Nanoreactors based on hydrophobized tubular aluminosilicates decorated with ruthenium: Highly active and stable catalysts for aromatics hydrogenation

Architectural design of core-shell nanotube systems based on aluminosilicate clay

A nanoarchitectural approach to the design of functional nanomaterials based on natural aluminosilicate nanotubes and their catalysis, and practical applications are described in this paper. We focused on the buildup of hybrid core-shell systems with metallic or organic molecules encased in aluminosilicate walls, and nanotube templates for structured silica and zeolite preparation. The basis for such an architectural design is a unique Al₂O₃/SiO₂ dual chemistry of 50 nm diameter halloysite tubes. Their structure and site dependent properties are well combined with biocompatibility, environmental safety, and abundant availability, which makes the described functional systems scalable for industrial applications. In these organic/ceramic hetero systems, we outline drug, dye and chemical inhibitor loading inside the clay nanotubes, accomplished with their silane or amphiphile molecule surface modifications. For metal-ceramic tubule composites, we detailed the encapsulation of 2-5 nm Au, Ru, Pt, and Ag particles, Ni and Co oxides, NiMo, and quantum dots of CdZn sulfides into the lumens or their attachment at the outside surface. These metal-clay core-shell nanosystems show high catalytic efficiency with increased mechanical and temperature stabilities. The combination of halloysite nanotubes with mesoporous MCM-41 silica allowed for a synergetic enhancement of catalysis properties. Finally, we outlined the clay nanotubes' self-assembly into organized arrays with orientation and ordering similar to nematic liquid crystals, and these systems are applicable for life-related applications, such as petroleum spill bioremediation, antimicrobial protection, wound healing, and human hair coloring.

Halloysite Nanotubes: Smart Nanomaterials in Catalysis

The use of clay minerals as catalyst is renowned since ancient times. Among the different clays used for catalytic purposes, halloysite nanotubes (HNTs) represent valuable resources for industrial applications. This special tubular clay possesses high stability and biocompatibility, resistance against organic solvents, and most importantly be available in large amounts at a low cost. Therefore, HNTs can be efficiently used as catalysts themselves or supports for metal nanoparticles in several catalytic processes. This review reports a comprehensive overview of the relevant advances in the use of halloysite in catalysis, focusing the attention on the last five years.

Clay nanotube-metal core/shell catalysts for hydroprocesses

Catalytic hydroprocesses play a significant role in oil refining and petrochemistry. The tailored design of new metal nanosystems and optimization of their support, composition, and structure is a prospective strategy for enhancing the efficiency of catalysts. Mesoporous support impacts the active component by binding it to the surface, which leads to the formation of tiny highly dispersed catalytic particles stabilized from aggregation and with minimized leaching. The structural and acidic properties of the support are crucial and determine the size and dispersion of the active metal phase. Currently, research efforts are shifted toward the design of nanoscale porous materials, where homogeneous catalysts are displaced by heterogeneous. Ceramic materials, such as 50 nm diameter natural halloysite nanotubes, are of special interest for this. Much attention to halloysite clay is due to its tubular structure with a hollow 10-15 nm diameter internal cavity, textural characteristics, and different chemical compositions of the outer/inner surfaces, allowing selective nanotube modification. Loading halloysite with metal particles or placing them outside the tubes provides stable and efficient mesocatalysts. The low cost of this abundant nanoclay makes it a good choice for the scaled-up architectural design of core-shell catalysts, containing active metal sites (Au, Ag, Pt, Ru, Co, Mo, Fe₂O₃, CdS, CdZnS, Cu-Ni) located inside or outside the tubular template. These alumosilicate nanotubes are environment-friendly and are available in thousands of tons. Herein, we summarized the advances of halloysite-based composite materials for hydroprocesses, focusing on the selective binding of metal particles. We analyze the tubes' morphology adjustments and size selection, the physicochemical properties of pristine and modified halloysite (e.g., acid-etched or silanized), the methods of metal clusters formation, and their localization. We indicate prospective routes for the architectural design of stable and efficient nanocatalysts based on this safe and natural clay material.

Ru Catalysts Supported on Commercial and Biomass-Derived Activated Carbons for the Transformation of Levulinic Acid into γ-Valerolactone under Mild Conditions

Ru catalysts (1 wt.%) supported on commercial and biomass-derived activated carbons (AC) have been prepared, thoroughly characterized, and used in the hydrogenation of levulinic acid to produce gamma-valerolactone (GVL). This is an important platform compound that plays a key role in the production of liquid fuels and that can also be used, for example, as a food flavoring agent, antifreeze, and solvent. The study focuses on the influence of the carbon support characteristics, such as porous texture and acidity, on the properties and performance (LA conversion and selectivity to GVL) of the catalysts. Catalytic activity tests have been carried out at 170 °C and also in noticeably milder conditions (70 °C) to implement a less energy-demanding process. All the catalysts show high LA conversion and GVL yield at 170 °C, while at 70 °C, important differences between them, related to the support properties, have been found. The catalysts prepared with more acidic supports show better catalytic properties: very good catalytic performance (98% LA conversion and 77% selectivity to GVL) has been obtained in mild temperature conditions.