Preparation And Deep Desulfurization Performance Of Reactive Adsorbents

Due to the increasingly stringent environmental regulations, deep desulfurizationtechnology for gasolines is becoming a more and more important research subject all over theworld. Ni/ZnO reactive adsorbent combines the catalytic hydrodesulfurization activity of Ni0with the adsorption ability of ZnO, therefore it is capable of both removing organic sulfurcompounds like thiophenes deeply and achieving a high sulfur capacity, thus becoming a hotresearch subject in recent years. However, studies on the rective adsorption have just started,and the reactive adsorption mechanisms of Ni/ZnO have not final conclusion.The dissertation aims at studying the reactive adsorption mechanisms of Ni/ZnO and thefactors affecting its activity. Besides, the preparation methods and procedures of the Ni/ZnOadsorbents are also explored, to a goal of obtaining NiO/ZnO composite materials withoutstanding physiochemical properties, in order to significantly improve the desulfurizationperformance of the Ni/ZnO adsorbents.The preparation chemistry of nanosized ZnO was investigated employing methods suchas homogeneous precipitation, homogeneous precipitation-hydrothermal and low-temperaturesolid-state reaction, in which the influence of synthesis conditions on ZnO morphology andphysiochemical properties was investigated in detail, with the formation mechanism of PEGassisted3D-structured ZnO precursor being proposed. It is found that both the hydrothermalcondition and PEG-induced function are the key factors of uniform structured ZnO perared byhomogeneous precipitation-hydrothermal method. Moreover, the ZnO materials prepared vialow-temperature solid-state reaction are uniform in size distribution with good dispersivity,and have large BET surface area and pore volume, being 44.2 m²·g^-1 and 0.17 cm³·g^-1,respectively.Ni/ZnO adsorbents were prepared via incipient impregnation method, and thedesulfurization performances of the Ni/ZnO adsorbents were related to the physiochemicalproperties of the corresponding ZnO matterials. It is found that the Ni/ZnO adsorbentsprepared using ZnO with larger BET surface area and pore volume as active components showhigher desulfurization activity and stability. The reactive adsorption mechanisms of Ni/ZnO were studied by means of investigationof thiophene adsorption action on ZnO, Ni and Ni/ZnO adsorbents. The results indicate thatthe ZnO component itself does not have the capability of removing organic sulfur compoundslike thiophenes via reactive adsorption, meaning that the ZnO does not react directly withorganic sulfur compounds. In fact, the adsorption of thiophene on Ni-based adsorbent belongsto chemisorption essentially, in which the sulfur is immobilized onto adsorbents in the form ofNi₃S₂. The desulfurization activity and stability of Ni/ZnO adsorbents are evidently higherthan that of single component adsorbents, either ZnO-based or Ni-based. Duringdesulfurization process, thiophene is first decomposed on Ni0 surface, and then the produced Sspecies is adsorbed by ZnO, the sulfur is finally immobilized onto adsorbents in the form ofZnS while Ni remaining its reduced state Ni0. The nature of the reactive adsorptiondesulfurization mechanisms of Ni/ZnO adsorbents are a synergistic effect of desulfurizationcatalyzed by Ni0 and chemisorption of S on ZnO, which may be carried out by means ofspillover of S species.The Ni/ZnO materials were synthesized employing methods such as mechanical mixed,mechanical mixed-thermal decomposition and low-temperature solid-state reaction. Theexperimental results show that the higher dispersibility of Ni and ZnO contributes to thehigher desulfurization activity of Ni/ZnO adsorbents. On the basis of optimizating preparationparameters of nanosized ZnO and NiO/ZnO composite materials, low-temperature solid-statereaction was employed to investigate the preparation procedure of NiO/ZnO materials. Theinfluence of the component ratio on the desulfurization performance was also investigated indetail, using model gasoline as feedstock. It is found that the optimum conditions are asfollows: Ni/Zn/Al molar ratio 1:3:1, thermo-decomposing temperature 400 oC and thermodecomposingtime 2 h. Under these conditions, the physiochemical properties anddesulfurization activity of Ni/ZnO-Al adsorbents reach optimum. The addition of Al cansignificantly improve the pore structure of NiO/ZnO materials. The BET surface area and porevolume increase significantly, while the grain sizes of NiO and ZnO decrease and thedispersity improves, which are favorable to the improvement of adsorption performance.The desulfurization performances of the Ni/ZnO-Al adsorbents were evaluated, usingstraight gasoline as feedstock. The optimal process parameters are as follows: reactiontemperature 300 oC, reaction pressure 0.5 MPa, LHSV 4 h-1 and H2/Oil ratio 200/1. Underthese conditions, the adsorbent is able to reduce the sulfur content of gasoline from 109.7μg·g^-1 to 1.0μg·g^-1. The breakthrough sulfur capacity and saturation sulfur capacity are 37.3mg·g^-1 and 213.0 mg·g^-1, respectively. Meanwhile, the utilization rate of activity components is 91.3 %. Therefore, the adsorbent displays a good potential application.