Synthesis And Properties Of Clay-based Nano-H-ZSM-5 Zeolite

Nano-ZSM-5 zeolites have shorter diffusion pathways and higher catalytic site accessibility,leading to longer catalytic lifetimes and good product selectivity than conventional ZSM-5 zeolites,which have attracted much attention.However,the nanoH-ZSM-5 zeolite is obtained in the hydrothermal environment by strict control of the nucleation and growth behavior of the synthetic material.The complexity of the synthesis process and the synthesis system of this strategy leads to great challenges in the preparation of green and sustainable synthesis of nano-H-ZSM-5 zeolite single crystals and the in-depth research of its crystallization mechanism.For scientific research and industry products,it is necessary to develop a facile and green sustainable route for the synthesis of nano-H-ZSM-5 zeolite single crystals with high crystallinity and defect-free crystals.Recently,the direct preparation of nano-H-ZSM-5 zeolite via the "silica-alumina homologation" of depolymerized natural clay has attracted much attention since the use of industrial chemical silica-alumina products and homogenization steps can be avoided,thereby rendering this approach a green and sustainable route.The key to the industrialization of this synthetic route is to overcome the high energy consumption and pollution problems in the process of depolymerization of natural silica-alumina clay into highly active silica and aluminum species and to precisely regulate the physical and chemical properties of the synthesized products(such as the size of the synthesized crystals)in a greener route.The literature shows that there is a certain structure-activity relationship between the highly reactive silica and aluminum species formed by the depolymerization of natural silica-alumina clay and the ZSM-5 zeolite crystals which are prepared from them in terms of crystal morphology,pore structure,and acid properties compared with chemical raw materials i.e.,the "genetic effect" of clay-based ZSM-5 zeolite compared.If the crystallization mechanism of clay-based nano-H-ZSM5 zeolite can be investigated by building a simple synthesis system,it will certainly reveal and elucidate the "genetic effect" at the molecular level,thus providing data for the research of "genetic engineering of zeolite materials".To achieve the above research objectives,a series of works have been carried out in this study,and the detailed results are as follows:Firstly,a green and sustainable minimalist synthesis system was developed to directly synthesize highly homogeneous nano-H-ZSM-5 zeolite with high silicaalumina ratios based on a solid-phase-like synthesis system by using alkali-meltingacid-washing treated illite clay as the synthetic silica and aluminum source with a very small amount of TPAOH.The chemical activity of the leached illite(ISR)was analyzed by crystallization kinetics and the effect of the amount of TPAOH used on the crystallinity,and morphology of the synthesized samples was carefully investigated.In addition,the crystallization behavior of the synthesized products was also carefully studied.The catalytic experiments for methanol to aromatics(MTA)showed that the synthesized clay-based nano-H-ZSM-5 zeolite(～300 nm),with higher specific surface area and uniformly distributed acidic sites,was superior in BTX product selectivity(benzene,toluene,xylene)and catalytic lifetime compared with commercial ZSM-5 zeolite catalysts.This strategy not only improves the yield of nano-ZSM-5 zeolite,but also completely avoids the ion-exchange process step with ammonium salts in the conventional nano-ZSM-5 zeolite synthesis process,which greatly reduces the synthesis cost of nano-ZSM-5 zeolite catalysts with high catalytic performance.Secondly,to avoid energy consumption and pollution problems in the depolymerization of natural silica-alumina clays.A green and low-energy method for depolymerization of natural silica and aluminous clays by acid vapor treatment in a solid-phase-like system is proposed,and based on the "ternary green synthesis system",the active silica and aluminous species produced by depolymerization of illite are used as raw materials to prepared highly crystalline,monodisperse nano-H-ZSM-5 zeolite catalysts with monolithic full skeleton hierarchical structure with a very small amount of TPAOH.The experimental results showed that the nucleation and growth behavior of the clay-based ZSM-5 zeolite could be decoupled by regulating the aging time,and the grain size of the synthesized ZSM-5 zeolite could be precisely controlled(56-176 nm).The effect of the synthesis conditions(such ais aging time and amount of TPAOH)on the physical and chemical properties of the synthesized nano-H-ZSM-5 zeolite single crystal was discussed in detail.The catalytic behavior of the synthesized nanoH-ZSM-5 single-crystal catalyst was evaluated using methanol to propylene(MTP)as the reaction probe.The experimental results show that compared with commercial ZSM-5 zeolite catalyst,the synthesized nano-H-ZSM-5 zeolite single crystal shows higher catalytic life and selectivity of propylene products(154 h,50.97%;56h,32.83%)due to its highly penetrating pore structure,uniformly distributed acid sites,uniform nano crystal size and full skeleton monolithic hierarchical pore structure.Finally,the "top-down" depolymerization behavior of natural illite clay was analyzed and summarized in terms of crystal structure,morphology,elemental composition,and the chemical environment of Si and Al elements by carefully characterizing the active silica-alumina species produced by depolymerizing natural illite at different depolymerization times.Then,the chemical activity of silica-alumina species with natural clay topology obtained by acid steam treatment of natural clays was calculated by using the crystallization kinetics of zeolite.It was found that the solid silica-alumina species obtained by acid vapor depolymerization of natural clays have similar high chemical activity as chemical silica-alumina chemical products and natural clays depolymerized by conventional thermal activation-acid/alkali treatment.Furthermore,based on the characterization of synthetic samples obtained at different crystallization times,an in-situ reconstitution mechanism based on the interaction of natural clay topology with organic templating agents(TPAOH)is proposed to elucidate the crystallization mechanism of clay-based nano-H-ZSM-5 zeolite single crystals and to reveal the relationship between clay-based active silica-alumina species and claybased nano-H-ZSM-5 zeolite single crystals in terms of crystal structure transformation,elemental composition,etc.The "genetic effect" between clay-based reactive silicaalumina species and clay-based H-ZSM-5 zeolite single crystals is revealed.Finally,the generalizability of the synthesis strategy is extended to provide experimental data to enrich the "material genetic engineering" of clay-based nano-H-ZSM-5 zeolites.