NH4Y沸石分子筛上质子的运动(Ⅰ)——用CNDO/2法计算氨在HY沸石上的吸附热及质子在NH_(4)Y内的迁移

PROTON MOBILITY IN NH_(4)Y ZEOLITE-I——THE HEAT OF ADSORPTION OF NH3 ON HY ZEOLITE AND PROTON MOBILITY IN NH_(4)Y OBTAINED BY THE CNDO/2 METHOD

用CNDO/2方法定量计算氨在HY(Si/Al=2.4)沸石上的吸附热,其值24.4kcal/mol与实验值符合较好。质子在表面与NH3间迁移,有两个能量极小值;吸附时,质子从接近表面的极小迁移到接近NH3极小时,要越过势垒9.40kcal/mol;反之要越过势垒11.9kcal/mol。400℃时,质子在表面与NH3间跳动频率为1.92×10^(9)sec^(-1)。NH3吸附后,NH键长增加,加以质子迁移,导致NH8上负电荷密度向沸石表面迁移,使H上的电子集居数减少,H更质子化。

The heat of adsorption of NH3 on HY zeolite has been obtained by CNDO/2 method using(AO)_(3) AlOHSi(AO)_(3) clusters.The result is closed to the experimental data.It is demonstrated that the proton moving from oxygen on surface of HY zeolite to nitrogen of adsorbate would prefer some site for energetic reason.Appering in the curve of potential energy were two minima.One of minima with distance-1.138A is near oxygen and the other one-1.720A is far away from oxygen but near nitrogen.From one minimum to the other the barriar must be climbed over.The value of potential barriar is 9.4kcal/mol from the side of oxygen and 11.9 kcal/mol from the side of nitrogen.Energy difference between the two minima is 2.5 kcal/mol.When NH3 is adsorbed on cation site,the proton moves a distanc from 1.085A(between the oxygen and proton)to 1.138A;The system was evolved energy 21.9kcal/mol.and then the proton climbed over the barriar to arrive at a lower minimum,evolved energy 2.5kcal/mol again,hence the total heat of the adsorption was 24.4 kcal/mol(experiment:25.7kcal/mol).The turnover number for the proton from the side of nitrogen to the side of the oxygen of the framework in the supercage is 1.92×10^(8) sec^(-1) at 400℃.