Lewis Acidity Regulation Of Matrix And Its Effects On The Catalytic Cracking Of Light Hydrocarbons

Catalytic cracking（FCC）plays an important role in oil refining and is one of the most important secondary processing technologies.Due to the increasing market demand for basic organic chemicals such as light olefins,to increase the production of light olefins in FCC reaction has received widespread attention.Research and development of efficient FCC catalysts is one of the most effective ways to improve resource utilization and adapt to the transformation of energy structure.At present,most studies on Lewis acid of FCC matrix are indirect studies on the regulation of Br（?）nsted/Lewis acid content ratio and the analysis of its influence on cracking reaction performance.Therefore,the preparation of matrix materials with only surface Lewis acid sites and adjustable Lewis acid properties is the key to further study the mechanism of matrix Lewis acid sites in catalytic cracking reaction.In this thesis,alumina and silicon oxide,the most widely used FCC catalysts,were studied.By heteroatomic doping modification of mesoporous materialsγ-Al₂O₃ and ordered mesoporous materials KIT-6,the Lewis acid properties of the materials were regulated without introducing Br（?）nsted acid sites.The results show that all mesoporous materials did not contain Br（?）nsted acid sites.The loading of boron decreased the Lewis acid content and strength ofγ-Al₂O₃,while the loading of zinc increased the Lewis acid content and strength ofγ-Al₂O₃.The loading of tin and cobalt introduced Lewis acid sites into KIT-6 materials,and the Lewis acid content and strength increased with the increase of atomic load.The loading of tin was more conducive to the formation of strong Lewis acid sites.And then,n-heptane and 1-hexene were used as model compounds for light hydrocarbons catalytic cracking by using a fixed bed microreactor.Whenγ-Al₂O₃ and KIT-6 were used as catalysts,the enhancement of Lewis acidity（increasing acid content and strength）on the catalyst surface reduced the activation energy of cracking reaction between n-heptane and 1-hexene and promoted cracking reaction.Whenγ-Al₂O₃ and KIT-6 were used in combination with ZSM-5 zeolites,the pre-activation of light hydrocarbons by matrix Lewis acid sites promoted the cracking reaction,and the cracking reaction activity of light hydrocarbons increased with the increase of Lewis acidity.In the cracking reaction of n-heptane,the increase of Lewis acid content in the matrix promoted the formation of light olefins,while the increase of Lewis acid strength reduced the selectivity of light olefins.The increase of Lewis acid content inhibited the total hydrogen transfer reactions to a certain extent,while the increase of Lewis acid strength intensified the total hydrogen transfer reactions.Both of them exacerbated the non-selective hydrogen transfer reactions and promoted the formation of carbon deposition.The cracking mechanism of n-heptane is mainly protolytic cracking mechanism.Its proportion in cracking reaction decreased with the increase of Lewis acid content ofγ-Al₂O₃ matrix,increased with the increase of Lewis acid strength ofγ-Al₂O₃ matrix,and increased with the increase of Lewis acid content and strength of KIT-6 matrix.In the cracking reaction of 1-hexene,the change of Lewis acidity of matrix had no significant effect on the 1-hexene conversion.The increase of Lewis acidity ofγ-Al₂O₃ matrix aggravated the total hydrogen transfer reactions and reduced the selectivity of light olefins.The introduction of Lewis acid sites in KIT-6 matrix promoted the formation of light olefins;the increase of Lewis acid content inhibited the total hydrogen transfer reactions and contributed to the formation of light olefins;the increase of Lewis acid strength intensified the total hydrogen transfer reactions and reduced the selectivity of light olefins;the increase of Lewis acidity in each matrix intensified the non-selective hydrogen transfer reactions and promoted the formation of carbon deposition.1-Hexene cracking reaction follows bimolecular cracking mechanism,which can be divided into monomolecular cracking reaction and bimolecular cracking reaction.The introduction of Lewis acid sites in KIT-6 matrix promoted 1-hexene to react in the monomolecular cracking reaction path,but the enhancement of Lewis acidity aggravated the bimolecular cracking reaction.Comparing the catalytic cracking performance ofγ-Al₂O₃ and KIT-6 matrix,takingγ-Al₂O₃ and 7.5Sn/KIT-6 with similar Lewis acid properties as examples and selecting the catalyst loading mode with the matrix in the upper layer,Cat-1_7.5Sn/KIT-6had higher catalytic activity in n-heptane cracking reaction,but lower yield of light olefins.The cracking activity of Cat-1_7.5Sn/KIT-6 and Cat-1_γ-Al2O3 in 1-hexene cracking reaction was little different,but the yield of propylene was higher in Cat-1_7.5Sn/KIT-6.These results indicate thatγ-Al₂O₃ and KIT-6 matrix have great performance in promoting light hydrocarbons cracking and increasing light olefins production.The regulation of matrix Lewis acidity has a significant effect on the cracking path and result of light hydrocarbons.It is of high research value to explore the mechanism of matrix Lewis acid in catalytic cracking reaction.