Nuclear Magnetic Resonance as a Tool for On-Line Catalytic Reaction Monitoring

Nuclear Magnetic Resonance (NMR) has become a well-established method in many different areas of research. The scope of the disciplines involved is extremely broad, but the power of NMR, lies in its ability to combine and extend the available techniques for a more thorough solution of problems which cannot be assigned to one of the popular categories. In the world of chemical engineering, every chemical process is designed to produce economically a desired product from a variety of starting materials through a succession of treatment steps. Frequently, the chemical treatment step (typically areaction taking place inside a reactor) is the hearth of the process, that makes or breaks the process economically. Heterogeneous Catalytic reactions play an important role in many industrial processes. It is estimated that well over 50 % of all the chemieals produced today are made with the use of catalysts. The rate constants of the heterogeneous catalytic reactions, and therefore the efficiency of the reaction, depends on several parameters. Therefore, the development and optimization of the catalyst becomes an important part of the design of a chemical process, and much effort and money are invested in that direction. The work presented here consists in a combination of different aspects of Nuclear Magnetic Resonance, focused on providing a reliable tool for the optimization of chemical processes, via the on-line monitoring of catalytic reactions. For that purpose, the following decomposition, was studied as a model reaction: 2H202 (liquid) -> + 2H20 + O2 (gas). The election of the decomposition of aqueous hydrogen peroxide solutions relies on two main reasons. Firstly, the fact that the reaction can be carried out in a simple laboratory glass tube, under room temperature and atmospheric pressure with occurrence of the gas phase in form of bubbles, makes it an excellent example of a simple liquid-gas reaction. Secondly, hydrogen peroxide has itself a huge importance as a chemical compound. It is one of the most versatile and environmentally desirable chemieals available today, used in a wide variety of industrial applications, from chemical synthesis to the treatment of pollutants, also including the synthesis of conjugated polymers and its use as a green propellant for space propulsion. The results presented along this work include: the feasibility of using the time dependence of the effective diffusion coefficient in the vicinity of the catalysts to monitor the decomposition with relatively high temporal resolution for several hours; the possibility of making use of the influence of two-site chemical exchange between protons in water and hydrogen peroxide on the transverse relaxation time, together with pH, to obtain a quantification of the H202 concentration during the reaction, in the liquid surrounding the catalyst; the use of the chemical exchange as a source of contrast in NMR Imaging, providing a tool for monitoring the reaction with spatial resolution inside a porous particle.