Interpenetration, Porosity, and High-Pressure Gas Adsorption in Zn₄O(2,6-naphthalene dicarboxylate)₃

Microporous coordination polymers (MCPs) have emerged as strong contenders for adsorption-based fuel storage and delivery in large part because of their high specific surface areas. The strategy of increasing surface area by increasing organic linker length has shown only sporadic success; as demonstrated by many members of the iconic Zn₄O-based IRMOF series, for example, accessible porosity is often limited by interpenetration or pore collapse upon guest removal. In this work, we focus on Zn₄O­(ndc)₃ (IRMOF-8, ndc = 2,6-naphthalene dicarboxylate), which exhibits typical surface areas of only 1000–2000 m²/g even though a surface area of more than 4000 m²/g is expected from geometric analysis of the originally reported crystal structure. We recently showed that a high surface area could be produced with zinc and ndc by room-temperature synthesis followed by activation with flowing supercritical CO₂. In this work, we investigate in detail the porosity of both the low- and high-surface-area materials. Positron annihilation lifetime spectroscopy (PALS) is used to show that the low-surface-area material suffers from near-complete interpenetration, explaining why traditional synthetic routes have failed to yield materials with the expected porosity. Furthermore, the high-pressure hydrogen and methane sorption properties of noninterpenetrated Zn₄O­(ndc)₃ are examined, and PALS is used to show that pore filling is not operative during room-temperature CH₄ sorption even at pressures approaching 100 bar. These results provide insight into how gas adsorbs in high-surface-area materials at high pressure and reinforce previous contentions that increasing surface area alone is not sufficient for the simultaneous optimization of deliverable gravimetric and volumetric gas uptake in MCPs.