Methane Activation by Iron-Carbide Cluster Anions FeC6– - The

Letter pubs.acs.org/JPCL

Methane Activation by Iron-Carbide Cluster Anions FeC6− Hai-Fang Li,†,‡ Zi-Yu Li,†,‡ Qing-Yu Liu,†,‡ Xiao-Na Li,† Yan-Xia Zhao,*,† and Sheng-Gui He*,† †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: Laser-ablation-generated and mass-selected iron-carbide cluster anions FeC6− were reacted with CH4 in a linear ion trap reactor under thermal collision conditions. The reactions were characterized by mass spectrometry and density functional theory calculations. Adsorption product of FeC6CH4− was observed in the experiments. The identified large kinetic isotope effect suggests that CH4 can be activated by FeC6− anions with a dissociative adsorption manner, which is further supported by the reaction mechanism calculations. The large dipole moment of FeC6− (19.21 D) can induce a polarization of CH4 and can facilitate the cleavage of C−H bond. This study reports the CH4 activation by transition-metal carbide anions, which provides insights into mechanistic understanding of iron−carbon centers that are important for condensedphase catalysis.

thermal collision conditions.32,38 Herein, we present the first example of metal carbide cluster anions FeC6− that can activate CH4 at room temperature. Moreover, a new driving force to activate methane is proposed on the basis of the theoretical calculations. Recently, a condensed-phase study suggested that carbide species containing a single Fe atom is the active site for CH4 activation and transformation under oxygen-free conditions.5 In this study, the laser ablation method was used to generate the FeCn− anions, in which FeC4− and FeC6− that had high signal intensities were mass-selected, thermalized, and then reacted with CH4 in a linear ion trap (LIT) reactor. No reactivity has been identified for FeC4− cluster (Figure S1, Supporting Information), while the FeC6− system is reactive and the result is shown in Figure 1. Upon the interaction of FeC6− toward CH4 in the reactor (Figures 1b,c), a product peak assigned as FeC6CH4− is observed, which suggests that an association reaction shown as reaction 1 takes place

M

ethane, the principal component of natural and shale gases, constitutes one of the major energy sources and an important feedstock for the synthesis of value-added chemicals. Owing to its tetrahedron symmetry (Td), the very strong C−H bond (4.50 eV), and low polarizability, C−H bond activation of CH4 is a big challenge in contemporary chemistry.1−3 Much effort has been devoted to exploring CH4 activation in condensed-phase system;4−7 however, the nature of the active sites and reaction mechanisms is not clear. Although great progress has been made in experimental technologies, the characterization of bonding and reactivity of the reactive sites in a condensed-phase system is still experimentally difficult.8−11 Thus, exploring an alternative method to study the mechanism of CH4 activation is of great importance. Study of gas-phase atomic clusters under well-controlled conditions serves as a bottom-up strategy to uncover the mechanism of CH4 activation.12−21 The reactions of CH4 with metal ions such as Os+,22 Ptn±,23 Au2+,24 Ta+,25 and others26−28 and metal oxide (hydride) ions such as MgO+,29 Al2O7+,30 V4O10+,31 La6O10−,32 NiH+, PdH+,33,34 and so on,14,35 were extensively studied experimentally and theoretically. Methane activation by a few heteronuclear metal oxide clusters has also been reported, and the available examples are AuNbO3+,36 VAlO4+,37 PtAl2O4−,38 [V2O5(SiO2)1−4]+,39 and [Ga2Mg2O5]+.40 Metal carbide species are also proposed to be present as promising centers for CH4 activation in the condensed-phase system;41,42 however, the extensive studies of metal carbide species mainly focused on the structure and stability,43−45 and only two gas-phase examples, TaCy+ (y = 1− 14)46 and MoC+,47 have been reported to be capable of activating CH4. These carbide clusters are positively charged, and CH4 activation by anions is considered to be difficult under © 2015 American Chemical Society

FeC6− + CH4 → FeC6CH4 −

(1)

Reaction 1 is confirmed by using the isotopic labeling experiments with 13CH4 (Figure 1d) as well as CD4 (Figure 1e). Figure 2 plots the signal variation of the reactant and product ions in FeC6− + CH4 with respect to the CH4 pressure (P). The relative intensity of the reactant cluster FeC6− (IR) deceases from ∼1.0 to 0.6 when the P value increases from 0.1 to ∼0.8 Received: May 6, 2015 Accepted: June 3, 2015 Published: June 3, 2015 2287

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were reactive with CH4, and their relative intensity follows a single exponential decay. It is noteworthy that the absolute k1 (FeC6− + CH4) value (4.3 × 10−13 cm3 molecule−1 s−1) can be over- or underestimated by 30% due to systematical errors in determining the tR and P in eq 2.49 The theoretical collision rate (kcollision)50,51 between FeC6− and CH4 is calculated to be 9.8 × 10−10 cm3 molecule−1 s−1, which means that the possibility of reaction upon each collision (k1/kcollision) is ∼0.04% for the reactive component of FeC6−. The kinetic isotopic effect (KIE) of k1 (FeC6− + CH4)/k1 (FeC6− + CD4) amounts to 11 ± 3. It has been reported that metal-mediated C−H bond cleavage exhibits larger KIE, typically >2.0,2,8 which implies that dissociative adsorption rather than molecular association has taken place in the reaction of FeC6− with CH4. Density functional theory (DFT) calculations with the M06L functional were performed to study the geometric structures of FeC6− and the reaction mechanism of reaction 1. The FeC6− cluster has four low-lying isomers (within 0.1 eV): one linear (Figure 3a, IS1, denoted as L−FeC6−) and three cyclic (Figure

Figure 1. Time-of-flight mass spectra for the reactions of FeC6− with CH4 (b,c), 13CH4 (d), and CD4 (e) in the reactor for 14.6 ms. The FeC6X− (X = CH4, 13CH4 and CD4) ions are labeled as +X. A weak signal marked by the asterisk is due to the contribution of H2O impurity in the gas-handling system.

Figure 2. Variations of the relative ion intensities with respect to the CH4 pressures in the reaction of FeC6− with CH4. The solid lines are fitted to the experimental data points by using eq 2 derived with the approximation of the pseudo-first-order reaction mechanism for the reactive species.

Pa. The decrease of IR becomes subdued with the increase of P, indicating that some of the experimentally generated FeC6− ions were inert toward CH4. It turned out that the IR value could be well-fitted by eq 2 (see Supporting Information) ⎛ P ⎞ tR ⎟ IR = x inert + (1 − x inert) × exp⎜ −k1 kBT ⎠ ⎝

Figure 3. (a) DFT-calculated structural isomers of FeC6−. Dipole moment (D) and zero-point vibration corrected energies (eV) are shown. (b) Reaction mechanism of L−FeC6− with CH4. The structures of I1−I7 and P1−P3 are given. The transition-state structures can be found in Figure S4 in the Supporting Information. Relative energies (eV) with respect to the separated reactants are given in parentheses. Bond lengths are given in pm.

(2)

in which xinert is the relative intensity of the unreactive component of FeC6− and k1 is the pseudo-first-order rate constant48 of the reactive component of FeC6−, kB is the Boltzmann constant, T is the temperature (∼300 K),49 and tR is the reaction time (∼14.6 ms). It is noteworthy that the experimentally generated cluster may have different structural isomers that can have very different reactivity. The xinert and k1 in eq 2 were determined to be (48 ± 5)% and (4.3 ± 0.7) × 10−13 cm3 molecule−1 s−1, respectively. The uncertainties ±5% and ±0.7 × 10−13 cm3 molecule−1 s−1 are the one standard errors in the least-squares fitting. It can be seen that (52 ± 5)% of the experimentally generated FeC6− ions

3a, IS2−IS4, denoted as C−FeC6−) structures. This well supports the experimental suggestion that the FeC6− cluster does not have a uniform structure populated in the reactor. In the ion mobility mass spectrometric experiments, the coexistence of linear and cyclic isomers for FeC6− cluster has also been identified.52 Therefore, L−FeC6− and C−FeC6−, which possess quartet electronic state, are all of the candidates in the reactions with CH4. The reaction mechanism calculations have demonstrated that the dissociative adsorption of CH4 by L−FeC6− is thermody2288

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Methane activation by FeC6− anions can be associated with the high dipole moment of the linear structure. Because of the good separation of Feδ+ (+0.34 |e|) and C6δ− (−1.34 |e|), L− FeC6− has a very large dipole moment of 19.21 D (Figure 3a, IS1). The polarization of C−H bond can occur by the high dipole moment when CH4 approaches L−FeC6− cluster. This results in the elongation of the three C−H bonds pointing toward Fe from 109 pm in free CH4 to 111 pm in intermediate I1 (Figure 3b) and a large binding energy release (0.37 eV). In sharp contrast, the dipole moments of C−FeC6− are not greater than 3.71 D (Figure 3a, IS2−IS4), and no significant polarization of CH4 can be observed (Figure S5, Supporting Information). The adsorption energies for C−FeC6− + CH4 are much smaller than the binding energy of I1 (≤0.13 eV versus 0.37 eV), and the C−H bonds at the Fe site still remain intact (109 pm). A similar result has also been identified for the reaction of Fe with CH4 (Figure S10, Supporting Information). To further confirm the dipole-moment-dependent reactivity, the reactivity of FeC4−, which has been verified to have a linear isomer populated in the cluster,52,59,60 is experimentally investigated with CH4. The FeC4− has been observed to be inert with CH4 under thermal collision conditions (Figure S1, Supporting Information). The DFT-calculated dipole moment of FeC4− amounts to 11.22 D (Figure S11, Supporting Information), which is much smaller than the value of L− FeC6−. As a result, it can be concluded that a high dipole moment is required for CH4 activation by the FeCn− cluster. Methane activation by gas-phase atomic clusters was extensively studied, and two mechanistic scenarios have been proposed: (i) hydrogen atom abstraction by oxygen-centered radicals (O·−)12,13,32,61 and (ii) oxidative addition by metal centers.38,46,47 The reactivity of the clusters toward CH4 can be significantly influenced with the variations of the local charges around the O·− centers.62 This study has identified that the gasphase L−FeC6− anion, capable of activating CH4 through oxidative addition, possesses a high dipole moment. The high dipole moment can lead to a strong local electric field along the geometrically most favorable direction (see I1 in Figure 3b) and can facilitate the charge transfer,63,64 resulting in a polarization of CH4 and then facile cleavage of C−H bond. This driving force can parallel similar behaviors of C−H bond activation promoted by external electric field.65 It was shown that the electric field can lower the reaction barriers and can enhance the cluster reactivity. In conclusion, methane activation mediated by transitionmetal carbide anions (FeC6−) under thermal collision conditions has been identified for the first time. The linear isomer with a high dipole moment is the reactive species for C−H bond cleavage of methane. This study provides a novel structure model to activate methane.

namically and kinetically favorable along the quartet potential energy surface, as shown in Figure 3b. In contrast, the reactions of C−FeC6− with CH4 are all subject to a significant overall barrier of ≥0.79 eV in the rate-determining step of first C−H bond cleavage (Figures S5 and S6, Supporting Information). This indicates that the C−FeC6− isomers correspond to the unreactive component (48%) of FeC6− in the experiments (Figure 2). The detailed reaction mechanism of L−FeC6− with CH4 is described in the text below. In the initial stage of the reaction, the CH4 interacts with Fe atom rather than the linear carbon chain to form the encounter complex (I1 in Figure 3b) of η3 configuration with a binding energy of 0.37 eV. The reaction proceeds through oxidative addition (I1 → TS1 → I2) with a barrier of 0.28 eV, resulting in the formation of one Fe−H bond and one Fe−C bond and the concomitant cleavage of one C−H bond. After the oxidative addition, the H atom transfers from Fe atom to the carbon chain to make a chemical bond with a carbon atom adjacent to the Fe (I2 → TS2 → I3), and an additional energy of 0.92 eV is released. It is noteworthy that the intermediate I7 is more stable than I3. Therefore, the transformation of I3 → I7 is also calculated, and a ring-closing rearrangement (I3 → TS3 → I4) and a ring-opening isomerization (I5 → I6 → I7) are involved. It turns out that such a process has an overall positive barrier of 0.11 eV (TS3). The sum of center-of-mass collisional energy (0.04 eV) and vibrational energy of the reactants (0.12 eV) is larger than the barrier, so it is possible to pass over TS3 from I3 to form I4; however, the rate of I3 → I4 estimated by Rice− Ramsperger−Kassel−Marcus (RRKM) theory53 is as small as 2.5 × 10−1 s−1, which is far smaller than the rate of I3 → I2 (3.9 × 104 s−1). This indicates that the transformation of I3 → I4 cannot take place. The rate54 for cluster collision with the He carrier gas in the LIT reactor is estimated to be ∼106 s−1, and it is larger than the rate of I3 → I2 by about two orders of magnitude. As a result, the collisions by the carrier gas can stabilize intermediate I3. The reaction pathways for the second C−H bond cleavages (Figure S7, Supporting Information) and the methyl radical liberations (P1 and P2) from I2 and I3 have also been tested, and it turns out that these processes are unfavorable both thermodynamically and kinetically. Thus, it concludes that the experimentally observed FeC6CH4− complex is the dissociative adsorption product of intermediate I3, which is in agreement with the high KIE value (11) in the reaction of L−FeC6− with CH4. To understand the importance of the linear carbon chain in FeC6− for CH4 activation, we also considered the reactivity of carbon-free Fe atom and Fe+ ion toward CH4. In the reaction of Fe + CH4, the oxidative addition of C−H bond to Fe atom is subject to a high positive barrier of 1.37 eV (Figure S10, Supporting Information). This indicates that the bare Fe atom cannot activate CH4 under thermal collision conditions, and such behavior is consistent with the result predicted by Sun and coworkers.55 Fe+ ion is more reactive than the neutral counterpart,38,56 and physical adsorption of CH 4 was experimentally observed.57,58 It is noteworthy that the rate constant for Fe+ + CH4 (4.0 × 10−13 cm3 molecule−1 s−1)57 is quite close to that for L−FeC6− + CH4 (4.3 × 10−13 cm3 molecule−1 s−1), in which the L−FeC6− cluster gives rise to dissociative adsorption of CH4, while Fe+ ion only brings about physical adsorption of CH4. These results imply that in the reaction of negatively charged FeC6− with CH4 the linear carbon chain should play a very important role.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed description of experimental and theoretical methods. Table giving related bond dissociation energies by experiments and DFT calculations and Figures giving additional mass spectra and DFT-calculated isomers and reaction mechanisms. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b00937. 2289

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AUTHOR INFORMATION

Corresponding Authors

*Y.-X.Z.: E-mail: [email protected]. Phone: +86-1062568330. Fax: +86-10-62559373. *S.-G.H.: E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Chinese Academy of Sciences (Strategic Priority Research Program, No. XDA09030101), the National Natural Science Foundation of China (Nos. 21325314 and 21203208), and the Major Research Plan of China (Nos. 2013CB834603 and 2011CB932302).

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DOI: 10.1021/acs.jpclett.5b00937 J. Phys. Chem. Lett. 2015, 6, 2287−2291