Matías
Blanco
a,
Patricia
Álvarez
a,
Clara
Blanco
a,
M. Victoria
Jiménez
*b,
Jesús J.
Pérez-Torrente
b,
Luis A.
Oro
b,
Javier
Blasco
c,
Vera
Cuartero
d and
Rosa
Menéndez
*a
aInstituto Nacional del Carbón (INCAR) – CSIC, P.O. Box 73, 33080, Oviedo, Spain. E-mail: rosmenen@incar.csic.es
bDepartment of Inorganic Chemistry, Instituto de Síntesis Química y Catálisis Homogénea (ISQCH-CSIC), University of Zaragoza, 50009, Zaragoza, Spain. E-mail: vijimenez@unizar.es
cInstituto de Ciencia de Materiales de Aragón (ICMA), Departamento de Física de la Materia Condensada, CSIC – Universidad de Zaragoza, 50009 Zaragoza, Spain
dESRF – The European Synchrotron, 71, Avenue des Martyrs, Grenoble, France
First published on 21st March 2016
CVD-grown multiwalled carbon nanotubes were purified by applying four different treatments with increasing oxidation severity. The growing severity of the treatment results in progressive oxygen functionalization of the surface along with introduction of an increasing quantity of defects on the carbon nanotube walls. Iridium–N-heterocyclic carbene complexes were covalently anchored to those oxidized surfaces through their surface carboxylic acids via acetyl linkers. The carbon nanotube-based iridium–NHC hybrid materials developed are active in the hydrogen-transfer reduction of cyclohexanone to cyclohexanol with 2-propanol/KOH as hydrogen source but with rather different activity. The catalytic activity of the hybrid catalysts is strongly influenced by the type and amount of oxygenated functionalization resulting from the treatment applied to the support, being the most active and the most oxidized material.
Carbon nanotubes (CNT),5 among other carbon materials, present excellent electronic, thermal, chemical and mechanical properties6–8 which make them suitable to support molecular transition metal complexes. All the synthetic methods used in their production, i.e. arc discharge, laser vaporization or CVD,9 result in impurities that join every produced CNT batch, which are mainly based on amorphous carbon particles and traces of the metals employed for the growth of the nanotubes.10 Purification methods involve thermal annealing, electrochemical or magnetic treatments, but the most applied procedures are based on different acid/oxidant treatments in order to dissolve the metals and eliminate the carbon impurities.11 It is known that these treatments also affect the carbon walls of the CNTs. In fact, the oxidation surface chemistry of CNTs is reasonably well established,12 and surface oxygen groups are deployed after the purification by oxidation processes, mainly carboxylic acids on the edges, tips and defects and hydroxylic and epoxy groups on the basal planes.13 depending on the grade of oxidation.14–16 Several proposals about the mechanism involving the oxidation of the CNT walls have been reported in the literature, but all of them agree that the severity of the treatments is directly related to the degree of oxidation.
Iridium–NHC complexes are efficient catalyst precursors for the reduction of CO and CN bonds to generate alcohols or amines under mild transfer hydrogenation conditions.17–19 The covalent immobilization of Ir–NHC complexes on CNTs can be achieved by taking advantage of their oxygenated surface chemistry, but their development as hybrid catalysts is under expansion in comparison with other suitable supports.20 In this regard, we have developed synthetic protocols for the covalent immobilization of Ir–NHC complexes to carbon surfaces through the carboxylic acid or the hydroxyl surface groups by using hydroxy-functionalized imidazolium salt. This functionalization approach results in the formation of ester and carbonate functions, respectively, as linkers to the Ir–NHC species. Interestingly, the hybrid materials were found to be efficient hydrogen-transfer catalysts exhibiting good recyclability.
The aim of this work is the covalent anchoring of Ir–NHC complexes, via acetyl linkers, through the carboxylic acids of different oxidized CNT supports resulting from oxidation treatments of increasing severity. We are aware that only a few N-acyl-substituted NHC–transition metal complexes have been reported to date.21 In addition, it has been found that these complexes usually react quickly and irreversibly with a great variety of mild nucleophiles, including water and alcohols, to give the corresponding protic NHC complexes.21a However, to our delight, we have found that these iridium hybrid catalysts exhibit an outstanding catalytic activity in hydrogen transfer, which is probably a consequence of the stabilizing effect exerted by the carbon nanotube support. Thus, the catalytic activity, stability and recyclability of the resultant hybrid materials in transfer hydrogenation reactions have been studied as a function of the surface chemistry of their corresponding support. In this context, it is worth noting that detailed catalytic studies based on supported catalysts with a gradual oxidation level of their surfaces are scarce.
X-ray absorption measurements at the Ir L3-edge were performed using a Si(111) double crystal monochromator at the BM23 beam line of the ESRF (Grenoble, France) and using a Si(311) double crystal monochromator at the Claess beam line of the Alba (Barcelona, Spain) synchrotron facilities. The spectra collected from both setups were equivalent. The energy resolution ΔE/E was estimated to be about 8 × 10−5 at the Ir L3-edge, and a pellet of Ir metal and cellulose, the complex [IrCl(cod)(MeIm(CH2)3OH)] and the salt IrCl3 were simultaneously measured for energy calibration as references for Ir(0), Ir(I) and Ir(III), respectively. The extended X-ray absorption fine structure (EXAFS) spectra were analysed using the ARTEMIS program,25 which makes use of theoretical phases and backscattering amplitudes calculated from FEFF6.26 The fits were carried out in R space using a Hanning window for filtering purposes.
Sample | XPS (atom%) | ICP (wt%) | |||||
---|---|---|---|---|---|---|---|
C/O | C | Cl | N | O | S | Ir | |
CNT-HCl | 28 | 96.6 | 0.0 | 0.0 | 3.4 | 0.0 | |
CNT-HCl-MI | 23 | 93.7 | 0.5 | 0.9 | 4.0 | 0.8 | |
CNT-HCl-MI-Ir | 3.1 | ||||||
CNT-LT | 20 | 95.2 | 0.0 | 0.0 | 4.8 | 0.0 | |
CNT-LT-MI | 20 | 92.4 | 0.6 | 1.4 | 4.6 | 1.0 | |
CNT-LT-MI-Ir | 6.1 | ||||||
CNT-MT | 16 | 94.1 | 0.0 | 0.0 | 5.9 | 0.0 | |
CNT-MT-MI | 15 | 91.4 | 0.8 | 1.7 | 6.1 | 0.0 | |
CNT-MT-MI-Ir | 10.0 | ||||||
CNT-ST | 4 | 80.8 | 0.0 | 0.0 | 19.2 | 0.0 | |
CNT-ST-MI | 3.5 | 73.9 | 1.3 | 2.3 | 21.0 | 1.5 | |
CNT-ST-MI-Ir | 12.3 |
The generated surface groups on the CNT-X materials were analysed by deconvolution of the C 1s band in the XPS spectra (Fig. 1), which also confirm the progressive introduction of oxygen groups following the increase in oxidation severity. Additionally, the diminution in the Csp2 band and the intensification in the Csp3 are both consequences of those oxidation processes. Interestingly, in accordance with this argument, the most oxidized carbon nanotube material, CNT-ST, shows the largest amount of carboxylic acids and hydroxylic groups. Additionally, the large percentage of oxygen in this material, also observed in graphene oxides, is also attributed to an increase in unreactive epoxy or ether groups.28
Fig. 1 XPS C 1s fitting of the parent materials CNT-X: a) CNT-HCl, b) CNT-LT, c) CNT-MT and d) CNT-ST. |
Applying a known carbon nanotube functionalization method, the treatment of the parent oxidized nanotubes (CNT-X) with thionyl chloride followed by reaction with N-methylimidazole allow the formation of hanging methylimidazolium salts by alkylation of the heterocycle with the more reactive pendant acyl groups. Operating in such a way, it is possible to obtain the imidazolium functionalized carbon nanotube materials named CNT-X-MI (MI = 1-acyl-3-methylimidazolium, Scheme 1).
The high-resolution XPS N 1s bands of the functionalized samples reveal a single band at 401.5 eV that is indicative of a unique functionalization mode. The atomic percentages obtained from XPS data (Table 1) show a steady increase in the amount of nitrogen, which corresponds to increasing functionalization of the carbon nanotube materials with the imidazolium groups. In addition, a higher ID/IG Raman ratio was observed in every sample and also when comparing the parent-oxidized with imidazole-treated materials, that indicates positive functionalization, which is in accordance with the correlation observed in related materials (see the ESI†).29 In addition, the correct N:Cl atomic ratio of ca. 2:1 was obtained which is in agreement with the presence of the imidazole ring (see the ESI†). All these data confirm that the more oxidation treatment applied, the more functionalized support is obtained, which is CNT-ST-MI.
The quantification of the imidazolium groups present in these materials has also been determined by means of thermogravimetric analysis (TGA) as the weight decreases at 400 °C (mainly associated to the elimination of the imidazolium fragments).30 As expected, more pronounced decays in the TGA profiles were observed for the materials coming from the more oxidized CNTs, which is in accordance with a larger number of imidazole units in these materials (see the ESI†).26 In fact, the nitrogen weight percentages determined from the TGA curves compare well with those obtained from the XPS data (see the ESI†).
Fig. 2 HRTEM images of the carbon nanotube–NHC–iridium hybrid catalysts: a) CNT-HCl-MI-Ir, b) CNT-LT-MI-Ir, c) CNT-MT-MI-Ir, and d) CNT-ST-MI-Ir. |
Although larger iridium particles of 1.2–1.4 nm are also detected, the Ir 4f region of the XPS spectra shows, for all samples, two maxima centred at 62.4 and 66.5 eV, corresponding to Ir 4f7/2 and Ir 4f5/2, according to Ir(I) species.31 Those larger spots, more marked on the unprotected outer walls, could be clusters or nanoparticles possibly formed by electron beam irradiation inside the microscope chamber.32 Similar size distributions were observed for other supported molecular iridium catalysts or even for graphene-based hybrid catalysts.3,4
Additionally, all the materials exhibited the same appearance in terms of number of layers and interlayer distances compared to their parent carbon nanotubes (see the ESI†), which indicates that functionalization has not caused any damage to the nanotube layers.
The amount of iridium in the nanotubes, determined by means of ICP-MS, varies progressively from 3.1% for the less oxidized material, CNT-HCl-MI-Ir, to 12.3% for the most oxidized material, CNT-ST-MI-Ir (6.1% for CNT-LT-MI-Ir and 10.0% for CNT-MT-MI-Ir, Table 1). The nitrogen weight percentages determined from the TGA curves allow for the calculation of the maximum iridium load in the CNT-X-MI-Ir materials assuming that each imidazole-2-ylidene is involved in the formation of an Ir–NHC bond. The ratio between the iridium found and the maximum calculated is 48%, 71% and 98% for CNT-HCl-MI-Ir, CNT-LT-MI-Ir, and CNT-ST-MI-Ir, respectively, although a slight metal overload (106%) was found for CNT-MT-MI-Ir. In this context, it is worth mentioning that the iridium load could be in some way influenced by a poor dispersion capacity of the materials in the polar reaction media, together with the accessibility to the reactive acyl chloride groups.33
Fig. 3 κ2-Weighted EXAFS spectra for the hybrid catalysts and the [IrCl(cod)(MeIm(CH2)3OH)] reference compound. The data are shifted in the vertical scale for the sake of comparison. |
The first coordination shell of the Ir atom in the reference compound, [IrCl(cod)(MeIm(CH2)3OH)], as it was previously reported35 is composed of a Cl atom, a C atom of the imidazol-2-ylidene (C1) ring and 4 C atoms of both CC bonds present in the cyclooctadiene ligand (Fig. 5a). Our analysis yields the following distances: Ir–Cl = 2.369(9) Å, Ir–C1 = 2–016(14) Å, Ir–C2,3 = 2.089(13) Å and Ir–C3,4 = 2.120(13) Å. The four latter distances correspond to the interatomic distances to the 1,5-cyclooctadiene carbons, while C1 refers to the carbon atom belonging to the NHC. In contrast, the Cl atom is missing in the Ir coordination shell of the supported catalysts and it is probably substituted with an O atom from the oxidized CNT materials.
In order to model the Ir local environment in the catalysts, we have replaced the Cl atom by oxygen, preserving the rest of the Ir–C bonds. In fact, the Ir–O distance was matched to the Ir–C1 bond length because this approach strongly stabilizes the fitting procedure and minimizes the number of free parameters. Moreover, it agrees with isostructural compounds like [Ir(NCCH3)(cod)(MeIm(CH2)3OH)][BF4], whose molecular structure, determined by X-ray diffraction (Fig. 5b), shows a N atom of the acetonitrile ligand bonded at 2.032(3) Å while the Ir–C1 bond length is 2.031(3) Å.35 We have observed a strong correlation between the values of Ir–C2,3 and Ir–C4,5; therefore, these interatomic distances were refined with a single parameter (ΔR2) starting from the theoretical value of the reference compound. Therefore, a total of 4 parameters are refined: an average inner potential correction of the threshold (ΔE0), an average Debye–Waller factor (σ2) and two distance parameters, ΔR1 for Ir–C1 and Ir–O and ΔR2 for Ir–C2,3 and Ir–C4,5. The amplitude reduction factor S02 is fixed to 1 in agreement with the value obtained for the reference compounds and previous studies.
Fig. 4a compares the best fit and experimental spectra corresponding to the moduli of the FTs of the κ2-weighted EXAFS shown in Fig. 3. Fig. 4b shows the back-Fourier filtered spectra in k-space corresponding to the first coordination shell together with the best fit results. These best fit results for CNT-X-MI-Ir and CNT-ST-1E-Ir34 are summarized in Table 2. Our analysis confirms a local environment composed of six light elements around the Ir atom in all the hybrid catalysts. The distances of Ir to the carbons in the diolefin range between 2.078 and 2.22 Å, while the shortest distances are between 2.025 and 2.045 Å. Finally, the higher value of the Debye–Waller factor is observed for CNT-MT-MI-Ir and especially for CNT-ST-MI-Ir suggesting a stronger structural disorder in the first coordination shell for these catalysts.
Catalyst | ΔE0 (eV) | R 1 (Å) | Ir–C2,3 (Å) | Ir–C4,5 (Å) | σ 2 (103, Å2) | R F |
---|---|---|---|---|---|---|
a The residual factor accounts for the misfit between the actual data and the theoretical calculations.25 Numbers in parentheses are the errors estimated from different analyses to the best significant digit. R1 stands for Ir–C1 and Ir–O. | ||||||
CNT-HCl-MI-Ir | 8.9(9) | 2.025(16) | 2.104(13) | 2.190(13) | 1.6(9) | 0.004 |
CNT-LT-MI-Ir | 9.0(9) | 2.037(16) | 2.114(14) | 2.199(14) | 1.7(9) | 0.006 |
CNT-MT-MI-Ir | 8.8(9) | 2.033(63) | 2.078(53) | 2.164(53) | 4.0(30) | 0.014 |
CNT-ST-MI-Ir | 8.8(9) | 2.046(12) | 2.118(11) | 2.204(11) | 5.0(30) | 0.011 |
CNT-ST-1E-Ir | 9.3(9) | 2.045(13) | 2.135(11) | 2.221(11) | 1.1(9) | 0.005 |
The hybrid catalysts derived from gradually oxidized carbon nanotubes having anchored Ir–NHC complexes via acetyl linkers (CNT-X-MI-Ir) were active in the transfer hydrogenation of cyclohexanone. Reaction times required reaching a conversion of more than 90%, and the turnover frequencies (TOF), at initial time and at 50% conversion, for all the examined catalysts are summarized in Table 3. The four hybrid materials showed similar kinetic profiles, but appreciable differences in the catalytic activity were found (Fig. 6a). Interestingly, the cyclohexanone reduction was observed immediately after the thermal equilibration of the reactant mixture with no detectable induction period. The hybrid catalyst CNT-ST-MI-Ir is the most active in this series reaching 91% conversion in 1.6 h with a TOF50 of 3000 h−1. CNT-MT-MI-Ir, CNT-LT-MI-Ir and CNT-HCl-MI-Ir were considerably less active with TOF50 values of 789, 428 and 254 h−1, although CNT-MT-MI-Ir showed an initial TOFo of 6500 h−1 which is the highest in the series.
Catalyst | Time (min) | Conv. (%) | TOF0 (h−1)c | TOF50 (h−1)d |
---|---|---|---|---|
a Reaction conditions: catalyst/substrate/KOH ratio of 1/1000/5, 0.1 mol% of catalyst in 2-propanol (5 mL) at 80 °C. b The reactions were monitored by GC using mesitylene as internal standard. c TOFs, turnover frequencies [(mol product/mol catalyst)/time (h)], were calculated at initial time (60 s), TOF0. d TOFs, turnover frequencies [(mol product/mol catalyst)/time (h)], were calculated at 50% conversion, TOF50. | ||||
CNT-HCl-MI-Ir | 900 | 91 | 1050 | 250 |
CNT-LT-MI-Ir | 300 | 93 | 1500 | 430 |
CNT-MT-MI-Ir | 200 | 90 | 6500 | 790 |
CNT-ST-MI-Ir | 100 | 91 | 5400 | 3000 |
CNT-ST-1E-Ir | 80 | 94 | 6300 | 3350 |
CNT-ST-1C-Ir | 210 | 89 | 3000 | 1300 |
TRCNT-ST-1C-Ir | 100 | 92 | 6000 | 3000 |
Ir-ImidO | 200 | 94 | 3000 | 1500 |
The pattern of catalytic activity shown by these hybrid catalysts CNT-X-MI-Ir correlates with the oxidation degree of the corresponding parent carbon nanotubes which points to a surface effect derived from the intensity of the oxidation treatment. According to the solid characterization techniques, those supports treated with weaker oxidizing purification procedures (HCl, LT and MT treatments) result in supports with an extended sp2 structure with higher C/O ratios, low intrinsic porosity and tips scarcely opened. The low level of hydrophilic functional groups could also imply a lower dispersion capacity in the polar catalytic reaction medium which makes the access of the reactants to the active sites and, in particular, to those settled in the inner cavity of the nanotube difficult. On the other hand, the severe oxidation treatment (ST treatment in particular) leads to a nanotube with a defective structure, plenty of holes, oxygen moieties, structural defects and opened tips.36,37 The defective structures contain suitable functional groups exposed to form active centres situated in the holes and open channels where reactants can arrive more easily in the diffusional stages of the heterogeneous catalytic process. Under these circumstances, the nanotube can also act as a “nano-reactor” offering a confinement effect that can occasionally enhance the catalysis activity.38–40
The molecular complex [IrCl(cod)(MeImCOCH3)] has been revealed as extremely moisture and alcohol sensitive which precluded the comparison of its catalytic activity with that of the carbon nanotube–NHC–iridium hybrid catalysts. [IrCl(cod)(MeImCOCH3)] is formed in the reaction of [{Ir(μ-OMe)(cod)}2] with the corresponding N-acetyl imidazolium salt along with complex [IrCl(cod)(MeImH)] resulting from the methanolysis of the acyl fragment. In fact, [IrCl(cod)(MeImCOCH3)] was cleanly converted to [IrCl(cod)(MeImH)] and methyl acetate upon reaction with methanol (NMR evidence).
It is noteworthy that neither the iridium-free carbon nanotube-based materials nor iridium supported materials without an imidazolium ligand have shown significant catalytic activity, which indirectly supports the covalent anchoring of the Ir–NHC complexes to the carbon nanotube-based materials via acetyl linkers. In this context, it has been recently reported that iridium oxide nanoparticles supported on nanoparticulate cerium oxide exhibit moderate catalytic activity in the transfer hydrogenation of cyclohexanone.41 However, the presence of IrO2 nanoparticles in our NHC–Ir hybrid catalysts is negligible because the fit of the first shell of our EXAFS spectra to a model with a rutile crystal structure (IrO2) renders a poor model. In fact, the EXAFS signal of IrO2 shows some structures in the oscillation between 7 and 9 Å−1 that are not found in the spectra or our catalysts, and the Fourier transform shows an important peak around 3 Å corresponding to the Ir–Ir path that is also absent in our spectra, and the white line (very sensitive to the number of d-holes) is significantly higher for the IrO2 spectrum.42
The catalytic performance of CNT-ST-MI-Ir has been compared with that of the molecular acetoxy-functionalized NHC complex [IrCl(cod)(MeIm(CH2)3OCOCH3] (Ir-ImidO)3 and of related hybrid catalysts having supported Ir–NHC complexes on carbon nanotubes obtained under the most severe oxidation conditions through the flexible 3-alkoxipropyl linker 1. In particular, the hybrid catalysts CNT-ST-1E-Ir3 and CNT-ST-1C-Ir, with ester and carbonate linking surface functions, result from the nanotube functionalization through the carboxylic acid and the hydroxyl surface groups, respectively, whereas TRCNT-ST-1C-Ir comes from the surface hydroxylic group functionalization of thermally reduced nanotubes (Scheme 3).35
As can be observed in Fig. 6b, the hybrid catalyst CNT-ST-1E-Ir is only slightly more active than CNT-ST-MI-Ir reaching 94% conversion in 1.3 h with a TOF50 of 3350 h−1 (Table 3). Thus, the flexibility imparted by the carbon chain linking the Ir–NHC supported complexes in CNT-ST-1E-Ir appears to influence positively the catalytic activity as both hybrid catalysts have a similar degree of oxidation. Interestingly, both hybrid catalysts are more active than the homogeneous catalyst [IrCl(cod)(MeIm(CH2)3OCOCH3] (Ir-ImidO) and the hybrid catalysts prepared by covalent functionalization of the hydroxyl surface groups CNT-ST-1C-Ir and TRCNT-ST-1C-Ir (Fig. 6b).
In contrast with the hybrid catalysts CNT-X-MI-Ir, for which the catalytic performance improves with the level of oxidation of the parent carbon nanotube material, the thermally reduced TRCNT-ST-1C-Ir was found to be more active than CNT-ST-1C-Ir (Fig. 6b).35 The structural characterization of the catalysts based on thermally reduced carbon nanotube materials showed an increase in the amount of Csp2 and a decrease in the oxygen functional groups, as a consequence of the partial restoring of the aromatic structure after the thermal treatment. Having in mind that EXAFS measurements showed a similar first-neighbour coordination shells for all the hybrid Ir–NHC catalysts,35 that difference should be ascribed to the different localization of the iridium centres in the hybrid catalysts. The active centres are located at the basal planes of the nanotubes for the catalyst with –OH functionalization, while for the –COOH functionalized materials, the Ir–NHC complexes are located at the holes and defects, where the approach of the reactants is easier because these centres are more exposed. However, in the case of a cleaner surface, which is provided for the thermal reduction, the diffusional stages can be faster, which enhances the activity of the reduced samples. These observations could be of a great importance because it gives a clear idea of how the functionalization should be in order to obtain more active catalysts.
Recycling studies (Fig. 7) were also carried out with the four hybrid catalysts CNT-X-MI-Ir based on gradually oxidized carbon nanotubes. The black solids obtained after each catalytic run were recovered, washed with fresh 2-propanol (4–5 mL) and subjected to another catalytic cycle. The recycling processes render conversions above 90% after five catalytic runs without any loss of catalytic activity. In addition, the kinetic profiles in the successive experiments are very similar to those plotted in Fig. 6a, even for the last cycle which was performed under air, which demonstrate the air stability of the hybrid catalysts3,35 (see the ESI†).
Fig. 7 Recyclability of the hybrid catalysts CNT-X-MI-Ir: conversion to cyclohexanol in five catalytic runs with recycling reaction times shown in Table 3. The 5th cycle was performed under air. |
In sharp contrast with the behaviour exhibited by a related N-acyl-substituted Ir–NHC molecular complex, the catalytic activity exhibited by the iridium hybrid catalysts suggests that the acetyl linkers in these materials do not undergo methanolysis under hydrogen transfer conditions which could be a consequence of the protection and stabilizing effect exerted by the nanocarbon walls of the support.
The local structure of the iridium atoms in the hybrid catalyst determined by EXAFS has shown a common local environment for all of them implying a coordination iridium sphere formed by two olefin bonds of the cod ligand, the carbon atom of the NHC carbene ligand, and an O atom from the oxidized carbon matrix as a result of the iridium–support interaction after ionization of the chlorido ligand.
The covalent functionalization through carboxylic acid moieties in the hybrid Ir–NHC catalyst makes possible both a confinement effect due to the porosity of the material and a surface effect based on the potential cooperation of hydroxyl functional groups on the nanotube walls. In this context, it has been found that the catalytic activity is enhanced by the presence of a high concentration of oxygen functionalities in the periphery of the active centers predominantly located at the structural defects including holes or opened tips.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cy01998b |
This journal is © The Royal Society of Chemistry 2016 |