Phase Transition Behavior Of Zeolite Y Under Hydrothermal Conditions

Zeolites were widely used in industry,due to their excellent properties in adsorption,separation,catalysis and ion exchange.However,the synthesis of zeolites requires structure-directing agents,some zeolites with high application value need expensive organic structure-directing agents(OSDAs),which greatly increases the cost of synthesis.The seed-assisted synthesis solves this problem.In particular,taking advantage of the crystallization mechanism of hetro-structural seed-assisted synthesis,we may discover several new zeolites,extend the composition of zeolites,obtain the zeolites with extremely high or low Si/Al ratio,and develop the hetro-structural seed-assisted non-organic structure-directing agent hydrothermal synthesis route for the zeolites having industrial applications.The transition of zeolites is a special case of seed assisted synthesis.It is of great significance to study the crystallization mechanism of hetro-structural seed assisted synthesis.In this paper,zeolite Y was used as raw material,we investigated the effects of various structure-directing agents on the transformation of zeolite Y.The main results are summarized as follow:1.We systematically investigated the phase transition behavior of zeolite Y(HY and NaY)to zeolite MER in the KOH solution under hydrothermal conditions.Zeolite MER has four types of 8-membered ring channels(3.1×3.5?2.7×3.6?3.4×5.1?3.3×3.3?)and important potential applications in small molecule catalysis and separation.Seven to ten days are needed to synthesize highly crystalline zeolite MER with traditional hydrothermal synthesis.With phase transition of zeolite Y in the KOH solution under hydrothermal treatment,highly crystalline zeolite MER can be obtained within two days.Hydrothermally treating the equivalent amorphous aluminosilicate gel resulted in the mixture of zeolite MER and zeolite LTL(for HY)or zeolite CHA(for NaY)with low degree of crystallinity.Phase transition of HY can be conducted at either 100 or 150°C,whereas that of NaY can only be conducted at150°C.KOH/SiO2 and H2O/SiO2 have significantly influence on the phase transition behavior of zeolite Y.Highly crystalline zeolite MER can only be obtained with the optimized KOH/SiO2 and H2O/SiO2.With this method,the synthesis period of zeolite MER is greatly reduced,which might be applied to the synthesis of other industrially important zeolites in a shortened time.At the same time,the structure-directing effect of the mixed metal cation was investigated,we found that the greatly influence of K~+/Na~+on the topological structure,size and morphology of products obtained by phase transition of zeolite Y.2.High silica zeolite Y(USY(FAU),Si/Al=12.4)was used as the sources of silicon and aluminum sources,we investigated that the structure-directing effect of inorganic structure-directing agents(Na~+),organic structure-directing agents agent(tetraethyl ammonium hydroxide),seed(zeolite FER,Si/Al=12.5)on transformation of zeolite USY.When Na~+is only used as the inorganic structure-directing agent,Adjusting the alkalinity of the synthesis system,zeolite USY can be transformed into MFI,MOR,ANA zeolites.Moreover,the higher alkalinity in the synthesis system,the denser structure of and the lower yield of products obtained by transformation of zeolite USY;When tetraethyl hydroxide was used as the organic structure-directing agent,adjusting the alkalinity of the synthesis system,zeolite USY can be transformed into BEA and MOR type zeolite.While MOR type zeolite obtained is with high Si/Al ratio(Si/Al=5.1).Hydrothermally treating the equivalent amorphous aluminosilicate gel resulted in the mixture of zeolite MOR and zeolite P(for BEA)or zeolite P and zeolite ANA(for MOR)with low degree of crystallinity;FER seed as the structure-directing agent,zeolite USY was successfully transformed into zeolite FER,but there was a little zeolite USY in the product.In the system of high alkalinity,zeolite USY was transformed into the mixture of zeolite FER and zeolite MOR.Zeolite USY is unique in the synthesis of high silica zeolite.