Experimental and Theoretical Investigation of Intercalation and

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Experimental and Theoretical Investigation of Intercalation and Molecular Structure of Organo-Iron Complexes in Montmorillonite C. I. Sainz-Díaz,*,† F. Bernini,‡ E. Castellini,‡ D. Malferrari,‡ M. Borsari,‡ A. Mucci,‡ and M. F. Brigatti‡ †

Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Av. de las Palmeras, 4, 18100-Armilla, Granada, Spain Dipartimento di Scienze Chimiche e Geologiche, Universita’ degli Studi di Modena e Reggio Emilia, Via Campi 183, I-41125 Modena, Italy

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‡

S Supporting Information *

ABSTRACT: The intercalation of the μ-oxo Fe(III)phenanthroline 1:1 complex [(OH2)3(Phen)FeOFe(Phen)(OH2)3]+4 inside montmorillonite yielded a nanostructured material with strong and selective entrapping ability toward thiol molecules and hydrogen sulfide. In this work, experiments and computational molecular modeling by means of quantum mechanical calculations has been applied to study the molecular structure and interactions between this complex and the interlayer of montmorillonite. This approach allowed the identification of the geometrical disposition of the complexes inside the interlayer, the characterization of the hydration and coordination water molecules, and the explanation of the physico-chemical properties of these functionalized materials. The antiferromagnetic spin configuration of the Fe(III) ions results in the most stable state. Two conformers of the complex have been considered, having the phenanthroline rings in twisted or in parallel planes, respectively, and the transition of one conformer into the other has been explored by molecular dynamics simulations. The conformer with phenanthroline rings in parallel planes is found to be the favored species for intercalation in montmorillonite. Both experimental nuclear magnetic resonance analysis and adsorption isotherms are consistent with the modeling results. Different complex amount, equal and double of the cation exchange capacity (CEC) of montmorillonite, and hydration states inside the interlayer have been investigated reproducing faithfully the experimental d(001) spacing of the montmorillonite in the different conditions. The complex molecules intercalated over the CEC of montmorillonite adopt a disposition of the phenanthroline rings perpendicular to that of the complex already introduced by cation exchange.

1. INTRODUCTION Design and development of new materials able to selectively immobilize gaseous pollutants is an important challenge for the chemical research due to the increasingly prominent role played by gaseous emissions of industrial and civil origin among the environmental concerns.1 Entrapping sulfur derivatives such as H2S and thiols attracts quite an interest for a long time, not only for their toxicity and for the odoriferous effect but also due to the need for their removal from liquid and gaseous hydrocarbons intended for fuels or chemical production.2,3 There are only few examples of materials able to immobilize selectively a specific sulfur derivative from gas mixtures.4,5 Recently, our research group has developed a new hybrid material consisting of montmorillonite (Mt) intercalated with the μ-oxo Fe(III)-phenanthroline complex [(OH2)3(Phen)FeOFe(Phen)(OH2)3]+4 that is able to trap a large amount of thiols and H2S.2,3 This iron complex can be immobilized inside the interlayer of montmorillonite up to an amount double of the cation exchange capacity (CEC), being the excess of positive charge neutralized by an equivalent amount of sulfate anions.6 The © XXXX American Chemical Society

property of the hybrid material to trap gaseous molecules, however, strongly depends on the amount of complex intercalated. In particular, XRD measurements show that, above a critical value of intercalated amount corresponding to the CEC of the montmorillonite, the host interlayer gradually begins to be structured and, contemporary, the immobilization of thiols becomes extremely selective: only completely hydrophobic thiols are rapidly entrapped in a large amount, while those carrying a hydrophilic tail are not at all. At molecular level, the structural/functional details of these unusual and intriguing behaviors are still without explanations and must be explored to understand in depth when and how the functionalization of the interlayer allows inducing a projected entrapping ability and selectivity in montmorillonite. In this work, we study the intercalation of the μ-oxo Fe(III)phenanthroline 1:1 complex [Fe2O(Phen)2(H2O)n]4+ (FePhen) inside the interlayer of montmorillonite by means of Received: August 14, 2018 Revised: October 11, 2018 Published: October 16, 2018 A

DOI: 10.1021/acs.jpcc.8b07912 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

O2−(bridge) → Fe3+ charge transfer transition.6 Then suspensions were made mixing 20 mg of Mt with 4 mL of a solution of the complex whose concentrations met in the range 0.2−6 mM. The obtained suspensions were shaken in an orbital incubator (Stuard Scientific Orbital Incubator SI50) for 30 min at 20 °C using a Haake k20 thermocryostat; after a rest period, once the solid−liquid separation was obtained, the supernatant was centrifuged at 14 000 rpm for 1 min and then subjected to UV−vis measurements. The amount of adsorbed complex was obtained by difference between the concentration of the starting FePhen solution and the concentration of the solution separated after the adsorption process. 2.3. Models and Methodology. The initial model of the complex was taken from experimental crystallographic data.9 The model of montmorillonite was taken from previous optimizations10 with the unit-cell formula: Na(Al3Mg)Si8O20(OH)4. This model is close to the montmorillonite STx-1 experimentally used, 6 (Ca 0.27 Na 0.04 K 0.01 )(Al 2.41 Fe 3+ 0.09 Mg0.71Ti0.03)Si8O20(OH)4, reported by The Clay Mineral Society Web page.11 A 3 × 2 × 1 supercell was generated placing the Mg cations in a maximally dispersed disposition along the octahedral sheet according to previous work.12 The cation exchange capacity (CEC) of this clay was 0.844 eq/kg. Being the investigated complex +4 charged, the CEC of montmorillonite will be reached with 0.211 mol/kg. Taking into account that the amount of smectite in this clay is 67%,13 1 kg of clay will be 1.106 unit-cell-g and the CEC will be 0.191 mol of Fe complex by unit-cell/g of smectite. Then at complete CEC level, there will be 1.15 molecules of Fe(III) complex per each 3 × 2 × 1 supercell of smectite. Therefore, our model will be one Fe(III) complex per 3 × 2 × 1 supercell of smectite. First-principles calculations based on the Density Functional Theory (DFT) method were carried out by means of the SIESTA14 and DMOL315 codes applying periodical boundary conditions in 3-D dimensions. Both codes are based on localized atomic orbitals, and double-ζ extended basis sets with polarization functions were used with spin polarization. The generalized gradient approximation (GGA) and Perdew− Burke−Ernzerhof (PBE) parametrization of the exchangecorrelation function were applied. Pseudopotentials with semicore correction (DSPP) were used in DMOL3.15 Normconserving pseudopotentials were used in SIESTA including scalar-relativistic effects, whereas nonlinear partial-core correction was added for Fe atoms. The convergence threshold criterion for the self-consistent field was 1 × 10−6. The optimization of atomic positions and crystal lattice cell parameters was performed at 0 K. In all structures, all atoms and the cell parameters were relaxed by means of conjugated gradient minimizations. These conditions are consistent with previous studies with organics4 and phyllosilicates.16 Preliminary calculations were set up to explore different values of mesh cutoff energy and different numbers of k-points in the irreducible wedge of the Brillouin zone to optimize the calculations level. The larger values of these parameters produce a higher level of calculations, though with a much higher computational effort. The mesh cutoff energy does not represent the energy of the structure, but it is only a control parameter for improving the level of the calculation.17 A wide range of mesh cutoff energy values in steps of 50 Ry was explored following a similar procedure in previous work18 by calculating the total energy of the experimental molecular structure of the Fe complex in the Γ point of the Brillouin zone.8 With a mesh cutoff energy value higher than 150 Ry, the

computational molecular modeling at the quantum mechanical level based on Density Functional Theory and periodical boundary conditions, along with calculations based on empirical interatomic potentials, and experimental studies of Nuclear Magnetic Resonance (NMR). Experimental adsorption isotherms were also performed at 25 °C adsorbing two polymorphs of the FePhen complex onto montmorillonite, obtained preparing the complex in aqueous and in alcoholic solutions. Different amounts of the complex and different hydration states of the interlayer were considered and the results compared to experimental data. The aim of this work is the identification of the geometrical disposition of the complexes and the distribution of the water molecules inside the interlayer in order to understand the relationship between structuring of the interlayer and trapping ability of these functionalized materials.

2. MATERIALS AND METHODS 2.1. Materials. Texas Montmorillonite (Mt) STx-1a was used (provided by the Clay Minerals Society) with a cation exchange capacity (CEC) of 0.844 eq/kg.11 All chemicals used in this work were of analytical grade and purchased from Carlo Erba (Fe2(SO4)3·8H2O, and NaOH), from Sigma-Aldrich (1,10-phenanthroline, Phen), and from Fluka (EtOH). 2.2. NMR and Adsorption Measurements. NMR analysis were performed at 300 K using an AVANCE III HD 600 Bruker spectrometer equipped with a 2.5 mm H/X CPMAS probe operating at 600.13 and 150.90 MHz for 1H and 13C, respectively. Samples were packed into 2.5 mm zirconia rotors and spun at a magic angle spinning (MAS). 1H NMR spectra were obtained at a 30 or 33 kHz MAS rate, using single-pulse excitation with a 125 kHz spectral width, 10 s relaxation delay, 2.6 μs 90° pulse, 4k data points, and 32−128 scans. The empty rotor 1H spectrum was subtracted to compensate for baseline distortions and background effects. Cross-polarization-MAS (CP-MAS) 13C NMR spectra were obtained at a 16 kHz MAS rate, using the standard Bruker CP sequence with 139 kHz spectral width, 1 s relaxation delay, 2.6 μs 90° 1H pulse, radio frequency (rf) field strength of about 62 kHz for Hartmann−Hahn match, 0.1 ms contact time, 4k data points, and 1k−32k scans. All chemical shifts were referenced by adjusting the spectrometer field to the value corresponding to 38.48 ppm chemical shift for the deshielded line of the adamantine 13C NMR signal, as previously reported.7 For comparison purpose, adsorption isotherms of two polymorphs of the FePhen complex onto montmorillonite were performed at 25 °C; in the crystal of one polymorph, prepared in aqueous solution as described previously,8 the two phenanthroline rings are in a parallel disposition one to the other, while in the crystal of the other polymorph, prepared in ethanol following the procedure of Zhao et al.,9 the two phenanthroline rings are in a twisted position. Adsorption measurements highlighted how this main difference in structure affects the adsorption behavior. The adsorption procedure of the complex in aqueous solution, extensively reported elsewhere,6 was maintained also for the polymorph prepared in ethanol, except for the fact that the iron salt and phenanthroline were both dissolved in ethanol solution instead of water. The concentration of FePhen complex in the starting solutions was quantified by means of spectrophotometric UV− vis measurements (Jasco V-570 spectrophotometer), following the signal of the FePhen complex at 356 nm, attributed to the B


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The crystal structure of the PhenFeS1 polymorph was fully (relaxing all atomic positions and cell parameters) optimized with SIESTA and the twisted conformation was maintained, yielding a crystal structure (a = 8.3, b = 12.0, c = 14.1 Å, α = 104.1°, β = 98.8°, γ = 93.3°) consistent with the experimental one (a = 8.7, b = 12.2, c = 14.7 Å, α = 103.3°, β = 100.0°, γ = 93.0°)9 (Figure 1a). Different spin configurations were

total energy decreases asymptotically with the increase of the cutoff energy, reaching a stability value with cutoff energy values greater than 600 Ry (slope lower than 0.0004 eV/Ry). Therefore, the total energy of our models can be considered independent of the mesh cutoff energy parameter for values higher than 800 Ry. Another calculation parameter is the number of k-points for the sampling of the Brillouin zone. The higher the number of sampling k-points used for the wave function calculation, the more accurate the electronic structure is, but the computational effort is also greater. With two kpoints, the decrease of energy was significantly lower than in the Γ point, and a higher number of k-points did not produce a great improvement. Therefore, in this work, our calculations were performed at 800 Ry of cutoff energy and two k-points. In order to overcome possible difficulties to describe the weak dispersion interactions with quantum mechanical methods, modeling using empirical interatomic potentials was also used in some cases for comparison. Several force fields (FF) were explored, such as Compass19 and Dreiding FF with the Discover and Forcite programs within Materials Studio package20 and the recently optimized CVFFH, based on the consistent valence force field (CVFF),21 that has provided good results in previous studies with phyllosilicates.22,23 Different conditions were tested for calculating van der Waals and Coulomb interactions. Atom based interactions with a cutoff of 18.5 Å were used for Dreiding and Compass FFs. However, with CVFFH, the Ewald method with a cutoff of 15.5 Å yielded better results. Therefore, these conditions have been defined to undertake this work. Though CVFFH was adequate for phyllosilicates, only the Dreiding FF was able to calculate the Fe complex structure properly. Molecular dynamics (MD) simulations were performed in the NVE and NVT ensembles. Powder X-ray diffraction patterns were simulated from the crystal structures using the REFLEX module implemented in the Materials Studio package.20

3. RESULTS AND DISCUSSION 3.1. Structure Modeling for FePhen. Two possible polymorphs of the sulfate salt of the μ-oxo Fe(III)phenanthroline 1:1 complex have been reported: one is the μ-oxo-di-μ-sulfato-bis[aqua(1,10-phenanthroline-κ2N,N′)iron(III)]tetrahydrate with formula [Fe 2 O(SO 4 ) 2 (Phen) 2 (H2O)2]*4H2O (PhenFeS1 hereafter) where the phenanthroline rings are twisted9 and the other is the μ-oxo-di-fac[triaqua-(1,10-phenanthroline-κ2N,N′)iron(III)]bis(sulfate) with formula [Fe2O(Phen)2(H2O)6]*(SO4)2 (PhenFeS2 hereafter) where the phenanthroline rings are in parallel planes.8,23 In both polymorphs, six water molecules per complex are present and each Fe center is coordinated with the bridging O atom, both N atoms of phenanthroline and, at least, one coordination water molecule. So a basic common structure made by the same atoms can be identified in the two polymorphs, the [Fe2O(Phen)2(H2O)2]4+ moiety, even if the disposition of the Phen rings can be different (twisted or parallel). In the first polymorph PhenFeS1 for each Fe center, this coordination is completed by two O atoms of the sulfate groups and the rest of the water molecules act as crystallization water, whereas, in the second polymorph PhenFeS2, the coordination of each Fe center is completed by two water molecules and the sulfate groups are out of the first coordination sphere of Fe.

Figure 1. Crystal polymorphs PhenFeS1 (a) and PhenFeS2 (b) of the sulfate salt of the μ-oxo Fe(III)-phenanthroline 1:1 complex. Crystal structures optimized with SIESTA. The H, O, C, N, Fe, and S atoms are in white, red, gray, blue, purple, and yellow colors, respectively, in all figures of this work.

explored, diamagnetic and the ferromagnetic and antiferromagnetic states of the paramagnetic form. The most stable state was the antiferromagnetic one (ΔE = −0.6972 eV). The crystal structure of the PhenFeS2 polymorph recently refined by Brigatti et al.8 was also fully optimized at variable volume with SIESTA obtaining a crystal structure (a = 8.15, b = 17.20, c = 9.45 Å, α = 90.0°, β = 91.3°, γ = 93.1°) according to the experimental one (a = 8.52, b = 17.60, c = 9.97 Å, α = 90.0°, β = 90.1°, γ = 90.0°) (Figure 1b). In both cases, the calculations yielded shorter values of the cell axis parameters, specially the axis a. This polymorph PhenFeS2 is 0.9519 eV/unit-cell more stable than PhenFeS1. A PhenFeS1 molecule was extracted from the crystal lattice and optimized as an isolated complex with the Dreiding FF obtaining a similar geometry to that in the experimental crystal form. Twenty water molecules were added surrounding the sulfate groups in this optimized structure, and MD simulations C


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Figure 2. Geometry of the tetrahydroxy salt of the Fe(III) complex optimized as isolated complex (PhenFeOH) (a) and the conformation extracted from the MD trajectory (PhenFeOHpar) (b).

Figure 3. Molecular structure optimized with DMOL3 of the tetrahydroxy salt of the Fe(III) complex in the parallel conformer (PhenFeOHpar) (a), the intermediate (PhenFeOHpar1) (b), and the twisted conformer (PhenFeOH) (c).

both phenanthroline rings in coplanar planes and displacing one of them 90° with respect to the other, an unstable structure is created (PhenFeOHpar1) whose optimization trajectory yielded a structure similar to the twisted conformer, where both heterocyclic rings form an angle of 45−50°. This twisted conformer is 4.83 kcal/mol more stable than the parallel one (at DMOL3 level). This energy difference can decrease in a polar medium due to the higher polarity of the parallel conformer structure. This can give one possible path between both PhenFeOH and PhenFeOHpar conformations: from PhenFeOHpar, when the π−π interaction between rings disappears, both rings tend to separate twisting to each other, and vice versa, from the twisted PhenFeOH conformer when both rings tend to approach, for example in a aqueous media, they fall in a minimum energy state by π−π interactions forming the PhenFeOHpar conformer (Figure 3). 3.2. Intercalation of FePhen in the Montmorillonite Interlayer: The Semi-saturated System. In previous experiments of intercalation of the FePhen complex as a cation in montmorillonite, two steps were found. In the first step, montmorillonite was intercalated with the FePhen complex in an amount equivalent to the CEC. The resulting material was named as the semi-saturated system (MtFePhensemi‑sat).6 Under these conditions, no structuring of the interlayer is observed, and a recent work has shown that thiols can be easily entrapped without any selectivity with respect to the nature of the tail group.2 The Fe complex cation with the twisted conformation was extracted from the above tetrahydroxy salt (PhenFeOH) without the hydroxyl anions. This cation was placed in the center of the interlayer of a 3 × 2 × 1 supercell of a montmorillonite model with an initial dspacing of 18 Å as a starting parameter. This initial d-spacing provides enough movement freedom to the complex during

were performed with steps of 1 fs during 5 and 10 ps in the NVE and NVT ensembles, respectively. In both cases, the geometry of the complex remained constant and the water molecules went out from sulfate groups. This behavior can be consistent with the low solubility of this salt in water and the formation of this polymorph only in nonaqueous media.9 Nevertheless, to address the case of an aqueous dissolution of both of these complexes, a possible scenario can occur, where both cations and anions are solvated and then an anion exchange of sulfate by hydroxyl anions can be produced. Changing the sulfate groups by hydroxyl anions, the tetrahydroxy salt of the complex (PhenFeOH) was generated and optimized with the Dreiding FF yielding the same geometry of the experimental sulfate complex with both phenanthroline rings in twisted planes (Figure 2a). MD simulations were performed with this structure with steps of 1 fs during 10 ps in the NVT ensemble. The geometry of the complex remained constant. However, exploring the trajectory of the simulations, a change of conformation was observed where the phenanthroline rings are in coplanar planes (Figure 2b). This configuration was extracted and fully optimized separately, remaining in the coplanar conformation (PhenFeOHpar). This result indicates that this conformation is possible; however, this coplanar conformation is 12.45 kcal/ mol less stable (with Dreiding FF calculations) than the twisted conformation as isolated molecule without solvent. This energy difference can be due to the intramolecular H bond interactions between the OH groups in the twisted conformer with d(OH···OH) = 2.4−2.5 Å, enhancing stability. Both conformers were optimized at quantum-mechanical level with DMOL3 and both geometries remained stable. In the PhenFeOHpar conformer, the aromatic rings are in front and π−π interactions give stability to the complex. Maintaining D


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Figure 4. Intercalation in amount equivalent to the CEC of Fe complex cations with the phenanthroline ligands in twisted configuration in the interlayer space of montmorillonite optimized with CVFFH, views from planes (010) (a) and (001) (b). The Fe complex cation was extracted from the tetrahydroxy salt PhenFeOH without the hydroxyl anions. The Al, Si, and Mg atoms are in pink, ochre, and green colors, respectively.

the Fe complex cation was released.6 This behavior is consistent with previous thermodynamic studies of dehydration of montmorillonite, where the noncoordinated water is released before the coordinated water of interlayer cations during the temperature increasing.24 In this structure, the phenanthroline rings of the Fe complex cation remained in coplanar planes and the axial coordination water molecules form H bonds with the basal tetrahedral O atoms (Ob) of montmorillonite, with d(OH···Ob) = 1.52−2.34 Å. In the interlayer space, more space between Fe complex cations is observed, being great enough for additional adsorption processes (Figure 5a). Previous experimental work found that the d(001) spacing of this structure changed from 16.1 Å at 80 °C to 15.2 Å after heating at 260 °C.6 This behavior can be interpreted as a partially high dehydration of the Fe complex cation. The water molecules coordinated with Fe cation, which are interacting strongly with the tetrahedral O atoms of the solid surface, are more stable and they are more likely to remain after this thermal treatment. Then, an almost completely dehydrated derivative of this intercalated Fe(III) complex cation was considered, maintaining only two coordination water molecules per complex, one per each Fe(III) cation, which are interacting with the tetrahedral O atoms, one with each tetrahedral sheet. This new structure was completely optimized with SIESTA at variable volume (atom positions and cell parameters), remaining the Fe(III) complex cation stable with the phenanthroline rings in coplanar conformation. The Fe coordination was slightly distorted, and the coordination water molecule forms H bonds with the basal tetrahedral O atoms with d(OH···Ob) = 1.52−1.79 Å (Figure 5b). The d-spacing of this optimized crystal structure was 15.2 Å, reproducing the experimental value after treating the system at 260 °C.6 On the other hand, the thermogravimetric analysis (TGA) of Mt-FePhensemi‑sat, collected in a He atmosphere with a heating rate of 20°/min showed an amount of water of 4.6%.6 We can derive from this percentage the number of water molecules per complex, being 12 water molecules. Six water molecules are coordinating the complex, forming a dioctahedral coordination, and the remaining six molecules are filling

the optimizations in order to explore other possible orientations. The phenanthroline rings of the Fe complex cation have the twisted conformation and one coordination water molecule per each Fe center. No more water molecules are included in these initial models for a better exploration of the conformations of the Fe complex. Hence, this model corresponds to a semi-saturated material with the intercalated complex partially dehydrated. The whole system was optimized with CVFFH and the phyllosilicate structure remained close to experimental data, whereas the Fe complex cation was deformed. Then, this Fe complex cation was considered as a rigid unit and the whole system was optimized at variable volume obtaining crystal lattice parameters (a = 15.5, b = 17.9, c = 16.6 Å, α = 106.0°, β = 105.1°, γ = 90.1°) close to experimental data of the semi-saturated system partially dehydrated (a = 15.5, b = 17.9, c = 15.8 Å, α = 91.2°, β = 100.5°, γ = 89.6°) and the phenanthroline rings remained in twisted planes. Taking into account the surface of the supercell (277.68 Å2), the adsorption of one of this conformer represents 66.2% of the surface coverage. Although the dspacing is close to the experimental one (16.0 Å vs 15.4 Å),6 this Fe complex cation conformer occupies almost the whole supercell space during the intercalation (Figure 4) and there is not enough space for additional intercalation of more Fe complex cations, as observed experimentally (see later). Hence, the other conformer with phenanthroline rings in coplanar planes will be more likely to be intercalated in montmorillonite than the twisted one. Therefore, the intercalation in montmorillonite of the conformer of the Fe complex cation with phenanthroline rings in coplanar planes and 3 coordination water molecules per each Fe(III) cation was also studied. This conformer was placed in the center of the interlayer of our montmorillonite model, one molecule per 3 × 2 × 1 supercell of phyllosilicate with the phenanthroline rings parallel to the layers of montmorillonite. The whole structure was optimized at variable volume with SIESTA, yielding a d-spacing of 16.1 Å. This is consistent with the previous experimental value obtained after treatment of the Mt-FePhensemi‑sat system at 80 °C where the rest of the interlayer water not-associated with E


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Figure 5. Intercalation in amount equivalent to the CEC of the coplanar conformer of Fe complex cation in the interlayer space of montmorillonite optimized with SIESTA with three (a) and one (b) coordination water molecules per Fe center; and with the experimental water content at room temperature (c).

the interlayer space between complexes. The whole system (montmorillonite + Fe complex cation + water molecules) was optimized at variable volume with SIESTA, yielding a d(001) spacing of 17 Å, reproducing the experimental data of MtFePhensemi‑sat at 25 °C6 (Figure 5c, Supporting Information, Figure S1). The axial coordination water molecules form H bonds with the basal tetrahedral O atoms of phyllosilicate, with d(OH···Ob) = 1.67−2.06 Å. Therefore, with these last three structures, the sequence of the dehydration process of Mt-FePhensemi‑sat can be explained. Heating at 80−100 °C, the six hydration water molecules freely placed between Fe complexes are desorbed, remaining only the coordination water molecules to the Fe complex. This process produces a decrease of d(001) spacing from 17.0 to 16.1 Å. Due to a further heating to 260 °C, for each Fe complex, four coordination water molecules are lost, while two

coordination water molecules remain in the axial coordination strongly joined with the Fe centers and forming H bonds with the basal tetrahedral O atoms of montmorillonite. This dehydration process reduces the d(001) spacing from 16.1 to 15.2 Å. In this semidehydrated complex, the Fe coordination is distorted and will be decomposed at higher temperatures. This coplanar conformer of the Fe complex intercalated in montmorillonite as a semi-saturated system can be distributed more or less ordered along the interlayer space. Two main ordered configurations can be defined in our system considering a double supercell: the same orientation of the Fe complex in each 3 × 2 × 1 cell (AAAA ordering) (Figure 6a,b) or the opposite orientation (ABAB ordering) (Figure 6c). The ABAB system is more stable, and the hydrophobic moieties are oriented toward the hydrophobic moieties of vicinal Fe complexes. Hence, this distribution allows having F


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Figure 6. Optimized crystal structures of the hydrated Fe(III) complex intercalated in a 6 × 4 × 1 supercell of montmorillonite in two highly ordered configurations: pattern (AAAA) with 3 coordination water molecules per Fe center (a) and with the experimental water content at room temperature (b); and pattern (ABAB) with the experimental water content at room temperature (c) (representation of a 6 × 8 × 1 supercell for observing the hydrophobic and hydrophilic zones). The H, O, N, C, Si, Al, and Mg atoms are in white, red, blue, gray, ochre, pink, and green colors.

illonite, or one coplanar and other perpendicular to the phyllosilicate layers. The first option is not possible because there is not enough space in a 3 × 2 × 1 supercell for both Fe complexes in one monolayer. The adsorption of one of this coplanar conformer represents a 50% of the surface coverage. Besides, the behavior of the d(001) spacing of this saturated system with the temperature (see Figure 10 in ref 6) is different from that of the semi-saturated system, and this accounts for a different interlayer structure. The high stability of d(001) with the temperature increasing in the saturated one can be explained only by a different disposition of the additional sulfate FePhen that can be in a perpendicular orientation with respect to the previous FePhen. The adsorption of this second Fe complex in a perpendicular disposition yields a 37% of surface coverage; then the adsorption of both Fe complexes will represent a 87% of surface coverage of the montmorillonite interlayer. Therefore, this relative disposition of both Fe complexes is likely to occur in this surface. Besides, the full optimization of this system yielded a d(001) spacing of 18.0 Å (Figure 7a,b), reproducing the experimental behavior and validating this model (Figure S1). This saturated system is a pillared clay with organic pillars. 3.4. Adsorption of Different Conformers of FePhen onto Montmorillonite. Two crystal polymorphs of the sulfate salt of FePhen have been reported: PhenFeS19 and PhenFeS2,25 and the structure of PhenFeS2 has been recently refined.8 The main difference between these polymorphs is the molecular structure of the conformers of the FePhen complex. The PhenFeS1 polymorph has the phenanthroline rings in

hydrophobic channels for adsorption of organics as found experimentally,2,3 acting as a surfactant with hydrophilic and hydrophobic parts. 3.3. Intercalation of FePhen in the Montmorillonite Interlayer: The Saturated System. In the second intercalation step, montmorillonite was intercalated with FePhen in a double amount compared to the CEC, to form the saturated system, Mt-FePhensat. Under these conditions, an evident structuring of the interlayer occurs6 and the thiols are strongly entrapped, but only when the tail group is hydrophobic.2 In the saturated system, the Fe(III) complexes exceeding the CEC amount will enter as a sulfate salt, i.e., with sulfate counterions. Besides, taking into account the water content (5.4% at T < 300 °C) measured by TGA6 and that the amount of Fe complex is 0.0434 per 100 g of solid, the relative proportion of water/complex is close to 7 water molecules per Fe complex. Hence, we can consider a model of a 3 × 2 × 1 supercell with a first FePhen as a cation and a second FePhen as sulfate salt, 3 water molecules for completing the coordination of each Fe center (3 waters × 4 Fe centers = 12 water molecules) and 2 water molecules solvating the sulfate groups. Our above calculations showed that only the coplanar conformer of Fe complex can be adsorbed in the interlayer of a 3 × 2 × 1 supercell of montmorillonite and no twisted conformer can be accepted in this scenario. Hence, two possible orientations can be considered: both Fe complexes, the initial cation and the additional sulfate salt, are with the phenanthroline rings coplanar with the layers of montmorG


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Figure 8. Plot of the moles of FePhen complex adsorbed for mass unit of montmorillonite, q, as a function of the FePhen complex equilibrium concentration, [FePhen], for the complexes prepared in water (diamond symbols, FePhenparallel) and in ethanol (square symbols, FePhentwisted), respectively. [FePhen]0 = 0.2−6 mM, T = 25 °C.

prepared from water will keep the phenanthroline rings in parallel position (FePhenparallel), like PhenFeS2 crystal, also in the intercalated state. It is a matter of fact that, in saturation conditions in ethanol, FePhentwisted reaches a value of maximum adsorbed amount (0.216 mol/kg), which is less than half of the maximum adsorbed amount in aqueous medium, FePhenparallel (0.434 mol/kg). The isotherm at 25 °C of the complex prepared in water strictly resembles that obtained at 20 °C for the same investigated system.6 The different adsorption is consistent with the bigger structure and steric hindrance of the polymorph with phenanthroline rings in a twisted position, and confirms that the model of the twisted polymorph does not suit for the description of the investigated system, as found in the modeling section. 3.5. NMR Spectroscopy. The NMR analysis can give us some information related to the disposition of the FePhen in Mt. In order to emphasize 1H signals due to FePhen, 1H MAS NMR spectra were recorded on samples of Mt exchanged with D2O and Ca2+ (Mt_D_Ca) with and without intercalation of FePhen complex in the montmorillonite interlayer. Both semisaturated (Mt-FePhensemi‑sat) and saturated (Mt-FePhensat) systems were studied and compared with the FePhen complex out of montmorillonite and the deuterated montmorillonite (Figure 9). In Mt_D_Ca, we still observe signals due to structural AlOH groups at 1.7 and 3.0 ppm and a residue of coordinated water molecules at 5.1 ppm (Figure 9a). The insertion of FePhen in the Mt lattice moves slightly to higher ppm these three signals (1.9, 3.2, and 5.3 ppm) and adds to the 1H NMR spectrum signals coming from the ligand (Figure 9c,d). Coordinated phenanthroline proton signals are found at about 30 (very broad), 18, 12, and 8.5 ppm, both in MtFePhensemi‑sat and Mt-FePhensat. In general, a higher intensity of 30, 18, and 12 ppm bands can be observed in Mt-FePhensat, with respect to Mt-FePhensemi‑sat, whereas the signal at 8.5 ppm is higher in Mt-FePhensemi‑sat than in Mt-FePhensat. This picture seems to point out a mixture of two forms, one dominant in Mt-FePhensemi‑sat, mainly characterized by shielded signals at about 8.5 ppm, and a second one, which

Figure 7. Optimized crystal structure of the saturated system with the Fe(III) complex cations intercalated in montmorillonite along with the additional Fe(III) complex with sulfate anions, viewed from planes (001) (a) and (010) (b).

twisted planes each other, whereas the PhenFeS2 one has both rings in parallel planes. In the intercalation experiments, it is not possible to know which conformers exist in the interlayer of montmorillonite. Our above calculations showed that the twisted conformer has a limited intercalation capacity and the parallel conformer is more likely to be present in the interlayer. The PhenFeS1 polymorph is obtained in ethanol solutions, and the PhenFeS2 one is crystallized in aqueous solutions. Then the twisted conformer will be more stable in ethanol and the parallel one in water. Therefore, we can intercalate each conformer in montmorillonite maintaining the crystallization solvent of each one. Then, adsorption measurements were performed on montmorillonite of both complexes prepared in the same conditions used to obtain the PhenFeS1 and PhenFeS2 crystals in ethanol and water solutions, respectively (Figure 8). Even if it is not possible to ascertain from isotherms the structure of the complex inside the interlayer, it is likely that the structure of the FePhen complex conformers prepared in the same conditions of the crystals is maintained. When adsorption is made from their crystallization mother solutions, the complexes are more likely to reassemble from their mother solutions in the interlayer; i.e., the structure of the adsorbed complex prepared from the alcoholic mother solution will keep the phenanthroline rings in twisted position (FePhentwisted), like PhenFeS1 crystal, while the structure of the complex H


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Figure 9. 1H MAS NMR spectra of (a) Mt_D_Ca, (b) PhenFeS2 crystal, (c) Mt-FePhensemi‑sat, and (d) Mt-FePhensat recorded at 30 kHz MAS, except for PhenFeS2 that was recorded at 33 kHz MAS. Signals from structural hydroxyls and coordinated phenanthroline are labeled. Asterisks denote spinning side bands. # denotes residual coordinated water signal.

signal is present at 149 ppm, a high one at 136 ppm, and a medium one at 126 ppm. These two spectra are both different with respect to that of pure PhenFeS2 crystal, where a sharper peak at 138 ppm (and a broader one at 210 ppm) is detected in the same conditions. We can explain the 13C NMR spectrum of Mt-FePhensat as due to the superimposition of two subspectra, one similar to that of Mt-FePhensemi‑sat and the second similar to that of pure PhenFeS2, with only one dominating signal at about 136 ppm. These NMR results show two different orientations of FePhen in Mt-FePhensat where the deshielded protons of phenanthroline ligands either are interacting with the O atoms of the montmorillonite surface or are close to the plane of an adjacent phenanthroline ring and deshielded by aromatic ring currents. These interactions are weak in the complex configuration of Mt-FePhensemi‑sat where both phenanthroline rings are coplanar to the mineral surface and more distant from one another. However, the FePhen complex exceeding the CEC value in Mt-FePhensat should be with the phenanthroline rings in a perpendicular orientation with respect to the mineral surface, where the phenanthroline H atoms interact more strongly with O atoms of the mineral surface. Besides, the chemical shift in the 13C NMR signal in Mt-FePhensat can be due to the field effect of the negatively charged O atoms of sulfate groups on the nuclear shielding of the C atoms, confirming the presence of sulfate groups in a similar form as the initial PhenFeS2 complex. This interpretation matches the NMR results and the above atomic models.

enhances in Mt-FePhensat, characterized by more deshielded signals (18 and 12 ppm). In any case, signals from the intercalated FePhen units are different from those observed in pure PhenFeS2 crystal that are found at 34.0, 14.5, and 9.0 ppm (Figure 9b). It is difficult to infer unambiguously the geometry of the intercalated complex in the two hybrid materials from 1H NMR data, but the appearance of the deshielded signal at 18.0 ppm suggests structural differences in the arrangements of the phenanthroline units in the two intercalated systems. The 13C CP-MAS NMR spectra were also acquired for MtFePhensemi‑sat and Mt-FePhensat and are reported in Figure 10 in comparison with that of PhenFeS2 crystal. A different situation is evidenced by 13C CP-MAS NMR spectra for the two intercalated systems. Three groups of 13C signals are clearly distinguished, at about 147, 137, and 126 ppm, in the case of Mt-FePhensemi‑sat, whereas, for Mt-FePhensat, a low

4. CONCLUSIONS Computational molecular modeling allowed getting insight in the nanostructuration of the montmorillonite interlayer in the presence of the FePhen complex. The geometrical disposition of the complexes and the hydration coordination of the water molecules determined by this computational approach fit very well with the experimental results and allow the explanation of physical-chemical properties of these functionalized materials. In fact, the calculated models of montmorillonite function-

Figure 10. 13C CP-MAS NMR spectra of (a) PhenFeS2 crystal, (b) Mt-FePhensemi‑sat, and (c) Mt-FePhensat recorded at 16 kHz MAS with a contact time of 0.1 ms. Asterisks denote spinning side bands. The dotted lines at 149, 138, and 126 ppm were inserted as guide for the eyes. I


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The Journal of Physical Chemistry C alized with FePhen reproduce the powder XRD d(001) spacing of the semi-saturated montmorillonite (with the CEC amount of FePhen) and the saturated montmorillonite (with the double amount of FePhen with respect to the CEC). The calculated model of the montmorillonite saturated by FePhen explains the different spatial configurations of the phenanthroline rings with respect to the mineral surface observed in the XRD and NMR experimental results. Moreover, the presence of hydrophobic and hydrophilic zones in the montmorillonite interlayer in the model of the semi-saturated system explains why it works as a gas trap for both hydrophobic and hydrophilic thiols; vice versa, the predominance of hydrophobic regions in the model of the saturated system (clusters exposing out the aromatic rings and keeping inside the hydrophilic regions) explains why it works as a gas trap only toward thiols having hydrophobic tails. These Fe(III) complexes act as surfactants in the interlayer space of montmorillonite. Besides, in the saturated montmorillonite, these FePhen complexes act also as organic pillars, forming a pillared montmorillonite. This theoretical−experimental complementary study can be applied to other similar nanostructurated systems with different ligands, cations, and phyllosilicates.

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FePhentwisted=μ-oxo Fe(III)-phenanthroline 1:1 complex [Fe2O(Phen)2(H2O)2]4+, with the Phen rings in twisted position. FePhenparallel=μ-oxo Fe(III)-phenanthroline 1:1 complex [Fe2O(Phen)2(H2O)6]4+, with the Phen rings in parallel position. Mt-FePhensemi‑sat=solid hybrid material made by Mt intercalated with FePhen complex in an amount equivalent to the CEC (semi-saturation conditions) Mt-FePhensat=solid hybrid material made by Mt intercalated with FePhen complex in an amount double than CEC (saturation conditions)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b07912.

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REFERENCES

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Figure S1: Experimental powder XRD patterns of Mt, Mt-FePhensat, and Mt-FePhensemi‑sat, along with a short explanation (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

C. I. Sainz-Díaz: 0000-0001-8612-3826 E. Castellini: 0000-0002-2933-4405 D. Malferrari: 0000-0002-0879-1703 M. Borsari: 0000-0002-3612-4764 A. Mucci: 0000-0003-3303-8761 M. F. Brigatti: 0000-0002-7526-9931 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the University of Modena and Reggio Emilia for the Visiting Professor programme, to the Computational Centre of University of Granada and CINECA of Bologna for the high performance computing service, and the Andalusian project RMN1897 and Spanish projects FIS2013-48444-C2-2-P and FIS2016-77692-C2-2-P for financial support.

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ABBREVIATIONS Mt=Montmorillonite STx-1a Phen=1,10-phenanthroline (C12H8N2) FePhen=μ-oxo Fe(III)-phenanthroline 1:1 complex [Fe2O(Phen)2(H2O)n]4+ J


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Experimental and Theoretical Investigation of Intercalation and

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