Thermodynamic Study of Water Adsorption in High-Silica Zeolites

Surface Structure, Hydration and Cationic Sites of Nanohydroxyapatite

In this work, we studied the surface/water interaction properties of a pure calcium hydroxyapatite (HA) and their modifications as a consequence of the partial Ca2+/Mg2+ (MHA) substitution by means of IR spectroscopy and microcalorimetry of adsorbed water. IR data indicated that water molecules in direct contact with the surface of HA are coordinated to surface cations and experience H-bond significantly stronger than in liquid water. The heats of adsorption associated to such interactions are very high, being twice-triple the heat of liquefaction of water. Interestingly, water experiences H-bond higher than in its bulk liquid state also in the second layer. Finally the entering in the material of Mg2+ ions was shown to significantly affect the affinity of the material toward water and the properties of its hydration layers.

Functionalization of Zeolitic Cavities: Grafting NH2 Groups in Framework T Sites of B-SSZ-13 A Way to Obtain Basic Solids Catalysts?

Insertion of B atoms into an Al-free zeolitic framework with CHA topology results in the formation of B-SSZ-13 zeotype with Si/B = 11. B K-edge NEXAFS testifies that B forms [B(OSi)4] units in a Td-like geometry (sp3-hybridized B atoms). According to B K-edge NEXAFS and IR, template burning results in the formation of [B(OSi)3] units in a D3h-like geometry (sp2-hybridized B atoms) with a break of a B-O-Si bond and the formation of a Si-OH group. The activated material contains B(III) Lewis acid centers able to specifically coordinate bases like NH3. Such [B(OSi)3] units are reactive toward ammonia, resulting in the formation of B-NH2 surface functionality inside the pores of B-SSZ-13 already under mild conditions, i.e., 35 mbar of NH3 at 373 K for 30 min and without crystallinity degradation. A minor fraction of Si-NH2 cannot be excluded owing to the presence of two IR doublets at 3500 and 3430 cm-1 and at 1600 and 1550 cm-1. Ab initio B3LYP/6-31+G(d,p) calculations on a cluster model, supported by a single-point MP2 on B3LYP/6-31+G(D,P) optimized structures, found the break by NH3 of a B-O-Si bond of the [B(OSi)3] unit with formation of [SiOH] and [H2N-B(OSi)2] species to be energetically favored. Comparison between experimental and computed frequency shifts shows them to be in semiquantitative agreement. The high stability of the B-NH2 surface functionality is probed by N K-edge NEXAFS spectra collected under UHV conditions. These findings can open a new route in the preparation of shape selective solid basic catalysts.

Aluminosilicate surfaces as promoters for peptide bond formation: an assessment of Bernal's hypothesis by ab initio methods

The role in prebiotic chemistry that Brønsted and Lewis sites, both present at the surface of common aluminosilicates, may have played in favoring the peptide bond formation has been addressed by ab initio methods within a cluster approach. B3LYP/6-31+G(d,p) free energy potential energy surfaces have been fully characterized for the model reaction glycine + NH3 --> 2-NH2 acetamide (mimicking the true 2 Gly --> GlyGly one) occurring on (i) a Lewis site, (ii) a Brønsted site, and (iii) a combined action of Lewis/Brønsted sites. Compared to the gas-phase (gp) activation free energy of 50 kcal/mol, the Lewis site alone reduces the gp barrier to 41 kcal/mol, whereas the activation by the Brønsted site dramatically reduces the barrier to about 18 kcal/mol. Nevertheless, formation of the prereactant complex in this latter case will rarely occur, since water will easily displace the glycine molecule interacting with the Brønsted site. However, if a realistic feldspar surface with neighboring Brønsted and Lewis sites is considered, the proper prereactant complex is highly stabilized by a simultaneous interaction with the Lewis and the Brønsted sites, in such a way that the Lewis site strongly attaches the glycine molecule to the surface whereas the Brønsted site efficiently catalyzes the condensation reaction, showing that the interplay between Lewis/Brønsted sites is an important issue. The free energy barrier computed for the realistic feldspar surface model is 26 kcal/mol. The role of dispersive interactions on the free energy barrier and the stabilization of the final product, not accounted for by the B3LYP functional, have been estimated and shown to be substantial. Speculations about further elongation of the formed dipeptide have been put forward on the basis of the relatively strong interaction energy of the formed GlyGly dipeptide with the aluminosilicate surface.

Molecular Water on Exposed Al3+ Cations Is a Source of Acidity in Silicoaluminas

Evidence about the remarkable acidic nature of molecular water adsorbed at the surface of mesoporous MCM-41 silicoaluminas (MSA) with a ratio of Si/Al = 25 is reported, coming from both infrared spectroscopy and ab initio calculations. By using CO as a probe, OH stretching and HOH bending modes of water adsorbed on coordinatively unsaturated Al ions (Lewis sites) at the surface of MSA have been detected for the first time. CO adsorption causes OH stretching frequencies to suffer a bathochromic shift of about -200/-250 cm(-1), whereas the HOH bending frequency undergoes a hypsochromic shift by about +10/+20 cm(-1). B3LYP/6-31+G(d,p) calculations on model clusters designed to mimic both Brønsted and Lewis sites validate the assignments, showing that water adsorbed on the surface Lewis site, when interacting with CO, shows an acidity comparable to that of a classical zeolitic Brønsted site.