The role of superficial radicals in the kinetic modeling of 3-pyridinemethanol and 3-pyridinecarboxyaldehyde selective oxidation to vitamin B3 in water by means of a TiO2/Cu(II)/UV-solar photocatalytic system.

This study reports the kinetic modeling of a nicotinic acid (vitamin B3) production process in batch condition by means of selective UV-solar TiO2-photocatalytic oxidation of 3-pyridinemethanol (3-PMA) in an acidic aqueous solution, using Cu(II) ions as electron acceptors. Since 3-PMA does not adsorb onto the TiO2 surface, it cannot react directly with the photogenerated positive holes. The selective oxidation of 3-PMA proceeds through a reaction with the surface radicals ({≡TiIVOH·}+{≡TiIVOH·}+) generated from the migration of the positive holes on the TiO2 surface, where they are trapped by surface titanol groups ({≡TiIVOH}{≡TiIVOH}). A detailed reaction mechanism, which justifies the selectivity of the photocatalytic oxidation process of 3-PMA to 3-pyridinecarboxyaldehyde (3-PCA) and nicotinic acid (NA), by means of superficial hydroxyl radicals, has been reported. On these bases, a kinetic scheme that represents the whole process has been proposed and used for an optimization procedure which has allowed the values of eight unknown kinetic constants to be estimated by means of the Marquardt approach. Among the unknown parameters, the kinetic constants values of the reactions between {≡TiIVOH·}+{≡TiIVOH·}+ radicals and 3-PMA (1.76·104 ± 1.6·103 M−1 s−1) and 3-PCA (1.57·104 ± 1.4·103 M−1 s−1) have been estimated.