Computational Discovery of Stable Metal-Organic Frameworks for Methane-to-Methanol Catalysis

The challenge of direct partial oxidation of methane to methanol has motivated the targeted search of metal-organic frameworks (MOFs) as a promising class of materials for this transformation because of their site-isolated metals with tunable ligand environments. Thousands of MOFs have been synthesized, yet relatively few have been screened for their promise in methane conversion. We developed a high-throughput virtual screening workflow that identifies MOFs from a diverse space of experimental MOFs that have not been studied for catalysis, yet are thermally stable, synthesizable, and have promising unsaturated metal sites for C-H activation via a terminal metal-oxo species. We carried out density functional theory calculations of the radical rebound mechanism for methane-to-methanol conversion on models of the secondary building units (SBUs) from 87 selected MOFs. While we showed that oxo formation favorability decreases with increasing 3d filling, consistent with prior work, previously observed scaling relations between oxo formation and hydrogen atom transfer (HAT) are disrupted by the greater diversity in our MOF set. Accordingly, we focused on Mn MOFs, which favor oxo intermediates without disfavoring HAT or leading to high methanol release energies─a key feature for methane hydroxylation activity. We identified three Mn MOFs comprising unsaturated Mn centers bound to weak-field carboxylate ligands in planar or bent geometries with promising methane-to-methanol kinetics and thermodynamics. The energetic spans of these MOFs are indicative of promising turnover frequencies for methane to methanol that warrant further experimental catalytic studies.

Selective Oxidation of Methane to Methanol over Au/H-MOR

Selective oxidation of methane to methanol by dioxygen (O₂) is an appealing route for upgrading abundant methane resource and represents one of the most challenging reactions in chemistry due to the overwhelmingly higher reactivity of the product (methanol) versus the reactant (methane). Here, we report that gold nanoparticles dispersed on mordenite efficiently catalyze the selective oxidation of methane to methanol by molecular oxygen in aqueous medium in the presence of carbon monoxide. The methanol productivity reaches 1300 μmol g_cat^-1 h^-1 or 280 mmol g_Au^-1 h^-1 with 75% selectivity at 150 °C, outperforming most catalysts reported under comparable conditions. Both hydroxyl radicals and hydroperoxide species participate in the activation and conversion of methane, while it is shown that the lower affinity of methanol on gold mainly accounts for higher methanol selectivity.

A Silver-Based Integrated System Showing Mutually Inclusive Super Protonic Conductivity and Photoswitching Behavior

Photoinduced electricity and proton conductivity led fuel cells have emerged, inter alia, as highly promising systems for unconventional energy harvesting. Notwithstanding their individual presence with widely acclaimed results, an integrating system with mutually inclusive manifestation of both features has hitherto not been reported in the literature. To achieve this objective, our approach was to design a ligand system incorporating prerequisite features of both systems, like extended conjugation instigating photophysical activity and functional groups facilitating ionic conduction. As such, we report herein the design, synthesis, and characterization of a pyridyl-pyrazole-based silver compound that exhibits an excellent photocurrent generation and very high proton conductivity. The X-ray single-crystal structure of the Ag complex fully supports our notion, showing extensive π-π conjugated aromatic rings with a protruding free sulfonic group, facing toward solvent-filled channels with numerous supramolecular interactions. The nanoscopic silver metallogel induces semiconductive features in the system which ultimately result in photoresponse behavior in terms of photocurrent generation with an whopping photocurrent gain (I_on/I_off) of 21.2. To complete the idea of an integrated system, the proton conductivity values were also measured for both gel and crystalline states, while the former state yields a better result. The maximum proton conductivity value turns out to be 1.03 × 10^-2 S cm^-1 at 70 °C, which is higher than or comparable to those of well-known systems in the literature for proton conductivity.

An Air-Rechargeable Zn Battery Enabled by Organic-Inorganic Hybrid Cathode

Self-charging power systems collecting energy harvesting technology and batteries are attracting extensive attention. To solve the disadvantages of the traditional integrated system, such as highly dependent on energy supply and complex structure, an air-rechargeable Zn battery based on MoS2/PANI cathode is reported. Benefited from the excellent conductivity desolvation shield of PANI, the MoS2/PANI cathode exhibits ultra-high capacity (304.98 mAh g-1 in N2 and 351.25 mAh g-1 in air). In particular, this battery has the ability to collect, convert and store energy simultaneously by an air-rechargeable process of the spontaneous redox reaction between the discharged cathode and O2 from air. The air-rechargeable Zn batteries display a high open-circuit voltage (1.15 V), an unforgettable discharge capacity (316.09 mAh g-1 and the air-rechargeable depth is 89.99%) and good air-recharging stability (291.22 mAh g-1 after 50 air recharging/galvanostatic current discharge cycle). Most importantly, both our quasi-solid zinc ion batteries and batteries modules have excellent performance and practicability. This work will provide a promising research direction for the material design and device assembly of the next-generation self-powered system.

Understanding W/H-ZSM-5 catalysts for the dehydroaromatization of methane

Tungsten is the most interesting and promising metal to replace molybdenum in methane dehydroaromatization (MDA) catalysis.

Hydrogenolysis of Polyethylene and Polypropylene into Propane over Cobalt-Based Catalysts

The development of technologies to recycle polyethylene (PE) and polypropylene (PP), globally the two most produced polymers, is critical to increase plastic circularity. Here, we show that 5 wt % cobalt supported on ZSM-5 zeolite catalyzes the solvent-free hydrogenolysis of PE and PP into propane with weight-based selectivity in the gas phase over 80 wt % after 20 h at 523 K and 40 bar H2. This catalyst significantly reduces the formation of undesired CH4 (≤5 wt %), a product which is favored when using bulk cobalt oxide or cobalt nanoparticles supported on other carriers (selectivity ≤95 wt %). The superior performance of Co/ZSM-5 is attributed to the stabilization of dispersed oxidic cobalt nanoparticles by the zeolite support, preventing further reduction to metallic species that appear to catalyze CH4 generation. While ZSM-5 is also active for propane formation at 523 K, the presence of Co promotes stability and selectivity. After optimizing the metal loading, it was demonstrated that 10 wt % Co/ZSM-5 can selectively catalyze the hydrogenolysis of low-density PE (LDPE), mixtures of LDPE and PP, as well as postconsumer PE, showcasing the effectiveness of this technology to upcycle realistic plastic waste. Cobalt supported on zeolites FAU, MOR, and BEA were also effective catalysts for C2-C4 hydrocarbon formation and revealed that the framework topology provides a handle to tune gas-phase selectivity.

Recent Insights into Cu-Based Catalytic Sites for the Direct Conversion of Methane to Methanol

Direct conversion of methane to methanol is an effective and practical process to improve the efficiency of natural gas utilization. Copper (Cu)-based catalysts have attracted great research attention, due to their unique ability to selectively catalyze the partial oxidation of methane to methanol at relatively low temperatures. In recent decades, many different catalysts have been studied to achieve a high conversion of methane to methanol, including the Cu-based enzymes, Cu-zeolites, Cu-MOFs (metal-organic frameworks) and Cu-oxides. In this mini review, we will detail the obtained evidence on the exact state of the active Cu sites on these various catalysts, which have arisen from the most recently developed techniques and the results of DFT calculations. We aim to establish the structure–performance relationship in terms of the properties of these materials and their catalytic functionalities, and also discuss the unresolved questions in the direct conversion of methane to methanol reactions. Finally, we hope to offer some suggestions and strategies for guiding the practical applications regarding the catalyst design and engineering for a high methanol yield in the methane oxidation reaction.

Elucidation of radical- and oxygenate-driven paths in zeolite-catalysed conversion of methanol and methyl chloride to hydrocarbons

Understanding hydrocarbon generation in the zeolite-catalysed conversions of methanol and methyl chloride requires advanced spectroscopic approaches to distinguish the complex mechanisms governing C-C bond formation, chain growth and the deposition of carbonaceous species. Here operando photoelectron photoion coincidence (PEPICO) spectroscopy enables the isomer-selective identification of pathways to hydrocarbons of up to C₁₄ in size, providing direct experimental evidence of methyl radicals in both reactions and ketene in the methanol-to-hydrocarbons reaction. Both routes converge to C₅ molecules that transform into aromatics. Operando PEPICO highlights distinctions in the prevalence of coke precursors, which is supported by electron paramagnetic resonance measurements, providing evidence of differences in the representative molecular structure, density and distribution of accumulated carbonaceous species. Radical-driven pathways in the methyl chloride-to-hydrocarbons reaction(s) accelerate the formation of extended aromatic systems, leading to fast deactivation. By contrast, the generation of alkylated species through oxygenate-driven pathways in the methanol-to-hydrocarbons reaction extends the catalyst lifetime. The findings demonstrate the potential of the presented methods to provide valuable mechanistic insights into complex reaction networks.

Electrocatalytic methane oxidation to formate on magnesium based metal-organic frameworks

The electrochemical methane oxidation reaction is a potential approach for upgrading the nature-abundant methane (CH₄) into value-added chemicals, while the activity and selectivity have remained substantially low due to the extremely inert chemical property of CH₄. Inspired by the methane mono-oxygenase in nature, we demonstrated Mg-substituted metal-organic frameworks (Mg-MOF-74) containing a uniform distribution of Mg-oxo-Mg nodes as efficient catalytic sites. Compared to MgNi-MOF-74 and Mg(OH)₂ without the Mg-oxo-Mg nodes, the Mg-MOF-74 presented a much enhanced CH₄ electrooxidation performance, with a unique selectivity of producing formate. The maximum Faradaic efficiency of all liquid products reached 10.9% at 1.60 V versus reversible hydrogen electrode (RHE), corresponding to the peak production rate of 126.6 μmol·h^-1·g^-1.

Exploring mechanistic routes for light alkane oxidation with an iron-triazolate metal-organic framework

In this work, we computationally explore the formation and subsequent reactivity of various iron-oxo species in the iron-triazolate framework Fe₂(μ-OH)₂(bbta) (H₂bbta = 1H,5H-benzo(1,2-d:4,5-d')bistriazole) for the catalytic activation of strong C-H bonds. With the direct conversion of methane to methanol as the probe reaction of interest, we use density functional theory (DFT) calculations to evaluate multiple mechanistic pathways in the presence of either N₂O or H₂O₂ oxidants. These calculations reveal that a wide range of transition metal-oxo sites - both terminal and bridging - are plausible in this family of metal-organic frameworks, making it a unique platform for comparing the electronic structure and reactivity of different proposed active site motifs. Based on the DFT calculations, we predict that Fe₂(μ-OH)₂(bbta) would exhibit a relatively low barrier for N₂O activation and energetically favorable formation of an [Fe(O)]²⁺ species that is capable of oxidizing C-H bonds. In contrast, the use of H₂O₂ as the oxidant is predicted to yield an assortment of bridging iron-oxo sites that are less reactive. We also find that abstracting oxo ligands can exhibit a complex mixture of both positive and negative spin density, which may have broader implications for relating the degree of radical character to catalytic activity. In general, we consider the coordinatively unsaturated iron sites to be promising for oxidation catalysis, and we provide several recommendations on how to further tune the catalytic properties of this family of metal-triazolate frameworks.

Porous nanographene formation on γ-alumina nanoparticles via transition-metal-free methane activation

γ-Al₂O₃ nanoparticles promote pyrolytic carbon deposition of CH₄ at temperatures higher than 800 °C to give single-walled nanoporous graphene (NPG) materials without the need for transition metals as reaction centers. To accelerate the development of efficient reactions for NPG synthesis, we have investigated early-stage CH₄ activation for NPG formation on γ-Al₂O₃ nanoparticles via reaction kinetics and surface analysis. The formation of NPG was promoted at oxygen vacancies on (100) surfaces of γ-Al₂O₃ nanoparticles following surface activation by CH₄. The kinetic analysis was well corroborated by a computational study using density functional theory. Surface defects generated as a result of surface activation by CH₄ make it kinetically feasible to obtain single-layered NPG, demonstrating the importance of precise control of oxygen vacancies for carbon growth.

Oxygen vacancies on the (100) surface of γ-Al₂O₃ nanoparticles catalyse CH₄-CVD for single-layered nanoporous graphenes with no transition metal reaction centre. The rate-limiting step is the proton transfer (PT) in the activation of CH₄ on them.

Sustainable Coproduction of Two Disinfectants via Hydroxide-Balanced Modular Electrochemical Synthesis Using a Redox Reservoir

Challenges posed by the sacrificial auxiliary reactions and expensive ion-exchange membranes in conventional electrosynthesis necessitate developing new electrochemical processes to enable efficient and sustainable distributed electrochemical manufacturing. Modular electrochemical synthesis (ModES) using a redox reservoir (RR) offers a promising membrane-free approach to improve energy efficiency and reduce waste through the pairing of multiple independent oxidative and reductive half-reactions; however, undesired ion-imbalance and induced pH changes in the existing ModES process limit sustained production. Here we present Ni(OH)₂ as a heterogeneous RR that can selectively store and transport the hydroxide ions involved in the target half-reactions by reversible conversion with NiOOH to enable an ion-balanced ModES of two common disinfectants, hydrogen peroxide (H₂O₂) and sodium hypochlorite (NaClO). This hydroxide-balanced ModES realizes stable operation without appreciable pH swing to accumulate practically useful concentrations of H₂O₂ and NaClO up to 251 and 481 ppm, respectively. These results illustrate additional design principles for electrosynthesis without sacrificial auxiliary reactions and the need for ion-selective RRs for modular electrochemical manufacturing.

Gas to Liquids Techno-Economics of Associated Natural Gas, Bio Gas, and Landfill Gas

Methane is the second highest contributor to the greenhouse effect. Its global warming potential is 37 times that of CO2. Flaring-associated natural gas from remote oil reservoirs is currently the only economical alternative. Gas-to-liquid (GtL) technologies first convert natural gas into syngas, then it into liquids such as methanol, Fischer–Tropsch fuels or dimethyl ether. However, studies on the influence of feedstock composition are sparse, which also poses technical design challenges. Here, we examine the techno-economic analysis of a micro-refinery unit (MRU) that partially oxidizes methane-rich feedstocks and polymerizes the syngas formed via Fischer–Tropsch reaction. We consider three methane-containing waste gases: natural gas, biogas, and landfill gas. The FT fuel selling price is critical for the economy of the unit. A Monte Carlo simulation assesses the influence of the composition on the final product quantity as well as on the capital and operative expenses. The Aspen Plus simulation and Python calculate the net present value and payback time of the MRU for different price scenarios. The CO2 content in biogas and landfill gas limit the CO/H2 ratio to 1.3 and 0.9, respectively, which increases the olefins content of the final product. Compressors are the main source of capital cost while the labor cost represents 20–25% of the variable cost. An analysis of the impact of the plant dimension demonstrated that the higher number represents a favorable business model for this unit. A minimal production of 7,300,000 kg y−1 is required for MRU to have a positive net present value after 10 years when natural gas is the feedstock.

Ethane-Based Catalytic Process for Vinyl Chloride Manufacture

The use of ethane as a platform molecule for the manufacture of polyvinyl chloride (PVC) is a longstanding challenge, which would allow to reduce the raw material costs and CO₂ emissions to produce this plastic. Herein, we discover that rare earth oxychlorides catalyze in a selective (up to 90 %) and stable (>50 h on stream) manner the reaction of ethane and molecular chlorine into 1,2-dichloroethane, which, upon established cracking, will translate into an order of magnitude higher vinyl chloride productivity compared to ethane oxychlorination technologies. In addition, representative europium oxychloride was supported on suitable carriers and was demonstrated to be selective (up to 90 %) and stable (>40 h on stream) in extrudate form. These findings bring the ethane-based production of PVC one step closer to implementation.