Insights into Mass Transfer Barriers in Metal–Organic Frameworks

Identifying mass transfer limitations is imperative for the practical application of nanoporous solids in adsorptive separations and catalysis. In particular, metal–organic frameworks (MOFs) with a staggering assortment of unique pore architectures and chemical binding sites are one class of materials where understanding structure–property relationships can facilitate material design. Here, we performed volumetric physisorption measurements and collected n-hexane adsorption isotherms of nine Zr-MOFs with unique pore architectures, textural properties, and crystal sizes. We collected measurements on a commercially available adsorption instrument and used a generalized mass transfer model that includes intracrystalline diffusion as well as a possible mass transfer resistance at the crystal boundary. The results indicate that uptake rates in all of the MOFs considered here are limited by mass transfer through crystallite surfaces. Moreover, the severity and guest concentration dependence of these surface resistances differ for each MOF. The identification of surface permeability as the rate-limiting mass transfer process within MOFs will aid the design of next-generation adsorbents and catalysts.