Solid Foam Ru/C Catalysts for Sugar Hydrogenation to Sugar Alcohols-Preparation, Characterization, Activity, and Selectivity

Abstract

Sugar alcohols are obtained by hydrogenation of sugars in the presence of ruthenium catalysts. The research effort was focused on the development of solid foam catalysts based on ruthenium nanoparticles supported on active carbon. This catalyst was used in kinetic experiments on the hydrogenation of l-arabinose and d-galactose at three temperatures (90, 100, and 120 °C) and two hydrogen pressures (20 and 40 bar). Kinetic experiments were carried out with binary sugar mixtures at different d-galactose-to-l-arabinose molar ratios to study the interactions of these sugars in the presence of the prepared solid foam catalyst. The solid foam catalyst preparation comprised the following steps: cutting of the open-cell foam aluminum pieces, anodic oxidation pretreatment, carbon coating, acid pretreatment, ruthenium incorporation, and ex situ reduction. The carbon coating method comprised the polymerization of furfuryl alcohol, followed by a pyrolysis process and activation with oxygen. Incorporation of ruthenium on the carbon-coated foam was done by incipient wetness impregnation (IWI), using ruthenium(III) nitrosyl nitrate as the precursor. By applying IWI, it was possible to prepare an active catalyst with a ruthenium load of 1.12 wt %, which gave a high conversion of the sugars to the corresponding sugar alcohols. The catalysts were characterized by SEM, HR-TEM, TPR, and ICP-OES to interpret the catalyst behavior in terms of activity, durability, and critical parameters for the catalyst preparation. Extensive kinetic experiments were carried out in an isothermal laboratory-scale semibatch reactor to which gaseous hydrogen was constantly added. High selectivities toward the sugar alcohols, arabitol and galactitol, exceeding 98% were obtained for both sugars, and the sugar conversions were within the range of 53-97%, depending on temperature. The temperature effect on the reaction rate was very strong, while the effect of hydrogen pressure was minor. Regarding the sugar mixtures, in general, l-arabinose presented a higher reaction rate, and an acceleration of the hydrogenation process was observed for both sugars as the ratio of d-galactose to l-arabinose increased, evidently because of competitive interactions on the catalyst surface.

Figure 1

Sugar alcohols from lignocellulosic biomass: catalytic hydrogenation.

Figure 2

Optical microscope image of an open-cell metallic foam.

Figure 3

Overview of the solid foam catalyst preparation process.

Figure 4

Overview of the setup for sugar hydrogenation experiments.

Figure 5

Changes in the open-cell foam catalyst through the preparation stages: (a) Al-untreated foam, (b) anodized Al foam, (c) foam coated with poly(furfuryl alcohol), (d) pyrolyzed/oxygen-treated carbon-coated foam, and (e) carbon-coated, Ru-impregnated, and reduced catalyst.

Figure 6

SEM micrographs of the oxide texture generated in catalyst C10. (a) Untreated foam (30X), (b) untreated foam (50 kX), (c) anodized foam (30X), (d) anodized foam (50 kX), (e) anodized and calcined foam (30 kX), and (f) anodized and calcined foam (50 kX).

Figure 7

Surface structure of a carbon-coated foam substrate: (a) C2 (∼12 wt % carbon), obtained from golden-colored poly(furfuryl alcohol); (b) C8 (∼50 wt % carbon), obtained from foamy dark poly(furfuryl alcohol); and (c) C10 (∼50 wt % carbon), preanodized and obtained from foamy dark poly(furfuryl alcohol).

Figure 8

TEM images of catalyst C10 and Ru nanoparticle size distribution.

Figure 9

Hydrogen-TPR profiles of catalyst C10 (before ex situ reduction).

Figure 10

Deactivation of catalyst C8 during hydrogenation of (a) l-arabinose and (b) d-galactose at 120 °C and 20 bar.

Figure 11

TEM images of catalyst C8 after 100 h of use and particle size distribution.

Figure 12

Effect of temperature on the hydrogenation rates at 20 bar for (a) l-arabinose and (b) d-galactose.

Figure 13

Effect of the hydrogen pressure on the hydrogenation rates at 20 bar for (a) l-arabinose and (b) d-galactose.

Figure 14

Effect of the d-galactose-to-l-arabinose molar ratio on the hydrogenation rates at 120° of (a) l-arabinose and (b) d-galactose.