3.6
-66.5±5.3
‡
-1
ꢀ
ꢀ
H (kJ mol )
‡
-1 -1
S (J mol K )
Conflict of interest
There are no conflicts to declare.
6
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adsorbed species. Dissolution spectra were collected following
the same procedure on a blank crystal in the absence of the
Raney Ni catalyst film.
Acknowledgements
R.R. acknowledges the financial support provided by the ERC
Consolidator Grant (LIGNINFIRST, Project Number: 725762).
Experimental
DFT calculations
H-Transfer reactions
The molecular geometry of phenol and the methoxyphenols
Typically, the substrate (1.8 mmol),
n
-hexadecane (internal was optimised using the gas-phase B3LYP/def2-SV(P) method
4
8
standard for GC analysis, 0.1 mmol), RANEY® Ni 2800 (1.0 g, as implemented in Turbomole 6.3. The optimised structure
washed with three 5-mL portions of 2-PrOH to remove water), was confirmed as minima by a full gas-phase frequency
-PrOH (6 mL) and a magnetic stirrer were placed in a 20-mL calculation. Vibrational modes were calculated within the
2
glass vial. The vial was flushed with Ar and then tightly sealed. harmonic approximation using gas-phase frequency
The experiment was performed under magnetic stirring (800 calculations.
rpm) at 80 or 120°C for 2 h. Aliquots of the reaction mixture
were taken at the indicated times.
Notes and references
Hydrogenation
1
2
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GC analysis
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–1
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