Adsorption/desorption of substituted pyridines as probe molecules for surface acidities studied by Fourier transform infrared spectroscopy

摘要

We determined what kind of original information (in comparison with the classical pyridine adsorption) could bring substituted pyridines (2,6-dimethyl pyridine called DMP, and 2,6-di-tert-butyl pyridine called DTBP) as probe molecules for surface acidities studied by Fourier-transform infrared spectroscopy in our industrial research laboratory. The three probe molecules were adsorbed on pure compound pellets and studied by transmission in a vacuum cell with variable temperature placed in the sample compartment of a NICOLET 7199 FTIR spectrometer. By performing single and competitive adsorptions, and desorptions at increasing temperatures for the three probe molecules on various compounds ($gamma and flash alumina), we obtained a very useful tool. After an important work of spectral attribution, the main observations were: (1) Because of its stronger basicity, DMP detects Broensted (proton donor sites unseen by pyridine (phenomenon ruled by basicity). (2) DMP can also detect simultaneously Lewis (electron acceptor) sites on which it can be displaced by pyridine, which then recovers all its Lewis sites. This phenomenon has been interpreted as being ruled by steric effects. (3) DTBP allowed us to differentiate two types of Lewis sites on $gamma and flash alumina: (a) a first type of acid sites due to electron-deficient oxygens on which DTBP molecules can be chemisorbed and can be displaced by pyridine (type I), and (b) a second type of acid sites only accessible to pyridine through the Al cation (type II). Pyridine, DMP, and DTBP adsorptions, with the help of variable temperature desorptions and also competitive adsorptions, studied by transmission Fourier-transform infrared spectroscopy were then shown to constitute a very informative tool for acid surface expertise in our industrial research laboratory because of chemisorption phenomena ruled by basicity and steric hindrance.