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This postdoctoral Marie Curie project dealt with the application of metal-organic frameworks as a platform for selective catalytic processes. From the outset, and following a previous experience by the fellow O. Gutov, Zr-based MOFs were chosen as a system that would provide the highest mechanical and chemical stability. This family of MOFs is constructed using octahedral Zr6 clusters linked by poly-carboxylate struts. The initial report of such a construct came from the laboratories of K. P. Lillerud (Lillerud et al, 2008) and described a metal-organic framework assembled from Zr6 clusters linked by terephthalic acid struts. The 12 cluster edges are each formed by a μ2 carboxylate units, and the 8 faces are capped, in alternating fashion, by μ3 -OH and μ3 -O units; this leads the material formulation as Zr6(O)4(OH)4L6. The terephthalate-based prototype is called UiO-66 (reflecting its Universitet i Oslo origins), and a large number of analogues have been prepared, many of which (such as UiO-67, UiO-68) are isoreticular with UiO-66. This family of MOFs is currently one of the sturdiest, both chemically and mechanically, making it an attractive platforms for the design of catalytic (and, overall, functional) materials. Particularly important is a relatively high (though, perhaps, not as high as was originally hoped for) resistance of Zr-based based MOFs to hydrolysis.

Most commonly, UiO-66 is prepared solvotehrmally by storing (undisturbed) a mixture of a Zr(IV) chloride with the terephthalic acid in DMF at 100-120 o C. Though not fully understood, the assembly is thought to begin with an initial formation of the Zr6 octahedra, followed by their assembly into the 3D cubic network. Importantly, these syntheses are commonly performed in the presence of an excess (≥30 equiv) of mono-dentate carboxylic acid, such as acetic or benzoic acid, which acts as a modulator in the crystal growth. The modulator acid is assumed to aid in the initial formation of the Zr6Mod12 species, which would then slowly assemble through carboxylate exchange (Scheme 1). In fact, in his previous work (Northwestern U) Dr Gutov had investigated the use of formic acid in UiO-67 synthesis, showing it to be a superior modulator (Hupp, Farha et al). Arguing in favor of the stepwise mechanism is the report by Gross, Schubert et al. on the synthesis of a UiO-type MOF using preformed and isolated Zr6 methacrylate clusters.

At this point, it was clear that any further development of this MC project would require a detailed understanding of the defect phenomenon, unavailable at the time this work began. Therefore, an ample study was conducted aiming to understand the relationship between the modulator employed and the properties of the resulting material. The study also facilitated the group’s entry into the MOF field. Necessary equipment was also acquired at this point, including a temperature-controlled oven and a medium-potency centrifuge for sample work up. At this stage, we also established fluid contacts with the facilities (ICIQ, URV, others) for MOF characterization. The study covered the three common structures, namely that based on the terephthalic acid (UiO-66), the biphenyl-dicarboxylic acid (UiO-67) and the methyl-substituted terphenyl dicarboxylic acid (UiO-68Me). The modulators explored were benzoic acid, acetic acid, trifluoroacetic and, formic acid and HCl. The study allowed, for the first time, to establish a relationship between the modulator employed and the defect content, porosity and crystallinity of the resulting MOF. Thus, while the canonical defect-free MOF formulation is Zr6(O)4(OH)4L6, the samples obtained were better described as [Zr6O4(OH)4]L(6-0.5n)Modn (L = ditopic linker acid; Mod = modulator anion). Hence, the n parameter here describes the defect contents, and varies widely across the systems studies; this, in turn, has a profound effect on particle size and morphology, as well as on gas uptake (Table 1, Figure 2).

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