Mechanochemical Synthesis of a Carboxylated Carbon Catalyst and Its Application in Cellulose Hydrolysis

Mechanochemical Synthesis of Porous Carbons and Their Applications in Catalysis

Porous carbons have shown considerable potential in catalysis as either as supports or metal-free catalysts. Various methods based on solution chemistry have been intensively developed for the preparation of porous carbon-based catalysts with controllable morphology, pore structure, surface chemical property as well as the desired active sites. Nowadays, mechanochemical synthesis, a re-emerging strategy, has become more and more popular in the synthesis of porous carbons, due to its feasibility and high synthetic efficiency under solvent-free condition. This Minireview presents recent advances in the mechanochemical synthesis of porous carbons by ball milling, and their applications in catalysis. It starts a brief introduction of the characteristics and work mechanism of ball milling, and then discuss the preparation of porous carbons as metal-free catalysts and carbon-supported metal catalysts. Finally, some issues and further opportunities for the mechanochemical synthesis of porous carbon-based catalysts are proposed and discussed.

Unraveling the hydrolysis of β-1,4-glycosidic bonds in cello-oligosaccharides over carbon catalysts

Larger cello-oligosaccharides undergo faster hydrolysis over carbon catalysts. This is attributed to reduction in activation energy caused by conformational change in the structure of oligosaccharides as they adsorb within the micropores of carbon.

Mechanochemistry-assisted hydrolysis of softwood over stable sulfonated carbon catalysts in a semi-batch process

The hydrolysis of lignocellulose is the first step in saccharide based bio-refining. The recovery of homogeneous acid catalysts imposes great challenges to the feasibility of conventional hydrolysis processes. Herein, we report a strategy to overcome these limitations by using stable sulfonated carbons as solid acid catalysts in a two-step process, composed of mechanocatalytic pretreatment and secondary hydrolysis in a semi-batch reactor. Without mechanocatalytic pre-treatment the hydrolysis of the insoluble substrate largely occurs through homogeneously catalyzed reactions. Ball-milling induced amorphization promotes a substantially higher substrate reactivity, because homogeneous hydrolysis occurs preferentially from less ordered structural domains in cellulose. In contrast, concerted ball-milling (CBM) of cellulose with the sulfonated carbon promotes a heterogeneously catalyzed hydrolysis to soluble oligosaccharides. By performing an in-depth physicochemical characterization of cellulose subjected to CBM treatment with different carbons, we reveal the crucial role of strong Brønsted acid sites in facilitating mechanocatalytic depolymerization. Recyclability experiments confirmed that despite being subject to profound structural changes during repeated pre-treatment/semi-batch hydrolysis cycles, the sulfonated carbon retained its catalytic activity. The combination of mechanocatalytic pretreatment with strong solid acids and hydrolysis in the semi-batch reactor was successfully extrapolated for the first time to the hydrolysis of real lignocellulose to achieve quantitative yields in C5 and high yields in C6 derived products.

Mechanochemical-assisted hydrolysis of pretreated rice straw into glucose and xylose in water by weakly acidic solid catalyst

In this work, carbonaceous materials bearing only weakly acidic COOH and phenolic OH groups were directly prepared by the pyrolysis of lignin and KOH in black liquor generating from rice straw that was pretreated with KOH aqueous solution. The synthesized carbon materials were used for the hydrolysis of cellulose or the alkali pretreated rice straw in water, after mixed ball-milling pretreatment, and provided a high glucose yield of 76.3% for cellulose, high yields of 52.1% glucose and 66.5% xylose for alkali pretreated rice straw, respectively, in 0.015 wt% HCl aqueous solution at 200 °C for 60 min. The weakly acidic catalyst showed good stability and recyclability in the aqueous reaction system. This work provides an efficient process for the hydrolysis of lignocellulose by biomass-derived weakly acidic catalysts in water and should have wide applications in biomass utilization.

Deactivation and Regeneration of Sulfonated Carbon Catalysts in Hydrothermal Reaction Environments

The deactivation pathways of sulfonated carbon catalysts prepared from different carbons were studied during the aqueous-phase hydrolysis of cellobiose under continuous-flow conditions. The sulfonation of carbon materials with a low degree of graphitization introduced sulfonic acid groups that are partially stable even during prolonged exposure to harsh hydrothermal treatment conditions (180 °C). The physicochemical characterization of hydrothermally treated materials coupled with the treatment of model compounds for sulfonic acids demonstrated that the stability is related to the presence of activating and deactivating substituents on the aromatic system. Besides sulfonic acid group leaching, a hitherto unknown mode of deactivation was identified that proceeds by the ion exchange of cations contained in the aqueous feed and protons of the sulfonic acid groups. Proton leaching is a fully reversible mode of deactivation by the treatment of the spent catalysts with strong Brønsted acids. Through a combined approach of physicochemical characterization, catalytic testing, and hydrothermal treatment, a methodology for the preparation of catalytically stable carbon materials that bear sulfonic acid groups was established.

Acid-Catalyzed Conversion of Carbohydrates into Value-Added Small Molecules in Aqueous Media and Ionic Liquids

Biomass is the only realistic major alternative source (to crude oil) of hydrocarbon substrates for the commercial synthesis of bulk and fine chemicals. Within biomass, terrestrial sources are the most accessible, and therein lignocellulosic materials are most abundant. Although lignin shows promise for the delivery of certain types of organic molecules, cellulose is a biopolymer with significant potential for conversion into high-volume and high-value chemicals. This review covers the acid-catalyzed conversion of lower value (poly)carbohydrates into valorized organic building-block chemicals (platform molecules). It focuses on those conversions performed in aqueous media or ionic liquids to provide the reader with a perspective on what can be considered a best case scenario, that is, that the overall process is as sustainable as possible.

Potassium Tethered Carbons with Unparalleled Adsorption Capacity and Selectivity for Low-Cost Carbon Dioxide Capture from Flue Gas

Carbons are considered less favorable for postcombustion CO₂ capture because of their low affinity toward CO₂, and nitrogen doping was widely studied to enhance CO₂ adsorption, but the results are still unsatisfactory. Herein, we report a simple, scalable, and controllable strategy of tethering potassium to a carbon matrix, which can enhance carbon-CO₂ interaction effectively, and a remarkable working capacity of ca. 4.5 wt % under flue gas conditions was achieved, which is among the highest for carbon-based materials. More interestingly, a high CO₂/N₂ selectivity of 404 was obtained. Density functional theory calculations evidenced that the introduced potassium carboxylate moieties are responsible for such excellent performances. We also show the effectiveness of this strategy to be universal, and thus, cheaper precursors can be used, holding great promise for low-cost carbon capture from flue gas.

Air Oxidation of Activated Carbon to Synthesize a Biomimetic Catalyst for Hydrolysis of Cellulose

Oxygenated carbon catalyzes the hydrolysis of cellulose present in lignocellulosic biomass by utilizing the weakly acidic functional groups on its surface. Here we report the synthesis of a biomimetic carbon catalyst by simple and economical air-oxidation of a commercially available activated carbon. Air- oxidation at 450-500 °C introduced 2000-2400 μmol g(-1) of oxygenated functional groups on the material with minor changes in the textural properties. Selectivity towards the formation of carboxylic groups on the catalyst surface increased with the increase in oxidation temperature. The degree of oxidation on carbon catalyst was found to be proportional to its activity for hydrolysis of cellulose. The hydrolysis of eucalyptus in the presence of carbon oxidized at 475 °C afforded glucose yield of 77 % and xylose yield of 67 %.