【摘要】 用CNDO/2方法计算了NH₃在HY原子簇上的吸附热和质子在NH₃和HY沸石间的迁移频率。计算结果指出:质子的迁移频率随着结构硅铝比的增加而增加;硅铝比的递增使迁移频率的增加大于相应羟基浓度的减少。质子迁移的活化能也随着晶胞常数的变小而下降。更多还原

【Abstract】 The cluster, (OA)3Si (OH) A1(OA)3, was taken from a single crystal structure of faujasite zeolite, where A is the closed atom. The S (II) was selected as the adsorption position for NH3, and the various Si/Al ratios of framework were replaced by different lattice constants under the condition that the crystal symmetry was kept unchanged. The adsorption heats of NH3 adsorbed on the hydroxyl groups of the cluster with various lattice constants and the potential energy curves of proton transfer between NH3 and clusterwere calculated by an approximate quantum chemical method--CNDO/2scheme, and an adsorption model of "hydrogen bonding". It is shown that there are values of two minima and a maximum on the potential energy curves. The first and second minimum values, E1 and E3,are of molecular adsorption of ammonia and ionic adsorption of ammonium respectively. The maximum value,E2,is the potential barrier, which must be overcome by proton when it transfers from molecular adsorption to ionic adsorption and vice versa. According to the Erying’s absolute reaction rate equation, the proton transfer frequencies between the cluster and NH3 were calculated.Calculated results show that the protonic concentration is in agreement with the order of(AlO2)- unit in zeolite framework, i.e. HY2.4>HY2.99>HY3.69> HY5.15.but this order is just contrary to the order of the strength of protonic acid, this is because the acid strength increases with the increasing of proton transfer frequency (T1 or T3), or increases with the decreasing of potential barrier E2.When the Si/Al ratio increases from 2.4 to 5.15, the decrease of the protonic concentration is less than one fold, but the increase of the proton transfer frequency is more than four times. If the mechanism of cumene cracking is considered as a carbonium mechanism and the reaction rate is in keeping with the strength of the protonic acid, then, in view of the above results, the reason why the conversion of the cumene cracking is 60% at Si/Al = 2.4, and 90% at Si/Al = 5.15 is quite simple and clear.更多还原