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

Abstract

The potential of valence to core Al X-ray emission spectroscopy to determine aluminum distribution in ferrierite zeolites was investigated. The recorded emission spectra of four samples prepared with different structure directing agents exhibit slight variations in the position of the main emission peak and the intensity of its low energy shoulder. Theoretical calculations indicate that an increased intensity of the Kb x shoulder in the Al emission spectra can be linked to a predominant occupation of the T3 site by a single aluminum atom. This study thus suggests that valence to core X-ray emission spectroscopy can be applied to help determine the occupation of aluminum at crystallographic T-sites in zeolites.Zeolites are microporous crystalline aluminosilicates whose frameworks are composed of corner sharing TO 4 tetrahedra (T = either silicon or aluminum). The mutual arrangement of these subunits defines the framework structure which is characterized by periodical pores and channels. Replacing silicon atoms with aluminum atoms at the T-sites introduces a local negative charge to the framework which can be stabilized by protons or cations, which confer the catalytic activity to the zeolite. The catalytic activity is therefore directly related to the positioning of aluminum atoms within the zeolite framework.The determination of both the location of aluminum atoms at individual T-sites (aluminum sitting) and the mutual arrangement of several aluminum atoms (aluminum distribution) remains an open question in zeolite structural analysis.1 The Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR) spectroscopy of 29Si represents a standard method to address the Si-Al connectivity, however, not providing information about the absolute position of aluminum and silicon atoms inside the framework.27 Al MAS NMR is sensitive to the averaged Al-O-Si angle and therefore partially yields aluminum occupation. sequences, where at least one aluminum atom is located in the second coordination shell of another aluminum atom, has been determined. Due to a large penetration depth of X-rays, X-ray-based techniques are suited for the characterization of catalysts under reaction conditions. X-ray absorption fine structure (EXAFS) analysis combined with DFT simulations showed a preference of aluminum for the T sites in the 4-ring of zeolite beta. 7 X-ray absorption near-edge structure spectroscopy (XANES) was successfully applied to monitor aluminum coordination as a function of temperature and gas composition. 8 X-ray diffraction (XRD) techniques are, in general, not suitable for distinguishing silicon and aluminum atoms because of their similar scattering power. The structural changes induced by introducing aluminum, mainly an elongation of the T-O distance by ca. 0.1 Å, can in some cases indicate the position of aluminum. 9 However, this difference is obscured by the low Al/Si proportion at that specific T-site. Using the X-ray standing waves, the location of aluminum in scolecite (NAT framework type) has been determined. 10 Alt...

Pinpointing and Quantifying the Aluminum Distribution in Zeolite Catalysts Using Anomalous Scattering at the Al Absorption Edge

“…Protons associated with the O atoms around T2 are accessible for reaction in the 10-ring channel. The concentration of aluminum at the T1 and T3 sites in the FER-PYRR structure is completely in line with the structural study mentioned above and previous X-ray emission spectroscopy, MQ MAS NMR, , heat of adsorption, and computational studies performed on the same samples. The advantage of the AXPD characterization is that it is also quantitative.…”

“…Although their chemical compositions (|H 2.2 |[Si 33.8 Al 2.2 O 72 ]) and framework structures are identical, they were synthesized under different conditions (specifically tailored to influence the Al distribution), and indeed they exhibit different catalytic properties . Conventional structure analyses and other techniques − suggest that the aluminum atoms are differently distributed over the four T sites in the two materials (Figure ), so these samples appeared to be ideal candidates for testing not only the viability of the Al AXPD method but also, because of their high Si:Al ratios (ca. 15), its limits of detection.…”

Inelastic Neutron Scattering Study of the Aluminum and Brønsted Site Location in Aluminosilicate LTA Zeolites

“…27 Al MQ MAS NMR has also been used in combination with other techniques such as EXAFs and density functional theory (DFT)-based molecular dynamics, and with results of cracking of n -hexane and 3-methylpentane using the concept of transition-state shape selectivity, to find the positions of the population of Al. Al X-ray emission spectroscopy, with the information about valence-to-core transitions and DFT cluster calculations, has been used to identify T3 as the most populated Al site in ferrierite …”

“…Al X-ray emission spectroscopy, with the information about valence-to-core transitions and DFT cluster calculations, has been used to identify T3 as the most populated Al site in ferrierite. 11 In a previous study, 12 we have investigated the location of the Brønsted acid sites in LTA-40 (Si/Al = 40) by using inelastic INS has been used previously to characterize relevant vibrations 13 and Brønsted acid sites in zeolites 14 as well as relevant catalytic and adsorption properties of zeolites. 15−17 In the present work, we aim to fully characterize the aluminum and proton distributions of LTA-5 (Si/Al = 5) using INS, 29 Si MAS NMR, and periodic DFT calculations.…”

Modeled Influence of a Structure Directing Agent on Al Siting Energetics: N,N,N-trimethyl-1-adamantyl ammonium (TMAda+) in Chabazite

“…27 Rietveld refinement of powder X-ray diffraction is another technique which can locate OSDAs inside zeolite cavities after crystallization. [28][29][30] Other characterization techniques, including fluorescence spectroscopy, 31 valence-to-core X-ray emission spectroscopy (XES), 32 and atom probe tomography (APT) 33 have been applied to provide information on the relationship between Al siting and the type of OSDA used in synthesis.…”