(339a) Synthesis and Characterization of Isomorphically Substituted Alpo-5 Zeolites for Oxygen Adsorption

Zeolites are inexpensive, hydrothermally stable, and well-studied heterogeneous catalysts and sorbents. The effectiveness of zeolites can be improved by isomorphic substitution, the substitution of one framework ion with a non-framework ion during synthesis. Aluminophosphates are a class of zeolites that are more amenable to isomorphic substitution, due to the lack of Si ions that form more rigorous framework structures. One specific aluminophosphate, Aluminiophosphate-5 (AlPO-5) is of particular interest for the study of isomorphic substitution during oxygen adsorption processes, as its unsubstituted framework cannot achieve high quantities of oxygen adsorption. The substitution of framework metals has been theoretically shown to improve the adsorption of oxygen gas in AlPO-5 isomorphically substituted with Si, Ge, Sn, Pd, Pt, Ti, V, Cr, Mn, Zr, Mo, Hf, W, Ce, and Pr. This work developed and evaluated multiple synthesis routes for the production of isomorphically substituted AlPO-5 crystals that preferentially adsorb oxygen at ambient temperature and low pressures.

Five hydrothermal synthesis routes were evaluated in which the composition of the synthesis gel, mixing time, crystallization temperature, or crystallization time were varied to determine the optimal procedure for isomorphic substitution in the AlPO-5 framework. The effect of substituting different metals (including Ce, Mn, Mo, Sn, and Si) into the AlPO-5 framework on oxygen adsorption was studied. The isomorphic substitution was verified with powder X-ray diffraction and X-ray photoelectron spectroscopy. AlPO-5 crystallinity, crystal morphology, and porosity/surface area were characterized via powder X-ray diffraction, scanning electron microscopy, and nitrogen physisorption, respectively. Oxygen uptake and release characteristics of the substituted AlPO-5 samples were evaluated using a thermogravimetric analyzer. Oxygen chemisorption or physisorption was verified by the calculated heat of adsorption, obtained from oxygen adsorption experiments at 273 K, 298 K, 323 K, and 348 K. Oxygen adsorption experiments were conducted at 298 K and 1 bar on a Micromeritics 3 Flex adsorption apparatus after degassing the samples for 24 hours at 673 K. At ambient temperature, substituted AlPO-5 achieved a maximum oxygen capacity of 0.1 mmol O₂/g sample. This work has elucidated the hydrothermal synthesis variables necessary for AlPO-5 framework substitution and demonstrated the feasibility of oxygen sorption on isomorphically substituted AlPO-5 frameworks. Additionally, the understanding gleaned from this research can be leveraged to other microporous material synthesis routes to create a larger class of oxygen-adsorbing sorbent materials.