Surface chemistry of H-ZSM5 studied by time-resolved IR spectroscopy

By means of IR and mass spectroscopy, the interactions of a series of frequently used probe molecules (water, benzene, ammonia, pyridine and propan-2-ol) with H-ZSM5 were studied over five decades of pressure (10^-51 mbar) to exemplify the surface chemistry at isolated strong Brönsted acid sites of zeolites. Equilibration with the adsorbate or dynamic changes of the adsorption system were followed by time-resolved IR spectroscopy. Thermal stability of the adsorbates were probed by temperature-programmed desorption/reaction. At partial pressures below 10^-3 mbar of the adsorbate/ reactant, interaction took place at strong Brönsted acid sites with high selectivity. It was found that the interaction of the probe molecules with the oxygen of the lattice was extremely weak. The stoichiometry of the adsorbate complexes and the role of the proton varied with the probe molecule used and the equilibrium pressure. All molecules tend to initially form 1:1 complexes with the adsorption sites. Benzene was concluded to be hydrogen bonded, whereas all other molecules investigated are protonated upon adsorption. With propan-2-ol, higher hydrocarbons were formed at the surface, the molecular weight depending subtly upon reaction temperature and reactant pressure. The specific adsorbate structures and the reaction intermediates are discussed in detail.