Gas-Phase Mechanism of O.- /Ni2+ -Mediated Methane Conversion to Formaldehyde.

The gas-phase reaction of NiAl2 O4 + with CH4 is studied by mass spectrometry in combination with vibrational action spectroscopy and density functional theory (DFT). Two product ions, NiAl2 O4 H+ and NiAl2 O3 H2 + , are identified in the mass spectra. The DFT calculations predict that the global minimum-energy isomer of NiAl2 O4 + contains Ni in the +II oxidation state and features a terminal Al-O.- oxygen radical site. They show that methane can react along two competing pathways leading to formation of either a methyl radical (CH3 ⋅) or formaldehyde (CH2 O). Both reactions are initiated by hydrogen atom transfer from methane to the terminal O.- site, followed by either CH3 ⋅ loss or CH3 ⋅ migration to an O2- site next to the Ni2+ center. The CH3 ⋅ attaches as CH3 + to O2- and its unpaired electron is transferred to the Ni-center reducing it to Ni+ . The proposed mechanism is experimentally confirmed by vibrational spectroscopy of the reactant and two different product ions.

There is a growing interest in tailoring the properties of pristine and doped aluminas as well as alumina‐supported transition metal oxides, because of their widespread use in heterogeneous catalysis, in general, and for C−H bond activation, in particular. A promising approach is the doping of alumina with 3d transition metal ions, since those ions can act as redox‐active catalytic centers allowing for more efficient conversions. Moreover, most transition metals of interest are largely available and mostly non‐toxic, in contrast to noble metal catalysts that are currently used. Nickel ions, for example, were recently shown to be particularly successful in activating C−H bonds of alkanes and alkenes.

In order to improve the performance of existing materials and to rationally design new single‐site catalysts, studies on gas‐phase clusters have proven quite helpful. Clusters serve as models for a catalyst's isolated active site. They can be studied under isolated and well‐defined conditions and, in combination with quantum chemical calculations, allow researchers to uncover reaction mechanisms operative at the molecular level.

Since methane is the simplest hydrocarbon and at the same time economically highly relevant, it is often used as prototypical substrate for studying the basic reactivity of heteronuclear metal oxide clusters. Recently, metal‐doped aluminum oxide ions, such as the MAl2O4 + model systems shown in Figure 1, have attracted considerable attention.[ , ] Whereas Al3O4 + (M=Al) exhibits a cone‐like geometric structure with a closed‐shell electronic structure and is unreactive towards methane, substitution with a transition metal (TM) ion, e.g. Fe3+, results in Fe3+(d5)/O2−↔Fe2+(d6)/O.− valence isomerism accompanied by structural rearrangement with formation of a highly reactive terminal oxygen radical anion (Figure 1). Both FeAl2O4 +  and ZnAl2O4 +  have been found to abstract hydrogen from methane under formation of a methyl radical.

Figure 1

The two most relevant MAl2O4 + isomers (aluminum: gray, oxygen: red) and their respective electron configurations for M=Al, Fe, Ni and Zn. While in the tricyclic cone‐like structure (left) the metal site M3+ (dark blue) is found to be trivalent, it is reduced to M2+ in the ladder‐like isomer (right) resulting in a terminal oxygen radical.

The question thus arises, how these insights can be generalized to other TM ions.

Here, we investigate the reactivity of MAl2O4 + with M=Ni towards methane and demonstrate that it is characteristically different from the systems with M=Fe, Zn. Typically, reactivity studies involving gas‐phase clusters rely solely on the combination of mass spectrometry with quantum chemical calculations, the vast majority of them using density functional theory (DFT). Consequently, the relevant structures are not experimentally verified, which is problematic, since DFT alone is not predictive, in particular when partially filled d‐shells of transition metal ions are involved. Therefore, in addition to the mass spectrometric investigation of the NiAl2O4 ++CH4 reaction, we use cryogenic ion trap vibrational spectroscopy to determine both the reactant and product ion structures predicted by DFT. Based on these experimentally confirmed structures we propose a reaction mechanism that involves a terminal oxygen‐centered radical anion separated from a redox active Ni2+ center and that rationalizes why NiAl2O4 + activates methane yielding both formaldehyde and a methyl radical.

NiAl2O4 + is formed by laser ablation and its reactivity towards CH4 is studied in a temperature‐controllable ion trap reactor under multiple‐collision conditions at 100 and 300 K. Figure 2a shows the reference mass spectrum at 100 K obtained after trapping of NiAl2O4 + in pure He (5 Pa) for 200 ms (see Figure S1 for He tagging at 10 K). As expected, only the parent ion is observed in the absence of methane. Figures 2b and c show the corresponding ion distribution for a ≈0.01 % methane in He gas mix (5 Pa) at 100 and 300 K, respectively. Two reaction channels are observed which we assign, based on DFT calculations (see below), to methyl radical or formaldehyde formation:

Figure 2

Time‐of‐flight mass spectra obtained after storing mass‐selected 58NiAl2O4 + ions up to 200 ms in the ion trap filled with a) He at 100 K, b), c) ≈0.01 % CH4 in He at 100 K (b) and 300 K (c), and d) ≈0.03 % CD4 in He at 300 K. The reactant ion NiAl2O4 + is labelled as R, the product ions NiAl2O4H+ and NiAl2O3H2 + are indicated as P1 and P2, respectively. The deuterium‐labelled product ions NiAl2O4D+ and NiAl2O3D2 + are indicated as P1′ and P2′, respectively.

The additional mass peaks observed in Figures 2b and c at mass‐to‐charge ratios m/z of 177, 162, 193 and 178 correspond to NiAl2O4H+ (P1), NiAl2O3H2 + (P2) as well as their complexes with methane, respectively. Those peaks are absent for the reaction of undoped Al3O4 + with CH4 (Figure S3). Formation of weakly bound complexes with methane is favoured at lower temperatures and higher methane partial pressures. Increasing the ion‐trap temperature to 300 K i) removes these weakly bound methane complexes from the mass spectrum and ii) enhances the formation of complexes with H2O, which is a common impurity in mass spectrometers at room temperature. The assignment is further confirmed by isotopic labelling experiments with CD4 (Figure 2d). The P1 (+CH3⋅) and P2 (+CH2O) formation efficiencies (both including CH4 adsorption complexes) expectedly depend on temperature and methane partial pressure. In more detail, the P1/P2 yield ratio increases from 2.3 to 3.8 (with ±20 % error) when increasing the ion trap temperature from 100 to 300 K.

Mass spectrometry typically only yields information on the mass‐to‐charge ratios, but little structural information. In order to characterize the geometric structure of the relevant ions, we therefore performed IRPD spectroscopy on R, P1 and P2. Here, the formation of the corresponding weakly bound methane complexes (cf. Figures 2 and S2) comes in handy, since these allow measuring IRPD spectra in the linear absorption regime, i.e. the IRPD cross section is typically directly proportional to the relative intensities in the computed spectra derived from a harmonic frequency analysis.

Figure 3 compares the experimental IRPD spectrum of He‐tagged NiAl2O4 + (R⋅He) at 10 K with the harmonic IR spectra of the two most stable isomers predicted by DFT (B3P86 functional /TZVP basis sets ), labelled IS01 and IS02. It is obvious that the spectrum predicted for IS01 shows much better agreement than the one predicted for IS02. This is also reflected in the values of 0.91 (IS01) and 0.31 (IS02) for the cosine similarity score S, which provides a quantitative measure for the agreement between experimental and theoretical spectra. The score can vary from zero to one, with a value closer to one indicating greater similarity. This comparison unambiguously confirms that NiAl2O4 +, like FeAl2O4 +, prefers a planar ladder‐like C s structure containing a terminal oxygen‐centred radical anion over the cone‐like structure observed for Al3O4 +.

Figure 3

Experimental IRPD spectrum (dark red) of helium‐tagged NiAl2O4 + (R⋅He). See Table S1 for positions and assignments of peaks a–e. Unscaled harmonic DFT (B3P86/TZVP) IR spectra (green), convolved using a Gaussian line shape function with a full‐width‐at‐half‐maximum of 15 cm−1, and minimum‐energy structures (blue, Ni; pink, Al; red, O) of the two lowest energy isomers IS01 ( R) and IS02 of NiAl2O4 + in its doublet state (see Figure S4 for additional information on low energy isomers). For the IS01 ( R) structure, the spin distribution is shown; green indicates β‐electron excess, red indicates α‐electron excess.

For the ground state of the reactant ion R, two quasi‐degenerate spin states (see Figures S4, S5) are found, a doublet R (IS01) and a quartet R (IS07), which result from the parallel and antiparallel orientation of the spin at the terminal oxygen radical and the spin of the two unpaired electrons of the Ni2+(d8) ion. The two states differ by 0.3 kJ mol−1 in energy (B3P86/TZVP) and have virtually identical structures and harmonic IR spectra. The situation is comparable to FeAl2O4 +.

After identifying the structure of the reactant ion R, we performed a comprehensive study of the low‐lying minimum‐energy structures of the two product ions, NiAl2O4H+ (P1) and NiAl2O3H2 + (P2). The results are summarized in Figure S6, see Figures S7 and S9 for a comparison of the corresponding harmonic IR spectra of these isomers to the IRPD spectra of CH4‐tagged P1 and P2. The influence of the tagging site on the IR spectrum was also considered (see Figures S8 and S10) and found to be negligible. Figure 4 summarizes all results (see Table S1 for peak positions and assignments). The structures of P1 and P2 can be unambiguously assigned to the corresponding spin states P1 (IS01) and P2 (IS07), respectively. The spectra of the corresponding isomers with alternative spin multiplicity ( P1, P2) result in significantly lower S values.

Figure 4

Comparison of the experimental IRPD spectra (dark red) of the two methane‐tagged product ions NiAl2O4H+ (P1⋅CH4) and NiAl2O3H2 + (P2⋅CH4) at 100 K to the unscaled harmonic DFT(B3P86/TZVP) spectra (green) of triplet P1⋅CH4, triplet P1, and singlet P1 (upper panels), as well as doublet P2⋅CH4, doublet P2, and quartet P2 (lower panels), respectively. The harmonic spectra were convolved with a Gaussian line shape function with a full‐width‐at‐half‐maximum of 15 cm−1. The cosine similarity score S is given in parentheses and the minimum‐energy structure is shown on the right (blue, Ni; pink, Al; red, O; grey, C; white, H). See Table S1 for peak assignments.

To rationalize the formation of P1 and P2, we explored the potential energy surface (PES) of the NiAl2O4 ++CH4 reaction. Figure 5 shows the lowest energy intermediates and transition structures. Methane is activated by the terminal oxygen radical site of R, forming a stable intermediate with an O−H bond and a weakly coordinated ⋅CH3 radical:

Figure 5

Energy profile for the reaction of NiAl2O4 + (doublet) with CH4 calculated with DFT (B3P86/TZVP). The relative enthalpies at 0 K (equal to the sum of electronic and zero‐point vibrational energies), ΔH 0, are given in kJ mol−1 and bond lengths are given in pm. Along with the minimum‐energy structures (blue, Ni; pink, Al; red, O; gray, C; white, H) the spin distributions are shown, green indicates β‐electron excess, red indicates α‐electron excess. See Figure S11 for depictions of the transition structures.

This hydrogen atom transfer (HAT) step does not necessarily require the formation of an encounter complex ( 1 c→ 1 c/→ 2 c, Figure S13) but can also proceed without a barrier ( R→, Figure 5). Both pathways eventually join in the intermediate 2, after overcoming a small barrier of 10 and 6 kJ mol−1, respectively. The methyl radical can move from the Al‐OH moiety in 2 to the second, central Al site, 3, where it is marginally more stable. The product P1 (NiAl2O4H+)+⋅CH3 can be formed from intermediates 1, 2 or 3. Note that dissociation of ⋅CH3 is always entropically favored (vide infra).

Besides adsorption at an Al site, the methane molecule can also attach to the Ni atom which leads to the most stable encounter complex (Figure S13). However, C−H bond activation of methane attached to the Ni‐site features, without exception, barriers that are higher in energy than the entrance channel (Figure S14). Consequently, HAT at the terminal oxygen radical anion site is the only possibility for initial C−H bond activation of methane, in contrast to the related systems ZnAl2O4 +, where the initial C−H bond activation can also occur at the [Zn2+,O2−] site as heterolytic splitting into CH3 − and H+, similar to what happens in the oxidative coupling of CH4 on MgO.

After the first C−H bond activation, the products P2+CH2O are formed in the following way. From 3 the most stable intermediate 4 is formed in which the ⋅CH3 radical has split into a CH3 + ion attached to the bridging O2− ion between Al and Ni and an electron that reduces Ni2+(d8) to Ni+(d9):

For the change of the Ni oxidation state passing from 3 to 4, see Supporting Information Section 4.3 and Table S2. This internal redox process is similar to addition of an H atom to a transition metal oxide species where the proton attaches to the O2− ion and the electron reduces the metal ion (proton coupled electron transfer).[ , ]

The next step, 4→ 5, is hydride transfer from the CH3O− species to Ni+:

Cleavage of the Ni−CH2 bond then forms the formaldehyde (H2C=O) moiety ( 5→ 6). Finally, the intermediate 6 rearranges into a more stable structure 9, with the hydride moved from Ni to Al and CH2=O from Al to Ni ( 6→ 7→ 8→). Desorption of formaldehyde from intermediate 9 requires 174 kJ mol−1 yielding P2, which lies well below the energy of the entrance channel (−147 kJ mol−1). Modelling of the direct desorption of CH2O from the Al site in 7 or 8 following a minimum energy pathway resulted in a transfer of the formaldehyde molecule to the Ni atom indicating a much higher barrier for the Al−O bond vs. Ni−O bond cleavage.

We find no evidence that syngas (CO+H2) was generated instead of H2C=O, but also cannot exclude it. DFT predicts that CO+H2 formation starting from intermediate 5 (Figure S15) is less favorable by about 21 kJ mol−1, an energy difference that is within the uncertainty limits of DFT. Moreover, neither the H2 nor the CO adduct of P2, which correspond to NiAl2O3H4 + and NiAl2O4CH2 +, respectively, were detected mass spectrometrically. However, this is also not conclusive, since mass spectrometric methods typically only detect the product ions and not the neutral product(s), although there are fortitudinous exceptions in which the neutral product molecule remains bound to the product ion.

Although the formation of P1 is less exothermic than that of P2, it is entropically preferred due to release of the ⋅CH3 moiety. At 298 K and an assumed pressure of 1 atm the Gibbs free energy for P1, −107 kJ mol−1, is lower than that of the highest barrier ( 1/ 2) in the pathway for the P2 generation, −82 kJ mol−1, see Supporting Information Section 4.4 Table S3. However, due to the small partial pressures in the present gas‐phase experiments, the collision‐induced thermalization is slow and, hence, we cannot assume that a transition state is in thermal equilibrium with the corresponding reactants. Instead, we have to consider constant total energy conditions. Therefore, we use RRKM theory to estimate rate constants for the conversion of 1→ 1/ 2 vs. 1→ P1 (see Supporting Information Section 4.5 Table S4) which indeed confirm the experimentally observed, favored formation of P1 (kinetic control).

In summary, the gas‐phase reaction of NiAl2O4 + with methane has been studied using ion trap mass spectrometry combined with IRPD spectroscopy and DFT. Both the reactant and product ion structures are spectroscopically identified. NiAl2O4 + converts methane to formaldehyde, which distinguishes it from many small (transition) metal oxide radical clusters that were previously studied. Before, selective formation of formaldehyde was observed for the reaction of Al2O3 + with CH4. However, the first step for H2C=O formation is different: the terminal radical O atom inserts into the C−H bond of methane yielding methanol and leaving the electron at one of the Al atoms. Other TM‐doped alumina clusters like FeAl2O4 + and ZnAl2O4 + do not produce formaldehyde. Hence the reactivity of NiAl2O4 + is attributed to the particular redox properties of the Ni site, especially its stable d‐shell configurations and the resulting oxidation states.

Conflict of interest

The authors declare no conflict of interest.

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