Exploring Delicate Structural Features on Zeolites via Combined Experimental and Computer Simulation Methods
PQ Corporation R&D Center
Researchers usually have assumptions and conjectures about the structural cause when see some unexpected phenomena pertaining to a just made zeolite. Yet, when it seems not to be critical, they often skip or postpone the tedious work needed to a thorough exploration of the cause especially in industrial R&D environment. Here I will show three examples when PQ R&D researchers along with some non-PQ colleagues indeed went after the deeper understanding of some peculiar phenomena pertaining to variously made and treated zeolites.
One example relates to an unexpectedly large difference between the FTIR spectra of the Brønsted acidic hydroxyls (BA-OH) on a commercial SAPO-34 material when measured with diffuse reflectance (DRIFT) and transmission (TR) sampling techniques. We proved that DRIFT emphasizes sites from a 15-20 Å deep surface layer of crystallites while TR emphasizes the average of bulk sites. Also ascertained that, contrary to suggestions in the literature, the surface specific BA-OH sites are not phosphorous related. They belong to an Al atom on the surface positioned into a geometrically specific lattice position.
In the second example we deal with the cause of the strong hydrophobicity of two commercial, proton exchanged, Si/Al ~ 40 ratio Y zeolites from Zeolyst International, CBV 901 and CBV 780. The former one adsorbs about one order of magnitude less water than a Si/Al ~ 4330 ratio MFI structured Silicalite while the latter one adsorbs about twice as much water than the Silicalite. Employing the above-described DRIFT versus TR FTIR comparisons, we show that the CBV 901 crystallites are surrounded with a thin hydrophobic layer, which has no BA-OH groups. Using computer modeling, we compared the role of the Al associated BA-OH groups and the Si-OH containing lattice defects to explain why two hydrophobic Y zeolites with identical Si/Al ratios adsorb substantially different amounts of H2O.
The third example is about the stability of hydroxyl nests [Si-OH]4 in acid leached zeolites. We could not detect with thermogravimetry and FTIR spectroscopy any sign of the presence of such nests when about 20% of Al was “rived” out from a Si/Al ~ 2.55 ratio commercial Y zeolite (CBV 100), even when the material remained at room temperature during the whole process. Despite the wide spread belief that a thermally stable (up to 400 C!) [Si-OH]4 would remain on the place of the removed tetrahedral Al atoms, we could not find any solid proof for their existence in the literature. By creating such entities artificially in computer models, we demonstrate that FTIR would be able to show their presence. Molecular dynamics calculations indicate that even if they form their existence may be ephemeral on a picosecond scale at 25 C.