PNNL

Abstract

Porous organic cage compounds are emerged with remarkable structural diversity and functionality that have applications in gas separation, catalysis and energy storage. Fundamental understanding of nucleation and growth of such materials have significant implications for understanding molecularly directed self-assembly phenomena. Herein we followed the structural evolution of a prototypical type of porous organic cage, CC3 as a function of synthesis time. Three distinctive crystal formation stages were identified: at short synthesis times, a rapid crystal growth stage in which amorphous agglomerates transformed into larger irregular particles was observed. At intermediate synthesis times, a decrease in crystal size over time was observed presumably due to crystal fragmentation, redissolution and/or homogeneous nucleation led. Finally, at longer synthesis times, a regrowth process was observed in which particles coalesced through Ostwald ripening leading to a continuous increase in crystal size. Molecular simulation studies, based on the construction of in silico CC3 models and simulation of XRD patterns and nitrogen isotherms, confirm the samples at different synthesis times to be a mixture of CC3a and CC3 amorphous phases. The CC3a phase is found to contract at different synthesis times, and the amorphous phase is found to essentially disappear at the longest synthesis time. Nitrogen and carbon dioxide adsorption properties of these CC3 phases were evaluated, and were highly dependent on synthesis time.