Quantitatively Probing the Al Distribution in Zeolites

Abstract

The degree of substitution of Si(4+) by Al(3+) in the oxygen-terminated tetrahedra (Al T-sites) of zeolites determines the concentration of ion-exchange and Brønsted acid sites. Because the location of the tetrahedra and the associated subtle variations in bond angles influence the acid strength, quantitative information about Al T-sites in the framework is critical to rationalize catalytic properties and to design new catalysts. A quantitative analysis is reported that uses a combination of extended X-ray absorption fine structure (EXAFS) analysis and (27)Al MAS NMR spectroscopy supported by DFT-based molecular dynamics simulations. To discriminate individual Al atoms, sets of ab initio EXAFS spectra for various T-sites are generated from DFT-based molecular dynamics simulations, allowing quantitative treatment of the EXAFS single- and multiple-photoelectron scattering processes out to 3-4 atom shells surrounding the Al absorption center. It is observed that identical zeolite types show dramatically different Al distributions. A preference of Al for T-sites that are part of one or more 4-member rings in the framework over those T-sites that are part of only 5- and 6-member rings in an HBEA150 zeolite has been determined using this analysis.

Investigating the Influence of Mesoporosity in Zeolite Beta on Its Catalytic Performance for the Conversion of Methanol to Hydrocarbons

“…Recently, Vjunov et al used ultrahigh-field 27 Al MAS NMR to probe the distribution of Al among the nine crystallographically distinct T-sites in zeolite Beta (T1-T9 sites, see Figure 3), and their analysis was based on the deconvolution of the spectra according to the DFT-predicted 27 Al NMR chemical shifts. [52][53] Using this established method, we probed the Al distribution in Beta-MS and Beta-C. Figure 3 shows the 27 Al NMR spectra measured at 900 MHz, where by fitting the experimental spectra using the DFT-calculated NMR chemical shifts, [52][53] We tested Beta-MS and Beta-C with the MTH reaction using a fix-bed reactor at 330 °C and 101 KPa total feed pressure. The initial conversion of methanol was controlled by varying the hydrocarbon selectivity increased with decreasing crystallite size of the ZSM-5 catalyst.…”

Impact of Zeolite Aging in Hot Liquid Water on Activity for Acid-Catalyzed Dehydration of Alcohols

“…The prominent tetrahedral Al feature is attributed primarily to excitations from Al 1s to a mixture of O 3p and Al 3p states. 8 In the case of lab-synthesized HBEA, both parent and water-treated, there is no indication of 5-or 6-coordinated Al species, which are typically observed at 1567.5 eV and 1568 eV, respectively. 15 In the case of the commercial HBEA150 and HBEA150w, the XANES suggest 7 and 8 % octahedral Al 3+ , respectively.…”

Exploring Low-Temperature Dehydrogenation at Ionic Cu Sites in Beta Zeolite To Enable Alkane Recycle in Dimethyl Ether Homologation

“…Based on a previous report by Vjunov et al probing the Al distribution in BEA with a similar SiO 2 /Al 2 O 3 ratio of 25, a T7 site was chosen for the Al substitution site in our model. 30 The minimum energy location of the Cu(I) ion in the BEA zeolite model lies in the six-membered ring associated with the T7 site (Table S3 in the Supporting Information). The Cu(I) ion is located slightly off-center, shifted toward the Al atom ( Figure 4B).…”