Insights into Mass Transfer Barriers in Metal-Organic Frameworks

Identifying mass transfer limitations is imperativefor the practical application of nanoporous solids in adsorptiveseparations and catalysis. In particular, metal-organic frameworks(MOFs) with a staggering assortment of unique pore architecturesand chemical binding sites are one class of materials whereunderstanding structure-property relationships can facilitatematerial design. Here, we performed volumetric physisorptionmeasurements and collectedn-hexane adsorption isotherms of nineZr-MOFs with unique pore architectures, textural properties, andcrystal sizes. We collected measurements on a commerciallyavailable adsorption instrument and used a generalized masstransfer model that includes intracrystalline diffusion as well as apossible mass transfer resistance at the crystal boundary. Theresults 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 ofsurface permeability as the rate-limiting mass transfer process within MOFs will aid the design of next-generation adsorbents andcatalysts.