(276d) Solvation Dynamics and Energetics of Hydride Transfer Reactions in Cellulosic Biomass Conversion

Cellulose and its sugar derivatives need to be dissolved in a solvent for their catalytic conversion to chemicals and fuels intermediates. Experimental studies have shown that the addition of a co–solvent to water or replacing water with an alternate solvent can significantly alter conversion and selectivities in these reactions. [1] It is believed that solvents can play dual role in biomass reactions. (i) A solvent can either preferentially solvate an “active” functional group of a biomass molecule or its derivative and protect it from taking part into the reaction [2], or (ii) on the other hand, solvent can also directly participate in the reaction.[3] Hydride transfer is a commonly observed, and usually the rate limiting step in some of the key biomass reactions like isomerization and dehydration.[4] Solvent dynamics and non–equilibrium solvation can play a major role in altering the energetics of charge transfer steps like hydride transfer.[5,6] In the present paper, we perform Car–Parrinello molecular dynamics-metadynamics simulations of intramolecular hydride transfer in glucose molecule, as a model system, in the presence of explicit quantum mechanically treated solvent molecules. Free energy change and activation free energy barriers for the hydride transfer step are computed as a function of solvent composition. It is observed that hydride transfer can be either exergonic or endergonic in different solvents. Similarly the free energy barrier for hydride transfer is a strong function of the solvent environment. Analyses of changes in the electronic structure of the reacting system and in solvent orientation along the trajectory suggest that changes in the charge structure of the sugar molecule polarizes the solvent and that hydride shift takes place in a non–equilibrium solvation environment. Orientational relaxation of the solvent, after the hydride transfer, is observed to be a significantly slower process. The free energy barrier is a result of non–equilibrium solvation and the exergonicity or endergonicity of the hydride transfer step depends on the relaxation dynamics of the solvent.

References:

1. Y. Román-Leshkov et al., Science, 2006, 312, 1933-1937.
2. S. H. Mushrif et al., Phys Chem Chem Phys, 2012, 14, 2637-2644.
3. G. Li et al., Catalysis Science & Technology, 2014, 4, 2241-2250.
4. R. Bermejo-Deval et al., P Natl Acad Sci USA, 2012, 109, 9727-9732
5. S. H. Mushrif et al., Chem. Eng. Sci. 2015, 121, 217-235.
6. S. H. Mushrif et al., Phys Chem Chem Phys, 2015, 17, 4961-4969.