Improving the Catalytic Performance of Keggin [PW12O40]3- for Oxidative Desulfurization: Ionic Liquids versus SBA-15 Composite

Selective Oxidation Cleavage of Cα-Cβ Bonds between Lignin Aromatic Units via Phosphomolybdic Acid-Ionic Liquid

Lignin, the most abundant renewable resource of aromatics in nature, is recognized as an alternative to fossil-based fuels and chemicals. In this study, a phosphomolybdic acid-ionic liquid (PMA-IL) [BSHMIM]H2PMo12O40 was synthesized and used as a catalyst for further oxidative depolymerization of extracted lignin to prepare valuable chemicals. Under optimized conditions, the conversion of lignin reached 61.33%, and the total yield of target products (vanillin, vanillic acid, and methyl vanillate) was 28.00 mg·g-1, where the selectivity of methyl vanillate reached 56.43%. The liquid products and residues obtained under the optimum conditions were analyzed, further confirming the advantages of PMA-IL compared with single PMA. In addition, this method also has an excellent catalytic effect for other types of lignin and presents a good substrate applicability. Finally, based on the research results of model compounds, the possible reaction pathway of the formation of vanillin, vanillic acid, and methyl vanillate was proposed.

A Novel Ternary Catalyst PW4@MOF-808@SBA-15 for Deep Extraction Oxidation Desulfurization of Model Diesel

In this work, a novel heterogeneous catalyst consisting of peroxophosphotungstate, microporous MOF-808, and mesoporous SBA-15 was synthesized, characterized, and used to remove sulfides from model fuel. The prepared material, PW4@MOF-808@SBA-15, exhibits excellent catalytic activity with a desulfurization efficiency of 99.8% in 60 min for multicomponent simulated fuel, and the desulfurization rate can reach more than 90% after ten consecutive cycles. The excellent catalytic activity and reusability are attributed to the hierarchically porous hybrid material MOF-808@SBA-15, which can effectively encapsulate PW4 and provide a site for the oxidation of sulfides.

Tailoring of Mesoporous Silica-Based Materials for Enhanced Water Pollutants Removal

The adsorptive performance of mesoporous silica-based materials towards inorganic (metal ions) and organic (dyes) water pollutants was investigated. Mesoporous silica materials with different particle size, surface area and pore volume were prepared and tailored with different functional groups. These materials were then characterised by solid-state techniques, namely vibrational spectroscopy, elemental analysis, scanning electron microscopy and nitrogen adsorption-desorption isotherms, allowing the successful preparation and structural modifications of the materials to be confirmed. The influence of the physicochemical properties of the adsorbents towards the removal of metal ions (Ni²⁺, Cu²⁺ and Fe³⁺) and organic dyes (methylene blue and methyl green) from aqueous solutions was also investigated. The results reveal that the exceptionally high surface area and suitable ζ-potential of the nanosized mesoporous silica nanoparticles (MSNPs) seem to favour the adsorptive capacity of the material for both types of water pollutants. Kinetic studies were performed for the adsorption of organic dyes by MSNPs and large-pore mesoporous silica (LPMS), suggesting that the process follows a pseudo-second-order model. The recyclability along consecutive adsorption cycles and the stability of the adsorbents after use were also investigated, showing that the material can be reused. Current results show the potentialities of novel silica-based material as a suitable adsorbent to remove pollutants from aquatic matrices with an applicability to reduce water pollution.

Heteropolyacid Ionic Liquid-Based MCF: An Efficient Heterogeneous Catalyst for Oxidative Desulfurization of Fuel

A new type of catalyst was synthesized by immobilizing heteropolyacid on ionic liquid-modified mesostructured cellular silica foam (denoted as MCF) and applied to the oxidative desulfurization of fuel. The surface morphology and structure of the catalyst were characterized by XRD, TEM, N₂ adsorption-desorption, FT-IR, EDS and XPS analysis. The catalyst exhibited good stability and desulfurization for various sulfur-containing compounds in oxidative desulfurization. Heteropolyacid ionic liquid-based MCF solved the shortage of the amount of ionic liquid and difficult separation in the process of oxidative desulfurization. Meanwhile, MCF had a special three-dimensional structure that was not only highly conducive to mass transfer but also greatly increased catalytic active sites and significantly improved catalytic efficiency. Accordingly, the prepared catalyst of 1-butyl-3-methyl imidazolium phosphomolybdic acid-based MCF (denoted as [BMIM]₃PMo₁₂O₄₀-based MCF) exhibited high desulfurization activity in an oxidative desulfurization system. The removal of dibenzothiophene could achieve levels of 100% in 90 min. Additionally, four sulfur-containing compounds could be removed completely under mild conditions. Due to the stability of the structure, sulfur removal efficiency still reached 99.8% after the catalyst was recycled six times.

Solvent-Free Desulfurization System to Produce Low-Sulfur Diesel Using Hybrid Monovacant Keggin-Type Catalyst

Two quaternary ammonium catalysts based on the monovacant polyoxotungstate ([PW₁₁O₃₉]⁷⁻, abbreviated as PW₁₁) were prepared and characterized. The desulfurization performances of the PW₁₁-based hybrids (of tetrabutylammonium and trimethyloctadecylammonium, abbreviated as TBA[PW₁₁] and ODA[PW₁₁], respectively), the corresponding potassium salt (K₇PW₁₁O₃₉, abbreviated as KPW₁₁) and the peroxo-compound (TBA-PO₄[WO(O₂)₂], abbreviated as TBA[PW₄]) were compared as catalysts for the oxidative desulfurization of a multicomponent model diesel (2000 ppm S). The oxidative desulfurization studies (ODS) were performed using solvent-free systems and aqueous H₂O₂ as oxidant. The nature of the cation in the PW₁₁ catalyst showed to have an important influence on the catalytic performance. In fact, the PW₁₁-hybrid catalysts showed higher catalytic efficiency than the peroxo-compound TBA[PW₄], known as Venturello compound. TBA[PW₁₁] revealed a remarkable desulfurization performance with 96.5% of sulfur compounds removed in the first 130 min. The reusability and stability of the catalyst were also investigated for ten consecutive ODS cycles without loss of activity. A treated clean diesel could be recovered without sulfur compounds by performing a final liquid/liquid extraction diesel/EtOH:H₂O mixture (1:1) after the catalytic oxidative step.

Confining Polyoxometalate Clusters into Porous Aromatic Framework Materials for Catalytic Desulfurization of Dibenzothiophene

Removal of notorious sulfur compounds to produce low-sulfur-content (≤10 ppm) diesel is necessary and vital for modern industry and environmental protection. A new type of inorganic-organic hybrid material has been designed and synthesized via confining molybdenum-containing polyoxometalate (POM) clusters within porous aromatic framework-1 (PAF-1) cavities named POM-PAF-1. Deep oxidative desulfurization experiments reveal that POM-PAF-1 possesses excellent reactivity under mild conditions, exemplified by a sulfur removal degree of 98.5% dibenzothiophene within 30 min at 30 °C. The improvement in oxidative desulfurization reactivity from traditional porous POM-based catalysts is owing to uniform POMs and lipophilic and porous PAF-1. The high performance of POM-PAF-1 in terms of excellent reactivity and good stability means it has potential in new heterogeneous catalysis.

Mesoporous Silica vs. Organosilica Composites to Desulfurize Diesel

The monolacunary Keggin-type [PW₁₁O₃₉]⁷⁻ (PW₁₁) heteropolyanion was immobilized on porous framework of mesoporous silicas, namely SBA-15 and an ethylene-bridged periodic mesoporous organosilica (PMOE). The supports were functionalized with a cationic group (N-trimethoxysilypropyl-N, N, N-trimethylammonium, TMA) for the successful anchoring of the anionic polyoxometalate. The PW₁₁@TMA-SBA-15 and PW₁₁@TMA-PMOE composites were evaluated as heterogeneous catalysts in the oxidative desulfurization of a model diesel. The PW₁₁@TMA-SBA-15 catalyst showed a remarkable desulfurization performance by reaching ultra-low sulfur levels (<10 ppm) after only 60 min using either a biphasic extractive and catalytic oxidative desulfurization (ECODS) system (1:1 MeCN/diesel) or a solvent-free catalytic oxidative desulfurization (CODS) system. Furthermore, the mesoporous silica composite was able to be recycled for six consecutive cycles without any apparent loss of activity. The promising results have led to the application of the catalyst in the desulfurization of an untreated real diesel supplied by CEPSA (1,335 ppm S) using the biphasic system. The system has proved to be a highly efficient process by reaching desulfurization values higher than 90% for real diesel during three consecutive cycles.

SBA-15 Anchored Metal Containing Catalysts in the Oxidative Desulfurization Process

Recalcitrant sulfur compounds are common impurities in crude oil. During combustion they produce SOx derivatives that are able to affect the atmospheric ozone layer, increasing the formation of acid rains, and reducing the life of the engine due to corrosion. In the last twenty years, many efforts have been devoted to develop conventional hydrodesulfurization (HDS) procedures, as well as alternative methods, such as selective adsorption, bio-desulfurization, oxidative desulfurization (ODS) under extractive conditions (ECODS), and others. Among them, the oxidative procedures have been usually accomplished by the use of toxic stoichiometric oxidants, namely potassium permanganate, sodium bromate and carboxylic and sulfonic peracids. As an alternative, increasing interest is devoted to selective and economical procedures based upon catalytic methods. Heterogeneous catalysis is of relevance in industrial ODS processes, since it reduces the leaching of active species and favors the recovery and reuse of the catalyst for successive transformations. The heterogenization of different types of high-valent metal transition-based organometallic complexes, able to promote the activation of stoichiometric benign oxidants like peroxides, can be achieved using various solid supports. Many successful cases have been frequently associated with the use of mesoporous silicas that have the advantage of easy surface modification by reaction with organosilanes, facilitating the immobilization of homogeneous catalysts. In this manuscript the application of SBA-15 as efficient support for different active metal species, able to promote the catalytic ODS of either model or real fuels is reviewed, highlighting its beneficial properties such as high surface area, narrow pore size distribution and tunable pore diameter dimensions. Related to this topic, the most relevant advances recently published, will be discussed and critically described.

Cold-Cured Epoxy-Based Organic⁻Inorganic Hybrid Resins Containing Deep Eutectic Solvents

The development of improved cold-cured resins, to be used as either adhesives or matrices for FRP (fiber reinforced polymer) composites employed in the construction industry, has become the focus of several academic and industrial research projects. It is expected that the use of nano-structured organic–inorganic hybrid materials could represent a realistic alternative to commercial epoxy-based resins due to their superior properties, especially in terms of higher durability against: moisture, temperatures, harsh environments, and fire. In this context, organic–inorganic epoxy hybrids were synthesized by a modified sol–gel method without the addition of water. The experimental formulations were prepared starting from a mixture of a silane-functionalized epoxy resin, alkoxysilane components and a deep eutectic solvent (DES) based on a blend of choline chloride and urea. The latter was added in two different loads in order to analyze in depth its effect as a promoter for an effective dispersion of silica nano-phases, formed through hydrolysis and condensation reactions, into the cross-linked epoxy network. The produced formulations were cold-cured for different time spans in the presence of two hardeners, both suitable for a curing process at ambient temperature. In this first part of a wider experimental program, several analyses were carried out on the liquid (rheological and calorimetric) and cold-cured (calorimetric, thermogravimetric, dynamic-mechanical, flexural mechanical, and morphological) systems to evaluate and quantify the improvement in properties brought about by the presence of two different phases (organic and inorganic) in the same epoxy-based hybrid system.