Synthesis Of Three-dimensional Ordered Hierachical MFI Zeolites And Their Catalytic Performance On Methanol To Olefins

Low-carbon olefins such as ethylene and propylene are important chemical raw materials,and traditional olefin production relies heavily on petroleum resources.Due to the limited petroleum resource in China and the relatively abundant coal resource,it is very important to obtain low-carbon olefins from the methanol chemical products.Zeolites such as MFI and CHA are important catalysts for methanol to olefin reaction(MTO).However,these zeolite are prone to carbon deposition due to the narrow pore size and limited molecular diffusion,resulting in rapid deactivation.The introduction of mesopores into zeolite to form hierarchical porous structure is an effective means to improve mass transfer and enhance catalyst activity and lifetime.At present,although many methods and techniques for preparing hierarchical porous zeolites have been developed,limited by the capability of preparation methods,there is no systematic study of the role of the hierachical porous structure in the MTO reaction.The hard template method can be used to exactly control the zeolite catalyst particle size,and the number of acidic sites can be modulated to facilitate the systematic study of the performance of the hierarchical porous zeolite catalyst.In this thesis,three-dimensional ordered close packing silica nanospheres(35 nm)and polymethyl methacrylate(PMMA)microspheres(200 nm,300 nm,600 nm)are used as sarcrifce templates to prepare three-demensionally ordered porous carbon.Three-dimensional ordered hierarchical(3DOH)MFI with regular crystalline domain sizes(35 nm-600 nm)and controllable Si/Al ratios are prepared by inverse replication of the ordered carbon templates.The obtained 3DOH MFI catalysts and the carbon templates are characterized by SEM,nitrogen adsorption/desorption,ICP,and NH3-TPD,etc.The results show that the morphologies of the catalysts are inverse replica of the templates and the number of acid sites can be controlled precisely.The results of methanol-to-olefins(MTO)catalysis show:for the samples with Si/Al ratios of 25 and 40,the catalyst life as well as the selectivity to ethylene and propylene decrease with increase of crystalline domain size;for the sample with Si/Al ratio of 70,both the catalyst life and the selectivity of olefines first increase and then decrease with increase of crystalline domain size;when the particle size of catalyst is fixed,as the silica-alumina ratio increases,the catalyst life and the selectivity to olefins first increases and then decrease,and the optimal Si/Al ratio is 40.Overall,among all the samples we prepared,35 nm 3DOH MFI Si/Al=40 is the best catalyst for MTO.