(543a) Designation of a Self-Catalytic Mixed-Conducting Membrane Reactor for Partial Oxidation of Methane to Syngas

Increasing
concern about world dependence on petroleum oil has generated interest in the
more efficient use of nature gas. Methane (CH₄), the main component
of natural gas, can be converted to liquid fuels, hydrogen, and other
value-added chemicals through a syngas intermediate, a mixture of CO and H₂.
Currently, partial oxidation of methane (POM) [1] with pure oxygen in the
presence of a catalyst is established to be the most potential process for
methane conversion because of its greater selectivity to syngas, its
exothermicity, and more desirable CO/H₂ ratio. As for POM, recently,
an important advance in directly using air as a feedstock resulted from the
application of a mixed-conducting oxide (with high oxygen ionic and electronic
conductivities) membrane reactor, which integrated the oxygen separation and POM
processes in a single unit. [2] However, one of the formidable problems for
this method is that the performance of the membrane reactor is always
restricted by the membrane permeability and catalytic activity. For a dense
membrane, the oxygen permeability can be improved via decreasing the membrane
thickness and/or increasing the surface oxygen exchange rate [3]. In addition,
the catalytic performance of the membrane reactor can be improved by increasing
the dispersity of catalyst.

According to above ideas to
design a high performance
membrane reactor, herein we designed a novel self-catalytic membrane reactor based on an
asymmetric mixed-conducting membrane for POM reaction. A mixed-conducting oxide
La₂NiO_4+d (LNO) was used not only for the
membrane support but also for the catalyst precursor and La₂Ni_0.9Co_0.1O_4+d (LNCO) oxide was selected
as the membrane. At the initial of POM reaction, CH₄ was oxidized by
LNO to CO and H₂, and simultaneously parts of LNO on the surface of
membrane support were reduced to Ni⁰ and La₂O₃,
which forms a self-catalytic reaction process because the catalyst (Ni⁰)
for POM reaction was produced during this process. Moreover,
the improved bulk diffusion and surface exchange rate of oxygen ion and high dispersity
of catalyst on the surface of membrane support make the membrane reactor
possess high catalytic performance for POM reaction. The CO selectivity (S_CO)
is higher than 95%, the conversion of CH₄ is kept at about 60% and
the oxygen permeation flux is about 2.3 ml°¤cm^-2°¤min^-1,
which is nearly 3 times that of the symmetrical LNCO membrane. In addition, the concept for the self-catalytic
membrane reactor can be widely used to instruct the design and preparation of high
efficiency catalytic membrane reactor for conversion of hydrocarbon.

Keywords: Self-Catalytic; Membrane
Reactor; POM

Acknowledgments: This work was supported
by Program for Changjiang Scholars and Innovative Research
Team in University (PCSIRT 0732), the National Basic
Research Program of China (No. 2003CB615702) and National Natural Science
Foundation of China (No.20576051, 20436030).

References:

[1] Hickman DA, Schmidt LD. Production of syngas by direct
catalytic oxidation of methane. Science 1993; 259:343-346.

[2] Jin WQ, Li SG, Huang P, Xu NP, Shi J, Lin YS. Tubular lanthanum cobaltite
perovskite-type membrane reactors for partial oxidation of methane to syngas. J.
Membr. Sci. 2000; 166:13-22.

[3] X. F. Chang, C. Zhang, W. Q. Jin, N. P. Xu, "Match of
thermal performances between the membrane and the support for supported dense
mixed-conducting membranes", J. Membr. Sci. 2006; 285:232-238.