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Heterogeneous catalysis of glucose mutarotation by alumina Dunstan, T. D.


The kinetics and mechanism of the heterogeneous catalysis of the mutarotation of glucose by alumina have been investigated. Various types of aluminas held in suspension, in dimethyl sulfoxide, were used. At 25.0°C, the first order kinetic plots for mutarotation by alumina neutral (for thin layer chromatography; y-form) were curved due, first, to relatively slow adsorption of glucose on alumina and, second, to progressive deactivation of the catalyst. Partially deactivated catalysts produced linear first order plots over three half lives and hence, glucose mutarotation by alumina is a first order reaction. Further, there were <1% side products formed during the surface reaction. Dehydration of the catalyst at low temperatures (i.e. upto 600°C) decreased the catalytic activity, unlike the other reactions studied on alumina surfaces. On further dehydration at higher temperatures the catalytic activity increased, and the activity per unit area of α-alumina (= 3.6 x 10⁻⁵ sec⁻¹ m⁻² ) formed at 1250°C was about 26 times that of the standard alumina neutral. High catalytic activity for the ct-form of alumina compared with the y-form was previously virtually unknown. Further, this a-alumina did not deactivate during catalysis and produced linear first order plots over three half lives. Adsorption studies showed the presence of (0.70 ± 0.02) x 10⁻⁴ moles of irreversible adsorption sites on the surface of a gram of alumina neutral. The isotherm for adsorption of glucose on alumina neutral showed only monolayer adsorption. The Langmuir plot for reversible adsorption of glucose sites; (1.0 ± 0.1) x 10⁻⁴ moles of strong adsorption sites with an equilibrium constant for adsorption = (8.2 ± 1.2) x 10² litre mole⁻¹ , and (1.4 ± 0.2) x 10⁻⁴ moles of weak adsorption sites with an equilibrium constant for adsorption = 44 ± 3 litre mole The study of the variation of initial rate with concentration of a-D-glucose showed that only the weak adsorption sites are catalytically active. Hence the active site density on alumina neutral was obtained as (5.4 ± 0.6) x 10⁻¹³ sites/cm². The turnover number of a catalytic site was determined to be 2 x 10⁻³ molecules/site/sec. This is one of the highest turnover numbers for a reaction catalyzed by an alumina surface. The observed first order rate constant for the surface reaction, [Figure 1] was shown to be k[sub=obs] = {(k₃ + k₄) [catalystic sites]}/{(k₂/k₁) + [Glucose]}, where k₁/k₂ = k₂ . The catalytic constant (k₃ + k₄) for the interconversion of α-D-glucose (G[sub=α]) and β-D-glucose (G[sub=β]) on the surface was determined to be 5 x 10⁻³ sec⁻¹. Comparison with the catalytic constant for mutarotation in pure water (= 4 x 10⁻⁴ sec⁻¹) showed that the alumina surface offers a better medium for mutarotation than water. Further, the activity of the catalytic sites on alumina neutral is about 9 times that of strong acids in water. The inhibitory effects of 'neutral' molecules (water, methanol, methyl glucoside, inositol etc.) indicate that the glucose adsorption sites on alumina neutral are relatively specific for adsorption of polyhydroxy compounds. In addition, aldehyde (e.g. hexanal) groups seem to interact preferentially with the catalytic sites on the alumina surface. Studies with acidic (carbon dioxide) and basic (e.g. pyridine, n-butylamine) inhibitor molecules suggest that the catalytic activity of alumina towards glucose mutarotation is due to the presence of basic oxide ions and weak Bronsted acid sites on the surface. About 85% of the activity of α-alumina formed at 1250°C is due to these basic sites, while the weak Bronsted acid sites give rise to about 90% of the activity of alumina neutral. The observed high catalytic activity of these weak Bronsted acid sites is probably due to the stabilization of the transition state leading to acyclic intermediate by the polar alumina surface. Normal deuterium isotope effects were observed with alumina neutral (k[sub=H]/K[sub=D] = 1.3) and the other aluminas prepared by dehydration of alumina neutral (e.g. k[sub=H]/K[sub=D] = 1.9) with α-alumina formed at 1250°C). There was no isotope effect on the adsorption-desorption process and hence, the observed isotope effect is due to the catalytic reaction on the surface. Therefore, the observed normal isotope effects indicate that glucose mutarotation on alumina surface is a general acid-base catalyzed reaction, and occurs by a consecutive mechanism, via the acyclic intermediate. There is probably no bifunctional catalysis of the glucose mutarotation on the alumina surface. Further, the acid sites seem to show an isotope effect of 1.2 and the basic sites an isotope effect of 2.1. Hence, these studies have shown that glucose mutarotation differs (e.g. higher catalytic activity of the hydrated surface, high catalytic activity of a-alumina) from many other reactions studied on alumina surfaces. This difference in behaviour under mild conditions is probably due to the high sensitivity of the mutarotation reaction to the weak acidic and basic sites on the alumina surface.

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