Substituted ALPO-5 Zeolites as Promising O₂Sorption Pump Materials: A Density Functional Theory Study

Many chemical processes depend on having an environment that is low in oxygen partial pressure (PO2 < 100 Pa); sorption pumps are a promising route to establishing that environment by either oxygen pumping or oxygen separation from an inert gas. Near-ambient sorption-based processes rely on either pressure or thermal swings, requiring no moving parts, no electricity, and neither very high nor very low temperatures. In this work, we use ab initio calculations to explore zeolites as a class of materials for sorption-based oxygen pumping/separation. Our calculations indicate that while O2 does not adsorb on the neat ALPO-5 zeolite under ambient conditions, itdoes adsorb on zeolites selectively substituted with transition metals and metalloids and, hence, can enable separation and pumping. ALPO-5 substituted with Si, Ge, Sn, Pd, Pt, Ti, V, Cr, Mn, Zr, Mo, Hf, W, Ce, and Pr provides adsorption energies ranging from -0.19 to -3.92 eV, where (-) indicates an exothermic process. Additionally, we provide a comprehensive understanding of what controls the adsorption energy: (1) the substitutions must be able to adopt an oxidation state that is more positive than the cation it replaces, and (2) the size of the pore into which the O2 adsorbs to the wall. Additionally, we conduct a thermodynamic analysis of a thermal swing cycle to approximate the optimal O2 binding energy for low-energy O2 pumping/separation. We find that the minimum energy cost likely occurs when the adsorption energy is in the range of 0.75-1.00 eV (72-97 kJ·mol-1), which corresponds to Ge-, V-, Pt-, or Ce-substituted ALPO-5.