La^(3+)-NaY型沸石的水热交换反应

THE MECHANISM OF La^(3+)-NaY ION-EXCHANGE REACTION

作者根据在100℃,180℃时La^(3+)-NaY型沸石交换反应等温线的测定和十三种单一稀土(Ln)元素在180℃时的交换度与相应Ln^(8+)离子水合焓关系的研究以及在不同温度下交换反应速率常数、反应级数与交换反应表观活化能的测定,比较系统地研究了La^(3+)-NaY型的水热交换反应,从而比较有据地提出这类交换反应的机理。

As is well known,the mechanism of La^(3+)-NaY ion-exchange reaction reviewed by H.S.Sherry has been mainly ascribed into two steps.The second step of them called the slow step is proposed as the rate-controlling step in the whole reaction with respect to the stripping of water molecules from the hydrated La^(3+) ion in the large cavity in relation with the replacement of Na+ by La^(3+) in the β-cage.The corresponding activation energy can be estimated by the enthaly of hydration of La^(3+) ion.Although the assumption of the rate-controlling step is reasonable,there is still lack of exact experimental evidence.It is to make a deeper understanding of the slow step by means of experimental devices.The arrangement of measurement condition at elevated temperature may facilitate the systematical study of the behaviour of this kind of ion-exchange reaction.The present authors have investigated the slow step at an elevated temperature in three aspects;1.To measure the ion-exchange isotherm,for La^(3+)-NaY system at 100℃and 180℃ respectively.2.To investigate the degree of ion-exchange of individual LnY at 180℃ and to compare the results with the enthalpy of hydration of Ln3+ ions.3.To evalute the rate-constant and apparent activation energy of the slow step at elevated temperature from our experimental data.As a result,a more rational mechanism of La^(3+)-NaY ion-exchange reaction is proposed.Ⅰ.La(H_(3)O)_(9)^(3+)+ NaY(s)→La(H_(2)O)_(9)^(3+)Na(SI)F(s)+Na+Ⅱ.La(H_(2)O)_(9)^(3+)(in large cavity)+La(H_(2)O)_(9)^(3+)Na (SI)Y(s)→La(H2O)93+LaY(s)+Na+Eq-Ⅱ should consist of the following two consecutive reactions;Ⅱ-A.La(H_(2)O)_(9)^(3+) (in large cavity)→La^(3+)+9H_(2)OⅡ-B.La^(3+) + La (H_(3)O)_(9)^(3+)Na(SI)F(s)→La(H_(2)O)3+LaY(s)+Na+Eq-Ⅱ-A is the rate-controlling step for this ion exchange reaction.