Rate-Controlling Mechanisms in the Photo-degradation of 5-Hydroxymethylfurfural

Rate-controlling mechanisms in the photo-degradation of 5-hydroxymethylfurfural (HMF) were studied applying kinetic and thermodynamic compensations. Aqueous solutions of HMF were prepared at a concentration of 100 mg L−1 and at pH values of 3, 3.4, 4, and 5. The UV irradiation of samples was performed in an installation consisting of a black chamber containing the reactor and a mid-pressure mercury lamp with emission wavelengths between 250 and 740 nm. Every sample was irradiated at 12, 25, 35, and 45 °C for 120 min, analyzing their HMF content each 10 min. The photo-degradation data fitted well to zero-order kinetic model, and the constant values were used to study whether the kinetic and thermodynamic compensation could be applied. The isokinetic temperature was very similar for kinetic compensation (TB = 278.0 K) and thermodynamic compensation (TB = 277.8 K). Applying the Leffler’s criterion, the HMF photo-degradation was entropically controlled, probably as a consequence of hydrophobic interactions. In order to check the entropical control, two experiments were repeated at pH 3 but avoiding agitation. As the new obtained kinetic constants were highly different from the values previously obtained using agitation, it can be concluded that the HMF photo-degradation is an entropy-controlled process and can be speeded up by changing non-thermal parameters, like agitation.