Synthesis, Modification And Catalytic Performance Of Hierarchically Structured Zeolites

Microporous zeolites are wildly used in petrochemical industry as adsorbents and catalysts owing to their high hydrothermal stability,unique acidity and microporous structures.However,suffering from the sub-nano channel/window sizes of traditional microporous zeolites,molecular diffusions in the microporous structures are highly impeded,especially for bulky molecules which can hardly access the active sites within micropores,leading to greatly deteriorated overall catalytic performance of the catalysts.Hierarchically structured zeolites（HSZ）have been largely developed to solve the problems caused by the microporous systems with traditional zeolites by introducing additional mesoporous channels in the bulky crystals.The additional interconnected mesoporous networks may greatly facilitate the mass transportion and consequently enhanced adsorption and catalytic performances would be achieved.Besides,the large pore volume and specific surface area of HSZ are beneficial to loading other active components to construct multi-functional catalysts.However,the extra-introduced mesoporous channels can also be regarded as“defects”within the zeolite crystals,which may lead to the relatively degraded hydrothermal stability of HSZ as compared to pristine microporous zeolites.In the meantime,the mesoporous structures bring more external surface acid sites to HSZ.Unlike the acid sites located within the micropore structure of zeolites,the acid sites located on external surface shows no shape-selectivity for certain reactions,which may lead to undesired side reactions.Thus,the post-synthesis modification of HSZ for improving and modifying their catalytic performances is of great importance.In this thesis,we focus on the preparation,modification and catalytic applications of HSZ.The main results are as follows:（1）Iron-functionalized HSZ（Fe-HSZ）were synthesized as heterogeneous Fenton catalysts by desilication combined with ion exchange.The HSZ was firstly prepared from ZSM-5 zeolites via desilication in the mixture of NaOH and TPAOH solution.The as-synthesized HSZ has large specific surface area(471 m²g^-1)and pore volume(0.40 cm³g^-1).Then HSZ was modified with iron species through an ion exchange method.HSZ shows much higher iron species loading capacity than ZSM-5zeolites due to its larger pore volume.The introduced mesopore structure endows the obtained Fe-HSZ with greatly enhanced adsorption capability for reactant,and as a result,Fe-HSZ showed excellent catalytic activity in the degradation of acid blue 74（AB74）.In cyclic experiments and leaching tests,Fe-HSZ showed high structural stability and catalytic stability.The excellent catalytic performance of Fe-HSZ and the facile preparation make it a promising catalyst for industrial application.（2）HSZ was synthesized and then modified with element phosphorus（P）by an impregnation method.The P modified HSZ（P-HSZ）showed much enhanced hydrothermal stability as compared to parent HSZ.The main textural properties of P-HSZ kept nearly unchanged after being treated at 750℃for 4 hours in steam.It was found that the P modification could induce the dealumination from tetrahedral framework and consequently eliminate the strong acid sites of HSZ.In the reaction of1-butene cracking,P-HSZ showed elevated yield of propylene and superior anti-coke ability as compared to HSZ.We believe that the elimination of strong acid sites suppresses the hydrogen transfer reactions and formation of coke,thus enhancing the yield of propylene by preventing the consumption of propylene in the undesirable side reactions.Furthermore,P-HSZ was modified with iron（Fe）through an impregnation method.The bi-element modified HSZ（Fe/P-HSZ）showed excellent hydrothermal stability in high temperature steam.In the reaction of 1-butene,Fe/P-HSZ showed markedly higher catalytic activity and propylene selectivity than P-HSZ.It has been found that the introduction of Fe species results in the increased acid site density,which contributes to the enhanced catalytic activity of Fe/P-HSZ.In the meantime the as-synthesized Fe/P-HSZ demonstrates remarkably weakened adsorption of propylene molecules due to the Fe modification,preventing propylene molecules from secondary reaction and thus consequently increasing the propylene selectivity in1-butene cracking.（3）The pore structure and external surface acidity of HSZ were fine-tuned bydealumination in oxalic acid solution and chemical liquid deposition of tetra-ethoxysilane（CLD of TEOS）.It was found that the above post-synthesis treatments adopted here could both selectively passivate the external surface of HSZ.Besides,CLD of TEOS also narrowed the pore-openings of HSZ in addition to passivating its external surface,which is beneficial to enhancing the para-Xylene（p-X）selectivity in the reaction of ortho-xylene（o-X）isomerization,too.The optimized catalyst HSZ（Si0.5）showed significantly higher o-X conversion（⁴2%）and p-X selectivity（⁵0%）than microporous ZSM-5 zeolites（¹3%and⁴5%,respectively）in the reaction of o-X isomerization.