C-H Activation Research Articles

CO-Assisted Methane Oxidation into Oxygenates over Surface Platinum-Titanium Alloyed Layers.

Methane oxidation using molecular oxygen remains a grand challenge in which the obstacle is not only the activation of methane but also the reaction with oxygen, considering the mismatch of the ground spin states. Herein, we report TiO2-supported Pt nanocrystals (Pt/TiO2) with surface Pt-Ti alloyed layers that directly convert methane into oxygenates by using O2 as the oxidant with the assistance of CO. The oxygenate yield reached 749.8 mmol gPt-1 in a H2O aqueous solution over 0.1% Pt/TiO2 under 31 bar of mixed gas (20:5:6 CH4:CO:O2) at 150 °C for 3 h, while the CH3OH selectivity was 62.3%. On the basis of the control experiments and spectroscopic results, we identified the surface Pt-Ti alloy as the active sites. Moreover, CO promoted the dissociation of O2 on the surface of Pt-Ti alloyed layers and the subsequent activation of CH4 to form oxygenated products.

MOFs-derived Ni@ZrO2 catalyst for dry reforming of methane: Tunable metal-support interaction

Metal-support interaction (MSI) plays an important role in heterogeneous catalysis, that could be well regulated by many strategies. Here, Ni@ZrO2 catalysts were controllably synthesized by sacrificial Zr-MOFs template strategy, and were applied to the dry reforming of methane (DRM). Amongst, the Ni/ZrO2-B catalyst shows the best performance toward CH4 and CO2 conversion, H2/CO ratio, and stability, following by Ni/ZrO2-A and Ni/ZrO2C catalysts. The difference in catalytic performance should be ascribed to different synthetic strategy, resulting in different properties of Ni species and ZrO2 matrix, and interface interaction. It revealed that small Ni particles were benefit for CH4 activation, and strong MSI boosted CO2 activation, that could remain the Ni dispersion and thermal stability. This study is beneficial to the catalyst design in DRM reaction.

Unlocking exceptional diffusion of porous Ni@SiO2 nanocapsule catalysts for enhanced dry reforming of methane

This paper introduces innovative porous Ni@SiO2 nanocapsule catalysts for high-temperature dry reforming of methane, synthesized through a selective etching method in a mild alkaline solution with varying CTAB concentrations. While core-shell catalysts have been proven effective in addressing issues like active metal sintering and coking, their dense inert oxide shells can impede intrinsic kinetics and hinder gas diffusion. The optimized catalyst, M-NSNC-2, showcases exceptional performance, featuring small Ni nanoparticles (5.3 nm) within a porous SiO2 nanocapsule (>900 m2g−1). M-NSNC-2 demonstrates outstanding methane conversion (>82%), stability (50 hours), and resistance to sintering, agglomeration, and higher-ordered carbon deposition. In comparison to dense Ni@SiO2 nanocapsules (NSNC), M-NSNC-2 exhibits superior performance with nearly 13-fold enhanced mass transfer diffusivity. Additionally, the porous Ni@SiO2 nanocapsule catalyst exhibits an apparent CH4 activation energy of 67.9 kJ/mol at a kinetic regime of 1800,000 mL gcat−1h−1, emphasizing its efficacy in catalyzing dry reforming of methane.

Superexchange-stabilized long-distance Cu sites in rock-salt-ordered double perovskite oxides for CO2 electromethanation

Cu-oxide-based catalysts are promising for CO2 electroreduction (CO2RR) to CH4, but suffer from inevitable reduction (to metallic Cu) and uncontrollable structural collapse. Here we report Cu-based rock-salt-ordered double perovskite oxides with superexchange-stabilized long-distance Cu sites for efficient and stable CO2-to-CH4 conversion. For the proof-of-concept catalyst of Sr2CuWO6, its corner-linked CuO6 and WO6 octahedral motifs alternate in all three crystallographic dimensions, creating sufficiently long Cu-Cu distances (at least 5.4 Å) and introducing marked superexchange interaction mainly manifested by O-anion-mediated electron transfer (from Cu to W sites). In CO2RR, the Sr2CuWO6 exhibits significant improvements (up to 14.1 folds) in activity and selectivity for CH4, together with well boosted stability, relative to a physical-mixture counterpart of CuO/WO3. Moreover, the Sr2CuWO6 is the most effective Cu-based-perovskite catalyst for CO2 methanation, achieving a remarkable selectivity of 73.1% at 400 mA cm−2 for CH4. Our experiments and theoretical calculations highlight the long Cu-Cu distances promoting *CO hydrogenation and the superexchange interaction stabilizing Cu sites as responsible for the superb performance.

Synthesis and Photochemistry of Tris(trimethoxysilyl)acyl-silanes and 1,4-Tetrakis(silyl)-1,4-bisacylsilanes.

In this contribution, we present the synthesis of two groups of novel acylsilanes 1-6. Compounds 1 and 2 represent tris(trimethoxysilyl)acylsilanes, and compounds 3-6 are 1,4-tetrakis(silyl)-1,4-bisacylsilanes. All isolated compounds were characterized by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography. Additionally, these compounds were further analyzed by ultraviolet/visible (UV/vis) spectroscopy and their longest wavelength absorption bands were assigned by density functional theory (DFT) calculations. On the basis of the well-known Brook rearrangement of acylsilanes, we irradiated 1-6 in benzene solutions at 405 nm (λ) for several hours. Photolysis of compounds 1 and 2 afforded the same silene rearrangement products as found in previous investigations of structurally related acylsilanes. In addition, trapping experiments with MeOH further support our proposed mechanism for silene formation. The photolysis of tetrakis(trimethylsilyl)bisacylsilane 3 gave rise to the formation of a monosilene intermediate 10; upon prolonged irradiation, the subsequently formed bissilene undergoes a fast dimerization to bicyclic product 11. Interestingly, unlike the expected head-to-head dimerization of Brook-type silenes, this bissilene undergoes a selective head-to-tail dimerization. In contrast, tetrakis(trimethylsilyl)bisacylsilane 4 undergoes a selective and completely stereoselective double CH activation to air stable bicyclic system 12. The mechanism of this rearrangement is fully described by DTF calculations. Unfortunately, tetrakis(trimethoxysilyl)bisacylsilanes 5 and 6 underwent unselective photochemical rearrangements.

Boosting Reactive Oxygen Species Formation Over Pd and VOδ Co-Modified TiO2 for Methane Oxidation into Valuable Oxygenates.

Direct photocatalytic methane oxidation into value-added products provides a promising strategy for methane utilization. However, the inefficient generation of reactive oxygen species (ROS) partly limits the activation of CH4. Herein, it is reported that Pd and VOδ co-modified TiO2 enables direct and selective methane oxidation into liquid oxygenates in the presence of O2 and H2. Due to the extra ROS production from the in situ formed H2O2, a highly improved yield rate of 5014µmolg-1h-1 for liquid oxygenates with a selectivity of 89.3% is achieved over the optimized Pd0.5V0.2-TiO2 catalyst at ambient temperature, which is much better than those (2682µmolg-1h-1, 77.8%) without H2. Detailed investigations also demonstrate the synergistic effect between Pd and VOδ species for enhancing the charge carrier separation and transfer, as well as improving the catalytic activity for O2 reduction and H2O2 production.

Directional Activation of Oxygen by the Au-Loaded ZnAl-LDH with Defect Structure for Highly Efficient Photocatalytic Oxidative Coupling of Methane.

Photocatalytic oxidative coupling of CH4 (OCM) is a promising CH4 conversion process that can achieve efficient methane conversion with the assistance of O2. It remains to be highly challenging to improve the photocatalytic OCM activity from catalyst design and to deepen the understanding of the reactant activation in the OCM process. In this work, the Au-loaded ZnAl-layered double hydroxides (LDHs) with and without oxygen vacancy are constructed (denoted as Au/ZnAl and Au/ZnAl-v), respectively. When applied for photocatalytic OCM, the Au/ZnAl-v shows a CH4 conversion rate of 8.5 mmol g-1 h-1 with 92% selectivity of C2H6 at 40°C, outperforming most reported photocatalytic OCM systems at low temperature reported in the literature. Furthermore, the catalytic performance of Au/ZnAl-v can be stable for 100h. In contrast, the An/ZnAl exhibits a CH4 conversion rate of 0.8 mmol g-1 h-1 with 46% selectivity of C2H6. Detailed characterizations and DFT calculation studies reveal that the introduced Ov sites on Au/ZnAl-v are able to activate O2, and the resulting superoxide radical O2·- greatly promotes the activation of CH4. The coupling of CH3· groups with the assistance of Au cocatalyst leads to the formation of C2H6 with high photocatalytic activity.

Forked Vein Structure W/WO3- x with Dual Active Sites in W and Oxygen Vacancies to Enhance Methylene Self-Coupling for Efficient Conversion of Methane to Ethylene.

The directional conversion of methane to ethylene is challenging due to the dissociation of the C─H bond and the self-coupling of methyl intermediates. Herein, a novel W/WO3- x catalyst with the fork vein structure consisting of an alternating arrangement of WO3- x and W is developed. Impressively, the catalyst achieves an unprecedented C2H4 yield of 1822.73µmolg-1h-1, with a selectivity of 82.49%. The enhanced catalytic activity is ascribed to the multifunctional synergistic effect induced by oxygen vacancies and W sites in W/WO3- x. Oxygen vacancies provide abundant coordination of unsaturation sites, which promotes the adsorption and activation of CH4, thus reducing the dissociation energy barrier of the C─H bond. The CH2 coupling barrier on the metal W surface is significantly lower compared to WO3, so CH2 can migrate to the W site for coupling. Importantly, the W/WO3- x with high periodicity provides multiple ordered local microelectric fields, and CH2 intermediates with dipole moments undergo orientation polarization and displacement polarization driven by the electric field, thus enabling CH2 migration. This work opens a new avenue for the structural design and modulation of photocatalysts, and provides new perspectives on the migration of methylene between multiple active sites.

Multicomponent remote C(sp2)[sbnd]H bond addition by Ru catalysis: An efficient access to the alkylarylation of 2H-imidazoles

Sequential CH bond addition with two different coupling partners is a powerful method for the rapid and modular construction of complex molecules based on simple starting materials. Herein, an efficient ruthenium-catalysed multicomponent long-range CH functionalization of 2H-imidazoles was developed. This protocol showed good substrate suitability and yielded alkyl arylation products with potential biological activity.

Designing a Ce/In-CHA OXZEO catalyst for high-efficient selective catalytic reduction of nitrogen oxide with methane

Selective catalytic reduction of nitrogen oxides with methane (CH4-SCR) has emerged as a promising technology for mitigating exhaust emissions from natural gas vehicles, while its insufficient catalytic activity and durability are key barriers to application. In this study, novel Ce/In-CHA OXZEO catalysts were prepared, demonstrating exceptional catalytic performance and stability in the CH4-SCR reaction. Over these OXZEO catalysts, indium species predominantly existed in the form of InO+ and InxOy within the CHA zeolite framework, while CeO2 nanoparticles primarily resided as an external layer. During the CH4-SCR reaction, NO oxidation to form NO2 and nitrate species was triggered by CeO2, which further enhanced the activation of CH4 on InO+ sites, thus boosting the low-temperature CH4-SCR process by facilitating the formation of the key intermediate CH3NO2. The synergy between InO+ and CeO2 underscores the exceptional performance and stability of the Ce/In-CHA with a low SiO2/Al2O3 ratio in reducing NOx and CH4 emissions.

Atomically isolated Sb(CN)3 on sp2-c-COFs with balanced hydrophilic and oleophilic sites for photocatalytic C-H activation.

Activation of carbon-hydrogen (C-H) bonds is of utmost importance for the synthesis of vital molecules. Toward achieving efficient photocatalytic C-H activation, our investigation revealed that incorporating hydrophilic C≡N-Sb(CN)3 sites into hydrophobic sp2 carbon-conjugated covalent organic frameworks (sp2-c-COFs) had a dual effect: It simultaneously enhanced charge separation and improved generation of polar reactive oxygen species. Detailed spectroscopy measurements and simulations showed that C≡N-Sb(CN)3 primarily functioned as water capture sites, which were not directly involved in photocatalysis. However, the potent interaction between water molecules and the Sb(CN)3-modified framework notably enhanced charge dynamics in hydrophobic sp2-c-COFs. The reactive species ·O2- and ·OH (ad) subsequently combined with benzyl radical, leading to the formation of benzaldehyde, benzyl alcohol, and lastly benzyl benzoate. Notably, the Sb(CN)3-modified sp2-c-COFs exhibited a 54-fold improvement in reaction rate as compared to pristine sp2-c-COFs, which achieved a remarkable 68% conversion rate for toluene and an 80% selectivity for benzyl benzoate.

Charge transfer boosts up methane adsorption and activation on three-coordinated metal sites

The activation and conversion of CH4 has attracted great interest due to its mitigation impact on the greenhouse gas and contribution to the high value-added chemicals. However, the activation of CH4 is the rate-limiting step due to the high symmetry of the molecule and its chemical inertness. In this study, we constructed active sites with specific geometry for methane activation based on the Nickel and other post-transition metals, where the interactions were significantly enhanced by 0.2∼0.6 eV varying with different VIII group metal types. With advanced electronic structure analysis methods, the origin of this interaction was unraveled, that is the Coulomb attraction induced by charge transfer from hydrogen of methane to three-coordinated metal Ni sites, rather than the covalent bonding or dispersion interaction. This enhancement not only increases the collision possibility by several orders of magnitude, but also lowers the barrier of CH bond breaking. These findings provide deep insight on the methane activation on metal catalysts and shed the light to design more efficient methane conversion catalysts.

Enhanced mono-aromatics production by the CH4-assisted pyrolysis of microalgae using Zn-based HZSM-5 catalysts

This study presents the catalytic pyrolysis of microalgae, Chlorella vulgaris (C. vulgaris), using pure CH4 and H2-rich gas evolved from CH4 decomposition on three different HZSM-5 catalysts loaded with Zn, Ga, and Pt, aimed specifically at producing high-value mono-aromatics such as benzene, toluene, ethylbenzene, and xylene (BTEX). In comparison with that for the typical inert N2 environment, a pure CH4 environment increased the bio-oil yield from 32.4 wt% to 37.4 wt% probably due to hydrogen and methyl radical insertion in the bio-oil components. Furthermore, the addition of bimetals further increased bio-oil yield. For example, ZnPtHZ led to a bio-oil yield of 47.7 wt% in pure CH4. ZnGaHZ resulted in the maximum BTEX yield (6.68 wt%), which could be explained by CH4 activation, co-aromatization, and hydrodeoxygenation. The BTEX yield could be further increased to 7.62 wt% when pyrolysis was conducted in H2-rich gas evolved from CH4 decomposition over ZnGaHZ, as rates of aromatization and hydrodeoxygenation were relatively high under this condition. This study experimentally validated that the combination of ZnGaHZ and CH4 decomposition synergistically increases BTEX production using C. vulgaris.

Adventures in CH-Arylation Chemistry

AbstractThe present article provides a personalized account on CH-arylation reactions employed for the synthesis of heterocycles. The presence of a nitro group allowed for direct and regioselective CH-arylations of pyrazoles, imidazoles, indoles and a variety of purine analogues. Direct CH-arylations without the presence of an activating nitro-group were employed for inter- and intramolecular reactions of purine derivatives, which allowed for the synthesis of a great variety of polycyclic systems. Domino C–N coupling / hydroamination / CH-activation reactions of diarylacetylenes with anilines allowed for the synthesis of polycondensated N-heterocycles. Products include indolo- and azaindolo[1,2-f]phenanthridines, quinolino[3′,4′:4,5]pyrrolo[1,2-f]phenanthridines, pyrimido[5′,4′:4,5]pyrrolo[1,2-f]phenanthridines, and benzothieno[2′,3′:4,5]pyrrolo[1,2-f]phenanthridines. The reaction of N-heterocycles, such as indoles, with 1,1-difluoroalkenes resulted in a twofold addition-elimination reaction to give 1,1-diaminoalkenes, which were transformed by CH-arylation into various polycondensated heterocycles, such as indoloisoquinolines, thienoindolizines, oxepines and helicenes. Pyridofluoranthenes, diindenopyrene and azadiindenopyrenes were prepared by a combination of Pd-catalyzed cross-coupling reactions with acid-mediated cycloisomerizations and Pd-catalyzed intramolecular CH-arylations. Bis(carbazoles), benzodithiazoles, benzodithiophenes and 2,5-diarylpyrroles were prepared by inter- and intramolecular CH-arylation reactions.

Discovery of novel oridonin sulfamide derivatives as potent NLRP3 inhibitors by a visible-light photocatalysis-enabled peripheral editing

The progress of organicsyntheticmethod can promote late-stage lead compound modification and novel active compound discovery. Molecular editing technology in the field of organic synthesis, including peripheral and skeletal editing, facilitates rapid access to molecular diversity of a lead compound. Peripheral editing of CH bond activation is gradually used in lead optimization to afford novel active scaffolds and chemical space exploitation. To develop oridonin derivatives with high anti-inflammatory potency, novel oridonin sulfamides had been designed and synthesized by a scaffoldhopping strategy based on a visible-light photocatalysis peripheral editing. All novel compounds revealed measurable inhibition of IL-1β and low cytotoxicity in THP-1 cells. The docking study indicated that the best active compound ZM640 was accommodated in thebinding site of NLRP3 with two hydrogen bond interaction. These preliminary results confirm that α, β-unsaturated carbonyl of oridonin is not essential for NLRP3 inhibitory effect. This new oridonin scaffold has its potential to be further developed as a promising class of NLRP3 inhibitors.

Metastable nickel–oxygen species modulate rate oscillations during dry reforming of methane

AbstractWhen a heterogeneous catalyst is active, it forms metastable structures that constantly transform into each other. These structures contribute differently to the catalytic function. Here we show the role of different metastable oxygen species on a Ni catalyst during dry reforming of methane by combining environmental scanning electron microscopy, near ambient pressure X-ray photoelectron spectroscopy, on-line product detection and computer vision. We highlight the critical role of dissociative CO2 adsorption in regulating the oxygen content of the catalyst and in CH4 activation. We also discover rate oscillations during dry reforming of methane resulting from the sequential transformation of metastable oxygen species that exhibit different catalytic properties: atomic surface oxygen, subsurface oxygen and bulk NiOx. The imaging approach allowed the localization of fluctuating surface regions that correlated directly with catalytic activity. The study highlights the importance of metastability and operando analytics in catalysis science and provides impetus towards the design of catalytic systems.

Ca and Sr co-doping induced oxygen vacancies in 3DOM La2−xSrxCe2−yCayO7−δ catalysts for boosting low-temperature oxidative coupling of methane

It is urgent to develop catalysts with application potential for oxidative coupling of methane (OCM) at relatively lower temperature. Herein, three-dimensional ordered macroporous (3DOM) La2−xSrxCe2−yCayO7−δ (A2B2O7-type) catalysts with disordered defective cubic fluorite phased structure were successfully prepared by a colloidal crystal template method. 3DOM structure promotes the accessibility of the gaseous reactants (O2 and CH4) to the active sites. The co-doping of Ca and Sr ions in La2−xSrxCe2−yCayO7−δ catalysts improved the formation of oxygen vacancies, thereby leading to increased density of surface-active oxygen species (O2−) for the activation of CH4 and the formation of C2 products (C2H6 and C2H4). 3DOM La2−xSrxCe2−yCayO7−δ catalysts exhibit high catalytic activity for OCM at low temperature. 3DOM La1.7Sr0.3Ce1.7Ca0.3O7−δ catalyst with the highest density of O2− species exhibited the highest catalytic activity for low-temperature OCM, i.e., its CH4 conversion, selectivity and yield of C2 products at 650 °C are 32.2%, 66.1% and 21.3%, respectively. The mechanism was proposed that the increase in surface oxygen vacancies induced by the co-doping of Ca and Sr ions boosts the key step of CH bond breaking and CC bond coupling in catalyzing low-temperature OCM. It is meaningful for the development of the low-temperature and high-efficient catalysts for OCM reaction in practical application.

Unraveling the effect of particle size of active metals in Ni/MgO on methane activation and carbon growth mechanism.

For dry reforming of methane, the active metal particle size of the catalyst has a significant effect on both the reaction activity and the resistance to carbon deposition. In this study, nickel particles of different sizes (Ni13, Ni25, and Ni37) supported on the MgO(100) slab are used to study the mechanism of CH4 activation and carbon growth based on DFT theoretical calculations. According to the results, the energy of adsorption for reaction intermediates changes depending on the size of the active metal. The adsorption process of CH3, CH2, CH and C on Ni25/MgO has a maximum exothermic value. Furthermore, the energy barriers of CH4 four-step dehydrogenation are lowest on Ni25/MgO during the CH4 activation process. The growth process of carbon deposition on the catalysts is also investigated in this work. The results indicate that the growth of carbon from C5 to C6 is difficult to proceed on Ni13/MgO due to size and active site limitation. Additionally, with an increase in particle size of the active metal, the absolute value of growth energy and average carbon binding energy of Cn increase on both Ni25/MgO and Ni37/MgO. It is proved that smaller particle size presents better resistance to carbon deposition. In the studied size range, Ni25/MgO is demonstrated to have greater catalytic activity and better resistance to carbon deposition.

Cooperative activation of carbon-hydrogen bonds by heterobimetallic systems.

The direct activation of C-H bonds has been a rich and active field of organometallic chemistry for many years. Recently, incredible progress has been made and important mechanistic insights have accelerated research. In particular, the use of heterobimetallic complexes to heterolytically activate C-H bonds across the two metal centers has seen a recent surge in interest. This perspective article aims to orient the reader in this fast moving field, highlight recent progress, give design considerations for further research and provide an optimistic outlook on the future of catalytic C-H functionalization with heterobimetallic complexes.

Dual Pt atoms stabilized by an optimized coordination environment for propane dehydrogenation

Tailoring atomically dispersed catalysts by various methods has been the subject of intense current research in heterogeneous catalysis. In this work, the structural stability and propane dehydrogenation performance of Pt-TiO2 catalysts with single, dual, and triple Pt atoms doped on the reduced TiO2 surface have been examined. Experimental results indicate the atomically dispersed Pt is thermally very stable and shows exceptionally high activity (TOFpropane = 4.93 s−1) and selectivity (∼98.5 %) for the dehydrogenation reaction. The recently developed machine-learning-based SSW-NN method is then employed to explore the global potential energy surface of the atomically dispersed Pt atom-doped TiO2, which suggests that the single and dual Pt atoms can be tightly held by the vacancies created on the reduced transition-metal oxide, as confirmed by DFT + U calculated energetics. After that, advanced characterization techniques are used in conjunction with microkinetic analysis to establish the quantitative structure–activity relationships. It is found that the dual-Pt-atom structure with two Pt atoms embedded in adjacent Ti and O vacancies is the only active ensemble that dramatically promotes the CH bond activation and hydrogen recombination, as compared to single Pt atoms accommodated by either Ti or O vacancies. These results provide new and direct evidence in support of the idea that specific function can be imparted to specific active sites by tuning their coordination environments, which makes it possible to tailor the catalytic properties of metal oxides to a particular reaction such as propane dehydrogenation.