Determining the aluminium occupancy on the active T-sites in zeolites using X-ray standing waves

Abstract

Zeolites are microporous crystalline materials that find wide application in industry, for example, as catalysts and gas separators, and in our daily life, for example, as adsorbents or as ion exchangers in laundry detergents 1 . The tetrahedrally coordinated silicon and aluminium atoms in the zeolite unit cell occupy the so-called crystallographic T-sites. Besides their pore size, the occupation of specific T-sites by the aluminium atoms determines the performance of the zeolites 2 . Despite its importance, the distribution of aluminium over the crystallographic T-sites remains one of the most challenging, unresolved issues in zeolite science. Here, we report how to determine unambiguously and directly the distribution of aluminium in zeolites by means of the X-ray standing wave technique 3 using brilliant, focused X-rays from a thirdgeneration synchrotron source. We report in detail the analysis of the aluminium distribution in scolecite, which demonstrates how the aluminium occupancy in zeolites can systematically be determined.Zeolites are widely used materials with a very wide range of applications. They are used in households in laundry detergents and in industry as catalysts for petrochemical refining and they have become increasingly important in fine-chemical production and in environmental applications; they are used as air and water purifiers and in heating and refrigerating, and they are used in agriculture as selective absorbents. With natural zeolites being discovered more than 200 years ago, intensive research is nowadays aimed at developing new, synthetic zeolites with tailored properties for specific tasks. The functionality of a zeolite is determined basically by just a few parameters: the size and connectivity of the pores and cages, the Si/Al ratio and the placement of the trivalent aluminium atoms at specific crystallographic T-sites. Among them, the placement of aluminium is still unknown for many zeolites because no generally applicable experimental method has been reported to solve this problem. This seriously hampers the understanding of the function of some technologically important zeolites and hinders the development of new zeolites with novel functionalities.In a zeolite, the replacement of a tetravalent silicon atom by a trivalent aluminium atom on a T-site places a negative charge on the framework, which is compensated by non-framework cations. These charge-compensating cations are loosely held in the pores and cages of a zeolite and can be exchanged by other cations. This property makes zeolites good cation exchangers, which are useful in water softening and in the removal of nuclear waste. When the charge-compensating species are protons or elements that are involved in oxidation or reduction reactions, zeolites become catalytically active in Brønsted and redox reactions, respectively. The Si/Al ratio and the aluminium distribution over the framework T-sites thus determine the species and the positions of the chargecompensating cations in a zeolite, which strongly affect its cata...

Probing zeolites by vibrational spectroscopies

“…581 Fig. [575][576][577]582,583 The shifts in n(N-H) frequency are dominated by the framework oxygen basicity; moreover, these theoretical calculations also show that the cation-p-ring interaction, which is clearly seen in the complexes shown in Fig. The n(N-H) band clearly exhibits two main components I and II at high and low frequency, respectively, which both increase with the concentration of framework aluminum and hence with zeolite basicity.…”

Distribution of aluminum over different T-sites in ferrierite zeolites studied with aluminum valence to core X-ray emission spectroscopy

“…Using the X-ray standing waves, the location of aluminum in scolecite (NAT framework type) has been determined. 10 Although the method yields an absolute occupancy, the dependence of this technique on X-ray photon flux, detector efficiency, and X-ray focusing makes it not applicable for studying nano-sized crystals. Like X-ray absorption spectroscopy, X-ray emission spectroscopy (XES) is an element selective method.…”

Experimental Basics

“…In this way, XSW studies can be increasingly extended to real system, and the recent study of a zeolite crystal by XSW is here just an example. 52 Using crystals or grains of a crystal of a few micrometer size needs an even smaller beam which must also be sufficiently collimated thus requiring specific focusing setups as described recently. 53,54 Other future trends will involve the combination of XSW with other spectroscopic techniques such as Mössbauer spectroscopy, high-resolution resonant or non-resonant inelastic scattering, and angular resolved photoelectron spectroscopy.…”