What are the catalytically active species for aqueous-phase isomerization of D-glucose into D-fructose in the presence of alkaline earth metal (hydr)oxides?

Enhanced catalytic activity of carbonate buffer for isomerization of D-galactose into D-tagatose

D-tagatose (Tag) is a rare monosaccharide with health benefits. In this work, catalytic activity of CAPS (pH 10.4), carbonate (pH 10.4), triethylamine (pH 11.2), quinuclidine (pH 11.5), and L-arginine (pH 12.5) was examined for isomerization of D-galactose (Gal) to Tag. The maximum yields of Tag were 15.0 % with CAPS, 15.2 % with carbonate, 19.3 % with triethylamine, 19.6 % with quinuclidine, and 18.1 % with L-arginine. Despite identical pH, the Tag formation rate with carbonate buffer was 3-8 times higher than with CAPS. For carbonate buffer, the reaction orders for hydroxide and carbonate anions were ∼ 1 and ∼ 0, respectively. Operando NMR studies of Gal-1-13C isomerization in carbonate and CAPS buffers indicate similar tautomeric distributions of the substrate in both buffers. The deuterium kinetic isotope effect demonstrated that carbonate facilitates isomerization through a proton transfer mechanism, with hydroxide anions acting as the catalytically active species whereas carbonate anions stabilize the enediolate anion and/or the transition state.

Ball billing induced highly dispersed nano-MgO in biochar for glucose isomerization at low temperatures

The isomerization of glucose is a crucial step for biomass valorization to downstream chemicals. Herein, highly dispersed MgO doped biochar (BM-0.5@450) was prepared from rice straw via a solvent-free ball milling pretreatment and pyrolysis under nitrogen conditions. The nano-MgO doped biochar demonstrated enhanced conversion of glucose in water at low temperatures. A 31 % yield of fructose was obtained from glucose over BM-0.5@450 at 50 °C with 80.0 % selectivity. At 60 °C for 140 min, BM-0.5@450 achieved a 32.5 % yield of fructose. Compared to catalyst synthesized from conventional impregnation method (IM@450), the BM-0.5@450 catalyst shows much higher fructose yields (32.5 % vs 25.9 %), which can be attributed to smaller crystallite size of MgO (11.32 nm vs 19.58 nm) and homogenous distribution. The mechanism study shows that the activated MgOH+·OH- group by water facilitated the deprotonation process leading to the formation of key intermediate enediol.

Tuning of MgO's base characteristics by blending it with amphoteric ZnO facilitating the selective glucose isomerization to fructose for bioenergy development

Fructose serves as an important intermediate in the preparation of liquid fuel compounds. Herein, we report its selective production via a chemical catalysis method over ZnO/MgO nanocomposite. The blending of an amphoteric ZnO with MgO reduced the latter's unfavorable moderate/strong basic sites that can influence the side reactions in the sugar interconversion, reducing fructose productivity. Of all the ZnO/MgO combinations, a 1 : 1 ratio of ZnO and MgO showed a 20% reduction in moderate/strong basic sites in MgO with ∼2-2.5 times increase in weak basic sites (overall), which is favorable for the reaction. The analytical characterizations affirmed that MgO settles on the surface of ZnO by blocking the pores. The amphoteric ZnO undertakes the neutralization of the strong basic sites and improves the weak basic sites (cumulative) by the Zn-MgO alloy formation. Therefore, the composite afforded as high as 36% fructose yield and 90% selectivity at 90 °C; especially, the improved selectivity can be accounted for by the effect of both basic and acidic sites. The favorable action of acidic sites in controlling the unwanted side reactions was maximum when an aqueous medium contained 1/5^th methanol. However, ZnO's presence regulated the glucose's degradation rate by up to 40% compared to the kinetics of pristine MgO. From the isotopic labelling experiments, the proton transfer pathway (or LdB-AvE mechanism by the formation of 1,2-enediolate) is dominant in the glucose-to-fructose transformation. The composite exhibited a long-lasting ability based on the good recycling efficiency of up to 5 cycles. The insights into the fine-tuning of the physicochemical characteristics of widely available metal oxides would help develop a robust catalyst for sustainable fructose production for biofuel production (via a cascade approach).

Kinetic Salt Effect on Base-Catalyzed Isomerization of d-Glucose into d-Fructose

Isomerization of d-glucose (Glc) into d-fructose (Fru) is of great importance for food sector as well as for valorization of lignocellulosic biomass. Soluble and solid bases exhibit high catalytic activity for the isomerization. Here, we report a salt effect on the base-catalyzed aqueous-phase Glc-Fru isomerization. Addition of soluble salts (Na₂ SO₄ , NaNO₃ , K₂ SO₄ , and NaCl) results in an increased apparent reaction rate (factors of 1.5 to 6). The salt effect was observed both in the presence of soluble base NaOH at constant pH value and solid bases MgO, Li₃ PO₄ , and Mg-Al hydrotalcite. Apparent activation energy and UV absorption spectra were not significantly influenced by addition of salts. Potentiometric titration showed that the acidity constants of the saccharides increase in the presence of electrolytes. Since the rate of the isomerization depends on the thermodynamic acidity constant of Glc, the isomerization is accelerated by the presence of electrolytes.

Study of base-catalyzed isomerization of d-glucose with a focus on reaction kinetics

We explored the isomerization of d-glucose into d-fructose using the simplest possible base catalyst, aqueous NaOH, to maintain a constant pH value during the reaction. Under the applied mild conditions (T 50–90 °C, pH 9.5–11.5), yields of d-fructose of up to 31% were observed. Selectivity-conversion plots were not significantly influenced by variation of the temperature, pH value or substrate concentration. A reaction network for kinetic modelling includes d-glucose-d-fructose interconversion, co-production of d-mannose and d-allulose (also known as d-psicose) as well as decomposition paths after deprotonation of the hexoses. All four hexoses were employed as substrates in the isomerization. Thermodynamic ionization constants of the saccharides were measured by means of potentiometric titration. In the kinetic studies, pH-independent rate constants as well as activation energies were determined. The obtained kinetic and thermodynamic results as well as selectivity-conversion correlations present a useful benchmark for soluble and solid base catalysts.

Stannate-catalysed glucose-fructose isomerisation in alcohols

Isomerisation of glucose to fructose is a crucial step in the valorisation of biomass-derived carbohydrates to renewable chemicals, polymers and fuels. Glucose isomerisation is base-catalysed but superior catalytic activity can...

Revealing the contributions of homogeneous and heterogeneous catalysis to isomerization of d-glucose into d-fructose in the presence of basic salts with low solubility

Hydroxide anions are identified as catalytically active species for the isomerization of d-glucose to d-fructose over low soluble basic salts. The highest selectivity for d-fructose was obtained for catalysis by MgCO3.