Study on the Si,Al Distribution in Dealuminated Zeolite Omega

Based on first-order kinetics, the dealuminization process of zeolite omega, which contains two erystallographically unequivalent tetrahedral sites, is simulated in terms of Monte Carlo method. The continuous dealuminization process is approximated by the discrete Markov chain with stationary transitions and countable state space. The fact that comparing with T_B sites the aluminum ions on T_A sites are removed preferentially from the zeolite frame work during the dealuminization, which has been found by ²⁷Al MAS NMR observations, is considered in the simulation. For the given Si/Al ratios, the relative populations of the Si building units Si_k(nAl);k=A,B;n=o-4 calculated by Monte Carlo method coincide with the analytical data of ²⁹Si MAS NMR observations. And so the correctness of the distribution model and reasonableness of the setting values of the coefficients in the simulation are justified. Along with the same approach the next-nearest-neighbor Al-Al distribution, which relates to the acidic properties of zeolite omega, can be calculated. The evolutions of the relative populations of the strong acid on T_A and T_B sites during the dealuminization are simulated theoretically. The calculation predicts that the number of the strong acid sites onT_A and T_B come up to their maximum around Si/Al ratio of 4.24 and 18.44 respectively.
By contrasting the paralleled experiments which scale the acidity of the dealuminated zeolite omega samples with the calculated results of corresponding Si/Al ratios, the acidic strengths between T_A and T_B sites can then be compared. It will reveal the distinct contributions of both T_A and T_B acid sites that the different catalytic activities of zeolite omega with Si/Al ratios 18.44 and 4.24.