(560a) Reactivity of Ni-Co/?-Al2O3 Catalysts for Hydrodeoxygenation of Guaiacol | AIChE

(560a) Reactivity of Ni-Co/?-Al2O3 Catalysts for Hydrodeoxygenation of Guaiacol

Authors 

Raikwar, D. - Presenter, Indian Institute of Technology Hyderabad
Shee, D., Indian Institute of Technology Hyderabad
Majumdar, S., Indian Institute of Technology Hyderabad

Reactivity of
Ni-Co/ γ-Al2O3 catalysts for hydrodeoxygenation of
guaiacol

Deepak Raikwar 1,
Saptarshi Majumdar 1, and Debaprasad Shee 1*

1Department of Chemical Engineering,
Indian Institute of Technology Hyderabad, Kandi, Sangareddy, 502 285,

Telangana, India

Email: ch13m15p000001@iith.ac.in, saptarshi@iith.ac.in, dshee@iith.ac.in

Abstract:

Keywords:
Guaiacol, Hydrodeoxygenation, Nickel-Cobalt, Cyclohexane, Benzene

1.     
Introduction:

Lignin
is one of the primary component of lignocellulosic biomass and the most
abundant aromatic polymer available in nature. It has been considered as an
alternative and renewable feedstock to petroleum-based fossil fuel for the
production of platform chemicals such as benzene, xylene, toluene (BTX). However,
the lignin depolymerized bio-oil primarily contains array of oxygenated
aromatic monomers which makes bio-oil unsuitable for direct utilization. Therefore,
further upgradation of lignin-derived bio-oil is essential to eliminate such
oxygen functionalities and to refine their properties. Catalytic
hydrodeoxygenation (HDO) is considered as the most effective route for upgrading
lignin-derived bio-oil during which platform chemicals (aromatics) and blending
agents (cycloalkanes) can be produced. Guaiacol is often selected as a
representative model compound for upgradation studies mainly due to its
presence in bio-oil in high fraction and existence of three types carbon-oxygen
linkages: C(sp3)-OAr, C(sp2)-OCH3 (methoxy),
and C(sp2)-OH (hydroxyl). The presence of three different linkages in
molecular structure with different bond dissociation energies poses a specific
challenge for development of active and selective catalyst to produce specific
product from guaiacol.

In
recent years, several catalysts have been utilized in the HDO of guaiacol which
completely or partially removes oxygen functionality from the compound1. Supported monometallic/bimetallic noble
metal catalysts (Rh, Ru, Pt, Pd) has been extensively studied for HDO of guaiacol.
However, the application of these catalysts is limited due to their high cost
and scarcity. Recently, Ni-based catalysts have been applied in HDO reactions
mainly because of high hydrogenation activity and low cost1. The high hydrogenation activity of
Ni can be tailored with the addition of a second metal. Therefore, a
combination of Ni and various other transition metals such as Fe, Mo, and Cu have
been evaluated for guaiacol HDO2–4. The major drawbacks
associated with these reports are either incapability of catalysts
to break C(sp2)–OH group giving partially deoxygenated products or
excessive hydrogenation capability and coke formation. In present study, HDO of
guaiacol over less expensive bimetallic nickel-cobalt supported catalyst was
examined for the production of valuable chemicals. The main objective of
present study was to: (i) Optimization of Ni and Co mole ratio for complete
removal of oxygen functionality from guaiacol (ii) Understand the coordinative effect
Ni and Co species on physiochemical characteristics and structural properties
(iii) Elucidate the structure-reactivity relationship of Ni-Co bimetallic
catalysts.

2.     
Experimental:

Promoted
and non-promoted Ni, Co, and Ni-Co catalysts supported over pretreated γ-Al2O3
were synthesized using co-impregnation method. All prepared
catalysts were characterized by BET, XRD, TPR, and FT-IR3. HDO
of guaiacol was carried out in a 300 mL stainless steel
make high-pressure and temperature batch reactor (Parr Instruments, USA, Model
no:4566HT).  

3.     
Result and Discussion:

BET
surface area of calcined and reduced Ni, Co and Ni-Co catalysts were obtained
in the range of 90-220 m2g-1. The
powder XRD patterns of calcined xNiyCoAl bimetallic
catalyst with different Ni(x)/Co(y) mole ratio at a constant total metal
loading of 9.2 mmolg-1cat are
displayed in Figure 1. The XRD patterns
of oxide catalysts gave a clear indication for the formation of nickel-cobalt
mixed oxide (NiCo2O4) due to shifting and weakening of
diffraction peaks corresponding to Co3O4 and NiO species5. Therefore, subsequent formation of
an oxidized bimetallic phase could provide an easier way for the formation of
Ni-Co alloy during H2 reduction. The H2-TPR profile of
alumina supported xNiyCo bimetallic catalyst with various
Ni/Co mole ratio calcined at 723 K for 6h is shown in Figure 2. The presence of a sharp reduction peak with high H2
consumption at 573 K in H2-TPR profile of bimetallic catalysts with
Ni/Co mole ratio of 1:2 (3.05Ni6.1CoAl) further confirms the
presence of pure NiCo2O4
spinel. Therefore, the possibility of formation of Ni-Co alloyed species during
reduction is higher in 3.05Ni6.1CoAl as compared to other
catalysts in the series at an optimal reduction temperature of 723 K. Metal
alloy formation may be the reason for enhanced catalytic activity because of
the change in electron density as a result of formation of heteronuclear
metal-metal bond6.

The
effect of variation in Ni/Co mole ratio for xNiyCoAl
during guaiacol conversion and product distribution is demonstrated in Figure 3. A guaiacol conversion of 95% with
cyclohexane (21%), cyclohexene (33%), phenol (20%) and benzene (10%) were obtained
over monometallic 9.2CoAl. Whereas, monometallic 9.2NiAl
resulted in a guaiacol conversion of 79% with cyclohexane (44%), phenol (17%)
and benzene (13%) as major product. The higher selectivity of aromatic ring
saturated product over 9.2NiAl clearly demonstrate high
hydrogenation capability of metallic Ni than Co. The bimetallic catalyst had a
better performance during guaiacol HDO for the formation of cyclohexane and
benzene as compared to monometallic ones. The conversion of guaiacol was observed
to increase with the increase in Ni content up to Ni/Co mole ratio of 1:2, but
decreased with further replacement by Ni. The bimetallic catalyst 3.05Ni6.1CoAl
(Ni/Co mole ratio=1:2) showed substantially higher guaiacol conversion (99%)
with cyclohexane (56%) and benzene (29%) as the major product. A closer
analysis of variation in selectivity of aromatic product (benzene) represents
that it passed through a maximum at the Ni/Co mole ratio of 1:2. From these
results, it can be inferred that Ni/Co mole ratio influences molecular and
electronic structure and catalytic activity of xNiyCoAl.
The higher activity of 3.05Ni6.1CoAl suggests that there
exists an optimal Ni/Co ratio close to the value required for the formation of
NiCo2O4 spinel. Upon reduction in H2, NiCo2O4
spinel transformed to Ni-Co alloy of different electronic properties. A further
increase or decrease in mole ratio of Ni/Co breaks the stoichiometry required
for the formation of NiCo2O4 spinel and therefore Ni-Co
alloyed domains, as a result, activity of catalyst decreases. It is also
consistent with XRD and H2-TPR data. H2-TPR profile of 3.05Ni6.1CoAl
suggests the presence of a higher proportion of NiCo2O4
spinel than NiO and Co3O4. Ni-Co alloyed domains provides
a different adsorption site than Ni and Co with new electron density which in
turn effects the chemisorption capacity of guaiacol and other intermediates during guaiacol HDO7. The
metallic Ni activates H2 molecule whereas metallic Co and Ni-Co
alloyed domains provides oxophilic sites for the adsorption of guaiacol coordinating
with an oxygen atom. The activated H2 molecule then interacts with
the oxygen atom and facilitates cleavage of CAr-O linkage. The Ni-Co
alloyed species promote the activation of CAr-OCH3 and CAr-OH
linkage thereby enhances the selectivity towards aromatic products. The
presence of higher fraction Ni-Co alloyed species on 3.05Ni6.1CoAl
is attributed to its superior activity during guaiacol HDO. The further
optimization of reaction parameters such as reaction temperature, initial H2
pressure, catalyst loading leads to a maximum selectivity of 57% and 32% for cyclohexane
and benzene with complete conversion of guaiacol.

Therefore,
in terms of selectivity of deoxygenated products (cyclohexane and benzene) and
guaiacol conversion, Ni-Co bimetallic catalyst can be a suitable catalyst for HDO
of compounds present in lignin-derived bio-oil.

 

Figure 1          XRD
patterns of oxidized xNiyCoAl catalysts of varying Ni/Co
mole ratio.

 

Figure 2          H2-TPR profile of xNiyCoAl
catalysts of varying Ni/Co mole ratio.

 

Figure 3          Effect of nickel to cobalt mole ratio on guaiacol
conversion and product distribution.

Reaction
conditions: Guaiacol= 5 ml, Tetraline= 95 ml, Catalyst loading= 0.25% (w/v),
Temperature=

573
K, Initial hydrogen pressure= 10 bars, Reaction time= 4h.

 

4.     
Conclusion:

The
following inference can be drawn from present study:

·        
Co
has been used as a modifier with Ni to develop alumina supported non-noble
bimetallic catalyst for controlled HDO of guaiacol in a high temperature and
pressure batch reactor under a wide range of process parameter.

·        
The
molar ratio of Ni and Co species in supported xNiyCoAl
catalyst is governing factor for generation of catalytically active NiCo2O4
and Ni-Co alloyed species.

·        
 The
characterization results suggest the presence of these species significantly in
catalyst with a Ni/Co mole ratio of 1:2 (3.05Ni6.1CoAl).

·        
A
superior guaiacol conversion (99%) and significant selectivity of cyclohexane
(57%) and benzene (32%) was obtained under optimal reaction conditions over 3.05Ni6.1CoAl.

 

5.     
Acknowledgments

The
authors are grateful for financial support from Department of Science and
Technology (DST), Govt. Of India.

6.     
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