Insights into the isomerization of xylose to xylulose and lyxose by a Lewis acid catalyst.

We present electronic structure calculations on the isomerization and epimerization of xylose to xylulose and lyxose, respectively, by a zeolite Sn-BEA model at the MP2 and B3LYP theory levels. Benchmarking calculations at the CCSD(T) theory level are also presented. We show that lyxose is formed from a stable intermediate in the xylose-xylulose isomerization pathway. In agreement with experimental observations, we predict that xylulose is thermodynamically and kinetically favoured over lyxose. We find that the slowest step for both reactions involves hydrogen transfer from the C2 to the C1 carbon of the carbohydrate molecule and we characterize it using natural population analysis. We conclude that the hydrogen transfer does not take place as a hydride ion but rather as concerted neutral hydrogen-electron transfer that involves different centres for the hydrogen and electron transfer.