A dry milling approach for the synthesis of highly active nanoparticles supported on porous materials

Exploiting the Potential of Biosilica from Rice Husk as Porous Support for Catalytically Active Iron Oxide Nanoparticles

Biomass-derived materials are put forward as eco-friendly alternatives to design heterogeneous catalysts. To contribute in this field, we explored the potential of mesoporous biogenic silica (RH-Silica) obtained from lignocellulosic waste, in particular from rice husk, as an inorganic support to prepare heterogenized iron oxide-based catalysts. Mechanochemistry, considered as a green and sustainable technique, was employed to synthetize iron oxide nanoparticles in pure hematite phase onto the biosilica (α-Fe₂O₃/RH-Silica), making this material a good candidate to perform catalyzed organic reactions. The obtained material was characterized by different techniques, and its catalytic activity was tested in the selective oxidation of styrene under microwave irradiation. α-Fe₂O₃/RH-Silica displayed a good catalytic performance, achieving a conversion of 45% under optimized conditions, and more importantly, with a total selectivity to benzaldehyde. Furthermore, a good reusability was achieved without decreasing its activity after multiple catalytic cycles. This work represents a good example of using sustainable approaches and green materials as alternatives to conventional methods in the production of high-added value products.

Mechanochemical Synthesis of Catalytic Materials

The mechanochemical synthesis of nanomaterials for catalytic applications is a growing research field due to its simplicity, scalability, and eco-friendliness. Besides, it provides materials with distinct features, such as nanocrystallinity, high defect concentration, and close interaction of the components in a system, which are, in most cases, unattainable by conventional routes. Consequently, this research field has recently become highly popular, particularly for the preparation of catalytic materials for various applications, ranging from chemical production over energy conversion catalysis to environmental protection. In this Review, recent studies on mechanochemistry for the synthesis of catalytic materials are discussed. Emphasis is placed on the straightforwardness of the mechanochemical route-in contrast to more conventional synthesis-in fabricating the materials, which otherwise often require harsh conditions. Distinct material properties achieved by mechanochemistry are related to their improved catalytic performance.

Metal doping of porous materials via a post-synthetic mechano-chemical approach: a general route to design low-loaded versatile catalytic systems

Mesoporous silica materials doped with metals (Fe/Zr) presented superior catalytic performance for oxidations. The use of MOFs as metal sources improved the metal distribution, and allowed a fine-tuning of Brønsted–Lewis acidity depending on the MOF.

Synthesis and Characterization of Novel Pyridine Periodic Mesoporous Organosilicas and Its Catalytic Activity in the Knoevenagel Condensation Reaction

The preparation of novel organic-inorganic hybrid mesoporous organosilica containing pyridinedicarboxamide functional groups uniformly distributed inside the nanostructured pore walls has been addressed. The mesoporosity and uniformity of the synthesized nanomaterials were characterized by different techniques such as nitrogen adsorption/desorption measurements and powder X-ray diffraction (PXRD). Additionally, the presence of the pyridinedicarboxamide in the pore walls of the nanomaterials was assessed by Fourier-transform infrared spectroscopy (FT-IR), as well as ²⁹Si and ¹³C solid-state cross-polarization and magic angle spinning nuclear magnetic resonance (CP/MAS-NMR). The Knoevenagel condensation of aldehydes with active methylene compounds was carried out over the pyridinedicarboxamide functionalized mesoporous organosilica, which has been proven to be an efficient heterogeneous basic catalyst in the presence of ethanol as solvent. The catalytic activity of the investigated materials was investigated in the Knoevenagel condensation between malononitrile and several benzaldehyde derivatives exhibiting a high conversion (>90%) and excellent selectivity toward the final condensation products under very mild reaction conditions. Furthermore, the catalyst stability is noteworthy as it could be recycled and reused at least twelve times without any significant change in the performance.

Tuneable Acidity in Fluorinated Al-SBA-15 Materials for the Esterification of Valeric Acid to Alkyl Valerates

The acidity of Al-SBA-15 materials functionalized by ball milling with several niobium loadings (0. 25-1 wt.%) as well as with several fluorine loadings (by wet impregnation using NH₄F as a precursor) was characterized and materials investigated in the esterification of valeric acid to alkyl valerates. The parent Al-SBA-15 support as well as the modified materials loaded with Nb and/or F have been catalysts synthesized characterized by X-ray diffraction (XRD), N₂ physisorption measurements, and diffuse reflection infrared spectroscopy (DRIFT) among others. A special interest was paid on the acidity of the materials that was investigated by temperature-programmed desorption of pyridine. Interestingly, the characterization results for the materials containing fluorine showed up an increase in the acidity strength despite of a reduction in the number of acid sites. The catalytic performance of the as-prepared catalysts was investigated in the microwave-assisted esterification reaction of valeric acid to valerate esters. Thus, while the materials modified with niobium exhibited a lower catalytic activity as compared with the catalytic support (Al-SBA-15), the materials loaded with fluorine either onto Al-SBA-15 or on Nb1%/Al-SBA-15 materials presented enhanced conversion values of valeric acid. Therefore, it can be said that the new acid sites with enhanced strength formed by the incorporation of fluorine boost the esterification of valeric acid with alcohols to form the respective valerate ester.

Fe-Containing MOFs as Seeds for the Preparation of Highly Active Fe/Al-SBA-15 Catalysts in the NAlkylation of Aniline

We have successfully incorporated iron species into mesoporous aluminosilicates (Al-SBA-15) using a simple mechanochemical milling method. The catalysts were characterized by nitrogen physisorption, inductively coupled plasma mass spectrometry (ICP-MS), pyridine (PY) and 2,6-dimethylpyridine (DMPY) pulse chromatography titration, powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX). The catalysts were tested in the N-alkylation reaction of aniline with benzyl alcohol for imine production. According to the results, the iron sources, acidity of catalyst and reaction conditions were important factors influencing the reaction. The catalyst showed excellent catalytic performance, achieving 97% of aniline conversion and 96% of imine selectivity under optimized conditions.

Mechanochemically Synthesized Supported Magnetic Fe-Nanoparticles as Catalysts for Efficient Vanillin Production

Magnetically separable nanocatalysts were synthesized by incorporating ironnanoparticles on a mesoporous aluminosilicate (Al-SBA-15) through a mechanochemical grindingpathway in a single step. Noticeably, magnetic features were achieved by employing biomass wasteas a carbon source, which additionally may confer high oxygen functionalities to the resultingmaterial. The resulting catalysts were characterized using X-ray diffraction, X-ray photoelectronspectroscopy, transmission electron microscopy, scanning electron microscopy, porosimetry, andmagnetic susceptibility. The magnetic nanocatalysts were tested in the selective oxidative cleavagereaction of isoeugenol and vanillyl alcohol to vanillin. As a result, the magnetic nanocatalystsdemonstrated high catalytic activity, chemical stability, and enormous separation/reusabilityqualities. The origin of catalytic properties and its relationship with the iron oxide precursor wereanalyzed in terms of the chemical, morphological, and structural properties of the samples. Suchanalysis allows, thus, to highlight the superficial concentration of the iron entities and the interactionwith Al as key factors to obtain a good catalytic response.

Co-Aromatization of n-Butane and Methanol over PtSnK-Mo/ZSM-5 Zeolite Catalysts: The Promotion Effect of Ball-Milling

The ball-milling (BM) method benefits the stabilization and dispersion of metallic particles for the preparation of the PtSnK–Mo/ZSM-5 catalyst. Based on the TPR, H2-TPD, XPS, and CO-FTIR results, the Pt–SnOx and MoOx species were formed separately on the BM sample. During the aromatization of cofeeding the n-butane with methanol, the yield of the aromatics is 59 wt.% at a n-butane conversion of 86% at 475 °C over the Pt Mo BM catalyst. The more weak acid sites also contribute to the aromatics formation with the less light alkanes formation. For the Pt Ga catalysts, the slow loss of activity suggests that the BM method can restrain the coke deposition on the Pt-SnOx species, because of a certain distance between the Pt–SnOx and GaOx species on the surface of ZSM-5.

Efficient Mechanochemical Bifunctional Nanocatalysts for the Conversion of Isoeugenol to Vanillin

A bifunctional nanocatalyst composed of iron containing SBA-15 material modified with sulfonic acid groups was synthesized by a mechanochemical approach. A full characterization of the obtained nanocatalyst was performed by N₂ physisorption isotherms analysis, transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and Fourier-Infrared Spectroscopy (FT-IR). The mechanochemically synthesized nanocatalyst displays a high isoeugenol conversion to vanillin under mild conditions using H₂O₂ as oxidizing agent. Interestingly, this conversion resulted to be higher than that one obtained with the same material synthesized by an impregnation method. Additionally, the nanocatalyst showed excellent reusability over four successive runs under the studied reaction conditions.

A comprehensive study on the continuous flow synthesis of supported iron oxide nanoparticles on porous silicates and their catalytic applications

The present work demonstrates an efficient continuous flow methodology for the preparation of iron oxide nanoparticles supported on porous silicates.

New bio-nanocomposites based on iron oxides and polysaccharides applied to oxidation and alkylation reactions

Polysaccharides from natural sources and iron precursors were applied to develop new bio-nanocomposites by mechanochemical milling processes. The proposed methodology was demonstrated to be advantageous in comparison with other protocols for the synthesis of iron oxide based nanostructures. Additionally, mechanochemistry has enormous potential from an environmental point-of-view since it is able to reduce solvent issues in chemical syntheses. The catalytic activity of the obtained nanocatalysts was investigated in both the oxidation of benzyl alcohol to benzaldehyde and in the alkylation of toluene with benzyl chloride. The microwave-assisted oxidation of benzyl alcohol reached 45% conversion after 10 min. The conversion of the alkylation of toluene in both microwave-assisted and conventional heating methods was higher than 99% after 3 min and 30 min, respectively. The transformation of benzyl alcohol and toluene into valuable product in both the oxidation and alkylation reaction reveals a potential method for the valorization of lignocellulosic biomass.

Mechanochemically synthesized Ag-based nanohybrids with unprecedented low toxicity in biomedical applications

A simple and innovative mechanochemical approach was employed to synthesize Ag-polysaccharide nanohybrid materials that were proved to exhibit remarkable surface properties and structures for biomedical applications. The synthesized Ag nanomaterials possessed an unprecedented low cytotoxicity against human cell lines A549 and SH-SY5Y as compared to similarly reported Ag nanomaterials due to the stability and low release of Ag⁺ and high biocompatibility of the nanohybrids.

Advances in Solid-State Transformations of Coordination Bonds: From the Ball Mill to the Aging Chamber

Controlling the formation of coordination bonds is pivotal to the development of a plethora of functional metal-organic materials, ranging from coordination polymers, metal-organic frameworks (MOFs) to metallodrugs. The interest in and commercialization of such materials has created a need for more efficient, environmentally-friendly routes for making coordination bonds. Solid-state coordination chemistry is a versatile greener alternative to conventional synthesis, offering quantitative yields, enhanced stoichiometric and topological selectivity, access to a wider range of precursors, as well as to molecules and materials not readily accessible in solution or solvothermally. With a focus on mechanochemical, thermochemical and “accelerated aging” approaches to coordination polymers, including pharmaceutically-relevant materials and microporous MOFs, this review highlights the recent advances in solid-state coordination chemistry and techniques for understanding the underlying reaction mechanisms.

Heterogeneously catalyzed lignin depolymerization

Biomass offers a unique resource for the sustainable production of bio-derived chemical and fuels as drop-in replacements for the current fossil fuel products. Lignin represents a major component of lignocellulosic biomass, but is particularly recalcitrant for valorization by existing chemical technologies due to its complex cross-linking polymeric network. Here, we highlight a range of catalytic approaches to lignin depolymerisation for the production of aromatic bio-oil and monomeric oxygenates.

Hierarchical zeolites and their catalytic performance in selective oxidative processes

Hierarchical ZSM-5 zeolites prepared using a simple alkali treatment and subsequent HCl washing are found to exhibit unprecedented catalytic activities in selective oxidation of benzyl alcohol under microwave irradiation. The metal-free zeolites promote the microwave-assisted oxidation of benzyl alcohol with hydrogen peroxide in yields ranging from 45-35 % after 5 min of reaction under mild reaction conditions as well as the epoxidation of cyclohexene to valuable products (40-60 % conversion). The hierarchically porous systems also exhibited an interesting catalytic activity in the dehydration of N,N-dimethylformamide (25-30 % conversion), representing the first example of transition-metal free catalysts in this reaction.

Mechanochemical preparation of advanced catalytically active bifunctional Pd-containing nanomaterials for aqueous phase hydrogenation

The preparation of supported Pd nanoparticles on porous materials was successfully achieved using a simple, fast and efficient reactive milling (mechanochemical) method. The catalytic activity of different Pd/Al-SBA-15 and Pd/Al-MCF materials was investigated in the hydrogenation of p-nitrophenol (PNP) to p-aminophenol (PAP) using NaBH₄ as a reducing agent.

Mechanochemical synthesis of advanced nanomaterials for catalytic applications

Mechanochemical synthesis emerged as the most advantageous, environmentally sound alternative to traditional routes for nanomaterials preparation with outstanding properties for advanced applications.

Fractionation of organosolv lignin from olive tree clippings and its valorization to simple phenolic compounds

Lignin valorization practices have attracted a great deal of interest in recent years due to the large excess of lignin produced by the pulp and paper industry, together with second-generation bioethanol plants. In this work, a new lignin valorization approach is proposed. It involves ultrafiltration as a fractionation process to separate different molecular weight lignin fractions followed by a hydrogen-free, mild, hydrogenolytic, heterogeneously catalyzed methodology assisted by microwave irradiation to obtain simple phenolic, monomeric products by depolymerization using a nickel-based catalyst. The main products obtained were desaspidinol, syringaldehyde, and syringol; this proves the efficiency of the depolymerization conditions applied. The concentration of these observed compounds increased when the molecular weights of the lignin fractions increased. The applied depolymerization conditions, which take advantage of the use of formic acid as a hydrogen-donating solvent, did not generate any biochar in the systems.