Studies On Synthesis Of MOF And Its Adsorption Performance For Normal Paraffins

Due to the larger surface area and pore volume,better controllability and stability of framework structures than the traditional porous materials,zeolitic imidazolate frameworks(ZIF)show their potential applications in adsorption separation process.Current work was focused on the preparation of ZIF-8 by using the solvothermal method.The optimal synthesis conditions were obtained and the adsorption performance for normal paraffins was evaluated.Current study provided the theory basis to the application of metal organic frameworks(MOF)in the adsorption separation of normal paraffins in naphtha for improving the separation efficiency.At first,ZIF-8 was synthesized through a solvothermal process and characterized using a variety of techniques.The optimal synthesis conditions were observed as follows:the ratio of raw materials of n(Zn(N03)2·6H20:n(C4H6N2):n(HCOONa):n(MeOH)= 2:1:2.63:29.14,crystallization temperature of 90℃,crystallization time of 15 h,agitating rate of 800 r/min.Under optimal conditions,it was indicated that synthesized ZIF-8 product exhibited the sodalite crystal structure,the yield was 75.5%and the average crystal diameter was of 1.58μm.At 298 K,the static saturated adsorption capacity of n-hexane onto synthesized ZIF-8 was 225.1 mg/g,which not only was over 85%larger than that on 5A molecular sieves but also had superior adsorption capacity and adsorption rate to the commercial ZIF-8 product from BASF.Subsequently,the adsorption equilibrium behavior and dynamic characteristics of the main normal paraffins contained in naphtha onto synthesized ZIF-8 were studied.It was indicated that the Langmuir-Freundlich adsorption model can explain the adsorption isotherms of normal paraffins onto synthesized ZIF-8 very well.As the temperature increases from 283 to 313 K,the adsorption enthalpies of n-hexane,n-heptane and n-octane onto synthesized ZIF-8 were-27.90 kJ/mol,-28.00 kJ/mol and-28.70 kJ/mol,respectively.The free enthalpy changes were(-5.81～-3.46)kJ/mol,(-6.15～-3.82)kJ/mol and(-6.46～-4.08)kJ/mol,respectively.And the entropy changes were-77.56(J/(mol·K)),-76.67(J/(mol·K)),and-78.03(J/(mol·K)),respectively.The adsorption isotherms obtained from molecular simulation calculation were found in good agreement with the experimental results.At the temperature ranging from 283 to 313 K,the diffusion coefficient for n-hexane,n-heptane and n-octane adsorption onto synthesized ZIF-8 were(3.38～5.33)×10-12 cm2/s,(2.02～3.30)×10-12 cm2/s and(1.21～2.09)×10-12 cm2/s,and diffusion activation energies were 11.12 kJ/mol,12.11 kJ/mol and 13.41 kJ/mol,respectively.The results of dynamic adsorption experiments using model oil as the raw material indicated an increase in Thomas adsorption rate constant and a slight decrease in dynamic adsorption capacity with the carbon number of adsorbates increasing.Finally,ZIF-8 was formed into pellet product and the adsorption performance of normal paraffins onto shaped ZIF-8 was studied.It was found that shaped ZIF-8 product possessed great mechanical strength of 17 N/cm and adsorption capacities of 209.7 mg/g for n-hexane,207.6 mg/g for n-heptane and 204.3 mg/g for n-octane,respectively.The shaped ZIF-8 showed a constant static saturated adsorption capacity for total normal paraffins of around 205 mg/g as the number of adsorption/desorption loops goes up,indicating a good regenerability.Through the adsorption process based on ZIF-8 adsorbent,normal paraffins can be effectively separated from naphtha.Furthermore,the potential aromatic content of the raffinate oil was 11 percentage points higher than that of naphtha,suggesting that ZIF-8 had the equivalent adsorption separation performance but greater adsorption capacity and adsorption rate as compared with 5A molecular sieve.