Water in Acid Boralites: Hydration Effects on Framework B Sites

Abstract

Properties and behaviour of protonated boron-containing zeolites at different hydration degree have been investigated by means of periodic DFT approaches. Geometry optimization and room-temperature Car-Parrinello molecular dynamics results, in line with experimental findings, indicate that the BO 3-bound silanolic acid site typical of dry boralites should convert to a solvated H 3 O + hydrogen bonded to tetrahedral BO 4 at moderate water content. By increase of the water loading, the tetrahedral structure of the B site is stabilized and the physicochemical properties of the water molecules solvating the acid proton gradually approach the liquid-phase ones. A relevant role of structural and vibrational properties of the zeolite framework in the water-induced trigonal-to-tetrahedral transition at the B site is highlighted by simulation results.

Supramolecular Organization in Confined Nanospaces

“…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 . Such weaker Brønsted acidity of B‐sites guarantees a high selectivity for ethylene from methane and is relevant for the environmentally friendly production of styrene .…”

“…[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. [244] Aluminophosphates (ALPO) [245,246] generally have neutral framework and large pore sizes, like in VPI-5.…”

Template burning effects on stability and boron coordination in boron levyne studied by in situ time resolved synchrotron powder diffraction

“…The experimental work as reviewed by Millini et al [1] and further performed by means of 11 B solid-state NMR [10,11] and IR spectroscopy [12], along with theoretical studies [13][14][15] have shown that the boron change in coordination upon heating is strongly related to the nature of the counterions (protons, large counterions, cationic organic structure directing agent (SDA)) and to the hydration/dehydration level. In particular, in fully hydrated proton forms, boron is tetrahedrally coordinated, whereas upon dehydration boron assumes a trigonal coordination [10,11,16].…”

“…Removal of organic complexes from the zeolite pores by template burning leaves behind one proton per each SDA complex for charge-balancing and hence boron atoms are transformed to trigonally coordinated boron. This tetrahedral-to-trigonal boron conversion is fully reversible upon rehydration [10][11][12]15,16,19]. The questions remain as to whether the boron atoms with trigonal coordination are in non-framework positions, or anchored in the framework [10].…”

Excited‐State Prototropic Equilibrium Dynamics of 6‐Hydroxyquinoline Encapsulated in Microporous Catalytic Faujasite Zeolites

“…[8] Host-guest systems are applicable in various fields, such as ion exchange, catalysis, sensing, and supramolecular organization. [9][10][11][12][13][14] These widespread applications of zeolites are mainly attributed to their catalytic activity, [15][16][17][18][19] which is mainly due to the presence of Brønsted acid/base sites. The acid/base sites of zeolites are capable of adsorbing guest molecules by strong hydrogenbonding interactions and lead to protonation or deprotonation of the guest molecules.…”