Pickering Interfacial Catalysis-Knoevenagel Condensation in Magnesium Oxide-Stabilized Pickering Emulsion

Particle shape anisotropy in pickering interfacial catalysis for Knoevenagel condensation

Dispersions of two immiscible liquids stabilized by solid particles are termed as Pickering emulsions. Stability of such emulsions is affected by various parameters such as amount of solid particle, method of emulsification, size, and shape of particles, etc. In this study, MgO samples prepared by different methods and characterized by XRD, FESEM, HRTEM, DLS, and CO2-TPD techniques were utilized for stabilizing o/w Pickering emulsions. The effect of particle shape on Pickering Interfacial Catalysis (PIC) for Knoevenagel condensation was investigated. It was found that in the case of rod and plate shaped particles, emulsion stability and catalytic activity were higher as compared to those obtained with other MgO samples prepared. The applicability of the MgO-PIC system is also successfully demonstrated for gram scale synthesis (85 % yield in 30 min). The MgO-PIC system was found to be reusable for at least five cycles without substantial loss in activity.

Improved Thermophysical and Mechanical Properties in LiNaSO4 Composites for Thermal Energy Storage

Solid-solid phase-change materials have great potential for developing compact and low-cost thermal storage systems. The solid-state nature of these materials enables the design of systems analogous to those based on natural rocks but with an extraordinarily higher energy density. In this scenario, the evaluation and improvement of the mechanical and thermophysical properties of these solid-solid PCMs are key to exploiting their full potential. In this study, LiNaSO4-based composites, comprising porous MgO and expanded graphite (EG) as the dispersed phases and LiNaSO4 as the matrix, have been prepared with the aim of enhancing the thermophysical and mechanical properties of LiNaSO4. The characteristic structure of MgO and the high degree of crystallinity of the EG600 confer on the LiNaSO4 sample mechanical stability, which leads to an increase in the Young's modulus (almost three times higher) compared to the pure LiNaSO4 sample. These materials are proposed as a suitable candidate for thermal energy storage applications at high temperatures (400-550 °C). The addition of 5 wt.% of MgO or 5% of EG had a minor influence on the solid-solid phase change temperature and enthalpy; however, other thermal properties such as thermal conductivity or specific heat capacity were increased, extending the scope of PCMs use.

Green Synthesis of Electrophilic Alkenes Using a Magnesium Catalyst under Aqueous Conditions and Mechanistic Insights by Density Functional Theory

A green approach for the synthesis of electrophilic alkenes has been developed via Knoevenagel condensation between active methylene compounds and carbonyl compounds using Mg powder under aqueous conditions. In this strategy, Mg(OH)2 acts as a catalyst, which was generated in situ by the reaction between metallic Mg (20 mol %) and water. Mg was found to be an efficient, nontoxic, and inexpensive metal catalyst system for producing a range of electrophilic alkenes in excellent yields (≤98%). A gram-scale synthesis of electrophilic alkenes has been developed, and Mg metal was recovered and recycled up to three times without an appreciable loss of catalytic activity. A catalytic cycle was proposed, and the reaction mechanism was investigated using density functional theory. The key steps are enolization of ethyl cyanoacetate, C-C bond formation, and then regeneration of the catalyst via metathesis with H2O. The overall reaction occurs easily with a maximum ΔG°⧧ value of 7.9 kcal/mol for the rate-determining C-C bond formation step. Our protocol has several advantages and can be further extended to one-pot sequential Knoevenagel condensation and Michael addition, and one-pot sequential Knoevenagel condensation and chemoselective reduction can be used for the synthesis of valuable precursors of pharmaceutical products under green and aqueous conditions.

High-Performance Heterogeneous Thermocatalysis Caused by Catalyst Wettability Regulation

Catalyst wettability regulation has emerged as an attractive approach for high catalytic performance for the past few years. By introducing appropriate wettability, the molecule diffusion of reactants and products can be enhanced, leading to high activity. Besides this, undesired molecules are isolated for high selectivity of target products and long-term stability of catalyst. Herein, we summarize wettability-induced high-performance heterogeneous thermocatalysis in recent years, including hydrophilicity, hydrophobicity, hybrid hydrophilicity-hydrophobicity, amphiphilicity, and superaerophilicity. Relevant reactions are further classified and described according to the reason for the performance improvement. It should be pointed out that studies of utilizing superaerophilicity to improve heterogeneous thermocatalytic performance have been included for the first time, so this is a comparatively comprehensive review in this field as yet.

Surface hydroxyl groups: the key to a CrOx/TiO2 catalyst for efficient catalytic oxidation of 2,2′-hydrazine diisobutyronitrile

Surface hydroxyl groups could contribute to the formation of Cr–O–Ti bonds on the surface of the CrOx/TiO2 catalyst, which thus promote the oxidation of 2,2′-hydrazobis-isobutyronitrile.

Ag2CO3 containing magnetic nanocomposite as a powerful and recoverable catalyst for Knoevenagel condensation

In this paper, the synthesis, characterization and catalytic application of a novel magnetic silica-supported Ag2CO3 (MS/Ag2CO3) with core-shell structure are developed. The MS/Ag2CO3 nanocomposite was prepared through chemical modification of magnetic MS nanoparticles with AgNO3 under alkaline conditions. The structure, chemical composition and magnetic properties of MS/Ag2CO3 were investigated by using VSM, PXRD, FT-IR, EDX and SEM techniques. The MS/Ag2CO3 nanocomposite was used as an effective catalyst for the Knoevenagel condensation under solvent-free conditions at 60 °C in an ultrasonic bath. The recovery and leaching tests were performed to study the nature of the MS/Ag2CO3 catalyst under applied conditions.

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.

Catalysis in Pickering emulsions

Particle-stabilised or Pickering emulsions are versatile systems. In the past 10 years a new application has emerged in the field of catalysis to use them as vehicles to carry out catalytic reactions, allowing a more environmentally friendly process with high conversions and selectivities and important advantages for catalyst recovery. As the area has advanced rapidly, the intention of this review is to summarize the latest innovations being reported. An overview is given regarding the kinds of liquid phases comprising the emulsion system, the different types of solid particle stabilizers (whether they contain catalyst or not) and the catalytic reactions studied. A section describing methods for recovering the catalyst is also included, in which various stimuli are discussed. Finally, the importance of using Pickering emulsions to carry out reactions in flow and in multi-step cascade systems is highlighted with various examples to support the benefits of transferring this technology to industrial processes.