The Synthesis And Scr Performance Of Hierarchical SSZ-13 Zeolite

Hierarchical zeolites are widely used in catalytic reactions due to the advantages of multiple pore structures,low mass transfer resistance and high transfer rates.Recently,the small-pore SSZ-13 is an effective commercial catalyst for NH₃ selective catalytic reduction（NH₃-SCR）,but suffers from the high diffusion resistance and the excessive oxidation of NH₃at high temperature.Therefore,the hierarchical SSZ-13 has gradually attracted people’s attention,and it is crucial to determine the role and function of the mesopore of porous zeolite catalysis for NH₃-SCR.In this paper,a pore-controllable hierarchical SSZ-13 was assembled using dual-template combining N,N,N-trimethyl-1-adamantammonium hydroxide（TMAda OH）as the structure-directing agent with C₁₆H₃₃-[N⁺-methylpiperdine](C₁₆MP)as the mesoporogen.The crystallization process was studied and it was found that the introduction of C₁₆MP leads to the transition of morphology of SSZ-13 from layered stacking to blocky stacking since the arrangement of organic and inorganic species in the crystallization gel.During induction and early crystallization stage,amorphous porous material dissolves and recrystallizes into CHA nucleus,followed by partial mesopores to micropores transformation.The Horvath-Kawazoe（HK）and Barrett-Joyner-Halend（BJH）pore size distribution curves indicate that C₁₆MP may induce SSZ-13 to form smaller intracrystal mesopores of about 2 nm first,and 0.38 nm CHA characteristic micropores gradually form at 36 h.At 48 h,the intracrystal mesopores gradually grow to 3 nm.Different dimensions and sizes of mesopores are created by controlling the ratio of dual-template TMAda OH/C₁₆MP.For SSZ-13（15:5）,it not only has the CHA microporous structure,but also includes 3 nm intracrystal mesopores and intercrytsal mesopores around 10 nm.With the increase of C₁₆MP,the content of 4Si species in the hierarchical SSZ-13 increases while the Si/Al ratio is maintained unchanged,suggesting that aluminum distribution may be regulated by the mesoporogen.The hierarchical pore Cu-SSZ-13 catalyst was studied for NH₃-SCR and it was found that at low temperature,Cu-SSZ-13（20:0）exhibits better NH₃-SCR catalytic performance due to the confined NO activation of the micropores.In contrast,the activity of hierarchical Cu-SSZ-13（15:5）at high temperature decreases only 8%,which is half（16%）of the microporous catalyst,mainly because the presence of mesopores inhibited the occurrence of non-selective ammonia oxidation side reactions.At a high space velocity of 200000 h^-1,the micro-mesoporous Cu-SSZ-13（15:5）has the high catalytic activity,and it can maintain more than 90%NO_x conversion within the temperature window of 300-510°C.Under the severe condition of hydrothermal aging at 800°C for 8 h,Cu-SSZ-13（15:5）can still maintain more than 90%catalytic activity in the range of 270-540°C.In order to further analyze the reason for the excellent activity and hydrothermal stability of hierarchical Cu-SSZ-13,the properties of copper active sites and acid sites in the catalyst were studied.Compared with Cu-SSZ-13（20:0）,Cu-SSZ-13（15:5）has a higher Cu²⁺/[Cu（OH）]⁺ratio and fewer Cu O_xclusters.The interaction between micropore and mesopore improves the adsorpbility of NO and NH₃.The distribution of Bronsted acid sites（B acid sites）and Lewis acid sites（L acid sites）in mesopores are relatively wide,including weak acid,medium strong acid,and strong acid.In addition,mesopores increase the total amount of acid in the catalyst,but the distributions of B acid and L acid in the mesopores are essentially similar for different micro-mesoporous catalysts.