Studies On Adsorption Desulfurization Of Metal-organic Framework Materials

Deep desulfurization of transportation fuels has been receiving more and more attention worldwide due to more stringent environmental regulations and increasing demand for ultra-clean fuels.Hydrogenation of olefin inevitably leads to a significant loss in octane number when hydrodesulfurization technology is employed to desulfurize the gasoline distillate at deeper level?10 ppmw S or less?.To overcome the above limitations,adsorption desulfurization has been extensively investigated in recent years,due to the advantages such as moderate operating conditions and maintenance of the quality of gasoline distillate.Some metal-organic framework materials?MOFs?have been widely studied in adsorption desulfurization due to the extra-high specific surface areas and high porosity.This work presented the synthesis of five kinds of porous MOFs and their adsorption desulfurization performances were studied.This dissertation firstly studied thiophene adsorption performance by batch method ofNi₃?BTC?₂,which displays isostructure as HKUST-1 but with different central metal.The results showed that Ni₃?BTC?₂ displayed a slightly higher TP adsorption capacity than HKUST-1 in model gasoline?MG?without competitors.However,in MGs containing competitors,toluene led to a more evident decline in sulfur capacity of Ni₃?BTC?₂ than cyclohexene.The adsorption isotherms of TP,cyclohexene and toluene fit Langmuir model,S-type model and Temkin model well,respectively,indicating that the adsorption mechanisms of TP and the two competitors are different from one another.The enthalpy of TP adsorption on Ni₃?BTC?₂ was estimated to be-80.01±0.86 k J/mol by Van't Hoff equation.Since no new sulfides were detected in the desulfurized MGs by GC-FPD measurement,TP adsorption was viewed as a physical adsorption process.Ni₃?BTC?₂ is sensitive to the air,leading to its poor reusability in batch experiment.However,the desulfurization performance of Ni3?BTC?2 only decreased slightly after 5 cycles when Ni₃?BTC?₂ was regenerated in fixed-bed reactor by N₂ flow without contacting the air.To investigate the effect of the second type of metal on the structure and desulfurizationperformance of HKUST-1,bimetallic MOFs?Cu/Ni-BTC?were synthesized and it displayed good stability in air.The results showed that the ratio of the two metal salts in the feed evidently affected the purity of the products.When n_Cu:n_Ni<1 in the feed,two kinds of crystals with different structure and morphology were obtained.On the other hand,a pure phase crystal Cu/Ni-BTC with the same structure as HKUST-1 was successfully synthesized when n_Cu:n_Ni>1in the feed.In this case,some parameters including the dissolving mode of the precursors,the feed ratio and the heating rate had effect on the amount of Ni²⁺in Cu/Ni-BTC,but had no effect on the structure and morphology.The adsorption desulfurization in fixed bed showed that the breakthrough volume of the effluent firstly increased then decreased as the doping amount of Ni increased.The maximum breakthrough volume of the effluent was measured to be 127 mL/g for Cu/Ni-BTC with n_Cu:n_Ni ratio of 136,showing an increase by 34%compared with HKUST-1.H₃BTB?1,3,5-Tri?4-carboxyphenyl?benzene?,the ligand with larger size wasemployed to synthesize Cu₃?BTB?₂,its activation condition was optimized and then its adsorption desulfurization was studied.Cu₃?BTB?₂ displayed a sulfur removal of 72%and a lower sulfur capacity of 0.82 mg S/g due to the interweave of the ligands.In addition,two kinds of MOFs synthesized by single metal and bi-ligands with paddle-wheel structure,Ni₂?BDC?₂?DABCO?and?BDC?₂?BPY?,were preliminary studied in adsorption desulfurization.Their poor desulfurization performances could be ascribed to the flexible skeleton?the former one?and the interweave of the ligands?the later one?,the decreased pore size led to the difficult access of TP and other sulfides to the channel and therefore a poor desulfurization performance.