Improved Pd/Ru metal supported graphene oxide nano-catalysts for hydrodeoxygenation (HDO) of vanillyl alcohol, vanillin and lignin

Article abstract of DOI:10.1039/d0gc00052c

Pd and Ru nanoparticles supported on graphene oxide (GO) [Pd?GO and Ru?GO] and bimetallic [Pd/Ru?GO] were prepared and well characterized by XRD, FT-IR, EDS, TEM, XPS and ICP-AES analyses. The prepared nano-catalysts were tested for hydrodeoxygenation (HDO) of lignin monomer molecules-vanillyl alcohol and vanillin. In comparison with previously reported methods, Ru?GO and bimetallic Pd/Ru?GO catalysts showed high activity and selectivity, under milder conditions, at room temperature and 145 psi H₂ pressure, for the formation of p-creosol, a value added product, as a potential future biofuel with antibacterial and anti-insecticidal properties. The multifold advantages of both these catalysts are in terms of reduced catalyst loading with a lower metal content and ambient temperture conditions resulting in higher conversion of the starting material. Furthermore, the efficacy of the developed methodology using Ru?GO and bimetallic Pd/Ru?GO catalysts under the optimized conditions was tested on the phenolic components of commercial lignin obtained by photo-catalytic fragmentation using TiO₂, to obtain a mixture after HDO which contained vanillyl alcohol and p-creosol among others, as indicated by HPLC-MS analysis.

Full text of DOI:10.1039/d0gc00052c

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DOI: 10.1039/D0GC00052C
ARTICLE
Received 00th January
20xx,
Improved Pd/Ru metal supported Graphene oxide nano-
catalysts for hydrodeoxygenation (HDO) of Vanillyl
alcohol, Vanillin and Lignin
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Shalini Arora,a Neeraj Guptab and Vasundhara Singha*
Pd and Ru nanoparticles supported on graphene oxide (GO) [Pd@GO and Ru@GO] and bimetallic [Pd/Ru@GO] were
prepared and well characterized by XRD, FT-IR, EDS, TEM, XPS and ICP-AES analysis. The prepared nano-catalysts were
tested for hydrodeoxygenation (HDO) of lignin monomer molecules- vanillyl alcohol and vanillin. In comparison to
previously reported methods, Ru@GO and bimetallic Pd/Ru@GO catalyst showed high activity and selectivity, under
milder conditions, at room temperature and 145 psi H2 pressure, for the formation of p-creosol, a value added product, as
a potential future biofuel along with having antibacterial and anti-insecticidal properties. The multifold advantages of both
these catalysts are in terms of reduced catalyst loading with lower metal content and ambient temperture conditions
resulting in higher conversion of the starting material. Further the efficacy of the developed methodology using Ru@GO
and bimetallic Pd/Ru@GO catalyst under the optimized conditions, was tested on the phenolic components of
commercial lignin obtained by photo-catalytic fragmentation using TiO2, to obtain a mixture after HDO which contained
vanillyl alcohol and p-creosol among others, as indicted by HPLC-MS analysis.
KEYWORDS: Vanillyl alcohol, Vanillin, Commercial Lignin, Pd/Ru@GO, Ru@GO, Hydrodeoxygenation
Hydrodeoxygenation (HDO) is a hydrogenolysis process of
INTRODUCTION
removing oxygen from oxygen containing compounds by the
The replacement of petroleum-derived fuels and chemical
addition of H2.5 Hydrodeoxygenation of lignin has the advantage of
increasing the fuel value of the product by adding H2 and offers
opportunities to not only break apart the complex structure of
lignin but also to increase the overall energy density and value of
the resulting products.6 Catalytic hydrodeoxygenation of lignin and
its model compounds has been reviewed and is considered to be
one of the most important and feasible strategy due to its high
conversion efficiency and reusability.7,8 However, such harsh
reaction conditions, including high H2 pressure and high
temperature, greatly increase the energy consumption and the
requirement of special equipment, making the process quite
expensive. 9-11 Therefore, much milder HDO processes are invariably
expected to give an economically feasible lignin conversion to value
added products.
feedstock is being motivated by environmental issues which include
climate concerns, growing energy demand and energy security.
Lignocellulosic biomass is one such material that exhibits promising
properties as a source of renewable carbon feedstock.1 Thus, it has
become an attractive alternative for many processes designed to
0.convert biomass into fuels and chemicals.2 Among the biomass
components, lignin recalcitrance has been looked upon as a major
obstacle in the production of bioethanol, phenolic compounds and
other alcohols.3, 4
a Department of Applied Sciences (Chemistry), Punjab Engineering College (Deemed
to be University), Chandigarh-160012, INDIA Tel: +91-9888222214; E-mail:
vasundhara@pec.ac.in
In literature, different catalytic systems have been
b
School of Chemistry, Faculty of Basic Sciences, Shoolini University, Solan (H.P.)-
explored which include metal carbides, metal nitrides supported
173212, INDIA, Tel: +918894211891; E-mail: neerajgupta@shooliniuniversity.com
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transition metals/ noble metals and bi-functional catalysts with processes for oxygen reduction reactions,35 coupling reactions,36
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varying structural features of importance such as active sites and and in particular, for hydrodeoxygenation rDeOacI:t1io0n.1s0.3379/D0GC00052C
surface area.12-15 Various lignin model molecules, such as guaiacol,16
Based on the literature reports and to address the issue of
anisole,17 phenol18 and vanillin19 have been investigated for HDO
milder HDO conditions for enhanced efficiency including high
reaction in literature. Among them, magnetically separable Ru
conversion and selectivity, in the present work, we herein report
nanoparticles were synthesized for HDO of eugenol at 275oC
the preparation of three different nano-catalysts Pd@GO, Ru@GO
temperature and 5 Mpa hydrogen pressure, showing remarkable
and bimetallic Pd/Ru@GO choosing the best metals previously
activity towards the formation of fully deoxygenated phenols.20, 21
reported, in desired oxidation states and further investigated the
The hydrogenation and hydrodeoxygenation reaction of eugenol
advantage offered by graphene oxide as a support providing acidic
(lignin model) using various transition metals (noble and non-noble)
sites and high surface area for valorizing lignin molecules- vanillyl
on carbon as support at 275oC temperature and 5 Mpa hydrogen
alcohol and vanillin to p-creosol. Encouraging results were achieved
pressure, resulted in the formation of alkanes with good
in terms of catalyst loading and room temperature conditions with
conversion.22
Ru@GO and bimetallic Pd/Ru@GO on the model compounds. These
In particular for vanillin, Pd/C,23 Ru/C,24 Pt/AC,25 Cu- results facilitated the extension of the work to test the efficacy of
Ni/SiO2,26 Pd/KIT-6,27 Pd/carbon nitride,28 Ru/CNT,29 Au/CNT,30 the methodology and catalytic activity on the post photo-catalytic
MO2C,31 Ni-SiO2-ZnO2,32 catalysts were recently reported under fragmentation of commercial lignin to obtain phenolic molecules
various reaction conditions including catalyst loading, temperature, including vanillin, to obtain vanillyl alcohol and p-creosol as an
pressure, time of reaction and solvent used, to yield vanillyl alcohol, indicator of the catalytic efficiency under milder HDO conditions.
creosol and hydrocarbons (Table S1, supplementary information).
EXPERIMENTAL
The HDO of vanillin has been performed with catalyst loading in the
range of 10 -70 mg using various catalysts and among them the best
Materials: Vanillin, Vanillyl alcohol, Graphene oxide, Palladium (II)
results in terms of catalyst loading were achieved in case of
chloride, Ruthenium (II) chloride and Lignin were purchased from
Pd/carbon-nitride (10 mg) (entry 6) with 99% conversion to p-
Sigma Aldrich and TCI. Tetrahydrofuran and concentrated
creosol and in Cu-Ni/SiO2 (10 mg) (entry 4, S1) with 96% conversion
hydrochloric acid were procured from Alfa Aesar. All the reagents
to p-creosol. HDO has previously been performed in the
were used without any further purification. Perkin Elmer RXI-FTIR
temperature range from 50-300oC with Pd/carbon nitride (entry 6,
and Panalytical’s X’Pert Pro- XRD were used to record the FTIR and
S1) with 99% conversion to p-creosol at 70oC and Ru/C (entry 2, S1)
XRD data of the catalysts. JSM 6100 (JEOL) analytical instrument for
at 55oC with 100 % conversion to vanillyl alcohol. Among the metals
EDS analysis, Nexsa base, XPS instrument for recording the X-Ray
used and reported in literature, Ru and Pd are highly active and
photoelectron spectroscopy, ARCOS, simultaneous ICP-AES
carbon as a support material has shown promising results.
(induced coupled plasma) spectrometer for measuring the metal
However, scope for further improvement in terms of lower catalyst
content. Waters 515 HPLC pumps, Waters 2998 PDA detector,
loading and ambient temperature is needed for this important HDO
manual single injection system and Empower 2 software were used
reaction.
for developing the calibration curves and for monitoring the
reaction.
Graphene oxide is known to be a promising support material
for future metal catalysis due to its high surface area, good
adsorption performance, strong mechanical strength, excellent
stability and electrical conductivity.33 Moreover, it can easily form
close contact or have improved electronic interaction with the
substrate molecules due to its surface functionalization.34 Use of
Graphene oxide as a support has been reported in catalytic
Catalyst preparation: A 20 ml solution containing 2.5 wt% each of
palladium chloride/ ruthenium chloride was prepared by adding 0.5
mL HCl and remaining deionized water. The resulting 0.01M
solution (0.5 mL) was mixed with GO (0.3 g) and ultra-sonicated to
completely disperse and exfoliate GO. Subsequently, the mixture
was transferred to a borosil dish and placed in the center of the
2 | J. Name., 2012, 00, 1-3
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tube furnace. The temperature of the furnace was raised to 200oC the solvent was evaporated under reduced pressure tVoiegwivAertimcleixOtnulirnee
at a rate of 10oC/min under argon atmosphere. After attaining a of products mainly comprising of pheDnOoIl:ic10.c10o3m9p/Do0uGnCd0s.00T52hCe
temperature of 200oC, hydrogen gas was purged into the furnace phenolic compounds obtained were analyzed using HPLC-MS.
and the atmosphere was maintained for 2 h. The furnace was
Results and Discussion
cooled to room temperature and the treated sample was labeled as
Pd/Ru@GO. Further, Pd@GO and Ru@GO were also prepared by
In continuation to the well documented performance of Pd and Ru
the same protocol using 5 wt% of Pd and Ru metal solutions
metals in the various reactions,39 we were motivated to prepare
respectively.
more efficient Pd@GO, Ru@GO and Pd/Ru@GO nano-catalysts by
reduction under an atmosphere of hydrogen at a temperature of
200oC in a tube furnace. Figure 1 shows the XRD pattern of all the
three synthesized nano-catalysts, showing the 2Ɵ values to identify
the crystalline states of Pd/Ru and also to observe the aggregation
of GO with Pd/Ru nanoparticles. The absence of peak at 11.1o (002
reflection), which otherwise appears because of oxygenated
functional groups of GO and the appearance of characteristic
diffraction peak at 26.5o (002 reflection) strongly indicated the
reduction of GO to rGO, during the thermal reductive preparation
of catalyst under H2 atmosphere.
Hydrodeoxygenation (HDO) of Vanillyl alcohol and Vanillin: The
conversion of vanillyl alcohol (4-hydroxy-3-methoxy benzylalcohol)/
vanillin (4-hydroxy-3-methoxy benzaldehyde) was performed in a
high pressure stainless steel reactor of 100 mL capacity as shown in
Scheme 1. Vanillyl alcohol (0.1 g, 0.649 mmol), catalyst (0.01 g) and
10 mL of methanol were stirred at ambient temperature in the
reactor for 12 h at 145 psi H2 pressure. Similarly, for HDO of vanillin
(0.1 g, 0.657 mmol), it was loaded with catalyst (0.02 g) and 10 mL
of methanol and stirred at ambient temperature in the reactor for
24 h at 145 psi H2 pressure. H2 gas was released and the catalyst
was allowed to settle, filtered and the solution was analyzed by
HPLC.
5000
(a)
Pd@GO
rGO (002)
HO
4000
3000
2000
1000
0
Catalysts
O
O
OH
OH
OCH3
+
vanillin
145 psi H2, Methanol (10 mL)
Room Temperature
HO
O
p-creosol
p-cresol
Pd (111)
Pd (200)
Pd (102)
Major product
Minor product
OH
Catalysts
Pd (220)
vanillyl alcohol
20
30
40
50
60
70
Scheme 1 Schematic representation of HDO of Vanillyl alcohol and
2 Theta (degree)
Vanillin.
Photo-catalytic
experiment
of
Lignin:
Photo-catalytic
8000
7000
6000
5000
4000
3000
(b)
Ru@GO
fragmentation of commercial lignin was carried out at room
temperature in an immersion photo-chemical reactor made of
Pyrex glass equipped with a circulating cooling water jacket using
0.3 g Degussa P-25 TiO2 added to 200 mL of lignin solution (0.2 g in
200 mL H2O) and the solution was kept in the dark for adsorption
for 30 min.38 The suspension was then subjected to irradiation
under UV light (125 W) with magnetic stirring for 5 h. After the UV
treatment the sample was centrifuged to recover the catalyst. The
clear aqueous fraction was extracted using dichloromethane and
rGO (002)
Ru (101)
Ru (012)
20
30
40
50
60
70
2 Theta (degree)
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of GO. This also confirms that GO surface has been reVdieuwceAdrtictloe OrGnliOne.
DOI: 10.1039/D0GC00052C
The absorption peak at 648.9 cm-1 and 583.7 cm-1, signified the
3000
2500
2000
1500
1000
500
(c)
Pd/Ru@GO
formation of Pd-O and Ru-O bond in case of Pd/Ru@GO. The
stretching vibration at 634.8 cm-1 for Pd-O bond in Pd@GO and
599.5 cm-1 is for Ru-O bond in Ru@GO.
rGO(002)
Pd@GO
Ru@GO
Ru(101)
Ru(103)
Pd(311)
Pd(105)
Pd/Ru@GO
120
0
2894.7
2894.7
1285.5
1301.7
583.7
648.9
20
40
60
80
2 Theta (degree)
1633.3
1633.3
3436.3
3429.3
100
80
634.8
599.5
Fig. 1 XRD plot of (a) Pd@GO, (b) Ru@GO and (c) Pd/Ru@GO.
The diffraction patterns of Pd@GO clearly shows the peaks related
to fcc Pd at (111), (102) and (220) planes at 2Ɵ values of 40.07o,
54.9o and 68.28o (JCPDS (05-0681) and Ru (101) and (012) planes at
2Ɵ values of a 44.0o and 66.3o, in case of Ru@GO. The XRD profile
providing the diffraction pattern for Pd/Ru@GO showing peaks
related to fcc PdO (200), (102), (220) and (311) planes at 2Ɵ values
of 44.12o, 54.9o, 68.28o and 82.1o respectively JCPDS (05-0681) and
RuO (101) and (103) planes at 2Ɵ value of 44.0o, 78.1o JCPDS (04-
003-0364). No characteristic diffraction peak at 41.259o associated
with Pd-Ru metal, indicating no alloy formation was found in the
XRD pattern of bimetallic Pd/Ru@GO. The average crystalline sizes
of the synthesized catalysts were calculated from diffraction plane
(002) corresponds to 2Ɵ value of 26.5o, applying Debye Scherrer’s
equation and was found to be 10 nm for Pd@GO, 9.32 nm for
Ru@GO and 9 nm for Pd/Ru@GO.
2885.9
1285.8
60
3408.1
3000
1633.3
2000
1000
-1
Wavenumber (cm
)
Fig. 2 FT-IR spectra of Pd@GO, Ru@GO and Pd/Ru@GO.
As reported earlier in the literature, bimetallic catalysts show
synergistic effect in heterogeneous catalysis.40 Therefore, X-Ray
Photoelectron Spectroscopy (XPS), a surface scan, was performed
for all the synthesized catalysts (Pd@GO, Ru@GO and Pd/Ru@GO)
in order to confirm the valence states of Pd, Ru and their synergistic
effect. The high resolution photoluminescence spectra of Pd@GO,
Ru@GO and Pd/Ru@GO (Figure S1, supplementary information),
showed the oxidation state of the Pd (3d), Ru (3p), O (1s) and C (1s).
The binding energies in range the of 329- 348.98 eV and 448-495 eV
are attributed to Pd(0) and Pd(II) 3d3/2, Pd 3d5/2 and Ru (0), Ru (III)
Kc × λ
=
Dp
훽 × cos ɵ
3p , 3p3/2 respectively. The XPS scan of Ru@GO is inherently
½
Where λ is wavelength of X-Ray (0.15418); Ɵ 002 is peak position of
002 bands at Ɵ = 26.4. β002 is full width half maxima (FWHM) for the
corresponding peak, kc is constant depending on X-ray diffraction
plane.
difficult to interpret due to overlap of C (1s) and Ru (3d) core levels
indicating asymmetric nature of Ru 3d core levels, therefore, a plot
was drawn to observe the combination of 3d and 3p core levels.41 In
case of bimetallic Pd/Ru@GO catalysts, the binding energy peaks of
Pd and Ru metals were shifted slightly, which indicates the
synergistic interaction of Pd and Ru by gaining of electrons from
each other on the surface of GO.42 The peaks in the range of 279-
298 eV and 525-534 eV are attributed to C 1s and O 1s respectively.
The binding energies of O 1s spectra correspond to O=C-O and C-OH
groups present on the surface of the support as confirmed from
literature reports.43
In order to see the nature of functional groups on the surface of
support, IR spectrum of the synthesized nano-catalysts were
recorded as shown in Figure 2. The absorption band for O-H is
present in the range of 3400-3500 cm-1 in the FT-IR spectrum of
Pd@GO, Ru@GO and Pd/Ru@GO. The absence of peak in the
region 1700-1800 cm-1 in the IR spectrum of Pd@GO, Ru@GO and
Pd/Ru@GO, indicating the absence of >C=O groups on the surface
4 | J. Name., 2012, 00, 1-3
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The relationship between the active components and catalytic Representative TEM images of the (Pd@GO, Ru@GO and
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performance was analyzed by plotting a graph between the Pd/Ru@GO) nano-catalysts, depicts the sDuOrfIa: c1e0.1m03o9r/pDh0oGlCo0g0y0a52nCd
percentage yield and the content of metals on the surface with the particle size distribution as shown in Figure 3(a-c), wherein the
various oxidation states of Pd and Ru as indicated by XPS, showing a uniform distribution of spherical shiny bright nanoparticles of Pd/Ru
similar trend wherein with increase in the content of various on the surface of GO and different layers of GO stacked over one
species the percentage yield increases, as represented in Figure S2, other were observed. As shown in TEM images, the particle size of
supplementary information.
the nano-catalysts was found to be in the range of 6-14 nm, which
is in good agreement with crystalline size obtained from XRD using
Debye Scherer equation.
The chemical composition of the catalysts prepared was
confirmed by EDS analysis as depicted in Figure S3, supplementary
information which showed the complete removal of chlorine and
presence of Pd/Ru metal on the surface of catalyst.
In addition to EDS analysis performed for confirming the
chemical composition, absence of chlorine and quantity of metal
ions on the surface of graphene oxide, inductively coupled plasma
spectroscopy (ICP-AES) analysis was also done for all the
synthesized catalysts. Both the analysis confirmed the complete
removal of chlorine ions from the surface of graphene oxide and
the actual metal content in the synthesized catalysts was also
measured by ICP-AES analysis. Lower values were obtained for all
the metals in case of ICP-AES as compare to EDS and the results are
summarized in Table 1.
Fig. 3b TEM images of Ru@GO.
Table 1 Metal content obtained from ICP-AES and EDS.
Entry
1.
Sample
Pd@GO
Wt.% Metal (ICP)
1.213
Wt.% Metal (EDS)
3.74
2.
Ru@GO
1.456
4.37
3.
Pd/Ru@GO
0.640/0.826
1.85/2.48
Fig. 3c TEM images of Pd/Ru@GO.
The catalytic activity for HDO of vanillyl alcohol was investigated
with all three nano-catalysts (Pd@GO, Ru@GO and Pd/Ru@GO)
and better results with bimetallic Pd/Ru@GO were obtained
compared to other two catalysts (Pd@GO and Ru@GO) with 100 %
conversion, as shown in Table 2. TON and TOF values were also
calculated from the content of metals (Pd and Ru) obtained from
Fig. 3a TEM images of Pd@GO.
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ICP-AES, number of moles of reactant, amount of catalyst, duration Following the same procedure as used for HDO of vanillyl alcohol,
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of reaction and percentage yield of product formed.
Table 2 HDO of vanilllyl alcohol using three different nano-catalysts another molecule vanillin, as it is one of the phenolic products
the reaction was also investigated with alDl OthI:e10t.h10re3e9/Dca0tGaCly0s0t0s5f2oCr
(Pd@GO, Ru@GO, Pd/Ru@GO) under optimized conditions
obtained by photo-catalytic fragmentation of lignin (confirmed by
LCMS), represented in Table 5 indicating both Ru@GO and
bimetallic Pd/Ru@GO gave comparable results for HDO of vanillin.
Entry
Catalysts
Conversion
(%)
Selectivity (%)
TON
TOF
p-creosol
84.5
94.6
96
42.5
1
2
Pd@GO
Ru@GO
Pd/Ru@GO
90
97
100
2920.5
2601.3
2670.8
-
243.4
216.8
222.5
-
3
4a
Pd/Ru@GO
55.5
Table 5 HDO of vanillin using three different nano-catalysts
5b
6c
7d
Pd/Ru@GO
Pd/Ru@GO
Pd/Ru@Al2O3
35.3
39.5
8.4
27.3
12.5
6.6
-
-
-
-
-
-
(Pd@GO, Ru@GO, Pd/Ru@GO) under optimized conditions
Entry
Catalysts
Conversion
(%)
Selectivity (%)
TON
TOF
Vanillyl
p-
creosol
88.8
93.2
92.3
88.5
13.6
2.9
Reaction conditions: Vanillyl alcohol 0.1g, 0.649 mmol), methanol as solvent
10 mL., catalyst loading 0.01 g and time 12 h, room temperature, 145 psi
pressure
The products was quantified by HPLC using PDA detector with a Waters
Spherisorb 5µm ODS2 (4.6 X 250mm mm) C-18 reverse phase column. The
HPLC conditions for analysis of products were; column temperature; 30oC,
methanol: water (1:1) as an eluent with flow rate of mobile phase 1.0
mL/min, injection volume 10μL with 40 minutes injection runtime
aBenzopheneone 0.01g, bBenzopheneone 0.015g, cTEMPO 0.01g
dPd/Ru@AL2O3
alcohol
5.943
0
0
0
51.7
15.7
4.2
1.
2.
3.
4*.
5 a.
6b.
7c.
Pd@GO
Ru@GO
100
100
100
100
65.3
32.2
9.7
1642.5
1357.4
1349.0
2703.7
-
-
-
68.4
56.5
56.2
112.6
-
-
-
Pd/Ru@GO
Pd/Ru@GO
Pd/Ru@GO
Pd/Ru@GO
Pd/Ru@Al2O3
5.4
Reaction conditions: Vanillin (0.1g, 0.657 mmol), catalyst loading (0.02g)
methanol as solvent 10 mL,
4* Pd/Ru@GO (0.01g, time 24 h), room temperature, 145 psi pressure
aBenzopheneone 0.01g, bPd/Ru@AL2O3
cTEMPO 0.01g
The reaction conditions were then optimized in terms of catalyst
loading and time of reaction using bimetallic Pd/Ru@GO catalyst as
represented in Table 3 and 4. On increasing the amount of catalyst
from 0.0025 g to 0.015 g, the conversion of vanillyl alcohol has
increased from 54% to 86%. Further on varying the reaction time
from 2 h to 12 h, the conversion of vanillyl alcohol increased from
71% to 100%. The optimized conditions for HDO of vanillyl alcohol
were 0.01g catalyst loading(Pd/Ru@GO), at room temperature for
12 h reaction time converting vanillyl alcohol completely to p-
creosol.
The optimization results of HDO of vanillin are summarized in
Tables 6, 7 and 8 using both Ru@GO and bimetallic Pd/Ru@GO. On
increasing the amount of catalyst Pd/Ru@GO, the conversion of
vanillin has increased from 74% to 100% at 0.01g ( Table 6, entry 3)
for vanillyl acohol and on further increasing catalyst loading up to
0.02g, 100% p-creosol was formed (Table 6, entry 5). On increasing
the time of the reaction, the conversion of vanillin was 100%.
Further as shown in Table 7 the time of reaction was varied and
maximum selectivity for p-creosol was achieved in 24 h.Table 8
illustrates the catalytic activity with Ru@GO by varying catalyst
loading and time of reaction and showed comparable results with
Pd/Ru@GO. The results show that selectivity to vanillyl alcohol
decreases and selectivity of p-creosol increases as a function of
conversion of starting material.
Table 3 Optimization of reaction conditions for HDO of vanillyl
alcohol in terms of catalyst loading (Pd/Ru@GO)
Entry
Catalyst loading
(g)
Conversion
Selectivity (%)
p-creosol
38
TON
TOF
(%)
54
68
84
84
1
2
3
4
0.0025
0.005
0.01
1442.2
1816.1
2243.4
1493.58
360.5
454.03
560.8
373.4
54.3
72
72
0.015
Reaction conditions: Vanillyl alcohol (0.1g, 0.649 mmol), methanol as
solvent 10 mL, 4 h reaction time, room temperature, 145 psi pressure
Table 6 Optimization of reaction conditions for HDO of vanillin in
terms of catalyst loading (Pd/Ru@GO)
Entry
Catalysts
loading
(g)
Conversion
(%)
Selectivity (%)
TON
TOF
Table 4 Optimization of reaction conditions for HDO of vanillyl
Vanillyl
Alcohol
86
p-
creosol
0
alcohol in terms of time of reaction using 0.01g Pd/Ru@GO catalyst
1.
2.
3.
4.
5.
6.
0.0025
0.005
0.01
0.015
0.020
0.025
74
87
100
100
100
100
8049.3
4692.8
2703.7
1800.0
1349.0
1077.0
670.7
391.0
112.6
150.0
56.2
Entry
Time (h)
Conversion (%)
Selectivity (%)
TON
TOF
92
0
p-creosol
100
0
1
2
3
4
2
4
8
71
84
93
53
72
86.1
96
1896.2
2243.4
2483.8
2670.8
948.1
560.8
310.5
222.5
42
24
24
38.7
63.3
63
89.7
12
100
Reaction conditions: Vanillin (0.1g, 0.657 mmol), methanol as solvent 10 mL,
12 h reaction time, room temperature, 145 psi pressure
Reaction conditions: Vanillyl alcohol (0.1g, 0.649 mmol), methanol as
solvent 10 mL, room temperature, 145 psi pressure
6 | J. Name., 2012, 00, 1-3
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Page 7 of 10
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Journal Name
ARTICLE
7.
8.
4-(3-methoxy-5-
vinylphenoxy)-
3-vinylphenol
3.43
267.16
Table 7 Optimization of reaction conditions for HDO of vanillin in
View Article Online
O
O
DOI: 10.1039/D0GC00052C
terms of time of reaction using 0.02g Pd/Ru@GO catalyst
H
O
Entry
Time (h)
Conversion
(%)
Selectivity (%)
TON
TOF
O
3,3-(ethane-1,2-
diylbis(oxy))bis
(4-
hydroxybenzald
ehyde) enol
7.01
3.26
307
Vanillyl
p-
OH
Alcohol
creosol
O
O
1.
2.
3.
4.
6h
12
18
24
100
100
100
100
44.2
24
12
0
37.8
63.3
78.5
92.3
1349.0
1349.0
1349.0
1349.0
224.8
112.4
74.9
OH
O
H3CO
OH
9
2,2’(oxybis(4,1-
phenylene))
bis(5-methoxy-
2.,3-
dihydrobenzofur
an-7-ol
503.49
56.2
O
Reaction conditions: Vanillin (0.1g, 0.657 mmol), methanol as solvent 10 mL,
0.02g catalyst (Pd/Ru@GO), room temperature, 145 psi pressure
OH
O
O
OCH3
Table 8 Optimization of reaction conditions for HDO of vanillin in
terms of catalyst loading and time of reaction using Ru@GO
The phenolic products obtained after photo-catalytic fragmentation
of commercial lignin were subjected to hydrodeoxygenation (HDO)
under the optimized conditions as for vanillin using 0.02 g of both
Ru@GO and bimetallic Pd/Ru@GO at room temperature for 24 h.
The activity of both the catalysts were found to be comparable in
terms of cleaving the monomer of lignin i.e vanillin, wherein
formation of some similar and few different phenolic molecules
were observed as confirmed from HPLC-MS after HDO of phenolic
molecules obtained by photo-catalytic fragmentation of lignin. The
formation of different products can be attributed to the selectivity
of a particular catalyst to efficiently transform some functional
groups in comparison to others. The different feasible HDO
products obtained using both Ru@GO and bimetallic Pd/Ru@GO
catalysts are summarized in Table 10 and 11.
Entry
Catalysts
loading
(g)/ time
Conversion
(%)
Selectivity (%)
TON
TOF
Vanillyl
p-
creosol
0
30.8
60.1
76.8
93.2
alcohol
100
49.6
27
1.
2.
3.
4.
5.
0.01/ 12
0.015/12
0.02/12
0.02/18
0.02/24
100
100
100
100
100
2714.8
1809.9
1357.4
1357.4
1357.4
226.2
150.8
113.1
75.4
12.6
0
56.5
Reaction conditions: Vanillin (0.1g, 0.657 mmol), methanol as solvent 10 mL,
room temperature, 145 psi pressure
Further, to check the efficacy of prepared metal based GO nano-
catalysts and methodology adopted the photo-catalytic
fragmentation study of commercial lignin (0.2 g) in aqueous
medium has been undertaken with 0.3 g of Degussa-P25 (photo-
catalyst) for 5 h under UV light. The fragmented products formed in
aqueous medium were extracted in dichloromethane (DCM),
analyzed by HPLC-MS and the phenolic products obtained are
shown in Table 9.
Table 10 Possible phenolic products obtained using Pd/Ru@GO
after HDO of phenolic compounds obtained by photo-catalytic
fragmentation of commercial Lignin
Table
9
Possible products obtained from photo-catalytic
fragmentation of commercial Lignin
Entry
Compound
identified in DCM
extract
Chemical Structure
Retention
time (min)
LCMS m/z
[M+1]
Entry
Compound
identified in
DCM extract
Tetrahydro-
2Hpyran
4-hydroxy-
2methoxy
benzaldehyde
Chemical Structure
Retention
time (min)
LCMS
m/z
[M+1]
1.
2.
Phenol
7.18
7.18
94.00
OH
OH
1.
2.
3.43
3.43
88.15
O
4-methyl phenol
109.02
HO
153.10
169.12
OH
O
O
3.
4.
5.
6.
7.
8.
9.
4-
4.77
3.52
4.77
7.18
4.77
7.26
7.076
123.04
139.02
153.10
154.00
167.07
211.00
275.02
O
hydroxybenzaldeh
yde
2-methoxy-4-
methyl phenol
O
3.
2,6-
3.43
dimethoxybenz
oquinone
OH
O
O
O
O
O
HO
4.
5.
4-formyl-2-
methoxyphenyl
acetate
2-methoxy-1,4-
phenylene
3.43
3.43
195.76
223.16
4-hydroxy-3-
methoxy
benzaldehyde
4-hydroxy-3-
methoxy
benzylalcohol
4-(1-hydroethyl)-2-
methoxyphenol
O
O
O
O
O
O
HO
O
O
OH
O
diacetate
O
O
OCH3
6.
Syringylglyoxalic
Acid
5.90
242.00
O
HO
OH
HO
OMe
HO
O
4-(3-hydroxyprop-
1-en-1-yl)-2,6-
dimethoxyphenol
2-hydroxy-4-
OH
O
O
OH
MeO
HO
O
methylphenoxy-
4methoxyphenyl
ethanone
O
O
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Table 11 Possible phenolic products obtained using Ru@GO after assisted the adsorption of reactant molecules. The metal(s) present
View Article Online
at the surface of graphene oxide dissociateDs OthI:e10h.y1d03ro9/gDe0nGmC0o0le0c5u2lCe
HDO of phenolic compounds obtained by photo-catalytic
fragmentation of commercial Lignin
into atomic hydrogen. Hydrogenation of vanillin by the reduction of
Entry
Compound
identified in
DCM extract
4-
Chemical Structure
Retention
time (min)
LCMS m/z
[M+1]
the aldehydic group (O=C group) takes place with the H+ ions
present at the surface of GO, resulting in the formation of vanillyl
alcohol, which further interacts with the carboxyl group of GO,
undergo dehydration and then decomposes to give the reduced
product (p-creosol) by hydrogenolysis. Hydrogenolysis of benzylic
alcohols are known to follow nucleophilic substitution reaction
mechanism.46 The dehydration during the hydrogenolysis of vanillyl
alcohol occurred by cleaving the alcoholic hydroxyl group, taking off
alkyl-OH bond, removing water molecule by H atom previously
generated by metal and leads to the formation of p-creosol
molecule as the major product.
OH
1.
4.77
123.04
hydroxybenzald
ehyde
O
OH
2.
3.
2-
12.66
4.77
137.46
152.04
methoxyphenol
OCH3
HO
4-hydroxy-3-
methoxy
benzaldehyde
4-hydroxy-3-
methoxy
O
O
HO
4.
5.
7.06
4.77
154.00
167.07
OH
O
benzylalcohol
OCH3
OH
4-(1-
hydroethyl)-2-
methoxyphenol
HO
H3CO
6.
7.
4-(3-hydroprop-
1-en-1-yl)-2-
methoxyphenol
2-(4-
hydroxyphenyl)-
5—methoxy-2,3-
dihydrobenzofur
an-7-ol
12.66
12.66
181.48
261.51
HO
OH
OH
O
HO
O
The course of the reaction is decided by the combination of
metal, support and the surface functional groups that can yield the
partially hydrogenated or fully hydrogenated products.44 It is
evident from the literature that high temperature along with the
noble metal or the transition metal are the important
hydrogenation component in a catalytic system. However, in the
presence of a strong acid component, low reaction temperature can
be sufficient for efficient progress of the reaction.45
Hydrodeoxygenation reaction proceeds via hydrogenation of
vanillin to vanillyl alcohol followed by hydrogenolysis to p-creosol.
The GO based catalysts contains -COO- group as evident from FT-IR
analysis, which is the active acidic group. Beside this group, the
phenolic group present on the surface support also contributes to
the acidity. To confirm this hypothesis, we prepared Pd and Ru
supported on basic alumina (Pd/Ru@Al2O3) using the same
impregnation method of preparing Pd/Ru@GO. The conversion
with Pd/Ru@Al2O3 basic catalyst under the same optimized
conditions was less than 10% as shown in Table 2 (entry 7d) and
Table 5 (entry 7c) confirming that not only the metal but the acidic
support also plays an important role in HDO. Figure 4 illustrates the
stepwise conversion of reactants and formation of the products
during the hydrodeoxygenation reaction. As depicted from XPS
results Pd and Ru both metals are present on the surface of
graphene oxide undergoing the reversible redox process and
Fig. 4 Step-wise conversion of reactants during hydrodeoxygenation
reaction.
In literature, a free-radical based mechanism has also been
proposed for the hydrogenolysis and hydrodeoxygenation
reaction.47 In order to investigate the formation of free radicals in
the present work, preliminary tests were conducted for the possible
involvement of free radicals during the process using
benzophenone and TEMPO as radical quenching agents formed in
the reaction, under optimized conditions for vanillin and vanillyl
alcohol which showed fall in conversion with varying selectivity of
the products formed, as shown in Table 2 (entry 4a, 5b and 6c) and
Table 5 (entry 5a, 6b). However, in future, the detailed study of the
free radical formation during the hydrodeoxygenation reaction
under the present catalytic conditions can be confirmed by EPR
8 | J. Name., 2012, 00, 1-3
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Page 9 of 10
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Journal Name
ARTICLE
studies, but has not been included within the scope of the present
work.
Conclusions
View Article Online
DOI: 10.1039/D0GC00052C
Three metal based GO nano-catalysts (Pd@GO, Ru@GO and
Pd/Ru@GO) were prepared and used for hydrodeoxygenation of
lignin model monomers- vanillyl alcohol and vanillin to p-creosol
with good selectivity. The reaction seems to proceed with the
involvement of surface active metals with synergistic effect of
surface active groups. The catalyst is recycled up to seven catalytic
cycles with appreciable catalytic activity. The methodology
developed had direct applications for the chemical conversion of
commercial lignin after its photo-catalytic fragmentation to vanillin
as one of the phenolic intermediates, to obtain vanillyl alcohol and
p-creosol. Further, detailed studies on commercial and extracted
lignin from agricultural waste biomass are underway.
Recycling Experiments: The recycling studies of the both catalysts
Ru@GO and bimetallic Pd/Ru@GO for HDO of vanillyl alcohol and
vanillin were conducted and performed under the optimized
conditions. The catalyst was recycled up to seven consecutive runs
without activating it to check the loss in activity which showed
comparable results up to three consecutive runs without any
activation and activity was found to be reduced after the third
cycle. However on activating the catalysts which were filtered and
dried by heating at 80o C for 4 h before reusing it and the results of
recycling are represented in Figure 5 and Figure 6. FT-IR analysis of
the used catalysts after seventh recycle was similar to the unused
catalyst and the spectra have been incorporated in the
supplementary information (Fig. S24, S25).
Conflicts of Interest
There are no conflicts to declare.
Pd/Ru@Go for vanillyl alcohol
Pd/Ru@Go for vanillin
100
80
60
40
20
0
Acknowledgements
The authors are thankful to Punjab Engineering College
(Deemed to be University) Chandigarh for necessary Facilities
and SAIF/CIL, Punjab University, IIT Mandi and IIT Bombay for
spectroscopic analysis. Dr. Sanjay Trehan, Panacea Biotech,
Mohali for fruitful discussions.
1
2
3
4
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