(663g) Oxygen Selectivity of RPM3-Zn Gate-Opening Metal-Organic Frameworks

Commercial cryogenic distillation process is applied to air separation, based on different normal boiling points of nitrogen (77.4 K) and oxygen (90.2 K), but the process comprises of 30% of capital costs due to significant energy consumption. This study begins to explore the potential for kinetic separations of air via exploiting the gate-opening (GO) process of various metal organic frameworks (MOFs). Here, we report the gas adsorption capacities of nitrogen and oxygen on the GO-MOF RPM3-Zn (a.k.a. Zn₂(bpdc)₂(bpee); bpdc = 4,4â??-biphenyldicarboxylate; bpee = 1,2-bipyridylethene) at temperatures ranging from 77 K to 273 K up to 20 bar. Given sufficient equilibration time, the nitrogen and oxygen GO pressures are indistinguishable at 77 K, 87 K, 195 K, and 273 K. Yet, somewhat unexpectedly, RPM3-Zn exhibits higher oxygen adsorption capacity than nitrogen at low temperatures. The heats of adsorption in gate opening RPM3-Zn with oxygen and nitrogen are calculated as 12 kJ/mol and 8 kJ/mol, respectively, further evidence for oxygen selectivity. The selectivity is enhanced significantly when kinetic effects are also included. Efforts to deduce the role of various adsorption sites on oxygen selectivity will be discussed.