Photocatalytic degradation of β-blockers in TiO2 with metoprolol as model compound. Intermediates and total reaction mechanism

The photocatalytic degradation of beta-blockers in TiO2 was investigated. Metoprolol (MET) was selected as a model compound since its structure is quite similar to several compounds utilized as beta-blockers. MET degradation as a function of time was investigated by means of several analytical techniques. Analyzing the degradation mixtures by total organic carbon (TOC) analyzer and high-performance liquid chromatography (HPLC), it was evident that after one hour of irradiation, MET was transformed into other more recalcitrant intermediates. Latter intermediates were slowly degraded achieving 80% mineralization after four hours of irradiation. Gas chromatography-mass spectrometry (GC-MS) analysis of mixtures obtained upon irradiation at short reaction times (an hour or less) allowed the identification of several new intermediate organic compounds. Among those, several aromatic and low molecular weight amines. Polyhydroxylated compounds were also identified but only in trace amounts. Infrared (IR) and ultraviolet-visible (UV-vis) spectroscopy studies indicated that MET is first transformed into several hydroxylated aromatic compounds. Eventually, after several hours of irradiation, these compounds are transformed into low molecular weight unsaturated acids. HPLC analysis of reaction mixtures and coinjection of standard solutions allowed the detection of several intermediates, namely 1,4-hydroquinone, 4-(2-methoxyethyl)phenol, 2-(4-hydroxyphenyl)ethanol, 4-hydroxybenzandehyde, 1,2,4-benzenetriol, catechol and oxalic acid. Nuclear magnetic resonance (NMR) studies in deuterated solvents allowed the unequivocal detection of some of the aromatics in the reaction mixture. These NMR studies also demonstrated the transformation of aromatics into low molecular alkenes and acids. MET degradation routes and a detailed reaction mechanism is presented based on the intermediates identified. Fundamental aspects about MET degradation will be quite useful for future applications of photocatalysis in the degradation of other beta-blockers, pharmaceuticals and organic compounds with structures similar to MET.