Synthesis Of Aluminum Phosphate Molecular Sieve AlPO-18 With Different Templates

AlPO-18 molecular sieve is a kind of molecular sieve with AEI configuration.Four-membered and six-membered rings exist in its skeleton,it has a three-dimensional channel with eight-membered rings,and the pore size is 0.38×0.38 nm.AlPO-18 shows obvious advantages in adsorption and separation due to its special structure and pore size.For example,AlPO-18 molecular sieve shows high separability in the removal of CO₂from natural gas,achieving the purpose of adsorption and separation in the most economical and environment-friendly way.At present,most of the reports are about the study of AlPO-5 molecular sieves or molecular sieve membranes for AlPO-n molecular sieves,while there are relatively few studies on the synthesis and scale-up of AlPO-18 molecular sieves.Moreover,AlPO-5 is often a kind of heterocrystalline phase in the preparation of AlPO-18 molecular sieve.Therefore,synthesis of pure phase AlPO-18 with high crystallinity and systematic study on the influence of its synthesis parameters are the focus of this paper.AlPO-18synthesized by hydrothermal synthesis method was studied using tetraethyl ammonium hydroxide and N,N-diisopropylethylamine as templates in this paper.The main research results are as follows:（1）The influence of aluminum source and gel ratio,such as HCl/Al₂O₃,TEA⁺/Al₂O₃,H₂O/Al₂O₃and P₂O₅/Al₂O₃on the synthesis of AlPO-18 are investigated when TEAOH is used as template agent.The results show that the pure AlPO-18 can be synthesized by using pseudo-boehmite,while the product synthesized by using aluminum isopropoxide contains a small amount of AlPO-5 heterocrystalline phase and the relative crystallinity is as low as 24.07%.The synthesis of AlPO-18 is sensitive to the acidity of the system.When the ratio of HCl/Al₂O₃is equal to 0.08,pure phase AlPO-18 can be synthesized;otherwise,heterocrystalline phase AlPO-5 will be produced.Even when the ratio of HCl/Al₂O₃is 0.58,the main crystalline phase has changed into spherical AlPO-5.The content of template and water also have great influence on the crystal phase of the synthesized product:When the ratios of TEA⁺/Al₂O₃is lower than 1.34,spherical AlPO-5heterocrystalline phase appears;hexagonal AlPO-18 can be synthesized within the rang of TEA⁺/Al₂O₃from 1.34 to 1.39;and when the ratios of TEA⁺/Al₂O₃is higher than 1.39,heterocrystalline phase AlPO-5 is also produced.When the ratios of H₂O/Al₂O₃is less than 42.86,there are a large number of heterocrystalline phase AlPO-5,so that the relative crystallinity of AlPO-18 is as low as 30.1%.When the ratios of H₂O/Al₂O₃is between42.86 and 50,AlPO-18 with pure phase can be synthesized.With the increase of the ratios of H₂O/Al₂O₃,the relative crystallinity increases gradually.When the ratios of H₂O/Al₂O₃is equal to 50,the relative crystallinity reaches 146.3%.When the ratios of P₂O₅/Al₂O₃is less than or equal to 0.8,there are a small amount of AlPO-5 heterocrystalline phase,and when it’s more than or equal to 1.05,there are also a small amount of AlPO-5heterocrystalline phase.（2）The influences of crystallization conditions on the synthesis of AlPO-18 are investigated when TEAOH is used as template.The experimental results show that pure phase AlPO-18 can be obtained with small particle size when the order of feeding is phosphorus source,aluminum source and template agent.The crystallization temperature and crystallization time have great influence on the crystal phase of the AlPO-18.The pure phase AlPO-18 can be synthesized when the crystallization temperature is 170℃,but the heterocrystalline phase AlPO-5 will appear when the crystallization temperature is 160℃or 180℃.With the increase of crystallization time,AlPO-5 of large grain is gradually transformed to tetragonal AlPO-18.The pure AlPO-18 can’t be synthesized until the crystallization time is 72 h.In addition,the pure product can be synthesized when the seed content is 2%～5%of the Al₂O₃mass fraction in the aluminum source,and its relative crystallinity increases gradually with increasing of the amount of crystal seeds.The highest relative crystallinity is 119.96%when the seed content is 5%of the Al₂O₃mass fraction.While the amount of seed is too high（more than 5%）,a small amount of heterocrystalline phase AlPO-5 will appear in the synthesis of AlPO-18.Finally,AlPO-18with pure phase and high crystallinity has successfully been synthesized in a 1 L reactor by adjusting the rotating speed.（3）The effects of aluminum source,template agent,water and phosphoric acid on the synthesis of AlPO-18 are explored in DIPEA system.The results show that pure phase AlPO-18 can be synthesized with pseudo-boehmite or aluminum isopropoxide as aluminum sources,but the relative crystallinity of AlPO-18 synthesized with pseudo-boehmite is much higher（92.45%）than aluminum isopropoxide（44.5%）.The content of template agent affects the crystal phase and relative crystallinity of the product:when the template content is too low（DIPEA/Al₂O₃=1.5）,AlPO-5 crystal phase can be obtained.Pure phase AlPO-18 can be synthesized within the range of DIPEA/Al₂O₃from1.75 to 2.5,and the relative crystallinity of the product gradually increases with the increase of the template content.Water content has little effect on the synthesized crystal phase.AlPO-18 can be synthesized within the range of H₂O/Al₂O₃from 52.5 to 60,and its relative crystallinity increases with increasing H₂O/Al₂O₃.In addition,when DIPEA is used as template agent,the ratios of P₂O₅/Al₂O₃can be adjusted in a certain range.The pure phase AlPO-18 can be synthesized in the range of P₂O₅/Al₂O₃from 0.9 to 1.05,and when the ratio of P₂O₅/Al₂O₃is 0.95,the relative crystallinity of the AlPO-18 synthesized is the highest（106.86%）.