Confirmation of the Nanopore Inner-Sphere Enhancement (NISE) Effect

1. Confirmation of the Nanopore Inner-Sphere Enhancement (NISE) Effect Using Nuclear Magnetic Resonance Spectroscopy and Calorimetry Daniel R. Ferreira* Cristian P. Schulthess Marcus V. Giotto Nadine J. Kabengi University of Connecticut, Storrs, CT, USAUniversity of Connecticut, Storrs, CT, USA University of Connecticut, Storrs, CT, USA University of Kentucky, Lexington, KY, USA An equal opportunity employer and program provider. Introduction Recent research into the adsorption of cations on zeolite minerals shows that nanopore channels ≤0.5 nm can enhance the adsorption of ions, especially weakly hydrated ones.1,3 This occurs due to the removal of hydrating water molecules inside the small nanopore channel. A new adsorption model, called the nanopore inner-sphere enhancement (NISE) effect explains this unusual adsorption mechanism. Objective:To confirm the predictions made by the NISE model concerning inner-sphere and outer-sphere adsorption mechanisms by Na on different zeolites. Nuclear Magnetic Resonance (NMR) spectroscopy gathers data on the nature of an ion’s local chemical environment and how it is bonded to neighboring molecules. NMR can determine if an ion is adsorbed via an outer-sphere or inner-sphere mechanism.2 Calorimetry gathers data on the energy of adsorption reactions. Materials & Methods NMR Study Samples: Zeolite slurry with 20 mM Na, pH 7. Equilibrated overnight, centrifuged, liquid decanted, water content adjusted to 150%. 23Na MAS-NMR, 7.05 T, 100,000 scans. Calorimetry Study Flow Adsorption Calorimetry (FAC) at room temperature. 0.054 g of solid saturated with 10 mM Ca(AcO)2 at pH=4.91 carrier solution. Injection of 20 mM Na(AcO)2 at pH = 4.91 The Nanopore Inner-Sphere Enhancement (NISE) Effect: The adsorption mechanism of cations is affected by nanopore channel diameter and the hydration strength of the ion. * ZSM-5: Has the strongest negative chemical shift indicating a denser e- cloud (i.e., it is close to the electronegative surface and inner-sphere adsorbed). * Zeolite Y: Has a positive chemical shift indicating a thinner e- cloud (i.e., the hydrated bond length is longer and it is far from the electronegative surface and outer-sphere adsorbed).Decreased solvent density due to interference from nanopore channel walls4 increases the Na-H2O bond length. * Mordenite: Has a weaker negative chemical shift than ZSM-5. It contains both large & small pores, and the NMR reading is a motional average of outer-sphere & inner-sphere adsorption (90% inner-sphere, 10% outer-sphere): δo =(δis tis + δos tos) / ttot where δo = obs. chem. shift, δis = i.s. chem. shift, δos = o.s. chem. shift, tis = time as i.s. complex, tos = time as o.s. complex Zeolite Y (FAU) Pore dimensions = 0.74 nm x 0.74 nm Surface area = 700 m2 g-1 SiO2:Al2O3 Ratio = 80:1 Calorimetry Study of Na exchange with Ca on ZSM-5 * Sorption of Na on ZSM-5 resulted in an endothermic peak. * The absence of a corresponding Ca exotherm indicates that Na sorption on ZSM-5 is not reversible with Ca and is not just ion exchange, but is most likely an inner-sphere reaction. *The presence of an exothermic peak in the beginning and a small shoulder at the bottom of the endotherm indicate the potential existence of two separate reactions overlaying each other. *The small exotherm at 17 minutes could correspond to the beginning of Na sorption on the surface while the larger endotherm at 22 minutes may be Na dehydration inside the small nanopores. Dehydration reactions are endothermic. *The molar enthalpy measured for Na sorption on ZSM-5 is: DH = 2.95 kJ/mol. ZSM-5 (MFI) Pore dimensions = 0.51 nm x 0.55 nm 0.53 nm x 0.56 nm Surface area = 425 m2 g-1 SiO2:Al2O3 Ratio = 80:1 Mordenite (MOR) Pore dimensions = 0.70 nm x 0.65 nm 0.26 nm x 0.57 nm Surface area = 500 m2 g-1 SiO2:Al2O3 Ratio = 90:1 NOTE:Zeolite nanopores are interconnected and ions can travel between different pore channels. Conclusions * Based on experimental adsorption results, the NISE model predicts that Na adsorbs via an outer-sphere mechanism on zeolite Y, via an inner-sphere mechanism on ZSM-5, and via a mixture of the two mechanisms on mordenite. * NMR spectroscopy of adsorbed Na on the three zeolites validate the predictions made by the NISE model regarding the mechanism of Na adsorption in different sized nanopore channels. * Calorimetry data indicate that Na is replacing Ca on the exchange complex of ZSM-5 at equimolar concentrations. This validates the NISE model prediction that Na adsorption on ZSM-5 is stronger than that of Ca, mainly due to Ca’s higher hydration energy. Citations: 1: Ferreira, D.R., and C.P. Schulthess. 2011. The nanopore inner sphere enhancement effect on cation adsorption: Sodium, potassium, and calcium. Soil Sci. Soc. Am. J. 75:389-396. 2: Kim, Y., and R.J. Kirkpatrick. 1997. 23Na and 133Cs NMR study of cation adsorption on mineral surfaces: Local environments, dynamics, and effects of mixed cations. Geochim. Cosmochim. Acta 61:5199-5208. 3: Schulthess, C.P., R.W. Taylor, and D.R. Ferreira. 2011. The nanopore inner sphere enhancement effect on cation adsorption: Sodium and nickel. Soil Sci. Soc. Am. J. 75:378-388. 4: Vaitheeswaran, S., G. Reddy, and D. Thirumalai. 2009. Water-mediated interactions between hydrophobic and ionic species in cylindrical nanopores. J. Chem. Phys. 130: 094502. daniel.ferreira@uconn.edu c.schulthess@uconn.edu mgiotto@ims.uconn.edu nadine.kabengi@uky.edu