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EXPERIMENTAL AND THEORETICAL STUDIES ON GLUCOSE

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EXPERIMENTAL AND THEORETICAL STUDIES ON GLUCOSE Powered By Docstoc
					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)
                            INTRODUCTION

 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
        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
Experimental conditions used in the hydrogenation reactions

 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



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

             Reactor: 100 ml parr reactor

             Reactant: 50% glucose solution
             Product
             analysis: HPLC
                                                                      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
The most stable conformations of the glucose molecule and their       to be oriented in a way to yield a cooperative
respective heat of formation calculated with the semi-empirical AM1   hydrogen bonding chain that is as efficient as
method
                                                                      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




                                 Geometric parameters for the metallic clusters




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).
EADS= adsorption energy
EGlucose/Cluster= energy of glucose adsorbed on the metallic cluster

Eglucose
Ecluster   =energies of the glucose molecule and of the cluster
             individually, meaning at infinite separation

  The 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.

				
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