Studies of mordenite by the Monte Carlo method

Abstract

A model on the distributions of Al atoms in the framework of parent modernites is suggested. The distribution of Al atoms in the framework of dealuminated mordenite is determined by both the parent sample and its follow-up dealumination. The continuous dealumination is simulated by a discrete stochastic process (Markov chain). The relative populations of the building units {Si( n-Al)} and the types of Al atom {Si( m-Al)} for parent mordenites with different Si Al ratios and the evolutions of the relative populations of {Si( n-Al)} and {Si( m-Al)} during the dealumination can then be calculated by Monte Carlo method. Since the relative populations of both {Si( n-Al)} and {Al( m-Al)} are obtained from the same sample space of the Monte Carlo simulation, their correlation is established from a reliable theoretical foundation. The former, calculated {Si( n-Al)}, can be compared directly with 29Si MAS n.m.r. observations of parent and dealuminated mordenites to prove the reasonability of the present theory. The latter, calculated {Al( m-Al)}, predicts the acidic properties of the parent and the dealuminated mordenites. The definition of the efficiency parameter of type acid site, which scales the contribution of a type acid site toward the acidity, is introduced. Based on this definition, the acid amount and the efficiency parameter α o (introduced by Beaumont and Barthomeuf for faujasites) in a mordenite sample can be expressed quantitatively. The present theory satisfactory explains the ammonia t.p.d. acidity measurement results for both the parent and dealuminated mordenite samples.