Pentasil zeolites from Mt Adamson (Northern Victoria Land): a key for the synthesis of new materials for eterogeneous catalysis and for the confinement of nano-materials.

Zeolites are a group of minerals and synthetic materials that have increased enormously in industrial and economic importance in recent decades. In addition to traditional applications in the purification of waste water and as additives in detergents (as molecular sieves and ion exchangers), valuable new economic applications have emerged. These include use in heterogeneous catalysis (for example, in the cracking of petroleum) and environmental protection (for example, the reduction of nitric oxide emissions). Moreover, their intrinsic pore architecture, with tuneable pore sizes and narrow pore size distributions, provides a unique platform to study the grafting of highly reactive organometallic compounds. Zeolitic voids can be regarded as nanoreactors i.e. uniform, nanometer-scale vessels in which highly specific reactions can occur. This in turn suggests that such nanovessels may ultimately have a role as molecular assemblers. While the possibilities are exciting and some progress in synthesizing these materials has been made, research in the area of nanopore host/guest chemistry has been hampered by difficulties in characterizing the final composite structure, and a clear structural characterization of the nanosized guests is still a challenge. Despite considerable research in industry into the potential uses of synthetic zeolites, natural phases retain their primary position even in this sector.

From a methodological point of view the complexity of natural systems and advanced technological systems can be advantageously compared, working simultaneously on synthetic products (similar or not to natural ones) and natural materials, and using complementary technical procedures diversified according to an innovative multidisciplinary approach. During field-works at Mt. Adamson (northern Victoria Land), a number of rare and important pentasil zeolites (gottardiite, mutinaite, terranovaite, tschernichite, and boggsite) were found and then structurally characterized. The first three are newly identified crystalline phases, and the other two represent second instances found in the world. All these minerals were studied in detail from the chemical, physical and structural point of view, and all of them, for different specific reasons, appeared to be potentially innovative materials for heterogeneous catalysis and nanoconfinement. Mutinaite is the natural counterpart of the synthetic zeolite ZSM-5, the best known and widely used zeolite in heterogeneous catalysis. The most relevant difference between the natural and the synthetic phase is the chemical composition, and in particular the Si/Al ratio (about 7 in mutinaite, and much higher than 10 in the synthetic phase). This suggests that mutinaite could even be considered a distinct catalyst with particular properties different from those of ZSM-5. An important point is that mutinaite, not withstanding its low Si/Al ratio, is characterized by high thermal stability. Gottardiite is the natural equivalent of the synthetic NU-87 and Terranovaite (from the name of the Italian base at Terranova Bay) has been found to date only at Mt. Adamson and does not yet have a synthetic analogue. It is characterized by an interesting two-dimensional ten-membered ring channel system, connected by ten-membered ring windows. Such a system is very similar to that present in the synthetic zeolite ZSM-11, which is of significant applicative interest.

Moreover, this two-dimensional ten-membered ring channel system seems to be an excellent host for nanowires developing in two directions. Boggsite, characterized by high thermal and structural stability and by the presence, in the dehydrated phase, of large and void linear channels, could be an ideal matrix for confinement and stabilization of nanomaterials like metals, semiconductor clusters and molecular chains. Tschernichite is the natural counterpart of the synthetic zeolite beta, largely used in many important catalytic processes. Zeolite beta is a highly faulted intergrowth of two polymorphs, strictly connected to each other and not separable. On the contrary, in the Mt. Adamson tschernichite, the two polymorphic phases crystallize in different distinct crystals. This peculiarity provided a unique opportunity to structurally and technologically characterize the two polytypes, thus providing precious information for the synthesis of the most efficient form for technological processes. The zeolites discovered in Antarctica have not only significantly improved the knowledge of the corresponding synthetic phases, but have also indicated the possibility of new methods for the synthesis of zeolitic materials. In particular the lower Si/Al ratio compared with analogous synthetic products, their formation in absence of an organic template, and the presence of calcium as a predominant extra framework cation, suggest the possibility of different conditions of synthesis from those currently applied in industry, and potential alternative applications for these materials. There is, however, no doubt that the uniqueness of the Antarctic finds is related to the particular conditions of genesis that permitted the concentration of such a large number of unusual zeolites in such a small area. It is therefore essential to learn the chemical-physical parameters underlying the crystallization of these minerals in order to transfer them for synthesis.

These phases were subject to the following studies:
1.Crystal-chemical and structural characterization.
2.Analysis of thermal properties.

For the identification of the mineralogical phases binocular and electronic microscopy (SEM) and X-ray powder diffraction (Gandolfi camera for materials present in small quantities or powder diffractometer in the case of abundant mineralogical phases) were utilized. Chemical characterisation of all the phases was carried out by means of electron microprobe and thermal analyses (thermogravimetric and thermodifferential) and structural analysis by means of 4-circle diffractometer for single crystals with a conventional generator and CCD detector or rotating-anode generator, or using synchrotron radiation. Examination of thermal properties was performed by means of: thermogravimetric and thermodifferential analysis and in-situ powder and single-crystal diffraction. Transmission Electron Microscopy (TEM) and computational methods were also applied.