Tighter Confinement Increases Selectivity of d‐Glucose Isomerization Toward l‐Sorbose in Titanium Zeolites

Abstract

Aqueous-phase isomerization of d-glucose to d-fructose and l-sorbose is catalyzed in parallel by Lewis acidic Ti sites in siliceous frameworks.G lucose isomerization rates (per Ti,3 73 K) are undetectable when Ti sites are confined within mesoporous voids (Ti-MCM-41, TiO 2-SiO 2) and increase to detectable values when Ti sites are confined within the smaller 12-membered ring (12-MR) micropores of Ti-Beta. Isomerization rates decrease to lower values (by % 20)with further decreases in micropore sizeasT isites are confined within 10-MR pores (Ti-MFI, Ti-CON), likely because of intrapore reactant diffusion restrictions,a nd reach undetectable values within the 8-MR pores of Ti-CHA as size exclusion prevents glucose from accessing active sites.R emarkably,t he selectivity toward l-sorbose over d-fructose increases systematically as spatial constraints around Ti sites become tighter,a nd is > 10 on Ti-MFI. These findings demonstrate the marked influence of confinement around Ti active sites on the selectivity between parallel stereoselective sugar isomerization pathways.

Toward Understanding Base-Catalyzed Isomerization of Saccharides

“…124,125 Glc has a Stokes diameter of 0.72 nm; 126 however, due to flexibility of this molecule, it can diffuse into MFI and Beta zeolites with nominal pore diameters of 0.55 and 0.7 nm, respectively. 127 Though the confinement effect can potentially influence isomerization performance, 127 no such effects have been reported for zeolites with basic sites yet. Interestingly, utilization of hierarchical zeolites results in an improved isomerization rate.…”

“…In recent years, an interest toward utilization of some classes of solid bases, such as zeolites, supported amines, and Mg–Al hydrotalcites has increased. Catalytic activity of zeolites in Na + , K + , Mg 2+ , or Ca 2+ forms was reported; ,,− in addition, MgO supported on zeolites was tested. , Glc has a Stokes diameter of 0.72 nm; however, due to flexibility of this molecule, it can diffuse into MFI and Beta zeolites with nominal pore diameters of 0.55 and 0.7 nm, respectively . Though the confinement effect can potentially influence isomerization performance, no such effects have been reported for zeolites with basic sites yet.…”

“…Catalytic activity of zeolites in Na + , K + , Mg 2+ , or Ca 2+ forms was reported; ,,− in addition, MgO supported on zeolites was tested. , Glc has a Stokes diameter of 0.72 nm; however, due to flexibility of this molecule, it can diffuse into MFI and Beta zeolites with nominal pore diameters of 0.55 and 0.7 nm, respectively . Though the confinement effect can potentially influence isomerization performance, no such effects have been reported for zeolites with basic sites yet. Interestingly, utilization of hierarchical zeolites results in an improved isomerization rate. , A significant leaching of metals, especially alkaline metal cations, was reported, but no kinetic studies on contribution of homogeneous and heterogeneous catalysis for zeolites with basic sites have been performed.…”

Making more with less: confinement effects for more sustainable chemical transformations

“…Another example is the work of Gounder and co-workers, who studied the conversion of glucose to sorbose, the precursor to synthetic ascorbic acid. 62 The process can be catalyzed by Lewis acids, so the authors screened Ti-modified zeolites, mesoporous silica, and non-porous silica as catalysts. The mechanism (Scheme 3) involves the adsorption of glucose to the Ti site at the O5 position, followed by ring-opening to the open-chain form followed by coordination of the carbonyl in the C1 position to the Ti site.…”

Comparative First-Principles Study on Tetravalent Ion-Incorporated MTW-Type Zeolites

“…Tetravalent metal ions (Ti 4+ , Ge 4+ , Zr 4+ , Sn 4+ ) can be incorporated into the zeolite by means of isomorphous substitution to introduce the well-defined Lewis acid center for the zeolite, which can enhance the catalytic activity of the zeolite for a designated reaction. − Mal et al reported Sn-ZSM-12 zeolite with a Si/Sn >70 that could be used as an effective catalyst for the oxidation of phenol, m -cresol, and m -xylene, which performed better than the Sn-free ZSM-12. They also found that Sn-ZSM-12 exhibited better activity than mesoporous Sn-MFI/MEL in the oxidation of naphthalene and 2-methylnaphthalene to naphthol, 2-naphthalenemethanol and 2-naphthaldehyde .…”