Refluxing the Naked Clay powder with methanolic solution of MnTPyP+ ions, with magnetic stirring, caused grinding of the clay powder into micro- or nano-scale powders. Intercalation was then confirmed by different methods. The three solids, Naked Clay, MnTPyP@Nano-Clay and MnTPyP@Micro-Clay have been characterized by different techniques.
FT-IR spectral analysis
The presence of the MnTPyP+ ions inside the composite material was confirmed by FT-IR spectra. Fig. (1b, c) show that each of MnTPyP@Micro-Clay and MnTPyP@Nano-Clay has two new major bands at 1650 and 1385 cm−1. The two bands correspond to the two bands (1656 and 1388 cm−1) observed for the homogeneous MnTPyP+ ions dissolved in methanol solvent here. The two bands also resemble the ones at 1600 and 1380 cm−1 reported for other polysiloxane supported tetra(4-pyridyl)porphyrinato manganese(III) complexes [33, 34].
A closer look at the spectra shows other evidence in favor of intercalation. The band at 1026 cm−1 observed for the Naked Clay powder in Fig. 1a was shifted to 1010 cm−1 after introduction of the MnTPyP+ ions. This indicates that the bonds at the clay layer surfaces were affected by intercalation of the MnTPyP+ ions in between.
The FT-IR spectra thus confirm intercalation of MnTPyP+ ions inside the clay. Presence of additional MnTPyP+ ions adsorbed at the outer surface of the clay cannot be ruled out, despite the careful rinsing of the resulting composite after reflux.
Electronic absorption spectral analysis
Electron absorption spectra, Fig. 2, also confirmed the presence of the MnTPyP+ ions inside the MnTPyP@Micro-Clay and MnTPyP@Nano-Clay. Fig. 2a shows the spectrum for the Naked Clay in the absence of MnTPyP+ ions. With its white color, the clay is expected to show no bands in the visible region. The two bands at 470 and 490 nm in Fig. 2c correspond to the Soret band for the in solution MnTPyP+ ions shown in the inset. In the free or solution forms of MnTPyP+, the Soret band typically occurs at about 462 nm, depending on type of solvent [9, 33, 35]. The composite MnTPyP@Micro-Clay, Fig. 2b, shows the same two bands at 470 and 490 nm for MnTPyP+ ions, with weaker intensities due to lower concentration of intercalated ions. The difference in MnTPyP+ bands in Fig. 2b, c is consistent with the color difference discussed in “Intercalation of MnTPyP+ ions inside clay particles” section above. The measured concentrations discussed below also confirm these results.
Both Fig. 2b, c involve a red shift in Soret band for the MnTPyP+ ions. Such a red shift confirms intercalation. The presence of two bands indicates different types of intercalated MnTPyP+, as will be further discussed.
AAS analysis
AAS was used to calculate the exact amount of [MnIII(Tpyp)]+(SO4)1/2 intercalated inside different clay powders. The amounts of excess solution MnTPyP+ and those eluted from the surface of the clay were grouped together and subtracted from the initial nominal amount originally used. The uptake of MnTPyP+ inside MnTPyP@Micro-Clay was 2.37 mg/g clay (3.12 × 10−3 mmol/g). Higher MnTPyP+ uptake occurred in the MnTPyP@Nano-Clay with 3.78 mg/g (5.25 × 10−3 mmol/g). This is expected as the nano-scale particles have higher relative surface area, vide infra. With smaller clay particle sizes, the MnTPyP+ ions also have shorter path length to travel inside. The difference in color intensity between the MnTPyP@Nano-Clay and MnTPyP@Micro-Clay further confirms these results.
Refluxing the Naked Clay with methanolic solution of MnTPyP+ ions for prolonged times may still yield higher uptake. Saturation uptake value may increase with further grinding in the clay particles. However, for practical handling purposes and to avoid further grinding of the clay into smaller sizes, the reflux/stirring time was not extended for longer than 30 h.
Ion exchange study was performed on the Naked Clay powder in its H-form as described by common procedures described earlier using aqueous Na+ ion solutions [36]. The exchange capacity was ~0.8 mmol/g (18.4 mg Na+/g). The value is lower than other literature values [30] because the clay here involves layered montmorillonite and biotite in addition to other non-layered phases, as described below. Moreover, the MnTPyP+ ion uptake values in both nano- and micro-scale clay powders are lower than the Na+ cation exchange capacity of the clay, as discussed above. This is not unexpected, as the MnTPyP+ ions may not be able to reach all negative sites inside the clay. Similar behavior has been reported earlier [30].
Specific surface area
The specific surface area for each of the three solids was measured by acetic acid adsorption from organic solvents [32]. From literature, the specific surface area for clay is not easy to measure accurately [37]. The values may have wide variations, depending on type of the clay, the particle size and the technique used. The approximate values measured here were 90, 130 and 200 m2/g for Naked Clay, MnTPyP@Micro-Clay and MnTPyP@Nano-Clay powders, respectively. Despite being only rough estimates, the values still give indication that the MnTPyP@Nano-Clay has highest specific surface area among the series, which explains why it exhibited higher MnTPyP+ ion uptake, as discussed above.
SEM micrographs
The naked and intercalated clay surfaces were studied with FE-SEM, Fig. 3. SEM micrographs were used to measure the sizes of the three types of clay particles. The Naked Clay showed particle sizes in the range 200–1000 nm with an average value of 625 nm. The MnTPyP@Micro-Clay particles showed a size range of 100–600 nm, with an average radius of 316 nm. The MnTPyP@Nano-Clay particles showed sizes in the range 10–140 nm with an average radius of 50 nm.
The SEM images clearly confirm grinding of the Naked Clay particles (under 6 h magnetic stirring and reflux) into smaller particles of MnTPyP@Micro-Clay. Further grinding into nano-scale particles has been achieved by longer stirring (30 h) under reflux conditions. Clay particle grinding during reflux made it not possible to observe size expansion due to intercalation with SEM.
XRD patterns
Figure 4 shows the XRD patterns measured for Naked Clay, MnTPyP@Micro-Clay and MnTPyP@Nano-Clay powders. The XRD patterns were mainly used to confirm the intercalation of MnTPyP+ ions inside the clay particles. The intercalation orientation was also studied by the patterns.
Comparison of the XRD patterns in the Figure with earlier reports [38] shows that the Naked Clay contains Kaolinite (with 2θ = 12.65°, 25.48°, 40.36° and 55°). Signals for quartz appear (at 2θ 20.70°, 26.69°, 50.26° and 60.11°). Illite also exists inside the Naked Clay (with 2θ 8.89° and 36.60°). Montmorillonite presence is evident in the Naked Clay (with 2Ɵ 6.58, 19.88 and 28.47 degree). Furthermore, the Naked Clay involves biotite (with 2θ 10.25°). The Montmorillonite and Kaolinite are the dominant phases, while the others are minor phases. All such phases are confirmed by comparing Fig. 4a with literature.
Figure 4a shows relatively sharp and high signals for Naked Clay, which means that its particles are relatively more crystalline than the other two powders. The signals became shorter and broader due to grinding by stirring under reflux for 6.0 h as shown in Fig. 4b. After longer treatment (30.0 h) the MnTPyP@Nano-Clay signals were further broadened and shortened. The signals are typical for nano-scale particles with lower crystallinity. The XRD patterns are consistent with the SEM micrographs discussed above.
The XRD patterns gave insight on orientation of MnTPyP+ ions intercalated inside the expandable montmorillonite phase. Figure 4a–c show shift in value of 2θ from (6.58 degree, for Naked montmorillonite) to three new values (6.00, 5.00 and 3.66 degree), after refluxing with MnTPyP+ for both MnTPyP@Micro-Clay and MnTPyP@Nano-Clay. This indicates interlayer distance expansion as a result of metalloporphyrin penetration between clay layers. The three different shift values indicate three intercalation orientations: horizontal, perpendicular and diagonal. The shifts in montmorillonite signal are re-summarized in Fig. 5.
Unlike earlier reports discussed above, the XRD patterns here indicate that the montmorillonite phase exhibited three different types of intercalation. Based on Braggs’ law, and taking into consideration λ = 1.54 Å, n = 1, and 2θ = 6.58 degrees, the original distance between tops of two adjacent layers (d) equals 13.4 Å. This value is consistent with earlier reports [39–41].
The shifting from 6.58° to 6.00° indicates that d increased from 13.4 Fig. 6a to 14.7 Å Fig. 6b. The net spacing expanded by about 1.3 Å. The MnTPyP ion has a thickness of about 1.09 Å. The results indicate that a monolayer of metalloporphyrin ions is sandwiched horizontally between two adjacent layers as shown in Fig. 6b. The logic is based on the concept of size matching effect reported earlier [42].
The shifting from 6.58° to 5.00° means that d increased from 13.4 to 17.3 Å, with net expansion of 3.9 Å. This indicates that the metalloporphyrin ions intercalated between two adjacent layers in a diagonal manner, as shown in Fig. 6d. By this way, the metalloporphyrin intercalation caused expansion by 3.9 Å.
Shifting the signal from 6.58° to 3.66° indicates increase in the distance between the tops of two adjacent layers (d) from 13.4 to 23.9 Å by intercalation, as shown in Fig. 6c with net expansion of ~10.5 Å. This means that the metalloporphyrin ions are perpendicularly inserted between two adjacent layers. Similar logic has been used by Constantino et al. for other metalloporphyrin systems perpendicularly intercalated in non-clay solids [19] as discussed above. Such expansion of space between adjacent layers is expected to facilitate entrance of different reactants inside the clay and consequently speed up catalytic organic reactions therein. The resulting supported catalyst will thus have an added value, as reported earlier [17].
Horizontal and perpendicular intercalations of metalloporphyrins inside different materials are known [42, 44, 45]. Diagonal orientations have also been reported inside niobite Nb3O8
− [19]. To our knowledge, perpendicular intercalations have not been described in montmorillonite in earlier reports. This work manifests perpendicular intercalation inside montmorillonite for the first time.
With its planar structure, biotite is expected to host MnTPyP+ ions by intercalation. RXD pattern, Fig. 4a–c, shows shifting in 2θ from 10.25° to 9.01°. The results are re-summarized in Fig. 7 a,b and c. The d value expanded from 8.6 to 9.8 Å, with only 1.2 Å expansion. This indicates that MnTPyP+ ions intercalated into biotite layers only horizontally. Both micro- and nano-clays underwent intercalation as shown in Fig. 7.
Kaolinite and Illite phases did not exhibit intercalation with MnTPyP+ ions. This is due to their well-known relatively small interlayer distances, as they belong to the non-expandable clays. Due to this reason they are used in ceramic industry, because they do not absorb water molecules. Quartz does not have separate layer structure, and consequently does not allow MnTPyP+ intercalation.
The catalytic activity of the intercalated MnTPyP+ ions, described here, in olefin hydrosilylation reaction has been reported recently. Reactions were conducted using Naked Clay, MnTPyP@Nano-Cla and homogeneous MnTPyP+ ions. While the Naked Clay showed no catalytic activity, the intercalated catalysts showed higher activity and selectivity than the homogeneous metalloporphyrin ion. The MnTPyP@Nano-Clay catalyst exhibited a turnover frequency (up to 1200 min−1) and exceptionally high selectivity to produce terminal hydrosilylation reaction products. Soundly high activity on recovery and reuse for third time was also observed for the MnTPyP@Nano-Clay system. An explanation for these behaviors has been discussed based on a proposed mechanism [46]. With its expanded layer structure, the clay allowed the reactant molecules, tri(ethoxy)silane and 1-octene, to reach the supported MnTPyP+ catalyst sites. Moreover, the cavities inside the clay support exhibited solvent like behavior and increased the catalyst efficiency. Unlike the homogeneous catalyst system, the supported catalyst showed high selectivity to produce terminal hydrosilylation product only based on steric effect.
Work is under way to investigate catalytic activity of clay supported metalloporphyrins in other types of reactions. Using single pure forms of clays, such as montmorillonite and biotite, as supports for different metallporphyrin ion catalysts is also underway (Additional file 1).