Weak-field ligands enable inert early transition metal oxides to convert methane to methanol: the case of ZrO

Electronic structure of metal oxide dications with ammonia ligands and their reactivity towards the selective conversion of methane to methanol

High-level quantum chemical calculations are performed for the (NH3)MO2+ and (NH3)5MO2+ species (M = Ti-Cu), extending our previous work on the bare MO2+ ions. The potential energy curves along the M-O distance are constructed for the ground and multiple excited electronic states of (NH3)MO2+ and are compared to those of MO2+. We see that ammonia stabilizes the oxo states (M4+O2-) over the oxyl (M3+O⋅-) ones. This trend is intensified in the (NH3)5MO2+ species. We then examined the reaction of the latter species with both methane and methanol. We find that the oxyl states activate a C-H bond easily with barriers smaller than 10 kcal/mol across all first-row transition metals, while the barriers for the oxo states start from about 50 kcal/mol for M = Ti and decrease linearly to 10 kcal/mol going toward M = Ni. This is attributed to the increasing spin density on the oxygen atom observed for the oxo states. The most important finding is that the formation of hydrogen bonds between the OH group of methanol and the N-H bonds of the ammonia ligands increases the activation barriers for methanol considerably, making them comparable to and slightly higher than those of methane. This finding suggests a new strategy to slow the oxidation of methanol, leading to the long-desired higher methane-to-methanol selectivity.

Transition metal oxide complexes as molecular catalysts for selective methane to methanol transformation: any prospects or time to retire?

Transition metal oxides have been extensively used in the literature for the conversion of methane to methanol. Despite the progress made over the past decades, no method with satisfactory performance or economic viability has been detected. The main bottleneck is that the produced methanol oxidizes further due to its weaker C-H bond than that of methane. Every improvement in the efficiency of a catalyst to activate methane leads to reduction of the selectivity towards methanol. Is it therefore prudent to keep studying (both theoretically and experimentally) metal oxides as catalysts for the quantitative conversion of methane to methanol? This perspective focuses on molecular metal oxide complexes and suggests strategies to bypass the current bottlenecks with higher weight on the computational chemistry side. We first discuss the electronic structure of metal oxides, followed by assessing the role of the ligands in the reactivity of the catalysts. For better selectivity, we propose that metal oxide anionic complexes should be explored further, while hydrophylic cavities in the vicinity of the metal oxide can perturb the transition-state structure for methanol increasing appreciably the activation barrier for methanol. We also emphasize that computational studies should target the activation reaction of methanol (and not only methane), the study of complete catalytic cycles (including the recombination and oxidation steps), and the use of molecular oxygen as an oxidant. The titled chemical conversion is an excellent challenge for theory and we believe that computational studies should lead the field in the future. It is finally shown that bottom-up approaches offer a systematic way for exploration of the chemical space and should still be applied in parallel with the recently popular machine learning techniques. To answer the question of the title, we believe that metal oxides should still be considered provided that we change our focus and perform more systematic investigations on the activation of methanol.

Partial Methane Oxidation in Fuel Cell-Type Reactors for Co-Generation of Energy and Chemicals: A Short Review

The conversion of methane into chemicals is of interest to achieve a decarbonized future. Fuel cells are electrochemical devices commonly used to obtain electrical energy but can be utilized either for chemicals’ production or both energy and chemicals cogeneration. In this work, the partial oxidation of methane in fuel cells for electricity generation and valuable chemicals production at the same time is reviewed. For this purpose, we compile different types of methane-fed fuel cells, both low- and high-temperature fuel cells. Despite the fact that few studies have been conducted on this subject, promising results are driving the development of fuel cells that use methane as a fuel source for the cogeneration of power and valuable chemicals.

Electronic Structure of RhO2+, Its Ammoniated Complexes (NH3)1-5RhO2+, and Mechanistic Exploration of CH4 Activation by Them

High-level electronic structure calculations are initially performed to investigate the electronic structure of RhO²⁺. The construction of potential energy curves for the ground and low-lying excited states allowed the calculation of spectroscopic constants, including harmonic and anharmonic vibrational frequencies, bond lengths, spin-orbit constants, and excitation energies. The equilibrium electronic configurations were used for the interpretation of the chemical bonding. We further monitored how the Rh-O bonding scheme changes with the gradual addition of ammonia ligands. The nature of this bond remains unaffected up to four ammonia ligands but adopts a different electronic configuration in the pseudo-octahedral geometry of (NH₃)₅RhO²⁺. This has consequences in the activation mechanism of the C-H bond of methane by these complexes, especially (NH₃)₄RhO²⁺. We show that the [2 + 2] mechanism in the (NH₃)₄RhO²⁺ case has a very low energy barrier comparable to that of a radical mechanism. We also demonstrate that methane can coordinate to the metal in a similar fashion to ammonia and that knowledge of the electronic structure of the pure ammonia complexes provides qualitative insights into the optimal reaction mechanism.

Electronic structure of the dicationic first row transition metal oxides

Multi-reference electronic structure calculations combined with large basis sets are performed to investigate the electronic structure of the ground and low-lying electronic states of the MO²⁺ diatomic species with M = Ti-Cu. These systems have shown high efficiency in the activation of the C-H of saturated hydrocarbons. This study is the first systematic and accurate work for these systems and our results and discussion provides insights into the reactivity and stability of MO²⁺ units. We find that they can be divided in three groups. The early transition metals (Ti, V, Cr) have very stable and well separated oxo (M⁴⁺O^2-) character ground states, the middle transition metals (Mn, Fe) have oxyl (M³⁺O˙^-) ground states with low-lying oxo excited states, and the late transition metals (Co, Ni, Cu) have well separated oxyl states. The reported spectroscopic constants will aid future experimental investigations, which are sparse in the literature. Periodic trends for the bond lengths, energetics, excitation energies, and wavefunction composition are discussed in detail. Complete basis set limit results indicate the high accuracy of the quintuple-ζ basis sets.

Methane to Methanol Conversion Facilitated by Anionic Transition Metal Centers: The Case of Fe, Ni, Pd, and Pt

Density functional theory and high-level ab initio electronic structure calculations are performed to study the mechanism of the partial oxidation of methane to methanol facilitated by the titled anionic transition metal atoms. The energy landscape for the overall reaction M^- + N₂O + CH₄ → M^- + N₂ + CH₃OH (M = Fe, Ni, Pd, Pt) is constructed for different reaction pathways for all four metals. The comparison with earlier experimental and theoretical results for cationic centers demonstrates the better performance of the metal anions. The main advantage is that anionic centers interact weakly with the produced methanol. This fact facilitates the fast removal of methanol from the catalytic center and prevents the overoxidation of methane. Moreover, a moderate or high energy barrier for the M^- + CH₄ → HMCH₃^- reaction step is observed, which protects the metal center from deactivation. Future work should focus on the identification of proper ligands, which stabilize the negative charge on the metal (electronic factors) and prevent the formation of the global CH₃MOH^- minimum (steric factors). Finally, a composite electronic structure method (combining size extensive coupled clusters approaches and accurate multireference configuration interaction) is proposed for computationally demanding systems and is applied to Fe^-.

Radical abstraction vs. oxidative addition mechanisms for the activation of the SH, OH, and CH bonds using early transition metal oxides

Quantum chemical calculations are performed to study the S-H, O-H, and C-H bond activation of H2S, H2O, and CH4 by bare and ligated ZrO+ and NbO+ units. These representative oxides bear low energy oxo and higher energy oxyl units. S-H and C-H bonds are readily activated by metal oxyl states (radical mechanism), but the O-H bond is harder to activate with either the oxyl or oxo states. Our results suggest that known practices for the C-H bond activation can be applied to S-H, but not to O-H bonds. The identified trends are rationalized in terms of the HS-H, HO-H, and H3C-H dissociation energies to the homolytic or heterolytic fragments. We also found that these dissociation energies drop to about half after coordination of H2S or H2O to the metal oxide unit. In addition, chlorine ligands are shown to stabilize the higher energy oxyl states of the transition metal oxygen unit enhancing the reactivity of the formed complexes.

Ab initio investigation of the ground and excited states of RuO+,0,- and their reaction with water

High-level quantum chemical calculations on RuO0,± elucidate the electronic structure of their low-lying electronic states. For thirty-two states, we report the electronic configurations, bond lengths, vibrational frequencies, spin-orbit splittings, and excitation energies. The electronic states of RuO can be generated from those of RuO+ by adding one electron to the σ non-bonding orbital closely resembling the 5s atomic orbital of Ru. The ground states for RuO and RuO- are clearly identified as 5Δ and 4Δ, but the two states (4Δ and 2Π) compete for RuO+. The difficulty of calculations is revealed by our small binding energies compared to the experimental values. In addition, we studied the reaction of the three species with water in their ground and selected low-lying electronic states. We found a consistent decrease of the activation energy barriers and higher exothermicity as we add electrons to the system. RuO- is found to facilitate the reaction for both kinetic and thermodynamic reasons.