EXPERIMENTAL AND THEORETICAL STUDIES ON GLUCOSE HYDROGENATION TO PRODUCE SORBITOL

M.Banu (26-09-2009)

Marcia C. Martins Castoldi, React.Kinet.Catal.Lett. Vol. 91, No. 2, 341−352 (2007)

Sorbitol and mannitol are highly important chemical compounds as they can be used in several industrial applications.

Approximately 60% of the available sorbitol is used as additive in foods, medicines, cosmetics and toothpastes.

The industrial production of sorbitol and mannitol consists in the catalytic hydrogenation of sucrose, glucose or fructose

The Raney-Ni type catalyst has been employed in this process due to its high activity and low cost

However, ruthenium catalysts present higher activity than nickel in the hydrogenation of glucose in aqueous solutions

INTRODUCTION

Mechanism for glucose and fructose hydrogenation

The surface of the catalyst may possess two types of sites, an acidic (the metal) and a basic one (the support), it may be rationalized that the saccharide adsorbs on the acidic sites through the C=O bond while the dissociative adsorption of hydrogen occurs in the basic sites

The interaction between the adsorbed composites leads to the final product sorbitol.

Basically, the monosaccharide family may consist of polyhydroxy aldehydes or ketones, i.e., aldoses or ketoses.

In solution these compounds cyclize to produce five- and/or six- membered rings (furanoses and pyranoses, respectively), which are much more stable than their open chain counterparts

Depending on the position of the OH group at the C1 atom, there are two stereochemical species (anomers) for a pyranose or furanose. The anomers are termed α and β when the OH group at C1 is below or above the ring plane of the Haworth formula, respectively. Each of them having characteristic hydrogenation rates

Based on those rates, it can be determined which anomer is preferentially adsorbed and hydrogenated With this goal, the joint utilization of experimental and theoretical methods provides feasible tools to study the structural and molecular properties in chemical systems.

Quantum calculations

In the first stage of the quantum study a conformational analysis of the α and β anomers of glucose with the semi-empirical AM1 (Austin model 1) method was carried out

The most stable confirmation in each case was re-optimised using the DFT methodology with the B3LYP functional and the D95V basis set

In the second stage the optimized geometry of the most stable α anomer, was then adsorbed on a metallic cluster of Ru4 and Pt4

The metallic clusters, in a planar arrangement, were maintained fix at their initial arrangement, with Ru-Ru distances of 2.70A and Pt-Pt distances of 2.77A while the sugar structure was fully optimized

For calculation of the sugar-metal interaction the D95V basis set for the sugar and the LANL2DZ pseudopotential for the metal were employed

The interaction energy is calculated as the difference between the energy of the sugar-metal complex, and the energy of the sugar and the metallic clusters calculated at infinite separation

Catalyst Temperature(0C) Pressure(atm) Conversion(%)

10% Pt/C 110 7 3.0

5% Ru/C 110 7 8.0

5% Ru/C 100 80bar 99

10%Ru-Pt/C 100 80bar 15

Experimental conditions used in the hydrogenation reactions

Catalyst: Pt/C, Ru/C, Ru-Pt/C

Reactor: 100 ml parr reactor

Reactant: 50% glucose solutionProduct analysis: HPLC

The most stable conformations of the glucose molecule and theirrespective heat of formation calculated with the semi-empirical AM1 method

Pyranose structures are more stable than the corresponding furanose ones, since pyranoses have been found to predominate at equilibrium conditions in solution.

Da Silva et al. concluded that glucose occurs in aqueous solution with more than 99% as a six-membered pyranosic ring.

Once the α-pyranose anomer was identified as being the most stable form, it was taken for the interaction studies with the metallic clusters.

However, in order to eliminate any deficiency of the semi-empirical AM1 method, the two most stable conformations of the α-pyranose anomer were re-optimized at the B3LYP/D95V level.

The essential difference between these two conformations is that in one of them the hydroxyl groups are oriented clockwise, while, in the other one, they are oriented in a counter-clockwise way.

For the isolated molecule, the hydroxyls prefer to be oriented in a way to yield a cooperative hydrogen bonding chain that is as efficient as possible.

For a glucopyranose the counterclockwise conformation is 0.87 kcal/mol more stable than the corresponding clockwise conformation.

Interaction studies with the metallic surfaces

In the first stage the calculations were carried out for a fixed geometry of the M4 clusters (M = Ru or Pt). Changing the multiplicity of the metallic clusters allows us to determine its electronic state of lower energy.

The data indicate that the lowest energy state for the Pt4 cluster is that with multiplicity 5 (S=2), while for the Ru4 cluster the lowest energy state is obtained with multiplicity 13 (S=6).

Geometric parameters for the metallic clusters

EADS= adsorption energyEGlucose/Cluster= energy of glucose adsorbed on the metallic cluster

Eglucose

Ecluster =energies of the glucose molecule and of the cluster individually, meaning at infinite separationThe adsorption energy of glucose on Ru4 is 12.4 kcal/mol The adsorption energy of glucose on Pt4 is 18.3 kcal/mol

The parameter more intimately related to the efficiency of a catalyst should therefore be its capacity to promote changes in the geometry, which may at the end lead to a reduction in the activation barrier for the reaction in the rate determining step.

In this way the geometric changes that occur upon glucose adsorption may have stronger influence on the reaction profile, especially if changes in bond lengths close to the contact point with the metal are observed.

In this respect changes in some C-O bond lengths are noteworthy. The C2-O1 bond in the isolated glucose molecule has a length of 1.44 Ǻ. It increases to 1.47 Ǻ after adsorption, a clear indication that the process of adsorption weakens this bond, consequently reducing the energy necessary to break it.

These results show that by adsorption the anomeric carbon becomes more susceptible to attack by hydrides.

For the case of platinum, small changes in the bonds length of the glucose molecule had been observed, but these changes were not significant as compared with the changes promoted by the ruthenium cluster.

CONCLUSIONS

The conformational analysis of the glucose molecule shows that the α-pyranose anomer is the most stable one.

After optimization, the most stable conformation is that with the hydroxyl groups oriented counter-clockwise.

This is 3.88 kcal/mol more stable than the corresponding anomer in the clockwise orientation.

The admixture of ruthenium and platinum catalyst supported on carbon (Ru-Pt/C) presents conversion lower than 50% showing that platinum reduces the performance of ruthenium.

The calculations show that glucose adsorbs more intensely on a Pt4 cluster than on Ru4. However, geometric changes observed after adsorption on Ru4 indicate that it may promotes the break of the C2-O1 bond, thereby, facilitating the attack by hydrides.