Preparation Of Kaolin-based ZSM-5 Zeolite By VPT And Its Application In MTA

ZSM-5 molecular sieve is a commonly used methanol to aromatics?MTA?catalyst because of its unique physical and chemical structure.However,currently,most of the molecular sieves are synthesized using chemical reagents.The use of abundant and cheap kaolin as the sole silicon and aluminum source for synthetic molecular sieves can effectively reduce the production cost of molecular sieves.The vapor phase transport?VPT?has been paid attention by researchers because it can reduce the number of separation steps in the preparation process,but it is currently less used in the preparation of natural mineral to molecular sieves.Therefore,in this paper,kaolin-based ZSM-5molecular sieves were prepared by using modified kaolin as raw material and based on the solid-phase transformation mechanism,using vapor phase transport?VPT?,and evaluating their MTA catalytic activity.First,kaolin clay was modified and pretreated by two methods:Acid treatment and Acid Alkali excitation.Then samples were characterized by XRD,XRF,and N₂-adsorption-desorption.It was found that acid treatment could not affect the crystals with complete crystal structure.The main involved in the reaction is the associated amorphous silicon aluminum in kaolin.Due to the limitation of the active aluminum content of kaolin,the silicon-aluminum ratio of kaolin after pickling and extraction is about 25,and the optimal acid treatment time is 2 h.The decrease in the crystallinity of the sample after alkali excitation indicates that the activity of kaolin powder was increased.The optimal excitation ratio is m?NaOH?/m?Kaolin?=0.075.From the perspective of pore structure,acid extraction is the main factor for pore formation,and alkali excitation mainly affects the micropore structure.However,the kaolin pore structure after pickling extraction and alkali excitation treatment is still underdeveloped and is a non-porous material.When the kaolin-based ZSM-5 molecular sieve was synthesized by the VPT method,the effects of time,alkali addition,and the presence or absence of seed crystals on the crystallization process were studied,and compared with hydrothermal crystallization.It was found that in the VPT method,there is also an induction period for crystal growth.After the induction period,the crystals grow rapidly.The mullite crystal in the raw material is the nucleation growth site of ZSM-5;Appropriate addition of alkali does help the formation of ZSM-5,but at the same time excess alkali will inhibit the formation of ZSM-5 and even even the formation of other crystal.The addition of seeds can effectively shorten the induction period of crystal growth,but after it is fully crystallized,it does not have a significant impact on the pore structure.By comparing the crystallization products synthesized by hydrothermal synthesis and VPT,it can be seen that VPT can complete the synthesis of ZSM-5 molecular sieve with less use of template and reduce the synthesis cost of molecular sieve.Finally,Zn was loaded on the prepared ZSM-5 molecular sieves and commercial ZSM-5 molecular sieves by equal volume impregnation,and the activity of MTA catalyst was evaluated.The two catalysts were characterized by EDS,XPS and NH₃-TPD.It was found that the amount of acid is the main reason for the difference in catalytic performance of the two catalysts;the methanol conversion rates of the two catalysts are close.But the kaolin-based ZSM-5 molecular sieve with more acid content has higher aromatic selectivity.From the perspective of catalytic life,the kaolin-based ZSM-5 molecular sieve has a higher catalytic life.According to DTG analysis,it was found that the kaolin-based ZSM-5 molecular sieve with a higher acid content instead has a lower carbon deposition amount,and the carbon deposition species of the two catalysts are different.The main reason is that the kaolin-based ZSM-5 molecular sieve uses mullite as the growth site.The low-acidity mullite structure modifies the surface acidity of kaolin-based ZSM-5,prevents the occurrence of deep side reactions and further inhibits the formation of carbon deposits.