Unraveling Radical and Oxygenate Routes in the Oxidative Dehydrogenation of Propane over Boron Nitride

In Situ Formation of B═O Bond in Oxidative Dehydrogenation of Propane and as a Hunter for Alkoxyl Radicals: A Computational Study

B═O multiple bonds are fundamentally important owing to the unique property of B and its potential as a tool in catalysis. Herein by means of DFT calculations, we investigated the in situ generation of transient -B═O species in the nonmetallic inorganic boron oxides and demonstrated its superior ability to capture alkoxyl radicals under the conditions of oxidative dehydrogenation of propane (ODHP). Boron-containing materials are emerging as promising catalysts for ODHP, while an extensive understanding of the underlying mechanisms remains challenging. Through systematic mechanistic and characterization calculations, we show the feasibility for the presence of -B═O species under ODHP conditions and propose that its π nature dictates its inertness in the C-H activation of propane (ΔG⧧ = 57 kcal/mol) but offers its strong ability in capturing alkoxyl radicals (barrierless with ΔG = -30 kcal/mol), which can then transform to the ·C3H6OH radical. This explains the observation of enol in the experimental study. This specific behavior of the -B═O species makes it one of the key players in the complex reaction network of ODHP and also implicates the rational design of scavengers for reactive oxygen species (ROS) and other active oxygenated species.

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.

Laccase Immobilized on Arginine-Functionalized Boron Nitride Nanosheets for Enhanced Atrazine Degradation

Enzyme-mediated systems have been widely employed for the biotransformation of environmental contaminants. However, the catalytic performance of free enzymes is restricted by the rapid loss of their catalytic activity, stability, and reusability. In this work, we developed an enzyme immobilization platform by elaborately anchoring fungal laccase onto arginine-functionalized boron nitride nanosheets (BNNS-Arg@Lac). BNNS-Arg@Lac showcased ∼75% immobilization yield and enhanced stability against fluctuating pH values and temperatures, along with remarkable reusability across six consecutive cycles, outperforming free natural laccase (nlaccase). A model pollutant, atrazine, was selected for a proof-of-concept demonstration, given the substantial environmental and public health concerns in agriculture runoff. BNNS-Arg@Lac-catalyzed atrazine degradation rate was nearly twice that of nlaccase. Moreover, BNNS-Arg@Lac consistently demonstrated superior atrazine degradation in synthetic agricultural wastewater and various mediator systems compared to nlaccase. Comprehensive product analysis unraveled distinct degradation pathways for BNNS-Arg@Lac and nlaccase. Overall, this research provides a foundation for the future development of enzyme-nanomaterial hybrids for degrading environmental chemicals and may unlock new potential for green and efficient resource recovery and waste management strategies.

Mechanochemically-derived iron atoms on defective boron nitride for stable propylene production

Single-atom catalysts (SACs), possessing a uniform metal site structure, are a promising class of materials for selective oxidations of hydrocarbons. However, their design for targeted applications requires careful choice of metal-host combinations and suitable synthetic techniques. Here, we report iron atoms stabilised on defective hexagonal boron nitride (h-BN) via mechanochemical activation in a ball mill as an effective catalyst for propylene production via N2O-mediated oxidative propane dehydrogenation (N2O-ODHP), reaching 95% selectivity at 6% propane conversion and maintaining stable performance for 40 h on stream. This solvent-free synthesis allows simultaneous carrier exfoliation and surface defect generation, creating anchoring sites for catalytically-active iron atoms. The incorporation of a small metal quantity (0.5 wt%) predominantly generates a mix of atomically-dispersed Fe2+ and Fe3+ species, as confirmed by combining advanced microscopy and electron paramagnetic resonance, UV-vis and X-ray photoelectron spectroscopy analyses. Single-atom iron favours selective propylene formation, while metal oxide nanoparticles yield large quantities of CO x and cracking by-products. The lack of acidic functionalities on h-BN, hindering coke formation, and firm stabilisation of Fe sites, preventing metal sintering, ensure stable operation. These findings showcase N2O-ODHP as a promising propylene production technology and foster wider adoption of mechanochemical activation as a viable method for SACs synthesis.

A solution for 4-propylguaiacol hydrodeoxygenation without ring saturation

We investigate solvent effects in the hydrodeoxygenation of 4-propylguaiacol (4PG, 166 amu), a key lignin-derived monomer, over Ru/C catalyst by combined operando synchrotron photoelectron photoion coincidence (PEPICO) spectroscopy and molecular dynamics simulations. With and without isooctane co-feeding, ring-hydrogenated 2-methoxy-4-propylcyclohexanol (172 amu) is the first product, due to the favorable flat adsorption configuration of 4PG on the catalyst surface. In contrast, tetrahydrofuran (THF)-a polar aprotic solvent that is representative of those used for lignin solubilization and upgrading-strongly coordinates to the catalyst surface at the oxygen atom. This induces a local steric hindrance, blocking the flat adsorption of 4PG more effectively, as it needs more Ru sites than the tilted adsorption configuration revealed by molecular dynamics simulations. Therefore, THF suppresses benzene ring hydrogenation, favoring a demethoxylation route that yields 4-propylphenol (136 amu), followed by dehydroxylation to propylbenzene (120 amu). Solvent selection may provide new avenues for controlling catalytic selectivity.

Modulating Electron Density of Boron-Oxygen Groups in Borate via Metal Electronegativity for Propane Oxidative Dehydrogenation

The robust electronegativity of the [BO3]3- structure enables the extraction of electrons from adjacent metals, offering a strategy for modulating oxygen activation in propane oxidative dehydrogenation. Metals (Ni 1.91, Al 1.5, and Ca 1.0) with varying electronegativities were employed to engineer borate catalysts. Metals in borate lacked intrinsic catalytic activity for propane conversion; instead, they modulated [BO3]3- group reactivity through adjustments in electron density. Moderate metal electronegativity favored propane oxidative dehydrogenation to propylene, whereas excessively low electronegativity led to propane overoxidation to carbon dioxide. Aluminum, with moderate electronegativity, demonstrated optimal performance. Catalyst AlBOx-1000 achieved a propane conversion of 47.5%, with the highest propylene yield of 30.89% at 550 °C, and a total olefin yield of 51.51% with a 58.92% propane conversion at 575 °C. Furthermore, the stable borate structure prevents boron element loss in harsh conditions and holds promise for industrial-scale catalysis.

CO Cofeeding Affects Product Distribution in CH3Cl Coupling over ZSM-5 Zeolite: Pressure Twists the Plot

C1 coupling reactions over zeolite catalysts are central to sustainable chemical production strategies. However, questions persist regarding the involvement of CO in ketene formation, and the impact of this elusive oxygenate intermediate on reactivity patterns. Using operando photoelectron photoion coincidence spectroscopy (PEPICO), we investigate the role of CO in methyl chloride conversion to hydrocarbons (MCTH), a prospective process for methane valorization with a reaction network akin to methanol to hydrocarbons (MTH) but without oxygenate intermediates. Our findings reveal the transformative role of CO in MCTH at the low pressures, inducing ketene formation in MCTH and boosting olefin production, confirming the Koch carbonylation step in the initial stages of C1 coupling. We uncover pressure-dependent product distributions driven by CO-induced ketene formation, and its subsequent desorption from the zeolite surface, which is enhanced at low pressure. Inspired by the above results, extension of the co-feeding approach to CH3OH as another simple oxygenate showcases the additional potential for improved catalyst stability in MCTH at ambient pressure.

Efficient conversion of propane in a microchannel reactor at ambient conditions

The oxidative dehydrogenation of propane, primarily sourced from shale gas, holds promise in meeting the surging global demand for propylene. However, this process necessitates high operating temperatures, which amplifies safety concerns in its application due to the use of mixed propane and oxygen. Moreover, these elevated temperatures may heighten the risk of overoxidation, leading to carbon dioxide formation. Here we introduce a microchannel reaction system designed for the oxidative dehydrogenation of propane within an aqueous environment, enabling highly selective and active propylene production at room temperature and ambient pressure with mitigated safety risks. A propylene selectivity of over 92% and production rate of 19.57 mmol mCu-2 h-1 are simultaneously achieved. This exceptional performance stems from the in situ creation of a highly active, oxygen-containing Cu catalytic surface for propane activation, and the enhanced propane transfer via an enlarged gas-liquid interfacial area and a reduced diffusion path by establishing a gas-liquid Taylor flow using a custom-made T-junction microdevice. This microchannel reaction system offers an appealing approach to accelerate gas-liquid-solid reactions limited by the solubility of gaseous reactant.

Tracking Active Phase Behavior on Boron Nitride during the Oxidative Dehydrogenation of Propane Using Operando X-ray Raman Spectroscopy

Hexagonal boron nitride (hBN) is a highly selective catalyst for the oxidative dehydrogenation of propane (ODHP) to propylene. Using a variety of ex situ characterization techniques, the activity of the catalyst has been attributed to the formation of an amorphous boron oxyhydroxide surface layer. The ODHP reaction mechanism proceeds via a combination of surface mediated and gas phase propagated radical reactions with the relative importance of both depending on the surface-to-void-volume ratio. Here we demonstrate the unique capability of operando X-ray Raman spectroscopy (XRS) to investigate the oxyfunctionalization of the catalyst under reaction conditions (1 mm outer diameter reactor, 500 to 550 °C, P = 30 kPa C3H8, 15 kPa O2, 56 kPa He). We probe the effect of a water cofeed on the surface of the activated catalyst and find that water removes boron oxyhydroxide from the surface, resulting in a lower reaction rate when the surface reaction dominates and an enhanced reaction rate when the gas phase contribution dominates. Computational description of the surface transformations at an atomic-level combined with high precision XRS spectra simulations with the OCEAN code rationalize the experimental observations. This work establishes XRS as a powerful technique for the investigation of light element-containing catalysts under working conditions.

Rationalizing kinetic behaviors of isolated boron sites catalyzed oxidative dehydrogenation of propane

Boron-based catalysts exhibit high alkene selectivity in oxidative dehydrogenation of propane (ODHP) but the mechanistic understanding remains incomplete. In this work, we show that the hydroxylation of framework boron species via steaming not only enhances the ODHP rate on both B-MFI and B-BEA, but also impacts the kinetics of the reaction. The altered activity, propane reaction order and the activation energy could be attributed to the hydrolysis of framework [B(OSi≡)3] unit to [B(OSi≡)3-x(OH···O(H)Si≡)x] (x = 1, 2, "···" represents hydrogen bonding). DFT calculations confirm that hydroxylated framework boron sites could stabilize radical species, e.g., hydroperoxyl radical, further facilitating the gas-phase radical mechanism. Variations in the contributions from gas-phase radical mechanisms in ODHP lead to the linear correlation between activation enthalpy and entropy on borosilicate zeolites. Insights gained in this work offer a general mechanistic framework to rationalize the kinetic behavior of the ODHP on boron-based catalysts.

Photoion Mass-Selected Threshold Photoelectron Spectroscopy to Detect Reactive Intermediates in Catalysis: From Instrumentation and Examples to Peculiarities and a Database

Photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) is a synchrotron-based, universal, sensitive, and multiplexed detection tool applied in the areas of catalysis, combustion, and gas-phase reactions. Isomer-selective vibrational fingerprints in the ms-TPES of stable and reactive intermediates allow for unequivocal assignment of spectral carriers. Case studies are presented on heterogeneous catalysis, revealing the role of ketenes in the methanol-to-olefins process, the catalytic pyrolysis mechanism of lignin model compounds, and the radical chemistry upon C-H activation in oxyhalogenation. These studies demonstrate the potential of ms-TPES as an analytical technique for elucidating complex reaction mechanisms. We examine the robustness of ms-TPES assignments and address sampling effects, especially the temperature dependence of ms-TPES due to rovibrational broadening. Data acquisition approaches and the Stark shift from the extraction field are also considered to arrive at general recommendations. Finally, the PhotoElectron PhotoIon Spectral Compendium (https://pepisco.psi.ch), a spectral database hosted at Paul Scherrer Institute to support assignment, is introduced.

Off-Stoichiometric Restructuring and Sliding Dynamics of Hexagonal Boron Nitride Edges in Conditions of Oxidative Dehydrogenation of Propane

Boron-containing materials, such as hexagonal boron nitride (h-BN), recently shown to be active and selective catalysts for the oxidative dehydrogenation of propane (ODHP), have been shown to undergo significant surface oxyfunctionalization and restructuring. Although experimental ex situ studies have probed the change in chemical environment on the surface, the structural evolution of it under varying reaction conditions has not been established. Herein, we perform global optimization structure search with a grand canonical genetic algorithm to explore the chemical space of off-stoichiometric restructuring of the h-BN surface under ambient as well as ODHP-relevant conditions. A grand canonical ensemble representation of the surface is established, and the predicted 11B solid-state NMR spectra are consistent with previous experimental reports. In addition, we investigated the relative sliding of h-BN sheets and how it influences the surface chemistry with ab initio molecular dynamics simulations. The B-O linkages on the edges are found to be significantly strained during the sliding, causing the metastable sliding configurations to have higher reactivity toward the activation of propane and water.

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.