Direct conversion of methane to formaldehyde and CO on B2O3 catalysts

Enhancing Efficiency and High-Value Chemicals Generation through Coupling Photocatalytic CO2 Reduction with Propane Oxidation

Conversion of CO2 into high-value chemicals using solar energy is one of promising approaches to achieve carbon neutrality. However, the oxidation of water in the photocatalytic CO2 reduction is kinetically unfavorable due to multi-electron and proton transfer processes, along with the difficulty in generating O-O bonds. To tackle these challenges, this study investigated the coupling reaction of photocatalytic CO2 reduction and selective propane oxidation using the Pd/P25 (1 wt%) catalyst. Our findings reveal a significant improvement in CO2 reduction, nearly fivefold higher, achieved by substituting water oxidation with selective propane oxidation. This substitution not only accelerates the process of CO2 reduction but also yields valuable propylene. The relative ease of propane oxidation, compared to water, appears to increase the density of photogenerated electrons, ultimately enhancing the efficiency of CO2 reduction. We further found that hydroxyl radicals and reduced intermediate (carboxylate species) played important roles in the photocatalytic reaction. These findings not only propose a potential approach for the efficient utilization of CO2 through the coupling of selective propane oxidation into propylene, but also provide insights into the mechanistic understanding of the coupling reaction.

Unidentical Twins: Geometrically Similar but Chemically Distinct Metal-Free Sites in Boron Oxide for Methane Oxidation to HCHO, CO and CO2

Metal-free boron-based catalysts such as boron oxide (B2O3) and boron nitride (h-BN) are promising catalysts for methane oxidation to HCHO and CO. The B2O3 catalyst contains various probable boron sites (B1 to B6), which may be responsible for methane oxidation. In this work, we utilized density functional theory to compare two relevant geometrically identical boron sites (B2 and B4) for their reactivities. The two sites are explored in-detail for the conversion of methane to formaldehyde (M2F), carbon monoxide and carbon dioxide. The B4 site activates the methane C-H bond easily as compared to the B2 site. In M2F conversion, the rate-determining step for the B2 site is the co-activation of dioxygen and methane, whereas over the B4 site, formaldehyde formation is the rate-determining step. The computationally-determined RDS for the B4 site coincides well with the reported experiments. It is further revealed that this site also prefers the formation of CO over CO2, which is in-line with the experiments in literature. It is also shown through orbital analysis that methanol formation does not occur during methane oxidation. We employed descriptors such as condensed Fukui functions and global electrophilicity index to chemically distinct these twin sites.

Converting CO2 Into Natural Gas Within the Autoclave: A Kinetic Study on Hydrogenation of Carbonates in Aqueous Solution

Catalytic conversion of carbon dioxide (CO2) into value-added chemicals is of pivotal importance, well the cost of capturing CO2 from dilute atmosphere is super challenge. One promising strategy is combining the adsorption and transformation at one step, such as applying alkali solution that could selectively reduce carbonate (CO3 2-) as consequences of CO2 adsorption. Due to complexity of this system, the mechanistic details on controlling the hydrogenation have not been investigated in depth. Herein, Ru/TiO2 catalyst was applied as a probe to elucidate the mechanism of CO3 2- activation, in which with thermodynamic and kinetic investigations, a compact Langmuir-Hinshelwood reaction model was established which suggests that the overall rate of CO3 2- hydrogenation was controlled by a specific C-O bond rupture elementary step within HCOO- and the Ru surface was mainly covered by CO3 2- or HCOO- at independent conditions. This assumption was further supported by negligible kinetic isotope effects (kH/kD≈1), similarity on reaction barriers of CO3 2- and HCOO- hydrogenation (ΔH≠ hydr,Na2CO3 and ΔH≠ hydr,HCOONa) and a non-variation of entropy (ΔS≠ hydr≈0). More interestingly, the alkalinity of the solution is certainly like a two sides in a sword and could facilitate the adsorption of CO2 while hold back catalysis during CO3 2- hydrogenation.

Methane-H2S Reforming Catalyzed by Carbon and Metal Sulfide Stabilized Sulfur Dimers

H2S reforming of methane (HRM) provides a potential strategy to directly utilize sour natural gas for the production of COx-free H2 and sulfur chemicals. Several carbon allotropes were found to be active and selective for HRM, while the additional presence of transition metals led to further rate enhancements and outstanding stability (e.g., Ru supported on carbon black). Most metals are transformed to sulfides, but the carbon supports prevent sintering under the harsh reaction conditions. Supported by theoretical calculations, kinetic and isotopic investigations with representative catalysts showed that H2S decomposition and the recombination of surface H atoms are quasi-equilibrated, while the first C-H bond scission is the kinetically relevant step. Theory and experiments jointly establish that dynamically formed surface sulfur dimers are responsible for methane activation and catalytic turnovers on sulfide and carbon surfaces that are otherwise inert without reaction-derived active sites.

Mapping the Catalytic-Space for the Reactivity of Metal-free Boron Nitride with O2 for H2O-Mediated Conversion of Methane to HCHO and CO

Transition-metal based catalysts have been widely employed to catalyze partial oxidation of light alkanes. Recently, metal-free hexagonal-boron nitride (h-BN) has emerged as a promising catalyst for the oxidation of CH4 to HCHO and CO; however, the intricate catalytic surface of h-BN at molecular and electronic levels remains inadequately understood. Key questions include how electron-deficient boron atoms in h-BN reduce O2, and whether the partial oxidation of methane over h-BN exhibits similarities to traditional transition-metal catalysts. In our study, we computationally-mapped in-detail the surface catalytic-space of h-BN for methane oxidation. We considered different structures of h-BN and show that these structures contain numerous sites for O2 binding and therefore various routes for methane oxidation are possible. The activation barriers for methane oxidation via various paths varies from ~83 to ~123 kcal mol-1. To comprehend the differences in activation barriers, we employed geometrical, orbital and distortion/interaction analysis (DIA). Orbital analysis reveals that methane activation over h-BN in presence of dioxygen follows a standard hydrogen atom transfer mechanism. It is also shown that water plays an intriguing role in reducing the barrier for HCHO and CO formation by acting as a bridge.

PdCu nanoalloy decorated photocatalysts for efficient and selective oxidative coupling of methane in flow reactors

Methane activation by photocatalysis is one of the promising sustainable technologies for chemical synthesis. However, the current efficiency and stability of the process are moderate. Herein, a PdCu nanoalloy (~2.3 nm) was decorated on TiO2, which works for the efficient, stable, and selective photocatalytic oxidative coupling of methane at room temperature. A high methane conversion rate of 2480 μmol g-1 h-1 to C2 with an apparent quantum efficiency of ~8.4% has been achieved. More importantly, the photocatalyst exhibits the turnover frequency and turnover number of 116 h-1 and 12,642 with respect to PdCu, representing a record among all the photocatalytic processes (λ > 300 nm) operated at room temperature, together with a long stability of over 112 hours. The nanoalloy works as a hole acceptor, in which Pd softens and weakens C-H bond in methane and Cu decreases the adsorption energy of C2 products, leading to the high efficiency and long-time stability.

Nearly 100% selective and visible-light-driven methane conversion to formaldehyde via. single-atom Cu and Wδ

Direct solar-driven methane (CH₄) reforming is highly desirable but challenging, particularly to achieve a value-added product with high selectivity. Here, we identify a synergistic ensemble effect of atomically dispersed copper (Cu) species and partially reduced tungsten (W^δ+), stabilised over an oxygen-vacancy-rich WO₃, which enables exceptional photocatalytic CH₄ conversion to formaldehyde (HCHO) under visible light, leading to nearly 100% selectivity, a very high yield of 4979.0 μmol·g^-1 within 2 h, and the normalised mass activity of 8.5 × 10⁶ μmol·g^-1_Cu·h^-1 of HCHO at ambient temperature. In-situ EPR and XPS analyses indicate that the Cu species serve as the electron acceptor, promoting the photo-induced electron transfer from the conduction band to O₂, generating reactive •OOH radicals. In parallel, the adjacent W^δ+ species act as the hole acceptor and the preferred adsorption and activation site of H₂O to produce hydroxyl radicals (•OH), and thus activate CH₄ to methyl radicals (•CH₃). The synergy of the adjacent dual active sites boosts the overall efficiency and selectivity of the conversion process.

Mechanistic Insights into Radical-Induced Selective Oxidation of Methane over Nonmetallic Boron Nitride Catalysts

Boron-based nonmetallic materials (such as B₂O₃ and BN) emerge as promising catalysts for selective oxidation of light alkanes by O₂ to form value-added products, resulting from their unique advantage in suppressing CO₂ formation. However, the site requirements and reaction mechanism of these boron-based catalysts are still in vigorous debate, especially for methane (the most stable and abundant alkane). Here, we show that hexagonal BN (h-BN) exhibits high selectivities to formaldehyde and CO in catalyzing aerobic oxidation of methane, similar to Al₂O₃-supported B₂O₃ catalysts, while h-BN requires an extra induction period to reach a steady state. According to various structural characterizations, we find that active boron oxide species are gradually formed in situ on the surface of h-BN, which accounts for the observed induction period. Unexpectedly, kinetic studies on the effects of void space, catalyst loading, and methane conversion all indicate that h-BN merely acts as a radical generator to induce gas-phase radical reactions of methane oxidation, in contrast to the predominant surface reactions on B₂O₃/Al₂O₃ catalysts. Consequently, a revised kinetic model is developed to accurately describe the gas-phase radical feature of methane oxidation over h-BN. With the aid of in situ synchrotron vacuum ultraviolet photoionization mass spectroscopy, the methyl radical (CH₃^•) is further verified as the primary reactive species that triggers the gas-phase methane oxidation network. Theoretical calculations elucidate that the moderate H-abstraction ability of predominant CH₃^• and CH₃OO^• radicals renders an easier control of the methane oxidation selectivity compared to other oxygen-containing radicals generally proposed for such processes, bringing deeper understanding of the excellent anti-overoxidation ability of boron-based catalysts.

Antiexfoliating h-BN⊃In2O3 Catalyst for Oxidative Dehydrogenation of Propane in a High-Temperature and Water-Rich Environment

Hexagonal boron nitride (h-BN) is regarded as one of the most efficient catalysts for oxidative dehydrogenation of propane (ODHP) with high olefin selectivity and productivity. However, the loss of the boron component under a high concentration of water vapor and high temperature seriously hinders its further development. How to make h-BN a stable ODHP catalyst is one of the biggest scientific challenges at present. Herein, we construct h-BN⊃xIn₂O₃ composite catalysts through the atomic layer deposition (ALD) process. After high-temperature treatment in ODHP reaction conditions, the In₂O₃ nanoparticles (NPs) are dispersed on the edge of h-BN and observed to be encapsulated by ultrathin boron oxide (BO_x) overlayer. A novel strong metal oxide-support interaction (SMOSI) effect between In₂O₃ NPs and h-BN is observed for the first time. The material characterization reveals that the SMOSI not only improves the interlayer force between h-BN layers with a pinning model but also reduces the affinity of the B-N bond toward O^• for inhibiting oxidative cutting of h-BN into fragments at a high temperature and water-rich environment. With the pinning effect of the SMOSI, the catalytic stability of h-BN⊃70In₂O₃ has been extended nearly five times than that of pristine h-BN, and the intrinsic olefin selectivity/productivity of h-BN is well maintained.

Enabling Specific Photocatalytic Methane Oxidation by Controlling Free Radical Type

Selective CH₄ oxidation to CH₃OH or HCHO with O₂ in H₂O under mild conditions provides a desired sustainable pathway for synthesis of commodity chemicals. However, manipulating reaction selectivity while maintaining high productivity remains a huge challenge due to the difficulty in the kinetic control of the formation of a desired oxygenate against its overoxidation. Here, we propose a highly efficient strategy, based on the precise control of the type of as-formed radicals by rational design on photocatalysts, to achieve both high selectivity and high productivity of CH₃OH and HCHO in CH₄ photooxidation for the first time. Through tuning the band structure and the size of active sites (i.e., single atoms or nanoparticles) in our Au/In₂O₃ catalyst, we show alternative formation of two important radicals, ^•OOH and ^•OH, which leads to distinctly different reaction paths to the formation of CH₃OH and HCHO, respectively. This approach gives rise to a remarkable HCHO selectivity and yield of 97.62% and 6.09 mmol g^-1 on In₂O₃-supported Au single atoms (Au₁/In₂O₃) and an exceptional CH₃OH selectivity and yield of 89.42% and 5.95 mmol g^-1 on In₂O₃-supported Au nanoparticles (Au_NPs/In₂O₃), respectively, upon photocatalytic CH₄ oxidation for 3 h at room temperature. This work opens a new avenue toward efficient and selective CH₄ oxidation by delicate design of composite photocatalysts.

Auto-accelerated dehydrogenation of alkane assisted by in-situ formed olefins over boron nitride under aerobic conditions

Oxidative dehydrogenation (ODH) of alkane over boron nitride (BN) catalyst exhibits high olefin selectivity as well as a small ecological carbon footprint. Here we report an unusual phenomenon that the in-situ formed olefins under reactions are in turn actively accelerating parent alkane conversion over BN by interacting with hydroperoxyl and alkoxyl radicals and generating reactive species which promote oxidation of alkane and olefin formation, through feeding a mixture of alkane and olefin and DFT calculations. The isotope tracer studies reveal the cleavage of C-C bond in propylene when co-existing with propane, directly evidencing the deep-oxidation of olefins occur in the ODH reaction over BN. Furthermore, enhancing the activation of ethane by the in-situ formed olefins from propane is successfully realized at lower temperature by co-feeding alkane mixture strategy. This work unveils the realistic ODH reaction pathway over BN and provides an insight into efficiently producing olefins.

Synthesis and Characterization of Catalytically Active Au Core─Pd Shell Nanoparticles Supported on Alumina

A two-step seeded-growth method was refined to synthesize Au@Pd core@shell nanoparticles with thin Pd shells, which were then deposited onto alumina to obtain a supported Au@Pd/Al₂O₃ catalyst active for prototypical CO oxidation. By the strict control of temperature and Pd/Au molar ratio and the use of l-ascorbic acid for making both Au cores and Pd shells, a 1.5 nm Pd layer is formed around the Au core, as evidenced by transmission electron microscopy and energy-dispersive spectroscopy. The core@shell structure and the Pd shell remain intact upon deposition onto alumina and after being used for CO oxidation, as revealed by additional X-ray diffraction and X-ray photoemission spectroscopy before and after the reaction. The Pd shell surface was characterized with in situ infrared (IR) spectroscopy using CO as a chemical probe during CO adsorption-desorption. The IR bands for CO ad-species on the Pd shell suggest that the shell exposes mostly low-index surfaces, likely Pd(111) as the majority facet. Generally, the IR bands are blue-shifted as compared to conventional Pd/alumina catalysts, which may be due to the different support materials for Pd, Au versus Al₂O₃, and/or less strain of the Pd shell. Frequencies obtained from density functional calculations suggest the latter to be significant. Further, the catalytic CO oxidation ignition-extinction processes were followed by in situ IR, which shows the common CO poisoning and kinetic behavior associated with competitive adsorption of CO and O₂ that is typically observed for noble metal catalysts.

Elevating Photooxidation of Methane to Formaldehyde via TiO2 Crystal Phase Engineering

Photocatalytic conversion of methane to value-added products under mild conditions, which represents a long sought-after goal for industrial sustainable production, remains extremely challenging to afford high production and selectivity using cheap catalysts. Herein, we present the crystal phase engineering of commercially available anatase TiO₂ via simple thermal annealing to optimize the structure-property correlation. A biphase catalyst with anatase (90%) and rutile (10%) TiO₂ with the optimal phase interface concentration exhibits exceptional performance in the oxidation of methane to formaldehyde under the reaction conditions of water solvent, oxygen atmosphere, and full-spectrum light irradiation. An unprecedented production of 24.27 mmol g_cat^-1 with an excellent selectivity of 97.4% toward formaldehyde is acquired at room temperature after a 3 h reaction. Both experimental results and theoretical calculations disclose that the crystal phase engineering of TiO₂ lengthens the lifetime of photogenerated carriers and favors the formation of intermediate methanol species, thus maximizing the efficiency and selectivity in the aerobic oxidation of methane to formaldehyde. More importantly, the feasibility of the scale-up production of formaldehyde is demonstrated by inventing a "pause-flow" reactor. This work opens the avenue toward industrial methane transformation in a sustainable and economical way.

W Single-Atom Catalyst for CH4 Photooxidation in Water Vapor

Solar-driven high-efficiency and direct conversion of methane into high-value-added liquid oxygenates against overoxidation remains a great challenge. Herein, facile and mass fabrication of low-cost tungsten single-atom photocatalysts is achieved by directly calcining urea and sodium tungstate under atmosphere (W-SA-PCN-m, urea amount m = 7.5, 15, 30, and 150 g). The single-atom photocatalysts can manage H₂ O₂ in situ generation and decomposition into ·OH, thus achieving highly efficient CH₄ photooxidation in water vapor under mild conditions. Systematic investigations demonstrate that integration of multifunctions of methane activation, H₂ O₂ generation, and decomposition into one photocatalyst can dramatically promote methane conversion to C1 oxygenates with a yield as high as 4956 µmol g_cat ^-1 , superior to that of the most reported non-precious photocatalysts. Liquid-solid phase transition can induce the products to facilely switch in from HCOOH to CH₃ OH by pulling the catalyst above water with CH₃ OH/HCOOH ratio from 10% (in H₂ O) to 80% (above H₂ O).

Synergy of Pd atoms and oxygen vacancies on In2O3 for methane conversion under visible light

Methane (CH4) oxidation to high value chemicals under mild conditions through photocatalysis is a sustainable and appealing pathway, nevertheless confronting the critical issues regarding both conversion and selectivity. Herein, under visible irradiation (420 nm), the synergy of palladium (Pd) atom cocatalyst and oxygen vacancies (OVs) on In2O3 nanorods enables superior photocatalytic CH4 activation by O2. The optimized catalyst reaches ca. 100 μmol h-1 of C1 oxygenates, with a selectivity of primary products (CH3OH and CH3OOH) up to 82.5%. Mechanism investigation elucidates that such superior photocatalysis is induced by the dedicated function of Pd single atoms and oxygen vacancies on boosting hole and electron transfer, respectively. O2 is proven to be the only oxygen source for CH3OH production, while H2O acts as the promoter for efficient CH4 activation through ·OH production and facilitates product desorption as indicated by DFT modeling. This work thus provides new understandings on simultaneous regulation of both activity and selectivity by the synergy of single atom cocatalysts and oxygen vacancies.

Partial oxidation of methane to methanol on boron nitride at near critical acetonitrile

Direct catalytic conversion of methane to methanol with O₂ has been a fundamental challenge in unlocking abundant natural gas supplies. Metal-free methane conversion with 17% methanol yield based on the limiting reagent O₂ at 275 °C was achieved with near supercritical acetonitrile in the presence of boron nitride. Reaction temperature, catalyst loading, dwell time, methane–oxygen molar ratio, and solvent-oxygen molar ratios were identified as critical factors controlling methane activation and the methanol yield. Extension of the study to ethane (C2) showed moderate yields of methanol (3.6%) and ethanol (4.5%).

Property-activity relations of multifunctional reactive ensembles in cation-exchanged zeolites: a case study of methane activation on Zn2+-modified zeolite BEA

The reactivity theories and characterization studies for metal-containing zeolites are often focused on probing the metal sites. We present a detailed computational study of the reactivity of Zn-modified BEA zeolite towards C-H bond activation of the methane molecule as a model system that highlights the importance of representing the active site as the whole reactive ensemble integrating the extra-framework Zn_EF²⁺ cations, framework oxygens (O_F^2-), and the confined space of the zeolite pores. We demonstrate that for our model system the relationship between the Lewis acidity, defined by the probe molecule adsorption energy, and the activation energy for methane C-H bond cleavage performs with a determination coefficient R² = 0.55. This suggests that the acid properties of the localized extra-framework cations can be used only for a rough assessment of the reactivity of the cations in the metal-containing zeolites. In turn, studying the relationship between the activation energy and pyrrole adsorption energy revealed a correlation, with R² = 0.80. This observation was accounted for by the similarity between the local geometries of the pyrrole adsorption complexes and the transition states for methane C-H bond cleavage. The inclusion of a simple descriptor for zeolite local confinement allows transferability of the obtained property-activity relations to other zeolite topologies. Our results demonstrate that the representation of the metal cationic species as a synergistically cooperating active site ensembles allows reliable detection of the relationship between the acid properties and reactivity of the metal cation in zeolite materials.

Gas-Phase Selective Oxidation of Methane into Methane Oxygenates

Methane is an abundant resource and its direct conversion into value-added chemicals has been an attractive subject for its efficient utilization. This method can be more efficient than the present energy-intensive indirect conversion of methane via syngas, a mixture of CO and H2. Among the various approaches for direct methane conversion, the selective oxidation of methane into methane oxygenates (e.g., methanol and formaldehyde) is particularly promising because it can proceed at low temperatures. Nevertheless, due to low product yields this method is challenging. Compared with the liquid-phase partial oxidation of methane, which frequently demands for strong oxidizing agents in protic solvents, gas-phase selective methane oxidation has some merits, such as the possibility of using oxygen as an oxidant and the ease of scale-up owing to the use of heterogeneous catalysts. Herein, we summarize recent advances in the gas-phase partial oxidation of methane into methane oxygenates, focusing mainly on its conversion into formaldehyde and methanol.

In situ growth of phosphorus-doped boron nitride on commercial alumina as a robust catalyst for direct dehydrogenation of ethylbenzene

The obtained PBN@Al2O3(N) synthesized by in situ growth of thin PBN layers on commercial Al2O3 exhibited a significantly improved stability with relatively high ethylbenzene conversion and styrene selectivity.

Methane Activation by (MoO3 )5 O- Cluster Anions: The Importance of Orbital Orientation

The reactivity of the molybdenum oxide cluster anion (MoO₃ )₅ O^- , bearing an unpaired electron at a bridging oxygen atom (O_b ^.- ), towards methane under thermal collision conditions has been studied by mass spectrometry and density functional theory calculations. This reaction follows the mechanism of hydrogen atom transfer (HAT) and is facilitated by the O_b ^.- radical center. The reactivity of (MoO₃ )₅ O^- can be traced back to the appropriate orientation of the lowest unoccupied molecular orbitals (LUMO) that is essentially the 2p orbital of the O_b ^.- atom. This study not only makes up the blank of thermal methane activation by the O_b ^.- radical on negatively charged clusters but also yields new insights into methane activation by the atomic oxygen radical anions.

Study on the selective oxidation of methane over highly dispersed molybdenum-incorporated KIT-6 catalysts

The isolated MoO_x species contribute to the highly selective formation of formaldehyde.