Ni/KIT-6 catalysts for hydrogenolysis of lignin-derived diphenyl ether

Article abstract of DOI:10.1007/s12039-018-1512-6

Abstract.: A series of mesoporous silica KIT-6 supported Ni catalysts were prepared by wet impregnation method and evaluated for the lignin-derived diphenyl ether hydrogenolysis under atmospheric pressure in the continuous process. All the catalysts were characterized by powder XRD, N ₂-physisorption, SEM, TEM, H ₂-TPR analysis H ₂-pulse chemisorption studies. Among the catalysts, 20-wt%-Ni/KIT-6 exhibited better activity and selectivity due to the well dispersion of Ni particles through uniform pore channel of KIT-6. 20Ni/KIT-6 shows consistent selectivity to aromatics with a decrease in conversion of diphenyl ether during the time on stream studies. Graphical Abstract: SynopsisNi/KIT-6 catalysts are active in cleavage of C _aryl-O-C _aryl bond of Diphenyl ether, a lignin model compound, in a continuous process at atmospheric pressure [Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-018-1512-6

J. Chem. Sci. (2018) 130:106
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1512-6
REGULAR ARTICLE
Ni/KIT-6 catalysts for hydrogenolysis of lignin-derived diphenyl
ether
JYOTHI YADAGIRIa,b,∗, KUMARA SWAMY KOPPADIa, SIVA SANKAR ENUMULAa,
VENKATESHWARLU VAKATIa, SEETHA RAMA RAO KAMARAJUa, DAVID RAJU BURRIa
and PUPPALA VEERA SOMAIAHb,∗
aCatalysis Laboratory, CSIR- Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500 607, India
bDepartment of Chemistry, University College of Science, Osmania University, Hyderabad 500 007, India
E-mail: jyothichemou@gmail.com; profvspuppala@gmail.com
MS received 13 March 2018; revised 23 May 2018; accepted 3 June 2018
Abstract. A series of mesoporous silica KIT-6 supported Ni catalysts were prepared by wet impregnation
method and evaluated for the lignin-derived diphenyl ether hydrogenolysis under atmospheric pressure in the
continuous process. All the catalysts were characterized by powder XRD, N2-physisorption, SEM, TEM, H2-
TPR analysis H2-pulse chemisorption studies. Among the catalysts, 20-wt%-Ni/KIT-6 exhibited better activity
and selectivity due to the well dispersion of Ni particles through uniform pore channel of KIT-6. 20Ni/KIT-6
shows consistent selectivity to aromatics with a decrease in conversion of diphenyl ether during the time on
stream studies.
Keywords. KIT-6; nickel; hydrogenolysis; lignin; diphenyl ether.
1. Introduction
The cleavage of Caryl–O–Caryl linkage of lignin results
in the production of phenols and arenes which makes
lignin a sustainable source for aromatic compounds.
The cleavage of Caryl–O–Caryllinkage can be achieved
by hydrolysis or hydrogenolysis reactions. In order to
investigate the cleavage of Caryl–O–Caryl bond, Diphenyl
ether (DPE) has been selected as a model compound.
It is a difficult task to cleave the C–O bond with high
dissociation energy (∼218 to 314 kJ mol−1) without
disturbing aromatic rings. Many researchers have exam-
ined noble and non-noble metal catalytic systems for
hydrogenolysis/hydrodeoxygenation (HDO) of lignin
model compounds related to the lignin up gradation.
Several noble metal catalytic systems such as supported
Ru, Pd, Pt, Rh catalysts and non-noble catalytic sys-
tems supported Cu, Co, Ni catalysts are reported.6–10
Among those catalytic systems Ni-based catalysts are
more prominent and inexpensive compared to other
noble metal catalysts in reductive depolymerization of
lignin. Sergeev et al., examined Ni(COD)2 complex for
cleavage of DPE m-xylene as solvent.11 Fang et al.,
studied chemo- and region-selective hydrogenolysis of
C–O bonds over Ni/C and found that embedded sta-
ble metal nanoparticles could cleave aryl ether linkages
even in substituted diaryl ethers.12 He et al., investi-
gated aqueous phase cleavage of aryl ethers by Ni/SiO2
The ever-increasing consumption of non-renewable
sources results in a search for alternative sources of
energy. During this quest for sustainable production
of chemicals, lignocellulosic biomass has evolved as a
renewable source for the production of chemicals and
fuels. The cellulosic part (65–85% by weight) with sim-
ple chemical structures can be upgraded into chemicals
via a series of reactions. However, an efficient conver-
sion of complex structured lignin (15–30% by weight)
is a challenging one.1,2 The pyrolysis of lignin leads to
the formation of highly viscous bio-oil which has low
heating value, thermal instability and is complicated to
utilize.3 An alternative approach to convert lignin in an
efficient way is its reductive depolymerization. Lignin
is a network polymer consisting of phenylpropane units
that are connected via C–C and/or C–O–C linkages;
among them more than 80% linkages are ether link-
ages. Most of the phenylpropane units are connected
α
through –O–4, β–O–4, 4–O–5 linkages and their bond
α
dissociation energies are in the−o1rder of –O–4 < β–
O–4 < 4–O–5 (∼314 kJ mol ).4,5 The cleavage of
the strongest 4–O–5 ether bond (Caryl–O–Caryl) of lignin
is one of the choices for productive depolymerization.
*
For correspondence
0123456789().: V,-vol

106 Page 2 of 7
J. Chem. Sci. (2018) 130:106
catalyst and reported that simultaneous hydrogenolysis reduction of hydrogen (H2-TPR) was performed on a home-
and hydrolysis reactions occur during the cleavage.13
made TPR system. Typically, About 50 mg of the catalyst
was placed in a quartz reactor and pre-treated in Helium flow
The same group has reported mechanistic aspects of
(30 mL min−1) at 300 C for 1 h. Later, the catalyst was
◦
aqueous phase cleavage of the aryl ethers and has also
studied the influence of substitution.14 Wang et al., have
explored Raney Ni as a catalyst for hydrogenolysis
of DPE along with the specific role of solvents. The
basicity of solvent has profound influence on product
selectivity in hydrogenolysis of DPE.15 In view of the
excellence of Ni catalysts in the hydrogenolysis of DPE,
thepresentstudywasundertakentodevelopmesoporous
silica (KIT-6)-supported Ni catalysts for hydrogenol-
treated with 5% H2–Ar mixture gas (30 mL min−1) while
◦
increasing the temperature up to 800 C with a temperature
ramp of 10 C min−1 and it was monitored using a thermal
◦
conductivity detector. H2-pulse chemisorption studies were
carried out on an Autosorb-iQ automated gas sorption anal-
yser (Quantachrome Instruments, USA) at 40 ◦C. The catalyst
◦
was pre-reduced at 500 C for 2 h and evacuated for 2 h
to remove physisorbed hydrogen. The temperature is cooled
◦
down to 40 C and H2 uptake was determined by injecting
ysis of lignin model compounds. The application of H2 (100 μl) in pulses subsequently. SEM images of the cata-
lysts were recorded on a scanning electron microscope (M/s.
JEOL, Switzerland) and TEM images were obtained on a JEM
2000EXII apparatus (M/s. JEOL, Switzerland) respectively.
Prior to TEM analysis, the catalyst sample was ultrasonicated
in ethanol and a drop was placed onto the carbon coated cop-
per grid. The solvent was then dried in an air oven at 80 ◦C
for 6 h.
mesoporous silica as support offers high surface area,
uniform pore channels to accommodate active species,
hydrophobic nature and high thermal stability.16,17 Here
in we report hydrogenolysis of DPE over Ni/KIT-6 cat-
alysts in continuous process at atmospheric H2.
2.3 Activity studies
2. Experimental
The hydrogenolysis of DPE was conducted in a fixed bed
downflow reactor (14 mm i.d. and 300 mm length) at atmo-
spheric conditions. In a typical experiment, about 1 g of the
catalyst was mixed with the same amount of quartz particles
and sandwiched between two plugs at the centre of the reactor.
Before the reaction, the catalyst was a reduced inflow of H2
(30 mL min−1) at 500 ◦C for 4 h and then the reaction temper-
ature is fixed. The liquid feed was fed into the reactor by using
syringe feed pump (M/s. B. Braun, Germany). The product
mixture collected periodically from an ice-cooled trap was
analyzed by FID equipped GC-17A (M/s. Shimadzu Instru-
ments, Japan) with EB–5 capillary column (30 m × 0.53 mm
× 5.0 μm) and confirmed by GC–MS, QP-2010 (M/s. Shi-
madzu Instruments, Japan) with EB–5 MS capillary column
(30 m × 0.25 mm × 0.25 μm).
2.1 Catalyst preparation
Mesoporous silica (KIT-6) was synthesized by following a
reported procedure.18 In a typical synthesis, a mixture of
4.0 g of P123 (M/s. Sigma-Aldrich, USA) and 7.9 g of 35-wt%
HCl in 144 g of deionized water was stirred for 8 h at 35 ◦C
and then 4 g of 1-butanol was added and stirred for 1 h at
35 ◦C. To this solution, 8.6 g of TEOS (M/s. Sigma-Aldrich,
USA) was added dropwise and continuously stirred for 24 h.
The resultant solution has undergone static hydrothermal age-
ing at 100 ◦C for 24 h and washed with deionized water. The
solid was kept in a hot oven at 100 ◦C for 12 h and calcined at
550 ◦C for 8 h inflow of air. The white solid mass was used as
support (KIT-6). Ni/KIT-6 catalysts were prepared via sim-
ple wet impregnation method. For this method, the requisite
amount of Ni(NO3)2·6H2O as a nickel precursor dissolved
in water was added to the KIT-6 support. The mixture was
then allowed to complete dryness on a hot plate and dried in
an oven at 100 ◦C for 12 h. Subsequently, calcined at 500 ◦C
for 5 h in air and denoted as XNK6 where X denotes weight
percentage of Ni and X is equal to 15, 20, 25 and 30.
3. Results and Discussion
The low-angle XRD patterns of KIT-6 and NK6 cata-
lysts were presented in Figure 1(a). It can be observed
that all the NK6 catalysts show three diffraction peaks
at 2 around 0.92, 1.62, 1.89◦ corresponds to the (211),
θ
2.2 Catalyst characterization
(220) and (420) planes, respectively, which indicate
the existence of 3D-cubic mesoporous structure of the
parent KIT-6 of Ia3d symmetry.18,19 The existence of
mesoporous structure of KIT-6 can be found even after
deposition of highest amount (30 wt%) of Ni. The wide-
angle XRD patterns of NK6 catalysts were shown in
The X-ray diffraction patterns were recorded on an Ulti-
maIV (M/s. Rigaku Corporation, Japan) operated at 40 kV
and 40 mA equipped with nickel-filtered Cu K radiation
(λ = 1.54056 Å). The BET surface areas, pore volume and
pore sizes were determined by using the N2 physisorption-
desorption studies (M/s. Quantachrome Instruments, USA,
samples were degassed at 150 ◦C for 2 h) by nitrogen adsorp-
α
Figure 1(b). The broad diffraction peak at 2 around 15–
θ
30◦ is representative for amorphous silica. All reduced
tion at liquid N2 temperature. Temperature programmed NK6 catalysts show XRD reflections at 44.49◦(111),

J. Chem. Sci. (2018) 130:106
Page 3 of 7 106
Figure 1. (a) Low-angle and (b) Wide angle XRD patterns of NK6 catalysts.
Table 1. Textural/structural parameters KIT-6 and various NK6 cat-
alysts.
Catalysts Surface area Pore volume Pore diameter Crystallite size
(m2/g)
(cc/g)
(nm)
(nm)
KIT-6
651
394
412
335
331
0.87
0.62
0.68
0.53
0.51
11.2
–
15NK6
20NK6
25NK6
30NK6
8.7
8.8
8.7
8.7
15
21
29
35
51.85◦ (200) and 76.38◦ (220) corresponding to metallic increase in the metal content. This decrease might be
Ni with a face-centered cubic geometry of space group due to higher density of nickel blocking the mesopores
Fm3m (225) (ICDD. No. 87-0712). The crystallite sizes of KIT-6. The pore size distribution of Ni/KIT-6 sam-
of the catalysts are measured by using Debye–Scherrer ples obtained from adsorption shows bi-model pore size
formula from their full width at half maximum (FWHM) distribution with average pore sizes 5–9 nm.
values and are presented in Table 1. It can be observed
The morphological features of Ni/KIT-6 samples of
that the crystallite size of Ni is increasing with metal fresh and spent catalysts were assessed by SEM and
loading. However, the retention of mesoporous struc- TEM techniques and corresponding images are pre-
ture of KIT-6 even after 30 wt% of Ni deposition shows sented in Figures 3 and 4, respectively. SEM image of
that the high surface area KIT-6 is more advantageous fresh 20NK6 catalyst shows agglomerated squares and
in dispersing the active metal.
EDX spectrum which makes the presence and compo-
Figure 2(a) shows N2 adsorption-desorption sition of Ni evident. From the EDX analysis, elemental
isotherms of NK6 samples. All the samples exhibit type composition is calculated for the fresh 20NK6 catalyst
IV isotherms with H2 hysteresis loop which is indicative and it is found to be ∼21 wt % of Ni. TEM images dis-
of the presence of mesoporous structure with cage-like play uniform pore channels and dispersed Ni particles
pores in accordance with the IUPAC classification and through that channels. From XRD, N2-physisorption,
provide evidence to the retention of mesoporous struc- SEM and TEM analysis, it could be confirmed that the
ture.20 The decrease in height of the loops with an mesoporous structure of KIT-6 is preserved and Ni parti-
increase in the weight loading is due to the decrease in cles are well-dispersed over a high surface area of KIT-6.
the pore volume.21 The structural and textural properties
In order to investigate the metal support interac-
of the samples are presented in Table 1. The surface area tions, the location of the metal particles (inside the
and pore volumes of the samples are decreasing with an pores or on the surface) and reduction behavior of the

106 Page 4 of 7
J. Chem. Sci. (2018) 130:106
Figure 2. (a) N2 adsorption-desorption isotherms for parent KIT-6 and NK6 catalysts; (b) Pore Size distribution profiles of
NK6 catalysts.
Figure 3. SEM and EDX images of (a) Fresh 20NK6 cata-
lyst.
Figure 5. H2-TPR profiles of NK6 catalysts.
peak between 350–580 ◦C. It is well-known that nickel
catalysts supported on silica showed different reduc-
tion patterns depending on the nature of the interaction
between nickel oxide and mesoporous silica KIT-6 sup-
port. Bulk nickel oxide gets reduced at around 420 ◦C.
Among the catalysts, 20NK6 exhibit high Tmax value
compared to other catalysts due to either strong inter-
Figure 4. TEM images of (a) Fresh 20NK6 and (b) Spent
20NK6 catalysts.
action of nickel particles with the support, or due to
the presence of uniformity in Ni particles over the sup-
port KIT-6. Tmax of the catalysts follows the order,
catalyst, temperature programmed reduction (TPR) was 20NK6>15NK6>25NK6>30NK6.
performed and the results are depicted in Figure 5.
H2-pulse chemisorption studies were performed and
All the NK6 samples show the single stage reduction the results are presented in Table 2. Among the Ni/KIT-6

J. Chem. Sci. (2018) 130:106
Page 5 of 7 106
Table 2. H2 Chemisorption studies of NK6 catalysts.
Catalyst H2uptake (m moles/g) Metal surface areaa (m2/g cat) Ni0 particle sizeb (nm) Dispersionc (%)
15NK6
20NK6
25NK6
30NK6
82
90
78
75
6.5
7.2
6.2
6.0
15.4
18.6
26.8
33.5
6.4
5.3
3.7
2.9
a,b,cObtained from H2 uptake. aActive metal surface area (ASA) = Nm × S × Am × 6.023 × 1023; where ASA is in
m2/g of the sample; Nm = the number of adsorbed gas molecules (Mole × 10−6/g), S = adsorption stoichiometric,
(Ni/ H2 = 2), Am = the cross-sectional area occupied by each active nickel surface atom (6.623×10−20m2), bAverage
particle size (d) = 10 × L×f /ASA×Z; Where L = percentage nickel loading, f is a particle ‘Shape correction factor’
(6 for spherical particles), Z = density of the nickel metal (8.9 g/cm3), d is in nm. c% Ni Dispersion (D) = Nm×S×M
/100xL; Where M is the molecular weight of nickel (58.7 g).
catalysts and the results are presented in Figure 6. Dur-
ing the hydrogenolysis of DPE reaction, the possible
products obtained are shown in Scheme 1.
Beyond 20 wt% Ni loading, there is a decrease in the
conversion of DPE due to the increase in the crystallite
size of Ni atoms. In all the cases, the selectivity towards
benzene and phenol is more or less same. Hence, 20NK6
is found to be the better one among the other catalysts.
This is due to the 20NK6 catalyst that shows the max-
imum amount of H2 uptake, which also indicates an
increased number of active sites and a greater active
metal surface area than other catalysts.
The effect of temperature on the hydrogenolysis◦of
DPE was carried out within the range of 275– 350 C
over 20NK6 and the results are presented in Figure 7.
TheconversionofDPEdecreasesgraduallywitharisein
Figure 6. Influence of Nickel loading on the hydrogenol-
temperature from 275 to 350 ◦C and reaches a minimum
ysis of DPE. Reaction conditions: catalyst: 1 g, Reaction
◦
of 58% at 350 C. In addition, the selectivity towards
phenol decreases, and benzene increases with the rise in
temperature from 275 to 350 ◦C. These results suggest
that higher than 275 ◦C of reaction temperature leads to
a lower yield of aromatics.
temperature: 275 ◦C, H2 flow: 20 mL min−1, liquid feed flow:
1 mL h−1
.
catalysts, 20NK6 shows more H2 uptake and greater
metal surface area than the other catalysts.
The influence of H2 flow on the hydrogenolysis of
DPE was studied over 20NK6 and the results are pre-
sented in Figure 8. There is an increase in the conversion
of DPE from 59 to 86% with H2 flow from 15 to
20 mL/min due to the increase in the number of available
H2 molecules. A further increase in flow decreases the
3.1 Activity results
In order to evaluate the optimum Ni loading, the
hydrogenolysis of DPE is conducted over various NK6
Scheme 1. Plausible reaction pathway for the hydrogenolysis of DPE over NK6 catalysts.

106 Page 6 of 7
J. Chem. Sci. (2018) 130:106
Figure 9. Time-on-stream study of 20NK6 catalyst on the
hydrogenolysis of DPE. Reaction conditions: catalyst: 1 g,
Reaction temperature: 275 ◦C, H2 flow: 20 mL min−1, liquid
Figure 7. Influence of the reaction temperature on the
hydrogenolysis of DPE. Reaction conditions: catalyst: 1 g,
H2 flow: 20 mL min−1, liquid feed flow: 1 mL h−1
.
feed flow: 1 mL h−1
.
4. Conclusions
The mesoporous silica KIT-6 supported Ni catalysts
were successfully prepared by the wet impregnation
method. All the catalysts retained their mesoporous
structure even after the highest amount of Ni deposition.
Among the catalysts, 20Ni/KIT-6 was found to be the
mosteffectiveoneinthehydrogenolysisofDPEatatmo-
spheric pressure in a continuous process. 20Ni/KIT-6
suffers from a decrease in the conversion of DPE due
to the accumulation of coke over the active sites of the
catalyst.
Acknowledgements
The authors J.V., K.K.S., S.S.E. and V.V. thank UGC, New
Delhi for sponsoring the fellowships.
Figure 8. Effect of H2 flow on the hydrogenolysis of DPE.
Reaction conditions: catalyst t: 1 g, Reaction temperature:
275 ◦C, liquid feed flow: 1 mL h−1
.
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