Carbon-Doped BN Nanosheets for the Oxidative Dehydrogenation of Ethylbenzene

Methanol conversion on borocarbonitride catalysts: Identification and quantification of active sites

Borocarbonitrides (BCNs) have emerged as highly selective catalysts for the oxidative dehydrogenation (ODH) reaction. However, there is a lack of in-depth understanding of the catalytic mechanism over BCN catalysts due to the complexity of the surface oxygen functional groups. Here, BCN nanotubes with multiple active sites are synthesized for oxygen-assisted methanol conversion reaction. The catalyst shows a notable activity improvement for methanol conversion (29%) with excellent selectivity to formaldehyde (54%). Kinetic measurements indicate that carboxylic acid groups on BCN are responsible for the formation of dimethyl ether, while the redox catalysis to formaldehyde occurs on both ketonic carbonyl and boron hydroxyl (B─OH) sites. The ODH reaction pathway on the B─OH site is further revealed by in situ infrared, x-ray absorption spectra, and density functional theory. The present work provides physical-chemical insights into the functional mechanism of BCN catalysts, paving the way for further development of the underexplored nonmetallic catalytic systems.

Relationship between the physicochemical properties of sludge-based carbons and the adsorption capacity of dissolved organic matter in advanced wastewater treatment: Effects of chemical conditioning

Pyrolysis carbonisation is a promising technology to convert organic waste into valuable carbon-based materials. However, sludge is generally highly compressible and difficult to dewater because of its high concentrations of biopolymers; the bound water of sludge is trapped in a network composed of biopolymers. Therefore, chemical conditioning is an indispensable step for improving sludge dewaterability performance. In the present work, the effects of different chemical conditioning agents (polymeric aluminium chloride (PACl), iron(III) chloride (FeCl₃), KMnO₄-Fe(II) and Fenton's reagent) on the physicochemical properties of sludge-based carbons (SBCs) were systematically studied and the SBCs were further used in advanced wastewater treatment. The adsorption mechanisms of dissolved organic matters (DOMs) by different SBCs were also investigated. The results showed that conditioning with KMnO₄-Fe(II) and Fenton's reagent improved the specific surface area of the SBCs, whereas inorganic salt flocculation conditioning reduced the porosity of the SBCs. In addition, we found that the Fenton-SBC and Mn/Fe-SBC performed better than the other investigated SBCs in the removal of organic compounds from secondary effluent and that the pseudo-second-order kinetic model could better describe the process of DOMs adsorption by all of the investigated SBCs. Moreover, three-dimensional fluorescence excitation-emission matrix spectroscopy in combination with an analysis of the physical and chemical fractionation of DOMs showed that all of the SBCs performed well in the adsorption of aromatic substances, hydrophobic acids and hydrophobic neutrals, whereas the Mn/Fe-SBC and Fenton-SBC performed better than the other SBCs in the removal of weakly hydrophobic acids.

B-MWW Zeolite: The Case Against Single-Site Catalysis

Boron-containing materials have recently been identified as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins. It has previously been demonstrated by several spectroscopic characterization techniques that the surface of these boron-containing ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B₂ (OH)_x O_(3-x/2) (x=0-6) layer. Yet, the precise nature of the active site(s) remains elusive. In this Communication, we provide a detailed characterization of zeolite MCM-22 isomorphously substituted with boron (B-MWW). Using ¹¹ B solid-state NMR spectroscopy, we show that the majority of boron species in B-MWW exist as isolated BO₃ units, fully incorporated into the zeolite framework. However, this material shows no catalytic activity for ODH of propane to propene. The catalytic inactivity of B-MWW for ODH of propane falsifies the hypothesis that site-isolated BO₃ units are the active site in boron-based catalysts. This observation is at odds with other traditionally studied catalysts like vanadium-based catalysts and provides an important piece of the mechanistic puzzle.

Nanoscale boron carbonitride semiconductors for photoredox catalysis

The conversion of solar energy to chemical energy achieved by photocatalysts comprising homogeneous transition-metal based systems, organic dyes, or semiconductors has received significant attention in recent years. Among these photocatalysts, boron carbon nitride (BCN) materials, as an emerging class of metal-free heterogeneous semiconductors, have extended the scope of photocatalysts due to their good performance and Earth abundance. The combination of boron (B), carbon (C), and nitrogen (N) constitutes a ternary system with large surface area and abundant activity sites, which together contribute to the good performance for reduction reactions, oxidation reactions and orchestrated both reduction and oxidation reactions. This Minireview reports the methods for the synthesis of nanoscale hexagonal boron carbonitride (h-BCN) and describes the latest advances in the application of h-BCN materials as semiconductor photocatalysts for sustainable photosynthesis, such as water splitting, reduction of CO₂, acceptorless dehydrogenation, oxidation of sp³ C-H bonds, and sp² C-H functionalization. h-BCN materials may have potential for applications in other organic transformations and industrial manufacture in the future.

A Mechanochemical-Assisted Synthesis of Boron, Nitrogen Co-Doped Porous Carbons as Metal-Free Catalysts

A green and convenient solid-state method assisted by mechanical energy is employed for the synthesis of boron (B) and nitrogen (N) co-doped porous carbons (B,N-Cs). Glutamic acid (Glu) and boric acid (H₃ BO₃ ) are used as the N-containing carbon precursor and boron source, respectively. This method is easy to perform and proved to be efficient towards co-doping B and N into the carbon matrix with high contents of B (7 atom %) and N (10 atom %). By adjusting the molar ratio of H₃ BO₃ to Glu, the surface chemical states of B and N could be readily modulated. When increasing H₃ BO₃ dosage, the pore size of B,N-Cs could be tuned ranging from micropores to mesopores with a Brunauer-Emmett-Teller (BET) surface area up to 940 m²  g^-1 . Finally, the B,N-Cs were applied as metal-free catalysts for the cycloaddition of CO₂ to epoxides, which outperform the N-doped carbon catalyst (NC-900) and the physically mixed catalyst of NC-900/B₄ C. The enhanced activity is attributed to the cooperative effect between B and N sites. X-ray photoelectron spectroscopy (XPS) analysis reveals that BN₃ in the B,N-Cs serves as a critical active site for the cooperative catalysis.

Molten salt assisted assembly growth of atomically thin boron carbon nitride nanosheets for photocatalytic H2 evolution

Atomically thin boron carbon nitride nanosheets (BCNNS) with high quality and large lateral size are synthesized via a novel molten salt assisted assembly growth strategy. The BCNNS show enhanced photocatalytic H₂ evolution activity in visible light.

Nonmetallic boron nitride embedded graphitic carbon catalyst for oxidative dehydrogenation of ethylbenzene

Nonmetallic BN embedded graphitic carbon hybrid had abundant carbonyl groups as the active site and enriched BO species as the oxygen adsorption sites, exhibiting a high catalytic performance for oxidative dehydrogenation of ethylbenzene to styrene with high yield.

An Organic Molecular Photocatalyst Releasing Oxygen from Water

Artificial photosynthesis employing solar energy is one of the best means of reaching a sustainable energy cycle, in which water is oxidized to oxygen and supplies electrons for fuel generation. The development of suitable oxygen evolution materials driven by sunlight is hence the most challenging research topic in the field of photocatalysis. Herein, photocatalytic O₂ production from water at a rate of 22.6 μmol h^-1 was performed by using a pyrene-based organic molecule constructed through the Ullmann coupling reaction. Its significantly improved light-harvesting ability and notably accelerated separation and transfer of photogenerated charges enhanced the O₂ evolution efficiency. This study highlights the structural design of organic molecules applied to photocatalytic water splitting.

Removal of organic dye by biomass-based iron carbide composite with an improved stability and efficiency

The efficiency of zero-valent iron (Fe⁰) for the degradation of contaminants in water or soil can be highly reduced by side reactions with oxygen or water. This work was conducted to test whether this drawback can be effectively suppressed by the carbonation of Fe⁰ with pyrolyzed biomass, which forms a Fe₃C composite. The composite Fe₃C was characterized and its reactivity and stability were assessed in batch tests with methyl orange (MO) as a model pollutant. The results indicated that the removal rate of MO on Fe₃C composite was higher than that of Fe⁰ (7.587 mg/(g·min) vs. 4.306 mg/(g·min)) at pH 4, where the degradation mechanism was confirmed by high-performance liquid chromatography-mass spectrometry. More importantly, the produced iron oxide in the Fe₃C composite was highly suppressed. Regeneration studies showed that after three times of cycling, the removal efficiency of MO on Fe₃C composite was kept to 99.42%, but Fe⁰ almost lost its reactivity. In situ chemical reduction of a colorimetric redox probe (indigo-5, 5'-disulfonate, I2S) quantitatively demonstrated that Fe₃C composite has the reduction kinetics of I2S obviously slower than Fe⁰, indicating that Fe₃C composite improved the stability of incorporated Fe⁰ to resist the side oxidation.

Propane oxidative dehydrogenation over highly selective hexagonal boron nitride catalysts: The role of oxidative coupling of methyl

Hexagonal boron nitride (h-BN) catalyst has recently been reported to be highly selective in oxidative dehydrogenation of propane (ODHP) for olefin production. In addition to propene, ethylene also forms with much higher overall selectivities to C2-products than to C1-products. In this work, we report that the reaction pathways over the h-BN catalyst are different from the V-based catalysts in ODHP. Oxidative coupling reaction of methyl, an intermediate from the cleavage of C─C bond of propane, contributes to the high selectivities to C2-products, leading to more C2-products than C1-products over the h-BN catalyst. This work not only provides insight into the reaction mechanisms involved in ODHP over the boron-based catalysts but also sheds light on the selective oxidation of alkanes such as direct upgrading of methane via oxidative upgrading to ethylene or CH _x O _y on boron-based catalysts.

Doping Nanoscale Graphene Domains Improves Magnetism in Hexagonal Boron Nitride

Carbon doping can induce unique and interesting physical properties in hexagonal boron nitride (h-BN). Typically, isolated carbon atoms are doped into h-BN. Herein, however, the insertion of nanometer-scale graphene quantum dots (GQDs) is demonstrated as whole units into h-BN sheets to form h-CBN. The h-CBN is prepared by using GQDs as seed nucleations for the epitaxial growth of h-BN along the edges of GQDs without the assistance of metal catalysts. The resulting h-CBN sheets possess a uniform distrubution of GQDs in plane and a high porosity macroscopically. The h-CBN tends to form in small triangular sheets which suggests an enhanced crystallinity compared to the h-BN synthesized under the same conditions without GQDs. An enhanced ferromagnetism in the h-CBN emerges due to the spin polarization and charge asymmetry resulting from the high density of CN and CB bonds at the boundary between the GQDs and the h-BN domains. The saturation magnetic moment of h-CBN reaches 0.033 emu g^-1 at 300 K, which is three times that of as-prepared single carbon-doped h-BN.

BN Pairs Enriched Defective Carbon Nanosheets for Ammonia Synthesis with High Efficiency

Electrochemical synthesis has garnered attention as a promising alternative to the traditional Haber-Bosch process to enable the generation of ammonia (NH₃ ) under ambient conditions. Current electrocatalysts for the nitrogen reduction reaction (NRR) to produce NH₃ are comprised of noble metals or transitional metals. Here, an efficient metal-free catalyst (BCN) is demonstrated without precious component and can be easily fabricated by pyrolysis of organic precursor. Both theoretical calculations and experiments confirm that the doped BN pairs are the active triggers and the edge carbon atoms near to BN pairs are the active sites toward the NRR. This doping strategy can provide sufficient active sites while retarding the competing hydrogen evolution reaction (HER) process; thus, NRR with high NH₃ formation rate (7.75 µg h^-1 mg_cat. ^-1 ) and excellent Faradaic efficiency (13.79%) are achieved at -0.3 V versus reversible hydrogen electrode (RHE), exceeding the performance of most of the metallic catalysts.

A comparative study of nitrobenzene reduction using model catalysts

A zigzag-type quinone plays an important role in the reduction of nitrobenzene even in the co-existence of other functional groups.

Effects of ball-milling treatment on physicochemical properties and solid base activity of hexagonal boron nitrides

Hexagonal boron nitride solid base catalysts were prepared by simple ball-milling at various rotation speeds of a commercial low-surface area boron nitride.

Structure-Mediated Charge Separation in Boron Carbon Nitride for Enhanced Photocatalytic Oxidation of Alcohol

Boron carbon nitride (BCN) is a promising earth-abundant photocatalyst for solar energy conversion. However, the photocatalytic activities of BCN materials remain moderate because of the fast electron-hole recombination. Herein, an ordered BCN structure is fabricated by a facile one-step thermal treatment strategy. The ordered structure of BCN is directly evident from powder X-ray diffraction and high-resolution transmission electron microscopy. Importantly, it is found that the long-period ordered structure can intrinsically accelerate the separation and transfer kinetics of photogenerated charge carriers. Benefiting from these advantages, the ordered BCN structure exhibits remarkable performance for photoinduced selective oxidation of benzyl alcohol compared with the pristine BCN counterpart. This work highlights the important role of the crystal structure of light-harvesting materials in affecting electron-hole separation and at the same time points to the ample potential for improving the photocatalytic performance.

Serendipity in Catalysis Research: Boron-Based Materials for Alkane Oxidative Dehydrogenation

Light olefins such as ethylene and propylene form the foundation of the modern chemical industry, with yearly production volumes well into the hundreds of millions of metric tons. Currently, these light olefins are mainly produced via energy-intensive steam cracking. Alternatively, oxidative dehydrogenation (ODH) of light alkanes to produce olefins allows for lower operation temperatures and extended catalyst lifetimes, potentially leading to valuable process efficiencies. The potential benefits of this route have led to significant research interest due to the wide availability of natural gas from shale deposits. Advances in this area have still not yielded catalysts that are sufficiently selective to olefins for industrial implementation, and ODH still remains a holy grail of selective alkane oxidation research. The main challenge in selective oxidation lies in preventing the overoxidation of the desired product, such as propylene during propane oxidation, to CO and CO₂. Research into selective heterogeneous catalysts for the oxidative dehydrogenation of propane has led to the extensive use of vanadium oxide-based catalysts, and studies on the surface mechanism involved have been used to improve the catalytic activity of the material. Despite decades of research, however, selectivity toward propylene has not proven satisfactory at industrially relevant conversions. It is imperative for new catalytic systems that minimize product overoxidation to be developed for future applications of oxidative dehydrogenation processes. While rational catalyst design has been successful in developing homogeneous catalyst systems, its practical use in heterogeneous catalyst development remains modest. The complexity of surfaces with a variety of terminations and bulk structures, let alone their modification by the chemical potential of a reaction mixture, makes heterogeneous catalyst discovery serendipitous in many cases. The catalyst family presented in this Account is no exception. The importance of catalysis research lies in exploring the science behind serendipity. In this Account, we will first present our initial discovery of boron nitride (BN) as an unexpected catalyst for the oxidative dehydrogenation of light alkanes. Beyond its surprising activity, BN also drew interest due to its low selectivity to carbon oxides. This observation made BN distinct from previously studied metal oxide catalysts for selective alkane oxidation. We narrowed down its unique reactivity to the oxygen functionalization of the catalyst surface, particularly the formation of B-O species as probed by various spectroscopic techniques. In investigating the critical role of each of the structural elements during ODH, we discovered that not only BN but an entire class of boron-containing compounds are active and selective for the formation of propylene from propane. All these materials form a complex oxidized surface with a distribution of BO _x surface sites. This discovery opens the doors to a new field of boron-based oxidation chemistry that currently has more questions than answers. We aim to make this Account a starting point for the research community to explore these new materials to understand their surface mechanisms and the surface species that offer a unique selectivity toward olefinic products. Effective use of these materials may lead to novel processes for efficient use of abundant light alkane resources by oxidation chemistry.

A "waiting" carbon nitride radical anion: a charge storage material and key intermediate in direct C-H thiolation of methylarenes using elemental sulfur as the "S"-source

Potassium poly(heptazine imide), a carbon nitride semiconductor, in the presence of hole scavengers and visible light gives stable radical anion with the specific density of unpaired electrons reaching 112 mmol g^–1.

Potassium poly(heptazine imide), a carbon nitride based semiconductor with high structural order and a valence band potential of +2.2 V vs. NHE, in the presence of hole scavengers and under visible light irradiation gives the corresponding polymeric radical anion, in which the specific density of unpaired electrons reaches 112 μmol g^–1. The obtained polymeric radical anion is stable under anaerobic conditions for several hours. It was characterized using UV-vis absorption, time resolved and steady state photoluminescence spectra. The electronic structure of the polymeric radical anion was confirmed by DFT cluster modelling. The unique properties of potassium poly(heptazine imide) for storing charges were employed in visible light photocatalysis. A series of substituted dibenzyldisulfanes was synthesized in 41–67% yield from the corresponding methylarenes via cleavage of the methyl C–H bond under visible light irradiation and metal-free conditions.

Gas-exfoliated porous monolayer boron nitride for enhanced aerobic oxidative desulfurization performance

Hexagonal boron nitride has been regarded to be an efficient catalyst in aerobic oxidation fields, but limited by the less-exposed active sites. In this contribution, we proposed a simple green liquid nitrogen gas exfoliation strategy for preparation of porous monolayer nanosheets (BN-1). Owing to the reduced layer numbers, decreased lateral sizes and artificially-constructed pores, increased exposure of active sites was expected, further contributed to an enhanced aerobic oxidative desulfurization (ODS) performance up to ∼98% of sulfur removal, achieving ultra-deep desulfurization. This work not only introduced an excellent catalyst for aerobic ODS, but also provided a strategy for construction of some other highly-efficient monolayer two-dimensional materials for enhanced catalytic performance.

Phosphor-doped hexagonal boron nitride nanosheets as effective acid–base bifunctional catalysts for one-pot deacetalization–Knoevenagel cascade reactions

Phosphor doping creates well-defined Brønsted acid and base sites on h-BN nanosheets, ensuring the smooth proceeding of deacetalization–Knoevenagel cascade reaction.

Photocatalytic CO2 conversion by polymeric carbon nitrides

CO₂ is a vital compound for life, and its concentration significantly affects the living environment of the Earth. By mimicking nature photosynthesis, we herein discusses the uses of polymeric carbon nitrides to balance CO₂ concentration by artificial photocatalysis.

Polymeric carbon nitride nanomesh as an efficient and durable metal-free catalyst for oxidative desulfurization

Polymeric carbon nitride nanomesh has been prepared and demonstrated to be a novel and outstanding catalyst for H₂S selective oxidation.

Progress in selective oxidative dehydrogenation of light alkanes to olefins promoted by boron nitride catalysts

We highlight recent progress on a newly-developed catalyst system, boron nitride, for selective oxidative dehydrogenation of light alkanes.

Tailoring N-Terminated Defective Edges of Porous Boron Nitride for Enhanced Aerobic Catalysis

Tailoring terminated edge of hexagonal boron nitride (h-BN) for enhancing catalysis has turned to be an imperative for the rational design of a highly active aerobic catalyst. Herein, a tailoring N-terminated porous BN (P-BN) strategy is reported with a zinc (Zn) salt as a dual-functional template. The Zn salt acts as both an in situ template and an N-terminated defective edges directing agent. The zinc salt template turns to Zn nanoparticles (Zn NPs) and functions as physical spacers, which are subsequently removed at a higher temperature, for producing P-BN, whose high surface area is high to 1579 m² g^-1 . Moreover, because of the lower electronegativity of Zn, boron (B) atoms are partly replaced by Zn atoms and ultimately preferred to N-terminated edges with the volatilization of Zn NPs. Owing to the moderate dissociative energy of oxygen atoms on N-terminated edges, the N-terminated edges are proved to be the origin of an enhanced aerobic catalytic activity by density functional theory (DFT) calculations. Moreover, the DFT calculation result is experimentally verified.