Acidity Modification Of Mesoporous Alumina Matrix And Its Effect On Catalytic Cracking Performance

Nowadays as the crude oil feedstock becomes increasingly heavier and inferior,the fluid catalytic cracking?FCC?process,an important secondary processing technology for heavy oil,confronts new challenges.In this work,the surface acidity and pore structure of mesoporous alumina matrix were modified to improve the catalytic performance of FCC catalyst.The alumina matrix was modified by two different kinds of modifiers,i.e.the fluorine-containing compounds?NH₄BF₄,?NH₄?₂SiF₆?and silicon-containing agents?zeolite Y structure-directing agent?SDA?,the synthesis mother liquor of zeolite Y,silica sol?using pseudo-boehmite as a raw material.The effect of synthesis conditions on the surface acidity and textual properties of alumina was investigated.Catalytic activity tests were conducted to correlate the relationship between surface acidity and catalytic performance.Results showed that mesoporous alumina with rich Br?nsted acid sites was obtained using NH₄BF₄ and?NH₄?₂SiF₆ as modifiers.However,the modification decreased the crystallinity,and reduced the specific surface area and pore volume of alumina.In order to improve the pore structure of modified alumina,a pore-expanding agent was added and the synthesis mixture was dried without filtration.Thus obtained sample had a pore volume of up to 1.01 cm³/g with a average pore size of 17.3 nm.The ratio of Br?nsted-to-Lewis acid sites was up to 3.0.In addition,the hydrothermally treated SDA of zeolite Y was used to modify the alumina matrix.An alumina matrix with rich Br?nsted acid sites and reduced Lewis acid sites was synthesized under neutral hydrothermal conditions.Modified sample had a good pore structure with a high specific surface area?337 m²/g?and a wide pore size distribution.Similarly,the mesoporous alumina modified by the crystallized synthesis mother liquor of zeolite Y had a high pore volume of 0.82 cm³/g.The pore size distribution was wide with a average pore size of 8¹2 nm.It had a high concentration of surface Br?nsted acid sites?6.02?mol/g?,and a large amount of acid sites with medium strength.In addition,the silica sol was used as a silicon source to modify alumina at different pH values.When prepared at the isoelectric point of SiO₂?pH value of 2³?,the modified alumina had a high ratio of Br?nsted-to-Lewis acid sites of 1.2.At pH value of 10¹1,the synthesized alumina had a large pore volume of 1.00 cm³/g and a pore size of 11.9 nm.The micro-activity test results showed that the yield of gasoline increased by 1.63%,the yield of LPG increased by 1.67%and the total conversion was improved by 4.04%,when the B/L value of matrix material increased from 0 to 0.3.These results indicated that the type and strength distribution of acid sites on alumina matrix play a critical role in the heavy oil conversion and product distributions.The Br?nsted acid sites and medium strength acidity facilitated the cracking of heavy oil macromolecules to improve the conversion,but did not accelerate the deep cracking of low carbon hydrocarbons in gasoline,resulting in an improved gasoline yield.