Experimental Techniques

To investigate the reaction mechanisms of the MTO process, several in-situ spectroscopic techniques were employed. “In-situ” is Latin for “in the place.” In Chemistry the term is often used to refer to the characterization of reacting species as the reactions are actually taking place. Spectroscopy is the study of the interaction of radiation with matter and can be used to deduce certain structural aspects of chemical species.

In-situ spectroscopic methods

UV/Visible spectroscopy was used to detect aromatic compounds, which strongly absorb visible and ultraviolet radiation. In UV/Vis spectroscopy, white light is applied to a sample and the wavelength and intensity of the reflected light are measured to obtain a spectrum. This spectrum can then be used to characterize and identify chemical species.

Magic Angle Spinning (MAS) is a technique used in the Nuclear Magnetic Resonance (NMR) spectroscopy of solids in which the sample is rotated at a high frequency (ω) and tilted at an angle (theta) relative to a magnetic field (B0), as illustrated in the diagram below. The structure of species adsorbed to the catalyst surface can be characterized using NMR spectra, in which signals are detected for chemically distinct atoms of a given isotope. The main isotope employed in the summary by Wang et. al. was 13C.



The reactions were carried out in a microreactor equipped for in-situ MAS NMR and UV/Vis spectroscopy. The reactants were introduced through an injector and into an annular region packed with catalyst. A diagram of the experimental apparatus is shown below. The reactor was rotated at 3 to 10 kHz for MAS spectroscopy. A quartz window at the end of the reactor allowed monitoring by UV/Vis spectroscopy. The product stream flowed out through the top of the annular region and was monitored via gas chromatography.


Diagram of the MAS-NMR/UV-Vis microreactor as given by Wang et. al.