Adsorption and Diffusion of Fructose in Zeolite HZSM-5: Selection of Models and Methods for Computational Studies

Abstract

The adsorption and protonation of fructose in HZSM-5 have been studied for the assessment of models for accurate reaction energy calculations and the evaluation of molecular diffusivity. The adsorption and protonation were calculated using 2T, 5T, and 46T clusters as well as a periodic model. The results indicate that the reaction thermodynamics cannot be predicted correctly using small cluster models, such as 2T or 5T, because these small cluster models fail to represent the electrostatic effect of a zeolite cage, which provides additional stabilization to the ion pair formed upon the protonation of fructose. Structural parameters optimized using the 46T cluster model agree well with those of the full periodic model; however, the calculated reaction energies are in significant error due to the poor account of dispersion effects by density functional theory. The dispersion effects contribute −30.5 kcal/mol to the binding energy of fructose in the zeolite pore based on periodic model calculations that include dispersion interactions. The protonation of the fructose ternary carbon hydroxyl group was calculated to be exothermic by 5.5 kcal/mol with a reaction barrier of 2.9 kcal/mol using the periodic model with dispersion effects. Our results suggest that the internal diffusion of fructose in HZSM-5 is very likely to be energetically limited and only occurs at high temperature due to the large size of the molecule.

Adsorption of Glucose within M(IV)‐Incorporated Zeolites: Insights from Periodic Density Functional Theory Calculations

“…In line with previous reports, [3][4][5][6][7][8][9][10][11][12][31][32][33] the optimal conformer of GP was selected for the adsorption studies. The acyclic glucose (AG) that emerges during the isomerization processes [3][4][5][6][7]34] was also investigated, along with the linear form (LG) that exists in equilibrium with GP albeit with relatively a small proportion.…”

“…The D ad values of GP within Sn-BEA are close to those of a-D-fructofuranose within Sn-BEA (-123 kJ/mol) [13] and within H-ZSM-5 (-127 kJ/mol). [12] For a specific O site, GP has resembling adsorption conditions in Sn-, Ti-, Zr-and Ge-BEA and hence their dispersion effects are close to each other; nevertheless, a clear opposite trend is detected between dispersion interactions (Ge > Ti > Sn > Zr) and adsorption energies (Zr > Sn > Ti > Ge), probably due to that GP molecules that bind more tightly to framework M(IV) ions rely less on the interactions with zeolite lattices. As compared to other pore topologies of zeolites, FER shows surprisingly more pronounced dispersion effects for GP adsorption, due to the comparable size of pore structure and GP molecule that induces the strongest non-covalent interactions as discussed above.…”

“…Isomerization of glucose to fructose has been recognized as one of the key reactions for the conversion of cellulosic biomass, where Lewis-acid zeolites such as Sn-BEA display unprecedented catalytic performances. [1] In contrast to the focusing studies on catalytic mechanisms, [2][3][4][5][6][7][8][9][10][11] scant attention has been given to the adsorption of related sugars, [12,13] which is known to be an essential first step for the isomerization and other heterogeneous catalytic processes.…”

“…Cheng et al [12] investigated the adsorption of a-D-fructofuranose via O 1 site within H-ZSM-5, finding that structural parameters of 46-T rather than 2-T and 5-T clusters achieve fine agreements with those of periodic model, while reaction thermodynamics cannot be correctly derived from B3LYP calculations due to the improper account for dispersion interactions. Although the isomerization reaction of glucose to fructose catalyzed by Sn-BEA can be triggered via O 1 or O 5 site, the former (O 1 ) is significantly preferred by an apparently lower energy barrier (85 vs. 194 kJ/mol for Step 1), [4] indicating that catalytic performances are closely associated with adsorption modes.…”

Adsorption, reduction and storage of hydrogen within ZSM-5 zeolite exchanged with various ions: A comparative theoretical study

“…Density functional theory (DFT) methods are known to be deficient to describe long-range dispersion interactions [51][52][53][54][55]. The hybrid MP2:DFT scheme was proposed to account for the dispersion energies reproducing the reaction barriers with near chemical accuracy [51,52].…”