Solid Guests in Metal–Organic Frameworks: Capacity Limits and Structural Effects on Loading Kinetics

A method that quantifies the maximum loading of solid guests in metal–organic frameworks (MOFs) is demonstrated. The approach relies on the fact that included guests do not crystallize and quantifies the residual unincorporated guest heat of melting relative to that of the pure substance. Changes in MOF structure and metal center dramatically change inclusion rates. Whereas loading solid guests of varying melting points in MOF-5 was readily achieved at percent loadings between 31.1% and 56.7%, for Mg-MOF-74, the loadings were between 13.7% and 44.5%, with the lowest melting compounds exhibiting the greatest deviation from the theoretical maximum loading. Increasing the temperature significantly above the melting point of the lower melting guests led to dramatic incorporation improvements. The rate of inclusion was even slower in Zn-MOF-74, as demonstrated by variable temperature infrared spectroscopy. Achieving full guest loading resulted in guest@MOF composites that were near the calculated maximum loading based on pore volume considerations. This method has particular utility for making drug delivery systems and solid electrolytes.