Post-treatment Of SAPO-34 Zeolite And Its Catalytic Performance In Methnol To Olefins

Light olefins including ethylene and propylene are most important materials in modern petrochemical industries.Nowadays,the main route of producing light olefins in our country is naphtha cracking.With the increasing shortage of petroleum resources,the traditional routes for producing light olefins are limited by raw materials.Methanol to olefins?MTO?process is a promising non-oil route which can replace the traditional route to increase the production of light olefins.SAPO-34 is considered an effective catalyst of the MTO process,however,the exclusively presence of micropores in conventional SAPO-34,which favors the selectivity to light olefins,but imposes severe limitations for mass transfer and thereby gives rise to rapid deactivation of SAPO-34 based MTO catalysts.Attempts have been devoted to prepare highly-efficient SAPO-34-based catalysts,such an approach is to improve the catalyst accessibility by introducing mesopores into SAPO-34 to yield so-called hierarchical zeolites containing both micropores and mesopores.In this thesis the hierarchical SAPO-34 were prepared via three post-treatment methods including acid treatment,alkali treatment and hydrothermal treatment.The physicochemical properties and its catalytic performance for MTO of the resultant SAPO-34 zeolites were investigated,the main results are:?1?Compared to hydrochlorid acid and oxalic acid treatment,citric acid treatment can introduce the mesopore to SAPO-34 zeolite,resulting a micro-mesoporous hierarchical SAPO-34 with larger extra surface area and mesoporous pore volume.When used as catalysts in MTO process,compared to its counterparts including non-treated SAPO-34 and hydrochlorid acid and oxalic acid treated ones,it shows much better catalytic performance of increasing the life of catalysts from 125 min to 385 min with the selectivity of light olefins including ethelane and propylene increased from 74.8% to 80.4%.The XRD,FT-IR and SEM characterization results show that with the increase of concentration of citric acid,the crystal structure of the SAPO-34 remained stable while the surface of the zeolite were etched with a regular hourglass-like etching region occured.XRF,EDS and ²⁹Si MAS NMR results show that after being acid treated,the silicon content and the molar SiO₂/Al₂O₃ ratio of the zeolite were reduced.It was considered that the decrease in silicon content was the result of indirect desilication.Acid-treatment can selectively deprive the aluminum atoms near the silicon islands in SAPO-34,resulting the decomposition of them and the indirect desilication.The pore size distribution was measured by TEM,N₂ adsorption-desorption,and mercury porosimetry techniques.It was found that the acid treatment can introduce mesopore and even macropore to SAPO-34 zeolites.The pore size of mesoporous is concentrated in 5-8 nm,and that of the macropore is in 150-300 nm.The MTO evaluation results show that the life of the zeolite and the selectivity of ethylene and propylene increased to some extent with the increase of acid concentration.The reaction lifetime of the acid-treated SAPO-34 was up to 385 min and its selectivity of ethylene and propylene reached 80.4%.?2?Chosing TEAOH as the alkali treatment medium.The results of XRD and FT-IR showed that the molecular sieve was still pure SAPO-34 after alkaline treatment,but the relative crystallinity decreased with the increase of alkali concentration.The SEM photos show that the pores on the surface of SAPO-34 also show an hourglass-like distribution after alkali treatment.Through the pore structure characterization,the alkali treatment can introduce the mesoporous structure into the SAPO-34 zeolites,but as the alkali treatment concentration increases,the micropore loss of the molecular sieve is larger.The XRF and MAS-NMR results show that the alkali treatment directly removes the silicon species from SAPO-34 zeolite.Especially in the high concentration alkali treatment,the SAPO-34 zeolite completely lost medium and strong acid,which also affected the performance of the MTO reaction.With the increase of the alkali concentration,the initial methanol conversion rate quickly from 100% Up to 23%,the reaction life and diene selectivity are also greatly reduced.?3?Post-treatment of SAPO-34 with different hydrothermal treatment times.After a long time of hydrothermal treatment,SAPO-34 zeolite can still maintain a high relative crystallinity,indicating that SAPO-34 zeolite has good hydrothermal stability.Hydrothermal treatment changes the surface morphology of the molecular sieve.Hydrothermal treatment does not have a significant effect on the acidity of the SAPO-34.From the BET results,as the treatment time increases,the extra surface area increases from 67.9 m²/g to 83.6 m²/g,and the mesopore volume also increases from0.062 cm³/g to 0.078 cm³/g.In the evaluation of MTO reaction,the reaction lifetime and diene selectivity of the hydrothermally treated SAPO-34 sample increased to varying degrees.It can be seen that the introduction of the mesoporous structure can improve the life and selectivity of the molecular sieve by improving the mass transfer under the condition of consistent acidity.