Tin-containing zeolites are highly active catalysts for the isomerization of glucose in water

Abstract

The isomerization of glucose into fructose is a large-scale reaction for the production of high-fructose corn syrup (HFCS; reaction performed by enzyme catalysts) and recently is being considered as an intermediate step in the possible route of biomass to fuels and chemicals. Here, it is shown that a large-pore zeolite that contains tin (Sn-Beta) is able to isomerize glucose to fructose in aqueous media with high activity and selectivity. Specifically, a 10% (wt∕wt) glucose solution containing a catalytic amount of Sn-Beta (1∶50 Sn: glucose molar ratio) gives product yields of approximately 46% (wt∕wt) glucose, 31% (wt∕wt) fructose, and 9% (wt∕wt) mannose after 30 min and 12 min of reaction at 383 K and 413 K, respectively. This reactivity is achieved also when a 45 wt% glucose solution is used. The properties of the large-pore zeolite greatly influence the reaction behavior because the reaction does not proceed with a medium-pore zeolite, and the isomerization activity is considerably lower when the metal centers are incorporated in ordered mesoporous silica (MCM-41). The Sn-Beta catalyst can be used for multiple cycles, and the reaction stops when the solid is removed, clearly indicating that the catalysis is occurring heterogeneously. Most importantly, the Sn-Beta catalyst is able to perform the isomerization reaction in highly acidic, aqueous environments with equivalent activity and product distribution as in media without added acid. This enables Sn-Beta to couple isomerizations with other acid-catalyzed reactions, including hydrolysis/isomerization or isomerization/dehydration reaction sequences [starch to fructose and glucose to 5-hydroxymethylfurfural (HMF) demonstrated here].glucose isomerization | heterogeneous catalysis T he isomerization of sugars is a key reaction used in various relevant industrial processes. For instance, the conversion of glucose into fructose for the production of high-fructose corn syrups (HFCS) has become the largest immobilized biocatalytic process worldwide. HFCS have reached a global production exceeding 8 × 10 6 tons∕year (in the United States alone, per capita consumption of HFCS reached 37.8 lbs∕year in 2008) (1-3). In addition, the recent drive to use biomass as an alternative to petroleum for the production of fuels and chemical intermediates has triggered a renewed interest in carbohydrate chemistry. In this respect, glucose isomerization is a crucial step in the efficient production of valuable chemical intermediates, such as 5-hydroxymethylfurfural (HMF) and levulinic acid, from biomass; however, a heterogeneous isomerization catalyst (biological or inorganic) that can easily integrate glucose isomerization with the transformation of fructose into these intermediates is lacking (4, 5). Here, we present highly active heterogeneous inorganic catalysts for the isomerization of glucose that resemble the performance of enzymatic catalysts by generating remarkably high-fructose yields at glucose conversions near the reaction equilibrium. Furthermore, unlike enzymatic cat...

Competitive Adsorption of Substrate and Solvent in Sn‐Beta Zeolite During Sugar Isomerization

“…Based on the 13 C intensities of the reactant and products, we estimate that about 50 % of the starting d ‐glucose was converted in 12 h, mostly into d ‐fructose (33 %). This approaches the glucose/fructose equilibrium ratio of 1.13 at room temperature 8, 32. The remainder of the 13 C labels were found in the carbonyl region (170–180 ppm), which should be related to secondary reaction products such as 5‐hydroxymethylfurfural 33.…”

“…This is surprising, as in batch experiments the isomerization reaction goes to equilibrium (glucose/fructose≈0.75 at 100 °C)8, 32 with comparatively little formation of byproducts 8. Instead, the spectrum of the heated sample contained signals owing to carbonyl groups ( δ ≈180 ppm) and CH 3 groups ( δ ≈20 ppm).…”

“…Glucose isomerization by bases is known, but suffers from by‐product formation and waste issues 5. Recently, Lewis acids have been identified as the preferred chemocatalysts for glucose isomerization 6, 7, 8, 9…”

“…Sn‐Beta can isomerize hexoses, pentoses, and trioses by intramolecular hydride and carbon shift reactions in various solvents 8, 10, 11, 12. Another platform molecule is lactic acid, which can be obtained from fructose via retro‐aldol condensation13, 14 and hydride shift reactions involving 1,3‐dihydroxyacetone and glyceraldehyde as intermediates (Scheme 1).…”

Dehydration of Glucose to 5‐Hydroxymethylfurfural Using Nb‐doped Tungstite

“…Both base and Lewis acid catalysts have been explored for this purpose 3. Lewis acids are preferred because their activity is not affected by strong Brønsted acids 4. Selective dehydration of fructose to HMF can be readily achieved in the presence of Brønsted acid catalysts in biphasic organic–water solvents 5, 6, 7…”