Adsorption of Ferrocene on Carbon Nanotubes, Graphene, and

Nov 23, 2016 - In favorable cases the chemical shift anisotropy (CSA) and the dipolar and quadrupolar interactions were reduced to a degree that allow...
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Adsorption of Ferrocene on Carbon Nanotubes, Graphene, and Activated Carbon Kyle J. Cluff and Janet Blümel* Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, United States ABSTRACT: Ferrocene could be adsorbed on activated carbon, carbon nanotube surfaces, graphite, and graphene in the absence of a solvent at room temperature in spite of its high melting point (174 °C). In each case only monolayers formed via self-adsorption and the transition to surplus polycrystalline material was abrupt, with no multiple layers occurring. Variable-temperature multinuclear solid-state NMR spectroscopy was applied to study the mobilities of the surface-adsorbed ferrocene molecules. It has been demonstrated that on favorable supports the major anisotropic interactions that usually broaden the solid-state NMR signals of polycrystalline and amorphous materials were reduced or completely eliminated due to the mobility of the adsorbed ferrocene. In favorable cases the chemical shift anisotropy (CSA) and the dipolar and quadrupolar interactions were reduced to a degree that allowed the recording of the spectra of the solid materials on a conventional solution NMR spectrometer.

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INTRODUCTION The importance and potential of carbon allotropes in modern and future advances in science and engineering can hardly be overestimated. Exciting applications range from the popular palladium catalyst supported on carbon that is in general use to the nanometer-scale electronics of the future.1 In addition, organic and inorganic pollutants can be very efficiently adsorbed onto activated carbon, graphene oxide, and carbon nanotubes (CNTs) from aqueous solution and gas streams.2−9 Activated carbon is a large-scale commodity and a particularly important sorbent in medicine, in catalysis, and for purification procedures.10 Because of its relatively low price in comparison to other carbon allotropes, it is widely employed in many forms.10 Activated carbon can, for example, be derived from biomass, a feature that increases its importance.11 It is also amenable to functionalization which can, e.g., be used to render the material magnetic to further facilitate separations.11 As another application, biologically produced H2 and CH4 both suffer from the presence of CO2, which may be removed via adsorption on special activated carbons.12 Activated carbon is also very important in wastewater treatment to remove heavy metals, including Hg2+, Cr6+, and Cd2+ species, as well as organic contaminants such as toxic dyes.2,3 NMR spectrocopic adsorption studies involving activated carbon as the support have focused mainly on liquids such as water, alcohols, and benzene.13−15 For example, 2H solid-state NMR shows that the presence of toluene on activated carbon inhibits the mobility of D2O.14 2H NMR has also been used to study alcohols confined in activated carbon fibers at low temperatures.15 Spectroscopic studies on adsorbed solids are more scarce. One example would involve phenanthrene adsorbed on biochars.16 It has been demonstrated that some biochars are able to adsorb more phenanthrene because of its mobility on the surface, while other adsorbents with higher © XXXX American Chemical Society

aromatic content lead to immobile adsorbates and therefore lower loadings.16 Like activated carbon, carbon nanotubes, which were recently highlighted,17 have been the subject of many adsorption studies aimed at purification and environmental remediation.18−22 For example, divalent heavy-metal ions18 and toxic dyes19,20 can be removed from solution using CNTs. Futhermore, multiwalled carbon nanotubes (MWCNTs) in combination with nanoparticles of iron oxide give magnetically separable adsorbents for chromium pollutants and dyes.21,22 In addition to purification, CNTs also have great potential in the electronics industry, where efforts are focused on producing nanometer-scale transistors, diodes, and other electronic components.1 In order to accomplish this, it is desirable to tune the properties of the CNTs by modifying them with organic, inorganic, and organometallic compounds. The novel materials generated in such studies may have interesting electronic1,23−29 or catalytic properties.30 Most research thus far has focused on filling the interior of single-walled carbon nanotubes (SWCNTs). For example, it has been shown that endohedral doping of nanotubes with fullerenes can successfully tune the electronic parameters of the materials.1,31 Even more dramatic changes are noticed upon filling the SWCNT with metal-containing dopants such as metallofullerenes.23 Additionally, various metals and metal oxides have been filled into SWCNTs in an effort to further tune their electronic parameters and to explore novel materials.28,29 Metallocenes are much more accessible than metallofullerenes, and they have many interesting properties that make them targets for encapsulation in SWCNTs. This was first accomplished in 2002, when ferrocene, chromocene, ruthReceived: August 30, 2016

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Organometallics enocene, and vanadocene were successfully filled into SWCNTs.32 Filled nanotubes are usually characterized with SEM33 and HR-TEM, which has shown that the metallocenes move one-dimensionally along the nanotubes.32,33 These nanotubes modified with metallocenes are also relevant to the energy industry, and metallocenes have been used recently to dope SWCNT thin films for making solar cells.34 Furthermore, encapsulated ferrocene can be decomposed at 700 °C to yield iron nanoparticles inside the nanotubes.35,36 Before metallocenes or other adsorbates could be introduced into CNTs, the nanotubes had to be opened by burning them at 440−570 °C for 20−30 min.32,35,37−39 Subsequently, the nanotubes were subjected to metallocenes in the liquid or vapor phase and heated to 100−200 °C under vacuum for 24 h to remove any exterior adsorbed metallocene.32 An alternative but still demanding method of opening the ends of the nanotubes to expose the interior by treatment with nitric acid has been described.28 Adsorbing species on the outside of the nanotubes avoids these additional synthetic steps requiring harsh conditions. The adsorption on the outer surface of nanotubes appears even more interesting, since the tubes are strongly held together in bundles. These may be dispersed by adsorbing polymers, PAH (polycyclic aromatic hydrocarbons), or surfactants on the outside.40,41 Bundles of nanotubes have several possible adsorption sites, including in the channel between two parallel tubes (groove site), in the space between three closely packed parallel tubes (interstitial site), and on the surface of a single tube (exterior site).40 There are some examples of Zn, Cu, Ni, and Co porphyrins which are adsorbed onto the exterior surface of nanotubes.42 Cp2ZrCl2 has also been adsorbed on the exterior of MWCNTs to form an interesting polymerization catalyst which produces polyethylene (PE) with a molecular weight (Mw) of 1000000 and polydispersity index (PDI) of 1.95, while the metallocene alone produces PE with a Mw value of only 300000.30 One of the most obvious benefits of exterior versus interior adsorption is that both SWCNTs and MWCNTs are readily accessible for modification by this method whereas endohedral doping is mainly limited to SWCNTs and double-walled carbon nanotubes (DWCNTs).37 MWCNTs are commercially available in many sizes, which will allow the adsorption on nanotubes of various curvatures to be compared. This may be interesting, since benzene is predicted by calculations to adsorb more strongly to larger nanotubes with less curvature.40 Exterior adsorption furthermore avoids other potential complicating factors. For example, Li and co-workers filled cobaltocene and derivatives thereof into SWCNTs but found that the cobaltocene only adsorbed into nanotubes into which it fit perfectly.43 They supposed that this is due to maximized interaction with the nanotubes that is strong enough to hold the cobaltocene down while it would otherwise sublime away under the reaction conditions.43 As will be demonstrated in this contribution, harsh conditions can be avoided in the case of external adsorption and much milder conditions suffice. Therefore, we deemed it important to investigate the adsorption and mobility of metallocenes on the exterior surface of SWCNTs and MWCNTs and to experimentally determine the strength of the metallocene−nanotube interaction. Since ferrocene is the least reactive and especially the most stable metallocene with respect to oxidation, it will be the focus of the presented study.

Exohedrally adsorbed ferrocene has previously been generated as a byproduct of filling the nanotubes with ferrocene and characterized by IR spectroscopy.37 Interestingly, the adsorption was so strong that ferrocene could not be completely washed off by ethanol or acetone and had to be removed by either annealing or washing with toluene.37 Ferrocene on CNTs opens interesting venues in electronics applications.1 It has previously been adsorbed onto MWCNTmodified electrodes and studied via electrochemistry.44,45 Therefore, we sought to explore the adsorption and mobility of the metallocene ferrocene (Cp2Fe, (C5H5)2Fe) and its deuterated version (Cp2Fe-d2, (C5H4D)2Fe) on carbon-based supports such as activated carbon and carbon nanotubes and the gained insights will be described in this contribution. These studies are of fundamental interest and have wide-ranging potential in fields such as catalysis, electronic devices, and the separation sciences. In general, the adsorption of small molecules on surfaces is important for all processes in academia and industry that rely on separation sciences or catalysis. The adsorbed species might display dynamic effects, and we could recently show with multinuclear dia- and paramagnetic solid-state NMR that metallocenes46−50 and phosphine oxides51 are mobile on the surface within the pores of amorphous silica. Hereby, one crucial point was the solvent-free method for forming monolayers on silica surfaces.46−51 Using different support materials provides a deeper insight into the adsorption process itself and the ensuing mobilities of the surface species. Furthermore, expanding the earlier findings from the silica surface46−51 to surfaces with entirely different characteristics would underline the general nature of the phenomenon of surface mobility in the absence of solvents. Carbon nanotubes have been investigated by 13C solid-state NMR spectroscopy previously.52−54 In the following, we describe a multinuclear solid-state NMR study of the adsorption of ferrocene on activated carbon and carbon nanotubes. The temperature dependence of the dynamic effects will be investigated. It will be demonstrated that anisotropic line-broadening effects that prevail in polycrystalline solids55 are effectively reduced or even eliminated by quasiisotropic mobility of the adsorbed molecules across favorable surfaces. In fact, the chemical shift anisotropy (CSA),55,56 quadrupolar interactions, and even dipolar interactions55 are reduced to a degree that measuring the solid materials with a conventional solution NMR spectrometer without magic angle spinning (MAS) or high-power decoupling is possible. These findings should have far-reaching implications for analytics, the separation sciences, and catalysis.

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RESULTS AND DISCUSSION In this contribution, molecular species with high melting points have been adsorbed on different carbon allotropes as supports. Different brands of activated carbon, SWCNTs, MWCNTs, graphite, graphene, and C60 fullerene have been tested as adsorbents for ferrocene. Unfortunately, dynamic effects cannot be studied by IR spectroscopy. Furthermore, ferrocene does not have strong and characteristic Raman or IR bands57−59 that would lie outside of the region where the support materials activated carbon60,61 and CNTs62,63 show intense and broad absorption bands. The only prominent ferrocene bands (811, 1002, 1108, 1411, 3085 cm−1)58 are, for example, overlapping with the broad and intense bands of activated carbon (800− 1450, 3000−3600 cm−1).60,61 Since ferrocene is very dilute in B

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Organometallics the samples, being adsorbed only in a monolayer on the support surfaces (see below), its absorption bands are rendered invisible underneath the intense IR bands of the supports. It needs to be emphasized that the IR spectra of the support materials60−63 do not change upon adsorption of ferrocene, as expected, since ferrocene is adsorbed reversibly, and the supports are not chemically modified under the conditions used, at ambient pressure and temperature. TEM did not indicate the presence of ferrocene clusters or nanoparticle formation on the surface. The latter would require the presence of a strong reducing agent and can also be excluded by XPS and the fact that ferrocene can be desorbed intact from the surface (see below). Fortunately, solid-state NMR spectroscopy provides insight into all aspects of the presented adsorption phenomenon. Variable-temperature multinuclear solid-state NMR spectroscopy has been applied to study the dynamic effects of the mobile surface-adsorbed species. It could be shown that indeed on selected supports the major anisotropic interactions that render solid-state NMR signals broad55 can practically be removed. Therefore, meaningful NMR measurements can be made without magic angle spinning (MAS)55 or high-power proton decoupling. It will be demonstrated that recording the spectra of the solid materials on a high-resolution solution NMR spectrometer is feasible. This feature allows the rapid screening of numerous samples in short periods of time and facilitates variable-temperature measurements. Most importantly, all NMR spectra prove that no chemical reaction of ferrocene with any of the carbon support materials takes place. The reversibility of the adsorption has been shown previously50 by desorption experiments, which showed that unchanged ferrocene is released from the supports under vacuum at elevated temperatures. Activated Carbon as Support Material. Activated carbon (DARCO KB-G; see the Experimental Section) turned out to be a surprisingly favorable support for adsorbing Cp2Fe-d2. The loadings for maximal surface coverage and submonolayers could be determined as described previously,46−50 and typical procedures and amounts are given in the Experimental Section. The maximum loading of Cp2Fe-d2 on this brand of activated carbon used is 614 mg/g. This is even higher than the 274 mg/ g for silica49 and the 356 mg/g adsorbed on SWCNTs (see below and the Experimental Section), mainly reflecting the different values for the specific surface area and available open space for each support. The high loading renders activated carbon an ideal sorbent not only for purification processes but also for analytical purposes, because recording, for example, NMR spectra with high loadings of adsorbed species is much faster than that for materials with lower loadings (see below). When polycrystalline, solid ferrocene is combined and ground with activated carbon at room temperature and in the absence of any solvent, a monolayer forms on the carbon surface within minutes. As has been found previously with silica as the support,46−50 the transition from polycrystalline material to a monolayer is abrupt, and no stacking or formation of multiple layers on the surface occurs. This can be deduced from the 2H MAS NMR spectra, on comparison of polycrystalline (Figure 1, bottom) and surface-adsorbed Cp2Fe-d2 on activated carbon (Figure 1, middle and top). Polycrystalline deuterated ferrocene displays the typical Pake pattern55 described previously, with the characteristic quadrupolar coupling constant QCC of 96.1 kHz.49 When Cp2Fe-d2 is adsorbed on activated carbon in a submonolayer, the molecules are mobile

Figure 1. 2H MAS spectra of Cp2Fe-d2: (a) polycrystalline; (b, c) adsorbed with the given amounts on activated carbon. The bottom two spectra were recorded with rotational speeds of 4 kHz and the top spectrum with 6 kHz.

enough to average out the large anisotropic quadrupolar interactions. The Pake pattern collapses, and one single symmetric line is visible in the spectrum (Figure 1, top). A similar effect had been described previously for D2O included in different pockets within solid polymers64,65 and for plasticization effects.66−68 The residual half-width of the signal is only 1.26 kHz. According to theory, this can only occur in case the molecules undergo a fast quasi-isotropic reorientation within the pores in the time range of nanoseconds.69 As described earlier for silica, the molecules undergo a spiraling translational motion across the walls of the pores.46,49 With a higher loading residual polycrystalline material remains after a densely packed monolayer has formed. Therefore, the Pake pattern becomes visible, in addition to the narrow signal of the adsorbed species (Figure 1, middle). There are no multiple layers of ferrocene stacking on the surface because the transition from Pake pattern to narrow line is abrupt and not gradual. Furthermore, the amount of adsorbed ferrocene is always constant and independent of the excess of polycrystalline ferrocene offered. The experimentally determined amount of ferrocene adsorbed corresponds well with the calculated amount necessary for a monolayer using the surface area of the activated carbon support (see the Experimental Section) and the footprint of ferrocene. The quadrupolar coupling constant QCC of the Pake pattern (Figure 1, middle) has been determined70 to be 96.9 kHz. The representative case in Figure 2 demonstrates that the deconvolution and simulation of the spectra incorporating Pake patterns and central lines give very accurate results. The slight deviation from the value for pure Cp2Fe-d249 is most likely due to issues regarding the different signal-to-noise ratios

Figure 2. 2H MAS spectrum of Cp2Fe-d2 adsorbed with 1881 mg/g on activated carbon (Figure 1b, black) and simulated spectrum (green). The spectrum was recorded with a rotational speed of 4 kHz. C

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Organometallics and the inherent difficulty of optimally phasing and baselinecorrecting a spectrum that was recorded with an echo pulse sequence.55 The residual line width of the resonance of the adsorbed ferrocene in the center of the spectrum amounts to 2.91 kHz. The increase of the half-width as compared to the submonolayer signal indicates that the movement of the adsorbed molecules is slower in this case. This result is in accordance with the scenario found previously with silica as the support.49,50 The denser the adsorbed molecules that are packed on the support, the more their mobility is impeded. Unfortunately, the obtained resolution of the narrowest signal of adsorbed ferrocene (Figure 1c) did not allow us to distinguish between adsorption via one or two Cp rings, as was possible in the case of silica as the support.50 Some theoretical calculations for adsorbed metallocenes show only a small difference between upright and horizontal orientation of the molecules on the surface. Other studies demonstrate that for selected metallocenes and nanotube diameters it is slightly more favorable for the metallocene to “lie down” on the surface and interact with it via both Cp rings.23,24,71,72 To summarize the results from the 2H MAS study, it has been proven that ferrocene is readily adsorbed and mobile on the surface of activated carbon within the pores. Cp2Fe-d2 adsorbs with high loadings on activated carbon in the absence of any solvent to form a well-defined monolayer. There is a slight dependence of the residual line width of the adsorbed ferrocene on the density of the packing on the surface. However, the transition from a densely packed monolayer to residual polycrystalline material is abrupt and the molecules do not stack or form multiple layers. Encouraged by the line-narrowing effect of adsorption for ferrocene on activated carbon, we explored whether the solid materials would be amenable to measurements with conventional high-resolution NMR spectrometers.50 The 2H MAS spectra discussed above showed that the adsorbed molecules were mobile enough to average out even the massive quadrupolar interactions. For 1H and 13C NMR investigations of solids, the main anisotropic interactions that lead to broad lines are dipolar interactions and the CSA,56 which are orders of magnitude smaller.55 Indeed, both 1H and 13C NMR measurements of the solid samples can be performed on a routine solution NMR spectrometer without MAS or high-power proton decoupling. Preparing the samples and recording the spectra without any sample spinning was fast and, for example, the 13C NMR signals in the spectra displayed in Figure 3 only required 512 scans, corresponding to 20 min of measurement time. The 1H NMR spectra depicted in Figure 4 were obtainable with as few as 16 scans. While the actual measurement time at the solution NMR spectrometer was comparable with the time requirement for recording the spectra at the solid-state NMR instrument, the preparation of larger numbers of samples was faster and hence the throughput of samples was swift. Furthermore, variabletemperature measurements could conveniently be performed at a conventional solution NMR spectrometer (Figures 3 and 4). As expected, pure solid activated carbon, which does not contain any mobile components such as adsorbed impurities or residual solvents, did not give any signal (Figure 3a). The same accounts for the 1H and 13C NMR spectra of polycrystalline ferrocene, which only contains immobile molecules fixed at their positions in the crystal lattice. The anisotropic homo- and heteronuclear dipolar interactions and the CSA are too large to allow the recording of the signals. The same accounts for the

Figure 3. 13C NMR spectra of (a) activated carbon and (b−e) Cp2Fed2 adsorbed in a monolayer on the surface of activated carbon at the indicated temperatures. All spectra were recorded on a conventional solution NMR spectrometer. Half-widths/chemical shifts are (b) 1920/72.6, (c) 810/71.5, (d) 480/70.9, and (e) 410/71.2 Hz/ppm.

Figure 4. 1H NMR spectra of Cp2Fe-d2 adsorbed in a monolayer on the surface of activated carbon at the indicated temperatures. All spectra were recorded on a conventional solution NMR spectrometer. The half-widths (kHz) are listed next to the signals; the chemical shifts are (a) 5.7, (b) 5.6, (c) 5.6, and (d) 5.7 ppm.

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C NMR signals of all carbon supports. Since there are no mobile molecules or moieties present in activated carbon or CNTs, the signals described earlier52−54 did not emerge in the spectra recorded on the solution NMR spectrometer. However, even at ambient temperatures, Cp2Fe-d2 adsorbed in a monolayer resulted in a resonance with a half-width of only 480 Hz (Figure 3d). Heating the sample to 60 °C narrowed the resonance slightly to 410 Hz, while cooling it to −20 and −50 °C led to substantially broadened lines with half-widths of 810 Hz and 1.92 kHz, respectively. Therefore, adsorbed ferrocene resides in a mobility regime that can be influenced within an easily accessible temperature window. On the high-temperature end, once the mobility exceeds that required to average out the anisotropic line-broadening solid-state NMR interactions, there is no further reduction of the half-widths. Interestingly, analogous results have previously been obtained for O PPh2(Ph-d5) adsorbed on silica,51 which represents a very different adsorbate/adsorbent system. For the chemical shifts there is no clear trend; they vary randomly between 72.6 and 70.9 ppm (Figure 3). D

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value in the spectra (a)−(d), proving again that the molecules of the residual polycrystalline material are not mobile within the applied temperature window, and the molecules are firmly fixed in the crystal lattice. Again, the residual NMR signal line width for the adsorbed ferrocene increases at low temperatures and becomes smaller at elevated temperatures. The fraction of adsorbed versus polycrystalline Cp2Fe-d2 decreases at lower temperatures. This effect is reversible, and when the sample is warmed again, the scenario (d) in Figure 5 is regained. Figure 6 displays the data obtained from all variabletemperature measurements graphically. The 13C, 2H, and 1H

In this context it should be pointed out that the linenarrowing effect observed can only be due to the fast motion of the ferrocene molecules adsorbed on the surface within the pores and that any formation of a polymorph or amorphous phase of ferrocene can be excluded. First of all, ferrocene has a high melting point of 174 °C, and it cannot be melted by grinding it with a mortar and pestle. All solid-state NMR spectra of ground ferrocene are identical independent of the grinding time, and no phase changes are observed. Furthermore, even amorphous solid material would not result in signals in high-resolution NMR spectrometers in the absence of mobility. The solid polymorphs or amorphous forms of ferrocene would retain a large CSA and a Pake pattern in the solid-state NMR spectra, because the underlying anisotropic interactions in the solid state, the chemical shift anisotropy and quadrupolar interactions, are intramolecular and 2H-intrinsic in nature.55 Furthermore, the adsorption process also takes place without grinding and by simply mixing ferrocene crystals with activated carbon. Forming polymorphs or amorphous ferrocene upon simple contact with the support, instead of monolayers, would not explain why this happens to a reproducibly well defined amount of ferrocene. Most importantly, polymorphs of ferrocene only come into existence at temperatures below 164 K or at high pressures above 3 GPa, and these conditions are clearly not met by manual grinding or just mixing the components.73,74 Regarding the 1H NMR spectra recorded at a conventional solution NMR spectrometer, the same temperature-dependent trend for the half-widths is observed (Figure 4). As for the 13C NMR resonances, the residual lines became broader at lower temperatures, ranging from 3.74 kHz at −50 °C to 930 Hz at 60 °C. It should be noted that the degree of deuteration of ferrocene in Cp2Fe-d2 is too low to show any “spin dilution” line-narrowing influence on the half-widths of the resonances in comparison to those of nondeuterated ferrocene.55 The deuterated adsorbent was simply chosen for application in the following 2H MAS measurements (Figure 5).

Figure 6. Line widths of Cp2Fe-d2 adsorbed on activated carbon. The 13 C and 1H measurements were performed on a 500 MHz solution NMR spectrometer on a sample with submonolayer coverage. The 2H data were obtained from an Avance 400 solid-state NMR spectrometer.

NMR spectra of Cp2Fe-d2 adsorbed on activated carbon in a submonolayer coverage show a similar trend with respect to the signal line widths. With increasing temperature the lines become narrower due to the higher translational mobilities of the adsorbed Cp2Fe-d2 molecules. However, once the mobility exceeds that required to completely average out the present line-broadening solid-state NMR interactions, there is no further reduction of the half-widths.50,51 Finally, the change in the residual line widths with the external magnetic field was investigated in order to distinguish between the line-narrowing effects of the translational mobility on the CSA56 and on dipolar interactions.55 While the CSA is dependent on the external magnetic field strength and actually increases with higher fields, the dipolar interactions are independent of the magnetic field strength.55 Interestingly, the residual line widths of the 13C NMR signals change only slightly with the field strengths, while for the 1H resonances the line widths increase more substantially with the external magnetic field (Table 1). Therefore, it can be concluded that in the case of 13C the magic angle spinning and surface mobility have already efficiently averaged out most of the anisotropic

Figure 5. 2H MAS spectra of Cp2Fe-d2 (95 mg) adsorbed on activated carbon (200 mg), recorded at the indicated temperatures with a rotational speed of 4 kHz. The residual line widths of the broad center lines are (a) 6.04, (b) 4.06, (c) 3.47, and (d) 2.85 kHz.

Table 1. Residual Line Widths of 1H and 13C NMR Resonances of Ferrocene Adsorbed in Sub-Monolayer Coverage on Activated Carbon at Different Field Strengths and Larmor Frequencies νLa

The variable-temperature 2H MAS spectra of Cp2Fe-d2, adsorbed from an excess of polycrystalline material, are displayed in Figure 5. The surplus polycrystalline ferrocene is responsible for the Pake patterns. The quadrupolar coupling constants QCC all have the same value of 96.1 kHz reported for solid polycrystalline Cp2Fe-d2.49 There is no variation of this

nucleus 13

C 1 H

half-width (Hz) (νL (MHz))

half-width (Hz) (νL (MHz))

416 (125.65) 1253 (499.68)

391 (75.43) 841 (299.96)

a

Conventional 500 and 300 MHz solution NMR instruments were used for the measurements.

E

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Organometallics dipolar and CSA interactions. For the 1H wide-line NMR spectra, however, there is still a CSA line-broadening component that responds to the change of the field strength in the expected manner with a broader line at higher external magnetic field strength. The measurements on conventional high-resolution NMR spectrometers allowed the rapid throughput of many samples in short periods of time. We took advantage of this by testing other brands of activated carbon in addition to DARCO KB-G (specific surface area 1700 m2/g, pore volumes for micro-, meso-, and macropores of 0.36, 0.95, and 0.45 mL/g). For example, in order to investigate the influence of the pore size on the mobility of adsorbed Cp2Fe-d2 on the surface, we applied the brand Norit SA 2 as the activated carbon support. This material has a specific surface area of 900−1050 m2/g and pore volumes for micro-, meso-, and macropores of 0.28, 0.3, and 0.4 mL/g, respectively. Interestingly, no NMR signal of any adsorbed Cp2Fe-d2 could be obtained even after prolonged measurement times, after drying or wetting the material thoroughly, or after treating it with NaOH. Presumably, the high contents of micropores of this support material impedes the translational mobility of the ferrocene molecules and therewith the adsorption process. Therefore, in addition to the incentives described in the Introduction, since pore size variation to study its effect on the mobility of adsorbed ferrocene molecules was not an option with activated carbon, carbon nanotubes with different and well-defined diameters were next investigated as support materials. Carbon Nanotubes as Support Materials. In a first attempt, two samples of filled CNTs were used to adsorb Cp2Fe-d2. The 2H wide-line spectra prove that Cp2Fe-d2 is indeed adsorbed and mobile on the surface. However, the intensity of the central signal, corresponding to the adsorbed molecules, was very small. One major impediment for MAS measurements of filled CNTs was that due to the conductivity of the samples a stable rotation with higher frequencies was not possible. The overall low signal-to-noise ratios were also due to the radio frequency shielding effect of the conducting filled CNTs. To avoid these problems, while still offering the same outside surface, MWCNTs and SWCNTs have been investigated next. Fortunately, Cp2Fe-d2 is also adsorbed with larger surface coverages on the surfaces of MWCNTs and SWCNTs. When polycrystalline ferrocene is just combined with SWCNTs without grinding, initially the orange ferrocene crystals are well visible, interspersed in the black fine powder of the nanotubes. The crystals vanish during a short time period of 12 h, leaving a smooth and homogeneous black texture of the fine SWCNT powder behind. The obtained spectra are identical with those recorded after grinding the components. Since the SWCNTs were not pretreated, and therefore still closed at the ends, the ferrocene was expected to be adsorbed at the outside of the nanotubes only. This is desired because it prevents a blurring of the results by being composed of outside and inside adsorption. Additionally, carbon nanotubes are more reactive on the outside (tending to make covalent bonds when possible because it relieves strain) than the inside because of the curvature, which causes a slight pyramidalization and affects the hybridization such that the π orbitals on the outside are larger and softer.40 It also generates some misalignment of the π orbitals.40 In spite of this “outside only” advantage, the adsorption on the exterior of nanotubes has not yet been explored in detail.

In contrast to polycrystalline ferrocene, the 2H MAS spectrum of Cp2Fe-d2 adsorbed on SWCNTs shows an intense signal in the center and only sidebands with minor intensity from residual polycrystalline material (Figure 7). The half-

Figure 7. 2H MAS spectra of polycrystalline Cp2Fe-d2 (bottom) and Cp2Fe-d2 adsorbed on 1−2 nm diameter SWCNTs (top).

width of the center signal is only 1.51 kHz, and therefore it can be concluded that the surface curvature of the SWCNTs provides a favorable template for quasi-isotropic rearrangement of the ferrocene molecules and consequently narrowed lines. Even the large quadrupolar interactions are averaged out by the translational mobility of the molecules on the CNT surfaces. In a next step an attempt was undertaken to correlate the reorientation time of the adsorbed ferrocene with the diameters of the carbon nanotubes. For this purpose, SWCNTs and MWCNTs with different diameters were applied as adsorbents. The larger diameter nanotubes were anticipated to lead to longer reorientation times and thus line narrowing to a lesser degree than that for the CNTs with smaller diameters, which should allow for fast circling and reorientation of the ferrocene molecules. Figure 8 displays a selection of the obtained 2H wide-line NMR spectra for deuterated ferrocene adsorbed on three different SWCNTs. The quadrupolar coupling constants QCC of the residual Pake patterns are practically the same for all three samples, and they are about equal to the QCC of

Figure 8. 2H wide-line NMR spectra of Cp2Fe-d2 adsorbed on different CNTs: (top) 40−60 nm MWCNTs, (middle) 10−20 nm MWCNTs; (bottom) 1−2 nm SWCNTs. Quadrupolar coupling constants from top to bottom: QCC = 96.3, 95.6, and 96.4 kHz. F

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Organometallics polycrystalline ferrocene (Figure 8).49 Therefore, the outer lines in the spectra can be assigned to the Pake patterns of residual polycrystalline ferrocene. All spectra display a comparatively narrow center line, indicating that adsorbed and mobile ferrocene led to collapsed Pake patterns. The residual line widths are 3.8, 1.7, and 5.1 kHz (Figure 8, bottom to top). While the largest CNT diameters (top) indeed lead to the broadest residual line in the center, there is no clear trend when the smallest and medium diameters of the CNTs are included. Therefore, no unequivocal correlation of the reorientation time with the diameters of the CNTs can be established. Factors such as the packing or stacking of the CNTs in the solid, which might influence the mobilities of the molecules by impeding their translational progress in different ways, might dominate the scenario and allow for more or less restricted cruising of the ferrocene molecules on the outside of the CNTs. In this respect, porous materials with their open spaces within the pores are more favorable, as they allow for basically unrestricted movement of the adsorbed ferrocene molecules on the surface. The intensities of the center lines are very different, and a clear trend can be observed in the spectra displayed in Figure 8. The SWCNT support with the smallest diameter has the largest specific surface area (857 m2/g) and therefore can accommodate more ferrocene molecules on the surface than the CNTs with larger diameters (427 m2/g for CNTs in the 12−20 nm range and 189 m2/g for CNTs with diameters from 40 to 60 nm). This consistent trend in the amount of adsorbed Cp2Fe-d2 is another indicator that the ferrocene is adsorbed only on the outside of the carbon nanotubes. However, it is remarkable that ferrocene adsorbs on CNTs with different diameters. In 2005 cobaltocene and derivatives thereof were filled into SWCNTs.34 It was found that the cobaltocene only adsorbed into nanotubes into which it fit perfectly. This was supposed to be due to maximized interaction with the nanotubes that is strong enough to hold the cobaltocene down while it would otherwise sublime out of the sample under the reaction conditions.34 Next, the influence of the temperature on the line widths was studied using 1H NMR spectroscopy (Figure 9). While the residual line widths increase at low temperatures, the hightemperature line-narrowing limit was already reached around 60 °C. This means that dipolar interactions are already efficiently averaged out by the quasi-isotropic reorientation of

the ferrocene molecules on the SWCNT surfaces at ambient temperature. Graphite, Graphene, and Fullerene as Supports. Encouraged by the vigorous adsorption of ferrocene on activated carbon and on carbon nanotubes, other types of carbon surfaces were probed. Since CNTs turned out to be excellent sorbents, an attempt was undertaken to adsorb ferrocene on graphite. Contemplating this scenario, one has to acknowledge that there is no short-range curvature of the surface, and therefore, translational mobility of Cp2Fe-d2 will not result in isotropic reorientation assuming that the molecule is not “tumbleweeding” or isotropically reorienting on one surface site46,49 but sliding forward, being attached via only one Cp ring.50 Another question arising is whether Cp2Fe-d2 might be able to intercalate between the graphite layers and what mobility effects would arise from this. Unfortunately, when graphite was ground with ferrocene for 5 min, no 13C NMR signal that would indicate adsorption could be detected. Even prolonged exposure, elevated temperatures, and measurements in the wide-line mode without MAS to exclude potential problems due to the conductivity of graphite did not result in a narrow NMR signal indicative of adsorption and fast translational mobility. The 2H wide-line NMR spectrum of Cp2Fe-d2, combined with graphite, remained identical with that of polycrystalline Cp2Fe-d2.49,50 While it cannot be excluded that Cp2Fe adsorbs on the outer graphite surface, due to the expected low signal-to-noise ratio with this low surface coverage, it can be safely assumed that intercalation between the graphite layers does not occur. Since in addition there is no short-range curvature of the outside graphite surface, translational mobility of Cp2Fe-d2 does not result in isotropic reorientation. An attempt to increase the available surface by exposing graphene, which possesses a much larger specific surface area than graphite, to Cp2Fe-d2 did not lead to adsorption on a scale measurable by 13C solid-state NMR. In 2H NMR only a trace amount of adsorbed and mobile Cp2Fe-d2 is visible. In this case of graphene one might speculate that the potentially folded or “crumpled”, nonplanar character of the thin graphene layers, in contrast to the overall more planar graphite layers, might allow a limited degree of quasi-isotropic rearrangement on these twodimensional surfaces. To complete our investigation of the most prominent carbon allotropes as support materials, Cp2Fe-d2 was mixed with C60 fullerene. In this case only a Pake pattern with a quadrupolar coupling constant QCC of 96.2 kHz70 was obtained by 2H MAS, which matches the literature value for polycrystalline deuterated ferrocene.49 The absence of an averaged signal in the middle of the spectrum indicates that ferrocene was not mobile enough on the surface to average out the anisotropic interactions in the solid state.55 Since adsorption was expected for C60 as the support on the basis of its tendency to form ferrocene/fullerene cocrystals,75 we assume that between the fullerene molecules there is just not enough open space to allow for unimpeded translational mobility of Cp2Fe-d2. Next, the importance of an uninterrupted extended surface area of the support material was probed by mixing ferrocene with anthracene. No adsorption occurred, indicating that the surface area of one single anthracene molecule does not allow for a long enough undisturbed path for translational mobility and that the ferrocene molecules cannot bridge the gaps between the stacked anthracene molecules in the crystal

Figure 9. Variable-temperature 1H wide-line NMR line widths of Cp2Fe adsorbed on SWCNTs (1−2 nm diameter). The spectra were recorded on an Avance 400 solid-state NMR instrument with a rotational speed of 2 kHz and 32 scans for each spectrum. G

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Organometallics lattice.76 Furthermore, no cocrystals were obtained from solutions of anthracene and ferrocene from different solvents. Finally, an array of experiments described earlier50 was performed to obtain estimates for the adsorption strengths of Cp2Fe on different support materials. For this purpose the onset temperatures needed to desorb ferrocene from the support surfaces were measured.50 The onset temperatures of desorption, as given in Table 2, were all very well-defined,

this research topic should have major implications for analytics, the separation sciences, and catalysis.

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All experiments were carried out under an inert atmosphere of purified nitrogen and in the absence of any solvents. The deuterated ferrocene Cp2Fe-d2 was synthesized by double lithiation and subsequent quenching with D2O according to a literature method.80 The 1H and 2H solid-state NMR spectra were recorded with a Bruker Avance 400 wide-bore spectrometer using a 4 mm MAS probe head. Typically 32 and 2400 transients were recorded for all 1H and 2 H solid-state NMR measurements, respectively. All samples were densely packed into the insert-free rotors or NMR tubes as finely ground powders. Compressed air could be used as both the bearing and drive gas for the MAS measurements. For the 2H MAS NMR spectra a standard echo pulse program for solids55 with a τ delay of 29.5 μs, a 90° pulse length of 4 μs, and a pulse repetition rate of of 3 s was applied. The quadrupolar coupling constants were derived from the 2H MAS NMR spectra using the NMR simulation program Dmfit.51 The error margin in this fitting procedure is ±0.5 kHz. The 13C solid-state NMR spectra were recorded using single pulse programs without cross-polarization or proton decoupling, and a pulse delay of 10 s was applied. For measurements in a standard 5 mm glass NMR tube a conventional high-resolution 500 MHz Varian spectrometer (additionally a 300 MHz Varian instrument for the data in Table 1) was used without sample spinning or 1H decoupling. Typically, 32 and 256 scans were applied for all 1H and 13C NMR spectra at the solution NMR instruments, along with pulse delays of 1 and 2 s, respectively. Deconvolution and processing of all spectra, e.g., to determine the half-widths of signals, was accomplished using the ACD/NMR Processor Academic Edition.81 The signals were referenced relative to the external standards C6D6 (δ(13C) 128.00 ppm) and D2O (δ(1H) = δ(2H) 4.75 ppm). The activated carbon DARCO KB-G was obtained from SigmaAldrich, and it has a specific surface area of 1700 m2/g and pore volumes for micro-, meso-, and macropores of 0.36, 0.95, and 0.45 mL/g, respectively. Batches were predried and degassed for 3 h at 200 °C in vacuo (0.035 mmHg) prior to being weighed in and used for adsorbing ferrocene. The activated carbon brand Norit SA 2 was purchased from Acros Organics. It has a surface area of 900−1050 m2/g and pore volumes for micro-, meso-, and macropores of 0.28, 0.3, and 0.4 mL/g, respectively. In order to determine the maximum loading of Cp2Fe on the activated carbon brand DARCO KB-G, 105 mg was ground with Cp2Fe (144 mg, 0.774 mmol) and Cp2Fe-d2 (54 mg, 0.287 mmol) in a glovebox for 1 min and left undisturbed overnight to fully adsorb. The 2 H MAS signals of adsorbed and polycrystalline ferrocene in the sample were then deconvoluted and integrated to give the maximum loading of 614 mg (3.300 mmol) of Cp2Fe on 1 g of activated carbon. A representative sample of Cp2Fe adsorbed on activated carbon in a submonolayer amount was prepared as follows. The activated carbon DARCO KB-G (405 mg) was ground with ferrocene (202 mg, 1.09 mmol) for 1 min in the glovebox. This should give 81% of the experimentally determined maximum loading of 614 mg (3.300 mmol) ferrocene per 1 g of activated carbon. The sample was left in the glovebox in a tightly sealed vial to avoid ferrocenium formation overnight. For the solid-state NMR measurements the material was packed into a 4 mm rotor or filled into a regular 5 mm glass NMR tube for measurements on a high-resolution instrument. For the temperature-dependent 2H MAS measurements with excess polycrystalline material, DARCO KB-G (99 mg) and Cp2Fe-d2 (132 mg, 0.703 mmol) were ground for 1 min in the glovebox and then left overnight to ensure complete adsorption. The ferrocene was present in a 2.15-fold excess of the amount required for a monolayer to ensure that there was both adsorbed and polycrystalline material at every temperature for the purpose of integration.

Table 2. Desorption Temperatures (±2 °C)50 of Cp2Fe Adsorbed on Different Support Materials support material

temp (°C)

polycrystalline Cp2Fe dry silica77 wet silica77 activated carbon MWCNTs (10−20 nm) SWCNTs (1−2 nm)

37 52 52 103 61 65

EXPERIMENTAL SECTION

ruling out any pore diffusion effects. Interestingly, Cp2Fe needs a much higher temperature of 103 °C to get desorbed from activated carbon in comparison to the temperatures needed to desorb it from any of the other supports. Therefore, one can assume that ferrocene is more strongly adsorbed on activated carbon. Ferrocene was attached to SWCNTs and MWCNTs about equally firmly within the error margins of the measurements. This is another indication that ferrocene is adsorbed only on the outside of the nanotubes and the inside of the tubes does not play a role. The comparison with wet and dry silica surfaces77 shows that the interactions of Cp2Fe with the various surfaces are not based on hydrogen bonding but are rather van der Waals in nature. Importantly, Cp2Fe, being more strongly adsorbed on activated carbon than on either silica or CNTs, renders it an ideal support for generating a single-atom catalyst (SAC)78,79 or a nanoparticle catalyst later by decomposing a precursor adsorbed in a well-defined monolayer at higher temperatures.50

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CONCLUSION In this contribution a multinuclear solid-state NMR study of the adsorption of ferrocene (Cp2Fe) and its deuterated version Cp2Fe-d2 on activated carbon, carbon nanotubes with different diameters, graphite, graphene, and C60 fullerene has been described. In all cases, except graphite and C60, the anisotropic solid-state interactions that prevail in polycrystalline solids were effectively reduced or even completely eliminated by quasiisotropic mobility of the adsorbed molecules across favorable surfaces. It was demonstrated that the chemical shift anisotropy (CSA), quadrupolar interactions, and even dipolar interactions were reduced to a degree that measuring the solid materials in a conventional high-resolution NMR spectrometer was possible. The temperature dependence of the dynamic effects was investigated. Comparing different types of support materials allowed the conclusion that the interactions of ferrocene with the support is of a van der Waals nature. The highest monolayer surface coverages have been achieved with activated carbon as the support. The monolayers form by self-adsorption of solid ferrocene in the absence of a solvent. Once the densest packaging in the monolayer is reached, the transition to surplus polycrystalline material is abrupt. The new insights gained on H

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The smallest SWCNTs and MWCNTs were obtained from Strem Chemicals and used without any pretreatment. For adsorbing ferrocene on the nanotubes, the solid components can simply be combined without grinding. The SWCNTs had diameters of 1−2 nm and lengths of 5−30 μm with an estimated surface area of 857 m2/g. The medium-sized MWCNTs were purchased from TCI with diameters of 10−20 nm, lengths of 5−15 μm, and an estimated surface area of 427 m2/g. The largest MWCNTs were obtained from TCI with diameters of 40−60 nm, lengths of 5−15 μm, and an estimated specific surface area of 189 m2/g.

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AUTHOR INFORMATION

Corresponding Author

*J.B.: fax, (+1)-979-845-5629; tel, (+1)-979-845-7749; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by The Robert A. Welch Foundation (A-1706) and the National Science Foundation (CHE-1300208). Furthermore, we thank Jordon W. Benzie for performing valuable NMR and crystallization experiments and Prof. Dr. S. Banerjee for providing a sample of graphene.

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