Degradation of β-O-4 model lignin species by vanadium Schiff-base catalysts: Influence of catalyst structure and reaction conditions on activity and selectivity

Article abstract of DOI:10.1016/j.cattod.2015.08.045

In the pursuit of value-added products from the degradation of the abundant aromatic biopolymer lignin, homogeneous catalysis has the potential to provide a mild, selective route to monomeric phenols. Homogeneous vanadium catalysts have previously been shown to effectively cleave dimeric β-O-4 model lignin compounds, with selectivity for C-C or C-O cleavage, or benzylic oxidation, depending on the ligand structure and oxidation state of the metal. In this study, a systematic kinetic investigation was undertaken in order to gain further understanding of the role of ligand structure and reaction conditions on the activity of vanadium Schiff-base catalysts towards a non-phenolic β-O-4 model lignin dimer, and the selectivity of these species towards C-O bond cleavage. Catalytic activity was found to be increased by the addition of bulky, alkyl substituents at the 3′-position of the phenolate ring, whereas electron-withdrawing substituents were found to dramatically reduce activity irrespective of their size. Selective depolymerization of a phenolic β-O-4 dimer was also achieved.

Full text of DOI:10.1016/j.cattod.2015.08.045

G Model
CATTOD-9776; No. of Pages8
ARTICLE IN PRESS
Catalysis Today xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Degradation of ␤-O-4 model lignin species by vanadium Schiff-base
catalysts: Influence of catalyst structure and reaction conditions on
activity and selectivity
Heather J. Parkera,b, Christopher J. Chuckc, Timothy Woodmana, Matthew D. Jonesa,∗
a Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK
b Doctoral Training Centre, Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, UK
c Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 June 2015
Received in revised form 25 August 2015
Accepted 31 August 2015
Available online xxx
In the pursuit of value-added products from the degradation of the abundant aromatic biopolymer lignin,
homogeneous catalysis has the potential to provide a mild, selective route to monomeric phenols. Homo-
geneous vanadium catalysts have previously been shown to effectively cleave dimeric ␤-O-4 model lignin
compounds, with selectivity for C C or C O cleavage, or benzylic oxidation, depending on the ligand
structure and oxidation state of the metal. In this study, a systematic kinetic investigation was under-
taken in order to gain further understanding of the role of ligand structure and reaction conditions on
the activity of vanadium Schiff-base catalysts towards a non-phenolic ␤-O-4 model lignin dimer, and
the selectivity of these species towards C O bond cleavage. Catalytic activity was found to be increased
by the addition of bulky, alkyl substituents at the 3ꢀ-position of the phenolate ring, whereas electron-
withdrawing substituents were found to dramatically reduce activity irrespective of their size. Selective
depolymerization of a phenolic ␤-O-4 dimer was also achieved.
Keywords:
Lignin depolymerization
Vanadium
Homogeneous catalysis
Model compound
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
and complex product mixtures are formed which generally require
further upgrading and/or separation to produce higher value prod-
The valorization of lignin is widely recognized as an important
contributor to the economic viability of the biorefinery concept;
despite this lignin is not currently widely exploited as a bioresource,
largely due to its recalcitrant nature and resistance to degradation
[1–4]. In its native polymeric form lignin is of little commercial
value and is routinely burnt to recover process heat, for instance
in the paper and pulping industry [1]. It is, however, the most
abundant source of renewable aromatic functionality available and
effective lignin depolymerization could provide a sustainable route
to potentially valuable monomeric phenolic species. Substituted
phenolic species obtainable from lignin, such as vanillin, have
potential applications as antioxidants [5].
Thermochemical techniques such as pyrolysis and gasification
have been employed in the degradation of lignin and have been
carried out on both isolated lignin and whole biomass. These pro-
cesses result in unselective degradation, often partially or fully
destroying the intrinsic aromatic structure of the lignin polymer,
ucts [1,6–8].
In order to selectively obtain valuable aromatic products from
lignin, a milder and more elegant deconstruction technique is
required; homogeneous catalysts could be employed to improve
selectivity whilst potentially avoiding the need for harsh reac-
tion conditions.[2,9,10] The inherent diversity of organometallic
complexes allows catalytic activity and selectivity to be tuned, for
example to target specific linkages or to produce particular prod-
ucts. Homogeneous species are less likely than their heterogeneous
counterparts to promote over-reduction resulting in a loss of aro-
maticity and are potentially capable of accessing linkages within
the three-dimensional structure of the lignin polymer which would
be beyond the reach of a traditional heterogeneous catalyst [10].
Most examples of homogeneous lignin depolymerization catal-
ysis to date have been demonstrated solely on model lignin
compounds [2]. The relative simplicity of these species compared
to native lignin increases the ease of analysis, thereby facilitating
mechanistic understanding of the degradation. Whilst a diverse
library of model compounds is available, the most common are
dimeric compounds containing the ␤-aryl ether (␤-O-4) linkage;
as well as being the most abundant linkage in lignin, it is also
∗
Corresponding author.
E-mail address: mj205@bath.ac.uk (M.D. Jones).
http://dx.doi.org/10.1016/j.cattod.2015.08.045
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: H.J. Parker, et al., Degradation of ␤-O-4 model lignin species by vanadium Schiff-
base catalysts: Influence of catalyst structure and reaction conditions on activity and selectivity, Catal. Today (2015),
http://dx.doi.org/10.1016/j.cattod.2015.08.045

G Model
CATTOD-9776; No. of Pages8
ARTICLE IN PRESS
2
H.J. Parker et al. / Catalysis Today xxx (2015) xxx–xxx
Fig. 1. Representative structures of vanadium-based lignin depolymerization catalysts reported by Hanson [17] and Toste [18].
Fig. 2. Degradation of ␤-O-4 model lignin compounds by vanadium catalysts (a) C C cleavage and benzylic oxidation, Hanson [17]; (b) C O cleavage and benzylic oxidation,
Toste [18].
amongst the most susceptible to cleavage and therefore represents
a desirable target for depolymerization [11].
process, with the model compound coordinating to vanadium
through the benzylic hydroxyl group. Subsequent studies by this
group demonstrated the partial depolymerization of Organosolv
lignin derived from Miscanthus giganteus using vanadium catalysts
[21].
In this current study, the role of the Schiff-base ligand on both
the activity and selectivity of homogeneous vanadium catalysts
was further probed via a systematic synthetic and kinetic investi-
gation in order to inform the development of an improved catalytic
system.
A wide range of metal complexes have been shown to effectively
cleave ␤-O-4 model lignin compounds [12–16]. For the production
of monomeric phenols, vanadium(V) complexes with O/N donor
ligands are amongst the most promising homogeneous catalysts,
Fig. 1 [17–20]. Changing the ligand structure has been shown to dra-
matically affect the selectivity of the vanadium complexes towards
C
O or C C bond cleavage, or benzylic oxidation. Hanson et al.
reported the C C bond cleavage in ␤-O-4 models by dipicolinate
vanadium(V) using air as the oxidant. Degradation occurred via
benzylic oxidation to the oxidation product (OP) followed by sub-
sequent C C cleavage to form benzoic acid, phenol and formic acid,
Fig. 2(a) [17].
2. Experimental
Son and Toste reported the depolymerization of a dimeric ␤-O-4
model compound in the presence of a variety of oxo-vanadium(IV)
and (V) species, Fig. 2(b). The former were observed to significantly
favour benzylic oxidation to form the oxidation product (OP), over
the desired C O bond cleavage to form the phenol (in this case
guaiacol) [18]. The highest selectivity of the vanadium(IV) catalysts
for C O cleavage was 25% in the case of the tridentate ligand.
For the vanadium(V) Schiff-base species, the ligand backbone
and functionalization were found to be very important for tuning
the activity and selectivity towards C O bond cleavage over oxida-
tion. Selectivity for C O cleavage could be improved by extending
carbon backbone of the tridentate Schiff-base ligands (from n = 1 to
n = 2, Fig. 1), thereby decreasing the O–V–N bite angle. Both activity
and selectivity for C O cleavage were favoured by the addition of
tert-butyl substituents at the 3- and 5-positions of the phenolate
ring.
2.1. Materials and methods
Where preparative details are not provided, reagents were pur-
chased from Sigma Aldrich, TCI Chemicals, Fluka, Lancaster, Acros
Organics or Alfa Aesar and used without additional purification.
NMR spectroscopy: NMR spectra were obtained on one of
Bruker Advance 300, 400 or 500 MHz spectrometers at 298 K in
(CD3)2SO, CD3OD or CDCl3 as solvent. Chemical shifts are reported
in parts per million (ppm) relative to the residual solvent peak and
coupling constants are reported in Hertz (Hz).
ESI-MS: ESI-MS analysis was recorded on a Bruker Daltonic
micrOTOF electrospray time-of-flight (ESI-TOF) mass spectrome-
ter coupled to an Agilent 1200 LC system as an autosampler. 10 ␮L
of sample was injected into a 30:70 flow of water:acetonitrile at
0.3 mL/min into the mass spectrometer.
Elemental analysis: Elemental compositions were obtained by
Mr Stephen Boyer at the Microanalysis Service, London Metropoli-
tan University, UK.
The mechanism for C O bond cleavage was proposed to proceed
via
a formally non-oxidative one-electron V(V)–V(IV) redox
Please cite this article in press as: H.J. Parker, et al., Degradation of ␤-O-4 model lignin species by vanadium Schiff-
base catalysts: Influence of catalyst structure and reaction conditions on activity and selectivity, Catal. Today (2015),
http://dx.doi.org/10.1016/j.cattod.2015.08.045

G Model
CATTOD-9776; No. of Pages8
ARTICLE IN PRESS
H.J. Parker et al. / Catalysis Today xxx (2015) xxx–xxx
3
X-ray crystallography: All data were collected on a Nonius
kappa CCD diffractometer with MoK␣ radiation (ꢀ = 0.71073 A).
(100 MHz, CDCl3) ı 20.9, 62.6, 117.9, 126.7, 127.9, 129.2, 129.4,
130.9, 131.0, 132.8, 144.2, 152.2.
˚
T = 150(2) K throughout and all structures were solved by direct
methods and refined on F2 data using the SHELXL-97 suite of
programmes.[22] Hydrogen atoms, were included in idealized
positions and refined using the riding model. The crystal data
is straightforward and the following CCDC numbers 1404037-
1404041 contain the information.
3-Trityl-5-methylsalicylaldehyde: 2-Trityl-4-methylphenol
(3.5 g, 0.01 mol), hexamethylenetetramine (2.80 g, 0.02 mol) and
trifluoroacetic acid (10 mL) were stirred together for 4 h at 120 ◦C.
The mixture was cooled to 80 ◦C, 33% aq. H2SO4 (15 mL) was added
and the reaction was heated for a further 2 h at 130 ◦C. After cooling
to room temperature, ethyl acetate (20 mL) and water (30 mL)
were added. The organic layer was separated and the water layer
extracted with ethyl acetate (3× 20 mL). The combined organic
extracts were washed with water (50 mL) and brine (30 mL) and
dried over MgSO4. The solvent was removed in vacuo and the
residue was washed with diethyl ether to yield the product as a
pale yellow powder in 49% yield. 1H NMR: (400 MHz, CDCl3) ı 2.26
(s, 3H, CH3), 7.10–7.25 (m, 15H, Ar-H), 7.27–7.30 (m, 1H, Ar-H),
7.36 (d, J = 2.0 Hz, 1H, Ar-H), 9.80 (s, 1H, CHO), 11.11 (d, J = 0.5 Hz,
2.2. 1H NMR depolymerization studies
2-Phenoxy-1-phenylethanol (33 mg, 0.15 mmol), catalyst
(0.5–7 mol%) and hexamethylbenzene (internal standard, 2 mg,
0.01 mmol) were dissolved in 1 mL DMSO-d6 in an NMR tube
(uncapped) and heated (70–120 ◦C) for 4 days.
1
2.3. Synthetic procedures
1H, OH). 13C{ H} NMR (100 MHz, CDCl3): ı 20.7, 62.9, 120.6, 125.7,
127.2, 127.8, 128.0, 128.2, 129.4, 130.8, 130.9, 132.7, 135.4, 138.8,
144.8, 158.5, 196.5.
2.3.1. Non-phenolic model compound synthesis
2-Phenoxy-1-phenylethanone: To a solution of 2-bromo-1-
phenylethanone (9 g, 45 mmol) in dimethylformamide (150 mL)
was added phenol (5 g, 53 mmol) and K2CO3 (7.3 g, 53 mmol). The
solution was stirred overnight and a colour change from yellow
to orange was observed. The reaction mixture was then poured
into warm water and left to recrystallize. The crystals were filtered
and redissolved in toluene; this solution was dried over MgSO4,
filtered and the solvent removed in vacuo to give the product as a
cream solid in 83% yield. 1H NMR (CDCl3, 300 MHz): ı 5.29 (s, 2H,
CH2), 6.92–7.06 (m, 3H, Ar-H), 7.28–7.35 (m, 2H, Ar-H), 7.48–7.56
(m, 2H, Ar-H), 7.60–7.68 (m, 1H, Ar-H), 7.98–8.08 (m, 2H, Ar-H).
3-(1-Adamantyl)-5-methylsalicylaldehyde: 2-(1-Adamantyl)-
4-methylphenol (1.0 g, 4.1 mmol), hexamethylenetetramine
(1.16 g, 8.3 mmol) and trifluoroacetic acid (7 mL) were stirred
together for 4 h at 120 ◦C. The mixture was cooled to 80 ◦C, 33%
aq. H2SO4 (70 mL) was added and the reaction was heated for a
further hour at 130 ◦C. After cooling to room temperature, ethyl
acetate (20 mL) and water (30 mL) were added. The organic layer
was separated and the water layer extracted with ethyl acetate (3×
20 mL). The combined organic extracts were washed with water
(50 mL) and brine (30 mL) and dried over MgSO4. The solvent was
removed in vacuo and the residue was washed with diethyl ether
to yield the product as a pale yellow powder in 66% yield. 1H NMR:
(400 MHz, CDCl3) ı 1.74–1.82 (m, 6H, Ad-H), 2.05–2.10 (m, 3H,
Ad-H), 2.11–2.18 (m, 6H, Ad-H), 2.31 (s, 3H, CH3), 7.13–7.19 (m,
1H, Ar-H), 7.27 (d, J = 2.3 Hz, 1H, Ar-H), 9.81 (s, 1H, OH), 11.64 (s,
1
13C{ H} NMR (CDCl3, 75 MHz): ␦ 70.8, 114.8, 121.7, 128.2, 128.9,
129.6, 133.9, 158.0, 194.6. ESI-MS: m/z calcd for [C14H12O2Na]+:
235.0735; found: 235.0792.
2-Phenoxy-1-phenylethanol: To a solution of 2-phenoxy-1-
phenylethanone (7 g, 33 mmol) in methanol (200 mL) was added
NaBH4 (2.5 g, 66 mmol) portion wise. The reaction was stirred for
3 h, after which time the solvent was removed in vacuo. The residue
was redissolved in ethyl acetate (50 mL) and the reaction was
quenched by the addition of aqueous HCl (50 mL). The resulting
solution was filtered to remove insoluble salts and the product
was extracted into ethyl acetate (3× 20 mL), washed with brine
(1× 30 mL), dried over MgSO4 and the solvent removed in vacuo to
afford a waxy, cream solid in 91% yield. 1H NMR (CDCl3, 300 MHz):
ı 4.02 (dd, J = 9.8, 9.0 Hz, 1H, CH2), 4.13 (dd, J = 9.8, 3.4 Hz, 1H,
CH2), 5.15 (dd, J = 8.9, 3.2 Hz, 1H, CH), 6.91–7.02 (m, 3H, Ar-H),
1
1H, CHO). 13C{ H} NMR: (CDCl3, 75 MHz): ı 20.6, 28.9, 37.0, 40.1,
120.3, 128.2, 131.3, 135.5, 159.4, 197.2.
2.3.3. Ligand synthesis
General procedure: To a solution of the aldehyde (1 g) in
methanol (70 mL) was added Na2SO4 (8 eq.) and 3-amino-1-
propanol (1 eq.), the reaction was heated to reflux and stirred
overnight. The mixture was then cooled to room temperature, fil-
tered and concentrated in vacuo to afford the product.
N-(3-Hydroxypropyl)-3,5-di-chlorosalicylaldimine: (1H2) 1H
NMR: (CDCl3, 300 MHz): ı 1.96 (quin, J = 6.3 Hz, 2H, CH2), 3.78
(t, J = 6.0 Hz, 2H, CH2-OH), 3.77 (t, J = 5.7 Hz, 2H, N-CH2), 7.11 (d,
J = 2.6 Hz, 1H, Ar-H), 7.40 (d, J = 2.6 Hz, 1H, Ar-H), 8.22 (s, 1H,
1
7.27–7.51 (m, 7H, Ar-H). 13C{ H} NMR (CDCl3, 75 MHz): ı 72.6, 73.3,
114.6, 126.3, 128.3, 128.6, 129.6, 144.5, 158.4. ESI-MS: m/z calcd
for [C14H14O2Na]+: 237.0891; found: 237.0894. Elemental Analy-
sis: Anal. Calcd for C14H14O2: C, 78.48; H, 6.59. Found: C, 78.31; H,
6.49.
1
N = CH). 13C{ H} NMR: (CDCl3, 75 MHz): ı 32.7, 53.9, 59.6, 118.4,
121.3, 123.9, 129.1, 132.8, 159.9, 163.9. ESI-MS: m/z calcd for
[C10H12Cl2NO2]+: 248.0245; found: 248.0237.
N-(3-Hydroxypropyl)-3,5-di-bromosalicylaldimine: (2H2) 1
H
2.3.2. Bulky alcohol and salicylaldehyde synthesis [23]
NMR: (CDCl3, 300 MHz): ı 1.98 (quin, J = 6.3 Hz, 2H, CH2), 3.75–3.82
(m, 4H, CH2-OH, N-CH2), 7.32 (d, J = 2.3 Hz, 1H, Ar-H), 7.71 (d,
2-Trityl-4-methylphenol: p-Cresol (25 g, 0.2 mmol) was heated
to 100 ◦C under a flow of argon. Sodium metal (1.1 g, 0.05 mmol)
was added slowly with vigorous stirring to form a cresolate melt.
To this was added triphenylchloromethane (10.0 g, 0.036 mmol)
and the mixture was heated at 140 ◦C for 3 h. The reaction mixture
was cooled to room temperature and subsequently treated with
7% aq. NaOH (100 mL) and ether (100 mL). The organic layer was
separated, washed with 7% aq. NaOH (5× 50 mL), water (100 mL),
and brine (50 mL), dried over MgSO4 and the solvent removed
in vacuo. The resulting solid was recrystallized from hot diethyl
ether to afford the product as a creamy solid in 46% yield. 1H
NMR: (400 MHz, CDCl3) ı 2.19 (s, 3H, CH3), 4.33 (s, 1H, OH), 6.74
(d, J = 8.0 Hz, 1H, Ar-H), 6.86 (d, J = 2.3 Hz, 1H, Ar-H), 7.04 (dd,
1
J = 2.3 Hz, 1H, Ar-H), 8.23 (s, 1H, N = CH). 13C{ H} NMR: (CDCl3,
75 MHz): ı 32.7, 53.7, 59.5, 108.0, 113.9, 118.8, 132.9, 138.3, 161.0,
163.8. ESI-MS: m/z calcd for [C10H12Br2NO2]+: 337.9209; found:
337.9527.
N-(3-Hydroxypropyl)-3,5-di-iodosalicylaldimine: (3H2) 1H
NMR: (CDCl3, 300 MHz): ı 1.98 (quin, J = 6.4 Hz, 2H, CH2), 3.75–3.80
(m, 4H, CH2-OH, N-CH2), 7.49 (d, J = 2.3 Hz, 1H, Ar-H), 8.05 (d,
1
J = 2.3 Hz, 1H, Ar-H), 8.13 (t, J = 1.0 Hz, 1H, N = CH). 13C{ H} NMR:
(CDCl3, 75 MHz): ı 32.7, 53.5, 59.6, 90.1, 118.8, 140.0, 149.1, 163.5.
ESI-MS: m/z calcd for [C10H12I2NO2]+: 431.8957; found: 431.8967.
N-(3-Hydroxypropyl)-salicylaldimine: (4H2) 1H NMR: (CDCl3,
300 MHz): ı 2.01 (quin, J = 6.4 Hz, 2H, CH2), 3.77 (t, J = 6.7 Hz, 2H,
CH2), 3.81 (t, J = 6.2 Hz, 2H, CH2), 6.92 (t, J = 7.5 Hz, 1H, Ar-H), 7.00
J = 8.0, 2.3 Hz, 1H, Ar-H), 7.16–7.34 (m, 15H, Ar-H). 13C{ H} NMR:
1
Please cite this article in press as: H.J. Parker, et al., Degradation of ␤-O-4 model lignin species by vanadium Schiff-
base catalysts: Influence of catalyst structure and reaction conditions on activity and selectivity, Catal. Today (2015),
http://dx.doi.org/10.1016/j.cattod.2015.08.045

G Model
CATTOD-9776; No. of Pages8
ARTICLE IN PRESS
4
H.J. Parker et al. / Catalysis Today xxx (2015) xxx–xxx
1
CH-CH3), 7.58 (br. s, 1H, Ar-H), 8.20 (br. s, 2H, Ar-H, N = CH). 13C{ H}
(d, J = 8.3 Hz, 1H, Ar-H), 7.29 (dd, J = 7.7, 1.4 Hz, 1H, Ar-H), 7.32–7.38
1
(m, 1H, Ar-H), 8.42 (s, 1H, N = CH). 13C{ H} NMR: (CDCl3, 75 MHz):
NMR: (CDCl3, 75 MHz): ı 24.2, 24.5, 25.6, 32.6, 63.4, 78.9, 80.5,
141.4, 150.8, 161.8. 51V NMR (CDCl3, 132 MHz): ı −562.4. Elemen-
tal Analysis: Anal. Calcd for C13H16I2NO4V: C, 28.13; H, 2.91; N,
2.52. Found: C, 28.00; H, 2.79; N, 2.57.
ı 33.5, 55.8, 60.2, 117.1, 118.5, 131.3, 132.3, 161.4, 165.3. ESI-MS:
m/z calcd for [C10H14NO2]+: 180.1025; found: 180.1025.
N-(3-Hydroxypropyl)-3,5-di-tert-butylsalicylaldimine: (5H2)
1H NMR: (CDCl3, 250 MHz): ı 1.32 (s, 9H, CH3), 1.45 (s, 9H, CH3),
1.98 (quin, J = 6.3 Hz, 2H, CH2), 3.72 (td, J = 6.6, 1.3 Hz, 2H, N-CH2),
3.79 (t, J = 6.3 Hz, 2H, CH2-OH), 7.10 (d, J = 2.5 Hz, 1H, Ar-H), 7.39
(d, J = 2.5 Hz, 1H, Ar-H), 8.40 (t, J = 1.3 Hz, 1H, N = CH), 13.82 (br. s,
VO(4)(OiPr): 1H NMR (CDCl3, 300 MHz): ı 1.37 (d, J = 6.4 Hz, 3H,
CH-CH3), 1.43 (d, J = 6.4 Hz, 3H, CH-CH3), 1.82–1.99 (m, 1H, CH2),
2.19–2.31 (m, 1H, CH2), 3.91 (d, J = 12.4 Hz, 1H, CH2), 4.42 (tt, J = 12.3,
2.2 Hz, 1H, CH2), 4.74–4.90 (m, 1H, CH2), 5.40–5.63 (m, 2H, CH2, CH-
CH3), 6.77–6.86 (m, 1H, Ar-H), 6.89 (d, J = 8.3 Hz, 1H, Ar-H), 7.26 (dd,
J = 7.5, 1.9 Hz, 1H, Ar-H), 7.38 (ddd, J = 8.6, 7.1, 1.7 Hz, 1H, Ar-H), 8.29
1
1H, Ar-OH). 13C{ H} NMR: (CDCl3, 75 MHz): ı 29.4, 31.5, 33.5, 34.2,
35.1, 55.9, 60.3, 117.8, 125.8, 126.9, 136.7, 140.0, 158.2, 166.4. ESI-
MS: m/z calcd for [C18H28NO2]−: 290.2120; found: 290.2121.
N-(3-Hydroxypropyl)-3-(1-adamantyl)-5-
(br. s., 1H, N-CH). 13C{ H} NMR: (CDCl3, 75 MHz): ı 24.1, 32.7, 63.4,
1
80.1, 83.0, 118.9, 132.8, 134.9, 163.1. 51V NMR (CDCl3, 105 MHz): ı
−556.8. Elemental Analysis: Anal. Calcd for C13H18NO4V: C, 51.49;
H, 5.98; N, 4.62. Found: C, 51.32; H, 5.84; N, 4.66.
methylsalicylaldimine: (6H2) 1H NMR: (CD3OD, 300 MHz): ı
1.85–1.91 (m, 6H, Ad-H), 1.99 (quin, J = 6.7 Hz, 2H, CH2), 2.09–2.16
(m, 3H, Ad-H), 2.22–2.27 (m, 6H, Ad-H), 2.32 (s, 3H, CH3), 3.75
(t, J = 6.4 Hz, 4H, CH2-OH, N-CH2), 7.02 (d, J = 1.3 Hz, 1H, Ar-H),
VO(5)(OiPr): 1H NMR (CDCl3, 300 MHz): ı 1.25 (s, 9H, C(CH3)3),
1.37–1.43 (m, 15H, C(CH3)3 (9H), CH(CH3)2 (6H)), 1.82–1.91 (m,
1H, CH2), 2.13–2.24 (m, 1H, CH2), 3.80–3.91 (m, 1H, CH2), 4.35 (t,
J = 11.7 Hz, 1H, CH2), 4.70–4.85 (m, 1H, CH2), 5.43 (t, J = 12.4 Hz, 1H,
CH2), 5.63 (spt, J = 6.0 Hz, 1H, CH(CH3)2), 7.09 (d, J = 2.6 Hz, 1H, Ar-
1
7.11 (d, J = 1.8 Hz, 1H, Ar-H), 8.46 (s, 1H, N = CH). 13C{ H} NMR:
(CD3OD, 75 MHz): ı 20.9, 30.7, 35.0, 38.1, 38.4, 41.6, 56.9, 60.5,
120.0, 128.0, 130.7, 131.3, 138.4, 159.7, 167.9. ESI-MS: m/z calcd
for [C21H29NO2Na]+: 350.2096; found: 350.2089.
1
H), 7.46 (d, J = 2.6 Hz, 1H, Ar-H), 8.27 (br. s, 1H, N = CH). 13C{ H}
N-(3-Hydroxypropyl)-3-trityl-5-methylsalicylaldimine:
(7H2) 1H NMR: (DMSO-d6, 400 MHz): ı 1.71 (quin, J = 6.6 Hz, 2H,
CH2), 2.21 (s, 3H, CH3), 3.42 (t, J = 6.3 Hz, 4H, CH2-OH, N-CH2), 7.06
(d, J = 2.0 Hz, 1H, Ar-H), 7.13–7.21 (m, 9H, Ar-H), 7.23 (d, J = 1.8 Hz,
NMR (CDCl3, 75 MHz): ı 24.6, 24.9, 25.4, 29.5, 31.5, 32.1, 32.9, 34.2,
35.2, 63.4, 127.0, 129.6, 141.0, 163.8. 51V NMR (CDCl3, 105 MHz): ı
−568.8. Elemental Analysis: Anal. Calcd for C21H34NO4V: C, 60.71;
H, 8.25; N, 3.37. Found: C, 60.51; H, 8.88; N, 4.10.
1
1H, Ar-H), 7.28 (t, J = 7.3 Hz, 6H, Ar-H), 8.50 (s, 1H, N = CH). 13C{ H}
VO(6)(OiPr): 1H NMR (CDCl3, 300 MHz): ı 1.45 (d, J = 6.0 Hz, 3H,
CH-CH3), 1.53 (d, J = 6.0 Hz, 3H, CH-CH3), 1.76 (d, J = 11.7 Hz, 3H, Ad-
H), 1.86 (d, J = 11.7 Hz, 3H, Ad-H), 1.89–2.00 (m, 1H, CH2), 2.04–2.11
(m, 3H, Ad-H), 2.14–2.22 (m, 6H, Ad-H), 2.22–2.29 (m, 1H, CH2),
2.32 (s, 3H, CH3), 3.92 (dt, J = 12.3, 3.5 Hz, 1H, CH2), 4.43 (tt, J = 12.6,
1.9 Hz, 1H, CH2), 4.82–4.93 (m, 1H, CH2), 5.50–5.59 (m, 2H, CH2,
CH-CH3), 6.99 (d, J = 1.6 Hz, 1H, Ar-H), 7.23 (d, J = 2.2 Hz, 1H, Ar-
NMR: (CDCl3, 75 MHz): ı 20.8, 21.5, 33.3, 55.8, 60.3, 63.2, 118.6,
125.3, 125.5, 126.1, 127.2, 128.3, 129.1, 130.7, 131.1, 134.4, 134.5,
145.6, 158.0. ESI-MS: m/z calcd for [C30H29NO2Na]+: 458.2096;
found: 458.2102.
2.3.4. Catalyst synthesis
H), 8.28 (br. s., 1H, N-CH). 13C{ H} NMR (CDCl3, 125 MHz): ı 19.7,
1
General procedure: Catalyst syntheses were conducted
using glove box and Schlenk line techniques under an atmo-
sphere of argon. In a glove box, equimolar amounts of the
ligand and VO(OiPr)3 were dissolved separately in anhydrous
dichloromethane. The ligand solution was added dropwise to the
metal solution and the reaction mixture was stirred for 0.5 h. The
solvent was removed in vacuo and recrystallization was attempted
from hexane, toluene or dichloromethane.
23.5, 23.9, 28.1, 31.8, 36.0, 39.2, 62.4, 78.7, 84.1, 118.4, 126.9, 129.4,
132.1, 162.3. 51V NMR (CDCl3, 105 MHz): ı −556.4.
VO(7)(OiPr): 1H NMR (CDCl3, 500 MHz): ı 1.06 (d, J = 6.0 Hz, 3H,
CH-CH3), 1.26 (d, J = 6.0 Hz, 3H, CH-CH3), 1.80–1.93 (m, 1H, CH2),
2.17 (d, J = 13.2 Hz, 1H, CH2), 2.27 (s, 3H, CH3), 3.86 (d, J = 11.7 Hz,
1H, CH2), 4.33 (t, J = 12.0 Hz, 1H, CH2), 4.66–4.79 (m, 1H, CH-CH3),
4.88–4.98 (m, 1H, CH2), 5.33 (t, J = 11.0 Hz, 1H, CH2), 7.04–7.40 (m,
VO(1)(OiPr): Recrystallized from dichloromethane. 1H NMR
(CDCl3, 300 MHz): 1.46 (d, J = 6.0 Hz, 3H, CH-CH3), 1.55 (d, J = 6.4 Hz,
3H, CH-CH3), 1.93–2.11 (m, 1H, CH2), 2.32–2.43 (m, 1H, CH2),
3.95–4.10 (m, 1H, CH2), 4.55 (t, J = 12.2 Hz, 1H, CH2), 4.92 (d,
J = 7.9 Hz, 1H, CH2), 5.65 (t, J = 11.1 Hz, 1H, CH2), 5.83 (spt, J = 6.8 Hz,
1H, CH-CH3), 7.22 (d, J = 2.6 Hz, 1H, Ar-H), 7.54 (d, J = 2.3 Hz, 1H, Ar-
17H, Ar-H), 8.25 (br. s., 1H, N = CH). 13C{ H} NMR (CDCl3, 75 MHz):
1
ı 20.8, 24.1, 24.4, 32.3, 63.5, 79.1, 84.9, 125.3, 127.1, 127.3, 131.1,
132.1, 136.8, 163.3. 51V NMR (CDCl3, 132 MHz): ı −562.6. Elemental
Analysis: Anal. Calcd for C33H34NO4V: C, 70.83; H, 6.12; N, 2.50.
Found: C, 70.71; H, 6.24; N, 2.57.
1
H), 8.31 (br. s., 1H, N = CH). 13C{ H} NMR: (CDCl3, 75 MHz): ı 24.0,
32.6, 63.6, 80.4, 130.3, 134.2, 162.1. 51V NMR (CDCl3, 105 MHz):
ı −563.1. Elemental Analysis: Anal. Calcd for C13H16Cl2NO4V: C,
41.96; H, 4.33; N, 3.76. Found: C, 41.83; H, 4.46; N, 3.84.
3. Results and discussion
3.1. Catalyst synthesis and characterization
VO(2)(OiPr): Recrystallized from dichloromethane. 1H NMR
(CDCl3, 300 MHz): ı 1.39 (d, J = 6.0 Hz, 3H, CH-CH3), 1.48 (d,
J = 6.0 Hz, 3H, CH-CH3), 1.86–2.04 (m, 1H, CH2), 2.22–2.35 (m, 1H,
CH2), 3.94 (d, J = 12.4 Hz, 1H, CH2), 4.47 (t, J = 12.2 Hz, 1H, CH2),
4.84 (d, J = 9.0 Hz, 1H, CH2), 5.56 (td, J = 11.2, 2.8 Hz, 1H, CH2),
5.75–5.88 (m, 1H, CH-CH3), 7.32 (d, J = 2.3 Hz, 1H, Ar-H), 7.76 (d,
Tridentate salen ligands 1H2–7H2 were synthesized via con-
densation of the relevant aldehyde with 3-amino-1-propanol.
Subsequent reaction of the ligands with vanadium oxytriiso-
propoxide under an inert atmosphere gave rise to vanadium(V)
complexes VO(1–7)(OiPr), Fig. 3. The catalysts were analyzed by
1
1
J = 2.3 Hz, 1H, Ar-H), 8.20 (br. s., 1H, N = CH). 13C{ H} NMR: (CDCl3,
1H, 13C{ H} and 51V NMR spectroscopy, and solid state structures
75 MHz): ı 24.2, 32.6, 63.5, 80.5, 109.3, 134.1, 139.7, 162.0. 51V
NMR (CDCl3, 105 MHz): ı −563.2. Elemental Analysis: Anal. Calcd
for C13H16Br2NO4V: C, 33.87; H, 3.50; N, 3.04. Found: C, 33.72; H,
3.40; N, 2.98.
were obtained for several of the species by single-crystal X-ray
diffraction.
The complexes were isolated as red crystalline samples in mod-
erate to good yields. For complexes VO(1–5)(OiPr) the solid state
structures were determined by single crystal X-ray diffraction,
Fig. 4. In the solid state the complexes are dimeric with the aliphatic
alcohol bridging between the two vanadium centres. In all cases the
vanadium atoms are in pseudo octahedral environments and the
metric data are consistent with vanadium(V) complexes, Table 1.
VO(3)(OiPr): Recrystallized from toluene. 1H NMR (CDCl3,
500 MHz): ı 1.50 (d, J = 6.0 Hz, 3H, CH-CH3), 1.58 (d, J = 6.0 Hz, 3H,
CH-CH3), 2.03 (q, J = 12.3 Hz, 1H, CH2), 2.35 (d, J = 12.9 Hz, 1H, CH2),
4.00 (d, J = 12.0 Hz, 1H, CH2), 4.52 (t, J = 12.6 Hz, 1H, CH2), 4.90 (d,
J = 9.8 Hz, 1H, CH2), 5.61 (t, J = 10.6 Hz, 1H, CH2), 5.90–6.03 (m, 1H,
Please cite this article in press as: H.J. Parker, et al., Degradation of ␤-O-4 model lignin species by vanadium Schiff-
base catalysts: Influence of catalyst structure and reaction conditions on activity and selectivity, Catal. Today (2015),
http://dx.doi.org/10.1016/j.cattod.2015.08.045

G Model
CATTOD-9776; No. of Pages8
ARTICLE IN PRESS
H.J. Parker et al. / Catalysis Today xxx (2015) xxx–xxx
5
Table 1
Selected bond distances (Å) and angles (◦) for complexes VO(1–5)(OiPr).
VO(1)(OiPr)
VO(2)(OiPr)
VO(3)(OiPr)
VO(4)(OiPr)
VO(5)(OiPr)
V(1)-O(1)
V(1)-O(2)
V(1)-O(3)
V(1)-O(4)
V(1)-N(1)
N(1)-V(1)-O(1)
O(3)-V(1)-O(1)
1.781(2)
1.8731(18)
1.9193(19)
1.596(2)
2.192(2)
172.96(9)
93.39(9)
1.7815(19)
1.8757(19)
1.9218(19)
1.5955(19)
2.196(2)
1.781(2)
1.879(2)
1.925(2)
1.595(3)
2.199(3)
1.7905(18)
1.8867(18)
1.8983(19)
1.6013(17)
2.178(2)
1.7957(11)
1.8870(10)
1.9011(10)
1.5954(11)
2.1616(13)
172.37(5)
173.26(9)
93.68(8)
173.38(12)
94.29(11)
173.97(8)
93.60(9)
94.73(5)
To probe the solution state behaviour further, DOSY NMR
were obtained for VO(4,6,7)(OiPr) in CDCl3. Measured dif-
fusion constants were found to be 8.4 × 10−1 m2 s−1 (H),
7.8 × 10−10 m2 s−1 (Ad) and 6.8 × 10−10 m2 s−1 (CPh3) respectively.
These are consistent with the dimeric solid state structure being
maintained in solution. However, when the sample was run in d8-
THF (a coordinating solvent) a significantly more complex NMR
spectrum was obtained with two clear species being present in
solution, indicating that the dimer is dissociating into a monomer.
Thus, in the presence of the coordinating lignin model compound
the active catalyst is the monomeric species.
3.2. 1H NMR depolymerization studies
Kinetic investigations in this study were carried out on
the non-phenolic model compound 2-phenoxy-1-phenylethanol.
Breakdown of the model compound to phenol and acetophenone,
and 2-phenoxy-1-phenylethanone (by C O bond cleavage and
benzylic oxidation respectively) was monitored by 1H NMR spec-
troscopy, Fig. 5 [17]. Conversion of the model compound and yields
of acetophenone and 2-phenoxy-1-phenylethanone were quanti-
fied by integration of the NMR spectra with respect to an internal
standard (hexamethylbenzene). Direct quantification of phenol by
this method was not possible due to overlapping resonances, there-
fore selectivity for C O bond cleavage was quantified from the yield
of co-product, acetophenone.
Fig. 3. Synthesis of vanadium Schiff-base catalysts.
Both acetophenone and phenol were found to be stable in the
presence of the catalyst and did not undergo further reaction. In
agreement with previous studies, the benzylic oxidation product
(OP), 2-phenoxy-1-phenylethanone, was not broken down under
the reaction conditions (results not shown) confirming that degra-
dation occurs directly from the alcohol rather than via the OP [18].
This is in contrast to the C C cleavage reported by Hanson et al.
which proceeds via oxidation [17]. No conversion of the model
compound was observed in the absence of a catalyst.
Fig. 4. Solid state structure of VO(2)(OiPr), ellipsoids are shown at the 30% proba-
bility level and hydrogen atoms have been removed for clarity.
The overall activity of catalysts VO(1–7)(OiPr) for the conversion
of the model compound was investigated and pseudo first order
rate constants were determined by monitoring the concentration
of the model compound as a function of time, Fig. 6. As steric bulk
on the phenolate ring had previously been reported to improve
catalyst activity [18], possibly by preventing the aggregation of
active catalytic intermediates into inert dimeric species, the size
of the substituent at the 3ꢀ-position was systematically increased.
The unsubstituted catalyst VO(4)(OiPr) was found to have an
observed rate constant, kꢀ, of 0.24 days−1. As in the literature, a
Interestingly the only previously reported solid state structure
of analogous Schiff-base complexes are based on a vanadium-
methoxy moieties.[18] In those cases it is observed that the OMe
is bridging (with the H-substituted ligand) or monomeric species
(tBu-substituted ligand) are isolated in the solid-state. However,
all examples in this study are dimeric. From 1H solution-state
NMR spectroscopic investigations the ligand is “locked” in place
as evidenced by the presence of distinct diastereotopic doublets
for the CH2 for the propyl bridge.
Fig. 5. Degradation of ␤-O-4 lignin model compound 2-phenoxy-1-phenylethanol to C O cleavage products acetophenone and phenol, and benzylic oxidation product (OP)
2-phenoxy-1-phenylethanone.
Please cite this article in press as: H.J. Parker, et al., Degradation of ␤-O-4 model lignin species by vanadium Schiff-
base catalysts: Influence of catalyst structure and reaction conditions on activity and selectivity, Catal. Today (2015),
http://dx.doi.org/10.1016/j.cattod.2015.08.045

G Model
CATTOD-9776; No. of Pages8
ARTICLE IN PRESS
6
H.J. Parker et al. / Catalysis Today xxx (2015) xxx–xxx
Fig. 6. ln([A]0/[A]t ) vs. time for VO(1–7)(OiPr), where [A]t is the concentration of
model compound 2-phenoxy-1-phenylethanol (in mol dm−3) at time, t and kꢀ is the
pseudo first order rate constant (5 mol% catalyst, DMSO-d6, 100 ◦C). {[A]0 = initial
concentration of model compound = 0.15 mol dm−3, [A]t = concentration of model
compound at time t as determined from 1H NMR spectroscopic analysis.}
Fig. 7. Effect of ligand substituents on conversion and selectivity for C O bond
cleavage in the degradation of model compound 2-phenoxy-1-phenylethanol. Con-
ditions: 5 mol% catalyst, DMSO-d6, 100 ◦C.
catalyst up to 1.80 for the bulkiest trityl substituent. This trend in
selectivity could be a result of the increased steric hindrance imped-
ing the access of molecular oxygen to the active site of the catalyst;
thus favouring C O cleavage over C O oxidation. This signposts
towards potential future avenues of research in this field. Analo-
gous investigations using a dipicolinate vanadium(V) catalyst were
reported to achieve relatively high selectivity for depolymeriza-
tion (via C C cleavage) over benzylic oxidation, however activity
was lower, with only 95% conversion after 7 days at 10 mol% load-
ing as compared to >95% in under 4 days at 5 mol% loading for
VO(6)(OiPr).
dramatic increase in activity was observed on addition of tert-
butyl groups at the 3ꢀ and 5ꢀ positions, with kꢀ = 0.68 days−1 for
VO(5)(OiPr). Adamantyl-substitution at the 3ꢀ-position produced
no further improvement in rate {VO(6)(OiPr): kꢀ = 0.67 days−1},
whilst increasing the size of the substituent further again to a
trityl group resulted in a significant decrease in activity back to the
level of the unsubstituted catalyst {VO(7)(OiPr): kꢀ = 0.23 days−1}.
If the role of steric bulk in increasing activity is related to the
prevention of dimerization, it appears that the tert-butyl group is
large enough to achieve this. The drop in activity observed with
the trityl substituent could be a result of reduced access of the
model compound to the catalyst active site.
Catalytic activity towards conversion of the ␤-O-4 model lignin
compound was found to increase with increasing catalyst loading.
In order to probe the electronic effects of the ligand on cata-
lyst performance, a range of halogen substituted catalysts were
also subjected to investigation. These species were significantly
less active than their bulky alkyl-substituted counterparts and
there was a very minor decrease in activity with increasing sub-
stituent size going down the group (kꢀ = 0.16, 0.14, 0.12 days−1 for
VO(1–3)(OiPr) respectively).
Whilst catalytic activity is important, the selectivity of the
catalyst for carbon oxygen bond cleavage over benzylic oxida-
tion is the priority, as breaking the C O bonds in lignin is more
likely to facilitate depolymerization, Fig. 7. Catalysts VO(1–3)(OiPr)
with electron-withdrawing ligand substituents were found to
favour oxidation and demonstrated low selectivity towards C
O
bond cleavage, whilst selectivities for the alkyl-substituted species
VO(5–7)(OiPr) were significantly higher. The unsubstituted cata-
lyst VO(4)(OiPr) displayed an intermediate selectivity. The major
difference between the trends in activity and selectivity was
in the performance of the three bulky alkyl-substituted species.
Selectivity of the catalyst appeared to be directly related to the
size of the substituent, with the ratio of C O cleavage to oxidation
Fig. 8. Arrhenius plots for the degradation of 2-phenoxy-1-phenylethanol by
VO(5,7)(OiPr). Conditions: 5 mol% catalyst, DMSO-d6, 70–120 ◦C.
t
increasing as H ꢁ Bu < Ad < CPh3, from 0.30 for the unsubstituted
Please cite this article in press as: H.J. Parker, et al., Degradation of ␤-O-4 model lignin species by vanadium Schiff-
base catalysts: Influence of catalyst structure and reaction conditions on activity and selectivity, Catal. Today (2015),
http://dx.doi.org/10.1016/j.cattod.2015.08.045

G Model
CATTOD-9776; No. of Pages8
ARTICLE IN PRESS
H.J. Parker et al. / Catalysis Today xxx (2015) xxx–xxx
7
Fig. 9. Depolymerization of phenolic ␤-O-4 model lignin compound guaiacylglycerol-␤-guaiacyl ether.
This effect was seen to tail off above 7 mol%, which could be due
to insolubility of the catalyst at higher loadings. It is more likely,
however, that it is a result of mass transfer limitations resulting
from inefficient mixing in the narrow, unstirred reaction vessel.
The selectivity was found to be largely unaffected by changes in
the catalyst loading.
␤-O-4 model lignin compounds. For the non-phenolic model,
kinetic investigations revealed that a subtle balance of steric and
electronic effects is responsible for tuning the activity and selectiv-
ity of these catalyststowardsC O bond cleavage. The unsubstituted
catalyst, VO(4)(OiPr) and those with electron-withdrawing ligand
substituents demonstrated low activities and selectivities. Bulky,
electron-donating ligand substituents were found to produce the
most selective catalysts, with the bulkiest trityl group effecting
the highest selectivity. Catalytic activity was also improved by
the addition of bulky aliphatic substituents such as tert-butyl and
adamantyl groups, however the trityl-substituted complex was less
active, possibly as a result of increased steric hindrance preventing
access of the model compound to the metal centre.
The next step in this work is to assess the applicability of these
catalysts to the depolymerization of real lignin substrates. Amongst
the major challenges to be overcome in the use of homogeneous
catalysts for lignin depolymerization is that of catalyst stability
and recyclability, however the potential to achieve highly selective
degradation to value-added products makes this a highly interest-
ing avenue of research.
The tert-butyl and trityl substituted catalysts VO(5)(OiPr) and
VO(7)(OiPr) were subjected to further investigation at a range
of temperatures. Unsurprisingly, catalyst turnover was found to
improve with increasing temperature; more interestingly, how-
ever, the selectivity for C O bond cleavage over oxidation was
also significantly enhanced, increasing from 0.88 at 70 ◦C to 3.73
at 120 ◦C. This trend was also observed for VO(5)(OiPr), with a rise
in C O cleavage selectivity from 0.51 to 1.89 over the same tem-
perature range. Again, this highlights potential for increasing the
selectivity in vanadium-catalyzed systems.
Rate constants for VO(5)(OiPr) and VO(7)(OiPr) as a function
of temperature were plotted in a classic Arrhenius form, Fig. 8.
From this data, two different regimes were observed; below around
400 K, the rate appears to be limited by the kinetics of the reaction
itself, however above this temperature an external influence domi-
nates the kinetics. This is most likely a mass transfer limited regime.
Regression analysis of the linear low temperature regime provided
values for the activation energy (Ea) for conversion of the ␤-O-4
model compound; in the case of VO(4)(OiPr) Ea was calculated to
be 96 6 kJ mol−1, whilst for VO(7)(OiPr) the value was found to be
Acknowledgements
We thank the EPSRC for funding (DTC studentship for HJP,
Grant EP/G03768X/1). CJC would like to thank Rodger and Sue
Whorrod for their generous donation to the University of Bath
resulting in the Whorrod Research Fellowship. Data created dur-
ing this research are openly available from the University of Bath
data archive at 10.15125/BATH-00135 (Link: http://dx.doi.org/10.
15125/BATH-00135).
66 8 kJ mol−1
.
It had previously been noted that oxygen is not required for
catalyst turnover, but that activity was reduced under anaero-
bic conditions [18]. Degradation of the model compound in the
presence of VO(7)(OiPr) was investigated under standard and
low oxygen concentrations (100 ◦C, 4 days, uncapped and capped
NMR tubes). As expected, a significant reduction in catalytic activ-
ity was observed under low oxygen conditions (kꢀ = 0.23 0.02,
0.10 0.02 days−1) for standard and low oxygen respectively).
Despite the reduced activity, restricting the availability of oxygen
further increased selectivity for C O bond cleavage to 2.29 (from
1.80 under standard conditions).
To further assess the suitability of these vanadium cat-
alysts for lignin depolymerization, catalyst VO(6)(OiPr) was
tested for activity on phenolic ␤-O-4 model lignin compound
guaiacylglycerol-␤-guaiacyl ether under analogous conditions,
Fig. 9. 1H NMR analysis confirmed complete conversion of the
model compound within 24 h and indicated 100% selectivity for
References
[1] J.E. Holladay, J.J. Bozell, J.F. White, D. Johnson, Top Value-Added Chemicals
from Biomass. Results of Screening for Potential Candidates from Biorefinery
Lignin, vol. II, U.S. Department of Energy (DOE) by PNNL, Richland, WA, USA,
2007.
[2] J. Zakzeski, P.C.A. Bruijnincx, A.L. Jongerius, B.M. Weckhuysen, Chem. Rev. 110
(2010) 3552–3599.
[3] M. Kleinert, T. Barth, Energy Fuels 22 (2008) 1371–1379.
[4] A.J. Ragauskas, G.T. Beckham, M.J. Biddy, R. Chandra, F. Chen, M.F. Davis, B.H.
Davison, R.A. Dixon, P. Gilna, M. Keller, P. Langan, A.K. Naskar, J.N. Saddler, T.J.
Tschaplinski, G.A. Tuskan, C.E. Wyman, Science 344 (2014) 1246843.
[5] A. Tai, T. Sawano, F. Yazama, H. Ito, Biochim. Biophys. Acta 1810 (2011)
170–177.
[6] M.P. Pandey, C.S. Kim, Chem. Eng. Technol. 34 (2011) 29–41.
[7] C. Xu, R.A.D. Arancon, J. Labidi, R. Luque, Chem. Soc. Rev. 43 (2014) 7485–7500.
[8] T. Yoshikawa, T. Yagi, S. Shinohara, T. Fukunaga, Y. Nakasaka, T. Tago, T.
Masuda, Fuel Process. Technol. 108 (2013) 69–75.
C
O bond cleavage, with no evidence of the ketone oxidation prod-
uct. Products were confirmed by GC–MS. Further work is ongoing
to investigate the effect of these catalysts on real lignin substrates.
Our catalyst appears to be more active for the breakdown of
this model compared to the previous one utilized in this study. We
[9] J.G. de Vries, P.J. Deuss, K. Barta, Catal. Sci. Technol. 4 (2014) 1174–1196.
[10] P.J. Deuss, K. Barta, Coord. Chem. Rev. (2015), http://dx.doi.org/10.1016/j.ccr.
2015.02.004.
[11] S. Kim, S.C. Chmely, M.R. Nimlos, Y.J. Bomble, T.D. Foust, R.S. Paton, G.T.
Beckham, J. Phys. Chem. Lett. 2 (2011) 2846–2852.
only observe evidence for C O cleavage and no evidence for C
C
cleavage in agreement with previous studies by Toste and Hanson
[18,19].
[12] J.M. Nichols, L.M. Bishop, R.G. Bergman, J.A. Ellman, J. Am. Chem. Soc. 132
(2010) 12554–12555.
[13] A. Wu, B.O. Patrick, E. Chung, B.R. James, Dalton Trans. 41 (2012)
11093–11106.
[14] A.G. Sergeev, J.F. Hartwig, Science 332 (2011) 439–443.
[15] M.C. Haibach, N. Lease, A.S. Goldman, Angew. Chem. Int. Ed. 53 (2014)
10160–10163.
[16] R.G. Harms, I.I.E. Markovits, M. Drees, h.c.m.W.A. Herrmann, M. Cokoja, F.E.
Kühn, ChemSusChem 7 (2014) 429–434.
4. Conclusions
Vanadium Schiff-base complexes have been shown to be effec-
tive catalysts for the degradation of both phenolic and non-phenolic
Please cite this article in press as: H.J. Parker, et al., Degradation of ␤-O-4 model lignin species by vanadium Schiff-
base catalysts: Influence of catalyst structure and reaction conditions on activity and selectivity, Catal. Today (2015),
http://dx.doi.org/10.1016/j.cattod.2015.08.045

G Model
CATTOD-9776; No. of Pages8
ARTICLE IN PRESS
8
H.J. Parker et al. / Catalysis Today xxx (2015) xxx–xxx
[17] S.K. Hanson, R.T. Baker, J.C. Gordon, B.L. Scott, D.L. Thorn, Inorg. Chem. 49
(2010) 5611–5618.
[21] J.M.W. Chan, S. Bauer, H. Sorek, S. Sreekumar, K. Wang, F.D. Toste, ACS Catal. 3
(2013) 1369–1377.
[18] S. Son, F.D. Toste, Angew. Chem. Int. Ed. 49 (2010) 3791–3794.
[19] S.K. Hanson, R.L. Wu, L.A. Silks, Angew. Chem. Int. Ed. 51 (2012) 3410–3413.
[20] B. Sedai, C. Dia´z-Urrutia, R.T. Baker, R. Wu, L.A.P. Silks, S.K. Hanson, ACS Catal.
1 (2011) 794–804.
[22] G. Sheldrick, Acta Crystallogr. A 64 (2008) 112–122.
[23] A.I. Kochnev, I.I. Oleynik, I.V. Oleynik, S.S. Ivanchev, G.A. Tolstikov, Russ.
Chem. Bull. 56 (2007) 1125–1129.
Please cite this article in press as: H.J. Parker, et al., Degradation of ␤-O-4 model lignin species by vanadium Schiff-
base catalysts: Influence of catalyst structure and reaction conditions on activity and selectivity, Catal. Today (2015),
http://dx.doi.org/10.1016/j.cattod.2015.08.045