Oxygen-selective adsorption in RPM3-Zn metal organic framework

Publication date: 29 June 2017
Source:Chemical Engineering Science, Volume 165
Author(s): Cheng-Yu Wang, Linxi Wang, Andrew Belnick, Hao Wang, Jing Li, Angela D. Lueking
Development of an oxygen selective adsorbent is anticipated to reduce the material and energy requirements for adsorptive separations of air by a factor of four, due to the relative concentrations of N2 and O2 in air, thereby decreasing the parasitic energy losses, carbon dioxide emissions, and cost of oxygen purification via pressure-swing adsorption. Here, we report that RPM3-Zn (a.k.a. Zn2(bpdc)2(bpee); bpdc=4,4′-biphenyldicarboxylate; bpee=1,2-bipyridylethene) is oxygen selective over nitrogen at temperatures from 77K to 273K, although the oxygen capacity of the sorbent decreased markedly at increasing temperatures. Due to an oxygen diffusivity that is ∼1000-fold greater than nitrogen, the effective oxygen selectivity increases to near infinity at low temperature at equal contact times due to N2 mass transfer limitations for gate-opening. The kinetic limitation for N2 to open the structure has a sharp temperature dependence, suggesting this effective kinetic selectivity may be "tuned in" for other flexible metal-organic-frameworks. Although the low temperature oxygen selectivity is not practical to displace cryogenic distillation, the results suggest a new mechanism for tailoring materials for kinetic selectivity, namely, capitalizing upon the delayed opening process for a particular gas relative to another.



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