The Inconsistency in Adsorption Properties and Powder XRD Data of MOF-5 Is Rationalized by Framework Interpenetration and the Presence of Organic and Inorganic Species in the Nanocavities

MOF-5 is the archetype metal−organic framework and has been subjected to numerous studies
the past few years. The focal point of this report is the pitfalls related to the MOF-5 phase identification
based on powder XRD data. A broad set of conditions and procedures have been reported for MOF-5
synthesis. These variations have led to materials with substantially different adsorption properties (specific
surface areas in the range 700 to 3400 m2/g). The relatively low weight loss observed for some as
synthesized samples upon solvent removal is also indicative of a low pore volume. Regrettably, these
materials have all been described as MOF-5 without any further comments. Furthermore, the reported
powder XRD patterns hint at structural differences:  The variations in surface area are accompanied by
peak splitting phenomena and rather pronounced changes in the relative peak intensities in the powder
XRD patterns. In this work, we use single-crystal XRD to investigate structural differences between low
and high surface area MOF-5. The low surface area MOF-5 sample had two different classes of crystals.
For the dominant phase, Zn(OH)2 species partly occupied the cavities. The presence of Zn species makes
the hosting cavity and possibly also adjacent cavities inaccessible and thus efficiently reduces the pore
volume of the material. Furthermore, the minor phase consisted of doubly interpenetrated MOF-5 networks,
which lowers the adsorption capacity. The presence of Zn species and lattice interpenetration changes the
symmetry from cubic to trigonal and explains the peak splitting observed in the powder XRD patterns.
Pore-filling effects from the Zn species (and partly the solvent molecules) are also responsible for the
pronounced variations in powder XRD peak intensities. This latter conclusion is particularly useful for
predicting the adsorption properties of a MOF-5-type material from powder XRD.