Search Results (11)

The role of p-benzoquinone (BQ) as a photocatalyst in the synthesis of solketal under UV irradiation has been studied, along with the combined use of BQ/TiO₂ P25 as a photocatalytic system for the process. The presence of the O₂/O₂^−• redox couple is essential for the reaction to take place. However, experiments with p-benzoquinone as a superoxide radical scavenger failed, with the opposite effect of enhancing the reaction being observed. It was found that p-benzoquinone and oxygen compete for photogenerated electrons in the conduction band of titania. A redox equilibrium between p-benzoquinone and hydroquinone (H₂Q), mediated by the O₂/O₂^−• system, was identified as a key factor in enabling the reaction. Furthermore, EPR spin-trapping experiments confirmed the presence of the carbon-centered radical 2-hydroxypropan-2-yl, which was determined to be the main radical species involved in the process. Either acetone or 2-propanol can generate this radical, with the BQ/H₂Q redox system being pivotal in the formation of the hemiacetal intermediate. This intermediate is subsequently converted into the final acetal (solketal), with H₂Q acting as a photoacid through an excited-state proton transfer (ESPT) mechanism. The photoacid behavior of hydroquinone was confirmed using pyridine as a basic probe, as the formation of hydroquinone–pyridine adducts was detected by Raman spectroscopy. Full article

Glycerol is a biogenic waste that is generated in both the biodiesel and oleo-chemical industries. The value addition of surplus glycerol is of utmost importance for making these industries economically profitable. In line with this, glycerol is converted into glycerol carbonate, a potential candidate for the industrial production of polymers and biobased non-isocyanate polyurethanes. In addition, glycerol can also be converted into solketal, which is the protected form of glycerol with a primary hydroxyl functional group. In this contribution, we developed a microwave-assisted solvent and catalyst-free method for converting solketal into solketal carbonate. Under conventional heating conditions, the reaction of solketal with dimethyl carbonate resulted in 70% solketal carbonate in 48 h. However, under microwave heating, 90% solketal carbonate was obtained in just 30 min. From the perspective of sustainability and green chemistry, biomass-derived heterogeneous catalysts are gaining importance. Therefore, in this project, several green catalysts, such as molecular sieves (MS, 4Å), Hβ-Zeolite, Montmorillonite K-10 clay, activated carbon from groundnut shell (Arachis hypogaea), biochar prepared from the pyrolysis of sawdust, and silica gel, were successfully used for the carbonyl transfer reaction. The obtained solketal carbonate was thoroughly characterized by ¹H NMR, ¹³C NMR, IR, and MS. The method presented here is facile, clean, and environmentally benign, as it eliminates the use of complicated procedures, toxic solvents, and toxic catalysts. Full article

The use of activated carbon-based catalysts for the production of solketal and γ-valerolactone (GVL), two products of interest for biorefinery processes, was investigated. Activated carbons (ACs) were prepared by chemical activation of olive stones, an agricultural byproduct, using H₃PO₄ to olive stone mass impregnation ratios (IRs) of 1:1 and 3:1, and under nitrogen or air atmosphere. The ACs showed S_BET values of 1130–1515 m²/g, owing to the presence of micropores (0.45–0.60 cm³/g). The use of an IR of 3:1 delivered a wider pore size distribution, with mesopore volume increasing up to 1.36 cm³/g. XPS confirmed the presence of phosphorus groups with surface concentrations of 2.2–3.2 wt% strongly bonded the AC surface through C-O-P bonds. The ACs were tested as acid catalysts for the acetalization of glycerol in a stirred batch reactor at temperatures of 30–50 °C, glycerol concentrations of 1.5 to 3.4 mol/L, and 1–3 wt% catalytic loading. The catalytic activity was clearly correlated with the quantity of C-O-P acid groups determined by TPD, which increased when ACs were prepared under air atmosphere. The AC prepared with IR 3:1 under air achieved full selectivity to solketal, with activation energy of 49 kJ/mol and conversion of up to 70%, matching the equilibrium conversion value under the optimum reaction conditions. A bifunctional catalyst was prepared over this AC by deposition of 5 wt% zirconium and tested in stirred batch reactor for the hydrogenation of levulinic acid (LA) using isopropyl alcohol (IPA) as solvent and H₂ donor, with LA:IPA ratios from 1:1 to 1:7 and temperatures between 160–200 °C. The catalyst reached full LA conversion and a GVL yield higher than 80% after only 12 h at 200 °C. A test conducted in the presence of water revealed that it was an inhibitor of the reaction. The identification of isopropyl levulinate as an intermediate suggests that the most likely reaction pathway was dehydration, followed by hydrogenation and cyclization, to obtain GVL. Kinetic modelling of the results showed a value of 42 kJ/mol for the hydrogenation step. The reusability of the catalyst was tested for five consecutive reaction cycles, maintaining most of the activity and selectivity towards GVL. Full article

Solketal, a widely used glycerol-derived solvent, can be efficiently synthesized through heterogeneous catalysis, thus avoiding the significant product losses typically encountered with aqueous work-up in homogeneous catalysis. This study explores the catalytic synthesis of solketal using solid acid catalysts derived from recovered carbon blacks (rCBs), which are obtained through the pyrolysis of end-of-life tires. This was further converted into solid acid catalysts through the introduction of acidic functional groups using concentrated H₂SO₄ or 4-benzenediazonium sulfonate (BDS) as sulfonating agents. Additionally, post-pyrolytic rCB treated with glucose and subsequently sulfonated with sulfuric acid was also prepared. Comprehensive characterization of the initial and modified rCBs was performed using techniques such as elemental analysis, powder X-ray diffraction, thermogravimetric analysis, a back titration method, and both scanning and transmission electron microscopy, along with X-ray photoelectron spectroscopy. The catalytic performance of these samples was evaluated through the batch mode glycerol acetalization to produce solketal. The modified rCBs exhibited substantial catalytic activity, achieving high glycerol conversions (approximately 90%) and high solketal selectivity (around 95%) within 30 min at 40 °C. This notable activity was attributed to the presence of -SO₃H groups on the surface of the functionalized rCBs. Reusability tests indicated that only rCBs modified with glucose demonstrated acceptable catalytic stability in subsequent acetalization cycles. The findings underscore the potential of utilizing end-of-life tires to produce effective acid catalysts for glycerol valorization processes. Full article

Biomass allows us to obtain energy and high-value-added compounds through the use of different physical and chemical processes. The glycerol obtained as a by-product in the synthesis of biodiesel is considered a biomass compound that has the potential to be used as a raw material to obtain different chemical products for industry. The development and growth of the biodiesel industry allows for the projection of glycerol biorefineries around these plants that efficiently and sustainably integrate the biodiesel production process together with the glycerol transformation processes. This work presents a review of the ketalization and esterification of glycerol to obtain solketal and acetylglycerols, which are considered products of high added value for the chemical and fuel industry. First, the general aspects and mechanisms of both reactions are presented, as well as the related chemical equilibrium concepts. Subsequently, the catalysts employed are described, classifying them according to their catalytic nature (zeolites, carbons, exchange resins, etc.). The reaction conditions used are also described, and the best results for each catalytic system are presented. In addition, stability studies and the main deactivation mechanisms are discussed. Finally, the work presents the kinetic models that have been formulated to date for some of these systems. It is expected that this review work will serve as a tool for the advancement of studies on the ketalization and esterification reactions that allow for the projection of biorefineries based on glycerol as a raw material. Full article

Fossil additives are a primary energy source and their contribution is around 80% in the world. Therefore, bioadditives that reduce their impact are each very important. This article discusses the chemical transformation of glycerol to carbonate, ethers, esters, ketals, and acetals, compounds with high technological applications, especially in the fuel sector as bioadditives. Mainly, heterogeneous catalysts are important in the production of more than 80% of chemicals in the word. The focus is on demonstrating how the Keggin heteropolyacids (HPAs) are efficient catalysts in the reactions of syntheses of glycerol-derived bioadditives, either in homogeneous or heterogeneous phases. Although solid, HPAs have a low surface area and are soluble in polar solvents, hampering their use as heterogeneous catalysts. Alternatively, they have been successfully used supported on solid matrixes with a high surface area. Another option is converting the Keggin HPAs to insoluble salts simply by exchanging their protons with large cations like potassium, cesium, or ammonium-derivatives. Therefore, solid heteropoly salts have reduced the cost and the environmental impact of bioadditive synthesis processes, being an alternative to traditional mineral acids or solid-supported catalysts. This review describes the most recent advances achieved in the processes of synthesis of glycerol-derived bioadditives over solid-supported HPAs or their solid heteropoly salts. Full article

The worldwide rise in biodiesel production has generated an excess of glycerol, a byproduct of the process. One of the most interesting alternative uses of glycerol is the production of solketal, a bioadditive that can improve the properties of both diesel and gasoline fuels. Even with its promising future, not much research has been performed on its toxicity in aqueous environments. In this work, solketal adsorption has been tested with two different commercial adsorbents: an activated carbon (Hydrodarco 3000) and a metal–organic framework (MIL-53). Diclofenac and caffeine were also chosen as emerging contaminants for comparison purposes. The effect of various parameters, such as the adsorbent mass or initial concentration of pollutants, has been studied. Adsorption kinetics with a better fit to a pseudo-second-order model, intraparticle diffusion, and effective diffusion coefficient were studied as well. Various isotherm equation models were employed to study the equilibrium process. The results obtained indicate that activated carbon is more effective in removing solketal from aqueous solutions than the metal–organic framework. Full article

Solketal is one of the most used glycerol-derived solvents. Its production via heterogeneous catalysis is crucial for avoiding important product losses typically found in the aqueous work-up in homogeneous catalysis. In this work, we present a study of the catalytic synthesis of solketal using sulfonated hydrothermal carbons (SHTC). They were prepared from glucose and cellulose resulting in different textural properties depending on the hydrothermal treatment conditions. The sulfonated hydrothermal carbons were also coated on a graphite microfiber felt (SHTC@GF). Thus, up to nine different solids were tested, and their activity was compared with commercial acidic resins. The solids presented very different catalytic activity, which did not correlate with their physical-chemical properties indicating that other aspects likely influence the transport of reactants and products to the catalytic surface. Additionally, the SHTC prepared from cellulose showed better reusability in batch reaction tests. This work also presents the first results for the production of solketal in a flow reactor, which opens the way to the use of SHTC@GF for this kind of reactions. Full article

The acetalization of glycerol with acetone represents a strategy for its valorization into solketal as a fuel additive component. Thus, acid carbon-based structured catalyst (SO₃H-C) has been prepared, characterized and tested in this reaction. The structured catalyst (L = 5 cm, d = 1 cm) showed a high surface density of acidic sites (2.9 mmol H⁺ g⁻¹) and a high surface area. This catalyst is highly active and stable in the solketal reaction production in a batch reactor system and in a continuous downflow reactor, where several parameters were studied such as the variation of time of reaction, temperature, acetone/glycerol molar ratio (A/G) and weight hourly space velocity (WHSV). A complete glycerol conversion and 100% of solketal selectivity were achieved working in the continuous flow reactor equipped with distillation equipment when WHSV is 2.9 h⁻¹, A/G = 8 at 57 °C in a co-solvent free operation. The catalyst maintained its activity under continuous flow even after 300 min of reaction. Full article

The depletion of fossil fuels has heightened research and utilization of renewable energy such as biodiesel. However, this has thrown up another challenge of significant increase in its byproduct, glycerol. In view of the characteristics and potentials of glycerol, efforts are on the increase to convert it to higher-value products, which will in turn improve the overall economics of biodiesel production. These high-value products include biofuels, oxygenated fuel additives, polymer precursors and other industrial bio-based chemicals. This review gives up-to-date research findings in the conversion of glycerol to the above high-value products, with a special focus on the performance of the catalysts used and their challenges. The specific products reviewed in this paper include hydrogen, ethanol, methanol, acetin, glycerol ethers, solketal, acetal, acrolein, glycerol carbonate, 1,3-propanediol, polyglycerol and olefins. Full article

Biodiesel is one of the most attractive sources of clean energy. It is produced by the transformation of vegetable oils with up to 10% formation of glycerol as a by-product. Therefore, development of new approaches for processing bio-glycerol into such value-added chemical compounds as solketals is necessary. Thus, various six- and five-membered cyclic compounds can be prepared by acetalization of glycerol with aldehyde or ketone. The resulting glycerol oxygenates are excellent fuel additives that increase viscosity, octane or cetane number, and stability to oxidation. In addition, these products significantly reduce carbon monoxide emissions from standard diesel fuel. In this review, we highlight recent advances in the glycerol valorization for the sustainable production of bio-additives. The review includes a discussion of the innovative and potential catalysts to produce solketals. Full article