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?

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

“…Minor amounts of glycolic acid were also observed. 15,20 In addition, oxidation of enediol species with atmospheric oxygen cannot be excluded. We measured the initial rates of Fru formation to estimate its dependency on the concentration of OH − anions.…”

“…1 shows the simplest reaction network used to describe the reaction kinetics. 20,71 Based on this network, Kooyman et al proposed the following equation for the initial reaction rate of Fru formation: 71…”

“…15 Solid bases 16 and solid Lewis acids 17 exhibit comparable catalytic activity for the isomerization though the Lewis acids catalyze co-formation of D-mannose in somewhat higher amounts. A broad range of solid bases exhibit catalytic activity for the isomerization, which includes MgO 14,18,19 and other alkaline earth metal (hydr)oxides, 20 MgO-doped ordered mesoporous carbon, 21 Mg-Al hydrotalcites, 14,15,18,[22][23][24][25][26][27][28][29][30] Mg-or Ca-impregnated or exchanged zeolites, [31][32][33][34][35] attapulgite, 36,37 MgO-Nb phosphate, 38 Mg-doped carbon nitride, 39 Mg-containing titanosilicates, 40 CaO-ZrO 2 , 41 MgO-ZrO 2 , 42 MgO/biochar, 43 CaO-Al 2 O 3 , 44 CaO-MgO, 45 CaO/ C, 46 alkaline earth metal titanates, 47 soluble amines [48][49][50][51][52][53][54][55]…”

“…Our results suggest that OH − is generated in situ by partial dissolution of the materials as catalytically active species. 20 The oxides are readily transformed into hydroxides, and the latter release hydroxide anions according to eqn (1):…”

Stannate-catalysed glucose–fructose isomerisation in alcohols

“…4 Chemocatalytic glucose-fructose isomerisation (Scheme 1) traditionally involves the use of bases with hydroxide anions (OH − ) functioning as catalytically active species. 5 However, recently several promising directions for the development of heterogeneous glucose-fructose isomerisation catalysts and processes have been identified, where solid bases catalyse the isomerisation mainly heterogeneously with only a minor contribution of catalysis by OH − . 6,7 Importantly, tailoring of the structure of the catalyst along with the reaction conditionsespecially the utilisation of alcohols as solventsseems to be a requirement for high fructose yields.…”