Abstract
Ab initio and density functional calculations are conducted to investigate the radicalization processes and radical catalysis of biomass sugars. Structural alterations due to radicalization generally focus on the radicalized sites, and radicalization affects H-bonds in D-fructofuranose more than in D-glucopyranose, potentially with outcome of new H-bonds. Performances of different functionals and basis sets are evaluated for all radicalization processes, and enthalpy changes and Gibbs free energies for these processes are presented with high accuracy, which can be referenced for subsequent experimental and theoretical studies. It shows that radicalization can be utilized for direct transformation of biomass sugars, and for each sugar, C rather than O sites are always preferred for radicalization, thus suggesting the possibility to activate C-H bonds of biomass sugars. Radical catalysis is further combined with Brønsted acids, and it clearly states that functionalization fundamentally regulates the catalytic effects of biomass sugars. In presence of explicit water molecules, functionalization significantly affects the activation barriers and reaction energies of protonation rather than dehydration steps. Tertiary butyl and phenyl groups with large steric hindrances or hydroxyl and amino groups resulting in high stabilities for protonation products drive the protonation steps to occur facilely at ambient conditions.
Highlights
Petroleum resources are expected to be exhausted in the few decades, and alternative energy sources are being sought
The Brønsted-acid catalysis of glucose is driven towards the formation of humin precursors and reversion products, while under similar conditions, fructose can be activated readily and with a sequence of facile reaction steps converted to 5-(hydroxymethyl)furfural (HMF) or other platform chemicals[16,17,18]
Electron magnetic resonances were used to study the irradiation of crystalline D-glucose, D-fructose and sucrose[27,28,29,30], and several C-centered radicals were assigned with help of density functional calculations[30,31,32]
Summary
In line with previous works[8,9,10,11,12,13,16,17,18], the lowest-energy conformers of α,β-D-glucopyranose and α,β-D-fructofuranose were the choice for studies, which were respectively referred to as αG, βG and αF, βF (Fig. 1). Structural optimizations of sugars and their radicals corresponding to the various O/C sites were performed at MP2/aug-cc-pVTZ (denoted as bs4) level[39]. The two-layer ONIOM methodology (MP2/bs4//M06L/bs3)[51,52] that has been validated sufficiently (Tables S1, S5 and Figures S3 and S4) was employed for energy calculations, on basis of B3LYP/bs[2] optimized structures. The radicalized C/O sites and neighbouring C/O groups as well as adsorbents (proton and water molecules) were defined as the high-level regions, while the rest were treated as the low-level regions. This methodology can be applied for the accurate energy calculations of other sugar and larger catalytic systems
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