Visible light-mediated and water-assisted selective hydrodeoxygenation of lignin-derived guaiacol to cyclohexanol

Article abstract of DOI:10.1039/c8gc03951h

A magnetically separable bimetallic AgPd/Fe@CN_X catalyst has been synthesized and utilized for upgrading of lignin-derived guaiacol to cyclohexanol via hydrodeoxygenation using formic acid as a source of hydrogen under visible light irradiation. A nitrogen-enriched carbonaceous material, carbon nitride (CN_X), is obtained via calcination of chitosan, a marine-derived abundant waste material and subsequently utilized as a support for the immobilization of nano-ferrites and silver and palladium nanoparticles. Hydrogen generation from renewable sources such as formic acid and its ideal utilization for the upgrading of biomass-derived guaiacol are the main findings. The synergetic effect of Ag and Pd on the CN_X support enhances the decomposition of formic acid which is aptly utilized for the selective defunctionalization of guaiacol at ambient pressure.

Full text of DOI:10.1039/c8gc03951h

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DOI: 10.1039/C8GC03951H
Journal Name
ARTICLE
Visible
light-mediated
and
water-assisted
selective
hydrodeoxygenation of lignin-derived guaiacol to cyclohexanol
a
b
c*
Sanny Verma , Mallikarjuna N. Nadagouda and Rajender S. Varma
Received 00th January 20xx,
Accepted 00th January 20xx
A magnetically separable bimetallic AgPd/Fe@CN
guaiacol to cyclohexanol via hydrodeoxygenation using formic acid as a source of hydrogen under visible light irradiation.
Nitrogen-enriched carbonaceous material, carbon nitride (CN ), is obtained via calcination of chitosan, a marine-derived
X
catalyst has been synthesized and utilized for upgrading of lignin-derived
DOI: 10.1039/x0xx00000x
X
www.rsc.org/
abundant waste material and subsequently utilized it as a support for the immobilization of nano-ferrites, silver and
palladium nanoparticles. Hydrogen generation from renewable sources such as formic acid and its ideal utilization for the
X
upgrading of biomass-derived guaiacol are the main findings. The synergetic effect of Ag and Pd on CN support enhances
the decomposition of formic acid which is aptly utilized for the selective defunctionalization of guaiacol at ambient pressure.
Guaiacol (2-methoxyphenol), a lignin-derived model molecule
and a typical bio-oil component, can be upgraded to
cyclohexanol via hydrodeoxygenation (HDO) process.
Introduction:
The lignocellulosic biomass is abundant and does not affect the
11-13
1
food chain. It has mostly been used in Kraft process to produce
Cyclohexanol has various industrial applications where it can be
used as a solvent, or utilized in the synthesis of cyclohexanone
pulp and paper2 wherein energy is produced by the
conventional burning of lignin and hemicellulosic residues in
black liquor. However, for more efficient and ideal utilization of
lignin and hemicellulose fractions of biomass, innovative
methods and technologies are sought. Due to the depletion of
fossil-based feedstock and striving for carbon-neutral
fuels/chemicals, biomass transformations have been
14
and polymers. Due to the presence of stable hydroxyl and
methoxyl group on the aromatic ring in guaiacol, simple
hydrogenation is inadequate for the synthesis of
15
cyclohexanol. Hence, the selective removal of methoxyl group
and hydrogenation of the aromatic ring are required for
conversion to cyclohexanol. Hitherto, HDO of guaiacol has been
investigated using noble catalysts (Pd, Pt, Ru), conventional
hydro-treating sulfide catalysts (CoMoS and NiMoS) and solid
3
,4,5
investigated intensively.
has been produced via flash pyrolysis of biomass in the
temperature range 300-600 °C where lignin fractions have been
utilized to produce valued products and fuels.
However, bio-oils are highly viscous, corrosive, acidic (pH=2.8-
Thus, transportation fuel (bio-oil)
acids which lead to hydrogenated oxygen containing
6
16-21
compounds namely cyclohexanol and phenol,
besides fully
hydrogenated hydrocarbons such as cyclohexane and
3
.8) and show low thermal stability due to the presence of
15
benzene.
oxygen-containing moieties such as phenols, aldehydes, and
acids.7 Typically, bio-oils contain 40% wt oxygen, which
necessitates upgrading by transformations of aromatic or
naphthenic hydrocarbons via hydrodeoxygenation (HDO)
Wang and Rinaldi developed a protocol for the HDO of bio-oil
derived phenolic compounds to cyclohexanols using RANEY@
,8
22
Ni catalyst at 120 °C with isopropanol as a hydrogen donor.
Recently, Jiang et al. developed HDO of guaiacol with
9
reaction. Catalytic HDO has been proficiently employed for the
cyclohexanol as the primary product in aqueous medium over
improvement of products heating value and upgrading of bio-
oils as it reduces oxygen content, especially aromatic -OCH and
3
23
2 3
bimetallic Ni and Co catalysts on γ-Al O and ZSM-5 supports.
Nakagawa et al. reported that the presence of MgO in
combination with Ru/C promotes demethoxylation, hence,
increases transformation of guaiacol to cyclohexanol with 80%
1
0
-OH containing scaffolds typified by guaiacol.
2
4
yield at 160 °C and 1.5 MPa hydrogen pressure. However,
basic MgO is not stable in aqueous media although base helps
in demethoxylation and diminishes the unselective C-O bond
dissociation during HDO process. Tomishinge et al. developed
Ru-Mn combination for the selective exclusion of methoxyl
group on the substituted phenol to give cyclohexanol with 81%
a. Oak Ridge Institute for Science and Education, P. O. Box 117, Oak Ridge TN,
3
7831, USA
b.
c.
Department of Mechanical and Materials Engineering, Wright State University,
Dayton, Ohio 45324, United States
Water Regional Centre of Advanced Technologies and Materials, Department of
Physical Chemistry, Faculty of Science, Palacky University, Šlechtitelů 27, 783 71 yield at elevated temperature and under hydrogen pressure.
Olomouc, Czech Republic. E-mail: varma.rajender@epa.gov
2
5
Recently, Zhang et al. reported the selective conversion of
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
guaiacol to cyclohexanol in water using a metal-solid base
2 3
bifunctional Ru/ZrO -La(OH) catalyst at 170 °C and 4 MPa
2
6
hydrogen pressure in an autoclave. However, aforementioned
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Journal Name
reports require high hydrogen pressure, elevated temperature
and a prolonged reaction time. Hence, the development of new
efficient catalytic system is still needed to convert guaiacol to
View Article Online
DOI: 10.1039/C8GC03951H
2
7, 28
cyclohexanol with high stability and selectivity.
In continuation our efforts towards development of sustainable
2
9-33
protocols using heterogeneous catalysts,
herein, we report
a magnetically separable bimetallic AgPd catalyst supported on
marine waste chitosan-derived nitrogen-enriched carbon
nitride (CN ), AgPd/Fe@CN ; the catalyst generates hydrogen
x X
from formic acid which can be utilized in-situ for the selective
hydrodeoxygenation of lignin-derived guaiacol under visible
light irradiation at ambient pressure.
X X
Figure 2. a) EDX analysis of AgPd/Fe@CN ; b) SAED image of AgPd/Fe@CN
Synthesis and characterization
Firstly, nitrogen-enriched carbon nitride support (CN ) was
x
obtained by the calcination of chitosan at 600ꢀ°C for 4ꢀhours
under nitrogen atmosphere. Then, it was dispersed in water
using sonication. The solution of FeSO
room temperature to the mixture of CN
hours.32 The aqueous solution of AgNO
and Pd(NO )
3 2
4
was added dropwise at
and stirred for 2
were
x
3
added to the colloidal solution at room temperature and
reaction mixture was further stirred for 4 hours. Sodium
borohydride (NaBH ) was added, in portions, at 50 °C to reduce
4
the iron sulfate, Ag (I) and Pd (II) to magnetic ferrites, Ag (0) and
Pd (0), respectively. This reaction mixture was stirred for next
1
2 hours. The ensuing black colored solid was isolated as
AgPd/Fe@CN using an external magnet, washed with
methanol and finally dried at 60 °C in vacuum.
AgPd/Fe@CN composite powder was further analyzed by
X
3
2, 34
The obtained
X
various instrumental techniques such as X-ray diffraction (XRD),
scanning electron microscope (SEM), transmission electron
microscopy (TEM), X-ray photoelectron spectroscopy (XPS),
Brunauer-Emmett-Teller (BET) surface area analysis, FT-IR and
inductive-coupled plasma atomic emission spectroscopy (ICP-
X X
AES). SEM analysis of CN support and AgPd/Fe@CN catalyst
showed visual differences in images and confirmed the
immobilization of silver (Ag), palladium (Pd) nanoparticles and
nano-ferrites over the surface of nitrogen-enriched carbon
nitride (Figure 1a and 1b). TEM analysis validates the formation
of single phase Ag nanoparticles, Pd nanoparticles and nano-
ferrites with spherical morphology in the size range of 10-15 nm
X X
Figure 3. XRD pattern of CN and AgPd/Fe@CN
The XRD and XPS analysis also authenticated the formation of
metallic silver and palladium nanoparticles along with nano-
ferrites over CN
X
support. Figure 3 depicts the XRD patterns of
(Figure 1c and 1d) which is further supported by EDX and SAED
CN and carbon nitride immobilized bimetallic AgPd/Fe@CNX
catalyst. The nitrogen-enriched carbon nitride displayed a
diffraction peak at 26.51° which corresponds to graphitic
X
analysis of bimetallic AgPd/Fe@CN catalyst (Figure 2a and 2 b).
X
4
6
peak.
Loading of metal nanoparticles leads to the
disappearance of the diffraction peak at 26.51°. The observed
peaks at 5.56°, 27.78°, and 43.01°, respectively, are
representing the (100), (110), and (210) Bragg reflection as
compared with JCPDS standards (00-050-1250) which
confirmed the presence of carbon nitride (CN
Additionally, the XRD pattern of AgPd/Fe@CNX corroborated
that the bimetallic catalyst was enriched with FeO/Fe
X
) support.
2 3
O
[
2Ɵ=18.21° (111), 30.11° (220), 35.36° (311), 43.16° (400),
5
3.56° (422), 62.79° (440); JCPDS#00-065-0731], Ag [2Ɵ= 38.1°
(
[
111), 44.27° (200), 64.42° (220); JCPDS#00-004-0783] and Pd
2Ɵ= 40.11° (111), 46.66° (200), 68.08° (220); JCPDS#00-005-
Figure 1. a) SEM image of CN
X X
; b) SEM image of AgPd/Fe@CN ;
c) TEM image of CN ; d) TEM image of AgPd/Fe@CN
X
X
2
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0
68] nanoparticles (ESI; Figure S3). Although, the intensity of nano-ferrites which is known to ease the hydrodeoxygenation
View Article Online
DOI: 10.1039/C8GC03951H
3
4
silver and palladium nanoparticles is not high, but their of lignin-derived component. Accordingly, it is crucial to
presence have been further confirmed by XPS, EDX and ICP-AES modify the catalyst by adding and utilizing a booster which
analysis.
speeds up the degradation of formic acid without interfering in
4
4
A high resolution XPS analysis of CN
X
and AgPd/Fe@CN
X
was the reaction. It has also been reported that bimetallic catalysts
show enhanced formic acid decomposition rather than single
support resembles the formation of tri-s-triazine-based metallic catalysts presumably due to the charge redistribution
performed (ESI; Figure S4 - Figure S9). The XPS analysis of CN
X
3
5,36
graphitic carbon nitride.
In C1S spectrum, the peak at 284.7 between metals which leads to strong adsorption of formate
4
5
eV (1s-π*C=C) is attributed to a typical transition of graphite and ions via back donation. Consequently, bimetallics namely
confirms the presence of graphitic carbon in good covenant Pd/TiO
2
/Fe@CN
X
,
Pd-Pt/Fe@CN
X
,
Au-Pd/Fe@CN
X
,
Cu-
were
with literature.3
7,38
The peak at 288.1 eV (1s-π* C-N-C) is Pd/Fe@CN
Co-Pd/Fe@CN
and AgPd/Fe@CN
,
X
,
X
,
X
ascribed to the characteristic resonance of nitrogen atom synthesized and tested for hydrodeoxygenation under visible
3
9
bonded with two carbon atoms in s-triazine. In N1S spectrum, light (Table 1, entries 8-13). The reaction of guaiacol with
2
the signal at 398.5 eV is ascribed to sp nitrogen atoms having Pd/TiO
2 X X X
/Fe@CN , Pd-Pt/Fe@CN , Cu-Pd/Fe@CN and Au-
two carbon neighbors (–N–C coordination in one tri-s-triazine Pd/Fe@CN
unit) and 400.8 eV is attributed to sp nitrogen atoms respectively (Table 1, entries 8-10). However, Co-Pd/Fe@CN
X
, gave cyclohexanol in 36%, 38%, 46% and 55% yield,
2
X
surrounded by three carbons (N–3C bridging between three tri- gave a better result as product yield was 62% after 8 hours of
s-triazine moieties).4
0, 41
The XPS analysis of (ESI; Figure S5) exposure under visible light (Table 1, entry 12). Interestingly,
gave the best result in 98% yield within 6 hours
). Two peaks were observed, first one at at room temperature (Table 1, entry 13). For better
38.0eV with a shoulder at 339.0eV and the second one at understanding, the photoactivity of the bimetallic
43.3eV with a shoulder at 344.8eV. These peaks were the AgPd/Fe@CN catalyst, the control and thermal experiments
showed the formation of Pd and Ag nanoparticles on the surface AgPd/Fe@CN
of carbon nitride (CN
X
X
3
3
X
result of Pd 3d5/2 and Pd 3d3/2 (ESI; Figure S9 (e)). The XPS peaks were performed in dark at room temperature (30 °C), 50 °C and
at 367.9eV and 373.8eV could be credited to Ag 3d5/2 and Ag 80 °C (Table 1, entries 14-16). Reaction in dark at room
3
(
d
3/2 confirming the presence of silver in zero oxidation state temperature did not give any traces of product within 24 hours
ESI; Figure S8 (d)).31 It is specified in the literature that the (Table 1, entry 14). Reactions at 50 °C and 80 °C gave
binding energy of the metal increases with the increase in the cyclohexanol in 35% and 67% yield, respectively after 24 hours
oxidation state, hence, the binding energy at 710.8eV of thermal heating in oil bath. These experiments show that the
2
+
corresponds to Fe 2p3/2 in Fe in the form of FeO. The satellite bimetallic AgPd/Fe@CN
X
catalyst works as photo-catalyst under
visible light irradiation and rather sluggishly under thermal
conditions. Moreover, the bimetallic AgPd/Fe/CN catalyst have
3
+
2 3
peak at 718.1eV is a characteristic peak of Fe in γ-Fe O ,
confirming that the catalyst is a combination of FeO/γ-Fe
O
2 3
X
2, 43
(
ESI; Figure S9 (f)).4
Furthermore, the presence of Pd and Ag shown higher activity for HDO of guaiacol to cyclohexanol due
was analyzed using ICP-AES and its contents were found to be to the synergistic effect leading to mutual interaction of Ag and
Pd (2.51%) and Ag (2.48%)
Pd nanoparticles with CN
X
support; our claim is corroborated by
a report that silver nanoparticles along with palladium atoms
4
5
Result and discussion
significantly enhances the dissociation of formic acid.
Undoubtedly, Pd and Ag have proven to be an ideal metallic
To upgrade guaiacol, a model component of lignin-derived bio- combination for in-situ hydrogen generation for upgrading of
oil, via selective hydrodeoxygenation, a series of catalysts were guaiacol under visible light.
assessed for in-situ generation of hydrogen from formic acid
under visible light. For this transformation, an array of mono- Based on GC-MS analysis outcomes, we have proposed a
and bi-metallic catalysts were synthesized on magnetic nano- tentative reaction pathway for the synthesis of cyclohexanol
ferrites supported nitrogen enriched CN
Commercially available homogeneous catalyst such as conversion by GC-MS during 6 hours of the reaction time, the
Pd(NO , Cu(NO and AgCl are ineffective even after 24 hours formation of two intermediates namely phenol and
of exposure (Table 1, entries 1-3). Monometallic TiO /Fe@CN cyclohexanone (ESI) was noticed; this has also been recently
Ag/Fe@CN , Cu/Fe@CN and Pd/Fe@CN were synthesized and observed corroborating our proposed mechanism (Figure 4).
X
support (Table 1). from guaiacol via HDO process. A careful monitoring of the
)
3 2
3 2
)
2
X
,
2
3
X
X
X
screened for HDO of 2-methoxyphenol (Table 1, entries 4-7);
the first three gave only the trace amounts of desired product,
while Pd/Fe@CN (Table 1, entry 7) delivered only ~5%, not a
X
practically feasible method. We presumed that the lack of in-
situ hydrogen concentration may be responsible for this
sluggish reaction due to the slow dissociation of the formic acid.
It became apparent that only monometallic supported catalyst
may not be sufficient for HDO under visible light. Hence, in bio-
oil upgrading, it is imperative to increase the degree of
dissociation of HCOOH by using Pd in bimetallic form along with
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DOI: 10.1039/C8GC03951H
Figure 4. A tentative reaction pathway for hydrodeoxygenation of
guaiacol
For the industrial application, it is imperative to study the
viability, recyclability and stability aspects of the bimetallic
AgPd/Fe@CN
using guaiacol as the substrate, formic acid as a source of
hydrogen and the magnetic AgPd/Fe@CN as a catalyst under
X
catalyst. Recycling experiments were conducted
X
photochemical conditions. After completion of the reaction, the
catalyst was isolated using an external magnet, washed with
methanol, dried at 60 °C and recycled for subsequent HDO
reaction of guaiacol. The catalyst was found to be durable and
could be reused at least six times without losing its catalytic
activity. These recycling experiments confirm that porous
Table 1: Reaction optimization for upgrading of guaiacol
OH
OH
OCH3
Catalyst, H O
2
HCOOH, Visible light
X
nitrogen enriched CN support holds metal nanoparticles firmly
via non-covalent interactions. This is further supported by the
ICP-AES analysis of mother liquor which does not show any
traces of metals; ICP-AES analysis of Fe, Pd and Ag were found
to be 4.88%, 2.51% and 2.48% before the reaction and 4.87%,
Yieldb, c Conversionb
Entry
Catalyst
Time
48 h
48 h
48 h
1
2
3
Pd(NO3)2
Cu(NO3)2
AgCl
-
-
-
-
-
2
.50% and 2.47%, respectively after the six-reaction cycle.
-
4
5
6
7
TiO /Fe@CNX
2
48 h
48 h
48 h
48 h
Trace
Trace
Trace
5%
2%
Conclusions
Ag/Fe@CNX
Cu/Fe@CNX
Pd/Fe@CNX
2%
An efficient, sustainable and stable magnetically separable
bimetallic AgPd/Fe@CN catalyst has been synthesized utilizing
X
an abundant waste material as a support. This heterogeneous
catalyst proficiently dehydrogenates formic acid, a renewable
chemical hydrogen storage system, to hydrogen and then
utilizes this in-situ generated H for upgrading of lignin-derived
2
bio-oil containing guaiacol, via hydrodeoxygenation under
visible light irradiation. Furthermore, the bimetallic catalyst
shows high reactivity towards dissociation of formic acid to H
due to the synergic effect of silver and palladium nanoparticle
with nano-ferrites and nitrogen-enriched carbon nitride
2%
8%
8
9
Pd/TiO /Fe@CN
2
X
24 h
24 h
24 h
(90%) 36%
(95%) 38%
40%
Pd-Pt/Fe@CNX
40%
52%
1
0
Au-Pd/Fe@CNX
Cu-Pd/Fe@CNX
Co-Pd/Fe@CNX
(88%) 46%
11
12
13
24 h
24 h
6 h
(92%) 55%
(95%) 62%
(>99%) 98%
-
60%
65%
2
AgPd/Fe@CNX
AgPd/Fe@CNX
AgPd/Fe@CNX
99%
-
4d
support (CN
reaction parameters. The magnetic retrieval quality of
AgPd/Fe@CN facilitates the catalyst separation during the
X
), hence, leading to the fast HDO under ambient
1
24 h
24 h
24 h
1
5e
6f
(87%) 35%
(93%) 67%
40%
X
1
AgPd/Fe@CNX
72%
reaction workup and demonstrates high recyclability with
negligible loss of catalytic activity. This highly active catalytic
method could be beneficial for the upgrading of renewable
lignin to valued components useful in industrial processes.
a) Reaction conditions: Guaiacol (1 mmol), formic acid (5 mmol),
catalyst (20 mg), water (1.0 mL), 20 W domestic bulb, room
temperature; b) Conversion and yield are based on GC-MS analysis
using n-hexadecane (5.0 mmol) as an internal standard; c) The % in
paranthesis indicate the level of selectivity in upgrading of guaiacol; d)
Reaction was performed in dark at room temperature; e) Reaction was
Conflict of Interest: There are no conflicts of interest to
o
o
declare.
performed in dark at 50 C; f) Reaction was performed in dark at 80 C
4
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3
2. S. Verma, R. B. Nasir Baig, C. Han, M. N. NaVdieawgAortuicdleaOanlninde
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Green Chemistry
Page 6 of 6
View Article Online
DOI: 10.1039/C8GC03951H
Visible light-mediated and water-assisted selective hydrodeoxygenation of lignin-derived
guaiacol to cyclohexanol
a
b
c*
Sanny Verma , Mallikarjuna N. Nadagouda and Rajender S. Varma
aOak Ridge Institute for Science and Education, P. O. Box 117, Oak Ridge TN, 37831, USA.
b
Department of Mechanical and Materials Engineering, Wright State University, Dayton, Ohio
4
5324, United States
cRegional Centre of Advanced Technologies and Materials, Department of Physical Chemistry,
Faculty of Science, Palacky University, Šlechtitelů 27, 783 71 Olomouc. E-mail:
varma.rajender@epa.gov
Hydrodeoxygenation (HDO) of lignin-derived guaiacol has been accomplished by magnetically
separable bimetallic AgPd supported on carbon nitride, AgPd/Fe@CN , catalyst and using formic
X
acid as a hydrogen source under visible light irradiation; carbon nitride support has been
synthesized via calcination of chitosan, a marine-derived abundant and renewable waste material.