Interaction Mechanism Between Components Of TEAOH Templated SAPO-34 Zeolite

SAPO-34 zeolite is a CHA structure silica-aluminophosphate microporous material.Due to its suitable acidity,large specific surface area and high hydrothermal stability,it has been successfully used in methanol to olefin process.The crystallization process of SAPO-34zeolite involves the interaction between silicon source,phosphorus source,aluminum source and template agent.Interactions between these components are the basis of regulating the properties of SAPO-34 zeolite,but it is still a lack of research in this regard.In this work,the interactions between components of SAPO-34 zeolite were studied with pseudoboehmite,H₃PO₄?Phosphoric acid?,TEOS?Tetraethylorthosilicate?and TEAOH?Tetraethyl ammonium hydroxide?as aluminum source,phosphorus source,silicon source and template,respectively.The acidity and MTO performance of samples were also studied.Main conclusions are as follows:?1?Pseudoboehmite is insoluble in water.However,crystallite size increases a little while the content of adsorbed water decreases under hydrothermal conditions.Pseudoboehmite can be dissolved in TEAOH solution at room temperature,but it recrystallized and precipitated under hydrothermal conditions.The size of precipitated microcrystals increased significantly and the amount of adsorbed water decreased.Moreover,certain amount of pseudoboehmite dissolved in TEAOH solution at 200?.For tetraethylorthosilicate,it is hydrolyzed to form Si?OH?₄ at first,and then Si?OH?₄dehydrolyzed to form solid precipitate.This process is slow in water,and complete hydrolysis takes 5 minutes under hydrothermal conditions.The initial solid product is hydroxyl-rich gel,and then the gel further dehydrolyzed to form hydroxyl-poor products with about 20 nm size.In TEAOH solution,the hydrolyzation of tetraethylorthosilicate was accelerated and tetraethylorthosilicate was fully hydrolyzed at room temperature.However,N?Et?⁴⁺slowed down the dehydroxylation of Si?OH?₄ and delayed the formation of precipitation.Crystallites from TEAOH solution are about 50 nm.Under hydrothermal conditions,silica source can be dissolved in water and TEAOH solution by 80%?85%.?2?In water,reactions between pseudoboehmite and phosphoric acid lead to the formation of spherical Al PO₄ withan 80%or so yield.In TEAOH solution,lamellar Al PO₄-18was produced with yields of about 50%?TPA?and 30%?TAP?.TEAOH promoted the hydrolysis of aluminum phosphate and acted as a template in the formation of precipitation.Pseudoboehmite and TEOS don't react with each other in water.Precipitation from pseudoboehmite-TEOS-water solution is same as the mixture of preciptitations from pseudoboehmite-water and TEOS-water.However,in TEAOH solution,a stable gel was obtained and almost no precipitation was formed.For phosphorus sourcesilicon source mixtures,Si O₂ is the main precipitate.Phosphoric acid has no obvious influence on the precipitation of TEOS under hydrothermal conditions in water,but has a certain inhibition effect on the precipitation of Si O₂ in TEAOH solution.?3?Without TEAOH,the propduct of pseudoboehmite-TEOS-phosphoric acid was Al PO₄ which formed via the reaction between pseudoboehmite and phosphoric acid.SAPO-34 can be obtained for all interaction sequence while TEAOH is used as template.However,the reaction between TEAOH and phosphoric acid will inhibit the hydrolysis of pseudoboehmite and TEOS,in which leads to the presence of undissolved pseudoboehmite in the sample as well as hinds the substitute of Si for P into the framework of the zeolite.TEAOH first interacts with pseudoboehmite or TEOS benefits the synthesis of SAPO-34zeolite with high crystallinity,high acid content,high olefin selectivity and long catalytic life.The TAPS sample shows the best catalysis life time and ethene,propene selectivity in MTO process.