How to Tailor Porous Boron Nitride Properties for Applications in Interfacial Processes

The research of new porous materials for applications in interfacial processes is key to addressing global energy and sustainability challenges. For example, porous materials can be used to store fuels such as hydrogen or methane or to separate chemical mixtures reducing the energy currently required by thermal separation processes. Their catalytic properties can be exploited to convert adsorbed molecules into valuable or less hazardous chemicals, thereby reducing energy consumption or pollutants emissions. Porous boron nitride (BN) has appeared as a promising material for applications in molecular separations, gas storage, and catalysis owing to its high surface area and thermal stability, as well as its tunable physical properties and chemistry. However, the production of porous BN is still limited to the laboratory scale, and its formation mechanism, as well as ways to control porosity and chemistry, are yet to be fully understood. In addition, studies have pointed toward the instability of porous BN materials when exposed to humidity, which could significantly impact performance in industrial applications. Studies on porous BN performance and recyclability when employed in adsorption, gas storage, and catalysis remain limited, despite encouraging preliminary studies. Moreover, porous BN powder must be shaped into macrostructures (e.g., pellets) to be used commercially. However, common methods to shape porous materials into macrostructures often cause a reduction in the surface area and/or mechanical strength. In recent years, research groups, including ours, have started addressing the challenges discussed above. Herein, we summarize our collective findings through a selection of key studies. First, we discuss the chemistry and structure of BN, clarifying confusion around terminology and discussing the hydrolytic instability of the material in relation to its structure and chemistry. We demonstrate a way to reduce the instability in water while still maintaining high specific surface area. We propose a mechanism for the formation of porous BN and discuss the effects of different synthesis parameters on the structure and chemistry of porous BN, therefore providing a way to tune its properties for selected applications. While the syntheses covered often lead to a powder product, we also present ways to shape porous BN powders into macrostructures while still maintaining high accessible surface area for interfacial processes. Finally, we evaluate porous BN performance for chemical separations, gas storage, and catalysis. While the above highlights key advances in the field, further work is needed to allow deployment of porous BN. Specifically, we suggest evaluating its hydrolytic stability, refining the ways to shape the material into stable and reproducible macrostructures, establishing clear design rules to produce BN with specific chemistry and porosity, and, finally, providing standardized test procedures to evaluate porous BN catalytic and sorptive properties to facilitate comparison.

Combining Soft Polysilazanes with Melt-Shear Organization of Core-Shell Particles: On the Road to Polymer-Templated Porous Ceramics

The preparation of ordered macroporous SiCN ceramics has attracted significant interest and is an attractive area for various applications, e.g., in the fields of catalysis, gas adsorption, or membranes. Non-oxidic ceramics, such as SiCN, own a great stability based on the covalent bonds between the containing elements, which leads to interesting properties concerning resistance and stability at high temperature. Their peculiar properties have become more and more important for a manifold of applications, like catalysis or separation processes, at high temperatures. Within this work, a feasible approach for the preparation of ordered porous materials by taking advantage of polymer-derived ceramics is presented. To gain access to free-standing films consisting of porous ceramic materials, the combination of monodisperse organic polymer-based colloids with diameters of 130 nm and 180 nm featuring a processable preceramic polymer is essential. For this purpose, the tailored design of hybrid organic/inorganic particles featuring anchoring sites for a preceramic polymer in the soft shell material is developed. Moreover, polymer-based core particles are used as sacrificial template for the generation of pores, while the preceramic shell polymer can be converted to the ceramic matrix after thermal treatment. Two different routes for the polymer particles, which can be obtained by emulsion polymerization, are followed for covalently linking the preceramic polysilazane Durazane1800 (Merck, Germany): (i) Free radical polymerization and (ii) atom transfer radical polymerization (ATRP) conditions. These hybrid hard core/soft shell particles can be processed via the so-called melt-shear organization for the one-step preparation of free-standing particle films. A major advantage of this technique is the absence of any solvent or dispersion medium, enabling the core particles to merge into ordered particle stacks based on the soft preceramic shell. Subsequent ceramization of the colloidal crystal films leads to core particle degradation and transformation into porous ceramics with ceramic yields of 18–54%.

Pulsed UV Laser Processing of Carbosilane and Silazane Polymers

Freestanding SiCNO ceramic pieces with sub-mm features were produced by laser crosslinking of carbosilane and silazane polymer precursors followed by pyrolysis in inert atmosphere. Three different pulsed UV laser systems were investigated, and the influence of laser wavelength, operating power and scanning speed were all found to be important. Different photoinitiators were tested for the two lasers operating at 355 nm, while for the 266 nm laser, crosslinking occurred also without photoinitiator. Pre-treatment of glass substrates with fluorinated silanes was found to ease the release of green bodies during solvent development. Polymer crosslinking was observed with all three of the laser systems, as were bubbles, surface charring and in some cases ablation. By focusing the laser beam several millimeters above the surface of the resin, selective polymer crosslinking was observed exclusively.

Molecular-Level Processing of Si-(B)-C Materials with Tailored Nano/Microstructures

The design of Si-(B)-C materials is investigated, with detailed insight into the precursor chemistry and processing, the precursor-to-ceramic transformation, and the ceramic microstructural evolution at high temperatures. In the early stage of the process, the reaction between allylhydridopolycarbosilane (AHPCS) and borane dimethyl sulfide is achieved. This is investigated in detail through solid-state NMR and FTIR spectroscopy and elemental analyses for Si/B ratios ranging from 200 to 30. Boron-based bridges linking AHPCS monomeric fragments act as crosslinking units, extending the processability range of AHPCS and suppressing the distillation of oligomeric fragments during the low-temperature pyrolysis regime. Polymers with low boron contents display appropriate requirements for facile processing in solution, leading to the design of monoliths with hierarchical porosity, significant pore volume, and high specific surface area after pyrolysis. Polymers with high boron contents are more appropriate for the preparation of dense ceramics through direct solid shaping and pyrolysis. We provide a comprehensive study of the thermal decomposition mechanisms, and a subsequent detailed study of the high-temperature behavior of the ceramics produced at 1000 °C. The nanostructure and microstructure of the final SiC-based ceramics are intimately linked to the boron content of the polymers. B₄ C/C/SiC nanocomposites can be obtained from the polymer with the highest boron content.

Polymer-Derived Silicoboron Carbonitride Foams for CO2 Capture: From Design to Application as Scaffolds for the in Situ Growth of Metal-Organic Frameworks

A template-assisted polymer-derived ceramic route is investigated for preparing a series of silicoboron carbonitride (Si/B/C/N) foams with a hierarchical pore size distribution and tailorable interconnected porosity. A boron-modified polycarbosilazane was selected to impregnate monolithic silica and carbonaceous templates and form after pyrolysis and template removal Si/B/C/N foams. By changing the hard template nature and controlling the quantity of polymer to be impregnated, controlled micropore/macropore distributions with mesoscopic cell windows are generated. Specific surface areas from 29 to 239 m(2)  g(-1) and porosities from 51 to 77 % are achieved. These foams combine a low density with a thermal insulation and a relatively good thermostructural stability. Their particular structure allowed the in situ growth of metal-organic frameworks (MOFs) directly within the open-cell structure. MOFs offered a microporosity feature to the resulting Si/B/C/N@MOF composite foams that allowed increasing the specific surface area to provide CO2 uptake of 2.2 %.

Pure &crystallized 2D Boron Nitride sheets synthesized via a novel process coupling both PDCs and SPS methods

Within the context of emergent researches linked to graphene, it is well known that h-BN nanosheets (BNNSs), also referred as 2D BN, are considered as the best candidate for replacing SiO₂ as dielectric support or capping layers for graphene. As a consequence, the development of a novel alternative source for highly crystallized h-BN crystals, suitable for a further exfoliation, is a prime scientific issue. This paper proposes a promising approach to synthesize pure and well-crystallized h-BN flakes, which can be easily exfoliated into BNNSs. This new accessible production process represents a relevant alternative source of supply in response to the increasing need of high quality BNNSs. The synthesis strategy to prepare pure h-BN is based on a unique combination of the Polymer Derived Ceramics (PDCs) route with the Spark Plasma Sintering (SPS) process. Through a multi-scale chemical and structural investigation, it is clearly shown that obtained flakes are large (up to 30 μm), defect-free and well crystallized, which are key-characteristics for a subsequent exfoliation into relevant BNNSs.

Boron nitride ceramics from molecular precursors: synthesis, properties and applications

Hexagonal boron nitride (h-BN) attracts considerable interest particularly when it is prepared from borazine-based single-source precursors through chemical routes suitable for the shaping and the nanostructuration of the final ceramic.

Ceramic Nanocomposites from Tailor-Made Preceramic Polymers

The present Review addresses current developments related to polymer-derived ceramic nanocomposites (PDC-NCs). Different classes of preceramic polymers are briefly introduced and their conversion into ceramic materials with adjustable phase compositions and microstructures is presented. Emphasis is set on discussing the intimate relationship between the chemistry and structural architecture of the precursor and the structural features and properties of the resulting ceramic nanocomposites. Various structural and functional properties of silicon-containing ceramic nanocomposites as well as different preparative strategies to achieve nano-scaled PDC-NC-based ordered structures are highlighted, based on selected ceramic nanocomposite systems. Furthermore, prospective applications of the PDC-NCs such as high-temperature stable materials for thermal protection systems, membranes for hot gas separation purposes, materials for heterogeneous catalysis, nano-confinement materials for hydrogen storage applications as well as anode materials for secondary ion batteries are introduced and discussed in detail.

Monodisperse platinum nanoparticles supported on highly ordered mesoporous silicon nitride nanoblocks: superior catalytic activity for hydrogen generation from sodium borohydride

Synthesis and characterization of ordered mesoporous silicon nitride nanoblocks as efficient supports of platinum nanoparticles for the hydrolysis of sodium borohydride.

Polymer-Derived Boron Nitride: A Review on the Chemistry, Shaping and Ceramic Conversion of Borazine Derivatives

Boron nitride (BN) is a III-V compound which is the focus of important research since its discovery in the early 19th century. BN is electronic to carbon and thus, in the same way that carbon exists as graphite, BN exists in the hexagonal phase. The latter offers an unusual combination of properties that cannot be found in any other ceramics. However, these properties closely depend on the synthesis processes. This review states the recent developments in the preparation of BN through the chemistry, shaping and ceramic conversion of borazine derivatives. This concept denoted as Polymer-Derived Ceramics (PDCs) route allows tailoring the chemistry of precursors to elaborate complex BN shapes which cannot be obtained by conventional process. The effect of the chemistry of the molecular precursors, i.e., borazine and trichloroborazine, and their polymeric derivatives i.e., polyborazylene and poly[tri(methylamino)borazine], in which the specific functional groups and structural motifs determine the shaping potential by conventional liquid-phase process and plastic-forming techniques is discussed. Nanotubes, nano-fibers, coatings, monoliths and fiber-reinforced matrix composites are especially described. This leads to materials which are of significant engineering interest.