Catalytic activity and stability of hydrophobic Mg–Al hydrotalcites in the continuous aqueous-phase isomerization of glucose into fructose

Abstract

The aqueous-phase isomerization of glucose into fructose, catalyzed by Mg-Al hydrotalcites, has been investigated under batch and continuous conditions. A commercial hydrotalcite with a hydrophobic surface modification and two hydrophilic hydrotalcites in carbonate form, or with OH − anions in the interlayer space, served as catalysts. With the hydrophobic hydrotalcite a lower conversion but superior selectivity to fructose could be demonstrated, reaching above 92% selectivity at 30% conversion. The observed by-products confirm retroaldolization of glucose and fructose as the main side reactions causing catalyst deactivation via adsorption. Additionally, acidic degradation products such as lactic acid cause neutralization of the hydrotalcites facilitating leaching of the Mg 2+ ions. Fructose contributes a greater extent to by-product formation. Applying continuous operation conditions, fructose is removed from the reaction mixture.Therefore, by-product formation is notably suppressed and catalyst stability increases. During 70 to 100 h time-on-stream a slow deactivation of the hydrophobic hydrotalcite occurs. Regeneration can be achieved via calcination and treatment in an aqueous sodium n-dodecyl sulfate solution to introduce dodecyl sulfate anions to the interlayer space of the hydrotalcite, restoring the hydrophobic material properties.

Catalytic Isomerization of Biomass‐Derived Aldoses: A Review

“…The primary particlesi nteractw ith each other to form materials with different morphologies, for example, hydrotalcites with a" sand-rose" structure, or the materials are formed by stacking of basal planes. [33] At the end, the majority of the carbonate anions are no longer accessible to the substrates (Figure 4). Previous studies have suggested that two locations of carbonates provide good access to the active sites.…”

“…[24,33] We have tested commercial Mg 6 Al 2 (CO 3 )(OH) 16 ·4H 2 Of or glucose isomerization under continuous conditions and have observed ag raduald ecrease in catalytic activity.N evertheless, the catalytic activity can be restored by calcination and rehydration of hydrotalcite. [24] Importantly,s imilar to zeolites,h ydrotalciteso utperform soluble hydroxides in terms of selectivity.I nt he presence of hydrotalcites, the number of byproducts is much lower than that formed with the use of soluble bases.D ihydroxyacetone, glyceraldehyde, glycolaldehyde,a nd lactic acid have been identified as side products that are formed in thep resence of [Mg 1Àx Al x (OH) 2 ]…”

“…[32] Later on, we have also found ad ependency of glucose conversion on basicity of the hydrotalcites and have proposed protocols for the synthesisofh ydrotalciteswith ah igh concentration of basic sites. [33] Precipitation in aqueous medium at the isoelectric point of the hydrotalcite (pH 10) gives rise to materials with a" sand-rose"s tructure. These materials exhibit ah igher concentration of basic sites than materials prepared at pH valuesb elow 10 and demonstrate stacking of the basal planes.…”

“…[24,33] An interlayer anion of hydrotalcitesi sr egarded as an active center for isomerization. x + (CO 3 ) x/2 ·m H 2 Oh ydrotalcites.…”

“…In this respect, solid catalysts are beneficial owing to facile catalysts eparation and recycling. Materialss uch as hydrotalcites, [30][31][32][33]39] immobilized amines, [34] zeolites in alkaline-exchange form, [30,40] mesoporous ordered molecular sieves of the M41S family, [41] zirconium carbonate, [42] and anion-exchanged resins [43] have been reported as efficient solid base catalysts. In the interest of productivity, the experiments are conducted at elevated temperatures, thoughs accharides, especially ketoses, are not stable under harsh conditions.…”