Identification of the Soluble Byproducts Formed during the Hydrothermal Conversion of Cellulose Catalyzed by Solid Tungstated Alumina

The soluble byproducts formed during the hydrothermal conversion of cellulose catalyzed by solid tungstated alumina (AlW) were analyzed by LC-MS and LC-MS² to determine their formulas and possible structures. These identified soluble compounds could be roughly divided into four species of carboxylic acids, α-carbonyl aldehydes, carbocyclic compounds, and furanic compounds with molecular mass in the range of 90-220 Da. Compared with the noncatalytic condition, the addition of AlW could increase the selectivity of carboxylic acids (especially α-hydroxy acid) from cellulose and suppress the formation of furanic compounds, carbocyclic compounds, and hydrochar. Based on the product distribution, the hydrothermal conversion route of glucose was proposed by regarding the formed α-carbonyl aldehydes as the key intermediates for formation of carboxylic acids, carbocyclic compounds, and furanic compounds.

Advances in catalytic routes for the production of carboxylic acids from biomass: a step forward for sustainable polymers

Polymers are ubiquitously present in our daily life because they can meet a wide range of needs and fields of applications. This success, based on an irresponsible linear consumption of plastics and the access to cheap oil, is creating serious environmental problems. Two lines of actions are needed to cope with them: to adopt a circular consumption of plastics and to produce renewable carbon-neutral monomers. This review analyses the recent advances in the chemocatalytic processes for producing biomass-derived carboxylic acids. These renewable carboxylic acids are involved in the synthesis of relevant general purpose and specialty polyesters and polyamides; some of them are currently derived from oil, while others can become surrogates of petrochemical polymers due to their excellent performance properties. Polyesters and polyamides are very suitable to be depolymerised to other valuable chemicals or to their constituent monomers, what facilitates the circular reutilisation of these monomers. Different types of carboxylic acids have been included in this review: monocarboxylic acids (like glycolic, lactic, hydroxypropanoic, methyl vinyl glycolic, methyl-4-methoxy-2-hydroxybutanoic, 2,5-dihydroxypent-3-enoic, 2,5,6-trihydroxyhex-3-enoic acids, diphenolic, acrylic and δ-amino levulinic acids), dicarboxylic acids (2,5-furandicarboxylic, maleic, succinic, adipic and terephthalic acids) and sugar acids (like gluconic and glucaric acids). The review evaluates the technology status and the advantages and drawbacks of each route in terms of feedstock, reaction pathways, catalysts and economic and environmental evaluation. The prospects and the new research that should be undertaken to overcome the main problems threatening their economic viability or the weaknesses that prevent their commercial implementation have also been underlined.

Catalytic cycle of carbohydrate dehydration by Lewis acids: structures and rates from synergism of conventional and DNP NMR

Structures and rates in the catalytic cycle of carbohydrate dehydration by Lewis acidic salt are determined through the systematic use of complementary NMR approaches.

Discovery and Exploration of the Efficient Acyclic Dehydration of Hexoses in Dimethyl Sulfoxide/Water

Current gaps in the development of sustainable processes include a lack of strategies to systematically identify and optimize the formation of new products. The dehydration of hexoses to 5-hydroxymethylfurfural (HMF) is a particularly widely studied process. In an attempt to identify a new high-selectivity conversion of glucose, quantitative NMR spectroscopy is used to screen conditions that have been reported to yield high conversions of glucose but low formation of HMF. In this manner, an olefinic six-carbon byproduct is identified. By adding water, selectivity for the compound was nearly tripled relative to previous reports. The detection of high-yielding side reactions in the formation of HMF is remarkable, considering how extensively HMF formation has been studied. High selectivity for the acyclic pathway allows hitherto unobserved intermediates in this pathway to be identified by using in situ NMR spectroscopy. An additional, presumably cyclic, pathway contributes to HMF formation.

Condensation of α-Carbonyl Aldehydes Leads to the Formation of Solid Humins during the Hydrothermal Degradation of Carbohydrates

Catalytic hydrothermal conversion of carbohydrates could provide a series of versatile valuable platform chemicals, but the formation of solid humins greatly decreased the efficiency of the process. Herein, by studying the hydrothermal degradation behavior and analyzing the degradation paths of kinds of model compounds including carbohydrates, furan compounds, cyclic ketone derivatives, and some simple short carbon-chain oxy-organics, we demonstrate that α-carbonyl aldehydes and α-carbonyl acids are the key primary precursors for humin formation during the hydrothermal conversion process. Then, we analyzed the hydrothermal degradation paths of two simple α-carbonyl aldehydes including glyoxal and pyruvaldehyde and found that the α-carbonyl aldehydes could undergo aldol condensation followed by acetal cyclization and dehydration to form solid humins rich of furan ring structure or undergo Cannizaro route (hydration followed by 1,2-hydride shift) to form corresponding α-hydroxy acids. On the basis of the hydrothermal behavior of the α-carbonyl aldehydes, we mapped the hydrothermal degradation routes of carbohydrates (glucose, fructose, and xylose) and illuminated the formation details of α-carbonyl aldehydes, α-hydroxy acids, γ-lactones, furfural derivatives, and humins. Finally, we deduced the typical structure fragments of humins from three α-carbonyl aldehydes of pyruvaldehyde, 2,5-dioxo-6-hydroxy-hexanal, and 3-deoxyglucosone, all of which could be formed during the hydrothermal degradation of hexose.

Catalytic Processes from Biomass-Derived Hexoses and Pentoses: A Recent Literature Overview

Biomass is a plentiful renewable source of energy, food, feed and chemicals. It fixes about 1–2% of the solar energy received by the Earth through photosynthesis in both terrestrial and aquatic plants like macro- and microalgae. As fossil resources deplete, biomass appears a good complement and eventually a good substitute feedstock, but still needs the development of relatively new catalytic processes. For this purpose, catalytic transformations, whether alone or combined with thermal ones and separation operations, have been under study in recent years. Catalytic biorefineries are based on dehydration-hydrations, hydrogenations, oxidations, epimerizations, isomerizations, aldol condensations and other reactions to obtain a plethora of chemicals, including alcohols, ketones, furans and acids, as well as materials such as polycarbonates. Nevertheless, there is still a need for higher selectivity, stability, and regenerability of catalysts and of process intensification by a wise combination of operations, either in-series or combined (one-pot), to reach economic feasibility. Here we present a literature survey of the latest developments for obtaining value-added products using hexoses and pentoses derived from lignocellulosic material, as well as algae as a source of carbohydrates for subsequent transformations.

Effects of Alkali-Metal Ions and Counter Ions in Sn-Beta-Catalyzed Carbohydrate Conversion

Alkali-metal ions have recently been shown to strongly influence the catalytic behavior of stannosilicates in the conversion of carbohydrates. An effect of having alkali-metal ions present is a pronounced increase in selectivity towards methyl lactate. Mechanistic details of this effect have remained obscure and are herein addressed experimentally through kinetic experiments and isotope tracking. The presence of alkali-metal ions has a differential effect in competing reaction pathways and promotes the rate of carbon-carbon bond breakage of carbohydrate substrates, but decreases the rates of competing dehydration pathways. Further addition of alkali-metal ions inhibits the activity of Sn-Beta in all major reaction pathways. The alkali-metal effects on product distribution and on the rate of product formation are similar, thus pointing to a kinetic reaction control and to irreversible reaction steps in the main pathways. Additionally, an effect of the accompanying basic anions is shown, supposedly facilitating the cation exchange and eliciting a different concentration-dependent effect to that of neutral alkali-metal salts.

Kinetic analysis of hexose conversion to methyl lactate by Sn-Beta: effects of substrate masking and of water

Strategies to tailor the Sn-Beta-catalysed methyl lactate process are identified by kinetic and mechanistic insights.

Facile and benign conversion of sucrose to fructose using zeolites with balanced Brønsted and Lewis acidity

A simple and robust two-step process with zeolites as catalysts converts sucrose in high yield into the versatile monosaccharide fructose.