Solid-state silicon n.m.r. studies of the zeolite mordenite and its dealumination

Abstract

A series of synthetic mordenites from different sources and covering a range of bulk SiO 2 Al 2O 3 ratios of between 10 and 52, have been measured with solid-state 29Si n.m.r. This series of experiments shows that the 29Si resonances at −100, −106 and −113 ppm are due to silicon atoms tetrahedrally bound via oxygen bridges to two, one and zero aluminium atoms, respectively. The effects of cation exchange, acid leaching and steam calcination have also been followed by 29Si n.m.r. It is shown that acid leaching as such results in aluminium removal from the framework, as does steam calcination. Acid leaching after steaming, however, does not substantially remove any more framework aluminium, but does remove non-framework aluminium produced by steam calcination. Additional information about the structure of these mordenites during ultrastabilization (the formation of silanol groups in lattice defects) can be obtained from 1H- 29Si cross polarization experiments (which are vital in any 29Si solid-state n.m.r, study of zeolite dealumination). Repeated acid leaching and steam calcination results in a basically aluminium-free framework, which yields a three-line 29Si n.m.r. spectrum. These peaks (−112.6, −113.5 and −115.3 ppm) reflect different crystallographic sites present in mordenite in a similar fashion to multiple Si(0AI) resonances in spectra of well-defined silicalite or ZSM-5 zeolite samples. These highly dealuminated mordenites display very long 29Si T 1 relaxation times (⩾240 s). This strongly suggests an extremely regular framework structure, without paramagnetic impurities.