545671 Plasma-CaO Coupling for CH4-CO2 transformation into Syngas and/or Hydrocarbons, Oxygenates. Catalyst Structural and Textural Modifications Under Plasma Discharge

Tuesday, June 4, 2019: 11:03 AM
Texas Ballroom D (Grand Hyatt San Antonio)
Nassim Bouchoul1, Elodie Fourre2, Jean-Michel Tatibouët2, Alysson Duarte3, Nathalie Tanchoux3 and Catherine Batiot-Dupeyrat2, (1)University of poitiers, Poitiers, France, (2)University of Poitiers, Poitiers, France, (3)University of Montpellier, Montpellier, France

Introduction

Non-thermal plasma is considered as an attractive alternative method for methane and carbon dioxide transformation to overcome carbon deposition limitation in classical catalytic processes. Many reactions are initiated at low temperature under the plasma discharge leading to valuable chemicals such as hydrocarbons or oxygenates [1]. However the selectivity towards targeted products is often difficult to achieve due to the occurrence of a large number of reactions in gaseous phase. The coupling of catalyst and plasma was proposed to overcome this drawback [2]. In many studies, the selected catalysts were similar to those used in conventional thermal catalytic processes, such as Ni/Al2O3, but it was shown that the presence of conductive Ni active sites decreased the electric field strength and consequently the electron density reducing thus reactant conversion [3]. The combination of non-thermal plasma with different metal oxides was investigated, while alumina possesses no catalytic activity in the thermal dry reforming of methane reaction, a significant improvement in CH4 and CO2 conversion over Al2O3 was observed when coupled with plasma [4]. The combination of plasma and catalyst requires the development of specific materials able to transform reactants present under active species form, such as excited species, radicals and ions. In this respect, the influence of calcium oxide located in a packed bed DBD reactor was studied. This material was chosen due to its relatively low dielectric constant (9-30), since as shown by Wang et al. [5] a too high dielectric constant of the packing material limits the discharge to the contact point of the catalyst beads.

  • Experimental part

The reaction was performed at room temperature and atmospheric pressure in a coaxial dielectric barrier discharges (DBD) reactor. The non-thermal plasma reactor was composed of an alumina tube (ID: 4mm; ED: 6mm), a stainless steel electrode inside the reactor (1.0mm) and a copper electrode wrapped around the quartz tube (100 mm long). Helium, methane and carbon dioxide were flown through the plasma reactor at a total flow rate of 40mL.min-1 using different ratio CH4/CO2 (from 0.5 to 2) with a constant concentration in He: 75%, corresponding to a contact time of 1.6s.

A sinusoidal supply of power was applied across the electrodes (TG1010A Aim-TTi, Thurlby Thandar Instruments Brand). The discharge power, calculated from the Lissajous figures, was fixed at 8 watts (frequency at 800Hz and voltage at 13.5kV).

The oxide used in this study we prepared according to the alginate way after optimization of the synthesis procedure allowing to obtain CaO after calcination at 600°C with a high purity [6].

2 Results and discussion

2.1 Influence of the grain size and role of CaO material

CaO oxide was sieved to diameters d1: 250-355µm, d2: 355-650µm, d3:650-800 and d4: 800-1000µm. The only parameters that are affected by the grain size are the accessible surface, the average size of space and the number of contact point between the particles. Results showed that conversion of CO2 and CH4 increased with decreasing particle size. Kasinathan et al. [7] obtained a similar effect for CH4 conversion into C2, C3 and C4 hydrocarbons over MgO/Al2O3. They conclude that the smaller the catalyst particle, the more diffuse the plasma streamers on the catalyst surface.

The selectivity to products depends on the particle size. Ethane formation is favored when the biggest particles are used, corresponding to the largest space between grains, suggesting that the recombination of CH3 radicals to form C2H6 is favored in gaseous phase and not at the surface of the solid. As expected, CO is the main product formed, while the amount of oxygenated products remains low whatever the grain size. The carbon balance is always higher than 90%. It appears that the presence of CaO does not lead to significant catalytic effect in the experimental conditions used. It is believed that the reactivity is governed by the characteristics of the discharge at the surface of the packing material possessing a low dielectric constant (≈11 [8]). CaO is characterized by the presence of meso and macropores with porous volume of 0.01 cm3/g and an average pore diameter of 10 nm with a specific surface area of 15 m2/g. The formation of micro-discharges in pores is possible when their diameter is larger than the Debye length. This value depends on electron density and temperature in the plasma streamer but is typically in the order of 100 nm to 1µm. According to calculations of A. Bogaerts group, plasma can penetrate in pores within 50nm but at very short time [9, 10]. Consequently, using CaO, the penetration of micro-discharges within pores can be excluded.

2.2. Influence of the ratio CH4/CO2

The carbon balance and the selectivity to CO is maximum with an excess of CO2. Besides CO and hydrocarbons, oxygenated products (formaldehyde, methanol, acetaldehyde and ethanol) were quantified, but with a low selectivity. An opposite trend is visible concerning formaldehyde and methanol formation: CH3OH is slightly favored when an excess of CO2 is used, while the formation of HCOH is favored in an excess of methane. These results are in agreement with the reported research of De Bie et al. using a one-dimensional fluid model [11]. The dominant reaction pathways was through the following three-body reaction between CH3 and OH radicals: CH3 + OH + M → CH3OH + M. It is also supported by the work of Wang et al. based on density functional theory (DFT) calculations [12] and Istadi using a hybrid artificial neural network – genetic algorithm technique [13]. The production of formaldehyde comes from the reaction between CO2 and CH2 radicals and acetaldehyde is obtained by O atom (from CO2 dissociation) reaction with ethyl radical [11].

2.3 Influence of temperature

The results obtained for grain size: 355 µm < d < 650 µm, P= 8W, from 100 to 300°C and a CH4/CO2 ratio of 2 show that methane conversion is relatively stable during time on stream and is closed to 20% whatever the reaction temperature, while for CO2 the conversion is strongly increased at the beginning of the experiment performed at 300°C, reaching 40%, then decreasing regularly with time and stabilizing at around 15% after 30 minutes. The carbon balance follows an opposite trend suggesting a reaction of CO2 with the basic CaO material. The selectivity to the main products: CO and C2H6 is not modified by the increase in the reaction temperature. Little modifications are observable on oxygenates, an increase in the reaction temperature lead to a decrease of the selectivity to methanol and formaldehyde while the selectivity to acetaldehyde is not strongly modified.

2.3 Characterization of the oxide after reaction at different temperatures

No significant differences (from TGA analysis) are observed before reaction and after reaction at room temperature and 100°C. The weight loss between 500-700°C is attributed to the presence of stable carbonate species corresponding to 7 monolayers.

When the reaction is performed at 200°C, TGA profile exhibits an important weight loss between 300-500°C (6.1%) corresponding to an endothermic peak. It is attributed to the decomposition of Ca(OH)2. The formation of calcium hydroxide can result from H2O dissociation to H+, OH- in gas phase. After reaction at 300°C, the presence of carbonate oxide is preponderant. This result shows, for the first time, that CO2 or excited CO2 under plasma discharge can diffuse from the gas phase deep into porous CaO.

Conclusion

This study shows that plasma-catalysis can be considered as an alternative method for the production of oxygenated compounds and CO2 valorization, despite the low selectivity to methanol. CaO particle size, CH4/CO2 ratio or temperature had little effect on methanol yield. However, the presence of the catalyst in the plasma discharge led to the formation of carbonate oxide, proof of the CO2 diffusion into CaO pores.

References

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