DFT investigation on thermochemical analyses of conversion of xylose to linear alkanes in aqueous phase.

Xylose is an integral part of hemicellulose fraction of lignocellulosic biomass. Its abundance in the lignocellulose makes it a desirable component for converting into various value-added compounds. In this study, conversion of xylose to four linear alkanes has been discussed by five different schemes including their thermochemistry under the framework of density functional theory. Main products are butane, pentane, octane and tridecane whereas the intermediate products include furfural, tetrahydrofuran, pentane-1,5-diol, etc. The simulations have been performed at B3LYP/6-31 + g(d,p) and M06-2X/6-31 + g(d,p) level of theories in aqueous phase using SMD solvation model. Thermochemical parameters (ΔG, ΔH and K) are obtained at a wide range of temperature, i.e. 298-698 K. Single point energy change (ΔE) of all the conversion steps has also been calculated at M05-2X/6-311++g(3df,2p) level of theory in the aqueous phase. It is observed that temperature plays a vital role in the formation of products. At high temperature, only scheme RS 1 (i.e. xylose to butane) can proceed to produce butane. The absolute difference between two functionals, B3LYP and M06-2X, was found to be small (<2 kcal/mol) for ring opening reactions making both the functionals suitable for a qualitative study. For saturation of cyclic compounds, a large difference (>10 kcal/mol) was observed between the two functionals making higher accuracy method more suitable for them. For all other reactions, use of M06-2X can be preferred.