| ZIF-8, as the typical representative of Zeolitic Imidazolate Frameworks (ZIFs), possesses attractive prospects in practical industrial applications of adsorption and separation, energy storage as well as catalysis fields by virtue of the enormous specific surface area, excellent hydrothermal stability, controllable structure and adjustable pore size.While the newly synthetic crystalline nature of powdery ZIF-8was confronted with some practical problems,such as large fluid resistance due to the limitation of pressure drop, vulnerable to blow away etc.Thus, powdery ZIF-8needs to be shaped to resolve the key bottlenecks prior to industrial application. For this purpose, we focused our study on the effect of the shaping methods on its microstructure and properties by means of XRD, SEM technique, N2adsorption-desorption analysis and compressive strength testing, meanwhile, combined with the five gases static adsorption behavior research at the different temperature on the ZIF-8and shaped ZIF-8.finally, the optimal shaped formula was determined, and the dynamic adsorption performance of CO2on the ZIF-8powders and shaped product were explored by constant pressure method. Based on the solve-thermal reaction, white powder ZIF-8crystals which have a better performance were obtained by batch amplified synthesis method. Then adopted the pressing shaping technology, pelleted ZIF-8particles with the diameter of9mm and the thickness of6mm in different shaping formulas were prepared. Through the analysis of characteristic methods, the BET specific surface areas of the shaped ZIF-8samples can be minimized with the SB powder and talcum powder as the appropriate binder, trichloroacetic acid as the better solvent. Moreover, the BET specific surface area losses of the shaped samples being adopted single binder are smaller than the preparation with the composite binder particles. In addition,the results show that the shaped products have the stable crystal structures, higher compressive strength and the BET specific surface area loss rates was kept less than7%compared to the unshaped ZIF-8when10wt%of single binder and2.5ml/g of solvent in the concentration of10wt%were added.Adsorption isotherms of CO2, CH4, C3H8, C3H4and H2on ZIF-8powder, as well as those on shaped samples (ST-ZIF-8, SC-ZIF-8, SN-ZIF-8and TT-ZIF-8, TC-ZIF-8, TN-ZIF-8) were measured by static volumetric measurement under different pressures and temperatures (298,323and348) K. The adsorption capacities of gases on both ZIF-8powder and shaped Tablets follow the order:CO2>C3H6> C3H8> CH4> H2. Furthermore, whether it is for powdery or shaped ZIF-8, the adsorption equilibrium isotherms can be well described by the Langmuir equation for CO2, CH4, C3H8, C3H6and H2 with the correlation coefficient R2>0.946, and the adsorption amounts on the shaped ST-ZIF-8and TT-ZIF-8tablets are lower, decreasing around5-10%, in comparison with the powdery ZIF-8under the same adsorption condition, thereinto ST-ZIF-8is more optimal shaped sample.Besides, it was found that the temperature has a significant influence on the adsorption capacities of gases, and the higher temperature leads to the lower adsorption capacity.The breakthrough curves of10%CO2with90%N2on powdery ZIF-8and shaped ST-ZIF-8were measured at298K and0.5MPa in order to investigate their dynamic adsorption performance.the results show that compared with the excess adsorption capacities of Qe, the amounts of CO2accumulated in the pore spaces of the ZIF-8or ST-ZIF-8and the packing gap spaces are generally smaller and can be ignored,while that in the system dead space (especially for piping, valves, etc.) is larger and must be eliminated to accurate calculate the excess adsorption quantity. Moreover, the dynamic adsorption rate of the ZIF-8or ST-ZIF-8samples need to be revised with the apparent adsorption amounts adsorbed by unsaturated adsorbent ratio to that of the total samples. Then a constant apparent adsorption rate is obtained by the ratio value multiply the slope of the tangent in C (t)/C=50%of the breakthrough curve. |
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