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

L. BULJUBASICH

Shaker Verlag

Año: 2010 p. 136

978-3-8322-9126-6

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 ad is still expanding,  encompassing  chemical,  petrochemical,  biological  and  medical  research,  plant physiology, aerospace engineering, process engineering, industrial food processing, materials and polymer sciences. 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. The raw materials undergo a number of physical treatment steps to put them in the form in which they can be reacted chemically. Frequently, the chemical treatment step (typically a reaction or a series of reactions taking place inside a reactor) is the hearth of the process, which makes or breaks the process economically.  Heterogeneous Catalytic reactions play an important role in many industrial processes, such  as  the  production  of  methanol,  sulphuric  acid,  ammonia,  and  various  petrochemicals, polymers, paints and plastics. It is estimated that well over 50 % of all the chemicals 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 the local environment of the catalyst surface including concentration  and  distribution  of  reagents  and  of  possible  deactivating  substances  in  the vicinity of the active sites, as well as on the rate of molecular transport as influenced by the topology  and  pore  space  geometry.  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 in the presence of metal containing porous media as a catalyst, was studied as a model reaction:   2H2O2  (liquid) --> 2H2O+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  chemicals  available  today.  It  is 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. Due to the large number of applications involving hydrogen peroxide  decomposition  either  in  equilibrium  reaction  processes,  or  supported  by  catalysts, there is increasing interest in the development of techniques which permit monitoring those reactions.  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   rate,   together   with   pH,   to   obtain   a quantification  of  the  H2O2   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 the porous particle.  Possible extensions of the results and experiments to different kind of samples and set ups are suggested at the end of the work.