Influence Of Alcohol Molecules And Autogenous Pressure On The Crystallization Process Of Open-Framework Aluminophosphates

Zeolites and related microporous crystalline materials with well-defined pore structures,good thermal stability and chemical stability,and large specific surface area have been widely used in chemical industry and our daily life.Since the development of such materials,the crystallization mechanism has attracted widespread attention among researchers.The reaction within the synthesis system of microporous crystalline materials is relatively complex.Although many researches have been carried on crystallization mechanism and many viewpoints have been put forward,however a general mechanism has not been formed yet.At present,the synthesis of microporous crystalline materials is still based on trial and error.Therefore,in order to reduce the blindness of the synthesis and achieve the directional design and synthesis of microporous materials with specific function,it is necessary to have a clearer understanding of the crystallization mechanism.As an important member of the microporous crystalline materials,open-framework aluminophosphates exhibit a rich structure chemistry.Compared with the traditional molecular sieves,the synthesis system of open-framework aluminophosphates is relatively simple and has less adjustable parameters,which is very suitable for the investigation of crystallization mechanism.Studying the crystallization mechanism of open-framework aluminophosphates can help us to further understand the crystallization process of zeolites and related microporous crystalline materials,so as to better guide the synthesis.In this thesis,we chose open-framework aluminophosphates as subject investigated and organic alcohol solvents were introduced into the synthesis system as reaction solvents or reaction additives.The influence of alcohol solvent molecules on the synthesis of aluminophosphate molecular sieves and the influence of autogenous pressure on the crystallization process of open-framework aluminophosphates were systematically investigated.The main achievements are summarized as follow:1.Three aluminophosphate molecular sieve AlPO₄-5s were synthesized using water,ethylene glycol,and triethylene glycol as solvents.The products were thoroughly characterized by powder X-ray diffraction analyses,thermogravimetric analyses,elemental analyses,scanning electron microscope analyses,and solid-state cross polarization magic angle spinning nuclear magnetic resonance analyses.The results showed that solvothermally synthesized aluminophosphate molecular sieve AlPO₄-5from triethylene glycol was much less stable than that hydrothermally synthesized from water.Thermogravimetric analyses and ¹³C cross polarization magic angle spinning nuclear magnetic resonance analyses confirmed that the bulky solvent molecules of ethylene glycol or triethylene glycol were encapsulated within the channels of AlPO₄-5 during the solvothermal synthesis.This is the first observation that the bulky organic solvent molecules are included in the organic structure-directing agents occupied microporous aluminophosphate molecular sieves.Upon heating,small molecules can be easily escaped from the channel without the strong interaction with the channel wall,whereas big molecules will have strong interaction with the channel wall during the escape,which can result in the collapse of the framework.More big molecules inside the channel,more significant impact on the thermal stability of framework are expected.2.Highly crystalline aluminophosphate molecular sieve AlPO₄-11 was synthesized using both water and triethylene glycol as solvents.The products were thoroughly characterized by powder X-ray diffraction analyses,thermogravimetric analyses,scanning electron microscope analyses,and solid-state cross polarization magic angle spinning nuclear magnetic resonance analyses.The results showed that both AlPO₄-11s had high degree of crystallinity and good thermal stability but a different morphology.Thermogravimetric analyses and ¹³C cross polarization magic angle spinning nuclear magnetic resonance analyses confirmed that the bulky organic solvent molecules were encapsulated in the organic-amine-occupied channels of AlPO₄-11.The solvent molecules may act as a co-structure-directing agent in the formation of AlPO₄-11 under solvothermal condition.3.Aluminophosphate molecular sieve AlPO₄-18 with a three-dimensional channel system of 8-membered rings was obtained by heating the initial gel with a molar composition of 1Al₂O₃:1.5P₂O₅:3.5TEA:80H₂O at 180?C for two days.When a small amount of isopropanol solvent was introduced to the system above,aluminophosphate molecular sieve AlPO₄-5 with a one-dimensional channel system of 12-membered rings was crystallized.The crystallization products and crystallization process of two systems were thoroughly characterized by powder X-ray diffraction analyses,pH value change,thermogravimetric analyses,elemental analyses,scanning electron microscope analyses,and solid-state cross polarization magic angle spinning nuclear magnetic resonance analyses.The results showed that the solvent molecules of isopropanol can change the crystallization direction of the synthesis system by affecting the pH of the reaction system in the crystallization process and type and distribution of the species formed in the liquid phase and the movement of the reaction balance between these species.Finally,isopropanol affected the structure-directing effect of triethylamine,leading to the formation of different aluminophosphate molecular sieves.4.The crystallization behavior of the initial gels containing the structure-directing agents of triethylamine,cyclohexylamine,tetraethylammonium hydroxide,propylamine,dipropylamine,and triethylenediamine under ambient pressure and autogenous pressure at elevated temperature was investigated.At 300?C or 180?C under ambient pressure,highly crystalline aluminophosphate molecular sieve AlPO₄-5was rapidly crystallized from the initial gels containing triethylamine,cyclohexylamine,tetraethylammonium hydroxide.When the same initial gels were loaded into autoclaves and heated at 180?C in an oven,three-dimensional open-framework aluminophosphate JDF-20,layered aluminophosphate UT-5,and a mixture containing aluminophosphate molecular sieve AlPO₄-5,was obtained,respectively.The initial gels containing propylamine and dipropylamine produced dense aluminophosphate when heating at 300?C or 180?C under ambient pressure was applied,while they produced layered aluminophosphate[C₃NH₁₀]₃·[Al₃P₄O₁₆]and aluminophosphate molecular sieve AlPO₄-11,respectively,when heating at 180?C under autogenous pressure was applied.Only amorphous was obtained when the initial gel containing triethylenediamine was heated at 300?C or 180?C under ambient pressure,while AlPO-CJB2,a 3D anionic open-framework aluminophosphate with Al/P of 11/12,was produced when such initial gel was heated at 180?C under autogenous pressure.The crystallization process of the initial gel containing triethylamine under ambient pressure was investigated with in situ thermogravimetric and mass spectrometric analysis and confirmed that the formation of aluminophosphate molecular sieve AlPO₄-5 under ambient pressure was mainly occurred in the stage of 150-300?C.We found for the first time that the pressure could significantly affect the structure-directing effect of amines.This finding greatly enhanced our understanding on the formation process and crystallization mechanism of zeolites and open-framework crystals.