Calculated magnetic properties for the characterization of zeolite active sites

Abstract

Detailed structures of zeolite catalysts, including Al and cation distribution, framework and catalytic site geometries, are not fully accessible from experiment. Since the magnetic properties of framework elements and extra-framework cations are strongly dependent on their environment, the combined use of magic angle spinning NMR or ESR techniques with quantum chemical calculations is very useful to establish the local structure around specific sites. General effects of Al and B substitution in the zeolite framework, coupled with H(+) or Na(+) counter-ions, on the (29)Si, (27)Al and (11)B NMR spectra were studied, using a density functional theory (DFT)-based methodology, for a model of the zeolite mazzite. In agreement with experiment, the exchange of Na(+) by H(+) in the boron compound is accompanied by a change of B coordination from tetrahedral to trigonal, with a characteristic downfield shift of around 10 ppm, whereas the presence of water restores the tetrahedral boron and its NMR chemical shift. Further, ESR spectra of zeolites exchanged with open-shell cations provide useful data on the metal coordination and its reactivity compared with calculated model ESR properties. The ESR hyperfine coupling constants, calculated using DFT, for models of different Cu sites of a Cu(II)-Y zeolite, with and without H(2)O or NH(3), show a clear correlation between the Cu spin population and its coordination, involving the participation of the zeolite framework in the reactivity.

First Principles Studies on Boron Sites in Zeolites

“…28 Structure, acidity, and relative stability of B-sites have been the subject of several theoretical studies based on cluster models. Calculations on cluster models of acid B-ZSM5, [29][30][31] Bmazzite, 32,33 and B-MCM22 34 agree in predicting a trigonal structure for the B-site, with a silanol group formed on the adjacent Si atom. Properties such as proton affinities, stretching frequencies, and NMR chemical shifts have also been calculated on model sites, with an overall agreement with experimental trends.…”

Supramolecular Organization in Confined Nanospaces

“…Zeolites where atoms other than Si and Al occupy tetrahedral sites (zeotypes) are important for catalysis: for example, the acid strength of proton‐containing zeolites, can be tuned by replacing Al with Ga or B . For instance, boron zeolites are less acidic than Al‐ones due to the tendency of boron to be trigonal . The change of B coordination from tetrahedral to trigonal leads to interruptions in the framework and is accompanied by a 10 ppm downfield shift of 11 B NMR signal,, whereas tetrahedral boron geometry is recovered upon hydration .…”

“…[235] For instance, boron zeolites are less acidic than Al-ones due to the tendency of boron to be trigonal. [236][237][238] The change of B coordination from tetrahedral to trigonal leads to interruptions in the framework and is accompanied by a 10 ppm downfield shift of 11 B NMR signal, [237,239] whereas tetrahedral boron geometry is recovered upon hydration. [240][241][242] Such weaker Brønsted acidity of B-sites guarantees a high selectivity for ethylene from methane [243] and is relevant for the environmentally friendly production of styrene.…”

Water in Acid Boralites: Hydration Effects on Framework B Sites

“…In the past, calculations have been carried out by using a variety of first-principles techniques (both wavefunction-or electronic density-based) mainly on cluster models of zeolite acid sites. [27][28][29][30][31] First principles studies accounting for the periodicity of the crystal structure are available as well. [32][33][34][35] Theoretical results agree in predicting that proton transfer from an Al-zeolite Brønsted acid site to a single water molecule does not occur; indeed, solvation of the hydronium ion by at least one water molecule is needed.…”

“…28,29,32 In the case of boron sites, whose acidity is lower, stabilization of hydronium species should require a larger number of solvating water molecules. Actually, a recent DFT study on a B-zeolite cluster model 30 evidenced that two water molecules are not effective in deprotonating the silanol group.…”