427110 Syngas Production over Nickel-Based Catalysts Via Dry Reforming of Methane: Influence of Catalyst Support

Thursday, November 12, 2015: 2:10 PM
355D (Salt Palace Convention Center)
Bruna Rego De Vasconcelos1, Doan Pham Minh1, Ange Nzihou2 and Patrick Sharrock3, (1)RAPSODEE, Ecole des Mines d'Albi-Carmaux, Albi, France, (2)RAPSODEE, Mines Albi, ALBI, France, (3)IUT Castres - Département de Chimie, Université Paul Sabatier, 81104 Castres, France

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 gcat. 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.

(a)

(b)

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

Ni/Pural MG70f, 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/Al2O3f, 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

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