The effect of temperature in the reaction rate and yield of vanillin production from lignin oxidation is shown in Fig. 12.9d for the range 372-414 K: higher initial temperatures led to higher vanillin yields in a shorter reaction time; however, the vanillin degradation is also higher. In fact, for pH 14, the temperature has an important effect on the rate of vanillin degradation as shown in Fig. 12.10c: at 414 K, for 40 min of reaction time about 20% of the initial vanillin was consumed while at 393 K the decrease was only about 4%. This effect is even clearer for long reaction times.

The work of Mathias, Fargues and Rodrigues [36, 116, 118, 149] allowed calculating the activation energies (Ea) for the vanillin production and oxidation:

29.1 and 46.0 kJ/mol, respectively. The kinetic constant for vanillin production can be expressed as:

3502

k = 1.376 x 101exp — (l/mol)175min—1. (12.2)

For vanillin oxidation, the following kinetic law was found for pH >11.5:

— rv = k'[O2][Cv] (12.3)

with,

k’ = 4.356 x 106exp^ —^7—^ (l/mol. min). (12.4)

Other authors reported the kinetic laws and activation energy for vanillin and syringaldehyde production from hardwood lignin [112, 113]. Similar Ea values were found for vanillin and syringaldehyde, 70.5 and 62.6 kJ/mol, respectively [113]. However, the rate constant for syringaldehyde production is higher than that for vanillin, as depicted in the kinetic laws for syringaldehyde (Sy) and vanillin (v) [113]:

rsy = 1.3 x 105exp——— — [O2][OH—]L4[L] (12.5)

rv = 5.4 x 105exp ^ — 8479^ [O2][OH—]19[L] (12.6)