Stepwise Dealumination of Zeolite Beta at Specific T-sites Observed with 27Al MAS and 27Al MQ MAS NMR.

Abstract

Using 27Al MAS and 27Al MQ MAS NMR, dealumination of zeolite H-Beta has been observed at specific T-positions in the framework. 27Al MQ MAS NMR is able to resolve aluminum in the T1 and T2 positions from the other (T3 - T9) positions in the framework. A quantitative analysis of the 27Al MQ MAS NMR spectra shows that aluminum atoms in positions T1 and T2 resist dealumination and do not adopt an octahedral oxygen coordination. Moreover, it is shown that a heat treatment of 450 C of NH4-Beta gives a single type of fairly symmetric framework octahedral aluminum, which can be reconverted to framework tetrahedral aluminum by ammonia treatment. A more severe heat treatment (550 C under steam) causes some of the tetrahedral framework aluminum atoms on positions T3-T9 to convert to at least two different types of octahedral aluminum that are connected to the framework. These sites are proposed to be different consecutive steps in the process of framework dealumination of zeolite Beta.