Catalytic consequences of borate complexation and pH on the epimerization of l-arabinose to l-ribose in water catalyzed by Sn-Beta zeolite with borate salts

Abstract

Sn-Beta zeolite with sodium tetraborate cooperatively catalyzes the epimerization of aldoses via an intramolecular 1,2 carbon-shift mechanism. l-Arabinose is one of the seven common sugars and its epimer, l-ribose, is a valuable rare sugar with applications in antiviral and anticancer agents. Here, a full factorial experimental design is performed to demonstrate the catalytic consequences of varying key reaction parameters such as pH, borate to sugar ratio, and reaction time. Reactivity data revealed that isomerization is favored under acidic pH conditions (pH<7.0); that epimerization dominates under neutral conditions (pH ranging from 7.0 to 7.8); and that drastic inhibition of both epimerization and isomerization rates occur under basic conditions (pH>7.8). Using a 5wt% arabinose feed and 100:1 sugar–metal ratios at 343K, conversions ranging from 20% to 30% were obtained with selectivities of 75%, 84%, and 91% for boron–sugar ratios of 0.2:1, 0.5:1, and 1:1, respectively. The predominance of epimerization over isomerization products with substoichiometric borate suggests that one borate can influence the reactivity of several sugar molecules and may influence the Sn active site directly. Reaction data obtained under differential conditions revealed that the epimerization reaction follows first order kinetics over a wide temperature range with an apparent activation energy of 98kJ/mol and pre-exponential factor of 1.9×1014LmolSn−1s−1.