Synthesis , structure and selective adsorption property of Zn metal-organic framework

Zn metal-organic framework material [Zn2(tib)(HBTB)2(H2O)]·2H2O (1) [tib = 1,3,5-tris(1imidazolyl)benzene, H3BTB = 4,4′,4′′-benzene-1,3,5-triyl-tribenzoic acid] was synthesized and characterized by Thermogravimetric analyses (TGA) , Powder X-ray diffraction (PXRD) analyses and Bruker D8 Advance X-ray diffractometer. The results showed that 1 was a 2D network to be joined together by hydrogen bonds to generate a 3D supramolecular structure. Gas, vapor adsorption behavior of the material was studied. It is meaningful that 1 can selective sorption of CO2 and MeOH.


Introduction
The rational design and synthesis of functional metalorganic frameworks (MOFs) materials have caused remarkable attention in the past decades due to their rich and varied structures as well as potential application, such as gas storage and separation, ion exchange, heterogeneous catalysis, drug delivery and luminescence [1][2][3][4][5][6][7][8][9][10].However, It still remains challenging by controlsynthesizing and predicting structures of the functional MOFs because of many factors which can affect the structure and property of MOFs [11,12].Among these factors, the selection of organic linkers is very crucial for formation of MOFs with definite framework structure and desired property.Therefore, proper organic bridging linkers are significant in building the desired MOFs materials.To our knowledge, 4,4′,4″-benzene-1,3,5-triyltribenzoic acid (H 3 BTB) may act as a good bridging ligand due to its various coordination modes and rigid multicarboxylic groups [13][14][15][16].

Materials and methods
All commercially available chemicals and solvents are of reagent grade and were used as received without further purification.Ligand tib was prepared according to the procedures reported previously [23].Elemental analyses for C, H and N were performed on a FLASH EA 1112 elemental analyzer.Thermogravimetric analyses (TGA) were carried out on a Mettler-Toledo (TGA/DSC1) thermal analyzer under nitrogen with a heating rate of 10 °C min -1 .FT-IR spectra were recorded in the range of 400-4000 cm -1 on a BRUKER TENSOR 27 spectrophotometer using KBr pellets.Powder X-ray diffraction (PXRD) analyses were performed on a Bruker D8 Advance X-ray diffractometer with Cu Kα (λ = 1.5418Å) radiation.Sorption experiments were carried out on a Belsorp-max volumetric gas sorption instrument.

X-ray crystallography
Crystallographic data of 1 was collected on a Bruker Smart Apex II CCD area-detector diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) using the ω-scan technique.The diffraction data were integrated using the SAINT program [24], which was also used for the intensity corrections for the Lorentz and polarization effects.Semi-empirical absorption correction was applied using the SADABS program [25].The structures were solved by direct methods using SHELXS-2014 and all the non-hydrogen atoms were refined anisotropically on F 2 by the full-matrix leastsquares technique with the SHELXL-2014 [26,27].All the hydrogen atoms, except for those of water molecules, were generated geometrically and refined isotropically using the riding model.The hydrogen atoms of the coordinated water molecules were found from the Fourier map directly, while those of free water molecules were not found.

Gas and vapor adsorption properties.
The porosity and high stability of the frameworks prompted us to examine their gas/vapor sorption behavior.The sorption performances of the activated samples 1′ for N 2 at 77 K, CO 2 at 195 K, CH 3 OH and CH 3 CH 2 OH at 298 K were discussed here.The activated sample 1′ was got by directly heating the fresh assynthesized samples at 160 o C for 10 h under high vacuum.The PXRD pattern of activated samples implied their good crystallinity (Fig. 5).The results showed that 1 had good stability and the desolvated samples 1′ maintained the framework structures without destroying.As shown in Fig. 6 and Fig. 7, the sorption isotherms for 1' suggest that almost no adsorption of N 2 at 77 K and CH 3 CH 2 OH at 298 K.The final value of CO 2 adsorption at 195 K is 32.14 cm 3 •g -1 at P = 0.99 atm corresponding to about 1.9 CO 2 molecules per formula unit for 1'.As for CH 3 OH adsorption of 1' at 298 K (Fig. 7), the final values of 47.3 cm 3 •g -1 (71.8 mg•g -1 ) at P = 0.99 atm are corresponding to 2.8 CH 3 OH molecules per formula unit for 1'.The hysteresis and incomplete desorption suggests the strong interactions between the adsorbate and adsorbent [28], which includes the hydrogen bond interactions between the guest molecules and framework, as well as the guest molecules and the guest molecules.Given all the above analysis results, we can see that complex 1 can select adsorption of CO 2 molecules.

Conclusions
In conclusion, one new Zn (II)-based MOF with mixed ligands had been synthesized.By using tricarboxylate as the auxiliary ligand, we had obtianed one 2D network.Furthermore, gas, vapor adsorption behavior of 1 was investigated and the results indicated that the frameworks showed the ability to selectively adsorb CO 2 and MeOH, suggesting a possible application in selective gas/vapour adsorption and separation.

Fig. 1 .
Fig. 1.Coordination environment of Zn(II) in 1. Hydrogen atoms and free solvent molecules are omitted for clarity.