Post-treatment And High-throughput Syntheses Of Hierarchical SAPO-34 Zeolites And Their MTO Catalytic Performance

As important chemical raw materials,low-carbon olefins such as ethylene and propylene have important and wide applications in the petrochemical and fine chemical synthesis.However,with the increasing shortage of petroleum resources,people are constantly searching for a way to produce low-carbon olefins independent of petroleum resources.In the past,ethylene and propylene were obtained by the means of petroleum cracking process under high temperature conditions.Up to now,the methanol to olefin reaction(MTO)is considered to be the most successful approach for producing low carbon olefins by using non-petroleum routes.The reaction utilizes abundant coal,natural gas,and renewable biomass as raw materials,and methanol as an intermediate to produce low-carbon olefins such as propylene,which has greatly eased the shortage of global oil.Therefore,the MTO conversion has aroused considerable research interest.The silicoaluminophosphate SAPO-34 zeolite(CHA-type structure)has proven to be an excellent catalyst for MTO reaction to produce ethylene and propylene owing to its large cavities connected by 8-ring pore openings,and moderate acidity.However,the hydrocarbon cokes deposited in SAPO-34 catalysts during the methanol conversion reaction easily block the channels/cages and cover the acid sites of the SAPO-34 catalysts,which restrict the mass transportation and lead to rapid deactivation of the catalysts.In order to retard coke deposition and solve the inherent diffusion limitation,great efforts have been made in recent decades,such as reducing particle size,the addition of oxidatives,optimizing the operating conditions,and modifying the zeolite pore structures.Among the reported methods,incorporating larger(meso or macro)pores into microporous zeolite crystals to prepare hierarchical SAPO-34 catalysts is considered to be one of the most effective strategies to enhance the mass transfer and prolong the lifetime of SAPO-34 catalysts in the MTO reactions.The hierarchical structure can significantly improve the diffusion of reactants and products through the zeolite catalysts as well as retard the coke deposition,thus prolonging the lifespan and improving the selectivity of light olefins.With the deepening of research,the methods of synthesizing SAPO-34 zeolites are gradually enriched.The current methods include: traditional hydrothermal synthesis,solvothermal synthesis,dry gel synthesis,ultrasound-assisted synthesis method,microwave assisted synthesis and solvent-free grinding synthesis,etc.This thesis is mainly focused on a highly efficient synthesis method for the synthesis of SAPO-34 zeolites-high-throughput synthesis.The combinatorial synthesis method involves simultaneous large amounts of parallel micro-syntheses.This method greatly saves manpower and allows large batches of experiments to be completed in a short period of time,covering all possible initial gel compositions and reaction conditions.This thesis is devoted to the study of the synthesis methods of hierarchical pore SAOP-34 zeolites.And a series of characterization methods,such as X-ray diffraction(XRD),scanning electron microscopy(SEM),nitrogen adsorption/desorption of synthetic SAPO-34 zeolite and MTO catalytic reaction test,are used to characterize the phase,morphology,pore structure and catalytic performance of the crystals,respectively.The main research results are as follows:1.A facile cost-effective post-treatment strategy has been developed for the fabrication of hierarchical SAPO-34 catalysts by etching zeolite crystals in triethylamine(TEA)aqueous solution coupled with a recrystallization process under hydrothermal conditions.Such an approach ensures less mass loss while retaining the similar chemical composition with the parent zeolite crystals.Moreover,the obtained hierarchical SAPO-34 catalysts display improved catalytic activity in MTO reactions—the obtained hierarchical SAPO-34 zeolites exhibit about 4-fold prolonged lifetime,and nearly 7% increased selectivity for ethylene and propylene.2.The high-throughput synthesis technique was been applied to the hydrothermal synthesis of SAPO-34 zeolite system,and 64 parallel reactions are designed and performed in a single experiment.The effect of the ratio of the morpholine and tetraethylammonium hydroxide in the initial gel on the crystal morphology of SAPO-34 zeolites is investigated,and the crystallization region diagram is completed according to the XRD and SEM results.In addition,through the synthesis system,hierarchical SAPO-34 zeolites have been successfully synthesized by adjusting the ratio of morpholine and tetraethylammonium hydroxide,which show the prolonged lifetime in the MTO reaction.