Rigorous Examination of the Crystal Structures of the As-made Forms of ITQ-55: What Might a Zeolite Look Like Near Nucleation?

Allen Burton

ExxonMobil Research and Engineering Company

We have used a combination of 29Si NMR and high-resolution synchrotron powder x-ray diffraction to examine the structure of the as-made forms of zeolite ITQ-55 prepared both from hydroxide media (OH-ITQ-55) and from fluoride media (F-ITQ-55). Our presentation will review the unusual behavior of this zeolite with respect to calcination. Each as-made material has interrupted bonds at a specific crystallographic tetrahedral (T) site (Si3-O-Si3) that are broken to create a siloxy/silanol pair Si3-OH•••- O-Si3. Upon calcination these siloxy/silanol pairs anneal to form Si3-O-Si3 bonds and the unit cell volume shrinks by about 8%.

In addition to the interrupted Si3-O-Si3 bonds of the as-made structure, OH-ITQ-55 has a high concentration of vacancies at other crystallographic T sites that cannot anneal upon calcination. These vacancies provide siloxy/silanol nests that compensate half of the charge of the occluded SDA cations. The vacancies are concentrated near the Si6 site, whose O atoms are closest to the other N atom of the SDA dication. In contrast, F-ITQ-55 has a low level of vacancies because negative charge is provided by the occluded fluoride that is bonded to the framework at the Si6 site to create a pentacoordinated silicon. Because the vacancies of the OH-ITQ-55 do not allow the structure to anneal, upon calcination the powder diffraction pattern of the material is broadened from a of loss of structure integrity (i.e., partial amorphitization) or from likely disorder due to random condensation of a portion of the silanols that exist within each nest. Each vacancy site is created by the removal of two adjacent T sites to create a nest composed of 1 siloxy and 5 silanol species. This contrasts with the normally anticipated case in which a single T atom is removed to create a nest of 1 siloxy and 3 silanol groups. Although the T sites are removed as adjacent pairs, we find that the fractional occupancy of the O atoms between those T sites (the O in the T-O-T linkage) refines to full occupancy. We discuss the implications of this result.