In situ solid-state NMR studies of catalytic reactions on zeolites and sulfated zirconia.

摘要

The surface acidic properties of sulfated zirconia catalyst were evaluated by multinuclear NMR using a number of NMR probe molecules. It has been demonstrated that sulfated zirconia is an inhomogeneous acid with one kind of Brønsted acid site and at least two kinds of Lewis sites. Most acid sites have acid strengths comparable to that of zeolites and a small portion of these acid sites possess acidity similar to that of 100% sulfuric acid. The strength of the Brønsted acid sites is stronger than that of Lewis sites. The types of Lewis sites were calcination temperature dependent. No Brønsted sites on zirconia surface were observed and the adsorption of water could not produce Brønsted acid sites on pure zirconia.; The basicity of sulfated zirconia and zirconia was also studied by NMR probe molecules. Both zirconia and sulfated zirconia are acid-base bi-functional solids. The oxidizing ability of sulfated zirconia is due to S6+ sulfur species. It can oxidize TMP to TMPO with a reduction product of SO ₂.; The SO₃ treatment of oxides produces solid Brønsted acids that are thermally stable. NMR probe molecule results show that SO ₃/zirconia and sulfated zirconia have very similar acidic properties. Both solid acids possess similar Brønsted and Lewis acid sites and similar average acidic strength. These similarities may imply a similarity in structure.; Surface ethyl sulfite and cyclopentenyl cation were observed as reaction intermediates during ethylene reactions on sulfated zirconia, and surface carbonate was observed during the decomposition of CCl₄ on sulfated zirconia.; Photochemical reactions in zeolite media were studied by 13C NMR. Photoproducts observed in the zeolite by NMR were identical to those found ex situ in previous studies, but this need not be the case in future work where acid digestion of the zeolite might cause degradation of the photoproducts.; A number of long-lived carbenium ions were detected and characterized as reaction intermediates by NMR spectroscopy. These carbenium ions formed on solid catalysts include the trityl cation, diphenylchlorocarbenium ion, diphenylbromocarbenium ion, and methyl-substituted cyclopentenyl cation. Reaction mechanisms are proposed based on in situ NMR observation.