Unraveling the Reaction Mechanism and Active Sites of Metal–Organic Frameworks for Glucose Transformations in Water: Experimental and Theoretical Studies

Abstract

The catalytic performance of two different metalorganic frameworks, UiO-66 and MOF-808, containing Lewis acid active sites has been evaluated for the transformation of glucose in water, and compared with that of analogous Lewis acid Zr-Beta zeolite. While fructose is the main product obtained on Zr-Beta, the mannose production increases when using Zr-MOFs as catalysts. Kinetic studies reveal a lower activation energy barrier for glucose epimerization to mannose when using Zr-MOF catalysts (83-88 and 100 kJ/mol for glucose epimerization and isomerization, respectively). A 13 C NMR study using 13 C1-labelled glucose allows confirming that on Zr-MOF catalysts mannose is exclusively formed following the glucose epimerization route through 1,2intramolecular carbon shift, whereas the two step glucose → fructose → mannose isomerization via 1,2-intramolecular proton shifts is the preferred pathway on Zr-Beta. A computational study reveals a different mode of adsorption of deprotonated glucose on Zr-MOFs that allows decreasing the activation barrier for the 1,2intramolecular carbon shift. The combination of spectroscopic, kinetic and theoretical studies allows unraveling the nature of the metal sites in Zr-MOFs and Zr-Beta catalysts and to propose a structure-activity relationship between the different Lewis acid sites and the glucose transformation reactions. The results presented here could permit new rationalized MOF catalyst designs with the specific active sites to facilitate particular reaction mechanisms.

Metal–Organic Framework-Based Solid Acid Materials for Biomass Upgrade

“…Density functional theory (DFT) calculations suggested that the activation energy of the 1,2-intramolecular hydride shift is similar among the three Zr-based materials. However, as for the 1,2-intramolecular carbon shift, the order of activation energy is UiO-66(Zr) < MOF-808(Zr) < < Zr-β zeolite, consistent with the corresponding selectivity order toward glucose [94].…”

Tailoring Lewis/Brønsted acid properties of MOF nodes via hydrothermal and solvothermal synthesis: simple approach with exceptional catalytic implications

“…On the other hand, TMPO is a probe molecule that can diffuse through the MOF-808 pores and interact with both Lewis and Brønsted acid sites present in the metal-containing nodes, so the two types of acid sites can be distinguished by analysis of the 31 P MAS NMR spectra after TMPO adsorption. 41,45,46 Indeed, clear differences are observed between the 31 P MAS NMR spectra of the two TMPO-adsorbed Hf-MOF-808 samples (see Fig. 4 ).…”

“…38 For the simulation of NMR parameters, the isotropic absolute chemical shielding constants ( σ ) were obtained using the gauge including atomic orbitals (GIAO) approach 39,40 and the 31 P chemical shifts were calculated as δ iso ( 31 P) = σ ref − σ , using phosphoric acid as reference. To improve accuracy, all values were corrected with an equation obtained by fitting δ iso ( 31 P) values at 6-31G(d,p) level against δ iso ( 31 P) values at 6-311++G(d,p) level for a series of acid–base TMPO adducts, described in reference 41 .…”

Isomerization of glucose to fructose catalyzed by metal–organic frameworks

“…Numerous studies have focused on glucose isomerization employing protic solvents; however, in this work, we utilized a nonprotic solvent (γ-valerolactone). The reaction mechanism of glucose isomerization on Lewis acid sites has been studied by several authors, showing that intra- Please do not adjust margins Please do not adjust margins hydride transfer (C2 to C1) was the dominant reaction pathway in heterogeneous and homogenous systems 4,9,35,40 . The use of protic or nonprotic solvents has a strong effect on the balance of Lewis and Brønsted acid sites in the catalyst 27 ; thus, protic solvents generate Brønsted acid sites due to solvent molecules being ligated to metal atoms 27 .…”

“…Furthermore, they have numerous modifiable compositions and can be easily functionalized, making them excellent and promising materials in heterogeneous catalysis applications, such as the oxidative desulfurization of fuels 19,20 , electrocatalysis [21][22][23][24] , gas adsorption and separation, hydrogen storage, drug delivery 25,26 and, in particular, catalytic and biomass valorization 10,[27][28][29][30][31][32] . A few studies have evaluated the catalytic activity of MOFs in glucose isomerization 10,[33][34][35] . But, the main effort has been done to modify MOFs (via metal clusters or organic ligands) to be applied for the one-pot synthesis of 5-HMF from glucose [36][37][38][39] .…”