Study On Two Kinds Of Molecular Sieves For Catalytic Reaction Of Methanol To Olefin

Ethylene and propylene are important chemical raw materials.These two kinds of low carbon olefins were widely used and demanded,which occupying an important position in the filed of chemical industry and national economic.The raw materials for the production of olefins are mainly derived from coal,petroleum,natural gas,biomass and other earth energy sources,and are produced by steam cracking,catalytic cracking,catalytic dehydrogenation and the like.Methanol to olefin?MTO?technology is the most potential olefin production method in recent years.It accords with the basic condition of our country which is rich in coal,poor in oil and little in gas.SAPO-18 molecular sieves have similar spatial structures to SAPO-34molecular sieves,while the performances for MTO is different.It is of great significance to study the crystallization mechanism of SAPO-18 molecular sieve and its MTO catalytic performance.There are some disadvantages about SAPO-34 molecular sieves,such as short catalytic life and easy carbon deposition.It is critical to solve these problem with a simple and lose-cost method.This paper mainly does the following work:1.By exploring the synthesis conditions of SAPO-18 molecular sieve,the best experimental scheme was selected.SAPO-18 molecular sieves with the different crystallization times were prepared by hydrothermal method.The samples at different crystallization times and its MTO?methanol to olefins?catalysis were characterized by X-ray diffraction?XRD?,scanning electron microscope?SEM?,NH₃-TPD,²⁹Si MAS NMR,²⁷Al MAS NMR,FT-IR and other methods.The characterization results combined with the catalytic performance of MTO indicated that the growth process of SAPO-18 molecular sieve in the hydrothermal system can be divided into three stages:initial crystallization??2h?,rapid growth?6h¹2h?,and stable growth?1³d?.In the early stage of crystallization,only a small amount of silicon entered the Al?PO?₄ intermediates.The silicon atoms then enter the Al?PO?₄ intermediates and SAPO-18 crystal replace phosphorus,aluminum atoms.In the fast growing period is given priority to with SM?mechanism,SM?mechanism as the auxiliary pole,while the two substitution mechanisms of growth stabilization are in the opposite order.2.Using triethylamine as the main template,the SAPO-34 molecular sieve synthesized by direct addition of sucrose to molecular sieve,semi-carbonization and full carbonization were investigated.It is characterized by XRD,SEM,NH₃-TPD,N₂-adsorption desorption and other methods.The results showed that the morphologies of SAPO-34 molecular sieves were changed by adding sucrose of different forms.The number and pore size of mesopores were increased after roasting,and the catalytic life,low carbon olefin selectivity and the distribution ratio of the two main products were changed.It is found that the semi-carbonized sucrose sample has the best catalytic performance.3.Using tetraethylammonium hydroxide as the template,the crystallization solutions with different crystallization temperatures and times were added to SAPO-34 molecular sieve during the synthesis process.It is characterized by XRD,SEM,and other methods.The results showed that this method could effectively change the morphology,extend the service life of the catalyst to different degrees,and change the selectivity of diene.