|
Glucose is used in
pharmaceutical applications as a filler and binder for
tablets and sweetening agents. The thermal decomposition is
investigated by means of thermogravimetric
analysis.
Conditions of Measurements
|
Instrument
|
NETZSCH TG
439
|
|
Sample
mass/mg
|
2.7...
2.9
|
|
Heating
rate/(K/min)
|
1,2,5,10
|
|
Atmosphere
|
Argon
|
|
Gas flow
rate/(ml/min)
|
20
|
Thermal decomposition
of Glucose
The first decomposition step produces mainly water. The
mass loss of this step is independent from the heating rate.
The mass loss of the second step, 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 process (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.
Model-free analysis
according to Ozawa-Flynn-Wall
The energy-plot indicates for the first step an activation
energy of approximately 110 kJ/mol and for the dominant path
of the second step an activation energy of 180
kJ/mol.
Results
of model fit
Model-fit of Thermal
Decomposition of Glucose
The triple-step reaction, containing competitive reaction
paths, allows a high quality fit of all TGA
measurements.
Kinetic Parameters
as Result of Nonlinear Regression
# |
Parameter |
Value |
Standard
Dev. |
0 |
lg
A1/s^-1 |
10.60 |
0.31 |
1 |
Act.Energy
1/(kJ/mol) |
122.83 |
2.75 |
2 |
React.order
1 |
1.58 |
0.16 |
|
|
|
|
3 |
lg
A2/s^-1 |
14.42 |
0.74 |
4 |
Act.Energy
2/(kJ/mol) |
182.14 |
7.92 |
5 |
React.order
2 |
1.80 |
0.05 |
|
|
|
|
6 |
lg
A3/s^-1 |
- 3.54 |
0.72 |
7 |
Act.Energy
3/(kJ/mol) |
5.44 |
7.88 |
8 |
React.order
3 |
1.20 |
0.68 |
|
|
|
|
9 |
Follow.React.1 |
0.27 |
0.012 |
10 |
CompReact.
2 |
1.00 |
constant |
11 |
CompReact.
3 |
0.03 |
0.06 |
|
|
|
|
12 |
MassLoss
1 |
84.01 |
0.35 |
13 |
MassLoss
2 |
84.01 |
equal
12 |
14 |
MassLoss
3 |
84.01 |
equal
12 |
15 |
MassLoss
4 |
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.
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