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

Abstract

Soil Chemistry S chulthess and Taylor (2007) proposed a new theory of cation adsorption to describe the adsorption of Na and Ni within zeolite minerals. This theory, called the nanopore inner sphere enhancement (NISE) theory, explains the unusual adsorption selectivity patterns observed for Na and Ni for Na, K, and Ca (Ferreira and. The NISE theory states that ions can dehydrate to fit into confining nanopore channels that are smaller than their hydrated diameters. In such a case, ions with lower hydration energies are more easily stabilized in their dehydrated states on adsorption than ions with higher hydration energies. 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. It is important to point out that the monovalent cations competed equally with a divalent cation at equimolar concentrations in these ion exchange reactions, which is very unusual. The most interesting results, however, The nanopore inner sphere enhancement (NISE) theory provides a new theoretical model of cation adsorption within confining nanopore channels. Inside nanopore channels, hydrated ions can dehydrate and more easily adsorb via an inner sphere mechanism. Adsorption data showed that in certain nanopores, weakly hydrated monovalent cations adsorbed more strongly than divalent cations, which tend to be strongly hydrated. Flow adsorption calorimetry is a valuable tool for directly measuring the heats of the ion exchange process and was used to measure the heats of Na and Ca exchange on three zeolite minerals: zeolite Y, mordenite, and ZSM-5. The data collected showed equal and reversible exchange reactions on mordenite but a strong endothermic Na adsorption and weak exothermic Ca adsorption on ZSM-5. On zeolite Y, the calorimetric signal was below the instrument detection limit of 5 to 7.5 mV. These differences coincide with the adsorption mechanisms and relative competitiveness predicted by the NISE theory for these two ions on the three zeolites studied. These data elucidate an exchange reaction where Ca is outcompeted by Na, which is often considered to be a weak background electrolyte.

Adsorption of copper (II) on mesoporous silica: the effect of nano-scale confinement

“…However, with decreasing pore size, to a pore regime where nano-scale confinement effects may become apparent, it is unclear if the adsorption mode and Cu adsorption complex geometry would remain constant. Ferreira et al demonstrated that Cu contains two electron orbitals of similar energy, therefore is subject to Jahn–Teller distortion in its octahedral coordination environment—creating a weak point in the hydration shell [ 15 , 17 ]. As a consequence, Cu was shown to change its coordination environment from outer-sphere to inner-sphere when adsorbed in narrow pores of mordenite (limiting pore diameter 0.26 nm) [ 15 , 17 ].…”

“…Ferreira et al demonstrated that Cu contains two electron orbitals of similar energy, therefore is subject to Jahn–Teller distortion in its octahedral coordination environment—creating a weak point in the hydration shell [ 15 , 17 ]. As a consequence, Cu was shown to change its coordination environment from outer-sphere to inner-sphere when adsorbed in narrow pores of mordenite (limiting pore diameter 0.26 nm) [ 15 , 17 ]. As shown by the significantly enhanced surface area normalized adsorption of Cu on SBA-15-4 over SBA-15-6 and SBA-15-8, our data may suggest changes in the adsorption complex geometry due to changes in water properties and steric strain of pore confinement.…”

“…Research has demonstrated these effects, showing an enhanced adsorption of arsenic (As) on ordered mesoporous alumina compared to non-porous activated alumina [ 12 ], and enhanced copper (Cu) and uranium (U) adsorption with decreasing hematite particle size [ 13 , 14 ]. Furthermore, the adsorption mode for various adsorbed metals on zeolites with nanopores changed from outer sphere to inner sphere as the pore became smaller [ 15 – 17 ]. The adsorption of Cu on zeolites with narrow pores (0.26–0.74 nm) has been investigated with electron paramagnetic resonance spectroscopy and demonstrated an enhanced adsorption and increased inner sphere coordination in the zeolites with the smallest pores [ 15 ].…”

Ion Exchange Thermodynamics at the Rutile–Water Interface: Flow Microcalorimetric Measurements and Surface Complexation Modeling of Na–K–Rb–Cl–NO3 Adsorption

“…The heat of exchange reflects the net summation of several reactions, a principal one being the difference between the partial dehydration of the incoming exchanging species (an endothermic process), and rehydration of the desorbed exchanged species, which is an exothermic process. 41 For example, the [Na/K] exchange includes the exothermic rehydration of Na + as it leaves the surface and enters the bulk solution and the endothermic dehydration of K + as it adsorbs on the surface as an inner-sphere species. From our previous experimental and molecular dynamics simulation results, 9,42 we know that Na + and Rb + adsorb predominantly as inner-sphere species at negatively charged rutile surfaces, and we infer K + does as well.…”

“…Both K + and Rb + replacing Na + was exothermic, while Na + replacing either K + or Rb + was endothermic. The heat of exchange reflects the net summation of several reactions, a principal one being the difference between the partial dehydration of the incoming exchanging species (an endothermic process), and rehydration of the desorbed exchanged species, which is an exothermic process . For example, the [Na/K] exchange includes the exothermic rehydration of Na + as it leaves the surface and enters the bulk solution and the endothermic dehydration of K + as it adsorbs on the surface as an inner-sphere species.…”