Synthesis Of Aluminophosphate Zeolites For Bifunctional Catalysts Used In Conversion Of Syngas To Olefins

Low carbon olefins are important basic chemical raw materials.With the increasing scarcity of petroleum resources and the sharp increase in demand for olefins,the development of non petroleum technology routes such as C1 conversion to olefins is of great significance.Applying a bifunctional catalyst based on metal oxide-molecular sieve to the one-step synthesis of light olefins（STO）has broad application prospects.In this project,a new method of liquid seed assisted synthesis of SAPO-n molecular sieves with low silicon and lamellar morphology was developed for high-performance bifunctional catalyst systems of oxide-molecular sieve.By combining with Zn Cr Ox metal oxides,bifunctional catalysts can be constructed and applied to STO reactions,which shows good catalytic performance.The main research contents and results are as follows:（1）The effects of the amount of liquid phase seed,the aging time of the liquid seed and hydrothermal crystallization time on the physicochemical properties of the synthesized SAPO-34 molecular sieves and reaction performances were investigated.The results show that with the gradual increase of the addition amount of liquid phase seeds,the proportion of SAPO-34 crystal structure in the generated molecular sieve increases.Due to the metastable nature of the liquid phase seeds,the state of the seeds can change at room temperature.The acidity amount and acidity strength of SAPO-34 molecular sieve synthesized with the aid of liquid phase seed aged for 0～14 days tend to be idealized.The CO conversion and the space-time yield（STY）was increased.In addition,the bifunctional catalyst maintains a good stability during the reaction process.（2）With the addition of liquid seeds,the effects of template dosage,water content,and Si O₂/Al₂O₃ in different template systems on the physicochemical properties and catalytic performance of synthesized molecular sieves were investigated.The results show that when TEA was used as the main template,the structure of the synthesized molecular sieve is easily affected by TEA/Al₂O₃ and Si O₂/Al₂O₃.Otherwise,when MOR was used as the main template,the amount of template and Si O₂/Al₂O₃ have little impact on the crystal structure and morphology of the synthesized molecular sieve.In addition,there are significant differences in the catalytic performance of molecular sieves synthesized with two different types of templates.The bifunctional catalyst composed of molecular sieves synthesized with the TEA template system has excellent catalytic performance in the conversion reaction of synthesis gas（the selectivity of C₂～C₄⁼is basically maintained at 80%,and the CO conversion rate is about 30%）.However,molecular sieves synthesized using MOR as the main template are prone to carbon deposition and deactivation during the reaction process.（3）Modifying the acidity and textural properties of SAPO-34 zeolites by post-treatment with Al（NO₃）₃.The results show that the crystallinity,BET specific surface area,and acid content of the treated SAPO-34 molecular sieve decrease.The catalytic results showed that the SAPO-34 molecular sieve treated with an appropriate concentration of Al（NO₃）₃ solution increased the CO conversion and C₂～C₄⁼selectivity.However,the bifunctional catalyst composed of molecular sieves treated with high concentration of Al（NO₃）₃ solution and Zn Cr Ox oxide can lead to a decrease in CO conversion during the reaction caused by severe acidity lost.（4）Adjusting the structure,acidity,morphology and textural properties of SAPO-n zeolite by adding Ga during synthesis.The results show that the introduction of a small amount of Ga can improve the crystallinity of molecular sieves and improve their acid properties.When combined with Zn Cr Ox metal oxides as a bifunctional catalyst,the CO conversion rate is increased from 10.1%to 30%,C₂～C₄⁼selectivity increased from 48.5%to64.7%.However,introducing excessive Ga into the molecular sieve synthesis process will destroy the crystal structure of SAPO-34 molecular sieve,leading to the appearance of SAPO-5 crystals and reducing its catalytic performance.