One-pot Synthesis of Porous Coordination Polymers

Generally, the properties of crystals are strongly determined not only by their chemical composition, but also by their morphology, size and surface structure.^1-3 Over the past decade, crystals with specific crystal facets have attracted great research interest because the exposed facets on the crystals finely affect the chemical and physical properties of the functional materials.^4-6 For example, noble-metallic nanocrystals having high-index facets generally show much higher plasmonic and catalytic activity than that of nanocrystals with low-energy facets.^7-9 Therefore, the design and synthesis of crystals with specific crystal facets is desired, but is still a challenge.

Porous coordination polymers (PCPs) or metal-organic frameworks (MOFs), which are considered as a unique class of inorganic–organic hybrid materials with metal centers and organic linkers,¹⁰ have received growing attention in recent years because of their tailored pore structure, high surface area and many exceptional properties such as gas adsorption and storage,^11,12 drug release,^13,14 sensing,^15,16 membranes,^17,18 and catalysis.^19,20 To date, many research efforts have been focused on the design, synthesis and applications of bulk MOFs materials; however, nanosized MOFs (NMOFs) materials have become increasingly interesting in the last decade because their microstructure, morphology and size also determine the properties and applications of NMOFs materials.^21-22 Likewise, aspect ratio and exposed crystal facets of NMOFs crystals play an important role on the properties of materials such as sorption properties, and oriented film formation as well.^23-25 Therefore, controllable synthesis of well-defined NMOF crystals with specific exposed crystal facets in the nano/micrometer range is highly desirable. However, it is still difficult to design a universal synthetic method to achieve above target due to the variety in MOFs structures and various experimental parameters affecting the crystal shape and size.

To date, several synthetic strategies have been employed by research groups for the synthesis of NMOFs such as room-temperature precipitation,^26-27 hydro/solvothermal techniques,^22,28 microwave-assisted synthesis,^29,30 sonochemical synthesis^31,32 and electrochemical synthesis method.³³ Among them, room-temperature precipitation, in which the solutions of metal salts and organic linkers are mixed, is the simplest system to produce NMOFs. Compared with other methods, solution-based precipitation syntheses at room temperature possess some advantages, such as safe reaction process, easily controllable reaction conditions, low energy consumption, and short reaction time. On the other hand, the use of various additives is an important synthesis approach for the fabrication of NMOFs materials with tunable morphology and size because the additives usually suppress nucleation and crystal growth during the reaction process. Monocarboxylic acids and their salts have been widely used as the additives or capping agents for the preparation of NMOFs which can alter the coordination equilibrium at the crystal surface during the nucleation and growth process.^29,30,34 Kitagawa and coworkers have used the combination of the coordination-modulation method and microwave synthesis to control the size and morphology of HKUST-1 in which dodecanoic acid was employed as the coordiantion modulator.^29,30 Huber and Fischer have yielded stable and size-selected MOF-5 colloids by adding p-perfluoroethylbenzoic acid as capping reagent.³⁴ Besides, N-heterocycles and alkylamines also are used as additives in the synthesis of MOFs and ZIFs. Oh et al. reported that pyridine was added in the solvothermal synthesis of In-MIL-53 to manipulate the morphology of the microsized crystals.³⁵ Huber and Wiebcke reported a rapid room-temperature production of ZIF-8 crystals with different sizes by employing an excess of auxiliary ligands such as 1-methylimidazole and n-Butylamine.³⁶ Another kind of additives including surfactants and polymers also has been used to control morphology and size of NMOFs which could suppress the growth of specific crystal facets. Our previously reported HKUST-1 nanocrystals with controllable size and morphology from nanocube to microoctahedron have been readily synthesized at room temperature by adjusting the concentration of CTAB (CTAB = cetyltrimethylammonium bromide).²⁶ Zeng and Eddaoudi reported highly monodisperse M(III)-soc-MOFs with a morphological evolution from simple cubes to complex octadecahedra have been achieved using a series of surfactants and structure-directing agents.³⁷ Porous coordination polymer [Cu₂(PZDC)₂(PYZ)] crystals (PZDC = pyrazine-2,3-dicarboxylate, PYZ = pyrazine) with tunable size were prepared in a predictable manner via addition of organic polymer Poly(vinylsulfonic acid, sodium salt) by Kitagawa’s group.³⁸ However, to the best of our knowledge, the study on other types of additives such as acids or bases allowing control the morphology of NMOFs, especially inducing the crystal morphological evolution of polyhedron, has not been realized so far. Although the role of the pH of the reaction medium, which is to deprotonate the ligands, accelerate nucleation rate and consequently cause smaller size of particles has been studied systematically,³⁹ the detailed studies on the morphological evolution of crystal induced by acid−base environment of the reaction medium are scarce.

Herein in this work, we develop a simple and straightforward method for the one-pot synthesis of porous coordination polymers with controllable shape by using different bases (sodium acetate, aqueous ammonia, triethylamine and NaOH) as deprotonation reagents in the ethanol-water mixture at room temperature. We specifically selected porous coordination polymer MOF-14 ([Cu₃(BTB)₂], H₃BTB = 4,4′,4”-benzene-1,3,5-triyl-tribenzoic acid) as candidate material for the investigations which possesses a dicopper paddle wheel secondary building unit that exhibits unsaturated metal centers upon activation and has received great attention due to its high porosity and large specific surface areas.⁴⁰ Its benzenetribenzoate-based analogue HKUST-1 ([Cu₃(BTC)₂], H₃BTC = 1,3,5-benzenetricarbocylic acid) also was studied. In this paper, a morphological evolution of MOF-14 from rhombic dodecahedron to truncated rhombic dodecahedron, cube with truncated edges and to cube was achieved by increasing the concentration of sodium acetate, likewise, a morphological transformation of HKUST-1 from octahedron to cuboctahedron and finally to cube was obtained in the similar process. We further investigated the proposed crystal formation mechanism and found that the higher-energy surface of crystals would appear when more amount of bases were added, indicating that the exposed surface facets of porous coordination polymers could be controlled by acid−base environment of the reaction medium. The gas adsorption properties of MOF-14 with different shapes were also studied and it reveal that properties of MOF-14 display a strong dependence on the facets exposed on the surface.