(560ei) Catalyst Development for Oxidative Coupling of Methane in a Gas-Solid Vortex Reactor

Catalyst development for oxidative
coupling of methane in a gas-solid vortex reactor

Saashwath Swaminathan
Tharakaraman¹, Guy B. Marin¹, Mark Saeys^1*

1. Laboratory for Chemical Technology, Ghent University,
Technologiepark 125, 9052 Gent, Belgium;

*Corresponding author:
Mark.Saeys@ugent.be

Oxidative
coupling of methane (OCM) is a promising one-step reaction for the direct
conversion of natural gas to C₂ hydrocarbons according to the
following reactions.^[1]

The presence of
oxygen and the high reaction temperature can facilitate the overoxidation of reactants
and products to CO and CO₂, typically limiting the C₂
hydrocarbon yields to below 30%.^[2]

The high reaction exothermicity makes
isothermal operation challenging on an industrial scale, and quasi-adiabatic
reactors are therefore evaluated. Experimental studies have shown that
adiabatic operation can be sustained using feed at ambient temperature by employing
catalysts that combine high activity and good selectivity to C₂
products.^[3] Bifurcation
studies indicate that reactors with good thermal back mixing combined with
limited species back mixing can potentially improve C₂ yields.^[4] The
gas-solid vortex (GSVR) reactor satisfies both of the aforementioned conditions
and hence emerges as a reactor technology to enhance C₂ yields.^[5]

In the GSVR, a dense
rotating bed of solids is obtained by tangential injection of gas through 1 mm
slots at velocities exceeding 100 m s^-1. Centrifugal forces on the
solid particles counteract the drag forces, resulting in the formation of an
uniform bed of solids with high gas-solid slip velocities. The very high
gas-solid slip velocities intensify interfacial transfer of mass, energy and
momentum, allowing a reduction in gas phase residence time.

The high reaction
temperature, the high solid velocities (exceeding 6 m s^{-1 [5]}) and the
low space times in the GSVR also require the development of catalysts with a combination
of high mechancial and thermal stability and high catalytic activity. Among OCM
catalysts, strontium promoted lanthanum oxide has been shown to be highly
active for OCM reaction.^[6] In this
work, we report the development of rare earth oxide based catalysts with a high
mechanical stability under elevated temperatures and examine the role of strontium
as a catalyst promoter.

To elucidate the
promoting effect of Sr on lanthanum oxide, a series of Sr/La₂O₃
catalysts with varying Sr loadings (1 to 11 wt% ) were prepared^[7] and the catalytic
performance of the catalyst was evaluated in a fixed bed reactor operating under
intrinsic kinetic conditions.^[8] Figure 1 shows the
kinetic performance of the tested catalysts. It can be seen that the promotion
of lanthanum oxide with strontium enhances the CH₄ activity of the
catalyst. Additionally, the selectivity towards C₂ hydrocarbons is
improved when the Sr loading is more than 1 wt%. This effect is more pronounced
when the reaction temperature exceeds 800 °C. For all the tested catalysts, the
C₂ selectivity increases with temperature and stabilizes above 800 °C.
Interestingly, addition of 1 wt% Sr slightly reduces the selectivity of the
catalysts. To understand the promoting effect of Sr on lanthanum oxide, the
catalysts were characterized using XRD, CO₂-DRIFT, CO₂-TPD
and XPS showing a connection between the high temperature CO₂
desportion temperature (Figure 2) and the
catalyst selectivity.

Figure 1: a)
Activity and b) C₂ selectivity of unsupported Sr/La₂O₃
catalysts vs. temperature. Other experimental conditions: P_total
=1.8 bar (50% He), inlet CH₄:O₂ ratio =4:1, space time =
0.2 kg_cat s mol^-1_CH4,0.

Figure 2: CO₂ TPD
profiles of various lanthana and Sr-promoted La₂O₃
catalysts. Experimental conditions: CO₂ desorption was carried out
in a He flow rate 60 ml min^-1 and temperature ramp of 10 °C min^-1.
CO_2­ adsorption was carried out 50 °C.

In the next step rare earth oxide based catalysts
were synthesized and their mechanical and thermal stability was tested in the
GSVR at a temperature of 700 °C with an inlet air flow of 26 kg h^-1.
A stable rotating bed of solids with a thickeness of 8 mm was sustained for 1 h
at these high temperatures. The introduction of standard Sr/La₂O₃
catalysts in the GSVR resulted in immediate attrition and entrainment.

Parallely, the new catalyst was tested in a
fixed bed reactor under quasi-isothermal conditions at very short space times. It shows a
reasonable CH₄ conversion ranging at low space times and a good C₂
selectivity. Experiments to test this promising catalyst under reactive OCM
conditions in the GSVR are currently underway and will be reported.

Acknowledgements:

This project has received funding from the
European Union’s Horizon 2020 research and innovation programme under grant
agreement No 680777.

The assistance of ir.
Manuel Nunez, ir. Shekhar Kulkarni and ir. Sepher Madanikashani while running
GSVR experiments is gratefully acknowledged.

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