Catalytic C–O bond cleavage in a β-O-4 lignin model through intermolecular hydrogen transfer

Article abstract of DOI:10.1016/j.ica.2021.120305

A base-free and redox neutral approach for the selective breaking of aryl ether bond (C–O) contained by a lignin model compound mimicking a β-O-4 linkage is reported. A palladium loaded metal-organic framework (MOF) was used as a catalyst for this purpose. The reaction proceeds through dehydrogenation of benzylic alcohol moiety followed by the hydrogenolysis of the ether bonds. Therefore, no external hydrogen source is required for the reaction to take place.

Full text of DOI:10.1016/j.ica.2021.120305

Inorganica Chimica Acta 521 (2021) 120305
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Catalytic C–O bond cleavage in a β-O-4 lignin model through
intermolecular hydrogen transfer
Muhammad Ahsan Usman, Maham Naeem, Muhammad Saeed*, Muhammad Zaheer*
Department of Chemistry and Chemical Engineering, Syed Babar Ali School of Science and Engineering, Lahore University of Management Sciences (LUMS), Lahore
54792, Pakistan
A R T I C L E I N F O
A B S T R A C T
Keywords:
A base-free and redox neutral approach for the selective breaking of aryl ether bond (C–O) contained by a lignin
model compound mimicking a β-O-4 linkage is reported. A palladium loaded metal-organic framework (MOF)
was used as a catalyst for this purpose. The reaction proceeds through dehydrogenation of benzylic alcohol
moiety followed by the hydrogenolysis of the ether bonds. Therefore, no external hydrogen source is required for
the reaction to take place.
Redox-neutral
Hydrogenolysis
Lignin model
Heterogeneous catalysis
MOF
1. Introduction
dissociation energy of C–O bond from 69.2 kcal/mol to 55.9 kcal/mol
[17]. So obtained oxidized lignin has been shown to provide high yield
Lignocellulosic biomass composed of cellulose, hemicellulose and
lignin, is the most abundant, cheap, and inexpensive renewable organic
resource that can potentially replace the fossil feedstocks in the sus-
tainable production of fuels and carbon-based chemicals [1–4]. Lignin
comprises 25–30% of the total biomass and is considered as one of the
renewable feedstocks for the sustainable production of value-added ar-
omatic chemicals [5–7] and materials [8]. However, as compared to
other lignocelluloses (cellulose and hemicellulose), this biopolymer has
been underutilized to its maximum potential for energy production, for
instance, as a boiler fuel to generate heat in paper and pulp industries
[3,9]. This is mainly because of recalcitrant nature of C–O bonds and
recondensation reactions of highly reactive degraded-products that
render depolymerization of lignin quite challenging [10]. Lignin depo-
lymerization, however, is considered as an entry point towards the
valorization of lignin to obtain low-molecular-weight compounds
(lignin platform compounds), which can be chemically modified into
carbon-containing chemicals and fuels [11,12]. Thereby deconstruction
of lignin via C–O bond cleavage utilizing catalytic depolymerization [6]
of lignin via hydrogenation [13], hydrogenolysis [14], and catalytic
transfer hydrogenolysis (CTH) [15,16] has been utilized which provide
low-yield of ill-defined products. However, these approaches require a
base to proceed at an appreciable rate.
(60 wt%) of low molecular weight products [18]. However oxidation of
lignin required in the first step of aforementioned approach in the
presence of green and inexpensive oxidants and reusable catalysts has
not been achieved yet [19,20]. Dehydrogenation of benzylic alcohol at
C
α of β-O-4 linkage to obtain oxidized lignin is another attractive
strategy that doesn’t require any external oxidant [21]. Recently,
several groups have reported a number of heterogeneous [22,23 24 25
26 16 27] and homogeneous [28 29 23] catalysts operating under mild
and redox-neutral conditions and regardless of the progress, a hetero-
geneous catalyst for completely redox-neutral cleavage of C–O bonds in
β-O-4 linkages in model compounds as well as in native lignin is still
desirable. In this work, we report a complete redox-neutral cleavage of
C–O bond in β-O-4 lignin model compounds using a reusable Metal-
Organic Framework (MOF) integrated with palladium nanoparticles
(NPs) labeled as Pd@MOF. MOFs has emerged as a promising class of
template material because of their high surface area, ordered pores,
tunable pore window and inherent host–guest chemistry [30]. The
synergistic effect between the metal NPs and MOFs matrix is reported to
promote the performance of a catalyst [31]. Herein, MIL-101-Cr [32]
was selected as a catalyst support due to its high crystallinity, stability in
water, thermal stability and Lewis acidity which may promote the
cleavage of O–H bonds [33].
Studies show that oxidation of benzylic hydroxyl group contained by
the most abundant motif (a β-O-4 linkage) of lignin, reduces the bond
* Corresponding authors.
E-mail addresses: muhammad.saeed@lums.edu.pk (M. Saeed), muhammad.zaheer@lums.edu.pk (M. Zaheer).
https://doi.org/10.1016/j.ica.2021.120305
Received 23 November 2020; Received in revised form 17 February 2021; Accepted 17 February 2021
Available online 23 February 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

M. Ahsan Usman et al.
Inorganica Chimica Acta 521 (2021) 120305
Scheme 1. Synthesis of β-O-4 model compounds 4a-e.
2. Experimental
concentrated in vacuo. The resulting solid was recrystallized in hot pe-
troleum ether to obtain phenoxyphenylethanones (3a-e) in excellent
(~90%) yields
2.1. Synthesis of MIL-101 (Cr)
MIL-101-Cr was synthesized by following a procedure reported in
literature [34]. Typically, Cr(NO3)3⋅9H2O (2.50 mmol, 1 g), terephthalic
acid (2.50 mmol, 0.415 g), and concentrated HCl (12 M, 0.25 mL) were
dissolved in deionized (DI) water (25 mL). The resulting solution was
placed in a Teflon-lined stainless-steel autoclave reactor and heated at
200 ◦C for 12 h. After the autoclave reactor was cooled down to room
temperature, green precipitates were separated by centrifugation (5000
rpm, 20 min) and washed with hot DMF to remove unreacted organic
linker. The synthesized MOF was activated in ethanol at 80 ◦C for 12 h.
Finally, the MOFs was separated by centrifugation and dried overnight
at 80 ◦C in a vacuum oven.
2.3.2. General method for the synthesis of β-O-4 model compounds
Phenoxyphenylethanones (3a-e, 10-mmol) were dissolved in a
mixture of THF/water (5:1) (25 mL) and NaBH4 (0.74g, 20 mmol) was
added into the solution. The reaction mixture was stirred at room tem-
perature for 5 h. Afterward, aqueous NH4Cl solution (30 mL) was added
to quench the excess NaBH4 in the reaction. The mixture was filtered
directly into a separatory funnel containing ethyl acetate to extract the
product (3 × 30 mL). Combinned ethyl acetate layer was washed with
water and dried over MgSO4. Finally, the extract was concentrated in
vacuo to obtain an off-white solid product.
2.3.3. 2-phenoxy-1-phenylethanol (4a):(85% yield)
1H NMR (600 MHz, CDCl3) δ (ppm): 7.46 (d, J = 7.4 Hz, 2H), 7.40 (t,
J = 7.5 Hz, 2H), 7.34 (t, J = 7.3 Hz, 1H), 7.29 (t, J = 5.3 Hz, 2H), 6.98 (t,
J = 7.3 Hz, 1H), 6.93 (d, J = 8.0 Hz, 2H), 5.13 (dd, J = 8.8, 3.0 Hz, 1H),
4.11 (dd, J = 9.5, 3.1 Hz, 1H), 4.02 (t, J = 9.2 Hz, 1H).
2.2. Synthesis of Pd@MOF
Palladium nanoparticles (NPs) were encapsulated into the MOFs
using well established double solvent approach [35]. In this method, n-
hexane and water waere used as hydrophobic and hydrophilic solvents
respectively. For 3 wt% metal loading, 300 mg of activated MOFs was
dispersed in n-hexane (60 mL), the resultant mixture was sonicated for
20 min until it became homogenous and stirred for additional 2 h. An
aqueous solution (0.4 mL) of K2PdCl4 (0.084 mmol, 35 mg) was drop-
wise added over 20 min under constant and vigorous stirring which
was continued for 6 h. The volume of water containing metal salt must
be less than or equals to pore volume of MOFs so that it could easily be
absorbed within hydrophilic pores. The product was separated through
centrifugation and dried overnight at 100 ◦C in a vacuum oven. Over-
whelming reduction (OWR) approach using an aqueous solution of
NaBH4 was used to reduce Pd(II) in the MOFs. In a typical experiment,
freshly prepared (0.6 M) aqueous NaBH4 solution (5 mL) was added to
Pd(II)/MOF dispersion under continuous stirring, which resulted in the
generation of catalysts as a dark-colored suspension. The synthesized
materials were separated by centrifugation and dried overnight at 80 ◦C
in a vacuum oven.
2.3.4. 2-(2-methoxyphenoxy)-1-phenylethanol (4b), (80% Yield)
1H NMR (600 MHz, CDCl3) δ (ppm): 7.44 (d, J = 7.3 Hz, 2H), 7.37 (t,
J = 7.5 Hz, 2H), 7.31 (t, J = 7.3 Hz, 1H), 7.07–6.98 (m, 1H), 6.95–6.92
(m, 2H), 6.91–6.88 (m, 1H), 5.11 (dd, J = 9.4, 2.8 Hz, 1H), 4.19 (dd, J =
10.0, 2.8 Hz, 1H), 3.98 (t, J = 9.8 Hz, 1H), 3.88 (s, 3H).
2.3.5. 2-(4-methoxyphenoxy)-1-phenylethanol (4c): (83% Yield)
1H NMR (600 MHz, CDCl3) δ (ppm): 7.46 (d, J = 7.2 Hz, 2H), 7.40 (t,
J = 7.5 Hz, 2H), 7.34 (t, J = 7.3 Hz, 1H), 6.87 (d, J = 9.3 Hz, 2H), 6.84
(d, J = 9.3 Hz, 2H), 5.10 (dd, J = 8.8, 3.1 Hz, 1H), 4.07 (dd, J = 9.7, 3.1
Hz, 1H), 3.97 (t, J = 9.5, 9.2 Hz, 1H), 3.78 (s, 3H).
2.3.6. 2-(2,6-dimethoxyphenoxy)-1-phenylethanol (4d) (75% Yield)
1H NMR (600 MHz, CDCl3) δ (ppm): 7.39 (d, J = 7.6 Hz, 2H), δ: 7.34
(t, J = 7.6 Hz, 2H), 7.27 (d, J = 7.2 Hz, 1H), 7.04 (t, J = 8.4 Hz, 1H), 6.61
(d, J = 8.4 Hz, 2H), 4.96 (dd, J = 9.9, 2.5 Hz, 1H), 4.44 (dd, J = 10.9, 2.6
Hz, 1H), 3.72 (t, J = 10.4 Hz, 1H), 3.88 (s, 6H).
2.3. Synthesis of β–O-4′ lignin model compounds
2.3.7. 2-(4-methylphenoxy)-1-phenylethanol (4d) (77% Yield)
1H NMR (600 MHz, CDCl3) δ (ppm): 7.46 (d, J = 7.4 Hz, 2H), 7.40 (t,
J = 7.6 Hz, 2H), 7.34 (t, J = 7.3 Hz, 1H), 7.09 (d, J = 8.5 Hz, 2H), 6.82
(d, J = 8.5 Hz, 2H), 5.12 (dd, J = 8.9, 3.0 Hz, 1H), 4.09 (dd, J = 9.6, 3.0
Hz, 1H), 3.98 (t, J = 9.3, 1H), 2.29 (s, 3H).
2.3.1. General method of synthesis of 2-phenoxy-1-phenylethanol (2a)
Lignin model substrates were synthesized according to a modified
procedure reported in the literature (Scheme 1) [36]. A 100-mL round
bottom flask, equipped with a reflux condenser and magnetic stirrer,
was charged with phenol (2a-e, 12 mmol) and K2CO3 (15 mmol) in
acetone (50 mL). 2-Bromoacetophenone (1, 10 mmol, 1.99 g) in 50 mL
acetone was then added dropwise into the reaction mixture with con-
stant stirring at room temperature. While stirring, the suspension was
refluxed for 6 h. After complete consumption of 1 (TLC monitoring), the
reaction mixture was filtered to remove K2CO3 and the filtrate was
2.4. Catalytic reaction
In general procedure for redox neutral C–O bond cleavage in 4a, a
reaction tube (Schlenk Flask) charged with a magnetic stirrer bar and
catalyst (20 mg) was used. The reaction tube was evacuated and refilled
2

M. Ahsan Usman et al.
Inorganica Chimica Acta 521 (2021) 120305
Fig. 1. SEM images of (pristine MOF (a) and Pd-loaded MOF (b). EDX elemental maps of a Pd-loaded MOF (c) are shown in (d-f). PXRD diffraction patterns of the
MOFs are shown in (g) along with a simulated pattern.
Fig. 1b. Incorporation of Pd within the MOF crystals was confirmed
Table 1
through Energy Dispersive X-ray (EDX) analysis. EDX elemental maps of
Solvent screening for the redox-neutral C–O bond cleavage.
a MOF crystal (Fig. 1c) are presented in Fig. 1d-f that confirm the suc-
cessful incorporation and uniform distribution of Pd within the MOF.
Crystallinity of pristine and Pd@MOF was also analyzed by Powder
X-Ray diffraction (PXRD) analysis (Fig. 1g). In diffraction patterns of
pristine MOF, peaks at of 8.5◦, 9.1◦, 10.3◦, 16.5◦, and 18◦ (2θ) matched
Sr.
#
Catalyst
Solvent
Tem.
(◦C)
Conv.
(%)
Yield (%)
closely with the simulated patterns of MIL-101. No significant change in
diffraction pattern was observed upon the incorporation of Pd within the
MOF and the peaks matched well with the simulated patterns of MIL-
101. An extra peak, however, at 28.3◦ (2θ) was observed in the dif-
fractogram Pd@MOF which could not be explained.
5a
2a
3a
1
2
3
4
5
6
7
8
Pd@MOF
Pd@MOF
Pd@MOF
Pd@MOF
Pd@MOF
Pd@MOF
Pd@MOF
MIL-101
(Cr)
EtOH
MeCN
H2O
120
120
120
120
120
120
120
120
18
0
–
–
–
>99
–
–
56
5
39
–
39
–
61
>99
–
Dioxane
Xylene
Diglyme
THF
Pd@MOF was tested in redox-neutral cleavage of aryl ether bonds in
lignin model compounds. For screening experiments, the compound
PhCH(OH)CH2-OPh (4a) containing β–O-4′ ether linkage to generate
phenol (2a) and aryl-ketone (3a) was used.
0
–
–
0
–
–
–
0
–
–
–
EtOH/
H2O
0
–
–
–
The reactions were performed without adding any base or external
hydrogen source in order to demonstrate the utilization of intra-
molecular hydrogen that promote the cleavage of C–O bonds (Table 1).
When reaction was performed in EtOH only 18% conversion to 3a along
with cleaved products 2a and 3a was observed (entry 1). No conversion
of the substrate was observed in acetonitrile (entry 2), dioxane (entry 4),
xylene (entry 5), diglyme (entry 6) and THF (entry 7). However, H2O
showed significantly higher conversion, with the 39% yield of aimed
products, respectively (entry 3). Pristine MOF was unable to convert the
substrate either (entry 8) which shows the role of Pd in promoting both
dehydrogenation and C–O bond cleavage reactions. Surprisingly, with
the mixture of H2O and EtOH (1:1) compound 4a was almost quantita-
tively converted into desired products 5a and 2a with negligible amount
of 3a (entry 9). Keeping in mind the detection of small amount of 3a, we
hypothesized that cleavage of C–O bond in 4a may proceed through a
Pd-catalyzed dehydrogenation of Cα-OH bond followed by cleavage of
Cβ-O bond via intramolecular hydrogen transfer. To prove this hypoth-
esis firstly, the reactions were conducted without a catalyst and with
pristine MOF (with no Pd NPs), but the substrate remained unchanged
and no desired products were observed (entry 9–10). To further
demonstrate the mechanism, several other reactions were conducted
with the same reaction conditions. Performing the reaction with 3a as
substrate favored our hypothesis because the formation of 5a and 2a was
not observed. In order to omit the possibility of dissolved oxygen that
might facilitate dehydrogenation, the reaction was performed in
completely deaerated solvent. The dissolved oxygen was removed with
argon purging for half an hour. The results were incredibly good with
complete conversion into desired products indicating the dissolved ox-
ygen playing no role in this reaction.
9
Pd@MOF
EtOH/
H2O
120
120
>99
>99
>99
<1
10
No catalyst
EtOH/
H2O
0
–
–
–
Reaction conditions: substrate (0.1 mmol), catalyst (2.8 mol% based on Pd),
solvent (10 mL each). The reaction tube was purged with nitrogen and the re-
action mixture was stirred under nitrogen atmosphere for 6 h.
with nitrogen gas (three times). Lignin model substrate 4a (0.1 mmol,
21 mg) and ethanol/water (1:1, 10 mL) was added in the reaction tube
under a continuous flow of nitrogen. The reaction mixture was refluxed
at 120 ◦C in an oil bath for 6 h. After cooling the reaction mixture to
room temperature, it was centrifuged to separate the catalyst. The
products were extracted with DCM or ethyl acetate (30 mL). The extract
was concentrated under reduced pressure and analyzed with GC–MS.
3. Results and discussions
To achieve homogeneously dispersed and cavity-fit Pd NPs into the
cages of MOFs, we employed a double solvent approach (DSA) [37] in
which two solvents, water (hydrophilic) in small amount and n-hexane
(hydrophobic) in large amount are used (experimental section). The
presence of n-hexane in excess over-spreads the external surface of
MOFs, ensuring the diffusion of aqueous Pd-salt into the pores through
capillary action.
Surface morphology and crystal size of as-prepared catalyst was
analyzed through scanning electron microscopy (SEM). SEM image of
pristine MOF (MIL-101-Cr) revealed octahedral shape of crystals with an
average size of 2 μm and a rather broad size distribution. A few rod-like
The experiments were performed to screen the best temperature
conditions for the redox-neutral cleavage of C–O bond (Table S1). The
reactions carried out at 30 ◦C,60 ◦C and 90 ◦C lead to no conversion. The
reaction performed at 120 ◦C within 2 h offered excellent yield of aimed
crystals spotted at different places (Fig. 1a) that might correspond to
MIL-88B geometry [38]. The morphology of the MOFs remained intact
even after the loading of Pd NPs (3 wt%) as indicated in SEM images in
3

M. Ahsan Usman et al.
Inorganica Chimica Acta 521 (2021) 120305
breaking of C–O bonds in a series of β–O-4 lignin model compounds. The
studies revealed that, C–O bond cleavage occurred through a simulta-
neous dehydrogenation of side-chain alcoholic groups and intra-
molecular hydrogen transfer reaction and therefore doesn’t require any
external hydrogen. Our catalyst is reusable and offers a broad substrate
scope.
Table 2
Redox-neutral cleavage of C–O bond in methoxy-substituted β–O-4′ ether
linkages.
CRediT authorship contribution statement
Muhammad Ahsan Usman: Investigation. Maham Naeem: Inves-
tigation. Muhammad Saeed: Resources, Conceptualization. Muham-
mad Zaheer: Conceptualization, Supervision, Project administration,
Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
M.Z. is thankful to Higher Education Commission of Pakistan for
financial support under NRPU(grant number 4130).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120305.
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