Thermal Decomposition of Glucose

Glucose is used in pharmaceutical applications as a filler and binder for tablets and sweetening agents. The thermal decomposition of Glucose is investigated by means of thermogravimetric analysis.

Conditions of Measurements
InstrumentNETZSCH TG 209
Sample mass/mg2.7 ... 2.9
heating rate/(K/min)1,2,5,10
AtmosphereArgon
Gas flow rate/(ml/min)20

Thermal decomposition of Glucose
The first thermal decomposition step produces mainly water. The mass loss of this step is independent from the heating rate. The mass loss of the second thermal decompostiton step of Glucose, however, is strongly dependent on the heating rate. This is sufficient proof that after the first step the later decomposition follows at least two competing reaction paths. 
Already now a proposal can be given for the kinetic model of the whole thermal decomposition process of Glucose (see result of model fit).

It should be emphasized once again that this model has a more formal character. In particular, no information on the proceeding chemistry can be given from thermogravimetric data only. Considerably more information would be obtained by coupling thermogravimetry with mass spectroscopy.

Fig.1. Thermal decomposition of glucose: final mass depends on the heating rate

Model-free analysis according to Ozawa-Flynn-Wall
The energy-plot indicates for the first thermal decomposition step of Glucose an activation energy of approximately 110 kJ/mol and for the dominant path of the second step an activation energy of 180 kJ/mol.

Fig.2. Friedman energy plod for decomposition of glucose. Error bars are shown for activation energy

Results of model fit

Model-fit of Thermal Decomposition of Glucose
The triple-step reaction of the thermal decomposition of Glucose, containing competitive reaction paths, allows a high quality fit of all TGA measurements. 

Fig.3. Measured data(symbols) and Kinetic model (solid lines) for decomposition of glucose.

Kinetic Parameters as Result of Nonlinear Regression

#ParameterValueStandard Dev.
0

lg A1/s^-1

10.600.31
1

Act.Energy 1/(kJ/mol)

122.832.75
2

React.order 1

1.580.16
3

lg A2/s^-1

14.410.74

4

Act.Energy 2/(kJ/mol)182.147.92

5

React.order 21.800.05

6

lg A3/s^-1- 3.540.72

7

Act.Energy 3/(kJ/mol)5.447.88

8

React.order 3

1.200.68
9

Follow.React.1

0.270.012
10

CompReact. 2

1.00constant
11

CompReact. 3

0.030.06
12

MassLoss1

84.010.35
13

MassLoss2

84.01 equal 12
14

MassLoss3

84.01 equal 12
15

MassLoss4

84.01 equal 12
Correlation Coeff.0.99973

The branched reaction path results in a specific behavior: by changing of the reaction temperature the parts of product C and D are varied.

At temperature of 210 ¡C the largest part of B is transformed to the product D. Inversely, at a temperature of 280 ¡C the largest part is transformed to the product C. This behavior is caused by the large difference of the activation energies for step 2 and step 3, respectively.