An Investigation of Strong Sodium Retention Mechanisms in Nanopore Environments Using Nuclear Magnetic Resonance Spectroscopy

Abstract

Recent experimental research into the adsorption of various cations on zeolite minerals has shown that nanopore channels of approximately 0.5 nm or less can create an effect whereby the adsorption of ions, especially those that are weakly hydrated, can be significantly enhanced. This enhanced adsorption occurs due to the removal of hydrating water molecules which in turn is caused by the nanopore channel's small size. A new adsorption model, called the nanopore inner-sphere enhancement (NISE) effect, has been proposed that explains this unusual adsorption mechanism. To further validate this model a series of nuclear magnetic resonance (NMR) spectroscopy studies is presented here. NMR spectra were gathered for Na adsorbed on three zeolite minerals of similar chemical composition but differing nanoporosities: zeolite Y with a limiting dimension of 0.76 nm, ZSM-5 with a limiting dimension of 0.51 nm, and mordenite with a limiting dimension of 0.26 nm. The NMR experiments validated the predictions of the NISE model whereby Na adsorbed via outer-sphere on zeolite Y, inner-sphere on ZSM-5, and a combination of both mechanisms on mordenite. The strong Na adsorption observed in these nanoporous minerals conflicts with sodium's general designation as a weak electrolyte.

Calorimetric Evidence in Support of the Nanopore Inner Sphere Enhancement Theory on Cation Adsorption

“…The adsorption studies showed the selectivity of Na, K, and Ca on those zeolite minerals. Figure 1 illustrates the NISE theory, as well as the inner sphere and outer sphere orientation of the ions as confirmed by previous spectroscopy data (Ferreira et al, 2012a(Ferreira et al, , 2012b. It also shows the process by which Na and Ca exchange proceeds in the different nanopore channels.…”

“…The NMR and EPR spectroscopy studies (Ferreira et al, 2012a(Ferreira et al, , 2012b confirmed the NISE theory of how monovalent and divalent cations behave in the constrained environments of different zeolite nanopores. The adsorption studies showed the selectivity of Na, K, and Ca on those zeolite minerals.…”

“…The predictions of the NISE theory were confirmed with nuclear magnetic resonance (NMR) spectroscopy for Na adsorption (Ferreira et al, 2012b) and electron paramagnetic resonance (EPR) spectroscopy for Mn and Cu adsorption (Ferreira et al, 2012a). Ferreira et al (2012b) conducted a series of NMR scans of adsorbed Na ions on the zeolite minerals zeolite Y, ZSM-5, and mordenite. These spectra confirmed that Na adsorbed via an outer sphere mechanism in the large nanopores of zeolite Y and via an inner sphere mechanism in the medium nanopores of ZSM-5.…”

“…Accordingly, in certain nanopore channels, monovalent ions are able to outcompete divalent ions because monovalent ions tend to have lower hydration energies than divalent ones (Collins, 1997). While hydration energy generally correlates well with the dehydration potential needed by the NISE theory, there are cases (such as Cu) where ions with high hydration energies can also dehydrate easily (Ferreira et al, 2012b).…”

“…Accordingly, in certain nanopore channels, monovalent ions are able to outcompete divalent ions because monovalent ions tend to have lower hydration energies than divalent ones (Collins, 1997). While hydration energy generally correlates well with the dehydration potential needed by the NISE theory, there are cases (such as Cu) where ions with high hydration energies can also dehydrate easily (Ferreira et al, 2012b).On zeolite Y, which contained the largest nanopore channels (0.74-nm limiting diameter), Ferreira and Schulthess (2011) observed that all three cations (Na, K, and Ca) adsorbed weakly and in similar amounts. On mordenite, with the smallest nanopore channels (0.26-nm limiting diameter), all three cations adsorbed strongly and in similar amounts.…”