INSITU VARIABLE-TEMPERATURE MAS C-13 NMR-STUDY OF THE REACTIONS OF ISOBUTYLENE IN ZEOLITES HY AND HZSM-5

The low-temperature chemistry of isobutylene in zeolites HY and HZSM-5 was probed by in situ variable-temperature C-13 NMR spectroscopy with magic-angle spinning (MAS). Isobutylene interacts with Bronsted acid sites of the zeolites at a temperature of ca. 123 K to form a hydrogen-bonded species, as indicated by a 2-C-13 DELTA-delta value (relative to the 2-C-13 resonance of isobutylene) of 12 ppm in HY and a DELTA-delta value of 16 ppm in HZSM-5. The greater DELTA-delta value observed for hydrogen-bonded isobutylene in HZSM-5 is consistent with the acid sites in HZSM-5 being more acidic than those in HY. The maximum number of hydrogen-bonded species detected did not exceed the number of Bronsted acid sites in the zeolites. As the temperature was raised from 123 K, hydrogen-bonded isobutylene was presumably converted to an unobserved symmetrical intermediate (a tert-butylcarbenium ion) that either underwent deprotonation to re-form isobutylene and the acid site or reacted with another isobutylene molecule to form a dimer. Evidence for this process included scrambling between the 1- and 3-C-13 labels of isobutylene. In all of the spectra obtained by either cross polarization or single-pulse excitation, no peaks were observed that could be assigned to a surface-bound silyl ether species. However, in experiments performed on the 2-methyl-2-propanol-2-C-13/HZSM-5 system, the formation of a surface-bound tert-butyl silyl ether species was observed, as was reported earlier by another laboratory.