A new approach for the synthesis of uniform metal-organic framework (MOF) nanocrystals with controlled sizes and aspect ratios has been developed using simultaneously the non-ionic triblock co-polymer F127 and acetic acid as stabilizing and deprotonating agents, respectively. The alkylene oxide segments of the triblock co-polymer can coordinate with metal ions and stabilize MOF nuclei in the early stage of the formation of MOF nanocrystals. Acetic acid can control the deprotonation of carboxylic linkers during the synthesis and, thus, enables the control of the rate of nucleation, leading to the tailoring of the size and aspect ratio (length/width) of nanocrystals. Fe-MIL-88B-NH(2), as an iron-based MOF crystal, was selected as a typical example to illustrate our approach. The results reveal that this approach is used for not only the synthesis of uniform nanocrystals but also the control of the size and aspect ratio of the materials. The size and aspect ratio of nanocrystals increase with an increase in the concentration of acetic acid in the synthetic mixture. The non-ionic triblock co-polymer F127 and acetic acid can be easily removed from the Fe-MIL-88B-NH(2) nanocrystal products by washing with ethanol, and thus, their amine groups are available for practical applications. The approach is expected to synthesize various nanosized carboxylate-based MOF members, such as MIL-53, MIL-89, MIL-100, and MIL-101.

A new rational approach has been developed for the synthesis of a mixed metal MIL-88B metal-organic framework based on a neutral mixed metal cluster, such as Fe(2)Ni(μ(3)-O). Unlike the conventional negative charged single metal cluster, the use of the neutral mixed metal cluster as nodes in the framework avoids the need of a compensating anion inside the porous MIL-88B system; thus the mixed metal MIL-88B becomes porous. The flexibility of the mixed metal MIL-88B can be controlled by terminal ligands with different steric hindrance. This allows us to reversibly customize the porosity of the MIL-88B structure at three levels of specific surface area as well as the pore volume.

A B S T R A C T : N a n o c u b e s a n d n a n o s h e e t s o f [Cu 2 (ndc) 2 (dabco)] n metal−organic framework (ndc = 1,4-naphthalene dicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]-octane) were synthesized by using simultaneously acetic acid and pyridine or only pyridine, respectively, as selective modulators. This approach can tailor crystal growing on different directions for the size-and shape-controlled synthesis of metal−organic framework (MOF) nanocrystals whose structure is composed of two or more types of linkers using selective modulators. These MOF nanocrystals exhibit high crystallinity and higher CO 2 uptakes compared to that of the bulk MOF material or of the [Cu 2 (ndc) 2 (dabco)] n nanorods generated by using only acetic acid as the selective modulator, which may be due to the morphology effect on their gas sorption properties.

The direct synthesis of Fe 3 -MIL-88B and Fe 2 Ni-MIL-88B was analyzed using different characterization techniques including UV-vis, IR, and Raman spectroscopies and XRD. It was found that single metal Fe 3 -MOF-235 seeds which were formed from the first stage of synthesis are precursors for the formation of MIL-88B. Fe 3 -MOF-235 seeds formed in the first stage of synthesis were then transformed to Fe 3 -MIL-88B in the case of single metal, and to mixed Fe 2 Ni-MIL88B in the case of mixed metal synthesis. In the both cases of Fe 3 -MIL-88B and Fe 2 Ni-MIL-88B, the FeCl 4 − anion is a key feature for the formation of MOF-235. An anion-mediated mechanism for the formation of the MOF-235 structure is also suggested. Experimental section ChemicalsFeCl 3 •6H 2 O (99%) and Fe(NO 3 ) 3 •9H 2 O (98%), 1,4-bezenedicarboxylic acid (bdc, 98%), NaOH (99%), and N,N-dimethylformamide (DMF) were used as purchased.