Kinetic spectrophotometric method for real-time monitoring of ultraviolet photoreactions: A mini-photoreactor

摘要： Photoreactors in research laboratories generally consist of UV-C or UV-A mercury lamps and vessels of volume ranging up to about 1.0 L of solutions (suspensions, solvents, reactants and catalysts). This photoreactor’s design implies in large amounts of chemicals - cost problems, considerable amount of co-generated waste - environmental issues, safety problems and the need of large laboratory space. Currently, there is an effort by the scientific community to apply the principles of “green chemistry”, tending to miniaturize reactors and formats [1-8] and use modern sources of radiation such as light-emitting diodes (UV-LEDs), which are much more environmentally friendly than mercury lamps [9-13]. Another disadvantage of classical photochemical reactors is the withdrawal of aliquots at several times intervals. This procedure modifies the total volume of solutions and, consequently, the total number of absorbed photons per chromophore. Besides that, this methodology is time and labor consuming, and sometimes it is unsafe. In this way, a low amount of experimental points can limit the full kinetic description and lead to misinterpretations in kinetic parameters such as reaction order and rate constants. Accurate determinations are also limited by temperature control, which is not commonly effective in classical photoreactors. Here in it is proposed an interesting mini-photoreactor constituted by one UV-LED and a traditional quartz cuvette of 1.00 cm optical length that allows the reaction execution and, simultaneously, continuous monitoring via UV-Vis spectrophotometry (real-time kinetic analysis). In order to validate the use of this new mini-photoreactor we employed the photodegradation of methylene blue (MB) and salicylic acid (SA) in homogeneous and heterogeneous systems. For the proposition, it is necessary a specific spectrophotometer apparatus with a detection system that is not influenced by external light [14] and a Peltier system for controlling temperature with a mini-bar magnetic agitation. Continuous kinetic monitoring was performed in a spectrophotometer with a xenon flash lamp and silicon diode detectors, which emit flashes only during data collection, which prevents extra photodegradation for very photosensitive samples. Another spectrophotometer’s type that can be employed is that one based on diode-array system, whose external light also does not affect the spectrophotometric measurement.