Synthesis And Catalytic Performance Of Hierarchical Beta And ZSM-5 Zeolites

Zeolites are important crystalline silicate and aluminosilicate materials,which are widely applied in many modern industrial processes related with heterogeneous catalysts,adsorption and separation etc.,due to great hydrothermal stability,strong intrinsic acidity,shape selectivity and ordered microporous structure.Beta?*BEA topology?with three-dimensional twelve-membered ring channel structure and ZSM-5?MFI topology?zeolites with two-dimensional ten-membered ring cross channel are two important industrial catalysts,which are widely used in hydrogen cracking,isomerization,aromatization and other petrochemical processes.However,traditional microporous zeolites?pore size<2 nm?will lead to limited diffusion of reactant/product molecules,which brings many problems,such as:reduced reaction rate,inactivation of reactive sites,especially reactions involving large molecules.The hierarchical zeolite is a porous material with at least two pore sizes of micropores,mesopores and macropores,which can greatly reduce the resistance of mass transfer and diffusion,reduce the rate of carbon deposition and promote the access of large molecules to the active site,while maintaining the acidity and crystallinity of microporous zeolite.Hierarchical zeolite has good application prospects in the field of catalytic reactions involving macromolecules.Some hierarchical zeolites with special morphological structures?such as hollow and nanosheets?have better application prospects because of their unique advantages.The catalytic reaction in ordinary hierarchical zeolite crystal reduces the utilization rate of the catalyst to a certain extent due to the diffusion limitation inside the crystals.The hollow-structured hierarchical zeolite with large inner cavity space and thin meso/micropore wall can significantly promote the rapid mass transfer of reactants and products,and improve the utilization rate and catalytic performance of the catalyst.However,the synthesis steps of the hierarchical zeolite with hollow structure had been reported to be complicated.The layer-by-layer assembly method needed to first charge the surface of the template to make the surface of the template positively charged,and then adsorbed the prepared suspension layer by layer on the template.In response to the above problems,this paper carried out the following work:Hierarchical Beta zeolite with hollow structure was synthesized by using template via one-step hydrothermal.The structure-directing ability of N₂-p-N₂ was studied by the DFT calculations.With the help of X-ray diffraction,Fourier transform infrared spectroscopy,nitrogen adsorption-desorption isotherm,scanning electron microscope and transmission electron microscope,the synthesized Beta samples were characterized.The results showed that the positively charged quaternary ammonium groups could interact with anionic aluminosilicate materials to induce the formation of Beta zeolite.The synthesized zeolites were hierarchical zeolite with hollow-structured.By adjusting the crystallization time,a series of hollow zeolites HHM-Beta?n??n=0,12,24,36,48,120 h?were synthesized.By studying the crystallization process,a possible synthesis mechanism was proposed.The formation of the hollow hierarchical Beta zeolite followed the"surface to core"crystallization process.Based on the sample HHM-Beta?120 h?,the effects of different template dosages,different alkalinity and different Si/Al ratios on the morphology and pore property parameters of the hierarchical zeolite with hollow structure were studied.Hollow-structured hierarchical zeolite with large inner cavity space and thinner meso/micropore wall can significantly promote the rapid mass transfer of reactant/product and the accessibility to the active site.Results of FTIR spectra of adsorbed pyridine on H⁺form of HHM-Beta?120 h?showed that the HHM-Beta?120 h?sample had more external Br?nsted acid sites than the traditional microporous zeolite.The HHM-Beta?120 h?sample showed excellent catalytic performance in the benzylation of macromolecular 1,3,5-mesitylene with benzyl alcohol.The conversion rate of benzyl alcohol was close to 100%at reaction 5 h.Compared with ordinary hierarchical zeolite,nanosheets zeolite have a larger external surface area,can provide more active sites,have a thin thickness,and have a short mass transfer path,which is conducive to the proximity of the reactants to the active sites,and can reduce the diffusion resistance,the formation of coke,prolong the service life of the catalyst.If we encapsulate the metal in the layered zeolie matrix,the metal active component is not easy to sinter and leaching,and has good dispersion.Methods for loading metal on a layered zeolite include solid or liquid ion exchange,chemical vapor deposition,and wet impregnation.These above methods have some problems,such as poor repeatability,complicated preparation process,and the formation of large particles oxidation during calcination,which is not conducive to industrial application.In response to the above problems,this paper carried out the following work:A series of Fe-encapsulated hierarchical ZSM-5 zeolite with nano-sheet assembly structure with different Fe contents?0.33,0.57,0.94 wt%?were successfully prepared using tetrabutylphosphonium hydroxide as the template agent and ferric citrate as the Fe source.The results of XRD and EDS element distribution showed that Fe species had good dispersion in the zeolite matrix and no large iron oxides are present.UV-Vis and EPR characterization results indicated that the Fe species mainly existed in the form of catalytic performance in a wide temperature range.The NO_x removal rate was>90%in 250?425?.The DFT calculation results showed that capturing NH₃ was the rate-determining step in the whole NH₃-SCR reaction process.The Br?nsted acid site can promote the adsorption of NH₃,and the synergy between the Fe active species and the zeolite framework made the Fe@ZSM-5 zeolite with a high Br?nsted acid concentration have excellent NH₃-SCR performance.In addition,the Fe@ZSM-5 zeolite also showed high resistance to SO₂ and H₂O due to the good dispersion of the Fe species.The DFT calculations confirmed that the interaction between the Fe@ZSM-5 zeolite catalyst and NH₃ was greater than H₂O and less than SO₂,which means that the catalyst deactivation caused by H₂O was reversible,and the SO₂ deactivation was irreversible to some extent.