Studies on Mordenite by Monte Carlo Method Ⅱ. Si, Al Distribution in Framework of Dealuminated Mordenite

The dealumination process is non-equilibrium which ends up with a metastable state. Therefore the \mathrmSi, \mathrmAl distribution in the framework of the dealuminated mordenites is quite different from the parent mordenite. It has been found that the ^29 \mathrmSi MAS NMR spectra of the dealuminated mordenite samples were evidently different from the parent mordenite samples with corresponding \mathrmSi / \mathrmAl framework ratios.
In order to describe the \mathrmSi, \mathrmAl distribution in the framework of dealuminated mordenites, it is necessary to follow with the starting state of parent material a simulation of the dealumination.
In the present work, the continuous dealumination is studied by a discrete stochastic process i.e. the Markov chain with stationary transitions and countable state space. The simulation is based on the assumptions of first-order kinetics and that the strong-acid \mathrmAlO_4^-sites are preferentially removed for mordenites. By this procedure the state variables, \operatornameSi(n-A l) and \mathrmAl(n-\mathrmAl), of the metastable states with different degree of dealumination can be calculated.
The calculated evolutions of \mathrmSi(n-\mathrmAl) upon the Al content of dealuminated mordenites can then explain the ^29 \mathrmSi MAS NMR spectra of the dealuminated mordenite samples for corresponding Si / Al framework ratios.
For the mordenite, that the isolated strong-acid sites, \mathrmAl(0-\mathrmAl), are preferentially removed which is different from the faujasite. Based on the fact, the Monte Carlo simulation shows that the number of strong-acid will decrease rapidly during the dealumination process. A further predication that the acidity is always weakened in dealuminated mordenite samples compared with mordenites synthesized with the same aluminium content will be easily checked by experiments.