Simultaneous in Situ X-ray Diffraction and Calorimetric Studies as a Tool to Evaluate Gas Adsorption in Microporous Materials

Combined application of in situ X-ray diffraction (XRD) and differential scanning calorimetry (DSC) is a novel technique for rapidly evaluating the suitability of microporous materials for postcombustion CO₂ capture. Further, while many microporous materials show promise for CO₂ capture, most are not evaluated in the presence of water vapor, a major component of postcombustion flue gas. As a demonstration of the versatility of XRD-DSC techniques, representatives of the classes of materials typically proposed for CO₂ capture, zeolites, and metal-organic frameworks (MOFs) were studied: zeolite NaX, Ni-MOF-74 [Ni₂(dobdc); dobdc = 2,5-dihydroxyterephthalate], ZIF-7 [ZIF: zeolitic imidazole framework, Zn(phim)₂; phim: benzimidazole], and SBMOF-1 [Ca(sdb); sdb: 4,4′-sulfonyldibenzoate]. Although NaX and Ni-MOF-74 show very high affinity toward CO₂ under idealized dry conditions, they are also very sensitive to the presence of water vapor and experience significant performance loss above 25% relative humidity (RH) at room temperature. Relative to NaX and Ni-MOF-74, ZIF-7 and SBMOF-1 show strong CO₂ affinity even in the presence of 75% RH and may be more ideally suited for postcombustion flue gas CO₂ capture than compounds with unsaturated metal sites. XRD-DSC is particularly powerful for evaluating the consequences of framework flexibility, with XRD providing the signature indicative of the structural rearrangement and the DSC providing the enthalpies of adsorption for each structure. This kind of detailed energy evaluation is not possible with other noncalorimetric methods.