Reports: ND553873-ND5: Reaction Pathways for Methane on Metal Oxide Surface - Influence of Lewis Acidity and Redox Activity
Carsten Sievers, PhD, Georgia Institute of Technology
SCOPE AND PURPOSE
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
RESULTS AND DISCUSSION
Ceria zirconia was
chosen as a support to stabilize small metal oxide clusters, which provide
Lewis acid sites for methane activation. AlOx,
FeOx and NiOxwere 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
Concentration of Lewis acid site (LAS) on ceria zirconia (CZ) based catalysts determined
by pyridine adsorption followed by IR spectroscopy.
IR spectroscopic studies on surface reactions of methane
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 Ce0.75Zr0.25O2
(CZ) at 50 °C (Figure 2a). However, no chemisorbed species were formed because
Ce0.75Zr0.25O2 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 CoxOy/Ce0.75Zr0.25O2
indicating the formation of surface methyl groups (Figure 2b) . 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.
Difference IR spectra of products from CH4 on (a) CZ (b) 2 wt% CoOx/CZ ceria zirconia.
Difference IR spectra of products from CH4 on (a) 2 wt% FeOx/CZ (b) 2 wt% NiOx/CZ.
additional CH2 stretching vibration bands at ~2920and
2850 cm-1 were observed, when Fe or Ni oxide clusters on CZ were exposed
to methane (Figure 3) . 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 NiOxspecies 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.
Catalytic performance of NiOx/CZ and FeOx/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 NiOx/CZ at 450 oC (c) C2selectivities
at 450 oC (d) mole fraction of products
over NiOx/CZ at 350 oC.
of Active Sites
X-ray absorption spectra were taken at reaction
conditions to elucidate the transformation of NiOx/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.
In-situ XANES of NiOx/CZ during conversion
of methane (a) Ni K-edge (b) Ce L3-edge.
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).