Exploring the Parameter Space of p-Cresyl Sulfate Adsorption in Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) have high porosity and surface area, making them ideal candidates for adsorption-mediated applications. One high-value application is the removal of uremic toxins from solution for dialysis. Previous studies have reported adsorptive removal of the uremic toxin p-cresyl sulfate from solution via zirconium-based MOFs, but a specific analysis of parameters contributing to adsorptive uptake is needed to clarify differences in uptake performance between MOFs. We synthesized zirconium 1,3,5-benzenetricarboxylate (MOF-808) and an iron-based analog, MIL-100(Fe), and compared their adsorptive uptake with previously reported values of other zirconium-based MOFs. MIL-100(Fe) adsorbed three times more p-cresyl sulfate from solution on a per mass basis than MOF-808 and had a greater adsorption efficiency than 75% of previously reported Zr-based MOFs. We compared p-cresyl sulfate uptake by MOFs as a function of BET surface area, number of aromatic carbons in the organic linker, internal cage diameter, and pore window diameter. There is poor correlation between p-cresyl sulfate uptake and each of the variables considered, but the number of aromatic carbons of the MOF linker was a better predictor of uptake than BET surface area (R2 = 0.7034 and 0.1430, respectively), and pore window aperture was a better predictor of uptake than the pore cage diameter (R2 = 0.4780 and 0.0383, respectively). We hypothesize that the greater adsorptive capacity of MIL-100(Fe) compared to MOF-808 results from direct coordination of p-cresyl sulfate to vacant metal sites in the MOF, and the total adsorption may be accounted for by some combination of adsorptive interactions occurring at both metal and organic linker sites near to the exterior particle surface. The adsorptive uptake of p-cresyl sulfate by MIL-100(Fe) was observed to increase with p-cresyl sulfate content, mass of MIL-100(Fe), and volume of p-cresyl sulfate solution; the mass of MIL-100(Fe) had the greatest effect on total adsorption.