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 La2NiO4+d (LNO) was used not only for the membrane support but also for the catalyst precursor and La2Ni0.9Co0.1O4+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 (SCO) 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.