Catalysis of Organic..

236Deactivation of Catalyst in Sugar Hydrog.Experimental SectionAqueous lactose (40 wt-% in water) and xylose (50 wt-%) solutions werehydrogenated batchwise in a three-phase laboratory reactor (Parr Co.). Reactions withlactose were carried out at 120 ˚C and 5.0 MPa H 2 . Xylose hydrogenations wereperformed at 110 ˚C and 5.0 MPa. The stirring rate was 1800 rpm in all of theexperiments to operate at the kinetically controlled regime.For lactose hydrogenations were used 5 wt-% (dry weight) sponge nickel and 2wt-% (dry weight) Ru/C catalyst of lactose amount. In case of xylose, 2.5 wt-% (dryweight) sponge nickel and 1.5 wt-% (dry weight) Ru/C catalyst of xylose amount wereused. Prior to the first hydrogenation batch, the Ru/C catalyst was reduced in thereactor under hydrogen flow at 200 ˚C for 2 h (1.0 MPa H 2 , heating and cooling rate 5˚C/min). The reactor contents were analysed off-line with an HPLC, equipped with aBiorad Aminex HPX-87C carbohydrate column.Results and discussionXylose hydrogenation gave xylitol as a main product (selectivity typically over 99 %)and arabinitol, xylulose and xylonic acid as by-products. In lactose hydrogenation, themain product was lactitol (selectivity typically between 97 and 99 %) and lactulitol,galactitol, sorbitol and lactobionic acid were obtained as by-products.Studies about xylose hydrogenation to xylitol suggested that the main reasons forthe sponge nickel deactivation were the decay of accessible active sites through theaccumulation of organic species in the catalyst pores and by poisoning of the nickelsurface. Deactivation during consecutive xylose hydrogenation batches over Ru/Ccatalyst was insignificant (Fig. 1A). Catalyst deactivation during consecutive lactosehydrogenation batches occurred faster than during the xylitol manufacture (Fig. 1B).One of the problems encountered in the catalytic hydrogenation of aldose sugarsis the formation of harmful by-products, such as aldonic acids. E.g. formation of D-gluconic acid is known to deactivate glucose hydrogenation catalysts by blocking theactive sites (9,10). Furthermore, aldonic acid formation increases leaching of metals,since they are strong chelating agents. Under non-optimized conditions, lactobionicacid is formed as a by-product during lactose hydrogenation and xylonic acid in xylosehydrogenation.