C-13 NMR-STUDY OF ACETYLENE CYCLOTRIMERIZATION ON PLATINUM ALUMINA CATALYSTS

The reactions of acetylene on supported 10% Pt/alumina catalysts have been studied using variable temperature solid-state C-13 NMR with magic-angle spinning. In the vicinity of room temperature, high loadings of acetylene cyclotrimerize to form benzene. No obvious differences were observed between catalysts prepared from eta- or gamma-alumina. Benzene also forms from dehydrogenation of cyclohexane on these catalysts. At 123 K the motionally averaged C-13 shielding tensor is accounted for by a model involving rapid rotation about the C6 axis and libration of that axis with a root mean square angle of 33-degrees. This motion is independent of whether it is directly adsorbed or prepared from acetylene in situ. Quantitative adsorption of acetylene and spin-counting measurements revealed that an amount of C-13 corresponding to ca. 0.3-monolayer coverage of the platinum particles is invisible in C-13 MAS spectra, presumably due to susceptibility broadening. This observation extends the work of Slichter et al. to higher pressures and reconciles the conflicting conclusions of two previous NMR investigations of the system. The present report underscores the need to apply complementary NMR techniques to the study of adsorbates on metals with large susceptibility anisotropies and the need to control adsorbate loadings.