Carbon materials such as activated coal, nanotubes, nanofibers, or graphene nanoplatelets were functionalized with sulfonic acid moieties by a diazonium coupling strategy. High acidity was obtained for the majority of the carbon solids except for the carbon nanofibers. The activity of these acidic catalysts for the hydrolysis of cellobiose, as model molecule for cellulose, into glucose in neutral water medium was studied. The conversion of cellobiose is increasing with the acidity of the catalyst. We found that a minimum threshold amount of acidic functions is required for triggering the hydrolysis. The selectivity toward glucose is very high as soon as sulfonic functions are present on the catalyst. The robustness of the sulfonic functions grafted on the carbons has been highlighted by successful recyclability over six runs.
Ruthenium nanoparticles supported on carbon black were coated by mesoporous protective silica layers (Ru/CB@SiO2) with different textural properties (SBET: 280–390 m2/g, pore diameter: 3.4–5.0 nm) and were tested in the selective hydrogenation of glucose into sorbitol. The influence of key parameters such as the protective layer pore size and the solvent nature were investigated. X-ray photoelectron spectroscopy (XPS) analyses proved that the hydrothermal stability was highly improved in ethanolic solution with low water content (silica loss: 99% in water and 32% in ethanolic solution). In this work, the strong influence of the silica layer pore sizes on the selectivity of the reaction (shifting from 4% to 68% by increasing the pores sizes from 3.4 to 5 nm) was also highlighted. Finally, by adding acidic co-catalyst (CB–SO3H), sorbitol was obtained directly through the hydrolytic hydrogenation of cellobiose (used as a model molecule of cellulose), demonstrating the high potential of the present methodology to produce active catalysts in biomass transformations.
Supported catalysts were prepared from water-soluble molecular clusters by pH controlled impregnations in order to probe the interactions occurring between the supports and the clusters and to maximize them. The PZC of different nano-carbon solids (nanotubes and nanofibers) was determined. The EpHL method of measuring the PZC could be successfully extended for the first time to these nano-carbon supports. When impregnating these nano-carbons with water-soluble Ru clusters by varying the pH, we found that two adsorption mechanisms were taking place. We postulate that interactions in the form of π-bond coordination or reactions with higher reactivity zones of the carbon surface occur at all pH values. Electrostatic interactions coexist with the latter and play a determining role, allowing or hindering maximal adsorption. Our water-impregnated samples exhibit smaller and better distributed nanoparticles in comparison to an organic-solvent-impregnated sample. Sintering of the particles at higher activation temperature led to nanoparticles with a bimodal size distribution on the nanofibers. The bimodal size distribution is a strong indication of two different adsorption mechanisms. The obtained Ru/nano-C catalysts present a valuable activity and selectivity in the hydrogenation of lactose into lactitol.
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