(710f) Syngas Production over Nickel-Based Catalysts Via Dry Reforming of Methane: Influence of Catalyst Support

Introduction

Increases
in energy demand, depletion of carbon-based energy resources and environmental
issues have driven scientists to study the substitution of fossil fuels for
renewable energies (1).

Natural
gas, hydrogen and bio-fuels are reputable to be good fuel alternatives. The
economic aspect is also propitious. Natural gas reserves, for example, are
larger than petroleum reserves. (1) (2) (3)

An
attractive way to use the natural gas is the conversion of methane into syngas,
from which many other chemicals, such as methanol, dimethyl ether, hydrogen and
liquid hydrocarbons, can be produced (4). Currently, steam reforming is the
most used process to convert methane and produce syngas. However, this process
has two main disadvantages: (1) it is highly endothermic ( (5), which means high amount of energy required and (2) the H₂/CO ratio is
higher than the one required for GTL process (3).

For
these reasons, dry reforming of methane (DRM) has been subject of many
researches. This reaction consumes greenhouse gases such as CO₂ and
methane (CH₄) to produce syngas with H₂/CO ratio close to
one, which is ideal for Fisher-Tropsh synthesis (4). DRM reaction and its side
reactions are expressed in equations (1-5):

Dry
reforming of methane:

Water-gas shift reaction:

Boudouard reaction:

Methane cracking reaction:

Carbon gasification:

Supported
noble metal (Ru, Pd, Pt and Rh) catalysts showed to be very active and stable
towards this reaction, however their elevated cost and limited availability are
significant drawbacks. Transition metals, especially Ni, were widely studied due
to their lower cost, higher accessibility and proven activity. Nevertheless,
they were reported to be more prone to carbon deposition on the catalyst active
surface (equations 3 and 4), which considerably reduces their stability over time
(4) (6) (7).

In
order to prevent carbon deposition and consequent loss of catalytic activity,
the addition of alkali promoters to catalysts has been reported. Alkali
promoters increase the basicity of the support which favors strong CO₂
adsorption. A large concentration of adsorbed CO₂ reduces carbon formation
by changing equilibrium concentrations in the Boudouard reaction (equation 3) (8) (9) (10). Addition of water to the feed can also inhibit carbon formation via
equation (5) (8).

Bradford
and Vannice (8) also showed that the formation of NiO-MgO solid solution could
stabilize Ni particles, which prevented carbon deposition over Ni/MgO catalyst
in DRM reaction.

In
this work, Ni supported on Mg-doped Al₂O₃ catalysts, with
different amounts of Mg, were tested in the DRM reaction. The aim was to
understand the influence of the support basicity on the catalyst activity and
stability and the role of the water formed during the reaction. Since carbon
gasification (equation 5) is a side reaction of the process, it is important to
connect the amount of water formed during the reaction to the activity and
stability of the catalysts.

Materials and methods

Three different alumina-based materials were used as catalyst
support: Al₂O₃, Pural MG30 (30%MgO + 70%Al₂O₃)
and Pural MG70 (70%MgO + 30%Al₂O₃), in order to test the
influence of the basicity in the catalyst activity. The supports Pural MG30 and
Pural MG70 were purchased from SASOL Germany.

Initially the supports were calcined at 1200°C for 5h in order to stabilize
them and avoid later catalyst deactivation by sintering. Then 5wt.%Ni/Al₂O₃,
5wt.%Ni/Pural MG30 and 5wt.%Ni/Pural MG70 catalysts were prepared by incipient
wetness impregnation method. Aqueous solution of Ni(NO₃)₂
was added drop wise to the support powder in order to get a consistent mixture.
After the impregnation, the powder was dried at 105°C.

The dry reforming of methane reaction was carried out in a fixed bed
reactor (i.d. of 8 mm) operated at atmospheric pressure. The experimental
apparatus is represented in figure 1.

Figure 1.
Schema of experimental apparatus used for the dry reforming reaction.

In these experiments, 300 mg of fresh catalyst particles diluted ten
times with inert alumina beads were fixed at the center of the reactor. A
thermocouple placed inside the reactor allowed a good reaction temperature
regulation. The position of the catalyst bed was adjusted to remain within the
constant temperature zone (4.5 cm) of the reactor. Inert alumina beads (400-500
µm) were used to hold the catalyst bed in a fixed position.

Prior to the reaction, the catalyst was reduced in-situ in 4% H₂/N₂
flow (70 mL/min) at 700 °C for 2h. After the reduction step, the DRM reaction
was performed at 700 °C for 50 h time on stream (TOS) with a space velocity of
18,000 mL/h g_cat. The feeding gas mixture was composed of 20% of methane,
20% of carbon dioxide and 60% of nitrogen. A silica gel tube was placed at the
exit of the reactor to serve as water trap. The weight difference between tube
before and after the reaction allowed the quantification of the water formed
during the reaction. Gas products were analyzed by a µ-GC A3000 (Agilent).

The determination of methane and carbon dioxide conversion and water
selectivity were calculated as follows:

Results

Figure
2 (a) and (b) shows the CH₄ and CO₂ conversion,
respectively, for the studied catalysts at 700°C and total flow rate equal to
90 mL/min.

Figure 2. CH₄
(a) and CO₂ (b) conversion over the studied catalysts at 700°C,
90mL/min and 50h of TOS.

Ni/Pural
MG70_f, the catalyst with the higher support basicity, had the best
activity and stability, showing methane and carbon dioxide conversion of 80%
over 50h of time on stream (TOS). However, the Ni/Al₂O_3f,
the catalyst with lower support basicity, showed low methane and carbon dioxide
conversion (≈20-30%)
after 20h of TOS. Clearly the basicity of the support has a large influence on
the catalysts activity.

Figure
3 shows the water selectivity for the studied catalysts during the DRM
reaction.

Figure 3. H₂O
selectivity for the studied catalysts during DRM reaction at 700°C, 90mL/min
and 50h of TOS.

The
catalysts with higher support basicity showed very low water selectivity during
the DRM reaction. In fact, the water produced via the water-gas shift reaction
(equation 2) can react with the carbon deposited over the catalysts surface via
equation 5, which could explain the higher activity of these two catalysts.

Ongoing
SEM (Scanning Electron Microscopy), TEM (Transmission Electron Microscopy), RAMAN
spectroscopy, X-ray diffraction characterizations of the catalysts before and
after catalytic tests will allow the better understanding of the catalysts
activity in relation with the syngas production.

 
References

1. Thermal and
catalytic gasification of bio-oils in the Jiggle Bed Reactor for syngas
production. Latifi, M., Berruti, F. and Briens, C. 2015,
International Journal of Hydrogen Energy, pp. 5856-5868.

2. Reforming of CH4 with CO2
on Pt-supported catalysts: Effect of the support on the catalytic behaviour. Ballarini, A. D., et al., et al. 2005, Catalysis Today, pp.
481-486.

3. Platinum supported on
alkaline and alkaline earth metal-doped alumina as catalysts for dry reforming
and partial oxidation of methane. Ballarini, A., et al., et al.
2012, Applied Catalysis A: General, pp. 1-11.

4. Highly stable and active
Ni-mesoporous alumina catalysis for dry reforming of methane. Newnham,
J., et al., et al. 2012, International Journal of Hydrogen Energy, pp.
1454-1464.

5. Catalytic activity and
characterizations of Ni/K2TixOy-Al2O3 catalyst for steam methane reforming. Lee,
S. Y., Lim, H. and Woo, H. C. 2014, International Journal of Hydrogen
Energy, pp. 17645-17655.

6. Evaluation of the economic
and environmental impact of combinig dry reforming with steam reforming of
methane. Gangadharan, P., Kanch, K. C. and Lou, H. H. 2012, Chemical
Engineering Research and Design, pp. 1956-1968.

7. Methane dry reforming on Ni
loaded hydroxyapatite and fluoroapatite. Boukha, Z., et al., et al.
2007, Applied Catalysis A: General, pp. 299-309.

8. CO2 Reforming of CH4. Bradford,
M. C. J. and Vannice, M. A. 1999, Catalysis Reviews: Science and
Engineering, pp. 1-42.

9. Gao, Jing, Hou, Zhaoyin and
Zheng, Xiaoming. Chapter 7 - Dry (CO2) Reforming. [book auth.] II
Shekhawat. Fuel Cells : Technologies forfuel processing. s.l. :
Elsevier, 2011, pp. 191-221.

10. Carbon dioxide reforming
of methane to produce synthesis gas over metal-supported catalysts : State of
the art. Wang, S., Lu, G. Q. and Millar, G. J. M. 1996, Energy &
Fuels, pp. 896-904.