沸石分子筛转晶的研究——4A转晶为NaY型沸石的机理

THE RECRYSTALLIZATION OF ZEOLITE MOLECULAR SIEVES-RECRYSTALLIZATION MECHANISM OF4A TO NaY ZEOLITE

采用扫描电子显微镜、X-光衍射分析和高真空苯吸附量的测定等方法,研究了在一定的化学环境下沸石分子筛的介稳状态;探讨了不同转晶条件对沸石重新晶化的影响;分别研究了在Y型、AG_(4)型或Ω型沸石的合成条件下,加入其它型号的沸石转变为Y型、AG_(4)或Ω型沸石的过程;并研完了在Y型沸石合成的化学环境下,A型沸石向Y型沸石转变的过程,并探讨其机理。

It is well known that the change of metastable zeolite to a morr stable one depends upon the intrinsic properties of the zeolite and also the chemical environment in which it exists.We studied the probability of transformation of zeolite from one type to another when changing the chemical environment.We investigated respectively the transformation of added zeolite to F,AG_(4)orΩtype in its own formation environment and also studied the process and mechanism of the transformation of NaA to NaF zeolite in the chemical environment of NaF type formation.Scanning electron microscope,X-ray diffraction and high vacuum adsorption methods were employed in these studies.By adding NaA zeolite to the crystallization environment of NaF zeolite formation,the process of recrystallization may be represented as:Amorphous aluminosilicate gelIn fact,the recrystallization of NaAzeolite does not occur until the gel crystallization is nearly complete.Scanning electron microscopic observation and the recrystallization curves are consistent with a mechanism that includes a degradation and dissolution of surface NaA crystal.The nucleation and crystal growth of NaF take place directly at the surface of NaA crystals.Scanning electron micrographs also show that the formation of the smaller spherical NaF crystals(-0.5μm)occurred at the surface of the larger cubic NaA crystals(-5μm).The nutrient for the NaF crystallization-is being supplied by the dissolution of NaA crystal surface.During recrystallization,the reaction proceeds from the surface to the interior of the NaA crystals,finally the NaA crystals is converted completely to smaller NaF crystals.