Porous carbon in catalytic transformation of cellulose

Design of highly active Ni catalysts supported on carbon nanofibers for the hydrolytic hydrogenation of cellobiose

The direct transformation of cellulose into sugar alcohols (one-pot conversion) over supported nickel catalysts represents an attractive chemical route for biomass valorization, allowing the use of subcritical water in the hydrolysis step. The effectiveness of this process is substantially conditioned by the hydrogenation ability of the catalyst, determined by design parameters such as the active phase loading and particle size. Herein, mechanistic insights into catalyst design to produce superior activity were outlined using the hydrolytic hydrogenation of cellobiose as a model reaction. Variations in the impregnation technique (precipitation in basic media, incipient wetness impregnation, and the use of colloidal-deposition approaches) endowed carbon-nanofiber-supported catalysts within a wide range of Ni crystal sizes (5.8–20.4 nm) and loadings (5–14 wt%). The link between the properties of these catalysts and their reactivity has been established using characterization techniques such as X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma-optical emission spectroscopy (ICP-OES). A fair compromise was found between the Ni surface area (3.89 m²/g) and its resistance against oxidation for intermediate crystallite sizes (∼11.3 nm) loaded at 10.7 wt%, affording the hydrogenation of 81.2% cellobiose to sorbitol after 3 h reaction at 190°C and 4.0 MPa H₂ (measured at room temperature). The facile oxidation of smaller Ni particle sizes impeded the use of highly dispersed catalysts to reduce the metal content requirements.

Acid Properties of GO and Reduced GO as Determined by Microcalorimetry, FTIR, and Kinetics of Cellulose Hydrolysis-Hydrogenolysis

Graphene oxide addresses increasing interests as a solid acid catalyst working in water for carbohydrate conversion. If there is a general agreement to correlate its unique catalytic performances to its ability to adsorb sugars, the origin of its acidity remains controversial. In this article, we study the acid strength of graphene oxide (GO) prepared by modified Hummers method and that of reduced GO by calorimetry of NH3 adsorption and by FTIR of pyridine adsorption. Very strong acid sites are detected on GO by calorimetry, while reduced graphene oxide (reGO) is not very acidic. The FTIR of pyridine adsorption shows the prevailing presence of Br∅nsted acid sites and a unique feature, the presence of pyridine coordinated by hydrogen bonds. This exceptionally strong Br∅nsted acidity is tentatively explained by the presence of graphene domains decorated by hydroxyl, carboxylic, or sulfonated groups within the GO sheet, resulting in a high mobility of the negative charges which makes the proton free and explains its strong acidity. Accordingly, only GO is active and selective for native cellulose hydrolysis, leading to 27% yield in glucose. Finally, we show that sugar alcohols cannot be formed directly from cellulose using GO combined with Pt/re-GO under hydrogen, explained by the reduction of oxygenated functions of GO. The instability of the functional groups of GO in a reducing atmosphere is the weak point of this peculiar solid acid.

The effect of physical morphology and the chemical state of Ru on the catalytic properties of Ru–carbon for cellulose hydrolytic hydrogenation

Ru–carbon catalysts with different physical morphologies and chemical states of Ru were prepared by different methods and used to catalyze the hydrolytic hydrogenation of cellulose at high temperatures.

Size-controllable ultrafine palladium nanoparticles immobilized on calcined chitin microspheres as efficient and recyclable catalysts for hydrogenation

In the present work, chitin microspheres were impregnated at different concentrations of palladium salt solution to generate the precursor-Pd2+/chitin, and then a series of size-controllable palladium nanocatalysts (Pd@chitin) were successfully constructed by calcining the composite microspheres. Transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) provided reliable evidence for well-dispersed and ultrafine palladium nanoparticles (Pd NPs) with mean diameters from about 1 to 3 nm. Chitin microspheres, as the supporting framework of these catalysts, played a significant role for stabilizing the highly dispersed Pd NPs based on their abundant functional groups and large surface areas. Moreover, the chitin matrix acted as a reductant for the precursor-Pd2+ during calcination, and the calcination process made Pd@chitin more stable. These Pd@chitin catalysts were further tested for the hydrogenations of styrene and benzaldehyde, and they displayed superior catalytic activities compared to commercial Pd/C and unsupported homogeneous Pd(OAc)2 catalysts. Notably, the most active catalyst of 1.2 wt% Pd@chitin had a highly competitive turnover frequency (TOF) of 50 000 h-1 in the hydrogenation of styrene, and the catalyst could be repeatedly used for more than 10 cycles with no decay of the catalytic activity, suggesting potential industrial applications.

Simultaneous catalytic conversion of cellulose and corncob xylan under temperature programming for enhanced sorbitol and xylitol production

Sorbitol and xylitol yields can be improved by converting cellulose and xylan simultaneously, due to a synergetic effect between both substrates. Furthermore, both yields can be greatly enhanced by simply adjusting the reaction conditions regarding the optimum for the production of each product, since xylitol (from xylan) and sorbitol (from cellulose) yields are maximized when the reaction is carried out at 170 and 205°C, respectively. Therefore, the combination of a simultaneous conversion of cellulose and xylan with a two-step temperature approach, which consists in the variation of the reaction temperature from 170 to 205°C after 2h, showed to be a good strategy for maximizing the production of sorbitol and xylitol directly from mixture of cellulose and xylan. Using this new and environmentally friendly approach, yields of sorbitol and xylitol of 75 and 77%, respectively, were obtained after 6h of reaction.

Direct catalytic production of sorbitol from waste cellulosic materials

Cotton wool, cotton textile, tissue paper and printing paper, all potential waste cellulosic materials, were directly converted to sorbitol using a Ru/CNT catalyst in the presence of H₂ and using only water as solvent, without any acids. Conversions up to 38% were attained for the raw substrates, with sorbitol yields below 10%. Ball-milling of the materials disrupted their crystallinity, allowing reaching 100% conversion of cotton wool, cotton textile and tissue paper after 4h, with sorbitol yields around 50%. Mix-milling these materials with the catalyst greatly enhanced their conversion rate, and the materials were efficiently converted to sorbitol with a yield around 50% in 2h. However, ball- and mix-milled printing paper presented a conversion of only 50% after 5h, with sorbitol yields of 7%. Amounts of sorbitol of 0.525, 0.511 and 0.559g could be obtained from 1g of cotton wool, cotton textile and tissue paper, respectively.