Synthesis Studies Of ZSM-5 Molecular Sieve And Evaluation Of NH₃-SCR Denitrification Performance

ZSM-5 is a kind of molecular sieve with unique and regular 3D connected pore structure,and its silicon-aluminum ratio can be effectively regulated in the range of 10-3000.Among them,the high silicon ZSM-5 molecular sieves possess not only good hydrothermal stability and suitable acidity,but also excellent hydrophobic oil affinity and adsorption separation characteristics due to the absence of skeleton aluminum atoms.In the environmental catalysis field such as NH₃-SCR denitration reaction,the use of them as the supporter is beneficial to promote the good dispersion of active components and inhibit the catalyst deactivation due to its structure collapse from skeleton dealumination as well.Meanwhile,the encapsulation limiting effect of the pore structure from the molecular sieve can alleviate the toxic effect of H2 O and SO2 on the active sites of the catalyst,and prolong the life of the catalyst.Therefore,high silicon ZSM-5 molecular sieve can be used as an ideal catalyst supporter.The Si O4 tetrahedral framework of the high silicon molecular sieve,however,has a weak binding ability with the aqueous cation and the template cation in the solution during nucleation and crystallization due to the lack of more negative electricity,usually required under more stringent synthesis conditions.Based on this,the effects of synthesis parameters such as silicon source,aluminum source,crystallization time,crystallization temperature,water content and type of template on the crystallinity and surface morphology of ZSM-5 molecular sieves were investigated.At the same time,the ZSM-5 sieves of ellipsoidal morphology with high crystallinity,good stability and better hydrophobic ability were synthesized by increasing the ratio of silicon to aluminum.Moreover,the in situ modification to the selected molecular sieves with polyacrylic acid were carried out to construct anchoring sites having strong adsorption to metal cations in pore channels.Using Cu as the active component,Cu/ZSM-5-PAD catalysts with limited domain dispersion of active sites were prepared by induced deposition strategy for active components and compared with Cu/ZSM-5-CIM catalysts prepared by ordinary impregnation method.The effects of the molecular sieve structure and the way of Cu species introduced on NH₃-SCR catalytic reaction of Cu/ZSM-5 catalysts were investigated.The contents and main conclusions of this manuscript are as follows:?1?Adopting n-butylamine as template,the effect of synthesis conditions on the crystal structure of ZSM-5 molecular sieves with hexagonal sheet-like morphology was investigated by controlling variable method of silicon source,aluminum source,alkalinity,crystallization time and type of template.The results shown that ZSM-5 molecular sieves with good crystallinity can be synthesized from three silicon sources?Xinyuanxiang,silica and silica sol?and two aluminum sources?aluminum sulfate octadecahydrate and sodium metaluminate?.Among them,the ZSM-5 molecular sieves synthesized from silica sol and aluminum sulfate octadecahydrate exhibited smooth surface,uniform particle size distribution and hexagonal plate structure.As proper increasing crystallization time and decreasing of solution alkalinity,the crystallinity of the synthesized ZSM-5 molecular sieve increases,but impurity crystal phase will appear as excessive increase of crystallization time.In addition,ZSM-5 zeolite molecular sieve with hexagonal structure and single/pure phase can be synthesized in the range of template-to-silicon ratio of 0.11⁰.19,and their differences in crystallinity and particle size are small.?2?Using tetrapropyl ammonium bromide as a template,by controlling the synthesis parameters such as crystallization time,crystallization temperature,water content,silicon source and template content,the differences of crystal structures for the synthesized ellipsoidal high silicon ZSM-5 molecular sieve were investigated by means of basic characterization techniques.The results showed that the high silicon ZSM-5 molecular sieve can be obtained by using three silicon sources?Meigao,Baisha River,silica sol?,among which the whole silicon ZSM-5 molecular sieve with silica sol as silicon source has a more regular ellipsoid shape with uniform particle size.The crystallization time and crystallization temperature have great influence on the nucleation and crystallization process of molecular sieve.Prolonging the crystallization time or increasing the crystallization temperature can effectively increase the particle size and crystallinity of the molecular sieve,and also make the surface of the molecular sieve smoother and more regular.As alkali content increases,the crystallinity of the obtained molecular sieves changes from first increase to decrease,and the grain morphology also changes from larger cubes to smaller elliptical spherical twins.Both the water content and the amount of template agent have insignificant effect on the synthesis.The crystallinity of the product is high and the morphology is regular,but the amount of template agent can increase particle size.?3?A series of ZSM-5 molecular sieves with different silicon-aluminum ratios were prepared and their dynamic water adsorption properties were investigated.The results show that the pure phase ZSM-5 molecular sieve can be synthesized in the selected Si/Al ratio range,and there is a reciprocal relationship between the ratio of silica-aluminum to water absorption,that is,the water absorption decreases with the increase of Si/Al ratio.when the Si/Al ratio reaches 576,the hydrophobic ability of the ZSM-5 molecular sieve is very similar to that of pure silicious molecular sieve,indicating a dynamic equilibrium being achieved.?4?The Cu/ZSM-5-PAD catalysts with stable dispersion of Cu based active sites were constructed using polyacrylic acid induced deposition strategy.Compared with the Cu/ZSM-5-CIM catalysts prepared by conventional impregnation method,the Cu/ZSM-5-PAD showed better catalytic performance.This can be attributed to the feature that Cu Ox species of Cu/ZSM-5-PAD are uniformly dispersed in the zeolite channels,making the sufficient exposure of acidic sites on the surface of the catalyst and thus enhancing the redox ability.These are beneficial to the adsorption and activation process for NO,NH₃ and O2,and presumedly promoting the formation and rapid transformation of reaction intermediates,and then enhancing the NH₃-SCR denitrification performance.In addition,owing to the limited-domain encapsulation effect of high-silicon ZSM-5 molecular sieves on Cu Ox species and the excellent hydrophobic ability of the Si O4 tetrahedral framework structure,which inhibit formation of sulfate being toxic to active sites,these characters make the Cu/ZSM-5-PAD to be excellent in hydrothermal stability and water resistance to sulfur.