The Si, Al Distribution in Faujasites Studied by Monte Carlo Method

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 ²⁹Si 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 ²⁹Si MAS NMR. Monte Carlo simulation reveals the correlation between two distributions and indicates that the ²⁹Si MAS NMR is able to bring to light the information on Al-Al ions distribution of zeolites.