Vinícius W. Faria,
Marcos F. Brunelli and
Carla W. Scheeren*
Laboratory of Catalysis, School of Chemistry and Food, Universidade Federal do Rio Grande-FURG, Rua Barão do Caí, 125, 95500-000 Santo Antônio da Patrulha, RS, Brazil. E-mail: carlascheeren@gmail.com
First published on 1st October 2015
Iridium nanoparticles (Ir(0) NPs) of 2.1 ± 0.5 nm were synthesized from [Ir(cod)Cl]2 (cod = 1,5-cyclooctadiene) in the ionic liquid (IL) 1-n-butyl-3-methylimidazolium hexafluorophosphate [BMI.PF6]. The Ir(0) NPs were subsequently supported on polymeric membranes using cellulose acetate (CA) in acetone and the IL 1-n-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide [BMI.N(Tf)2]. Polymeric membranes with thicknesses of 20 μm were prepared with 10 mg of Ir(0) NPs. The polymeric membranes were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). The analysis showed that the Ir(0) NPs are homogeneously distributed over the entire membrane, which has a compact structure. The presence of small Ir(0) NPs induced increases in the surface areas of the polymeric membranes. The presence of the IL in the membrane structure increases the separation between the cellulose macromolecules, which results in greater flexibility and durability of the polymeric membranes. The CA/LI/Ir(0) combination exhibits excellent synergistic effects that increase the activity of the catalyst in hydrogenation reactions.
The use of active polymeric membranes5–9 has received little attention when compared with catalytically active inorganic membranes.10–13 However, inorganic membranes (albeit with high chemical and thermal stability)14 may be substituted by less expensive and more versatile polymeric organic membranes.6
The Supported Ionic Liquid Phase (SILP) is emerging as an interesting protocol for the immobilization of metal nanoparticles (MNPs) because it may combine the advantages of ionic liquids (ILs) with those of heterogeneous support materials.15,16 These materials are prepared via the covalent attachment of ILs to the support surface or by simply depositing the IL phases containing catalytically active species, usually transition metal complexes15 or metal nanoparticles16 on the surface of the support, which is usually a silica or polymeric material. Therefore, the combination of metal nanoparticles dispersed in an IL with a polymeric organic membrane, such as cellulose derivatives,17–22 may generate new and versatile catalytic materials.
For example, cellulose polymeric membranes were prepared via the combination of Rh or Pt NPs and the IL [BMI.N(Tf)2]. The catalyst showed superior activity in cyclohexene hydrogenation and possess higher stability than the metal NPs only.23 In another work, Pd NPs were supported in cellulose polymeric membranes. The material formed was applied in Suzuki coupling reactions with effective results.24 In another case, a poly(ionic liquid) was chemically grafted to a microPESs support membrane to stabilize Pd NPs. The well-defined Pd NPs immobilized inside the membrane provided catalytically active sites for the organic transformations.25
Iridium NPs synthesized in ILs with catalytic activity have been described in the literature.26–31 Different supports, such as silica, carbon, alumina and polyvinylpyrrolidone, have been studied to immobilize Ir NPs due the attractive properties of these materials.32–36
We report iridium NPs synthesized in the IL [BMI.PF6] combined with cellulose acetate in acetone and the IL 1-n-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide [BMI.N(Tf)2] for the generation of new polymeric membrane-supported Ir(0) NPs hydrogenation catalysts.
The Ir(0) NPs were irregularly shaped with a monomodal size distribution of 2.7 ± 0.5 nm (Fig. 2). The NPs were dispersed in the IL [BMI.N(Tf)2] and transferred as a syrup in cellulose acetate (CA) in acetone. This homogeneous solution was spread over a glass plate, and polymeric membranes with thicknesses of 20 μm were obtained using a spacer. The combination of CA/IL/Ir(0) may have several advantages, such as potentially using fixed-bed reactors for continuous reactions44,45 reducing IL levels and allowing for facile and efficient separation of products from catalyst. In this work, we have prepared polymeric membranes of cellulose acetate in acetone and the IL [BMI.N(Tf)2] containing supported Ir(0) NPs. The polymeric membranes formed were characterized using TEM, SEM/EDS, BET analyses, and the mechanical properties were investigated.
The XRD pattern (Fig. 1) confirmed the crystallinity of the iridium, and the mean diameter could be estimated from the XRD diffraction pattern by means of the Debye–Scherrer equation calculated from the full width at half-maxima (fwhm) of the (111), (200), (220), (311), and (222) planes. The diameter obtained from the XRD analysis was 2.7 ± 0.5 nm.
TEM analysis of the Ir(0) NPs show that the particles display a spherical shape; the evaluation of their characteristic diameter results in a monomodal particle size distribution (Fig. 2A). A mean diameter of 2.1 ± 0.5 nm Ir(0) NPs was estimated from an ensembles of 300 particles found in an arbitrarily chosen area of the enlarged micrographs. Fig. 2B shows the particle size distributions, which can be reasonably well fitted by a Gaussian curve.
The diameters of the Ir(0) NPs determined from the XRD (2.7 ± 0.4 nm) and TEM (2.1 ± 0.5 nm) analyses were different. The use of the full width at half maximum (fwhm) of a peak to estimate the size of the crystalline grain via the Scherrer equation has serious limitations because it does not take into account the existence of a distribution of sizes and the presence of defects in the crystalline lattice. Therefore, the calculation of average diameter of grain from the fwhm of the peak can overestimate the real value because the larger grains contribute strongly to the intensity, while the smaller grains only increase the base of the peak.
The polymeric membranes containing supported Ir(0) NPs were characterized by TEM (Fig. 3).
The TEM micrographs of CA/IL/Ir(0) polymeric membranes (Fig. 3) show that the Ir(0) NPs are homogeneously distributed over the membrane (black points). This is an indication that the immobilization of the NPs does not significantly change the aggregation and size distribution of the nanoparticles in the IL.
The scanning electron micrographs (SEM) and energy dispersive X-ray spectroscopy (EDS) of the cross sections of the CA/IL/Ir(0) are shown in Fig. 4.
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Fig. 4 (A) SEM micrographs of CA/IL/Ir(0) and (B) EDS showing iridium, S and F signals, indicating the presence of the NPs and IL [BMI.N(Tf)2] in the polymeric membranes. |
The SEM micrographs (Fig. 4A) show that the CA/IL/Ir(0) polymeric membranes contain stains that are heterogeneously distributed over the entire membrane cross-section, indicating the presence of Ir(0) NPs (represented by the clear points (BSE method)). The CA/IL/Ir(0) polymeric membrane cross-section in Fig. 4A shows that the thickness was approximately 20 μm. The EDS analysis (Fig. 4B) shows iridium, S and F signals, indicating the presence of the NPs and the IL [BMI.N(Tf)2] in the polymeric membranes. The CA and CA/IL polymeric membranes were also analyzed by SEM/EDS. The micrographs showed the compact structure of the polymeric membrane, and EDS showed the S and F signals from the anion of the IL (Fig. S1†). It is clear that the morphological structure of the polymeric membranes changes with or without the IL. In particular, the CA polymeric membrane (Fig. S2, A†) seems to have a scaled structure. In contrast, the addition of the IL seems to generate a fibrous structure in the longitudinal direction of the CA/IL polymeric membrane (Fig. S2, B†).
The BET surface areas of the (CA), CA/IL and CA/IL/Ir(0) polymeric membranes are summarized in Table 1.
Entry | Polymeric membrane | Ir(0) | BMI.N(Tf)2 | SBET |
---|---|---|---|---|
1 | CA | — | — | 192 m2 g−1 |
2 | CA/Ir(0) | 10 mg | — | 114 m2 g−1 |
3 | CA/IL | — | 1.0 g | 24 m2 g−1 |
4 | CA/IL/Ir(0) | 10 mg | 1.0 g | 92 m2 g−1 |
The surface area of the pure cellulose polymeric membrane (CA) was 192 m2 g−1 (±10%); after the addition of the [BMI.N(Tf)2] IL (1.0 g), it was 24 m2 g−1 (±10%), demonstrating a reduction in the superficial area when the IL was added. This result suggests that the addition of the IL results the occupation of the free pores in the polymeric membrane, especially in the predominant fraction. The N2 adsorption–desorption isotherms at very low relative pressures (Pie/Po < 0.2) exhibited high adsorption, confirming the microporous structure. In the case of the polymeric membrane containing Ir(0) NPs, the surface area obtained was 114 m2 g−1 (±10%), and after the addition of the IL, a surface area of 92 m2 g−1 (±10%) was obtained. This result indicates that the presence of small Ir(0) nanoparticles induces an increase in the CA/IL polymeric membrane surface area (compare entries 3 and 4, Table 1). The concentration of Ir(0) NPs incorporated in the polymeric membrane was determined using FAAS. The concentrations were 762 μg g−1 and 0.08% (m m−1) for Ir(0). The metal concentration incorporated in the polymeric membrane is related to the thickness of the membrane; it was observed that, for thickness up to 20 μm, the materials is saturated by 10 mg of nanoparticles.46
The presence of the IL in the polymeric membranes was confirmed by the stretching band at 3170 cm−1, which is due to presence of aromatic C–H groups. After impregnation of the IL in the polymeric membranes, a significant decrease is observed in the intensity of the band at 3400 cm−1, which is attributed to the –OH stretching of the pure cellulose acetate, indicating participation of the –OH group in the interaction with the IL (see Fig. S3†).
The tensile stress versus strain at break curves of the pure and modified polymeric membranes are shown in Fig. 5. Some of the main parameters that can influence the stress–strain curve profiles are the polymer structure, molecular weight, degree of cross-linking, chain orientation, ionic interaction, processing conditions and temperature, among others. The stress versus strain curves supplies important information about the Young's modulus (slope of linear region of the plot), tenacity, and stress and strain at break of polymeric membranes.
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Fig. 5 Stress versus strain curves of the modified membranes showing the effects of the different compositions of the cellulose acetate membranes. |
The stress–strain curves in Fig. 5 are similar for the CA/IL, CA/Ir(0) and CA/IL/Ir(0) polymeric membranes when compared to the pure CA polymeric membrane. These results show increases in elasticity, decreases in tenacity and toughness (area under the stress–strain curve), and reductions of the Young's moduli for the CA/Ir(0), CA/IL and CA/IL/Ir(0) polymeric membranes. The stress–strain curve profiles suggest that the addition of the IL leads to drastic decreases in the Young's modulus and the elongation at break. This result indicates that an increase in the distance between two cellulose macromolecules resulted in greater flexibility of the cellulose.
The plasticizer effect of the IL [BMI.N(Tf)2] in the CA-1.0 g IL polymeric membranes reduces the intermolecular forces that are usually present in the cellulose acetate. In other words, it is possible that the bis(trifluoromethane sulfonyl)imide anion of the IL strongly interacts with the hydrogen bond networks formed in the cellulose acetate chains through the nitrogen atom. These results are corroborated by the infrared spectra (Fig. S3†).
The catalytic activities of the heterogeneous catalysts studied in this work were expressed using the turnover frequency (TOF); the TOF values were estimated for low substrate conversions (10%). These activities should also be corrected by the number of exposed surface atoms using the metal atom's magic number approach.47
The catalytic properties of the Ir(0), CA/Ir(0) and CA/IL/Ir(0) polymeric membranes were evaluated in hydrogenation reactions of 1-hexene at 4 bar of H2 and 75 °C (Table 2). The catalytic activity (TOF) was strongly influenced by the applied catalyst.
Entry | Sample | Substrate | Ir(0) (mg) | Timea (h) | TOFb (h−1) | TOFc (h−1) |
---|---|---|---|---|---|---|
a Time for 10% substrates conversion (determined by GC).b TOF values for 10% conversion.c TOF corrected values for surface exposed atoms of iridium (45%).d Conditions: IL: [BMI.N(Tf)2]; TON = 250 mmol substrate/mmol Ir(0); 75 °C; 4 bar H2. | ||||||
1 | Ir(0) | 1-Hexene | 5 | 0.04 | 638 | 1417 |
2 | CA/Ir(0) | 1-Hexene | 1.6 | 0.04 | 592 | 1317 |
3 | CA/IL/Ir(0) | 1-Hexene | 1.6 | 0.03 | 750 | 1667 |
4 | Ir(0) | Cyclohexene | 5 | 0.04 | 578 | 1284 |
5 | CA/Ir(0) | Cyclohexene | 1.6 | 0.09 | 40 | 89 |
6 | CA/IL/Ir(0) | Cyclohexene | 1.6 | 0.09 | 270 | 600 |
7 | Ir(0) | Benzene | 5 | 0.1 | 266 | 591 |
8 | CA/Ir(0) | Benzene | 1.6 | 1.46 | 18 | 40 |
9 | CA/IL/Ir(0) | Benzene | 1.6 | 0.31 | 78 | 173 |
The catalytic properties of the Ir(0) NPs and the CA/Ir(0), and CA/IL/Ir(0) polymeric membranes were evaluated in hydrogenation reactions of 1-hexene, cyclohexene and benzene at 4 bar H2 and 75 °C (Table 2). The catalytic activity was strongly influenced by the catalyst used. For example, a higher catalytic activity (TOF) was observed for 1-hexene, cyclohexene and benzene using Ir(0) NPs (see entries 1, 4 and 7, Table 2). The activity of the Ir(0) NPs was lower than that of CA/IL/Ir(0) (compare entries 1 and 3, Table 2); this fact may be related to nanoparticle agglomeration, reducing the exposed area of the catalytically active species and causing a loss of catalytic activity. The TOF data indicate that the CA/IL/Ir(0) polymeric membranes are more active than those without the ionic liquid (compare entries 2 and 3, 5 and 6, 8 and 9, Table 2). This is an indication that there is a synergistic effect between the cellulose and the IL on the stabilization of the NPs. This result is in line with those results observed for other supported IL phase catalysts.48 The cellulose/IL ratio should be observed because membrane pore saturation may occur with higher molar concentrations of the IL. Therefore, levels of 1.0 g/5.0 g IL/cellulose guarantee NP stabilization and, most likely, the establishment of a more efficient porous contact region between the gas and the liquid phases within the polymeric membrane structure.49
Fig. 6 shows the hydrogenation curves of 1-hexene using Ir(0), CA/Ir(0) and CA/IL/Ir(0).
The Ir(0) NPs and CA/Ir(0), CA/IL/Ir(0) polymeric membranes were evaluated in recyclability studies using 1-hexene as the substrate (Fig. 7).
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Fig. 7 Recyclability of the Ir(0), CA/Ir(0), and CA/IL/Ir(0) nanocatalysts as well as the commercial Crabtree catalyst in the hydrogenation of 1-hexene. |
The recyclability studies included a commercial catalyst (Crabtree). Of note, the CA/IL/Ir(0) material can be reused up to 7 times in 1-hexene hydrogenation without losing catalytic activity (Fig. 7).
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16426e |
This journal is © The Royal Society of Chemistry 2015 |