摘要
用Monte carlo方法计算设定硅铝比下八面沸石中五种结构单元Si(nAl)(n=0,1,2,3,4)的数量分布,与^(29)SiMASNMR实验结果直接比较,有令人满意的吻合. 八面沸石中四种类型铝离子Al(mAl)(m=0,1,2,3)的数量分布关系到八面沸石酸性质.在低铝浓度情况下,Monte Carlo计算结果与基于二项分布统计假设的计算结果相近;在高铝浓度(低硅铝比)情况下,Monte Carlo方法较只适于高度稀释前提的二项分布假设为优。在Monte Carlo模拟八面沸石脱铝过程的研究中,基于一级动力学把连续的脱铝过程用离散、可列状态的马尔科夫链作近似,其优点是考虑了过程因素从而更逼近真实;同时又计算了脱铝过程亚稳态的铝-铝次邻分布{Al(mAl)}和硅-铝近邻分布{Si(nAl)}.前者涉及脱铝八面沸石的酸性质;后者可通过^(29)SiMAsNMR实验观测。Monte Carlo模拟把二者联系起来,从而开拓了用^(29)SiMASNMR实验推断铝-铝次邻分布的途径.
The distributions of five building units, Si(nAl) (n=0,1,2,3,4), in faujasites were calculated in terms of Monte Carlo Simulation for the given Si/Al ratios. Theoretical results coincide with ^(29)Si MAS NMR experiments satisfactorily.According to Loewenstein rule, there is no nearest-neighbour relation among Al ions in zeolites. However, it is possible that two Al ions form the next-nearest-neignbour relation. It is obvious that any one of the Al ions on faujasite lattice may has 0,1,2 ot 3 next-nearest-neighbour Al ions, by which four types of Al ions can be identified. The relative number of all the types of Al ions, which concern the acidic behaviour of the faujasite,was calculated by Monte Carlo Simulation based on the distribution of the random sitting model. If the aluminum concentration is low, the calculated results of Monte Carlo simulation will be close to the results based on the binomial distribution calculation. For the high aluminum concentration cases, Monte Carlo simulation is more reliable than the results obtained from the binomial distribution, because the later is only appropriate for those dilute cases.The dealuminization process of faujasite was studied with the stochastic process approach, in which the continuative dealuminization process was approximated by the discrete Markov chain. Both the next-nearest-neighbour Al-Al ions distribution, Al(mAl) and the nearest-neighbour Si-Al ions distribution, Si(nAl) for the dealuminated faujisite were calculated simultaniously from the same sample space simulating the sequence of metastable states. And so an important advantage of Monte Carlo simulation is that the correlation between the two calculated distribution is close to the nonequilibrium reality of the dealuminization.The Al-Al ions next-nearest-neighbour distribution of the dealuminated faujasite is related to its acidic behaviour and the Si-Al ions nearest-neighbour distribution of the dealuminated faujasite can be observed experimently by ^(29)Si MAS NMR. Monte Carlo simulation reveals the correlation between two distributions and indicates that the ^(29)Si MAS NMR is able to bring to light the information on Al-Al ions distribution of zeolites.
