Creation Of Hierarchical Pore Structure In Molecular Sieve Materials

Molecular sieve materials have been regarded as the most attractive candidates in a variety of applications,such as chemistry,chemical engineering,energy,medicine,and advanced materials.On the basis of the various porestructures,they are normally worked well in the fields from the separation on atomic size to the macromolecular delivery.When taking into account of the more and more complicated reactants used nowadays,hierarchical porestructures are widely accepted as an effective option to avoid mass limitation.Therefore,the exploration of a simple method to prepare hierarchical molecular sieve materials is still of interesting and be a great challenge.In this thesis,we studied the creation of hierarchical porestructures in the zeolitic molecualr sieve,ZSM-5,and ordered mesoporous molecular sieve,Al-SBA-15,by direct synthesis and post-streatment strategies,respectively.The main contents are summarized as follows:1.Microporous molecular sieve-ZSM-5Morphology control of ZSM-5 molecular sieve nanoparticle aggregates.The cuplike ZSM-5 nanocrystal agglomerates were directly synthesized following an organic templatefree strategy with the assistance of a preformed MFI precursor(the seed solution)and a nonionic-type polyacrylamide(PAM).The morphological adjustment approach,hierarchical mesopore structure evolution,and the derived acidic properties of the resulting ZSM-5 nanocrystal agglomerates were systematically investigated.The cuplike agglomerates possessing a thin-wall structure which is composed of monolayer ZSM-5 nanocrystals can be synthesized by simply prolonging the high-temperature(448 K)crystallization.Due to the morphological characteristics,the diffusion pathway in the resulting cuplike ZSM-5 nanocrystal agglomerates(CZNA)is perpendicular to the cup wall and the length is identical to the wall thickness,around 200 nm.As for the formation of the cuplike morphology,the MFI composition building units(CBU)in the center nanocrystals continuously dissociate,migrate,rearrange,and assemble onto the nanocrystals close to the external surface of the preformed agglomerates,thereby forming a “shell-like” structure.After random collapse takes place on the “shell” through prolonging the high-temperature hydrothermal treatment,a cuplike morphology is obtained.As the consequence,the CZNA simultaneously possesses a high crystallinity and a highly open mesostructure,especially as well as the extremely short diffusion pathway.Because of these advantages,CZNA exhibits very high catalytic activity in the Low Density Polyethylene(LDPE)catalytic cracking reaction.2.Mesoporous molecular sieve-Al-SBA-151)Al-SBA-15 adjusted pore structure by alkali treatment.The effect of alkaline treatment on the mesoporous material Al-SBA-15 was investigated.It is observed that elemental Si can be readily extracted from wall structures and the Al-rich surface that is consequently exposed protects the uniform hexagonal arrays.The extracted substances rapidly accumulate inside the Al-SBA-15 mesopores,and continuously dissolve and migrate into the alkaline solution upon prolonged treatment time.Therefore,alkalinetreated Al-SBA-15 exhibits a larger pore size and a thinner wall than before treatment.More importantly,the secondary-scale mesopores inside the Al-SBA-15 walls were remarkably expanded,allowing efficient penetration of the carbon precursor.Its structural replica,CMK-3,has a highly ordered mesostructure,which is attributed to the formation of enhanced carbon sticks that connect the primary carbon rods.For the same reason,CMK-3 templated from alkaline-treated Al-SBA-15 exhibits strong pressure resistant capability.2)Synthesis of high stability and strong acid PHTS type Al-SBA-15 without acid system.In this work,ordered mesoporous molecular sieve materials,Al-SBA-15 was respectively synthesized from the strong acidic and the acid-free media by using a very small quantity of surfactants because of the economic and environmental considerations.The effects of various Al sources on the structural and acidic properties of the final materials were investigated.When following the acid-free strategy,a highly ordered mesostructure can be facilely constructed even with a very low surfactant concentration,extremely regardless of the kind of Al source that used.The resulting Al-SBA-15 exhibited a much higher aluminum content,a stronger acid strength and especially the predominant Br?nsted acidic properties in comparison with those synthesized under the traditional strong acidic conditions,thanks to the high-quality crosslinking and self-assembly of silica and alumina species.In addition,the morphology of the product can also be modulated by simply altering the type of aluminum sources.The Al-SBA-15 synthesized with the aluminum nitrate as Al source exhibited very high catalytic activity in the Low Density Polyethylene(LDPE)cracking reaction,and the temperature corresponding to the maximum cracking rate was even comparable to the zeolitic catalyst.