Elucidating the Roles of Zeolite H-BEA in Aqueous-Phase Fructose Dehydration and HMF Rehydration

Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites

The Brønsted acidity of proton-exchanged zeolites has historically led to the most impactful applications of these materials in heterogeneous catalysis, mainly in the fields of transformations of hydrocarbons and oxygenates. Unravelling the mechanisms at the atomic scale of these transformations has been the object of tremendous efforts in the last decades. Such investigations have extended our fundamental knowledge about the respective roles of acidity and confinement in the catalytic properties of proton exchanged zeolites. The emerging concepts are of general relevance at the crossroad of heterogeneous catalysis and molecular chemistry. In the present review, emphasis is given to molecular views on the mechanism of generic transformations catalyzed by Brønsted acid sites of zeolites, combining the information gained from advanced kinetic analysis, in situ, and operando spectroscopies, and quantum chemistry calculations. After reviewing the current knowledge on the nature of the Brønsted acid sites themselves, and the key parameters in catalysis by zeolites, a focus is made on reactions undergone by alkenes, alkanes, aromatic molecules, alcohols, and polyhydroxy molecules. Elementary events of C-C, C-H, and C-O bond breaking and formation are at the core of these reactions. Outlooks are given to take up the future challenges in the field, aiming at getting ever more accurate views on these mechanisms, and as the ultimate goal, to provide rational tools for the design of improved zeolite-based Brønsted acid catalysts.

Selective Electrosynthesis of 2,5-Diformylfuran in a Continuous-Flow System

The gram-scale selective oxidation of biomass-based chemicals, in particular 5-hydroxymethylfurfural (HMF), into value-added 2,5-diformylfuran (DFF) has a high application potential but suffers from high cost, low selectivity, and harsh reaction conditions. Besides, the electrooxidation strategy requires the usage of expensive electrodes and struggles with low selectivity and efficiency, which restricts its further scaled-up application. In this regard, a continuous-flow system was developed through redox mediator I^- /I₂ for the efficient synthesis of DFF, which could accelerate the mass transfer of I^- (I₂ ) to aqueous (organic) phase and avoid over-oxidation to achieve high selectivity. After the solvent system, iodine concentration, and reaction time were optimized, highly efficient DFF synthesis (selectivity >99 %) could be achieved in the electrochemical flow system using inexpensive graphite felt (GF) as electrode. Moreover, selective HMF oxidation was paired with the hydrogen evolution reaction with increased efficiency after using in-situ-loaded GF-CoS₂ /CoS and GF-Pt electrodes. As a result, the required energy to achieve the gram-scale synthesis of DFF was significantly reduced, demonstrating outstanding potential for large-scale production of the target product.

Dehydration of Fructose to 5-Hydroxymethylfurfural: Effects of Acidity and Porosity of Different Catalysts in the Conversion, Selectivity, and Yield

There is a demand for renewable resources, such as biomass, to produce compounds considered as platform molecules. This study deals with dehydration of fructose for the formation of 5-hydroxymethylfurfural (HMF), a feedstock molecule. Different catalysts (aluminosilicates, niobic acid, 12-tungstophosphoric acid—HPW, and supported HPW/Niobia) were studied for this reaction in an aqueous medium. The catalysts were characterized by XRD, FT-IR, N2 sorption at −196 °C and pyridine adsorption. It was evident that the nature of the sites (Brønsted and Lewis), strength, quantity and accessibility to the acidic sites are critical to the conversion and yield results. A synergic effect of acidity and mesoporous area are key factors affecting the activity and selectivity of the solid acids. Niobic acid (Nb2O5·nH2O) revealed the best efficiency (highest TON, yield, selectivity and conversion). It was determined that the optimum acidity strength of catalysts should be between 80 to 100 kJ mol−1, with about 0.20 to 0.30 mmol g−1 of acid sites, density about 1 site nm−2 and mesoporous area about 100 m2 g−1. These values fit well within the general order of the observed selectivity (i.e., Nb2O5 > HZSM-5 > 20%HPW/Nb2O5 > SiO2-Al2O3 > HY > HBEA).

Selective Conversion of Scenedesmus into Lactic Acid over Amine-Modified Sn-β

Amine-modified Sn-β was synthesized to improve the yield of lactic acid produced from Scenedesmus. After studying the growth of Scenedesmus, we selected Scenedesmus with the highest sugar content of 46.7% after 8 days of culture as the reaction substrate. The results showed that the yield of lactic acid from Scenedesmus was greatly increased after being catalyzed by 3-aminopropyltrimethoxysilane (APTMS)-modified Sn-β. After the pretreatment of Scenedesmus in an ice bath ultrasound, under the optimal reaction conditions (190 °C and 5 h), the yield of lactic acid reached the highest (37%). The acid-base characterization results of the catalyst confirmed that there are both Lewis acidic sites and medium-strength basic sites in the catalyst. Both of these sites can promote the hydrolysis of Scenedesmus, while the Lewis acidic sites can promote the production of lactic acid and the basic sites can effectively inhibit the production of the byproduct 5-hydroxymethylfurfural (HMF). This study proved that this amination catalyst is a useful strategy to increase the yield of lactic acid.

Catalytic Activity of Mixed Al2O3-ZrO2 Oxides for Glucose Conversion into 5-Hydroxymethylfurfural

In the present work, a series of catalysts based on aluminum and zirconium oxides was studied for the transformation of glucose into 5-hydroxymethylfurfural. These catalysts were characterized by using experimental techniques, such as X-ray diffraction, N2 adsorption–desorption at −196 °C, X-ray photoelectron spectroscopy, temperature-programmed desorption of NH3 and CO2, and scanning transmission electron microscopy. The catalytic behavior in glucose dehydration was evaluated in a water-methyl isobutyl ketone biphasic system, in the presence of CaCl2, in order to minimize losses due to unwanted secondary reactions. High glucose conversion and 5-hydroxymethylfurfural (HMF) yield values were obtained in the presence of an Al(Zr)Ox catalyst with an Al:Zr molar ratio of 7:3, reaching 97% and 47%, respectively, at 150 °C after 120 min. Under tested conditions, this catalyst retained most of its catalytic activity for four reuses.

Catalytic Upgrading of Biomass-Derived Sugars with Acidic Nanoporous Materials: Structural Role in Carbon-Chain Length Variation

Shifting from petroleum-based resources to inedible biomass for the production of valuable chemicals and fuels is one of the significant aspects in sustainable chemistry for realizing the sustainable development of our society. Various renowned biobased platform molecules, such as 5-hydroxymethylfurfural, furfural, levulinic acid, and lactic acid, are successfully accessible from the transformation of biobased sugars. To achieve the specific reaction routes, heterogeneous nanoporous acidic materials have served as promising catalysts for the conversion of bio-sugars in the past decade. This Review summarizes advances in various nanoporous acidic materials for bio-sugar conversion, in which the number of carbon atoms is variable and controllable with the assistance of the switchable structure of nanoporous materials. The major focus of this Review is on possible reaction pathways/mechanisms and the relationships between catalyst structure and catalytic performance. Moreover, representative examples of catalytic upgrading of biobased platform molecules to biochemicals and fuels through selective C-C cleavage and coupling strategies over nanoporous acidic materials are also discussed.

Theoretical investigation on the mechanism of glucose-to-fructose isomerization synergistically catalyzed by MnCl2 and [C4SO3HMIM][CH3SO3] in [BMIM]Cl

The mechanism of glucose-to-fructose isomerization catalyzed by manganese chloride (MnCl₂) and 1-methyl-3-(3-sulfobutyl)-imidazolium methylsulfonate ([C₄SO₃HMIM][CH₃SO₃]) in a 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) ionic liquid (IL) was investigated computationally.

Preparation of highly active phosphated TiO2catalystsviacontinuous sol–gel synthesis in a microreactor

Highly efficient TiO₂based catalysts for biomass conversion were obtained through optimised and well-controlled sol–gel synthesis in a multi-mixer microreactor.

Fabrication of mesoporous POMs/SiO2nanofibers through electrospinning for oxidative conversion of biomass by H2O2and oxygen

The oxidation process for mesoporous H₅PMo₁₀V₂O₄₀/SiO₂nanofiber catalyst.

Tailoring nanoscopic confines to maximize catalytic activity of hydronium ions

Acid catalysis by hydronium ions is ubiquitous in aqueous-phase organic reactions. Here we show that hydronium ion catalysis, exemplified by intramolecular dehydration of cyclohexanol, is markedly influenced by steric constraints, yielding turnover rates that increase by up to two orders of magnitude in tight confines relative to an aqueous solution of a Brønsted acid. The higher activities in zeolites BEA and FAU than in water are caused by more positive activation entropies that more than offset higher activation enthalpies. The higher activity in zeolite MFI with pores smaller than BEA and FAU is caused by a lower activation enthalpy in the tighter confines that more than offsets a less positive activation entropy. Molecularly sized pores significantly enhance the association between hydronium ions and alcohols in a steric environment resembling the constraints in pockets of enzymes stabilizing active sites.

The rates of acid-catalysed reactions vary in constrained environments. Here the authors show that molecularly sized pores greatly promote aqueous phase alcohol dehydration by enhancing the association between substrate and hydronium ions, and even by lowering the free energy barrier.

External surface and pore mouth catalysis in hydrolysis of inulin over zeolites with different micropore topologies and mesoporosities

External surface and pore mouth catalysis for inulin hydrolysis over zeolite catalysts.

Heterogeneously Catalyzed Hydrothermal Processing of C5-C6 Sugars

Biomass has been long exploited as an anthropogenic energy source; however, the 21st century challenges of energy security and climate change are driving resurgence in its utilization both as a renewable alternative to fossil fuels and as a sustainable carbon feedstock for chemicals production. Deconstruction of cellulose and hemicellulose carbohydrate polymers into their constituent C₅ and C₆ sugars, and subsequent heterogeneously catalyzed transformations, offer the promise of unlocking diverse oxygenates such as furfural, 5-hydroxymethylfurfural, xylitol, sorbitol, mannitol, and gluconic acid as biorefinery platform chemicals. Here, we review recent advances in the design and development of catalysts and processes for C₅-C₆ sugar reforming into chemical intermediates and products, and highlight the challenges of aqueous phase operation and catalyst evaluation, in addition to process considerations such as solvent and reactor selection.

HMF etherification using NH4-exchanged zeolites

The reversible dissociation of NH₄⁺ ions in the intra-cages of zeolites is correlated with their catalytic reactivity for HMF etherification.

Catalytic behaviour of TiO2–ZrO2 binary oxide synthesized by sol–gel process for glucose conversion to 5-hydroxymethylfurfural

A combination of TiO₂–ZrO₂ and Amberlyst 70 remarkably catalyzes glucose conversion to HMF in a biphasic media.

One-pot synthesis of 5-hydroxymethylfurfural from carbohydrates using an inexpensive FePO4 catalyst

A “double character” FePO₄ catalyst, which acts as a homogeneous catalyst at high temperature, was used to produce 5-HMF starting from cellulose and woody biomass without the addition of homogeneous acids.

Hydrophobic microporous and mesoporous oxides as Brønsted and Lewis acid catalysts for biomass conversion in liquid water

Microporous and mesoporous silicates with internal or external hydrophobic surfaces show differences in catalytic reactivity and stability in liquid water.

Adsorption of the compounds encountered in monosaccharide dehydration in zeolite beta

A comprehensive study of the adsorption of the compounds involved in the reaction of dehydration of fructose to 5-hydroxymethyl furfural (HMF) on the zeolite H-BEA with SiO2/Al2O3 = 18 has been carried out. Furthermore, a method for the estimation of the real adsorption loading from the experimentally measured excess adsorption is developed and applied to calculate the adsorption isotherms both in the case of single-solute and multisolute mixtures. It was found that zeolite H-BEA adsorbs HMF and levulinic acid from water mixtures to greater extent than sugars and formic acid, which prefer to partition in the aqueous phase. HMF and levulinic acid adsorption isotherms could be fitted in a Redlich-Peterson isotherm model, while the adsorption of formic acid is better fitted using the Freundlich model and sugars via the Henry model. Adsorption loadings decreased with increasing temperature (0, 25, and 40 °C), which is characteristic of an exothermic process. From the temperature dependence of the isotherms, the limiting heat of adsorption at zero coverage was determined using van't Hoff equation. Given the importance and the complexity of multicomponent systems, several experiments of adsorption of multisolute solutions have been carried out. In most of the cases, the ideal adsorbed solution theory (IAST) has been proven to satisfactorily predict adsorption from multisolute mixtures using as input the single-solute isotherms.