The Design And Characterization Of Si-modified Alumina As The FCC Matrix

Fluid catalytic cracking?FCC?is one of the most important conversion technologies for petroleum refinery,which is widely used to convert high-boiling and high-molecular weight petroleum hydrocarbons into high-value products such as olefins,gasoline,and diesel.However,as crude oils around the world are becoming heavier,treating light feeds with traditional FCC technology is unprofitable.As is known to all,FCC catalyst plays an important role in this process and the change of its formulation could lead to significant differences in product distribution.Thus,the rational design of effective and efficient FCC catalyst to promote the conversion of inferior and heavy feedstock with various kinetic diameters from 1.0 nm to 10.0nm are always urgent and profitable subjects.Mesoporous alumina with pseudo-boehmite phase was prepared by using the cation–anion double hydrolysis method from mixed aqueous solution of aluminum sulfate and sodium aluminate.The effect of synthetic conditions on the crystal structure and textual properties of pseudo-boehmite was investigated,such as synthetic temperature,p H value and the addition of sodium silicate.With the assistance of characterization techniques,such as X-ray diffraction?XRD?,N₂ adsorption–desorption isotherms?BET?and ²⁷Al magic-angle spinning nuclear magnetic resonance?MAS NMR?,the relationship between textural properties of the mesoporous alumina samples and their synthetic conditions was discussed.The results displayed that an increase in synthesis temperature promoted the formation of higher crystalline pseudo-boehmite with the increase of its surface area and pore volume.Pure pseudo-boehmite phase could be obtained in the pH value range from 6.0 to 9.0,while bayerite phase occurred when the pH value was over 10.0.The introduction of sodium silicate could greatly improve the pore volume(1.20 cm³·g^-1)and surface area(480.2 m²·g^-1)of pseudo-boehmite.Interestingly,pure pseudo-boehmite phase was obtained at very high pH value without formation of bayerite phase when sodium silicate was initially added into the aluminum hydroxide colloid.A strategy for adjusting surface acidity including Bro?nsted acid sites?BAS?and Lewis acid sites?LAS?in silica-modified alumina while maintaining the textural structure of?-Al₂O₃is proposed,with using cation-anion double hydrolysis?CADH?method and water glass as a modifier.Pyridine-IR spectra showed that the ratio of BAS/ALS could be controlled between2.37 and 0.11 by tuning the pH value from 6.0 to 12.0 in initial gel.In combination with ²⁷Al and ²⁹Si MAS NMR,it is concluded that the most strongly acidic LAS are assigned to penta-coordinated Al,which are considered to be located at the interface that connects mixed silica-alumina phase with the domains of alumina-typed phase.However,distinguished from the bridging hydroxyl groups in zeolite,the BAS in our materials arise from surface silanol groups.The weakly acidic hydroxyl groups are isolated surface silanol groups while the strongest BAS are the surface silanol groups with the Al located in the bulk phase.