Role of Lewis and Brønsted Acidity in Metal Chloride Catalysis in Organic Media: Reductive Etherification of Furanics

Assessing Activation Quality through Evaporative Drying Patterns of Zr-MOF (UiO-66) Colloidal Droplets

We demonstrate a simple droplet diagnostic approach to monitor the UiO-66 MOF (metal-organic framework) synthesis and its quality using the sessile droplet drying phenomenon. Drying a sessile droplet involves evaporation-driven hydrodynamic flow and particle-nature-dependent self-assembled deposition. In general, the MOF synthesis process involves different sizes and physicochemical nature of particles in every synthesis stage. Equivalent quantities of each of purified pore-activated UiO-66 MOF, yet-to-be-purified pore-inactivated UiO-66 MOF, and reaction precursors of UiO-66 MOF give different deposition patterns when a well-dispersed aqueous droplet of these materials undergoes drying over substrates of varying stiffness and wettability. Yet-to-be-purified, pore-inactivated UiO-66 MOF nanoparticles undergo transport toward the droplet periphery, leading to a thick ring-like deposition at the dried droplet edge. Under appropriate drying conditions, such a deposit leads to desiccation-type mud-like reticular cracking. We study the origin of such ring-like deposits and cracks to understand how the surface charge density of UiO-66 particles controls their stability. We demonstrate that ZrOCl2 salt trapped in a nonpurified pore-inactivated UiO-66 MOF moiety is the principal reason for ring-like deposit formation and subsequent cracking in its dried aqueous droplet edge. Qualitatively, we identified Lewis acid salts that are capable of acting as Bro̷nsted acid upon hydrolysis (like FeCl3, SnCl2, and ZrOCl2), influence surface charge density and colloidal stability of dispersed UiO-66 MOF particles. As a result, immediate particle coagulation is avoided, so those travel to the droplet edge, forming ring-like deposition and subsequent cracking upon drying. Further, we show that crack patterns on such deposits are highly dependent on the stiffness and temperature of depositing substrates via a competition between axial and lateral strains at the deposit-substrate interface.

Iodine-Modified Pd Catalysts Promote the Bifunctional Catalytic Synthesis of 2,5-Hexanedione from C6 Furan Aldehydes

Currently, low intimacy between hydrogenation sites and acidic sites causes unsatisfactory catalytic activity and selectivity for the synthesis of 2,5-hexanedione from C₆ furan aldehydes (5-methylfurfural, 5-hydroxymethylfurfural). Herein, iodine(I) modification of Pd-supported catalysts (such as PdI/Al₂ O₃ and PdI/SiO₂ ) was investigated to modulate the hydrogenation sites and acidic sites. Unlike Pd catalysts that produced 71.4 % yield of 2-hydroxymethyl-5-methyl tetrahydrofuran via an overhydrogenation route of 5-methylfurfural, PdI catalysts showed a high efficiency for 2,5-hexanedione with 93.7 % yield by a hydrogenative ring-opening route. More importantly, the selective synthesis of 2,5-hexanedione from 5-hydroxymethylfurfural with a high yield of 50.2 % by the hydrogenolysis and subsequent ring-opening route was reported for the first time. I-modified Pd nanoparticles produced in-situ hydrogen spillover, which promoted the selective C=O hydrogenation and ring-opening steps by regulating the adsorption configuration of the reactants and the transformation of Lewis to Brønsted acidity, respectively.

Ru(dppbsa)-catalyzed hydrodeoxygenation and reductive etherification of ketones and aldehydes

Ru(dppbsa)-catalyzed hydrodeoxygenation and reductive etherification of ketones and aldehydes were developed. The carbonyl substrates without β-CH functionality follow the hydrogenation-hydrogenolysis path, wherein the hydrogenolysis of the alkanol intermediates presents as...

Batch and Continuous-Flow Preparation of Biomass-Derived Furfural Acetals over a TiO2 Nanoparticle-Exfoliated Montmorillonite Composite Catalyst

Furfural acetals with high octane value, high calorific value and high oxidation resistance are considered promising biofuels or fuel precursors with huge potential demand. However, there are few studies on efficient scalable catalyst systems, including continuous-flow catalyst systems, for their preparation. In this work, TiO₂ nanoparticles supported on exfoliated montmorillonite, with strong Lewis acid sites and abundant accessible Brønsted acid sites, is used to catalyze the acetalization reactions of biomass-derived furfural and alcohols. Low dosage of the catalyst made the reaction reach equilibrium in a very short time (TOF=690-1305 min^-1 ) at room temperature with the acetal as the only product. In continuous-flow reactions, the catalyst showed a stable product output with conversion close to that for the batch reaction with a short catalyst-reactant contact time of 150 s. Contrast experiments revealed that both Lewis and Brønsted acid sites on the catalyst were indispensable for maximizing the catalytic performance, and simultaneously activating both furfural and alcohol on the adjacent Lewis and Brønsted acid sites was proposed to be responsible for the high catalytic performance.

Zirconium and hafnium polyhedral oligosilsesquioxane complexes – green homogeneous catalysts in the formation of bio-derived ethers via a MPV/etherification reaction cascade

Newly designed POSS supported zirconium and hafnium isopropoxides are robust, durable and selective homogeneous catalysts for the conversion of hydroxymethylfurfural to bis(isopropoxymethyl)furane via MPV/etherification reaction with isopropanol as a green solvent/reagent.

Ruthenium on phosphorous-modified alumina as an effective and stable catalyst for catalytic transfer hydrogenation of furfural

Supported ruthenium was used in the liquid phase catalytic transfer hydrogenation of furfural. To improve the stability of Ru against leaching, phosphorous was introduced on a Ru/Al₂O₃ based catalyst upon impregnation with ammonium hypophosphite followed by either reduction or calcination to study the effect of phosphorous on the physico-chemical properties of the active phase. Characterization using X-ray diffraction, solid state ³¹P nuclear magnetic resonance spectroscopy, X-ray absorption spectroscopy, temperature programmed reduction with H₂, infrared spectroscopy of pyridine adsorption from the liquid phase and transmission electron microscopy indicated that phosphorous induces a high dispersion of Ru, promotes Ru reducibility and is responsible for the formation of acid species of Brønsted character. As a result, the phosphorous-based catalyst obtained after reduction was more active for catalytic transfer hydrogenation of furfural and more stable against Ru leaching under these conditions than a benchmark Ru catalyst supported on activated carbon.

Phosphorous induces structural changes in Ru/Al₂O₃ that make it more active and more stable for liquid phase hydrogenation of furfural.

On demand production of ethers or alcohols from furfural and HMF by selecting the composition of a Zr/Si catalyst

Silica is used to tailor the acid–base properties of ZrO₂ to selectively transform furfural and HMF into alcohols or ethers.

Performance-advantaged ether diesel bioblendstock production by a priori design

Lignocellulosic biomass offers a renewable carbon source which can be anaerobically digested to produce short-chain carboxylic acids. Here, we assess fuel properties of oxygenates accessible from catalytic upgrading of these acids a priori for their potential to serve as diesel bioblendstocks. Ethers derived from C₂ and C₄ carboxylic acids are identified as advantaged fuel candidates with significantly improved ignition quality (>56% cetane number increase) and reduced sooting (>86% yield sooting index reduction) when compared to commercial petrodiesel. The prescreening process informed conversion pathway selection toward a C₁₁ branched ether, 4-butoxyheptane, which showed promise for fuel performance and health- and safety-related attributes. A continuous, solvent-free production process was then developed using metal oxide acidic catalysts to provide improved thermal stability, water tolerance, and yields. Liter-scale production of 4-butoxyheptane enabled fuel property testing to confirm predicted fuel properties, while incorporation into petrodiesel at 20 vol % demonstrated 10% improvement in ignition quality and 20% reduction in intrinsic sooting tendency. Storage stability of the pure bioblendstock and 20 vol % blend was confirmed with a common fuel antioxidant, as was compatibility with elastomeric components within existing engine and fueling infrastructure. Technoeconomic analysis of the conversion process identified major cost drivers to guide further research and development. Life-cycle analysis determined the potential to reduce greenhouse gas emissions by 50 to 271% relative to petrodiesel, depending on treatment of coproducts.

Niobium pentoxide nanomaterials with distorted structures as efficient acid catalysts

Niobium pentoxides are promising acid catalysts for the conversion of biomass into fuels and chemicals. Developing new synthesis routes is essential for designing niobium pentoxide catalysts with improved activity for specific practical processes. Here we show a synthesis approach in acetophenone, which produces nanostructured niobium pentoxides with varying structure and acidity that act as efficient acid catalysts. The oxides have orthorhombic structures with different extents of distortions and coordinatively unsaturated metal atoms. A strong dependence is observed between the type and strength of the acid sites and specific structural motifs. Ultrasmall niobium pentoxide nanoparticles, which have strong Brønsted acidity, as well as Lewis acidity, give product yields of 96% (3 h, 140 °C, 100% conversion), 85% (3 h, 140 °C, 86% conversion), and 100% (3 h, 110 °C, 100% conversion) in the reactions of furfuryl alcohol, 5-(hydroxymethyl)furfural, and α-angelica lactone with ethanol, respectively.

Solid Acids for the Reaction of Bioderived Alcohols into Ethers for Fuel Applications

The use of solids acids in the synthesis of ethers suitable to be used as fuels or fuel additives were reviewed in a critical way. In particular, the role of Brønsted and Lewis acid sites was highlighted to focus on the pivotal role of the acidity nature on the product distribution. Particular emphasis is given to the recently proposed ethers prepared starting from furfural and 5-hydroxymethyl furfural. Thus, they are very promising products that can be derived from lignocellulosic biomass and bioalcohols and possess very interesting chemical and physical properties for their use in the diesel sector.

Synergistic Catalysis of Brønsted Acid and Lewis Acid Coexisted on Ordered Mesoporous Resin for One-Pot Conversion of Glucose to 5-Hydroxymethylfurfural

A novel bifunctional ordered phenolic resin with Brønsted acid and Lewis acid sites (Yb(OTf)2/PhSO3H-MPR) was prepared for the first time by a two-step sulfonation and postgrafting protocol. The Brønsted acids (benzenesulfonic acids) were transformed from the phenyl groups that existed in the skeleton of ordered mesoporous phenolic resin. Meanwhile, the benzenesulfonic acids can coordinate with Yb(OTf)3 compound, resulting in the generation of Lewis acids in the pore channels of ordered phenolic resin. Yb(OTf)2/PhSO3H-MPR sample retained large specific surface and well-ordered hexagonal mesopores. As expected, it can promote one-pot cascade reaction by using glucose as the reactant to produce 5-hydroxymethylfurfural with good conversion and moderate selectivity. This synergistic catalytic performance could be attributed to its uniformly distributed Brønsted-Lewis acids. Meanwhile, the intrinsic hydrophobic pore surface can decrease the interference of water solvent, leading to enhanced catalytic efficiency. Besides, it was reused more than five times, showing good stability in water.

Adjusting the acidity of sulfonated organocatalyst for the one-pot production of 5-ethoxymethylfurfural from fructose

A novel sulfonated organocatalyst bearing stable double H-bonds shows high catalytic performance and good reusability for the tandem production of 5-ethoxymethylfurfural (EMF), a biofuel candidate, from biomass-derived fructose.

Synthesis of well-defined yttrium-based Lewis acids by capturing a reaction intermediate and catalytic application for cycloaddition of CO2 to epoxides under atmospheric pressure

Single-site yttrium complexes were prepared by immobilization of an intermediate of cycloaddition of CO₂ to epoxides and applied in catalysis.

Catalytic transfer hydrogenation of biomass-derived 5-hydroxymethylfurfural into 2,5-bis(hydroxymethyl)furan over tunable Zr-based bimetallic catalysts

ZrBa-SBA is highly efficient for the MPV reduction of HMF to BHMF, as the etherification of BHMF was suppressed.