Study On Synthesis And Acidity Of Ga-MFI Molecular Sieve

Facing the depleting fossil energy,how to improve the utilization rate of fossil energy and maximize their application value has become an important topic of scholars’attention.Molecular sieve is a typical porous inorganic material with huge surface area,regular pore structure and high structural stability,and some application properties can be produced by adjusting the skeleton element composition.In particular,the introduction of group ⅢA elements into the Si-based skeleton of molecular sieves has given the material excellent acid properties,making it an extremely important solid acid catalyst,which is widely used in traditional petrochemical fields.Among them,MFI type molecular sieve is one of the most important molecular sieve catalyst materials,which are widely used in petrochemical,coal chemical and fine chemical industries due to its good thermal and hydrothermal stability and excellent shape selection properties.However,most of the MFI type molecular sieves are mainly used in Br（?）nsted acid catalytic reactions nowadays,and less used as Lewis acid catalysts.Therefore,they cannot be used efficiently in reactions such as aromatization,isomerization,methane conversion,macromolecular cracking,and syngas/biomass conversion.The main idea of this thesis is to introduce Ga element into the MFI molecular sieve backbone to enhance its Lewis acid characteristics while maintaining its acid properties.Molecular sieves are more suitable for Lewis acid-catalyzed reactions.In terms of skeleton construction,although Ga and Al belong to the same ⅢA group elements and have the same valence electron structure,but the ionic radius is larger,and the bond angle range with the molecular sieve skeleton Si elements is more flexible,theoretically Ga elements are easier to enter the molecular sieve skeleton and form a variety of skeleton structures.At the same time,the structural diversity obtained from Ga-doped skeleton is also more likely to lead to orientation growth during the crystallization of molecular sieve and generate MFI molecular sieve with special morphology,so as to obtain special catalytic properties.A series of Ga-MFI zeolites were one-step synthesized following the seed-assisted strategy without using additional organic template.The effects of synthesis parameters on the framework fabrication,morphology evolution,and gallium incorporation were systematically investigated by XRD,N₂-adsorption,UV-Raman,STEM,TEM,solid state NMR,NH₃-TPD,and Py-IR analyses.The Si/Ga seed exhibited a highly efficient structure directing capability in the assembly of MFI framework and the amount of the costly tetrapropylammonium hydroxide（TPA⁺）was thus decreased to less than one-tenth in comparison with the traditional synthesis.Crystallization temperature and pre-aging conditions played the key roles in the Ga-MFI zeolite synthesis.Only with the help of Si/Ga seed,the synthesized Ga-MFI nanocrystal agglomerates possess a highly accessible hierarchical mesopore structure that consisted solely of interparticle-type mesopores.Simultaneously,it also exhibited a remarkable gallium incorporation,which ensured the great acidic properties,including strong acidic strength and high acid amount.As a consequence,the Ga-MFI nanocrystal agglomerates performed the excellent catalytic activity in the LDPE degradation.Both of the morphology and the intraframework gallium concentration can be facilely controlled by adding the mineralizer and surfactant.The addition of NH₄F and CTAB was favorable for the formation of the slabs-form product but against to the gallium substitution.although the entry of Ga into the skeleton is more difficult,the synthesis can be carried out under more demanding thermodynamic conditions,and Ga still exists mainly in the skeleton in the tetra-ligand form,ensuring the products with extremely high crystallinity and strong acid strength.The catalytic activity of the Ga-MFI slaps catalysts for LDPE is slightly lower than that of the Ga-MFI nanocrystal agglomerates catalysts due to the lack of a multistage mesoporous structure.