Highly Efficient And Controllable Engineering Of ZSM-5 Zeolite Onto SS-fiber Felt And Structure-performance Relationship In The MTP Reaction

The methanol-to-propylene?MTP?reaction process has attracted intensive attention because it can adapt alternative carbon sources for the olefins production.It is well-known that the practical use of ZSM-5 pellet catalyst in a fixed bed reactor faces severe problems like poor mass/heat transfer,high pressure drop,and even adverse effects of the used binders.Microstructured design can remarkably improve the hydrodynamics and enhance the heat/mass transfer of zeolite catalyst,which has become an important research aspect of zeolite.Our previous work has reported a stainless-steel-fiber structured ZSM-5 catalyst prepared via hydrothermal synthesis method,which demonstrated dramatic selectivity and stability improvement in the MTP process.The kinetic and modeling study results show that ZSM-5/SS-fiber shows higher diffusion efficiency and narrower residence time distribution,not only promoting the propylene formation but also improving the utilization efficiency of the structured ZSM-5.Therefore,based on the“Top-Down”preparation strategy,a microstructured ZSM-5/SS-fiber catalyst is essentially designed and engineered from micro-to macro-scale by caramel-assistant hydrothermal synthesis.Then,ZSM-5/SS-fiber catalysts are synthesized by a combined washcoating and dry-gel conversion?DGC?method,in order to greatly improve the raw materials utilization and synthesis efficiency.In MTP process,the stability of as-obtained ZSM-5/SS-fiber catalysts can be markedly improved by a series of controllable synthesis strategy.Firstly,ZSM-5/SS-fiber catalyst is essentially designed and engineered from micro-to macro-scale by caramel-assistant hydrothermal synthesis.The caramel not only creates the mesoporosity in the ZSM-5 crystals,but also releases acid under hydrothermal synthesis conditions which lowers the zeolite crystallinity.Hence,ZSM-5/SS-fiber catalyst synthesized with caramel/SiO₂ molar ratio of 1.8 achieves best stability in MTP reaction,while the single-run lifetime of ZSM-5/SS-fiber synthesized with caramel/SiO₂ molar ratio of 2.4 decreases due to its ultra-low crystallinity although it contains more mesopores.Then,thin-felt ZSM-5/SS-fiber catalyst is highly efficiently synthesized via a washcoating with steam-assisted crystallization?SAC?method.Such method realizes the full use of raw materials with less wastes generating,and the zeolite content can be flexibly controlled.Impact of synthesis conditions on the morphology,textural/acidic properties and MTP performance of microstructured zeolite is investigated.A volcano-like relationship for lifetime is observed against the SAC time,which is governed first by the crystallinity improvement as well the mesoporosity development?0-12 h crystallization?,and then by the mesoporosity-atrophy with a distinct crystal size evolution?>12 h crystallization?.After that,thin-felt ZSM-5/SS-fiber catalyst is developed through a combined washcoating and vapor-phase transport?VPT?synthesis method.The use of silicalite-1seeds and ethylenediamine for zeolite structure directing reduces the TPAOH amount used,which favorably reduces the preparation costs of the ZSM-5/SS-fiber catalyst.The silicalite-1 seeding gel plays a key role in transforming the amorphous dry gel into a ZSM-5 zeolite layer,which is predominantly governed by the seed surface crystallization?SSC?mechanism.Inspired by this,the crystal size and mesoporosity of zeolite layer are controllably tuned toward enhanced diffusion by increasing the silicalite-1 seeding gel amount used,which markedly prolongs the single-run lifetime of ZSM-5/SS-fiber with a remarkable decrease in the coking rate in the MTP process.The adverse effect of non-uniform aluminum in the zeolite layer on the MTP stability is revealed.The use of aluminum-containing seeding gel to replace the pure-silica one makes the acid sites of as-obtained ZSM-5 mounted on SS-fiber distributed homogeneously thereby leading to a marked suppression of hydrogen transfer,and as a result,the catalyst single-run lifetime is further increased.Finally,a hollow-B-ZSM-5/SS-fiber catalyst is developed through alkali leaching of the full-silica core of the Silicalite-1@B-ZSM-5 in situ structured onto a thin-felt stainless-steel fiber,which is synthesized by a seed-assisted dry-gel VPT method.As-obtained catalyst shows remarkable stability improvement in the MTP reaction because of the enhanced diffusion of the nano-hollow-structure and mesoporosity development.A boron-free hollow-ZSM-5/SS-fiber is also obtainable with the textural properties comparable to the hollow-B-ZSM-5/SS-fiber but shows undesired degeneration of the tetra-coordinated aluminum.Boron incorporation is essential for preventing the framework dealumination during alkali leaching treatment due to the increased framework negative charges.The preferential generation of coke in the micropores of the hollow-ZSM-5/SS-fiber causes a rapid deactivation even compared to the parent Silicalite-1@ZSM-5/SS-fiber,due to the existence of extra-framework aluminum.The hollow-B-ZSM-5/SS-fiber catalyst achieves an ultra-long single-run lifetime of 2040 h??29?90%conversion?under industrial-relevant conditions.