60th Annual Report on Research 2015

The enormous scale of methane reserves has motivated significant research activities focused on its conversion to fuels and chemicals [1-6]. Since large amounts of natural gas are located in remote areas and transporting gases in pipelines is difficult, processes for producing denser products are desirable. Unfortunately, such processes have proven to be challenging. This work addresses the need for a new technology for direct methane conversion by developing catalysts for the selective activation of methane at temperatures below 500 °C. A specific focus was the direct non-oxidative coupling of methane into ethane and ethylene. In the final year of this grant, combined coupling and partial oxidation of methane into oxygenates such as ethanol will be studied.

RESULTS AND DISCUSSION

Catalysts Design

Ceria zirconia was chosen as a support to stabilize small metal oxide clusters, which provide Lewis acid sites for methane activation. AlO_x, CoO_x, PdO_x, FeO_x and NiO_x were individually deposited on ceria zirconia with a loading of 2 wt%, which resulted in the formation of a substantial amount of Lewis acid sites (Figure 1).

Figure 1: Concentration of Lewis acid site (LAS) on ceria zirconia (CZ) based catalysts determined by pyridine adsorption followed by IR spectroscopy.

In-situ IR spectroscopic studies on surface reactions of methane

The formation of surface species on the catalysts was tracked through their characteristic C-H stretching modes in in-situ IR spectra. Only a small amount of physisorbed methane was observed on Ce_0.75Zr_0.25O₂ (CZ) at 50 °C (Figure 2a). However, no chemisorbed species were formed because Ce_0.75Zr_0.25O₂ does not have sufficient Lewis acidity for methane activation. Catalysts with added metal oxide clusters activated methane starting at 150 °C. Specifically, CH₃ stretching vibrations at ~ 2950 and 2879 cm^-1 were observed for Co_xO_y/Ce_0.75Zr_0.25O₂ indicating the formation of surface methyl groups (Figure 2b) [7]. The same observation was made in the cases of other metal oxides supported on ceria zirconia. Thus, it is concluded that these materials are capable of chemisorbing methane as surface methyl species. This constitutes the first step of methane conversion over oxide surfaces.

Figure 2: Difference IR spectra of products from CH₄ on (a) CZ (b) 2 wt% CoO_x/CZ ceria zirconia.

Figure 3: Difference IR spectra of products from CH₄ on (a) 2 wt% FeO_x/CZ (b) 2 wt% NiO_x/CZ.

Interestingly, additional CH₂ stretching vibration bands at ~2920 and 2850 cm^-1 were observed, when Fe or Ni oxide clusters on CZ were exposed to methane (Figure 3) [8]. This indicates the formation of longer alkyl chains on the surface and illustrates that these samples can catalyze the growth of higher alkyl chains, in addition to the cleavage of the C-H bonds of methane. The nickel based catalyst appears to be the most active one. The surface species were removed within 3 hours in high vacuum at 250 °C (Figure 3).

Reactivity Studies

      Motivated by the observation of higher alkyl chains in the IR spectra of surface species on Ni/CZ, we studied non-oxidative coupling of methane in a packed bed reactor. Ethane, ethylene and hydrogen were observed as the main products at 350-450 °C (Figure 4). The conversion of methane went through a maximum at the initial stage of the reaction at which time only limited amounts of C₂ products were observed. These observations could be explained by the conversion of NiO_x species into a different active phase during the initial stages of the reaction, which provides the active sites for steady non-oxidative methane coupling. After about 3500 min, the conversion of methane reached the thermodynamic equilibrium of 0.4%. Note that substantially higher values will be obtained when hydrogen is removed continuously using a membrane reactor. Additional experiments will be performed, in which the surface alkyl group will be hydrolyzed with steam to convert them into alcohols.

Figure 4: Catalytic performance of NiO_x/CZ and FeO_x/CZ for the non-oxidative coupling of methane in a fixed-bed reactor at 1 atm (a) conversion of methane at 450 ^oC (b) mole fraction of products over NiO_x/CZ at 450 ^oC (c) C₂ selectivities at 450 ^oC (d) mole fraction of products over NiO_x/CZ at 350 ^oC.

Characterization of Active Sites

X-ray absorption spectra were taken at reaction conditions to elucidate the transformation of NiO_x/CZ during the conversion of methane (Figure 5). The white line at the Ni K-edge decreased abruptly after 4 hours indicating a decrease of the oxidation state of Ni. Cerium underwent a mild reduction in the same period. It is therefore suggested that ceria-zirconia supplies oxygen to keep Ni oxidized during the initial stage after which Ni reduces and forms the active sites for non-oxidative coupling of methane in steady state.

Figure 5: In-situ XANES of NiO_x/CZ during conversion of methane (a) Ni K-edge (b) Ce L₃-edge.

SIGNIFICANCE

Our findings show that ceria zirconia supported metal oxide clusters have great potential for the production of higher hydrocarbons and alcohols from methane in a single reactor. In-situ IR spectroscopy provided us with fundamental insight into the surface chemistry involved in these reactions. Although the activation of methane has been studied extensively [1, 5-7], the coupling of methane with the potential of directly producing ethanol has never been reported at low temperatures (i.e. below 500 °C).

REFERENCES

[1] Periana R.A., Taube D.J., Evitt E.R., Luffer D.G., Wentreck P.R., Voss G., Masuda T. (1993) Science 259: 340.

[2] Liu H-F., Lui R-S., Liew K.Y., Johnson R.E., Lunsford J.H. (1984) J. Am. Chem. Soc. 106: 4117.

[3] Li. F., Yuan G. (2005) Chem. Common 17: 2238.

[4] The world Factbook 2012. CIA, Washington, DC, 2012.

[5] Sinev M.Y., Korchak V.N., Krylov O.V. (1986) Kinet. And Catal. 27: 1110.

[6] Joubert J., Salameh A., Krakoviack V., Delbecq F., Sautet P., Coperet C., Basset J.M. (2006) J. Phys. Chem. 110: 23944-23950.

[7] Joubert J., Salameh A., Krakoviack V., Delbecq F., Sautet P., Coperet C., Basset J.M. (2006) J. Phys. Chem. 110: 23944-23950.

[8] Socrates G. (2001) Infrared and Ramen Characteristic Group Frequencies. Middlesex UK: John Wiley & Sons LTD.