Solvent Effect in Hydrogenolysis of Xylitol over Bifunctional Ru/MnO/C Catalysts under Alkaline-Free Conditions

Abstract

The hydrogenolysis reaction of biomass-derived xylitol to glycols and glycerol has been carried out in different aqueous solvents over a bifunctional Ru/MnO/C catalyst under alkaline-free conditions at 60 bar H 2 and 200°C. In pure water, the retro-aldol reaction takes place. However, decarbonylation and epimerization are the dominant reactions and produce C4 and C5 alditols, which limits the overall selectivity to glycols and glycerol (30%). When 90:10 vol.% water:1,4dioxane and water:2-PrOH solutions are used as solvent, the product distribution is very similar to the one in water. Meanwhile, in 90:10 vol.% water:primary alcohol (ROH with R = Me, Et, nPr, nBu) the overall selectivity to glycols and glycerol is greatly enhanced (up to 70%), whereas the selectivity to C4 and C5 alditols is reduced. In a solution with higher MeOH proportion of 20 vol.%, the glycols are detected with even higher selectivity, however, some deactivation of the catalyst is observed. TGA analysis of the used catalysts shows that during the process some coke is deposited on the catalyst via a dehydrogenation step of ROH. The coke selectively poisons the Ru sites that are active for the undesired reactions of decarbonylation and epimerization.

Ru-(Mn-M)OX Solid Base Catalysts for the Upgrading of Xylitol to Glycols in Water

“…The Ru-(Mn-Zn)O X material was not active at all and no conversion was measured after a reaction time of 30 h. A comparable result was observed in our previous study using Ru/ZnO/C catalyst; the catalytic activity decreased from 220 to 13 h −1 in comparison with Ru/MnO/C catalyst [25]. Yet, previous studies have reported that the combination of Cu and ZnO was active and selective in the hydrogenolysis of alditols to glycols [21,[36][37][38].…”

“…Moreover, the products transferred into the gas phase (determined as the difference of initial TOC and TOC measured in the liquid phase) were produced in lower amounts over Ru-(Mn-Al)O X . This observation suggests that the side reactions of cascade decarbonylation leading to the degradation of glycols, particularly EG, were minimized [25]. This was confirmed by the calculation of the molar ratio of C 3 to C 2 products, which was 1.3 and 1.9 for the reactions in the presence of Ru-(Mn-Al)O X and Ru/(Mn-Al)O X , respectively, while the theoretical molar ratio is 1.0 for a totally selective hydrogenolysis reaction of xylitol to EG plus PG.…”

“…The results of activity and selectivity to the main products are shown in Table 2. The selectivities are given at 80% conversion when the maximum concentration of the desired products was reached, as described previously [10,25]. The total carbon balance (comparison between TOC measured and TOC calculated from HPLC analysis) was around 90%.…”

“…The material was dried and calcined, as previously. Ru/(Mn-Al)O X catalyst was then synthetized by wet impregnation of the aqueous solution of Ru precursor over (Mn-Al)O X , followed by reduction and passivation, according to the previous procedure [25].…”

“…Moreover, we observed that the activity increased with the Mn-loading up to 384 h −1 and the selectivity to glycols and glycerol to 50% over 3.1%Ru/MnO(19.3%)/C catalyst. The selectivity over the same 3%Ru/MnO(4.5%)/C catalyst was then enhanced up to 70% in a water/alcohol mixture solvent, which avoided the competitive decarbonylation reaction on the Ru sites [25]. Nevertheless, the catalyst had a poor hydrothermal stability.…”